Microscopic analysis

The properties of a material depend on its chemical composition, phase composition, micro and nanostructures. The phase composition and microstructure characteristics play a key role in determining the properties of materials with similar chemical compositions. Thus it is essential to gain an understanding of the phase composition, microstructure or nanostructure of materials, chemicals or products through microscopic analysis. There are three main microscopic techniques that are used: Optical microscopy can be used to analyze and determine optical parameters of various crystals such as refractive index, maximum birefringence and brightness. It also measures various special phenomena such as cleavage, color, etc. Electron microscopy involves the diffraction, reflection, or refraction of electromagnetic radiation/electron beams interacting with the specimen, and the collection of the scattered radiation or another signal in order to create an image. Scanning probe microscopy involves the interaction of a scanning probe with the surface of the object of interest.

Optical microscopy

Optical microscopy refers to the observation of the surface topography and internal structure of an object using visible light to identify the optical properties of the crystal. Observation of the transparent crystal can be performed using a transmission microscope such as a polarized light microscope. Reflective microscope or metallographic microscopy is used for opaque objects. The use of polarized light microscopy and metallographic microscopy for crystals identification is one of the important material analysis methods.

The morphology, crystal shape and degree of self-shape of the mineral can be observed under a single polarized light using an optical microscope. Observe the cleavage and cleavage angle of minerals, the color and the phenomenon of pleochroism absorption; determine the approximate range of mineral refractive index according to the contour, the roughness of surface. Interferograms of minerals can be used to determine the axial and optical properties of the mineral. Metallographic microscope is primarily used to identify and analyze the structure of various metals, alloys and non-metallic materials. Its industrial applications include: Observe surface characteristics of opaque materials, chips, printed circuit boards, liquid crystal panels, fibers, coatings and other non-metallic materials; Study and analyze some surface conditions; Test the performance of some products, such as mechanical properties, product defects, provide advice for production, and help improve processes.

Scanning electron microscopy (SEM)

Scanning electron microscopy uses a finely focused electron beam to scan point-by-point on the surface of sample, interacting with the sample to produce various physical signals that are received by the detector, amplified and converted into a modulated signal, and finally displayed on the screen to reflect various features of the sample surface.

SEM has the characteristics of large depth of field, strong stereoscopic image, large magnification range, continuous adjustable, high resolution, large sample chamber space and simple sample preparation. It is an effective analytical tool for sample surface research. Other instruments can be assembled simultaneously in the SEM, such as spectrometer, energy spectrometer, electron backscattering diffractometer, etc. Achieve the simultaneous analysis of the surface morphology, micro-area composition, phase structure and other aspects of the sample.

The application of SEM in materials science includes surface morphology and fracture morphology analysis, in-situ observation of the surface morphology. Because of its large depth of field, it is widely used in the characterization of various fracture morphology, with clear imaging and strong stereoscopic effect.

Transmission electron microscopy (TEM)

Transmission electron microscope is similar to the optical microscope in imaging principle. The basic imaging process is as follows: The anode emits electrons, and the electrons are accelerated to act on the sample, amplified by the objective lens, the intermediate mirror and the projection mirror, and finally imaged in the phosphor screen.

The preparation of the sample is critical to the function of the electron microscope. The appropriate preparation method must be selected according to the requirements of different instruments and the characteristics of the samples. A 50-200 KV electron beam is commonly used in TEM, and the thickness of the sample should be controlled at about 100-200 nm.

Atomic force microscopy (AFM)

AFM relies on the force of the probe and the surface of the sample to perform the imaging process. Since the forces between atoms are ubiquitous, atomic force microscopy can not only detect conductors, but also biological structures such as biomaterials and polymer materials that are not conductive. AFM can also be operated in gas, liquid, high and low temperatures, vacuum or controlled atmosphere for a wide range of applications.

The main working modes of AFM are contact mode, non-contact mode and tapping mode. Surface morphology, surface remodeling, surface electronic states, and charge density of metals and semiconductors can be studied through AFM analysis. In the microscopic field, research on friction, lubrication, wear and tribochemical reactions at the nanoscale can be performed. In the field of electrochemistry, in-situ electrochemical studies can be carried out, and the characterization and kinetics of interface structures can be studied. Applications in polymer molecules, LB membranes, and biological samples are also common.

Electron Probe Microanalysis

A finely-aggregated electron beam is an incident on the surface to excite characteristic X-rays of the element, and the wavelength of X-ray is analyzed to determine the type of element in the sample (qualitative analysis); analyze the intensity of X-rays and calculate the amount of corresponding elements in the sample (quantitative analysis). Electron probe analysis has three basic modes of operation. Point analysis is used for full-spectrum qualitative analysis or quantitative analysis of selected points. Line analysis is used to display the concentration change of elements along the selected line. Surface analysis is used to observe elements concentration distribution in the selected microdomain.

Through the combination of the electron probe with scanning electron microscopy and transmission electron microscopy, researchers can analyze the chemical composition of the microdomain while imaging the microstructure of the microdomain.

X-ray diffractometry (XRD)

X-rays are electromagnetic waves of very high frequency. X-ray diffraction, as electromagnetic waves, is projected into the crystal and scattered by atoms inside the crystal. The atoms are periodically arranged. There is a fixed phase relationship between the scattered spherical waves. The spherical waves in some scattering directions get reinforced and get cancelled in some other directions, so that diffraction occurs.

The arrangement of atoms inside each crystal is unique, so the corresponding diffraction pattern is unique, similar to human fingerprints, so this technique can be used for phase analysis. Among them, the distribution of diffraction lines in the diffraction pattern is determined by the size, shape and orientation of the unit cell, and the intensity of the diffraction line is determined by the type of atoms and their position in the unit cell.

The main applications of XRD technology are as follows: Phase identification or quantitative analysis of the crystal structure, crystal perfection, crystalline or amorphous state; Precise determination of the lattice parameters can be used to study the thermal expansion coefficient of the substance, the type and composition of the solid solution, solid phase solubility curve, macroscopic stress, analysis of chemical heat treatment layer, supersaturated solid solution decomposition process, etc.

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