Conductividad térmica

Capacity of a material to conduct heat

Para conocer los valores de conductividad térmica, consulte la Lista de conductividades térmicas .

La conductividad térmica de un material es una medida de su capacidad para conducir calor . Se denota comúnmente por , o .${\displaystyle k}$${\displaystyle \lambda }$${\displaystyle \kappa }$

La transferencia de calor ocurre a una tasa menor en materiales de baja conductividad térmica que en materiales de alta conductividad térmica. Por ejemplo, los metales suelen tener una alta conductividad térmica y son muy eficientes para conducir el calor, mientras que lo contrario es cierto para los materiales aislantes como la espuma de poliestireno . En consecuencia, los materiales de alta conductividad térmica se utilizan ampliamente en aplicaciones de disipadores de calor y los materiales de baja conductividad térmica se utilizan como aislamiento térmico . El recíproco de la conductividad térmica se llama resistividad térmica .

La ecuación que define la conductividad térmica es , donde es el flujo de calor , es la conductividad térmica y es el gradiente de temperatura . Esto se conoce como Ley de Fourier para la conducción de calor. Aunque comúnmente se expresa como un escalar , la forma más general de conductividad térmica es un tensor de segundo rango . Sin embargo, la descripción tensorial solo se hace necesaria en materiales que son anisotrópicos .${\displaystyle \mathbf {q} =-k\nabla T}$${\displaystyle \mathbf {q} }$${\displaystyle k}$${\displaystyle \nabla T}$

Definición

Definición simple

La conductividad térmica se puede definir en términos del flujo de calor a través de una diferencia de temperatura.${\displaystyle q}$

Considere un material sólido colocado entre dos ambientes de diferentes temperaturas. Sea la temperatura a y sea ​​la temperatura a , y suponga . Una posible realización de este escenario es un edificio en un día frío de invierno: el material sólido en este caso sería la pared del edificio, separando el ambiente exterior frío del ambiente interior cálido.${\displaystyle T_{1}}$${\displaystyle x=0}$${\displaystyle T_{2}}$${\displaystyle x=L}$${\displaystyle T_{2}>T_{1}}$

De acuerdo con la segunda ley de la termodinámica , el calor fluirá del ambiente caliente al frío en un intento de igualar la diferencia de temperatura. Esto se cuantifica en términos de un flujo de calor , que da la tasa, por unidad de área, a la que el calor fluye en una dirección dada (en este caso menos la dirección x). En muchos materiales, se observa que es directamente proporcional a la diferencia de temperatura e inversamente proporcional a la distancia de separación : ^[1]${\displaystyle q}$${\displaystyle q}$${\displaystyle L}$

${\displaystyle q=-k\cdot {\frac {T_{2}-T_{1}}{L}}.}$

La constante de proporcionalidad es la conductividad térmica; es una propiedad física del material. En el escenario actual, dado que el calor fluye en la dirección negativa xy es negativo, lo que a su vez significa eso . En general, siempre se define como positivo. La misma definición de también puede extenderse a gases y líquidos, siempre que se eliminen otros modos de transporte de energía, como la convección y la radiación .${\displaystyle k}$${\displaystyle T_{2}>T_{1}}$${\displaystyle q}$${\displaystyle k>0}$${\displaystyle k}$${\displaystyle k}$

Por simplicidad, hemos asumido aquí que no varía significativamente cuando la temperatura varía de a . Los casos en los que la variación de temperatura de no es insignificante deben abordarse utilizando la definición más general de que se analiza a continuación.${\displaystyle k}$${\displaystyle T_{1}}$${\displaystyle T_{2}}$${\displaystyle k}$${\displaystyle k}$

Definición general

La conducción térmica se define como el transporte de energía debido al movimiento molecular aleatorio a través de un gradiente de temperatura. Se distingue del transporte de energía por convección y trabajo molecular en que no implica flujos macroscópicos o tensiones internas que realicen el trabajo.

El flujo de energía debido a la conducción térmica se clasifica como calor y se cuantifica mediante el vector , que da el flujo de calor en la posición y el tiempo . Según la segunda ley de la termodinámica, el calor fluye de alta a baja temperatura. Por tanto, es razonable postular que es proporcional al gradiente del campo de temperatura , es decir${\displaystyle \mathbf {q} (\mathbf {r} ,t)}$${\displaystyle \mathbf {r} }$${\displaystyle t}$${\displaystyle \mathbf {q} (\mathbf {r} ,t)}$${\displaystyle T(\mathbf {r} ,t)}$

${\displaystyle \mathbf {q} (\mathbf {r} ,t)=-k\nabla T(\mathbf {r} ,t),}$

donde la constante de proporcionalidad,, es la conductividad térmica. Esto se llama ley de conducción de calor de Fourier. En realidad, no es una ley, sino una definición de conductividad térmica en términos de cantidades físicas independientes y . ^{[2] [3]} Como tal, su utilidad depende de la capacidad de determinar para un material dado en determinadas condiciones. La constante misma suele depender y, por tanto, implícitamente del espacio y el tiempo. También podría ocurrir una dependencia explícita del espacio y el tiempo si el material no es homogéneo o cambia con el tiempo. ^[4]${\displaystyle k>0}$${\displaystyle \mathbf {q} (\mathbf {r} ,t)}$${\displaystyle T(\mathbf {r} ,t)}$${\displaystyle k}$${\displaystyle k}$${\displaystyle T(\mathbf {r} ,t)}$

En algunos sólidos, la conducción térmica es anisotrópica , es decir, el flujo de calor no siempre es paralelo al gradiente de temperatura. Para dar cuenta de tal comportamiento, se debe utilizar una forma tensorial de la ley de Fourier:

${\displaystyle \mathbf {q} (\mathbf {r} ,t)=-{\boldsymbol {\kappa }}\cdot \nabla T(\mathbf {r} ,t)}$

donde es simétrico, tensor de segundo rango llamado tensor de conductividad térmica. ^[5]${\displaystyle {\boldsymbol {\kappa }}}$

Un supuesto implícito en la descripción anterior es la presencia de equilibrio termodinámico local , que permite definir un campo de temperatura .${\displaystyle T(\mathbf {r} ,t)}$

Otras cantidades

En la práctica de la ingeniería, es común trabajar en términos de cantidades derivadas de la conductividad térmica e implícitamente tener en cuenta características específicas del diseño, como las dimensiones de los componentes.

