The erosion of CVD diamond by diamond particles

This paper describes the damage features generated by the high-velocity impact of diamond particles on diamond produced by chemical vapour deposition (CVD). Two types of diamond grit - angular and cubo-octahedral - were used to impact the surface of the diamond target at a mean impact velocity of 268?m?s^?1. The main damage features were elastic-plastic radial and lateral cracks. Distorted Hertzian ring cracks were also seen on samples impacted with cubo-octahedral diamond grit. This indicates that, in contrast to previous experiments by the authors using silica sand erodent, Hertzian damage can be generated at this velocity when the mechanical properties of the erodent and target are comparable. Fragmentation of the diamond grit occurred on impact; however, the extent of the degradation was much reduced compared with the silica sand erodent.     

Newly observed microscopic planar defects on {111} in natural diamond

Abstract

Circular disc-crack-like defects randomly distributed on {111} planes have been discovered by 300kV transmission electron microscopy in diamond that had crystallized on non-faceted growth surfaces of mean orientation {100}. In a sample of ～ 100 defects, disc diameters were narrowly distributed about a mean of 1·2 μm and one in seven discs were located on a coplanar stacking fault of average diameter 2 μm. Discs totally enclosed within the specimen exhibit concentric moiré fringes covering their image areas when viewed obliquely, showing them to be dilated into thin lenticles. Internal pressures, deduced from moiré-fringe counts on six lenticles, were in the range 1·5 to 1·8 GPa.     

Long-range interaction of radiation-generated self-interstitial atoms and adatoms on tungsten surface

ABSTRACT

The morphological modification of the W surface induced by specimen irradiation with 5?keV He atoms was studied at the atomic level using low-temperature ?eld ion microscope techniques. The major impact-induced damage mode on W {110} surface was found to be the formation of adatom sets produced by radiation-generated self-interstitial atoms emerging at the surface. The long-range interaction was revealed by evaluating the experimental probability density of finding two radiation-generated adatoms at a given separation in comparison with the probability density corresponding to a random distribution of adatoms. The statistically significant difference of these distributions consists of the absence or severe reduction of observed adatom pairs with separations in the range 5–11?Å. The experimental distribution was characterised by a relatively open arrangement of adatoms with the closest separation of about 11?Å. Our field ion microscope observations also reveal the self-organisation of radiation-generated W adatoms into atomic chains with the lateral interatomic separation substantially larger than the nearest-neighbor distance. These effects indicate the existence of long-range interactions between radiation-generated adatoms and radiation-generated self-interstitial atoms in the subsurface region.     

The alignment of Bi(Pb)-Sr-Ca-Cu-O near the Ag-(Bi(Pb)-Sr-Ca-Cu-O) interface in silver-sheathed multifilamentary Bi(Pb)2Sr2Ca2Cu3O

The Ag-(Bi(Pb)-Sr-Ca-Cu-O (BSCCO)) interface region has been studied by scanning electron microscopy and transmission electron microscopy to a distance of 2µm away from the silver in the Ag-sheathed multifilamentary Bi(Pb)₂Sr₂Ca₂Cu₃O (Bi(Pb)-2223) tapes. Owing to undulation of the silver surface, silver was found to play an important but not always positive role in the alignment of BSCCO grains. Based on the kink angle variation, the effective thickness of multifilamentary Bi(Pb)-2223 tapes capable of carrying a large current has been defined. We believe that the difference in effective thickness is the key reason leading to different critical current densities J c for these tapes.     

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.     

EELS analysis of vacuum arc-deposited diamond-like films

Abstract

Electron energy-loss spectroscopy measurements have been made on amorphous diamond-like carbon films produced by condensing the plasma stream from a filtered vacuum arc. The results are compared with spectra from diamond, graphitized carbon and amorphous carbon. Although the energy-loss spectra reveal the presence of a small quantity of sp²-bonded material in the diamond-like film, the high plasmon frequency and shape of the K-edge show that the material is essentially an amorphous form of diamond. The fraction of sp²- bonded carbon was quantified and found to be of the order of 15%. It was not possible to determine if the sp² carbon was on the surface or throughout the bulk.     

Low-energy boron implants into silicon: A molecular dynamics simulation study of implants into a stepped surface

Molecular dynamics simulations have been conducted to study implantation of boron with a low kinetic energy (E _k in the range 5 eV) into the silicon surface. The bombarded surface has a realistic stepped structure and the geometry of the step edge is taken from the theory of Chadi (1987, Phys. Rev. Lett., 59, 1691). It is found that reflection is the more common event. However subsurface implants and adatom formations are also observed. The effect of the step morphology is important and different trajectories are observed if the ions hit steps of different shapes.     

