bluuebunny/crossref_metadata_2025 huggingface dataset:
Periodic fluctuation of Ba/Nd ratio in single crystals of high-Jc NdBa2Cu3O7−δ superconductor
Authors: Hirayama Tsukasa , Ikuhara Yuichi , Nakamura Masaru , Yamada Yasuji , Shiohara Yuh
Published: March 2008

A single crystal of NdBa2Cu3O7−δ was synthesized by the Top Seeded Solution Growth method (TSSG method). Then, the crystal was heat-treated 500 °C for 400 h and then 340 °C for 200 h in a pure oxygen gas flow. This sample showed critical current density (Jc) as high as 15,000 A/cm2 under a magnetic field, of 5 T, applied along the c-axis of the crystal. Electron microscopic studies with energy dispersive x-ray spectroscopy (EDS) have revealed that the Ba/Nd ratio fluctuates between 2.0 and 0.7 with a wavelength of a few tens of nanometer. This implies that superconducting phase of Nd1.0Ba2.0Cu3O7−δ and nonsuperconducting phase of Nd1.8Ba1.2Cu3O7−δ mingle with each other, which is the ideal structure for high-Jc superconducting material.
Trapping levels in hydrothermal and solution grown bismuth titanium oxide
Authors: Davies D. Eirug , Harris M. T.
Published: March 2008

Deep trapping levels in Bi12TiO20 obtained by top seeded solution growth and by the hydrothermal technique have been compared. This was undertaken as such levels directly influence the photorefractive behavior of the material. It is found that the most predominant of the peaks revealed by thermally stimulated conductivity measurements represents two rather than a single defect level and that the deeper of the two becomes more significant in hydrothermally grown material. One defect found in the solution pulled material is notably absent from that produced hydrothermally. The consequence of adding phosphorus doping and the manner in which it affects the deep levels has also been examined.
Anharmonicity in metals from the universal energy equation
Authors: Girifalco L. A. , Kniaz K.
Published: March 2008

A theoretical computation of vibrational anharmonicity is presented which is a generalization of the simple Gruneisen approach. The calculation was based on a model that defines a simple relationship between the binding energy of a solid and the variation of vibration frequencies with volume. The agreement between calculated and experimental Gruneisen constants is good.
Metastable liquid phase separation in undercooled molten Pd40.5Ni40.5P19
Authors: Yuen C. W. , Lee K. L. , Kui H. W.
Published: March 2008

It was demonstrated that molten Pd40.5Ni40.5P19 undergoes liquid state phase separation in the undercooling regime δT = T1 − T where T1 is the liquidus of Pd40.5Ni40.5P19 and T is the kinetic crystallization temperature. Liquid state phase separation by nucleation and growth takes place for δT ≤ 60 K while that by spinodal decomposition occurs for δT ≥ 100 K. Microstructural analysis of the undercooled specimen obtained in the undercooling regime of 60 ≤ δT ≤ 100 K indicates that it is the transition regime. Finally, it was found that when undercooled molten Pd40.5Ni40.5P19 undergoes liquid state spinodal decomposition, it first decomposes into two liquid networks, which is finally replaced by a system of three liquid networks.
Metallization schemes for dielectric thin film capacitors
Authors: Al-Shareef H. N. , Dimos D. , Tuttle B. A. , Raymond M. V.
Published: March 2008

A detailed analysis of Pt/Ti, Pt/TiO2, and Pt/ZrO2 electrodes was carried out to develop a bottom electrode stack for sol-gel derived thin film capacitors. For the Pt/Ti stack, the choice of layer thickness and deposition temperature is found to affect adhesion to the SiO2/Si substrate as well as the extent of hillock formation and Pt–Ti interaction. By using elevated temperature deposition, Pt films close to 1 μm in thickness can be produced with relatively good adhesion and morphological stability using Ti adhesion layers. In addition, Pt films grown on ZrO2 and TiO2 adhesion layers exhibit little morphological change and no degradation in sheet resistance after annealing at 650 °C. However, neither ZrO2 nor TiO2 are as effective as Ti metal in promoting Pt adhesion. Experiments aimed at establishing a correlation between hillock formation and capacitor yield revealed two important results. First, the behavior of Pt/Ti stacks during annealing in air is markedly different from their behavior during PZT film crystallization. Second, preannealing of the Pt/Ti in air prior to PZT film growth actually improves capacitor yield, even though hillock formation occurs during the preannealing treatment. Implications of these results regarding the role of hillocks in controlling capacitor yield are discussed.
Microstructure of nitrate polycrystals solidified under ultrasonic vibration
Authors: Enomoto Naoya , Iimura Yasushi , Nakagawa Zenbe-e
Published: March 2008

Molten nitrates in the system (1 −x)NaNO3−xBa(NO3)2were solidified in the presence of a power ultrasound of 20 kHz. Their microstructures were compared with those of controlled samples which were solidified normally. Grain size in the controlled sample of monolithic NaNO3(x= 0) was reduced by sonication. In the hypo- (x= 8 wt. %) and the hypereutectic (x= 28 wt. %) binary samples, the sonication completely eliminated the dendritic structure of the primary crystals and induced equiaxed particles of the primary phase. At eutectic (x= 18 wt. %), the sonication removed oriented structures of the eutectic lamellae. Several mechanisms of the microstructural modification were mentioned.
Crystal structure and defects of Zr4Co4Si7 (V-phase) investigated by high resolution transmission electron microscope
Authors: Mao J. F. , Ye H. Q. , Ning X. G. , He L. L. , Yang D. Z.
Published: March 2008

