Lecture%2002%20Atoms%20and%20Bonds

Lecture%2002%20Atoms%20and%20Bonds - MECH 340 Engineering...

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Unformatted text preview: MECH 340 Engineering Material Structure and Bonds Objectives The goal of this chapter is to describe the underlying physical concepts related to the structure of matter. To examine the relationships between structure of atoms-bonds-properties of engineering materials. Learn about different levels of structure i.e. atomic structure, nanostructure, microstructure, and macrostructure. Arrangement of atoms in materials that defines interaction among tronic structure of individualhe same atoms can have (interatomic bonding).(for t ong s that defines interactionifferent properties, e.g. two forms of d am s (interatomic bonding).arbon: graphite and diamond) c , Chapter 1, Introduction Structure of Material ngement of atoms in materials ual properties, e.g. two Arrangement of small grains of ent forms of the same atoms can have aterial that can be identified by m among nent properties, diamtwo forms of : graphite and e.g. ond) r microscopy. on: graphite and diamond) ic re level (Chapters 2 & 3) gement elf (Chaptersaterials) ic lev o atoms in m 2 & 3 copic structure (Ch. 4) • 2)same atoms can haveMicros e Structural elemtoms in materialsbe that a ents that may irials can ibe identified of al that of ndividual atoms by ngement small grains iewed with the naked eyatoms define interaction (for the same e. v sial that canamong atoms (interyou can have r copy. be identified by atomic bonding) University ofdVirginia, Dept. of Materials Science and Engineering ifferent properties) orms of. oscopy ) Subatomic le el Atomic level oscopic structure (Ch.v4) • s Macros structure gement ofssmall grain(Ch. 4) of 3copic Electronic structure of copic Arrangement of ) ros tructure Microscopic level Arrangement of small grains of materials than can be identified by microscopy Monarch butterfly ~ 0.1 m 9 oscopic structure uralcopic structure be ros elements that may d 4)ith lements thatymay be . w e the naked e e. tural Macroscopic level f ed withf theinia, Dept. ef Materials Science and Engineering niversity o Virg naked oye. 9 by Monarch butterfly ~ 0.1 m Monarch butterfly ~ 0.1 m University of Virginia, Dept. of Materials Science and Engineering Objects fashioned from metals, ceramics, glasses, polymers ... Things Natural Cat ~ 0.3 m Monarch butterfly ~ 0.1 m Bee ~ 1 5 mm Dust mite 300 µm 10-2 m 10-2 m 0.01 m 1 cm 10 mm 1 cm 10 mm 1 millimeter (mm) Head of a pin 1-2 mm Things Manmade Microelectronics 10-3 m 10-3 m Head of a pin 1-2 mm The 21st century challenge -- Fashion materials at the nanoscale with desired properties and functionality The Scale of Things – Nanometers and More The 100 mm Progress in miniaturizatio The Challenge Ant ~ 5 mm 1,000,000 nanometers = 1 millimeter (mm) Microwave MEMS (MicroElectroMechanical Systems) Devices 10 -100 µm wide Dust mite Human hair ~ 200 m mde 50 µ ! wi Fly ash ~ 10-20 µm 10-4 m The Microworld 10-4 m 0.1 mm 100 µm 0.1 mm 100 !m 0.01 mm 10 µm 0.01 mm 10 !m Infrared MicroElectroMechanical (MEMS) devices 10 -100 !m wide Human hair ~ 60-120 !m wide Fly ash ~ 10-20 !m Microworld 10-5 m 10-5 m O P O O Red blood cells Red blood cells Pollen g Pollen grainrain O O O O O O O O O O O O O O O O O O O O 10-6 m Visible spectrum Visible Magne o do cell s i Redgbrltncodilmains a et f m 11(~m widem)ipes µ 7-8 ! str S S S S S S S S Red blood cells with white cell ~ 2-5 µm ATP synthase 10-6 m 1 micrometer (µm) 1,000 nanometers = 1 micrometer (!m) 0.1 µm 100 nm 0.1 !m 100 nm 0.01 µm 10 nm 0.01 !m 10 nm Ultraviolet Zone plate x-ray “lens” Outer ring spacing ~35 nm Indium arsenide quantum dot Quantum dot array -germanium dots on silicon Progress in atomic-level understanding Schematic, central core 10-7 m The Nanoworld 10-7 m Nanoworld Fabricate and combine nanoscale building blocks to make useful devices, e.g., a photosynthetic reaction center with integral semiconductor storage. 