Genki Saito

Microencapsulation of Metal-based Phase Change Material for High-temperature Thermal Energy Storage

Latent heat storage using alloys as phase change materials (PCMs) is an attractive option for high-temperature thermal energy storage. Encapsulation of these PCMs is essential for their successful use. However, so far, technology for producing microencapsulated PCMs (MEPCMs) that can be used above 500°C has not been established. Therefore, in this study, we developed Al-Si alloy microsphere MEPCMs covered by α-Al2O3 shells. The MEPCM was prepared in two steps: (1) the formation of an AlOOH shell on the PCM particles using a boehmite treatment, and (2) heat-oxidation treatment in an O2 atmosphere to form a stable α-Al2O3 shell. The MEPCM presented a melting point of 573°C and latent heat of 247 J g−1. The cycling performance showed good durability. These results indicated the possibility of using MEPCM at high temperatures. The MEPCM developed in this study has great promise in future energy and chemical processes, such as exergy recuperation and process intensification.

Synthesis of copper/copper oxide nanoparticles by solution plasma

This paper describes the synthesis of copper/copper oxide nanoparticles via a solution plasma, in which the effect of the electrolyte and electrolysis time on the morphology of the products was mainly examined. In the experiments, a copper wire as a cathode was immersed in an electrolysis solution of a K2CO3 with the concentration from 0.001 to 0.50 M or a citrate buffer (pH = 4.8), and was melted by the local-concentration of current. The results demonstrated that by using the K2CO3 solution, we obtained CuO nanoflowers with many sharp nanorods, the size of which decreased with decreasing the concentration of the solution. Spherical particles of copper with/without pores formed when the citrate buffer was used. The pores in the copper nanoparticles appeared when the applied voltage changed from 105 V to 130 V, due to the dissolution of Cu2O.

A new CaCO3-template method to synthesize nanoporous manganese oxide hollow structures and their transformation to high-performance LiMn2O4 cathodes for lithium-ion batteries

This paper presents a new CaCO3-template synthesis of highly nanoporous manganese oxide hollow structures and their transformation to high-performance LiMn2O4 cathodes for lithium-ion batteries via the facile coprecipitation of Mn–Ca-carbonates and temperature-controlled decomposition of MnCO3 and CaCO3, followed by the selective removal of the carbonates by washing with HCl. The as-prepared Mn2O3 nanostructures showed very high specific surface area with their subunit particle size of 100 nm. The transformation to uniformly porous LiMn2O4 hollow structures was successfully achieved by the facile impregnation of LiOH into the porous Mn2O3 hollow precursors, followed by a conventional solid-state reaction. The LiMn2O4 hollow structures deliver a discharge capacity of about 120 mA h g−1 at a 1 C rate and 115 mA h g−1 at a 10 C rate with excellent cycling stability. The capacity retention approached 94% after up to 800 cycles of charging–discharging at a 10 C rate.

Nanomaterial synthesis using plasma generation in liquid

Over the past few decades, the research field of nanomaterials (NMs) has developed rapidly because of the unique electrical, optical, magnetic, and catalytic properties of these materials. Among the various methods available today for NM synthesis, techniques for plasma generation in liquid are relatively new. Various types of plasma such as arc discharge and glow discharge can be applied to produce metal, alloy, oxide, inorganic, carbonaceous, and composite NMs. Many experimental setups have been reported, in which various parameters such as the liquid, electrode material, electrode configuration, and electric power source are varied. By examining the various electrode configurations and power sources available in the literature, this review classifies all available plasma in liquid setups into four main groups: (i) gas discharge between an electrode and the electrolyte surface, (ii) direct discharge between two electrodes, (iii) contact discharge between an electrode and the surface of surrounding electrolyte, and (iv) radio frequency and microwave plasma in liquid. After discussion of the techniques, NMs of metal, alloy, oxide, silicon, carbon, and composite produced by techniques for plasma generation in liquid are presented, where the source materials, reaction media, and electrode configurations are discussed in detail.

A New Route to Synthesize β-SiAlON:Eu2+ Phosphors for White Light-Emitting Diodes

A simple, highly efficient method of synthesizing β-SiAlON:Eu2+ phosphors for white light-emitting diodes was reported. The new route was carried out via a combustion synthesis method using Si, SiO2, and Al raw materials, and NaCl as a diluent under nitrogen pressure. The phase purity, photoluminescence properties, and thermal quenching of the phosphors were investigated. The results revealed that the synthesized β-SiAlON:Eu2+ powders, which comprised rodlike crystals, exhibited high thermal stability: The emission intensity at 160 °C was 84% of that measured at room temperature, which indicates that the synthesized β-SiAlON:Eu2+ phosphors have high potential for application to white LEDs.

