1. NCSU The World of Atoms Instructor: Dr. Gerd Duscher http:// www4.ncsu.edu/~gjdusche http:// www4.ncsu.edu/~gjdusche email: gerd_duscher@ncsu.edugerd_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

2. What is an Atoms? Bohr Model that is too simple

3. NCSU Ionic Bonding + - Covalent Bonding shared electrons from carbon atom shared electrons from hydrogen atoms H H H H C CH 4 arises from interaction between dipoles -ex: liquid HCl asymmetric electron clouds + - + - van der Waals bonding H Cl H van der Waals bonding Van Der Waals Bonding How do they bond?

4. NCSU bond length, r bond energy, E o melting temperature, T m T m is larger if E o is larger. What properties does that imply? F F r r larger T m smaller T m Energy (r) r o E o = “bond energy” Energy (r) r o r unstretched length

5. NCSU 18 Ceramics (Ionic & covalent bonding): Metals (Metallic bonding): Polymers (Covalent & Secondary): large bond energy large T m large E small  variable bond energy moderate T m moderate E moderate  directional Properties van der Waals bonding dominates small T small E large  Summary: Primary Bonds secondary bonding

6. NCSU Non dense, random packing Dense, regular packing Dense, regular-packed structures tend to have lower energy. Energy And Packing r typical neighbor bond length typical neighbor bond energy energy r typical neighbor bond length typical neighbor bond energy energy

7. NCSU atoms pack in periodic, 3D arrays typical of: Crystalline materials... -metals -many ceramics -some polymers atoms have no periodic packing occurs for: Noncrystalline materials... -complex structures -rapid cooling crystalline SiO 2 noncrystalline SiO 2 "Amorphous" = Noncrystalline Materials And Packing

8. NCSU 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... Metallic Crystals

9. NCSU rare due to poor packing (only Po has this structure) close-packed directions are cube edges. Coordination # = 6 (# nearest neighbors) Simple Cubic Structure (sc)

10. NCSU Coordination # = 8 Close packed directions are cube diagonals. --Note: All atoms are identical; the center atom is shaded differently only for ease of viewing. Body Centered Cubic Structure (bcc)

11. NCSU Coordination # = 12 Close packed directions are face diagonals. --Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing. Face Centered Cubic Structure (fcc)

12. NCSU 11 ABCABC... stacking sequence 2D projection fcc unit cell fcc Stacking Sequence A sites B sites C B B B B B BB C C C A A

13. NCSU 12 Coordination # = 12 ABAB... Stacking Sequence APF = 0.74 3D Projection 2D Projection Hexagonal Close-Packed Structure (hcp)

14. NCSU Hexagonal Close-Packed Structure (hcp) graphite

15. NCSU Diamond Structure silicon, diamond ZnS – type (GaAs)

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

17. NCSU Perovskite Strucutre SrTiO 3 Applications: non-linear resistors (PTC), SMD capacitors, piezoelectric sensors and actuators, ferroelectric memory.

18. NCSU 16 Why? Metals have... close-packing (metallic bonding) large atomic mass Ceramics have... less dense packing (covalent bonding) often lighter elements Polymers have... poor packing (often amorphous) lighter elements (C,H,O) Composites have... intermediate values Densities Of Material Classes  metals  ceramics  polymers  (g/cm 3 ) Graphite/ Ceramics/ Semicond Metals/ Alloys Composites/ fibers Polymers 1 2 20 30 Based on data in Table B1, Callister *GFRE, CFRE, & AFRE are Glass, Carbon, & Aramid Fiber-Reinforced Epoxy composites (values based on 60% volume fraction of aligned fibers in an epoxy matrix). 10 3 4 5 0.3 0.4 0.5 Magnesium Aluminum Steels Titanium Cu,Ni Tin, Zinc Silver, Mo Tantalum Gold, W Platinum Graphite Silicon Glass-soda Concrete Si nitride Diamond Al oxide Zirconia HDPE, PS PP, LDPE PC PTFE PET PVC Silicone Wood AFRE* CFRE* GFRE* Glass fibers Carbonfibers Aramid fibers

19. NCSU Some engineering applications require single crystals: Crystal properties reveal features of atomic structure. --Ex: Certain crystal planes in quartz fracture more easily than others. --diamond single crystals for abrasives --turbine blades Crystals as Building Blocks

20. NCSU Most engineering materials are polycrystals. 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). 1 mm POLYCRYSTALS

21. NCSU Single Crystals -properties vary with direction: anisotropic. -example: the modulus of elasticity (E) in bcc iron: Polycrystals -properties may/may not vary with direction. -if grains are randomly oriented: isotropic. (E poly iron = 210 GPa) -if grains are textured, anisotropic. 200 mm Single vs Polycrystals E (diagonal) = 273 GPa E (edge) = 125 GPa

22. NCSU TEMs at NCSU The NEW JEOL 2010F This is a TEM/STEM, which can do everything

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

24. 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.

25. NCSU Why? before deformation after tensile elongation slip steps That is what happens when pulling wires. Dislocation move, more dislocation get generated and entangle (interact) with themselfs, and other defects.

26. NCSU Dislocations slip planes incrementally... The dislocation line (the moving red dot)......separates slipped material on the left from unslipped material on the right. Simulation of dislocation motion from left to right as a crystal is sheared. Incremental Slip push fixed

27. NCSU 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. Atomic view of edge dislocation motion from left to right as a crystal is sheared. Bond Breaking And Remaking push fixed

28. NCSU Vacancies: -vacant atomic sites in a structure. Self-Interstitials: -"extra" atoms positioned between atomic sites. Point Defects

29. NCSU Vacancy in Silicon

30. NCSU 8 Two outcomes if impurity (B) added to host (A): Solid solution of B in A (i.e., random dist. of point defects) Solid solution of B in A plus particles of a new phase (usually for a larger amount of B) OR Substitutional alloy (e.g., Cu in Ni) Interstitial alloy (e.g., C in Fe) Second phase particle --different composition --often different structure. Point Defects In Alloys

31. NCSU Imaging of Single Bi Atoms in Si(110) A. Lupini, VG HB501UX with Nion Aberration Corrector, 100 kV

32. NCSU 2 Vacancy atoms Interstitial atoms Substitutional atoms Anti-site defects Dislocations Grain Boundaries Point defects (0 dimensinal) Line defects (1 dimensional) Area defects (2dimensional) Types of Imperfections