The results of high resolution transmission electron microscope (HRTEM) observation and image simulation show that Zr4Co4Si7 possesses the same structure type of Zr4Co4Ge7. Addition of Fe or Ni into the Zr4Co4Si7 compound, except that the dimensions changed slightly, does not change the lattice type and coordination in the crystal structure, maintaining the V-phase structure. Also, twins with coherent boundaries and partially coherent interfaces are observed. The image conditions of Zr4Co4Si7 and the structure differences between Zr4Co4Si7 and tetrahedral close-packed phases are also discussed.
Effect of calcium modification on the microstructure and oxidation property of submicron spherical palladium powders
Authors: Che Shenglei , Sakurai Osamu , Funakubo Hiroshi , Shinozaki Kazuo , Mizutani Nobuyasu
Published: March 2008

Ca-modified spherical palladium particles were prepared from the mixed solution of Pd(NO3)2 and Ca(NO3)2 by ultrasonic spray pyrolysis. Pure palladium powder and that modified with less than 55 ppm Ca were composed of single crystal particles. However, Ca addition of more than 500 ppm resulted in polycrystalline particles. Crystallite size of the particles decreased with the increase of Ca addition and changed dramatically at the addition of some hundred ppm. Ca additive did not form solid solution with palladium but formed CaPd3O4 on the surface and grain boundary of the particles. 50 ppm−1% of Ca addition significantly reduced the oxidation of palladium powder. More addition of Ca resulted in excess oxidation due to the reaction between palladium and calcium oxide.
Synthesis of pure amorphous Fe2O3
Authors: Cao X. , Prozorov R. , Koltypin Yu. , Kataby G. , Felner I. , Gedanken A.
Published: March 2008

A method for the preparation of pure amorphous Fe2O3 powder with particle size of 25 nm is reported in this article. Pure amorphous Fe2O3 can be simply synthesized by the sonication of neat Fe(CO)5 or its solution in decalin under an air atmosphere. The Fe2O3 nanoparticles are converted to crystalline Fe3O4 nanoparticles when heated to 420 °C under vacuum or when heated to the same temperature under a nitrogen atmosphere. The crystalline Fe3O4 nanoparticles were characterized by x-ray diffraction and M¨ossbauer spectroscopy. The Fe2O3 amorphous nanoparticles were examined by Transmission Electron Micrography (TEM), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and Quantum Design SQUID magnetization measurements. The magnetization of pure amorphous Fe2O3 at room temperature is very low (<1.5 emu/g) and it crystallizes at 268 °C.
Auger valence electron spectra in Ca-silicides
Authors: Abe S. , Nakayama H. , Nishino T. , Iida S.
Published: March 2008

CaSi2 and CaSi have been investigated by Auger Valence Electron Spectroscopy (AVES). Some drastic differences of the Auger peak due to 3s states in the Si atoms were observed in the Si[2s, 2p, V] Auger spectra. The peak that arised from valence electron states in the Ca atoms was observed in the Ca[2p, 3p, V] Auger spectra for both Ca-silicides. This result suggests that the Ca–Si bonds are partially ionic. However, the number of the valence electrons in Ca atoms for CaSi was larger than that for CaSi2. This result implies that the part of homopolar bonds between the Si and Ca atoms in CaSi is stronger than that in CaSi2. Based on these results, it has been concluded that the change of the Si [2s, 2p,V] Auger spectra is associated with the difference of the part of homopolar bonds between the Si and Ca atoms.
Selective chemical etching of hexagonal boron nitride compared to cubic boron nitride
Authors: Harris Stephen J. , Weiner Anita M. , Doll Gary L. , Meng Wen-Jin
Published: March 2008

A BN film containing comparable amounts of sp2 and sp3 phases was subjected to a gas-phase chemical etch in a hot-filament environment containing 1% CH4 in H2. After a partial etch, examination by FTIR shows that the sp2 was preferentially etched, leaving a larger sp3 fraction than in the unetched film. The possibility that preferential etching could be used to increase the purity of cBN films is discussed.
Particle aggregation in alumina aerogels
Authors: Keysar S. , De Hazan Y. , Cohen Y. , Aboud T. , Grader G. S.
Published: March 2008

Alumina aerogels were synthesized by low temperature CO2 supercritical drying (SCD) of gels via the Yoldas process. The aerogels have a surface area of ˜425 m2/gr, similar to that obtained under high pressure/temperature SCD. The surface area and the cluster size of the aerogels are strongly influenced by the amount of acid during gelation. Gels and aerogels were studied by small angle x-rays scattering (SAXS), and the data were analyzed using the Fisher–Burford equation. The SAXS results along with TEM observations support the existence of a hierarchical aggregation at the gelation stage, having a mass fractal dimension of Dm = 2.6−2.8. During the SCD the morphology collapses to form a structure with surface fractal dimension Ds = 2.6−2.9.
High temperature x-ray and calorimetric studies of phase transformations in quasicrystalline Ti–Zr–Ni alloys
Authors: Stroud R. M. , Kelton K. F. , Misture S. T.
Published: March 2008

