MATERIALS TRANSACTIONS
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ONLINE ISSN: 1347-5320
PRINT ISSN: 1345-9678

MATERIALS TRANSACTIONS Vol. 43 (2002), No. 7

  • Theoretical Formation Energy of Oxygen-Vacancies in Oxides

    pp. 1426-1429

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    DOI:10.2320/matertrans.43.1426

    Formation energies of neutral and charged oxygen vacancies in MgO, ZnO, Al2O3, In2O3 and SnO2 have been calculated by a first principles plane-wave pseudopotential method. Two kinds of polymorphs, i.e., an ordinary phase and a high-pressure or an hypothetical negative pressure phase, have been chosen in order to see the effects of crystal structure. Supercells composed of 54 to 96 atoms were employed, and structural relaxation around the vacancy within second nearest neighbor distances was taken into account. Defect levels were obtained from the difference in total energies of the neutral and charged supercells that contain a vacancy. Ionization energies of the vacancy were calculated as the difference in the bottom of the conduction band and the defect levels. They are found to be proportional to band-gaps with a factor of approximately 0.5, which are prohibitively large for the n-type conduction.
  • Ab-initio Calculation of Si-K and Si-L ELNES Edges in an Extended Inactive Defect Model of Crystalline Silicon

    pp. 1430-1434

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    DOI:10.2320/matertrans.43.1430

    The Si–K and Si–L2,3 edges of the electron energy-loss near-edge (ELNES) spectra of a model of an extended inactive defect in Si with no dangling bonds were calculated using an ab-initio method which includes the electron-hole interaction. In this method, atom-by-atom excitation is possible. The calculated results are discussed in the context of the subtle structural differences in the local atomic environment. Comparison of the results with measured data shows satisfactory agreement. The method can be applied to other more complicated defective systems such as grain boundaries and interfaces for effective materials characterization.
  • First-principles Calculation of Transition-metal L2,3-edge Electron-energy-loss Near-edge structures Based on Direct Diagonalization of the Many-electron Hamiltonian

    pp. 1435-1438

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    DOI:10.2320/matertrans.43.1435

    First-principles relativistic configuration-interaction (CI) calculations of cation L2,3-edge electron-energy-loss near-edge structures (ELNES) of SrTiO3, NiO and CaF2 have been carried out based on the direct approach where the ground state (GS) configuration and the excited state (ES) configuration were calculated simultaneously by direct diagonalization of the many-electron Hamiltonian. The obtained theoretical spectra were compared with our recent results based on the indirect approach where the GS configuration and the ES configuration were calculated separately and their energy separation was adjusted to the value predicted by the Slater’s transition-state calculation. Although both approaches well reproduced the overall features of the experimental spectra, the direct approach tend to slightly overestimate the absolute transition energies. In the case of Ti L2,3-edge of SrTiO3, however, the absolute transition energies calculated by the direct approach were comparable to those by the indirect approach and were also in good agreement with experimental values, indicating sufficient inclusion of electron correlations through pure configuration interactions.
  • First-Principles Calculations of Co Impurities and Native Defects in ZnO

    pp. 1439-1443

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    DOI:10.2320/matertrans.43.1439

    First-principles plane-wave pseudopotential calculations have been conducted to investigate Co impurities and native defects associated with oxygen excess in ZnO. The electronic states and formation energies are evaluated using the total energies of supercells. The electronic states indicate that Co impurities are donor-like, while native defects associated with oxygen excess are acceptor-like. Among the native defects, Zn vacancies are likely to be a dominant species in view of the much lower formation energy than those of the others. Calculations for Co impurity-Zn vacancy complexes imply that Co impurities stabilize Zn vacancies and hence oxygen excess in their vicinities. This is suggested to be a role of Co impurities at grain boundaries in ZnO varistor ceramics.
  • Energetics of Hydrogen States in SrZrO3

    pp. 1444-1450

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    DOI:10.2320/matertrans.43.1444

    The optimized geometries and formation energies of hydrogen in the perovskite-type oxide, SrZrO3, have been calculated using the density functional theory under the generalized gradient approximation (GGA). The stable hydrogen sites in the oxide are examined with special interests. It is shown that the hydrogen is tightly bound to an oxygen ion and located slightly outside the ZrO6 octahedron in SrZrO3. The O–H bond orientates towards the second-nearest-neighbor oxygen ion, which induces large local displacements of adjacent oxygen ions. For example, the nearest-neighbor oxygen ions tend to approach the hydrogen. The addition of an acceptor dopant, yttrium, modifies such local distortion of oxygen ions around the hydrogen as to increase protonic conductivity. In addition, the calculated hydrogen formation energy shows that the hydrogen behaves as a shallow donor in SrZrO3. The fundamental principles of doping and atmosphere control in proton conducting oxides are also given in view of energetics.
  • Theoretical Calculation of Positron Lifetimes in CoAl and CoTi

    pp. 1451-1455

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    DOI:10.2320/matertrans.43.1451

    Positron states for the bulk and vacancy in intermetallic compounds CoAl and CoTi crystallizing in the B2 structure have been calculated using the DV-Xα electronic structure calculation and compared with experimental results. The calculated positron lifetimes in CoAl can explain the compositional dependence of the positron lifetimes in CoAl. The calculated positron lifetime at the Ti vacancy in CoTi is longer than that at the Co vacancy. The charge transfer from Ti to Co leads to the difference of the localization of the positron wave function at the vacancy. At the Co vacancy site surrounded by the positively charged Ti atoms, the positron wave function does not localize as well as at the Ti vacancy site, which decrease the positron lifetime. The calculated results suggest that the positron lifetimes in intermetallic compounds can not be directly correlated to those of the pure component and the charge transfer between the constituent atoms affects the localization of the positron wave function.
  • Theoretical Solution Energy of Alkaline Earth Ions in Lanthanum Chromites

    pp. 1456-1459

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    DOI:10.2320/matertrans.43.1456

    First principles calculations by a plane-wave basis pseudopotential method have been made for Mg, Ca, and Sr-doped lanthanum chromites (LaCrO3). A supercell composed of 40 atoms of a high temperature phase of LaCrO3 having a cubic-perovskite structure has been used. Solution energies of a neutral solute and a compensated solute by an oxygen vacancy were systematically computed. They were obtained for two kinds of cation sites and four thermodynamical conditions with different chemical potentials of constituent atoms. Mg shows lowest solution energy when it substitutes for the Cr ion. On the other hand, both Ca and Sr prefer to be located at La site. The charge neutral states are preferred by all of them. The results are consistent to experimental results regarding the site preference and the charge state.
  • Evaluation of Migration Energy of Lithium Ions in Chalcogenides and Halides by First Principles Calculation

    pp. 1460-1463

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    DOI:10.2320/matertrans.43.1460

    Migration energies of Li ion in Li3N, Li2X (X=O, S, Se, and Te) and LiX (X=F, Cl, Br, I) via vacancy mechanism have been calculated by first principles pseudopotential method using plane-wave basis. The energy was obtained as the difference in total energies of supercells by two separate calculations; one with a Li+ ion at the normal point and the other with a Li+ ion at the saddle point. Positions of atoms within the second nearest neighbor of the jumping ion were fully relaxed. Two kinds of diffusion paths, i.e., direct and indirect jumps, were considered. Results show rough agreement with available experimental data. The migration energies for the indirect jump in both halides and chalcogenides show a tendency to decrease with the increase in the periodic number in the Periodic table. This is consistent with the widely accepted view. However, the migration energies for the direct jump of chalcogenides do not obey the rule. Comparison of two polymorphs of LiF implies that not only the anionic species but also the crystal structure plays an important role in determining migration energy.
  • Refinement of X-ray Fluorescence Holography for Determination of Local Atomic Environment

    pp. 1464-1468

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    DOI:10.2320/matertrans.43.1464

