Search

In this study, soft X-ray emission spectroscopy of an aqueous colloidal dispersion of multi-walled carbon nanotubes modified via the plasma process in an aqueous solution was performed for investigating the electronic state of water molecules on the colloidal particles. In the aqueous dispersion, reconstruction of the hydrogen-bonded network was implied by the O 1s spectral changes in the 1b1' and 1b1'' peaks. Furthermore, the O 1s spectral intensity around the 3a1 state was enhanced to an unusually broad energy range in comparison with previous studies. This unusual spectral change might be attributed to the hybridization of the electronic states of oxygen-containing functional groups on the surface of the plasma-modified multi-walled carbon nanotubes and that of the surrounding water molecules. Our observation indicates not only reconstruction of the hydrogen-bonded network in the aqueous dispersion but also a significant interaction of the electronic states between the water molecules and the plasma-modified multi-walled carbon nanotubes.

The development of stretchable elastomer composites with considerable mechanical strength and electrical conductivity is desired for future applications in communication tools, healthcare, and robotics. Herein, we have developed a novel stretchable elastomer composite by employing a slide-ring (SR) material as a matrix for restoration and graphene oxide (GO) as a precursor for a conductive filler. Highly dispersed GO in an organic solvent, prepared via a new method developed by the authors, allowed the uniform dispersion of GO into the matrix by simply mixing the solvent and SR. The resultant SR/GO composite exhibited considerably high mechanical toughness and cyclic durability. These properties were approximately maintained after pulse laser irradiation to add electrical conductivity on the composite by photoreducing of the dispersed GO, and its electrical conductivity was higher than that of the SR/graphene, carbon nanotubes, or graphite composites. The potential of the SR/GO composite as a stretchable base substrate for wearable devices was demonstrated by producing a prototype humidity sensor, a human motion monitoring sensor, and an electrical heater based on the composite with conductive circuits drawn using pulse laser patterning.

Ferroelectric nanoparticles (NPs) have attracted considerable attention owing to their size effect on the ferroelectricity and their possible application toward future electronic devices such as mu...

The low stability of graphene oxide (GO) in organic solvents renders it unsuitable for several applications. Thus, an effective dispersion technique must be rapidly developed in order to allow the ...

Hexagonal boron nitride (h-BN) sheets have numerous applications, but current low-cost, environmentally friendly techniques for h-BN exfoliation yield undesirably small sheet sizes. We have developed a novel chemical-free method to prepare large h-BN nanosheets (BNNSs) by the exfoliation of h-BN using plasma with cavitation bubbles in water, named here as cavitation-bubble plasma. Unlike ordinary liquid exfoliation methods, which reduce the sheet size of BNNSs compared with that of the starting h-BN, the cavitation-bubble plasma process exfoliated h-BN without reducing the sheet size. Furthermore, the plasma dramatically improved the exfoliation yield.

Prussian blue analogs (PBA) are widely studied for radioactive cesium decontamination. However, there are fewer works related to their post use storage. Considering the oxidative stabilization of the material after the selective uptake of Cs, the thermogravimetric properties in powder and bead form, with various Cs and other alkali metal ions adsorbed, and various heating rates were studied. TG-DTA taken in dry air condition shows an exothermic decomposition at ~270 °C. This temperature varied with the heating rate, mass, and the proportion of adsorbed ions. The best condition for complete oxidation of Prussian blue (PB) is found to be a gradual oxidative decomposition by heating in the temperature range of 200–220 °C until the total mass is decreased by >35%. After this, the temperature could be safely increased to >300 °C for the complete oxidative decomposition of PB that formed iron oxide and salt of the adsorbed Cs. A pilot scale test conducted using the radioactive Cs adsorbed Prussian blue microbeads (PB-b) confirmed that no Cs was released in the effluent air during the process.

