CHAPTER 6 DIELECTRIC AND CONDUCTIVITY STUDIES OF ZIRCONIUM TIN TITANATE (ZST)

Transcription

1 123 CHAPTER 6 DIELECTRIC AND CONDUCTIVITY STUDIES OF ZIRCONIUM TIN TITANATE (ZST) 6.1 INTRODUCTION We know that zirconium tin titanate ceramics are mostly used in microwave frequency applications. Previous researchers measured the microwave dielectric properties of zirconium tin titanate (Ki Hyun Yoon et al 1995), (Heiao et al 1988), and (Kim et al 1995). In this investigation, the dielectric properties, like the dielectric constant, dielectric loss, and conductivity of zirconium tin titanate were calculated in the low frequency region [G Hz]. 6.2 INTRODUCTION TO DIELECTRIC MATERIALS Dielectrics are non-metallic materials of high specific resistance with negative temperature coefficient, and are also called insulators. In dielectric materials there is no free electrons in the conduction band. All the electrons are tightly bound in the valence band. Based on the acceptance of the electricity, we can classify the dielectrics in to two categories, (i.e) active and passive dielectric materials. On applying an external electric field a few dielectric materials accept electricity. (i.e) active dielectrics (e.g) the Piezoelectric effect, Ferroelectrics etc.; passive dielectrics do not accept electrical energy through them (e.g), glass, and mica, rubber.

2 POLARISATION MECHANISM IN DIELECTRICS The displacement of the charged particles under the action of the external electrical field is called dielectric polarization. There are four types of polarization mechanisms. i) Electronic polarisation ii) iii) iv) Ionic polarisation Orientation polarisation Space-charge polarisation Electronic Polarisation Figure 6.1 (a) Position of positive and negative charges in an atom without field (b) Position of positive and negative charges in an atom with field

3 125 The electronic polarisation is defined as an electric strain produced in an atom due to the application of an electric field. It is the result of the displacement of the positively charged nucleus, and the (negative) electron cloud of an atom in opposite directions, on the application of the electric field. This kind of polarisation is present in all materials. Further, it is proportional to the volume of the atoms in the material, and it is independent of temperature. When the electric field is applied, a dipole moment is created in the dielectric. The induced dipole moment µ = e E where e - electronic polarisability The Electronic polarisability e = 4 0R 3 (Farad-m 2 ) (6.1) Ionic Polarisation Figure 6.2 Displaced ions by the electric field (a) without field (b) with field The Ionic polarisation is due to the displacement of cations and anions in opposite directions, which occurs in an ionic solid. This displacement is also independent of temperature. Example: NaCl crystal.

4 126 The ionic polarisability i is = µ 2 /3K B T = (e )[1/M + 1/m] (6.2) where M is equal to the mass of the ve ion,and m is equal to the mass of +ve ion Orientation Polarization The orientation polarization is due to the presence of polar molecules in the dielectric medium. Polar molecules have the permanent dipole moment even in the absence of an electric field. When an electric field is applied on the dielectric medium with polar molecules, the electric field tries to align those dipoles along with its direction. Due to that, there is a resultant dipole moment in that material, and this process is called orientation polarization. Therefore, orientation polarisation is inversely proportional to the temperature of the material. Orientation Polarization P 0 = [Nµ 2 B/3K B T] = [Nµ 2 E/3kT] (6.3) Figure 6.3 Orientational polarisation (a) without field (b) with field

5 Space Charge- polarization This type of polarization occurs due to the accumulation of charges at the electrode or at the interface in multiphase dielectrics. This is possible when one of the phases present has a much higher resistivity than the other. It is found in semiconductors and ferrites. Figure 6.4 Space-charge polarisation (a) without field (b) with field Total polarisation P=NE [4 R 3 + (e 2 2 )(1/M+1/m) + µ 2 /3kT]. (6.4) 6.4 FREQUENCY AND TEMPERATURE EFFECTS ON THE POLARIZATION MECHANISM When an a.c field is across the material, polarisation occurs as a function of time. The polarization P(t), as a function of time t is given by P(t) = P[1 - e -1/t ], where P is the maximum polarization, which occurs at a static field, applied for a long time, and t r is the relaxation time,ie.,the time taken for polarization. The relaxation times are different for different kinds of polarization mechanisms.

