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In physics,
thermal conductivity, k, is the property of a material that indicates its ability to conduct heat. It is used primarily in Heat conduction for heat conduction.
It is defined as the quantity of heat, Δ
Q, transmitted during time Δ
t through a thickness
L, in a direction normal to a surface of area
A, due to a temperature difference Δ
T, under steady state conditions and when the heat transfer is dependent only on the temperature gradient.
: thermal conductivity = heat flow rate × distance / (area × temperature difference)
: k=\frac{\Delta Q}{\Delta t}\times\frac{L}{A\times\Delta T}
: Alternately, it can be thought of as a
flux of heat (energy per unit area per unit time) divided by a temperature gradient (temperature difference per unit length)
: k=\frac{\Delta Q}{A\times{} \Delta t}\times\frac{L}{\Delta T}
Typical units are SI: W/(m*·K) and English units: Btu·ft/(h·ft²·°F). To convert between the two, use the relation 1 Btu·ft/(h·ft²·°F) = 1.730735 W/(m·K). Chemical Engineers' Handbook, 7th Edition, Table 1-4
Examples
In metals, thermal conductivity approximately tracks electrical conductivity, as freely moving
valence electrons transfer not only electric current but also heat energy. However, the general correlation between electrical and thermal conductance does not hold for other materials, due to the increased importance of phonon carriers for heat in non-metals. As shown in the table below, highly electrically conductive
silver is less thermally conductive than diamond, which is an
electrical insulator.
Thermal conductivity depends on many properties of a material, notably its structure and temperature. For instance, pure
crystalline substances exhibit very different thermal conductivities along different crystal axes, due to differences in phonon coupling along a given crystal axis.
Sapphire is a notable example of variable thermal conductivity based on orientation and temperature, for which the CRC Handbook reports a thermal conductivity of 2.6 W/(m·K) perpendicular to the
c-axis at 373 K, but 6000 W/(m·K) at 36 degrees from the
c-axis and 35 K (possible typo?).
Air and other gases are generally good insulators, in the absence of convection. Therefore, many insulating materials function simply by having a large number of gas-filled pockets which prevent large-scale convection. Examples of these include expanded and extruded polystyrene (popularly referred to as "styrofoam") and silica
aerogel. Natural, biological insulators such as fur and
feathers achieve similar effects by dramatically inhibiting convection of air or water near an animal's skin.
Thermal conductivity is important in
building insulation and related fields. However, materials used in such trades are rarely subjected to chemical purity standards. Several construction materials'
k values are listed below. These should be considered approximate due to the uncertainties related to material definitions.
The following table is meant as a small sample of data to illustrate the thermal conductivity of various types of substances. For more complete listings of measured
k-values, see the references.
List of thermal conductivities
This is a list of approximate values of thermal conductivity, k, for some common materials. Please consult the list of thermal conductivities for more accurate values, references and detailed information.
{]!Thermal conductivity
Watt/(Metre·Kelvin)|-|
Air|0.04 – 0.4|-|[Alcohol or oil|0.15|-|[Rubber (unfilled)|0.19|-|[Epoxy (silica-filled)] (liquid)|0.6|-|Thermal grease|1.1|-|[Ice|2.4|-|[Stainless steel|35.3|-|[Aluminium|318|-|[Copper|429|-|[Diamond|900 – 2320|}
Measurement
For good conductors of heat,
Searle's bar method can be used . For poor conductors of heat, Lees' disc method can be used . An alternative traditional method using real thermometers is described at . A brief review of new methods measuring thermal conductivity, thermal diffusivity and specific heat within a single measurement is available at A thermal conductance tester, one of the instruments of
gemology, determines if
gemstone are genuine
diamonds using diamond's uniquely high thermal conductivity.
