bluuebunny/crossref_metadata_2025 huggingface dataset:
The calculation of stacking fault energies in close-packed metals
Authors: Crampin S. , Hampel K. , Vvedensky D. D. , MacLaren J. M.
Published: September 2007

The one-electron theory of metals is applied to the calculation of stacking fault energies in face-centered cubic metals. The extreme difficulties in calculating fault energies of the order of 0.01 eV/(interface unit-cell area) are overcome by applying the Force theorem and using the layer–Korringer–Kohn–Rostoker method to determine the charge density of isolated defects. A simple scheme is presented for accommodating deviations from charge neutrality inherent in this approach. The agreement between theoretical and experimental values for the stacking fault energy is generally good, with contributions localized to within three atomic planes of the fault, but suggest the quoted value for Rh is a significant overestimation.
Brazing of aluminum nitride substrates
Authors: Grant Norton M. , Kajda Jacek M. , Steele Brian C.H.
Published: September 2007

Aluminum nitride (AlN) is currently under investigation as a potential candidate for replacing aluminum oxide (Al2O3) as a substrate material for electronic circuit packaging. The requirements for such a material are that it can be metallized and joined to produce hermetic enclosures for semiconductor devices. A technique for brazing AlN using a nonactive metal braze has been investigated. The process involves the in situ decomposition of an active metal hydride. This process improves the wetting of the AlN and led to the development of strong bonding between braze and ceramic. The ceramic-braze interface was studied using scanning electron microscopy (SEM). The nature of the interfacial reactions and the reaction products have been identified using x-ray diffraction (XRD). The progress of the reaction has been followed using differential thermal analysis (DTA). The experimental results have been correlated with thermodynamic predictions of the reaction processes. In addition to joining ceramic to ceramic, braze joints of AlN to copper and to a low expansion iron-nickel lead frame alloy were made. Residual stress resulting from a mismatch of thermal expansion coefficients between AlN and copper caused cracking in the ceramic upon cooldown from the brazing temperature. No cracking occurred in the ceramic when joined to the iron-nickel alloy. The results obtained are important for the realization of AlN as a ceramic packaging material for semiconductor devices.
Reaction between niobium and silicon carbide at 1373 K
Authors: Yaney D. L. , Joshi A.
Published: September 2007

The reaction zone formed between niobium and silicon carbide during heating for 4 h at 1373 K was examined by transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). The typical reaction layer sequence is SiC/Nb5Si4C/Nb5Si3/Nb2C/NbO/Nb. However, in one area of the specimen, the first reaction layer was NbC rather than Nb5Si4C. The high oxygen and carbon concentrations near the outer surface were shown by AES depth profiling to result from carbon and oxygen contamination from the vacuum system during annealing. In order to determine if the observed reaction layer sequence is consistent with conditions of local thermodynamic equilibrium, the quaternary Nb–Si–C–O phase diagram was calculated from available thermodynamic data. A minimum (most negative) free energy of formation for the ternary compound Nb5Si4C of −582 kJ/mole was estimated assuming that the equilibrium between NbSi2 and SiC observed experimentally at 1573 K1 also exists at 1373 K. Except for the region immediately adjacent to the substrate, the observed reaction layer sequence was in agreement with the calculated quaternary phase diagram. However, it was noted that agreement with the quaternary phase diagram would be obtained if a thin layer of either SiO2 or NbC were present at the substrate surface.
Quantitative analysis of elastic moduli of tellurite glasses
Authors: El-Mallawany Raouf
Published: September 2007

The estimated bulk moduli and Poisson's ratio of binary, ternary, and quaternary rare earth tellurite glasses have been calculated using the bond compression model according to the cation-anion bond of each oxide present in the glass. Information about the structure of the glass can be deduced after calculating the number of network bonds per unit volume, the value of the average stretching force constant, the average ring size, the structure sensitivity factor, and the mean crosslink density. Comparisons between the calculated and the experimental elastic moduli and Poisson's ratio have been carried out. Also, the longitudinal and shear elastic stiffnesses of binary rare earth tellurie glasses are compared and analyzed with those of other binary rare earth glasses.
Adsorption and bonding of C1Hx and C2Hy on unreconstructed diamond(111). Dependence on coverage and coadsorbed hydrogen
Authors: Mehandru S. P. , Anderson Alfred B.
Published: September 2007

The adsorption and bonding of CH3, CH2, CH, C2H, and C2H2 fragments to clean and hydrogenated diamond(111) surfaces are investigated in the framework of the atom superposition and electron delocalization molecular orbital method. Low coverage calculations are performed using large cluster models for the surfaces, and high coverages are examined with band calculations on thick two-dimensional slabs with every surface carbon covered by a hydrocarbon fragment (i.e., 1:1 surface coverage). For low coverage adsorption on clean and H-covered surfaces the adsorption energies are in the order C2H>CH ≃ CH2>CH3. In each case, the predominant component of bonding is covalent in character and is a result of overlaps between the sp-hybridized singly occupied dangling surface state orbital on the surface carbon and the sp-hybridized orbital on the fragment carbon atom. While the charge transfer contribution to bonding is nearly the same for CH3, CH2, and CH fragments, it is significantly larger for C2H due to a comparatively stable radical orbital on C2H. C2H2 binds to the surface on the di-σ site where both its ends form bonds to the surface atoms. Onefold adsorption to a H-covered surface is predicted to be unstable. The 1:1 CH3 coverage on diamond(111) is highly unstable because of steric repulsions between adsorbate fragments due to their spacial proximity. This finding is supported by a calculation of the cis-trans isomerization energy of di-t-butyl ethylene, including full structure relaxations. At low coverage CH3 can bind on adjacent surface sites by tilting away from one another. The 1:1 coverage for CH2, CH, and C2H fragments is predicted to be stable on this surface.
Activity of tungsten and rhenium filaments in CH4/H2 and C2H2/H2 mixtures: Importance for diamond CVD
Authors: Sommer M. , Smith F. W.
Published: September 2007

