Intercalation-Desorption Studies of Graphite Nitrate
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STUDIES OF GRAPHITE NITRATE
T. DZIEMIANOWICZ, K. LEONG, AND W.C. FORSMAN Department of Chemical Engineering, University of Pennsylvania Philadelphia, PA 19104
ABSTRACT A new model for intercalation kinetics is presented whick takes into account both nucleation of the interlaminar spaces and intercalant layer growth. For nitric acid intercalation, the nucleation constant, 8, ranges from 1.25xi0-3 to 10-4 sec-I depending on graphite type (HOPG > powder > fibers); the corresponding range of effective diffusivities, 5 2 D, is 2.5xi0- to 10-10 cm /sec. It is emphasized that effective diffusivities obtained from sorption kinetics are reaction enhanced. Stepwise isothermal desorption experiments show that the diffusivities of HNO 3 neutrals are lower than the effective diffusivities on sorption. At desorptive equilibrium, intercalant retention is in the order HOPG > fibers > powder. In HOPG, resistance is constant over the desorptive transition stage II - III - IV; during the stage IV + V + VI regime (dynamic vacuum), resistance doubles. A similar resistance gain is noted on desorption of neturals from nitrated graphite fibers.
INTRODUCTION Graphite intercalation compounds (GIC's) have been shown to possess a number of interesting physical and chemical properties, not the least of which is the potential for extraordinary a-axis (i.e., along the carbon planes) electrical conductivity. It remains to be seen, however, how effectively such "laboratory curiosities" may be parlayed into useful engineering materials. In this regard, two central issues are (1) how, and in what form can intercalation compounds be combined with other materials to yield a useful product, and (2) how can GIC's be stabilized to mitigate the effects of hostile environments. In a sense it is the latter question which is the more urgent, since it is well known that GIC properties may be radically altered once removed from an equilibrium environment. Earlier work in this laboratory has focused on the chemistry and properties of graphite nitrate compounds. Though lacking in at least one important practical consideration -- thermal stability -- graphite nitrate is in many other respects a suitable model system for studies of potential applications. The synthesis [1], chemistry [2], structure and properties [3) have been scrutinized as closely as in most other acceptor-type compounds. Relatively 5 high conductivities (4x10 (f-cm)-I for stage 2) have been reported [4]; furthermore, preliminary studies show that the electrical properties are relatively insensitive to intercalant concentration at least over the range stage 2-4 [5]. Finally, the reaction is an economical one, both in terms of time and energy requirements -- it proceeds rapidly at room temperature -- and in that the raw material cost of the intercalant (or its precursors) is small relative to most other intercalants. The purpose of the present study is to
Hat. Res. Soc. Symp. Proc. Vol. 20 (1983) 0Elsevier Science Publishing Co.,
Inc.
278 investigate the nature of isothermal intercalation-
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