Quantitative Evaluation of Thermal Conductivity of Single‐Bent Microwire Using Vanadium Dioxide Temperature Tag

Abstract

Stress/strain engineering is believed to be an effective way to adjust the thermal conductivity of materials dynamically or as needed. Compared with bulk materials, micro‐/nanoscale structures can withstand greater stress/deformations that lead to evident changes in their thermal conductivity after undergoing stress/strain; this phenomenon has been predicted by theoretical simulations. Nevertheless, measuring the effective thermal conductivity of a single wire of a small size upon controllable bending angles has faced major challenges. Herein, a method using VO2 tag as a temperature indicator is developed to achieve the in situ quantitative measurement of the thermal conductivity of bent silicon microwires (MWs), where thermally insulated spider silk is used to adjust the position of the suspended end of wires for different bending angles. It is found that the thermal conductivity of Si wires increases and then decreases upon subsequent bending; it indicates that the thermal conductivity of MWs can be dynamically tuned by bending. Further studies reveal that the variation of thermal conductivity is reversible with small bending (elastic) and irreversible with large bending (plastic). With this setup, new thermophysical properties of materials are explored at small scales, and possible stress/strain‐gated thermal switches emerge.

VO2 Nanoparticles‐Based Thermochromic Smart Windows

“…When VO 2 is heated above MIT temperature, it will produce a rapid increase in conductivity, thus becoming a metal, accompanied by lattice relaxation to Rutile structure. [48][49][50][51][52] At the phase transition temperature of 68 °C, VO 2 has a coupled structure electronic phase transition from monoclinic insulating phase (I) to Rutile metal phase (M), which has a huge transmittance contrast in the infrared range, and the change in the visible range can be ignored. [53] However, there are still several challenges to pure VO 2 -based smart windows existing, including high τ c, inherent yellowish-brown color, and low oxidation resistance; more importantly, the adverse combination of low T lum and ~Tsol were less than 50 % and 20 % respectively in most studies.…”

“…At temperatures below MIT, VO 2 is insulated and monoclinic, and the dimerization of vanadium ions leads to the localization of vanadium electrons in the outer shell. When VO 2 is heated above MIT temperature, it will produce a rapid increase in conductivity, thus becoming a metal, accompanied by lattice relaxation to Rutile structure [48–52] . At the phase transition temperature of 68 °C, VO 2 has a coupled structure electronic phase transition from monoclinic insulating phase (I) to Rutile metal phase (M), which has a huge transmittance contrast in the infrared range, and the change in the visible range can be ignored [53] .…”