A STUDY ON THE THERMAL STABILITY TO 1000°C OF VARIOUS CARBON ALLOTROPES AND CARBONACEOUS MATTER BOTH UNDER NITROGEN AND IN AIR

ABSTRACT

The thermal behavior of graphite, C₆₀ fullerene, fullerene black (carbon soot containing fullerenes), extracted fullerene black and diamond has been analyzed to 1000°C by TGA–DTA (thermogravimetric analysis and differential thermal analysis) under a nitrogen flow at a heating rate of 20°C/min. Very small weight losses have been recorded in the case of graphite and diamond. Furthermore no diamond graphitization has been observed. The sublimation of pure C₆₀ and the fullerene fraction of fullerene black (both pristine and extracted) has been observed and discussed.

The combustion reaction in air flow of graphite, C₆₀ and C₇₀ fullerenes, fullerene black (both unextracted and extracted), carbon nanotubes and diamond has been studied by TGA–DTA at a heating rate of 20°C/min. C₇₀ fullerene and fullerene black have been found to be the most reactive carbon materials with O₂. The role played by C₇₀ in the degradation of fullerites has been discussed. Among the carbon materials examined, the best resistance to O₂ attack has been shown by diamond and carbon nanotubes. The behavior of graphite is intermediate between diamond and fullerene blacks. The behavior of C₆₀ fullerene appears closer to that of graphite although it appears to be more reactive with O₂. Samples of graphite and carbon blacks N375 and N234 have been studied by TGA–DTA in air flow before and after a radiation treatment with neutrons or γ radiation. The effect of the radiation damage in the combustion reaction of these carbon materials has been discussed.     

Special features of structural transformations of fullerite C<Subscript>60</Subscript> at high pressures and temperatures

The results of experimental studies of special features of fullerite C₆₀ structural transformations at high pressures and temperatures are discussed. It has been found that under the action of a pressure of ∼ 7.0 GPa and a temperature of 2000–2200 K on fullerite graphite forms and with the liquid phase present diamond forms.     

Thermodesorption states of fullerene C60 in the polyimide-fullerene system

The data of thermodesorption mass spectrometry indicate that fullerene C₆₀ molecules are desorbed from a polyimide (PI) surface at temperatures below the PI decomposition onset temperature, while the desorption of C₆₀ from the bulk begins in the temperature region of the polymer decomposition. It is suggested that strong chemical bonds between C₆₀ and PI macromolecules are formed in the bulk in the stage of the polyamic acid preparation and are broken upon destruction of the polymer macromolecules. The character of C₆₀ thermodesorption from the PI surface depends on thickness of the surface film of fullerene C₆₀.     

Volumetric measurements under pressure for 2,2,4-trimethylpentane at temperatures up to 353.15 K and for benzene and three of their mixtures at temperatures up to 348.15 K

(p, V, T) data have been obtained in the form of volume ratios relative to 0.1 MPa for benzene (298.15 to 348.15 K), 2,2,4-trimethylpentane (TMP) (313.15 to 353.15 K), and their mixtures near 0.25, 0.5, and 0.75 mole fraction of benzene (313.15 to 348.15 K) for pressures up to near the freezing pressures for benzene and the mixtures, and up to 400 MPa for TMP. Isothermal compressibilitiesκ _T, isobaric expansivitie α, changes in heat capacity at constant pressureΔC _p, and excess molar volumesV ^E have been determined from the data. Literature data at atmospheric pressure have been used to convert theΔC _p toC _p at several temperatures. The isobars for α over the temperature range 278.15 to 353.15 K for TMP intersect near 47 MPa and reverse their order in temperature when plotted against pressure; normalization of the α's by dividing the values at each temperature by the α at 0.1 MPa prevents both the intersection and the reversal of the order. TheV ^E are positive and have an unusual dependence on pressure: they increase with temperature and pressure so that the order of the curves for 0.1, 50, and 100 MPa changes in going from 313.15 to 348.15 K.     

