bluuebunny/crossref_metadata_2025 huggingface dataset:
High-Temperature Dielectric Properties Measurements of Ceramics
Authors: Bringhurst Shane , Andrade Octavio M. , Iskander Magdy F.
Published: March 2011

This paper dicusses experimental arrangements, describes measurement techniques and presents experimental results for hightemperature broadband dielectric properties measurements of ceramics. The cavity perturbation technique and the open-ended coaxial line method are used in these experimental measurements. The design and construction details of cavities and probes are described and representative results of measurements on zirconia and alumina samples (green and sintered) are presented. Results of measurements made on insulating materials are shown. In general measurements are made in the frequency band 1 to 10 GHz and temperatures up to 1000°C.
Calibration Method for Open-Ended Coaxial Probe/Nector Network Analyzer System
Authors: Blackham David
Published: March 2011

The vector network analyzer readily lends itself to a variety of techniques for measuring material properties. The open-ended coaxial probe is one technique that allows swept frequency measurements of permittivity. The open-ended coaxial probe technique requires little or no sample preparation and can make non-destructive measurements on a variety of materials. It is especially suited for materials in liquid or semi-solid form. An understanding of network analyzer calibration and the coaxial probe model provides insight into calibration techniques that minimize systematic errors. This understanding also.facilitates the extension of measurement conditions beyond normal measurement constraints leading to a simple procedure which provides a first order correction of perturbations to the systematic errors.
Modeling The Microwave Frequency Dielectric Properties of Thermoplastic Composite Materials
Authors: Jackson Mitchell L. , Stern Curtis H.
Published: March 2011

Mixture models were studied in an effort to predict the microwave frequency permittivities of unidirectional-fiber-reinforced thermoplastic-matrix composite materials as a function of fiber volume fraction, fiber orientation relative to the electric field, and temperature. The permittivities of the constituent fiber and plastic materials were measured using a resonant cavity perturbation technique at 9.4 GHz and at 2.45 GHz. The permittivities of the composite specimens were measured using a reflection cavity technique at 9.4 GHz and at 2.45 GHz. Simple “rule-of-mixtures” models that use the fiber and plastic permittivities have been found to approximate the complex dielectric properties of the composite for varied fiber volume fractions. The permittivities of oriented composites were modeled using a tensor rotation procedure. Composite permittivities were modeled with temperature up to the glass transition temperature of the thermoplastic matrix.
Ceramic Materials for High-Quality Dielectric Resonators of Microwave Range
Authors: Nenasheva Yelizaveta A.
Published: March 2011

The dielectric properties of ceramic materials and dielectric resonators on the basis of oxides of Ba,Ti and alkaline earth elements, and also of the calcium titanate-lanthanum aluminate system, with ε =82 and ε =40…48, frequency temperature coefficient (TCf) from - 10ppm/°C to + 10ppm/°C have been studied. Ceramics with ε =82 ensures the DR Q-factor of 3000 and more at f=3GHz. The Q-factor of DR made of ceramics with ε=42, TCf =0 reaches 8000 at f=7GHz.
Highlights from Panel Discussion on “Key Issues for Future Developments in Microwave Processing”
Authors: Gac Frank D. , Iskander Magdy F.
Published: March 2011

This proved to be a very insightful discussion, thanks to an enthusiastic audience and the team of panelists listed below:

Peter Angelini, Oak Ridge National Laboratory

Raymond F. Decker, Wavemat/Paton Tek Inc.

E. Eugene Eves, III, Raytheon

Frank D. Gac, Los Alamos National Laboratory

John F. Gerling, Gerling Laboratories

Magdy F. Iskander, University of Utah

Bernard Krieger, Cober Electronics

Richard Smith, Electric Power Research Institute

William B. Snyder, Microwave Materials Technology, Inc.

Wayne R. Tinga, University of Alberta

Although the panelists represented a mix of individuals from government, academia, and industry, only one aspect of industry was represented, namely microwave system manufacturers. For future panel discussions, it is recommended that the materials manufacturing (i.e., microwave user) sector also be represented.
Electromagnetic Field Modeling of a Loaded Microwave Cavity
Authors: Chaussecourte P. , Lamaudiere J.F. , Maestrali B.
Published: March 2011

