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N. M. Rodrigueza) Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802-4801 (Received 29 March 1993; accepted 15 July 1993)

A combination of in situ transmission electron microscopy and thermogravimetric techniques has been used to follow the manner by which phosphorus addition to graphite influences its interaction with oxygen. Direct observation of the process shows that the additive completely inhibits the reaction at temperatures below 830 °C. At higher temperatures phosphorus species are found to bond preferentially to the graphite "armchair" {1120} faces leaving the "zigzag" {1010} faces vulnerable to attack by oxygen. In situ electron diffraction analysis indicates the formation of a chemical bond between the phosphorus and graphite edge atoms at high temperatures, which involves the formation of a complex believed to become an integral part of the structure. This unique type of chemical bonding is believed to be responsible for the observed thermal stability of P - 0 species on the graphite atoms at temperatures up to 1050 °C. In a further series of experiments, phosphorus was found to poison the catalytic activity of cobalt, which in its unadulterated state is a very effective promoter of the graphite-oxygen reaction.

I. INTRODUCTION Carbon in the form of graphite has become a very attractive material for use in high temperature applications where strength, toughness, and low weight are desired; however, its use is restricted to nonoxidizing environments. Graphite starts to undergo gasification at temperatures as low as 700 °C in oxygen, and the presence of catalytically active impurities can decrease this onset temperature and also increase the rate of oxidation. 12 This behavior imposes serious limitations on the use of graphite or other carbonaceous solids in processes where such materials come into contact with air at high temperatures. These include aerospace applications where materials are expected to survive and maintain their integrity in a variety of environmentally hostile conditions. The advantages of carbon as catalyst support are frequently difficult to realize since its use is restricted to low temperature reactions or systems where oxygen is not one of the reactive components. One of the various approaches that have been used to decrease the rate of oxidation of carbon has been the introduction of inhibitors to the substrate,3 and in this regard phosphorus has been the most effective additive.4"8 Rakszawski and Parker4 investigated the effect of various elements and their oxides on the graphite oxygen reaction. In their thermogravimetric studies using "'Author to whom all correspondence should be addressed. J. Mater. Res., Vol. 8, No. 11, Nov 1993

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pelletized graphite samples mixed with 0.1mol% of additive, they found that phosphorus in the form of P 2 O 5 behaved as a very effective inhibitor and that the oxidation reaction was first order with respect to oxygen pressure. They suggest