Multilayer piezoelectric devices based on PZT

Transcription

1 JOURNAL OF MATERIALS SCIENCE: MATERIALS IN ELECTRONICS 7 (1996) Multilayer piezoelectric devices based on PZT E. NIETO, J. F. FERNANDEZ, C. MOURE, P. DURAN Instituto de Ceramica y Vidrio, CSIC, Electroceramics Department, Arganda del Re],,, Madrid, Spain A PZT 53/47 was obtained by a modified chemical route. Small-particle-size powders were allowed to obtain high solid content, non-aqueous slips with Newtonian behaviour. The use of both the tape-casting technique and the common multilayer ceramic fabrication process led to a high-density multilayer ceramic body without cracks or delaminations, and a homogeneous microstructure. The architecture of multilayer piezoelectrics has an important effect on the clamping of resonance modes, particularly on planar modes, by modifying the nature of the electrode and the effect of the polarization procedure. This fact is corroborated by the dielectric and piezoelectric properties measured. 1. Introduction By means of the tape-casting method, sheet ceramics of large area and small thickness can be prepared quickly and reliably. This method is the most widely used in the manufacture of ceramic multilayer materials, such as capacitors, piezoelectric devices, actuators, microelectronic substrates, function gradient materials and so on [1-3]. In this method the first objective is to obtain a ceramic slip of adequate viscosity. So besides water or another disolvent and the ceramic powder, other elements such as binders, plasticizers and dispersants [4, 5] are added, to give stability to the suspension [6-8] and to determine the strength and flexibility of the tape [9]. Layers of predetermined thickness are obtained when the slip is cast onto a substrate by using a fix or a moving doctor blade on a planar table. Drying in order to eliminate the disolvent, produces grain coalescence which leads to green tapes with high densities [5]. Before sintering, the layers are printed with electrodes, stacked, laminated, pressed and finally diced into chips. Low Pd-content electrodes must be used as internal electrodes in order to reduce the cost and suppress the formation of poor dielectric phases produced by the Pd-PbO reaction [10, 11]. For a 70 Ag/30 Pd electrode, the sintering temperature has to be lowered to 1130~ by (a) increasing the reactivity of powders, which also improves the properties [12]; (b) using sintering aids such as V205, to achieve a 95% d, PZT at ~ [6]. Other sintering aids make lower sintering temperatures possible but give an inferior homogeneity [13] in comparison with ceramic without adding aids; and (c) improving the layer processing, by controlling electrode and ceramic thickness, delaminations and interaction between ceramic and electrode, thus preventing interdiffusion [14, 15]. The present work deals with the preparation of PZT ceramic slips with a high solid content and good rheological properties. On this basis, high-green Chapman & Hall 1995 density layers and lower sintering temperatures were obtained. Thus, a lower-cost internal electrode will be attained. The dielectric and piezoelectric properties of the prepared ceramic multilayers will be studied. 2. Experimental procedure PbZro.s3Tio.4703 powders were obtained by a modified conventional ceramic procedure using the corresponding oxides as raw materials [16]. The weighted amounts of lead oxide, titania and zirconia were homogenized in an alcohol-water medium plus oxalic acid. A highly dispersed slurry was attained by continuously stirring and heating the mixture. Then the homogeneously mixed slurry was washed, filtered and dried at 65 ~ As a result, an almost amorphous yellowish powder was obtained. Traces of crystalline titania and zirconia were found by XRD analysis. Synthesis was performed at 700~ for 4 h. After an attrition milling process, the PZT powder obtained was characterized by using BET, SEM, XRD and Laser Coulter techniques. The high-solid-content slip formulation was prepared according to a procedure described elsewhere [ 17, 18]. The final ratio of solid content was as high as 84.5% which ensures the high green density of the sheets. In the tape-casting process (Fig. 1) the different liquid components were incorporated through different steps in order to reduce mutual interaction and to increase the physical properties of the subsequent sheets [19, 20]. The rheology of the slip was monitored by a viscosimeter Rotovisco Haake RV-20. Tape-casting was carried out in a system with a moving doctor blade on a fixed table [21, 22]. The blade speed was 1 cms -1. Square layers of 5.2 x 5.2 cm were punched from sheets and electroded by screen-printing with an ink of Ag/Pd 70/30 (Dupont 6142D) that contains lead to improve solderability: 55 84

