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

MATERIALS TRANSACTIONS Vol. 57 (2016), No. 7

  • Special Issue on Recent Progress in Thermoelectrics -New Analyses and New Materials-

    pp. 1017-1017

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    DOI:10.2320/matertrans.MPR2016903

  • Enhanced Thermoelectric Properties of Silicon via Nanostructuring

    pp. 1018-1021

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    DOI:10.2320/matertrans.MF201601

    The effectiveness of thermoelectric (TE) materials at converting heat gradients into electricity, and vice versa, is quantified by the dimensionless figure of merit, ZT. Current TE materials, such as Bi2Te3 and PbTe, have ZT values of approximately 1, but contain highly toxic and/or rare elements, which limits their widespread use. However, Si is a non-toxic, inexpensive, and abundant element. Even though bulk Si exhibits good electrical properties, its lattice thermal conductivity (κlat) is high (>100 Wm−1 K−1), which results in a ZT value of approximately 0.01 at room temperature. If it were possible to lower the κlat of Si without altering the electrical properties, Si would be an ideal TE material. These changes can be realized by nanostructuring Si. In this review, we discuss the recent achievements in the enhancement of the TE properties of Si via nanostructuring. Based on these recent results, we also indicate some potential topics to investigate to enhance the TE properties of Si further.
  • Control of Phonon Transport by Phononic Crystals and Application to Thermoelectric Materials

    pp. 1022-1028

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    DOI:10.2320/matertrans.MF201606

    Phonon transport and thermodynamic properties of nanostructured materials have been investigated and utilized to improve thermoelectric performance for various materials. In nanostructures, phonon transport is completely different from that in bulk materials and results in dramatic enhancement in the thermoelectric performance. This article reviews the impact of nanostructuring on the phonon transport and mainly focuses on phononic crystal nanostructures, in which the wave nature of phonons also plays an important role. We demonstrate that it is important to efficiently scatter thermal phonons, which distribute to wide range of frequencies, with different phonon scattering mechanisms in the spatial domain. We also demonstrate an enhancement of thermoelectric property of polysilicon thin films by phononic crystal patterning.
  • Development of Thermoelectric Materials Consisting Solely of Environmental Friendly Elements

    pp. 1029-1034

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    DOI:10.2320/matertrans.MF201610

    A guiding principle for developing practical thermoelectric materials was constructed on the basis of simulations of thermoelectric properties using linear response theory. Al-Mn-Si C54-phase, Al-Mn-Si C40-phase and higher manganese silicide (HMS), all of which consist solely of cheap, environmentally friendly elements, were selected using the guiding principle. The validity of the strategy to develop practical thermoelectric material was clearly proved by our newly prepared HMS possessing a dimensionless figure of merit exceeding unity.
  • Material Dependence of the Thermoelectric Figure of Merit

    pp. 1035-1039

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    DOI:10.2320/matertrans.MF201612

    First-principles calculations were used to perform a cross-material investigation on the electronic structures of 13 parent compounds for thermoelectric materials. Boltzmann transport equations were used to calculate carrier doping level dependence of the Seebeck coefficient, electrical conductivity with respect to relaxation time, effective Lorenz numbers and the maximum possible thermoelectric figure of merit (ZeT) in the limit of zero phonon thermal conductivity. High ZeT was obtained only in semiconductors with finite band gaps. High ZeT for high doping level was achieved in compounds that had a steep density of states at the band edge. Calculations were combined with experimental transport properties to evaluate electron relaxation time of the samples. These analyses can be used to understand the nature of electron scattering mechanisms in specific thermoelectric materials and reduce the number of experiments required to develop new thermoelectric materials.
  • Angle-Resolved Photoemission Analysis of Electronic Structures for Thermoelectric Properties of Off-Stoichiometric Fe2–xV1+xAl Alloys

    pp. 1040-1044

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    DOI:10.2320/matertrans.MF201603

    The electronic states of Heusler (L21)-type off-stoichiometric Fe2–xV1+xAl alloys have been investigated by soft x-ray angle-resolved photoelectron spectroscopy (ARPES) to clarify the origin of the remarkable increase in thermoelectric power, which cannot be explained by the rigid-band model. In off-normal and normal ARPES, Fe2.05V0.95Al shows weakly dispersive bulk bands from the binding energy of 0.3 eV at the Γ point in the Γ–X and Γ–L directions, and an almost dispersionless one around 0.3 eV in the gap of dispersive bulk bands in the Γ–L direction, which is attributed to antisite Fe defects. At the Γ point, the bulk bands do not appear to cross the Fermi level EF, which is consistent with the rigid-band model for excess Fe content, but no band crossing EF is found at the X point. The antisite Fe defect states near EF might push up the band at the X point and cause p-type thermoelectric properties, which is unexpected for the rigid-band model. The change in electronic structures and thermoelectric properties for the off-stoichiometry and substitution by a forth element is discussed.
  • Effect of Anomalous Crystal Structure of Iron Aluminides Fe2Al5 and Fe4Al13: Low Phonon Thermal Conductivity and Potentiality as Thermoelectric Materials

    pp. 1045-1049

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    DOI:10.2320/matertrans.MF201607

