Thermal Behavior Chapter 7
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Questions and Answers

What is the coefficient of thermal expansion for copper?

  • 16.5 x 10^(-6) (°C)^-1 (correct)
  • 16.5 x 10^(-5) (°C)^-1
  • 16.5 x 10^(-7) (°C)^-1
  • 16.5 x 10^(-4) (°C)^-1

    • What is the change in length of a copper wire that is cooled from 40°C to -9°C?

  • 0.018 m
  • 0.01 m
  • 0.012 m (correct)
  • 0.015 m

    • What is the unit of thermal conductivity?

  • W/m°C
  • J/m²s
  • J/m°C
  • W/m-K (correct)

    • What is the mechanism of heat transfer in metals?

    <p>atomic vibrations and motion of free electrons</p> Signup and view all the answers

    What is the thermal conductivity of aluminum?

    <p>247 W/m-K</p> Signup and view all the answers

    What is the thermal conductivity of silica (SiO2)?

    <p>0.4 W/m-K</p> Signup and view all the answers

    What is the direction of heat flux in a material?

    <p>from higher temperature to lower temperature</p> Signup and view all the answers

    What is the purpose of Fourier's Law?

    <p>to calculate heat flux</p> Signup and view all the answers

    What is the ability of a material to absorb heat quantitatively measured by?

    <p>The energy required to produce a unit rise in temperature for one mole of a material</p> Signup and view all the answers

    What are the two ways to measure heat capacity?

    <p>Cp and Cv</p> Signup and view all the answers

    What is the unit of heat capacity?

    <p>J/mol-K</p> Signup and view all the answers

    What happens to heat capacity as temperature increases?

    <p>It increases with temperature</p> Signup and view all the answers

    What is the value of Cv at high temperatures for solids?

    <p>3R</p> Signup and view all the answers

    What is the energy stored as in a material from an atomic perspective?

    <p>Atomic vibrations</p> Signup and view all the answers

    What happens to materials when their temperature is changed?

    <p>They change size</p> Signup and view all the answers

    What is the formula to calculate the change in length of a material due to thermal expansion?

    <p>ℓfinal - ℓinitial = aℓ(Tfinal - Tinitial)</p> Signup and view all the answers

    What type of curve is observed when there is an increase in interatomic separation with an increase in temperature?

    <p>Asymmetric curve</p> Signup and view all the answers

    Why do polymers have larger αλ values compared to metals?

    <p>Because of weak secondary bonds</p> Signup and view all the answers

    What is the unit of the linear coefficient of thermal expansion?

    <p>1/K or 1/°C</p> Signup and view all the answers

    Which of the following materials has the highest αλ value?

    <p>Teflon</p> Signup and view all the answers

    What is the αλ value of aluminum at room temperature?

    <p>23.6</p> Signup and view all the answers

    What is the αλ value of silica at room temperature?

    <p>9.5</p> Signup and view all the answers

    What is the thermal stress equation?

    <p>s = EaℓDT</p> Signup and view all the answers

    What is the main cause of thermal stresses?

    <p>Restrained thermal expansion and contraction</p> Signup and view all the answers

    What is the unit of thermal shock resistance (TSR)?

    <p>no unit</p> Signup and view all the answers

    What is the purpose of the thermal protection system in the Space Shuttle Orbiter?

    <p>To protect the shuttle from heat during re-entry</p> Signup and view all the answers

    What is the thermal expansion coefficient of brass?

    <p>20 x 10^-6/°C</p> Signup and view all the answers

    What is the thermal stress in the brass rod when heated to a temperature of 106°C?

    <p>-172 MPa</p> Signup and view all the answers

    What is the thermal shock resistance (TSR) equation?

    <p>(T1 - T2) = Eaℓ / k</p> Signup and view all the answers

    What is the main cause of thermal shock resistance?

    <p>Rapid quenching</p> Signup and view all the answers

    What is the purpose of the thermal expansion coefficient?

    <p>To calculate thermal stress</p> Signup and view all the answers

    What is the material with the highest thermal shock resistance?

    <p>Reinforced carbon-carbon</p> Signup and view all the answers

    At what temperature range were silica tiles used in the Shuttle Orbiter Thermal Protection System?

    <p>400-1260C</p> Signup and view all the answers

    What is the approximate percentage of porosity in the silica fibers used in the Shuttle Orbiter Thermal Protection System?

    <p>90% porosity</p> Signup and view all the answers

    What process bonded the silica fibers together during heat treatment?

    <p>Bonding</p> Signup and view all the answers

    Who adapted Fig. 19.2W?

    <p>L.J. Korb, C.A. Morant, R.M. Calland, and C.S. Thatcher</p> Signup and view all the answers

    Where was Fig. 19.4W courtesy of?

    <p>Lockheed Aerospace Ceramics Systems</p> Signup and view all the answers

    What was the title of the publication where the article by L.J. Korb, C.A. Morant, R.M. Calland, and C.S. Thatcher was published?

    <p>Ceramic Bulletin</p> Signup and view all the answers

    Study Notes

    Heat Capacity

    • Heat capacity is the ability of a material to absorb heat, quantitatively measured as the energy required to produce a unit rise in temperature for one mole of a material.
    • Heat capacity is measured in J/mol-K and has units of J/mol × K.
    • There are two ways to measure heat capacity: Cp (heat capacity at constant pressure) and Cv (heat capacity at constant volume), with Cp usually being greater than Cv.
    • Heat capacity increases with temperature, and for solids, it reaches a limiting value of 3R, where R is the gas constant (8.31 J/mol-K).

    Thermal Expansion

    • Thermal expansion occurs when materials change size due to temperature changes.
    • The linear coefficient of thermal expansion (α) is measured in 1/K or 1/°C and represents the change in length per unit change in temperature.
    • From an atomic perspective, thermal expansion occurs due to the increase in interatomic separation as temperature increases.
    • Polymers have larger α values than metals and ceramics due to their weak secondary bonds.

    Thermal Conductivity

    • Thermal conductivity is the ability of a material to transport heat, measured in J/m-K-s.
    • Fourier's Law states that heat flux (q) is proportional to the negative temperature gradient (dT/dx) and thermal conductivity (k).
    • Atomic perspective: thermal conductivity occurs due to atomic vibrations and the motion of free electrons in hotter regions transporting energy to cooler regions.

    Thermal Stresses

    • Thermal stresses occur due to restrained thermal expansion or contraction, or temperature gradients leading to differential dimensional changes.
    • Thermal stress (σ) is calculated as σ = EαΔT, where E is the modulus of elasticity, α is the linear coefficient of thermal expansion, and ΔT is the temperature change.

    Thermal Shock Resistance

    • Thermal shock resistance occurs due to nonuniform heating or cooling, leading to the development of tension at the surface.
    • The critical temperature difference for fracture (ΔT) is calculated as ΔT = σf / (Eα), where σf is the fracture strength.
    • Thermal shock resistance (TSR) is a measure of the quench rate required to produce fracture, calculated as TSR = σf / (Eα).

    Thermal Protection System

    • Application: Space Shuttle Orbiter thermal protection system, which uses silica tiles, nylon felt, and silicon rubber coating to withstand high temperatures during re-entry.
    • Silica tiles have a microstructure with ~90% porosity, made up of Si fibers bonded during heat treatment.

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    Explore the thermal properties of materials, including heat capacity, thermal expansion, thermal conductivity, and thermal shock resistance.

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