bluuebunny/crossref_metadata_2025 huggingface dataset:
Improved uniformity and selected area deposition of diamond by the oxy-acetylene flame method
Authors: von Windheim Jesko A. , Glass Jeffrey T.
Published: September 2007

The role of SiO2 in nucleation of diamond has been investigated in an oxy-acetylene flame. It was found that growth methods that minimize SiO2 formation enhance diamond nucleation. A short pretreatment of a scratched Si surface in a low oxygen-to-acetylene ratio flame, at a distance 1.5 cm from the flame core, significantly improved uniformity of subsequent diamond growth. When scratched surfaces were intentionally oxidized, nucleation of diamond was completely inhibited. By using a mask to controllably deposit SiO2 on a scratched Si surface, highly selective deposition of diamond was achieved with resolution below 5 μm. These results are discussed with reference to competing oxidation and carbon formation processes that take place during the nucleation of diamond. During the nucleation stage, carbon may be deposited on the scratched Si via a route in which the Si surface catalyzes carbon formation reactions that are otherwise kinetically unfavorable. The formation of an oxide layer, on the other hand, would act to passivate the surface, and thus inhibit carbon formation via a catalytic route. The decomposition of CO to C and CO2 is given as an example of a reaction that is favored at temperatures below 1000 K, but requires surface catalysis to proceed because it remains frozen out in the gas phase due to a very slow reaction rate.
Characterization of extremely thin epitaxial layers and films by new interference fringes
Authors: Taijing Lu , Toyoda Koichi , Lian Li , Nango Nobuhito , Ogawa Tomoya
Published: September 2007

The existence and imperfection of several types of epitaxial layers and films on the substrate crystals were successfully detected and characterized nondestructively using a new interference fringes method. It was found that the interference fringes generated by amorphous thin films showed many very tiny bright spots that became discontinuous, which was quite different from the case of fringes generated by a crystalline epitaxially grown layer (smooth fringes). Multiple layers on a substrate were also observed by the fringes that were not concentric, but interlacing.
Composite Si/C/N powder production by laser induced gas phase reactions
Authors: Borsella E. , Botti S. , Fantoni R. , Alexandrescu R. , Morjan I. , Popescu C. , Dikonimos-Makris T. , Giorgi R. , Enzo S.
Published: September 2007

Ultrafine amorphous Si/C/N ternary powders have been prepared in a CO2 laser assisted process. We demonstrate the possibility to drive the C/N ratio in the powder by properly choosing the experimental conditions and the gaseous reactant ratio in the initial mixture containing silane, dimethylamine, and ammonia. A kinetic model that accounts for reaching the equilibrium between the gaseous reaction intermediates and the solid products is proposed.
Studies of theoretical and experimental precipitation conditions of Y+3, Ba+2, and Cu+2 ions in oxalate-diethylamine media in the preparation of YBa2Cu3Oy superconductor
Authors: Chen D.H. , Sheen S.R. , Chang C.T. , Shei C.Y. , Hurng W-M.
Published: September 2007

In the preparation of YBa2Cu3Oy (Y-123) superconductor, the coprecipitation method was frequently used in various solution systems due to advantages such as good homogeneity, low reaction temperature, fine and uniform particle size, easy scale-up, and economical concerns. In this paper, a detailed study of different competitive reactions such as dissociation, precipitation, complex formation, and association involving metal ions, oxalate ion, and diethylamine species is performed. The study aids in understanding the precipitation and yields an optimal coprecipitation condition for the title system. A series of experiments are also conducted which confirm that the as-derived optimal condition may be extensively applied to other systems.
Microstructure, microhardness, and superconductivity of CeO2-added Y–Ba–Cu–O superconductors
Authors: Kim Chan-Joong , Kim Ki-Baik , Hong Gye-Won , Won Dong-Yeon , Kim Byoung-Hwan , Kim Chun-Taik , Moon Hong-Chul , Suhr Dong-Soo
Published: September 2007

The CeO2-added Y–Ba–Cu–O oxides were prepared by two different processes, the conventional solid-state reaction process and the partial melt process using powders, to examine the effect of the dopant on microstructure, microhardness, and superconductivity. In the solid-state reacted sample, most of the added CeO2 was converted to a form of BaCeO3, but some might enter into the 1-2-3 phase, resulting in the orthorhombic-to-tetragonal phase transition that accompanied the disappearance of twin structure in 1-2-3 grains. In the partially melted sample, however, the phase change was not observed up to 5 wt. % of CeO2. All the added CeO2 in these samples was consumed to form only BaCeO3 which was finely dispersed in large 1-2-3 grains during the peritectic reaction stage. The zero resistance temperature (Tc) of the solid-state reacted sample gradually decreased with increasing CeO2 content due to the phase change and the formation of BaCeO3, whereas the Tc of the partially melted sample was nearly constant regardless of CeO2 content up to 5 wt. %, owing to the separation of the second phase from the 1-2-3 superconducting phase. Microhardness of the partially melted sample increased with increasing CeO2 content. The strengthening effect appears to come from the composite system where the fine BaCeO3 particles are dispersed in a 1-2-3 matrix.
Compressive creep of dense Bi2Sr1.7CaCu2Ox
Authors: Routbort J.L. , Goretta K.C. , Miller D.J. , Kazelas D.B. , Clauss C. , Domínguez-Rodríguez A.
Published: September 2007