    Iron Aluminide Fe2Al5 has a rigid framework of both fully occupied aluminum and iron sites and channels of partially occupied aluminum sites. On the other hand, Fe4Al13 possesses a large unit cell with 102 atoms. These complex and peculiar crystal structures bring a low phonon thermal conductivity. Here, we report the thermoelectric properties and discuss how the proposed chain structure and large unit cell can lead to a low phonon thermal conductivity. The calculated room-temperature phonon thermal conductivity by using the Wiedemann-Franz law is approximately 1.5 W/mK and 0.8 W/mK for Fe2Al5 and Fe4Al13, respectively. From the comparison with other Fe-Al alloys, which have neither plural partially occupied sites nor a large unit cell, we found that (1) the specific heat does not decrease at high temperature, i.e. aluminum atoms at partially occupied sites seem to be fixed rather than liquid behavior. (2) the speed of sound for Fe2Al5 and Fe4Al13 are almost identical among Fe-Al alloys, i.e. the average phonon group velocity of acoustic modes for Fe2Al5 and Fe4Al13 are not slower than that of Fe-Al alloys, (3) the electrical conductivities of Fe2Al5 and Fe4Al13 are lower than those of the other Fe-Al alloys. These results suggest that the low phonon thermal and electrical conductivities are brought by short relaxation times of both phonons and electrons due to chemical disorder such as the partially occupied sites.
  • First-Principles-Based Phonon Calculation and Raman Spectroscopy Measurement of RuGa2 and RuAl2 with High Thermoelectric Power Factors

    pp. 1050-1054

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    DOI:10.2320/matertrans.MF201605

    TiSi2-type intermetallic compounds RuGa2 and RuAl2 have narrow band gaps of approximately 0.3 eV and relatively large power factors of between 600–900 K. However, the maximum values of the dimensionless figure of merit, ZT, for RuGa2 and RuAl2 are 0.5 and 0.2, respectively, due to a high lattice thermal conductivity. We investigated the phonon properties of these compounds using first-principles calculation and Raman spectroscopy and developed a method to reduce the lattice thermal conductivity of both compounds. Phonon dispersion relations and density of states were obtained from a real-space force constants method based on supercells with finite displacements. The calculated zone-center wavenumbers were comparable to the experimental Raman wavenumbers. The phonon group velocities of the acoustic branches agreed with the experimental transverse and longitudinal speeds of sound. Differences between the phonon dispersion of RuGa2 and RuAl2 were attributed to the fact that Ga is a heavier and weaker bonding element than Al. According to the calculated partial phonon density of states, acoustic phonon modes of RuGa2 and RuAl2 are strongly influenced by the Ga site and Ru site, respectively. Substitution of heavier atoms onto these sites would effectively reduce the averaged phonon group velocity and lattice thermal conductivity of these compounds.
  • Thermoelectric Properties of Al-Mn-Si Based C54 Phase Containing Small Amount of C40 Phase

    pp. 1055-1058

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    DOI:10.2320/matertrans.MF201613

    The dimensionless figure of merit, ZT, of Al-Mn-Si based C54 phase was effectively increased using the precipitation of a small amount of Al-Mn-Si based C40 phase, which possesses slightly smaller magnitude of both electrical resistivity and Seebeck coefficient than those of C54 phase. We prepared a series of samples by simply alloying Al37(Mn26Ru3Re3Fe1)Si30) C54 phase and Al27.5(Mn29W3Fe1)Si39.5 C40 phase as [Al37(Mn26Ru3Re3Fe1)Si30]1-x[Al27.5(Mn29W3Fe1)Si39.5]x at x = 0, 0.01, 0.02, 0.03, 0.05, and 1, and found that the magnitude of Seebeck coefficient remained unchanged at x < 0.03, while the lattice thermal conductivity and electrical resistivity were moderately decreased with increasing x. The maximum ZT has consequently increased up to ZT = 0.42 at x = 0.03 from ZT = 0.38 of the C54-phase at x = 0.
  • Thermoelectric Properties of Cr1-xWxSi2

    pp. 1059-1065

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    DOI:10.2320/matertrans.MF201611

    The thermoelectric properties of W-substituted CrSi2 were studied. Polycrystalline samples of hexagonal C40-structured Cr1-xWxSi2 (x = 0.05, 0.10) were synthesized by arc melting and spark plasma sintering followed by heat treatment. Compared to non-substituted CrSi2, the lattice thermal conductivity was reduced from 9.0 to 4.6 W m−1 K−1 by W substitution because of enhanced phonon–impurity scattering. However, the power factor was slightly reduced by W substitution, as with Mo substitution. The thermoelectric figure of merit, zT, increased with increasing W content because of the reduced thermal conductivity, reaching 0.19 at 670 K. We analyzed the observed electrical properties using a theoretical model based on the Boltzmann transport equation. The effective mass of CrSi2 increases from 2.5 to ~3.5 by W or Mo substitution, indicating that W or Mo substitution affects the band structure of CrSi2, which reduces the power factor.
  • Improvement in Thermoelectric Performance of Cu-Doped β-Rhombohedral Boron

    pp. 1066-1069

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    DOI:10.2320/matertrans.MF201609

