1 / 67

Understanding Electrical Properties of Materials

Learn about electrical conductance, resistance, conductivity, and the effects of imperfections and temperature on metals and semiconductors. Explore band structures, resistivity, relative permittivity, and electron transport in materials.

cdailey
Download Presentation

Understanding Electrical Properties of Materials

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. EED2013 Engineering Materials Non-Mechanical Properties of Materials

  2. Overview Electrical Properties Magnetic Properties Thermal Properties Durability

  3. Electrical Properties ISSUES TO ADDRESS... • How are electrical conductance and resistance characterized? • What are the physical phenomena that distinguish conductors, semiconductors, and insulators? • For metals, how is conductivity affected by imperfections, temperature, and deformation? • For semiconductors, how is conductivity affected by impurities (doping) and temperature?

  4. View of an Integrated Circuit (a) (d) Al (d) Si (doped) 45mm 0.5mm • A dot map showing location of Si (a semiconductor): -- Si shows up as light regions. (b) • A dot map showing location of Al (a conductor): -- Al shows up as light regions. (c) Figs. (a), (b), (c) from Fig. 18.27, Callister & Rethwisch 8e. • Scanning electron micrographs of an IC: Fig. (d) from Fig. 12.27(a), Callister & Rethwisch 3e. (Fig. 12.27 is courtesy Nick Gonzales, National Semiconductor Corp., West Jordan, UT.)

  5. Electrical Conduction • Resistivity, r: -- a material property that is independent of sample size and geometry surface area of current flow current flow path length • Conductivity, s • Ohm's Law: V = I R voltage drop (volts = J/C) C = Coulomb resistance (Ohms) current (amps = C/s)

  6. Electrical Properties • Which will have the greater resistance? • Analogous to flow of water in a pipe • Resistance depends on sample geometry and size. 2 D 2D

  7. J =  (V/ ) Electron fluxconductivityvoltage gradient Definitions Further definitions J=   <= another way to state Ohm’s law J  current density   electric field potential = V/

  8. Conductivity: Comparison CERAMICS Soda-lime glass 10 -9 Concrete 10 -13 Aluminum oxide <10 SEMICONDUCTORS POLYMERS -14 -4 Polystyrene <10 Silicon 4 x 10 -15 -10 -17 -11 0 Polyethylene 10 -10 -10 Germanium 2 x 10 -6 GaAs 10 insulators semiconductors • Room temperature values (Ohm-m)-1 = ( - m)-1 METALS conductors 7 Silver 6.8 x 10 7 Copper 6.0 x 10 7 Iron 1.0 x 10 Selected values from Tables 18.1, 18.3, and 18.4, Callister & Rethwisch 8e.

  9. Example: Conductivity Problem What is the minimum diameter (D) of the wire so that V < 1.5 V? I = 2.5 A + - Cu wire V 100 m < 1.5 V 2.5 A 6.07 x 107 (Ohm-m)-1 Solve to get D > 1.87 mm

  10. Relative Permittivity, εr • Property governs the electro-static charge stored on an electric capacitor. • The main equation that this is found is: C = εoεrA/d where C = capacitance in Farads A = area of the capacitor plate d = distance between the capacitor plates εo = absolute permittivity (8.85 x 10-12)

  11. Relative Permittivity, εr Here is a table of some example values of εr:

  12. Band Structure Representation Adapted from Fig. 18.3, Callister & Rethwisch 8e.

  13. Partially filled band Overlapping bands Energy Energy empty band empty GAP band partly filled filled band band filled states filled states filled filled band band Conduction & Electron Transport • Metals (Conductors): -- for metals empty energy states are adjacent to filled states. -- thermal energy excites electrons into empty higher energy states. -- two types of band structures for metals - partially filled band - empty band that overlaps filled band

  14. Energy Band Structures: Insulators & Semiconductors • Semiconductors: -- narrow band gap (< 2 eV) -- more electrons excited across band gap empty empty Energy conduction conduction band band ? GAP filled valence band filled states filled band • Insulators: -- wide band gap (> 2 eV) -- few electrons excited across band gap Energy GAP filled valence band filled states filled band

