Structural Analysis and Materials Characterisation - Essay Example

?Structural Analysis and Materials Characterisation 493225) Introduction Detection of cracks in machinery, buildings or various tools are significant so as to ascertain whether the crack or fracture that has been generated is due to a defective material or due to nature of the process that induced the crack. These cracks can be detected using light-optical or electro-optical microscopes. Since the surface where a fracture has occurred tends to be rough the image obtained by a light-optical microscope is not very clear. Light Optical Microscopy-Limitations The smallest gap r1 between two points that can be ascertained is called the resolution of a microscope. This usually depends on wavelength of the radiation. The beams that enter the lens form an image by overlapping on each diffraction pattern. As per the below diagram, Rayleigh found out that distinction between two points would be possible if maximum of the first diffraction pattern matched with the minimum or the beginning of the second diffraction pattern. Therefore the distance d1 indicated is inversely proportional to diameter of the lens opening. In brief the gap r1 is dependent on wavelength ?, refractive index of the medium µ and the angle formed by the beam ?. r1=d1/2=0.61 ?/ (µ sin?). Therefore a high resolution or a lower value of r1 can be obtained by a shorter wavelength, a higher refractive index of the traversing medium and a smaller distance to the specimen causing a larger value of (µsin?). When ordinary light-optical microscopes are used in air with refractive index=1, wavelengths of light being 400-700nm the maximum resolution that can be achieved is up to 200nm. Hence a magnification ratio above 1000 would be difficult to achieve. It is in these areas where a high amount of magnification is required that Electron microscopes prove the most beneficial. The Scanning Electron Microscope De Broglie’s relation describes the basic working principle of an electron microscope. The equation derived is ?= [1.5/ (V+ 10-6 V2)] 1/2 nm. Hence the wavelength can be adjusted by controlling the voltage of the electron beam. Electrons tend to get highly scattered in air and therefore a vacuum atmosphere needs to be maintained. Specimens also need to be made electrically conductive to avoid getting overcharged with electrons during testing. The diagram shows the main components of a Scanning Electron Microscope (SEM). These function in close cohesion in the running of seven prominent systems which are notably 1. Vacuum system- To prevent the scattering of electron beams a vacuum atmosphere is maintained to prevent dispersion. To achieve this two classes of pump are used. A low vacuum pump brings down the air pressure from atmosphere to 10-3 Torr and a high vacuum pump bring it further down from 10-3 Torr to 10-6 Torr. 2. Electron beam Generation system- This system produces the ‘illuminating’ or the primary electron beam for impingement on the sample. An electron gun generates the beam in a SEM. It is composed of a filament made of tungsten wire, Cerium Hexaboride or Lanthanum Hexaboride. A grid cap that directs the flow of electrons and a positively charged anode that accelerates the electrons onto the surface of the specimen. 3. Electron beam manipulation system- a system of lenses and coils control the shape, size and position of the electron beam to be directed on the sample surface. Electrostatic and magnetic fields control electron motion Electrostatic fields are found in the electron gun while magnetic field is present in the rest of the SEM. By passing electric current through a copper wire a magnetic field is made to form an electron microscope lens. A series of these lenses also known as condenser lens removes any kind of spherical aberration or astigmatism in the image. When the beam traverses the final condenser lens two sets of magnetic scanning coils move the beam thereby scanning in the X and Y direction in a raster pattern i.e the specimen is scanned from the upper left hand corner to the right corner after which it drops to the next line for the next scan. SEM thus derives its name by this scanning technique it follows. 4. Beam specimen interaction system- The interaction on the specimen generates different type of signals which can be detected. The electrons on specimens are typically accelerated in voltage ranges of 2 to 40kV. This interaction in a zone called the ‘interaction volume’ zone produces Secondary electrons (SE) and back scattered electrons (BSE). When primary beams are directed on specimens at angles less than 90? secondary electrons are produced but manage to escape from the surface layers. Its escape energy is limited to 50eV. Back scattered electrons escaping from the surface have an energy of >50eV. The higher the potential energy and smaller the atomic number of the specimen the BSE production is of a higher quantity. This results in greater resolution. 5. Detection System- Different detectors that are sensitive to emissions of different energy levels are used to detect emissions from sample. The nature of specimen topography forms the basis of most SEM’s. This information is provided by the secondary electrons. A 200 volt applied to the secondary electron detector attracts these secondary electrons and is further accelerated by a voltage of 10kV on to the scintillator. The secondary electrons hitting the scintillator release photons which travel through the light pipe onto the photomultiplier (PM). This amplifies the original signal. Every striking photon generates number of electrons and hence the amplification produced is high. The X rays released from the specimens are characterised to the nature of the material from which it is released. Background X rays on the other hand lose this characteristic after losing its energy on striking other particle. An EDS X ray detector collects the entire spectrum of x rays from 0eV to 30eV and is generally used when dealing with large concentration of x rays. However when wavelengths of smaller wavelengths need to be detected a WDS detector is used. This offers greater accuracy in dealing with more sensitive detection. 