bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of the dislocations in Si-doped GaAs comparing diluted Sirtl light etching, electron-beam-induced current, and micro-Raman techniques
Authors: Martín P. , Jiménez J. , Frigeri C. , Sanz L. F. , Weyher J. L.
Published: March 2008

Impurity atmospheres around dislocations have been studied in n-type Si-doped liquid encapsulated Czochralski (LEC) GaAs substrates by micro-Raman spectroscopy, diluted Sirtl-like etching with light (DSL) method, and electron-beam-induced current (EBIC). A complete morphological study of the recombinative atmospheres revealed by photoetching was achieved by phase stepping microscopy (PSM), which is an optical interferometry technique allowing to obtain the surface topography with a high vertical resolution (in the nanometer range). The minority carrier diffusion length was measured by EBIC at different points of the atmospheres. Structural distortion at the regions surrounding the dislocation core were observed by micro-Raman spectroscopy. The carrier depletion depth and the recombination of the photogenerated carriers were also studied by Raman spectroscopy, obtaining a good agreement with the EBIC data and the photoetching rates. Impurity gettering and diffusion and defect reactions involving As interstitials are assumed to play a major role in the formation of the recombinative atmospheres.
Piezoelectric and pyroelectric behavior of lead zirconate titanate–lead sulfide–glass nanocomposites
Authors: Kundu T. K. , Chakravorty D. , Cross L. E.
Published: March 2008

Composites containing micron-sized lead zirconate titanate and nanometer-sized lead sulfide in a glass matrix were prepared using a sol-gel technique. The PbS size ranged from 3.5 to 15.3 nm. It was possible to pole some of these nanocomposites. The measured values of saturation polarization, remanent polarization, and coercive field showed an increase as the PbS particle size was enhanced. The composites exhibited improvement in the piezoelectric and pyroelectric behavior as compared to that of the base glass-ceramic.
Chemical synthesis and characterization of polyaniline-molybdenum trisulfide composite
Authors: Fusalba Florence , Bélanger Daniel
Published: March 2008

A novel polyaniline-molybdenum trisulfide composite has been prepared by chemical polymerization from an acidic (1 M HCl) aqueous solution containing aniline and ammonium tetrathiomolybdate. The presence of molybdenum trisulfide in the polyaniline matrix induces morphological change to the polymer as evidenced by scanning electron micrographs. X-ray diffraction and differential scanning calorimetry indicate that polyaniline-molybdenum trisulfide is slightly less crystalline than polyaniline-HCl. X-ray photoelectron spectroscopy (XPS) and elemental analysis have been used to confirm the presence of molybdenum trisulfide in the polymer matrix. The XPS data also confirm that molybdenum trisulfide and tetrathiomolybdate anions are present with polyaniline to form a new inorganic-organic composite.
Calorimetric and high-resolution transmission electron microscopy study of nanocrystallization in zirconia gel
Authors: Málek Jiří , Mitsuhashi Takefumi , Ramírez-Castellanos Julio , Matsui Yoshio
Published: March 2008

The formation of metastable tetragonal zirconia nanophase by thermal treatment of a zirconia gel derived from zirconyl chloride has been studied by high-resolution transmission electron microscopy (HRTEM) and differential scanning calorimetry (DSC). HRTEM observations revealed that a fully crystallized sample consists of nanocrystals, around 13 nm in size. This nanocrystalline t-ZrO2 has practically the same crystal structure as that of the high-temperature tetragonal zirconia phase. The nonisothermal crystallization rate is very fast in as-prepared zirconia gel. DSC data at various heating rates can be described by a two-parameter model which predicts the crystallization kinetics in isothermal conditions very well. The Johnson–Mehl–Avrami (JMA) model can be used, however, in partially crystalline samples (crystallinity >30%) where the rate of crystallization process is considerably slower. The kinetic exponent of the JMA model (m = 1.0 ± 0.1) then corresponds to linear dependence of the crystallization rate as a function of the fraction crystallized.
Morphological evolution of barium titanate synthesized in water in the presence of polymeric species
Authors: Bagwell Roger B. , Sindel Jürgen , Sigmund Wolfgang
Published: March 2008

The synthesis of barium titanate (BaTiO3) was investigated in water at 90 °C in the presence of polymeric additives. Homopolymers (polyacrylic acid) and block copolymers (polyethylene oxide-block-polymethacrylic acid) were added during synthesis to influence particle morphology and size distribution. The polymers affected the morphological evolution of the forming powder by adsorbing preferentially on specific planes. The polymeric species additionally slowed the formation of barium titanate. The barium concentration also changed the morphology, particle size, and other powder characteristics.
Residual stress fields in sol-gel-derived thin TiO2 layers
Authors: Teeuw D. H. J. , de Haas M. , De Hosson J.Th. M.
Published: March 2008

This paper discusses the induction of residual stresses during the curing process of thin titania layers, which are derived using a sol-gel process. During this process, stresses may build up in the spinning stage, the drying stage, and the consolidation stage. The magnitude and character of these stresses depend heavily on the morphology of the layers in the various stages and the processing conditions. Dried layers are densified using two different processes, conventional furnace heating and laser heating. X-ray analysis and scanning electron microscopy are used as tools to study crystallization, grain growth, phase transformation, and the evolution of residual stress fields in the thin titania layers. Through an extensive study of the residual stress state in the layers, more insight is gained in the evolution of stresses during the curing process of sol-gel-derived thin titania layers.
Microstructural evolution and mechanical properties of gas-pressure-sintered Si3N4 with Yb2O3 as a sintering aid
Authors: Lee Ki-Min , Lee Won-Ho , Koh Young-Hag , Choi Jong-Jin , Kim Hyoun-Ee , Baek Seung Su
Published: March 2008

