PZT ceramic matrix nanocomposites

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Materials Chemistry and Physics 109 (2008) 1–4

Materials science communication

Enhancement of dielectric permittivity in the ZnO/PZT ceramic matrix nanocomposites Zhi-Min Dang a,b,∗ , Dan Xie c , Yan-Fei Yu a,b , Hai-Ping Xu a,b , Yu-Dong Hou d a

Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China b Ministry of Education, Key Laboratory for Nanomaterials, Beijing University of Chemical Technology, Beijing 100029, China c Institute of Microelectronics, Tsinghua University, Beijing 100084, China d College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, China Received 10 July 2007; accepted 15 October 2007

Abstract Nanosized ZnO/lead zirconate titanate (ZnO/PZT) ceramic matrix composites were prepared by a traditional sintering process in this study. Dependence of the dielectric permittivity of the ZnO/PZT composites on frequency and temperature was studied. We observed the remarkable dependence of dielectric permittivity of the ZnO/PZT composites on the concentration of ZnO, frequency and temperature. A relatively high dielectric permittivity, which is more twice higher than that of pure PZT ceramics, was found in the ZnO/PZT composites. The results showed that the addition of nanosized ZnO had an obvious influence on the dielectric permittivity of the ZnO/PZT ceramics matrix composites. © 2007 Published by Elsevier B.V. Keywords: ZnO; PZT; Composites; Dielectric permittivity; Ceramic

1. Introduction Lead zirconate titanate (PZT) with the perovskite structure is being studied widely due to its excellent piezoelectric properties [1–2]. Recently, the application of piezoelectric ceramics for piezoelectric transformers was vigorously investigated for lighting liquid-crystal display (LCD) backlights [3–4]. It is well known that the dielectric permittivity of the materials has a crucial influence on deciding its piezoelectric property. In order to improve the dielectric property of ferroelectric ceramics, ceramic matrix composites consisting of two or more materials with different macroscopic properties are more attractive than their single-phase counterparts due to the improvement in both the physical and mechanical properties [5–7]. However, most composites studied are derived from a combination of piezoelectric ceramics and polymers [8–11]. Little work has been undergone on the preparation of piezoelectric/semiconductor composites with high dielectric permittivity because a large amount of other elements in a ceramic matrix would produce an effect on the dielectric property due to an assembly of impurities at the grain

∗

Corresponding author. Tel.: +86 10 6445 2126; fax: +86 10 6445 2126. E-mail address: [email protected] (Z.-M. Dang).

0254-0584/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2007.10.025

boundaries [12]. It has been recently found, however, that the presence of large amounts of semiconducting nanosized ZnO particles in the PZT matrix ceramics can result in a significant increase of dielectric permittivity. In this communication, the PZT was chosen as the matrix phase due to its wide applications. The nanosized ZnO was chosen as the semiconducting phase since it has a direct-band gap of 3.3 eV and showed a negligible reaction with PZT when the ZnO/PZT composites were sintered at high temperature. 2. Experimental The ZnO/PZT composites were prepared by the conventional solid-state reaction and subsequent sintering process. All raw materials for preparing the PZT and nanoscaled ZnO powders were from China. The PbO, ZrO2 , and TiO2 were weighted according to the stoichiometric formula Pb(Zrx Ti1−x )O3 with an atomic Zr/Ti ratio 55/45. An excess of 0.5 wt% PbO is added to the starting materials to compensate for the evaporation of Pb during sintering. The mixture oxides were milled in a planetary mill for 24 h at a speed of ∼250 min−1 , using ethanol as milling fluid. The oxides with the ethanol solvent were then dried at 150 ◦ C in oven. The mixture powder was calcined at 850 ◦ C for 2 h to achieve the PZT powders. The ZnO powders with a diameter about 50–90 nm, which was prepared by employing a wet chemical method as reported in our past work [13], was mixed ultrasonically with the calcined PZT powders in ethanol solvent to break down the agglomeration of ZnO particles. Dense bulk compacts were made of the mixture ZnO/PZT powders by slight uniaxial pressing in a mold with inner diameter about 10 mm, following by isostatic pressing at 10 MPa. The

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Fig. 2. Dependence of the dielectric permittivity ε of the ZnO/PZT composites on the ZnO volume fraction (a) at 103 and 106 Hz (25 ◦ C) and (b) at −100 and 100 ◦ C (103 Hz).

