Free Electron Model What kind of interactions hold metal atoms together? How does this explain high electrical and thermal conductivity?

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1 Electrical Good conductors of heat & electricity Create semiconductors Oxides are basic ionic solids Aqueous cations (positive charge, Lewis acids) Reactivity increases downwards in family Free Electron Model What kind of interactions hold metal atoms together? Mechanical Lustrous (reflect light) Most are solids (which one is not?) How does this explain high electrical and thermal conductivity? Malleable (hammer into thin sheets) Ductile (drawn out into wires) Defects/Hardening affect strength Mixtures are Alloys How does this explain malleability and ductility? CHEM 112 LRSVDS Material Properties 1 CHEM 112 LRSVDS Material Properties 2

2 W Mo Cr Atomic orbitals (AOs) mix to form Start with 2 AOs, end with MOs Start with n AOs, end up with MOs In metals energy difference between orbitals in valence band is small. MOs form a continuous band of allowed energy states. As strength of metallic bond increases, What happens to the melting point? Electron Sea model cannot explain:!!bp, MP trends!!heat of fusion!!hardness of metals CHEM 112 LRSVDS Material Properties 3 CHEM 112 LRSVDS Material Properties 4

3 Most METALS are Conductors Band is partially filled by Valence electrons Some materials are Insulators or Semiconductors Valence band is full (or completely empty).!!to conduct electricity, the e - needs to be able to get to an empty orbital CHEM 112 LRSVDS Material Properties 5 CHEM 112 LRSVDS Material Properties 6

4 Insulators Some elements (diamond, sulfur) Many Ionic Solids Network Covalent Solids Organic Compounds (polymers) Ceramics! inorganic, nonmetallic solids! can be covalent network and/or ionic bonded Examples: Aluminates (Al 2 O 3 ) Carbides (SiC, Ca 2 C) Oxides (BeO, ZrO 2, Al 2 O 3 ) Nitrides (BN) Silicates (SiO 2 /metal oxides) Si Ge GaP, GaAs Similar Inorganic Compounds Silicon; essential to integrated circuits in computers, electronic devices Properties: shiny, silvery gray brittle Poor thermal conductor SEMI-METAL Uses: alloy (with Al, Mg) Silicone polymers Electronic applications for these applications very pure silicon (<1ppb) is required. Zone refining to get pure Si CHEM 112 LRSVDS Material Properties 7 CHEM 112 LRSVDS Material Properties 8

5 Semiconductors: conductivity band gap *! Increase conductivity by: 1.! 2.! 3.! Insulators: conductivity band gap E g ~ D Sn 8 C(graphite) Si Diamond GaAs CHEM 112 LRSVDS Material Properties 9 CHEM 112 LRSVDS Material Properties 10

6 CHEM 112 LRSVDS Material Properties 11 CHEM 112 LRSVDS Material Properties 12

7 Improving Conduction in Semiconductors (Doping) 1) Add impurities that donate extra electrons: n-type semiconductor 2) Add impurities that accept electrons: p-type semiconductor Examples What elements would you use to dope Si to make it an n-type semiconductor? What elements would you use to dope Si to make it a p-type semiconductor? Classify each of the following as n-type, p-type, or not doped B-doped Si P-doped Si As-doped Ge GaAs Te-doped GaAs Si-doped GaAs CHEM 112 LRSVDS Material Properties 13 CHEM 112 LRSVDS Material Properties 14

8 p-type material bonded to an n- type material. LEDs opposite of solar cells Produce light from the from the flow of electrons through a semiconductor diode.!!produce IR light using Si based semiconductors!!produce visible light using doped Aluminum-Gallium-Arsenide (AlGaAs) semiconductors CHEM 112 LRSVDS Material Properties 15 When no current flows: Depletion Zone CHEM 112 LRSVDS Material Properties 16

9 Changing the dopants changes the size of the depletion zone and the color of visible light produced Wavelength nm Color Material and structure of LEDs 700 red GaP: Zn-O/GaP 660 red GaAI0.35 As/GaAs 630 red GaAs0.35PO.65: N/GaP 610 orange GaAs0.25Po.75: N/GaP 590 yellow GaAs0.15PO.85: N/GaP 565 green Gap: N/GaP When electrons combine with holes, light is emitted. 555 green GaP/GaP The energy of light (E = h!) is the same as the band gap energy E g ( which varies with the diode material) CHEM 112 LRSVDS Material Properties 17 CHEM 112 LRSVDS Material Properties 18

10 Metals aren t infinitely conductive; there is some resistance to electron flow. Superconductors show no resistance to flow of electricity (flow of electrons). Superconducting behavior starts when cooled below the superconducting transition temperature, T c. Meissner effect: permanent magnets levitate over superconductors. The superconductor excludes all magnetic field lines, so the magnet floats in space. CHEM 112 LRSVDS Material Properties 19 CHEM 112 LRSVDS Material Properties 20

