Strain building and correlation with grain nucleation during silicon growth

This work is dedicated to the grain structure formation in silicon ingots with a particular focus on the crystal structure strain building and its implication in new grain nucleation process. The implied mechanisms are investigated by advanced in situ X-ray imaging techniques during silicon directional solidification. It is shown that the grain structure formation is mainly driven by S3 twin nucleation. Grain competition phenomena occurring during the growth process lead to the creation of higher order twin boundaries, localised strained areas and associated crystal structure deformation. On the one hand, it is demonstrated that local strain building can be directly related to the characteristics of the twin boundaries created during silicon growth due to grain competition. On the other hand, space restriction due to competition during growth can be at the origin of local strain building as well. Finally, the accumulation of all these factors generating strain is responsible for spontaneous new grain nucleation. When occurring, both grain nucleation and subsequent grain structure reorganisation contribute to lower the strain in the growing ingot. It is demonstrated as well that the local distribution of the strained areas created during silicon growth is retrieved after cooling down, from melting temperature to room temperature, on top of an additional larger scale deformation of the sample due to the cooling down only

In situ investigation of the structural defect generation and evolution during the directional solidification of 〈110〉 seeded growth Si

This work is dedicated to the advanced in situ X-ray imaging and complementary ex situ investigations of the growth mechanisms when silicon solidifies on a monocrystalline seed oriented ⟨110⟩ in the solidification direction. It aims at deepening the fundamental understanding of the phenomena that occur throughout silicon crystal growth with a particular focus on mechanisms of formation of defects detrimental for photovoltaic applications. Namely, grain nucleation, grain boundary formation and evolution, grain competition, twining occurrence, dislocation generation and interaction with structural defects are explored and analysed. Nucleation of twin crystals preferentially occurs on {111} facets at the edge of the sample where solid e liquid e vapor triple point lines exist in interaction also with the crucible as well as, at grain boundary grooves at the solid e liquid interface (solid e solid e liquid triple lines), where two grains are in competition, either on the {111} facets of the groove or in the groove. Enhanced undercooling and/or stress accumulation levels are found to act as driving forces for grain nucleation. Additionally, it is demonstrated that twin formation has the property to relax stresses stored in the crystal during the growth process. However, grains formed initially in twin position can undergo severe distortion when they are in direct competition or when they are squeezed in e between grains. Moreover, we show by X-ray Bragg diffraction imaging that on the one hand, coherent S3 ⟨111⟩ grain boundaries efficiently block the propagation of growth dislocations during the solidification process, while on the other hand, dislocations are emitted at the level of incoherent and/or asymmetric S27a ⟨110⟩ at the encounter with either S3 ⟨111⟩ or S9 ⟨110⟩ grain boundaries. Indeed, grain boundaries that deviate from the ideal coincidence orientation act as dislocation sources that spread inside the surrounding crystals

Effect of Zn- and Ca-oxides on the structure and chemical durability of simulant alkali borosilicate glasses for immobilisation of UK high level wastes

Compositional modification of United Kingdom high level nuclear waste (HLW) glasses was investigated with the aim of understanding the impact of adopting a ZnO/CaO modified base glass on the vitrified product phase assemblage, glass structure, processing characteristics and dissolution kinetics. Crystalline spinel phases were identified in the vitrified products derived from the Na2O/Li2O and the ZnO/CaO modified base glass compositions; the volume fraction of the spinel crystallites increased with increasing waste loading from 15 to 20 wt%. The spinel composition was influenced by the base glass components; in the vitrified product obtained with the ZnO/CaO modified base glass, the spinel phase contained a greater proportion of Zn, with a nominal composition of (Zn0.60Ni0.20Mg0.20)(Cr1.37Fe0.63)O4. The addition of ZnO and CaO to the base glass was also found to significantly alter the glass structure, with changes identified in both borate and silicate glass networks using Raman spectroscopy. In particular, these glasses were characterised by a significantly higher Q3 species, which we attribute to Si–O–Zn linkages; addition of ZnO and CaO to the glass composition therefore enhanced glass network polymerisation. The increase in network polymerisation, and the presence of spinel crystallites, were found to increase the glass viscosity of the ZnO/CaO modified base glass; however, the viscosities were within the accepted range for nuclear waste glass processing. The ZnO/CaO modified glass compositions were observed to be significantly more durable than the Na2O/Li2O base glass up to 28 days, due to a combination of the enhanced network polymerisation and the formation of Ca/Si containing alteration layers

L'Erodium brachycarpum (Godr.) Thell., espèce méditerranéenne méconnue de la série évolutive de l'E. botrys (Cav.) Bertol.

9 páginas, 2 figuras.Peer reviewe

Crystal distortions and structural defects at several scales generated during the growth of silicon contaminated with carbon

Carbon is a major impurity when one wants to produce silicon for efficient photovoltaic solar cells. Its impact on the generation of distortions and defects was studied by X-ray imaging at ESRF both in situ, during solidification, and ex situ

Crystal distortions and structural defects at several scales generated during the growth of silicon contaminated with carbon

International audienceFor all fabrication processes of the photovoltaic (PV) industry based on silicon, grain boundaries, dislocations, and impurity contamination control during solidification remains a major challenge to improve the electrical properties. In particular, carbon (C) is a major deleterious impurity for solar cells. The combination of X-ray radiography and Bragg diffraction imaging (topography) achieved in situ during silicon solidification allowed us to characterise the dynamics of the growth mechanisms involved in the formation of the grain structure and of defects, related to the presence of C. Ex situ techniques were used to characterise the grain structure and for a more precise analysis of the defects and their associated distortion fields. In the presence of C, it is shown that the resulting grain structure is constituted by a higher proportion of high-order and incoherent twin boundaries compared with the case of pure samples. Crystal distortion is characterised at the grain scale level and at a lower scale, both in situ and after cooling-down. The highest distortion at the grain scale corresponds to the position of the high order twin boundaries and is accentuated during cooling-down following solidification. Locally distorted regions and sub-grains are distributed all over the samples. They are observed in situ during the solidification from various seeds containing C (mono-crystals, industrial ribbons and multi-crystalline samples) and are retrieved after solidification. A model implying the presence of SiC precipitates at the solid-liquid interface is proposed to explain the formation during solidification of these sub-grains and of the associated local distortion