MS44-P03 Yttrium substituted Bismuth oxides as high-temperature thermochromic materialsPolymorphs of bismuth oxide and its yttrium substituted solid solutions (Bi1-xYx)2O3 (0.00 £ x £ 0.25) exhibit excellent thermochromic properties in the range from room temperature to 1050 K. The colors change mostly from yellow at low temperatures to various brown hues at high temperatures. The compounds in this nominal series were examined between 293 K and 1050 K using X-ray powder diffraction, UV-Vis spectroscopy, and dynamic scanning calorimetry. A combination of Tauc and DASF methods were applied to determine the band gap energies and types from the diffuse UV-Vis spectra for these semiconducting oxides. It is well known that reversible monoclinic (low temperature) to defect fluorite-type cubic (high temperature) or tetragonal (low temperature) to defect fluorite-type cubic (high temperature) phase-transitions occur on heating and cooling for pure bismuth oxide and the solid solution with 10% cation substitution, while none in the solid solutions with x > 0.1 without annealing. Thermochromic behavior is observed for all samples studied in this series to be generally a gradual darkening as the temperature increases at the regions without any phase transitions, and a more abrupt color change at the stage where a phase-transition happens (Figure 1). The UV-Vis reflectance spectra show the room-temperature absorption edges of all samples in the range between 2.4 eV to 2.8 eV. The spectrum of pure a-Bi2O3 show a sharper threshold at the absorption edge comparing to the rest samples, which contain high concentration of vacancies on the anion sites. At higher temperatures, the absorption edges extend into longer wavelength regions, resulting in darker colors.
Figure 1: Optical color change of (Bi0.95Y0.05)2O3 between 300 k and 1050 K.References:
Keywords: thermochromic materials, high-temperature powder diffraction, phase-transition