Session: 17-01-01: Research Posters

Paper Number: 150404

150404 - Design, Fabrication and Characterization of Self-Switchable Thermochromic Coating for All-Day Thermophotonic Power Generation 

Vanadium dioxide (VO₂) is a phase transition metal oxide known to change optical properties with change in temperature. Such material undergoes transition from Lorentz oscillator to Drude model at 68^oC. This thermal switching property can be used to modulate thermal radiative properties. For instance, thin layer of VO₂ on Fabry-Perot cavity results in thermal emitter at hot temperature and infrared reflector at cold temperature with metal backings. On the contrary, infrared transparency to reflectance switching is used to trun on/off the radiative cooling by stacking VO₂ on selective emitter. This allows for all-day thermophotonic power generation by producing temperature difference at both daytime and nighttime by interacting with Sun and the outer Space. Such coatings require large transition in optical properties, where purity of VO₂ becomes significant. In this work, VO₂ fabrication that includes oxidation and reduction is introduced such that produced film does not contain non-desired surface oxides.  

 

The thermochromic VO₂ film is grown on infrared selective LiF substrate by oxidizing sputtered vanadium film in a muffle furnace at 350^oC. It is followed by reduction of over-oxidized surface layer, V₂O₅. The optimal oxidation time is studied with Silicon wafer based on the transmittance spectra, which maximizes the transmittance difference between insulating and metallic phase. Optimal time is determined for various thickness where time increases exponentially with increase in thickness while V₂O₅ ratio on the surface grows. Hence, it requires reduction back to VO₂ so that pure transparency can be obtained at insulating phase. This is done by annealing the film at 500^oC with forming gas. Due to introduction of reduction agency, V₂O₅­ is reduced to VO₂ at certain time. Transmittance difference is further maximized from the oxidation process. The work demonstrates the relationship between time and reduction of V₂O₅ back to VO₂ but also over-reduction back to vanadium. Therefore, full schematic of vanadium oxides between vanadium to V₂O₅ to VO₂ is described in respect to the optical properties with oxidation and reduction. On top of the fabricated VO₂ film on LiF wafer, antireflection coatings made of SiO₂ and SiN are also deposited by PECVD to enhance solar absorptance. FTIR with reflectance accessory and home-made temperature stage is utilized to measure temperature-dependent reflectance in the range of -20 ^oC to 120 ^oC. Tunable light source and a PTFE integrating sphere is used to measure solar absorptance from room temperature to 120^oC. Moreover, dielectric function of fabricated VO₂ on both insulating and metallic phase are determined which is assisted by Machine learning.

Presenting Author: Ken Araki Arizona State University

Presenting Author Biography: Dr. Ken Araki is a postdoctoral researcher at Arizona State University working under the supervision of Prof. Liping Wang. His research is focused on thermophotonic power generation with thermochromic coating. He received Doctoral of Philosophy in Mechanical Engineering at University of North Texas. He has conducted research on thermal radiation and nanophotonics under the supervision of Prof. Richard Zhang. His research interests are both far-field and near-field thermal radiation, nanophotonics, 2D materials, transition metal oxides, and passive radiative cooling.

Authors:

Ken Araki Arizona State University
Liping Wang Arizona State University