Dense polycrystalline Bi2Sr1.7CaCu2Ox (2212) was deformed from 780–835 °C in oxygen partial pressures, Po2, of 103 to 2 × 104 Pa. Results could be divided into two stress regimes: one at lower stress in which the steady-state creep rate, ∊, was proportional to stress, γ, having an activation energy of 990 ± 190 kJ/mole and being independent of PO2, and another at higher stress in which ∊ was proportional to σn, with n ≍ 5–6. Transmission electron microscopy supported the interpretation that in the lower-stress viscous regime, creep was controlled by diffusion, whereas dislocation glide and microcracking were responsible for strain accommodation at higher stresses.
Structural studies of Al-based powders prepared by chemical methods
Authors: Lenarda M. , Ganzerla R. , Storaro L. , Frattini R. , Enzo S.
Published: September 2007

Finely dispersed metal powders have been obtained after chemical reduction of Ni and Co acetylacetonate by lithium aluminum hydride in tetrahydrofuran at low temperature. The Al/Ni and Al/Co stoichiometry of the as-reduced powders was 1.1 and 1.2, respectively. The structure and thermal stability of the as-reduced powders were affected by the temperature of reduction. For the NiAl powders it was found that the thermal treatment initially induces a separation of highly unstable Ni(Al) and Al(Ni) solid solutions, which subsequently react to give a single NiAl phase of cubic structure not reported in the equilibrium phase diagram. Conversely, the reduction of cobalt acetylacetonate directly gives a cubic metastable phase, from which precipitates some hexagonal form of Co after treatment at 450 °C.
Nucleation and growth of diamond on carbon-implanted single crystal copper surfaces
Authors: Ong T.P. , Xiong Fulin , Chang R.P.H. , White C.W.
Published: September 2007

The nucleation and growth of diamond crystals on single crystal copper surfaces has been studied. Microwave plasma enhanced chemical vapor deposition (MPECVD) was used for diamond nucleation and growth. Prior to diamond nucleation, the single crystal copper surface is modified by carbon ion implantation at an elevated temperature (∊820 °C). This procedure leads to the formation of a graphite film on the copper surface, resulting in an enhancement of diamond crystallite nucleation. A simple lattice model has been constructed to describe the mechanism of diamond nucleation on graphite as 〈111〉diamond parallel to 〈0001〉graphite and 〈110〉diamond parallel to 〈11$\overline 1$0〉graphite. This leads to a good understanding of diamond growth on carbon-implanted copper surfaces.
Kinetics of interfacial reaction between borosilicate glass and sapphire substrate
Authors: Jean Jau-Ho , Gupta Tapan K.
Published: September 2007

Reaction kinetics between borosilicate glass (BSG) and sapphire has been studied at temperatures from 850 °C to 950 °C. Microstructural and chemical analyses show that the nonporous interdiffusion layer is formed with Al+3 ion dissolving from sapphire and K+ diffusing from BSG onto the interface of sapphire/BSG, and that both ions are always coupled together in the reaction layer. The interdiffusion layer moves toward BSG with time and the reaction starts immediately at temperatures investigated without incubation period. The growth kinetics for the interdiffusion layer follows a parabolic rate law in the temperature range investigated, and shows an apparent activation energy in the range of 176 k–/mol. The diffusion coefficient of aluminum ion is determined from EDX analysis, and the values range from 0.7–1.4 × 10−12 at 850 °C to 3.0–6.0 × 10−12 cm2/s at 950 °C. The above results also show an activation energy close to that determined from the parabolic growth rate constants, suggesting that the mass-transport kinetics of aluminum ion from sapphire into the interdiffusion layer controls the formation process.
The formation ofa–Fe2O3monodispersed particles in solution
Authors: Morales M.P. , González-Carreño T. , Serna C.J.
Published: September 2007

Uniform α–Fe2O3particles of varying axial ratios have been prepared from hydrolyzed ferric chloride solutions at 100 °C. In the absence of phosphate anions, spherical particles were obtained by a mechanism that follows the classical LaMer and Dinegar scheme. However, in the presence of phosphates ellipsoidal particles were observed, with their formation taking place through an aggregation process from smaller primary particles of α–Fe2O3. It is also shown that all particles are monocrystalline irrespective of their formation mechanism.
Crystallization behavior in a low temperature acetate process for perovskite PbTiO3, Pb(Zr, Ti)O3, and (Pb1−x, Lax) (Zry, Ti1−y)1−x/4O3 bulk powders
Authors: Lin C.T. , Scanlan B.W. , McNeill J.D. , Webb J.S. , Li Li , Lipeles R.A. , Adams P.M. , Leung M.S.
Published: September 2007

An aqueous, metallo-organic precursor for PbTiO3 (PT), Pb(Zr, Ti)O3 (PZT), and (Pb1−x, Lax) (Zry, Ti1−y)1−x/4O3 (PLZT) ceramic powders has been developed employing the acetate complexes of Pb2+, La3+, Zr4+, and Ti4+. The crystallization behavior (e.g., degree of crystallization and crystallite size) was examined over the temperature range of 400–700 °C with concentrations of excess lead varying from 0–5% using x-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy techniques. It was found that PT, PZT, and PLZT crystallize at temperatures as low as 450 °C, 500 °C, and 500 °C, respectively, and the excess lead required for crystallization of the perovskite phase is ⋚1%. Intermolecular mixing among acetate precursors via chelating and bridging structures was evident and appeared to be responsible for the low crystallization temperatures. The process is compared to the current sol-gel methods for its possible use in thin film ferroelectric manufacture.
Topographic/structure changes of implanted Si3N4
Authors: Oblas D.W. , Sarin V.K. , Ostreicher K.
Published: September 2007