    The effects of Cu doping on the thermoelectric properties of β-rhombohedral boron have been systematically investigated using nominal compositions of CuxB105 (x = 0–5). The electrical conductivity increases with increasing x up to 5 at.%, whereas the positive Seebeck coefficient decreases because of an increase in the carrier concentration. Consequently, the power factor is enhanced by a factor of four from 22 μW m−1 K−2 (x = 0) to 90 μW m−1 K−2 (x = 4) at 973 K. In addition, Cu doping of the interstitial D and E sites contributes a ~50% reduction in the thermal conductivity from 4.3 W m−1 K−1 (x = 0) to 2.1 W m−1 K−1 (x = 5) at 973 K because of increased numbers of phonon scattering events. The dimensionless figure of merit ZT is also enhanced by a factor of six from 0.006 (x = 0) to 0.038 (x = 4) at 973 K in the p-type material. The ZT value obtained is higher than that of the conventional thermoelectric boride B4C.
  • Role of Nanoscale Precipitates for Enhancement of Thermoelectric Properties of Heavily P-Doped Si-Ge Alloys

    pp. 1070-1075

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    DOI:10.2320/matertrans.E-M2016805

    Thermoelectric (TE) devices can save energy by generating electricity from waste heat. For industrial application of TE power generation, TE materials with high conversion efficiency and that are non-toxic and inexpensive are required. The nanostructured silicon-germanium (Si-Ge) alloy is one possible candidate for such TE materials. Nanostructuring is an effective way to improve the conversion efficiency of materials because it dramatically reduces the lattice thermal conductivity (κlat). Here, we experimentally demonstrate effective phonon blocking without carrier mobility deterioration in bulk Si-Ge alloys containing phosphorous-rich nanoscale precipitates connected coherently or semi-coherently with the matrix phase. When the Ge content was less than 5%, the nanoscale precipitates effectively scattered heat-carrying phonons, leading to a sufficient reduction in κlat. However, at higher Ge content compositions, phonon scattering by Ge substitution with Si was more predominant than phonon scattering by nanoscale precipitates for the reduction of κlat. As a result, significant enhancement of zT was achieved at low Ge contents.
  • Thermoelectric Properties of (100) Oriented Silicon and Nickel Silicide Nanocomposite Films Grown on Si on Insulator and Si on Quartz Glass Substrates

    pp. 1076-1081

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    DOI:10.2320/matertrans.E-M2016807

    We have grown (100) oriented composite films of Si and Ni silicide nanocrystals (Ni–Si NC film) on substrates of Si on insulator (SOI) and Si on quartz glass (SOQ). Owing to improvement of carrier transport properties and reduction of the thermal conductivity in the oriented films, they have higher dimensionless figures of merit, ZT of 0.22–0.42 for p-type Ni–Si NC film and 0.08–0.13 for n-type Ni–Si NC film—than that of bulk Si (ZT < 0.01) at 30℃. The ZT values of p-type and n-type Ni–Si NC films were increased to 0.65 and 0.40 at 500℃, respectively.
  • Microstructures and Optical Properties of Silicon Spheres for Solar Cells

    pp. 1082-1087

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    DOI:10.2320/matertrans.M2016066

    Silicon (Si) spheres were prepared by a powder melting method, and their microstructures and optical properties were investigated. The lattice constant of the Si spheres increased upon phosphorus (P) diffusion, compared to that before P diffusion. This was attributed to the presence of interstitial P atoms. A fluorine-doped tin oxide (SnOx:Fy) anti-reflection film was then coated on the surface of the Si spheres. Changes in the lattice constant and bandgap of the SnOx:Fy film occurred upon subsequent annealing. This was attributed to changes in the composition of the SnOx:Fy film.
  • Influence of Pulverization and CaO Nanoparticles Addition on Thermoelectric Properties and Grain Growth of Mg2Si Based Compound

    pp. 1088-1093

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    DOI:10.2320/matertrans.Y-M2016812

