Atomic and electronic structure of a carbon nitride compound prepared by magnetron sputtering

The atomic structure, chemical composition and chemical bonding state of a carbon nitride compound, synthesized by rf magnetron sputtering, have been studied by high-resolution electron microscopy, nanobeam diffraction and electron energy loss spectroscopy. It is revealed that the grain sizes of a crystalline compound in an amorphous matrix lie in the range 5-10 nm, and that the compound has a hexagonal structure with a = 0.324 nm and c = 0.404 nm. The C and N K edges are observed in the high-energy region of the electron-energy-loss spectrum. Only features corresponding to C sigma* bonds are found in the C K edge spectrum, which provides evidence for sp³ hybridization of C orbitals in the compound. Quantifying the electron-energy-loss spectra suggests that the compound is C₇N₁₀.     

Reflectance of multilayer amorphous semiconductor films with random thickness layers

Abstract

Reflectance and transmittance spectra of amorphous semiconductor multilayer films were computed for layers of random thickness. This choice was prompted by the experimental results recently reported on hydrogenated amorphous silicon/silicon-nitride films in which the thickness of the lower bandgap material layers is randomly chosen. The calculation clearly elucidates the origin of a peculiar behaviour of the reflectivity observed in the spectral region of strong absorption.     

The infrared absorption spectra of diamonds expected to contain voidites

Abstract

The infrared spectra of several type Ia diamond crystals expected to contain voidites have been examined in the wavenumber range 4000–2700 cm^?1. The absence of any absorption which could possibly be attributable to the stretching of N─H bonds makes unlikely recent suggestions that the voidites may contain a solid phase of NH₃.     

Unusual stress behaviour of CeO2-doped diamond-like carbon nanofilms

CeO₂-doped diamond-like carbon (DLC) films with thicknesses of 180–200 nm were deposited by unbalanced magnetron sputtering. When the CeO₂ concentration is in the range 5–8%, the residual compressive stress of the deposited films is reduced by 90%, e.g. from about 4.1 GPa to 0.5 GPa, whereas their adhesion strength increases. These effects are attributed to the dissolution of CeO₂ within the DLC amorphous matrix and a widening interface between the DLC film and the Si substrate, respectively.     

Thermal aging effects on magnetisation reversals in a pre-deformed Fe-1wt%Cu alloy studied via first-order reversal curves

Abstract

The ability of amorphous metallic alloys to resist irradiation-induced structural changes will depend on the composition of the alloy and the particle fluences used. In this paper we show that a fast neutron fluence of 10²⁰ cm^?2 can induce local crystallization in a refractory NiTi glass. For specimens irradiated with lower fluences, the mode of crystallization of the amorphous matrix is changed from that of a primary process to a eutectic mechanism.     

Absence of photodegradation in amorphous chalcogenide films with a narrow optical bandgap

The time-dependent change in photocurrent during illumination has been studied in amorphous arsenic tritelluride (a-As Te ), which is an amorphous 2 3 chalcogenide material with a narrow optical bandgap. No photodegradation in the photocurrent is observed in this material, although photodegradation occurs in most wide-bandgap amorphous chalcogenides.     

Growth of diamond crystals by combustion synthesis

Abstract

Low-pressure synthesis of diamond by activated chemical vapour deposition processes is attractive for producing diamond films for optical and electronic (i.e. non-abrasive) applications because of the ability of these processes to produce these films in large dimensions. The low growth rate (about 1 μm h^?1) has, however, precluded these processes for abrasive grain production on simple economic grounds. With the introduction of combustion synthesis of diamond at growth rates exceeding 100μmh^?1, this barrier seems less formidable than it once did. The synthesis of large (about 200 μm) well formed single crystals of diamond by the combustion process with growth rates of about 100μm h^?1, reported here, offers an opportunity to explore the potential of this process for abrasive grain production.     

Quasi-two-dimensional or hcp palladium nanoparticles with host-guest interaction prepared at room temperature

Quasi-two-dimensional palladium nanoparticles with an average lateral dimension of 7 nm have been prepared by reduction of a PdCl₂ graphite intercalation compound precursor by lithium-diphenylide in tetrahydrofuran at room temperature. Selected-area electron diffraction patterns provide evidence that the palladium nanoparticles are hcp single-crystal particles. Owing to the template effect of the graphite lattice, the lattice parameter of palladium was found to have a strong relation with the graphite, and a 3a _graphite superstructure was inferred. The palladium structure is rotated by 30^o with regard to the carbon host lattice. Raman spectroscopy on this sample showed that a charge transfer between carbon and palladium occurs. The sample can be considered as a common Pd-graphite intercalation compound with palladium nanoparticles as guest. This behaviour is different from palladium nanoparticles prepared by hydrogen reduction at higher temperatures from the same precursor material. These particles may represent an early stage of nanoparticle formation.     

