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Encyclopedia > Electron Microscope

An electron microscope is a type of microscope that uses electrons as a way to illuminate and create an image of a specimen. It has much higher magnification and resolving power than a light microscope, with magnifications up to about two million times, compared to about two thousand that can be achieved with visible light. Robert Hookes microscope (1665) - an engineered device used to study living systems. ... For other uses, see Electron (disambiguation). ... Magnification is the process of enlarging something only in appearance, not physical size. ... Resolving power is the ability of a microscope or telescope to measure the angular separation of images that are close together. ... A 1879 Carl Zeiss Jena Optical microscope. ...


This high magnification allows electron microscopes to see smaller objects and greater detail in these objects. Unlike a light microscope, which uses glass lenses to focus light, the electron microscope uses electrostatic and electromagnetic lenses to control the illumination and imaging of the specimen.

Contents

History

An old model transmission electron microscope.
An old model transmission electron microscope.
An image of an ant from a scanning electron microscope
An image of an ant from a scanning electron microscope

The first electron microscope prototype was built in 1931 by the German engineers Ernst Ruska and Max Knoll.[1] It was based on the ideas and discoveries of French physicist Louis de Broglie. Although it was primitive and not fit for practical use, the instrument was still capable of magnifying objects by four hundred times. Download high resolution version (290x1212, 65 KB) Wikipedia does not have an article with this exact name. ... A photo of an ant head taken with a Scanning Electron Microscope [1] File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... A photo of an ant head taken with a Scanning Electron Microscope [1] File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... For other uses, see Ant (disambiguation). ... Year 1931 (MCMXXXI) was a common year starting on Thursday (link will display full 1931 calendar) of the Gregorian calendar. ... Ernst August Friedrich Ruska (December 25, 1906–May 25, 1988) was a German physicist. ... Max Knoll (1897-1969) was a German inventor. ... Louis-Victor-Pierre-Raymond, 7th duc de Broglie, generally known as Louis de Broglie (August 15, 1892–March 19, 1987), was a French physicist and Nobel Prize laureate. ...


Reinhold Rudenberg, the research director of Siemens, had patented the electron microscope in 1931, although Siemens was doing no research on electron microscopes at that time. In 1937 Siemens began funding Ruska and Bodo von Borries to develop an electron microscope. Siemens also employed Ruska's brother Helmut to work on applications, particularly with biological specimens.[2][3] Siemens redirects here. ... Helmut Ruska (June 7, 1908 - August 30, 1973) was a German physician and biologist from Heidelberg. ...


In the same decade of 1930s Manfred von Ardenne pioneered the scanning electron microscope and his universal electron microscope.[4] Manfred von Ardenne (January 20, 1907 - May 26, 1997) was a German inventor. ... SEM Cambridge S150 at Geological Institute, University Kiel, 1980 SEM opened sample chamber The scanning electron microscope (SEM) is a type of electron microscope capable of producing high-resolution images of a sample surface. ...


Siemens produced the first commercial TEM in 1939, but the first practical electron microscope had been built at the University of Toronto in 1938, by Eli Franklin Burton and students Cecil Hall, James Hillier and Albert Prebus.[5] The University of Toronto (U of T) is a public research university in the city of Toronto, Ontario, Canada. ... Year 1938 (MCMXXXVIII) was a common year starting on Saturday (link will display the full calendar) of the Gregorian calendar. ... Eli Franklin Burton, FRSC born February 14, 1879, died July 6, 1948, was a physicist. ... James Hillier) James Hillier OC, Ph. ...


Although modern electron microscopes can magnify objects up to two million times, they are still based upon Ruska's prototype. The electron microscope is an integral part of many laboratories. Researchers use it to examine biological materials (such as microorganisms and cells), a variety of large molecules, medical biopsy samples, metals and crystalline structures, and the characteristics of various surfaces. The electron microscope is also used extensively for inspection, quality assurance and failure analysis applications in industry, including, in particular, semiconductor device fabrication. For other uses, see Prototype (disambiguation). ... A microorganism or microbe is an organism that is so small that it is microscopic (invisible to the naked eye). ... Drawing of the structure of cork as it appeared under the microscope to Robert Hooke from Micrographia which is the origin of the word cell being used to describe the smallest unit of a living organism Cells in culture, stained for keratin (red) and DNA (green) The cell is the... In science, a molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ... Brain biopsy A biopsy (in Greek: bios = life and opsy = look/appearance) is a medical test involving the removal of cells or tissues for examination. ... For alternative meanings see metal (disambiguation). ... Crystal (disambiguation) Insulin crystals A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... It has been suggested that Wafer fabrication be merged into this article or section. ...


