Impurity controlled phase formation at platinum–sapphire interfaces

Diffusion-bonding of high purity polycrystalline platinum (Pt) and single crystal sapphire (α-Al2O3) at 1200°C in N2/O2 (air), CO2/CO and argon atmospheres results in the formation of aluminum borate (Al18B4O33) at the interface. The interfacial phase, previously reported as aluminum peroxide oxide (AlO2), was identified using X-ray diffraction, micro-Raman spectroscopy, and 27Al and 11B magic angle spin NMR. The aluminum borate formed in layers as thick as 10μm according to the reaction:11Al2O3(s)+4B[Pt]→Al18B4O33(s)+4Al[Pt]This reaction proceeds through the diffusion of boron impurities present in the Pt (at the ppm level), oxygen provided by the dissociation of sapphire and the dissolution of Al from Al2O3 into Pt. This study demonstrates the strong influence of trace levels of impurities on the formation of near bulk quantities of an interfacial phase.