Dielectric and pyroelectric characteristics of PZT doped with gadolinium

Materials Science and Engineering, B25 (1994) 29-33

29

Dielectric and pyroelectric characteristics of PZT doped with gadolinium H. D. Sharma, A. K. T r i p a t h i , V i j a y a r a g h a v a n C h a r i a r , T. C. G o e l a n d P. K. C. Pillai Department of Physics, Indian Institute of Technology, New Delhi 110016 (India) (Received June 25, 1993; in revised form November 4, 1993)

Abstract This paper reports on the dielectric and pyroelectric properties of Gd 3+ modified PZT ceramic (Pbl_3x/2Gd3~/2) (Zro.6Tio.4)O3 (x in the range 0.0-0.1). It is found that Gd 3÷ modified PZT ceramic has a low dielectric constant and high pyroelectric coefficient. It is shown that the PGZT ceramic possesses the characteristics required by a good IR detector. The calculated parameters of interest, based on measured properties, are comparable with those of other ceramic and pyroelectric materials.

1. Introduction There is a great need for the development of suitable pyroelectric detectors for a variety of applications. Pyroelectric detectors using ferroelectric ceramics are simple in construction and do not require cooling. They are robust and non-hygroscopic, and their useful temperature range can be tailored. The ferroelectric properties of PbTiO3, and PZT ceramics modified by partial substitution: of La 3+, Sm 3+, Nd 3+, and Gd 3+ have been investigated by several workers and interesting results have been reported [1-5]. Pyroelectric studies of PZT incorporating the trivalent rare earth Gd 3÷ have not been reported in the literature. Therefore, it was thought worthwhile to investigate the dielectric and pyroelectric properties of PGZT.

2. Experimentaldetails (Pbl_3x/2Gd3x/a)(Zro.6Tio.4)O 3 samples with x=O, 0.025, 0.05.0.075 and 0.1 were prepared using PbO, ZrO2, TiO2 and Gd203 obtained from Aldrich Chem. Co., USA. The oxides were wet mixed in stoichiometric proportions in a solution of alcohol, and ball milled using zirconia balls for 16 h. Extra lead oxide (4% by weight over and above the stoichiometric ratio) was also added to the mixture during ball milling. This is to counteract lead losses during calcination and sintering. The mixture was dried and calcined at 850 °C for 2 h. 4% by weight polyvinyl alcohol was added as a 0921-5107/94/$7.00 SSD10921-5107 (93)01038-6

binder to the calcined powder and mixed thoroughly. The mixture was compacted to form pellets of 1 cm diameter using a die and hydraulic press under a load of 10 tons. The pellets were then heated very slowly (at 2 °C min-1) up to 600 °C to avoid bending or cracking of the samples. The samples were then taken to a final sintering temperature of 1200 °C at a heating rate of 5 °C min-1 and were maintained at that temperature for 2 h. The sintering was done in a closed alumina crucible in lead atmosphere. An atmosphere of lead was generated by smearing the inner surface of the crucible with a layer of lead oxide powder. X-ray diffractograms of these samples were taken using CuKa radiation on Rigaku diffractometer. The voltage and current ratings used for the present experiment were 40 kV and 30 mA respectively and the X-ray scan speed was 10 °min-1. Dielectric measurements were taken using an LF Hewlett Packard impedance analyzer (model 4192A). Quick drying silver paint was used for these and other electrical measurements. Charge field hysteresis phenomena were studied using a Sawyer Tower circuit [6]. Samples were corona poled at a voltage of 7 k V c m -1 and current of 50/~A for measurement of the pyroelectric coefficient. The pyroelectric current was measured at a heating rate of 4°C m i n -1 o n a Keithley 610C electrometer. The pyroelectric coefficient was calculated using the relation

Pi=(I/A)(dO/dT )-' where I is the pyroelectric current, A is the electroded area of the sample and d O/d T is the heating rate. © 1994 - Elsevier Sequoia. All rights reserved

