Review - ENGR 220 Review Final Exam: Friday, June 11,...

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Unformatted text preview: ENGR 220 Review Final Exam: Friday, June 11, 1:00-3:00 pm, Main Auditorium Accommodations: LeBow 348; 1:00-4:00 pm (1.5X); 1:00-5:00 pm (2X) Review - 1 Exam Coverage etc. Final Exam Coverage: Chapters 17, 18, 20, 21 + some topics from previous chapters/earlier material… Style: Like Midterms…1 x multipart, short answer/circle the answer question + 5 recitation problem style questions, plus one 15 point (9%) bonus Q Keys to Success: Read the textbook & posted lecture notes Read your own lecture notes Practice solving problems…recitation, example & other Take advantage of TA and/or RI office hours Review - 2 ANNOUNCEMENTS Course Evaluations (AEFIS): Opening…soon Closing, after 2-3 weeks Please complete an evaluation of the course, the various instructors…either before or after the final exam etc. o Your feedback provides us with useful information which we use to make changes in the future Review - 3 MSE Program Promotion • If you are still “undecided” about your major, and maybe interested in the exciting field of Materials Science and Engineering then check out our web page at www.materials.drexel.edu for more details about the program • A minor (~21 credits) in Materials is also possible… Review - 4 Past Short Answer Examples (1) i. The bond in a compound AB, where the electronegativitiy of A is much less than that of B, will be: metallic ____ ionic ____ covalent ____ or van der ? Waals ii. In a BCC metal, the number of atoms per unit cell is ________. iii. The property of a crystalline material such as iron to exist, at different temperatures, as different crystal structures is called _________________. iv. An atactic polymer has its side groups arranged in the following way: All on the same side________, on alternate sides _______ or randomly ______ . v. A possible cathodic reaction for zinc in hydrochloric acid (HCl) is: _____________+ ->_____________. vi. For a solid material to be either ferromagnetic or paramagnetic some of the atoms of the solid need to have____________________________. vii. For an material (insulator or semiconductor) to be transparent to visible light the size of its energy gap Eg must be: _______________________. (Red light has wavelength R and frequency R Blue light has a wavelength B and frequency B. R > B and R < B). Review - 5 Past Short Answer Examples (2) viii. ix. x. xi. Doping of Si with B results in an intrinsic/extrinsic/superextrinsic (choose one) _ -type semiconductor. 18 karat gold is more/less/the same (circle one) conductive than 24 karat gold because it is ______, which implies that the mobility of electrons is higher/lower/the same (choose one) in the 24 karat gold. There are one/two/three/four (choose one) atomic diffusion mechanisms in solids. These are _________________. The concentration of vacancies in a solid increases/decreases/stays the same (choose one) with increasing temperature. xii. xiii. Most solids are/are not (choose one) single crystals. Silicon wafers used in the semiconductor industry are/are not (choose one) single crystals because _________________. The atomic packing factor, APF, of the HCP and FCC arrangements of atoms are the highest/lowest (choose one) possible for equisized spheres. The value of this APF is ______. The way I got to this number is (show details of calculation) ______________ Review - 6 Ch. 14: ORGANIC CHEMISTRY IN 6 LINES or LESS Covalent Bonds between C, H, N, O, S, Cl, F Similar electronegativities Carbon - 4 bonds Nitrogen - 3 bonds Oxygen and Sulfur - 2 bonds Hydrogen, Chlorine and Fluorine - 1 bond Review - 7 Chapter 14 - Polymers What is a polymer? Poly mer many repeat unit repeat unit repeat unit repeat unit HHHHHH CCCCCC HHHHHH HHHHHH CCCCCC H Cl H Cl H Cl Polyethylene (PE) Polyvinyl Chloride (PVC) H C H HH CC CH3 H HH CC CH3 H H C CH3 Polypropylene (PP) Adapted from Fig. 14.2, Callister 7e. Review - 8 Polymer Composition Most polymers are organic hydrocarbons i.e. comprised of H and C only Strong covalent intramolecular bonds Weak hydrogen and van der Waals