The Power of X-Rays
X-ray fluorescence (XRF), which operates by irradiating a sample with a beam of high energy X-rays and exciting characteristic X-rays from those elements present in the sample, is a very established technique for analyzing the total elemental/oxide composition of raw materials, as well as intermediate and final products in the ceramic industry. In addition to its speed and simplicity, XRF offers a reliable method that covers a broad variety of materials and matrices. This technology—and especially wavelength dispersive XRF (WDXRF), which sorts the individual X-ray wavelengths using a system of crystals and detectors, and accumulates specific intensities for each element—has several advantages over other types of materials characterization techniques, including:
- a wide dynamic range (from traces in the parts-per-million [ppm] range to major elements up to 100%);
- excellent precision (both short- and long-term repeatability);
- excellent response to light elements, including boron, carbon and nitrogen, which are often present in engineering ceramics but can be hard to detect; and
- overall accuracy with suitable sample preparation.
Over the past decade, both technologies have been significantly improved. Today’s WDXRF instruments can now be operated at medium to low power with no external peripheral dependence, such as water cooling or gas supply. Additionally, the cost of both ownership and maintenance has been reduced, which has resulted in a significantly lower cost per analysis compared to other techniques. Advances have also been made in XRD instrumentation, such as the ability to integrate XRD and XRF technology into a single instrument. These and other advances are enabling ceramic manufacturers to fully optimize their materials characterization processes.
Instrumentation and Analytical TechniquesSince every application is different, the type of XRF and/or XRD instrument selected typically depends on the desired analytical flexibility and performance. Options include entry-level systems, calibration programs, sequential XRF, integrated XRF-XRD instruments, and stand-alone powder XRD systems.
Calibration Programs. When an accurate quantitative analysis covering a wide range of material types and concentrations is required, the appropriate sample preparation method and calibration programs must be used. The particle size and mineralogical effects must be minimized or eliminated to ensure accurate and reliable analyses. It is also important to consider the wide dynamic range of individual oxide concentrations when different types of materials are mixed within the same calibration curve. The most appropriate sample preparation method that satisfies both of these requirements is the fusion bead technique. This procedure basically consists of heating a mixture of the sample and borate flux until the flux melts; continuing the heating process until the sample dissolves into the molten flux; agitating the mixture to homogenize the melt, pouring the molten glass into a hot mold; and finally cooling to obtain a solid glass disc, ready for X-ray measurement, without any additional treatment. The fusion bead technique eliminates the effects of particle size and mineralogy and produces a homogeneous specimen for analysis.
Stand-Alone Powder XRD. The powder X-ray diffraction method is ideally suited for characterizing and identifying polycrystalline phases. The main use of this tool is to identify and possibly quantify phases or minerals in a sample. One such instrument,5 which is built around a vertical theta-theta goniometer, offers convenient geometry for handling powder samples by providing easy sample preparation, sample changer options and the use of specialized sample holders. The system is equipped with a Peltier-cooled Si(Li) solid state detector, which provides superior energy resolution compared to a scintillation detector. The high resolution allows the system to remove K-beta and fluorescence radiation from the sample and thereby eliminate the need for filters and monochromators. As a result, the diffraction peak intensities are substantially higher than for other available configurations, and excellent resolution is achieved even at very low angles.
- Peak finding and profile fitting
- Qualitative and quantitative analysis
- Percent crystallinity determination
- Crystallite size determination
- Texture and residual stress analysis
- Indexing and least squares unit cell determination