# Thermal conductivity

Last updated

The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by ${\displaystyle k}$, ${\displaystyle \lambda }$, or ${\displaystyle \kappa }$.

## Contents

Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity. For instance, metals typically have high thermal conductivity and are very efficient at conducting heat, while the opposite is true for insulating materials like Styrofoam. Correspondingly, materials of high thermal conductivity are widely used in heat sink applications, and materials of low thermal conductivity are used as thermal insulation. The reciprocal of thermal conductivity is called thermal resistivity.

The defining equation for thermal conductivity is ${\displaystyle \mathbf {q} =-k\nabla T}$, where ${\displaystyle \mathbf {q} }$ is the heat flux, ${\displaystyle k}$ is the thermal conductivity, and ${\displaystyle \nabla T}$ is the temperature gradient. This is known as Fourier's Law for heat conduction. Although commonly expressed as a scalar, the most general form of thermal conductivity is a second-rank tensor. However, the tensorial description only becomes necessary in materials which are anisotropic.

## Definition

### Simple definition

Consider a solid material placed between two environments of different temperatures. Let ${\displaystyle T_{1}}$ be the temperature at ${\displaystyle x=0}$ and ${\displaystyle T_{2}}$ be the temperature at ${\displaystyle x=L}$, and suppose ${\displaystyle T_{2}>T_{1}}$. A possible realization of this scenario is a building on a cold winter day: the solid material in this case would be the building wall, separating the cold outdoor environment from the warm indoor environment.

According to the second law of thermodynamics, heat will flow from the hot environment to the cold one as the temperature difference is equalized by diffusion. This is quantified in terms of a heat flux ${\displaystyle q}$, which gives the rate, per unit area, at which heat flows in a given direction (in this case minus x-direction). In many materials, ${\displaystyle q}$ is observed to be directly proportional to the temperature difference and inversely proportional to the separation distance ${\displaystyle L}$: [1]

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

The constant of proportionality ${\displaystyle k}$ is the thermal conductivity; it is a physical property of the material. In the present scenario, since ${\displaystyle T_{2}>T_{1}}$ heat flows in the minus x-direction and ${\displaystyle q}$ is negative, which in turn means that ${\displaystyle k>0}$. In general, ${\displaystyle k}$ is always defined to be positive. The same definition of ${\displaystyle k}$ can also be extended to gases and liquids, provided other modes of energy transport, such as convection and radiation, are eliminated.

For simplicity, we have assumed here that the ${\displaystyle k}$ does not vary significantly as temperature is varied from ${\displaystyle T_{1}}$ to ${\displaystyle T_{2}}$. Cases in which the temperature variation of ${\displaystyle k}$ is non-negligible must be addressed using the more general definition of ${\displaystyle k}$ discussed below.

### General definition

Thermal conduction is defined as the transport of energy due to random molecular motion across a temperature gradient. It is distinguished from energy transport by convection and molecular work in that it does not involve macroscopic flows or work-performing internal stresses.

Energy flow due to thermal conduction is classified as heat and is quantified by the vector ${\displaystyle \mathbf {q} (\mathbf {r} ,t)}$, which gives the heat flux at position ${\displaystyle \mathbf {r} }$ and time ${\displaystyle t}$. According to the second law of thermodynamics, heat flows from high to low temperature. Hence, it is reasonable to postulate that ${\displaystyle \mathbf {q} (\mathbf {r} ,t)}$ is proportional to the gradient of the temperature field ${\displaystyle T(\mathbf {r} ,t)}$, i.e.

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

where the constant of proportionality, ${\displaystyle k>0}$, is the thermal conductivity. This is called Fourier's law of heat conduction. In actuality, it is not a law but a definition of thermal conductivity in terms of the independent physical quantities ${\displaystyle \mathbf {q} (\mathbf {r} ,t)}$ and ${\displaystyle T(\mathbf {r} ,t)}$. [2] [3] As such, its usefulness depends on the ability to determine ${\displaystyle k}$ for a given material under given conditions. The constant ${\displaystyle k}$ itself usually depends on ${\displaystyle T(\mathbf {r} ,t)}$ and thereby implicitly on space and time. An explicit space and time dependence could also occur if the material is inhomogeneous or changing with time. [4]

In some solids, thermal conduction is anisotropic, i.e. the heat flux is not always parallel to the temperature gradient. To account for such behavior, a tensorial form of Fourier's law must be used:

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

where ${\displaystyle {\boldsymbol {\kappa }}}$ is symmetric, second-rank tensor called the thermal conductivity tensor. [5]

An implicit assumption in the above description is the presence of local thermodynamic equilibrium, which allows one to define a temperature field ${\displaystyle T(\mathbf {r} ,t)}$.