  15. 6 r • Resistivity increases with: Cu + 3.32 at%Ni 5 Ohm-m) 4 -- temperature deformed Cu + 1.12 at%Ni Resistivity, -- wt% impurity 3 Cu + 1.12 at%Ni d -8 -- %CW 2 (10 i  = thermal “Pure” Cu 1 t + impurity 0 -200 -100 0 T (ºC) + deformation Adapted from Fig. 18.8, Callister & Rethwisch 8e. (Fig. 18.8 adapted from J.O. Linde, Ann. Physik5, p. 219 (1932); and C.A. Wert and R.M. Thomson, Physics of Solids, 2nd ed., McGraw-Hill Book Company, New York, 1970.) Metals: Influence of Temperature and Impurities on Resistivity • Presence of imperfections increases resistivity -- grain boundaries -- dislocations -- impurity atoms -- vacancies These act to scatter electrons so that they take a less direct path.

  16. Adapted from Fig. 18.9, Callister & Rethwisch 8e. 180 50 r 160 Ohm-m) 40 140 125 30 30 Resistivity, 120 Yield strength (MPa) -8 20 100 (10 21 wt% Ni 10 80 0 60 0 10 20 30 40 50 0 10 20 30 40 50 wt% Ni, (Concentration C) wt% Ni, (Concentration C) From step 1: CNi = 21 wt% Ni Estimating Conductivity • Question: -- Estimate the electrical conductivity  of a Cu-Ni alloy that has a yield strength of 125 MPa. Adapted from Fig. 7.16(b), Callister & Rethwisch 8e.

  17. Charge Carriers in Insulators and Semiconductors Two types of electronic charge carriers: Free Electron – negative charge – in conduction band Hole – positive charge – vacant electron state in the valence band Adapted from Fig. 18.6(b), Callister & Rethwisch 8e. Move at different speeds - drift velocities

  18. Intrinsic Semiconductors • Pure material semiconductors: e.g., silicon & germanium • Group IVA materials • Compound semiconductors • III-V compounds • Ex: GaAs & InSb • II-VI compounds • Ex: CdS & ZnTe • The wider the electronegativity difference between the elements the wider the energy gap.

  19. • Concept of electrons and holes: valence electron hole electron hole Si atom electron pair creation pair migration - - + + no applied applied applied • Electrical Conductivity given by: # holes/m3 hole mobility electron mobility # electrons/m3 Intrinsic Semiconduction in Terms of Electron and Hole Migration electric field electric field electric field Adapted from Fig. 18.11, Callister & Rethwisch 8e.

  20. for intrinsic semiconductor n = p = ni  = ni|e|(e+ h)  • Ex: GaAs For GaAs ni = 4.8 x 1024 m-3 For Si ni = 1.3 x 1016 m-3 Number of Charge Carriers Intrinsic Conductivity

  21. Intrinsic Semiconductors: Conductivity vs T • Data for Pure Silicon: -- s increases with T -- opposite to metals material Si Ge GaP CdS band gap (eV) 1.11 0.67 2.25 2.40 Selected values from Table 18.3, Callister & Rethwisch 8e. Adapted from Fig. 18.16, Callister & Rethwisch 8e.

  22. Intrinsic vs Extrinsic Conduction 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + • n-type Extrinsic: (n >> p) • p-type Extrinsic: (p >> n) 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + Phosphorus atom Boron atom hole conduction electron 5+ 3 + valence electron no applied no applied Si atom Adapted from Figs. 18.12(a) & 18.14(a), Callister & Rethwisch 8e. electric field electric field • Intrinsic: -- case for pure Si -- # electrons = # holes (n = p) • Extrinsic: -- electrical behavior is determined by presence of impurities that introduce excess electrons or holes -- n ≠ p