The unavoidable way of a better conception of cavities for industrial applications is electromagnetic field numerical computation with the aim of placing the fields E and H into the material, and furthermore to obtain some characteristics like the input impedance of the cavity or the dissipated power in the sample. From Maxwell's equations and using a Finite Element Method with 3D edge elements we get a linear system from which, after resolution, we can obtain several kinds of results: research of eigen modes of an empty or partially dielectric loaded cavity, that is, for a bounded domain, all the frequencies that could be excited, using only dimensionnal (L,l,h) and physical (εʹ,εʺ) characteristics, or response of a loaded cavity to an external excitation, that is, setting a TE10 mode in a waveguide feeding a cavity, obtain the distribution of field inside the oven and the dielectric. An original approach allows us, by projection of the excitation on eigen modes, to understand the behavior of the cavity and the interaction wave-product. Our computation is in very good agreement with analytical results we can obtain in some specific configurations (eigenvalues of an empty cavity, or transmission of only the TE10 mode in a correctly shaped waveguide).
Coupling with an Adjustable Compound Iris in a Single Mode Applicator
Authors: Sa'adaldin Hussamaldin S. , Black W. Murray , Ahmad Iftikhar , Silberglitt Richard
Published: March 2011

Coupling of the input power to a dielectric in a rectangular microwave cavity by using a compound iris having both inductive and capacitive slits, has been investigated. The compound iris is used in combination with a variable tuning plunger to maintain a resonant frequency of 2.45 GHz and minimize the reflected power. Under certain conditions, the compound iris has been observed to achieve an improved coupling of the input power, resulting in higher temperature of cylindrical 0.95 cm rods of silicon carbide (SiC) and alumina (A12O3). By placing the SiC perpendicular to the applied E field (with SiC enclosed in a porous insulation partially filling the applicator), coupling with the compound iris by adjusting both inductive and capacitive slits was achieved for a wide range of values compared to the use of a single pair of inductive slits. The coupling was more sensitive to a precise positioning of the single inductive slit. Such a difference was not observed for the case where the sample was placed parallel to the E field. In temperatures over 1600°C have been reached with a 600 W of power injected into the cavity. In tests with low loss alumina samples (99.8% pure) placed parallel to the E field, the compound iris permitted a greater coupling and higher temperature. The presence of alumina insulation had a significant effect in achieving higher temperatures, and occassionally preventing the initiation of arcing.
Synthesis of Fullerenes (C60 AND C70) by Combustion of Hydrocarbons in A Flat Flame Burner
Authors: Mitra S. , Pope C. J. , Gleason K. K. , Makarovsky Y. , Lafleur A. L. , Howard J. B.
Published: March 2011

Fullerene (C60 and C70) synthesis by combustion of ethylene and benzene in a flat flame burner was investigated. This method of fullerene synthesis is particularly attractive because of its potential of scale up. Also the ability to change the flame conditions andcontrol the yield of C60 and C70 makes this method versatile. No fullerenes were found in soot samples collected from the ethylene flame. However, fullerenes were formed in a benzene flame with C/O = 0.88 and operated at 40 torr, with cold gas velocity of 25.3 cm/s (273 K) and containing 10% argon. The concentration of fullerenes in this flame was found to depend strongly on the height above the burner surface. It exhibited a strong maxima at about 1.0 cm above the burner height suggesting the presence of both growth and destruction mechanisms.
Temperature Dependent Structural Studies of K- and RB- Doped C60
Authors: Zhou Otto , Zhu Qing , Vaughan Gavin B.M. , Fischer John E. , Heiney Paul A. , Coustel Nicole , Mccauley John , Smith Amos B , Cox David E.
Published: March 2011

The temperature dependent structural evolutions of RbxC60 (x = 3, 5, 6) and K4C60 were studied using both in-house andsynchrotron x-ray powder diffraction and thermal analysis techniques over a temperature range of 10K - 673K. The superconducting face centered-cubic (fcc) Rb3C60 and the body centered-tetragonal (bct) M4C60(M = K, Rb) phases are found to be line compounds in this temperature range, while the body centered-cubic (bcc) phase forms a solid solution in which the solubility of vacant M sites increases with temperature. The orientation of the C60 molecules in the K4C60 phase was analyzed. A crystalline fcc Rb1C60 phase is stable only above room temperature.
Nucleation, Cluster Growth, and Structure of Crystalline C60 and C70
Authors: Yao Nan , Behal S. K. , Klein C. F. , Disko M. M. , Fung S. C. , Sherwood R. D. , Creegan K. M. , Cox D. M.
Published: March 2011

