XRD (X-ray diffraction) is an X-ray based method to characterize primarily crystalline material. This method is already established for more than 50 years in R&D covering a wide spectrum of applications, ranging from the simple determination of atomic layer distances, determination of crystal structures, phase determination of polycrystalline materials to full phase quantification of materials. The introduction of fast detection technologies created the possibility to use full XRD, besides single peak quantifications like the classical free lime application, in an industrial production environment. In parallel the demand for increasing knowledge about composition of materials in industrial processes did grow, where XRD could really contribute in a better understanding of the process and the materials. The additional gained knowledge results in process improvements and in advanced steps in quality control.
Requirements for Sample Preparation
The requirements for sample preparation depends on the available XRD analyzer and the applied applications.
A good portion of the industrial used XRD applications require samples with small particle sizes to achieve a representative sample with statistical representation of the various mineralogical phases. Where the crystallite size is ideally between a couple of micron to a maximum of 100 micron.
But it is essential to the outcome of the analysis and the subsequent data application to take care that during the process of reducing the particle/crystallite size, predominantly grinding, no changes to the material characteristics will be applied. This is not always very easy to achieve, because many materials show phase transformations during the grinding process. The phase transformations are initiated by changes in the temperature and by the applied pressure on the individual grains.
The next step after grinding is to produce a sample that can be handled and measured by the analyzer. It strongly depends on the application how such a sample will look like. Pressed pellets in steel rings did form the standard over the past decades for industrial and academic polycrystalline powder samples. The reason for that is mainly the capability of most X-ray diffractometers to process such samples and the ease of use of these steel rings. The fine powder is mixed with a binder and pressed into the steel ring to get a mechanical stable sample.
In particular for quality and process control applications are the desired attributes high repeatability, high accuracy and the independence of individual operators. This could be easily realized by the use of (semi-)automated equipment. In the majority of these cases could the sample preparation equipment be shared for both XRF (X-ray fluorescence) and XRD. Sometimes it is even possible to use one sample for the two complementary analytical techniques.