2: MASS TRANSFER MEDIA

2: MASS TRANSFER MEDIA

1. ATOMIC STRUCTURE & INTERATOMIC BONDING

   • Electrons have wavelike and particulate properties
     • Wavelike characteristics
       • Electrons are in orbitals defined by a probability
       • Each orbital at discrete energy level is determined by quantum numbers
   • Determined by electronic structure
     • Chemical
     • Electrical
     • Thermal
     • Optical
   • Bonding
     • Ionic
       • Large bonding energy
         • Large Tm, large E, small a
       • Nondirectional (ceramics)
     • Covalent
       • Variable bonding energy
         • Diamond - large
         • Bismuth - small
         • Moderate Tm, moderate E, moderate a
       • Directional (semiconductors, ceramics, polymer chains)
     • Metallic
       • Variable bonding energy
         • Tungsten - large
         • Mercury - small
       • Nondirectional (metals)
     • Secondary
       • Smallest bonding energy
         • Small Tm, small E, large a
       • Directional - interchain (polymer), intermolecular
     • Properties
       • Bond length r
       • Melting Temperature Tm
         • Tm is larger when Eo is larger
       • Bond Energy Eo
       • Coefficient of thermal expansion a
         • a is larger when Eo is smaller
   • Energy and Packing
     • Non dense, random packing
     • Dense, ordered packing
       • Tend to have lower energies
     • Crystalline materials
       • Atoms pack in periodic 3D arrays
       • Metals, ceramics, some polymers
     • Noncrystalline materials
       • No periodic packing
       • Complex structures, rapid cooling
       • Amorphous
   • Crystal Systems
     • Unit cell - smallest repetitive volume which contains the complete lattice pattern of a crystal
   • Atoms may assemble into crystalline or amorphous structures
   • Crystallographic points, directions and planes are specified in terms of indexing schemes
     • Related to atomic linear and planar densities
   • Anisotopic: material properties vary with single crystal orientation
   • Iso-tropic: material properties are non-directional
   • Metallic Crystal Structure
     • Densely packed
       • Only one element present - all atomic radii the same
       • Metallic bonding is not directional
       • Neighbor distances small in order to lower bond energy
       • Electron cloud shields cores from each other
     • Simplest crystal structures!!!!!!!
     • SC (Simple Cubic)
       • Only Po has this structure
       • Close-packed directions are cube edges
       • Low packing density
       • Coordination # = 6
     • BCC (Body Centered Cubic)
       • Atoms touch each other along cube diagonals
       • e.g. Cr, W, Tantalum, Molybdenum
       • Coordination # = 8
     • FCC (Face Centered Cubic)
       • Atoms touch each other along face diagonals
       • e.g. Al, Cu, Au, Pb, Ni, Pt, Ag
       • Coordination # = 12
       • Stacking sequence ABCABC in 2D projection
     • HCP (Hexagonal Close-Packed)
       • ABAB stacking sequence
       • Coordination # = 12
       • 2D or 3D projection
         • 2D e.g. Cd, Mg, Ti, Zn

2. IMPERFECTIONS IN SOLIDS

   • Solidification - result of casting of molten material
     • 1. Nuceli form
     • 2. Nuclei grow to form crystals - grain structure
       • Crystals grow until they join
   • Polycrystalline Materials
     • Grain boundaries
       • Transition from lattice of one region to that of another
       • Slightly disordered
       • Low density in grain boundaries
         • High mobility
         • High diffusivity
         • High chemical reactivity
   • Point defects
     • Vacancy atoms
       • Vacant atomic sites causing distortion of planes
     • Interstital atoms
       • Extra atoms in atomic sites causing distortion of planes
     • Substitutional atoms
       • Different composition
       • Different structure
   • Line defects
     • Dislocations
       • Slip between crystal planes when dislocations move
       • Permanent plastic deformation
       • 1D defects around which atoms are misaligned
       • Edge dislocation
         • Extra half-plane of atoms inserted in a crystal structure
         • b perpendicular to dislocation line
           • Burger's vector b: measure of lattice distortion
             • Also known as slip direction
       • Screw dislocation
         • Spiral planar ramp from shear deformation
         • b parallel to dislocation line
       • In metals plastic deformation is occurs by slip
         • An edge dislocation (extra half-plane of atoms) slides over adjacent half-planes of atoms
   • Strengthening
     • 1. Reduce grain size
       • Grain boundaries are barriers to slip
       • Hall-Petch equation
     • 2. Form solid solutions
       • Impurity atoms distort lattice and generate strains
         • Barriers to dislocation motion
       • Smaller/larger substitutional impurities generate local stresses
         • Large impurities concentrate at regions of tensile strains
         • Small impurities concentrate at regions of compressive strains
     • 3. Precipitation strengthening
       • Hard precipitates are difficult to shear
       • e.g. Ceramics in metals (SiC in Iron)
       • Precipitates act as "pinning" sites with spacing S
     • 4. Cold work (strain hardening)
       • Deformation at room temperature for most metals
       • Common forming operations reduce cross-sectional area
       • Dislocations entangle with one another during cold work
         • Dislocation motion becomes more difficult
       • Dislocation density = total dislocation length/unit volume
     • Shot peening
       • 1. Part to be hardened placed in chamber
       • 2. Hard find particles strike surface at high speeds
       • 3. Energy of particles causes local plastic deformation
       • Low cost
       • Increase in crack propagation resistance
     • Surface phase transition diagram

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