This article was downloaded by: [115.118.165.60] On: 22 July 2015, At: 23:30 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: 5 Howick Place, London, SW1P 1WG Click for updates Phase Transitions: A Multinational Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gpht20 The effect of CuO and NiO doping on dielectric and ferroelectric properties of Na 0.5 Bi 0.5 TiO 3 lead-free ceramics Sunanda Kakroo a, Arvind Kumar b, S.K. Mishra c, Vijay Singh d & Pramod K. Singh ae a Research and Technology Development Center, School of Basic Sciences & Research, Sharda University, Greater Noida, India b G. L. Bajaj, Institute of Technology & Management, Greater Noida, India c Group Director(Research), Rameesh Group of Institutions, Greater Noida, India d Department of Chemical Engineering, Konkuk University, Seoul, Republic of Korea e Institute of Micro and Nanosystem Technology, Vestfold University College, Tonsberg, Norway Published online: 21 Jul 2015. To cite this article: Sunanda Kakroo, Arvind Kumar, S.K. Mishra, Vijay Singh & Pramod K. Singh (2015): The effect of CuO and NiO doping on dielectric and ferroelectric properties of Na 0.5 Bi 0.5 TiO 3 lead-free ceramics, Phase Transitions: A Multinational Journal, DOI: 10.1080/01411594.2015.1063631 To link to this article: http://dx.doi.org/10.1080/01411594.2015.1063631 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
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Phase Transitions, 2015 http://dx.doi.org/10.1080/01411594.2015.1063631 RESEARCH ARTICLE The effect of CuO and NiO doping on dielectric and ferroelectric properties of Na 0.5 Bi 0.5 TiO 3 lead-free ceramics Sunanda Kakroo a *, Arvind Kumar b, S.K. Mishra c, Vijay Singh d and Pramod K. Singh a,e a Research and Technology Development Center, School of Basic Sciences & Research, Sharda University, Greater Noida, India; b G. L. Bajaj, Institute of Technology & Management, Greater Noida, India; c Group Director(Research), Rameesh Group of Institutions, Greater Noida, India; d Department of Chemical Engineering, Konkuk University, Seoul, Republic of Korea; e Institute of Micro and Nanosystem Technology, Vestfold University College, Tonsberg, Norway (Received 6 May 2015; accepted 15 June 2015) In the present work, lead-free piezoelectric ceramics (Na 0.5 Bi 0.5 )TiO 3 xcuo ynio (for x D 0.0, 0.02, 0.04 and 0.06) have been prepared by a conventional solid-state reaction method. An investigation of CuO and NiO doping in bismuth sodium titanate (BNT) and a study of the structure, morphology, and dielectric and ferroelectric properties of the NBT CuNi system have been conducted. Phase and microstructural analysis of the (Na 0.5 Bi 0.5 )TiO 3 (NBT) based ceramics has been carried out using X- ray diffraction and scanning electron microscopy (SEM) techniques. Field emission scanning electron microscopy (FE-SEM) images showed that inhibition of grain growth takes place with increasing Cu and Ni concentration. The results indicate that the co-doping of NiO and CuO is effective in improving the dielectric and ferroelectric properties of NBT ceramics. Temperature-dependent dielectric studies have also been carried out at room temperature to 400 C at different frequencies. The NBT ceramics co-doped with x D 0.06 and y D 0.06 exhibited an excellent dielectric constant e r D 1514. The study suggests that there is enormous scope of application of such materials in the future for actuators, ultrasonic transducers and high-frequency piezoelectric devices. Keywords: ceramics; X-ray diffraction; dielectric properties; piezoelectricity; ferroelectric 1. Introduction Lead zirconate titanate (PZT) based ceramics are widely used piezoelectric materials for actuator, sensor and transducer applications due to their excellent piezoelectric properties. However, the hazardous nature of piezoelectric materials due to lead content raises serious environmental concerns.[1 3] In recent years, intensive research has been carried out to search for an alternative environmental-friendly lead-free material. Lead-free piezoelectric ceramics are classified into ferroelectrics with a bismuth layered structure, tungsten bronze and perovskite structures.[4,5] Lead-free piezoelectric materials with a perovskite structure are found to be suitable for actuator and high-power applications. In this structure, cations based on their valence states and coordination numbers occupy the A or B sites. The structure may be described by a simple cubic unit cell with a large cation (A) at the corners, a small cation (B) at the body center and oxygen (O) at the center of the faces.[5,6] *Corresponding author. Email: sunandakakroo@gmail.com Ó 2015 Taylor & Francis
