bluuebunny/crossref_metadata_2025 huggingface dataset:
The effect of electrode composition on rf magnetron sputtering deposition of Pb[(Mg1/3Nb2/3)0.7Ti0.3]O3 films
Authors: Jiang M.C. , Wu T.B.
Published: September 2007

Ferroelectric Pb[(Mg1/3Nb2/3)0.7Ti0.3]O3 (abbreviated PMNT) thin films were prepared on silicon substrates by rf magnetron sputtering deposition with PbO-enriched PMNT targets. The effects of electrode composition and thin film growth conditions were investigated with grazing-incidence x-ray diffraction, secondary ion mass spectrometry, and scanning electron microscopy. The dielectric property of perovskite films was also measured. The usage of a Pt/Ti electrode was observed to enhance the formation of perovskite PMNT films; in addition, the TiO2 rutile phase was formed at the interface between the PMNT film and Pt electrode due to the oxidation of out-diffused Ti atoms from the inner Ti electrode. It was then noticed that if a target containing a larger excess of PbO was used, a higher consumption of TiO2 occurred and more perovskite phase would be formed in the deposited films. Consequently, perovskite PMNT films having a uniform microstructure and satisfactory dielectric property close to the bulk value were obtained by rf magnetron sputtering deposition on the Pt/Ti/SiO2/Si substrate at 640 °C under appropriate working conditions.
Development of an economical electron backscattering diffraction system for an environmental scanning electron microscope
Authors: Thaveeprungsriporn V. , Mansfield J.F. , Was G.S.
Published: September 2007

A low cost, highly versatile electron backscattering diffraction system has been developed for an ElectroScan E3 Environmental Scanning Electron Microscope (ESEM). A P20 phosphor coated screen is placed in the microscope environment to image electron backscattering diffraction patterns (EBSP's). A CCD TV camera is used to view the patterns through a leaded glass port in the microscope specimen chamber. This system has a spatial resolution approaching 5 μm and yields analyzable patterns at pressures of up to 6 Torr. With minor modifications this system may be adapted to fit any scanning electron microscope. Comparison of analyses of the grain boundary misorientation of a Ni-16Cr-9Fe alloy with this system and by selected area channeling patterns (SACP's), performed in a standard SEM, yielded excellent agreement between the two techniques. Owing to the capability to perform on-line analysis of EBSP's, the EBSP technique required only half the time as the SACP technique. The capabilities of the system are illustrated in a study of the grain boundary character distribution of Ni-16Cr-9Fe following thermomechanical treatment.
Thermal conductivity and diffusivity of KH2PO4 and NH4H2PO4 polycrystalline samples near their phase transitions
Authors: Suleiman B.M. , Karawacki E. , Gustafsson S.E.
Published: September 2007

Simultaneous measurements of thermal conductivity and diffusivity of ferroelectric polycrystalline samples of the KDP family type have been performed with the transient plane source (TPS) technique over the temperature range 88-300 K. The thermal conductivity behavior resembles that of the corresponding single crystals, but with values a factor of three lower. This decrease in thermal conductivity of the polycrystalline form is attributed to scattering mechanisms due to microstructural imperfections such as point defects, grain boundaries, etc. Values of porosity have been estimated using the dimensions of the unit cell deduced from the x-ray analysis and the apparent densities. The data indicate that the phase transition mechanisms (displacement type and order-disorder type) still dominate the conduction processes in spite of the presence of other scattering mechanisms due to the imperfection of the microstructure in the polycrystalline form of these materials.
Glass transition: A unified treatment
Authors: Shi Frank G.
Published: September 2007

A unified kinetic and thermodynamic description of the glass transition in undercooled liquids at normal pressure is established. The following results are obtained for the first time: (1) The glass transition temperature Tg is determined to be in the range of Ts < Tg < Tn. Both Ts and Tn are material-dependent and each of them is characterized by a different Ω(T) = TΔslc(T)/Δslc(T) with Δhlc as the excess enthalpy and Δslc the excess entropy. (2) Being above Kauzmann's isentropic temperature, the lowest limit Ts is determined by Ω(Ts) = 1 −2/(3γ) with γ being the ratio between the total energy and the free energy of the liquid-crystal interface. (3) Although a glass preserves the entropy and enthalpy values of the liquid at Tg, the ratio Ω(Tg) is found to be bound by a Tg-independent material constant 1 −2/(3γ). (4) Tg increases linearly with the logarithm of the cooling rate and such a linear relationship is found to be not always valid. (5) The observed cooling-rate dependent glass transition at Tg is the kinetically modified reflection of an underlying cooling-rate independent transition at Ts, and the underlying transition at Ts is kinetically equivalent to the sudden and strong divergence of the structure relaxation time of the liquid. (6) It is shown that if the cooling rate exceeds a minimum value determined here as a function of temperature, the atoms of an undercooled liquid will not have sufficient time to rearrange themselves into the corresponding crystalline configuration; consequently, crystalline nucleation can be prevented. The results are supported by the available experimental evidence. A systematic test of the results on different systems is possible since the results are in terms of experimentally accessible quantities.
High dielectric constants in BaTiO3 doped 90V2O5−10Bi2O3 oxide glasses obeying Debye-type dielectric relaxation behavior
Authors: Chakraborty Sahana , Bera A.K. , Mollah S. , Chaudhuri B.K.
Published: September 2007

