bluuebunny/crossref_metadata_2025 huggingface dataset:
Tribology of diamond-like carbon sliding against itself, silicon nitride, and steel
Authors: Jia K. , Li Y.Q. , Fischer T.E. , Gallois B.
Published: September 2007

Diamond-like carbon (DLC) films were deposited on (100) silicon wafers and silicon nitride balls by RF plasma-assisted chemical vapor deposition at a pressure of 700 mTorr and a substrate temperature of 360 K. The friction coefficient and the wear rates were measured using a pin-on-disk tribometer in 40% humid and dry air. Friction coefficients are near 0.05 in all cases measured. In dry air, the wear of silicon nitride and steel against DLC is below measurement capability because of a protecting DLC transfer layer, and wear of DLC is 2.5 ± 10−8 mm3/Nm against silicon nitride and 6.5 ± 10−9 mm3/Nm against steel. In humid air, the DLC transfer layer does not adhere to the solids, and wear of both bodies is larger. Unmeasurable wear is obtained when DLC slides against itself in humid air; the wear rate is 5 ± 10−9 mm3/Nm in dry air. These results are interpreted in terms of the properties of a friction-induced transformation of the surface layer of DLC.
Effect of grain size and mechanical processing on the dielectric properties of BaTiO3
Authors: Lee B.W. , Auh K.H.
Published: September 2007

Dielectric properties of polycrystalline BaTiO3 ceramics having grain sizes of 1 to 40 μm have been studied. Fine-grained ceramic BaTiO3 of 1 μm average grain size has 90°domains and has shown higher dielectric constant, lower ferroelectric transition temperature (Tc), and lower transition energy than coarser-grained material. 90°domain switching was preferentially produced in the fine-grained BaTiO3 as a result of abrasion. For the fine-grained BaTiO3, the dielectric constant decreased with one-dimensional pressure, whereas, for the coarse-grained material, the dielectric constant increased before decreasing with the pressure. The one-dimensional pressure resulted in increased Tc of both the fine- and coarse-grained BaTiO3, with the effect being the greatest for the coarse-grained material. The relationship between these results and internal stress, and the effect of external pressure imposed on internally stressed lattice, were discussed.
Structural, morphological, and magnetic study of nanocrystalline cobalt-copper powders synthesized by the polyol process
Authors: Chow G.M. , Kurihara L.K. , Kemner K.M. , Schoen P.E. , Elam W.T. , Ervin A. , Keller S. , Zhang Y.D. , Budnick J. , Ambrose T.
Published: September 2007

Nanocrystalline CoxCu100−x (4 ⋚ x ⋚ 49 at. %) powders were prepared by the reduction of metal acetates in a polyol. The structure of powders was characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), extended x-ray absorption fine structure (EXAFS) spectroscopy, solid-state nuclear magnetic resonance (NMR) spectroscopy, and vibrating sample magnetometry (VSM). As-synthesized powders were composites consisting of nanoscale crystallites of face-centered cubic (fcc) Cu and metastable face-centered cubic (fcc) Co. Complementary results of XRD, HRTEM, EXAFS, NMR, and VSM confirmed that there was no metastable alloying between Co and Cu. The NMR data also revealed that there was some hexagonal-closed-packed (hcp) Co in the samples. The powders were agglomerated, and consisted of aggregates of nanoscale crystallites of Co and Cu. Upon annealing, the powders with low Co contents showed an increase in both saturation magnetization and coercivity with increasing temperature. The results suggested that during preparation the nucleation of Cu occurred first, and the Cu crystallites served as nuclei for the formation of Co.
Estimation of phase separation rates of PbO–B2O3 melts
Authors: Inoue S. , Wada K. , Nukui A. , Yamane M. , Shibata S. , Yasumori A. , Yano T. , Makishima A. , Inoue H. , Uo M. , Fujimori Y.
Published: September 2007

The rate of liquid-liquid phase separation was experimentally studied in the PbO-B2O3 system. The in situ measurements were made by observing the melts with a videocamera continuously as the melts were cooled down from homogenization temperatures at a rate of 2.5 °C/min. The time interval between the beginning and the completion of the darkening of the visual field was determined as a measure of the separation rate. The phase-separation rate was estimated to be at least 900 times larger than that of the metastable phase separation below the liquidus.
Control of Y2BaCuO5 size and morphology in melt-processed YBa2Cu3O7−δ superconductor
Authors: Sakai N. , Yoo S.I. , Murakami M.
Published: September 2007

Important factors governing the size and the morphology of Y2BaCuO5 (Y211) at 1100 °C in air were investigated for three different starting materials having the same nominal composition of Y:Ba:Cu = 1.8:2.4 : 3.4, Y2BaCuO5-BaCuO2-CuO-Pt. Y2O3-BaCuO2-CuO-Pt, and melt-quenched materials from 1400 °C in a Pt crucible. With various amounts of Pt doping, the heating rate and the holding time were employed as the processing parameters. While, with the aid of Pt doping as the effective growth inhibitor, fine round Y211 grains could be obtained by simply employing a refined round Y211 precursor and a rapid healing, there were several important factors for obtaining fine acicular (or needle-like) Y211 grains as follows: (i) The Pt dopants dissolved in the liquid phase should act as the effective heterogeneous nucleation sites. (ii) Y211 grains should grow into the acicular shapes before the system reaches an equilibrium amount of Y211 in the liquid (i.e., reaction-controlled). (iii) A large amount of the liquid phase should be supplied instantly at the partial melt stage with a rapid heating.
Crystallization and in-plane alignment behavior of YBa2Cu3O7−y films on MgO(001) prepared by the dipping-pyrolysis process
Authors: Manabe T. , Yamaguchi I. , Nakamura S. , Kondo W. , Kumagai T. , Mizuta S.
Published: September 2007

