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

MATERIALS TRANSACTIONS Vol. 51 (2010), No. 10

  • A Reliability Study of Nanoparticles Reinforced Composite Lead-Free Solder

    pp. 1720-1726

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    DOI:10.2320/matertrans.MJ201002

    This work looks at the development and investigation of a reinforced composite solder with low melting point. The composite solder was prepared by adding Sn-3.0Ag-0.5Cu nanoparticles into Sn-58Bi solder paste. The Sn-3.0Ag-0.5Cu nanoparticles were manufactured using a self-developed Consumable-electrode Direct Current Arc (CDCA) technique. The test FR-4 Printed Circuit Board (PCB) with Cu pad and Electroless Nickel Immersion Gold (ENIG) surface finish were fabricated, and fifty SR1206 chip resistors were mounted on pads of test PCB with the reinforced composite solder paste by using conventional surface mount technology. The differential scanning calorimetry (DSC) was used to analyze the constituent of the composite solder joint after reflow. A scanning electron microscope (SEM), transmission electron microscope (TEM) and optical microscope (OM) were employed in order to observe the morphology of nanoparticles; the microstructure of reinforced composite solder joint; the crack initiation and propagation in solder joint; and the fracture mode after shear test. The thermal cycling (TC) was carried out with a temperature range of −40°C and 125°C. The contact resistance of the solder joint was measured during thermal cycling, and the shear test of solder joints was performed before and after 500 thermal cycles. After the shear test, all fracture surfaces were inspected to identify the fracture mode of the composite solder joint. The results of the experiments detailed in this work indicate that the shear strength of the composite solder increased 2 times in comparison to Sn-58Bi. Meanwhile, the thermomechanical fatigue (TMF) resistance of the composite solder with 1 mass% nanoparticles was 16 times stronger than Sn-58Bi and 4 times stronger than Sn-3.0Ag-0.5Cu. However, the tendency of forming micro-cracks between nanoparticles and solder matrix and the fracture within solder was increased for solder joints with more than 3 mass% nanoparticles after thermal cycling.
  • Effects of Zn-Bearing Flux on Joint Reliability and Microstructure of Sn-3.5Ag Soldering on Electroless Ni-Au Surface Finish

    pp. 1727-1734

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    DOI:10.2320/matertrans.MJ201001

    Joint reliability and microstructure for Sn-3.5Ag soldering on an electroless Ni-P/Au surface finish were investigated, using the flux bearing Zn(II) stearate. The content of the zinc compound in the flux varied from 0 mass% to 50 mass%. The results of both shock and pull strength tests for as-reflowed solder joints showed that Zn-bearing fluxes gave higher joint strength than did flux without Zn compound. Additionally, the use of flux containing 50 mass% of Zn(II) stearate provided the smallest reduction rate in pull strength in regard to aging treatment at 150°C for 1000 h. According to the microstructure of the aged joints, both an interfacial intermetallic layer and a P-rich layer of the joint obtained by the use of the flux without Zn(II) stearate became thick with prolonged aging. On the other hand, a slow growth of the interfacial reaction layer was observed for the joint with Zn(II) stearate in flux during annealing. It is presumed that the formation of the interfacial intermetallic compound with Zn at reflow works effectively in suppressing the diffusion of Ni into the solder matrix during subsequent aging, and this can maintain a relatively high joint strength for those joints with Zn(II) stearate in flux. After damp heat treatment with 85°C/85% RH up to 500 h, no sign of corrosion was observed in either joint. Moreover, there was no significant change of the microstructure, nor of the pull strength, with or without Zn(II) stearate in flux.
  • Effect of In Addition on Wetting Properties of Sn-Zn-In/Cu Soldering

    pp. 1735-1740

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    DOI:10.2320/matertrans.MJ201004

    In this study, trace amounts (0.5–2 mass%) of indium (In) are added to the binary Sn-9Zn alloy to examine the effect of such addition on wetting properties of Sn-9Zn-In soldering. Results show that addition of In to Sn-9Zn alloy leads to decrease in eutectic temperature; and the greater the amount of In added, the larger the decrease is. On the other hand, the maximum wetting force, an indicator of wettability and solderability, increases with increasing mass% of In added. In addition, the higher the eutectic temperature, the better the wettability of solder is. Increasing In content in the solder also enhances the dissolution of Cu. Moreover, the higher the In content, the faster the dissolution rate is. At wetting temperatures of 220°C and 240°C, the wetting time achieved is shorter than 3 s, which can meet practical needs for industrial applications. In sum, the addition of 0.5 mass% of In is optimal in that it can achieve good wettability while maintaining low dissolution rate, which would also enhance solderability.
  • Effects of Post-Plating Reflow on Changes in Crystal Grain Size of Sn-2 mass%Bi Alloy Plating with Thermal Cycling Treatment

    pp. 1741-1746

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    DOI:10.2320/matertrans.MJ201021

    Changes in the grain size of tin (Sn) crystals associated with a thermal cycling treatment were investigated for layers of Sn-2 mass%Bi electroplated on Fe-42Ni substrates in order to clarify the effects of post-plating reflow. Morphology of plated surface was observed by scanning electron microscopy (SEM) and scanning ion microscopy (SIM), and the Sn-grains were observed by electron beam back scattered diffraction (EBSD). Microstructures were also analyzed with a transmission electron microscope (TEM) in order to investigate the process of whisker formation. It was found that the Sn grains in Sn-Bi specimen are smaller than those in pure Sn specimen at the initial state of as-plating, after the reflow treatment and after the thermal cycling treatment of 1500 cycles. On the Sn-Bi specimen without a reflow treatment, short whiskers had grown after 100 cycles of thermal cycling treatment, but on those with the reflow treatment, relatively-long whiskers were formed after 500 cycles. The grain sizes increase temporarily after the reflow treatment and also after the very early stage of thermal cycling treatment, but gradually decreased with the thermal cycling treatment. The phenomenon is explained by the “deformation and recrystallization” effects of the thermal cycling treatment.
  • Effect of Small Addition of Zinc on Creep Behavior of Tin

    pp. 1747-1752

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    DOI:10.2320/matertrans.MJ201023

