Investigations On The Preparation Of Lanthanum Lead Zirconate Titanate Ceramic Powders Via A Hydrothermal Synthesis

PLZT ceramics can possess good piezoelectric,ferroelectric,dielectric,pyroelectric and electro-optic properties by adjusting their compositions,and are widely used as the electronic components,e.g.,actuators,capacitors,electro-optical switchers,pyroelectric detectors and optical filters.The performance of bulk ceramics greatly depends on the properties of the ceramic powders as a start raw materials.The preparation process of the ceramic powder has become a major concern for improving the performance of the bulk ceramic.In this thesis,PLZT ceramic powders with high performance were prepared via a two-step hydrothermal process.The main research contents and results are listed as follows.(1)One-step,two-step and sol solution-hydrothermal methods were used to prepare the PLZT ceramic powders.One-step hydrothermal method derived PLZT ceramic powders have a narrow particle size distribution,and average particle size of 1.1 μm.This method is the simpler,but the average particle size of is larger.Two-step hydrothermal method derived PLZT ceramic powders have also a narrow particle size distribution,,and the average particle size is about 0.8 um.The sol solution hydrothermal method derived PLZT ceramic powders have a narrow particle size distribution as well,the average particle size is about 2.1 um.the particles are more uniform.(2)PLZT ceramic powders with three zirconium/titanium ratios were prepared using a two-step hydrothermal method,i.e.,PLZT(6/52/48),PLZT(6/63/37),PLZT(6/95/5).Their average particle sizes are 0.22 μm,0.61 μm,and 0.81 μm,respectively.With the increase of zirconium/titanium ratio,the concentration of KOH required for hydrothermal process is larger,and the average particle size of the prepared PLZT powder becomes larger.Using the PLZT powder prepared by the two-step hydrothermal method,the PLZT bulk ceramics are procured using ball-milling,drying,granulation,pressing,bind burn-out,and two-step sintering processes.The average grain sizes of the bulk ceramics of the compositions of PLZT(6/52/48),PLZT(6/63/37),PLZT(6/95/5)are 2.7 μm,2.8 μm,and 3.4 μm,respectively.The saturation polarizations are 43.3μC/cm2,46.63 μC/cm2,and 25.93 μC/cm2.The largest applied electric fields for PLZT(6/52/48)and PLZT(6/63/37)at room temperature are 8 MV/m and 7 MV/m,.The largest applied electric field for PLZT(6/95/5)bulk ceramic at 100 ° C is 9 MV/m(3)Two-step hydrothermal method was used to prepare PLZT ceramic powders with different contents of lanthanum,i.e.,PLZT(7/76/24),PLZT(9/76/24),and PLZT(11/76/24).Under the condition of low concentration of KOH,the solubility of PLZT precursor in the solution is low,and the crystallite concentration in the solution is also low,thus the PLZT crystallites are probably directly precipitated from the surface of the precursor,and the crystallization mechanism of the powders is the in-situ growth mechanism.Hence,higher temperature and longer time of reaction is required to obtain pure powders.At high concentrations of KOH,the PLZT precursor is completely dissolved in the solution,the crystallite concentration in the solution is large,and the PLZT crystallites are directly precipitated from the solution during the hydrothermal process.The crystallization mechanism is the dissolution and precipitation mechanism When the PLZT powders are precipitated as per the dissolution and precipitation mechanism,the low temperature period is too short,which will cause the powder to agglomerate.Increasing the holding time of the low temperature section is helpful to reduce the agglomeration phenomenon.Under the same hydrothermal conditions,with the increase of the lanthanum content,the agglomeration of PLZT powder increases When the KOH concentration is 2.5 mol/L,the first hydrothermal temperature is set as 100℃,and the temperature as 10 h,and the second hydrothermal temperature as 200℃,and the temperature as 3 h,one can obtain the lead lanthanum zirconate titanate powders of different lead/lanthanum ratios with fine particles,uniform particle size distribution,and good crystallite morphologies.The average particle sizes of PLZT(7/76/24),PLZT(9/76/24),PLZT(11/76/24)prepared under these conditions were 0.82μm,0.58 μm,and 0.86 μm,respectively(4)The PLZT(9/76/24)powder with an average particle size of 0.58 μm prepared by the two-step hydrothermal method was sintered using a two-step approach to procure bulk ceramics with an average grain size of 1.5 μm.The saturation polarization of the bulk ceramic at room temperature can reach 39.34 μC/cm2,the largest applied electric field is 7 MV/m,and the transition from single hysteresis loop to double hysteresis loop was observed at 80℃.According to the dielectric temperature curve,the maximum dielectric constants of PLZT(9/76/24)bulk ceramics at three test frequencies of 1k Hz,10k Hz,and 100k Hz are 11497,11185,and 10458,respectively.These peaks all appear in the temperature range of 80℃～100℃.With the increase of frequency,the peak of dielectric constant versus temperature moves towards high temperatures,indicating that the ceramic is of a relaxor ferroelectric.