The use of thermal rectifier devices to control heat, so that it can be transmitted on demand and in an orderly manner, which is of great significance for improving energy utilization and achieving high-precision temperature control. A thermal diode is a rectifier device with a positive heat flow greater than the reverse heat flow under a given temperature difference. Thermal diodes can be implemented by asymmetric nanostructures, which require fine nanostructures that typically involve complex synthesis/preparation processes and have limited rectification properties, or by "Junction" structures, or by constructing heterojunctions of two materials with different thermal conductivity trends with temperature. Phase change material phase transition processes usually involve sudden thermal conductivity mutations, and good thermal rectification performance can be obtained by constructing thermal diodes from phase change materials. However, the phase change thermal diodes that have been reported are rigid and do not apply to surfaces, and even if prefabricated to accommodate surfaces, they will fail due to the gap between the expansion/contraction caused by temperature changes and the surface during use.
In view of the above problems, the aerogel team of the Suzhou Institute of Nanotechnology and Nanobionics of the Chinese Academy of Sciences proposed a flexible thermal diode strategy for the construction of aerogel thin films with limited phase change fluids (Figure 1). In order to further improve its mechanical strength and flexibility, densification treatment obtained DANF, two phase change fluids with phase change fluids with close phase change temperature, thermal conductivity with temperature change trend and opposite surface wettability (N-isopropyl acrylamide) (PNIPAM) solution and eicosane (C20) were screened out, and the two phase change fluids were supported by the characteristics of strong capillary force and high porosity of the aerogel film. Two aerogel limiting phase change fluids OANF-PNIPAM and DANF-C20 membranes were prepared, and the thickness of the two aerogel limiting phase change fluids was optimized and the thermal diode was constructed by contacting each other. The thermal diode has good flexibility (radius of curvature of 420 μm) and mechanical strength (tensile strength≥7.2 MPa), and at the same time, exhibits excellent thermal rectification performance, with a thermal rectification ratio of up to 2.0. This study is the first to achieve flexible thermal diode preparation, which will accelerate the process of thermal diodes from theoretical research to practical application.
Thermal conductivity increases with increasing temperature (positive temperature coefficient) There are few types of materials, PNIPAM aqueous solution as a critical solution temperature (LCST, 32 °C) type phase change fluid can increase thermal conductivity by gel condensation when the temperature rises, so this study uses ONF limited domain PNIPAM aqueous solution to obtain a positive temperature coefficient aerogel limiting phase change fluid OANF-PNIPAM. Thanks to the OANF's own flexibility and PNIPAM fluid properties, OANF-PNIPAM films remain flexible at ambient temperatures below or above LCST, and can be repeatedly curled, twisted, knotted, and even folded (Figure 2a-b). Scanning electron microscopy photos of OANF and OANF-PNIPAM can be analyzed that when the temperature exceeds LCST, the PNIPAM molecules are coated on the aramid nanofibers due to phase separation, which improves the mechanical properties of the ONF-PNIPAM film (Figure 2c-e). In addition, OANF-PNIPAM films exhibit hydrophilic properties (Figure 2f).
In order to match the ONF-PNIPAM to construct the "Junction" structure, C20 with PNIPAM phase change temperature close to the PNIPAM, hydrophobic opposite, thermal conductivity with temperature change trend (i.e., negative temperature coefficient) as the phase change fluid, in order to further improve the flexibility of the film, danf limit C20 was selected, and finally the DANF-C20 film with good flexibility before and after C20 melting was obtained (Figure 3a). The micromorphology of danf aerogel film and DANF-C20 film reveals the reason for its excellent flexibility performance, that is, densification eliminates large pores (>50 nm), the initial 3D porous network is compressed into layer structures, and each layer is densely woven from aramid nanofibers (Figure 3b-d), which makes danf-C20 tensile strength 3 times higher than OANF-C20 and higher than other aerogel phase change materials. The hydrophobicity of C20 also gives the DANF-C20 membrane (Figure 3g), allowing it to form a stable interface in direct contact with the hydrophilic OANF-PNIPAM, avoiding mutual osmosis and affecting the stability of the device.
