5 - ECE331 Imperfections in Solids Why study this Crystal...

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1 Lu ECE331 Imperfections in Solids Why study this? Crystal is regular, repeatable lattice of atoms. BUT like everything, not perfect. Point Defects Vacancy Missing atom from lattice. Sometimes produced during solidification and growth, sometimes by physical factors, (heating, neutrons, working) but present in all crystals. Usually low, but reaches 1 vacancy in 10,000 atoms as temperature rises to melting point. Lu ECE331 N v = N exp (-Q v /kT) N v = no. of vacancies per m 3 . N = no. of lattice points per m 3 . Q v = energy required to produce a vacancy. K = Boltzmann’s constant. T = Temperature in degrees Kelvin. Self-Interstitial An extra atom is inserted into lattice between normal atom positions and produced distortion to surrounding lattice.
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2 Lu ECE331 Example The fraction of Al sites vacant at 400°C is 2.29×10 -5 . ρ = 2.70 g/cm 3 , at.mass = 26.98 amu. Calculate the density of vacancy sites. The density of Al atoms 3 28 23 3 6 / 10 02 . 6 10 023 . 6 / 98 . 26 / 10 70 . 2 .
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5 - ECE331 Imperfections in Solids Why study this Crystal...

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