X-ray fluorescence and X-ray diffraction can be used to provide rapid, accurate materials characterization, leading to higher quality finished products.
Entry-level XRF instruments, like the ARL Optim'X shown above, can be used to analyze the major and minor elements/oxides in various materials related to traditional ceramics.
The quality of raw materials in the ceramic industry is judged by the level of impurities the materials contain. Purifying raw materials is an expensive process, and it can be even more costly to miss impurities, since they often lead to defects in the finished product. For these reasons, it is important to be able to determine these elements or oxides with a high level of accuracy and precision.
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.
X-ray diffraction (XRD) uses X-rays to identify the unique diffraction pattern (or “fingerprint”) of a given material. Unlike XRF, which is limited to measurements relating to a material’s chemical composition, XRD can analyze compounds and/or phases. For this reason, it is often used in conjunction with XRF to obtain a more complete characterization of materials. In some specific areas of ceramic processing, where it is necessary to obtain the correct phase to ensure the corresponding physio-chemical properties, XRD can be a powerful tool.
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 Techniques
Since 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. Entry-Level XRF.
Entry-level XRF instruments1
are designed for dedicated analysis of major and minor elements/oxides in various materials related to traditional ceramics. These instruments, which are particularly useful in small to medium-scale operations, generally do not require water cooling (external or internal) and can be operated without an external detector gas supply, depending on the configuration. Typical results of an entry-level XRF obtained using a series of limestone standards are shown in Tables 1 and 2. The samples were analyzed as pressed pellets.
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.
The XRF intensity from the analyte (a given element in the specimen) is influenced by the other elements in the matrix either by absorption or enhancement. These inter-element effects can be calculated using fundamental parameters and can be used as correction factors to obtain the true intensity-concentration relationship (calibration).
Table 3 shows different ceramic materials that can be analyzed using a calibration program that follows these principles,2
while Table 4 summarizes the typical accuracies obtained on various oxides and their corresponding ranges.
Figure 1. A high-powered XRF instrument equipped with a universal goniometer, such as the ARL Advant'XP shown above, can be used in a range of applications.
In addition to analyzing the oxides present in conventional ceramics, XRF instruments can also be used in high-tech applications to characterize materials such as carbides and nitrides. In these applications, the system must be configured with suitable dispersive devices (typically a crystal-collimator-detector combination) and the correct X-ray power to obtain the best possible analysis. Figure 1 shows a list of materials and applications that can be analyzed using a high-power XRF instrument with a universal goniometer.3
Certified reference materials are available for most common oxide-based materials listed in Table 3, and these reference materials can be used to set up the quantitative analysis program. When a sample is totally unknown (neither the elements nor their concentration range) or when a sample is non-routine (it does not fit into any calibration range or has elements that are not calibrated) and perhaps irregular, “standard-less” analysis programs such as QuantAS and UniQuant™, can be used to obtain the sample composition.
Integrated XRF-XRD Instruments.
Figure 2. Integrated XRF and XRD instruments can enable the user to more easily identify both phases and constituent elements.
Integrated XRF-XRD instruments can be very useful in identifying and quantifying the elements/oxides and the corresponding mineral phases within the same sample using one analytical program. One such instrument3
is capable of performing both chemical analysis for total elemental/oxide composition and specific phase analysis (see Figure 2).
Figure 3. Example of differentiation of phases in a limestone sample using an integrated XRF-XRD instrument (the ARL 9800).
In ceramic manufacturing, it is often necessary to be able to monitor the sintering process both by controlling the total oxide content and by checking the completion of specific phases or structures. For example, when alumina and magnesia are sintered to form spinel, the residual oxides should ideally be measured and quantified to control the process. This level of analysis can be accomplished using one commercially available integrated XRF-XRD instrument.4
Figure 3 shows the example of the calcination of limestone as monitored by an integrated XRF-XRD instrument. In addition to quantifying the total calcium oxide content by XRF, it is possible to clearly differentiate the related phases of calcium, such as CaCO3, CaO, Ca(OH)2
, etc., using XRD within the same instrument and in the same sample.
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.
Figure 4. An XRD scan obtained on a clay mineral sample using the ARL X’TRA. Three scans with different scan speeds are shown to demonstrate the high sensitivity and excellent peak to background ratio obtained even at high data acquisition speeds.
Figure 4 shows an example of an XRD scan obtained on a mineral sample at three different scan speeds. Clear diffraction peaks can be seen even with a one-minute scan time, providing a basis for rapid screening of materials for their phase content. To obtain a quantitative phase analysis, Rietveld-based programs, such as SiroQuant™, can be used. Various data processing routines can also be used to obtain the following information of the ceramic sample subject to different physical conditions:
- 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
Rapid, Accurate Analyses
Thanks to a new generation of instruments that facilitate the analysis of elements/oxides and provide additional phase analyses, today’s ceramic manufacturers can benefit from the advantages of advanced XRF and XRD techniques. Basic oxide analysis in raw materials or the final products can be done with entry-level XRF instruments, while more comprehensive information on the sample can be obtained using integrated X-ray instruments. Using a high-power sequential XRF in conjunction with a stand-alone powder diffractometer can enable more investigative work related to both conventional and advanced ceramics. Additionally, higher sensitivities and “standard-less” analysis programs in both XRF and XRD areas can be effectively combined to perform rapid materials characterization.
All instruments referenced in this article are supplied by Thermo ARL.
For More Information
For more information about XRF and XRD instruments, contact Thermo ARL, ARL Applied Research Laboratories S.A., En Vallaire Ouest C, Case Postale, Ecublens CH-1024, Switzerland; (41) 21-694-71-11; fax (41) 21-694-71-12; e-mail Ravi.Yellepeddi@thermoarl.com
; or visit http://www.thermo.com/eThermo/CDA/BU_Home/BU_Homepage/0,1285,121,00.html