bluuebunny/crossref_metadata_2025 huggingface dataset:
Crystallization of YBa2Cu3O7-y films on SrTiO3(100) by postannealing of precursors prepared by dipping-pyrolysis process
Authors: Manabe T. , Kondo W. , Mizuta S. , Kumagai T.
Published: September 2007

The effects of annealing temperature and oxygen partial pressure [p(O2)] were investigated on the crystallization and orientation of superconducting YBa2Cu3O7-y (YBCO) films on SrTiO3(100) prepared by the dipping-pyrolysis process. Annealing conditions for precursor films consisting of a Y2O3—BaCO3—CuO mixture were varied in the temperature range 700–850 °C and initial p(O2) range 2 × 10-4-10-2 atm followed by pure O2 treatment. Three different types of crystallization behavior were observed. The c-axis-oriented epitaxial YBCO films were obtained on the lower p(O2) and higher temperature side in the p(O2)-T diagram. The c/a-axis-oriented growth was observed in films heat-treated at 750–800 °C under p(O2) of 10-3 atm. In addition, mixtures of YBCO and YBa2Cu4O8 resulted when the films were annealed at 700 °C.
Synthesis of Al-based metastable alloys by mechanical milling Al and amorphous Fe78Si12B10powders
Authors: Wang K. Y. , He A. Q. , Shen T. D. , Quan M. X. , Wang J. T.
Published: September 2007

Syntheses of Al-based metastable alloys from powder mixtures of elemental Al and amorphous Fe78Si12B10 [x at. % Al + (100 — x) at. % (Fe78Si12B10)] alloy by mechanical milling (MM) using a planetary ball mill are investigated. X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are used to characterize their structure during the MM process. For the powder mixture with low content of Al (x = 75, 82), fully amorphous material can be obtained by MM, while for the milled product with a high content of Al (x = 90), nanocrystalline Al and amorphous phases are obtained. During the initial milling stage, the Al atoms are dissolved into the amorphous Fe78Si12B10 matrix by heavy deformation. Consequently, the Al-enriched homogeneous amorphous alloys are produced with the disappearance or shrinkage of diffraction peaks of Al in the XRD pattern. Further milling of the powder mixture with 75 at. % Al results in the crystallization of amorphous phase and the formation of nanocrystalline Al3Fe type phase. The crystallization products of all as-milled samples are very similar, composed of Al13Fe4 and AlFe3 phases. It is suggested that the kinetics of nucleation and growth favor the formation of amorphous phase due to the existence of amorphous phase initially. The amorphization reaction by mechanical milling is diffusion process, but defects and strain also play an important role.
Impression creep of lead
Authors: Chiang Donyau , Li J. C. M.
Published: September 2007

The impression creep behavior of lead was investigated using a 100 μm diameter punch at ambient and elevated temperatures (433 K-563 K) under punching stresses of 6–70 MPa. The results were compared with the data obtained from conventional creep tests reported in the literature. Unlike the indentation creep test, the impression creep test showed a steady-state velocity after a short transient period when the flat-end cylindrical punch was pushed against the lead surface by a constant load. Both the temperature and stress dependences were comparable to those of the constant stress tensile creep tests under similar conditions. A master curve for lead was established by collecting data from the impression creep tests and the constant stress tensile creep tests. The indentation creep measurements for lead were included also. However, the indentation data depend on the load applied.
Growth of diamond films on copper
Authors: Hartsell M. L. , Piano L. S.
Published: September 2007

Copper and diamond both have cubic crystal structures and similar lattice constants, making Cu an excellent candidate substrate for the heteroepitaxial growth of electronic device quality diamond. In this study a Cu substrate preparation method was developed to obtain large-area, free-standing polycrystalline diamond films. Concurrently, a statistically designed experiment was used to maximize the diamond film quality and resulted in (111) and (100) faceted films of quality comparable to or better than those grown on Si. The diamond films were analyzed by Raman spectroscopy, x-ray diffraction, and scanning electron microscopy.
Effects of dopants in PZT films
Authors: Chang Jhing-Fang , Desua Seshu B.
Published: September 2007

Undoped and lanthanide (La and Nd) doped Pb(ZrxTi1-x)03, i.e., PZT, ferroelectric thin films were prepared by metallorganic decomposition (MOD) and spin-coating. The precursors for making the undoped PZT films were derived from lead acetate, zirconium n-propoxide, and titanium isopropoxide. In addition, lanthanum acetylacetonate and neodymium acetate were introduced into the precursor solution to accomplish the doping of the corresponding elements. Both undoped and doped PZT films were coated onto Pt/Ti/Si02/Si and single-crystal sapphire substrates to various thicknesses and annealed at a range of temperatures and times. The effects of lanthanide dopants in PZT films were studied with regard to microstructure, Curie temperatures, crystal distortion, optical properties, and electrical properties. The results indicate that the addition of La and Nd dopants tends to enhance perovskite phase formation and improve the packing densities and electrical properties of PZT films.
Tribo-oxidation of a TiN coating sliding against corundum
Authors: Vancoille E. , Blanpain B. , Xingpu Ye , Celis J-P. , Roos J. R.
Published: September 2007

