Graphite Melting Research Articles

Explicit Gibbs free energy equation of state applied to the carbon phase diagram

We provide a simple explicit form for the Gibbs free energy $G(P,T)$ of diamond, graphite, and liquid carbon. The Gibbs free energy function is shown to reproduce the known equation of state properties of carbon up to pressures of 600 GPa (6 MBar) and temperatures of 15 000 K. Recent experiments on graphite melting suggest the presence of a first-order liquid-liquid phase transition at roughly 6 GPa. We show that such a transition is consistent with shock compression data at higher pressures. We reanalyze experiments on the diamond-liquid melting line with our equation of state. Our analysis suggests that the diamond-liquid melting line may have a more positive slope as a function of pressure than previously estimated. A maximum in the diamond melting line is predicted by the model, in agreement with recent ab initio simulations.

Liquid-Liquid Phase Transformation in Carbon

A first order phase transition is found in liquid carbon using atomistic simulation methods and Brenner's bond order potential. The phase line is terminated by a critical point at 8801 K and 10.56 GPa and by a triple point on the graphite melting line at 5133 K and 1.88 GPa. The phase change is associated with density and structural changes. The low-density liquid is predominantly $\mathrm{sp}$ bonded with little ${\mathrm{sp}}^{3}$ character. The high-density liquid is mostly ${\mathrm{sp}}^{3}$ bonded with little $\mathrm{sp}$ character. This is the first nonempirical evidence of a liquid-liquid transition between thermodynamically stable fluids.

Graphite melting and properties of liquid carbon

The results of fast heating (by electrical pulse current) of highly oriented pyrolytic graphite specimens are presented. Experimental data are obtained for graphite melting: the enthalpy of solid and liquid phases under melting, the heat of melting, and the heat capacity of solid and liquid phases near melting. Liquid carbon resistivity is measured under fast heating of cylindrical graphite specimens in thick-walled sapphire capillary tubes. Preliminary data for the isobaric heat capacity Cp for liquid carbon up to 10,000 K and for the isochoric heat capacity Cv up to 8000 K are presented.

The molten state of graphite: An experimental study

A subsecond laser heating technique has been successfully applied for graphite melting under controlled isobaric conditions. During the applied pulses, relatively large amounts of graphite were melted and subsequently solidified under good stability conditions of the liquid mass. The solid–liquid-vapor triple point was determined. From metallographic analysis of the quenched liquid, the expansion upon melting could be estimated. A mathematical model was then applied to analyze the measured thermograms and the thermal conductivity of liquid carbon was deduced. Both experimental observations and calculation results indicate a nonmetallic nature of liquid carbon in the pressure range of 110–2500 bar. Finally, an analysis of the melting line Tm(p) based on Simon’s empirical equation of state confirms the self-consistency of all results obtained.

Formation of amorphous carbon on melting of microcrystalline graphite by picosecond laser pulses

Observations of microcrystalline graphite subjected to picosecond laser pulses reveal the formation of a liquid phase with a subsequent transition to a uniform amorphous state of a surface layer upon solidification. This phenomenon is observed on a definite type of graphite and with the radiation incident on a plane parallel to the sixfold symmetry axis, and only for certain parameters of the laser pulse. A structural analysis of the amorphous phase is performed by electron microscopy and Raman scattering spectroscopy. A periodic structure with a period of the order of the wavelength of the heating pulse is formed in the heating region. The “rulings” of this periodic structure are oriented in the direction of polarization of the heating pulse. A study of the reflection kinetics of the probe laser pulse showed that the characteristic existence time of the liquid phase and of the solidification process is ∼10−10 s.

Entropy of melting: Its effect on the liquid-liquid phase change of carbon

Graphite melting experiments reported recently suggest that the enthalpy of melting of graphite decreases slowly and linearly with pressure, unlike our previous suggestion, which included a liquid-liquid phase change like those observed in other systems. We have investigated the possibility that this smooth pressure dependence could eliminate the liquid-liquid phase change. Our analysis shows that the available experimental enthalpies of melting cannot rule out such a phase change. An accurate determination of the (P,T) melting line curvature can shed light on the presence or absence of such a phase change.

