bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydride structures in Ti-aluminides subjected to high temperature and hydrogen pressure charging conditions
Authors: Legzdina D. , Robertson I.M. , Birnbaum H.K.
Published: September 2007

The distribution and chemistry of hydrides produced in single and dual phase alloys with a composition near TiAl have been investigated by using a combination of TEM and x-ray diffraction techniques. The alloys were exposed at 650 °C to 13.8 MPa of gaseous H2 for 100 h. In the single-phase gamma alloy, large hydrides preferentially nucleated on the grain boundaries and matrix dislocations and a population of small hydrides was distributed throughout the matrix. X-ray and electron diffraction patterns from these hydrides indicated that they have an fcc structure with a lattice parameter of 0.45 nm. EDAX analysis of the hydrides showed that they were enriched in Ti. The hydrides were mostly removed by vacuum annealing at 800 °C for 24 h. On dissolution of the hydrides, the chemistry of hydride-free regions of the grain boundary returned to the matrix composition, suggesting that Ti segregation accompanied the hydride formation rather than Ti enrichment causing the formation of the hydride. The hydrogen content in the two-phase (γ-α2) alloy was approximately three times that of the single phase alloy, which was presumably a consequence of the presence of the α2-Ti3Al phase in the two-phase alloy. The hydrides in the two-phase material were shown by x-ray diffraction to have an fcc structure and were removed on annealing in vacuum at 800 °C for 24 h.
Near-interface characterization of diamond films on silica and silicon
Authors: Pickrell D.J. , Zhu W. , Badzian A.R. , Newnham R.E. , Messier R.
Published: September 2007

The near-interface structure of diamond films grown from a methane and hydrogen gas mixture by microwave plasma enhanced chemical vapor deposition has been studied. Freestanding diamond films grown on both silica and silicon at two different methane concentrations were analyzed by scanning and transmission electron microscopies, electron diffraction, Raman spectroscopy, and secondary ion mass spectroscopy. It was found that the substrate chemistry greatly influenced the nature of the carbon initially deposited on the substrate surface. Diamond formed large flat contact areas on silicon, whereas on silica a particulate type of intermediate layer formed first because of the chemical reactions occurring on and/or with the surface. It was found that the phase content of the films was greatly affected by the methane concentration in hydrogen. At the low (1.0% or less) methane concentrations in hydrogen, phase pure diamond formed; while at the high (5.0%) methane concentration in hydrogen, graphite and disordered carbon were codeposited along with diamond during the early growth stages. Silicon carbide was detected at the diamond interfaces which appeared in discrete areas on silica as opposed to a rather continuous layer as is believed to form on silicon.
Characterization of electrochemical photovoltaic cells using polycrystalline CdSe and CdTe electrodes grown by a liquid metal-vapor reaction
Authors: Deng Zhitsing , Cinquino Michelina , Lawrence Marcus F.
Published: September 2007

Polycrystalline CdSe and CdTe layers were fabricated by putting liquid Cd in contact with Se or Te vapors under constant Ar flow. The crystalline structure, surface properties, and semiconducting properties of these materials have been determined. Both materials were found to be n-type semiconductors. The results show that, under the proper experimental conditions, the liquid metal-vapor reaction enables the synthesis of polycrystalline CdSe photoelectrodes with a 6.9% energy conversion efficiency when used in an electrochemical photovoltaic cell under 80 mW/cm2 of white light illumination. This efficiency rates amongst the highest ones measured under similar conditions using polycrystalline CdSe. These CdSe layers have a majority charge carrier density of ND = 2.6 × 1017 cm−3 and possess a highly textured surface which is assumed to be mainly responsible for the high photovoltaic efficiency. The highly textured CdTe samples obtained by this process, however, show a photovoltaic conversion efficiency of only 0.2%, and this is seen to be mainly due to their high majority charge carrier density of ND = 7.8 × 1019 cm−3.
Thin films of rf-magnetron sputtered InN on mica: Crystallography, electrical transport, and morphology
Authors: Kistenmacher T.J. , Bryden W.A. , Morgan J.S. , Dayan D. , Fainchtein R. , Poehler T.O.
Published: September 2007

Reactive rf-magnetron sputtering has been employed for the growth of thin films of InN on the (001) face of mica at a variety of substrate temperatures from 50 to 550 °C. These films have been characterized by x-ray scattering, stylus profilometry, and electrical transport measurements, and their topography has been studied by SEM and STM. At low deposition temperatures, the InN films exhibit texture [(00.1)InN‖ (001)mica], while at higher deposition temperatures a large fraction of the grains are heteroepitaxial [(00.1)InN‖(001)mica, (2.0)InN · (060)mica]. The utility of the x-ray precession method in the determination of this heteroepitaxial relationship is highlighted. The films exhibit a local mobility maximum near a substrate temperature of 350 °C, beyond which a sharp increase in resistivity associated with voids and cracks owing to the onset of secondary grain growth leads to a dramatic decrease in electrical mobility. At the highest growth temperatures, however, the interconnection between grains improves and lower resistivity and higher mobility are re-established.
Chemical reactions of lead magnesium niobate titanate in the presence of a glass
Authors: Srikanth V. , Subbarao E.C.
Published: September 2007