Thermal-Mechanical and Thermodynamic Properties of Graphene Sheets using a Modified Nosé-Hoover Thermostat

The investigation assesses the thermal-mechanical and thermodynamic properties of various graphene sheets using a modified Nosé-Hoover (NH) thermostat method incorporated with molecular dynamics (MD) simulation. The investigation begins with an exploration of their thermal-mechanical properties at atmospheric pressure, including Young’s modulus, shear modulus, Poisson’s ratio, specific heats and linear and volumetric coefficients of thermal expansion (CTE). Two definitions of the line change ratio (ΔL/L) are utilized to determine the linear CTE of graphene sheets, and the calculations are compared with each other and data in the literature. To estimate the volumetric CTE values, the Connolly surface method is applied to predict the volume of the deformed graphene sheets in the free relaxation state and under temperature loading. Their specific heats are also determined by estimating the ratio of the amount of heat energy per unit mass that is required to raise the temperature by one degree. Finally, the dependences of the size and temperature on the thermal-mechanical and thermodynamic properties are examined. The calculations are validated by comparison with the results obtained from the existing thermostats and with the literature experimental and theoretical data The results indicate that the presently calculated thermal-mechanical and thermodynamic properties of graphene sheets are very similar to the published experimental and theoretical results. The graphene sheets tend to have a negative linear CTE at temperatures below 300 K. Additionally, the calculated linear CTE of graphene sheets depends strongly on the line-change-ratio assumptions. The modified NH thermostat is the only one of five thermostats that can accurately reproduce the Debye T3-dependent specific heat at temperatures below the Debye temperature.     

AN FT-ICR STUDY ON LASER ABLATION OF GRAPHITE AND CARBON BLACK TARGETS: COSMOCHEMICAL IMPLICATIONS

Graphite and carbon black samples were laser ablated in an FT-ICR (Fourier transform-ion cyclotron resonance spectrometer) by Nd-YAG laser under high vacuum in conditions recalling those existing in the interstellar medium. It is shown that graphite gives a regular sequence of polyyne and cyclopolyyne chains from C₁₀ to C₂₇ and additionally produces 2% of C₆₀ fullerene. When carbon black is used in place of graphite under the same conditions, no C₆₀ fullerene was produced and also the sequence of polyyne species generated was very irregular. If really carbon black structure and elemental composition approaches more closely than pure graphite the structure of the carbon dust, we can predict that C₆₀ fullerene should not be produced from the thermal decomposition of this dust in high vacuum.     

Heat capacity of VO2 single crystals

The heat capacity of VO₂ was remeasured both above and below the transition temperature range of 330 – 345 K. For 150 < T < 320 K the results could be fit to the standard Debye theory with a Debye temperature θ_D = 750 K; for 360 < T < 720 K no Debye temperature could be assigned to the data, which exceeded the equipartition value for C_v above 580 K. The enthalpy of transition was found to be 1.12 k cal/mole VO₂.     

Nonlinear Stern-Volmer Fluorescence Quenching of Pyrene by C60/70

Abstract

The Stern-Volmer plot of fluorescence quenching of pyrene by C₆₀ (or C₇₀) is upward, and non linearly proportional to the fullerene concentration. Electronic absorption measurements show that pyrene and C₆₀ do not form static complexes under the experimental condition. a novel “inner filter effect” model is proposed to rationalize the observed non linear upward Stern Volmer behavior. At a given C₆₀ concentration, increasing either solvent polarity or temperature (T > 280 K) causes diminishment in the I₀/I ratio. the data strongly suggest that even at concentrations far below saturation, fullerenes form aggregates in organic solvents.     

Mechanical milling of fullerene with carbide forming elements

Mechanical alloying of Ti, V, Cr, Mo and W with fullerene (C₆₀(C₇₀)) and graphite reveals that fullerene is more reactive than graphite. The formation heat of carbide is the driving force for reaction in the mechanical alloying process. Higher heat of formation results in the direct formation of carbide in Ti-C systems, and the formation of carbide in V-C systems during the subsequent heating of milled powder. In the systemsc with lower carbide heat of formation, a mixture of metal with carbon is obtained by ball milling. No carbide was obtained even after heating the milled powders up to 973 K. Small amount of fullerene remained when milled with Mo and W for 10 hours.     

