Oxygen, an usual impurity of photovoltaic silicon, is known to be detrimental for the electrical performances of this material. In this investigation, solidification experiments are performed in coated silica crucibles in order to study the influence of crucible size, crucible coating, argon flow and crucible material on silicon contamination by oxygen. In order to interpret the results, an analytical, semi-quantitative model is established taking into account two oxygen flows: the contamination flow at the crucible/silicon interface and the evacuation flow at the silicon free surface.

Surface and bulk interactions between a metal M (M = Au, Cu or Ni) and monocrystalline α-SiC are studied. Surface interactions are quantified by the work of adhesion measured by the sessile drop technique under high vacuum. Products of M-SiC bulk reactions are characterised by electron microscopy and microprobe analysis. Interpretation of experimental results is based on classical thermodynamics for equilibria of bulk phases and on interfacial thermodynamics for adsorption phenomena and wetting.

Graphite crucibles are potentially interesting for the directional solidification processing of photovoltaic silicon, because, contrarily to standard silica crucibles, they can be used many times. In the present work, two types of graphite crucibles are studied: i) graphite directly coated with the Si3N4 powder classically used as a releasing layer for standard silica crucibles, and ii) graphite protected by a dense SiC layer before deposition of the releasing coating. In both cases, spontaneous detachment of the silicon ingot is observed to occur during cooling. Results are given and discussed concerning the impurities introduced in the silicon during the melting-solidification cycle, as well as their consequences on the photo-electrical characteristics of silicon. These results are compared with those obtained using the standard silica crucible. It is shown that re-useable SiC protected graphite crucibles lead to the same performances as standard silica. Reproducible results are found after four runs with the same crucible, and no degradation of the crucible is observed.

The crystallization of silicon for photovoltaic applications is currently performed by directional solidification in amorphous silica crucibles. In order to avoid sticking, silica crucibles are coated with a layer of silicon nitride which acts as an interface releasing agent between the silicon and the crucible. Due to silica softening and subsequent transformations during the melting-solidification cycle, Si3N4-coated silica crucibles can be used only one time. A more interesting solution would be to have a graphite crucible which could be used several times in view of its high mechanical performances at elevated temperatures. The goal of this study is to determine in which way the Si3N4 coating can also be used for graphite crucibles. The study is conducted by means of the sessile drop technique and microstructure characterizations carried out by optical microscopy. For comparison purposes, experiments are also performed for the standard configuration of Si3N4-coated silica.

Photovoltaic silicon ingots are currently grown in silica crucibles coated with a porous silicon nitride layer which acts as an interface releasing agent between the silicon and the crucible. The interactions between Si and the Si3N4 coating determine the infiltration and sticking phenomena occurring at the interface and also affect the pollution of Si by the components of the coating. In this investigation the interfacial interactions and microstructure are studied in crystallization experiments performed in crucibles involving high silicon masses (tens of kg) and long contact time between the silicon and the coated silica (tens of hours). It is shown that for long times, a dramatic change in the nature of the coating/Si interface takes place, with the formation of a self-crucible which prevents the direct contact between the silicon and the coating. The stability of the self-crucible is modeled taking into account the capillary and hydrostatic pressures. The influence of the self-crucible on different practical aspects of the photovoltaic silicon crystallization process is discussed.

Processing of photovoltaic silicon by solidification is currently carried out under argon flow in silica crucibles coated with an oxidized silicon nitride powder. A series of experiments was performed to study the reactions between coating components under argon flow by varying the temperature, the holding time and the oxygen content in the coating. The results are discussed with the help of a simple analytical model taking into account the diffusive transport of gaseous reaction species from the inside of the porous coating to the flowing argon. The conclusions drawn are used to discuss different practical aspects of the photovoltaic silicon crystallization process.

The wettability of ceramics by liquid metals is discussed from both the fundamental point of view and the point of view of applications. The role of interfacial reactions (simple dissolution of the solid in the liquid or formation of a layer of a new compound) is illustrated and analysed. Several results are presented in order to illustrate the role of wettability in materials processing, namely infiltration processing, joining dissimilar materials by brazing and selecting crucibles for crystallising liquid metals and semiconductors. The review includes results obtained during the last 15 years mainly, but not only, by the Grenoble group.

Processing of photovoltaic quality silicon (PV Si) starting from metallurgical Si involves contact between liquid silicon and the refractory materials used as crucibles for melting and crystallisation. The interactions (i.e. wetting, infiltration and sticking) between silicon and two types of crucible used in the course of PV Si processing, namely graphite and coated silica, are described and interpreted. A model of the wetting and infiltration phenomena, coupling thermodynamics and kinetics, is used to analyse the influence of the main parameters of the material (microstructure, chemistry) and the process (temperature and atmosphere). Examples are given of the ways to use the understanding of elementary processes at silicon/crucible interfaces in order to select crucible material and determine the desired process parameters.