30

H. D. Sharma et al.

/

3. Results and discussions

PZT doped with Gd

density with increasing La content in PLZT for all Zrto-Ti ratios has also been reported by Haertling and Land [8]. Further, our observations of a decrease in bulk density can be explained by the difference in ionic radii of Pb and Gd. Since the ionic radius of G d 3÷ (0.94) is smaller than that of Pb 2÷ (1.20), the intralattice porosity in Gd modified PZT increases with increasing Gd 3÷ doping in the PZT sample. The differential scanning calorimetry (DSC) thermogram (Fig. 2) of the PGZT sample was obtained using a DuPont DSC meter at a heating rate of 10 °C min-'. The transition temperature of the Gd modified PZT sample is found to be 323.97 °C. Figures 3(a) and 3(b) show typical plots of the variation in dielectric constant and dielectric loss tangent with temperature f o r (Pbl_3x/2Gd3x/2)(Zro.6Tio.4)O3 where x = 0 . 0 5 at several frequencies. It is observed that the value of e' increases gradually with temperature, while the value of tan 6 decreases up to 60 °C and then increases slightly. It is also observed that at lower frequencies (up to 1 kHz), the rise in tan 6 is significant

X-ray diffractograms of PZT and PGZT with different percentages of Gd 3÷ doping are shown in Fig. 1 and the peaks are identified. The presence of well resolved peaks due to (002) and (200) planes indicates that the material is in tetragonal form. It can be observed from the diffractograms that the heights of the peaks due to the different planes increase at 5% Gd 3+ doping and decline at 7.5% Gd 3÷ doping without any appreciable change in peak position. The lattice parameters were calculated and are tabulated in Table 1. Some peaks due to titanium oxide and zirconium oxide are also observed in the diffractograms. This suggests the presence of trace quantities of unreacted TiO2 and ZrO 2 in the samples, as has been reported by others [7]. Table 1 shows the variation of lattice parameters a and c as a function of Gd 3÷ concentration in PZT. As can be seen from the table, the substitution of Pb 2÷ by Gd 3+ leads to a strong reduction in a and c parameters. A similar effect is observed on the parameters c/a and (a 2c)]/3. These results indicate that a solid solution was formed in the Gd203--PZT system. It is pertinent to note here that although 4% by weight extra lead was introduced in the samples prior to sintering, the possibility of the creation of lead vacancies due to evaporation cannot be ruled out. This is essential to maintain charge neutrality in the ceramic and can explain the decrease in unit cell volume and bulk density with increase in Gd 3+ concentration. A decrease in bulk

TABLE 1. Lattice parameters of P G Z T Lattice parameter (A°)

Pure PZT

PGZT with 5% Gd

PGZT with 7.5% Gd

a c c/a (a 2c)1/3

4.058 4.171 1.0278 4.095

4.026 4.t26 1.0243 4.059

4.024 4.120 1.0239 4.058

(11o)

C

(110) (110)

t-

C

C t II

(100) I

20

(a)

I

It

11111 1200) (002) I

I 40 20

i ;

111,2) (211) I

ii

ZrOz

] •00)Ti02]l

11121 II11) (200) 121111

(1001

zr II]] l[

I T °2, Ill

(1111 (200)

I

I

(1121

(21111

I

I 60

I 20

(b)

I 40 2e

60

20

(c)

I

40 20

Fig. 1. X-ray diffractograms of (a) PZT, (b) PGZT, Gd 3÷ concentration 5%, and (c) PGZT, Gd 3÷ concentration 7.5%.

60

H. D. Sharma et al.

/ PZT doped with Gd

31

0.00 410

323.97'C

390

-0.25

370 -0.50

1. 2.

100 HZ 316 HZ

3 • 4.

1 kHz 3.16 kHz

2

5.

10

3,

6. 7.

31.6 KHz 100 KHz

8.

316 KHz

1

KHz

350

a. --035

C

g

3:

/

330

1"6~3J/9

3~0

-1.00

0

290 -1.25 50

I 100

I 150

I I 200 250 Temp.('C )

