bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation of resistance and interfacial reaction of contacts to n-type InP
Authors: Huang J. S. , Vartuli C. B. , Nguyen T. , Bar-Chaim N. , Shearer J. , Fisher C. , Anderson S.
Published: March 2008

Metal contact of compound semiconductors has been extensively studied using various characterization tools. However, there was little work of scanning transmission electron microscopy (STEM) done previously for the study of the InP contact systems. Using STEM and Auger electron spectroscopy analyses, we correlated the resistance with interfacial reactions in the AuGe/Ni/Au/Cr/Au contacts to n-InP. Detailed nanoscale structural and chemical information was uniquely revealed by the STEM compared to other techniques. For the high-resistance samples, little interfacial reaction between n-InP and Au occurred. For the low-resistance samples, significant out-diffusion of P in the Au and Ni layers occurred, forming Au–P and Ni–P metallic compounds. Accumulation of Ge in the Ni layer was also detected. We suggest that Ni–P may be very critical in obtaining low contact resistance for n-InP.
Pressure-induced nanocrystallization of Zr55Al10Ni5Cu30 bulk metallic glass
Authors: Zhang Jia , Qiu K. Q. , Wang A. M. , Zhang H. F. , Quan M. X. , Hu Z. Q.
Published: March 2008

The effect of pressure on the crystallization behavior of Zr55Al10Ni5Cu30 bulk metallic glass was investigated by differential scanning calorimetry, x-ray diffraction, and transmission electron microscopy. Although the crystallization products under high pressure were about the same as those under ambient pressure, the evident changes in the relative crystallization fraction of each phase were observed. The applied pressure enhanced the crystallization temperature. Pressure annealing of the bulk metallic glass produced a composite with dispersion of very fine nanocrystallites in the amorphous matrix. A full nanocrystallization was obtained for the sample annealed under 5 GPa at 793 K. The mechanism for the pressure-induced nanocrystallization is discussed.
Thermal conductivity of AlN ceramic with a very low amount of grain boundary phase at 4 to 1000 K
Authors: Watari Koji , Nakano Hiromi , Urabe Kazuyori , Ishizaki Kozo , Cao Shixun , Mori Katsunori
Published: March 2008

An AlN ceramic fired at 2173 K for 100 h under a reduced N2 atmosphere with carbon possessed a room-temperature conductivity of 272 Wm−1K−1, slightly lower than the value for high-purity, single-crystal AlN. However, the thermal conductivity of the ceramic at temperatures below 100 K was much lower than that of single crystal. This is mainly due to phonon scattering by grain junctions that possess an amorphous film with a thickness of under 1 nm. At 500 to 1000 K, no significant difference in the conductivity was observed between the ceramic and the single crystal.
Defect clustering in GaN irradiated with O+ ions
Authors: Wang C. M. , Jiang W. , Weber W. J. , Thomas L. E.
Published: March 2008

Transmission electron microscopy (TEM) was used to study microstructures formed in GaN irradiated with 600-keV O+ ions at room temperature. Three types of defect clusters were identified in the irradiated GaN: (i) basal-plane stacking faults with dimensions ranging from 5 to 30 nm, (ii) pyramidal dislocation loops, and (iii) local regions of highly disordered material. High-resolution TEM imaging clearly revealed that one type of the basal-plane stacking faults corresponded to insertion of one extra Ga–N basal plane in the otherwise perfect GaN lattice. The interpretation of these results indicated that interstitials of both Ga and N preferentially condensed on the basal plane to form a new layer of Ga–N under these irradiation conditions. The formation of these extended defects and their interactions with the point defects produced during irradiation contributed to a dramatic increase in the dynamic recovery of point defects in GaN at room temperature.
Intermediate-temperature oxidation behavior of Ti2AlC in air
Authors: Wang X. H. , Zhou Y. C.
Published: March 2008

The isothermal oxidation behavior of Ti2AlC at intermediate temperatures of 500, 600, 700, 800, and 900 °C in flowing air was investigated by means of thermogravimetric analysis, x-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM)/energy dispersive spectroscopy. An anomalous oxidation with higher kinetics at lower temperatures of 500 and 600 °C was observed although Ti2AlC exhibited good oxidation resistance at higher temperatures. As revealed by SEM scale morphology observation, oxidation-induced cracks present at temperatures of 500 and 600 °C resulted in poor protectivity of scales and accounted for the anomalous oxidation. The weight gain data for the remaining temperatures were analyzed with an instantaneous parabolic rate constant method by assuming a parabolic rate law. The variations of instantaneous parabolic rate constant with time reflected the complexity of the oxidation behavior of Ti2AlC at intermediate temperatures. These variations were discussed from the viewpoint of phase transformation of oxidation products on the basis of XRD, Raman spectroscopy, and SEM scale morphology observation. As indicated by XRD and Raman spectra, the scale formed on Ti2AlC at higher temperature was stable α–Al2O3 and rutile TiO2, while transition aluminas of γ–Al2O3, δ–Al2O3, θ–Al2O3, and TiO2 (anatase) were detected at lower temperatures. The whisker morphology of θ–Al2O3 at 800 °C and ridged morphology of α–Al2O3 at 800 and 900 °C were confirmed by SEM.
Selenium oxyanions: Highly selective uptake by a novel anion exchanger
Authors: Kozai Naofumi , Ohnuki Toshihiko , Komarneni Sridhar
Published: March 2008