A series of investigations were carried out to evaluate the topographic and structural effects of ion implantation on monolithic Si3N4 ceramic surfaces. Implantations were performed with N2, Ne, or Ar in the fluence range of 2.0 × 1016 to 4 × 1017 particles/cm2 and implant voltages of 125 to 200 keV, depending on the mass of the implanted species. Single crystal Si and SiC were also examined for comparative purposes. Noble gases produced blisters on Si3N4 and significant increases in surface expansion. TEM examination of the Si3N4 blister shell showed a distribution of small bubbles, ranging in size from 5 to 300 nm, depending upon the type of Si3N4, and a transformation of the original crystalline structure into an amorphous phase. Analysis of the blister shell, using electron energy loss spectrometry (EELS) and energy dispersive x-ray spectrometry (EDXS) showed that a significant quantity of the implanted Ar was still present in the blister (skin).
High temperature reactions within SiC–Al2O3 composites
Authors: Gadalla Ahmed , Elmasry Mohamed , Kongkachuichay Paisan
Published: September 2007

Composites of SiC–Al2O3 and SiC–mullite are unstable at high temperatures. The reactions occurring within the composites between 1700 and 1850 °C in stagnant inert atmospheres were characterized. Gaseous products cause excessive weight losses which cannot be attributed to passive oxidation. These losses can be successfully retarded by processing under high pressures. Compatible phases were determined by x-ray analysis for mixtures lying in the section SiC–Al4C3−Al2O3−SiO2. The reactions produced condensed phases of Al2OC and Al4O4C as well as gaseous SiO and CO. The condensed phases have high vapor pressures above 1700 °C. The effect of these reactions on densification of composites by firing at different temperatures for various periods under different pressures was studied. Dense materials prepared under high pressures at 1825 °C were tested at 1700 °C under normal pressure in argon, where active oxidation is expected, and weight losses were insignificant.
Synthesis of nanocomposites: Organoceramics
Authors: Messersmith Phillip B. , Stupp Samuel I.
Published: September 2007

We report here on the synthesis of new materials termed organoceramics in which polymers are molecularly dispersed within inorganic crystalline phases. These nanocomposite materials may not only have unique morphologies and physical properties but may also lead to new processing methods for ceramic-based materials. In organoceramics polymer molecules could opportunistically occupy sites such as grain boundaries or other two-dimensional defects, nanopores, lattice channels, or interlamellar spaces. Our synthetic approach to get macromolecules to those sites is to nucleate and grow inorganic crystals from homogeneous solutions containing the polymer chains as co-solutes. The new materials discussed in this manuscript were synthesized by growing calcium aluminate crystals in the presence of water soluble polymers and were characterized by x-ray diffraction, scanning electron microscopy, elemental analysis, and diffuse reflectance infrared spectroscopy. The macromolecules used in organoceramic synthesis included poly(vinyl alcohol), poly(dimethyldiallyl ammonium chloride), and poly(dibutyl ammonium iodide). We found that the chemistry of polymer repeats can impact on the spatial distribution of the dispersed organic chains and also on the morphology of organoceramic powders. In the case of the poly(vinyl alcohol) organoceramic the polymer is intercalated in “flattened” conformations in Ca2Al(OH)6[X] ·nH2O, thus increasing the distance between ionic layers from 7.9 Å to ∊ 18 Å (X is a monovalent or divalent anion). Such a layered nanocomposite can be formed only by intercalating the poly(vinyl alcohol) during growth of the Ca2Al(OH)6[X] · nH2O crystal. The synthetic pathway is therefore able to overcome large entropic barriers and incorporate significant amounts of polymer in the organoceramic product, in some cases up to 38% by weight. The particles of this nanocomposite are spheroidal aggregates of thin plate crystals whereas the use of a polycationic polymer in the synthesis leads to rod-like particles in which organic chains may reside in channels of the inorganic crystal.
Environmental embrittlement of γ titanium aluminide
Authors: Takasugi T. , Hanada S. , Yoshida M.
Published: September 2007

The environmental embrittlement for the nearly stoichiometric TiAl compound, the microstructure of which consists of monophase γ with equiaxed grains, was evaluated by tensile tests, to determine the effects of the atmospheres used (vacuum, O2 gas, air, and H2 gas) and the testing temperatures (R.T. to 1173 K). At room temperature, the highest elongation and UTS values were observed in the samples tested in vacuum, while the worst values were observed in the samples tested in H2 gas. Transgranular cleavage fracture was dominant and primarily independent of the environmental media. At intermediate temperatures, the samples tested in vacuum exhibited higher elongation and UTS values than those tested in air. Intergranular fracture became more dominant as temperature increased but was insensitive to the environmental media. Based on these results, the mechanism responsible for the observed environmental embrittlement and the implications were discussed.
The application of transmission electron microscopy to the study of high nuclearity carbonyl clusters (HNCC's)
Authors: Devenish R.W. , Mulley S. , Heaton B.T. , Longoni G.
Published: September 2007