Microstructural evolution and mechanical properties of gas-pressure-sintered Si3N4 with 4 wt% Yb2O3 as a sintering aid were investigated. The microstructure was not uniform throughout the specimen. Extremely large elongated grains were formed at the outer region near the surface, while relatively small elongated grains were formed at the inner region of the specimen. The outer region expanded inward with the sintering time. Mechanical properties, such as flexural strength, fracture toughness, and R-curve behavior of the specimens were strongly influenced by these variations in microstructure. The fracture toughness and the R-curve behavior of the outer region were higher than those of the inner region of the same specimen. On the other hand, the strength of the inner region was higher than that of the outer region. By controlling the relative thickness of each region, Si3N4 specimens having functionally graded microstructure were obtained. The Si3N4 with such microstructure exhibited high strength, high fracture toughness, and good flaw tolerance at the same time.
Experimental assessment of the inhibition of reduction of Ca2+-doped barium titanate in a reducing atmosphere
Authors: Fang Tsang-Tse , Shuei Jyh-Tzong
Published: March 2008

Several experiments have been conducted to assess the possible mechanisms for the inhibition of reduction of Ca2+-doped barium titanate in a reducing atmosphere. Three methods, i.e., conventional, semiwet, and citrate processes, have been used to prepare the powders. It was found that the formation mechanism would influence the occupation of Ca2+ in the B site. Moreover, it was concluded that Ca2+ occupation of the B site is the major cause for the inhibition of reduction of Ca2+-doped barium titanate. Ti deficiency would be the driving force for Ca2+ to occupy the B site.
Synthesis and luminescence of the nanosized Ce-doped silica particles dispersed within the pores of mesoporous silica host
Authors: Cai Weiping , Zhang Ye , Zhang Lide
Published: March 2008

Nanosized Ce-doped silica particles (without and with Al addition) dispersed within pores of mesoporous silica host were synthesized by soaking and sol-gel technique. It was found that the dispersed phosphor particles are mainly located within the pores that are less than 4 nm in diameter. There exist two luminescence peaks at about 350 and 700 nm, respectively, for this phosphor in the dispersed or aggregated state. For the dispersed system, the luminescence intensities of both peaks are more than 14 times higher than those of the aggregated one, the shoulders on the lower sides of the luminescence peaks disappear, and no effect of Al addition on the luminescence occurs which is in contrast to the aggregated system. This is mainly attributed to the high dispersity of nanosized phosphor particles in the host and the effective avoidance of a clustering of Ce3+ ions.
Microstructure and thermokinetics analysis of combustion synthesized AlN
Authors: Chen Kexin , Ge Changchun , Li Jiangtao , Cao Wenbin
Published: March 2008

The mechanism for combustion synthesis (CS) of AlN was investigated in the present paper. A novel quenching technique was developed to retain the high-temperature structure formed in the sample during the combustion process. Based on the microstructural observation of the quenched specimen and thermodynamic analysis, a two-step reaction mechanism for CS AlN, i.e., vaporization of Al and subsequent reaction with N2, is proposed.
Modeling studies of the effect of thermal and electrical conductivities and relative density of field-activated self-propagating combustion synthesis
Authors: Carrillo-Heian E. M. , Graeve O. A. , Feng A. , Faghih J. A. , Munir Z. A.
Published: March 2008

The role of the electrical conductivity of the product and of the thermal conductivities of the reactants on self-propagating combustion synthesis was investigated through modeling studies. Similar studies were made to investigate the role of the relative density of the reactants. The effect of an imposed electric field on the results of the modeling analysis was considered. For any given imposed field, the wave velocity exhibited a maximum at a given normalized thermal conductivity, electrical conductivity, and relative density. The results are discussed in terms of the Joule heat contribution of the field and are compared with experimental observations.
Silicon nitride: Enthalpy of formation of the α- and β-polymorphs and the effect of C and O impurities
Authors: Liang Jian-jie , Topor Letitia , Navrotsky Alexandra , Mitomo Mamoru
Published: March 2008

High-temperature oxidative drop solution calorimetry was used to measure the enthalpy of formation of α− and β−Si3N4. Two different solvents, molten alkali borate (48 wt% LiBO2 · 52 wt% NaBO2) at 1043 and 1073 K and potassium vanadate (K2O · 3V2O5) at 973 K, were used, giving the same results. Pure α− and β−Si3N4 polymorphs have the same molar enthalpy of formation at 298 K of −850.9 ± 22.4 and −852.0 ± 8.7 kJ/mol, respectively. The unit cell dimensions of impure α−Si3N4 samples depend linearly on the O and C impurity contents, and so does the molar enthalpy of formation. The energetic stability of the α−Si3N4phase decreases when the sample contains O and C impurities. The experimental evidence strongly suggests that the impurities dissolve into the α−Si3N4 structure to form a (limited) isostructural solid solution series but that this solid solution series is energetically less stable than a mechanical mixture of pure (α or β) Si3N4, SiO2, and SiC. Thus, the α-phase is not stabilized by impurities and is probably always metastable.
Programmed substrate temperature ramping to increase nucleation density and decrease surface roughness during metalorganic chemical vapor deposition of aluminum
Authors: Jonnalagadda R. , Yang D. , Rogers B. R. , Hillman J. T. , Foster R. F. , Cale T. S.
Published: March 2008

