bluuebunny/crossref_metadata_2025 huggingface dataset:
Solid-state synthesis of multiwalled carbon nanotubes
Authors: Doherty S. P. , Buchholz D. B. , Li B-J. , Chang R. P. H.
Published: March 2008

A modified high-temperature arc furnace was used to produce carbon nanotubes from carbon black by a solid-state transformation without a catalyst. The layer of carbon nanotubes thus formed was nearly pure with only a minor amount of carbon black particles. The properties of these nanotubes were found to be very similar to those produced by the conventional arc synthesis. Based on this process, a mechanism for the growth of these nanotubes is proposed. In addition, field-emission properties of these nanotubes were comparable to the properties of arc-grown carbon nanotubes.
Microstructural characterization of Yba2Cu3O7–δ thick films grown at very high rates and high temperatures by pulsed laser deposition
Authors: Berenov A. , Malde N. , Bugoslavsky Y. , Cohen L. F. , Foltyn S. J. , Dowden P. , Ramirez-Castellanos J. , Gonzalez-Calbet J. M. , Vallet-Regi M. , MacManus-Driscoll J. L.
Published: March 2008

Microstructural and magnetic characterization were undertaken on high-rate, high-temperature grown YBa2Cu3O7–δ (YBCO) films. The films were of approximately 1 μm thickness and were grown by pulsed laser deposition on (100) SrTiO3 using a high-power industrial laser at growth temperatures between 750 °C and 870 °C and at growth rates of up to 4 μm/min. Two YBCO layers with different c-lattice parameters were observed in the films, the higher c value occurring near the substrate interface and arising from cation disorder and oxygen nonstoichiometry, and the lower one near the film surface arising from cation disorder alone. BaCuO2 precipitates were present near the surface of the films, indicative of partial melting during growth. The amount of BaCuO2 increased with growth temperature. Epitaxial Y2O3 also formed in increasing amounts suggestive of a different partial melting reaction in the films compared to bulk YBCO, where Y2BaCuO5 coexists with liquid. Around 1 MA/cm2 values of high critical current density (Jc) were observed in the films, and the in-field Jc improved with growth temperature despite the fact that the superconducting transition width increased significantly.
Processing and microstructural characterization of sputter-deposited Ni/Ni3Al multilayered thin films
Authors: Sperling Evan A. , Banerjee Rajarshi , Thompson Gregory B. , Fain Jason P. , Anderson Peter M. , Fraser Hamish L.
Published: March 2008

The crystallographic texture, orientation relationships, coherency stress, and thermal stability of sputter-deposited Ni/Ni3Al multilayered thin films were studied as a function of bilayer period (Λ) as well as processing parameters such as substrate type, deposition temperature, and prebake conditions. Deposition onto oxidized Si or single-crystal Cu(001), NaCl(001), or KBr(001) substrates near room temperature produces multilayers with a [111] crystallographic texture along the Ni/Ni3Al interface normal and a disordered face-centered cubic structure for the Ni3Al phase. In contrast, deposition at 673 K onto NaCl(001) or KBr(001) substrates that are prebaked in vacuum at 693 K produces a chemically ordered L12 structure for the Ni3Al phase and (001) epitaxial growth. X-ray diffraction measurements of (001) multilayers with equal volume fraction of Ni and Ni3Al reveals a transition from a nearly incoherent state at Λ=40 nm to a semicoherent one at Λ 40 nm. Remarkably, (001) multilayers were observed to solutionize at 1373 K, which is approximately 100 K below that predicted by the Ni–Al phase diagram.
Microstructures and mechanical properties of ultrafine-grained Ti foil processed by equal-channel angular pressing and cold rolling
Authors: Zhu Y. T. , Kolobov Y. R. , Grabovetskaya G. P. , Stolyarov V. V. , Girsova N. V. , Valiev R. Z.
Published: March 2008

We processed coarse-grained Ti and equal-channel angular pressing (ECAP) processed ultrafine-grained (UFG) Ti into 20-μm-thick Ti foils by cold rolling and intermediate annealing. The foils produced from rolling the UFG Ti exhibit a homogeneous nanostructure, while foils produced from rolling the coarse-grained Ti exhibit heterogeneous structures with a mixture of nanostructured regions and coarse-grained regions. The former foils also have higher strength and ductility and exhibit uniform deformation over a larger strain range at room temperature than the latter ones. This work demonstrated the advantage and viability of producing nanostructured Ti foil by rolling ECAP-processed UFG Ti stock.
What types of grain boundaries can be passed through by persistent slip bands?
Authors: Zhang Z. F. , Wang Z. G. , Eckert J.
Published: March 2008

Three typical interactions of persistent slip bands (PSBs) with different types of grain boundaries (GBs) were investigated and analyzed in fatigued copper crystals. The results show that PSBs cannot transfer through all types of large-angle GBs, regardless of their orientation with respect to the stress axis. Secondary slip was often observed near the GBs, leading to strain incompatibility. When the slip systems of the two adjacent crystals are coplanar, the transmission of a PSB across a GB strongly depends on the slip directions of the two adjacent crystals. It was found that only the low-angle GBs can be passed through by PSBs, and accordingly they are insensitive to intergranular fatigue cracking. For a special copper bicrystal with coplanar slip systems, the ladderlike dislocation arrangements within the adjacent PSBs become discontinuous and a dislocation-affected-zone appears near the GB due to the difference in the slip direction of the two adjacent crystals. Therefore, the necessary conditions for the transmission of a PSB across a GB are that the neighboring grains have a coplanar slip system and identical slip directions.
Temperature dependence of the soft magnetic character of Fe73.5Cu1Nb3Si13.5B9 amorphous and nanocrystalline alloys
Authors: Gonzáz J.
Published: March 2008

