Sonochemical decomposition effects of nickelocene aiming for low-temperature and dispersant-free synthesis of nickel fine particle.

Sonochemical decomposition effects of nickelocene, which sublimates easily were investigated to synthesize dispersant-free nickel fine particles at low temperature. In a hydrazine monohydrate and 2-propanol mixed solvent, the reduction of nickelocene was promoted by ultrasound irradiation, and nickel fine particles were synthesized while precluding the sublimation of nickelocene. Unlike the common hydrazine reduction of nickel salts, which requires multiple-step reactions, nickelocene was reduced directly without forming intermediates. The effect of the water-bath temperature (20-60 °C) was investigated, where larger fine particles were synthesized using a higher water-bath temperature (60 °C). When irradiated at 20 °C, the reduction rate of nickelocene was low, leading to the formation of nickel fine particles and organic nanoparticles via the reduction and decomposition of nickelocene. The ultrasound frequency was also investigated, where fine nickel particles were synthesized using low-frequency ultrasound irradiation. The formation of high-temperature hotspots led to the diffusion and growth of nickel on the surface of the nickel fine particles; therefore, raspberry-like nickel fine particles were synthesized. In this study, the difficult-to-handle nature of nickelocene, owing to its sublimation properties, was easily overcome by ultrasound irradiation. Instantaneous and localized reactions at hotspots contributed to inhibiting particle growth. Furthermore, Ni fine particles were synthesized via a direct reduction pathway, which differs from previous reactions. This method represents a new, dispersant-free, low-temperature process for synthesizing Ni fine particles.

Facile room temperature synthesis of size-controlled spherical silica from silicon metal via simple sonochemical process.

The waterglass or St o¨ ber method is commonly used to synthesize spherical colloidal silica; however, these methods have some disadvantages, such as complicated processes for the removal of sodium ions and expensive and energy-consuming raw materials such as tetraethoxysilane (TEOS). In this study, size-controlled spherical colloidal silica was synthesized from silicon metal at room temperature using an ultrasound process with hydrazine monohydrate as the solvent. Silicon metal dissolves easily in hydrazine monohydrate under ultrasound irradiation, and spherical colloidal silica can be synthesized by adding alcohol to this precursor solution. By changing the concentration or type of alcohol, size-controlled colloidal silica 20-200 nm in size could be easily obtained. In addition, finer and more monodisperse particles were produced by low-frequency ultrasound irradiation, which had a higher stirring effect at the particle formation stage. The present method is effective because size-controlled colloidal silica can be synthesized at room temperature using only silicon metal, hydrazine, and alcohol as raw materials, without complicated processes or expensive and energy-consuming raw materials such as TEOS or tetramethoxysilane (TMOS).

Sonochemical decomposition of noble metal oxides and sonochemical alloying of gold-silver systems.

Recently, environmental problems, such as global warming, have become more severe; thus, there is a requirement to implement sustainable development goals in materials processing. In this study, we investigated a low-cost and environmentally-friendly sonochemical process for the synthesis of metal nanoparticles with large specific surface areas and catalysis effects. Au2O3 hydrate and Ag2O were reduced to Au and Ag, respectively, at room temperature in a short time when irradiated with ultrasound in ethanol. Furthermore, when a mixed powder of Au2O3 hydrate and Ag2O was irradiated in ethanol, Au-Ag alloys were obtained in only 10 min. This fast and environmentally friendly alloying technique, known as sonochemical alloying, is promising for alloy syntheses.

Sonochemical synthesis of supersaturated Ga-Al liquid-alloy fine particles and Al3+-doped γ-Ga2O3 nanoparticles by direct oxidation at near room temperature.

In this study, we investigated the fabrication of supersaturated gallium (Ga)-aluminum (Al) liquid alloy and Al3+-doped γ-Ga2O3 nanoparticles (NPs) at near room temperature (60 °C) using sonochemical and sonophysical effects. Supersaturated Ga-Al liquid alloy microparticles (Dav = 1.72 µm) were formed and stabilized at 60 °C by the thermal nonequilibrium field provided by sonochemical hot spots. Compared with liquid Ga, supersaturated Ga-Al liquid alloy was rapidly oxidized to a uniform oxide without Al2O3 or Al deposition. Thus, ultrafine Al3+-doped γ-Ga2O3 NPs were obtained after only 1 h of ultrasonic irradiation at 60 °C. The oxidation of liquid Ga was remarkably accelerated by alloying with metallic Al and ultrasonic irradiation, and the time was shortened. The average diameter and surface area of the γ-Ga2O3-based NPs were 59 nm and 181 m2/g, respectively. Compared with γ-Ga2O3, the optical bandgap of the Al3+-doped γ-Ga2O3 NPs was broadened, and the thermal stability improved, indicating Al3+-doping into the γ-Ga2O3 lattice. However, the lattice constant of γ-Ga2O3 was almost unchanged with or without Al3+-doping. Al3+ was introduced into the defect sites of Ga3+, which were massively induced in the defective spinel structure during ultrasonic processing. Therefore, sonochemical processing, which provides nonequilibrium reaction fields, is suitable for the synthesis of supersaturated and metastable materials in metals and ceramics fields.

