Physical Properties of Materials Let s get physical!! density Physical Properties electrical thermal expansion shock Density Which Ones? melting point What is density? = THEORETICAL DENSITY, ρ Concept Question As you heat a block of aluminum from 0 C to 100 C its density T = 0 C 1. Increases 2. Decreases CORRECT 3. Stays the same M, V 0 ρ 0 = M / V 0 T = 100 C What would make a material dense? +HEAT M, V 100 ρ 100 = M / V 100 14 < ρ 0 1
Densities of Materials Classes Bond length Bond Strength vs density r 0 Data from Table B1, Callister 6e. Essentially a measure of bond strength E 0 16 Material property- charts: Stiffness - Density Structural Effects Electrical Conductivity STIFFNESS 1000 100 10 1 0.1 Composites Woods Foams Ceramics Polymers Elastomers 0.01 0.1 1 10 Density Metals 100 Densities of common glass forming oxides are less than the corresponding crystalline forms Glasses have more open network structures. v-sio 2 2.20 g/cm 3 α-quartz 2.65 g/cm 3 (Room T. form) Why do some materials conduct electricity and some don t? Materials that allow electron flow Conductors! Those that don t -!!! 2
Bohr s Atom Obstacles? nucleus electrons in orbits scattering events Nett electron motion Resistance is futile Temperature -Why??????? as T so do thermal vibrations resistivity at temp T initial resisitivity temp coefficient of resistivity temp Impurity atoms -Why??????? Disruption in lattice Dislocations -Why??????? Disruption in lattice Grain boundaries -Why?????? Disruptions again Thermal Conduction Free electrons The contribution from k e increase with increasing free electron concentrations Thus in metals k e >> k Phonons l Energy associated with Ceramics k l >>k e travelling lattice waves produced from vibrating k = k l + k e atoms Mechanism Not as effective heat transfer as k e, 3
Free and mobile electrons Space Shuttle During Re-entry Rigidized Silica Fibrous Insulation High electrical High thermal No band gap Metallic lustre The space shuttle Columbia, tore apart killing all seven of its crew Silica fibers magnified hundreds of times Electrical-thermal White hot at 1260 C, the glow from a cube of LI-900 held in bare hand provides the only light in this photo, taken 10 seconds after removal from the oven. Thermal Conduction Summary Conduction happens mainly in solids Free & mobile electrons Results in All atoms vibrate, but vibrate more when heated. Heat spreads by conduction when atoms increase their vibrations, and pass this energy on to those nearby. High electrical High thermal conducticvity In metals, free electrons carry the heat energy faster than the atomic vibrations......and transfer it by colliding with other electrons and atoms. Because of this, metals are the best conductors of heat energy. No band gap Metallic lustre due to Wiedemann -Franz Law Microstructure can effect this 4
Thermal Expansion Concorde most sources indicate that the airframe stretched by 12 to 30 centimetres at Mach 2 Thermal Expansion amount of expansion depends on change in temperature, T original length, L o coefficient of thermal expansion a L 0 + L = L 0 + α L 0 T L = α L 0 T (linear expansion) Temp: T Example: Short days in winter Long days in summer!! Temp: T+ T L 0 L The distance between Sydney and Melbourne is approximately 1000km. What would be the increase in length if the temperature went from 20 C to 40 C? The CTE for steel is 10.8 x 10-6 C -1 Explain why a brass lid on a glass jar will loosen when heated? CTE for brass is 20 x 10-6 C -1 and CTE for glass is 9 x 10-6 C -1. From an atomic point of view Thermal expansion is reflected by an increase in average distance between atoms Thermal expansion due to the asymmetric nature of curve c.f (a) & (b)} Vibrational amplitude Example Equilibrium spacing, bond length 5
CTE vs Bond Strength Effect of atomic bond strength CTE vs Material Type The > the atomic bonding The deeper & narrower the trough So r does not change as much CTE per C 500-1000 kj/mol 70-900 kj/mol 7-30 kj/mol Ceramics Metals Polymers Composites Overview of Thermal Properties of Materials Caution!!! Complex systems with more than one material must have matched or compensated thermal expansions Thermal Expansion of Materials 6
Bi-metallic Switch Longer summer days? Thermal Expansion: Volume The increase in volume (a threedimensional measure of expansion) can be expressed in a similar fashion: Thermal Toughening --puts surface of glass part into compression --suppresses growth of cracks from surface scratches. --sequence: Tempered Glass The strength of glass can be enhanced by inducing compressive residual stresses at the surface. Small Scratches Coefficient of volume thermal expansion --Result: surface crack growth is suppressed. The surface stays in compression - closing small scratches and cracks. Volume expansion coefficient is 3 times larger than linear expansion 11 7
Thermal Shock thermal better = less chance of thermal shock coefficient of thermal expansion lower coefficient = less chance of thermal shock Material Thermal Conductivity CTE Aluminium 222 23.6 Soda glass 1.7 9 Pyrex 1.4 3.3 2 most important properties that determine resistance to thermal shock Melting Point clip Macroscopic Properties Inferred from Bonding U F T m r 0 E 0 r Smaller T m Larger T m goes as E 0 increases in size Bond Energy and Melting Point Bond Type Material Bonding Energy, kj/mol T m, C Ionic NaCl 640 801 MgO 1000 2800 Covalent Si 450 1410 C (diamond) 713 >3550 Metallic Hg 0.7-39 Al 3.4 660 Fe 4.2 1538 W 8.8 3410 Van der Waals Ar 0.08-189 Cl 2 0.32-101 Hydrogen NH 3 0.36-78 H 2 O 0.52 0 What is relationship between Bonding Energy and T melt? 8
Bond strength Recapping.. Recapping.. determines Bond stretching Hooke s law Elastic modulus due to Is this correlation reasonable? due to Melting point Solid to liquid phase transformation from Greater interatomic spacing A number of physical and mechanical properties can be inferred from the atomic bond. Electrical Properties Conduction Thermal Properties: Melting point Conduction Expansion Density Ceramics Ionic and Covalent bonds Metals Metallic bonding Polymers Covalent and Secondary Large bond energies large T m, E Small α Varying bond energy intermediate T m, E, α secondary dominates outcome small T m, E large α Recapping These physical properties depend on the properties of the atoms: Bond strength Atomic weight Bond type 9