Scanning Electron Microscopy (SEM) is a versatile and powerful tool extensively utilized for examining the surface characteristics of materials. In SEM, a sample is bombarded with a high-energy electron beam, and the emitted electrons and X-rays are analyzed. This analysis provides crucial information about a material’s topography, morphology, composition, grain orientation, and even crystallographic data in three dimensions. Morphology pertains to the shape and size of features, while topography describes the surface texture, including smoothness or roughness. Composition refers to the elements and compounds present in the material, and crystallography relates to the atomic arrangement within the material (Akhtar et al., 2018).
Based on the image below, the operational principle of SEM involves generating an electron beam through electromagnetic fields and lenses. The objective lens focuses this electron beam onto the specimen. Subsequently, the focused beam scans a specific area on the specimen’s surface in a raster pattern, guided by deflection coils controlled by a scan generator. The magnification level dictates the size of this raster pattern. Altering the magnification adjusts the scanned area size on the sample. When the electron beam interacts with the material, it generates a multitude of signals, including emitted electrons and X-rays. These signals are detected and converted into an image, providing detailed information about the material’s external morphology (texture), chemical composition, and crystal structure.
How to Read SEM Analysis: Interpreting SEM Images for Material Characterization
Understanding SEM images is crucial for materials analysis, especially in fields like automotive engineering where material integrity and performance are paramount. Figure 4 presents a typical SEM analysis result, in this instance, of a Lithium-Ion Battery material. Here’s how to interpret such an image:
Figure 4. SEM Analysis Results of a Lithium Ion Battery
From Figure 4, several key aspects can be evaluated to understand the material properties:
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Particle Size and Distribution: Observe the scale bar in the SEM image, usually indicated as a white line with a length marker (e.g., 10 µm, 1 µm, 100 nm). This scale allows you to visually estimate the size of the particles in the image. Note whether the particles are uniformly sized (monodisperse) or if there is a range of sizes (polydisperse). Also, assess the distribution of particles – are they evenly spread out, clustered together (agglomerated), or showing any patterns of arrangement? In battery materials, smaller particles can offer a larger surface area for reactions, but might also lead to unwanted side reactions.
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Particle Shape and Structure (Morphology): Examine the shape of individual particles. Are they spherical, cubic, rod-shaped, irregular, or plate-like? The shape can significantly influence material properties. For example, in Figure 4, the particles appear angular and irregular, which might be due to manufacturing processes like milling. Ideally, for battery applications, more homogenous and smoother graphite particles are often preferred for better stability during battery cycling. Look for surface texture – are the particles smooth or rough? Porous or solid? These morphological features are critical for understanding material behavior.
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Particle Density and Arrangement (Topography): Observe how densely the particles are packed together. Some areas might show closely packed particles, while others might appear more dispersed. The arrangement and contact between particles are vital in applications like batteries and catalysts, as they affect electrical conductivity and ion transport. In SEM images, density can be inferred from the relative brightness and spacing between features. Denser areas might appear brighter due to more electron scattering.
The cost for SEM testing at UNS (PUI) ranges as follows:
- SEM Top view (sample): IDR 250,000 (2 points, 3 magnifications)
- SEM-EDX (Energy Dispersive X-Ray) (sample): IDR 300,000 (1 point)
- SEM (cross section & top view) (sample): IDR 500,000 (2 points, 2 magnifications)
- Elemental Mapping SEM (sample): IDR 500,000
- SEM data interpretation (image): IDR 150,000 (particle size, roughness)
- Preparation/coating for SEM (sample/minute): IDR 50,000
Other Essential Material Analysis Techniques
While SEM is invaluable for surface characterization, other techniques provide complementary information about material properties. Here’s a brief overview of XRD, FTIR, TEM, and TGA:
1. X-Ray Diffraction (XRD)
X-ray Diffraction (XRD) is a technique used to determine the crystallographic structure of a material. By analyzing the diffraction pattern of X-rays when they interact with a crystalline sample, XRD can identify the phases present, determine lattice parameters, and assess crystal quality.
Figure 1. XRD pattern of a membrane.
How to Analyze XRD Results:
- Identify Diffraction Peaks: Peaks on the XRD graph indicate diffraction from specific crystal lattice planes.
- Diffraction Angle (2θ): The position of peaks on the horizontal axis (2θ) provides information about the spacing between crystal lattice planes.
- Peak Intensity: Higher peak intensity suggests a higher degree of crystallinity or preferred orientation of the crystal planes.
- Peak Width: Broader peaks can indicate smaller crystallite size or crystal defects. The Full Width at Half Maximum (FWHM) is used to quantify peak broadening.
- Crystal Structure Interpretation: Compare the obtained diffraction pattern with database references (like ICDD or JCPDS) to identify the crystalline phases present in the sample.