    X-ray fluorescence holography (XFH) is a promising method for determination of a local environment around a particular element. Since the holographic signal is about 0.3% of the background isotropic fluorescent radiation, it takes a few months to record a set of complete XFH data using a conventional energy dispersive detector. In order to overcome this difficulty, we designed an XFH setup with a combination system of a cylindrical LiF analyzer and an avalanche photo diode (APD) for bulk samples, and with a multi-element SSD for impurity samples. The holography experiments of an Au single crystal and Zn atoms doped in a GaAs wafer performed with these systems at the synchrotron radiation facility SPring-8 enables us to obtain high quality hologram data within a practical measurement time.
  • Behavior of Impurities In and Cd in the LiNbO3-LiTaO3 System

    pp. 1469-1474

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    DOI:10.2320/matertrans.43.1469

    The temperature dependences of the nuclear-electric-quadrupole frequency ωQ of 117In and 111Cd doped in ferroelectrics LiTaO3, Li1−xInx⁄3TaO3 with x=0.2, and Li1−xCdx⁄2TaO3 with x=0.167, measured by the perturbed-angular-correlation technique, indicate that In behaves like Li; further, in a certain temperature range above the phase-transition temperature, a local system consisting of In (Li) and oxygen ions is very stable, by taking resonance structures, whereas Cd does not behave like Li, and a system consisting of Cd and oxygen ions does not take resonance structures, because the ionic size of Cd is large. It is considered that the resonance structures are due to a disordering of the oxygen ions, and that the order-disorder of oxygen ions is the driving mechanism for ferroelectric instability in the LiNbO3–LiTaO3 system.
  • Feasibility Study of Local Structure Analysis of Ultrathin Films by X-ray Fluorescence Holography

    pp. 1475-1479

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    DOI:10.2320/matertrans.43.1475

    The feasibility of the determination of local atomic structure was tested by computing Ge X-ray fluorescence hologram patterns of a Ge film 3 atomic layers thick on a Si substrate. An atomic image obtained from a single-energy hologram pattern exhibits many small oscillations and a precise atomic image is difficult to evaluate. By summing plural images reconstructed from holograms obtained at several different energies, the final atomic image is improved due to the disappearance of the oscillations. The full-width at half-maximum of peaks of the atomic image reconstructed from six holograms calculated at 27.0–29.5 keV with 0.5 keV steps is 0.03 nm. This clearly demonstrates that the XFH method has strong potential as an experimental tool for evaluating the local atomic structure.
  • 140Ce (←140La) Time-Differential Perturbed Angular Correlation Study of CeRu2Ge2

    pp. 1480-1485

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    DOI:10.2320/matertrans.43.1480

    The hyperfine interaction at 140Ce nucleus in CeRu2Ge2 has been investigated, using the 140Ce Time-Differential Perturbed Angular Correlation (TDPAC) method. 140Cs ions, which are fission products from 235U, have been implanted into a single crystal of CeRu2Ge2 at room temperature. 140Cs decays to 140Ce 4+ intermediate state through 140Ba with the life-time of 12.752 days and 140La with a life-time of 1.6781 days. TDPAC spectrum at 4.2 K clearly shows the existence of the magnetic perturbation, which is absent at room temperature. Larmor frequency observed at 4.2 K is 1.32(1) Grad/s corresponding to the hyperfine magnetic field of 24.2(2) T at 140Ce nucleus. Magnetization measurements of CeRu2Ge2 performed by using the SQUID magnetometer revealed the magnetic moment of 1.848 μB which indicates that the hyperfine coupling constant of Ce is 13.1(3) T/μB.
  • Excess Vacancies Induced by Disorder-Order Phase Transformation in Ni3Fe

    pp. 1486-1488

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    DOI:10.2320/matertrans.43.1486

    The order-disorder transformation and lattice defects in Ni3Fe have been studied by positron lifetime measurements. Anomalous vacancy-generation during ordering transformation, which was originally found on the ordering process of super-cooled disordered Cu3Au, has been confirmed on the ordering transformation of Ni3Fe. Disordered fcc solid solution of Ni3Fe was brought to room temperature by quenching the specimen from temperatures above the order-disorder transformation point TC. The ordering process into L12 structure was promoted by heating the sample isochronally or isothermally. It has been found that vacancies are generated in both heating processes, i.e., during the ordering process of super-cooled disordered Ni3Fe. Generated vacancies are not stable up to TC and annealed out at temperatures below TC.
  • Anomalous Temperature Changes of Positron Lifetime and Electrical Resistivity in B2-NiTi Alloys

    pp. 1489-1493

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    DOI:10.2320/matertrans.43.1489

    The pre-martensitic phenomena in Ni-rich NiTi alloys have been studied by means of positron lifetime spectroscopy and electrical resistivity measurement. We have found anomalous positron lifetime changes above the martensitic transformation temperatures of Ni50Ti50–Ni51Ti49 alloys. Positron lifetime increases anomalously with decreasing temperature even at temperatures 100 degrees higher than Ms of Ni51Ti49 alloys. Almost the same change in positron lifetime has been observed on the subsequent heating run with little hysteresis. We have also found the anomalous behavior of positron lifetime and negative temperature dependence of electrical resistivity even in NiTi alloys that do not exhibit martensitic transformation. The observed pre-martensitic changes of positron lifetime and electrical resistivity directly reflect the intrinsic electronic-structural changes of NiTi alloys that cause so-called pre-martensitic phenomena.
  • Recovery of Hydrogen Induced Defects and Thermal Desorption of Residual Hydrogen in LaNi5

    pp. 1494-1497

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    DOI:10.2320/matertrans.43.1494

    Lattice defects induced by initial hydriding and their effect on residual hydrogen content in LaNi5 have been studied by means of positron lifetime spectroscopy and hydrogen thermal desorption measurement. Component analyses of positron lifetime spectra show that surprising amount of vacancies together with dislocations are generated by the initial hydriding. Vacancy migration in LaNi5 after hydrogen desorption at room temperature is observed around 423–673 K, while dislocations in LaNi5 are much more stable. Hydrogen thermal desorption measurement shows that the release of residual hydrogen occurs mainly in the temperature range from 450 to 650 K, and it ceases at about 800 K . The release temperature of residual hydrogen closely corresponds with the temperature of vacancy migration and annihilation in LaNi5. Residual hydrogen in LaNi5 is most likely trapped by vacancies and vacancy clusters, which are induced by hydriding.
  • Positron Lifetime Study of Defect Structures in B2 Ordered Co-Al Alloys

    pp. 1498-1501

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    DOI:10.2320/matertrans.43.1498

    The positron lifetimes in the B2 intermetallic compounds Co100−XAlX (X=46.9 to 54.1) water-quenched, air- and furnace-cooled from high temperatures have been measured to reveal their defect structures. The positron mean lifetime depends entirely on the composition, irrespective of the cooling rate, and steadily increases from 161 to 180 ps as the Al concentration increases from 46.9 to 54.1 at%. A water-quenched Co53.1Al46.9 was isochronally annealed for 900 s successively at 25 K intervals up to 1073 K, but its positron lifetime remained unchanged by the annealing. These results clearly show that vacancies are formed not only on the Co-sites but also on the Al-sites, and the total vacancy concentration is more than 10−4 in both the Co-rich and the Al-rich CoAl. It was furthermore found that the population ratio of Al-vacancies to Co-vacancies gradually increases with the Co concentration, and the number of Al-vacancies is comparable with that of Co-vacancies in the vicinity of stoichiometric composition.
  • Hyperfine Interaction of 140Ce(←140La) in CaB6

    pp. 1502-1505

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    DOI:10.2320/matertrans.43.1502

