Assist. Prof. Bilge Imer

1. MSN 504 Phase Transformations & Diffusion in Materials Assist. Prof. Bilge Imer

2. Phase • A phase is a physically distinct, homogeneous portion of a thermodynamic system delineated in space by a bounding surface, the interphase interface, and distinguished by its state of aggregation (solid, liquid or gas), its crystal structure, composition and/or degree of order. Each phase generally exhibits a characteristic set of physical, mechanical and chemical properties and is conceivably mechanically separable from the whole. Bilkent University Institute of Materials Science and Nanotechnology

3. Phase Transformation • A Phase transformation is a change in the state of an assembly of interacting particles (atoms, molecules, electrons, etc.) as indicated by qualitative changes in the physical, mechanical and chemical properties induced by small quantitative changes in the thermodynamic variables such as T, P, E (electric field), H (magnetic field), etc. The rearrangement of the constituent particles carries the system from one configuration to another of lower free energy which can be described generally by one or several so-called order parameters which define the particular state of the system. Bilkent University Institute of Materials Science and Nanotechnology

4. Diffusion • It is a form of mass transport. In liquids and gases mass transport occurs in the form of convection and diffusion while in solids it only occurs with diffusion. • It can be said that diffusion is the movement of particles/atoms/electrons/defects in a matter from high to low concentration in the presence of gradient until equilibrium is reached. Bilkent University Institute of Materials Science and Nanotechnology

5. Materials Nanomaterials Bilkent University Institute of Materials Science and Nanotechnology

6. Types of Materials • Materials can be classified according to structural, physical, electrical, optical and magnetic properties, area of use, etc. All these properties are closely related with bonding type and energies between atoms. • However if a group of material shows close resemblance in all properties we can classify them in one category. So according to this: Metals, Polymers, Ceramics and Composites can be the general classification of materials. Bilkent University Institute of Materials Science and Nanotechnology

7. Periodic Table Bilkent University Institute of Materials Science and Nanotechnology

8. Courtesy of Prof. Erman Bengu, CHEM 201 Atomic Configuration • Most elements: Electron configuration not stable. Adapted from Table 2.2, Callister 7e. • Valance electrons determine chemical, electrical, thermal and optical properties, and they are responsible for bonding 5

9. THE PERIODIC TABLE Courtesy of Prof. Erman Bengu, CHEM 201 • Columns: Similar Valence Structure, Similar Properties Electropositive elements: Readily give up electrons to become + ions. Electronegative elements: Readily acquire electrons to become - ions.

10. Bonding types Bilkent University Institute of Materials Science and Nanotechnology

11. Courtesy of Prof. Erman Bengu, CHEM 201 Atomic Bonding in Solids • Start with two atoms infinitely separated • Attractive component is due to nature of the bonding (minimize energy thru electronic configuration) • Repulsive component is due to Pauli exclusion principle; electron clouds tend to overlap • Essentially atoms either want to give up (transfer) or acquire (share) electrons to complete electron configurations; minimize their energy • Transfer of electrons => ionic bond • Sharing of electrons => covalent • Metallic bond => sea of electons r

12. METALLIC BONDING Courtesy of Prof. Erman Bengu, CHEM 201 • Arises from a sea of donated valence electrons (1, 2, or 3 from each atom). Ion cores in the “sea of electrons”. Valance electrons belong no one particular atom but drift throughout the entire metal. “Free electrons” shield +’ly charged ions from repelling each other… Adapted from Fig. 2.11, Callister 6e. • Primary bond for metals and their alloys

13. IONIC BONDING Courtesy of Prof. Erman Bengu, CHEM 201 • Occurs between + and – ions (anion and cation). • Requires electron transfer. • Large difference in electronegativity required. • Example: Na+ Cl-

14. COVALENT BONDING Courtesy of Prof. Erman Bengu, CHEM 201 • Requires shared electrons • Example: CH4 C: has 4 valence e, needs 4 more H: has 1 valence e, needs 1 more Electronegativities are comparable. Adapted from Fig. 2.10, Callister 6e.

15. Summary: Primary Bonds secondary bonding Courtesy of Prof. Erman Bengu, CHEM 201 Ceramics Large bond energy large Tm large E small a (Ionic & covalent bonding): Metals Variable bond energy moderate Tm moderate E moderate a (Metallic bonding): Polymers Directional Properties Secondary bonding dominates small Tm small E large a (Covalent & Secondary):

16. SECONDARY BONDING ex: liquid H 2 asymmetric electron H H 2 2 clouds + - + - H H H H secondary secondary bonding bonding + - + - Cl Cl H H secondary bonding Courtesy of Prof. Erman Bengu, CHEM 201 Arises from interaction between dipoles • Fluctuating dipoles Adapted from Fig. 2.13, Callister 7e. • Permanent dipoles-molecule induced secondary -general case: bonding Adapted from Fig. 2.14, Callister 7e. secondary -ex: liquid HCl bonding -ex: polymer secondary bonding

17. Summary: Bonding Courtesy of Prof. Erman Bengu, CHEM 201 Comments Type Bond Energy Ionic Large! Nondirectional (ceramics) Directional (semiconductors, ceramics polymer chains) Covalent Variable large-Diamond small-Bismuth Metallic Variable large-Tungsten Nondirectional (metals) small-Mercury Secondary smallest Directional inter-chain (polymer) inter-molecular

18. Energy and Packing Energy typical neighbor bond length typical neighbor r bond energy • Dense, ordered packing Energy typical neighbor bond length r typical neighbor bond energy Courtesy of Prof. Erman Bengu, CHEM 201 • Non dense, random packing COOLING Dense, ordered packed structures tend to have lower energies.

