FERROELECTRIC PROPERTIES AND PHASE STABILITY IN POTASSIUM NITRATE-POLYMER COMPOSITE FILMS

Transcription

1 FERROELECTRIC PROPERTIES AND PHASE STABILITY IN POTASSIUM NITRATE-POLYMER COMPOSITE FILMS A THESIS Submitted in fulfilment of the requirements for the award of the degree of DOCTOR OF PHILOSOPHY In PHYSICS By NEERAJ KUMAR DEPARTMENT OF PHYSICS INDIAN INSTITUTE OF TECHNOLOGY ROORKEE ROORKEE (INDIA) JUNE, 2005

2 INDIAN INSTITUTE OF TECHNOLOGY ROORKEE CANDIDATE'S DECLARATION I hereby certify that the work, which is being presented in the thesis, entitled "FERROELECTRIC PROPERTIES AND PHASE STABILITY IN POTASSIUM NITRATE POLYMER COMPOSITE FILMS" in fulfillment of the requirements for the award of the Degree of Doctor of Philosophy and submitted in the Department of Physics of the Institute, is an authentic record of my own work carried out during the period from January, 2000 to June, 2005 under the supervision of Dr. Rabinder Nath. The matter embodied in this thesis has not been submitted by me for the award of any other degree in this institute or any other Institute / University. (NEERAJ KUMAR) This is to certify that the above statement made by the candidate is correct to the best of my knowledge and belief. Date: till Dr. Rabinder Nath Professor (Supervisor) Department of Physics Indian Institute of Technology, Roorkee The Ph.D. Viva-Voce examination of Mr. NEERAJ KUMAR, Research Scholar has been held on 0 C. at /0.9 a avzia r co6( Signature of 41S'uperviso r (s) Signat Idir) e of H.O.D. Signature of External Examiner (0

3 ABSTRACT Ferroelectric materials have attracted an immense interest due to their vital role in electronic industry with their wide variety of applications as ferroelectric devices. Many ferroelectric materials, such as, barium titanate (BaTiO3), barium strontium titanate (BST), potassium-di hydrogen phosphate (KDP), potassium nitrate (KNO3), Lithium niobate (LiNb03), lead zirconate titanate (PZT), strontium bismuth tantalate (SBT), strontium barium niobate (SBN) and tri-glycine sulphate (TGS) etc, have been studied in a variety of forms like, single crystal, pellet and thin film for the use of these materials in memory devices operated at low voltage. Usually the ferroelectric materials has very high coercive field ( few kv/cm). Therefore it is necessary to fabricate these materials in thin film form. These ferroelectric materials are usually brittle and show difficulties for fabrication in thin film form. In the recent past, thin films of many ferroelectric materials have been fabricated by various methods viz., sputtering, pulse laser deposition (PLD), pulse laser ablation (PLA), metal organic chemical vapor deposition (MOCVD), chemical solution deposition (CSD), metal organic decomposition (MOD), molecular beam epitaxy (MBE) and sol-gel. Another advantage of obtaining ferroelectrics in thin film form is the possibilities of its integrate to the CMOS. This has been exploited to fabricate dynamic random access memories (DRAMs), nonvolatile random access memories (NV-RAMs) and ferroelectric random access memories (FRAMs). Thin film transistors such as metal ferroelectric semiconductor field effect transistor (MFSFET) has also been fabricated and studied. There are many other applications of thin film ferroelectric materials as voltage tunable dielectric capacitor in resonators and filters. Also recently thin film ferroelectrics arc finding increasing use in micro

4 electronic mechanical systems (MEMS) in producing highly sensitive actuators and sensors with low noise. The polymer-ferroelectric composite materials are emerging as a new class of electronic and dielectric materials. The composite materials for piezoelectric and pyroelectric application have gained enormous attention in the past because of their potential to be produced with desirable properties by choosing a proper combination of the constitutes phases. The ceramic-polymer composite with ferroelectric properties combines many useful properties of the polymer such as flexibility of the polymer and electroactivity of the ceramic to produce large area device with reasonable mechanical strength. Furthermore, the composites made of electroactive ceramics and polymers are suitable for many applications since they can be easily prepared in a variety of shapes. The composite materials have been processed using mixtures of ceramic as filler in a matrix of polymer, where the fillers are included in the matrix in order to modify its physical properties in a high range. The ceramic fillers such as PZT, BaTiO3, PbTiO3 in polyvinylidene fluoride (PVDF) with different doping have been produced in the past. The composites in the film form are becoming important from the application point of view in large area devices operated at low voltages. The work in the thesis has been focused on the composite films of potassium nitrate (KNO3) with different polymers such as polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), and polyvinyl alcohol (PVA). Potassium nitrate (KNO3) is well known to exist in three phases at atmospheric pressure. It exists in phase II in orthorhombic crystal structure at room temperature. The phases III (rhombohedral crystal structure) of KNO3 is ferroelectric and exist in the temperature range C upon cooling. Several attempts have been made to obtain ferroelectric phase

5 III at room temperature, either by applying hydrostatic pressure or by mixing the KNO3 powder with carbon black, SiC or starch. The stabilization of the ferroelectric phase III in thin films of KNO3 at room temperature has also been reported and was attributed to the existence of surface field effects. The work presented in thesis is an attempt to study to ferroelectric phase stability in KNO3 - polymer composite films at room temperature using DSC, Raman and X-ray studies. Further the ferroelectric switching and dielectric studies have also been included in the thesis. The work presented in the thesis can be broadly outlined into various chapters as given below. Chapter I presents the introduction part of the thesis and contains the outline of the concepts needed to understand the work and survey of literature on the chosen topic of the thesis. The current trends in research on the ferroelectric composite materials has been given. The scope of the present work and its motivation are also included. The general discussion on the potential application of the composite films in various devices and particularly in memory devices is presented. The material properties of KNO3 and PVDF and other polymers are also given. Chapter II contains the experimental details of the sample preparation and the various techniques of measurements employed during this study. The fabrication and design of the experimental set-ups for measuring different electrical properties such as ferroelectric hysteresis loop characteristics, polarization switching and electrical conductivity have also been included in this chapter. The details of vacuum deposition of electrodes onto the composite films are also described. Chapter III has been devoted to the Structural and thermal Studies and ferroelectric phase stabilization in the composite films. X-rays diffraction experiments have been performed to obtain the structural information and evaluation of the presence of phases at different doping levels. The X-ray diffraction measurements (iv)

6 have been useful in quantifying the crystalline phases in the composite films. The X- ray diffraction scans have been observed in different weight percent composition of the composite sample. The diffraction peaks of phase III in different composition composite films were found to occur at 20 = 29.8 and that of phase II occurred within 20 = The relative intensity of the phase III and phase II was estimated. The relative intensity ratio exhibits maximum around 50 wt. % of KNO3 and drops down for higher percentage of KNO3 in the composite. The X-rays diffraction scan of pure KNO3 in pellet form was also taken and analyzed. Raman studies of the composites at the room temperature have been included and analyzed. The DSC thermograms of different compositions of KNO3: PVDF composite samples were observed for heating and cooling cycles. The peaks have been identified and phase transition enthalpies were measured. The changes in the DSC scans of the composites have been compared with the pure components. The SOwt.% composite samples provide the best phase stability for the phase ferroelectric III. This chapter contains the full analysis and correlation of the x-ray and the DSC results. Chapter IV presents the ferroelectric and dielectric characteristics observed in the composite films under variety of conditions and optimized composition has been determined with respect to the remanent polarization (Pr). The ferroelectric polarization versus voltage (P-V) and polarization current density versus voltage (J-V) characteristics were simultaneously investigated at room temperature using 50 Hz sinusoidal signal for the composite films of different wt.% KNO3 composition. The hysteresis loop of SOwt.% KNO3 shows the largest value of remanent polarization. The dependence of remanent polarization upon KNO3 weight percent in the composite was also investigated. The ferroelectric hysteresis loop characteristics were also investigated in 50 wt.% composite films of other polymers like PVF and PVA. (v)

7 The dependence of remanent polarization on the number of reversal cycles was also tested in 50wt.cY0 sample. The dependence of the dielectric constant on temperature during heating and cooling cycles measured at frequency of 100 khz. The dielectric constant dependence upon temperature in the composite film shows the transition from the paraelectric to the ferroelectric phase. The dielectric constant shows two-fold increase in the temperature range 134 C C as compared to the room temperature values. This corresponds to the transition of phase II to phase I with the transition starting at 134 C during heating cycle. During the cooling cycle, the dielectric constant shows sharp change in the temperature range 120 C 104 C which is attributed to the phase change from phase I + phase III. Therefore, the dielectric variation in the temperature range 120 C *104 C in the cooling mode can be related to the phase change I III. Studies of the switching current transient characteristics are discussed in details describe in chapter V. The switching current response was studied using square wave signal and Ishibashi and Takagi theory was applied to the switching current transients. The needle like domain growth is believed to be involved in the switching process of the composite films of PVDF: KNO3. The effective of voltage and doping percent of KNO3 (by weight) on switching transients has also been investigated. The switching current characteristics were also observed in the 10, 50 and 80 wt.% composite films of PVDF: KNO3. The occurrence of switching current peak supports the presence of ferroelectric polarization, which corroborates the existence of ferroelectric phase III in the composite films. However, the switching current characteristics in the composite films of 50wt.% of PVF:KNO3 were also studied using triangular voltage sweep. The existence of switching current pulse supports ferroelectric nature of the composite films. (vi)

8 Chapter VI deals with the evaluation of electrical properties in the composite films. The results of various electrical measurements viz., capacitance-voltage (C-V), conductance voltage (G-V), and current- voltage (I-V) are included in this chapter. The temperature dependence of electrical conductivity has been discussed in heating and cooling cycles. The de conductivity variation with temperature has been found to provide knowledge of the phase transition in the composite films. The activation energies calculated for different phases during heating and cooling modes in the composite films are lower than observed by other workers in pure KNO3. The observed C-V characteristics show the existence of ferroelectric phase in the composite films. The sharp features of C-V curve can be attributed to the switching of ferroelectric polarization. The contribution of the space charge effects has been discussed in ferroelectric C-V characteristics. The strong bias dependence of capacitance of composite films may be exploited to be used as tunable dielectric material in high frequency ferroelectric varactors. The conductance versus bias voltage (G-V) characteristics were also measured along with the C-V characteristics. These curves clearly show the butterfly shape of G-V curves as observed in other ferroelectric materials. The space charge peaks of C-V characteristics are not seen in the G-V curves. Chapter VII contains an overall summary, correlations, and conclusions arrived at from various studies like, structural, thermal, ferroelectric, dielectric and switching transients in the composite films.

