Highly anisotropic polyolefin elastomer (POE)/natural graphite (NG) composites with high through‐plane thermal conductivity and excellent mechanical properties, in which NG sheet perfectly aligns ...along one direction, are prepared by two‐roll milling, hot compression, and mechanical cutting. The through‐plane thermal conductivity coefficient of POE/NG composites is markedly improved to be 13.27 W m−1 K−1 at an NG loading of 49.30 vol%. When the composite is used as a thermal management material, it shows excellent heat dissipating capability in the through‐plane direction, which is very important for thermal management in electronic applications. This effective method is highly probable to be widely used for the facile fabrication of polymer‐based thermal management materials.
Highly through‐plane thermally conductive polyolefin elastomer based composites filled with commercial nature graphite (NG) without any treatments are fabricated by a conventional melt‐mixing method of a two‐roll milling method. The as‐fabricated composites with NG flakes perfectly vertically aligning in the matrix reveal high through‐plane thermal conductivity (13.27 W m−1 K−1).
The integration of intrinsic thermal conductivity and intrinsic flame retardancy of epoxy resins shows wider application prospects in electricals and electronics. Discotic liquid crystal epoxy ...(D‐LCE) is synthesized from pyrocatechol, 2‐allyloxyethanol, and 3‐chloroperoxybenzoic acid. P/Si synergistic flame‐retardant co‐curing agent (DOPO‐POSS, DP) is synthesized from p‐hydroxybenzaldehyde, 9, 10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene 10‐oxide (DOPO), and amino terminated polysilsesquioxane (POSS). Finally, D‐LCE is cured within liquid crystal range with 4, 4′‐diaminodiphenyl methane (DDM) and DP, to obtain intrinsic highly thermal conductive/flame‐retardant epoxy resins (D‐LCERDP). D‐LCERDP‐10.0 (10.0 wt% DP) synchronously possesses excellent intrinsic thermal conductivity and intrinsic flame retardancy, with thermal conductivity coefficient in vertical and parallel direction (λ⊥ and λ∥) of 0.34 and 1.30 W m−1 K−1, much higher than that of general bisphenol A epoxy resin (E‐51, λ⊥ of 0.19 W m−1 K−1, λ∥ of 0.65 W m−1 K−1). The limiting oxygen index (LOI) value of D‐LCERDP‐10.0 reaches 31.1, also better than those of E‐51 (19.8) and D‐LCER (21.3).
A new method is offered to achieve the integration of intrinsic thermal conductivity and intrinsic flame retardancy. Novel liquid crystal epoxy and co‐curing agent are synthesized by introducing discotic liquid crystal structure and P/Si synergistic flame‐retardant groups, respectively. The prepared epoxy resins synchronously possess excellent intrinsic high thermal conductivity and intrinsic flame retardancy.
•Preparation of hybrid nano-composites and nanofluids.•Stability measurement and enhancement methods of hybrid nanofluids.•Factors affecting thermal conductivity of hybrid nanofluids.•Mechanisms ...involving in thermal conductivity enhancement of hybrid nanofluids.•Correlations to predict thermal conductivity to hybrid nanofluids.
Innumerable studies are conducted on nanofluids containing single type nanoparticles and attributes of such colloidal mixture have been well elucidated and prospected. Furtherance in nano-composites has entitled production of hybrid nanomaterials (nanoparticles) and remarkable researchers are exploring hybrid nanofluid characteristics. The cardinal objective of this study is to provide a comprehensive review on thermal conductivity of hybrid nanofluids by overviewing experimental, numerical and ANN (artificial neural networking) studies. Assorted factors that affect thermal conductivity such as nanoparticle type, concentration of nanoparticles, types of base fluid, size of nanoparticle, temperature, addition of surfactant, pH variation and sonication time are analyzed in present paper. Additionally, synthesis of hybrid nano-composites, preparation of hybrid nanofluids, approaches for stability measurement and enhancement, methods of thermal conductivity measurement and reasons for thermal conductivity enhancement are discussed. Miscellaneous empirical correlations developed by researchers for thermal conductivity prediction of hybrid nanofluids are also compiled and presented. Results suggest that enhancing temperature and concentration increases thermal conductivity and proper selection of hybrid nanoparticles plays a prime role in attaining stability of hybrid nanofluids.
