scispace - formally typeset
Search or ask a question

Showing papers in "Journal of Materials Research in 1989"


Journal ArticleDOI
TL;DR: The Raman spectrum of hexagonal diamond (lonsdaleite) is distinct from that of the cubic diamond and allows it to be recognized as discussed by the authors, and the Raman line width varies with mode of preparation of the diamond and has been related to degree of structural order.
Abstract: As the technology for diamond film preparation by plasma-assisted CVD and related procedures has advanced, Raman spectroscopy has emerged as one of the principal characterization tools for diamond materials. Cubic diamond has a single Raman-active first order phonon mode at the center of the Brillouin zone. The presence of sharp Raman lines allows cubic diamond to be recognized against a background of graphitic carbon and also to characterize the graphitic carbon. Small shifts in the band wavenumber have been related to the stress state of deposited films. The effect is most noticeable in diamond films deposited on hard substrates such as alumina or carbides. The Raman line width varies with mode of preparation of the diamond and has been related to degree of structural order. The Raman spectrum of hexagonal diamond (lonsdaleite) is distinct from that of the cubic diamond and allows it to be recognized.

2,300 citations


Journal ArticleDOI
TL;DR: In this article, the size-dependent physical and chemical properties of clusters, the methods of synthesis of macroscopic amounts of size-selected clusters with desired properties, and most importantly, the possibility of their controlled assembly into new materials with novel properties.
Abstract: The Panel was charged with assessing the present scientific understanding of the size-dependent physical and chemical properties of clusters, the methods of synthesis of macroscopic amounts of size-selected clusters with desired properties, and most importantly, the possibility of their controlled assembly into new materials with novel properties. The Panel was composed of both academic and industrial scientists from the physics, chemistry, and materials science communities, and met in January 1988.In materials (insulators, semiconductors, and metals) with strong chemical bonding, there is extensive spatial delocalization of valence electrons, and therefore the bulk physical properties which depend upon these electrons develop only gradually with cluster size. Recent research using supersonic-jet, gas-aggregation, colloidal, and chemical-synthetic methods indeed clearly establishes that intermediate size clusters have novel and hybrid properties, between the molecular and bulk solid-state limits. A scientific understanding of these transitions in properties has only been partially achieved, and the Panel believes that this interdisciplinary area of science is at the very heart of the basic nature of materials. In Sec. V (Future Challenges and Opportunities), a series of basic questions for future research are detailed. Each question has an obvious impact on our potential ability to create new materials.Present methods for the synthesis of useful amounts of size-selected clusters, with surface chemical properties purposefully controlled and/or modified, are almost nonexistent, and these fundamentally limit our ability to explore the assembly of clusters into potentially novel materials. While elegant spectroscopic and chemisorption studies of size-selected clusters have been carried out using molecular-beam technologies, there are no demonstrated methods for recovery and accumulation of such samples. Within the past year, the first reports of the chemical synthesis of clusters with surfaces chemically modified have been reported for limited classes of materials. Apparatus for the accumulation and consolidation of nanophase materials have been developed, and the first promising studies of their physical properties are appearing. In both the chemical and nanophase synthesis areas, clusters with a distribution of sizes and shapes are being studied. Progress on macroscopic synthetic methods for size-selected clusters of controlled surface properties is the most important immediate goal recognized by the Panel. Simultaneous improvement in physical characterization will be necessary to guide synthesis research.Assuming such progress will occur, the Panel suggests that self-assembly of clusters into new elemental polymorphs and new types of nanoscale heterogeneous materials offers an area of intriguing technological promise. The electrical and optical properties of such heterogeneous materials could be tailored in very specific ways. Such ideas are quite speculative at this time; their implementation critically depends upon controlled modification of cluster surfaces, and upon development of characterization and theoretical tools to guide experiments.The Panel concluded that a number of genuinely novel ideas had been enunciated, and that in its opinion some would surely lead to exciting new science and important new materials.

