10. In Situ Visualization of the Ammonothermal Crystallization Process by X-ray Technology

X-ray based in situ monitoring techniques for ammonothermal processes are reviewed. Technological aspects are discussed, including general aspects of in situ X-ray visualization technology as well as the corrosion resistance of prospective materials for X-ray transparent windows under ammonothermal conditions (for more comprehensive information on corrosion resistance see Chap. 11). In situ X-ray visualization methods have proven to be extremely useful for gaining insights that are inaccessible through other methods to date. Results obtained by in situ X-ray imaging comprise information on solubility and dissolution kinetics as well as insights into the transport of solutes and local concentration changes of solutes. Moreover, phase changes of the fluid can be monitored and the technique has been adapted for solubility studies of novel materials that are unavailable as bulk materials to date. Findings of particular interest include a revision of solubility data for GaN, insights into face-selective and mineralizer-selective dissolution kinetics of GaN, and the visualization of Ga transport within the fluid upon dissolution of GaN. The observed extremely slow motion of Ga-containing species suggests a pronounced influence of solutes on the viscosity of the fluid, which has largely been treated as negligible due to the lack of data for the respective mixtures so far.