I 300

I 350

1 400

270

7

Fig. 2. DSC thermogram of PGZT, Gd ~+ concentration 5%. 250

above 100 °C. Importantly, it is seen that the value of e' is low and does not vary much with frequency, This property of the ceramic is central to obtaining a higher and fairly constant pyroelectric voltage responsivity and detectivity over a wide frequency range. The variation of the dielectric constant with temperature and frequency shows the normal behavior of a dipolar material. The low value of the dielectric constant compared with that of undoped PZT which was prepared under conditions similar to those for the PGZT ceramic (e'-- 850 at room temperature, 10 kHz) can be understood in terms of the anisotropy in the material. The c/a ratio in PbTiO3 is 1.06 while in PZT with 60% Zr it is 1.03. The c/a ratio for the PGZT samples as calculated from the X-ray data is found to be 1.0243 for 5% Gd doping and 1.0239 for 7.5% Gd doping. This indicates that the structure in Gd modified PZT is less tetragonal than those of PbTiO 3 and PZT. The decrease in tetragonality may be one of the reasons for the decrease in the dielectric constant of the PGZT ceramic. Among the various factors that determine the dielectric constant of a ceramic are the dipole moment, domain wall mobility and internal stresses in the material. We would also like to point out that the lattice parameters obtained by us are slightly different from those reported by Jaffe et al. [9]. This appears to be due to the presence of traces of unreacted TiO 2 and ZrO2, which could alter the exact composition of the sample. Figures 4(a) and 4(b) show the variation in dielectric constant e' and imaginary component of the dielectric constant e" for different concentrations of Gd 3+ for various temperatures at 10 kHz. It is observed that the dielectric constant increases gradually as the G d 3+ doping increases up to 7.5%, and at 10% Gd 3+ doping

t J 30 40

I

I

60

80

(a)

I

100

I

I

I

120

140

160

180

Temp ('C)

1 2 3 4

.-. ... ..

/

100Hz 316Hz 1 kHz 3.16 kHz

5 ,.. 10

kHz

6 ... 31.6 kHz 7

..,

100kHz

8 .,. 316 kHz

/

j o

3.0-

~

o

o2.0 1.51 Y'--Y~

T

I

30

60

80

(b)

40

I 100 Temp. ("C)

I 120

I 140

I

160

180

Fig. 3. (a) Dielectric constant as a function of temperature at different frequencies for 5% PGZT; (b) variation of tan 6 with teh~perature at different frequenciesfor 5% PGZT.

the dielectric constant actually decreases at all frequencies. The behavior of e" is similar. This type of behavior has also been observed for Yb 3+ doping i n P Z T [10]. A charge field hysteresis curve of a Gd 3÷ doped sample is shown in Fig. 5. As can be seen from the figure, the hysteresis loop is symmetric about the origin. The values of saturation polarization P, and remnant polarization Pr are found to be 8.3/~C cm-2 and 5 . 8 3 9 p C c r n -2 at room temperature., The coercive field at room temperature is found to be 5.9375 k V c m - i.

32

H. D. Sharma et al. 1200

75 % Gd 3+ ~000

% 800 0

600

o~ (J .u 400

i5

5'1, Gd 3÷

200 "

2 5"1, Gd 3+

o 0

I 20

(a)

~

.c

--&

I 40

I 60

I 80

I 100

I 1~0

I lt.,O

Temp (*C)

/

PZT doped with Gd

Figure 6 shows the variation in pyrocurrent and pyrocoefficient with temperature. It is observed that both the pyrocurrent I and pyrocoefficient Pi increase intially with temperature up to 55 °C and then remain constant up to 125 °C. Above 125 °C, both increase linearly with temperature. The variation in pyroelectric coefficient with temperature for four different concentrations of Gd 3+ is shown in Fig. 7. It is seen that the maximum value of pyroelectric coefficient for P G Z T ceramic is between 5% and 7.5% doping. However, the pyrocoefficient increases almost linearly for the 2.5% Gd 3+ doped sample and as such can be used for detection from 40 °C to 125 °C [11-13]. The important characteristics of pyroelectric detectors are (a) the current responsivity F~, (b) the voltage responsivity Fv, and (c) the detectivity FD. The current and voltage responsivity are given by Fi = Pi/C'

60L

Fv =pi/c' c'

3+

F D = p i / c ' g."

'v

~

°-----°~~~" 2o

-

~

where e" = e' tan 6, tan 6 being the loss tangent in the dielectric. The values of these pyroelectric parameters are plotted in Fig. 8.

~

o ~

20

(b)

respectively, where Pi is the pyroelectric coefficient of the ceramic, c' is the volume specific heat and e' is the dielectric constant of the material. The detectivity FD is given by

~

40

~

60

~

80

y

100

T2.s',.

120

140

Temp. ( ' C )

Fig. 4. (a) Variation of the dielectric constant e' with temperature for different Gd 3÷ concentrations at 10 kHz; (b) variation of the imaginary component of the dielectric constant e" with temperature for different G d 3 ÷ concentrations at 10 kHz.

36

E o

24

o

7o 12

O

u 2

I

i

70

% 4O

1C

20

I

I

I

60

100

140

180

Temp.('C)

Fig. 5. Hysteresis loop for PGZT with 5% Gd 3+ doping.