Tribology at high temperatures

The paper reviews publications, primarily of the Institute of Machinery Science of the Russian Academy of Sciences. The techniques of investigations and the results are briefly outlined as to how various high-melting point materials withstand friction in vacuum and in gaseous atmospheres (within a wide temperature range) in like or unlike combinations, such as metals, metal-like compounds, oxides, graphite materials and molybdenum disulphide. The results of measurements of adhesion, including that between diamond and various metals, are given. The friction drop of graphite material on corundum ceramics caused abnormally by tribochemical interaction, is described. The ‘eutectic wear’ accompanied by a sharp friction reduction is based on an investigation of the phenomenon of eutectic melting of diamond and graphite with experimental validation. The results of studies of vacuum ion-plasma deposited CrO and MoS 2 coatings are demonstrated and a technique is suggested as to how to make the friction of MoS 2 very low.

The Pressure-temperature Phase and Reaction Diagram for Carbon

ABSTRACTCarbon atoms form very strong bonds to each other, yielding materials like: (i) crystalline graphite, diamond and their many “amorphous” hybrids; (ii) crystalline forms of giant closed–surface molecules such as the fullerenes; and (iii) liquid and gas phases which have molecular contents which are complicated and not yet defined or understood. Because of the high bonding energy the melting and vaporization temperatures of the solid forms are very high, and the activation energies required to transform one solid form to another are large. One consequence is that at lower temperatures the different solid phases may continue to exist metastably far into aP, Tregion in which another solid phase is the thermodynamically stable one.In the thermodynamic sense the vapor pressure line of graphite, the graphite/liquid/vapor triple point, the graphite melting line, the graphite/diamond equilibrium line, and the graphite/diamond/liquid triple point are quite well established. Data for the melting temperature of diamond vs. pressure are sparse and rough, but they indicate that the melting temperature increases with pressure,-in agreement with some theories. Although carbon should transform to a solid metallic state at very high pressures, experimental evidence shows diamond to be stable to over 400GPa, and theoretical calculations indicate that it could be the stable form up to pressures of 1200 to 2300GPa. Attention is given to the solid state transformations which can take place when graphite is compressed and heated along differentP, Tpaths under different conditions.

Wagner interaction coefficients and modified margules equations

The relationships between the Wagner interaction and Margules coefficients are derived to discuss their limitations and to clarify incorrect construction of the excess Gibbs energy from the partial excess Gibbs energies, expressed as linear functions of mole fractions. Margules-type equations with Henrian reference states are obtained, and their significance is discussed. An estimate for the Gibbs energy of melting of graphite is obtained, and the use of activity coefficients for deriving the Gibbs energy of melting of refractory elements is suggested. This research is part of the effort in materials properties at the U.S. Bureau of Mines.

Comments on the activity of carbon in liquid iron

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High-pressure liquid-liquid phase change in carbon.

The likelihood of a first-order liquid-liquid phase change in carbon from a mostly graphitic configuration to one that is more tetrahedral in character is explored. Liquid-phase changes in other materials are noted. Pertinent data on liquid carbon are reviewed and the model is briefly described. The model is consistent with a positive diamond-melting-line slope. Constraints on the strain energy between graphitic and diamondlike liquid clusters allow a phase diagram with a liquid-liquid transition and a graphite-liquid-liquid triple point. An upper limit on the strain energy is the constraint that the graphite melting line should have a reasonable curvature that agrees with melting-line data. The transformation of liquid structure from graphitic to diamondlike under compression significantly increases the compressibility.

Graphite melting under laser pulse heating

For a long time, the solid/liquid/vapor triple point of graphite has been a subject of debate, and despite the progress in the experimental techniques adopted, the uncertainties became even greater in recent years. In the experiments described in this paper spherical graphite specimens were heated by four tetrahedrally oriented laser beams to produce a fairly uniform temperature on the surface. A sufficiently large amount of molten graphite was produced to make it possible to identify the liquid/solid transition by a conventional thermoanalytical method. The temperature was measured by multichannel pyrometry, which enabled a detailed analysis of the perturbations and errors to be carried out. The triple point of graphite was determined to be Tt=4100±50 K and pt=110±10 bar.

