Fabrication of niobium doped Pb(Zr,Ti)O₃ fibers by viscous polymer processing

1. Institute of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China;
2. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,Wuhan University of Technology, Wuhan 430070, China;

3. Interdisciplinary Research Centre in Materials Processing University of Birmingham,B15 2TT, United Kingdom

Received 10 April 2006; accepted 25 April 2006

Key words:

viscous polymer processing; PZT-5 fiber; perovskite structure;

1 Introduction

PZT fibers have been used in ultrasonic transducers[1], hydrophones[2] and so-called active fiber composites (AFCS)[3] because of their fine scales and good piezoelectric properties. During the past 15 years, three major techniques have been developed to produce fine PZT fibers: extrusion[4], viscous suspension spinning (VSSP)[5] and sol-gel processing[6]. Although alkoxide-based sol-gel processing is a traditional technique to produce fine PZT fibers, it has the drawback of high contraction ratio during sintering. Microscopic investigations revealed that fibers produced via VSSP are inhomogeneous and to date, only the extrusion process method offers the mean to manufacture high quality fibers[7].

In this work, the study on fine scale ceramic fibers based on viscous polymer processing (VPP)[8] was reported, and a technique was developed originally to make macro-defect-free structural ceramics. One advantage of VPP over conventional slurry extrusion is the increase of the strength of ceramic green bodies by at least an order of magnitude because of the macromolecular polymer binder system and high shear stress used during processing. Another advantage is the highly plastic nature of the ceramic slurry or dough which is thus amenable to various forming methods. We then outline the microstructures we have made. Experimental results are reported. We conclude that VPP is a promising technique to produce high ceramic content PZT green fibers, and results in high directional and compact fibers.

2 Experimental

Niobium doped Pb(Zr,Ti)O₃ (PZT-5) piezoelectric ceramic powder is required with mean particle size lower than 1 μm for the VPP-based fabrication.

VPP-based fabrication was a three-stage process.First, PZT-5 powders were ground for 16 h with alcohol, using a ball-mill (an agate pot and agate balls with sizes of 10-15 mm). Second, Ball-milled PZT-5 ceramic powders added with PVA binder and glycerin were mixed and churned up for 30 min. Here the mass ratio was 30∶5∶1 (powder∶binder∶glycerin), and the ceramic content was up to 83.3%. After ground on a high shear twin-roll, a combination of ceramic powder, solvent and plasticizer suitable for extrusion were made. Third, the mixed slurry was aged for 2-4 d and then were extruded to green PZT-5 fibers. After dried at 100 ℃ for 24 h, the green fibers were cured to remove polymer and then sintered.

The differential scanning calorimetry (DSC) and thermal gravity (TG) curves of PZT-5 slurry, as shown in Fig.1, were implemented at a heating rate of 10 ℃/min in N₂ atmosphere (Germany NETZSCH STA 409C TG-DSC analyzer). At around 90 ℃ the endothermic peak accorded to evaporation of water. The polymer burning reacted at 170-240 ℃. According to TG-DSC, the polymer remover of the green fiber was performed at a heating rate of 1 ℃/min from room temperature to 850 ℃ and maintaining 2 h (to remove a sufficient amount of the residual solvent and organic groups). The TG curve shows dramatic decrease of fiber mass after 1 100 ℃, so the PZT-5 ceramic fiber should be sintered under a lead excess atmosphere.

Fig.1 TG-DSC curves of PZT-5 slurry

Particle size analysis of PZT-5 powders was carried out to determine the average particle size and surface area (English Malvern Mastersizer X). Zeta potential was executed on a Brookhaven Instrument. X-ray diffraction (XRD) (Panalytical X-pert Pro) and scanning electron microscopy (SEM) (Japan JSM-5610 LV Electron Microscopy) were used for the analysis of the formed phase and the observation of the morphology of the green and sintered fibers. Tensile strengths of green and fired fibers were inspected (Super Duper Multi National Conglomerates RUS). Ferroelectric property was performed at a frequency of 1 Hz at room temperature (the Radiant Precision Workstation).