Phase transitions of Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass at pressures up to 27 GPa

The phase transitions of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass have been investigated under high pressures and at room temperature. Direct resistance measurements in a diamond anvil cell provide evidence of a reversible transition between amorphous and crystalline phases; crystallization events occurred at 24 and 26.2 GPa on uploading, and crystalline-to-amorphous phase transitions were observed at 16 and 10.6 GPa on downloading. The phase transitions were confirmed by transmission electron microscopy observations.     

Density-functional and shell-model calculations of the energetics of basal-plane stacking faults in sapphire

We report the generation of fullerenes and nanotubes using an arc discharge on graphite in a high-pressure nozzle. The vapour from the arc is quenched via collisions with helium gas, forming carbon clusters within a localized highpressure region. The carbon molecules are entrained in the gas jet as it expands into the vacuum and deposited onto a silicon substrate in a low-pressure (6 mT) environment. Mass spectroscopy measurements of the plasma reveal the presence of C molecules in the expanding plume. Microstructural examination of films deposited using this method revealed clustered regions of larger fullerenes and nanotubes surrounded by an amorphous matrix. Films containing fullerenes and nanotubes were found to be significantly harder and more elastic than amorphous carbon films deposited under identical parameters but without conditions for fullerene and/or nanotube formation.     

Analytical derivation of the effective temperature for field-dependent hopping conductivity

The electric-field enhancement of hopping conductivity in amorphous solids can be described in terms of an effective temperature T _eff given by for hopping, at a temperature T and field F, within a uniform distribution of electronic states with localization length γ^?1. We have derived this expression analytically and find at low fields a value for C of 1/2^1/2, which is consistent with the F-independent value C = 0.67 previously obtained numerically for band-tail hopping. With increasing electric field, the decrease in C(F) matches the hopping transport characteristics in amorphous semiconductors (hydrogenated amorphous silicon and hydrogenated amorphous carbon nitride) better than previous numerical simulations.     

Effects of ball milling and annealing of an alloy in the Al-Fe-Cu system: Implications for phase equilibria

An icosahedral quasicrystalline alloy in the Al-Fe-Cu system has been mechanically milled in a high-energy ball mill (Szegvari attritor) for 1, 3, 6 and 10 h. Samples were characterized by X-ray diffraction and transmission electron microscopy. The evolution of nanosize crystallites of the disordered B2 phase (bcc; a = 0.29 nm), coexisting with either the parent icosahedral phase or an amorphous phase, occurs during milling. Isothermal heat treatment of milled powder at various temperatures (200, 500, 600, 700, 800 and 850°C) leads in all cases, except at 200°C, to the transformation from disordered B2 and amorphous phases to an ordered B2 phase with a high degree of long-range ordering. The maximum degree of superlattice ordering was found after isothermal treatment at 800^oC. The implications of these results are discussed with reference to phase equilibria existing between crystalline and quasicrystalline phases in the Al-Fe-Cu system.     

High-resolution in situ electron microscopy of a silicon surface modification by molten aluminium at high temperatures

High-resolution in situ heating experiments in a transmission electron microscope have been applied to a study of the surface modification of Si by molten Al in a vacuum of 1 10^-5 Pa. Transformation from an atomically rough to an atomically flat surface was induced by wetting of molten Al atoms. Si {110}, {112}, {115} and {773} surfaces show sawtooth-like structures composed of nanofacets. It is concluded that molten Al atoms remove a native amorphous oxide layer which existed on the original Si surface, making the surface 'clean' even in a non-ultra-high vacuum such as 10^-5Pa.     

A 2-categorial Generalization of the Concept of Institution

After defining, for each many-sorted signature Σ = (S, Σ), the category Ter(Σ), of generalized terms for Σ (which is the dual of the Kleisli category for \mathbb T_S{\mathbb {T}_{\bf \Sigma}}, the monad in Set ^S determined by the adjunction T_S \dashv G_S{{\bf T}_{\bf \Sigma} \dashv {\rm G}_{\bf \Sigma}} from Set ^S to Alg(Σ), the category of Σ-algebras), we assign, to a signature morphism d from Σ to Λ, the functor d_à{{\bf d}_\diamond} from Ter(Σ) to Ter(Λ). Once defined the mappings that assign, respectively, to a many-sorted signature the corresponding category of generalized terms and to a signature morphism the functor between the associated categories of generalized terms, we state that both mappings are actually the components of a pseudo-functor Ter from Sig to the 2-category Cat. Next we prove that there is a functor Tr^Σ, of realization of generalized terms as term operations, from Alg(Σ) × Ter(Σ) to Set, that simultaneously formalizes the procedure of realization of generalized terms and its naturalness (by taking into account the variation of the algebras through the homomorphisms between them). We remark that from this fact we will get the invariance of the relation of satisfaction under signature change. Moreover, we prove that, for each signature morphism d from Σ to Λ, there exists a natural isomorphism θ ^d from the functor Tr^L °(Id ×d_à){{{\rm Tr}^{\bf {\bf \Lambda}} \circ ({\rm Id} \times {\bf d}_\diamond)}} to the functor Tr^S °(d^* ×Id){{\rm Tr}^{\bf \Sigma} \circ ({\bf d}^* \times {\rm Id})}, both from the category Alg(Λ) × Ter(Σ) to the category Set, where d* is the value at d of the arrow mapping of a contravariant functor Alg from Sig to Cat, that shows the invariant character of the procedure of realization of generalized terms under signature change. Finally, we construct the many-sorted term institution by combining adequately the above components (and, in a derived way, the many-sorted specification institution), but for a strict generalization of the standard notion of institution.     

