bluuebunny/crossref_metadata_2025 huggingface dataset:
Degradation of YBC critical current as a result of annealing in air
Authors: Bransky Judith
Published: September 2007

The present study was aimed at determining the rate of degradation of the transport critical current density Jc of ceramic YBCO stored in atmosphere, as a function of temperature. The transport critical current of ring-shaped samples was measured by the inductance technique. The samples were prepared by the standard method. The results show that annealing at 100 °C for over 500 h caused practically no degradation of Jc. Above 100 °C the rate of degradation increases rapidly with temperature.
Formation of quasicrystalline phases in an Al–Cu–Co–Fe alloy obtained by using a gravity chill-casting technique
Authors: Perez R. , Arizmendi A. , Juarez-Islas J.A. , Martinez L.
Published: September 2007

A study of the quasicrystalline phases obtained from a quaternary alloy of Al–Cu–Co–Fe is carried out. The formation of these phases is based on a gravity chill-casting technique. The alloy was cast in a wedge-shaped copper mold. Optical and scanning electron micrographs show a characteristic dendritic growing in regions close to the wedge edge. These types of dendrites display apparent symmetries that resemble the tenfold, fivefold, and eightfold symmetries obtained in quasicrystalline materials. Electron diffraction patterns, in addition to x-ray diffraction patterns, show two kinds of quasicrystalline phases. The icosahedral and decagonal phases coexist in these types of alloys also with binary and ternary compounds of intermetallic nature.
The properties of thermal hillocks as a function of linewidth and process parameter in Al-on-chemical-vapor-deposited W films
Authors: Pico Carey A. , Bonifield Tom D.
Published: September 2007

The formation of hillocks has been studied as a function of process parameter in patterned and unpatterned Al98.5wt. %Si1.0wt. %Cu0.5wt. % films deposited on chemical-vapor-deposited W-coated substrates. The effects of linewidth, substrate temperature during film deposition, and sintering time and temperature on hillock size were investigated. Three types of hillocks are found: the “surface hillocks”, the “side hillock”, and the “line hillock”. These are further classified by their shapes. The surface hillock and side hillock, which have been seen previously, form on patterned metal lines having linewidths greater than the larger Al alloy grain sizes (~3 μm). None is seen on linewidths between 0.9 and 2 μm where long-range grain boundary diffusion cannot occur. A new type of hillock, the line hillock, is seen to occur on metal structures having linewidths of 0.6 μm. The line hillock is inconsistent with the current understanding of hillock formation and may present severe restrictions on the downsizing of ultra-large-scale integrated devices.
Epitaxical nucleation of polycrystalline silicon carbide during chemical vapor deposition
Authors: Sheldon Brian W. , Besmann Theodore M. , More Karren L. , Moss Thomas S.
Published: September 2007

Polycrystalline silicon carbide was deposited from methyltrichlorosilane in cold-walled and hot-walled reactors, on (100) SiC surface layers that were formed on (100) Si wafers. The initial stages of the process were studied by electron microscopy after relatively short deposition times. Submicron surface features nucleated with a specific crystallographic orientation with respect to the substrate, where h111j planes in th—SiC substrate coincided with h0001j planes in the a–SiC features. These a–SiC features occurred only at twins on h111j planes of the b–SiC substrate. This demonstrates that nucleation under these conditions is controlled by defects in the substrate. Surface contamination and the reactor configuration also had substantial effects on nucleation.
Kinetics and mechanisms of high-temperature creep in polycrystalline aluminum nitride
Authors: Vasudev A. , More K.L. , Ailey-Trent K.S. , Davis R.F.
Published: September 2007

The operative and controlling mechanisms of steady-state creep in hot-pressed AlN have been determined both from kinetic data within the temperature and constant compressive stress ranges of 1470 to 1670 K and 100 to 370 MPa, respectively, and from the microstructural results of TEM. No secondary phases were detected in the bulk or at the grain boundaries using Raman spectroscopy and HREM. The stress exponent was ≍1.0 at all temperatures. The activation energies ranged between 558 and 611 kJ/mol. The most prominent microstructural features of the crept samples were elongated grains, strain whorls, and triple-point folds. Dislocations were generated only at the strain whorls in order to relieve the localized stress caused by intraboundary mechanical interaction among the grains. They contributed little to the observed deformation. The controlling mechanism for creep was diffusion-accommodated grain-boundary sliding. This mechanism was accompanied in parallel by relatively small amounts of unaccommodated grain-boundary sliding. Cavitation was not observed.
Closed porosity aluminosilicate for electronic packaging applications
Authors: Hietala S.L. , Smith D.M. , Hietala V.M. , Brinker C.J.
Published: September 2007

The electrical properties of sol-gel prepared aluminosilicate films were investigated for suitability in electronic applications. The aluminosilicate films exhibited apparent closed porosity and a dielectric constant as low as ≍5 with processing temperatures from 373 K to 873 K. The porosity was inaccessible to nitrogen at 77 K, helium at 293 K, and water vapor at 293 K. Both bulk and thin-film samples were analyzed for hydroxyl and carbon contents to elucidate the relative dependence of the measured electrical properties on processing conditions. Experiments indicate it is possible to vary the porosity in bulk material in ways that should improve electrical properties.
Hydrotalcites with an extended Al3+-substitution: Synthesis, simultaneous TG-DTA-MS study, and their CO2 adsorption behaviors
Authors: Tsuji Masamichi , Mao Gang , Yoshida Takashi , Tamaura Yutaka
Published: September 2007

