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The ability of a material to transport heat.
temperature gradient
thermal conductivity (J/m-K-s)
heat flux (J/m^2 -s)
- Atomic perspective: Atomic vibrations and free electrons in
hotter regions transport energy to cooler regions.
T 2
T 2 > T 1
T 1
x 1 heat flux x 2
Thermal Conductivity
dx
dT q k
Fourier’s Law
Thermal Conductivity
• Thermal conductivity, k, is the ability of a
material to conduct heat, and is an
intensive property of that material.
• Thermal conductivity can be calculated
from a number of measured quantities, but
is normally defined as: the quantity of
heat, Q, transmitted through a thickness L,
in a direction normal to a surface with area
A, due to a temperature difference ΔT,
under steady state conditions and when
the heat transfer is dependent only on the
temperature gradient.
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- All-Clad Metalcrafters bond together different metals to capitalize on unique properties.
- (^) Because the raw materials are critical to performance, All-Clad metallurgists specify the metal formulations down to the chemical composition and microstructure.
- (^) Since quality always takes precedence over convenience, the metals are formulated for optimal cooking performance; not for ease of manufacturing.
- The “stay-cool” handle is cast from solid stainless steel, and is ergonomically-designed for comfort during long cooking sessions.
- (^) Rivets are formed from high-yield-strength stainless steel, and treated to remove trace elements of iron that could otherwise cause corrosion.
Engineering Materials for Thermal Behavior
Easy-to-clean stainless steel interior will not react with food; Thick copper-core distributes heat evenly; Stainless steel exterior. Solid cast stainless steel handles; High quality white porcelain.
Hand-hammered 1.2mm- gauge copper for superior heat conductivity and temperature control. Nonreactive tin interior is easy to clean. Heavy porcelain insert prohibits scorching of contents. Copper lid with bronze knob. Riveted bronze handles
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- Thermal stresses occur due to: -- restrained thermal expansion/contraction -- temperature gradients that lead to differential dimensional changes
Thermal Stresses (Ex 1)
Thermal stress: E ( T 0 Tf ) E T
- (^) 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 - Tf
Tf = 20 °C - [-1.72 x 10^8 Pa / (1 x10^11 Pa) (20 x 10-6^ / °C)]
Tf = 20 °C + 86 °C
α = 106 °C
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- Occurs due to: nonuniform heating/cooling
- Ex: Assume top thin layer is rapidly cooled from T 1 to T 2
Tension develops at surface
E l ( T 1 T 2 )
Critical temperature difference for fracture (set = f )
( T 1 T 2 )fracture
f
E l
set equal
f k E l
Thermal Shock Resistance
Temperature difference that can be produced by cooling:
k
T T
quench rate
rapid quench
resists contraction
tries to contract during cooling T^2 T 1
E
k
- (quench^ rate)forfracture ^ ThermalShock Resistance(^ TSR^ )^ f
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.
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NASA Space Environment and Experiments Branch
- (^) Thermal Performance of an Annealed Pyrolytic Graphite Solar Collector
- (^) A solar collector having the combined properties of high solar absorptance, low infrared emittance, and high thermal conductivity is needed for applications where solar energy is to be absorbed and transported for use in minisatellites.
- (^) Electrical and Thermal Conductivity of Carbon Fiber-Polymer Composites Plates
- (^) Composite thermal conductivity was measured using an optical heating technique and infrared scanning of the surface as well as being calculated from the rule of mixtures.
- (^) Multi-Layer Thermal Control Coatings
- (^) Thermal control coatings on spacecraft will be increasingly important as spacecraft grow smaller and more compact. New thermal control coatings will be needed to meet the demanding requirements of next generation spacecraft. http://www.nasa.gov/index.html^10
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Space Shuttle Atlantis
- Silica tiles (400-1260C):
-- large scale application -- microstructure:
Fig. 19.2W,and C.S. Thatcher, "The Shuttle Orbiter Thermal Protection System", Callister 6e. (Fig. 19.2W adapted from L.J. Korb, C.A. Morant, R.M. Calland, Ceramic Bulletin , No. 11, Nov. 1981, p. 1189.)
Thermal Protection System
reinf C-C (1650°C)
Re-entry T
Distribution
silica tiles (400-1260°C)
nylon felt, silicon rubber coating (400°C)
~90% porosity! Si fibers bonded to one another during heat treatment. 100 m
Who cares about thermal
properties?
- (^) Ryton®^ PPS (polyphenylene sulfide) is produced
by Chevron Phillips Chemical as a high performance engineering resin known for dimensional stability and resistance to corrosive and high-temperature environments. With a thirty-plus year history, Ryton®^ PPS is recognized as the world’s premier product for demanding plastic components in automotive, electrical, appliance and industrial applications.
- (^) Spec sheet for thermal properties:
- (^) Thermal Conductivity
- (^) Specific Heat
- Differential Thermal Analysis
- (^) Coefficient of Linear Thermal Expansion
- Thermal Degradation
- (^) NASA Outgassing Test
http://www.cpchem.com/enu/docs_ryton/RytonThermalPr^13 operties.pdf
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- Thermoplastics: -- little cross linking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene
- Thermosets: -- significant cross linking (10 to 50% of repeat units) -- hard and brittle -- do not soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin
Thermoplastics and Thermosets
Callister, Fig. 16.
T
Molecular weight
Tg
mobile Tm
liquid
viscous liquid
rubber
tough plastic
partially crystalline crystalline solid solid
Melting
(^) The melting of a polymer crystal corresponds to the transformation
of a solid material: an ordered structure of aligned molecular chains becomes a viscous liquid where the structure is highly random.
(^) This phenomenon occurs, upon heating, at the melting
temperature, Tm.
(^) There are several features peculiar to the melting of polymers that
are not normally observed with metals and ceramics.
(^) Melting of polymers takes place over a range of temperatures.
(^) The melting behavior depends on the crystallization temperature.
(^) The melting behavior is a function of the rate of melting; increasing
this rate results in an elevation of the melting temperature. Annealing also raises the Tm by decreasing vacancies and other imperfections.
(^) The glass transition occurs in amorphous (or glassy) and
semicrystalline polymers. (^) Caused by a reduction in motion of large segments of molecular
chains with decreasing temperature. (^) Upon cooling, the glass transition corresponds to the gradual
transformation from a liquid to a rubbery material, and finally to a rigid solid. (^) The temperature where polymer experiences the transition from
rubbery to rigid is termed the glass transition temperature.
Silica Aerogels
- (^) One of the extraordinary properties that was discovered about first silica aerogels was their very low thermal conductivity.
- (^) In the1980s it was apparent that silica aerogels were an attractive alternative to traditional insulation due to their high insulating value and environment-friendly production methods.
- (^) Aerogel materials are open cell, nanoporous materials that have a very high proportion of free void volume (typically >90%) compared to conventional solid materials.
- (^) Silica aerogels prepared via sol-gel processing have some of the best thermal properties of any solid insulation material known.
- (^) Excellent thermal insulation properties have also been reported in organic and carbon based aerogels as well as other inorganic metal oxides produced using sol-gel processing.
- (^) The passage of thermal energy through an insulating material occurs through three mechanisms; solid conductivity, gaseous conductivity and radiative (infrared) transmission. The sum of these three components gives the total thermal conductivity of the material. For dense silica, solid conductivity is relatively high (a single-pane window transmits a large amount of thermal energy). However, silica aerogels possess a very small (~1-10%) fraction of solid silica.
Aerogel
Titan
