Search

Copyright of Journal of Polytechnic is the property of Journal of Polytechnic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This study investigated the effect of Sb additions (0, 0.4, 0.8, 1.2, 1.6, and 2.0 wt.%) on the microstructure, mechanical properties, and thermal conductivity of hypereutectic Al-20Si-0.3Zr alloy. The aim was to refine and homogenize the Si phases to improve the alloy's thermal conductivity, casting fluidity, and mechanical properties. Various techniques, including SEM, EDS, XRD, DSC, a universal testing machine, and a conductivity tester, were used to analyze the as-cast specimens. The results showed that adding Sb up to 1.6 % significantly refined the morphology of primary Si, reducing its average size by 87.2 % from 376.17 to 48.30 µm, and modified the eutectic Si structure from seagrass-like to vane-like. This refinement and homogenization led to an increase in optimal elongation (El) by 183 % from 1.2 to 3.4 % and the ultimate tensile strength (UTS) by 29.4 % from 132.1 to 170.9 MPa. Furthermore, the addition of 1.6 % Sb improved the alloy's thermal conductivity and conductivity while also reducing its solidification temperature range from 442.2 to 328.5 K, improving casting fluidity. These findings suggest that Sb modification can be a useful method to improve the thermal conductivity, casting fluidity, and mechanical properties of Al-20Si-0.3Zr alloy. [ABSTRACT FROM AUTHOR]

Polymer composites as thermal interface materials have been widely used in modern electronic equipment. In this work, we report a novel method to prepare highly through-plane thermally conductive silicone rubber (SR) composites with vertically aligned silicon carbide fibers (VA-SiCFs) entangled by SiC nanowires (SiCNWs) networks. First, a series of carbon fibers (CFs) skeletons were fabricated in sequence of coating poor thermally conductive polyacrylonitrile-based CFs with polydopamine, ice-templated assembly, and freeze-drying processes. Furthermore, VA-SiCFs networks, i.e., long-range continuous SiCFs-SiCNWs networks, based on the prepared CFs skeletons, were in-situ obtained via template-assisted chemical vapor deposition method. The thermal conductivity enhancement mechanism of VA-SiCFs networks on its SR composites was also intensively studied by finite element simulation, based on the first principles investigation of SiC, and Foygel's theory. The in-situ grown VA-SiCFs networks possess high intrinsic thermal conductivity without the thermal interface between fillers, acting as the high-efficiency through-plane long-range continuous thermal conduction path, in which the SiCNWs were the in-plane "thermal spreader". The VA-SiCFs/SR composites reached a high through-plane thermal conductivity, 2.13 W/(m·K), at the filler loading of 15 vol.%, which is 868.2%, and 249.2% higher than that of pure SR sample, and random-CFs@polydopamine (PDA)/SR composites at the same content, respectively. The VA-SiCFs/SR composites also exhibited good electrical insulation performance and excellent dimensional stability, which guaranteed the stable interfacial heat transfer of high-power density electronic devices. [ABSTRACT FROM AUTHOR]

Thermoelectric performance of a material is determined using a figure of merit (FOM) determined as ZT (ZT = σS2T/κ where σ is the electrical conductivity, S is the Seebeck coefficient, κ is the thermal conductivity, and T is the temperature). In the case of a thin film, it is normal in the first approach to consider calculating the FOM by using the thermal conductivity of the film. However, both the thermal influence of the substrate and the emissivity of the film must also be taken into account. In the present work, the heat transfer model is used in order to study the influence of the thermal conductivity, the thickness, and the emissivity of the film on the thermal gradient of the stack (substrate + thin film). The limits of these three parameters are determined in order to have the temperature variation due to the presence of the film compared to the substrate alone that remains less than 1%. Under these limits, the thermal conductivity of the substrate can be taken into account instead of the thermal conductivity of the thin film, and a modified FOM (Z'T) can be calculated. The present study leads to the determination of the validity of modified ZT. In the case of CuCrO2:Mg thin films, the model shows that the use of Z'T is valid. The calculated value of Z'T with the measured Seebeck coefficient and the electrical conductivity as a function of the temperature for 100 nm thick films and the temperature dependent thermal conductivity taken from the literature reached 0.02 at 210 °C. A thermoelectric module made with this material showed 10.6 nW when 220 °C is applied at the hot side. [ABSTRACT FROM AUTHOR]

