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Working Principles of several Instruments used in Experimental Condensed Matter Physics
Field Emission Scanning Electron Microscope
Working Principle: A series of electromagnetic coils are used as lenses to focus and manipulate the electron beam. The electrons generated by a field emission source are accelerated in a field gradient. The high energetic electron beam passes through electromagnetic lenses, focusing on the specimen. The electron beam is scanned over the specimen surface by deflection coils. As a result, there is an interaction of electrons with the atoms that make up the sample and produces signals that contain information about the samples surface topography and morphology.
The type of signals produced includes secondary electrons (ejected by low energy electron striking), backscattered electrons (ejected by high energy electron striking) and photons. The secondary electron is emitted from one of the orbitals of the incident atom and has energy less than 50 eV. As the energy of secondary electron is less, it is responsible for the topological contrast which provides information mainly about the surface morphology. The backscattered electrons are the high energy beam electrons (incident electrons) that are elastically scattered from the sample. For imaging sample in the FE-SEM, surface of the sample must be electrically conductive, otherwise there would be accumulation of charges at the surface and images become blurred. Therefore, surface of the insulating samples are usually coated with an ultrathin coating of electrically conducting material (graphite or gold or platinum) to make the sample conducting.
Vibrating Sample Magnetometer:
Working Principle: In this magnetic measurement instrument, a sample to be studied is tightly bound in a Teflon tape and placed inside the uniform magnetic eld. According to Faraday's law of induction, the induced electromotive force (e.m.f.)/voltage is given by the rate of change of magnetic flux.
The VSM uses uses a linear motor to vibrate the sample sinusoidally at 40 Hz with a completely automated DC magnetometer and self-adjusting centering operation. The magnetization measurement is executed by oscillating the sample near a detection (gradiometer pick-up) coil and simultaneously the voltage induced is detected from the pick-up coils based on Faradays law of induction. The voltage induced by the pick-up coil is given by
Vemf = 2pifCmASin(2pift)
This induced voltage due to time-varying magnetic flux is amplied and lock-in detected in the VSM detection module.
Spark Plasma Sintering:
Working Principle: Spark plasma sintering (SPS) is an ecient non-equilibrium technique that enables sintering of bulk materials from powder using a rapid heating rate with short holding time and at low sintering temperature compared to most of the conventional sintering techniques. The schematic diagram of SPS is shown in Fig. In SPS process, a high electric-pulsed direct current (DC) is applied across the graphite die with simultaneous axial pressure, which leads to the generation of plasma due to the electrical discharge in the gaps between the powder particles placed inside the chamber in a graphite die. This spark discharge lead to the elimination of adsorptive gases or impurities present on the surface of the sample. This removes the formation of oxide lms on the sample surface.
This improves the thermal diffusivity of the sintered sample. In this process, densification process is enhanced owing to Joule's heating and plastic deformation effects. In SPS process, grain growth is restricted due to rapid heating compared to hot pressing and green sintering approaches.
X-ray diffraction: X-ray diffraction (XRD) method is an analytical technique which gives the detailed information about the structural properties such as lattice parameters, crystallite size, space group, lattice strain, preferred orientation, phase composition, etc.
Working Principle: A collimated beam of X-ray with a single wavelength is incident on a powder sample which is composed of a large number of crystallites that have random orientations and hence all possible orientations of the crystals are simultaneously present. If the magnitude of the atomic spacing of the specimen is comparable to the wavelength of the X-ray, the X-rays are scattered in random directions. At a specified angle, X-rays reflected from a particular set of the crystallographic plane give a diffraction pattern. However, the only central maximum is observed in the XRD pattern as other diffraction intensities are comparable to the background signal. The crystallographic diffraction is explained by Bragg's law which is expressed as:
2d_{hkl}sin(theta) = n$\lambda$
where $\lambda$ is the wavelength of X-ray, n is an integer known as the order of diffraction, d_{hkl} is the inter-planer spacing within the same family of planes that cause constructive interference and 2$\theta$ is the diffraction angle known as Braggs angle.
Physical Properties Measurement System:
Working Principle:
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