We studied the dynamics of solvated electrons in the early stage of plasma generation in water induced with an intense femtosecond laser pulse. According to the decay kinetics of solvated electrons, fast recombination process of solvated electrons (geminate recombination) occurred with a more prolonged lifetime (500 ps to 1 ns) than that observed in previous pulse photolysis studies (10-100 ps). This unusually longer lifetime is attributed to additional production of solvated electrons due to abundant free electrons generated with the laser-induced plasma, implying significant influence of free electrons on the dynamics of solvated electrons., Comment: 24 pages, 5 figures

In this paper we report about the particle size controlling factors on various perovskite metal oxide, XTiO3 (X = Ca, Sr, Ba, Pb), nanoparticles by hydrothermal synthesis in supercritical water of ...

The slide-ring (SR) effect in composite materials is demonstrated by comparing the mechanical properties of plasma-modified boron nitride (p-BN)/SR elastomers and p-BN/fixed crosslinked (FC) elastomer composite materials that include a large amount of plasma-surface-modified BN (∼56 vol%). The toughness of p-BN/SR was ∼7 times that of p-BN/FC. Such SR effect was not observed in the raw BN/SR composite material but only in the p-BN/SR composite material. Time-domain nuclear magnetic resonance measurements suggested that the plasma-surface modification of BN formed crosslinking points to the SR and suppressed the decrease in the molecular mobility of SR in the composite materials.

To develop flexible and tough composite materials with out-of-plane aligned thermally conductive pathways, a novel freeze-casting method was developed using dimethyl sulfoxide (DMSO), carbon nanofibers (CFs), slide-ring polymers (SRs), and cross-linkers. Porous composite materials composed of SR and plasma-surface-modified CFs were obtained. Using DMSO, which has a high melting point (19 °C) and high polymer solubility, the SR cross-linking reaction was performed during freeze-casting. The CFs were treated with plasma before composite preparation for better affinity and dispersibility to the composites. The obtained porous composites contained thermally conductive pathways in the out-of-plane direction and exhibited a low Young's modulus (0.14 MPa). The composite with a thermoplastic polyurethane (TPU) elastomer embedded into its pores exhibited a low Young's modulus (3.62 MPa), high toughness (10.0 MJ/m3), and high thermal conductivity in the out-of-plane direction (0.57 W/mK). The proposed freeze-casting method can be applied for the fabrication of various polymers and fillers and is promising for the development of flexible porous composite elastomers with out-of-plane aligned thermally conductive pathways.

Plasma processing in solution is effective for the preparation of composites. To optimize the plasma processing in solution, a method that can be used to sufficiently evaluate the surface modifications is necessary. In this study, to identify such a method, hexagonal boron nitride (hBN) particles modified by plasma processing in solution and exhibiting different zeta potentials were used to prepare a polyrotaxane composite. Plasma processing in a hydroquinone solution yielded hBN particles with a high zeta potential, named HQpBN, and X-ray computed tomography showed a uniform dispersion of the HQpBN/polyrotaxane composite. The tensile strength and elongation of the HQpBN/polyrotaxane composite were twice as high as those of an unmodified hBN/polyrotaxane composite, in which hBN particles aggregated. These results demonstrate that the zeta potential of the modified particles can be used as an indicator to guide optimal surface modification during plasma processing in solution for the preparation of homogeneous composites.

Continuous hydrothermal synthesis of Pr-doped CaTiO3 nanoparticles from Pr(NO3)3, Ca(NO3)2, a TiO2 sol (crystallite diameter of 5 nm), and various aqueous solutions of KOH was carried out at 673 K and 30 MPa. The synthesis at a very short residence time of 0.02 s could be examined by using a T-type micromixer for rapid heating of the aqueous solutions to 673 K. Pr-doped CaTiO3 nanoparticles having an average particle diameter of 22 nm and a strong red emission peak of about 613 nm were continuously produced at a residence time of 5.0 s. Further, the effects of residence time and KOH molality on the Ca/Ti ratio, particle diameter, crystallite diameter, and crystal structure of the products were carefully studied in order to discuss the formation mechanism of mainly the CaTiO3 structure in Pr-doped CaTiO3 nanoparticles from a system containing a solid oxide (TiO2). With increasing residence time and also KOH molality, the Ca/Ti ratio of the product increased up to a stoichiometric ratio of CaTiO3 (1.0), maj...