We discuss substrate temperature ramping effects during chemical vapor deposition of aluminum on nucleation density, texture, surface roughness, and resistivity of the resulting films. Results from three different process protocols are presented. Ramping the temperature down during the deposition from 673 K resulted in a larger fraction of small nuclei compared to deposition at a constant temperature of 573 K. From among the protocols studied, the lowest surface roughness was obtained by initially depositing for a short time while ramping the temperature down from 673 K, followed by deposition at 573 K, compared to all the other films. The same process protocol resulted in the highest Al(111) texturing, highest reflectivity, and lowest resistivity.
Elastic modulus measurement of multilayer metallic thin films
Authors: Cho Ki-Hyun , Kim Youngman
Published: March 2008

Two- and three-layer composite models were developed using a beam vibration theory, and the models were applied for measuring Young's moduli of thin metallic films. The Cr, Ni, and Co-coated Si wafer composites (two-layer composite) and (Cr/Ti/Si) composites (three-layer composite) were produced by radio-frequency (rf) magnetron sputtering and used to test the developed models. Young's moduli of (Cr) films obtained by the three-layer composite model agree well with those of (Cr) films obtained by the two-layer composite model, considering (Ti/Si) as the one layer and (Cr) as the other layer. This suggests that moduli of multilayer films may be obtained by using a two-layer composite model repeatedly.
Microcharacterization of liquid delivery metalorganic chemical vapor deposition processed thin film materials exhibiting giant magnetoresistance
Authors: Weaver M. L. , Brandao L. P. M. , Garmestani H. , Gillman E. S. , Dahmen K. H. , Morrone A.
Published: March 2008

Thin films of La0.6Ca0.4MnO3 (LCMO) have been produced on (001) oriented LaAlO3 (LAO) and yttrium-stabilized zirconia (YSZ) substrates by liquid-delivery metalorganic chemical vapor deposition (LD-MOCVD). X-ray diffraction (XRD) analyses showed that the films were epitaxially grown on LAO substrates and were monocrystalline at a thickness of less than 500 Å. At a thickness of greater than 500 Å, the films became polycrystalline but maintained their high texture (preferred crystalline orientation). Films grown on YSZ were always polycrystalline but were also highly oriented. Regardless of the substrate, the 1500 Å thick polycrystalline films exhibited substantially significant magnetoresistance ratios even above room temperature.
Microstructural evolution of dense and porous pyroelectric Pb1−xCaxTiO3 thin films
Authors: Seifert Andreas , Sagalowicz Laurent , Muralt Paul , Setter Nava
Published: March 2008

Pb1−xCaxTiO3 thin films with x = 0−0.3 for pyroelectric applications were deposited on platinized silicon wafers by chemical solution processing. Ca-substitution for Pb in PbTiO3 results in a reduced c/a ratio of the unit cell, which, in turn, leads to better pyroelectric properties. Control of nucleation and growth during rapid thermal annealing to 650 °C allowed the formation of either highly porous or dense (111) oriented films. The inclusion of pores creates a matrix-void composite with the low permittivity desired for pyroelectric applications, resulting in a high figure of merit. The growth mechanisms for the microstructural evolution of both dense and porous films were analyzed by x-ray diffraction, transmission electron microscopy, scanning electron microscopy, and Rutherford backscattering spectrometry and allowed establishment of microstructure/property relationships.
Micropyretic synthesis of MoSi2 powders through an aluminothermic reaction
Authors: Fu Ming , Penumella S. , Sekhar J. A.
Published: March 2008

An aluminothermic reaction starting with inexpensive MoO3, SiO2, and Al powders was utilized to prepare molybdenum disilicide (MoSi2) powders by the micropyretic/combustion synthesis process and leaching. The combustion-synthesized product was porous and could readily be crushed into powders. X-ray diffraction (XRD) analysis revealed that the product of such a reaction consisted of α–Al2O3, MoSi2, and a small amount of Mo(Si,Al)2 and Mo5Si3. The reason for the formation of Mo(Si, Al)2 phase is discussed. MoSi2 powders were obtained by leaching out the Al2O3 from the synthesized powder mixtures in boiling phosphoric acid solution. The synthesized MoSi2 powders, including a small amount of Mo(Si, Al)2 and Mo5Si3, were very fine with an average particle size of about 1 μm.
Bombarding energy dependence of bonding structure in amorphous carbon interlayer and its effect on diamond nucleation
Authors: Oh U. C. , Cheng De Gang , Lu Fan Xiu , Je Jung Ho
Published: March 2008

The bombarding energy dependence of bonding structure in amorphous carbon interlayer and its effect on diamond nucleation density (Nd) were studied. Amorphous carbon (a-C) interlayer was deposited by magnetron sputtering. Interestingly, the intensity ratio (ID/IG) of the D band (∼1400 cm−1) to the G band (∼1570 cm−1) in the Raman spectra and the optical band gap of the a-C film were found to be inversely proportional to the sputtering power, that is, to bombarding energy. When diamond was subsequently deposited at 800 °C by microwave plasma chemical vapor deposition (CVD), diamond could be grown only on the interlayers with higher ID/IG (≥2.20), and Nd was increased up to 2 × 106/cm2 with the increase of ID/IG ratio, that is, with the decrease of the bombarding energy. We experimentally confirmed that the amount of the sp3 bonded carbon clusters within the interlayer was dependent on the bombarding energy of the particles, determining the diamond nucleation density. We suggest that the transformation of the amorphous carbon into graphitic carbon should be effectively prevented for the diamond nucleation on the a-C interlayer.
The effect of growth condition on the structure of 2H – AlN films deposited on Si(111) by plasma-assisted molecular beam epitaxy
Authors: Kaiser U. , Brown P. D. , Khodos I. , Humphreys C. J. , Schenk H. P. D. , Richter W.
Published: March 2008