### Other quantities

In engineering practice, it is common to work in terms of quantities which are derivative to thermal conductivity and implicitly take into account design-specific features such as component dimensions.

For instance, thermal conductance is defined as 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 kelvin. For a plate of thermal conductivity ${\displaystyle k}$, area ${\displaystyle A}$ and thickness ${\displaystyle L}$, the conductance is ${\displaystyle kA/L}$, measured in W⋅K−1. [6] The relationship between thermal conductivity and conductance is analogous to the relationship between electrical conductivity and electrical conductance.

Thermal resistance is the inverse of thermal conductance. [6] It is a convenient measure to use in multicomponent design since thermal resistances are additive when occurring in series. [7]

There is also a measure known as the heat transfer coefficient: the quantity of heat that passes per unit time through a unit area of a plate of particular thickness when its opposite faces differ in temperature by one kelvin. [8] In ASTM C168-15, this area-independent quantity is referred to as the "thermal conductance". [9] The reciprocal of the heat transfer coefficient is thermal insulance. In summary, for a plate of thermal conductivity ${\displaystyle k}$, area ${\displaystyle A}$ and thickness ${\displaystyle L}$, we have

• thermal conductance = ${\displaystyle kA/L}$, measured in W⋅K−1.
• thermal resistance = ${\displaystyle L/(kA)}$, measured in K⋅W−1.
• heat transfer coefficient = ${\displaystyle k/L}$, measured in W⋅K−1⋅m−2.
• thermal insulance = ${\displaystyle L/k}$, measured in K⋅m2⋅W−1.

The heat transfer coefficient is also known as thermal admittance in the sense that the material may be seen as admitting heat to flow.[ citation needed ]

An additional term, thermal transmittance, quantifies the thermal conductance of a structure along with heat transfer due to convection and radiation.[ citation needed ] It is measured in the same units as thermal conductance and is sometimes known as the composite thermal conductance. The term U-value is also used.

Finally, thermal diffusivity ${\displaystyle \alpha }$ combines thermal conductivity with density and specific heat: [10]

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

As such, it quantifies the thermal inertia of a material, i.e. the relative difficulty in heating a material to a given temperature using heat sources applied at the boundary. [11]

## Units

In the International System of Units (SI), thermal conductivity is measured in watts per meter-kelvin (W/(mK)). Some papers report in watts per centimeter-kelvin (W/(cm⋅K)).

In imperial units, thermal conductivity is measured in BTU/(hft°F). [note 1] [12]

The dimension of thermal conductivity is M1L1T−3Θ−1, expressed in terms of the dimensions mass (M), length (L), time (T), and temperature (Θ).

Other 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 measures such as the R-value (resistance) and the U-value (transmittance or conductance). Although related to the thermal conductivity of a material used in an insulation product or assembly, R- and U-values are measured per unit area, and depend on the specified thickness of the product or assembly. [note 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

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

The thermal conductivities of common substances span at least four orders of magnitude. [15] 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. [16] 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). [17]

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.

SubstanceThermal conductivity (W·m−1·K−1)Temperature (°C)
Air [18] 0.02625
Styrofoam [19] 0.03325
Water [20] 0.608926.85
Concrete [20] 0.92
Copper [20] 384.118.05
Natural diamond [17] 895–135026.85

## 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. [21] 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. [21]

### 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). [22]

Even more dramatically, the thermal conductivity of a fluid diverges in the vicinity of the vapor-liquid critical point. [23]

### 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. [24] 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. [25]

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

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. [26] 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. [26]

### 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. [27] The thermal conductivity of 99% isotopically enriched cubic boron nitride is ~ 1400 W·m −1·K −1, [28] which is 90% higher than that of natural boron nitride.

## 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. [29] 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. [30]

### 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. [31] 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. [32] 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. [33] [34] 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. [35] 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. [36]

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. [35] [37]

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. [38]

### Metals

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

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. [39] [40]

The phonon mean free path has been associated directly with the effective relaxation length for processes without directional correlation. If Vg 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, [41] Vlong is much greater than Vtrans, 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. [39] [42]

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. [43] [44] [45] [46]

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 λL (${\displaystyle \kappa }$L) is small. [47]

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 λL.

