Synthesis of AlN Nanopowder from c-Al 2 O 3 by Reduction–Nitridation in a Mixture of NH 3 –C 3 H 8 Tomohiro Yamakawa, w Junichi Tatami, Toru Wakihara, Katsutoshi Komeya, and Takeshi Meguro Graduated School of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan Kenneth J. D. MacKenzie School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand Shinichi Takagi and Masahiro Yokouchi Kanagawa Industrial Technology Research Institute, Evina 243-0435, Japan Aluminum nitride (AlN) powders were synthesized by gas re- duction–nitridation of c-Al 2 O 3 using NH 3 and C 3 H 8 as the re- actant gases. AlN was identified in the products synthesized at 11001–14001C for 120 min in the NH 3 –C 3 H 8 gas flow confirm- ing that AlN can be formed by the gas reduction–nitridation of c-Al 2 O 3 . The products synthesized at 11001C for 120 min con- tained unreacted c-Al 2 O 3 . The 27 A1 MAS NMR spectra show that Al–N bonding in the product increases with increasing re- action temperature, the tetrahedral AlO 4 resonance decreasing prior to the disappearance of the octahedral AlO 6 resonance. This suggests that the tetrahedral AlO 4 sites of the c-Al 2 O 3 are preferentially nitrided than the AlO 6 sites. AlN nanoparticles were directly formed from c-Al 2 O 3 at low temperature because of this preferred nitridation of AlO 4 sites in the reactant. AlN nanoparticles are formed by gas reduction–nitridation of c- Al 2 O 3 not only because the reaction temperature is sufficiently low to restrict grain growth, but also because c-Al 2 O 3 contains both AlO 4 and AlO 6 sites, by contrast with a-Al 2 O 3 which con- tains only AlO 6 . I. Introduction A LUMINUM NITRIDE (AlN) ceramics have attracted consider- able attention as IC substrates, packages, heat-sinks, and fillers, because of their high intrinsic thermal conductivity (330 W/mK), high electric insulation (410 14 O Á cm), and low thermal expansion coefficient (3.2 Â 10 À6 /K), which is close to that of Si. 1,2 Several investigations have been conducted on the synthe- sis of AlN by various techniques, including direct nitridation, 3,4 chemical vapor deposition, 5–7 and carbothermal reduction–nit- ridation. 8–14 In particular, Al 2 O 3 –C–N 2 system has found in- dustrial application, as the morphology of the products can be controlled because of the endothermic reaction. Furthermore, it is well known that the AlN powder synthesized by this technique is easy to be densified the sintered body. However, a longer re- action period and higher temperature is needed to fully nitride a- Al 2 O 3 . In order to synthesize AlN at lower temperatures and shorter times, several researchers have investigated the use of various Al 2 O 3 polymorphs as starting materials. They reported that transition aluminas are easier to be nitrided than a- Al 2 O 3 . 12–14 In recent years, the synthesis of AlN nanoparticles has actively been pursued because of their properties. Plasma syn- thesis, 15–19 direct nitridation using NH 3 gas, 20,21 vapor phase synthesis, 22,23 and electron beam heating 24 have been proposed. More recently, Suehiro et al. 25,26 used gas reduction–nitridation in the Al 2 O 3 –NH 3 –C 3 H 8 system to synthesize AlN particles at lower nitridation temperatures which are thermodynamically advantageous compared with the Al 2 O 3 –C–N 2 system. It has also been reported that AlN nanoparticles can be synthesized from d, g-Al 2 O 3 . 27 The formation mechanism of AlN from tran- sition alumina has not yet been elucidated. Studies on the synthesis mechanism of AlN from Al 2 O 3 have been carried out, based on phase analysis and microstructural observations using X-ray diffraction (XRD), transmission elec- tron microscopy (TEM), and scanning electron microscopy (SEM). 27 Al MAS NMR has also been used to investigate the formation mechanism of oxynitride ceramics such as AlON 28,29 and SiAlON. 30–33 NMR spectroscopy should also provide useful information on the formation of AlN by gas reduction–nitridat- ion of g-Al 2 O 3 . The purpose of the present work is to investigate the formation mechanism of nano AlN particles from a transi- tion alumina (g-Al 2 O 3 ), by gas reduction–nitridation using several analytical techniques including 27 Al MAS NMR. II. Experimental Procedure Commercial nanocrystalline g-Al 2 O 3 powder (AKPG015, Sumi- tomo Chem. Co., Tokyo, Japan) was used as the starting material. The main characteristics of the raw powder are sum- marized in Table I. The raw Al 2 O 3 powder was weighed into an Al 2 O 3 boat, placed in an electric furnace with a high-purity Al 2 O 3 work tube, and fired to 7001C at a heating rate of 51C/ min in Ar gas (99.999% purity) to eliminate oxygen in the sys- tem and remove the surface water of the g-Al 2 O 3 . We initially confirmed that the characteristics of the g-Al 2 O 3 did not change during the pre-heating treatment up to 7001C. Heating was con- tinued in a flowing gas mixture (4 L/min) of NH 3 (99.999% purity) and 0.5 vol% C 3 H 8 (99.99% purity). The sample was heated to the reaction temperature of 11001–14001C at a rate of 81C/min and held for 0–120 min before being cooled in NH 3 . After the heating, the sample was removed the reactor in short- term contact with ambient air, it was vacuum-encapsulated the samples. The phases present in the products were identified by X-ray diffractometry (RINT2500, Rigaku, Tokyo, Japan) using CuKa radiation operated at 50 kV and 300 mA. Their morphologies were observed with a transmission electron microscope (JEM- 2000FX, JEOL, Tokyo, Japan). The specific surface areas of the 171 J ournal J. Am. Ceram. Soc., 89 [1] 171–175 (2006) DOI: 10.1111/j.1551-2916.2005.00693.x r 2005 The American Ceramic Society L. Klein—contributing editor w Author to whom correspondence should be addressed. e-mail: d03tb005@ynu.ac.jp Manuscript No. 20406. Received April 12, 2005; approved July 19, 2005.