Yu-ki Taninouchi

Dehydration behavior of the superprotonic conductor CsH2PO4 at moderate temperatures: 230 to 260 °C

The dehydration behavior of caesium dihydrogen phosphate CsH2PO4 was investigated in the temperature range of 230 °C to 260 °C under high humidity, conditions of particular relevance to the operation of fuel cells based on this electrolyte. The onset temperature of dehydration was determined from changes in ionic conductivity on heating and confirmed by weight change measurements under isothermal conditions. The relationship between the onset temperature of dehydration (Tdehy) and water partial pressure (pH2O) was determined to be log(pH2O/atm = 6.11(±0.82) − 3.63(±0.42) × 1000/(Tdehy/K), from which the thermodynamic parameters of the dehydration reaction from CsH2PO4 to CsPO3 were evaluated. The dehydration pathway was then probed by X-ray powder diffraction analysis of the product phases and by thermogravimetric analysis under slow heating. It was found that, although the equilibrium dehydration product is solid caesium metaphosphate CsPO3, the reaction occurs via two overlapping steps: CsH2PO4 → Cs2H2P2O7 → CsPO3, with solid caesium hydrogen pyrophosphate, Cs2H2P2O7, appearing as a kinetically favored, transient phase.

Electrical breakdown of short multiwalled carbon nanotubes

Electrical breakdown of short (∼50nm) multiwalled carbon nanotubes (MWNTs) is studied utilizing mechanically controllable break junction technique with gold electrodes. Measurements of the conductance-bias characteristics near the breakdown point revealed two different types of breakdown behavior for the short MWNTs. In one type designated as type I, the conductance increases nearly linearly with the bias and, over the breakdown point, decreases stepwise. On the other hand, in the type-II breakdown, the conductance shows a transient and irreversible increase right before the breakdown and subsequently jumps to zero. We consider that the type-II breakdown in vacuum is contact related, whereas the type-I breakdown occurs through the usual MWNT heating.

Phase Relationship of CsH2PO4 – CsPO3 System and Electrical Properties of CsPO3

The dehydration behavior of the paraelectric phase of CsH 2 PO 4 was investigated by thermogravimetric and X-ray diffraction analyses, and then the phase diagrams of CsH 2 PO 4 -CsPO 3 system were established. The relationship between the onset temperature of dehydration (T dehy /K) and the partial pressure of water (pH 2 O/atm) is logpH 2 O = 7.62(±1.18) - 4.42(±0.56)(1000/T dehy ) below 228°C. The thermodynamically stable phase just above T dehy is the fully dehydrated product CsPO 3 (s), although partially dehydrated products transiently appeared in the course of the dehydration to CsPO 3 . Such developments allowed us to complete the temperature-humidity phase diagram and to regard the composition-temperature phase diagram as the eutectic type. This paper also reports the phase transition and electrical properties of CsPO 3 examined by differential thermal analysis, ac impedance spectroscopy, and dc polarization measurement. The just-synthesized CsPO 3 showed the relatively high electrical conductivity in unhumidified Ar. Ionic conductivity as high as 5 X 10 -4 S cm -1 was observed on heating from 150 to 450°C. Such high ionic conductivity disappeared after first heating up to 620°C and was explained by the proton diffusion through the absorbed H 2 O. At around 600°C, a high-temperature phase of CsPO 3 showed the electrical conductivity as high as 10 -3 S cm -1 . However, this conductivity was not purely ionic.

Effective temperature of Au nanocontacts under high biases

The effective local temperature of gold nanocontacts under high biases has been evaluated at 77 K by studying the two-level fluctuations (TLFs) of conductance. Upon varying bias from 0.2 to 0.6 V, TLF frequency increases exponentially while the effective contact temperature remains unchanged, which indicates negligible current-induced heating in Au nanocontacts at least up to 0.6 V at 77 K, in accordance with a theoretical model proposed by Todorov et al. [Phys. Rev. Lett. 86 (2001) 3606].

Bias-induced local heating effects on multi-walled carbon nanotube-Au contacts

Exploiting a mechanically controllable break junction (MCBJ) technique, we prepared multi-walled carbon nanotubes (MWNTs) bridging over Au electrodes and investigated their conductance?bias voltage (G?V) characteristics in a high-bias regime. Above a threshold bias voltage, we observed a rapid and irreversible increase in conductance, which appears only with increasing bias. This phenomenon can be attributed to irreversible contact annealing due to contact heating. Observation of two-level fluctuations in the MWNT conductance at high biases also implies defect annealing inside Au electrodes. The conductance increment is occasionally observed at one bias polarity but not at a reverse polarity. The asymmetry in the heating effect with respect to the bias polarity suggests that the electron transport in MWNTs is ballistic, and the energy dissipation takes place only at the anode contact.