Electron microscope manufacturers

Major manufacturers include:

FEI Company (NASDAQ: FEIC), founded in 1971, is a supplier of electron microscopy tools to researchers, developers and manufacturers working on the nanoscale. ... Philips HQ in Amsterdam Koninklijke Philips Electronics N.V. (Royal Philips Electronics N.V.), usually known as Philips, (Euronext: PHIA, NYSE: PHG) is one of the largest electronics companies in the world, founded and headquartered in the Netherlands. ... It has been suggested that Hitachi Works be merged into this article or section. ... A manufacturer of scanning electron microscopes, transmission electron microscopes, electron microprobes, electron beam lithography systems. ... TESCAN, s. ... Carl Zeiss in middle age. ...

Types

Transmission Electron Microscope (TEM)

The original form of electron microscopy, Transmission electron microscopy (TEM) involves a high voltage electron beam emitted by a cathode, usually a tungsten filament and focused by electrostatic and electromagnetic lenses. The electron beam that has been transmitted through a specimen that is in part transparent to electrons carries information about the inner structure of the specimen in the electron beam that reaches the imaging system of the microscope. The spatial variation in this information (the "image") is then magnified by a series of electromagnetic lenses until it is recorded by hitting a fluorescent screen, photographic plate, or light sensitive sensor such as a CCD (charge-coupled device) camera. The image detected by the CCD may be displayed in real time on a monitor or computer. A section of a cell of Bacillus subtilis, taken with a Tecnai T-12 TEM. The scale bar is 200nm. ... A section of a cell of Bacillus subtilis, taken with a Tecnai T-12 TEM. The scale bar is 200nm. ... International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ... Diagram of a copper cathode in a Daniells cell. ... Electromagnetism is the physics of the electromagnetic field: a field, encompassing all of space, composed of the electric field and the magnetic field. ... A specially developed CCD used for ultraviolet imaging in a wire bonded package. ...


Resolution of the TEM is limited primarily by spherical aberration, but a new generation of aberration correctors have been able to partially overcome spherical aberration to increase resolution. Software correction of spherical aberration for the High Resolution TEM HRTEM has allowed the production of images with sufficient resolution to show carbon atoms in diamond separated by only 0.89 ångström (89 picometers) and atoms in silicon at 0.78 ångström (78 picometers)[6][7] at magnifications of 50 million times.[8] The ability to determine the positions of atoms within materials has made the HRTEM an important tool for nano-technologies research and development. Focal plane Longitudinal sections In optics, spherical aberration is an image imperfection that occurs due to the increased refraction of light rays that occurs when rays strike a lens or mirror near its edge, in comparison with those that strike nearer the center. ... High Resolution Transmission Electron Microscopy (HRTEM) is an imaging mode of the transmission electron microscope (TEM) that allows the imaging of the crystallographic structure of a sample at an atomic scale. ... An Ã¥ngström or aangstroem (the official transliteration), or angstrom (symbol Ã…) is a non-SI unit of length that is internationally recognized, equal to 0. ... Picometre (American spelling: picometer) is an SI measure of length that is equal to 10−12 of a metre. ...


Scanning Electron Microscope (SEM)

Unlike the TEM, where electrons of the high voltage beam form the image of the specimen, the Scanning Electron Microscope (SEM)[9] produces images by detecting low energy secondary electrons which are emitted from the surface of the specimen due to excitation by the primary electron beam. In the SEM, the electron beam is rastered across the sample, with detectors building up an image by mapping the detected signals with beam position. SEM Cambridge S150 at Geological Institute, University Kiel, 1980 SEM opened sample chamber The scanning electron microscope (SEM) is a type of electron microscope capable of producing high-resolution images of a sample surface. ... SEM Cambridge S150 at Geological Institute, University Kiel, 1980 SEM opened sample chamber The scanning electron microscope (SEM) is a type of electron microscope capable of producing high-resolution images of a sample surface. ...


