Introduction

Bismuth (Bi) is considered as the most massive element that can stably exist in nature. It is a brittle, crystalline, white metal, Somewhat pink with a high lustre. It has an atomic number of 83 and at the position of the sixth period, Ⅴ_A group of the periodic table. High purity (above 99.99% content) Bismuth can form bismuth crystal with fascinating shape and colour. More detail information, including properties, processing, manufacturing, and applications are shown in the website link and the QR code below. Specific characterisation techniques should be applied to have a deeper understanding of the material's microstructures and properties. Material characterisation plays a significant role in material science and engineering, which help us figure out the mechanism behind the external performance.

As an elementary metal crystal substance, the purity, grain size, crystal structure, and metallographic composition contributes the most to the properties of Bismuth. Thus, XRF (X-ray Flourescence), EBSD (Electron Backscattered Diffraction), and XRD (X-ray Diffraction Microscopy) are recommended to characterise the material. XRF is used to test the purity and detect the impurities composition, EBSD is used to observe the grain size, and XRD is used to test the crystal structure and metallography. Detailed proposals are given in the following pages.

Remarks:

Due to the particularity of the pure Bismuth crystal, few teams have done research on pure Bismuth or Bismuth crystal. Thus, Some examples of Bismuth alloys and Bismuth oxides are displayed to show the principles and results of the characteriasation methods.

X-ray Flourescence (XRF)

X-ray fluorescence spectrometer (XRF) is a relatively effective qualitative and quantitative technique which is fast, accurate, and nondestructive to identify and detect the element content of the material. By changing the diffraction angles, the intensity of the fluorescent X-rays produced by different elements can be gotten, while the intensity of these rays is proportional to the abundance of the elements. It is compatible with solid, liquid, and powdered samples, and can be considered as minimal sample preparation.

When the sample content is greater than 1%, the relative standard deviation can be less than 1%. XRF has high resolution(<220ev) with trace element abundances at the ppm level. Besides, XRF images have high spectral resolution but low spatial resolution. Thus, XRF is a superior technology to detect the elements with high precision.

Fig. 1, XRF Spectrum of Pure Bismuth, taken with an Innov-X a-2000 X-Ray Fluorescence spectrometer with a Si-PiN detector (Hardware settings: Source: Ta; Voltage: 40 kV; Current: 24 uA; Filter: 250 uM Cu, Analytical Mode-FP algorithm, acquisition time 34s).

The spectrum shows a few information about different excitation process and respective energy radiated. For example, in the highest peak, which says “ 13.02 keV”, “L” means the electron in outer shell finally come to L energy level, “β” means the outer shell is two levels higher than L level, which is N level, and “1” means electron comes from the lower sub-level of N level, and the energy “13.02 keV” means the energy of fluorescence radiation of this process. [1] The positions of peaks implies that the sample has Bi within it, and the purity of Bi in this sample can be obtained by analyzing the width of each peak.

Reference

[1] XRF Spectrum Bismuth. http://www.xrfresearch.com/xrf-spectrum-bismuth/. Accessed June 8，2020

Electron Backscattered Diffraction (EBSD)

The electron backscattered diffraction (EBSD) is a very powerful tool for material science. Accelerated electrons in the primary beam of a scanning electron microscope (SEM) can be diffracted by atomic layers in crystalline materials. These diffracted electrons can be detected when they impinge on a phosphor screen and generate visible lines, called  EBSP (electron backscatter patterns). These patterns are effectively projections of the geometry of the lattice planes in the crystal [1], and they give direct information about the grain size of purity bismuth crystal. Analyzing grain size of a bismuth crystal is important for quality control. When a metal or alloy is processed, the atoms within each growing grain are lined up in a specific pattern, depending on the crystal structure of the sample. [2]

^{Through the calculation of the average value, we can use computer software or direct measurement to obtain qualitative information about the grain size of the bismuth crystal. Furthermore, the image shows that with the time of annexed deformation increasing, the grain size of 20% Bi2Te3-80% Sb2Te3 alloys also increases.}

^{For the crystal orientation, the parts of the same color represent the same crystal orientation, which is shown in the images.}

Fig. 1. EBSD images of mechanically deformed 20% Bi₂Te₃-80% Sb₂Te₃ alloys annealed at 380°C [3]

Fig. 2 shows the EBSD chrysanthemum-like image of BSCCO (Bi-Sr-Ca-Cu-Co), which is an EBSD feature image detected based on SEM technology. We can use computer software technology to convert the chrysanthemum-like image into a color image containing grain size and grain oritation, and then detect more about the nature of the material.

