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					                                                     NCSU
The World of Atoms

Instructor:      Dr. Gerd Duscher
                 http://www4.ncsu.edu/~gjdusche
                 email: gerd_duscher@ncsu.edu

Office:          2156 Burlington Nuclear Lab.
Office Hours:    Tuesday: 10-12pm

Objective today: How do atoms arrange themselves ?
                 Why is symmetry important ?
                  Why do atoms break symmetry?
                              NCSU
What is an Atoms?
Bohr Model




         that is too simple
 How do they bond?                                                   NCSU


Ionic Bonding Covalent Bonding
                                                         shared electrons
                                              H          from carbon atom
                                     CH4
     +            -
                                      H       C        H

                                                         shared electrons
                                              H          from hydrogen
                                                         atoms
Van Der Waals Bonding
 arises from interaction between dipoles
         asymmetric electron                  -ex: liquid HCl
              clouds
                                                  van der Waals
         +    -           +     -          H Cl     bonding       H Cl
              van der Waals
                 bonding
                                                                       NCSU


What properties does that imply?
 • bond length, r                    • melting temperature, Tm
  F
                             F
                                        Energy (r)
              r
 • bond energy, Eo                          ro
                                                                r
      Energy (r)
                                                 smaller T m
        unstretched length
       ro                                        larger T m
                                 r
                   Eo =                Tm is larger if Eo is larger.
              “bond energy”
                                                                 NCSU
Summary: Primary Bonds
 Ceramics                        large bond energy
 (Ionic & covalent bonding):           large Tm
                                        large E
                                        small a

 Metals                         variable bond energy
 (Metallic bonding):                 moderate Tm
                                     moderate E
                                     moderate a

 Polymers                         directional Properties
 (Covalent & Secondary):       van der Waals bonding dominates
                                           small T
                                           small E
                                           large a
                                                                             NCSU
Energy And Packing
• Non dense, random packing




                                            energy
                                                      typical neighbor
                                                       bond length

                        typical neighbor                                 r
                         bond energy


• Dense, regular packing


                                           energy
                                                     typical neighbor
                                                      bond length


                        typical neighbor                                 r
                         bond energy

 Dense, regular-packed structures tend to have
  lower energy.
                                                            NCSU
Materials And Packing
Crystalline materials...
• atoms pack in periodic, 3D arrays
• typical of:    -metals
                 -many ceramics
                 -some polymers         crystalline SiO2



Noncrystalline materials...
 • atoms have no periodic packing
 • occurs for: -complex structures
                -rapid cooling
"Amorphous" = Noncrystalline          noncrystalline SiO2
                                                          NCSU
Metallic Crystals
• tend to be densely packed.
• have several reasons for dense packing:
 -Typically, only one element is present, so all atomic
  radii are the same.
 -Metallic bonding is not directional.
 -Nearest neighbor distances tend to be small in
  order to lower bond energy.

• have the simplest crystal structures.

  We will look at three such structures...
                                                                   NCSU
Simple Cubic Structure (sc)
• rare due to poor packing (only Po has this structure)
• close-packed directions are cube edges.

                                         • Coordination # = 6
                                           (# nearest neighbors)
Body Centered Cubic Structure                                               NCSU


(bcc)
• Close packed directions are cube diagonals.
  --Note: All atoms are identical; the center atom is shaded
    differently only for ease of viewing.


                                                     • Coordination # = 8
Face Centered Cubic Structure                                             NCSU


(fcc)
• Close packed directions are face diagonals.
  --Note: All atoms are identical; the face-centered atoms are shaded
    differently only for ease of viewing.

                                                  • Coordination # = 12
                                  NCSU
fcc Stacking Sequence
• ABCABC... stacking sequence
• 2D projection
                   A
                      B    B
                         C
                A
      A sites      B     B    B
                      C     C
      B sites         B     B
      C sites


• fcc unit cell
Hexagonal Close-Packed                             NCSU


Structure (hcp)
• ABAB... Stacking Sequence
• 3D Projection             • 2D Projection

                  A sites

                  B sites

                  A sites


• Coordination # = 12
• APF = 0.74

                                              12
 Hexagonal Close-Packed   NCSU


 Structure (hcp)
graphite
                                        NCSU
Diamond Structure




silicon, diamond    ZnS – type (GaAs)
                                                             NCSU

Structure Of Compounds: Nacl
• Compounds: Often have similar close-packed structures.
• Structure of NaCl              • Close-packed directions
                                  --along cube edges.
                                                               NCSU
Perovskite Strucutre


SrTiO3




  Applications: non-linear resistors (PTC), SMD capacitors,
  piezoelectric sensors and actuators, ferroelectric memory.
                                                                                              NCSU
Densities Of Material Classes
rmetals rceramics rpolymers                Metals/
                                                        Graphite/
                                                        Ceramics/ Polymers
                                                                                  Composites/
                                           Alloys                                   fibers
                                                        Semicond
Why?                                 30
                                                             B ased on data in Table B1, Callister
                                           Platinum
Metals have...                       20    Gold, W
                                           Tantalum
                                                              *GFRE, CFRE, & AFRE are Glass,
                                                             Carbon, & Aramid Fiber-Reinforced
 • close-packing                                            Epoxy composites (values based on
                                                            60% volume fraction of aligned fibers
    (metallic bonding)               10    Silver, Mo
                                           Cu,Ni
                                                                     in an epoxy matrix).

