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'''Heat transfer physics''' describes the kinetics of energy storage, transport, and transformation by principal energy carriers: [[phonon]]s (lattice vibration waves), [[electron]]s, [[Maxwell–Boltzmann distribution|fluid particles]], and [[photon]]s.<ref name=GernerBook>{{cite book|last=Edited by Tien|first= C.-L.; Majumdar, A.; Gerner, F. M.|title=Microscale energy transport|year=1998|publisher=Taylor & Francis|location=Washington, D.C.|isbn=978-1560324591}}</ref><ref name=ChenBook>{{cite book|last=Chen|first=G.|title=Nanoscale energy transport and conversion : a parallel treatment of electrones, molecules, phonons, and photons|year=2004|publisher=Oxford|location=New York|isbn=978-0195159424}}</ref><ref name=ZhangBook>{{cite book|last=Zhang|first=Z. M.|title=Nano/microscale heat transfer|year=2007|publisher=McGraw-Hill|location=New York, NY|isbn=978-0071436748|edition=[Online-Ausg.].}}</ref><ref name=VolzBook>{{cite book|last=Volz|first=S.|title=Microscale and Nanoscale Heat Transfer (Topics in Applied Physics)|year=2010|publisher=Springer|isbn=978-3642071584}}</ref><ref name=HTPbook>{{cite book|last=Kaviany|first=M.|title=Heat transfer physics|year=2008|publisher=Cambridge University Press|location=Cambridge|isbn=9780521898973}}</ref> 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 also transformed (converted) among various carriers.
The heat transfer processes (or kinetics) are governed by the rates at which various related physical phenomena occur, i.e., the rate of particle collisions in [[classical mechanics]]. These various states and kinetics determine the heat transfer, e.g., the net rate of energy storage or transport. Governing these process from the atomic level (atom or molecule length scale) to macroscale are the [[laws of thermodynamics]], including [[conservation of energy]].

== Introduction ==
[[File:Equilibrium Particle distribution function.jpg|thumbnail|right|Variation of equilibrium particle distribution function with respect to energy for different energy carriers.]]
[[File:Kinetics of atomic-level energy transport and transition interaction, Interaction times spectrum1.jpg|thumbnail|right|Kinetics of atomic-level energy transport and transition interaction<ref name=HTPbook />]]
[[File:Time-length scale regimes.jpg|thumbnail|right|Length-time scale regimes for ab initio, MD, Boltzmann transport, and macroscopic treatments of heat transfer.<ref name=HTPbook />]]
Heat is thermal energy stored in temperature-dependent motion of particles. The, macroscopic energy equation for infinitesimal volume used in heat transfer analysis is<ref name=EHTAppB>{{cite book|last=Kaviany|first=M.|title=Essentials of heat transfer : principles, materials, and applications|year=2011|publisher=Cambridge University Press|location=Cambridge|isbn=9781107012400}}</ref>

:<math>\nabla \cdot \mathbf{q} = -\rho c_p \frac{\partial T}{\partial t} + \sum_{i,j} \dot s_{i-j},</math>

where '''q''' is heat flux vector, -''ρc<sub>p</sub>''(''∂T''/''∂t'') is temporal change of internal energy (''ρ'' is density, ''c<sub>p</sub>'' is [[specific heat capacity]] at constant pressure, ''T'' is temperature and ''t'' is time), and <math>\textstyle\dot s</math> is the energy conversion to and from thermal energy (''i'' and ''j'' are for principal energy carriers).  So, the terms represent energy transport, storage and transformation.  Heat flux vector q is composed of three macroscopic fundamental modes, which are [[Thermal conduction|conduction]] ('''q'''<sub>''k''</sub> = -''k''∇''T'', ''k'': thermal conductivity), [[convection]] ('''q'''<sub>''u''</sub> = ''ρc<sub>p</sub>'''''u'''''T'', '''u''': velocity), and [[radiation]] ('''q'''<sub>''r''</sub> = <math>2\pi\textstyle\int_{0}^{\infty} \int_{0}^{\pi}</math>'''s''' ''I<sub>ph,ω</sub>'' sin''θdθdω'', ''ω'': angular frequency, ''θ'': polar angle, ''I<sub>ph,ω</sub>'': spectral, directional radiation intensity, '''s''': unit vector), i.e., '''q''' = '''q'''<sub>''k''</sub> + '''q'''<sub>''u''</sub> + '''q'''<sub>''r''</sub>.

Once states and kinetics of the energy conversion and thermophysical properties are known, the fate of heat transfer is described by the above equation. These atomic-level mechanisms and kinetics are addressed in heat transfer physics. The microscopic thermal energy is stored, transported, and transformed by the principal energy carriers: phonons (''p''), electrons (''e''), fluid particles (''f''), and photons (''ph'').<ref name=HTPReview>{{cite journal|last=Carey|first=V. P.|coauthors=Chen, G.; Grigoropoulos, C.; Kaviany, M.; Majumdar, A.|title=A Review of Heat Transfer Physics|journal=Nanoscale and Microscale Thermophysical Engineering|year=2008|volume=12|issue=1|pages=1–60|doi=10.1080/15567260801917520}}</ref>

== Length and time scales ==

Thermophysical properties of matter and the kinetics of interaction and energy exchange among the principal carriers are based on the atomic-level configuration and interaction.<ref name=GernerBook />     Transport properties such as thermal conductivity are calculated from these atomic-level properties using classical and [[Quantum mechanics|quantum physics]].<ref name=HTPbook /><ref name=Oligschleger1999>{{cite journal|last=Oligschleger|first=C.|coauthors=Schön, J.|title=Simulation of thermal conductivity and heat transport in solids|journal=Physical Review B|year=1999|volume=59|issue=6|pages=4125–4133|doi=10.1103/PhysRevB.59.4125|arxiv = cond-mat/9811156 |bibcode = 1999PhRvB..59.4125O }}</ref>  Quantum states of principal carriers (e.g.. momentum, energy) are derived from the [[Schrödinger equation]] (called first principle or ''ab initio'') and the interaction rates (for kinetics) are calculated using the quantum states and the quantum [[Perturbation theory (quantum mechanics)|perturbation theory]] (formulated as the [[Fermi golden rule]]).<ref name=Pisani1996>{{cite book|last=Pisani|first=C.|title=Quantum-mechanical ''ab-initio'' calculation of the properties of crystalline materials|year=1996|publisher=Springer-Verlag|location=Berlin|isbn=3540616454}}</ref>   Variety of ''ab initio'' (Latin for from the beginning) solvers (software) exist (e.g., [[ABINIT]], [[CASTEP]], [[Gaussian (software)|Gaussian]], [[Q-Chem]], [[Quantum ESPRESSO]], [[SIESTA (computer program)|SIESTA]], [[VASP]], [[WIEN2k]]). Electrons in the inner shells (core) are not involved in heat transfer, and calculations are greatly reduced by proper approximations about the inner-shells electrons.<ref name=Sholl2009>{{cite book|last=Sholl|first=D. S.|title=Density functional theory : a practical introduction|year=2009|publisher=Wiley|location=Hoboken, N.J.|isbn=0470373172|edition=[Online-Ausg.].|coauthors=Steckel,  J. A.}}</ref>

The quantum treatments, including equilibrium and nonequilibrium ''ab initio'' molecular dynamics (MD), involving larger lengths and times are limited by the computation resources, so various alternate treatments with simplifying assumptions have been used and kinetics.<ref name=Marx2009>{{cite book|last=Marx|first=D.|title=''Ab initio'' molecular dynamics : basic theory and advanced methods|year=2009|publisher=Cambridge University Press|location=Cambridge, UK|isbn=0521898633|edition=1. publ., repr.|coauthors=Hutter, J}}</ref>  In classical (Newtonian) MD, the motion of atom or molecule (particles) is based on the empirical or effective interaction potentials, which in turn can be based on curve-fit of ''ab initio'' calculations or curve-fit to thermophysical properties. From the ensembles of simulated particles, static or dynamics thermal properties or scattering rates are derived.<ref name=Haile1997>{{cite book|last=Haile|first=J.M.|title=Molecular dynamics simulation : elementary methods|year=1997|publisher=Wiley|location=New York|isbn=047118439X|edition=Reprinted.}}</ref><ref name=Frenkel2002>{{cite book|last=Frenkel|first=D|title=Understanding molecular simulation from algorithms to applications|year=2002|publisher=Academic Press|location=San Diego|isbn=0122673514|edition=2nd ed.|coauthors=Smit, B}}</ref>

At yet larger length scales (mesoscale, involving many mean free paths), the [[Boltzmann transport equation]] (BTE) which is based on the classical Hamiltonian-statistical mechanics is applied. BTE considers particle states in terms of position and momentum vectors ('''x''', '''p''') and this is represented as the sate occupation probability.  The occupation has equilibrium distributions (the known boson, fermion, and Maxwell-Boltzmann particles)  and transport of energy (heat) is due to nonequilibrium (cause by a driving force or potential). Central to the transport is the role of scattering which turn the distribution toward equilibrium). The scattering is presented by the relations time or the mean free path.  The relaxation time (or its inverse which is the interaction rate) is found from other calculations (''ab initio'' or MD) or empirically.  BTE can be numerically solved with [[Monte Carlo method]], etc.<ref name=Lundstrom2009>{{cite book|last=Lundstrom|first=M.|title=Fundamentals of Carrier Transport.|year=2009|publisher=Cambridge Univ Press|location=Cambridge [u.a.]|isbn=0521637244|edition=2. ed., digitally pr. version.}}</ref>

Depending on the length and time scale the proper level of treatment (''ab initio'', MD, or BTE). Heat transfer physics analyses may involve multiple scales (e.g., BTE using interaction rate from ''ab initio'' or classical MD) with states and kinetic related to thermal energy storage, transport and transformation.