The properties of a variety of HNCC's supported on holey carbon films have been investigated by transmission and analytical electron microscopy; a review of how these techniques can be used to study these crystallographically well-characterized clusters is also presented. Images of the metallic skeleton corresponding to a single cluster unit have been observed for [Ni38Pt6(CO)48H]5−, [Ni34C4(CO)38H]5−, [Pt38(CO)44Hx]2−, [Rh17S2(CO)32]3−, [Ni12Sn(CO)22]2−, and [Ni22Ge(CO)22]2−, and analytical data at low electron beam intensities are consistent with their crystallographically established structures. At increasing current densities, agglomeration into larger particles occurs and these larger particles (2–5 nm) gradually lose the chemical identity of the precursor. Ultimately at current densities of the order of 107 A/m2 metal segregation and/or metal alloy particles having close-packed crystal structures are formed together with graphitic carbon from decomposition of the CO ligands. Possible mechanisms for this beam damage are discussed. There is evidence of cluster/substrate interaction resulting in increased stability compared to that observed in solution and in the normal crystalline state.
A Mössbauer spectroscopy study of nanoscale Ge–Sn dispersions prepared by ball milling
Authors: Boolchand P. , Koch C.C.
Published: September 2007

Nanoscale mixtures of Ge/Sn, a nominally immiscible binary system, prepared by mechanical attrition have been studied by 119Sn Mössbauer spectroscopy. The Mössbauer measurements in general reveal two sites for the Sn atoms, a tetragonal β–Sn site and another site designated as “A”. The β-Sn integrated intensity decreases in magnitude with Ge concentration, 1 – x, in Ge1−xSnx as the second A-site intensity increases. The isomer shift and the small/negligible quadrupole splitting of site A suggests it represents Sn in solid solution in the Ge lattice. This in turn represents a large (12–24 at. %) nonequilibrium solid solubility of Sn in Ge prepared by mechanical milling, compared to the equilibrium value of <1.0 at. %. Oxidation of the Sn was detected by Mössbauer spectroscopy at Sn-poor concentrations (x ⋚ 0.10) when the milling vial was not totally free of oxygen (i.e., milling in impure argon). This may be due to rapid oxidation of finely divided Sn film particles possessing a large surface-to-volume ratio at these compositions.
Interaction between atomic hydrogen and YBa2Cu3O7
Authors: Ponce Angel L.G. , Fripiat Jose J.
Published: September 2007

Atomic hydrogen obtained from dissociative chemisorption of molecular H2 on Pt particles deposited on the surface of YBa2Cu3O7 reacts with the oxide in producing O vacancies and intercalating H at 82 °C under a H2 pressure of about 400 Torr. An induction period which extends over 1 h is observed as long as the concentration in O vacancies is below 0.1. Above this approximate limit the reaction proceeds quickly until about 1 mol H2 has been consumed. It then slows down progressively, but it is not completed even after 27 h of reaction and ∼1.4 H2 consumed, under these experimental conditions. The enthalpy for the creation of the O vacancy is 143 kcal/g mol O, while the H intercalation enthalpy is −57.5 kcal/g mol H. It appears that the reaction of molecular H2 over YBa2Cu3O7 not coated with Pt proceeds similarly, but the rate is nearly one order of magnitude slower, under identical conditions. The stoichiometry of the reaction agrees with the earlier suggestion that O1 is a labile oxygen which can be replaced by intercalated H. This site and the interstitial vacant sites in the copper chains square plane would be the first ones to be occupied by intercalated H.
Synthesis of YBa2Cu3O7−x composite superconductor by the spray drying method
Authors: Takao Yasumasa , Awano Masanobu , Takagi Hiroyoshi
Published: September 2007

YBa2Cu3O7−x–Ag and YBa2Cu3O7−x–SiO2 composite superconductors were prepared by the spray drying method. The composite powder was prepared by calcination of the precursor powder, which was synthesized by spray drying the oxalate slurries and dissolved components. Fine Ag, Ba2SiO4, and other phases were distributed in composite sintered bodies, with the added Ag or SiO2 showing enhanced magnetic properties.
Bi2Sr2CaCu2O8+x films prepared by fluorinated precursors in dry ambient
Authors: Hung L.S.
Published: September 2007

Bi2Sr2CaCu2O8+x films were prepared by mixing 2-ethylhexanoate precursors with heptafluorobutyric acid and annealing at 820 °C in dry ambient. The films underwent substantial mass transport and exhibited abnormal grain growth, indicating that some liquid phases might be involved in the process. It was found that the high atomic mobilities during annealing significantly improved in-plane orientations and consequently increased critical current densities.
Disordering and amorphization of L12-type alloys by mechanical attrition
Authors: Benameur T. , Yavari A.R.
Published: September 2007

X-ray diffraction patterns obtained during the grinding of Ni3Ge and Ni3Al alloys which at equilibrium exhibit the L12 ordered fcc structures show the emergence of a nanocrystalline structure and transformation to the disordered fcc form but little amorphization. Furthermore, the non-L12 Al2Pt alloy which also has a more strongly negative heat of mixing is easier to amorphize than the Ni3Ge and Ni3Al with L12 superstructure. This is in contrast to the Zr3Al compound (also L12-type) for which a short milling time is sufficient for obtaining complete amorphization. Variations in the aptitudes toward amorphization of the three L12-type alloys under ball-milling conditions are attributed in part to the differences in the lattice stability terms of their disordered fcc phases.
Mechanical properties of vapor-grown carbon fibers prepared from benzene in Linz–Donawitz converter gas by floating catalyst method
Authors: Ishioka Munehiro , Okada Toshihiko , Matsubara Kenji
Published: September 2007