We present the first high temperature x-ray diffraction (HTXRD) studies of in situ quasicrystal-crystal and crystal-crystal transformations in Ti–Zr–Ni alloys. Together with differential scanning calorimetry studies, these x-ray measurements indicate three separate paths for the Ti–Zr–Ni quasicrystal-crystal transformation: single exothermic, single endothermic, or multiple endothermic. The mode of transformation depends on the alloy composition and the level of environmental oxygen. The crystalline products include the Ti2Ni, MgZn2 Laves, α−(Ti, Zr), and β−(Ti, Zr) phases. In the absence of oxygen, the endothermic transformation of the quasicrystal demonstrates that it is the lowest free energy (stable) phase at the Ti53Zr27Ni20 composition. Oxygen stabilizes the Ti2Ni phase, eliminating both the quasicrystal and the MgZn2 Laves phase, at partial pressures as low as a few hundred ppm.
Processing and characterization of compositionally modified PbTiO3 thin films prepared by pulsed laser deposition
Authors: Lee B. W. , Cook L. P. , Schenck P. K. , Wong-Ng W. , Chiang C. K. , Brody P. S. , Bennett K. W.
Published: March 2008

Modified lead titanate of 0.9PbTiO3 −0.1Pb(Mg0.5 W0.5)O3 thin films have been deposited onto Pt-coated Si substrates by pulsed laser deposition. Films were crystallized in situ during deposition or by post-depositional heat treatment (post-annealing). Compositional and structural characterization showed that the phase formation and microstructure of the films were highly sensitive to deposition conditions. Perovskite single phase films were formed in situ at 650 °C, PO2 = 40 Pa as well as by post-annealing amorphous films at 650 °C. In the post-annealing process, the amorphous as-deposited phase was crystallized to perovskite and/or pyrochlore, and the ratio of perovskite to pyrochlore was found to be influenced by the depositional PO2. Depending on the deposition temperature, the grain structures of the crystallized films were columnar or equiaxed. A relatively homogeneous surface morphology was obtained by deposition at a lower pressure (PO2 = 13 Pa). The in situ crystallized films showed variable crystallographic orientation. The more (111) oriented films had the lowest remanent polarizations and the highest coercive fields. A method for preparing randomly oriented films, via a two-step deposition process with intermediate annealing, is believed to give the most consistent results and the best ferroelectric properties at the present level of development.
Dielectric properties of barium titanium niobates
Authors: Roberts G. L. , Cava R. J. , Peck W. F. , Krajewski J. J.
Published: March 2008

The results of measurements of dielectric constants, in the vicinity of ambient temperature, are presented for eight barium titanium niobium oxides (BaTi1+2nNb4O13+4n for n = 0, 1, 2, 3, 4; Ba3Ti4Nb4O21, Ba3Ti5Nb6O28, and Ba6Ti2Nb8O30) in polycrystalline ceramic form. The dielectric constants are in the range of 30 to 70. The results of dielectric measurements on solid solutions obtained by partial substitution of Ta for Nb are also reported. These substitutions do not dramatically increase the dielectric constants. One material, Ta-substituted Ba3Ti5Nb6O28, has a very low temperature coefficient of dielectric constant at K ≈ 45.
Influence of texture on the switching behavior of Pb(Zr0.70Ti0.30)O3 sol-gel derived thin films
Authors: Brooks Keith G. , Klissurska Radosveta D. , Moeckli Pedro , Setter N.
Published: March 2008

Rhombohedral Pb(Zr0.70Ti0.30)O3 thin films of four different well-defined textures, namely, (100), (111), bimodal (110)/(111), and (100)/(111), were prepared by a sol-gel method. The films were characterized in terms of grain size, presence of second phases, surface roughness, columnarity of grains, and other microstructural features. The dielectric, ferroelectric, and fatigue properties were investigated, with emphasis on the hysteresis switching characteristics. Results are discussed from the reference point of the allowable spontaneous polarization directions available for the different textures. The values of coercive field, remanent and saturation polarization, and slope of the loop at the coercive field, at saturating fields can be qualitatively explained based on the texture, independent of microstructural differences. The occurrence of surface pyrochlore, however, is observed to affect the functionality of the saturation curves, particularly for the samples of bimodal texture. Shearing of the hysteresis curves of the bimodal films is also attributed to surface microstructural features. The occurrence of nonswitching 71° or 109° domains in the (111) and (110)/(111) textured films is hypothesized based on a comparison with the data from the (100) textured film. Corrected saturation polarization values agree with the spontaneous polarization values of rhombohedral PZT single crystals and published calculated values for rhombohedral PZT ceramics. The fatigue characteristics show increases in the switching component of polarization in the range 103−107 bipolar cycles, particularly for the (111) textured sample. Onset of fatigue is observed for all samples between 107 and 108 switching cycles.
Gas-phase coating of TiO2 with SiO2 in a continuous flow hot-wall aerosol reactor
Authors: Powell Quint H. , Fotou George P. , Kodas Toivo T. , Anderson Bruce M. , Guo Yongxiang
Published: March 2008

The feasibility of in situ coating of titania particles with silica using a high-temperature, gas-phase process was demonstrated. Titania was produced from the reaction of TiCl4 and O2 in a hot-wall, tubular, aerosol reactor and directly coated in the gas phase via the reaction of O2 with SiCl4 vapor. Rough SiO2 coatings were obtained at 1300 °C while uniform, dense coatings were obtained at all conditions examined for 1500 °C. The presence of water in the reactor significantly influenced the morphology of the coatings and resulted in smooth, dense, and uniform coatings at 1300 °C. Coating thicknesses could be controlled from 5 nm to roughly 100 nm, corresponding to growth rates on the order of 10–100 nmys. The characteristics of the coatings depended upon the concentration of SiCl4 and the coating temperature. These process variables influenced the coating mechanism, growth rate, and densification which directly influenced the coating uniformity and thickness.
Synthesis, structure, and gas sensitivity properties of SnO2–CuO mixture phase obtained by pyrolysis of an aerosol
Authors: Román J. , Fabian J. C. , Labeau M. , Delabouglise G. , Vallet-Regí M.
Published: March 2008