A relaxor ferroelectric of composition 0.93Pb(Mg1/3Nb2/3)O3-0.07PbTiO3 was sintered with 3 wt.% commercial sealing glass at 750 °C for 30 min to achieve ≥95% of theoretical density and a nearly pure perovskite phase. At higher glass additions (up to 20 wt.%), higher sintering temperatures (up to 800 °C), and longer sintering times (up to 4 h), the amount of perovskite (PMN type) decreases and that of pyrochlore (6PbO · MgO · 3Nb2O5 or 3PbO · 2Nb2O5) increases. On sintering at 800 °C for 4 h no perovskite phase is present in compositions with even 1% glass addition. The reaction of glass with the PMN phase was found to lead to the disappearance of the perovskite. Addition of 0.1 to 0.6 wt.% MgO to compositions containing 1 and 3 wt.% glass (and balance PMN-PT) results in essentially pure PMN perovskite phase on sintering at 700–800 °C for 30–240 min, confirming that the reaction of glass with PMN and depletion of MgO from PMN can be arrested. The sintered ceramics exhibit relaxor behavior and possess dielectric properties essentially commensurate with the phase composition.
Post deposition excimer laser processing of MoSx thin films
Authors: Jervis T.R. , Hirvonen J-P. , Nastasi M.
Published: September 2007

We have examined the effect of excimer laser surface processing and doping with Au on the mechanical, tribological, and bonding properties of MoSx thin films. We find that the effects of processing are manifested primarily in the surface of the films, but that there is also some film-substrate interaction during high fluence processing. The changes are sufficient to dramatically alter the wear life of the films. At low loads, laser processing alone reduces the run-in period and increases wear life. At higher loads, processing reduces wear life, although the run-in period is still short. These results are understood in terms of changes in the hardness of the surface of the films. Laser processing renders deposited films insensitive to high humidity storage. After laser processing, Raman spectroscopy shows changes in bonding to that characteristic of single crystal MoS2. Laser doping with Au results in wear life comparable to or greater than that of unprocessed films even at the highest loads used. Thus, laser doped MoSx films show lower initial friction, comparable wear life, and may exhibit greater environmental stability than as-deposited films.
TlBaCaCuO and YBaCuO superconductor thin films via an electrodeposition process
Authors: Bhattacharya R.N. , Parilla P.A. , Mason A. , Roybal L.L. , Ahrenkiel R.K. , Noufi R. , Hellmer R.P. , Kwak J.F. , Ginley D.S.
Published: September 2007

Thin-film superconductors of TlBaCaCuO (TBCCO) and YBaCuO (YBCO) were fabricated via an electrodeposition process. The precursors of the superconducting TBCCO films were codeposited at a constant potential of −4 V onto a silver-coated SrTiO3 substrate. The YBCO precursors also were codeposited but under pulsed-potential conditions (in order to improve the film morphology) and onto a silver-coated MgO substrate. The pulsed-potential cycle consisted of 1 s at −4 V followed by 1 s at −1 V. The post-annealed TBCCO film showed zero resistance at about 102 K and critical current density at 76 K of 20 000 A/cm2 in zero magnetic field and 5000 A/cm2 in a 10 kOe field parallel to the film plane. The post-annealed YBCO film showed zero resistance at approximately 80 K and critical current density of 5160 A/cm2 at 4 K in zero magnetic field.
CO2 decomposition kinetics of YBa2Cu3O7−x via in situ electrical conductivity measurements
Authors: Cooper E.A. , Gangopadhyay A.K. , Mason T.O. , Balachandran U.
Published: September 2007

In situ electrical conductivity measurements were employed to study the kinetics of decomposition of YBa2Cu3O7−x in flowing 5% CO2/95% O2 atmosphere at 815 °C. Three regimes could be observed in the decay of the conductivity with time, interpreted as corresponding to nucleation over the initial 5–10 min, rapid formation of Y2Cu2O5, BaCO3, and CuO between 10 min and 1 h, and finally transition to the formation of Y2BaCuO5, BaCO3, and CuO at longer times. The absence of dispersion in impedance measurements as a function of frequency together with the gradual decay of conductivity (roughly corresponding to the remaining volume fraction of YBa2Cu3O7−x) indicates that not all grain boundaries are involved in the decomposition process.
Microstructures of the melt-powder-melt-growth processed YBaCuO
Authors: Yamaguchi K. , Murakami M. , Fujimoto H. , Gotoh S. , Oyama T. , Shiohara Y. , Koshizuka N. , Tanaka S.
Published: September 2007

Microstructures of two MPMG processed YBaCuO materials with and without Y2BaCuO5 (211) inclusions were investigated by transmission electron microscopy. Using the MPMG process, it is possible to change the quantity of the 211 inclusions in the YBa2Cu3O7 (123) matrix. We prepared two YBaCuO samples with 0 and 30 vol. % 211 and with respective critical current density values of 2000 and 30 000 A/cm2 at 77 K and 1 T (magnetic field parallel to the c-axis). As possible pinning centers, we found stacking faults in the 123 matrix. However, we observed no appreciable change in their number and structure by introducing the 211 inclusions. Therefore, the difference in Jc values can be attributed to the 211 inclusion itself.
A comparative study of the electrical properties and microstructure of polycrystalline YBaCuO and thallium-based thin films
Authors: Grovenor C.R.M. , Romano L.T. , Mingard K.P. , Lai H-C. , Vernon-Parry K.D.
Published: September 2007