Tensile strength and Young's modulus were determined for vapor-grown carbon fibers (VGCF's) prepared from benzene in Linz–Donawitz converter gas (LDG) by the floating catalyst method. The tensile strength decreased with increasing diameter and volume and depended strongly on whether the fiber was straight or crooked. The average tensile strength of straight fibers was 2.05 GPa, while that of crooked fibers was 1.09 GPa. The latter broke at bends in the fiber where they acted as defects. The average Young's modulus of straight fibers was 163 GPa, while that of crooked fibers was a little higher at 197 GPa. VGCF's prepared in LDG appeared to have comparable mechanical properties to those grown in hydrogen.
Phase transformation and preferred orientation in carboxylate derived ZrO2 thin films on silicon substates
Authors: Chu Peir-Yung , Campion Isabelle , Buchanan Relva C.
Published: September 2007

Phase transformation and preferred orientation in ZrO2 thin films, deposited on Si(111) and Si(100) substrates, and prepared by heat treatment from carboxylate solution precursors were investigated. The deposited films were amorphous below 450 °C, transforming gradually to the tetragonal and monoclinic phases on heating. The monoclinic phase developed from the tetragonal phase displacively, and exhibited a strong (111) preferred orientation at temperature as low as 550 °C. The degree of preferred orientation and the tetragonal-to-monoclinic phase transformation were controlled by heating rate, soak temperature, and time. Interfacial diffusion into the film from the Si substrate was negligible at 700 °C and became significant only at 900 °C, but for films thicker than 0.5 μm, overall preferred orientation exceeded 90%.
Mechanochemical effects on synthesis and pulverization of Ba2YCu3O7−y, superconductor and Ba2YCu3O7−y–Ag composite
Authors: Awano Masanobu , Kani Keiko , Takao Yasumasa , Kuwahara Yoshitaka , Takagi Hiroyoshi
Published: September 2007

Mechanochemical effects on the synthesis and pulverization process of the Ba2YCu3O7−y superconductor (abbreviated to BYCO) and BYCO–Ag composite were examined. Grinding of the precursor of BYCO promoted the homogeneous distribution of each element and subsequently the crystallinity of the precursors was decreased. The sintered body obtained by using the ground and calcined powder had complicated crystal phases consisting of the BYCO phase with secondary phases and resulted in the impairment of the superconducting transport current property. Grinding of crystallized BYCO powder decreased its grain size and crystallinity; the crystallinity was recovered by annealing. On the other hand, grinding contributed to the synthesis of the BYCO–Ag superconducting composite by minimizing the particle sizes and mixing of components. The BYCO–Ag composite so obtained displayed improvement of superconducting properties.
Grain growth kinetics and microstructures of the highTcGdBa2Cu3O7−δsuperconductor
Authors: Shin M.W. , Hare T.M. , Kingon A.I. , Koch C.C.
Published: September 2007

Grain growth in the GdBa2Cu3O7−δhighTcsuperconductor was investigated. The composition Gd1.09Ba1.91Cu3O7−δ, within the solid solubility region, was selected for the present grain growth study. Differential thermal analysis did not reveal any thermal event except the incongruent melting point, which is indicative of the absence of a liquid second phase during grain growth. The final densities of isothermally annealed samples ranged from 91.3% to 93.7% of theoretical density. The microstructure observation showed a greater grain aspect ratio in this material than in YBa2Cu3O7−δ. The average grain aspect ratio was about 4.7. A very low grain growth exponent of 0.07 was found in the isothermal annealing temperature range from 965 °C to 1020 °C. By comparison with the results on the YBa2Cu3O7−δsystem previously reported, it was concluded that the grain growth kinetics in these materials are strongly controlled by the anisotropy of the grain boundary energy. The activation energy of grain growth was calculated to be about 77 kJ/mole.
Self-diffusion along twist grain boundaries in Cu
Authors: Nomura Miki , Adams James B.
Published: September 2007

In a previous paper we studied vacancy diffusion in two high-angle twist grain boundaries in Cu, using the EAM. In this paper, we discuss vacancy diffusion along four additional twist grain boundaries, from 8.8–43.6°. Vacancy formation energies in all the possible sites were calculated (0.14–1.42 eV) and found to be directly related to the degree of coincidence with the neighboring crystal planes. The optimal migration paths were found to coincide with the screw dislocations which comprise the boundary. Vacancy migration energies were found to be low (0.02–0.52 eV). The activation energies for self-diffusion at the boundaries were found to be less than half of the bulk value, in general agreement with experiment. Calculated diffusion rates, δD, for medium-high angle twist grain boundaries were in reasonable agreement with experimental data for polycrystalline material. Diffusion rates were found to decrease with increasing twist angle, in contrast with two sets of conflicting experimental data.
Vacancies and antisite defects in ordered alloys
Authors: Johnson R.A. , Brown J.R.
Published: September 2007

Equations for the concentrations of vacancies and antisite defects in ordered alloys in thermodynamic equilibrium at and near stoichiometry have been derived as functions of defect energies and a Lagrangian parameter. While the resulting equations cannot be solved analytically and in general require iterative calculations, an approximation is given that permits simple numerical evaluation with just a minor loss of accuracy. Using defect energies obtained from an embedded-atom method calculation for Cu3Au, it is found that the adjustment for off-stoichiometric compositions is accounted for primarily by the creation of antisite defects rather than vacancies, and the vacancy concentration on Au sites is orders of magnitude less than that on Cu sites. There is a significant increase in the Au vacancy concentration but a slight decrease in the net vacancy content with increasing Cu fraction.
Effect of oxidation on the mechanical properties of a NbAl3alloy at intermediate temperatures
Authors: Raj S.V. , Hebsur M. , Locci I.E. , Doychak J.
Published: September 2007