The nucleation and cluster growth of C60 and C70 crystallites on various substrates at ambient temperature have been investigated using electron microscopy. It was found that the initial nucleation is closely associated with surface defects, and the fullerenes are much more strongly bonded to each other than to the substrate. Sublimed C60 or C70 crystallites nucleate at the step edge in the liquid state and are aligned with the step walls and terraces through the process of coalescence. Reflection Electron Microscopy (REM) studies have shown an abnormal profile of C60 grown crystals as a result of the interaction of C60 molecules with the surface strain field during crystal growth. Transmission electron diffraction patterns reveal a twin structure with (110) habit plane for the low temperature ordered phase.
Calculation of the Stability and the Polarizability of Isolated Fullerene Molecules as a Function of Charge State
Authors: Quong Andrew A. , Pederson Mark R.
Published: March 2011

We present first-principles local density functional calculations of the electronic structure and energetics of neutral and negatively charged fullerene molecules. We find thatthe negatively charged -1 state is stable relative to the neutral molecule and that the -2 state is stable relative to the neutral molecule but not to the -1 state of the molecule. We have also performed calculations of the electronic polarizabilities for different charged states and developed a simple model to estimate the dielectric constant of fullerene based crystals.
Structural Models of Negatively Curved Graphitic Carbon
Authors: Townsend S. J. , Lenosky T. J. , Muller D. A. , Nichols C. S. , Elser V.
Published: March 2011

We have developed an automatic computational method for generating graphitic carbon structures. We propose models for crystalline and amorphous forms of carbon consisting of a single negatively curved graphitic sheet forming an extended structure. Stability and elastic properties are computed using an energy estimate fit to local density approximation(LDA) calculations. These forms of carbon are found to be more stable than C60. The radial distribution function for the random structures closely matches that of films of amorphous carbon grown on NaCI substrates from sublimated graphite.
Preparation and Properties of Low Density Replica Carbon Foams
Authors: Hulsey S. S. , Poco P. K.
Published: March 2011

Low density foams are used for a variety of applications, including catalytic supports, battery anodes, microporous membranes, and laser fusion targets. The technique for making replica carbon foams described in this paper has been previously reported[I] and involves a process in which an inorganic substrate (sodium chloride) is infused with a carbonizable polymer. After carbonization, the substrate is removed by a leaching process and the wet foam is dried; the resultant foam is referred to as replica carbon. This paper describes improvements in the processing which result in a smaller pore size and improved foam homogeneity.

The original substrate is the single most important factor affecting the resultant structure. Techniques to improve the uniformity of the substrate and the translation of substrate anomalies into the final product are described.
Production and Characterization of Metal-Encapsulated Fullerenes
Authors: Johnson Robert D. , Yannoni Costantino S. , Hoinkis Mark , Vries Mattanjah de , Salem Jesse R. , Crowder Mark S. , Bethune Donald S.
Published: March 2011

We report here the arc-production and spectroscopic characterization of fullerene-encapsulated metal atoms and metal-atom clusters. In particular, both solution and solid-stateelectron paramagnetic resonance (EPR) spectra of LaC82', YC82', SCC82' and Sc3 C82 have been obtained. Additional species containing rareearth atoms and clusters have been produced. The results suggest, for example, that the three scandium atoms in Sc3C82 form a molecule In the shape of an equilateral triangle — as was previously suggested for Sc3 molecules Isolated In a cryogenic rare-gas matrix. The spectraof the MC82 species (M =La, Y, Sc) exhibit small hyperfine couplings and g-values close to 2, suggesting that they can be described as a +3 metal cations within – 3 fullerene radical anion cages. Sc2C2n species — the most abundant metailofuilerenes In the scandlum-fullerene mass spectrum — are EPR-silent, even though Sc2 is EPR-actlve In a rare-gas matrix at 4.2K. A broader Implication of this work Is that production of macroscopic quantities of metal-containing fullerenes may make possible the fabrication of exotic new structures with regular arrays of metal atoms or clusters Isolated in fullerene molecules, resulting in new typesof host/guest nanostructured materials.
Mass Spectrometry of Metallofullerenes
Authors: Ross Mark M. , Nelson H.H. , Callahan J.H. , Mcelvany S.W.
Published: March 2011