    Magnesium silicide is a promising eco-friendly thermoelectric material. Its constituent elements are non-toxic and exist in abundance in the Earth's crust. To improve thermoelectric performance of the material, reduction of thermal conductivity is required. In this study, we attempted to lower the thermal conductivity of Al-doped Mg2Si by milling of the powder and addition of calcium oxide nanoparticles (CONP). Grain size distribution and thermoelectric properties in the sintered samples were measured. Furthermore, influence of CONP addition on grain growth was investigated after heat treatment at 500 ℃ for 50 h in an argon gas atmosphere. Milling process was effective enough to grain refinement, but CONP addition had little effect on it. The thermal conductivity was reduced in the milling treated samples regardless of CONP addition. The milling treated sample without CONP addition had the best thermoelectric performance of ZT = 1.15 at 600 ℃. The samples with CONP addition, however, resulted in degradation of thermoelectric performance at high temperature. The heat treatment brought to the grain growth for the sample without CONP addition and resulted in degradation of thermoelectric performance, while the sample with CONP addition inhibited the grain growth after the heat treatment and maintained the thermoelectric performance.
  • Microstructural Evolution through Uniaxial Hot Pressing before Age Hardening of AZ91D Alloy

    pp. 1094-1100

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    DOI:10.2320/matertrans.L-M2016813

    In recent years, magnesium alloys are attracting attention from aircraft and automotive industries because of their low density, high specific strength and high damping capacity. However, magnesium alloys suffer from low ductility at room temperature. The improvement of ductility in AZ91D alloy through a new thermo-mechanical treatment (TMT) process was reported. This TMT process is based on simple uniaxial hot pressing in atmosphere. Uniaxial hot pressing was carried out at 673 K up to 67% compressive strain. After the hot pressing, the specimen was held isothermally at 673 K for various times. Hot pressed specimens were aged at 473 K. The specimen isothermally held for 3.6 ks showed tensile strength of 358 MPa and elongation to failure of 9.6%. Microstructural observation revealed that both high strength and ductility in AZ91D alloy were caused by the homogeneous distribution of fine intermetallic precipitates inside grains.
  • Effects of Precipitation State on Serrated Flow in Al-Mg(-Zn) Alloys

    pp. 1101-1108

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    DOI:10.2320/matertrans.L-M2016814

    The effects of the aging conditions on the serrated flow in Al-Mg(-Zn) alloys were investigated, focusing on the precipitation states. Al-6%Mg and Al-6%Mg-3%Zn alloys were naturally and artificially aged after solution treated and quenched. The serrated flow was examined by tensile test. As for the Al-6%Mg-3%Zn alloy, the naturally aged specimens show the increase in the critical strain for serrated flow. The Zn-Mg clusters are formed during natural aging. Furthermore, the increase in the number density and the size of these clusters corresponds to the increases in the critical strain, which reveals that the formation of these coherent clusters is one of the dominant factors for delaying the onset of serrated flow. On the other hand, as for the artificially aged specimens in the under-aged conditions, the transformation of the clusters to the incoherent meta-stable precipitates brings about the decrease in the critical strain. Furthermore, the increase in the volume fraction of these precipitates in the over-aged conditions reduce the stress amplitude of serrated flow, which are brought about by the decrease in the solute Mg content.
  • Grain Refinement Mechanism and Evolution of Dislocation Structure of Co–Cr–Mo Alloy Subjected to High-Pressure Torsion

    pp. 1109-1118

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    DOI:10.2320/matertrans.M2016052

    Ultrafine-grained materials often possess superior mechanical properties owing to their small grain size. The high-pressure torsion (HPT) process is a severe plastic deformation method used to induce ultra-large strain and produce ultrafine grains. In this study, the grain refinement mechanisms in the Co–28Cr–6Mo (CCM) alloy, evolution of dislocation density as a result of HPT and its effects on mechanical properties were investigated. The dislocation density and subgrain diameter were also calculated by X-ray line profile analysis. The microstructure of the CCM alloy subjected to HPT processing (CCMHPT) was evaluated as a function of torsional rotation number, N and equivalent strain, εeq. Strain-induced γ→ε transformation in neighboring ultrafine grains is observed in CCMHPT processed at εeq = 2.25 and εeq = 4.5. Low-angle crystal rotation around the [110] fcc direction occurs in different locations in the same elongated grain neighboring ultrafine grains, which suggests the formation of low-angle grain boundaries in CCMHPT processed at εeq = 2.25 and εeq = 4.5. Two possible grain refinement mechanisms are proposed. The maximum dislocation densities, which are 2.8 × 1016 m−2 in γ phase and 3.8 × 1016 m−2 in ε phase, and maximum subgrain diameters, which are 21.2 nm in γ phase and 36 nm in ε phase, are achieved in CCMHPT processed at εeq = 9. HPT processing causes a substantial increase in the tensile strength and hardness owing to the grain refinement and a significant increase in the volume fraction of ε phase and dislocation density.
  • Aging Effect of Mg-Sc Alloy with α+β Two-Phase Microstructure

    pp. 1119-1123

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    DOI:10.2320/matertrans.M2016093

