Boron and phosphorous have been added to CC composite as inhibitors to slow the oxidation of carbon. Apparently oxides of these elements block active surface sites, slowing the phase boundary controlled oxidation process. The oxides accomplish this without changing the activation energy of the process.
Early inhibition studies were concerned primarily with the oxidation protection of nuclear reactor graphites. Workers found that CL, CCL4, POCL3, P2O5, and B4C slowed the reaction of carbon with oxygen. McKee lists these substances and their derivatives as the only known inhibitors of the carbon oxidation reaction.
PhosphorusOxychloride (POCL3) has been studied by several workers and several mechanisms have been proposed. Arthur and Bankham suggest that POCL3 may slow reactions between CO and O2 since CO/CO2 ratios are larger when the inhibitors are present. Wicke on the other hand proposes that the inhibitor acts as a physical barrier between the oxygen and carbon. The physical barrier was thought to occur even though the concentration of POCL3 was not large enough to create a monolayer on the surface.
Magne et.al studied the effects of phosphates on the oxidation of graphite. The phosphates had two effects on the reaction. One effect was the neutralization of catalytic impurities on the surface. The other effect was the decreased reactivity of carbon atoms by direct reaction with the phosphates. The authors state that the phosphates may not impede very potent catalysts like lead. These workers also found that CO/CO2 retios increased when inhibitors were present.
Mckee examined the morphology of POCL3-inhibited single crystal graphite after oxidation. Earlier workers had found that single crystal graphite oxidized at edges of basal planes and at dislocations that emerged from basal plane. The attack was shown by etch pits that had formed. Mckee, however, did not observe etch pits on an inhibited sample at these sites at temperatures of up to 1000C. The strong adsorption of POCL3 at these sites was thought to block the oxidation process.
In a later study Mckee used various organophosphates to inhibit graphite powders. He found that a similar activation energy existed for the inhibited powder and the uninhibited powder. This suggested that the inhibitors blocked reactive surface sites.
Boron-based inhibitors have followed a similar evolution. Early workers found that B additives slowed the reaction of carbon with oxygen. Woodley did work on boronated graphite. The boron was added as boron carbide to two grades of graphite, Black and grey. The black graphite exhibited increasing oxidation rates as a function of time at temperatures between 500 and 800C similar to ordinary nuclear graphite. The grey graphite, however, exhibited a maximum in oxidation rate with time followed by a decrease in rate. Woodley attributed this effect to the formation of more B2O2 in the grey material as a result of a more uniform distribution of the carbide. The B2O3 formed at rates slower than the oxidation of C into CO2. The build-up of B2O3 reduced surface areas 25-50% by filling pores and providing a physical barrier between reactants leading to inhibition. The activation energies for the oxidation reaction were between 31 and 38 Kcal/mol. Woodley also found that B altered the CO/CO2 ratio such that little CO formed between 580 and 780C.
Walker and Alldarice studied the effect of B doping of graphite on oxidation kinetics. These workers observed a maximum in oxidation rate for doped and undoped samples at 1% burn off. The rate decayed into a steady state rate at 6% burn off. Activation energies varied between 45 and 52 Kcal/mol. Lower activation energies were associated with higher levels of doping. The authors attributed the reduction in oxidation rate of the doped samples to the blocking of active oxidation sites. The surface area as determined by BET exhibited a maximum at 10% burn off for the same conditions. For the undoped sample a constant value of surface area was observed after the maximum, while the doped sample exhibited a decrease in surface area.
The same authors examined the effect of water vapor on the oxidation of B doped graphite. Water vapor caused B2O3 removal from external surfaces by the formation of volatile boric acids. No such effects were observed on internal surfaces. cfccarbon.com
The morphology of molten B2O3 on single crystal graphite has been studied by Thomas who found that the liquid B2O3 had a globular form on the graphite surface but also filled hexagonal holes on the surface. As these underlying holes widen the liquid reverts to the globular form. Splitting of the boron oxide globules was seen to occur.