In 1912, Laue and others confirmed thatCrystalline materialThe atoms between tens and hundreds of picometers (pm) are periodically arranged;This periodically arranged atomic structure can become the "diffraction grating" of X-ray diffraction;X-rays have wave characteristics, are electromagnetic waves with a wavelength of tens to hundreds of picometers, and have the ability to diffract.[1]This experiment became the first milestone of X-ray diffraction.When a beam of monochromatic X-ray is incident on the crystalatomRegularUnit cellThe distance between these regularly arranged atoms and the wavelength of incident X-ray areX-ray diffraction analysisThe same order of magnitude, so the X-rays scattered by different atoms interfere with each other and produce strong X-ray diffraction in some special directionscrystal structureThe diffraction pattern produced by each crystal reflects the atomic distribution law within the crystal.This is the basic principle of X-ray diffraction.
Although neutron diffraction, electron diffraction, infrared spectroscopy, Mossbauer spectroscopy and other methods can be used for the analysis of material structure, X-ray diffraction is the most effective and widely used means, and X-ray diffraction is the first method used by humans to study the microstructure of materials.The application range of X-ray diffraction is very wide, and it has penetrated into various engineering technology sciences such as physics, chemistry, earth science, material science, etc. It has become an important experimental method and structural analysis means, with the advantages of non-destructive samples.[2]
X-ray is a kind of short wavelength (about 20~0.06 angstrom)electromagnetic waveIt can penetrate a certain thickness of material, and can make fluorescent material glow, photographic emulsion photosensitive, and gas ionization.Bombarding metals with high-energy electron beams“target”The material produces X-ray, which has a specific wavelength corresponding to the elements in the target, called characteristic (or identification) X-ray.in consideration ofX-rayWavelength and crystal interioratomThe distance between faces is similar, German physicist in 1912Laue(M. von Laue) proposed an important scientific prediction: crystal can be used as the spatial diffraction of X-raygratingThat is, when a beam of X-ray passes through the crystal, it will be diffracted. The superposition of diffracted waves makes the intensity of the ray strengthen in some directions and weaken in other directions.Analyze the diffraction pattern obtained on the photographic negative to determinecrystal structure。This prediction was immediately verified by experiments.
principle
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When a beam of monochromatic X-rays incident on a crystal, because the crystal is composed of cells regularly arranged by atoms, the distance between these regularly arranged atoms is the same order of magnitude as the wavelength of the incident X-rays, so the X-rays scattered by different atoms interfere with each other, producing strong X-ray diffraction in some special directions, and the orientation and intensity of the diffraction rays in space distribution,It is closely related to the crystal structure.This is the basic principle of X-ray diffraction.
Bragg equation
In 1913, the British physicist W.H. Bragg and his son (W.L. Bragg)LaueBased on the discovery, we not only successfully measured thecrystal structure, and proposedCrystal diffractionThe famous formula of foundation -Bragg equation：2dsinθ=nλ
Where d is the crystal plane spacing;N is the reflection series;θ is the grazing angle;λ is the wavelength of X-ray.The Bragg equation isX-ray diffraction analysisThe fundamental basis of.[3]
Kinematic diffraction theory
Darwin's theory is called X-ray diffraction kinematics theory.The theory treats the diffraction phenomenon as a three-dimensional Frannhofer diffraction problem, and considers that the scattering of each volume element of the crystal is independent of the scattering of other volume elements, and the scattered rays will not be scattered when passing through the crystal.Although the diffraction direction and intensity can be obtained by such treatment, the fundamental assumption of the kinematic theory is not completely reasonable.Because the scattered lines must be scattered again in the crystal, in addition to combining with the original rays, the scattered rays can also combine with each other.Darwin soon realized this and made multiple scattering corrections in his theory.
Dynamic diffraction theory
Ewald's theory is called kinetic theory.The theory takes into account the interaction of all waves in the crystal, and considers that the incoming rays and the diffracted rays coherently combine in the crystal, and can exchange energy back and forth.The strength formula obtained by the two theories is the same for fine crystal powder, but for large and complete crystal, the correct result must be obtained by using the kinetic theory.
application
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X-ray diffraction technology has become the most basic and important means of structural testing. Its main applications are as follows:
Phase analysis
Phase analysis is the most widely used aspect of X-ray diffraction in metals, which can be divided into qualitative analysis and quantitative analysis.The former measures thelatticeThe plane spacing and diffraction intensity are compared with the diffraction data of the standard phase to determine the phase in the material;The latter determines the content of each phase in the material according to the intensity of the diffraction pattern.It has been widely used in studying the relationship between the properties and the content of each phase, and checking the composition ratio of materials and whether the subsequent treatment procedures are reasonable.
Determination of crystallinity
The crystallinity is defined as the percentage of the ratio of the weight of the crystalline part to the total sample weight.Amorphous alloys are widely used, such asSoft magnetic materialThe crystallinity directly affects the performance of materials, so the measurement of crystallinity is particularly important.There are many methods to determine the crystallinity, but either method is determined by the diffraction pattern area of crystalline phase and that of amorphous phase.
Precise measurement of lattice parameters
Precision measurementlatticeParameters are commonly used inphase diagramSolid state ofSolubility curveDetermination of.The change of solubility often causes the change of lattice constant;When the solution limit is reached, the continuous increase of solute will cause the precipitation of new phase and no longer cause the change of lattice constant.This turning point is the dissolution limit.In addition, the unit cell can be obtained by precise measurement of lattice constantAtomic numberTo determineSolid solutionType;It can also calculate the density, expansion coefficient and other usefulPhysical constant。
X-ray diffractometer
Basic composition of X-ray diffractometer
Basic composition
(1) The high stability X-ray source provides the X-ray required for measurement. Changing the anode target material of the X-ray tube can change the wavelength of the X-ray, and adjusting the anode voltage can control the intensity of the X-ray source.
(2) The sample and the adjustment mechanism of sample position orientation The sample must be a solid block of single crystal, powder, polycrystalline or microcrystalline.
(3) The X-ray detector detects the diffraction intensity or the diffraction direction at the same time, and the polycrystalline diffraction pattern data can be obtained through the instrument measurement and recording system or the computer processing system.
(4) Modern diffraction pattern processing and analysis systemX-ray diffractometerThey are attached with computer systems equipped with special diffraction pattern processing and analysis software, which are characterized by automation and intelligence.
Latest progress
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Since the discovery of Laue in 1912Copper sulfate crystalIn the past 100 years, X-ray diffraction, an important means of detection, has played an active role in people's understanding and exploration of nature, especially in condensed matter physics, material science, life medicine, chemistry and chemical engineering, geology, mineralogy, environmental science, archaeology, history and many other fields. New fields have been constantly opened up and new methods have emerged in endlessly,Especially with the rise of synchrotron radiation light source and free electron laser, X-ray diffraction research methods are still expanding, such as ultrafast X-ray diffraction, soft X-ray microscopy, X-ray absorption structure, resonant inelastic X-ray diffraction, synchrotron radiation X-ray tomography microscopy, etc.These new X-ray diffraction detection technologies will certainly inject new vitality into various disciplines.
