Analysis principle of X ray diffraction performance

Analysis principle of X-ray diffraction performance

X-ray diffraction (XRD) analysis is a method to determine the phase and crystal structure of solid samples by observing the X-ray scattering patterns of crystal planes composed of regularly arranged atoms for lithium iron phosphate battery.

In the solid crystal, the lattice spacing is generally several image1X-ray beams, which are electromagnetic waves with a wavelength of order of 1image1, similar to the atomic size. When the ray beam shoots at the solid crystal plane, the X rays scattered by each atom interfere with each other to produce a diffraction pattern. Assume that d (image1) is the atomic crystal plane spacing, θ Is the angle between the reflected light and the reflecting plane, the optical path difference of the corresponding scattered waveimage2, when it is equal to the wavelength( λ) The so-called Bragg rule can be expressed as follows:

image3

The Bragg rule can be used to derive the plane spacing (d), which refers to the crystal plane spacing with a diffraction peak of 20 (see Figure 4.48) [1-3]. By analyzing the positions of all the peaks in the diffraction pattern, we can predict the crystal plane distribution in the crystal and learn more about the structure.

image4

Generally speaking, the following information can be obtained through XRD analysis:

1) Crystal phase identification: the crystal structure can be determined by determining the space group and unit cell. The crystal phase of the sample can be detected by using the peak position and peak intensity, and the diffraction pattern information library collected by the Joint Com mittee on Powder Diffraction Standards (JCPDS) and the International Center for Diffraction Data (ICDD) can be referred to. JCPDS card includes crystal phase, space group, unit cell, diffraction peak and other information of known substances, and can also be used for quantitative analysis of impurities. For an actual sample, a small amount of impurities may overlap the spectrum of the main phase material. Because the impurity detection limit of XRD is 2 wt%, it is difficult to detect lower content with XRD analysis. For nanostructured materials or thin film samples, the size of the diffraction peak corresponding to the preferred crystal plane is often larger than other peaks.

2) Crystallinity: The diffraction pattern of substances with high crystallinity shows sharp peaks (narrow half width), while amorphous liquid or glassy substances show broad peaks. Because there are some crystals in the polymer sample, its diffraction pattern shows the characteristics of semi crystalline. Therefore, the crystallinity of the material can be deduced from the peak type and peak strength of the powder XRD diffraction peak.

3) Grain size: grain size can be obtained by Scherrer formula, which is based on the linewidth effect of X-ray peak. For the average particle size of 0.2~20 μ We can clearly observe the diffracted rays of the highly crystallized primary particles of the m powder sample. When the size of primary particle decreases to 0.2 μ When m is up to tens of nanometers, the width of the diffracted rays becomes larger. If the particle size is further reduced to 20, the diffraction pattern shows amorphous characteristics. As shown in equation (4.67), Scherrer formula calculates the average particle size through the above phenomenon.

image5

Where t is the particle size; K is the kurtosis function (generally 0.9); λ Is the X-ray wavelength; B is half height width (in radians); θ It’s a human shooting angle.

In addition, we can detect random strain and non-uniform distortion of particles. Localization distortion of crystal will change the spacing between crystal planes, thus increasing the width of diffracted rays. Because the effect becomes more significant with the increase of diffraction angle, the inhomogeneity of crystal can be determined by studying the correlation between angle and diffraction width.

As shown in Fig. 4.49, a set of XRD equipment consists of an X-ray source, a goniometer, a filter, a sample stage and a detector for collecting X-ray. When high pressure is applied to the target wire, the X-ray target will generate X-ray, and usually Cu is used as the target wire material.

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When high-speed electrons collide with atoms, electrons in the inner shell near the nucleus will jump and create vacancies, which will be occupied by electrons from the outer layer. When electrons move from high energy level orbit to low energy level orbit, they will produce a kind of electromagnetic wave called X-rays, which corresponds to the energy difference between orbits. The X-ray generated by filling the vacancy in layer K with electrons in layer L is K α, K is generated by M-layer electrons β。 Cu-K α X-ray is usually used as X-ray diffraction source, K β The rays can be filtered. To remove K β X-ray can be filtered by materials with atomic number lower than 1-2. E.g. Cu-K filtered by Ni α It is usually used as a light source in XRD, because the Ni film has a β X-ray has strong adsorption.

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