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 λL. [48] [ 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. [47]

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 λL can be determined. The temperature dependence for λL originates from the variety of processes, whose significance for λL depends on the temperature range of interest. Mean free path is one factor that determines the temperature dependence for λL, 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 ${\displaystyle \Lambda =v\tau }$. [49]

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 λL is determined by the specific heat and is therefore proportional to T3. [49]

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 λL, [47] as it means that momentum is not conserved. Only momentum non-conserving processes can cause thermal resistance. [49]

At high temperatures (T > Θ), the mean free path and therefore λL 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. [47] [49]

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

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". [50] 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 constant (W/(K·m), or W/(°C·m))
${\displaystyle P}$ = power (W)
${\displaystyle A}$ = the cross-sectional area (m2) which is perpendicular to the direction of heat flow = 1 m2 in the definition above
${\displaystyle t}$ = thickness (m) = 1 m in the definition above
${\displaystyle \Delta T}$ = temperature difference (K, or °C) = 1 K in the definition above

The above equation is often written in this alternate form:

${\displaystyle P={\frac {Q}{t}}={\frac {kA\Delta T}{d}}}$ [51]

where:

${\displaystyle Q}$ = the amount of heat (energy, in J) being transferred per unit time t
${\displaystyle t}$ = time (sec)
(therefore ${\displaystyle {\frac {Q}{t}}}$ = power (W), and is identical to the P term in the previous equation)
${\displaystyle k}$ = the specific thermal conductivity constant (W/(K·m), or W/(°C·m)), and is identical to ${\displaystyle \lambda }$ in the previous equation
${\displaystyle A}$ = the cross-sectional area (m2) which is perpendicular to the direction of heat flow (same as previous equation)
${\displaystyle \Delta T}$ = temperature difference (K, or °C) (same as previous equation)
${\displaystyle d}$ = the distance, or thickness (m) of the material, and is identical to t in the previous equation

If you consider ΔT and d together, you can look at them as a single temperature gradient entity:

${\displaystyle {\frac {\Delta T}{d}}}$ = temperature gradient (temperature change per unit distance of heat flow through a material) (K/m, or °C/m)

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]), [52] 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". [53] Note that lower thermal resistance values correspond to higher thermal conductivity, and therefore the lower the thermal resistance, the better the heat transfer. For cooling electronics, thermal conductive pads and heat sinks with thermal resistance values as low as possible (thermal conductivity values as high as possible) are desired.

## 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 TH and TL 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

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

## Related Research Articles

Electrical resistivity is a fundamental property of a material that measures how strongly it resists electric current. Its inverse, called electrical conductivity, quantifies how well a material conducts electricity. A low resistivity indicates a material that readily allows electric current. Resistivity is commonly represented by the Greek letter ρ (rho). The SI unit of electrical resistivity is the ohm-meter (Ω⋅m). For example, if a 1 m solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1 Ω, then the resistivity of the material is 1 Ω⋅m.

Thermal conduction is the transfer of internal energy by microscopic collisions of particles and movement of electrons within a body. The colliding particles, which include molecules, atoms and electrons, transfer disorganized microscopic kinetic and potential energy, jointly known as internal energy. Conduction takes place in all phases: solid, liquid, and gas.

In mathematics and physics, the heat equation is a certain partial differential equation. Solutions of the heat equation are sometimes known as caloric functions. The theory of the heat equation was first developed by Joseph Fourier in 1822 for the purpose of modeling how a quantity such as heat diffuses through a given region.

The Knudsen number (Kn) is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. This length scale could be, for example, the radius of a body in a fluid. The number is named after Danish physicist Martin Knudsen (1871–1949).

The Drude model of electrical conduction was proposed in 1900 by Paul Drude to explain the transport properties of electrons in materials. Basically, Ohm's law was well established and stated that the current J and voltage V driving the current are related to the resistance R of the material. The inverse of the resistance is known as the conductance. When we consider a metal of unit length and unit cross sectional area, the conductance is known as the conductivity, which is the inverse of resistivity. The Drude model attempts to explain the resistivity of a conductor in terms of the scattering of electrons by the relatively immobile ions in the metal that act like obstructions to the flow of electrons.

In solid-state physics, the free electron model is a quantum mechanical model for the behaviour of charge carriers in a metallic solid. It was developed in 1927, principally by Arnold Sommerfeld, who combined the classical Drude model with quantum mechanical Fermi–Dirac statistics and hence it is also known as the Drude–Sommerfeld model.