The effects of substrate cleaning, nitridation time, and substrate temperature in the range 800–1000 °C on the microstructure of AlN/Si(111) films grown by simultaneous plasma-assisted molecular beam epitaxy have been investigated. It has been demonstrated, using a combination of conventional and high-resolution transmission electron microscopy, that the interface structure, the film defect structure, and the film surface roughness are strongly related. The formation of single crystal 2H–AlN films with atomically flat surfaces occurs at 800 °C for conditions of 2.5 nm/min growth rate on very pure, atomically flat Si substrates.
Structure and optical properties of diamondlike carbon synthesized by plasma immersion ion processing
Authors: He Xiao-Ming , Lee D. H. , Walter K. C. , Li D. Q. , Nastasi M.
Published: March 2008

Hard and transparent diamondlike carbon (DLC) films have been prepared on room-temperature substrates by using a C2H2–Ar plasma immersion ion processing (PIIP) method. The optical properties of the DLC films with different thicknesses deposited on PMMA (polymethyl methacrylate), silicon wafers, and glass plates were systematically examined. It was found that careful control of substrate bias voltage was needed for favorable growth of DLC films with low atomic hydrogen content, high hardness and wear resistance, and excellent optical properties. The resultant DLC films exhibited a low friction coefficient, high optical gap energy, and very high optical transmittance both in infrared and visible light ranges. The study confirmed that C2H2–Ar PIIP with low negative bias voltages and suitable C2H2/Ar gas ratios can produce optically transparent and hard DLC films on optical materials.
Structure-property behavior during aging of sol-gel-derived silica modified with Si–H and Si–CH3 groups
Authors: Sglavo Vincenzo M. , Diré Sandra , Ferrari Maurizio
Published: March 2008

Unsupported films and thin rods were prepared by the sol-gel method from solutions of triethoxysilane and methyldiethoxysilane. Measurements of elastic modulus, density, glass transition temperature, and refractive index were performed at different aging times. Results showed a large increase of the elastic modulus with aging time, which was related to the progressive condensation taking place in the xerogel network, as shown by solid-state nuclear magnetic resonance (NMR). The stiffness increase was shown to correspond to higher glass transition temperature and larger density and refractive index. Different increasing rates of the physical and mechanical characteristics were observed for films and rods during aging. This behavior was related to the different loss rate of the pore liquid, which affects the network evolution in the wet gel.
Bulk amorphous Pd–Ni–Fe–P alloys: Preparation and characterization
Authors: Shen T. D. , He Y. , Schwarz R. B.
Published: March 2008

Bulk amorphous alloys of PdxNiyFe80−x−yP20 (25 ≤ x ≤ 60, 20 ≤ y ≤ 55, x + y ≥ 60) were prepared by a flux-melting and water-quenching method. Seven-mm diameter glassy rods of Pd40Ni40−xFexP20 (0 ≤ x ≤ 20) were studied in greater detail. For these alloys, the difference between the crystallization and glass transition temperatures ranges from 102 K for x = 0 to 53 K for x = 20. In this composition range, the reduced glass transition temperature, Trg, ranges from 0.66 to 0.57. The change in density upon crystallization ranges from 0.24 ± 0.04% for x = 0 to 1.33 ± 0.24% for x = 10. The partial molar volume of Fe in amorphous Pd40Ni40−xFexP20 alloys is significantly larger than the molar volume of (metastable) fcc Fe. This, as well as a comparison with the molar volumes of crystalline compounds, suggests chemically selective Fe–Pd bonding in these glasses.
Pyrolysis of self-assembled organic monolayers on oxide substrates
Authors: Shin Hyunjung , Wang Yuhu , Sampathkumaran Uma , De Guire Mark R. , Heuer Arthur H. , Sukenik Chaim N.
Published: March 2008

The pyrolysis of siloxy-anchored, organic self-assembled monolayers (SAMs) on oxide substrates [titanium dioxide powder; hydrolyzed silicon dioxide on (100) silicon] was studied using x-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and mass spectroscopy (MS). Pyrolysis in air began on heating at 200 °C and was complete by 400 °C for both octadecyltrichlorosilane (OTS) and C16-thioacetate (TA) SAMs, as observed in TGA of SAM-coated TiO2 powders, and in XPS studies of TA-SAM-coated TiO2 powders and Si wafers after various heat treatments. In low-oxygen environments, pyrolysis of SAMs began at higher temperatures: between 250 and 400 °C for heating in ultrahigh vacuum (10−8 Torr) as observed in XPS studies of TA-SAMs on Si, and between 300 and 400 °C in nitrogen, as observed in TEM analysis of sulfonate SAMs under a TiO2 thin film on Si substrates.
Irreversible hydrogenation of solid C60 with and without catalytic metals
Authors: Brosha Eric L. , Davey John , Garzon Fernando H. , Gottesfeld Shimshon
Published: March 2008

The dehydrogenation of C60 · H18.7 was studied using thermogravimetric and powder x-ray diffraction analysis. C60 · H18.7 was found to be stable up to 430 °C in Ar at which point the release of hydrogen initiated the collapse of a fraction of fullerene molecules. X-ray diffraction analysis performed on C60 · H18.7 samples dehydrogenated at 454, 475, and 600 °C displayed an increasing volume fraction of amorphous material. The decomposition product comprises randomly oriented, single-layer graphite sheets. Evolved gas analysis using gas chromatograph (GC) mass spectroscopy confirmed the presence of both H2 and methane upon dehydrogenation. Attempts to improve reversibility or reduce hydrogenation/ dehydrogenation temperatures by addition of Ru and Pt catalysts were unsuccessful.
Electrodeposition of lithium phthalocyanine thin films: Part I. Structure and morphology
Authors: Brinkmann M. , Graff S. , Chaumont C. , André J-J.
Published: March 2008