  23. doped undoped 3 2 freeze-out extrinsic intrinsic concentration (1021/m3) Conduction electron 1 0 0 200 400 600 T (K) Adapted from Fig. 18.17, Callister & Rethwisch 8e. (Fig. 18.17 from S.M. Sze, Semiconductor Devices, Physics, and Technology, Bell Telephone Laboratories, Inc., 1985.) Extrinsic Semiconductors: Conductivity vs. Temperature • Data for Doped Silicon: -- s increases doping -- reason: imperfection sites lower the activation energy to produce mobile electrons. • Comparison:intrinsic vs extrinsic conduction... -- extrinsic doping level: 1021/m3 of a n-type donor impurity (such as P). -- for T < 100 K: "freeze-out“, thermal energy insufficient to excite electrons. -- for 150 K < T < 450 K: "extrinsic" -- for T >> 450 K: "intrinsic"

  24. p-n Rectifying Junction + - + - + + - - + - p-type - n-type + + + - + - - + - - + n-type - p-type + + - - + + - + - + - • 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. p-type n-type -- No applied potential: no net current flow. Adapted from Fig. 18.21 Callister & Rethwisch 8e. -- Forward bias: carriers flow through p-type and n-type regions; holes and electrons recombine at p-n junction; current flows. -- Reverse bias: carriers flow away from p-n junction; junction region depleted of carriers; little current flow.

  25. Properties of Rectifying Junction Fig. 18.22, Callister & Rethwisch 8e. Fig. 18.23, Callister & Rethwisch 8e.

  26. Summary • Electrical conductivity and resistivity are: -- material parameters -- geometry independent • Conductors, semiconductors, and insulators... -- differ in range of conductivity values -- differ in availability of electron excitation states • For metals, resistivity is increased by -- increasing temperature -- addition of imperfections -- plastic deformation• For pure semiconductors, conductivity is increased by -- increasing temperature -- doping [e.g., adding B to Si (p-type) or P to Si (n-type)] • Other electrical characteristics -- ferroelectricity -- piezoelectricity

  27. Magnetic Properties ISSUES TO ADDRESS... • What are the important magnetic properties? • How does magnetic memory storage work?

  28. Generation of a Magnetic Field -- Vacuum • Computation of the applied magnetic field, H: • Created by current through a coil: B0 N = total number of turns  = length of each turn (m) I= current (ampere) H H= applied magnetic field (ampere-turns/m) B0 = magnetic flux density in a vacuum (tesla) I • Computation of the magnetic flux density in a vacuum, B0: B0=0H permeability of a vacuum(1.257 x 10-6 Henry/m)

  29. Generation of a Magnetic Field -- within a Solid Material • Relative permeability (dimensionless) • A magnetic field is induced in the material B B = Magnetic Induction (tesla) inside the material applied magnetic field H B=H permeability of a solid current I

  30. Relative Permeability, μr • Property that governs the magnetic strength of a material. • The main equation that this property occurs is B/H = μoμr where B = flux density in Tesla H = magnetising force μo = absolute permeability (12.566 x 10-7)

  31. Types of Magnetism (3) ferromagnetic e.g. Fe3O4, NiFe2O4 (4) ferrimagnetic e.g. ferrite(), Co, Ni, Gd cm (2) paramagnetic ( e.g., Al, Cr, Mo, Na, Ti, Zr ~ 10-4) cm ( as large as 106 !) cm (1) diamagnetic ( ~ -10-5) cm vacuum ( = 0) e.g., Al2O3, Cu, Au, Si, Ag, Zn B (tesla) H (ampere-turns/m) Plot adapted from Fig. 20.6, Callister & Rethwisch 8e. Values and materials from Table 20.2 and discussion in Section 20.4, Callister & Rethwisch 8e.

