Spent automobile catalysts are the most important secondary resource for platinum group metals (PGMs), and their recycling is essential not only to ensure a steady supply of PGMs but also to preserve the natural environment. However, several factors limit the efficiency of PGM recovery in current recycling processes. Specifically, PGMs represent a small proportion of the entire catalyst and are difficult to dissolve in aqueous solutions for subsequent separation. In recent decades, multiple technologies have been developed to make the recycling of PGMs more efficient and environmentally-friendly. Herein, we introduce the typical industrial processes for recovering PGMs from spent catalysts, along with established and emerging trends in the technological development of PGM recycling. Furthermore, we review novel recycling techniques for converting PGMs in spent catalysts into more soluble states and/or physically concentrating PGMs from spent catalysts.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 294–304.

For Mg-doped Zn₂P₂O₇, this systematic investigation of co-doping onto Zn sites has elucidated specific effects on negative thermal expansion (NTE). The low-cost and low-environmental-impact NTE material Zn_2−xMg_xP₂O₇ shows large NTE in a temperature range including room temperature for x = 0.4. Although Mg doping broadens the operating-temperature window, it remains several dozen degrees wide. Moreover, the total volume change related to NTE becomes less than that of the Zn₂P₂O₇ parent material. Findings obtained from this study demonstrate that co-doping of Mg and of another element onto the Zn site is effective for achieving simultaneous expansion of the operating-temperature window and maintenance of the volume change related to NTE. One illustrative case is that Zn_1.64Mg_0.30Al_0.06P₂O₇ has a large negative coefficient of linear thermal expansion of about −65 ppm/K at temperatures of 300–375 K. In fact, at temperatures high above room temperature, Zn_1.64Mg_0.30Al_0.06P₂O₇ powder shows better thermal expansion compensation capability than the composition without Al. The Al-doped phosphates are expected to have broad practical application because of their performance, cost, and environmental load characteristics.

In this paper, the influences of ROH (R = Li, Na, K) additions on the preparations of Tungsten (W)/Molybdenum (Mo) powders by hydrogen reduction of WO₃/MoO₂ in the temperature range of 800°C to 1100°C were studied in detail. In the absence of ROH addition, the prepared W/Mo powders basically maintained the morphology of raw oxide. However, with the assistance of ROH, the morphology and particle size of the products had been greatly changed. The grains of W and Mo were obviously coarsened, and micron-sized Mo powder and W powder with good dispersion can be prepared. In addition, the addition of ROH can promote the reduction rate of WO₃ but inhibit the reduction of MoO₂. Moreover, the effect of LiOH on grain growth is more significant than that of NaOH and KOH.

In order to clarify plastic deformation behavior and mechanism of fracture energy absorption of Ti₃SiC₂, Vickers indentation tests were conducted for Ti₃SiC₂ sintered bodies with various textured orientations. Textured Ti₃SiC₂ sintered bodies were fabricated by slip casting in a strong magnetic field and spark plasma sintering (SPS), and their orientation distribution were analyzed by SEM/EBSD. It was found that for the textured Ti₃SiC₂, the plastic deformation behavior around Vickers indents such as an indent shape and a grain pile-up were strongly affected by basal slip and kink deformation. Furthermore, the fracture energy absorption mechanism around the indents also depended on the texture orientations. From our results, it is concluded that the most effective factor for suppressing the crack propagation was the grain pile-up, and the second one was crack deflections.

This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 67 (2020) 607–614

Alumina particles were added in the adhesive layer to stabilize the welding process and enhance the load capacity for weld-bonding joints. In this study, the strength of alumina particles reinforced weld-bonding joints of DP780 dual-phase steel sheets was investigated by conducting experiments based on the Box–Behnken Design (BBD). The multivariate regression models between process parameters (welding current, welding time and electrode pressure) and response values (failure load, nugget diameter) were established, and the optimal combination of weld-bonding process parameters was obtained. The findings indicate that welding current has the biggest impact on the strength of joints. The optimum process parameters ranges are: welding current of 8.5 kA, welding time of 125 ms, and electrode pressure of 0.5 MPa. Developed regression models were validated by experiments.

In this study, polyvinyl alcohol (PVA) - polytetrafluoroethylene (PTFE) - vapor grown carbon fiber (VGCF) sheets are fabricated as a new paper-like thermal interface material (TIM), which is a potential substitute for traditional TIMs. Two types of PVA-PTFE-VGCF sheets were fabricated by using 120 nm and 300 nm PTFE particles. The microstructure shows that the VGCFs were arranged in random directions inside the sheet and interconnected via the aggregate of PVA. 300 nm PTFE particles were well distributed within the sheet, while 120 nm PTFE particles aggregated partially and formed pores nearby. The fabricated sheets have a low thickness of 31.2 um and 28.2 um, and lightweight properties with a density of 0.79 × 10⁶ g·m⁻³ and 0.91 × 10⁶ g·m⁻³, respectively. With the addition of 300 nm PTFE particles, the fabricated sheet has higher thermal conductivities of 9.81 W·m⁻¹·k⁻¹ in the in-plane direction and 2.11 W·m⁻¹·k⁻¹ in the through-plane direction. In the high temperature and humidity test, the thermal conductivities of the fabricated sheet were increased due to the rearrangement of PVA and PTFE particles.

