Measurement of the hysteretic thermal properties of W-doped and undoped nanocrystalline powders of VO2

C L Gomez-Heredia,J A Ramirez-Rincon,D Bhardwaj,J Drevillon,K Joulain,A M Umarji,Y Ezzahri,I J Tadeo,J Ordonez-Miranda,J L Cervantes-Lopez,O Ares,P Rajasekar,J J Alvarado-Gil

doi:10.1038/s41598-019-51162-4

Abstract

Hysteresis loops exhibited by the thermal properties of undoped and 0.8 at.% W-doped nanocrystalline powders of VO2 synthesized by means of the solution combustion method and compacted in pellets, are experimentally measured by photothermal radiometry. It is shown that: (i) the W doping reduces both the hysteresis loops of VO2 and its transition temperature up to 15 °C. (ii) The thermal diffusivity decreases (increases) until (after) the metallic domains become dominant in the VO2 insulating matrix, such that its variation across the metal-insulation transition is enhanced by 23.5% with W-0.8 at.% doping. By contrast, thermal conductivity (thermal effusivity) increases up to 45% (40%) as the metallic phase emerges in the VO2 structure due to the insulator-to-metal transition, and it enhances up to 11% (25%) in the insulator state when the local rutile phase is induced by the tungsten doping. (iii) The characteristic peak of the VO2 specific heat capacity is observed in both heating and cooling processes, such that the phase transition of the 0.8 at.% W-doped sample requires about 24% less thermal energy than the undoped one. (iv) The impact of the W doping on the four above-mentioned thermal properties of VO2 mainly shows up in its insulator phase, as a result of the distortion of the local lattice induced by the electrons of tungsten. W doping at 0.8 at.% thus enhances the VO2 capability to transport heat but diminishes its thermal switching efficiency.

Highlights

Www.nature.com/scientificreports accompanied by a considerably reduction in Tc at W = 0 at.% (~57 °C) thanks to the capability of the powdered material to get adapted to the temperature-induced-changes, due to the presence of void spaces among neighboring grains, which lessen the stress due to the phase changes[32,36]
The signal measured at the rear surface is normalized with that obtained at the front face, leading in the thermally thick limit a linear dependency between the normalized phase φSN and the square can be obtained from the slope/fitting parameter m =
Normalized phase φCN has been calculated from the phase signal measured for the sample/air and sample/

Summary

Introduction

In the middle- and far-infrared spectral range, the thermochromic transition in optical transmittance of the VO2 nanocrystals significant falls when tungsten doping increases and when the crystals’ temperature rise, leading mostly hysteresis loops with narrow ΔH and lower transition temperatures[32,33,35,36,39,42,43,44] In this optical approach, it has been found that infrared emissivity of VO2 nanopowders falls in about 33% across their phase transition, remaining constant in the extreme states[14,15]. W doping leads to poor heat conduction switching efficiency in the VO2 transition, but interestingly this doping enhances the heat propagation in the insulating state and modifies the path that the thermal properties follow during the MIT This behavior makes vanadium dioxide doped with tungsten a material with a greater flexibility than the undoped one, and this opens ways for novel applications

Methods

Results

Conclusion