464 citations


Journal ArticleDOI
TL;DR: In this paper, the requirements for fitting bcc metals within the EAM format are discussed and, for comparative purposes, a general embedding function is defined and an analytic first-and second-neighbor model is presented.
Abstract: The requirements for fitting bcc metals within the EAM format are discussed and, for comparative purposes, the EAM format is cast in a normalized form. A general embedding function is defined and an analytic first- and second-neighbor model is presented. The parameters in the model are determined from the cohesive energy, the equilibrium lattice constant, the three elastic constants, and the unrelaxed vacancy formation energy. Increasing the elastic constants, increasing the elastic anisotropy ratio, and decreasing the unrelaxed vacancy formation energy favor stability of a close-packed lattice over bcc. A stable bcc lattice relative to close packing is found for nine bcc metals, but this scheme cannot generate a model for Cr because the elastic constants of Cr require a negative curvature of the embedding function.

460 citations


Journal ArticleDOI
TL;DR: In this article, the activation energies for self-diffusion of transition metals (Au, Ag, Cu, Ni, Pd, Pt) have been calculated with the Embedded Atom Method (EAM).
Abstract: The activation energies for self-diffusion of transition metals (Au, Ag, Cu, Ni, Pd, Pt) have been calculated with the Embedded Atom Method (EAM); the results agree well with available experimental data for both mono-vacancy and di-vacancy mechanisms. The EAM was also used to calculate activation energies for vacancy migration near dilute impurities. These energies determine the atomic jump frequencies of the classic “five-frequency formula,” which yields the diffusion rates of impurities by a mono-vacancy mechanism. These calculations were found to agree fairly well with experiment and with Neumann and Hirschwald's “Tm” model.

256 citations


Journal ArticleDOI
TL;DR: In this paper, the surface morphology of the diamond films was a function of position on the sample surface and the methane concentration in the feedgas, which was determined to be similar to natural diamond in terms of composition, structure, and bonding.
Abstract: Thin carbon films grown from a low pressure methane-hydrogen gas mixture by microwave plasma enhanced CVD have been examined by Auger electron spectroscopy, secondary ion mass spectrometry, electron and x-ray diffraction, electron energy loss spectroscopy, and electron microscopy. They were determined to be similar to natural diamond in terms of composition, structure, and bonding. The surface morphology of the diamond films was a function of position on the sample surface and the methane concentration in the feedgas. Well-faceted diamond crystals were observed near the center of the sample whereas a less faceted, cauliflower texture was observed near the edge of the sample, presumably due to variations in temperature across the surface of the sample. Regarding methane concentration effects, threefold {111} faceted diamond crystals were predominant on a film grown at 0.3% CH4 in H2 while fourfold {100} facets were observed on films grown in 1.0% and 2.0% CH4 in H2. Transmission electron microscopy of the diamond films has shown that the majority of diamond crystals have a very high defect density comprised of {111} twins, {111} stacking faults, and dislocations. In addition, cross-sectional TEM has revealed a 50 A epitaxial layer of β3–SiC at the diamond-silicon interface of a film grown with 0.3% CH4 in H2 while no such layer was observed on a diamond film grown in 2.0% CH4 in H2.

232 citations


Journal ArticleDOI
TL;DR: In this paper, the Raman spectra for consolidated nanophase TiO2 particles in their as-compacted state and after annealing at a variety of temperatures up to 1273 K were reported.
Abstract: Raman spectra are reported for consolidated nanophase TiO2 particles in their as-compacted state and after annealing at a variety of temperatures up to 1273 K. The Raman-active bands normally observed for the rutile form of TiO2 were present in as-compacted samples having average grain sizes in the range from about 10 to 100 nm. However, significant broadening of these bands was found, which was uncorrelated with initial grain size, but not necessarily with other synthesis-related factors. This broadening decreased upon isochronal annealing at elevated temperatures in air. Based upon these observations, it is concluded that nanophase TiO2 in the as-consolidated state contains significant defect concentrations within the rutile grains and that these intragrain defects and the grain-boundary regions as well have local atomic structures with the rutile symmetry, albeit with some short-range displacements. Some sporadic sample regions containing small amounts (<5%) of the anatase form of TiO2 were also found; these traces of anatase transformed to rutile upon annealing in air at temperatures above 883 K.

206 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the Al-Cu-Fe stable icosahedral crystal has an almost perfect symmetry and a reciprocal lattice of the body-centered cubic type.
Abstract: We show that the Al–Cu–Fe stable icosahedral crystal has an almost perfect icosahedral symmetry, and a reciprocal lattice of the body-centered cubic type.