Por ejemplo, la conductancia térmica se define como la cantidad de calor que pasa en unidad de tiempo a través de una placa de un área y espesor particulares cuando sus caras opuestas difieren en temperatura en un kelvin. Para una placa de conductividad térmica , área y espesor , la conductancia es , medida en W⋅K ⁻¹ . ^[6] La relación entre conductividad térmica y conductancia es análoga a la relación entre conductividad eléctrica y conductancia eléctrica .${\displaystyle k}$${\displaystyle A}$${\displaystyle L}$${\displaystyle kA/L}$

La resistencia térmica es la inversa de la conductancia térmica. ^[6] Es una medida conveniente para usar en el diseño de componentes múltiples ya que las resistencias térmicas son aditivas cuando ocurren en serie . ^[7]

También existe una medida conocida como coeficiente de transferencia de calor : la cantidad de calor que pasa por unidad de tiempo a través de una unidad de área de una placa de espesor particular cuando sus caras opuestas difieren en temperatura en un kelvin. ^[8] En ASTM C168-15, esta cantidad independiente del área se denomina "conductancia térmica". ^[9] El recíproco del coeficiente de transferencia de calor es el aislamiento térmico . En resumen, para una placa de conductividad térmica , área y espesor , tenemos${\displaystyle k}$${\displaystyle A}$${\displaystyle L}$

• conductancia térmica = , medida en W⋅K ⁻¹ .${\displaystyle kA/L}$
  • resistencia térmica = , medida en K⋅W ⁻¹ .${\displaystyle L/(kA)}$
• coeficiente de transferencia de calor = , medido en W⋅K ⁻¹ ⋅m ⁻² .${\displaystyle k/L}$
  • aislamiento térmico = , medido en K⋅m ² ⋅W ⁻¹ .${\displaystyle L/k}$

El coeficiente de transferencia de calor también se conoce como admitancia térmica en el sentido de que se puede considerar que el material admite el flujo de calor. ^{[ cita requerida ]}

Un término adicional, transmitancia térmica , cuantifica la conductancia térmica de una estructura junto con la transferencia de calor debido a la convección y la radiación . ^{[ cita requerida ]} Se mide en las mismas unidades que la conductancia térmica y, a veces, se la conoce como conductancia térmica compuesta . El término valor U también se utiliza.

Finalmente, la difusividad térmica combina la conductividad térmica con la densidad y el calor específico : ^[10]${\displaystyle \alpha }$

${\displaystyle \alpha ={\frac {k}{\rho c_{p}}}}$.

Como tal, cuantifica la inercia térmica de un material, es decir, la dificultad relativa para calentar un material a una temperatura determinada utilizando fuentes de calor aplicadas en el límite. ^[11]

Unidades

En el Sistema Internacional de Unidades (SI), la conductividad térmica se mide en vatios por metro-kelvin ( W / ( m ⋅ K )). Algunos artículos informan en vatios por centímetro-kelvin (W / (cm⋅K)).

En unidades imperiales , la conductividad térmica se mide en BTU / ( h ⋅ ft ⋅ ° F ). ^{[nota 1] [12]}

La dimensión de la conductividad térmica es M ¹ L ¹ T ⁻³ Θ ⁻¹ , expresada en términos de las dimensiones masa (M), longitud (L), tiempo (T) y temperatura (Θ).

Otras unidades que están estrechamente relacionadas con la conductividad térmica son de uso común en las industrias textil y de la construcción. La industria de la construcción utiliza medidas como el valor R (resistencia) y el valor U (transmitancia o conductancia). Aunque están relacionados con la conductividad térmica de un material utilizado en un producto o ensamblaje de aislamiento, los valores R y U se miden por unidad de área y dependen del espesor especificado del producto o ensamblaje. ^{[nota 2]}

Likewise the textile industry has several units including the tog and the clo which express thermal resistance of a material in a way analogous to the R-values used in the construction industry.

Measurement

Main article: Thermal conductivity measurement

There are several ways to measure thermal conductivity; each is suitable for a limited range of materials. Broadly speaking, there are two categories of measurement techniques: steady-state and transient. Steady-state techniques infer the thermal conductivity from measurements on the state of a material once a steady-state temperature profile has been reached, whereas transient techniques operate on the instantaneous state of a system during the approach to steady state. Lacking an explicit time component, steady-state techniques do not require complicated signal analysis (steady state implies constant signals). The disadvantage is that a well-engineered experimental setup is usually needed, and the time required to reach steady state precludes rapid measurement.

In comparison with solid materials, the thermal properties of fluids are more difficult to study experimentally. This is because in addition to thermal conduction, convective and radiative energy transport are usually present unless measures are taken to limit these processes. The formation of an insulating boundary layer can also result in an apparent reduction in the thermal conductivity.^[13][14]

Experimental values

Experimental values of thermal conductivity^{[clarification needed]}

Main article: List of thermal conductivities

The thermal conductivities of common substances span at least four orders of magnitude. Gases generally have low thermal conductivity, and pure metals have high thermal conductivity. For example, under standard conditions the thermal conductivity of copper is over 10000 times that of air.

Of all materials, allotropes of carbon, such as graphite and diamond, are usually credited with having the highest thermal conductivities at room temperature.^[15] The thermal conductivity of natural diamond at room temperature is several times higher than that of a highly conductive metal such as copper (although the precise value varies depending on the diamond type).^[16]

Thermal conductivities of selected substances are tabulated here; an expanded list can be found in the list of thermal conductivities. These values should be considered approximate due to the uncertainties related to material definitions.

Substance	Thermal conductivity (W·m−1·K−1)	Temperature (°C)
Air[17]	0.026	25
Styrofoam[18]	0.033	25
Water[19]	0.6089	26.85
Concrete[19]	0.92	–
Copper[19]	384.1	18.05
Natural diamond[16]	895–1350	26.85

Influencing factors

Temperature

The effect of temperature on thermal conductivity is different for metals and nonmetals. In metals, heat conductivity is primarily due to free electrons. Following the Wiedemann–Franz law, thermal conductivity of metals is approximately proportional to the absolute temperature (in kelvins) times electrical conductivity. In pure metals the electrical conductivity decreases with increasing temperature and thus the product of the two, the thermal conductivity, stays approximately constant. However, as temperatures approach absolute zero, the thermal conductivity decreases sharply.^[20] In alloys the change in electrical conductivity is usually smaller and thus thermal conductivity increases with temperature, often proportionally to temperature. Many pure metals have a peak thermal conductivity between 2 K and 10 K.