Thermal conductivity changes with temperature is a key parameter of thermal diode rectifier performance. Figure 4a shows the thermal conductivity and thermal diffusion rate of the OANF-PNIPAM film as a function of temperature, which is 0.66 W/m·K at 15°C (below the phase change temperature) and increases to 1.11 W/m·K at 45°C (above the phase change temperature). When the temperature is lower than the phase change temperature, the OANF-PNIPAM thermal conductivity is attributed to the phonon transport in the aerogel skeleton and the molecular interaction in the PNIPAM solution, when the temperature is higher than the phase change temperature, the PNIPAM is coated on the aerogel skeleton, the phonon transmission is enhanced, and the phonon is more effective than the molecular heat transfer, so the ONF-PNIPAM thermal conductivity is improved (Figure 4b). The thermal conductivity and thermal diffusion rate of DANF-C20 film vary with temperature as shown in Figure 4c. When the temperature is lower than the C20 phase change temperature (C20 is in the crystalline state), the thermal conductivity of DANF-C20 is about 0.22W/m·K, while when the temperature is higher than the C20 phase change temperature (C20 is in the molten state), the thermal conductivity of DANF-C20 is reduced to 0.10W/m·K, which is due to the fact that when C20 is in the crystalline state, the thermal conductivity of DANF-C20 is all attributed to phonon transmission, and when C20 is in the molten state, the thermal conductivity changes from phonon transmission to inefficient molecular interaction. Therefore, the thermal conductivity of DANF-C20 is reduced (Figure 4d).
According to theoretical derivation, the thermal rectification ratio of the phase change thermal diode under optimal conditions is related to the ratio before and after the thermal conductivity phase change, and the thermal rectification ratio is calculated to be about 2.0. At the same time, the laboratory has set up a thermal rectification performance test device, as shown in Figure 5a, the cold end and the hot end are precisely controlled by the circulating water cooling device and bi2Te3 thermoelectric ceramic sheet, respectively. Both the forward and reverse heat flow increase with the increase in temperature difference between the two ends, but the forward heat flow increases more, and when the temperature difference between the two ends increases to 40 °C, the heat rectifier ratio reaches a maximum value of 2.0 (Figure 5b-c). In order to characterize reliability, the thermal diode thermal rectification cycle stability was tested, the thermal rectification cycle stability of this thermal diode was tested, the cycle was 50 times, the thermal rectification ratio fluctuated between 1.94 and 2.03, and the stability was good (Figure 5d), thanks to the excellent nano-limiting function of OANF and DANF aerogel films, and the stability of the hydrophilic-hydrophobic interface between OANF-PNIPAM and DANF-C20 films. Further, the feasibility of this flexible thermal diode in practical thermal management applications was confirmed by building a simple model (Figure 5e-f). The strategy of flexible thermal diode construction will promote the technological advancement of thermal diodes.
The research results were published in Advanced Functional Materials under the title of Nanoporous Kevlar aerogel confined phase change fluids enable super-flexible thermal diodes. His research work is supported by the National Natural Science Foundation of China and the Royal Society-Newton Senior Scholars Fund.
Figure 1. Schematic diagram of a flexible thermal diode constructed by aerogel limiting phase change fluid
Fig. 2.Structure and performance characterization of positive temperature coefficient aerogel limiting phase change fluid OANF-PNIPAM
Fig. 3.Structure and performance characterization of negative temperature coefficient aerogel limiting phase change fluid DANF-C20
Figure 4:Thermal properties and thermal conductivity mechanism of aerogel limiting phase change fluid
Figure 5:Aerogel limited phase change fluid thermal diode thermal rectification performance and application
Source: Suzhou Institute of Nanotechnology and Nanobionics, Chinese Academy of Sciences