201 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of chromium on several properties of Fe3Al, including tensile strength and ductility, fracture behavior, and slip and dislocation characteristics, were studied.
Abstract: The effects of the addition of chromium on several properties of Fe3Al, including tensile strength and ductility, fracture behavior, and slip and dislocation characteristics, were studied. Alloying with up to 6 at. % chromium results in an increase in room temperature ductility from approximately 4% to 8–10%. Along with this increase in ductility, the addition of chromium produces a change in fracture mode from transgranular cleavage to a mixed mode of intergranular-transgranular cleavage, and a change in slip behavior from coarse straight slip to fine wavy slip. These phenomena are discussed in terms of the effect of chromium on the antiphase boundary energies and dislocation characteristics.

194 citations


Journal ArticleDOI
TL;DR: In this article, the primary state of damage produced by displacement cascades is controlled by replacement collision sequences during the ballistic phase, and melting and resolidification during the thermal spike.
Abstract: Molecular dynamics simulations of energetic displacement cascades in Cu and Ni were performed with primary-knock-on-atom (PKA) energies up to 5 keV. The interatomic forces were represented by the Gibson II (Cu) and the Johnson-Erginsoy (Ni) potentials. Our results indicate that the primary state of damage produced by displacement cascades is controlled basically by two phenomena: replacement collision sequences during the ballistic phase, and melting and resolidification during the thermal spike. The thermal-spike phase is of longer duration and has a more marked effect in Cu than in Ni. Results for atomic mixing, defect production, and defect clustering are presented and compared with experiment. Simulations of “heat spikes” in these metals suggest a model for “cascade collapse” based on the regrowth kinetics of the molten cascade core.

188 citations


Journal ArticleDOI
TL;DR: Transparent silica gel-polymer composites have been prepared by the impregnation of porous gels with organic monomer and polymerization in situ as mentioned in this paper, and properties such as density, refractive index, modulus of rupture, compressive strength, abrasion rate and Vickers hardness have been measured over the compositional range of 100% silica to 100% polymethyl methacrylate (PMMA).
Abstract: Transparent silica gel–polymer composites have been prepared by the impregnation of porous gels with organic monomer and polymerization in situ. The relative amount of each phase was adjusted by varying the porosity of the silica gel prior to impregnation. These materials constitute a new class of transparent composites. Properties, such as density, refractive index, modulus of rupture, compressive strength, abrasion rate, and Vickers hardness, have been measured over the compositional range of 100% silica to 100% polymethyl methacrylate (PMMA).

187 citations


Journal ArticleDOI
TL;DR: The properties of graphite intercalation compounds (GICs) are discussed with respect to possible applications as mentioned in this paper and some problems connected with the production and use of these sheets are discussed.
Abstract: The properties of graphite intercalation compounds (GIC’s) are discussed with respect to possible applications. Five families of intercalates give high electrical conductivity to GIC’s: pentafluorides leading to high conductivity, 108 S/m (higher than metallic copper); metal chlorides; fluorine and alkali metals with bismuth giving relatively high conductivity of the order of 107 S/m plus stability in air; and residual halogens showing extremely high stability under severe conditions, though the conductivity is only of the order of 106 S/m. Electrodes of different GIC’s have been tried in primary and secondary batteries, where their characteristics are high electrical conductivity and easy diffusion of electrochemically active species between the graphite layers. Primary lithium batteries of a covalent graphite fluoride are now widely used commercially. Secondary batteries using different host graphites and intercalates give interesting results. Large amounts of hydrogen can be stored in the functional space in alkali metal-GIC’s. The same GIC’s show high coefficients of isotope separation of hydrogen at liquid nitrogen temperature. The structure and texture of the host graphite play a decisive role in the absorption and separation behaviors of GIC’s. Exfoliated graphite prepared by rapid heating of GIC’s or their residue compounds leads to flexible graphite sheets which have great industrial applications. Some problems connected with the production and use of these sheets are discussed.