Standard Measurement Techniques
- IEEE Standard 442-1981, "IEEE guide for soil thermal resistivity measurements"
- IEEE Standard 98-2002, "Standard for the Preparation of Test Procedures for the Thermal Evaluation of Solid Electrical Insulating Materials", ISBN 0-7381-3277-2
- ASTM Standard D5470-06, "Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials"
- ASTM Standard E1225-04, "Standard Test Method for Thermal Conductivity of Solids by Means of the Guarded-Comparative-Longitudinal Heat Flow Technique"
- ASTM Standard D5930-01, "Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source Technique"
- ASTM Standard D2717-95, "Standard Test Method for Thermal Conductivity of Liquids"
Related terms
The reciprocal of thermal conductivity is
thermal resistivity, measured in
kelvin-metres per
watt (K·m·W−1).
When dealing with a known amount of material, its
thermal conductance and the reciprocal property,
thermal resistance, can be described. Unfortunately there are differing definitions for these terms.
First definition (general)
For general scientific use,
thermal conductance is the quantity of heat that passes in unit time through a plate of
particular area and thickness when its opposite faces differ in temperature by one degree. For a plate of thermal conductivity
k, area
A and thickness
L this is
kA/L, measured in W·K−1. This matches the relationship between electrical conductivity (A·m−1·V−1) and electrical conductance (A·V−1).
There is also a measure known as
heat transfer coefficient: the quantity of heat that passes in unit time through
unit area of a plate of particular thickness when its opposite faces differ in temperature by one degree. The reciprocal is
thermal insulance. In summary:
- thermal conductance = kA/L, measured in W·K−1
- thermal resistance = L/kA, measured in K·W−1
- heat transfer coefficient = k/L, measured in W·K−1·m−2
- thermal insulance = L/k, measured in K·m²·W−1.
The heat transfer coefficient is also known as
thermal admittance
Second definition (buildings)
When dealing with buildings,
thermal resistance or
R-value (insulation) means what is described above as thermal insulance, and
thermal conductance means the reciprocal. For materials in series, these thermal resistances (unlike conductances) can simply be added to give a thermal resistance for the whole.
A third term,
thermal transmittance, incorporates the thermal conductance of a structure along with heat transfer due to
convection and thermal radiation. It is measured in the same units as thermal conductance and is sometimes known as the
composite thermal conductance. The term
U-value is another synonym.
In summary, for a plate of thermal conductivity
k (the
k value Definition of
k value from Plastics New Zealand), area
A and thickness
L:
- thermal conductance = k/L, measured in W·K−1·m−2;
- thermal resistance (R value) = L/k, measured in K·m²·W−1;
- thermal transmittance (U value) = 1/(Addition(L/k)) + convection + thermal radiation, measured in W·K−1·m−2.
Textile industry
In textiles, a
tog (unit) value may be quoted as a measure of thermal resistance in place of a measure in SI units.
Origins
The thermal conductivity of a system is determined by how atoms comprising the system interact. There are no simple, correct expressions for thermal conductivity. There are two different approaches for calculating the thermal conductivity of a system.The first approach employs the Green-Kubo relations. Although this expression is exact*, in order to calculate the thermal conductivity of a dense fluid or solid using this relation requires the use of molecular dynamics computer simulation.
- The term exact is applied to mean that the equations are solvable.
The second approach is based upon the relaxation time approach. Due to the anharmonicity within the crystal potential, the
phonons in the system are known to scatter. There are three main mechanisms for scattering:
- Boundary scattering, a phonon hitting the boundary of a system;
- Mass defect scattering, a phonon hitting an impurity within the system and scattering;
- Phonon-phonon scattering, a phonon breaking into two lower energy phonons or a phonon colliding with another phonon and merging into one higher energy phonon.
Further information can be found in the publication "The Physics of Phonons" by G P Srivastava.
