Study On Synthesis And Reaction Mechanism Of Ultra-fine Zirconium Diboride Ceramic Powders

Zirconium diboride and its composites have excellent physicochemical properties including high melting point, high hardness, good resistance to wear, oxidation and corrosion, high thermal and electrical conductivity and outstanding capability of neutron-controlling. Thus they have extensive application in the fields such as ultra-temperture ceramics, wear-resistant coating, high-temperature electrodes, neutron absorber and so on. However, conventional synthesized zirconium diboride ceramic powders have large grain size and low acticvity, and are difficult to be sintered, which limit its application. In order to improve the performance of Zirconium diboride and its composites, the synthesis of ultra-fine Zirconium diboride ceramic powders has been paid more attention.In this dissertation, using nanometric ZrO2 and amorphous B as raw materials, ultra-fine ZrB2 powders were synthesized rapidly via borothermal reduction reaction. Ball milling condition for Boron powders, rapid synthesis process of ZrB2 powders and heat treatment process were investigated systematically. The composition and micro structure of ZrB2 powders were characterized by XRD, FES EM, XPS, etc. And the synthesis mechanism was also discussed.Boron powders were milled by agitating ball milling. The optimum conditions are found as follows:the mass ratio of ZrO2 balls to B powders is 30:1, speed of rotation 25Or/min, milling time 160min. The milled B powders have homogeneous particle sizes and normal distribution.Ultra-fine ZrB2 powders were fabricated by fast synthesis technique. In the process of borothermal reduction reaction, ZrO2 was reduced by B powders to ZrB2 and B2O3, and then the reaction of by-product B2O3 and B formed low-valent boron oxide compound. Under the condition of vacuum, the reaction of B2O3 and B is unfavorable to the conversion from ZrO2 to ZrB2. In the case of existing gas pressure, the conversion from ZrO2 to ZrB2 at lower temperature can be promoted. By investigating the relationship of borothermal reduction reaction rate and temperature, gas pressure, heat treatment process was determined as follows:synthesis temperature 1700℃, holding for 5min, vacuum at 15Pa, heating rate at 200℃/min. Purification treatment for ultra-fine ZrB2 powders were carried out. The optimum chemical purification conditions are found as follows:water washing temperature 80℃, water washing for 1h; acid washing temperature 25℃, acid washing for 0.5h, acid concentration at 0.5 mol/L. The results of heat treatment showed ZrO2 can be reduced completely under the following conditions:the molar ratio of B to ZrO2 equals to 4.3, synthesis temperature 1400℃, holding for 30min, Ar pressure at 0.05bar, heating rate at 200℃/min. After heat treatment, the oxygen content of Zirconium diboride powders is 1.07wt%. The average grain size is about 1.9μm...