bluuebunny/crossref_metadata_2025_split huggingface dataset:
Structural Isomers of C70
Authors: Raghavachari Krishnan 
Published: March 2011

ABSTRACTAlternative isomeric structures of C70 have been investigated using semiempirical and ab initio quantum chemical techniques. As in the case of C60, these isomers are characterized by the presence of pentagonal rings adjacent to each other. The lowest energy alternative isomer of C70 has only one pair of edge-sharing pentagons and lies ≈ 1.4 eV higher in energy than the ground state. This energy difference is smaller than that for the lowest energy alternative isomer of C60 which contains two pairs of adjacent pentagons and lies ≈ 2.0 eV higher in energy than its ground state.
The Preparation of C60/C70 and the Characterization by Different Techniques
Authors: Häussler P. , Bestgen H. , Meer H.-U. ter 
Published: March 2011

ABSTRACTWe report on the preparation of C60/C70 by the arc method. Evidence of monofunctional C60O, and C60-CH2 has been observed by mass spectrometry. Dependent on their preparation conditions, high resolution transmission electron microscopy of crystals consisting of mixtures of C60/C70 show different structure. Needle-like crystals grown from toluene show superlattice formation while single crystals obtained by sublimation have fcc structure. UV-VIS-absorption spectra in toluene are compared to those of films of various thickness and crystallinity.
Optical Spectra of Fullerenes: C60 a new amorphous semiconductor?
Authors: Skumanich Andrew 
Published: March 2011

ABSTRACTThe optical absorption spectra for thin films of sublimed C60 and C70 are presented. Both transmission spectroscopy and photothermal deflection spectroscopy are used to obtain the spectrum over a wide energy range (0.4 to 6.2 eV) for the same film. The optical gap varies for the different fullerenes. The value for C60 is ∼ 1.6 eV and the corresponding transition appears to be optically forbidden given the weakness of the absorption. Structure is observed on the “gap edge” which can be attributed to vibronic transitions. The gap region can be described in terms used for amorphous semiconductors, having features such as an Urbach edge, and sub-gap defect absorption. The similarity suggests that the nature of fullerene thin films may be analogous to amorphous silicon and carbon films. These films have both a molecular and an amorphous semiconductor nature.
Positron state and Lifetime in C60/C70 Fullerites
Authors: Schaefer H.-E. , Forster M. , WÜrschum R. , Banhart F. 
Published: March 2011

ABSTRACTThe present paper reports on positron lifetime measurements on Ceo/C 70 fullerite powder as a function of temperature and quasi-hydrostatic pressure in order to give an estimate of the positron annihilation site in the fullerite lattice. A single-component positron lifetime of 402 ps is observed which significantly decreases under quasi-hydrostatic pressure. From this and the soft intermolecular properties of the fullerites one can conclude that the positron is annihilated rather in the intermolecular space than inside the fullerene molecules. However, positron trapping at lattice defects, which are observed by high-resolution electron microscopy, cannot be ruled out.
The Early Stages of Microwave-Assisted Chemical Vapor Deposition of Diamond on Fused Silica Substrates
Authors: Rankin J. , Shigesato Y. , Boekenhauer R.E. , Csencsits R. , Paine D.C. , Sheldon B.W. 
Published: March 2011

ABSTRACTThe early stages of the microwave assisted chemical vapor deposition of diamond on fused silica and silicon substrates were examined with Raman spectroscopy and scanning electron microscopy. Grain size as a function of time was determined for both substrates. Grains formed on fused silica were larger, with smoother growth surfaces than those formed on silicon substrates under the same conditions. For deposition on silica, the particle morphology changes from cuboid to cubo-octahedral for deposition times between 5 and 15 minutes. Also, the glass surfaces were etched during the pretreatment and deposition stages. These results are discussed in terms of mass transport limited growth, and chemical interactions between the gas-phase and the substrate surface.
Growth of Diamond from Sputtered Atomic Carbon and Atomic Hydrogen
Authors: Olson Darin S. , Kelly Michael A. , Kapoor Sanjiv , Hagstrom Stig B. 
Published: March 2011