Fig. 2. EBSD chrysanthemum-like image of BSCCO (Bi-Sr-Ca-Cu-Co) [4]

Fig. 3 shows that EBSD image of the Sn-Bi alloy with the detected grain size. At the same time, we can conclude that in Sn-Bi alloys based on Sn, as the content of Bi gradually increases, the grain size of the alloy gradually decreases.

Fig. 3. Grain size observations of as-plated films using FIB microscopy: (a) pure Sn (b) Sn-0.5Bi (c) Sn-1Bi, and (d) Sn-2Bi [5]

Fig. 4(a) shows the EBSD microstructure reconstruction of the longitudinal section (along the extrusion direction) of the extruded BAZ711 alloy. It can be seen from Fig. 5(a) that the extruded alloy is completely recrystallized, with an average grain size of about 2.9 μm. [6]

Fig. 4. EBSD analysis of as-extruded BAZ711 alloy: (a) EBSD orientation map; (b) Inverse pole figure [6]

References:

[1] Geochemical Instrumentation and Analysis. Electron Backscatter Diffraction (EBSD). [R/EB]. [2020-06-03.] https://serc.carleton.edu/research_education/geochemsheets/ebsd.html.

[2] Olympus Industrial Resources. Application Notes. Grain Size Analysis in Metals and Alloys. [Z/EB]. [2020-06-03.]   https://www.olympus-ims.com.cn/en/applications/grain-size-analysis/.

[3] ResearchGate. EBSD images of mechanically deformed 20%Bi₂Te₃-80%Sb₂Te₃ alloys [R/EB]. [2020-06-04.]  https://www.researchgate.net/figure/EBSD-images-of-mechanically-deformed-20Bi-2-Te-380Sb-2-Te-3-alloys-annealed-at-380C_fig1_262953746.

[4] EDAX.AMETEK.MATERIALS ANALYSIS DIVISION. [R/EB]. [2020-06-09]. https://www.edax.com/resources/interactive-periodic-table/bismuth.

[5] ResearchGate. Mitigation of Sn Whisker Growth by Small Bi Additions. [R/EB]. [2020-06-09]. https://www.researchgate.net/publication/260723436_Mitigation_of_Sn_Whisker_Growth_by_Small_Bi_Additions.

[6] Meng Shua-ju, YU Hui, CUI Hong-wei, ZHANG Jing, ZHAO Wei-min, Wang Zhi-feng, QIN Chun-ling. Microstructure and mechanical properties of a new mG-Bi-al-Zn deformed magnesium alloy [J]. The Chinese Journal of Nonferrous Metals. [2020-06-09]. DOI：10.19476/j.ysxb.1004.0609.2017.05.003

X-ray Diffraction Microscopy (XRD)

X-ray diffraction (XRD) is a rapid analytical technique that is mainly used for phase identification of crystalline materials and provides information such as the composition of the material, the structure or morphology of atoms or molecules within the material. [1]

