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PYROELECTRIC AND PIEZOELECTRIC STUDIES ON BaTi03: SILICA

Ani1 GOVINDANIA.K.TRIPATHIIT.C.GOEL AND P.K.C.PILLA1 Department of Physics, I.I.T.Delhi, New Delhi, India

Abstract

BaTi03: Silica-glass composites were prepared in different weight ratio and their dielectric constant was studied over a wide temperature and frequency range. The results indicate that with increasing amount of silica the dielectric constant of the composites goes on decreasing, while their Curie temperature increases. Pyroelectric current and charge field hysteresis effects were also studied in these composites. It was observ d that the pyroelectric coefficient decreases from

composite is increased. The piezo coefficient d33 also shows a decrease with increasing glass percentage but the piezo- voltage coefficient peaks at 1% glass content.

GLASS COMPOSITES

230 &C/ m 5 K to 2 0 0 h C /m2K as the percentage of glass in the

1. INTRODUCTION - In recent years much work is being done on Ceramics and ceramic polymer composites. This is mainly due to the possibility of their use in piezo- and-pyro-electric transducers [1-3].But very little has been done of glass ceramic composites[4].

In the present paper we are reporting the results obtained on BaTi03 : Silicon oxide glass composites. The studies that were carried out are dielectric constant variation with temperature and frequency, pyro-electric coefficient and piezo-electric coefficients of these composites.

2. EXPERIMENTAL - Capacitor grade BaTiO obtained from Koch-Light ( U . K . ) was used in the present 3study. The samplos were prepared by mixing Si02 (microfined 2 micro meter particle ) and BaTi03 manually using Acetone as thinner in the appropriate weight percentage i.e. l%, 2%,3% of Si02. 4 small percentage of PVA (poly vinyl alchohol) was added as a binder to prepare the pellets (1 cm diameter and 1 . 5 mm thickness). These pellets were sintered at llOO°C for one hour. The sintered pellets were then ground to uniform thickness and electroded with quick drying silver paint. These samples were corona poled at 6KV, the poling conditions were maintained uniform for all pellets. The poled samples were kept short circuited for 24 hours in order to stabilize the current in the samples before pyro-electric measurements were taken. A heating rate of 3.3OC per minute was maintained during these measurements.

CH3029-6/91/oooO-0524 $01 .OO Copyright 1991 IEEE

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d33 measurements were carried out on corona poled samples using APC Pennebaker Model 8000 d33 meter. Apart from d 3 3 ~ dh (hydrostatic piezo-coefficient) was also measured.

For hysteresis measurement a circuit similar to Sawyer-Tower circuit was used. A sinusoidal field of 15kV/cm at 60 Hz was applied on the samples. The dielectric constant was measured on a HP 4192 A impedance analyser.

3. Result and discussion

A typical plot of the variation of dielectric constant pure BaTi03 and composites is shown in Fig. 1. It can be seen that while the curie peak is found at about 150 C. for pure BaTi03 , it. shifts towards higher temperature with increasing amount of Si02 in the composites. Moreover the peak also broadens. This shift and broadening may be due to the stress exerted by glass on BaTi03 grains. A similar shift in BaTi03 curie peak was observed in BaTi03: PVDF composites also [ 5 ] .

The pyro-coefficients of the composites are shown in Fig.2. It can be seen that the pyro-coefficient goes on decreasing withincreasing amount of glass in the composites. This could be due to the decrease in the coercive field with increasing amount of silica, which makes it easier for the composites to polarize and depolarize.

The results obtained on piezo-electric measurements of these samples are shown in Table 1. It can be seen that although piezo-electric coefficient d33 decreases with increasing amount of Si0 in the composite, the piezo-electric figure of merit i.e. 2ahgh s h o w s a p e a k a t 1% of S i 0 i n t h e composites. This could be due to the considerable lowering of dielectric constant of this composite compared to pure BaTi03. While the value of dh had not changed considerably.

Hysteresis loops of pure BaTi03 and a composite are shown in Figs. 3 and 4 respectively. The results are tabulated in Table 3. Here again it was observed that PSatl P and EC goes on decreasing with increasing amount of $io2 in the composites. The decrease of Ec indicates that it is easy to pole these samples.

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4 CONCLUSIONS - It can be concluded on the basis of these studies that although BaTi03 glass composite have lower pyro-and piezo- coefficient compared to pure BaTi03 I the piezo-electric figure of merit dhgh is higher in 14 Si02 : BaTiOj composite. Moreover the curie temperature of BaTi03 also shifts towards higher temperature in these composites and hence the working temperature range is increased.

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REFERENCE

[l] H. Sanno and S . Saito, Japan. J. Appl. Rhys. Suppl. 22-2 67 ( 1 9 8 3 ) .

[ 2 ] J.Runt and E.C. Galoci, J. Appl. Polym. Sci. 29, 611

[ 3 ] H . Takeuchi and C. Nakaya, Ferroelectrics 68, 53 ( 1 9 8 6 ) . [ 4 ] R.E. Newnham, Ann. Rev. Mater. Sci. 16, 47 ( 1 9 8 6 ) . [ 5 ] A. Halliyal et. al. Mater. Res. Bull., 18, 1007 ( 1 9 8 3 ) .

( 1 9 8 4 )

Table 1. Piezo-electric co-efficient --

Mat er i a 1 E ’ d3 3 9 3 3 d h gh dh gh pc/N pc/N pc/N

Table 2. Pyro-coefficients --

BaTi03 (pure) 238.14 1437

BaTi03/Si02 (1%) 220.5 8 73

BaTi03/Si03 ( 2 % ) 200.31 7 68

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Table 3. Charge Field hysteresis & BaTi03 : SiO, composites - - --

%aTi03 room .0124 .0073 8.26 1.59 temp. 75 .0121 .0072 ? . a ? 1.67 100 .0115 -0071 7.71 1 .63 1 2 0 -0114 .0069 7.42 1 . 6 4

l % S i l i c a room .0116 .007 7.56 1 .65

50 -0114 -0074 7.42 1 .53 100 .0093 .0066 7.43 1 .42 1 5 0 .0090 .0063 7.01 1 .43

2 % S i l i c a room .0109 .0069 7.39 1.58

50 -0095 .0064 7.39 1 . 4 3 100 .009 .0063 7.42 1 .42 125 .0087 .0069 7.01 1 . 3

3 % S i l i c a room .0095 .0062 7.03 1 .53

50 .0093 -0061 7.09 1 . 5 3 100 .00801 .0052 7.01 1 .53 1 2 0 .007a -0053 6 - 0 8 1.47

temp.

temp

temp.

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Dlel. const (Tnousenos) 2.6

1.6

0.6 80 106 130 166 180 206

Temp C - Barium Titanate + Slllca 1%

* Silica 2% -a- Silica 35

Fig.1 Variation of dielectiic constant with temp.at 10kHz.

U G a, .A

U

U a, 0 0

; E r D

c ~ r e E e - 3 , ~ Ti ,eng~e - 7 P : :: - - 2% 3 I 'CC

Fig.2 Pyro electric coefficient of composites.

?i3.4 Charge field hysteresis of BaTi03:15 Sio2

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Fig.3 Charge field hysteresis of BaTi03