The effects of annealing temperature and oxygen partial pressure [p(O2)] were investigated on the crystallization and orientation of YBa2Cu3O7−y (YBCO) films on MgO(001) prepared by the dipping-pyrolysis process. The c-axis oriented films without in-plane alignment were prepared by annealing in the YBCO-unstable region, i.e., at low initial p(O2) of 10−4−10−3 atm and 950 °C followed by O2 treatment, through intermediate Y2BaCuO5 and liquid phase. In-plane aligned c- or c/a-axis films were prepared by similar heat treatment with an initial p(02) of 10−4 atm and 900–925 °C through a mixture of BaCu2O2 and YBa3Cu2O6+x. In contrast, nonoriented YBCO films were obtained by annealing at higher initial p(O2)'s and lower temperatures, i.e., by direct reaction among Y2O3, BaCO3, and CuO.
Aspects of the Shi–Seinfeld–Okuyama theory of transient nucleation
Authors: Sundar G. , Hoyt J.J.
Published: September 2007

The analytic solution to the time-dependent nucleation problem by Shi-Seinfeld-Okuyama (SSO) [Phys. Rev. A 41, 2101 (1990)] is reviewed. The singular perturbation solution employed by SSO is extended to examine the effect of initial quench position on the incubation time. Two cases are discussed. The first investigates the role of excess vacancies from the high temperature quench on the transient kinetics. The second case examines the change in the incubation time due to the effects of a preexisting subcritical cluster size distribution which forms during the high temperature anneal.
Microwave-hydrothermal processing of metal powders
Authors: Komarneni Sridhar , Pidugu Rajyalakshmi , Li Qing Hua , Roy Rustum
Published: September 2007

Novel microwave-hydrothermal processing has been developed by us recently for the synthesis of a wide variety of ceramic powders. Herein, we report the use of microwave-hydrothermal processing to synthesize several metal powders such as Cu, Ni, Co, and Ag by reducing their corresponding metal salts or hydroxides with ethylene glycol. Metal powders have been produced extremely rapidly a (few minutes) by microwave catalysis. The kinetics of metal powder synthesis have been increased by at least an order of magnitude by microwave-hydrothermal processing compared to the conventional refluxing process in ethylene glycol at about 195 °C.
A study of texture in diamond films as functions of methane concentration during chemical vapor deposition and post-growth hydrogen treatment
Authors: Ganesana D. , Sharma S.C.
Published: September 2007

We have studied effects of hydrogen on texture in diamond films grown by hot filament assisted chemical vapor deposition by utilizing x-ray diffraction (XRD). We present results for the relative intensities of the XRD peaks originating from the (111). (220), and (400) crystallographic planes as functions of CH4/H2 makeup during growth and post-growth H2 treatment of the films. The texture of the films can be controlled by varying composition of the CH4/H2 mixture during growth and also by subjecting films to hydrogen treatment. The complementary characterization of these films by XRD, Raman spectroscopy, and positron annihilation techniques exemplifies a correlation among film texture, diamond contcnt, and dcnsity of the microvoids in the films.
Crystal structure systematics from oxide phase diagrams by contouring them with Zoltai's tetrahedral sharing coefficient
Authors: Chakoumakos B.C.
Published: September 2007

For crystal structures of oxides with tetrahedral coordination polyhedra, the average number of tetrahedra participating in the sharing of a corner, i.e., Zoltai's tetrahedral sharing coefficient, provides a measure of the degree of polymerization of the tetrahedra. By contouring oxide phase diagrams with Zoltai's tetrahedral sharing coefficient, crystal structure systematics can be conveniently displayed and correlated with other physical and thermochemical properties. The advantages of this analysis are (i) a structural map guides exploration for new compounds, (ii) possible structures for existing compounds that are not known are suggested, (iii) the internal consistency of the chemistry of specific compounds is tested by structural constraints, (iv) the physical behavior and properties of a family of compounds in a chemical system can be correlated with the degree of polymerization of the tetrahedra, and (v) the analysis lends itself to computer programming, in that contour templates of tetrahedral sharing coefficients for different types of oxide systems can be easily determined and overlaid on traditional phase diagrams. Shortcomings to this approach are that the tetrahedral sharing coefficient does not define a unique tetrahedral anion topology, ambiguities arise if some of the oxygen atoms are not part of the tetrahedral anion, and many chemical systems contain oxides where one or more of the tetrahedral cations adopt other coordination geometries.
Low temperature/low pressure hydrothermal synthesis of barium titanate: Powder and heteroepitaxial thin films
Authors: Chien A.T. , Speck J.S. , Lange F.F. , Daykm A.C. , Levi C.G.
Published: September 2007

Barium titanate powder and heteroepitaxial thin films were successfully produced by hydrothermal routes at ambient pressure and temperatures less than 100 °C. This processing method provides a simple low temperature route for producing epitaxied barium titanate thin films on single-crystal SrTiO3 substrates and powders which could also be extended to other systems. A dissolution/reprecipitation growth mechanism also was proposed for the formation of barium titanate by this route using previously published aqueous stability diagrams. Repeated hydrothermal treatments improved film thickness and surface coverage at the expense of increased surface roughness.
Removal of residual chromium from aluminum oxide by containerless liquid-phase processing
Authors: Biswas A. , Richard Weber J.K. , Nordine Paul C.
Published: September 2007