SnO2–CuO mixture phase has been prepared by pyrolysis of an aerosol produced by ultrahigh frequency of different precursor solutions. As-received samples were annealed in different conditions to study the influence of the temperature on the microstructure. Scanning electron microscopy showed that the samples consisted of hollow spherical particles and rings, depending on the precursor solution utilized. The evolution of conductance under both pure and polluted air is discussed.
Crystallographic structure of S′ precipitates in Al–Li–Cu–Mg alloys
Authors: Pérez-Landazábal J. I. , Nó M. L. , Madariaga G. , Juan J. San
Published: March 2008

The crystallographic structures of some phases present as precipitates in Al–Li–Cu–Mg alloys are not perfectly resolved. In particular, several structural models have been proposed for the S′ phase, and this controversy is not yet resolved. New x-ray powder diffraction measurements have been performed to compare the theoretical powder diffraction patterns associated with each proposed model with the measured spectra obtained from two different kinds of alloys. The structural model of the S′ phase described by Mondolfo [L. F. Mondolfo, Aluminum Alloys Structure and Properties (Butterworths, London, 1976)] agrees with the experimental results and allows us to reject the two other proposed crystallographic models.
Coexistence of 1 : 2 and 1 : 1 long-range ordering types in La-modified Ba(Mg0.33Ta0.67)O3 ceramics
Authors: Youn Hyuk-Joon , Hong Kug Sun , Kim Hwan
Published: March 2008

The ordering structure of lanthanum-substituted Ba(Mg0.33Ta0.67)O3 was investigated using x-ray diffraction. It was observed that the ordering type of this solid solution transformed from 1: 2 to 1: 1 with 10 mol% substitution of La(Mg0.67Ta0.33)O3. These two types of ordering coexist over a wide range of composition. This coexistence of ordering types was considered as phase coexistence and its physical origin was investigated using microwave measurements. Based on the intensity of superlattice reflections and the discontinuity in dielectric properties, it was suggested that the coexistence of two ordering types originates from the overlap of the two stability regions.
Preparation of TiO2-based powders with high photocatalytic activities
Authors: Kasuga Tomoko , Hiramatsu Masayoshi , Hirano Masayoshi , Hoson Akihiko , Oyamada Kyoko
Published: March 2008

TiO2-based powders doped with a small amount of SiO2 were prepared by a sol-gel method and subsequently were heated to precipitate fine anatase crystals. Although the obtained powders have large specific surface areas (≈200 m2 · g−1), they showed poorer activity in a photocatalytic property than the undoped TiO2 powders which have the area of 50 m2 · g−1. The SiO2-doped TiO2 powders were treated chemically with aqueous NaOH. Infrared reflection spectra showed that the treatment reduced the amount of SiO2 in the powders. The photocatalytic property of the powders was extremely improved by the treatment, and the powders showed higher activity than the undoped TiO2 powders.
Evidence of critical scaling behavior during vapor phase synthesis of continuous filament composites
Authors: Kinney J. H. , Haupt D. L.
Published: March 2008

We present experimental measurements of the accessible pore fraction in ceramic matrix composites during consolidation by vapor phase infiltration. For two topologically distinct filament architectures, the accessible pore fraction decreased during consolidation with a power law decay and a critical scaling exponent of 0.41 (R2= 0.97). A three-dimensional analysis of the percolating pores revealed that the structures became topologically equivalent and simply connected near the critical density.
Carbon nanotubes grown in situ by a novel catalytic method
Authors: Peigney A. , Laurent Ch. , Dobigeon F. , Rousset A.
Published: March 2008

Carbon nanotubes can be produced by the catalytic decomposition of hydrocarbons on small metal particles. However, nanotubes are generally produced together with non-tubular filaments and tubes coated by pyrolytic carbon. We propose a novel catalyst method for the in situ production, in a composite powder, of a huge amount of single- and multiwalled carbon nanotubes, having a diameter between 1.5 and 15 nm and arranged in bundles up to 100 μm long. We anticipate that dense materials prepared from such composite powders could have interesting mechanical and physical properties.
Thermochemistry of combustion reaction in Al–Ti–C system during mechanical alloying
Authors: Ye L. L. , Liu Z. G. , Li S. D. , Quan M. X. , Hu Z. Q.
Published: March 2008

The combustion reaction while mechanical alloying (MA) the Al–Ti–C system has been detected by in situ thermal analysis and the results of x-ray diffraction (XRD). Based on the information provided by in situ thermal analysis, the reaction temperature is estimated to be 1677 K, which is in good agreement with the value of the adiabatic temperature of 1700 K. It is considered that the formation reaction of Ti–C, which ignited by the heavy collisions of milling balls, induced the following reaction between Ti and Al at high temperature.
Growth of (11n) oriented Bi–Ca–Sr–Cu–O films by liquid phase epitaxial method
Authors: Raina K. K. , Pandey R. K.
Published: March 2008

