Presentation on theme: "Electrical Properties"— Presentation transcript:
1 Electrical Properties • How are electrical conductance and resistancecharacterized?• What are the physical phenomena that distinguishconductors, semiconductors, and insulators?
2 Electrical Conduction • Ohm's Law:DV = I Rvoltage drop (volts = J/C)C = Coulombresistance (Ohms)current (amps = C/s)Ie-A(crosssect.area)DVL• Resistivity, r and Conductivity, s:-- geometry-independent forms of Ohm's Lawconductivity-- Resistivity is a material property & is independent of sample
3 Conductivity: Comparison -1-1• Room T values (Ohm-m)= ( - m)METALSconductorsPolystyrene <10-14Polyethylene-15-10-17Soda-lime glass 10Concrete-9Aluminum oxide <10-13CERAMICSPOLYMERSinsulators-117Silver6.8 x 107Copper6.0 x 107Iron1.0 x 10Silicon4 x 10-4Germanium2 x 10GaAs10-6SEMICONDUCTORSsemiconductorsSelected values from Tables 18.1, 18.3, and 18.4, Callister 7e.
4 Energy States: Insulators & Semiconductors -- Higher energy states notaccessible due to gap (> 2 eV).• Semiconductors:-- Higher energy states separatedby smaller gap (< 2 eV).Energyfilledbandvalenceemptyfilled statesGAP?Energyfilledbandvalenceemptyfilled statesGAP
5 Intrinsic vs Extrinsic Conduction # 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)no appliedelectric field5+4+Phosphorus atomvalenceelectronSi atomconductionhole• p-type Extrinsic: (p >> n)no appliedelectric fieldBoron atom3+4Adapted from Figs (a) & 18.14(a), Callister 7e.
6 p-n Rectifying Junction • Allows flow of electrons in one direction only (e.g., usefulto convert alternating current to direct current.• Processing: diffuse P into one side of a B-doped crystal.• Results:+-p-typen-typeAdapted from Fig , Callister 7e.--No applied potential:no net current flow.--Forward bias: carrierflow through p-type andn-type regions; holes andelectrons recombine atp-n junction; current flows.+-p-typen-type--Reverse bias: carrierflow away from p-n junction;carrier conc. greatly reducedat junction; little current flow.+-p-typen-type
9 Integrated Circuit Devices Fig , Callister 6e.Integrated circuits - state of the art ca. 50 nm line width> 100,000,000 components on chipchip formed layer by layerAl is the “wire”
10 Ferroelectric Ceramics Ferroelectric Ceramics are dipolar below Curie TC = 120ºCcooled below Tc in strong electric field - make material with strong dipole momentFig , Callister 7e.
11 Piezoelectric Materials Piezoelectricity – application of pressure produces currentat restcompression induces voltageapplied voltage induces expansionAdapted from Fig , Callister 7e.
12 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 forconductance 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).
13 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?
14 Applied Magnetic Field • Created by current through a coil:Appliedmagnetic field Hcurrent IN = total number of turnsL = length of each turn• Relation for the applied magnetic field, H:applied magnetic fieldunits = (ampere-turns/m)current
15 Response to a Magnetic Field • Magnetic induction results in the materialcurrent IB = Magnetic Induction (tesla)inside the material• Magnetic susceptibility, c (dimensionless)c measures thematerial responserelative to a vacuum.HBvacuumc= 0> 0< 0
16 Magnetic Susceptibility • Measures the response of electrons to a magnetic field.• Electrons produce magnetic moments:Adapted from Fig. 20.4, Callister 7e.magnetic momentselectronnucleusspin• Net magnetic moment:--sum of moments from all electrons.• Three types of response...
17 3 Types of Magnetism Magnetic induction B (tesla) permeability of a vacuum:(1.26 x 10-6 Henries/m)Magnetic inductionB (tesla)ferromagnetic e.g. Fe3O4, NiFe2O4ferrimagnetic e.g. ferrite(), Co, Ni, Gd(3)(cas large as106!)(2)paramagnetice.g., Al, Cr, Mo, Na, Ti, Zr(c~ 10-4)vacuum(c= 0)-5diamagnetic(c~ -10)(1)e.g.,Al2O3, Cu, Au, Si, Ag, ZnStrength of applied magnetic field (H)(ampere-turns/m)Plot adapted from Fig. 20.6, Callister 7e. Values and materials from Table 20.2 and discussion in Section 20.4, Callister 7e.