The influence of the thickness of a thin (1.5–30 nm) copper layer on the thermal boundary conductance (TBC) at the interface between gold and silicon, sapphire and diamond, respectively, was studied using Time Domain Thermoreflectance. Overall, a monotonic increase in the TBC was observed over the first 10 nm, before reaching a plateau. In some cases, it was also observed that an interlayer reduces the TBC as compared to the reference system. This is rationalized by assuming that the TBC evolution as a function of the interlayer thickness is controlled by (i) a contribution of the gold layer that has to be taken into account for all phonons having a wavelength larger than the interlayer thickness and (ii) a thickness-dependent resistance within the interlayer that appears when the electron-phonon coupling is incomplete, i.e., typically over the first 10 nm. A model is proposed in which the contribution to thermal boundary conductance by phonons coming directly from the gold layer is estimated using a simple Debye approximation, while the resistance that appears within the interlayer is estimated by g(T) times h with g(T) the electron-phonon coupling factor and h the interlayer thickness. This results in a system with three resistances in series, i.e., the metal-metal and metal-dielectric interfacial resistances and the interlayer resistance, and a contribution due to phonons of the gold layer. A reasonably good agreement between this model and experimental data is observed. [ABSTRACT FROM AUTHOR]

In this work, we investigate the electronic transport properties of a graphene allotrope composed of 5–6-7 carbon aromatic rings called phagraphene and compare with the results of the transition-voltage spectroscopy (TVS) and propose the behavior at low voltage characteristic of a topological insulator. Phagraphene properties were compared to those of graphene in a zigzag nanoribbon configuration, zigzag graphene vs zigzag phagraphene nanoribbon (zzGNR and zzPGNR). The molecular geometry and the electronic properties were calculated by density functional theory (DFT) without spin, and the electronic transport and TVS were obtained by means of DFT combined with non-equilibrium Green´s function when we couple the optimized geometry of zzGNR and zzPGNR to the leads (left and right), forming the molecular junction that will be subjected to the action of an external bias voltage (Ve) to generate the molecular device. The results exhibit (i) a metal-insulator transition when Ve is increased until Ve = 1.4 V which corresponds to the nonlinear region (resonance), showing the field effect transistor behaviour for zzGNR junctions; and (ii) two nonlinear regions (two negative differential resistances), showing a resonant tunnel diode behaviour with two operation windows (Ve = 0.5 V and Ve = 1.7 V) for the zzPGNR junction. In addition, the zzPGNR junction exhibits topological insulator characteristics upon introducing topological defects such as pentagons and heptagons in the hexagonal lattice of graphene, and when Ve = 1.7 V, there occurs a topological insulator-metal transition that can be seen in the behaviour of the density of states, transmittance, and frontier molecular orbitals with Ve. [ABSTRACT FROM AUTHOR]

This study aims to evaluate the effective thermal conductivity of graphite flake/aluminum composites using two-dimensional (2D) image simulations. However, the effective thermal conductivity calculated from the two-dimensional microstructure images may not be equivalent to that measured using the experimental methods. The reason is that the two-dimensional microstructure image cannot reveal depth information based on the observation surface, which leads to the orientation difference between the graphite flakes in the 2D microstructure image and the experimental sample. Here, the orientation of the graphite flakes relative to heat flow direction was characterized by the angle between the graphite flake basal plane and the heat flow direction. The relationship between the angles in the 2D cross-sections extracted from threedimensional (3D) models, angles in the 3D models, and aspect ratios of graphite flakes displayed in the 2D cross-sections were studied by computer simulation. We found that the angle in the 2D cross-section was larger than that in the corresponding 3D model, and the difference between the angles can result in a thermal conductivity error of up to 840Wm-1 K-1. In addition, all the angles and aspect ratios were distributed on a curved surface, and the curved surface function could convert the angle in the cross-section into the corresponding angle in the 3D model. Finally, the effective thermal conductivity of the graphite flake/aluminum composite with 10 vol% graphite flakes was determined using a 2D image simulation, and the interfacial thermal conductance was calculated by the reversed method. [ABSTRACT FROM AUTHOR]