On the other hand, heat conductivity in nonmetals is mainly due to lattice vibrations (phonons). Except for high-quality crystals at low temperatures, the phonon mean free path is not reduced significantly at higher temperatures. Thus, the thermal conductivity of nonmetals is approximately constant at high temperatures. At low temperatures well below the Debye temperature, thermal conductivity decreases, as does the heat capacity, due to carrier scattering from defects at very low temperatures.^[20]

Chemical phase

When a material undergoes a phase change (e.g. from solid to liquid), the thermal conductivity may change abruptly. For instance, when ice melts to form liquid water at 0 °C, the thermal conductivity changes from 2.18 W/(m⋅K) to 0.56 W/(m⋅K).^[21]

Even more dramatically, the thermal conductivity of a fluid diverges in the vicinity of the vapor-liquid critical point.^[22]

Thermal anisotropy

Some substances, such as non-cubic crystals, can exhibit 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, with 35 W/(m⋅K) along the c axis and 32 W/(m⋅K) along the a axis.^[23]Wood generally conducts better along the grain than across it. Other examples of materials where the thermal conductivity varies with direction are metals that have undergone heavy cold pressing, laminated materials, cables, the materials used for the Space Shuttle thermal protection system, and fiber-reinforced composite structures.^[24]

When anisotropy is present, the direction of heat flow may not be exactly the same as the direction of the thermal gradient.

Electrical conductivity

In metals, thermal conductivity approximately tracks electrical conductivity according to the Wiedemann–Franz law, 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. Highly electrically conductive silver is less thermally conductive than diamond, which is an electrical insulator but conducts heat via phonons due to its orderly array of atoms.

Magnetic field

The influence of magnetic fields on thermal conductivity is known as the thermal Hall effect or Righi–Leduc effect.

Gaseous phases

Exhaust system components with ceramic coatings having a low thermal conductivity reduce heating of nearby sensitive components

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 obstruct heat conduction pathways. Examples of these include expanded and extruded polystyrene (popularly referred to as "styrofoam") and silica aerogel, as well as warm clothes. Natural, biological insulators such as fur and feathers achieve similar effects by trapping air in pores, pockets or voids, thus dramatically inhibiting convection of air or water near an animal's skin.

Low density gases, such as hydrogen and helium typically have high thermal conductivity. Dense gases such as xenon and dichlorodifluoromethane have low thermal conductivity. An exception, sulfur hexafluoride, a dense gas, has a relatively high thermal conductivity due to its high heat capacity. Argon and krypton, gases denser than air, are often used in insulated glazing (double paned windows) to improve their insulation characteristics.

The thermal conductivity through bulk materials in porous or granular form is governed by the type of gas in the gaseous phase, and its pressure.^[25] At lower pressures, the thermal conductivity of a gaseous phase is reduced, with this behaviour governed by the Knudsen number, defined as ${\displaystyle K_{n}=l/d}$, where ${\displaystyle l}$ is the mean free path of gas molecules and ${\displaystyle d}$ is the typical gap size of the space filled by the gas. In a granular material ${\displaystyle d}$ corresponds to the characteristic size of the gaseous phase in the pores or intergranular spaces.^[25]

Isotopic purity

The thermal conductivity of a crystal can depend strongly on isotopic purity, assuming other lattice defects are negligible. A notable example is diamond: at a temperature of around 100 K the thermal conductivity increases from 10,000 W·m⁻¹·K⁻¹ for natural type IIa diamond (98.9% 12C), to 41,000 for 99.9% enriched synthetic diamond. A value of 200,000 is predicted for 99.999% 12C at 80 K, assuming an otherwise pure crystal.^[26]

Theoretical prediction

The atomic mechanisms of thermal conduction vary among different materials, and in general depend on details of the microscopic structure and atomic interactions. As such, thermal conductivity is difficult to predict from first-principles. Any expressions for thermal conductivity which are exact and general, e.g. the Green-Kubo relations, are difficult to apply in practice, typically consisting of averages over multiparticle correlation functions.^[27] A notable exception is a dilute gas, for which a well-developed theory exists expressing thermal conductivity accurately and explicitly in terms of molecular parameters.

In a gas, thermal conduction is mediated by discrete molecular collisions. In a simplified picture of a solid, thermal conduction occurs by two mechanisms: 1) the migration of free electrons and 2) lattice vibrations (phonons). The first mechanism dominates in pure metals and the second in non-metallic solids. In liquids, by contrast, the precise microscopic mechanisms of thermal conduction are poorly understood.^[28]

Gases

See also: Kinetic theory of gases

In a simplified model of a dilute monatomic gas, molecules are modeled as rigid spheres which are in constant motion, colliding elastically with each other and with the walls of their container. Consider such a gas at temperature ${\displaystyle T}$ and with density ${\displaystyle \rho }$, specific heat ${\displaystyle c_{v}}$ and molecular mass ${\displaystyle m}$. Under these assumptions, an elementary calculation yields for the thermal conductivity

${\displaystyle k=\beta \rho \lambda c_{v}{\sqrt {\frac {2k_{\text{B}}T}{\pi m}}},}$

where ${\displaystyle \beta }$ is a numerical constant of order ${\displaystyle 1}$, ${\displaystyle k_{\text{B}}}$ is the Boltzmann constant, and ${\displaystyle \lambda }$ is the mean free path, which measures the average distance a molecule travels between collisions.^[29] Since ${\displaystyle \lambda }$ is inversely proportional to density, this equation predicts that thermal conductivity is independent of density for fixed temperature. The explanation is that increasing density increases the number of molecules which carry energy but decreases the average distance ${\displaystyle \lambda }$ a molecule can travel before transferring its energy to a different molecule: these two effects cancel out. For most gases, this prediction agrees well with experiments at pressures up to about 10 atmospheres.^[30] On the other hand, experiments show a more rapid increase with temperature than ${\displaystyle k\propto {\sqrt {T}}}$ (here ${\displaystyle \lambda }$ is independent of ${\displaystyle T}$). This failure of the elementary theory can be traced to the oversimplified "elastic sphere" model, and in particular to the fact that the interparticle attractions, present in all real-world gases, are ignored.