    A 140La(Iπ=3, T1⁄2=40.3 h)-doped layer has been produced in CaB6 by means of radioactive isotope (RI) beam technique: 140Cs(Iπ=1, T1⁄2=63.7 s) was implanted into CaB6 and the radioactive equilibrium of 140Ba–140La was achieved. The concentration of La in CaB6 was La/Ca∼0.001 and ∼0.005. Obtained TDPAC spectra of the 2083 keV level of 140Ce (Iπ=4+, T1⁄2=3.4 ns, μ=+4.35±0.10μN) followed by the β decay of 140La showed the existence of hyperfine magnetic fields: Bhyp=−15.0±0.5 T and −1.00±0.15 T for La/Ca∼0.001 and Bhyp=−1.51±0.12 T for La/Ca∼0.005.
  • Atomic Structure and Diffusion in Amorphous Si-B-C-N by Molecular Dynamics Simulation

    pp. 1506-1511

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    DOI:10.2320/matertrans.43.1506

    We carried out molecular dynamics simulation of amorphous silicon nitride containing boron and carbon, in order to investigate the short-range atomic arrangement and diffusion behavior. In amorphous Si–B–N, boron atoms are in a nearly threefold coordinated state with nitrogen atoms, while boron atoms in amorphous Si–B–C–N have bonding with both carbon and nitrogen atoms. Carbon atoms in Si–B–C–N are also bonded to silicon atoms. The self-diffusion constant of nitrogen in Si–B–N becomes much smaller than that in amorphous Si3N4. Also, amorphous Si–B–C–N exhibits smaller self-diffusion constants of constituent atoms, even compared to Si–B–N. Addition of boron and carbon is important in decreasing atomic mobility in amorphous Si–B–C–N. This may explain the increased thermal stability of the amorphous state observed experimentally.
  • Local Atomic Structure and Electronic State of ZnS Films Synthesized by Using CBD Technique

    pp. 1512-1516

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    DOI:10.2320/matertrans.43.1512

    We have succeeded in synthesizing ZnS films on substrates such as glass or Si wafer by using CBD (chemical bath deposition) method. These semiconductor films were photocatalytic and showed high productivity of hydrogen gas from the solution containing HS ions. From observation of SEM/TEM, it was found that the cluster film consisted of 100 nm sized agglomerated particles composed of nano-crystals. The result of EDS analysis for the cross-section of the film prepared by using FIB showed the presence of small amounts of oxygen along with zinc and sulfur. The information of the local quantum structures obtained from EXAFS/XANES analysis suggested that the cluster film has unique atomic-distribution and metastable electronic state different from bulk ZnS and ZnO. Based on the results of the above analysis, the relation between local quantum structures and properties was proposed.
  • Transmission Electron Microscopic Studies of LiNb0.5Ta0.5O3 Films Deposited on Sapphire Substrates by Thermal Plasma Spray CVD (Microstructure of LiNb0.5Ta0.5O3 Films Deposited by Thermal Plasma Spray CVD)

    pp. 1517-1524

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    DOI:10.2320/matertrans.43.1517

    Cross sections and plan views of LiNb0.5Ta0.5O3 films were investigated mainly by high-resolution transmission electron microscopy. These films were deposited on (0001) sapphire substrates by thermal plasma spray chemical vapor deposition method at various feeding rates of liquid raw materials. It was found that the crystallinity and the preferential orientation of the LNT films depend on the feeding rate. The LNT film formed at the feeding rate of 7 mL/min was epitaxially grown on the substrate, and the orientation relationship between the film and the substrate was (0001)LNT⁄ ⁄(0001)sapphire, [11\\bar20]LNT⁄ ⁄[11\\bar20]sapphire. The LNT films fabricated at the higher and the lower feeding rate were polycrystalline. These films included twin crystals and other phases such as Li(Nb, Ta)3O8. The calculations based on the coincidence of reciprocal lattice points revealed that the epitaxial orientation relationships observed by transmission electron microscopy satisfied the geometrically optimal coherency across the interface.
  • Local Atomic Structure and Catalytic Activities in Electrodeposited Mo-Ni Alloys

    pp. 1525-1529

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    DOI:10.2320/matertrans.43.1525

    The atomic structures of Mo–Ni alloys electrodeposited from aqueous solutions containing various molybdenum and nickel concentrations were studied by X-ray diffraction. The films are arranged with respect to molybdenum concentrations in the films. That is, the Group I (Mo<10%), the Group II (20%<Mo<30%) and the Group III (Mo≈40%). In the Group I, the structures of the films are fcc crystals of nickel solid solutions. The films in the Groupes II and III are amorphous. The hydrogen evolution as an electrode in alkaline water electric cell was also compared. At about 20% molybdenum, the nickel film shows the maximum hydrogen evolution.
  • Fullerene and Sulfur Compounds

    pp. 1530-1532

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    DOI:10.2320/matertrans.43.1530

    A new method to synthesize fullerene and sulfur compounds, C60S16 and C70S16 compounds of about 1 mm length, has been developed. The C60S16 crystal is a C-centered monoclinic structure of a=2.0874(1) nm, b=2.1139(1) nm, c=1.05690(6) nm and β=111.9 degree. The C70S16 compound has a primitive monoclinic structure of a=1.5271(2) nm, b=1.49971(7) nm, c=2.18024(9) nm and β=109.791(1) degree. The crystalline structure of these compounds is constructed by fullerene and S8-rings. A very small amount of charge-transfer between fullerenes and sulfur rings are expected by first-principle calculation.
  • Preparation of a TiO2 Film Coated Si Device for Photo-Decomposition of Water by CVD Method Using Ti(OPri)4

    pp. 1533-1536

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    DOI:10.2320/matertrans.43.1533

    Growth of a TiO2 film on a Si wafer by CVD method was studied for a new design of chemically stable device which is useful in the visible region and increases the efficiency of the photocatalytic decomposition process of water. This design is made up of a monolithic structure of a TiO2 thin film and a Si solar cell. The TiO2 film was deposited on a Si substrate by the vapor phase reaction of Ti(OPri)4 with water at temperatures from room temperature to 473 K. At room temperature, a rough TiO2 film with a nano-sized cone shape deposit was observed by AFM. The smooth TiO2 film of ∼10 nm thickness was obtained by the reaction of 80 Pa Ti(OPri)4 and 493 Pa water vapors at 473 K for 10.8 ks. The surface and the thickness of the film became rougher and thicker, respectively, with increasing pressure of Ti(OPri)4.
  • Current-Voltage Characteristics Across Small Angle Symmetric Tilt Boundaries in Nb-Doped SrTiO3 Bicrystals

    pp. 1537-1541

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    DOI:10.2320/matertrans.43.1537

    Grain boundary structure and current–voltage (IV) characteristics were investigated for Nb-doped SrTiO3 bicrystals having small angle tilt boundaries with misorientation angles of 2° and 4° against [001]. The bicrystals were fabricated by using hot-joining technique at 1400°C for 10 h under a pressure of 0.4 MPa. High resolution transmission electron microscopy study revealed that the joined boundaries are free from any secondary phases such as amorphous phases even on an atomic scale. The structures of the two boundaries are composed of edge type dislocations whose Burgers vector is [010]. But the density of boundary dislocations differs between the two boundaries. They exist at an interval of 10 nm in the 2°-boundary and 5.2 nm in the 4°-boundary. On the other hand, it was found that non-linearity in IV relation across the boundary increases with an increase in the misorientation angle. This result clearly indicates that the potential barrier height is closely related to the density of boundary dislocations in the case of small angle type boundaries.
  • Structure Analysis of GaN Thin Film with Inversion Domains by High Voltage Atomic Resolution Microscopy

    pp. 1542-1546

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    DOI:10.2320/matertrans.43.1542

    The atomic structures and surface morphologies of three types of GaN films were investigated by high voltage atomic resolution microscopy (HVARM). By HVARM, each atomic column of Ga and N could clearly be resolved and the polarity of the film and inversion domains could be directly determined. The GaN film was grown on a sapphire substrate by molecular beam epitaxy (MBE) after nitridation of the sapphire surface. Inversion domains (IDs) crossed the whole film to the surface and made small pyramids on the surface. The small pyramids had Ga-polarity and the rest had N-polarity. A GaN film with In exposure during film growth had an almost Ga-polarity flat surface. In exposure process reduced the density of inversion domains that have a N-polarity. While a GaN film grown on an AlN buffer layer was unipolar, with a Ga-polarity. HVARM observation revealed that the density of the IDs determine the qualities and the polarity of the film.
  • HRTEM Observation and Atomic Modeling of α/β Interphase Boundary in a Ti-22V-4Al Alloy

    pp. 1547-1551

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    DOI:10.2320/matertrans.43.1547