    Mg-20.5 at% Sc alloy with hcp (α)+bcc (β) two-phase was investigated to understand the effects of aging treatment at 473 K on microstructure, hardness and tensile properties. A Mg-Sc alloy ingot was prepared by induction melting in an Ar atmosphere, and then hot rolled at 873 K followed by cold rolling into a sheet. The rolled specimens were annealed at 873 K to obtain an α+β two-phase microstructure. The annealed specimens were then aged at 473 K for various time. Vickers hardness of the α+β two-phase alloy drastically increased after a certain incubation time and then reached a maximum hardness of 142.8 HV. The incubation time of the Mg-20.5 at% Sc alloy with the α+β two-phase was longer than that of the same alloy with a β single-phase. Ultimate tensile strength (UTS) and elongation of the as-annealed specimen were 280 MPa and 28.2%, respectively. Meanwhile, the specimen aged at 473 K for 14.4 ks showed a UTS of 357 MPa and an elongation of over 12%. The specimen aged for 18 ks showed a higher UTS of 465 MPa, while keeping a better elongation of 6.9%. The age hardening of the Mg-Sc alloys was found to be due to the precipitation of very fine α phase in β phase.
  • Effects of Partial Remelting on Microstructure of Al-Si-Ti Bulk Alloy Prepared by Cold Pressing Mixed Powders

    pp. 1124-1133

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    DOI:10.2320/matertrans.M2016070

    The microstructural evolution and phase transformations have been investigated during the partial remelting of a bulk alloy prepared by the cold pressing of A356, pure Ti and pure Al powders. A dropping experiment was used to investigate the reaction kinetics of Ti powders and the Al matrix simultaneously. The results show that a semi-solid microstructure with fine and spheroidal primary α-Al particles suspended in the liquid phase could be obtained after the bulk alloy was heated to 595℃ for 30 min. The microstructural evolution process was divided into four stages, involving the transformation of the powders into primary particles, the formation of a liquid phase and the increase in its amount, which results in the formation of a continuous liquid layer, the rapid coarsening of the primary particles and the increase in the liquid phase amount, and the final coarsening of the primary particles. Chemical reactions between the Ti powders and the Al element in the matrix occurred simultaneously. Next, core-shell structured, reinforced particles composing both an intermetallic shell and a soft Ti metal core formed in situ. The compact shell subsequently ruptured and peeled off when its thickness increased to a given value for a given size of Ti powder particles. Finally, the Ti powders were consumed completely because of the formation and the subsequent peeling of the shell. Results of the dropping simulation experiment show that the reaction product layer grows in a linear kinetic manner characterized by an activation energy of 374 kJ/mol.
  • Microstructure and Anticorrosion Property of High-Entropy Alloy AlFeNiCrCoTi0.5Vx

    pp. 1134-1137

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    DOI:10.2320/matertrans.M2015443

    The influence of vanadium element on the microstructure, microhardness, and electrochemical corrosion resistance properties of as-cast AlFeNiCrCoTi0.5Vx (x = 0, 0.5, 1, 1.5, 2) was studied. The microstructures of the alloys with V contents lower than 1.0 are mainly comprised of the simple body-centered cubic (BCC) and face-centered cubic (FCC) composite structures. With V mole ratio increased to 1.5, the FCC structure plays the leading role in the alloys, which enhances the plasticity and toughness of the alloys. The microhardness of the alloys increased with the increasing of V content in the alloys. Cr, Ni, Ti, and Al elements in the alloys facilitate passivation in 3.5 M NaCl solution. V element mainly enriched in interdendritic region of the alloys, which altering the distribution of Cr and Ti. Cr and Ti is benefit to the passivation, which enlarges the corrosion potential of the alloys and lowers the passive current. Consequently, the alloys present a protective effect in 3.5 M NaCl solution.
  • Effects of Heat Exposure Time and Temperature on the Delamination Behavior of Air Plasma-Sprayed Thermal Barrier Coatings under Shear Loading

    pp. 1138-1146

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    DOI:10.2320/matertrans.M2016077

    The delamination behavior of air plasma-sprayed thermal barrier coatings (APS-TBCs) exposed to heat in air at different temperatures was evaluated under mode II loading conditions. The TBC layer, BC layer, and substrate were composed of 8 mass% Y2O3 partially stabilized ZrO2, NiCoCrAlY alloy, and Inconel 738 nickel base superalloy. The heat exposure was performed at 1173 K or 1423 K for 10 to 200 h. During the heat exposure, the thickness of thermally grown oxide (TGO) increased and the hardness of the bond coat (BC) layer near the TGO decreased with increasing heat exposure time. The delamination toughness decreased monotonically with increasing heat exposure time when the TBCs were heat exposed at 1173 K. In this case, delamination occurred at the TBC layer. The average thickness of the remaining TBC on the substrate side decreased with increasing exposure time. As for the TBCs exposed at 1423 K, the delamination toughness increased over the first 50 h, but then began to decrease with further exposure time. The delamination pathway has transferred to near the TGO layer. The fraction of TBC layer fracture decreased, whereas those of the TBC/TGO interface and TGO/BC interface fracture increased with increasing exposure time. The change in delamination toughness may have been caused by the interaction between the TGO thickening and the reduction of BC layer hardness. The increase in TGO thickness decreased the delamination toughness due to an increase in residual stresses. The decrease in hardness of the BC layer near the TGO may have increased the delamination toughness by increasing the plastic dissipation.
  • Piecewise Modeling of Flow Stress of 7075-T6 Aluminum Alloy in Hot Deformation

    pp. 1147-1155

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    DOI:10.2320/matertrans.M2015465

