ส ปร ยา ตร ว จ ตรเกษม และ กฤช โกยวาณ ชย Supreya Trivijitkasem and Krit Koyvanich บทค ดย อ

Transcription

1 การศ กษาเซราม กเลดแลนทาน มเซอร โคเนตไตตาเนต (PLZT) ซ นเทอร ท อ ณหภ ม แตกต างก น Characterization of Lead Lanthanum Zirconate Titanate (PLZT) Ceramics Sintered at Various Temperatures ส ปร ยา ตร ว จ ตรเกษม และ กฤช โกยวาณ ชย Supreya Trivijitkasem and Krit Koyvanich บทค ดย อ การศ กษาโครงสร างจ ลภาคและสมบ ต ทางไฟฟ าของเซราม กเลดเซอร โคเนตไตตาเนต(PZT) ท โดพด วย แลนทาน มออกไซด (La 2 O 3 )ปร มาณต างๆด งน Pb 1 x La x (Zr Ti )O 3 โดย x = 0.01, 0.03 และ 0.05 ซ งเตร ยมด วยว ธ ผสมออกไซด ท อ ณหภ ม ซ นเทอร แตกต างก น 3 ค า ค อ 1200 C,1250 C และ1300 C จาก เทคน คการเล ยวเบนร งส เอกซ พบว าเฟสเททระกอนอลม ค าแลตท ชพาราม เตอร a และ c ระหว าง Å และ Å ตามล าด บ โดยท กอ ตราส วนม ค า c/a = 1.22 สมบ ต ไดอ เล กทร ก สมบ ต เฟอโรอ เล กทร กและสมบ ต เฟ ยโชอ เล กทร กของระบบเซราม กส วนใหญ ข นอย ก บปร มาณการโดพและอ ณหภ ม ซ นเทอร ABSTRACT This article characterizes microstructure and electrical properties of lead zirconate titanate, PZT, ceramics doped with lanthanum oxide (La 2 O 3 ). The Pb 1 x La x (Zr Ti )O 3 ( x =0.01, 0.03 and 0.05) ceramics were prepared by a conventional mixed-oxide method, three sintering temperatures were performed : 1200 C, 1250 C and 1300 C. X-ray diffraction techniques was employed to determine the lattice parameter of the ceramics, the tetragonal lattice parameter a and c were between Å and Å, respectively, the ratio c/a = 1.22 was obtained. Dielectric, ferroelectric and piezoelectric properties of the ceramic system were largely controlled by the dopant content and sintering temperature. Key words : PZT, La 2 O 3, Piezoelectric E- mail address : ku.ac.th ภาคว ชาฟ ส กส คณะว ทยาศาสตร มหาว ทยาล ยเกษตรศาสตร กทม Department of Physics, Faculty of Science, Kasetsart University, Bangkok

2 INTRODUCTION Perovskite lead zirconate titanate (PZT) system is most widely used in ceramic form, with composition close to the morphotropic phase boundary (MPB) of high piezoelectric coupling. Many researchers have been studied the effect of additive oxides to PZT ceramics, in the vicinity of the MPB between rhombohedral and tetragonal phases, on the piezoelectric properties and microstructure (Garg and Agrawal, 2001 ; Ryu et al., 2001). The addition of lanthanum to PZT system, or simply PLZT, gives a high density transparency product with useful electro-optical properties, which is used as the basis of the optical memories (Santos et al., 2001). The effect of lead zirconate niobate (PZN) additive on the sintering behavior and piezoelectric properties of PZT ceramics was also reported to improved sinterability and maximum piezoelectric properties (Fan et al., 2001; Lee et al.,2004; Zeo et al., 2004). In present work, effect of lanthanum additive at various sintering temperatures on the dielectric and piezoelectric properties of PZT (a Zr/Ti ratio of 54/46) ceramics was investigated. MATERIALS AND METHODS The studied ceramics of three nominal compositions were prepared by a conventional ceramic mixing method, the general formula : Pb 1-x La x (Zr 0.54 Ti 0.46 )O 3 where x = 0.01, 0.03 and 0.05 were produced. The commercially available oxide powders of high purity (all 99.9% purity) were used, PbO, ZrO 2, TiO 2 and La 2 O 3 (Fluka chemika) of the required amounts were carefully weighed and mixed by wet ball milling for 24 h using zirconia balls and acetone as media. The mixture powder was dried at 100 o C for 12 h. After drying, the mixture powder was calcined in a closed alumina cup at 800 o C for 4 h at a constant heating rate of 300 o C/h. The calcined powder was then ground in a mortar and pestle to crush agglomerates to pass through an 100 mesh sieve and was cold isostatically pressed in a 13 mm die at a pressure of 150 MPa into pellets using deionization water as a binder. The pellets were placed into the covered alumina crucible, a Pb(Zr 0.54 Ti 0.46 )O 3 atmosphere powder were placed on the top, bottom and in the vicinity of the pellets in order to maintain stoichiometries as closely to the nominal compositions as possible. Moisture contents and binder were get rid by heating the sealed alumina crucible at 500 o C for 1 h at the constant heating rate of 300 o C/h. Then, the pellets in the sealed alumina crucible were sintered for 1 h, three sintering temperatures were carried out: 1200 o C, 1250 o C and 1300 o C, thus yielding three pellet samples for each composition, hence there were altogether 9 samples. The samples were characterized for weight loss, sintered density by using density determination (AG 204, Mettler Toledo). X-ray

