Quality Control

From laboratory sample to pellet.

The term ceramics was originally used for pottery and earthenware collectively. Today, it comprises a much wider range of materials, including metallic oxides, nitrides and carbides. These materials are used in application areas ranging from household items (porcelain, sanitaryware, artware) to high-performance tools for industrial use (ball bearings, cutting tools, isolators, catalysts). In addition to their great hardness, ceramics are also resistant to thermal and chemical influences, making them highly suitable for applications where the product is subjected to high mechanical or thermal stress.

Another important factor is the purity of the material, as even slight impurities can lead to rejects during the manufacturing process. Such impurities may not only influence the physical and chemical properties of the product but can also prove to be harmful to the health of the user. Therefore, quality control with regard to the composition of the ceramic material is a challenge for each manufacturer. It is necessary to prepare ceramic materials quickly and reproducibly in order to carry out representative and reliable quality checks.

Analysis Requirements

The successful use of spectrometers in the analytical laboratory requires an understanding of the method used and a great deal of practical experience, and routine tasks often do not leave users with enough time to develop and optimize the methods involved. Some analysis technologies require extensive external training for the laboratory analysts, which delays the implementation and acceptance of the method by the laboratory staff.

X-ray fluorescent analysis (XRF) is an exception because the sample is analyzed in solid form and the measurements are easy to carry out. XRF is therefore well established in areas where quick results are essential, such as for quality checks during production. Since XRF measurements are so simple to carry out, the importance of reliable sample preparation is often neglected. This can lead to poor reproducibility and even incorrect analysis results.

For XRF analysis, the laboratory sample of a few grams often has to represent a total amount that could be several tons. In addition to the quality of the spectro-meter, the quality of the sample preparation has a decisive influence on the precision and reproducibility of the analysis results. It is important to consider which mill and size reduction principle is suitable for a particular material.

Figure 1. Saturation depth. Only a part of the fluorescent light leaves the sample.

Saturation Depth

The deeper the X-ray enters the sample, the more it interacts with the sample’s atoms. An increasing portion of the X-ray is absorbed by the sample until a specific depth is reached beyond which the X-ray light can no longer penetrate. This also applies to the fluorescent light that must exit the sample in order to be detected.

The lowest detectable sample layer is called the saturation depth (see Figure 1). The saturation depth depends on the intensity of the X-rays, the wavelength (i.e., the type of detected atom) and the density of the sample’s surroundings (the matrix). If different elements are analyzed in the same surroundings, the saturation depth increases according to the atomic number of the element in question. Table 1 illustrates this correlation for porcelain.

Table 1. X-ray saturation depth of different elements in a porcelain sample.

Saturation depth generally decreases with the atomic number, which means that the element becomes more difficult to detect and explains why elements such as carbon and boron emit very weak fluorescent signals. Changing the matrix, by analyzing iron in zirconium oxide or tungsten carbide instead of porcelain, for example, has a great influence on the saturation depth. Heavy elements in the matrix decrease the saturation depth considerably, making a correct analysis much more difficult.

Figure 2. Vibratory disc mill. Inside the grinding jar, the grinding tools (usually a puck and a ring) are moved in such a way that the sample is crushed by impact and friction effects.

Sample Preparation

When preparing samples for XRF analysis, care should be taken to ensure that the size of the particles to be examined lies within the saturation depth of the X-rays in order to obtain a representative analysis result. For a porcelain sample, for example, a fineness of 80 microns is only necessary if elements lighter than potassium have to be analyzed. Otherwise, a grind size of 100 microns, which can be quickly and easily obtained with any suitable laboratory mill, is sufficient.

Sample materials frequently come in large amounts with large feed sizes, making preliminary size reduction necessary. Jaw crushers are very suitable as a preliminary grinder for ceramic materials; they crush the material through pressure and friction between two breaking jaws, one moving and one stationary.

After preliminary size reduction, a part of the sample is subjected to fine grinding. This part of the sample must have the same properties as the original bulk material in order to obtain reliable information about the composition of the total sample. The selection of the sample division method and instrument depends on the material and the amount. Dry, pourable bulk samples can be fed to rotating dividers via vibratory feeders, whereas sample splitters are suitable for sticky and non-flowing materials.

