Hard and superhard carbon phases synthesized from fullerites under pressure

A review has been presented on the structural and mechanical properties of hard carbon phases synthesized from fullerite C₆₀ under pressure. The density and nanostructure have been recognized as the key parameters defining the mechanical properties of hard carbon phases. By suggesting a version of the transitional high-pressure diagram of C₆₀ (developed up to 20 GPa), the three areas of the formation of hard carbon phases have been highlighted. The corresponding phases of superhard carbon are (1) disordered sp₂-type atomic structures at moderate pressures and high temperatures (> 1100 K), (2) three-dimensionally polymerized C₆₀ structures at moderate temperatures and high pressures (> 8 GPa), and (3) sp₃-based amorphous and nanocomposite phases at high pressures and temperatures. First region can be in turn separated into 2 subparts with different peculiarities of sp² structures and properties: low pressure part (0.1–2 GPa) and high-pressure part (2–8 GPa). Temperature can be recognized as a factor responsible for the formation of nanostructures by the partial destruction of molecular phases, whereas pressure is a factor responsible for stimulating the formation of rigid polymerized structures consisting of covalently bonded C₆₀ molecules, whereas the combination of both factors leads to the formation of atomic-based phases with dominating sp³ bonding.     

Mechanical properties and deformation of fullerites

Mechanical tests of C₆₀/C₇₀ single crystals allow one to assign them to the category of soft and fragile materials. Their mechanical properties are comparable with those of graphite but, in contrast, they are isotropic. The dependence of C₆₀/C₇₀ microhardness on the temperature in the range 77–570 K and on the influence of solvent residues has been examined. Preliminary data on microhardness of pure C₆₀ and C₇₀ have been obtained.     

Dynamics of carbon nanotube growth from fullerenes

The growth of double-walled carbon nanotubes from peapods was studied. The transformation was monitored by the decrease of fullerene Raman lines, the growth of inner tube Raman lines, and the development of X-ray diffraction patterns. A visual check of the growth process by HRTEM provided additional information. From the difference in time constants for the bleaching of fullerene Raman lines and for the growth of nanotube Raman lines, the existence of an intermediate phase was concluded that was eventually observed in X-ray diffraction and HRTEM. Time constants for the growth of large diameter inner tubes were up to a factor two larger than for small diameter inner tubes. The results fully support the fullerene coalescence growth model triggered by Stone-Wales transformations.     

Ultrafast method of fullerenes extraction from carbon condensate

The Soxhlet extraction method (SE) only used in the extraction of fullerenes from carbon condensate (CC) obtained in the fullerene production facility, which in the literature is often referred to as carbon black, does not provide complete extraction. Fullerenes can also be extracted from the CC suspension and the solvent by the mechanical extraction method (ME) and show higher degree of extraction efficiency. This method combined with filtration process takes only a few minutes and allows a significant reduction in solvent consumption. Extraction via the ME method application makes it possible to carry out extraction of fullerenes twenty times faster than by the SE method. It can be used as an independent method, bypassing the extraction by the SE method, and also as an additional one. Application of the ME method allows additional extraction of fullerenes weight percentage from the CC, after extracting fullerenes from it applying the SE method. In this instance the higher fullerenes prevail in the essences extracted by the ME method with their content reaching as high as 39.50?wt%. The obtained samples of fullerene mixtures (FM) are characterized by the methods of high-performance liquid chromatography and mass spectrometry. Analysis of the results shows that the composition of the FM, isolated by the ME method, differs from the compositions extracted via the SE method.     

Synthesis of encapsulating and hollow onion-like fullerenes from coal

Onion-like fullerenes (OLFs) with different structures were synthesized from coal by radio frequency plasma. The morphologies and structures of the products were characterized by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS) techniques. The results clearly indicated that two types of uniform OLFs with high purity could be prepared from coal, and Fe in coal favored the growth of OLFs.     

Ultrasound induced growth of C60 fullerites over KBr

C₆₀ fullerites of millimeter to centimeter sizes were synthesized by ultrasound induced charging of C₆₀ over KBr surface in C₆₀/benzene solution at room temperature in the absence of ultrahigh vacuum. C₆₀ fullerites prepared by this method are found to be extremely brittle. C₆₀ fullerites can be prepared in large scale in a cost effective way using this method.     

The melting curve of helium from 4 to 25 K

New data for the melting curve of He⁴ for temperatures between 4 and 25 K agree with earlier results by Mills and Grilly and by Crawford and Daniels, but disagree with the relationship given by Glassford and Smith [CRYOGENICS 6 (1966) 193] who carried out equation of state studies on fluid and solid He⁴. The reasons for the disagreement are not clear, but can be interpreted in terms of temperature scale inaccuracies which could have influenced all of their results.     

The test measurements of TOF in small and large scintillation counters

We tested the time resolution of “small” scintillation counters (0.8 (height) × 2.6 (width) cm² and 0.7 (height) × 2.1 (width) cm², with 1.0 cm thickness) and of a “large” counter (21 (height) × 96 (width) cm² with 2 cm thickness). The time walk correction using software (off-line) was compared with that using hardware (constant fraction discriminator). It was found that the two methods gave nearly equal time resolution. Intrinsic time resolutions of 155 and 370 ps fwhm were obtained for the small and the large counters, respectively.     