The resistance R, spectral emissivity ∊, and power consumption of W and Re filaments heated to 2500 °C in mixtures of CH4 or C2H2 in H2 have been measured in a series of experiments focusing on the state of the filament activity, i.e., its ability to dissociate the reactant gases. It has been found that these properties of the filaments, as well as the partial pressures of CH4 and C2H2 in the reaction chamber, depend critically on both the filament temperature and the reactant ratio, e.g., C2H2/H2. Specifically, both W and Re filaments show sharp jumps in power consumption at essentially the same temperature, signaling strong increases in filament activity and, hence, production of atomic hydrogen. These results are proposed to be due to the removal of nonreactive carbon from the surface of the filament via etching by atomic hydrogen and are consistent with the predictions of our thermodynamic model for the C–H system. Evidence for gas phase reactions is presented and the role of thermal diffusion is discussed. The emissivities of the W and Re filaments are observed to have significantly different temperature dependences which are attributed to differences in the phase diagrams for the W–C and Re–C systems. The implications of these results for hot-filament diamond CVD are discussed.
Diamond-like carbon films prepared by rf substrate biasing in an ECR discharge
Authors: Varhue W. , Pastel P.
Published: September 2007

The optical band gap, density, and hydrogen content of diamond-like carbon films have been controllably varied by rf biasing the substrate in an ECR discharge of pure methane. The optical band gap varied from 2.7 to 1.2 eV, the density from 1.4 to 2.2 g/cm3, and the atomic fraction of hydrogen from 50 to 5%. The range of measured values is in agreement with those predicted by both the random covalent network and defected graphite models.
Structure and bonding studies of the C:N thin films produced by rf sputtering method
Authors: Torng C. J. , Sivertsen J. M. , Judy J. H. , Chang C.
Published: September 2007

Thin C:N films were prepared by rf diode sputtering of a graphite target in a mixed argon/nitrogen plasma. We have observed a systematic variation of the properties of these C:N films with an increase in the nitrogen partial pressure. XPS, AES, and TEM studies show that nitrogen will stabilize the diamond sp3 bonding. From XPS studies, we found that the density of our C:N films is increased from 1.37 × 1023 atoms/cm3 to 1.63 × 1023 atoms/cm3 using a 100% nitrogen plasma. The energy gap of our nitrogen carbon also shows an increase from 1.1 eV to 1.4 eV using a 100% nitrogen plasma. The mechanical properties also are shown to be enhanced for certain applications. By using the same method, we can also show that it can produce 100% amorphous C:N films which are more diamond-like as compared with other methods.
Chemical structure and physical properties of diamond-like amorphous carbon films prepared by magnetron sputtering
Authors: Cho N-H. , Krishnan K. M. , Veirs D. K. , Rubin M. D. , Hopper C. B. , Bhushan B. , Bogy D. B.
Published: September 2007

Thin films of amorphous carbon (a–C) and amorphous hydrogenated carbon (a–C:H) were prepared using magnetron sputtering of a graphite target. The chemical structures of the films were characterized using electron energy loss spectroscopy (EELS) and Raman spectroscopy. The mass density, hardness, residual stress, optical band gap, and electrical resistivity were determined, and their relation to the film's chemical structure are discussed. It was found that the graphitic component increases with increasing sputtering power density. This is accompanied by a decrease in the electrical resistivity, optical band gap, mass density, and hardness. Increasing the hydrogen content in the sputtering gas mixture results in decreasing hardness (14 GPa to 3 GPa) and mass density, and increasing optical band gap and electrical resistivity. The variation in the physical properties and chemical structures of these films can be explained in terms of the changes in the volume of sp2-bonded clusters in the a–C films and changes in the termination of the graphitic clusters and sp3-bonded networks by hydrogen in the a–C:H films.
Effect of grain size on superplastic behavior of Al2O3/YTZ
Authors: Nieh T. G. , Wadsworth J.
Published: September 2007

Grain size strongly influences the superplastic properties of a fine-grained, 20 wt.% Al2O3-reinforced, Y2O3-stabilized tetragonal ZrO2 (Al2O3/YTZ). The elongation to failure of the material decreases, and the flow strength increases, with increasing grain size. The flow stress at a constant true strain rate and at a given strain is proportional to the grain size raised to a 0.75 power. As a result, the superplastic deformation rate in Al2O3/YTZ is inversely proportional to the grain size raised to a 1.5 power.
The formation of superconducting phases in Bi(Pb)–Sr–Ca–Cu oxide/Ag microcomposites produced by oxidation of metallic precursor alloys
Authors: Gao Wei , Li Songcun , Parrella Ron , Rudman David A. , Vander Sande John B.
Published: September 2007