    Sn-based alloys with high creep resistance are required for soldering applications. This paper describes the effect of solid solution strengthening on the creep resistance of Sn-Zn alloys. The maximum solubility limit of Zn is 0.34 mass% in Sn. The creep behaviors of Sn, Sn-0.1 mass%Zn and Sn-0.4 mass%Zn were examined at 298 and 398 K under constant strain rates ranging from 1×10−4 to 1×10−2 s−1. The creep resistance of Sn was improved significantly by the addition of a small amount of Zn owing to the solid solution strengthening. The creep resistance of Sn-0.4 mass%Zn was at the same level as that of Sn-37 mass%Pb. We obtained a stress exponent of about 7 and an activation energy of 41–45 kJ/mol, which indicates that the creep behavior was climb-controlled dislocation creep controlled by pipe diffusion. The finding that a small addition of Zn improves the creep resistance is useful for developing new Pb-free solders.
  • Fluxless Bonding of Ni-P/Cu Plated Al Alloy and Cu Alloy with Lead-Free Sn-Cu Foil

    pp. 1753-1758

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    DOI:10.2320/matertrans.MJ201010

    To improve the joint strength of the joint of the Ni-P/Cu plated Al alloy and the Cu alloy for a water cooling system, fluxless bonding was conducted with Sn-Cu solder foil in a vacuum. In the joint bonded at 250°C, fracture easily occurs at the joint interface due to insufficient interfacial reaction and shear strength is relatively low. In the joint bonded at 350°C, the excess growth of the Cu3Sn layer in the Cu6Sn5/Cu interface and the large void formation in the joint degrade shear strength. On the contrary, in the joint bonded at 300°C, both the excess growth of the Cu3Sn layer and the large void formation are prevented, and thus shear strength becomes relatively high. When bonding was conducted under the optimum conditions, which are bonding temperature of 300°C, bonding time of 20 min and applied stress of 0.2 MPa, shear strength became 33.4 MPa. This value is approximately four times that of the joint bonded with flux.
  • Effect of Formic Acid Surface Modification on Bond Strength of Solid-State Bonded Interface of Tin and Copper

    pp. 1759-1763

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    DOI:10.2320/matertrans.MJ201019

    The effect of formic-acid surface modification on the bond strength of the solid-state bonded interface of tin and copper has been investigated by SEM observation of the interfacial microstructures and fractured surfaces. Formic-acid surface modification was carried out by boiling a tin and a copper surface in formic acid for 600 s. Solid-state bonding was carried out in a vacuum chamber at a bonding temperature T of 403–473 K and a bonding pressure P of 7 MPa (bonding time: t=1800 s). The bond strength increased with an increase in the bonding temperature, independently of the formic-acid surface modification. Because of the surface modification, bonded joints were obtained at a bonding temperature that was 40 K less than the typical temperature required, and the bond strength was comparable to that of the base metal. When surface modification is applied, the oxide film is destroyed by the improved plastic deformability of the tin. Thus, the tensile strength of the joint is increased. On the other hand, when surface modification is applied, a high-tensile-strength joint is obtained at a low temperature because metallic tin and copper are exposed at the bond interface as a result of the decomposition of formate in the bond interface at a low temperature.
  • Synthesis of Highly Concentrated Ag Nanoparticles in a Heterogeneous Solid-Liquid System under Ultrasonic Irradiation

    pp. 1764-1768

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    DOI:10.2320/matertrans.MJ201005

    Ag nanoparticles were synthesized at a high concentration (about 10–100 times higher than traditional process) without condensation, by ultrasonic irradiation of a heterogeneous solid-liquid system. This process offers reduced costs of condensation of the prepared solution and waste liquid treatment, compared to the traditional process. Besides, Ag conductive films were prepared with a relatively good electrical resistivity of 3.30 μΩ cm after brief, low temperature sintering.
  • Synthesis of Carbon Nanotube/Silver Nanocomposites by Ultrasonication

    pp. 1769-1772

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    DOI:10.2320/matertrans.MJ201012

    Carbon nanotube (CNT)/Ag nanocomposites were synthesized by an ultrasonic process. Dispersion of aggregated CNTs and synthesis of both Ag nanoparticles and Ag/CNT composite particles were executed by ultrasonic treatment. Dispersibility of Ag nanoparticles improved with the combined use of a dispersant, which led to improved densification upon sintering. In addition, synthesis of nanosize Ag particles allowed sintering of the CNT/Ag nanocomposites at 300°C, with a resulting electrical resistivity of 2.46 μΩ·cm.
  • Transmission Electron Microscopy Study on Microstructure of Ag-Based Conductive Adhesives

    pp. 1773-1778

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    DOI:10.2320/matertrans.MJ201022

    We used advanced electron microscopy techniques to study the microstructure of Ag-based conductive adhesives, which are promising alternatives to Pb-free solders. Systematic microscopy observations revealed the development of connections between Ag agglomerates, when the weight density of Ag (Ag content) was increased up to 92 mass%. However, in a sample containing 94 mass% Ag, disconnection between well-defined Ag agglomerates and/or formation of voids between Ag-filler particles were observed. These results appeared to be consistent with the plot of resistivity versus Ag content: the resistivity decreased to a minimum value at 92 mass% Ag. We also demonstrated the formation of very small Ag particles in a thin-foil specimen subjected to electrical breakdown. This microscopic observation may be useful for understanding the process of electric breakdown in thin-foil specimens.
  • Influence of Temperature and Dwelling Time on Low-Cycle Fatigue Characteristic of Isotropic Conductive Adhesive Joint

    pp. 1779-1784

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    DOI:10.2320/matertrans.MJ201018

    The effects of temperature and dwell time on the low-cycle fatigue life of Ag-epoxy based conductive adhesive have been investigated by using a micro-joint specimen. The low cycle fatigue life of the conductive adhesive increases when test temperature is elevated beyond the glass transition point. On the other hand, the dwell time at 398 K reduces the fatigue life, which, however, is increased by the dwell time at 348 K. The cross-sectional image suggests the embrittlement of epoxy resin during the dwell time at 398 K, which reduces the fatigue endurance of the conductive adhesive. In contrast, some kind of recovery effects exists during dwell time near the glass transition temperature, which improves the fatigue life.
  • Effects of Silver Coating Covered with Copper Filler on Electrical Resistivity of Electrically Conductive Adhesives

    pp. 1785-1789

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    DOI:10.2320/matertrans.MJ201020

