Purification of metallurgical grade silicon is one of the methods used to produce photovoltaic grade silicon. In our study, particles of metallurgical grade silicon were purified using a hydrogenated argon thermal plasma. During their residence time in the plasma, the particles were purified by partial evaporation and then sprayed into liquid droplets on the surface of a ceramic substrate. The in-flight purification of powder depends essentially on their evaporation rate, which is directly related to the temperature and chemical properties of the plasma zones crossed by the particles. It was, therefore, important to characterize the plasma parameters: electron density and temperature profiles. Excited states of atomic hydrogen, neutral and ionized silicon and impurity lines were detected in the plasma flow. Those lines were then used to estimate the electron density and temperature, which are around 2.4 × 1016 cm−3 and 10 500 K in the inductive zone. Finally, we estimate the silicon evaporated fraction X of the particles during their thermal treatment in the hydrogenated argon plasma. The results show that the loss of mass is weak (X = 2.5 × 10−4) but nevertheless sufficient for the elimination of the superficial impurities in the powders. This conclusion was confirmed by inductively coupled plasma analyses.
The recycling of the Si powder resulting from the kerf loss during silicon ingot cutting into wafers for photovoltaic application shows both significant and achievable economic and environmental benefits. A combined x-ray photoelectron spectroscopy (XPS), attenuated total reflection (ATR)-Fourier transform infrared (FTIR) and micro-Raman spectral analyses were applied to kerf-loss Si powders reclaimed from the diamond wire cutting using different cutting fluids. These spectroscopies performed in suitable configurations for the analysis of particles, yield detailed insights on the surface chemical properties of the powders demonstrating the key role of the cutting fluid nature. A combined XPS core peak, plasmon loss, and valence band study allow assessing a qualitative and quantitative chemical, structural change of the kerf-loss Si powders. The relative contribution of the LO and TO stretching modes to the Si-O-Si absorption band in the ATR-FTIR spectra provide a consistent estimation of the effective oxidation level of the Si powders. The change in the cutting media from deionized water to city water, induces a different silicon oxide layer thickness at the surface of the final kerf-loss Si, depending on the powder reactivity to the media. The surfactant addition induces an enhanced carbon contamination in the form of grafted carbonated species on the surface of the particles. The thickness of the modified surface, depending on the cutting media, was estimated based on a simple model derived from the combined XPS core level and plasmon peak intensities. The effective nature of these carbonated species, sensitive to the water quality, was evidenced based on coupled XPS core peak and valence band study. The present work paves the way to a controlled process to reclaim the kerf-loss Si powder without heavy chemical etching steps.
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