This paper is aimed at understanding the tribo-oxidation of a physical vapor-deposited TiN coating when sliding against a corundum ball. This is achieved through a compositional and structural analysis of the wear debris. Wear debris particles generated at three different sliding speeds were analyzed with micro-Raman spectroscopy, transmission electron microscopy, and electron probe microanalysis. The analysis showed that the wear debris when formed at the low and medium sliding speed consist of TiO2 with a nanocrystalline structure containing both anatase and rutile structural elements. Only rutile structural elements could be observed in the debris formed at the high sliding speed. These results on the characterization of the wear debris are interpreted with calculations of the flash temperature in the tribo-contact and with recent ball-on-disk results on the wear rate of TiN as a function of the sliding speed to propose a mechanistic view of the tribo-oxidation and wear process. The relation with previous and recent experimental results on the static oxidation of TiN is also given.
Alkali resistance enhancement of basalt fibers by hydrated zirconia films formed by the sol-gel process
Authors: Jung T. H. , Subramanian R. V.
Published: September 2007

Basalt fibers were dip-coated in zirconium-n-propoxide, unstabilized or stabilized by chelation with ethyl acetoacetate. The thermal transformations of the hydrated zirconia coatings formed were investigated by dynamic x-ray diffraction and differential thermal analysis. The changes in the surface chemical compositions of coated and uncoated fibers, following alkali immersion extending to 90 days, were characterized by EDXA and IR spectral analysis. Fiber strengths were also measured after immersion in 0.1 M NaOH for different durations. It was found that the transition of the amorphous zirconia coating to the tetragonal crystalline phase is shifted to higher temperatures by chelation of the zirconium alkoxide. Alkali corrosion of the uncoated basalt fibers results in dissolution of the oxides of Si, Al, and Ca, and the formation of unsoluble hydroxides of Fe, Mg, and Ti from the chemical constituents of basalt. These reactions are suppressed by the protective zirconia coating on basalt fibers formed by the unstabilized zirconium alkoxide. However, the coating formed from zirconium propoxide stabilized by ethyl acetoacetate does not form an effective barrier against alkali attack since it is easily detached from the fiber surface during alkali immersion. The tensile strength of uncoated basalt fibers is drastically reduced by alkali attack. But the strength of zirconia-coated basalt fibers is maintained even after 90 days of alkali immersion. The vastly improved alkaline durability of the coated fibers shows the potential of zirconia-coated basalt fibers for cement reinforcement.
Electroless copper films deposited onto laser-activated aluminum nitride and alumina
Authors: DeSilva M. J. , Pedraza A. J. , Lowndes D.H.
Published: September 2007

Metallization of ceramic substrates by laser activation and subsequent electroless deposition has been demonstrated recently in aluminum nitride and alumina. However, the bond strength between the electroless copper and the ceiamic substrate is weak (less than 14 MPa). Low temperature annealing of electroless copper films deposited on substrates activated at low laser energies strongly increases the adhesion strength. The effectiveness of the annealing for improving the metal-ceramic bonding is dependent upon the laser treatment performed on the substrate prior to deposition. Faster deposition kinetics are obtained for both substrates by increasing the laser energy density. On the other hand, an increase in the laser energy density leads to poor adhesion strengths. The dislocation microstructure produced during laser irradiation in aluminum nitride is analyzed as a possible cause of laser activation. Free aluminum produced by laser irradiation of aluminum nitride and of alumina is discussed as another factor of laser activation. The chemical and microstructural changes taking place in the near-surface region as a consequence of laser-induced processes are correlated with adhesion enhancement promoted by the annealing treatment.
Preparation and growth mechanism ofa-axis-oriented YBa2Cu3O7-δ films by laser metal-organic chemical vapor deposition
Authors: Ushida Takahisa , Higa Hiroyuki , Higashiyama Kazutoshi , Hirabayashi Izumi
Published: September 2007

Recently we have found a very effective method for controlling the orientation of YBCO films by metal-organic chemical vapor deposition (MOCVD) using uv laser irradiation during deposition onto a MgO(100) substrate. The irradiated part was stronglya-axis-oriented normal to the surface of the substrate for films prepared at 650-700 °C, whereas the unirradiated parts showedc-axis or (110) orientation. This phenomenon occurs not only on MgO(100) substrates but also on other substrates. We obtained aTcabove 81 K on thea-axis oriented part. The critical current density was over 105A/cm2at 4.2 K and O T. The surface morphology depends on the laser power density and the repetition rate. A high degree of thea-axis orientation is obtained only by using uv light during deposition of YBCO film. IR or a visible laser causes only surface melting and the destruction of film orientation. We propose that the nuclei fora-axis orientation are formed by the aggregation of Ba caused by uv laser irradiation near the film surface.
Optical emission spectroscopic studies on the growth of YBCO thin films by dc-94.92 MHz hybrid plasma sputtering
Authors: Ito W. , Mahajan S. , Okayama S. , Yoshida Y. , Morishita T.
Published: September 2007

Optical emission spectroscopic studies of dc-94.92 MHz hybrid plasma generated in the newly developed magnetron sputtering system were performed during the growth of YBa2Cu3O7−δ (YBCO) thin films. All the detectable species showed uniform spatial distribution along the radial direction of the target more than 8 mm above the target surface. High cathode current conditions in the hybrid plasma were found to make the plasma with high ion density. The high ionizing efficiency was concluded to be essential in obtaining an excellent crystalline film. This result is explained by the ion acceleration mechanism through the ion sheath formed near the substrate surface. Oxygen gas introduced into the growing chamber during deposition was found to be responsible for the oxidation of the target surface.
Smooth surface of Bi-Sr-Ca-Cu-O thin films on the Y3Al5O12 (YAG) substrate
Authors: Kubota Nobuhiko , Shiohara Yuh , Tanaka Shoji
Published: September 2007

