Thermal conductivity measurements of porous dust aggregates: I. Technique, model and first results

Abstract

We present a non-invasive technique for measuring the thermal conductivity of fragile and sensitive materials. In the context of planet formation research, the investigation of the thermal conductivity of porous dust aggregates provide important knowledge about the influence of heating processes, like internal heating by radioactive decay of short-lived nuclei, e.g. 26 Al, on the evolution and growth of planetesimals. The determination of the thermal conductivity was performed by a combination of laboratory experiments and numerical simulations. An IR camera measured the temperature distribution of the sample surface heated by a well-characterized laser beam. The thermal conductivity as free parameter in the model calculations, exactly emulating the experiment, was varied until the experimental and numerical temperature distributions showed best agreement. Thus, we determined for three types of porous dust samples, consisting of spherical, 1.5 µm-sized SiO 2 particles, with volume filling factors in the range of 15% to 54%, the thermal conductivity to be 0.002 to 0.02 W m −1 K −1 , respectively. From our results, we can conclude that the thermal conductivity mainly depends on the volume filling factor. Further investigations, which are planned for different materials and varied contact area sizes (produced by sintering), will prove the appropriate dependencies in more detail.

Thermal conductivity model for powdered materials under vacuum based on experimental studies

“…Moreover, our model is also applicable to the sintered dust materials in terms of the contact radius, r c appeared in Eq. (12). The evaluation of the applicability of our model to such sintered materials and their effect on the thermal evolution of planetesimals is currently in progress.…”

“…In addition to theoretical studies, the effects of the individual parameters on the thermal conductivity have been investigated experimentally. [9][10][11][12][13] For example, the particle size dependence was investigated by Merrill, 9 Huetter et al, 11 and Gundlach and Blum, 13 and the density (or porosity) dependence by Fountain and West 10 and Krause et al 12 However, the estimated values obtained by these studies often differed from each other by some orders of magnitude. There has been no systematic investigation with the same experimental configuration and conditions (the conductivity is especially sensitive to the volume or height of the sample 14 ).…”

Thermal evolution and sintering of chondritic planetesimals

“…0.9 (e.g. Yang et al 2000;Krause et al 2011a). The number of contacts of a particle to neighbouring particles then is on the average equal to about two, i.e., the fine dust grains form essentially chain-like structures.…”

“…For low porosities from the range 0 < φ ≤ 0.2 we used data measured for a number of ordinary H and L chondrites by Yomogida & Matsui (1983). For high porosities φ > 0.4 we used the data for silica powder derived from laboratory measurements by Krause et al (2011a). All these measurements were conducted under vacuum conditions to exclude any contribution from heat transport from gas-fillings in the pores.…”

“…5 shows the conductivity K for a silica powder that consists of equal-sized spheres with 1.5 μm diameter. By the nature of the experimental method the data of Krause et al (2011a) do not refer to a well-defined temperature but the heat-conductivity was derived by analysing the cooling behaviour of their sample. Therefore the value of K is some average value over the raise and fall of the temperature in the experiments, which is somewhat above 300 K temperature.…”