Results on microstructure and coercivity of current-annealed Fe73.5Cu1Nb3Si13.5B9 amorphous alloy treated at different current densities (12–56 A/mm2) and duration (0.5–720 min) are presented. Saturation magnetization and coercivity dependencies with the current density of the nanocrystalline samples is explained by considering the presence of two phases: nanocrystals of Fe(Si) body-centered cubic (bcc) grains and the residual amorphous matrix. An increase in the magnetic hardness observed when the sample was heated by current densities, giving rise to an increase in the sample temperature above the Curie point of the residual amorphous matrix, could be ascribed to exchange and dipolar decoupling of the Fe(Si)-bcc grains.
Use of combined elastic modulus in depth-sensing indentation with a conical indenter
Authors: Fischer-Cripps A. C.
Published: March 2008

Conventional methods of analysis for depth-sensing indentation test data use the slope of the elastic unloading portion of the load–displacement response in conjunction with the elastic equations of contact for a rigid cone. It is common practice to incorporate the combined modulus of the indenter and specimen in these equations although the validity of this practice never appears to have been verified. This work demonstrates the validity of using the combined elastic modulus in depth-sensing indentation testing in conjunction with the elastic equations of contact for a conical indenter.
Partial melt processing of solid-solution Bi2Sr2CaCu2O8+δthick-film conductors with nanophase Al2O3additions
Authors: Haugan T. , Wong-Ng W. , Cook L. P. , Vaudin M. D. , Swartzendruber L. , Barnes P. N.
Published: March 2008

Partial-melt processing of Bi2+xSr2-x-yCa1+yCu2O8+δ(Bi-2212) thick-film conductors with additions of nanophase Al2O3was studied for dual purposes of increasing flux pinning and inhibiting Sr—Ca—Cu—O phase defect formation. Nanophase Al2O3(<50% mole fraction) was added to Bi:Sr:Ca:Cu:O powders with four different compositions: three with Bi:Cu approximately 2:2 and one (Bi2Sr2.38Ca1.15Cu2.92O9.7+δ) closer to the ideal Bi-2223 composition. The effect of Al2O3addition on film microstructural and superconducting properties was studied for a range of partial-melt temperatures (865 to 900 °C). Results were compared to Al2O3-free films with compositions lying within the single-phase solid-solution 2212 region. Nanophase Al2O3reacted with 2212-type precursors to form a composite of micron size or smaller particles of solid-solution (Sr,Ca)3Al2O6in a solid-solution 2212 superconducting matrix. The Ca content of the (Sr,Ca)3Al2O6particles formed approximated that of the 2212 precursor (≤6% mole fraction difference). Addition of 6–25% volume fraction of (Sr,Ca)3Al2O6to Bi-2212 (by reaction between Al2O3and Bi-2212) only slightly reduced superconducting transition temperatures andc-axis texturing; however this addition improved film quality by reducing Sr—Ca—Cu—O defect volume fraction by factors of 2 to 6 and significantly increased the critical current density by over one order of magnitude for 0 to 2 T applied fields at 20 to 30 K.
Preparation and structure of organic-inorganic hybrid low-melting phosphite glasses from phosphonic acid H3PO3
Authors: Niida Haruki , Takahashi Masahide , Uchino Takashi , Yoko Toshinobu
Published: March 2008

An organic–inorganic, hybrid, low-melting glass free of pollution elements such as Pb and F was prepared through nonaqueous acid-base reactions of Si—Cl + P—OH ⇉ Si—O—P + HCl⇈ and Sn—Cl + P—OH ⇉ Sn—O—P + HCl⇈ using phosphonic acid (H3PO3), dimethyldichlorosilane (Me2SiCl2), and tin(II)chloride (SnCl2) as starting materials. Transparent, colorless, and homogeneous phosphite glass was successfully obtained. The formation of P—O—Si linkage and the disappearance of P—OH were confirmed by 31P nuclear magnetic resonance spectra. With increasing Si content, the glass-transition temperature decreased, reflecting the decrease of the average linkage number per one cation.
Observation and formation mechanism of stable face-centered-cubic Fe nanorods in carbon nanotubes
Authors: Kim Hansoo , Kaufman Michael J. , Sigmund Wolfgang M , Jacques David , Andrews Rodney
Published: March 2008

The crystallographic structure and orientation of iron nanoparticles present in carbon nanotubes (CNTs) was studied when iron was used as a catalyst. It was found that while most of the nanoparticles encapsulated inside the CNTs had the expected α–Fe (body-centered-cubic) phase, a significant number of them formed and retained the γ–Fe (face-centered-cubic) phase that is not the normal bulk phase at room temperature (nor even expected to form at the growth temperature used). It was also found iron particles at the tips of the nanotubes were either α–Fe or cementite (Fe3C). On the basis of these observations and thermodynamics, a mechanism for the formation of these particles and insights into CNT growth is proposed.
Monodispersed Sc2O3 precursor particles via homogeneous precipitation: Synthesis, thermal decomposition, and the effects of supporting anions on powder properties
Authors: Li Ji-Guang , Ikegami Takayasu , Mori Toshiyuki , Yajima Yoshiyuki
Published: March 2008