Table 1 lists the statistical results of PZT-5 ceramic powders after 16 h ball-milling. The d₅₀ of the sample was 0.54 μm. High specific surface area of 3.55 m²/g was measured, too. The PZT-5 solution used to measure zeta potential had PH value of 6 and concentration of 0.4 g/L. The ceramic powder had positive zeta potential of 8.81 mV. This powder showed high dispersibility as preparing ceramic slurry, which was suitable for VPP processing.

Table 1 Properties of PZT-5 ceramic powder after 16 h milling

Fig.2 shows the XRD patterns of PZT-5 fibers sintered under various conditions. An intermediate phase, pyrochlore, lied in fibers sintered at both 1 270 ℃ and 1 280 ℃ for 2 h. When the sintering conditions changed to 1 290 ℃ for 2 h or 1 280 ℃ for 4 h, the pure perovskite phase formed.

Fig.2 XRD pattern of PZT-5 ceramic sintered under various conditions

Fig.3 shows the SEM micrographs of the sintered PZT-5 fibers under various sintering conditions. As increasing temperature or prolonging time, the pure perovskite structure forms. This is consistent with the XRD analysis shown in Fig.2. Further more, it can also be seen that the particle size of the fibers sintered at

1 280 ℃ for 4 h (Fig.3(e) and Fig.3(f)) is 2-5 μm without pores or cracks, which is more uniformity than that sintered at 1 290 ℃ for 2 h (Fig.3(g) and Fig.3(h)). And its crystal grains are broken, which exhibit that the interaction between the grains is stronger for a lead excess atmosphere.

Fig.3 SEM micrographs of sintered PZT-5 fibers under different conditions: (a), (b) 1 270 ℃ for 2 h; (c), (d) 1 280 ℃ for 2 h; (e), (f) 1 280 ℃ for 4 h; (g), (h) 1 290 ℃ for 2 h

Fig.4 shows the tensile strength vs strain of the green PZT-5 fiber after dried and the sintered one at 1280 ℃ for 4 h. It can be seen that the green fiber behaves nonlinear from Fig.4(a). It is caused by the change with strain. The slim bend may be caused by the glue used to adhibit the fiber on paper for testing. The ultimate tensile strength of the fired fiber is about 13.84 MPa which is far higher than that of the green one from Fig.4(b). The high value of tensile strength for sintered fiber also illuminates the dense structure as shown in Fig.3(e) and Fig.3(f). And the elastic modulus of it is calculated to be 6.9 GPa which is consistent with PZT-5 ceramic.

Fig.4 Tensile strength-strain curves of PZT-5 fiber: (a) Green fiber; (b) Fiber sintered at 1 280 ℃ for 4 h

The fiber sintered at 1280 ℃ for 4 h is embedded in epoxy and then cured. The epoxy with fiber is sliced to a plate with thickness of 340 μm. Air-dry silver paste was used to one whole side of the plate. However, only the fiber has the silver paste at the other side. Fig.5 shows the P-E loop of the fiber. It can be seen that the polarization can be saturation at 9 kV/mm. And the remnant polarization (P_r) and coercive field (E_c) are 50.65 μC/cm² and 2.45 kV/mm, respectively. The coercive field is the same with the PZT-5 ceramic. Nevertheless, this fiber can withstand a much higher electric field than the ceramic (about 5 kV/mm), and the remnant polarization is larger than the ceramic (42 μC/cm²). Both of them show well directional and high compact of green fiber during extrusion[10]. And after sintering, high quality fibers with far fewer defects than ceramic are achieved.

Fig.5 P-E loop of PZT-5 fiber sintered at 1280 ℃ for 4 h

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(Edited HE Xue-feng)

Foundation item: Project (50402014) supported by the National Natural Science Foundation of China

Corresponding author: CHEN Wen; Tel: +86-27-87651107; Fax: +86-27-87864580; E-mail: chenw@mail.whut.edu.cn