    The flow stress behavior of the 7075-T6 aluminum alloy was studied through single-pass compression experiment by using simulator within temperature range of 573–723 K and strain rate range of 0.01–10 s−1. The stress-strain curves are presented as three stages including work hardening, dynamic recrystallization and relative steady state. The stress increases to a peak value firstly and then decrease nonlinearly to an initial steady state within some strain. Subsequently, the stress shows as linear change after initial steady state. In order to describe the change of flow stress in the range of peak stress to initial steady state, a new model has been developed based on phenomenological representation of the shape and the boundary conditions of the curves. Furthermore, a new material parameter C2 and k which are sensitive to the temperature and strain rate were proposed in the developed constitutive model. The equations expressed in terms of peak stress and strain, steady state stress and strain and additional parameters. The stress-strain curves of 7075-T6 aluminum alloy obtained by this model are in good agreement with experimental results consequently that the method proposed is valid and the model is reliable.
  • Removal of Ca from Magnesium Melt by Flux Refining

    pp. 1156-1164

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    DOI:10.2320/matertrans.M2015426

    Magnesium refining processes are typically based on the use of fluxes, most of which contain MgCl2, KCl, NaCl, CaCl2, or BaCl2. These fluxes separate the surface oxides, gases, or other contaminates from the metal. The most important physical and chemical properties of a salt melt are the fusibility, density in the operating temperature range (920–1023 K), and viscosity. Therefore, in this study, the physical and chemical properties of various molten salts were examined to find a new refining flux with a low melting point, low viscosity, high reactivity, and good melt protection using thermodynamic calculations and various experiments using inductively coupled plasma emission spectroscopy, scanning electron microscopy-energy dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. In addition, the effects of the chemical composition of the flux, amount of flux, fluxing temperature, and stirring time and intensity on the refining process of molten magnesium were studied. The optimized conditions for the Mg refining process resulted in a ternary flux system with 31 mass% MgCl2, 60 mass% KCl, and 9 mass% NaCl, through the addition of 1 mass% CaF2 and 5 mass% MgO, and stirring at 150 RPM for 15 min at 953 K.
  • Electrodeposition of Zn–Zr Oxide Composite from Dispersed-Particle-Free Solution

    pp. 1165-1170

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    DOI:10.2320/matertrans.M2016084

    Electrodeposition of Zn–Zr oxide composite from an unagitated sulfate solution containing Zn2+ and Zr ions was investigated at pH 1–2 and at 313 K under galvanostatic conditions. The Zr content was higher in deposits formed from the solution at pH 2 than in those formed from the solution at pH 1 and initially decreased with increasing current density; however, when the current density was increased further, the Zr content in the deposits increased. This increase in Zr content was attributed to the acceleration of the hydrolysis of Zr ions by an increase in hydrogen evolution in the solution in the vicinity of cathode. In solutions containing Zr ions, Zn deposition was substantially polarized because of the electric resistance of film of the Zr oxide formed by the hydrolysis of Zr ions. The pH in the vicinity of the cathode, as measured using an Sb microelectrode, was approximately 2.2, which is similar to the critical pH for the formation of ZrO2. Scanning electron microscopy and energy-dispersive X-ray spectroscopy point analysis of the deposits revealed that granular Zr oxide was deposited at the surfaces of the Zn platelet crystals and in the voids between the crystals. Polarization curves in 3 mass% NaCl solution revealed that the corrosion potential of the deposited Zn–1.1 mass% Zr oxide films was more noble than that of Zn films and that the corrosion current density of the Zn–1.1 mass% Zr oxide films was lower than that of Zn films.
  • Binary Systems of Kaolinite and Hematite: Preparation, Characterization and Adsorption Characteristics for Fluoride

    pp. 1171-1176

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    DOI:10.2320/matertrans.M2015448

