Extended polymerization in ABC-stacked C70 fullerite

This letter reports on the extended polymerization occurring in ABC-stacked C₇₀ fullerite, which up to now was thought to occur only in the AB-stacked phase. Frustration effects on ABC close-packing of C₇₀ molecules were believed to prevent extended polymerization based on 2 + 2 cycloaddition bonds, but here we show that an extended polymeric phase is obtained at 10 GPa and 270 °C. Its structure, obtained by Rietveld analysis of the X-ray diffraction data combined with density functional theory methods, consists of zigzag chains running within the dense hexagonal (1 0 1) pyramidal planes.     

Mechanical properties of different types of diamond

The mechanical properties of different types of diamond (synthetic diamonds with different nitrogen impurity concentrations 0.3 and 200 ppm) have been investigated by sclerometry hardness and wear resistance measurements. Diamond (111) and (100) faces in the 〈100〉 and 〈110〉 directions were tested. It was found the synthetic diamond with nitrogen impurity concentration of 0.3 ppm exceeds other diamond types with respect to hardness and wear resistance, and reveals anisotropy of the mechanical properties, different from other diamond types. The hardness measured on the (111) face for synthetic diamond was 175±5 GPa for 0.3 ppm of nitrogen impurities and 151±5 GPa for 200 ppm of nitrogen impurities. The hardness measurements were performed using an ultrahard fullerite indenter exceeding diamond in hardness and the diamond faces were deformed plastically under scratching conditions.     

Experimental study on the stability of graphitic C3N4 under high pressure and high temperature

The stability and decomposition of graphitic C₃N₄ (g-C₃N₄) were studied in the pressure and temperature range of 10–25 GPa and up to 2000 °C by multi-anvil experiments and phase characterization of the quenched products. g-C₃N₄ was found to remain stable at relatively mild temperatures, but decomposes to graphite and nitrogen at temperatures above 600–700 °C and up to 15 GPa, while it decomposes directly to diamond (plus nitrogen) above 800–900 °C and between 22 and 25 GPa. The estimated decomposition curve for g-C₃N₄ has a positive slope (~ 0.05 GPa/K) up to ~ 22 GPa, but becomes inverted (negative) above this pressure. The diamond formed through decomposition is characterized by euhedral crystals which are not sintered to each other, but loosely aggregated, suggesting the crystallization in a liquid (nitrogen) medium. The nitrogen release from the graphitic CN framework may also play an important role in lowering the activation energy required for diamond formation and enhancing the grain growth rate. No phase transition of g-C₃N₄ was found in the studied P–T range.     

Phase transitions in single crystal tubes formed from C60 molecules under high pressure

Pure single crystal tubes formed from C₆₀ molecules, with a face-centered cubic (fcc) structure were fabricated by a liquid–liquid interfacial precipitation method using C₆₀ powder. A bulk transition from fcc to a simple cubic structure and a surface transition from (1 × 1) to (2 × 2) have been observed around 246 K (bulk transition temperature T_B) and 214 K (surface transition temperature T_S), respectively, during the measurement of the temperature dependence of electrical resistance. The initiation of the two transitions under pressure was investigated using a piston cylinder high pressure apparatus and it was found that both T_B and T_S increase with increasing pressure. And the C₆₀ molecules at the surface of the tube exhibit the same behavior of that in the bulk at a pressure of about 2.1 GPa.     

Microstructure,mechanical, thermal,and oxidation properties of a Zr2[Al(Si)]4C5–SiC composite prepared by in situ reaction/hot-pressing

The microstructure, mechanical and thermal properties, as well as oxidation behavior, of in situ hot-pressed Zr₂[Al(Si)]₄C₅–30 vol.% SiC composite have been characterized. The microstructure is composed of elongated Zr₂[Al(Si)]₄C₅ grains and embedded SiC particles. The composite shows superior hardness (Vickers hardness of 16.4 GPa), stiffness (Young's modulus of 386 GPa), strength (bending strength of 353 MPa), and toughness (fracture toughness of 6.62 MPa m^1/2) compared to a monolithic Zr₂[Al(Si)]₄C₅ ceramic. Stiffness is maintained up to 1600 °C (323 GPa) due to clean grain boundaries with no glassy phase. The composite also exhibits higher specific heat capacity and thermal conductivity as well as better oxidation resistance compared to Zr₂[Al(Si)]₄C₅.     