The effectiveness of K2Zn3[Fe(CN)6]2 (KZnHCF) for the radioactive cesium (R-Cs) decontamination from wood ash extract is studied. Because the ash-washing solution is of high pH and high K+-concentration, adsorbent with high pH tolerance and selectivity for Cs+ over coexistent K+ is required. For this purpose, we evaluated the effectiveness of KZnHCF, Prussian blue (PB), and zeolite. With 10,000 mg/L of K+-concentration, the distribution coefficient, Kd of KZnHCF, PB, and zeolite were 719, 4400, and 210 mL/g, respectively. Regarding the pH-effect, KZnHCF shows good Cs adsorption even at pH 12, in contrary, PB is effective only at pH

Plasma processing in solution with hydroquinone additive was performed for aqueous dispersion of hexagonal boron nitride, and the effect of hydroquinone additive was determined by analyzing the zeta potential distribution. The hydroquinone additive enhanced plasma modification in solution to produce well-dispersed h-BN particles separated by centrifugation. The mean zeta potential improved up to -50 mV at neutrality and maintained in a pH range 6–9. Infrared absorption spectra suggested that the dispersed h-BN particles were modified with hydroxyl functional groups by plasmas in solution.

Nonporous, submicrometer spherical particles can be fabricated by pulsed laser melting in liquid. In this process, particles dispersed in liquid absorb laser light, instantaneously heated above melting point, and quenched to form submicrometer spherical particles. The dispersed raw particles can only be heated from the laser beam direction, and thus the surface layer facing laser beam is reaction zone. Two automated iterative batch processing apparatuses with different cell sizes were developed to fabricate large quantities of submicrometer spherical particles by efficient use of liquid surface layer. Model calculations indicated that the formation rate will be higher when smaller cells and frequent cell exchange are adopted. This is experimentally confirmed with the fabrication of submicrometer spherical silver particles. The overall production rate of the iterative process with smaller cells was 5.15 mg h−1. However, the smaller cells imposed several limitations, including difficult sample agitation and stray light effects.

Thermally conductive flexible materials must be mechanically compatible with flexible electronic devices. They must have a low Young's modulus, high tensile strength, and toughness, in addition to a high thermal conductivity. These requirements have motivated researchers to develop composites of flexible rubber combined with a large amount of high thermally conductive carbon fibers/nanotubes. However, such composites become brittle because of the poor affinity between the rubber and carbon fibers/nanotube and local stress concentrations in the polymer matrix around the carbon fibers/nanotubes. Herein, to reduce this brittleness and achieve a high thermal conductivity, composite materials containing a slide-ring (SR) material and plasma-surface-modified carbon nanofiber (CNF)/carbon nanotube (CNT) aligned via the application of an electric field are described. The addition of a small amount of plasma-surface-modified CNTs to plasma-surface-modified CNF composites and the electric field alignment of these carbon materials produces a clear synergistic effect, facilitating an increase in the thermal conductivity. Furthermore, although this composite contains 45 wt% CNF and 5 wt% CNT, the toughness and tensile strength are no lower than in the case of raw SR. This composite material has a thermal conductivity similar to that of some metals, a low Young's modulus typical of elastomers, and a high tensile strength.

Graphene nanoplates (GNP) were modified via the plasma in liquid process (PLP) with two types of aqueous solutions for the fabrication of hydroxylated GNP and Ag-nanoparticles-physisorbed GNP. GNP were hydroxylated using the PLP in NaCl and hydroquinone aqueous solution. During the PLP employing AgNO3 aqueous solution, crystalized Ag nanoparticles with a diameter of around 10–100 nm were physisorbed on GNP with little functionalization. It was found that the amount of hydroxyl groups and Ag nanoparticles increased with the plasma treatment time.