    In soils and sediments kaolinite (Kao) and hematite (Hem) are often cemented together as binary systems, which have a significant influence on the mobility of nutrients and pollutants in natural environments. In this study, the binary systems of kaolinite-hematite complex (KHC) and kaolinite-hematite mixture (KHM) were prepared, and the surface properties and fluoride adsorption of the samples were investigated. Compared to KHM, the XRD peaks for both kaolinite and hematite in KHC were lower. The crystallinity of hematite in KHM and KHC was 94.13% and 80.99% respectively. This indicates that the surface of kaolinite in KHC was largely coated by poorly crystalline hematite, and the coating in KHM was relatively weak. The total pore volume of the samples followed the sequence of Hem > KHC > KHM > Kao, and the specific surface area (SSA) decreased in the order of KHC > Hem > KHM > Kao. The isoelectric point (IEP) of Kao, Hem, KHM and KHC was 3.3, 7.6, 5.9 and 6.5, and the zeta potential at pH 5.5 was −18.6, 25.3, 2.8 and 10.3 mV, respectively. The fluoride adsorption data of the samples were fitted using one-site Langmuir, two-site Langmuir and Freundlich models, and results indicated that the binary systems possessed the surface with various adsorption sites for fluoride. At pH 5.5, the adsorption capacity (qmax) of Kao, Hem, KHM and KHC was 1.92, 13.79, 7.48 and 10.46 mg·g−1, respectively. Compared to the average of Kao and Hem, the qmax of KHC increased significantly (P < 0.05), whereas that of KHM had no significant change (P > 0.05).
  • Application of the Taguchi Method to Optimize Graphene Coatings on Copper Nanoparticles Formed Using a Solid Carbon Source

    pp. 1177-1182

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    DOI:10.2320/matertrans.M2016011

    Graphene has attracted much recent interest as an electronic material due to its large electron mobility. Large-area graphene has been synthesized using chemical vapor deposition (CVD). However, it is difficult to apply this process to grow graphene on nanoparticles (NPs) because of their small radius of curvature, which results in a large defect density. In this work, we used the Taguchi method to optimize the deposition of graphene on nanoparticles. We used polyvinylpyrrolidone (PVP) to coat copper NPs via CVD and optimized the process conditions using a minimal number of experiments. The PVP served as the solid carbon source, forming graphene when heated to 875℃. To improve the quality of the graphene coatings on the Cu NPs, the following process parameters were varied: gas conditions (ratio of Ar to H2), process time and temperature, the amount of PVP solution, and the molecular weight of PVP. We identified optimal process conditions using only eight experiments. Raman spectroscopy was used to analyze the quality of the graphene coatings by comparing two-dimensional (2D) spectra and ID/IG ratios of the different coatings. A decrease in ID/IG, in combination with sharper Raman bands, is indicative of the thickness and crystal quality of the graphene layer. The quality of the graphene layer was also evaluated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM).The optimal conditions for the formation of graphene-coated Cu NPs were: a temperature of 875℃, a deposition time of 2 minutes, an Ar-to-H2 ratio of 1:1, PVP with a molecular weight of MW = 3,500 (K-12) during the polyol process, and a 50-wt.% PVP solution with MW = 45,000 (K-30). Using the Taguchi method, we identified trends relating defect density versus process conditions and successfully obtained a graphene coating with a minimal defect density.
  • Fundamental Studies on Oxidizing Roasting of the “Bioselenium”

    pp. 1183-1191

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    DOI:10.2320/matertrans.M2016060

    A method has been developed for the recovery and recycling of selenium (Se) from Se-containing wastewater using the Se reducing bacterium, Pseudomonas stutzeri NT-I. The treatment of Se-containing wastewater with strain NT-I produced the bioselenium, which was composed mainly of organic matter and 11–14 mass% of Se. Specifically, the recovery of Se from the bioselenium by oxidizing roasting followed by wet reduction was studied. After thermodynamic calculations were performed to estimate the oxidizing behavior during roasting, experiments were conducted on the bioselenium for various roasting conditions. Selenium in the bioselenium was recovered in the form of solid SeO2 with a purity of 99% (metal basis) and a maximum yield of 97 mass% after roasting at 700℃. Wet reduction of SeO2 to metallic Se was achieved with a purity of 99% (metal basis).
  • Reliability of Ag Nanoporous Bonding Joint for High Temperature Die Attach under Temperature Cycling

    pp. 1192-1196

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    DOI:10.2320/matertrans.MD201518

    The long-term reliability of a Ag nanoporous bonding joint for high temperature die attach under temperature cycling from −55℃ to 150℃ was investigated. A Ag nanoporous sheet was adopted as a bonding layer for the die attach of a Si chip on an active metal brazed Cu Si3N4 substrate. The initial joint strength was similar to that with Pb-5Sn die attach. There was no significant change in the joint strength after temperature cycling up to 1500 cycles. It was possible to confirm that the shear strength of the Ag nanoporous bonding joint had good stability under temperature cycling.
  • High-Entropy Alloys Including 3d, 4d and 5d Transition Metals from the Same Group in the Periodic Table

    pp. 1197-1201

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    DOI:10.2320/matertrans.M2016121