2 TiO2 + ZrO2 + PrOH / Oxalic acid + PrOH/H20 ~ [ 1:1 Heating and continuous stirring PbO ~ l Amorphous powder Crystallization Milling Figure 1 Tape-casting process for PZT. PZT powder ( degrees ) Cast tape Figure 3 XRD pattern of calcined (250~ (700 ~ 4 h) PZT powder. 2 h) and synthesized Print electrode Stack layers Laminate layers Dice into chips Burn out Sintering External electrode Poling Test Figure 2 Steps to attain electroded MLP. Multilayers were prepared by stacking eight layers, five of them active layers. The stacks were pressed at 70~ with a final pressure of 113 Pa (Fig. 2). The pressed laminates were diced into chips of 0.7 x 0.7 cm and fired according to the following schedule: (a) burn-out: 700~ for 1 h at a heating rate of 0.5 ~ min- 1. (b) sintering: soaking temperatures ranging from 1050~ to 1175~ with a heating and cooling rate of 3 ~ min-1. The sintering process was performed in a buffered sealed high-alumina crucible in order to control PbO losses. The density of multilayer devices was measured by the Archimedes method. The nature of the phases was established by XRD surface analysis. Microstructure development was observed by Reflexion Optical Microscopy (ROM) and SEM on polished samples. MLP were poled in a silicone oil bath at approximately 120~ with electrical fields between 17 and 25 kv cm- 1 applied to the samples in the perpendicular direction of the MLP stacking; d33 was measured with a Berlincourtmeter, whereas the electromechanical coupling factors were calculated by the resonance-antiresonance method according to the IRE Standards [23]. The dielectric constant and losses were measured in the 1-10 khz frequency range. In order to compare results, electroded and unelectroded piezoelectric multilayers were studied following the same characterization process. 3. Results and discussion Fig. 3 shows the XRD pattern of the synthesized powder where PZT perovskite phase is formed but traces of PbO and ZrO2 remain unreacted. This effect is attributed to the lower reactivity of ZrO2 [-243. The average particle size, measured by both XRD analysis and Laser Coulter, is 0.4 Ilm. The small amount of particles, ca. 10%, higher than 1.5 I~m corresponds to small agglomerates, as was corroborated by SEM analysis (Fig. 4). The slip presents Newtonian behaviour and a viscosity of 4000 cps at 10 s-1 which decreases as the shear rate augments. The thixotropic behaviour of the slip is associated with the cracking of polymeric bonds between binder and plasticizers. The high solid content and careful control of casting parameters led to high-green-density tapes, ~ 100 pm. Fig. 5 represents the apparent density of active multilayers against temperature and sintering time. The density of electroded samples is calculated by taking into account the presence of noble metals in the MLP by the formula given by W. Wessing et al. [253. In our case, with Pm= 10.9 g cm- 3 and Pc g cm- 3 dm= 6 I~m and dc = 60 I~m, the following formula is obtained: P* = Pc x