See also
External links
- Thermal Impedence vs. Thermal Conductivity
- http://physics.nist.gov/Pubs/SP811/appenB9.html
- http://www.npl.co.uk/thermal/faq_index.html#heat%20transfer%20property thermophysics FAQ5
- http://www.ornl.gov/roofs+walls/research/detailed_papers/rastra/dynamic.htm
- http://www.tak2000.com/data2.htm
- http://thermophys.savba.sk
- Calculation of the Thermal Conductivity of Glass Calculation of the Thermal Conductivity of Glass at Room Temperature from the Chemical Composition
- http://www.mathisinstruments.com/index.asp?pathinfo=/html/content/technology/tech_glossary.asp&dbbypass=
References
-
- Halliday, David; Resnick, Robert; & Walker, Jearl(1997). Fundamentals of Physics (5th ed.). John Wiley and Sons, INC., NY ISBN 0-471-10558-9.
- TM 5-852-6 AFR 88-19, Volume 6 (Army Corp of Engineers publication)
- Srivastava G. P (1990), "The Physics of Phonons." Adam Hilger, IOP Publishing Ltd, Bristol.
In physics,
thermal conductivity, k, is the property of a material that indicates its ability to conduct heat. It is used primarily in
Heat conduction for
heat conduction.
It is defined as the quantity of heat, Δ
Q, transmitted during time Δ
t through a thickness
L, in a direction normal to a surface of area
A, due to a temperature difference Δ
T, under steady state conditions and when the heat transfer is dependent only on the temperature gradient.
: thermal conductivity = heat flow rate × distance / (area × temperature difference)
: k=\frac{\Delta Q}{\Delta t}\times\frac{L}{A\times\Delta T}
: Alternately, it can be thought of as a
flux of heat (energy per unit area per unit time) divided by a temperature gradient (temperature difference per unit length)
: k=\frac{\Delta Q}{A\times{} \Delta t}\times\frac{L}{\Delta T}
Typical units are
SI: W/(m*·K) and English units: Btu·ft/(h·ft²·°F). To convert between the two, use the relation 1 Btu·ft/(h·ft²·°F) = 1.730735 W/(m·K). Chemical Engineers' Handbook, 7th Edition, Table 1-4
Examples
In metals, thermal conductivity approximately tracks
electrical conductivity, as freely moving valence electrons transfer not only electric current but also heat energy. However, the general correlation between electrical and thermal conductance does not hold for other materials, due to the increased importance of phonon carriers for heat in non-metals. As shown in the table below, highly electrically conductive
silver is less thermally conductive than diamond, which is an
electrical insulator.
Thermal conductivity depends on many properties of a material, notably its structure and temperature. For instance, pure
crystalline substances exhibit very different thermal conductivities along different crystal axes, due to differences in phonon coupling along a given crystal axis.
Sapphire is a notable example of variable thermal conductivity based on orientation and temperature, for which the CRC Handbook reports a thermal conductivity of 2.6 W/(m·K) perpendicular to the
c-axis at 373 K, but 6000 W/(m·K) at 36 degrees from the
c-axis and 35 K (possible typo?).
Air and other gases are generally good insulators, in the absence of
convection. Therefore, many insulating materials function simply by having a large number of gas-filled pockets which prevent large-scale convection. Examples of these include expanded and extruded polystyrene (popularly referred to as "styrofoam") and silica
aerogel. Natural, biological insulators such as fur and feathers achieve similar effects by dramatically inhibiting convection of air or water near an animal's skin.
Thermal conductivity is important in
building insulation and related fields. However, materials used in such trades are rarely subjected to chemical purity standards. Several construction materials'
k values are listed below. These should be considered approximate due to the uncertainties related to material definitions.
The following table is meant as a small sample of data to illustrate the thermal conductivity of various types of substances. For more complete listings of measured
k-values, see the references.
List of thermal conductivities
This is a list of approximate values of thermal conductivity, k, for some common materials. Please consult the list of thermal conductivities for more accurate values, references and detailed information.