6 128 Figure 6.5 (a) & (b) Frequency versus power loss of various polarization processes and the peak power losses corresponding to each process Frequency Dependence Polarization Mechanism Electronic polarisation is very rapid and it will be completed once the moment of the voltage is applied. The reason for this is that the electrons are more elementary particles than ions, even for very high frequency of the applied voltage, i.e, in the optical range ( Hz), as shown in Figure 6.5(a).

7 129 Ionic Polarization occurs slightly slower than electronic polarization, because the ions are heavier than the electrons. Ionic polarization does not occur in optical frequency, where as it occurs in the frequency Hz) Orientation or dipolar polarization This type of polarisation is much slower than ionic polarization. The relaxation time for the orientation polarisation is much higher than ( sec). This type of polarization occurs in audio and radio frequencies (10 6 Hz), as shown in Figure 6.5(a) Space charge polarization This type of polarization occurs at low frequency in the order of 10 2 Hz. The dielectric constant against frequency in nanocrystalline materials is attributed to four types of polarization, viz., interfacial, dipolar, atomic, and electronic. At a lower frequency, all four types of polarizations contribute to the dielectric constant. The gradual decrease in the dielectric constant with frequency is mainly due to interfacial and dipolar polarization. Moreover, in the higher frequency range, the dielectric constant would be saturated, because of the electronic exchange of the ions charged state, and cannot follow the ac field beyond a certain critical frequency. The power loss during various polarization processes as a function of frequency is also given in Figure 6.5(b). By the application of an external electrical field on a dielectric material, energy loss will occur during each cycle, and heat will be produced. The energy loss is directly proportional to the product of the voltage and current. But, in a real dielectric, the energy loss is appreciable, depending on the frequency of the applied field, as shown in Figure 6.5(b).

8 130 The electronic and ionic polarizations are independent of temperature, but orientation and space charge polarization are temperature dependent. Hence, they diffuse through the randomizing action of thermal energy which decreases the tendencies of the permanent dipoles aligned along the field direction and the dielectric constant decreases. In the space charge polarisation, if we increase the temperature the ions can easily overcome the activation barrier. Hence, they diffuse through the inter-atomic distances, this gives rise to polarization; so, in this polarization,the dielectric constant increases with an increase in temperature. 6.5 CLASSIFICATION OF DIELECTRIC CERAMICS Dielectric ceramics are very important electrical materials in our day- to -day life. Larger quantities of dielectric ceramics are manufactured than other electronic electric and electro optic materials. The main applications of dielectric ceramics are in capacitors and microwave resonators High Quality factor (Q) Materials The dielectric constant ( ) of this group ranges from 4 to 400. The temperature coefficient ranges from 4120 to 4700 ppm / C. The high Quality factor value is defined as the reciprocal number of a dissipation factor tan, in the range of 1000 to A few high quality materials for microwave applications need very high soaking time at 1400 C. But, glass ceramics are widely used as ceramic multilayer substrates, with Ag and Cu as inner conductors.