Verneuil sapphire was purified of Cr3+ by containerless melting and processing at ca. 2550 K in argon, dry air, and pure oxygen. Recovered material was examined by laser induced fluorescence and Raman spectroscopy. The Cr3+ fluorescence intensity decreased in processed specimens at rates proportional to the chromium concentration and p(O2)0.21. The initial chromium concentration was ca. 5 ppm and decreased by factors of ca. 50, 3000, and 2 × 105 after processing for 300 s in argon, air, and oxygen, respectively. Evidence is presented that the Cr3+ was removed predominantly as CrO2(g) and not by conversion to other oxidation states of chromium in the condensed phase.
A micromechanistic model of the combined combustion synthesis-densification process
Authors: Zhang Yangsheng , Stangle Gregory C.
Published: September 2007

A series of computer experiments has been conducted in order to study the combined combustion synthesis-densification process, in which a mechanical load is applied to a sample as it undergoes a combustion synthesis process. The current work is an extension of a theoretical model of the combustion synthesis process that was developed previously.1,2In this work, the appropriate constitutive equations for sample deformation have been incorporated, in order to account for the pore-volume change that may take place when the mechanical load is applied, thus densifying the sample. It was shown that the brief appearance of a liquid phase in the combustion wave front provides an important opportunity for densification when the self-propagating combustion synthesis process is conducted in conjunction with an applied mechanical load. That is, the concomitant decrease in the (local) total volume fraction of the solid phases—due to the elementary melting and dissolution processes that also occur (locally)—effectively lowered the (local) apparent yield strength of the sample, thus allowing for the compaction and densification of the sample (i.e., locally). Results indicated that the mechanical load should be applied at the instant at which the sample is ignited, in order to ensure that articles whose density is uniform throughout the sample can be fabricated. This work provided a more detailed and quantitative understanding of this unique process for preparing dense articles by the self-propagating combustion synthesis process, that is, when it is conducted in conjunction with an applied mechanical load.
Proton-induced fluorescence properties of terbium gallium garnet
Authors: Hollerman W.A. , Fisher J.H. , Ila D. , Jenkins G.M. , Holland L.R.
Published: September 2007

The authors completed a 3 MeV proton irradiation test on a terbium gallium garnet crystal sample. The main goal was to determine the proton dose required to reduce the fluorescence intensity to half its original value (half-brightness dose) at ambient temperature. The 3 MeV proton half-brightness dose was found to be 1.25 × 1015 p/cm2 using the Birks and Black relation. This decay is comparable to other fluors irradiated by the authors. The sample exhibited a yellow glow when irradiated in a 3 MeV beam. The fluorescence spectrum was composed of four peaks at wavelengths of 487.2 nm, 542.4 nm, 589.92 nm, and 624.1 nm.
Post-annealing induced defects and their influences on Bi2Sr2CaCu2Oy superconducting thin films
Authors: Zhang Xiao Feng
Published: September 2007

Defects in Bi2Sr2Can-1CunOy superconducting thin films annealed in an oxygen atmosphere are examined by high-resolution electron microscopy (HREM). In addition to the majority 2212 (n = 2) phase, subsequent slabs of other homologous phases with n values up to n = 10 are found intergrown in the films. Large-angle tilt grain boundaries and various secondary phases such as CuO, Sr-Cu-O oxides are formed in the films. The occurrence of these defects is attributed to an inhomogeneous Sr, Ca, and Cu distribution induced by the post-annealing. Superconducting transition temperature (Tc) is increased by the annealing under suitable conditions.
Phase relations in the system Na2ZnP2O7-Zn2P2O7
Authors: Grebenshchikov R.G. , Mikirticheva G.A. , Shitova V.I. , Petrova M.A. , Novikova A.S. , Kuchaeva S.K. , Grabovenko L.U.
Published: September 2007

Data on phase relations in the system Na2ZnP2O7-Zn2P2O7 reported in the literature are scarce and contradictory. The system was first studied by Berul and Voskresenskaya [Russ. J. Inorg. Mater. (English transl.) 4 (12), 2129 (1968)], who established the existence of a congruently melting intermediate compound, Na8Zn6(P2O7)5. In the paper by Majling et al. [Chem. Zvesti. 28 (3), 294 (1974)], the system Na2ZnP2O7-Zn2P2O7 is considered to be eutectic. The subsolidus phase relations are represented by a mixture of two phases, terminal members of the system. The existence of the above-mentioned compound was not confirmed for this system. In the present work we studied the phase relations in the system Na2ZnP2O7-Zn2P2O7 on samples obtained by solid-state synthesis and glass crystallization. The sequence of crystallization of phases was investigated, and the equilibrium and metastable phase diagrams were constructed for the system.
On the evolution of structure and composition in sol-gel-derived lead zirconate titanate thin layers
Authors: Lakeman Charles D.E. , Xu Zhengkui , Payne David A.
Published: September 2007

The evolution of structure and chemical distribution in sol-gel derived Pb(Zr0.53T10.47)O3 thin layers was monitored by x-ray diffraction, analytical electron microscopy, and diffuse reflectance Fourier transform infrared spectroscopy. Electron microscopy confirmed the as-deposited coatings were amorphous with short-range order. Medium-range order developed on heat treatment, and chemical heterogeneity was observed at the nanoscale. The extent of compositional heterogeneity decreased with increasing temperature. Above 500 °C, the coatings crystallized into an intermediate phase which converted to the perovskite phase above 600 °C.
Conventional and microwave sintering studies of SrTiO3
Authors: Chang Horng-Yi , Liu Kuo-Shung , Lin I-Nan
Published: September 2007