6. Signal Processing system- This processes the signal that has been generated and offers additional electronic modification of the image to enhance its quality. The amplifier in the detector amplifies the signals and is converts it into an image to be viewed on the cathode ray tube (CRT). The brightness control, contrast all form part of this processing system. 7. Display and recoding system- A system allows for the recording of the end results using the photographic medium. The well defined image on the CRT is transferred to a film by making certain adjustments in brightness and contrast. Specifications and Features There are number of companies that offer SEM machines. These have been listed in the last segment of this report. However an ideal SEM should cover the minimum requirements that have been listed below and form a part of the specifications of a Hitachi S-4800 model used at the Institute of Energy Technology, Norway. Specifications 1. Secondary electron image resolution: 1.0nm (at 15kV) and 2nm (at 1 kV) 2. Backscattered electron image resolution: 3.0 nm 3. Electron optics: Electron gun 4. Cold Field emission electron source Acceleration voltage: 0.5 ? 30kV (variable at 0.1 kV/step) 5. Magnification: x30 ? x 800,000 6. Detectors: Secondary electron detector, YAG BSE detector, Transmitted electron detector, Energy dispersive X-ray detector. 7. Specimen stage: PC controlled 5 axis motor drive. Transverse X: 0-110mm, Y: 0-110mm, Z: 1.5-40mm, R: 0-360?, T: -5 ? +70 degrees 8. Frame memory: 640 x 480 pixels, 1280 x 960 pixels, 2560 x 1920 pixels, 5120 x 3840 pixels. 9. Image file format: BMP, TIFF, JPEG Sample Preparation Guidelines and Accessories The following guidelines need to be followed in the preparation of a sample to be viewed through a SEM. 1. The size of the sample should be limited to 5-8mm in size. 2. The sample needs to be kept on a circular sample holder. 3. Biological samples dehydrate under vacuum thus affecting the performance of the microscope. These samples therefore need to be stabilized by freezing or chemical methods. Liquefied carbon dioxide can also be used to dry specimens without affecting its integrity. 4. Colloidal silver paste or graphite paste should be used in fixing the sample onto this holder. 5. Samples are to be arranged in a circular pattern to avoid wastage of time in the location of samples. This also facilitates easy viewing of specimen to specimen. 6. To make the samples electrically conducting the samples need to be coated with 20-50nm thick gold or silver using the sputter technique. In this method ionized argon gas under partial vacuum is accelerated to the coating metal. This displaces the metal ions which are then accelerated to the anode where the test sample has been placed. When the metal molecules strike the specimen these splash like paint and hence the name of the technique. Available Market Products There are a number of products that are available in the market. The kind of budget and the degree of accuracy one aspires in the usage of this SEM’s determines the product that is to be purchased. Some of the leaders in SEM’s are 1. CAMSCAN have a number of variants of SEM. ‘Apollo 300’ marketed by them is used for high resolution performances and operations upto 30kV. Magnification achieved is in the range of 10x to 2500,000x 2. FEI products have launched the ‘Quanta’ series of products operating in voltages of 200V to 30kV. Magnification achieved is of the range 20x to 1000000x. 3. Hitachi is also a leading player in SEM’s. Its specifications have been listed in the previous section. 4. ZEISS technologies have developed the ‘Merlin’ SEM which has a double condenser system and achieves an image resolution of 0.8nm 5. ASPEX Corporation has brought about the PSEM express desktop SEM which offers reliable solutions at great speed. It offers a image resolution of 25nm. It other variant called PSEM extreme offering higher resolution was conferred the 2008 R&D award for Scientific Innovation. 6. JOEL offers under its brand JSM-6510 a resolution of 3.0nm while achieving a magnification of 5x to 300,000. It also offers a whole range of products for sample preparation like the cross section polisher for sample too sensitive to mechanical polishing and the Ion slicer for preparing very thin films without the use of solvents or chemicals. Reference Lists 1. Boughton, 2002, Japans new dawn, Popular Science and Technology, Available at :http://www.popsci.com [Accessed 22 June 2009] 2. Boughton, 2002,’The Bretton woods proposal’, Political Science Quarterly,May 97, p.564 3. Fishman, R,2005,’The rise and fall of suburbia’,[e-book] Chester Press, Available through: Anglin University library, ,http://libweb.anglica.uk>, [Accessed on 5 june 2010] 4. Baron P, 2008, Business and Organiation, 1998, Bloomington university press 5. Tawancy M Hani et al, Quantitative WDS analysis using electron probe microanalyzer, Materials Characterisation 56v(2006), p.p 192-199 6. Scanning Electron Microscopy, Technical Faculty of Christian Albrechts University, n.d, [Accessed on 12th January 2010] 7. IFE’s scanning Electron Microscope (SEM), Materials and Corrosion Technology Department, Institute for Energy Technology, Available at www.ife.com, [Accessed on 10th January 2010] 8. Dunlap Michael & Adaskaveg, Introduction to the Scanning Electron Microscope, Facility for Advanced Instrumentation 1997, p.p 1-28 9. Jeol, 2009, Sems’s Available at ,http://www. Jeol.com>[Accsessed on 13th Jnauary 2010] 10. Dehe “ber 2008, p.p 3-7 11. Department of Trade and industry 1998, “Rethinking Construction”, Report of the Construction Task Force, Available at: http://www.constructingexcellence.org.uk , [Accessed 1st jan 2011] , p.p 3-6. 12. “Hughes Will & Maeda Yauyoshi”, “ Construction Contract Policy: Do we mean what we say?”, July 2002, p.6 13. “Housing Grants Construction and Regeneration Act 1996 (Part II) “, capital & Asset management, Building Contract Directive, April 2010, p.p 1-7 14. Report by the Comptroller and Auditor General, “Modernising Construction”, , January 2001, p.3 15. Tilbury Nadine, “Introduction to English Contract Law”, Bielefeld University, 2006, p.32 Read More