bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase Equilibria and Dielectric Properties in Perovskite-like (1 − x)LaCa0.5Zr0.5O3–xATiO3 (A = Ca, Sr) Ceramics
Authors: Levin Igor , Vanderah Terrell A. , Coutts Rachel , Bell Steven M.
Published: March 2008

Phase equilibria and dielectric properties were analyzed for selected compositions in both LaCa0.5Zr0.5O3–CaTiO3 (LCZ-CT) and LaCa0.5Zr0.5O3–SrTiO3 (LCZ–ST) systems using x-ray powder diffraction and transmission electron microscopy. The end-member LaCa0.5Zr0.5O3 does not occur as a single phase but rather as a mixture of a perovskite-type phase with approximate composition La0.94Ca0.53Zr0.53O3 plus a minor amount of La2O3. This perovskite phase exhibited a combination of 1:1 ordering of Ca and Zr on the B-sites and octahedral tilting. In the (1 – x)LCZ– xCT system, the compositions x = 1/3 and x = 1/2 yielded single phases with perovskite-like structures featuring similar 1:1 B-site ordering superimposed onto octahedral tilting. The x = 1/2 composition in the LCZ–ST system resides in a two-phase field and contains a major perovskite phase and La2O3; the B-cations in the perovskite phase remain disordered at all temperatures. The approximate boundaries of perovskite-like phase fields in the La2O3–ATiO3–CaZrO3 (A = Ca, Sr) systems were outlined, as well as a schematic diagram for perovskite B-cation ordering transitions in the LCZ–CT system. The dielectric properties of the compositions investigated were measured at microwave frequencies and were correlated with the observed structural behavior.
Effect of Lanthanum-doping on the Dielectric Properties of Pb(Fe1/2Ta1/2)O3 Relaxor Ferroelectrics
Authors: Zhu W. Z. , Yan M. , Mantas P. Q. , Baptista J. L. , Kholkin A. L.
Published: March 2008

This paper reports the influence of lanthanum-doping on the dielectric behavior of Pb(Fe1/2Ta1/2)O3 relaxor ferroelectrics, where the La ions are incorporated into the B-site of the perovskite constructing unit and the resultant charge imbalance is compensated by the presence of free charge carriers (holes). The formulated compositions were fabricated using one-step preparation method, and a nearly pure perovskite phase was attained, as verified by XRD analysis. Both the dielectric permittivity maximum (ε′max) and the temperature at which ε′max occurs (Tm) are progressively reduced by La-doping. The degree of frequency dispersion, however, is promoted, indicating the weakened ferroelectric couplings among the oxygen octahedra as a result of lanthanum incorporation. Moreover, the breadth of paraelectric to ferroelectric phase transition is broadened with an increase in lanthanum content, due to enhanced inhomogeneity and increase in the number of B-site cations. Frequency spectroscopy at a certain temperature below Tm reveals a widening of the size distribution of ferroelectric nanodomains, which is characteristic of relaxor microstructure, by La-doping.
Oxidation Behavior of TiAlN Barrier Layers with and without Thin Metal Overlayers for Memory Capacitor Applications
Authors: Song Jaewon , Kim Hye Ryeong , Park Jaehoo , Jeong Seehwa , Hwang Cheol Seong
Published: March 2008

The oxidation behavior of sputtered TiAlN thin-film barrier layers was studied by cross-section transmission electron microscopy. Bare 100-nm-thick TiAlN films on SiO2/Si began to oxidize from the surface after annealing in air for 10 min from about 550 °C. Annealing at 700 °C oxidized half of the layer thickness. A 100-nm-thick Pt overlayer on the barrier layer retarded macroscopic oxidation at 650 °C. However, a 10-nm-thick Pt overlayer accelerated oxidation as a result of the catalytic dissociation of O2 molecules to form O atoms, which oxidized the barrier layer at 550 °C to the same extent as without the thin Pt overlayer at 650 °C. The effects of other thin metal overlayers, such as Ru and Ir, were also investigated. Ru and Ir did not accelerate TiAlN oxidation due to the absence of catalytic activity.
Crystal Structure–microwave Dielectric Property Relations in R2BaZnO5 (R= Sm, Eu, Gd, Dy, Ho, Er, and Tm) Ceramics
Authors: Kan Akinori , Ogawa Hirotaka
Published: March 2008

R2BaZnO5 (R = Sm, Eu, Gd, Dy, Ho, Er, and Tm) were synthesized, and the effects of the differences in ionic radii of R ions on the microwave dielectric properties and crystal structure have been investigated by using x-ray powder diffraction. It was shown that the R2BaZnO5compounds have an orthorhombic crystal structure with Pnma (No. 62) and the lattice parameters of the samples increase linearly with the increase in the ionic radii of the R ions. The dielectric constants (εr) of R2BaZnO5sintered at the optimum temperatures vary linearly from 16.1 to 19.3, suggesting that these variations in εr are the result of the differences in the ionic polarizabilities of R ions. Moreover, it is suggested that the variations in the valences of R ions determined by using bond valence sum may exert an influence on quality factor (Qf ), because the values of valences of R ions in R(1)O7 polyhedra and Qf exhibit similar tendencies with changes in the ionic radii of R ions. In the R2O3–BaO–ZnO system, Dy2BaZnO5showed the appropriate microwave dielectric properties: εr = 17.1, Qf = 29669 GHz, and τf = −1.5 ppm/°C.
Effect of Dislocation Core Spreading at Interfaces on Strength of Thin-films
Authors: Baker Shefford P. , Zhang Lin , Gao Huajian
Published: March 2008

