The absolute vapour pressures over pure Cm CT0, fullerite and over the binary Ca/C70 system are calculated from ion intensities vs. temperature up to 800 K using the Knudsen effusion mass spectrometric method. The vapour pressures ( Since 1990 when Kratschmer and co-workers' reported a procedure for the preparation of macroscopic quantities of solid fullerenes, containing primary C, and C70, a spectacular effort has been devoted to this third allotropic form of carbon. Mass spectrometry has been the common technique used in the investigation of fullerenes from the beginning. Recently, several review papers have been published dealing with the role of mass spectrometry in the characterization of f u l l e r e n e~.~-~ In contrast to this, there are only a few papers dealing with the thermodynamics of the evaporation properties and vapour pressures of these compounds. In one of the earlier experiments' the enthalpies of sublimation of Cm and C70 in their mixtures, i.e. the fullerite composition, were estimated to be respectively 167.6 kJ/mol and 179.7 kJ/mol, though no absolute pressures were given. At the same time Mathews and co-workers6 mentioned the effect of heat pretreatment of fullerite samples on the heat of sublimation, and reported slightly higher values for this thermodynamic quantity. Later, the absolute ressure of pure C, was measured by the same group using the Knudsen effusion mass spectrometry (KEMS) method. The reported value at 700 K was 4.12 X Pa and 0.229 Pa at 800K. Over this temperature interval, the heat of sublimation was slightly revised to 181.4 kJ/mol for pure Cm. In 1992, Margrave and co-workers' measured the total pressure over C, / C70 solution as 0.11 Pa at 800K using both the 'transpiration' and the 'quartz crystal microbalance' techniques. The apparent heat of sublimation was found to be 193 kJ/mol. Early in 1993, Mathews et al.' measured the absolute pressure of pure C70 as 9.35 X low3 Pa at 750 K and calculated the corresponding heat of sublimation as 195.7 kJ/mol. All the authors mentioned the effect of heat pretreatment of the samples on the reproducibility of the results, but P Author for correspondence. none of them investigated its origin. In our work, an attempt was made to reinvestigate values for the absolute pressures of C, and C70 and to study the evaporation properties of these two compounds. EXPERIMENTALIn this work, KEMS and high resolution transmission electron microscopy (HRTEM) were used for the determination of absolute vapour pressures and structural investigations of fullerenes prior to and after evaporation measurements. A Nier-type low resolution mass spectrometer was used to detect and analyse the molecular beam effusing from the Knudsen cell.Molecules were ionized by electron bombardment. To ensure extremely stable and reproducible ion currents, the anode electron current was held constant at 20.00 f 0.05 PA, independent of the energy of the ionizing electrons which was also held constant (30 eV) during the measurement. Positive ions were accelerated to a po...
The analytical description of excess functions was made possible using more advanced treatment of experimental data with respect to literature.
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