Size-Controlled Ni Nanoparticles Formation by Solution Glow Discharge

We report the size control of Ni nanoparticles generated via solution glow discharge and focus on the effect of electrolyte concentration on Ni nanoparticles. In our experiments, voltage was applied to generate a plasma in NaOH electrolytes with concentrations ranging from 1.0 to 0.001 kmol m -3 . The applied voltage strongly depended on the electrolyte concentration, and interestingly, product size decreased with electrolyte concentration; for example, (mean diameter, applied voltage, electrolyte concentration) = (148 nm, 90 V, 0.5 kmol m -3 ), and (70 nm, 590 V, 0.001 kmol m -3 ). These results suggested the possibility of using plasma electrolysis for synthesizing size-controlled nanoparticles by changing only electrolyte concentration.

Solution plasma synthesis of bimetallic nanoparticles

This paper describes the facile solution plasma synthesis of bimetallic nanoparticles, including solid solution alloys (Ni-Cu and Ni-Cr system), eutectic alloys of Sn-Pb, and intermetallic alloys (SnSb and Ni₃Sn), by using metallic alloy wire as the cathode and Pt wire as the anode. In the typical process, the cathode was melted by the local-concentration of current, upon applying a DC voltage between the two electrodes immersed in the electrolyte. The solid solution alloys of Ni-Cu and Ni-Cr prepared in this study have a uniform distribution of composition. On the other hand, the uniformity in the composition of the eutectic Sn-Pb alloy depends on the microstructure of the electrode. The use of quenched electrode with small crystal grains favors the formation of Sn-Pb alloy nanoparticles, in which the Sn-rich and Pb-rich phases coexist in each particle. The formation of intermetallic SnSb and Ni₃Sn alloy nanoparticles is accompanied by the formation of colloidal oxide. These results demonstrate that the solution plasma technique is applicable not only for the synthesis of pure metals but can also be used for the synthesis of various alloy nanoparticles.

Excitation temperature of a solution plasma during nanoparticle synthesis

Excitation temperature of a solution plasma was investigated by spectroscopic measurements to control the nanoparticle synthesis. In the experiments, the effects of edge shielding, applied voltage, and electrode material on the plasma were investigated. When the edge of the Ni electrode wire was shielded by a quartz glass tube, the plasma was uniformly generated together with metallic Ni nanoparticles. The emission spectrum of this electrode contained OH, Hα, Hβ, Na, O, and Ni lines. Without an edge-shielded electrode, the continuous infrared radiation emitted at the edge created a high temperature on the electrode surface, producing oxidized coarse particles as a result. The excitation temperature was estimated from the Boltzmann plot. When the voltages were varied at the edge-shielded electrode with low average surface temperature by using different electrolyte concentrations, the excitation temperature of current-concentration spots increased with an increase in the voltage. The size of the Ni nanopart...

Improved electrochemical properties of LiMn2O4 with the Bi and La co-doping for lithium-ion batteries

A series of LiBixLaxMn2−2xO4 (x = 0, 0.002, 0.005, 0.010, 0.020) samples were synthesized by solution combustion synthesis in combination with calcination. The phase structure and morphology of the products were characterized by X-ray diffraction, scanning electron microscopy, and transition electron microscopy. The results demonstrated that a single-phase LiMn2O4 spinel structure was obtained for the LiBixLaxMn2−2xO4 (x = 0, 0.002, 0.005) samples, whereas impurities were observed for the LiBixLaxMn2−2xO4 (x = 0.010, 0.020) samples as a result of the doping limit. The electrochemical properties were investigated by galvanostatic charge–discharge cycling and cycling voltammetry in a voltage range of 3.2–4.4 V. The substitution of Mn3+ by equimolar Bi3+ and La3+ could significantly improve the structural stability and suppress the Jahn–Teller distortion, thereby resulting in improved electrochemical properties for the Bi and La co-doped samples in contrast with the pristine LiMn2O4 sample. In particular, the LiBi0.005La0.005Mn1.99O4 sample delivered a high initial discharge capacity of 130.2 mA h g−1 at 1C, and following 80 cycles, the capacity retention was as high as 95.0%. Moreover, it also presented the best rate capability among all the samples, in which a high discharge capacity of 98.3 mA h g−1 was still maintained at a high rate of 7C compared with that of 75.8 mA h g−1 for the pristine LiMn2O4 sample.

Surface morphology of a glow discharge electrode in a solution

This paper describes the surface morphology of a glow discharge electrode in a solution. In the experiments detailed in the paper, the effects of electrolysis time, solution temperature, voltage, electrolyte concentration, and surface area on the size of nanoparticles formed and their amount of nanoparticles produced were examined to study the surface morphologies of the electrodes. The results demonstrated that the amount of nanoparticles produced increased proportionally with the electrolysis time and current. When the voltages were below 140 V, surfaces with nanoparticles attached, called “Particles” type surfaces, were formed on the electrode. These surfaces changed and displayed ripples, turning into “Ripple” type surfaces, and the nanoparticle sizes increased with an increase in the amount of nanoparticles produced. In contrast, at voltages over 160 V, the surfaces of the electrodes were either “Random” or “Hole” type and the particle sizes were constant at different amount of nanoparticles produced.