We successfully produced water-dispersible reduced graphene oxide (rGO) by pH tuning liquid-phase photoreduction. In this method, the stabilizers and chemical modification usually used for dispersing rGO are not required. The stable carboxyl groups continue to ionize throughout the photoreduction process under alkaline conditions and continue to provide water-dispersible rGO. Moreover, the decomposition of GO into CO2 is prevented, and the production of defects is largely avoided. This is because the epoxide groups on the GO nanosheets that lead to decomposition are converted into hydroxide groups under alkaline conditions. Thus, this simple aggregation-, defect-, and stabilizer-free method is potentially important for the future application of rGO.

Corrosion resistant microfluidic devices with Ti-lined structure were fabricated for hydrothermal synthesis of high purity metal oxide nanoparticles under high-temperature and high-pressure condition. Pr-doped Ca 0.6 Sr 0.4 TiO 3 red phosphor nanoparticles without contaminations were continuously and safely synthesized using the devices at 673 K and 30 MPa within short residence time of 20 ms in spite of corrosive environment. The nanoparticles having an average particle size of 10 nm and a perovskite structure showed a strong red emission peak around 613 nm.

As the demand for graphene oxide (GO) increases, the need for a simple and practical GO preparation method that does not involve a chemical agent and time-consuming procedures increases. Herein, we propose an approach to the direct oxidation of multilayer graphene and natural graphite via the simple ozone bubbling method using atmospheric pressure barrier discharge. Although this method does not require chemical agents or long and complex procedures, it enabled users to achieve the successful oxidation of multilayer graphene and natural graphite. X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, atomic force microscopy, UV–vis spectroscopy and Raman spectroscopy results indicate that the products obtained from the ozone-based oxidation comprised oxygenated functional groups, mainly the carboxyl group, located at the edges of few-layer graphene structures characterised by low defect density. This simple preparation method can widen and accelerate the practical use of GO.

Melting temperatures of organic solids are often depressed by high-pressure CO 2 due to a dissolution of CO 2 in the molten organic compounds. For thermodynamic analysis of the melting point depression, solubilities of CO 2 in molten biphenyl and naphthalene were measured by near-infrared spectroscopy at various temperatures and pressures up to 20 MPa. Molarity of the organic component was determined from the 3 ν C H absorption band, and that of CO 2 from the 2 ν 1 + ν 3 band. Mole fraction of CO 2 in the liquid phase was found to be an increasing function of the pressure up to 0.6 at 20 MPa and a weakly decreasing function of the temperature. The solubility data were used for modeling of the mixtures by the Peng–Robinson equation of state with a binary interaction parameter k 12 . Calculation of the solid–liquid–gas phase equilibrium for the model fluid qualitatively described a large decrease in the melting temperature with increasing pressure up to 10 MPa followed by a small change at higher pressures. The melting point change was interpreted by the two competing effects: hydrostatic pressure effect increases the melting point by ca. 8 °C at 20 MPa, whereas CO 2 solubility effect reduces it by ca. 30 °C. Decomposition of the solubility effect into ideal and non-ideal mixing parts revealed that the non-ideality increases the melting point by more than 10 °C.

In this work, a continuous hydrothermal synthesis method in supercritical water was applied to environmentally benign production of Ca1−xSrxTiO3 (x = 0.0–1.0) solid-solution nanoparticles as key materials in conducting, electric, magnetic, and photocatalytic fields. A T-type micromixer (330 μm id) was introduced for rapid heating of stating solutions of Ca(NO3)2, Sr(NO3)2, and TiO2 sol using turbulent flow mixing with preheated NaOH aqueous solution and also for exact control of reaction temperature. At 673 K and 30 MPa for 5.0 s mean residence time, Ca1−xSrxTiO3 solid-solution nanoparticles having crystallite diameters of around 20 nm with monomodal diameter distributions were obtained without byproducts and production of CaTiO3 and SrTiO3 separately over the whole composition range. Effects of NaOH molality, Ca and Sr compositions in the starting solutions, and mean residence time on the reaction were examined. The results showed that TiO2 sol dissolution and Ca1−xSrxTiO3 precipitation were almost finished within 0.25 s mean residence time, and after that Ostwald ripening proceeded.