Fig. 1. (a) The XRD patterns of pure PZT, the nanosized ZnO and the ZnO/PZT (80/20) composite, (b) morphology of the ZnO/PZT ceramic composite with 20 vol% nanosized ZnO. bulks were sintered at ∼1150 ◦ C for 2 h. The as-sintered samples were about 10 mm in diameter and 1 mm in thickness. Phase analysis was performed on X-ray diffraction (XRD) (Siemens D5000) with 2θ range from 20◦ to 60◦ . Alternating current (AC) dielectric measurements of pure PZT and the ZnO/PZT composites were performed with the Precision LCR (Inductance Capacitance Resistance) Dielectric Meter (Agilent 4284A). The dielectric permittivity ε of the samples was calculated by the formula of a parallel plate capacitor as ε = Ct/ε0 A; where C is the capacitance of the metalinsulator-metal element, ε0 is the vacuum dielectric permittivity, t is the thickness of the samples and A is the area of the electrode, respectively. The dielectric permittivity was obtained at frequency ranges from 103 to 106 Hz and temperature ranges from −150 to 150 ◦ C as a decrease in temperature, and under inert N2 atmosphere to prevent the adsorption of moisture during the experiment. Electrodes were painted with silver paste prior to measurement.

3. Results and discussion The XRD patterns of pure PZT, the nanosized ZnO, and the ZnO/PZT ceramic composite with 0.20 volume fraction of the nanosized ZnO are shown in Fig. 1a, respectively. A series of characteristic peaks of pure PZT and nanosized ZnO are observed as presented in Fig. 1a. However, the characteristic peaks of nanosized ZnO disappear in the ZnO/PZT ceramic composite. The result illuminates that the size of nanometer ZnO grows

during the high temperature sintering process and might have some chemical reactions between PZT and ZnO which is not still known. Fig. 1b shows the typical morphology of ZnO/PZT ceramic composite with 20 vol% nanosized ZnO powders. The porous nature of PZT matrix is derived from the etching effect HF acid solution. In addition, it can be seen that the PZT particles are made of a lot of flakes, which shows the process of PZT crystal development during the sintering. Fig. 2 shows the dependence of the dielectric permittivity ε of the ZnO/PZT composites investigated on the volume fraction of ZnO at different frequencies (Fig. 2a) and temperatures (Fig. 2b). It is clear to see that the dielectric permittivity of the ceramic composites increases with an increase in the volume fraction of nanosized ZnO. According to our knowledge, the enhancement of dielectric permittivity in insulator-conductor can be explained using an effective cluster model [14]. The clusters of nanosized semiconducting ZnO will be formed slowly with increasing the concentration of ZnO in the composites. The dielectric permittivity of the composites increases with a formation of more ZnO clusters. A rapid increase in dielectric permittivity can be found when the volume fraction of ZnO is close to percolative threshold, in which the interconnect clusters of ZnO are formed in the ZnO/PZT composites. Mathematically, the effective dielectric permittivity of composites can be described as follows

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Fig. 3. (a) The temperature dependence of the dielectric permittivity ε of the ZnO/PZT (80/20) composite at different frequencies. The inset in (a) shows a temperature dependence of ε of pure PZT. (b) The frequency dependence of ε of the ZnO/PZT (80/20) composite at different temperatures, −100, −50, 0, 50, 100 and 150 ◦ C, respectively.

[15–18]: ε ∝ εPZT (fc − fZnO )−s , for fZnO < fc where fZnO is the volume fraction of nanosized ZnO fillers, fc is the percolation threshold, and s is the critical exponent of dielectric dispersion, s ≈ 1. It can worth noting that the dielectric permittivity of the ZnO/PZT (80/20) composite is twice larger than that of pure PZT ceramics at concentration of ZnO near to the percolation threshold. However, the increase in the dielectric permittivity of the insulator-conductor transition system is very significant in past results [10,12,17,19]. The result would be attributed to a poor conductivity of ZnO used (band gap: 3.3 eV). In addition, high dielectric permittivity can be observed at low frequency, 103 Hz (Fig. 2a) and high temperature, 100 ◦ C (Fig. 2b), which could be the results of more than one causes, such as electrode polarization, interfacial polarization and conductivity effects. In order to study the temperature and frequency dependence of the dielectric permittivity carefully, Fig. 3 show the dependence of dielectric permittivity of pure PZT and the ZnO/PZT (80/20) ceramic composite on temperature from −150 to 150 ◦ C (Fig. 3a) and frequency from 103 to 106 Hz (Fig. 3b). The dielectric permittivity of pure PZT (the inset in Fig. 3a) increases