    Electrically conductive adhesives (ECAs) are usually composed of conductive metal fillers and polymeric resin. For the metals fillers, silver is the most commonly used due to its high electrical and thermal conductivities, and chemical stability. Recently copper can be a promising candidate for conductive filler metal due to its low resistivity, low cost and good electro-migration performance. In this study, to overcome the problem of high electrical resistance associated with the oxidation of copper, copper particles were coated with silver, and the silver-coated copper was tested as a filler metal. In particular, the effect of silver coating on the electrical resistivity of ECAs just after curing and after reliability tests was investigated. It was found that the electrical resistivity of ECA using silver-coated copper filler was much lower and more stable than that of ECA using pure copper filler after curing and after reliability tests.
  • Characteristics of Thermosonic Anisotropic Conductive Adhesives (ACFs) Flip-Chip Bonding

    pp. 1790-1795

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    DOI:10.2320/matertrans.MJ201016

    In recent years, Anisotropic Conductive Films (ACFs) have been widely used in microelectronic packaging applications due to their potential advantages, which include low processing temperatures, high-resolution capabilities, and compatibility for nonsolderable materials. In this study, we investigated a novel, thermosonic ACF flip-chip bonding process using ultrasonic vibration. The thermosonic ACF flip-chip bonding was characterized in terms of adhesion strength and electrical property in comparison with conventional thermo-compression ACF bonding. ACF has a nickel particle with a 4-μm diameter, and its recommended thermo-compression bonding conditions were 453 K for bonding temperature and 10 s for bonding time. The investigated thermosonic flip-chip bonding conditions were 433 K and 453 K for bonding temperature and 0.5 s and 1 s for bonding time at each temperature, respectively. In addition, a cross-section of the ACF joint was observed using scanning electron microscopy. As a result, the developed thermosonic flip-chip bonding process showed good electrical and mechanical characteristics, as compared with those of the conventional thermo-compression bonding process. In the future, we expect that a thermosonic flip-chip bonding method with ACFs can be effectively applied to reduce the bonding temperature and time.
  • Structural Study of Zr50Cu50 Amorphous Alloy by Anomalous X-ray Scattering Coupled with Reverse Monte-Carlo Simulation

    pp. 1796-1801

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    DOI:10.2320/matertrans.MAW201016

    Anomalous X-ray scattering (AXS) analysis was applied for the structural analysis of a rapidly quenched Zr50Cu50 amorphous alloy. The present AXS analysis provided the environmental structural information round Cu and Zr, and the subsequent reverse Monte Carlo (RMC) simulation allowed us to obtain three partial pair distribution functions together with a three-dimensional structural model. The Voronoi polyhedral analysis of the nearest-neighbor region confirms that the structural features are similar to those of the dense random packing of hard spheres (DRPHS) model. The chemical oscillatory structure composed of alternating Cu-rich and Zr-rich coordination shells was realized in the middle range, up to 0.8 nm. The shell structure is suggested to be one of the fundamental structural features of medium-range ordering in the Zr50Cu50 amorphous alloy.
  • Effect of P Content on Stress Relaxation and Clustering Behavior in Cu-Ni-P Alloys

    pp. 1802-1808

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    DOI:10.2320/matertrans.M2010190

    Stress relaxation behavior and cluster distributions in Cu-P alloy and Cu-Ni-P alloys with different P content have been investigated by means of three-dimensional atom probe (3DAP). The overall improvement in the stress relaxation performance is considered in terms of dislocation pinning by solute atoms and clusters. The Cu-Ni-P alloy with low P content forms a low density of Ni-P clusters during annealing and shows a greater improvement in stress relaxation resistance than the Cu-P alloy. It is shown that the pinning effect of solute P has much less impact on the stress relaxation behavior in Cu alloys than the effect of the clusters. It is demonstrated that the clusters play a key role in the stress relaxation in Cu alloys and that the stress relaxation performance can be related to the volume fraction of the clusters.
  • Diffusion of Aluminum in β-Titanium

    pp. 1809-1813

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    DOI:10.2320/matertrans.M2010225

    Interdiffusion coefficient \\ ildeD in the β-phase of Ti-Al alloys has been determined by Matano’s method in the temperature range 1323 to 1823 K with (pure Ti)-(Ti-8.5 at% Al alloy), (pure Ti)-(Ti-16.5 at% Al alloy) and (Ti-8.5 at% Al alloy)-(Ti-16.5 at% Al alloy) couples. In the whole temperature range the value of \\ ildeD increases gradually with increasing aluminum content. The Arrhenius plot of \\ ildeD up to 6 at% Al shows an upward curvature similar to that recognized in the self-diffusion in β-Ti. The curvature becomes small with increasing aluminum content, and it is nearly linear in the concentration more than 10 at% Al. The activation energies for the impurity diffusion in β-Ti are proportional to the square of radius of the diffusing atom. This suggests that the size effect is dominant in the impurity diffusion in β-Ti.
  • Elasto-Plasticity Behavior of High Strength Steel Sheet in Biaxial Stress Path Change

    pp. 1814-1818

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    DOI:10.2320/matertrans.P-M2010817

    The elasto-plasticity behavior of a high-strength steel sheet of 980 MPa-TS was investigated by performing biaxial tension experiments. In order to evaluate the accuracy of constitutive models of plasticity in describing such elasto-plasticity behavior, numerical simulations of stress-strain responses were conducted for the same stress paths as those in the experiments using two types of constitutive models: the isotropic hardening model (IH model) and the kinematic hardening model proposed by the present authors (Yoshida-Uemori model). In this work, special emphasis is placed in the cases of stress-path change. In experiments on radial loadings after equi-balanced tension preloading, it was found that flow stresses are considerably lower than uniaxial tension flow stress. The IH model can hardly describe this phenomenon, although it is sufficient for stress-strain analysis of proportional loading cases. In contrast to this, the Yoshida-Uemori model can well predict every stress-strain response in biaxial stress-path changes.
  • Prediction on Nominal Stress-Strain Curve of Isotropic Polycrystal Ti-15-3-3-3 Sheet by FE Analysis

    pp. 1819-1824

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    DOI:10.2320/matertrans.M2010033