intermolecular bonds: Review - 9 Chemistry of Polymers • Polymer molecules…v. large…macromolecules – – – – Covalent bonds within each molecule Backbone = string of C atoms…singly bonded to adjacent C Remaining 2 valence e-….side-bond to atoms, radicals… e.g. ethylene (C2H4)…a gas at room temp… Adapted from Fig. 14.1, Callister 7e. C-C bond angle = 109° Review - 10 Bulk or Commodity Polymers • Generalized Formula: – Where R = atom (H, Cl, etc.) or group (CH3, C2H5 or C6H5 - methyl, ethyl, phenyl, etc.) • Chain Repeat Units: – If all same… Homopolymers – If different… Copolymers Review - 11 The GREAT DIVIDE THERMOPLASTICS THERMOSETS Linear polymers: (2 links/MER) They can be SOFTENED (and melted) REPEATEDLY by raising the temperature. Weak (secondary) bonds between chains. Strong bonds within chain. EXAMPLES??? ARE CROSSLINKED (3 links/MER) It is a RIGID 3-D molecule. ONCE a THERMOSET is formed it CANNOT BE RESHAPED OR REMELTED. EXAMPLES? Review - 12 Molecular Weight Calculation Review - 13 Factors Affecting Properties of Polymers 1) 2) 3) 4) 9) Molecular Weight Degree of crystallinity Chain alignment/entanglement Side group chemistry Glass Transition Temperature (Tg) Review - 14 CHAPTER 17: CORROSION AND DEGRADATION ISSUES TO ADDRESS... Why does corrosion occur? Which metals are most likely to corrode? How do temperature and the environment affect corrosion rate? How do we suppress corrosion? Review - 15 INTRODUCTION Background: Most materials interact with their environment Interactions can result in deterioration of mechanical properties: Impaired usefulness of material… Consequences of ignored degradation? Deterioration Mechanisms: Metals: – Loss of material via dissolution (corrosion) – Formation of oxide scale/films (oxidation) Ceramics: – Relatively deterioration-resistant – Issue mainly at high temps & v. extreme environments Polymers: – Degradation… – Dissolve in solvents…absorb & swell – UV radiation & heat can break covalent bonds Review - 16 METALLIC CORROSION Electrochemical Process: Chemical reaction with transfer of e- from one material to another Metals lose or give up valence eOxidation M Mn+ + neReaction (“half-reaction”) occurs at the anode e- generated from oxidized metal atom become part of another chemical species: Reduction e.g. Mn+ + neM, or 2H+ +2e- H2 (in acids) Reaction occurs at the cathode 2 or more simultaneous reduction reactions possible Overall…MUST have at least 1 oxidation + 1 reduction reaction (“redox”) Review - 17 EFFECT OF CONCENTRATION & TEMP. • emf series highly idealized: pure metals, 1M solutions, 25°C • Changing T and conc. will change V, possibly the reaction direction… n+ n M1 + + M2 • Consider the generalized reaction: M1 + M2 V depends on T and molar ion concentrations… n RT [M1 + ] ln n+ nF [M2 ] aka the Nernst equation T = temp (K); R = gas const.; n = # electrons partic’n.; F = Faraday constant (96,500 C/mol), or… V = V2o V1o V = V2o V1o n 0.0592 [M1 + ] log n + n [M2 ] V must be +ve for reaction to proceed spontaneously in direction assumed…implication? Review - 18 POLYMER DEGRADATION Polymers: interact with their environment known as “degradation” not corrosion metals…electrochemical Polymers…physiochemical: – swelling and dissolution – covalent bond rupture…heat, chemical, radiation – loss of mechanical integrity – more complex – e.g. polyethylene, when heated in O2 atm., becomes brittle… Review - 19 SWELLING & DISSOLUTION Polymers exposed to liquids Liq. solute diffuses into polymer: macromolecules forced apart… material expands or swells consequences of increased chain separation? – reduced secondary bonding between chains – material softer and more ductile – lowers Tg…rubbery & weak Swelling = partial dissolution w. limited solubility Dissolution = complete solubility Review - 20 BOND RUPTURE Degradation of Polymers by Scission: breakage/rupture of bonds within molecular chain usually due to heat, radiation or chemical reaction Consequences? chain separation reduction in MW reduction of mechanical properties (brittleness, cracking, discoloration) Causes: Radiation (e-beams, X-rays, gamma rays, etc.) UV, Oxygen & Ozone Heat Weathering