Fabrication Of Si₃N₄ Ceramic Bulk And Substrate Based On In-situ Nitridation Of Si/Si₃N₄ Powder

Silicon nitride?Si₃N₄?ceramics,due to their excellent mechanical properties,good thermal conductivity and thermal shock resistance,have been widely studied and considered as potential candidate materials for engineering components applications such as electronic substrate material for the new generation of wide band-gap high power semiconductor devices.However,the fabrication cost is one of the major barriers in the direct manufacturing of Si₃N₄ components because of the combination of high cost of high purity Si₃N₄ raw powders and forming process.Thus,a shape forming method was developed to fabricate Si₃N₄ electronic substrate via tape casting based on sintered reaction-bonded Si₃N₄?SRBSN?.Because of the huge amount of heat generate during the reaction process,the melting of silicon could result in the distortion and cracking,especially in the large size substrates,consequently this would lead to the decrease in the properties and production yield of Si₃N₄ ceramic substrates.Therefore,Si₃N₄ powder was introduced and acted as diluents,which could inhibit the violent exothermic nitridation reactions,which might provide an alternative approach to fabricate cost-effective Si₃N₄ ceramic substrates with high mechanical properties and high yield.Therefore,the objective of this study is to systematically investigate the effects of Si:Si₃N₄ molar ratio and the amount of sintering additives on phase compositions,densification behavior,microstructures,and mechanical properties of SRBSN.Commercial Si and?-Si₃N₄ powders were selected as the based starting materials,and ZrO₂+Gd₂O₃+MgO powders were used as sintering additives in this study.Results showed that,the ZrO₂ additives,which was added as catalyst,exhibited an effective nitridation reaction.The?-Si₃N₄ was observed as main phase in all the samples,and no Si phase was detected,indicative of complete nitridation of Si powders.The final density of the samples decreased from 3.39 g/cm³ to 2.92 g/cm³ with the increase of Si content,except for the sample without the initial Si₃N₄ powder addition,while the thermal diffusivity of the samples exhibited no apparent difference.The four point flexure strength,fracture toughness,and thermal conductivity were about 924±54 MPa,10.3±1.2 MPa·m^1/2,and⁶0W·m^-1·K^-1 respectively,by adding 25 mol%of Si in the starting mixtures.The effect of content of the sintering additives,which varied from 6 wt%to 12 wt%,on the Si₃N₄ properties was also studied based on the starting material with 25 mol%Si powder.Results showed that,the density of Si₃N₄ ceramics,the aspect ratio of?-Si₃N₄ grains,the four-point flexure strength,and thermal conductivity increased with the increase in sintering additives,when the content of sintering additives was less than 10 wt%.However,all of these properties were degraded when the contents of the sintering additives increased from10 wt%to 12 wt%.The four-point flexure strength,fracture toughness,and thermal conductivity were about 949±54 MPa,10.8±0.7 MPa·m^1/2,and⁶1 W·m^-1·K^-1,respectively,for the Si₃N₄ ceramics with 10 wt%sintering additives.On the other hand,the effect of sintering additives contents on Si₃N₄ substrate by the means of tape casting was investigated.Results revealed that,when the contents of the sintering additives increased from 8 wt%to 12 wt%,the density of Si₃N₄ ceramics,the aspect ratio of?-Si₃N₄ grains,the four-point flexure strength and thermal conductivity were increased and then decreased with the increase of sintering additives.The Si₃N₄ substrate was obtained when the content of sintering adds was 10 wt%,the four-point flexure strength and thermal conductivity were about 453±46 MPa and⁵5 W·m^-1·K^-1,respectively.Results from this research indicated that good properties of Si₃N₄ ceramics and substrates could be successfully obtained based on in-situ nitridation of Si powder combined with Si₃N₄ powder.Therefore,it provides a promising alternative fabrication route to reduce the manufacturing cost of Si₃N₄ ceramic substrates,without compromising the properties and performances.