In this review paper, recent studies on low-temperature sintering strategies of copper materials for conductive layer preparation have been summarized. Coinage metals, gold, silver, and copper have been used as materials for conductive inks and pastes for printed electronics. Copper is a highly electrically and thermally conductive material that can be used in electronic circuits and die-attach materials. Recently, copper-based inks/pastes have gained significant attention of researchers and industries as conductive materials. However, copper is readily oxidized under air, especially, at the nanoscale, and copper particles may catch fire because of the rapid oxidization. To overcome this issue, copper nanoparticles and fine particles are coated with organic molecules which act as insulators after sintering. Some interesting surface treatments or activation strategies have been investigated in this regard. In this paper, different perspectives on the applications of copper in conductive and die-attach materials have been presented.

Zr-based metallic glasses have been extensively studied owing to their high glass forming ability and superior mechanical properties. The glass forming ability of Zr–Ni–(Al, Ti, Nb) and Zr–Cu–(Al, Ti, Fe, Co, Ni) bulk metallic glasses was studied based on the bond parameter function and formation enthalpy model. Elliptical curves can be used to distinguish the glass forming regions from other regions in two-dimensional model diagrams. The elliptic parameters can be defined by the relative parameters of the constituent elements. The reproducibilities of this model for Zr–Ni- and Zr–Cu-based metallic glasses were 84.4% and 93.4%, respectively, indicating good agreement of this model with available experimental data. The predicted results provide a new strategy for designing excellent-performance metallic glasses.

Three-point bending tests were performed on pure zinc single crystals with different crystal orientations to investigate orientation dependence on bending deformation behavior. With basal planes parallel to the neutral planes, the specimens deformed due to basal slips and formed a gull shape after deformation. On the other hand, specimens whose neutral planes are perpendicular to the basal planes and neutral axes are perpendicular to [0001] deformed due to second order pyramidal 〈c + a〉 slips, {1012} twinning and basal slips within {1012} twins, displaying a V shape. The bending deformation behavior of pure zinc single crystals was found to show strong orientation dependence. Also, the bending deformation behavior of pure zinc single crystals was found to differ from that of pure magnesium single crystals when the neutral plane and neutral axis are respectively perpendicular to the basal planes and [0001].

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 86 (2022) 8–16. Abstract, Fig. 1 and the caption of Fig. 11 are slightly modified.

The factors representing the stability of retained austenite (γ_R) in low-alloy TRIP steel at a plastic strain of up to 0.04 were statistically analyzed by evaluating the size, strain level, crystal orientation, morphology, and precipitation site of a large number of γ_R grains (>1000). The ratio (R_area) of the area of γ_R after deformation to that before deformation was adopted as a representative parameter for the stability of γ_R. The frequency of low R_area in fine γ_R was significantly high, indicating that they were unstable. Moreover, it was indicated that γ_R in the high-strain regions tended to be unstable, whereas γ_R in the bainite region was stable. The crystal orientation and aspect ratio did not correlate with R_area in TRIP steel. The strain level and precipitation sites in coarse γ_R were uncorrelated with R_area, indicating that the size of the γ_R has a greater effect on its stability. The stability of γ_R was not dominated by the carbon concentration but was affected by other factors. The size of γ_R may represent the stress conditions at the γ_R/ferrite or bainite boundaries, which is why this factor represents the stability of γ_R in the TRIP steel at a plastic strain of up to 0.04.

Improving the oxidation resistance and mechanical strength of Mg₂Si is an especially critical issue for practical use in thermoelectric devices. To solve this problem, a sintered sample was prepared by adding 1.1 mol% to 11.0 mol% of TiB₂ powder to Mg₂Si (0.2 at% Sb) powder, and the oxidation resistance and mechanical strength were evaluated. The oxidation resistance was evaluated by raising the temperature from 300 K to 1023 K in a 200 Pa H₂O atmosphere using an environmental scanning electron microscope and the sample surface condition was evaluated. In addition, the composition of the sample surface layer before and after oxidation was evaluated by energy dispersive X-ray spectroscopy (EDX). Mechanical strength was evaluated by a three-point bending test from 300 K to 823 K. Thermoelectric properties were measured from 300 K to 773 K.

