Influence of low frequency impurity modes in thermal conductivity

AbstractThermoelectrics enable waste heat recovery, holding promises in relieving energy and environmental crisis. Lillianite materials have been long-term ignored due to low thermoelectric efficiency. Herein we report the discovery of superior thermoelectric performance in Pb7Bi4Se13 based lillianites, with a peak figure of merit, zT of 1.35 at 800 K and a high average zT of 0.92 (450–800 K). A unique quality factor is established to predict and evaluate thermoelectric performances. It considers both band nonparabolicity and band gaps, commonly negligible in conventional quality factors. Such appealing performance is attributed to the convergence of effectively nested conduction bands, providing a high number of valley degeneracy, and a low thermal conductivity, stemming from large lattice anharmonicity, low-frequency localized Einstein modes and the coexistence of high-density moiré fringes and nanoscale defects. This work rekindles the vision that Pb7Bi4Se13 based lillianites are promising candidates for highly efficient thermoelectric energy conversion.

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Onset of size effects in lattice thermal conductivity and lifetime of low-frequency thermal phonons

MRS Proceedings ◽

10.1557/opl.2012.269 ◽

2012 ◽

Vol 1404 ◽

Author(s):

A.A. Maznev

Keyword(s):

Thin Film ◽

Thermal Conductivity ◽

Size Effects ◽

Lattice Thermal Conductivity ◽

Low Frequency ◽

Square Root ◽

Relaxation Rates ◽

Frequency Limit ◽

Conductivity Of Thin Films ◽

Closed Form Formula

ABSTRACTThe onset of size effects in phonon-mediated thermal transport along a thin film at temperatures comparable or greater than the Debye temperature is analyzed theoretically. Assuming a quadratic frequency dependence of phonon relaxation rates in the low-frequency limit, a simple closed-form formula for the reduction of the in-plane thermal conductivity of thin films is derived. The effect scales as the square root of the film thickness, which leads to the prediction of measurable size-effects even at “macroscopic” distances ~100 μm. However, this prediction needs to be corrected to account for the deviation from the ω−2 dependence of phonon lifetimes at sub-THz frequencies due to the transition from Landau-Rumer to Akhiezer mechanism of phonon dissipation.

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The Key Factors of Low-Frequency Electric Heating Assisted Depressurization Method in the Exploiting of Methane Hydrate Sediments

10.2118/205119-ms ◽

2021 ◽

Author(s):

Ermeng Zhao ◽

Jian Hou ◽

Yunkai Ji ◽

Lu Liu ◽

Yongge Liu ◽

...

Keyword(s):

Thermal Conductivity ◽

Heat Capacity ◽

Specific Heat ◽

Specific Heat Capacity ◽

Low Frequency ◽

Electric Heating ◽

Gas Production ◽

Hydrate Decomposition ◽

Hydrate Saturation ◽

Production Behavior

Abstract Natural gas hydrate is widely distributed in the permafrost and marine deposits, and is regarded as an energy resource with great potential. The low-frequency electric heating assisted depressurization (LF-EHAD) has been proven to be an efficient method for exploiting hydrate sediments, which involves complex multi-physics processes, i.e. current conduction, multiphase flow, chemical reaction and heat transfer. The physical properties vary greatly in different hydrate sediments, which may profoundly affect the hydrate decomposition in the LF-EHAD process. In order to evaluate the influence of hydrate-bearing sediment properties on the gas production behavior and energy utilization efficiency of the LF-EHAD method, a geological model was first established based on the data of hydrate sediments in the Shenhu Area. Then, the influence of permeability, porosity, thermal conductivity, specific heat capacity, hydrate saturation and hydrate-bearing layer (HBL) thickness on gas production behavior is comprehensively analyzed by numerical simulation method. Finally, the energy efficiency ratio under different sediment properties is compared. Results indicate that higher gas production is obtained in the high-permeability hydrate sediments during depressurization. However, after the electric heating is implemented, the gas production first increases and then tends to be insensitive as the permeability decreases. With the increasing of porosity, the gas production during depressurization decreases due to the low effective permeability; while in the electric heating stage, this effect is reversed. High thermal conductivity is beneficial to enhance the heat conduction, thus promoting the hydrate decomposition. During depressurization, the gas production is enhanced with the increase of specific heat capacity. However, more heat is consumed to increase the reservoir temperature during electric heating, thereby reducing the gas production. High hydrate saturation is not conducive to depressurization because of the low effective permeability. After electric heating, the gas production increases significantly. High HBL thickness results in a higher gas production during depressurization, while in the electric heating stage, the gas production first increases and then remains unchanged with the increase of thickness, due to the limited heat supply. The comparison results of energy efficiency suggest that electric heating is more advantageous for hydrate sediments with low permeability, high porosity, high thermal conductivity, low specific heat capacity, high hydrate saturation and high HBL thickness. The findings in this work can provide a useful reference for evaluating the application of the LF-EHAD method in gas hydrate sediments.