A maximum Al3+-substitution has been demonstrated to be 45 mole% of (M + Al) in the brucite layer of hydrotalcites. The chemical composition of the highly substituted hydrotalcites can be typically represented by [M0.55Al0.45(OH)2] [(CO3)0.225 · 0.50H2O] where M = Mg, Ni, Zn, and Co. It showed the small lattice parameters of a0 3.05–2.98 A in the hexagonal lattice, which corroborates Al3+-substitution in the brucite layer. The simultaneous thermal analyses (TG and DTA) and mass spectrometry (MS) study have been performed. The highly Al3+-substituted hydrotalcites also showed quite different isotherms for the CO2 adsorption. These materials adsorbed CO2 gas by removing water within the interlayer and showed the selectivity for CO2 adsorption: Cu–Al ∼Zn—Al < Co—Al < Mg—Al < Ni—Al. The Mg—Al and Co—Al hydrotalcite-like compounds showed a doubled amount of CO2 by removing carbonate ions within the interlayer.
Synthesis, characterization, and properties of new polystyrene-SiO2 hybrid sol-gel materials
Authors: Wei Yen , Yang Dachuan , Tang Liguang , Hutchins MaryGail K.
Published: September 2007

A new family of organic-inorganic hybrid materials has been prepared by incorporating polystyrene structure units covalently into the SiO2 glass network via the sol-gel approach. The polymer precursors were synthesized by free-radical copolymerization of styrene with 3-(trimethoxysilyl)propyl methacrylate (MSMA) at various feeds. These copolymers were then hydrolyzed and co-condensed with tetraethyl orthosilicate in tetrahydrofuran at room temperature to afford monolithic polystyrene-SiO2 hybrid sol-gel materials having SiO2 contents of 15 to 84% by weight. The hybrid materials derived from the copolymers with MSMA contents greater than 22 mol% have excellent optical transparency. In these transparent hybrid materials, the polymer chains should be uniformly distributed in and covalently bonded to the amorphous SiO2 matrices. The bulk properties of these materials including density, refractive index, and hardness were found to be related to their molecular compositions and can be tailored by varying the polymer contents.
Hard sphere modeling of the effect of slip on interstitial sites in the B2 structure
Authors: Baker I.
Published: September 2007

Geometrical modeling of the B2 structure indicates that the tetrahedral interstitial site is always the largest both before and after an a/2〈111〉 translation on {110}, such as occurs during the slip of a partial dislocation in some B2 compounds. The tetrahedral site within the APB which trails a gliding a/2〈111〉 dislocation is larger than in the unslipped lattice, suggesting that interstitial atoms will segregate there. Also, some interstitial sites in a B2 lattice are larger than those in a bcc lattice of the same lattice parameter, suggesting that interstitials may have greater solubility in B2 compounds.
High dielectric permittivity in glass-ceramic metal nanocomposites
Authors: Roy B. , Chakravorty D.
Published: September 2007

Strands of nanometer-sized silver particles have been grown at the glass-crystal interface of a suitable glass-ceramic system. The samples exhibit high values of dielectric constant of the order of 500 at temperatures in the range of 80 to 200 K. This is believed to arise due to the Gorkov–Eliashberg anomaly. Above 200 K, a dielectric dispersion is observed, which is explained on the basis of a diagonal-layer heterogeneous conductor model.
Ferroelectric behavior of pulsed laser deposited BaxSr1−xTiO3 thin films
Authors: Mehrotra Vivek , Kaplan Simon , Sievers Albert J. , Giannelis Emmanuel P.
Published: September 2007

Ba0.75Sr0.25TiO3 thin films have been deposited on single-crystal MgO substrates by pulsed laser deposition with the objective of forming ferroelectric films with a low Curie temperature. The films have been characterized by capacitance measurements and by transmission electron microscopy, x-ray diffraction, and Rutherford backscattering spectrometry (random and channeled). Films deposited with the substrate at 500 °C are polycrystalline, while those deposited at 650 °C are highly aligned and possibly epitaxial. The films are transparent in the visible region with an optical absorption edge at about 300 nm. Capacitance measurements on the polycrystalline films reveal a Curie transition at 283 K. The lowering of Curie temperature from the corresponding bulk sample is attributed to the films being under compression, as verified by Raman spectroscopy.
Phase evolution in boron nitride thin films
Authors: Kester D.J. , Ailey K.S. , Davis R.F. , More K.L.
Published: September 2007

Boron nitride (BN) thin films were deposited on monocrystalline Si(100) wafers using electron beam evaporation of boron with simultaneous bombardment by nitrogen and argon ions. The effect of film thickness on the resultant BN phase was investigated using Fourier transform infrared (FTIR) spectroscopy and high resolution transmission electron microscopy (HRTEM). These techniques revealed the consecutive deposition of an initial 20 Å thick layer of amorphous BN, 20–50 Å of hexagonal BN having a layered structure, and a final layer of the polycrystalline cubic phase. The growth sequence of the layers is believed to result primarily from increasing biaxial compressive stresses. Favorable surface and interface energy and crystallographic relationships may also assist in the nucleation of the cubic and the hexagonal phases, respectively. The presence of the amorphous and hexagonal regions explains why there have been no reports of the growth of 100% cubic boron nitride on Si.
Structural transformations and metastable phases produced by mechanical deformations in the Bi–Sr–Ca–Cu–O superconducting system
Authors: Tessier P. , Trudeau M.L. , Ström-Olsen J.O. , Schulz R.
Published: September 2007