    Exact equi-atomic senary alloys including three elements from 3d, 4d and 5d transition metals (TMs) were investigated for their ability to form solid solutions as high-entropy alloys (HEAs). Three alloys of CoCuPdTiZrHf, CoCuFeTiZrHf and AgAuCuNiPdPt were selected by focusing on (Ti, Zr, Hf) from Early-TMs, and (Cu, Ag, Au) and/or (Ni, Pd, Pt) from Late-TMs based on an alloy design with a help of Pettifor map for binary compounds with several stoichiometries and binary phase diagrams, together with a marginal Al4CoNiPdPt alloy. The XRD analysis revealed that the CoCuPdTiZrHf alloy was formed into a bcc, whereas both the CoCuFeTiZrHf and Al4CoNiPdPt alloys were a CsCl, and the AgAuCuNiPdPt alloy was dual fcc structures. The observations with optical and scanning-electron microscopes and analysis with energy dispersive X-ray for chemical composition revealed the homogeneous morphologies of these alloys in micrometer scale. The types of crystallographic structures of the CoCuPdTiZrHf, CoCuFeTiZrHf and AgAuCuNiPdPt HEAs and the Al4CoNiPdPt alloy can be principally explained by valence electron concentration. Three constituent elements from TMs in the same group enhance the increase in the number of complete solid solutions in the constituent binary systems, leading to forming these HEAs.
  • Tensile Strength of a Ti/Thermoplastic ABS Matrix CFRTP Joint Connected by Surface Activated Carbon Fiber Cross-Weave Irradiated by Electron Beam

    pp. 1202-1208

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    DOI:10.2320/matertrans.M2016017

    A new method with extremely large friction force taking advantage of the high interface area of carbon fiber (CF: 6 µm-diameter) cross weave cloth, surface activated by homogeneous low voltage electron beam irradiation (HLEBI) on the ABS half-length prior to dipping in ABS resin to enhance the ability of difficult to adhere thermoplastic with CF has been suggested for a joint (Ti/EBCF/ABS) of carbon fiber reinforced thermoplastic ABS polymer and titanium (Ti). The joint was strengthened by the HLEBI over that without HLEBI. Experimental results showed ultimate tensile strength (σb) of the (Ti/EBCF/ABS) was 18.2 MPa: 2.1 and 9.1 and 4.2 times higher than that: without HLEBI (Ti/CF/ABS) (8.64 MPa); with glue (Ti/Glue/ABS) (2.00 MPa); and without glue (Ti/ABS) (4.32 MPa), respectively. Since the cross sectional area of CF impregnated ABS portion was about 1/12 that of the entire ABS wrapped CFRTP, the corrected σb (cσb) value of (Ti/EBCF/CFRTP) (139 MPa) could be estimated and was 2.5 times higher than cσb of (Ti/CF/CFRTP) (56.5 MPa). X-ray diffraction (XRD) and wavelength dispersive X-ray spectroscopy (WDS) analysis showed titanium carbide TiC was not detected. A “strain hardening” model was constructed to predict deformation mechanisms in the (Ti/ABS) and (Ti/EBCF/ABS) joints. The new joint method of applying HLEBI to the carbon fiber cloth remarkably enhanced the safety level of lightweight material with high resistance to fracture for airplanes and automobiles over that without HLEBI.
  • Pressureless Bonding by Micro-Sized Silver Particle Paste for High-Temperature Electronic Packaging

    pp. 1209-1214

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    DOI:10.2320/matertrans.MD201513

    Lead-free bonding in high-temperature electronic components is desirable for realizing eco-friendly technology. A pressureless process is more appropriate for electronic packaging because it enables a more automated manufacturing process and avoids any potential damage caused by application of pressure. Recently, Ag nanoparticles were used without pressure to join materials for high-temperature electronic applications. In this study, a Ag paste of micro-sized particles was proposed for electroless nickel immersion gold (ENIG) finished Cu pressureless bonding owing to its advantages of both cost effectiveness and easy manufacturing process compared to Ag nanoparticle paste. The micro-sized Ag paste was composed of both chestnut-burr-like (CBL) and spherical particles. The weight ratios of CBL to spherical particles were 10:0, 7:3, and 5:5. The bonding process was carried out at 573 K for 60 min in a nitrogen atmosphere. The experimental results showed that all of the sintered layers had an open porous structure. ENIG-finished Cu joint using the Ag paste of the 5:5 weight ratio exhibited the shear strength of 18.6 MPa, which is comparable to that of a conventional Pb-5Sn joint.
  • Hydrogen Ab/Desorption of LiH-KH Composite and Ammonia System

    pp. 1215-1219

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    DOI:10.2320/matertrans.M2016071

    Lithium hydride-Potassium hydride (LiH-KH) composite prepared by ball-milling is focused in order to modify the kinetic properties of the reaction between LiH and gaseous ammonia (NH3). The LiH-NH3 system is recognized as one of the most promising hydrogen storage system because it generates hydrogen at room temperature by ammonolysis reaction, is regenerated below 300℃, and possesses more than 8.0 mass% hydrogen capacity. From the experimental results, it is confirmed that the hydrogen generation from the reaction between NH3 and the LiH-KH composite shows much higher reaction rate than that of the simple summation of each component, which can be recognized as a synergetic effect. Then, double-cation metal amide (MNH2) phases such as LiK(NH2)2, which are not assigned to any amides reported before, are formed as the reaction product. Moreover, it is confirmed that hydrogenation of the generated amide can proceed to form LiH-KH composite and NH3.

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