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A Graphene Chain Acts as a Long-Distance Ballistic Heat Conductor

2010 14th International Heat Transfer Conference, Volume 6 ◽

10.1115/ihtc14-22289 ◽

2010 ◽

Author(s):

Koji Takahashi ◽

Yohei Ito ◽

Tatsuya Ikuta

Keyword(s):

Thermal Conductivity ◽

Carbon Nanofiber ◽

Soft Drink ◽

Three Dimensional ◽

Low Frequency ◽

Phonon Transport ◽

Dynamics Simulation ◽

Infinite Length ◽

Unexpected Result ◽

Long Distance

A carbon nanofiber material, consisting of bottomless graphene cups inside on each other in a line, like a set of soft-drink cups, has been discovered to have the potential to conduct heat ballistically over a long distance. Its longitudinal heat transport ability had been forecast to be extremely poor due to the weak van der Waals force operating between the graphene cups, but our measurements using nano thermal sensor showed that its thermal conductivity is much higher than that along the c-axis of bulk graphite. This unexpected result can be understood by its similarity to a one-dimensional (1D) harmonic-chain where no phonon is scattered even for an infinite length. The current graphene-based nanofiber resembles this type of “superconductive” chain due to the huge difference between the stiff covalent bonding in each cup and the weak inter-cup interaction. A non-equilibrium molecular dynamics simulation is conducted to explore the phonon transport in this fiber. The simulation results show that the thermal conductivity varies with the fiber length in a power law fashion with an exponent as large as 0.7. The calculated phonon density of states and atomic motions indicate that a low-frequency quasi-1D oscillation occurs there. Our investigations show that treating the current nanofiber as a 1D chain with three-dimensional oscillations explains well why this material has the most effective ballistic phonon transport ever observed.

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Violation of the T−1 Relationship in the Lattice Thermal Conductivity of Mg3Sb2 with Locally Asymmetric Vibrations

Research ◽

10.34133/2020/4589786 ◽

2020 ◽

Vol 2020 ◽

pp. 1-10

Author(s):

Yifan Zhu ◽

Yi Xia ◽

Yancheng Wang ◽

Ye Sheng ◽

Jiong Yang ◽

...

Keyword(s):