Although a NbAl3alloy containing Cr, W, and Y has excellent oxidation resistance above 1440 K, it suffers from severe environmental attack during deformation at intermediate temperatures between 900 and 1100 K. Specimens tested in constant velocity and constant load direct compression tests showed varying degrees of degradation depending on environment (i.e., air or argon), surface finish, stress, and temperature. As a result, there were corresponding differences in mechanical behavior and in the observed microstructures. At high stresses and strain rates, specimens with as-machined surfaces were brittle at and below 1100 K when tested in air but showed fracture strains above 4% when deformed in argon. However, reproducible compressive ductility of 2–3% was attained on polished specimens tested in air. At intermediate stresses, the creep curves showed sudden and periodic increases in strain before the specimens failed catastrophically after about 80 h. Microstructural examination of these specimens revealed extensive oxidation within cracks. Constant load tests conducted at lower stresses below 100 MPa showed an apparent incubation period where the change in the length of the specimen was immeasurably small. Following the incubation period, which typically lasted between 10 and 110 h depending on stress, temperature, and surface condition, specimens increased significantly in length due to oxide growth. In this case, considerable oxide spalling occurred during the course of the test, often leading to a substantial decrease in the cross-sectional area of the specimen. Microstructural observations revealed extensive cracking in the oxide layer and in the matrix, where the cracks had originated at the oxide-metal interface. The effects of environment on the mechanical properties are rationalized with the help of a schematic environmental-deformation mechanism map.
Inhibiting effect of AlPO4 and SiO2 on the anatase → rutile transformation reaction: An x-ray and laser Raman study
Authors: Debnath R. , Chaudhuri J.
Published: September 2007

The anatase → rutile transformation reaction of TiO2 prepared in contact with amorphous aluminum phosphate and with fume silica has been investigated as a function of temperature by x-ray diffraction methods. It is observed that the anatase → rutile transformation in such systems becomes greatly inhibited and the Raman spectra of titania on such surfaces exhibit features that indicate substrate-TiO2 interfacial reactions.
An investigation of the isotropy of epoxy polymers
Authors: Odom Edwin M. , Adams Donald F.
Published: September 2007

Measurements of the engineering constants E, G, and v are routinely made for polymeric materials. If these materials are isotropic, these measurements should satisfy the relationship G = E/2(1 + v). However, many past measurements have indicated that this relationship is not satisfied. This raises questions about the assumptions of material isotropy and the applicability of Hooke's law. The methods used to measure these engineering constants for a number of different polymers are first described. Then, new results obtained in the current investigation are presented, indicating that the elastic constants do in fact satisfy the isotropic relationship for strains up to 0.5%. However, it is shown that at strain levels above this level, the relationship between stress and strain is nonlinear.
Thermoelectric power of YBa2Cu3O7−δ superconductive thin films as a function of oxygen contents
Authors: Cornejo Iván A. , Trefny John U. , Yarar Baki
Published: September 2007

Experimental correlations of thermoelectric power (TEP), oxygen deficiency (δ), and c-lattice constant in YBa2Cu3O7−δ (YBCO) c-oriented, polycrystalline, thin-film, high-temperature oxide superconductors are presented. The TEP values in the normal state were observed to be positive for samples with oxygen deficiencies between 0.08 ≤ δ ≤ 1.00 and negative for 0.00 ≤ δ ≤ 0.08. Furthermore, the TEP magnitudes were highly sensitive to the amount of oxygen and were found to increase with increasing oxygen deficiency, ranging from values of −2 μ V/K to more than two orders of magnitude higher. In addition, relatively temperature-independent TEP values were observed for hole-conductive samples with low oxygen deficiencies (δ ≤ 0.30). The lattice constants (c) of the same samples were measured by XRD. These were found to increase with oxygen deficiency and to correlate systematically with the TEP values. We conclude that TEP is a reliable indicator of the oxygen content in YBCO materials.
A simple predictive model for spherical indentation
Authors: Field J.S. , Swain M.V.
Published: September 2007

A simple model is described with which the entire force versus penetration behavior of indentation with a sphere, during loading and unloading, may be simulated from knowledge of the four test material parameters, Young's modulus, Poisson's ratio, flow stress at the onset of full plastic flow and strain hardening index, and the elastic properties of the indenter. The underlying mechanisms are discussed and the predictions of the model are compared with data produced by an ultra low load, penetration measuring instrument.
Structural and optical properties of laser-synthesized copper oxide films
Authors: Wautelet M. , Roos A. , Lazzaroni R. , Hanus F. , Lambin G.
Published: September 2007

Thin copper films are deposited onto glass and irradiated, in air, by means of a scanned Ar+ laser beam. The kinetics of oxidation are measured by an interferometric method. The laser-synthesized oxide films are characterized by optical microscopy, scanning tunneling microscopy, and optical spectroscopy. It is shown that the oxidation kinetics, as well as the structural and optical properties of the oxide, depend on the laser beam power and on the initial thickness of the copper film. It is argued that these facts are related to the thermal history of the samples under laser irradiation.
Low temperature perovskite formation of lead zirconate titanate thin films by a seeding process
Authors: Kwok Chi Kong , Desu Seshu B.
Published: September 2007

