Role of defects, resonances, anharmonicities and electron–phonon scattering processes on thermal conductivity of YBa2Cu3O7−δ

2016 ◽  
Vol 30 (26) ◽  
pp. 1650360 ◽  
Author(s):  
Vinod Ashokan ◽  
B. D. Indu ◽  
A. Kr. Dimri
Keyword(s):  
Thermal Conductivity ◽  
Quantum Dynamics ◽  
Phonon Scattering ◽  
High Temperature Superconductors ◽  
Relaxation Times ◽  
Line Widths ◽  
Double Time ◽  
New Formulation ◽  
Callaway Model

In this work, thermal conductivity of high temperature superconductors (HTS) has been analyzed on the basis of modified Callaway model. In the new formulation, the relaxation times of various contributing processes have been observed in newer perspectives of electron and phonon line widths. To obtain line widths, the quantum dynamics of electron and phonon is carried out by using double time thermodynamic Green’s functions method via a general Hamiltonian. The outcome of this heuristic approach is utilized to successfully explain the spectacular behavior of thermal conductivity of HTS, and particularly in the vicinity of transition temperature.

THEORY OF THERMAL CONDUCTIVITY OF HIGH TEMPERATURE SUPERCONDUCTORS: A NEW APPROACH

2011 ◽  
Vol 25 (09) ◽  
pp. 663-678 ◽  
Author(s):  
VINOD ASHOKAN ◽  
B. D. INDU
Keyword(s):  
Thermal Conductivity ◽  
High Temperature ◽  
Phonon Scattering ◽  
High Temperature Superconductors ◽  
Relaxation Times ◽  
Impurity Scattering ◽  
Resonance Scattering ◽  
Perturbative Approach ◽  
Conductivity Curve ◽  
Line Widths

An ab initio formulation of relaxation times of various contributing processes have been observed with newer understanding in terms of electron and phonon line widths. This is dealt with the help of double time temperature-dependent Green's function via a non-perturbative approach using a crystal Hamiltonian which comprises of the effects of electrons, phonons, impurities, anharmonicities and interactions thereof. The frequency line widths is observed as an extremely sensitive quantity in the transport phenomena of high temperature superconductors (HTS) as a collection of a large number of scattering processes, namely: boundary scattering, impurity scattering, multi-phonon scattering, interference scattering, electron–phonon processes and resonance scattering. The behavior of electrons and phonons is then investigated to describe the thermal conductivity of a variety of HTS samples specially in the vicinity of transition temperature to successfully explain the spectacular dip region of thermal conductivity curve which was lacking in explanation earlier with a sound physical justification.

Lattice Thermal Conductivity Modelling of a Diatomic Nanoscale Material

2020 ◽  
Vol 10 (5) ◽  
pp. 602-609
Author(s):  
Adil H. Awad
Keyword(s):  
Thermal Conductivity ◽  
Lattice Thermal Conductivity ◽  
Phonon Scattering ◽  
Correction Term ◽  
Nanoscale Material ◽  
New Approach ◽  
Two Samples ◽  
Conductivity Modelling ◽  
Callaway Model

Introduction: A new approach for expressing the lattice thermal conductivity of diatomic nanoscale materials is developed. Methods: The lattice thermal conductivity of two samples of GaAs nanobeam at 4-100K is calculated on the basis of monatomic dispersion relation. Phonons are scattered by nanobeam boundaries, point defects and other phonons via normal and Umklapp processes. Methods: A comparative study of the results of the present analysis and those obtained using Callaway formula is performed. We clearly demonstrate the importance of the utilised scattering mechanisms in lattice thermal conductivity by addressing the separate role of the phonon scattering relaxation rate. The formulas derived from the correction term are also presented, and their difference from Callaway model is evident. Furthermore their percentage contribution is sufficiently small to be neglected in calculating lattice thermal conductivity. Conclusion: Our model is successfully used to correlate the predicted lattice thermal conductivity with that of the experimental observation.

THERMAL CONDUCTIVITY OF HIGH TEMPERATURE SUPERCONDUCTORS

1991 ◽  
Vol 05 (12) ◽  
pp. 2003-2035 ◽  
Author(s):  
MANUEL D. NUÑEZ REGUEIRO ◽  
DARÍO CASTELLO
Keyword(s):  
Thermal Conductivity ◽  
High Temperature ◽  
Transition Temperature ◽  
Phonon Scattering ◽  
High Temperature Superconductors ◽  
Low Energy ◽  
Disordered Materials ◽  
Superconducting Transition ◽  
Universal Pattern ◽  
Heat Carriers

We review and analyze the data on the thermal conductivity of both ceramic and single crystal samples of high temperature superconductors. A universal pattern can be extracted and interpreted in the following way: phonons are the main heat carriers in these materials, and in the high temperature range the thermal conductivity κ is almost constant due to phonon scattering against disorder; below the superconducting transition temperature κ increases as phonon scattering against carriers condensing into the superconducting state decreases and at still lower temperatures there is a region in which a T2 law is obeyed that most probably is due to resonant phonon scattering against low energy excitations, i.e. tunneling systems similar to those found in disordered materials. The origin of the relevant disorder is discussed.