The structural transformations occurring during the mechanical alloying of mixtures of elemental oxides in the Bi–Sr–Ca–Cu–O system have been studied. Metastable cubic structures with complete chemical disorder and amorphous phases can be synthesized by strong mechanical deformations. With proper thermal treatments, these metastable phases transform into the high Tc superconducting structures. Discussions on the mechanisms of formation and on the thermal stability of the various metastable phases are presented.
Solution-based synthesis routes to (Bi1−xPbx)2Sr2Ca2Cu3O10+δ
Authors: Ruiz M.T. , de la Fuente G.F. , Badía A. , Blasco J. , Castro M. , Sotelo A. , Larrea A. , Lera F. , Rillo C. , Navarro R.
Published: September 2007

A comparison of several solution syntheses with a solid-state route to the lead-containing high Tc phase of the Bi–Sr–Ca–Cu–O system has been performed using DSC, TGA, x-ray diffraction, electron microscopy, magnetic ac susceptibility, and critical-current measurements. A novel polymer solution synthesis route is shown to yield an increased percentage of the 2223 high Tc phase when sintering is performed at the low temperature end of this phase's stability range. Under the best preparative conditions given in the literature, however, the properties of samples obtained with the different synthetic methods described in the present study are shown to be similar. Samples derived from solution syntheses have been found to contain higher amounts of carbon precipitates, while samples derived from conventional solid-state synthesis contain CuO precipitates, according to energy dispersive spectroscopy analyses.
Effect of internal fission-fragment irradiation on critical currents and flux creep in Bi–Sr–Ca–Cu–O superconductors doped with UO2
Authors: Luborsky F.E. , Arendt R.H. , Fleischer R.L. , Hart H.R. , Tkaczyk J.E. , Orsini D.A.
Published: September 2007

Fission fragment damage was introduced into samples of Bi2Sr2CaCu2Ox and Bi1.7Pb0.3Sr2Ca2Cu3Oy, doped with various levels of UO2, by irradiation with thermal neutrons. The critical temperatures were unchanged. Concurrent with an increase in intragranular Jc previously reported, a decrease in flux creep was observed. The apparent pinning potential for creep at 10 K and 0.8 T increased on irradiation by about two to three times for both the 2212 and 2223 compounds. This increased apparent pinning potential is attributed to the strong pinning introduced by the damage caused by the travel of the fission fragments through the crystal. Pinning potential after irradiation increased with an increase in the amount of UO2 in the sample. The increase in bulk pinning potential on irradiation was proportional to the increase in intragranular critical currents on irradiation, qualitatively as expected theoretically.
Microscopy and microindentation mechanics of single crystal Fe−3 wt. % Si: Part I. Atomic force microscopy of a small indentation
Authors: Harvey S. , Huang H. , Venkataraman S. , Gerberich W.W.
Published: September 2007

Atomic force microscope measurements of elastic-plastic indentation into an Fe−3 wt. % Si single crystal showed that the volume displaced to the surface is nearly equal to the volume of the cavity. The surface displacement profiles and plastic zone size caused by a 69 nm penetration of a Vickers diamond tip are reasonably represented by an elastic-plastic continuum model. Invoking conservation of volume, estimates of the number of dislocations emanating from the free surface are reasonably consistent with the number of dislocations that have formed in the plastic zone to represent an average calculated plastic strain of 0.044.
Microscopy and microindentation mechanics of single crystal Fe−3 wt. % Si: Part II. TEM of the indentation plastic zone
Authors: Zielinski W. , Huang H. , Gerberich W.W.
Published: September 2007

Direct observations of dislocation arrangements about a range of microindentations into the [100] face of an Fe−3 wt. % Si single crystal have been accomplished. Dislocations of both large loop character initiated from the indenter and small loop character initiated as secondary reactions are found. Analysis of these allows the various contributions to the plastic strain gradient around the indentation to be assessed, with the experimental observations being reasonably consistent with continuum models.
Influence of boron content on the amorphization rate of Co–B mixtures by mechanical alloying
Authors: Corrias A. , Ennas G. , Marongiu G. , Musinu A. , Paschina G.
Published: September 2007

Amorphous cobalt-boron alloy powders have been prepared by a high energetic ball mill at room temperature starting from different Co/B ratios. They were characterized by means of x-ray diffraction, scanning and transmission electron microscopy, and differential scanning calorimetry. Ball milling of Co–B mixtures induces solid-state amorphization which becomes faster with increasing boron content. After maximum amorphization ball milling leads to crystallization of t-Co2B in all the binary samples.
Highly oriented, textured diamond films on silicon via bias-enhanced nucleation and textured growth
Authors: Stoner B.R. , Sahaida S.R. , Bade J.P. , Southworth P. , Ellis P.J.
Published: September 2007

Highly oriented diamond films were grown on single-crystal silicon substrates. Textured films were first nucleated by a two-step process that involved the conversion of the silicon surface to an epitaxial SiC layer, followed by bias-enhanced nucleation. The nucleation stage, which produced a partially oriented diamond film, was immediately followed by a (100) textured growth process, thus resulting in a film surface where approximately 100% of the grains are epitaxially oriented relative to the silicon substrate. The diamond films were characterized by both SEM and Raman spectroscopy. Structural defects in the film are discussed in the context of their potential effect on the electrical characteristics of the resulting film.
High purity isotopically enriched 70Ge and 74Ge single crystals: Isotope separation, growth, and properties
Authors: Itoh Kohei , Hansen W.L. , Haller E.E. , Farmer J.W. , Ozhogin V.I. , Rudnev A. , Tikhomirov A.
Published: September 2007