Generally, the TEM resolution is about an order of magnitude greater than the SEM resolution, however, because the SEM image relies on surface processes rather than transmission it is able to image bulk samples and has a much greater depth of view, and so can produce images that are a good representation of the 3D structure of the sample.


Reflection Electron Microscope (REM)

In addition there is a Reflection Electron Microscope (REM). Like TEM, this technique involves electron beams incident on a surface, but instead of using the transmission (TEM) or secondary electrons (SEM), the reflected beam is detected. This technique is typically coupled with Reflection High Energy Electron Diffraction and Reflection high-energy loss spectrum (RHELS). Another variation is Spin-Polarized Low-Energy Electron Microscopy (SPLEEM), which is used for looking at the microstructure of magnetic domains.[10] RHEED stands for Reflection High Energy Electron diffraction. ... Ferromagnetism is a phenomenon by which a material can exhibit a spontaneous magnetization, and is one of the strongest forms of magnetism. ...


Scanning Transmission Electron Microscope (STEM)

The STEM combines the high resolution of the TEM with the beam rastering functions of the SEM, allowing a range of analytical techniques to be used that are not possible with conventional TEM. A scanning transmission electron microscope (STEM) is a type of transmission electron microscope. ...


Sample Preparation

An insect coated in gold, having been prepared for viewing with a scanning electron microscope.
An insect coated in gold, having been prepared for viewing with a scanning electron microscope.

Materials to be viewed under an electron microscope may require processing to produce a suitable sample. The technique required varies depending on the specimen and the analysis required: Image File history File links Metadata Size of this preview: 496 × 600 pixelsFull resolution (1859 × 2248 pixel, file size: 698 KB, MIME type: image/jpeg) Photo credit: Peter Halasz. ... Image File history File links Metadata Size of this preview: 496 × 600 pixelsFull resolution (1859 × 2248 pixel, file size: 698 KB, MIME type: image/jpeg) Photo credit: Peter Halasz. ...

  • Cryofixation – freezing a specimen so rapidly, to liquid nitrogen or even liquid helium temperatures, that the water forms vitreous (non-crystalline) ice. This preserves the specimen in a snapshot of its solution state. An entire field called cryo-electron microscopy has branched from this technique. With the development of cryo-electron microscopy of vitreous sections (CEMOVIS), it is now possible to observe virtually any biological specimen close to its native state.
  • Dehydration – replacing water with organic solvents such as ethanol or acetone.
  • Embedding – infiltration of the tissue with a resin such as araldite or epoxy for sectioning. After this embedding process begins, the specimen must be polished to a mirror-like finish using ultra-fine abrasives. The polishing process must be done accordingly, or it may lead to scratches imposing on the image quality.
  • Sectioning – produces thin slices of specimen, semitransparent to electrons. These can be cut on an ultramicrotome with a diamond knife to produce very thin slices. Glass knives are also used because they can be made in the lab and are much cheaper.
  • Staining – uses heavy metals such as lead, uranium or tungsten to scatter imaging electrons and thus give contrast between different structures, since many (especially biological) materials are nearly "transparent" to electrons (weak phase objects). In biology, specimens are usually stained "en bloc" before embedding and also later stained directly after sectioning by brief exposure to aqueous (or alcoholic) solutions of the heavy metal stains.
  • Freeze-fracture or freeze-etch – a preparation method particularly useful for examining lipid membranes and their incorporated proteins in "face on" view. The fresh tissue or cell suspension is frozen rapidly (cryofixed), then fractured by simply breaking or by using a microtome while maintained at liquid nitrogen temperature. The cold fractured surface (sometimes "etched" by increasing the temperature to about -100°C for several minutes to let some ice sublime) is then shadowed with evaporated platinum or gold at an average angle of 45° in a high vacuum evaporator. A second coat of carbon, evaporated perpendicular to the average surface plane is often performed to improve stability of the replica coating. The specimen is returned to room temperature and pressure, then the extremely fragile "pre-shadowed" metal replica of the fracture surface is released from the underlying biological material by careful chemical digestion with acids, hypochlorite solution or SDS detergent. The still-floating replica is thoroughly washed from residual chemicals, carefully fished up on EM grids, dried then viewed in the TEM.
  • Ion Beam Milling – thins samples until they are transparent to electrons by firing ions (typically argon) at the surface from an angle and sputtering material from the surface. A subclass of this is Focused ion beam milling, where gallium ions are used to produce an electron transparent membrane in a specific region of the sample, for example through a device within a microprocessor. Ion beam milling may also be used for cross-section polishing prior to SEM analysis of materials that are difficult to prepare using mechanical polishing.
  • Conductive Coating – An ultrathin coating of electrically-conducting material, deposited either by high vacuum evaporation or by low vacuum sputter coating of the sample. This is done to prevent the accumulation of static electric fields at the specimen due to the electron irradiation required during imaging. Such coatings include gold, gold/palladium, platinum, tungsten, graphite etc. and are especially important for the study of specimens with the scanning electron microscope. Another reason for coating, even when there is more than enough conductivity, is to improve contrast, a situation more common with the operation of a FESEM (field emission SEM). When an osmium coater is used, a layer far thinner than would be possible with any of the previously mentioned sputtered coatings is possible.[1]

