Shoichi Yoshihara

High-speed observation of the piston effect near the gas-liquid critical point

We investigated adiabatic changes in a near-critical fluid on acoustic time scales using an ultrasensitive interferometer. A sound emitted by very weak continuous heating caused a stepwise adiabatic change at its front with a density change of order 10(-7) g/cm(3) and a temperature change of order 10(-5) K. Very small heat inputs at a heater produced short acoustic pulses with width of order 10 micros, which were broadened as they moved through the cell and interacted with the boundaries. The pulse broadening became enhanced near the critical point. We also examined theoretically how sounds are emitted from a heater and how applied heat is transformed into mechanical work. Our predictions agree well with our data.

Ultra-sensitive high-speed density measurement of the “piston effect” in a critical fluid

In order to conduct a detailed investigation of the piston effect, the peculiar heat transportation phenomenon in critical fluids, an ultra-sensitive high-speed density measurement system was developed using a very thin heater, a sensitive interferometer and a large-capacity high-speed data acquisition system. As the first step of the investigation, an experiment was conducted to measure the velocity of sound in CO2 near its critical point. Short heat pulses suppressing the disturbance of natural convection were applied to a small cell filled with almost-critical CO2 fluid and were detected by the measurement system. Because the pulses propagated through the cell at the velocity of sound and were reflected several times between the cell walls, the velocity could be precisely determined by measuring the time intervals between successive reflections. The resulting velocity profile versus temperature showed good agreement with theoretical prediction and numerical simulations. This result validated the density measurement system, and it is considered that the measurement system will be a very effective tool for further studies on critical phenomena with the aid of numerical simulation.

Study for the Reduction of Carbon Dioxide Using Sabatier Reaction

The Sabatier reaction with a titanium dioxide (TiO2)-supported ruthenium (Ru) catalyst (Ru/TiO2) was investigated with the goal of scaled-up carbon dioxide (CO2) methanation at lower temperatures than conventional catalysts. The catalytic reaction of stoichiometric amounts of CO2 and hydrogen (H2) without any diluting was conducted over a 0.75 wt% Ru/TiO2 (Ru/TiO2(0.75)) at several flow rates. The CO2 reduction was achieved with catalyst temperatures less than 300°C in good to excellent conversions. Transmission electron microscopy (TEM) and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX) images of Ru/TiO2(0.75) revealed highly dispersed Ru nanoparticles having one to a few nanometers’ diameter on the TiO2 supports. The physical properties of Ru metals are considered to contribute the decrease of the reaction temperature.