Dielectric and piezoelectric properties of low-temperature sintered lead zirconate titanate ceramics with 0.78PbO-0.22CuO flux addition

Abstract

Liquid phase sintering of lead zirconate titanate (abbreviated as PZT) ceramics with a 0.78 PbO-0.22 CuO (10:1 in weight ratio) flux was investigated. A small amount sintering flux consisting of a stoichiometrically mixed oxide of PbO-CuO with a eutectic composition successfully accelerated densification of the PZT ceramics far below the conventional sintering temperature. With the 3 wt% flux additions, full densifications of the PZT ceramics were achieved at temperatures as low as 825 °C. The results obtained in the crystal structure, microstructure, and electrical property analyses suggest that the Cu^sup 2+^ impurity ion substitutes as an acceptor center for the perovskite B-site without forming isolated secondary phases. Defect associates of the Cu^sup 2+^ impurities and charge compensating oxygen vacancies appear to affect dielectric and piezoelectric properties of the sintered PZT samples in both positive and negative ways by forming defect dipoles. When the Cu^sup 2+^ content is small, slightly improved dielectric and piezoelectric performances were achieved, though ferroelectric relaxation was obviously developed at the higher Cu^sup 2+^ contents. For the PZT samples sintered at 825 °C with 3 wt% flux addition, relative dielectric permittivity was 2200 and dielectric loss was less than 2%. Remnant polarization, coercive fields, and piezoelectric coefficient (d^sub 33^) were 32 μC/cm^sup 2^, 8.9 kV/cm and 373 pC/N, respectively.[PUBLICATION ABSTRACT]

Met. Mater. Int., Vol. 18, No. 6 (2012), pp. 1067~1072 doi: 10.1007/s12540-012-6022-3

Dielectric and Piezoelectric Properties of Low-Temperature Sintered Lead Zirconate Titanate Ceramics with 0.78PbO-0.22CuO Flux Addition

Baek Hyun Kim1, Jong Hoon Park2, Byungkyu Kim3,*, and Do-Kyun Kwon1,*

1Department of Materials Engineering (Materials Research Institute), Korea Aerospace University, Korea

2Department of Materials Science and Engineering, Seoul National University, Korea

3School of Aerospace and Mechanical Engineering, Korea Aerospace University, Korea

(received date: 5 December 2011 / accepted date: 12 January 2012)

Liquid phase sintering of lead zirconate titanate (abbreviated as PZT) ceramics with a 0.78 PbO-0.22 CuO (10:1 in weight ratio) flux was investigated. A small amount sintering flux consisting of a stoichiometrically mixed oxide of PbO-CuO with a eutectic composition successfully accelerated densification of the PZT ceramics far below the conventional sintering temperature. With the 3 wt% flux additions, full densifications of the PZT ceramics were achieved at temperatures as low as 825 C. The results obtained in the crystal structure, microstructure, and electrical property analyses suggest that the Cu2+ impurity ion substitutes as an acceptor center for the perovskite B-site without forming isolated secondary phases. Defect associates of the Cu2+ impurities and charge compensating oxygen vacancies appear to affect dielectric and piezoelectric properties of the sintered PZT samples in both positive and negative ways by forming defect dipoles. When the Cu2+ content is small, slightly improved dielectric and piezoelectric performances were achieved, though ferroelectric relaxation was obviously developed at the higher Cu2+ contents. For the PZT samples sintered at 825 C with 3 wt% flux addition, relative dielectric permittivity was 2200 and dielectric loss was less than 2%. Remnant polarization, coercive fields, and piezoelectric coefficient (d33)

were 32 C/cm2, 8.9 kV/cm and 373 pC/N, respectively.

Key words: dielectrics, ferroelectric materials, sintering, scanning electron microscopy (SEM), piezoelectricity

1. INTRODUCTION

In general, the sintering temperature of conventional Lead zirconate titanate (PZT) ceramics is high as approximately 1200-1250 C. Ceramic processing at such a high temperatures brings some Problems for manufacturing PZT ceramics, such as compositional change due to PbO volatilization, high manufacturing cost, and a restriction of the electrode materials for multilayer type device fabrications. In particular, the control of PbO content in PZT ceramics is very critical because loss of PbO during the sintering process results in deterioration of piezoelectric properties of PZT [1-3].