Shock compressibility and shock-induced phase transitions of C60 fullerite

Shock compressibility of C₆₀ fullerite and sound velocities in shock-compressed fullerite were experimentally studied at the pressures range up to 50 GPa. In our experiments, we used polycrystalline C₆₀ specimens with a density of 1.64 g/cc. The Hugoniot of C₆₀ fullerite had a set of peculiarities, which may be attributed to a series of polymorphic phase transitions. The jump of sound velocity in shocked C₆₀ at pressure 9 GPa indicates the formation of a rather hard carbon phase. It is possible to assume that it is a polymerized C₆₀ phase. In the region of pressures 9–25 GPa, destruction of this phase and formation of a graphite-like carbon occurs. With further growth of shock pressure, phase transition of the graphite-like carbon to a diamond-like phase is observed with a transition onset pressure 25 GPa. If shock pressures are higher than 33 GPa, Hugoniot of C₆₀ fullerite is determined by the thermodynamic properties of the diamond-like phase.     

Spontaneous nucleation of diamond in the system MgCO3–CaCO3–C at 7.7 GPa

High-pressure and high-temperature experiments were carried out to determine the minimum temperature required for spontaneous nucleation of diamond in the system comprising a carbonate mixture (60 mol% MgCO₃ and 40 mol% CaCO₃) and graphite at 7.7 GPa, for 1 h and longer reaction times up to 12.5 h, and also in the systems MgCO₃–graphite and CaCO₃–graphite for comparison. The above carbonate mixture melted at a temperature between 1600°C and 1700°C. The minimum temperature for spontaneous nucleation of diamond was 1900°C for a reaction time of 1 h; it was lowered to 1700°C for 11 h, but no diamond was formed at 1600°C for 12.5 h. These results indicate that the molten state of the solvent-catalyst and enough reaction time are necessary for diamond nucleation in the systems examined at 7.7 GPa.     

Nanoindentation with spherical tips of single crystals of YBCO textured by the Bridgman technique: Determination of indentation stress–strain curves

The mechanical properties of superconductor ceramics are of interest in the manufacture of superconducting devices. The current trend is to produce smaller devices (using, e.g., thin films), and the correct characterization of small volumes of material is critical. Nanoindentation is used to assess mechanical parameters, and several studies determine hardness and Young's modulus by sharp indentation. However, studies on the elasto-plastic transition with spherical indentation are scare. Here we used, spherical diamond tip indenter experiments to explore the elasto-plastic transition and to measure the yield strength of the orthorhombic phase of YBa₂Cu₃O_7?δ (YBCO or Y-123) at room temperature. The study was carried out for a range of monodomains on the (1 0 1)-plane for Bridgman samples. Inspection of the load–unload curves for penetration depths lower than 200 nm allows for observation of the elasto-plastic transitions. Focused ion beam (FIB) trenches showed no cracking due to the indentation, although oxygenation cracks were apparent. The mean pressure for the onset of elasto-plastic deformation is 3.5 GPa, and the elastic modulus, E, calculated using the Hertzian equations is 123.5 ± 3.4 GPa.     

