Daisuke Nagai

Effect of grain size on thermoelectric properties of n-type nanocrystalline bismuth-telluride based thin films

bismuth-telluride-based thin films with the average grain size of 60 nm by a flash-evaporation method and shown the re- duction of thermal conductivity. 12 To further reduce the thermal conductivity of the nano- crystalline bismuth-telluride-based thin films and investigate the effect of grain size on thermoelectric properties, we pre- pare the nanocrystalline thin films by the flash-evaporation method with an improved annealing condition. The grain size of the thin films is estimated using an x-ray diffraction XRD and atomic force microscopy AFM. The thermo- electric properties, in terms of the electric conductivity, the Seebeck coefficient, and the thermal conductivity, are mea- sured at room temperature. The measured thermal conductiv- ity is divided into the lattice thermal conductivity l and electronic thermal conductivity e. Then we investigate the grain size dependence of the lattice thermal conductivity of the nanocrystalline thin films using a simplified phonon transport model.

Reversible chain association/dissociation via a CO2 responsive crosslinking/decrosslinking system

Reversible chain association/dissociation phenomenon via CO(2) responsive crosslinking/decrosslinking was detected in aqueous solutions of polyallylamine (PAA). The chain association/dissociation behavior was reversible and useful in the synthesis of porous crosslinked polystyrene, which suggested potential utility in the area of CO(2)-responsive separable adhesives, switches and sensors.

Synthesis of Hydrogels from Polyallylamine with Carbon Dioxide as Gellant: Development of Reversible CO2 Absorbent

Hydrogels were successfully synthesized utilizing CO(2) as a gellant. A cross-linking reaction of polyallylamine (PAA) with CO(2) in the presence of 1,8-Diazabicyclo[5,4,0]-undec-7-ene (DBU) provided hydrogels bearing urea cross-linking points and residual amino groups in the side chains. The obtained hydrogels absorbed CO(2) at 25 °C and gave a maximum absorption four times larger than that of PAA aqueous solution and 2.8 times larger than that of the most commonly used absorbent, monoethanolamine. The PAA hydrogels desorbed the absorbed CO(2) completely under a N(2) atmosphere at 120 °C, and could be repeatedly recycled without loss of efficiency, indicating their potential application as recyclable CO(2) absorption materials.

A facile, selective, high recovery system for precious metals based on complexation between melamine and cyanuric acid

We developed a facile, selective, high recovery system for precious metals based on complexation between melamine and cyanuric acid (denoted as MC) through hydrogen bonding. The addition of melamine to an aqueous solution of metal ions allows for homogeneous coordination to metal ions with high efficiency at a high rate. After coordination, subsequent addition of cyanuric acid to the metal–melamine complexes results in precipitation of the metal–MC complex, which can be separated by filtration. The recovery of PdII using an MC system was fast and the recovery efficiency was greater than 90% within 1 min. The maximum amount of Pd recovered by the MC system (0.595 gPd/gMC) was greater than other materials in the literature. The system was also capable of selective recovery of PdII from other metal solutions. Pd recovered by the MC complex could be separated quantitatively by reductive treatment, and the MC system was recyclable at least 4 times. This system is therefore promising as a selective and high recovery system for precious metals.

Heavy metal adsorptivity of calcium-alginate-modified diethylenetriamine-silica gel and its application to a flow analytical system using flame atomic absorption spectrometry.

This study aimed to evaluate the heavy metal adsorptivity of calcium-alginate-modified diethylenetriamine-silica gel (CaAD) and incorporate this biosorbent into a flow analytical system for heavy metal ions using flame atomic absorption spectrometry (FAAS). The biosorbent was synthesized by electrostatically coating calcium alginate onto diethylenetriamine (dien)-silica gel. Copper ion adsorption tests by a batch method showed that CaAD exhibited a higher adsorption rate compared with other biosorbents despite its low maximum adsorption capacity. Next, CaAD was packed into a 1mL microcolumn, which was connected to a flow analytical system equipped with an FAAS instrument. The flow system quantitatively adsorbed heavy metals and enriched their concentrations. This quantitative adsorption was achieved for pH 3-4 solutions containing 1.0×10(-6) M of heavy metal ions at a flow rate of 5.0 mL min(-1). Furthermore, the metal ions were successfully desorbed from CaAD at low nitric acid concentrations (0.05-0.15 M) than from the polyaminecarboxylic acid chelating resin (Chelex 100). Therefore, CaAD may be considered as a biosorbent that quickly adsorbs and easily desorbs analyte metal ions. In addition, the flow system enhanced the concentrations of heavy metals such as Cu(2+), Zn(2+), and Pb(2+) by 50-fold. This new enrichment system successfully performed the separation and determination of Cu(2+) (5.0×10(-8)M) and Zn(2+) (5.7×10(-8) M) in a river water sample and Pb(2+) (3.8×10(-9) M) in a ground water sample.