There are three different processing ways of Bi discussed: (1) boiling Bi (NO₃) ₃ in Na₂ EDTA aqueous solution added ascorbic acid and PVP with an environment of PH 10-11 (by NaOH) about 12 h [2]; (2) high-purity (99.999%) Bi fabricated with C-templates after the samples were annealed at 200 °C for 8h under flowing N2 [3]; (3) synthesised by reducing NaBiO₃ with EG for 24 h at different temperatures [4]. Their XRD patterns are shown in Fig. 1,2,3 respectively. In Fig. 1, the sample was characterised by powder X-ray diffraction at a scanning rate of 2°/min in the 2θ range from 10° to 80°, using Cu KR radiation (λ) 1.5418 Å) on Philips ARL X'TRA diffractometer. Analysed qualitatively, all the diffraction peaks can be indexed to be hexagonal rhomb-centred phase bismuth (JCPDS card no. 05-0519) [space group, R3 hm (166)] with cell parameters of a=4.545 Å and c=11.83 Å, which are in good agreement with the literature values (a =4.546 Å, c = 11.852 Å). The intensities and positions of the peaks match those data reported in the literature very well. No peaks of any other phases are detected, indicating the high purity of the product.[2]

Fig. 1, A typical XRD pattern of the obtained Bismuth [2]

Fig. 2, XRD patterns of different Bi sizes [3]

Fig. 3, XRD patterns of Bi under different processing T [4]

There are three different processing ways of Bi discussed: (1) boiling Bi (NO₃) ₃ in Na₂ EDTA aqueous solution added ascorbic acid and PVP with an environment of PH 10-11 (by NaOH) about 12 h [2]; (2) high-purity (99.999%) Bi fabricated with C-templates after the samples were annealed at 200 °C for 8h under flowing N2 [3]; (3) synthesised by reducing NaBiO₃ with EG for 24 h at different temperatures [4]. Their XRD patterns are shown in Fig. 1,2,3 respectively. In Fig. 1, the sample was characterised by powder X-ray diffraction at a scanning rate of 2°/min in the 2θ range from 10° to 80°, using Cu KR radiation (λ) 1.5418 Å) on Philips ARL X'TRA diffractometer. Analysed qualitatively, all the diffraction peaks can be indexed to be hexagonal rhomb-centred phase bismuth (JCPDS card no. 05-0519) [space group, R3 hm (166)] with cell parameters of a=4.545 Å and c=11.83 Å, which are in good agreement with the literature values (a =4.546 Å, c = 11.852 Å). The intensities and positions of the peaks match those data reported in the literature very well. No peaks of any other phases are detected, indicating the high purity of the product. [2]

In Fig. 2, (a)(b)(c)(d) are different sizes of Bi. The arrows point to peaks of the metastable phase, while marked above the individual peaks are Miller indices corresponding to the lattice planes of bulk.[2] The XRD patterns also show that Bi has a rhombohedral crystal structure. Fig. 3 shows the XRD patterns of Bi under different processing temperatures. All the diffraction peaks for the samples at 413 K, 433 K and 453 K could be indexed based on the diffraction data of JCPDS 85-1330, indicating that these samples with different conditions have the same structure. From the results above, all the XRD patterns show that the Bismuth produced in different ways, whether for polycrystal in Fig. 1 or single crystal in Fig. 2 have a hexagonal rhomb-centred crystal structure.

References:

[1] Andrei A. Bunaciu, Elena gabriela Udriştioiu & Hassan Y. Aboul-Enein (2015) X-Ray Diffraction: Instrumentation and Applications, Critical Reviews in Analytical Chemistry, 45:4, 289-299, DOI: 10.1080/10408347.2014.949616.

[2] Ruiling F.; Shu X.; Yi-Nong L.; Jun-Jie Z., Synthesis and Characterization of Triangular Bismuth Nanoplates, CRYSTAL GROWTH & DESIGN, 2005, VOL.5, NO.4 1379-1385.

[3] Zhibo Z., Dmitry G., Mildred S. D., Jackie Y. Y., Processing and Characterisation of Single-Crystalline Ultrafine Bismuth Nanowires, Chem. Mater. 1999, 11, 1659-1665.

[4] Xin M., Wen-Yu Z., Dan-Qi H., Hong-Yu Z., Wan-Ting Zhu, Qing-Jie Z., Synthesis and Characterization of High-Purity Bismuth Nanowires via Seed-Assisted Growth Approach, Journal of ELECTRONIC MATERIALS, 2015, Vol. 44, No. 6, 2015.