70Ge and 74Ge isotopes were successfully separated from natural Ge and zone purified. Several highly enriched, high purity 70Ge and 74Ge single crystals were grown by the vertical Bridgman method. The growth system was designed for reliable growth of low dislocation density, high purity Ge single crystals of very small weight (∼4 g). A 70Ge and a 74Ge crystal were selected for complete characterization. In spite of the large surface to volume ratio of these ingots, both 70Ge and 74Ge crystals contain low electrically active chemical net-impurity concentrations of ∼2 × 1012 cm−3, which is two orders of magnitude better than that of 74Ge crystals previously grown by two different groups.1,2 Isotopic enrichment of the 70Ge and the 74Ge crystals is 96.3% and 96.8%, respectively. The residual donors and acceptors present in both crystals were identified as phosphorus and copper, respectively. In addition, less than 1011 cm−3 gallium, aluminum, and indium were found in the 70Ge crystal.
Deposition and characterization of ZrO2 thin films on silicon substrate by MOCVD
Authors: Hwang Cheol Seong , Kim Hyeong Joon
Published: September 2007

ZrO2 thin films were deposited at 1 atm on Si substrates by oxidation-assisted thermal decomposition of zirconium-trifluoroacetylacetonate in the temperature range of 300–615 °C. Above a deposition temperature of 400 °C, the deposited thin films have a columnar grain structure, where each grain is perpendicular to the substrate surface with a c-axis preferred crystallographic orientation, and have poor electrical characteristics as a dielectric thin film. But the thin film deposited at 350 °C has a fine equiaxed microcrystalline structure and has superior electrical characteristics of a breakdown field of 1 MV/cm and a relative dielectric constant of 27.
Postfailure subsidiary cracking from indentation flaws in brittle materials
Authors: Padture Nitin P.
Published: September 2007

Vickers indentation sites in ceramics have been examined after specimen failure from median/radial indentation cracks. Evolution of a new cracking pattern of “ortho-lateral” cracks, originating at the intact corners of the Vickers indentation and running orthogonal to the classic-lateral cracks and parallel to the new fracture surface, has been observed. In some instances postfailure extension of the classic-lateral cracks toward the surface was also observed. Enhanced residual tensile stress from relaxation of constraints on the indentation-plastic cavity by the generation of a fracture surface is postulated to drive the subsidiary cracking. A simple qualitative model to explain this phenomenon is presented. Possible implications of such postfailure subsidiary cracking on residual-stress-driven flaws, postmortem fractography, and wear in ceramics are discussed.
Tailored MoSi2/SiC composites by mechanical alloying
Authors: Jayashankar S. , Kaufman M.J.
Published: September 2007

MoSi2-based composites have been synthesized through the mechanical alloying (MA) of elemental molybdenum and silicon powders with and without carbon additions. The interplay between the phase formation sequence in the powders and the microstructural evolution in the consolidated samples is described. It is shown that the glassy SiO2 phase characteristic of conventional powder processed MoSi2 can be effectively eliminated by combining mechanical alloying, carbon additions, and an in situ carbothermal reduction reaction. Using this approach, composites consisting of uniformly distributed micron-size SiC in an MoSi2 matrix can be formed. The effect of important processing variables such as the extent of carbon additions, extraneous iron pickup during MA, partial pressures of oxygen, consolidation temperatures, and consolidation atmospheres is discussed based on the evidence obtained from DTA, TGA, TEM, and XRD.
Growth of highly oriented SnO2 thin films on glass substrate from tetra-n-butyltin by the spray pyrolysis technique
Authors: Yagi I. , Hagiwara Y. , Murakami K. , Kaneko S.
Published: September 2007

Highly oriented SnO2 thin films have been grown successfully from tetra-n-butyltin on heated glass substrates by a pneumatic spraying system. The effects of film growth rate and substrate temperature on the microstructures of the films were investigated by x-ray diffraction and scanning electron microscopy. The SnO2 films of preferentially oriented (110) crystal plane were grown under the optimum growth conditions.
Synthesis of superconductors from soluble metal oxo alkoxide precursors
Authors: Rupich M.W. , Liu Y.P. , Ibechem J. , Hachey J.P.
Published: September 2007

Soluble metal oxo alkoxide precursors are useful for the production of homogeneous, micron-sized YBa2Cu3O7−x powders and the fabrication of superconducting YBa2Cu3O7−x films on large area and nonplanar substrates. This paper describes the synthesis and chemistry of a series of soluble metal oxo alkoxide precursors for the YBa2Cu3O7−x superconducting material and the effect of the thermal processing conditions on the formation of the superconducting phase. We have found that the direct conversion of the metal oxo alkoxide precursors to an amorphous or crystalline YBa2Cu3O6.5 phase without the intermediate formation of the individual metal oxides or carbonates allows the formation of the superconducting phase at temperatures as low as 720 °C. The prevention of the intermediate formation of the metal oxide and carbonate phases is achieved by the controlled thermolysis of the metal oxo alkoxide precursors in an inert atmosphere. The precursors have potential use in the low-temperature deposition of films on large area or nonplanar surfaces and for the production of homogeneous powders.
Sol-gel coating of YBa2Cu3O7−x with TiO2 for enhanced anisotropic grain growth
Authors: Fagan James G. , Amarakoon V.R.W.
Published: September 2007

