Sublimation Of Graphite Research Articles

7507. Radiation-enhanced sublimation of graphite in PISCES experiments: R E Nygren et al,J Vac Sci Technol, A8, 1990, 1778–1782

7507. Radiation-enhanced sublimation of graphite in PISCES experiments: R E Nygren et al,J Vac Sci Technol, A8, 1990, 1778–1782

The effect of pyrolytic carbon coating on gas emission and sublimation of graphite induced by pulse laser beams

The emission and sublimation of graphites coated with pyrolytic carbon (PyC) has been measured under irradiation with pulse laser beams to examine the effects of pulse heat load on the role of the coating. Under a heat load density of 640 MW/m 2 and a pulse length of 1 ms, gas emission, sublimation and vacuum degradation are enhanced by the PyC coating. This is mainly due to very low thermal conductivity of the PyC film in the surface normal direction resulting in an enhanced temperature rise during the heat load. On the other hand, the gas emission, sublimation and vacuum degradation of the coated specimens were small relative to the non-coated specimens when compared on the basis of the measured peak surface temperature. Thus the surface coating with PyC may be effective in reducing the gas emission when the carbon temperature increases gradually but can be harmful to vacuum and surface stability in the case of pulse high heat load.

Low energy data on radiation enhanced sublimation of graphite

Erosion of POCO graphite by helium in PISCES-A was measured by carbon spectroscopy at temperatures from 550°-2000°C, energies of 30–250 eV and fluxes of 1−2×10 22 m −2 s −1, Yields at low energies were higher than predicted in current models. The effect of redeposition is discussed.

Radiation-enhanced sublimation of graphite in PISCES experiments

Radiation enhanced sublimation (RES) of graphite leads to potentially high erosion rates in advanced fusion devices (Compact Ignition Tokamak, CIT) and (the International Thermonuclear Experimental Reactor ITER) and may be important in the ‘‘carbon blooms’’ observed in Tokamak Fusion Test Reactor (TFTR) and the Joint European Torus (JET). Erosion rates measured by weight loss are reported for POCO graphite exposed to helium plasmas for a temperature range from 900–2000 °C, ion energies of 30–300 eV, ion fluxes of 1–6×1018 cm−2 s−1, densities of 2–10×1012 cm−3, and electron temperatures of 4–10 eV. These are the first data on RES at fusion-relevant particle fluxes. The data show little reduction from a direct linear dependence upon flux. For these conditions, the amount of redeposition and carbon self-sputtering was minimal. Over 1700 °C, there is evidence of electron emission from the sample.

Flux dependence of radiation induced sublimation of graphite at elevated temperatures

Above 1200 K ion bombardment of graphite results in an anomalous radiation induced sublimation (RISU) which has been attributed to the diffusion of C self-interstitials to plane graphite surfaces. In the present paper, new experimental results on the RISU yield are presented with special emphasis on its flux dependence. In the main parameter range investigated (5 keV Ar +, 1800 K, 5 × 10 16 to 3 × 10 19 Ar + m −2 s −1 ) the yield is about 5 and appears to be independent of the ion flux. Simple models based on the assumption that the RISU yield is controlled by the internal sink density for interstitials predict a correlation between flux dependence, temperature dependence and absolute values of the yield which is not confirmed by our experimental data. Possible explanations for this discrepancy are discussed. It is concluded that the RISU yields to be considered for fusion reactor conditions (10 22 to 10 23 ions m −2 s −1) are comparable with those observed under present ion beam conditions.

Material technologies for fusion reactor. In-vessel materials.

Chemical sputtering and radiation-enhanced sublimation of graphite have been the subject of many intense researches. Chemical form of desorbing species and synergism between thermal atoms and energetic atoms are among the controversial problems of surface erosion of graphite. Self-sustaining surface segregated low-Z layers have been proposed as a new approach to impurity control. Hydrogen isotope dynamics between plasma and first wall is very important in controlling the plasma, and hence the fusion reactor itself. In this sense, surface recombination coefficient of hydrogen must be evaluated for various materials with various surface conditions. Its theory and surface impurity effect are described.

On the influence of impurities on the high-temperature sputtering yield of graphite

The influence of near-surface impurities on the enhanced sputtering of graphite near 600°C and above 1000° Chas been investigated under various doping conditions. The chemical sputtering yield of methane molecules for 2 keV D+ ions on graphite at 600°C has been determined for several coverages of Si, Fe, Ni, Ti, Mo, and Au. Graphite samples were pre-deposited with ~ 500 Å of a given impurity, and weight loss was monitored in-situ using a vacuum balance while sputtering through the deposited layer. Small amounts of impurities remaining on the surface after prolonged sputtering reduced chemical erosion by 30–50%. The radiation-enhanced sublimation of graphite observed for energetic ion bombardment above 1000°C was investigated during 50 keV Ar sputtering and simultaneous Ti evaporation. For very low surface concentrations of Ti, graphite sputtering increased while for concentrations above 10 at% the yields were significantly reduced. A graphite sample containing 4 at% bulk Si was also investigated and showed greatly reduced chemical erosion at 600° Cbut no reduction in radiation-enhanced sublimation at 1500°C due to surface enrichment and depletion of Si for the low- and high-temperature irradiation, respectively.