2 S. Kakroo et al. (Na 0.5 Bi 0.5 )TiO 3 (NBT) and Na 0.5 K 0.5 NbO 3 (KNN) are currently two important leadfree piezoelectric compositions which exhibit perovskite structures. 5KNN-based ceramics have shown a piezoelectric constant of 300 pc/n. Its transition temperature is found to be»400 C. KNN-based compositions exhibit better piezoelectric and electromechanical properties, as reported in the literature.[1,5 10] However, KNN ceramics are difficult to synthesize due to high volatilization of the Na and K elements. They also require high sintering temperatures.[10 13] In the recent period, lead-free bismuth sodium titanate (BNT) has attracted a lot of interest due to its excellent dielectric and piezoelectric properties. BNT ceramic discovered by Smolenskii et al. in 1960 [7,8] is considered as one of the promising lead-free piezoelectric ceramics. BNT belongs to the perovskite family (ABO 3 ) with a bismuth cation (Bi 3C ) and a sodium cation (Na C ) at the A sites and a titanium cation (Ti 4C ) at the B site. BNT along with its solid solution and alkali niobate compounds has received considerable attention due to their better piezoelectric properties and coupling coefficients than any other lead-free-based piezo-ceramics. It has a relatively large remnant polarization and coercive field at room temperature and high Curie temperature (T c» 340 C). Therefore, BNT piezoelectric ceramics are not only environment friendly, but also find applications in the field of science and technology. However, the major disadvantages of BNT ceramics are their (1) high coercive field, and (2) high conductivity, creating difficulties during the poling of samples, which can be minimized with suitable doping. It has been reported that BNT-based compositions modified with different dopants, e.g. BNT-ATiO 3 (A D Ca, Sr, Ba and Pb), BNT-Bi 0.2 Sr 0.7 TiO 3, BNT-KNbO 3 (KN), (Bi 1-x La x ) 0.5 (Na 1-y Li y ) 0.5 TiO 3 (BLNLT), BNT-Bi 0.5 Li 0.5 TiO 3 (BNLT), BNT- Bi 0.5 K 0.5 TiO 3 (BNKT), BNT-K 0.5 Na 0.5 NbO 3 (BNT-KNN), (Bi 1-x Nd x ) 0.5 Na 0.5 TiO 3 (BNNT), BNT-BKT-KNN, (1-x)(Bi 0.96 La 0.04 ) 0.5 Na 0.5 TiO 3 -(x)(ba 0.90 Ca 0.10 )TiO 3 (BLNT-BCT), BNT-BKNN, BNT-BKT-BiFeO 3, and BNT-BKT-BaTiO 3 -SrTiO 3 ceramics, play a significant role in the improvement of dielectric and piezoelectric properties.[14 23] Vijayeta Pal et al. synthesized the [1 {(Bi 1 x La x ) 0.5(Na 1 y Li y ) 0.5TiO 3 } zbatio 3 ] (BLNLT BT), where x D 0.04, y D 0.025 and z D 0, 0.02, 0.04 and 0.06, ceramic by a semi-wet route and reported that dopants (La/Li/Ba) affect the microstructure, phase transition temperatures (T d and T m ), and dielectric, ferroelectric and piezoelectric properties.[18] Similarly characteristic reported that the addition of an NiO additive in the (y) BNT-yBaTiO 3 (BNT-BT) ceramics influences the dielectric property as well as the phase transition temperature.[24] Tian et al. synthesized the CuO-doped lead-free BNT (Bi 0.5 Na 0.5 TiO 3 ) and barium zirconate titanate (Ba(Zr 0.07 Ti 0.93 )O 3 ceramics by a multi-step solid-state reaction process. At room temperature, the sample doped with 2 mol% of CuO showed excellent piezoelectric properties (d 33 D 156.5 pc/n) and also a high-electromechanical-coupling factor (kt D 52%).[25] In the present work, an effort has been made to obtain single-phase powders of (Na 0.5 Bi 0.5 )TiO 3 xcuo ynio (NBT CuNi) for x D y D 0, 0.02, 0.04 and 0.06. Also, a detailed study of the microstructure, temperature-dependence dielectric properties, dielectric loss and ferroelectric properties has been carried out. 2. Experimental procedure 2.1. Materials High-purity raw materials (AR grade Merck) of Bi 2 O 3 (99%), Na 2 CO 3 (99.9%), TiO 2 (99%), CuO (98%) and NiO (98%) (Merck KGaA, Darmstadt, Germany) were used as starting materials in the present work.
