- THE MAGAZINE
Ceramics can be found in almost every aspect of our daily lives. In fact, our society has come to rely on their versatility as a material. Ceramics are sought after for their compressive strength, which makes them a good structural material, as well as their insulating properties for both heat and electricity. Other beneficial attributes include hardness, abrasion resistance and chemical resistance, especially to caustic applications. Glass-ceramics are also used to make optical equipment and fiber insulation.
Some of the oldest known ceramics, found in Asia, date back to some 24,000 years ago, making it one of the first materials processed and used by man. From those early beginnings to its uses today in fuel cell technology and the aerospace industry, we have engineered ceramics for many purposes. As the complexity of these materials evolves, techniques and technologies are needed to study, identify, and measure them.
X-ray spectrometry is a technique that lends itself well to this need. Generally speaking, this technique is nondestructive, rapid and can address some very important issues such as elemental composition, the arrangements of those elements, and quantity of elements and compounds. The need for this information starts with the very mining of the raw materials through to the finishes applied to them as final products.
The two aspects of X-ray spectroscopy with application in the ceramic industry are X-ray diffraction (XRD) and X-ray fluorescence (XRF). These technologies are well-established and used in many other industries to answer the same questions of elemental composition and how those elements are connected.
A simplified example can be found in diamond and graphite. Most people could tell you that diamonds and graphite are made of the element carbon, but clearly they are two very different compounds having very different physical properties. XRF spectroscopy can identify that both compounds are 100% carbon, but XRD is able to reveal how the carbons are connected to each other, which is what gives these two materials their very different properties and values. These techniques provide different information but can complement each other.
All ceramics have one thing in common: They did not start out in their final form. Raw materials were processed to create them. This is where we will begin investigating. If we think about some of the historic ceramics, we find that they began as clay and were processed into ceramic materials and objects. These starting materials need to be mined, identified and refined.
XRD is suited to identify the types of clays. Each type of clay has a particular elemental composition and a particular arrangement of those elements. This gives each clay type a unique diffraction pattern that we can think of as a fingerprint. XRD is used in mining industries to identify and quantify the clays contained in the mine. Samples are taken and processed by different techniques to achieve a powder sample that can be analyzed.
Figure 1 shows a diffraction pattern of a sample of bentonite, a clay consisting mostly of montmorillonite and other minerals. Bentonite has many industrial applications, including as a component in drilling mud, an additive to cement, in the making of green sand for sand casting, and as a protein absorber in wine making. Depending on the specific application, certain ratios of particular minerals are more favorable than others; it is therefore useful to be able to quantify those different minerals.
Quantification can be achieved by a few different techniques that are applied to the spectra. The first technique is empirical calibration, where a response for a measureable signal is related to a concentration of that particular analyte. As this relates to XRD, quantification of the particular minerals is performed by identifying peaks for the minerals of interest and then measuring the responses of the signal for those minerals at different concentrations. Plotting the concentration vs. response generates a calibration curve, which is then used to quantify that compound in unknown samples of similar matrices.
The other technique is referred to as refinement. This process models the identified minerals onto the spectra collected and uses ratios of the intensities of the minerals to calculate the relative concentrations of each mineral to give a semi-quantitative determination.
XRF spectroscopy has been applied as a tool for the formulations of ceramics and for the impurity testing of raw materials. One example can be found in the manufacturing of medical implants because of their sensitivity, wide elemental analysis range and simple sample preparation techniques. Systems can have elemental ranges for elements as light as beryllium and as heavy as uranium, with sensitivity to the parts per million (ppm) level. Analysis times for XRF measurements are based on the desired level of sensitivity, and can range from tens of seconds to a few hundred seconds for trace-level determinations. For the analysis of materials where light elements are of interest at lower concentrations, removal of atmosphere is required, either by vacuum system or displacement using a light element gas purge.
Figure 2 shows the spectra from the analysis of a sample of mixed oxides used in the manufacturing of a medical implant device composed mostly of zirconia oxide. The analysis was performed under a helium atmosphere for improved sensitivity of light elements. The analysis identified the expected oxides of zirconia, yttrium and hafnium; in addition, iron was identified, determined to be an impurity in the material and was quantified to a trace level of 0.16%.
Additional desirable characteristics of XRF spectroscopy include flexibility of sample forms and simplistic sample preparation that can achieve good results. A variety of sample forms can be analyzed with XRF systems, ranging from bulk items to powder samples. Smaller samples such as powders are placed or packed in sample cups or pressed into discs or pellets for analysis.
Packing and pressing help to reduce scattered X-rays by increasing the density of the sample, which improves sensitivity. Figure 3 shows the preparation of a powdered sample hand-pressed into a sample cup.
X-ray spectroscopy can also measure the finishes applied to products for performance purposes or safety reasons, such as metal leaching, particularly as it relates to heavy metals. XRF is well-suited for this analysis because finished products can be measured with little to no sample preparation. Figure 4 shows a drinking mug placed in the sample compartment for analysis. Some XRF systems can accommodate fairly large samples, which increases their flexibility by
enabling a variety of samples to be analyzed.
The XRF analysis of the mug identified that the mug contained lead at a level of 2,352 ppm. Lead is a metal that can be leached out of materials with water, and elevating the temperature of the water or reducing its pH increases the rate of leachability. Measurements were conducted on the finished and non-finished surfaces of the mug, which determined that lead was contained in the mug’s glaze. This is of additional concern, as the likelihood of exposure is increased when the lead is contained in a contact surface.
XRF can also be used in developing finishes and monitoring the manufacturing and application of these finishes, which can impart specific attributes to products. Desired functions can be imparted by adding specific components to the ceramic products’ finishes.
For example, materials can be added to glazes to make their surfaces anti-microbial. These materials contain elements that can be measured by XRF. X-ray spectroscopy can also be used to ensure that appropriate levels of microbial-inhibiting materials are added to the glazes, and that the application of the glazes to the products will achieve the desired effect.
A Complete Picture
X-ray spectroscopy is used in the ceramic industry in many different ways, from identifying raw materials to quantifying minerals and contaminants in those materials, as well as in the finishes that are applied to the finished ceramic products. These techniques are rapid and nondestructive, and provide information about elemental composition and the arrangements of those elements. XRF and XRD answer different questions, but the two techniques can be used together to provide a more complete picture of the materials used in the ceramic industry.
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