Theoretical study on the effective thermal conductivity of silica aerogels based on a cross-aligned and cubic pore model

Aerogel nanoporous materials possess high porosity, high specific surface area, and extremely low density due to their unique nanoscale network structure. Moreover, their effective thermal conductivity is very low, making them a new type of lightweight and highly efficient nanoscale super-insulating material. However, prediction of their effective thermal conductivity is challenging due to their uneven pore size distribution. To investigate the internal heat transfer mechanism of aerogel nanoporous materials, this study constructed a cross-aligned and cubic pore model(CACPM) based on the actual pore arrangement of SiO_(2) aerogel. Based on the established CACPM, the effective thermal conductivity expression for the aerogel was derived by simultaneously considering gas-phase heat conduction, solid-phase heat conduction, and radiative heat transfer. The derived expression was then compared with available experimental data and the Wei structure model. The results indicate that, according to the model established in this study for the derived thermal conductivity formula of silica aerogel, for powdery silica aerogel under the conditions of T = 298 K, a_(2)= 0.85, D_(1)= 90 μm, ρ = 128 kg/m^(3), within the pressure range of 0–10^(5)Pa, the average deviation between the calculated values and experimental values is 10.51%. In the pressure range of 10^(3)–10^(4)Pa, the deviation between calculated values and experimental values is within 4%. Under these conditions, the model has certain reference value in engineering verification. This study also makes a certain contribution to the research of aerogel thermal conductivity heat transfer models and calculation formulae.

Multifunctional tubular carbon nanofibers/polyurethane electromagnetic wave absorber with room-temperature self-healing and recyclable performance

Incorporating self-healing and recyclable capabilities into microwave absorbing materials is expected to facilitate the life extension,cost reduction,and performance stability,thereby meeting the practical applications.In this research,a recycling and room-temperature self-healing electromagnetic wave(EMW)absorber is designed,in which linear polyurethane cross-linked by aromatic disulfide bonds is used as healable matrix,and tubular carbon nanofibers(TCNFs)are employed to attenuate microwave.The resultant composites with a TCNFs content of 7 wt.%harvest a minimum reflection loss(RLmin)of−39.0 dB and an effective absorption bandwidth(EAB)of 2.9 GHz at a matching thickness of 4.0 mm.Driven by the reversible dynamic bonds including hydrogen bonds and aromatic disulfide bonds,the high healing efficiency of 88.7%at 25°C and 93.2%at 60°C is presented.Impressively,even after three repairing cycles at room temperature,a healable efficiency of 86.4%is acquired,and RLmin can still reach−39.1 dB at the same thickness,together with an EAB of 3.0 GHz.Additionally,the results of solvent recycling experiment manifest that the recycled specimen achieves the almost similar mechanical and microwave dissipation properties as original one.These attractive characteristics make the designed self-healing and recyclable composites promising for next-generation EMW consumption devices.