    In order to study interface boundary structures, a three dimensional Near-Coincidence Site lattice (Near-CSL) model based on Bollmann’s O-lattice method has been proposed. Previous studies have shown that intragranular α-phase precipitates in a Ti–22V–4Al alloy have a morphology of lath-like, with two well defined facet planes. A detailed TEM observation of the laths has been made in this study and the results correlated to the present analysis. Accommodation mechanism of misfit dislocations and the Burgers vector of the defects observed at the broad face and side facet plane of the intragranular α-phase precipitates have been determined. The structural ledges at the side facet, as predicted by the present analysis, is consistent with HRTEM observation. The Burgers vector of misfit dislocations at the broad face is in good agreement with that previously reported for a Zr–2.6Nb alloy, that has almost the same lattice parameter ratio as the Ti–22V–4Al alloy used in the present study. Although the Burgers vector of the defects on the broad face can be determined by HRTEM observation, the boundary plane is not well defined. However, it can be demonstrated that the interface boundary structure of the broad face can be rationalized from the results of both the Near-CSL analysis and the HRTEM observations.
  • Internal Friction Analysis of CaO-Doped Silicon Carbides

    pp. 1552-1556

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    DOI:10.2320/matertrans.43.1552

    Internal friction originated from sliding and diffusional flow along grain boundaries was monitored (as a function of both temperature and damping frequency) in model SiC polycrystals with an intergranular SiO2 glassy film. Emphasis was placed on the change of the internal friction characteristics upon additional Ca cation. It is expected that Ca cation segregates to the intergranular glass phase and modifies its network structure. The presence of a relaxation peak of internal friction due to grain-boundary sliding enabled quantitative evaluation of the activation energy for viscous flow of the intergranular glass and the related viscosity magnitude. A peak-analysis procedure, according to the peak-shift method (i.e., monitoring peak shift upon damping frequency change), is proposed, which quantitatively revealed the activation energy for viscous flow in various impurity-doped intergranular glasses. The presence of chemical gradients at grain boundaries, namely the presence of families of boundaries within the SiC polycrystal with different chemical characteristics, has also been analyzed by taking into account the dependence of peak morphology on damping frequency.
  • Internal Friction Behavior of Alumina Polycrystals with Engineered Grain Boundaries

    pp. 1557-1560

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    DOI:10.2320/matertrans.43.1557

    Internal friction of alumina polycrystals with engineered grain boundaries was measured in the low frequency range of the torsional forced-vibration. The maximum shear stress amplitude was 50 MPa. Owing to the very high temperature reached during these experiments (1900 K), internal friction spectra could be detected which reveal new important features of the intrinsic rheological behavior of alumina grain boundaries. The monotonically rising internal friction background and the torsional creep behavior, both of a diffusive origin, were systematically characterized upon modifying the grain-boundary structure of alumina with the addition of selected cation dopants (e.g., Titanium and Lutetium). The combined characterizations of internal friction spectrum and torsional creep rate enabled us to precisely assess the role of different cation dopants on “locking” or promoting the diffusive grain-boundary flow, which governs polycrystal deformation.
  • Grain Boundary Sliding and Atomic Structures in Alumina Bicrystals with [0001] Symmetric Tilt Grain Boundaries

    pp. 1561-1565

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    DOI:10.2320/matertrans.43.1561

    Orientation controlled alumina bicrystals were fabricated by a hot joining technique at 1773 K in air to obtain [0001] symmetric tilt boundaries including coincidence grain boundaries. The grain boundary energies were measured by the thermal grooving technique, and they were found to strongly depend on the grain boundary character. Atomic structures of those grain boundaries were observed by high-resolution electron microscopy (HREM). It was found that the atomic structures did not always correlate with the grain boundary energies. This finding indicates that the grain boundary energy originates not only from the atomic bonding on the grain boundary but also from the strain of the grain interior in the vicinity of the boundary. The grain boundary sliding was also investigated by the high-temperature creep test. As the results, the grain boundaries with the same energy showed different sliding behavior. The occurrence of grain boundary sliding is considered to depend on the atomic bonding of a grain boundary.
  • Effect of Chemical Bonding State on High-temperature Plastic Flow Behavior in Fine-grained, Polycrystalline Cation-doped Al2O3

    pp. 1566-1572

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    DOI:10.2320/matertrans.43.1566

    High-temperature plastic deformation characteristics such as flow stress and elongation to failure in polycrystalline Al2O3 with an average grain size of about 1 \\micron is remarkably changed by the doping of 0.1 mol% YO1.5, SiO2, TiO2 or ZrO2 at 1400°C under an initial strain rate of 1.2×10−4 s−1. The difference in the flow stress and the tensile ductility is considered to be originated from change in the grain boundary chemistry in Al2O3 due to segregation of the dopant cation in the vicinity of the grain boundary. A change in the chemical bonding state in the cations-doped Al2O3 is examined by first-principle molecular orbital calculations using DV-Xα method based on [Al5O21]27− cluster model. Chemical shift observed in electron energy-loss spectra (EELS) at the grain boundary due to the dopant segregation can be roughly reproduced by the molecular orbital calculations. A correlation is found between the flow stress and product of net charges of aluminum and oxygen ions. Moreover, the tensile elongation to failure seems to correlate with bond overlap population between Al and O. The change in the chemical bonding strength at the grain boundary must dominantly affect to the grain boundary diffusion and bond strength at the grain boundary, and thus seems to be an important factor to determine the plastic flow behavior in polycrystalline Al2O3.
  • Impact of Cu Local Interconnect on LSI Performance

    pp. 1574-1576

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    DOI:10.2320/matertrans.43.1574

    The effect of Cu local interconnect on intrinsic gate delay is analyzed. Beyond the 70-nm node, the intrinsic gate delay is limited by the local interconnect by taking account of electromigration in fine routings. Developments of materials and processes with improved electromigration for local interconnect are required.
  • Integration of High Performance CMOS Logic LSI by Applying Cu Wiring to SiLKT.M./SiO2 Hybrid Structure

    pp. 1577-1584

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    DOI:10.2320/matertrans.43.1577

    This paper describes a 0.13-\\micron CMOS made by using highly reliable copper and SiLKT.M. (DOW CHEMICAL) interconnection technologies. We propose a hybrid interlayer structure with SiLKT.M. at the trench level and SiO2 at the via level to improve electrical properties, mechanical strength, and reliability. Using these technologies, we made a fully functional 1.5-Mbit SRAM macro and investigated the reliability of its copper wiring in terms of electromigration.
  • Tungsten Nitride Deposition by Thermal Chemical Vapor Deposition as Barrier Metal for Cu Interconnection

    pp. 1585-1592

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    DOI:10.2320/matertrans.43.1585

    A CVD-WN film is deposited as a barrier metal for copper interconnection using thermal CVD (Chemical Vapor Deposition) of WF6/NH3/SiH4 gases. Deposition of CVD-WN film with a resistivity of less than 300 \\microΩ·cm at a temperature of 400°C or less has been realized for the first time in the world. The deposited WN film is proved to be excellent in barrier properties and able to prevent Cu diffusion even with the film thickness of 6 nm as well. It is also proved that the CVD film is superior to a sputtered barrier metal in coverage and ECD (Electro-chemical deposition) filling properties. XPS analysis showed that adhesion of the CVD film to low-k materials is deteriorated by the existence of F at the interface between the WN film and low-k. Chemical surface pretreatment for low-k materials proves to restrain the pile-up of F at the interface and improve adhesion.
  • Electrochemical and Simulative Studies of Trench Filling Mechanisms in the Copper Damascene Electroplating Process

    pp. 1593-1598

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    DOI:10.2320/matertrans.43.1593