Monodispersed Sc2O3 precursor particles were synthesized by the urea-based homogeneous precipitation method, with an investigation into the effects of supporting anions (NO3−, Cl−, and SO4 2−) on powder properties. Characterizations of the powders were achieved by elemental analysis, x-ray diffractometry, Fourier transform infrared, differential thermal analysis/thermogravimetry, high-resolution scanning electron microscopy, and the Brunauer-Emmett-Teller method. Unlike other rare earths, Sc3+ does not precipitate as basic carbonate but instead forms hydrated γ-ScOOH from either nitrate or chloride solution. Particles of the hydrated γ-ScOOH are pumpkin-shaped (approximately 1.0 μm) and are made up of thin-platelike crystallites emanating from a common axis. The presence of complexing SO42− changes the reaction chemistry toward Sc2O3 powders, leading to basic sulfate [Sc(OH)1.6(SO4)0.7 η H2O] precursor particles having hexagonal morphology (approximately 10 μm in diameter and 0.5 μm in thickness). The hydrated γ-ScOOH directly converts to Sc2O3 by calcination at 400 °C or above, while the basic sulfate transforms to oxide at temperatures ≥ 900 °C via an amorphous state and a Sc2(SO4)3 intermediate. The effect of SO42− on powder morphologies and reaction chemistry is discussed. Nanocrystalline Sc2O3 powders comprising monodispersed particles were obtained via thermal decomposition of the precursors.
Experimental and theoretical characterization of the surface acoustic wave propagation properties of GaN epitaxial layers on c-plane sapphire
Authors: Choi Kook Hyun , Joon Hyeong , Chung Su Jin , Kim Jin Yong , Lee Tae Kun , Kim Young Jin
Published: March 2008

Surface acoustic wave (SAW) propagation properties of gallium nitride (GaN) epitaxial layers on sapphire were theoretically and experimentally characterized. GaN thin films were grown on a c-plane sapphire substrate using a metalorganic chemical vapor deposition system. The experimental characterization of SAW propagation properties was performed with a linear array of interdigital transducer structures, while SAW velocities were calculated by matrix methods. Experimentally, we found pseudo-SAW and high-velocity pseudo-SAW modes in the GaN/sapphire structure, which had a good agreement with calculated velocities.
An ab initio study of the ideal tensile and shear strength of single-crystal β–Si3N4
Authors: Ogata Shigenobu , Hirosaki Naoto , Kocer Cenk , Shibutani Yoji
Published: March 2008

In this study, the ideal tensile and shear strength of single-crystal β–Si3N4 was calculated using an ab initio density functional technique. The stress-strain curve of the silicon nitride polymorph was calculated from simulations of uniaxial strain deformation. In particular, the ideal strength calculated for an applied ∈11 tensile strain was estimated to be approximately 57 GPa. Recently, a good correlation was reported between the shear modulus of high-strength materials and the experimentally determined Vickers indentation hardness value. Using the reported correlation an estimate was made of the Vickers indentation hardness of single-crystal β–Si3N4: approximately 20.4 GPa.
X-ray photoelectron spectroscopy study of the metal/polymer contacts involving aluminum and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) derivatives
Authors: Jönsson S. K. M. , Salaneck W. R. , Fahlman M.
Published: March 2008

The contact formed between aluminum and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) (PEDOT-PSS) derivatives was studied using x-ray photoelectron spectroscopy. The aluminum/PEDOT-PSS contact contains an interfacial layer formed by chemical reactions between aluminum and mainly poly(styrenesulfonic acid) (PSSH). These chemical interactions were studied with the help of model systems (PSSH, benzenesulfonic acid, and sodium benzenesulfonate). The preferred reaction site of aluminum is the SO3− and SO3−H+ groups of the PSS chains, giving rise to C-S-Al(-O) and C-O-Al species. The resulting contact formed consists of an insulating aluminum/PSS layer and a thin region of partially dedoped PEDOT-PSS. There is significant aluminum diffusion into films of the highly conducting form of PEDOT-PSS that have substantially less PSS at the surface. Hence, no (thick) aluminum/PSS layer is formed in this case, though the PEDOT chains close to the aluminum contact will still be partially dedoped as for the aluminum/PEDOT-PSS case.
Effect of poly(acrylic acid) and poly(vinyl alcohol) on the solubility of colloidal BaTiO3 in an aqueous medium
Authors: Paik Ungyu , Hackley Vincent A. , Lee Jaeho , Lee Sangkyu
Published: March 2008

The influence of poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA) on Ba dissolution from the BaTiO3-aqueous solution interface was investigated. Incongruent dissolution of Ba impacts the colloidal stability, microstructure, and electrical properties of BaTiO3 and related perovskite dielectric materials used in the manufacture of ceramic capacitors. The solubility characteristics of BaTiO3 were influenced significantly by the presence of PAA and PVA. PAA, which forms weak monodentate complexes with Ba2+, acted as both a passivating and a sequestering agent, depending on pH. Both PAA and PVA provided some degree of passivation in the acidic pH region. Above pH 8, where BaTiO3 solubility decreases sharply, PVA had a moderate passivating effect, whereas solubility was enhanced by PAA with a positive linear dependence on concentration. The adsorptive and electrokinetic behavior of colloidal BaTiO3 with respect to PAA and PVA are correlated with the observed passivating and sequestering properties of these polymers.
Decomposition of NiMn2O4 spinels
Authors: de Györgyfalva G. D. C. Csete , Reaney I. M.
Published: March 2008