11 Superconductors hexagonal close packing (hcp) Unit cell is bcc cubic close packing (ccp) Unit cell is fcc Close packed arrangements of spheres maximize IMF s: common in metals Both structures have 12 nearest neighbors (coordination # = 12) Superconducting Ceramic Oxides; Unit Cell of YBa2Cu3O7 CHEM 112 LRSVDS Material Properties 21 CHEM 112 LRSVDS Material Properties 22

12 Structures of metallic solids Face centered cubic Most metals crystallize as one of three structures: Body Centered Cubic (bcc); CN=8 Hexagonal close packed (hcp); CN=12 Cubic Close-packed (ccp, unit cell fcc);cn=12 "! Every metal atom has 8 or 12 neighbors Not enough electrons for e- pair bonds to each neighbor; share e- between all atoms, non-directional hcp (CN=12) CHEM 112 LRSVDS Material Properties ccp (CN=12) 23 CHEM 112 LRSVDS Material Properties 24

13 Malleability of Metals and Alloys Some metals are soft and ductile. Others are hard. Why? Corrugation Close-packed structures: Non-close-packed structures: Defects in Metallic Crystals Defects are responsible for important mechanical properties of metals: malleability, yield stress, etc. Metals have:!!non-directional bonding!!large number of nearest neighbors Defect Types: Hexagonal (hcp) Cubic (ccp) CN=12 Cu (ccp) Zn (hcp) Body centered cubic CN=8 CuZn alloy (brass) CHEM 112 LRSVDS Material Properties 25 1.! Vacancy (missing atom) point defect 2. Dislocation (extra line of atoms) line defect 3. Grain Boundary 2-D interface between two crystal grains CHEM 112 LRSVDS Material Properties 26

14 Dislocations Move Under Stress Moving a dislocation breaks/makes a line of metal-metal bonds (easy) shear force Hardening of Metals Structural materials need to be strong (girders, knife blades, airplane wings). How can we minimize movement of dislocations? 1. Use single crystals (expensive) 2. Anneal and Work Harden: a. Annealing; heat makes crystal grains grow, dislocations move (metal becomes more malleable) b. Work hardening - hammering moves dislocations (line defects) to grain boundaries "creates a planar defect (stronger under stress) Shearing a perfect crystal means we have to break a plane of bonds (requires much more force) Cold working or drawing of a metal increases strength and brittleness (e.g., iron beams, knives, horseshoes) Perfect Crystal CHEM 112 LRSVDS Material Properties 27 CHEM 112 LRSVDS Material Properties 28

15 Formation of Alloys to Harden Metals 3.! Alloying add other components to the metal resulting in a mixture with more strength Impurity atoms or phases pin dislocations; stops the motion because of different atomic size or stronger bonds to the metal atoms ALLOYS Alloys: Mixtures of metals -! often have improved physical properties 1)! Homogeneous (Solution) alloy Mixed at the atomic level 3)! Heterogeneous alloy non-homogeneous dispersions. (e.g. pearlite steel has two phases: almost pure Fe and cementite, Fe 3 C). 3)! Intermetallic compounds compounds of two different metals having definite compositions Examples: brass, solder, sterling silver, dental amalgam Cr 3 Pt razor blades Ni 3 Al jet engines, lightweight and strong Co 5 Sm permanent magnets in headsets Au 3 Bi, Nb 3 Sn superconductors, low T high field magnets CHEM 112 LRSVDS Material Properties 29 CHEM 112 LRSVDS Material Properties 30

16 SOLUTION ALLOYS There are two types of solution alloy: Substitutional alloy when one metal substitutes for another in the structure.! atoms must have similar atomic radii! elements must have similar bonding characteristics. Iron and Steel Below 900 o C, iron has bcc structure (not close packed); hard as nails Above 900 o C, iron has ccp structure (close packed); soft and malleable Can be worked into various shapes when hot Rapid Quenching; plunge into cold water. No time to return to bcc, still malleable, can be shaped Steelmaking: Carbon steel contains ~ 1% C by weight (dissolves well in ccp iron but not in bcc) Interstitial alloy when a non-metal is present in the interstices of the metal.!interstitial atoms are smaller!the alloy is much stronger than the pure metal (increased bonding between nonmetal and metal).!example steel (contains up to 3 % carbon). CHEM 112 LRSVDS Material Properties 31 Tempering of steel (slow cooling): ccp Fe can t dissolve the extra C, forms a new phase of Fe 3 C (cementite); crystalline mixture of Fe grains and cementite grains Fe 3 C grains stop movement of dislocation in high carbon steel; very hard, brittle material CHEM 112 LRSVDS Material Properties 32

17 Steel: Fe (pig iron) + small amounts of C Mild Steel: <0.2% C malleable and ductile used in cables, nails, and chains. Medium Steel: % C tough used in girders and rails. High Carbon Steel: % C very tough used in knives, tools, and springs. Stainless Steel: 73% Fe, 18% Cr, 8% Ni, 1% C. CHEM 112 LRSVDS Material Properties 33 CHEM 112 LRSVDS Material Properties 34

18 Metals are typically polycrystalline Amorphous alloys have superior mechanical properties because dislocations cannot move. CHEM 112 LRSVDS Material Properties 35