Thin films of YBCO and thallium-based superconductor compounds have been deposited with very similar polycrystalline structures. The critical current densities carried by the YBCO films are much lower than measured in the thallium films (>103, as opposed to >104 A/cm2). Grain boundaries in these films have been studied to correlate microstructure with the measured electrical properties. High defect densities and frequent microcracking have been observed at and around the boundaries in the YBCO films, but these defects are not seen in the thallium-based films. We suppose that this difference is because of the higher differential thermal expansion stresses set up in YBCO during cooling. Our observations imply that eventual application of polycrystalline superconducting films prepared by an ex situ process is more likely for the thallium-based materials than for YBCO.
Erosion of diamond films and graphite in oxygen plasma
Authors: Joshi A. , Nimmagadda R.
Published: September 2007

The nature and rates of erosion of diamond, graphite, and diamond-like carbon (DLC) films exposed to oxygen plasmas were evaluated by comparison of surface morphological changes and weight losses. The RF plasma oxidation at ambient temperature caused severe etching of graphite and DLC specimens, while causing only minor damage to diamond film surfaces. The results suggest that erosion by low energy ions is very selective to the sp2 and sp states compared to the sp3 state of carbon. The selectivity of etching by oxygen plasma is significant, as compared to what has been reported for hydrogen in atomic and ionic states, or for oxidation at elevated temperatures in molecular oxygen. These observations have significant implications to the synthesis of diamond films by chemical vapor deposition (CVD) as well as to the application of diamond film coatings on graphite or other substrates for protection against energetic atomic oxygen prevailing in the low earth orbits (LEO).
Growth of diamond particles in chemical vapor deposition
Authors: Iijima Sumio , Aikawa Yumi , Baba Kazuhiro
Published: September 2007

An early stage of diamond growth in hot-filament chemical vapor deposition on silicon substrates was examined by the high resolution electron microscope. “Pretreatment” of the substrate surfaces by diamond powder abrading was found to plant diamond seed crystals with a density of as high as 1011/cm2. These crystals provide sites for subsequent growth of diamond films. The CVD grown diamond particles tend to be cuboctahedra. Smaller particles in nanometer size are faultless, but larger ones of several tens of nanometers develop crystal faults. Some of them may originate from the seed crystals. Degradation of the diamond seed crystals due to the electron beam irradiation is discussed in terms of fabrication of diamond film.
RBS and XPS analyses of phosphor packages for laser-heat thermoluminescence dosimetry
Authors: Stern S.H. , Price J.L. , Simons D.G. , Land D.J. , Mathur V.K. , Anderson C.R. , Beard B.C.
Published: September 2007

Phosphor packages for a laser-heat thermoluminescence radiation-dosimetry system have been analyzed with Rutherford Backscattering Spectrometry and X-ray Photoelectron Spectroscopy. Samples consist of 20–50 μm diameter powder grains of CaSO4: Tm and LiF: (Mg, Ti) phosphor embedded in a transparent silicone matrix about 60 μm thick. Our principal finding with regard to layer morphology indicates an inhomogeneous outer layer of areal density at least ∼300 μg/cm2 depleted of phosphor and contaminated with boron.
Highly mobile oxygen hole-type charge carriers in fused silica
Authors: Freund Friedemann , Masuda Michael M. , Freund Minoru M.
Published: September 2007

Some peculiar positive charge carriers are thermally generated in fused silica above 500 °C. These charge carriers appear to be positive holes, chemically O− states, probably arising from dissociation of peroxy defects. The charge carriers give rise to a pronounced positive surface charge which disappears upon cooling but can be quenched by rapid quenching from ≍800 °C. Reheating to ≍200 °C remobilizes these charge carriers and causes them to anneal below 400 °C. The generation of positive holes charge carriers may be important to understand failure mechanisms of SiO2 insulators.
Effects of alloying and treatment on void swelling of 316 stainless steels
Authors: Liu B.X. , Lai S.L. , Sun J.G.
Published: September 2007

Several samples of type 316 stainless steels, which were treated differently and modified with minor elements (such as Ti, Nb, etc.), were irradiated by 1 MeV electrons in HVEM at temperatures ranging from 823 K to 883 K. Void swelling behavior of the steels was investigated, and three parameters, i.e., swelling, void density, and void size, were measured. The results show that prior cold work improves the swelling resistance of type 316 stainless steels more effectively than solid-solution treatment. It is also shown that Ti is the best alloying element studied that can suppress void nucleation and its growth drastically by acting as sinks and impeding dislocation climb, resulting in the reduction of void swelling.
Structural and electrical investigation of amorphous-to-crystalline transformation in iron disilicide alloy thin films
Authors: Michelini M. , Nava F. , Galli E.
Published: September 2007