This study proposed a new metal-based material design with a modulus of elasticity and thermal conductivity comparable to that of wood by architecturing of metal sheets. The proposed new material is designed in a form in which metal sheets of the same shape with beads are repeatedly stacked. In order to find a design with the target modulus of elasticity and thermal conductivity values, designs were derived using the Design of Experiment (DOE) and the material properties were predicted accordingly. For the prediction of material properties designed in the shape of a metal sheet architecture, finite element analysis combined with the homogenization method was used in consideration of the repeatability of the material microstructure. The reliability of the prediction of material properties based on the finite element analysis using a unit cell was verified by comparison with the results obtained from the compression test and the temperature wave method for the specimen. By analysing the modulus of elasticity and thermal conductivity data corresponding to the designs derived by DOE, we evaluated the effect of the design variables of the metal sheet architecture on the material properties. In addition, we investigated whether the material properties comparable to wood or leather were included within the derived design domain, and presented detailed design data of a metal sheet architecture that provides targeted material properties. It can be inferred from this study that the use of architecturing of metal sheets enables the development of new metal-based materials that can simulate the properties of other materials while utilizing the advantages of fire resistance and recyclability inherent in metals. [ABSTRACT FROM AUTHOR]

The electronic structure of the (La0.8Sr0.2)0.98Mn1-xCrxO3 model series (x = 0, 0.05, or 0.1) was measured using soft X-ray synchrotron radiation at room and elevated temperature. O K-edge nearedge X-ray absorption fine structure (NEXAFS) spectra showed that low-level chromium substitution of (La,Sr)MnO3 resulted in lowered hybridisation between O 2p orbitals and M 3d and M 4sp valance orbitals. Mn L3-edge resonant photoemission spectroscopy measurements indicated lowered Mn 3d-O 2p hybridisation with chromium substitution. Deconvolution of O K-edge NEXAFS spectra took into account the effects of exchange and crystal field splitting and included a novel approach whereby the pre-peak region was described using the nominally filled t2g ↑ state. 10% chromium substitution resulted in a 0.17 eV lowering in the energy of the t2g ↑ state, which appears to provide an explanation for the 0.15 eV rise in activation energy for the oxygen reduction reaction, while decreased overlap between hybrid O 2p-Mn 3d states was in qualitative agreement with lowered electronic conductivity. An orbital-level understanding of the thermodynamically predicted solid oxide fuel cell cathode poisoning mechanism involving low-level chromium substitution on the B-site of (La,Sr)MnO3 is presented. [ABSTRACT FROM AUTHOR]

The electronic structure of the (La0.8Sr0.2)0.98Mn1-xCrxO3 model series (x = 0, 0.05, or 0.1) was measured using soft X-ray synchrotron radiation at room and elevated temperature. O K-edge nearedge X-ray absorption fine structure (NEXAFS) spectra showed that low-level chromium substitution of (La,Sr)MnO3 resulted in lowered hybridisation between O 2p orbitals and M 3d and M 4sp valance orbitals. Mn L3-edge resonant photoemission spectroscopy measurements indicated lowered Mn 3d-O 2p hybridisation with chromium substitution. Deconvolution of O K-edge NEXAFS spectra took into account the effects of exchange and crystal field splitting and included a novel approach whereby the pre-peak region was described using the nominally filled t2g ↑ state. 10% chromium substitution resulted in a 0.17 eV lowering in the energy of the t2g ↑ state, which appears to provide an explanation for the 0.15 eV rise in activation energy for the oxygen reduction reaction, while decreased overlap between hybrid O 2p-Mn 3d states was in qualitative agreement with lowered electronic conductivity. An orbital-level understanding of the thermodynamically predicted solid oxide fuel cell cathode poisoning mechanism involving low-level chromium substitution on the B-site of (La,Sr)MnO3 is presented. [ABSTRACT FROM AUTHOR]

