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Thermal transport in high-strength polymethacrylimide (PMI) foam insulations is described, with special emphasis on the density and temperature effects on the thermal transport performance. Measurements of the effective thermal conductivity are performed by a freestanding sensor-based $$3\omega $$ method. A linear relationship between the density and the effective thermal conductivity is observed. Based on the analysis of the foam insulation morphological structures and the corresponding geometrical cell model, the quantitative contribution of the solid conductivity and the gas conductivity as well as the radiative conductivity to the total effective thermal conductivity as a function of the density and temperature is calculated. The agreement between the curves of the results from the developed model and experimental data indicate the model can be used for PMI foam insulating performance optimization. [ABSTRACT FROM AUTHOR]

A three-dimensional reticular macro-porous SiOC ceramics structure, made of spherical agglomerates, has been thermally characterized using a freestanding sensor-based $$3\omega $$ method. The effective thermal conductivity of the macro-porous SiOC ceramics, including the effects of voids, is found to be $$0.041\,\hbox { W}\cdot \hbox { m}^{-1}\cdot \hbox { K}^{-1}$$ to $$0.078\,\hbox { W}\cdot \hbox { m}^{-1}\cdot \hbox { K}^{-1}$$ at room temperature, comparable with that of alumina aerogel or carbon aerogel. These results suggest that SiOC ceramics hold great promise as a thermal insulation material for use at high temperatures. The measured results further reveal that the effective thermal conductivity is limited by the low solid-phase volume fraction for the SiOC series processed at the same conditions. For SiOC ceramics processed under different pyrolysis temperatures, the contact condition between neighboring particles in the SiOC networks is another key factor influencing the effective thermal conductivity. [ABSTRACT FROM AUTHOR]

A practical and reusable freestanding sensor based on the 3 ω technique is fabricated by encapsulating a nano-thick nickel detector between two pieces of Kapton film. Because of the protection of the Kapton layer, the mechanical strength and applicability of the sensor are greatly improved compared with the traditional 3 ω sensor. Furthermore, the nondestructive thermal-conductivity characterization for solids, liquids, and nanopowder specimens based on the 3 ω technique is first realized. However, the introduction of the additional protective film also induces an undesirable decrease in accuracy for the thermal-conductivity measurement. This article discusses the underlying reason for this decreased accuracy and accordingly proposes an error correction scheme for accurate determination of the thermal conductivity. [ABSTRACT FROM AUTHOR]

This paper presents an inverse analysis for simultaneous estimation of relaxation parameters and thermal diffusivity with a universal heat conduction model by using temperature responses measured at the surface of a finite medium subjected to pulse heat fluxes. In the direct analysis, the temperature responses in a finite medium subjected to a pulse heat flux are derived by solving the universal heat conduction equation. The inverse analysis is performed by a nonlinear least-squares method for determining the two relaxation parameters and thermal diffusivity. Here, the nonlinear system of algebraic equations resulting from the sensitivity matrix is solved by the Levenberg–Marquardt iterative algorithm. The inverse analysis is utilized to estimate the relaxation parameters and the thermal diffusivity from the simulated experimental non-Fourier temperature response obtained by direct calculation. [ABSTRACT FROM AUTHOR]

This work investigates the effect of the response delay of a measuring system on a thermal diffusivity measurement. A model of an m th-order delay in the measuring system is introduced, and a general expression for the output of the system with temperature response as input is derived. The effect on the temperature response caused by such a system is discussed. As a practical example, a third-order measuring system is considered. The measured temperature responses of stainless steel foils are compared with those calculated with the model of a third-order delay system. Good agreement between the two results is shown. [ABSTRACT FROM AUTHOR]