To incorporate more complex interparticle interactions, a systematic approach is necessary. One such approach is provided by Chapman–Enskog theory, which derives explicit expressions for thermal conductivity starting from the Boltzmann equation. The Boltzmann equation, in turn, provides a statistical description of a dilute gas for generic interparticle interactions. For a monatomic gas, expressions for ${\displaystyle k}$ derived in this way take the form

${\displaystyle k={\frac {25}{32}}{\frac {\sqrt {\pi mk_{\text{B}}T}}{\pi \sigma ^{2}\Omega (T)}}c_{v},}$

where ${\displaystyle \sigma }$ is an effective particle diameter and ${\displaystyle \Omega (T)}$ is a function of temperature whose explicit form depends on the interparticle interaction law.^[31][32] For rigid elastic spheres, ${\displaystyle \Omega (T)}$ is independent of ${\displaystyle T}$ and very close to ${\displaystyle 1}$. More complex interaction laws introduce a weak temperature dependence. The precise nature of the dependence is not always easy to discern, however, as ${\displaystyle \Omega (T)}$ is defined as a multi-dimensional integral which may not be expressible in terms of elementary functions. An alternate, equivalent way to present the result is in terms of the gas viscosity ${\displaystyle \mu }$, which can also be calculated in the Chapman–Enskog approach:

${\displaystyle k=f\mu c_{v},}$

where ${\displaystyle f}$ is a numerical factor which in general depends on the molecular model. For smooth spherically symmetric molecules, however, ${\displaystyle f}$ is very close to ${\displaystyle 2.5}$, not deviating by more than ${\displaystyle 1\%}$ for a variety of interparticle force laws.^[33] Since ${\displaystyle k}$, ${\displaystyle \mu }$, and ${\displaystyle c_{v}}$ are each well-defined physical quantities which can be measured independent of each other, this expression provides a convenient test of the theory. For monatomic gases, such as the noble gases, the agreement with experiment is fairly good.^[34]

For gases whose molecules are not spherically symmetric, the expression ${\displaystyle k=f\mu c_{v}}$ still holds. In contrast with spherically symmetric molecules, however, ${\displaystyle f}$ varies significantly depending on the particular form of the interparticle interactions: this is a result of the energy exchanges between the internal and translational degrees of freedom of the molecules. An explicit treatment of this effect is difficult in the Chapman–Enskog approach. Alternately, the approximate expression ${\displaystyle f=(1/4){(9\gamma -5)}}$ was suggested by Eucken, where ${\displaystyle \gamma }$ is the heat capacity ratio of the gas.^[33][35]

The entirety of this section assumes the mean free path ${\displaystyle \lambda }$ is small compared with macroscopic (system) dimensions. In extremely dilute gases this assumption fails, and thermal conduction is described instead by an apparent thermal conductivity which decreases with density. Ultimately, as the density goes to ${\displaystyle 0}$ the system approaches a vacuum, and thermal conduction ceases entirely. For this reason a vacuum is an effective insulator.

Liquids

The exact mechanisms of thermal conduction are poorly understood in liquids: there is no molecular picture which is both simple and accurate. An example of a simple but very rough theory is that of Bridgman, in which a liquid is ascribed a local molecular structure similar to that of a solid, i.e. with molecules located approximately on a lattice. Elementary calculations then lead to the expression

${\displaystyle k=3(N_{\text{A}}/V)^{2/3}k_{\text{B}}v_{\text{s}},}$

where ${\displaystyle N_{\text{A}}}$ is the Avogadro constant, ${\displaystyle V}$ is the volume of a mole of liquid, and ${\displaystyle v_{\text{s}}}$ is the speed of sound in the liquid. This is commonly called Bridgman's equation.^[36]

Metals

Main article: Free electron model

For metals at low temperatures the heat is carried mainly by the free electrons. In this case the mean velocity is the Fermi velocity which is temperature independent. The mean free path is determined by the impurities and the crystal imperfections which are temperature independent as well. So the only temperature-dependent quantity is the heat capacity c, which, in this case, is proportional to T. So

${\displaystyle k=k_{0}\,T{\text{ (metal at low temperature)}}}$

with k₀ a constant. For pure metals such as copper, silver, etc. k₀ is large, so the thermal conductivity is high. At higher temperatures the mean free path is limited by the phonons, so the thermal conductivity tends to decrease with temperature. In alloys the density of the impurities is very high, so l and, consequently k, are small. Therefore, alloys, such as stainless steel, can be used for thermal insulation.

Lattice waves

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Heat transport in both amorphous and crystalline dielectric solids is by way of elastic vibrations of the lattice (i.e., phonons). This transport mechanism is theorized to be limited by the elastic scattering of acoustic phonons at lattice defects. This has been confirmed by the experiments of Chang and Jones on commercial glasses and glass ceramics, where the mean free paths were found to be limited by "internal boundary scattering" to length scales of 10⁻² cm to 10⁻³ cm.^[37][38]

The phonon mean free path has been associated directly with the effective relaxation length for processes without directional correlation. If V_g is the group velocity of a phonon wave packet, then the relaxation length ${\displaystyle l\;}$ is defined as:

${\displaystyle l\;=V_{\text{g}}t}$

where t is the characteristic relaxation time. Since longitudinal waves have a much greater phase velocity than transverse waves,^[39] V_long is much greater than V_trans, and the relaxation length or mean free path of longitudinal phonons will be much greater. Thus, thermal conductivity will be largely determined by the speed of longitudinal phonons.^[37][40]

Regarding the dependence of wave velocity on wavelength or frequency (dispersion), low-frequency phonons of long wavelength will be limited in relaxation length by elastic Rayleigh scattering. This type of light scattering from small particles is proportional to the fourth power of the frequency. For higher frequencies, the power of the frequency will decrease until at highest frequencies scattering is almost frequency independent. Similar arguments were subsequently generalized to many glass forming substances using Brillouin scattering.^{[41][42][43][44]}

Phonons in the acoustical branch dominate the phonon heat conduction as they have greater energy dispersion and therefore a greater distribution of phonon velocities. Additional optical modes could also be caused by the presence of internal structure (i.e., charge or mass) at a lattice point; it is implied that the group velocity of these modes is low and therefore their contribution to the lattice thermal conductivity λ_L (${\displaystyle \kappa }$_L) is small.^[45]

Each phonon mode can be split into one longitudinal and two transverse polarization branches. By extrapolating the phenomenology of lattice points to the unit cells it is seen that the total number of degrees of freedom is 3pq when p is the number of primitive cells with q atoms/unit cell. From these only 3p are associated with the acoustic modes, the remaining 3p(q − 1) are accommodated through the optical branches. This implies that structures with larger p and q contain a greater number of optical modes and a reduced λ_L.