Thermal Conductivity ◽

Temperature Dependence ◽

Lattice Thermal Conductivity ◽

Elevated Temperatures ◽

Low Frequency ◽

Strong Temperature Dependence ◽

Depth Analysis ◽

Force Profile ◽

Phonon Renormalization ◽

Increasing Temperature

Most crystalline materials follow the guidelines of T−1 temperature-dependent lattice thermal conductivity (κL) at elevated temperatures. Here, we observe a weak temperature dependence of κL in Mg3Sb2, T−0.48 from theory and T−0.57 from measurements, based on a comprehensive study combining ab initio molecular dynamics calculations and experimental measurements on single crystal Mg3Sb2. These results can be understood in terms of the so-called “phonon renormalization” effects due to the strong temperature dependence of the interatomic force constants (IFCs). The increasing temperature leads to the frequency upshifting for those low-frequency phonons dominating heat transport, and more importantly, the phonon-phonon interactions are weakened. In-depth analysis reveals that the phenomenon is closely related to the temperature-induced asymmetric movements of Mg atoms within MgSb4 tetrahedron. With increasing temperature, these Mg atoms tend to locate at the areas with relatively low force in the force profile, leading to reduced effective 3rd-order IFCs. The locally asymmetrical atomic movements at elevated temperatures can be further treated as an indicator of temperature-induced variations of IFCs and thus relatively strong phonon renormalization. The present work sheds light on the fundamental origins of anomalous temperature dependence of κL in thermoelectrics.

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Molecular Dynamics Simulations on Influence of Defect on Thermal Conductivity of Silicon Nanowires

Frontiers in Energy Research ◽

10.3389/fenrg.2021.664891 ◽

2021 ◽

Vol 9 ◽

Author(s):

Hao Li ◽

Qiancheng Rui ◽

Xiwen Wang ◽

Wei Yu

Keyword(s):

Thermal Conductivity ◽

Molecular Dynamics ◽

Silicon Nanowires ◽

Surface Defects ◽

Low Frequency ◽

Simulation Method ◽

Dynamics Simulation ◽

Non Equilibrium Molecular Dynamics ◽

Dynamics Simulations ◽

The Impact

A non-equilibrium molecular dynamics simulation method is conducted to study the thermal conductivity (TC) of silicon nanowires (SiNWs) with different types of defects. The impacts of defect position, porosity, temperature, and length on the TC of SiNWs are analyzed. The numerical results indicate that SiNWs with surface defects have higher TC than SiNWs with inner defects, the TC of SiNWs gradually decreases with the increase of porosity and temperature, and the impact of temperature on the TC of SiNWs with defects is weaker than the impact on the TC of SiNWs with no defects. The TC of SiNWs increases as their length increases. SiNWs with no defects have the highest corresponding frequency of low-frequency peaks of phonon density of states; however, when SiNWs have inner defects, the lowest frequency is observed. Under the same porosity, the average phonon participation of SiNWs with surface defects is higher than that of SiNWs with inner defects.

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Sound absorption and thermal insulation properties of composite thermoplastic polyurethane/polystyrene (TPU/PS) nanofiber web and TPU nanofiber web and PS-extracted TPU/PS microfiber web

Journal of Industrial Textiles ◽

10.1177/15280837211039568 ◽

2021 ◽

pp. 152808372110395

Author(s):

Neslihan Karaca ◽

İlkay Özsev Yüksek ◽

Nuray Uçar ◽

Ayşen Önen ◽

Cafer Kirbaş

Keyword(s):

Thermal Conductivity ◽

Thermal Insulation ◽

Sound Absorption ◽

Composite Nanofibers ◽

Fiber Diameter ◽

Thermoplastic Polyurethane ◽

Low Frequency ◽

Soxhlet Extraction ◽

Nanofiber Web ◽

Potential Use

In this study, composite thermoplastic polyurethane (TPU)/polystyrene (PS) nanofiber web and TPU nanofiber web and PS-extracted TPU/PS microfiber web have been experimentally investigated with regard to sound absorption and thermal conductivity coefficients to observe a potential use in sound and thermal insulation areas. Moreover, other properties such as surface area, morphology, tensile strength/elongation, air permeability, and thermal degradation have been analyzed. It has been observed that nanofiber web properties such as fiber diameter, extensibility, pore volume, and porosity have been clearly changed by Soxhlet extraction of PS from the composite TPU/PS nanofibers. PS-extracted TPU/PS fibers can be preferred for the low frequency (600–800 Hz) due to higher SAC (0.7). On the other hand, TPU nanofibers were more effective at medium frequencies (around 3000 Hz, SAC 0.6). Both TPU and PS-extracted TPU/PS composite fibers had similar thermal conductivities, whereas TPU/PS composite nanofibers had lowest thermal conductivity (0.05 W/mK) with moderate maximum SAC value (around 1000 Hz, SAC 0.5–0.6).

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