XRD
X-ray diffraction is the main method to study the phase and crystal structure of a substance. When a substance (crystalline or non-crystalline) is subjected to diffraction analysis, the substance is irradiated with X-rays to produce different degrees of diffraction. The production of the unique diffraction pattern can be determined by the composition, crystal form, intramolecular bonding method, molecular configuration, and conformation.
The principles of X-ray diffraction are as follows: When a beam of monochromatic X-rays enters the crystal, since the crystal is composed of unit cells of regularly arranged atoms and the distance between these atoms is of the same order as the incident X-ray wavelength, the X-rays scattered by different atoms interfere with each other, producing strong X-ray diffraction in some special directions. The orientation and intensity of the diffraction lines in the space are closely related to the crystal structure. The Bragg equation reflects the relationship between the direction of the diffraction line and the crystal structure. For a particular crystal, only the angle of the ray that satisfies the Brag equation can produce interference enhancement, and only show diffraction fringes. 2dsinθ=nλ, where d is the spacing between crystal planes; n is the reflection order; θ is the glancing angle; λ is the wavelength.
With the PDF standard card, XRD could determine whether the sample is SiC or not and also the crystal structure of the sample. XRD is measuring the angular position of peaks to those much less than 1 degree and determining lattice constants to those less than 0.01nm. X-ray diffraction analysis is qualitative or semiquantitative.
Figure 1 X-ray diffraction pattern of single crystal SiC [1]
Figure 1 is an example of X-ray diffraction pattern of single crystal SiC. The sample is manufactured by the method of solvothermal synthesis with a system of SiCl4, CCl4 and potassium. The corresponding relationship between 2θ and crystal face is shown in table 1.
|
2θ |
33.5º |
35.6 º |
41.4 º |
60.0 º |
71.8 º |
75.5 º |
|
Phase |
α |
β |
β |
β |
β |
β |
Table 1 The result of XRD
According to the JCPDS Card No. 29-1131, 73-1708, and 73-1664, the signal * in the figure denotes the diffraction of α-SiC and other peaks are all β-SiC with FCC structure. [2] Sharp peaks indicate that SiC has distinct crystal structure.
SEM
Scanning electron microscope was widely used in observing the morphology and composition of the surface ultra-structure of various solid substances. [3]
The principles of SEM are as follows: A high-energy electron beam is formed under the acceleration voltage, and it bombards the surface of the sample point by point in a raster-like scan, and at the same time excites electronic signals of different depths. [4] Then the electronic signal will be received by the probe of the signal receiver, synchronously transmitted to the computer screen through the amplifier and imaged. [5]
SEM provides micrograph of the sample in high resolution, to the precision of 6nm or even lower with better devices. Meanwhile, SEM could be used as material analysis platform, where different electronic probe devices are attached to obtain composition information with quantitative data. Therefore, the precision and accuracy of SEM analysis mostly is determined by its obtained images. For this SiC sample, SEM is used to observe and analyze its structure and micromorphology.
Here are two typical examples of SEM to characterize the microstructure of SiC. Figure 2 shows the microstructure observation of sintered SiC by liquid-state sintering (LPS) at 1900℃ and solid-state sintering (SSS) at 2100℃. [6]
Figure 2 SEM photos for the SSSed sample: (a) polished surface (b) fracture section;
for the LPSed samples: (c) polished surface (d) fracture section (e) elongated grains of SiC ceramics after annealing [6]
The microstructures of SSSed samples contain coarse and ununiform grains, vague grain boundaries and bad bonded SiC grains, which means high brittle, low toughness and low density of sample material. The microstructures of LPSed samples contain clear grain boundaries, visible grain shape, and equal axis grains is clear, which illustrates brittle is improved and toughness raised.
Figure 3 SEM micrograph of SiC powder (a) sub-μm (b) >1μm [7]
Figure 3 shows the SEM micrograph of SiC powder. It can be seen that morphologies of β-SiC particles are diverse with prismatic, plate, strip, equal-axis, polygon, of which most are equiaxed. The general particle size is sub-micron grade, in which most are below 500 nm; some with more than 1μm. [7]
EDS
The X-ray energy spectrum analysis (EDS) method can be used to analyze the elemental composition and content of materials. It is a basic and reliable method and widely used in the field of material analysis.
