The Studies On The Preparations And Applications Of Functional Ceramic Ultrafine Powders

In this thesis, three kinds of functional ceramics with representative characters were chosen as main studying materials to investigate the preparation and application of ultrafine ceramic powders. These materials are basic material BaTiO3 of dielectric and semiconductor ceramic, high temperature optical material MgAl2O4 and Bi0.5Na0.5TiO3 based lead-free piezoelectric ceramic material, respectively.The effects of technical conditions, such as reactant Ba/Ti mole ratio, synthesis temperature, medium environment, PH value and synthesis method, on the purity, Ba/Ti mole ratio and dielectric properties of product BaTiO3 powder were systematically studied, aiming at the technical imperfection of oxalate coprecipitation method by which BaTiO3 powder has been manufactured in commercial quantity at present in our country. The BaTiO3 powder synthesized according to the optimal condition has few detrimental impurities and high purity not less than 99.8 weight percent. The primary particle has an average size of about 0.1 to 0.2 micron with approximate orbicular appearance, and the product Ba/Ti mole ratio can be controlled in the range of 0.998 to 1.002. The pure BaTiO3 ceramic prepared by this improved oxalate coprecipitation method in our work has the properties of high room temperature dielectric constant not less than 4000 and low dielectric loss less than 1.0 percent. Dielectric constant temperature curve, XRD, SEM and TEM were used to investigate the effects of different doping processes, such as micron oxides doping, solution doping and nanometer oxides doping, on the dielectric properties, sintering behavior, microstructure and crystal structure of BaTiO3-Nb2O5-Co3O4 ternary system X7R ceramic, and high performance X7R dielectric material was prepared by doping Nb2O5-Co3O4 nanometer composite oxide to BaTiO3 powder, with a dielectric constant higher than 5000 and dielectric loss lower than 1.5%.In the aspect of preparation of MgAl2O4 powder and spinel transparent ceramic, monolithic, ultrafine MgAl2O4 spinel powders were obtained via biology polysaccharide assisted sol-gel process and improved semi-alkoxide sol-gel process,respectively, at comparatively low calcining temperatures. IR, DTA/TG, XRD and TEM were adopted to investigate the precursor gel structure, chemical process, crystal structure and microstructure of MgAfeC^ powders. At the same time, the transparent MgAl2C>4 ceramic was prepared using the MgAfeC^ powder synthesized by semi-alkoxide sol-gel process, with limited transmittance not more than 64 percent in ultraviolet wave bond. The ultraviolet spectroscopy and electro-optic analytical balance were used to measure the transmittance, bulk density and water absorption.Aiming at the A-site complex ferroelectric Bio.sNao.sTiOs (abbreviated to BNT) with ABO3-type perovskite structure, BNT powders and corresponding ceramics were synthesized by traditional oxides solid phase method and Pechini method. The relations between the excessive amount of Bi3+ and properties, microstructure of BNT ceramic were investigated by means of dielectric, piezoelectric, ferroelectric properties test and microstructure analyse. Two kinds of new BNT based lead-free piezoelectric ceramics were prepared by traditional ceramic process via the complex replacement of A-site and B-site ions of BNT. XRD and SEM were used to analyze the relations of the compositions, crystal structures and microstructures of these ceramics. The relations between the compositions and properties were systematically investigated by means of measurement of dielectric, piezoelectric and ferroelectric properties, and the optimum compositions of these ceramics were determined. The phase transition mechanism and the effect of dopants of these ceramics were also discussed by means of testing the relations between dielectric constant, dielectric loss and temperature, as well as the P-E hysteresis loops at different temperatures.The main innovative achievements obtained from the present work are as follows:1. The conventional oxalate coprecipitation method is improved to synthesize highpure, ultrafine BaTiC>3 powder, including following two aspects:1) The fineness and controllable accuracy of Ba/Ti mole ratio of BaTiC>3powder are improved effectively by means of introducing the surfactant andhigher primary alcohol to the synthesis medium and controlling the feed ratein the initial synthesis stages.2) The secondary agglomeration of BaTiOi. powder is effectively lessened by the introduction of hydrogen bond disconnecting reagent so that the purity and fineness of product powder achieve the best level of the BaTiC>3 powders synthesized by the same technics at home. The room temperature dielectric constant of pure BaTiC>3 ceramic in this study is not less than 4000, which also achieves the best level in our country.2. It is the first time to draw the conclusion that it is most helpful to prepare high performance BT-Nb-Co system X7R material by doping Nb2Os, CO3O4 to BaTiC>3 powder in the form of composite nanometer oxides.3. A new technology named as "biology polysaccharide assisted sol-gel process" was invented to synthesize ultrafine powders. The chemical composition homogeneous, monodisperse, ultrafine and monolithic MgAfeC^ spinel powder was prepared by this method using cheap natural polysaccharide polymer and nitrates as starting raw materials. This method can be applied to synthesize those ultrafine powders insensitive to Na, K and Ca elements.4. The semi-alkoxide sol-gel process reported in literature was improved in this thesis. Tetrahydrofuran was adopted as the solvent and stabilizer of aluminum isopropanol for the first time. It can improve the dissolubility and stability of aluminum isopropanol. Moreover, monolithic, chemical homogeneous, high pure and ultrafine MgA^C^ spinel powder can be synthesized at the temperature of 900°C by this means. Therefore, the problem of the semi-alkoxide sol-gel process reported before that monolithic spinel powder could not be prepared at the calcing temperature of 900 "C was solved in this study.5. It was the first time to synthesize BNT powder successfully by Pechini method. Dense BNT ceramic was also prepared using this BNT powder.6. The effects of the excessive amount of Bi3+ on the microstructure and dielectric, piezoelectric and ferroelectric properties of BNT ceramic were systematically investigated and disciplinary experiment results were obtained. The results show that appropriate excessive amount of Bi3+ is favorable to obtain dense and fine microstructure and better piezoelectric, ferroelectric properties.