3 / \ A? E 0 v 7 0~ t- E3 2 hours Figure 4 SEM micrograph of PZT synthesized powder at 700 ~ 4 h. Piezoelectric multilayer (a) ' Temperature(~ 1200 where p* represents the apparent density of electroded MLP and Pc the ceramic density. The maximum density was attained at 1100 ~ for 2 h. This temperatur e is lower than those used in sintering PZT ceramic synthesized from classical mixed and calcined oxides [26]. Non-electroded samples show higher densities than electroded samples except in the case of short sintering times and temperatures of up to 1125 ~ The lower density above 1125~ could be associated with weight losses, mainly PbO, during sintering. In the case of MLP with internal electrodes, weight losses are controlled and reduced at high temperatures. The interaction between electrode and ceramic could retain PbO by reaction with Pd [10]. The XRD pattern showed the evolution of the phase symmetry and it was observed that the ceramic crystallizes as a perovskite phase which transforms from rhombohedral to tetragonal as the temperature rises. Transformation could be attributed to the PbO losses in the ceramic matrix which produce a displacement of the composition towards the PbTiO3-rich region. The reaction, according to Saha and Agrawal [27], can be represented as Pb(ZrxTi(1 -x)o3 ~ Pb~l -a)(zr(x_ ~) Ti. -x)) O(3 -A2~) + APbO + 8ZrO2. Lead oxide could be trapped at the electrodes or lost by volatilization, and zirconia phases could not be confirmed by XRD analysis in the ceramic matrix. Fig. 6 shows the ROM micrographs corresponding to polished transversal sections of the MLPs sintered at temperatures of between 1050~ and 1150~ Good continuity in the internal electrode microstructure and a clear interface without reaction between the metal alloy and the ceramic phases can be observed at these temperatures. No delamination phenomena were found. Fig. 7 corresponds to the SEM micrographs obtained on surfaces sintered at 1150~ ll00~ and? E o v 7 t- r~ llo0~ 6 I I I I I I ' (b) Time ( hours ) Figure 5 Variation of density against (a) temperature and (b) time. (A) Electroded, (9 calculated, ([]) unelectroded ~ Microstructures reveal highly homogeneous ceramics with low porosity and no secondary phases. The grain size increases slightly as the sintering temperature rises. As was previously indicated, the best microstructure was found in the denser sample, i.e. at 1100 ~ The microstructure of active samples did not show electrode-ceramic interaction in spite of temperature treatment, close to the liquid temperature in the Ag-Pd diagram system [.28]. The fracture behaviour shows a predominately transgranular crack propagation mechanism in all the samples, and some regions of intergranular propagation which breaks up the bigger grains were also observed. As shown in Tables I and II, high dielectric constants were reached in the samples sintered at 1100 ~ for 2 h. The permittivity values of multilayer samples correspond, according to the relation K~3 = I;eff/n 2 (n is the number of active layers), to a dielectric constant of 550 for each individual layer. These values 57

4 Figure 6 ROM photograph of polished electroded samples at different temperatures: (a) 1050~ (b) 1100~ and (c) 1150 ~ are very much in line with the tetragonal symmetry of ceramic layers. Electroded samples sintered in the ~ range show the highest piezoelectric coupling parameters, the highest d33 constants, and the lowest Qm. These values could be explained by estimating the higher porosity, the different compressive stresses and the partial clamping of the electroded multilayer ceramic due to the presence of inactive electrode layers [29]. The high poling field, 25 kv cm- 1, gives rise to the appearance of a planar response with Kt/Kp ~ 2. The anisotropy of samples poled at 17kVcm-1, Kt/K p,~ (~3, can be explained by the clamping phenomena between layers and the low polarization field affecting unsaturated regions. The reduction of the frequency constant of the transversal mode seems to be more pronounced as the Figure 7 SEM micrographs of sintered surface of electroded MLP at different temperatures: (a) 1050~ (b) l l00~ and (c) 1150 ~c. 58

