Preparation Of ZrB₂-SiC Composite Powders By Molten-Salt-Mediated Magnesiothermic Reduction Synthesis

ZrB₂-SiC?ZS?are the most promising materials in ultra-high temperature field.Molten salt route is an efficient and practical method for preparation of ceramic powders.In this thesis,a novel molten-salt-mediated magnesiothermic reduction method was used to prepare Zr B₂,2H-Si C and ZS composite powders by using magnesium powders as reducing reagent.The influences of firing temperatures,soaking time and the dosage of raw materials on the synthesis of ZrB₂,2H-SiC and ZS composite powders were investigated.The phase composition and microstructure of as-prepared there kinds of powders were characterized by using XRD,FE-SEM,EDS and TEM.Moreover,the oxidation kinetics of prepared ZrB₂,2H-SiC and ZS composite powders were also studied.The results indicate that:?1?ZrB₂ ultrafine powders were synthesized via molten-salt-mediated magnesiothermic reduction in NaCl-KCl molten salt using zirconium dioxide and boron carbide as raw materials,and magnesium powders as reducing reagent.ZrB₂ can be formed at 1173 K,and the optimal synthesis temperature is 1473 K.ZrB₂ ultrafine powders can be synthesized under following conditions:the B/Zr molar ratio was 2.2,the excess Mg amount was 50 wt%and 1473 K for 3 h,the crystalline size of the prepared ZrB₂ powders is about 30?300 nm.?2?A novel molten-salt-mediated magnesiothermic reduction method was used to prepare 2H-SiC ultrafine powders by using silicon dioxide,activated carbon and magnesium powders as raw materials.Si O₂ could be completely converted into SiC after 3 h at 1073 K,this temperature was about 600 K lower than required by the conventional methods,and the relatively content of 2H-SiC in the final Si C sample reached about 45 wt%.Furthermore,the content of 2H-SiC could be increased to 72wt%upon firing at 1373 K for 3 h and using about 60 wt%excessive Mg.The average crystalline sizes of 2H-SiC in final product powders resultant from 3 h firing at 1073and 1373 K were about 24 and 39 nm,respectively.Both reducing reagent Mg and molten salt medium played decisive roles on the synthesis of Si C and 2H-SiC.?3?ZS composite powders were synthesized after 3 h at 1473 K via molten-salt-mediated magnesiothermic reduction,this temperature was about 400 K lower than that required by the conventional methods.The optimum molar ratios of B/Zr and C/Si were 2.4 and 1.0,respectively.The grain size of the prepared ZS composite powders was about several microns,and SiC were well distributed around the Zr B₂ particles.The crystalline sizes of Zr B₂ and SiC in prepared composite powders were both about several hundred nanometers.A back propagation artificial neural networks?BP ANNs?is used to establish the non-liner relationship between the relative content of ZrB₂ and SiC in the perpared powders and the heating temperature,the soaking time,and the dosage of B,C and Mg.The test experiment verifies BP ANNs works well for predication.?4?Ultrafine ZS composite powders were synthesized after 3 h at 1473 K via molten-salt-mediated magnesiothermic reduction in NaCl-KCl molten salt using zircon,boron oxide and activated carbon as raw materials,and magnesium powders as reducing reagent,and the B₂O₃ amount as well as Mg amount showed significant effects on the formation of ZS composite powders.The grain size of the prepared ZS composite powders was about several microns.ZrB₂ particles with three different morphologies of globularity,rod and square formed in final powder product.SiC were well distributed around the ZrB₂ particles,and the crystalline sizes of ZrB₂ and SiC in the prepared composite powders were about several hundred nanometers and dozens of nanometers,respectively.?5?The oxidation kinetics of the prepared Zr B₂,2H-Si C and ZS composite powders were investigated via non-isothermal oxidation method.The results showed that the apparent activation energy of Zr B₂ and 2H-SiC was 325.27 kJ·mol^-11 and269.15 kJ·mol^-1,respectively.Nevertheless,the apparent activation energy of ZrB₂ and SiC in the composite powders were 380.96 kJ·mol^-11 and 263.58 kJ·mol^-1,respectively,indicating that the addition of SiC could improve the oxidation resistance of ZrB₂.