Fig. 6. (a) Variation of pyrocurrent with temperature, 5% C-d; (b) temperature dependence of the pyroelectric coefficient, 5% Gd.

H. D. Sharma et al.

IO

--=--

7.5/s0/~,0

x

--.a--

5 / 60/t,O

~'E u ~, 60 -

/ I

---o.-- 2.5160 / 40

-o-

oo

,,~

10 1501,,.0

/ / ~

°Q.

•.u_ - 65 U .:-U

&

A &

PZT doped with Gd

33

It is seen that F i, F V and F D for P G Z T are almost independent in the t e m p e r a t u r e range 4 0 - 1 2 5 ° C . A b o v e 125 *C, Fi, F v and F D increase linearly with temperature. This suggests that the material can be used as a pyroelectric detector in the t e m p e r a t u r e range up to 125 °C. A n o t h e r useful advantage of doping P Z T with Gd3÷ is that it relieves the internal stresses in the ceramic and thereby facilitates poling. Work is now in progress with respect to optimizing the G d 3 + doping concentration with varying proportions of Z r and Ti.

75!

A ¢0

/

I~

Acknowledgments

n

T h e authors would like to thank the Defence Research and D e v e l o p m e n t Organization, Ministry of Defence, G o v e r n m e n t of India for the financial assistance provided to carry out this work.

15

I

0

I

t,O

20

I

60

I

80 Temp.

I

100

I

120

140

r60

('C)

Fig. 7. Variation of the pyroelectric coefficient with temperature for different percentages of Gd 3+ in PGZT.

7.C

5.0

3.0 y

1.0 I

I

I

I

I

I

7.( H

5.0

3.0

1.0 I

I

I

I

I

I

I

1.6' 1.2 u-~ 0.8 0.~

~

0, 20

I

I

I

"-0

60

80

Temp

I

I

I

I

100

120

1~'0

160

180

('C)

Fig. 8. Variation of pyroelectric parameters of PGZT with temperature.

References

1 M. E. Lines and A. M. Glass, Principles and Applications of Ferroelectrics and Related Materials, Clarendon Press, Oxford, 1979. 2 R.W. Whatmore, Rep. Prog. Phys., 49 (1986) 1335-1386. 3 A. A. Bogomolov and T. A. Dabizha, Study of the pyroelectric properties of PLZT ceramics by dynamic method, Ferroelectrics, 74 (1987) 81-85. 4 E Duran, J. F. Fdez Lozano, F. Capel and C. Mouro, Large electromechanical anisotropic modified lead titanate ceramics, Part I: processing, J. Mater Sci., 23 (1988) 4463-4469. 5 P. Duran, J. F. Fdez Lozano, F. Capel and C. Mouro, Large electromechanical anisotropic modified lead titanate ceramics, part II: dielectric and mechanical properties, J. Mater. Sci., 24 (1989) 447-452. 6 M. Chaturvedula, PhD Thesis, liT Delhi, 1987, p. 45. 7 S. Sugihara and T. Fakuyama, Synthesis of electronic ceramics by using CO 2 laser, Proc. Second European Conf. on the applications of Polar Dielectrics, in Ferroelectrics, 133 (1992) 307-311. 8 G. H. Haertling and C. E. Land, Hot pressed (Pb, La)(Zr, Ti)O 3 ferroelectric ceramics for electrooptic applications, J. Am. Ceram. Soc., 54 (1971) 1. 9 B. Jaffe, W. R. Cook, Jr, and H. Jaffe, Piezoelectric Ceramics, Academic Press, London, 1971, pp. 137-142. 10 H. D. Sharma, A. K. Tripathi, A. Govindan, T. C. Goel and P. K. C. Pillai, Studies on the dielectric and pyroelectric properties of (Pb l_ xYbx)(Zr0.65Ti0,35)03, Ferroelectrics, in press. 11 K. K. Deb, Pyroelectric characteristics of (Pb0.9 Sm0.1)TiO3 ceramics, Ferroelectrics, 82 (1988) 45-53. 12 K. K. Deb, Investigation of pyroelectric characteristics of modified PbTiO 3 ceramics for improved IR detector performance, Ferroelectrics, 88 (1988) 167-176. 13 R. W. Whatmore, A. Patel, N. M. Shorrocks and F. W. Ainger, Ferroelectric materials for thermal IR sensors: state of the art and perspectives, Ferroelectrics, 104 (1990) 269-283.