Using the nonconventional sintering technique, such as microwave sintering, it is observed to enhance the densification rate of SrTiO3 materials as effectively as employing the highly active powders prepared by the chemical route. Although the chemically derived powders demonstrate better sinterability than the mixed oxide powders, the thermal analysis indicates that the segregation of Ti4+-containing clusters during decomposition of precursors in the direct pyrolysis (DP) process induces the occurrence of TiO2 particles (anatase phase) prior to the formation of SrTiO3 phase. These particles retard the necking process required to sinter the materials. The spray pyrolysis (SP) process can circumvent the preferential nucleation of TiO2 phase and, therefore, produce powders exhibiting superior sintering behavior to the DP-derived powders. The microwave sintering technique, on the other hand, substantially enhances the rate of diffusion of the ions in the materials such that even the mixed oxide powders can be sintered at a temperature about 200 °C lower than that needed to achieve the same density in a conventional sintering process. Fine grain (∼4 μm) microstructure is obtained for the materials microwave sintered at 1220 °C for 10 min. The migration of grain boundaries requires higher temperature to initiate than the formation of neckings between the grains. The grain growth occurs only when the material was sintered at temperatures higher than 1250 °C.
The influence of monomer and polymer properties on the removal of organic vehicle from ceramic and metal moldings
Authors: Matar S.A. , Evans J.R.G. , Edirisinghe M.J. , Twizell E.H.
Published: September 2007

This paper describes the effects of monomer and polymer properties on the competition between degradation of organic vehicle and transport of degradation products in ceramic moldings during pyrolysis. An experimentally tested model is studied systematically for ranges of material and process parameters characteristic of known polymers and their degradation products. The work highlights the properties having the greatest influence on the successful removal of organic vehicle from molded ceramics. The polymer properties controlling the diffusion constant are linked to the temperature dependence of viscosity of the molten suspension. Enthalpy of vaporization of the organic vehicle and the activation energy for the diffusion coefficient have a commanding influence on the critical heating rate for avoidance of defects. Preliminary guidelines emerge for the design of polymers for plastic forming of ceramic suspensions.
Glass formation from low molecular weight organic melts
Authors: Kim Seong-Jin , Karis T.E.
Published: September 2007

Glass formation from the melt of organic monomers was studied for a variety of different organic molecular structures with Tg near ambient temperature. Crystallization is suppressed by one or more of the molecular properties, hydrogen bonding, interlocking, dipolar, and hydrogen bonding, combined with hindered rotational isomerism. Examples of materials in each category are presented for illustration. The viscosity of undercooled liquids was characterized by the Vogel-Tammon-Fulcher (VTF) equation, η = A cxp[DT0/(T - T0)], where A, D, and T0 are experimentally determined parameters. Our experimental D values are discussed in relation to the molecular structure and glass formation mechanism. The insight provided by our interpretation is intended to assist in the design of new molecular structures with controlled viscosity-temperature characteristics, as well as glass-forming ability by cooling from melts.
The effects of melting reactions on laboratory-scale waste vitrification
Authors: Smith Peter A. , Vienna John D. , Hrma Pavel
Published: September 2007

At the U.S. Department of Energy's Hanford Site, processes are being developed to vitrify waste generated during nuclear materials processing. One of the wastes slated for vitrification is known as neutralized current acid waste (NCAW). The batch chemistry of simulated NCAW was varied with oxidants and reductants. Untreated, formated, nitrated, or sugar-added samples were combined with frit to produce melter feed. Offgas measurements of the formated melter feed showed that formates decomposed at temperatures too low for participation in melt redox reactions. Sugar pyrolyzed and produced CO and H2 at temperatures exceeding 665 °C. For the sugar-added samples, the glass quenched from 1200 °C produced an Fe2+ /ΣFe of 0.79. The measured iron redox ratios from the glasses made from untreated, formated, and nitrated wastes were essentially indistinguishable (0.0024 at 1000 °C and 0.032 at 1200 °C). However, the batch chemistry affected volume expansion and the reaction paths.
Mechanistic and processing studies in combustion synthesis of niobium aluminides
Authors: Kachelmyer C.R. , Rogachev A.S. , Varma A.
Published: September 2007

Combustion synthesis of NbAl3 and Nb2Al was studied using the volume combustion mode. The effects of heating rate and green density were examined for NbAl3 synthesis. The effect of green density was also investigated for Nb2Al. Greater reaction completion was achieved at higher heating rates and green densities. In both NbAl3 and Nb2Al samples, the reaction was initiated above the melting point of Al. Quenching (Nb+3Al) samples pressed at relatively high and low densities below the ignition temperature, and results of a particle-foil experiment, identified the spreading characteristic of molten Al over Nb, providing mechanistic details about niobium aluminide product formation.
Carbon nitride films produced by high-energy shock plasma deposition
Authors: Bursill L.A. , JuLin Peng , Gurarie V.N. , Orlov A.V. , Prawer S.
Published: September 2007

High-energy shock plasma deposition techniques are used to produce carbon-nitride films containing both crystalline and amorphous components. The structures are examined by high-resolution transmission electron microscopy, parallel-electron-energy loss spectroscopy, and electron diffraction. The crystalline phase appears to be face-centered cubic with a unit cell parameter approx. a = 0.63 nm, and it may be stabilized by calcium and oxygen at about 1–2 at. % levels. 85 at. % of the carbon atoms appear to have trigonal bonding for the crystalline phase, the remaining 15 at.% having tetrahedral bonding. The amorphous carbon-nitride film component varies from essentially nanocrystalline graphite, containing virtually no nitrogen, to amorphous carbon-nitride containing up to 10 at. % N, where the fraction of sp3 bonds ranges up to approx. 85 at. %. There is PEELS evidence that the nitrogen atoms have sp2 trigonal bonds in both the amorphous and crystalline phases.
Identification of oxidation mechanisms in silicon nitride ceramics by transmission electron microscopy studies of oxide scales
Authors: Backhaus-Ricoult M. , Gogotsi Yu. G.
Published: September 2007

