chapter-2.doc - CHAPTER-2 EXPERIMENTAL METHODS TECHNIQUES...

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CHAPTER-2 EXPERIMENTAL METHODS / TECHNIQUES USED 2.0 INTRODUCTION In this chapter, all the experimental techniques/methods involved in the present work are described briefly. Materials are made of atoms. Knowledge of how atoms are arranged into solids is known as crystal structure of materials. The crystal structures are the foundation on which we build our understanding of the synthesis and physical properties of materials. There are also many ways for measuring chemical compositions of materials and methods based on inner-shell electron spectroscopes are covered here. The larger emphasis is on measuring spatial arrangements of atoms in the range from 10 -8 to 10 -4 cm, bridging from the unit cell of the crystal to the microstructure of the material. To date, most of our knowledge about the spatial arrangements of atoms in materials has been gained from diffraction experiments. In a diffraction experiment, an incident wave either an electromagnetic wave e. g. X-rays or matter waves i.e. electron / neutron wave is directed into a material and a detector is typically moved about to record the directions and intensities of the outgoing diffracted waves. The basic aims of the scientist in the area of micro analytical analysis have been to unravel the structural details of materials down to the atomic level. Keeping in view, one of the fundamental aspects of physics that resolution can never be better than wavelength of illumination used. One needs to replace the visible light (wavelength = 4000-8000 Å) by some other waves e.g. X-rays, electrons or neutrons. Where as the wavelength of X-ray, depend upon the target element e. g. CuKα = 1.5418 Å, in the case of electrons it can be controlled by accelerating voltage e.g. wavelength λ = 0.037 Å for 100kV electrons. One of the major themes of the present work is to study the structural and microstructural techniques employed for this purpose correspond to the “X-ray diffraction and Electron microscopy”. So it will be appropriate to describe these techniques in some details. The following sections 41
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embody the descriptions for characterization of as synthesized (bulk form) as well as the transformed materials. 2.1 THE POWDER X-RAY DIFFRACTOMETER More than 95% of all solid materials can be described as crystalline. When X- rays interact with a crystalline substance, X-ray diffraction results from an electromagnetic wave (the X-ray) impinging on a regular array of scattering sites (repeating arrangement of atoms within the crystal). An x-ray diffraction pattern of the pure substance is therefore like a fingerprint of the substance. The powder diffraction method is thus ideally suited for characterization and identification of polycrystalline phases. An instrument for studying crystalline and nanocrystalline materials by measurements of the ways in which they diffract x-rays of known wavelength has been aptly called a diffractometer. An X-ray diffractrometer [1-3] consist of two elements: X-ray tube and a goniometer. X-rays are generated in a cathode ray tube by
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