We prepared the superconducting Bi-Sr-Ca-Cu-O thin films on Y3Al5O12 (YAG) single crystal substrates by metal-organic chemical vapor deposition (MOCVD). This film of 50 nm thickness on a YAG substrate showed c-axis orientation clearly, and the zero-resistivity (Tc-zero) was achieved at 66 K. The film has the advantageous property of surface smoothness compared with the film fabricated on MgO and SrTiO3 single-crystal substrates. The surface property of the YAG substrate seems to influence the film quality.
The stability of YBa2Cu3O7−x in contact with silver
Authors: Lin Sern-Hau , Wu Nae-Lih
Published: September 2007

The stability of YBa2Cu3O7−x (the 123 compound) in contact with silver (Ag) at temperatures below 900 °C was investigated by conducting SEM and EDX analyses on 123 agglomerates that were enclosed in a dense Ag matrix and subjected to various thermal treatments. The stability of the 123 agglomerates was found to depend heavily on the oxygen content in the Ag matrix. In the case of insufficient oxygen content in Ag, the 123 agglomerates, which were as large as 150 μm thick, decomposed readily at temperatures above 500 °C. The complete decomposition process can be summarized as continuous extraction of Ba from the 123 oxide into the surrounding Ag matrix and is proposed to be driven by high mutual solubilities between Ag and Ba under the oxygen-lean condition. Prolonged preoxygenation of the (123 + Ag) mixture powders at temperatures above 400 °C prevents the occurrence of 123 decomposition in compacted samples during subsequent heat treatment, suggesting that the critical oxygen content in Ag for stabilizing the 123 compound to be no higher than 10−3 at. % (the oxygen saturation solubility at 400 °C). The findings may have implications for the processing of other Ba-containing high-Tc superconducting oxides as well.
Di- and tri-carboxylic-acid-based etches for processing high temperature superconducting thin films and related materials
Authors: Ginley D.S. , Barr L. , Ashby C.I.H. , Plut T.A. , Urea D. , Siegal M.P. , Martens J.S. , Johansson M.E.
Published: September 2007

The development of passive and active electronics from high-temperature superconducting thin films depends on the development of process technology capable of producing appropriate feature sizes without degrading the key superconducting properties. We present a new class of chelating etches based on di- and tri-carboxylic acids that are compatible with positive photoresists and can produce submicron feature sizes while typically producing increases in the microwave surface resistance at 94 GHz by less than 10%. This simple etching process works well for both the Y-Ba-Cu-O and Tl-Ba-Ca-Cu-O systems. In addition, we demonstrate that the use of chelating etches with an activator such as HF allows the etching of related oxides such as LaAlO3, which is a key substrate material, and Pb(Zr0.53Ti0.47)O3 (PZT) which is a key ferroelectric material for HTS and other applications such as nonvolatile memories.
The stability of the surface of La2CuO4 to reactions with adsorbed n-butyl amine: X-ray photoelectron spectroscopy study
Authors: Badrinarayanan S. , Mandale A.B. , Sainkar S.R. , Pavaskar N.R. , Ramaswamy V.
Published: September 2007

The reaction of n-butyl amine adsorbed on the ternary oxide La2CuO4 has been studied by x-ray photoelectron spectroscopy (XPS), paying special attention to the surface composition. We suggest that n-butyl amine reacting with La2CuO4 reduces CuO to Cu2O. The reaction is confined to the surface because the original composition of the material could be restored after in situ scraping with a stainless steel blade.
Template synthesis of metal microtubule ensembles utilizing chemical, electrochemical, and vacuum deposition techniques
Authors: Brumlik Charles J. , Menon Vinod P. , Martin Charles R.
Published: September 2007

Microtubules are an interesting type of microstructure that resemble miniature drinking straws. Such tubular microstructures are found in nature. In addition, we and others have been investigating strategies for making synthetic analogs. We are especially interested in the idea of making metal microtubules. Four procedures for preparing metal microtubules are described in this paper. The general approach, called template-synthesis, entails using the pores in a microporous membrane as templates for forming the tubules. Microporous anodic aluminum oxide membranes and nuclear track-etch membranes are used as the template membranes. Gold and silver microtubules are made with outer diameters as small as 200 nm. These microstructures are characterized by scanning electron microscopy.
Mechanistic studies in combustion synthesis of Ni3Al and Ni3Al-matrix composites
Authors: Lebrat Jean-Pascal , Varma Arvind , McGinn Paul J.
Published: September 2007