Electrical and structural properties of coevaporated Fe–Si thin alloy films as a function of annealing temperature have been studied by in situ electrical resistivity measurements and structural analysis including 2 MeV 4He+ ion backscattering, x-ray diffraction, scanning microscopy, and roughness measurements. In the as-deposited state the coevaporated alloy film was amorphous. Upon annealing a sharp increase in resistivity occurred near 400 °C and the increase was determined to be amorphous-to-crystalline β–FeSi2 phase transformation. In cooling, the resistivity increased monotonically with decreasing temperature. Surface roughness is well pronounced for films heat treated at temperatures higher than 700 °C and is due to the difference in thermal expansion coefficients of substrate and β–FeSi2. The crystalline β–FeSi2 was determined to be a semiconductor with an energy gap of 0.80 eV. It is p-type, having a hole concentration and a Hall mobility of 1.4 × 1018 cm−3 and 4 cm2/V s, respectively, at room temperature. The kinetics of the transformation was determined by isothermal heat treatment over the temperature range of 362 °C to 390 °C, and an activation energy of 2.3 eV was measured. The nucleation and growth kinetics in the crystallization process show a change in the power of time dependence from 2.1 to 3.4.
The surface topography of non-shear treated pitch and PAN carbon fibers as viewed by the STM
Authors: Hoffman W.P. , Hurley W.C. , Liu P.M. , Owens T.W.
Published: September 2007

The atomic structure and roughness on the surface of a carbon fiber have a great effect on the degree of bonding of that fiber in a carbon fiber composite. Although there have been many studies on the bulk structure of these fibers, this is the first study dealing with the atomic surface structure of several carbon fibers. With the advent of the scanning tunneling microscope (STM), it is now possible to study both the roughness and structure of these fibers on the atomic scale. Type II PAN based fibers were found to have a rougher surface than type II pitch-based fibers. Similar to what has been observed in the interior of pitch fibers, the percentage of graphitic structure on the surface increased with the degree of heat treatment and with the modulus of the fiber.
Electron microscopy characterization of monoclinic SiAs precipitates in heavily As+-implanted silicon
Authors: Armigliato A. , Parisini A.
Published: September 2007

Silicon wafers have been implanted with As+ ions at an energy of 100 keV and a dose of 1 × 1017 cm−2 and subsequently annealed at 1050°for 15 min. This results in a peak As concentration of 7 × 1021 cm−3, which is far beyond the solid solubility value of arsenic in silicon at this annealing temperature. Rod-like precipitates, dislocations, and small precipitate-like defects have been observed by transmission electron microscopy. From the analysis of several diffraction patterns taken on a number of rod-like particles at different tilt angles, it has been unambiguously found that they have the structure of the monoclinic SiAs compound previously reported in literature. The stoichiometry of the precipitates has been confirmed by x-ray microanalysis. To our knowledge, this is the first time that this SiAs phase is detected in As+-implanted silicon.
Photopyroelectric (PPE) determination of thermal diffusivity of Bi2Te2.85Se0.15 sintered thermoelectric semiconductors
Authors: Wada Hideo , Watanabe Masahito , Morimoto Jun , Miyakawa Toru
Published: September 2007

Thermal diffusivity of the sintered semiconductors was measured by the photopyroelectric (PPE) method. The measurement based on the phase-modulation frequency characteristics was shown to give superior results, eliminating errors expected in the conventional signal amplitude-distance characteristics measurements. Thermal diffusivities of the melt-grown and hot-pressed samples were found to be αmelt(c⊥) = 0.014 cm2/s, αmelt(c//) = 0.011 cm2/s, αhot(c⊥) = 0.012 cm2/s, and αhot(c//) = 0.008 cm2/s, depending on the relation between the c-axis direction of grain and thermal flow direction. The thermal diffusivity of the hot-pressed samples shows a strong dependence on the hot-press pressure through the orientation factor.
Shielding of cracks in a plastically polarizable material
Authors: Zhou S.J. , Thomson Robb
Published: September 2007

In this paper, we address some fundamental questions regarding the response of a crack to externally generated dislocations. We note that since dislocations that formed at external sources in the material must be in the form of loops or dipoles, the theory must be couched in terms of crack shielding in a plastically polarizable medium. There are strong analogies to dielectric theory. We prove two general theorems: (1) Dipoles formed in the emission geometry relative to a crack tip always antishield the crack and (2) when dipoles are induced during uniform motion of a crack through a uniformly plastically polarizable material, then the net shielding is always positive. We illustrate these general theorems with a number of special cases for fixed and polarizable sources. Finally, we simulate the self consistent time dependent response of a crack to a polarizable source as the crack moves past it. The results show that the crack is initially antishielded, but that positive shielding always dominates during later stages of configuration evolution. The crack may be arrested by the source, or it may break away from it, depending upon the various parameters (source strength and geometry, dislocation mobility, Griffith condition for the crack, etc.). The results indicate that the time dependence of crack shielding in the presence of a nonuniform density of sources will be very important in practical cases of brittle transitions in materials.
A review of recent developments in Fe3Al-based alloys
Authors: McKamey C.G. , DeVan J.H. , Tortorelli P.F. , Sikka V.K.
Published: September 2007

Fe3Al-based iron aluminides have been of interest for many years because of their excellent oxidation and sulfidation resistance. However, limited room temperature ductility (<5%) and a sharp drop in strength above 600 °C have limited their consideration for use as structural materials. Recent improvements in tensile properties, especially improvements in ductility produced through control of composition and microstructure, and advances in the understanding of environmental embrittlement in intermetallics, including iron aluminides, have resulted in renewed interest in this system for structural applications. The purpose of this paper is to summarize recent developments concerning Fe3Al-based aluminides, including alloy development efforts and environmental embrittlement studies. This report will concentrate on literature published since about 1980, and will review studies of fabrication, mechanical properties, and corrosion resistance that have been conducted since that time.
Infrared zone melting process for YBa2Cu3O7−δ wires
Authors: Ozkan N. , Glowacki B.A. , Robinson E.A. , Freeman P.A.
Published: September 2007

