Hydrothermal Synthesis Of High-quality PZT Powders And Properties Of Relevant Piezoelectric Ceramics

Lead zirconate titanate (PZT) is a widely used piezoelectric functional material, which owns great advantages such as excellent piezoelectric, dielectric and ferroelectric properties, flexible doping and high stability. Especially, the PZT piezoelectric ceramics with a composition around the morphotropic phase boundary (MPB) exhibit high piezoelectric constants and electromechanical coupling factors, and are the base materials for a variety of applications such as ultrasound transducers, piezoelectric transformers, filters and piezoelectric buzzers. Preparation of high-purity, uniform-sized, well-crystallized, and stoichiometry-appropriate PZT powders is a key technology towards high-performance piezoelectric devices. The conventional solid-state reaction method that is widely used nowadays involves a complicated high temperature calcination process, which may lead to a stoichiometry deviation and thus deteriorated electrical properties of the PZT ceramics due to the high volatility of Pb. Meanwhile, powders synthesized by the conventional solid-state reaction method are generally uneven in size, easy to agglomerate and contain more impurities that are introduced in the ball milling process. The hydrothermal method that was developed in recent decades can produce highly pure and well-crystallized powders directly under relatively low temperature without calcination. In addition, the hydrothermal derived powders have well controlled stoichiometry and superior sinterability. So the hydrothermal method has been extensive studied and applied for preparation of ultra-fine powders. However, previous studies on the hydrothermal synthesis of PZT powders were mainly focused on the influence of hydrothermal process parameters on the morphology, phase structure, and grain size distribution of the powders, whereas the influence of hydrothermal-derived powders on the final ceramic properties was rarely concerned.To overcome the problems of preparing piezoceramics by conventional solid-state reaction method, and advance the hydrothermal synthesis of piezoelectric powders, we studied in this work the production of PZT powders by hydrothermal synthesis. The phase structure and morphology of PZT powders were controlled by regulating the hydrothermal conditions, such as the mineralizer concentration and reaction temperature. The growth mechanism of PZT powders synthesized under different hydrothermal conditions was discussed preliminarily, and the optimal synthesis condition for the PZT powders was determined based on the phase structure, morphology of the resulting PZT powders and the performance of the final ceramics. In addition, we also prepared PZT ceramics of the same composition by conventional solid-state sintering, and compared the crystal structure, morphology, and electrical properties of the conventionally prepared PZT powders and ceramics with those of their hydrothermal derived counterparts. Moreover, PZT rods were synthesized successfully by hydrothermal synthesis in this work, implying potentials for the development of nano-piezoelectric devices. The main contents and achievements of the work are as follows:(1) Tetragonal perovskite PbZr0.52Ti0.48O3powders were synthesized by a two-step hydrothermal approach using ZrC102(8H20),(C4H9O)4Ti, Pb(NO3)2as precursors, and KOH as mineralizer to prepare ceramic samples. The effect of hydrothermal conditions, such as the mineralizer concentration and reaction temperature, on the crystallization and morphology of the resulting PZT powders and on the piezoelectric performance of the final PZT ceramics have been systematically studied. Reaction mechanisms in the formation of PZT powders under different hydrothermal conditions were discussed preliminary. The results showed that a hydrothermal environment of low alkalinity and low temperature would help to obtain mono-dispersed, well-crystallized PZT powders with a cubic morphology and high sinterability. With a reactant concentration of0.2M, mineralizer concentration of1M, reaction temperature of180℃, and reaction time of12hours, the final PZT ceramic samples exhibit the best electric properties (d33=310pC/N, Kp=53.2%, ε33T/ε0=1358, tan δ=0.005), which are much superior to those of the piezoceramics fabricated by conventional solid-state reaction method (d33=223pC/N, Kp=52%, ε33T/ε0=730, tan δ=0.004), with the piezoelectric constant d33and mechanical coupling factor Kp increased by39%and33%, respectively.(2) PZT rods were produced by hydrothermal synthesis while using PVA as surfactant. The influence of PVA’s concentration on the crystallization and morphology of the PZT rods was discussed while keeping the reactant concentration, mineralizer concentration, reaction temperature, and the reaction time fixed at0.1M,0.5M,200℃, and12hours, respectively. Experiments showed that PZT rods represent the best crystallization and morphology when the concentration of PVA is2.1mg/ml. Moreover, we synthesized PZT rods, typically 200～500nm in diameter and10～50μm long, without adding any surfactant. Experimental results show that the phase structure of the hydrothermal products are all body-centered tetragonal, i.e., the so-called PX phase, irrespective of PVA addition. The structure of the PZT rods could transform into perovskite through the annealing treatment in atmosphere at650℃for20min.