19. MATERIALS AND PACKING Courtesy of Prof. Erman Bengu, CHEM 201 Crystalline materials... • atoms pack in periodic, 3D arrays • typical of: -metals -many ceramics -some polymers crystalline SiO2 Adapted from Fig. 3.18(a), Callister 6e. LONG RANGE ORDER Noncrystalline materials... • atoms have no periodic packing • occurs for: -complex structures -rapid cooling "Amorphous" = Noncrystalline noncrystalline SiO2 Adapted from Fig. 3.18(b), Callister 6e. SHORT RANGE ORDER

20. SIMPLE CUBIC STRUCTURE (SC) Courtesy of Prof. Erman Bengu, CHEM 201 • Rare due to poor packing (only Po has this structure) • Close-packed directions are cube edges. Closed packed direction is where the atoms touch each other • Coordination # = 6 (# nearest neighbors) (Courtesy P.M. Anderson)

21. BODY CENTERED CUBIC STRUCTURE (BCC) Courtesy of Prof. Erman Bengu, CHEM 201 • Close packed directions are cube diagonals. --Note: All atoms are identical; the center atom is shaded differently only for ease of viewing. ex: Cr, W, Fe (), Tantalum, Molybdenum • Coordination # = 8 2 atoms/unit cell: 1 center + 8 corners x 1/8 (Courtesy P.M. Anderson)

22. FACE CENTERED CUBIC STRUCTURE (FCC) Courtesy of Prof. Erman Bengu, CHEM 201 • Close packed directions are face diagonals. --Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing. ex: Al, Cu, Au, Pb, Ni, Pt, Ag • Coordination # = 12 Adapted from Fig. 3.1, Callister 7e. 4 atoms/unit cell: 6 face x 1/2 + 8 corners x 1/8 (Courtesy P.M. Anderson)

23. FCC STACKING SEQUENCE Courtesy of Prof. Erman Bengu, CHEM 201 • ABCABC... Stacking Sequence • 2D Projection • FCC Unit Cell

24. HEXAGONAL CLOSE-PACKED STRUCTURE (HCP) Courtesy of Prof. Erman Bengu, CHEM 201 • ABAB... Stacking Sequence • 3D Projection • 2D Projection Adapted from Fig. 3.3, Callister 6e. 6 atoms/unit cell • Coordination # = 12 ex: Cd, Mg, Ti, Zn • APF = 0.74 • c/a = 1.633

25. COORDINATION # AND IONIC RADII Courtesy of Prof. Erman Bengu, CHEM 201 • Coordination # increases with Adapted from Fig. 12.4, Callister 6e. Adapted from Fig. 12.2, Callister 6e. Adapted from Fig. 12.3, Callister 6e. Adapted from Table 12.2, Callister 6e.

26. Courtesy of Prof. Erman Bengu, CHEM 201 Imperfections in Solids Is it enough to know bonding and structure of materials to estimate their macro properties ? BONDING + STRUCTURE + DEFECTS PROPERTIES Defects do have a significant impact on the properties of materials

27. Courtesy of Prof. Erman Bengu, CHEM 201 Imperfections in Solids Crystals in nature are never perfect, they have defects ! Defects in Solids 0-D, Point defects Vacancy Interstitial Substitutional 1-D, Line Defects / Dislocations Edge Screw 2-D, Area Defects / Grain boundaries Tilt Twist 3-D, Bulk or Volume defects Crack, pore Secondary Phase Atoms in irregular positions MATERIALS PROPERTIES Planes or groups of atoms in irregular positions Interfaces between homogeneous regions of atoms

28. Courtesy of Prof. Erman Bengu, CHEM 201 Imperfections in Solids Atomic Composition Bonding Microstructure: Materials properties Thermo-Mechanical Processing X’tal Structure Addition and manipulation of defects

29. POINT DEFECTS Courtesy of Prof. Erman Bengu, CHEM 201 • Vacancies: -vacant atomic/lattice sites in a structure. • Self-Interstitials: -"extra" atoms positioned between atomic sites.

30. Courtesy of Prof. Erman Bengu, CHEM 201 Point Defects: Vacancies & Interstitials • Most common defects in crystalline solids are point defects. • At hightemperatures, atoms frequently and randomlychange their positions leaving behind empty lattice sites. • In general,diffusion (mass transportby atomic motion) - can only occur because of vacancies.

31. Courtesy of Prof. Erman Bengu, CHEM 201 Point Defects: Vacancies & Interstitials Schematic representation of a variety of point defects: (1) vacancy; (2) self-interstitial; (3) interstitial impurity; (4,5) substitutional impurities The arrows represent the local stresses introduced by the point defects. less distortion caused