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

Abstract

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°C, and form an amorphous phase containing Al, N, C, and O around 400°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°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°C product at the AlN precursor stage as the filler material, and is successfully achieved at temperatures above 1000°C. The shear strengths of the joints formed at 1500°C are 88 ± 29 MPa.

Low‐Temperature Synthesis of Aluminum Nitride from Transition Alumina by Microwave Processing

“…There have been some reports on the use of urea as the carbon source for realizing nitridation reaction at low temperatures between 900°C and 1500°C. [11][12][13] However, in these cases, it was also observed that it was the NH 3 generated from urea that enhanced the nitridation reaction. Therefore, it appears that NH 3 gas is indispensable for the formation of AlN at low temperatures.…”

Thermodynamic and kinetic analyses of vacuum synthesis of AlN by the alumina carbothermal reduction nitridation method

“…When AlN is prepared by the CRN method, ball milling can shorten the reaction time and reduce the reaction temperature, but even the high-energy ball milling process requires up to 20 h of the milling time to activate the raw materials. 13 Sue-hiro et al [14][15][16] used NH 3 and C 3 H 8 mixtures to successfully produce AlN at 1100-1400 • C, while Paseuth et al 8 used NH 3 and C 2 H 2 mixtures to prepare AlN below 1400 • C. Other researchers [17][18][19] used urea as a carbon source to realize the CRN process between 900 and 1500 • C. However, such a reaction would involve the use or generation of toxic NH 3 and even the generation of highly toxic HCN. Chikami et al 20 attempted to lower the reaction temperature by microwave heating.…”

Kinetics of microwave synthesis of AlN by carbothermal‐reduction‐nitridation at low temperature

“…[7][8][9] Other research groups used urea as the source of carbon and ammonia for synthesizing AlN at low temperatures between 900 and 1500°C. [10][11][12][13] Although these methods can yield AlN at low temperatures, they use toxic NH 3 and generate toxic HCN, 12 which render them unsuitable for actual production.…”