Review - 21 SUMMARY • Corrosion due to: natural tendency of metals to give up electrons electrons given up by an oxidation reaction electrons then used in reduction reaction • Metals with a more negative Standard Electrode Potential more likely to corrode relative to other metals • Galvanic Series ranks the reactivity of metals in seawater • Increasing T speeds up oxidation/reduction reactions • Corrosion may be controlled by: using metals which form a protective oxide layer reducing T adding inhibitors painting using cathodic protection Review - 22 Chapter 18: 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, T, and deformation? • For semiconductors, how is conductivity affected by impurities (doping) and T? Review - 23 KEY equation for this chapter = n e μe The KEY is n…to understand n, we need to understand band theory in solids… Review - 24 Possible Band Structures (0 K) Ef = Fermi energy (level): energy corresponding to highest filled e- state Metals Insulators Semiconductors (Eg >2 eV) (Eg <2eV) Review - 25 For Metals = 1 1 = n e μe Anything that decreases e- mobility will increase resistivity Review - 26 SEMICONDUCTORS TWO TYPES: INTRINSIC & EXTRINSIC Review - 27 Second Most Important Equation in This Chapter • Electrical conductivity of semiconductors: # holes/m = n e μe + p e μh 3 # electrons/m 3 μh always < μe hole mobility electron mobility For an intrinsic semiconductor, by definition, n = p = ni e (μ e + μ h ) Review - 28 Summary: Intrinsic vs Extrinsic Conduction • Intrinsic: # electrons = # holes (I.e. n = p) - case for pure Si • Extrinsic: -n p - occurs when impurities with different # valence ethan Si host are added, e.g. P or B • n-type Extrinsic: (n >> p) • p-type Extrinsic: (p >> n) Phosphorus atom 4+ 4+ 4+ 4+ n e μe 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 p e μh Review - 29 Ch.18 Summary • Electrical conductivity and resistivity are: - material parameters - geometry independent • Electrical resistance is: - geometry and material dependent parameter • Conductors, semiconductors, and insulators... - differ in accessibility of energy states for conduction electrons • For metals, resistivity is increased by: - deformation - increasing defects; imperfections, alloying, etc. - increasing temperature • For semiconductors, conductivity is increased by - increasing temperature - doping (e.g., adding B to Si (p-type) or P to Si (n-type) Review - 30 Chapter 20: Magnetic Properties ISSUES TO ADDRESS.. • How do we characterize 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?) Review - 31 Applied Magnetic Field (Field Vector) • Created by current flow through a coil: Externally applied magnetic field H current I N = total number of turns L = length of solenoid H also called magnetic field strength • Relation for the applied magnetic field, H: # Turns NI H= L Applied magnetic field units = (Ampere-turns/m) Current (A) Coil length (m) Review - 32 Magnetic Flux Density (Field Vector) Flux Density or Induction, B Magnitude of internal field strength within material subjected to H-field Units: Tesla B = μH μ = permeability…property of medium through which Hfield passes and B-field is measured In vacuum: B0 = μ0H μ0 = permeability of vacuum (4 x 10-7 H/m) μr = μ μ0 μr = relative permeability (unitless) Review - 33 Magnetic Permeability, Magnetization… • μr = measure of degree to which material can be magnetized, or ease B-field can be induced in presence of external H-field • Magnetization of solid, M Defined by B = μ0H + μ0M In presence of H-field, magnetic moments within material align with field and reinforce it by virtue of their magnetic fields (μ0M) Magnitude of M proportional to applied H-field: • M = mH • m = magnetic susceptibility (also unitless) It can be easily shown that: • m = μr - 1 Review - 34 Magnetic Moments • 2 sources of electron magnetic moments: – orbital motion of e- around nucleus: • • • • e- = moving charge consider as small current loop current loop generates small magnetic field magnetic moment along axis of rotation – electron spin: • spin up or down (± 1/2) around an axis – consider each e- as small magnet with orbital and spin magnetic