{]!Thermal conductivity
Watt/(
Metre·
Kelvin)|-|Air|0.04 – 0.4|-|[Alcohol or oil|0.15|-|[Rubber (unfilled)|0.19|-|[Epoxy (silica-filled)] (liquid)|0.6|-|Thermal grease|1.1|-|[Ice|2.4|-|[Stainless steel|35.3|-|[Aluminium|318|-|[Copper|429|-|[Diamond|900 – 2320|}
Measurement
For good conductors of heat,
Searle's bar method can be used . For poor conductors of heat, Lees' disc method can be used . An alternative traditional method using real thermometers is described at . A brief review of new methods measuring thermal conductivity,
thermal diffusivity and specific heat within a single measurement is available at A thermal conductance tester, one of the instruments of
gemology, determines if gemstone are genuine
diamonds using diamond's uniquely high thermal conductivity.
Standard Measurement Techniques
- IEEE Standard 442-1981, "IEEE guide for soil thermal resistivity measurements"
- IEEE Standard 98-2002, "Standard for the Preparation of Test Procedures for the Thermal Evaluation of Solid Electrical Insulating Materials", ISBN 0-7381-3277-2
- ASTM Standard D5470-06, "Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials"
- ASTM Standard E1225-04, "Standard Test Method for Thermal Conductivity of Solids by Means of the Guarded-Comparative-Longitudinal Heat Flow Technique"
- ASTM Standard D5930-01, "Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source Technique"
- ASTM Standard D2717-95, "Standard Test Method for Thermal Conductivity of Liquids"
Related terms
The reciprocal of thermal conductivity is
thermal resistivity, measured in
kelvin-metres per
watt (K·m·W−1).
When dealing with a known amount of material, its
thermal conductance and the reciprocal property,
thermal resistance, can be described. Unfortunately there are differing definitions for these terms.
First definition (general)
For general scientific use,
thermal conductance is the quantity of heat that passes in unit time through a plate of
particular area and thickness when its opposite faces differ in temperature by one degree. For a plate of thermal conductivity
k, area
A and thickness
L this is
kA/L, measured in W·K−1. This matches the relationship between electrical conductivity (A·m−1·V−1) and electrical conductance (A·V−1).
There is also a measure known as
heat transfer coefficient: the quantity of heat that passes in unit time through
unit area of a plate of particular thickness when its opposite faces differ in temperature by one degree. The reciprocal is
thermal insulance. In summary:
- thermal conductance = kA/L, measured in W·K−1
- thermal resistance = L/kA, measured in K·W−1
- heat transfer coefficient = k/L, measured in W·K−1·m−2
- thermal insulance = L/k, measured in K·m²·W−1.
The heat transfer coefficient is also known as
thermal admittance
Second definition (buildings)
When dealing with buildings,
thermal resistance or
R-value (insulation) means what is described above as thermal insulance, and
thermal conductance means the reciprocal. For materials in series, these thermal resistances (unlike conductances) can simply be added to give a thermal resistance for the whole.
A third term,
thermal transmittance, incorporates the thermal conductance of a structure along with heat transfer due to convection and
thermal radiation. It is measured in the same units as thermal conductance and is sometimes known as the
composite thermal conductance. The term
U-value is another synonym.
In summary, for a plate of thermal conductivity
k (the
k value Definition of
k value from Plastics New Zealand), area
A and thickness
L:
- thermal conductance = k/L, measured in W·K−1·m−2;
- thermal resistance (R value) = L/k, measured in K·m²·W−1;
- thermal transmittance (U value) = 1/(Addition(L/k)) + convection + thermal radiation, measured in W·K−1·m−2.
Textile industry
In textiles, a
tog (unit) value may be quoted as a measure of thermal resistance in place of a measure in SI units.
Origins
The thermal conductivity of a system is determined by how atoms comprising the system interact. There are no simple, correct expressions for thermal conductivity. There are two different approaches for calculating the thermal conductivity of a system.The first approach employs the Green-Kubo relations. Although this expression is exact*, in order to calculate the thermal conductivity of a dense fluid or solid using this relation requires the use of molecular dynamics computer simulation.