Fourier transform infrared and Raman spectroscopy studies on a - and ß-C3N4 films

Crystalline carbon nitride films have been synthesized on Si(100) substrates by the microwave plasma chemical vapour deposition technique. In order to understand their structure, Raman and Fourier transform infrared spectra of aand b-C3N4 were calculated using a scale factor and the observed Raman and infrared spectra of a- and b-Si N . A scale factor (1.47) for the stretching 3 4 frequencies from the tetrahedral C-N to Si-N bonds was calculated through Hooke's law. Experimental Raman and infrared spectra conform to the calculated results: so we conclude that the film contain a-C3N4 and b-C3N4     

Stress-corrosion cracking in YBa2Cu3O7−x ceramic superconductor

Abstract

Thin-foil transmission electron microscopy specimens of orthorhombic YBa²Cu³O^7?x subjected to residual elastic strains have shown slow crack growth in an atmosphere with low moisture content. Microcracks, initiated by amorphization at the strained edges, propagate alternately along (110) twin boundaries and (001) stacking faults, where strain energy is accumulated locally. The formation of carbon-rich amorphous layers along the strained lattice planes, followed by residual stress-assisted crack propagation, resembles stress-corrosion cracking in this ceramic superconductor.     

An explanation for the low-temperature H evolution peak in hydrogenated amorphous silicon films deposited 'on the edge of crystallinity'

The puzzling existence of a sharp low-temperature (T = 400°C) H evolution peak in compact hydrogenated amorphous silicon (a-Si : H) films deposited 'on the edge of crystallinity' is examined. From infrared absorption and X-ray diffraction (XRD) measurements, we show that none of the standard methods used to explain the existence of this peak in a-Si : H materials is applicable to the present films. From the Si-H wag-mode peak frequency, we postulate the existence of very small Si crystallites contained within the amorphous matrix. While the crystallite volume fraction is too small to be detected by XRD in the as-grown films, crystallization is observed for this material at anneal temperatures as low as 500°C. It is proposed that these crystallites catalyse the crystallization of the remainder of the amorphous matrix upon moderate annealing, enabling H surface desorption and H₂ out-diffusion to the sample surface along newly formed grain boundaries at low anneal temperatures.     

Electron spin resonance of hydrogenated amorphous silicon-carbon alloy films prepared by magnetron sputtering

Abstract

New electron spin resonance (ESR) lines with g¹ = 2·0017 and g₂ and g₃ = 2·0006 have been found in the ESR spectra of as-deposited a-Si_1–x C_x:H films prepared by magnetron sputtering of silicon in the gas mixtures of methane and argon. Similarities between the observed spectra and those for the E′ centre in glassy SiO₂ are discussed.     

A thermodynamic model for nanocrystallization in bulk metallic glasses

The structural complexity of glass-forming alloys, which generally contain more than three components, can lead by partial crystallization during annealing to a dispersion of nanocrystals in an amorphous matrix, giving the material a very high mechanical strength. In the present study, the evolution of the driving force for crystallization is expressed as a function of the composition and the chemical potentials of the components. Application to Zr₆₀Al₁₀Cu₃₀ and Zr₆₀Al₁₀Cu₂₀Pd₁₀ bulk metallic glasses shows that the first crystallization step leads to a metastable equilibrium between nanocrystals of an intermetallic and a percolating amorphous phase. The effects of the number of components and of chemical bonding on the fraction crystallized is analysed and discussed.     

Crystallization behaviour of Al-based amorphous alloy and nanocomposites by rapid quenching

The microstructure and crystallization behaviour of melt-spun Al₈₈Ni₉Ce₂Fe₁ amorphous alloy and nanophase composites have been studied by means of X-ray diffraction, transmission electron microscopy and scanning and isothermal calorimetry. The diffraction patterns from Al₈₈Ni₉Ce₂Fe₁ amorphous alloys are diffuse, indicating a basically amorphous structure but contain two rings presumed to be associated with quenched-in nuclei. In the cases of Al₈₈Ni₉Ce₂Fe₁ nanophase composites, nanoscale precipitated particles are homogeneously dispersed in an amorphous matrix, and the crystallite diameter and volume fraction are sensitive to quenching conditions. During thermal crystallization, a two-step phase transformation occurs in the amorphous alloy and nanocomposites, which is characterized by a diffusion-controlled precipitation of nanoscale Al particles and the growth of a Al₃(Ni, Fe) nanophase prior to a Al₁₁Ce₃ nanophase. This study gives insight into structure-control for obtaining nanophases dispersed in an amorphous matrix by rapid quenching.