Nucleation and melting from nanovoids

Melting in solids containing nanovoids is studied via molecular dynamics simulation, and the finite size effects are treated with different size systems. For the first time, we have found four typical stages in void melting that are different from the melting of bulk materials and nanoparticles. Melting in each of the stages is governed by the interplay among different thermodynamic mechanisms arising from the changes in the interfacial free energies, the curvature of the interface, and the elastic energy induced by the density change at melting. As a result, the local melting temperatures show a strong dependence on the void size, which is the root cause of the observed complex hierarchical melting sequence.     

The shape of a sessile drop for small and large surface tension

Asymptotic solutions for large and small surface tension are developed for the profile of a symmetric sessile drop. The problem for large surface tension (i.e., small Bond number) is a regular perturbation problem, where the solution may be written as a uniformly valid asymptotic expansion. The problem for small surface tension (i.e., large Bond number) is a singular perturbation problem with boundary-layer behaviour in the edge region. The solution is a matched asymptotic expansion, where some care is to be taken for the matching. The respective ranges of validity are established by comparing the asymptotic results with solutions obtained by numerical integration of the full equations.     

Thermal stability of fullerenes

Technical Physics Letters - We have studied the thermal stability of fullerenes, for which there are discrepancies in available literature. A critical temperature of the thermal stability of C60...     

Development of a method for integrating time-dependent constitutive equations with large,small or negative strain rate sensitivity

A unified numerical method to integrate stiff time-dependent constitutive equations has been developed. This method is a stable, non-iterative and self-correcting solution procedure which works successfully over a wide range in strain rate sensitivity. Time steps are automatically controlled during integration to achieve a user-specified accuracy. This method is implemented in the program package NONSS whose dual purpose is examination of the behaviour of unified constitutive models by themselves (‘one-element behaviour’) as well as providing a computationally efficient subroutine for utilizing such models with existing finite element programs for non-linear structural and metal forming analyses. This paper first reviews the relation between the numerical characteristic of constitutive equations and the choice of integration methods. Then the paper presents the derivation of the governing basic equations in the new method, and also derives a special algorithm which permits large integration steps within the negative strain rate sensitivity (‘serrated yielding’) regime. Examples of the program's performance are given, including plasticity at high and low temperatures, cyclic deformation and multiaxial straining.     

温度对洋葱状富勒烯形成的影响

在化学气相沉积法(CVD)制备洋葱状富勒烯(OLFs)过程中,温度是最主要的参数之一.以烃类为碳源,二茂铁为催化剂,在不同温度下制备OLFs,分析了温度对形成机理的影响,通过场发射扫描电镜(FESEM)对产物的形貌和微观结构进行了表征.采用热重(TGA)考察了OLFs的热稳定性,表明温度越高,形核所需的最大吉布斯自由能越小,越容易形核,形成的晶核越稳定.当温度超过1000℃时,晶粒之间的融合能力也随之增强,产物出现粘连现象,从而1000℃为制备OLFs的最佳温度.     

Microstructure and strength of a tantalum-tungsten alloy after cold rolling from small to large strains

Microstructural evolution of a refractory tantalum-tungsten alloy(Ta-4％W)after cold rolling from small to large von-Mises strains(0.12-2.7)was quantitatively studied using transmission electron microscopy.Grain subdivision was observed to take place at two levels.Geometrically necessary boundaries nearly paralleling to slip planes enclosed volumes further divided by diffuse cells and by remnants of Taylor lattices.With increasing strain,the diffuse cells evolved into clear incidental dislocation boundaries enclosing cells,while the Taylor lattices disappeared.Grain subdivision was thus intermediate between those observed in cell forming and in non-cell forming alloys.Meanwhile,the average misorientation angle across all boundaries increased while the average boundary spacing decreased.Distributions of the microstructural parameters at each strain level were found to exhibit universal scaling laws.The microstructural evolution was found closely linking to the observed high strength and strain hardening of this alloy.Based on measured microstructural parameters,the flow stress was calculated utilizing linearly addition of the strengthening by solutes,incidental dislocation boundaries(Taylor strengthening)and geometrically necessary boundaries(Hall-Petch equation).The relative contribution of each strength mechanism evolved with increasing strain and with microstructural evolution:solutes and friction stress dominated at small strains while boundaries dominated at larger strains.Calculated strengths were in close agreement with experimental tension tests and demonstrated an unexpectedly high and continuous parabolic hardening without transition across this large strain range.     

13C enrichment of fullerenes

It is shown experimentally that fullerenes can be enriched with ¹³C as a constituent of CO₂ gas molecules. A qualitative model of the processes that correctly reflects the experimental laws is proposed. Center for Laser Technology, Scientific-Research Institute for Electrophysical Apparatus Pis’ma Zh. Tekh. Fiz. 25, 12–18 (September 26, 1999)