So, heat transfer physics covers the four principal energy carries and their kinetics from classical and quantum mechanical perspectives. This enables multiscale (''ab initio'', MD, BTE and macroscale) analyses, including low-dimensionality and size effects.<ref name=ChenBook />

==Phonon==

Phonon (quantized lattice vibration wave) is a central thermal energy carrier contributing to heat capacity (sensible heat storage) and conductive heat transfer in condensed phase, and plays very important role in thermal energy conversion. Its transport properties are represented by the phonon conductivity tensor '''K'''<sub>''p''</sub> (W/m-K, from the Fourier law  '''q'''<sub>''k,p''</sub> = -'''K'''<sub>''p''</sub>⋅∇ ''T'') for bulk materials, and the phonon boundary resistance ''AR<sub>p,b</sub>'' [K/(W/m<sup>2</sup>)] for solid interfaces, where ''A'' is the interface area. The phonon specific heat capacity ''c<sub>v,p</sub>'' (J/kg-K) includes the quantum effect.  The thermal energy conversion rate involving phonon is included in <math>\dot{s}_{i\mbox{-}j}</math>.  Heat transfer physics describes and predicts, ''c<sub>v,p</sub>'', '''K'''<sub>''p''</sub>, ''R<sub>p,b</sub>'' (or conductance ''G<sub>p,b</sub>'') and <math>\dot{s}_{i\mbox{-}j}</math>, based on atomic-level properties.

For an equilibrium potential ⟨''φ''⟩<sub>o</sub> of a system with ''N'' atoms, the total potential ⟨''φ''⟩ is found by a Taylor series expansion at the equilibrium and this can be approximated by the second derivatives (the harmonic approximation) as

:<math>\qquad \qquad \langle\varphi\rangle = \langle\varphi\rangle_\mathrm{o} + \sum_i\sum_\alpha\frac{\partial\langle\varphi\rangle}{\partial d_{i\alpha}}|_\mathrm{o}d_{i\alpha} + \frac{1}{2}\sum_{i,j}\sum_{\alpha,\beta}\frac{\partial^2\langle\varphi\rangle}{\partial d_{i\alpha}\partial d_{j\beta}}|_\mathrm{o}d_{i\alpha}d_{j\beta}+ \frac{1}{6}\sum_{i,j,k}\sum_{\alpha,\beta,\gamma}\frac{\partial^3\langle\varphi\rangle}{\partial d_{i\alpha}\partial d_{j\beta}\partial d_{k\gamma}}|_\mathrm{o} d_{i\alpha}d_{j\beta}d_{k\gamma}+... + </math>
:<math>\qquad \qquad  \ \ \ \ \ \  \ \ \  \approx \langle\varphi\rangle_\mathrm{o} + \frac{1}{2}\sum_{i,j}\sum_{\alpha,\beta}\Gamma_{\alpha\beta}d_{i\alpha}d_{j\beta}, </math>

where '''d'''<sub>''i''</sub> is the displacement vector of atom ''i'', and Γ is the spring (or force) constant as the second-order derivatives of the potential. The equation of motion for the lattice vibration in terms of the displacement of atoms ['''d'''(''jl'',''t''): displacement vector of the ''j''-th atom in the ''l''-th unit cell at time ''t''] is

:<math>\qquad \qquad m_j\frac{d^2\mathbf{d}(jl,t)}{dt^2} = -\sum_{j'l'} \boldsymbol{\Gamma} \binom{j \ j^\prime}{l \ l^\prime}\cdot \mathbf{d} (j^\prime l^\prime, T), </math>

where ''m'' is the atomic mass and '''Γ''' is the force constant tensor. The atomic displacement is the summation over the [[normal mode]]s  ['''s'''<sub>''α''</sub>: unit vector of mode ''α'', ''ω<sub>p</sub>'': angular frequency of wave, and '''κ'''<sub>''p''</sub>: wave vector]. Using this plane-wave displacement, the equation of motion becomes the eigenvalue equation<ref name=AMSolidStatePhysics>{{cite book|last=Ashcroft|first=N. W.; Mermin, N. D.|title=Solid state physics|year=1977|publisher=Holt, Rinehart and Winston|location=New York|isbn=0030839939|edition=27. repr.}}</ref><ref name=ZimanSolid>{{cite book|last=Ziman|first=J.M.|title=Principles of the theory of solids|year=1985|publisher=Cambridge University Press|location=Cambridge|isbn=0521297338|edition=2nd ed.}}</ref>

:<math>\qquad \qquad \mathbf{M} \omega_p^2 (\boldsymbol{\kappa}_p,\alpha) \mathbf{s}_\alpha(\boldsymbol{\kappa}_p) = \mathbf{D} (\boldsymbol{\kappa}_p) \mathbf{s}_\alpha(\boldsymbol{\kappa}_p), </math>

where '''M''' is the diagonal mass matrix and '''D''' is the harmonic dynamical matrix. Solving this eigenvalue equation gives the relation between the angular frequency ''ω<sub>p</sub>'' and the wave vector '''κ'''<sub>''p''</sub>, and this relation is called the phonon [[Phonon#Dispersion_relation|dispersion relation]].  Thus, the phonon dispersion relation is determined by matrices '''M''' and '''D''', which depend on the atomic structure and the strength of interaction among constituent atoms (the stronger the interaction and the lighter the atoms, the higher is the phonon frequency and the larger is the slope ''dω<sub>p</sub>''/''d''κ''<sub>p</sub>'').  The Hamiltonian of phonon system with the harmonic approximation is<ref name=AMSolidStatePhysics /><ref name=DoveLD>{{cite book|last=Dove|first=M. T.|title=Introduction to lattice dynamics|year=2005|publisher=Cambridge University Press|location=Cambridge|isbn=0521398940|edition=Digitally printed 1st pbk. version.}}</ref><ref name=Greegor1979>{{cite journal|last=Greegor|first=R.|coauthors=Lytle, F.|title=Extended x-ray absorption fine structure determination of thermal disorder in Cu: Comparison of theory and experiment|journal=Physical Review B|year=1979|volume=20|issue=12|pages=4902–4907|doi=10.1103/PhysRevB.20.4902|bibcode = 1979PhRvB..20.4902G }}</ref>

:<math>\qquad \qquad \mathrm{H}_p = \sum_x \frac{1}{2m} \mathbf{p}^2(\mathbf{x}) + \frac{1}{2}\sum_{\mathbf{x},\mathbf{x}^\prime}\mathbf{d}_i(\mathbf{x})D_{ij}(\mathbf{x}-\mathbf{x}^\prime)\mathbf{d}_j(\mathbf{x}^\prime),</math>

where ''D<sub>ij</sub>'' is the dynamical matrix element between atoms ''i'' and ''j'', and '''d'''<sub>''i''</sub> ('''d'''<sub>''j''</sub>) is the displacement of ''i'' (''j'') atom, and '''p''' is momentum. From this and the solution to dispersion relation, the phonon [[annihilation operator]] for the quantum treatment is defined as

:<math>\qquad \qquad b_{\kappa,\alpha} = \frac{1}{N^{1/2}}\sum_{\kappa_p,\alpha} e^{-i(\boldsymbol{\kappa}_p\cdot\mathbf{x})}\mathbf{s}_\alpha(\boldsymbol{\kappa}_p)\cdot[(\frac{m\omega_{p,\alpha}}{2\hbar})^{1/2}\mathbf{d}(\mathbf{x})+i(\frac{1}{2\hbar m\omega_{p,\alpha}})^{1/2}\mathbf{p}(\mathbf{x})],</math>

where ''N'' is the number of normal modes divided by ''α'' and ''ħ'' is the [[Planck_constant#Reduced_Planck_constant|reduced Planck constant]]. The [[creation operator]] is the adjoint of the annihilation operator, <br />
:<math> \qquad \qquad b_{\kappa,\alpha}^\dagger = \frac{1}{N^{1/2}}\sum_{\kappa_p,\alpha} e^{i(\boldsymbol{\kappa}_p\cdot\mathbf{x})}\mathbf{s}_\alpha(\boldsymbol{\kappa}_p)\cdot[(\frac{m\omega_{p,\alpha}}{2\hbar})^{1/2}\mathbf{d}(\mathbf{x})-i(\frac{1}{2\hbar m\omega_{p,\alpha}})^{1/2}\mathbf{p}(\mathbf{x})].</math>
The Hamiltonian in terms of ''b<sub>κ,α</sub>''<sup>†</sup> and ''b<sub>κ,α</sub>'' is H''<sub>p</sub>'' = ∑<sub>''κ,α''</sub>''ħω<sub>p,α</sub>''[''b<sub>κ,α</sub>''<sup>†</sup>''b<sub>κ,α</sub>'' + 1/2] and ''b<sub>κ,α</sub>''<sup>†</sup>''b<sub>κ,α</sub>'' is the phonon [[number operator]]. The energy of quantum-harmonic oscillator is ''E<sub>p</sub>'' = ∑<sub>''κ'',''α''</sub> [''f<sub>p</sub>''(''κ'',''α'') + 1/2]''ħω<sub>p,α</sub>''('''κ'''<sub>''p''</sub>), and thus the quantum of phonon energy ''ħω<sub>p</sub>''.

The phonon dispersion relation gives all possible phonon modes within the [[Brillouin zone]] (zone within the [[primitive cell]] in [[reciprocal space]]), and the phonon [[density of states]] ''D<sub>p</sub>'' (the number density of possible phonon modes). The phonon [[group velocity]] ''u<sub>p,g</sub>'' is the slope of the dispersion curve, ''dω<sub>p</sub>''/''d'''''κ'''<sub>''p''</sub>. Since phonon is a boson particle, its occupancy follows the [[Bose-Einstein distribution]] {''f<sub>p</sub>''<sup>o</sup> = [exp(''ħω<sub>p</sub>''/''k''<sub>B</sub>''T'')-1]<sup>-1</sup>, ''k''<sub>B</sub>: [[Boltzmann constant]]}.  Using the phonon density of states and this occupancy distribution, the phonon energy is ''E<sub>p</sub>''(''T'') = '''∫'''''D<sub>p</sub>''(''ω<sub>p</sub>'')''f<sub>p</sub>''(''ω<sub>p</sub>,T'')''ħω<sub>p</sub>dω<sub>p</sub>'', and the phonon density is ''n<sub>p</sub>''(''T'') = '''∫'''''D<sub>p</sub>''(''ω<sub>p</sub>'')''f<sub>p</sub>''(''ω<sub>p</sub>,T'')''dω<sub>p</sub>''. Phonon heat capacity ''c<sub>v,p</sub>'' (in solid ''c<sub>v,p</sub>'' = ''c<sub>p,p</sub>'', ''c<sub>v,p</sub>'' : constant-volume heat capacity,  ''c<sub>p,p</sub>'': constant-pressure heat capacity) is the temperature derivatives of phonon energy for the Debye model (linear dispersion model), is<ref name=KittelSolidStatePhysics>{{cite book|last=Kittel|first=C.|title=Introduction to solid state physics|year=2005|publisher=John Wiley & Sons|location=Hoboken, NJ|isbn=047141526X|edition=8th ed.}}</ref>

:<math>\qquad \qquad  c_{v,p} = \frac{dE_p}{dT}|_v = \frac{9k_\mathrm{B}}{m}(\frac{T}{T_D})^3n\int_0^{T_D/T}\frac{x^4e^x}{(e^x-1)^2}dx\qquad (x = \frac{\hbar\omega}{k_\mathrm{B}T}),</math>

where ''T''<sub>D</sub> is the [[Debye model|Debye temperature]], ''m'' is atomic mass, and ''n'' is the atomic number density (number density of phonon modes for the crystal 3''n''). This gives the [[Debye T3 law|Debye ''T''<sup>3</sup> law]]  at low temperature and [[Dulong-Petit law]] at high temperatures.

