Photon thermal conductivity

The equation of radiative transfer will not be solved here since solutions to some approximations of the equation are well known. In photon radiation, it has served as the framework for photon radiative transfer. It is well known that in the optically thin or ballistic photon limit, one gets the heat flux as q = g T[ - T ) from this equation for radiation between two black surfaces [13]. For the case of phonons, this is known as the Casimir limit. In the optically thick or diffusive limit, the equation reduces to q = -kpVT where kp is the photon thermal conductivity. The same results can be derived for phonon radiative transfer [14,15]. 

When a substance is transparent to visible light, such as single crystals of simple inorganic solids which do not contain uansition metal ions, or some glasses, anotlier significant component in the thermal conductivity is the transmission of photons in the infra-red region, which becomes more important with... 

Coupling the motion of the mosaic cell (TLS and boson peak) to phonons is necesssary to explain thermal conductivity therefore the interaction effects discussed later follow from our identification of the origin of amorphous state excitations. The emission of a phonon followed by its absorption by another cell will give an effective interaction, in the same way that photon exchange leads to... 

Amorphous materials have no long-range structural order, so there is no continuous lattice in which atoms can vibrate in concert in order for phonons to propagate. As a result, phonon mean free paths are restricted to distances corresponding to interatomic spacing, and the (effective) thermal conductivity of (oxide) glasses remains low and increases only with photon conduction (Figure 8.2). 

In the case of solid crystalline oxides, thermal conductivity decreases with increasing temperature but begins to rise above 1500— 1600 °C because transmission of heat by radiation (photons) begins to take a significant part besides the conduction of heat (phonon mechanism). In completely transparent materials (the coefficient of absorption a = O), no interaction with the radiation occurs in an opaque body (a = oo) the heat is transferred by conduction alone. With translucent materials, each element of the substance absorbs some of the incident radiation, and emits simultaneously,This internal radiation mechanism of heat transmission is characteristic for glasses. At high temperatures, a considerable proportion of heat is therefore transmitted by radiation the so-called apparent thermal conductivity is a sum of true conductivity with radiation conductivity ... 

Thermal conductivity and expansion are important properties of adhesives used in electronics. Both properties influence the performance of computer chips. Generally, the chip has a protective cover which is attached by an adhesive. The adhesive bond must be maintained during thermally induced movement in the chip. The chip is bonded to its base with an adhesive which must also take thermal movement and, in addition, transfer heat from the chip. Two epoxy adhesives were used in the study silica filled epoxy (65 and 75 wt% SiO2 epoxy) and epoxy containing 70 wt% Ag. Figure 15.6 shows their thermal conductivities. The behavior of both adhesives is completely different. The silver filled adhesive had a maximum conductivity at about 6()"C whereas the maximum for SiOz filled adhesive was 120"C. The Tg of both adhesives was 50 and 160 C, respectively. Below its Tg, the thermal conductivity of the adhesive increases at the expense of increased segmental motions in the chain molecules. Above the Tg the velocity of photons rapidly decreases with increasing temperature and the thermal conductivity also decreases rapidly. 

A radioisotope battery is one of the choice for energy source of meteorological obseiwation and development of undersea and space[l]. We have considered a strontium-90 (half-life 28.8y) heat-source model of a radioisotope battery and improved it in two aspects—radiation dose reduction and improvement of thermal conductivity—adding graded structure to the model[2]. The present study reports the dose reduction of bremsstrahlung photons from -ray of - "Sr and its daughter nuclide yttrium-90. The calculation was carried out by a continuous energy Monte Carlo code, MCNP 4A[3]. 

Not only is the "grain" of AgBr a remarkable amplifier, it has to function as a coincidence counter. If grains responded to the generation of single thermal conduction electrons, false development of unexposed grains would quickly follow (1). The action of a single photon (or a thermal event) produces one elec-... 

The first three represent true absorption because in each case a photon is absorbed. The fourth only changes the direction of a photon. The contribution from thermal conduction by electrons must also be considered. 

In very dense stellar material, the mean free path of the photon becomes so small that it is no longer the most efficient carrier of energy. Thermal conduction by electrons becomes the dominant transport mechanism, controlled by a conductive opacity Cond in much the same way as conduction by photons is controlled by the radiative opacity. From the diffusion approximation we obtain... 

The characteristics of the energy carrier are included in the heat capacity C, velocity v, and the mean free path i. Neglecting photons for now, the thermal conductivity can be written as... 

Phonon velocity is constant and is the speed of sound for acoustic phonons. The only temperature dependence comes from the heat capacity. Since at low temperature, photons and phonons behave very similarly, the energy density of phonons follows the Stefan-Boltzmann relation oT lvs, where o is the Stefan-Boltzmann constant for phonons. Hence, the heat capacity follows as C T3 since it is the temperature derivative of the energy density. However, this T3 behavior prevails only below the Debye temperature which is defined as 0B = h( DlkB. The Debye temperature is a fictitious temperature which is characteristic of the material since it involves the upper cutoff frequency ooD which is related to the chemical bond strength and the mass of the atoms. The temperature range below the Debye temperature can be thought as the quantum requirement for phonons, whereas above the Debye temperature the heat capacity follows the classical Dulong-Petit law, C = 3t)/cb [2,4] where T is the number density of atoms. The thermal conductivity well below the Debye temperature shows the T3 behavior and is often called the Casimir limit. 

The rapid increase in the thermal conductivity of liquid selenium with temperature can be attributed to the photon component of the thermal conductivity. For liquids with a small absorption coefficient, this radiation term should rise as a third power of the absolute temperature. From the results of the thermal conductivity data we can, therefore, get information about two optical parameters, i.e., the optical absorption coefficient, a, and the refractive index, n, in the form a/n. ... 

Thermal conductivity and electrical conductivity The movement of mobile electrons around positive metallic cations makes metals good conductors. The delocalized electrons move heat from one place to another much more quickly than the electrons in a material that does not contain mobile electrons. Mobile electrons easily move as part of an electric current when an electric potential is applied to a metal. These same delocalized electrons interact with light, absorbing and releasing photons, thereby creating the property of luster in metals. 

Berman, R. (1979) in The Properties of Diamond, edited by J.E. Field, Academic Press, London, p. 3. Discusses the thermal conductivity of diamond and the effect different impurities have on this property. Kingery, W.D., Bowen, H.K., and Uhlmann, D.R. (1976) Introduction to Ceramics, 2nd edition, Wiley, New York, pp. 583-645. A very detailed chapter on thermal properties. The discussion of photon conductivity and the thermal properties of glasses are covered in more depth than we do. 

Photovoltaic and photoconductive effects result from direct conversion of incident photons into conducting electrons within a material. The two effects differ in the method of sensing the photoexcited electrons electrically. Detectors based on these effects are called photon detectors, because they convert photons directly into conducting electrons no intermediate process is involved, such as the heating of the material by absorption of photons in a thermal detector which causes a change of a measurable electrical property. 

In addition to these, there has been considerable amount of research danonstrat-ing the use of NCD thin film in packaging and thermal applications [47]. Active devices, like transistors, have been mounted on diamond carriers thereby improving their performance due to better thermal management. The high thermal conductivity exhibited by thin film diamond has been a positive impact in packaging approaches in the MEMS devices, as well. Furthermore, NCD has been used for many optical applications to fabricate resonators [48], two-dimensional photonic crystals [49], UV transparent electrodes on SiC [50] and in some sensor applications [51],... 

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