From these ideas, it can be concluded that increasing crystal complexity, which is described by a complexity factor CF (defined as the number of atoms/primitive unit cell), decreases λ_L.^{[46][failed verification]} This was done by assuming that the relaxation time τ decreases with increasing number of atoms in the unit cell and then scaling the parameters of the expression for thermal conductivity in high temperatures accordingly.^[45]

Describing anharmonic effects is complicated because an exact treatment as in the harmonic case is not possible, and phonons are no longer exact eigensolutions to the equations of motion. Even if the state of motion of the crystal could be described with a plane wave at a particular time, its accuracy would deteriorate progressively with time. Time development would have to be described by introducing a spectrum of other phonons, which is known as the phonon decay. The two most important anharmonic effects are the thermal expansion and the phonon thermal conductivity.

Only when the phonon number ‹n› deviates from the equilibrium value ‹n›⁰, can a thermal current arise as stated in the following expression

${\displaystyle Q_{x}={\frac {1}{V}}\sum _{q,j}{\hslash \omega \left(\left\langle n\right\rangle -{\left\langle n\right\rangle }^{0}\right)v_{x}}{\text{,}}}$

where v is the energy transport velocity of phonons. Only two mechanisms exist that can cause time variation of ‹n› in a particular region. The number of phonons that diffuse into the region from neighboring regions differs from those that diffuse out, or phonons decay inside the same region into other phonons. A special form of the Boltzmann equation

${\displaystyle {\frac {d\left\langle n\right\rangle }{dt}}={\left({\frac {\partial \left\langle n\right\rangle }{\partial t}}\right)}_{\text{diff.}}+{\left({\frac {\partial \left\langle n\right\rangle }{\partial t}}\right)}_{\text{decay}}}$

states this. When steady state conditions are assumed the total time derivate of phonon number is zero, because the temperature is constant in time and therefore the phonon number stays also constant. Time variation due to phonon decay is described with a relaxation time (τ) approximation

${\displaystyle {\left({\frac {\partial \left\langle n\right\rangle }{\partial t}}\right)}_{\text{decay}}=-{\text{ }}{\frac {\left\langle n\right\rangle -{\left\langle n\right\rangle }^{0}}{\tau }},}$

which states that the more the phonon number deviates from its equilibrium value, the more its time variation increases. At steady state conditions and local thermal equilibrium are assumed we get the following equation

${\displaystyle {\left({\frac {\partial \left(n\right)}{\partial t}}\right)}_{\text{diff.}}=-{v}_{x}{\frac {\partial {\left(n\right)}^{0}}{\partial T}}{\frac {\partial T}{\partial x}}{\text{.}}}$

Using the relaxation time approximation for the Boltzmann equation and assuming steady-state conditions, the phonon thermal conductivity λ_L can be determined. The temperature dependence for λ_L originates from the variety of processes, whose significance for λ_L depends on the temperature range of interest. Mean free path is one factor that determines the temperature dependence for λ_L, as stated in the following equation

${\displaystyle {\lambda }_{L}={\frac {1}{3V}}\sum _{q,j}v\left(q,j\right)\Lambda \left(q,j\right){\frac {\partial }{\partial T}}\epsilon \left(\omega \left(q,j\right),T\right),}$

where Λ is the mean free path for phonon and ${\displaystyle {\frac {\partial }{\partial T}}\epsilon }$ denotes the heat capacity. This equation is a result of combining the four previous equations with each other and knowing that ${\displaystyle \left\langle v_{x}^{2}\right\rangle ={\frac {1}{3}}v^{2}}$ for cubic or isotropic systems and ${\displaystyle \Lambda =v\tau }$.^[47]

At low temperatures (< 10 K) the anharmonic interaction does not influence the mean free path and therefore, the thermal resistivity is determined only from processes for which q-conservation does not hold. These processes include the scattering of phonons by crystal defects, or the scattering from the surface of the crystal in case of high quality single crystal. Therefore, thermal conductance depends on the external dimensions of the crystal and the quality of the surface. Thus, temperature dependence of λ_L is determined by the specific heat and is therefore proportional to T³.^[47]

Phonon quasimomentum is defined as ℏq and differs from normal momentum because it is only defined within an arbitrary reciprocal lattice vector. At higher temperatures (10 K < T < Θ), the conservation of energy ${\displaystyle \hslash {\omega }_{1}=\hslash {\omega }_{2}+\hslash {\omega }_{3}}$ and quasimomentum ${\displaystyle \mathbf {q} _{1}=\mathbf {q} _{2}+\mathbf {q} _{3}+\mathbf {G} }$, where q₁ is wave vector of the incident phonon and q₂, q₃ are wave vectors of the resultant phonons, may also involve a reciprocal lattice vector G complicating the energy transport process. These processes can also reverse the direction of energy transport.

Therefore, these processes are also known as Umklapp (U) processes and can only occur when phonons with sufficiently large q-vectors are excited, because unless the sum of q₂ and q₃ points outside of the Brillouin zone the momentum is conserved and the process is normal scattering (N-process). The probability of a phonon to have energy E is given by the Boltzmann distribution ${\displaystyle P\propto {e}^{-E/kT}}$. To U-process to occur the decaying phonon to have a wave vector q₁ that is roughly half of the diameter of the Brillouin zone, because otherwise quasimomentum would not be conserved.