A two-step seeding process has been developed to lower the transformation temperature and modify the grain structure of ferroelectric lead zirconate titanate (PZT) thin films with high Zr/Ti ratio. Previous study has shown that nucleation is the rate-limiting step for the perovskite formation. Therefore, any process that enhances the kinetics of nucleation is likely to decrease the transformation temperature. In this process, a very thin (45 nm) seeding layer of PbTiO3, which has a low effective activation energy for perovskite formation, was used to provide nucleation sites needed for the low temperature perovskite formation. In this study, we have shown that the pyrochlore-to-perovskite phase transformation temperature of PbZrxTi1−xO3 films of high Zr/Ti ratio (e.g., x = 53/47) can be lowered by as much as 100 °C. The grain size of these films can also be substantially modified by this two-step approach.
Devitrification inhibitor in binary borosilicate glass composite
Authors: Jean Jau-Ho , Gupta Tapan K.
Published: September 2007

When an appropriate mixture of a low-softening borosilicate glass (BSG) and a high-softening high silica glass (HSG) is sintered at temperatures ranging from 800 to 1000 °C, a crystalline phase, identified as cristobalite by XRD, is known to precipitate out of the initial amorphous binary mixture of glasses as the sintering continues. The precipitation of cristobalite is found to originate in HSG, and is controlled by the transport of alkali ions (e.g., K+, Na+, and Li+) from BSG to HSG.1 In this paper, we report that when a small amount of alumina is present as a dopant in the above binary mixture of BSG and HSG, the cristobalite formation is completely prevented at the sintering temperatures investigated. The above result is attributed to a strong affinity between Al+3 from alumina and alkali ions from BSG, which diverts the diffusion of alkali ions from HSG to alumina, thus forming a K+ and Al+3-rich reaction layer adjacent to the alumina particles far too rapidly compared to that of cristobalite formation.
Model for the temperature dependent transmission of optically transparent poly(methyl methacrylate) composites
Authors: Lin Hongy , Day Delbert E. , Stoffer James O.
Published: September 2007

An infinite cylinder model was used to predict the optical transmission of transparent composites containing unidirectionally aligned glass fibers. The parameters used in the model are the volume content and diameter of the glass fibers, the refractive index of the fiber and matrix, the nonwet fiber content, the thickness of the composite, and the temperature coefficient of the refractive index of the matrix. The transmission calculated from the model agreed well with the measured temperature-dependent transmission of a composite from 20 to 70 °C for thin specimen (<1.0 mm) containing fiber less than 10 vol. %. The model showed that a small amount of nonwet fiber (0.7% of fiber content) could cause a substantial reduction in the maximum transmission. The content of nonwet fibers should be as small as possible for making a highly transparent composite.
Microstructure of CuInS2films prepared by sulfuration of Cu–In–O films
Authors: Wada Takahiro , Negami Takayuki , Nishitani Mikihiko
Published: September 2007

Polycrystalline CuInS2thin films are prepared by sulfuration of Cu–In–O films. The Cu–In–O films are deposited from a sintered Cu2In2O5target by using a pulsed laser deposition (PLD) method. Then, the Cu–In–O films are converted into CuInS2films by means of a subsequent annealing in an H2S gas atmosphere. The characteristics of the films are determined by using an x-ray diffractometer (XRD), an energy dispersive x-ray spectrometer (EDX), and a scanning electron microscope (SEM). The effects of the deposition and sulfuration temperatures of the Cu–In–O films on the structural and microstructural properties of CuInS2films are examined experimentally. Single-phase CuInS2films with a chalcopyrite structure are obtained when Cu–In–O films are sulfurated at a temperature higher than 400 °C. Grain size of CuInS2is larger when a lower deposition temperature and a higher sulfuration temperature of Cu–In–O films are employed.
The effects of natural convection and conduction in a zone-melting-recrystallization chamber
Authors: Heilman Bradley D. , Marston Matthew A. , Wong Peter Y. , Miaoulis Ioannis N.
Published: September 2007

Zone-melting recrystallization (ZMR) with a graphite strip heater is used to improve the material quality of thin film structures for microelectronic applications. The process takes place in a sealed chamber filled with an inert gas such as argon or helium. The effect of natural convection and conduction at the interface between the gas and film structure was studied both numerically and experimentally. Numerical simulations of the temperature profile in the film structure, and the flow pattern and temperature field in the gas were developed. Experimental observations in a scaled setup using a liquid medium verified the flow patterns calculated from the numerical model of the gas flow in the chamber. Results indicated that the gas is stagnant in the region below the strip heater; consequently, conduction from the strip heater to the wafer is prevalent. Outside the stagnant region, natural convection cools the film structure. These two effects combine to create a steeper thermal gradient across the entire wafer which can increase the thermal stresses in the film. The magnitude of this thermal gradient depends strongly on the thermal diffusivity of the gas. The configuration of the strip heater may significantly affect the amount of heat conduction in the stagnant region.
Filament formation in switching devices based on V2O5 gel films
Authors: Zhang J-G. , Eklund P.C.
Published: September 2007

Filament formation in V2O5 gel films leading to a two-terminal switching device has been observed. In a previous paper, we have identified the switching with a metal-insulator transition within a permanent, current-induced channel between the electrical contacts. Here, we describe the reversible formation of a filament inside the channel, and obtain a static solution of the heat transport equation for this device which indicates a large temperature variation within the filament, and further reveals the failure mechanism for the reversible switching. The principle of least entropy production has been used to analyze the filament growth, which results in a successful simulation of the “on” state I-V characteristic of the switching process, including its negative resistance region.
Phase transformations and microstructure evolution in sol-gel derived yttrium-aluminum garnet films
Authors: Hay R.S.
Published: September 2007