Speed-of-Sound Measurements in n-Nonane at Temperatures between 293.15 and 393.15 K and at Pressures up to 100 MPa

Speed-of-sound measurements in liquid phase n-nonane (C₉H₂₀) are reported along six isotherms between 293.15 and 393.15 K and at pressures up to 100 MPa. The experimental technique is based on a double reflector pulse-echo method. The acoustic path lengths were obtained by comparison with measurements carried out at atmospheric pressure and ambient temperature in pure water. The values of the speed of sound are characterized by an overall estimated uncertainty of less than 0.2 %. These results were compared with literature values and with predictions of a dedicated equation of state.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Equation of State for <Emphasis Type="Italic">C</Emphasis><Subscript>60</Subscript> Fullerene Aqueous Solutions

In the present work PVT data of the C₆₀ fullerene aqueous solution (C₆₀ FAS) were measured as a function of the concentration of C₆₀ molecules using the variable cell method with metal bellows in the temperature range from 295 to 360 K at pressures from 0.1 to 103.2 MPa. On the liquid–vapor equilibrium line the density (ρ) of the C₆₀ FAS was measured using the pycnometer method. As a result, numerical values for the isothermal modulus of elasticity (K_T), isobaric expansivity (α_P), isothermal deviation of entropy factor (TΔS), enthalpy (ΔH), and internal energy (ΔU) have been determined. Finally, an equation of state for the C₆₀FAS was obtained for the first time.     

A new route to graphene starting from heavily ozonized fullerenes: Part 3 – an electron spin resonance study

It is shown that graphite oxide (GO) and both heavily ozonized C₆₀ and C₇₀ fullerenes, known as “fullerene ozopolymers,” are paramagnetic materials with a very strong electron spin resonance (ESR) signal at room temperature. When thermally annealed, the paramagnetic centers are gradually lost in large part. This occurs at 350°C in the case of GO, while for fullerene ozopolymers, a higher temperature is required, reaching the same results in the end. The half-width of ESR signal is linked to the distribution of paramagnetic centers. Once again, striking analogies were found in the half-width of the ESR signal measured on GO and fullerene ozopolymers, at least in the temperature range of 25–450°C. Similarly, the same g-factor values, which are diagnostic for understanding the chemical nature of paramagnetic centers, were found on both GO and fullerene ozopolymers in all ranges of temperature considered.     

ON THE MECHANISM OF CARBON CLUSTERS FORMATION UNDER LASER IRRADIATION. THE CASE OF DIAMOND GRAINS AND SOLID C60 FULLERENE

ABSTRACT

It is shown by FT-ICR (Fourier transform ion cyclotron resonance) mass spectrometry that carbon clusters considered to be the superior homologues of C₆₀ fullerene are formed by laser irradiation of both synthetic diamond grains or from pure C₆₀ fullerene crystals. The surfaces of the laser irradiated diamond or C₆₀ have been examined by Raman spectroscopy. In the case of diamond the Raman spectrum suggests the superficial formation of mixed carbon nanostructures consisting of disordered graphite, fullerenic nanostructures, onion-like carbon nanostructures and diamond-like carbon. Based on the Raman spectra of the surface and on data taken from the phase diagram of carbon, it is shown that the graphitization is needed in order to produce fullerenes from diamond under laser ablation conditions. In the case of C₆₀ fullerene, it is shown by Raman spectroscopy that the laser irradiation of the crystals causes initially their photopolymerization and after further irradiation their transformation into disordered graphite. Based on these results and on a literature survey on the formation of fullerenes from more than 15 completely different substrates, it is concluded that fullerenes are formed always when laser ablation leads to a graphitization of the laser-irradiated substrate. Some astrochemical implications of the conclusions have been discussed.     

A new route to graphene starting from heavily ozonized fullerenes: Part 1—thermal reduction under inert atmosphere

It is shown that heavily ozonized C₆₀ or C₇₀ fullerenes (known also as “fullerene ozopolymers”) are suitable substrates for the preparation of graphene or nanographene in place of graphite oxide (GO) by thermal reduction in inert atmosphere. TGA-FTIR study shows that the release profile of CO₂ and CO from fullerene ozopolymers in the temperature range between 25°C and 900°C is comparable to that shown by GO. Furthermore, the FT-IR spectral evolution of fullerene ozopolymers from room temperature to 630°C under inert atmosphere is once again strikingly comparable to that observed on GO under the same conditions.     

Normal-state transport properties of fullerene superconductors

The normal-state transport properties of alkali-metal-doped fullerene crystals are explored. The Hall effect has been measured in K_xC₆₀ from room temperature toT _c . The electrical resistivity for K₃C₆₀ and Rb₃C₆₀ has been measured over a wide temperature range (20–650 K), and notable differences are observed for the materials at both low and high temperatures. The electrical resistivity of Rb₃C₆₀ has been measured in hydrostatic pressures up to 9 kbar. The resistivity is highly pressure sensitive. The transport results give an insight into the normal-state conduction mechanism and thus have consequences for the superconductivity mechanism.     