Critical strain arguments are often used to model the thickness dependence of the strength of thin films on substrates. In these arguments, plastic deformation occurs when the stress in a film is high enough that the strain energy relieved by the introduction of a misfit dislocation is sufficient to generate the line energy of that misfit. Such models typically assume compact dislocation cores. However, experimental evidence suggests that, under certain circumstances, dislocation cores may spread out into the interface between the film and the substrate. If this happens, the energy of the misfit dislocation, and the critical stress needed for its propagation, will be lowered. In this paper, the effect of dislocation core spreading on the critical stress has been modeled. The effects of interface strength, film thickness, and misfit dislocation spacing are considered.
Synthesizing Nanocrystalline Carbon Thin Films by Hot Filament Chemical Vapor Deposition and Controlling Their Microstructure
Authors: Gupta S. , Weiner B. R. , Morell G.
Published: March 2008

Nanocrystalline carbon (n-C) thin films were deposited on Mo substrates using methane (CH4) and hydrogen (H2) by the hot-filament chemical vapor deposition (HFCVD) technique. Process parameters relevant to the secondary nucleation rate were systematically varied (0.3–2.0% methane concentrations, 700–900 °C deposition temperatures, and continuous forward and reverse bias during growth) to study the corresponding variations in film microstructure. Standard nondestructive complementary characterization tools such as scanning electron microscopy, x-ray diffraction, atomic force microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy were utilized to obtain a coherent and comprehensive picture of the microstructure of these films. Through these studies we obtained an integral picture of the material grown and learned how to control key material properties such as surface morphology (faceted versus evenly smooth), grain size (microcrystalline versus nanocrystalline), surface roughness (from rough 150 rms to smooth 70 rms), and bonding configuration (sp3 C versus sp2 C), which result in physical properties relevant for several technological applications. These findings also indicate that there exist fundamental differences between HFCVD and microwave CVD (MWCVD) for methane concentrations above 1%, whereas some similarities are drawn among films grown by ion-beam assisted deposition, HFCVD assisted by low-energy particle bombardment, and MWCVD using noble gas in replacement of traditionally used hydrogen.
Acidic Dispersion of Fused SiO2 Particles
Authors: Sabia Robert , Ukrainczyk Ljerka
Published: March 2008

The acidic dispersion behavior of fused SiO2 particles was investigated and compared to two fumed SiO2 particles. In contrast, the fused SiO2 particles have a larger particle size, broader size distribution, and lower surface area. Fluoride adsorption was used to study surface activity, and acid–base titration was used to study surface charge in 10−1 to 10−3 M NaCl solutions over the pH range of 2–7.5. Each of the three SiO2 particles exhibited similar titration behavior, with the fused SiO2 particles displaying a higher intrinsic pKa2 value of 7.0 as compared to 6.8 and 6.1 for the two fumed SiO2 particles. Rheological experiments designed to test for dispersion and agglomeration/ gellation at 3 and 6 wt% solids loading in 10−3 M NaCl solutions adjusted to pH 2, 4, and 6 showed the fused SiO2 particles to be more stable in suspension, exhibiting lower viscosity results for all test conditions. Results show that the fused SiO2 particles tested in this report display superior dispersion properties as compared to conventional fumed SiO2 particles for slurry applications under acidic conditions.
Fatigue-free La-modified Pb(Zr, Ti)O3 capacitors using aseed layer
Authors: Shannigrahi Santiranjan R. , Lee Sun-Hwa , Jang Hyun M.
Published: March 2008

The development of lead zirconate titanate (PZT)-based capacitors using common Pt electrodes has been a long-time goal of ferroelectric random access memories (FRAM).In this work, a series of Pb1−xLax(Zr0.55Ti0.45)O3 capacitors (for 0.01 < x < 0.05) having fatigue-free characteristics have been grown on Pt/ Ti/SiO2/Si substrates. Typically 2–3 mol% La-modified PZT capacitors fabricated at 580 °C by applying a PZT seed layer exhibited fatigue-free behavior up to 6.5 × 1010 switching cycles, a low coercive field of 50–55 kV/cm, and a stable charge retention profile with time, all of which assure their suitability for the future nonvolatile FRAM.
X-ray photoelectron spectroscopic studies on nanoquasicrystalline powders of Al70Cu20Fe10 obtained by mechanical alloying
Authors: Barua P. , Srinivas V. , Dhabal S. , Ghosh T. B.
Published: March 2008

Surface chemical properties of nanoguasicrystalline powders of Al–Cu–Fe synthesized by the ball-milling technique have been investigated using x-ray photoelectron spectroscopy (XPS). The samples were exposed to ambient conditions at room temperature as well as higher temperatures. Our XPS results reveal that the surfaces of nanopowders of Al70Cu20Fe10 are coated with an Al2O3 layer within which the quasicrystalline phase resides. It also appears that the thickness of this layer does not increase significantly on either heating below 873 K or prolonged exposure to ambient conditions.
Improving sol-gel Yba2Cu3O7−δ film morphology using high-boiling-point solvents
Authors: Dawley J. T. , Clem P. G. , Siegal M. P. , Tallant D. R. , Overmyer D. L.
Published: March 2008

The effect of high-boiling-point solvent addition on the morphology of low-p(O2) processed, sol-gel YBa2Cu3O7−δ (YBCO) films is discussed. Proper selection and addition of a high-boiling-point solvent prevents film roughening during precursor pyrolysis, while permitting at least a tenfold reduction in pyrolysis time compared to standard film processing in air or O2. Use of such solvents appears to increase film plasticity, avoiding elastic compressive stress related buckling. High-quality YBCO films on 〈100〉 LaAlO3 with a critical current density (Jc) ∼ 3–4 MA/cm2 at 77 K, are routinely crystallized with this new sol-gel process. Diethanolamine-based, sol-gel YBCO films deposited on sol-gel SrTiO3-buffered 〈100〉 Ni have Jc(77 K) > 1 MA/cm2, demonstrating a route toward all-sol-gel superconducting wires.
Multiphase microstructure evolution model including dislocation plasticity
Authors: Cleri Fabrizio , D'Agostino Gregorio
Published: March 2008

