Morphology control of aluminum nitride (AlN) for a novel high-temperature hydrogen sensor
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Morphology control of aluminum nitride (AlN) for a novel high-temperature hydrogen sensor Angga Hermawan, Yusuke Asakura, and Shu Yin Institute of Multidisciplinary Research for Advanced Material (IMRAM), Tohoku University, Sendai 980-8577, Japan (Received: 23 March 2020; revised: 3 July 2020; accepted: 13 July 2020)
Abstract: Hydrogen is a promising renewable energy source for fossil-free transportation and electrical energy generation. However, leaking hydrogen in high-temperature production processes can cause an explosion, which endangers production workers and surrounding areas. To detect leaks early, we used a sensor material based on a wide bandgap aluminum nitride (AlN) that can withstand a high-temperature environment. Three unique AlN morphologies (rod-like, nest-like, and hexagonal plate-like) were synthesized by a direct nitridation method at 1400°C using γ-AlOOH as a precursor. The gas-sensing performance shows that a hexagonal plate-like morphology exhibited p-type sensing behavior and showed good repeatability as well as the highest response (S = 58.7) toward a 750 ppm leak of H2 gas at high temperature (500°C) compared with the rod-like and nest-like morphologies. Furthermore, the hexagonal plate-like morphology showed fast response and recovery times of 40 and 82 s, respectively. The surface facet of the hexagonal morphology of AlN might be energetically favorable for gas adsorption–desorption for enhanced hydrogen detection. Keywords: aluminum nitride; controllable morphology; direct nitridation; γ-AlOOH; hydrogen sensor
1. Introduction Hydrogen energy has gained significant interest in recent years and is expected to replace hydrocarbon-based fuels to achieve sustainability. Hydrogen can be produced at high temperatures (greater than 400°C) in a solid oxide electrolysis cell (SOECs) through the decomposition of water molecules (H2O) [1–2]. However, hydrogen leaks may occur during production that can cause explosions that threaten human safety. Moreover, hydrogen is a colorless and odorless gas that makes it difficult to detect using human senses, and therefore a responsive gas sensor is required for rapid hydrogen monitoring [3]. Metal oxides, such as SnO2, ZnO, and CuO, have been used for hydrogen gas-sensing materials [4]. Nevertheless, their sensing properties diminish at high temperatures due to depression of the surface reaction [5]. The availability of suitable materials that can withstand such a harsh environment is very limited. A recent study showed that group-III nitrides, especially GaN-based sensors, exhibited an excellent response toward the ppm level of H2 gas and good chemical stability at elevated temperatures [6]. Interestingly, the sensing property of GaN gas has dramatically exceeded its respective oxide (β-Ga2O3) performance. Addi-
tionally, wide bandgap semiconductors are promising candidates for high-temperature sensor applications due to their resilience to harsh environments (extreme temperatures, acid/basic conditions, high humidity) chemical stability, and mechanical
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