From the kinetic theory of gases,<ref name=MillatBook>{{cite book|last=Edited by Millat|first=J. (IUPAC)|title=Transport properties of fluids : their correlation, prediction and estimation|year=1996|publisher=Univ. Press|location=Cambridge|isbn=0521461782}}</ref> thermal conductivity of principal carrier ''i'' (''p'', ''e'', ''f'' and ''ph'') is

:<math>\qquad \qquad k_i = \frac{1}{3}n_ic_{v,i}u_i\lambda_i,</math>

where ''n<sub>i</sub>''  is the carrier density and the heat capacity is per carrier, ''u<sub>i</sub>'' is the carrier speed and ''λ<sub>i</sub>'' is the [[mean free path]] (distance traveled by carrier before an scattering event). Thus, the larger the carrier density, heat capacity and speed, and the less significant the scattering, the higher is the conductivity. For phonon ''λ<sub>p</sub>'' represents the interaction (scattering) kinetics of phonons and is related to the scattering relaxation time ''τ<sub>p</sub>'' or rate   (= 1/''τ<sub>p</sub>'') through ''λ<sub>p</sub>''= ''u<sub>p</sub>τ<sub>p</sub>''. Phonons interact with other phonons, and with electrons, boundaries, impurities, etc., and ''λ<sub>p</sub>'' combines these interaction mechanisms through the [[Matthiessen's rule|Matthiessen rule]]. At low temperatures, scattering by boundaries is dominant and with increase in temperature the interaction rate with impurities, electron and other phonons become important, and finally the phonon-phonon scattering dominants for ''T''  >  0.2''T<sub>D</sub>''. The interaction rates are reviewed in<ref name=Holland1963>{{cite journal|last=Holland|first=M.|title=Analysis of Lattice Thermal Conductivity|journal=Physical Review|year=1963|volume=132|issue=6|pages=2461–2471|doi=10.1103/PhysRev.132.2461|bibcode = 1963PhRv..132.2461H }}</ref>  and includes quantum perturbation theory and MD.

A number of conductivity models are available with approximations regarding the dispersion and ''λ<sub>p</sub>''.<ref name=DoveLD /><ref name=KittelSolidStatePhysics /><ref name=Holland1963 /><ref name=Nilsson1971>{{cite journal|last=Nilsson|first=G.|coauthors=Nelin, G.|title=Phonon Dispersion Relations in Ge at 80 °K|journal=Physical Review B|year=1971|volume=3|issue=2|pages=364–369|doi=10.1103/PhysRevB.3.364|bibcode = 1971PhRvB...3..364N }}</ref><ref name=Tiwari1971>{{cite journal|last=Tiwari|first=M.|coauthors=Agrawal, B.|title=Analysis of the Lattice Thermal Conductivity of Germanium|journal=Physical Review B|year=1971|volume=4|issue=10|pages=3527–3532|doi=10.1103/PhysRevB.4.3527|bibcode = 1971PhRvB...4.3527T }}</ref><ref name=McGaughey2004>{{cite journal|last=McGaughey|first=A.|coauthors=Kaviany, M.|title=Quantitative validation of the Boltzmann transport equation phonon thermal conductivity model under the single-mode relaxation time approximation|journal=Physical Review B|year=2004|volume=69|issue=9|doi=10.1103/PhysRevB.69.094303|bibcode = 2004PhRvB..69i4303M }}</ref><ref name=Ziman1972>{{cite book|last=Ziman|first=J.M.|title=Electrons and phonons : the theory of transport phenomena in solids|year=1972|publisher=Oxford University Press|location=London|isbn=019851235X|edition=[2e éd. corrigée]}}</ref> Using the single-mode relaxation time approximation (∂''f<sub>p</sub>''<sup>′</sup>/∂''t''|<sub>''s''</sub> = -''f<sub>p</sub>''<sup>′</sup>/''τ<sub>p</sub>'') and the gas kinetic theory, Callaway phonon (lattice) conductivity model as<ref name=Holland1963 /><ref name=Callaway1959>{{cite journal|last=Callaway|first=J.|title=Model for Lattice Thermal Conductivity at Low Temperatures|journal=Physical Review|year=1959|volume=113|issue=4|pages=1046–1051|doi=10.1103/PhysRev.113.1046|bibcode = 1959PhRv..113.1046C }}</ref>

:<math>\qquad \qquad k_{p,\mathbf{s}} = \frac{1}{8\pi^3}\sum_{\alpha}\int c_{v,p}\tau_p(\mathbf{u}_{p,g}\cdot\mathbf{s})^2d\kappa \ \ \ \ \ \mathrm{ for \ component \ along\ } \mathbf{s},</math>
:<math>\qquad \qquad k_p = \frac{1}{6\pi^3}\sum_{\alpha}\int c_{v,p}\tau_p {u}_{p,g}^2\kappa^2d\kappa \ \ \ \ \ \ \ \  \mathrm{for \ isotropic \ conductivity}.</math>

With the Debye model (a single group velocity '''u'''<sub>''p,g''</sub>, and a specific heat capacity calculated above),  this becomes <br />
:<math>\qquad \qquad k_p = (48\pi^2)^{1/3}\frac{k_\mathrm{B}^3T^3}{ah_\mathrm{P}^2T_\mathrm{D}}\int_0^{T/T_\mathrm{D}}\tau_p\frac{x^4e^x}{(e^x-1)^2}dx,</math>

where ''a'' is the lattice constant ''a'' = ''n''<sup>-1/3</sup> for a cubic lattice, and ''n'' is the atomic number density. Slack phonon conductivity model mainly considering acoustic phonon scattering (three-phonon interaction) is given as<ref name=BermanConductivity>{{cite book|last=Berman|first=R.|title=Thermal conduction in solids|year=1979|publisher=Clarendon Press|location=Oxford|isbn=978-0198514305}}</ref><ref name=SlackModel>{{cite book|last= Edited by Seitz|first= F.; Ehrenreich, H.; Turnbull, D.|title=Solid state physics : advances in research and applications.|year=1979|publisher=Academic Press|location=New York|isbn=0-12-607734-7|pages=1–73}}</ref>

:<math>\qquad \qquad  k_p = k_{p,S} = \frac{3.1\times10^{12}\langle M\rangle V_a^{1/3}T_{D,\infty}^3}{T\langle\gamma_G^2\rangle N_o^{2/3}}\qquad \mathrm{\ high\ temperatures}\ ( T > 0.2 T_D,\mathrm{\ phonon\mbox{-} phonon\ scattering\ only)},</math>

where ⟨M⟩ is the mean atomic weight of the atoms in the primitive cell, ''V<sub>a</sub>''=1/''n'' is the average volume per atom, ''T<sub>D,∞</sub>'' is the high-temperature Debye temperature, ''T'' is the temperature, ''N''<sub>o</sub> is the number of atoms in the primitive cell, and ⟨γ<sup>2</sup><sub>G</sub>⟩ is the mode-averaged square of the Grüneisen constant or parameter at high temperatures. This model is widely tested with pure nonmetallic crystals, and the overall agreement is good, even for complex crystals.

Based on the kinetics and atomic structure consideration, a material with high crystalline and strong interactions, composed of light atoms (such as diamond and graphene) is expected to have large phonon conductivity. Solids with more than one atom in the smallest [[unit cell]] representing the lattice have two types of phonons, i.e., acoustic and optical. (Acoustic phonons are in-phase movements of atoms about their equilibrium positions, while optical phonons are out-of-phase movement of adjacent atoms in the lattice.) Optical phonons have higher energies (frequencies), but make smaller contribution to conduction heat transfer, because of their smaller group velocity and occupancy.

Phonon transport across hetero-structure boundaries (represented with ''R<sub>p,b</sub>'', [[Interfacial thermal resistance|phonon boundary resistance]]) according to the boundary scattering approximations are modeled as acoustic and diffuse mismatch models.<ref name=SwartzBoundary>{{cite journal|last=Swartz|first=E.|coauthors=Pohl, R.|title=Thermal boundary resistance|journal=Reviews of Modern Physics|year=1989|volume=61|issue=3|pages=605–668|doi=10.1103/RevModPhys.61.605|bibcode = 1989RvMP...61..605S }}</ref> Larger phonon transmission (small ''R<sub>p,b</sub>'') occurs at boundaries where material pairs have similar phonon properties (''u<sub>p</sub>'', ''D<sub>p</sub>'', etc.), and in contract large ''R<sub>p,b</sub>'' occurs when some material is softer (lower cut-off phonon frequency) than the other.