We present the recent extensions of our stochastic microstructure evolution model including multiphase domain evolution and dislocation plasticity. The model was implemented by means of numerical simulations based on the velocity Monte Carlo algorithm. It describes the evolution of a two-dimensional microstructure by tracking the motion of triple junctions, i.e., the vertices where three grain boundaries (GBs) meet. GBs can be modeled as straight, curved, or discretized segments; the misorientation dependence of both grain-boundary energies and mobilities can be included to represent different textures.
Structural and optical properties of lithium bismuthate glasses
Authors: Pan A. , Ghosh A.
Published: March 2008

Structural and optical properties of ion - conducting lithium bismuthate glasses are reported here. The structure of these glasses has been explored from the compositional variation of the density, molar volume, and glass transition temperature. The optical study in the visible and infrared region indicates a large transmission window for these glasses. The BiO6 octahedra were identified as the main structural unit from the Raman spectra of these bismuthate glasses.
Preparation and characteristics of 90° rotated biepitaxial Fe3O4 thin films
Authors: Matsuda Hiroshi , Sakakima Hiroshi , Adachi Hideaki , Odagawa Akihiro , Setsune Kentaro
Published: March 2008

In-plane 90° rotated biepitaxial Fe3O4 thin films have been successfully prepared onto MgO (110) substrates using a CeO2 seed layer and their microstructure, electric, and magnetic properties were investigated. From the x-ray φ-scan measurements, the in-plane epitaxial relations were determined as 〈110〉Fe3O4//〈110〉MgO and 〈001〉Fe3O4//〈001〉MgO for the no-seeded Fe3O4 layer, and 〈001〉Fe3O4//〈110〉MgO and 〈110〉Fe3O4//〈001〉MgO for the CeO2 (110) seeded Fe3O4 layer. The CeO2 seed layer was found to rotate the upper Fe3O4 lattice at 90° upon normal axis to the layer against the no-seeded Fe3O4. The transmission electron microscopy and electron diffraction analyses revealed that the transition region of the biepitaxial Fe3O4 boundary between CeO2-seeded and no-seeded portions consisted of columnarlike polycrystalline grains. The Fe 3O4 films exhibited single-crystallinelike electric and magnetic properties, however, substantial spin-dependent-tunneling magnetoresistance across the 90° grain boundary was not observed even in the antiparallel situation for each Fe3O4 portion.
Dislocations around precipitates in AlGaN epilayers
Authors: Kang Junyong , Tsunekawa Shin , Kasuya Atsuo
Published: March 2008

Dislocations around precipitates in undoped AlGaN were investigated by transmission electron microscopy. The dislocation images were taken under different diffraction conditions. The dislocations are classified into two types, a pure edge dislocation loop and a close-;coiled helical dislocation. Both types of dislocations were found to depend on the shape and size of the precipitate sources. It is suggested that the pure edge dislocation loop results from homogeneous shear stress and the close-;coiled helical dislocation is caused by spherically symmetrical stress concentration at round ends of the precipitates and chemical force due to defect concentration change.
Aqueous processing of SiC green sheets II: Binder and plasticizer
Authors: Zhang J. X. , Jianga D. L. , Tan S.H. , Gui L. H. , Ruan M. L.
Published: March 2008

Well-dispersed SiC slurries in the presence of binder and plasticizer were prepared and investigated in this series of work. In this part, poly(vinyl alcohol) (PVA) 1788 was investigated as a potential binder for aqueous tape casting process. The minimum amount of binder was determined through a primary calculation. Effects of the binder on SiC slurries properties were analyzed in term of zeta potential measurement and rheological test. Coupled with PEI as dispersant and glycerol as plasticizer, they lead to homogeneous systems which seem compatible. The suspensions (formulated with 47.9 wt% SiC powder, 1 wt% dispersant, 3.5 wt% binder, and 3.5 wt% plasticizer) exhibited a shear-thinning behavior with a very limited time-dependent character. After tape casting and drying process, the properties of green tapes were evaluated in term of solid content and gap height. The optimal solid content of SiC was found to be near 22.25 vol%, and the gap height, near 200 μm. The microstructure of green tape was characterized by TEM. Results showed that preparation of homogeneous green tapes with relative densities at about 51 vol% was feasible.
Predictions of strontium accommodation in A2B2O7 pyrochlores
Authors: Pirzada Mohsin , Grimes Robin W. , Maguire John , Sickafus Kurt
Published: March 2008

A2B2O7 pyrochlore oxides are being considered as potential host materials for the immobilization of fission products. It is therefore important to establish the relative ability of these compounds to accommodate fission product ions. We address this issue by using computer simulations to predict the structures and relative equilibrium energies associated with solution of Sr2+ over an extensive compositional range. Results indicate that strontium is accommodated via substitution for A host cations with oxygen vacancy compensation. This results in a nonstoichiometric composition. Optimum compositions and defect clusters structures are identified by constructing contour energy maps.
Step bunching in potassium dihydrogen phosphate crystal growth: Phenomenology
Authors: Booth N. A. , Chernov A. A. , Vekilov P. G.
Published: March 2008