Thermodynamic Considerations of Direct Oxygen Removal from Titanium by Utilizing the Deoxidation Capability of Rare Earth Metals

Oxygen removal from metallic Ti is extremely difficult and, currently, there is no commercial process for effectively deoxidizing Ti or its alloys. The oxygen concentration in Ti scraps is normally higher than that in virgin metals such as in Ti sponges produced by the Kroll process. When scraps are remelted with virgin metals for producing primary ingots of Ti or its alloys, the amount of scrap that can be used is limited owing to the accumulation of oxygen impurities. Future demands of an increase in Ti production and of mitigating environmental impacts require that the amount of scrap recycled as a feed material of Ti ingots should also increase. Therefore, it is important to develop methods for removing oxygen directly from Ti scraps. In this study, we evaluated the deoxidation limit for β-Ti using Y or light rare earth metals (La, Ce, Pr, or Nd) as a deoxidant. Thermodynamic considerations suggest that extra-low-oxygen Ti, with an oxygen concentration of 100 mass ppm or less can be obtained using a molten salt equilibrating with rare earth metals. The results presented herein also indicate that methods based on molten salt electrolysis for producing rare earth metals can be utilized for effectively and directly deoxidizing Ti scraps.

Magnetic Concentration of Platinum Group Metals from Catalyst Scraps Using Iron Deposition Pretreatment

Spent automobile catalysts are the most important secondary source of platinum group metals (PGMs). However, effective recovery of PGMs from catalyst scraps is difficult because they are present in only small quantities as chemically stable substances. In this study, in order to improve the efficiency of the existing recycling processes, the authors experimentally investigated a novel physical concentration pretreatment process for PGMs using samples that simulate an automobile catalyst. In order to magnetically separate PGMs directly from the catalysts, ferromagnetic Fe was deposited on the PGM particles (or the porous catalyst layer) using an electroless plating technique. By using a plating bath containing sodium borohydride and potassium sodium tartrate as the reducing and complexing agents, respectively, Fe was successfully deposited on the sample without requiring complicated pretreatments such as sensitization and activation. After Fe deposition and subsequent pulverization, the PGMs could be extracted and concentrated in the form of magnetic powder using a magnet. The proposed magnetic concentration process was demonstrated to be feasible, and it has the potential to make the recycling of PGMs more efficient and environmentally friendly.

Vapor Treatment for Alloying and Magnetizing Platinum Group Metals

Among platinum group metals (PGMs), Pt, Pd, and Rh are the essential constituents of automotive catalysts. Recovery of PGMs from catalyst scrap is important, but it is difficult because PGMs are chemically stable and are present as minor components in the scrap. In this study, the authors investigated the reaction between PGMs and FeCl2 vapor in order to develop a novel method for the separation and concentration of PGMs directly from catalyst scrap. The wire samples of Pt, Pd, and Rh were reacted with FeCl2 vapor in a steel vessel maintained at 1200 K. After the heat treatment, the surfaces of PGM wires were alloyed with Fe, and these samples became magnetized. The results obtained in this study suggest that a FeCl2 vapor treatment followed by magnetic separation can be utilized as an effective technique for the separation of PGMs directly from catalyst scrap.

Enhanced Dissolution of Platinum Group Metals Using Electroless Iron Deposition Pretreatment

In order to develop a new method for efficiently recovering platinum group metals (PGMs) from catalyst scraps, the authors investigated an efficient dissolution process where the material was pretreated by electroless Fe deposition. When Rh-loaded alumina powder was kept in aqua regia at 313 K (40 °C) for 30 to 60 minutes, the Rh hardly dissolved. Meanwhile, after electroless Fe plating using a bath containing sodium borohydride and potassium sodium tartrate as the reducing and complexing agents, respectively, approximately 60 pct of Rh was extracted by aqua regia at 313 K (40 °C) after 30 minutes. Furthermore, when heat treatment was performed at 1200 K (927 °C) for 60 minutes in vacuum after electroless plating, the extraction of Rh approached 100 pct for the same leaching conditions. The authors also confirmed that the Fe deposition pretreatment enhanced the dissolution of Pt and Pd. These results indicate that an effective and environmentally friendly process for the separation and extraction of PGMs from catalyst scraps can be developed utilizing this Fe deposition pretreatment.

Recycling Titanium and Its Alloys by Utilizing Molten Salt

It is commonly believed that the deoxidation of titanium (Ti), or the direct removal of oxygen (O) dissolved in metallic Ti, is practically impossible when magnesium (Mg) is used as the deoxidizing agent. In recent years, it has been experimentally demonstrated that O dissolved in Ti can be directly removed using MgCl2 molten salt electrolysis. By the electrochemical deoxidation technique, Ti wires containing 0.12 mass% O were deoxidized to less than 0.02 mass% O. In some cases, the concentration of O in the Ti wires was reduced to the level of 0.01 mass% O, which cannot be attained using the current Kroll process. The possible application of this deoxidation technique to practical industrial recycling processes is also discussed.