Application of Laser-Flash Method to Porous Media: Problems and Challenges

Abstract

The further development of porous media requires fast and simple ways for measuring their thermodynamic properties. Given the major role they play in insulating systems, however this is of importance. One of these properties is the thermal diffusivity, which can be determined by a number of test methods. This experiment takes a closer look at the Laser Flash Method (LFM). The LFM is a transient method, which was developed by Parker and others for measuring thermal diffusivity of metals and metals alloys [1]. Nowadays, this is a widely-used method to determine the thermal diffusivity of different materials also for porous media like ceramics [2] or metal foam [3]. The simplicity of both, the measuring and handling of the arrangement and the fast way of getting data contribute to the fact that this method is very attractive. Furthermore when using this method, it is possible to obtain a lot of data at various temperatures. Samples used in this method only need to be small, which simplifies the procedure. Also the samples have to be fully dense, homogeneous and isotropic solid materials that are opaque to the applied energy pulse [4]. They should also have an even surface. Porous materials are neither opaque, nor made of dense material, nor do they have an even surface. According to american and german standards [4, 5] of LFM: Deposition of opaque materials like metals (silver, gold copper, etc.) followed by a thin film of high emissivity coating like graphite on both faces of the sample should enhance the performance of the measurement. However, these standards do not contain any parameters even also no restriction about the coating layer and coating way. The present paper deals with the task of changing the properties of the porous materials to fit the requirements of the test method LFM, because it has so many advantages over other methods. Therefore it is of interest to find a way to apply it to porous materials as well. In this context, the aim of these experiments was to find a way of applying the LFM to porous insulating material by coating the samples with a layer of different materials to make them less transparent and to even out their surface.