BaTiO3 doped 90V2O5−10Bi2O3 (VB) oxide glasses show a larger dielectric constant (102-104) compared to that of the base glass VB (∼102). The VB glass with 15 wt.% BaTiO3 shows the largest dielectric constant, and all the glasses show a Debye-type dielectric relaxation behavior. The increase in dielectric constant appears to be mainly due to the formation of microcrystalline clusters of BaTiO3 (1.8–8 μm, depending on the BaTiO3 concentration present in the base glass). Other than the dilution effect with BaTiO3, the glass-former oxide Bi2O3 also influences the dielectric behavior of BaTiO3 doped VB glasses.
Aluminizing for TiAl turbocharger rotor by reduced pressure metal-organic chemical vapor deposition (RPMOCVD) and the efficiency of its oxidation resistance
Authors: Suzuki Takakazu , Umehara Hiroyuki , Hino Haruki
Published: September 2007

A complex-shaped TiAl turbine rotor has been uniformly aluminized by a metal-organic chemical vapor deposition under reduced pressure (RPMOCVD), and a thick TiAl3 layer, which affects the oxidation resistance, can be formed on the surface by subsequent heat treatment. The oxidation resistance has been studied with an oxidation test at 1173 K for 760 ks in static air. The microstructure has been investigated by SEM, EPMA, AES, and XRD. A heat treatment at above 933 K, which is the melting point of Al, is required to enhance the oxidation resistance of TiAl. With increasing the surface roughness of TiAl, the formation of TiAl3 increases, and consequently the oxidation resistance is more improved.
Effect of alumina on densification of binary borosilicate glass composite
Authors: Jean Jau-Ho , Gupta Tapan K.
Published: September 2007

The effect of alumina on densification of the binary borosilicate glass composite, containing low-softening borosilicate glass (BSG) and high-softening high silica (HSG) glass, has been investigated. It is found that with a small amount of alumina, 2-10 vol. %, present as a dopant in the binary glass mixture of BSG and HSG, both densification and densification rate are significantly reduced, but the activation energy of densification at a given densification is dramatically increased. However, no significant change in densification behavior with increasing alumina content from 2 to 10 vol. % is observed. These results are attributed to a chemical reaction taking place at the interface of alumina/BSG, forming a reaction layer adjacent to alumina. Since the composition of the reaction layer is known to be rich in aluminum and alkali ions and poor in silicon, the alkali ions content in BSG is continuously decreased during sintering. Accordingly, the resultant loss of alkali ions from BSG causes a rise in viscosity of BSG, thus slowing down the densification kinetics and increasing the activation energy of densification.
Preparation of fine multicomponent oxide ceramic powder by a combustion synthesis process
Authors: Zhang Yangsheng , Stangle Gregory C.
Published: September 2007

An important, rather novel procedure for the synthesis of submicron crystalline multicomponent oxide ceramic powders has been studied. The synthesis of CuFe2O4 powder, a ferrite material, has been used as a model system for understanding the synthesis process. The effect of the fuel content, powder packing, and surface heat loss has been investigated in terms of the maximum reaction temperature and reaction period, phase formation, and particle size and morphology. It has been shown that the maximum temperature and reaction period can be tailored to produce different phases. The submicron features of the synthesized powders are indicated by the large surface area values obtained from BET measurement.
Crystallography of surface nucleation and epitaxial growth of lithium triniobate on congruent lithium niobate
Authors: McCoy M.A. , Dregia S.A. , Lee W.E.
Published: September 2007

The decomposition of congruent lithium niobate (LN) crystals proceeds by surface nucleation and growth of LiNb3O8 precipitates which exhibit an epitaxial orientation relationship. The same orientation relationship is observed on x-, y-, and z-cut LN substrates. The epitaxy arises from similarities between the two crystal structures and provides for an essentially continuous oxygen-ion framework from parent to product. On y-cut LN, the precipitates have a well-defined habit plane, and the interfacial misfit between the two structures is accommodated by a rectangular grid of misfit dislocations. The density, geometry, and imaging behavior of the misfit dislocations suggest that their Burgers vectors serve to accommodate the disregistry in the oxygen-ion framework. Based on these observations, it is concluded that the mechanism of formation of LiNb3O8 consists of preferential nucleation on the surface and subsequent growth, possibly by counterdiffusion of cations in a closed system where the oxygen-ion framework remains essentially fixed in space.
Growth of lithium borate crystals from the vitreous state
Authors: Katsumata T. , Yoshimura T. , Kanazawa K. , Aizawa H.
Published: September 2007

The morphology and the growth mechanism of lithium borate crystals from the vitreous state have been studied for various compositions, X = B/(Li + B), from 0.62 to 0.75. Crystalline phases and morphology of grown crystals varied with the composition. Octahedral and/or spherical Li2B4O7 crystals are seen in the specimen with X = 0.62, 0.64, 0.67, and 0.68. The spherical crystal is composed of an octahedral core and fibrous crystals. The size of the core varied with the composition of the starting glass, X. The fibrous crystal growth is supposed to arise from the morphological instability due to the compositional variation caused by the solute pileup at the growth interface.
A new synthesis of β'-SiAION using the vapor phase technique reduction of kaolin
Authors: Seron A. , Béguin F. , Thébault J.
Published: September 2007

