Their Thermal Conductivity Research Articles

Multi-scale analysis of thermal conductivity of graphene foam/PDMS composites

In this paper, multiscale modeling of polymer composites consisting of Graphene Foam (GF) and Polydimethylsiloxane (PDMS) is conducted, and their Thermal Conductivity (TC) is investigated through the use of nano-to-microscale analyses. The TC of the PDMS matrix and GF is calculated using the Molecular Dynamics (MD) method. The effective properties of the composites are computed utilizing the Mechanics of Structure Genome (MSG) coupled with Carrera Unified Formulation (CUF) as a novel micromechanical method, which allows an accurate description of the problem resulting in a high-fidelity analysis. Due to the unique interconnected structure of GF, the TC of GF/PDMS composite reaches 0.406 Wm–1 K–1 for GF with 63% porosity, which is about 69% ± 2% higher than that of neat PDMS.

Application of Soft Computing Techniques for Predicting Thermal Conductivity of Rocks

Due to the different challenges in rock sampling and in measuring their thermal conductivity (TC) in the field and laboratory, the determination of the TC of rocks using non-invasive methods is in demand in engineering projects. The relationship between TC and non-destructive tests has not been well-established. An investigation of the most important variables affecting the TC values for rocks was conducted in this study. Currently, the black-boxed models for TC prediction are being replaced with artificial intelligence-based models, with mathematical equations to fill the gap caused by the lack of a tangible model for future studies and developments. In this regard, two models were developed based on which gene expression programming (GEP) algorithms and non-linear multivariable regressions (NLMR) were utilized. When comparing the performances of the proposed models to that of other previously published models, it was revealed that the GEP and NLMR models were able to produce more accurate predictions than other models were. Moreover, the high value of R-squared (equals 0.95) for the GEP model confirmed its superiority.

Proposing an Adaptive Neuro-Fuzzy System-Based Swarm Concept Method for Predicting the Physical Properties of Nanofluids

This paper employs dispersed nanoparticles (NPs) to build an adaptive neuro-fuzzy system (ANFIS) for predicting their thermal conductivity (TC) and viscosity according to the most important input data including concentration, size, the thickness of the interfacial layer, and intensive properties of NPs. In this regard, we gather an extensive and comprehensive data set from different sources. Here, the ANFIS model factors are optimized by using the particle swarm optimization (PSO) technique. Afterward, the obtained results are compared with previously published models which did a better job in predicting target values. In the following, to investigate the validity of our proposed model, statistical and graphical techniques are employed and it was proved that this model is efficient to evaluate the output values. Amounts of results obtained from the PSO-ANFIS model evaluation are 0.988 and 0.985 for the R2 and 0.0156 and 0.0876 for root mean squared error (RMSE) of TC ratio and viscosity ratio values, respectively, letting out a valid forecast of targets. Finally, by performing various statistical analyzes, it can be said that this model shows a high ability to predict target values and can be considered a good alternative to previous models.

Highly thermally conductive polystyrene/polypropylene/boron nitride composites with 3D segregated structure prepared by solution-mixing and hot-pressing method

Continuous heat conduction networks are very significant for filler-filled polymer composites to enhance their thermal conductivity (TC). 3D segregated structure has been considered as an effective approach to construct continuous heat conduction networks. Herein, we prepared the polystyrene/polypropylene/boron nitride (PS/PP/BN) ternary composites with 3D segregated filler networks by solution-mixing and hot-pressing method. The ternary PS/PP/BN composite containing 50 wt% BN achieves TC of 5.57 W/m·K, which is 29 times than that of PS resin (0.19 W/m·K) and 2.76 folds of the PS/BN binary counterpart (2.02 W/m·K). More importantly, the ternary PS/PP/BN composite exhibits favorable thermal stability after 50 hot-cold cycles or thermal ageing test for 50 h, whose TC drops slightly by 3.2% and 3.4%, respectively. The typical 3D segregated structure has been constructed in our PS/PP/BN ternary composites, which can provide effective heat conduction pathways. Moreover, BN platelets align along the PP microspheres in the PS matrix during the hot-pressing process. These two factors make the TC of the PS/PP/BN ternary composites increase sharply. Our strategy can potentially pave the way for the easy, low-cost and large-scale preparation of highly thermally conductive polymer-based composites as thermal management materials.

