Effects of acoustic softening on thermal conductivity beyond group velocity

2020 ◽  
Vol 127 (20) ◽  
pp. 204302
Author(s):  
M. D. Gerboth ◽  
D. G. Walker
Keyword(s):  
Thermal Conductivity ◽  
Group Velocity ◽  
Acoustic Softening

Umklapp collisions and thermal conductivity

2021 ◽  
pp. 309-321
Author(s):  
Geoffrey Brooker
Keyword(s):  
Thermal Conductivity ◽  
Heat Conduction ◽  
Heat Flow ◽  
Group Velocity ◽  
Thermodynamic Equilibrium ◽  
Reciprocal Lattice ◽  
Reciprocal Lattice Vector ◽  
Momentum Balance ◽  
Lattice Vector ◽  
Phonon Group Velocity

“Umklapp collisions and thermal conductivity” deals with heat conduction in a dielectric solid. Collisions of phonons are divided into Umklapp and normal according as a reciprocal lattice vector is or is not involved in the phonon momentum balance. A local temperature is defined by appeal to local thermodynamic equilibrium. An equilibrium phonon distribution can be off-centred, yet non-decaying, if the only collisions are “normal”, conserving the total phonon momentum. Then heat flow does not decay, even if a representative collision reverses the phonon group velocity. Conversely, in an Umklapp collision it is the non-conservation of phonon momentum that causes heat flow to decay.

THE EFFECT OF SPONTANEOUS AND PIEZOELECTRIC POLARIZATION ON THERMAL CONDUCTIVITY OF InN

2013 ◽  
Vol 27 (09) ◽  
pp. 1350031 ◽  
Author(s):  
BIJAYA KUMAR SAHOO ◽  
SUSANT KUMAR SAHOO ◽  
SUKDEV SAHOO
Keyword(s):  
Thermal Conductivity ◽  
Thermal Properties ◽  
Group Velocity ◽  
Room Temperature ◽  
Phonon Scattering ◽  
Polarization Property ◽  
Optical And Electrical Properties ◽  
High Quality ◽  
Phonon Group Velocity ◽  
Debye Frequency

The spontaneous (SP) and piezoelectric (PZ) polarization present in the wurtzite III nitrides influence the optical and electrical properties of these materials. The effects of SP and PZ polarization on thermal properties of III nitrides have yet to be investigated. Here we have investigated the SP and PZ effects on thermal conductivity of InN . Inclusion of polarization property modifies the group velocity of phonons. The combined phonon scattering rates and thermal conductivity k of InN are calculated using modified phonon group velocity, Debye frequency and Debye temperature. Without SP and PZ polarization, the room temperature thermal conductivity of InN is found to be 132.55 W/m.K. Inclusion of SP and PZ polarization property enhances the room temperature thermal conductivity from 132.55 to 134.32 W/m.K. Our predicted thermal conductivity values are closer to the recent experimental value 120 W/m.K measured by Levander et al. for a high quality irradiated InN films at room temperature.

Lattice Dynamics Study of Anisotropic Heat Conduction in Superlattices

2000 ◽  
Vol 626 ◽  
Author(s):  
B. Yang ◽  
G. Chen
Keyword(s):  
Thermal Conductivity ◽  
Group Velocity ◽  
Lattice Dynamics ◽  
Phonon Scattering ◽  
Diffuse Interface ◽  
Phonon Confinement ◽  
Dynamics Model ◽  
Phonon Group Velocity ◽  
Anisotropic Heat Conduction ◽  
The Cross

ABSTRACTPast studies on the thermal conductivity suggest that phonon confinement and the associated group velocity reduction are the causes of the observed drop in the cross-plane thermal conductivity of semiconductor superlattices. In this work, we investigate the contribution of phonon confinement to the in-plane thermal conductivity of superlattices and the anisotropic effects of phonon confinement on the thermal conductivity in different directions, using a lattice dynamics model. We find that the reduced phonon group velocity due to phonon confinement may account for the dramatic reduction in the cross-plane thermal conductivity, but the in-plane thermal conductivity drop, caused by the reduced group velocity, is much less than the reported experimental results. This suggests that the reduced relaxation time due to diffuse interface phonon scattering, dislocation scattering, etc, should make major contribution to the in-plane thermal conductivity reduction.

Tuning thermal conductivity of crystalline polymer nanofibers by interchain hydrogen bonding

RSC Advances ◽  
2015 ◽  
Vol 5 (107) ◽  
pp. 87981-87986 ◽  
Author(s):  
Lin Zhang ◽  
Morgan Ruesch ◽  
Xiaoliang Zhang ◽  
Zhitong Bai ◽  
Ling Liu
Keyword(s):  
Thermal Conductivity ◽  
Hydrogen Bonding ◽  
Hydrogen Bonds ◽  
Group Velocity ◽  
Thermal Conduction ◽  
Crystalline Polymer ◽  
Polymer Chains ◽  
Torsional Motion ◽  
Polymer Nanofibers

Interchain hydrogen bonds enhance thermal conduction in crystalline polymer nanofibers by confining torsional motion of polymer chains and by increasing the group velocity of phonons.

