Thermal properties

Material science

© Su-Jin Kim, GNU

Thermal Properties(열적성질)

Professor Su-Jin Kim

Gyeongsang National University

Thermal properties

Material science

© Su-Jin Kim, GNU

OCW

Motion of molecule (TED 5min) Temperature, Radiation

https://youtu.be/b0IbXG0hnOk

Thermal conductivity

https://youtu.be/TmeJFYJcdaw

Thermal properties

Material science

© Su-Jin Kim, GNU

KOCW

재료의열적성질

http://www.kocw.net/home/cview.do?lid=a5a36e3b7fdebee6

Thermal properties

Material science

© Su-Jin Kim, GNU

Melting point (녹는점)• Depends on the energy required to separate its atoms.

• Plastic < Lead < Al < Cu < Fe < Ti < Ceramic < Tungsten

Thermal properties

Material science

© Su-Jin Kim, GNU

Energy & Atomic vibration

• Energy is stored as atomic vibrations.

• Energy storage and thermal expansion.

Interatomic distance

Vib

rati

on

en

ergy

Automic Vibration

Thermal properties

Material science

© Su-Jin Kim, GNU

• Heat capacity(열용량) is the ability of a material to absorb heat.• The energy required to increase the temperature of the material.

dQ = C dT

Specific heat (비열)

• Specific heat(비열) has typical units of (J/kgK)energy required to increase a unit mass by a unit T.Energy input(J) = Specific heat(J/kgK) x Mass(kg) x Temperature change (K)

dQ = c m dT

Thermal properties

Material science

© Su-Jin Kim, GNU

Specific heatMaterial cp (J/kgK)

Liquid Water 4186

Oil 2300

Polymers

1600

Polypropylene 1925

Polyethylene 1850

Polystyrene 1170

Teflon 1050

Ceramics

800

Magnesia(MgO) 940

Alumina(Al2O3) 775

Glass 840

Metals

400

Aluminum 900

Steel 486

Tungsten 138

Gold 128

Thermal properties

Material science

© Su-Jin Kim, GNU

Energy cost, Excel

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Air Water Aluminum Iron Plastic Alumina

Cost $ Volume Temperature change Energy cost

V(m3) dT(ºC) ($/kWh) $/kJ

1 25 0.2 5.55556E-05

Density Mass Specific Heat Energy input Cost

Material ρ(kg/m3) m(kg) c(J/kgºC) dQ(kJ) $

Air 1.225 1.225 1000 31 0.0

Water 1000 1000 4200 105000 5.8

Aluminum 2700 2700 900 60750 3.4

Iron 7874 7874 448 88189 4.9

Plastic 920 920 1800 41400 2.3

Alumina 3950 3950 775 76531 4.3

m = ρ V dQ = c m dT

Thermal properties

Material science

© Su-Jin Kim, GNU

Thermal Expansion (열팽창)

• Materials expand upon heating and contract when cooled.

• Thermal expansion coefficient α(K-1)

∆l = α l0 ∆T

Linear extension (m/m)

Temperature increase (K)

l0 ∆l∆T

Thermal properties

Material science

© Su-Jin Kim, GNU

Thermal Expansion Coefficient

Material α(10-6/K)

Polymers

100

Polypropylene 145-180

Polyethylene 106-198

Polystyrene 90-150

Teflon 126-216

Metals

10

Aluminum 23.6

Steel 12

Tungsten 4.5

Gold 14.2

Ceramics

5

Magnesia(MgO) 13.5

Alumina(Al2O3) 7.6

Soda-lime Glass 9

Silica (cryst. SiO2) 0.4

Thermal properties

Material science

© Su-Jin Kim, GNU

Thermal Conductivity (열전도율)

• The ability of a material to transfer heat.

• Thermal conductivity, k (W/mK)

Q = -k A (Th-Tl) / x

Q: Thermal Conduction (W) (Th-Tl) / x : Temperature gradient (K/m)

x

Th

Q

Tl k

A

Thermal properties

Material science

© Su-Jin Kim, GNU

Thermal Conductivity

Material k (W/mK)

Metals

100

Aluminum 247

Steel 52

Tungsten 178

Gold 315

Ceramics

20

Magnesia(MgO) 38

Alumina(Al2O3) 39

Soda-lime Glass 1.7

Silica (cryst. SiO2) 1.4

Polymers

0.2

Polypropylene 0.12

Polyethylene 0.48

Polystyrene 0.13

Teflon 0.25

Thermal properties

Material science

© Su-Jin Kim, GNU

Thermal Stress (열응력)

• Thermal stress occurs due to uneven heating & cooling and mismatch in thermal expansion.

Thermal stress (MPa)

Modulus of elasticity (GPa)

Thermal expansion coefficient (K-1)

Temperature increase (K)

∆T

σ = E α ∆T

Thermal properties

Material science

© Su-Jin Kim, GNU

Thermal Shock (열충격)

• Thermal shock occurs due to uneven heating & cooling.

• Assume top thin layer is rapidly cooled ∆T

• Tension stress at the surface :

• Quench rate : dq/dx = k ∆T

TE

∆Tqdx

• Thermal shock resistance is the ability of a material to be rapidly cooled and not crack. Maximize

E

kf

1500℃