Phase ID and Quantification, Lattice Parameter, Crystallinity, Crystallite Size, and Microstrain
The main use of XRD in the facility is typically phase identification. This is an incredibly important basic function of XRD because you want to ensure that the samples you study with other measurement techniques are the phase(s) that you think they are. Otherwise, you could waste a great deal of your time. Additionally, phase quantification, lattice parameter, crystallinity, crystallite size, and microstrain can all greatly affect material properties.
High-temperature XRD (HTXRD) – Kinetic
Kinetic experiments are great for investigating how systems change with temperature, time, or both. The picture to the left shows a synthesis experiment. The bottom of the pattern shows the peaks for the starting materials. As the temperature is increased, you’ll notice horizontal lines that mark regions of change. From these changes, we can determine reaction equations, reaction temperatures, how the new phases are forming, etc.
High-temperature XRD (HTXRD) – Equilibrium
Equilibrium experiments are great for getting more detailed information on phases changes. Temperature step size and scan time are much larger/longer than in a kinetic experiment so that the data are good enough to perform Rietveld refinement. The example in the picture shows a part of an equilibrium experiment that showcases anisotropic thermal expansion. The (002) and (200) peaks move to the left with increasing temperature much more than the (020) peak. Among other properties, lattice parameters for each temperature and, subsequently, thermal expansion coefficients can be determined through Rietveld refinement.
Grazing Incidence (GIXRD)
GIXRD is primarily used for thin films or ion-irradiated samples. Depending on your phase, conventional XRD can penetrate on the order of microns into your sample. If you want to probe a shallower depth without getting results from what lies beneath (e.g., only want diffraction from a thin film while avoiding peaks from the substrate), you will need to use GIXRD. If you know the phase(s) contained within the surface and the depth that you want to investigate, software in the lab can calculate what grazing angle you should use.
Texture refers to the situation in which crystallites (and subsequently the unit cells) tend to align themselves preferentially in certain directions within the sample. The example to the left is a pole figure that corresponds to a single diffraction peak in a cold-rolled piece of metal (note that pole figures are not limited to cold-rolled materials). The act of cold rolling the metal causes the crystallites within the sample to rotate in particular ways in response to the applied stress. For this reason, the pole figure shows regions of different intensity.
Residual Stress (Video Coming Soon)
Residual stress is the stress that is left behind when the forces that caused the stress have been removed. Residual stresses can be intentionally added to a material in order to improve performance, or they can develop with disastrous consequences. In residual stress measurements, we actually measure strain and convert to stress using the elastic constants and Poisson’s ratio. We determine the strain by measuring the location of a peak or peaks at different sample orientations. The top panel of the picture to the left is an example of a peak measured at a particular sample orientation, and the bottom panel shows the linear fit of multiple sample orientations that results in a compressive stress of ~2.3 GPa. Shear stresses can also be measured if present in the sample, but additional steps must be taken in the measurement process.
Micro-diffraction and Sample Mapping (Video Coming Soon)
Our combination of a focusing mirror, microbeam masks, and microbeam collimators allows us to achieve beam sizes as small as 0.46 mm x 0.135 mm at an incident angle of 5 degrees. As the incident angle increases, that 0.46 mm number decreases (e.g., the beam size is 0.15 mm x 0.135 mm at 15 degrees). Combined with our motorized XYZ sample stage and a batch program (with which you can tell the system each sample position you want to test), these microbeam optics allow for automated sample mapping.
While X-ray fluorescence as a technique is coming soon to the Diffraction Facility, this section deals with the problems caused by fluorescence. When using a copper radiation source to study samples with large amounts of iron and/or cobalt, the resulting diffraction pattern is commonly plagued by high background levels and weak peaks (as shown by the blue pattern in Figure 1). Thankfully, utilization of cobalt radiation can greatly improve the pattern (see the red pattern in Figure 1). Figure 2 shows how strongly various elements fluoresce under copper, cobalt, and chromium radiation. The Diffraction Facility has both copper and cobalt X-ray tubes.