CO2-expanded liquid (CXL) is a mixture of organic solvent with high-pressure CO2 whose volume is increased by CO2 dissolved in it. CXLs have attracted attention as tunable solvents, because the solvent properties can be widely controlled by the pressure. The volume expansion and the solubility of CO2 were measured by near-infrared spectroscopy for 6 CXLs at various pressures up to 55 bar and 40 °C. The molarity of organic solvent was determined from the absorbance of the 3ν and 2ν + δ bands, and that of CO2 was obtained from the area of the 3ν3 band, whose peak shifted to higher frequency with increasing pressure due to a decrease in the molecular interaction around CO2. The expansion coefficient was shown to be an increasing function of the pressure with larger slope at higher pressure, and the mole fraction of CO2 in the liquid phase was an almost linearly increasing function of the pressure. The results were in quantitative agreement with the literature data measured by conventional sampling method, indicating the validity of the spectroscopic method.

Leakage current is a serious problem for the use of ferroelectricity in room-temperature multiferroics BiFeO3, and oxygen nonstoichiometry is considered as one of its principal origins. In order to establish a method to control oxygen content in the compound, we investigated the annealing process of stoichiometric BiFeO3 grains in air and revealed that oxygen diffusion occurs in two steps: (1) the weight of the sample decreases in a short time, which originates from the generation of oxygen deficiency near the surface of the grains; and then (2) it increases gradually and slowly, which originates from oxygen diffusion toward equilibrium in the inner part of the grains, introducing excess oxygen there. Step 1 causes the leakage current, and step 2 tends to cause inhomogeneity of oxygen content as well as the leakage current. Steps 1 and 2 are related to oxygen deficiency and excess oxygen often observed in thin films and bulk crystals, respectively. For the synthesis of homogeneous and highly insulating bulk sample, it is important to avoid these annealing processes, and it is a good way to grow a crystal with stoichiometric oxygen content by the control of atmospheric oxygen partial pressure and taking out its inner part.

We report a continuous production of Perovskite-type metal oxide nano-phosphor CaTiO3:Pr by hydrothermal synthesis using a flow type reaction system. Aqueous mixing solution of Ca(NO3)2, TiO2 sols, and Pr(NO3)3 were used as stating materials. Reaction temperatures and pressure were set to 200 to 400 °C and 30 MPa, respectively. Characteristics were performed by powder X-ray diffractometry, transmission electron microscopy and spectrophotometer. We investigated the effects of the reaction temperature on crystal structure, particle size and photoluminescence properties. From the results, the particle obtained at 400°C was single crystal with around 20 nm in a diameter and had Perovskite structure. The particles shows a typical red emission peak at around 612 nm, which is assigned to the transition 1D2 →3H4 of Pr3+. The intensity of the red luminescence increased with reaction temperature and then it showed the maximum at 400°C of supercritical water condition.

Prussian blue is a historical pigment synthesized for the first time at the beginning of 18th century. Here we demonstrate that the historical pigment exhibits surprising adsorption properties of gaseous ammonia. Prussian blue shows 12.5 mmol/g of ammonia capacity at 0.1 MPa, whereas standard ammonia adsorbents show only 5.08–11.3 mmol/g. Dense adsorption was also observed for trace contamination in atmosphere. Results also show higher adsorption by Prussian blue analogues with the optimization of chemical composition. The respective capacities of cobalt hexacyanocobaltate (CoHCC) and copper hexacyanoferrate (CuHCF) were raised to 21.9 and 20.2 mmol/g, the highest value among the recyclable adsorbents. Also, CoHCC showed repeated adsorption in vacuum. CuHCF showed regeneration by acid washing. The chemical state of the adsorbed ammonia depends on the presence of the water in atmosphere: NH3, which was stored as in the dehydrated case, was converted into NH4+ in the hydrated case.

