Anisotropic Nanoclustered Carbon Phases Prepared from Fullerite C60 Under Non‐hydrostatic High Pressure

Abstract

We show that application of the non‐hydrostatic pressure to the cluster‐based molecular material, like fullerite C₆₀, provides a new opportunity to create elastically and structurally anisotropic carbon materials, including 2D polymerized rhombohedral C₆₀ and graphite‐type (sp ²) disordered atomic‐based phases. The elastic anisotropy, detected by the difference in the ultrasound velocities propagating along and across the loading axis, is directly confirmed by the results of x‐ray diffraction.     

Structural (XRD and HRTEM) investigations of fullerite C60 and C70 samples

Since the carbon states on the basis of C₆₀ and C₇₀ exhibit very interesting physical properties such as hardness exceeding diamond, we focused our investigations on the comparison of structures of the C₆₀ and C₇₀ hard phases synthesized at high pressure and high temperatures. X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) investigations have revealed that two states (amorphous and nanocrystalline) co-exist in samples of both kinds of fullerites. The difference between the diffraction patterns of C₆₀ and C₇₀ has been ascribed mainly to the grade of disorder. The XRD and HRTEM results have been correlated with the transport properties (electrical and thermal conductivity) of the samples.     

Spectroscopic Studies of p-H2 to Above 200 GPa

Raman and infrared vibrational spectra of H ₂ have been measured to pressures in excess of 200 GPa and at liquid helium temperatures using new high sensitivity techniques. Detailed study of the pressure dependence of o-p conversion rate reveals an initial increase followed by a decrease above 1 GPa. The conversion rate then increases dramatically with pressure, and this continues to above 50 GPa. New sets of vibron, phonon, roton, and libron excitations in converted para samples are documented as a function of pressure through phases I, II, and III. The results provide important information on the crystal structures, molecular orientational state, and vibrational dynamics of the high-pressure phases.     

Diamond-like a-C:H coatings deposited in a non-self-sustained discharge with plasma cathode

Hydrogenated amorphous carbon (a-C:H) coatings have been obtained by means of acetylene decomposition in a non-self-sustained periodic pulse discharge (2A, 50 kHz, 10 μs) with hollow cathode. The discharge operation was maintained by plasma cathode emission with grid stabilization based on dc glow discharge. Using the proposed method, it is possible to control the deposition conditions (total pressure of the Ar + C₂H₂ mixture, partial pressure of C₂H₂, ion current density, carbon ion energy) within broad limits, to apply a-C:H coatings onto large-area articles, and to perform deposition in one technological cycle with ion etching and ion implantation treatments aimed at improving the adhesion of coatings to substrates (Ti, Al, stainless steel, VK8 hard alloy) at temperatures below 150°C. Results of determining the deposition rate (1–8 μm), the nanohardness of coatings (up to 70 GPa), and the fraction of sp ³ bonds (25–70%) in the diamond-like coating material are presented.     

Synthesis of boron-carbon phases with the α-tetragonal boron structure at 8–9 GPa

Our results demonstrate that phases with the structure of α-tetragonal boron can be obtained in the B-C-H system at pressures of 8–9 GPa and temperatures only below 1600–1700°C. The formation of these phases takes place at carbon content up to 20% relative to the boron in the growth system. Increasing the carbon concentration in the growth system reduces the length of the tetragonal cell diagonal. No B₅₀C₂ has been obtained in this study, which can be accounted for by the specifics of the doping process at high pressures.     

Equilibrium Phase Diagram of Polymerized C60 and Kinetics of Decomposition of the Polymerized Phases

Abstract

Equilibrium phase diagram (T=0–1000 K; p=0–8 GPa) of pressure‐temperature polymerized C₆₀ has been constructed. It included rhombohedral; orthorhombic and tetragonal polymerized phases of C₆₀ along with fcc monomeric phases of C₆₀. The isothermal kinetics of depolymerization (T=373–513 K) of the O‐, R‐, T‐ and D‐phases was studied by means of the IR‐spectroscopy. The isothermal rate of decomposition goes up along the series: (T‐)<(O‐)<(R‐)<(D‐).     

