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Thermal conductivityFrom Wikipedia, the free encyclopedia

In physics, thermal conductivity, (or denoted ), is the property of a material's ability to conductheat. It appears primarily in Fourier's Law for heat conduction.

Heat transfer across materials of high thermal conductivity occurs at a higher rate than across materialsof low thermal conductivity. Correspondingly materials of high thermal conductivity are widely used in

heat sink applications and materials of low thermal conductivity are used as thermal insulation. Thermalconductivity of materials is temperature dependent. The reciprocal of thermal conductivity is thermalresistivity.

Contents

1 Units of thermal conductivity

2 Measurement 3 Experimental values 4 Definitions

4.1 Conductance 4.2 Resistance

4.3 Transmittance 5 Influencing factors

5.1 Temperature

5.2 Material phase 5.3 Material structure 5.4 Electrical conductivity

5.5 Convection 6 Physical origins

6.1 Lattice waves 6.2 Electronic thermal conductivity

7 Equations

8 See also 9 References

10 Further reading

Units of thermal conductivityIn the International System of Units (SI), thermal conductivity is measured in watts per meter kelvin (W/

(mK)) or Wm1

K1

.

In the imperial system of measurement thermal conductivity is measured in Btu/(hrftF) where 1 Btu/

(hrftF) = 1.730735 W/(mK).[1]

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Experimental values of thermal conductivity.

ther units which are closely related to the thermal conductivity are in common use in the construction

and textile industries. The construction industry makes use of units such as the R- alue (resistancevalue) and the U- alue (thermal transmittance). lthough related to the thermal conductivity of aproduct R and U-values are dependent on the thickness of a product.

Likewise the textile industry has several units including the Tog and the Clo which express thermalresistance of a material in a way analogous to the R-values used in the construction industry.

ote: R- alues and U- alues quoted in the US (based on the imperial units of measurement) do notcorrespond with and are not compatible with those used in Europe (based on the SI units ofmeasurement).

Measurement

Main article: Thermal conductivity measurement

There are a number of ways to measure thermal conductivity. Each of these is suitable for a limitedrange of materials, depending on the thermal properties and the medium temperature. There is adistinction between steady-state and transient techniques.

In general, steady-state techniques are useful when the temperature of the material does not change withtime. This makes the signal analysis straightforward (steady state implies constant signals). The

disadvantage is that a well-engineered experimental setup is usually needed. The Divided Bar (varioustypes) is the most common device used for consolidated rock samples.

The transient techniques perform a measurement during the process of heating up. Their advantage is

quicker measurements. Transient methods are usually carried out by needle probes. method describedby ngstrom involves rapidly cycling the temperature from hot to cold and back and measuring the

temperature change as the heat propagates along a thin strip of material in a vacuum.

erimental values

Main

article:List of


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thermal conductivities

Thermal conductivity is important in material science, research, electronics, building insulation andrelated fields, especially where high operating temperatures are achieved. However, materials used insuch trades are rarely sub ected to chemical purity standards. Several materials are shown in the list of

thermal conductivities. These should be considered approximate due to the uncertainties related tomaterial definitions.

n the one hand solutions for computer cooling or turbine blades usually use high thermal conductivematerials such as silver, copper and aluminium, to cool down specific components. n the other hand inconstruction or furnaces low thermal conductive materials such as polystyrene and alumina are used to

separate warm / hot parts from cold ones.

efinitions

The reciprocal of thermal conductivity is thermal resistivity, usually measured in kelvin-meters per watt

(KmW1). 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 theseterms.

Conductance

For general scientific use, thermal conductance is the quantity of heat that passes in unit time through aplate ofparticular area and thickness when its opposite faces differ in temperature by one kelvin. For a

plate of thermal conductivity k, areaA and thicknessL this is kA/L, measured in WK1 (equivalent to:

W/ C). Thermal conductivity and conductance are analogous to electrical conductivity ( m1

1

) and

electrical conductance ( 1).

There is also a measure known as heat transfer coefficient: the quantity of heat that passes in unit timethrough unit area of a plate of particular thickness when its opposite faces differ in temperature by one

kelvin. The reciprocal is thermal insulance. In summary:

thermal conductance = kA/L, measured in WK1

thermal resistance =L /(kA), measured in KW1 (equivalent to: C/W)

heat transfer coefficient= k/L, measured in WK1m2

thermal insulance =L /k, measured in Km W1.