    In order to predict the nominal stress-strain curve of an isotropic polycrystal Ti-15V-3Al-3Cr-3Sn (hereafter, Ti-15-3(ST)) sheet in tension test, three-dimensional finite element (FE) analysis is elucidated considering three boundary conditions. According to the constraint at both ends of specimen, three boundary conditions are a simulated case based on the empirical data, a full case and a free case. In the simulated case, the nominal stress-strain curve can be well predicted until the fracture strain. Maximum load point does not mean the termination of uniform deformation in the present study. The onset of localized necking is intensively discussed and the origin of localized necking is concluded to be not due to the plastic instability but due to the deformation constraint at both ends of specimen.
  • Prediction on Nominal Stress-Strain Curve of Anisotropic Polycrystal with Texture by FE Analysis

    pp. 1825-1832

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    DOI:10.2320/matertrans.M2010094

    In order to predict nominal stress-strain (S-S) curve in tensile test for a commercial purity titanium (hereafter, CP-Ti) with texture, three boundary conditions are elucidated as well as plastic anisotropy in three-dimensional finite element (FE) analysis. Three boundary conditions are a free case, a full constraint case at both ends of specimen, and a simulated case based on empirical data. In the simulated case, the shape change and the nominal S-S curve can be well predicted with deviation smaller than 10%. The origin of localized necking is concluded not due to the plastic instability but due to the constraint at both ends of specimen. Deviation point of the nominal S-S curve at the simulated case from that of the free case has been suggested as the onset of localized necking of CP-Ti, which is determined as 0.25 in transverse direction and 0.50 in rolling direction.
  • Effect of Pretreatment Film Composition on Adhesion of Organic Film on Zinc Coated Steel Sheet

    pp. 1833-1841

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    DOI:10.2320/matertrans.M2010185

    It is known that chromate pretreatment films with dry-type silica adhere well to organic films such as paint and laminate films on a zinc coated steel sheet. The role of surface polarity in the adhesion mechanism has been investigated for a long time, but remains unclear. We examined the correlation between the composition of chromate pretreatment films with dry-type silica and their adhesion to a laminate film on a zinc coated steel sheet, for both the wet silica and without silica types of chromate pretreatment film. We also investigated the adhesion mechanism by analyzing the pretreatment films using SEM, AFM, BET, TEM and EDX.
    The results were as follows. Not only the type and particle size of silica, but also the silica/chromate ratio and the coexistence of chromate affected the adhesion of laminate films. The highest adhesive strength was obtained for the pretreatment films formed in a pretreatment solution with SiO2/Cr ratio of 4.0 and dry-type silica with a particle diameter of 7 nm. The good adhesion of chromate pretreatment films with dry-type silica is due to an anchor effect by sub-micron surface roughness and an increased number of interface bonding sites by a nano-porous structure. The formation of an insoluble Cr(III) compound layer plays an important role in binding silica particles to the zinc coating surface.
  • Electrodeposition of Silver-Nickel Thin Films with a Galvanostatic Method

    pp. 1842-1846

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    DOI:10.2320/matertrans.M2010126

    NiAg thin films were deposited by galvanostatic electrodeposition in an electrolyte containing NiSO4, AgNO3 and C6H5Na3O7. The variation of composition and crystallography of electrodeposited NiAg thin films with current density and electrolyte concentration was investigated. At a low current density, electrodeposition of silver was dominant, which could be induced by a comparably low reduction potential. However, nickel electrodeposition became the dominant component at a higher current density because of the mass transfer limitation of Ag ions. When 50 mA/cm2 was applied, the FCC (200) phase was observed, which implies significant enhancement of the nucleation rate by increasing the reduction potential at a high current density condition.
  • Concept of Furnace for Metal Refining by Microwave Heating —A Design of Microwave Smelting Furnace with Low CO2 Emission—

    pp. 1847-1853

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    DOI:10.2320/matertrans.M2010026

    A concept of ore refining by microwave heating and a model of the microwave smelting furnace were proposed in order to build the first prototype furnace for efficient iron ore refining. The repose and wall friction angles were measured for powdery ores (mined in Robe River, Australia), and the electromagnetic field distribution was analyzed through numerical calculation to design the furnace.
    The hearth angle needs a certain amount of degrees, and the degrees must be less than 19.8-degrees in perspective of the mass transfer. A reverberator type furnace was adequate and utilizeed the energy efficiently. The reflected output reached the maximum when the electric wall was made at the cone surface.
  • Formation of Internal Crack in High-Speed Twin-Roll Cast 6022 Aluminum Alloy Strip

    pp. 1854-1860

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    DOI:10.2320/matertrans.L-M2010818

    An Al-Mg-Si based 6022 aluminum alloy, which represents a wide freezing temperature range in solidification, was cast through high-speed twin-roll casting (HSTRC). Under a roll rotation speed of 60 m/min and an initial roll separating force of 14.1 kN, the cast strip showed a large central crack, which deteriorated a quality of the cast strip. The strip exhibited an equiaxed grain structure at the near-surface region and a band of fine globular grains at the mid-central region. The large central crack was mostly observed at a boundary between the central band and dendritic solidified shell. It was considered that decrease of cooling rate near the minimum roll gap caused the cracking due to residual liquid. In order to find the condition for defect-free strip casting, HSTRC was carried out under various casting conditions. Large-scale central cracking was prevented when a sufficient roll separating force was applied by increasing a solidification time, or increasing an initial roll separating force. From a direct temperature measurement during the HSTRC using a sensitive thermocouple, it was confirmed that a cooling rate increased with increasing the roll separating force.
  • Vertical and Horizontal Directional Solidification of Zn-Al and Zn-Ag Diluted Alloys

    pp. 1861-1870

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    DOI:10.2320/matertrans.M2010036

