Koji Tajiri

Porosity dependence of sound propagation in liquid-4He-filled aerogel

Longitudinal sound-wave propagation has been studied in an aerogel-liquid 4He system for various porosities of aerogel. The superfluid transition was identified as the absorption peak, whose magnitude was suppressed by aerogel. The sound velocity was analyzed within a hydrodynamic theory in both normal and superfluid phases. The absorption peak due to phonon-roton interaction around 1 K was not observed even with the most porous aerogel. The low-temperature sound velocity and attenuation show that direct collisions of phonons with aerogel strands play an important role in the acoustic properties.

Superior thermal stability and oxygen storage capacity of ceria nanoparticles dispersed on alumina cryogel

Cerium-dispersed alumina cryogel was prepared from cerium nitrate and aluminum sec-butoxide by a combination of one-pot sol–gel with freeze drying. The cryogel catalyst exhibited higher thermal stability of ceria and larger oxygen storage capacity than the corresponding xerogel and impregnation catalysts. The better stability and capacity were owing to the homogeneous distribution of cerium in boehmite gel by the one-pot synthesis of cerium-boehmite gel, and also to the suppression of aggregation of the cerium during the subsequent elimination of water from frozen gel by sublimation. As a result, cerium was finely dispersed and stabilized throughout the alumina cryogel, retarding the sintering of ceria at high temperatures. The destruction of fine structures of the catalyst at high temperatures was less pronounced on the cryogel, which also contributed to the stability of ceria.

Palladium-alumina Cryogel with High Thermal Stability and CO Oxidation Activity

Palladium-alumina cryogel was prepared from palladium nitrate and aluminum sec-butoxide through a sol–gel processing and subsequent freeze drying. The cryogel showed higher thermal stability of palladium and catalytic CO oxidation activity than the corresponding xerogel and impregnation catalysts. The superior stability and activity were ascribed to the uniform distribution of palladium ions in the boehmite gel by the one-pot preparation of palladium-boehmite co-gel, and also to the suppression of transfer and aggregation of the metal during the subsequent freeze drying. As a result palladium ions were finely distributed and stabilized throughout the alumina cryogel support, which consequently suppressed the sintering of palladium at elevated temperatures.Graphical Abstract

Long-Term Optical and Thermal Examinations of Ceramic Wall System with Solar-Altitude Dependent Reflectance

Many ceramic tiles are used as outer skins of office and residential buildings and they prevent severe weathers of solar ray, temperature, wetness and wind. The solar irradiance to the outer walls is a significant factor to influence the heat flow into room and consequently the energy load due to air conditioning. Particularly in the region where clear seasonal change is experienced, optical and thermal properties of the wall surfaces are desired to be controlled as a function of season; solar reflective in hot days in summer; solar absorptive in chilly days in winter. The ceramic walls with solar-altitude dependent reflectance are expected to be a simple and efficient method for autonomous control of heat flow into the room and in this study they were investigated from the viewpoints of optical property and shape of the tiles. The correlation between solar absorption and luminous reflectance revealed that there are some ceramic tiles whose solar absorption slightly depends on solar altitude. The triangular-shaped ceramic tiles that have reflective-upward and absorptive-downward surfaces were investigated in optical and thermal properties, based on one-year exposure of the tiles to solar radiation and ray-tracing simulation. These measurement and simulation found that the triangular-shaped tiles are an effective approach to the control of solar absorption and suggested the effective shape and surface property of the tiles such as optical reflectance, surface specularity and triangular angle.

Solidification and Melting of 4He in Aerogel Observed by Ultrasound Propagation

Helium in small confined geometries remains a liquid at considerably higher pressures than bulk melting. In the present work, we have studied solidification and melting of 4He in aerogel whose structure is different from that of Vycor glass, for which the p‐T phase diagram has been established. The velocity and attenuation of longitudinal ultrasound were measured to detect solidification and to determine the solidification pressure. The solidification pressure in 94% porous aerogel is elevated by about 0.3 MPa from that of bulk. This value is considerably smaller than in Vycor glass. We have observed smaller hysteresis on melting and freezing in aerogel than that in Vycor glass. Transmission of sound through the solid‐liquid interface in aerogel has been observed.

The Relation between Catalytic Activity of CO Oxidation and Support Structure in Oxidation Catalysts using Gold Nanoparticles

Thiol-passivated gold nanoparticles were adsorbed on several kinds of support materials such as titania-coated silica aerogels and xerogels etc., and then the thiol was removed by heat treatment. The catalytic activity of the prepared composites for CO oxidation reaction was measured, and the effects of the support on the catalytic activity were investigated. Density of the supports, namely, whether aerogel supports or xerogel ones, hardly affected the catalytic activity. It was found that the catalysts having high catalytic activity could be obtained by this preparation method, even using the xerogels as the support. Calcination of the supports before adsorption of the gold nanoparticles affected the activity. The difference of the catalytic activity was observed between the composites with same gold nanoparticle size, so it was considered that the surface condition of the support materials affects the state of gold nanoparticles in composite.