A novel process for the continuous densification of YBa2Cu3O7−δ wires by infrared zone melting is presented. Superconducting ceramic wires with moderate porosity (20–40%) were passed through a high temperature infrared zone at a speed of 1–2 cm/min, which resulted in zone melting followed by fast solidification to give near zero porosity wires. The atmospheric conditions during zone melting had a significant effect upon the resultant properties and microstructure of the wires. The best results were obtained for infrared zone melting under a vacuum that resulted in the decomposition of the superconducting phase into four phases without significant macroscopic chemical segregation across the wire. Subsequent conventional sintering and annealing treatment of such wires fully restored them to polycrystalline YBa2Cu3O7−δ wires having 96% of the theoretical density. The maximum transport critical current density obtained for such wires at 77 K in zero magnetic field was 2.65 × 103 A cm−2. Normalized critical current values as a function of magnetic field indicated that the quality of the intergrain connections for vacuum infrared zone melted wires was significantly better than that for conventionally processed wires and tapes.
Application of fractal geometry measurements to the evaluation of fracture toughness of brittle intermetallics
Authors: Fahmy Y. , Russ J.C. , Koch C.C.
Published: September 2007

Determination of the boundary fractal dimension of planar sections through particles produced by fracturing a series of brittle intermetallic V3Au materials with increasing oxygen content provides a single parameter that describes the surface roughness. This is observed to correlate with the measured fracture toughness, K1c, as determined with an indentation cracking test. The correlation agrees quantitatively with results obtained on several brittle ceramics by other workers.
Melt-spin processing of YBa2Cu3O7−x
Authors: Folkerts T.J. , Kramer M.J. , Dennis K.W. , McCallum R.W.
Published: September 2007

We introduce a novel containerless melt-spin processing technique for YBa2Cu3O7−x using prereacted oxide powders that are rapidly melted and subsequentially quenched in a controlled atmosphere. This results in flakes with typical dimensions of 3000 × 150 × 15 μm3. Powder XRD indicates that all samples contain Y2O3, but that the Ba–Cu–O phases present depend on the processing parameters. SEM and TEM studies show these phases are finely dispersed: the typical grain size for Y2O3 is 1–2 μm, and for the Ba–Cu–O phases it is <0.5 μm.
Residual stress measurements of thin aluminum metallizations by continuous indentation and x-ray stress measurement techniques
Authors: LaFontaine W.R. , Paszkiet C.A. , Korhonen M.A. , Li Che-Yu
Published: September 2007

Stress relaxation in aluminum films of several thicknesses was characterized by using both continuous indentation and x-ray diffraction techniques. Results of the indentation and x-ray stress measurements compare closely for films of small thicknesses. Indentation data from thicker films do not compare well to the x-ray data due to the presence of a residual stress distribution.
Examination of the chemistry involved in microwave plasma assisted chemical vapor deposition of diamond
Authors: Weimer W.A. , Cerio F.M. , Johnson C.E.
Published: September 2007

Chemical reaction products formed in a microwave plasma assisted chemical vapor deposition apparatus for diamond film deposition are detected using mass spectrometry. Carbon source gases CH4, C2H6, C2H4, or C2H2 produce CH4, C2H2, CO, and H2O as major stable reaction products when introduced into a H2/O2 plasma under diamond deposition conditions. The effect of oxygen addition is similar for all carbon source gases with respect to reaction product formation, indicating that a common reaction mechanism is active in all cases. On a qualitative basis, these observations are consistent with a mechanism describing the oxidation of CH4 in flames. No beneficial effects were observed using alternating growth/etch cycles to deposit films. Films grown using CH4 as the carbon source gas consistently produce higher quality diamond films compared to films grown from C2H2.
Sintering characteristics of nanocrystalline TiO2—A study combining small angle neutron scattering and nitrogen absorption–BET
Authors: Wagner W. , Averback R.S. , Hahn H. , Petry W. , Wiedenmann A.
Published: September 2007

Small angle neutron scattering (SANS) was employed to characterize the pore structure of nanophase TiO2 ceramic materials compacted at different temperatures. Nanophase samples, produced by inert gas condensation, were compacted at 25, 290, 413, and 550 °C using a pressure of ≍1 GPa. The pore size distribution of the sample compacted at room temperature was very broad, with sizes ranging from ≍3–30 nm and pores comprising 38% of the sample volume. Compaction at 290 and 413 °C reduced the pore volume to 25% and 20%, respectively, by eliminating pores at both the small and large ends of the distribution. Compaction at 550 °C resulted in a pore volume that was less than 8%. Complications in the SANS analysis arising from the scattering from grain boundaries are discussed. The results from SANS are compared with those derived from nitrogen absorption, BET, measurements.
Processing of Bi2.1Sr1.8Ca1.1Cu2O8 source material for float-zone fiber growth
Authors: Peszkin P.N. , Raymakers R.J. , Feigelson R.S. , Moulton L.V. , Lu Z.
Published: September 2007

