Dual Phase Change Thermal Diodes for Enhanced Rectification Ratios: Theory and Experiment

Abstract

Thermal diodes are materials that allow for the preferential directional transport of heat and are highly promising devices for energy conservation, energy harvesting, and information processing

Engineering the Thermal Conductivity of Functional Phase‐Change Materials for Heat Energy Conversion, Storage, and Utilization

“…The performance of thermal diodes is typically defined as the ratio of the forward ( J + ) and the reverse ( J − ) steady‐state heat flux (or thermal conductivities, k + and k − ): ɛ = J + / J − = k + / k − . In this definition of rectification, the larger the ɛ value, the higher the thermal rectification efficiency, with ɛ = 1 meaning there no thermal rectification effect . Another definition of thermal rectification can be quantified as ɛ = (J + − J − ) / J − = (k + − k − ) / k − , thus there is no thermal rectification phenomenon when ɛ = 0 …”

“…The feasible selection of PCM is crucial for obtaining a high thermal rectification ratio. According to the trends of the thermal conductivity change, PCM can be divided into positive temperature coefficient (PTC) thermal‐switching materials and negative temperature coefficient (NTC) thermal‐switching materials . PTC thermal‐switching materials refer to materials with an increasing trend in thermal conductivity with respect to temperature, whereas NTC thermal‐switching materials present the opposite trend.…”

“…The option of PTC thermal‐switching materials and their phase transition points is somehow limited within the phase change thermal diodes reported, including poly( N ‐isopropylacrylamide) (PNIPAM), nitinol, and VO 2 . PNIPAM is a representative thermos‐responsive polymers, whose surface property can translate from hydrophilic to hydrophobic at near 32 °C with an increasing temperature.…”

“…The combination of two PCMs with opposite thermal conductivity saltant trends and similar phase change temperatures possesses the potential to greatly enhance the thermal rectification ratio. Cottrill et al introduced the concept of a dual‐phase‐change thermal diode using a junction of a paraffin‐based polystyrene foam hybrid and an aqueous PNIPAM solution, as shown in Figure . The cooperation of increased thermal conduction of PNIPAM in the forwarding bias and decreased thermal conduction of octadecane in the reverse bias heightened the difference of the heat flux between the two stages, showing a significantly enhanced thermal rectification ratio up to 2.6, which was the highest value among experimentally demonstrated thermal diodes reported in the literature.…”

“…b) Experimental and theoretical heat fluxes in the forward and reverse biases as a function of the temperature difference of the dual‐phase‐change thermal diodes. a,b) Reproduced with permission . Copyright 2018, Wiley‐VCH.…”