Picosecond laser heating and melting of graphite

Ultrashort laser pulses impinging onto solid, strongly absorbing surface deposit their energy within an absorption depth from the surface. This localized energy deposition may result in rapid and very efficient heating of the surface to temperatures far exceeding the melting or boiling point. Temperature evolution at the surface of samples and their electronic structure may be studied with nonperturbing, time-resolved optical diagnostic techniques. Picosecond laser pulses provide the fastest means for heating matter at high temperature, since the characteristic energy transfer times from photoexcited electrons to the lattice occur in this time scale. Surface evaporation is not affecting the observations in this time scale, simply because there is no time for the surface atoms to escape. As an illustration, measurements on graphite are presented here. The complex index of refraction of highly oriented pyrolytic graphite (HOPG) during picosecond laser irradiation has been measured at 1.06 μm via time-resolved ellipsometry at angles of incidence up to 80∘. In particular, a value of the complex index of refraction for the liquid phase has been derived.

Femtosecond Laser Melting of Graphite and Diamond

ABSTRACTFemtosecond time-resolved reflectivity measurements performed on highly oriented pyrolytic graphite (HOPG) and diamond elucidate the nature of the phase transition from solid to liquid carbon. In HOPG, we find that a high-reflectivity phase lasting as long as 10 ps appears when the surface is irradiated with pulse fluences in excess of 0.13 J/cm2, the critical fluence for melting. This transforms within 30 ps into an equilibrium low-reflectivity phase lasting hundreds of ps, similar to behavior observed in picosecond reflectivity experiments. The results suggest the occurrence of a two-step phase transition (graphite -> liquid metal -> liquid insulator) when HOPG is excited above the critical fluence. Similar results are obtained with diamond.

A model for pulsed laser melting of graphite

A model for laser melting of carbon at high temperatures to form liquid carbon has been developed. This model is solved numerically using experimental data from laser irradiation studies in graphite consistent with a melting temperature for graphite of 4300 K. The parameters for high-temperature graphite are based on the extension of previously measured thermal properties into the high-temperature regime. A simple classical free electron gas model is used to calculate the properties of liquid carbon. There is very good agreement between the model calculation and experimental results for laser pulse fluences below 2.0 J/cm2. Modifications to the model for larger laser pulse fluences are discussed.

Experimental measurements of melting and vaporization due to simulated plasma disruptions

The first wall and limiter surfaces of magnetic confinement fusion devices will be subjected to intense pulsed thermal loads due to plasma disruptions which can result in vaporization and melting limiting the lifetime of these components. This paper presents experimental data on the vaporization and melting of AXF-5Q graphite, Type 316 stainless steel, OFHC copper and tantalum due to thermal loads varying up to 2400 J/cm2 applied for times between 1 and 50 ms. The surface thermal loads were simulated with a 120-keV electron beam in the electron beam surface heating (ESURF) facility. The analysis procedure consisted of determining vaporization losses through weight-loss measurements and evaluation of the melt zone thickness through a metallographic sectioning procedure. Comparisons with some of the more recent theoretical predictions are made. The data trends (i.e., increasing melt and vaporization with energy density, existence of threshold energy densities, and melt thickness limits at high energy density due to increased vaporization) are generally found to agree with the analytical predictions. However, the magnitude of the melt zone thicknesses and vaporization losses show differences significant enough to warrant investigation of both modeling assumptions and the experimental techniques and conditions.

Pulsed Laser Melting of Graphite

ABSTRACTExperimental evidence for laser melting of graphite, by irradiation with 30ns pulses from a ruby laser, is presented. RBS-channeling analysis, Raman scattering and TEM measurements reveal that the surface of graphite melts at a threshold energy density of about 0.6 J/cm2. For laser pulse energy densities above 0.6 J/cm2, the melt front penetration depth increases nearly linearly with increasing energy density. An intense emission of carbon particles during and after irradiation is observed. The thickness of the carbon layer removed in this process also increases nearly linearly with increasing pulse fluence. A dramatic redistribution of ion implanted impurities is also observed. Furthermore, the crystalline structure of the resolidified material is shown to depend on the energy density of the laser pulse. In order to explain these phenomena, a model for laser melting of graphite at high temperatures to form liquid carbon has been developed in which a free electron gas approximation is used to describe the properties of liquid carbon. The model is solved numerically to give the time and depth dependences of the temperature as a function of the laser pulse energy density. Very good agreement is found between the observed melt depth dependence on laser pulse energy density, as determined by RBS-channeling, and the model calculations. The redistribution of ion implanted impurities and the modification of the crystalline structure, caused by the pulsed laser irradiation, are also consistent with the model and permit the determination, for the first time, of interfacial segregation coefficients for impurities in liquid carbon. The model also predicts that liquid carbon at low pressure (p < 1 kbar) has metallic properties.