There have been many different approaches used to reduce the sintering temperature of PZT ceramics like pressure assisted sintering, utilization of chemically prepared fine PZT powders, and addition of sintering aids, etc. [4-8]. For large-scale, low-cost commercial applications, the use of sintering aids is most appropriate. Typical conventional sintering aids are oxides of metals such as Li, Na, B, V, Bi, and Pb, or their flu-

orides. A liquid phase sintering aid is typically chosen due to its low melting temperature, which should be such that there is a liquid phase present during second stage of the sintering process. This liquid phase makes it easier for particle rearrangement and for material transport enabling rapid and enhanced low temperature sintering. Combinations of two or more metal oxides are also chosen as sintering aids [9-11]. Normally, the compositions of these types of sintering aids are determined as eutectic compositions of metal oxides, which form liquid phases at sufficiently low temperatures to promote liquid phase sintering of the PZT ceramics. For high dielectric constant compositions, such as PZT, it is preferred that no secondary phase exist after sintering, since the electrically lossy secondary phase can cause severe deterioration of dielectric properties when it forms a series equivalent circuit with the major phase [12]. Therefore, a compromise must be found between the accelerated densification and the detrimental effect of the associated secondary phase. PbO-based compositions have been reported as promising liquid phase sintering aids for PZT ceramics, since the PbO-based additives in principle could compensate for the losses of PbO from the PZT, which is important to maintaining the

*Corresponding author: [email protected], [email protected] KIM and Springer, Published 20 December 2012

1068 Baek Hyun Kim et al.

original electrical characteristics. PbO-based sintering aids such as PbO-SiO2 glassy phase [13] and PbO-Cu2O eutectic compositions [14] have been explored for liquid phase sintering of the PZT ceramics, resulting in sufficient densification of the PZT ceramics at low sintering temperatures of 850 C. However, electrical deteriorations, i.e. reduction in dielectric permittivity and piezoelectric coefficient, were exhibited due to the formation of unwanted secondary phases, in both cases. Therefore, it is necessary to develop a novel flux composition which suppresses the secondary phase formation for an improved electrical performance while still maintaining equivalent densification.

In the present work, a PbO-CuO binary system was chosen as the flux material for the liquid phase sintering of the PZT ceramics. Eutectic composition exists in the PbO-CuO which is 78% PbO-22% CuO, and it forms a eutectic liquid phase at 789 C [15]. Since the ionic content of copper ion is reduced by half compared with the PbO-Cu2O eutectic composition, it is anticipated that the secondary phase formation might be restrained, and consequently achieve improvement in electrical performance of the low temperature processed PZT ceramics. Sintering behavior, microstructure, and related dielectric and piezoelectric properties over a wide frequency range of PbO-CuO added PZT ceramics are discussed.

2. EXPERIMENTAL PROCEDURES

The ceramic samples were synthesized by conventional mixed metal oxide methods using chemical reagent-grade PbO (99+%, Sigma-Aldrich), CuO (99.9%, High Purity Chemical Laboratory), and commercial PZT powders (Hizirco AD-0, Hayashi Chemical Industry Co.LTD., Japan). The proper amount of PbO and CuO powders were added into Hizirco AD-0 by the exact molar ratio of eutectic composition (78% PbO + 22% CuO), as the sintering aids to promote liquid-phase sintering. The powders were mixed homogeneously by ball-milling with ZrO2 balls using ethanol as a solvent for 24 h and then dried and granulated. The granulated-mixed powders were pressed at 1000 kg/cm2 to form disk-type pellets with a diameter of 8 mm and a thickness of 1mm, approximately. The pellets were sintered at various temperatures from 800 to 900 C for 2 h with a heating rate of 5 C/min.

The sintered densities of the samples were measured by the Archimedes method. X-ray diffraction (XRD, M18XHF, Mac science Instruments, and D-8 advance, Bruker Miller co.) was used to determine the crystalline phases of the sintered samples. Polished and thermally etched surfaces of the pellets were examined by field emission scanning electron microscopy (FESEM, JSM-6330F, Jeol) to observe the micro-structure and the porosity of the samples.

In order to obtain the electrical properties of the samples, silver paste was coated on both sides of the pellets as an elec-

trode and fired at 650 C for 1 h. The relative permittivity of the sintered samples was measured using impedance analyzer (HP4194A, Hewlett Packard) with a temperature controllable furnace. The samples were poled at 70 C with 20 kV/cm of applied DC-fields for 10 min. The polarization versus electric field hysteresis loop and the induced strain of the samples was measured by Radiant Precision LC system. The piezoelectric coefficient d33 was identified using d33-meter (ZJ-6B, Institute of Acoustic Academia Sinica).

