Thermal Conductivity of Cellular Metals Measured by the Transient Plane Sour Method

Abstract

Cellular metals combine the advantages of a metal (strong, hard, tough, conductive both electrically and thermally, etc) with the functional properties of a foam (lightweight and adjustable properties by selecting the density). Because of this, metal foams are interesting in a number of engineering applications such as structural panels, energy absorption devices, acoustic damping panels, compact heat exchangers, etc. [1] Particularly, the thermal transport properties of metal foams are attractive in a wide collection of applications, [2][3] from thermal insulation to heat transfer, being also important for the later processing of the metal matrix, for instance in the thermal treatments of the base alloy.In the last 10 years the fabrication routes to produce aluminium foams have been deeply studied and continuously improved. [4][5][6] Nevertheless, these materials are often non homogeneous materials, characterised by a density gradient related to both the presence of an outer skin and the presence of internal defects (big pores, wide cell-size distributions, fractured cell walls and cell-wall corrugations and misalignments [7] ). These deviations from homogeneity affect the physical properties and as a consequence an important aspect of aluminium foaming technology is to have experimental techniques able to characterise the in-homogeneity.Previous investigations have analysed the thermal conductivity of closed cell aluminium foams using a steady state method [8] and the effect of temperature on thermal conductivity. [9] Aluminium PM metal foams were previously characterized by the authors using the TPS method. [10] Additionally, the influence of different fillers [11] and the influence of cell size [12] on the thermal conductivity of high-porosity open cell aluminium foams has been studied. The cited experimental papers have mainly dealt with open cell aluminium and copper foams and closed cell aluminium foams. Moreover, in most of the previous works the results are those obtained for the bulk sample.The aims of this paper are firstly to carry out the experimental analysis of the thermal conductivity of diverse typos of metal foams in a wide porosity range; secondly to determine the influence of density, metallic matrix and type of cellular structure on this property and finally to show the ability of the TPS method to measure the local thermal conductivity of metal foams and to detect in-homogeneities. MaterialsClosed cell foams based on aluminium and pure zinc foams produced by the powder metallurgical (PM) route have been studied. Al-7Si foams with 0.5 wt-% TiH 2 as blowing agent and Zn foams with 1 wt-% TiH 2 have been produced. Consolidation to a foamable precursor material was reached by means of cold isostatic pressing and subsequent hot extrusion. On the other hand, open cell aluminium cast alloy AlSi9Cu3 foams have been produced by aluminium melt infiltration of polymer granulates using a high pressure die-casting process.To produce PM foam specimens, suitable pieces of the foamable precursor material we...

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

“…Reducing the cell size at constant density introduces a higher number of cell walls in the heat flow path and walls are the main cause of the scattering and absorption of the radiative heat flow. The predictions plotted are in close agreement with the experimental data [19] (relative density 0.046 and f s ¼ 0.3) validating the above statements.…”

“…Thus, the total heat transference can be predicted as the sum of the four mechanisms considered separately. [1][2][3][4][5][6] In order to evaluate the percentage of contribution of each one of these mechanisms in the overall thermal conductivity, models evaluated in previous research works [5,6,9,19] have been used. These models are considered in the next sections:…”

“…This is a valid procedure as previous works have already demonstrated. [19][20][21] It is important to mention that the non-linear change of f s [18] with density makes both equations similar at high porosities. [19] l gs ¼ l gas V gas þ l sol V 1:55…”

“…Figure 6 shows the normalized thermal conductivity for ERG open cell aluminum foams, [24,25] alporas foams, [26] aluminum foams, and zinc foams produced by the powder metallurgical route (PM route), [19][20][21]27] open cell infiltrated porous aluminum, [19,20] AZ91 magnesium foams, [21] and copper lotus foams. [28] It is appreciated the influence of cellular structure on the thermal conductivity is much smaller than that of relative density because all these data follow the same trend.…”

“…As proposed in other previous studies, and suggested at the beginning of this paper, the trend of thermal conductivity with relative density fits well to Equation 2. [19] Thus, from this analysis it can be stated that relative density is playing the main role for these materials being smaller (although nonnegligible) the influence of the cellular structure. A noteworthy result is related to lotus copper foams with a relative regular structure showing values out of the expected trend-although the measuring procedure on these highly conductive materials is complex due to the high value of its thermal conductivity and values could be shifted.…”