Everyday ice is a crystal, which means its molecules are lined up in a repeating pattern. ... Cryo-electron microscopy (sometimes called cryoEM or electron cryomicroscopy) is a form of electron microscopy (EM) where the sample is studied at cryogenic temperatures (generally liquid nitrogen temperatures). ... Cryo-electron microscopy (sometimes called cryoEM or electron cryomicroscopy) is a form of electron microscopy (EM) where the sample is studied at cryogenic temperatures (generally liquid nitrogen temperatures). ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... Grain alcohol redirects here. ... The chemical compound acetone (also known as propanone, dimethyl ketone, 2-propanone, propan-2-one and β-ketopropane) is the simplest representative of the ketones. ... This article does not cite any references or sources. ... Araldite is a registered trademark of Huntsman Advanced Materials (previously part of Ciba) referring to their range of engineering and structural epoxy, acrylic, and polyurethane adhesives. ... In chemistry, epoxy or polyepoxide is a thermosetting epoxide polymer that cures (polymerizes and crosslinks) when mixed with a catalyzing agent or hardener. Most common epoxy resins are produced from a reaction between epichlorohydrin and bisphenol-A. The first commercial attempts to prepare resins from epichlorohydrin occurred in 1927 in... A microtome is a mechanical instrument used to cut very thin slices for microscopic examination. ... This article is about the mineral. ... In Electron Microscopy glass knives are used to make the ultrathin sections needed for imaging. ... This article is about the metal. ... General Name, symbol, number uranium, U, 92 Chemical series actinides Group, period, block n/a, 7, f Appearance silvery gray metallic; corrodes to a spalling black oxide coat in air Standard atomic weight 238. ... For other uses, see Tungsten (disambiguation). ... ... General Name, symbol, number argon, Ar, 18 Chemical series noble gases Group, period, block 18, 3, p Appearance colorless Standard atomic weight 39. ... Focused ion beam, also known as FIB, is a scientific instrument that resembles a scanning electron microscope. ... General Name, Symbol, Number gallium, Ga, 31 Chemical series poor metals Group, Period, Block 13, 4, p Appearance silvery white   Standard atomic weight 69. ...

Disadvantages

Pseudocolored SEM image of the feeding basket of Antarctic krill. Real electron microscope images do not carry any color information, they are greyscale. The first degree filter setae carry in v-form two rows of second degree setae, pointing towards the inside of the feeding basket. The purple ball is one micrometer in diameter. To display the total area of this structure one would have to tile 7500 times this image.