This investigation focused on the effects of TiO2 additions (2.5–5 mol%) incorporated via sol-gel coating of powders on sintering behavior, microstructure development, electrical resistivity, and magnetic susceptibility of YBa2Cu3O7−x. Specimens were sintered at ≤ 960 °C for 6 h in all cases. TEM analysis indicated the sol-gel coating to be uniformly distributed around each particle and of thickness ∼ 20 to 40 nm. The addition of TiO2 was seen to reduce the sintering temperature by up to ∼ 200 °C, indicating the formation of a liquidus phase below that normally reported for YBa2Cu3O7-x. Grain growth and grain anisotropy behavior were influenced by TiO2 addition. A maximum in density, grain size, and anisotropy was achieved with 2.5 mol% addition sintered at 930 °C. TiO2 addition was shown to result in the formation of greater amounts of secondary phases such as Y2BaCuO2, and BaCuO2, particularly at higher TiO2 levels and sintering temperatures. The use of TiO2 additions also altered the magnetic susceptibility with the optimum response occurring for samples with 2.5 mol% TiO2 addition when sintered at 930 °C.
A new method for measuring the strength and ductility of thin films
Authors: Read David T. , Dally James W.
Published: September 2007

A new method of measuring the mechanical strength of thin films is described. We prepare miniature arrays of four tensile specimens, each 0.25 mm wide, 1 mm long, and 2.2 μm thick, using deposition, patterning, and etching processes common to the semiconductor industry. Each array of four specimens is carried on and protected by a rectangular silicon frame. Thirty-six such specimens are produced on a single wafer. After a specimen frame is mounted, its vertical sides are severed without damaging the specimens. The load is applied by micrometers through a special tension spring. Tensile properties of a 2.2 μm thick Ti–Al–Ti film were determined.
Hot-hardness comparisons among isostructural Be12X intermetallic compounds
Authors: Bruemmer S.M. , Arey B.W. , Brimhall J.L. , Hirth J.P.
Published: September 2007

Hot hardness has been measured in seven isostructural Be12X compounds: Be12Cr, Be12Mn, Be12V, Be12Mo, Be12Ti, Be12Ta, and Be12Nb. All vacuum-hot-pressed (VHP) materials exhibited similar hardness levels (800 to 1000 kg/mm2) at low temperatures (<600 °C), but sharply diverged at higher temperatures. Most compounds showed a sharp decrease in hardness above about one-half their melting temperatures except for Be12Nb and Be12Ti which maintained strength until higher temperatures. Ductile-brittle transition temperatures (DBTT's) of 600, 625, 690, 700, 700, and 850 °C were determined for Be12Cr, Be12Mn, Be12V, Be12Mo, Be12Ti, and Be12Nb, respectively. Hot-isostatically-pressed (HIP) and VHP Be12Nb heats both showed comparable behavior, indicating that the temperature required for macroscopic plasticity is not dependent on processing conditions. Differences in slip characteristics were observed between beryllides with similar melting temperatures, but differing DBTT's. Dislocations were activated on multiple slip systems in Bei2V during compressive deformation at 800 °C. Similar behavior was not observed in Be12Nb until test temperatures above 1000 °C. It is proposed that dislocation mobility may be related to the stability between Be12X and Be17X2 phases. Larger refractory metal atoms such as Nb may form faulted layers related to Be17X2 structures and thereby impede deformation at intermediate temperatures. Smaller atoms such as Cr, Mn, V, and Mo do not form a stable Be17X2 phase and thereby allow enhanced dislocation mobility at these temperatures.
Epitaxial growth of Fe/Mo/Fe(111) and Fe/Cr/Fe(111) on Si(111)
Authors: Cheng Yang-Tse , Chen Yen-Lung
Published: September 2007

Epitaxial body-centered cubic Mo and Cr films have been grown on the (111) surface of α–Fe films on Si(111) at 300 and 575 K by electron beam evaporation in ultrahigh vacuum. X-ray diffraction and transmission electron microscopy show that the Mo films are oriented with the (111) plane parallel to the α-Fe(111) plane and with the Mo[1$\overline 1$0] direction parallel to the Fe[1$\overline 1$0] direction in the plane of the substrate. The same orientation relationship holds for the Cr films epitaxially grown on α-Fe(111) surfaces. Epitaxial Fe(111)/Mo(111)/Fe(111) and Fe(111)/Cr(111)/Fe(111) films have also been grown on Si(111). This work provides new examples of low temperature epitaxy which can occur at a substrate temperature as low as 0.1 times the melting temperature of the deposited materials.
A model for disorder in fluorine-intercalated graphite
Authors: di Vittorio S.L. , Dresselhaus M.S. , Dresselhaus G.
Published: September 2007

The structural and electronic properties of fluorine- and bromine-intercalated graphite fibers and HOPG are summarized. In contrast to the bromine intercalate, which is purely ionic for any experimentally attainable intercalate concentration, fluorine has a dual ionic and covalent behavior in graphite. Furthermore, whereas bromine-intercalated graphite is ordered, fluorine-intercalated graphite is disordered. The stiff graphene planes are buckled and islands of various fluorine concentrations are formed. A thermodynamic model is proposed that accounts for the differences between fluorine- and bromine-intercalated graphite materials. The model describes the competition between ionically bonded and covalently bonded intercalate phases of fluorine in graphite. Covalent bonding is more favorable energetically, but an important nucleation barrier exists due to strain and to the destruction of the conjugation of the double bonds.
Deposition of silicon carbide using the chemical vapor composites process: Process characterization and comparison with RASSPVDN model predictions
Authors: Allendorf M.D. , Hurt R.H. , Yang N. , Reagan P. , Robbins M.
Published: September 2007

In this work, we explore the use of the chemical vapor composites (CVC) process to increase the rates of silicon carbide (SiC) growth on graphite substrates. Large SiC seed particles are used that are deposited by gravity-driven sedimentation. The results show that addition of large (dp = 28 μm) SiC seed particles to a gas phase containing hydrogen and methyltrichlorosilane increases the deposition rate of SiC by amounts substantially higher than that expected from the addition of the particle volume alone. Insight into the mechanism of this deposition rate enhancement is obtained through analysis of SEM photographs of deposits. Growth rates and deposit structures are consistent with the trends predicted by the previously developed random-sphere model of simultaneous particle-vapor deposition (RASSPVDN), which is used here to interpret the data.
Nanocrystalline α–Fe, Fe3C, and Fe7C3 produced by CO2 laser pyrolysis
Authors: Bi Xiang-Xin , Ganguly B. , Huffman G.P. , Huggins F.E. , Endo M. , Eklund P.C.
Published: September 2007