Diphasic yttrium-aluminum garnet (Y3Al5O12, YAG) sols were made by hydrolysis of aluminum and yttrium isopropoxides. The sols were gelled across TEM grids to make films. The films were heat-treated up to 1550 °C for as long as 300 h. Heat-treatments of bulk gel were also done. Microstructure and phase evolution were observed by TEM. Some observations were donein situin a TEM hot-stage. YAG fraction and grain size, matrix grain size, nuclei/area, and film thickness were measured. Bulk samples were characterized by x-ray, DTA, and TGA. Yttrium-aluminum monoclinic (YAM) and transition alumina appeared at 800 °C. YAG nucleated between 800 °C and 950 °C. Nucleation was weakly correlated with the transient presence of YAlO3garnet, and was eventually site-saturated at 0.3/μm3. The change in grain growth rate of the YAM and transition alumina matrix correlated with the change in the growth rate of YAG. Between 850 °C and 1000 °C YAG growth hadt1/2dependence and 280 kJ/mole activation energy. Below 850 °C nucleation was continuous, and growth hadt0.85dependence. Above 1000 °C YAG growth hadt1/4dependence, and the matrix grains coarsened witht1/4dependence. Thicker films reacted faster because the nuclei/area and the growth rate after nucleation scaled with thickness. YAG growth was accompanied by formation of 20–100 nm subgrains. In the late stages of matrix grain coarsening there was also some reaction to YAG by a different process. Nucleation and growth kinetics are compared with other systems. Possible mechanisms are discussed.
Mechanical properties of hot isostatically pressed Si3N4 and Si3N4/SiC composites
Authors: Unal O. , Petrovic J.J. , Mitchell T.E.
Published: September 2007

The mechanical properties of hot isostatically pressed monolithic Si3N4 and Si3N4−20 vol. % SiC composites have been studied by microindentation at temperatures up to 1400 °C. Indentation crack patterns and microstructures have been examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. It is shown that dense Si3N4 base materials can be synthesized by HIPing without densification aids. Both the monolithic Si3N4 and the Si3N4/SiC composites exhibit high hardness values which gradually decrease with increasing temperature. Both types of material show low fracture toughness values apparently because of strong interfacial bonding. On the other hand, the fracture toughness of the composite is about 40% higher than that of the monolithic material, due to the presence of the 20 vol. % SiC whiskers. A crack deflection/debonding mechanism is likely to be responsible for the higher toughness observed in the composite. High resolution electron microscopy shows that the grain boundaries in both samples contain a thin SiO2 layer.
Continuous microindentation of passivating surfaces
Authors: Venkataraman Shankar K. , Kohlstedt David L. , Gerberich William W.
Published: September 2007

Continuous microindentation tests performed on the electropolished surfaces of single crystal Fe (3 wt.% Si), known to have a thin passivation film, show a sharp discontinuity at a load of 1.8 mN. To this point, there was no apparent plastic deformation in the metal in that the loading and unloading curves exactly overlay each other. Stresses at the discontinuity were close to the theoretical strength of the metal. Elastic contact theories of Hertz and Love reproduced the elastic portion of the load-displacement curves. On removing the passivation film with a HCl solution, indentation tests yielded strengths nearly two orders of magnitude smaller. The strength recovered to near its initial value after the liquid evaporated and the passivation film re-formed.
Glass-metal nanocomposites in bulk form by sol-gel route followed by hot pressing
Authors: Roy S. , Chatterjee A. , Chakravorty D.
Published: September 2007

Glass-metal nanocomposites involving copper and nickel, respectively, have been synthesized in bulk form by hot pressing sol-gel derived silica-metal nanoparticle composite powders. The particle diameters range from 9 to 17.5 nm. The specimens exhibit the characteristic behavior of the metallic species in their nanocrystalline forms.
Resistivity measurement of an a-axis-oriented YBa2Cu3O7–δ thin film at 450-650 °C
Authors: Usui Toshio , Oishi Akira , Teshima Hidekazu , Ohata Kazumi , Morishita Tadataka
Published: September 2007

Resistivity measurement of an a-axis-oriented YBa2Cu3O7-δ (YBCO) thin film was carried out in a stream of O2/Ar mixtures at 450–650 °C. Rapidly reversible change in the resistivity of the YBCO film was observed as a function of the oxygen partial pressure in the ambient atmosphere due to the oxygen in-/out-diffusion in the YBCO film. Resistivity measurement of YBCO thin films at higher temperatures of 450–650 °C was found to be quite effective to predict the influence of fabricating processes on the superconducting properties of YBCO films.
Optical spectroscopic study of the radio-frequency oxygen thermal plasma evaporated YBa2Cu3O7−x
Authors: Yuhya Shigenori , Tsujino Jiro , Tatsumi Noriyuki , Shiohara Yuh
Published: September 2007

Optical emission spectroscopy of the rf thermal plasma evaporated YBa2Cu3O7−x has been investigated. The deposited films with the nearly stoichiometric composition could be obtained by using Y-rich powder. From the optical emission spectra, O, Y, Y+, Ba, Ba+, Cu, and YO as the emitting species were observed. The excitation temperature was lower for Y+ than for the other dissociated elements. The relative intensity for YO of a suboxide became larger with a lower plasma torch. It is thought that the deposited film that is poor in Y results from YO sticking to the substrate rather than Y or Y+.