Thermodynamic and kinetic effects of oxygen removal on the thermal conductivity of aluminum nitride

High thermal conductivity, low dielectric constant, high electrical resistivity, low density, and a thermal expansion coefficient that matches well with that of silicon are the principal attributes of AlN that have attracted much attention over the past decade. It is also now well established that oxygen as an impurity lowers the thermal conductivity of AlN. Processing techniques have been developed which not only facilitate pres-sureless densification of AlN but also enhance its thermal conductivity. The present work explores the thermodynamics and the kinetics of oxygen removal and the resultant enhancement of thermal conductivity. Polycrys-talline AlN ceramics were fabricated with Y2O3, Dy2O3, Yb2O 3, CaO, BaO, or MgO as additives. Samples were sinter/annealed at 1850°C for up to 1000 min. The AlN grain size of sintered samples ranged between 2 and 9 μm. The samples typically contained two or three phases with the predominant phase being AlN. Secondary phases in Y2O 3-doped AlN consisted of yttrium aluminates which were along three grain junctions and along grain facets. The presence of Y3AI 5O12, YAIO3, and Y4AI 2O9, as well as Y2O3, depending upon the Y2O3/Al2O3 ratio, was revealed by X-ray diffraction. Thermal conductivity increased with the amount of additive and annealing time. Thermal conductivity also depended on the type of additive. Samples with thermal conductivity up to 200 W/(m · K) were fabricated. The variation in thermal conductivity with the type and the amount of the additive is explained on the basis of the thermodynamics of oxygen removal. In particular, the higher thermal conductivity of CaO-doped, in comparison with MgO-doped, samples is rationalized on the basis that the free energy of formation, AG°, of CaAI2O4 is less than that of MgAI2O4. It is proposed that the higher the |δG°|, with °G°