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Introduction of Graphene
Published in Abhay Kumar Singh, Tien-Chien Jen, Chalcogenide, 2021
Abhay Kumar Singh, Tien-Chien Jen
Thermal management is one of the key factors for reliable performance of electronic devices at a time when considerable amount of heat is generated during the operation. Since graphene can be a major component in electronic devices in the future due to its high thermal conductivity (up to 5000 W/mK) at room temperature, in the case of single layer defect-free graphene [116]. Their strong CAC covalent bonds and phonon scattering can also contribute in high thermal conductivity performance. It was also reported that the thermal conductivity of pure single-layer graphene is much higher than the past reported thermal conductivity of other carbon allotropes at room temperature, such as carbon nanotubes (3000 W/mK for MWCNT and 3500 W/mK for SWCNT) [118, 119]. The graphene thermal conductivity may also be affected by factors such as defects, edge scattering and isotopic doping [120, 121]. Usually, these factors can harm the conductivity due to phonon scattering at defect and phonons modes localization due to doping.
Benefits of the Selection and Use of High Entropy Alloys for High-Temperature Thermoelectric Applications
Published in T.S. Srivatsan, Manoj Gupta, High Entropy Alloys, 2020
Where L is the Lorentz factor (2.4·10−8J2K−2C−2) for free electrons, and e the electronic charge [2]. From the Wiedemann–Franz law, it is therefore evident that on the one hand, the κe will be difficult to minimize efficiently since it is coupled directly to S and σ, while on the other hand, the κLatt, which is directly related to the phonon transport of heat, can be separately minimized. This becomes true since the majority of phonons exist at longer wavelengths (lower frequencies) than the electrons, and can therefore be affected by defects that are not affecting the electrons to the same extent; hence, more directed phonon scattering can be achieved while maintaining a high electron mobility through the material. It is therefore important to find materials that are able to decouple these terms.
Zinc Oxide (ZnO)
Published in Zbigniew Galazka, Transparent Semiconducting Oxides, 2020
Thermal conductivity of hexagonal ZnO is described by the second-order tensor with two independent thermal conductivity coefficients perpendicular λ2016c and parallel λ||c to the c-axis. Wolf and Martin [247] reported thermal conductivities (with a steady-state longitudinal heat-flow apparatus) of bulk ZnO crystals (obtained by the hydrothermal and vapor phase methods) in a temperature range of 3.5–300 K with a maximum value of about 1000 W/mK below 50 K. At 300 K, the thermal conductivity is just below 100 W/mK. Also, the ratio of λ||c/λ2016c was found to be 1.15, indicating a minor anisotropy of the thermal conductivity. The thermal conductivity is limited by phonon–phonon scattering at elevated and high temperatures.
Tuning the electrical, thermal, and mechanical properties of SiC-BN composites using sintering additives
Published in Journal of Asian Ceramic Societies, 2020
Rohit Malik, Young-Wook Kim, Kwang Joo Kim, B. V. Manoj Kumar
Figure 9 shows the thermal diffusivity and heat capacity of SiC-4 vol% BN composites sintered with various additive systems. Thermal diffusivity is a material property that determines the rate of heat transfer. Because the heat is primarily conducted via phonons in ceramics, the rate of heat transfer or thermal diffusivity is determined by the amount of phonon scattering, which includes defect-phonon scattering, phonon-grain-boundary scattering, and temperature-activated phonon-phonon scattering. A high density of phonon scattering sites leads to high phonon scattering and deteriorates the thermal diffusivity. In the present study, the thermal diffusivity of SiC-4 vol% BN composites decreased from 35.3 mm2/s to 31.7 mm2/s with a decrease in grain size from 5.4 ± 2.8 µm to 3.7 ± 1.0 µm for SYbM and SAAY, respectively, due to the increased phonon scattering at grain boundaries. Collins et al. [41] reported an increase in thermal diffusivity from 132 mm2/s to 338 mm2/s with an increase in grain size from 6.8 to 17.2 µm for the chemical-vapor-deposited β-SiC, due to the decrease in the phonon-grain boundary scattering with an increase in grain size. The measured heat capacities for SYSc, SYbM, SYbC, and SAAY were 0.742, 0.684, 0.652, and 0.704 J∙g−1 K−1, respectively.
