Synthesis Mechanism And Application Properties Of High Performance Si₃N₄-SiC Refractory

The thesis researched the synthesis mechanism and application properties of high performance Si3N4-SiC refractory by comparing silicon nitridation mechanism of Si-SiC with pure silicon powder and Si-Fe3Si-Si3N4 in flowing nitrogen,discussing the manufacturing technique improvement of Si3N4-SiC,analyzing the high temperature stability and oxidation of Si3N4-SiC at different atmosphere.Bricks pressed and molded with pure silicon powder were sintered at 1180?and 1420? in flowing nitrogen.Test result showed that the nitridaiton rate of silicon increased along temperature.However,silicon was hardly fully nitrided by directly heating up to target temperature.The reason is that the nitridation product,such as SiO2,Si2N2O and Si3N4,forms a compact protective film on the surface of silicon powder and blocks the contact of silicon inner part with nitrogen.In order to realize full nitridation of silicon,it is necessary to use a stepped sintering technique to break the reaction equilibrium repeatedly and provide continuous reaction kinetics.The thesis analyzed the synthesis mechanism of Si3N4-SiC refractory in flame-isolation nitridation shuttle kiln.There are two modes of silicon nitridation,one of which is direct nitridation at low oxygen partial pressure and the other is indirect nitridation at high oxygen partial pressure.In flowing nitrogen with a purity of 99.999%,Si reacts with oxygen first and generates great amount of gaseous SiO,which consumes oxygen and decreased the oxygen partial pressure.Only when p(O2)reaches a rather low level,silicon can react with nitrogen directly and generate column ?-Si3N4.SiO reacts with nitrogen and generates fibrous ?-Si3N4.The flowing of SiO is the cause of nonuniform distribution of a-Si3N4 in Si3N4-SiC.The effusion of SiO from the bricks causes the material loss and low nitridation rate of silicon in Si3N4-SiC.By analyzing the nitridation of Si-SiC in graphite vacuum furnace and the preparation of Si3N4 bonded ferrosilicon nitride,the possibilities of Si3N4-SiC manufacturing technique improvement was discussed.In graphite vacuum furnace,Si is mainly nitrided to ?-Si3N4 directly with nitrogen,for almost all the oxygen transforms to CO and oxygen partial pressure is low.Ferrosilicon in raw Fe3Si-Si3N4 increases the liquid amount and decreases the temperature of liquid appearance,which favors the ?-to ?-Si3N4 transformation and ?-Si3N4 generation.?-Si3N4/?-Si3N4 ratio can be increased by lowering oxygen partial pressure,increasing sintering temperature or adding ferrosilicon alloys to raw materials.Otherwise,during the production of Si3N4 bonded ferrosilicon nitride,Si3N4 in raw materials reacts with oxygen taking precedence over Si and generates Si2N2O on surface,which leads to low oxygen partial pressure and direct nitridation of silicon without generating SiO.As a result,there is no material loss and silicon nitridation rate is increased.In order to analyze the high temperature stability of Si3N4 and SiC,Si3N4-SiC was resintered in low oxygen partial pressure at 1600 ?&1800 ?,and Si-SiC and Si3N4-SiC were carburized and sintered at 1500?&1800?.The results indicated that Si3N4 and silicon transformed to SiC in an environment containing carbon.There are two Si3N4 to SiC transformation modes.When the temperature is below 1537?,Si3N4 transforms to SiC by reacting with CO and generating gaseous SiO.When the temperature is above 1537 ?,Si3N4 transforms to SiC by reacting with C directly.When the temperature is up to 1800?,S13N4 is unstable and resolves into gaseous Si and nitrogen,which reacts with CO and generates SiC.?-Si3N4 is more stable than ?-Si3N4 at high temperature.An Si3N4-SiC saggar brick after-use was sliced to 5 layers and the oxidation of Si3N4 and SiC in an gradually varied air-nitrogen atmosphere during long-term service was analyzed.The outer layer of saggar brick worked in air with high oxygen partial pressure and Si3N4 and SiC were passively oxidized to SiO2 and Si2N2O.The inner layer of saggar brick worked in nitrogen and Si3N4 and SiC were actively oxidized to gaseous SiO.SiO generated by silicon oxidation in the saggar infiltrated into the saggar brick breaking the equilibrium of active oxidation,and reacts with nitrogen generating fibrous ?-Si3N4,which transformed to column ?-Si3N4 filling and closing some pores of the brick during long-term usage.Therefore,Si3N4-SiC saggar bricks after-use had a lower open porosity and higher bulk density,especially the inner layer working in nitrogen.A great amout of SiO2 glass was generated in the outer layer,affecting its high temperature properties,and might cause material failure.