Phase Transitions 3 2.2. Synthesis method Lead-free piezoelectric ceramics (Na 0.5 Bi 0.5 )TiO 3 xcuo ynio (for x D y D 0, 0.02, 0.04, 0.06 and 0.08) were prepared by a conventional solid-state reaction process. A stoichiometric amount of all the powders was thoroughly mixed and ball milled using zirconia balls for 12 hours. Ethyl alcohol was used as a mixing medium. The mixed slurry was kept in a dust-free environment to evaporate ethyl alcohol. The dried mixture was then put in an alumina crucible and calcined at 750 C for two hours in a muffle furnace. The calcined powder formed in lumps was then ground to remove lumps, obtaining fine powder. The X-ray diffraction (XRD) technique was used to investigate the nature of calcined powders. The powders were then mixed with 2 wt.% of polyvinyl alcohol (PVA), which was used as a binder to obtain material in the form of circular discs. The compacted circular discs obtained at a load of 5 kn were cleaned with an 8/0 class of emery paper in order to remove the iron contamination. The green circular pellets were heated at 500 C for one hour to remove the PVA binder. The samples were then sintered for densification in a programmable muffle furnace at 1100 C for two hours with a heating rate of 2 C/minute. The samples were then cooled in the furnace to room temperature. The density of the sintered samples was determined using the Archimedes principle. 3. Characterizations 3.1. X-ray diffraction Powder XRD patterns were recorded at room temperature using a Rigaku X-ray diffractometer having Cu Ka radiations. The diffraction patterns were recorded at a scan rate of 2 per minute for 2u varying from 20 to 70. 3.2. Microstructure The sintered samples were gold coated for SEM studies. The surface morphology was studied using the SEM model FE-SEM Quanta 200 FEG. 3.3. Dielectric and ferroelectric measurements The dielectric constant (e r ) and tangent loss (tand) were obtained as a function of temperature using a HIOKI-LCR (Model-3532-50) meter in a proportional-integral-derivative (PID)-controlled heating chamber. A polarization vs. electric field hysteresis (P E) loop tracer (Marine India) based on a modified Sawyer Tower circuit was used to study the variation of polarization with electric field. 4. Results and discussions 4.1. X-ray diffraction analysis Figure 1 shows the XRD patterns of sintered samples of (Na 0.5 Bi 0.5 )TiO 3 xcuo ynio ceramics for x D y D 0.0, 0.02 and 0.06. It is clear from the XRD patterns that NBT CuNi ceramics are similar to the pure BNT ceramic, which shows a rhombohedral structure.[26] It can be seen from the patterns that the peaks do not change with increasing values of x and y. The indexing of XRD peaks obtained from various compositions suggests that the structure is rhombohedral in nature. However, it is also evident from the XRD patterns that an extra peak appears, which may be due to the presence of impurities
4 S. Kakroo et al. Figure 1. XRD patterns of (Na 0.5 Bi 0.5 )TiO 3 xcuo ynio for x D y D 0.0, 0.02 and 0.04. in the material. Several investigations reveal that there are numerous controversies with regard to the crystallographic symmetry of BNT ceramics. A large group of researchers have reported that BNT has a rhombohedral structure at room temperature.[27 29] However, few researchers suggest that the BNT shows a monoclinic phase at room temperature.[30,31] In order to confirm the structure and symmetry of the pure and modified BNT system, further study has been carried out with the help of some structural software, which will be presented later. 4.2. Microstructural analysis SEM micrographs of (Na 0.5 Bi 0.5 )TiO 3 xcuo ynio ceramics for x D y D 0, 0.02, 0.04 and 0.06 sintered at 1000 C for two hours are shown in Figure 2. It is evident from the micrograph that the pure Na 0.5 Bi 0.5 TiO 3 samples are porous. But all samples doped with CuO and NiO exhibit dense and uniform microstructures. The average grain size of the samples has been calculated with the help of the line intercept method and found to be 2 4 mm. Grains and grain boundaries are well distributed and clearly visible in the doped samples. 4.3. Dielectric analysis The variations of dielectric constant (e r ) and dielectric loss (tand) with temperature at frequencies 1, 10 and 100 khz of (Na 0.5 Bi 0.5 )TiO 3 xcuo ynio ceramics for x D y D 0, 0.02, 0.04 and 0.06 are shown in Figure 3. The temperature dependence of dielectric constant (e r ) shows that e r increases to a certain temperature (T m ) and exhibits broad dielectric maxima around T m, and with further increase in temperature above T m, the dielectric
