PowerPoint PresentationThermal PropertiesThermal EnergyAtomic VibrationsHeat CapacitySpecific Heat: ComparisonInternal Energy ComparisonSlide 8c17tf01Lorentz ConstantThermal Expansion EffectsThermal ExpansionCoefficient of Thermal Expansion: ComparisonThermal “Expansion”Slide 15Orton/Harrop DilatometerSlide 17Thermal ConductivitySlide 19Thermal Conductivity: ComparisonSlide 21Thermal Stresses (Ex 1)Thermal Shock ResistanceThermal CyclingSlide 26NASA Space Environment and Experiments BranchThermal Protection SystemThermal Protection System (TPS)Who cares about thermal properties?Thermoplastics and ThermosetsMelting & Glass Transition Temps.Meltingc11tf03Aerogel PropertiesSilica AerogelsSlide 37A 2.5 kg brick is supported on top of a piece of aerogel weighing only 2 gramsSlide 39InvarSlide 41SummaryChapter 17:Thermal PropertiesThermal Properties•Heat capacity•Specific Heat•Thermal Energy Mechanism•Coefficient of Thermal Expansion•Thermal Conductivity•Thermal Stresses•Thermal Shock•Applications where these parameters are significant2Thermal Energy•The energy needed to raise the temperature of an object depends on the mass and composition of the object. •The heat capacity measures the combined effect of mass and composition. Heat capacity, C, as distinct from specific heat capacity, is the measure of the energy required to increase the temperature of an object by a given temperature interval. Heat capacity is an extensive property dependent on the amount of material.•The specific heat, c, or specific heat capacity, is a property of the composition only. It measures the energy required to increase the temperature of a unit quantity of a specific substance by a specific temperature interval.•An object's temperature is a measure of the random molecular motions. Individual atoms and molecules are never still. 3Atomic Vibrations• Faster molecules striking slower ones at the boundary in elastic collisions will increase the velocity of the slower ones and decrease the velocity of the faster ones, transferring energy from the higher temperature to the lower temperature region. • With time, the molecules in the two regions approach the same average kinetic energy (same temperature) and in this condition of thermal equilibrium there is no longer any net transfer of energy from one object to the other. • The atoms and ions that are bonded together with considerable interatomic forces, are not motionless. • Due to the consistent vibrating movements, they are permanently deviating from their equilibrium position.•Atomic vibrations are in the form of lattice waves or phonons.5• Quantitatively: The energy required to produce a unit rise in temperature for one mole of a material.heat capacity(J/mol-K)energy input (J/mol)temperature change (K)Heat Capacity• Two ways to measure heat capacity:Cp : Heat capacity at constant pressure.Cv : Heat capacity at constant volume.Cp usually > Cv• Heat capacity has units of  FmollbBtu KmolJdTdQC The ability of a material to absorb heat.6increasing cp• PolymersPolypropylene Polyethylene Polystyrene Tefloncp (J/kg-K)at room T• CeramicsMagnesia (MgO)Alumina (Al2O3)Glass• MetalsAluminum Steel Tungsten Gold1925 1850 1170 1050900 486 138 128cp (specific heat): (J/kg-K)Material940 775 840Specific Heat: ComparisonCp (heat capacity): (J/mol-K) More heat energy is required to increase the temperature of a substance with high specific heat capacity than one with low specific heat capacity.  For instance, compare the specific heat energy required to increase the temperature of glass (cp = 840 J/kg-K) with that required for gold of the same mass (cp = 128 J/kg-K) .  The symbols for specific heat capacity are either C or c depending on how the quantity of a substance is measured.Internal Energy Comparison•When the sample of water and copper are both heated by 1°C, the addition to the kinetic energy is the same, since that is what temperature measures. •But to achieve this increase for water, much more energy must be added to the potential energy portion of the internal energy. •So the total energy required to increase the temperature of the water is much larger; its specific heat is much larger.Lorentz ConstantLorentz constant relates electrical and thermal conductivity. The Lorentz constant is proportional to the ratio of the thermal conductivity to the electrical conductivity: L = k/σT For example, if the electrical conductivity of aluminum is 3.8 x107/ Ωm, estimate it's thermal conductivity (Lorentz constant from Table 17.1).Thermal conductivity: k = σLT = LT/ρ; (T = 293K)Given k = 245 W/m KThermal Expansion Effects11•The most easily observed examples of thermal expansion are size changes of materials as they are heated or cooled.•Almost all materials (solids, liquids and gases) expand when they are heated and contract when they are cooled. •Increased temperature increases the frequency and magnitude of the molecular motion of the atoms and produces more energetic collisions. •Increasing the energy of the collisions forces the molecules further apart and causes the material to expand.12Thermal ExpansionLength increases when temperature increases.)(αinitialfinalinitialinitialfinalTT linear coefficient ofthermal expansion (1/K or 1/°C)TinitialTfinal initial finalTfinal > Tinitial13Coefficient of Thermal Expansion: ComparisonWhy does  generally decrease with increasingbond energy?Polypropylene 145-180 Polyethylene 106-198 Polystyrene 90-150 Teflon 126-216• Polymers• CeramicsMagnesia (MgO) 13.5Alumina (Al2O3) 7.6Soda-lime glass 9Silica (cryst. SiO2) 0.4• MetalsAluminum 23.6Steel 12 Tungsten 4.5 Gold 14.2 (10-6/C)at room TMaterialPolymers have larger  values because of weak secondary bondsincreasing 14Thermal “Expansion” Ex: A copper wire 15 m long is cooled from 40 to -9°C. How much change in length will it experience? l16.5 x 10 6(oC) 1mm 12m 012.0 ]C40)C9[()m 15)](C/1(10 x5.16[ 60TThe linear coefficient of thermal expansion (CTE) of iron changes abruptly at temperatures where a phase transformation occurs.Orton/Harrop Dilatometer16©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.The relationship between the