moments – Fundamental magnetic moment: • • • • Bohr magneton… μB = 9.27 x 10-24 A-m2 spin magnetic moment = ± μB orbital magnetic moment = ml μB where ml = magnetic quantum # Review - 35 Magnetic Moments Indicates response of electrons to a magnetic field Net magnetic moment: sum of magnetic moments from all electrons orbital moments of some e- pairs will cancel each other same true for spin moments… • e.g. e- with spin up will cancel e- with spin down net magnetic moment = sum of magnetic moments of all e- (both orbital and spin), taking cancellation into account For atom with completely filled electron shells/subshells: • total cancellation of both orbital and spin moments • such materials cannot be permanently magnetized! • e.g. He, Ne, Ar, etc. • Three types of response... Review - 36 Unpaired Electrons! No unpaired electrons means no magnetism… You cannot have magnetism without unpaired electrons If the electrons are paired… no magnetism! If you want magnetism make sure there are unpaired electrons Review - 37 3 Types of Magnetism B = (1+ m )μ oH Permeability of vacuum: (1.26 x 10-6 H/m) Magnetic Induction B (Tesla) (3) Ferromagnetic e.g. Ferrite ( ), Co, Ni, Gd Ferrimagnetic e.g. Fe3O4, NiFe2O4 ( m as large as 106!) (2) Paramagnetic ( m ~10-4) e.g., Al, Cr, Mo, Na, Ti, Zr Vacuum ( = 0) (1) Diamagnetic ( m ~ -10-5) e.g. Al2O3, Cu, Au, Si, Ag, Zn Applied Magnetic Field H (Ampere-turns/m) Review - 38 Hysteresis & Permanent Magnets Review - 39 Hard vs. Soft Magnets Area within loop = magnetic energy loss/unit vol. of material per magnet’n./demagnet’n. cycle…heat gen. within sample Review - 40 Summary • Magnetic field (H) can be produced by: – passing a current through a coil • Magnetic Induction (B): – occurs when material subjected to a magnetic field – is a change in magnetic moment from UNPAIRED electrons • Types of material response to a field include: – ferri- or ferromagnetic (large magnetic induction) – paramagnetic (poor magnetic induction) – diamagnetic (opposing magnetic moment) • Hard magnets: large coercivity & remanence • Soft magnets: small coercivity & remanence • Magnetic storage media: – particulate -Fe2O3 in polymeric film (tape or floppy) – thin-film CoPtCr or CoCrTa on glass disk (hard drive) Review - 41 Chapter 21: Optical Properties ISSUES TO ADDRESS... What happens when light shines onto a material? Why do some materials have characteristic colors? Why are some materials transparent and others not? Optical applications: o o o o luminescence photoconductivity solar cells optical communications fibers Review - 42 The Electromagnetic Spectrum Visible Light: v. narrow range… 0.4-0.7 m (400-700 nm) perceived color = fn.( ) “white” light = mix of all colors Review - 43 Optical Properties Light has both particulate and wave-like properties – Photons…with mass – Photon energy E =h = hc = energy (J) = wavelength (m) = frequency (Hz) h = Planck's constant (6.62 x 10-34 J.s) c = speed of light (3 x 108 m/s) Review - 44 Light Interaction with Solids • Incident light either transmitted, absorbed, reflected or scattered: i .e. Io = IT + IA + IR + IS Reflected: IR Absorbed: IA Transmitted: IT Incident: I0 Scattered: IS • Optical classification of materials: Transparent Translucent Opaque Single crystal Polycrystalline & dense Adapted from Fig. 21.10, Callister 6e. (Fig. 21.10 is by J. Telford, with specimen preparation by P.A. Lessing.) Polycrystalline & porous Review - 45 Atomic & Electronic Interactions Two Key Optical Phenomena… • Electronic Polarization: em wave includes fluctuating E-field E-field interacts with e- cloud around each atom perturbs or shifts e- cloud relative to nucleus: – some radiation energy may be absorbed – light waves retarded in vel. in medium…refraction • Electron Transitions: excitation e- from one occupied state to vacant higher energy state only specific values of E allowed all photon energy absorbed e- cannot remain in excited state forever…decays back to ground state re-emits photon(s) Review - 46 Optical Properties of Metals: Absorption • All frequencies visible light absorbed: continuously available empty e- states above Fermi level Ef total