351. Experimental study of the sublimation of graphite at high temperatures: P D Zavitsanos,J Chem Phys, 59 (6), 1973, 2966–2973 ( in English)

351. Experimental study of the sublimation of graphite at high temperatures: P D Zavitsanos,J Chem Phys, 59 (6), 1973, 2966–2973 ( in English)

Experimental study of the sublimation of graphite at high temperatures

The equilibrium vaporization of pyrolytic graphite was studied using rf induction heating and a time-of-flight mass spectrometer. rf induction heating was demonstrated for the first time as a compatible and practical heating technique for high temperature mass spectrometry. Relative ion intensities were measured for C1(g), C2(g), C3(g), and C4(g) in the temperature range 2320 K-3000 K and partial vapor pressures were obtained. The vapor pressures of C2 and C3 in the temperature range 2400 K-3000 K can be represented by logP(C2) (atm) = 9.971 − 44 230/T(K) and logP (C3) (atm) = 9.944 − 40 670/T(K). Heats of vaporization obtained by the second and third law methods are as follows: SpeciesSecond law ΔH°298(kcal/mole)Third law ΔH°298(kcal/mole)C1170.3 ± 1.1C2206.0 ± 1.3202.7 ± 0.6C3194.4 ± 1.2202.1 ± 1.5C4242.7Thermodynamic functions calculated by Strauss and Thiele were used forC3. JANAF thermodynamic data were used for all other species. The entropy change in the vaporization of C3 determined from this work is 45.55 eu at 2740 K.

Spectres optiques de lames minces de carbone. effets de traitments thermiques

The optical study of turbostratic carbons was undertaken after having succeeded in making transparent carbon films of different thicknesses. The fundamental optical spectra ϵ 1 and σ have been plotted as a function of the energy hω of the incident photons in the infrared range. The X-ray study has shown that the sublimation of graphite in vacuum leads to a real ‘degraphitization’ if the vapour is collected on a cold surface; such films have optical properties quite different when compared with graphite. This type of carbon can be compared to a semiconductor with a gap of 0·2 eV but it contains some permanent free carriers. When these films are heated in vacuum up to 900°C, their structure changes into an oriented model of turbostratic carbon (1000°C). Their electronic properties can be compared with those of graphite: the optical conductivity σ comes close to that of graphite while ϵ 1 takes lower (and always negative) values than before the heat treatment. This fact shows that the free carriers are more numerous than in graphite, because of the presence of a great number of defects which act as electron traps. These results agree fairly well with the schema proposed by Mrozowski for graphitizable carbons.

Particle emission during the sublimation of graphite

Particle emission during the sublimation of graphite

Time dependence of carbon species distribution during the sublimation of graphite

Time dependence of carbon species distribution during the sublimation of graphite

Asymptotic Solution for Viscous, Absorbing, Emitting Shock Layer

16 Rhinehardt, J., private communication, Dec. 1969, Air Force Weapon-Lab. Albuquerque. 16 Langmuir, I., Reactions at Low Pressures, Proceedings of the American Chemical Society, 1915, p. 1139. 17 Matting, F. W. and Chapman, D. R., Analysis of Surface Albation of Noncharring Materials With Description of Associated Computing Program, TN-D-3758, 1966, NASA. 18 Hirschfelder, J. O., Curtiss, C. F., and Bird, R. B., Molecular Theory of Gases and Liquids, Wiley, New York, 1954, p. 283. 19 Altman, R. L., Unpublished Paper, 1970, NASA. 0 Fogaroli, R. P. and Brant, D. N., Re-Evaluation of Thermochemical Ablation, Fundamentals Memo TFM-9151-060, 1968, Thermodynamics Lab., General Electric. 21 Doltori, T. A., Maurer, R. E., and Goldstein, H. E., Thermodynamic Performance of Carbon in Hyperthermal Environment, AIAA Paper 68-754, Los Angeles, Calif., 1968. Schoessow, G. J., Graphite Triple Point and SolidusLiquidus Interface Experimentally Determined up to 1000 Atmospheres, CR-1148, 1968, NASA. 23 Zavitsanos, P. D., Experimental Study of the Sublimation of at High Temperatures, Final Report Contract AT(929-1 >789, 1969, Sandia Labs., Albuquerque. 24 Boehringer, J. C. and Spindler, R. J., Radiant Heating of Semi-Transparent Materials, AIAA Journal, Vol. 1, No. 1, Jan. 1963, p. 84.

Particle emission during the sublimation of graphite

Particle emission during the sublimation of graphite

A study of radiation properties of oxide-metal composites.

