ИСТИНА

Unique thermal stability and chemical inertia determine promising application of exfoliated graphite (EG) as a basis of heat-insulating layer especially in high temperature region [1]. The main disadvantage of low density heat isolators based on EG is low durability. For low density (pressed to 0.03-0.20 g/cm3) EG samples of such kind no investigations have been carried out, that's why the objective of this work is the estimation of pyrocarbon (PC) usage efficiency for creation of low density, thermoinsulating carbon/carbon composites with enhanced characteristics. Two types of EG were prepared for EG-based carbon/carbon composites. The nitrate exfoliated graphite (EG(N)) with a bulk density of 2-4 g/l was obtained by graphite interaction with 98% nitric acid followed by hydrolysis and thermal exfoliation at 1000oC. The preparation of electrochemical exfoliated graphite (EG(E)) with the bulk density of 0.9-1.5 g/l was carried out by anodic oxidation in 60% nitric acid solution followed by hydrolysis and exfoliation at 1000oC. CVI technique through methane decomposition at temperature of 950-1100oC and methane pressure of 140-500 mbar was used. Infiltration time was varied from 2 min to 120 min. Some patterns of PC deposition to low-density EG are described in our previous work [2]. It was shown that thermal conductivity coefficients of EG(E) are from 0.7 W/m∙K for EG(E) and from 1.7 W/m∙K for EG(N) respectively at 30 and 900oC. It was observed that pyrocarbon deposition leads to increase thermal conductivity of the composite. Nevertheless changing of ratio EG/PC and density of composites allows to produce materials with low thermal conductivity of the same as that of similar materials without pyrocarbon. It was shown that elastic modulus characteristics of the EG-based materials improve as low-density PC matrix is saturated. Samples containing 10-15 wt.% of PC with thermal conductivity about 0.7 W/m∙K have 5-6 times higher elastic modulus in comparison to EG-samples.