Metallic alloy ribbons of Bi–Sr–Ca–Cu with nominal compositions of 2-2–1-2, 2-2–2-3, 2-2–2-4, and 2-2–3-4, with or without Pb, were made by vacuum induction melting followed by melt-spinning. Ag, at the level of 20–80 wt. %, was added to aid in the alloy melting and metallic ribbon formation processes. These ribbons were then subjected to a controlled atmosphere oxidation and annealing to produce superconducting oxide/silver microcomposites. Among the four alloy groups investigated in the present work, 2-2–3-4 alloys with 0.6Bi replaced by Pb possess the best superconducting properties after suitable treatment. These specimens exhibited zero resistance at T = 104–110 K and a critical current density of 600 A/cm2 at 77 K in zero field with excellent reproducibility. The annealing conditions, namely the annealing temperature, time, and atmosphere, are discussed in terms of the formation kinetics of the superconducting phases. While the 85 K Tc “2212” phase formed over a wide temperature range (740 to 845 °C) after an ∼2 h anneal, the 110 K high-Tc transition appeared only after an ∼10 h anneal at 810–840 °C, suggesting that the “2223” phase grew from the 2212 phase controlled by diffusion. The microstructures of these superconducting microcomposites were observed and analyzed by a combination of optical and scanning electron microscopy, x-ray powder diffraction, and electron probe microanalysis. The high Tc “2223” phase exhibited plate-like grains which were distributed randomly in orientation. The silver addition improved the mechanical properties of the metallic precursor ribbons and the superconducting microcomposites, promoted the formation of the 2223 phase, and influenced the oxidation and the annealing conditions.
The formation of single-phase equiatomic MnBi by rapid solidification
Authors: Guo X. , Zaluska A. , Altounian Z. , Ström-Olsen J. O.
Published: September 2007

The low temperature phase (LTP) of MnBi, which is of interest because of its large magnetic anisotropy, has been obtained in almost single-phase form (>95%) by rapid solidification, followed by thermal annealing. X-ray and electron microscope studies indicate that the melt-spun ribbons are amorphous, contrary to the accepted rules for glass formation. The transformation of the amorphous phase is a complex process. The amorphous phase crystallizes first into Bi, Mn3Bi, and ferrimagnetic MnBi. The formation of the LTP begins immediately after the eutectic melting of these phases around 540 K.
Application of chemical vapor deposited yttria for the protection of silicon carbide fibers in a SiC/Ni3Al composite
Authors: Larkin D. J. , Interrante L. V. , Bose A.
Published: September 2007

A CVD process has been developed for coating Textron-Avco SCS-6 SiC fiber with yttria. Both Y(fod)3·H2O and Y(thd)3 (fod = 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionato; thd = 2,2,6,6-tetramethyl-3,5-heptanedionato) were examined as potential Y2O3 CVD precursors. Analysis of the deposits by Auger spectroscopy indicated significant F and C'incorporation in the case of Y(fod)3 · H2O whereas, under appropriate conditions, Y(thd)3 gave a deposit which was essentially free of C and other impurities. GCFTIR analysis of the volatile products of the CVD process indicated isobutylene, tetrafluoroethylene, 1,1-difluoroethylene, fluoroform, and fluoroethylene for Y(fod)3 · H2O and mainly isobutylene and propylene for Y(thd)3. The precursor Y(thd)3 was chosen to deposit 1–2 μm of yttria on short lengths of silicon carbide fibers. The coated fibers were then incorporated into a nickel aluminide (Ni3Al) matrix by reactive sintering, with yttria affording protection from the known SiC + 2Ni ⇉ Ni2Si + C degradation process. The SiC/Ni3Al composites, before and after annealing at 1000 °C for up to 100 h, were studied by using SEM and EMPA to determine the extent of reaction. With the exception of certain portions of the fibers that were inadequately coated with yttria, complete protection of the fibers was indicated.
Compressive creep of YBa2Cu3Ox
Authors: Goretta K. C. , Routbort J. L. , Biondo A. C. , Gao Y. , de Arellano-López A. R. , Domínguez-Rodríguez A.
Published: September 2007

YBa2Cu3Oxwas deformed from 850 to 980 °C in oxygen partial pressures of 103to 105Pa. Steady-state creep rate, ̇, forP(O2) from 104to 105Pa could be expressed as ̇ = Aσ1.0(GS)−2.8±0.6exp −(970 ± 130 kJ/mole)/RT, whereAis a constant, σ the steady-state stress, GS the average grain size, and R andThave their usual meanings, ForP(O2) from 103to 3 ⊠ 103Pa, the activation energy decreased to about 650 kJ/mole and for a given temperature creep kinetics were much faster. The data and microscopic observations indicated that creep occurred by diffusional flow. Comparisons with diffusion data for YBa2Cu3Oxsuggested that Y or Ba may be rate-controlling diffusing species.
Particle and grain size effects on the dielectric behavior of the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3
Authors: Papet Philippe , Dougherty Joseph P. , Shrout Thomas R.
Published: September 2007