ABSTRACTDiamond thin films were grown on a scratched silicon crystal surface by a novel CVD technique. The substrate was exposed to a bombardment of sputtered carbon atoms from a graphite target in a helium DC glow discharge, and subsequently exposed to atomic hydrogen generated by a hot tungsten filament. The resulting diamond films were characterized by Raman spectroscopy and SEM. Deposited film quality, and growth rate are presented as a function of carbon flux, and atomic hydrogen flux. The observed increase in growth rate with atomic hydrogen indicates that a surface reaction mechanism may be responsible for growth. The saturation of the utilization of carbon confirms that the diamond growth is probably a surface reaction. Based on this work we propose that the growth of diamond films in the sequential CVD reactor is most likely governed by surface reactions, and that the necessity of gas phase precursors can be precluded.
Homoepitaxial Growth Rate Studies on Diamond (110), (111), and (100) Surfaces in a Hot-Filament Reactor
Authors: Chu C. Judith , Bai Benjamin J. , Komplin Norma J. , Patterson Donald E. , D'evelyn Mark P. , Hauge Robert H. , Margrave John L. 
Published: March 2011

ABSTRACTGrowth rates of homoepitaxial (110), (111), and (100) diamond films were experimentally determined, for the first time, in a hot filament reactor using methane and carbon tetrachloride as the carbon source. Methane concentrations from 0.07 % to 1.03 % in H2 were studied at a substrate temperature of 970°C. Growth rates were found to be crystal-face dependent with respect to methane concentration, being linear or first order for the (100)-orientation, sublinear for (110), and sigmoidal for (111). The observed growth kinetics of (111) suggest the viability of an acetylene mechanism for (111), along with the methyl radical mechanism at methane concentrations above 0.73%. CC14 concentrations from 0.06% to 0.69% in H2 were also investigated at a substrate temperature of 970°C. Growth rate behavior was similar to that of methane for all three crystal faces.The temperature dependence of the growth rates was also crystal-orientation dependent. At substrate temperatures above 730°C, growth rates are thought to be mainly transport limited, yielding effective activation energies of 8±3, 18±2, and 12±4 kcal/mole for (100), (110), and (111) orientations, respectively. At substrate temperatures below 730°C, growth rates are thought to be surface reaction rate-limited, with an overall effective activation energy of 50±19 kcal/mole for the three crystal-orientations studied.
Formulation of a Mathematical Process Model for the Foaming of a Mesophase Carbon Precursor
Authors: Sandhu S. S. , Hager J. W. 
Published: March 2011

ABSTRACTMathematical equations have been formulated to guide an experimental effort to produce an open-celled mesophase pitch foam. The formulation provides an analytical description of homogeneous bubble nucleation and growth, diffusion of the blowing gas through the liquid to the bubble surface, and the average material thickness between bubbles. Implications of the formulation for the experimental production of mesophase pitch foam are discussed.
The Role of Atomic Hydrogen and Oxygen in Low Temperature Growth of Diamond by Microwave Plasma Assisted Cvd
Authors: Muranaka Y. , Yamashita H. , Miyadera H. 
Published: March 2011

ABSTRACTDiamond films grown in the microwave plasmas of CO(7–8%)-O2(0–2.2%)-H2 systems in the range of 130–750°C were characterized by scanning electron microscopy, Raman spectroscopy, and cathodoluminescence (CL) studies. The films grown in the CO-O2-H2 system had much better crystallinity than those grown in the CO-H2 system. This was because oxygen extremely purified diamond films by suppressing polyacetylene inclusion, and prohibited the vacancy formation in the crystallites. These oxygen functions have indicated the possibility that high quality diamond films (FWI-tM of the diamond Raman peak =4.0–4. lcm−1) close to natural diamond (FWHM=3.0cm−1) were obtained in the CO(8%)-O2(2.2%)-H2 system between 400 and 750°C. Though crystallinity deterioration occurred at 130°C, the obtained film (FWHM=10.2cm−1) in the CO(8%)-O2(2.2%)-H 2 system was of good crystallinity comparable to those (FWHM=7–21cm−1) grown by conventional CVD processes and gas systems between 590 and 1327°C. The CO-O2-H2 microwave plasma was concluded to be one of the best environment for the low temperature growth of highly purified diamond films of good crystallinity.
Near-Surface Optical Detection of CH3 During Diamond Growth
Authors: Corat E. J. , Goodwin D. G. 
Published: March 2011