Silicon-aluminum oxynitrides and/or aluminum nitride were prepared by the reduction-nitridation of kaolin in graphite crucibles under hydrogen/nitrogen flow at temperatures in the range 1100 °C–1450 °C. Almost pure β'-SiAION was obtained in less than 24 h at 1200 °C. At high temperatures (e.g., 1450 °C) and for long reaction times (e.g., 10 h), β'-SiAION is fully reduced to AlN. In most preparations, β'-SiAION is formed together with small amounts of AlN. However, the formation of AlN can be limited by using short reaction times and/or by adjusting the reducing power of the atmosphere, i.e., the N2/H2 ratio. Compared to the carboreduction of aluminosilicates which always yields mixtures, the present method leads to almost pure products under appropriate conditions. The temperature for the formation of β'-SiAION is at least 200 to 400 °C lower than the temperatures used for the carboreduction of aluminosilicates or sintering of powders, respectively. A gaseous species formed by the reaction of hydrogen with graphite is suspected to be responsible for the nitriding reduction of kaolinite.
Preparation of a multilayer and a compositional gradient layer composite by the aerosol filtration method
Authors: Takao Yasumasa , Awano Masanobu , Kuwahara Yoshitaka , Murase Yoshio
Published: September 2007

A multilayer composite of an oxide superconductor and a compositional gradient layer composite of a ferrite chip component was prepared by the aerosol filtration method. Homogeneous, minute, and arbitrary control of the composition profile was possible for the multilayer composite of the Bi system superconductor. High controllability for designing the composition profile of the compositional gradient layer of nickel zinc ferrite was confirmed by EPMA analysis.
X-ray diffraction analysis for thin films of PbTiO3-La2/3TiO3 prepared by the sol-gel method
Authors: Yoshioka Hideki
Published: September 2007

Thin films in the system (1 - x) PbTiO3−xLa2/3TiO3 were prepared by the sol-gel and dip-coating methods. Phases deposited in the films and the lattice parameters as a function of the composition were investigated by the x-ray diffraction method. The solid solutions with a perovskite structure were formed as a single phase with x up to 0.9. For the composition of x = 1.0, metastable La-Ti-O perovskite phase with a small amount of the impurity phase, La2Ti2O7, was obtained. Simulation of x-ray diffraction patterns based on the defect structure model shows that the structure of the La-Ti-O perovskite phase includes randomly distributed cation vacancies at the A-site, namely (La2/3□1/3)TiO3.
Compositional dependence of infrared to blue and red conversion luminescence in oxyfluoride glass-ceramics co-doped with Tm3+ and Yb3+ ions
Authors: Denis J.P. , Özen G. , Wu Xu , Kermaoui A. , Pellé F. , Blanzat B.
Published: September 2007

The upconversion of infrared radiation into visible light has been studied in heavy metal oxyfluoride glass-ceramics co-doped with Yb3+ and Tm3+ ions at 300 K. The general composition of the compounds is 69.9PbF2 + 7.5WO3 + 7.5MO2 + 15YbF3 + 0.1TmF3 (M = Si, Ge, Zr, Te, and Th). Two main upconversion emissions were observed. They are centered at 477 and 775 nm, corresponding to the 1G4 → 3H6 and 3F4 → 3H6 transitions, respectively. Slopes of the emission intensity versus excitation power measurements indicate that the blue emission is due to three-photon absorption, while two-photon absorption processes are responsible for the red emission. A comparative method was used to measure the upconversion efficiencies under 16.5 mW/cm2 excitation power. Measurements were made after the compounds were annealed at 450 °C for 4 h. The best conversion efficiencies were obtained for the compound having silicon (Si). They are 28 × 10−6 for the blue and 18 × 10−2 for the red emission.
A pretreatment process for enhanced diamond nucleation on smooth silicon substrates coated with hard carbon films
Authors: Feng Z. , Komvopoulos K. , Brown I.G. , Bogy D.B.
Published: September 2007

Diamond nucleation on unscratched silicon substrates coated with thin films of hard carbon was investigated experimentally with a microwave plasma-assisted chemical vapor deposition system. A new pretreatment process was used to enhance the nucleation of diamond. Relatively high diamond nucleation densities of ∼108 cm−2 were achieved by pretreating the carbon-coated silicon substrates with a methane-rich hydrogen plasma at a relatively low temperature for an hour. Scanning electron microscopy and laser Raman spectroscopy studies revealed that diamond nucleation occurred from nanometer-sized spherical particles of amorphous carbon produced during the pretreatment. The nanoparticles possessed a structure different from that of the original hard carbon film, with a broad non-diamond Raman peak centered at ∼1500 cm−1, and a high etching resistance in pure hydrogen plasma. The high diamond nucleation density is attributed to the significant percentage of tetrahedrally bonded (sp3) atomic carbon configurations in the nanoparticles and the presence of sufficient high-surface free-energy sites on the pretreated surfaces.
Diamond growth on carbide surfaces using a selective etching technique
Authors: Grannen K.J. , Chang R.P.H.
Published: September 2007

Microwave plasma-enhanced chemical vapor deposition of diamond films on silicon carbide and tungsten carbide (with 6% cobalt) surfaces using fluorocarbon gases has been demonstrated. No diamond powder pretreatment is necessary to grow these films with a (100) faceted surface morphology. The diamond films are characterized by scanning electron microscopy and Raman spectroscopy. The proposed nucleation and growth mechanism involves etching of the noncarbon component of the carbide by atomic fluorine to expose surface carbon atoms and diamond nucleation and growth on these exposed carbon atoms. Hydrogen is necessary in the growth process to limit the rapid etching of the carbide substrates by corrosive fluorine atoms.
Donor doping of Ga in ZnO varistor grain boundary
Authors: Gupta Tapan K.
Published: September 2007