The role of particle and grain size on the dielectric behavior of the perovskite relaxor ferroelectric Pb(Mg1/3Nb2/3)O3[PMN] was investigated. Ultrafine powders of PMN were prepared using a reactive calcination process. Reactive calcination, the process by which morphological changes take place upon reaction of the component powders, produced particle agglomerates less than 0.5 μm. Through milling, these structures were readily broken down to ∼70 nanometer-sized particulates. The highly reactive powders allowed densification as low as 900 °C, but with corresponding grain growth in the micron range. Such grain growth was associated with liquid phase sintering as a result of PbO–Nb2O5second phase(s) pyrochlore. Sintering, assisted by hot uniaxial pressing, below the temperature of liquid formation of 835 °C, allowed the fabrication of highly dense materials with a grain size less than 0.3 μm. The dielectric and related properties were determined for samples having grain sizes in the range of 0.3 μm to 6 μm. Characteristic of relaxors, frequency dependence (Kand loss) and point ofTmaxwere found to be related to grain and/or particle size and secondarily to the processing conditions. Modeling of particle size/dielectric behavior was performed using various dielectric properties of 0–3 composites comprised of varying size powder in a polymer matrix. An intrinsic-microdomain perturbation concept was proposed to interpret observed scaling effects of the relaxor dielectric behavior in contrast to normally accepted extrinsic grain boundary models.
Superconductive (Y1−xCax)Ba2Cu4O8 (x = 0.0 and 0.05) ceramics prepared by low and high oxygen partial pressure techniques
Authors: Wada Takahiro , Suzuki Nobuo , Yamaguchi Koji , Ichinose Ataru , Yaegashi Yuji , Yamauchi H. , Koshizuka Naoki , Tanaka Shoji
Published: September 2007

Both YBa2Cu4O8 and (Y0.95Ca0.05)Ba2Cu4O8 were successfully prepared by firing for 160 h at 850 °C and at oxygen partial pressure of 3 atm without using any catalysts. These samples were characterized in terms of the crystallographic structure and thermal and superconducting properties. The x-ray powder diffraction patterns and superconducting properties for these samples were little changed after post-annealing for 6 h at 1070 °C and at oxygen partial pressure of 400 atm. However, when heat-treated at 700 °C in air and then quenched into liquid nitrogen, samples without post-annealing showed broader superconducting transitions than those post-annealed. Actually, the sharpness of the superconducting transition for a post-annealed sample was little affected by quenching. These observations were in agreement with the results of both transmission electron microscopy and thermal analyses.
Bi–Sr–Ca–Cu–O superconducting films fabricated using metal alkoxides
Authors: Katayama Shingo , Sekine Masahiro
Published: September 2007

Superconducting films in the Bi–Sr–Ca–Cu–O systems were made using metal alkoxides. To prepare a dip-coating solution using a mixed alkoxide solution, insoluble Cu and Bi alkoxides were dissolved by modification with 2-dimethylaminoethanol and formation of a double alkoxide, respectively. Formation of the double alkoxides of Bi with Ca or Sr was confirmed using FT-IR and 1H-NMR. Bi–Sr–Ca–Cu–O films on yttria-stabilized ZrO2 and single crystal MgO(100) substrates were made using this solution. The films were closely oriented along the c-axis perpendicular to the substrate. The film on MgO(100) fired at 850 °C for 48 h showed two resistance drops around 115 and 85 K, corresponding to the high-Tc and low-Tc phases, respectively, and zero resistance at 72 K.
Oxidation protection for a variety of transition metals and copper via surface silicides formed with silane containing atmospheres
Authors: Cabrera A.L. , Kirner J.F. , Armor J.N.
Published: September 2007

The reaction of SiH4/H2 mixtures with iron, cobalt, nickel, copper, chromium, molybdenum, and tungsten, at temperatures between 350 and 800 °C and 1 atm of total pressure, was studied. When the amount of water vapor in the gas mixture is carefully controlled, a metal silicide diffusion coating forms at the appropriate treatment temperature. Cu silicide coatings form at temperatures as low as 350 °C. Metal silicide coatings for Fe, Ni, Co, and Cr form at intermediate temperatures (500–700 °C), and higher temperatures (above 700 °C) are required for W and Mo. Composition and structure of the metal silicide coatings were determined by Auger depth profiling and x-ray diffraction. Kinetics of the surface reaction between SiH4 and the metal substrate as well as the behavior of these coatings in oxidizing environments at high temperatures were studied by a microgravimetric technique. The metal silicide coatings provide oxidation protection for Fe, Ni, and Cr in pure O2 up to 1000 °C, for W and Mo in air up to 900 °C, and for Cu exposed to air up to 700 °C.
Internal friction due to oxygen relaxation in superconducting YBa2Cu3O7−δ above Tc
Authors: Cost J.R. , Stanley J.T.
Published: September 2007

YBa2Cu3O7−δ has an internal friction peak near 210 °C measured at 1.18 Hz. The shift in the temperature of this peak with frequency as measured with different internal friction techniques indicates that the relaxation process has an average activation energy of 1.14 ± 0.05 eV and a pre-exponential factor, τ0 = 1.5 × 10−13s (log τ0 = −12.82 ± 0.23). The peak height decreases during annealing at above 375 °C in helium and increases when annealed in oxygen. The mechanism for the relaxation is believed to be the stress-induced motion of oxygen atoms in the Cu–O plane. Detailed analysis shows that the internal friction peak is determined by a spectrum of activation energies; this spectrum is a symmetrical single peak 0.12 eV wide at half-height. Models for the relaxation are considered and the resulting diffusivities are compared to tracer diffusion results.
Ni/YBa2Cu3O7−xand Ni/Bi2Sr2Ca0.8Y0.2Cu2Oxinterface formation: Reactivity, segregation, and chemical trapping
Authors: Meyer H.M. , Hill D.M. , Weaver J.H. , Goretta K.C. , Balachandran U.
Published: September 2007