In plasmas and electrolytes, the Debye length, is a measure of a charge carrier's net electrostatic effect in a solution and how far its electrostatic effect persists. A Debye sphere is a volume whose radius is the Debye length. With each Debye length, charges are increasingly electrically screened. Every Debye‐length , the electric potential will decrease in magnitude by 1/e. Debye length is an important parameter in plasma physics, electrolytes, and colloids. The corresponding Debye screening wave vector for particles of density , charge at a temperature is given by in Gaussian units. Expressions in MKS units will be given below. The analogous quantities at very low temperatures are known as the Thomas–Fermi length and the Thomas–Fermi wave vector. They are of interest in describing the behaviour of electrons in metals at room temperature.

A heat current is a kinetic exchange rate between molecules, relative to the material in which the kinesis occurs. It is defined as , where is heat and is time.

In physics, the Wiedemann–Franz law states that the ratio of the electronic contribution of the thermal conductivity (κ) to the electrical conductivity (σ) of a metal is proportional to the temperature (T).

The rate of heat flow is the amount of heat that is transferred per unit of time in some material, usually measured in watt. Heat is the flow of thermal energy driven by thermal non-equilibrium, so that 'heat flow' is a redundancy. Heat must not be confused with stored thermal energy, and moving a hot object from one place to another must not be called heat transfer. But, in spite of all these remarks, it is common in normal parlance to say ‘heat flow’, to talk of ‘heat content’, etc.

In mesoscopic physics, ballistic conduction is the unimpeded flow of charge carriers, or energy-carrying particles, over relatively long distances in a material. In general, the resistivity of a material exists because an electron, while moving inside a medium, is scattered by impurities, defects, thermal fluctuations of ions in a crystalline solid, or, generally, by any freely-moving atom/molecule composing a gas or liquid. Without scattering, electrons simply obey Newton's second law of motion at non-relativistic speeds.

Dielectric loss quantifies a dielectric material's inherent dissipation of electromagnetic energy. It can be parameterized in terms of either the loss angleδ or the corresponding loss tangent tan δ. Both refer to the phasor in the complex plane whose real and imaginary parts are the resistive (lossy) component of an electromagnetic field and its reactive (lossless) counterpart.

In thermodynamics, the thermal effusivity, thermal inertia or thermal responsivity of a material is defined as the square root of the product of the material's thermal conductivity and its volumetric heat capacity.

Relativistic heat conduction refers to the modelling of heat conduction in a way compatible with special relativity. This article discusses models using a wave equation with a dissipative term.

As the devices continue to shrink further into the sub-100 nm range following the trend predicted by Moore’s law, the topic of thermal properties and transport in such nanoscale devices becomes increasingly important. Display of great potential by nanostructures for thermoelectric applications also motivates the studies of thermal transport in such devices. These fields, however, generate two contradictory demands: high thermal conductivity to deal with heating issues in sub-100 nm devices and low thermal conductivity for thermoelectric applications. These issues can be addressed with phonon engineering, once nanoscale thermal behaviors have been studied and understood.

Chapman–Enskog theory provides a framework in which equations of hydrodynamics for a gas can be derived from the Boltzmann equation. The technique justifies the otherwise phenomenological constitutive relations appearing in hydrodynamical descriptions such as the Navier–Stokes equations. In doing so, expressions for various transport coefficients such as thermal conductivity and viscosity are obtained in terms of molecular parameters. Thus, Chapman–Enskog theory constitutes an important step in the passage from a microscopic, particle-based description to a continuum hydrodynamical one.

Phonons can scatter through several mechanisms as they travel through the material. These scattering mechanisms are: Umklapp phonon-phonon scattering, phonon-impurity scattering, phonon-electron scattering, and phonon-boundary scattering. Each scattering mechanism can be characterised by a relaxation rate 1/ which is the inverse of the corresponding relaxation time.

The vaporizing droplet problem is a challenging issue in fluid dynamics. It is part of many engineering situations involving the transport and computation of sprays: fuel injection, spray painting, aerosol spray, flashing releases… In most of these engineering situations there is a relative motion between the droplet and the surrounding gas. The gas flow over the droplet has many features of the gas flow over a rigid sphere: pressure gradient, viscous boundary layer, wake. In addition to these common flow features one can also mention the internal liquid circulation phenomenon driven by surface-shear forces and the boundary layer blowing effect.