Effect of film thickness and grain size on fatigue-induced dislocation structures in Cu thin films

This letter presents systematic experimental observations of fatigue damage and corresponding dislocation structures in thin Cu films as a function of film thickness made using transmission electron microscopy and focused-ion-beam microscopy. It is found that, in thick films and grains of at least 3.0 μm diameter, coarse surface extrusions and dislocation wall and cell structures occur whereas, in thin films or in small-diameter grains, finer extrusions occur but no clearly defined dislocation structures are present. This minimum required dimension of 3.0 μm for fatigue damage formation may be caused by constrained dislocation motion in small dimensions.     

Revealing texture architecture in a surface gradient nanostructured Al-Cu-Mg alloy

ABSTRACT

A multiscale crystallographic texture architecture in a surface gradient nanostructured Al-Cu-Mg alloy after surface sliding friction treatment (SSFT) has been revealed by a combination of electron backscatter diffraction and precession electron diffraction (PED)-assisted transmission electron microscopy (TEM) orientation mapping. Accompanying a grain structure variation from lamellar coarse grains to equiaxed nanograins, the major texture components evolve from brass {110}<112> in the coarse-grain matrix, Goss {110}<001> at a depth of ～80?μm, E {111}<011> and F {111}<112> at a depth of ～20?μm, to a mixture of rotated cube {001}<110>, E and F in the topmost surface layer. The through-thickness textural development and evolution are attributed to the cyclic loading of concurrent shear and compression during the SSFT processing. The PED-assisted orientation mapping shows good capability in mapping severe plastic-deformation-induced nanostructures with large residual strains and high defect density.     

Epifluorescence optical microscopy: a sensitive tool for determining the spatial distribution of europium-dihalide precipitates in KCl : KBr : Eu2+ crystals

The spatial distribution of europium-dihalide-type precipitates in KCl?:?KBr?:Eu²⁺ single crystals annealed at 200°C for long periods of time was determined by epifluorescence optical microscopy. Laue-type patterns and optical spectroscopy were used to test the degree of long-range ordering of the specimens and to monitor the precipitation during annealing, respectively. Precipitates smaller than about 0.08?µm were found (2.84?×?10¹² precipitates cm^?3) all across the host, whereas larger precipitates of about 0.17?µm were found (2.9?×?10⁴ precipitates cm^?1) along certain linear structural singularities of the matrix. These singularities, identified as crystal dislocations, were observed to terminate always either at a triple node of singularities or at the crystal surface, forming a three-dimensional network of crystal singularities (2.0?×?10⁶ singularities cm^?2), identified as the Frank network. The presence of europium exhaustive matrix zones accompanying the observed europium-decorated paths indicates that impurity segregation processes, occurring during annealing, favour the precipitation phenomena.     

Frank's 'cubic' hexagonal phase: An intermetallic cluster compound as an example

Frank introduced in 1965 the novel idea of projection from a four-dimensional cube to recover the points of a hexagonal lattice with a special c / a ratio of ( 3 - 2 ½. This was called a cubic hexagonal crystal, as there was a similarity to the conventional cubic crystals in that directions were perpendicular to planes with the same Miller-Bravais indices. While a number of crystals in the NiAs-Ni 2 In family have been reported with this special axial ratio, the number of atoms in the unit cell is small. As the first example of an intermetallic cluster compound, we identify µ-Al 4 Mn, µ-Al 4 Cr, Zn-Mg-Sm and a host of related intermetallics featuring a special aggregate of icosahedra as Frank's 'cubic' hexagonal phase or its variant. The metric is generated by the Friauf polyhedra and the icosahedral linkages and leads to a multimetric crystal and several interesting connections with hexagonal quasicrystals, hexagonal phases and derivative orthorhombic and lower-symmetry structures.     

Photoinduced volume changes related to photoinduced anisotropy in chalcogenide glasses

We report metastable photoinduced volume expansion (hyperexpansion) of up to 10 µm in height and several millimetres in diameter for the chalcogenide glass As 2 S 3 . This is an order of magnitude larger than reported to date in any material, including chalcogenide glasses. We suggest a model for the effect and propose that its origin is similar to the origin of photoinduced anisotropy in glassy chalcogenides proposed earlier. The model is based on the anisotropic chemical bonding found in most chalcogenide glasses; the anisotropy arises from the fact that chalcogen atoms exhibit both strong covalent bonding and weak van der Waals bonding. The origin of the photoinduced volume changes (expansion and contraction) is in a thermally induced diffusion, or athermal tunnelling, of photoexcited chalcogen atoms, with possible subsequent sublimation from the sample surface. The primary role of hyperpolarizable van der Waals bonds is supported by the absence of photoinduced volume changes in the chalcogenide glass Ga 1.4 La 0.6 S 3, in which chalcogen atoms do not bond via van der Waals bonds.     