Bi2.1Sr1.8Cu2O8 fibers having excellent superconducting properties can be grown by a laser-heated float zone process. In order to maintain stable growth conditions and thereby obtain fibers free of diameter fluctuations and voids, dense ceramic starting material containing only the 2212 phase is required. In this study various processing parameters, including calcining and sintering temperatures and times, grain size of the powders used, and pressing pressures were optimized to yield dense, chemically homogeneous starting material. It was found that under most conditions there was no increase in the density on sintering. Retrograde densification was the usual situation except at higher pressures and was found to depend on pressing pressure, calcination history, and sintering temperature. Cold-pressing at higher pressures (100000 psi) yielded denser but chemically inhomogeneous material. Ceramic samples sintered for long times (>48 h) yielded source rods that produced instabilities during fiber growth, presumably due to preferential loss of mass during sintering.
High-resolution electron microscopy of diamond hexagonal silicon in low pressure chemical vapor deposited polycrystalline silicon
Authors: Cerva Hans
Published: September 2007

Thin poly-Si layers deposited at 625 °C by LPCVD that are used in silicon technology for microelectronics exhibit a pronounced additional x-ray diffraction peak at about 0.334 nm. High-resolution electron microscopy (HREM) reveals that this peak stems from {010} reflections of a diamond hexagonal (dh) Si phase, which occurs as small inclusions with the orientation relationship (01) ‖ (0001), [011] ‖ [20] to the diamond cubic (dc) Si matrix. Due to the high density of planar faults on {111}, the dh-Si phase also exists in the form of the 2H silicon polytype with the orientation relationship (1) ‖ (0001), [011] ‖ [20]. In the first case the formation of the dh-Si phase may be understood by a multiple twinning transformation process, and in the second case by glide of Shockley partial dislocations on {111} planes. Various other hexagonal polytypes occur, which have all the {010} reflections in common and make a major contribution to the 0.334 nm peak. The medium temperature of 625 °C for layer deposition leads to a 〈011〉 preferential orientation and a high density of twins as well as to high compressive stress in the poly-Si layer itself. This seems to promote the formation of dh-Si. The strong twinning behavior produces a typical tilt grain boundary between adjacent dh-Si grains: [20], (016), Θ = 35°with a translation vector t = 1/2[031] parallel to it. The dh-Si phase vanishes in this poly-Si film after annealing at temperatures above 1000 °C due to grain growth by recrystallization.
Constitutive modeling of the effects of oxygen on the deformation behavior of silicon
Authors: Maroudas Dimitris , Brown Robert A.
Published: September 2007

A systematic theory is presented that models the effects of interstitial oxygen on the deformation behavior of silicon. The theory is based on calculation of the dependence of the dislocation velocity on the applied stress in the crystal and determination of the locking and unlocking stresses for dislocation motion. Internal stresses in the oxygen-hardened crystals are modeled by the superposition of the unlocking stress, a back stress due to the interaction between mobile dislocations, and an internal stress that arises from the interaction between a dislocation and the oxygen cloud around other dislocations. The initiation of dislocation multiplication is modeled as a two-step thermally activated process; the first step is the unlocking of the dislocation and the second step is the formation of jogs along the dislocation line. The coupled model for oxygen transport and dislocation motion is used to simulate crystal deformation in dynamic experiments and to reproduce stress-strain curves. The predictions of the initial stage of deformation are in good agreement with the experimental data of Yonenaga et al. [J. Applied Phys. 56, 2346 (1984)].
Application of the bond valence method to Si/NiSi2 interfaces
Authors: O'Keeffe M.
Published: September 2007

It is shown how the bond valence method can be used to estimate expected interatomic distances in coherent interfaces. The method is illustrated by application to the Si/NiSi2 (111) interface with results generally in accord with experimental data.
Microstructure of TiO2 rutile thin films deposited on (110) α−Al2O3
Authors: Gao Y. , Merkle K.L. , Chang H.L.M. , Zhang T.J. , Lam D.J.
Published: September 2007

TiO2 rutile thin films grown on (110) sapphire (α−Al2O3) by the MOCVD technique have been characterized by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). The TiO2 rutile thin films grew on the sapphire with two epitaxial orientations. The epitaxial orientation relationships between the rutile films (R) and the sapphire substrate (S) were found to be (1) (101)[010]R ‖ (110)[0001]s. Detailed atomic structures of near-interface regions have been investigated by HREM, providing a clear picture of the initial stage of film growth. HREM images show that about 70% of the nuclei at the interface are the (101) rutile, but most of them are very small, about 5 nm (or 2% of the film thickness) in the growth direction. The film growth was dominated by the (200) orientation. Nucleation and growth of the films will be discussed in terms of the lattice mismatch at the interface and growth rates along the two orientations. Planar defects such as twin boundaries and special grain boundaries are commonly observed in the films, especially in regions close to the substrate. The twin plane and twinning direction are {101} and 〈101〉, respectively. Special grain boundaries are found to be correlated with nucleation and twinning.
The hardness and Young's modulus of bulk YBa2Cu3O7−x (1:2:3) and YBa2Cu4O8 (1:2:4) as determined by ultra low load indentation
Authors: Lucas B.N. , Oliver W.C. , Williams R.K. , Brynestad J. , O'Hern M.E.
Published: September 2007