We report the extremely high and selective uptake of selenium oxyanions by a novel anion exchanger, Ni1-xZn2x(OH)2(OCOCH3)2xnH2O (0.15<x<0.25). The tested Ni–Zn basic salt (x=0.24) exhibited very high selectivity for Se(IV) [Kd = 9.0 × 104 cm3/g with an initial Se(IV) concentration of 1 × 10-4 M] in the presence of 0.1 M C1− solution. The uptake of Se(IV) on the Ni–Zn basic salt was irreversible when treated with solutions containing 1 N C1− 1N NO3− or 1 N PO43−This novel exchanger also showed high Kd (2.6 × 103 cm3/g) for Se(VI), and therefore it is expected to be useful for decontamination and removal of selenium oxyanions from contaminated water.
The use of polymeric milling media in the reduction of contamination during mechanical attrition
Authors: De Castro Claudio L. , Mitchell Brian S.
Published: March 2008

Aluminum samples were milled in a high-energy ball mill at times ranging from 10 to 500 min in stainless vial/media and nylon vial/media. The crystallite size of aluminum milled in the stainless steel setup reached a plateau at 25 nm around 100 min, whereas the crystallite size of aluminum milled in the nylon setup reached the same plateau at 500 min. Contamination studies were conducted using x-ray fluorescence, hydrogen–carbon–nitrogen analysis, and thermogravimetric analysis. Although organic contamination due to the nylon milling media was high at 12 wt% when milled for 500 min, it is shown that the contaminant could be easily removed by thermal treatment.
Microstructural variations in mullite ceramics derived from diphasic sols using different forming techniques
Authors: Kaya C. , Boccaccini A. R.
Published: March 2008

Mullite ceramics with controlled microstructure in terms of residual glassy phase and density were produced from diphasic silica/boehmite sols using pressure filtration, die-pressing, and extrusion techniques. The effect of the compaction process on the green and sintered densities and on the final mullite microstructure was investigated. It is shown that glassy-phase-free mullite with high green and sintered densities can be produced from engineered diphasic colloidal suspensions using pressure filtration. The results show that the green body formation technique strongly determines the final sintered density, but it has no effect on the final phase composition of diphasic sol-derived sintered mullites.
Major phase transformations and magnetic property changes caused by electromagnetic fields at microwave frequencies
Authors: Roy Rustum , Peelamedu Ramesh , Grimes Craig , Cheng Jiping , Agrawal Dinesh
Published: March 2008

We demonstrate in this paper that common crystalline phases can be made noncrystalline and hard magnets can be converted to soft magnets in the solid state in several seconds at temperatures far below the melting points. New crystal structures and magnetic structures of ferromagnetic oxides (ferrites such as BaFe12O19, CoFe2O4, Fe3O4, and ZnFe2O4, etc.) are formed by reacting either the stoichiometric mixture of oxides or the preformed phase-pure crystalline material in a pure H field (or E field) at microwave (2.45 GHz) frequencies. These major changes in the magnetic properties as well as major structural phase changes are caused by the magnetic field.
Overview: Damage in brittle layer structures from concentrated loads
Authors: Lawn Brian R. , Deng Yan , Miranda Pedro , Pajares Antonia , Chai Herzl , Kim Do Kyung
Published: March 2008

In this article, we review recent advances in the understanding and analysis of damage initiation and evolution in laminate structures with brittle outerlayers and compliant sublayers in concentrated loading. The relevance of such damage to lifetime-limiting failures of engineering and biomechanical layer systems is emphasized. We describe the results of contact studies on monolayer, bilayer, trilayer, and multilayer test specimens that enable simple elucidation of fundamental damage mechanics and yet simulate essential function in a wide range of practical structures. Damage processes are observed usingpost mortem(“bonded-interface”) sectioning and directin situviewing during loading. The observations reveal a competition between damage modes in the brittle outerlayers—cone cracks or quasiplasticity at the top (near-contact) surfaces and laterally extending radial cracks at the lower surfaces. In metal or polymeric support layers, yield or viscoelasticity can become limiting factors. Analytical relations for the critical loads to initiate each damage mode are presented in terms of key system variables: geometrical (layer thickness and indenter radius); material (elastic modulus, strength and toughness of brittle components, hardness of deformable components). Such relations provide a sound physical basis for the design of brittle layer systems with optimal damage thresholds. Other elements of the damage process—damage evolution to failure, crack kinetics (and fatigue), flaw statistics, and complex (tangential) loading—are also considered.
Characterization of CdTe:Zn:V crystals grown under microgravity conditions
Authors: Corregidor V. , Babentsov V. , Castaño J. L. , Fiederle M. , Feltgen T. , Benz K. , Dieguez E.
Published: March 2008

CdTe:Zn:V crystals grown by the seeded Bridgman method in microgravity conditions during the STS95-Spacelab-AGHF-1 mission and in the ground laboratory (l-g) were analyzed and compared. The results obtained clearly show that the structural quality of the space crystal is better. Density of inclusions, concentration of dislocations, and presence of stresses are lower in the microgravity-grown (μ-g) crystal. The l-g crystal contains twins and grains from the beginning of the growth process, that is, from the near-seed region. In general, the concentration of inclusions and amount of segregated impurities on the l-g crystal are larger than in the μ-g crystal. X-ray rocking curves and low-temperature photoluminescence spectra demonstrate the relatively high quality of both crystals on a microscale at the beginning of the growth and show that the l-g conditions were worse at the end. The results of this investigation demonstrate a positive role of contactless growth and μ-g conditions in the melt in suppressing the creation of inclusions and dislocations.
Effects of germanium on the electronic transport mechanism in Co20(Cu1-xGex)80 nanogranular ribbons
Authors: He J. , Zhang Z. D. , Liu J. P. , Sellmyer D. J.
Published: March 2008

