Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userRevisiting the Reduction of Thermal Conductivity in Nano- to Micro-Grained Bismuth Telluride: The Importance of Grain-Boundary Thermal Resistance
106
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Nanograined bulk alloys based on bismuth telluride (Bi2Te3) are the dominant materials for room-temperature thermoelectric applications. In numerous studies, existing bulk phonon mean free path (MFP) spectra predicted by atomistic simulations suggest sub-100 nm grain sizes are necessary to reduce the lattice thermal conductivity by decreasing phonon MFPs. This is in contrast with available experimental data, where a remarkable thermal conductivity reduction is observed even for micro-grained Bi2Te3 samples. In this work, first-principles phonon MFPs along both the in-plane and cross-plane directions are re-computed for bulk Bi2Te3. These phonon MFPs can explain new and existing experimental data on flake-like Bi2Te3 nanostructures with various thicknesses. For polycrystalline Bi2Te3-based materials, a better explanation of the experimental data requires further consideration of the grain-boundary thermal resistance that can largely suppress the transport of high-frequency optical phonons.
rate research
Read More
Micro/nano porous polymeric material is considered a unique industrial material due to its extremely low thermal conductivity, low density, and high surface area. Therefore, it is necessary to establish an accurate thermal conductivity prediction model suiting their applicable conditions and provide a theoretical basis for expanding of their applications. In this work, the development of the calculation model of equivalent thermal conductivity of micro/nano porous polymeric materials in recent years was summarized. Firstly, it reviews the process of establishing the overall equivalent thermal conductivity calculation model for micro/nano porous polymers. Then, the predicted calculation models of thermal conductivity are introduced according to the conductive thermal conductivity models and the radiative thermal conductivity models separately. In addition, the thermal conduction part is divided into the gaseous thermal conductivity model, solid thermal conductivity model and gas-solid coupling model. Finally, it is concluded that, compared with other porous materials, there are few studies on heat transfer of micro/nano porous polymers, especially on the special heat transfer mechanisms such as scale effects at the micro/nanoscale. In particular, the following aspects of porous polymers still need to be further studied: micro scaled thermal radiation, heat transfer characteristics of special morphologies at the nanoscale, heat transfer mechanism and impact factors of micro/nano porous polymers. Such studies would provide a more accurate prediction of thermal conductivity and a wider application in energy conversion and storage systems.
Nanofluids are known to have significantly different thermal properties relative to the corresponding conventional fluids. Heat transfer at the solid-fluid interface affects the thermal properties of nanofluids. The current work helps in understanding the role of two nanoscale phenomena, namely ordering of fluid layer around the nanoparticle (nanolayer) and thermal resistance at the interface of solid-fluid in the enhancement of thermal conductivity of Al2O3 - CO2 nanofluid. In this study, molecular dynamics (MD) simulations have been used to study the thermal interfacial resistance by transient non-equilibrium heat technique and nanolayer formed between Al2O3 nanoparticle (np) and surrounded CO2 molecules in the gaseous and supercritical phase. The nanoparticle diameter (dNP) is varied between 2 and 5 nm to investigate the size effect on thermal interfacial resistance (TIR) and thermal conductivity of nanofluid and the results indicate that the TIR for larger diameters is relatively high in both the phases. The study of the effect of surface wettability and temperature on TIR reveals that the resistance decreases with increase in interaction strength and temperature, but is entirely independent at higher temperatures, in both gaseous and supercritical nanofluid. A density distribution study of the nanolayer and the monolayer around the nanoparticle revealed that the latter is more ordered in smaller diameter with less thermal resistance. However, nanolayer study reveals that the nanoparticle with bigger diameters are more suitable for the cooling/heating purpose, as the system with larger diameters has higher thermal conductivity. Results show that the nanolayer plays a significant role in determining the effective thermal conductivity of the nanofluid, while the influence of TIR appears negligible compared to the nanolayer.
We report on experimental investigation of thermal contact resistance of the noncuring graphene thermal interface materials with the surfaces characterized by different degree of roughness. It is found that the thermal contact resistance depends on the graphene loading non-monotonically, achieving its minimum at the loading fraction of ~15 wt.%. Increasing the surface roughness by ~1 micrometer results in approximately the factor of x2 increase in the thermal contact resistance for this graphene loading. The obtained dependences of the thermal conductivity, thermal contact resistance, and the total thermal resistance of the thermal interface material layer on the graphene loading and surface roughness indicate the need for optimization of the loading fraction for specific materials and roughness of the connecting surfaces. Our results are important for developing graphene technologies for thermal management of high-power-density electronics.
We report a record low thermal conductivity in polycrystalline MoS2 obtained by varying grain sizes and orientations in ultrathin films. By optimizing the sulphurisation parameters of nanometre-thick Mo layer, we could grow MoS2 films with tuneable morphologies. The thermal conductivity is extracted from a Raman laser power-dependent study on suspended samples. The lowest value of thermal conductivity of 0.27 Wm-1K-1, which reaches a similar value as teflon, is obtained in a polycrystalline sample formed by a combination of horizontally and vertically oriented grains, with respect to the bulk (001) monocrystal. Analysis by means of molecular dynamics and finite element method simulations confirm that such grain arrangement leads to lower grain boundary conductance. We discuss the possible use of these thermal insulating films in the context of electronics and thermoelectricity.
We report measurements and Monte Carlo simulations of thermal conductivity of porous 100nm- thick silicon membranes, in which size, shape and position of the pores were varied randomly. Measurements using 2-laser Raman thermometry on both plain membrane and porous membranes revealed more than 10-fold reduction of thermal conductivity compared to bulk silicon and six-fold reduction compared to non-patterned membrane for the sample with 37% filling fraction. Using Monte Carlo solution of the Boltzmann transport equation for phonons we compared different possibilities of pore organization and its influence on the thermal conductivity of the samples. The simulations confirmed that the strongest reduction of thermal conductivity is achieved for a distribution of pores with arbitrary shapes that partly overlap. Up to 15% reduction of thermal conductivity with respect to the purely circular pores was predicted for a porous membrane with 37% filling fraction. The effect of pore shape, distribution and surface roughness is further discussed.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