    Zinc-Aluminum and Zinc-Silver alloys were solidified in horizontal and vertical form and the results obtained analyzed. Thermal parameters such as the cooling rates, the liquidus and solidus interphase velocities, and the temperature gradients were determined. The velocity of the liquidus interphase is greater than or sometimes equal to the velocity of the solidus interphase. In the vertical setup, columnar-to-equiaxed (CET) grain transition was obtained. CET does not occur in the horizontal one-directional solidification when critical minimum gradients reach values greater than 0.1 K/mm.
  • Pore Morphology of Porous Al-Ti Alloy Fabricated by Continuous Casting in Hydrogen Atmosphere

    pp. 1871-1877

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    DOI:10.2320/matertrans.M2010223

    Porous Al-5 mass%Ti alloy was fabricated by unidirectional solidification in hydrogen atmosphere, using a continuous casting technique. The porous Al-Ti alloy was prepared at different transfer (solidification) velocities, and the effect of transfer velocity on the pore morphology was investigated. It was found that the pore shape changes with increasing transfer velocity, while the porosity does not change with increasing transfer velocity. In the case of a low transfer velocity (0.5 mm·min−1), elongated pores surrounded by the columnar microstructure are formed, which indicates that the pores grow along the solidification direction together with the solid phase. In the case of a middle transfer velocity (5.0 mm·min−1), elongated pores surrounded by the columnar microstructure and needle or plate-like Al3Ti alloys are formed. In the case of a high transfer velocity (10.0 mm·min−1), spherical pores surrounded by the equiaxed microstructure are formed, because the primary crystals formed in the solidification front prevent the growth of elongated pores. It is suggested that the pore morphology is closely related with the solidification rate.
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  • Crystallization Behaviour of Electroless Ni-P UBM with Medium Phosphorous Induced by Single and Step Heat Treatment

    pp. 1878-1882

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    DOI:10.2320/matertrans.M2010178

    This study examined the crystallization behavior of electroless Ni-P UBM with an medium phosphorous content (∼15 at%) induced by a single and step heat treatment using in-situ transmission electron microscopy (TEM). Different heat treatment processes affected the crystallization behavior of electroless Ni-P UBM. After single heat treatment at 300°C for 60 min, the electroless Ni-P UBM contained Ni and Ni3P. In addition to Ni and Ni3P, more complex Ni-P compounds, such as Ni12P5 and Ni5P2 formed in the electroless Ni-P UBM resulting from a step heat treatment at 150°C for 60 min followed by 300°C for 60 min.
  • Magnetic Properties of Rapidly Solidified Ribbon of Fe49Co49V2 and Spark-Plasma-Sintered Pellet of Its Powder

    pp. 1883-1886

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    DOI:10.2320/matertrans.M2010047

    Both the coercive force and the saturation magnetostriction of rapidly solidified Fe49Co49V2 thin ribbons increase with increases in the circumferential velocity during the melt-spinning process because of the formation of a preferred orientation texture. Fe49Co49V2 pellets produced by spark plasma sintering of the powder prepared by milling the rapidly solidified ribbon also exhibit a coercive force higher than those of ordinary alloy ingot. In particular, the coercive force significantly increases after the milling process, reaching a maximum value of 86 Oe, or 10 times higher than an ingot with same composition. The change in the magnetic properties results from the effects of rapid solidification and of the stress induced during the milling process. The magnetostriction of a pellet is slightly smaller than the value for the ingot and is comparable to the value of rapidly solidified ribbons produced by low circumferential velocities.
  • A New Non-PRM Bumping Process by Electroplating on Si Die for Three Dimensional Packaging

    pp. 1887-1892

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    DOI:10.2320/matertrans.M2009314

    A new bumping process on a Si die by electroplating without a photo-resist-mold (PRM) was assessed for the three dimensional (3D) stacking of Si dice. In this process, solder bumps were deposited selectively onto the surface of a Cu plugged-through silicon-via (TSV) in a Si die. Since lithography related processes to make a PRM for solder bumping were omitted, it can reduce the production time and cost for bumping. The substrate was a Si wafer, and TSVs were produced by deep-ion-reactive-etching (DRIE). As an insulation layer, SiO2 was formed by high-density-plasma-chemical-vapor-deposition (HDPCVD) or wet oxidation method. A Ti adhesion and an Au seed layers were deposited by sputtering. Cu was plugged into the vias by pulsed direct-current (DC) electroplating. The backside of the Si wafer was ground by chemical-mechanical-polishing (CMP). Sn bumps were electroplated on the Cu plugged vias by a current supplied through a Cu-overplated layer (COL) on the vias surface. As experimental results, wet oxidation provided a uniform SiO2 thickness through the via depth, whereas the SiO2 thickness by HDPCVD decreased with the via depth. The Ti and Au thicknesses also decreased along the via depth. In electroplating for Cu plugging, defect-free Cu plug was achieved by pulsed DC for 18 h, where one cycle composed of a 5 s cathodic term with −12.2 mA·cm−2 and a 25 s anodic term with 2.3 mA·cm−2. Sn bumping using COL without PRM was accomplished successfully at a current density of −30 mA·cm−2 for 15 min. The diameter and height of the Sn bumps ranged from 48.5 to 52 μm and 22 to 25 μm, respectively. The bumps had a rivet head shape without a columnar part and showed facets on the bump surface. The COL was removed by CMP without damage to the bumps.
  • Effect of Alumina Content on the Mechanical Properties of Alumina Particle Dispersion Magnesium

    pp. 1893-1900

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    DOI:10.2320/matertrans.M2010060

