Influence of tailored MLI for complex surface geometries on heat transfer

Complex, non-developable surfaces require a tailored multi-layer insulation (MLI) for lowest heat load. The most experiments showing the heat transfer through MLI are performed under quasi-ideal conditions determining the principle insulation quality. But the surface to be insulated in real cryostats implies feed-throughs and other non-developable surface parts. The thermal performance of MLI is degraded significantly at cutting points. To investigate this degrading effect a LN$_{2}$-filled cylinder with a diameter of 219 mm and a length of 1820 mm was insulated with MLI and the heat load was measured by means of calorimetry. In addition the heat load to an insulated cylinder with eighteen branches was measured. Both cylinders have the same surface of 1.37 m$^{2}$ for a comparison of the results. This article describes the experiments with different ways of tailoring the MLI for the cylinder with branches and discusses their results. It was shown that the cutting points at the branches have a significant degrading influence on the thermal performance of MLI

Surface energies of AlN allotropes from first principles

In this Letter we present first principle calculation of surface energies of rock-salt (B1), zinc-blende (B3), and wurtzite (B4) AlN allotropes. Out of several low-index facets, the highest energies are obtained for mono-atomic surfaces (i.e. only by either Al or N atoms): \gamma_{\{111\}}^{\rm B1}=410\uu{meV/\AA}^2, \gamma_{\{100\}}^{\rm B3}=346\uu{meV/\AA}^2, \gamma_{\{111\}}^{\rm B3}=360\uu{meV/\AA}^2, and \gamma_{\{0001\}}^{\rm B4}=365\uu{meV/\AA}^2. The difference between Al- and N-terminated surfaces in these cases is less then 20\uu{meV/\AA}^2. The stoichiometric facets have energies lower by 100\uu{meV/\AA}^2 or more. The obtained trends could be rationalised by a simple nearest-neighbour broken-bond model.Comment: 7 pages, 2 figure