Nickel aluminides exhibit limited ductility and toughness at room temperature. One way to improve these characteristics is by adding ceramic reinforcements to the matrix. In this paper, we have studied the combustion synthesis of Ni3Al and Ni3Al-matrix composites, using the self-propagating high-temperature synthesis (SHS) mode. First, studies of the Ni3Al synthesis were carried out by quenching the reaction during its progress, which revealed the mechanism of the synthesis. The influence of Al2O3and SiC whiskers or particulates, and B4C particulates added to the reaction mixture prior to combustion synthesis, was investigated next. It was found that, in general, reinforcements are heat sinks and limit the propagation of the reaction. Also, whiskers impede the flow of formed liquid to a larger extent than do particulates. Al2O3is inert and matrices reinforced with up to 2 wt. % Al2O3are composed essentially of Ni3Al grains. However, both B4C and SiC react with the Ni-Al matrix and lead to complex phases. In particular, B4C readily forms a Ni-Al-B liquid phase and disrupts dramatically the progress of the Ni3Al matrix synthesis.
Microstructure evolution and mechanical strength of silicon nitride ceramics
Authors: Iwamoto Y. , Nomura H. , Sugiura I. , Tsubaki J. , Takahashi H. , Ishikawa K. , Shinohara N. , Okumiya M. , Yamada T. , Kamiya H. , Uematsu K.
Published: September 2007

Microstructure evolution was studied in silicon nitride ceramics by a novel characterization method, and its relevance to the strength was discussed. The characterization method involves an immersion liquid for making green and partially sintered bodies transparent, and a subsequent direct optical microscopic examination. Granules for compaction process were prepared with the spray-drying process and were found to contain pores or deep dimples. Green bodies formed by CIP with these granules contain regularly arrayed pores at the center of granules and also crack-like voids at the boundaries of granules. These pores were preserved in the sintering process and resulted in large pores in the sintered body. They behave as fracture origin in ceramics and reduce the fracture strength. The Weibull modulus was high due to the presence of uniformly distributed pores.
Crystallization of ion beam deposited calcium phosphate coatings
Authors: Chen Tse S. , Lacefield William R.
Published: September 2007

Amorphous calcium phosphate coatings on the order of 1 μm thick were deposited onto titanium and silicon substrates using an ion-beam sputtering technique. The target material utilized for sputter deposition was plasma-sprayed fluorapatite [Ca10(PO4)6F2; FA]. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were employed to analyze the coatings. The amorphous as-deposited coatings were annealed in air (at 500 °C or 600 °C) to a crystalline state consisting of a polycrystalline FA matrix with a small amount of microcrystallites of a different composition. The higher annealing temperature (600 °C) tended to produce coarser FA and microcrystallite grains; however, the coatings buckled on the titanium substrates as a result of the heat treatment. Attempts to form the FA phase by in situ annealing in the vacuum chamber at a substrate temperature of 500 °C were not successful. The average bond strength for the as-deposited and 500 °C post-annealed coatings was comparable, while the lowest bond strength was observed in the 600 °C post-annealed coatings. The results suggest that the 500 °C post-annealed coatings have a suitable structure and possess sufficient adherence to be acceptable for use in certain medical and dental implant applications, and further tests under physiologic conditions will be conducted.
Defect mapping of a synthetic diamond single crystal by cathodoluminescence spectroscopy
Authors: Robins Lawrence H. , Black David R.
Published: September 2007

Cathodoluminescence (CL) spectroscopy in a scanning electron microscope was used to identify and to map the spatial distribution of luminescent defects in a synthetic diamond single crystal. Several defect CL bands were observed in the 1.5-3.5 eV region: (i) a band with a zero-phonon line at 2.156 eV, attributed to a center containing nitrogen and atomic vacancies; (ii) a broadband centered at ∼2.2 eV, tentatively attributed to a boron-containing center; (iii) a doublet line at 2.33 eV, attributed to a nitrogen-containing center; (iv) a zero-phonon line at 2.555 eV, attributed to a nickel-containing center; (v) a broadband centered at ∼2.85 eV, attributed to a dislocation-related center; and (vi) a zero-phonon line at 3.188 eV, attributed to a center containing nitrogen and a carbon interstitial. Lines due to free and acceptor-bound excitons were observed in the 5.0-5.4 eV region. The spatial variation of the CL was examined in the vicinity of regions of relatively high dislocation density (∼106 dislocations cm−2), which had been found in a previous x-ray diffraction imaging experiment. A quantitative analysis was made of the spatial variation of the band intensities. Upon moving from a relatively defect-free region to the center of a high dislocation density region, the intensities of defect bands (i) and (v) increased by very large factors (these bands were observed only within the high dislocation density regions); the intensity of defect band (vi) increased by a factor of ∼2; the acceptor-bound exciton intensity increased by a factor of 1.3; the intensities of defect bands (ii)-(iv) decreased by a factor of ∼2; and the free exciton intensity decreased by a factor of ∼7.5.
A hybrid method for determining material properties from instrumented micro-indentation experiments
Authors: Chen Y-M. , Ruff A.W. , Dally J.W.
Published: September 2007

The impact code EPIC was employed to study the relationship between the applied force and the penetration depth in a micrometer-scale indentation experiment with oxygen free high conductivity (OFHC) copper. EPIC is an elastic-plastic finite element code that uses a Lagrangian formulation and triangular mesh, which can accommodate large deformation without the need to remesh during the computation process. By fitting the force-penetration curves for a triangular indenter with second degree polynomials, it was demonstrated that the fit changed with two material constants in the constitutive equation. A systematic procedure for determining the material constants is described that is based on matching either the slope or the curvature of the force penetration depth curves from numerical simulation and experiments. It is concluded that material constants can be determined from indentation data obtained using pyramidal or spherical indenters as well as a flat-ended indenter.
Further observations on the effects of nonhydrostatic compression on the FR1 → AO polymorphic phase transformation in niobiumdoped, lead-zirconate-titanate ceramic
Authors: Zeuch David H. , Montgomery Stephen T. , Keck Jeffrey D.
Published: September 2007