ABSTRACTResonance-enhanced multiphoton ionization (REMPI) has been used to detect the methyl radical CH3 within 0.5 mm of the substrate during CVD diamond growth. A strong dependence of the CH3 REMPI signal near the surface on substrate temperature is observed, which is not seen further from the surface. Below 1000 K, the observed temperature dependence may be characterized by an activation energy of approximately 4 ± 1 kcal/mole. The cause of the methyl depletion at low temperatures is not yet clear, but may be due to either gas-phase recombination near the surface or surface chemistry. The same qualitative behavior is observed for different substrate-filament distances and for gas compositions from 0.5% to 5% CH4 in H2.
Widealized Strut Geometries for Open-Celled Foams
Authors: Hager Joseph W. 
Published: March 2011

ABSTRACTOpen-celled foam is modeled as a network of struts formed by the nucleation, growth and intersection of uniform spherical bubbles arranged in a regular three-dimensional array. The number of struts per unit cell and their characteristic geometry and orientations depend on the bubble-centering algorithm chosen to define the array and on the degree of bubble intersection. A foam model based on a body-centered cubic arrangement of uniform spherical bubbles is used to compare strut geometries over a range of bubble interferences. The model predictions are compared to the microstructures observed in amorphous carbon foam. The significance of these idealized foam results for graphitic carbon foams is discussed.
High Temperature Rectifying Contacts on Semiconducting Diamond Using Doped Silicon
Authors: Venkatesan V. , Thompson D.G. , Das K. 
Published: March 2011

ABSTRACTHigh temperature rectifying contacts have been fabricated on naturally occurring lib semiconducting diamond crystals using highly doped Si. Polycrystalline Si deposited by low pressure chemical vapor deposition (LPCVD) and amorphous Si deposited by sputtering were investigated. Following LPCVD deposition, the polycrystalline Si filn was doped with P by solid state diffusion at a temperature of 900°C using a POCI3 source. Boron doped and As doped Si films were deposited by sputtering from highly doped Si targets. Current-voltage measurements were performed on the fabricated P doped, B doped and As doped Si contacts from room temperature up to ∼400∼C. In all cases, the contacts yielded excellent rectification in the temperature range investigated. Current conduction in doped Si/diamond systems appears to be space charge limited. The position and concentration of deep levels in a natural lib diamond crystal have been determined from an analysis of space charge limited current-voltage (I-V) characteristics.
Shock-Induced Martensitlc Transformation of Highly Oriented Graphite to Diamond
Authors: Erskine David. J. , Nellis William. J. 
Published: March 2011

ABSTRACTShock-wave profiles of highly ordered pyrolytic graphite shocked normal to the basal plane of the graphite crystal structure have been measured. For graphite with sufficient orientational order a martensitic transformation to a diamond-like phase is observed with a transition onset pressure 19.6±0.7 GPa, the stability limit of the graphite structure under shock compression. The minimum overpressure required for the transformation is not more than 6 GPa.
Modelling and Measurements of the Thermal Diffusivity of Diamond Thin Film Samples
Authors: Siang Tan Hong , Ping Hui , Ahn Jaeshin , Hock Tan Fong 
Published: March 2011

ABSTRACTA new method for the modelling and measurement of the thermal diffusivity of very thin films is proposed, and is applied to diamond thin film samples grown by chemical vapor deposition using a mixture of methane and hydrogen. This method is based on a two-dimensional model of the heat conduction process in a thin film to which a delta-function heat pulse is applied as a line source. The mathematical solution is specialized to obtain expressions for the temperature pulse propagating through the plane of the film. With suitable approximations, it is shown that the thermal diffusivity of the film can be deduced directly from a measurement of the temperature excursion with time at various points on the films. The advantages and difficulties of the measurement using this method will be discussed.
Hard Amorphous Carbon Films Deposited by ArF Pulse Laser Ablation of Graphite at Room Temperature
Authors: Xiong Fulin , Chang R. P. H. 
Published: March 2011

ABSTRACTHard amorphous carbon films have been deposited by ArF pulsed laser ablation of graphite at room temperature, with the laser power density of 5x108 W/cm2. The films prepared in the high vacuum environment possess remarkable diamond-like properties with a hardness up to 38 GPa and an optical energy band gap of 2.4 eV. The properties of the films doped with nitrogen vary with the nitrogen content, but improve interface adhesion, resulting in the extension of the film thickness limit to a greater range. The results suggest that the properties of the laser ablation deposited diamond-like carbon films depend not only on the laser power density, but also strongly on the laser wavelength or photon energy.
An X-Ray Scattering Study of Amorphous Hydrogenated Carbon
Authors: Burke T.M. , Huxley D.W. , Newport R.J. , Bushnell-Wye G. , Franks J. 
Published: March 2011