    To enhance the mechanical properties of magnesium (Mg) alloys, powders of pure Mg (Al2O3/Mg) dispersed with over 10 vol% alumina (Al2O3) particles were made by ball milling powder mixtures of pure Mg and Al2O3 particles with particle sizes of 0.3 and 1 μm. The effect of the Al2O3 content of hot-pressed discs formed from the Al2O3/Mg powders on the mechanical properties of the discs was investigated. The Al2O3 particles were found to be uniformly dispersed in the Al2O3/Mg discs. The hardness of the discs increased sharply at an Al2O3 content of 20 vol%. This is considered to be because of the excellent interfacial bonding between Mg and Al2O3 when the Al2O3/Mg discs contain a sufficient amount of Al2O3 particles and have a sufficient interparticle distance. The maximum hardness of a 20 vol% Al2O3/Mg disc (with an Al2O3 particle size of 0.3 μm) was 220 HV, which is much higher than the hardness of AZ91 Mg alloys. The bending strength of the Al2O3/Mg discs increased from 0 to 10 vol% Al2O3 and then decreased from 20 vol% Al2O3. It is considered that the 0 and 10 vol% discs have similar fracture deflections and that the 10 vol% discs with Al2O3 particles can withstand higher compression and tension forces than the 0 vol% discs which do not contain any particles. The reason for the lower bending strength of the 20 vol% Al2O3 disc is thought to be because it is harder and more brittle, making it easier for voids to form in the discs, and thus making it easier for cracks to propagate on the specimen surface.
  • Influence of Nd Oxide Phase on the Coercivity of Nd-Fe-B Thin Films

    pp. 1901-1904

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    DOI:10.2320/matertrans.MAW201019

    To understand the coercivity mechanism of Nd-Fe-B sintered magnets, the microstructure of grain boundary composed of Nd2Fe14B and Nd-rich phases has been studied. However, the influence of Nd-rich phase, which contains some amount of oxygen, on microstructure and coercivity has not been clear. In this study, the influence and the interfacial microstructure between the Nd2Fe14B phase and the Nd-rich phase were investigated using Nd-Fe-B thin films. Furthermore, the microstructural change of Nd-oxide (Nd-O) phase was investigated using oxidized Nd thin films. The coercivity (HcJ) of the Nd-Fe-B thin films decreased by about 80% from the level of as-deposited film (HcJ(as-depo)) after oxidation and annealing at low temperature (∼350°C). From TEM observation of the Nd-Fe-B film, some steps along the surface of the Nd2Fe14B phase contacting with the hcp Nd2O3 phase were observed. Investigation of the microstructural change of Nd oxide phase was carried out using Nd thin films. The as-deposited Nd film was composed of the dhcp Nd (α-Nd) phase, and the fcc NdOx phase formed at the surface of α-Nd phase after oxidation. After annealing at 350°C, the hcp Nd2O3 phase crystallized from the fcc NdOx phase, and it resulted in large roughness at the boundary with the α-Nd phase. From the results described above, the crystallization of hcp Nd2O3 phase causes damage at the surface of Nd2Fe14B phase during the annealing at low temperature, which results in the decrease of coercivity.
  • Characteristics of Printed Thin Films Using Indium Tin Oxide (ITO) Ink

    pp. 1905-1908

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    DOI:10.2320/matertrans.MAW201017

    Characteristics of the indium tin oxide (ITO) thin films fabricated by a direct printing method were investigated. For the direct printing, 30 mass% ITO ink was formulated by dispersing ITO nanoparticles of less than 10 nm in diameter into alcohol based solvent with additives. Then, the ITO ink was printed onto a glass substrate, which was followed by the heat treatment at 350, 400, 500 and 600°C to fabricate the sintered films. Then, cross-section of sintered film was observed. As a result, the ITO thin film showed porous nanostructure. Also, sheet resistance of the ITO thin film decreased with increasing heating temperature. The decrease of the resistance was attributed to the improvement of carrier concentration in the ITO thin film. In the meanwhile, the optical transmittance increased in proportional to the heating temperature. In case of the ITO thin film heated at 600°C, the improved characteristics of the electrical resistance and the optical transmittance were achieved as 2.19×103 Ω/sq and 78.6%, respectively.
  • Thermoelectric Characteristics of the Thermopile Sensors with Variations of the Width and the Thickness of the Electrodeposited Bismuth-Telluride and Antimony-Telluride Thin Films

    pp. 1909-1913

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    DOI:10.2320/matertrans.M2010122

    Thermopile sensors were processed on glass substrates by using successive electrodeposition of p-type Sb-Te and n-type Bi-Te thin films, and their thermoelectric characteristics were measured. The thermopile sensor, consisting of the p-n legs of 2 μm-thickness and 50 μm-width, exhibited the sensitivity of 24.8 mV/K. By changing the width of the p-n thin-film legs from 50 to 100 μm, the sensitivity decreased to 15.4 mV/K because of less pairs of the p-n thin-film legs in the thermopile. With increasing the thickness of the thin-film legs from 2 to 5 μm, the sensitivity was improved to 36.5 mV/K due to higher Seebeck coefficients of the 5 μm-thick Bi-Te and Sb-Te films than those of the 2 μm-thick films.
  • Equal Channel Angular Extrusion Technique for Controlling the Texture of n-Type Bi2Te3 Based Thermoelectric Materials

    pp. 1914-1918

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    DOI:10.2320/matertrans.M2010065

    The repetitive equal channel angular extrusion (ECAE) technique was applied to prepare n-type Bi1.9Sb0.1Te2.7Se0.3 thermoelectric materials prepared from rapidly solidified and stacked foils. Each ECAE process was performed at temperatures ranging from 693 to 773 K in an argon atmosphere. The number of passes in the ECAE process was varied up to 6 passes via route CY. The texture of the specimens after ECAE was observed using an Orientation Imaging Microscopy (OIM). Observing the texture revealed that basal planes are satisfactory aligned, and that the direction of the basal planes tends to rotate in the extrusion direction with an increasing number of passes. A highly orientated texture was gradually formed by repetitive ECAE processing. Both the number of passes and extrusion temperature were found to affect the texture characteristics. Formation of twin boundaries in the initial stage of the repetitive ECAE plays a key role in releasing the shear strain imposed during ECAE operation. A strongly orientated texture was observed in the specimen after 6 passes of ECAE via route CY extruded at 773 K. The grain size of the specimen after ECAE, however, was found to be unrelated to the number of passes. The average grain size was within the range 9.0 to 19.2 μm in specimens extruded at 693 to 773 K. Measurements of thermoelectric properties revealed that carrier mobility was strongly dependent on the degree of orientation. Results also showed that a higher power factor of 4.01×10−3 Wm−1·K−2 was measured mainly due to increased carrier mobility. The Z value of the specimen after 6 passes of ECAE via route CY at 773 K reached 3.04×10−3 K−1. This result indicates that the ECAE technique largely improves thermoelectric properties of bismuth telluride based compounds.
  • Optimization of Sintering Temperature for Maximizing Dimensionless Figure of Merit of La-Doped Strontium Titanate Thermoelectric Material in the Combination of Combustion Synthesis with Post Spark Plasma Sintering

    pp. 1919-1922

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    DOI:10.2320/matertrans.M2010102