    The role of additives in copper electroplating baths in the damascene process has been investigated. We proposed a bottom-up filling model and confirmed it by comparing the experimental and simulation results. Janus Green B and Basic Blue 3 which absorb on the copper surface and suppress copper deposition were examined for additive use to improve filling capability. Damascene copper grew uniformly in the bath that contained Basic Blue 3. But it grew preferentially from the bottom of the trench for Janus Green B. Addition of Janus Green B produced a continuous concentration gradient in the sub-micron trench when the additive’s diffusion rate and consumption rate on the copper surface were well balanced. We estimated filling profiles from numerical simulation using parameters that were determined by an electrochemical method. These profiles agreed well with the experimental results.
  • Copper Wires for High Speed Logic LSI Prepared by Low Pressure Long Throw Sputtering Method

    pp. 1599-1604

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    DOI:10.2320/matertrans.43.1599

    Copper sputtering method for fabrication of high performance logic LSI was studied. Extension of target to substrate distance is effective to improve step coverage of sputtered film combined with reduced operation pressure. Step coverage of low pressure long throw sputtering method also strongly depends upon the feature size of trenches and holes which are formed on silicon wafer. Sub-micron holes and trenches are successfully filled with copper by using this sputtering process followed by re-flow annealing process. Hydrogen annealing process prior to the sputtering deposition on via openings is also investigated to realize good conductivity through the via. This process results in the reduction of copper oxide at the surface of copper film. Using these newly developed processes, 0.2 \\micron node BiCMOS LSI with 4 level copper interconnects was successfully fabricated and high performance of the copper interconnect system was clearly demonstrated.
  • Application of High-pressure Annealing Process to Dual Damascene Copper Interconnections

    pp. 1605-1614

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    DOI:10.2320/matertrans.43.1605

    We have carried out a detailed study of the relevant process techniques by using high-pressure annealing in order to embed Cu interconnections flawlessly into via holes or trenches, and have thereby identified a process technique increase the reliability of electrical circuits. In the present study, Cu interconnections were subjected to heat treatment in a high-pressure argon gas atmosphere (150 MPa) with the prospect that such a process might lead to optimized conditions for creating the embedded Cu interconnection by preventing the occurrence of minute voids, improving the distribution of poly-crystalline orientation and improving the adhesive strength between Cu and TaN (barrier layers), thereby increasing the reliability of Cu interconnections and consequently improve the yield. This paper describes the effects of the high-pressure annealing process for dual damascene Cu interconnections in some detail.
  • Ni-B Electroless Plating as Cap Layer for Ag Multi-Level Metallization

    pp. 1615-1620

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    DOI:10.2320/matertrans.43.1615

    It is essential to decrease RC delay in order to increase the IC device speed. Cu is now adopted to decrease the resistivity of interconnects. Several low k materials have been studied to decrease the dielectric constant. Ag is one of the candidates after Cu as a low resisitivity interconnect metal although it has not yet been adopted in the actual products. It is also studied to decrease the effective dielectric constant values by considering the device configuration such as the cap layer only on the metal. Co–W–P is reported as one of the cap material candidates for Cu. Ni–B is proposed herein as the cap material for Cu and Ag. The study results confirmed that an Ni–B layer can be selectively deposited on Ag metal by the electroless plating method, using DMAB (dimethylamine borane) as the reducing agent; that a deposition rate of 150 nm/min and a B content of 3.2 at% can be obtained under conditions of pH 10 and 353 K; that an Ni–B layer with a 3.2 at%B content provides a barrier effect that prevents Ag and Cu diffusion through thermal processes (in this case, Ni–B remained crystalline before and after a thermal process); and that an Ni–B layer with a 13.5 at%B content does not provide a barrier effect that would prevent Cu diffusion through thermal processes (in this case, the Ni–B structure changed from amorphous to crystalline after the thermal process). Damascene interconnects with Ag metal and a Ni–B cap layer were also formed as part of the trial effort.
  • Increase in Electrical Resistivity of Copper and Aluminum Fine Lines

    pp. 1621-1623

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    DOI:10.2320/matertrans.43.1621

    The resistivities of thin films and fine lines of copper (Cu) and aluminum (Al) were measured by a resistance ratio method (referred to as “RR method” hereafter), which measures the ratio of room-temperature resistance to liquid-nitrogen-temperature resistance. This method can predict resistivity exactly without the need for precise and detailed line or film dimension measurements. Thinner films and finer lines have higher resistivities in the case of both Cu and Al, with Cu showing larger resistivity increases (films and lines) than Al . From the present data, we estimated the electron mean free path of Cu to be 55 nm, which is close to most of the previously reported values, and that of Al to be 22 nm. Line resistivities depend not only on the line width but also on line thickness. We propose a simple equation for expressing line resistivity in terms of line thickness and width.
  • Effect of Annealing Atmosphere on Void Formation in Copper Interconnects

    pp. 1624-1628

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    DOI:10.2320/matertrans.43.1624

    In order to understand the void formation mechanism in electroplated Cu interconnects used in Si-semiconductor devices, microstructure of Cu/CuO/Cu layered films which were prepared on the Si3N4/Si substrates by the sputter-deposition technique was observed by transmission electron microscopy (TEM) and scanning ion microscopy (SIM). A high density of macro and micro voids were observed in the samples annealed in atmosphere containing hydrogen, whereas no voids were observed in the samples annealed in Ar atmosphere. TEM observation suggested that a small amount of oxygen contained in the Cu films (even a native oxide layer) formed water vapor at elevated temperatures, causing formation of the micro-voids when the samples were annealed in hydrogen atmosphere. The present result suggested that the void formation in the electroplated Cu films was induced by existence of impurities such as oxygen in the Cu films, and that the void growth was strongly enhanced by annealing in hydrogen atmosphere.
  • Evolution of Grain and Micro-Void Structure in Electroplated Copper Interconnects

    pp. 1629-1632

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    DOI:10.2320/matertrans.43.1629

    A detailed investigation has been carried out into the evolution of grain size and grain orientation in electroplated Cu interconnection lines. The specimens were annealed to produce a range of different grain size distributions. Very accurate grain size distributions were obtained from extensive TEM observations of a large number of specimens, followed by computer tracing of grain boundaries. Although annealing causes the average grain size to increase, very small grains are found to persist in the distribution, even when the average grain size is large. These small grains have been investigated in detail to determine the reason for their thermal stability. In addition, the occurrence and redistribution of small micro-voids has been investigated for wafers with different grain sizes. These voids are believed to be associated with seams which are formed in the Cu lines during plating.
  • Formation of Slit-Like Voids at Trench Corners of Damascene Cu Interconnects

    pp. 1633-1637

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    DOI:10.2320/matertrans.43.1633

    Stress voiding was investigated in damascene Cu lines embedded in Ta/TaN/SiO2/Si. Microstructure was observed before and after heat treatment at 723 K using a focused ion beam (FIB) technique. The distribution of thermal stress was calculated using a three-dimensional finite element method (FEM). FIB observation revealed that slit-like voids were formed at trench shoulders both before and after heat treatment. FEM calculation indicated that a large shear stress concentration occurred at the voided sites. The coincidence between the FIB observation and the FEM calculation suggests that the slit-like voids were formed by shear-mode delamination of Cu from the Ta/TaN barrier layer.
  • Dry Sliding Wear Behavior of A2218/Al3Fe Composites Fabricated by Plasma Synthesis Method

    pp. 1638-1646

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    DOI:10.2320/matertrans.43.1638