The dependency of giant magnetoresistance (GMR) on the nonmagnetic matrix in nanogranular Co20(Cu1-xGex)80 ribbons was studied. When the matrix Cu is substituted with semiconductor Ge, the magnetoresistance transitioned from negative to positive at low temperatures. The positive GMR effect is closely related to the quantity of Co/Co3Ge2/Co junctionlike configurations. This result provides evidence for the competition between two types of electronic transport mechanisms in the magnetic granular ribbons: (i) electronic spin-dependent scattering, inducing a negative magnetoresistance and (ii) Coulomb blockade of the electronic tunneling, inducing a positive magnetoresistance.
Thermodynamics and phase stability in the In–N system
Authors: Onderka B. , Unland J. , Schmid-Fetzer R.
Published: March 2008

Results of a chemical vapor deposition crystal growth method intended to produce large amounts of InN needed for thermodynamic experiments are reported. Polycrystalline films of InN were grown by reaction of InCl3 and NH3 in a hot-wall silica reactor under nearly atmospheric pressure. Samples were analyzed using x-ray diffraction and chemical analysis. The decomposition of InN was studied in both thin film and powder form. InN films were investigated by isothermal heating under nitrogen and subsequent microscopic inspection. The removal of the nucleation barrier of forming the first liquid phase was emphasized. InN powder decomposition experiments involved two different customized thermogravimetric methods: (i) dynamic oscillation thermogravimetric analysis (TGA), and (ii) isothermal stepping TGA for a higher resolution of the decomposition start. The decomposition start was found consistently at (773 ±5) K under 1 bar of nitrogen. Nevertheless, it is suggested that InN may be metastable even below room temperature based on Computer Coupling of Phase Diagrams and Thermo chemistry-type thermodynamic analysis of all available phase equilibrium and thermodynamic data. This included the determination of the absolute entropy of InN, 31.6 ±3 J/mol-formula K, based on a Debye and Einstein analysis of the experimental data on the heat capacity. All calculations of pressure are corrected for the fugacity of nitrogen, which becomes crucial above 1000 bar. The contradictory literature data in the In–N system are discussed based on three different internally consistent thermodynamic analyses of the system that highlight the consequences of different choices made on the decomposition temperature of InN. Widely reproduced data in the literature are shown to produce thermodynamically impossible negative absolute entropy of InN. Complete P-T-x phase diagrams are given, which strongly suggest that solid InN is metastable under ambient conditions. To find out that InN crystals could be reproducibly superheated more than 500 K before they actually decompose comes as a surprise compared to other III-V systems, especially Ga–N.
Amorphization of crystalline orthoboric acid on a vitreous B2O3 substrate
Authors: McCalla Eric , Brüning Ralf
Published: March 2008

Time-resolved x-ray diffraction measurements were carried out during and after the exposure of anhydrous boron oxide glass films to humid air. B(OH)3 crystals grew on the glass substrate with planar sheets of B(OH)3 molecules aligned parallel to the surface. After the space above the sample was evacuated, the crystal peaks began to disappear. During the decay, new crystals of orthoboric acid grew with the orientation of the B(OH)3 sheets perpendicular to the substrate. Then crystals with all orientations decayed completely, and an amorphous scattering pattern remained. The water acquired by the sample during the exposure eventually reacted with the anhydrous B2O3 glass to produce amorphous BO(3+y)/2Hy. The decay of the crystals took from one to several days, depending on the length of the exposure to humid air. Measurements of the metastable equilibrium phases in the B2O3–H2O binary system showed that at room temperature the amorphous phase could reach the composition BO1.85H0.7. At higher water concentrations this saturated amorphous phase coexisted with crystalline B(OH)3. This result is consistent with the instability of the crystalline B(OH)3 film in contact with anhydrous glass, and we present a thermodynamic model for this process. In the present case, amorphization is driven by the outdiffusion of water from a crystalline phase rather than interdiffusion in conventional solid-state amorphization.
Dislocations in SrTiO3thin films grown on LaAlO3substrates
Authors: Qin Y. L. , Jia C. L. , Urban K. , Hao J. H. , Xi X. X.
Published: March 2008

The dislocation configurations in SrTiO3thin films grown epitaxially on LaAlO3(100) substrates were studied by conventional and high-resolution transmission electron microscopy. Misfit dislocations had, in most cases, a Burgers vectora〈100〉 and line directions of 〈100〉 These dislocations constitute orthogonal arrays of parallel dislocations at the interface, relieving the lattice mismatch between SrTiO3and LaAlO3. Threading dislocations were found to be the major defects in the films. Two types of threading dislocations with the Burgers vectorsa〈100〉?anda〈100〉?were identified. The relations of these threading dislocations with the misfit dislocations were investigated and are discussed in this paper.
Tribological behavior of CrN coating on aluminum alloys deposited by arc ion plating
Authors: Zhou Fei , Suh Chang-Min , Kim Seock-Sam , Murakami Ri-Ichi
Published: March 2008