9 LIST OF PUBLICATIONS A part of the studies presented and referred in this thesis has also resulted in the following research papers: 1. Neeraj Kumar and R.Nath, "Ferroelectric Polarization Switching in KNO3 : PVDF Films", J. Phys. D: Appl. Phys., Vol. 36, pp (2003). 2. Neeraj Kumar and R. Nath "Ferroelectric Phase Stability Studies in Potassium Nitrate: Polyvinylidene Fluoride Composite Films" J. Appl. Phys., Vol., 97, pp (1)- (6) (2005). 3. Neeraj Kumar and R. Nath "Ferroelectric Properties of Polyvinyl Alcohol: Potassium Nitrate Composite Films", "Ferroelectrics" Journal (In Press) 4. Neeraj Kumar and R.Nath "Thermal and Ferroelectric Properties of Potassium Nitrate: Polyvinyl Fluoride Composite Films", IEEE Trans. Dielctr. Electr. Insul. (Accepted). PRESENTATION AND PARTICIPATION IN CONFERENCES 1. National Conference on Sensor Technology, Sept 26-27, 2002 CEES, Defence Research & Development Organization (DRDO), Ministry of DefenCe, Metcalfe House, Delhi International Conference on Smart Materials, Structure & System, Dec 12-14, 2002, in J.N.Tata Auditorium at Indian Institute of Science (I.I.Sc.) Bangalore Twelfth National Seminar on Ferroelectrics and Dielectrics (NSFD-XII), Dec 16-18, 2002 in Material Science Centre at Indian Institute of Science (I.I.Sc.) Bangalore th Asian Meetings on Ferroelectrics (AMF-4), Dec 12-15, 2003 in J.N.Tata Auditorium at Indian Institute of Science (I.I.Sc.) Bangalore Thirteenth National Seminar on Ferroelectrics and Dielectrics (NSFD-XIII), Nov 23-25, 2004 in Department of Physics and Astrophysics at University of Delhi.

10 ACKNOWLEDGEMENT The author wishes to express his profound and sincere gratitude to Dr. Rabinder Nath, Professor, Department of Physics, Indian Institute of Technology Roorkee, Roorkee for their expert guidance, invaluable advice, constructive criticism, valuable suggestions and keen interest throughout the period of this research work. In fact, words would fail to describe the invaluable help and the unending encouragement which the author was highly privileged to receive from them on many occasions. The author has his deep sense of gratitude to Dr. J. Rai and Dr. I. S. Tyagi, the present and Ex Heads Department of Physics, for providing me all the necessary facilities required for the accomplishment of this work. I would like to express my sincere thanks to Dr. (Mrs.) Tara Kaura for their invaluable, generous encouragement and moral support during the entire work. The author also acknowledges his deep sense of gratitude to all faculty members of Physics Department due to their continuous inspiration and timely encouragement. The author is highly obliged and wishes to express his sincere thanks to the technical and official staff of the Department of Physics. The direct and indirect help and the cooperation rendered by the faculty members and technical staff of the Institute Instrumentation Center (IIC), I.I.T., Roorkee, National Physical Laboratory (NPL), New Delhi, and Indian Institute of Science (IISc.), Bangalore throughout the period of the experimental facilities is duly acknowledged.

11 It is a pleasure to acknowledge the support extended by Dr. N. K. God, Prof. Department of Hydrology, Dr. R.P. Maheshwari, Department of Electrical Engineering of this Institute, and Dr. A.S. Ahlawat Scientist and Director (Control Systems), ISRO, Bangalore. The author also wants to express his sincere thanks to all those who directly or indirectly helped him at various stages of his work. The constant encouragement, appreciation and timely help from many of my friends and colleagues is also unforgettable. Last, but not the least, the author would like to express his profound gratitude and sincere sentiments to his parents, his wife, his son "Aashutosh", his brother, his sister, and his jeeja ji for their constant encouragement, whole hearted moral support and never-ending endurance, which boosted the author's morale and inspired his motivation for the hard work and the painstaking patience, without which the work presented herein, would not have been completed. The financial support provided by Indian Institute of Technology, Roorkee (Formerly University of Roorkee, Roorkee) is gratefully acknowledged. DATE ),IreS (NEERAJ KUMAR) (x)

12 LIST OF FIGURES Figure No. Title Page No. FIGURES FOR CHAPTER-I 1.1(a) Shows schematic of the different phases of KNO (b) Crystal structure changes when KNO3 was passed through the heating and cooling cycles Phase diagram of KNO3 as a function of pressure and 5 temperature measured by Bridgman (1916). 1.3 Rhombohedral unit cell in the phase - III of KNO DSC signals of KNO3, both for heating and cooling 8 procedures. 1.5 DSC curve of KNO3 containing carbon black for heating and cooling cycles. 1.6 Variation of linear thermal expansion coefficient with 13 temperature of 40 }im thick KNO3 film. 1.7 Show the room temperature diffraction pattern from a 14 powder specimen of KNO3.The vertical bars indicate the expected peak positions and relative intensities for reflections from KNO Shape variation of the (003) reflection for III-KNO The temperature dependence of the dielectric constant 18 measured at 100 khz in KNO DC conductivity versus 1000/T plot of KNO3 crystal Typical switching transient currents in ferroelectrics Shape of reversed domains and a dimensionality. (a), (b) and 25 (c) correspond to the one, two and three dimensional cases, respectively.

13 FIGURES FOR CHAPTER-II Figure No. Title Page No. 2.1 Flow chart of the fabrication steps involved in making of the 41 composite films. 2.2 Melt-Press machine for ferroelectric composite films Shows the lay out of the vacuum deposition apparatus Measurement cell with sample holder for composite films Circuit diagram for measurement of P-V characteristics 46 [switch position "b"] and J-V characteristics [switch position 2.6 Typical ferroelectric hysteresis loop of the composite films Typical butter-fly loop for 3-V characteristics of the 47 composite films. 2.8 Ferroelectric polarization switching measuring circuit Typical switching pulse of the composite films using square 49 wave signals. FIGURES FOR CHAPTER-III 3.1 Shows the x-ray diffraction pattern of pure KNO Shows the x-ray diffraction pattern of pure PVDF films Shows the x-ray diffraction scans of 10, 30, 40, 70, and wt.% KNO3 composite films. 3.4 Shows the x-ray diffraction scans of SOwt. % composite films Expanded x-ray diffraction scans of pure KNO3, pure PVDF, 56 and different wt.% composite films. 3.6 Peak intensity ratio [phase III ( ) /phase II 57 ( )] versus KNO3 weight percent. 3.7 Crystallite size at of reflection (003) versus doping 58 percentage of KNO3 in PVDF. 3.8 X-ray diffraction scans of pure PVF and 50wt.% KNO3 : PVF 59 composite films.

14 Figure No. Title Page No. 3.9 X-rays diffraction scans of pure PVA film and composite 60 (KNO3: PVA) film Endotherm DSC scans pure KNO3, pure PVDF and different 62 wt.% composite film Exotherm DSC scans pure KNO3, pure PVDF and different 64 wt.% composite film Transition enthalpy ih versus wt.% of KNO3 in the 66 composite Endotherm DSC scans for PVF, KNO3 and the PVF: KNO3 67 composite samples Exotherm DSC scans for PVF, KNO3 and the PVF: KNO3 69 composite samples Endotherm DSC scans of PVA film, KNO3 powder and PVA: 70 KNO3 composite film Exotherm DSC scans of PVA film; PVA: KNO3 composite 70 film and KNO3 powder Endotherm DSC thermograms of pure polyethylene and wt.% PE: KNO3 composite films Exotherm DSC thermograms of pure polyethylene and wt.% PE: KNO3 composite films Raman spectra of 50 wt.% KNO3 : PVDF composite films at 73 room temperature. FIGURES FOR CHAPTER-IV 4.1 Ferroelectric hysteresis loops of different KNO3 composition 77 in composite (a: 10, b: 50, c: 80wt. %). 4.2 Current density versus voltage (J-V) characteristics in the 77 composite films. 4.3 Plot of remanent polarization versus weight percentage of 78 KNO3 in the composite films. 4.4 Shows the variation of coercive field with weight percent 79 composition of KNO3.

15 Figure No. Title Page No. 4.5 Ferroelectric hysteresis loop in 50% KNO3: PVF composite 80 films. 4.6 Current density versus electric field in 50% KNO3: PVF 81 composite films. 4.7 Shows a typical ferroelectric loop observed in the poled 82 KNO3: PVA composite sample. 4.8 Shows the current density versus applied voltage in the poled 83 KNO3: PVA composite sample. 4.9 Shows the P-E hysteresis loops of 50 wt.% KNO3 : PVDF 84 composites for different temperature Remanent polarization versus temperature of KNO3 : PVDF 85 composite films Dependence of Remanent polarization versus reversal cycles 86 in 35 µm thick films Frequency dependence of (.1-V) characteristics in 87 KNO3: PVDF composite films Plot of log V, versus log f in the composite of KNO3: PVDF 87 films Shows the dependence of the dielectric constant on 89 temperature during heating and cooling cycles in KNO3: PVDF composite films Shows the dependence of the dielectric constant on 90 temperature during heating and cooling cycles in KNO3: PVF composite films Shows the plot of log a versus 71 in KNO3: PVDF composite 92 films.

16 FIGURES FOR CHAPTER-V Figure No. Title Page No. 5.1 (a)- 5.4 (d) Theoretical fit (eq.5.1) to the experimental switching response 99 at 10 V for different wt.% KNO3 doped composite films: (a) 10%; fitting parameters: n= 1.62, to=100 (b) 20%; fitting parameters: n= 1.6, t0=120 (c) 30%; fitting parameters: n = 1.46, to=180 ICs. (d) 50%; fitting parameters: n= 1.35, to=400 ps. 5.5 Theoretical fit (eq.5.1) to the experimental switching response 102 for 20% KNO3 composite films at: 8V; fitting parameters: n=1.6, to= Theoretical fit (eq.5.1) to the experimental switching response 103 for 20%. KNO3 composite films at: 16V; fitting parameters: n=1.8, to= The semilog plot of ts versus applied voltage V in 20% 106 KNO3: PVDF composite films. 5.8 Shows the im tm versus electric field in 50% KNO3: PVDF 108 composite films. 5.9 Theoretical fit (eq.5.1) to the experimental switching response 108 for 50% KNO3: PVDF (35 µm ) composite films at: 28V; fitting parameters: n=2.1, 4)=270 ps Theoretical fit (eq.5.1) to the experimental switching response 110 for 50% KNO3:,PVF (35 pm) composite films; fitting parameters: n=1.8, to-400 i.ts Switching current in 50wt. KNO3: PVDF composite films 111 using sine wave input pulse Switching current in 50wt. % KNO3: PVF composite films 113 using triangular wave input pulse. (xv)

17 FIGURES FOR CHAPTER-VI Figure No. Title Page No. 6.1 C-V characteristics of KNO3: PVDF films C-V characteristics of KNO3: PVF films Conductance versus bias dependence characteristics in wt.% KNO3: PVDF composite films. 6.4 Conductance versus bias dependence characteristics in wt.% KNO3: PVF composite films. 6.5 The existence of the current voltage characteristics in the composite films. 121 (xvi)

18 LIST OF TABLES Table No. Title Page No. TABLES FOR CHPTER-I 1.1 Unit-cell data on phase I, II, and III of KNO The values of the Raman shift at different temperature X-rays reflection for different phases of KNO The physical properties of KNO3, PVDF, PVF, PVA, and PE. 31 TABLES FOR CHAPTER- III 3.1 Summarizes the x-ray reflections observed in PVDF Shows the peak intensity ratio of phase III and phase-ii Shows the Raman shift at different phases of KNO3 in the 74 composite films. TABLES FOR CHAPTER- IV 4.1 Shows the comparison of polarization and coercive fields 83 between the different polymers in the 50wt.% composite films. 4.2 The activation energies calculated for different phases during 94 heating and cooling modes in the composite films. TABLES FOR CHAPTER- V 5.1 Effect of KNO3 doping on switching transient at 10 V Effect of voltage on i, t, for different KNO3 doped samples Switching parameters for different voltage switching 104 transients for 20% KNO3 composite film.

