(

f

Indian 10urnal of Pure & Applied Physics VoL 37. April 1999. pp. 359-362

Structural, electrical and piezoelectric properties of rare-earth doped PZT ceramics

S R Shannigrahi, R N P Choudhary*, H N Acharya & T P Sinha +

Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721 302

+Department of Physics, Bose Institute, Calcutta 700 009

Received 3 February 1999

Polycrystalline samples of (PbO.93Ro07)(ZrO.60 Ti040)0982S03 (R = Y, La, Nd, Sm, Od, Dy, Eu, Pr. Er and Yb) have been prepared by sol-gel technique. Structural and microstructural parameters have been. determined using XRD and SEM techniques. Studies of dielectric constant as a function of temperature and frequency suggest that the compounds have diffuse phase transition of second order. From the studies of polarisation, and piezoelectric properties, some interesting results have been obtained.

1 Introduction Because of their potentially low cost and greater

durability under adverse atmospheric conditions lead zirconate-titanate (PZT) ceramics have come into prominence during the last several decades as substitute for crystal in various transducer applications, namely in the measurement and transmission of sound and shock waves, vibration, pressure, etc IA. The substitution of modifier ions on PZT produces ceramics which have highly improved piezoelectric properties than that of the parent compositions.7. In this context, a systematic study for the development of PZT compositions have been carried out on (Pb093Ro07 )(Zr0 60 Ti040)o982s0 3 (R= Y, La, Nd, Sm, Gd, Dy, Eu, Pr, Er and Vb) (abbreviated as PRZT) prepared by metal- alkoxide sol-gel process in­itiated in our laboratory on which not much have been reported .

2 Experimental Procedure The precursors used to form the polycrystalline sam­

ples of PRZT were lead acetate trihydrate Pb(CH3COO)2.3H20 (99.999%, E. Merck, Germany), rare-earth acetate hydrate R(CH3COO)3. xH20 (99.9%, Aldrich, USA) , zirconium propoxide Zr(C3H70)4 (99 .99%, Fluka, Switzerland) and titanium isopropox­ide Ti[(CH3)2CHO]4 (99.99%, E. Merck, Germany). Glacial acetic acid and distilled water were used as solvents while ethylene glycol was used as an additive in order to get a monolithic gel. First of all lead acetate and individual rare-earth acetate were dissolved sepa­rately in acetic acid in the ratio of 2 gm of salt in I ml

*Corresponding author

acid and were heated at I 10°C for hal f an hour to remove the water content before cooling them down to 80°(, These two solutions were mixed in a vessel and stirred. During stirring first zirconium propoxide and then tita­nium isopropoxide were added in the mixture. Ethylene glycol was then added in the proportion of I ml to 10 gm of lead acetate in the solution. The initial reaction had to be completed before the glycol was added since residual titanium isopropoxide and zirconium propoxide alcolyzed with ethylene glycol to form a condensed solid. A small amount of distilled water was added to get the final sol. The sol was kept at 60°C for 24 hr to get the clear transparent gel. The gels were then dried at 100°C for 72 hr and then powders were obtained. The oven dried powdered gels were calcined at 550°C for 15 hL The powders were cold pressed into discs (pellets) at 6 x 107 kg.m-2 pressure using an uniaxial hydraulic press. The pellets were then sintered for 7 hr at 1300°C. In order to prevent PbO loss due to vaporization during sintering, an equilibrium PbO vapour pressure was es­tablished with PbZr03 as setter and placing everything in the covered platinum crucible to maintain the stoichiometry of the compounds. The density of the sintered pale yellow pellets was measured by the Ar­chimedes' method and was found 97-98 % of the theo­retical density. The formation and quality of the desired compounds were checked by X-ray diffraction (XRD) technique with powder diffractometer (Philips PW 1877) using CuKa radiation (A = 0.15418 nm) in a wide range of Bragg angle (20° ~ 208 ~ 60°) at room tempera­ture with the scanning rate 3°min-1 on powders as well

360 INDIAN J PURE & APPL PHYS, VOL 37, APRIL 1 999

as on s intered pellets. High purity s i lver-paste was then painted on flat polished surfaces of the sintered pel lets to work as e lectrode and then dried at 1 00°C before taking any electrical measurements. The dielectric con­stant (E) and tangent loss (tan 8) of the samples were obtained using a GR 1 620 AP capacitance measuring assembly with a three-terminal sample holder, as a func­tion of frequency (0 .4- 1 0 kHz) at room temperature and as a function of temperature (30o-350°C) at frequency 1 0 kHz.

