38 Questions
What is the coefficient of thermal expansion for copper?
16.5 x 10^(-6) (°C)^-1
What is the change in length of a copper wire that is cooled from 40°C to -9°C?
0.012 m
What is the unit of thermal conductivity?
W/m-K
What is the mechanism of heat transfer in metals?
atomic vibrations and motion of free electrons
What is the thermal conductivity of aluminum?
247 W/m-K
What is the thermal conductivity of silica (SiO2)?
0.4 W/m-K
What is the direction of heat flux in a material?
from higher temperature to lower temperature
What is the purpose of Fourier's Law?
to calculate heat flux
What is the ability of a material to absorb heat quantitatively measured by?
The energy required to produce a unit rise in temperature for one mole of a material
What are the two ways to measure heat capacity?
Cp and Cv
What is the unit of heat capacity?
J/mol-K
What happens to heat capacity as temperature increases?
It increases with temperature
What is the value of Cv at high temperatures for solids?
3R
What is the energy stored as in a material from an atomic perspective?
Atomic vibrations
What happens to materials when their temperature is changed?
They change size
What is the formula to calculate the change in length of a material due to thermal expansion?
ℓfinal - ℓinitial = aℓ(Tfinal - Tinitial)
What type of curve is observed when there is an increase in interatomic separation with an increase in temperature?
Asymmetric curve
Why do polymers have larger αλ values compared to metals?
Because of weak secondary bonds
What is the unit of the linear coefficient of thermal expansion?
1/K or 1/°C
Which of the following materials has the highest αλ value?
Teflon
What is the αλ value of aluminum at room temperature?
23.6
What is the αλ value of silica at room temperature?
9.5
What is the thermal stress equation?
s = EaℓDT
What is the main cause of thermal stresses?
Restrained thermal expansion and contraction
What is the unit of thermal shock resistance (TSR)?
no unit
What is the purpose of the thermal protection system in the Space Shuttle Orbiter?
To protect the shuttle from heat during re-entry
What is the thermal expansion coefficient of brass?
20 x 10^-6/°C
What is the thermal stress in the brass rod when heated to a temperature of 106°C?
-172 MPa
What is the thermal shock resistance (TSR) equation?
(T1 - T2) = Eaℓ / k
What is the main cause of thermal shock resistance?
Rapid quenching
What is the purpose of the thermal expansion coefficient?
To calculate thermal stress
What is the material with the highest thermal shock resistance?
Reinforced carbon-carbon
At what temperature range were silica tiles used in the Shuttle Orbiter Thermal Protection System?
400-1260C
What is the approximate percentage of porosity in the silica fibers used in the Shuttle Orbiter Thermal Protection System?
90% porosity
What process bonded the silica fibers together during heat treatment?
Bonding
Who adapted Fig. 19.2W?
L.J. Korb, C.A. Morant, R.M. Calland, and C.S. Thatcher
Where was Fig. 19.4W courtesy of?
Lockheed Aerospace Ceramics Systems
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?
Ceramic Bulletin
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.
Explore the thermal properties of materials, including heat capacity, thermal expansion, thermal conductivity, and thermal shock resistance.
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