19 Table No. Title Page No. 5.4 Shows the various parameters as ts, t m, i m, imtm, Qs, and imtm /Qs 107 in 50 wt.% KNO3: PVDF composite films. 5.5 Various switching parameters obtain from the present studies 110 and from the literature are summarized. 5.6 The values of Ps obtained from the hysteresis loop and the 112 switched charge measurements for different compositions.

20 GLOSSARY OF SYMBOLS a A a BMF BST C CMOS CVD Activation field for ferroelectric switching. Lateral area of memory cell Lattice constant Barium Magnesium Fluoride BaMgF4 Barium Strontium Titanate Ba Sr1,TiO3 Capacitance Complementary Metal-Oxide Semiconductor Chemical Vapour Deposition D Displacement vector; Dimension of domains (1,2, or 3) d dc DRAM e E Film thickness Direct current Dynamic Random Access Memory Charge of the electron Electric field Relative dielectric constant EB Ec EEPROM Breakdown field Coercive field Electrically Erasable Programmable Read-Only Memory FeRAM, FRAM G Ferroelectric Random Access Memory Conductance Displacement current density Leakage current density

21 JFET k Junction FET Exponent characterizing frequency dependence of coercive field K MBE MOD MOSFET Ns NDRO NV Thermal conductivity Molecular Beam Epitaxy Metal-Organic Decomposition Metal Oxide Silicon FET Number of bipolar switching cycles Non Destructive Read Out Non Volatile, P. Delta-function polarization at ferroelectric surfaces P, Polarization due to charged defects Pr Ps PLD PLZT PT PZT Q R RAM READ Remanent Polarization Spontaneous polarization Pulsed Laser Deposition PZT with lanthanum doping Lead Titanate Lead zirconate titanate PbZr,,Tii. 03 Total electric charge Nucleation rate Random Access Memory To decode and output the information stored in a memory RTA p Rapid Thermal Annealing Charge per unit volume

22 Electrical conductivity o SBN SBT SBTN SCLC SRAM t Electrical conduction in the high temperature limit Strontium Bismuth Niobate SrBi2Nb209 Strontium Bismuth Tantalate SrBi2Ta209 Strontium Bismuth Tantalate-Niobate SrBi2Ta2.xNb,(09 Space Charge Limited Current Static Random Access Memory Exponent characterizing field dependent of switching time tans to Tc TDDB tn is U v V WRITE XRD z Loss tangent Breakdown time Curie Temperature Time Dependent Dielectric Breakdown Nucleation time Switching time Potential energy Domain wall speed Voltage To encode and input data in to a memory X-Ray Diffraction Depth in to film from surface or interface

23 CONTENTS Page No. CANDIDATES DECLARATION (1) ABSTRACT (ii) LIST OF PUBLICATION (viii) ACKNOWLEDGEMENT (ix) LIST OF FIGURES (xi) LIST OF TABLES (xvii) GLOSSARY OF SYMBOLS (xix) CONTENTS (xxii) CHAPTER I INTRODUCTION BRIEF HISTORICAL REVIEW ON POTASSIUM 3 NITRATE (KNO3) Existence of Different Phases of KNO Phase Diagram Unit Cell Phase Transition Experiments Differential Scanning Calorimetry (DSC) Measurements Raman Spectroscopy and Infrared (IR) Experiments Thermal Expansion Experiment X-Ray Diffraction (XRD) Determination Phase Stabilization Electrical Studies 17

24 Dielectric Measurements Electrical Conductivity Measurements Ferroelectric Hysteresis Loop Studies Capacitance-Voltage (C-V) Studies Polarization Switching and Parameters Phenomenological Theory for Ferroelectric 24 Domain Switching Overview of KNO3Investigations STUDIES IN OTHER FERROELECTRIC THIN FILMS STRUCTURE OF POLYMERS Structure of Poly (vinylidene fluoride) Structure of Poly (vinyl fluoride) Structure of Poly (vinyl alcohol) Structure of Polyethylene POLYMER - FERROELECTRIC COMPOSITE MATERIALS SCOPE AND MOTIVATION OF THE PRESENT WORK 35 CHAPTER-II : EXPERIMENTAL: DESIGNED, FABRICATION, 38 MATERIALS, AND MEASUREMENTS 2.1 INTRODUCTION MATERIALS USED AND THEIR SPECIFICATIONS DETAILS OF THE INSTRUMENTS USED 39 IN VARIOUS EXPERIMENTS 2.4 SAMPLE PREPARATION Purification of Potassium Nitrate (KNO3) Mixing with Polymer and Fabrication of Composite Films 42

25 2.4.3 Fused Composite Films Preparation of the Solvent Cast- Films of KNO3: PVA Composite Electrode Deposition INSTRUMENTATION AND MEASUREMENT TECHNIQUES Measurement Cell Ferroelectric Hysteresis Loop Measurements Ferroelectric Switching Transient Measurements Electrical Conductivity Measurements Capacitance-Voltage (C-V) and Dielectric Measurements DIFFERENTIAL SCANNING CALORIMETRY (DSC) 50 CHAPTER- III: STRUCTURAL AND THERMAL PROPERTIES INTRODUCTION RESULTS AND DISCUSSION X-Ray Diffraction (XRD) Pure KNO Pure PVDF Films KNO3:PVDF Composite Films Expanded X-Ray Diffraction Scan for Phase-III Crystallite Size X-Ray Diffraction of PVF, and PVA Composite Films DIFFERENTIAL SCANNING CALORIMETRY (DSC) Endothermic DSC Exothermic DSC DSC with Other Polymer Composites 67

26 3.4 RAMAN SPECTROSCOPY In the Composite Films of KNO3 :PVDF CONCLUSIONS 74 CHAPTER-IV: FERROELECTRIC AND DIELECTRIC STUDIES INTRODUCTION RESULTS AND DISCUSSION Ferroelectricity in KNO3: PVDF Composite Films Ferroelectricity in Different KNO3: Polymer Composite Films KNO3. PVF Composite Films KNO3: PVA Composite Films Temperature Dependence of PR in KNO3 : PVDF Fatigue Measurements Frequency Dependence Dielectric Measurements KNO3: PVDF Composite KNO3: PVF Composite Electrical Conductivity CONCLUSIONS 95 CHAPTER-V: SWITCHING CURRENT TRANSIENT 97 CHARACTERISTICS 5.1 INTRODUCTION RESULTS AND DISCUSSION Switching in KNO3: PVDF Composite Films Switching Response of 5-µm Thick Films 98

27 Switching Response of 35-um Thick Films Switching in KNO3: PVF Composite Films Switching Response of 35-um Thick Films Switching Current Response to Different Input Wave Signals KNO3: PVDF Composite Films KNO3: PVF Composite Films CONCLUSIONS 114 CHAPTER-VI : ELECTRICAL PROPERTIES INTRODUCTION RESULTS AND DISCUSSSION Capacitance Voltage (C-V) and Conductance- Voltage (G-V) 116 characteristics PVDF: KNO3 Composite Films PVF: KNO3 Composite Films Current-Voltage (I-V) Characteristics PVDF: KNO3 Composite Films CONCLUSIONS 122 CHAPTER-VII : SUMMARY,CORRELATIONS, AND CONCLUSIONS STRUCTURAL AND THERMAL PROPERTIES FERROELECTRIC AND DIELECTRIC PROPERTIES POLARIZATION SWITCHING C-V, G-V, AND I-V MEASUREMENTS MAIN CONCLUSIONS 131 REFERENCES 133

28 I - INTRODUCTION CHAPTER -I INTRODUCTION 1.1 INTRODUCTION In recent years, there has been phenomenal growth in the field of ferroelectric thin film materials with potential wide ranging applications in the field of electronics [1-8] as ferroelectric memories [9], ferroelectric capacitors [10-18], ferroelectric memory cells [19-26], ferroelectric random access memories (FeRAMs) [9,27], nonvolatile random access memories (NVRAMs)[28], dynamic random access memories (DRAMs) [29,30], metal ferroelectric semiconductor field effect transistors (MFSFET) [31,32], static random access memories (SRAM) [1,9,28], and microelectronic mechanical systems ( MEMS) [1,9]. Traditionally, ceramic materials have been used for these applications. These ceramics materials are usually brittle, heavy and difficult to produce in large area devices, therefore, the polymer-ferroelectric composite materials are emerging as a new class of electronic and dielectric materials. The added advantages of polymers are their flexibility, low density, low cost and ease in processing in a desired shape. The composite materials for piezoelectric and pyroelectric applications [2,5,6] have gained enormous attention in the past because of their potential to be produced with desirable properties by choosing a proper combination of the constituent phases. The ceramic-polymer composite with ferroelectric properties combines many useful properties of the polymer such as flexibility of the polymer and electroactivity of the ceramic to produce large area device with reasonable mechanical strength. Furthermore, the composites made of electroactive ceramics and polymers are 1

29 I - INTRODUCTION suitable for many applications since they can be easily prepared in a variety of shapes and also in film form. The composite materials have been processed using mixtures of ceramic as filler in a matrix of polymer, where the fillers are included in the matrix in order to modify its physical properties in a high range. The ceramic fillers such as PZT, BaTiO3, PbTiO3 in PVDF with different doping have been produced in the past. The composites in the film form are becoming important from the application point of view in large area devices operated at low voltages. A technological interest in potassium nitrate (KNO3) as a ferroelectric material arises because of the attractive large signal switching properties of the thin films [33]. The square hysteresis loop, low switching potential (5V), and fast switching times (20ns) make KNO3 thin films promising material as a permanent storage medium in large scale integrated random access memories (RAMs)[33-35]. The primary goal of this chapter is to describe the electrical, structural, and thermal properties of the ferroelectric potassium nitrate studied in the past. A brief historical background of potassium nitrate (phase-iii) with a description of structure and preparation methods are included. The physical and chemical structure of the polymers with which the composite films were prepared mixing with KNO3.The choice of the polymer for making the composite has been based on its advantage in retaining the ferroelectric phase-iii up to room temperature and suppression of the conversion of phase-iii to phase-ii in the composite. The current trends in the research on the ferroelectric composite materials has been presented. The scope of the present work and its motivation are also included. The general discussion on the potential application of the composite films in various devices and particularly in memory devices is presented. The material properties of 2

30 I - INTRODUCTION KNO3 and the polymers used for the fabrication of composite have also been included. 1.2 BRIEF HISTORICAL REVIEW ON POTASSIUM NITRATE (KNO3) It has been known since the late 1960s that KNO3 exists in one of the several crystallographic phases depending on external state variables. Only one phase is ferroelectric (phase-iii) which is known to exist at elevated temperature while cooling from higher temperature [36]. This ferroelectric phase-iii would be of technological interest if it can be stabilized at room temperature.' It has been reported by Nolta, et. al., [37] that KNO3 in thin film form was stable at room temperature Existence of Different Phases of KNO3 Potassium nitrate was discovered in 1958 as a ferroelectric material by Sawada et al., [36] since then KNO3 has been investigated by many workers [37-42] from the ferroelectric [37], dielectric [39], and structural [33,40,42] view point. KNO3 has been found to possess aragonite structure ( D21,16 ) known as phase-ii by many workers [38-42] at room temperature. KNO3 has been found to exist in variety of phases [43] as described in Figurel.1 (a) and (b). PHASE- II PARAELEICRIC PHASE -II FERROELECTRIC PHASE (III) 130 C 110 C 124 C COOLING PHASE-I PARAELECIZIC 1 COOLING WITH PHASE - I ike PRESSURE Figure 1.1(a): Shows schematic of the different phases of KNO3. 3