Dry sliding wear and friction tests of CrN coating on two types of aluminum alloy substrates, 6061 Al and 7075 Al, deposited by arc ion plating were performed with a ball-on-disk tribometer. The effects of normal load and the mechanical properties of the substrate on the friction coefficient and wear resistance of CrN coating were investigated. The worn surfaces were observed by scanning electron microscopy. The results show that surface microhardness of CrN-coated 7075 Al is higher than that of CrN-coated 6061 Al. With an increase in normal load, wear volume increases, while the friction coefficient decreases. The friction coefficient of CrN-coated 6061 Al is higher than that of CrN-coated 7075 Al, while the wear resistance of CrN-coated 6061 Al is lower than that of the CrN-coated 7075 Al. This indicates that the substrate mechanical properties have strong influence on the friction coefficient and wear of CrN coating. The main wear mechanism was fragments of CrN coating, caused by apparent plastic deformation of substrate during wear tests.
Improvements of microstructure, wettability, tensile and creep strength of eutectic Sn–Ag alloy by doping with rare-earth elements
Authors: Wu C. M. L. , Yu D. Q. , Law C. M. T. , Wang L.
Published: March 2008

To improve the properties of the eutectic Sn–Ag lead-free solder alloy, various amounts of mixed rare-earth (RE) elements, mainly Ce and La, were added. The microstructure, wetting properties, melting behavior, mechanical properties, and creep behavior were studied. It was revealed that RE elements can refine the intermetallics, and with 0.5% RE addition, the RE-bearing phase can be detected in the microstructure of the slow-cooled alloy. The results of differential scanning calorimetry indicate that the melting points of the RE-doped alloys are slightly lower than that of the Sn–3.5Ag and have a eutectic peak. The wetting property and creep resistance of the Sn–3.5Ag–0.25RE alloy are better than those of the Sn–3.5Ag alloy. The creep properties were studied at the temperatures of 303, 348, and 393 K, at various stress levels between 8 and 34 MPa. The stress exponents of the Sn–3.5Ag and Sn–3.5Ag–0.25RE were obtained at these temperatures. Tensile, creep, and wetting properties were found to improve with the addition of RE elements. The improvement of creep resistance is due to the fine dispersion of intermetallics and the decrease in interface energy between matrix and intermetallics. The wettability improvement is mainly due to the accumulation of RE elements at the solder/flux interface, leading to the reduction of the interfacial tension between solder and flux.
Hydrothermal synthesis of perovskite and pyrochlore powders of potassium tantalate
Authors: Goh Gregory K. L. , Haile Sossina M. , Levi Carlos G. , Lange Fred F.
Published: March 2008

Potassium tantalate powders were hydrothermally synthesized at 100 to 200 °C in 4 to 15 M aqueous KOH solutions. A defect pyrochlore, Kta2O5(OH).nH2O (n≈ 1.4), was obtained at 4 M KOH, but at 7–12 M KOH, this pyrochlore was gradually replaced by a defect perovskite as the stable phase. At 15 M KOH, there was no intermediate pyrochlore, only a defect perovskite,0.85Ta0.92O2.43(OH)0.570.15H2O. Synthesis at higher KOH concentrations led to greater incorporation of protons in the perovskite structures. The potassium vacancies required for charge compensation of incorporated protons could accommodate water molecules in the perovskite structure.
Nickel–alumina nanocomposite powders prepared by novel in situchemical reduction
Authors: Huang Zheng-Ren , Jiang Dongliang , Michel Daniel , Mazerolles Léo , Ferrand Alain , Costanzo Thomas di , Vignes Jean-Louis
Published: March 2008

Nanocomposite powders of Ni–Al2O3 (5 and 10 vol% Ni) were prepared from porous alumina preforms with high specific area (100 m2 g?1) impregnated with nickel nitrate. Samples were obtained by reduction under a controlled oxygen partial pressure either directly or through an intermediate step, giving nickel aluminum spinel. In both cases, homogeneous dispersions of nickel particles in the alumina matrix were achieved. The Ni particle size ranged from 10 to 100 nm, depending on the preparation conditions and temperature.
Novel approaches for controlling stage structure of metal chloride–graphite intercalation compounds
Authors: Shioyama H. , Saman M. B. H. M. , Sanpanich A.
Published: March 2008

Intercalation of EuCl3 TbCl3, and AlCl3 into graphite was carried out in the presence of chlorine. The observation of products by x-ray diffraction showed that the extent of chloride intercalation could be controlled through adjustment of the pressure of chlorine; increasing pressure tended to increase the extent of intercalation. In the case of EuCl3, the extent of intercalation varied to show a stage transition. In contrast, TbCl3 intercalation gave a mixture of stage 2 or stage 4 graphite intercalation compound (GIC) and remaining graphite, where the extent of intercalation is revealed by the ratio of GIC to graphite. With respect to AlCl3 intercalation, although the preparation of stage 1, 2, and 4 GICs was successful, stage 3 and 5 GICs could not be obtained.
Optimization of red phosphor for plasma display panel by the combinatorial chemistry method
Authors: Sohn Kee-Sun , Kim Chang Hae , Park Joon Taik , Park Hee Dong
Published: March 2008

Combinatorial chemistry was applied to the optimization of red phosphors used for plasma display panels. Quaternary and ternary combinatorial libraries were developed for (Y, Gd, Lu, Sc)BO3 and (Y, Gd)(BO3 PO4) systems. Our combinatorial chemistry system consists of solution-based combinatorial synthesis and characterization, enabling the swift scanning of luminance and of Commission Internationale de l'Eclairage (CIE) chromaticity under vacuum ultraviolet light excitation. As a consequence of the combinatorial approach, several new candidates were found to show higher luminance than the commercially available red phosphor for plasma display panels.
Poly(meta-phenylene isophthalamide) nanofibers: Coating and post processing
Authors: Liu Wenxia , Graham Matthew , Evans Edward A. , Reneker Darrell H.
Published: March 2008