Journal ArticleDOI
TL;DR: In this article, a boron-compound layer was developed consisting of a surface-adjacent "FeB" sublayer on top of an "Fe2B", and the extent of penetration of the two sublayers as a function of boriding time and temperature in the range 1025-1275 K.
Abstract: Specimens of pure Fe and of Fe-0.8 mass % C, Fe-0.5 mass % Cr, Fe-4.0 mass % Cr, Fe-4.0 mass% Ni, and Fe-10.0 mass% Ni alloys were borided in boriding powder. A boron-compound layer developed consisting of a surface-adjacent “FeB” sublayer on top of an “Fe2B” sublayer. Layer-growth kinetics were analyzed by measuring the extent of penetration of the “FeB” and “Fe2B” sublayers as a function of boriding time and temperature in the range 1025–1275 K. Layer growth is dominated by B diffusion through “FeB/Fe2B”. This diffusion process is of strongly anisotropic nature. Consequently, ragged interfaces occur between the substrate and the boride layers. The depths of the tips of the most deeply penetrated “FeB” and “Fe2B” needles have been taken as measures for diffusion in the easy [001] diffusion directions. Assuming unidirectional B diffusion and parabolic growth, a simple model of layer growth has been given. It accounts for the specific volume difference between “FeB” and “Fe2B”. In contrast with earlier work, the model provides values for the kinetic parameters for growth of each of the phases in the boron-compound layer.

Journal ArticleDOI
TL;DR: In this paper, the surface and grain boundaries of Ni, Al, and Ni3Al were modeled using local volume potentials and the simulations showed that with appropriately fit potentials, the surfaces and grain boundary structure can be realistically calculated.
Abstract: We have used “local volume” (embedded atom) type potentials to study the surfaces and grain boundaries of Ni, Al, and Ni3Al. The simulations show that with appropriately fit potentials, the surface and grain boundary structure can be realistically calculated. The surface rippling and relaxation show good agreement with experiments. The energies of most surfaces and grain boundaries also agree with existing data. The structural unit model for grain boundaries in Ni3Al shows the same generic units as in pure metals, but with large variations due to distortions and multiplicity. The utility of the structural unit model is thus more limited for alloys. The grain boundary energies were found to be the highest for Al-rich Ni3Al grain boundaries, and depend significantly on the local composition of the grain boundary. The cusps in the grain boundary energy as a function of misorientation angle are different for different grain boundary stoichiometries. The Ni3Al grain boundaries have approximately the same grain boundary energy and cohesive energy as that of Ni.

Journal ArticleDOI
TL;DR: In this article, the chemical interactions between the YBa2Cu3O7−x high Tc superconductor and various substrates (Si, Al2O3, ZrO2, SrTiO3 and MgO, Ag, Cu, and Nb) were studied.
Abstract: The chemical interactions between the YBa2Cu3O7−x high Tc superconductor and various substrates (Si, Al2O3, ZrO2, SrTiO3, MgO, Ag, Cu, and Nb) were studied. Powders of the orthorhombic YBa2Cu3O7−x (123) phase and each of the various substrates were mixed, pressed into pellets, and subjected to either one of the following heat treatments in flowing oxygen: 600 °C for 11 h, 800 °C for 10 h, 945 °C for 5 h, and 945 °C for 17 h. The reacted samples were characterized by qualitative and quantitative x-ray diffraction, scanning electron microscopy with EDAX, electrical resistivity, and magnetic susceptibility measurements. For all substrates studied, the extent of reaction is much more significant at 945 °C than at 800 °C and 600 °C. The reaction products, in general, are some Ba compounds, CuO (and Cu2O), and the Y2BaCuO5 (211) phase. The reaction with Si results in a morphology of layers of 123/211/Ba2SiO4/CuO/Si. Reactions with Al2O3 and ZrO2 result in similar products and morphologies. Reaction with SrTiO3 results in the replacement of Ba by Sr and the formation of a Ba–Ti–Y–Cu unknown phase. With MgO, the chemical reaction results in the enrichment of MgO with Cu and the formation of an apparently glassy Ba–Cu phase. With the metallic substrates Ag, Cu, and Nb, Ag shows no interaction at all, Cu results in the formation of CuO (and Cu2O), whereas the Nb samples completely disintegrated. By comparing the percentage of remaining 123 phase in samples reacted at 945 ° for 17 h, a reactivity scale in decreasing order was obtained as Nb > Si > ZrO2  Al2O3 > SrTiO3 > MgO  Cu > Ag. It is suggested that the chemical reactivity of the Y–Ba–Cu oxide superconductor is mostly controlled by the highly electropositive Ba2+ ions.

