Physical Characteristics

Heat transfer through foam material goes through a matrix, by means of radiation and through filling medium. Heat conductivity in temperature range from 20 to 800 ⁰Ñ can be expressed by the equation:

Where  — heat conductivity coefficient of a compact material and filling gas;  — relative density of a foam material; k — empirical constant (k = 0,9 *10–12 W/m-K); Ò — average absolute temperature.

Electrical conduction of foam materials independently on production method, in the range of relative densities can be found with the help of a simplified equation quite precisely:

Where  — specific conductivity of a foam material and a compact material,  — relative density of HPCM.

Mechanical characteristics of foam materials are defined by three-dimensional isotropic structure and behaviour of separate structural elements — bridges. There are four sites in the diagram of material compression. In the first site extent of which depends on the weakest elements of the skeleton and edge discontinuity, deformation appears at small loadings. In the second site there is an elastic deformation. In the third site bridges lose their stability, there develop plastic deformations and the compression stress-strain diagram goes to the flat site (compression plateau). The process has a cyclic chain character: stability loss in one of the bridges results in deformation development in the next ones going on through the whole layer, gradually material layers collapse to compactification limit when pores close down and the deformed material starts to have characteristics of a solid compact material. In the fourth site pressure inside the material increases again and gradually approaches the diagram of a solid compact material’s compression. The process is presented in the scheme below.

For brittle materials (carbon, ceramics, nonplastic metals) the compression diagramme in the third site has a fragmentary image.It is caused by abrupt relaxation of pressure at bridges breakdown. At further loading the whole layer collapses, up to full colouring of the sample. Pressure at destruction can be more or less than in the previous cycle.

Acoustic properties of foam materials are determined by internal structure, properties of a compact material, as well as by external conditions. With thickness growth the bounds of frequency range in which the material is most effective extent. Sound absorption coefficient value and frequency value depend on porosity and cells size. Maximum of sound absorption of a foam material is located at hard wall, as a rule, in the range of high frequencies 3–4 KHz. Offset of the maximum of absorption can be achieved by increasing air clearance between the blocks and the wall. Sound absorption characteristic of a foam-aluminium block at different air clearances is presented on the diagram below, and there is also a characteristic of continuous aluminium sheet for comparison.