Nanocrystalline α–Fe, Fe3C, and Fe7C3, particles with narrow size distributions were produced by CO2 laser pyrolysis of vapor mixtures of Fe(CO)5 and C2H4. Details of the synthesis procedure are discussed. Mossbauer spectroscopy and x-ray diffraction were used to identify the structural phases and the former was used also to study the magnetism of the nanoparticles. All the nanoparticles were observed to be ferromagnetic in this size range. If excess C2H4 appears in the reactant gas mixture, several monolayers of pyrolytic carbon were observed to form on the particle surface, as deduced from transmission electron microscopy and Raman scattering studies. Results of thermo-gravimetric analysis/mass spectroscopy studies of this carbon coating indicate it is gasified in hydrogen at temperatures T ∼ 250 °C.
The effect of Li-salt additions on the densification of tin oxide
Authors: Yuan D.W. , Wang S.F. , Huebner W. , Simkovich G.
Published: September 2007

Sintering of pure SnO2 to high densities is difficult due to its high vapor pressure, and hence, additives are typically used to enhance densification. In this study, the effects of two lithium compounds, LiF and LiNO3, on the densification behavior of SnO2 were evaluated. While LiF resulted in only a modest improvement in densification, LiNO3 additions resulted in densities of ≥ 95% theoretical at 1500 °C in air. Thermal, x-ray, and SEM/TEM microstructural analyses indicated no liquid phase formation. From these studies we attribute the enhanced sintering behavior to the ionic-compensation of Li+ as an acceptor dopant, i.e., 3[Li‴sn] = 2[Voö], which in turn increased the diffusivity of oxygen.
Hydrothermal synthesis and sintering of ultrafine CeO2 powders
Authors: Zhou Y.C. , Rahaman M.N.
Published: September 2007

Undoped CeO2 and Y2O3-doped CeO2 powders, with particle sizes of ≍10–15 nm, were prepared under hydrothermal conditions of 10 MPa at 300 °C for 4 h. The compacted powders were sintered freely in air or in O2 at constant heating rates of 1–10 °C/min up to 1350 °C. The undoped CeO2 started to sinter at ≍800–900 °C and reached a maximum density of 0.95 of the theoretical at 1200 °C, after which the density decreased slightly. Isothermal sintering at 1150 °C produced a sample with a relative density of ≍0.98 and an average grain size of ≍100 nm. The samples sintered above 1200 °C exhibited microcracking. The decrease in density and the microcracking above 1200 °C are attributed to a redox reaction leading to the formation of oxygen vacancies and the evolution of O2 gas. Doping with Y2O3 produced an increase in the temperature at which measurable sintering commenced and an increase in the sintering rate, compared with the undoped CeO2. Sintered samples of the doped CeO2 showed no microcracks. The CeO2 doped with up to 3 mol% Y2O3 was sintered to almost full density and with a grain size of ≍200 nm at 1400 °C.
Ignition criteria for self-propagating combustion synthesis
Authors: Zhang Yangsheng , Stangle Gregory C.
Published: September 2007

Ignition criteria for gasless self-propagating combustion synthesis have been investigated through an ignition temperature analysis. The calculations were based on the dimensionless energy and mass continuity equations where the dimensionless parameters associated with the rate of local heat generation (β), activation energy (γ), the rate of surface heat loss by convection (ω), the rate of surface heat loss by radiation (δ), and the rate of reaction (λ) were incorporated. The relative significance of each of these parameters on the ignition of the self-propagating combustion reaction was evaluated to be γ > β > δ > ω. The ignition region, transition region, and nonignition region were identified for selected conditions. The correlations between ignition behavior and the material properties, the thermodynamic and kinetic properties, as well as the experimental conditions were discussed. The calculations indicated that only those systems with δH/Cp > 1.5 × 103 (K) will give rise to a self-propagating combustion reaction without external energy input. Thus, this value can be used as an approximate guide for the existence of self-sustaining combustion. The calculations provide a sound basis toward interpreting experimental observations and developing a fundamental understanding of the process.
Transmission electron microscopy of GeSe2+δ thin films
Authors: Maghsoudlou H. , Salamanca-Riba L. , Haro-Poniatowski E.
Published: September 2007

GeSe2 can exist in both amorphous and crystalline phases. Although most semiconductor devices are constructed from crystalline materials, the use of amorphous materials in devices has high potential. The study of GeSe2 is especially interesting since it has been established that an amorphous-to-crystalline transition can be induced by laser irradiation. In order to better understand this phenomenon, it is necessary to study the microstructure of GeSe2 glass. Therefore, transmission electron microscopy studies were undertaken to investigate the degree of crystallinity of GeSe2 glass. It was found that small microcrystallites with diameters in the range of 100–300 Å were embedded in a glass matrix. These microcrystallites formed larger clusters in some areas.
Dielectric properties of aerogels
Authors: Hrubesh L.W. , Keene L.E. , Latorre V.R.
Published: September 2007