Films of Bi–Ca–Sr–Cu–O (BCSCO) superconductor of the Bi2CaSr2Cu2Ox composition have been grown by the liquid phase epitaxy method (LPE) using a partially closed growth chamber. The films were grown on (110) NdGaO3 substrates by slow cooling under optimized conditions below the peritectic melting point (885 °C) of Bi2CaSr2Cu2O8. Optimization of parameters, such as seed rotation, soak of initial growth temperature, and growth period, results in the formation of the 2122 phase of BCSCO. X-ray diffraction (XRD) measurements show that the films grown on (110) NdGaO3 have a preferred (11n) orientation. The best values of zero resistance transition (Tc0) and critical current density (Jc0) obtained for films grown on (110) NdGaO3 substrates are 87 K and 5.7 × 104 A/cm2 (at 20 K), respectively. The films grown at rotation rates of less than 30 and more than 80 rpm are observed to be associated with a subphase in the Bi2CaSr2Cu2O8 system. Electron microprobe analysis indicates the composition of this subphase to be Bi0.07Ca0.93Sr2Cu5O8. Higher growth temperatures (>860 °C) also encourage the formation of this phase.
Formation of p-type Cu3BiS3 absorber thin films by annealing chemically deposited Bi2S3–CuS thin films
Authors: Nair P. K. , Huang L. , Nair M. T. S. , Hu Hailin , Meyers E. A. , Zingaro R. A.
Published: March 2008

Formation of the ternary compound Cu3BiS3 during annealing of chemically deposited CuS (∼0.3 μm) films on Bi2S3 film (∼0.1 μm on glass substrate) is reported. The interfacial atomic diffusion leading to the formation of the compound during the annealing is indicated in x-ray photoelectron depth profile spectra of the films. The formation of Cu3BiS3 (Wittichenite, JCPDS 9-488) is confirmed by the x-ray diffraction (XRD) patterns. The films are optically absorbing in the entire visible region (absorption coefficient 4 × 104 cm−1 at 2.48 eV or 0.50 μm) and are p-type with electrical conductivity of 102−103 Ω−1 cm−1. Potential applications of these films as optical coatings in the control of solar energy transmittance through glazings and as a p-type absorber film in solar cell structures are indicated.
SiC (SCS-6) fiber reinforced–reaction formed SiC matrix composites: Microstructure and interfacial properties
Authors: Singh M. , Dickerson R. M. , Olmstead Forrest A. , Eldridge J. I.
Published: March 2008

Microstructural and interfacial characterization of unidirectional SiC (SCS-6) fiber reinforced–reaction formed SiC (RFSC) composites has been carried out. Silicon–1.7 at.% molybdenum alloy was used as the melt infiltrant, instead of pure silicon, to reduce the activity of silicon in the melt as well as to reduce the amount of free silicon in the matrix. Electron microprobe analysis was used to evaluate the microstructure and phase distribution in these composites. The matrix is SiC with a bi-modal grain-size distribution and small amounts of MoSi2, silicon, and carbon. Fiber push-outs tests on these composites showed that a desirably low interfacial shear strength was achieved. The average debond shear stress at room temperature varied with specimen thickness from 29 to 64 MPa, with higher values observed for thinner specimens. Initial frictional sliding stresses showed little thickness dependence with values generally close to 30 MPa. Push-out test results showed very little change when the test temperature was increased to 800 °C from room temperature, indicating an absence of significant residual stresses in the composite.
Characterization of iron oxide-silica nanocomposites in flames: Part II. Comparison of discrete-sectional model predictions to experimental data
Authors: Biswas Pratim , Wu Chang Yu , Zachariah Michael R. , McMillin Brian
Published: March 2008

A discrete-sectional model accounting for particle formation by chemical reaction and growth by coagulation and condensation is developed to predict the evolution of the nanocomposite aerosol size distribution in a multicomponent iron-silicon system in a flame. Particle formation by nucleation of the vapor is represented by an Arrhenius-type rate expression, with the rate constant being obtained from experiments and simulation results reported in the literature. Precursor vapor concentrations and the second aerosol volume moment predictions are compared to laser-induced fluorescence (LIF) and light scattering intensity measurements from experiments described in Part I20 of the paper. The results elucidate the important formation and growth mechanisms of nanocomposite ferric oxide-silica particles in flame reactors. The role of operating parameters such as precursor characteristics and temperature profiles on the final product characteristics is established.
Model of chemical vapor infiltration using temperature gradients
Authors: Skamser Daniel J. , Jennings Hamlin M. , Johnson D. Lynn
Published: March 2008

An optimized chemical vapor infiltration (CVI) process has conditions that promote complete densification at the fastest allowable reaction rate. In order to help define optimum conditions, a model has been developed to simulate the CVI of a fibrous specimen for determining the effects of temperature gradients along with the other processing parameters such as pressure, size, chemistry, rate of reaction, and porosity on the resulting deposition profiles. This model simulates the deposition of alumina matrix within fibers wrapped around a tube. This symmetry reduces the model to a simple one-dimensional problem. Parameters for transport properties, calculated using a local microstructure model, are used in this macroscopic model. The model is applied as a guideline for choosing optimum conditions for producing a dense ceramic matrix composite. From this model, process diagrams are constructed that can help an experimentalist to choose the best conditions for the CVI process using temperature gradients.
Effect of Mo microstructure on the critical volume fraction for conduction in Mo-alumina cermets
Authors: Kelso J. F. , Higgins R. R. , Krivda F. J.
Published: March 2008

The microstructure of Mo in an alumina/frit matrix was found to be dependent on the initial particle sizes of the alumina and Mo powders, the glass content in the matrix ceramic, and the amount of moisture in the firing atmosphere. The Mo microstructure had a significant influence on the critical volume fraction for conductivity in these cermets. Coarser alumina powder, finer Mo powder, and higher glass content promoted coalescence of Mo into conductive networks at lower metal contents. Drier firing atmospheres produced a more coarsened Mo microstructure with a slight decrease in the amount of network contiguity, causing an increase in the amount required for electrical percolation.
Surface composites: A new class of engineered materials
Authors: Singh Rajiv , Fitz-Gerald James
Published: March 2008