A new thin film synthesis route based on the electrochemical oxidation of PcLi2 and deposition of lithium phthalocyanine (PcLi) onto indium tin oxide (ITO) substrate is demonstrated. The effects on the thin film morphology of various parameters such as the electrolysis time, the nature of the solvent, and the oxidation potential are investigated. The thin film growth is studied via x-ray diffraction, potential step experiments, and ex situ scanning electron microscopy. Various morphologies of the x-form thin films are observed for different electrolysis times and solvents. Thin films grown in acetonitrile of thickness above 1 μm consist in unidirectionally oriented needle-shaped crystallites.
Fracture toughness of diamondlike carbon coatings
Authors: Nastasi M. , Kodali P. , Walter K. C. , Embury J. D. , Raj R. , Nakamura Y.
Published: March 2008

The fracture behavior of diamondlike carbon (DLC) coatings on Si substrates has been examined using microindentation. The presence of DLC coatings reduces the radial crack length to less than one-half the crack length observed in uncoated Si at the same indenter load. A total work of fracture analysis of the radial cracks formed in the DLC-coating/Si-substrate system gives 10.1 MPa m1/2 as the average fracture toughness for DLC alone. A bond-breaking calculation for DLC suggests that the elastic limit fracture toughness should be 1.5 MPa (m)1/2. The higher value obtained from experiment and total work analysis suggests that plastic work and/or a tortuous path crack evolution occurred during DLC fracture process.
Magnetic properties of Tb2(Fe,Si)17 single crystals
Authors: Li Y. X. , Gao S. X. , Du J. , Tang C. C. , Cai C. , Wu G. H. , Zhan W. S.
Published: March 2008

The magnetic properties of single-crystalline Tb2Fe17−xSix (x = 0, 1, 2, 3, and 3.3) have been investigated. The Si substitution constricted the lattices by 1.5% and caused the Th2Ni17 transfer to Th2Zn17. The Curie temperature increased from 413 to 526 K, and the spontaneous magnetic moment decreased from 82.6 to 46.4 emu/g with the increase of Si. The stronger anisotropy and coercivity were generated by Si occupying the Fe sublattices. A domain wall pinning-dominated mechanism was responsible for increasing the coercivity force from 0.01 T (x = 1) to about 0.36 T (x = 3.3) at 1.5 K.
Sintering behavior of nanophase MoSi2 with and without a ZrO2 additive
Authors: Tan Qi , Zangvil Avigdor
Published: March 2008

It has been shown that sintering temperature can be lowered and the density, hardness, and toughness of MoSi2 raised by using nanophase MoSi2 and ZrO2 additives. A minimum hot-pressing temperature of 1350 °C is needed for complete densification without a significant increase in grain size.
InGaN/GaN/AlGaN-based laser diodes grown on epitaxially laterally overgrown GaN
Authors: Nakamura Shuji
Published: March 2008

Epitaxially laterally overgrown GaN on sapphire was used to reduce the number of threading dislocations originating from the interface of the GaN epilayer with the sapphire substrate. The GaN layer above the SiO2 mask area surrounding the window and corresponding to the lateral overgrowth was nearly free of threading dislocations. A high density of threading dislocations was observed in the vicinity of GaN grown in the window regions. InGaN multiquantum well-structure laser diodes (LDs) grown on pure GaN substrates, which were fabricated by removing the sapphire substrate, were demonstrated. The LDs with an output power of 5 mW exhibited a lifetime of more than 290 h and an estimated lifetime of 10,000 h despite a relatively large threshold current density. The far-field pattern of the LDs with a cleaved mirror facet revealed single-mode emission without any interference effects.
Microstructures and solidification behavior in Y–Ba–Cu–O/Ag superconducting leads
Authors: Maeda J. , Shiohara Y.
Published: March 2008

The microstructures and the solidification behavior in unidirectionally solidified Y–Ba–Cu–O/Ag (YBCO/Ag) superconducting current leads were investigated. The solidification model of this system was discussed by the Y2Ba1Cu1O5–Ba3Cu5Ox–Ag quasiternary phase diagrams constructed by analysis of the solute compositions in the melt. The diffusion fields and the spacing between silver particles in the YBCO/Ag system were discussed. The relationship between the shapes of entrapped silver particulates, the miscibility gap in the phase diagram, and the wetting behavior at the growth interface in two-dimensional monotectic reactions were also discussed.
Dislocation structure of low-angle grain boundaries in YBa2Cu3O7−δ/MgO films
Authors: Oktyabrsky S. , Kalyanaraman R. , Jagannadham K. , Narayan J.
Published: March 2008

Grain boundaries in laser-deposited YBa2Cu3O7−δ(YBCO)/MgO thin films have been investigated by high-resolution transmission electron microscopy. The films exhibit perfect texturing with YBCO(001)/MgO(001) giving rise to low-angle [001] tilt grain boundaries resulting from the grains with thecaxis normal to the substrate surface and with misorientation in thea-bplane. The atomic structure of the grain boundaries was analyzed by using a dislocation model. Low-angle grain boundaries have been found to be aligned along (100) and (110) interface planes. For the (110) boundary plane, the low-energy dislocation configuration was found to consist of an array of alternating [100] and [010] dislocations. We have calculated the energy of various configurations and shown that the energy of the (110) boundary with dissociated dislocations is comparable to that of the (100) boundary, which explains the coexistence of (100) and (110) interface facets along the boundary. We have also modeled critical current transport through grain boundaries with various structures and found that the low-energy (110) grain boundary with dissociated dislocation array is expected to transport a lower superconducting current (by 25% for 6° misorientation) than (100) boundaries.
On temperature dependence of deformation mechanism and the brittle–ductile transition in semiconductors
Authors: Pirouz P. , Samant A. V. , Hong M. H. , Moulin A. , Kubin L. P.
Published: March 2008