Phase Transitions 5 Figure 2. 0.06. SEM micrographs of (Na 0.5 Bi 0.5 )TiO 3 xcuo ynio for x D y D 0.0, 0.02, 0.04 and constant starts to decrease. Our investigation shows that the samples are by nature highly dependent on frequencies. A careful look at the dielectric constant vs. the temperature graph for various compositions of NBT reveals that there are signatures for two broad dielectric anomalies presented as T d and T m. In general, two broad dielectric anomalies are obtained in the pure BNT system, which are known to be T d and T m, where T d is referred to as the depolarization temperature, corresponding to the transition from the ferroelectric to intermediate phase, and T m refers to the temperature at which the dielectric constant is maximum and the phase transition corresponds to a transition from the antiferroelectric to paraelectric phase. The ferroelectric-to-paraelectric phase transition becomes broader with doping concentration of NiO and CuO in NBT. It is also observed from Figure 3 that T d and T m merge into single with increasing Ni and Cu contents in NBT ceramics. This behavior occurs due to the doping of multiple ions in NBT ceramics, which affects the microstructures and crystallinity of the samples.[32 35] From the figure, it is also evident that the phase transition at T m is a diffuse-type phase transition. The room temperature values of e r, tand and Tc (max) at a frequency of 1 khz are given in Table 1 for different compositions. The tangent loss decreased compared to the pure NBT in all samples, but it increased with further doping of NiO and CuO in NBT ceramics. The increase in the tangent loss of samples implies that the samples have higher conductivity. The diffuse-like behavior of the (Bi 0.5 Na 0.5 )TiO 3 xcuo ynio samples may be explained on the basis of the presence of micro-heterogeneities. Micro-heterogeneities arise due to random occupation of A and B sites by different ions. Such heterogeneous distributions of cations lead to different states of polarization and hence have different
6 S. Kakroo et al. Figure 3. Variation of dielectric constant and dielectric loss with temperature of (Na 0.5 Bi 0.5 ) TiO 3 xcuo ynio for x D y D 0.0, 0.02, 0.04 and 0.06.
Phase Transitions 7 Table 1. Density, dielectric constant, dielectric loss and T c of (Na 0.5 Bi 0.5 ) TiO 3 xcuo ynio ceramics at 1 khz. x D y Density (g cm 1 ) (e r ) at room temperature tand at room temperature T c ( C) (e r )att c g 0.0 4.87 638 0.86 340 12815 1.29 0.02 5.21 1074 0.19 340 8169 1.36 0.04 5.32 1143 0.13 345 8344 1.43 0.06 5.62 1514 0.53 350 12047 1.51 relaxation times in different regions. Due to this, the dielectric maxima get diffused. [36,37] The degree of diffuseness of phase transition can be calculated with the help of the following relation described by Uchino and Namura [38]: 1=e r D 1=e rm C C 1 ðt TcÞ g ; (1) where C D 2 e rm d 2, e rm D dielectric constant at T C and d indicates the deviation from the Curie Weiss temperature, and g is the degree of diffuseness of the phase transition. When g D 1, the materials are normal ferroelectrics, for 1 < g < 2, they are known as relaxor ferroelectrics, whereas g D 2 corresponds to the diffuse phase. The value of g is calculated from the plot log [1/e r 1/e rm ] vs. log [(T Tc) g ], which is shown in Figure 4. The figure suggests that the compound does not obey the Curie Weiss law. As the value of x and y increases, g increases and the (Bi 0.5 Na 0.5 )TiO 3 -xcuo ynio compound becomes disordered in nature, which suggests that the diffuseness can mainly be attributed to the structural disordering and compositional fluctuations during the arrangement Figure 4. 100 khz. log (1/e r 1/e rmax ) vs. log (T T m ) plot of (Na 0.5 Bi 0.5 )TiO 3 xcuo ynio ceramics at