A C20-based 3D carbon allotrope with high thermal conductivity

2020 ◽  
Vol 22 (1) ◽  
pp. 306-312 ◽  
Author(s):  
Yupeng Shen ◽  
Fancy Qian Wang ◽  
Jie Liu ◽  
Qian Wang
Keyword(s):  
Thermal Conductivity ◽  
Group Velocity ◽  
Structural Unit ◽  
High Thermal Conductivity ◽  
Carbon Allotrope ◽  
Large Group

A new 3D carbon allotrope composed of C20 as its structural unit is proposed, which is stable and possesses high thermal conductivity due to the weak anharmonicity and large group velocity of phonons in this structure.

scholarly journals Uniform Strain-Dependent Thermal Conductivity of Pentagonal and Hexagonal Silicene

2021 ◽  
Vol 8 ◽  
Author(s):  
Huake Liu ◽  
Guangzhao Qin ◽  
Ming Hu
Keyword(s):  
Thermal Conductivity ◽  
Detailed Analysis ◽  
Group Velocity ◽  
Phonon Transport ◽  
Two Dimensional ◽  
Transport Behavior ◽  
Out Of Plane ◽  
The Difference ◽  
Silicon Based ◽  
Strain Dependent

Two-dimensional (2D) pentagonal monolayer structures have shown promising characteristics and fascinating physical and chemical properties. The disparate strain-dependent thermal conductivity of two-dimensional penta-structures was reported, but the difference between the silicon-based pentagonal and hexagonal structures is barely researched. In this work, based on first-principles calculations, we studied the strain-modulated phonon transport behavior of two 2D pentagonal (penta-SiH and bilayer penta-Si) and one hexagonal silicene structures (H-silicene), of which the penta-SiH and H-silicene mean the structures are hydrogenated for the purpose of thermodynamical stability. We found that the silicon-based pentagonal structure also presented a different strain-dependent thermal conductivity from other pentagonal materials, such as penta-graphene, penta-SiC, or penta-SiN. Moreover, even with the similar strain-dependent thermal transport behavior in penta-SiH and bilayer penta-silicene, we find that the governing mechanism is still different. For both pentagonal silicene structures, the thermal conductivity presents a large improvement at first as the tensile strain increases from 0 to 10% and then stabilizes with a strain larger than 10%. A detailed analysis shows that the in-plane modes contributed the most part to the group velocity enhancement under strains in penta-SiH which is opposite from the bilayer penta-graphene, although the phonon group velocity and phonon lifetime of both structures increase with applied strain. On the other hand, a similarity was found in pentagonal silicene and hexagonal silicene despite the differences in geometry structures. Furthermore, based on the detailed analysis between the pentagonal (penta-SiH) and hexagonal silicene structures (H-silicene), the difference in out-of-plane phonon scattering cannot be ignored: different major scattering channels of the out-of-plane flexural modes result in different thermal conductivity sensitivity to strains, and the disparity in anharmonicity leads to different thermal conductivity under no strain.

scholarly journals Lattice thermal conductivity in isotope diamond asymmetric superlattices

2021 ◽  
Author(s):  
Hsu Kai Weng ◽  
Akira NAGAKUBO ◽  
Hideyuki Watanabe ◽  
Hirotsugu OGI
Keyword(s):  
Thermal Conductivity ◽  
Specific Heat ◽  
Group Velocity ◽  
Lattice Thermal Conductivity ◽  
Trade Off ◽  
Number Of Layers ◽  
Phonon Group Velocity ◽  
Wide Range ◽  
Superlattice Structures ◽  
Superlattice Period

Abstract We study lattice thermal conductivity of isotope diamond superlattices consisting of 12C and 13C diamond layers at various superlattice periods. It is found that the thermal conductivity of a superlattice is significantly deduced from that of pure diamond because of the reduction of the phonon group velocity near the folded Brillouin zone. The results show that asymmetric superlattices with different number of layers of 12C and 13C diamonds exhibit higher thermal conductivity than symmetric superlattices even with the same superlattice period, and we find that this can be explained by the trade-off between the effects of phonon specific heat and phonon group velocity. Furthermore, impurities and imperfect superlattice structures are also found to significantly reduce the thermal conductivity, suggesting that these effects can be exploited to control the thermal conductivity over a wide range.

Interferometric (Fourier-Spectroscopic) Measurement of Electron Energy Distributions

1990 ◽  
Vol 48 (2) ◽  
pp. 110-111 ◽  
Author(s):  
F. Hasselbach ◽  
A. Schäfer
Keyword(s):  
Group Velocity ◽  
Wave Packets ◽  
Index Of Refraction ◽  
Electron Wave ◽  
Fringe Contrast ◽  
Faraday Cage ◽  
Optical Index ◽  
Wien Filter ◽  
Coherent Wave ◽  
Electric And Magnetic Fields

Möllenstedt and Wohland proposed in 1980 two methods for measuring the coherence lengths of electron wave packets interferometrically by observing interference fringe contrast in dependence on the longitudinal shift of the wave packets. In both cases an electron beam is split by an electron optical biprism into two coherent wave packets, and subsequently both packets travel part of their way to the interference plane in regions of different electric potential, either in a Faraday cage (Fig. 1a) or in a Wien filter (crossed electric and magnetic fields, Fig. 1b). In the Faraday cage the phase and group velocity of the upper beam (Fig.1a) is retarded or accelerated according to the cage potential. In the Wien filter the group velocity of both beams varies with its excitation while the phase velocity remains unchanged. The phase of the electron wave is not affected at all in the compensated state of the Wien filter since the electron optical index of refraction in this state equals 1 inside and outside of the Wien filter.

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