Thermal Degradation of Tungsten Nanowire-Based Hyperbolic Metamaterial Emitters for Near-Field Thermophotovoltaic Applications

A nanowire-based hyperbolic metamaterial emitter is widely used in a near-field thermophotovoltaic system. However, it is generally hypothesized that the emitter is stable enough to be less subject to the thermal degradation. To make an improvement, the thermal degradation of a W-Al 2 O 3 metamaterial emitter was investigated. On this basis, an improved energy transfer model of the near-field thermophotovoltaic system with the thermal degradation was constructed, and the effects of the heating time, emitter temperature and partial pressure of the oxygen on the emitter and system performances were analyzed. The results show that the thermal degradation in the emitter is mainly caused by the oxidation of the metal W. The produced WO 3 is volatile at a high temperature, leading to the reduction of the W nanowire diameter. The diameter reduction rate decreases from 6.67 nm·h −1 to 0.04 nm·h −1 by changing the emitter temperature and partial pressure of the oxygen. It is noted that the increasing temperature has the dual effects on the spectral efficiency. On one hand, it can make the oxygen atoms diffuse more easily, resulting in the increase in the oxidation rate and leading to the decrease in the spectral efficiency. On the other hand, the increasing temperature is beneficial for the spectral efficiency of the emitter according to the Wien's displacement law. Compared to the system without the oxidation, there exhibits a reduction rate of the spectral efficiency up to 13.47 % for the system involving the oxidation, meaning that the system efficiency without the oxidation is seriously overestimated. Therefore, to promote the development of the near-field TPV system applications, the prevention of the thermal degradation is a key point.