Transformers silicon-based liquid

For transformer windings immersed in oil, hydrocarbon oil is the most widely used, whereas in areas where fire risk is a problem, then air-cooled transformers (AN) or synthetic silicon-based liquid cooling (SN) can be specified. Silicon-based liquids do not have any of the disadvantages identified with the chlorinated biphenyls. Air-cooled transformers can be provided with Class C insulation or be cast resin insulated. The relative costs per unit of each type are ... 

Liquid immersed types use various forms of oil and special synthetic liquids. The chlorinated liquids, e.g. polychlorinated-biphenyl, have been banned in most countries because they are very strong pollutants and are almost impossible to destroy, except by intensive burning in a special furnace. Modern liquids are synthetic compounds typically silicone based, and are usually specified to be flame retardant. 1EC60296, 60836, and 60944 describe suitable liquids. These transformers are the type normally used in oil and gas plants. Resin insulated transformers are very suitable for indoor locations and off-shore plants because they contain no flammable liquid, produce no spillage and require minimal maintenance. They are usually more expensive than conventional liquid immersed transformers. 

Silica-based materials obtained by the sol-gel process are perhaps the most promising class of functional materials capable to meet such a grand objective. In the sol-gel process liquid precursors such as silicon alkoxides are mixed and transformed into silica via hydrolytic polycondensation at room temperature. Called soft chemitry or chimie douce, this approach to the synthesis of glasses at room temperature and pressure and in biocompatible conditions (water, neutral pH) has been pioneered by Livage and Rouxel in the 1970s, and further developed by Sanchez, Avnir, Brinker and Ozin. 

The properties of the dual-film electrode were characterized by in situ Fourier transform infrared (FTIR) reflection absorption spectroscopy [3]. The FTIR spectrometer used was a Shimadzu FTIR-8100M equipped with a wide-band mercury cadmium teluride (MCT) detector cooled with liquid nitrogen. In situ FTIR measurements were carried out in a spectroelectro-chemical cell in which the dual-film electrode was pushed against an IR transparent silicon window to form a thin layer of solution. A total of 100 interferometric scans was accumulated with the electrode polarized at a given potential. The potential was then shifted to the cathodic side, and a new spectrum with the same number of scans was assembled. The reference electrode used in this experiment was an Ag I AgCl I saturated KCl electrode. The IR spectra are represented as AR/R in the normalized form, where AR=R-R(E ), and R and R(E ) are the reflected intensity measured at a desired potential and a base potential, respectively. 

The transformation of the epitaxial layer polytype to a structure different from 3C, 6H or 15R was first obtained with the use of liquid phase epitaxy by the travelling solvent method [63]. The authors employed scandium-based solutions and they observed the 4H-SiC layer growth on substrates of the 6H polytypes. Later, the detailed studies of Vodakov et al [64] showed that the 6H to 4H polytype transition occurs when the growth is performed onto the carbon face and only if the melt contains excess carbon. No polytype transformation occurred if the melts were free from excess carbon or contained excess silicon. In [63] no carbon was added to the solvent intentionally, so most probably it entered the melt via reaction with the graphite... 

Several types of phase transformations can occur during aging. We have already mentioned mlcrosyneresis, in which solid phase separates from the liquid on a local scale. There may also be segregation of the liquid into two or more phases. For example, in base-catalyzed hydrolysis of silicon alkoxides, there may be isolated regions of unreacted alkoxide [14]. When a gel of that type is soaked in pure water, it turns white and opaque [15], apparently (as explained in Section 2) from segregation of droplets of partially reacted alkoxide. Aging may also lead to crystallization, as in the precipitation of nitrate crystals from alumina gel made from Al(NO)3 [16],... 

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