Electron microscopes are expensive to buy and maintain. They are dynamic rather than static in their operation: requiring extremely stable high-voltage supplies, extremely stable currents to each electromagnetic coil/lens, continuously-pumped high-/ultra-high-vacuum systems, and a cooling water supply circulation through the lenses and pumps. As they are very sensitive to vibration and external magnetic fields, microscopes aimed at achieving high resolutions must be housed in buildings (sometimes underground) with special services. Newer generations of TEM operating at lower voltages (around 5 kV) do not have stringent voltage supply, lens coil current, cooling water or vibration isolation requirements and as such are much less expensive to buy and far easier to install and maintain. krill filter (Photo by Gerd Alberti & Uwe Kils) GFDL larger images and links http://www. ... krill filter (Photo by Gerd Alberti & Uwe Kils) GFDL larger images and links http://www. ... Binomial name Dana, 1850 Antarctic krill are eaten by penguins(Euphausia superba) is a species of krill found in the Antarctic waters of the Southern Ocean. ... In computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample. ... A seta is a stiff hair, bristle, or bristle-like process or part of an organism. ...


The samples have to be viewed in vacuum, as the molecules that make up air would scatter the electrons. Recent advances have allowed hydrated samples to be imaged using an environmental scanning electron microscope Look up Vacuum in Wiktionary, the free dictionary. ...


Scanning electron microscopes usually image conductive or semi-conductive materials best. Non-conductive materials can be imaged by an environmental scanning electron microscope. A common preparation technique is to coat the sample with a several-nanometer layer of conductive material, such as gold, from a sputtering machine; however this process has the potential to disturb delicate samples. GOLD refers to one of the following: GOLD (IEEE) is an IEEE program designed to garner more student members at the university level (Graduates of the Last Decade). ...


The samples have to be prepared in many ways to give proper detail, which may result in artifacts purely the result of treatment. This gives the problem of distinguishing artifacts from material, particularly in biological samples. Scientists maintain that the results from various preparation techniques have been compared, and as there is no reason that they should all produce similar artifacts, it is therefore reasonable to believe that electron microscopy features correlate with living cells. In addition, higher-resolution work has been directly compared to results from X-ray crystallography, providing independent confirmation of the validity of this technique. Recent work performed on unfixated, vitrified specimens has also been performed, further confirming the validity of this technique. In natural science and signal processing, an artifact is any perceived distortion or other data error caused by the instrument of observation. ... Biology studies the variety of life (clockwise from top-left) E. coli, tree fern, gazelle, Goliath beetle Biology (from Greek: βίος, bio, life; and λόγος, logos, knowledge), also referred to as the biological sciences, is the study of living organisms utilizing the scientific method. ... X-ray crystallography, also known as single-crystal X-ray diffraction, is the oldest and most common crystallographic method for determining the structure of molecules. ...


Electron Microscopy Application areas

Semiconductor & Data Storage

Biology & Life Sciences Failure analysis is the process of determining the cause of failure, collecting and analyzing data, and developing conclusions to eliminate the failure mechanism causing specific device or system failures. ...

Research Cryobiology is the study of living organisms, organs, biological tissues or biological cells at low temperatures. ... Protein targeting or protein sorting is the mechanisms by which a cell transports proteins to the appropriate positions in the cell or outside of it. ... Electron Tomography (ET) is a tomography technique for obtained detailed 3D structures of macromolecular objects. ... Cryo-electron microscopy (sometimes called cryoEM or electron cryomicroscopy) is a form of electron microscopy (EM) where the sample is studied at cryogenic temperatures (generally liquid nitrogen temperatures). ... Toxicology (from the Greek words toxicos and logos [1]) is the study of the adverse effects of chemicals on living organisms [2]. It is the study of symptoms, mechanisms, treatments and detection of poisoning, especially the poisoning of people. ... Viral load is a measure of the severity of a viral infection, and can be estimated by calculating the amount of virus in an involved body fluid, e. ... Virology, often considered a part of microbiology or of pathology, is the study of organic viruses: their structure and classification, their ways to infect and exploit cells to reproduce and cause disease, the techniques to isolate and culture them, and their potential uses in research and therapy. ... A vitrification experiment for the study of nuclear waste disposal at Pacific Northwest National Labs Vitrification is a process of converting a material into a glass-like amorphous solid which is free of any crystalline structure, either by the quick removal or addition of heat, or by mixing with an...