The effect of Ga doping on the grain boundary properties of a ZnO varistor has been presented in this note. Within the range of doping level studied, it is shown that Ga acts as a donor at the grain boundary and behaves similar to that of Al.1 The grain boundary doping behavior of both Ga and Al, when acting as donors, can therefore be represented by the same defect model.
Heteroepitaxial growth of chemically derived ex situ Ba2YCu3O7−x thin films
Authors: McIntyre Paul C. , Cima Michael J.
Published: September 2007

Heteroepitaxial growth of Ba2YCu3O7−x (BYC) thin films prepared by postdeposition annealing on (001) LaAlO3 was characterized by TEM and x-ray diffraction studies of specimens rapidly cooled from various points in the growth heat treatment. Heteroepitaxial nucleation of BYC occurred between 720 and 770 °C during heating at 25 °C/min to the annealing temperature of 830 °C. The c-axis normal BYC rapidly coalesced into a continuous film with nearly complete coverage of the substrate surface after growth of a film of several unit cells thickness. The experimental results were not consistent with purely solid phase heteroepitaxial nucleation and growth or epitaxial grain growth, mechanisms for microstructural evolution of other chemically derived epitaxial oxide thin films. The nature of the transformation and the microstructure of the final superconducting films were consistent, instead, with growth of epitaxial BYC from a liquid phase that is present transiently during the anneal. This hypothesis was supported by thermal analysis results obtained from the precursor material of which the films are composed prior to transformation to BYC.
An optimum heating program for fabricating MTG Bi1.6Pb0.4Sr2Ca2Cu3Oy superconductors
Authors: Du Z.L. , Yu T.F. , Chow J.C.L. , Fung P.C.W.
Published: September 2007

Following our recent report on the successful fabrication of “stationary” MTG Bi1.6Pb0.4Sr2Ca2Cu3Oy pellets in a simple tube furnace, we have tried many heating programs with various “crystal growth starting temperatures”, Ts, and sintering temperatures, Te; the samples are sintered for 20–40 h before quenching in air. It is found that Ts-Te should be rather narrow to produce good MTG samples, and the typical value of Jc is enhanced from 100 A cm−2 (for a polycrystalline sample) to about 3 × 103 A cm−2 for our good ones. SEM and TEM results indicate that the samples are full of packs which contain highly oriented mica-like layers.
X-ray photoelectron spectroscopy of uv laser irradiated sapphire and alumina
Authors: Pedraza A.J. , Park J.W. , Meyer H.M. , Braski D.N.
Published: September 2007

X-ray photoelectron spectroscopy (XPS) was performed in as-received, thermally annealed, and laser-irradiated sapphire and alumina specimens in order to study the effects of the different treatments on surface chemistry and properties. Laser irradiations with a 308 nm wavelength laser were performed in air and in a reducing atmosphere consisting of a mixture of Ar and 4% of hydrogen. The atomic percentages of carbon, aluminum, and oxygen were measured in all the specimens. Particular attention was paid to the percentages of oxygen in the oxide and in a hydroxyl state. The XPS analyses clearly established that a very thin film of metallic aluminum is formed on the surface of both alumina and sapphire substrates when they are irradiated under a reducing atmosphere. However, the film is discontinuous because it is electrically insulating. Substrates irradiated in air have metallic aluminum only for fluences below 0.4 J/cm2. The valence band photoemission spectra of as-received, annealed, and laser-irradiated specimens were measured. In irradiated specimens, the width of the valence band spectra was found to decrease by ∼10%. One possible cause of this decrease is the generation of point defects during laser irradiation. Electroless copper deposition occurs on sapphire and alumina substrates if their surface has been activated by laser irradiation. The time required for copper deposition was monitored by measuring the electrical resistivity in the irradiated area while the substrates were immersed in an electroless bath. The kinetics of deposition on laser-activated substrates and the XPS results show that the presence of metallic aluminum accelerates the deposition process. However, the presence of aluminum is not the sole reason for laser activation in alumina. Very strong metal-ceramic bonding is produced after thermal annealing of samples having preirradiated substrates. This result is explained in terms of the excess oxygen that is present at the ceramic surface after irradiation.
Preparation of barium hexa-aluminate aerogel
Authors: Mizushima Yasuyuki , Hori Makoto
Published: September 2007

Barium hexa-aluminate (BaO · 6Al2O3) aerogels were prepared using a supercritical drying method and their properties examined. A barium hexa-aluminate aerogel prepared from a double alkoxide of barium and aluminum showed a high specific surface area of 421 m2/g. Monolithic barium hexa-aluminate formed. No BaO · Al2O3 or alumina was observed, as is often the case in powder processing. The specific surface area of the monolithic barium hexa-aluminate fired at 1400 °C for 2 h was 12 m2/g, while that of the barium hexa-aluminate xerogel was only 0.8 m2/g. A barium hexa-aluminate aerogel was also prepared using barium by using a chelating agent. This aerogel showed a specific surface area of 454 m2/g as-dried. In the case of the chelating agent, BaO · Al2O3 was also detected along with barium hexa-aluminate after firing.
Temperature and compositional dependence of optical absorption edge in glasses containing PbO and TeO2
Authors: Li Guangming , Nogami Masayuki , Abe Yoshihiro
Published: September 2007

