spectroscopy; micro-area energy dispersionX-ray diffraction; polycapillary X-ray optics; mico-beam X-ray fluorescence spectrometer; ore; micro-area; small sample

Micro energy dispersive X-ray diffraction (ElDXRD) analysis has importance application prospects in measuring the phase structure of small samples or sample micro-area. A novel method of micro EDXRD analysis using a self-development micro X-ray fluorescence spectrometer is proposed. The two-dimensional scan of a 4 mm×4 mm mico-area on the surface of a RMB-5-Jiao coin is made by a portable micro-beam X-ray fluorescence spectrometer based on polycapillary X-ray optics focusing (focal spot diameter is 190.7 μm). After the obtained data are processed, the mappings of Cu₃ Sn (0 8 3), CuO(2 0 2) and other crystal phases are presented. Simultaneously, the two-dimensional scan of an ore particle with diameter of about 1 mm is also made by a desktop micro-beam X-ray fluorescence spectrometer based on polycapillary X-ray optics focusing (focal spot diameter is 31 gm). The mappings of SiO₂(3 2 9), Fe₂ O₃ (1 1 6) and other crystal phases are also presented. The results show that the micro-beam X-ray fluorescence spectrometer based on polycapillary X-ray optics focusing has a potential application in the micro energy dispersion X-ray diffraction analysis of small samples or micro-areas of samples.

X-ray diffraction (XRD) is an important method for analyzing the structure of matter. It was used to study the surface structure and composition distribution of materials in the late 1970s and early 1980s. It has been widely used in physics, chemistry, earth science and materials. science and other fields. With the development of scientific fields such as environment, geology and materials, it is necessary to analyze the crystal phase structure of small particle samples or sample micro-regions with a diameter of less than 1 mm, but the length × width is generally about 1 mm × 10 mm, which is difficult to meet the analysis needs of small samples. The capillary X-ray lens is an optical device that converges the X-ray beam, which can condense the X-ray beam excited by the X-ray tube into an X-ray beam spot with a diameter of several tens of microns, and can make the X-ray beam in the energy range of 2-12 keV. The energy of X-rays at the focal spot is amplified by 2 to 3 orders of magnitude. Energy Dispersive X-ray Diffraction (EDXRD) is to irradiate the sample with a multi-energy X-ray beam under the condition that the angle between the X-ray beam irradiating the sample and the X-ray detector is constant. According to the Bragg principle, when the X-ray detector is used to detect the X-ray beam diffracted by the sample to obtain the information of the phase structure of the sample, compared with the conventional angle scattering XRD, EDXRD has many advantages and has been widely used in material analysis. , identification of physical evidence, etc. In this paper, we try to combine the technology of X-ray condensing by capillary X-ray lens with EDXRD method, carry out the method research of energy dispersive microbeam XRD analysis, and explore the feasibility of microbeam XRD analysis method based on energy dispersion.

The capillary X-ray lens is an X-ray optical device designed using the principle of total reflection. It is composed of millions of hollow glass tubes with an inner diameter of several microns fused under the action of high temperature. The schematic diagram is shown in Figure 1. Among them, F₁ is the front focal length of the lens, that is, the distance from the X-ray point light source to the entrance end of the lens; L is the length of the lens; F₂ is the back focal length of the lens, that is, the distance from the exit end of the lens to the X-ray focal spot. The capillary X-ray lens makes the X-ray beam emitted from the point light source enter the hollow capillary glass tube, and transmits it through total reflection on its inner wall, and then uses the bending of the hollow glass tube to change the transmission direction of the X-ray, so as to focus the X-ray beam. A focal spot with a diameter of up to several tens of microns can be formed, and the intensity of the X-ray beam in the micro area is increased by 2 to 3 orders of magnitude.

This experiment was completed by a self-developed microbeam energy dispersive X-ray fluorescence spectrometer. The main equipment includes: micro-focal spot X-ray tube (Rontgen Company, Germany), SDD (Silicon Drift Detector) X-ray detector (Amptek Company, USA, the energy resolution is 145eV when the energy is 5.9keV, and the effective area of the beryllium window is 25mm² ), capillary X-ray lens, laser pointer, high-precision laser displacement sensor, CCD (Charge-Coupled Device) camera with 14 million pixels and 20 times magnification function, three-dimensional sample stage and Siemens programmable logic controller (PLC) control system Wait. Among them, the angle between the micro-focal spot X-ray tube, X-ray detector and the horizontal plane is all 45°, which are distributed on both sides of the CCD camera. The X-ray beam axis, the detector axis, the laser pointer beam axis and the CCD axis converge at the X-ray focal spot. The spectrometer control software is developed based on the LabVIEW language environment.

Fig. 1 Schematic of polycapillary X-ray optics

In actual measurement, factors such as the content of the element of interest in the sample and the size of the sample area to be scanned and analyzed will cause differences in the speed of analyzing the sample and the total scanning time. If the scanning area of the sample is large but the focal spot of the X-ray irradiated sample is small, the total sample measurement time will be too long; if the scanning area is small and the detection focal spot is large, the resolution of the element scanning imaging will be higher. Poor wait. Therefore, in the experiment, the appropriate capillary X-ray lens is generally selected according to the needs of sample analysis. In order to meet the analysis requirements of scanning areas of different sizes, a portable microbeam X-ray fluorescence spectrometer equipped with a capillary X-ray lens with a focal spot diameter of 190.7 μm and a desktop equipped with a capillary X-ray lens with a focal spot diameter of 31 μm were selected in the experiment. Microbeam X-ray fluorescence spectrometer was used for measurement analysis.