10 nm 10-8 m 10-8 m ~10 nm diameter Self-assembled, Nature-inspired structure Self-assembled BiomManys1ng ATfPnm otor u i 0s o “mushroom” Cell membrane Nanotube electrode ATP synthase 10-9 m 10-9 m 1 nanometer (nm) 1 nanometer (nm) Soft x-ray DNA ~2 nm wide DNA ~2-1/2 nm diameter meter centimeter millimeter micrometer nanometer m cm mm µm nm 100 10-2 10-3 10-6 10-9 Atoms of silicon spacing ~tenths of nm 10-10 m 10-10 m 0.1 nm 0.1 nm Atoms of silicon spacing ~tenths of nm Quantum corral of 48 iron atoms on copper surface Quanioned coeraalaoifme8wiironnaSomspon copper surface posit tum on r t t 4 th a tTM ti positioneid one 14t na time with an STM tip Corral d ameter a m Carbon buckyball ~1 nm diameter Carbon nanotube ~1.3 nm diameter Office of Basic Energy Sciences Office of Science, U.S. DOE Version 01-18-05, pmd Corral diameter 14 nm University of Virginia, Dept. of Materials Science and Engineering 1m 0.01 m 0.001 m 0.000001 m 0.000000001 m Chart from http://www.sc.doe.gov/production/bes/scale_of_things.html 12 Metals Metals • valence electrons are Introduction To Materials Science, Chapter 1, Introduction Metals detached from atoms, and spread in an 'electron sea' that "glues" the ions together. Strong, ductile, conduct electricity and heat well, are shiny if Several uses of steel and polished. pressed aluminum. Several uses of steel and pressed aluminum. Ceramics Ceramics • Introduction To Atoms behave like eitherMaterials Science, Chapter 1, Introduction positive or negative ions, Ceramics and are bound by Coulomb forces. They are usually combinations of metals or semiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and carbides). Hard, brittle, insulators. Examples of ceramic materials ranging Examples: glass, porcelain. high performance combustion engines w metals and ceramics. Polymers • are bound by covalent forces and also by weak vander Waalsforces, and usually based on C and H. They decompose at moderate temperatures (100 –400 C), and are lightweight. Examples: plastics rubber. Composites Introduction To Materials Science, Chapter 1, Introduction Composites Polymer composite materials: reinforcing glass fibers in a polymer matrix. University of Virginia, Dept. of Materials Science and Engineering 18 Polymer composite materials: reinforcing glass fibers in a polymer matrix. Atom Models The Structure of the Atom The Electronic Structure of the Atom The Periodic Table Atomic Bonding Binding Energy and Interatomic Spacing Atom Model The atomic number of an element is equal to the number of electrons or protons in each atom. The atomic mass of an element is equal to the average number of protons and neutrons in the atom. The Avogadro number of an element is the number of atoms or molecules in a mole. The atomic mass unit of an element is the mass of an atom expressed as 1/12 the mass of a carbon atom. Bohr Model orbital electrons n= principal quantum number n=3 2 1 Adapted from Fig. 2.1, Callister 6e. Nucleus: Z = number of protons (1 for hydrogen to 94 for plutonium) N = number of neutrons Atomic mass A≈Z+N The Electronic Structure of the Atom Quantum numbers are the numbers that assign electrons in an atom to discrete energy levels. A quantum shell is a set of fixed energy levels to which electrons belong. Pauli exclusion principle specifies that no more than two electrons in a material can have the same energy. The two electrons have opposite magnetic spins. The valence of an atom is the number of electrons in an atom that participate in bonding or chemical reactions. Electronegativity describes the tendency of an atom to gain an electron. Energy States Electrons... • have discrete energy states • tend to occupy