Recent deformation experiments on semiconductors have shown the occurrence of a break in the variation of the critical resolved shear stress of the crystal as a function of temperature. These and many other examples in the literature evidence a critical temperature at which a transition occurs in the deformation mechanism of the crystal. In this paper, the occurrence of a similar transition in two polytypes of SiC is reported and correlated to the microstructure of the deformed crystals investigated by transmission electron microscopy, which shows evidence for partial dislocations carrying the deformation at high stresses and low temperatures. Based on these results and data in the literature, the explanation is generalized to other semiconductors and a possible relationship to their brittle–ductile transition is proposed.
First principles study of influence of alloying elements on TiAl: Lattice distortion
Authors: Song Y. , Yang R. , Li D. , Wu W. T. , Guo Z. X.
Published: March 2008

The influence of ternary additions Cr, Fe, Mn, Ni, Zr, Nb, Mo, Hf, Ta, Si, Ga, Ge, In, and Sb, as well as the anti-site defects of both Ti and Al, on lattice parameters of TiAl were studied by the first principles electronic structure calculations with a discrete variational cluster method. The results of the calculation show that the effect of ternary additions on the distortion of TiAl lattice varies with the substitution behavior of the individual alloying element involved. The addition of alloying elements in TiAl causes a change in the electronic structure and the density of states of the system and results in variation of the bond strength between the atoms. The total and partial density of states (DOS) of binary TiAl and of ternary TiAl–M, M = Cr, Zr, and Sb, etc., were comparatively examined. The relationship between the DOS and the bond strength is discussed. The present work suggests that the origin of the lattice distortion of the ternary TiAl–M systems lies in the variation of the electronic structure.
Dependence of the mechanical and structural properties of (Ti,Al)N films on the nitrogen content
Authors: Jiménez C. , Sánchez-Fernández C. , Morant C. , Martínez-Duart J. M. , Fernández M. , Sánchez-Olías J.
Published: March 2008

(Ti,Al)N films with increasing nitrogen content were grown by reactive cosputtering and characterized by auger electron spectroscopy, grazing x-ray diffraction, polarization curves, electrochemical impedance spectroscopy, nanoindentation, and atomic force microscopy. For Ti/Al ≈ 1 the Ti1−x AlxN phase is always present, but lower nitrogen contents lead to an additional phase, probably α–Ti(Al), which causes a decrease in hardness and Young's modulus. The increase of nitrogen content results ina decrease of surface roughness or a more compacted surface coating, according to the impedance results.
Phase stability of Ni–Al nanoparticles
Authors: de Debiaggi S. Ramos , Campillo J. M. , Caro A.
Published: March 2008

The phase stability of Ni–Al clusters of nanometer size was studied by using the embedded atom model and Monte Carlo simulation techniques. For temperatures of 500 and 1000 K and for a range of compositions below 70 at.% Al, the equilibrium structures of the system were determined and compared with the bulk results. We found that the bulk NiAl (B2) and Ni3Al (L12) phases were stable phases in the nanoparticle system; however, for deviations from ideal composition, the analysis revealed that, because of the surface effect, the composition of the clusters was not uniform. There was a core region in which the structure was ordered, B2 or L12, with a composition very close to the ideal, and a chemically disordered mantle region that allocated the deviations from ideal stoichiometries; in this way, a larger phase field appeared, indicating trends similar to those found in experiments on nanocrystalline Ni–Al powder [S.K. Pabi and B.S. Murty, Mater. Sci. Eng. A214, 146 (1996)]. For concentrations between 37 and 51 at.% Al, an intermediate phase, similar to the tetragonal L10 martensite, appeared.
Synthesis of magnetic particle/organic hybrid from metalorganic compounds
Authors: Yogo Toshinobu , Nakamura Tomoyuki , Sakamoto Wataru , Hirano Shin-ichi
Published: March 2008

A nanocrystalline magnetic particle/oligomer hybrid was successfully synthesized by polymerization of iron(III) 3-allylacetylacetonate (IAA) followed by in situ hydrolysis. An iron oxide particle/oligomer hybrid was synthesized by hydrolysis of the IAA oligomer under alkaline and reducing conditions by the addition of hydrazine or methylhydrazine. Crystalline particles of approximately 10 nm were found to be dispersed in the oligomeric matrix. The nanocrystalline particles were identified to be iron oxide spinel by x-ray diffraction analysis and electron diffraction. The nanometer-sized ferrimagnetic iron oxide particle/oligomer hybrid showed a typical superparamagnetic behavior.
Assessment of single-fiber fragmentation using scanning acoustic microscopy
Authors: Sathish Shamachary , Madhukar Madhu S. , Cantrell John H. , Yost William T.
Published: March 2008

Scanning acoustic microscopy (SAM) was used to obtain the critical fragmentation length of a 7-μm-diameter carbon fiber embedded in an optically transparent epoxy matrix and was subjected to a standard fiber fragmentation test. The SAM-assessed critical fragmentation length of 356 ± 59.5 μm compared favorably with the value 341 ± 52.3 μm obtained independently from commonly used photoelastic techniques. Additionally, the SAM images allowed an assessment of regions of fiber-matrix debond, including the measurement of an average debond of 61.0 ± 11.8 μm along the fiber from the fragment ends, which could not be obtained with photoelastic methods. The application of SAM to the assessment of optically opaque composite materials is explained in this paper.
Synthesis of surface-metallized polymeric films by in situ reduction of (4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionato) silver(I) in a polyimide matrix
Authors: Southward Robin E. , Bagdassarian Carey K. , Sudol Christopher J. , Wasyk Jennifer L. , Sproul Susanna H. , Broadwater Sharon T. , Scott Joseph L. , Thompson David W.
Published: March 2008

