Ray Powder Diffraction

X-ray powder diffraction (XRPD) techniques are used to characterize samples in the form of loose powders or aggregates of finely divided material. These techniques cover various investigations, including qualitative and quantitative phase identification and analysis, determination of crystallinity, microidentification, lattice-parameter determinations, high-temperature studies, thin film characterization, and, in some cases, crystal structure analysis. The powder method, as it is referred to, is perhaps best known for its use as a phase characterization tool partly because it can routinely differentiate between phases having the same chemical composition but different crystal structures (polymorphs). Although chemical analysis can indicate that the empirical formula for a given sample is FeTiO3, it cannot determine whether the sample is a mixture of two phases (FeO and one of the three polymorphic forms of TiO2) or whether the sample is the single-phase mineral FeTiO3 or ilmenite. The ability of XRPD to perform such identifications more simply, conveniently, and routinely than any other analytical method explains its importance in many industrial applications as well as its wide availability and prevalence.

In general, an x-ray powder diffraction characterization of a substance consists of placing a powder sample in a collimated monochromatic beam of x-radiation. The diffraction pattern is recorded on film or using detector techniques, then analyzed to provide x-ray powder data that can be used to solve such problems as those listed in Table 6.

In XRPD analysis, samples usually exist as finely divided powder (usually less than 44 /'m in size) or can be reduced to powder form. The particles in a sample comprise one or more independently diffracting regions that coherently diffract the x-ray beam. These small crystalline regions are termed crystallites. Consolidated samples, such as ceramic bodies or as-received metal samples. will likely have crystallites small enough to be useful for powder diffraction analysis, although they can appear to have considerably larger particle sizes. This occurs because a given grain or particle can consist of several crystallites (independently diffracting regions). Although larger grain sizes can sometimes be used to advantage in XRPD, the size limitation is important because most applications of powder diffraction rely on x-ray signals from a statistical sample of crystallites. The angular position, 0, of the diffracted x-ray beam depends on the spacings, d, between planes of atoms in a crystalline phase and on the x-ray wavelength A:

The intensity of the diffracted beam depends on the arrangement of the atoms on these planes.

X-ray powder diffraction techniques usually require some sample preparation. This can involve crushing the sample to fit inside a glass capillary tube, rolling it into a very thin rod shape for the Debye-Scherrer camera technique, spreading it as a thin layer on a sample holder, or packing it into a sample holder of a certain size for other XRPD techniques. In some cases, samples compatible with metallographic examination can be accommodated in powder diffractometers, but some form of sample preparation will usually be necessary. Preparation will depend on the equipment available and the nature of the examination.

Source: Materials

A diffraction pattern can be recorded using film, analog, or digital methods. Whether film, analog, or digital data collection is used, the final data can be displayed as a graph of intensity, as a function of interplanar distance d, or as a function of diffraction angle 20. Many modem automated powder diffractometers can provide further data reduction, including peak finding. a tabular listing of peak intensity versus interplanar spacing, search/match software, and other computer utilities.

A powder pattern from a single-phase material will have maxima at positions dictated by the size and shape of its unit cell and will increase in complexity as the symmetry of the material decreases. For example, many metal patterns and those from simple compounds that tend to be mostly of cubic symmetry and have small unit cell edges will produce powder patterns having fewer lines or maxima than would be expected from a compound of lower symmetry or one having a very large unit cell. A pattern of a mixture of phases in which all the individual patterns are superimposed will produce a complex experimental pattern, especially when the number of phases present in the mixture exceeds approximately three or when the phases constituting the mixture are all of very low symmetry or have very large unit cell dimensions.

Phase identification using XRPD is based on the unique pattern produced by every crystalline phase. Much as a fingerprint is unique for each person, the diffraction pattern can act as an empirical fingerprint for that phase, and qualitative identification of phases can be accomplished by pattern-recognition methods that include established manual techniques and the newer methods that use computers. most of which implement programs based on heuristic algorithms. All of these methods make use of the database maintained by the JCPDS International Centre for Diffraction Data (Ref

Inert gas fusion is used to determine the quantitative content of gases in ferrous and nonferrous materials. These gases, such as hydrogen, nitrogen, and oxygen, are introduced to a material by physical and chemical adsorption. Surface metals deposited on base materials physically adsorb atmospheric gases to form metal-bearing compounds. The type of metal atom available and its compatibility with the types and concentrations of atmospheric gases available under the temperatures and pressures to which the material is exposed determines the types of metallic compounds formed. Although the bond at low temperatures is unstable, the metallic compounds formed become more stable as temperature increases and the physical adsorption bond becomes a chemical adsorption bond.

As base materials are smelted, the adsorbed gases are chemically absorbed into the melt. The less stable compounds are reduced in the melt of the material, allowing each to recombine and form a more complex stable compound. The metallic elements will absorb atmospheric gases based on the affinity of the metal for each gas at various temperatures during the melt.

Processed materials are again subject to gas adsorption as the material is initially cooled and worked. Stages of drawing, rolling, heat treating, or annealing provide favorable conditions for physical and chemical adsorption of atmospheric gases. Strict control of hydrogen, nitrogen, and oxygen levels minimizes their adverse effects on material strength.

Hydrogen causes internal cracks that generally appear during cooling in such processes as drawing, rolling, or forging of the material. Materials with large cross sections can break under high or continuous stress due to internal cracks. Hydrogen is adsorbed and diffused often during working of the material. Hydrogen that is adsorbed will generally diffuse during cooling or aging. Materials used under high temperatures and pressures and exposed to high-hydrogen environments can develop structural problems due to hydrogen embrittlement.

Nitrogen in some materials can provide strength. For example, nitrogen is added to austenitic manganese steels to increase yield strength. A material can be subjected to nitriding to increase hardenability. Nitrogen may decrease ductility.

The effects of oxygen on a material are similar to those of hydrogen in that inclusions and blowholes can appear in the material. Oxygen, when combined with the carbon and nitrogen in a material, will also cause increased hardness with age. Oxygen is often the hardest to control because it is available from various sources and reactive with many metals. Smelting and processing regulate the levels of gases introduced into materials by controlling temperature, pressure, and environment, which create adsorption and absorption of gases.

Gases introduced into the material are commonly quantitatively determined using inert gas fusion. Inert gas fusion reverses the physical and chemical bonding between the gases and the metals to dissociate the gases and sweep them from the fusion area with an inert carrier gas. Resistance or induction heating of a sample in a pure graphite crucible dissociates the gas/metal bonds. Because they are formed over a wide range of temperatures, bonds can be broken only by heating the specimen above the highest temperature at which the gas/metal bonding occurred. More information on these operations can be found in "Inert Gas Fusion" in Materials Characterization, Volume 10, ASM Handbook.

0 0

Post a comment