Extracting optical parameters of Cu-Mn-Fe spinel oxide nanoparticles for optimizing air-stable, high-efficiency solar selective coatings

TL;DR

This paper develops a method to extract optical parameters of Cu-Mn-Fe spinel oxide nanoparticles, enabling the design of high-efficiency, air-stable solar selective coatings with optimized optical properties for high-temperature applications.

Contribution

A novel inverse problem-solving approach based on four-flux-radiative method to evaluate absorption spectra of nanoparticle coatings, aiding in optimization and analysis of solar selective materials.

Findings

Cu-Mn-Fe spinel oxides are indirect bandgap materials with 1.7-2.1 eV.

Iron-free CuMn2O4 exhibits a direct bandgap of 1.84 eV.

Optimized CuMn2O4 coating achieves 97% solar absorptance and 93.5% energy conversion efficiency.

Abstract

High-temperature Cu-Mn-Fe spinel-oxide nanoparticle solar selective absorber coatings are investigated experimentally and theoretically. A reliable, general approach to evaluate absorption coefficient spectra from the optical measurements of the nanoparticle-pigmented coatings is developed based on solving the inverse problem using four-flux-radiative method. The derived absorption properties of NP materials can be directly applied to predict the solar absorptance, optimize the nanoparticle-pigmented coatings, and analyze the thermal degradation, which agree well with the experimental results. The analysis reveals that the Cu-Mn-Fe spinel oxides are fundamentally indirect bandgap ranging from 1.7 to 2.1 eV, while iron-free CuMn2O4 is a direct bandgap material with Eg=1.84 eV. With the same coating thickness and nanoparticle load, the solar absorptance ranks in the order of Mn2O3 < MnFe2O4 < CuFe2O4 < CuFeMnO4 < CuMn2O4. The optimized spray-coated iron-free CuMn2O4 NP-pigmented coating demonstrates a high solar absorptance of 97%, a low emittance of 55%, a high optical-to-thermal energy conversion efficiency of ~93.5 % under 1000x solar concentration at 750 degrees C, and long-term endurance upon thermal cycling between 750{\deg}C and room temperature in air. The optical parameter analysis approach can be easily extended to other material systems to facilitate the searching and optimizing high-temperature pigmented-solar selective coatings.