Topic 3: Structure of Materials Five levels of structure: electronic structure, atomic structure, crystal structure, microstructure and macrostructure Bonding between atoms (electronic & atomic structure) determines structure at higher levels (crystal, micro- and macro-structure) Structure & defects in structure ultimately determine material properties Outline: –Types of bonding (Ch. 5) –Crystal structures, non-crystallinity (Ch. 6 & 7) –Defects in crystals (Ch. 8) Reading assignment: Ch. 9 & 10

Bonding (Electronic & Atomic Structure) Example: Sodium (Na) atomic number electrons Each shell can hold a total of 2n 2 electrons. Most atoms strive to achieve 8 electrons in their outermost shell. In the case of Na, it is much easier for Na to lose the 1 electron (resulting in 8 electrons in the n = 2 shell) rather than grab 7 electrons for the n = 3 shell Electrons in outermost shell are called valence electrons electron Nucleus (protons + neutrons) Shell (indexed by n) Valence electron

Types of bonding Depending on the type of each atom and the environment each atom sees, several bonding types arise Primary bonds (strong): metallic, ionic, covalent Secondary bonds (weak): hydrogen & van der Waals Generally, combination of more than one type of bonding exists in a solid

Metallic bonds Metallic elements have only one, two, or at most three valence electrons. In a metallic solid, the atoms give up all valence electrons to a “common pool” or “sea” of electrons. The remaining non-valence electrons and nuclei form positive ion “cores”. The valence electrons are free to drift throughout the entire lattice and act as electron glue holding the material together Metallic bonds have no directionality and are strong in all directions. They are good electrical and thermal conductors because of the free electrons Atoms can pack closely, as a result, metals are heavy and dense Atoms can move over each other and change position relatively easily (more about this later). So, metals can be formed by pressing, forging, rolling, etc. Na + Positive ion cores electrons

Ionic bonds Examples: NaCl, Al 2 O 3, MgO, CaO, H 2 O In ionic materials, atoms achieve their eight electrons in the outer shells by giving up valence electrons (e.g., Na) or by accepting electrons (e.g., Cl). The resulting positively and negatively charged ions attract each other and produce an ionic bond. Bonds very strong and ions difficult to move As no free electrons, poor electrical and poor thermal conductors Na + Cl - Na + Cl - Na + Cl - Na + Cl - Na + Cl - Na + Cl - Na + Cl - Na + Cl -

Examples: C, Si, GaAs, SiC, H 2 While metal atoms share their electrons throughout the piece of metal, in covalent materials, atoms share electrons with specific nearby atoms to achieve 8 electrons in their outer shells. For example, atomic number of carbon is 6  4 valence electrons; in the diamond form of carbon, each atom gets its 8 electrons by sharing its 4 valence electrons with 4 neighboring atoms Bonds are directional, because electrons at one bond repel electrons at other bonds. In diamond, there is a degree angle between neighboring bonds Covalent bonds are strong: hardest material is diamond Poor electrical and thermal conductor because of no free electrons Covalent bonds Carbon: 4 valence electrons

Bond responsible for holding water molecules in ice In water, H atoms are covalently bonded to O, but the electrons are somewhat shifted towards the O atoms creating polarity (combination of ionic and covalent) Hydrogen bond: Water molecules will arrange in a hexagonal symmetry in ice; this accounts for the shape of snowflakes that have the form of a complex 6-pointed star Hydrogen bond is weak as charges involved are small Loose packing in ice which is why density of ice lower than water O H H ++ - Hydrogen bond Hydrogen bonds Oxygen: 6 valence electrons Hydrogen: 1 valence electrons

van der Walls bonds Weakest of all chemical bonds Result from weak electrostatic interaction; polarity or dipole of molecules is not permanent or fixed in direction Closed shell atoms interact through van der Walls bonding; example: solids of inert gas atoms, graphite, polymers Materials with both primary and secondary bonds: graphite (covalent in plane, van der Waals between planes), polymers (covalent along chain, van der Waals between chains) In summary, chemical bonds affect: strength, ductility, melting point, electrical and thermal conductivity, density, crystal structure, shape of solids, etc.

Close packed crystals A plane B plane C plane A plane …ABCABCABC… packing…ABABAB… packing

Close packed structures Cubic close packed (CCP) or Face centered cubic (FCC) Hexagonal close packed (HCP)

Three common crystal structures in metals: –Face centered cubic (fcc): ABCABC… packing: Ni, Cu, Ag, Al, Au –Hexagonal close packed (hcp): ABABAB … packing: Mg, Zn, Co, Ti –Body centered cubic (bcc): Fe, Cr, W, Ta, Mo Easy for close packed planes to slide over each other: slip planes (plays an important role in determining deformation & strength) Crystal Structures of Metals BCC unit cell shear

Crystal Structure Larger scale packing of atoms, molecules or other building blocks; primarily determined by the bonding between atoms and molecules Lattice: infinite array of points in space; all points have identical surroundings; mathematical construct Crystal structure: Associate each lattice point with one or more atoms

Crystal Lattices

Other Crystal Structures Diamond (carbon, silicon) lattice: FCC lattice, with 2 atoms per lattice point Graphite: hexagonal lattice Sodium Chloride (NaCl): FCC lattice with 2 atoms (1 sodium & 1 chlorine) per lattice point Polyethylene:

Amorphous Materials No long range order, only short range order Created by heating (atoms break away bonds and get dislodged from their equilibrium positions), followed by quenching – rapid cooling so that atoms do not have time to return to their equilibrium crystal structures Amorphous glass (SiO 2 ) occurs naturally Sometimes we intentionally create amorphous metals (although very difficult) to create very strong (but brittle) materials [remember, no close-packed slip planes in amorphous materials], or novel magnetic materials

Defects in Crystals Dislocations (line defects: 1-dimensional) Point defects (0-dimensional) Grain boundaries (planar defects: 2- dimensional)

Dislocation: line defect (1-dimensional) Extra “half-plane” of atoms in a crystal Dislocations make slip 1000 times easier, which is why metals deform easily Slip of atom planes over each other due to deformation occurs one atom row at a time, analogous to caterpillar motion or moving a pile of bricks one at a time mediates

Point Defects (0-dimensional) An avenue for atomic motion within the lattice, in response to an external mechanical or electrical load In stainless steel, carbon, which makes it a steel, is an interstitial impurity in the iron lattice (and chromium, which makes it stainless, is a substitutional impurity) In semiconductors, substitutional impurities are called dopants, and control the amount of charge carriers Intrinsic (vacancies) Extrinsic (interstitial and substitutional impurity atoms) Alter the mechanical properties (by affecting slip and dislocation motion), electronic properties (doping in semiconductors), etc.

Grain Boundaries, Microstructure & Macrostructure: planar defects (2-dimensional) During solidification … Boundary between two different phases or materials, or between two crystallites of the same material but oriented wrt each other Affects mechanical properties by affecting point and line defect motion Other defects: voids, porosity, precipitates, secondary inclusions

Topic 3: Structure of Materials Five levels of structure: electronic structure, atomic structure, crystal structure, microstructure and macrostructure Bonding between atoms (electronic & atomic structure) determines structure at higher levels (crystal, micro- and macro-structure) Structure & defects in structure ultimately determine material properties Outline: –Types of bonding (Ch. 5) –Crystal structures, non-crystallinity (Ch. 6 & 7) –Defects in crystals (Ch. 8) Reading assignment: Ch. 9 & 10