Prussian blue is a historical pigment synthesized for the first time at the beginning of 18th century. Here we demonstrate that the historical pigment exhibits surprising adsorption properties of gaseous ammonia. Prussian blue shows 12.5 mmol/g of ammonia capacity at 0.1 MPa, whereas standard ammonia adsorbents show only 5.08-11.3 mmol/g. Dense adsorption was also observed for trace contamination in atmosphere. Results also show higher adsorption by Prussian blue analogues with the optimization of chemical composition. The respective capacities of cobalt hexacyanocobaltate (CoHCC) and copper hexacyanoferrate (CuHCF) were raised to 21.9 and 20.2 mmol/g, the highest value among the recyclable adsorbents. Also, CoHCC showed repeated adsorption in vacuum. CuHCF showed regeneration by acid washing. The chemical state of the adsorbed ammonia depends on the presence of the water in atmosphere: NH3, which was stored as in the dehydrated case, was converted into NH4(+) in the hydrated case.

We propose the preparation of thin films on the porous substrate using one component Ca 0.8 Sr 0.2 Ti 0.7 Fe 0.3 O 3 − δ (CTO) perovskite-type oxide with the same composition, prepared by using different procedures. An asymmetric membrane with a 20 μm thickness was prepared by using CTO synthesized by the supercritical hydrothermal synthesis method. This asymmetric membrane was used as a one component membrane reactor (OCMR) comprising the membrane and the partial oxidation catalyst (Ni supported on Ca 0.8 Sr 0.2 Ti 0.9 Fe 0.1 O 3 − δ ) to produce synthesis gas from methane. The performance of OCMR has been investigated for the partial oxidation of methane in synthesis gas in natural gas conversion process. For this OCMR, the conversions of methane and selectivity to CO were 82.3% and 99.8% at 1173 K, respectively, and were significantly improved compared with the unsupported solid disk type membrane with a 500 μm thickness. Moreover, the activation energy of the oxygen permeation process and the relationship between oxygen permeation and film thickness were confirmed to investigate oxygen permeation properties of the CTO thin film under OCMR.

Boehmite nanoparticles with different shapes and lengths were used in the preparation of the boehmite films and coatings, and as the fillers for an organic polymer. Then, it was investigated how the shapes and lengths of the particles reflected the characteristics of the films, coatings and fillers. The films were prepared by casting an aqueous sol of the nanoparticles on a Teflon-lined plate, drying and peeling. The coatings were fabricated on porous alumina tubes or graphite sheets by dipping the substrates into a sol of the nanoparticles. The boehmite-polyvinyl alcohol (PVA) composite films, in which the nanoparticles are fillers, were prepared by casting a mixture of a boehmite nanoparticle sol and PVA. The films, coatings and composites were characterized by TEM, TG-DTA, gas permeability, pore size distribution and thermal expansion measurements. In the films, fibrous particles with aspect ratios over 50 aggregated in parallel to each other to form slit-like micropores, on the other hand, plate- and rod-like particles with aspect ratios below 10 randomly aggregated to form meso pores. Corresponding to this, for the coatings on porous alumina tubes and graphite sheets, the fibrous particles gave much low gas permeability and high thermostability, respectively, compared with the plate-like and rod-like particles. For the boehmite-PVA composite films, the fibrous particles gave lower thermal expansion than the plate-like and rod-like particles.

