Few techniques have contributed as much to the understanding of matter as X-Ray diffraction (XRD). XRD is a constantly-evolving analytical technique that can provide fundamental information about the structure of matter, and also provide direct support in resolving many industrial problems. X-ray diffraction has given us knowledge of the internal structure of all compounds and materials, including those in common use, such as steel and ceramics, and of the distinction between polymorphs, i.e. substances with the same ‘chemistry’ but a different crystal habit. For example, graphite and diamond, as well as other forms of carbon recently identified, such as graphene and nanotubes; or austenite and ferrite in steels; but also glass and quatz, which are respectively the amorphous and crystalline forms of silicon dioxide. XRD has also contributed fundamentally in the field of biology, for example in the discovery of the double helix structure of DNA, and it is now the standard technique used to understand and develop new pharmaceutical compounds, and in the study of proteins and viruses.
In parallel to this fundamental contribution on the structure of materials, X-ray diffraction is used in a variety of industrial technologies and activities. Through XRD it is possible to identify and quantify the presence of crystalline phases, the ‘components’ of various materials, or to know the form and dimension of the crystals that they consist of, as well as the type and density of defects, such as the density of dislocations caused by the plastic deformation of a metal. In the area of technology, XRD can be used to measure residual stresses in mechanical components or in electronics, and the orientation or arrangement of the crystals in coatings and thin films, fibres or various types of surface layers.
XRD is available at laboratory scale, but also at the big synchrotron facilities, for cutting edge research on materials, on the chemistry and physics of matter. XRD is used increasingly in industry, and it is estimated that over 2/3 of XRD machines are in company research and development departments.
In our laboratory we have the equipment and skills to do a variety of types of XRD measurements. The X-ray diffraction laboratory (XRDlab) has been operating for over thirty years at the University of Trento in the Department of Civil, Environmental and Mechanical Engineering, and deals with various aspects of scientific and technological research on materials for engineering and for solid state physics, with a particular emphasis on nanotechnologies and materials for energy applications. In the following, we give some examples of the use of XRD at the XRDlab.
Analysis of the residual austenite in steel components is essential for the manufacture and quality control of steel products, for example in gears that are ‘carburized’, where carbon is diffused into the surface layer, which adds hardness and mechanical resistance, but which in some cases, due to an excess of this iron polymorph, can lead to fracture. Using XRD with a method developed at XRDlab, we can routinely identify and quantify the presence of austenite through the carburized layers up to a depth of several millimetres.
This analysis is often done together with an analysis of residual stresses, the system of self-balancing forces that remains after a mechanical or thermal treatment. These forces can cause failure, for example due to cyclic mechanical fatigue, as happens from shear forces induced by cutting and/or welding of steel components, but they can also have beneficial effects, such as the compression forces induced by peening. The XRDlab also has shot-peening apparatus, which is used in research and the production of industrial test samples.
XRD is particularly suitable for studying powders and loose materials, such as cements, various metallic and ceramic powders, and pharmaceuticals. For example it is possible to determine the density of defects and the size of the crystallites (crystalline domains at the nanometric scale) in powders obtained through high energy milling. This has been done for powders of steel and of fluorite, which have been the subject of a number of studies and simulations regarding both the milling process and the microstructure produced. Using this technique we determine the type and number of defects, and the size of the crystallites obtained through the milling. Recently we have applied milling and characterisation using XRD to strategic pharmaceuticals that are poorly soluble, such as Efavirenz, which is used in treatment against HIV. Using this and other techniques we were able to demonstrate the mechanical activation of the drug, which allowed us to alter (accelerate or slow) the dissolution kinetics of the biological medium, as well as to increase the efficacy of the active ingredient.
XRD is used in the development of materials for energy applications. Some of the many examples are ceramic absorbers in thin film photovoltaic cells, membranes and solid electrolytes for fuel cells, special cements for building and for thermal energy storage, and the most modern nanostructured metallic catalysts. In this area XRD techniques are combined with those of modelling at the atomic scale, a constantly evolving area, such as Molecular Dynamics; for example, to follow the kinetics of the uptake and release of hydrogen in palladium, or the catalytic action of palladium nanocrystals.
XRDlab has numerous collaborations with industry and with public and private institutions, supported by a network of contacts and international projects. Studies conducted in the lab have been published in high impact international journals. A wide variety of topics have been studied (detailed in the bibliography). In addition to those mentioned above – the mechanical activation of drugs through high energy milling, materials for energy applications, the structure ad microstructure of mechanical components and atomistic modelling – there are studies on crystallographic methodologies and the uses of X-rays, for which the XRDlab is well-known internationally.
