Investigations On Graphite Anti-oxidation Coating Technology Of High Temperature Composite Heat Storage Materials

Efficiency and capacity of thermal storage system can be improved greatly by adopting properly high-temperature thermal energy storage materials,which is becoming a hot research spot in the field of thermal energy storage.Graphite as an effectively thermal conductivity additive,has been reported to be widely used in composite phase change heat storage materials serviced at medium and low temperature.However,due to its serious oxidation at high temperature,application of graphite in high-temperature thermal energy storage materials is usually limited.In order to further extend its application to the hightemperature heat storage area,a preliminary study on the oxidation and anti-oxidation behavior of graphite at high temperature is carried out in this paper.After effective surface treatments and anti-oxidation coating on different graphite substrates by the solution impregnation route,typical composite phase change heat storage materials with inorganic salts and ceramic skeleton as phase change and supported materials,respectively,are fabricated via form-stable technique.Based on this,the hightemperature oxidation and anti-oxidation behavior of graphite were systematically studied.The research results suggest that composite with the impregnated graphite as additive can increase its initial oxidation temperature about 200℃than that doped by the uncoated flake graphite with a mass loss rate as only 8%after being calcinated at 700℃ for 8h and a reduction of about 30%in the oxidation amount of graphite.Meanwhile,SEM observations on the thermal cycled samples revealed unreacted graphite flakes still can be observed after 548 thermal cycles,which suggested excellent oxidation resistance and thermal cycling property performance.Based on the possible influences of experimental variations,such as the impregnation ratio,impregnation time and different graphite matrix,etc.on the coating results,the researcher pointed out in this paper that the impregnation time should be kept as long as 3 hours with the mass ratio of C/Al(H2PO4)3 as 1:3 for achieving the best impregnation performance.For different types of graphite,better impregnation results can be more easily achieved in expanded graphite and flake graphite.Considering that the used phase change material in the composite is usually molten salt,based on the previous solution impregnation method,researchers further attempted to synthesize TiC coating on flake graphite surface at high temperature using the molten salts as reaction medium.The research results indicate that TiC have good compatibility with the eutectic carbonate below 750℃.Using the molten salt as reaction medium,carbontitanium carbide with a typical core-shell structure can be in-situ synthesized at 1300℃by effectively controlling the experimental processing parameters,such as ratio of Ti and C,molten salt types and reaction temperature,etc.Compared with uncoated flake graphite,the quality loss of the as-prepared graphite powder can be reduced by more than 80%with the calcined temperature as 900℃.Further investigations on the thermal energy storage composite doping by this core-shell structured graphite suggested the oxidation amount can be reduced by more than 20%,and good thermal stability in properties can be obtained under the conditions of thermal-cycling.