The formation of transition aluminas during oxidation of AlN

Abstract

The thermodynamically stable form of aluminum oxide is the rhombohedral α-Al2O3 or corundum phase [1]. This is the terminal product when AlN and alumina compounds such as boehmite, γ -alumina, gibbsite, and bayerite are oxidized at temperatures ∼1200 ◦C [2–6]. In many cases the transition to α-Al2O3 is not straightforward and often involves the formation of one or more intermediate phases such as the monoclinic θ -alumina and the tetragonal δ-alumina. These intermediate phases are called “transition” aluminas. The transition aluminas have partially disordered crystal structures all based on a close-packed oxygen sublattice with varying interstitial aluminum configurations. As equilibrium is approached, the structures become more ordered forming a hexagonal oxygen sublattice until stable α-Al2O3 is formed [7]. The sequence of transition aluminas that form is strongly dependent on the starting material and how it was formed. For example, in the oxidation of boehmite (AlOOH) one proposed sequence is γ→ δ→ θ→α [7]. However, in a recent study of the oxidation of boehmite at temperatures in the range 800–1000 ◦C δ-alumina was found to be the predominant phase, whilst γ -alumina and α-alumina were not observed until an oxidation temperature of 1050 ◦C [5]. In this present study, the sequence of transition phases formed during the oxidation of AlN have been studied using diffuse reflectance Fourier transform infra-red spectroscopy (DRIFTS). The AlN powder used in this study was obtained from Aldrich Chemical Company, Inc. of Milwaukee, WI, USA and is 98+% pure according to the vendor’s specification. The particle size of the powder is<10μm and the particles have a range of shapes. The powder was stored under nitrogen at all times to minimize hydration of the AlN surface. The only exposure to air occurred when samples were transferred from the glove box to the furnace. The samples were oxidized in air at temperatures in the range 850–1150 ◦C for times up to 4 h. After oxidation, a small sample of each of the powders was mixed with KBr. The powder mixture was ground using an agate pestle and mortar to achieve an overall particle size in the range 10–20 μm. These samples were placed in a microcup of a Perkin-Elmer diffuse reflectance accessory that had been aligned in the sample compartment of a Perkin-Elmer 1760 FTIR. After adjusting the sample height to give maximum energy throughput, 128 scans were collected and ratioed against a KBr background. The spectra were obtained with a glowbar source, Ge coated KBr beam splitter, and TGS detector at 4 cm−1 resolution (2 points/resolution interval). Data were Kubelka-Munk transformed using the instrument software. A powder sample of 99+% pure α-alumina (surface area 80–100 m2 g−1) was obtained from Alfa Products, Ventron Division, Danvers, MA, USA. The DRIFT spectra, in the range 4000– 400 cm−1, for the as-received AlN and AlN samples oxidized at 850 ◦C for 0.5, 2, and 4 h are shown in Fig. 1. The band positions together with their assignments are given in Table I. It is evident from these data that the major phase present in the samples oxidized at 850 ◦C is AlN. In an effort to see if the NH/OH region changed as a function of oxidation time at 850 ◦C the spectral region