The wettability of ceramic and metallic solids by liquid metals is discussed with emphasis on the effect of interfacial reactions on the spreading kinetics and the final degree of wetting. Two types of reaction are considered, simple dissolution of the solid in the liquid and dissolution followed by formation of a new compound at the interface. The review includes results obtained during the last 15 years mainly by the Grenoble group.

Photovoltaic silicon ingots are currently grown in SiO2 crucibles coated with a porous silicon nitride layer which acts as an interface-releasing agent between silicon and the crucible. The present investigation focuses on the initial stages of Si–crucible interactions, involving infiltration of the porous coating by molten silicon. In this study, performed using the sessile drop technique in flowing argon, three types of infiltration are considered: infiltration under the drop, which is representative of infiltration occurring at the crucible/bulk silicon interface; infiltration at the coating surface in front of the nominal triple line, leading to the formation of a silicon-rich film; and infiltration under this film. The experimental results, obtained by varying the infiltration duration, the temperature of the coating heat treatment prior to the infiltration experiment and the argon flow, are interpreted with the help of an analytical model, taking into account diffusive transport in the infiltrated coating, in the bulk liquid and in the flowing gas.

The wetting of Cu–Fe two-phase composites by molten Sn is studied by the sessile drop technique under high vacuum at 400 °C. In this system Sn reacts with both solid components, forming intermetallic compounds. It is found that the curve of contact angle vs. the surface fraction of components passes through a minimum, behaviour that cannot be interpreted by existing models describing wetting of heterogeneous surfaces and/or reactive wetting. It is shown that the observed enhanced wetting can be explained by the dissolution contrast of Cu and Fe phases, leading to interfacial microroughness, thus providing an additional driving force for wetting. In order to take into account this new effect of interfacial reactions on wettability, an equation similar to Wenzel’s equation is established. It is shown that this equation can explain the change in wettability of composites when Sn is replaced by SnPb eutectic presenting a lower reactivity than pure Sn, as well as the effect on wettability observed when the scale of composite microstructure is changed with the surface fraction of components remaining constant.

The fundamental equations of wetting and adhesion are presented and the contact angles of non-reactive liquid metals and molten oxides on different types of solids are interpreted. The main features of reactive wetting are briefly described and illustrated. The two types of brazing used in practice (capillary brazing and sandwich brazing) are defined and the thermodynamics and kinetics of capillary infiltration are presented. The different configurations expected to occur in ‘sandwich brazing’ at varying intrinsic contact angles are described and illustrated. Three examples of brazing of metals and ceramics in non-reactive and reactive systems are discussed to show how wetting can affect brazability and the properties of brazed joints.

Sixty years after the first measurement of capillary properties of silicon, experimental results on the surface tension and temperature coefficient of liquid silicon are still divergent. The reason for this persisting divergence is discussed by examining the effect on these quantities of (i) oxygen contained as an impurity in the gas, (ii) impurities in Si and (iii) contamination by the supporting material (substrate or crucible). From the analysis of experimental data, the following expression is derived for the temperature dependence of the surface tension σ: σ(mN/m)=840(±45)−0.19(±0.09)(T(K)−1685).

The fundamental issues of the reaction at liquid Si/graphite interfaces between Si melting point (1412°C) and 1600°C are studied on the basis of results obtained with polycrystalline graphite concerning the growth kinetics of the interfacial reaction layer and the microstructure and morphology of this layer. Experiments were also performed using vitreous carbon substrates. Results are also reported for Si–Al alloys at 1000°C. The elementary process controlling the growth kinetics is determined and a model is proposed to describe the different stages of the interfacial reaction.

The surface of silicon is very sensitive to interactions with oxygen present as impurity in furnace atmosphere. It is shown that three types of Si surfaces can be obtained depending on the oxygen's partial pressure in the furnace and on temperature: oxidized, oxide-free but containing adsorbed oxygen and adsorption free. The influence of oxygen on the surface tension of molten Si is also discussed. Wetting by Si and Si alloys is then described and analysed for three types of ceramics: (i) ionocovalent oxides (Al2O3, SiO2, MgO, etc.), with a particular emphasis on the Si/silica couple, (ii) the different types of carbons where wetting is assisted by the reaction between Si and carbon, and (iii) the predominantly covalent ceramics (SiC, Si3N4, AlN, BN, etc). The role of wetting in the processing of silicon or silicon-based multimaterials is also illustrated.