We report preparation of Tb3+ or Eu3+ doped zirconia nanoparticles by sub- and supercritical hydrothermal reaction and luminescence properties. Experiments were performed by a flow-type hydrothermal reaction system. We investigated the effect of the reaction temperature on crystal structure, particle size and photo-, electro-luminescence properties. Reaction temperature was varied in the ranges of 150 to 400 C and reaction pressure was fixed to 30MPa. Characteristics of the products were performed by XRD, TEM, SEM-EDS and PL, EL measurements. The product transformed from amorphous zirconium hydroxide to tetragonal ZrO2 with an increase in the reaction temperature, regardless of the doped ion species. The ZrO2:Eu particles obtained at 400C were spherical with around 5 nm in diameter. ZrO2:Eu nanoparticles obtained under supercritical water condision showed typical photoluminescence spectra of f-f transition of Eu3+; however ZrO2:Tb nanoparticle obtained under the same condition showed no photoluminescence. Furthermore, the ZrO2:Eu nanoparticles obtained at 400 C showed high electroluminescence properties at electric field of 350V and 1kHz.

The solubility of terephthalic acid (TPA) in subcritical water was measured as a function of the temperature from (349 to 547) K at a pressure of 10.0 MPa. A semibatch flow apparatus was developed for the solubility measurement. N-Methylpyrrolidone was used as a good solvent for collection of TPA saturated in subcritical water without deposition upon cooling. The solubility of TPA in water showed an exponential increase with increasing temperature from 1.25·10–5 at 349 K to 2.99·10–2 at 547 K. The aqueous solubility of TPA was found to be lower than those of m- and o-phthalic acids by one and three orders of magnitude, respectively. The Gibbs free energy of solution (ΔGsol) as well as the enthalpic and entropic components (ΔHsol and –TΔSsol) were calculated from the temperature dependence of the solubility. The thermodynamic analysis showed that the ΔGsol gradually decreases with increasing temperature, as a result of the competition between a large increase in ΔHsol and a large decrease in –TΔSsol. The Δ...

Continuous hydrothermal synthesis of nickel ferrite nanoparticles from Fe(NO3)3 and Ni(NO3)2 was performed using a central collision-type micromixer developed for rapid heating of a starting soluti...

Continuous hydrothermal synthesis of Fe2O3, NiO, and CuO nanoparticles from starting solutions of metal nitrates was carried out at 673 K and 30 MPa with a flow-through apparatus. A T-type micro mixer was used for superrapid heating of the starting solutions. Residence time was varied from very short time of 0.002–2 s. Conversion of metal nitrates to products, average particle size (APS), crystal phase and crystallinity were characterized by ICP, TEM, XRD and TG, respectively. In the Fe2O3 synthesis, the conversion became 97.9% at shortest residence time of 0.002 s and APS slightly increased from 4.0 to 6.7 nm with increasing residence time. In the NiO synthesis, the conversion was 11.4% at 0.002 s and increased to 70.1% at 2 s. In parallel, APS increased from 15.4 to 18.7 nm. In the CuO synthesis, Cu(NO3)2 was not converted to CuO at 0.002 s. The conversion and APS sharply increased with increasing residence time to 0.257 s. After that, the conversion, and APS gradually increased from 25.6 to 42.5% and from 28.7 to 34.3 nm, respectively. Mechanism of particle nucleation and growth at different metal species was discussed on the basis of the experimental results and the metal oxide solubility.

Barium titanate (BaTiO 3 : BT) nanoparticles were synthesized by the hydrothermal method in the presence of dispersants using a continuous supercritical flow reaction system. The reactants of TiO 2 sol/Ba(NO 3 ) 2 mixed solution and KOH solution were used as starting materials and that was heated quickly up to 400 °C under the pressure of 30 MPa for 8 ms as reaction time. The dispersant solution such as polyacrylic acid (PAA) and polyoxyethylene(20) sorbitan monooleate (Tween 80) was injected in the cooling process after the reaction. The crystal phase of the obtained particles was identified as perovskite cubic BaTiO 3 by X-ray diffractometry (XRD) and Raman spectroscopy. Raman spectra and thermogravimetric data revealed that PAA and Tween 80 fabricated hybrid BT nanoparicles. Primarily particle size of the BaTiO 3 nanoparticle was determined by means of BET surface area, as small as less than 10 nm irrespective of dispersants. In contrast, dispersed particle size in solution measured by dynamic light scattering (DLS) technique decreased from 282 nm to less than 100 nm depending on the dispersant. Aggregation of BaTiO 3 nanoparticles might be depressed in the presence of dispersants, especially PAA is the most effective among the dispersants examined.

