Fundamental studies of the influence of boron on the graphite-oxygen reaction using in situ electron microscopy techniques

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Abstract
Controlled atmosphere electron microscopy coupled with in situ electron diffraction has been used to follow the manner by which the addition of boron oxide influences the graphite-oxygen reaction. Continuous observations of the process show that at about 450 °C the boron oxide undergoes a strong interaction with both the graphite edge and the basal plane regions, and this results in a spreading of the oxide to form a uniform thin adherent film over the entire substrate. The coated graphite specimens appear to be impervious to attack by oxygen at temperatures below 815 °C. Above this temperature, however, there is a weakening of the additive-graphite interaction with the “armchair” {11$\overline 1$0} faces, and these regions then become vulnerable to attack by oxygen. At the same time very shallow pits are observed to develop in the basal plane, and this action coincides with the appearance of boron carbide species in the electron diffraction pattern. In a complementary series of experiments, it is found that boron carbide is an extremely active catalyst for the graphite-oxygen reaction even at temperatures as low as 100 °C. The impact of these low pressure studies on the behavior of carbon structures used in aerospace applications is discussed.