Journal ArticleDOI
TL;DR: The epitactic nature of the growth of YBa2Cu3O6+x (YBCO) superconducting thin films on ceramic substrates has been studied using high-resolution electron microscopy (HREM) and selected-area diffraction (SAD) of cross-sectional specimens as discussed by the authors.
Abstract: The epitactic nature of the growth of YBa2Cu3O6+x (YBCO) superconducting thin films on ceramic substrates has been studied using high-resolution electron microscopy (HREM) and selected-area diffraction (SAD) of cross-sectional specimens. The films were grown in situ on (001)-oriented MgO and (001)-oriented Y2O3-stabilized cubic ZrO2 (YSZ) single-crystal substrates by electron beam evaporation. Both of these materials have large lattice misfits with respect to YBCO. Different orientation relationships were observed for films grown on the two types of substrates. These orientation relationships are shown to provide the best matching of the oxygen sublattices across the substrate-film interfaces. A crystalline intermediate layer, 6 nm thick, between the YBCO film and YSZ substrate was observed by HREM and shown by EDS to be a Ba-enriched phase, possibly barium zirconate formed by a reaction. In contrast, the YBCO–MgO interface was found to be sharp and free of any intermediate layers.

Journal ArticleDOI
TL;DR: In this article, the room temperature deformation behavior and microstructure of Ti 48 Al 52 and Ti 52 Al 48 alloys are compared and the possible reasons for the differences in micro-structure and mechanical behavior between these two alloys were discussed.
Abstract: The room temperature deformation behavior and microstructure of Ti 48 Al 52 and Ti 52 Al 48 alloys are compared. The material was fabricated by rapid solidification melt spinning, and examined in both as-cast and consolidated forms. The Ti 52 Al 48 alloy exhibited enhanced strength and ductility in both forms in bend tests compared with the Ti 48 Al 52 alloy. The microstructure of the Ti 52 Al 48 alloy was two-phase γ–TiAl and α 2 –Ti 3 Al. The Ti 48 Al 52 alloy was single-phase γ–TiAl and had a larger grain size than the previous alloy. The microstructure of the Ti 52 Al 48 alloy after room temperature deformation consisted primarily of {111} mechanical twins and a /2〈110〉 perfect dislocations. The comparable Ti 48 Al 52 alloy microstructure contained fewer twins, and many more a 〈101〉 and a /2〈112〉 superdislocations were present in addition to a /2〈110〉 dislocations. The superdislocations had dissociated and formed sessile faulted dipoles. The possible reasons for the differences in microstructure and mechanical behavior between these two alloys are discussed.

Journal ArticleDOI
TL;DR: In this paper, the depth dependence of hardness in a well-annealed single crystal of silver has been characterized in nanoindentation experiments, based on similar experiments performed by Chen and Hendrickson, but extends their results to indent depths on the nanometer scale.
Abstract: The depth dependence of hardness in a well-annealed single crystal of silver has been characterized in nanoindentation experiments. The work is based on similar experiments performed by Chen and Hendrickson, but extends their results to indent depths on the nanometer scale. The hardness is generally found to increase with decreasing depth, with a rather sharp increase observed at depths of less than 50 nm. Using etch pitting to reveal the surface dislocation structure after indentation, the sharp rise in hardness is found to be associated with the disappearance of dislocation rosette patterns and any signs of near-surface dislocation activity, thereby suggesting that very small scale indentation plasticity may take place by nondislocation mechanisms. However, order of magnitude calculations show that possible alternatives, specifically, diffusional mechanisms, are too slow to make significant contributions. It is suggested that for very small indents, either the surface dislocation debris is quickly annealed out before it can be observed or indentation plasticity is accommodated entirely by subsurface dislocation activity.