The optical absorption edge of the sol-gel derived borosilicate, silica, and phosphosilicate glasses containing heavy-metal oxides PbO and TeO2 exhibits a strong dependence on the temperature and composition. As the temperature is varied, a significant reversible thermochromic effect is observed in these glasses with a temperature coefficient as high as 10−3 eV/°C for most compositions, which is in general one magnitude of order greater than those of conventional semiconductor materials. The absorption edge of the glasses containing 10.24 wt. % PbTeO3 at room temperature was located at 3.76, 3.86, and 4.33 eV for the borosilicate, silica, and phosphosilicate glasses, respectively. With an increase in the heavy-metal concentration from 5.36 up to 18.62 wt. % in the borosilicate glasses, the absorption edge moved from 3.80 to 3.31 eV. On further increasing the concentration, the absorption edge no longer showed obvious red-shift. The observed optical characteristics were explained in view of the bond polarizability and structural coordination.
Carbothermal reduction and nitridation reaction of SiOx and preoxidized SiOx: Formation of α-Si3N4 fibers
Authors: Ramesh P.D. , Rao K.J.
Published: September 2007

The chemical composition of amorphous SiOx has been analyzed by oxidation studies and is found to be SiO1.7. SiO1.7 appears to be a monophasic amorphous material on the basis of 29Si nuclear magnetic resonance, high resolution electron microscopy, and comparative behavior of a physical mixture of Si and SiO2. Carbothermal reduction and nitridation reactions have been carried out on amorphous SiO1.7 and on amorphous SiO2 obtained from oxidation of SiO1.7. At 1623 K reactions of SiO1.7 lead exclusively to the formation of Si2N2O, while those of SiO2 lead exclusively to the formation of Si3N4. Formation of copious fibers of α-Si3N4 was observed in the latter reaction. It is suggested that the partial pressure of SiO in equilibrium with reduced SiO1.7 and SiO2 during the reaction is the crucial factor that determines the chemistry of the products. The differences in the structures of SiO2 and SiO1.7 have been considered to be the origin of the differences in the SiO partial pressures of the reduction products formed prior to nitridation. The effect of the ratios, C:SiO1.7 and C:SiO2, in the reaction mixture as well as the effect of the temperature on the course of the reactions have also been investigated.
Microstructures of GaN films deposited on (001) and (111) Si substrates using electron cyclotron resonance assisted-molecular beam epitaxy
Authors: Basu S.N. , Lei T. , Moustakas T.D.
Published: September 2007

The microstructures of GaN films, grown on (001) and (111) Si substrates by a two-step method using Electron Cyclotron Resonance assisted-Molecular Beam Epitaxy (ECR-MBE), were studied by electron microscopy techniques. Films grown on (001) Si had a predominantly zinc-blende structure. The GaN buffer layer, grown in the first deposition step, accommodated the 17% lattice mismatch between the film and substrate by a combination of misoriented domains and misfit dislocations. Beyond the buffer layer, the film consisted of highly oriented domains separated by inversion domain boundaries, with a substantial decrease in the defect density away from the interface. The majority of defects in the film were stacking faults, microtwins, and localized regions having the wurtzite structure. The structure of the GaN films grown on (111) Si was found to be primarily wurtzite, with a substantial fraction of twinned zinc-blende phase. Occasional wurtzite grains, misoriented by a 30°twist along the [0001] axis, were also observed. A substantial diffusion of Si was seen in films grown on both substrates.
Formation of the secondary phases in the Pb-containing perovskite films by pulsed laser deposition
Authors: Yeh M.H. , Liu K.S. , Lin I.N.
Published: September 2007

The growth behavior of Pb2+-containing ferroelectric thin films has been systematically examined. The kinetics of the formation of perovskite phase were successfully enhanced by using a material containing no Zr4+-ions, viz., Pb0.95La0.05Ti0.9875O3 (PLT) films, and by utilizing platinum coating on silicon substrate. Meanwhile, the formation of TiO2 phase (rutile) on PLT/Pt(Si) films has been observed and was ascribed to both the outward diffusion of Ti4+-ions from the Ti-layer underneath the Pt-coating and the loss of Pb2+-ions on the surface of the films. The perovskite materials, which were free of either pyrochlore, Zr-rich phase, or TiO2 phase, can be obtained by in situ depositing the PLT films at 450 °C substrate temperature and 1 mbar oxygen pressure. Thus obtained thin films possessed high dielectric constant, ∊r = 1346 and tan δ = 0.071 at 10 kHz, and large charge storage density, Qc = 5.4 μC/cm2 at 50 kV/cm.
Preferred orientation and microstructure of Ni-Zn-Cu ferrite thin films deposited by rf magnetron sputtering
Authors: Cho Hae Seok , Kim Min Hong , Kim Hyeong Joon
Published: September 2007

We have investigated the effects of process parameters such as rf power, substrate, and gas pressure PAr on preferred orientation, microstructure, and magnetic properties of Ni-Zn-Cu ferrite thin films deposited by conventional rf magnetron sputtering. The texture structure was developed in the ferrite films deposited on the SiO2/Si(100) substrate at low rf power conditions. The ferrite film on the Si(111) substrate always had (111) texture irrespective of process parameters due to lattice matching, but the texture of the ferrite film on SiO2/Si(100) changed from (111) to (100) and finally returned to (111) orientation again with decreasing PAr. Such behavior would occur presumably due to the characteristic atomic stacking sequence corresponding to a given condition of the ion bombardment. The ferrite films deposited at low PAr had a denser microstructure consisting of tightly packed columnar grains with a smoother surface, better adhesion to the substrate, and better crystallinity than those at high PAr. Hc‖ of ferrite film deposited at low PAr was larger than that at high PAr and also larger than Hc⊥ of that deposited at the same PAr because larger compressive stress was induced at low PAr than at high PAr.
Ultrasonic velocity and attenuation in a TaH0.51 hydride single crystal
Authors: Di Masso L. , Biscarini A. , Coluzzi B. , Mazzolai F.M.
Published: September 2007

