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Chapter 20 - New Technologies Research Center (ntrc)

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Electrical Properties • How are electrical conductance and resistance characterized? • What are the physical phenomena that distinguish conductors, semiconductors, and insulators? Chapter 20 - 1 Electrical Conduction • Ohm's Law: DV = I R voltage drop (volts = J/C) resistance (Ohms) current (amps = C/s) C = Coulomb A (cross sect. area) e- I DV L • Resistivity, r and Conductivity, s: -- geometry-independent forms of Ohm's Law -- Resistivity is a material property & is independent of sample conductivity  1 s r Chapter 20 - 2 Conductivity: Comparison • Room T values (Ohm-m)-1 = ( - m)-1 METALS CERAMICS conductors -10 Silver 6.8 x 10 7 Soda-lime glass 10 -10-11 Copper 6.0 x 10 7 Concrete 10 -9 Iron 1.0 x 10 7 Aluminum oxide <10-13 SEMICONDUCTORS POLYMERS Polystyrene Silicon 4 x 10 -4 Polyethylene Germanium 2 x 10 0 GaAs 10 -6 semiconductors -14 <10 10 -15-10-17 insulators Selected values from Tables 18.1, 18.3, and 18.4, Callister 7e. Chapter 20 - 3 Energy States: Insulators & Semiconductors • Insulators: • Semiconductors: -- Higher energy states not -- Higher energy states separated accessible due to gap (> 2 eV). by smaller gap (< 2 eV). Energy Energy empty band filled valence band filled band ? GAP filled states filled states GAP empty band filled valence band filled band Chapter 20 - 4 Intrinsic vs Extrinsic Conduction • Intrinsic: # electrons = # holes (n = p) --case for pure Si • Extrinsic: --n ≠ p --occurs when impurities are added with a different # valence electrons than the host (e.g., Si atoms) • n-type Extrinsic: (n >> p) • p-type Extrinsic: (p >> n) Phosphorus atom 4+ 4+ 4+ 4+ s  n e e 4+ 5+ 4+ 4+ 4+ 4+ 4+ 4+ Adapted from Figs. 18.12(a) & 18.14(a), Callister 7e. no applied electric field Boron atom hole conduction electron 4+ 4+ 4+ 4+ valence electron 4+ 4+ 4+ 4+ Si atom 4+ 3+ 4+ 4+ no applied electric field s  p e h Chapter 20 - 5 p-n Rectifying Junction • Allows flow of electrons in one direction only (e.g., useful to convert alternating current to direct current. • Processing: diffuse P into one side of a B-doped crystal. Adapted from Fig. 18.21, • Results: p-type n-type + + + + + Callister 7e. --No applied potential: no net current flow. --Forward bias: carrier flow through p-type and n-type regions; holes and electrons recombine at p-n junction; current flows. --Reverse bias: carrier flow away from p-n junction; carrier conc. greatly reduced at junction; little current flow. - - - - - p-type + - + - n-type + ++- - + - + p-type + + + + n-type - - - - + - Chapter 20 - 6 Properties of Rectifying Junction Fig. 18.22, Callister 7e. Fig. 18.23, Callister 7e. Chapter 20 - 7 Transistor MOSFET • MOSFET (metal oxide semiconductor field effect transistor) Fig. 18.24, Callister 7e. Fig. 18.25, Callister 7e. Chapter 20 - Integrated Circuit Devices Fig. 18.26, Callister 6e. • Integrated circuits - state of the art ca. 50 nm line width – > 100,000,000 components on chip – chip formed layer by layer • Al is the “wire” Chapter 20 - 9 Ferroelectric Ceramics Ferroelectric Ceramics are dipolar below Curie TC = 120ºC • cooled below Tc in strong electric field - make material with strong dipole moment Fig. 18.35, Callister 7e. Chapter 20 - 10 Piezoelectric Materials Piezoelectricity – application of pressure produces current at rest compression induces voltage applied voltage induces expansion Adapted from Fig. 18.36, Callister 7e. Chapter 20 - 11 Summary • Electrical conductivity and resistivity are: -- material parameters. -- geometry independent. • Electrical resistance is: -- a geometry and material dependent parameter. • Conductors, semiconductors, and insulators... -- differ in accessibility of energy states for conductance electrons. • For metals, conductivity is increased by -- reducing deformation -- reducing imperfections -- decreasing temperature. • For pure semiconductors, conductivity is increased by -- increasing temperature -- doping (e.g., adding B to Si (p-type) or P to Si (n-type). Chapter 20 - 12 Chapter 20: Magnetic Properties ISSUES TO ADDRESS... • How do we measure magnetic properties? • What are the atomic reasons for magnetism? • How are magnetic materials classified? • Materials design for magnetic storage. • What is the importance of superconducting magnets? Chapter 20 - 13 Applied Magnetic Field • Created by current through a coil: Applied magnetic field H N = total number of turns L = length of each turn current I • Relation for the applied magnetic field, H: N I H L current applied magnetic field units = (ampere-turns/m) Chapter 20 - 14 Response to a Magnetic Field • Magnetic induction results in the material B = Magnetic Induction (tesla) inside the material current I • Magnetic susceptibility, c (dimensionless) B c >0 vacuum c = 0 c<0 H c measures the material response relative to a vacuum. Chapter 20 - 15 Magnetic Susceptibility • Measures the response of electrons to a magnetic field. • Electrons produce magnetic moments: magnetic