Decrease in the Crystallite Diameter of Solid Crystalline Magnetite around the Curie Temperature by Microwave Magnetic Fields Irradiation.

A decrease in the crystallite diameter of ferrites irradiated with microwaves has been considered as a non-thermal effect of so-called de-crystallization; however, its mechanism has not been elucidated. We hypothesized that a decrease in the crystallite diameter is caused by interaction between the ordered spins of ferrite and the magnetic field of microwaves. To verify this, we focused on magnetite with a Curie temperature of 585 °C. Temperature dependence around this temperature and time dependence of the crystallite diameter of the magnetite irradiated with microwaves at different temperatures and durations were investigated. From the X-ray diffraction data, the crystallite diameter of magnetite exhibited a minimum value at 500 °C, just below the Curie temperature of magnetite, where the energy loss of the interaction between magnetite's spins and the microwaves takes the maximum value. The crystallite diameter exhibited a minimum value at 5 min irradiation time, during which the microwaves were excessively absorbed. Transmission electron microscopy observations showed that the microstructure of irradiated magnetite in this study was different from that reported previously, indicating that a decrease in the crystallite diameter is not caused by de-crystallization but its similar phenomenon. A decrease in coercivity and lowering temperature of Verwey transition were observed, evidencing decreased crystallite diameter. This study can thus contribute to the development of the theory of a non-thermal effect.

Sonochemical effect and pore structure tuning of silica xerogel by ultrasonic irradiation of semi-solid hydrogel.

Silica xerogels were prepared by the sol-gel method under ultrasonic irradiation, using tetraethylorthosilicate (TEOS) as the starting material. Hexamethyldisiloxane (HMDSO) was used as the hydrophobizing agent. When preparing silica xerogel, it is necessary to perform aging and hydrophobization to suppress shrinkage during ambient pressure drying, however, such treatments are time-consuming. In this study, the semi-solid hydrogel was irradiated with ultrasonic for the first time in order to accelerate aging and hydrophobic treatment, and the effect of ultrasonic frequency on structure was investigated. Firstly, ultrasonic irradiation was performed at frequencies of 100 kHz and 500 kHz, followed by hydrophobic treatment at a frequency of 500 kHz, in order to promote aging. The results identify optimum conditions for ultrasonic irradiation to promote aging and hydrophobization reactions, and it was found to be possible to prepare silica xerogels in less than 1/5 of the conventional time. The silica xerogels had a low density and the shrinkage was suppressed. In this study, it was found that ultrasonic irradiation of semi-solid hydrogel was very effective for promoting the reaction.

Controlling oxygen coordination and valence of network forming cations.

Understanding the structure-property relationship of glass material is still challenging due to a lack of periodicity in disordered materials. Here, we report the properties and atomic structure of vanadium phosphate glasses characterized by reverse Monte Carlo modelling based on neutron/synchrotron X-ray diffraction and EXAFS data, supplemented by Raman and NMR spectroscopy. In vanadium-rich glass, the water durability, thermal stability and hardness improve as the amount of P2O5 increases, and the network former of the glass changes from VOx polyhedra to the interplay between VOx polyhedra and PO4 tetrahedra. We find for the first time that the coordination number of oxygen atoms around a V4+ is four, which is an unusually small coordination number, and plays an important role for water durability, thermal stability and hardness. Furthermore, we show that the similarity between glass and crystal beyond the nearest neighbour distance is important for glass properties. These results demonstrate that controlling the oxygen coordination and valence of the network-forming cation is necessary for designing the properties of glass.

Microwave Irradiation Process for Al-Sc Alloy Production.

Scandium is being explored as an alloying element for aluminium alloys, which are gaining importance as high-performance lightweight structural alloys in the transportation industry. Sc-rich ScAlN thin films show strong piezoelectricity and can be fabricated on a hard substrate for use as wideband surface acoustic wave filters in next-generation wireless mobile communication systems. However, the use of ScAlN thin films in microelectromechanical system devices is limited by the high cost of metallic Sc, which is due to the difficulty in smelting of this material. Here, we propose a novel microwave irradiation process for producing Al-Sc alloys, with Mg ions as a reducing agent. Although scandium oxide is thermodynamically stable, intermetallic Al3Sc is obtained in high yield (69.8%) via a low-temperature (660 °C) reduction reaction under microwave irradiation. Optical spectroscopy results and thermodynamic considerations suggest a non-thermal equilibrium reaction with the univalent magnesium ions excited by microwave irradiation.

Enhancement of Fixed-bed Flow Reactions under Microwave Irradiation by Local Heating at the Vicinal Contact Points of Catalyst Particles.