9 High Dielectric Constant Material Barium Titanate is the main example for the high dielectric constant material; its dielectric constant value is less than 1000.The capacitance change increases due to the increase of the dielectric constant at room temperature. The dielectric constant of the lead based relaxer dielectrics is at room temperature, which provides better temperature bias, voltage, and performance than BaTiO3-based dielectric ceramics. In this situation, the development of the ceramic capacitor in the industry consists of miniaturization, improvement of volumetric efficiency, cost reduction, improvement in reliability and the design of new products with high performance Classification of Dielectrics Based on Permittivity In general, the relative permittivity (Dielectric constant) of dielectric materials varies from 6 to 20,000. Based on the relative permittivity, we can classify the dielectric materials in to three categories. i) Low permittivity ( up to15 ) ii) Medium permittivity ( between ) iii) Higher permittivity ( between ) Low permittivity dielectric ceramics are mostly used for electrical insulation in power transmission, where the dielectric break down voltage is an important electrical parameter. This type of dielectric ceramics stores the minimum amount of charge with in them. The dissipation factor of these dielectric ceramics is The electrical properties of a few low permittivity dielectrics are given in Table 6.1.The dielectric properties and applications of some ceramics are given in Table 6.2.

10 132 Table 6.1 Low permittivity dielectric materials and their electrical properties Material Dielectric tan Dielectric strength Resistivity at constant (x10-3 ) (Kv.mm -1 ) 25 C ( cm) r ) Porcelai Steatite Cordierite Zircon Alumina ZrO Mullite Applications of Low Dielectric Constant Materials i) The low permittivity dielectric ceramics can be used in semiconductor manufacturing industries.these types of dielectric ceramics are used to manufacture high performance Integrated circuit (IC) devices with minimal increase in the chip size Examples of Low Dielectric Constant Materials Nano porus silica Hydrogensilsesquioxanes (HSQ) Silicon oxy fluoride (FSG) Teflon

11 Medium- Permittivity Dielectric Ceramics These types of dielectric ceramics have dielectric constant values of 15 to 500. The approximate permittivity of the TiO 2 single crystal is 170 along the c-axis. The polycrystalline ceramic s permittivity is around The medium permittivity dielectric ceramics, with their dielectric properties and applications, are tabulated. These types of dielectric ceramics are used as low loss stable capacitors and microwave resonators. Titanium dioxide is used in capacitors. The other purpose medium dielectric ceramics are Titanates, zirconates, Ba 2 Ti 9 O 20, BaTi 4 O 9, Zr x Ti y Sn z O 4 (ZTS) and (Ba, Pb) Nd High Permittivity Dielectric Ceramics These dielectric ceramics were initially used in high permittivity capacitors, pyroelectric sensors, electro optic devices, PTC thermistors, and dielectric relaxers. They exhibit a high dielectric constant ( 10 5 ). Mostly relaxers are prepared by the solid state powder mixing method; e.g, Nb 2- xta x O 6 (Tsumari et al), Pb(Zn 0.65 Mg 0.35 ) 1/3 Nb 2/3 O 3, (Chen et al,1991), Pb(Zn x/3 Mg (1-x)/3 Nb 2/3 )O 3 - PbTiO 3 -(Ba, Sr)TiO 3 (Pilgrim et al 1992 (Kani et al 1993), Ba(Mg 1/3 Ta 2/3 )O 3, (Sagala et al 1993) (Chen et al 1994), Ba (Zn 1/3 Ta 2/3 ) O 3,( Kawashima et al 1983), Ba(Sn x Zn (1-x)/3 Nb (1-x)2/3 )O 3 (Sun 1993) and bsc 0.5Ta 0.5 O 3 based compositions [(Osbond et al 1993), Ba 4 Bi 2 Ti 4 Nb 6 O 30, (Pathumark et al 1994).

12 134 Table 6.2 Dielectric properties and applications of some ceramics Material Dielectric Constant tan (X 10-4 ) Applications r) TiO Sensor element Transformer capacitors MgTio Microwave filter Transformer capacitors CaTiO High power capacitors Pb ZrO High power capacitor BaTi 4 O Microwave filters Transformer capacitor ZrTi Sn O Dielectric resonator 6.6 EXPERIMENTAL PROCEDURE The powder was prepared by the sol-gel technique. The pellet was sintered in a conventional furnace at 1400 C for 3hrs dwelling time. The density measurements of the sintered samples were carried out, using the Archimedes principle. The scanning electron microscope images were captured, using the HITACHI model S-3400 JAPAN. The dielectric properties, such as the dielectric loss and dielectric constant, and the conductivity of the zirconium tin titanate sintered sample, were measured in the low frequency region (50 Hz - 5MHz), as a function of different temperatures ranging from 40 C to 550 C, using the LCR meter (HIOKI 3532 LCR HITESTER.)

