Structure-Property Correlation [2] Atomic bonding and material properties

Transcription

1 MME 297: Lecture 05 Structure-Property Correlation [2] Atomic bonding and material properties Dr. A. K. M. Bazlur Rashid Professor, Department of MME BUET, Dhaka Topics to discuss today... Review of atomic bonding Properties from bonding Energy diagrams and properties of materials References: 1. W D Callister, Jr. and D G Rethwisch. Materials Science and Engineering An Introduction 9th Ed, Wiley, pp /28

2 REVIEW OF ATOMIC BONDING Why Review Bonding? Atoms form the fundamental building blocks from which all matter is made. The properties of a solid depend exquisitely on the types of atoms from which it is made. It makes sense to begin any study of materials with a review of the chemistry and physics of atoms. 3/28 Bonding in Solids Chemical bonds between atoms occur when the valence electrons in the atoms interact with each other in such a way that the overall energy is more favorable than when the atoms are separated The energy is most favorable when each atom can obtain eight outershell electrons (an octet), which gives it a noble gas configuration Three primary ways for atoms to achieve an octet in the outer shell: give or take valence electrons (ionic bonding) share valence electrons with neighboring atoms (covalent bonding) share valence electrons with all atoms (metallic bonding) 4/28

3 Primary Bonds Electrons are transferred or shared Strong ( KJ/mol, or 1-10 ev/atom) 1. Metallic: atoms are ionized, loosing some electrons from the valence band. Those electrons form a electron sea, which binds the charged nuclei in place. 2. Covalent: electrons are shared between the molecules, to saturate the valence band. Example - H 2 3. Ionic: strong Coulombic interaction among negative atoms (have an extra electron each) and positive atoms (lost an electron). Example - Na + Cl - 5/28 Secondary Bonding No transfer or share of electrons Interaction of atomic/molecular dipoles Weak (< 100 KJ/mol or < 1 ev/atom) Exists between virtually all atoms or molecules Evidenced for inert gases, between molecules of covalently bonded structures Coulombic attraction between opposite charges Very weak bonding energy, typically of the order of only 10 kj/mol (0.1 ev) No electron sharing/transfer Asymmetric charge distribution Formation of atomic or molecular dipole (permanent, or temporary) 6/28

4 Mixed Bonding A very few compounds exhibit pure ionic or covalent bonding. Iron, for example, is bonded by a combination of metallic and covalent bonding Compounds formed from two or more metals (intermetallic compounds) may be bonded by a mixture of metallic and ionic bonds (particularly when there is a large electronegativity difference between the elements) Ceramics and semiconducting compounds having metallic and nonmetallic elements have a mixture of covalent and ionic bonds Most polymeric materials have a mixture of covalent and secondary bonds. 7/28 Covalent Bonding Bonding tetrahedron: each of the four extreme (or pure) bonding types is located at one corner of the tetrahedron four mixed bonding types are included along tetrahedron edges Metallic Bonding Ionic Bonding van der Waals Bonding Covalent semi-conductors Material-type tetrahedron: correlation of each material classification (metals, ceramics, polymers, etc.) with its type(s) of bonding Metals (Metallic) Molecular Solids (van der Waals) Metals Metallic Ceramics Ionic and/or Covalent Polymers Covalent and van der Waals Semiconductors Covalent or Covalent and Ionic Ionic 8/28

5 Summary : Properties of bonding Type Bond energy Properties of bonding Ionic Large! Non-directional (ceramics) Covalent Variable Directional Large Diamond Small Bismuth (semiconductor, ceramics, polymer chains) Metallic Variable Non-directional Large Tungsten Small Mercury (metals) Secondary Smallest Directional Inter-chain (polymers) Intermolecular (water) 9/28 PROPERTIES FROM BONDING Properties from Metallic Bond Atoms joined by metallic bond can shift their relative positions (without breaking) when the metal is deformed, permitting metals to have good ductility. 10/28

6 When voltage is applied to a metal, the electrons in the electron cloud can easily move and carry a current Brooks/Cole Publishing / Thomson Learning 11/28 Properties from Covalent Bond When a silicon rod is bent, the bonds must break if the silicon atoms are to permanently change their relationships to one another. For an electron to move and carry a current, the covalent bond must be broken, requiring high temperatures or voltage. Thus covalent materials are brittle rather than ductile, and behave as electrical insulators instead of conductors. Many ceramics, semiconductors, and polymers are fully or partially bonded by covalent bonds, explaining why glass shatters when dropped and why bricks are good insulating materials. 12/28