We have measured the real (dielectric constant) and imaginary (loss factor) components of the complex relative permittivity at 298 K using microwave frequencies (2, 10, and 18–40 GHz) for bulk SiO2-aerogels and for two types of organic aerogels, resorcinol-formaldehyde (RF) and melamine-formaldehyde (MF). Measured dielectric constants are found to vary linearly between values of 1.0 and 2.0 for aerogel densities from 10 to 500 kg/m3. For the same range of densities, the measured loss tangents vary linearly between values of 2 × 10−4 and 7 × 10−2. The observed linearity of the dielectric properties with density in aerogels at microwave frequencies shows that their dielectric behavior is more gas-like than solid-like. The dielectric properties of aerogels are shown to be significantly affected by the adsorbed water internal to the bulk material. For example, water accounts for 70% of the dielectric constant and 70% of the loss at microwave frequencies for silica aerogels. Because of their very high porosity, even with the water content, the aerogels are among the few materials exhibiting such low dielectric properties. Our measurements show that aerogels with greater than 99% porosity have dielectric constants less than 1.03; these are the lowest values ever reported for a bulk solid material.
Superconducting fibers from organometallic precursors: Part III. Pyrolytic processing of precursor fibers
Authors: Zhang Zhi-Fan , Kennish Richard A. , Youngdahl Blohowiak Kay A. , Hoppe Martin L. , Laine Richard M.
Published: September 2007

Sixty−70 μm diameter preceramic fibers, extruded from THF solutions containing 1:2:3 stoichiometric mixtures of yttrium, barium, and copper carboxylates, were pyrolytically transformed into ceramic fibers using controlled heating schedules and reactive atmospheres. The objectives of the work reported here were to identify appropriate processing conditions such that during pyrolysis the preceramic fibers would (1) eliminate the organic ligands without pore or void formation, (2) reach full density with a controlled grain size, and (3) form orthorhombic phase, 123 fibers with reasonable mechanical properties. The mechanisms of organic ligand decomposition and loss were examined using mass spectral fragmentation and TGA. Microstructural and phase evolution were correlated with heating schedules and atmospheres, using XRD, DTA, SEM, and limited flux exclusion studies. The mechanisms of decomposition of the spinnable mixtures suggest intermolecular rather than intramolecular decomposition pathways. Different pyrolysis atmospheres were also examined to explore methods of controlling the degradation process. Slow pyrolysis in air followed by oxygen anneals appears to give superior fibers in terms of controlled grain size and phase. The pyrolyzed fibers exhibit the appropriate orthorhombic phase according to x-ray powder diffractometry. Preliminary flux exclusion measurements demonstrate that the fibers are superconducting although the measured δTc is not exceptional.
Composition and crystal structure parameters of single crystals (Bi, Pb)2Sr2(Ca1−xRx)Cu2O8+δ (R = Y, Er, Ho, Tm, and Yb)
Authors: Ilyushin A.S. , Shi L. , Leonyuk L.I. , Mustafa B.M. , Nikanorova I.A. , Red'ko S.V. , Jia Y. , Vetkin A.G. , Zhou G. , Zubov I.V.
Published: September 2007

To correlate structural and compositional parameters in the bismuth 2212 system, single crystals with the composition (Bi1−yPby)2Sr2(Ca1−xRx)Cu2O8+δ (R = Y, Er, Ho, Tm, and Yb; y = 0, 0.1; 0 ≤ x ≤ 0.5) have been studied at room temperature by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The main results are as follows: (i) The actual content, x, of R (R = Y, Er) in samples is in significant excess over its content x′ in the melt for x′ < 0.5. The opposite effect (but several times smaller) takes place for Ca. At x′ = 0.5, the x value practically coincides with x′. (ii) For all R under examination and x′ = 0.1, the value of x is within the limits of 0.43 ≤ x ≤ 0.51; i.e., x exceeds x′ several times. (iii) The total content of Ca, R (R = Y, Er), and Sr is close to 3 through the whole range 0 ≤ x′ ≤ 0.5. At x′ < 0.5 Ca is partly substituted for Sr, while R occupies only Ca crystallographic positions. Thus the actual formula of the samples is (Bi1-yPby)2+∊Sr2-z(Ca1+z-xRx)Cu2O8+δ. (iv) The evidence was received that the nonlinear dependence c(x) at x < 0.5 is connected with the partial substitution of Sr with Ca. The dependence of c, namely on the R = Y content in the denoted range of x, is close to linear with the slope ∂c/∂x = −0.67(2) Å/at.
Stress development and relaxation in copper films during thermal cycling
Authors: Thouless M.D. , Gupta J. , Harper J.M.E.
Published: September 2007

The reliability of integrated-circuit wiring depends strongly on the development and relaxation of stresses that promote void and hillock formation. In this paper an analysis based on existing models of creep is presented that predicts the stresses developed in thin blanket films of copper on Si wafers subjected to thermal cycling. The results are portrayed on deformation-mechanism maps that identify the dominant mechanisms expected to operate during thermal cycling. These predictions are compared with temperature-ramped and isothermal stress measurements for a 1 μm-thick sputtered Cu film in the temperature range 25–450 °C. The models successfully predict both the rate of stress relaxation when the film is held at a constant temperature and the stress-temperature hysteresis generated during thermal cycling. For 1 μm-thick Cu films cycled in the temperature range 25–450 °C, the deformation maps indicate that grain-boundary diffusion controls the stress relief at higher temperatures (>300 °C) when only a low stress can be sustained in the films, power-law creep is important at intermediate temperatures and determines the maximum compressive stress, and that if yield by dislocation glide (low-temperature plasticity) occurs, it will do so only at the lowest temperatures (<100 °C). This last mechanism did not appear to be operating in the film studied for this project.
Micro-effects of resputtering due to negative ion bombardment of growing thin films
Authors: Kester Daniel J. , Messier Russell
Published: September 2007