We have developed a real-time phase-shifting interferometer capable of imaging interfacial morphology with a depth resolution of approximately 25 Å, with a lateral resolution of approximately 0.5 μm across a field of view of 2 × 2mm2, and with time resolution of 0.1 s. The method is applied in situ to the (101) face of potassium dihydrogen phosphate crystals growing from an aqueous solution. We image the formation and evolution of solution-flow-induced step bunches and determine their characteristic wavelength to be λc = 45 μm. This wavelength is within the range predicted by a stability theory on the basis of the balance between the diffusion interaction between steps and capillarity. The value of λc suggests that step–step interactions are the likely major factor for instability.
Microstructural evolution during pyrolysis of triol-based sol-gel single-layer Pb(Zr0.53Ti0.47)O3 thin films
Authors: Zhou Zhaoxia , Reaney Ian M. , Hind David , Milne Steven J. , Brown Andy P. , Brydson Rik
Published: March 2008

Advanced analytical transmission electron microscopy has been used to investigate microstructural evolution during pyrolysis in triol-based sol-gel thin films. At pyrolysis temperatures up to 300 °C, the films remained amorphous; however, nanometer-sized precipitates were observed in films heat-treated up to 400 °C for 10 min. Analytical transmission electron microscopy indicated that the precipitates were Pb-rich, as well as deficient in O, Ti, and Zr. Films pyrolyzed up to 500 °C for 10 min were composed of a nanocrystalline pyrochlore phase; however, pores could be observed, situated in the same position as the nanometer-sized precipitates at 400 °C. Face-centered cubic Pb-rich crystallites were also present on the surface of pyrolyzed films but absent in the fully crystallized films annealed at 650 °C. A tentative mechanism is proposed to explain these observations.
Structure and magnetic properties of Ni films obtained by pulsed laser ablation deposition
Authors: Vergara J. , Madurga V.
Published: March 2008

Pulsed laser ablation deposition was used to fabricate Ni films. The structure of the as-deposited films is hexagonal (a = 0.267 nm and c = 0.435 nm), and its resistivity is nearly one order of magnitude larger than the resistivity of bulk Ni. No spontaneous magnetization was observed in the as-deposited samples. Annealing above 200 °C induced an irreversible structural transformation from the metastable hexagonal phase to the stable face-centered-cubic (fcc) one. Parallel to this transformation, the magnetic moment of the annealed Ni samples increased, and their resistivity decreased. On annealing at 440 °C, the structural transformation from the hexagonal-close-packed to the fcc phase was completed, and the magnetic moment, Curie temperature, and electrical resistivity of the fcc Ni films were found similar to the ones observed in bulk Ni. A direct correlation between the volume fraction of the fcc Ni phase in our films and their magnetic moment was established.
Synthesis of nanocrystalline CeO2–Y2O3 powders by a nitrate–glycine gel-combustion process
Authors: Bianchetti Mario F. , Juárez Ricardo E. , Lamas Diego G. , de Reca Noemí E. Walsöe , Pérez Lidia , Cabanillas Edgardo
Published: March 2008

In this work, the synthesis of CeO2–10 mol% Y2O3 powders by a nitrate–glycine gel-combustion route was investigated. Special attention was given to the influence of the glycine/metal ratio and calcination temperature on powder morphology. In contrast to the usual reported behavior, the best powder properties (crystallite size, 4.5–7 nm; specific surface area; 25–40 m2/g) were obtained for slow combustion processes with glycine/metal ratios of 1.5–2, whereas energetic reactions resulted in large crystallite and particle sizes. Furthermore, it was found that the crystallite size increases considerably even at moderate calcination temperatures (350–550 °C), showing the high reactivity of these nanopowders.
Single buffer layers of LaMnO3 or La0.7Sr0.3MnO3 for the development of Yba2Cu3O7−δ-coated conductors: A comparative study
Authors: Aytug T. , Paranthaman M. , Zhai H. H. , Christen H. H. , Sathyamurthy S. , Christen D. D. , Ericson R. R.
Published: March 2008

Single, epitaxial buffer layers of insulating LaMnO3 (LMO) or conductive La0.7Sr0.3MnO3 (LSMO) have been grown by sputter deposition on biaxially textured Ni and Ni–alloy substrates. We report baseline investigations of their compatibility with the Yba2Cu3O7−δ (YBCO) coatings and demonstrate biaxially textured YBCO films grown by pulsed-laser deposition on these single-buffered tapes. Superconducting property characterizations revealed better properties for YBCO films on LMO-buffered tapes relative to those grown on LSMO layers. Self-field critical current densities (Jc) exceeding 1 × 106 A/cm2 at 77 K have been obtained for the YBCO (200 nm) films on LMO-buffer layers. These results offer prospects for the use of single, LMO-buffered metal tapes in the development of practical YBCO-coated conductors.
Large cryogenic storage of hydrogen in carbon nanotubes at low pressures
Authors: Pradhan B. B. , Harutyunyan A. A. , Stojkovic D. , Grossman J. J. , Zhang P. , Cole M. M. , Crespi V. , Goto H. , Fujiwara J. , Eklund P. P.
Published: March 2008

We report up to 6 wt% storage of H2 at 2 atm and T = 77 K in processed bundles of single-walled carbon nanotubes. The hydrogen storage isotherms are completely reversible; D2 isotherms confirmed this anomalous low-pressure adsorption and also revealed the effects of quantum mechanical zero point motion. We propose that our postsynthesis treatment of the sample improves access for hydrogen to the central pores within individual nanotubes and may also create a roughened tube surface with an increased binding energy for hydrogen. Such an enhancement may be needed to understand the strong adsorption at low pressure. We obtained an experimental isosteric heat qst = 125 ± 5 meV. Calculations are also presented that indicate disorder in the tube wall enhances the binding energy of H2.
Kinetic study of (Bi,Pb)2Sr2Ca2Cu3O10+x phase formation in KCl flux
Authors: Lee Sergey , Yamamoto Ayako , Tajima Setsuko
Published: March 2008