High hardness is desirable for thermal insulation materials in various applications to improve wear resistance. However, hard materials like diamond, Si3N4, and SiC are often accompanied by high ...thermal conductivity, due to their strong covalent bonds and high Young’s modulus, which increase sound velocity and benefit heat transportation. How to achieve concurrent high hardness and low thermal conductivity remains a challenge in thermal insulation materials. In this work, we report (Yb+Ca) co-doped α-SiAlON (Yb0.5Ca0.75Si7.5Al4.5O1.5N14.5, a solid solution of α-Si3N4) with a low thermal conductivity of 3.4 W/(m·K) and a hardness of 18.1 GPa at 25℃. The thermal conductivity of α-SiAlONs co-doped by (Yb+Nd/Lu) were also studied, and we found that phonon scattering could be significantly increased only when there was a remarkable difference in cation mass and radius between the co-doping cations, like that between Yb3+ and Ca2+. Additionally, high-entropy α-SiAlONs co-doped by five kinds of cations were studied (e.g., Nd0.1Gd0.1Dy0.1Er0.1Yb0.1Si9.5Al2.5ON15), and we found that the high-entropy strategy is not so effective in pursuing low thermal conductivity in α-SiAlONs. The contribution of different phonon scattering mechanisms in the α-SiAlONs was discussed based on inverse thermal diffusivity. Compared with most other ceramics (e.g., 8YSZ and rare-metal zirconates) with low thermal conductivity, (Yb+Ca) co-doped α-SiAlON has a lower bulk density, higher hardness, and higher toughness, which makes it a potential candidate material for applications requiring both thermal insulation and good mechanical properties.
•Concurrent high hardness (18.1 GPa) and low thermal conductivity (3.4 W/(m·K)) are realized in (Yb+Ca) co-doped α-SiAlON.•(Yb+Ca) co-doped α-SiAlON shows advantages in higher hardness, lower bulk density and higher toughness.•The high-entropy strategy seems not so effective in pursuing low thermal conductivity in α-SiAlONs.•The key to achieve low thermal conductivity in α-SiAlONs is to increase extrinsic phonon scattering.
·Random distribution of the inclusions in a composite material is simulated by a finite element model.·Effective thermal conductivity is calculated through finite element method.·Proposed numerical ...procedure is validated by experimental results and compared to empirical model results.
Many materials (e.g. soil) are multi-phase composites in engineering, and the thermal conductivity of mixtures is a critical parameter for analyzing the temperature field. Owing to its heterogeneity, the effective thermal conductivity of multi-phase materials is also not deterministic. The prediction of the thermal properties of a multi-phase material remains a challenging task. In this study, soil is considered to be a typical multi-phase material. A numerical simulation model is established by the finite element method to predict the meso-scale effective thermal conductivity of soil. Monte Carlo simulation is employed to account for the random distribution of voids. Comparisons among numerical, experimental and empirical results suggest that the proposed model can predict reasonably accurate results. In addition, the effective thermal conductivity of unfrozen soil, partially frozen soil and fully frozen soil is calculated. The effects of soil type, porosity and saturation degree on effective thermal conductivity are considered via parametric studies. The proposed numerical method can be used as an effective supplement to empirical model and experimental tests for evaluating the thermal conductivity of multi-phase materials.
•Inclusion of n-eicosane reduces temperature more significantly than paraffin wax.•A higher latent phase completion time was found in case of 3mm diameter pin-fin.•A higher enhancement in operation ...time is obtained in case of 3 mm diameter pin-fin.•The maximum thermal capacities of 2.24kJ/K and 2.90kJ/K are obtained for 3mm tpin-fin.•Thermal conductance of 6.95×10-1W/K and 5.69×10-1 are obtained for paraffin wax and n-eicosane.
The present paper covers the comparison of two different configurations (square and circular) pin-fin heat sinks embedded with two different phase change materials (PCMs) namely paraffin wax and n-eicosane having different thermo-physical properties were carried out for passive cooling of electronic devices. The pin-fins, acting as thermal conductivity enhancers (TCEs), of 2mm square and 3mm circular fin thickness of constant volume fraction of 9% are chosen and input heat fluxes from 1.2kW/m2 to 3.2kW/m2 with an increment of 0.4kW/m2 are provided. Two different critical set point temperatures (SPTs) 45°C and 65°C are chosen to explore the thermal performance in terms of enhancement ratios, enhancement in operation time, latent heating phase duration, thermal capacity and conductance. The results show that 3mm diameter of circular pin-fins has the best thermal performance in passive thermal management of electronic devices.
Controlling thermal properties is central to many applications, such as thermoelectric energy conversion and the thermal management of integrated circuits. Progress has been made over the past decade ...by structuring materials at different length scales, but a clear relationship between structure size and thermal properties remains to be established. The main challenge comes from the unknown intrinsic spectral distribution of energy among heat carriers. Here, we experimentally measure this spectral distribution by probing quasi-ballistic transport near nanostructured heaters down to 30 nm using ultrafast optical spectroscopy. Our approach allows us to quantify up to 95% of the total spectral contribution to thermal conductivity from all phonon modes. The measurement agrees well with multiscale and first-principles-based simulations. We further demonstrate the direct construction of mean free path distributions. Our results provide a new fundamental understanding of thermal transport and will enable materials design in a rational way to achieve high performance.
•EVM benefited the encapsulation of PEG and decreased its supercooling extent.•A theoretical calculation method was applicable to predict the thermal conductivity enhancement ability of Ag NW.•Latent ...heats and thermal conductivity of PEG–Ag/EVM ss-CPCMs maintained simultaneously reasonable.