Negative ion bombardment of an evolving thin film can cause changes in the film's surface due to resputtering of the already deposited material. Through the study of rf-sputtered perovskite (BaTiO3) thin films, we have found that surface micro-effects, i.e., changes in the surface morphology of the films at the μm-scale level, are dependent on the deposition conditions. Ripples, cones, ridges, and etch pits of various shapes and sizes were all observed on growing films. A transformation of the morphology of the top surface of the film as a function of both deposition time and location on the substrate has been observed. The type of surface morphology found at any point was found to be dependent on a number of factors, including deposition rates, flux and energy of bombarding ions, and the average angle of incidence of the bombarding ions. We have developed a qualitative model for the formation of these surface features, based on the resputtering yield as a function of the average angle of incidence of the bombarding ions. The model suggests that surface nonuniformities, often ripples, initiate the development of etch pits. Other mechanisms of the surface morphology development (such as clustering) are used to explain the formation of surface features other than etch pits.
X-ray and micro-Raman analysis of nanophase ZrO2 and Y2O3–ZrO2 produced by the gas-phase condensation technique
Authors: Foster C.M. , Bai G.R. , Parker J.C. , Ali M.N.
Published: September 2007

Nanophase (n-) ZrO2 was produced in its pure and partially stabilized form by the gas-phase condensation method. The material was examined by x-ray diffraction and Raman scattering to obtain information on the structural evolution of the material during sintering. Two types of Y2O3-doped ZrO2 nanophase material were made: one by co-deposition of n-Y2O3 and n-ZrO2 in a consecutive manner and the second by mechanically mixing n-Y2O3 and n-ZrO2. We have determined that the co-deposition process is the most effective means of doping the n-ZrO2.
The structure of aperiodic, metamict (Ca, Th)ZrTi2O7(zirconolite): An EXAFS study of the Zr, Th, and U sites
Authors: Farges François , Ewing Rodney C. , Brown Gordon E.
Published: September 2007

The structural environments of Zr, Th, and U in aperiodic (metamict) (Ca, Th)ZrTi2O7were examined using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Samples are aperiodic due to a radiation-induced transformation caused by alpha-decay event damage. In the aperiodic samples, Zr is mainly 7-coordinated [d(Zr−O) ≍ 2.14–2.17 ≍ 0.02 Å]; whereas, Th is mainly 8-coordinated [d(Th–O) ≍ 2.40−2.41 ≍ 0.03 Å]. Nearly identical bond lengths and coordination numbers for these elements were determined for an annealed, crystalline sample. The radiation-induced transition from the periodic to the aperiodic state is characterized by a significant broadening of the distribution of (Zr, Th)–O distances. In one metamict sample with ≍1.9 wt.% U3O8, U is essentially tetravalent. The absence of higher oxidation states (U6+) is consistent with the lack of evidence for alteration (samples are over 500 million years old). The reduced medium-range order around Zr, Th, and U is related to the increase of alpha-decay event damage and precludes decomposition of zirconolite into simple oxides of Zr, Th, or U. Comparison with other metamict (Zr, Th, U)-bearing phases (e.g., ZrSiO4and ThSiO4) suggests that Zr4+, Th4+, and U4+prefer 7-, 8-, and 6-coordinated sites, respectively, in aperiodic phases at ambient temperatures and pressures. Examination of the structure of crystalline (Ca, Th)ZrTi2O7demonstrates that M–O–M angles (M = Ca, Ti, Zr, and Th) are relatively small (≍100–120° for edge-sharing polyhedra). A limited relaxation of the constraints of periodicity around M cations caused by radiation damage (e.g., tilting of polyhedra) dramatically affects the distribution of these angles. This type of structural relaxation may be the mechanism by which long-range periodicity is lost and medium-range order is reduced with increasing radiation damage, while the major cations retain their nearest-neighbor environments. This relaxation may also help explain the lattice expansion observed in zirkelites when they undergo radiation damage.
Fabrication and properties of functionally graded NiAl/Al2O3 composites
Authors: Miller D.P. , Lannutti J.J. , Noebe R.D.
Published: September 2007

A modified sedimentation process was used in the production of a functionally gradient material (FGM), NiAl/Al2O3. A simple finite element model was used to guide our design and fabrication efforts by estimating residual stress states as a function of composite structure. This approach could lead to tailored designs that enhance or avoid specific residual stress states. Thermal cycling tests were factored into the model to predict time dependent or steady-state internal temperature and stress profiles. Four-point bend tests were conducted to establish the mechanical load-displacement behavior of a single interlayer FGM at room temperature, 800 and 1000 K. Room temperature bend strength of the FGM was 3–4 times that of the base NiAl. At elevated temperatures, composite fracture occurred in a gradual, noncatastrophic mode involving NiAl retardation of a succession of cracks originating in the alumina face.
An internal synthesis method for producing a mullite–chromium carbide composite
Authors: Ho C.T.
Published: September 2007

In situ formation of chromium carbide in mullite matrix through the reaction of Cr2O3, SiC, and Al2O3 has been studied. Three different chromium compounds, Cr3Si, Cr3C2, Cr7C3, and mullite were formed. The Cr3Si particles formed first and the Cr7C3 phase was only dominant above 1600 °C. The Cr3C2 phase was the main product chromium carbide at temperatures ranging from 1450–1550 °C. The mullite phase formed concurrently through the reaction of SiO2 with Al2O3. SiO2 is the product of the reaction between Cr2O3 and SiC. The composite hot-pressed at 1450 °C in argon gives a flexural strength and fracture toughness up to 407 MPa and 3.8 MPa m1/2, respectively.