Thermal analysis and x-ray diffraction have confirmed that single-phase, cubic-spinel-structured NiMn2O4 (2:1 Mn2O3:NiO) begins to decompose into a rocksalt and a second spinel-structured phase above 907 °C. The decomposition product in samples air-quenched from 900 and 1200 °C was therefore investigated using transmission electron microscopy. Samples quenched from 900 °C (below decomposition temperature) did not show the expected single-phase microstructure, but instead grains contained nanoregions of a lenticular fringe contrast. Samples quenched from 1050 to 1200 °C were generally composed of Mn-rich spinel grains in addition to grains containing Mn-rich spinel precipitates (30–50 nm) surrounded by a Ni-rich rocksalt matrix. As temperature increased, the spinel grains and precipitates became clearly tetragonal, exhibiting a ferroelastic domain structure arising from a cooperative Jahn–Teller distortion. A decomposition mechanism based on the degree of inversion is proposed to explain these microstructures. Slow cooling samples from 1250 °C resulted in partial recomposition, leading to a microstructure principally composed of cubic spinel and regions of much smaller spinel structured precipitates (50–120 nm) in a rocksalt structured matrix. The slow-cooled samples showed a larger increase in resistance over time than single-phase samples did when held at 400 °C. X-ray diffraction measurements carried out before and after electrical characterization showed a reduction in the amount of rocksalt structured material present in slow-cooled samples.
Biomimetic synthesis of superparamagnetic iron oxide particles in proteins
Authors: Sinha Arvind , Nayar Suprabha , Murthy G. V. S. , Joy P. A. , Rao V. , Ramachandrarao P.
Published: March 2008

Matrix-mediated in situ synthesis of monodispersed magnetite and maghemite nanoparticles (2–16 nm) was carried out using the cavities present in gels of globular proteins such as egg white and bovine serum albumin. Under stringent conditions, spatial-charge-distribution-assisted molecular recognition of proteins for inorganic ions led to the site- and polymorph-specific synthesis of superparamagnetic iron oxide particles. A transformation from magnetite to maghemite as a nucleating phase could be observed by partially denaturing the egg white protein, signifying the delicate role of quaternary structure of proteins under different reaction conditions, in determining the size and shape of the polymorph.
Unlubricated tribological performance of advanced ceramics and composites at fretting contacts with alumina
Authors: Basu Bikramjit , Vleugels Jozef , Van Der Biest Omer
Published: March 2008

Engineering ceramics and cermets are widely used for demanding tribological applications. In this perspective, the objective of this paper was to understand the friction and wear behavior of some of the potential tribomaterials, e.g., ZrO2–30 vol% TiB2 composite, sialon–40 vol% TiB2 composite, TiB2-based cermet with 16 vol% Ni3(Al,Ti) binder, and monolithic TiB2 in fretting contacts. Wear tests on the TiB2-containing materials under dry unlubricated conditions (23–25 °C, 50–55% relative humidity) were performed against corundum on a ball-on-flat tribometer. The obtained friction and wear data were critically analyzed to investigate how the binder phase in TiB2 matrix influences the tribological performance. Furthermore, morphological investigations of the transfer layers on the worn surfaces were performed and the wear mechanisms discussed. X-ray photoelectron spectroscopy analysis of the worn surfaces in the monolithic TiB2/alumina revealed the pronounced transfer of mixed oxides containing TiO2 and B2O3 to the alumina counterbody and also indicated the transfer of alumina to TiB2 flat. Tribochemical reactions and abrasion along with the material transfer between the counterbodies were observed to play a major role in the wear of the fretting couples.
Transformation kinetics for mullite in kaolin–Al2O3 ceramics
Authors: Chen Yung-Feng , Wang Moo-Chin , Hon Min-Hsiung
Published: March 2008

Transformation kinetics of mullite formation in kaolin–Al2O3 ceramics was studied by x-ray diffraction, transmission electron microscopy, and energy dispersion spectrometry. The mullitization process of kaolin–Al2O3 ceramics is described by two stages; one is the primary mullite transformation at 1273 to 1573 K, and the other is the secondary mullite formation at 1573 to 1873 K. The activation energy of 1164.6 kJ mol-1 obtained for the secondary mullite formation is lower than 1356.9 kJ mol-1 for the primary mullite transformation by the general form of the Johnson–Mehl–Avrami equation. The lower value of growth morphology parameter strongly supports that in the secondary mullite formation the added alumina is dissolved into glassy phase and the mullite is then precipitated.
Mechanical methods to determine layer compliances within multilayered composites
Authors: Malzbender J. , Steinbrech R. W.
Published: March 2008

A comparison of bending and compressive/tensile loading cases of multilayered composites is given. Bending perpendicular to the layer interfaces is suggested as a sensitive and experimentally convenient method for determining the elastic modulus of compliant isotropic layers within a multilayered composite. Different approaches to analyze the strain and compliance behavior of the composite and individual layers are derived. Ranking of the relevant equations with respect to strain sensitivity is made, which favors the bending test perpendicular to the layer interfaces. The derived relationships permit a prediction of the bending behavior and the flexural rigidity of the composite. Furthermore, the properties of a layer within the structure can be determined. Limitations that exist for the bending perpendicular to the layer interfaces in the case of low stiffness of the layers can be overcome using either a compressive or a tensile loading mode. Application of the formulas to buckling and interfacial delamination is considered. Materials, such as plasma-sprayed thermal barrier coatings, that are supposed to behave elastically different in tension and compression are given special consideration. Although determination of elastic moduli is the main interest, in the case of known elastic moduli, the formulas can be used to determine the thickness of individual layers within a composite structure.
Formation and mechanical properties of Cu–Hf–Al bulk glassy alloys with a large supercooled liquid region of over 90 K
Authors: Inoue Akihisa , Zhang Wei
Published: March 2008