 • large atomic mass                       Steels

                         r (g/cm3)
                                           Tin, Zinc
                                                         Zirconia
Ceramics have...                      5
                                           Titanium
                                      4                  Al oxide
 • less dense packing                                    Diamond
                                                         Si nitride
                                      3
    (covalent bonding)                     Aluminum      Glass-soda
                                                         Concrete
                                                                                   Glass fibers
                                                         Silicon      PTFE
 • often lighter elements             2    Magnesium     G raphite
                                                                                   GFRE*
                                                                                   Carbon fibers
                                                                      Silicone     CFRE*
Polymers have...                                                      PVC
                                                                      PET
                                                                                   A ramid fibers
                                                                                   AFRE *
 • poor packing                       1                               PC
                                                                      HDPE, PS
                                                                      PP, LDPE
     (often amorphous)
 • lighter elements (C,H,O)          0.5                                           Wood
Composites have...                   0.4
                                     0.3
 • intermediate values
                                                       NCSU
 Crystals as Building Blocks

• Some engineering applications require single crystals:
  --diamond single                  --turbine blades
     crystals for abrasives




• Crystal properties reveal features
   of atomic structure.
  --Ex: Certain crystal planes in quartz
    fracture more easily than others.
                                                       NCSU
POLYCRYSTALS
• Most engineering materials are polycrystals.




       1 mm


• Nb-Hf-W plate with an electron beam weld.
• Each "grain" is a single crystal.
• If crystals are randomly oriented,
   overall component properties are not directional.
• Crystal sizes typ. range from 1 nm to 2 cm
   (i.e., from a few to millions of atomic layers).
                                                              NCSU
Single vs Polycrystals
• Single Crystals                    E (diagonal) = 273 GPa
  -properties vary with
   direction: anisotropic.
  -example: the modulus
   of elasticity (E) in bcc iron:
                                    E (edge) = 125 GPa
• Polycrystals
  -properties may/may not                   200 mm
    vary with direction.
  -if grains are randomly
    oriented: isotropic.
    (Epoly iron = 210 GPa)
  -if grains are textured,
    anisotropic.
                                         NCSU

TEMs at NCSU
           The NEW JEOL 2010F




               This is a TEM/STEM,
               which can do everything
                                          NCSU

TEMs at NCSU
  TEM Lab Course at the OLD TEM: Topcon
                           NCSU



STEM at
ORNL
This STEM provides the
smallest beam in the world.

It uses the brightest source
in the universe,
1000 times brighter than
a supernova.
                                                              NCSU
Why?

 That is what happens when pulling wires.
• before deformation           • after tensile elongation


                                                 slip steps




Dislocation move, more dislocation get generated and
entangle (interact) with themselfs, and other defects.
                                                                          NCSU
Incremental Slip
• Dislocations slip planes incrementally...
• The dislocation line (the moving red dot)...
  ...separates slipped material on the left
      from unslipped material on the right.
             push



                                              Simulation of dislocation
                                              motion from left to right
                                              as a crystal is sheared.



                fixed
                                                            NCSU
Bond Breaking And Remaking
• Dislocation motion requires the successive bumping
   of a half plane of atoms (from left to right here).
• Bonds across the slipping planes are broken and
   remade in succession.
           push




                               Atomic view of edge
                               dislocation motion from
                               left to right as a crystal
                               is sheared.



            fixed
                                                                   NCSU
Point Defects
• Vacancies:
  -vacant atomic sites in a structure.

                                                      Vacancy
      distortion
      of planes

• Self-Interstitials:
  -"extra" atoms positioned between atomic sites.

                                                    self-
     distortion                                     interstitial
     of planes
                     NCSU
Vacancy in Silicon
                                                                        NCSU
Point Defects In Alloys
Two outcomes if impurity (B) added to host (A):
 • Solid solution of B in A (i.e., random dist. of point defects)


                                OR

       Substitutional alloy                 Interstitial alloy
         (e.g., Cu in Ni)                    (e.g., C in Fe)
 • Solid solution of B in A plus particles of a new
    phase (usually for a larger amount of B)
                                   Second phase particle
                                   --different composition
                                   --often different structure.


                                                                    8
                                                                                        NCSU
    Imaging of Single Bi Atoms in Si(110)
                                         40 00

                                         35 00

                                         30 00

                                         25 00

                                         20 00

                                         15 00
                                                 0   10 0   20 0   30 0   40 0   50 0



                                         45 00

                                         40 00

                                         35 00

                                         30 00

                                         25 00

                                         20 00

                                         15 00
                                                 0   10 0   20 0   30 0   40 0   50 0




A. Lupini, VG HB501UX with Nion Aberration Corrector, 100 kV
                                               NCSU
Types of Imperfections
• Vacancy atoms
• Interstitial atoms     Point defects
• Substitutional atoms   (0 dimensinal)
• Anti-site defects


• Dislocations           Line defects
                         (1 dimensional)

• Grain Boundaries       Area defects
                         (2dimensional)

                                           2

				
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posted:3/15/2013
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