We recently performed a series of experiments on Nb-doped lead-zirconate-titanate ceramic to investigate the influence of constant shear stresses on the displacive, first-order rhombohedral/ferroelectric → orthorhombic/antiferroelectric polymorphic transformation. In a previous paper and report we demonstrated that increasing shear stresses lowers the mean stress and confining pressure at which the transformation occurs, but we did not identify a criterion by which the transformation could be predicted to take place under nonhydrostatic stress. In this paper we use the dielectric anomaly which accompanies the transformation as an indicator of onset of the transition, and correct for the effects of high-pressure-seal friction on measurement of the maximum compressive stress applied to the test specimens during deviatoric loading. We show that a convincing case can be made that the transformation occurs when the maximum compressive stress equals the hydrostatic pressure at which the transformation would otherwise occur.
Single molecular precursor metal-organic chemical vapor deposition of MgAl2O4 thin films
Authors: Zhang Jiming , Stauf Gregory T. , Gardiner Robin , Van Buskirk Peter , Steinbeck John
Published: September 2007

MgAl2O4 films have been grown epitaxially on both Si(100) and MgO(100) by a novel single source metal-organic chemical vapor deposition (MOCVD) process. A single molecular source reagent [magnesium dialuminum isopropoxide, MgAl2(OC3H7)8] having the desired Mg: Al ratio was dissolved in a liquid solution and flash-vaporized into the reactor. Both thermal and plasma-enhanced MOCVD were used to grow epitaxial MgAl2O4 thin films. The Mg: Al ratio in the deposited films was the same as that of the starting compound (Mg: Al = 1:2) over a wide range of deposition conditions. The deposition temperature required for the formation of crystalline spinel was found to be significantly reduced and crystallinity was much improved on Si by using a remote plasma-enhanced MOCVD process. The epitaxial nature of the MgAl2O4 films was established by x-ray pole figure analysis.
Crystal orientation and growth anisotropy of YBa2Cu3O6+x fabricated by directional solidification method
Authors: Nakamura Y. , Furuya K. , Izumi T. , Shiohara Y.
Published: September 2007

The unidirectional solidification by a zone-melting method was performed to investigate the growth anisotropy of YBa2Cu3O6+x (123) superconductive oxides. Large faceted interfaces were obtained at growth rates lower than 2 mm/h. The three-dimensional interface morphology and the onset of growth were observed. The crystal faces toward the growth direction were revealed to be close to the {112} face, which was selected as the result of the preferential growth. The growth rate of the {100} faces was evaluated to be about 1.5 times faster than that of the {001} faces from the steady growth condition.
A modified citrate gel route for the synthesis of phase pure Bi2Sr2CaCu2O8 superconductor
Authors: Devi Sujatha P. , Maiti H.S.
Published: September 2007

A simple method for the bulk synthesis of 80 K bismuth-based superconducting phase (2212) is described. It employs a modified citrate gel route giving rise to an auto-ignited combustion process. The precursor ash obtained after combustion is further calcined at 800 °C to produce the desired superconducting powder with excellent homogeneity and fineness, leading to good sinterability and fairly sharp transition temperature. For a particular citrate-nitrate ratio, the nature of combustion of the gels containing two different bases has been compared. The thermal decomposition characteristics of the gels and the plausible mechanism of formation of the superconducting phase are also described.
Effect of oxygen partial pressure and melting temperature on Ag sheathed Bi-2212 wires
Authors: Kanai T. , Inoue N. , Kamo T.
Published: September 2007

A serious problem for Ag-sheathed Bi-2212 wires produced by a partial melt process is oxygen release which causes effluence of the oxide materials from the Ag sheath. Higher oxygen partial pressure during the melt process cuts the oxygen release. We investigated the effect of the oxygen partial pressure, melting temperature, and cooling rate on superconducting properties of Ag-sheathed wires. It was found that (i) the formation of the Bi-2212 phase under a higher oxygen partial pressure of 1 atm is slower than that with a lower oxygen partial pressure of 0.2 or 0.5 atm; therefore, the cooling rate should be as slow as 0.25 °C/min to get homogeneity of the intragrain superconductivity as well as formation of the Bi-2212 phase, and (ii) melting temperature should be just above the incipient melting point; otherwise, Jc significantly decreases due to the formation and growth of voids together with degradation of superconductivity.
Effects of milling and Ag doping on the fabrication of LaBa2Cu3Oy superconductor
Authors: Fang Tsang-Tse , Huang Jao-Wei , Wu Ma-Shine
Published: September 2007

The effects of milling and Ag addition on the decomposition of single-phase LaBa2Cu3Oy have been evaluated. It was found that the decomposition of milled single-phase LaBa2Cu3Oy powders when sintered in pure N2 is attributed to the introduction of strain in the lattice which causes the instability of the structure. The possible reasons why single-phase LaBa2Cu3Oy could be synthesized by sintering in pure N2 at high temperatures and its transition width is always broadened are proposed. The decomposition of compacts of unmilled powders of single-phase LaBa2Cu3Oy when sintered in pure N2 for a long time is due to the fact that oxygen diffuses along the grain boundaries and evolves through the surface of the specimens. Silver might segregate to the grain boundaries and prevent decomposition. It is suggested that to obtain a high-quality LaBa2Cu3Oy, sintering in the reduced atmosphere to achieve a proper oxygen content is required. Reduced atmosphere and Ag addition could enhance the densification rate. For Ag-doped specimens, Tc is highest for x = 0.0001, but decreases for x > 0.0001.
Al2O3 fiber strength degradation in metal and intermetallic matrix composites
Authors: Draper S.L. , Locci I.E.
Published: September 2007