The heat transfer coefficient is also known as thermal admittance

esistance

Main article: Thermal resistance

It is a thermal-property of a material to resist the flow of heat.

It is a resistance offered by a material (a metal in general and a heat sink material in particular) to theconduction or flow of heat through it.


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Thermal resistance is the reciprocal of thermal conductance, i.e., lowering its value will raise the heat

conduction and vice versa.

When thermal resistances occur in series, they are additive. So when heat flows through twocomponents each with a resistance of 1 C/W, the total resistance is 2 C/W.

common engineering design problem involves the selection of an appropriate si ed heat sink for a

given heat source. Working in units of thermal resistance greatly simplifies the design calculation. Thefollowing formula can be used to estimate the performance:

where:

Rhs is the maximum thermal resistance of the heat sink to ambient, in C/W (equivalent to K/W)

is the temperature difference (temperature drop), in C Pth is the thermal power (heat flow), in watts

Rs is the thermal resistance of the heat source, in C/W

For example, if a component produces 100 W of heat, and has a thermal resistance of 0.5 C/W, what is

the maximum thermal resistance of the heat sink Suppose the maximum temperature is 125 C, and theambient temperature is 25 C then the is 100 C. The heat sink's thermal resistance to ambient

must then be 0.5 C/W or less.

Transmittance

third term, thermal transmittance, incorporates the thermal conductance of a structure along with heattransfer due to convection and 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.

nfluencin factors

Tem erature

The effect of temperature on thermal conductivity is different for metals and nonmetals. In metalsconductivity is primarily due to free electrons. Following Wiedemann Fran law thermal conductivity

of metals is approximately proportional to the absolute temperature (in Kelvin) times electricalconductivity. In pure metals the electrical resistivity often increases proportional to temperature and thus

thermal conductivity stays approximately constant. In alloys the change in electrical conductivity isusually smaller and thus thermal conductivity increases with temperature, often proportional totemperature.

n the other hand conductivity in nonmetals is mainly due to lattice vibrations (phonons). Except forhigh quality crystals at low temperatures, the phonon mean free path of phonons is not reducedsignificantly at higher temperatures. Thus the thermal conductivity of nonmetals is approximately

constant at not too low temperatures. t low temperatures well below Debye-temperature thermalconductivity decreases ust like the heat capacity does.




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Ceramic coatings with low thermal

conductivities are used on exhaust

systems to prevent heat from reaching

sensitive components

Material hase

When a material undergoes a phase change from solid to liquid or from liquid to gas the thermal

conductivity may change. n example of this would be the change in thermal conductivity that occurswhen ice (thermal conductivity of 2.18 W/(mK) at 0 C) melts into liquid water (thermal conductivityof 0.58 W/(mK) at 0 C).

Material structure

Pure crystalline substances can exhibit different thermal conductivities along different crystal axes, dueto differences in phonon coupling along a given crystal axis. Sapphire is a notable example of variable

thermal conductivity based on orientation and temperature, with 35 W/(mK) along the c-axis and 32 W/

(mK) along the a-axis.[2]

lectrical conductivity

In metals, thermal conductivity approximately tracks electrical conductivity according to theWiedemann-Fran law, as freely moving valence electrons transfer not only electric current but also heatenergy. However, the general correlation between electrical and thermal conductance does not hold forother materials, due to the increased importance of phonon carriers for heat in non-metals. s shown in

the table below, highly electrically conductive silver is less thermally conductive than diamond, which isan electrical insulator.

Convection

ir 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 toas "styrofoam") and silica aerogel. atural, biological insulators such as fur and feathers achieve similareffects by dramatically inhibiting convection of air or water near an animal's skin.

Light gases, such as hydrogen and helium typically have high

thermal conductivity. Dense gases such as xenon anddichlorodifluoromethane have low thermal conductivity. n

exception, sulfur hexafluoride, a dense gas, has a relatively highthermal conductivity due to its high heat capacity. rgon, a gasdenser than air, is often used in insulated gla ing (double paned

windows) to improve their insulation characteristics.

hysical ori ins

Heat flux is exceedingly difficult to control and isolate in alaboratory setting. Thus at the atomic level, there are no simple,correct expressions for thermal conductivity. tomically, the

thermal conductivity of a system is determined by how atomscomposing the system interact. There are two different approaches for calculating the thermal

conductivity of a system.


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