Journal of the Japan Society of Powder and Powder Metallurgy Vol. 47, No. 4 391

Œ¤‹†

Improvement of Mechanical Properties of Piezoelectric Ceramics by Incorporating Nano Particles

Ken-ichi Tajima, Hae Jin Hwang, Mutsuo Sando and Koichi Niihara

•™1 Kyocera R&D Center Kagoshima, 1-4 Yamashita-cho, Kokubu 899-4312.

•™2 National Industrial Research Institute of Nagoya, 1-1 Hirate-cho, Kita-ku, Nagoya 462-8510.

•™3 The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki 567-0047.

Received December 27, 1999

SYNOPSISLead zirconate titanate (PZT) based composites were prepared from commercially available PZT powder and a

small amount (0.1 - 1.0vol%) of nano-sized alumina or magnesia, and their mechanical and piezoelectric properties were evaluated. The fracture strength of the composites increased with increasing second phase content due to grain size reduction, particularly in the case of PZT/MgO composites, which showed a significant improvement in strength. The planar electromechanical coupling factor, Kp, of the composites remained unchanged. As a consequence, highly reliable PZT composites with high strength (140 MPa) and Kp (60%) have been fabricated.

KEY WORDS

PZT, nano composites, mechanical properties, microstructure, piezoelectric properties

1 IntroductionBecause of their high piezoelectric properties, PZT and

related materials are widely used for electromagnetic filters, resonators, actuators and so on. In the application of actuators, high reliability is especially important, because high stresses are applied from the large displacement of the PZT. However, PZT with suitable piezoelectric

properties typically has poor mechanical properties, e.g., low Young's modulus (60- 80 GPa), fracture toughness (0.6 - 1.0 MPa ¥ m1/2) and fracture strength (50 - 70 MPa). Several attempts to improve the mechanical strength of PZT based composite systems have been reported in the literature. In PZT/Ag33, PZT/SiC4i and PZT/ZrO25) systems, high strength ceramics have been successfully prepared. However, the piezoelectric properties, in particular, the

planar electromechanical coupling factor, Kp, of the composites decreased to levels insufficient for their application as actuators. We have previously reported that microwave sintering and reinforcement by a small amount of second phase particles successfully maintain Kp and improve the mechanical properties of PZT based nanocomposites. It was found that as little as 0.1- 0.5 vol% of second phase is sufficient to improve the strength of PZT ceramics, and the reduction in Kp was minimal because the amount of second phase was small. However, as a result of using high purity and non-modified PZT

(Pb(Zro.52Tios)O3) with low Kp as the base material in these

studies, the Kp of the composites was less than 40%, which

is not large enough for high power actuators.

The purpose of this study is to obtain PZT based

composites with high strength and suitable Kp for actuator

applications. To achieve this target, commercially available

hard PZT powder, which is expected to have a higher Kp,

was selected as the matrix material. The effect of the small

amount of second phases on the mechanical/piezoelectric

properties and microstructure were investigated.

2 Experimental ProceduresThe starting material for the matrix was commercially

available PZT (PZTHQ, Sakai Chemical Industry Co. Ltd., Sakai, Japan). This PZT powder is modified to give a high mechanical quality factor (Qm; about 1500) and Kp (> 60 %). The mean particle size of the PZT was 0.3 Min and the Zr : Ti atomic ratio was 50:50. As the second phase, A1203

(TMDA-R, Taimei Chemicals Co. Ltd., Nagano, Japan) and MgO (100A, Ube Chemical Industries Ltd., Ube, Japan), with average particle sizes of 100 rim and 15nm respectively, were used in this study. The additive content of the second phase was between 0.1 and 1.0 vol% of the PZT matrix. The starting materials were mixed by wet ball-milling for 24h in isopropyl alcohol using ZrO2 balls. Disc shaped specimens (17 mm in diameter and 2.5 mm in thickness) were formed by uniaxial pressing at 5 MPa followed by cold isostatic pressing (CIP) at 196 MPa. The

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392Ken-ichi Tajima, Hae Jin Hwang, Mutsuo Sando, Koichi Niihara

specimens were sintered at 1200•Ž for 2 h in an alumina

crucible under PbO atmosphere using a conventional

electric furnace. For electrical measurements, sintered

specimens were ground and lapped to 1 mm in thickness,

and silver paste was printed on both sides of each disc,

followed by firing at 600•Ž for 10 min. Poling treatment

was carried out in silicon oil at 120•Ž for 30 min with an

electric field of 3 kV/mm.

The bulk density was determined by Archimedes' method

in water. The dielectric constant at 1 kHz was measured

using an impedance analyzer (Model 4194A, Hewlett-

Packard Co. Ltd., Tokyo, Japan). The planar electro-

mechanical coupling factor, Kp, was calculated by the

resonance method. The tetragonality (c/a) of the PZT phase

was determined by XRD analysis using the (200) and (002)

peaks of tetragonal PZT(JCPDS33-0784). The fracture

strength of the unpoled disc-shaped samples (1.7 mm in

thickness) was measured by the piston-on-ring biaxial

flexure test"). It has been confirmed that the strength value

by this method is almost the same as the 4-point bending

test using standard alumina specimens"). Fracture

toughness were measured by the indentation fracture (IF)

method. Fracture surfaces and microstructure were

observed with a scanning electron microscope (SEM). The

average grain size of each composite was estimated from

SEM micrographs by a linear intercept method .