Additive-free HIPSN and Y2O3 + Al2O3-doped HPSN are oxidized in air in the temperature range from 1300 to 1500 °C. TEM, SEM, EDS, and XRD are used to analyze the composition and microstructure of the oxide scales in order to determine the oxidation mechanisms. HIPSN exhibits excellent resistance to oxidation in air at temperatures up to 1480 °C due to the formation of a protective silica (cristobalite) scale. No formation of Si2N2O and oxygen-enriched β'-Si3N4 under the silica layer is observed for materials densified without additives. Oxidation rates of additive-containing HPSN are more important due to the formation of a viscous aluminosilicate phase, which easily penetrates along the grain boundaries in the material. Silicon nitride grains in contact with the viscous phase first become enriched in aluminum and oxygen and are then dissolved in the glassy phase. No Si2N2O intermediate layer is formed. The finding of the decisive role of the aluminosilicate in the oxidation process allows one to explain inconsistencies observed in the oxidation kinetics of silicon nitride ceramics. Effects of sintering additives, WC contamination and temperature on the oxidation mechanisms, and structure of oxide scales are discussed.
The synthesis and characterization of solid-state materials produced by high shear-hydrodynamic cavitation
Authors: Moser W.R. , Marshik B.J. , Kingsley J. , Lemberger M. , Willette R. , Chan A. , Sunstrom J.E. , Boye A.
Published: September 2007

A new method for the synthesis of complex metal oxides, based on hydrodynamic cavitation, was used to prepare pure phase, nanostructured solid-state materials. The continuous process afforded a wide variety of metal oxides in grain sizes of 1-10 nm. Catalysts, ceramics, superconductors, piezoelectrics, and zeolites were prepared by cavitational synthesis. The method enabled the synthesis of fine particles of metals and metal oxides supported on high surface area supports such as silica, and the synthesis of fine particles of cubic zirconia without ion modification.
A novel route for the synthesis of Sr1−xBaxNb2O6 thin films
Authors: Kashani A. , Tomar M.S. , Dayalan E.
Published: September 2007

Stoichiometric Sr1−xBaxNb2O6 (SBN) powder and thin films were prepared by a chemical method. The starting materials were niobium ethoxide and the hydroxides of strontium and barium. Powders were obtained by evaporation of the precursor solution, and thin films were deposited by spin coating. Annealing temperature required to obtain complete conversion to the crystalline material was about 700 °C. Stoichiometric polycrystalline films of Sr1−xBaxNb2O6 were deposited on quartz and silicon substrates. Leakage current-voltage and the capacitance-voltage measurements on a metal/SBN/n-silicon structure show a diode-type characteristic.
Anomalous electrical behavior of partially carbonized polyacrylonitrile fibers
Authors: Jang Bor Z. , Zhao Liren
Published: September 2007

Partially carbonized polyacrylonitrile fibers were observed to undergo a resistivity change of 2 to 4 orders of magnitude at a transition temperature typically in the range of 98 °C to 200 °C. The current-voltage curves exhibited an initial supercurrent-like increase, followed by a rapid drop to a high resistance state, and then a rise in current again at a later stage. These phenomena cannot be interpreted by existing theories on switching in inorganic amorphous semiconductors. They are explainable if the microstructure of the pyrolyzed fiber is viewed as comprising nanometer-scale superconducting phases interspersed with semiconducting phases, much like a large number of Josephson junctions connected in series.
Growth of highly oriented La0.84Sr0.16MnO3 perovskite films
Authors: Chung Brandon W. , Brosha Eric L. , Garzon Fernando H. , Raistrick Ian D. , Houlton Robert J. , Hawley Marilyn E.
Published: September 2007

We have grown thin films of La0.84Sr0.16MnO3 on SrTiO3 (100), MgO (100), CeO2 (100)/Al2O3, and (100) oriented yttria-stabilized zirconia (YSZ) substrates by using a 90°off-axis RF magnetron sputtering deposition. X-ray diffraction analysis and ion beam channeling experiments reveal that the deposited films grow epitaxially on SrTiO3, biaxially textured on MgO, and highly textured on YSZ. Scanning tunneling microscopy reveals that the thin films possess extremely smooth surfaces.
Neutron irradiation and annealing of 10B doped chemical vapor deposited diamond films
Authors: Khasawinah S.A. , Popovici Galina , Farmer J. , Sung T. , Prelas M.A. , Chamberlain J. , White H.
Published: September 2007

10B doped diamond films grown by hot filament chemical vapor deposition were neutron irradiated at moderately high fluence levels. The as-irradiated and annealed samples, along with an unirradiated sample, were analyzed using Raman spectroscopy and x-ray diffraction. It was found that a non-diamond amorphous phase was formed on irradiation. This phase transformed back to diamond on annealing. No graphite formation was observed. A comparison with nanodiamond powder was made. A similarity between irradiated diamond films and nanocrystalline diamond powder is discussed.
Oxidation of diamond windows
Authors: Johnson Curtis E. , Bennett Jean M. , Nadler Melvin P.
Published: September 2007