31 I - INTRODUCTION On heating, the crystal structure changes from orthorhombic (phase-ii) to rhombohedral (phase-i) and on cooling, the phase-i changes first to phase-iii at about 124 C and then III to II at about 110 C [36]. It was observed that the phase-iii always reverts back to phase-ii, but the rate at which this process occurred was strongly affected by the temperature of the specimen [38]. PHASE -II (a) ARAGONITE STRUCTURE ORTHORHOMBIC D12 (Pnma) ON HEATING 130 C PHASE -I (0) CALCITE STRUCTURE RHOMBOHEDRAL Dia (Rjm) PHASE -III (y) RHOMBOHEDRAL D;,(R3 m) FERROELECTRIC ON COOLING Figure 1.1 (b): Crystal structure changes when KNO3 was passed through the heating and cooling cycles [42,43] Phase Diagram The phase diagram of KNO3 as a function of pressure and temperature measured by Bridgman is shown in Figure 1.2 [44]. The ferroelectric phase-iii, is a metastable phase at atmospheric pressure. It always appears when the crystal is cooled from above 180 C (phase-i), but does not appear when the crystal is heated from phase-ii. The examination from the phase diagram (Bridgman Figure 1.2) shows that the stability of phase-i, II, and III meet a triple point at 113 bars and C. The pressure existing at the equilibrium boundary I-III at 133 C is 500 bars [44]. 4

32 I - INTRODUCTION 1130, Pa raelectric I Temperature (C) Pressure (103 kg/cm2) Figure 1.2: Phase diagram of KNO3 as a function of pressure and temperature measured by Bridgman (1916) [38,39,43,44]. The structure of the various phases of KNO3 were studied prior to the discovery of ferroelectricity in the phase-iii. The structure of phase-i (paraelectric phase) is closely related to the calcite structure (trigonal). It belongs to the space group D35d R3m and has one molecule per unit cell. From the projected electron density, the nitrate ion is oscillating along the c- axis with amplitude of about 0.4A.The structure of ferroelecti-ic phase (phase-iii) is closely related to that of the paraelectric phase belonging to the space group Diu R3in ; however; in this phase the nitrate ion is known to be displaced along the c-axis from the center of the unit cell by.0.5 A. The potassium nitrate has the aragonite structure (orthorhombic, D26h Pnma ) at room temperature. The crystal structure changes from orthorhombic to trigonal at about 130 C on heating. The low and high temperature phases being named the phase-ii and the phase-i, respectively [38,39,43,44]. 5

33 I - INTRODUCTION Unit Cell The crystal structures of the phase-i and phase-ii are non-polar, but that of the phase-iii is polar. A rhombohedral unit cell in the ferroelectric phase-iii is shown in Figurel.3 contains one molecule of KNO3 and K+1 ions occupy its corners whereas one NO3- ion lies near its body center. The NO3 group forms a regular triangle with the N atom at its center. The plane of the NO3 group is perpendicular to the c-axis and it does not exist exactly at the body center [39]. Potassium Nitrogen Oxygen Figure 1.3: Rhombohedral unit cell in the phase - III of KNO3 [39]. The crystal structure and related parameters of different phases of KNO3 are shown in Table

34 I - INTRODUCTION( Tablel.1: Unit-cell data on phase I, II, and III of KNO Phase of Temperature Unit cell data Crystal structure Space KNO3 ( C) (A) group a b C Phase-II Orthorhombic P IIITIR Phase-I Rhombohedral (cce 73 50' ) Phase-III Rhombohedral (ccr= 76 56' ) R3m R3m The phase transitions also accompany the change in volume of the unit cell. The relative volume changes of KNO3 unit cell associated with the II-41, phase transitions are +0.27%, , and %, respectively [45]. The transition may have increased the overall density of the loosely packed powder. However, the decrease in the volume during the transition may have allowed the powder to increase the packing density even further. Perhaps, this packing placed the KNO3 under an effective stress, which is known to affect the phase transition temperatures. This effective applied stress at elevated temperature may have enhanced the sintering of the KNO3 particles similar to the physical situation accomplished in hot pressing [45] Phase Transition Experiments A series of experiments have been performed to study the phase transitions in potassium nitrate. These experiments include the thermal expansion, differential scanning calorimetry, Raman spectroscopy and Infrared (IR) measurements. KNO3 sample in micron thickness range when cooled from the high temperature phase-i results in a metastable ferroelectric phase-iii structure, which is retained up to room temperature [41]. This structure is unstable and transforms slowly 7

35 I - INTRODUCTION to the equilibrium (non-ferroelectric) bulk form, the transformation being accelerated at slightly elevated temperatures. The retention of ferroelectric phase-iii at room temperature in thin films of KNO3 grown by fusion has aroused considerable interest [41] Differential Scanning Calorimetry (DSC) Measurements The investigations of phase transformations in KNO3 have been carried out by DSC measurements by many workers [45-51]. The endothermic (heating) curve of pure KNO3 shows a single peak near C, which may arise from the transition of phase-ii to I as shown in Figure 1.4. Heating C 7 1. c4 ba U cn Cooling 103 C C Temperature ( C) Figure 1.4: DSC signals of KNO3, both for heating and cooling procedures The exothermic DSC curve shows a peak at C corresponding to the transition of phase-i to phase-iii. The transition peak near 103 C has been attributed to the phase transition III to II [46]. The DSC results on pure KNO3 have also been reported by Westphal et al, [45-51] giving the enthalpies for the phase transitions 8

36 I - INTRODUCTION and III-41 as 5.065, 2.603, and kj/mole, respectively. The transition involves an existence rearrangement of the atoms but very little change in density. The existence of ferroelectric phase-iii down to room temperature was not observed by either heat flow calorimetry (HFC) or photo acoustic calorimetry for KNO3 powder. The DSC experiments were also carried out by Isaac and Philip [47] in pure KNO3. The endothermic curve shows a single peak near 132 C, which may arise from the transition of phase II to phase I. The transition temperature from phase-ii to phase-i for pure KNO3 was reported to be C, and the corresponding enthalpy of the transition of Pe61.18 J/g. The absence of a transition from phase-iii to phase-ii in KNO3: SiC mixture has been observed above 17vol. % of SiC. Isaac and Philip [47] have also observed similar results in the carbon doped KNO3 samples as shown in Figure 1.5. HEATING Temperature ( C) Figure 1.5: DSC curve of KNO3 containing carbon black for heating and cooling cycles [471. 9

37 I - INTRODUCTION The KNO3 / SiC mixture studies were also carried out by heat flow calorimetry by Westphal [48]. The result shows that the suppression of the phase-iii to phase-ii transition when SiC is mixed with KNO3. The surface effects as the source of the stabilization of the ferroelectric phase-iii of KNO3. The surface free energy of each individual particle would not have been affected by mixing the two materials, therefore surface free energy was probably not the mechanism which stabilized ferroelectric phase-iii. The transition characteristic for the 1I >1 and I >III phase transition were relatively unaffected by repeated thermal cycling [48]. Westphal has explained that the presence of SiC powder reduces the probability of nucleation of phase-h from phase-iii [48,49]. The ferroelectric phase-iii exists over a wide temperature range in a polycrystalline sample contrary to the behaviour exhibited by the bulk form where it exists only over a few degrees ( 120 C to 110 C). Hydrostatic pressure considerably enhances the temperature range over which ferroelectric phase-iii exists, Scott et.al., [34,35] have suggested based on Raman spectroscopic data, that the actual mechanism for the stability of ferroelectric phase-iii is the presence of large surface electric fields in the ferroelectric state. The effect of particle size on the phase transition temperature of KNO3 at atmospheric pressure has also been studied using the DSC experiment [45]. The phase H >1 transition temperature was relatively unaffected by particle size, whereas the phase I >III transition temperature was reduced up to 4 C for particle sizes < m. The phase III- III transition temperature decreased sharply for particle sizes < 240 [im, and for 38pm particles was 44 C lower than that reported for bulk material [45]. 10

38 I - INTRODUCTION Raman Spectroscopy and Infrared (IR) Experiments The Raman scattering studies for a complete phase transition cycle (II A-3III-4I) in KNO3 have been reported [52] with the intention to correlate the Raman-active modes in different crystalline structures. The Raman spectrum of a single KNO3 crystal was measured at both room temperature and liquid nitrogen temperature [52]. In order to understand the nature, stability and existence temperature width of phase-i, phase-ii, and phase-iii (ferroelectric phase) of KNO3, a detailed Raman investigations have been carried out with various heating cooling rates, repeated thermal cycles, and differential preheating temperatures by many workers [34,53-55]. The increase in the existence temperature width of the ferroelectric phase-iii of KNO3 was observed with the increase of heating/ cooling rate but the repeated thermal cycling reduced it. The phase-iii to phase-ii transformation extended to a lower temperature in the cooling process when the sample was subjected to a higher preheating temperature. The ferroelectric phase-iii could be retained down to room temperature with higher cooling rate and higher preheating temperature. This could have important implication in the viewpoints of its applications. The coexistence of phase-iii with phase-ii was observed with all the heating/cooling rates [53,54]. The values of the Raman shift at different temperature reported in literature [34,35,52,53] for different phases of KNO3 are summarized Table 1.2. Raman spectroscopy of KNO3 thin-film has been performed as a function of film thickness, temperature, annealing procedures, and read write cycles [34,53]. The results show that conversion to phase-ii was not the cause of the failure in these memory devices, which are in agreement with the x-ray results of Schaffer and Mikkola [33-35]. 11

39 I -INTRODUCTION{ Table 1.2: The values of the Raman shift at different temperature. Temperature ( C) KNO3 Phases Raman shift (cm-1) Ref. 22 KNO3 (II) 71,55,130 [34,35] (R.T.) KNO3 (III) 92,95,97,103,107,114,120, KNO3 (I) 714,836,1056,1428 [53,54] KNO3 (II) 50,55, 60, 65:83, 103, 122, 123, 133, 137,138,714,1054,1348, KNO3 (III) 50, 120,716,836,1057,1352 [34,54,53] 120 KNO3 (II + 111) 92, 97, 107, 113 [34,35] Thermal Expansion Experiment The thermal expansion experiment has been found to be useful to reveal the phase transitions in KNO3. The value of thermal expansion coefficient have been calculated from x-ray [42] and dialtometric method for different phases of KNO3 [42]. Figure 1.6 shows the linear thermal expansion coefficient of 40j_im thick melt-casted polycrystalline films of KNO3 [56]. The thermal expansion behaviour resembles closely with the bulk properties. The distinct sharp changes in the value of a are seen at the transition temperatures which indicates that these transitions are of the first order. The transitions was observed at C in the heating mode and the ferroelectric phase transition I -III was observed at C and phase II transition at 68.5 C was observed in the cooling mode. The average value of a was obtained in these measurements during heating. The drastic change in the value of a has been ascribed to the disordering in the position or orientation of atoms or radicals [56]. 12

