Subproject 5



In situ visualization of the ammonothermal crystallization process by X-ray measuring technology


The main objective of sub-project 5 is to establish an in situ X-ray visualization of the ammonothermal crystallization process in a high pressure autoclave (sub-projects 1, 2 and 3). This constitutes a complementary measurement technique to the in situ optical characterization (sub-project 3) in order to determine the crystallization rates for varying process parameters.

  1. This sub-project focuses on the digital two dimensional in situ X-ray imaging, which provides reliable results for structures higher than 50 µm. The feasibility could be proved in preliminary experiments on a model system with two sapphire windows (thickness: 10mm) to observe SiC semiconductor particles with diameters between 500 µm and 3 mm. A 60 kV tube (18 mA, Al-filter) was used as an X-ray source, which operated for a time period of 1.6 s. X-ray tubes with similar operating parameters are available as medical devices (e. g. dental X-ray tubes). We have already used these tubes with simple technical modifications regarding radiation protection in the laboratory.
  2. For the third year of the project, preliminary experiments for the in situ X-ray diffraction in (de-)focused Laue geometry are planned. This should already allow the measurement of the crystallization process on a sub-µm-scale. This pre-development builds the bridge to the second phase of the project.

In collaboration with sub-projects 1, 2 and 4 the crystallization rates in ammonobasic and ammonoacidic systems can be tracked as a function of the process parameters pressure, temperature and temperature difference for various mineralizers in the high pressure autoclave (sub-project 3). Initial experiments are expected in the middle of the second project year.

To establish the in situ X-ray visualization in ammonothermal high pressure autoclaves (pressure values in the kbar range), the method development by means of chemically related hydrothermal "medium pressure" autoclaves (pressure values in the 100 bar range) is planned for the first 1.5 years of the project. The scientifically well understood hydrothermal crystallization of zeolite and ZnO, which is also an established technology, acts as a model system in this context. Because of operating pressures below 30 bar and temperatures below 200 °C the crystallization parameters of zeolites are comparatively easy to realize. From an electronic point of view, ZnO is an interesting model substance. ZnO, like GaN has, a hexagonal crystal structure and it belongs to the hexagonal semiconductors with wide band gaps and high potential for optoelectronic applications. ZnO as a conductive substrate is a possibility for GaN heteroepitaxy. There is a lattice mismatch of only 1.8%, for SiC and sapphire as standard substrates the values for GaN are in the region from 3.4 % up to 13.4 %.

In some processes the hydrothermal crystallization of ZnO takes place at pressures below 100 bar. The temperature ranges from 300 ° C - 400 ° C, the temperature difference between source and crystal is approximately 10 ° C - 15 ° C. These process parameters allow autoclaves with a wall thickness in the mm range and significantly thinner windows than are used for example in the ammonthermal crystallization of GaN. There may even be the possibility of aluminum-autoclaves with an inside coating. The objective is the optimization of the X-ray contrast in the measurement and of the spatial resolution in the in situ X-ray visualization. The results should help to identify critical measurement aspects in order to reduce the development time for the definitely challenging ammonthermal system.

From the scientific point of view, the use of aluminum autoclaves enables us to view the complete system synchronously, i.e. the crystallization of the zeolite or the ZnO as well as the morphological changes of the source material. We hope to generate comprehensive crystallization data for the first time, which is of great technological importance for a scale-up of the process to obtain crystals with diameters in industrial dimensions. If we achieve quick results with ZnO synthesis, a further sub-project will be established. This group would be focused on hydrothermal crystallization with mineralizers, which enable a p-type doting of ZnO. Perhaps, this opens an additional, novel research perspective. The main focus of the project remains on the ammonothermal crystallization.

The construction of a portable X-ray in situ visualization (2D-topology) through sapphire windows in a high-pressure steel autoclave (sub-project Z / mobile optical cell) for use in different laboratories is planned. Therefore, the measurement equipment has to be implemented with a portable computer and a medical technology based X-ray tube. This enables operation in external laboratories without expensive and time-consuming X-ray certification.