Diamond windows were heated at 700-900 °C in air for short times to better define the limits of application at high temperature. The infrared (IR) transmission of polished chemical vapor deposition (CVD) diamond windows was reduced after heating in a furnace at 800 °C for 75 s, while heating at 700 °C for 75 s produced little change. The 800 °C heating caused increased visible light scatter, and increased scatter appears to be mainly responsible for the reduced IR transmission. The forward scatter at 10.6 μm for the CVD diamond samples was 0.8% before heating, 2.8% after heating at 800 °C for 75 s, and 6.2% after heating at 800 °C for 255 s. Single crystal (110) type IIa diamond exhibited little change in IR transmission when heated at 800 °C in air for 255 s, while heating for 555 s at 800 °C caused a significant drop in IR transmittance (6-12%). A slight drop in IR transmittance (1-5%) occurred for a type IIa diamond when heated at 900 °C for 45 s. The etched surfaces were characterized by differential interference contrast optical microscopy, scanning electron microscopy, and Talystep surface profiles. Etched features are concentrated at grain boundaries (for the CVD samples) and at sites with residual damage from mechanical polishing that could not be seen before the heat treatment. Deep etch pits formed at grain boundaries that extend into the bulk of samples.
Microstructure development of sol-gel derived epitaxial LiNbO3 thin films
Authors: Nashimoto Keiichi , Cima Michael J. , McIntyre Paul C. , Rhine Wendell E.
Published: September 2007

Film growth and microstructural evolution were investigated for sol-gel derived LiNbO3 thin films deposited on lattice-matched single-crystal substrates. Epitaxial LiNbO3 films of about 100 nm nominal thickness were prepared by spin coating a solution of the lithium niobium ethoxide on sapphire (0001) substrates and annealing at 400 °C or 700 °C in a humidified oxygen atmosphere. These films exhibited an epitaxial relationship with the substrate of the type LiNbO3 (0001) || α-Al2O3 (0001) and LiNbO3 [100] || α-Al2O3 [100] as determined by x-ray pole figure analysis. Transmission electron microscopy indicated the epitaxial films annealed at 400 °C consisted of slightly misoriented ∼5 nm subgrains and of numerous ∼10 nm enclosed pores. The microstructure and orientation development of these films was consistent with a heteroepitaxial nucleation and growth mechanism, in which epitaxial nuclei form at the substrate surface and grow upward into an amorphous and porous intermediate film: Epitaxial films annealed at 700 °C contained larger 150-200 nm subgrains and pinholes. Misorientations between adjacent subgrains appeared to be significantly smaller in films annealed at 700 °C than those in films annealed at 400 °C. Hydrolysis of the alkoxide precursor solution prior to spin coating promoted the development of polycrystalline films on single-crystal sapphire substrates. Infrared spectra and thermal analysis indicated that, independent of the degree of the solution hydrolysis, nucleation of LiNbO3 was immediately preceded by decomposition of an amorphous carbonate intermediate phase.
Oxygen incorporation in aluminum nitride via extended defects: Part III. Reevaluation of the polytypoid structure in the aluminum nitride-aluminum oxide binary system
Authors: Westwoord Alistair D. , Youngman Robert A. , McCartney Martha R. , Cormack Alasiair N. , Notis Michael R.
Published: September 2007

This paper extends the concepts that were developed to explain the structural rearrangement of the wurtzite AlN lattice due to incorporation of small amounts of oxygen, and to directly use them to assist in understanding the polytypoid structures. Conventional and high-resolution transmission electron microscopy, specific electron diffraction experiments, and atomistic computer simulations have been used to investigate the structural nature of the polytypoids. The experimental observations provide compelling evidence that polytypoid structures are not arrays of stacking faults, but are rather arrays of inversion domain boundaries (IDB's). A new model for the polytypoid structure is proposed with the basic repeat structural unit consisting of a planar IDB-P and a corrugated IDB. This model shares common structural elements with the model proposed by Thompson, even though in his model the polytypoids were described as consisting of stacking faults. Small additions (≃ 1000 ppm) of silicon were observed to have a dramatic effect on the polytypoid structure. First, it appears that the addition of Si causes the creation of a new variant of the planar IDB (termed IDB-P'), different from the IDB-P defect observed in the AlN-Al2O3 polytypoids; second, the addition of Si influences the structure of the corrugated IDB, such that it appears to become planar.
Nuclear magnetic resonance investigations of the structural phase transitions of AlPO4 tridymite
Authors: Xiao Yuehui , James Kirkpatrick R.
Published: September 2007

27Al and 31P NMR spectroscopic data are presented for the tridymite polymorph of AlPO4 (AlPO4−1) through its structural phase transition at about 80 °C. The RT 27Al and 31P spectra of AlPO4−t both contain doublets of broad peaks, indicating two well-separated groups of sites in the RT structure with mean Al-O-P bond angles per tetrahedron of ∼ 147.8°and 153.1°(±1). With increasing temperature, the doublets remain the same up to about 74 °C, where the relative intensities of the two peaks start to change. The peak corresponding to smaller Al-O-P bond angles disappears, and above ∼88 °C the 27Al and 31P spectra contain single symmetrical peaks, corresponding to a mean Al-O-P bond angle of 153.4°. This bond angle increases gradually with increasing temperature to 153.7°at ∼150 °C and remains constant to about 500 °C. 27Al quadrupole echo experiments suggests that the 27Al nuclear quadrupole coupling constant (QCC) is small and decreases with increasing temperature. QCC remains nonzero in the high temperature phase of AlPO4−t, consistent with the previously proposed 3m local symmetry of Al in the high-temperature structure.
Damage accumulation and cyclic fatigue in Mg-PSZ at Hertzian contacts
Authors: Pajares Antonia , Wei Lanhua , Lawn Brian R. , Marshall David B.
Published: September 2007