3 Results and Discussion

Table 1 shows some physical, dielectric, and piezoelectric

properties of monolithic PZT and PZT composites. Almost

fully dense bodies can be obtained by sintering at 1200•Ž

for 2h. Addition of the second phases did not degrade the

sinterability of the ceramics. The densities of composites

containing A1203 or MgO were slightly higher than those

of monolithic PZT. The dielectric constants of PZT/A1203

composites were higher than those of monolithic PZT; those

of PZT/MgO composites increased initially up to 1710 with

an addition of 0.1 vol% MgO, but thereafter decreased

gradually with increasing MgO content. These differences

Table 1 Characteristic of the PZT composites .

* measurement at 1 kHz

in density and dielectric constant with second phase addition seem to be due to microstructural changes, such as a change in grain size of the composites"). Fig. 1 shows the grain size dependence of the dielectric constants of monolithic PZT and PZT composites containing A1203 and MgO. As is evident from Fig. 1, the value of the dielectric constants depends significantly on the grain size of the specimens, and decreases as the grain size decreases, irrespective of the additive species. On the other hand, the reduction in Kp of the composites was not so significant, and it remained over 60 % in all samples. It is well known that the Kp of PZT ceramics is easily altered by compositional modifications12). If A1203 or MgO form a solid solution with Pb(Zr,Ti)03, the Al 3' and Mg 2' ions should substitute for Zr4+ or Ti4+ in the perovskite crystal structure. Since substitution of lower valence ions produces oxygen vacancies to maintain charge neutrality, it also results in the reduction of the tetragonality and hence piezoelectric behavior of PZT ceramicsl3-l). However, this was not found to be the case in the present study.

Fig. 2 shows the tetragonality (c/a) change with MgO and A1203 content of our PZT composites. There was no significant decrease in the tetragonality of the PZT/A1203 composites, while in the case of 0.1 vol% MgO, the tetragonality was actually a little bit higher than that of monolithic PZT. The small decrease of the tetragonality in 0.5 or 1.0 vol% MgO containing composites might be due to the grain size reduction"), because the tetragonality slightly decreased on further addition of MgO. These results suggest that A13+ or Mg2+ do not substitute for B-site atoms, and consequently, high Kp can be maintained even though non-ferroelectric particles such as A1203 or MgO are incorporated in the PZT matrix.

Fig.1 Grain size dependence of dielectric constants for PZT and

PZT composites.

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Improvement of Mechanical Properties of Piezoelectric Ceramics by Incorporating Nano Particles 393

The fracture strength of the composites increased with increasing second phase addition in all samples, as shown in Fig. 3. In the case of 0.5 and 1.0 vol% MgO containing composites, the strength was 140 MPa, which is much higher than that of monolithic PZT (93 MPa). This improvement in strength might be associated with the grain size reduction. In general, PZT ceramics with finer grain structure show higher fracture strength just as in other ceramic systems'). Fig. 4 shows the fracture surface of the monolithic PZT and PZT composites. A completely intergranular fracture mode was observed in monolithic PZT, but it switched to intragranular fracture in the PZT composites. This change in fracture mode suggests that the grain boundaries of the composites are reinforced by the dispersoids. Second phase dispersoids can be found in both A1203 and MgO added composites. Fig. 5 shows the second phase particles in a PZT/A1203 composite. The secondary particles were dispersed at the grain boundary

as well as inside of the PZT grain. As mentioned previously, although the grain size increased in A1203 containing composites, their strengths were slightly higher than that of monolithic PZT. It is assumed that grain boundary reinforcement and fracture toughness increase (Fig. 6) due to the secondary particles are responsible for the high strength. The fracture toughness increase in PZT/A1203 composites would be explained as an effect of crack deflection due to the enlarged grain size. Determination of

Fig.2 Tetragonality (c/a) of the PZT crystal lattice as a function of second phase content of PZT composites.

Fig.3 Flexural strength of PZT and PZT com posites.

Fig.4 Fracture surfaces for (a) monolithic PZT, (b) PZT/0.5 vol% A1203 composite and (c) PZT/0.5vol% MgO composite

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394Ken-ichi Tajima, Hae Jin Hwang, Mutsuo Sando, Koichi Niihara

Fig.5 SEM micrograph of PZT/0.5 vol% A1203 composite. Arrows indicate second phase dispersoids.

Fig.6 Fracture toughness of PZT and PZT composites as a function

of volume fraction of second phase.

the secondary phase composition and TEM analysis of the

interfaces between PZT and second phase particles are now

in progress.

4 ConclusionsCommercially available hard PZT based composites

containing small amounts of A1203 or MgO were prepared by a conventional route. In the A12O3 added composites, no degradation of the mechanical and piezoelectric

properties was found, although the grain size was increased. In contrast, small additions of MgO to hard PZT reduced the grain size significantly. In this case, a substantial improvement in strength (140 MPa) and very little reduction in Kp (60%) were achieved in the PZT/MgO composites compared to those of monolithic PZT (MOR 93 MPa, Kp 62%).

5 AcknowledgementThis work was promoted by AIST, MITI, Japan as part

of the Synergy Ceramics Project under the Industrial Science and Technology Frontier (ISTF) Program. Under this program, part of the work has been supported by NEDO. The authors are members of the Joint Research Consortium of Synergy Ceramics.

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