Ultrasonic propagation of longitudinal waves (CL mode) has been investigated as a function of temperature in a single crystal of a TaH0.51 hydride. Stepwise changes of the elastic constant CL and of the attenuation A have been observed in the vicinity of the β ↔ ∊ and ∊ ↔ α phase transitions. These changes occur over narrow temperature ranges corresponding to regions of coexistence of two phases. A relatively small temperature dependent softening is displayed by CL on approaching Tβ→∊ from the low temperature side. At the transition temperatures no divergency has been observed in the attenuation, which appears to originate from domain boundary motions, rather than from the transitions themselves. A determination of the H diffusion coefficient by a permeation method at high temperature supports a view that adiabatic tunneling is an effective mechanism even at temperatures as high as 1173 K.
Electron diffraction from cylindrical nanotubes
Authors: Qin L.C.
Published: September 2007

Electron diffraction intensities from cylindrical objects can be conveniently analyzed using Bessel functions. Analytic formulas and geometry of the diffraction patterns from cylindrical carbon nanotubes are presented in general forms in terms of structural parameters, such as the pitch angle and the radius of a tubule. As an example the Fraunhofer diffraction pattern from a graphitic tubule of structure [18,2] has been simulated to illustrate the characteristics of such diffraction patterns. The validity of the projection approximation is also discussed.
Low temperature mullite formation from sol-gel precursors by hot pressing
Authors: Pradhan S.K. , Chatterjee A. , Datta A. , De M. , Chakravorty D.
Published: September 2007

The mullite phase has been synthesized by subjecting a suitably chosen precursor gel to different pressure and temperature conditions. At ambient pressure mullite phase grows at a temperature of 1400 °C when the gel is heated for 2 h. At a pressure of 24 bar, on the other hand, similar growth occurs at a temperature of 750 °C for 5 min.
Synthesis, processing behavior, and characterization of bismuth superconductors using freeze dried nitrate precursors
Authors: Coppa N.V. , Hults W.L. , Smith J.L. , Brynestad J.
Published: September 2007

The synthesis of Bi2−xPbxSr2Ca2Cu3O10 (Bi2223) powders from a freeze dried nitrate precursor is reported here. We examine the composition and morphology of the precursor material, describe the chemistry and kinetics of product formation, and evaluate the phase composition and superconducting properties of the products. A nitrate solution containing the appropriate ratio of cations was rapidly frozen and then freeze dried at low temperatures to form an atomic mixture of the component salts. The thermal processing of the freeze dried material consisted of three steps: (i) dehydration, (ii) denitration, and (iii) solid state reaction to form the Bi2223 superconducting product. Calcium-substituted bismuthates and strontium-substituted calcium cuprate, not Bi2201, are the intermediates between the nitrates and the superconducting products. These highly disordered phases rapidly transform into Bi2212 or Bi2223 at higher temperatures (≍790 °C). The kinetics of product formation was studied using XRD analysis and magnetic susceptibility. The kinetics were shown to follow the nucleation and growth mechanism. Bi2223 formed after only 30 min at a few degrees below the melting point, and after 37 h Bi2223 products exhibited excellent phase composition and magnetic susceptibility characteristics.
Calculation of dielectric polarizabilities of perovskite substrate materials for high-temperature superconductors
Authors: Fratello V.J. , Brandle C.D.
Published: September 2007

Dielectric polarizabilities for most of the ions in known perovskites scale with the ionic volume and the valence. These ionic dielectric polarizabilities and the ion additivity rule have been used to calculate molecular dielectric polarizabilities for perovskite substrate materials used for high-temperature superconductors. Using the ion additivity rule to predict possible low permittivity compositions seems to suggest that the constraints of the perovskite structure and stoichiometry, lattice match to high-temperature superconductors, and congruent melting required for bulk growth limit the compositions to ones unlikely to be superior to the currently available materials. The most limiting factor on the relative permittivity of the perovskites is probably the close-packed nature and lack of voids in the structure. However, in nonferroelectric perovskites, the polarizabilities derived from relative permittivity data using the Clausius-Mossotti relation are significantly less than the calculated values, with deviations that correlate with degree of cation compression. Use of cation compression to reduce the polarizability shows some promise for improving dielectric constants.
Investigations of the chemistry and bonding at niobiumsapphire interfaces
Authors: Bruley J. , Brydson R. , Müllejans H. , Mayer J. , Gutekunst G. , Mader W. , Knauss D. , Rühle M.
Published: September 2007

Spatially resolved electron energy-loss data have been recorded at the interface between niobium and sapphire (α-Al2O3), a model metal/ceramic couple. The spatial-difference technique is used to extract interface specific components of the energy-loss near-edge structure (ELNES), which are dependent on the chemistry and bonding across the interface. Multiple scattering calculations of aluminum, oxygen, and niobium clusters were performed to simulate the measured Al L2,3 ELNES. Two samples fabricated by different techniques were examined. The first interface was made by diffusion bonding pure crystals. Its interface spectrum is identified with tetrahedral coordination of the Al ions at the interface. The calculations match the experimental edge structures, supporting the notion of aluminum to niobium metal bonding and concurring with a structural model in which the basal plane of sapphire at the interface is terminated by a full monolayer (i.e., 67% excess) of aluminum. The second sample was produced by molecular beam epitaxy. The spectrum of this interface is consistent with an atomistic structure in which the interfacial basal plane of sapphire is terminated by oxygen. An unoccupied band of states within the band gap of Al2O3 is observed, signifying chemical bonding between metal and ceramic.
Electrochemical codeposition of indium and antimony from a chloroindate molten salt
Authors: Carpenter Michael K. , Verbrugge Mark W.
Published: September 2007

