Other Processing - Molten Carbonate Method

High pressure and high temperature is a method used to grow diamond under an HPHT environment. The process is performed in the diamond stable region at pressures ranging from 7 to 10 GPa and temperatures ranging from 1700 to 2000 K. Graphite or other carbons are dissolved in molten transition metals and then precipitated in diamond form. 

The metal is produced on a massive scale by the Hall-Heroult method in which aluminum oxide, a nonelectrolyte, is dissolved in molten cryolite and electrolyzed in a large cell. The bauxite contains iron oxide and other impurities, which would contaminate the product, so the bauxite is dissolved in hot alkali, the impurities are removed by filtration, and the pure aluminum oxide then precipitated by acidification. In the cell, molten aluminum is tapped off from the base and carbon dioxide evolved at the graphite anodes, which are consumed in the process. The aluminum atom is much bigger than boron (the first member of group 13) and its ionization potential is not particularly high. Consequently aluminum forms positive AT ions. However, aluminum also has nonmetallic chemical properties. Thus, it is amphoteric and also forms a number of covalently bonded compounds. 

Other methods are also closely related to the resulting surfaces of the carbon materials. For example, an artificial passivating SEl film is formed when a suspension of n-butyl lithium/hexane, lithium naphthalene, or molten lithium is used, first to treat the graphite and then to react with electrolytes. This results in a higher reversible capacity of the graphite, 430 mAh/g, and higher initial coulombic efficiency. Apparently, the first treatment can passivate active defects in carbon materials. The merit of this process is that it is independent of the specific carbon materials. However, it is complicated and the formed SEl film is still not comparable with that obtained by electrochemical reduction. In another reported case, deposition of an inorganic ionic conductor, lithium phosphorus oxynitride (LiPON), can also lead to improvement in electrochemical performance. 

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