== Electron ==
{{see also|Thermoelectric effect}}

Quantum electron energy states for electron are found using the electron quantum Hamiltonian, which is generally composed of kinetic (-''ħ''<sup>2</sup>∇<sup>2</sup>/2''m<sub>e</sub>'') and potential energy terms (''φ<sub>e</sub>''). Atomic orbital, a [[Function (mathematics)|mathematical function]] describing the wave-like behavior of either an [[electron]] or a pair of electrons in an [[atom]], can be found from the [[Schrödinger equation]] with this electron Hamiltonian. [[Hydrogen-like atom]]s (a nucleus and an electron) allow for closed-form solution to Schrödinger equation with the electrostatic potential (the [[Coulomb law]]). The Schrödinger equation of atoms or atomic ions with more than one electron has not been solved analytically, because of the Coulomb interactions among electrons. Thus, numerical techniques are used, and an [[electron configuration]] in approximated as product of simpler hydrogen-like atomic orbitals (isolate electron orbitals). Molecules with multiple atoms (nuclei and their electrons) have [[molecular orbital]] (MO, a mathematical function for the wave-like behavior of an electron in a molecule),  and are obtained from simplified solution techniques such as  [[linear combination of atomic orbitals]] (LCAO). The molecular orbital is used to predict chemical and physical properties, and the difference between highest occupied molecular orbital ([[HOMO/LUMO|HOMO]]) and the lowest unoccupied molecular orbital ([[HOMO/LUMO|LUMO]]) is a measure of [[Excited state|excitability]] of the molecules.

In a [[crystal structure]] of  [[metal]]lic solids, the [[free electron model]] (zero potential, ''φ<sub>e</sub>'' = 0) for the behavior of [[valence electron]]s is used. However, in a [[Crystal structure|periodic lattice (crystal)]], there is periodic crystal potential, so the electron Hamiltonian becomes<ref name=KittelSolidStatePhysics />

:<math> \qquad \qquad \mathrm{H}_e = - \frac{\hbar^2}{2m_e}\nabla^2 + \varphi_c(\mathbf{x}),</math>

where ''m<sub>e</sub>'' is the electron mass, and the periodic potential is expressed as ''φ<sub>c</sub>'' (''x'') = ∑<sub>''g''</sub> ''φ<sub>g</sub>''exp[''i''('''g'''∙'''x''')] ('''g''': reciprocal lattice vector).  The time-independent Schrödinger equation with this Hamiltonian is given as (the eigenvalue equation)

:<math> \qquad \qquad \mathrm{H}_e \psi_{e,\mathbf{x}}(\mathbf{x}) = E_e(\boldsymbol{\kappa}_e) \psi_{e,\mathbf{x}}(\mathbf{x}),</math>

where the eigenfunction ''ψ<sub>e,κ</sub>'' is the electron wave function, and eigenvalue ''E<sub>e</sub>''('''κ'''<sub>''e''</sub>), is the electron energy ('''κ'''<sub>''e''</sub>: electron wavevector).  The relation between wavevector, '''κ'''<sub>''e''</sub> and energy ''E<sub>e</sub>'' provides the [[electronic band structure]].  In practice, a lattice as [[Many-body problem|many-body systems]]  includes interactions between electrons and nuclei in potential, but this calculation can be too intricate. Thus, many approximate techniques have been suggested and one of them is [[density functional theory]] (DFT), uses functionals of the spatially dependent [[Electronic density|electron density]] instead of full interactions. DFT is widely used in ''ab initio'' software ([[ABINIT]], [[CASTEP]], [[Quantum ESPRESSO]], [[SIESTA (computer program)|SIESTA]], [[VASP]], [[WIEN2k]], etc.). The electron specific heat is based on the energy states and occupancy distribution (the [[Fermi–Dirac statistics]]).  In general, the heat capacity of electron is small except at very high temperature when they are in thermal equilibrium with phonons (lattice). Electrons contribute to heat conduction (in addition to charge carrying) in solid, especially in metals. Thermal conductivity tensor in solid is the sum of electric and phonon thermal conductivity tensors '''K''' = '''K'''<sub>''e''</sub> + '''K'''<sub>''p''</sub>.

Electrons are affected by two thermodynamic forces [from the charge, ∇(''E''<sub>F</sub>/''e<sub>c</sub>'') where ''E''<sub>F</sub> is the [[Fermi level]] and ''e<sub>c</sub>'' is the [[Elementary charge|electron charge]] and temperature gradient, ∇(1/''T'')] because they carry both charge and thermal energy, and thus electric current '''j'''<sub>''e''</sub> and heat flow '''q''' are described with the thermoelectric tensors ('''A'''<sub>''ee''</sub>, '''A'''<sub>''et''</sub>, '''A'''<sub>''te''</sub>, and '''A'''<sub>''tt''</sub>) from the [[Onsager reciprocal relations]]<ref name=Onsager1931>{{cite journal|last=Onsager|first=L.|title=Reciprocal Relations in Irreversible Processes. I.|journal=Physical Review|year=1931|volume=37|issue=4|pages=405–426|doi=10.1103/PhysRev.37.405|bibcode = 1931PhRv...37..405O }}</ref> as

:<math> \qquad \qquad  \mathbf{j}_e = \mathbf{A}_{ee}\cdot\nabla\frac{E_\mathrm{F}}{e_c} + \mathbf{A}_{et}\cdot\nabla\frac{1}{T} ,\ \ \mathrm{and}</math>
:<math> \qquad \qquad  \mathbf{q}= \mathbf{A}_{te}\cdot\nabla\frac{E_\mathrm{F}}{e_c} + \mathbf{A}_{tt}\cdot\nabla\frac{1}{T}.</math>

Converting these equations to have '''j'''<sub>''e''</sub> equation in terms of electric field '''e'''<sub>e</sub> and ∇''T'' and '''q''' equation with '''j'''<sub>''e''</sub> and ∇''T'', (using scalar coefficients for isotropic transport, ''α<sub>ee</sub>'', ''α<sub>et</sub>'', ''α<sub>te</sub>'', and ''α<sub>tt</sub>'' instead of '''A'''<sub>''ee''</sub>, '''A'''<sub>''et''</sub>, '''A'''<sub>''te''</sub>, and '''A'''<sub>''tt''</sub>)

:<math> \qquad \qquad \mathbf{j}_e = \alpha_{ee}\mathbf{e}_e - \frac{\alpha_{et}}{T^2}\nabla T \qquad (\mathbf{e}_e = \alpha_{ee}^{-1}\mathbf{j}_e+\frac{\alpha_{ee}^{-1}\alpha_{et}}{T^2}\nabla T), </math>
:<math> \qquad \qquad \mathbf{q}= \alpha_{te}\alpha_{ee}^{-1}\mathbf{j}_e-\frac{\alpha_{tt}-\alpha_{te}\alpha_{ee}^{-1}\alpha_{et}}{T^2}\nabla T.</math>

Electrical conductivity/resistivity  ''σ<sub>e</sub>'' (Ω<sup>−1</sup>m<sup>−1</sup>)/ ρ<sub>''e''</sub> (Ω-m), electric thermal conductivity ''k<sub>e</sub>'' (W/m-K) and the Seebeck/Peltier coefficients ''α''<sub>S</sub> (V/K)/''α''<sub>P</sub> (V) are defined as,

:<math> \qquad \qquad \sigma_e = \frac{1}{\rho_e}=\alpha_{ee}, \ \ k_e = \frac{\alpha_{tt}-\alpha_{te}\alpha_{ee}^{-1}\alpha_{et}}{T^2},\mathrm{and} \ \alpha_\mathrm{S} = \frac{\alpha_{et}\alpha_{ee}^{-1}}{T^2} \ \ (\alpha_\mathrm{S} = \alpha_\mathrm{P}T).  </math>

Various carriers (electrons, [[magnon]]s, phonons, and [[polaron]]s) and their interactions substantially affect the Seebeck coefficient.<ref name=Emin1987>{{cite journal|last=Emin|first=D.|title=Icosahedral Boron-Rich Solids|journal=Physics Today|year=1987|volume=40|issue=1|pages=55|doi=10.1063/1.881112|bibcode = 1987PhT....40a..55E }}</ref><ref name=Kanatzidis2003>{{cite book|last=Edited by Kanatzidis|first=M.G.;Mahanti, S. D.;Hogan, T. P.|title=Chemistry, physics, and materials science of thermoelectric materials : beyond bismuth telluride|year=2003|publisher=Kluwer Academic/Plenum Publ.|location=New York [u.a.]|isbn=0306477386}}</ref>   The Seebeck coefficient can be decomposed with two contributions, ''α''<sub>S</sub> = ''α''<sub>S,pres</sub> + ''α''<sub>S,trans</sub>, where ''α''<sub>S,pres</sub> is the sum of contributions to the carrier-induced entropy change, i.e., ''α''<sub>S,pres</sub> = ''α''<sub>S,mix</sub> + ''α''<sub>S,spin</sub> + ''α''<sub>S,vib</sub> (''α''<sub>S,mix</sub>: entropy-of-mixing, ''α''<sub>S,spin</sub>: spin entropy, and ''α''<sub>S,vib</sub>: vibrational entropy). The other contribution ''α''<sub>S,trans</sub> is the net energy transferred in moving a carrier divided by ''qT'' (''q'': carrier charge). The electron’s contributions to the Seebeck coefficient are mostly in ''α''<sub>S,pres</sub>. The ''α''<sub>S,mix</sub> is usually dominant in lightly doped semiconductors. The change of the entropy-of-mixing upon adding an electron to a system is the so-called Heikes formula

:<math> \qquad \qquad  \alpha_\mathrm{S,mix} = \frac{1}{q}\frac{\partial S_\mathrm{mix}}{\partial N} = \frac{k_\mathrm{B}}{q}\mathrm{ln}(\frac{1 - f_e^\mathrm{o}}{f_e^\mathrm{o}}),</math>

where ''f<sub>e</sub>''<sup>o</sup> = ''N''/''N<sub>a</sub>'' is the ratio of electrons to sites (carrier concentration). Using the chemical potential (''µ''), the thermal energy (''k''<sub>B</sub>''T'') and the Fermi function, above equation can be expressed in an alternative form, ''α''<sub>S,mix</sub> =  (''k''<sub>B</sub>/''q'')[(''E<sub>e</sub>'' - ''µ'')/(''k''<sub>B</sub>''T'')].
Extending the Seebeck effect to spins, a ferromagnetic alloy can be a good example. The contribution to the Seebeck coefficient that results from electrons’ presence altering the systems spin entropy is given by ''α''<sub>S,spin</sub> = Δ''S''<sub>spin</sub>/''q'' = (''k''<sub>B</sub>/''q'')ln[(2''s'' + 1)/(2''s''<sub>0</sub> +1)], where ''s''<sub>0</sub> and ''s'' are net spins of the magnetic site in the absence and presence of the carrier, respectively. Many vibrational effects with electrons also contribute to the Seebeck coefficient. The softening of the vibrational frequencies produces a change of the vibrational entropy is one of examples. The vibrational entropy is the negative derivative of the free energy, i.e.,

:<math> \qquad \qquad  S_\mathrm{vib} = -\frac{\partial F_\mathrm{mix}}{\partial T} = 3Nk_\mathrm{B}T\int_0^\omega\{\frac{\hbar\omega}{2k_\mathrm{B}T}\mathrm{coth}(\frac{\hbar\omega}{2k_\mathrm{B}T}) - \mathrm{ln}[2\mathrm{sinh}(\frac{\hbar\omega}{2k_\mathrm{B}T})]\}D_p(\omega)d\omega,</math>

where ''D<sub>p</sub>''(''ω'') is the phonon density-of-states for the structure. For the high-temperature limit and series expansions of the hyperbolic functions, the above is simplified as ''α''<sub>S,vib</sub> = (Δ''S''<sub>vib</sub>/''q'') = (''k''<sub>B</sub>/''q'')∑''<sub>i</sub>''(-Δ''ω<sub>i</sub>''/''ω<sub>i</sub>'').