moments electron nucleus electron spin Adapted from Fig. 20.4, Callister 7e. • Net magnetic moment: --sum of moments from all electrons. • Three types of response... Chapter 20 - 16 3 Types of Magnetism B  (1  c)oH permeability of a vacuum: (1.26 x 10-6 Henries/m) Magnetic induction B (tesla) (3) ferromagnetic e.g. Fe3O4, NiFe2O4 ferrimagnetic e.g. ferrite(), Co, Ni, Gd ( c as large as 106 !) (2) paramagnetic ( c ~ 10 -4) e.g., Al, Cr, Mo, Na, Ti, Zr vacuum (c = 0) (1) diamagnetic ( c ~ -10 -5) e.g., Al 2 O3 , Cu, Au, Si, Ag, Zn Strength of applied magnetic field (H) Plot adapted from Fig. 20.6, Callister 7e. Values and (ampere-turns/m) materials from Table 20.2 and discussion in Section 20.4, Callister 7e. Chapter 20 - 17 Magnetic Moments for 3 Types none opposing (2) paramagnetic random aligned (3) ferromagnetic ferrimagnetic aligned Applied Magnetic Field (H) aligned No Applied Magnetic Field (H = 0) (1) diamagnetic Adapted from Fig. 20.5(a), Callister 7e. Adapted from Fig. 20.5(b), Callister 7e. Adapted from Fig. 20.7, Callister 7e. Chapter 20 - 18 Ferro- & Ferri-Magnetic Materials • As the applied field (H) increases... --the magnetic moment aligns with H. B sat H Magnetic induction (B) H H H H 0 • “Domains” with aligned magnetic moment grow at expense of poorly aligned ones! Adapted from Fig. 20.13, Callister 7e. (Fig. 20.13 adapted from O.H. Wyatt and D. DewHughes, Metals, Ceramics, and Polymers, Cambridge University Press, 1974.) Applied Magnetic Field (H) H=0 Chapter 20 - 19 Permanent Magnets • Process: B 3. remove H, alignment stays! => permanent magnet! Adapted from Fig. 20.14, Callister 7e. 4 . Coercivity, HC Negative H needed to demagnitize! B Soft • Hard vs Soft Magnets large coercivity --good for perm magnets --add particles/voids to make domain walls hard to move (e.g., tungsten steel: Hc = 5900 amp-turn/m) 2. apply H, cause alignment Applied Magnetic Field (H) 1. initial (unmagnetized state) Adapted from Fig. 20.19, Callister 7e. (Fig. 20.19 from K.M. Ralls, T.H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering, John Wiley and Sons, Inc., 1976.) Applied Magnetic Field (H) small coercivity--good for elec. motors (e.g., commercial iron 99.95 Fe) Chapter 20 - 20 Magnetic Storage • Information is stored by magnetizing material. • Head can... recording medium -- apply magnetic field H & align domains (i.e., magnetize the medium). -- detect a change in the magnetization of the Image of hard drive courtesy medium. Martin Chen. • Two media types: Reprinted with permission from International Business Machines Corporation. -- Particulate: needle-shaped g-Fe2O3. +/- mag. moment along axis. (tape, floppy) Adapted from Fig. 20.24, Callister 7e. (Fig. 20.24 courtesy P. Rayner and N.L. Head, IBM Corporation.) recording head Adapted from Fig. 20.23, Callister 7e. (Fig. 20.23 from J.U. Lemke, MRS Bulletin, Vol. XV, No. 3, p. 31, 1990.) --Thin film: CoPtCr or CoCrTa alloy. Domains are ~ 10 - 30 nm! (hard drive) Adapted from Fig. 20.25(a), ~2.5 m ~120 nm Callister 7e. (Fig. 20.25(a) from M.R. Kim, S. Guruswamy, and K.E. Johnson, J. Appl. Phys., Vol. 74 (7), p. 4646, 1993. ) Chapter 20 - 21 Superconductivity Hg Copper (normal) 4.2 K Adapted from Fig. 20.26, Callister 7e. • Tc = temperature below which material is superconductive = critical temperature Chapter 20 - 22 Limits of Superconductivity • 26 metals + 100’s of alloys & compounds • Unfortunately, not this simple: Jc = critical current density if J > Jc not superconducting Hc = critical magnetic field if H > Hc not superconducting Hc= Ho (1- (T/Tc)2) Adapted from Fig. 20.27, Callister 7e. Chapter 20 - 23 Advances in Superconductivity • This research area was stagnant for many years. – Everyone assumed Tc,max was about 23 K – Many theories said you couldn’t go higher • 1987- new results published for Tc > 30 K – ceramics of form Ba1-x Kx BiO3-y – Started enormous race. • Y Ba2Cu3O7-x Tc = 90 K • Tl2Ba2Ca2Cu3Ox Tc = 122 K • tricky to make since oxidation state is quite important • Values now stabilized at ca. 120 K Chapter 20 - 24 Meissner Effect • Superconductors expel magnetic fields normal superconductor Adapted from Fig. 20.28, Callister 7e. • This is why a superconductor will float above a magnet Chapter 20 - 25 Current Flow in Superconductors • Type I current only in outer skin - so amount of current limited • Type II current flows within wire Type I M Type II complete diamagnetism HC1 HC mixed state HC2 H normal Chapter 20 - 26 Summary • A magnetic field can be produced by: -- putting a current through a coil. • Magnetic induction: -- occurs when a material is subjected to a magnetic field. -- is a change in magnetic moment from electrons. • Types of material response to a field are: -- ferri- or ferro-magnetic (large magnetic induction) -- paramagnetic (poor magnetic induction) -- diamagnetic (opposing magnetic moment) • Hard magnets: large coercivity. • Soft magnets: small coercivity. • Magnetic storage media: -- particulate g-Fe2O3 in polymeric film (tape or floppy) -- thin film CoPtCr or CoCrTa on glass disk (hard drive) Chapter 20 - 27 Chapter 20 - 28