PHONON HEAT CONDUCTIVITY OF InSb AND CdS

2011 ◽  
Vol 25 (10) ◽  
pp. 1409-1418 ◽  
Author(s):  
M. ATAULLAH ANSARI ◽  
VINOD ASHOKAN ◽  
B. D. INDU
Keyword(s):  
Thermal Conductivity ◽  
Heat Conductivity ◽  
Lattice Thermal Conductivity ◽  
Relaxation Times ◽  
Temperature Range ◽  
Callaway Model ◽  
Theory And Experiments ◽  
Good Agreement

The lattice thermal conductivity of InSb and CdS has been analyzed on the basis of the most acquiescent Callaway model in the temperature range 2–300.779 K and 2.296–283.565 K. To reinvigorate the effects of phonon anharmonicities, more rigorous expressions for the phonon–phonon interactions, resonance, impurity and interference scattering relaxation times have been introduced to theoretically justify the experimentally observed results. A fairly good agreement between theory and experiments has been presented.

The nature and role of free radicals in coal conversion processes as revealed by e.s.r. studies

1981 ◽  
Vol 300 (1453) ◽  
pp. 83-87 ◽  
Keyword(s):  
Free Radicals ◽  
Relaxation Times ◽  
Systematic Investigation ◽  
Significant Information ◽  
Coal Conversion ◽  
Line Shapes ◽  
Solid Coal ◽  
Spin Resonance ◽  
Line Widths

Dr Ladner and Professor Pines in the previous two papers demonstrated how nuclear magnetic resonance can provide significant information on the structure of coals and coal extracts. We wish to provide evidence that electron spin resonance (e.s.r.) also can yield significant information on coal structure and coal conversion processes. Free radicals in coal have been known for several years (Uebersfeld et al . 1954; Ingram et al . 1954). In our own research group, we have during the last several years carried out a systematic investigation of the nature and role of free radicals in coals, coal components and in coal conversion processes, with the use of e.s.r. techniques (Petrakis & Grandy 1978, 1980 a, b : Grandy & Petrakis 1979; Petrakis et al . 1980). For the most part, we have studied residual free radicals, but very recently we have also constructed a special cavity that allows the study of free radicals under liquefaction and pyrolysis conditions. Professor Pines indicated in the previous paper that the very observation of the carbon-13 n.m.r. spectra of solid coal is possible because of the effects that free radicals in coal have on the relaxation times of the carbon-13 nuclei. E.s.r. allows the study of the free radicals in coal as they pre-exist in coals and during and after conversion. The significant e.s.r. parameters in these studies are spin concentrations, g values, line shapes and line widths.

scholarly journals Influence of Te Vacancies on the Thermoelectric Properties of n-type Cu0.008Bi2Te2.7-xSe0.3

2020 ◽  
Vol 58 (10) ◽  
pp. 721-727
Author(s):  
Yerim Yang ◽  
TaeWan Kim ◽  
Seokown Hong ◽  
Jiwoo An ◽  
Sang-il Kim
Keyword(s):  
Thermal Conductivity ◽  
Transport Properties ◽  
Point Defect ◽  
Thermoelectric Properties ◽  
Thermal Transport ◽  
Phonon Scattering ◽  
Band Model ◽  
Total Thermal Conductivity ◽  
Thermal Transport Properties ◽  
Callaway Model

In this study, we report the influence of Te vacancy formation on the thermoelectric properties of n-type Cu0.008Bi2Te2.7Se0.3 alloys, including their electronic and thermal transport properties. Te-deficient Cu0.008Bi2Te2.7-xSe0.3 (x = 0, 0.005, 0.01 and 0.02) samples were systematically synthesized and characterized. Regarding electronic transport properties, carrier concentration was increased with Te vacancies, while carrier mobility was maintained. As a result, the electrical conductivity significantly increased while the Seebeck coefficient reduced moderately, thus, the power factor was enhanced from 3.04 mW/mK<sup>2</sup> (pristine) to 3.22 mW/mK<sup>2</sup> (x = 0.02) at 300 K. Further analysis based on a single parabolic band model revealed that the weighted mobility of the conduction band increased, which is favorable for electron transport, as Te vacancies were generated. Regarding thermal transport properties, lattice thermal conductivity decreased with Te vacancies due to additional point defect phonon scattering, however, total thermal conductivity increased due to larger electronic contribution as Te vacancies increased. Analysis using the Debye-Callaway model suggests that the phonon scattering by the Te vacancies is as efficient as the substitution point defect scattering. Consequently, the thermoelectric figure of merit zT increased at all temperatures for x = 0.005 and 0.01. The maximum zT of 0.95 was achieved for Te-deficient Cu0.008Bi2Te2.69Se0.3 (x = 0.01) at 400 K.

Thermal Conductivity of Waste Forms and Geologic Media

1982 ◽  
Vol 15 ◽  
Author(s):  
R. O. Pohl ◽  
J. W. Vandersande
Keyword(s):  
Thermal Conductivity ◽  
Grain Boundaries ◽  
High Temperatures ◽  
Low Temperatures ◽  
Phonon Scattering ◽  
Lamellar Structures ◽  
Waste Forms ◽  
Thermal Conductivities ◽  
Geologic Media

ABSTRACTIn order to predict the range of thermal conductivities to be expected in waste forms and in geologic media, an understanding of the pertinent phonon scattering processes is required. It has been shown that grain boundaries in polycrystalline media are unimportant at low temperatures relative to lamellae which arise from twinning, exsolution, or foreign inclusions within the grains. The possible role of lamellar structures on the conductivity at high temperatures will be discussed.

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