Hidetoshi Nagamoto

Synthesis and textural properties of unsupported and supported rutile (TiO2) membranes

Two approaches were postulated for improving the stability of porous texture of titania membranes: (1) retarding the phase transformation and grain growth; (2) avoiding the phase transformation. Based on the second approach, rutile membranes were made directly from a rutile sol, prepared by the precipitation of titania on SnO2 nuclei. The rutile membranes were stable up to 800 °C, with a porosity of ca. 40%, whereas normal titania membranes (starting with anatase) show very little porosity above 600 °C. Alumina substitution retards grain growth and pore growth at 850 °C for unsupported as well as supported membranes.

Crack-free porous YSZ membrane via controlled synthesis of zirconia sol

Abstract Sol-gel processing is one of the most powerful tool to synthesize inorganic thin membranes. Several membranes, such as alumina, silica, titania, etc., have been prepared so far. The key in the sol-gel process is how to conquer cracking in the drying stage. We approach this problem in view of the control of sol-gel chemistry. The particulate zirconia sol is a useful precursor to synthesize nanostructured yttria stabilized zirconia (YSZ) at low temperature, but we could not prepare a crack-free film on a porous support due to its mechanical fragileness. Polymeric sol containing linear chains of zirconium and oxygen was synthesized via controlled hydrolysis and condensation of zirconium n -butoxide using triethanolamine (TEA, 2,2′,2″-nitrilotriethanol) as a chelating agent. Polymeric sol is useful to remove cracking, but the film formation property on a porous support is so poor since most of the sol is sucked into the support and no gel layer is formed on the top. By mixing two sols, we successfully balance the film formation property and the mechanical strength to cracking. Thus, crack-free porous YSZ thin membrane with a few-tens of nanometers pores are successfully prepared. The nanostructure is still stable after the post-calcination at 1173 K.

Formation mechanism of crack-free porous YSZ membrane

The key step in the sol-gel processing of thin films is the removal of cracking in the drying and calcining stage. We could successfully balance the film formation property and the mechanical toughness to cracking by mixing polymeric and particulate sols of the same material. Crack-free YSZ thin membranes were, thus, prepared on a porous support. In this paper, the detailed mechanism of the film toughening is studied. Small angle X-ray scattering of the sols and nitrogen adsorption/desorption of the calcined oxides were carried out, which corresponded well to each other. Based on these results and the FE-SEM observations of the supported thin film, we propose a networking model of spherical particle agglomerate by linear inorganic polymer to interpret the formation mechanism while keeping the mechanical toughness. This approach should be applicable to other metal oxides and mixed oxides systems.

Direct Alcohol Electro-oxidation in an Intermediate Temperature Fuel Cell

Anodic reactions in a direct alcohol fuel cell (DAFC) at intermediate temperature (250 oC) were investigated from the standpoint of achieving high performance. In order to evaluate the temperature effect on the electro-oxidation of small alcohols in the intermediate temperature region, a kinetic study on the electro-oxidation of methanol and ethanol on Pt/C was conducted using a single cell fabricated with CsH2PO4 proton conducting solid electrolyte. At 250 oC, the current density of ethanol electro-oxidation is comparable to that of methanol electro-oxidation in a low potential region (< 400 mV vs. RHE). In addition, the analysis of reaction products suggests that C-C bond cleavage in ethanol electro-oxidation can proceed at the intermediate temperature. Through the electrochemical measurements, characteristic features in the alcohol electro-oxidation at the intermediate temperature were discussed.

Synthesis of Nanostructured Diamond via Controlled Surface Pretreatment in Hexane Medium

The grain density of chemical vapor deposition (CVD) diamond film is greatly enhanced via controlled surface pretreatment in hexane medium under ultrasonic irradiation. Surface treatment of the substrate prior to the deposition is essential for continuous diamond film formation by CVD processing. Generally the pretreatment is carried out using diamond powder dispersed in alcohol or acetone medium under ultrasonic irradiation. In this study we used hexane as a dispersion medium, and synthesized diamond on a silicon wafer by conventional microwave plasma-enhanced CVD (MPCVD). The grain size of a continuous film whose substrate is treated in hexane is successfully reduced to ~100 nm. The corresponding grain density is enhanced to ~1010/cm2. The diamond film is homogeneously blue in color. When the substrate is treated in acetone, the density is in the range of 108/cm2 under the same synthesis conditions.