The microstructures of the samples were analyzed by scanning electron microscopy (SEM, CAMSCAN 1 80).

The temperature variation of polarization (P,) and coercive field (Ec) of the samples were recorded at 50 Hz with an aG field of 20 kV.cm-1 using a dual-trace oscil loscope attached to a modified Sawyer-Tower cir­cuit.

The electrical poling of the samples was carried out under a dc field of 1 5 kV.cm-1 for I hr in si l icone oi l bath below their trans ition temperature. d33 (piezoelec­tric strain coefficient) measurements were carried out using Berl incourt d33 meter.

3 Results and Discussion

The room temperature XRD patterns of sintered PRZT pel let samples show the formation of single­phase compounds. Al l the reflection peaks were indexed

22

20 10 kHz

-·- Er 1 8 -- le

-- V --Sm

.e -�-Nd -- Oy -.-Gd

1 4 --Pr

'0 -o- Eu -- Vb

.<:: 12 c: . i 0

1 0 "

� .i! .51 0

6

100 200

and lattice parameters of PRZT were determined using least-squares refinement method with the help of com­puter package (PowdMult). Compared to pure PZT, no shift in major peak was observed. From this, we con­clude that the basic crystal structure of pure PZT has not been affected by the incorporation of the R

3+ ions. How­ever, a minor shift is observed in the peak positions indicating a small change in the lattice parameters. The lattice parameters of the compounds are tabulated in Table I . All the R

3+ doped PZT compounds belong to

the tetragonal system at room temperature.

Table I - Some structural parameters of PRZT

R a c A v. Grain size d

(A) (A) ( SEM) (gm/cc) ( 11m)

Y 4.05 1 4 4.09 1 3 6.00 7 .749 La 4.080 1 4. 1 347 2.80 7 .792 Nd 4. 1 1 40 4.2035 2.40 7.546 Sm 4.0367 4.0 1 57 1 .80 8.093 Dy 4.0930 3.9675 6.00 8. 1 09 Gd 4.0827 4.076 1 2 .72 7.925 Eu 4.0783 4.0772 3 .80 7.77 1 Er 4 .0 1 6 1 4. 1 637 4. 1 4 8.034 Pr 4.0228 4.0 1 36 3.33 8.26 1 Yb 4 . 1 1 56 4.0953 6. 1 1 7.788

l O O . 0 0 5 0 0

Fig. I - Variation of E with temperature at 1 0 kHz for all different compositions

"

.,.

SHANNIGRAHI el at.: PZT CERAMICS 36 1

Tab le 2 - Compari son of die lectric and piezoelectric properties of PRZT

Emax Emax tan ~hc tan 8Tc Tc Tc Activation P, Ec d33 X 10-12

at I kH z at 10 kH z at I kH z at 10 kH z (OC) (0C) enc:rgy (~C/cm2) (kY/cm) (CIN )

at I kHz at 10 kH z (Ea)

y

La 20567 20384 0.029 0.024 193

Nd 6242 5878 0.033 0 .030 199

Sm 8955 76 13 0.042 0 .037 3 15

Dy 16468 13838 0.069 0.039 355

Gd 16300 14143 0.056 0.053 347

Eu 11490 8530 0.290 0.2 10 363

Er 148 11 0 .250

Pr 181 6 1620 0 .029 0.009 79

Yb 6562 0 .320

The average gra in size of PRZT compounds as cal­culated from SEM mi crographs is uniform and are nearly spherical. The average grain size is given in Table 1.

Fig. 1 - shows the va riation of E with temperature at 10kHz for all di fferent compositi ons. Here th e dielectric peak is broadened. This broadening of the die lectric peak and variation of EIIl . , can be attributed to the vari­ation of grain sizes in the range of 1 to 6 m and structural di sorder in the arrangement of the cations at the A-site whi ch is occupi ed by Pb1

+ , th ose are compensated by lattice site vacancies. and/or at the B-site occ upied by ZrJ wi th Ifltl ic c :. ite vaca ncies, lead ing to microscopic heterogeneity Ill lhe compos ition and thus a distribution of different loca l Curie points8

. The Curie temperature Tc fo r all the samp les except Y - doped PZT are presented in Table 2.