Thermal curing of the (4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionato)silver(I)-containing poly(amic acid) formed from 3,3′,4,4′-benzophenone tetracarboxylic acid dianhydride (BTDA) and 4,4′-oxydianiline (4,4′-ODA) in dimethylacetamide gives both polyimide films via cyclodehydration and reduction of silver(I) to the native metal. Silver(0) migrates to the surface resulting in surface metallized composite films, which can have excellent reflectivity, but do not exhibit surface electrical conductivity. The films retain mechanical and thermal properties similar to those of the parent polyimide. X-ray diffraction shows crystalline face-centered-cubic silver in the films after thermal curing. Microscopy data show that the surface particle sizes are in the range of approximately 50–100 nm. Significant silver remains in the bulk of the polyimide film with varying particles sizes generally less than approximately 15 nm. The interior of the metallized films is not electrically conducting. Films were characterized by x-ray diffraction, differential scanning calorimetry, thermal gravimetric analysis, x-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy, and mechanical measurements.
Growth of graphite nanofibers from the decomposition of CO/H2 over silica-supported iron–nickel particles
Authors: Anderson P. E. , Rodriguez N. M.
Published: March 2008

Extremely fine, tubular graphite nanofibers of varying geometries and degrees of crystallinity were produced by the decomposition of CO and hydrogen over various compositions of nickel–iron particles supported on silica. High-resolution transmission electron microscopy coupled with temperature programmed oxidation studies revealed that, as the iron content of the catalyst was increased, the bimetallic particles precipitated a chainlike graphitic fibrous structure in a stepwise mechanism. The high-iron-content system Fe–Ni (8:2) yielded a small amount of these chainlike graphite fibers that were extremely resilient to oxidation, suggesting a highly crystalline structure. When the catalyst particles consisted of a nickel–iron mixture, Fe–Ni (5:5), there was a decrease in the degree of crystallinity of the fibers (78% graphite) and a corresponding increase in the amount of amorphous carbon precipitated (22% amorphous) within the structure. The high-nickel catalyst Fe–Ni (2:8) generated the largest amount of the tubular nanofiber product. It was significant that there was an increase in the amorphous carbon content (58%) precipitated as opposed to graphitic carbon (42%) in the structures. In some cases, amorphous carbon was deposited inside the graphite core of the nanofibers. The influence of the catalyst composition and nature of the metal-support interaction are key factors in the continuing development of graphite nanofibers possessing desired structures for potential uses in a variety of applications.
Equilibrium phases in Mn–V–O system under ambient atmosphere
Authors: Chung S. K. , Shin S. J. , Andriiko A. A. , Rudenok P. V.
Published: March 2008

The products obtained from MnO2–V2O5 mixtures at the ambient oxygen pressure and temperatures up to 900 °C were studied by means of thermal analysis and x-ray diffraction methods. The following phases were found to exist in equilibrium, depending on the composition: (I) Mn2O3–Mn2V2O7, (II) Mn2V2O7–MnV2O6, and (III) V2O5–solid solution of VO2 in MnV2O6. A phase diagram was obtained for the composition region III. The data are instructive for syntheses of manganese vanadates by thermal methods.
Solution deposition processing and electrical properties of Ba(Ti1−xSnx)O3 thin films
Authors: Yoon Ki Hyun , Park Ji Hoon , Jang Jae Hyuk
Published: March 2008

Ba(Ti1−xSnx)O3 (0 ≤ x ≤ 0.3) thin films were deposited on a platinized silicon substrate by a solution deposition process with methoxyethanol, water, and propylene glycol as solvents. Dielectric properties and current–voltage characteristics of the thin films were investigated in conjunction with phase evolution and microstructures by varying heating temperatures and Sn contents (x). Thin films annealed above 700 °C showed a pure perovskite phase with nanoscaled grains (20–30 nm). The dielectric constant of the thin films depended on the Sn content and showed a maximum value of 330 at x = 0.15. The leakage current behavior of an optimum composition corresponding to x = 0.15 was examined by correlating with charge transport mechanisms. Schottky emission was found to be predominant at voltages less than 6.8 V, and Fowler–Nordheim tunneling appeared to be responsible above 6.8 V. The Schottky barrier of the Ba(Ti0.85Sn0.15)O3–Pt interface was determined to be 1.49 eV.
Thermally induced structural changes in epitaxial SrZrO3films on SrTiO3
Authors: Langjahr P. A. , Wagner T. , Rühle M. , Lange F. F.
Published: March 2008

Epitaxial, continuous, approximately 40-nm-thick films of SrZrO3on SrTiO3substrates prepared by a chemical solution deposition method including a postdeposition heat treatment at 900–1000 °C were subjected to further heat treatments at higher temperatures (approximately 1200–1300 °C) to investigate their high temperature stability. Experimental investigations included scanning electron microscopy, atomic force microscopy, and conventional transmission electron microscopy. The investigations have demonstrated a morphological instability of the films. Concentration profiles of the cations determined by energy dispersive x-ray spectroscopy, as well as investigations by x-ray diffraction, revealed that the film islands consisted of a solid solution. As shown by high-resolution electron microscopy, the reaction between film and substrate also led to an increase in the separation distance of the misfit dislocations that were introduced during the lower temperature heat treatment to relax the lattice mismatch strain. The morphological and structural changes of the films are reported and discussed in this paper.
Transmission electron microscopy observation of second-phase particles in β–Si3N4 grains
Authors: Hirosaki Naoto , Saito Tomohiro , Munakata Fumio , Akimune Yoshio , Ikuhara Yuichi
Published: March 2008