The Seebeck coefficient derived in the above Onsager formulation is the mixing component ''α''<sub>S,mix</sub>, which dominates in most semiconductors. The vibrational component in high-band gap materials such as B<sub>13</sub>C<sub>2</sub> is very important.<br />
Considering the microscopic transport ( transport is a results of nonequilibrium),

:<math> \qquad \qquad \mathbf{j}_e = -\frac{e_c}{\hbar^3}\sum_p\mathbf{u}_ef_e^\prime = -\frac{e_c}{\hbar^3k_\mathrm{B}T}\sum_p\mathbf{u}_e\tau_e(-\frac{\partial f_e^\mathrm{o}}{\partial E_e})(\mathbf{u}_e\cdot\mathbf{F}_{te}),</math>
:<math> \qquad \qquad  \mathbf{q}=\frac{1}{\hbar^3}\sum_p(E_e-E_\mathrm{F})\mathbf{u}_ef_e^\prime = \frac{1}{\hbar^3k_\mathrm{B}T}\sum_p \mathbf{u}_e\tau_e(-\frac{\partial f_e^\mathrm{o}}{\partial E_e})(E_e-E_\mathrm{F})(\mathbf{u}_e\cdot\mathbf{F}_{te}),</math>

where '''u'''<sub>''e''</sub> is the electron velocity vector, ''f<sub>e</sub>''’ (''f<sub>e</sub>''<sup>o</sup>) is the electron nonequilibrium (equilibrium) distribution, ''τ<sub>e</sub>'' is the electron scattering time, ''E<sub>e</sub>'' is the electron energy, and '''F'''<sub>''te''</sub> is the electric and thermal forces from ∇(''E''<sub>F</sub>/''e<sub>c</sub>'') and ∇(1/''T'').
Relating the thermoelectric coefficients to the microscopic transport equations for ''j<sub>e</sub>'' and q, the thermal, electric, and thermoelectric properties are calculated. Thus, ''k<sub>e</sub>'' increases with the electrical conductivity σe and temperature ''T'', as the [[Wiedemann-Franz law]] presents [''k<sub>e</sub>''/(''σ<sub>e</sub>T<sub>e</sub>'') = (1/3)(''πk''<sub>B</sub>/''e<sub>c</sub>'')<sup>2</sup> = 2.44×10<sup>−8</sup> W-Ω/K<sup>2</sup>].   Electron transport (represented as ''σ<sub>e</sub>'') is a function of carrier density ''n<sub>e,c</sub>'' and electron mobility ''μ<sub>e</sub>'' (''σ<sub>e</sub>'' = ''e<sub>c</sub>n<sub>e,c</sub>μ<sub>e</sub>'').   ''μ<sub>e</sub>'' is determined by electron scattering rates <math>\dot{\gamma}_e</math> (or relaxation time, <math>\tau_e = 1/\dot{\gamma}_e </math> ) in various interaction mechanisms including interaction with other electrons, phonons, impurities and boundaries.

Electrons interact with other principal energy carriers. Electrons accelerated by an electric field are relaxed through the energy conversion to phonon (in semiconductors, mostly optical phonon), which is called [[Joule heating]]. Energy conversion between electric potential and phonon energy is considered in [[thermoelectrics]] such as Peltier cooling and thermoelectric generator.  Also, study of interaction with photons is central in [[Optoelectronics|optoelectronic]] applications (i.e. [[light-emitting diode]], [[solar photovoltaic cells]], etc.). Interaction rates or energy conversion rates can be evaluated using the Fermi golden rule (from the perturbation theory) with ''ab initio'' approach.

== Fluid particle ==

Fluid particle is the smallest unit (atoms or molecules) in the fluid phase (gas, liquid or plasma) without breaking any chemical bond. Energy of fluid particle is divided into potential, electronic, translational, vibrational, and rotational energies. The heat (thermal) energy storage in fluid particle is through the temperature-dependent particle motion (translational, vibrational, and rotational energies). The electronic energy is included only if temperature is high enough to ionize or dissociate the fluid particles or to include other electronic transitions. These quantum energy states of the fluid particles are found using their respective quantum Hamiltonian. These are H''<sub>f,t</sub>'' = -(''ħ''<sup>2</sup>/2''m'')∇<sup>2</sup>, H''<sub>f,v</sub>'' = -(''ħ''<sup>2</sup>/2''m'')∇<sup>2</sup> + Γ''x''<sup>2</sup>/2 and H''<sub>f,r</sub>'' = -(''ħ''<sup>2</sup>/2''I<sub>f</sub>'')∇<sup>2</sup> for translational, vibrational and rotational modes. (Γ: [[Hooke's law|spring constant]], ''I<sub>f</sub>'': the [[moment of inertia]] for the molecule). From the Hamiltonian, the quantized fluid particle energy state ''E<sub>f</sub>'' and [[Partition function (statistical mechanics)|partition functions]] ''Z<sub>f</sub>'' [with the [[Maxwell-Boltzmann statistics|Maxwell-Boltzmann (MB) occupancy distribution]]] are found as<ref name=CareyBook>{{cite book|last=Carey|first=V. P.|title=Statistical thermodynamics and microscale thermophysics|year=1999|publisher=Cambridge University Press|location=Cambridge|isbn=0521654203}}</ref>

:<math> \qquad \qquad \mathrm{translational }\ \  \  \ \ \ \ \ \ \ \ \ E_{f,t,n} = \frac{\pi^2\hbar^2}{2m}(\frac{n_x^2}{L^2}+\frac{n_y^2}{L^2}+\frac{n_z^2}{L^2}) \ \ \ \mathrm{and} \ \ \ Z_{f,t}\sum_{i = 0}^\infty g_{f,t,i}\mathrm{exp}(-\frac{E_{f,t,i}}{k_\mathrm{B}T}) = V (\frac{mk_\mathrm{B}T}{2\pi\hbar^2})^{3/2},</math>
:<math> \qquad \qquad \mathrm{vibrational     }\  \ \ \ \ \ \ \ \ \ \ \ \ \  E_{f,v,l} = \hbar\omega_{f,v}(1+\frac{1}{2}) \ \ \mathrm{and} \ \ Z_{f,v}\sum_{j = 0}^\infty \mathrm{exp}[-(l+\frac{1}{2})\frac{\hbar\omega_{f,v}}{k_\mathrm{B}T}] = \frac{\mathrm{exp}(-T_{f,v}/2T)}{1-\mathrm{exp}(-T_{f,v}/T)},</math>
:<math> \qquad \qquad \mathrm{rotational}\  \ \ \ \ \ \ \ \ \ \ \ \ \  \ \   E_{f,r,j} = \frac{\hbar^2}{2I_f} \ \ \mathrm{and} \ \ Z_{f,r}\sum_{j = 0}^\infty (2j+1)\mathrm{exp}[-\frac{-\hbar^2j(j+1)}{2I_fk_\mathrm{B}T}] \approx \frac{T}{T_{f,r}}(1+\frac{T_{f,r}}{3T}+\frac{T_{f,r}^2}{15T^2}+...),</math>
:<math> \qquad \qquad  \mathrm{total }\  \ \ \ \ \ \ \ \ \ \ \  \ \  \ \  \ \ \ \ \ \ \ \ \ \ E_{f,i} = \sum_i E_{f,i} = E_{f,t} +  E_{f,v} + E_{f,r} + ...  \ \ \mathrm{and} \ \ Z_{f}=\prod_{i}Z_{f,i} = Z_{f,t}Z_{f,v}Z_{f,r}... .</math>

Here, ''g<sub>f</sub>'' is the degeneracy, ''n'', ''l'', and  ''j'' are the transitional, vibrational and rotational quantum numbers, ''T<sub>f,v</sub>'' is the characteristic temperature for vibration (= ''ħω<sub>f,v</sub>''/''k''<sub>B</sub>, : vibration frequency), and ''T<sub>f,r</sub>'' is the rotational temperature [= ''ħ''<sup>2</sup>/(2''I<sub>f</sub>k''<sub>B</sub>)]. The average specific internal energy is related to the partition function through ''Z<sub>f</sub>'',  <math> e_f = (k_\mathrm{B}T^2/m)(\partial \mathrm{ln}Z_f/\partial T)|_{N,V}.</math>

With the energy states and the partition function, the fluid particle specific heat capacity ''c<sub>v,f</sub>'' is the summation of contribution from various kinetic energies (for non-ideal gas the potential energy is also added). Because the total degrees of freedom in molecules is determined by the atomic configuration, ''c<sub>v,f</sub>''  has different formulas depending on the configuration,<ref name=CareyBook />