The basic principle of X-ray energy spectrum analysis is to detect the characteristic X-ray produced by the sample under the electron beam to analyze the elements and content of the sample. It is a qualitative and quantitative analysis method. The detail of the principle is that when the sample is irradiated by a high energy electron beam, the inner electrons of the inner atom will be excited to a higher energy level than Fermi energy level, thus creating a vacancy in the electron orbit. [8] When the outer electron fills the vacancy, its energy will be emitted in the form of an X-ray, which becomes the characteristic X-ray. [8] The principle of qualitative analysis of EDS is that the energy of characteristic X-ray is only related to the atomic number of the elements in the sample, so the types of elements in the sample can be obtained after the characteristic X-ray energy spectrum is made. [8] The principle of EDS quantitative analysis is that the content of an element in a sample is proportional to the intensity of the characteristic X-ray corresponding to the element. Therefore, the element content can be obtained by comparing the height of the energy spectrum peak with that of the standard sample.
The resolution of the X-ray energy spectrum (EDS) analysis method is measured by the index of energy resolution, which refers to the minimum energy interval that can be detected by the detector for two kinds of particles or rays with different energy. [9] The smaller the index of energy resolution, the stronger the ability of the spectrometer to identify the peaks. At present, the energy resolution of the advanced spectrometer is as low as 125eV, and the detectable elements are beryllium (atomic number 4) to uranium (atomic number 92). [9]
The following figure shows the energy spectrum analysis results of a silicon carbide ceramic sample.
Figure 4 Result of EDS Analysis of a SiC Ceramic [10]
It can be seen from the energy spectrum analysis that the SiC ceramic contains not only Si and C elements, but also Al, y, and Ti elements. After consulting the relevant literature, it is found that Al and Y elements are introduced by the sintering aids (Al 2O 3 and Y 2O 3) added in the preparation process, while Ti elements are introduced by tin particles used to improve the mechanical properties of the ceramic, so there are peaks of Al, y and Ti elements in the energy spectrum. [10]
References
[1] Ning J Q, Yang B F, Fu Z P, et al. Solvothermal synthesis and characterization of SiC single crystal materials [J]. Journal of Chemical Physics: English edition, 2004, 17 (5): 633-636
[2] Joint Committee on Powder Diffraction Standards (JCPDS), International Centre for Diffraction Data, 2002, Card No. 29-1131, 73-1708, and 73-1664.
[3] Goldstein, Joseph I., et al. Scanning Electron Microscopy and X-Ray Microanalysis. Kluwer Academic/Plenum Publishers, 1992.
[4] Wu K J. Principles and characteristics of scanning electron microscopy [J]. Science and technology information, 2010 (29): 113
[5] The application of electron backscatter diffraction and orientation contrast imaging in the SEM to textural problems in rocks [J]. American Mineralogist, 1999, 84(11-12):1741-1759.
[6] Jiang Y, Wu L E, Huang Z K. Microstructure Observation and Analysis of Sintered SiC Ceramics by SEM [J]. Key Engineering Materials, 2013, 544:200-204.
[7] Jiang Y, Wu L N, Chen Y H, et al. Microstructure Observation and Analysis of Powders, Green Bodies Used for Sintering SiC Ceramics by SEM [J]. Advanced Materials Research, 2012, 624:107-111.
[8] Zhou Y. Material analysis method [M]. China Machine Press, 2011.
[9] Shi Mingzhe, Zhang Hui, Zhu Huiji. Discussion on improving the accuracy of quantitative analysis of energy dispersive spectrometer [J]. Reliability and environmental test of electronic products, 2009,27 (S1): 14-21
[10] Zhang Lingjie, Guo Xingzhong, Yang Hui, Zhu Xiaoyi, Li haimiao. SEM / EDS analysis of the distribution of nano tin particles in SiC ceramics [J]. Journal of materials science and engineering, 2009,27 (06): 834-836