Formula for FWHM:
β = ½ (2θmax – 2θmin)
Scherrer Equation: (Used to estimate crystallite size from peak broadening, equation is missing in original text, if needed can be added but as per instructions keep original info).
XRD Testing Cost at UNS (PUI):
- Without data interpretation: IDR 200,000
- With data interpretation: IDR 350,000
2. Fourier Transform Infrared Spectroscopy (FTIR)
Fourier Transform Infrared Spectroscopy (FTIR) measures the absorption of infrared radiation by a material as a function of wavelength or frequency. FTIR spectroscopy is used to identify functional groups in a sample by analyzing the vibrational and rotational energy levels of molecules.
Figure 3. FTIR Spectra of Propanal, Acetone, and Propanoic Acid
How to Interpret FTIR Spectra:
- Identify X and Y Axes: The X-axis is wavenumber (cm⁻¹), ranging from approximately 4000 cm⁻¹ to 400 cm⁻¹. The Y-axis is % Transmittance, from 0% to 100%.
- Identify Characteristic Peaks: Peaks in the spectrum correspond to the absorption of IR radiation by specific functional groups.
- Spectral Regions: Divide the spectrum into regions:
- Region 1 (4000-2500 cm⁻¹): Single bonds of N-H, C-H, and O-H.
- Region 2 (2500-2000 cm⁻¹): Triple bonds.
- Region 3 (2000-1500 cm⁻¹): Double bonds like C=O, C=N, and C=C.
- Region 4 (1500-400 cm⁻¹): Fingerprint region, complex vibrations.
- Functional Group Identification: Correlate peak positions with known functional group absorption frequencies to identify the chemical bonds present in the sample.
FTIR Testing Cost at UNS (PUI): Approximately IDR 80,000 without data interpretation.
4. Transmission Electron Microscopy (TEM)
Transmission Electron Microscopy (TEM) is similar in principle to a light microscope but uses electrons instead of light and electromagnets instead of glass lenses. TEM provides high-resolution images of the internal microstructure of materials in 2D.
How to Analyze TEM Images:
TEM data is typically presented as images. Scale bars are crucial for determining actual sizes. Figure below shows a TEM image of a Fe3O4/SiO2/TiO2 composite material.
TEM Image of Fe3O4/SiO2/TiO2 Composite
From the image, you can observe:
- Structure: The composite shows a multi-layered or core-shell structure. Fe3O4 is the core, surrounded by a SiO2 middle layer and a TiO2 outer shell.
- Particle Size: Use the scale bar to measure particle diameters. In the example provided in the original text, manual measurement using a ruler and the scale bar is demonstrated to estimate the real particle diameter.
TEM Testing Cost at UNS:
- Standard TEM: IDR 1,100,000 per sample
- + HRTEM: IDR 300,000 per sample
- + EDX Spectrum: IDR 100,000 per spectrum
- + Electron Diffraction: IDR 100,000 per image
- + Biological Sample Preparation: IDR 2,000,000 per sample
5. Thermal Gravimetric Analysis (TGA)
Thermal Gravimetric Analysis (TGA) measures the change in mass of a sample as a function of temperature or time under a controlled atmosphere. TGA is used to determine the thermal stability and composition of materials by observing mass loss due to decomposition, oxidation, or other thermal events.
How to Read TGA Graphs:
TGA results are displayed as graphs showing weight change (usually as a percentage of initial weight) versus temperature or time. Below is an example of a TGA graph for a carbon-containing sample.
(Unfortunately, the TGA graph image was not correctly loaded in the provided text, and thus cannot be included here. A generic TGA graph description will be provided instead.)
General TGA Graph Interpretation:
- Weight Loss Steps: The TGA curve will show steps where the sample loses weight. Each step corresponds to a thermal event, such as:
- Moisture Loss: Weight loss at lower temperatures (e.g., below 100-200°C) often indicates the evaporation of adsorbed water.
- Decomposition: Weight loss at higher temperatures indicates the decomposition of organic components or other thermally unstable materials.
- Oxidation: In an oxidizing atmosphere, weight gain might occur due to oxidation, followed by weight loss as oxides decompose at higher temperatures.
- Temperature of Events: The temperature at which weight loss or gain occurs is crucial for identifying the thermal stability of different components in the material.
- Residual Weight: The final weight at the end of the TGA experiment can indicate the amount of non-volatile residue or inorganic components in the sample.
Conclusion
Understanding material properties is fundamental to effective car part repair and diagnostics. Techniques like SEM, XRD, FTIR, TEM, and TGA provide invaluable insights into the microstructure, composition, and thermal behavior of materials used in automotive components. By learning how to interpret the results from these analyses, automotive professionals can gain a deeper understanding of material degradation, failure mechanisms, and optimize repair strategies for enhanced vehicle performance and longevity.