3 diffraction (XRD) analysis was carried out at room temperature to determine phase structure and lattice parameters. The JEOL model JDX-3530,Ni filtered CuK α radiation at 30 kv, 40 ma were used. Data collection was performed in the 2θ ranging from 20 o -60 o of a step scan with a step size of 0.02 o and counting time of 0.5 s per step. Polished surface microstructure was examined by scanning electron microscope (SEM, JSM- 5410, JEOL). Samples were polished to a 1 µm finish and thermally etched at 100 o C lower than their sintering temperature. For the piezoelectric properties measurements, the sintered pellets were lapped and silver pastes was applied on the lapped surfaces and were fired at 100 o C for 12 h for electrode formation. The electrode specimens were poled in a silicone oil bath at 120 o C for 20 min by applying a DC field, according to coercive field of each specimen. The specimens were aged for 24 h prior to testing. The ferroelectric polarization versus electric field (P-E hysteresis) were measured by using an RT66 standard ferroelectric test system, and the piezoelectric constant (d 33 ) was measured by using a piezometer (PM 25, Piezometer system; UK). The electromechanical coupling coefficient (k p ) was determined by the resonant and antiresonant technique, The dielectric properties were measured at a frequency of 1 khz by using an impedance analyzer (HP 4194A, Impedance/Gain Phase Analyzer, Hewlett Packard, UK). 1. Weight loss and bulk density RESULTS AND DISCUSSION Three nominal ceramic compositions prepared by conventional solid-state method are listed in Table 1, the percent weight loss, shrunk diameter, shrunk thickness on sintering and sintering density are also reported. The discrepancy in weight loss was relatively small, it was less than 5% which was mostly due to the PbO loss during sintering (Zai et al., 2001). The shrunk diameter and thickness on sintering was about 11-15%. The sintering density was increased as the increased sintering temperature. When more lanthanum was doped, the density was increased from 7.5 g/cm 3 to 7.8 g/cm Phases and microstructure The phase formation behavior of the sintered ceramics was revealed by an XRD method. The XRD patterns of PLZT ceramics shown in Figure 1 were identified as a material with a perovskite structure having rhombohedral and tetragonal symmetry by using JCPDS card no and , respectively. The tetragonal lattice parameters were determined from the evolution of the (001)

4 and (110) tetragonal peaks by using least square method. The results given in Table 2 revealed that the tetragonal cell parameters and cell volume of PLZT ceramics were not varied as the sintered Table1. Some physical properties of the sintered Pb 1-x La x (Zr 0.54 Ti 0.46 )O 3 ceramics. Sample Lanthanum composition (x) Sintered temperature ( o C) PL 0.01 ZT PL 0.03 ZT PL 0.05 ZT Weight loss (%) Shrunk diameter (%) Shrunk thickness (%) Density (g/cm 3 ) Table 2. Effect of La 2 O 3 doped on tetragonal structure and average grain size of PLZT ceramics. Sample PL 0.01 ZT (1200 o C) PL 0.01 ZT (1250 o C) PL 0.01 ZT (1300 o C) PL 0.03 ZT (1200 o C) PL 0.03 ZT (1250 o C) PL 0.03 ZT (1300 o C) PL 0.05 ZT (1200 o C) PL 0.05 ZT (1250 o C) PL 0.05 ZT (1300 o C) a (Å) c (Å) V = a 2 c (Å 3 ) grain size (µm)

5 temperature and the amount of La 2 O 3, tetragonal lattice constant a = Å, c = Å, and cell volume V = (Å) 3 were obtained. There is small pyrochlore phase in each sample, the amount of lanthanum content does not effect the pyrochlore phase of the sample. Intensity (a.u.) 2θ(Degree) Figure 1. X-ray diffractogram of Pb 1-x La x (Zr 0.54 Ti 0.46 )O 3 ceramics. SEM micrographs of PLZT caramics are shown in Figure 2. The influence of sintering behavior and the amount of La 2 O 3 doped on the microstructure of PLZT system was observed. The grain was bigger at higher sintering temperature and less La 3 + dopant. The large grain size revealed hexagon shape indicated that surface grains grew extensively during sintering. Extensive grain of PLZT ceramics belonged to the 0.01 mol% La 2 O 3 doped sample sintered at 1300 o C. The average grain size reported in Table 2 were determined from line intercept method.

