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

摘要

Hysteresis loops exhibited by the thermal properties of undoped and 0.8?at.% W-doped nanocrystalline powders of VOsub2/sub 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 VOsub2/sub and its transition temperature up to 15?°C. (ii) The thermal diffusivity decreases (increases) until (after) the metallic domains become dominant in the VOsub2/sub 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 VOsub2/sub 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 VOsub2/sub 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 VOsub2/sub 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 VOsub2/sub capability to transport heat but diminishes its thermal switching efficiency.