40 I - INTRODUCTION The thermal expansion coefficient for ferroelectric phase-iii is = 7.5 x 104/ C in the range 100 C < T< 125 C [33,50]. The thermal conductivity measurements on KNO3 [57] also show a sudden decrease in the thermal conductivity at the transition temperature [57] CI 1200 ' "? 1000 ' d Heating Cooling t 0 40 I -Do 'I I I 1 I I I Temperature ( C) Figure 1.6: Variation of linear thermal expansion coefficient with temperature of m thick KNO3 film [ X-Ray Diffraction (XRD) Determination X-ray diffraction measurements have been done by various researchers [44,48,59-61] to know the crystal structure and values of different phases of KNO3. The x-ray study on the disordered structure [58] above the ferroelectric Curie point (125 C) in KNO3 was studied [58]. These studies provide important information about the thermal transformation ferroelectric to paraelectric phases. The phase determination study by the x-ray diffraction with pressure and temperature were also 13

41 I - INTRODUCTION reported by Davis and Adams [44]. The relationship between the structures of the a-, 13- and y-phases were also given [60]. All elements in KNO3 have relatively low atomic numbers; the scattered x-ray intensities are expected to be low. The typical x-rays diffraction spectra reported in literature [33] are shown in Figures 1.7 and r , c*a 1000 %-- z ;71 0 N z N O eh z. " 0 1I TWO THETA (DEGREES) Figure 1.7: Show the room temperature diffraction pattern from a powder specimen of KNO3. The vertical bars indicate the expected peak positions and relative intensities for reflections from KNO The measurements have been also performed by these authors on 5-gm thick film of containing phase III. The peak shapes of the (003) reflections prior to any polarization reversal and after polarization reversals are shown in Figurel.8. 14

42 I - INTRODUCTION INTENSITY (CPS) TWO-THETA (DEGREES) Figurel.8: Shape variation of the (003) reflection for Ill-KNO31331 Tablel.3 shows the x-rays reflections of different phases in KNO3 and their corresponding d and 20 values [33,44, 48, and 58-61]. Table 1.3: X-rays diffraction for different phases of KNO3. KNO3 Phases 20 Reflections d (A) (hkl) III-phase 27.3 (012) III-phase 29.8 (003) II-phase 23.3 (111) H-phase 23.7 (021) phase 29.2 (012) The pressure dependence of the x-ray diffraction studies has been carried out [61]. X-ray diffraction studies of Schaffer and Mikkola [33], revealed that the failure of KNO3 thin film ferroelectric switching devices after fatigue is not due to 15

43 I - INTRODUCTION conversion to phase-ii. The space charges have been attributed to the switching failure of the devices [33-35] Phase Stabilization Studies of phase stabilization in KNO3 are important in order to establish the occurrence and the stability of ferroelectric phase-iii in KNO3. The following techniques have been used by various workers to obtain ferroelectric phase-iii in KNO3. (i) By applying pressure [61] (i) By doping ( SiC, 12, KB,) [45,62,63] (ii) By heating effects [36-39] There have been several attempts have been made to obtain ferroelectric phase-iii at room temperature by applying hydrostatic pressure [61]. Taylor and Lechner [64] discovered the technique used for obtaining stable ferroelectric phase-iii in which the material was subject to high temperature and pressure simultaneously. Their results showed that ferroelectric phase-iii could be achieved over a wide ranges, of pressure and temperature. The hysteresis loops were measured on samples at pressure varying from 3000 to 20,000 psi and temperature from C [64]. Scott et.al., [34] indicated that the stabilization of ferroelectric phase-iii in a thin film was not due to stress but rather to surface charges [34]. Then came a study by Isaac and Philip [47], who mixed polycrystalline KNO3 with carbon black used as a medium to enhance optical absorption for photo acoustic studies. They also observed the existence of ferroelectric phase-iii at room temperature in these samples. Westphal [45] studied phase stabilization in KNO3 mixed with SiC powder using different sample preparation conditions. He concluded that the particles of KNO3 interacted with one another differently depending upon the distribution of SiC 16

44 I - INTRODUCTION between KNO3 granules and therefore, the presence of SiC powder can change the characteristics of the phase changes in KNO3 [45]. The presence of extraneous materials in KNO3 can suppress the conversion of phase-iii to phase-ii thus stabilizing ferroelectric phase-iii at room temperature. It has also been reported that KNO3 in a KBr matrix, when kept in a nonabsorbent liquid shows a very long stability of ferroelectric phase-iii at room temperature [63]. The appearance of ferroelectric phase-iii in KNO3 is somewhat dependent upon the rate of cooling. It has been known that upon heating, the ferroelectric phase- III does not appear under atmospheric pressure but does appear under high-pressure [42]. It has been observed that the phase-iii always reverted to the stable phase-ii, but that the rate at which this process occurred was strongly affected by the temperature [38]. The stability of ferroelectric phase-iii arises due to measuring the pressure-at constant temperature [44] Electrical Studies The electrical characterization of ferroelectric thin films includes _.the determination of property parameters through different electrical measurements. The ferroelectric thin films are generally characterized for both ac and de responses. The dielectric studies involve both the frequency- domain and the time domain response, while dc measurements are restricted to the leakage current flowing through the ferroelectric thin film under varying electric fields. These studies could be helpful in providing further understanding of the ferroelectric material system Dielectric Measurements The dielectric behaviour of KNO3 crystal as a function of temperature has been investigated by Sawada and Nomura [39] as shown in Figure 1.9 measured at 17

45 I - INTRODUCTION frequency of 100 khz. This type of behaviour of dielectric constant has been found in pure KNO3 by other workers [39] Temperature ( C) Figure 1.9:The temperature dependence of the dielectric constant measured at 100 khz in KNO3 [39]. The behaviour of dielectric constant in a single crystal of KNO3 has been reported to exhibit strong dependence near the phase transition temperature [68]. The dielectric constant shows two folds increase in the temperature range 134 C-0152 C as compared to the room temperature values. This corresponds to the transition of phase II to phase I with the transition starting at 134 C during heating cycle. It is also known that the crystal structure changes from orthorhombic (phase-ii) to trigonal (phase-i) phase at 130 C [39]. During the cooling cycle, the dielectric constant shows sharp change in the temperature range 120 C-0104 C and this change has been attributed to the phase change I --0 III. The dielectric study of KNO3-series mixed crystals (RbKNO3) has also been reported [59]. The dielectric constant vary from 5 to 66 in the 30 C to C. The transition temperature of C for the transition on heating and C for the I >I1I and 92 C for the III >II on cooling 18

46 I - INTRODUCTION have been mentioned [59]. The dielectric measurements have also been carried out in the single crystal of KNO3 mixed with (NH4) Ki_ x NO3. The results concluded that the value of maximum spontaneous polarization decreases and the ferroelectric temperature region get extended with increasing ammonium concentration. The cooling rate has been found to affect occurrence of the ferroelectric phase-iii transition [42]. The temperature dependence of the dielectric constant of KNO3 has been show to give Curie-Weiss behaviour [65]. A series of carbon- doped KNO3 thin layers were grown from the melt by a special technique. The changes of the dielectric constant, d.c.resistivity, energy of vacancy formation, pyroelectric current and ferroelectric hysteresis loop with carbon concentration were investigated [67] Electrical Conductivity Measurements The dc conductivity variation with temperature has also been found to provide knowledge of the phase transition in KNO3 [66,68]. The dc conductivity studies on single crystal KNO3 shown anomalies at about 130 C on heating and about 124 C and 108 C on cooling as shown in Figure 1.10 [66,68] :E E ' 10-10" Heating I T (e) Figure 1.10: DC conductivity versus 1000/T plot of KNO3 crystal [68]. 19

47 I - INTRODUCTION These measurements gave the activation energies of phase I, II and III as 0.89, 1.38 and 0.78 ev respectively [68] Ferroelectric Hysteresis Loop Studies The details of the nature of ferroelectricity in KNO3 have been discussed using the statistical theory and the thermodynamic theory [65]. It has been concluded that the ferroelectric transition at 125 C is a first order transition implying that the ferroelectricity in KNO3 crystals is an order-disorder phenomena in which the first order ferroelectric transition caused by a strong lattice-electric dipole interaction. The presence of ferroelectric loop confirms the presence of ferroelectric phase. There are many ferroelectric hysteresis loop studies on KNO3 performed by various workers in the past [36-39]. The hysteresis loop has been obtained in single crystal at different temperature. The D-E hysteresis loop has been observed at 121 C for a fused sample, from which the coercive field and the spontaneous polarization were obtained to be 4.5 kv/cm and /C/cm2, respectively [36]. It has also been reported that thin films of KNO3 provide much higher Tc than that of bulk KNO3 [35] Capacitance-Voltage (C-V) Studies The capacitance-voltage measurements have been used to study the polarization switching and asymmetric behaviour of the polarization states in the ferroelectric materials by many workers [69-79]. The usual C-V characteristics show butterfly curve but with additional peaks can also arise from the space charge injection effects [70]. The space charge effects have been studied in ferroelectric C-V characteristic upon irradiation [71-76]. The space charges at the interface of the crystallites, which can develop localized space charge field, can affect C-V measurements. The strong bias dependence of capacitance of composite films may be 20

48 I - INTRODUCTION exploited to be used as tunable dielectric material in high frequency ferroelectric varactors [1]. The C-V plots have been integrated to determine the quantity of displaced charge during switching [103]. The capacitance variation with applied bias is an indication of switching in a ferroelectric and the abruptness of the capacitance variation at the point of switching gives a good estimate of the threshold electric field (± Ea. The capacitance- voltage measurements have been carried out in a metalinsulator-semiconductor configuration by sandwiching a NaNbO3 film between Al and Si electrodes [70]. It has been observed that the hysteresis voltage width is not affected by different sweep rates [70]. The thin films of PZT have also been studied by observing capacitance and conductance variation with bias voltage [69] and by pulse switching measurements [69, 92,95,78]. The C-V measurements have been performed in a variety of materials like LiNbO3, metal ferroelectric semiconductor (MFS) capacitor [69,73], SBT films [1, 69,79, 83, 85], and other ferroelectric thin films [1, 69, ] Polarization Switching and Parameters Switching is the process by which the remanent polarization is reoriented in to a new position of remanent polarization (Pr). It is possible to induce switching both by an electric field and mechanical stress. At zero applied field, there are two states of polarization, ± Pr furthermore, these two states of polarization are equally stable. Either of these two states could be encoded as a "1" or a "0" and since no external field is required to maintain these states, the memory device is nonvolatile. To switch the state of the device, a threshold field greater than ± E, is required. Additionally, in order to reduce the required applied voltage (to within 5 V) for a given Ec, the ferroelectric materials need to be processed in thin films. 21