The laser welding of stainless steel and brass is difficult due to their different physical properties, including thermal conductivity and melting point. In this study, the laser welding of stainless steel 308 and brass was experimentally investigated. To systematically investigate the effects of different thermophysical properties, including the heat transfer rate, thermal conductivity, and laser beam interaction in relation to the beam absorption coefficient, the temperature of the melt pool zone was measured while changing welding conditions. Different temperature gradients were obtained by changing different parameters including the laser peak power, nozzle distance, and laser beam deviation. The results showed that an asymmetric melt pool formed, to which the melting of brass made a higher contribution. Due to the lower melting temperature and the higher thermal conductivity of brass, the measured temperature and the melt size of brass were higher. The microstructure of the melt pool consisted of intermetallic compounds. Variation of melt pool size (width and depth) and temperature could be affected by laser peak power and reductions in the nozzle distance than the other parameters. For instance, 1 mm increase in the nozzle distance reduced the melt pool zone temperature by approximately 20 °C for stainless steel. Furthermore, an increase in the laser peak power raised the maximum measured temperature to about 225 and 160 °C for brass and stainless steel sides, respectively. At the same time, the amount of temperature reduction at a period of 20 s for stainless steel is almost half that of brass. [ABSTRACT FROM AUTHOR]

Welding copper to itself and other metals is challenging using conventional welding techniques. The process window for welding copper with an infrared (IR) laser, resistance welder, or an ultrasonic welder is very narrow. In the case of the infrared laser, the high reflectivity at these wavelengths makes it difficult to couple the power into the material and control the temperature of the weld puddle. In the case of ultrasonic and resistive welding, the high thermal conductivity of the material and the tendency to create particles cause less than ideal welds. These fundamental problems can be overcome by using a laser with a wavelength that is highly absorbed by these materials. This paper will present recent welding results using a fiber coupled 500-W blue laser system coupled to a welding head to deliver a 215 μm spot size and an average power density of 1.6 MW/cm2. These results will be compared with the authors' previous results from a free space delivered laser system that was the prototype for the 500-W fiber coupled laser. The fiber coupled laser system performance exceeded the free space performance because of two factors: (1) the welding was able to be performed at normal incidence (90°) to the surface allowing for greater power coupling into the copper and (2) a smaller spot size with a higher power density was used (1.6 MW/cm2 vs 398 kW/cm2). Tests on welding battery components, including stacks of foils, buss bars, hairpins (for motors), and other components, with no porosity and no spatter will be covered. Both copper and mixed metals welding results will be presented. Tests have also been performed with 1 kW of laser power from a processing head with a 400 μm spot size and an average power density of 800 kW/cm2. Both systems have enough power density to initiate the keyhole welding process in copper, stainless steel, and aluminum. The difference in welding speeds for these two systems will be compared in this paper. [ABSTRACT FROM AUTHOR]

High-quality Al 2 O 3 porous ceramic planar membranes suffer from severe deformation and cracking, which occur during sintering process. This study reports on solving this problem, by introducing calcium hydroxide powder in the alumina slurry. Phase-inversion tape-casting technology, applied during molding, and sintering at 1550 °C, favored an in-situ expansion reaction, which effectively suppressed deformation, and well-formed and crack-free calcium hexaluminate porous planar membranes were obtained. The produced membranes had a low thermal conductivity (0.69 W·m−1 K−1 at 85 °C), ascribed to the in-situ formed plate-like structure of calcium hexaluminate (CA 6) and to the high porosity. After hydrophobic modification, the membranes were applied in membrane distillation processing. High rejection rate (>99.9%) and water flux (19.8 L·m-2 h−1) were achieved at 85 °C, using a 4 wt% NaCl solution as a feed solution. [ABSTRACT FROM AUTHOR]

We present a technique that retrieves the thermal diffusivity of metallic samples in a two-dimensional map with micrometer resolution. The technique uses a photothermal method based on the deflection of a probe beam after heating the surface with a modulated pump. After adequate calibration, the time delay between the pump modulation and the deflection modulation provides direct information on the local thermal diffusivity. The calibration is carried out by measuring the frequency dependence of the deflection signal at several sample locations. The capabilities of the technique are illustrated with the measurement of a cross section of a surface treated steel sample. [ABSTRACT FROM AUTHOR]