    The wear behavior of an A2218 alloy and an A2218/Al3Fe composite fabricated by the plasma synthesis method was investigated under dry sliding conditions. The wear tests were carried out at a sliding speed of 0.2 m/s and at the load range of 3–600 N. The variation of wear rate with applied load is composed of two regions for both materials and the two regions belong to mild wear. On the basis of observation and analysis on the worn surface, subsurface and the wear debris, the wear mechanism acting on each region was identified. At low loads, oxidative wear is the main wear mechanism and at high loads, the wear is controlled mainly by delamination and adhesive wear mechanism and oxidative wear is an additional wear mechanism. The A2218/Al3Fe composite exhibits an improved wear resistance at lower loads. On the contrary, at higher loads, due to the presence of the mechanical mixed layer on the worn surface of the A2218 monolithic alloy, both the monolithic alloy and composite exhibit similar wear rate.
  • Three Regions of Dislocation Creep in In Situ TiB Fiber-Reinforced α-Titanium Matrix Composite

    pp. 1647-1652

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    DOI:10.2320/matertrans.43.1647

    The steady-state creep behavior of metal matrix composites was analyzed via consideration of two accommodation processes, diffusion and plastic, which are inevitable for the materials to continue creep deformation. The creep experiments were performed using a model material, in situ TiB fiber-reinforced pure α-Ti matrix composite, which has a good interfacial bonding, a moderate diffusional-accommodation rate and no fine oxide dispersions. A sigmoidal curve of strain rate and stress relation in a double logarithmic plot was observed, indicating the presence of three deformation regions: plastic-accommodation-control region, diffusional-accommodation-control region and complete diffusional-accommodation region, at high, middle and low stresses, respectively. The activation energies in the three regions were close to those of volume, interface, and volume diffusion of α-Ti, respectively.
  • Volume Fraction Dependence of Plastic and Diffusional Accommodation in High-Temperature Deformation of Ti/TiB In Situ Composite: Appearance and Vanishing of Load Transfer Effect

    pp. 1653-1657

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    DOI:10.2320/matertrans.43.1653

    The appearance and vanishing of the load transfer effect in the steady-state condition of composites’ creep were analyzed via consideration of two accommodation processes, diffusion and plastic, which are inevitable for composites to continue creep deformation. Creep experiments were performed using model materials, Ti–5, 15 and 20 vol%TiB in situ composites, which have moderate diffusional-accommodation-controlled creeps, good interfacial bonding and no fine dispersions. With higher strain rates than the diffusional-accommodation-controlled creep, the stress exponent of all composites was 4.5, similar to that of α–Ti, 4.3, and the strain rates decreased with increasing volume fraction of the reinforcements. Variations of the strain rate agreed with the prediction by the self-consistent potential method, and thus the load transfer effect was confirmed in this region. With strain rates near the diffusional-accommodation-controlled creep, the stress exponents of Ti–15 and 20 vol%TiB decreased to 2. With a strain rate lower than the diffusional-accommodation-controlled creep, the strain rates of the composites did not decrease with increasing volume fraction, and vanishing of the load transfer effect was observed.
  • Numerical Study on Microstructures and Their Rheological Properties in Electrorheological Fluids

    pp. 1658-1662

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    DOI:10.2320/matertrans.43.1658

    We study the relation between microstructures of electrorheological (ER) fluids and their viscosity change by performing Brownian dynamics simulations of a model ER system both in a static state and in a simple steady shear. From large-scale three-dimensional simulations, it is found that (1) under no shear flow there are two principal phases in microstructural changes: first aggregation of particles into chains oriented along the field direction, and the subsequent slow coalescence of chains into columns, and (2) under a simple steady shear there are three stages in viscosity changes with increasing the field: Newtonian at a weak field, non-Newtonian at a moderate field, and Bingham plastic with yield stress at a high field.
  • Model Calculation of the Damage Rate Dependence of Yield Stress Change in an Irradiated Fe-Cu Model Alloy

    pp. 1663-1669

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    DOI:10.2320/matertrans.43.1663

    The damage rate dependence of the yield stress change in a neutron-irradiated Fe–Cu model alloy was analyzed by a model calculation. The model was based on the rate theory, and focused on the description of the nucleation and growth of point defect clusters and copper clusters. The binding between copper atoms and vacancies and the effect of cascade damage which directly creates small point defect clusters were incorporated in this model. The instability of small point defect clusters caused by thermal dissociation was also included. From the result of the calculation, the yield stress changes were estimated using the Orowan model and the Russel-Brown model. As a result of this calculation, it was clarified that copper clusters are the main factor of yield stress change in almost all irradiation stages below 0.1 dpa. The contribution of copper clusters to yield stress change increased with decreasing damage rate. The nature of the damage rate dependence is not affected by the copper-vacancy binding but by the sink strength, which changes dynamically throughout irradiation.
  • Cooling Rate Evaluation for Bulk Amorphous Alloys from Eutectic Microstructures in Casting Processes

    pp. 1670-1675

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    DOI:10.2320/matertrans.43.1670

    The structural features of metallic glasses depend on the cooling rate of the melt. The cooling rates for casting processes which are typically employed for preparation of bulk metallic glasses are suggested from microstructures of an eutectic Al–33 mass%Cu model alloy. The interlamellar spacing λ of eutectic Al–CuAl2 rod-shaped specimens of 50 mm in length and 2 to 5 mm in diameter has been determined by optical microscopy and scanning electron microscopy. From the measured interlamellar spacings ranging from λ=0.18 to 0.5 \\micron, the solidification front velocities and the cooling rates at different positions in the as-cast samples are derived. The decisive effect of the diameter of cast rods is confirmed. For a centrifugal casting technique, the maximum cooling rate decreases from 730 to 95 K/s when the diameter increases from 2 to 5 mm. Moreover, it is revealed that the local cooling rates decrease significantly from the bottom towards the top of the rods. From the estimated cooling rates at a fixed rod diameter the centrifugal casting technique is assessed as superior to other methods applied, namely copper-mould casting and suction casting. The estimated cooling rates are compared with literature data for glass-forming alloys.
  • Two-Way Shape Memory Effect Induced by Bending Deformation in Ductile Cu-Al-Mn Alloys

    pp. 1676-1683

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    DOI:10.2320/matertrans.43.1676

    Martensitic transformation temperatures and the two-way shape memory effect (TWME) in ductile Cu–Al–Mn alloys deformed at room temperature were investigated using differential scanning calorimetry (DSC) and the bending test. The Ms and Af temperatures were found to increase slightly with increasing degree of pre-deformation, while the Mf and As temperatures decreased. The TWME is strongly influenced by the Ms temperature and the grain size of the specimens, i.e., the highest degree of TWME is always obtained in the specimens with Ms≈−100°C and the TWME increases with increasing ratio of grain size to specimen width, where the maximum value of TWME=3.2% was obtained in the present study. In situ observation of the surface relief at several temperatures on the deformed specimens was also performed and it was found that some restricted martensite variants grow and shrink with temperature change without the typical self-accommodation microstructure in the specimens with large TWME.
  • Low Resistance TiAl Ohmic Contacts with Multi-Layered Structure for p-Type 4H-SiC

    pp. 1684-1688

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    DOI:10.2320/matertrans.43.1684

    The effect of the Al concentration and layer structure on the electrical and microstructural properties of TiAl Ohmic contacts for p-type 4H–SiC were investigated. The Al concentration was found to effect strongly on these contact properties, and the specific contact resistance of 1×10−5 Ω-cm2 was obtained for the TiAl contacts with the Al concentration higher than 77 at% after annealing at 1000°C. However, agglomeration of Al was observed after annealing, which caused the rough surface morphology. On the other hand, the TiAl contacts with the Al concentration lower than 75 at% showed non-Ohmic behavior and had smooth surface morphology after annealing at 1000°C. It was found from X-ray diffraction analysis that the interface structures were strongly influenced by the Al concentrations of the TiAl contacts. For the TiAl contact with high Al concentration, formation of Al3Ti, Ti3SiC2, and Al4C3 was observed, and for the TiAl contact with low Al concentration, formation of Al3Ti, Ti3SiC2, and Ti5Si3 was observed. It was concluded that the electrical property of the TiAl contact was not affected by the number of TiAl layers and was very sensitive to the Al concentration in the TiAl contacts.
  • New Chemical Layered Structure in Ti-Cr Alloys

    pp. 1689-1695

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    DOI:10.2320/matertrans.43.1689