The kinetics of (Bi,Pb)2Sr2Ca2Cu3O10+x phase formation in KCl flux was studied, and kinetic analysis using the Avrami relation for isothermal phase transformation gave the Avrami exponent n = 2.5 at 855 °C for the whole process of (Bi,Pb)-2223 phase formation. The estimated value of the activation energy Ea = 150 kJ/mol for the formation of (Bi,Pb)-2223 phase at 845–855 °C is the lowest among the previously reported values. The low value of activation energy explains the fast formation of single-phase (Bi,Pb)-2223 powder in the KCl flux.
Creep behavior of in situ dual-scale particles-TiB whisker and TiC particulate-reinforced titanium composites
Authors: Ma Z. Z. , Tjong S. S. , Meng X. X.
Published: March 2008

A titanium composite reinforced by in situ dual-scale particle, high-aspect-ratio TiB whiskers and fine TiC particulates was fabricated by a reactive hot pressing technique from a B4C–Ti system. The composite was subjected to creep investigations in compression at 873–923 K. This composite exhibited a stress exponent of 4.5–4.6 and a creep activation energy of 298 kJ/mol. By comparison, unreinforced Ti exhibited a stress exponent of 5.2–5.3 and a creep activation energy of 259 kJ/mol. No change in the stress exponent with varying creep rates was observed in both composite and unreinforced Ti under the investigated creep rates. The creep resistance of the composite was more than one order of magnitude higher than that of the unreinforced Ti. The load transfer mechanism accounted for this result. The creep of both composite and unreinforced Ti was controlled by lattice diffusion in the titanium matrix.
Nanoindentation measurement of hardness and modulus anisotropy in Ni3Al single crystals
Authors: Wang W. , Lu K.
Published: March 2008

Hardness and elastic modulus of (111), (110), and (001) oriented Ni3Al single crystals were determined by nanoindenter measurements. Obvious elastic modulus anisotropy and hardness anisotropy were observed. The modulus for (001) was about 17% smaller than that for (111), (110). The hardness was found to be strongly dependent on the indentation size and exhibited a small anisotropy at low indentation loads. When the indentation load was increased further, the hardness anisotropy became apparent. The hardness for (111) was observed to be higher compared to (001). The indentation hardness size effect was examined by using strain gradient plasticity theory.
Improved critical current in MgB2 tapes sheathed with carbon steels
Authors: Fujii H. , Kumakura H. , Togano K.
Published: March 2008

We fabricated MgB2 tapes by a powder-in-tube method using sheath materials of Fe and carbon steel (CS). In the as-rolled state, the CS-sheathed tapes showed higher transport critical current density (Jc) values than the Fe-sheathed tape did. This is due to the higher-density MgB2 layer associated with the high mechanical strength of the CS. Furthermore, heat treatment above 800 °C was very effective in increasing the Jc of these tapes. The heat-treated CS-sheathed tapes showed Jc values of 10 kA/cm2 at 4.2 K and 7.5 T and still above 2 kA/cm2 at 10 T. These values are the highest ever reported for MgB2 tapes. An extrapolation to 0 T in the Jc-B data gave about 1 MA/cm2, which was independent of the sheath materials. Microstructural observations suggest that the high Jc and the small field dependence of the Jc properties of the CS-sheathed tapes can be ascribed to the improved grain connectivity and grain alignment produced by the high mechanical strength of CS.
Mechanisms of ceramic coating deposition in solution-precursor plasma spray
Authors: Bhatia Tania , Ozturk Alper , Xie Liangde , Jordan Eric H. , Cetegen Baki M. , Gell Maurice , Ma Xinqin , Padture Nitin P.
Published: March 2008

The solution-precursor plasma spray (SPPS) method is a new process for depositing thick ceramic coatings, where solution feedstock (liquid) is injected into a plasma. This versatile method has several advantages over the conventional plasma spray method, and it can be used to deposit nanostructured, porous coatings of a wide variety of oxide and non-oxide ceramics for a myriad of possible applications. In an effort to understand the SPPS deposition process, key diagnostic and characterization experiments were performed on SPPS coatings in the Y2O3-stabilized ZrO2 (YSZ) system. The results from these experiments show that there are multiple pathways to SPPS coating formation. The atomized precursor droplets undergo rapid evaporation and breakup in the plasma. This is followed by precipitation, gelation, pyrolysis, and sintering. The different types of particles reach the substrate and are bonded to the substrate or the coating by sintering in the heat of the plasma. The precursor also reaches the substrate or the coating. This precursor pyrolyzes in situ on the substrate, either after it reaches a “cold” substrate or upon contact on a “hot” substrate and helps bond the particles. The coating microstructure evolves during SPPS deposition as the coating temperature reaches approximately 770 °C.
Dielectric and piezoelectric properties of textured Sr0.53Ba0.47Nb2O6ceramics prepared by templated grain growth
Authors: Duran Cihangir , Trolier-McKinstry Susan , Messing Gary L.
Published: March 2008

Fiber textured Sr0.53Ba0.47Nb2O6ceramics were reactively sintered to ≥95% of the theoretical density from a mixture of SrNb2O6and BaNb2O6powders. Texture in 〈001〉 was obtained by templated grain growth on 〈001〉-oriented acicular KSr2Nb5O15template particles. The most highly textured ceramics had a peak dielectric constant of 23,600 (atTc= 128 °C), a remanent polarization (Pr) of 20.3 µC/cm2, a saturation polarization (Psat) of 24 µC/cm2(69–96% of single crystal), and a piezoelectric charge coefficient (d33) of 84 pC/N (76–93% of single crystal). A model, correlating polarization with grain orientation, predicts thatPrincreases sharply when a percolating grain network forms to transfer charge between elongated grains.
Surface alloying of AZ91D alloy by diffusion coating
Authors: Zhang M-X. , Kelly P. M.
Published: March 2008