In order to measure the crystal phase distribution of the sample micro-area, a RMB 50 coin was selected as the experimental sample, the scanning area was the “角” part on the surface of the coin, and the area size was 4 mm × 4 mm, as shown in Figure 2.

Fig. 2 Schematic of scanning area of coin sample

Due to the large scanning area, in order to obtain a suitable total scanning time and imaging resolution, a portable microbeam energy dispersive X-ray fluorescence spectrometer with a detection focal spot diameter of 190.7 μm was selected for measurement. The experimental voltage is 30kV, the current is 0.6mA, the scanning step is 100μm, and the detection time of each point is 20s. The X-ray energy spectrum of the scanned area of the sample is shown in Fig. 3

Fig. 3 Energy spectrum in scanning area of coin sample

According to Planck’s formula:

where
h is Planck’s constant,
c is the speed of light, and
λ is the photon wavelength.
Combined with the Bragg formula, the energy E of the diffraction peak is converted into its corresponding interplanar spacing d, namely

where
θ is the diffraction angle and
n is the diffraction order.
The calculated d was compared with the PDF card of the corresponding phase, and the crystal plane index (h k l) was obtained. The results are shown in Table 1.

Table 1 Micro-area data of coin sample measured by our spectrometer and reference data

The 1600 energy spectra scanned in this experiment were processed by PyMca software, and the crystal phase distribution shown in Figure 4 was obtained.

Fig. 4 Crystal phase mappings in scanning area of coin sample. (a) Cus Sn (0 8 3); (b) Sn (6 4 2); (c) CuO (202)

In order to verify the accuracy of the analysis results of the spectrometer, the experimental samples were analyzed by X-pert-pro-MPD XRD instrument produced by PANalytical Corporation of the Netherlands. The results are shown in Figure 5.

Fig. 5 Energy spectrum of coin sample measured by X-pert-pro-MPD XRD spectrometer

It can be seen from Figure 3 that the EDXRD method can not only detect different elements such as Cu and Sn on the coin surface, but also can detect different crystal phases of Cu₃Sn, Cu, CuO and other substances. It can be seen from Fig. 4 that the relative contents of Cu₃Sn (0 8 3) and Sn(6 4 2) in the crystal phase of the “角” pattern area are relatively high, while the relative content of CuO (2 0 2) is relatively low. It can be seen from Figure 5 that by comparing the sample scanning XRD analysis data of the X-pert-pro-MPD XRD instrument with the EDXRD analysis results, it is found that the two are basically consistent.

In order to detect the crystal phase or crystal phase distribution of the small particle samples, the tiny ore particles with a diameter of about 1 mm as shown in Fig. 6 are selected as the experimental samples.

Fig. 6 Ore particle sample

The sample is small, therefore, in order to obtain a suitable total measurement time and imaging resolution, a desktop microbeam energy dispersive X-ray fluorescence spectrometer with a detection focal spot diameter of 31 μm was used to scan the whole ore particles in two dimensions. The measurement was carried out under the experimental conditions that the X-ray tube voltage was 30kV, the current was 0.7mA, the detection time of each point was 5s, and the scanning step was 30μm. The obtained X-ray energy spectrum of the scanning area is shown in Figure 7.

Fig. 7 Energy spectrum in scanning area of ore particle sample

The crystal plane spacing d corresponding to each diffraction peak was calculated by Planck’s formula and formula of interplanar spacing d, and compared with the PDF card of the corresponding phase to obtain the crystal plane index (h k l), and the results are shown in Table 2.

Table2 Data of ore particle sample measured by our spectrometer and reference data

Using PyMCA software to process all X-ray energy spectra obtained by scanning, the crystal phase distribution in the scanning area as shown in Fig. 8 can be obtained.

Fig. 8 Crystal phase mappings in scanning area of ore particle sample. (a) Fe2O3(1 1 6); (b) Fe2O3(0 2 10); (c) SiO2(3 2 9); (d) SiO2(3 1 4)

It can be seen from Fig. 7 that the analysis method of microbeam X-ray focused by capillary X-ray lens can not only measure the main elements Si and Fe in the ore particles, but also measure the different crystal phases of Fe₂O₃, SiO₂ and other substances. Among them, the existence of Fe₂O₃ explains the reason why the surface of ore particles is red. In addition, the distribution of different crystal phases such as Fe₂O₃ (1 1 6), Fe₂O₃ (0 2 10), SiO₂ (3 2 9) and SiO₂ (3 1 4) in the whole ore particles can be obtained from Fig 8. The formation process and its categories have certain reference value.

Using a microbeam energy dispersive X-ray fluorescence spectrometer focused by a capillary X-ray lens, energy dispersive microbeam XRD analysis was performed on the micro-area of the 5-cent coin and the tiny ore particles. The distribution of each crystal phase in the scanned area can also be obtained. The experimental results show that the microbeam EDXRD focused by the capillary X-ray lens has certain application prospects in the field of crystal phase analysis of small samples or sample micro-regions.