49 I - INTRODUCTION In most ferroelectric memories, the memory cell is read destructively by sensing the current transient that is delivered to a small load resistor when an external voltage is applied to the cell. For example, if a memory cell is in a negative state (-Pr ) and a positive switching voltage is applied to it, there will be a switching charge given by [4,8,9,104]. dp Q = A 6.06E + A f dt 0 dt r j fi(t)dt = E06E +2P,A (1.2) where A is the area of the cell, E is the dielectric constant of the ferroelectric materials, Ea is the applied field and P is the polarization of the ferroelectric. Figure 1.11, depicts both the switching current (curve b) and non-switching currents (curve c) with input pulse (curve a). The non-switching current arises from a linear dielectric response (AsosEa ) and the switching current response (curve b) arises from the displacement current. The kinetics of polarization reversal may be described by measuring the switching time is for different amplitude of electric pulse. Since the current decrease exponentially, it is difficult to measure the total switching time, and is is usually taken as the value where current falls to 0.1 imax [3,4] as in Figure 1.11 (b). The switching time can be expressed as [3,4] t = Aead` v (1.3) where A is constant, V is the applied voltage, a is the activation field and d is the sample thickness.. At very high fields this relation changes to a power law of the form [3,4,62] t, cc E-" (1.4) 22

50 I - INTRODUCTION where the exponent n depends on the material. It has been observed the value of n was about 1.5 for BaTiO3 and KNO3 [105,106] and 1 for NaNO2 [106]. The maximum switching current may be given by [105,106] 1max = (1.5) The constants a, n, to, and 1,0 in equations (1.3), (1.4) and (1.5) can be temperature dependent, with switching time decreasing as the Curie point is approached [4,8,9, ]. E Input pulse (a) t (C) Non switching current Figure 1.11: Typical switching transient currents in ferroelectrics [4]. 23

51 I - INTRODUCTION Phenomenological Theory for Ferroelectric Domain Switching There has been renewed interest in the polarization-reversal phenomena in ferroelectric materials in association with the development of thin-film ferroelectric memories [8,9,35,62]. It is well known that the polarization reversal in ferroelectrics does not proceed homogeneously throughout a specimen, but inhomogeneously by a nucleation growth mechanism. The reversal process has often been analyzed using a method based upon Kolmogorov-Avrami (KA) theory, which was originally a model of crystal growth [ ]. Relying on several key assumptions, Ishibashi and coworkers have developed this theory to study how the switching property is influenced by size and surface effects in ferroelectric films [109]. A detailed review on this work has been given by Nagaya et al (1993). This extended KA theory Ishibashi theory as it was called (Dennis 1993) has been compared with an earlier model of polarization reversal (Fatuzzo 1962), but no simple relationship between the two models has been found. It has recently been shown that after some mathematical manipulation the essential kinetics parameters in the KA theory, which describe the domain structure evolution, can be obtained from the measurements of the computer simulation and measurements on model ferroelectric samples [107]. Theoretical Background In Ishibashi and Takagi [109] model an infinite crystal was considered in which the ferroelectric switching occurs by motion of domain walls after nucleation. There are several typical types of domain patterns in ferroelectrics such as rochelle salt, TGS, and BaTiO3. Therefore in this model several cases according to the classifications by shapes of reversal domains, its initial size and nucleation rate has been considered as follows. 24

52 I - INTRODUCTION The one dimensional case shown in Figure 1.12 (a), where the domain boundary moves along one direction after the formation of plate-like nuclei. Plate-like nuclei Cylindrical nuclei Spherical nuclei (a) (b) (c) Figure 1.12: Shape of reversed domains and a dimensionality. (a), (b) and (c) correspond to the one, two and three dimensional cases, respectively [107,109]. The two-dimensional case is depicted in Fig (b) where the boundary moves two dimensionally after the formation of cylindrical nuclei. The case of three dimensional domain growth is shown in Fig (c), where the boundary moves three dimensionally after the formation of spherical nuclei. In the case of ferroelectric domain switching, the three dimensional case may not really occur. In each case one wall velocity 'v' is assumed which is constant under constant applied field 'E'. If a nucleus whose radius is 'rc' and whose center is at '0' (Fig. 1.12) is formed at an instant 'C', the domains originate from this nucleus covers at time t, the volume given by S(t, )=C{r, + v(t r)}" (1.6) where 'rc' is the radius of the nucleus, 'v' is the wall velocity. The effective dimensionality 'n' is related to the actual growth dimension of domain walls and 'C' is a factor determined by n (C=2, rt and 4 r for n=1,2 and 3, respectively). 3 25

53 I - INTRODUCTION One-dimensional growth (d=1) implies plate-like domains with walls moving in one direction perpendicular to the ferroelectric axis [Fig (a)]. Twodimensional growth (d=2) occurs when the nuclei are considered to be cylinders and the axis of all nuclei are parallel to the ferroelectric axis of the crystal [Fig (b)]. For three-dimensional growth (d=3) the domains are spherical and the wall is expanding in all three directions [Fig (c)]. However, in switching phenomenon two mechanisms for nucleation can be physically distinguished. In case of one step nucleation, all the nuclei involved in switching arise instantaneously at the beginning of the process from latent nuclei (e.g. defects) and there is no further nucleation after the growth of domains begins[109]. In this case the effective dimensionality is n=d. In another mechanism, the nucleation continues to occur at a constant rate during the time over which domains are growing. In this case the effective dimensionality is given by n=d+1. The time dependence of the fraction V (t) of the switched volume to the total volume is given by [109], (t) =1 exp[ (t /to )"] (1.7) where t is the time, to is the characteristic time and n is the effective dimensionality of the domain growth. If P is the polarization associated with Vs then the expression for the displacement current density can be written as [109], i(t) = 2P dv 3.(t) dt (1.8) Combining equations 1.7 and 1.8, we obtain, At) = 2P n [t f exp to to t o jn (1.9) 26

54 I - INTRODUCTION The value of the reduced characteristic time u = to /t in. [t. is the time at which the current density j(t) is maximum] and the effective dimensionality n enable us to examine Ishibashi's prediction and to ascertain information about the switching mechanism. This model has been successfully applied to various ferroelectric thin films such as TGS [8,9,109], PZT [8,9,78, ], and KNO3 [8,9,35,62,105]. This model will also be applied to analyze the switching transients in the composite films in chapter V Overview of KNO3 Investigations It has been known since the late 1960s that KNO3 exists in one of several crystallographic phases depending on external state variables. Only one phase, called phase-iii, is ferroelectric. This rhombohedral phase exists for pure bulk materialsonly between about 124 and 110 C, upon cooling from another rhombohedral phase-i at atmospheric pressure [65]. The ferroelectric phase III would be of technological interest were it only stable at room temperature. It was subsequently reported by Nolta, et al., that in thin film form KNO3 was stable at room temperature [115]. A Raman studies by Scott, et. al., indicated to those authors that the stabilization of the ferroelectric phase-iii in a thin film was not due to stress but rather to surface charges [34]. Then came a study by Isaac and Philip [47], authors who mixed polycrystalline KNO3 with carbon black used as a medium to enhance optical absorption for photo acoustic studies. Subsequent to this study, Westphal, et.al, [45,48,49] attempted to reconcile their photoacoustic results with those of Isaac and Philip [47]. Westphal, et.al, inferred that the carbon black had influenced the behaviour of the KNO3 particles in a fundamental manner. Westphal then studied KNO3 mixed with SiC powder using different sample preparation conditions. Westphal concluded that the particles of KNO3 interacted with one another differently depending upon the 27

55 I - INTRODUCTION distribution of SiC between KNO3 granules. The conclusion was drawn that the presence of SiC powder can change the characteristics of the phase changes in KNO3 [45,48,49]. 1.3 STUDIES IN OTHER FERROELECTRIC THIN FILMS In the recent past, thin films of many ferroelectric materials have been fabricated by various methods viz., sputtering [116,117], pulse laser deposition (PLD) [118], pulse laser ablation (PLA) [119], metal organic chemical vapor deposition (MOCVD) [ ], chemical solution deposition (CSD) [124,125], metal organic decomposition (MOD), molecular beam epitaxy (MBE), and sol-gel [ ]. Another advantage of obtaining ferroelectrics in thin film form is the possibilities of its integration to the CMOS. Attempts have been made to fabricate dynamic random access memories (DRAMs) [28-30], nonvolatile random access memories (NV- RAMs)[1], and ferroelectric random access memories (FeRAMs)[1,12,27]. Thin film transistors such as metal ferroelectric semiconductor field effect transistor (MFSFET) has also been fabricated and studied [31]. There are many other applications of thin film ferroelectric materials as voltage tunable dielectric capacitor in resonators and filters [1,3,4,24], ferroelectric memory cell [19-26], ferroelectric switching [ , and ferroelectric capacitors [10-17]. Also, recently thin film ferroelectrics are finding increasing use in micro electronic mechanical systems (MEMS) in producing highly sensitive actuators and sensors with low noise [1,4,24,104, ]. 1.4 STRUCTURE OF POLYMERS The nature of the polymer in the fabrication the composite can play key role in the modification of the ferroelectric properties [211,212]. Broadly speaking, polymers can be divided in two categories viz., polar and nonpolar. The polar polymers can 28

56 I - INTRODUCTION further be subdivided into two categories viz., ferroelectric and nonferroelectric. In this regard in the polar categories, we have chosen polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), and polyvinyl alcohol (PVA). Out of these PVDF is known to be ferroelectric and others PVF and PVA are nonferroelectric. Polyethylene (PE) is nonpolar and highly insulating. In this section, various relevant properties and chemical structure of the chosen polymer are described Structure of Poly (vinylidene fluoride): PVDF PVDF is well-known semicrystalline ferroelectric polymer and has been studied by many workers [ ]. The amorphous phase occupying about 50% of the polymer volume. It is clear that the amorphous part itself does not contribute to the residual polarization, but nevertheless, plays an important role in the ferroelectric polarization build-up in the ferroelectric crystallite [214]. Poly (vinylidene fluoride) (PVDF) has been intensively investigated because of its interesting ferroelectric properties and technological applications. PVDF can exist in several crystalline phases [215]. Of the five known modifications, the a- and 13-phases are most common. The a- form, is stable and arises usually from the melt when PVDF crystallizes in quiescent conditions. The form II is monoclinic and the unit cell of form II structure has no net dipole moment. Thus PVDF of form II structure may not be expected to be ferroelectric and piezoelectric. (a) Form 1 (II-phase) The [3-phase is all trans planar zigzag conformation with the dipole moments perpendicular to the chain axis. Form I has an orthorhombic structure and belongs to the space group Cm2m (C2v14) with lattice constants, a=8.58a, b=4.91a and c=2.56 A (chain axis). The dipoles of the monomer units in the polymer chain backbone are aligned in such a way that self-cancellation among them does not occur and therefore 29

57 I - INTRODUCTION spontaneous polarization occurs. The fl-phase PVDF is a polar phase associated with the highest piezo-, pyro and ferroelectric properties [216]. The magnitude of the dipole moment for each monomer unit is 7x Cm (2.1D). Assuming a rigid dipolar model, the spontaneous polarization of a single crystal of the form I type PVDF will be 13 lic/cm2, The typical values of polarization and coercive field are 2.5 lic/cm2 at 200 MV/m have been obtained from the ferroelectric hysteresis loop measurements [137] Structure of Poly (vinyl fluoride): PVF PVF polymer crystallizes much like polyethylene because the fluorine are close enough in size to hydrogen so as not to interference with regular packing. PVF has head-head and tail-tail defects, where successive repeat units are backwards. Typically, these amount to 5 % for PVDF and 25-32% for PVF. The dipole moment of PVF could be quite large in the transplanar conformation if all fluorines were on the same side on the C-C plane (isotactic) [219] Structure of Poly (vinyl alcohol): PVA Poly (vinyl alcohol) is manufactured through a process of addition polymerization, and the polymer is built on a carbon-chain backbone, with an OHgroup on very other carbon. The abundance of OH groups along each polymer strand allow it to form hydrogen bonds, making poly (vinyl alcohol) very soluble in water. Still, since the molecule is so large and bulky, the dissolving process is quite slow. PVA can exist amorphous as well as crystalline form depending upon the method of preparation. The x-ray diffraction peaks at 20=11.4, 19.4 and 40.4 have been found and corresponds to the crystalline phase. PVA was usually crystalline due to the strong intermolecular interaction between PVA chains through intermolecular hydrogen bonding [ ]. 30

