Inelastische Neutronenstreuung bei hohen Temperaturen und Drücken zur Aufklärung der Lösungs- und Transportmechanismen von Wasser in wasserhaltigen Silikatschmelzen

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We have successfully constructed a high temperature/pressure cell for the application on neutron scattering spectrometers provides temperatures up to 1250 K and pressures up to 2 kbar at the sample position. This has been achieved with an internally heated NbZr autoclave. So far experiments on hydrous Na2O•3SiO2 melts with 10 mol% water have been performed on the time-of-flight spectrometer TOFTOF at FRM II. The measured density correlation functions exhibit the incoherent dynamics of the proton. The quasielastic line width is used to derived the self diffusion coefficient. The Q2 dependence even holds up to 1.5 A−1. This might be an indication of the diffusion mechanism via channel structures, similar to the alkali ions in anhydrous alkali silicate melts. A contrast variation with H2O/D2O allows to measure pure hydrogen dynamics since the total scattering cross-section for H is much larger than for D. With this analysis we expect to gain new insight of rotational dynamics of the water species and of the influence of the water addition on the Na dynamics. For the hydrous SiO2 and Al2O3•Na2O•6SiO2 systems, the water diffusion is not fast enough to be resolved within the instrument energy resolution of TOFTOF at the working temperature range of the pressure cell. However, experiments scheduled on the neutron backscattering spectrometer with higher resolutions should allow to study diffusive dynamics and diffusion mechanisms of the water species.