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It is known that water exists in supercritical state above 374C and 22.1MPa, where gas phase and liquid phase are merged into a single phase. In the supercritical state, the density is controllable by changing the pressure. The properties such as the ionic product, solubilities of salts, gas and organic compounds, and dielectric constant of supercritical water are very different from those of water at room temperature.Recently much attention has been paid to supercritical water because many possible applications such as synthesis of functional materials, waste oxidation, and biomass conversion have been proposed by using above peculiar properties of the supercritical water. Thus, much intensive work is in progress all over the world. In addition, a new concept of nuclear reactor using the supercritical water as a coolant has been proposed, which has been chosen by DOE, USA as one of the forth generation nuclear reactors.Therefore, radiation chemistry study of the supercritical water seems inevitably important, yet little work has been done so far. We started radiation chemistry study of supercritieal water by pulse radiolysis and γ radiolysis five years ago. We have observed hydrated electrons as a first target. We also measured inorganic radicals, metal ions, organic radicals in high temperature and supercritical water by a pulse radiolysis technique. It was found that absorption spectra of the transient species are dependent on temperature. Some transients show red-shift, and some bands are blue-shifted, with increasing temperatures. A G-value evaluation of water decomposition products was also done by using methyl viologen as a scavenger and it was revealed that the values are significantly dependent not only on temperature but also on density in supercritical water. An extended study on the behavior of solvated eleclions in different alcohols at high temperatures and super critical state is also in progress.