Lec 22 - ENGR ENGR 4250 Advanced Materials Engineering...

Info iconThis preview shows pages 1–12. Sign up to view the full content.

View Full Document Right Arrow Icon
ENGR 4250 ENGR 4250 – Advanced Materials Engineering Lecture 22 Thermal Properties
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Chapter 19: hermal Properties SUES TO ADDRESS Thermal Properties ISSUES TO ADDRESS. .. How does a material respond to heat ? How do we define and measure. .. -- heat capacity -- coefficient of thermal expansion -- thermal conductivity -- thermal shock resistance How do ceramics, metals, and polymers rank?
Background image of page 2
Heat Capacity General: The ability of a material to absorb heat. Quantitative: The energy required to increase the mperature of the material temperature of the material. heat capacity /mol- ) energy input (J/mol) dQ C = (J/mol K) temperature change (K) Two ways to measure heat capacity: dT C p : Heat capacity at constant pressure. C v : Heat capacity at constant volume. C p > C v • Specific heat has typical units of g J K kg
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Energy Storage How is the energy stored? Phonons – thermal waves - vibrational modes Adapted from Fig. 19.1, Callister 7e .
Background image of page 4
Phonon • Vibrational thermal energy: quantum nature nly certain energy values are allowed • Only certain energy values are allowed • Single quantum is Phonon • Analogous to quantum of em waves: photon • Sometimes waves are called phonons • Cause thermal scattering of free electrons during conduction
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Heat Capacity vs T Heat capacity. .. -- increases with temperature aches a limiting value of 3 -- reaches a limiting value of 3 R 3 R C v = constant gas constant = 8.31 J/mol-K C v =AT 3 Adapted from Fig. 19.2, Callister 7e . ebye temperature T (K) θ D 0 0 Atomic view: -- Energy is stored as atomic vibrations. Debye temperature (usually less than T room ) -- As T goes up, so does the avg. energy of atomic vibr.
Background image of page 6
Other Heat Capacity Contributions Electronic Contribution he electrons (those that contribute to current) The electrons (those that contribute to current) absorb thermal energy by increase in K.E. – Only free electron above 0K (above Fermi energy) – Less in semiconductors, ceramics, and polymers • Insignificant at higher T andomization of Electron orbits Randomization of Electron orbits – Ferromagnetic materials bove Curie point Above Curie point
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Heat Capacity: Comparison Polymers c p (J/kg-K) at room T 925 material Polypropylene Polyethylene Polystyrene eflon 1925 1850 1170 050 c p : (J/kg-K) C p : (J/mol-K) c p Why is c p significantly larger for polymers? Teflon Ceramics Magnesia (MgO) lumina (Al 1050 940 75 reasing Alumina (Al 2 O 3 ) Glass Metals 775 840 inc r Selected values from Table 19.1, Callister 7e . Aluminum Steel Tungsten old 900 486 138 28 Gold 128
Background image of page 8
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Thermal Expansion • In many materials is anisotropic v α • Depends upon crystallographic directions • For isotropic materials is approximately = v l 3 • Thermal expansion Æ increase in average distance between atoms
Background image of page 10
Properties From Bonding: T m Bond length , r Melting Temperature , T m nergy Energy r Bond energy , E o r o r aller Energy larger T m smaller T m r o Equilibrium distance T m is larger if E o is larger.
Background image of page 11

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 12
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 10/18/2010 for the course ENGR 3360 taught by Professor Ahmadbarari during the Fall '10 term at UOIT.

Page1 / 45

Lec 22 - ENGR ENGR 4250 Advanced Materials Engineering...

This preview shows document pages 1 - 12. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online