Interfaces formed by condensing Ni atoms onto YBa2Cu3O7−x(Y-123) and Bi2Sr2Ca0.8Y0.2Cu2Ox(Bi-2212) have been studied with x-ray photoelectron spectroscopy. For both Y-123 and Bi-2212, the Ni 2p3/2 and O 1score level features indicate Ni–O reactions and changes in the Cu 2p3/2 emission that reflect reduction from nominal Cu2+to Cu1+oxidation states. Ni deposition onto Bi-2212 also reduces Bi–O bonding and releases Bi atoms. For Ni/Y-123, analysis of emission intensities as a function of coverage shows that O and Ba intermix in the growing metal overlayer but that Cu is trapped at the buried interface. For Ni/Bi-2212, similar analysis shows O and Bi intermixing but less effective Cu trapping.
Effect of ternary additions on the structural stability and electronic structure of intermetallic compounds: Al3Ti + Cu
Authors: Hong T. , Freeman A.J.
Published: September 2007

The effects of substituting Cu for either Ti or Al in Al3Ti were studied by the first-principles local density self-consistent linear muffin tin orbital method. The Cu atoms are found to strongly favor Al sites and to promote the stability of the L12 phase, in agreement with experiment. Surprisingly, the rigid band model is closely followed when Cu substitutes for Al. However, the structure of the density of states undergoes remarkable changes if Cu substitutes for Ti; a much weaker hybridization occurs in both L12- and DO22-like structures, giving rise to weaker binding effects. The modification of bond directionality upon Cu addition is determined by comparing the charge density for the structures calculated with those of pure Al3Ti. The effect of tetragonal distortion is also examined. By comparing with the bonding characteristics of pure Al3Ti in the L12 and DO22 structures, the addition of Cu to Al3Ti is found to be equivalent to the tetragonal distortion in DO22 Al3Ti as far as bonding is concerned, resulting in the stabilized L12-like structure for (AlCu)3Ti. The semi-empirical inverse relation between the structural stability and the density of states at Fermi energy is well established.
Site selectivity of Mn atoms in γ–TiAl alloys determined by x-ray scattering
Authors: Babu V. Suresh , Seehra Mohindar S.
Published: September 2007

The relative variations of the intensities of the superlattice peaks in the x-ray scattering of Ti50−xMnxAl50 alloys (x = 0, 0.77, 1.85, and 3.30) are used to determine the site occupancy by Mn atoms. It is concluded that up to x = 1.85, Mn occupies the Ti sites whereas for x = 3.30, some Al sites may also be occupied in the L10 structure of γ–TiAl alloys.
A microstructural study of a Ni2AlTi–Ni(Al, Ti)–Ni3(Al, Ti) three-phase alloy
Authors: Yang R. , Leake J.A. , Cahn R.W.
Published: September 2007

Early studies showed that the two-phase ordered alloy of semi-coherent β–Ni2AlTi (L21) and β–Ni(Al, Ti) (B2) exhibits excellent elevated-temperature creep strength, and the precipitation of the “rod-like” γ'–Ni3(Al, Ti) (L12) from either the β or the β' phase improves the room-temperature ductility of the phases concerned. In the present investigation an attempt is being made to combine the above microstructural features in β'–β–γ' three-phase alloys and for this purpose the composition Ni63Al22Ti15, near the β'–γ' edge of the three-phase region in the recently estimated Ni–Al–Ti isotherm at 900 °C, has been selected for detailed study. The expected precipitation of both the β and the γ' phases occurs in the dendritically solidified β' phase after a 1100 °C/3 h homogenization and a 900 °C/115 h anneal, although the original interdendritic γ' phase remains. The morphology of the two types of precipitates and their orientation relationships with the β' parent phase have been examined using transmission electron microscopy and diffraction, and the experimentally obtained data compared with those predicted by Khachaturyan's elastic strain energy theory. The β precipitates are nearly cuboidal in shape and are bounded by interface dislocations of aβ〈100〉 edge type. For the β precipitates, both morphology and orientation relation agree with those predicted by the theory. The γ' precipitates were found to obey the Nishiyama–Wassermann orientation relationship with the parent phase. These precipitates are about 0.5 μm thick and elongated along their 〈211〉 directions, and in all cases consist of two twin-related variants, giving a sword-like morphology. The {11} twin planes, parallel to the {10} of the parent phase, have been identified as the habits of the precipitation. The theory, however, predicts a habit of {0.732, 0, 0.681}γ' type and a Baker–Nutting orientation relationship. This discrepancy has been attributed to the inapplicability of some assumptions made in the theory: equal elastic moduli between parent and product phases and a tetragonal transformation strain based on Bain's model of the bcc → fcc transformation. The presence of diffuse streaks in the diffraction patterns of the parent phase, which can be correlated with the 〈110〉〈10〉 shear waves, suggests high elastic anisotropy and lends credit to Zener's model. Crystallographic consideration shows that this model is feasible for the L21 → L12 transformation and explains the observed morphological features of the γ' precipitates. Some earlier studies are also discussed.
A quasicrystalline transition state in an annealed Al65Cu20Fe15 alloy
Authors: Zhang Z. , Li N.C. , Urban K.
Published: September 2007

A quasicrystalline transition state was found in an Al65Cu20Fe15 alloy after heat treatment at 700 °C and subsequent slow cooling of samples originally containing the icosahedral quasicrystalline phase. The transition state is characterized by the simultaneous presence of two types of quasicrystalline domains. Their structure can be obtained by the introduction of one-dimensional periodicity along one of the fivefold axes of the original icosahedral quasicrystal. The two types of domains differ in their period lengths which are obtained by multiplying a fundamental length of the icosahedral structure by the Fibonacci numbers 5 and 8.
Influence of silver and excess copper on the formation and properties of 2223 (Bi, Pb)–Sr–Ca–Cu–O superconductor
Authors: Tripathi R.B. , Johnson D.W. , Fleming D.A.
Published: September 2007