We find that the reciprocal die lectric constant is a linear function of tempcrature on both sides of 7~ except within the range of ± 20°C around it. Moreover, th e temperature gradi cnt of the rec iprocal dielectric con­stant against temperature at low and hi gh temperature is about 2: 1 in our experiments. These results suggest that the phase transition is of second order9

.

The ae electrica l conductiv ity 0' is re lated to the dielectric constant and loss, by thc express ion 0' == CDE['o

tan 0 and 0' == 0'0 exp( -E./ Ks T) where, Eo is the dielectric constant at free space, CD is the angular frequency, Ks is the Boltzmann constant and Ea is the activation energylO. Activation energy has been calcu lated from the plot of

(eY)

0 0 0

19 1 0 .66 163 10. 58 259

196 0 .32 7.64 6.00 96

315 0 .52 7.27 4 .54 83

355 0.20 9.46 7. 11 73

343 0.62 8.33 8.67

364 0.64 4 . 15 5.29 46

359 0. 1 5.98 6.53

77 030 0.73 2. 12 2 1

378 0 .29 4 .87 3.79 83

InO' vs . 101,/ T in the parae lectric region. It has been observed that Eo increases with the increase of ion ic radiu s of the dopants with a fi xed concentration.

A hi gh electric fi e ld (- 20-25 kY.c m-l) was required to obtained saturation polari sati on. The remnant polari ­sation (P,) and coercive fi eld (Ec) 'vvere determined from the hysteres is loop. It has been observed that the coer­cive field is approx imately a linear functi on of tempera­ture at the Curie point. whereas polarisat ion appears to

be quadratic ; both re lat ions aga in ind icate a second order phase trans iti on at 7: (Ref. 11). The va lu es of E( and p, at 30°C are given in Ta ble 2.

Thc va lu es of the piezoelectric strain coefficient (eI,, )

ofPR ZT are g iven in Table 2. Here no systematic change has been observed with the var iation of ioni c radius or different R,1 ; dopants.

4 Conclusions PRZT ce rami c wi th the said co mpos ition synth esised

from ace tate- alkox ide so ls. were found to be ve r) fiil l'

and homogeneous. There is a ve ry good agreement bet ween observed and ca Icul ated d-val ues recorded from XRD . Therc is a var iati on in the grain size with the dopin g of different Rl, ' as observed fro m SEM . The acti vation energy increases with increasing of ion ic size of rare-earth ions ofa fixed concentration. With respect to severa l other properti es La-doped PZT shows highest va lue. However. the other rare-earth doped PZT except Y doping can be used as ferroe lectric material for certa in spec i fi c appl ieations with wide range of temperature.

362 INDIAN J PURE & APPL PHYS, VOL 37, APRIL 1999

References I Jaffe B, Roth R S & Marzullo S. J Appl Phys. 25 (1954) 909.

2 Jaffe B, Roth R S & Marzullo S, J Res Nat Bur Stds. 55 (1955) 239.

3

4

5

6

Berlincourt D A. Cmolik C & Jaffe H. Proc IRE. (1960) pp.

220-29.

Matsuo Y & Sasaki H. JAm C eram Soc. 48(6) ( 1965) 289 .

Miga S & Wojcik K. Ferroelectrics. 100 (1989) 167.

Jaffe B, Cook W R & Jaffe H. Piezoelectric ceramics, (Aca­demic Press, New York). 1971.

, I

( :' t \ ~ "., "

, ,

7

8

Tkeda T & Okano T. Jap J Appl Phys. 3 (1964) 263 .

Lines M E & Glass A M. Principles and applications of ferroelectrics and related materials. (Ox ford University Press. Oxford). 1977.

9 Smyth t P. Dielectric behaviour and structure. (McGraw Hill. New York). 1955.

10 Kingery W D. Introduction to ceramics. (Wiley . New York), 1960.

II Moulson A J & Herbet J M. Eiectroceramics. (Chapman and Hall), 1.990, p. 358.