Electrospun nanofibers have applications in the areas of filtration, composites, biomaterials, and electronics. Controlling the surface properties of these nanofibers is important for many applications. Nanofibers can also be used as unique substrates for observing the growth of deposited films and creating nanoscale structures. In this work, electrospun poly(meta-phenylene isophthalamide) (MPD-I) nanofibers were used as substrates for creating nanoscale structures out of carbon-based materials and metals. MPD-I was used because it can be electrospun into nanofibers with diameters smaller than 10 nm and it has good thermal stability. MPD-I nanofibers were coated with carbon, copper, and aluminum using plasma enhanced chemical vapor deposition and physical vapor deposition methods. Some of the aluminum-coated nanofibers were then converted into nanotubes. Transmission electron microscopy was used to determine the thickness, uniformity, and grain size of the coatings on the fibers and the nanotubes.
Agglomerate-free BaTiO3 particles by salt-assisted spray pyrolysis
Authors: Itoh Yoshifumi , Lenggoro I. Wuled , Pratsinis Sotiris E. , Okuyama Kikuo
Published: March 2008

Optimum conditions for the synthesis of nonagglomerated BaTiO3 particles by salt-assisted spray pyrolysis (SASP) were investigated. The effect of particle residence time in the reactor and salt concentration on the crystallinity and surface morphology of BaTiO3 was examined by x-ray diffraction and scanning electron microscopy. Mixtures of a metal chloride or nitrate salt, dissolved in aqueous precursor solutions, were sprayed by an ultrasonic atomizer into a five-zone hot-wall reactor. By increasing the salt concentration or the particle residence time in the hot zone, the primary particle size was increased, and its surface texture was improved compared to BaTiO3 particles prepared by conventional spray pyrolysis. The SASP-prepared BaTiO3 crystal was transformed from cubic to tetragonal by simply increasing the salt concentration at constant temperature and residence time. Further thermal treatments such as calcination or annealing are not necessary to obtain nonagglomerated tetragonal BaTiO3 (200–500 nm) particles with a narrow size distribution. Increasing the carrier gas flow rate and decreasing the residence time in the hot zone resulted in cubic BaTiO3 particles about 20 nm in diameter.
Decomposition and thermodynamic property of metastable Fe–Zn solid solutions produced by mechanical alloying
Authors: Zhou F. , Chou Y. T. , Lavernia E. J.
Published: March 2008

Thermal decomposition of supersaturated single-phase body-centered cubic (bcc) Fe100−xZnx(5≤x≤65 at.%) solid solutions, processed via mechanical alloying of high-purity metal powders, was investigated using x-ray diffraction and differential scanning calorimetry (DSC). At elevated temperatures the metastable solid solution decomposed into a stable equilibrium aggregate consisting of the pure bcc Fe phase and an intermetallic compound Fe4Zn9. The decomposition temperature decreased with increasing Zn concentration. The enthalpy of decomposition for various Fe–Zn solid solutions measured by the DSC was in the range of 1.2–3.5 kJ/mol. The enthalpy of mixing of the as-milled solid solutions from elemental Fe and Zn powders was estimated to be 0.5–1.7 kJ/mol. In addition, the activation energies of decomposition for these solid solutions were determined on the basis of the Kissinger analysis, and their values appeared to be independent of the Zn concentration in the alloy, with an average of 147 ± 17 kJ/mol.
Controlled growth of gallium nitride single-crystal nanowires using a chemical vapor deposition method
Authors: Han Song , Jin Wu , Tang Tao , Li Chao , Zhang Daihua , Liu Xiaolei , Han Jie , Zhou Chongwu
Published: March 2008

Chemical vapor deposition (CVD) using gold nanoparticles as the catalyst to grow high-quality single-crystal gallium nitride nanowires was developed. This method enables control over several important aspects of the growth, including control of the nanowire diameter by using monodispersed gold clusters, control of the nanowire location via e-beam patterning of the catalyst sites, and control of the nanowire orientation via epitaxial growth ona-plane sapphire substrates. Our work opens up new ways to use GaN nanowires as nanobuilding blocks.
Electrophoretic deposition of α-alumina particles in a strong magnetic field
Authors: Uchikoshi Tetsuo , Suzuki Tohru S. , Okuyama Hideo , Sakka Yoshio
Published: March 2008

The electrophoretic deposition of single-crystalline α-alumina particles dispersed in aqueous media was performed in a strong magnetic field of 10 T. The α-alumina particles in the stable suspension were aligned due to their anisotropic diamagnetic susceptibility and then deposited on a cathodic substrate. The orientation of the α-alumina crystallites was confirmed by x-ray diffraction of the sintered specimen.
Improvement in the crystallinity and electrical properties in Hg1−xCdxTe epilayers utilizing CdTe buffer layers grown on GaAs substrates by a two-step annealing process
Authors: Ryu Y.S. , Song B.S. , Kim H.J. , Kang T.W. , Kim T.W.
Published: March 2008

