Handbook of Modern Coating Technologies
X-ray diffraction
- Introduction
The wavelength of radiation emitted by an object depends on its temperature. Hotter objects emit radiation of shorter wavelengths and so, very hot sources, such as Sun's corona emit X-rays. Other interesting sources of X-rays in the Universe include pulsars and black-holes, while the credit for their laboratory production and detection goes to the German physicist Wilhelm Conrad Rontgen, who discovered X-rays on November 8, 1895, and was subsequently awarded the very first Nobel prize in Physics for this discovery [1]. It should however be noted that prior to their discovery and systematic study by Rontgen, many scientists working on partially evacuated discharge tubes had unknowingly produced X-rays and even found their evidence on photographic films. X-rays are high energy electromagnetic radiation, whose wavelength ranges from 10 nm for soft X-rays down to
- 01 nm on the other extreme of hard X-rays.
Owing to their ability to pass through various objects otherwise opaque to visible radiation, namely, human body, wood, or even metals up to a certain thickness, X-rays could be used for radiography—a variant of photography based on transmitted radiation. As a result, they soon became a favorite diagnostic tool among physicians and engineers, even though their exact character was not known until 1912, when their diffraction by regular crystals was discovered. A new dimension to the diagnostic techniques using X-rays was added with the development of computer—assisted tomography. This innovation revolutionized the field of medical diagnostic techniques by facilitating high-contrast scanning in three dimensions. Recognizing the importance of this development, Allan Cormack and Godfrey Hounsfield were awarded the 1979 Nobel Prize in Medicine [2—6].
Although the application of conventional radiography as a diagnostic tool continues to date, the diffraction techniques are credited with many breakthroughs in the field of medicine and materials science. Study of diffraction pattern allows investigation of internal structure of materials on a much finer scale and for a much higher degree of complexity. The technique has contributed significantly to our understanding of complex molecules of life, such as proteins. On materials science front, X-ray diffraction (XRD) is an indispensable nondestructive technique for various qualitative and quantitative investigations, namely, determination of
Handbook of Modern Coating Technologies. DOI: https://doi.org/10.1016/B978-0-444-63239-5.00003-2
© 2021 Elsevier B.V. All rights reserved.
crystal structure and lattice parameters, analysis of crystal texturing, quantitative phase analysis, and estimation of residual stresses. This understanding of the application of diffraction patterns for determination of crystal structures subsequently paved the way for application of electron diffraction patterns for similar studies through transmission electron microscopy, thereby further aiding our understanding of the materials.
Much of the recent technological progress in materials science, especially that in the fields of VLSI, cutting tools, and turbine vanes, has been made possible by advances in surface engineering, which involves working with thin films, whose physical, chemical, electrical, and mechanical properties are tailored to suit the given application. As the thickness of these films usually varies from just a few nanometers to a few tens of micrometers, it is generally not possible to make a sound assessment of their properties through conventional nondestructive or destructive techniques. This technological challenge has led to the development of various X-ray measurement techniques for thin films, which can be used for determining their thickness, composition, microstructure, density, residual stress, and so on. Since the quantity of material in a thin film is inherently small, its analysis using X-ray techniques poses a technological challenge and hence, often calls for specialized equipment, attachments and procedures. As a result, while XRD techniques for bulk materials had got established quite early, the advances in the field of thin films are relatively recent and have often been the result of technological push by equipment manufacturers.
In view of the necessity to develop a preliminary understanding of X-rays and the nature of their interaction with matter, the discussion in the following subsections has been arranged in a logical sequence—with discussion on thin film analysis appearing towards the end. However, depending on one's level of understanding, an individual may skip any number of sections and proceed to the section of interest. Furthermore in-line with the title of this handbook, the chapter has been written from the perspective of materials science and so, its usefulness for individuals working on biomolecules or tissues would be rather limited.
- Application areas of various X-ray techniques
Over the course of past few decades, various X-ray techniques have found application in diverse fields. For example, X-ray fluorescence (XRF) (refer Section 3.3.3 for a brief description of fluorescence) based Karatmeters have been finding increasing application in precious metal industry, thereby allowing speedy, reliable, and precise assessment of the composition of jewelry artifacts [7—9]. The technique also finds application in assessing food safety by detecting the presence of heavy metal contaminants [10,11]. A variant of this technique, known as total reflection XRF spectrometry, facilitates detection of trace amounts of elements in dried liquid samples such as water or milk by irradiating the sample carrier (usually quartz glass) at very small angle of incidence (~0.1 degree). The small angle of incidence (with respect to surface tangent) helps improve the signal-to-noise ratio by limiting the depth of penetration of X-rays into the underlying substrate [12], while facilitating their longer interaction with the thin layer of interest (refer Section 3.10.2.3). Different types of XRD
| techniques, which we shall study in subsequent sections, find application in soil characterization [13], mineralogy [14], textiles [15,16], gemology [17—19], and so on. Since XRD techniques are capable of characterizing very small samples, they are often helpful in performing forensic analysis [20].
XRD techniques had been successfully used for inferring the structure of proteins even before the structure of DNA was determined by Watson and Crick. Since then, the technique has undergone extensive refinement and is now capable of providing much more information about the structure of proteins [21,22]. Augmented by suitable computer hardware and software, the diffraction techniques are also used for analyzing other complex structures, such as those encountered in pharmaceuticals and electronics industry [23—25]. A recently developed technique benefitting VLSI fabrication is X-ray micro-CT, which is an extension over the well-developed CT scanning routinely used in medical diagnostics [26]. The application areas of various X-ray—based radiography, XRF, and XRD techniques are summarized in Fig. 3—1. |
| XRD in forensic examination |
| XRF in elemental analysis |
| X-Ray
techniques |
| XRD in life sciences, pharmaceuticals & drug research |
| XRD in gemology |
| FIGURE 3-1 Application areas of X-ray techniques. |
| XRD in
materials science |
| XRF in food safety |
| H
XRD in mineralogy ................ & soil analysis ; |
| XRD in integrated circuits |
| XRD in textile industry |
| Radiography & CT scans in medicine & industry |
While the application areas of various X-ray techniques have been constantly expanding, we shall mostly restrict our discussion in this chapter to XRD techniques, with primary focus on materials science.
