Formation Process of AlN Through Precursors and Its Application to Joining AlN Ceramics

Formation process of AlN from urea and aluminum chloride hexa-hydrate (N/Al = 60) under nitrogen is investigated. An aluminum hexa-urea complex crystallizes on mixing the two raw materials at room temperature. The complexes react with excess urea and/or its decomposed products, such as biuret above 160 degrees C, and form an amorphous phase containing Al, N, C, and O around 400 degrees C after releasing volatile products. The resultant amorphous phase undergoes changes as the temperature increases, and then crystalline carbon with a fullerene-like XRD pattern forms at 850 degrees C. AlN crystallization follows the decomposition of the fullerene-like crystals. At the AlN precursor stage, a large number of radicals (g=2.0029 and S=1/2) are detected by ESR. The radicals are assumed to be due to dangling bonds with an unpaired electron in amorphous carbon. Joining AlN ceramics is conducted using the 850 degrees C product at the AlN precursor stage as the filler material, and is successfully achieved at temperatures above 1000 degrees C. The shear strengths of the joints formed at 1500 degrees C are 88 +/- 29MPa.