4. Resistivity up to Melting and the Recording of Melting Area

Clue experimental data (starting with 1963 year, and up to 1996) by Francis Bundy (USA) are discussed in detail (under heating graphite by milliseconds electrical current pulse). Francis Bundy was the first who obtain imparted energy and resistivity of carbon near melting point. Bundy—may be the first who has been constructed phase diagram for carbon simultaneously with his experimental activity during many years. In spite of the fact that Bundy did not measure temperature, a thorough analysis of the experimental data (resistivity and enthalpy at different pressures, up to 100 kbar) gives him an advantage before other investigators. Experimental investigation of graphite at high temperatures under heating by pulse of electrical current within microseconds time interval actively started at the Institute for High Temperatures (IVTAN) in 1972, in Moscow, in the group headed by S.V. Lebedev (the pioneer of electrical explosion method). Specimens were made of isotropic graphite of low initial density, and of anisotropic pyrolytical graphite UPV-1T (like HOPG) of the high density. At the initial stage only resistivity and input energy were measured, but restricted volume around the specimen was used that gives a possibility to investigate heated graphite at high pressure. This method gives the estimation of the start of melting (in kJ/g units) and the heat of graphite melting (10 kJ/g) was obtained long before obtaining nearly the same value (10.5 kJ/g) under the temperature measurements. Next experiments of milliseconds heating by electrical current by M. Sheindlin with co-workers (Joint Institute for High Temperatures) are discussed. A special high-pressure chamber and fast pyrometer were used that give a start of graphite melting at ~ 5000 K at elevated pressure. It was obtained the dependence of emissivity for anisotropic graphite UPV1-TMO against temperature. The total error for temperature measurement ±4 %, error for emissivity measurements ±6 %. The original data of Ared Cezairliyan (USA) was obtained that the addition of oxygen into the chamber leads to an increase of the detected temperature of melting (oxygen reacts with the steam, forming a transparent carbon monoxide). It confirms that carbon vapor (sublimate) plays a leading role in underestimation of melting temperature measuring for graphite. The whole temperature plateau under graphite melting was obtained (for the first time) under microseconds heating of low density graphite as in Austria by Gernot Pottlacher as in Los Alsmos, USA by Robert Hixon. But the difficulties has appeared with the specimen density measurements because of low initial graphite density. The last point of this chapter devoted to the difficulties in recording melting temperature under laser heating.