High-temperature infrared optical properties of fused silica for selective emitter applications

Thermophotovoltaic (TPV) technology is an emerging method of energy harvesting and storage that operates by directly converting infrared (IR) thermal radiation into electricity through the photoelectric effect. A challenge in the advancement of TPV technology has been the improvement of efficiency through minimizing spectral mismatch between the emitter and photovoltaic cell. Fused silica is a promising candidate for selective emitter applications due to its strong thermomechanical properties and high emissivity in the IR. The high-temperature dependent refraction indices (n) and extinction coefficients (k) of fused silica have not yet been explored in the literature due to the need for specialized equipment. These constants are crucial for determining the emissivity of fused silica and thus the suitability of its applications in TPV systems. In this work n and k values are measured using an IR spectroscopic ellipsometer and a high-temperature heat cell (up to 1000 °C). All absorption peaks in the infrared k spectrum, due to vibrations of the Si-O bonds, are shown to experience a redshift and a broadening upon heating caused by Si-O bond softening and smearing. The presented data is applicable towards the development of selective TPV emitters.