The origin of the cracking of highly porous silicon layers during drying is investigated. Optical and scanning electron microscopy observation allow us to observe the cracking occurrence. In situ x-ray diffraction experiments, under controlled vapor pressure of pentane, reveal that large capillary stresses occur at a vapor pressure P* during the controlled drying. These stresses lead to the cracking of the highly porous layer, which occurs for samples thicker than a critical thickness hc. Taking into account the mechanical properties of the material, a model based on energy balance is presented. This model predicts a layer thickness hc of cracking occurrence, showing that hc varies as (1−p)3/γLV2 (where γLV is the surface tension of the drying liquid and p is the porosity). This model is in good agreement with experimental data obtained with two liquids, water, and pentane, which have very different surface tension and also for two different porosities.
Bulk-textured YBa 2 Cu 3 O 7−x single domains could be used for current-limiting applications by cutting and assembling long meanders, which would be submitted to network electric fields before using a breaker to interrupt a fault current. For that purpose, large YBaCuO single domains up to 93 mm in diameter can be isothermally grown by using a standard melt texturing growth (MTG) process with a SmBaCuO seed. The essential parameters that have to be controlled in order to reach this size are the temperature growth window, the substrate reactivity and the temperature homogeneity in the sample. Standard 45 mm diameter single domains show excellent superconducting properties, such as J c above 10 5 A cm −2 and a homogeneous superconducting-to-normal transition at 91.8 K for 20 cm long conductors cut in these pellets. These measurements demonstrate the long -range homogeneity of single domains regarding T c and J c . Nevertheless the high J c values lead to a too large a dissipation in the normal state at T = 77 K. Different methods to reduce the critical current density are described in order to fulfill the conditions for a safe recovery of the material after undergoing a magnetothermal transition.
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