Catalytic 3D polymerization of C60

The present study addresses the problems of catalytic synthesis of C₆₀-based 3D polymers in the presence of carbon disulfide CS₂. It is shown that CS₂ lowers the pressure of fullerite 3D polymerization at room temperature. In particular, the pressure at which the 3D ultrahard fullerite C₆₀ phase is formed decreases from 18 GPa to 7 GPa. The catalyst plays no significant role at room temperature and low pressures up to 6 GPa. When the C₆₀ + CS₂ is heated to 1200 K, the pressure of the 3D C₆₀ polymerization is further reduced to 2–4 GPa. Based on the TEM data, we propose a 3D polymer structure based on C₆₀ obtained by the catalytic synthesis.     

Anisotropic Carbon Phases Prepared from Fullerite C70 under High Pressure

Abstract

We present the structural and ultrasonic study of carbon phases prepared by quenching after heating of fullerite C₇₀ in the temperature range 300–1100°C at pressures 4 and 7.5 GPa. The main aspect of the work concerns the structural and elastic anisotropy of samples resulted as consequence of quasi hydrostatic pressure with an additional pressure component along the axis of pressure load. Structural anisotropy correlates with anisotropy of the tensor of effective elastic constants taken for the medium with single axis of anisotropy. Comparison of the data obtained with anisotropy effects, earlier observed in the phases synthesized from C₆₀ in the similar quasi hydrostatic conditions, clarifies the role of non‐spherical symmetry of C₇₀ molecule.     

Superior hardness and Young's modulus of low temperature nanocrystalline diamond coatings

Nanocrystalline diamond (NCD) coatings with thickness of about 3 μm were grown on silicon substrates at four deposition temperatures ranging from 653 to 884 °C in CH₄/H₂/Ar microwave plasmas. The morphology, structure, chemical composition and mechanical and surface properties were studied by means of Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD), Raman spectroscopy, nanoindentation and Water Contact Angle (WCA) techniques. The different deposition temperatures used enabled to modulate the chemical, structural and mechanical NCD properties, in particular the grain size and the shape. The characterization measurements revealed a relatively smooth surface morphology with a variable grain size, which affected the incorporated hydrogen amount and the sp² carbon content, and, as a consequence, the mechanical properties. Specifically, the hydrogen content decreased by increasing the grain size, whereas the sp² carbon content increased. The highest values of hardness (121 ± 25 GPa) and elastic modulus (1036 ± 163 GPa) were achieved in NCD film grown at the lowest value of deposition temperature, which favored the formation of elongated nanocrystallites characterized by improved hydrophobic surface properties.     

New Ordered Structure of Amorphous Carbon Clusters Induced by Fullerene–Cubane Reactions

As a new category of solids, crystalline materials constructed with amorphous building blocks expand the structure categorization of solids, for which designing such new structures and understanding the corresponding formation mechanisms are fundamentally important. Unlike previous reports, new amorphous carbon clusters constructed ordered carbon phases are found here by compressing C₈H₈/C₆₀ cocrystals, in which the highly energetic cubane (C₈H₈) exhibits unusual roles as to the structure formation and transformations under pressure. The significant role of C₈H₈ is to stabilize the boundary interactions of the highly compressed or collapsed C₆₀ clusters which preserves their long‐range ordered arrangement up to 45 GPa. With increasing time at high pressure, the gradual random bonding between C₈H₈ and carbon clusters, due to “energy release” of highly compressed cubane, leads to the loss of the ability of C₈H₈ to stabilize the carbon cluster arrangement. Thus a transition from short‐range disorder to long‐range disorder (amorphization) occurs in the formed material. The spontaneous bonding reconstruction most likely results in a 3D network in the material, which can create ring cracks on diamond anvils.     