    The new chemical layered structure, referred to as a chemical stripe structure, was found as a metastable state in the (bcc→hcp+C15) metastable phase separation in the Ti–Cr alloys. Note that a metastable separation is here defined as a separation characterized by a positive value of the second derivative of Gibbs free energy with respect to chemical composition. The experimental data showed that the layered structure had periodicity of 4×d002, about 0.72 nm, and chemical ordering of Ti:Cr=3:1, and that a lot of antiphase boundaries with phase shifts of π⁄2 and π were involved in the structure. The coherent length of the layered structure was estimated to be about 15 nm along the modulated direction. In addition to these features, it was found that the layered structure was formed from the local bct state as another metastable state, not directly from the zone structure consisting of one Cr layer. On the basis of these data, the physical origin of the formation of the chemical layered structure is also discussed.
  • Residual Thermal Phase Stresses in α-γ Fe-Cr-Ni Alloys Measured by a Neutron Diffraction Time-of-Flight Method

    pp. 1696-1702

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    DOI:10.2320/matertrans.43.1696

    Residual thermal phase elastic strains in rod specimens and powders with particle size from 150 to 250 \\micron of α-γ Fe–Cr–Ni alloys quenched from 1273 to 273 K, were measured by means of a high-resolution neutron powder diffractometer using a time-of-flight (TOF) method. Diffraction patterns were analyzed by the Rietveld method and the single-peak method. Residual thermal phase strains obtained in the rod specimens or the powders from the both methods are positive, i.e., tensile, for austenite (γ) phase and negative for ferrite (α) phase, being consistent with the prediction from the difference in thermal expansion coefficients of the constituent phases. The Rietveld analysis combined with the TOF method is considered to give the most reliable results of the average thermal phase strain. The results by the single-peak analysis suggest that thermal phase strain shows [hkl] dependence; the absolute value of thermal phase strain is decreased with increasing of the Young modulus along the [hkl] direction.
  • Fatigue Properties of the Ti-Ni Base Shape Memory Alloy Wire

    pp. 1703-1706

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    DOI:10.2320/matertrans.43.1703

    The effect of the alloy compositions on the cyclic bending fatigue behaviors of the Ti–Ni base shape memory alloy wires was investigated using a rotary bending fatigue tester specially designed for wires. The fatigue test results were discussed in connection with the static tensile properties. The results were summarized as follows. (1) The martensite inducing stress increased with the increasing of the Ni-content. (2) The fatigue life decreased with the increasing of the test temperature and the Ni-content. (3) The fracture pattern resulted from the rotary bending fatigue test has changed from the dimple pattern under the room temperature and small cyclic strain to the quasi-cleavage fracture under the high temperature and large cyclic strain.
  • Oxidation and Opening of Well-Aligned Carbon Nanotube Tips

    pp. 1707-1710

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    DOI:10.2320/matertrans.43.1707

    We report here that well-aligned carbon nanotubes with open tips can be directly obtained by introducing carbon dioxide during the chemical vapor deposition process. In-situ oxidation of carbon nanotubes by carbon dioxide results in the strip off of nanotube tips, however, without damaging the nanotube alignment. Such oxidation of aligned nanotube arrays, whose tips are all on the array surface, is more efficient than the oxidation of disordered nanotubes. Aligned carbon nanotube arrays with open tips have potential applications in field emission, filter membrane, and energy storage.
  • A Mechanism of Porosity Distribution in A356 Aluminum Alloy Castings

    pp. 1711-1715

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    DOI:10.2320/matertrans.43.1711

    The problem of porosity and shrinkage defects in metal casting is complex. Over the years there have been debates on the mechanisms responsible for their formation. In this study A356 aluminum alloy with different hydrogen contents in the melts were cast in a permanent mold and the porosity content and thermal parameters were measured. A simple mechanism was proposed to explain the porosity distribution in the casting. In the mechanism both the roles of interdendritic feeding resistance by Darcy’s law and kinetic hydrogen diffusion into the pore are involved. By differentiating one factor in the model, and comparing the prediction with the experimental data, the study suggests that both factors should be taken into consideration to satisfactorily explain the porosity distribution in the castings.
  • Dynamical Study of Spatio-Temporal Structural Fluctuations in the Intermetallic Compound Nickel-Titanium during Radiation-Induced Crystalline-to-Amorphous Transformation

    pp. 1716-1718

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    DOI:10.2320/matertrans.43.1716

    We have studied nanostructural fluctuations during electron irradiation in the ordered intermetallic compound NiTi by performing irradiation experiments with a high-resolution high-voltage electron microscope (HVEM) and molecular dynamics (MD) simulations. It was observed that atom clusters formed and disappeared repeatedly during irradiation, which results in the structural fluctuation. Comparing the experimental results with the MD simulations, we conclude that such spatio-temporal structural fluctuations under irradiation can be accounted for the evolution of metastable nanoclusters. The present study demonstrates the usefulness of high-resolution HVEM for in situ studies of dynamic, nanoscale fluctuation phenomena.
  • Thermoelectric Properties of Hot-pressed Boron Suboxide (B6O)

    pp. 1719-1723

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    DOI:10.2320/matertrans.43.1719

    Boron suboxide (B6O) sintered bodies were prepared by a solid state reaction and a hot pressing method. The thermoelectric properties of B6O were compared with those of B4C. The electrical conductivity was smaller than that of B4C, and the Seebeck coefficient was twice as large as that of B4C indicating p-type conduction. The hopping conduction of electronic charge carriers was suggested from the temperature dependencies of the electrical conductivity and mobility. The thermal conductivity was greater than that of B4C. The thermoelectric dimension-less figure-of-merit increased with increasing temperature, and was 0.62×10−3 at 1000 K. This value was almost in agreement with that of B4C.
  • Surface Tension and Wettability of Liquid Fe-16 mass%Cr-O Alloy with Alumina

    pp. 1724-1731

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    DOI:10.2320/matertrans.43.1724

    The surface tension of liquid Fe–16 mass%Cr alloys and its wettability with alumina substrate at 1823 K were determined as a function of oxygen activity which was less than 0.0070 using the sessile drop technique. Oxygen was found to be strongly surface active in liquid Fe–16 mass%Cr alloys. The variation of surface tension of Fe–16 mass%Cr alloys with oxygen activity can be described by the following equation: σlg=1750−304ln(1+383aO) mN/m. The addition of 16 mass%Cr to liquid iron caused an increase in the contact angle between the droplet and alumina substrate. The contact angle between liquid Fe–16 mass%Cr–O alloys and an alumina substrate remains almost constant (at about 150°) in the present experimental range of oxygen activity. The interfacial tension between liquid Fe–16 mass%Cr–O alloys and solid alumina, calculated using Young’s equation, can be expressed by the following equation: σsl=2270−315ln(1+298aO) mN/m. The work of adhesion between liquid Fe–16 mass%Cr–O alloys and alumina was virtually constant in the present experimental range of oxygen activity.
  • Hydriding-Dehydriding Properties of Mg-Rich Mg-Ni-Nd Alloys with Refined Microstructures

    pp. 1732-1736

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    DOI:10.2320/matertrans.43.1732

    The hydriding/dehydriding characteristics, pressure-composition isotherms and X-ray diffractographs of Mg-rich Mg–Ni–Nd alloys fabricated by different solidifying processes (mold-casting, water-quenching, or melt-spinning) are compared and discussed in relation with their microstructures observed by SEM . Among them, the alloy prepared by melt-spinning and subsequent annealing presents best hydrogen-storage properties. It can quickly absorb hydrogen up to ∼5.0 mass% above 423 K and can wholly desorb it above 453 K with moderate speeds. These enhanced absorption/desorption kinetics are exclusively caused by a refined microstructure of the alloy involving Mg2Ni and Nd2H5 precipitates in a Mg matrix as well as by a catalytic action of the Nd-hydride.
  • Bulk Glassy Pd-Cu-Ni-Cr-P Alloys Containing Dispersed Nanoparticles Prepared by Copper-Mold Casting

    pp. 1737-1740

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    DOI:10.2320/matertrans.43.1737

