Synthesis And Properties Of The Ultrafine Powder And Fine-grained Ceramics On Two Ferroelectric Materials

With the advent of information society, more and more functional materials withnovel properties are needed. Ferroelectric materials as extremely important functionalmaterials have received considerable attention due to their excellent ferroelectric,piezoelectric, pyroelectric, and electro-optic properties. (Pb, La)(Zr, Ti)O₃ (PLZT) andKTN are very important ferroelectrics. PLZT is considered to be the promising candidatefor capacitors, transducers, micro-actuator as well as optical storage devices because of itsexcellent ferroelectric, dielectric and electro-optic properties. KTN with attractivequadratic and nonlinear electrooptic properties, as well as a high dielectric permittivityand large piezoelectric and pyroelectric coefficients can be used in many important fieldssuch as volume phase holographic storage, real time optical information processing, andIR detectors electro-optic devices. For the purpose of application, challenge still exists inreducing the synthesis cost and improving the properties of the materials. Therefore,finding new synthesis strategies and investigating their new properties are very importantnot only for fundamental research but also for applied research of the materials.This thesis gives a brief review of the latest development of PLZT and KTN, andthen a description of the synthesis of PLZT and KTN ultrafine powders and fine-grainedceramics. The ferroelectric, dielectric and the optical properties have also beeninvestigated. The main content is described as follows:1. PLZT ultrafine powders in sizes of < 50 run were synthesized by sol-gel method.The new synthesis method not only reduced the cost of the raw materials but alsosimplified the preparing process. The improvement made the synthesis reaction occuramong the molecules which is very helpful for improving the properties and realizingindustrialization. The mechanisms of precursor hydrolysis and polymerization have beendiscussed.The preparing parameters such as reaction temperature and reaction time havealso been investigated in detail.2. Using the ultrafine powder, PLZT fine-grained ceramics with grain size of≤500nm were prepared. The grain size of the as-prepared ceramics is much smaller than that ofthe ceramics (1.56μm) derived from traditional solid state reaction. The optimizedsintering time and temperatures were obtained. The powders were annealed at 600℃for1.5 h and the ceramics were sintered at 1150℃for 1.5 h. The electric measurements revealed that the d₃₃= 595,ε_r= 5108, tgδ= 0.02347. The sintering temperature ofceramics fabricated by ultrafine powders was lower 50-80℃than that of traditional solidstate reaction.3. The modified PLZT ceramics were fabricated by doping BST nanopowders intoPLZT ultrafine powders. The ceramics with lmol% BST doping show good piezoelectricand dielectric properties: d₃₃= 508,ε_r= 8315, tgδ= 0.035. With the increase of the dopingcontent of BST, the PLZT ceramics exhibit lower spontaneous polarization and lowercoercive field comparing to the pure PLZT ceramics.4. The Curie temperature of the PLZT ceramics derived ultrafine powders is 212℃.Dielectric constant of those is up to 15404. The Curie temperature of the ceramics bydoping with Mg²⁺ shifts to high temperature. While it shifts to low temperature by dopingwith K⁺. These characters of PLZT materials are useful for its application.5. The grain size dependence of dielectric properties were investigated, and therelationship ofε_r and cla were obtained in theory.6. KTN nanomaterials were prepared by using hydrothermal method. The electronstructure, UV-Vis absorption and photoluminescence properties have been investigated.The phase structure of the KTN nanopowders transferred from cubic to tetragonal phasewith the increase of the KOH content. The cla value increases too. The precursors with 16M KOH were treated at 240℃for 24 h and KTN nanopowders of tetragonal structure canbe obtained. The band gaps of the KTN nanopowders increase with the increase of theKOH content. The KTN ceramics were prepared by the hydrothermal derived powdersand their optical properties were also investigated.7. The effects on the microstructures and electric properties of KTN by doping withW⁶⁺ ions have been investigated. The KTN conductivity decresed a half compared withthat of pure KTN by doping with 6mol% W⁶⁺ ions. The leakage current of KTN ceramicswhich were doped 8mol% W⁶⁺ions is 8.71×10^-4A. It is one-sixth times of that of pureKTN ceramis (5.14×10^-3A). It is clear that W⁶⁺ doping can reduce the leakage current ofthe KTN materials and improve their ferroelectric and piezoelectric properties.