Tem characterization of nanodiamond thin films

The microstructure of thin films grown by microwave plasma-enhanced chemical vapor deposition (MPCVD) from fullerene C₆₀ precursors has been characterized by scanning electron microscopy (SEM), selected-area electron diffraction (SAED), bright-field electron microscopy, high-resolution electron microscopy (HREM), and parallel electron energyloss spectroscopy (PEELS). The films are composed of nanosize crystallites of diamond, and no graphitic or amorphous phases were observed. The diamond crystallite size measured from lattice images shows that most grains range between 3–5 nm, reflecting a gamma distribution. SAED gave no evidence of either sp²-bonded glassy carbon or sp³-bonded diamondlike amorphous carbon. The sp²-bonded configuration found in PEELS was attributed to grain boundary carbon atoms, which constitute 5–10% of the total. Occasionally observed larger diamond grains tend to be highly faulted.     

Mechanical properties of nanostructed diamond-like carbon films synthesized by low energy cluster beam deposition

Mechanical properties of carbon-films obtained by low energy neutral cluster beam deposition (LECBD) have been measured using the nanoindentation technique. Three selected size distributions centered around C₂₀, C₆₀ and C₉₀₀ have been deposited on various substrates at room temperature. The films with a nanostructured morphology (grain size around 15 to 25 nm) conserve a memory of the specific character of the free clusters which is sp³ for C₂₀, sp² for C₉₀₀ and intermediate (sp^2.3) for C₆₀. The densities of the films (0.8 to 1.1 g.cm⁻³) are lower than most polymers but their hardnesses (3 to 12 GP a) are comparable to those of many DLC-films. These results are discussed taking into account the particular structures of the free clusters and the nucleation and growth mechanism specific for the LECBD technique, which corresponds nearly to a random stacking of incident clusters.     

类金刚石薄膜对轴承钢表面机械性能和滚动接触疲劳寿命的影响

使用等离子体浸没离子注入与沉积(PIII&D)技术在轴承钢基体表面合成类金刚石(DLC)薄膜,研究了薄膜的结构和性能,结果表明,所制备的DLC薄膜主要是由金刚石键(sp3)和石墨键(sp2)组成的混合无定形碳,且sp3键含量大于10%,DLC膜层致密均匀,与基体结合良好,DLC膜具有很高的硬度和杨氏模量,分别达到40 GPa和430 GPa;其最低摩擦系数由基体的0.87下降到0.2,被处理薄膜试件在90%置信区间下的L10、L50、La和平均寿命L较基体分别延长了10.1倍、4.2倍、3.5倍和3.4倍,PIII&D轴承钢滚动接触疲劳寿命的分散性得到了显著改善.     

Growth and thermal annealing of amorphous germanium carbide obtained by X-ray chemical vapor deposition

The growth of amorphous hydrogenated germanium carbide (a-GeCx:H) alloys was performed with high deposition rates by radiolysis chemical vapor deposition (X-ray) of germane/allene (GeH₄/C₃H₄, 70/30 %) mixtures at different irradiation times. The experimental deposition parameters were correlated to the composition, the structural features, and the optical coefficients of the films, as studied by different spectroscopic techniques, namely, IR, Raman, and UV–Vis. It was observed that the increase of irradiation time yields a more hydrogenated and more disordered material, with abundant formation of sp³ CH₂ groups, characterized by high band-gap values. In addition, we report the effects of thermal annealing on bonding structures and optical properties of the amorphous germanium carbon alloys. The decrease of hydrogen extent, together with the enhancement of sp² C bonds present and amorphous-to-crystalline germanium phase transition, contribute to a larger structural order of the material and to the reduction of the optical gap at higher temperatures.     

High-pressure polymorph of LaNbO4

A new polymorph of LaNbO₄ with a BaMnF_4-type structure(a = 0.3938(2) nm,b = 1.442(1) nm,c = 0.5682(3) nm,Z = 4) and octahedral oxygen coordination of Nb was prepared at high pressures and temperatures (8 GPa, 1570 K). At normal pressure, the reverse transformation, from high-pressure LaNbO₄ to theM-fergusonite form, occurs at temperatures above 870 K and is close to a second-order phase transition. The spontaneous polarization in high-pressure LaNbO₄, evaluated from the second-order nonlinear optical response, is about 2 ώC/cm².     