Highly thermally conductive flame retardant epoxy nanocomposites with multifunctional ionic liquid flame retardant-functionalized boron nitride nanosheets

Multifunctionality of ILFR-fBNNS, i.e. curing, thermal conduction and flame retardation.

The relevance of material and processing parameters on the thermal conductivity of thermoplastic composites

Thermoplastics composites show vast promise as an alternative for thermal management applications in the scope of the development of next‐generation electronics and heat exchangers. Their low cost, reduced weight, and corrosion resistance make them an attractive replacer for traditionally used metals, in case their thermal conductivity (TC) can be sufficiently increased by designing the material (e.g., filler type and shape) and processing (e.g., dispersion quality, mixing, and shaping) parameters. In the present contribution, the relevance of both types of parameters is discussed, and guidelines are formulated for future research to increase the TC of thermoplastic polymer composites. POLYM. ENG. SCI., 58:466–474, 2018. © 2017 Society of Plastics Engineers

Experimental investigation on the effect of SiO2 secondary phase on thermo-physical properties of SiC nanofluids

Nanofluids (NFs), wherein solid nanoparticles (NPs) are dispersed in traditional heat exchange fluids, are recognized for improving the performance of traditional fluids by enhancing their thermal conductivity (TC). The presence of impurities or undesired phases in commercial NPs may influence the thermo-physical properties of NFs including TC and viscosity, which makes it difficult to understand the real effect of NPs on heat transport characteristics of NFs. Moreover, the presence of these impurities in commercial NPs is unavoidable and their removal from commercial NPs with no negative impact on composition of NPs is challenging. To study the impact of impurities on thermo-physical properties of NFs a systematic experimental work was performed using commercial α-SiC and SiO2 NPs as the secondary phase as it commonly co-exists in commercial SiC batches. For this purpose, a series of NFs containing 9wt% of α-SiC/SiO2 NP mixture with different content of SiO2 NPs from 5 to 50% were fabricated and investigated. The results show that as the undesired impurity phase (SiO2) increases, TC of NFs decreases slightly while viscosity increases dramatically. This may be a sound path to tuning the viscosity of the NFs while the achieved high TC is mildly influenced by the secondary phase.

Growth of 4″ diameter polycrystalline diamond wafers with high thermal conductivity by 915 MHz microwave plasma chemical vapor deposition

Polycrystalline diamond (PCD) films 100 mm in diameter are grown by 915 MHz microwave plasma chemical vapor deposition (MPCVD) at different process parameters, and their thermal conductivity (TC) is evaluated by a laser flash technique (LFT) in the temperature range of 230–380 K. The phase purity and quality of the films are assessed by micro-Raman spectroscopy based on the diamond Raman peak width and the amorphous carbon (a-C) presence in the spectra. Decreasing and increasing dependencies for TC with temperature are found for high and low quality samples, respectively. TC, as high as 1950 ± 230 W m−1 K−1 at room temperature, is measured for the most perfect material. A linear correlation between the TC at room temperature and the fraction of the diamond component in the Raman spectrum for the films is established.

The fabrication of Al–diamond composites for heat dissipation by liquid–solid separation technology

Low-cost and good quality Al–diamond composites were prepared in the application of heat dissipation using an advanced liquid–solid separation technology. Their thermal conductivity (TC) and coefficient of thermal expansion (CTE) were measured, especially on the effect of diamond particle surface metallization. The TC of composites reinforced with Ni-coated diamond particles was found to have increased by almost 80 % compared to that of Al alloys. The diamond surface in the composites was characterized using an electron microprobe, X-ray diffraction and SEM. It was found that high TC of the composites was obtained due to the generation of Al4Ni3 and AlNi phases at their interfaces between Al and diamond. In addition, the theoretical calculation for TC and CTE of the composites show the similar tendency with their experimental data.