13 RESULTS AND DISCUSSION Dielectric Constant of ZST In general, the dielectric constant study of a ceramic material gives an outline about the nature of atoms, and ions, and their bonding in the material. From the analysis of the dielectric constant and dielectric loss as a function of frequency and temperature, the different polarization mechanisms in solids can be understood. Here, the dielectric constant was measured as a function of frequency (50Hz-5MHz) at temperatures ranging from 40 C, 100 C, 200 C, 300 C 400 C 500 C to 550 C, and is shown in Figure 6.6. The dielectric constant was evaluated by using the following relation, r = (Cd / 0 A) (6.5) where C- is the capacitance, d is the thickness of the sample, and A is the area of the sample, and 0 is the relative permittivity of free space. From this plot, it was observed that the value of the dielectric constant was high in the lower frequency region for all the temperatures, and then it is decreased with an increase in the frequency. The high value of the dielectric constant at a low frequency region is attributed to the space charge polarization due to charged lattice defects.

14 136 ( r) (Hz) Figure 6.6 Dielectric constant versus frequency The dielectric constant in ceramics was attributed to four types of polarizations: interfacial, dipolar, atomic and electronic. At lower frequencies as well as lower temperatures, as all the four types of polarization are involved, the dielectric constant and the dielectric loss are very high. The dielectric constant and the dielectric loss gradually decrease due to the increase in the frequency and the temperature, which is due to the presence of polarizations (interfacial and dipolar) and the disappearance of the other two types. This proves that the dielectric constant and dielectric loss are strongly temperature dependent. Beyond a certain temperature (350 C) the charges acquire adequate thermal energy to overcome the resistive barrier at the grain boundary, and conduction takes place resulting in a decrease in the polarization. The interfacial polarization occurs up to frequencies of around 3 MHz as well as above 500 C, with possibly some contribution from the dipolar polarization also, which slightly increases with the temperature. With further increase in the temperature above 500 C, the dielectric constant as well as dielectric loss would be saturated, because the electronic exchange cannot follow the a.c field beyond a certain critical frequency. At low

15 137 frequency, the dielectric constant of the ZST ceramics decreased slightly with increasing frequency; this agrees very well with the results reported in the literature (Verma et al 2005) Dielectric Loss & Conductivity Studies of ZST (Hz) Figure 6.7 Dielectric loss versus log f The dielectric loss of zirconium tin titanate was measured as a function of frequency at different temperatures ranging from 40 C, 100 C, 200 C, 300 C 400 C, 500 C and 550 C, as shown in fig.6.7. The dielectric loss in the present system is attributed to four types of polarisation: Interfacial, dipolar, atomic and electronic. At lower temperature, four types of Polaristion are contributed. The gradual decrease in dielectric loss with temperature is mainly due to the interfacial and dipolar polarisation. More over in the higher temperature range, the dielectric loss would be saturated because of the dielectric exchange between Zr 2+ and Zr 4+ ions cannot

16 138 follow the A.C field frequency beyond certain critical temperature. Figure 6.8 shows the conductivity versus temperature graph. From this measurement, it is seen that the conductivity increases in the temperature range of 100ºC 600ºC (Resistance also gradually decreases); It is seen that whenever there is an increase in temperature, the conductivity also increases. Figure 6.8 Temperature variation versus conductivity of the ZST sintered at 1400 C for 3 h 6.8 CONCLUSION The low frequency region dielectric properties of the Sol-gel derived zirconium tin titanate were studied. The conductivity, dielectric constant and dielectric loss plots were drawn for the ZST sample. The dielectric study confirms that ZST is suitable for low frequency applications.