6 PL 0.01 ZT (1200 o C) PL 0.01 ZT (1250 o C) PL 0.01 ZT (1300 o C) Lower Lacontent PL 0.03 ZT (1200 o C) PL 0.03 ZT (1250 o C) PL 0.03 ZT (1300 o C) PL 0.05 ZT (1200 o C) PL 0.05 ZT (1250 o C) PL 0.05 ZT (1300 o C) Higher sintered temperature Figure 2. Thermally etched polished surface SEM micrographs of PLZT ceramics. 3. Ferroelectric, dielectric and piezoelectric characterization The typical shapes of measured ferroelectric polarization versus electric field (P-E hysteresis) of PLZT ceramics sintered at different temperatures are shown in Figure 3. When more La 2 O 3 was added, normal ferroelectric behavior with rectangular loop was formed, and remanent polarization (Pr) was increased. The effect of sintering temperature on ferroelectric properties assessed from polarization field (P E) measurements are listed in Table 3. Remanent polarization (Pr) and coercive field (E) were also observed to decrease as increasing sintering temperature. This is probably due to smaller grain size at low sintering temperature, and polarization switching is thus suppressed.

7 E (kv/cm) (a) E (kv/cm) (b) P (µc/cm 2 ) P (µc/cm 2 ) P (µc/cm 2 ) E (kv/cm) (c) Figure 3. P E hysteresis as a function of sintering temperature for (a) PL 0.01 ZT (b) PL 0.03 ZT and (c) PL 0,05 ZT ceramics. Table 3. Summarize of ferroelectric, dielectric and piezoelectric properties for PLZT system. Sample PL 0.01 ZT (1200 o C) PL 0.01 ZT (1250 o C) PL 0.01 ZT (1300 o C) PL 0.03 ZT (1200 o C) PL 0.03 ZT (1250 o C) PL 0.03 ZT (1300 o C) PL 0.05 ZT (1200 o C) PL 0.05 ZT (1250 o C) PL 0.05 ZT (1300 o C) Pr (µc/cm 2 ) E (kv/cm) ε tan δ k P % d 33 (pc/n)

8 Dielectric and piezoelectric properties for PLZT ceramics measured at room temperature are given in Table 3. Both dielectric constant (ε) and dielectric loss (tan δ) were increased with the increased amount of La 2 O 3. Piezoelectric properties for PLZT system are also revealed in Table 3,both piezoelectric coefficient (d 33 ) and planar electromechanical coupling factor (k p ) for PLZT system were increased with the increased La 2 O 3 dopant, but when the sintering temperature were increased, both d 33 and k p were decreased. CONCLUSION Lead zirconate titanate doped with lanthanum La 3 + ceramics were obtained by employing mixed-oxide method sintered at 1200, 1250, and 1300 o C. Under the observation of SEM micrograph, the grain size was found to increase with the increased sintering temperature and less La 3+ dopant. XRD analysis indicated pyrochlore phase in each ceramics, the amount of lanthanum did not effect the tetragonal parameters. Ferroelectric, dielectric and piezoelectric measurements showed a dependence of those properties on the amount of lanthanum and sintering temperature. The Pb La (Zr Ti )O 3 ceramics sintered at 1200 o C showed the optimum condition to obtain better ferroelectric, dielectric and piezoelectric properties. LITERATURE CITED Garg, A. and D.C. Agrawal Effect of rare earth (Er, Gd, Eu, Nd and La) and bismuth additives on the mechanical and piezoelectric properties of lead zirconate titanate ceramics. Mater. Sci.4 Eng. B[86]: Ryu, J., J.J. Choi and H.E. Kim Effect of Heating Rate on the Sintering Behavior and the Piezoelectric Properties of Lead Zirconate Titanate Ceramics. J.Am. Ceram.Soc., 84 [4]: Santos, I.A., C. Endo, A.L. Zanin, M.H. Lente, J.A. Eiras and D. Garcia Hot-Pressed Transparent PLZT Ceramics from Low Cost Chemical Processing. Mater. Res., 4[4] : Fan, H., G.T. Park, J.J. Choi, J. Ryu and H.E.Kim Preparation and improvement in the electrical properties of lead zinc-niobate-based ceramics by thermal treatments. J, Mater. Res., 17 [1]:

9 Lee S.M., C.B. Yoon, S.H. Lee and H.E.Kim Effect of lead zinc niobate addition 0n sintering behavior and piezoelectric properties of lead zirconate titanate ceramics. J. Mater. Res., 19 [9] : Seo,S.B., S.H. Lee, C.B. Yoon, G.T. Park, and H.E. Kim Low-Temperature Sintering and Piezoelectric Properties of 0.6Pb(Zr 0.47 Ti 0.53 )O 3-0.4Pb(Zn 1/3 Nb 2/3 )O 3 Ceramics. J. Am. Ceram. Soc., 87 [7] : Zai, M.H.M., A. Akiba, H, Goto, M. Matsumoto, and E.M. Yeatman Highly (111) Oriented Lead Zirconate Titanate Thin Films Deposited using a Non-Polymeric Route. Thin solid films, 394 :