Crystalline carbon nitride films prepared by microwave plasma chemical vapour deposition

Crystalline carbon nitride films have been synthesized on Si (100) substrates by a microwave plasma chemical vapour deposition technique, using mixture of N₂, CH₄ and H₂ as precursor. Scanning electron microscopy shows that the films consisted of hexagonal bars, tetragonal bars, rhombohedral bars, in which the bigger bar is about 20 μm long and 6 μm wide. The X-ray photoelectron spectroscopy suggests that nitrogen and carbon in the films are bonded through hybridized sp² and sp³ configurations. The x-ray diffraction pattern indicates that the films are composed of α-, β-, pseudocubic and cubic C₃N₄ phase and an unidentified phase. Raman spectra also support the existence of α- and β-C₃N₄ phases. Vickers microhardness of about 41.9 GPa measured for the films.     

First-principles computational design and synthesis of hybrid carbon–silicon clathrates

Type I and Type II silicon clathrates (Si₄₆ and Si₁₃₆), which can be considered as analogs of carbon fullerene materials, are composed with face-sharing Si₂₀, Si₂₄, and Si₂₈ cages linked through sp ³-covalent bonds. Besides silicon clathrates, theoretical computations have shown that both Type I carbon clathrate (C₄₆) and Type II carbon clathrate (C₁₃₆) may exist as metastable phases under high pressures. However, the energies of formation for the Type I and Type II carbon clathrates are extremely high and neither Type I nor Type II carbon clathrates have been synthesized. The objective of this investigation was to develop Type I hybrid carbon–silicon clathrates by substituting atoms on the silicon clathrate framework with C atoms. A first-principles computational approach was first utilized to design the framework structure and to identify appropriate guest atoms that are amenable to the formation of hybrid carbon–silicon clathrate compounds. A new class of Type I clathrates based on the carbon–silicon system was discovered as potential candidates. Some of the promising candidate clathrates were synthesized using an industrial arc-melting technique. The yield and stability of these newly discovered clathrates were evaluated. In addition, the electronic properties of selected clathrate materials were predicted using first-principles computations, which showed profound influences of the electronic properties by C atom substitution on the Si framework and insertion of guest atoms into the cage structure.     

Solid-state Carbon Nitrides

Whether crystalline or amorphous sp³-bonded carbon nitrides containing substantial amounts of nitrogen can be synthesized remains an open question. Various deposition techniques and high pressure syntheses to 40 GPa have not been successful thus far. The use of appropriately designed molecular or solid-state precursors and low enough synthesis temperatures to insure kinetic control of reaction products appears to be a promising direction for future efforts. In addition, synthesis at higher pressures, which can effect larger shifts in free energies of reaction should be pursued. The importance of careful characterization to obtain structural (by means of electron or X-ray diffraction), compositional (by means of EELS, EDX, or RBS), and, especially, bonding (by means of EELS) information on single-phase material cannot be overemphasized. Because few examples are known of covalently bonded networks containing carbon, synthesis of a sp³-bonded carbon nitride would be a spectacular achievement.     

Synthesis of a new cubic phase in the B-C-N system

Phase formation in the B-C-N system has been studied at pressures from 6 to 15 GPa and temperatures from 1000 to 1600°C using mixtures of carbon nitride (C₃N₄) nanospheres and boron. A new ternary phase with the structure of cubic boron nitride in which some of the nitrogen and boron atoms are replaced by carbon has been obtained at p ? 8 GPa and t ? 1500°C. According to the Rietveld refinement results, its composition is BC_0.47N_0.85.     

Photoluminescence Study of the Two‐Dimensional Tetragonal and Rhombohedral Polymers of C60 at High Pressure

Abstract

The pressure behavior of the photoluminescence (PL) spectra of the planar polymeric phases of C₆₀ have been studied at pressure up to 4 GPa. The PL spectra and the pressure‐induced shift of the principal bands differ considerably for the pristine C₆₀, two‐dimensional tetragonal (2D‐T) and rhombohedral (2D‐R) polymers of C₆₀. The changes in the PL spectra may be related with the transformation of the electron energy spectrum of polymers.