New Cu-based bulk glassy alloys with large supercooled liquid regions and high mechanical strength were formed in Cu–Hf–Al ternary systems. The large supercooled liquid region exceeding 70 K was obtained in the composition range of 40 at.% Hf at 2.5% Al, 37.5–50% Hf at 5% Al, and 45% Hf at 7.5% Al. The largest supercooled liquid region ΔTx(= Tx – Tg) was 91 K for Cu50Hf45Al5 alloy, and the highest reduced glass-transition temperature was 0.63 for Cu50Hf42.5Al7.5 and Cu52.5Hf40Al7.5 alloys. The alloys with large ΔTx values above 50 K were formed into bulk glassy rods with diameters up to 3 mm by copper mold casting, and the glassy alloy rods exhibited high compressive fracture strength of 2260 to 2370 MPa and Young's modulus of 121 to 128 GPa combined with elastic elongation of 1.9% to 2.0% and plastic elongation of 0.2% to 0.6%. No bulk glassy alloys were formed in the Cu–Hf binary system by copper mold casting, and, hence, the addition of 2.5% to 7.5% Al to Cu–Hf alloys was very effective for increasing glass-forming ability as well as the stabilization of supercooled liquid. The effectiveness can be interpreted on the basis of the concept of the formation of a unique glassy structure in special multicomponent alloys with the three component rules.
Tribological properties of a B2-type Al-Pd-Mn quasicrystal approximant
Authors: Mancinelli Chris , Jenks Cynthia J. , Thiel Patricia A. , Gellman Andrew J.
Published: March 2008

The tribological properties of a B2-type Al–Pd–Mn quasicrystal approximant were investigated and compared with those of an Al–Pd–Mn icosahedral quasicrystal. The approximant was of the _ phase, having a crystalline CsCl-type structure and nominal composition Al48Pd42Mn10. Friction coefficients measured in ultrahigh vacuum between a pair of Al48Pd42Mn10 samples having truly clean surfaces were found to be twice as high as those reported for the Al70Pd21Mn9 quasicrystal. When the surfaces were oxidized by exposure to O2 or H2O, the friction coefficients decreased by roughly a factor of two for both materials but the friction coefficient for the approximant remained roughly twice that of the quasicrystal. The rate of oxidation of the approximant was found to be one order of magnitude higher than that of the quasicrystal. This corroborates findings that suggest that quasicrystals exhibit an inherent resistance to oxidation and corrosion. Vickers hardness measurements show that the quasicrystal is roughly three times as hard as the approximant.
Relationship between nanohardness and microstructures in high-purity Fe–C as-quenched and quench-tempered martensite
Authors: Ohmura T. , Hara T. , Tsuzaki K.
Published: March 2008

The relationship between the nanohardness and the microstructures in the Fe–C martensite was studied to understand the contributions of the matrix and the grain boundary to the macroscopic strength. As-quenched martensite was examined for five kinds of Fe–C alloys with various carbon contents in the range of 0.1–0.8 mass%, while quench-tempered martensite was investigated for an Fe–0.4% C alloy. The ratio of the nanohardness to the macrohardnessHn/Hvwas much smaller for the Fe–C martensite than those for the single crystals, indicating that there is a significant grain-boundary effect for the martensite. The ratioHn/Hvof the as-quenched martensite decreased with an increase in the carbon content since the size of the block structure decreased with increasing carbon content. For the quench-tempered specimens, a significant reduction of the grain-boundary effect occured at the tempering temperature of 723 K. It is mainly due to the depression of the locking parameter caused by the disappearance of the film-like carbides on the boundaries.
Residual strain field in indented GaAs
Authors: Puech Pascal , Demangeot François , Pizani Paulo Sergio , Wey Samuel , Fontaine Chantal
Published: March 2008

This paper presents an optical mean to probe carefully the strain field generated by a microindentation on [111]-oriented GaAs sample, using micro-Raman spectroscopy and microphotoluminescence spectroscopy. Raman and photoluminescence signals recorded from the same point of the sample are directly compared. The frequency shift of the longitudinal and transverse optical phonons was analyzed in great detail, revealing unambiguously the presence of both compressive and tensile strains within the indented area. Outside the indentation fingerprint, the magnitude of strain deduced from luminescence measurements was found to be lower than the one determined by Raman scattering. The Raman spectra revealed significant variations of the optical phonons polarizability with the deformation. Finally, atomic force microscope images of the indented zone aid in the interpretation.
Amorphous alloys synthesized by interface-assisted ion beam mixing in the Ag–W system with the largest positive heat of formation
Authors: Zhang R. F. , Liu B. X.
Published: March 2008

Amorphous alloys were synthesized by interface-assisted ion beam mixing in the W-rich portion of the Ag–W system. These alloys are characterized by the largest positive heat of formation among the transition-metal alloys and were formed through a two-step structural transition in the alternately deposited Ag–W films. First, during alternate deposition of the nano-sized Ag and W layers, the interfacial free energy drove the Ag–W interaction, which resulted in an intermediate body-centered-cubic (bcc) supersaturated solid solution. Second, the bcc solid solution transformed into an amorphous state upon irradiation at room temperature by 200-keV xenon ions. We report the experimental observations of the Ag–W amorphous alloy formation and a brief discussion concerning the effects of the interfacial free energy and ion irradiation dose on the structural transition observed in the Ag–W films.
Aromatic liquid-crystalline polyesters comprising a 2,5-thiophene unit synthesized and studied by the thin-film polymerization method
Authors: Xu Jingmei , Wang Yan , Chung Tai-Shung , Goh Suat Hong
Published: March 2008