Structural characterization of corrugated anisotropic graphene-based carbons obtained from the collapse of 2D C60 polymers

The structural organization and the transformation mechanism of the layered carbon phase produced from the tetragonal and rhombohedral variants of the 2D polymers of C₆₀ were investigated. High Resolution Transmission Electron Microscopy imaging combined with Electron Diffraction analysis were performed for samples synthesized at 2.6, 5.0 and 8.5 GPa and 1100 K. The structure of this layered carbon phase is composed of corrugated layers in a pseudo-epitaxial relationship with the {1 1 1}_cubic planes of the parent polymer phase. A pseudo-martensitic transformation mechanism is proposed with a partial C–C bond reconstruction, as indicated by measurements of the angles between textured 0 0 l reflections. The distribution of dense polymeric planes in tetragonal, rhombohedral and distorted pseudo-cubic 2D polymers as well as the crystallographic relationships between different layered carbon microstructures indicate that the carbon layers are generated from the polymerized dense planes of each structure.     

Effect of H2/Ar plasma on growth behavior of ultra-nanocrystalline diamond films: The TEM study

Incorporation of H₂ species into Ar plasma was observed to markedly alter the microstructure of diamond films. TEM examinations indicate that, while the Ar/CH₄ plasma produced the ultrananocrystalline diamond films with equi-axed grains (~ 5 nm), the addition of 20% H₂ in Ar resulted in grains with dendrite geometry and the incorporation of 80% H₂ in Ar led to micro-crystalline diamond with faceted grains (~ 800 nm). Optical emission spectroscopy suggests that small percentage of H₂-species (< 20%) in the plasma leads to partially etching of hydrocarbons adhered onto the diamond clusters, such that the C₂-species attach to diamond surface anisotropically, forming diamond flakes, which evolve into dendrite geometry. In contrast, high percentage of H₂-species in the plasma (80%) can efficiently etch away the hydrocarbons adhered onto the diamond clusters, such that the C₂-species can attach to diamond surface isotropically, resulting in large diamond grains with faceted geometry. The field needed to turn on the electron field emission for diamond films increases from E₀ = 22.1 V/μm (J_e = 0.48 mA/cm² at 50 V/μm applied field) for 0% H₂ samples to E₀ = 78.2 V/μm (J_e < 0.01 mA/cm² at 210 V/μm applied field) for 80% H₂ samples, as the grains grow, decreasing the proportion of grain boundaries.     

Elastic properties of translucent polycrystalline cubic boron nitride as characterized by the dynamic resonance method

Cubic boron nitride (cBN) is second only to diamond in a number of extreme material properties, and its performance exceeds diamond in many applications involving contact with ferrous alloys and/or high temperatures. However, its properties are less well understood. We have sintered cBN powder (2–4 μm or 8–12 μm particle size) into pure, translucent, polycrystalline compacts by pressing at a pressure of 7.7 GPa and temperatures from 2100 to 2350°C without any sintering agent. We have determined the Young's modulus E, shear modulus G, and Poisson's ratio ν of a number of translucent polycrystalline cBN compacts, in the form of free-standing disks, using the dynamic resonance method. The measured values for E, G, and ν lay in the ranges of 665–895 GPa, 295–405 GPa, and 0.11–0.15, respectively, depending on the grain size of the cBN starting material and the sintering temperature. These values may be compared with the theoretical values of E, G, and ν for pure, equiaxed, cBN of 909 GPa, 405 GPa, and 0.12, respectively. Combining the Young's modulus with previous Vickers hardness measurements, the fracture toughness K_IC of well-sintered translucent PCBN is evaluated as 6.8 MPa m^1/2. The dependence of the elastic properties on the synthesis conditions is discussed in the context of the microstructure and of related material properties.     

Solution-phase synthesis of dumbbell-shaped C120 by FeCl3-mediated dimerization of C60

In the presence of FeCl₃, C₆₀ in 1,1,2,2-tetrachloroethane reacted at 150 °C to produce a dumbbell-shaped dimer C₁₂₀. We propose a reaction mechanism that includes FeCl₃-mediated generation of an intermediate C₆₀ radical cation, which reacts with C₆₀ to form the dimer. The respectable chemical yield (21%) and quantity (106 mg) of the product will open new possibilities for investigating solution-phase polymerization of fullerenes as well as for conducting applied research on C₁₂₀.     

