Influence of Solid Phase Conductivity and Cellular Structure on the Heat Transfer Mechanisms of Cellular Materials: Diverse Case Studies

Abstract

Cellular materials can be considered as two-phase composites in which a gas has been dispersed in a continuous solid phase. Although some porous materials can be found in nature they are usually produced at industrial scale (polymer foams, cellular metals, cellular ceramics, etc.) to satisfy diverse requirements since sometimes required properties are not accessible by using solid materials-technical requirements and less amount of raw material is needed saving costseconomical requirements. Porous materials offer interesting and unique properties as a reduced thermal conductivity, high cushioning capabilities, and excellent noise absorption.The evaluation of the thermal conductivity of cellular materials from the conductivity data of the two component phases and the structure of the material is an interesting subject that has been approached by different authors. [1][2][3][4][5][6][7][8] However, considering the complex relationships between structural parameters, composition and heat transfer mechanism (HTM), further research in this field is still needed.It is expected that HTM (convection, radiation, and conduction) contribute with different weights depending on the relative density and thermal properties of the solid phase. Therefore, radical differences in the relative weights for each HTM are expected for low density materials based on an insulating solid (polymers) or for materials with medium/ high relative densities based on a heat conductor (metals). Although, the number of previous works analyzing each type of material (polymer foams or cellular metals) is largely high, [9][10][11][12][13][14][15][16][17][18] as far as we know, there are no publications considering and comparing the HTM for both types of materials in a single study keeping a similar perspective (i.e., using similar concepts and models).This work uses, on one hand, experimental data and well known analytical models to predict the conductivity of cellular materials and, on the other hand, finite element modeling to investigate the effect of diverse structural parameters on the HTM. The main aim of the paper is to discuss the relative weight of each HTM in the full density range and the effect of the properties of the matrix and the cellular structure on the thermal conductivity for these two types of materials. The key result of this analysis will be a practical knowledge on the mechanisms that have to be taken into account for both types of materials.

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

“…The evaluation of the thermal conductivity of porous materials from the conductivity data of the two component phases and the structure of the material is an interesting subject that has been approached by different authors [91][92][93][94]. In a porous material, it is assumed that there are four different contributions to the total thermal conductivity (k t ) (Eq.…”

“…In order to analyze the influence of the reduction in the pore size from the micrometer to the nanometer range on the thermal conductivity, it should be noted that convection plays a minor role in closed-pore materials with pore sizes below 4 mm in diameter [95] and in open-pore systems with pore sizes less than 2 mm [96] (and therefore will be negligible in closed and open micro-and nanoporous polymers). Likewise, the influence of the radiation term is well known in conventional and microporous materials, and this term is negligible for porous materials with relative densities over 0.2 [91]. However, the conventional models used to evaluate the radiation mechanism in porous polymers assumes that the wavelength of the infrared radiation is smaller than the pore size, but this presumption is incorrect in nanoporous polymers [97].…”

Applicability of Solid Solution Heat Treatments to Aluminum Foams

“…The particular approach selected to solve the temperature field and its evolution during the quenching process was the use of Finite Element Method (FEM). This technique has been already used in different problems involving thermal transfer in cellular materials [29][30][31][32] with accurate results. In this investigation FEM software COMSOL Multiphysics linked to MATLAB 6.5 was used to implement the model.…”

Production, cellular structure and thermal conductivity of microcellular (methyl methacrylate)–(butyl acrylate)–(methyl methacrylate) triblock copolymers

“…As can be seen, the thermal conductivity increases as a function of density. If the results for these materlals are compared with those previously publlshed for polyolefin-based foams, 32 ¡t can be observed that they follow a very similar trend in terms of relative thermal conductivity (thermal conductivity of the foam divided by the conductivity of the solid) versus relative density (Fig.6). The polyolefin-based foams of the cited paperwere not microcellular; therefore these results suggest that the microcellular structure does not play a significant role in the thermal conductivity for MAMfoams.…”

“…40 The results are collected in Table3, where a relative contribution of between 6 and 14%isobtained, which isa typical result for foams with relative densities in the range 0.11-0.17. 32 Therefore, the same conclusión reached when the results were compared with polyolefin foams isfoundhere;themicrocellularstructuredoesnot improvethethermalinsulationcapabilityofthefoamsunderstudy in comparison to foams with conventional cell sizes. This result can be understood taking into account that the relative densities of the foams are above 0.1, and in this density range the radiation contribution still plays a minor role in the final conductivity.…”