A series of novel liquid-crystalline polymer (LCP) systems, including a heterocyclic 2,5-thiophene unit, were synthesized and studied using a modified thin-film polymerization method as part of a continuing investigation of molecular structural effects on liquid crystallinity. The monomers used in the reaction includedp-acetoxybenzoic acid (ABA)/2,6-acetoxynaphtholic acid (ANA), 4,4′-biphenol (BP)/4,4′-diacetoxybiphenyl (DABP)/p-acetoxyacetanilide (AAA), and 2,5-thiophenedicarboxylic acid (TDA). Polarizing light microscopy and Fourier transform infrared spectra were used to study,in siturecord the evolution of morphological change in the thin-film polymerization process, and confirm the occurrence of polymerization. Experimental results indicated that monomer structure and composition as well as temperature influence the formation of the liquid crystalline (LC) phase greatly and revealed that 2,5-thiophene structure is a viable mesogenic core unit. It is more effective than isophthalic acid (IA) in assisting in the LC formation as polymers containing TDA have a better developed LC phase and lower critical ABA content than those with IA. Stripe texture was observed in the ANA/DABP/TDA system, possibly due to the structural characteristics and matching of the monomers. The decrease of isotropic round areas in the LC phase and the annihilation between two pairs of defects with reaction time were also studied.
Joint differential scanning calorimetry, electrical resistivity, and microhardness study of aging in two AlCuMg alloys
Authors: Riontino G. , Massazza M. , Abis S.
Published: March 2008

Two age-hardenable AlCuMg alloys (Cu 4.5 wt.% and Cu/Mg ratios of 2.5 and 8) were studied by electrical resistivity, differential scanning calorimetry, and microhardness measurements during natural and artificial aging at 110 °C. The results were interpreted in terms of Guinier–Preston/Guinier–Preston–Bagariatskij zone formation followed by a modification in θ″ and S″ intermediate phases.
Microstructures and mechanical properties of Sn–8.55Zn–0.45Al–XAg solders
Authors: Huang Chia-Wei , Lin Kwang-Lung
Published: March 2008

The microstructure, melting point, and mechanical properties of Sn–8.55Zn–0.45Al–XAg lead-free solders were investigated. The Ag content of the solders investigated was 0–3 wt.%. The results indicate that the AgZn3 and Ag5Zn8 compounds are formed at the addition of Ag to Sn–8.55Zn–0.45Al solders. The adding of Ag also results in the formation of hypoeutectic structure, increasing the melting point of the solders and decreasing the ductility. Results of thermal analysis reveal that the Sn–8.55Zn–0.45Al–XAg solder has eutectic temperature at 198 °C when the addition of Ag is 0.5 wt.%. The eutectic solder exhibits greater tensile strength and higher ductility than the 63–Sn–37Pb solder.
Evolution of microstructure in TiC/NiCr cermet induced by electropulsing
Authors: Zhang W. , Sui M. L. , Zhou Y. Z. , Guo J. D. , He G. H. , Li D. X.
Published: March 2008

Microstructures of a TiC/Ni80Cr20 cermet, subjected to single high-current-density electropulsing, were characterized by x-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy. Under the electropulsing, the shift of NiCr peaks versus the reverse change of TiC counterparts illustrates that the treatment gives rise to strong thermal stress impacting on the cermet. The stress, accompanied by the transient rise of temperature, led to microstructural evolutions of the cermet. Some nanostructured TiC grains, consisting of many nanocrystallites with small-angle grain boundaries, developed during electropulsing. Also, many regions teemed with coexisting nanosized TiC and NiCr crystallites, which possessed good bonding. Within the NiCr regions, large amounts of deformation twins were produced by the electropulsing.
Citrate-nitrate gel transformation behavior during the synthesis of combustion-derived NiO-yttria-stabilized zirconia composite
Authors: Marinšek Marjan , Zupan Klementina , Maček Jadran
Published: March 2008

NiO-yttria-stabilized zirconia powder mixtures were prepared from reactive citrate-nitrate gels using the combustion technique. The influence of the fuel/oxidant molar ratio in the precursor on the combustion rate and its thermal characteristics was studied by thermal analysis and evolved gas analysis. It was found that the precursor thermal decomposition properties depended strongly on the citrate/nitrate ratio prior to the combustion. Intermediate precursors and final powder ashes were also analyzed by x-ray diffraction.
Enhanced field emission from nitrogen-doped amorphous diamond
Authors: Kan Ming-Chi , Huang Jow-Lay , Sung James C. , Chen Kuei-Hsien , Lii Ding-Fwu
Published: March 2008