A study of limiter damage in a magnetic field error region of the ZT-40M experiment

A study has been initiated of material plasma interactions on the ZT-40M, Reversed Field Pinch (RFP) plasma physics confinement experiment at Los Alamos National Laboratory. Observations of the evaporation and cracking of TiC coatings, initially placed on an AXF-5Q Graphite mushroom limiter, installed in a high field error region (e.g. an experimental vacuum vessel/liner port) were investigated. A parametric study was performed of the thermal and stress behavior of the limiter and coating materials undergoing plasma material heat exchange processes, in order to infer the magnitude of heat flux necessary to explain the observed material damage. In addition the vacuum (liner) wall material behavior was studied parametrically using the same heat flux values as the limiter study. A one-dimensional conduction model was used with applied heat and radiation boundary conditions, for predicting temperature distributions in space and time, where the thermal stress was calculated using a restrained in bending only plate model. Wall loadings corresponding to first wall, limiter energy fluxes ranging between 1 × 10 2 W/cm 2 and 1 × 10 5 W/cm 2 were used as parameters with plasma material interaction times (τ QO) between 0.5 ms and 10 ms. Short plasma energy deposition time (τ QO ⩾ 10 ms) spacial and time histories of temperature and stress were calculated for SS-304, Inconel-625, TiC and AXF-5Q Graphite materials. The parametric study indicates that for τ QO ∼ 6 ms a wall loading of ⋍ 1 × 10 4 W/cm 2 results in the melting of Inconel and SS-304 first wall material and a loading of ⩽ 5 × 10 4 W/cm 2 results in the melting of TiC and Graphite. For Inconel, stresses in excess of the tensile strength occur for Q O greater 1 × 10 3 W/cm 2 . The same is true for SS-304. For Graphite AXF-5Q the tensile strength is exceeded for Q O ≳ 1 × 10 4 W/cm 2 . From the observations of damage and the parametric results calculated it can be inferred that an energy flux of 1 × 10 4 W/cm 2 ≲ Q O ≲ 5 × 10 4 W/cm 2 was observed over a time scale of 5 ms ≲ τ QO ≲ 10 ms in the field error region, if the model considered is relevant to the phenomenon in question.

Investigation of Carbon Vapor Pressure at Very High Temperatures and Pressures with the Aid of Laser Heating

For several decades the behavior of carbon and carbonaceous materials has been observed with unvarying interest. Well known are two polymorphous modifications of carbon, namely, graphite and diamond, each possessing unique physicochemical properties which make them attractive for extensive utilization in numerous industries.The history of carbon studies at high temperatures and pressures produced quite a few examples of ingenious experimental techniques and experimenter's skill, however, many of those studies only resulted in the emergence of new, hard-to-solve problems. This is precisely the case with the melting and vaporization of graphite.

Melting of Graphite at Very High Pressure

A method of flash-heating small rods of graphite inside a superpressure cell has been developed. The heating energy was inserted in less than 7 msec from a bank of electrolytic capacitors. This quick heating and cooling allowed fusion and freezing of the graphite to occur without serious melting or reaction of the surrounding wall material. Electrical data were recorded with oscillographs and cameras. The start of melting was found to be indicated by an abrupt downward trend of resistance. Polished cross sections of the samples showed clearly the part which melted. Melting temperatures increased from about 4100°K at 9 kbar to a maximum of about 4600°K in the region of 70 kbar, then decreased to about 4100°K at 125 kbar. A value of 25 kcal/mole for the heat of fusion at 48 kbar was determined. The graphite/diamond/liquid triple point is shown to be at about 4000 to 4200°K and 125 to 130 kbar.