The part sample is then subjected to pulverization. It is important to use a suitable mill and grinding tools that will not alter the material properties to be determined in any way during the sample preparation process. A thorough knowledge of the instruments is required, as is some experience in the preparation of different materials. Finally, care should be taken to ensure that possible abrasion from the grinding tools does not interfere with the analysis results.

The most frequently used mill for the size reduction of hard and brittle sample materials for subsequent XRF analysis is the vibratory disc mill (see Figure 2). Inside the grinding jar, the grinding tools (usually a puck and a ring) are moved in such a way that the sample is crushed by impact and friction effects. The required reproducible analytical fineness can be achieved after very short grinding times. The quick turnaround provides a decisive advantage when analysis results are needed quickly, such as for a product approval.

Grinding aids can be used during this stage, particularly if the material has a tendency to cake during grinding. One grinding aid that seems to have received wide acceptance is Vertrel©XF, a DuPont product that helps prevent caking and conveniently evaporates after grinding.

Small sample volumes can also be processed in a mixer mill. Here, the grinding jars perform radial oscillations in a horizontal direction. The inertia of the grinding balls causes them to impact with high energy on the sample material at the rounded ends of the grinding jars.

Pellet Production

For most XRF applications, pellets with a plane surface are used. In contrast to loose powder, a pellet is advantageous in that the element concentration detected by the X-ray is higher because the material is more compact. In addition, a smooth surface is preferable to a rough one from an optical point of view.

Pellets are usually produced either through fusion of the sample with salt or by pressing the sample into a pellet. Fusion of the sample with lithium tetra borate is a very effective method of producing a bead. The sample is weighed together with the flux in a platinum crucible, and then the crucible is heated in a fusion machine to more than 1000°C. This process, which destroys the original matrix and creates a homogeneous borate glass, yields highly reproducible results regardless of the original material.

Fusion has a few disadvantages, however. Volatile elements like thallium or cadmium tend to escape during the fusion process and cannot be detected. Moreover, the sample is heavily diluted with lithium salt (factor 10-50), which impairs the detection limit when compared to pellets. Certain elements (e.g., boron, iron, carbides) could even damage the very expensive platinum crucible. Finally, it takes much more time to produce a bead than a pellet (15 minutes compared to approximately two minutes).

Pellet pressing is the most common procedure for many applications, even though calibration of the spectrometer can be more involved due to the sample matrix. A pressed pellet should be homogeneous; absolutely solid, since loose particles pollute the X-ray tube; stable; and storable.

The pressing of a sample can be carried out with or without additives. Pressing without additives (free pressing) is not very common because the pellets are usually not sufficiently stable. The most frequently used materials are cellulose- or paraffin-based. Cellulose has the advantage of also acting as a grinding aid, which helps avoid the caking of the sample inside the grinding jar. Cellulose can be used in vibratory disc mills as well as mixer mills.

Wax is added after the sample has been ground, either manually or by mixing it with the help of polyamide balls in a plastic jar in the mixer mill. The addition of wax makes the pellet’s surface indelible. Moreover, wax is more inexpensive than cellulose and is not hygroscopic, which is important if the pellets are to be stored. Either steel rings or aluminum cups are used to stabilize the pellets. The cups can be labeled on the reverse side and are useful for storing the pellets.

Successful Analysis

The size reduction techniques described here are an essential precondition to producing representative samples for XRF analysis. Close attention should be paid to the particle size of the ground sample, since particles that are too coarse tend to impair the reproducibility of the analysis. However, it is not recommended that the material be ground to sizes finer than necessary, as this practice results in needless time and effort for sample preparation.

Jaw crushers are widely used for the preliminary size reduction of ceramic samples. A representative part sample is obtained from this first step with the help of a sample divider and is then subjected to fine grinding in a vibratory disc mill or mixer mill. The finely ground sample can then be pressed to pellet form and analyzed by XRF.

For additional information regarding the preparation of samples for XRF analysis, contact Retsch, Inc., 74 Walker Lane, Newtown, PA 18940; (866) 473-8724; fax (267) 757-0358; e-mail patricia.jung@retsch-us.com; or visit www.retsch-us.com.


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