3. RESULTS AND DISCUSSION

Figure 1 shows the relative bulk density of sintered PZT ceramics with 3~7 wt% of mixed metal oxide flux addition. The mixed metal oxide flux was composed of PbO and CuO in 10:1 weight ratio (i.e. 2.73 wt% of PbO and 0.273 wt% of CuO for 3 wt% of mixed oxide content), which corresponds to the PbO-CuO binary eutectic composition. With the PbOCuO addition, densification of the PZT started at ~775 C, and fully densified samples were achieved after sintering at 825 C for 2 h. Since the eutectic point of PbO-CuO is reported as 789 C [15], this sintering behavior agrees well with the liquid phase sintering promoted by the eutectic liquid phase. According to the phase diagram of PbO-CuO systems, it is believed that the eutectic liquid decomposes into PbO and CuO during the cooling process. However, the XRD profiles in Fig. 2(a) exhibited the sintered PZT sample with 3 wt% PbO-CuO addition to be composed only of perovskite phase PZT. For higher flux contents, only a tiny peak located at28.6 of 2-theta, corresponding to (101) reflection of PbO, was observed. And the peak-completely disappeared after sintering at higher temperatures (> 900 C). Therefore, it is believed that a considerable amount of excess PbO decomposed from the eutectic liquid has been consumed to compensate and/or prevent PbO loss from the main PZT phase.

Fig. 1. Relative bulk densities of the low-temperature sintered PZT samples with different flux contents as a function of sintering temperature.

Dielectric and Piezoelectric Properties of Low-Temperature Sintered Lead Zirconate Titanate Ceramics 1069

Fig. 2. (a) Room temperature XRD pattern of the PbO-CuO fluxed PZT ceramics, which were sintered at 825 C for 2 h, and (b) Magnification of the (002), (200) reflections of the PZT ceramics.

For the decomposed CuO phase, any trace of the copper species was not detected by the XRD analysis, but there was a change in the perovskite (002)/(200) peaks located at around 45 of 2-theta as shown in Fig. 2(b). The XRD pattern of pure PZT sample showed clear peak separation of the (002) and (200) reflections, which indicates tetragonality of the perovskite structure. The tetragonality gradually decreased with the increase in PbO-CuO flux content, which implies that a structural distortion of the perovskite unit cell might occur with incorporation of cation substitution by impurity ions such as Cu2+. B-site cation substitutions by Cu2+ in the perovskite structure have been reported by several researchers [16-18]. The similarity of effective ionic radii between Ti4+ (rTi4+ = 0.68 ), Zr4+ (rZr4+ = 0.81 ), and Cu2+ (rCu2+ = 0.72) supports this theory. For PZT, it is well established that an acceptor type dopant, such as Cu2+ for Ti4+, induces oxygen vacancy for charge compensation. The structural distortion of the CuO added PZT perovskite may attribute to the exist-

ence of the oxygen vacancies. A decrease in the tetragonality of perovskite BaTiO3 with Cu2+ substitution was also reported by Shukla et al. [19].

Figure 3 exhibits microstructures of PbO-CuO fluxed PZT ceramic samples sintered at 825 C for 2 h with different flux content. For all flux contents, very well densified micro-structures with grain sizes ranging 0.3~2 m were observed. This agrees with the XRD analysis results showing that any trace of the secondary phase, which possibly decomposed from the eutectic liquid, was not found in the microstructures. Therefore, it is believed that PbO-CuO mixed oxides in eutectic composition are very effective for sintering PZT ceramics at remarkably low temperatures without unwanted secondary phase formation.

Dielectric properties of the PbO-CuO fluxed PZT ceramic samples are shown in Fig. 4. Relative dielectric permittivity and dielectric loss were measured as a function of frequency. The 3 wt% PbO-CuO added PZT sample, which was sin-

Fig. 3. SEM micrographs of the PbO-CuO fluxed PZT ceramics, which were sintered at 825 C for 2 h. The flux contents are (a) 3 wt%, (b) 5 wt%, and (c) 7 wt%.

1070 Baek Hyun Kim et al.

tered at 825 C, exhibited superior dielectric properties (r = 2200, tan = 0.02 @ 1kHz) compare to pure PZT sintered at

1200 C. The slightly improved dielectric permittivity of the 3 wt% PbO-CuO added PZT sample might attribute to either reduced PbO loss due to the low processing temperature or additional polarization by defect dipoles associated with the B-site substitution. It was reported by Eichel et al. that a B-site substitution in PZT by Cu2+ introduces an oxygen vacancy for charge compensation, and consequently form defect dipole as the following description [18].