Mechanical properties of C58 materials and their dependence on thermal treatment

C₅₈ fullerene cages made by electron-impact induced fragmentation of C₆₀ fullerenes have been assembled into several micron thick solid films by low energy cluster beam deposition onto inert substrates held at room temperature under ultrahigh vacuum. The resulting as-prepared material, RT-C₅₈, behaves as an amorphous wide-band semiconductor. Nanoindentation was used to measure its mechanical properties revealing that RT-C₅₈ has a higher elastic modulus E and hardness H than the reference carbon allotropes solid C₆₀ and Highly Ordered Pyrolytic Graphite (HOPG): E(RT-C₅₈) = 14 GPa and H(RT-C₅₈) = 1.2 GPa. This effect can be explained by the unique intrinsic “functionalization” of C₅₈ cages: they comprise reactive surface sites constituted by annelated pentagon rings which give rise to covalently stabilized oligomers, –C₅₈–C₅₈–C₅₈, under our deposition conditions. Annealing, thick RT-C₅₈ films up to 1100 K in ultrahigh vacuum results in HT-C₅₈, a new material with considerably modified electronic and vibrational properties compared to the as-prepared RT-C₅₈ film. The associated molecular transformations, including also partial cage–cage coalescence reactions, raise the overall mechanical hardness of the material: H(HT-C₅₈) = 3.9 GPa.     

Microstructure and mechanical properties of Ti (C,N) based cermets reinforced with different ceramic particles processed by spark plasma sintering

The addition effect of different ceramic particles such as TiB₂, TiN and nano-Si₃N₄ on the microstructure and mechanical properties of TiCN-WC-Co-Cr₃C₂ based cermets, which are prepared by spark plasma sintering, was studied. Microstructural characterization of the cermets was done by scanning electron microscope. X-ray diffraction was performed to study the crystal structures. Mechanical properties such as hardness and fracture toughness were measured for the different developed cermets. The hardness and fracture toughness of the TiCN-WC-Co-Cr₃C₂ cermets without TiN, TiB₂, and nano-Si₃N₄ were 8.4 GPa and 3.4 MPa m^1/2, respectively. It was found that 5 wt% TiB₂ addition alone improved the corresponding hardness and fracture toughness to 19.2 GPa and 6.9 MPa m^1/2, respectively. The addition of 5 wt% TiN, improved the hardness and fracture toughness to 16.7 GPa and 6.9 MPa m^1/2, respectively. With the combination of 5 wt% TiN and 5 wt% TiB₂, the hardness and fracture toughness were improved to 15.5 GPa and 6.6 MPa m^1/2, respectively. But, the addition of 5 wt% Si₃N₄ showed a balanced improvement in both hardness (17.6 GPa) and toughness (6.9 MPa m^1/2). Fracture toughness did not change much for all the above cermets with different ceramic inclusions.     

Structure and stability of carbon nitride under high pressure and high temperature up to 125 GPa and 3000 K

The crystal structure of carbon nitride under high pressure and temperature was investigated up to megabar pressures using graphitic C₃N₄ as a starting material. It transformed to an orthorhombic phase above 30 GPa and 1600 K, which has a similar unit cell parameters (a = 7.6251(19), b = 4.4904(8), and c = 4.0424(8) Å at 1 atm) to those of reported hydrogen-bearing carbon nitride phases such as C₂N₂(NH) and C₂N₂(CH₂). Although the C:N ratio of this orthorhombic phase was carefully determined to be 3:4, FT-IR analysis showed a strong possibility of hydrogen contamination both in the starting and recovered samples. These results suggest that in the studied wide pressure and temperature range, hydrogen-bearing carbon nitride favors the orthorhombic structure with a fundamental composition of C₂N₂X where NH, CH₂, and even potentially vacancies can be flexibly accommodated in the X site.     