Simulation of Convective and Radiative Entry Heating, Advances in Hypervelocity Techniques, edited by A. M. Krill, Plenum, New York, 1962. 5 Marvin, J. G. and Pope, R. B., Laminar Convective Heating and Ablation in the Mars Atmosphere, AIAA Journal, Vol. 5, No. 2, Feb. 1967, pp. 240-248. 6 Wilson, G. R. (compiler), Thermophysical Properties of Six Charring Ablators from 140° to 700 °K and Two Chars from 800° to 3000°K, TN D-2991, Oct. 1965, NASA. 7 Parker, J. A., Needs for Characterization and Process Control of Low-Density Composites for Aerospace Application, presented at International Society of Plastics Institute, New York, June 1966. 8 Vojvodich, N. S. and Pope, R. B., /The Influence of Ablation on Stagnation Region Convective Heating for Dissociated and Partially Ionized Boundary Layer Flows, Proceedings of the 1965 Heat Transfer and Fluid Mechanics Institute, edited by A. F. Charwat et al., Stanford University Press, Stanford, Calif., 1965. 9 Pope, R. B., Stagnation-Point Convective Heat Transfer in Frozen Boundary Layers, Paper 68-15, Jan. 1968, AIAA. 10 Scala, S. M. and Gilbert, L. M., Sublimation of Graphite at Hypersonic Speeds, AIAA Journal, Vol. 3, No. 9, Sept. 1965, pp. 1635-1644. 11 Wakefield, R. M., Lundell, J. H., and Dickey, R. R., The Effect of Oxygen Depletion in Gas-Phase Chemical Reactions on the Surface Recession of Charring Ablators, Paper 68-302, April 1968, AIAA. 12 Emmons, H. W. and Leigh, D., Tabulation of the Blasius Function with Blowing and Suction, Combustion Aerodynamics Lab. Interim TR9 Nov. 1953, Div. of Applied Science, Harvard Univ.

Oxidation and sublimation of graphite in simulated re-entry environments.

Graphite thermochemical response tested in simulated environment for use as thermal shield for hypersonic reentry vehicles

Sublimation of graphite at hypersonic speeds

In this paper, a new theoretical model is presented for the sublimation of graphite at hypersonic flight speeds. The aerothermochemical interactions between dissociated air and graphite are treated by means of a nine-component model, including O, O2, N, N2, CO, CO2, C, C3, and CN. The mass-transfer rate, the heat-transfer rate, and the skin-friction coefficient are determined numerically and are then correlated by means of algebraic equations as a function of stagnation pressure, stagnation enthalpy, and wall temperature in the high-Reynoldsnumber stagnation-point flow regime.

Dissociation Energy of the CH Radical

The 4300-Å band of CH has been observed in emission using a carbon furnace above 2400°C which contains 1 atm of H2 gas. From the CH emission intensity variation with temperature and a thermochemical cycle involving the heat of sublimation of graphite, a CH dissociation energy of 80±5 kcal (3.5±0.2 eV) is derived for the 2Π state. The close agreement between this value and that obtained previously from a CH rotational predissociation, establishes not only that the CH dissociation energy is 80.0 kcal (3.47 eV) but further confirms that the heat of sublimation of graphite is 169.58 kcal. For this 2Π state the atomic dissociation products are identified experimentally as ground-state C(3P) and H(2S) atoms. In addition, for the process C2H2=2CH an acetylene dissociation energy of 228.1 kcal is derived.

Dissociation Energy of Cyanogen and Related Quantities by X-Ray Densitometry of Shock Waves

Density ratios across shock waves in a 0.85 Kr+0.15 C2N2 mixture at an initial pressure of 50 mm Hg and room temperature, have been determined with an x-ray densitometer as a function of shock velocity. The heat required to dissociate cyanogen into two CN radicals D(C2N2) has been determined to be 145±6 kcal/mole by comparing the experimental data with curves of density ratio vs shock velocity calculated as a function of D(C2N2). Dissociation energies of 174±3 kcal/mole for CN and 129±3 kcal/mole for HCN forming H and CN, and a heat of formation of 109±3 kcal/mole for CN, were obtained by the application of Hess's law to the appropriate chemical reactions using this value of D(C2N2) and the currently accepted values for the dissociation energy of nitrogen (225 kcal/mole) and the heat of sublimation of graphite (170 kcal/mole). The value of D(HCN) was confirmed by analogous density-velocity measurements on shock waves in a 0.85 Kr+0.15 HCN mixture. A rate constant for the recombination of CN to form C2N2 at 2900°K was deduced from the variation of density with time behind the shock. The value obtained was of the order of 1×109 (mole/liter)—2 sec—1.

Dissociation Energies of Diatomic Molecules

Force constant, dissociation energy, and equilibrium internuclear distance in a sequence of similar diatomic molecules have been found to be related by the expression kere/D0=const. The applicability of this relationship has been shown for eighteen sequences of similar molecules. With the help of this relationship, the dissociation energies of several important diatomic molecules have been determined. Evidence has been adduced in favor of the value 170 kcal for the heat of sublimation of graphite.