A new technique of surface modification by diffusion coating for AZ91D alloy was developed. A 1.0–2.0-mm alloy layer, which has hardness four to five times higher than the substrate metal, was formed after the treatment. Consequent solution treatment and aging could further improve the hardness of the alloy layer. Microstructure and chemical composition were investigated using optical microscope and electron probe.
Low-cost manufacturing process for nanostructured metals and alloys
Authors: Brown Travis L. , Swaminathan Srinivasan , Chandrasekar Srinivasan , Compton W. Dale , King Alexander H. , Trumble Kevin P.
Published: March 2008

In spite of their interesting properties, nanostructured materials have found limited uses because of the cost of preparation and the limited range of materials that can be synthesized. It has been shown that most of these limitations can be overcome by subjecting a material to large-scale deformation, as occurs during common machining operations. The chips produced during lathe machining of a variety of pure metals, steels, and other alloys are shown to be nanostructured with grain (crystal) sizes between 100 and 800 nm. The hardness of the chips is found to be significantly greater than that of the bulk material.
Effects of gamma radiation on poly(methyl methacrylate)/single-wall nanotube composites
Authors: Muisener P. A. O'Rourke , Clayton L. , D'Angelo J. , Harmon J. P. , Sikder A. K. , Kumar A. , Cassell A. M. , Meyyappan M.
Published: March 2008

Single-wall carbon nanotube (SWNT)/poly(methyl methacrylate) (PMMA) composites were fabricated and exposed to ionizing radiation for a total dose of 5.9 Mrads. Neat nanotube paper and pure PMMA were also exposed for comparison, and nonirradiated samples served as controls. A concentration of 0.26 wt% SWNT increased the glass transition temperature (Tg), the Vickers hardness number, and modulus of the matrix. Irradiation of the composite did not significantly change the Tg, the Vickers hardness number, or the modulus; however, the real and imaginary parts of the complex permittivity increased after irradiation. The dielectric properties were found to be more labile to radiation effects than mechanical properties.
Microwave dielectric properties of RETiTaO6 (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, Al, and In) ceramics
Authors: Surendran Kuzhichalil Peethambaran , Solomon Sam , Varma Manoj Raama , Mohanan Pezholil , Sebastian Mailadil Thomas
Published: March 2008

Microwave dielectric ceramics based on RETiTaO6 (RE = La, Cc, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, Al, and In) were prepared using a conventional solid-state ceramic route. The structure and microstructure of the samples were analyzed using x-ray diffraction and scanning electron microscopy techniques. The sintered samples were characterized in the microwave frequency region. The ceramics based on Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy, which crystallize in orthorhombic aeschynite structure, had a relatively high dielectric constant and positive τf while those based on Ho, Er, and Yb, with orthorhombic euxenite structure, had a low dielectric constant and negative τf. The RETiTaO6 ceramics had a high-quality factor. The dielectric constant and unit cell volume of the ceramics increased with an increase in ionic radius of the rare-earth ions, but density decreased with it. The value of τf increased with an increase in RE ionic radii, and a change in the sign of τf occurred when the ionic radius was between 0.90 and 0.92 Å. The results indicated that the boundary of the aeschynite to euxenite morphotropic phase change lay between DyTiTaO6 and HoTiTaO6. Low-loss ceramics like ErTiTaO6 (εr = 20.6, Quxf = 85,500), EuTiTaO6 (r = 41.3, Quxf = 59,500), and YtiTaO6 (εr = 22.1, Quxf = 51,400) are potential candidates for dielectric resonator applications.
Cold rolling and recrystallization textures in L12-type Co3Ti ordered alloys
Authors: Kaneno Yasuyuki , Nakaaki Itsuko , Takasugi Takayuki
Published: March 2008

The texture development during cold rolling and recrystallization of L12-type Co3Ti alloys was investigated as functions of alloy composition and rolling reduction. Two kinds of fully annealed Co3Ti alloys with different alloy compositions (Co77Ti23 and Co78Ti22), whose initial textures and grain sizes were almost identical, were used. After 70% reduction, the cold-rolling textures of both alloys were composed of strong α-fiber and weak β-fiber textures, in which the intensity of {011}〈211〉 orientation was higher than those of {112}〈111〉 and {123}〈634〉 orientations, indicating that texture transition from copper type to brass type occurred in cold-rolled Co3Ti alloys. This texture transition was more prominently recognized in the Co78Ti22 alloy, whose chemical composition was more off-stoichiometric. On the other hand, the recrystallization textures of both alloys were considerably weak, and their orientation spreads were more significant with increasing rolling reduction. Also, the recrystallization textures were basically independent of alloy composition in contrast to the rolling textures.
Microstructural investigation of four kinds of γ′–Fe4N nitrides in ion-nitrided pure iron
Authors: Liu Z. Q. , Hei Z. K. , Li D. X.
Published: March 2008

Conventional and high-resolution electron microscopies were used to investigate the microstructure and precipitation mechanism of four kinds of γ′–Fe4N nitrides, formed at the surface during nitriding of polycrystalline pure iron sheet. γ′ nitrides in the top columnar compound layer were thin plates (3–12 nm), which precipitated from ε–Fe2−3N grains with orientation relationship of (111)γ′//(0001)ε and [011]γ′//[1210]ε. Those in the transition compound layer remained as equiaxed grains (1–1.5 μm), which grew from the α–Fe substrate during nitriding. In the diffusion layer, striated γ′ nitrides nucleated from α″–Fe16N2 nitrides with orientation relationship of (100)γ′// (110)α″ and [011]γ′//[111]α″ while unstriated γ′ nitrides precipitated from the α–Fe substrate with the orientation relationship of (100)γ′//(110)α and [011]γ′//[111]α.
Microstructure and mechanical properties of Ti–Si–N coatings
Authors: Meng W. J. , Zhang X. D. , Shi B. , Tittsworth R. C. , Rehn L. E. , Baldo P. M.
Published: March 2008