According to EDX analysis, the oxygen concentration in the sample without TiB₂ before and after thermal oxidation was 0.26 at% and 37.15 at%, respectively, while the concentration in the sample containing 3.3 mol% of TiB₂ was 0.02 at% and 6.59 at%, respectively. The bending strength of the sample without TiB₂ was 152.8 MPa at 773 K, and that of the sample with 3.3 mol% TiB₂ was 207.7 MPa at 773 K. The dimensionless figure of merit of Mg₂Si (0.2 at% Sb) with 3.3 mol% TiB₂ was 0.49 at 773 K, which was the same as without addition of TiB₂. It was found that the addition of up to 4.4 mol% of TiB₂ to Mg₂Si can suppress oxidation and improve high temperature bending strength without affecting thermoelectric properties.

Magnetic Zn–Al hydrotalcite (mHT) was synthesized by co-precipitation of Zn²⁺/Al³⁺ salt mixture in the presence of Fe₃O₄. mHT nanomaterials were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, vibrating-sample magnetometer, and nitrogen adsorption isotherm. Then, mHT was used as support for immobilizing cellulase under an adsorption mechanism to form cell@mHT. The effect of pH, cellulase concentration, the amount of mHT, and immobilization time on the immobilization of cellulase onto mHT were carefully investigated. The enzyme activity of both free cellulase and cell@mHT as well as immobilization efficiency, was analyzed by determination of reduced glucose using DNS as a color indicator. The immobilization process obtained the highest loading efficiency with 99.0% when carried out at pH 5, with a cellulase concentration of 0.1 mg/ml and using 0.1 g of mHT. Cell@mHT shows good enzyme activity when reacting with 1 mass% CMC solution at 50°C after 45 minutes with relative enzyme activity of 80.8%.

Fe–Cu–C sintered steel is prone to rust, therefore rust preventive oil is used for storage. These rust preventive oils generally have a problem of reduced workability due to stickiness caused by high viscosity. However, few electrochemical studies have been conducted on the correlation between viscosity and corrosion resistance. Therefore, in this study, we investigated the effect of the base oil viscosity on the corrosion resistance of rust preventive oil by changing the base oil viscosity.

As an experimental method, electrochemical measurements were carried out using the water screen test established in our laboratory, which can quantitatively evaluate the rust preventing ability. According to the polarization curves measurements, corrosion current density I_corr is reduced by coating the rust preventive oil. It showed about 100,000 times better corrosion resistance with and without rust preventive oil. While, almost no difference in rust preventive ability was confirmed due to the difference in base oil viscosity. According to the results of the open circuit potential, the time required for the potential of Fe–Cu–C sintered steel to saturate was 505 ks for low viscosity, 651 ks for medium viscosity, and 340 ks for high viscosity. Therefore, it was found that there is no correlation between the time until rust occurs and the viscosity of the oil film. It was also found that the effects of the additives were fully exerted even if the oil viscosities were different. Therefore, the problem of reduced workability due to stickiness can be solved by using low viscosity base oil.

Identification of surface atom diffusion paths, as well as their activation energies, is important when analyzing the kinetics of surface phenomena, such as spillover in heterogeneous catalysis. This study developed a geometrical algorithm, using Voronoi tessellation, to derive diffusion paths connecting already identified reasonable adsorption sites. Existence of a diffusion path is presumed when the Voronoi cells of two reasonable adsorption sites share a Voronoi ridge and the line segment connecting the two adsorption sites intersect with the corresponding Voronoi ridge. The algorithm was applied on the rutile TiO₂ (110) and reconstructed CeO₂ (001) surfaces. Nudged elastic band calculations on the obtained paths showed that the activation energy required for hydrogen spillover was 1.2 and 0.6 eV for TiO₂ and CeO₂, respectively. The algorithm is, in principle, applicable to any type of surface and adsorbing atom species. Its ability to systematically obtain reasonable surface diffusion paths without eyeballing is valuable when studying the plausibility of surface diffusion in hydrogen spillover, and other situations, on both simple and complicated surfaces.