lowest available energy state. Increasing energy n=4 n=3 n=2 n=1 4p 4s 3s 2s 1s 3p 2p 3d 7s 6s 5s 4s 3s 2s 1s 7p 6p 5p 4p 3p 2p 7d 6d 6f 5d 5f 4d 4f 3d Stable electron configurations... • have complete s and p subshells • tend to be unreactive. Z 2 10 18 36 !Element !Configuration ! !He ! ! !1s2 ! !Ne ! ! !1s22s22p6 ! !Ar ! ! !1s22s22p63s23p6 ! !Kr ! ! !1s22s22p63s23p63d104s24p6 Valence Electrons • Most elements: Electron configuration not stable. Element Atomic # Hydrogen 1 Helium 2 Lithium 3 Beryllium 4 Boron 5 Carbon 6 ... Neon 10 Sodium 11 Magnesium 12 Aluminum 13 ... Argon 18 ... ... Krypton 36 Electron configuration 1s1 1s2 ! ! ! (stable) 22s1 1s 1s22s2 1s22s22p1 1s22s22p2 ... 1s22s22p6 ! ! (stable) 1s22s22p63s1 1s22s22p63s2 1s22s22p63s23p1 ... 1s22s22p63s23p6 ! ! (stable) ... 1s22s22p63s23p63d104s246 !(stable) • Why? Valence (outer) shell usually not filled completely. Periodic Table give up 1e give up 2e give up 3e Metal Nonmetal Intermediate H Li Be Na Mg K Ca Sc Rb Sr Cs Ba Fr Ra Y Electropositive elements: Readily give up electrons to become + ions. Electronegative elements: Readily acquire electrons to become - ions. accept 2e accept 1e inert gases He Ne O S F Cl Ar Se Br Kr Te I Xe Po At Rn • Columns: Similar Valence Structure Electronegativity • Ranges from 0.7 to 4.0, • Large values: tendency to acquire electrons. H 2.1 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7 Be 1.5 Mg 1.2 Ca 1.0 Sr 1.0 Ba 0.9 Ra 0.9 Ti 1.5 Cr 1.6 Fe 1.8 Ni 1.8 Zn 1.8 As 2.0 F 4.0 Cl 3.0 Br 2.8 I 2.5 At 2.2 He Ne Ar Kr Xe Rn - Smaller electronegativity Larger electronegativity Atomic Bonding Metallic bond, Covalent bond, Ionic bond, van der Waals bond are the different types of bonds. Ductility refers to the ability of materials to be stretched or bent without breaking Van der Waals interactions Glass temperature is a temperature above which many polymers and inorganic glasses no longer behave as brittle materials Intermetallic compound is a compound such as Al3V formed by two or more metallic atoms Ionic Bond • • • • Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required. Example: NaCl Cl (nonmetal) unstable electron Na (cation) stable Na (metal) unstable + Coulombic Attraction Cl (anion) stable An ionic bond is formed between two unlike atoms with different electronegativities. When sodium donates its valence electron to chlorine, each becomes an ion; attraction occurs, and the ionic bond is formed When voltage is applied to an ionic material, entire ions must move to cause a current to flow. Ion movement is slow and the electrical conductivity is poor Ionic Bonding • Predominant bonding in Ceramics H 2.1 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7 Be 1.5 Mg 1.2 Ca 1.0 Sr 1.0 Ba 0.9 Ra 0.9 Ti 1.5 Cr 1.6 NaCl MgO CaF2 CsCl Fe 1.8 Ni 1.8 Zn 1.8 As 2.0 O F 3.5 4.0 Cl 3.0 Br 2.8 I 2.5 At 2.2 He Ne Ar Kr Xe Rn - Give up electrons Acquire electrons Covalent Bonds • Requires shared electrons • Electronegativities are comparable. • Example: CH4 C: has 4 valence e, needs 4 more H: has 1 valence e, needs 1 more Covalent bonds are directional. In silicon, a tetrahedral structure is formed, with angles of 109.5° required between each covalent bond Covalent bonding requires that electrons be shared between atoms in such a way that each atom has its outer sp orbital filled. In silicon, with a valence of four, four covalent bonds must be formed Covalent Bond H2 H 2.1 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7 Be 1.5 Mg 1.2 Ca 1.0 Sr 1.0 Ba 0.9 Ra 0.9 column IVA H2O C(diamond) SiC Ti 1.5 Cr 1.6 Fe 1.8 Ni 1.8 Zn 1.8 Ga 1.6 F2 He O 2.0 F 4.0 Cl 3.0 Br 2.8 I 2.5 At 2.2 Ne Ar Kr Xe Rn - C 2.5 Si 1.8 Ge 1.8 Sn 1.8 Pb 1.8 Cl2 As 2.0 GaAs • • • • Molecules with nonmetals Molecules with metals and nonmetals Elemental solids (RHS of Periodic Table) Compound solids (about column IVA) Metallic