58 I - INTRODUCTION As PVA and KNO3 both are soluble in water; therefore, solvent cast film of PVA: KNO3 composite can be obtained Structure of Polyethylene: PE Commercial PE is a predominantly linear non polar polymer with the chemical composition roughly of polymethylene, (CH2) n. The only stable local chain conformation of PE at low temperature is the flat zigzag chain configuration with C-C bond length of nm, and C-C-C bond angle of 112 (93'). This local chain conformation also prevails in the melt and solution. The principal crystalline form of linear PE is orthorhombic, like the linear paraffins, with theoretical density of 1.00 g/cm3. PE is a very good insulator and is therefore widely used for wire and cable encapsulation [228]. Polymer density and molecular weight have little affect on electrical properties of PE. PE with density close to g/cm is known as HDPE. The physical and other properties of the potassium nitrate and above polymers are given in Table1.4 [39,56,66,68, ]. Material Table 1.4: The physical properties of KNO3, PVDF, PVF, PVA, and PE. Melting point ( C) Thermal conductivity (W/m.K) Linear coefficient of thermal expansion Density (g/cm3) Dielectric constant Dipole Moment (C.cm) KNO x10 4 C x10-27 PVDF x10 5 C x PVF x10 5 C x10-28 PVA x10 5 C x10-28 PE x10-5 C x

59 I - INTRODUCTION 1.5 POLYMER- FERROELECTRIC COMPOSITE MATERIALS The work on ferroelectric polymers has been carried out by many workers [ ]. Ferroelectric composites are usually a mixture of a ferroelectric material embedded in a polymer matrix. The aim of these materials is to exhibit and retaining the ferroelectric, pyroelectric and piezoelectric properties. Ferroelectric composites of ceramic and polymers have received considerable attention due to their advantages in piezoelectric properties for transducer applications. These composites thus constitute a new structure, which might combine a high ferroelectric activity of ceramics and high mechanical strength of polymers. Therefore, the design of the composites with optimum properties becomes very challenging since the electro-active properties not only depend on the materials and the compositions but also on their interconnection [230]. The ferroelectric- polymer composites can combine advantageous properties such as flexibility of the polymer with easy processing and electroactivity of the ferroelectric materials for piezo, pyro, and ferroelectric applications [231]. There are many approaches, which have been successfully tried in which both the constituent materials and their connectivity have been varied. The impact of the constituent materials on the resulting properties may be obvious but the connectivity pattern can be an important factor in determining the final properties of the composites. The connectivity is usually denoted by two digits in which the first digit specifies the dimensions in which the ceramic is self-connected while the second digit describes the same quantity for the polymer phase. There are ten possible combinations of connectivity for a diphasic material of which the most important configurations are the 0-3 and the 1-3 connectivity patterns [232,233]. Among the composite studies, the 0-3 connectivity is most common, which consists a three-dimensionally connected polymer 32

60 I - INTRODUCTION matrix loaded with ferroelectrically active ceramic particles. One of the most attractive features of the 0-3 design is its versatility in fabricating a variety of forms including thin films and certain molded shapes. This type of composite is also easy to fabricate and amenable to mass production [234]. PVDF based composites play an important role in sensor and actuator applications due to their high piezoelectric effect. In most of the composite studied, PVDF and its copolymer PVDF/TrFe [Poly (vinylidene fluoride-trifluoroethylene)] have been employed. The ferroelectric composites made of electroactive ceramics and ferroelectric polymer are very attractive for applications since they can be easily prepared in a variety of shapes. They exhibit high piezoelectric and pyroelectric response, and their properties can be tailored to various requirements. Recently the dielectric and calorimetric studies of such a composite, e.g., lead lanthanum zirconate titanate (PLZT)-P(VDF/TrFe) ceramics copolymer composite have been discussed [234]. A model is proposed to establish the relevant constitutional parameters of ferroelectric composite materials such as shape, composition and connectivity, which determine the piezoelectric, dielectric, and elastic properties of the composite. A proper Monte Carlo method is used to calculate the active volumetric fraction of ceramic [235]. The use of ferroelectric composite materials in thin- films form has a general advantage reduced size and weight. BaTiO3- polymer composite layers have been produced by the spin coating technique (thickness 3-104m), The dielectric permittivity of the layers at room temperature can be tuned from 2.8 to approximately 33 by varying the ceramic filling from 0 to 60% by volume [236]. The dielectric properties of the films are almost insensitive to temperature variations in the range C [236]. 33

61 I - INTRODUCTION New ferroelectric composite materials have been fabricated and studied using nanomaterials [237]. The nanoceramics can be obtained using the solgel technology [237]. These composite shows improvement towards higher operating temperature [2371. The flexible composites were fabricated in thin film form [ tm] by hot pressed method [238]. Thin films ceramic / polymer composite sensors with mixed connectivities possess high values of piezo and pyroelectric coefficients and the formability and flexibility which are not attainable in a single-phase ferroelectric materials [238,239]. The efficiency and the piezo-and pyroelectric figure of merit are influenced by the temperature dependence of the dielectric properties and the nature of the spatial distribution of polarization of the composite materials [239]. The composite with low acoustic impedance matching to water and tissue can be produced which can be used for biomedical and under water transducer applications. Moreover, the composites are suitable for pyroelectric sensors due to their low permittivity value and the fact that they can be easily prepared in a variety of shapes. It should be observed that the dielectric permittivity (E') and dielectric losses (E") are involved in figures of merit (FOM) of various physical quantities important for applications and thus the knowledge of dielectric dispersion and absorption is essential for tuning the properties of the composites [240]. A high dielectric constant polymer composite system has been given [241]. Investigation of the polarization mechanism in tungsten bronze relaxor ferroelectrics, Pb2..x Bax Nb206 (PBN [(1- x)%)], in particular, of the morphotropic phase boundary compositions have been carried out for both ceramics and bulk crystals by means of electrical, thermal, optical, and electron microscopic methods [242]. It has been found that for the same BaTiO3 loading dielectric characteristics of the composites strongly depend on the type of polymer [243]. Polar polymers increase the dielectric constant of the 34

62 1 - INTRODUCTION composites at low frequencies but have little effect at giga hertz frequencies. At fixed frequency and temperature, the composites follow a linear relationship between logarithm of their dielectric constant and volume fraction of the ferroelectric filler [243]. The advantages of the ferroelectric composite materials are high coupling factor, low acoustic impedance, good matching to water or human tissue, mechanical flexibility, broad bandwidth in combination with a low mechanical quality factor and the possibility of making undiced arrays by simply patterning the electrodes. Piezoelectric composite materials are especially useful for underwater sonar and medical diagnostic ultrasonic applications. The ferroelectric composite materials are also useful for acoustic transducer applications and actuator applications [244]. 1.6 SCOPE AND MOTIVATION OF THE PRESENT WORK The ferroelectric, dielectric and polarization switching properties of the ferroelectric materials in general and of composites in particular have become important from the point of view of scientific and technological interest. The polymer-ferroelectric composite materials are emerging as a new class of electronic and dielectric materials. The composite materials for piezoelectric and pyroelectric applications have gained enormous attention in the past because of their potential to be produced with desirable properties by choosing a proper combination of the constituent phases. The ceramic-polymer composite with ferroelectric properties combines many useful properties of the polymer such as flexibility of the polymer and electroactivity of the ceramic to produce large area device with reasonable mechanical strength. The composite materials have been processed using mixtures of ceramic as filler in a matrix of polymer, where the fillers are included in the matrix in order to modify its physical properties in a high range. The composites in film form are also 35

63 I - INTRODUCTION becoming important due to their applications as low voltage operated ferroelectric devices. It has been known since the late 1960s that KNO3 exists in one of several crystallographic phases depending on external state variables. Only one phase, called phase III, is ferroelectric. This rhombohedral phase III exists for pure bulk material only between about 124 and 110 C, upon cooling from another rhombohedral phase I at atmospheric pressure. Phase III would be of technological interest were it only stable at room temperature. There is considerable motivation to retain ferroelectric phase III at room temperature. Several groups have had such a goal as a focus of their research. Several attempts have been made to obtain phase III at room temperature, either by applying hydrostatic pressure or by mixing the KNO3 powder with carbon black, SiC or starch. The stabilization of the ferroelectric phase III in thin films of KNO3 at room temperature has also been reported and was attributed to the existence of surface field effects. The presence of extraneous materials in KNO3 can suppress the conversion of phase III to phase II thus stabilizing phase III at room temperature. For example, it has been reported that KNO3 in a KBr matrix, when kept in a nonabsorbent liquid shows a very long stability of phase III at room temperature. In order to obtain phase III of KNO3 at room temperature, we have thought to prepare the composite of KNO3 with different polymers. The nature of the polymer in the fabrication the composite can play a key role in the modification of the ferroelectric properties. Broadly speaking, polymers can be divided in two categories viz., polar and nonpolar. The polar polymers can further be subdivided into two categories viz., ferroelectric and nonferroelectric. In this regard in the polar categories, we have chosen polyvinylidene fluoride (PVDF), polyvinyl fluoride 36

64 I - INTRODUCTION (PVF), and polyvinyl alcohol (PVA). Out of these PVDF is known to be ferroelectric and others PVF and PVA- are nonferroelectric. Polyethylene (PE) is nonpolar and highly insulating. The presence of polymer can also reduce brittleness and increase cohesion to the composite films. The densities of PVDF / PVF and KNO3 are in the same range and this can be helpful in the formation of uniform composite layers. The dielectric constants of the polymers and KNO3 are also in the same order. The work in the thesis has been focused on the composite films of potassium nitrate (KNO3) with different polymers like PVDF, PVF, and PVA and PE. The retantivity of phase III at room temperature in the composite films has been analyzing using thermal (DSC) and structural studies (x-ray diffraction). The composites, which are favourable to the ferroelectric phase retantivity, have been chosen for further. studies of ferroelectric and electrical properties. 37

65 II- EXPERIMENTAL : DESIGNED, FABRICATION, MATERIALS, AND MEASUREMENTS CHAPTER-II EXPERIMENTAL: DESIGNED, FABRICATION, MATERIALS, AND MEASUREMENTS 2.1 INTRODUCTION This chapter is contains the details and description of materials used, fabrication process of the composite films, design of the circuits, and the measuring cell. In this chapter the sample preparation techniques, electrode deposition and the procedures adopted for various electrical measurements are also presented. 2.2 MATERIALS USED AND THEIR SPECIFICATIONS (i) Potassium Nitrate (KNO3): Source: Form: Solubility: Atomic weight: E-Merck, INDIA Powder Water-soluble g/mol (ii) Polyvinylidene fluoride (PVDF) Structure: Source: Form: -C142?--CF2- Aldrich, U.S.A. Powder (iii) Polyvinyl Alcohol (PVA) Structure: Source: Form: Solubility: [-CH2 CH (OH)-] n G.S. Chemical, INDIA Powder Water-soluble Average Molecular weight