Pop-in phenomenon in MgO and LiF: Observation of dislocation structures

This letter is based on recent progress in the observation of dislocation distributions around nanoindentations by chemical etching. This so-called nanoetching technique is used to determine the dislocation mechanisms associated with the pop-in phenomenon in MgO and LiF. Successive stages of highly controlled chemomechanical polishing have revealed that these dislocations are half-loops lying in the classical slip systems of MgO or LiF. However, they do not extend on the surface in the classical rosette-arms pattern but stay concentrated around the imprint. A mechanism of dislocation interactions, enhanced by the fact that the dislocations are suddenly nucleated in a small volume, is proposed to explain this specific distribution.     

Toughening of a brittle material by means of dislocation subboundaries

A novel technique to toughen brittle materials has been developed. Surfaces of a brittle material were damaged at room temperature, followed by annealing at a high temperature. During annealing, cracks, which were introduced during surface damage, disappear and dislocation subboundaries are formed. This treatment improves the fracture toughness K_Ic by a factor of two.     

Effect of severe plastic deformation on the coercivity of Co–Cu alloys

Cast Co–5.6 wt% Cu and Co–13.6 wt% Cu alloys were subjected to severe plastic deformation (SPD) by high-pressure torsion (HPT). The HPT treatment drastically decreases the size of the Co grains (from 20 µm to 100 nm) and the Cu precipitates (from 2 µm to 10 nm). As a result, the coercivity H _c of both the alloys radically increases. The saturation magnetization, M _s, remains almost unchanged. Thus, SPD of the bulk samples opens the way for drastic increase in the coercivity for the Co-based alloys.     

On the relativistic nature of predicted and real physical experiences: A field experiment

ObjectivesAn important yet unresolved question stemming from judgment and decision-making literature is whether individuals evaluate physical experiences in relative or absolute terms.Design and methodThe study examined 181 experienced basketball players in a 2 (type of experience: predicted versus real) × 2 (evaluation mode: separate versus joint) × 2 (type of activity: running versus shooting) experimental research design.ResultsWe demonstrated that individuals who were familiar with physical tasks evaluated predicted and real physical experiences in absolute terms. In addition, we showed that relativistic modes of evaluation applied to real physical experiences but not predicted physical experiences.ConclusionsThis research contributes to the debate concerning whether prior task experience influences formation of relative evaluations, and reveals that contexts that urge for relative evaluations undermine happiness with physical tasks.     

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.     

Spectroscopic studies of a thermochromic centre in synthetic diamonds grown by the temperature-gradient method

Abstract

Synthetic diamonds grown by the temperature-gradient method at temperatures just above the melting point of the solvent-catalyst often have an unattractive dull-green colour. This green colour, which is prominent at room-temperature, disappears if the diamond is cooled to below 120 K and is restored on warming the crystal back to room temperature. We show that the behaviour of the absorption band responsible for the colour is consistent with the transition occurring from the upper level of a centre with a split ground state, for which transitions from the lower level are forbidden. Temperature-dependence measurements indicate that the energy splitting in the ground state is about 50 meV. The broad featureless absorption band, with a maximum near 1·9eV, is assumed to be vibronic in origin. The absence of structure in the band indicates that the electronic transition is strongly coupled to the lattice and, as expected, the zero-phonon line, estimated to be at around 1eV, is too weak to detect. The same absorption band is also shown to be present in some individual crystals of synthetic diamond abrasive grit.     

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.     

Gratitude predicts hope and happiness: A two-study assessment of traits and states

Abstract

Gratitude is the appreciation of a gift received; happiness is the enjoyment of a present good; and hope is the desire for a valued future. Two studies investigated gratitude as a predictor of hope and happiness. In Study 1, hierarchical regressions (N = 181) revealed that trait gratitude exceeded other constructs (forgivingness, patience, self-control) in predicting trait hope and happiness. In Study 2, we experimentally tested the impact of a gratitude-related writing intervention on state hope and happiness. Participants (N = 153) first wrote about a current, meaningful, hoped-for outcome and completed state hope and happiness measures. Participants were randomly assigned to either (a) gratefully remember a past hope that had been fulfilled or (b) a control condition. The grateful remembering condition (vs. control) prompted significant increases in state hope and happiness, commending grateful remembering as a practice that can bolster present happiness and hope for the future.