:<math> \qquad \qquad \mathrm{monatomic\ ideal\ gas}\ \ \  \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ c_{v,f} = \frac{\partial e_f}{\partial T}|_V = \frac{3R_g}{2M},</math>
:<math> \qquad \qquad  \mathrm{diatomic\ ideal\ gas}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \  \ \  \ \ \ \ \ \ \ \ \ \ c_{v,f} = \frac{R_g}{M} \{ \frac{3}{2} + (\frac{T_{f,v}}{T})^2 \frac{\mathrm{exp}(T_{f,v,i}/T)}{[\mathrm{exp}(T_{f,v,i}/T)-1]^2}+1 +\frac{2}{15}(\frac{T_{f,v}}{T})^2\},</math>
:<math> \qquad \qquad  \mathrm{nonlinear,\ polyatomic\ ideal\ gas }  \ \ \ \ \ \ \ \ \ c_{v,f} = \frac{R_g}{M}\{3+ \textstyle\sum_{j=1}^{3N_o-6}\displaystyle (\frac{T_{f,v}}{T})^2\frac{\mathrm{exp}(T_{f,v,i}/T)}{[\mathrm{exp}(T_{f,v,i}/T)-1]^2}\} .</math>

where ''R<sub>g</sub>'' is the gas constant (= ''N''<sub>A</sub>''k''<sub>B</sub>, ''N''<sub>A</sub>: the Avogadro constant) and ''M'' is the molecular mass (kg/kmole). (For the polyatomic ideal gas, ''N''<sub>o</sub> is the number of atoms in a molecule.) In gas, constant-pressure specific heat capacity ''c<sub>p,f</sub>'' has a larger value and the difference depends on the temperature ''T'', volumetric thermal expansion coefficient ''β'' and the isothermal compressibility κ [''c<sub>p,f</sub>''  – ''c<sub>v,f</sub>''  = ''Tβ''<sup>2</sup>/(''ρ<sub>f</sub>κ''), ''ρ<sub>f</sub>'' : the fluid density].  For dense fluids that the interactions between the particles (the van der Waals interaction) should be included, and ''c<sub>v,f</sub>'' and ''c<sub>p,f</sub>'' would change accordingly.
The net motion of particles (under gravity or external pressure) gives rise to the convection heat flux '''q'''<sub>u</sub> = ''ρ<sub>f</sub>c<sub>p,f</sub>'''''u'''''<sub>f</sub>T''. Conduction heat flux '''q'''''<sub>k</sub>'' for ideal gas is derived with the gas kinetic theory or the Boltzmann transport equations, and the thermal conductivity is

:<math> \qquad \qquad \ \ \  \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \  k_{f} = \frac{1}{3}n_fc_{p,f}\langle u_f^2\rangle\tau_{f\mbox{-}f},</math>

where ⟨''u<sub>f</sub>''<sup>2</sup>⟩<sup>1/2</sup> is the RMS ([[root mean square]]) thermal velocity (3''k''<sub>B</sub>''T''/''m'' from the MB distribution function, ''m'': atomic mass) and ''τ<sub>f-f</sub>'' is the relaxation time (or intercollision time period) [(2<sup>1/2</sup>''π d''<sup>2</sup>''n<sub>f</sub>'' ⟨''u<sub>f</sub>''⟩)<sup>-1</sup> from the gas kinetic theory, ⟨''u<sub>f</sub>''⟩: average thermal speed (8''k''<sub>B</sub>''T''/''πm'')<sup>1/2</sup>, ''d'': the collision diameter of fluid particle (atom or molecule), ''n<sub>f</sub>'': fluid number density].

''k<sub>f</sub>'' is also calculated using [[molecular dynamics]] (MD), which simulates [[Motion (physics)|physical movements]] of the fluid particles with the [[Newton's laws of motion|Newton equations of motion]] (classical) and [[Force field (chemistry)|force field]] (from ''ab initio'' or empirical properties). For calculation of ''k<sub>f</sub>'', the equilibrium MD with [[Green–Kubo relations]], which express the transport coefficients in terms of integrals of time correlation functions (considering fluctuation), or nonequilibrium MD (prescribing heat flux or temperature difference in simulated system) are generally employed.

Fluid particles can interact with other principal particles. Vibrational or rotational modes, which have relatively high energy, are excited or decay through the interaction with photons. [[Gas laser]]s employ the interaction kinetics between fluid particles and photons, and laser cooling has been also considered in CO<sub>2</sub> gas laser.<ref name=Djeu1981>{{cite journal|last=Djeu|first=N.|coauthors=Whitney, W.|title=Laser Cooling by Spontaneous Anti-Stokes Scattering|journal=Physical Review Letters|year=1981|volume=46|issue=4|pages=236–239|doi=10.1103/PhysRevLett.46.236|bibcode = 1981PhRvL..46..236D }}</ref><ref name=Shin2009>{{cite journal|last=Shin|first=S.|coauthors=Kaviany, M.|title=Enhanced laser cooling of CO<sub>2</sub>–Xe gas using (02<sup>0</sup>0) excitation|journal=Journal of Applied Physics|year=2009|volume=106|issue=12|pages=124910|doi=10.1063/1.3273488|bibcode = 2009JAP...106l4910S }}</ref> Also, fluid particles can be [[Adsorption|adsorbed]] on solid surfaces ([[physisorption]] and [[chemisorption]]), and the frustrated vibrational modes in adsorbates (fluid particles) is decayed by creating ''e<sup>-</sup>''-''h<sup>+</sup>'' pairs or phonons.  These interaction rates are also calculated through ''ab initio'' calculation on fluid particle and the Fermi golden rule.<ref name=Sakong2008>{{cite journal|last=Sakong|first=S.|coauthors=Kratzer, P.; Han, X.; Laß, K.; Weingart, O.; Hasselbrink, E.|title=Density-functional theory study of vibrational relaxation of CO stretching excitation on Si(100)|journal=The Journal of Chemical Physics|year=2008|volume=129|issue=17|pages=174702|doi=10.1063/1.2993254|bibcode = 2008JChPh.129q4702S }}</ref>

== Photon ==
[[File:Spectral photon absorption coefficient.jpg|thumbnail|right|Spectral photon absorption coefficient for typical gas, liquid, and solid phases. For the solid phase, examples of polymer, oxide, semiconductor, and metals are given.]]
Photon is the quanta of [[Electromagnetic radiation|electromagnetic (EM) radiation]] and energy carrier for [[Thermal radiation|radiation heat transfer]]. The EM wave is governed by the classical [[Maxwell's equations|Maxwell equations]], and the [[Quantization of the electromagnetic field|quantization of EM wave]] is used for phenomena such as the [[Black-body radiation|blackbody radiation]] (in particular to explain the [[ultraviolet catastrophe]]). The quanta EM wave (photon) energy of angular frequency ''ω<sub>ph</sub>'' is ''E<sub>ph</sub> = ħω<sub>ph</sub>'', and follows the Bose-Einstein distribution function (''f<sub>ph</sub>''). The photon Hamiltonian for the quantized radiation field ([[second quantization]]) is<ref name=Sakurai1973>{{cite book|last=Sakurai|first=J.J.|title=Advanced quantum mechanics|year=1973|publisher=Benjamin/Cummings|location=Menlo Park, Calif.|isbn=0201067102|edition=4th printing, with revisions.}}</ref><ref name=Merzbacher1998>{{cite book|last=Merzbacher|first=E.|title=Quantum mechanics|year=1998|publisher=Wiley|location=New York [u.a.]|isbn=0471887021|edition=3. ed.}}</ref>

:<math> \qquad \qquad   \mathrm{H}_{ph} = \frac{1}{2}\int (\epsilon_\mathrm{o}\mathbf{e}_e^2 + \mu_\mathrm{o}^{-1}\mathbf{b}_e^2)dV = \sum_\alpha \hbar \omega_{ph,\alpha}(c_\alpha^\dagger c_\alpha + \frac{1}{2}),</math>

where '''e'''<sub>''e''</sub> and '''b'''<sub>''e''</sub> are the electric and magnetic fields of the EM radiation, ''ε''<sub>o</sub> and ''μ''<sub>o</sub> are the free-space permittivity and permeability, ''V'' is the interaction volume, ''ω<sub>ph,α</sub>'' is the photon angular frequency for the ''α'' mode and ''c<sub>α</sub>''<sup>†</sup> and ''c<sub>α</sub>'' are the photon creation and annihilation operators. The vector potential '''a'''<sub>''e''</sub> of EM fields ('''e'''<sub>''e''</sub> = -∂'''a'''<sub>''e''</sub>/∂''t'' and '''b'''<sub>''e''</sub> = ∇×'''a'''<sub>''e''</sub>) is

:<math> \qquad \qquad   \mathbf{a}_{e} (\mathbf{x},t) = \sum_\alpha(\frac{\hbar}{2\epsilon_\mathrm{o}\omega_{ph,\alpha}V})^{1/2}\mathbf{s}_{ph,\alpha} (c_\alpha e^{i \boldsymbol{\kappa}_\alpha \cdot \mathbf{x}} + c_\alpha^\dagger e^{-i\boldsymbol{\kappa}_\alpha\cdot\mathbf{x}}), </math>

where '''s'''<sub>''ph,α''</sub> is the unit polarization vector, '''κ'''<sub>''α''</sub>   is the wave vector.