ABSTRACTThe structure of “diamond-like” carbon, a-C:H has been studied using the Xray diffraction facilities available at the Synchrotron Radiation Source (Daresbury Lab, UK). The principle advantage of using the SRS is that high fluxes are available at short wavelengths (λ ∼ 0.6Å): it is therefore possible to extract the “diffuse” scattering profile associated with all amorphous systems with considerable precision over a very wide K-range. A novel diffraction technique is also described in which the X-ray beam is incident on an as-deposited thin film at or near the critical angle. In this geometry we demonstrate that it is possible to gather diffraction data on -1μm films in-situ.
Electron Collisions in the Plasma as a Major Factor in the Growth of Amorphous Hydrogenated Carbon
Authors: Lin S. , Feldman Bernard J. 
Published: March 2011

ABSTRACTWe have measured the dependence of the optical bandgap of amorphous hydrogenated carbon as a function of self-bias voltage and external bias voltage. It is observed that there is a much stronger variation in the optical bandgap with self-bias voltage than external bias voltage. We explain this observation in terms of collisions in the plasma between electrons and precursor molecules and ions that break carbon-hydrogen bonds. Increased self-bias voltage causes increased collisions, more broken CH bonds, precursor ions and molecules more deficient in hydrogen, leading to grown films with lower hydrogen concentrations and thus smaller optical bandgaps. This interpretation is further supported by plasma emission spectroscopy where the intensity ratio of CH to H lines decreases with increasing self-bias voltage, demonstrating that increased self-bias voltage does lead to increased electron collisions in the plasma and more broken CH bonds.
Hard Amorphous Carbon Pvd Coatings
Authors: KNOTEK O. , LÖffler F. , Brand J. 
Published: March 2011

ABSTRACTAmorphous diamond-like carbon (ADLC) coatings are deposited on cemented carbides, high speed steels (HSS) and other steels, using arc-ion plating. The paper describes both the problems and the potential of this process with its many influencing parameters. It also details a number of properties of hydrogen-free a-C films deposited at coating temperatures of 200°C; as an example, a tribological application is described.
Ligament Structure of Open-Cell Carbon Foams and the Construction of Models Based on That Structure
Authors: Anderson David P. , Gunnison Katie E. , Hager Joseph W. 
Published: March 2011

ABSTRACTThe ligament structure of several open-cell carbon foams was examined by optical and electron microscopy. The arrangement, sizes, and shapes of the ligaments were measured and analyzed according to the cell sizes. The ligament lengths and cross-sections vary with the cell sizes in a simply scaled fashion. A models based on the observed dodecahedral-like arrangement of ligaments was constructed consisting of 12-, 14-, and 15-faced polyhedra with five-edged faces dominating.
Thermal and Mechanical Properties of Diamond-Like a-C:H Films
Authors: Amato G. , Benedetto G. , Boarino L. , Demichelis F. , Pirri C. F. , Spagnolo R. , Tagliaferro A. 
Published: March 2011

ABSTRACTDiamond-like amorphous carbon and hydrogenated amorphous carbon films (DLC) prepared by rf sputtering have been characterized by means of measurements of optical gap, hardness and Young's modulus. Preliminary results of the application of the photothermal displacement technique (PTD) are also reported, confirming that this method can in principle be applied for a more complete characterization of DLC films at room temperature and low temperatures.
Internal Stress in Nitrogen Doped Diamond-Like a-C:H Films
Authors: Franceschini D. , Achete C. , Freire F. , Mariotto G. 
Published: March 2011

ABSTRACTResults of a study on internal stress, hardness and structure of nitrogen doped amorphous hydrogenated hard carbon films deposited by r.f. glow discharge on silicon are presented. Films obtained for different bias voltage (Vb), N2partial pressure and total gas pressure were characterized by Nuclear Reaction (NR), Elastic Recoil Detection (ERD), Rutherford Back-Scattering (RBS), Infrared Spectroscopy (IR) and Raman Scattering. The elemental composition, density and structure are correlated with Vickers hardness, and internal stress values obtained by the substrate bending method. It has been observed that internal stress considerably decreases with increasing nitrogen content, contrary to the behavior shown by hardness, structure and hydrogen concentration, which remain unchanged.
Characterization of the Structure of Carbon Material Through the sp3/sp2 Bonding Ratio Measurements
Authors: Demichelis F. , Martino C. De , Pirri C.F. , Tagliaferro A. 
Published: March 2011

