X ray Diffraction (XRD) Highlights in One Article

What is X-ray diffraction (XRD)

X-Ray Diffraction (XRD) is the main method to study the physical phase and crystal structure of a substance. When a substance (crystalline or amorphous) is analyzed by diffraction, the substance is irradiated by X-rays to produce different degrees of diffraction phenomena, and the composition of the substance, the crystal type, the way of bonding within the molecules, the conformation of the molecules, the conformation, and other characteristics of the substance determine the unique diffraction pattern of the substance.

The XRD technique has the advantages of no damage to the sample, no contamination, quickness, high measurement accuracy, and the ability to obtain a large amount of information about the integrity of the crystal. Therefore, XRD, as a modern scientific method for analyzing the structure and composition of materials, has been widely used in research and production in various disciplines.

Figure 1:X-Ray Diffraction.

X-ray diffraction (XRD) equipment

The X-ray diffractometer consists of an X-ray generating system (generating X-rays), a goniometer and detection system (measuring 2θ and obtaining diffraction information), and a recording and data processing system, which work together to output diffraction patterns. Among them, the goniometer is the core component, which is more complicated to fabricate and directly affects the accuracy of experimental data.

X-ray Diffraction (XRD) Application Scenarios

1. Physical analysis

Physical phase analysis is the most used aspect of X-ray diffraction in metals and is divided into qualitative and quantitative analysis. The former determines the phases present in a material by comparing the spacing of the dot planes and the diffraction intensity measured on the material with the diffraction data of a standard phase, while the latter determines the content of the phases in the material on the basis of the intensity of the diffraction pattern. It is widely used to study the relationship between properties and phase content, and to check whether the material's composition ratio and subsequent processing protocols are reasonable.

2. Measurement of crystallinity

The degree of crystallinity is defined as the percentage of the ratio of the weight of the crystallized portion to the total weight of the specimen. Amorphous alloys are used in a wide range of applications, such as soft magnetic materials, and the crystallinity directly affects the properties of the material, so the determination of crystallinity is particularly important. The degree of crystallinity is determined based on the area of the diffraction pattern of the crystalline phase versus the area of the pattern of the amorphous phase.

3. Precision Measurement of Spot Parameters

Precise determination of the dot parameter is often used for the determination of the solid state solubility curve of the phase diagram. Changes in solubility often cause changes in the fractional array constant; when the solubility limit is reached, the continued increase in solute causes the precipitation of a new phase, which no longer causes changes in the fractional array constant. This turning point is the dissolution limit. In addition, the precision determination of the fractional constant can be used to obtain the number of atoms per unit cell, thus determining the type of solid solution; useful physical constants such as density and expansion coefficient can also be calculated.

4. Characterization of nanomaterial particle size

The particle size of nanomaterials is closely related to their properties. Nanomaterials are highly susceptible to agglomerate formation due to their fine particle size, and the use of the usual particle size analyzers tends to give erroneous data. The average particle size of nanoparticles can be determined using the X-ray diffraction linewidth method (Scheller method).

5. Determination of crystal orientation and organization

Measurement of crystal orientation, also known as single-crystal orientation, is the process of finding the orientation of crystals in a crystal sample in relation to the external coordinate system of the sample. Although single crystal orientation can be determined by physical methods such as optical methods, the X diffraction method can not only accurately orient single crystals, but also obtain information on the internal microstructure of the crystal. Generally, the single crystal orientation by the Laue method is based on the positional relationship between the polarized ruddy projection of the Laue spot transition on the negative and the polarized ruddy projection of the sample's external coordinate axis. Transmission Lloyd's method is only applicable to samples with small thickness and small absorption coefficient, while back-emission Lloyd's method requires no special preparation of samples, and the size of the sample thickness, etc., is not limited, so this method is mostly used.

6. Stress Test

Macroscopic stress refers to the internal stress that is uniformly distributed over a large area in a component. Macrostress has a significant impact on the use of materials: negative (e.g. seawater stress corrosion, etc.) and positive (e.g. compressive stress improves fatigue life, etc.).

As long as stress exists, there will be strain, which will result in a change in the crystal face spacing; X-rays can measure the change in face spacing very well, so they can be utilized to measure stress.

Example of X-ray diffraction (XRD) analysis

1. Spectral comparison method: Compare the spectra of the sample to be tested and the known phases, the method can be intuitive and simple identification of the phases, but the spectra should be compared with each other under the same experimental conditions to obtain, the method is more suitable for the analysis of common phases and speculative phases.

2. Data comparison method: the measured data (2θ, d, I/I1) can be compared with the standard diffraction data to identify the phases.

3. automatic computer search identification method: the establishment of a database of standard phase diffraction data (PDF card), the measured data of the sample into the computer, the computer according to the appropriate procedures for retrieval, the main analytical software are JADE, Search Match and so on.

The following figure shows the XRD patterns of ZnO nanosheets loaded with different contents of Au nanoparticles. Compared to pure ZnO, the hybridized material showed obvious characteristic diffraction peaks of Au (JCPDS No. 04-0784) with the sequential increase of loading amount, so it is considered that Au was successfully loaded on the surface of ZnO.

X-ray Diffraction (XRD) Sample Requirements and Methods of Preparation

XRD can measure both bulk and powdered samples and has different requirements for different sample sizes and sample properties.

(1) Requirements and preparation of bulk samples

For non-fracture metal bulk sample, as flat as possible, smooth, clean, in order to remove the surface of the oxide film, to eliminate the surface strain layer, XRD swept through an area to get the diffraction peaks, the sample needs to be a certain size, the general area of the sample should be greater than 10mm × 10mm.

For thin-film specimens, its thickness should be greater than 20nm, and do the test before the test to determine the orientation of the substrate, if the surface is very uneven, according to the actual situation can be fixed with conductive adhesive or playdough on the sample, and make the sample surface as flat as possible.

For the flake, cylindrical specimens will exist serious merit orientation, resulting in diffraction intensity anomalies, at this time in the test should be a reasonable choice of response direction plane.

For fracture, cracked surface diffraction molecules, the port is required to be as flat as possible and provide the elements contained in the fracture.

(2) Requirements and preparation of powder samples

When the powder sample is analyzed by X-ray powder diffractometer, the suitable grain size should be within the order of magnitude of 320 mesh particle size (about 40um), so that the broadening of diffraction lines can be avoided and good diffraction lines can be obtained.