To integrate irreconcilable material properties into a single component, a new class of engineered materials termed “surface composites” has been developed. In this engineered material, the second phase is spatially distributed in the near surface regions, such that the phase composition is linearly graded as a function of distance from the surface. Surface composites are different from existing engineered materials such as “bulk composites” and “functionally graded materials” (FGM). Unlike bulk composites, the surface phase in surface composites is present only at the near surface regions. In contrast to FGM, the graded properties of surface composites are achieved by unique morphological surface modification of the bulk phase. To fabricate surface composites, the initial surface of the bulk material is transformed using a novel multiple pulse irradiation technique into truncated cone-like structures. The laser induced micro-rough structures (LIMS) possess surface areas which are up to an order of magnitude higher than the original surface. The second phase is deposited on the surface using thin or thick film deposition methods. A key characteristic of surface composites is the formation of a three-dimensional, compositionally and thermally graded interface, which gives rise to improved adhesion of the surface phase. Examples of various types of surface composites such as W/Mo, silica/SiC, diamond/steel, etc. are presented in this paper. The unique properties of surface composites make them ideal engineered materials for applications involving adherent thick film coatings of thermally mismatched materials, compositional surface modification for controlled catalytic activity, and creating adherent metal-ceramic and ceramic-polymeric joints.
SrBi2Ta2O9 thin films made by liquid source metal-organic chemical vapor deposition
Authors: Zhu Yongfei , Desu Seshu B. , Li Tingkai , Ramanathan Sasangan , Nagata Masaya
Published: March 2008

A liquid source metal-organic chemical vapor deposition system was installed to deposit SrBi2Ta2O9 (SBT) thin films on sapphire and Pt/Ti/SiO2/Si substrates. The process parameters such as deposition temperature and pressure, and ratio of Sr: Bi: Ta in the precursor solutions were optimized to achieve stoichiometric films with good reproducible ferroelectric properties. It was found that the nucleation of SBT started at a deposition temperature close to 500 °C and grain growth dominated at 700 °C and higher temperatures. With increasing deposition temperatures, the grain size of SBT thin films increased from 0.01 μm to 0.2 μm; however, the surface roughness and porosity of the films also increased. To fabricate specular SBT films, the films had to be deposited at lower temperature and annealed at higher temperature for grain growth. A two-step deposition process was developed which resulted in high quality films in terms of uniformity, surface morphology, and ferroelectric properties. The key to the success of this process was the homogeneous nucleation sites at lower deposition temperature during the first step and subsequent dense film growth at higher temperature. The two-step deposition process resulted in dense, homogeneous films with less surface roughness and improved ferroelectric properties. SBT thin films with a grain size of about 0.1 μm exhibited the following properties: thickness: 0.16–0.19 μm; 2Pr: 7.8–11.4 μC/cm2 at 5 V; Ec: 50–65 kV/cm; Ileakage: 8.0–9.5 × 10−9 Acm−2 at 150 kV/cm; dielectric constant: 100–200; and fatigue rate: 0.94–0.98 after 1010 cycles at 5 V.
Extended x-ray absorption fine structure study of incorporation of Bi and Pb atoms into the crystal structure of Ba4.5Nd9Ti18O54
Authors: Valant M. , Arčon I. , Suvorov D. , Kodre A. , Negas T. , Frahm R.
Published: March 2008

In the extended x-ray absorption fine structure (EXAFS) study of the local environment of Bi3+ and Pb2+ ions incorporated in Ba4.5Nd9Ti18O54, actual sites of Bi- and Pb-incorporation are determined. Evidence is given that dopant ions are not distributed randomly on all theoretically possible sites; Bi3+ selectively enters one out of three possible channels, corresponding to the sites x = 0.9484, y = 0.2500, z = 0.2939, and/or x = 0.0455, y = 0.2500, z = 0.6928 previously occupied by Nd3+, while Pb2+ selectively enters site x = 0.4940, y = 0.2500, and z = 0.4993 previously shared by Ba2+ and Nd3+.
Synthesis and characterization of sol-gel derived hexa-aluminate phosphors
Authors: Ravichandran D. , Roy R. , White W. B. , Erdei S.
Published: March 2008

Two refractory phosphors, BaMg2Al16O27: Eu2+ (BAM) and MgAl11O17.5: Ce3+, Tb3+ (MAO), have been synthesized both by the conventional solid-state processing route (using oxides as the starting materials) and by reacting precursors made by the sol-gel process using organic precursors. The phases formed were reacted at 1000 °C in (a) steam and (b) steam + AlF3. The phosphors were well crystallized with particle sizes in the range of 1–10 μm. The emission spectra showed the characteristic broad band blue emission of Eu2+ for BAM and a narrow band green luminescence of Tb3+ for MAO. The melting points of BAM and MAO were measured to be 1920 ± 20 °C and 1950 ± 20 °C, respectively, using an Ir-strip furnace and optical pyrometer. BAM and MAO phosphor materials are congruently and incongruently melting, respectively. Excellent crystallization via the sol-gel route was found even at 1220 °C. Enhancement of the luminescent output by the steam treatment by some 25% was determined.
Materials reliability issues with coaxial cable systems for the information superhighway
Authors: Comizzoli R. B. , Crane G. R. , Fiorino M. E. , Frankenthal R. P. , Krautter H. W. , Peins G. A. , Siconolfi D. J. , Sinclair J. D.
Published: March 2008