5 TABLE I Piezoelectric properties of electroded samples polarized at 17 kv cm 1 T" Kp Kt fn d33(x10-12) K3T3(10kHz) tan~(10khz) Qm(10kHz) ('C) C/N (Kt) (Kt) (K,) (K,) (Kt) (K (Kt) (K,) 1150 TABLE II Piezoelectric properties of electroded samples polarized at 25 kvcm - 1 T a K v Kt JN d33(x 10 12) K~3 (10 khz) tan ~ (10 khz) Qm (I0 khz) ('C) C//N (Kp) l12.5(kp) 1796(K, (Kp) (Kp) 1749(K (Kp) (Kp) 1900(K') 1125 TABLE III Piezoelectric properties of samples without internal electrodes T e' tan8 Kt d33( CC) C/N) sintering temperature increases. Such behaviour could be explained by the spread of clamping regions or, on the other hand, by the electrode-ceramic interaction acting as a thickness reducer. The piezoelectric properties of samples without an internal electrode are summarized in Table III. The decrease in K~3 as the sintefing temperature rises could be attributed to the PbO loss which displaces compositions towards the tetragonal-rich region in the phase diagram. The lowest sintering temperatures lead to lower piezoelectric constants, owing to a poling anisotropy. Longitudinal resonance modes are higher than planar ones. The poling field applied to the unelectroded MLPs was perpendicular to the layers. The degree of contact between layers modifies longitudinal and planar modes. In all the samples the da3 value, which decreases as the sintering temperature is increased, is lower than the d33 measured in electroded bodies. The best values of unelectroded samples are reached at a sintering temperature of l l00~ when the multilayer structure presents the highest density. 4. Conclusions With fine PZT powder, high-solid-content non-aqueous slips can be obtained. The MLP fabrication processes for these powders lead to multilayer ceramics which attain homogeneous microstructure in each layer, a clear interface and a high density without cracks or delaminations. The final MLP structure determines the clamping of resonance modes, mainly planar modes, depending upon the nature of the electrodes and the polarization procedure. The thickness coupling factor Kt attains high values for samples fired at 1100 ~ whereas the planar coupling factor Kp is smaller as the multilayer structure disturbs the vibration in the normal direction towards the poling axis. Acknowledgement This Research was supported by the Spanish Comision Interministerial de Ciencia Y Tecnologia (MAT ). References 1. C. WENTWORTH and G. W. TAYLOR, Am. Ceram. Soc. Bull. 46(12) (1967) R.E. MISTLER, Ceram. Bull. 69(6) (1990) J.V. BIGGERS, T. P., SHROUT and W. A. SHULZE, Am. Ceram. Soc. Bull. 58(5) (1979) R. MORENO and J. REQUENA, Bol. Soc. Esp. Ceram. Vidr. 31(2) (1992) R. E. MISTLER, D. J. SHANEFIELD, R. B. RUNK, "Ceramic Processing before firing", eds G. Y. Unoda and L. L. Hench (John Wiley & Sons, New York, USA, 1978) p R. MORENO, Am. Ceram. Soc. Bull. 71(10) (1992) K. NAGATA, J. Ceram. Soc. Jpn 100 (1992) 125l. 8. S. BHATTACHARJEE, M. K. PARIA and H. S. MA[TI, J. Mater. Sci. 28 (1993)

6 9. R. MORENO, Am. Ceram. Soc. Bull. 71(1) (1992) S.F. WANG and W. HUEBNER, J. Am. Ceram. Soc. 76(2) (1993) 474. l l. J.G. PEPIN, Adv. Ceram. Mater. 3(5) (1988) G. GUZMAN, M. A. AEGERTER, P. BARBOUX and J. LIVAGE, J. Mater. Sci. 28 (1993) C.H. WANG and L. WU, Ceram. Int. 19 (1993) W.R. BUESSEM and T. I. PROKOPOWICZ, Ferroelectrics 10 (1976) J. G. PEPIN, W. BORLAND, P. O'CALLAGHAN and R. J. S. YOUNG, J. Am. Ceram. Soc. 72(12) (1989) E. NIETO, J. F. FERNANDEZ, C. MOURE and P. DURAN, Proc. 8th CIMTEC World Ceramics Conyress, Florencia, 1994, in press. 17. W.R. CANNON, J.R. MORRISandW. R. M1KESKA, Adv. in Ceram. 19 (1986) J.F. FERNANDEZ, P. DURAN and C. MOURE, Ferroelectrics 127 (1992) K. MIKESKAandW. R. CANNON, Am. Ceram. Soc. 9(1984) M. DESCAMPS, P. TRONC, P. LAURENT and B. THIERRY, Silicates Industries (1992) R.B. RUNK and M. J. ANDREJCO, Ceram. Bull. 54(2) (1975) J. JIMENEZ, M. POYATO and J. F. FERNANDEZ, Bol. Soc. Esp. Ceram. Vidr. 33(1) (1994) "IRE Standards on Piezoelectric Crystals", Proc IRE 49 (1961) S.Y. CHEN, S.Y. CHENGandC. M. WANG,J. Am. Ceram. Soc. 73(2) (1990) W. WESSING, W. WAHL and M. SCHN(~LLER, Ferroelectrics 87 (1989) P. DURAN and C. MOURE, Mater. Chem. Phys. 15 (1986) S. K. SAHA and D. C. AGRAWAL, Am. Ceram. Soc. Bull. 71(9) (1992) I. KARAKAYA and W. T. THOMPSON, Bull. Alloy Phase Diag. 9(3) (1988) W. WESSING, M. SCHNOLLER and H. WAHL, Ferroelectrics 68 (1986) 145. Received 22 November 1994 and accepted 27 January