Journal ArticleDOI
TL;DR: In this article, the crystal structure of a single crystal, selected from the reaction product, was determined from three-dimensional x-ray diffraction data, and the stoichiometry was found to be Pb 0.71 Cu 0.29 Sr 2 (Y 0.73 Ca 0.27 )Cu 2 O 7.
Abstract: The reaction product from an oxide mixture of 1Pb:2Sr:0.75Y:25Ca:2Cu at 960 °C was nearly single phase. The crystal structure of a single crystal, selected from the reaction product, was determined from three-dimensional x-ray diffraction data. The stoichiometry was found to be Pb 0.71 Cu 0.29 Sr 2 (Y 0.73 Ca 0.27 )Cu 2 O 7 . The tetragonal lattice constants are a = 3.8207(4) A, c = 11.826(1) A, space group P 4/ mmm , M r = 655.6, Z = 1, D x , = 6.31 g cm −3 , λ(MoK α ) = 0.71069 A. R = 0.0269 for 179 unique observed reflections. The crystal structure is isomorphous with the 1212 phases. The Pb ion at the origin has an occupancy of 0.705 (8) and a Cu ion is located at 0.221, 0, 0 with an occupancy of 0.295. The oxygen ion in the plane of the Pb ions is disordered over a fourfold site with an occupancy of one. An ordered arrangement of this oxygen ion results in a distorted environment for periodically repeated Pb ions. The Pb–O bond lengths to the two oxygen ions above and below the Pb plane are 1.968(14) A. The Cu ion, displaced from the origin, can be considered as tetrahedrally coordinated. The four Cu–O bond distances in the CuO 2 plane are 1.917(6) A and the bond length to a fifth oxygen atom is 2.304(14) A at the apex of the square pyramid. A valence sum calculation for Cu in square coplanar coordination yields a value of 2.01. A similar calculation for Cu in the tetrahedral site yields 1.5. Sr is in a monocapped square antiprism coordination polyhedron. Y and Ca atoms occupy statistically the same site and are in cubic coordination. The structure can be described as consisting of CuO 2 –(Y, Ca)–CuO 2 layers with PbO 6 octahedra flanking the layers and Sr in the oxygen interstice. The compound is metallic at room temperature and shows no superconducting transition to 20 K.

Journal ArticleDOI
TL;DR: In this paper, the structural energy differences between cubic LI2 and tetragonal DO22 crystal structures are calculated for MAl3 compounds, where M is a group III, IV, or V transition metal.
Abstract: The structural energy differences between cubic LI2 and tetragonal DO22 crystal structures are calculated for MAl3 compounds, where M is a group III, IV, or V transition metal. The stability of the DO22 structure relative to L12 increases rapidly as the transition-metal d-electron count increases. Typical values of E(DO22) – E(L12) are 0.1–0.15 eV/atom (9600–14500 J/g-atom) for group III,  0.05 eV/atom ( 4800 J/g-atom) for group IV, and ∼ –0.2 eV/atom (∼ –19000 J/g-atom) for group V trialuminides. Similar trends are calculated for the DO23/L12 energy difference. The calculated electronic densities of states (DOS) show that each structure has a minimum in the DOS distribution at a characteristic d-electron count. The preferred crystal structure for a given compound is the one in which the Fermi level lies in the minimum.

Journal ArticleDOI
TL;DR: In this article, the structural intermediates in tetramethyl and tetraethyl orthosilicate (TMOS and TEOS) low pH sol-gel reactions were studied using silicon-29 NMR.
Abstract: Silicon-29 NMR is used to study the structural intermediates in tetramethyl and tetraethyl orthosilicate (TMOS and TEOS) low pH sol-gel reactions. Differences in the number of compact ring structures are related to differences in gel times. Reactions are followed at various silicon: water: acid molar ratios.

Journal ArticleDOI
TL;DR: In this article, the lattice mismatch plays a minor role in the formation of stacking faults and twin boundaries and a formation mechanism is proposed for these defects which is based on deposition errors during the adsorption of atoms on {111} facets of film nuclei.
Abstract: Films of three compound semiconductors with the zincblende structure grown epitaxially on {100} silicon substrates by chemical vapor deposition or metal-organic chemical vapor deposition were investigated by transmission electron microscopy. The three systems have similar thermal mismatches but cover a wide range of lattice mismatch. From the comparison of the observed microstructures as well as from the investigation of early stages of film formation it is concluded that the lattice mismatch plays a minor role in the formation of stacking faults and twin boundaries. A formation mechanism is proposed for these defects which is based on deposition errors during the adsorption of atoms on {111} facets of film nuclei. The observed microstructural features are discussed in terms of this model.

Journal ArticleDOI
Thomas M. Shaw1, S. L. Shinde1, Duane Dimos1, Robert F. Cook1, P. R. Duncombe1, C. Kroll1 
TL;DR: In this paper, the effect of grain size and heat treatment on polycrystalline Y1Ba2Cu3O7−δ was examined using transmission electron microscopy and optical microscopy.
Abstract: We have used transmission electron microscopy and optical microscopy to examine the effect that grain size and heat treatment have on twinning and microcracking in polycrystalline Y1Ba2Cu3O7−δ. It is shown that isothermal oxygenation heat treatments produce twin structures consisting of parallel twins, with a characteristic spacing that increases with increasing grain size. Slow cooling through the temperature range where the orthorhombic-to-tetragonal transformation induces twinning, however, produces a structure consisting of a hierarchical arrangement of intersecting twins, the scale of which appears to be independent of grain size. It is also shown that the microcracking induced by anisotropic changes in grain dimensions on cooling or during oxygenation can be suppressed if the grain size of the material is kept below about 1 μm. The results are examined in the light of current models for transformation twinning and microcracking and the models used to access the effect other processing variables such as oxygen content, doping or heat treatment may have on the microstructure of Y1Ba2Cu3O7−δ.

