No abstract

The objective of this work was to study the electrodeposition of silicon in molten KF-LiF (eut)-K2SiF6 (5 mol%) on Ag and Si substrates at 550oC and 800oC. In addition electrochemical studies of the system were performed on silver, tungsten and glassy carbon substrates at 800oC. The electrochemical measurements confirmed that the reduction of Si is diffusion controlled and occurs in two steps. On silver, dense, coherent films with good adhesion and no inclusions of salt were obtained at 800oC at the growth rate 52 µm/hour. On silicon, the microstructure seemed to consist of small "randomly" oriented crystals. At 550oC, the deposit became porous and powderish both on silver and silicon substrates. The low temperature combined with the relatively high current densities applied was believed to hinder crystal growth.

To make electrical energy from photovoltaic (PV) silicon (Si) solar cells competitive, the cost in each of the PV manufacturing process steps has to be diminished. Today, high-purity Si is produced by an energyintensive process exhibiting high irreversible thermodynamic losses. The purity of the product from this process (99,9999999 pct [9 N]) far exceeds what generally is accepted to be the requirements for PV purposes (4 to 6 N). Here we show a novel method for the purification of Si based on the principle of electrochemical refining in a molten high-melting-point fluoride electrolyte at temperatures above the melting point of silicon 1685 K (1412°C). The method comprised a vertical stack of three molten layers with a metal alloy at the bottom, an intermediate electrolyte layer, and purified metal at the top. The integrity of the layers being secured was through the immiscibility of the liquids and the careful tailoring of the individual densities. Boron (B), exhibiting similar thermodynamic properties to Si, effectively was not removed. A suitable low-B feedstock may be identified in kerf from the sawing of mono-or multicrystalline Si blocks into wafers. To produce purified metal in the 6 N range, practice from electrorefining of aluminum shows that long-term, stable operation in large-scale industrial reactors is needed. The trends and mechanisms observed in the laboratory scale indicate that high purity also can be achieved for Si provided that these criteria can be met.

No abstract

The electrochemical behaviour of dissolved Fe2O3 and the electrodeposition of iron in molten CaCl2-CaF2 (80-20 mol %) and other mixed chloride/fluoride electrolytes was studied at 827 {degree sign}C and 890 {degree sign}C by cyclic voltammetry, chronoamperometry and galvanostatic electrolysis, and deposits were characterized by XRD and SEM. Pure iron was obtained in these melts at current efficiencies greater than 90 %. High cathodic current densities (~0.5 A cm2) were achieved by using a rotating cathode. The work is related to studies of the possible development of new processes for the production of iron and steel with large reductions of CO2 emissions.