Blackbody radiation among various types of photon emission employs the [[photon gas]] model with thermalized energy distribution without interphoton interaction. From the linear dispersion relation (i.e., dispersionless), phase and group speeds are equal (''u<sub>ph</sub>'' =  ''d ω<sub>ph</sub>''/''dκ'' = ''ω<sub>ph</sub>''/''κ'', ''u<sub>ph</sub>'': photon speed) and the Debye (used for dispersionless photon) density of states is ''D<sub>ph,b,ω</sub>dω'' = ω<sub>ph</sub><sup>2</sup>''dω<sub>ph</sub>''/''π''<sup>2</sup>''u''<sub>ph</sub><sup>3</sup>. With ''D<sub>ph,b,ω</sub>'' and equilibrium distribution ''f<sub>ph</sub>'', photon energy spectral distribution ''dI<sub>b,ω</sub>'' or ''dI<sub>b,λ</sub>'' (''λ<sub>ph</sub>'': wavelength) and total emissive power ''E<sub>b</sub>'' are derived as

:<math> \qquad \qquad  dI_{b,\omega} = \frac{D_{ph,b,\omega}f_{ph}u_{ph}d\omega_{ph}}{4\pi} </math> <math> =\frac{\hbar\omega_{ph}^3}{4\pi^3u_{ph}^2} </math> <math> \frac{1}{e^{\hbar\omega_{ph}/k_\mathrm{B}T}-1} d\omega_{ph} \ \mathrm{or} \  d I_{b,\lambda} = \frac{4\pi\hbar u_{ph}^2 d \lambda_{ph}}{\lambda_{ph}^5(e^{2\pi\hbar u_{ph} / \lambda_{ph}k_\mathrm{B}T}-1)} \ \  	</math> ([[Planck's law|Planck law]])<math>,</math>

:<math> \qquad \qquad   E_b = \int_0^\infty d E_{b,\lambda} = \sigma_\mathrm{SB}T^4\ \mathrm{, \ where} \ \sigma_\mathrm{SB} = \frac{\pi^2 k_\mathrm{B}^4}{60 \hbar^3 u_{ph}^2} 	\ \ \ \ </math>  ([[Stefan–Boltzmann law]])<math>.</math>

Compared to blackbody radiation, [[laser]] emission has high directionality (small solid angle Δ''Ω'') and spectral purity (narrow bands Δ''ω''). Lasers range far-infrared to X-rays/γ-rays regimes based on the resonant transition ([[stimulated emission]]) between electronic energy states.<ref name=SiegmanLaser1986>{{cite book|last=Siegman|first=A. E.|title=Lasers|year=1986|publisher=University Science Books|location=Mill Valley, Calif.|isbn=0935702113|edition=8. print.}}</ref>

[[Near-field thermal radiation|Near-field radiation]] from thermally excited dipoles and other electric/magnetic transitions is very effective within a short distance (order of wavelength) from emission sites.<ref name=Ottens2011>{{cite journal|last=Ottens|first=R.|coauthors=Quetschke, V.; Wise, Stacy; Alemi, A.; Lundock, R.; Mueller, G.; Reitze, D.; Tanner, D.; Whiting, B.|title=Near-Field Radiative Heat Transfer between Macroscopic Planar Surfaces|journal=Physical Review Letters|year=2011|volume=107|issue=1|doi=10.1103/PhysRevLett.107.014301|arxiv = 1103.2389 |bibcode = 2011PhRvL.107a4301O }}</ref><ref name=Tatarskii1987>{{cite book|last=Tatarskii|first= V.I.; Rytov, S.M.; Kravtsov, Y. A.|title=Principles of statistical radiophysics|year=1987|publisher=Springer|location=Berlin u.a.|isbn=3540125620|edition=2. rev. and enl. ed.}}</ref><ref name=Domingues2005>{{cite journal|last=Domingues|first=G.|coauthors=Volz, S.; Joulain, K.; Greffet, J.-J.|title=Heat Transfer between Two Nanoparticles Through Near Field Interaction|journal=Physical Review Letters|year=2005|volume=94|issue=8|doi=10.1103/PhysRevLett.94.085901|bibcode = 2005PhRvL..94h5901D }}</ref>

The BTE for photon particle momentum '''p'''<sub>''ph''</sub> = ''ħω<sub>ph</sub>'''''s'''/''u<sub>ph</sub>'' along direction s experiencing absorption/emission <math> \textstyle \dot{s}_{f,ph-e}\ </math> (= ''u<sub>ph</sub>σ<sub>ph,ω</sub>''[''f<sub>ph</sub>''(''ω<sub>ph</sub>'',''T'') - ''f<sub>ph</sub>''('''s''')], ''σ<sub>ph,ω</sub>'': spectral [[absorption coefficient]]), and generation/removal  <math> \textstyle \dot{s}_{f,ph,i}</math>, is<ref name=Sampson1965>{{cite book|last=Sampson|first=D. H.|title=Radiative Contributions to Energy and Momentum Transport in a Gas|year=1965|publisher=Interscience}}</ref><ref name=Howell2010>{{cite book|last= Howell|first= J. R.; Siegel, R.;Mengüç, M. P.|title=Thermal radiation heat transfer.|year=2010|publisher=CRC|location=Boca Raton, Fla.|isbn=1439805334|edition=5th ed.}}</ref>

:<math> \qquad \qquad  \frac{\partial f_{ph}}{\partial t} + u_{ph}\mathbf{s}\cdot\nabla f_{ph} =  \frac{\partial f_{ph}}{\partial t}|_s + u_{ph}\sigma_{ph,\omega}[f_{ph}(\omega_{ph},T)-f_{ph}(\mathbf{s})]+ \dot{s}_{f,ph,i}. </math>

In terms of radiation intensity (''I<sub>ph,ω</sub>'' = ''u<sub>ph</sub>f<sub>ph</sub>ħω<sub>ph</sub>D<sub>ph,ω</sub>''/4''π'', ''D<sub>ph,ω</sub>'': photon density of states), this is called the equation of radiative transfer (ERT)<ref name=Howell2010 />

:<math> \qquad \qquad   \frac{\partial I_{ph,\omega}(\omega_{ph}, \mathbf{s})}{u_{ph} \partial t} + \mathbf{s}\cdot\nabla I_{ph,\omega} (\omega_{ph},\mathbf{s}) =  \frac{\partial I_{ph,\omega}(\omega_{ph}, \mathbf{s})}{u_{ph}\partial t}|_s + </math> <math> \sigma_{ph,\omega}[I_{ph,\omega}(\omega_{ph},T)-I_{ph}(\omega_{ph},\mathbf{s})]+  \dot{s}_{ph,i}. </math>

The net radiative heat flux vector is <math> \textstyle \mathbf{q}_r = \mathbf{q}_{ph} = \int_0^\infty\int_{4\pi}\mathbf{s} I_{ph,\omega}d \Omega d\omega.</math>

From the [[Einstein coefficients|Einstein population rate equation]], spectral absorption coefficient ''σ<sub>ph,ω</sub>'' in ERT is,<ref>{{cite book|last=Loudon|first=R.|title=The quantum theory of light|year=2000|publisher=Oxford Univ. Press|location=Oxford [u.a.]|isbn=978-0198501763|edition=3. ed.}}</ref> <br />
:<math> \sigma_{ph,\omega} = \frac{\hbar\omega\dot{\gamma}_{ph,a}n_e}{u_{ph}},</math>
where <math>\dot{\gamma}_{ph,a}</math> is the interaction probability (absorption) rate or the [[Atomic spectral line|Einstein coefficient]] ''B<sub>12</sub>'' (J<sup>−1</sup> m<sup>3</sup> s<sup>−1</sup>), which gives the probability per unit time per unit spectral energy density of the radiation field (1: ground state, 2: excited state), and ''n<sub>e</sub>'' is electron density (in ground state). This can be obtained using the transition dipole moment '''''μ'''<sub>e</sub>'' with the FGR and relationship between Einstein coefficients. Averaging ''σ<sub>ph,ω</sub>'' over ''ω'' gives the average photon absorption coefficient ''σ<sub>ph</sub>''.

For the case of optically thick medium of length ''L'', i.e., ''σ<sub>ph</sub>L'' >> 1,  and using the gas kinetic theory, the photon conductivity ''k<sub>ph</sub>''  is 16''σ''<sub>SB</sub>''T''<sup>3</sup>/3''σ<sub>ph</sub>'' (''σ''<sub>SB</sub>: [[Stefan–Boltzmann constant]], ''σ<sub>ph</sub>'': average photon absorption), and photon heat capacity ''n<sub>ph</sub>c<sub>v,ph</sub>'' is 16''σ''<sub>SB</sub>''T''<sup>3</sup>/''u<sub>ph</sub>''.

Photons have the largest range of energy and central in a variety of energy conversions. Photons interact with electric and magnetic entities. For example electric dipole which in turn are excited by optical phonons or fluid particle vibration, or transition dipole moments of electronic transitions. In heat transfer physics, the interaction kinetics of phonon is treated using the perturbation theory (the Fermi golden rule) and the interaction Hamiltonian. The photon-electron interaction is<ref name=Bartolo2010>{{cite book|last=Di Bartolo|first=B.|title=Optical interactions in solids|year=2010|publisher=World Scientific|location=New Jersey|isbn=9814295744|edition=2nd ed.}}</ref>

:<math> \qquad \qquad  \mathrm{H}_{ph-e} = -\frac{e_c}{m_e}(a+a^\dagger)\mathbf{a}_e\cdot\mathbf{p}_e = -(\frac{\hbar\omega_{ph,\alpha}}{2\epsilon_o V})^{1/2} (\mathbf{s}_{ph,\alpha}\cdot e_c \mathbf{x}_e)(a+a^\dagger)(ce^{i\mathrm{\kappa}\cdot\mathrm{x}}+c^\dagger e^{-i\mathrm{\kappa}\cdot\mathrm{x}}), 	\ \ \ \ </math>

where '''p'''<sub>''e''</sub> is the dipole moment vector and ''a''<sup>†</sup> and ''a'' are the creation and annihilation of internal motion of electron.  Photons also participate in ternary interactions, e.g., phonon-assisted photon absorption/emission (transition of electron energy level).<ref name=Garcia2006>{{cite journal|last=Garcia|first=H.|coauthors=Kalyanaraman, R.|title=Phonon-assisted two-photon absorption in the presence of a dc-field: the nonlinear Franz–Keldysh effect in indirect gap semiconductors|journal=Journal of Physics B: Atomic, Molecular and Optical Physics|year=2006|volume=39|issue=12|pages=2737–2746|doi=10.1088/0953-4075/39/12/009|bibcode = 2006JPhB...39.2737G }}</ref><ref name=Kim2008>{{cite journal|last=Kim|first=J.|coauthors=Kapoor, A.; Kaviany, M.|title=Material metrics for laser cooling of solids|journal=Physical Review B|year=2008|volume=77|issue=11|doi=10.1103/PhysRevB.77.115127|bibcode = 2008PhRvB..77k5127K }}</ref> The vibrational mode in fluid particles can decay or become excited by emitting or absorbing photons. Examples are solid and molecular gas laser cooling.<ref name=Phillips1998>{{cite journal|last=Phillips|first=W. D.|title=Nobel Lecture: Laser cooling and trapping of neutral atoms|journal=Reviews of Modern Physics|year=1998|volume=70|issue=3|pages=721–741|doi=10.1103/RevModPhys.70.721|bibcode = 1998RvMP...70..721P }}</ref><ref name=Chan2011>{{cite journal|last=Chan|first=J.|coauthors=Alegre, T. P. Mayer; Safavi-Naeini, Amir H.; Hill, Jeff T.; Krause, Alex; Gröblacher, Simon; Aspelmeyer, Markus; Painter, Oskar|title=Laser cooling of a nanomechanical oscillator into its quantum ground state|journal=Nature|year=2011|volume=478|issue=7367|pages=89–92|doi=10.1038/nature10461|arxiv = 1106.3614 |bibcode = 2011Natur.478...89C }}</ref><ref name=Hehlen2007>{{cite journal|last=Hehlen|first=M.|coauthors=Epstein, R.; Inoue, H.|title=Model of laser cooling in the Yb3+-doped fluorozirconate glass ZBLAN|journal=Physical Review B|year=2007|volume=75|issue=14|doi=10.1103/PhysRevB.75.144302|bibcode = 2007PhRvB..75n4302H }}</ref>

