Properties of Materials

Transcription

1 Properties of Materials

2 Thermal Properties

3 Thermal Conductivity Temperature Wall The Thermal Conductivity (k) is the measure of the ability of a material to transmit heat by conduction. The heat (Q) is measured in Watts. k is defined from the equation: Q = ka(t a - T b) Distance Real building Calculated d Where T a and T b are the temperatures either side of an element of material with thickness d and cross sectional area A. k has the units of Watts/(m o C)

4 Coefficient of Thermal Expansion This is defined as the proportionate length change per o C of temperature change. If the coefficient is x the length change will be: L = x L T where L is the length and T is the change in temperature.

5 Thermal stresses occur due to: Thermal Stresses (Ex 1) -- restrained thermal expansion/contraction -- temperature gradients that lead to differential dimensional changes Thermal stress: E T T ) E T f ( 0 A brass rod is stress-free at room temperature (20 C). It is heated up, but prevented from expanding in length. At what temperature does the stress reach -172 MPa? E = 100 GPa for brass α = 20 x 10-6 / C / (E α) = 20 C - T f T f = 20 C - [-1.72 x 10 8 Pa / (1 x10 11 Pa) (20 x 10-6 / C)] T f = 20 C + 86 C α = 106 C 5

6 Thermal Shock Resistance Occurs due to: nonuniform heating/cooling Ex: Assume top thin layer is rapidly cooled from T 1 to T 2 Temperature difference that can be produced by cooling: quench rate ( T1 T2) k Large TSR when rapid quench tries to contract during cooling T 2 resists contraction (quench rate) for fracture f k E T 1 set equal Tension develops at surface E (T 1 T 2 ) Critical temperature difference for fracture (set = f ) (T 1 T 2 ) fracture f E f k ThermalShock Resistance ( TSR) E is large 6

7 Thermal Cycling Satellites, spacecraft and all components must be able to withstand the rigors of a space environment while maintaining structural integrity throughout a mission that might last 10 years in low Earth orbit. 7

8 Steel Flammability Non-Flammable but often painted Loss of Strength in Fire High thermal conductivity leads to rapid failure. (note that a high thermal capacity would delay failure). Strength lost by 550 o C. Cold worked steel worse. Other hazards High thermal expansion will disrupt structure. High thermal conductivity may ignite other areas.

9 Plastics Thermoplastics melt and then burn. Thermosetting plastics char and then burn. Great variations between types but typical ignition at 400 o C Loss of Strength in Fire Thermoplastics may melt by 100 o C Thermosets OK to about 300 o C (varies) Other hazards Toxic fumes, Melts and drops.

10 Glass Loss of Strength in Fire Sheet glass shatters due to differential thermal expansion. Design Toughened/Double glazing no help for resistance but hazard from toughened is less. Wire glass, intumescent laminate can give resistance. Fibres often used for fire resistance (e.g. fire blankets)

11 Electrical Properties

12

13 Materials of Choice for Metal Conductors One of the best material for electrical conduction (low resistivity) is silver, but its use is restricted due to the high cost Most widely used conductor is copper: inexpensive, abundant, high σ, but rather soft cannot be used in applications where mechanical strength is important. Solid solution alloying and cold working inprove strength but decrease conductivity. Precipitation hardening is preferred, e.g. Cu-Be alloy When weight is important one uses aluminum, which is half as good as Cu and more resistant to corrosion. Heating elements require low σ (high R), and resistance to high temperature oxidation: nickelchromium alloy

14 Polymers Polymers are typically good insulators but can be made to conduct by doping. A few polymers have very high electrical conductivity - about one quarter that of copper, or about twice that of copper per unit weight.

15 Piezoelectricity In some ceramic materials, application of external forces produces an electric (polarization) field and viceversa Applications of piezoelectric materials is based on conversion of mechanical strain into electricity (microphones, strain gauges, sonar detectors).

16 Magnetic Properties All materials can be classified in terms of their magnetic behaviour falling into one of five categories depending on their bulk magnetic susceptibility. The two most common types of magnetism are diamagnetism and paramagnetism, which account for the magnetic properties of most of the periodic table of elements at room temperature (see figure ).

17 A magnetic field can be produced by: --putting a current through a coil. Magnetic induction: --occurs when a material is subjected to a magnetic field. --is a change in magnetic moment from electrons. Types of material response to a field are: --ferri- or ferro-magnetic (large magnetic induction) --paramagnetic (poor magnetic induction) --diamagnetic (opposing magnetic moment)

18

19 Diamagnetism and Paramagnetism Diamagnetic material in the presence of a field, dipoles are induced and aligned opposite to the field direction. c18f05 Paramagnetic material

20 FERROMAGNETISM mutual alignment of atomic dipoles even in the absence of an external magnetic field. coupling forces align the magnetic spins

21 Antiferromagnetism Antiparallel alignment of spin magnetic moments for antiferromagnetic manganese oxide (MnO) At low T Above the Neel temperature they become paramagnetic

22 FERRIMAGNETISM spin magnetic moment configuration for Fe 2+ and Fe 3+ ions in Fe 3 O 4. Above the Curie temperature becomes paramagnetic

23

24 Optical Properties Reflection, propagation and transmission of a light beam incident on an optical medium.

25 Optical Properties Material Light interaction Interaction of photons with the electronic or crystal structure of a material leads to a number of phenomena. The photons may give their energy to the material (absorption); photons give their energy, but photons of identical energy are immediately emitted by the material (reflection); photons may not interact with the material structure (transmission); orduring transmission photons are changes in velocity (refraction). At any instance of light interaction with a material, the total intensity of the incident light striking a surface is equal to sum of the absorbed, reflected, and transmitted intensities i.e.

26

27

28 Electromagnetic Radiation-Waves EM radiation travels in a vacuum at the speed of light (c=3*10 8 m/s). The speed of light is related to dielectric permittivity and magnetic permeability of a vacuum. 1 The frequency and wavelength c are also related to c. The energy of light (photons) with a given frequency (or wavelength) is related to Planck s constant (h=6.63*10-34 J*sec). Radiation can thus be defined in terms of energy, frequency, or wavelength. c o o E h hc E ν λ??????

29 EM Radiation Spectrum Spans from gamma rays (radioactive materials) to x-rays, UV, visible, IR, microwave, radio/tv

30 Applications of various waves/photons

31 Optical classification There are three primary ways to describe the optical quality of a material (color comes later) Transparent: you can see through it (but color may change). Glass Insulators Some semiconductors Translucent: light is transmitted diffusely (internal scattering), usually related to defects such as grain boundaries or pores. Polycrystalline insulators Opaque: you can t see through it. Bulk metals Some semiconductors

32 Optical Classification R A T I I I I I I I I I I R A T R A T Intensity of the incident beam=sum of the intensities of the transmitted, absorbed, and reflected beams. Materials with little absorption and reflection are transparent. You can see through them. Materials in which light is transmitted diffusely are translucent. Objects are not clearly distinguishable. Materials where light is absorbed and reflected are opaque.

33 Refraction Fish use this concept to see you standing on the shore trying to catch them.