Hydrodynamic Phonon Transport Perpendicular to Diffuse-Gray Boundaries
Published in Nanoscale and Microscale Thermophysical Engineering, 2019
Runqing Yang, Shengying Yue, Bolin Liao
Phonons are major carriers of heat in semiconductors and insulators, and the scattering between phonons is usually the main source of thermal resistance in single crystals of these materials [1]. Phonon–phonon scattering can be classified into two types: the momentum-conserving normal scattering processes and the momentum-destroying Umklapp scattering processes. It is understood [2] that Umklapp scatterings act as momentum sinks in the bulk of a material and directly contribute to the thermal resistance. Whereas normal scattering processes do not directly create thermal resistance per se, they perturb the phonon distributions and indirectly affect the thermal transport in presence of Umklapp processes [3, 4]. In different materials and under different external conditions, the dominating phonon–phonon scattering mechanism can vary, giving rise to distinct regimes of heat conduction [5–7]. When the characteristic size of the sample is smaller than the intrinsic phonon mean free path due to phonon–phonon scatterings, as is relevant in microelectronic devices [8] and nanostructured thermoelectric materials [9], the extrinsic phonon-boundary scattering dominates and the phonon transport approaches the ballistic regime [10, 11]. In macroscopic samples, where the intrinsic phonon–phonon scattering becomes the leading phonon scattering channel, the relative strength of the normal processes and Umklapp processes determines the characteristics of the phonon transport. In most three-dimensional bulk materials above their Debye temperature, the Umklapp processes prevail and the phonon transport is diffusive, where no net drift flow of phonons can be established and maintained due to the constant dissipation of the phonon momenta, leading to the familiar Fourier's law of heat conduction.
Thermoelectric materials and applications for energy harvesting power generation
Published in Science and Technology of Advanced Materials, 2018
Ioannis Petsagkourakis, Klas Tybrandt, Xavier Crispin, Isao Ohkubo, Norifusa Satoh, Takao Mori
Theoretical work to improve the thermoelectric conversion efficiency owing to quantum effects appearing in low-dimensional structures such as superlattices, proposed by Dresselhaus et al. [106], motivates thin-film researchers to investigate thermoelectric materials. The use of quantum-confinement phenomena enhances Seebeck coefficient (S) and control Seebeck coefficient and electrical conductivity (σ) somewhat independently. Phonon scattering becomes more effectively induced by numerous interfaces, resulting in lower thermal conductivity. The thermoelectric properties of superlattices based on various materials have been investigated. Enhanced Seebeck coefficient and power factor (S2σ) is demonstrated using (001) oriented Si/Ge superlattices reported by Koga et al. [129]. Venkatasubramanian et al. reported figure of merit (ZT) enhancement in superlattices composed of Bi2Te3 and Sb2Te3 layers, which are well-known high performance thermoelectric materials [130]. Reduction of lattice thermal conductivity in PbTe/PbTe0.75Se0.25 superlattices was found by Caylor et al. [131]. There are some oxides with non-toxic elements that are promising for use as thermoelectric materials. Ohta et al. demonstrated the enhancement of Seebeck coefficient in SrTiO3/Sr(Ti,Nb)O3 superlattices [132]. The thermoelectric properties of one-dimensional nanowires have also been studied [133]. One dimensional quantum confinement offers sharper density of states (DOS) of electrons than higher dimensional quantum confinements. Seebeck coefficient can be enhanced due to the sharper DOS [134]. The quantum confinement effects have not been confirmed experimentally, except maybe for a few works. Tian et al. fabricated InAs nanowires by chemical vapor deposition (CVD), and observed oscillations in the Seebeck coefficient and power factor concomitant with the stepwise conductance increases due to the one dimensional quantum confinements in InAs nanowires (Figure 15) [135].