Thermal Properties Flashcards

1
Q

What is thermal expansion?

A

Tells how much a material will expand or contract when subject to a change in temperature

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2
Q

The importance of thermal expansion

A

The freezing and melting of water in pipes can cause a decrease in the strength of the pipe and bursts

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3
Q

The equation for thermal expansion dependent on temperature

A

alpha = 1/l dl/dT

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4
Q

The equation for thermal expansion independent of temperature

A

alpha = (l - l0)/l(T-T0)

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5
Q

Typical values for thermal expansion of metals

A

10 - 20 *10-6 K-1

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6
Q

Typical values for thermal expansion of ceramics and glasses

A

1 - 10*10^-6 K-1

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7
Q

Typical values for thermal expansion of polymers

A

30 - 300 *10-6 K-1

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8
Q

How does bonding relate to the thermal expansion?

A

If not bonded tightly and then heated up then the atoms can move a lot more

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9
Q

How does Young’s modulus correlate with the thermal expansion?

A

Young’s modulus correlates inversely with the thermal expansion

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10
Q

What is differential thermal expansion?

A

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.

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11
Q

What is the thermal conductivity?

A

The thermal conductivity tells us how well a material transmits heat

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12
Q

What conducts heat better - wood or metal?

A

Metal conduct heat more than wood

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13
Q

What is the thermal conductivity equation?

A

Jq = - lambda * dT/dx
where lambda is the thermal conductivity
Jg is the heat flux density
dT/dx is the temperature gradient

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14
Q

What are the units of lambda?

A

W/mK

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15
Q

What is the ranking of thermal conductivity (increasing)?

A

Air –> Water and Glass –> Rocks and Minerals –> Metals –> Diamonds

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16
Q

What is the trend between the thermal expansion and the thermal conductivity?

A

There is a slight correlation of high thermal conductivity with low thermal expansion

17
Q

Which have a greater thermal conductivity - polymers or polymer foams?

A

Polymers. Foams contain pores of gases which are bad at conducting heat.

18
Q

What is the specific heat capacity?

A

The specific heat capacity is the amount of energy required to heat a unit mass of a substance by one degree in temperature

19
Q

The equation for the specific heat capacity?

A

Cp = (1/m)*dQ/dT
where m is the mass
Q is the heat energy
T is the temperature

20
Q

What are the specific units of specific heat capacity

21
Q

What is the change of density with specific heat capacity?

A

Low density materials have high specific heats

22
Q

When is thermal diffusivity used

A

In non steady state heat flow problems

23
Q

What is the equation for thermal diffusivity?

A

a = lambda / (rho*Cp)

24
Q

What is the melting temperature?

A

The melting temperature is the temperature at which there is a solid to liquid transition.

25
What sorts of materials is the melting temperature characteristic of?
It is characteristic of a single phase crystalline material
26
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
27
What temperatures are materials usable at?
Materials are usable well below the melting temperature. There is a maximum service temperature for materials
28
What is the homologous temperature?
Temperature at which the material is being used at/ melting point of the material
29
When does creep occur in relation to the homologous temperature?
T>0.7 T/Tm
30
Which materials are excellent at high temperatures?
Ceramics and metals
31
Which materials are poor at high temperatures?
Polymers
32
Does carbon increase or decrease the melting point of iron?
An increasing carbon content in an alloy decreases the melting temperature of the alloy.