Synthesis And Properties Of ZrO₂-C-ZrB₂ Composites

Zirconium diboride(ZrB₂)is an excellent material used at high temperature and has been applied in many fields because of its high melting point,high hardness and high thermal conductivity.As an additive in refractory,ZrB₂ is mostly applied in increasing the corrosion resistance to slag and improving the oxidation resistance for carbon-containing materials. In this paper,ZrB₂ was at first to be synthesized by carbothermic and alumino-thermic processes respectively.The ZrO₂-C-ZrB₂ composites were prepared by adding ZrB₂ that displaced partial ZrO₂ or C in ZrO₂-C composites.The General mechanical properties of the material were measured.The corrosion resistance of ZrO₂-C-ZrB₂ composites,which will be used in the place where the slag is contacting in the submerged entry nozzle,has been also studied.The microstructures,phase compositions were observed and analyzed by means of XRD,SEM and EDS.The mechanisms of corrosion were discussed.The isothermal oxidation kinetics and non-isothermal oxidation kinetics were studied by TG-DTG.The isothermal mathematical model was proposed and the apparent activation energy was obtained.The results show that: 1.ZrB₂ can be obtained up to 93.2%purity and the average size of the particles is about 5-20μm in the conditions of 1550℃for 2 hours by reactions among carbon,B₄C and ZrO₂.Another method to synthesize ZrB₂ is at 1550℃for 2 hours by alumino-thermic process with borax and zirconium silicate.The purity of ZrB₂ is only 40.2%accompanying Al₂O₃ and ZrO₂ as byproducts.2.The ability of corrosion resistance to slag is increased evidently by increasing the content of ZrB₂.The mechanism was analyzed that ZrB₂ is firstly oxidized to produce B₂O₃,and then B₂O₃ reacts further with the Al,Si or Ca in the slag to produce the borate with low melting point.The products surround the surface of the sample and keep the slag from infiltrating.The low carbon and carbon-free refractory can get the best corrosion resistance with high content of ZrB₂.3.The oxidation resistance of the composites with the increasing of the content of ZrB₂ can be enhanced significantly due to the formation of the protective coating of B₂O₃-rich glassy on the exposed surface of graphite.A mathematical model was proposed based on the isothermal kinetic equations. The oxidation process of the composites at 800℃can be controlled by two stages:chemical reaction-controlled process and then the mixture processes both chemical reaction-controlled and gas diffusion.However,the oxidation rate within 900～1100℃is controlled by chemical reaction at initial stage; then both chemical reaction and diffusion at intermediate stage and finally controlled only by diffusion.4.Non-isothermal oxidation kinetics of ZrO₂-C-ZrB₂ composites was investigated by model-fitting methods.The results show that the oxidation mechanism is random nucleation.The kinetic parameters are lg A= 8.760 min^-1and E_a= 95.7 kJ/mol.