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Advanced Sputtering Technologies of Flexible Hard Nanocoatings
Published in Sam Zhang, Jyh-Ming Ting, Wan-Yu Wu, Protective Thin Coatings Technology, 2021
The thermal conductivity is dominated by phonons at high temperatures and is given by the specific heat C per unit volume, the sound velocity v, and the phonon mean free path l as κ ≈ Cvl so τc≈10aB2/vl
Elemental Semiconductors
Published in Lev I. Berger, Semiconductor Materials, 2020
Lattice dynamics of a crystal is particularly interesting because the information regarding the character of interatomic interactions and magnitude and orientation of interatomic forces, together with the data on chemical composition of the crystal, determines a majority of its properties: thermal, mechanical, optical, electrical, etc. The experimental methods of the phonon dispersion study are, most commonly, infrared and Raman spectroscopy and neutron scattering. Stoneham3.163 reviewed the empirical models of interatomic forces which include the bond polarizabilities model introduced by Smith,3.171 the valence force potentials model (see, e.g., Musgrave and Pople3.172 and Bell3.173), the shell model (Dolling and Cowley3.174 and Peckman3.175), and the bond charge models (see, e.g., Phillips3.176 and Go et al.3.177). The inelastic scattering of slow neutron experiments on diamonds (Warren et al.3.179,3.180) was used to determine the shape of the phonon dispersion curves. Their results, together with the magnitudes calculated on the basis of the shell model, are presented in Figure 3.17. The data are in satisfactory agreement with the magnitudes of phonon frequency at specific points of the Brillouin zone, presented by Solin and Ramdas3.181 who used the Raman spectroscopy data (Table 3.3).
Detectors
Published in C. R. Kitchin, Astrophysical Techniques, 2020
Phonons are the quanta of sound (i.e., of the vibrational energy in a material). Like photons, their energies are related to their frequencies by Planck’s constant (h). Normal sound waves (like normal light beams) are formed from huge numbers of individual phonons – a 100-dB level sound has an intensity of 10 mW m−2 or about 1.5 × 1029 phonons s−1 m−2 at 100 Hz. However, atomic-level events, such as the elastic scattering of an atom’s nucleus by a DMP can excite individual phonons, and these in turn can be used in dark matter detectors.
An Investigation into the Roughness and Film Thickness Effects on the Interfacial Thermal Resistance
Published in Nanoscale and Microscale Thermophysical Engineering, 2023
Over the past decades, sustainable issues such as global warming and environmental protection have drawn great attention, prompting efforts to the development of renewable energy and waste energy recovery. Thermoelectric power generators (TEGs) can convert waste heat directly into electrical energy in an environmentally friendly way and therefore also receive attention. The efficiency of a TEG is largely determined by its material figure of merit [1], , where , , , and are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. Semiconductors are preferred as thermoelectric materials for their well electrical properties and low thermal conductivity. In semiconductors, thermal energy is transferred primarily by lattice vibration and secondarily by electrons. The vibrational waves propagate through crystals and the vibrational energy of atoms is quantized as phonons. Increasing doping or impurity concentration, embedding nanostructures/defects, adding grain boundaries/interfaces, etc. [2–4] are common ways to increase phonons’ scattering rates and consequently reduce the thermal conductivity, leading to an enhancement of the material figure-of-merit.
Understanding size and strain induced variabilities in thermal conductivity of carbon nanotubes: a molecular dynamics study
Published in Mechanics of Advanced Materials and Structures, 2022
Sushan Nakarmi, Vinu U. Unnikrishnan
The effect of the aforementioned parameters are also analyzed with respect to the study of dispersion relationship and phonon density of states during thermal transport under different ambient conditions. The thermal energy in the solid medium is carried by electrons and phonons. Electrons have a major contribution in heat conduction in metals whereas phonons (quantum of lattice vibration) are dominant heat carriers in insulators. In CNTs, phonons have a major influence on the thermal transport [24] and hence, electronic contribution can be neglected. In classical mechanics, phonon designates the normal modes of vibration where the energy carried by phonon with the frequency ω is given by Any lattice vibration can be assumed to be a constituent of (sum of) many sinusoidal waves. In a non-dispersive medium, every sinusoidal wave propagates at the same speed and hence there is no dispersion of vibrational waves. However, in a dispersive medium, different sinusoidal waves (i.e. waves with different wave number q) propagates at different speeds (ω). Thus, a fluctuation wave (or wave packet of energy) gets dispersed into multiple smaller waves. Phonon dispersion is the relationship between the frequency (ω) and wave vector (q) that represents normal modes of vibration in lattice dynamics.
Study on the interfacial thermal resistance between CNTs and Al with a TTM-MD model
Published in Molecular Physics, 2021
Gengler et al. [33] show that only phonons scattering considered in the simulation is one reason to explain the difference between simulation and experiment results. Therefore, there is also an electromagnetic contribution to the experimental result. Besides, there are many factors which account for the difference between them, such as surface roughness, different structures, topological characteristics and the contact stress [36, 37]. In simulation, two materials at the interface contact are ideally perfectly flat. However, the interface in the experiment would be much complicated. The interface would be no longer perfect and there are many defects or porosity [38]. Defects around the interface can scatter phonons and then prevent the transfer of heat flux. The creation of covalent bonds between CNT and substrate would be decreased due to interface defects, which could worsen the ability of the heat transfer through the vibration of phonons or electrons–phonons coupling. The contact stress can also influence the value directly. The larger the stress used in the experiment, the better two material contacted at the interface. Thus, more covalent bonds can be created at the interface. Thermal resistance also can be affected by different contents and properties of impurity at the interface.