Due to their high-porosity, nanoporous structure and pores, aerogel materials possess extremely low thermal conductivity and have broad potential in the thermal insulation field. Silica aerogel materials are widely used because of their low thermal conductivity and high temperature resistance. Pure silica aerogel is very fragile and nearly transparent to the infrared spectrum within 3–8 μm. Doping fibers and opacifiers can overcome these drawbacks. In this paper, the influences of opacifier type and content on the thermal conductivity of silica fiber mat-aerogel composite are experimentally studied using the transient plane source method. The thermal insulation performances are compared from 100 to 750 °C at constant pressure in nitrogen atmosphere among pure fiber mat, fiber mat-aerogel, 20% SiC-fiber mat-aerogel, 30% ZrO₂-fiber mat-aerogel and 20% SiC + 30% ZrO₂-fiber mat-aerogel. Fiber mat-aerogel doped with 20% SiC has the lowest thermal conductivity, 0.0792 W/m·K at 750 °C, which proves that the proper type and moderate content of opacifier dominates the low thermal conductivity. The pore size distribution indicates that the volume fraction of the micropore and mesopore is also the key factor for reducing the thermal conductivity of porous materials.

As an effective means to manipulate elastic wave energy, the defect mode of phononic crystals (PCs) has recently been employed to implement vibration energy harvesting by localizing waves within certain areas at defect frequencies. Prior research has indicated that the thermal environment affects the defect behavior and energy harvesting of single defects. However, the dynamic characteristics of multi-defect phononic crystals have not yet been discussed. This paper presents an investigation of the coupling behavior and energy harvesting of double defects in heated PC beams. The frequencies and mode shapes of defect modes are studied for different defect pairs within the first bandgap. Results indicate that the difference in the single-defect frequency between the two defects plays a dominant role in determining the intensity of the defect interaction which splits the defect modes. Meanwhile, the influence of the energy distribution around the defects on the coupling behavior becomes prominent when the two defects are quite close to each other. These two factors could cause degeneration of the defect modes. As an external factor, thermal load could enhance the defect coupling intensity, and may reactivate the degenerated defect modes. This effect would increase as the initial defect interaction gets stronger. For vibration energy harvesting, temperature rise may lead to a significant reduction in defect-based harvested energy, even though the number of defect-induced resonance peaks could be increased in the thermal environment. The thermosensitivity of harvesting performance varies greatly for different combinations of defects. Thermal effects should be considered in the design of defect-based harvesters.

This work presents an investigation of vibration and acoustic response characters of a clamped rectangular thin plate in thermal environments. The general form of governing equation of plate flexural vibration with considering thermal loads is established, and the influence of uniform temperature change on response characters is studied in detail. A set of double sinusoidal mode shape functions for fully clamped boundaries is used to describe the displacement distribution. The governing equation is solved with Fourier series expansion to discuss the natural vibration and dynamic responses of a clamped plate in thermal environments. Accordingly, acoustic radiation characteristics are obtained based on Rayleigh integral. Results show that natural frequencies decrease with the increment of plate temperature, and the first natural frequency is much more sensitive to thermal environment changes. Response curves of plate vibration and radiated sound power shift toward lower frequency range, and the response amplitudes of the first resonant peak of the two responses present opposite variation tendencies. These phenomena are verified with finite element and boundary element simulations. Thermal loads reduce the radiation efficiency of the plate obviously below the critical frequency, but the maximum value almost remains unchanged.