Undoped and nitrogen-doped amorphous diamond were deposited on n-type Si(100) substrate by a cathodic arc process. By varying nitrogen partial pressure of the gas mixture in the range of 0 to 55.6%, up to 23 at.% of nitrogen was incorporated in amorphous diamond. The electron spectroscopy for chemical analysis, Raman, and atomic force microscopy measurements indicated that most nitrogen atoms replaced carbon atoms in the amorphous structure. The electron concentration, mobility, and its resistivity were determined by the measurement of Hall effect. With the increasing of nitrogen content, the resistivity curve is U-shaped with a local minimum. The electron emission current in vacuum was characterized based on the diode design. It was found that the electron emission was highly reproducible. This is in contrast to the high variability observed for carbon nanotubes. The lowest resistivity coating had the lowest “turn-on” electrical field of 0.7 V/μxm at the current density of 10 μA/cm2.
Elastic recovery in the unloading process of pyramidal microindentation
Authors: Sakai M.
Published: March 2008

It is confirmed on the basis of extensive test results for various ceramic and metallic materials that the indentation load P versus penetration depth h curves (P–h curves) both in loading and unloading processes are well approximated with the quadratic formulas of P = k1h2 and P = k2(h – hr)2, respectively, and unloading parameter k2 is quantitatively related to the elastic modulus E′ of the material indented, where hr is the residual penetration depth after a complete unload. The unloading/reloading indentation processes for a locally deformed conical/pyramidal impression are well represented by the equivalent mechanical process of a conical/pyramidal indenter with the effective face angle of βeff = (β – βr) on a flat elastic half-space, in terms of the inclined face angles β and βr of the indenter used and of the residual impression formed, respectively. With utilization of the unloading parameter k2 and the relative residual depth of penetration ξr, a novel method is proposed for estimating E′. Theoretical considerations for a nonquadratic P–h unloading behavior are also made.
Chemical synthesis and characterization of boron/boron nitride core–shell nanostructures
Authors: Huo K. F. , Hu Z. , Fu J. J. , Xu H. , Wang X. Z. , Chen Y. , Lü Y. N. , Liu B. H. , Ding J.
Published: March 2008

A moderate chemical method [i.e., the reaction of diborane (B2H6) and a mixture gas of ammonia and nitrogen (NH3/N2) over nanoscale iron boride at 1100 °C] was developed to explore the boron nitride (BN) nanostructures. The products were well characterized by high-resolution electron microscopy and energy-dispersive x-ray spectroscopy. Two types of novel core–shell nanocapsules of amorphous boron core encapsulated in crystalline boron nitride shell were obtained. The first one looked like a peanut with an amorphous B core containing a trace of BN crystallites, a transition layer of BN nanofibers and amorphous B, and a thornlike shell of BN nanofibers. The second one looked like a perfect sphere consisting of a pure amorphous B core and a rather smooth crystalline BN shell. These results not only provided us a new chemical method for preparing BN nanostructures but also enriched the important BN nanostructures family. A growth mechanism is also briefly discussed.
Enhancement of thermoelectric figure of merit by incorporation of large single crystals in Ca3Co4O9 bulk materials
Authors: Funahashi Ryoji , Urata Saori , Sano Toyohide , Kitawaki Masaaki
Published: March 2008

Having recently succeeded in synthesizing large single crystals of (Ca2CoO3)CoO2 (Co-349) with superior thermoelectric properties using a modified flux method, we have prepared a composite material of Co-349 powder and single crystals and examined its thermoelectric properties. The electrical conductivity σ of this composite, which contained 20 wt.% single crystals, was higher than that of a sample without the single crystals. While the achievable effect has yet to be fully realized, improved grain alignment and the effect of current bypassing grain boundaries through the large single crystals in the composite are thought to cause the increasing σ, which consequently results in an enhanced thermoelectric figure of merit of about 0.56 at 973 K in air.
Mixing entropy and the nucleation of silicides: Ni–Pd–Si and Co–Mn–Si ternary systems
Authors: Detavernier C. , Qu X. P. , Van Meirhaeghe R. L. , Li B. Z. , Maex K.
Published: March 2008

Nucleation can play an important role during the formation of silicides, especially when the difference in Gibbs free energy ΔG between the existing and newly formed phase is small. In this work, it is shown that the addition of elements that form a solid solution with either the existing or nucleating phase influences the entropy of mixing and thus changes ΔG. In this way, the height of the nucleation barrier may be controlled, thus controlling the nucleation temperature. The influence of mixing entropy on silicide nucleation is illustrated by experiments for two ternary systems: Co–Mn–Si and Ni–Pd–Si. It is shown that the nucleation temperature of CoSi2 is increased by the addition of Mn, the nucleation temperature of MnSi1.7 is increased by the presence of Co, the nucleation temperature of NiSi2 is increased by the addition of Pd, and the nucleation temperature of PdSi is decreased by the addition of Ni. In all four cases, the effect of the alloying element on the nucleation temperature can be explained by a model on the basis of the concept of mixing entropy.
Synthesis of a KNbO3 particle/polymer hybrid from metalorganics
Authors: Yogo Toshinobu , Banno Kouichi , Sakamoto Wataru , Hirano Shin-ichi
Published: March 2008

A nanocrystalline KNbO3 particle/polymer hybrid was synthesized through controlled hydrolysis and polymerization of metalorganics below 100 °C. A KNbO3 precursor was synthesized from potassium ethoxide, niobium ethoxide, and acetoacetoxyethyl methacrylate. The K–Nb alkoxide precursor was hydrolyzed and polymerized yielding KNbO3 particle/polymer hybrid. The organic matrix included nanometer-sized crystalline particles depending upon the hydrolysis conditions. The nanocrystalline particles were identified to be potassium niobate by electron diffraction and energy dispersive x-ray analysis. The suspension of the hybrid in silicone oil revealed a yield stress dependent upon various conditions, such as concentration and applied field. The hybridization was found to have a pronounced effect on the electrorheological properties of the fluid, including the KNbO3 particle/polymer hybrid.
Mechanical property evaluation through sharp indentations in elastoplastic and fully plastic contact regimes
Authors: Mata M. , Alcalá J.
Published: March 2008