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monotonically with increasing the temperature; whereas a slight shift can be seen at ∼40 ◦ C for pure Pb(Zr0.55 Ti0.45 )O3 ceramics prepared in this study. The increase in dielectric permittivity with temperature from −150 to 150 ◦ C is attributed to dipole polarization process in pure PZT ceramics, which is accordant to the frequency change of AC field. However, the dielectric behavior of the ZnO/PZT (80/20) composite shows a great difference to that of pure PZT with increasing temperature (Fig. 3a), although the dielectric permittivity increases also with an increase in temperature. The distinct difference is that the dielectric permittivity of the composite increases remarkably at low frequency (103 Hz) from −150 to 0 ◦ C. The dielectric permittivity decreases gradually with increasing the frequency from 103 to 106 Hz. Therefore a bulge shape can be observed below 0 ◦ C in Fig. 3a. The dielectric permittivity of the ZnO/PZT (80/20) composite at 150 ◦ C (∼800) is about 4 times higher than that at −150 ◦ C (∼200). In comparison with pure PZT, the rapid increase in dielectric permittivity of the ZnO/PZT (80/20) composite would be from percolation effect, in which the semiconducting ZnO might form the percolation clusters among the interface of PZT crystal particles during the sintering process. Fig. 3b shows the frequency dependence of the ZnO/PZT (80/20) composite at several temperature points. It is very clear that the dielectric permittivity decreases with increasing the frequency, which can be explained using the Debye model. Furthermore, the dielectric permittivity at −100 ◦ C decreases about 50% with the change of frequency; whereas the decrease in dielectric permittivity at 150 ◦ C is very slow. To explain the influence of low temperature on the dielectric properties (the dielectric permittivity ε and dielectric loss tan δ) further, Fig. 4 shows the dielectric behavior of pure PZT and the ZnO/PZT (80/20) composite at 0, −50 and −100 ◦ C, respectively. The dielectric permittivity at 0 ◦ C is always higher than those at <0 ◦ C for both pure PZT and the ZnO/PZT (80/20) composite with changing frequency from 103 to 106 Hz. It is also observed from the inset of Fig. 4a that the decreasing trend in dielectric permittivity of pure PZT at three temperature points is same with the change of frequency. However, the decrease in dielectric permittivity of the ZnO/PZT (80/20) composite at −100 ◦ C is very remarkable, in which the dielectric permittivity decreases from ∼410 to ∼240 with changing frequency from 103 to 106 Hz. At 0 ◦ C, the dielectric permittivity falls ∼5.4%. The result shows the ZnO/PZT (80/20) composite shows a negligible frequency dependence of dielectric permittivity when the temperature is ≥0 ◦ C. In order to illustrate the change of the dielectric permittivity in Fig. 4a, Fig. 4b shows the frequency dependence of the dielectric loss of pure PZT and the ZnO/PZT (80/20) composite at 0, −50 and −100 ◦ C. The dielectric loss of pure PZT increases with increasing the frequency. And the dielectric loss at 0 ◦ C is always higher than those at <0 ◦ C, which the maximum value of the dielectric loss at 0 ◦ C is ∼0.025 at 106 Hz (the inset of Fig. 4b). However, the dielectric loss of the ZnO/PZT (80/20) composite is larger than that of pure PZT. Furthermore the dielectric loss of the composite shows a marked relaxation peak with changing frequency from 103 to 106 Hz. The relaxation peak shifts to high frequency with increasing temperature from −100 to 0 ◦ C. The result shows that the nano-

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mics increases monotonically with increasing the temperature; whereas the dielectric permittivity of the ZnO/PZT (80/20) composite increases remarkably at low frequency (103 Hz) from −150 to 0 ◦ C. The decrease in dielectric permittivity of the ZnO/PZT (80/20) composite at low temperature is very remarkable when the frequency is changed from 103 to 106 Hz. It is worth noting that the ZnO/PZT (80/20) composite displays a negligible frequency dependence of dielectric permittivity when the temperature is ≥0 ◦ C. The relaxation peak of the dielectric loss shifts to high frequency with increasing temperature from −100 to 0 ◦ C. The formation of ZnO percolative clusters along the interfaces among the PZT crystal particles plays a crucial role on improving the dielectric permittivity of the ZnO/PZT composites. Acknowledgements This work was financially supported by the NSF of China (grant no. 50677002), NSF of Beijing City (grant no. 2063031), Doctor Project of China Ministry of Education, Key Lab of the Ministry of Education for Nanomaterials (grant no. 2006-4), and Program for New Century Excellent Talents in University (NCET). References Fig. 4. (a) The frequency dependence of ε of the ZnO/PZT (80/20) composite at low temperatures, −100, −50 and 0 ◦ C, respectively. The inset in (a) shows a frequency dependence of ε of pure PZT at low temperatures. (b) The frequency dependence of the dielectric loss tan δ of the ZnO/PZT (80/20) composite at low temperatures, −100, −50 and 0 ◦ C, respectively. The inset in (b) shows a frequency dependence of tan δ of pure PZT at low temperature.

sized ZnO shows an obvious influence on the dielectric property of the PZT ceramics. 4. Conclusions The dielectric permittivity of the semiconductor-ferroelectric ZnO/PZT composites presented in this study was increased due to the presence of a large amount of ZnO particles dispersed in the PZT matrix. The dielectric permittivity of the composites was about 2 times higher than that of pure PZT ceramics at the experimental temperature and frequency when the concentration of ZnO in the composites is close to the percolation threshold. The percolation ZnO/PZT ceramic composite displays the high dielectric permittivity at low frequency and high temperature. The dielectric permittivity of pure PZT cera-

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