Migration of liquid phase in low temperature sintering of AlN

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In the process of low-temperature sintering of AlN ceramics, the reaction of the sintering aids YF3 and CaF2 with superficial Al2O3, inherently contained in AlN lattice, results in formation of liquid phase. Nevertheless, the uniformly dispersed liquid phase is prone to migrate from the bulk to the surface of the samples, opposing densification. The analysis of the experimental results indicates that fresh liquid phase can continuously arrive from the bulk to the surface due to chemical reactions and crystallization which occur at the surface as well as wettalibilty and capillarity phenomena. The surface is depleted of liquid phase since the latter is consumed due crystallization and carbothermal reduction reactions with the elements of the atmosphere of the furnace N2 and C, resulting in formation of a dense layer of crystals of Al2Y4O9, CaYAl3O7 and Y2O3, grown perpendicularly to the surface. The chemical and structural features of this newly formed crystalline surface layer generate a significant difference of the wetting regimes and the capillary forces between the surface and the bulk, favouring pumping of the liquid from the bulk to the surface.

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