Therefore, these phonons have to possess energy of ${\displaystyle \sim k\Theta /2}$, which is a significant fraction of Debye energy that is needed to generate new phonons. The probability for this is proportional to ${\displaystyle {e}^{-\Theta /bT}}$, with ${\displaystyle b=2}$. Temperature dependence of the mean free path has an exponential form ${\displaystyle {e}^{\Theta /bT}}$. The presence of the reciprocal lattice wave vector implies a net phonon backscattering and a resistance to phonon and thermal transport resulting finite λ_L,^[45] as it means that momentum is not conserved. Only momentum non-conserving processes can cause thermal resistance.^[47]

At high temperatures (T > Θ), the mean free path and therefore λ_L has a temperature dependence T⁻¹, to which one arrives from formula ${\displaystyle {e}^{\Theta /bT}}$ by making the following approximation ${\displaystyle {e}^{x}\propto x{\text{ }},{\text{ }}\left(x\right)<1}$^{[clarification needed]} and writing ${\displaystyle x=\Theta /bT}$. This dependency is known as Eucken's law and originates from the temperature dependency of the probability for the U-process to occur.^[45][47]

Thermal conductivity is usually described by the Boltzmann equation with the relaxation time approximation in which phonon scattering is a limiting factor. Another approach is to use analytic models or molecular dynamics or Monte Carlo based methods to describe thermal conductivity in solids.

Short wavelength phonons are strongly scattered by impurity atoms if an alloyed phase is present, but mid and long wavelength phonons are less affected. Mid and long wavelength phonons carry significant fraction of heat, so to further reduce lattice thermal conductivity one has to introduce structures to scatter these phonons. This is achieved by introducing interface scattering mechanism, which requires structures whose characteristic length is longer than that of impurity atom. Some possible ways to realize these interfaces are nanocomposites and embedded nanoparticles or structures.

Conversion from specific to absolute units, and vice versa

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Specific thermal conductivity is a materials property used to compare the heat-transfer ability of different materials (i.e., an intensive property). Absolute thermal conductivity, in contrast, is a component property used to compare the heat-transfer ability of different components (i.e., an extensive property). Components, as opposed to materials, take into account size and shape, including basic properties such as thickness and area, instead of just material type. In this way, thermal-transfer ability of components of the same physical dimensions, but made of different materials, may be compared and contrasted, or components of the same material, but with different physical dimensions, may be compared and contrasted.

In component datasheets and tables, since actual, physical components with distinct physical dimensions and characteristics are under consideration, thermal resistance is frequently given in absolute units of ${\displaystyle {\rm {K/W}}}$ or ${\displaystyle {\rm {^{\circ }C/W}}}$, since the two are equivalent. However, thermal conductivity, which is its reciprocal, is frequently given in specific units of ${\displaystyle {\rm {W/(K\cdot m)}}}$. It is therefore often necessary to convert between absolute and specific units, by also taking a component's physical dimensions into consideration, in order to correlate the two using information provided, or to convert tabulated values of specific thermal conductivity into absolute thermal resistance values for use in thermal resistance calculations. This is particularly useful, for example, when calculating the maximum power a component can dissipate as heat, as demonstrated in the example calculation here.

"Thermal conductivity λ is defined as ability of material to transmit heat and it is measured in watts per square metre of surface area for a temperature gradient of 1 K per unit thickness of 1 m".^[48] Therefore, specific thermal conductivity is calculated as:

${\displaystyle \lambda ={\frac {P}{(A\cdot \Delta T/t)}}={\frac {P\cdot t}{(A\cdot \Delta T)}}}$

where:

${\displaystyle \lambda }$ = specific thermal conductivity (W/(K·m))

${\displaystyle P}$ = power (W)

${\displaystyle A}$ = area (m²) = 1 m² during measurement

${\displaystyle t}$ = thickness (m) = 1 m during measurement

${\displaystyle \Delta T}$ = temperature difference (K, or °C) = 1 K during measurement

Absolute thermal conductivity, on the other hand, has units of ${\displaystyle {\rm {W/K}}}$ or ${\displaystyle {\rm {W/^{\circ }C}}}$, and can be expressed as

${\displaystyle \lambda _{A}={\frac {P}{\Delta T}}}$

where ${\displaystyle \lambda _{A}}$ = absolute thermal conductivity (W/K, or W/°C).

Substituting ${\displaystyle \lambda _{A}}$ for ${\displaystyle {\frac {P}{\Delta T}}}$ into the first equation yields the equation which converts from absolute thermal conductivity to specific thermal conductivity:

${\displaystyle \lambda ={\frac {\lambda _{A}\cdot t}{A}}}$

Solving for ${\displaystyle \lambda _{A}}$, we get the equation which converts from specific thermal conductivity to absolute thermal conductivity:

${\displaystyle \lambda _{A}={\frac {\lambda \cdot A}{t}}}$

Again, since thermal conductivity and resistivity are reciprocals of each other, it follows that the equation to convert specific thermal conductivity to absolute thermal resistance is:

${\displaystyle R_{\theta }={\frac {1}{\lambda _{A}}}={\frac {t}{\lambda \cdot A}}}$, where

${\displaystyle R_{\theta }}$ = absolute thermal resistance (K/W, or °C/W).

Example calculation

The thermal conductivity of T-Global L37-3F thermal conductive pad is given as 1.4 W/(mK). Looking at the datasheet and assuming a thickness of 0.3 mm (0.0003 m) and a surface area large enough to cover the back of a TO-220 package (approx. 14.33 mm x 9.96 mm [0.01433 m x 0.00996 m]),^[49] the absolute thermal resistance of this size and type of thermal pad is:

${\displaystyle R_{\theta }={\frac {1}{\lambda _{A}}}={\frac {t}{\lambda \cdot A}}={\frac {0.0003}{1.4\cdot (0.01433\cdot 0.00996)}}=1.5\,{\rm {K/W}}}$

This value fits within the normal values for thermal resistance between a device case and a heat sink: "the contact between the device case and heat sink may have a thermal resistance of between 0.5 up to 1.7 °C/W, depending on the case size, and use of grease or insulating mica washer".^[50]

Equations

In an isotropic medium, the thermal conductivity is the parameter k in the Fourier expression for the heat flux

${\displaystyle {\vec {q}}=-k{\vec {\nabla }}T}$

where ${\displaystyle {\vec {q}}}$ is the heat flux (amount of heat flowing per second and per unit area) and ${\displaystyle {\vec {\nabla }}T}$ the temperature gradient. The sign in the expression is chosen so that always k > 0 as heat always flows from a high temperature to a low temperature. This is a direct consequence of the second law of thermodynamics.