Journal ArticleDOI
TL;DR: In this paper, the activation energies of two refractory glass compositions in the barium aluminosilicate system were studied by differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM).
Abstract: Barium aluminosilicate glasses are being investigated as matrix materials in high-temperature ceramic composites for structural applications. Kinetics of crystallization of two refractory glass compositions in the barium aluminosilicate system were studied by differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). From variable heating rate DTA, the crystallization activation energies for glass compositions (wt percent) 10BaO-38Al2O3-51SiO2-1MoO3 (glass A) and 39BaO-25Al2O3-35SiO2-1MoO3 (glass B) were determined to be 553 and 558 kJ/mol, respectively. On thermal treatment, the crystalline phases in glasses A and B were identified as mullite (3Al2O3-2SiO2) and hexacelsian (BaO-Al2O3-2SiO2), respectively. Hexacelsian is a high-temperature polymorph which is metastable below 1590 C. It undergoes structural transformation into the orthorhombic form at approximately 300 C accompanied by a large volume change which is undesirable for structural applications. A process needs to be developed where stable monoclinic celsian, rather than hexacelsian, precipitates out as the crystal phase in glass B.

Journal ArticleDOI
TL;DR: In this article, β-SiC thin films have been epitaxially grown on 6H−SiC {0001} substrates via chemical vapor deposition (CVD), and the growth rate increased linearly with the source/carrier gas flow rate ratio.
Abstract: Beta–SiC thin films have been epitaxially grown on 6H–SiC {0001} substrates via chemical vapor deposition (CVD). The growth rate increased linearly with the source/carrier gas flow rate ratio. The activation energy for the growth of β–SiC grown on the Si face of the 6H–SiC substrate was 12 Kcal/mole. These observations are consistent with a surface reaction-controlled process. The as-grown surface morphology is dependent on the terminal layer of the substrate, the growth temperature, and the source/carrier gas flow rate ratio. The C face of a 6H–SiC {0001} substrate caused a higher growth rate and thus poorer surface morphology than the Si face under the same growth conditions. The optimum temperature range for growth of a flat, mirror-like β–SiC surface was determined to be 1773–1823 K in the present CVD system. The microstructure and nucleation of double positioning boundaries were investigated via transmission and scanning electron microscopies. Triangular defects and their modifications were also observed, and their origins have been discussed.

Journal ArticleDOI
TL;DR: The glass-forming region of the BiO1.5-CuO-Ca0.5O system has been determined by melting 5 g batches of glass rods 4 mm in diameter and 75 mm long as discussed by the authors.
Abstract: The glass-forming region of the BiO1.5–CuO–Ca0.5Sr0.5O system has been determined by melting 5 g batches. Glass rods 4 mm in diameter and 75 mm long have been made. The glass transition temperatures (TG) and the onset crystallization temperatures (Tx) are around 390 °C and 440 °C, respectively. Densities of these glasses range from 5.5 to 7.0 g/cm3. The glass structure consists of the [BiO3] and [BiO6] units, and the conversion between these two polyhedra mainly depends on the Ca0.5Sr0.5O amount. The molecular volume of the glasses indicated that increasing Ca0.5Sr0.5O caused loose glass structures.

Journal ArticleDOI
TL;DR: Using TEM, Auger spectroscopy, EDX, and convergent beam electron diffraction, a thorough characterization of the interphase region between SCS6 fibers and Ti-6Al-4V matrix in a metal matrix composite has been performed as discussed by the authors.
Abstract: Using TEM, Auger spectroscopy, EDX, and convergent beam electron diffraction, a thorough characterization of the interphase region between SCS6 fibers and Ti–6Al–4V matrix in a metal matrix composite has been performed. The interphase region is shown to be very complex, consisting of numerous layers of varying compositions and thicknesses. The chemical interaction of the fiber and matrix results in a 0.5–1.5 μm thick TiC layer. Evidence for the existence of a Tix Siy (C) layer is also presented. The SCS6 overlayer on the fibers has inhibited any chemical interaction between the matrix and the SiC filament itself, 60% of the interphase region originating from the SCS6 protective coating. In situ fracture experiments (in an Auger spectrometer) reveal that fracture takes place between the TiC and an amorphous carbon layer.