A frequently advocated design for theInformation Superhighwaythat will provide voice, video, and two-way digital communication for businesses and residences calls for transmission on optical fibers from the source (head end) to neighborhood nodes and coaxial cable for distribution from the neighborhood nodes to individual subscribers. Materials reliability challenges abound from the head end to the customer's premise. The maximum allowed service outage per customer is expected to be less than one hour per year. The fiber portion of the network, including lasers, amplifiers, lenses, and the fiber itself, carries two-way transmission to large neighborhoods. With one fiber serving hundreds of customers, cost/reliability tradeoffs emphasize reliability. At neighborhood nodes, where the network branches, coaxial components carry the signal to the street level, where each subscriber is served by at least one drop cable and one network interface unit. The volume of components is much higher. The challenge is to achieve high reliability at low cost. Indeed, one of the impediments to replacing coaxial distribution in the neighborhood with fiber is the difficulty of achieving low-cost, reliable packaging for lasers and other optical components, while maintaining reliability. Since covering all the many materials aspects of system reliability would fill an entire book, the important design considerations, materials issues, and test methods that bear on the reliability of hybrid fiber coaxial systems will be illustrated by reviewing several aspects of coaxial reliability. The intent is to illustrate the methods used to identify and prevent failure mechanisms and the information that must be assembled for making appropriate decisions on reliability/cost tradeoffs. While coaxial systems have been used for over 20 years to bring cable TV to homes across North America, the requirements for reliability and signal quality with these one-way analog rf systems are less demanding than those for two-way digital data transmission. Many of the reliability issues that could potentially degrade performance characteristics of coaxial cable systems are materials and environment related. This paper discusses these issues to the extent that they are currently understood, presents results from some ongoing research to improve our understanding of the potential materials/environment related causes of signal deterioration, describes some prevention strategies, and discusses some of the many materials challenges that remain.
Effects of Electroless Nickel Plating on Resistivity-temperature Characteristics of (Ba1-xPbx)TiO3thermistor
Authors: Wanping Chen , Longtu Li , Zhilun Gui
Published: March 2008

Effects of electroless nickel plating on resistivity-temperature (R-T) characteristics of (Ba1-xPbx)TiO3 thermistor were studied. Comparison experiments showed that not only the permeation of plating solution, but also the reaction of electroless nickel plating influences the positive temperature coefficient (PTC) effect, and that the two effects are different in nature. It is first proposed in this paper that hydrogen atoms generated in electroless nickel plating may reduce components of PTC ceramics.
Grain Size Dependence of Mechanical Properties in Nanocrystalline Selenium
Authors: Lu K. , Zhang H. Y. , Zhong Y. , Fecht H. J.
Published: March 2008

Porosity-free nanocrystalline element selenium (nc-Se) samples with the mean grain sizes ranging from 8 to 70 nm were synthesized by complete crystallization of the melt-quenched amorphous Se solid. Mechanical properties including microhardness (Hv) and elastic modulus (E) of the nc-Se samples were measured by means of nanoindentation tests and microhardness tests, respectively. With a reduction of grain size, the nc-Se samples were found to be substantially hardened. But the grain size dependence of Hv does not follow a simple Hall–Petch relation over the whole grain size range, exhibiting three distinct stages corresponding to three different Hall–Petch slopes. The maximum Hall–Petch slope was found to be in the grain size range of 15–20 nm, corresponding to large values of the elastic modulus. This behavior can be explained in terms of the lattice distortion in the nc-Se samples that was experimentally determined by using quantitative x-ray diffraction measurements. A conclusion is drawn that the lattice structure of the nm-sized crystallites may play an important role in mechanical properties of nanocrystalline materials.
The Spreading Kinetics of Ag–28Cu(L) on Nickel(S): Part II. Area of Spread on Surfaces Plated with Electrolytic Ni
Authors: Weirauch Douglas A. , Horvath Stephen F.
Published: March 2008

Furnace brazing is commonly used in the electronic industry to attach I/O pins, lid-seal rings, and heat spreaders to cofired multilayer ceramic packages. Despite the widespread industry usage of electrolytic and electroless Ni coatings to render base metal surfaces wettable by braze fillers, there is no fundamental treatment of “brazeability” in the published literature that can be used by the materials technologist to design coatings for a given application. In this study, dynamic hot stage microscopy is used to establish the parameters that control the spreading of eutectic Ag–Cu braze on surfaces plated with electrolytic Ni. The effects of plating thickness, substrate type, plating annealing, and the braze thermal cycle are considered. A braze spreading mechanism developed for polycrystalline Ni foil in Part I of this study is linked to Ni-plated surfaces through consideration of differences in microstructure. Eventual extension of this improved understanding to surfaces plated with electroless Ni–B and Ni–P deposits will result in shortened product design time and improved manufacturing process control.
Deposition Process and Property of Silica Films Containing Organic Groups from Aqueous Solution of Alkoxides
Authors: Oh Junrok , Imai Hiroaki , Hirashima Hiroshi , Tsukuma Koji
Published: March 2008

Silica thin films containing organic groups were directly deposited from aqueous solutions of silicon alkoxides. Effects of functional groups of the alkoxides and the deposition conditions on the preparation of the films were investigated in order to clarify the deposition process. Silica films containing methyl and phenyl groups were deposited from aqueous solutions of methyl- and phenyltrialkoxysilanes, respectively, in the pH region lower than 3 or higher than 6. The composition and the property of the deposited films were influenced by pH and the temperature of the solutions and the starting materials. The adhesion of the deposited films on hydrophobic substrates was lower than that on hydrophilic ones, although the films were formed on both surfaces. The refractive index of the methyl-containing films, ca. 1.44, decreased with removal of methyl groups by heating up to 500 °C.
A Study on the Microstructure of Preferred Orientation of Lead Zirconate Titanate (PZT) Thin Films
Authors: Kim Chang Jung , Yoon Dae Sung , Lee Joon Sung , Choi Chaun Gi , No Kwangsoo
Published: March 2008

