Messungen und berechnungen zur kinetik der einlagerung von FeCl 3 in graphit

Abstract

The kinetics of the formation of graphite intercalation compounds have been studied little so far. For graphite-FeCl 3, Barker and Croft[1] measured the FeCl 3 uptake and described this process by means of Fick's law of diffusion. Their considerations were based on the idea that the intercalation starts simultaneously at the edges of all graphite interspaces and that in each interspace the concentration gradient of the FeCl 3 molecules follows Fick's law. This seems to be improbable because of the big spatial requirements of the molecules and in view of the high reaction rate we measured. We studied gravimetrically the intercalation into graphite flakes (0.5 mm dia.) with the “double-furnace-method” [6]. Single crystals were analysed afterwards by X-ray diffraction methods and by an electron microprobe. At a saturation degree of 16% (related to the final content of the 2nd stage under these conditions) the FeCl 3 is still situated in the outer areas of the crystal (Fig. 1a). However, the X-ray diffraction diagram (Fig. 2a) shows already 00 l-reflections of areas with the enlarged distance of 9.38 Å beside areas of pure free graphite. The rotation photograph of the same crystal shows hk-reflections of the normal compact FeCl 3 layer. At a saturation degree of only 20–30%, a constant FeCl 3 distribution over the whole cross-section is already observed (Fig. 1b). The X-ray diffraction diagram of this crystal shows the 00 l-reflections of the disordered 5th stage in addition to those of small amounts of free graphite (Fig. 2b). These results, especially the absence of a noticeable concentration gradient far below the saturation uptake, show that the intercalation cannot be described by Fick's law of diffusion. Therefore we propose the following model: The formation of layer nuclei at the edges of the interspace determines the rate of reaction. The nuclei grow very quickly and become layer islands, which distribute themselves uniformly throughout the interspace with considerable velocity or fill up the whole interspace. From the beginning, these islands show the final structure of the compact FeCl 3 layer. In a simple calculation we assume that the probability of nucleus formation at the edges of a graphite crystal is constant with respect to time and to all positions. For the reaction rate, we obtain eqn (2). This equation is describing the experimental data quite good up to intermediate degrees of saturation (Fig. 3). With further intercalation, however, the rate of FeCl 3 uptake decreases more and more. The following reasons can be discussed: 1. (a) The probability of nucleation decreases, perhaps on account of steric reasons. 2. (b) The growth rate of a nucleus or the mobility of an insular FeCl 3 layer between the graphite layers decreases. 3. (c) Imperfections in the crystals favour the intercalation in the beginning. An apparent activation energy of 105 kJ/mole for nucleus formation at the beginning of the intercalation results from the Arrhenius plot (Fig. 4).