Thermal Properties Flashcards
What is thermal expansion?
Tells how much a material will expand or contract when subject to a change in temperature
The importance of thermal expansion
The freezing and melting of water in pipes can cause a decrease in the strength of the pipe and bursts
The equation for thermal expansion dependent on temperature
alpha = 1/l dl/dT
The equation for thermal expansion independent of temperature
alpha = (l - l0)/l(T-T0)
Typical values for thermal expansion of metals
10 - 20 *10-6 K-1
Typical values for thermal expansion of ceramics and glasses
1 - 10*10^-6 K-1
Typical values for thermal expansion of polymers
30 - 300 *10-6 K-1
How does bonding relate to the thermal expansion?
If not bonded tightly and then heated up then the atoms can move a lot more
How does Young’s modulus correlate with the thermal expansion?
Young’s modulus correlates inversely with the thermal expansion
What is differential thermal expansion?
If two or more different materials are in contact with a system that will change temperature then the change in expansion will result in mechanical forces in the system.
What is the thermal conductivity?
The thermal conductivity tells us how well a material transmits heat
What conducts heat better - wood or metal?
Metal conduct heat more than wood
What is the thermal conductivity equation?
Jq = - lambda * dT/dx
where lambda is the thermal conductivity
Jg is the heat flux density
dT/dx is the temperature gradient
What are the units of lambda?
W/mK
What is the ranking of thermal conductivity (increasing)?
Air –> Water and Glass –> Rocks and Minerals –> Metals –> Diamonds
What is the trend between the thermal expansion and the thermal conductivity?
There is a slight correlation of high thermal conductivity with low thermal expansion
Which have a greater thermal conductivity - polymers or polymer foams?
Polymers. Foams contain pores of gases which are bad at conducting heat.
What is the specific heat capacity?
The specific heat capacity is the amount of energy required to heat a unit mass of a substance by one degree in temperature
The equation for the specific heat capacity?
Cp = (1/m)*dQ/dT
where m is the mass
Q is the heat energy
T is the temperature
What are the specific units of specific heat capacity
J/ kg K
What is the change of density with specific heat capacity?
Low density materials have high specific heats
When is thermal diffusivity used
In non steady state heat flow problems
What is the equation for thermal diffusivity?
a = lambda / (rho*Cp)
What is the melting temperature?
The melting temperature is the temperature at which there is a solid to liquid transition.
What sorts of materials is the melting temperature characteristic of?
It is characteristic of a single phase crystalline material
Which materials do not have a defined melting temperature
- Alloys because they consist of metals and impurities which will melt at different temperatures
- Amorphous materials - they have a glass transition temperature
- Thermoset polymers do not have a liquid state because of their structure
What temperatures are materials usable at?
Materials are usable well below the melting temperature. There is a maximum service temperature for materials
What is the homologous temperature?
Temperature at which the material is being used at/ melting point of the material
When does creep occur in relation to the homologous temperature?
T>0.7 T/Tm
Which materials are excellent at high temperatures?
Ceramics and metals
Which materials are poor at high temperatures?
Polymers
Does carbon increase or decrease the melting point of iron?
An increasing carbon content in an alloy decreases the melting temperature of the alloy.