The effect of silver additions and excess copper on the sinterability and weak link behavior has been studied in the nominal composition Bi1.72Pb0.32Sr2Ca2Cu3Ox (Ag = 1−4 wt. %) and Bi1.72Pb0.32Sr2Cu3+yOx where y = 0.2−0.5, using magnetic susceptibility, XRD, and SEM methods. Ag additions enhance the weak link behavior, giving a magnetic susceptibility shoulder at 103 K. This shoulder became more prominent at high fields (35 Oe), and a low Tc phase became evident with 4 wt. % Ag additions. In contrast to stoichiometric compositions, samples containing excess copper demonstrated an enhancement of the sinterability and a large volume of 2223 phase even in calcined samples. XRD data and SEM analysis failed to detect any Cu-rich phase up to copper contents y = 0.4. However, in copper samples with y = 0.5, a small x-ray peak at 2θ = 48.5°has been observed corresponding to CuO.
Casting and machining of devices of high temperature superconducting BSCCO
Authors: Rayne R.J. , Toth L.E. , Bender B.A. , Lawrence S.H. , Miller M.M. , Soulen R.J. , Candella G.
Published: September 2007

Magnetic shields for SQUID applications were successfully fabricated using high Tc superconducting Bi–Sr–Ca–Cu–O (BSCCO). In order to produce shields with adequate superconducting properties and close dimensional control, it was necessary to develop several new processing techniques. Shields were produced by casting liquid BSCCO into molds, heat treating, and machining. A series of BSCCO alloys with different compositions were cast from the molten state into metal molds and subsequently heat treated to render the castings superconducting. The heat-treating cycles were studied with the aid of thermogravimetric analysis (TGA), differential thermal analysis (DTA), and dilatometer measurements. The phases and microstructures after various heat-treating cycles were monitored by x-ray diffraction (XRD), optical microscopy, and scanning electron microscopy (SEM). Superconducting properties were measured after various stages of heat treatment and machining. Prototype magnetic shields were machined from bulk castings and found to perform successfully. The most significant factor in shield quality was the nominal composition of the shield, which was shown by transmission electron microscopy (TEM) to affect the grain boundary chemistry.
The effect of electronic energy loss on the dynamics of thermal spikes in Cu
Authors: Prönnecke S. , Caro A. , Victoria M. , de la Rubia T. Diaz , Guinan M.W.
Published: September 2007

We present results of a molecular dynamics simulation study of the effect of electron-ion interactions on the dynamics of the thermal spike in Cu. Interatomic forces are described with a modified embedded atom method potential. We show that the electron-ion interaction acts to reduce the lifetime of the thermal spike and therefore the amount of atomic rearrangement that takes place in energetic displacement cascades in Cu. The results point toward the important effect that inelastic energy losses might have on the dynamics of displacement cascades in the subcascade energy regime where the lifetime of the thermal spike is expected to exceed the electron-phonon coupling time.
Alpha-recoil damage in titanite (CaTiSiO5): Direct observation and annealing study using high resolution transmission electron microscopy
Authors: Lumpkin Gregory R. , Eby Ray K. , Ewing Rodney C.
Published: September 2007

“Tracks” of alpha-recoil nuclei have been observed directly in titanite (CaTiSiO5). Recoil tracks in titanite are 4 to 6 nm in diameter and consist of a central aperiodic zone surrounded by a narrow (0–2 nm) outer zone that is essentially crystalline, but which exhibits modulated image contrast due to interstitial defects. Previous work has suggested that titanite is 2 to 3 times more sensitive to alpha-decay damage than other ceramic phases (e.g., zircon, ZrSiO4). We find, however, that track diameters in titanite are essentially the same as reported for other phases, including zircon (ZrSiO4), pyrochlore (NaCaTa2O6F), and zirconolite (CaZrTi2O7). An annealing study of titanite (300 to 700 °C, N2) shows a two-stage recovery process. Track diameter decreases at 400 °C. An intermediate phase develops at 500 °C, and nearly all tracks are epitaxially recrystallized. At 700 °C, all tracks and the intermediate phase are gone.
Improved surface properties of polymer materials by multiple ion beam treatment
Authors: Lee E.H. , Lewis M.B. , Blau P.J. , Mansur L.K.
Published: September 2007

Ion beam treatment studies have been carried out to investigate the potential for improvements in surface-sensitive properties of polymers. Kapton, Teflon, Tefzel, and Mylar have been implanted with boron, nitrogen, carbon, silicon, and iron ions, singly or simultaneously with dual or triple beams. The implanted materials were characterized by optical microscopy, transmission electron microscopy, nano-hardness indentation, wear testing, scanning tunneling microscopy, x-ray analysis, nuclear reaction analysis, Fourier transform infrared spectroscopy, and Raman spectroscopy. Although the polymers showed a color change and varying degrees of measurable surface depression in the bombarded area, the implanted surface revealed substantial improvements in surface smoothness, hardness, and wear resistance. In particular, B, N, C triple-beam implanted Kapton showed over 30 times larger hardness than unimplanted material, making it more than three times harder than stainless steel. Sliding wear properties were characterized using an oscillating nylon or high carbon steel wear ball. Severe wear tracks were observed in virgin Kapton, but no appreciable wear was observed in ion implanted Kapton. Mechanisms underlying the improved surface properties are addressed.
A model for the effect of line width and mechanical strength on electromigration failure of interconnects with “near-bamboo” grain structures
Authors: Arzt E. , Nix W.D.
Published: September 2007