New insights into thermal conduction mechanisms of multi-walled carbon nanotube/ionic liquid suspensions

Finely dispersed multi-walled carbon nanotube (MCNT)/ionic liquid suspensions spanning the dilute and semi-dilute regimes were prepared and their thermal conductivity (TC) was investigated. These suspensions showed remarkable TC enhancements only at a percolation concentration, i.e. 0.1 wt%, and at specific temperatures, indicating that the TC was both concentration and temperature dependent. Therefore, the results of the TC measurements demonstrated an interesting percolation thermal conduction phenomenon. Based on this, new insights into heat conduction mechanisms of MCNT/ionic liquid suspensions were proposed. At dilute concentration, the contribution of MCNT to the thermal conduction was negligible and base liquid played a dominant role. At or around percolation concentration, however, network structures of MCNT and temperature played synergistic effect. The former provided efficient thermal conduction paths and the latter dredged these paths. While at semi-dilute concentration, no marked TC enhancement was achieved due to the formation of MCNT aggregates.

Experimental study on preparation and base liquid effect on thermo-physical and heat transport characteristics of α-SiC nanofluids

Nanostructured solid particles dispersed in a base liquid are a new class of nano-engineered colloidal solutions, defined with a coined name of nanofluids (NFs). These fluids have shown potential to enhance heat transfer characteristics of conventional base liquids utilized in heat transfer application. We recently reported on the fabrication and thermo-physical property evaluation of SiC NF systems, containing SiC particles with different crystal structure. In this study, our aim is to investigate the heat transfer characteristics of a particular α-SiC NF with respect to the effect of α-SiC particle concentration and different base liquids on the thermo-physical properties of NFs. For this purpose, a series of NFs with various α-SiC NPs concentration of 3, 6 and 9wt.% were prepared in different base liquids of distilled water (DW) and distilled water/ethylene glycol mixture (DW/EG). Their thermal conductivity (TC) and viscosity were evaluated at 20°C. NF with DW/EG base liquid and 9wt.% SiC NP loading exhibited the best combination of thermo-physical properties, which was therefore selected for heat transfer coefficient (HTC) evaluation. Finally, HTC tests were performed and compared in different criteria, including equal Reynolds number, equal mass flow rate and equal pumping power for a laminar flow regime. The results showed HTC enhancement of NF over the base liquid for all evaluation criteria; 13% at equal Reynolds number, 8.5% at equal volume flow and 5.5% at equal pumping power. Our findings are among the few studies in the literature where the heat transfer enhancement for the NFs over its base liquid is noticeable and based on a realistic situation.

Nanofluids based on fluorinated graphene oxide for efficient thermal management

Nanofluids containing various concentrations of graphene oxide (GO), fluorinated graphene oxide (FGO) and highly fluorinated graphene oxide (HFGO) were prepared and their thermal conductivity (TC) was evaluated at different temperatures. Here we report the enhanced TC of stable suspensions of FGO (fluorine (F) ∼5.6%) and HFGO (F ∼ 23%) in comparison to benchmark GO and bare carrier fluids. Enhancement in TC of up to 15–25% is observed in these fluorinated suspensions even at lower filler fractions of 0.01–0.10 wt% and all fluids exhibit filler dependent enhancement in TC. The enhanced TC observed in the case of FGO and HFGO is explained based on liquid layering and percolation mechanism. Moreover, this investigation also unveils the role of F− in enhancing the TC of GO by modifying the surface properties and the effect of liquid layering in determining effective TC of nanofluids.

Development Insulation Performance of Concrete Masonry Units

Thermal insulation of buildings takes more of researchers' interest because of differenteconomical and ecological considerations. Thermal insulation of walls is a principal part ofthermal insulation system for any building.This paper includes production of four types of no-slump concrete including, normal weightconcrete, semi-lightweight concrete, no-fines concrete and lightweight concrete. Theirproperties including, density, compressive strength and absorption were studded and theirthermal conductivity were calculated. Concrete oven-dry densities range was 1664.2-2141.6kg/m3, compressive strength was 5.3-15.7 N/mm2, absorption was 6.3-16.7 % and their thermalconductivity was 0.576-1.047 W/m.K. In addition, the research includes production two types ofconcrete masonry units (CMU) from each type of concrete and studies their properties including,compressive strength and absorption. The overall heat transfer coefficient for each type wascalculated. The results show that the reduction in concrete density by 22% results a reduction inoverall heat transfer coefficient for CMU by about 28% for CMU type I and 15% for CMU type II,the results also show that the change in cavities sizes and their arrangements into CMUreduces the overall heat transfer coefficient between 11-28% depends on concrete density. Thecombination between the two strategies results to optimum reduction in overall heat transfercoefficient of about 36%.