High-quality Hg1−xCdxTe epilayers on CdTe buffer layers were grown by molecular beam epitaxy using various growth methods. The reflection high-energy electron diffraction, scanning electron microscopy, and atomic force microscopy measurements showed that the crystallinity and electrical properties of the Hg1−xCdxTe epilayers grown on CdTe buffer layers deposited by using a two-step annealing growth method were improved. These results indicate that high-quality Hg1−xCdxTe films can be obtained by using CdTe buffer layers grown by the two-step annealing growth method and that the grown Hg1−xCdxTe epilayers hold promise for potential applications in optoelectronic devices in the area of infrared detectors.
Evolution of metastable phases during crystallization of pulsed laser deposited amorphous Al–Fe films
Authors: Bysakh Sandip , Das Puspendu Kumar , Chattopadhyay Kamanio
Published: March 2008

Amorphous thin films of different Al–Fe compositions were produced by plasma/vapor quenching during pulsed laser deposition. The chosen compositions Al72Fe28, Al40Fe60, and Al18Fe82 correspond to Al5Fe2 and B2-ordered AlFe intermetallic compounds and α–Fe solid solution, respectively. The films contained fine clusters that increased with iron content. The sequences of phase evolution observed in the heating stage transmission electron microscopy studies of the pulsed laser ablation deposited films of Al72Fe28, Al40Fe60, and Al18Fe82 compositions showed evidence of composition partitioning during crystallization for films of all three compositions. This composition partitioning, in turn, resulted in the evolution of phases of compositions richer in Fe, as well as richer in Al, compared to the overall film composition in each case. The evidence of Fe-rich phases was the B2 phase in Al72Fe28 film, the L12- and DO3-ordered phases in Al40Fe60 film, and the hexagonal ε–Fe in the case of the Al18Fe82 film. On the other hand, the Al-rich phases were Al13Fe4 for both Al72Fe28 and Al40Fe60 films and DO3 and Al5Fe2 phases in the case of Al18Fe82 film. We believe that this tendency of composition partitioning during crystallization from amorphous phase is a consequence of the tendency of clustering of the Fe atoms in the amorphous phase during nucleation. The body-centered cubic phase has a nucleation advantage over other metastable phases for all three compositions. The amorphization of Al18Fe82 composition and the evolution of L12 and ε–Fe phases in the Al–Fe system were new observations of this work.
Promoting secondary nucleation using methane modulations during diamond chemical vapor deposition to produce smoother, harder, and better quality films
Authors: Ali N. , Neto V.F. , Gracio J.
Published: March 2008

In this paper, we present results obtained from a comparison study relating to the deposition of diamond films using two processes, namely, time-modulated chemical vapor deposition (TMCVD) and conventional CVD. Polycrystalline diamond films were deposited onto silicon substrates using both hot-filament CVD and microwave plasma CVD systems. The key feature of TMCVD is that it modulates methane (CH4) flow during diamond CVD, whereas in conventional CVD the CH4 flow is kept constant throughout the deposition process. Films grown using TMCVD were smoother, harder, and displayed better quality than similar films grown using constant CH4 flow during CVD. The advantage of using TMCVD is that it promotes secondary nucleation to occur on existing diamond crystals. Pulsing CH4, consecutively, at high and low concentrations allows the depositing film to maintain its quality in terms of diamond-carbon phase. Films grown under constant CH4 flow during diamond CVD displayed a columnar growth mode, whereas with the time modulated films the growth mode was different. The mechanism of film growth during TMCVD is presented in this paper. The growth rate of films obtained using the hot filament CVD system with constant CH4 flow was higher than the growth rate of time modulated films. However, using the microwave-plasma CVD system, the effect was the contrary and the time-modulated films were grown at a higher rate. The growth rate results are discussed in terms of substrate temperature changes during TMCVD.
Fibrous structures on diamond and carbon surfaces formed by hydrogen plasma under direct-current bias and field electron-emission properties
Authors: Kobashi Koji , Tachibana Takeshi , Yokota Yoshihiro , Kawakami Nobuyuki , Hayashi Kazushi , Yamamoto Kazuhiro , Koga Yoshinori , Fujiwara Shuzo , Gotoh Yasuhito , Nakahara Hironori , Tsuji Hiroshi , Ishikawa Junzo , Köck Franz A. , Nemanich Robert J.
Published: March 2008

Polycrystalline diamond films, single crystal bulk diamonds, and diamond powder were treated in microwave plasma of hydrogen at 1.6 torr under a negative direct-current bias of −150 to −300 V without metal catalyst. It was found that fibrous structures, uniformly elongated along the direction normal to the specimen surface, were formed on the diamond surfaces. Similar experiments for glasslike carbon resulted in conical structures with frizzy fibers at the tops. Transmission electron microscopy measurements indicated that the fibers formed on diamond consisted of randomly oriented diamond nanocrystals with diameters of less than 10 nm, while the conical structures formed on glasslike carbon consisted of graphite nanocrystals. Field emission measurements of the fibrous specimens exhibited better emission efficiency than untreated ones. The field emission electron microscopy of the fibrous glasslike carbon showed a presence of discrete electron emission sites at a density of approximately 10,000 sites/cm2.
Hydrothermal synthesis of KNbO3 and NaNbO3 powders
Authors: Goh Gregory K.L. , Lange Fred F. , Haile Sossina M. , Levi Carlos G.
Published: March 2008

