Relationship between pore size and the gas pressure dependence of the gaseous thermal conductivity

Abstract

In porous materials, such as foams or pressed powder boards, the thermal conductivity via the gas phase represents at ambient temperatures a significant or even dominant contribution to the total thermal conductivity. The variation of the thermal conductivity of an open porous material with gas pressure is a function of the pore size. In the pressure range below 1 bar, measurement of thermal conductivity offers a non-destructive probe of pore sizes larger than 100 nm. If the thermal conductivity setup also allows for measurements at higher pressures, the pore size range can be extended to even smaller values.Experimental data for a variety of different porous materials such as open porous foams, granular porous and nonporous materials and aerogels, porous solids with an adjustable mean pore size in the meso-or macro-pore range, are presented. The pore characteristics determined from thermal conductivity measurements are compared to morphological data derived from nitrogen sorption and small angle X-ray scattering data.The potential and limits of this method in terms of the extraction of an average pore size and a pore size distribution are discussed.

Cellulosic-crystals as a fumed-silica substitute in vacuum insulated panel technology used in building construction and retrofit applications

“…Literature shows thermal conductivity (λ) does not follow a linear dependence on gas pressure and is instead formulated as given in equation (3) [27]. This assertion was confirmed experimentally in studies by Isaia et al [14], Alam et al [17], Kwon et al [30], and Di et al [32]:…”

“…This was shown by Lorentzati et al [16], Alam et al [17], Chen et al [21], and Di et al [32]. In standard modelling thermal conductivity is separated into the following components, these dependent on underlying physical processes [27]:…”

“…2.1 Commercial and super insulation heat transfer Insulation thermal conductivity is the sum of internal solid-state conductivity, convection, and radiative (infrared) transmissions (Figure 4) [12,13,27]. The dominant component is gaseous heat transfer with standard commercial product values ranging 0.035 -0.040 W/m.K [28].…”

“…However, research to investigate reduction levels achievable in pursuit of improvement is recommended. Equation (4) dictates reduction of cellulosic core pore size would lead to lower thermal conductivity at atmospheric pressure [27]. There are likely to be physical limits on achievable performance, albeit routes to manufacture development are available.…”

Nanoporous polymeric materials: A new class of materials with enhanced properties

“…The confinement of the gas molecules within pores or a possible alteration of the phonon scattering mechanisms in the matrix are the mechanisms underlying this phenomenon. This expected reduction in the thermal conductivity has been previously experimentally demonstrated in aerogels [63,[106][107][108], porous metals [109], and porous ceramics [110] (these works will be discussed below for comparison). However, the thermal conductivity of polymeric foams is a recent issue because of the technical difficulties found in the production of nanoporous polymers with adequate densities, pore sizes, and external dimensions to allow the characterization of their properties as thermal insulators.…”