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Explain Schottky and Frenkel Defects

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1. Explain Schottky and Frenkel defects.

Schottky defect

This defect arises if some of the atoms or ions are missing from their normal lattice sites. The lattice sites, which are unoccupied are called lattice vacancies or holes. The number of missing positive and negative ions is the same in this case and the crystal remains neutral. This defect appears generally in ionic crystals in which the positive and negative ions do not differ much in size. Eg. NaCl.

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Na+ missing Cl- missing

Frenkel defect

AgBr is an example for this type of defect. The crystal remains electrically neutral because the number of anions and cations remain the same. In the above example one of the Ag+ ion occupies a position in the interstitial space rather than its own appropriate site. This defect generally occurs in ionic crystals in which anion is much larger in size than the cation.

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[pic]20. What is super conductivity? Give its uses.

The ability of certain ultra cold substances to conduct electricity without resistance is called super conductivity. This super conductivity state is a state in which a material has virtually zero electrical resistance. Substances having this property are called super conductors. At the extremely low temperatures, vibration of the nuclei of certain atoms slow down so much and they synchronise with the passing waves of electrons in a flow of electric current.

When this happen, resistance to electric current disappears.

e.g., Metals such as tungsten , mercury and lead act as super conductors at about 10K.

Uses:

(i) Super conductivity is the basis of new generation of energy saving power systems. Super conducting generators are smaller in size and weight when we compare with conventional generators. These generators consume low energy and we can save more energy.

(ii) High efficiency ore separating machines are built by using super conducting magnets.

(iii) Super conducting solenoids are used in Nuclear Magnetic Resonance imaging equipment, which is a whole body scan equipment.

[pic]23. Explain Bragg's spectrometer method.

* Solution:

Bragg's spectrometer method is used for studying crystals using X-rays.

The apparatus consists of an X-ray tube from which a narrow beam of X-rays is allowed to fall on the crystal mounted on a rotating table. The rotating table is provided with scale and vernier from which the angle of incidence q can be measured.

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An arm, which is rotating about the same axis as the crystal table, carries an ionization chamber. The rays reflected from the crystal enter into the ionization chamber and ionise the gas present inside. Due to the ionisation, current is produced which is measured by electrometer. The current of ionisation is a direct measure of intensity of reflected beam from the crystal. For different angles of incidence, the corresponding ionisation current is measured from the electrometer. These values are plotted in the form of graph. For sodium chloride, the maximum reflection or peaks for 100 plane occurred q = 5.9o, 11.85o and 18.15o. The sines of these angles are 0.103, 0.205 and 0.312, which are in the ratio 1:2:3. These peaks represent the first, second and order reflection. The ratio confirms the correctness of Bragg's equation.

. Write a short note on superconductivity. List some applications of superconductors.

The ability of certain metals, their compounds and alloys to conduct electricity with zero resistance at very low temperatures is called superconductivity. The materials, which exhibit this property, are called superconductors.

The phenomenon of superconductivity was first observed by Kammerlingh Onnes in 1911. He found that mercury suddenly showed zero resistance at 4.2 K. In 1957 Bardeen, Cooper and Schrieffer first explained super conductivity. This theory is called BCS theory.

The temperature at which electrical resistivity of the-material suddenly drops to zero and the material changes from normal conductor to a superconductor is called the transition temperature or critical temperature TC. At the transition temperature the following

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