Fullerenes with a metal atom inside of the cage, metallofulierenes, were produced by either laser vaporization or a graphite arc and characterized using a variety of mass spectrometric methods. First, yttrium-fullerene adducts were formed by direct laser vaporization of samples consisting of graphite, yttrium oxide and fullerenes. Fragmentation and oxidation ion/molecule reactions showed that the laser-generated adducts are endohedral complexes (Yx@Cn), in contrast to externally-bound Y(Cn)+ species formed by gas-phase reactions. In addition, evidence was obtained for laserinduced bulk coalescence reactions yielding the metallofullerenes. Second, negative ion/desorption chemical ionization mass spectrometry was used to characterize metallofullerenes in arc-generated soot, pyridine extracts and the extract residue. The pyridine extracts of La2O3/graphite soot contain mostlyLa@C82 and La2@C80', in addition to (empty) fullerenes. However, the raw soot and the extract residue contain a broader range of metallofullerenes with relative abundances different than those observed from the extract (e.g. abundant La@C60', Lax@Cn') and La@C74). The thermal desorption behavior of the doped and undoped fullerenes indicate an interaction between the Cn and Lax@Cn species. Analysis of aqueous solutions of dried pyridine extracts of La2O3/graphite soot show Cn and Lax;@Cn' which is consistent with the possible presence ofmetallofullerene/ fullerene ionic complexes, (Lax@Cn)+C-n.
Structural Isomers of C70
Authors: Raghavachari Krishnan
Published: March 2011

Alternative 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

We 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

The 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

The 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

The 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

Diamond 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

Growth 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

Mathematical 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

Diamond 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

Resonance-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

Open-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

High 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

Shock-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

A 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

Hard 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

The 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

We 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

Amorphous 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

The 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

Diamond-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

Results 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

Hydrogenated 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

Neutron 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

In 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

Neutron 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

Thermal 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.
The Structure of Amorphous Hydrogenated Silicon:Carbon Alloys Using X-Ray and Neutron Scattering and Computer Simulation - The Effect of Hydrogen Dilution
Authors: Burke T. M. , Honeybone P. J. R. , Huxley D. W. , Newport R. J. , Frauenheim Th. , Blaudeck P. , Kohler Th. , Hotham C.
Published: March 2011

Neutron and X-ray diffraction techniques have been applied to the study of two samples of a-Si:C:H. Both samples were prepared using conventional glow discharge methods, but the hydrocarbon/silane precursor gas was diluted with hydrogen in one case. Analysis of the X-ray diffraction data gives a clear picture of the silicon network, since the scattering profile is dominated by the Si-Si correlations. The high real-space resolution neutron diffraction data, however allows one to comment on the effect of this dilution on the silicon-carbon bonding morphology, and in particular on the degree to which the additional hydrogen enhances hetero-coordination. In addition we present the results of a preliminary computer simulation study of the structure of a-C:H and a-Si:H using an approximate molecular dynamic density functional theory, and discuss its viability in the study of the more complex a-Si:C:H ternary alloy.
Contact-Free Determination of Scattering Times in Heterojunction Device Structures
Authors: Omling P. , Linke H. , Ramvall P. , Meyer B. K.
Published: March 2011

In this paper we present contact-free measurement techniques which are important for the evaluation of heterojunctions of interest for optical as well as high-speed devices. The techniques are the microwave-detection of Shubnikov-de Haas oscillations, photoluminescence-detected magneto-oscillations, and optically detected cyclotron resonance using microwaves (ODCR) as well as far-infrared lasers (FIR-ODCR). The techniques are illustrated by several examples, and the possibilities to determine 2D carrier concentrations, effective masses, and scattering times in the heterojunction structures are discussed.
Photoreflectance Characterization of the Semi-Insulating InP Substrate Interface with InGaAs and InAlAs Epilayers
Authors: Zhou Weimin , Shen H. , Pamulapati J. , Dutta M. , Bennett B. R. , Perry C. H. , Weyburne D. W.
Published: March 2011

Photoreflectance (PR) has been performed on a series of undoped and n-type, InGaAs and InAlAs molecular beam epitaxy (MBE) grown layers with different In mole fractions, and epilayer thicknesses on Fe-doped semi-insulating (SI)-InP substrates. From investigations of the temperature dependence, time constant dependence and an additional cw light beam intensity dependence, three substrate peaks are identified as an excitonic transition from the substrate, a free electron transition near the interface which gives a Franz-Keldysh oscillation (KFO), and a transition from the spin-orbit split-off valence band. The results are indicative of a redistribution of charge near the substrate interface in the process of MBE growth; the associated PR signal (phase) could be used for in-situ monitoring of epilayer growth on SI-InP wafers.
On the Origin of the Contrast Inhomogeneities Found on In0.52Al0.48As Layers Grown on InP Substrates At High Temperatures
Authors: Peiro F. , Cornet A. , Morante J. R. , Georgakilas A.
Published: March 2011