A molecular dynamics investigation on thermal conductivity of graphynes

Graphyne is an allotrope of graphene with both sp and sp2 hybridized carbon atoms. In this paper, molecular dynamics simulations are carried out on four different graphynes (α, β, γ, and 6,6,12-graphenes) to explore their thermal conductivity (TC). The difference between the four graphyne structures lies in the different percentages of the acetylenic linkages (triple-bonded carbon linkages). The presence of the acetylenic linkages in graphynes is found to cause a significant reduction in the TC as a result of the associated low atom density in the structures and weak single bonds in the acetylenic linkages. Among the four graphynes considered, the 6,6,12-graphyne shows the most obvious directional anisotropy in the TC. Beside the structure, external strain and temperature are also found to affect the TC of graphynes.

Thermal conductivity and rheological properties of graphite/oil nanofluids

A series of colloidal graphite/oil nanofluids were prepared by mechanical ball milling method. Their thermal conductivity (TC) and rheological properties were detailedly investigated. The TC tests show that the TC enhancements of the nanofluids depend strongly on the volume fraction (vol.%) of graphite and increase nonlinearly with increasing loading but have a weak relationship with temperature, and promisingly, only adding 1.36vol.% of graphite into oil can result in a high TC enhancement of 36%. It is worth noting that the TC enhancement exhibits an abrupt variation, i.e., from 11% to 36%, when the amount of graphite increases from 0.68 to 1.36vol.%, which could be attributed to the formation of percolating aggregate structures. The rheological measurements were also conducted to demonstrate the microstructure and fluid behaviors of the colloidal graphite/oil nanofluids. Compared with the Newtonian fluid behaviors of base liquids and other nanofluids, the obvious shear thinning, significant viscosity increase, and slight viscoelasticity enhancement for a typical nanofluid sample containing 1.36vol.% graphite with dispersant can be created, offering the evidence for the formation of percolating aggregate structures. Our results may primarily provide a tremendous step on the way of finding abundant, low cost, commercially available, and high-performance nanomaterials for nanofluid production and secondarily determine if graphite-containing nanofluids toward practical applications are possible.

A gel–sol transition phenomenon of oxidation multi-walled carbon nanotubes–glycerol nanofluids induced by polyvinyl alcohol

Oxidation multi-walled carbon nanotubes (O-MWCNTs)–glycerol nanofluids were prepared, either with or without any polyvinyl alcohol (PVA), and their thermal conductivity (TC), stability, fluidity, and rheological properties were investigated. The results demonstrated that the TC enhancements of the nanofluids were almost linear with the nanotube concentration but showed little dependence on temperature. The largest TC enhancement was up to 21.0% when the volume concentration was only 1.4 vol%. The nanofluids exhibited high stability for more than 2 months and a very interesting gel–sol transition phenomenon induced by a low amount of PVA (about 0.06 wt%), which can be observed by an inverted and side tube method and supported by the rheological data. As the fluidity loss of carbon nanotube-based nanofluids has long been one of the major obstacles for their potential applications in energy transfer technologies, the novel gel–sol transition phenomenon introducing the fluidity recovery has great significance in both academic research and practical application fields of the nanofluids. Furthermore, a reasonably stable and disperse mechanism was also proposed to explain this phenomenon.

Effect of Si addition to Al–8Mg alloy on the microstructure and thermo-physical properties of SiCp/Al composites prepared by pressureless infiltration

Fifty-five volume percentage of SiCp/Al composites were prepared by pressureless infiltration to investigate the effect of Si addition to Al–8Mg alloy from 0 wt% to 18 wt% on the interfacial reaction between Al and SiC and the thermo-physical properties of the prepared composites. TEM and X-ray analysis showed that the degree of interfacial reaction decreased as the Si content increased, and that it ceased at 1273 K when the Si addition to the aluminum was greater than 12 wt%. The Si addition to Al–8Mg alloy reduced the CTE of the composites and increased their thermal conductivity (TC), but Si beyond 12 wt% led to the reduction of TC, though the CTE was lower. This is attributable to the combined action of the Si on the wettability, interfacial reaction and the TC and CTE of the matrix itself.