A series of Ti–Si–N coatings with 0 < Si < 20 at.% were synthesized by inductively coupled plasma assisted vapor deposition. Coating composition, structure, atomic short-range order, and mechanical response were characterized by Rutherford backscattering spectrometry, transmission electron microscopy, x-ray absorption near-edge structure spectroscopy, and instrumented nanoindentation. These experiments show that the present series of Ti–Si–N coatings consists of a mixture of nanocrystalline titanium nitride (TiN) and amorphous silicon nitride (a-Si:N); i.e., they are TiN/a-Si:N ceramic/ceramic nanocomposites. The hardness of the present series of coatings was found to be less than 32 GPa and to vary smoothly with the Si composition.
Thermally-induced phase changes in electrophoretically deposited titanate and niobate layered oxides
Authors: Unal Ugur , Matsuo Dan , Matsumoto Yasumichi , Koinuma Michio
Published: March 2008

Thin films of CsxTi(2−x/4)□x/4O4 (CsTiO), K4Nb6O17 (KNbO), and their proton-exchanged forms, i.e., HxTi(2−x/4)□x/4O4 (HTiO) and H4Nb6O17 (HNbO), were prepared using the electrophoretic deposition technique. The amine- and thiol-intercalated HTiO and HNbO films were prepared by exfoliation of powders in aqueous ethylamine and (mercaptoethyl)amine hydrochloride solutions, respectively. The heat-induced phase transformation of these films was investigated. Evidently, the CsTiO and thiol-intercalated HTiO films underwent phase transformation at relatively high temperatures due to the cations within the interlayer. CsTiO and HTiO films lost their layered structure and transformed, in turn, into the anatase and rutile phases with increasing temperature. However, the intercalated samples exhibited unidentified phases at in-between temperatures, eventually transforming to TiO2. The KNbO film transformed into a layered KNb3O8 structure, while the HNbO lost its layered structure completely to form Nb2O5. Thus, the phase change depended on the modification of the interlayers, and the heat treatment resulted in thin films with new crystal structures for the amine- and thiol-intercalated samples.
Electrical behavior of oxidized metal powders during and after compaction
Authors: Garino Terry J.
Published: March 2008

The electrical behavior during compaction of tantalum and aluminum powders was characterized before and after thermal oxidation. The resistivity of unoxidized powders decreased by >106 over a narrow range of stress between 1 and 10 MPa. Thermal oxidation of the powders to produce submicrometer thick oxide layers on the particles increased the precompaction electric breakdown strength from <0.2 to >5 kV/cm but did not have a significant effect on the low field resistivity of the powders during compaction. At higher fields, the decrease in resistivity during compaction occurred at lower stresses and over a much narrower stress range since catastrophic electrical breakdown occurred once a certain level of stress was reached. The breakdown field at constant stress also decreased as the stress was increased for the oxidized powders. These effects are caused by the cracking of the brittle oxide coatings at the contact points between the particles during compaction.
The effect of doping Ag on the microstructure of La2/3Sr1/3MnO3 films
Authors: Zhan Q. , Yu R. , He L. L. , Li D. X. , Li J. , Ong C. K.
Published: March 2008

The microstructure of Ag-doped La2/3Sr1/3MnO3 (LSMO) thin films deposited on (001) LaAlO3 single-crystal substrates was systematically investigated in cross section and plan view by high-resolution electron microscopy and analytical electron microscopy. The results showed that the films deposited at 750 °C were perfectly epitaxial with or without Ag-doping. No Ag in the doped film was detected. On the other hand, the LSMO films deposited at 400 °C were less perfect. With increasing Ag-doping level, the shape of LSMO grains became irregular, and the grain size increased gradually. Large polycrystalline clusters consisting of LSMO, AgO, and Ag grains formed in the doped films, and the amount and size of them increased with increasing Ag-doping level. Ag existed at the LSMO grain boundaries in its elemental state. A growth process for the LSMO-Ag system is discussed based on the experimental results. The enhancement of the magnetic spin disorders at the grain boundaries and interfaces caused by doping Ag could result in an improvement of low-field magnetoresistance.
Comparative study of nonequilibrium phase of A3B and AB3 types in the Ni–Mo system by first principles and thermodynamic calculations
Authors: Liu J. B. , Zhang R. F. , Liu B. X.
Published: March 2008

In the Ni–Mo system, two possible nonequilibrium states located near Ni3Mo were predicted through calculation of the total energies of eight possible structures of the A3B type as a function of their lattice constant(s) by applying the Vienna ab initio simulation package. Comparatively, Gibbs free energy diagrams were constructed from Miedema's model and, in general, the calculated results were in accordance with the prediction from ab initio calculation. Besides, the prediction of one of the nonequilibrium states was in agreement with the fact that a metastable hexagonal-close-packed Ni3Mo phase was indeed observed in Ni75Mo25 multilayer films upon thermal annealing, and calculated lattice constants matched well with those determined by diffraction analysis in experiments as well.
Nano-sizing titanium into titanium carbide by 1-chlorobutane
Authors: Chang Yu-Hsu , Chiu Hsin-Tien
Published: March 2008

TiC nanoparticles (average diameter 10–150 nm) were prepared by reacting bulk Ti powders (average diameter 20 μm) with gaseous 1-chlorobutane at a relatively low temperature (1073–1273 K). 1-Chlorobutane provided the carbon atom in the TiC and the chlorine atom that assisted the shrinking of the size of the original Ti powders. The apparently simple procedure is a complex heterogeneous process combining etching, deposition, and carburization reactions.
Laser-induced nanoparticle ordering
Authors: Pedraza A. J. , Fowlkes J. D. , Blom D. A. , Meyer H. M.
Published: March 2008