The mechanisms for fiber damage in single crystal Al2O3 fiber-reinforced composites were investigated. Both fiber fragmentation and fiber strength degradation were observed in composites with a variety of matrix compositions. Four mechanisms that may be contributing to the fiber strength loss have been proposed and include matrix reaction, reaction with binders, residual stress-induced damage, and pressure from hot pressing. The effect of matrix reaction was separated from the other three effects by sputter-coating the matrices on cleaned fibers and annealing with a temperature profile that simulates processing conditions. These experiments revealed that Y and Cr in FeCrAlY base alloys and Zr in NiAl alloys reacted with the fiber, and grooves and adherent particles were formed on the fiber surface which were responsible for the strength loss. The effects of the matrix reaction appeared to dominate over the other possible mechanisms, although evidence for reaction with binders was also found. Ridges on the fiber surface, which reflected the grain boundaries of the matrix, were also observed. In order for single-crystal Al2O3 to be used as a fiber in MMC's and IMC's, a matrix or protective coating which minimizes matrix reaction during processing will be necessary. Of the matrices investigated, the Thermo-spanTM alloy was the least damaging to fiber properties.
X-ray photoelectron spectroscopy investigation of MgAl2O4thin films for humidity sensors
Authors: Mattogno Giulia , Righini Guido , Montesperelli Giampiero , Traversa Enrico
Published: September 2007

MgAl2O4thin films, to be studied as active elements for humidity sensors, were deposited on Si/SiO2substrates by radio-frequency sputtering. This paper discusses the x-ray photoelectron spectroscopy (XPS) investigation of these films. XPS demonstrated that the thin films had a stoichiometry close to that of MgAl2O4. The evaluation of the modified Auger parameter α' for Al gave structural information about the order of the crystalline structure of the thin films. The combination of Ar+ion etching and XPS analysis showed the simultaneous presence of Mg, Al, and Si at the film-substrate interface. The thicknesses of the interfaces were calculated between 7 and 10 nm. The analysis of the binding energy (b.e.) values of the XPS peaks at different etching depths showed that O 1s and Si 2pb.e. values were characteristic of a silicate at the interface, whereas in the substrate they were typical of silica. This suggests a chemical interaction took place between film and substrate with the formation of a silicate layer at the interface, which may be the cause of the good adhesion of MgAl2O4films to silica, as observed by peel tests with Scotch tape.
Amorphous or nanocrystalline AlN thin films formed from AlN: H
Authors: Wang Xiao-Dong , Hipps K.W. , Dickinso J.T. , Mazur Ursula
Published: September 2007

This work describes the formation of stoichiometric AlN films by single ion-beam sputtering of Al, using an ionized N2 (75%) + H2 (25%) mixture, onto substrates heated to 200 °C or above. The role of substrate temperature on film composition and properties is followed in the substrate temperature range between ambient and 250 °C. Infrared spectra of freshly prepared and 2 month old (aged in air) films demonstrate that substrate heating significantly affects the chemical nature of the resulting films. SEM and STM data, combined with IR and UV-visible spectral results, indicate that films formed at a substrate temperature of ≥200 °C are very smooth and highly resistant to attack by atmospheric gases. X-ray diffraction data show no diffraction peaks, indicating that the film is either amorphous or crystalline on a scale of less than 4 nm.
Characteristics of titanium oxide films deposited by an activated reactive evaporation method
Authors: Fujii Tatsuo , Sakata Naoki , Takada Jun , Miura Yoshinari , Daitoh Yoshihiro , Takano Mikio
Published: September 2007

Titanium di- and sesquioxide films were epitaxially grown on the (001) surface of sapphire single-crystalline substrates by an activated reactive evaporation method. Formation range for each titanium oxide was determined as a function of oxygen pressure (Po2) by means of x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Films prepared at Po2 ≥ 2.0 × 10−4 Torr were stoichiometric (100)-oriented rutile of TiO2, and with decreasing Po2 they would accommodate more and more Ti3+ ions in the rutile structure. At Po2 = 0.6 × 10−4 Torr, on the other hand, (001)-oriented Ti2O3 was formed and an electrical transition was clearly detected at about 400 K. However, the large lattice mismatch between the substrate and these films leads to a periodic introduction of misfit dislocations in the case of the TiO2 films and a mixing of stacking sequences for the Ti2O3 films.
Preparation of MoS2 thin films by chemical vapor deposition
Authors: Lee Woo Y. , Besmann Theodore M. , Stott Michael W.
Published: September 2007