    This paper presents a finding of producing nanoparticle dispersed bulk glassy alloys in a Cr-modified Pd–Cu–Ni–P (PCNP) system by using a copper-mold casting technique. The PCNP alloys with the compositions near Pd40Cu30Ni10P20 possess high glass-forming ability (GFA) resulting from their exceptionally small nucleation rate. Cr additions in Pd40Cu30−xNi10P20Crx (x=1–5 at%) alloys result in homogenous nucleation of Ni33Cr33P34 when these alloys are solidified from liquid state. The composition of the remaining liquid changed to that near Pd40Cu30Ni10P20 by the precipitation of Ni3Cr33P34 and the matrix can be easily solidified to a glassy phase, leading to the formation of a mixed structure consisting of nanoscale Ni33Cr33P34 particles embedded in a glassy phase. The particle size is significantly affected by alloy composition and cooling rate.
  • Hydrogenation Properties of Partially Remelted Mg-Ni Alloys

    pp. 1741-1747

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    DOI:10.2320/matertrans.43.1741

    Thixotropic Mg alloys have been focused only on the aspect of semi liquid forming until now. In the present study, with the aim of improving hydrogen storage capacity, we kept an eye on the microstructural evolutions and chemical properties of thixotropic microstructure in Mg-(3, 6, 9, 12 mass%) Ni alloys by using a partial remelting process. According to the results of pressure-composition-isotherm (PCT) measurements and image analysis, Mg rich solid phases were regarded as the hydrogen absorbing phases and eutectic regions (quenched liquid phase) were considered as the catalyzer to improve hydrogenation kinetics. Especially, the hydrogenation properties depended on the properties of globules and liquid fractions.
  • The Microstructural Effects on Tensile Properties and Erosion Wear Resistance in Upper Bainitic ADI Related to Variation in Silicon Content

    pp. 1748-1757

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    DOI:10.2320/matertrans.43.1748

    The Effects of austempering duration and silicon content on the particle erosion wear resistance of high strength upper bainitic ADI were characterized. The results showed that the amount of retained austenite dominantly affects tensile strength and variation of wear resistance. For a sample with higher silicon content, the austempering duration should be prolonged to ensure complete bainitic transformation. In addition, a tiny increase in carbide formation can certainly play an important role in debasing tensile strength and erosion wear resistance. This results in an alternation in erosion behavior. However, hardness cannot be correlated to wear resistance. Erosion–induced phase transformation of retained austenite phase will eventually produce ε-carbide, which thus promotes the erosion wear rate.
  • Microstructural Analyses of Grain Boundary Carbides of Tempered Martensite in Medium-Carbon Steel by Atomic Force Microscopy

    pp. 1758-1766

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    DOI:10.2320/matertrans.43.1758

    The microstructures of JIS-SCM440 steel (containing 0.4%C, 1%Cr and 0.2%Mo; mass%) that had been tempered at 723 K were observed by atomic force microscopy (AFM). In an AFM image of an electropolished surface, carbides and blocks were clearly distinguished, since the rate of electropolishing depends on the crystal phase and crystal orientation on the surface. However, the prior austenite (γ) grain boundaries could not be clearly recognized. Therefore, an AFM image of the surface that had been etched with picric acid was taken at the same location on the surface. Since picric acid selectively etches prior γ grain boundaries, the boundaries were clearly visible in the image. The two AFM images were then superimposed to reveal prior γ grain boundaries, carbides, and blocks. Finally, the number and sizes of the carbides on the prior γ grain boundaries were examined.
  • Synthesis and Mechanical Properties of Cu-Based Cu-Zr-Ti Bulk Glassy Alloys Containing ZrC Particles

    pp. 1767-1770

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    DOI:10.2320/matertrans.43.1767

    Bulk glassy Cu60Zr30Ti10 composites containing ZrC particles up to 9 vol% were formed in a rod with a diameter up to 2 mm by copper mold casting. The ZrC particle size was about 4 \\micron. No appreciable second crystalline phases are observed in the bulk glassy composites. The glass transition temperature (Tg), crystallization temperature (Tx) supercooled liquid region (ΔTx=TxTg) and liquidus temperature (Tl) of the glassy matrix remain almost unchanged in the volume fraction (Vf) range of ZrC particles up to 9%. The Young’s modulus (E), compressive yield strength (σc,y) and compressive fracture strength (σc,f) for the composite alloys increase gradually from 114 to 123 GPa, 2020 to 2190 MPa and 2150 to 2220 MPa, respectively, with increasing Vf from 0 to 9%. The plastic elongation (εp) decreases from 1.3% at 0 vol% to 0.4% at 9 vol%. The present study shows a possibility of synthesizing the high strength composite Cu-based bulk glassy alloys containing ZrC particles with good ductility by a process of arc melting and copper mold casting.
  • Formation of Bulk Glassy Ni-(Co-)Nb-Ti-Zr Alloys with High Corrosion Resistance

    pp. 1771-1773

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    DOI:10.2320/matertrans.43.1771

    Ni60−xCoxNb20Ti10Zr10 (x=0–40 at%) glassy alloys were formed by melt spinning and a large supercooled region of over 40 K for the 0–20 at%Co alloys was found. The Ni60−xCoxNb20Ti10Zr10 (x=0–20 at%) alloys have high glass-forming ability leading to the formation of bulk glassy rods with diameters up to 1.5 mm by copper-mold casting. The bulk glassy alloys with 0–20 at%Co exhibited nearly the same anodic polarization behavior in 1 N HCl solution. They were spontaneously passivated with a low passive current density of the order of 10−2 A/m2 and no pitting corrosion due to anodic polarization with a potential sweeping up to 2.0 V vs. Ag/AgCl occurred, indicating their high corrosion resistance in the aggressive acid.
  • Influence of Soldering Conditions on Void Formation in Large-area Solder Joints

    pp. 1774-1777

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    DOI:10.2320/matertrans.43.1774

    This paper aims to clarify the process factors for applying the void free soldering process to practical use. Solder cleanliness lessens the influence of the heating rate on the void formation during soldering. The lowest vacuum for the purpose of void free soldering for cleaning and Ag coating, and for soldering were 2×10−3 Pa and 4×10−1 Pa, respectively. Cooling in N2 lessens the void formation during soldering. The allowable standing time from the Ag coating to soldering is 25 h. The reliability of power modules made with this new process was satisfactory.
  • Energy Absorption in Closed-Cell Al-Zn-Mg-Ca-Ti Foam

    pp. 1778-1781

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    DOI:10.2320/matertrans.43.1778

    Cellular metallic foams have interesting potential for impact energy absorption. In this study, modification of solid in a closed-cell Al–Ca–Ti foam was carried out, adding the strengthening elements of Zn and Mg for enhancement of energy absorption. Samples with dimensions of 100×100×100 mm3 were examined at a dynamic strain rate corresponding to the crashing speed of automobiles. The compression deformation behavior of the Al–7Zn–0.5Mg–1.5Ca–1.5Ti (by mass%) foam was found to be different from that of the original Al–1.5Ca–1.5Ti (by mass%) foam. Plateau stress could be effectively enhanced with the combined effect of strengthening solid alloy and increasing the aspect ratio of cell wall thickness against cell edge length. Plateau stress of the modified foam was independent of strain rate, while the stress in Al–Ca–Ti foam exhibited a certain strain rate sensitivity. Plateau strain, designated as the strain where the compressive stress reached 1.5 times higher value than the yield stress, was also enhanced by the present modification. As a result, absorption energy was also effectively enhanced but was independent of strain rate as a result of the present modification.

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