Nanoparticles were produced on the surface of silicon upon pulsed-laser irradiation in the presence of an inert gas atmosphere at fluences close to the melting threshold. It was observed that nanoparticle formation required redeposition of ablated material. Redeposition took place in the form of a thin film intermixed with extremely small nanoparticles possibly formed in the gas phase. Through the use of nonpolarized laser light, it was shown that nanoparticles, fairly uniform in size, became grouped into curvilinear strings distributed with a short-range ordering. Microstructuring of part of the surface prior to the laser treatment had the remarkable effect of producing nanoparticles lying along straight and fairly long (approximately 1 mm) lines, whose spacing equaled the laser wavelength for normal beam incidence. In this work, it is shown that the use of polarized light eliminated the need of an aiding agent: nanoparticle alignment ensued under similar laser treatment conditions. The phenomenon of nanoparticle alignment bears a striking similarity with the phenomenon of laser-induced periodic surface structures (LIPSS), obeying the same dependence of line spacing upon light wavelength and beam angle of incidence as the grating spacing in LIPSS. The new results strongly support the proposition that the two phenomena, LIPSS and laser-induced nanoparticle alignment, evolve as a result of the same light interference mechanism.
Processing and characterization of Ti–B-based functionally graded materials produced by microwave-activated combustion synthesis
Authors: Cirakoglu M. , Bhaduri S. , Bhaduri S. B.
Published: March 2008

Single as well as graded compositions were fabricated in the Ti–B system using a microwave-activated combustion synthesis (MACS) process. When synergistically combined with microwave processing, combustion synthesis offers great potential for the fabrication of ceramic structures and composites. Combustion waves were triggered using a SiC susceptor to initially absorb microwaves. The effects of processing variables, such as thermal insulation, and atmosphere on the process were investigated. Examination of reacted samples by means of x-ray diffraction indicated the presence of titanium monoboride and diboride along with unreacted titanium. Compared with conventional combustion synthesized products, MACS resulted in smaller pores. However, the total amount of porosity remained almost the same. The microstructure of the graded layers and interfaces were examined using optical and scanning electron microscopy. Over the entire cross section, the interfaces were continuous and crack free.
Improvement in tunability and dielectric loss of (Ba0.5Sr0.5)TiO3 capacitors using seed layers on Pt/Ti/SiO2/Si substrates
Authors: Jeon Young-Ah , Shin Woong-Chul , Seo Tae-Suck , Yoon Soon-Gil
Published: March 2008

The absence of a low dielectric constant layer at the barium strontium titanate (BST)/Pt interface and a decreased roughness are critical issues in the production of (Ba0.5Sr0.5)TiO3 thin films with high tunabilities and low losses. An improvement in dielectric properties was achieved by the insertion of seed layers at the BST/Pt interface by pulsed laser deposition. The higher tunability can be attributed to (100) texturing of the BST films, which is independent of grain size and grain morphologies, thus leading to a variation in seed layer thicknesses. The tunability and dielectric constant of 1600-Å-thick BST films showed a maximum of 53% and 720, respectively, at a seed layer thickness of 100 Å. Dielectric loss is dependent on the roughness of BST films and reached a minimum of 0.8% at a root mean square roughness of 28 Å. The maximum figures of merit, defined as the ratio of tunability to dielectric loss, of approximately 58 at 100 kHz and 198 kV/cm were obtained at a seed layer thickness of 70 Å. The optimized seed layer thickness for BST deposition onto Pt/Ti/SiO2/Si substrates plays an important role in maintaining the high tunabilities and low loss, which are suitable for microwave device applications.
Comparison of citrate–nitrate gel combustion and precursor plasma spray processes for the synthesis of yttrium aluminum garnet
Authors: Devi P. Sujatha , Lee Yongjae , Margolis Joshua , Parise John B. , Sampath Sanjay , Herman Herbert , Hanson Jonathan C.
Published: March 2008

The influence of synthesis conditions on the formation of yttrium aluminum garnet (YAG) powders starting from the same solution precursors was investigated by employing a citrate–nitrate gel combustion process and a precursor plasma spraying technique. YAG powders were formed at ≥500 °C, through the citrate–nitrate gel combustion process, without any intermediate phase formation. Time-resolved x-ray experiments were performed for the first time on these citrate–nitrate precursor materials to understand their mode of decomposition. The in situ data confirmed a single-step conversion to YAG from the precursor powder without any intermediate phase formation. Ex situ experiments also produced similar results. However, the use of the same citrate–nitrate precursor solution as a liquid feedstock material in the precursor plasma spraying technique revealed an entirely different transformation mechanism to YAG through intermediate phases like H–YalO3 and O–YalO3.
Positive temperature coefficient of resistivity effect in Pb-doped KnbO3
Authors: Pribošič Irena , Makovec Darko , Drofenik Miha
Published: March 2008

KnbO3 is a ferroelectric material with a Curie temperature (TC) at 415°C, thus giving it the potential to be a material for high-temperature positive temperature coefficient of resistivity (PTCR) applications. In this study, we investigated the PTCR effect in donor-doped KnbO3 ceramics containing 0, 0.1, 0.2, and 0.3 mol% PbO. The donor-doped KnbO3 ceramics exhibited a PTCR anomaly with a relatively low room-temperature resistivity. The temperature of the tetragonal-to-cubic phase transition (TC) of the KnbO3 decreased with the amount of added PbO, while the orthorhombic-to-tetragonal phase transition (TOT) remained unchanged.