The electrochemical codeposition of In and Sb from a novel low temperature molten salt electrolyte is reported. The melt, which consists of InCl3and 1-methyl-3-ethylimidazolium chloride, allows the codeposition to be accomplished at 45 °C. XPS shows that InSb can be deposited from this system. Electrochemical experiments are provided along with an interpretation that draws on the importance of In(I) species in the melt.
A micromechanistic model of the combustion synthesis process: Part I. Theoretical development
Authors: Zhang Yangsheng , Stangle Gregory C.
Published: September 2007

A theoretical model of the combustion synthesis process has been developed. In particular, a set of nonlinear and interrelated partial differential equations is given that accounts for all of the relevant physical and chemical processes that occur during the combustion synthesis process. The appropriate conservation equations for thermal energy, mass, and momentum densities are correctly described—for each phase at each point in the sample—at all times during the process. In addition, details of the necessary interphase transfer terms are expressed in a number of constitutive relationships, in which the dependence of an independent variable upon its dependent variable(s) is given explicitly. In doing so, microstructural details are accounted for, derived primarily from percolation concepts as applied to disordered porous media. All assumptions that are incorporated into the theoretical model have been tabulated in detail. This theoretical model establishes an approach to the development of a sound, quantitative, and fundamental understanding of the combustion synthesis process, particularly with respect to the processing-microstructure-properties relationship. It also provides a point of departure for conducting detailed, quantitative computer experiments of the combustion synthesis process.
A micromechanistic model of the combustion synthesis process: Part II. Numerical simulation
Authors: Zhang Yangsheng , Stangle Gregory C.
Published: September 2007

A series of computer experiments was conducted for the self-propagating combustion synthesis process in the Nb-C system, based on the general theoretical model that was developed previously.1A detailed and quantitative description was given for the various physical and chemical processes that take place during the combustion synthesis process. The results are presented at various length scales in order to provide an insight into understanding the mechanisms that are responsible for the self-propagating behavior. It was shown that a fundamental understanding and precise control of the process require a strong emphasis on the joint contributions of theratesof the various mass and energy redistribution processes that occur during the combustion synthesis process. A proper balance of each of the elementary process rates must be achieved to give rise to self-propagating behavior. This paper illustrates some of the capabilities of the general theoretical model in quantitatively describing the self-propagating combustion synthesis process.
Combustion synthesis of MoSi2-Mo5Si3 composites
Authors: Subrahmanyam J.
Published: September 2007

Thermochemical calculations were carried out for the Self-propagating High-temperature Synthesis (SHS) of MoSi2, Mo5Si3, and their composites. Adiabatic temperatures and the amounts of liquid phases formed at the adiabatic temperature were calculated for different initial temperatures. These materials were prepared from elemental powders of Mo and Si by the thermal explosion mode of SHS. The products were characterized by x-ray diffraction, scanning electron microscopy, and microhardness. The larger particle morphology of MoSi2 and the MoSi2-Mo5Si3 composite and the finer morphology of Mo5Si3 can be explained in terms of the presence of transient liquid phase at the adiabatic temperature. The morphology of fracture surface of MoSi2 shows intergranular fracture, while Mo5Si3 and the MoSi2-Mo5Si3 composite show transgranular cleavage fracture.
Synthesis and properties of strontium-doped yttrium manganite
Authors: Fu Bo , Huebner Wayne , Trubelja Mladen F. , Stubican Vladimir S.
Published: September 2007

The system Y1−xSrxMnO3 (x = 0.000, 0.005, 0.010, 0.050, and 0.100) was studied as a potential cathode material for solid oxide fuel cells. Powders were prepared using an organometallic precursor; however, achieving homogeneous compositions was complicated due to the presence of intermediate, metastable phases. The desired hexagonal Y1−xSrxMnO3 phase formed from the precursor at 800 °C, while small amounts of a metastable orthorhombic (Y,Sr)MnO3 phase formed in the temperature range between 850°and 960 °C, and another orthorhombic YMn2O5 phase between 840°and 1200 °C. The metastable (Y, Sr)MnO3 phase readily transformed into the stable hexagonal phase at about 960 °C. The other metastable intermediate phase, YMn2O5, was formed as a decomposition product of a portion of the major hexagonal YMnO3 at 840 °C, and subsequently reacted with Y2O3 back to the hexagonal YMnO3 at 1200 °C. For the studied compositions, densities higher than 95% theoretical could be obtained by sintering in air at temperatures above 1400 °C for 2 h. The investigated system was comparable in electrical conductivity with the current cathode material La1−xSrxMnO3, and had an average apparent thermal expansion coefficient between 5 and 7 ppm/°C in the temperature range between 200°and 1000 °C. Unfortunately microcracking was observed in all sintered specimens, possibly caused by a high-temperature phase transition between the hexagonal and cubic polymorphs of Y1−xSrxMnO3. The microcracking presents a major obstacle to the use of this material as a cathode in solid oxide fuel cells.
Processing of fullerene-reinforced composites
Authors: Barrera E.V. , Sims J. , Callahan D.L. , Provenzano V. , Milliken J. , Holtz R.L.
Published: September 2007

