Amorphous Fullerite Research Articles

Effect of synthesis parameters on the structure and properties of carbon particles formed from amorphous fullerites

The effect of high-pressure synthesis temperature on the structure and indentation characteristics of the superelastic hard carbon formed from amorphous fullerites and on the tribological properties of the Co-based composite materials (CM) reinforced by the particles of such carbon has been studied by Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), indentation measurements, and tribological tests. It is shown that ball milling (for 48 h) of C60 fullerite crystals results in the amorphization of the product of fullerite transformation upon their high-pressure treatment at temperatures above the stability limit of fullerene molecule (∼800°C). An increase in synthesis temperature at 8 GPa from 800°C to 1200°C leads to a gradual graphitization of the structure of amorphous fullerite derived carbon. This decreases its hardness and indentation modulus from 32 to 18 GPa and from 256 to 95 GPa, respectively, and increases the elastic recovery (from 80% to 86%). The best tribological characteristics of the CM are attained at the maximum particle hardness, which is realized in the CM synthesized at 800°C. When the synthesis temperature is elevated to 1200°C, the friction coefficient and wear rate of the CM increase, but they remain substantially lower than those of the matrix cobalt.

Indentation creep of superelastic hard carbon

Instrumented indentation methods have been used to study the effect of the loading rate and holding time at different maximum loads on the indentation characteristics of superelastic hard carbon particles reinforcing metal matrix composite materials. The properties of the carbon particles prepared in fullerite-metal powder mixtures by high-pressure synthesis substantially differ as a function of synthesis parameters (P, T) and the initial fullerite characteristics (C60 in crystalline or amorphous state). The indentation creep CIT decreases with increasing Fmax (500-2000 mN) and increases with holding time (60-600 s). The harder carbon particles formed from amorphous fullerite are characterized by higher indentation creep CIT and deeper penetration at constant load. Such creep behavior correlates with different elastic recovery characteristics of the particles upon indentation.

Neutron diffraction study of metal-matrix composite with fullerite

Interaction of amorphous fullerite C60 with austenitic Fe-33.2 wt. % Ni alloy at pressures 0-8 GPa and temperatures 600-1100 °C was studied by neutron diffraction. The amorphous fullerite was obtained by ball milling and mixed with the powder of the crystalline alloy. The interaction at sintering led to the dissolution of carbon in fcc Fe-Ni solid solution and the formation of carbide (Fe, Ni)3C, but the Fe-Ni-C alloy did not undergo phase transformations and preserved the original fcc structure. As a result, the alloy hardened, we could also witness a clear barometric effect: at the pressure of 2 GPa the amount of the dissolved carbon and the microhardness turned out to be significantly higher than those at 8 GPa. During sintering amorphous fullerite is undergoing phase transitions and its microhardness is higher than the microhardness of the metal component. At high temperatures of interaction graphite appears. The presence of Fe-Ni alloy in the composite reduces the temperature of graphite formation in comparison with transformations in the pure amorphous fullerene.

Interaction of amorphous fullerene C60 with austenite Fe–Ni alloy at high temperatures and pressures

Interaction of amorphous fullerite C60 with austenitic Fe-33.2 wt.% Ni alloy at pressures 0–8 GPa and temperatures 600–1100 °C was studied by neutron diffraction. The amorphous fullerite was obtained by ball milling and mixed with the powder of the crystalline alloy. The interaction at sintering led to the dissolution of carbon in fcc Fe–Ni solid solution and the formation of carbide (Fe, Ni)3C, but the Fe–Ni–C alloy did not undergo phase transformations and preserved the original fcc structure. As a result, the alloy hardened, we could also witness a clear barometric effect: at the pressure of 2 GPa the amount of the dissolved carbon, hardness at micro- and nanoindentation turned out to be significantly higher than those at 8 GPa.During sintering amorphous fullerite is undergoing phase transitions and its microhardness is higher than the microhardness of the metal component. At high temperatures of interaction graphite appears. The presence of Fe–Ni alloy in the composite reduces the temperature of graphite formation in comparison with transformations in the pure amorphous fullerene.

Phase transformations in amorphous fullerite C60 under high pressure and high temperature

First phase transformations of amorphous fullerite C60 at high temperatures (up to 1800K) and high pressures (up to 8GPa) have been investigated and compared with the previous studies on the crystalline fullerite. The study was conducted using neutron diffraction and Raman spectroscopy. The amorphous fullerite was obtained by ball-milling. We have shown that under thermobaric treatment no crystallization of amorphous fullerite into С60 molecular modification is observed, and it transforms into amorphous-like or crystalline graphite. A kinetic diagram of phase transformation of amorphous fullerite in temperature–pressure coordinates was constructed for the first time. Unlike in crystalline fullerite, no crystalline polymerized phases were formed under thermobaric treatment on amorphous fullerite. We found that amorphous fullerite turned out to be less resistant to thermobaric treatment, and amorphous-like or crystalline graphite were formed at lower temperatures than in crystalline fullerite.