CuO PbPb + (1) This defect associate can interplay with the direction of spontaneous polarization, since it contains an electric dipole moment pD = q l with q = 2e at the oxygen vacancy site and q = 2e at the Cu2+ site, both having distance of about half a lattice constant. Generally, defect dipoles tend to increase the overall polarization as to the spontaneous polarization a defect polarization is added [20]. For the higher flux contents (5 and 7 wt% flux additions), it was obvious that dielectric relaxation occurred at low frequencies. Dielectric permittivity data of the 7 wt% PbO-CuO added PZT as a function of temperature shown in Fig. 5(c) characterized as a typical ferroelectric relaxor behavior. The ferroelectric relaxor behavior, which shows a rounded permittivity peak with low-frequency dispersion, was first observed by Smolenskii et al. [21]. They proposed that the compositional heteroge-

Pb(Zr,Ti)O3 CuZr Ti

,

VO

( ) 2OO

+

Fig. 4. Room temperature dielectric properties of the PbO-CuO fluxed PZT ceramics as a function of measurement frequency.

Fig. 5. Temperature dependent dielectric properties of the the PbO-CuO fluxed PZT ceramics. The flux contents are (a) 3 wt%, (b) 5 wt%, and (c) 7 wt%.

Dielectric and Piezoelectric Properties of Low-Temperature Sintered Lead Zirconate Titanate Ceramics 1071

Fig. 6. Comparisons of (a) Polarization vs. electric field and (b) longitudinal strain vs. electric field plots of the pure and 3 wt% PbO-CuO fluxed PZT ceramics.

neity (or compositional fluctuation) at the cation site was responsible for this diffuse phase transition phenomenon. In addition, defect dipoles associated with the impurity cations and the induced oxygen vacancies can enhance the relaxor phenomenon by forming a nano-scale local domain structure[22]. With respect to the observed dielectric properties of the PbO-CuO fluxed PZT, the flux content has to be limited as 3 wt%, where the actual CuO content is only 0.273 wt%, to avoid unwanted dielectric anomaly of the PZT ceramics.

Figure 6 exhibits the polarization and strain responses to the applied electric fields of pure and 3 wt% PbO-CuO added PZT ceramic samples which were sintered at 1200 C and 825 C, respectively. Higher polarizations, which agreed with the dielectric permittivity data, were observed for the 3

wt% PbO-CuO added samples. The remnant polarization of the low-temperature sintered PZT with 3 wt% PbO-CuO addition was as high as 32 C/cm2, and the coercive field was 8.9 kV/cm. The enhanced polarization resulted in improvement of induced mechanical strain in the same direction of applied electric field. The piezoelectric coefficient (d33) of

the low-temperature sintered PZT ceramics was as high as 373 pC/N, whereas that of the pure PZT was 363 pC/N. The piezoelectric properties were also significantly degraded when the flux content exceeds the optimum value, as shown in Fig. 7.

4. CONCLUSIONS

A mixed oxide flux consisting PbO and CuO at eutectic composition (0.78 PbO-0.22 CuO) was found to be a very effective sintering aid for low-temperature sintering of lead zirconate titanate piezoelectric ceramics. With only 3 wt% flux addition (2.73 wt% PbO and 0.27 wt% CuO), the sintering temperature of the PZT ceramics was drastically reduced to 825 C without any unwanted secondary phase formation. With respect to the observations, it was considered that the impurity ion such as Cu2+, which decomposed from the eutectic liquid, tended to be incorporated into the perovskite PZT lattice rather than to form isolated secondary phases. Generally, the incorporation of the impurity ions into the lattice is regarded as unfavorable phenomenon since it may result in a modification of the crystal structure and consequently deterioration of electrical properties. However, limited amount Cu2+ impurities in the PZT lattice slightly improved the dielectric and piezoelectric properties by introducing additional polarization from the defect associates. Higher

Fig. 7. Longitudinal piezoelectric coefficients (d33) of the sintered PZT ceramics with different PbO-CuO flux contents.

1072 Baek Hyun Kim et al.

flux content beyond the optimum level obviously caused a detrimental effect to the dielectric and piezoelectric properties. Therefore, a compromise must be found between the accelerated densification and the detrimental effect, and the optimum flux content for the PZT ceramics found in this study was 3 wt% of PbO-CuO mixed oxide. These results anticipate the realization of monolithic actuator and/or capacitor device fabrication using the obtained fluxed PZT ceramics with metallic inner electrodes by co-firing at low temperatures below 850 C.

ACKNOWLEDGEMENTS

This research was supported by NSL (National Space Lab) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (Grant # 2010-0015077).

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