Fracture surface and correlation of buckling force with aspect ratio of C60 crystalline whiskers

The structure and mechanical properties of crystalline whiskers with an average diameter of 600 nm, composed of C₆₀ molecules were studied by transmission electron microscopy combined with nanonewton force measurements used in atomic force microscopy. C₆₀ nanowhiskers with a body-centered tetragonal structure are compressed along their long axis. Young's modulus of the C₆₀ nanowhiskers was estimated to be 28 ± 5 GPa. The buckling stress was correlated to the aspect ratio of length to diameter of the C₆₀ nanowhiskers. The (100) surface was the principal fracture surface of the C₆₀ nanowhiskers.     

Peculiarities of interaction in the Zn–C system under high pressures and temperatures

The wettability of graphite by a melt of zinc at 8.0 GPa in the temperature range 1900–3100 K has been investigated experimentally, and the temperature dependence of the wetting angle of graphite by the zinc melt determined. A polycrystalline diamond layer has been revealed at the interface in the zinc–graphite system in the diamond stability region. Metallographic and X-ray studies of zinc–carbon alloys obtained under high pressure have been carried out. The existence of the carbide ZnC₂ was found, which participates in two three-phase, non-variant equilibria in the system. It is determined that octahedral diamond crystals form in high-carbon alloys of zinc at the pressure of 8.0 GPa between 2100 and 2500 K.     

Fullerene (C60)–transitional metal (Ti) composites: Structural and biological properties of the thin films

Thin films of the binary C₆₀/Ti composites (with various phase ratios) were deposited on the Si(001) wafers and microscopic glass coverslips in continuous and micropatterned forms. The composites acquired a nanogranular structure with granules of about 50 nm in size. The RBS inspection confirmed homogeneous distribution of the phases and also the presence of oxygen. The Raman study suggested polymerization of the C₆₀/Ti composites into polymeric structures. The hybrid substrates were tested on biocompatibility—the films were seeded with human osteoblast-like MG 63 cells, and their proliferation was analyzed for 7 days. It has been found that the C₆₀/Ti composites can be counted as good supports for the adhesion of the selected MG 63 cells. The composites exhibited similar biocompatibility as the mix of amorphous carbon and titanium, but different than fullerene (C₆₀) solids.     

Diamond formation from graphite in the presence of anhydrous and hydrous magnesium sulfate at high pressures and high temperatures

The diamond forming regions in the anhydrous MgSO₄ and hydrous MgSO₄·H₂O–graphite systems, as well as the morphology of diamond synthesized from both of systems were investigated under the conditions of 6.5–7.7 GPa and 1600–2000 °C. The region was basically the same in both systems, although the melting point of MgSO₄ is at least 600 °C higher than that of MgSO₄·H₂O at 7.7 GPa. In the anhydrous system, the appearance of {111} faces is favored at higher temperatures and that of {100} faces at lower temperatures, while in the hydrous system, {111} faces are predominant at all examined conditions.     

Preparation and properties of B6O/TiB2-composites

B₆O/TiB₂ composites with varying compositions were produced by FAST/SPS at temperatures between 1850 and 1900 °C following a non-reactive or a reactive sintering route. The densification, phase and microstructure formation and the mechanical and thermal properties were investigated. The comparison to an also investigated pure B₆O material showed that the addition of TiB₂ in a non-reactive sintering route promotes the B₆O densification. Further improvement was obtained by sintering reactive B–TiO₂ mixtures which also results in materials with a finer grain size and thus in enhanced mechanical properties. The fracture toughness was significantly improved in all composites and is up to 4.0 MPa m^1/2 (SEVNB) and 2.6–5.0 MPa m^1/2 (IF method) while simultaneously a high hardness of up to 36 GPa (HV_0.4) and 28 GPa (HV₅) could be preserved. The high temperature properties at 1000 °C of hardness, thermal conductivity and CTE were up to 20 GPa, 18 W/mK and 6.63 × 10^?6/K, respectively.