Material Science Lecture 2, Basic Introduction

Material Science

1. Lecture 2 By Ch. Azmat Ali

2. Why Study Material Science Every scientist/engineer encounter problems in which they have to select materials Knowledge of properties of different materials comes handy for material selection

3. Materials used for coffee cup?

4. Material for airplane wing Should withstand high force (strong) Could bear cyclic application of force (fatigue property) Density and corrosion resistance Shock landing (impact), high temperature (creep) are not important W ood…Steel…Aluminum…Carbon Fiber

5. Optical properties are important Material must interact with light Must be resistant to corrosion and wear. What about solar cell on satellite? Material for solar cells

6. Type of Materials Metals (aluminum, magnesium, zinc, iron, nickel, steel etc ) Free electrons Metallic bonding Good conductivity, ductility, formability, shock resistance etc Used for electrical and structural or lead bearing application

7. Ceramics (brick, glass, tableware, insulators) Mostly ionic bonding or amorphous Complex structure Good strength and hardness Useful optical, electrical and thermal properties. Poor formability, ductility and shock resistance. They are less often used for structural or load-bearing applications Type of Materials

8. Semiconductors (Silicon, germanium, gallium arsenide) Electrical conductivity between conductors and ceramics Electrical conductivity can be controlled by doping Used in integrated circuits Usually made out of molten materials Type of Materials

9. Polymers (rubber, plastic and adhesives) Organic (covalent bonging) Produced by creating polymerization of organic molecules (obtained from petroleum or agricultural products) Low electrical and thermal conductivity Some have excellent ductility, formability and shock resistance; other have opposite properties They are lightweight and have excellent resistance to corrosion Type of Materials

10. Composite Materials (concrete, plywood, fiberglass etc ) Have properties that cant be obtained by a single material With composites we can produce lightweight, strong, ductile, high temperature-resistant materials Functional Classification

11. Structure-Property-Processing Relationship Structure, property and processing are inter-related. If one is changed, one or the others change We must determine how the three interrelate.

12. Properties Mechanical How a material responds to applied stress Most common mechanical properties are strength, ductility and stiffness Impact, creep and wear are also important Small change in processing effects mechanical properties greatly.

13. Properties Physical Electrical, magnetic, optical, thermal, elastic and chemical behavior Depend both on structure and processing of material. Small change in composition profoundly changes conductivity of semiconductors and cermaics . High firing temp greatly reduces thermal insolation properties of ceramic bricks. Small amount of impurities change the color of gas or polymer

14. The structure can be considered on several levels all of which influences the behavior of product. Atoms Arrangement of atoms Grain structure Phases Structure

15. Processing Processing produce desire shape from initial formless material. e.g for metals, by pouring liquid into mold, by welding or using high pressures (forging, drawing, extraction, rolling, bending) Ceramic’s processing is done by heat treatment. Polymers are produced by injection of softened plastic into molds, drawing, forming.

16. Structure-Property-Processing Relationship The processing of the material effects structure e .g processing of copper by casting and forming leads to very different shapes, size and orientations of grains. Cast structure may contain voids

17. Chapter 2: Atomic Structure (outline) Review of Atomic Structure Electrons, protons, neutrons, quantum mechanics of atoms, electron states, the periodic Table • Atomic Bonding in Solids Bonding energies and forces • Primary Interatomic Bonding Ionic Covalent Metallic • Secondary Bonding Different types of dipole-dipole bonds • Molecules and molecular solids

18. The structure can be considered on several levels all of which influences the behavior of product. Atoms Arrangement of atoms Grain structure Phases Structure

19. Electron in Atom The electrons form a cloud around the nucleus, of radius of 0.05 – 2 nm. This picture looks like a mini planetary system. But quantum mechanics tells us that this analogy is not correct : Only certain “orbits” or shells of electron probability densities are allowed. The shells are identified by a principal quantum number n , which can be related to the size of the shell , n = 1 is the smallest; n = 2, 3 .. are larger. The second quantum number, l , defines subshells within each shell. It defines shape of the orbital. The Magnetic quantum number, m, Specifies the orientation in space of an orbital of a given energy. The Spin quantum number, s, describes spin of an electron within a sub shell.

20. Electron in Atom Aufbau principle:   lower energy orbitals are filled first Pauli Exclusion Principle: no two electrons in an atom can have the same four quantum numbers Hund's rule:   greater total spin state usually makes the resulting atom more stable. Accordingly, it can be taken that if two or more orbitals of equal energy are available, electrons will occupy them singly before filling them in pairs.

21. The quantum numbers arise from solution of Schrodinger’s equation Each “orbit” or shell can accommodate only a maximum number of electrons, which is determined by quantum mechanics. In brief , the most inner K-shell can accommodate only two electrons , called s-electrons ; the next L-shell two s-electrons and six p-electrons ; the M-shell can host two s-electrons , six p electrons , and ten d-electrons ; and so on. Electron in Atom

22. Electrons in Atoms Subshells by energy: 1s,2s,2p,3s,3p,4s,3d,4s,4p,5s,4d,5p,6s,4f ,… Elec trons that occupy the outermost filled shell – the valence electrons – they are responsible for bonding. Electrons fill quantum levels in order of increasing energy (only n, l make a significant difference).

23. Electrons in Atoms

24. Electronegativity Electronegativity - a measure of how willing atoms are to accept electrons Subshells with one electron → low electronegativity Subshells with one missing electron → high electronegativity Electronegativity increases from left to right Metals are electropositive – they can give up their few valence electrons to become positively charged ions