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StudentLecture_28 (1)

StudentLecture_28 (1) - This week in CHM 11500 T/8/2009...

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This week in CHM 11500 T, Lecture 28, 12/8/2009 Check out of lab. Remember: Safety goggles Appropriate dress HW due Friday. Final Exam: Tuesday, December 15, 8:00 AM Hall of Music ~60 % cumulative, ~40 % new material Help Session: Sunday, 13 Dec. at 1 to 3 PM in Lilly Hall 1105 Sorry, I forgot to post notes. This set will be posted later today.
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Superconducting Ceramics Superconductivity is a phenomenon in which the electrical resistivity of a material drops to zero at some low temperature This is the “ critical temperature ”, written T c Some metals and metal alloys exhibit superconductivity at T c near 0-20 K! (very inconvenient temperatures, require liquid helium , bp 4 K) Some ceramics have T c around 100 K ( much nicer, liquid N 2 , bp 77 K) Complex formulas: YBa 2 Cu 3 O 7 with T c = 92 K HgBa Ca Cu O 8 with T = 153 K demo
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Superconducting Ceramics Lattice of a YBa 2 Cu 3 O 7 superconductor Ionic solid, difficult to form flexible wires.
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How does electrical conduction happen in solids? Band theory Recall atomic energy levels and orbitals Valence orbitals = outermost orbital with highest n value
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Band Theory As the number of atoms in a solid increases, the number of available energy levels increases. The number of energy levels from a single type of orbital in a cluster of atoms = the total number available from the individual atoms For Li 2s orbitals; 1 valence electron from each Li atom.
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Band Theory As the number of atoms in the group increases, the energy levels get closer and closer together, until they eventually make a continuous band. Electrons will fill energy levels from the bottom up, sometimes forming an unfilled band.
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