The reduction process of lunar regolith simulant was designed to obtain metallic iron (Fe) and utilize lunar resources, focusing mainly on the effect of titanium dioxide (TiO₂) on the recovery of metallic Fe from molten slag. Thermodynamic calculations were conducted to predict the equilibrium phases existing at high temperature upon heating the regolith simulant to 1600°C under an oxygen partial pressure of 10⁻¹⁵ atm, in which the metallic Fe coexisted with the molten slag. Based on these predictions, two steps were applied in the experimental procedure: a reduction step followed by a melting step. The results showed that metallic Fe droplets were obtained by aggregating fine metallic Fe particles in the melting step. The viscosity of molten slag was found to decrease when TiO₂ was added to the regolith simulant, but the recovery ratio—defined as the weight ratio of the metallic Fe obtained from the aggregation of fine metallic Fe particles to the total amount of Fe in the regolith simulant—also decreased. The wettability between liquid Fe and molten slag improved upon increasing the TiO₂ content in the latter. These results indicate that wettability exerts a substantially more dominant effect than the viscosity of molten slag does in the recovery of metallic Fe from regolith simulant.

The adhesive mechanism of thermal sprayed ceramic coating is not clear because of the complicated profile of the blasted surface and the microstructure of the coating. In particular, little research has been carried out on the adhesive mechanism on light metal substrates. The objective of this study was to investigate the effect of surface roughness on the adhesive strength of yttria-stabilized zirconia coatings applied by atmospheric plasma spraying on an aluminum alloy (JIS A5052) substrate. Tensile adhesive and interfacial indentation tests were performed to evaluate their bonding properties. The substrates were blasted at three different pressures and seven different blast angles using fused alumina abrasives of three different particle sizes. Our measurements revealed that the tensile adhesive strength decreased with increasing substrate roughness (root mean square slope, RΔq). After the tensile strength tests, the fracture surfaces were observed using an FE-EPMA analyzer and analyzed using an X-ray fluorescence analyzer. It was found from elemental analysis that the fractured part shifted to the substrate with increasing RΔq. It was concluded that the tensile adhesive strength decreased with increasing surface roughness owing to the low strength of the substrate.

This Paper was Originally Published in Japanese in J. Japan Thermal Spray Soc. 59 (2021) 19–26. The abstract is slightly modified from the original paper.

A rotating magnetic field was applied during the solidification of Al–Si–Fe alloys (Si = 5 mass% or 10 mass% and Fe = 0.5 mass% or 1 mass%) and the effect of electromagnetic stirring on the area and shape of the intermetallic compounds was studied. The phases formed during solidification of the alloys were not influenced by electromagnetic stirring. The alloys consisted of α-Al, Si and intermetallic β-Al₉Fe₂Si₂. At coil current frequencies above 10 Hz, electromagnetic stirring induced fragmentation of dendritic α-Al, giving rise to coarsening of the β-Al₉Fe₂Si₂ present between the fragmented α-Al grains. The application of electromagnetic stirring at coil current frequencies above 80 Hz decreased the aspect ratio of β-Al₉Fe₂Si₂ in Al–10 mass%Si–1 mass%Fe, while no such decrease was identified in Al–5 mass%Si–0.5 mass%Fe, Al–5 mass%Si–1 mass%Fe or Al–10 mass%Si–0.5 mass%Fe. It was found that the shape of the β-Al₉Fe₂Si₂ phase formed under forced flow of the melt depended significantly on the stirring intensity and the solid fraction of the melt at which intermetallic compounds were formed.

AlSi10Mg alloy products with a hydrogen content of approximately 3.9 or 5.6 cm³/100 g-Al of hydrogen were fabricated by selective laser melting (SLM) using normal (as-received) and moist powders, and their non-reversible dimensional changes during heat treatment at 473 or 803 K were investigated. The linear dimensional change arising from the heat treatment at 473 K was approximately 0.22% by 3.6 ks and remained constant thereafter. This behavior was independent of the amount of hydrogen in the SLM products, suggesting that the dimensional changes at 473 K were induced by precipitation of Si phase from the α-Al phase. However, the linear dimensional changes during the heat treatment at 803 K were comparatively large and continued to increase during the heat treatment. At the same time, the linear dimensional changes at 803 K also showed a dependence on the amount of hydrogen in the SLM products. These phenomena indicated that the porosity expansion and precipitation of Si phase occurred simultaneously at 803 K. For the SLM product with a hydrogen content of approximately 3.9 cm³/100 g-Al, the linear dimensional change during the heat treatment at 803 K was 0.867% at 18 ks, of which 0.116% and 0.751% were estimated to have been induced by the precipitation of Si phase and the porosity expansion, respectively. From gas analyses using different methods, it was elucidated that the hydrogen desorbed from the powder and was entrapped in the SLM products at the time of laser scanning, and then enriched to the porosities during the heat treatment at 803 K, causing the porosity expansion.

Microstructure of the materials is essential to design new functional materials, especially from atomic scale to micron order structures. Many attempts have been carried out to give nanostructures and such structures have been analyzed using cutting-edge methods both theoretically and experimentally. In this report, recent trends in research on functional materials related to the micro-structures in nano-meter scale are reviewed by mainly introducing the contents of the special issue published in Materials Transactions, vol. 61, No. 8, in which following 6 categories of the topics are included; computational materials science, magnetic materials, advanced functional oxides, carbon and organic materials, and optical materials.