Following the finite element simulations in our earlier work about the contact deformation regimes, mathematical formulations were derived to correlate hardness and the amount of pileup and sinking-in phenomena around sharp indenters with uniaxial mechanical properties. The formulations are applicable regardless of the deformation regime ruling the contact response of a strain-hardening solid. A methodology was devised where the use of these formulations in mechanical property assessments from indentation experiments was demonstrated. The current results make contact with existing methodologies using the II-theorem in functional analysis to extract uniaxial properties from instrumented indentation load depth of penetration curves. It is argued that since surface deformation is an essential feature of the contact response, it enters directly or indirectly in such existing methodologies. The paper considers how independent knowledge of surface deformation can be used to guide mechanical property assessments from load-depth of penetration curves. A discussion on the uniqueness of mechanical characterizations through indentation experiments is also provided.
Microstructural characterization of thick YBa2Cu3O7−δ films on improved rolling-assisted biaxially textured substrates
Authors: Leonard K. J. , Kang S. , Goyal A. , Yarborough K. A. , Kroeger D. M.
Published: March 2008

The microstructural changes associated with the reduced dependence of critical current density (Jc) versus thickness of thick, epitaxial YBa2Cu3O7–δ (YBCO) films on rolling-assisted biaxially textured substrates (RABiTS) were investigated. Pulsed laser deposited YBCO films varying in thickness from 1.0 to 6.4 ?m on RABiTS with an architecture of Ni–3 at.% W/Y2O3/yttrium-stabilized-zirconia/CeO2/YBCO were prepared for cross-sectional transmission electron microscopy studies. Dramatic improvements in physical properties and microstructural quality were observed resulting from the use of Ni–3 at.% W substrates, which provided a sharper texture over earlier Ni substrates, and replacement of CeO2 with Y2O3 as the seed layer within the buffers. The YBCO films showed exceptional orientation up to 6.4 μm thickness, with no misoriented grains or dead layers observed and only limited reaction between the YBCO and CeO2 cap layer. The high quality of the films was also attributed in part to the formation of a tungsten oxide layer forming at the top of the Ni–3% W substrate, limiting the growth of deleterious NiO into the conductor.
Effects of nanostructures on the fracture strength of the interfaces in nacre
Authors: Song F. , Bai Y. L.
Published: March 2008

A strengthening mechanism arising from the mineral bridges in the organic matrix layers of nacre (mother of pearl) is presented by studying the structural and mechanical properties of the interfaces in nacre. This mechanism not only increases the average fracture strength of the organic matrix interfaces by about five times, but also effectively arrests the cracks in the organic matrix layers and causes the crack deflection in this biomaterial. The present investigation shows that the main mechanism governing the strength of the organic matrix layers of nacre relies on the mineral bridges rather than the organic matrix. This study provides a guide to the interfacial design of synthetic materials.
A new route to bulk nanocrystalline materials
Authors: Su Jian-Qing , Nelson Tracy W. , Sterling Colin J.
Published: March 2008

Despite their interesting properties, nanostructured materials have found limited use as a result of the cost of preparation and the difficulty in scaling up. Herein, the authors report a technique, friction stir processing (FSP), to refine grain sizes to a nanoscale. Nanocrystalline 7075 Al with an average grain size of 100 nm was successfully obtained using FSP. It may be possible to further control the microstructure of the processed material by changing the processing parameters and the cooling rate. In principle, by applying multiple overlapping passes, it should be possible to produce any desired size thin sheet to nanostructure using this technique. We expect that the FSP technique may pave the way to large-scale structural applications of nanostructured metals and alloys.
Optimum tribological improvement of aluminum using oxygen plasma source ion implantation
Authors: Bolduc M. , Terreault B. , Reguer A. , Shaffer E. , St-Jacques R. G.
Published: March 2008

Aluminum and its alloys show poor tribological properties. Oxygen plasma source ion implantation is an emerging technology for the improvement of the surface mechanical properties of these materials. We found an optimum O ion dose, corresponding to 35 at.% O, for which we were able to obtain nanohardness enhancements by factors of 2× and 3× for pure and alloyed (AA7075) Al, respectively. Nanoscratch test results showed reductions in the scratch depths and the friction coefficients by nearly the same factors. It is also important to control the process temperature (∼160 °C). These improvements are due to the formation of a smooth, stiff, but nonbrittle metal–oxide (Al–Al2O3) nanocomposite.
High-temperature deformation of mullite and analysis of creep curves
Authors: Rhanim H. , Olagnon C. , Fantozzi G. , Azim A.
Published: March 2008

The creep behavior of mullite was studied under different stresses and in the temperature range 1200–1450 °C, and an analysis of creep curves was proposed. The study of creep behavior of mullite at high temperatures clearly indicates that this material exhibits concurrent creep and slow crack growth. An effective transition stress exists at each temperature. The analysis takes account of the total creep curve; in particular, the primary and stationary stages. It is now possible to determine by extrapolation the steady-state creep rate for specimens that break in the transient domain during tests. Thus, one can verify the influence of the stress on the steady-state creep rate over a wide stress range. On the other hand, this analysis clearly indicates the existence of two values of the activation energy around 1300 °C; this suggests a change of creep mechanism at this temperature.