Strong renormalization of Ba vibrations in thermoelectric type-IX clathrate Ba 24 Ge 100

International audienc

Silicon purity controlled under electromagnetic levitation (SPYCE): influences on undercooling

International audienceThe rapid evolution of photovoltaic Si production induced a shortage of high purity silicon raw material. The use of lowest purity silicon has a strong effect on the casting conditions and ingot structure and properties. During solidification, solute rejection at the growth interface leads to an increase of the impurities concentration in the liquid phase and then to the precipitation of silicon nitride and silicon carbide. As a consequence, the grain structure of the ingot changes from columnar to small grains, also known as grits. A new electromagnetic levitation setup which has been developed in order to measure the undercooling versus impurity concentration is presented. The impurity concentration in the levitated Si drop is controlled by the partial pressure of nitrogen or hydrocarbon gas. As nucleation is a random phenomenon, statistical measurements are presented, from samples which showed numerous heating/melting and cooling/solidification phases. The effect of carbon impurities on the undercooling of silicon droplet is discussed

One-dimensional model of the equiaxed grain formation in multi-crystalline silicon

International audienceDuring solidification of low purity silicon for photovoltaic (PV) cells, solute rejection at the growth interface leads to an increase of the carbon concentration in the liquid phase and then to the precipitation of silicon carbide (SiC). When the precipitate radius becomes higher than the silicon critical nucleus radius, SiC can act as a refining agent for the Si and Si equiaxed grains appear in the liquid. The grain structure of the ingot changes from columnar to small grains, also known as grits. We developed a one-dimensional analytical model of this series of phenomena, including C segregation, SiC nucleation and growth. Si nucleation on the SiC precipitates and subsequent growth of the Si equiaxed grains. The equations are implemented under Matlab software in order to predict the columnar to equiaxed transition (CET) during the directional solidification of PV Si. We carried out calculations of the position and thickness of the equiaxed areas and of the number and size of Si grits as a function of the main process parameters: thermal gradient and growth velocity. Recommendations in order to adapt the growth process parameters to the initial carbon content are given. It is expected that coupling this model to global 3D numerical simulation codes could help improving the yield of ingot solidification. (C) 2011 Elsevier B.V. All rights reserved

« Etude de la germination et de la croissance de SiC dans le Si photovoltaïque »

International audienc

Carbon reaction with levitated silicon - Experimental and thermodynamic approaches

International audienceMetallurgical grade silicon (MG-Si) has become a new source of raw material for the photovoltaic industry. The use of this material as an alternative feed stock has however introduced phenomena that are detrimental to both the yield of the manufacturing process and the performance of the photovoltaic cells produced. This is mainly related to the presence of carbon, which precipitates to silicon carbide (SiC) in the ingot. This article focuses on the effect of carbon on silicon nucleation. Statistical experimental results of silicon nucleation are obtained as a function of carbon concentration and are presented and compared to thermodynamic calculations

Thermoelectric properties and stability of nanostructured chromium disilicide CrSi 2

International audienceCrSi2 alloy presents a strong interest for thermoelectric applications; however, its thermal conductivity is still too high and limits strongly its figure of merit. By combining experiment and modeling, we show that the nanostructuring of CrSi2 leads to a strong decrease in the thermal conductivity without affecting much the electronic transport properties. The thermal conductivity of nanostructured CrSi2 (∼45 nm) has also been determined as a function of the density. We predict that it would be about 5.5 W/mK at RT for a 100% dense sample, i.e., twice smaller than in bulk samples. We also give for the first time, a prediction of the effect of alloying on the thermal conductivity using the Callaway-Klemens model. To consider final applications, it is necessary to also investigate the thermal stability of nanostructured compounds. We show that grain coarsening of nanostructured CrSi2, even if it remains limited, happens above 1073 K and will deserve further attention

Influence of self-substitution on the thermoelectric Fe2VAl Heusler alloy

International audienc