A simple analytical model for the effect of mechanical strength and line width (for the case of narrow lines) on the electromigration failure of metallic interconnects is presented. Because the line width/grain size ratio and the diffusivity enter differently in the model, application of the resulting failure time equation to published data can provide insight into the mechanisms of enhancement of electromigration resistance by grain structure optimization and alloying.
Laser interferometric measurement of the surface tension of thin foils
Authors: Jablonski Gregory A. , Sacco Albert
Published: September 2007

A technique has been developed to measure the surface tension of thin foils under their own vapor pressure. The zero creep method of surface tension measurement is used in conjunction with laser interferometry. This technique allows very small sample strains to be measured. Sample length changes of the order of 0.3 μm were measured with the laser interferometer. The sensitivity of the laser interferometer/zero creep system allows the surface tension to be measured much closer to the Tammann temperature, which is approximately one-half the melting temperature (≍1/2 Tm) of the material. The system has been tested on thin Sn foils in air, and on thin Al and Ni foils under their own vapor pressure (vacuum).
A mathematical model for chemical vapor infiltration with microwave heating and external cooling
Authors: Gupta Deepak , Evans James W.
Published: September 2007

A mathematical model has been used to compute temperature profiles in ceramic preforms that are heated by microwaves. The temperature profiles were then input to a second part of the model describing chemical vapor infiltration of the preform, that is the diffusion of gaseous reactants, heterogeneous reaction, and evolution of the pore structure. Equations were solved numerically for parameters corresponding to the infiltration of SiC preforms by pyrolysis of trichloromethylsilane. While based on some simplifications, the model leads to the conclusion that infiltration proceeds more rapidly, and to a greater extent, with microwave heating/external cooling than in isothermal infiltration. The model suggests that infiltration might be optimized by manipulation of microwave power and external cooling. The computed extent of infiltration is seen to be very sensitive to the initial pore size.
Thermodynamic instability of the YBa2Cu3O7−x phase at the 1:2:3 composition
Authors: Garzon Fernando H. , Raistrick Ian D. , Ginley D.S. , Halloran John W.
Published: September 2007

We have measured enthalpies of formation of YBa2Cu3O7−x, Y2BaCuO5, YBa2Cu4O8, and BaCuO2 using high precision isothermal solution calorimetry. Thermodynamic analysis of the calorimetric data indicates that YBa2Cu3O7−x at the pure-phase composition (1:2:3 cation ratio) is metastable at ≍873 K and 1 atm Po2.
Superconducting fibers from organometallic precursors. Part II: Chemistry and low temperature processing1
Authors: Laine Richard M. , Youngdahl Kay A. , Kennish Richard A. , Hoppe Martin L. , Zhang Zhi-Fan , Ray Jean
Published: September 2007

Working principles are developed as guidelines for the selection and/or design of organometallic polymers for processing fiber precursors to metal oxide fibers. These principles form the basis for the selection of metal carboxylate preceramics as an optimal approach to processing yttrium barium cuprate (123) ceramic superconducting fibers. A variety of candidate yttrium, barium, calcium, strontium, bismuth, and copper metal carboxylates were synthesized. Solubility and empirical rheology tests were conducted to screen these compounds to choose spinnable precursor systems. Simple extrusion studies confirmed that THF solutions of mixtures of yttrium, barium, and copper isobutyrates with some quantity of barium 2-ethyl-hexanoates can be used to successfully form 60–70 μm diameter 123 precursor fibers.
The nature of [001] tilt grain boundaries in YBa2Cu3O7−x
Authors: Gayle Frank W. , Kaiser Debra L.
Published: September 2007

Fifty-two faceted grain boundary segments ([001] tilt boundaries) in clusters of bulk-scale YBa2Cu3O7−xcrystals having coincident c-axes were characterized by optical microscopy techniques. Grain boundary orientations were widely distributed. All grain boundaries, except those far from the symmetric condition and those with {110} facets, exhibited well-developed matching of the twin domains across the boundary. It is suggested that this newly reported phenomenon of twin pattern matching occurs due to a local coordination of the tetragonal → orthorhombic transformation strain across grain boundaries and may be beneficial with regard to electrical transport in highly textured polycrystalline material.
Determination of yielding and debonding in Al–Cu thin films from residual stress measurements via diffraction
Authors: Shute C.J. , Cohen J.B.
Published: September 2007

The yield strength and interfacial bonding are properties of interest for understanding void formation in thin film interconnect and subsequent failure of VLSI devices. A method is presented to examine the mechanical properties of thin polycrystalline films attached to substrates by measuring the change in thermal residual stress, due to the difference in coefficient of expansion between the film and substrate, as a function of decreasing temperature of the sample. The yield strengths of passivated 0.5, 1.0, and 2.0 μm thin films of Al–2% Cu on oxidized Si wafer substrates have been determined with this method to be 325, 170, and 120 MPa, respectively. Unpassivated films of the same thicknesses were also examined, but yielding did not occur for these films even though the residual stress reached a value of over 400 MPa. The lack of yielding in the unpassivated samples and the thickness dependence of the passivated samples is attributed to the grain size of these materials, which is less than the film thickness for the unpassivated case and greater than the film thickness after passivation. Debonding occurred in the 2 μm unpassivated sample but in none of the others, indicating a thickness dependence of the energy for delamination.
Anomalous lattice expansion of metal-hydrogen thin films
Authors: Miceli P.F. , Zabel H. , Dura J.A. , Flynn C.P.
Published: September 2007