In the one-dimensional case, q = H/A with H the amount of heat flowing per second through a surface with area A and the temperature gradient is dT/dx so

${\displaystyle H=-kA{\frac {\mathrm {d} T}{\mathrm {d} x}}.}$

In case of a thermally insulated bar (except at the ends) in the steady state, H is constant. If A is constant as well the expression can be integrated with the result

${\displaystyle HL=A\int _{T_{\text{L}}}^{T_{\text{H}}}k(T)\mathrm {d} T}$

where T_H and T_L are the temperatures at the hot end and the cold end respectively, and L is the length of the bar. It is convenient to introduce the thermal-conductivity integral

${\displaystyle I_{k}(T)=\int _{0}^{T}k(T^{\prime })\mathrm {d} T^{\prime }.}$

The heat flow rate is then given by

${\displaystyle H={\frac {A}{L}}[I_{k}(T_{\text{H}})-I_{k}(T_{\text{L}})].}$

If the temperature difference is small, k can be taken as constant. In that case

${\displaystyle H=kA{\frac {T_{\text{H}}-T_{\text{L}}}{L}}.}$

See also

• Copper in heat exchangers
• Heat pump
• Heat transfer
• Heat transfer mechanisms
• Insulated pipes
• Interfacial thermal resistance
• Laser flash analysis
• List of thermal conductivities
• Phase-change material
• R-value (insulation)
• Specific heat
• Thermal bridge
• Thermal conductance quantum
• Thermal contact conductance
• Thermal diffusivity
• Thermal interface material
• Thermal rectifier
• Thermal resistance in electronics
• Thermistor
• Thermocouple
• Thermodynamics
• Thermal conductivity measurement

References

Notes

1. 1 Btu/(h⋅ft⋅°F) = 1.730735 W/(m⋅K)
2. R-values and U-values quoted in the US (based on the inch-pound units of measurement) do not correspond with and are not compatible with those used outside the US (based on the SI units of measurement).

References

1. Bird, R. Byron; Stewart, Warren E.; Lightfoot, Edwin N. (2007), Transport Phenomena (2nd ed.), John Wiley & Sons, Inc., p. 266, ISBN 978-0-470-11539-8
2. Bird, Stewart, and Lightfoot pp. 266-267
3. Holman, J.P. (1997), Heat Transfer (8th ed.), McGraw Hill, p. 2, ISBN 0-07-844785-2
4. Bejan, Adrian (1993), Heat Transfer, John Wiley & Sons, pp. 10–11, ISBN 0-471-50290-1
5. Bird, Stewart, & Lightfoot, p. 267
6. Bejan, p. 34
7. Bird, Stewart, & Lightfoot, p. 305
8. Gray, H.J.; Isaacs, Alan (1975). A New Dictionary of Physics (2nd ed.). Longman Group Limited. p. 251. ISBN 0582322421.
9. ASTM C168 − 15a Standard Terminology Relating to Thermal Insulation.
10. Bird, Stewart, & Lightfoot, p. 268
11. Incropera, Frank P.; DeWitt, David P. (1996), Fundamentals of heat and mass transfer (4th ed.), Wiley, pp. 50–51, ISBN 0-471-30460-3
12. Perry, R. H.; Green, D. W., eds. (1997). Perry's Chemical Engineers' Handbook (7th ed.). McGraw-Hill. Table 1–4. ISBN 978-0-07-049841-9.
13. Daniel V. Schroeder (2000), An Introduction to Thermal Physics, Addison Wesley, p. 39, ISBN 0-201-38027-7
14. Chapman, Sydney; Cowling, T.G. (1970), The Mathematical Theory of Non-Uniform Gases (3rd ed.), Cambridge University Press, p. 248
15. An unlikely competitor for diamond as the best thermal conductor, Phys.org news (July 8, 2013).
16. "Thermal Conductivity in W cm⁻¹ K⁻¹ of Metals and Semiconductors as a Function of Temperature", in CRC Handbook of Chemistry and Physics, 99th Edition (Internet Version 2018), John R. Rumble, ed., CRC Press/Taylor & Francis, Boca Raton, FL.
17. Lindon C. Thomas (1992), Heat Transfer, Prentice Hall, p. 8, ISBN 978-0133849424
18. "Thermal Conductivity of common Materials and Gases". www.engineeringtoolbox.com.
19. Bird, Stewart, & Lightfoot, pp. 270-271
20. Hahn, David W.; Özişik, M. Necati (2012). Heat conduction (3rd ed.). Hoboken, N.J.: Wiley. p. 5. ISBN 978-0-470-90293-6.
21. Ramires, M. L. V.; Nieto de Castro, C. A.; Nagasaka, Y.; Nagashima, A.; Assael, M. J.; Wakeham, W. A. (July 6, 1994). "Standard reference data for the thermal conductivity of water". NIST. Retrieved 25 May 2017.
22. Millat, Jürgen; Dymond, J.H.; Nieto de Castro, C.A. (2005). Transport properties of fluids: their correlation, prediction, and estimation. Cambridge New York: IUPAC/Cambridge University Press. ISBN 978-0-521-02290-3.
23. "Sapphire, Al2O3". Almaz Optics. Retrieved 2012-08-15.
24. Hahn, David W.; Özişik, M. Necati (2012). Heat conduction (3rd ed.). Hoboken, N.J.: Wiley. p. 614. ISBN 978-0-470-90293-6.
25. Dai, W.; et al. (2017). "Influence of gas pressure on the effective thermal conductivity of ceramic breeder pebble beds". Fusion Engineering and Design. 118: 45–51. doi:10.1016/j.fusengdes.2017.03.073.
26. Wei, Lanhua; Kuo, P. K.; Thomas, R. L.; Anthony, T. R.; Banholzer, W. F. (16 February 1993). "Thermal conductivity of isotopically modified single crystal diamond". Physical Review Letters. 70 (24): 3764–3767. Bibcode:1993PhRvL..70.3764W. doi:10.1103/PhysRevLett.70.3764. PMID 10053956.
27. see, e.g., Balescu, Radu (1975), Equilibrium and Nonequilibrium Statistical Mechanics, John Wiley & Sons, pp. 674–675, ISBN 978-0-471-04600-4
28. Incropera, Frank P.; DeWitt, David P. (1996), Fundamentals of heat and mass transfer (4th ed.), Wiley, p. 47, ISBN 0-471-30460-3
29. Chapman, Sydney; Cowling, T.G. (1970), The Mathematical Theory of Non-Uniform Gases (3rd ed.), Cambridge University Press, pp. 100–101
30. Bird, R. Byron; Stewart, Warren E.; Lightfoot, Edwin N. (2007), Transport Phenomena (2nd ed.), John Wiley & Sons, Inc., p. 275, ISBN 978-0-470-11539-8
31. Chapman & Cowling, p. 167
32. Bird, Stewart, & Lightfoot, p. 275
33. Chapman & Cowling, p. 247
34. Chapman & Cowling, pp. 249-251
35. Bird, Stewart, & Lightfoot, p. 276
36. Bird, Stewart, & Lightfoot, p. 279
37. Klemens, P.G. (1951). "The Thermal Conductivity of Dielectric Solids at Low Temperatures". Proceedings of the Royal Society of London A. 208 (1092): 108. Bibcode:1951RSPSA.208..108K. doi:10.1098/rspa.1951.0147. S2CID 136951686.
38. Chang, G. K.; Jones, R. E. (1962). "Low-Temperature Thermal Conductivity of Amorphous Solids". Physical Review. 126 (6): 2055. Bibcode:1962PhRv..126.2055C. doi:10.1103/PhysRev.126.2055.
39. Crawford, Frank S. (1968). Berkeley Physics Course: Vol. 3: Waves. McGraw-Hill. p. 215. ISBN 9780070048607.
40. Pomeranchuk, I. (1941). "Thermal conductivity of the paramagnetic dielectrics at low temperatures". Journal of Physics USSR. 4: 357. ISSN 0368-3400.
41. Zeller, R. C.; Pohl, R. O. (1971). "Thermal Conductivity and Specific Heat of Non-crystalline Solids". Physical Review B. 4 (6): 2029. Bibcode:1971PhRvB...4.2029Z. doi:10.1103/PhysRevB.4.2029.
42. Love, W. F. (1973). "Low-Temperature Thermal Brillouin Scattering in Fused Silica and Borosilicate Glass". Physical Review Letters. 31 (13): 822. Bibcode:1973PhRvL..31..822L. doi:10.1103/PhysRevLett.31.822.
43. Zaitlin, M. P.; Anderson, M. C. (1975). "Phonon thermal transport in noncrystalline materials". Physical Review B. 12 (10): 4475. Bibcode:1975PhRvB..12.4475Z. doi:10.1103/PhysRevB.12.4475.
44. Zaitlin, M. P.; Scherr, L. M.; Anderson, M. C. (1975). "Boundary scattering of phonons in noncrystalline materials". Physical Review B. 12 (10): 4487. Bibcode:1975PhRvB..12.4487Z. doi:10.1103/PhysRevB.12.4487.
45. Pichanusakorn, P.; Bandaru, P. (2010). "Nanostructured thermoelectrics". Materials Science and Engineering: R: Reports. 67 (2–4): 19–63. doi:10.1016/j.mser.2009.10.001.
46. Roufosse, Micheline; Klemens, P. G. (1973-06-15). "Thermal Conductivity of Complex Dielectric Crystals". Physical Review B. 7 (12): 5379–5386. Bibcode:1973PhRvB...7.5379R. doi:10.1103/PhysRevB.7.5379.
47. Ibach, H.; Luth, H. (2009). Solid-State Physics: An Introduction to Principles of Materials Science. Springer. ISBN 978-3-540-93803-3.
48. http://tpm.fsv.cvut.cz/student/documents/files/BUM1/Chapter16.pdf
49. https://www.vishay.com/docs/91291/91291.pdf
50. "Heatsink Design and Selection - Thermal Resistance".