The chemical vapor deposition (CVD) of MoS2 by reaction of H2S with molybdenum halides was determined to be thermodynamically favored over a wide range of temperature, pressure, and precursor concentration conditions as long as excess H2S was available. The thermochemical stability of H2S, MoF6, and MoCI5 was also assessed to address their suitability as precursors for the CVD of MoS2. The results from the thermodynamic analysis were used as guidance in the deposition of MoS2 thin films from MoF6 and H2S. The (002) basal planes of MoS2 films deposited above 700 K were preferentially oriented perpendicular to the substrate surface.
Wetting between prospective crucible materials and the Ba-Cu-O melt
Authors: Krauns Ch. , Tagami M. , Yamada Y. , Nakamura M. , Shiohara Y.
Published: September 2007

The reaction between the Ba-Cu-O melt (with a Ba to Cu ratio of 3-5 or 3-7) used for crystal pulling of Y-123 and several prospective crucible materials (MgO, Y2O3, Al2O3, and YSZ) has been investigated by dipping these materials into the melt at 985 °C and 1050 °C in air. For comparison, MgO, YSZ, LaAlO3, and SrTiO3 single crystals were also investigated. In the case of MgO single crystals, no wetting or reactive layer has been observed, although with increasing time an increasing amount of MgO has been dissolved into the melt. The contact angle between melt and MgO single crystal can be estimated to be about 40°, assuming a surface tension of 0.3 N/m. For all other materials, increasing time was accompanied by increasing wetting and/or an increasing reaction layer. For MgO polycrystals increased wetting with increasing time has been observed. For example, it takes about 30 min to wet the first 10 mm. The melt completely permeates samples with an apparent density of up to 85%. However, the permeated depth becomes very thin in samples with an apparent 95% density. It was found that Y2O3 is a better barrier to the melt at lower densities because even at 85% density the permeated layer is very thin, but the surface wetting occurs at a slightly faster rate for Y2O3 than for MgO.
The influence of NaCl on the reactivity of high alumina cement in water: Pore-solution and solid phase characterization
Authors: Goñi S. , Gaztañaga M.T. , Sagrera J.L. , Hernández M.S.
Published: September 2007

The influence of NaCl (3% of Cl−by weight of cement) on the reactivity of High Alumina Cement (HAC) in water has been studied over a period of one month. The changes in microstructure were monitored by x-ray diffraction and scanning electron microscopy. The pore-solution, extracted by the application of high mechanical pressure (500 MPa), was studied for the chemical composition and changes caused by chloride and sodium ions.
TEM study of the structure and chemistry of a diamond/silicon interface
Authors: Tzou Y. , Bruley J. , Ernst F. , Rühle M. , Raj R.
Published: September 2007

The interface between diamond and silicon, fabricated by growing diamond films on (001) silicon by microwave plasma assisted chemical vapor deposition (MPACVD), was characterized by high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). Two types of interface morphology were identified. Type A interfaces contain an amorphous transition layer composed of silicon, carbon, and oxygen; the diamond overgrowth on this layer consists of nanocrystalline grains with random orientations. Type B interfaces consist of large diamond grains having special orientations with respect to the silicon substrate, without an obvious presence of a glassy phase and with a much lower oxygen content than type A interfaces.
Magnetic susceptibility of buckytubes
Authors: Wang X.K. , Chang R.P.H. , Patashinski A. , Ketterson J.B.
Published: September 2007

The measured room temperature magnetic susceptibility of a bulk sample of buckytubes (buckybundle) is −10.75 × 10−6 emu/g for the magnetic field parallel to the buckybundle axis, which is approximately 1.1 times the perpendicular value and 30 times larger than that of C60. The experimental results are discussed in terms of a graphite-like electronic structure model.
Surface crystallographic structure compatibility between substrates and highTc(YBCO) thin films
Authors: Guo Ruyan , Bhalla A.S. , Cross L.E. , Roy Rustum
Published: September 2007

A review and some case studies on the interatomic distances of candidate substrates in comparison to the highTcsuperconductor (HTSC) phase YBa2Cu3O7−δ(YBCO) is presented, in an attempt to enhance the basis for substrate selection for YBCO film epitaxy. This preliminary study was carried out by examining a variety of interatomic distances in the structure rather than merely matching the lattice parameters. Interatomic structure matching planes of selected YBCO orientations in contact with substrates were identified. The surface termination of the substrate was found to be a crucial parameter in determining the oriented or epitaxial growth. Possible composition dependence of the orientation of the films at the nucleation stage was also anticipated depending on the comparison. Several currently most commonly used substrates are discussed in some detail.
Identification of fracture sequences during sharp indentation of polycrystalline Al2O3
Authors: Zeng Kaiyang , Rowcliffe David
Published: September 2007

The effects of loading and unloading rate on fracture features formed during sharp indentation of polycrystalline Al2O3 have been studied. It was found that there is a transition between intergranular and transgranular fracture if the loading rate is increased by a factor of 104 during the whole indentation cycle. By varying the loading or unloading rate during indentation, and correlating the corresponding dependence of fracture features, the part of the indentation cycle can be identified in which specific segments of cracks are generated. Hence, the fracture sequence during indentation is established. The fracture sequence for polycrystalline Al2O3 is compared with those reported from direct observation in optically transparent materials such as soda-lime glass and various single crystal ceramics.
Metal-organic chemical vapor deposition of ZrO2 films using Zr(thd)4 as precursors
Authors: Si Jie , Desu Seshu B. , Tsai Ching-Yi
Published: September 2007