    This paper describes thermoelectric properties of La-doped SrTiO3 prepared by using a combination of combustion synthesis (CS) with post spark plasma sintering (SPS), on which effects of sintering temperature were mainly examined. In experimental, combustion-synthesized (CSed) samples (Sr1−xLaxTiO3, x=0.08) were spark-plasma-sintered (SPSed) at temperatures from 1513 to 1663 K and the thermoelectric properties of sintered Sr0.92La0.08TiO3 were measured from room temperature to 1073 K. In experimental sintering temperature range, sintering temperature didn’t heavily affect the average grain size of sintered SLTO in the case that sintering temperature ranges from 1543 to 1603 K. However, when sintering temperature rose over 1603 K, the average grain size of sintered SLTO was dramatically affected and it increased with sintering temperature. With increasing sintering temperature, the effects of phonon scattering at grain boundaries decreased therefore, thermal conductivity of all sintered samples increased with sintering temperature. With increasing sintering temperature, electric conductivity of sintered SLTO increased and absolute value of Seebeck coefficient of sintered SLTO decreased because sintering at higher temperature caused the more oxygen defects which generate the larger carrier density. In conclusion, the product sintered at 1573 K showed the maximum of the dimensionless figure of merit of 0.16 at 1005 K.
  • New Development of Titanium Wire Ball as Vertebra Substitute

    pp. 1923-1926

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    DOI:10.2320/matertrans.M2010083

    Fabrication of a new ball-shaped titanium material for vertebral compression fracture treatment has been studied using micro-arc oxidation in an electrolyte containing hydroxyapatite (HAp). To prevent microstructural disturbance on the surface, ethanol was used as a co-solvent. The material thus obtained was found to have high porosity with many micropores, fixed HAp particles on the surface, and elasticity comparable to cancellous bone. Furthermore, it was demonstrated that the materials have osteoconductivity and the elasticity was kept under cycled stress. These indicate that they can be new bone filling materials for percutaneous vertebroplasty (PV).
  • Recovery of Platinum and Palladium from the Spent Petroleum Catalysts by Substrate Dissolution in Sulfuric Acid

    pp. 1927-1933

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    DOI:10.2320/matertrans.M2010218

    In this study, the recovery of platinum and palladium from petroleum catalysts has been elaborated using a method consisting mainly of dissolving alumina substrate in sulfuric acid thereby concentrating the precious metals in the residue. The effect of dissolution temperature and time, concentration of sulfuric acid, and pulp density on the dissolution of alumina substrate was investigated systematically. The optimum dissolution conditions for the platinum catalysts AR-405 and R-134 were: sulfuric acid 6.0 mol/L, dissolution temperature 100°C, dissolution time 2∼4 h, pulp density 220 g/L. The dissolution of R-134 catalyst the substrate consisting only of γ-Al2O3 phase was higher than that of AR-405 which contained the mixture of γ-Al2O3 or α-Al2O3. The optimum conditions for LD-265 are: sulfuric acid concentration 8.0 mol/L, dissolution temperature 100°C, pulp density 220 g/L and time 18 h. It was found that platinum and palladium which was impregnated to alumina substrate as fine particles, also dissolved to some extent during sulfuric acid treatment. They could be recovered by a cementation process using aluminum powder. The complete recovery of Pt from AR-405 and R-134 catalysts was possible by the proposed method. Nevertheless, the method may not be applicable to LD-265 catalyst due to the time-consuming substrate dissolution process.
  • Phase Separation from L21 to (B2+L21) in Fe-24.6Al-7.5Ti Alloy

    pp. 1934-1938

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    DOI:10.2320/matertrans.M2010167

    As-quenched microstructure of the Fe-24.6 at% Al-7.5 at% Ti alloy was a mixture of (A2+L21) phases. When the as-quenched alloy was aged at 1173 K for moderate times, the L21 domains grew considerably and B2 phase was formed at a/2⟨100⟩ anti-phase boundaries (APBs) as well as phase separation from well-grown L21 to (B2+L21*) occurred basically contiguous to the APBs, where L21* is also a L21-type phase. With continued aging at 1173 K, the phase separation would proceed toward the whole well-grown L21 domains. This microstructural evolution has not been reported in the Fe-Al-Ti alloy systems before.
  • Microstructure Analysis of High Coercivity PLD-Made Nd-Fe-B Thick-Film Improved by Tb-Coating-Diffusion Treatment

    pp. 1939-1943

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    DOI:10.2320/matertrans.M2010189

    Microstructure of high coercive pulsed laser deposition (PLD) made Nd-Fe-B thick-film magnet improved by “Tb-coating-diffusion” treatment has been investigated with an analytical transmission electron microscope (ATEM). It was found that the PLD-made film consists mainly of fine Nd2Fe14B grains about 200 nm in diameter and a large amount of Nd intergranular phases. Energy dispersive X-ray spectroscopy (EDS) analysis showed that each Nd2Fe14B grain is covered with a thin (Nd,Tb)2Fe14B layer about 20–30 nm in thickness and thin Nd-Tb grain-boundary-layer about 5–10 nm in width. The present results indicate that such a two-layered-wrapping structure enhances significantly the coercivity of the PLD-made thick-film magnet.
  • In Vitro Biocompatibility of Ni-Free Ti-Based Shape Memory Alloys for Biomedical Applications

    pp. 1944-1950

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    DOI:10.2320/matertrans.M2010038