Hertzian contact damage in as-fired, peak-aged, and over-aged Mg-PSZ is studied, in single-cycle and multiple-cycle loading. Indentation stress-strain curves reveal a monotonically increasing quasi-plasticity component in the contact deformation with increasing aging time. A bonded-interface technique is used to obtain surface and subsurface views of the damage zones beneath the spherical indenter. Analytical techniques, including optical and scanning electron microscopy, acoustic emission, Raman spectroscopy, and thermal wave imaging, are used to characterize the damage. The damage patterns are fundamentally different in the three aging states: microfracture dominated in as-fired; tetragonal-monoclinic phase-transformation-dominated in peak-aged; monoclinic-phase twinning-dominated in over-aged. The damage accumulates with increasing number of cycles, most strongly in the as-fired state. It also increases with increasing test duration in the as-fired and over-aged states, but not perceptibly in the peak-aged. The results imply predominantly mechanical fatigue effects, augmented by a chemical component in the as-fired and over-aged states. Broader implications in relation to the susceptibilities of zirconia ceramics to fatigue degradation in concentrated stress configurations, with special relevance to the evolution of flaws at the microstructural level, are considered.
Comments on “Ion beam sputter deposition of YBa2Cu3O7−δ thin films” [J. Mater. Res. 8, 3032 (1993)]
Authors: Gauzzi Andrea , Pavuna Davor
Published: September 2007

In this brief letter we want to comment on several statements published recently in the Journal of Materials Research by our former co-workers Kellett and James (KJ) in the paper entitled “Ion beam sputter deposition of YBa2Cu3O7−δ thin films”.
Reply to the comments of A. Gauzzi and D. Pavuna
Authors: Kellett Bruce John , James Jonathan Horton
Published: September 2007

We thank Dr. A. Gauzzi and Dr. D. Pavuna for bringing to our attention their comments on our article “Ion beam sputter deposition of YBa2Cu3O7−δ,” as it gives us the opportunity to clear up some discrepancies, make more explicit some of the results, and correct unintended misunderstandings.
Diamond growth on thin Ti wafers via chemical vapor deposition
Authors: Chen Qijin , Lin Zhangda
Published: September 2007

Diamond film was synthesized on thin Ti wafers (as thin as 40 μm) via hot filament chemical vapor deposition (HFCVD). The hydrogen embrittlement of the titanium substrate and the formation of a thick TiC interlayer were suppressed. A very low pressure (133 Pa) was employed to achieve high-density rapid nucleation and thus to suppress the formation of TiC. Oxygen was added to source gases to lower the growth temperature and therefore to slow down the hydrogenation of the thin Ti substrate. The role of the very low pressure during nucleation is discussed, providing insight into the nucleation mechanism of diamond on a titanium substrate. The as-grown diamond films were characterized by scanning electron microscopy (SEM), Raman spectroscopy, and x-ray analysis.
Investigation of the processing conditions for the growth of large cross section meet-powder-meet-growth YBCO samples in a bidirectional temperature gradient
Authors: Frangi F. , Monot I. , Murakami M.
Published: September 2007

A bidirectional gradient furnace with sample displacement was used for the growth of large meet-powder-meet-growth (MPMG) YBCO samples. The optimization of the macrogranular structure with consequent extension of the central part of the sample, characterized by large aligned domains, was studied. Considering the large cross section of our samples, we studied the combined effects of different ratios between the longitudinal and transversal temperature gradients, different pulling rates (R = 0, 0.5, 1, and 2 mm/h), and one or more stages at the beginning of the sample growth on grain nucleation, dimension, and orientation. Different mechanical pretreatments of sample surfaces were also tested in order to control domain nucleation. We obtained 20 × 25 × 45 mm3 samples with low misoriented extended domains. Moreover, we were able to vary their c-axis directions from perpendicular to parallel with respect to the sample length.
Formation mechanism and relaxor ferroelectric properties of lead lithium iron tungstate ceramics
Authors: Lu Chung-Hsin , Hwang Wen-Shin
Published: September 2007

The formation mechanism and ferroelectric properties of Pb(Li1/4Fe1/4W1/2)O3 prepared by solid-state reaction have been investigated in this study. The formation processes of Pb(Li1/4Fe1/4W1/2)O3 are characterized to be an initial reaction of lead tungstates PbWO4 and Pb2WO5 at a low temperature range, followed by a subsequent reaction to produce Pb(Li1/4Fe1/4W1/2)O3 from above 650 °C. Through a two-stage calcination (700 °C/quenching-regrinding-710 °C/8 h), a nearly single phase of Pb(Li1/4Fe1/4W1/2)O3 is obtained. This compound exhibits a cubic perovskite structure (α = 8.0113 Å) with a partial ordering arrangement of B-site cations. Above 720 °C, becomes Pb(Li1/4Fe1/4W1/2)O3 thermodynamically unstable and gradually decomposes in forming elongated Pb2WO5 grains, thereby resulting in a nonhomogeneous microstructure. As the ac frequency increases, the maximum dielectric permittivity of Pb(Li1/4Fe1/4W1/2)O3 significantly decreases; in addition, the corresponding temperature increases. The strong frequency dependence of dielectric properties, as well as the critical exponent and diffuseness evaluated through a modified permittivity-temperature equation proposed in this study, verify the relaxor characteristics of Pb(Li1/4Fe1/4W1/2)O3
The dielectric temperature characteristic of additives modified barium titanate having core-shell structured ceramics
Authors: Park Yung , Kim Yoonho
Published: September 2007