ABSTRACTHydrogenated amorphous carbon (a-C:H) films have been deposited by sputter assisted plasma chemical vapor deposition (CVD). The relative concentration of sp3 and sp2 hybridized carbon in samples is determined by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies and by a new method through the complex dielectric constant deduced from optical transmittance and reflectance. The results are compared and discussed.
The Structure of Amorphous Hydrogenated Carbon by Neutron Diffraction
Authors: Huxley D.W. , Honeybone P.J.R , Newport R.J. , Howells W.S. , Franks J. 
Published: March 2011

ABSTRACTNeutron diffraction data from a large, off-substrate sample of amorphous hydrogenated carbon (a-C:H) is presented and discussed. The material is prepared using a fast-atom deposition system using acetylene as the precursor gas. The experiments were performed on the ISIS pulsed neutron source (Rutherford Appleton Laboratory, UK) which is capable of yielding data over an exceptionally wide dynamic range; this ensures a real-space resolution sufficient to resolve directly, for the first time, contributions from the principle C-C bond types. Precise details on the C-H correlations are also revealed by the data, including the presence of molecular hydrogen trapped within distorted spheroidal cages. Quantitative complementary data on the vibrational states of the bonded hydrogen, derived from inelastic neutron scattering (INS) using a simple force-field model, is also presented. In particular, the INS data is used to provide a reliable estimate of the CH:CH2 ratio.
Production of Diamond-Like Carbon Films by Plasma Source Ion Implantation
Authors: Xie Ling , Worzala Frank J. , Conrad John R. , Dodd Richard A. 
Published: March 2011

ABSTRACTIn addition to being successfully used for ion implantation, the plasma source ion implantation (PSII) technique has been used to produce diamond-like carbon films. Homogeneous, adherent films were obtained on silicon and stainless steel substrates under 2 kV pulse bias voltages and 50 mtorr methane plasma pressure. Chemical composition analysis was made using Auger electron microscopy. Fretting wear tests and scratch tests were performed to study the tribological and adherent properties. Cross sectional TEM samples were prepared. The interfacial microstrucutres and chemical compositions were analyzed using transmission electron microscopy and scanning transmission electron microscopy.
A Computer Modelling Study of the Structure of a-C:H
Authors: Huxley D.W. , Newport R.J. , North A.N. , Walters J.K. 
Published: March 2011

ABSTRACTNeutron diffraction data has been used as input to a computer-modelling algorithm based on the “Reverse Monte Carlo” technique. Using this method the positions of ∼ 5000 “atoms” in a box, with full periodicity, are altered until the associated model structure factor agrees with the analogous experimental curve to within errors. It is then possible to estimate the partial pair distribution functions (i.e. those associated with C-C, C-H and H-H correlations), bond angle distributions, coordination number distributions, etc. Whilst X-ray data is well-conditioned for the study of the carbon-carbon network, neutrons are sensitive to the interference terms involving hydrogen. We present an exploratory study of the effectiveness of the RMC method in this context, and suggest viable options for the future use of the method in model building.
Thermal Fatigue Resistance of A Carbon-Carbon Composite
Authors: Gu Q.C. , LepistÖ T.K. , Kettunen P.O. 
Published: March 2011

ABSTRACTThermal fatigue resistance of carbon-carbon composites with three different fiber surface treatments is studied in thermal cycles between 100 °C and 1,700 °C up to a number of cycles of 100 in free and restricted expansion conditions. The effects of thermal cycles were studied by SEM paying attention especially to structural damage and interfacial debonding between fibers and matrix. Bending tests subsequent to 10 thermal cycles were used to study the effect of thermal cycling on mechanical properties of the composites. The effect depends on the surface treatment of fibers prior to pyrolization. In some cases, the bending strength decreased due to the thermal cycling, whereas a suitable surface treatment minimized the damaging effect and increased the pseudo-ductility of the composite.