    This study was undertaken to evaluate the in vitro biocompatibility of newly developed Ni-free Ti-based shape memory alloys (SMAs) in comparison to that of commercial pure titanium (cpTi). This study compared Ti-24 mol%Nb-3 mol%Al (Ti-Nb-Al) and Ti-7 mol%Cr-3 mol%Sn (Ti-Cr-Sn) to cpTi from a cell-compatibility perspective. In all, 63 samples (21 samples for each group) were prepared, which were machined into 10-mm-diameter, 0.15-mm-thick, mirror-polished disks. Their surface morphology was evaluated using scanning electron microscopy (SEM). The chemical composition of the sample surface was determined using an energy dispersive X-ray analyzer (EDX). Sample surface roughness was measured using a non-contact 3D profiler. After sample surface observations, the cell proliferation and viability of African green monkey kidney fibroblast cell line COS7 in direct contact with these new alloys were evaluated by DNA quantification, by live cell imaging using CelLuminate Red fluorescent cell stain as a new method, and by cytoskeletal observations by immunofluorescent actin labelling. Cell proliferation was examined after 1, 3, and 5 days of culture. Results were the following. (i) Each sample showed high purity and a very smooth surface, showing no morphological differences among groups. (ii) The COS7 cells took in sufficient CelLuminate Red to visualize the cells using epifluorescent microscopy, and (iii) cell proliferation with Ti-Cr-Sn was lower than with either cpTi or Ti-Nb-Al. These results suggest that Ti-Nb-Al alloy showed biocompatibility as high as that of cpTi and that it is more suitable for biomedical applications.
  • Effect of Oxide Dispersion on Dendritic Grain Growth Characteristics of Cast Aluminum Alloy

    pp. 1951-1957

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    DOI:10.2320/matertrans.M2010166

    The dispersion characteristics of the nano-sized Y2O3 powders in molten aluminum were investigated from the viewpoint of changes in microstructure and mechanical property as a function of oxide contents. As the solidification structure, the oxide nanoparticles dispersed in the columnar crystal was mainly segregated on the grain boundary, whereas the oxide nanoparticles dispersed in the equiaxed crystal was uniformly dispersed on both grain boundary and inside the crystal. The most uniform dispersion of oxide nanoparticles was observed at Y2O3 content of 2 mass%. As Y2O3 content of 3 mass%, the size of oxide nanoparticles in metal matrix increased due to the particle aggregation, as confirmed by SEM analysis. Moreover, it was found that the mechanical properties such as hardness and tensile strength were improved at Y2O3 content of 2 mass%, indicating the well-dispersion of nano-sized Y2O3 powders in cast aluminum. The hardness was increased by 1.2 times up to 57 HV and tensile strength was increased by 1.55 times up to 80 MPa, compared with the case of pure aluminum. However, at Y2O3 content of 3.0 mass%, tensile strength was sharply decreased by 0.6 times due to aggregation of oxide nanoparticles, while the hardness was increased to 57 HV, which is the same as the case of Y2O3 content of 2.0 mass%.
  • Microstructures and the Charge-Discharge Characteristics of Advanced Al-Si Thin Film Materials

    pp. 1958-1963

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    DOI:10.2320/matertrans.M2010070

    In this study, radio frequency magnetron sputtering was used to prepare Al-Si film negative electrodes and the effect of pre-sputtered Al thin film on the charge-discharge capacity characteristics are discussed. The pre-sputtered 40 nm Al thin film not only reduced the resistivity of the composite negative electrode film, but also prevented peeling between the Al-Si films and Cu foils. In addition, annealing in the vacuum led to an improvement on the index of crystalline (IOC) of the negative electrode matrix and enhanced the diffusion of the pre-sputtered Al film. The annealed Al-Si film with diffused Al film saw an enhancement in the bonding characteristics at the interface stability and the charge-discharge cycling life at high temperature (55°C).
  • Numerical Study of Fluid Flow and Heat Transfer Behaviors in a Physical Model Similar in Shape to an Actual Glass Melting Furnace and Its Experimental Verification

    pp. 1964-1972

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    DOI:10.2320/matertrans.M2010215

    In this study, a three dimensional numerical model based on the SOLA-VOF method, which incorporates a Quasi-two Phase method to consider the gas bubbling phenomena, was developed to investigate the fluid flow and heat transfer behaviors in a glass melting furnace. A reduced physical model with heating electrodes and air bubbling devices was also constructed to validate the numerical model. The reduced physical model was made of an acrylic tank similar in shape to an actual glass melting furnace, but reduced to one-tenth in size. The gas flow rate was determined at 6.67×10−7 Nm3/s by the similarity conversions. The electrode temperatures were set between 298 K to 373 K. The flow trajectories of tracer particles and temperature field were measured to validate the accuracy and reliability of the numerical model. The results show that the temperature and trajectories of tracer particles predicted by the numerical model were consistent with experimental observations/measurements from the physical model.
  • Effect of Denucleating Techniques on Undercooling of Co2NiGa Alloys

    pp. 1973-1976

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    DOI:10.2320/matertrans.M2010100

    By using the method of glass fluxing combined with superheating-cooling cycling, the undercooling behaviour of Co2NiGa alloy melts was investigated. The obtained degree of undercooling is very different with differently used glass due to different denucleating mechanisms. The denucleating agent composed by 70% Na2B4O7 + 30% NaSiCa showing a moderate viscosity and good physicochemical purification ability was optimized to be an ideal denucleating agent. By using this kind of denucleating agent, a large (>200 K) and stable undercooling can be obtained in Co2NiGa melts after superheating over 200 K for 1.5 min. The effects of the superheating and holding time on undercooling have been discussed.
  • Simulation of Deposition Behavior of Bulk Amorphous Particles in Cold Spraying

    pp. 1977-1980

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    DOI:10.2320/matertrans.M2010213

    Bulk amorphous Zr41.2Ti13.8Cu12.5Ni10Be22.5 powders were chosen as feedstock to investigate the deposition behavior of amorphous alloy particles in cold spraying. The constitutive equation of the particles was established. Using the finite element method, the effects of velocity and preheating temperature of the particles on their deformation behavior were studied, the temperature changes near the contact interface were calculated, the relationship between deposition behavior and effective bonding was analyzed, and finally the coating generation mechanism with the cooling conditions were discussed.

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