Heat transfer physics describes the kinetics of energy storage, transport, and energy transformation by principal energy carriers: phonons, electrons, fluid particles, and photons. Heat is energy stored in temperature-dependent motion of particles including electrons, atomic nuclei, individual atoms, and molecules. Heat is transferred to and from matter by the principal energy carriers. The state of energy stored within matter, or transported by the carriers, is described by a combination of classical and quantum statistical mechanics. The energy is different made (converted) among various carriers. The heat transfer processes are governed by the rates at which various related physical phenomena occur, such as the rate of particle collisions in classical mechanics. These various states and kinetics determine the heat transfer, i.e., the net rate of energy storage or transport. Governing these process from the atomic level to macroscale are the laws of thermodynamics, including conservation of energy.

Moving heat sources is a topic in transient heat transfer that is applicable to engineering problems, particularly welding engineering. In the early 20th Century, welding engineers began studying moving heat sources, both empirically and theoretically. Depending on welding parameters, plate geometry and material properties, the solution takes three different forms: semi-infinite, intermediate, or thin plate. The temperature distribution and cooling rates can be determined from theoretical solutions to the problem, allowing engineers to better understand the consequences of heat sources on weldability and end item performance.

## 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. "The thermal properties of porous andesite". Journal of Volcanology and Geothermal Research. 398: 106901. 2020-06-01. doi:10.1016/j.jvolgeores.2020.106901. ISSN   0377-0273.
16. An unlikely competitor for diamond as the best thermal conductor, Phys.org news (July 8, 2013).
17. "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.
18. Lindon C. Thomas (1992), Heat Transfer, Prentice Hall, p. 8, ISBN   978-0133849424
19. "Thermal Conductivity of common Materials and Gases". www.engineeringtoolbox.com.
20. Bird, Stewart, & Lightfoot, pp. 270-271
21. Hahn, David W.; Özişik, M. Necati (2012). Heat conduction (3rd ed.). Hoboken, N.J.: Wiley. p. 5. ISBN   978-0-470-90293-6.
22. 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". Journal of Physical and Chemical Reference Data. NIST. 24 (3): 1377–1381. doi:10.1063/1.555963 . Retrieved 25 May 2017.
23. 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.
24. "Sapphire, Al2O3". Almaz Optics . Retrieved 2012-08-15.
25. Hahn, David W.; Özişik, M. Necati (2012). Heat conduction (3rd ed.). Hoboken, N.J.: Wiley. p. 614. ISBN   978-0-470-90293-6.
26. 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.
27. 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.
28. Chen, Ke; Song, Bai; Ravichandran, Navaneetha K.; Zheng, Qiye; Chen, Xi; Lee, Hwijong; Sun, Haoran; Li, Sheng; Gamage, Geethal Amila Gamage Udalamatta; Tian, Fei; Ding, Zhiwei (2020-01-31). "Ultrahigh thermal conductivity in isotope-enriched cubic boron nitride". Science. 367 (6477): 555–559. doi:10.1126/science.aaz6149. hdl:. ISSN   0036-8075. PMID   31919128.
29. see, e.g., Balescu, Radu (1975), Equilibrium and Nonequilibrium Statistical Mechanics, John Wiley & Sons, pp. 674–675, ISBN   978-0-471-04600-4
30. Incropera, Frank P.; DeWitt, David P. (1996), Fundamentals of heat and mass transfer (4th ed.), Wiley, p. 47, ISBN   0-471-30460-3
31. Chapman, Sydney; Cowling, T.G. (1970), The Mathematical Theory of Non-Uniform Gases (3rd ed.), Cambridge University Press, pp. 100–101
32. 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
33. Chapman & Cowling, p. 167
34. Bird, Stewart, & Lightfoot, p. 275
35. Chapman & Cowling, p. 247
36. Chapman & Cowling, pp. 249-251
37. Bird, Stewart, & Lightfoot, p. 276
38. Bird, Stewart, & Lightfoot, p. 279
39. 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.
40. 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.
41. Crawford, Frank S. (1968). Berkeley Physics Course: Vol. 3: Waves. McGraw-Hill. p. 215. ISBN   9780070048607.
42. Pomeranchuk, I. (1941). "Thermal conductivity of the paramagnetic dielectrics at low temperatures". Journal of Physics USSR . 4: 357. ISSN   0368-3400.
43. 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.
44. 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.
45. 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.
46. 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.
47. 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.
48. 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.
49. Ibach, H.; Luth, H. (2009). Solid-State Physics: An Introduction to Principles of Materials Science. Springer. ISBN   978-3-540-93803-3.

• 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