Orthorhombic KNbO3 and NaNbO3 powders were hydrothermally synthesized in KOH and NaOH solutions (6.7–15 M) at 150 and 200 °C. An intermediate hexaniobate species formed first before eventually converting to the perovskite phase. For synthesis in KOH solutions, the stability of the intermediate hexaniobate ion increased with decreasing KOH concentrations and temperatures. This led to significant variations in the induction periods and accounted for the large disparity in the mass of recovered powder for different processing parameters. It is also believed that protons were incorporated in the lattice of the as-synthesized KNbO3 powders as water molecules and hydroxyl ions.
Glass transformation temperature and stability of tellurite glasses
Authors: El-Mallawany Raouf
Published: March 2008

The glass transformation (Tg) and onset crystallization temperatures (Tx) of (100 – x) TeO2–(x)V2O5, (x = 10, 35, and 50 mol%) glasses were measured in the temperature range 300–800 K by differential scanning calorimetry at different heating rates. From the variation of the heating rate, the glass transition activation energy was calculated by different methods. The glass stabilization range S = Tx – Tg was calculated for the whole glass series. Quantitative analysis of the glass transformation temperature was carried out using the calculated number of bonds per unit volume and oxygen packing density.
Continuous hydrothermal crystallization of α–Fe2O3 and Co3O4 nanoparticles
Authors: Hao Yalin , Teja Amyn S.
Published: March 2008

Iron oxide (α–Fe2O3) and cobalt oxide (Co3O4) were produced via precipitation reactions carried out in a continuous hydrothermal apparatus. The resulting particles were nanometer-sized because of the high supersaturations generated when metal nitrate solutions are combined with sodium hydroxide or with hot, compressed water. The average particle size increased with the metal nitrate feed concentration and with residence time. A logarithmic relationship was obtained between the particle size and feed concentration and between particle size and residence time in the apparatus. The production of nanoparticles with narrow size distribution was shown to require low metal nitrate feed concentrations and short residence times. In the range of temperatures studied in this work, temperature apparently had no effect on the size except when cobalt nitrate was contacted with supercritical water in the absence of sodium hydroxide. In this case, large cobalt oxide particles were obtained when the temperature was above the critical temperature of water.
Influence of deposition parameters on the stress of magnetron sputter-deposited AlN thin films on Si(100) substrates
Authors: Iriarte G.F. , Engelmark F. , Ottosson M. , Katardjiev I.V.
Published: March 2008

In this work, a systematic study of the influence of five deposition parameters, i.e., process pressure, substrate temperature, target power, and substrate bias, as well as gas composition on the residual stress in fully textured polycrystalline aluminum nitride thin films deposited on Si(100) wafers using the reactive sputtering method was performed. Post-growth residual stress measurements were obtained indirectly from radius of curvature measurements of the wafer prior to and after deposition. Two different techniques were used to determine the curvature: an optically levered laser beam and an x-ray diffraction method. Stresses in both cases were then evaluated using the Stoney formulation [G.G. Stoney, Proc. R. Soc. (London)A82,172 (1909)]. Both methods give similar results, with slight quantitative differences. The existence of a transition region between tensile and compressive stress previously reported in the literature is also confirmed. The transition is shown to be strongly dependent on the process parameters. Optimal films regarding stress were grown at 2 mtorr, 900 W at the target, a 20/45 Ar/N2gas mixture, and floating potential at the substrate. The substrate temperature did not influence the measured internal stress in the films.
Bioactive behaviors of porous apatite- and wollastonite-containing glass-ceramic in two kinds of simulated body fluid
Authors: Zhang Yunlong , Mizuno Mineo , Yanagisawa Masaaki , Takadama Hiroaki
Published: March 2008

Simulated body fluid (SBF) has been widely used for assessment of bioactivities of different materials. Those kinds of conventional SBF (C-SBF) have a higher Cl− concentration and a lower HCO3− concentration than those of human blood plasma. Therefore, a revised simulated body fluid (R-SBF), with ion concentrations including those of Cl− and HCO3− exactly equal to those of human plasma, was recently developed. Porous glass-ceramic was used as the testing material to evaluate the two kinds of SBF. Results showed that C-SBF is considered more active than R-SBF because the porous glass-ceramic had a higher apatite formation rate on its surface,while R-SBF is considered to simulate the human body fluid more precisely than C-SBF.
Self-propagating high-temperature synthesis of functionally graded materials as thermal protection systems for high-temperature applications
Authors: Bertolino N. , Monagheddu M. , Tacca A. , Giuliani P. , Zanotti C. , Maglia F. , Tamburini U. Anselmi
Published: March 2008

Self-propagating high-temperature synthesis was used to prepare boride-based functionally graded materials (FGMs) as thermal barriers for space re-entry vehicles. FGMs are characterized by inhomogeneous spatial composition, resulting in different spatial characteristics. In this work, the FGMs were composed of a ceramic [i.e., MB2 (M = Ti, Zr, Hf)] and a metallic (i.e., NiAl) side, joined together by composite layers of graded stoichiometries of the two components. Thus, in the same material, the boride end gives thermal insulation, while the intermetallic end offers an easy junction to the structure of the space aircraft. The prepared FGMs showed good adhesion between the layers and global compactness after preparation and thermal tests. The microhardness along the samples was measured, and their insulating capabilities were evaluated.
Structural transformation of a kaolinite and calcite mixture to gehlenite and anorthite
Authors: Traoré Karfa , Blanchart Philippe
Published: March 2008

