Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interactions when the incident electrons are decelerated in the solid sample. These signals include secondary electrons that produce SEM images , backscattered electrons BSE , diffracted backscattered electrons EBSD that are used to determine crystal structures and orientations of minerals , photons characteristic X-rays that are used for elemental analysis and continuum X-rays , visible light cathodoluminescence--CL , and heat.
Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples i. X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete ortitals shells of atoms in the sample.
As the excited electrons return to lower energy states, they yield X-rays that are of a fixed wavelength that is related to the difference in energy levels of electrons in different shells for a given element. Thus, characteristic X-rays are produced for each element in a mineral that is "excited" by the electron beam.
SEM analysis is considered to be "non-destructive"; that is, x-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyze the same materials repeatedly. The specific capabilities of a particular instrument are critically dependent on which detectors it accommodates. The SEM is routinely used to generate high-resolution images of shapes of objects SEI and to show spatial variations in chemical compositions: 1 acquiring elemental maps or spot chemical analyses using EDS , 2 discrimination of phases based on mean atomic number commonly related to relative density using BSE , and 3 compositional maps based on differences in trace element "activitors" typically transition metal and Rare Earth elements using CL.
Precise measurement of very small features and objects down to 50 nm in size is also accomplished using the SEM. Backescattered electron images BSE can be used for rapid discrimination of phases in multiphase samples. SEMs equipped with diffracted backscattered electron detectors EBSD can be used to examine microfabric and crystallographic orientation in many materials. There is arguably no other instrument with the breadth of applications in the study of solid materials that compares with the SEM.
The SEM is critical in all fields that require characterization of solid materials. While this contribution is most concerned with geological applications, it is important to note that these applications are a very small subset of the scientific and industrial applications that exist for this instrumentation.
Most SEM's are comparatively easy to operate, with user-friendly "intuitive" interfaces. Many applications require minimal sample preparation. Modern SEMs generate data in digital formats, which are highly portable. Samples must be solid and they must fit into the microscope chamber.
Maximum size in horizontal dimensions is usually on the order of 10 cm, vertical dimensions are generally much more limited and rarely exceed 40 mm. For most instruments samples must be stable in a vacuum on the order of 10 -5 - 10 -6 torr. Samples likely to outgas at low pressures rocks saturated with hydrocarbons, "wet" samples such as coal, organic materials or swelling clays, and samples likely to decrepitate at low pressure are unsuitable for examination in conventional SEM's.
However, "low vacuum" and "environmental" SEMs also exist, and many of these types of samples can be successfully examined in these specialized instruments. Most SEMs use a solid state x-ray detector EDS , and while these detectors are very fast and easy to utilize, they have relatively poor energy resolution and sensitivity to elements present in low abundances when compared to wavelength dispersive x-ray detectors WDS on most electron probe microanalyzers EPMA.
An electrically conductive coating must be applied to electrically insulating samples for study in conventional SEM's, unless the instrument is capable of operation in a low vacuum mode. Sample preparation can be minimal or elaborate for SEM analysis, depending on the nature of the samples and the data required. Minimal preparation includes acquisition of a sample that will fit into the SEM chamber and some accommodation to prevent charge build-up on electrically insulating samples.
Most electrically insulating samples are coated with a thin layer of conducting material, commonly carbon, gold, or some other metal or alloy. The choice of material for conductive coatings depends on the data to be acquired: carbon is most desirable if elemental analysis is a priority, while metal coatings are most effective for high resolution electron imaging applications. Desktop SEM.
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Don't have an account? Create Account. Sign in Quick Order. Search Thermo Fisher Scientific. Search All. Electron Microscopy Scanning Electron Microscopes Scanning electron microscopes for a wide range of surface analysis needs.
Scanning electron microscopes Since the introduction of electron microscopes in the s, scanning electron microscopy SEM has developed into a critical tool within numerous different research fields, spanning everything from materials science, to forensics, to industrial manufacturing, and even to the life sciences. Quick comparison of floor-model scanning electron microscopes. High-resolutions scanning electron microscopes.
High-resolutions scanning electron microscopes If you would like to maximize resolution and contrast, look no further than our revolutionary Verios and Apreo SEMs. Entry-level and environmental floor-model scanning electron microscopes. Desktop scanning electron microscopes. Desktop scanning electron microscopes The Thermo Scientific Phenom product line redefines speed, ease-of-use and performance thanks to its innovative, user-friendly design and software, which allows even novice users to obtain an SEM image in minutes.
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