Silicon nitride was fabricated by adding Y2O3 and Nd2O3 as sintering additives, sintering for 8 h at 1900 °C, and heat treating for 4 h at 2200 °C to enhance grain growth. The microstructure was investigated by scanning electron microscopy, high-resolution electron microscopy, energy dispersive x-ray spectroscopy (EDS), and electron microdiffraction. This material had a duplex microstructure composed of many fine grains and a few coarse grains. In β–Si3N4 grains, second-phase particles with the composition of liquid phase, Y–Nd–Si–O or Y–Nd–Si–O–N, in the size of 10–30 nm were observed. EDS spectra and microdiffraction patterns revealed that those were amorphous or crystalline particles of Y–Nd–apatite, (Y,Nd)10Si6O24N2. These particles were presumably formed during cooling by the precipitation of Y–Nd–Si–O–N, which was trapped in the β–Si3N4 grains as solid solution or trapped liquid. The results suggest that attention should be paid to the trace amounts of trapped elements in β–Si3N4 grains in trying to improve the thermal conductivity of sintered silicon nitride.
Microstructure in silicon nitride containing β-phase seeding: Part I
Authors: Lu Horng-Hwa , Huang Jow-Lay
Published: March 2008

A proper powder preparation technique was used to develop elongated β–Si3N4 particles as seeds from raw materials. The phase transformation and development of microstructure in Si3N4 ceramics containing β–Si3N4 seeds were investigated. The specimens of seeded silicon nitride had higher phase transformation rates than the specimens without seed. A core/shell structure was observed by transmission electron microscopy in seeded specimens owing to the difference in aluminum concentration in Si3N4 grains. The misfit between the core and the shell was accommodated by interfacial dislocation that has a rotation character. The growth mode was epitaxial, although there was some compositional difference between the core and the shell. A relatively larger grain size and wider grain size distribution in seeded Si3N4 specimens was due to the amount of β-phase seeds that could act as the nuclei, and the final modification of the microstructure was due to the coalescence process. The crack wake process characterized the mechanism of toughening.
Formation and characterization of hydroxyapatite coating layer on Ti-based metal implant by electron-beam deposition
Authors: Choi Jae-Man , Kong Young-Min , Kim Sona , Kim Hyoun-Ee , Hwang Cheol Seong , Lee In-Seop
Published: March 2008

A hydroxyapatite [HAp; Ca10(PO4)6(OH)2] coating layer was formed on a Ti-based alloy by the electron-beam deposition method. When pure HAp was used as a target for the deposition, an amorphous layer was formed on the metal substrate. By heat treatment in a vacuum at 630 °C, the layer was crystallized into tricalcium phosphate [Ca3(PO4)2]. The crystallization improved the dissolution rate of the layer remarkably; however, at the same time, it deteriorated the bond strength with the substrate. When extra CaO (up to 25 wt%) was added to the target and processed under the same conditions, a layer compositionally close to crystalline HAp was deposited. Before the heat treatment, even though the layer was in amorphous state, the dissolution rate in the physiological solution was extremely low. Furthermore, the bond strength increased remarkably compared to the layer formed by the pure HAp target. Compositional and structural resemblance of the layer with the crystalline HAp was attributed to these improvements in properties.
Morphology and growth mechanism of alumina whiskers in aluminum-based metal matrix composites
Authors: Li Y-F. , Qin C-D. , Ng D. H. L.
Published: March 2008

A process has been invented for growing alumina (Al2O3) whiskers in the matrix of the aluminum-based metal matrix composite by the internal oxidation reaction between aluminum (Al) and molybdenum oxide (MoO3) at 850 °C. The whiskers were formed during the firing stage and distributed uniformly throughout the metal matrix. The fractional volume of the whiskers was about 30%, and their average size was 1 μm in diameter and 10 μm in length. The growth mechanism of the Al2O3 whiskers was studied. It was found that the nutrient for the growth of the whiskers was supplied from the molten Al at the base of the whiskers where they reacted with MoO3 forming the Al2O3 whiskers and the Al–Mo intermetallic phases.
Powder synthesis of barium titanate and barium orthotitanate via an ethylene glycol complex polymerization route
Authors: Lee S. J. , Biegalski M. D. , Kriven W. M.
Published: March 2008

Pure and reactive barium titanate (BaTiO3) and barium orthotitanate (Ba2TiO4) powders have been synthesized by an ethylene glycol (EG), polymerizationcomplexation route. The EG content affected the crystallization behavior and powder morphology. The BaTiO3powder, which had a particle size of approximately 100 nm, crystallized from amorphous to tetragonal phase on calcining at 700 °C for 1 h. Ball-milled BaTiO3 powder sintered to 97% relative density at 1200 °C after 2 h, with a grain size of approximately 200 nm. Ba2TiO4 powder required longer holding times or higher temperatures to be crystallized from the amorphous phase than did BaTiO3. In Ba2TiO4, the phase transformation between low-temperature monoclinic symmetry to high-temperature orthorhombic symmetry was observed by dilatometry and differential scanning calorimetry. A volume decrease of ∼0.5% accompanied the monoclinic-to-orthorhombic transformation on heating. The high-temperature orthorhombic phase could be retained down to room temperature y the addition of at least 6 wt% magnesia (MgO) stabilizer.