  • Materials Qualification
  • Materials and Sample Preparation
  • Nanoprototyping
  • Nanometrology
  • Device Testing and Characterization

Industry Nanometrology is the science of measurement at the nanoscale level. ...

  • High-Resolution Imaging
  • 2D & 3D Micro- Characterization
  • Macro Sample to Nanometer Metrology
  • Particle Detection and Characterization
  • Direct Beam-Writing Fabrication
  • Dynamic Materials Experiments
  • Sample Preparation
  • Forensics
  • Mining (Mineral Liberation Analysis)
  • Chemical/Petrochemical

The word forensic (from Latin: forensis - forum) refers to something of, pertaining to, or used in a court of law. ... Chuquicamata, the second largest open pit copper mine in the world, Chile. ... A chemical substance is any material substance used in or obtained by a process in chemistry: A chemical compound is a substance consisting of two or more chemical elements that are chemically combined in fixed proportions. ... Petrochemicals are chemical products made from raw materials of petroleum (hydrocarbon) origin. ...

See also

Wikibooks
Wikibooks' [[wikibooks:|]] has more about this subject:
The Opensource Handbook of Nanoscience and Nanotechnology

Image File history File links Wikibooks-logo-en. ... Field emission microscopy (FEM) is an analytical technique used in materials science. ...

References

  1. ^ Ernst Ruska Nobel Prize autobiography
  2. ^ Ernst Ruska (1986). Ernst Ruska Autobiography (English). Nobel Foundation. Retrieved on 2007-02-06.
  3. ^ DH Kruger, P Schneck and HR Gelderblom (13). "Helmut Ruska and the visualisation of viruses" (in English). The Lancet 355 (9216): 1713-1717. doi:10.1016/S0140-6736(00)02250-9. 
  4. ^ M von Ardenne and D Beischer (1940). "Untersuchung von metalloxud-rauchen mit dem universal-elektronenmikroskop" (in German). Zeitschrift Electrochemie 46: 270-277. 
  5. ^ MIT biography of Hillier
  6. ^ OÅM: World-Record Resolution at 0.78 Å, (May 18, 2001) Berkeley Lab Currents.
  7. ^ P. D. Nellist, M. F. Chisholm, N. Dellby, O. L. Krivanek, M. F. Murfitt, Z. S. Szilagyi, A. R. Lupini, A. Borisevich, W. H. Sides, Jr., S. J. Pennycook (17). "Direct Sub-Angstrom Imaging of a Crystal Lattice" (in English). Science 305 (5691): 1741. doi:10.1126/science.1100965. 
  8. ^ The Scale of Things, DOE Office of Basic Energy Sciences (BES).
  9. ^ SCANNING ELECTRON MICROSCOPY 1928 - 1965
  10. ^ http://ncem.lbl.gov/frames/spleem.htm

Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 37th day of the year in the Gregorian calendar. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ...

External links


  Results from FactBites:
 
Electron microscope - Wikipedia, the free encyclopedia (1806 words)
The electron microscope is a type of microscope, which differs from a normal microscope due to it's immense magnification or resolving power of small objects or details through the use of electrons.
The first practical electron microscope was built at the University of Toronto in 1938, by Eli Franklin Burton and students Cecil Hall, James Hillier and Albert Prebus.
Unlike the TEM, where electrons are detected by beam transmission, the Scanning Electron Microscope (SEM) produces images by detecting secondary electrons which are emitted from the surface due to excitation by the primary electron beam.
Electron Microscope - MSN Encarta (818 words)
Electron microscopes are able to obtain much higher powers of magnification than standard visible light microscopes because electrons have much shorter wavelengths associated with them than light waves.
The vessels that house electron microscopes must be evacuated to very high vacuum to prevent the scattering of the electrons off of air molecules.
Electron microscopes have given scientific and lay media remarkable pictures, such as the “faces” of insects, the shapes of microscopic organisms, and the surface structure of molecules of new, high-tech alloys and other substances.
  More results at FactBites »

 
 

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