Using ''ab initio'' calculations based on the first principles along with EM theory, various radiative properties such as dielectric function ([[electrical permittivity]], ''ε<sub>e,ω</sub>''), spectral absorption coefficient (''σ<sub>ph,ω</sub>''), and the complex refraction index (''m<sub>ω</sub>''), are calculated for various interactions between photons and electric/magnetic entities in matter.<ref>{{cite journal|last=Bao|first=H.|coauthors=Ruan, X.|title=Ab initio calculations of thermal radiative properties: The semiconductor GaAs|journal=International Journal of Heat and Mass Transfer|year=2009|volume=53|issue=7-8|pages=1308–1312|doi=10.1016/j.ijheatmasstransfer.2009.12.033}}</ref><ref>{{cite journal|last=Bao|first=H.|coauthors=Qiu, B.; Zhang, Y.; Ruan, X.|title=A first-principles molecular dynamics approach for predicting optical phonon lifetimes and far-infrared reflectance of polar materials|journal=Journal of Quantitative Spectroscopy and Radiative Transfer|year=2012|volume=113|issue=13|pages=1683–1688|doi=10.1016/j.jqsrt.2012.04.018|bibcode = 2012JQSRT.113.1683B }}</ref> For example, the imaginary part (''ε<sub>e,c,ω</sub>'') of complex dielectric function (''ε<sub>e,ω</sub>'' = ''ε<sub>e,r,ω</sub>'' + ''i'' ''ε<sub>e,c,ω</sub>'') for electronic transition across a bandgap is<ref name=ZhangBook /> <br />
:<math> \qquad \qquad  \ \  \  \ \ \ \ \ \ \ \ \epsilon_{e,c,\omega} = \frac{4\pi^2}{\omega^2V}\sum_{i\isin \mathrm{VB},j\isin \mathrm{CB}}\sum_{\kappa}w_\kappa |p_{ij}|^2\delta(E_{\kappa,j}-E_{\kappa,i}-\hbar\omega),\ \ \ \qquad \ \  </math>
where ''V'' is the unit-cell volume, VB and CB denote the valence and conduction bands, ''w<sub>κ</sub>'' is the weight associated with a ''κ''-point, and ''p<sub>ij</sub>'' is the transition momentum matrix element.
The real part is ''ε<sub>e,r,ω</sub>'' is obtained from ''ε<sub>e,c,ω</sub>'' using the [[Kramers-Kronig relation]]<ref name=WootenBook>{{cite book|last=Wooten|first=F.|title=Optical Properties of Solids|year=1972|publisher=Academic Press|location=San Diego [etc.]|isbn=978-0127634500|edition=3. [Dr.]}}</ref> <br />
:<math> \qquad \qquad  \  \  \ \ \ \ \ \ \ \ \ \epsilon_{e,r,\omega} = 1 + \frac{4}{\pi}\mathbb{P}\int_{0}^\infty \mathrm{d}\omega'\frac{\omega'\epsilon_{e,c,\omega'}}{\omega'^2-\omega^2}.</math>
Here, <math>\mathbb{P}</math> denotes the [[Cauchy principal value|principal value of the integral]].

In another example, for the far IR regions where the optical phonons are involved, the dielectric function (''ε<sub>e,ω</sub>'') are calculated as  <br />
:<math> \frac{\epsilon_{e,\omega}}{\epsilon_{e,\infty}} = 1 + \sum_j\frac{\omega_{\mathrm{LO},j}^2 - \omega_{\mathrm{TO},j}^2}{\omega_{\mathrm{TO},j}^2 - \omega^2 - i\gamma\omega}  ,</math>
where LO and TO denote the longitudinal and transverse optical phonon modes, ''j'' is all the IR-active modes, and ''γ'' is the temperature-dependent damping term in the oscillator model. ''ε<sub>e,∞</sub>'' is high frequency dielectric permittivity, which can be calculated DFT calculation when ions are treated as external potential.

From these dielectric function (''ε<sub>e,ω</sub>'') calculations (e.g., [[Abinit]], [[VASP]], etc.), the complex refractive index ''m<sub>ω</sub>''(= ''n<sub>ω</sub>'' + ''i'' ''κ<sub>ω</sub>'', ''n<sub>ω</sub>'': refraction index and ''κ<sub>ω</sub>'': extinction index) is found, i.e., ''m<sub>ω</sub>''<sup>2</sup> = ''ε<sub>e,ω</sub>'' = ''ε<sub>e,r,ω</sub>'' + ''i'' ''ε<sub>e,c,ω</sub>''). The surface reflectance ''R'' of an ideal surface with normal incident from vacuum or air is given as<ref name=PedrottiBook>{{cite book|last=Pedrotti|first=F. L.|title=Introduction to optics|year=2007|publisher=Pearson Prentice Hall|location=Upper Saddle River, N.J.|isbn=978-0131499331|edition=3rd ed. --|coauthors=Pedrotti, L. S.; Pedrotti, L. M.}}</ref> ''R'' = [(''n<sub>ω</sub>'' - 1)<sup>2</sup> + ''κ<sub>ω</sub>''<sup>2</sup>]/[(''n<sub>ω</sub>'' + 1)<sup>2</sup> + ''κ<sub>ω</sub>''<sup>2</sup>]. The spectral absorption coefficient is then found from ''σ<sub>ph,ω</sub>'' = 2''ω'' ''κ<sub>ω</sub>''/''u<sub>ph</sub>''. The spectral absorption coefficient for various electric entities are listed in the below table.<ref name=BornBook>{{cite book|last=Born|first=M.|title=Principles of optics : electromagnetic theory of propagation, interference and diffraction of light|year=2006|publisher=Cambridge Univ. Pr.|location=Cambridge [u.a.]|isbn=978-0521642224|edition=7. expanded ed., repr. with corr., 4. print.|coauthors=, Emil Wolf. With contributions by A.B. Bhatia}}</ref>
{| class="wikitable"
|-
! Mechanism !! Relation (''σ<sub>ph,ω</sub>'')
|-
| Electronic absorption transition (atom, ion or molecule) || <math> \frac{n_{e,A}\pi^2 u_{ph}^2 \dot{\gamma}_{ph,e,sp}}{\omega_{e,g}^2\int_\omega\mathrm{d}\omega} = \frac{n_{e,A}\pi\omega_{e,g} |\boldsymbol{\mu}_e|^2}{3\epsilon_\mathrm{o}\hbar u_{ph}\int_\omega\mathrm{d}\omega}</math>, [''n<sub>e,A</sub>'': number density of ground state, ''ω<sub>e,g</sub>'': transition angular frequency, <math>\dot{\gamma}_{ph,e,sp}</math>: spontaneous emission rate (s<sup>−1</sup>), '''''μ'''<sub>e</sub>'': transition dipole moment,  <math> \textstyle\int_\omega\mathrm{d}\omega</math>: bandwidth]
|-
| Free carrier absorption (metal) || <math> \frac{2\omega\kappa_\omega}{u_{ph}} = (\frac{2n_{e,c}e_c^2\langle\langle\tau_e\rangle\rangle\omega}{\epsilon_\mathrm{o}\epsilon_em_eu_{ph}^2})^{1/2} </math> (''n<sub>e,c</sub>'': number density of conduction electrons, <math>\langle\langle\tau_e\rangle\rangle</math>: average momentum electron relaxation time, ''ε''<sub>o</sub>: [[Vacuum permittivity|free space electrical permittivity]])
|-
| Direct-band absorption (semiconductor) || <math> \frac{e_c^2|\langle\varphi_v|e_c\mathbf{x}|\varphi_c\rangle|^2D_{ph-e}[f_e^\mathrm{o}(E_{e,v})-f_e^\mathrm{o}(E_{e,c})]}{\epsilon_\mathrm{o}^2m_{e,e}^2u_{ph}n_\omega\omega} </math> (''n<sub>ω</sub>'': index of refraction, ''D<sub>ph-e</sub>'': joint density of states)
|-
| Indirect-band absorption (semiconductor) || with phonon absorption: <math> \frac{a_{ph\mathrm{-}e\mathrm{-}p,a}(\hbar\omega-\Delta E_{e,g}+\hbar\omega_{p})^2}{\mathrm{exp}(\hbar\omega_p/k_\mathrm{B}T)-1} </math> (''a<sub>ph-e-p,a</sub>'' phonon absorption coupling coefficient, Δ''E<sub>e,g</sub>'': bandgap, ''ω<sub>p</sub>'': phonon energy ) <br /> with phonon emission: <math> \frac{a_{ph-e-p,e}(\hbar\omega-\Delta E_{e,g}-\hbar\omega_{p})^2}{1-\mathrm{exp}(-\hbar\omega_p/k_\mathrm{B}T)} </math> (''a<sub>ph-e-p,e</sub>'' phonon emission coupling coefficient)
|}

== See also ==
*[[Energy transfer]]		
*[[Mass transfer]]
*[[Heat transfer]]	
*[[Energy transformation|Energy transformation (Energy conversion)]]

*[[Thermal physics]]
*[[Thermal science]]
*[[Thermal engineering]]

== References ==
{{Reflist}}

[[Category:Heat transfer]]
[[Category:Thermodynamics]]
[[Category:Condensed matter physics]]