8. Direct Determination of Viscosity of Supercritical Solutions

The following chapter is mainly aimed at simulators and crystal growers, as viscosity has an influence on the flow behaviour in the reactor and the diffusion coefficient in the crystal’s vicinity. So, the chapter gives an overview of influencing factors to viscosity in ammonothermal media, which are the pressure and temperature as well as the concentration of used mineralizers. Therefore, different possible viscometers are described and discussed in detail for its potential use in ammonothermal media. Hereby two promising options are presented in detail: a modified rolling ball viscometer as well as an adaptation of the ultrasonic pulse-echo method for viscosity measurement for ammonothermal systems. While the last is mostly based on literature research and only some general prove of principle are carried out, the first one is fully described and analysed during operation. This means for the adaptation of this principle four critical aspects have to be overcome. As a result, the viscosity of ammonia in the range above 400 °C up to 600 °C at maximum pressure of 252 MPa is shown. Additionally, some measurements of ammonia-ammonium-fluoride-mixtures are compared with pure ammonia, whereas the viscosity is about 1.4 times lager with ammonium fluoride then without.