Using a highly-spatially-resolved mechanical properties microprobe, the Young's modulus and hardness of bulk YBa2Cu3O7−x (1:2:3) and YBa2Cu4O8 (1:2:4) have been determined. The Young's modulus of a superconductor is an important parameter in determining critical grain sizes above which microcracking will occur due to anisotropic thermal stresses that arise during processing. This phenomenon of microcracking has been determined to cause a decrease in the attainable critical current densities in bulk superconductors. The mechanical properties data for these two materials show that the Young's modulus of 1:2:3 is approximately 35% greater than the modulus of 1:2:4. This along with available anisotropic thermal expansion data for 1:2:3 and 1:2:4 suggests that the critical grain size for 1:2:4 is about 7 times greater than the critical grain size for microcracking in 1:2:3.
Diffusional transformation and structural relaxation in Y1Ba2Cu3−xNixO7−z compounds
Authors: Raman S.V. , Sun Y.Y. , Matsuishi K. , Quan X.
Published: September 2007

In this paper the mechanism of phase transition and structural relaxation are elucidated by means of model experiments in the differential scanning calorimeter. Materials for this study were varied in structure and oxygen content by the introduction of Ni for Cu, through co-precipitation of oxolate gels, calcination, and sintering in ambient atmosphere. Both relaxation and phase transition are revealed by scrutinizing the heating rate dependence of heat capacity, and they seem to occur contemporaneously. Two higher order transitions are observed and are characterized by activation energies of 186 kJ/mole and 550 kJ/mole at the low and high temperatures, respectively. Perhaps the first transition is of a displacive type and occurs in anticipation of a reconstructive structural change from a fourfold chain to a sixfold sheet-like configuration. It is delineated by a change in relaxation kinetics from a localized type to a cooperative type with activation energies of 36 and 10 kJ/mole, respectively. The cooperative relaxation occurs at higher temperatures and is kinetically continuous across the reconstructive transition region. The structure characteristic exponent has a value of 0.70 for both localized and cooperative relaxation and resembles the two-dimensional relaxation often observed in depolymerized glasses. With kinetic impediment the transition region seems to approach a lambda-like behavior. In the subtransitional region, impediments in relaxation are useful in delineating oxygen stoichiometric differences between samples. The above discussion is supported by the structural and chemical characteristics that were gathered by x-ray powder diffraction, Raman spectroscopy, scanning electron microscopy and microprobe, and low temperature resistance measurements on Y123 samples containing 5 and 10 at. % Ni, respectively.
Generalized plane strain analysis of a bimaterial composite containing a free surface normal to the interface
Authors: Tewary V.K. , Kriz R.D.
Published: September 2007

The elastic plane strain Green's function calculated in earlier papers is modified to account for generalized plane strain and applied to calculating the stress and the displacement field in a bimaterial composite containing a free surface normal to the interface and subjected to an out-of-plane load. The result is obtained in terms of a closed integral representation which is evaluated numerically as well as analytically. The method is applied to a cubic solid containing a Σ-5 grain boundary and to fiber-reinforced laminated composites. The singularities in the stress are identified and discussed.
Growth of stable Al–Pd–Mn icosahedral phase
Authors: Dong C. , Dubois J.M. , de Boissieu M. , Boudard M. , Janot C.
Published: September 2007

We have carried out a detailed investigation of the Al71Pd19Mn10 alloy by using a combination of experimental techniques (neutron diffraction, electron diffraction, high resolution electron microscopy, and electron probe microanalysis). This alloy contains only the perfect icosahedral phase. This icosahedral phase is stable from room temperature up to its melting, though a transient cubic metastable phase coexists with it during the heating. It grows directly from liquid without involving any crystalline phase. Its isothermal growths at different temperatures can well be described by the Avrami equation with exponent n = 1.85. Comparisons with the Al–Cu–Fe I phase have also been made. The growth velocity of both I phases is approximately 1 μm/min with ΔT around 10 K, much slower than that of the crystalline phases growing under the same condition. After the sample was maintained at near liquidus temperature for more than 10 h, element losses and Si contamination from the crucible changed the sample composition. In consequence, crystalline phases have formed during the subsequent cooling and made the growth of the icosahedral phase a eutectic reaction.
Silicon oxycarbide glasses: Part II. Structure and properties
Authors: Renlund Gary M. , Prochazka Svante , Doremus Robert H.
Published: September 2007

Silicon oxycarbide glass is formed by the pyrolysis of silicone resins and contains only silicon, oxygen, and carbon. The glass remains amorphous in x-ray diffraction to 1400 °C and shows no features in transmission electron micrographs (TEM) after heating to this temperature. After heating at higher temperature (1500–1650 °C) silicon carbide lines develop in x-ray diffraction, and fine crystalline regions of silicon carbide and graphite are found in TEM and electron diffraction. XPS shows that silicon-oxygen bonds in the glass are similar to those in amorphous and crystalline silicates; some silicons are bonded to both oxygen and carbon. Carbon is bonded to either silicon or carbon; there are no carbon-oxygen bonds in the glass. Infrared spectra are consistent with these conclusions and show silicon-oxygen and silicon-carbon vibrations, but none from carbon-oxygen bonds. 29Si-NMR shows evidence for four different bonding groups around silicon. The silicon oxycarbide structure deduced from these results is a random network of silicon-oxygen tetrahedra, with some silicons bonded to one or two carbons substituted for oxygen; these carbons are in turn tetrahedrally bonded to other silicon atoms. There are very small regions of carbon-carbon bonds only, which are not bonded in the network. This “free” carbon colors the glass black. When the glass is heated above 1400 °C this network composite rearranges in tiny regions to graphite and silicon carbide crystals. The density, coefficient of thermal expansion, hardness, elastic modulus, index of refraction, and viscosity of the silicon oxycarbide glasses are all somewhat higher than these properties in vitreous silica, probably because the silicon-carbide bonds in the network of the oxycarbide lead to a tighter, more closely packed structure. The oxycarbide glass is highly stable to temperatures up to 1600 °C and higher, because oxygen and water diffuse slowly in it.
Deformation and microstructural changes in SiC whisker-reinforced Si3N4 composites
Authors: Koester D.A. , More K.L. , Davis R.F.
Published: September 2007