The thermal conductivity of molten NaCl and KCl was calculated through the Evans-Gillan nonequilibrium molecular dynamics (NEMD) algorithm and Green-Kubo equilibrium molecular dynamics (EMD) simulations. The EMD simulations were performed for a “binary” ionic mixture and the NEMD simulations assumed a pure system for reasons discussed in this work. The cross thermoelectric coefficient obtained from Green-Kubo EMD simulations is discussed in terms of the homogeneous thermoelectric power or Seebeck coefficient of these materials. The thermal conductivity obtained from NEMD simulations is found to be in very good agreement with that obtained through Green-Kubo EMD simulations for a binary ionic mixture. This result points to a possible cancellation between the neglected “partial enthalpy” contribution to the heat flux associated with the interdiffusion of one species through the other and that part of the thermal conductivity related to the coupled fluxes of charge and heat in “binary” ionic mixtures. [ABSTRACT FROM AUTHOR]

A class of materials known as superlattices has shown substantial promise for potential thermoelectric (TE) applications because of its low thermal conductivity. We have investigated natural superlattice Ruddlesden-Popper (RP) phases [S. N. Ruddlesden and P. Popper, Acta Crystallogr. 10, 538 (1957)] to elucidate their potential as TE materials. The TE properties of Nb-doped SrO(SrTiO3)n (n=1,2) with a RP structure were measured, and the origin of the TE properties is discussed from the viewpoint of the structure of the TiO6 octahedron. Compared with the cubic perovskite-type Nb-doped SrTiO3, the lattice thermal conductivity decreased by more than 50% (4.4–5 W m-1 K-1) at room temperature and by 30% (1.9–2.2 W m-1 K-1) at 1000 K. There was a decrease in electrical conductivity owing to the randomly distributed insulating SrO layers in polycrystalline RP phases, and it was found that large TE power can be obtained in conjunction with high symmetry TiO6 octahedra. The largest dimensionless figure of merit (ZT), 0.14 at 1000 K, was obtained in 5 at. % Nb-doped SrO(SrTiO3)2. [ABSTRACT FROM AUTHOR]

We measured the facet temperature profiles of GaInAs/AlInAs quantum cascade lasers (QCLs) operating in continuous wave mode by means of microprobe photoluminescence. These results were used to evaluate the in-plane (k∥) and the cross-plane (k⊥) thermal conductivities of the active region and to validate a two-dimensional model for the anisotropic heat diffusion in QCLs. In the temperature range of 80–250 K, k⊥ monotonically increases with temperature and remains one order of magnitude smaller than the thermal conductivities of bulk constituent materials. We found an excellent agreement between the calculated and experimental values of the thermal resistance of GaInAs/AlInAs QCLs operating in continuous wave up to 400 K. Comparison between the calculated thermal performances of QCLs sharing the same active region structure, but having either a buried or a ridge waveguide, shows that devices with Au contact layers thicker than 4 μm have better thermal properties than the buried structures. [ABSTRACT FROM AUTHOR]

We fabricate field-effect transistors (FETs) by depositing a regioregular poly(3-hexylthiophene) (RR-P3HT) active layer via different preparation methods. The solvent used in the polymer film deposition and the deposition technique determine the film microstructure, which ranges from amorphous or granular films to a well-defined fibrillar texture. The crystalline ordering of RR-P3HT into fibrillar structures appears to lead to optimal FET performances, suggesting that fibrils act as efficient “conduits” for the charge carrier transport. Treating the silicon oxide gate insulator with hexamethyldisilazane enhanced the FET performance. [ABSTRACT FROM AUTHOR]