This work has been motivated by the current interest in using fullerenes as a possible reinforcement in structural materials. The fullerenes (of which C60 is the most common) are nanometer in size and have been observed to be stable in contact with various metal systems. Therefore, they offer a nanosize reinforcement that is lightweight and hollow. In this research the emphasis was on processing metals with fullerene additions where the fullerenes were dispersed throughout the metal. Various processing approaches were employed to produce nanocrystalline materials, thin films, and powder-processed composites. Indications are that fullerenes remained unaltered with each processing approach in matrices of tin, copper, and aluminum. A key aspect of the processing of metals containing dispersed fullerenes was the use of fullerene sublimation. Along with the various processing methods identified, the methods of characterizing the fullerenes in the metals were also identified.
Properties of Al2O3 films prepared by argon ion assisted deposition
Authors: Al-Robaee Mansour S. , Krishna Ghanashyam M. , Subanna G.N. , Rao Narasimha K. , Mohan S.
Published: September 2007

Aluminum oxide films have been prepared by ion assisted deposition using argon ions with energy in the range 300 to 1000 eV and current density in the range 50 to 220 μA/cm2. The influence of ion energy and current density on the optical and structural properties has been investigated. The refractive index, packing density, and extinction coefficient are found to be very sensitive to the ion beam parameters and substrate temperatures. The as-deposited films were found to be amorphous and could be transformed into crystalline phase on annealing. However, the crystalline phases were different in films prepared at ambient and elevated substrate temperatures.
Nucleation and initial growth of diamond film on Si substrate
Authors: Jiang N. , Sun B.W. , Zhang Z. , Lin Z.
Published: September 2007

A high resolution electron microscopic (HREM) study of interface structure between diamond film and its silicon substrate is presented. The HREM images reveal that there is an amorphous intermediate layer between the diamond film and its substrate for samples grown by hot filament chemical vapor deposition (HF-CVD). In some cases, β-SiC crystallites and a few graphite microcrystallites may be embedded in this amorphous layer. The HREM images obtained from cross-sectional specimens reveal that the diamond crystallites nucleate directly either on the amorphous intermediate layer, at diamond seed crystallites that were left during pretreatment of Si substrate by diamond paste,β-SiC particles, or at some scratches of the Si substrate. HREM images also reveal that the quantity, distribution, and the size of β-SiC particles in the intermediate layer are different for different processes. Some β-SiC crystallites have certain orientation relationships with the Si substrate. A HREM study of cross-sectional specimens indicates that twins and microtwins in the HF-CVD diamond film are formed during nucleation of the film either from diamond seeds, β-SiC crystallites, or the amorphous intermediate layer. Multiple twins formed from different β-SiC crystallites have also been observed. High densities of “V” shaped microtwins formed during the initial growth of the diamonds and the formation mechanism of these twins are discussed.
Electron spectroscopy on the charge transfer complex [(7-amino-2,4 dimethyl-1,8 naphthyridine) (TCNQ)] (TCNQ = 7,7',8,8'-tetracyanoquinodimethane)
Authors: Arena A. , Lo Schiavo S. , Mezzasalma A.M. , Patané S. , Piraino P. , Saitta G.
Published: September 2007

Reflection electron energy loss spectroscopy (REELS) and optical reflectivity measurements have been used to characterize in the valence energy region the dielectric behavior of 7-amino-2,4 dimethyl-1,8 naphthyridine and of the charge transfer complex formed by this compound and the 7,7',8,8'-tetracyanoquinodimethane (TCNQ). The effects of the charge transfer interaction on the core energy levels have been investigated by comparing the line shape of the carbon and nitrogen Is core photoelectron peaks of the electron-donor and electron-acceptor molecules and of the charge transfer complex formed by them.
YBCO film growth on ultrathin Ag layers
Authors: Zhong C. , Ruggiero S.T. , Fletcher R. , Moser E.
Published: September 2007

We discuss our results on the growth of YBCO thin films on ultrathin (1-10 nm) Ag underlayers. Substrates were LaAlO3. YBCO was sputter deposited and Ag thermally evaporated. It was observed that Tc remained relatively unaffected by the Ag underlayers, ranging from 86-88 K. Critical currents were found to be consistent with YBCO grown on bulk Ag when the Ag underlayer film reached complete coverage (∼9 nm). Films grown on Ag showed a marked tendency for microcrystalline growth on the basis of atomic-force microscopy (AFM) results.
Application of the metallurgical route for synthesis of 110 K Bi-Pb(Sb)-Sr-Ca-Cu-O superconducting oxides
Authors: Konstantinov K. , Michel D. , Ochin P.
Published: September 2007

Bulk nearly single phase Bi-2223 materials doped with Pb and Sb were prepared using the metallurgical route. Li-doped samples were also synthesized, but Li leads to the 2212 phase and prevents the formation of the 2223 phase. The dependence of the sintering conditions on the phases obtained was studied and the influence of different factors on the melting temperature and the temperature required for the formation of the 2223 phase is discussed. For the materials with nominal composition Bi1.5Pb0.3Sb0.1Sr2Ca2Cu3Sy, Tc(zero) = 111 K was achieved. It was found that the Sb-rich phase segregates on the surface of the superconducting grains.