The dielectric temperature characteristics and microstructures of BaTiO3-based ceramics sintered with additives such as Sm2O3, CeO2, and Bi2O3:Nb2O5 were investigated using TEM, XRD, and EDS. For a Sm2O3-modified BaTiO3 ceramic whose additives were uniformly distributed in grains, the ferroelectric transition temperature (Tc) was shifted to a lower temperature, while the transition temperatures (T1 and T2) were shifted to a higher temperature. The additions of CeO2 and Bi2O3: PbO to BaTiO3 formed the chemical inhomogeneity which was composed of grain core, grain shell, and concentration gradient region. The dielectric curve versus temperature of CeO2-modified BaTiO3 has the shape of one strong peak, whereas BaTiO3 ceramics sintered with Bi2O3:Nb2O5 exhibit the broad dielectric constant at the low temperature region and 130 °C ferroelectric transition peak. The dielectric temperature characteristics of additives modified BaTiO3 were determined in terms of the chemical inhomogeneity and stress induced by the difference of the unit cell volume between grain core and grain shell.
Rheology of aqueous boehmite-coated silicon nitride suspensions and gels
Authors: Shih Wei-Heng , Pwu Leh-Lii
Published: September 2007

The rheological properties of boehmite-coated silicon nitride aqueous suspensions and gels are reported. In unidirectional rheological tests, it was found that the boehmite coating reduces the viscosity of the suspensions over a wide range of shear rates and volume fractions of particles. The suspension shear stress as a function of shear rate can be described by the Bingham model, and the Bingham yield stresses of boehmite-coated silicon nitride suspensions are lower than those of the uncoated suspensions. The reduction in the viscosity and the Bingham yield stress is attributed to a shallower secondary minimum in the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential between coated particles than that for uncoated silicon nitride particles. Moreover, at low values of pH, the coated silicon nitride suspensions gelled over time, and the viscoelastic behavior of the gels was studied by dynamic oscillatory tests. It was found that the shear modulus (G′) and loss modulus (G″) remain constant up to a certain strain amplitude, γ°, beyond which G′ and G″ begin to vary. The value of G′ in the linear region increases exponentially, whereas γ° decreases exponentially with the volume fraction of coated silicon nitride particles. The exponential behavior of the shear modulus G′ of the gels is similar to the exponential pressure-density relationship found in the previous pressure filtration study, indicating that particulate rearrangement occurs as volume fraction of particles is increased.
Morphology of α-hexathienyl thin-film-transistor films
Authors: Lovinger A.J. , Davis D.D. , Ruel R. , Torsi L. , Dodabalapur A. , Katz H.E.
Published: September 2007

We have studied the morphology of thin films of α-hexathienyl (α-6T), a hexamer of thiophene that is a very promising material for thin-film-transistor applications. Using electron- and atomic-force microscopies, we found that on both rigid (Si/SiO2 and glass) and flexible (polyimide) substrates, evaporated films show an apparently random, polycrystalline morphology. The crystals are lamellar, ca. 100-200 nm in lateral dimensions and 15-30 nm in thickness, and exhibit irregular boundaries. Nevertheless, electron-diffraction evidence from such films indicates that the constituent molecules are deposited preferentially end-on and assume a normal or nearly normal orientation with respect to their substrates. Rapid high-temperature annealing causes growth of much larger (μm-sized) crystalline grains and a partial transformation to a high-temperature polymorph; however, this process leads to formation of gaps in the film, which may cause deterioration of electronic performance.
Synthesis of functionally graded metal-ceramic microstructures by chemical vapor deposition
Authors: Lee W.Y. , Bae Y.W. , More K.L.
Published: September 2007

A composite microstructure consisting of small α-Al2O3 particles dispersed in a β-NiAl coating matrix was synthesized by chemical vapor deposition (CVD). White the surface of a pure Ni substrate was being reacted with AlCl3 and H2 to form β-NiAl at a temperature of 1050 °C, the partial pressure of CO2 in the reactor was controlled via pulsing to nucleate and disperse 50 to 500 nm α-Al2O3 particles in the β-NiAl matrix. The relative amount of the α-Al2O3 phase increased with coating thickness as the rate of the β-NiAl formation decreased with time. These experimental observations suggest that the synthesis of a graded composite microstructure by the CVD method is feasible.
Preparation of LaBa2Cu3Oy thick films in the La–Ba–Cu–O system by partial melting techniques
Authors: Dimesso L. , Hyun O.B. , Hirabayashi I.
Published: September 2007

The preparation of thick films of the LaBa2Cu3Oy (La-123) phase by using the partial melting technique (PMT) of powders deposited on MgO substrates by drying alcoholic suspensions is reported. The effects of the starting composition, processing time (t), and temperature (T) on the superconducting properties of the La1+xBa2Cu3Oy (−0.25 ⋚ x ⋚ 0.5) system were investigated. The presence of the La4Ba2Cu2O10 (La 422) phase was detected in the samples quenched in liquid N2 after heating up to 1200 °C. The crucial step of the processing is the cooling rate down to the crystallization temperature, Tp, determined by DTA analysis, in order to favor the formation of the La-123 phase from the reaction between the La-422 solid phase and the liquid. The superconducting properties, tested by magnetization measurements, showed a diamagnetic onset temperature (Tc,diam) as high as 89 K.