The present work deal with the coarse contrast modulation along the <010> directions observed in (100) planar view TEM observations of In0.52Al0.48As layers grown on InP substrates at temperatures higher than 550°C. The most important features of these contrast inhomogeneites are briefly reviewed. Besides, the appearance of the contrast in [011] cross-sectioned specimens is presented and its origin related to precipitates at the layer-substrate interface. The density of precipitates (ρp) has been found to increase as Tg rises, and an approximate value of the activation energy needed to induce the precipitation has been derived from the correlation of ρp and Tg.
Heavily Carbon Doped P-Type GaAs/InGaAs Strained-Layer Superlattices Grown by Mombe
Authors: Qi Ming , Luo Jinsheng , Shirakashi J. , Tokumitsu E. , Nozaki S. , Konagai M. , Takahashi K.
Published: March 2011

Heavily carbon doped ρ-type GaAs/In0.3Ga0.7 As strained-layer superlattices (SLSs) with hole concentrations as high as 1 × 1020 / cm3 were successfully grown for the first time by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMG), solid arsenic and solid indium. The single quantum well (SQW) and multiple quantum well (MQW) structures with different well width from 2 to 16nm were also designed to analyse the strain relaxation. The experimental results show that the SLS structures made of heavily carbon doped ρ—type GaAs and InGaAs have both of high hole concentration and small effective bandgap, which are satisfied for the application in heterojunction bipolar transistors (HBTs) as base layer.
Temperature Dependent Quenching Mechanisms of the Luminescence of InGaAs/GaAs Strained Quantum Wells
Authors: Martelli F. , D'Ottavi A. , Catania B. , Bruni M. R. , Simeone M. G. , Zugarini M.
Published: March 2011

In this work we report on detailed temperature dependent photoluminescence measures on InGaAs/GaAs strained layer quantum well structures. Excitation energies above and below the GaAs band gap have been used in order to separate the contributions of the carrier trapping to the generation-recombination processes from the recombination mechanisms proper of the quantum wells. The results show that a decrease of the trapping efficiency contributes to the photoluminescence quenching, and suggest that the strong photoluminescence quenching observed in these structures should be mainly attributed to defects present in the quantum wells. A clear dependence of the luminescence quenching on the excitation intensity has also been found.
Constant Temperature LEC Growth of Uniform Composition InGaAs Bulk Crystals through Continuous Supply of GaAs
Authors: Nakajima K. , Kusunoki T. , Kuramata K.
Published: March 2011

A 45 mm long In0.05 Ga0.95 As ternary bulk crystal with an extremely uniform composition has succeeded to be grown at a constant temperature by the liquid encapsulated Czochralski (LEC) method with a continuous supply of only GaAs to the pseudobinary InAs-GaAs growth melt. A double crucible which has both growth and source melt zones has been developed to improve the controllability of die supply of GaAs to the melt. These zones are separated by a cylindrical wall with a slit through which source elements are continuously supplied from die source melt zone to the growth melt zone. These results demonstrate mat large and uniform InGaAs bulk crystals can be grown using the constant temperature LEC method with a double crucible and with a continuous supply of only GaAs.
Optical Studies of InP/GaAs/InP Single Strained Layers
Authors: Hessman Dan , Pistol Mats-Erik , Olajos Janos , Seifert Werner , Liu Xiao , Samuelson Lars
Published: March 2011

Single strained layers of GaAs grown on InP have been studied using different optical techniques. Absorption has been measured on these structure which are believed to be type II. The absorption is found to increase slowly close to the threshold and the spectral shape is very similar to photoluminescence excitation spectra. No sharp excitonic peaks were found. The absorption has been modelled using K.p-theory including strain and confinement. The agreement with experiments is good with respect to the shape of the spectra but differs a factor of three with respect to the magnitude of the absorption. The strain and composition of the samples have been measured by a combination of photoluminescence and Raman spectroscopy.
Strain Analysis on MBE Grown InAs/AlSb Ultrathin-Layer Superlattices Using Raman Scattering
Authors: Yano Mitsuaki , Furuse Hiroshi , Okuizumi Masaru , Inoue Masataka
Published: March 2011

This paper describes an experimental analysis of strained InAs/AlSb ultrathin-layer superlattices (ULSs) grown by MBE. Interface bonds of InSb or AlAs were selectively made in the ULSs by controlling the beam supply sequence during growth. Structural analysis was performed for ULSs on both InAs and AlSb buffer layers using Raman scattering. This analysis revealed that each constituent in the ULSs was relaxed individually only when the AlAs interface bond was combined with the AlSb buffer layer. Other combinations of the interface bond and buffer layer, however, resulted in a free-standing superlattice structure. These experimental results are consistent with the interface dependence of electron mobility reported for InAs/AlSb single quantum well structures.