66 II- EXPERIMENTAL : DESIGNED, FABRICATION, MATERIALS, AND MEASUREMENTS (iv) Polyvinyl fluoride (PVF) Structure: Source: Form: -CH2=CHF- Aldrich, (U.S.A.) Powder (V) Polyethylene (PE) Structure: Source: Form: -(CH2)n- INDIA Powder 2.3 DETAILS OF THE INSTRUMENTS USED IN VARIOUS EXPERIMENTS (1) X-Ray Diffraction (XRD): The x-ray diffraction scans for ferroelectric composite films were undertaken using BRUKER AXS diffractometer with Nifiltered, CuKa radiation of wavelength 1.54 A. The detector was scanned at 0.05 per step. The goniometer speed was 10/ minute. (2) Differential Scanning Calorimetry (DSC): The DSC measurements were carried out at 10 C/min by using Perkin Elmer, Pyris Diamond Model for examining the phase transition temperatures of the composite films. (3) Polymer Film Making Press (Melt-Press Machine): Model: Source: KB, Press Model SL89 Spectra Lab, INDIA (4) Vacuum Coating Unit: Model: Source: 12 A 4D Hind High Vacuum Co., INDIA. 39

67 II- EXPERIMENTAL : DESIGNED, FABRICATION, MATERIALS, AND MEASUREMENTS (5) Storage Oscilloscope for measuring P-V, J-V Characteristics. Model: HM407-2Hameg Instruments, Germany (6) C-V Analyzer Model:. Keithley 590 (7) Capacitance Bridge Model: LCRQ Bridge 6018, Scientific, INDIA (8) Precision Electronic Micro Balance Model: HM 202, JAPAN (9) Scientific Function Generator Model: SM 5060, Scientific INDIA (10) Programmable Temperature Controller Model: 1570 Source: Digitech-Roorkee, INDIA (13) Standard Test Sieves Mesh NO. 240 Particle Size 60 [tm Source: Asian Scientific Inst. INDIA 2.4 SAMPLE PREPARATION The following procedures were adopted for the fabrication of the composite samples. The composite films were obtained by mixing of KNO3 and the polymer component in known proportion and then pressing the composite mixture at elevated temperature and pressure in a hot press machine. The fabrication process of the composite films is given below by the help of flow chart. 40

68 II- EXPERIMENTAL : DESIGNED, FABRICATION, MATERIALS, AND MEASUREMENTS FABRICATION PROCESS KNO3 powder Crystal growth in water Crystals dried in vacuum Fine powder Filtering through (Mesh No. 240) Particles size 60 i.tm Polymer: KNO3 mixture Hot Pressing Composite film Figure 2.1 : Flow chart of the fabrication steps involved in making of the composite films Purification of Potassium Nitrate (KNO3) The purification of KNO3 means to remove the unwanted impurities. For this purpose, the crystal grown method was adopted in which KNO3 powder was dissolved in double distilled water and saturated solution was prepared. The crystals were grown by keeping the solution at a constant temperature for about 100 hrs in closed environment for its slow growth. Good and larger size crystals were obtained and excessive water was removed by using the whatmann 1 filter paper. These crystals were kept in vacuum oven at pressure 10-3 mbar at 60 C for 2 days. The crystals were ground in a mortar and pestle to obtain the powder and this powder was passed through 41

69 II- EXPERIMENTAL : DESIGNED, FABRICATION, MATERIALS, AND MEASUREMENTS standard sieves [mesh No.240] (particle size m 60pm). This powder was further dried and used to fabricate the composite films with different polymer matrix Mixing with Polymer and Fabrication of Composite Films The purified potassium nitrate powder was uniformly mixed with known weight of polyvinylidene fluoride powder. The powder mixture was kept in a stainless steel die in the melt press machine shown in Figure 2.2. Figure 2.2: Melt-Press machine for ferroelectric composite films. The mixture was heated up to temperature of 218 C and then a stress of 250 Kg/cm2 was applied for 30 seconds. The temperature of die was brought down to room temperature and then the pressure was released. Using this method of preparation, different wt.% KNO3 (10, 30, 40, 50, 70 and 80%) composite films were prepared. The same procedure was adopted for preparing composite films of KNO3 with PVF and PE polymers. The composte films more than 50wt.% KNO3 doping were found to be more 42

70 EXPERIMENTAL : DESIGNED, FABRICATION, MATERIALS, AND MEASUREMENTS brittle. Therefore, 50wt.% KNO3 composite films were thought to be better composite films for ferroelectric applications Fused Composite Films The ferroelectric properties were also studied in the fused composite films. The fusing process gives thin composite films as discussed below. The composite films obtained by hot press method (section 2A.2} were used and then fused onto the copper substrate at 380 C and then cooled down to room temperature. The top copper electrode having an area of 7.85 x10-3 cm` was vacuum deposited on the top surface of the sample. The thickness of the fused film was measured to be 5 pm by capacitance method using dielectric constant, K-12. The same method of preparation was adopted for another percentages of KNO3 doped samples. The earlier reports on the pure fused KNO3 films quenched at the room temperature show ferroelectric behaviour [256]. This supports and justifies the making of the fused composite films for ferroelectric studies Preparation of the Solvent-Cast Films of KNO3 s PVA Composite The purified powders of potassium nitrate and PVA were used for the preparation of composite films. The powder of KNO3 (50% by weight) was mixed in PVA powder. The powder mixture was dissolved in doubled distilled water and stirred well while heating at 100 C. The composite solution of PVA: KNO3 was spread onto the glass plate and covered by a glass belzar for three days. After the solvent cast composite film has formed, it was removed from the glass plate and kept in a vacuum oven for six hours at 60 C to remove the moisture. Then the composite film was pressed in a stainless steel die at 118 C and stress 250 Kg/cm2 for 1 minute and cooled down to room temperature. The thickness of the composite film so obtained was measured to be x30 p.m. 43

71 11- EXPERIMENTAL : DESIGNED, FABRICATION, MATERIALS, AND MEASUREMENTS Electrode Deposition Figure 2.3 shows the vacuum unit used for electrode deposition at 10-5 mbar on to the composite films. The experimental studies of other workers have shown here that the indium electrode gave better performance with ferroelectric measurements [32,103,1451 Therefore, indium electrodes were chosen to carry out all the electrical measurements on KNO3 composite films. ('hamber Seal Baffle or high vacuum valve Gauges Inlet r valve I 4 i Roughing valve Cold trap High Vacuum Pump Backing Valve Rotary Pump (Diffusion pump) Figure 23: Shows the lay out of the vacuum deposition apparatus 44

72 H- EXPERIMENTAL DESIGNED, FABRICATION, MATERIALS, AND MEASUREMENTS 2.5 INSTRUMENTATION AND MEASUREMENT TECHNIQUES The present experimental research work involves various types of measurements viz., P-E loops, switching, and dielectric versus temperature, C-V and electrical conductivity measurements. A brief description of the measurement techniques with the experimental setups and the equipment involved is outlined below Measurement Cell Figure 2.4 shows the measurement cell designed and fabricated for the investigations of the P-V hysteresis loop, J-V characteristics and temperature dependent of electrical conductivity of the composite films. Figure 2.4: Measurement cell with sample holder for composite films. The cell consist of a brass cylindrical box of diameter (-8 cm) over which the BNC connections were taken. This cylinder is kept onto a metallic base fitted with heater and shielding. The heating can be operated in the temperature range 20 to 200 C with the help of a programmable temperature controller with adjustable heating rate from 1-5 C/min. The sample is held in a sandwich configuration by a sample holder 45

73 II- EXPERIMENTAL : DESIGNED. FABRICATION. MATERIALS, AND MEASUREMENTS made up of precise screw gauge for proper contact to the upper part of the sample with Teflon insulation. Proper shielding is also done by grounding the measurement cell to prevent the extranoise created by the external electrical disturbances. The electrical connections were made by shielded wires through BNC connectors with Teflon insulations. The temperature of the sample was monitored using a alumel-chromel thermocouple and could be controlled by programmable temperature controller within an accuracy of 1 C Ferroelectric Hysteresis Loop Measurements The measurements of P-V hysteresis loop are important because the existence of the loop together with the reversal of spontaneous polarization on application of electric field is commonly accepted as a proof of ferroelectricity. Figure 2.5 shows the modified Sawyer-Tower circuit designed and fabricated for the investigation of ferroelectric hysteresis loop of the composite films. Figure 2.5: Circuit diagram for measurement of P-V characteristics [switch position "b1 and J-V characteristics [switch position "al. 46

74 II- EXPERIMENTAL : DESIGNED, FABRICATION, MATERIALS, AND MEASUREMENTS The typical hysteresis loop and the corresponding J-V characteristics in the composite films , warm. Figure 2.6: Typical ferroelectric hysteresis loop of the composite films. Figure 2.7: Typical butter-fly loop for J-V characteristics of the composite films. The input signal of 50 Hz was applied to the sample using a function generator (Scientific SM 5060). The voltage applied to the sample can be varied up to 30 Vpp to obtain proper hysteresis loop. The hysteresis loop was recorded using a storage oscilloscope connected by the computer with standard software (SP107e Germany). A 47

75 II- EXPERIMENTAL : DESIGNED, FABRICATION, MATERIALS, AND MEASUREMENTS variable resistance potentiometer was connected in parallel to the computer for adjusting the hysteresis loop. The typical hysteresis loop obtain is shown in Figure 2.6. The corresponding J-V characteristics were also recorded in the composite files as shown in Figure Ferroelectric Switching Transient Measurements The polarization switching characteristics using sinusoidal, triangular and square wave signals were applied and the output current was observed across a resistance in series with the sample through the storage oscilloscope when kept in X-t mode. The storage oscilloscope was also connected to the computer with standard software (SP107e Germany). The schematic circuit diagram and set up for measuring the switching characteristics is shown in Figure 2.8. The typical out put of the switching current pulses obtain from composite film is shown in Figure 2.9 after the application of bipolar square wave signal. Pulse Train Input (Sine, S quare,and Triangular) Digital Storage oscilloscope Sample Resistance Figure 2.8 : Ferroelectric polarization switching Measuring Circuit 4g

76 II- EXPERIMENTAL : DESIGNED, FABRICATION, MATERIALS, AND MEASUREMENTS Figure 2.9 : Typical switching pulse of the composite films using square wave signals Electrical Conductivity Measurements The composite films, coated with circular electrodes were used in the present investigations. A fixed voltage applied with the help of constant voltage DC power supply to the sample and the current was measured at the different values of the temperature. The electrical conductivity was calculated using the expression _ AV (2.1) where d is the sample thickness, A is the area of the electrode and I is the current flowing under applied voltage of 10V. The conductivity was evaluated as a function of temperature for 50wt.% KNO3 composite sample for the heating and cooling modes (..:1 5 C/min). The dc conductivity variation with temperature has been found to provide knowledge of the phase transition in KNO3 as discussed in chapter IV Capacitance-Voltage (C-V) and Dielectric Measurements The C-V measurements were performed at 105 Hz on 50wt.% KNO3 composite films using a Keithley 590 CV analyzer in parallel mode. The voltage was swept from 49