The lead zirconate titanate (PZT) thin films were fabricated using sol-gel spin coating onto Pt/Ti/glass substrates. Effects of the holding time for pyrolysis and the coating cycle on the preferred orientation of the PZT thin films were studied. The films were fabricated with different coating cycles (3, 5, 7, 9, 11), dried at 330 °C for different holding times (5, 30, 60 min), and then annealed at the same temperature of 650 °C using rapid thermal annealing (RTA). The preferred orientations of the films were investigated using x-ray diffraction and glancing angle x-ray diffraction. The microstructure and the selected area diffraction pattern of the PZT thin films were also investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively.
Crystallization Mechanisms of some Se100–xTex Glassy Alloys
Authors: Calventus Y. , Suriñach S. , Baró M. D.
Published: March 2008

The coupling of calorimetric and microscopic techniques shows that the whole crystallization process for some Se100–x Tex (x = 10, 15) glassy alloys proceeds by two different mechanisms, which we call surface and bulk. These mechanisms are activated differently depending on the particular heating rate used and on the temperature of the isothermal heat treatment chosen. The nucleation frequencies and growth rates were determined from reflection polarized optical microscopy analysis, and a good agreement is found between these experimental results and predictions done by the classical nucleation and the normal growth theories. The apparent activation energy from the whole crystallization process which is obtained via differential scanning calorimetry is higher for Se85Te15 than for Se90Te10, and the relation with these values and those obtained from activation energies of nucleation and growth is established. A detailed discussion about the meaning of the different Avrami indexes found is presented.
Effects of Glass Elements on the Structural Evolution ofin situGrown Ferroelectric Perovskite Crystals in Sol-gel Derived Glass-ceramics
Authors: Yao Kui , Zhu Weiguang , Zhang Liangying , Yao Xi
Published: March 2008

Several ABO3perovskite ferroelectric crystals, PbTiO3, Pb(Zr, Ti)O3, and BaTiO3have beenin situgrown from amorphous gels with glass elements, and the structural evolution has been systematically investigated using x-ray diffraction (XRD), infrared spectra (IR), differential thermal analysis (DTA), thermogravimetric analysis (TGA), and dielectric measurements. It is found that in the Si-contained glass-ceramic systems, Si and B glass elements are incorporated into the crystalline structures, resulting in the variation of the crystallization process, change of lattice constant, and dielectric properties. Some metastable phases expressed by a general formula AxByGzOw(A = Pb and Ba; B = Zr and Ti; G for glass elements, especially for Si) have been observed and discussed.
Electrical and Structural Properties of SrTiO3 Thin Films Deposited by Plasma-enhanced Metalorganic Chemical Vapor Deposition
Authors: Kim Nam-Kyeong , Yoon Soon-Gil , Lee Won-Jae , Kim Ho-Gi
Published: March 2008

The microstructure and electrical properties were investigated for SrTiO3(STO) thin films deposited on Pt/Ti/SiO2/Si substrates by PEMOCVD. The SrF2 phase existing in the STO films deposited at 450 °C influences the dielectric constant, dissipation factor, and leakage current density of STO films. The dielectric constant and dissipation factor of STO films deposited at 500 °C were 210 and 0.018 at 100 kHz, respectively. STO films were found to have paraelectric properties from the capacitance-voltage characteristics. Leakage current density of STO films at 500 °C was about 1.0 × 10-8 A/cm2 at an electric field of 70 kV/cm. The leakage current behaviors of STO films deposited at 500 and 550 °C were controlled by Schottky emission with applied electric field.
Direct current electrical conductivity of Versicon™ blended in poly(vinyl chloride)
Authors: Speel Rodney , LeMaire Peter K.
Published: March 2008

The direct current (dc) electrical conductivity of the conducting polymer Versicon™ blended in poly(vinyl chloride) (PVC) was measured from 25 K to 310 K. The data were fitted to various electrical transport models, and the best fit was found with the fluctuation-induced tunneling model, suggesting that tunneling dominates in the mode of electron transport at low temperatures. The parameters, T1 and T0 from the fluctuation-induced tunneling model, were found to be 625 K and 129 K, respectively. The interparticle distance was estimated to be about 13 Å. At higher temperatures, the plot of the log of resistivity versus the reciprocal of the temperature was linear, indicating that thermally activated hopping dominated the mode of electrical transport at these temperatures. The results support earlier findings that VersiconTM forms continuous aggregates in blends. The results also support growing evidence in the literature that these types of aggregate formation tend to strongly influence the mode of electrical transport in composites.
The origin of an anomalous, low 2θ peak in x-ray diffraction spectra of MoS2 films grown by ion beam assisted deposition
Authors: Dunn D. N. , Seitzman L. E. , Singer I. L.
Published: March 2008

The origin of a previously reported anomalous low 2θ x-ray diffraction peak from MoS2 thin films grown by ion beam assisted deposition was investigated. The anomalous peak, observed in a film grown on Si(100), was removed by ion irradiating the film with 180 keV Ar++ ions to a dose of 1 × 1015 ions/cm2. Microstructures of the two films were investigated using x-ray diffraction and cross-section transmission electron microscopy. Diffraction data and bright-field images indicated that the low 2θ peak was due to a local interplanar expansion of the crystal structure normal to MoS2 basal planes. This expansion was attributed to molecular defects.