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

Abstract

Aluminum nitride (AlN) was synthesized by carbothermal reduction and nitridation method from a mixture of various transition alumina powders and carbon black using 2.45 GHz microwave irradiation in N2 atmosphere. We achieved the synthesis of AlN at 1300–1400°C using 2.45 GHz microwave irradiation for 60 min. Our results suggest that θ‐Al2O3 is more easily nitrided than γ‐, δ‐, and α‐Al2O3. On the other hand, nitridation ratio of samples synthesized in a conventional furnace under nitrogen atmosphere were zero or very low. These results show that 2.45 GHz microwave irradiation enhanced the reduction and nitridation reaction of alumina.

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

“…First, nitrogen diffusion was likely to proceed considerably when compared with the case in which α-Al 2 O 3 was used as the raw material; this occurred because AlN was topotactically synthesized by direct nitridation reaction from transitional alumina using the microwave method. 17 On the other hand, the difference in the standard Gibbs energy of α-Al 2 O 3 and γ-Al 2 O 3 is as small as about 5 kJ/mol at 1500 K, 27 and it is not a drastic advantageous reaction from a thermodynamic perspective. On the other hand, Janney and Kimrey reported that the activation energy for 18 710 kJ/mol in a conventional process, but it decreases to 409 kJ/mol in a microwave process.…”

“…The lower apparent activation energy in our study indicates that microwave irradiation promoted the diffusion of N and O through the reacted AlN layer. First, nitrogen diffusion was likely to proceed considerably when compared with the case in which α‐Al 2 O 3 was used as the raw material; this occurred because AlN was topotactically synthesized by direct nitridation reaction from transitional alumina using the microwave method . On the other hand, the difference in the standard Gibbs energy of α‐Al 2 O 3 and γ‐Al 2 O 3 is as small as about 5 kJ/mol at 1500 K, and it is not a drastic advantageous reaction from a thermodynamic perspective.…”

“…Figure 2 shows XRD patterns of samples synthesized at various temperatures for 60 minutes using a tube furnace or microwaves, and additional samples with the highest nitridation ratio at each temperature were synthesized using microwaves. Nitridation ratios were calculated from the XRD peak intensities of the samples, 17 and this calculation process was shown in Data S1. As shown in Figure 2A, the nitridation ratio of the sample (MW, 1100°C, 60 minutes) shown was almost equal to that of the sample synthesized by tube furnace heating (conv., 1500°C, 60 minutes).…”

“…Hence, microwaves have been widely applied for the carbothermal reduction in nickel oxide and iron oxide . Furthermore, we successfully synthesized AlN by a rapid CRN process via 2.45 GHz microwave irradiation at 1400°C using transition alumina as the starting material . However, there are few examples of thermodynamic verification against kinetics of these chemical reactions, and so far, details of the underlying mechanism are unclear.…”

急速炭素熱還元法によるTi₄O₇およびAlNナノ粒子のマイクロ波合成

“…However, the former has problems in view of safety and convenience, the latter has a problem that cannot obtain the desired morphology. In this review, we introduce the rapid carbothermal reduction process using microwave irradiation 20), 21) , which can solve the above problems.…”