Low Cost Preparation Of Aluminum Nitride Ceramic Substrate By Carbothermal Reduction Nitridation

Aluminum nitride has excellent potential for use as a substrate material for high-density, high thermal conductivity, excellent electrical insulation and low thermal expansion mismatch relative to Si. The thermal conductivity of AlN is several times higher than that of Al2O3 at room temperature and is almost equal to that of BeO at 150℃. But the development of aluminum nitride is restricted by high preparation cost,so low-cost preparation is a key technique to realize industrialization. In this research, a new fabrication technology is developed. Aluminum nitride substrate with thermal conductivity of ~59W/m?K is prepared by gelcasting and carbothermal reduction nitridation methods using alumina and carbon black as raw materials.The alumina-carbon suspensions were prepared and their rheology characteristics were studied systematically. The aqueous alumina-carbon suspensions of 56vol% solid loading were prepared in neutral and alkaline surroundings respectively for the first time. It is found that alumina-carbon suspension shows"shear thinning"behavior in lower shear rate (<150s-1), and"shear thickening"in higher shear rate. In given range, the critical shear rate of"shear thickening"is impervious to the solid loading in alkaline suspension,while it is increased with the decrease of the solid loading clearly in neutral suspension.The dispersant content has as remarkable effects on the viscosity of the alumina-carbon suspension. Under constant shear rate, the viscosity of the suspension first decreases with the increment of the dispersant content , and then begin to rise at a certain value. However, the dispersant content remarkably affects the suspension's critical shear rate of the"shear thickening". The critical shear rate is higher when the content is 1.0wt%. So it can be taken as the appropriate contents of dispersant to the suspension with the solid loading of 56vol%.Ball milling can smash the power in suspension, but this effect is limited. As the milling time prolongs, the average diameter of powder first decreases first, then the plateau appears and the average diameter keeps changeless. The finer the particle diameter is, the earlier the plateau appears. Furthermore, the critical shear rate of the"shear thickening"of the alumina-carbon suspension increases with the ball-milling time, also the stage of"shear thinning"disappears in long time ball-milling, and the suspension becomes thixotropic gradually.The polymer gelating procedure and mechanism of the alumina–carbon suspension were investigated. The different gelating ways such as heating, catalysis, oxidation-deoxidization and the effect of monomer content and initiator content on the gelating rate were studied in detail. It reveals that the gelling rate accelerates by the increase of the content of monomer and initiator. However, in view of more cavity in the green body caused by too much polymer, excessive content is not proposed.The drying procedure of alumina-carbon gel flan was studied carefully. Comprehensive analyses show that the drying procedure of the gel-casting ceramic flan is associated with sample size, surrounding temperature and humidity. Moreover, the shrinking rate and volume density of the ceramic flan are affected by solid loading of suspension. As the solid loading increases, the shrinking rate and density increases, and average pore diameter decreases.Aluminum nitride ceramics were successfully fabricated by carbothermal reduction-nitridation in flowing nitrogen gas using different reacting aids. In appropriate condition, the alumina-carbon ceramic flans can be completely converted to aluminum nitride. When CaF2 was chosen as reaction additive, the appropriate content was 2.0wt% and preferable reaction temperature was 1750℃. When Y2O3 was chosen as reaction additive, the appropriate content was 1.0wt% and preferable reaction temperature was 1750℃. But if CaF2 and Y2O3 were chosen as reaction additive, the appropriate composition was 2.0wt%CaF2 and 1.0wt%Y2O3, and in this condition, the preferable reaction temperature was 1650℃, and the reaction temperature was reduced by 100℃.In this paper, the effects of the carbothermal reduction-nitridation parameters on the reaction was studied, especially the additive contents, reaction temperature and soaking time. The appropriate content of different reaction additives such as oxide and fluoride were optimized respectively. The thermodynamics of the carbothermal reduction-nitridation procedures were analyzed and the model of the reaction was established.The aluminum nitride specimen has a thermal conductivity of (49~59) W/m?K, which is about as twice as that of alumina. There will be a widely application as promising substrate material.