7 Properties from Ionic Bond Solids that exhibit considerable ionic bonding are also often mechanically strong because of the strength of the bonds. Electrical conductivity of ionically bonded solids is very limited. A large fraction of the electrical current is transferred via the movement of ions and cause ionic conductivity. When voltage is applied to an ionic material, entire ions must move to cause a current to flow. Owing to their size, ions typically do not move as easily as electrons. Ions move slowly and, thus, the ionic conductivities of these material are poor Brooks/Cole Publishing / Thomson Learning 13/28 However, in many technological applications we make use of the electrical conduction that can occur via movement of ions as a result of increased temperature, chemical potential gradient, or an electrochemical driving force. Examples of these include: lithium ion batteries that make use of lithium cobalt oxide conductive indium tin oxide coatings on glass for touch sensitive screens for displays, and solid oxide fuel cells based on compositions based on zirconia (ZrO 2 ) 14/28

8 Properties from Secondary Bond (a) Within each chain, bonding between carbon is covalent. The individual chains are weakly bonded to one another (between chlorine and hydrogen atoms) by van der Waals bonds. This additional bonding makes PVC stiffer. (b) When a force is applied to the polymer, the van der Waals bonds are broken and the chains slide past one another Brooks/Cole Publishing / Thomson Learning Mixed bonding in polyvinyl chloride (PVC) (c) When heat is applied, the secondary bonds melts easily, thus making polymers as lowmelting materials. 15/28 Dislocations Controlling Deformation Line defects are imperfections in a crystal structure for which a row of atoms have a local structure that differs from the surrounding crystal. They are almost always present in a real crystals. In a typical material, about 5 out of every 100 million atoms ( %) belongs to a line defect. In a 10-cm 3 chunk of material (about the size of a six-sided die), there will be about atoms belonging to line defects! Transmission electron micrograph of nickel showing dislocations (dark lines and loops) The resistance offered by a material to the motion of dislocation determines the ductility of the material. the greater the resistance, the less ductile the material. 16/28

9 dislocation METAL Dislocation motion through the electron cloud is intrinsically easy in pure metals (though alloying to give solid solutions or precipitates can make it more difficult). 17/28 COVALENT CERAMIC Dislocation motion in covalent solids is intrinsically difficult because the interatomic bonds must be broken and reformed. 18/28

10 IONIC CERAMIC Dislocation motion in ionic crystals is easy on some planes, but hard on others. The hard systems usually dominate. 19/28 BINDING ENERGY AND MATERIALS PROPERTIES Interatomic spacing is the equilibrium spacing between the centers of two atoms. Binding energy is the energy required to separate two atoms from their equilibrium spacing to an infinite distance apart. Modulus of elasticity (E) is the slope of the stress-strain curve in the elastic region. Yield strength (s y ) is the level of stress above which a material begins to show permanent deformation. Coefficient of thermal expansion (a) is the amount by which a material changes its dimensions when the temperature changes. 20/28

11 Atoms or ions are separated by an equilibrium interatomic spacing r 0 that corresponds to the minimum inter-atomic energy (aka the binding energy) for a pair of atoms or ions (or when zero force is acting to repel or attract the atoms or ions) Binding energies for the four bonding mechanisms Ionic bond kcal/mol Covalent bond Metallic bond Van der Waals <10 21/28 Bonding Energies and Melting Temperatures for Various Substances 22/28

12 Binding Energy and the Melting Point Energy r 0 = equilibrium distance r E 0 = bond energy Energy DH F = T m DS F r 0 larger T m smaller T m r Melting temperature, T m, is larger, if the bond energy, E 0, at r 0 is larger, and the radius of curvature of the energy-distance curve is smaller. 23/28 Binding Energy and the Modulus of Elasticity Stiffness is directly related to the curvature of Force Distance or, Energy Distance curve Elastic modulus, E, is larger, if the bond energy, E 0, at r 0 is larger, and the radius of curvature of the energy-distance curve is smaller. 24/28

13 A steep df/da slope gives a high modulus Strong bonds are stiffer than weaker bonds 2003 Brooks/Cole Publishing / Thomson Learning The force (F) distance (a) curve for two materials 25/28 Binding Energy and the Coef. of Thermal Expansion Materials that display a steep curve (asymmetry in the curve) with a deep trough have low linear coefficients of thermal expansion 26/28

14 Summary : Properties from Bonding Ceramics (Ionic and covalent bonding) Metals (Metallic bonding) Polymers (Covalent and secondary) Large bond energy Large T m Large E Small a Intermediate/Variable bond energy Moderate T m Moderate E Moderate a Directional properties (Variable bond energy) Small T m Small E Large a 27/28 Next Class MME 297 Lecture 06 Structure-Property Correlation [3] The Structure of Solids 1