Journal ArticleDOI
TL;DR: In this article, the stability of the CsCl-type compounds AlRu, SiRu, NiTi, and CuEr under heavy deformation by high-energy ball milling was investigated.
Abstract: We investigated the stability of the CsCl-type compounds AlRu, SiRu, NiTi, and CuEr under heavy deformation by high-energy ball milling. As a result, nanocrystalline intermetallic compounds have been synthesized with a microstructure showing a mixture of highly strained crystallites (5–12 nm) with random orientation. Thermal analysis of the samples indicates stored deformation energies of 5–10 kJ/mole stored by incorporation of lattice defects, chemical disorder and grain boundaries, and an increase of the heat capacity of up to 13%. Chemical disordering of the crystal is monitored by means of the long-range order parameter. The observed decrease in chemical order can be explained by the high grain boundary density of nanocrystalline metals.

Journal ArticleDOI
TL;DR: In this paper, a laser-induced forward transfer (LIFT) was proposed for the rapid deposition and patterning in a clean environment of high superconducting thin films, where a stoichiometric high {ital T}{sub {ital c}} compound is initially deposited in a thin layer on an optically transparent support by laser evaporation or another conventional technique.
Abstract: We propose in this work a new approach, named laser-induced forward transfer, for the rapid deposition and patterning in a clean environment of high {ital T}{sub {ital c}} superconducting thin films. A stoichiometric high {ital T}{sub {ital c}} compound is initially deposited in a thin layer on an optically transparent support by laser evaporation or another more conventional technique. By irradiating under vacuum or in air the precoated layer with a strongly absorbed single laser pulse through the transparent support, we are able to remove the film from its support to be transferred onto a selected target substrate, held in contact to the original film. Using this technique, we have successfully transferred with one single pulse, provided by an excimer or a Nd:YAG laser, YBaCuO and BiSrCaCuO precoated thin films on various substrates. The Rutherford backscattering spectrometry experiments do not show any strong modification in the composition of the transferred layer against the source material, and the superconducting phases for the two types of compounds were obtained after subsequent thermal annealing carried out in a furnace around 850--900 {degree}C in O{sub 2}. For the BiSrCaCuO films transferred onto MgO, substrates we have measured an onset critical temperature of about 90more » K with a zero resistance at 80 K.« less

Journal ArticleDOI
TL;DR: In this paper, the deposition process of diamond was considered on the basis of these experimental results and a review of the relevant field, and the results suggest that the diamond synthesis proceeded by the deposition, with the etching process operating simultaneously, and that atomic hydrogen played a very important role as the etaching agent for non-diamond carbon.
Abstract: Diamond synthesis from the vapor phase has been studied using hot filament assisted CVD, microwave plasma assisted CVD in an open system, and chemical transport process in a closed system using a gas mixture of hydrocarbon diluted with hydrogen gas. The deposited materials were identified as a cubic diamond with x-ray diffraction and Raman scattering measurement. The deposition process of diamond was considered on the basis of these experimental results and a review of the relevant field. The results suggest that the diamond synthesis proceeded by the deposition process, with the etching process operating simultaneously, and that atomic hydrogen played a very important role as the etching agent for non-diamond carbon. Also, the deposition process would be considered as the method that utilizes the bonding energy difference on each crystal surface of the diamond. The complicated problems on diamond synthesis originate from the fact that a third allotropic form exists in carbon, that is, the carbyne group.

Journal ArticleDOI
TL;DR: In this article, the use of sulfur-containing organic monolayer films improves adhesion between gold and silicon dioxide, and the structures of these monolayers were analyzed using contact angle, ellipsometry, and XPS.
Abstract: The use of sulfur-containing organic monolayer films improves adhesion between gold and silicon dioxide. The structures of these monolayers were analyzed using contact angle, ellipsometry, and XPS. The zone of adhesive failure was at or near the gold-monolayer interface.