Introduction
In order to characterise the material, two characterisation techniques are introduced in the following parts. The first is X-ray diffraction, the other is X-ray fluorescence. The principal and precision of two techniques are stated. Some example data that is representative of neodymium magnet is shown.
Technique 1—X-ray diffraction
Principle：
X-ray diffraction (XRD) is a method using the diffraction phenomena of x-ray in crystal to analysis the crystal structure, lattice parameter, crystal defect, content of different structural phases in the material. X-ray diffraction analysis is an indirect method based on crystal structure. Each type of crystalline material has its own specific structure parameters, including lattice type, unit cell size, atomic number and relative atomic position. These parameters are reflected in x-ray diffraction pattern. Although there are million types of material, there is no two kinds of materials with the same diffraction pattern. So, x-ray diffraction analysis can be a method to identify the material [3]. The schematic figure of X--ray diffractometer is shown below:
Figure 1: Schematic of X-ray Diffractometer
Method：
X-ray diffraction can be used to do qualitative and quantitative analysis of phase. For qualitative analysis, it can compare measured values with standard values. The lattice spacing and diffraction intensity can be obtained by x-ray. By comparing these values with standard materials value, which is recorded as PDF cards, the composition of phases can be determined. For quantitative analysis, the content ratio of each phase can be obtained by using the relationship between the intensity of the diffraction peak and the composition content. However, this is a kind of semi-quantitative analysis.
Precision and accuracy
According to Bragg’s equation, lattice interplanar spacing are well determined when wavelength and incident angle are fixed value. As long as the process method is correct, the measurement result is accurate. In general, XRD test content precision is 5% and XRD angular accuracy is 0.01°.[4]
Example data
Figure2: XRD Spectrum of NdFeB [4]
Figure 2 is the XRD spectrum of NdFeB. It tells the relationship among incident angle, intensity and wavelength of its phases. By digital processing diffraction intensity and wavelength (change to distance) and comparing with standard data in the PDF card, the component can be determined. Once get a phase, do the same process for the remaining peaks and find the remaining phases. Finally, the material can be determined.
TextTechnique 2— X-ray Fluorescence
Principle
X-ray fluorescence analysis (XRF) is a technology which can detect the type and content of the elements, XRF is using origin X-Ray photon or other microscopic particles to excite atom un the substance, so as to produce secondary characteristic X-ray for material composition analysis and chemical state research.
Specific atom has specific level structure, the electrons outside the core all move on their fixed orbits with their own energy, then inner electron breaks away from the atom and becomes a free electron under the irradiation of X-ray with enough energy, and because atomic energy level structure of each element is specific, the energy of the X-ray emitted by the excited transition is also specific. Therefore, by measuring the energy of the characteristic X-ray, the existence of the corresponding element can be determined, and the strength of the characteristic X-ray represents the content of the element.
Method
- The principle of quantitative analysis [5]
The fluorescent X-rays of different elements have their own specific wavelength, so the composition of elements can be determined according to the wavelength of the fluorescent X-ray. If it is a wavelength dispersive spectrometer, the wavelength λ of the X-ray can be calculated from the rotation angle of the detector for the crystal with a certain crystal surface spacing, so as to determine the element composition.
- The principle of qualitative analysis [5]
The quantitative analysis is based on the fact that the fluorescent X-ray intensity li of the element is directly proportional to the content wi of the element in the sample, as shown in the equation below.
Precision and accuracy
The quantitative analysis is based on the fact that the fluorescent X-ray intensity li of the element is directly proportional to the content wi of the element in the sample, as shown in the equation below.
Fluorescence test can be used to test the element type and content of materials by wavelength. The measurement of element type can be more comprehensive and complete. There will be some errors when measuring the content. The result is more precise but not enough accurate, the relative standard deviation of neodymium is 0.36% - 0.49% [5]and there will be too many factors affecting the measurement, such as temperature, pH, etc.
Example
Fluorescence test can not only measure the main elements such as neodymium, iron and boron and their contents, but also measure other elements such as lanthanum, samarium and other rare earth elements in materials through its high sensitivity.
Table 1, Certified results of determined elements in 6 control samples(%, mass fraction) [6]
|
|
Sample 1 |
Sample 2 |
Sample 3 |
Sample 4 |
Sample 5 |
Sample 6 |
|
Nd |
18.55 |
18.06 |
17.98 |
16.52 |
16.96 |
16.83 |
|
B |
1.05 |
1.05 |
1.01 |
1.00 |
0.96 |
0..94 |
|
Fe |
77.86 |
76.96 |
75.11 |
74.41 |
74.43 |
70.00 |
|
Dy |
0.0071 |
0.10 |
0.21 |
0.33 |
0.48 |
0.62 |
Reference
[1] Yutaka Matsuura. Recent development of Nd2Fe2B sintered magnets and their applications[J ]. Journal of Magnetism and Magnetic Materials ,2006 ,303 :344 - 347.
[2] Wang Fang. (2018). Development of Nd-Fe-B permanent magnetic materials. Gansu Science and Technology Vertical and Transverse, v.47; No. 278 (08), 50-53
[3] Zhou Yu. Harbin Institute of Technology. Analysis Techniques in Materials Science. CHINA MACHINE PRESS. ISBN 978-7-111-34230-4.
[4] Shen Q.W, Zhou Y.Q, HAO C.S, Che Y.X. Calculation of the X-ray diffraction spectro of Ndfeb triplet system [J]. Journal of Inorganic Chemistry,1987(03):122-128.
[5] Han Ping, Wang Jihua, Lu Anxiang, et al. Determination of heavy metals in soil by portable X-ray fluorescence spectrometer [J]. Spectroscopy and spectral analysis, 2012, 32 (3): 826-829
[6] Liang Yu. X-ray fluorescence analysis of Nd-Fe-B permanent magnet materials [J]. Physical and chemical examination: chemical volume, 1990 (3): 157-158161