8 S. Kakroo et al. Figure 5. Hysteresis loop for (Na 0.5 Bi 0.5 )TiO 3 xcuo ynio ceramics for (a) x D y D 0.0, (b) x D y D 0.02, (c) x D y D 0.04 and (d) x D y D 0.06. of cations in one or more crystallographic sites of the structure. The finding suggests that all compositions of NBT exhibit good relaxor properties. 4.4. Ferroelectric behavior The P E loops for all samples of NBT ceramics have been measured at room temperature and the results are presented in Figure 5. The values of the saturation polarizationðp s Þ, the remnant polarization ðp r Þ and coercive field ðe c Þ are listed in Table 2. The table clearly depicts that the value of the remnant polarization ðp r Þ for pure BNT is very low. This may be due to the highly porous nature of the BNT sample. As reported earlier, CuO and NiO doping has a significant effect on the ferroelectric properties of (Na 0.5 Bi 0.5 )TiO 3 x- CuO ynio ceramics. A large change in the remnant polarization is observed when the Table 2. Values of the coercive field ðe c Þ and remnant polarization ðp r Þ at room temperature for (Na 0.5 Bi 0.5 ) TiO 3 xcuo ynio ceramics. Composition (x D y) Coercive field E c (kv cm 1 ) Remnant polarization ðp r Þ (mccm 2 ) Maximum polarization (P max )(mccm 2 ) 1 0.0 12.31 2.76 4.22 2 0.02 9.0 4.97 8.52 3 0.04 6.98 7.16 10.80 4 0.06 8.0 9.12 14.12
Phase Transitions 9 doping of CuO and NiO is increased in pure NBT ceramics. It is observed that P r increases and a well-saturated hysteresis loop are for the samples. 5. Conclusions In the present work, lead-free (Na 0.5 Bi 0.5 )TiO 3 xcuo ynio ceramics for x D y D 0, 0.02, 0.04 and 0.06 were prepared by a solid-state reaction method. The microstructure image showed homogeneous grain growth in NBT doped with NiO and CuO. The dielectric and ferroelectric properties of NBT CuNi ceramics were improved significantly and found to be 1514 and 9.12 mc cm 2, respectively, for the composition (Na 0.5 Bi 0.5 ) TiO 3 0.06CuO 0.06NiO. The temperature dependence dielectric behavior confirmed the diffuse nature of phase transition in NBT CuNi ceramics. It was also confirmed that the depolarization temperature vanishes with increase in the amount of doping of NiO and CuO in NBT ceramics. Acknowledgements The authors are grateful to the School of Material Science & Technology, Indian Institute of Technology, Banaras Hindu University, Varanasi, for XRD and SEM measurements. The authors also extend their thanks to Prof. R.K. Dwivedi, JIIT, Noida, India, for P E loop measurements. Disclosure statement No potential conflict of interest was reported by the authors. References [1] Jaeger RE, Egerton L. Hot pressing of potassium-sodium niobates. J Am Ceram Soc. 1962;45:209 213. [2] Dubok VA. Bioceramics yesterday, today, tomorrow. Powder Metallurgy Met. 2000;39:381 394. [3] Nettleton RE. Ferroelectric phase transitions: a review of theory and experiment. Ferroelectrics. 1970;1:87 91. [4] Takenaka T, Nagata H, Hiruma Y, et al. Lead-free piezoelectric ceramics based on perovskite structures. J Electroceramics. 2007;19:259 265. [5] Park SE, Shrout TR. Characteristics of relaxor-based piezoelectric single crystals for ultrasonic transducers. IEEE Trans Ultrason Ferroelectr Freq Control. 1997;44:1140 1147. [6] Jaffe B, Cook WR, Jaffe H. Piezoelectric ceramics. London: Academic Press; 1971. [7] Smolenskii GA, Isupov VA, Agranovskaya AI, et al. New ferroelectrics of complex composition. Sov Phys Solid State. 1961;2:2651 2654. [8] Takenaka T, Sakata K, Toda K. Piezoelectric properties of (Bi 1/2 Na 1/2 )TiO 3 -based ceramics. Ferroelectrics. 1990;106:375 380. [9] Guo Y, Kakimoto K, Ohsato H. Phase transition behavior and piezoelectric properties of (Na 0.5 K 0.5 )NbO 3 LiNbO 3 ceramics. Appl Phys Lett. 2004;85:4121 4123. [10] Saito Y, Takao H, Tani T, et al. Lead-free piezoceramics. Nature. 2004;432:84 87. [11] Hollenstein E, Damjanovic DM, Setter N. Piezoelectric properties of Li and Ta modified (Na 0.5 K 0.5 ) NbO 3 ceramics. Appl Phys Lett. 2005;87:182905. [12] Yoo J, Lee K, Chung K, et al. Piezoelectric and dielectric properties of (LiNaK)(NbTaSb)O 3 ceramics with variation in poling temperature. Jpn J Appl Phys. 2006;45:7444 7448. [13] Nagata H, Takenaka T. Lead-free piezoelectric ceramics of (Bi 1/2 Na 1/2 ) TiO 3 1/2(Bi 2 O 3 Sc 2 O 3 ) system. Jpn J Appl Phys. 1997;36:6055 6057. [14] Takenaka T, Nagata H, Hiruma Y. Phase transition temperatures and piezoelectric properties of (Bi 1/2 Na 1/2 )TiO 3 and (Bi( 1/2 )K( 1/2 ))TiO 3 -based bismuth perovskite lead-free ferroelectric ceramics. IEEE Trans Ultrason Ferroelectr Freq Control. 2009;56:1595 1612.
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