Publication: Analytical and experimental study of the wettability of silicon against different substrates
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2019-06-07
Defense date
2019-09-16
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Abstract
A causa de la crisis del petróleo se produjo un auge en la industrial fotovoltaica y, por tanto, del silicio solar. La obtención del silicio mediante vías metalúrgicas está marcada por su alta reactividad y, el relativo, alto punto de fusión del silicio (1414oC). Por tanto, en la producción de silicio se necesitan materiales que soporten altas temperaturas, pero que al tiempo no contaminen el silicio. Además, para evitar la pérdida de material y la posible rotura de los crisoles, se necesitan materiales con los que el silicio no interactúe ni infiltre. El material más usado es el grafito porque las impurezas no afectan a la efectividad de las placas solares. Sin embargo, la reactividad entre el silicio y el grafito facilita la mojabilidad y la infiltración del silicio, aumentando el coste del proceso. Por otro lado, el control de los procesos de mojado e infiltración es importante para otras aplicaciones como la fabricación de materiales cerámicos o incluso en soldadura. Por todo ello, esta tesis se centra en la búsqueda de un material que pueda ser usado como crisol en un proceso de purificación de silicio y en obtener información útil sobre la interacción con el silicio tanto para aplicaciones fotovoltaicas como para soldadura o la fabricación de refractarios. La investigación está dividida en materiales alternativos al grafito como crisoles, un estudio exhaustivo de substratos grafíticos y finalmente posibles recubrimientos para evitar o reducir la mojabilidad y la infiltración.
En este trabajo se ha comprobado que a pesar de su buen comportamiento frente al silicio la alúmina reacciona con el silicio, por lo que no puede ser utilizada como crisol, al igual que los recubrimientos de TiO2 y B4C, aunque en el caso de B4C se lograra reducir la infiltración significativamente. También, se demuestra que material compuesto carbón-carbón podría ser usado como crisol. Por otro lado, el mejor comportamiento frente al silicio se obtuvo por un grafito con granos y poros muy pequeños.
Due to the oil crisis, there has been an increase in interest in the photovoltaic industry, as well as in the demand the solar silicon. The key parameters for the purification of solar silicon through metallurgical routes are the relatively high melting point of the Si (1414oC) and its high reactivity. Therefore, materials should with stand high temperatures and do not get polluted in contact with silicon. Moreover, materials, which are not wetted and infiltrated by silicon in order to avoid the loss of material and the possible broke of the crucible, are sought. Graphite substrates are largely used because the effect of its impurities is negligible on the efficiency of solar cells. However, the reactivity between silicon and graphite boost the wetting and infiltration of the molten silicon, increasing the cost. On the other hand, the phenomena of wetting and infiltration are also a key factor in other applications such as the fabrication of refractories or brazing. As a result, this thesis is focused on finding a material which can be used in any process of silicon purification and on obtaining useful information about wetting and infiltration. The research is divided into the search for alternative materials for the crucibles, apart from the graphite for photovoltaic applications, an accurate analysis of the behavior of molten silicon against graphite substrates and finally, the use of coatings in order to avoid wetting and infiltration. In this work it has been proved that despite its good behavior against silicon, alumina reacts with silicon, so it can not be used as a crucible. Similarly, is with the TiO2 and B4C coatings, although in the case of B4C it is possible to reduce the infiltration significantly. Also, it is shown that carbon-carbon composite material could be used as a crucible. On the other hand, the best behavior against silicon was obtained by a graphite with very small grains and pores.
Due to the oil crisis, there has been an increase in interest in the photovoltaic industry, as well as in the demand the solar silicon. The key parameters for the purification of solar silicon through metallurgical routes are the relatively high melting point of the Si (1414oC) and its high reactivity. Therefore, materials should with stand high temperatures and do not get polluted in contact with silicon. Moreover, materials, which are not wetted and infiltrated by silicon in order to avoid the loss of material and the possible broke of the crucible, are sought. Graphite substrates are largely used because the effect of its impurities is negligible on the efficiency of solar cells. However, the reactivity between silicon and graphite boost the wetting and infiltration of the molten silicon, increasing the cost. On the other hand, the phenomena of wetting and infiltration are also a key factor in other applications such as the fabrication of refractories or brazing. As a result, this thesis is focused on finding a material which can be used in any process of silicon purification and on obtaining useful information about wetting and infiltration. The research is divided into the search for alternative materials for the crucibles, apart from the graphite for photovoltaic applications, an accurate analysis of the behavior of molten silicon against graphite substrates and finally, the use of coatings in order to avoid wetting and infiltration. In this work it has been proved that despite its good behavior against silicon, alumina reacts with silicon, so it can not be used as a crucible. Similarly, is with the TiO2 and B4C coatings, although in the case of B4C it is possible to reduce the infiltration significantly. Also, it is shown that carbon-carbon composite material could be used as a crucible. On the other hand, the best behavior against silicon was obtained by a graphite with very small grains and pores.
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Keywords
Wettability, Silicon purification, Alumina, Graphite materials, Photovoltaic properties, Reactivity, Coatings