A series of novel polyethylene glycol-silver nanowire/expanded vermiculite shape-stabilized composite phase change materials (PEG–Ag/EVM ss-CPCMs) were prepared by physical blending and impregnation method to overcome liquid leakage during phase transition and enhance the thermal conductivity of PEG. In these PEG–Ag/EVM ss-CPCMs, PEG served as the phase change material for thermal energy storage; Ag NW served as thermal conductivity enhancement filler; EVM acted as the supporting material to provide structural strength and prevent the leakage of melted PEG. SEM analysis results indicated that Ag NW wrapped with PEG was well dispersed and enwrapped inside the pores and surfaces of EVM due to capillary force and surface tension. It was found that the maximum encapsulation capacity of PEG in all PEG–Ag/EVM ss-CPCMs with good shape stability was 66.1wt.%. The thermal conductivity of PEG–Ag/EVM ss-CPCMs could be greatly enhanced by the prepared Ag NW with a length of 5–20μm and a diameter of 50–100nm. A theoretical calculation method was developed to predict and evaluate the thermal conductivity enhancement ability of Ag NW. The predictions were consistent with experimental results. The thermal conductivity of PEG–Ag/EVM ss-CPCM19.3 reached 0.68W/mK, which was 11.3 times higher than that of pure PEG, and corresponding phase change latent heat was 96.4J/g. The supercooling extent of PEG in PEG–Ag/EVM ss-CPCMs decreased approximate 7°C because the EVM could act as a heterogeneous nucleation center to promote the crystallization of PEG. FT-IR and TGA results showed that the PEG–Ag/EVM ss-CPCMs exhibited excellent chemical compatibility and thermal stability.
Se‐doped Mg3.2Sb1.5Bi0.5‐based thermoelectric materials are revisited in this study. An increased ZT value ≈1.4 at about 723 K is obtained in Mg3.2Sb1.5Bi0.49Se0.01 with optimized carrier ...concentration ≈1.9 × 1019 cm−3. Based on this composition, Co and Mn are incorporated for the manipulation of the carrier scattering mechanism, which are beneficial to the dramatically enhanced electrical conductivity and power factor around room temperature range. Combined with the lowered lattice thermal conductivity due to the introduction of effective phonon scattering centers in Se&Mn‐codoped sample, a highest room temperature ZT value ≈0.63 and a peak ZT value ≈1.70 at 623 K are achieved for Mg3.15Mn0.05Sb1.5Bi0.49Se0.01, leading to a high average ZT ≈1.33 from 323 to 673 K. In particular, a remarkable average ZT ≈1.18 between the temperature of 323 and 523 K is achieved, suggesting the competitive substitution for the commercialized n‐type Bi2Te3‐based thermoelectric materials.
By optimizing Se‐doping content in n‐type Mg3Sb1.5Bi0.5‐based thermoelectric materials, the carrier concentration is raised up. Introducing Mn into the matrix manipulates the carrier scattering mechanism below 550 K and suppresses the lattice thermal conductivity, resulting in a remarkably high average ZT near room temperature.
•Inclusion of paraffin wax reduces the base temperature of heat sink.•An enhancement ratio of 2.64 at ψ=1.00 is achieved for a SPT of 60°C at 3.174kW/m2 for 2mm thick pin-fin heat sink.•At heat input ...of 1.98kW/m2, it took 145min and 229min to reach SPTs of 60°C and 70°C respectively.•3mm Pin-fin thick heat sink outperform the 2mm pin-fin thick heat sink for volume fractions of ψ=0.33 and ψ=0.66.•An enhancement of 4.30 is achieved against heat flux of 3.174kW/m2 for 2mm thick pin-fin heat sink.
The present experimental investigation focuses on the passive cooling of electronic devices by using phase change material (PCM) based pin-fin heat sinks to increase reliability, to ensure sufficiently lower temperature, to stretch the operating duration and to improve the functionality of installed features. Paraffin wax is used as a PCM and filled in heat sinks made of aluminum. As the thermal conductivity of PCM is very low, aluminum square fins are used as thermal conductivity enhancer (TCE). A volume fraction of TCE is kept constant at 9% and the uniform heat flux is applied to finned and un-finned heat sinks. An un-finned heat sink is used for base line comparison. Fin thicknesses of TCE of 1mm,2mm, and 3mm with square cross sectional area are investigated with a constant height of 20mm. Volume fractions of PCM are varied as 0.00, 0.33, 0.66 and 1.00 for each heat sink to determine the thermal performance. The present study reports thermal performance at various heat fluxes to enhance the operating time for different set point temperatures (SPTs) and to compare the latent heat phase duration for various heat sinks tested. The results reveal that maximum thermal performance in operating time is achieved for 2mm thick pin-fin heat sink filled with PCM volumetric fraction of 1.00.
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