Two Alternative Ways for Formation of Mono‐atomic Carbon Layer on Ni(1 1 1): Organic‐Gas Cracking and Thermal Decomposition of Fullerenes in Thin Film

Abstract

Formation of graphite mono‐atomic layers (monolayer of graphite, MG) on Ni(1 1 1) single‐crystal surface by cracking of propylene C₃H₆ as well as by decomposition of fullerene molecules C₆₀ was investigated. Comparative analysis of carbon films was made in both cases by means of auger electron spectroscopy (AES), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM) techniques. It was shown with the LEED measurements, that quality of graphite mono‐atomic layer obtained in the two different ways is similar and high enough. For the system prepared by fullerene fragmentation some fullerene‐similar topography features were found in STM images. These observations are explained as a result of arrangement of single C₆₀ molecules underneath the MG on Ni(1 1 1).     

The Metal-Insulator Transition and Superconductivity in Allotropes of Carbon Intercalated with Copper: Prediction and Experiment

Abstract

The complex of studies carried out on Cu_nC₆₀ samples, including SQUID and microwave measurements clearly support the existence of a superconducting phase of Cu_1,5 C₆₀ with T_c=120K. Heating a samples of pyrolitic graphite intercalated with copper and oxygen above 77K initiates a Metal - Insulator transition. We assume the same idea that the superconducting transition temperature T_c in Cu_n C₆₀ samples changes near the M -I transition.     

Extraction equilibria in a fullerene-containing C<Subscript>60</Subscript>-C<Subscript>70</Subscript>-<Emphasis Type="Italic">o</Emphasis>-Xylene-<Emphasis Type="Italic">i</Emphasis>-butylamine-water system

Extraction equilibria of the liquid(I)-liquid(II) type have been studied for the first time in a system containing light fullerenes C₆₀ and C₇₀. The experiments were performed with a five-component mixture of fullerene C₆₀, fullerene C₇₀, o-xylene, ibutylamine, and water at 25°C. Isothermal diagrams describing the distribution of fullerene components between separating liquid phases have been obtained. The distribution coefficients of fullerenes (C₆₀ and C₇₀ concentrations in the xylene phase relative to those in the amine phase) are found to be almost constant and approximately equal to ～16.1 for both C₆₀ and C₇₀.     

Thermal Conductivity Measurements of Aqueous SrCl2 and Sr(NO3)2 Solutions in the Temperature Range Between 293 and 473 K at Pressures up to 100 MPa

Accurate high-pressure thermal conductivity measurements have been performed on H₂O+SrCl₂ and H₂O+Sr(NO₃)₂ mixtures at pressures up to 100 MPa over a temperature range between 293 and 473 K using a parallel-plate apparatus. The concentrations studied were 0.025, 0.05, 0.10, 0.15, and 0.20 mass fraction of the salts. The estimated accuracy of the method is about ±1.6%. The pressure, temperature, and concentration dependences of the thermal conductivity have been studied. Measurements were made on six isobars, namely, 0.1, 20, 40, 60, 80, and 100 MPa. The thermal conductivity shows a linear dependence on pressure and concentration for all isotherms. Along each isobar, a given concentration shows the thermal-conductivity maximum at a temperature of about 413 K. The measured values of thermal conductivity at atmospheric pressure are compared with the results of other investigators. Literature data at atmospheric pressure reported by Ridel and by Zaitzev and Aseev agree with our thermal conductivity values within the estimated uncertainty.     

Comparative analysis of two methods for synthesis of fullerenes at different helium pressures

Abstract

The results of the effect of helium pressure in the chamber on the amount and composition of the produced fullerenes (C₆₀, C₇₀, etc.) synthesized in the arc plasma with graphite electrodes are presented. The findings obtained when the arc is powered by a direct (DC) and alternating (AC) currents of low frequency were compared in the same chamber with the electrodes located at the same angle to each other. These two methods are drastically different. The complete conversion of graphite into fullerene soot in AC occurs, but a part of the graphite is converted into a cathode deposit that does not contain fullerenes in DC, the relative amount of which increases when decreasing the helium pressure in the chamber. The highest fullerene content in fullerene soot of 10.2?wt.% is produced at a pressure of 127.5?kPa in AC arc, but in DC arc, the highest content of fullerenes in fullerene soot of 8.3?wt.% is produced at a pressure of 33.3?kPa.