Constant stress compressive creep studies have been conducted on hot-pressed Si3N4 containing 30 vol. % SiC whiskers and an initial vitreous phase composed of Al2O3, Y2O3, and SiO2. The conditions of temperature and stress were 1470–1670 K and 50–350 MPa, respectively; the atmosphere was purified N2 at 1 atm. Significant changes in the stress exponent and activation energy indicate a change in the controlling creep mechanism at ≍225 MPa and 1570 K. Prolonged annealing in the unstressed condition reduced creep rates but had little effect on the stress exponent values. Transmission and scanning electron microscopy revealed that the break in the stress exponent curves was caused by the removal of the amorphous material at the grain boundary and resulting contacts between Si3N4 grains. The break in the activation energy curves is believed to be similarly related. Analysis of all the data indicates that the composite creeps via grain boundary sliding accommodated by viscous flow at low stresses and temperatures and by diffusion at high stresses and temperatures. The contributions of these two mechanisms varied measurably as a function of stress and temperature. No cavitation was observed. The presence of the SiC whiskers had no observable effect on deformation.
Crystal chemistry of the texture formation in superconductive YBa2Cu3O7−δ ceramics
Authors: Monot I. , Wang J. , Desgardin G. , Raveau B.
Published: September 2007

Classical sintering of YBa2Cu3O7−δ always leads to poor critical transport currents whatever the powder process and the thermal cycle are. Best results are obtained by synthesis of textured ceramics. Fabrication of YBa2Cu3O7−δ with well-oriented grain structure by the melt textured growth process was performed in order to understand the mechanism that controls the texture formation. The evolution of the microstructure was studied by quenching samples at different stages of the MTG cycle. The importance in the texture formation process of the presintering temperature, of the high temperature plateau, of the thermal gradient and its fluctuations, of the presence of second phase inclusions (essentially 211 phase), and of the substrate and its reactivity with Y–Ba–Cu–O is shown.
Reactions between liquid silicon and vitreous silica
Authors: Liu Zhensheng , Carlberg Torbjörn
Published: September 2007

Oxygen incorporation in silicon crystals during Czochralski growth is dependent on many factors, of which the dissolution of the silica crucible is of great importance. In this paper the reactions between vitreous silica and molten silicon have been analyzed, both in sealed ampoules and in Czochralski crucibles. It was found that the vitreous silica crystallizes to cristobalite by lateral growth. For this reaction to occur it is necessary that liquid silicon is present. The vitreous silica dissolves and the cristobalite grows with a thin layer of liquid silicon between them. Different oxygen concentrations in the melt in equilibrium with the amorphous and crystallized silica are necessary for the reaction to proceed. The oxygen flux in the melt is dependent upon the dissolution of both vitreous silica and cristobalite as well as the reaction between these phases.
TEM observations of the mechanism of delamination of chromium films from silicon substrates
Authors: Goyal D. , King A.H.
Published: September 2007

We have observed the complete delamination of polycrystalline chromium films from single crystal silicon substrates during deposition due to the formation of high internal stresses. These intrinsic stresses can give rise to interfacial defects which assist in the separation of the film from the substrate. Stresses in the film are balanced by stresses in the substrate, which cause mechanical failure in the substrate near the interface. Extensive arrays of dislocations and cracking of the substrate have been observed. We find that the delamination of the films from the substrate is initiated by the formation of damage in the substrate, rather than to the film or the interface.
On the role of diffusion in phase selection during reactions at interfaces
Authors: Thompson C.V.
Published: September 2007

It is argued that interdiffusion must precede nucleation of new phases during reactions at interfaces between compositionally different phases. The relative rates at which elemental components diffuse in the reacting phases control the sequence in which phases can form, and can also strongly affect the relative nucleation rates of alloy products, especially in the transient nucleation regime. While detailed predictions of the relative nucleation rates require usually unavailable knowledge of the energies of the relevant interfaces, in some cases, knowledge of the relevant diffusivities, along with a thermodynamic analysis, can lead to predictions of likely phase formation sequences. These concepts are used to explain the association of diffusional asymmetry with systems that undergo solid state amorphization, and to specify semiquantitatively the degree of asymmetry required for solid state amorphization.
The influence of water vapor on thermal transformation of boehmite
Authors: Hrabe Zdenek , Komarneni Sridhar , Pach Ladislav , Roy Rustum
Published: September 2007

Boehmite compacts and boehmite gels (seeded and unseeded) were annealed at various temperatures in nitrogen and 1 atm water vapor to determine the influence of water vapor on boehmite transformation to new phases, changes in porosity, and morphology. Water vapor was found to accelerate the phase transformations markedly compared to dry N2 treatment. The catalyzing effect of water vapor may be due to its interactions on grain surfaces and generation of fast diffusion paths, resulting in nucleation and growth of new phases. This result shows that control of the furnace atmosphere is a useful variable in thermally activated processes.