- The term exact is applied to mean that the equations are solvable.
The second approach is based upon the relaxation time approach. Due to the anharmonicity within the crystal potential, the
phonons in the system are known to scatter. There are three main mechanisms for scattering:
- Boundary scattering, a phonon hitting the boundary of a system;
- Mass defect scattering, a phonon hitting an impurity within the system and scattering;
- Phonon-phonon scattering, a phonon breaking into two lower energy phonons or a phonon colliding with another phonon and merging into one higher energy phonon.
Further information can be found in the publication "The Physics of Phonons" by G P Srivastava.
See also
External links
- Thermal Impedence vs. Thermal Conductivity
- http://physics.nist.gov/Pubs/SP811/appenB9.html
- http://www.npl.co.uk/thermal/faq_index.html#heat%20transfer%20property thermophysics FAQ5
- http://www.ornl.gov/roofs+walls/research/detailed_papers/rastra/dynamic.htm
- http://www.tak2000.com/data2.htm
- http://thermophys.savba.sk
- Calculation of the Thermal Conductivity of Glass Calculation of the Thermal Conductivity of Glass at Room Temperature from the Chemical Composition
- http://www.mathisinstruments.com/index.asp?pathinfo=/html/content/technology/tech_glossary.asp&dbbypass=
References
-
- Halliday, David; Resnick, Robert; & Walker, Jearl(1997). Fundamentals of Physics (5th ed.). John Wiley and Sons, INC., NY ISBN 0-471-10558-9.
- TM 5-852-6 AFR 88-19, Volume 6 (Army Corp of Engineers publication)
- Srivastava G. P (1990), "The Physics of Phonons." Adam Hilger, IOP Publishing Ltd, Bristol.
Thermal conductivity - Wikipedia, the free encyclopedia
In physics, thermal conductivity, k, is the property of a material that indicates its ability to conduct heat. It appears primarily in Fourier's Law for heat conduction.
List of thermal conductivities - Wikipedia, the free encyclopedia
In physics, thermal conductivity, k, is the intensive property of a material that indicates its ability to conduct heat. First, we define heat conduction by the formula:
Thermal Conductivity : Thermal Performance : Material Properties ...
Thermal conductivity and thermal resistance measurements on construction and engineering materials.
Thermal Conductivity
Material: Thermal conductivity (cal/sec)/(cm 2 C/cm) Thermal conductivity (W/m K)* Diamond... 1000: Silver: 1.01: 406.0: Copper: 0.99: 385.0: Gold... 314: Brass... 109.0
Epogee: Thermal Conductivity
Epogee Solar Water Heating Approved Training Course and other Renewable Energy Training Courses ... Reference Section: Thermal Conductivities OK, you all knew that copper was a ...
Thermal Conductivity
Thermal Conductivity (k-values) Units = W/mK . This is a measure of the rate at which heat is conducted through a particular material under specified conditions.
Thermal conductivity - What does TC stand for? Acronyms and ...
Acronym Definition; TC: Air Tanzania (IATA airline code) Tc: Case Temperature: Tc: Core Temperature: TC: Critical Temperature: TC: Tachometer Constant: TC: Tachycardia
Thermal Conductivity
Theory & Principle of Operation The sensor uses four matched filaments that change resistance according to the thermal conductivity of the gas passing over it.
e-Prints Soton - Thermal conductivity of molten lead-free solders
The paper reports measurements of the thermal conductivity of a number of molten solders for the electronics industry that are part of a group of materials designed to be free of ...
Research - Kingston University London
Adjali, M.H. and Laurent, M. (2007) Thermal conductivity estimation in non-linear problems. International Journal of Heat and Mass Transfer, 50 (23-24). pp. 4623-4628.