Neutron diffraction study of the interaction of iron with amorphous fullerite

The amorphous fullerite C60 has been prepared by mechanical activation (grinding in a ball mill), and its interaction with iron during sintering of powders with 0–95 at % Fe has been studied. After sintering in the range 800–1200°C under a pressure of 70 MPa, the samples have nonequilibrium structures different from the structures of both annealed and quenched steels. In this case, the carbon phase, i.e., amorphous fullerite, undergoes a polyamorphous transition to amorphous graphite. It has also been shown that the interaction of amorphous fullerite with iron is weaker compared to crystalline fullerite or crystalline graphite.

Neutron studies of structural changes in amorphous C60 fullerite under temperature, pressure, and thermobaric effects

The structural behavior of amorphous fullerites obtained as a result of mechanical activation and thermal, baric, and thermobaric effects is studied via neutron diffraction. It is shown that the phase transition between the molecular crystal (fullerite) and atomic crystal (graphite) phases in the nanoscale state occurs through intermediate amorphous phases.

P‐doped random C20 fullerite – model structure for fullerene‐like CPx

AbstractCarbon‐based amorphous and fullerene‐like (FL) thin films have a great potential due to their mechanical resiliency. TEM investigation and interpretation of results from FL structures embedded in bulk phases is not straightforward. Here, a model is presented for description of the structure of FL‐CP0.1 thin films and to describe all the rings in the electron diffraction pattern in a self‐consistent way. The model structure consists of random close‐packed and P‐doped C20 clusters resembling a kind of amorphous fullerite (crystals of fullerene cages). The 10% P doping was implemented by creating C18P2 cages by replacing two C atoms by P in the C20 cage. The simulated electron scattering is in agreement with the experimental electron diffraction pattern and reproduces all the observed diffuse diffraction rings of FL‐CP0.1 at ∼1.6, ∼2.6, and ∼5.9 Å. Simulation of HRTEM images confirmed the amorphous appearance of this nanostructured material. magnified imageA cluster of P‐doped random close‐packed C20 fullerite (right) and its calculated electron diffraction (left) for different average cluster size (expressed in number of atoms).

Polyamorphous transition in amorphous fullerites C70

Samples of amorphous fullerites C70 have been obtained by mechanical activation (grinding in a ball mill). The structure of the samples has been investigated by neutron and X-ray diffraction. The high-temperature (up to 1200°C) annealing of amorphous fullerites revealed a polyamorphous transition from molecular to atomic glass, which is accompanied by the disappearance of fullerene halos at small scattering angles. Possible structural versions of the high-temperature amorphous phase are discussed.

Polyamorphic transition in amorphous fullerites

Samples of amorphous fullerites have been prepared by mechanoactivation (grinding in a ball mill), and their structure has been studied using neutron diffraction. It has been found that the amorphous fullerites subjected to high-temperature (600–1700°C) annealing undergo a polyamorphic transition from the molecular glass to the atomic glass, which is accompanied by the disappearance of fullerene halos at small scattering angles.

Amorphous–crystalline films of fullerite C60

Fullerite C60 films obtained by evaporation and condensation of C60 molecules on a cooled glass surface are investigated by high-resolution electron-optical methods (including direct lattice resolution and selected area diffraction). It is found that the films have an amorphous–crystalline structure, the amount of amorphous phase reaching 70%. Diffraction patterns of amorphous fullerite are obtained. The values of 4π(sin θ)/λ are determined for the first three halos. The positions of halos indicate that the structure of amorphous fullerite can be described by the hard spheres liquid model.

Large Scale Atomistic Simulations using the Tight Binding Approach

ABSTRACTAtomistic modelling of Materials Science problems often requires the simulation of systems with an irreducibly-large unit cell, such as amorphous materials, fullerites, or systems containing extended defects, such as dislocations, cracks or grain boundaries. Large-scale simulations with the Tight-Binding approach must face the computational obstacle represented by the O(N3)-scaling of the diagonalization of the Hamiltonian matrix. This bottleneck can be overcome by parallel computing techniques and/or the introduction of faster, O(N)-scaling algorithms. We report the activities performed in the frame of a collaboration among several research groups on the porting of TBMD codes on parallel computers. In particular, we describe the porting of a O(N3) TBMD code on different MIMD computers, with either distributed or shared memory, by using appropriate software tools. Furthermore, preliminary results obtained in the porting of an O(N) TBMD code on an experimental, hybrid MIMD-SIMD computer architecture are reported. The new perspective of using specialized platforms to deal with large-scale TBMD simulation is discussed.