  32. (2) paramagnetic random aligned Adapted from Fig. 20.5(b), Callister & Rethwisch 8e. (3) ferromagnetic (4)ferrimagnetic Adapted from Fig. 20.7, Callister & Rethwisch 8e. aligned aligned Magnetic Responses for 4 Types No Applied Applied Magnetic Field (H = 0) Magnetic Field (H) (1) diamagnetic opposing Adapted from Fig. 20.5(a), Callister & Rethwisch 8e. none

  33. Domains in Ferromagnetic & Ferrimagnetic Materials H H H • “Domains” with aligned magnetic H moment grow at expense of poorly aligned ones! H H = 0 • As the applied field (H) increasesthe magnetic domains change shape and size by movement of domain boundaries. B sat Adapted from Fig. 20.13, Callister & Rethwisch 8e. (Fig. 20.13 adapted from O.H. Wyatt and D. Dew-Hughes, Metals, Ceramics, and Polymers, Cambridge University Press, 1974.) induction (B) Magnetic 0 Applied Magnetic Field (H)

  34. Hysteresis and Permanent Magnetization Stage 2. Apply H, align domains Stage 3. Remove H, alignment remains! => permanent magnet! Stage 4. Coercivity, HCNegative H needed to demagnitize! Stage 6. Close the hysteresis loop Stage 5. Apply -H, align domains • The magnetic hysteresis phenomenon B Adapted from Fig. 20.14, Callister & Rethwisch 8e. H Stage 1. Initial (unmagnetized state)

  35. Hard and Soft Magnetic Materials Hard Soft Hard magnetic materials: -- large coercivities -- used for permanent magnets -- add particles/voids to inhibit domain wall motion -- example: tungsten steel -- Hc = 5900 amp-turn/m) B H Soft magnetic materials: -- small coercivities -- used for electric motors -- example: commercial iron 99.95 Fe Adapted from Fig. 20.19, Callister & Rethwisch 8e. (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.) 35

  36. Magnetic Storage • Digitized data in the form of electrical signals are transferred to and recorded digitally on a magnetic medium (tape or disk) • This transference is accomplished by a recording system that consists of a read/write head -- “write” or record data by applying a magnetic field that aligns domains in small regions of the recording medium -- “read” or retrieve data from medium by sensing changes in magnetization Fig. 20.23, Callister & Rethwisch 8e.

  37. Magnetic Storage Media Types • Hard disk drives (granular/perpendicular media): -- CoCr alloy grains (darker regions) separated by oxide grain boundary segregant layer (lighter regions) -- Magnetization direction of each grain is perpendicular to plane of disk Fig. 20.25, Callister & Rethwisch 8e. (Fig. 20.25 from Seagate Recording Media) 80nm ~ 500nm -- Tabular (plate-shaped) ferrimagnetic barium-ferrite particles • Recording tape (particulate media): Fig. 20.24, Callister & Rethwisch 8e. (Fig. 20.24 courtesy Fuji Film Inc., Recording Media Division) ~ 500nm -- Acicular (needle-shaped) ferromagnetic metal alloy particles

  38. Summary • A magnetic field is produced when a current flows through a wire coil. • Magnetic induction (B): -- an internal magnetic field is induced in a material that is situated within an external magnetic field (H). -- magnetic moments result from electron interactions with the applied magnetic field • Types of material responses to magnetic fields are: -- ferrimagnetic and ferromagnetic (large magnetic susceptibilities) -- paramagnetic (small and positive magnetic susceptibilities) -- diamagnetic (small and negative magnetic susceptibilities) • Types of ferrimagnetic and ferromagnetic materials: -- Hard: large coercivities -- Soft: small coercivities • Magnetic storage media: -- particulate barium-ferrite in polymeric film (tape) -- thin film Co-Cr alloy (hard drive)

  39. Thermal Properties ISSUES TO ADDRESS... • How do materials respond to the application of heat? • How do we define and measure... -- heat capacity? -- thermal expansion? -- thermal conductivity? -- thermal shock resistance? • How do the thermal properties of ceramics, metals, and polymers differ?