Synthesis of zirconium tetramethylheptanedione [Zr(thd)4] was optimized. Purity of Zr(thd)4 was confirmed by melting point determination, carbon, and hydrogen elemental analysis and proton nuclear magnetic resonance spectrometer (NMR). By using Zr(thd)4, excellent quality ZrO2 thin films were successfully deposited on single-crystal silicon wafers by metal-organic chemical vapor deposition (MOCVD) at reduced pressures. For substrate temperatures below 530 °C, the film deposition rates were very small (⋚1 nm/min). The film deposition rates were significantly affected by (i) source temperature, (ii) substrate temperature, and (iii) total pressure. As-deposited films are carbon free. Furthermore, only the tetragonal ZrO2 phase was identified in as-deposited films. The tetragonal phase transformed progressively into the monoclinic phase as the films were subjected to a high-temperature post-deposition annealing. The optical properties of the ZrO2 thin films as a function of wavelength, in the range of 200 nm to 2000 nm, were also reported. In addition, a simplified theoretical model which considers only a surface reaction was used to analyze the deposition of ZrO2 films. The model predicated the deposition rates well for various conditions in the hot wall reactor.
Effects of boron on the deformation behavior of Ni3Al
Authors: Brusso J.A. , Mikkola D.E.
Published: September 2007

The effects of boron additions (0 to 4000 wppm B) on the room temperature deformation behavior of Ni-24Al have been examined with both quasi-static deformation and short pulse duration shock loading. Changes in compressive yield strength and hardness with the amount of boron suggest that strengthening effects are more complicated than predicted by usual interstitial solid-solution strengthening considerations. The nature of the dislocations changes from SISF-dissociated superdislocations in the Ni-24Al base alloy to APB-dissociated dislocations with the additions of small amounts of boron. Therefore, the observed variation in strength with boron additions reflects a solid-solution strengthening contribution from the interstitial boron coupled with a “softening” effect arising from the greater mobility of the APB-bounding partials relative to the SISF-bounding partials. It is suggested that this “softening” is a necessary prerequisite for enhanced ductility in Ni3Al. In addition, the grain boundary fracture strength must be increased by boron additions. While this may occur through an increased grain boundary cohesive strength due to boron segregation, it is also expected that the interaction of the dislocations with the grain boundaries will be significantly altered as the nature of the dislocations changes. The lack of a “boron effect” in Ni-25Al, the so-called stoichiometric effect, can be attributed to a diminished “softening”, combined with the rapid solid-solution strengthening observed in this alloy.
Effects of ternary additions on the twin energy and site preference in γ-TiAl
Authors: Xu Jian-hua , Freeman A.J.
Published: September 2007

Site preference and the effects of ternary additions (Mn, V, etc.) on the twin energy in γ-TiAl were studied by means of all-electron total energy self-consistent calculations based on the local density approximation. The results show that when Mn, Ti, or V substitute on the Al-sites in γ-TiAl, the twin energy is about 20–25% lower than that of stoichiometric γ-TiAl; this may explain observations of increased twinning activity in 48% Al TiAl. By contrast, when ternary additions of V (or Al) occupy Ti-sites, the twin energy has nearly the same value as that of pure γ-TiAl, which is consistent with the observation of only a few twins for Al-rich compositions. By comparing the total energy of Ti6(XAl5) and (Ti5X)Al6 supercells, it is found that the relative stability of Ti6(XAl5) over (Ti5X)Al6 (i.e., the preference for occupying Al-sites) is increased in going from Nb, V, Cr, and Ti to Mn, in agreement with observation that excess Ti occupies Al-sites, and Nb preferentially substitutes on the Ti-sites. The results indicate that Mn preferentially substitutes on the Al-sites, and V (or Cr) may occupy both Ti- and Al-sites.
Ion exchange and radiation response of H-related point defects in natural quartz crystals
Authors: Bahadur Harish
Published: September 2007

Infrared absorption measurements have been used to study the radiation response of various hydroxyl point defects present in natural quartz crystals in their unswept, Na-swept, and H-swept conditions for “low-H” and “high-H” samples with similar aluminum concentration. The irradiation was done at 77 K, 300 K, and finally again at 77 K in sequence. At low-temperature irradiation the Al-OH− bands equalize in their intensity-like H-swept or prior 300 K-irradiated cultured quartz. Among other major bands the alkali-related centers show a reduction in their strength at all stages of irradiation. Purely H-related centers show no steady-state room temperature effects; only the low-temperature irradiation reduces their band strength. The results have been discussed in terms of proton and alkali ion motion to shallow and deep traps and compared with cultured quartz.
Nickel/metal hydride batteries using rare-earth hydrogen storage alloy
Authors: Chen J. , Zhang Y.S.
Published: September 2007

Fine particles of a hydrogen storage alloy (LaNi3.8Co0.5Mn0.4Al0.3) were microencapsulated with a thin film of nickel of about 0.6 μm thickness. The microencapsulated alloy powders were used as an anode material in a sealed nickel/metal hydride battery. The battery characteristics were compared with those of a battery with a bare (uncoated) alloy anode. The battery using the bare alloy was less stable compared to the coated alloy due to the role of the coated nickel as an oxygen barrier for protecting the alloy surface from oxidation. In addition, charge-discharge characteristics were improved greatly by the nickel coating, especially at high rates and at low temperatures due to the role of nickel as a microcurrent collector. So the microencapsulation of the alloy powders improves the performances of the alloy electrode.