EXPERIMENT-4

Aim: Synthesis and Characterization of Carbon Nanotube/Polypyrrole (CNT/PAni) composite. Requirements: Formula used: Theory: X-ray powder diffraction is a powerful tool for characterizing the products of a solid state synthesis reaction. At the simplest level, diffraction patterns can be analyzed for phase identification, that is, determining what crystalline substances are present in a given sample. More quantitatively, the peak positions can be used to refine the lattice parameters for a given unit cell. Unit cells in three-dimensional repeating structures have different shapes based upon the symmetry of the structure. In all cases, the unit cells are parallelepipeds, but the different shapes arise depending on restrictions placed on the lengths of the three edges (a, b, and c) and the values of the three angles ( and , , ). Crystal System Lattice Parameter Restrictions a = b = c = = = 90 a = b c = = = 90 a b c = = = 90 a b c = = 90; 90 a b c 90 a = b c = = 90; = 120 a = b c = = 90; = 120

Cubic Tetragonal Orthorhombic Monoclinic Triclinic Hexagonal Trigonal*

Table :The seven crystal systems and the restrictions placed on the lattice parameters of the unit cell.

Power X-ray diffraction is also used to characterize crystallographic structure, grain size, and preferred orientation in polycrystalline or powder solid samples. This is a preferred method of analysis for characterization of unknown crystalline materials. Compounds are identified by comparing diffraction data against a database of known materials. It can be used to follow phase changes as a function of variable such as temperature, pressure. X-ray diffraction is a physical phenomenon as well as an experimental method for the characterization of materials. Principle: When matter is irradiated with a beam of X photons, it emits an X-ray beam with a wavelength equal or very close to that of the incident beam, which is an effect referred to as scattering. The scattered energy is very small, but in the case where scattering occurs without a modification of the wavelength (coherent scattering) and when the scattering centers are located at nonrandom distances from one another, we will see how the scattered waves interfere to give rise to diffracted waves with higher intensities. The analysis of the diffraction figure, that is, the analysis of the distribution in space of the diffracted intensity, makes it possible to characterize the structure of the material being studied. This constitutes the core elements of X-ray diffraction. Diffractogram: Each peak in a diffraction pattern arises from a unique set of repeating planes in the structure. These sets of planes are oriented in all different directions in three-dimensional space. However, in order to see diffraction from a specific set, the planes must be oriented relative to the incident Xray. Therefore, X-ray powder diffraction relies on a large number of crystallites in random orientations in order to observe the most diffraction peaks. Of course, the proper orientation is only one factor. Diffraction from a particular set of planes may not be observed or the peak intensity may be low due to symmetry (patterns of systematic absences) or other factors that contribute to low intensity. Instrument Design: The X-ray powder diffractometer consists of a copper X-ray source, a solid state detector, and a computer to control the diffractometer and collect and analyze the diffraction data. X-ray Source: Filters and monochromator crystals

Generally speaking, X-ray beams produced with a tube are polychromatic, in other words, they contain characteristic emission peaks in addition to a continuous spectrum over a wide range of wavelengths. In order to characterize the material to be studied, it is important to have a monochromatic beam which makes it possible to associate a single diffraction peak with each family of crystal planes. Therefore, it is necessary to select one peak among all of those emitted by the tube. Naturally, the most intense one is chosen. There are two common techniques used to solve this problem: the filter: this consists of a thin sheet of a material that absorbs all of the radiations emitted with wavelengths below the one we wish to select; the monochromator crystal (or monochromator): this consists of a single crystal cut with respect to a specific family of planes. This single crystal is oriented so as to have the chosen family of planes in the Bragg position for the wavelength we wish to select, which means that only the X photons with that wavelength are diffracted. The result is a monochromatic beam. Filters The idea behind filters functioning lies in the absorption characteristics of the materials used. The absorption varies as e- x, where is the linear absorption coefficient, is the density and x is the length traveled. Naturally, the absorption coefficient varies with the wavelength. Filters have to meet two conditions: first, to absorb as much as possible of the unwanted peaks, and so be thick enough, and second, to be as transparent as possible to the wavelength we wish to select (the K peak in practice), and so be thin enough. Therefore, it is important to choose a material whose absorption spectrum according to the wavelength shows a sharp decrease at a value very close to the wavelength of the anodes K peak. In practice, the appropriate material is an element very close in Mendeleyevs table to the one used in the anode. Figure 2.10 shows the example of a zirconium filter used for a molybdenum anode. Monochromator crystals As we have already mentioned, the underlying idea of such an instrument lies in the selective diffraction of a polychromatic beam by a single crystalMultilayered monochromators or mirrors In the early 1990s, a new type of monochromator elements appeared. These devices were made of stacks of alternate metal layers a few nanometers thick. The first layer is the same as the third, the fifth, the seventh, etc., whereas the even layers are different from the odd layers but all identical [ARN 90, SCH 88]. The resulting multi-layered object can be considered an artificial crystal with an interplanar distance equal to the distance between two layers of the same kind. The intensity diffracted by these artificial crystals will be all the greater if the deposited layers show a strong electronic contrast. The most common elements are made of an alternate stacking of tungsten and silicon layers [SCH 95]. More recently, some authors have designed similar multi-layered monochromators out of refractory materials. Stacks of aluminum oxide and tungsten layers were produced to create monochromators that could withstand high temperatures . Detectors Photographic film X-rays reduce silver halides the same way light does and therefore can produce images on photographic film. These films were the first X-ray detectors, first used by Rntgen to show the existence of X radiation. The darkening of the film is related to the intensity of the X-ray beam and, additionally, there is no illumination threshold. Therefore, even very low intensities can be detected, simply by lengthening the exposure time. The plotted curve in

Geometry of XDS 2000

detector

Figure :The geometry of the sample, X-ray source, and detector on the Scintag XDS 2000 X-ray powder diffractometer During data collection, the sample remains in a fixed position and the X-ray source and detector are programmed to scan over a range of 2values (2 is the sum of the angles between the X-ray source and the sample and the sample and the detector). Routinely, a 2range of 2 to 60 is sufficient to cover the most useful part of the powder pattern. Choosing an appropriate scanning speed (measured in 2/min) depends on balancing the desire to collect a powder pattern quickly with obtaining a reasonable signal-to-noise ratio for the diffraction peaks. Usually how fast we can scan depends on the crystallinity of the sample. In most cases, we will begin with a scanning speed of 2/min and recollect the diffraction pattern at a slower speed if the background is too noisy.