Further reading

Undergraduate-level texts (engineering)

• Bird, R. Byron; Stewart, Warren E.; Lightfoot, Edwin N. (2007), Transport Phenomena (2nd ed.), John Wiley & Sons, Inc., ISBN 978-0-470-11539-8. A standard, modern reference.
• Incropera, Frank P.; DeWitt, David P. (1996), Fundamentals of heat and mass transfer (4th ed.), Wiley, ISBN 0-471-30460-3
• Bejan, Adrian (1993), Heat Transfer, John Wiley & Sons, ISBN 0-471-50290-1
• Holman, J.P. (1997), Heat Transfer (8th ed.), McGraw Hill, ISBN 0-07-844785-2
• Callister, William D. (2003), "Appendix B", Materials Science and Engineering - An Introduction, John Wiley & Sons, ISBN 0-471-22471-5

Undergraduate-level texts (physics)

• Halliday, David; Resnick, Robert; & Walker, Jearl (1997). Fundamentals of Physics (5th ed.). John Wiley and Sons, New York ISBN 0-471-10558-9. An elementary treatment.
• Daniel V. Schroeder (1999), An Introduction to Thermal Physics, Addison Wesley, ISBN 978-0-201-38027-9. A brief, intermediate-level treatment.
• Reif, F. (1965), Fundamentals of Statistical and Thermal Physics, McGraw-Hill. An advanced treatment.

Graduate-level texts

• Balescu, Radu (1975), Equilibrium and Nonequilibrium Statistical Mechanics, John Wiley & Sons, ISBN 978-0-471-04600-4
• Chapman, Sydney; Cowling, T.G. (1970), The Mathematical Theory of Non-Uniform Gases (3rd ed.), Cambridge University Press. A very advanced but classic text on the theory of transport processes in gases.
• Reid, C. R., Prausnitz, J. M., Poling B. E., Properties of gases and liquids, IV edition, Mc Graw-Hill, 1987
• Srivastava G. P (1990), The Physics of Phonons. Adam Hilger, IOP Publishing Ltd, Bristol

External links

• Thermopedia THERMAL CONDUCTIVITY
• Contribution of Interionic Forces to the Thermal Conductivity of Dilute Electrolyte Solutions The Journal of Chemical Physics 41, 3924 (1964)
• The importance of Soil Thermal Conductivity for power companies
• Thermal Conductivity of Gas Mixtures in Chemical Equilibrium. II The Journal of Chemical Physics 32, 1005 (1960)