  40. Heat Capacity • Two ways to measure heat capacity:Cp : Heat capacity at constant pressure. Cv: Heat capacity at constant volume. Cpusually > Cv • Heat capacity has units of The ability of a material to absorb heat • Quantitatively: The energy required to produce a unit rise in temperature for one mole of a material. energy input (J/mol) heat capacity (J/mol-K) temperature change (K)

  41. Dependence of Heat Capacity on Temperature • Heat capacity... -- increases with temperature -- for solids it reaches a limiting value of 3R Cv= constant 3R R = gas constant = 8.31 J/mol-K Cv Adapted from Fig. 19.2, Callister & Rethwisch 8e. 0 T (K) 0 q D Debye temperature ) (usually less than T room • From atomic perspective: -- Energy is stored as atomic vibrations. -- As temperatureincreases, the average energy of atomic vibrations increases.

  42. Atomic Vibrations Atomic vibrations are in the form of lattice waves or phonons Adapted from Fig. 19.1, Callister & Rethwisch 8e.

  43. cp (J/kg-K) at room T Material • Polymers cp(specific heat): (J/kg-K) 1925 Polypropylene 1850 Polyethylene 1170 Polystyrene 1050 Teflon • Ceramics Magnesia (MgO) 940 Alumina (Al2O3) 775 Glass 840 • Metals Aluminum 900 Steel 486 Selected values from Table 19.1, Callister & Rethwisch 8e. Tungsten 138 Gold 128 Specific Heat: Comparison Cp(heat capacity): (J/mol-K) • Why is cpsignificantly larger for polymers? increasing cp

  44. Thermal Expansion Materials change size when temperature is changed Tinitial initial Tfinal> Tinitial Tfinal final linear coefficient of thermal expansion (1/K or 1/ºC)

  45. Symmetric curve: -- increase temperature, -- no increase in interatomic separation -- no thermal expansion Atomic Perspective: Thermal Expansion Asymmetric curve: -- increase temperature, -- increase in interatomic separation -- thermal expansion Adapted from Fig. 19.3, Callister & Rethwisch 8e.

  46. a (10-6/C)at room T Material • Polymers Polypropylene 145-180 Polyethylene 106-198 Polystyrene 90-150 Teflon 126-216 • Metals Aluminum 23.6 Steel 12 increasing  Tungsten 4.5 Gold 14.2 • Ceramics Magnesia (MgO) 13.5 Alumina (Al2O3) 7.6 Soda-lime glass 9 Silica (cryst. SiO2) 0.4 Coefficient of Thermal Expansion: Comparison Polymers have larger  values because of weak secondary bonds • Q: Why does a generally decrease with increasing bond energy? Selected values from Table 19.1, Callister & Rethwisch 8e.

  47. Answer: For Cu rearranging Equation 19.3b Thermal Expansion: Example Ex: A copper wire 15 m long is cooled from 40 to -9ºC. How much change in length will it experience?

  48. Thermal Conductivity The ability of a material to transport heat. Fourier’s Law temperature gradient heat flux (J/m2-s) thermal conductivity (J/m-K-s) T2 T1 T2>T1 x1 x2 heat flux • Atomic perspective: Atomic vibrations and free electrons in hotter regions transport energy to cooler regions.

  49. Aluminum 247 Steel 52 Tungsten 178 Gold 315 • Ceramics Magnesia (MgO) 38 Alumina (Al2O3) 39 increasing k atomic vibrations Soda-lime glass 1.7 Silica (cryst. SiO2) 1.4 • Polymers Polypropylene 0.12 Polyethylene 0.46-0.50 vibration/rotation of chain molecules Polystyrene 0.13 Teflon 0.25 Thermal Conductivity: Comparison Energy TransferMechanism Material k (W/m-K) • Metals atomic vibrations and motion of free electrons Selected values from Table 19.1, Callister & Rethwisch 8e.

  50. Thermal Stresses • Occur due to: -- restrained thermal expansion/contraction -- temperature gradients that lead to differential dimensional changes Thermal stress 

More Related