A highly efficient supramolecular adsorbent for precious metal: adsorption behavior of PdII by melamine cyanurate

Melamine cyanurate (M-CA) was found to be a highly recoverable and selective supramolecular adsorbent for recovery of PdII ion. Adsorption of PdII by M-CA was fast in the first 30 min, with 93% adsorption efficiency, and equilibrium was reached within 1 h. Langmuir and Freundlich isotherm models were employed to correlate the experimental data. The adsorption of PdII by M-CA is well supported by a Langmuir isotherm model, with a high maximum adsorption capacity of 0.874 gPd gM-CA−1. M-CA was also capable of selective adsorption of PdII from a solution containing other metals. Pd adsorbed on M-CA was separated quantitatively by reductive treatment, and the M-CA could be recycled at least 3 times. We propose an in-plane adsorption mechanism for M-CA, which is a π–π stacked layered complex, based on the results of X-ray spectroscopy and computational calculations. This study demonstrates that M-CA is promising as a selective and highly efficient adsorbent for recovery of precious metals.

Recovery of Cs(I) by a thermoresponsive metal recovery polymer consisting of an N-isopropylacrylamide unit and a sodium methacrylate unit

Recovery of CsI ions was achieved using a thermoresponsive metal recovery polymer consisting of a sodium methacrylate (SMA) unit for adsorption and an N-isopropylacrylamide (NIPAAm) unit for thermoresponsiveness. The water-soluble polymer efficiently provided homogeneous adsorption at ambient temperature. After adsorption, heating of the resulting solution precipitated CsI-adsorbed polymers that were separable by filtration. The kinetics, pH effect, and effect of the copolymer composition ratio on CsI recovery were investigated. Furthermore, the washing of soil containing CsI ions using an aqueous solution of the polymer was examined. The washed soil was separated by filtration, and heating of the filtrate precipitated CsI-adsorbed polymers that could be recovered by filtration. This indicated the potential utility of thermoresponsive metal recovery polymer in the recovery of the radioactive cesium from contaminated soil.

Temperature Dependence of Electrophoretic Mobility and Hydrodynamic Radius of Microgels of Poly(N-isopropylacrylamide)

Electrostatic interactions in charged microgels, which are dominated by the microgel net charge, play a crucial role in colloidal stabilization and loading of small, charged molecules. In this study, the temperature dependences of electrophoretic mobility μ and hydrodynamic radius Rh were measured for a slightly ionized poly(N-isopropylacrylamide) (PNIPA) microgel in a dilute suspension. A decrease in Rh was observed in the temperature range between 30 °C and 35 °C, corresponding to the lower critical solution temperature of PNIPA, and an increase in |μ| was observed in a higher temperature range between 34 °C and 37 °C. The analysis based on electrophoresis theory for spherical polyelectrolytes indicated that the net charge of the microgel decreased as the microgel was deswollen.

Molecular Dynamics Simulations of Heat Conduction in Nano-Structured Silicon

We carried out molecular dynamics simulations (MD) of heat conduction in Si thin film and Si films with a nano-hole to represent the nano-structure, in order to investigate the mechanism of the thermal conductivity reduction of nano-structured materials. The Stillinger-Weber potential is used in this study. Different temperatures are applied at the both sides of boundaries of the calculation domain in the z-direction, and periodic boundary conditions are applied in the x and y directions. The calculated temperature profile of a Si thin film of 10.86nm thickness is compared to that calculated by using the phonon Boltzmann transport equation (BTE). These agreed reasonably well with each other, and the phonon mean free path of Si is estimated to be several tens of nanometers. Molecular dynamics simulation of Si at the uniform temperature of 800K is also carried out. Phonon dispersion curves are calculated by using the time-space 2D Fourier transform. The phonon modes at high frequency are not present in nano-structures of Si. We discuss the mechanism of the reduction of the thermal conductivity of nano-structured material on the atomic scale.© 2007 ASME