Scattering phonon-defect

The third term in Eq. 7, K, is the contribution to the basal plane thermal resistance due to defect scattering. Neutron irradiation causes various types of defects to be produced depending on the irradiation temperature. These defects are very effective in scattering phonons, even at flux levels which would be considered modest for most nuclear applications, and quickly dominate the other terms in Eq. 7. Several types of in-adiation-induced defects have been identified in graphite. For irradiation temperatures lower than 650°C, simple point defects in the form of vacancies or interstitials, along with small interstitial clusters, are the predominant defects. Moreover, at an irradiation temperatui-e near 150°C [17] the defect which dominates the thermal resistance is the lattice vacancy. 

These carriers of heat do not move balistically from the hotter part of the material to the colder one. They are scattered by other electrons, phonons, defects of the lattice and impurities. The result is a diffusive process which, in the simplest form, can be described as a gas diffusing through the material. Hence, the thermal conductivity k can be written as ... 

The main scattering processes limiting the thermal conductivity are phonon-phonon (which is absent in the harmonic approximation), phonon defect, electron-phonon, electron impurity or point defects and more rare electron-electron. For both heat carriers, the thermal resistivity contributions due to the various scattering processes are additive. For... 

Since the number of phonons increases with temperature, the electron-phonon and phonon-phonon scattering are temperature dependent. The number of defects is temperature independent and correspondingly, the mean free path for phonon defect and electron defect scattering does not depend on temperature. 

Since at low temperatures, the electrical conductivity is determined by the scattering on defects, whereas at room temperature it depends on the scattering on phonons, the RRR is a measure of the limiting defect scattering. It indicates how pure a material is. The RRR of particularly pure materials can be as high as 104. 

Discontinuities in the lattice such as vacancies, impurities, or grain boundaries also act to scatter phonon propagation, hence a lower thermal conductivity is expected in solids containing these defects at cryogenic temperatures. Whichever mecha-... 

The thermal conductivity of a pure metal is lowered by alloying, whether the alloy formed is a single phase (solid solution) or multiphase mixture. There are several reasons for this. First, electrons are scattered by crystal imperfections and solute atoms (electron-defect scattering). Second, a substantial portion of the thermal conductivity in alloys, in contrast to that of pure metals, is by phonons, Kph (phonons are the sole contribution in electrically insulating solids) and phonons are also scattered by defects. Finally, electron-phonon interactions limit both Kei and Kp. ... 

In pure metals the electron-phonon interaction is inversely proportional to the number of thermal phonons Te-ph 7 [1]. This result is valid for pure limit 97- / 1 qr is thermal phonon wave vector, / is the electron mean free path) [2,3]. In dirty limit qt / 1) electrons mostly scatter from defects and impurities and the electron-phonon interaction demonstrates more complicated behavior. According to the theoretical analysis made by Thouless [4] and Reizer [3] the relaxation time is proportional to T Te.pf x T ) in the case of full phonon drag of scattering centers. 

Thermal conductivity of irradiated SiC decreases due to the accumulation of point defects that effectively scatter phonons in SiC. In the 500-800 C range of interest, the thermal conductivity of conventional SiC/SiC composites at 1 dpa could be in the range required for the FCI As the point defects accumulate, meeting the requirement of low conductivity becomes much easier. So as far as the FCI application is concerned, the critical time for SiC/SiC composites is the beginning of... 

The indices (e,d), (e,ph), (e,magn) indicate free-carrier scattering by defects, phonons and magnons, respectively. 

If a(300 K) is not limited by phonons, defect scattering or interchain processes due to the finite conjugation length could be responsible for the resistivity of the samples. This is in agreement with the observed [Pg.83]

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