The formation of local high temperature regions, or so-called "hot spots", in heterogeneous reaction systems has been suggested as a critical factor in the enhancement of chemical reactions using microwave heating. In this paper, we report the generation of local high temperature regions between catalyst particles under microwave heating. First, we demonstrated that reaction rate of the dehydrogenation of 2-propanol over a magnetite catalyst was enhanced 17- (250 °C) to 38- (200 °C) fold when heated with microwave irradiation rather than an electrical furnace. Subsequently, the existence of microwave-generated specific local heating was demonstrated using a coupled simulation of the electromagnetic fields and heat transfer as well as in situ emission spectroscopy. Specific high-temperature regions were generated at the vicinal contact points of the catalyst particles due to the concentrated microwave electric field. We also directly observed local high temperature regions at the contact points of the particles during microwave heating of a model silicon carbide spherical material using in situ emission spectroscopy. We conclude that the generation of local heating at the contact points between the catalyst particles is a key factor for enhancing fixed-bed flow reactions under microwave irradiation.

Formation of particle of bismuth-indium alloys and particle diameter by ultrasonic cavitation.

The miniaturization of electronic equipment requires fine bonding. Therefore, it is necessary to miniaturize the solder particles used for bonding different materials. Ultrasonic cavitation is a technique that uses ultrasonic irradiation to synthesize such microparticles. In this study, we investigated the effects of ultrasonic irradiation conditions on synthetic microparticles produced by this technique. Spherical particles were obtained by irradiating Bi-45 wt% In melted in a solvent with ultrasonic waves for 15 s, and the resultant metal composition was found to be equivalent to the raw material composition. We found a clear correlation between the ultrasonic irradiation time and particle size. When irradiated for 60 min, the average particle diameter was 3.3 µm. In addition, the particle division rate decreased as the irradiation time increased, which is probably due to attenuation of the vibration wave as the boundary surface increased with the refinement of the particle.

Effect of Aspect Ratio on the Permittivity of Graphite Fiber in Microwave Heating.

Microwave (MW) heating has received attention as a new heating source for various industrial processes. Some materials are expected to be a more effective absorber of MW, and graphite is observed as a possible candidate for high-temperature application. We investigated the dependence of the aspect ratio of graphite fibers on both their heating behavior and permittivity under a 2.45 GHz MW electric field. In these experiments, both loss tangent and MW heating behavior indicated that the MW absorption of conductive fibers increases with their aspect ratio. The MW absorption was found to be well accounted for by the application of a spheroidal model for a single fiber. The absorption of graphite fibers decreases with increasing aspect ratio when the long axis of the ellipsoid is perpendicular to the electric field, whereas it increases with the aspect ratio when the long axis is parallel to the electric field. The analytical model indicated that MW heating of the conductive fibers is expected to depend on both the shape and arrangement of the fibers in the electric field.

In situ analysis of reaction kinetics of reduction promotion of NiMn2O4 under microwave H-field irradiation.

Reaction kinetics during microwave H-field heating and electric furnace heating were investigated to verify the effect of rapid heating and the non-thermal microwave effect. This work is focused on the ferromagnetic material NiMn2O4, and the reduction reaction under rapid heating is investigated using an in situ analysis device. The results showed that the activation energy varied with the heating rate under microwave H-field irradiation, and it was the same as that in an electric furnace when the heating rate was the same. On the other hand, under microwave H-field irradiation, the reduction reaction began from a significantly low temperature compared with that of electric furnace heating. The results suggest that the transition theory during microwave irradiation should be expanded to include the effect of spin waves excited by a microwave H-field on magnetic materials.

Control of Magnetic Properties of NiMn2O4 by a Microwave Magnetic Field under Air.

NiMn2O4 prepared by conventional heating was irradiated with a microwave H-field using a single-mode cavity under air and magnetic properties of the microwave-irradiated material were investigated. X-ray diffraction and transmission electron microscopy demonstrated that the phase and microstructure are not affected by H-field irradiation. Measurements of the magnetization as a function of temperature revealed that the antiferromagnetic sublattice disappeared and electron spin resonance showed the existence of Mn2+, suggesting that Mn3+ is partially reduced. Moreover, the magnetization of NiMn2O4 was controlled from 35.3 to 18.2 emu/g and the coercivity from 140 to 750 Oe by changing the sample temperature during microwave irradiation. The reduction reaction of NiMn2O4 is controlled by microwave H-field irradiation, resulting in control over the magnetic properties.

Preparation of platinum nanoparticles in heterogeneous solid-liquid system by ultrasound and microwave irradiation.

Pt nanoparticles and Pt/spherical carbon (SC) were prepared in a heterogeneous (PtO2/SC, water/ethanol) solid-liquid system without dispersant using ultrasound and microwave irradiation. The Pt nanoparticles and Pt/SC were characterized using X-ray diffraction (XRD), transmission electron microscopy-energy-dispersive X-ray spectroscopy (TEM-EDX), and dynamic light scattering method for particle size determination. The average size of the Pt nanoparticles prepared by only ultrasound irradiation was smaller than that prepared by microwave irradiation. Pt nanoparticles were successfully prepared by microwave irradiation with a reaction time of several minutes at a specific concentration of diluted ethanol. Pt/SC prepared by combined ultrasound and microwave irradiation resulted in fine particles with good homogeneous distribution.