Currently, industrial processes that produce silicon occur in batch units which are
energy intensive, capital intensive, and emit harmful pollutants into the atmosphere. A
new technology, solid oxide membrane (SOM) processing, seeks to produce silicon
without direct emissions and with lower energy and capital costs. Previous studies have
shown that this technology can produce silicon; however, the proof-of- concept cell was
incapable of producing large volumes of silicon due to restrictions in the molten salt.
Current research has engineered an oxyfluoride molten salt to be more efficient in
four main ways: higher amount of silica in the molten salt, chemistry stable with the
yttria-stabilized zirconia (YSZ) membrane, low volatility, and high electrical
conductivity. The newly designed salt allows for up to 25 at% of silicon oxide to dissolve
into the flux, removing mass transfer limitations. The mixture utilizes calcium oxide to
stabilize the presence of silicon oxide, giving the flux a volatility of less than 0.1 µg/cm 2
*s. The presence of calcium oxide also increases the optical basicity of the system,
allowing the flux to be compatible with the YSZ membrane showing no signs of
corrosion. Lastly, the new flux composition has a conductivity of 2.87 and 4.38 S/cm, at
1050 °C and 1100 °C, respectively, which is above the desired value of 1 S/cm.
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Combining these improvements in the salt with pre-existing techniques, silicon
crystals were produced in the new SOM cell. Two distinct SOM cell configurations were
attempted, one with a liquid cathode (tin) and one with a solid cathode (molybdenum).
Both cells were able to successfully make silicon metal. The tin cathode was able to
produce high purity silicon crystals extracted via acid etching. The molybdenum cathode
produced a plated layer of molybdenum disilicide. Samples were examined by using
scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS).
An equivalent circuit model for the SOM process was developed to calculate polarization
losses during the electrolysis process.
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/30729 |
| Date | 03 July 2018 |
| Creators | Villalon Jr., Thomas Anthony |
| Contributors | Pal, Uday B. |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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