Kiwamu Sue

Continuous production of phosphor YAG:Tb nanoparticles by hydrothermal synthesis in supercritical water

Phosphor YAG:Tb ((Y{sub 2.7}Tb{sub 0.3})Al{sub 5}O{sub 12}) nano particles were synthesized by a hydrothermal method at supercritical conditions (400 deg. C and 30 MPa) using a flow reactor. Hydroxide sol solutions formed by stoichiometric aluminum nitrate, yttrium nitrate, terbium nitrate and potassium hydroxide solutions. The relationship between particle size and experimental variables including pH, concentration of coexistent ions and hydroxide sol were investigated. Particles were characterized by XRD, TEM and photo-luminescence measurements. Particle size of YAG:Tb became finer as pH was increased or potassium nitrate concentration of the starting metal salt solution was increased. By removing the coexisting ions (NO{sub 3}{sup -}, K{sup +}) from the metal salt solution, single phase YAG:Tb particles with 20 nm particle size were obtained. The emission spectra of YAG:Tb particles of 14 nm shows a blue shift.

Continuous synthesis of zinc oxide nanoparticles in supercritical water

Hydrothermal synthesis of nano-size zinc oxide particles was conducted using a flow type apparatus for rapid heating of zinc nitrate and potassium hydroxide aqueous solution to supercritical conditions at 30 MPa and temperatures ranging from 573 to 673 K.

Water density dependence of formaldehyde reaction in supercritical water

Abstract The water density dependence of formaldehyde (HCHO) reaction in supercritical water (SCW) was studied with batch experiments. Major products from the reaction were methanol (CH 3 OH), formic acid (HCOOH), hydrogen (H 2 ), carbon monoxide (CO) and carbon dioxide (CO 2 ). It was found that the Cannizzaro reaction mechanism was the preferred reaction pathway for HCHO reaction in SCW. At higher water densities, CH 3 OH yields increased confirming the predominance of the Cannizzaro reaction mechanism. At low water densities, CO yields increased and CH 3 OH yields decreased, which indicated that monomolecular decomposition became the main reaction pathway. Addition of base to the reacting mixtures was found to promote the Cannizzaro reaction path whereas addition of acid promoted monomolecular decomposition.

Near-infrared spectroscopic study of a water-in-supercritical CO2 microemulsion as a function of the water content.

A water-in-supercritical CO(2) microemulsion is a reverse micelle encapsulating a nanometer-size water droplet dispersed in supercritical CO(2). In the microemulsion solution, water exists not only in the reverse micelle but also in the solvent CO(2). For quantitative analysis of the water distribution, near-infrared spectra of water + CO(2) and water + surfactant + CO(2) mixtures were measured over a wide range of water/CO(2) ratios from 0.1 to 1.0 wt% at 60 °C and 30.0 MPa. The stretching combination band of water was decomposed into two components, a sharp one peaked at 7194 cm(-1) assigned to monomeric water dissolved in CO(2) and a broad one around 7000 cm(-1) corresponding to aggregated water in the microemulsion. Integrated molar absorptivities of these types of water were negligibly different from each other, despite the different hydrogen-bonding environments. The spectral decomposition revealed that water is distributed mainly into CO(2) at water contents smaller than 0.5 wt% and then is introduced into the microemulsion after saturation of water in CO(2) and full hydration of the surfactant headgroup.

Effect of Chromium Ion from Autoclave Material on Corrosion Behavior of Nickel-Based Alloys in Supercritical Water

Abstract Seven kinds of corrosion tests were carried out on thirteen kinds of nickel-based alloys in subcritical and supercritical water containing 0.01 mol/kg sulfuric acid (H2SO4) and 800 ppm oxygen (O2) using a Ni-44Cr-1Mo autoclave. The obtained corrosion rates were significantly lower than the previous experiments performed under the same conditions using a titanium autoclave. A compact oxide scale was found on the specimens tested in the Ni-44Cr-1Mo autoclave. The chromium-rich oxide layer would originate from the autoclave material and provide a protective function against corrosion. The corrosion retardation phenomenon has been explained by dependencies of chromia (Cr2O3) solubility on the phase state of water and temperature.

Potentiometric cell for measuring pH of supercritical aqueous solutions

A flow-through electrochemical cell for subcritical and supercritical water systems was developed based on the cell of Lvov et al. [J. Electroanal. Chem. 463, 146 (1999)]. The cell consisted of two flow-through platinum hydrogen electrodes and was constructed from a high nickel alloy (Hastelloy-B2). The inner surface of electrochemical cell was coated with Al2O3 adhesive and covered with the Al2O3 tube for corrosion resistance. Orifices were placed near the center of the cell to reduce concentration fluctuations around the test electrode. With the cell, a flow apparatus was used to measure the potentials for HCl+NaCl aqueous solutions at supercritical conditions. The cell potentials were measured at temperatures ranging from 23.9 to 400.2 °C and at pressures ranging from 25.0 to 35.1 MPa and the standard deviations of measured potentials were less than 0.4 mV for all tests. Comparison of the experimentally determined and theoretically calculated pH differences are better than ±0.03 logarithmic units over ...

Control of methanol oxidation by ionic behavior in supercritical water.

In supercritical water the rate of methanol oxidation was controlled by ionic behavior as follows: the oxidation rate of methanol decreased with increasing proton and hydroxide ion concentration, possibly due to stabilization of the reactant, while that of CO was suppressed by added protons and enhanced by added hydroxide ions.

Continuous Production of Quinacridone Nanocrystals and Control of Crystal Form by High-Temperature Water Crystallization Method

A high-temperature water (HTW) crystallization method was applied to produce size- and form-controlled organic nanocrystals without the use of any organic solvents. HTW and room-temperature water (RTW) were used as the extraction and cooling solvents, respectively. Nanocrystals are precipitated by mixing a high-temperature quinacridone aqueous solution in a newly developed micromixing unit with RTW from another line. Quinacridone nanocrystals having an average diameter of ca. 9 nm as determined from transmission electron microscopy observations and ca. 83 nm from dynamic light scattering measurements were produced at 250.6?C and 24 MPa with a flow rate of 10 g/min for HTW and 20 g/min for RTW. In addition, it was found that nanocrystals having different crystal sizes and forms could be obtained by changing the temperature and flow rate of RTW. This new method promises an environmentally sustainable process for the continuous production of organic nanocrystals.

Practical development of continuous supercritical fluid process using high pressure and high temperature micromixer

In the synthesis of metal oxide fine particles by continuous supercritical hydrothermal method, the particle characteristics are greatly affected by not only the reaction conditions (temperature, pressure, residence time, concentration, etc.), but also the heating rate from ambient to reaction temperature. Therefore, the heating method by direct mixing of starting solution at room temperature with supercritical water is a key technology for the particle production having smaller size and narrow distribution. In this paper, mixing engineering study through comparison between conventional T-shaped mixers and recently developed swirl mixers was carried out in the hydrothermal synthesis of NiO nanoparticles from Ni(NO3)2 aqueous solution at 400 °C and 30 MPa. Inner diameter in the mixers and total flow rates were varied. Furthermore, the heating rate was calculated by computational fluid dynamics (CFD) simulation. Relationship between the heating rate and the average particle size were discussed. It was clari...