Graphite sublimation pressure

White powder, hexagonal graphite-like form or cubic crystal cubic form similar to diamond in its crystal structure, and reverts to graphite form when heated above 1,700°C density 2.18 g/cm melts at 2,975°C (under nitrogen pressure) sublimes at 2,500°C at atmospheric pressure insoluble in water and acid attacked by hot alkalies and fused alkali carbonates not wetted by most molten metals or glasses. 

Data on the change in vapor pressure of carbon with temperature, obtained by measuring the rate of loss of carbon from a carbon filament heated to known temperatures, were reported by Wertenstein and Jedrzejewski,1 Brody and Millner,1-2 and Marshall and Norton.1 These data yield values ranging from —160 to —210 for the heat of sublimation of carbon at 18°. Marshall and Norton1 computed from their data the value —178 1 for the heat of the reaction, C (c, graphite) =C (g), at -273°. 

The reduction of tungsten oxides by carbon or carbon-containing compoimds can be easily performed. Statements about the starting temperature for the reaction between WO3 and solid carbon (carbon blacks, graphite) vary in the current literature between 655 °C and 783 °C. Differences in WO3 and C properties (particle size of the powders, preparation history, crystallinity, etc.) as well as in atmospheres may be responsible for that. The temperature range coincides with the begirming of self-conductivity and sublimation of WO3. Carbon monoxide starts to react witii WO3 at 535 °C (reduction pressure 1 bar, PcoJPco equilibrium ratio 8.52) [3.45]. 

The other films were produced to study the influence of the interface between the metal top contact and the organic semiconductor. Here, silicon wafers with a native oxide were used as substrates. The samples were provided by the group of Pflaum at the University of Stuttgart. To minimise impurities, like in the case of DIP, Pc (purchased from Fluka) is purified twice by gradient sublimation before being used as starting material. The films were prepared in UHV at a base pressure of about 7 x 10 mbar from a graphite effusion cell. The evaporation rate was about 3 A/s it was controlled by a quartz microbalance located next to the sample [8]. 

Rubrene was purchased from Aldrich (elemental purity > 98%) and additionally purified by gradient sublimation Freshly cleaved mica (001) was used as substrate. Rubrene thin films were deposited by hot wall epitaxy in a vacuum chamber with a base pressure below 10 Pa at different deposition rates and substrate temperatures (Ts). Pole figures were measured with a Philips X pert x-ray diffractometer using CrKa radiation and a secondary side graphite monochromator. Specular scans were performed on a Bruker D8-Discover diffractometer using CuKa radiation. POWDER CELL and STEREOPOLE were used for the evaluation of the specular scans and simulation of pole figures. 

The two-dimensional phase diagram of the monolayer of Xe adsorbed on graphite is displayed in Figs. 1 [6,8,14] and 2 [11,15,16] in two usital representations, coverage 0 vs. pressure and pressure vs. T. Fig. 2 extends the data of Fig. 1 to lower pressures and temperatures and allows the presentation of the second layer formation and of the bulk sublimation curve, as well. 

Graphite is an excellent refractory material with a melting point of about 4473°C, but it must be under a pressure of some 100 atm, otherwise it just simply sublimes. Since graphite oxidizes slowly in air at about 400°C, it should be protected in an inert atmosphere to operate successfully at elevated temperatures and, it is most important that the inert atmosphere at temperatures in the region of 2500°C is quiescent, since otherwise, the surface will be badly eroded as the gas flow sweeping across the surface continually removes the surface graphite by sublimation. 

A small-scale process has been described by Scaife and Wylie in which thorium carbide is used as the feed material. This is made by pelletting a mixture of finely divided thorium oxide with graphite and heating in a carbon resistance tube furnace above 2000°C, in an argon atmosphere. The rather pyrophoric carbide is then reacted with iodine at 500 C and 4 to 200 mm pressure, and sublimed under vacuum to give a yellow thorium tetra-iodide. This is then thermally decomposed on a tungsten filament at high temperature in the normal manner. 

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