Kaolinite mixed with calcite was sintered at low temperature (1100 °C; 5 °C/min). The successive phase transformations are metakaolinite to gehlenite and then anorthite, although the available phase diagram indicates a direct anorthite recrystallization. Transmission electron microscopy and electron diffraction studies of nanocrystallites revealed that the transformation path is favored by the structural similarities of phases. In particular, the pseudolayers of gehlenite have a major orientation relationship with the initial metakaolinite layers. The gehlenite axis, [001]G, is parallel to the metakaolinite axis, [001]A. This direct transition is favored by the existence of Si tetrahedral units and 4–fold–coordinated Al in both structures. Ca atoms, initially in the interlayer spacing of metakaolinite, remain in the interlayers of gehlenite. During the second transformation step, anorthite recrystallizes from gehlenite with axis [020]A parallel to [210]G. It is proposed that this orientation relationship is favored by the orientation and shape of Ca-atom channels through both structures, along [001]G and [100]A axes.
Preparation and characterization of poly(tetramethyl-p-phenylenediamine)/clay hybrids via intercalative polymerization
Authors: Chen Guangming , Iyi Nobuo , Fujita Taketoshi
Published: March 2008

New noncovalent bonding polymer/clay hybrids were prepared, including the polymer poly(tetramethyl-p-phenylenediamine) (poly-TMPD). Polymerization occurred in the interlayer space of clay mineral successively after intercalation of monomers. Two types of clay minerals with different surface properties—a hydrophilic lithium fluorotaeniolite (TN) and four kinds of organophilic fluorotaeniolites (org-TNs)—were used as the hosts. Powder x-ray diffraction results showed an increase of 0.7–1.0 nm in the basal spacings, indicating the formation of poly–TMPD in the interlayer space of the hosts. Intercalative polymerization was also supported by Fourier transform infrared spectroscopy. The orientation of the poly-TMPD and thermal behavior were also discussed.
Porous silica derived from chitosan-containing hybrid composites
Authors: Retuert J. , Quijada R. , Arias V. , Yazdani-Pedram M.
Published: March 2008

In this paper, we report the preparation by the sol-gel technique of organic–inorganic hybrid composites containing the biopolymer chitosan incorporated in a siloxane-based inorganic network. The hybrid xerogels were transformed into porous silica particles by elimination of the organic phase. Surface characteristics of the silica samples can be easily tailored. In this way Brunauer–Emmett–Teller areas, pore volume, and pore diameter of the prepared silica can be predetermined within a wide range. Morphology of the particles at longer length scales can be designed to obtain either irregularly shaped particles with layered morphology or spherical particles. The results are explained on the basis of the cationic polyelectrolytic properties of chitosan, which allows easy association with siloxane oligomers, the precursors of silica in forming hybrid nanocomposites.
Agglomerate formation during drying
Authors: Vertanessian Alejandro , Allen Andrew , Mayo Merrilea J.
Published: March 2008

The evolution of agglomerate structure during drying of particles from suspension has been studied for a nanocrystalline Y2O3 (8% mol fraction)-stabilized ZrO2 powder. Agglomerates in drying and dried suspensions were examined at the smallest size scales (1 nm to 1 μm) using ultra-small angle x-ray scattering (USAXS) and at the largest size scales (100 nm to 10 μm) using scanning electron microscopy. The results were correlated with the degree of particle dissolution in each suspension (measured by flame absorption spectroscopy of the suspension filtrate) and the zeta potential of the particles in suspension prior to drying. Results show that large agglomerates readily form across a pH range from 2 to 9. The fact that Y+3 ion dissolution varies by over four orders of magnitude in this range leads to the conclusion that there is little direct correlation between the degree of Y dissolution and agglomeration in this system (Zr ion dissolution was below the detection limit at all pH values studied). The observation of large agglomerates well before the introduction of air-water interfaces into the drying mass likewise leads to the conclusion that capillary forces are not essential to agglomerate formation. Instead, agglomerates appear to form as a direct consequence of increasing suspension concentration. Zeta potential also plays a role. Specifically, there was a notable change in agglomerate morphology as the isoelectric point was approached, at approximately pH 8. Here USAXS shows the particles in suspension to have a layered interior structure, with small primary particles aggregated in sheets to form each blocky particle. This is in contrast to the more rounded agglomerates formed away from the isoelectric point, which appear to be composed of the same primary particles arranged in chainlike structures. USAXS of powders from the dried suspensions confirms that the structures seen after drying are the same as those present in suspension. The two structural morphologies are attributed to diffusion-limited (sheets) versus reaction-limited (chains) aggregation, respectively.
Mechanical evaluation at high temperatures of hot gunning refractory mixtures
Authors: Gasillón G.I. , Martinez A.G.Tomba , Cavalieri A.L. , de Córdova M. , Topolevsky R.
Published: March 2008

The mechanical behavior at high temperatures of three hot gunning refractory mixtures used for coke oven repair was studied. The materials were characterized by chemical, mineralogical, granulometric, and microscopic analyses, exhibiting differences in composition and particle size distributions of both aggregates and bonding phases. The hot adhesion of the mixtures to silica bricks was determined using torsional forces, and the interphase between them was analyzed by scanning electron microscopy. Thermal expansion up to 1200 °C and creep under compressive load (172 kPa at 1200 °C, 3 h) were carried out on prisms prepared by ramming and sintering (1200 °C, 1 h). The creep strain–time data are well fitted using constitutive equations. The mixtures showed differences in both hot adhesion and thermal expansion–creep tests: the material based on calcium aluminates exhibited poorer properties than those based on calcium silicate. The results are explainded taking into consideration the characteristics of both aggregates and bonding phases.