Duplex stainless steel (DSS) can be used in harsh environments owing to its excellent strength and corrosion resistance. However, the sigma (σ) phase, which reduces the corrosion resistance, precipitates during manufacturing. To elucidate the deterioration mechanism of corrosion resistance, we prepared three types of DSS (F55) with different volume fractions of the σ phase. Polarization curves were measured to investigate the corrosion characteristics. The corrosion rate and corrosion potential (E_corr) were evaluated using Tafel extrapolation. Owing to the precipitation of the σ phase, E_corr was low, and the corrosion rate was high, which indicates that the σ phase precipitation promotes the anodic reaction. An electricity damage corrosion test at a constant quantity of electricity was conducted on the sample, which was heat-treated at 1173 K and had the largest amount of σ phase precipitation. The X-ray diffraction results before and after the corrosion test showed that the alpha (α) phase was reduced by corrosion. In addition, the results of optical micrographs and electron probe microanalysis maps confirmed that the σ phase was precipitated in the α phase. This indicates that the precipitation of the σ phase increases the corrosion rate of the surrounding α phase. Therefore, the precipitation of the σ phase, which is harmful to DSSs, deteriorates the corrosion resistance because the periphery of the σ phase is easily corroded.

Erosion testing was conducted on iron specimens in various aluminum-based molten binary alloys, and the correspondence between the intermetallic compounds formed at the contact interface and the thermodynamically stable phases was examined. On the basis of the results, the mechanism and dominant factor of the dissolution of the intermetallic layers were investigated. Although the erosion ratio of the specimens by molten Al–3%Mn alloy was significantly small, the ratios by molten alloys of the Al–Si system were great, in comparison to those by other melts. Meanwhile, the intermetallic compounds identified by EBSD corresponded to the stable phases based on the equilibrium calculation of the compositions analyzed by EDS and/or the pseudo binary phase diagrams of the aluminum alloy-Fe systems. In addition, there was a positive correlation between the experimental erosion rate and apparent saturation solubility of Fe to the melts based on the pseudo binary phase diagrams. These results suggest that the dissolution of the layers was caused by almost the same mechanism as the phenomena described by Noyes-Whitney-Nernst equation, and that the saturation solubility of Fe is a dominant factor affecting the diffusion-controlled dissolution from the solid-liquid interfaces under local equilibrium.

This Paper was Originally Published in Japanese in J. JFS 93 (2021) 541–550.

The role of retained austenite and martensite on impact-wear resistance of 27%Cr cast iron quenched at temperatures of 1223 K and 1423 K was investigated at various impact angles by impact wear tests using an air blasting machine. Measurement of the hardness and volume fraction of martensite in the matrix of both samples as well as SEM observation were performed. Quenching treatment at 1223 K resulted in a higher volume of initial martensite in the matrix, and wear rate reached maximum at 60°. On the contrary, the matrix with a high volume of retained austenite obtained by quenching at 1423 K showed a maximum impact-wear rate at 45°. The difference in the wear resistance between the two types of as-quenched samples can be explained by the transformation from metastable austenite to strain induced martensite and self-tempering of the initial martensite occurring under impact wear environment. As the volume of retained austenite increases, hardening due to strain induced martensite was found to overlap with softening due to tempering of the initial martensite, which contributed to the suppression of the wear rate.

This Paper was Originally Published in Japanese in J. JFS 93 (2021) 462–469. Some spelling errors were modified.

Silver nanoparticles (AgNPs) on the surface of cellulose nanofibers (CNF) were immobilized using a high-pressure wet-type jet mill to increase their stability while maintaining their catalytic activity. As a raw starting material, an aqueous silver nitrate solution was mixed with a CNF suspension. The mixture was then processed five times using the high-pressure wet-type jet mill at a discharge pressure of 100 MPa. AgNPs with an average particle size of 3.8 ± 0.8 nm were immobilized and well-dispersed on the surface of CNF, as observed by transmission electron microscopy. The catalytic activities of the AgNPs coated with PVP (Ag–PVP) and the prepared AgNPs-supported on CNF composite (Ag–CNF) were evaluated. As a result, the catalytic activity of Ag–CNF was discovered to be 2.32 times greater than that of Ag–PVP. The catalytic activity was measured in terms of the number of runs because Ag–CNF can be easily recovered by centrifugation or filtration. Even the catalytic activity of Ag–CNF after five runs was maintained at 62%, but the catalytic activity gradually decreased as the number of runs increased.