We report x-ray diffraction experiments investigating the lattice expansion due to hydrogen in Nb/Ta superlattices and thin Nb films. For low hydrogen concentrations (cH < 0.09 H/metal), the lattice expands only normal to the plane of the film due to the epitaxial constraints imposed by the substrate. At higher concentrations, corresponding to strains on the order of the critical strains expected for the formation of misfit dislocations, we find a structural instability which leads to the formation of two metal-H phases, one of which has a lattice expansion in the plane of the film. These results differ significantly from the expected phase diagram and properties of the bulk metal-H systems.
Contribution to the analysis of the α/β interface in some titanium alloys
Authors: Servant C. , Quesne C. , Baudin T. , Penelle R.
Published: September 2007

In (α + β) titanium alloys, the interface between the α and β phases has been widely studied, and many conflicting results have been published concerning the formation of an interface phase having an hcp or fcc crystalline structure. Several authors consider that this phase is an artifact and consists of a titanium hydride formed during thin foil preparation for TEM examination. This paper reports examinations by TEM and STEM of heat-treated Ti–6Al–4V alloy and two titanium molybdenum aluminides. The formation of an α/β interface phase is discussed as a function of composition heat-treatment and thin foil preparations. Particularly, for Ti–6Al–4V in a nonequilibrium state, a strong concentration gradient was revealed in the α/β interface, and a metastable interface phase enriched in beta-stabilizers with an fcc structure has been clearly characterized.
Photoconductivity of activated carbon fibers
Authors: Kuriyama K. , Dresselhaus M.S.
Published: September 2007

The conductivity and photoconductivity are measured on a high-surface-area disordered carbon material, i.e., activated carbon fibers, to investigate their electronic properties. This material is a highly disordered carbon derived from a phenolic precursor, having a huge specific surface area of 1000–2000 m2/g. Our preliminary thermopower measurements show that the dominant carriers are holes at room temperature. The x-ray diffraction pattern reveals that the microstructure is amorphous-like with Lc ≃ 10 Å. The intrinsic electrical conductivity, on the order of 20 S/cm at room temperature, increases by a factor of several with increasing temperature in the range 30–290 K. In contrast, the photoconductivity in vacuum decreases with increasing temperature. The magnitude of the photoconductive signal was reduced by a factor of ten when the sample was exposed to air. The recombination kinetics changes from a monomolecular process at room temperature to a bimolecular process at low temperatures, indicative of an increase in the photocarrier density at low temperatures. The high density of localized states, which limits the motion of carriers and results in a slow recombination process, is responsible for the observed photoconductivity.
Stress and thermal expansion of β–SiC films by the vibrating-membrane method
Authors: Berry B.S. , Pritchet W.C. , Fuentes R.I. , Babich I.
Published: September 2007

The internal tensile stress in heteroepitaxial films of cubic (β) SiC grown on Si substrates by chemical vapor deposition at 1520–1650 K has been determined from the resonant frequencies of membrane samples formed by removal of a central circular region of the substrate. At 293 K, the stress falls in the range 250–500 MPa, and tends to be larger for samples grown at higher temperatures. The temperature dependence of the stress has been studied over the range 90–670 K and found to exhibit a reversible, mild, but convoluted variation that includes a minimum near 160 K and a maximum near 400 K. Analysis of this behavior reveals that the thermal expansion of β–SiC is remarkably close to that of Si, with an expansion curve that intersects that of Si near 80 K and 500 K. This behavior is different from that obtained from available handbook data for α–SiC, and suggests that data for α–SiC should not be relied on to estimate the overall magnitude of the thermal stress component in β–SiC films. Annealing studies carried out in the range 673–1473 K on as-prepared membranes indicate a high degree of membrane stability, with no evidence of stress relaxation. It has also been observed that annealed membranes may exhibit very low levels of mechanical damping, corresponding to Q-values in the range 106−107.
H2O catalysis of aluminum carbide formation in the aluminum-silicon carbide system
Authors: Maruyama Benji , Ohuchi Fumio S.
Published: September 2007

Aluminum carbide was found to form catalytically at aluminum-silicon carbide interfaces upon exposure to water vapor. Samples, composed of approximately 2 nm thick layers of Al on SiC, were fabricated and reacted in vacuo, and analyzed using XPS. Enhanced carbide formation was detected in samples exposed to 500 Langmuirs H2O and subsequently reacted for 600 s at 873 K. The cause of the catalysis phenomenon is hypothesized to be the weakening of silicon-carbon bonds caused by very strong bonding of oxygen atoms to the silicon carbide surface. Aluminum carbide formation is of interest because of its degrading effect on the mechanical properties of aluminum/silicone carbide reinforced metal matrix composites, as well as its effect on the electrical properties of aluminum metallizations on silicon carbide layers in microelectronic components.
Mechanical properties of single-quasicrystalline AlCuCoSi
Authors: Wittmann R. , Urban K. , Schandl M. , Hornbogen E.
Published: September 2007

The mechanical properties of single-quasicrystals of decagonal AlCoCuSi have been studied for the first time by applying the Vickers indentation method. The hardness has been determined asH≃ 9.6 MPa. Estimates for the modulus of elasticity and the fracture toughness are given. The quasicrystals are highly strained internally. Scratching experiments show slight anisotropies in the abrasive friction coefficient. The dominant abrasive mechanism is microplowing.