Basic Research Of Carbon Material Modification And The Preparation Of Microporous Lightweight Materials In Molten Salt Media

The refractory materials contained carbon widely used in refractory industry has thedemand which still should be improved its hydrophilicity and antioxidant capacity, and foundnew carbon materials as substitution of non-renewable natural graphite. The energy-saving andcost-reducing of the high-temperature process industry need more high-performance lightweightmaterials. Based on the physical and chemical characteristics of the molten salt and the designtheory of the material composition and structure, the carbides or nitrides were produced by themolten salt method in natural graphite or wood-carbonized material through in situ reaction, sothe wettability and oxidation resistance of natural graphite were improved. The new direction ofthe research and development of carbon alternatives was issued. On the other hand, takingadvantage of the character of molten salt medium, we used the molten salt as a pore former andsintering aid for forsterite, and the lightweight materials of forsterite was successfully producedby the chemical reactivity between molten salt medium and forsterite, which could effectivelyremove iron oxide impurity. It is a new way to prepare lightweight materials, and expanding theapplications of the molten salt method.(1) Used NaF-NaCl as a molten salt and Si or Ti powders as raw material in argon, siliconcarbide nanowires and titanium carbide layer were successfully synthesized on the surfaces ofnatural graphite in the temperature range of1100°C to1400°C, respectively. The synthesizedsilicon carbide nanowires and titanium carbide should change the original structure andproperties of the graphite, and the graphite contained SiC nanowires or TiC layer had goodwettability with water. SiC nanowires or TiC layer were the antioxidant, and their oxide layers ofoxidation product would prevent further oxidation of the graphite. In nitrogen atmosphere andother same experimental conditions, TiN whiskers were successfully formed on the surface ofgraphite from1100°C to1400°C. TiN whiskers with different appearances such as rod, coneand star-shaped have grown with VLS mechanism, and the diameter of TiN whisker was about500-600nm. The generation of TiN whiskers has improved the oxidation resistance of thegraphite.(2) With the protection of argon, TiC/C composite ceramics which maintained natural porous structure of wood have successfully synthesized in KCl-KF molten salt at900°C, andTiC layer is formed on the surface of the charring carbon template. The morphology of thetitanium carbide crystals was controlled by the salt/Ti molar ratio and reaction temperature.Similarly, in the same atmosphere and molten salt system, the SiC/C composite ceramics havebeen compounded at1250°C, and retained the natural porous structure of wood. The diameter ofthe formed of SiC nanowires was from10nm to100nm on the surface of charring carbontemplate and inside its pores. The composite material of TiC/C or SiC/C was the potentialcandidate alternative material of natural graphite in the refractory industry.(3) Used NaCl and Na2CO3as molten salt medium respectively, the forsterite lightweightmaterials were prepared, and the dynamical models of the preparation process of the lightweightmaterials were issued. When NaCl acted as the molten salt, the proper temperature of preparationof the lightweight materials was1100°C, and the suitable content of the molten salt was40%to50%. NaCl didn’t participate in chemical reactions, and it just provided the environment for thesintering mechanism of the forsterite as liquid-phase sintering model, to promote the sinteringand grain growth of the forsterite material. The proper temperature to prepare forsteritelightweight materials in Na2CO3molten salt medium was900-1100°C. Na2CO3molten salt notonly provided the environment for forsterite sintering by the liquid phase model, but also reactedwith the iron oxide in forsterite and then generated a water-soluble compound and periclase.Iron-containing compound could be discharged when it dissolved in the water, which to achievethe purpose of removing iron selectively. The generated periclase phase is beneficial to improvethe refractory performances of the forsterite lightweight materials.