In the synthesis of metal oxide fine particles by continuous supercritical hydrothermal method, rapid mixing of starting solution with supercritical water is a key factor for producing nanoparticles that have a narrow size distribution. In this paper, continuous hydrothermal synthesis of NiO nanoparticles from Ni(NO3)2 aqueous solution at 400 °C and 30 MPa was carried out with T-shaped mixers and the effect of inner diameter, flow rate, and mixing directions on the particle size was examined. The computational fluid dynamics (CFD) simulation of the mixers was performed to evaluate the heating rate of the starting solution. When the inner diameter of the T-shaped mixer was decreased from 2.3 to 0.3 mm and the flow rate was increased from 30 to 60 g/min, the produced NiO particle size decreased remarkably from 54.3 to 20.1 nm. This trend of the decrease in particle size could be described as a function of the heating rate. The experimental and CFD results showed the detail regions of local heating that correlated with the NiO nanoparticle size.

This paper summarizes specific features of supercritical hydrothermal synthesis of metal oxide particles. Supercritical water allows control of the crystal phase, morphology, and particle size since the solvent's properties, such as density of water, can be varied with temperature and pressure, both of which can affect the supersaturation and nucleation. In this review, we describe the advantages of fine particle formation using supercritical water and describe which future tasks need to be solved.

Highly crystalline BaTiO 3 nanoparticle was synthesized rapidly by hydrothermal reaction in supercritical water using a continuous flow reactor. The reactants of TiO 2 sol (or TiCl 4 )/Ba(NO 3 ) 2 mixed solution and KOH solution were used as starting materials and that was heated quickly up to 400 °C under the pressure of 30 MPa for 8 ms as reaction time. The XRD results revealed that the crystal phase of the obtained particles was cubic BaTiO 3 , indicating that the hydrothermal reaction in supercritical water was successfully proceeded under present reaction conditions. Primarily particle size of the BaTiO 3 nanoparticle was determined by means of BET surface area, as small as less than 10 nm with decreasing the reaction pH. In contrast, dispersed particle size in solution measured by DLS (dynamic light scattering) technique decreased from 260 to 90 nm with increasing the reactants concentration. Aggregation of BaTiO 3 nanoparticles might be depressed in the presence of coexisting nitrate anions.

Crystallization of Mn-doped zinc silicate, α-Zn2SiO4:Mn2+ was studied using several Zn, Mn, and Si sources in supercritical water with and without ethylenediamine tetraacetic acid (EDTA). Oxalate, hydroxide, oxide, and sulfate were used as the Zn and Mn sources and amorphous, crystalline, and colloidal SiO2 and tetraethoxyorthosilicate (TEOS) were used as the Si sources. Zn and Mn sources acted as a trigger for Zn2SiO4 crystallization through dissolution, while the solubility of the Si source affected reaction velocity. The oxalate source gave single phase α-Zn2SiO4:Mn2+, while the hydroxide source provided mainly β-Zn2SiO4:Mn2+ with a small amount of α-Zn2SiO4:Mn2+. The oxide sources did not readily react and only gave small amounts of α- and β-Zn2SiO4:Mn2+. Although the sulfate source did not give any Zn2SiO4 phases, addition of EDTA as a complexing agent for Zn2+ ions caused α-Zn2SiO4 crystallization. Remarkably, addition of EDTA into the sulfate source and TEOS gave botryoidal druses of rod-like shaped α-Zn2SiO4 crystals, which are similar to natural α-Zn2SiO4 deposits (willemite) that only occur in La Calamine, Belgium (Moresnet). In conclusion, Zn and Mn oxalates and amorphous SiO2 were the best combinations for crystallizing single phase α-Zn2SiO4:Mn2+ from supercritical water.