18 Magnetic Moments for 3 Types No AppliedAppliedMagnetic Field (H = 0)Magnetic Field (H)(1) diamagneticnoneopposingAdapted from Fig. 20.5(a), Callister 7e.Adapted from Fig. 20.5(b), Callister 7e.(2) paramagneticrandomalignedAdapted from Fig. 20.7, Callister 7e.(3) ferromagneticferrimagneticaligned
19 Ferro- & Ferri-Magnetic Materials • As the applied field (H) increases...--the magnetic moment aligns with H.BsatHAdapted from Fig , Callister 7e. (Fig adapted from O.H. Wyatt and D. Dew-Hughes, Metals, Ceramics, and Polymers, Cambridge University Press, 1974.)HH• “Domains” withaligned magneticinduction (B)HMagneticmoment grow atexpense of poorlyHaligned ones!H = 0Applied Magnetic Field (H)
20 Permanent Magnets • Process: • Hard vs Soft Magnets B B 2. apply H, causealignment3. remove H, alignment stays!=> permanent magnet!Adapted from Fig , Callister 7e.4Negative H needed to demagnitize!. Coercivity, HCApplied MagneticField (H)1. initial (unmagnetized state)large coercivity--good for perm magnets--add particles/voids tomake domain wallshard to move (e.g.,tungsten steel:Hc = 5900 amp-turn/m)• Hard vs Soft Magnetssmall coercivity--good for elec. motors(e.g., commercial iron Fe)Adapted from Fig , Callister 7e. (Fig from K.M. Ralls, T.H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering, John Wiley and Sons, Inc., 1976.)Applied MagneticField (H)BHardSoft
21 Magnetic Storage • Information is stored by magnetizing material. • Head can...-- apply magnetic field H &align domains (i.e.,magnetize the medium).-- detect a change in themagnetization of themedium.recording headrecording mediumAdapted from Fig , Callister 7e. (Fig from J.U. Lemke, MRS Bulletin, Vol. XV, No. 3, p. 31, 1990.)Image of hard drive courtesy Martin Chen.Reprinted with permissionfrom International Business Machines Corporation.• Two media types:-- Particulate: needle-shapedg-Fe2O3. +/- mag. momentalong axis. (tape, floppy)Adapted from Fig , Callister 7e. (Fig courtesy P. Rayner and N.L. Head, IBM Corporation.)~2.5 mmAdapted from Fig (a), Callister 7e. (Fig (a) from M.R. Kim, S. Guruswamy, and K.E. Johnson, J. Appl. Phys., Vol. 74 (7), p. 4646, )~120 nm--Thin film: CoPtCr or CoCrTaalloy. Domains are ~ nm!(hard drive)
22 SuperconductivityHgCopper (normal)4.2 KAdapted from Fig , Callister 7e.Tc = temperature below which material is superconductive= critical temperature
23 Limits of Superconductivity 26 metals + 100’s of alloys & compoundsUnfortunately, not this simple:Jc = critical current density if J > Jc not superconductingHc = critical magnetic field if H > Hc not superconductingHc= Ho (1- (T/Tc)2)Adapted from Fig , Callister 7e.
24 Advances in Superconductivity This research area was stagnant for many years.Everyone assumed Tc,max was about 23 KMany theories said you couldn’t go higher1987- new results published for Tc > 30 Kceramics of form Ba1-x Kx BiO3-yStarted enormous race.Y Ba2Cu3O7-x Tc = 90 KTl2Ba2Ca2Cu3Ox Tc = 122 Ktricky to make since oxidation state is quite importantValues now stabilized at ca. 120 KSuddenly everyone was doing superconductivity. Everyone was doing similar work, making discoveries, & rushing to publish so they could claim to have done it first. Practically, daily new high temp. records were set.
25 Meissner Effect Superconductors expel magnetic fields This is why a superconductor will float above a magnetnormalsuperconductorAdapted from Fig , Callister 7e.
26 Current Flow in Superconductors Type I current only in outer skin- so amount of current limitedType II current flows within wireMHType IType IIcomplete diamagnetismmixed stateHC1HC2HCnormal
27 Summary • A magnetic field can be produced by: • Magnetic induction: -- 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)