Bond The metallic bond forms when atoms give up their valence electrons, which then form an electron sea. The positively charged atom cores are bonded by mutual attraction to the negatively charged electrons When voltage is applied to a metal, the electrons in the electron sea can easily move and carry a current Mixed Bonding Polymers diamond H bonded polymers graphite liquid crystals • The chemical bonding of atoms or ions can involve more than one type of primary complex slats bond and can also involve secondary dipole bonds. For primary bonding there ionic ceramics can be the following combinations of mixed-bond types ionic glasses doped semiconductor transition metals alloys alkali metals • • • • Ionic-Covalent Metallic-Covalent Metallic-Ionic Ionic-Covalent-Metallic %ionic character = (1 − e − 0.25( X A − X B ) )(100%) 2 Secondary Bonding Arises from interaction between dipoles • Fluctuating dipoles asymmetric electron clouds ex: liquid H2 H2 H2 + - secondary + bonding - HH secondary bonding HH • Permanent dipoles-molecule induced -general case: -ex: liquid HCl -ex: polymer + secondary bonding secondary bonding + - H Cl secon dary b H Cl ondin g 13 Van der Waal forces Illustration of London forces, a type of a van der Waals force, between atoms Summary Type Ionic Bond Energy Large Variable (large diamond, small Bismuth) Variable (large tungsten, small mercury) Smallest Comments Non-directional (Ceramics) Directional (Semiconductors, Ceramics, Polymer chains) Non-directional (metals) Directional (inter-chain polymers, intermolecular) Binding Energy (kcal/mol) 150-370 Covalent 125-300 Metallic Secondary 25-200 <10 Bonding Energy and Interatomic Spacing Interatomic spacing is the equilibrium spacing between the centers of two atoms. Binding energy is the energy required to separate two atoms from their equilibrium spacing to an infinite distance apart. Modulus of elasticity is the slope of the stress-strain curve in the elastic region (E). Yield strength is the level of stress above which a material begins to show permanent deformation. Coefficient of thermal expansion (CTE) is the amount by which a material changes its dimensions when the temperature changes. Bond Energy Atoms or ions are separated by and equilibrium spacing that corresponds to the minimum inter-atomic energy for a pair of atoms or ions (or when zero force is acting to repel or attract the atoms or ions) The force-distance curve for two materials, showing the relationship between atomic bonding and the modulus of elasticity, a steep dF/da slope gives a high modulus Thermal Expansion The inter-atomic energy (IAE)-separation curve for two atoms. Materials that display a steep curve with a deep trough have low linear coefficients of thermal expansion Melting Temperature • Bond length, r F • Melting Temperature, Tm F r Energy (r) ro r smaller Tm larger Tm • Bond energy, Eo Energy (r) ro unstretched length r Eo= “bond energy” Tm is larger if Eo is larger. Elastic Modulus • Elastic modulus, E length, Lo cross sectional area Ao undeformed deformed Elastic modulus F ΔL =E Ao Lo ΔL F • E ~ curvature at ro Energy unstretched length ro E is larger if Eo is larger. r smaller Elastic Modulus larger Elastic Modulus Thermal Expansion • Coefficient of thermal expansion, α length, Lo unheated, T1 coeff. thermal expansion ΔL heated, T2 ΔL Lo = α (T2-T1) • α ~ symmetry at ro Energy ro larger α smaller α r α is larger if Eo is smaller. Summary Ceramics (Ionic & covalent bonding): Metals (Metallic bonding): Large bond energy large Tm large E small α Variable bond energy moderate Tm moderate E moderate α Polymers (Covalent & Secondary): secon dary bond ing Directional Properties Secondary bonding dominates small T small E large α Assignment #1 Reading: Chapter 2 Assignment Problems: ...
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