Thermodynamic and Kinetic Effects of Oxygen Removal on the Thermal Conductivity of Aluminum Nitride

Abstract

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 AIN 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 AIN. Processing techniques have been developed which not only facilitate pressureless densification of AIN 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. Polycrystalline AIN ceramics were fabricated with Y 2 O 3 , Dy 2 O 3 , Yb 2 O 3 , CaO, BaO, or MgO as additives. Samples were sinter/annealed at 1850°C for up to 1000 min. The AIN grain size of sintered samples ranged between 2 and 9 μm. The samples typically contained two or three phases with the predominant phase being AIN. Secondary phases in Y 2 O 3 ‐doped AIN consisted of yttrium aluminates which were along three grain junctions and along grain facets. The presence of Y 3 Al 5 O 12 , YAIO 3 , and Y 4 Al 2 O 9 , as well as Y 2 O 3 , depending upon the Y 2 O 3 /Al 2 O 3 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, Δ G °, of CaAl 2 O 4 is less than that of MgAl 2 O 4 . It is proposed that the higher the |Δ G °|, with Δ G ° < 0, the higher is the resultant thermal conductivity. An increase in the thermal conductivity with annealing time is attributed to the kinetics of oxygen removal from AIN grains.