absorption in thin outer layer (< 0.1 m) metals opaque to all em radiation at low end of f-spectrum most absorbed radiation re-emitted from surface, gen. w. same …i.e. as reflected light Review - 47 Optical Properties of Non-Metals • Insulators & Semiconductors: • Band structure…with band gaps • May be transparent or opaque to visible light • Thus, we need to also consider refraction and transmission… • Refraction: • “bending” of light…e.g. bent stick in water • related to electronic polarization…perturbation of ecloud relative to nucleus • magnitude = fn. (size of atoms/ions) • larger atoms/ions greater e- polarization, slower velocity in material, greater refraction (n) • n affects both optical path and influences % of incident light reflected at the surface Review - 48 Refractive Index, n • Transmitted light distorts electron clouds no transmitted light + transmitted light + electron cloud distorts • Light is slower in a material vs. in vacuum n = refractive index c (velocity of light in vacuum) v (velocity of light in medium) - Adding large, heavy ions (e.g., lead can decrease the speed of light) - Light can be “bent” • Note: n = fn. ( ) Typical glasses ca. Polymers PbO (Litharge) Diamond 1.5 -1.7 1.3 -1.6 2.67 2.41 Selected values from Table 21.1, Callister 7e. Review - 49 Selective Absorption: Semiconductors • Absorption by electron transition occurs if h > Egap e- from VB excited across band gap into vacant states in CB Free e- in CB + hole in VB Electron energy Blue ( ~0.4 m) light: Eg(max) = hc/ min = h = 3.1 eV unfilled states Red ( ~0.7 m) light: Eg(min) = hc/ max = h = 1.8 eV Egap Incident photon energy h Io filled states Adapted from Fig. 21.5(a), Callister 7e. • If Egap < 1.8 eV, full absorption; opaque (Si, GaAs) • If Egap > 3.1 eV, no absorption; colorless/transparent (diamond) • If Egap in between, partial absorption; material has color Review - 50 Light Absorption & Transmission • Intensity of absorbed radiation = fn.(medium + path length) • Intensity of transmitted (non-absorbed) light: decreases with distance x light traverses in material/medium IT =e I0 IT ln = I0 ( = linear absorption coefficient (mm-1), a characteristic of the material, which varies with x = distance measured from incident surface into material x x ) 2 IT = I0 1- R e (See Example Problem 21.1) x I0 = intensity @ front surface x = thickness = absorption coefficient R = reflectance IT = transmitted intensity@ rear surface Review - 51 Solar Cells • Operation: • p-n junction: P-doped Si conductance Si electron Si P Si incident photons produce electron-hole pairs e- & holes drawn away from junction in opposite directions…external current flow typically ~0.5 V potential current increases w/light intensity light Si n-type Si p-n junction p-type Si n-type Si p-n junction p-type Si Si B - -- + + ++ • Solar powered weather station: Si hole creation of hole-electron pair Si Si B-doped Si polycrystalline Si Los Alamos High School weather station (photo courtesy P.M. Anderson) Review - 52 Optical Fiber Profiles Step-index Optical Fiber: ncladding < ncore Fig. 21.21, Callister 7e. Graded-index Optical Fiber: Impurities added to silica so n varies parabolically across Ø Fig. 21.22, Callister 7e. Review - 53 Ch 21 SUMMARY • When light (em radiation) shines on a material, it may be: – reflected, absorbed and/or transmitted • Optical classification: – transparent, translucent, opaque • Metals: – fine succession of available energy states above Ef results in absorption and reflection (re-emission) • Non-Metals: – may have full (Egap < 1.8 eV), no (Egap > 3.1 eV), or partial absorption (1.8 eV < Egap = 3.1 eV) – color determined by light wavelengths that are transmitted or re-emitted from electron VB/CB transitions – color may be changed by adding impurities which change the band gap (Eg) magnitude (e.g., Ruby) • Refraction: – speed of transmitted light varies among materials Review - 54 ...
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This note was uploaded on 07/27/2011 for the course ENGR 134 taught by Professor Marks during the Spring '11 term at Drexel.

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