Carbon at High Temperatures

Astrophysical and terrestrial aspects of carbon investigations are discussed. Carbon and diamond structures are shown in comparison. A practical role of carbon in industry is underlined. The clue steady state experimental studies (starting with 1911 year), was discussed up to 1963 year. Particularly the estimation data of Leipunskiy (Russia) are shown to obtain diamond at high pressure (more than 40 kbar) for carbon with dissolved iron and at elevated temperature (higher 1500 K).

The carefully executed experiments by Ared Cezairliyan (USA) are reviewed. Cezairliyan obtained resistivity and specific heat of graphite grade POCO up to 3000 K. The specimens were made of commercial graphite POCO; electrical resistivity and heat capacity were obtained against temperature. Temperature was measured with the help of a blackbody model under millisecond current heating (heating rate—from 600 up to 6500 K/s).

Determination of parameters (P, T) for the triple point of carbon (solid–liquid–gaseous) began in 1939 (Basset), and continued in 1959 (Noda). But the impressive result was obtained only in 1976 (Gokcen) for the pressure at the triple point of carbon (∼120 bar). Study of Shoessov at pressures up to 1000 bar, however, had no progress in temperature measurement. The study only by Haaland (1976) [18] gave a truthful value for the melting temperature (close to 5000 K). In addition, Haaland gave a brief analysis of the main results on different high-temperature carbon investigations, noting that the vapor (more precisely, carbon sublimate) affect the quality of the temperature measurements. Later researchers have taken a number of measures to avoid that of the steam jet (sublimate) has appeared in the optical path under temperature measurements. The determination of specific input energy under beginning of melting, enabled investigators to interpret the accuracy of the temperature measurements near the melting point of graphite. Millisecond heating current (M. Sheindlin) allowed us to obtain reliable dependence of enthalpy (input energy) against temperature of the graphite in the solid state up to 4500 K. This experiment is discussed in this chapter. Experiments of different duration (from seconds to nanoseconds) showed no dependence of the melting temperature on the heating rate (Table 3.3). The results of the various experiments performed as under heating by current or by laser heating give the matching results on the melting temperature of graphite (4800–4900 K), provided that the pressure is maintained above 110 bar.

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.

Experimental steady state data (by Buchnev et al.) on the enthalpy against temperature (steady state experiments up to 3800 K, and further calculation up to 4900 K) are discussed. These measurements is stated as the more reliable data among stationary investigations. The resulting data are close to the measurements under fast heating by pulse of electric current. The possible high expansion (50–70 %) of carbon under melting was obtained in some experiments.

The main discussed results were obtained at the high pressure (14–94 kbar) under slow (millisecond) heating by electrical current (Togaya et al.) during 1997–2010 years. In spite of the fact that only resistivity and enthalpy were measured the authors gave an important conclusion on the appearing liquid state of carbon and on the dependence of resistance versus pressure. Liquid carbon (at the melting point) diminishes its resistance at a pressure lower 50 kbar, and resistance rises at a pressure higher 50 kbar. American scientists (Gathers et al.) obtained experimental data under microseconds heating, that graphite is compacted even in a solid phase, reaching its maximum density near the input energy ∼7.5 kJ/g (that is before melting). Thus, graphite of low initial density compacts to the maximum density (2.2 g/cm³) in the time interval shorter than 1–2 microseconds, and then reaches melting point just as usual graphite of high initial density. The third part of this chapter (also microseconds heating) shows the behavior of resistance for different graphite grades under thermo-compression in sapphire capillary tubes. Korobenko and Savvatimskiy (JIHT, Russia) obtained the data on expansion of carbon at fast microseconds heating and estimated the resistivity of carbon near melting with an expansion included. A detailed data on resistivity versus input energy under fast heating of isotropic graphite MF-307 (Japan production) are discussed. It was mentioned a critical sensitivity of graphite to the start of melting under rising pressure. It was discussed the role of pinch pressure for graphite melting (except its surface). Discovered a critical sensitivity of graphite melting to the magnitude of applied pressure (experiments for graphite sticks heated inside sapphire capillary tubes).

Two installations are shown: with a small current pulse (upper limit 15 kA) for training students and the largest one with a high current pulse (upper limit 400 kA). The features of these two systems discussed in all the details that are the most important to the experimenters. It was given a choice of a blackbody design and fast pyrometer for recording melting and liquid state of carbon under fast heating in JIHT. A detailed description of the arrangements in pulse electrical heating is shown. Pulse heating results on measuring carbon temperature (up to 12,000 K) are shown.

Several diagram of Bundy are considered up to 1994 year, starting with combined diagram for C, Si and Ge (1964 year). The contribution to phase diagram by Motohiro Togaya for bulk carbon and modified diagram for nanocarbon of other investigators are also shown. A significant part of this chapter deals with the computational work on the melting of graphite and phase diagram of carbon, including high pressure level. Modeling of carbon phase diagram and the agreement with the experiments are discussed, including the new publications of 2014–2015 years.

The advantages of fast carbon heating to measure properties at high temperatures and some practical applications of carbon heating are considered. It was commented a complicated study of converting diamond—to liquid carbon under high pulse pressure with temperature recording (nanosecond experiment by Jon Eggert et al., USA). The more high pressure (terapascal value) is obtained in 2015 year with laser energy up to 0.76 MJ in a 20-ns time to convert diamond to the highest pressure (experiment by R.F. Smith et al. USA).

The calculation of Graphene melting is shown that gives melting temperature of one carbon layer equals 4900 K. It was shown the broad experimental and estimated methods to investigate Graphene: theory and the experiments of Alexander Balandin (USA). It was mentioned the first experiments of measuring melting temperature of the HAPG graphite under fast electrical heating in 2015 year.

Carbon has many different forms; it is durable and well handled. This ensures its application in many industries (engineering, aviation, aerospace, electronics and medicine). Due to the high melting temperature (4900 K) a study of the liquid state of carbon is only possible in the pulse heating processes. Liquid carbon is the area of our new knowledge and possible use in future pulse technology. In addition to carbon it is required for industry the knowledge of the high-temperature physical properties of carbon compounds, such as carbides. High temperature properties of these refractory compounds (as well as graphite),–are important for practice, because refractory carbides and nitrides are included in the coating composition used in gas turbine tracts and nozzles of aircraft, protective plates of missile systems, matrix of nuclear fuels.