Polarization has been identified as an important parameter in the generation of micro- and nano-structures during ultrafast laser machining. In this study, deep cavities (500 μm x 500 μm x 100 μm) were machined with a femtosecond laser. The influence of polarization on structure formation was investigated on a stainless steel, a commercially pure titanium, an aluminum alloy, and a silicon sample. Smooth surfaces relatively free of laser induced periodic surface structures (LIPSS or ripples) were achieved using a single set of laser parameters for all examined materials. We show that the influence of polarization on ripple formation in deep cavities is limited to metals with low thermal conductivity and a high electron-phonon coupling constant. A continuously rotating polarization was found to yield optically smooth cavities on both bottom and sidewalls. Obtaining smooth surfaces using ultrafast lasers is relevant in many applications where slower processes are currently used such as in sample preparation for electron microscopy, fabrication of microfluidic channels, 3D serial sectioning, micro-electro-mechanical systems, and micro-optical elements. [ABSTRACT FROM AUTHOR]

As a most important thermal management material, high thermal conductivity of ZrB 2 is expected. However, the reported values of thermal conductivity κ of ZrB 2 are quite scattering, and no consensus has been reached. The contribution from lattice separated by Wiedemann-Franz law is low and the relationship between electron and phonon contributions is still blurry. To explore the intrinsic κ of ZrB 2 , in this work, two approaches, i.e. analytical Debye-Callaway model and iterative solution to the Boltzmann transport equation (BTE), are used to simulate the temperature-dependent theoretical lattice κ of ZrB 2. Our work demonstrates that the lattice thermal conductivity of ZrB 2 has been underestimated. The intrinsic lattice thermal conductivity of ZrB 2 is estimated to be 91 and 88 W m−1 K−1 at 300 K, by two different models, respectively. The effects of low lying optical phonon modes and grain boundary on the thermal conductivity of ZrB 2 are discussed. The thermal conductivity of ZrB 2 is controllable by designing effective grain size and microstructure. By casting light on the micro mechanism on lattice heat conduction of ZrB 2 , our work will be constructive to the application of ZrB 2 as thermal management material. [ABSTRACT FROM AUTHOR]

• Developed room-temperature curable carbon electrodes with high conductivity (49.5 s/cm) and low sheet resistance (4.5 Ω/□). • Two-step perovskite sensitization method, with single cation was utilized. • Ambient processed room-temperature curable carbon electrodes for HTM-free PSC. • Devices made with the carbon PSC exhibited power conversion efficiency of 9.0% and exhibited good stability. Hole conductor and metal cathode free perovskite solar cells (PSC) were fabricated with room temperature curable carbon as the top electrode. Conductivity and sheet resistance was optimized by varying the graphite and carbon black content. Highly conductive (49.5 s/cm), low sheet resistance (4.5 Ω/□) and porous carbon electrodes (45–50 µm) were obtained by room temperature (36 ± 1 °C) curing, while most of the reported literature uses high temperature process (400 °C). Perovskite sensitization was done in ambient conditions (36 ± 1 °C, 35% humidity) by two step method which consists of spin coating and dip coating for 10 min. XRD results confirm the complete conversion of PbI 2 into MAPbI 3 (perovskite) throughout the carbon layer and the layers beneath it. Conventional PSC and hole transport material (HTM) free PSC were also fabricated in the similar conditions to serve as reference devices for the carbon-based PSC. Carbon-based HTM -free PSC exhibited a power conversion efficiency (PCE) of 9.0% with a current density of 21.4 mA/cm2 and open circuit voltage of 0.98 V. Simultaneously, PSC with conventional device architecture and HTM-free devices exhibited a PCE of 9.8% and 5.3%, respectively. A comparative study on charge transport properties and electron life time for all the three PSCs were carried out by electrochemical impedance spectroscopy (EIS) and found to be greater for carbon-based PSC. Steady-state photocurrent measurement under standard test conditions (AM 1.5G) were carried out for conventional and carbon-based PSC, and better stability and lower rate of degradation was observed for carbon-based PSC. [ABSTRACT FROM AUTHOR]

This study aims to estimate effective thermal conductivities of metal matrix nanocomposites (MMNCs) containing carbon nanotubes (CNTs) using a new hierarchical micromechanical method. Nano-filler agglomeration made of CNTs into the MMNCs, frequently encountered in real engineering situations, is considered as the main novelty of this approach. Also, two important parameters, including CNT curvature and interfacial thermal resistance between the CNT and metal matrix are taken into account in the micromechanical modeling. The developed hierarchical approach is validated through the comparison with the experimental measurements available in the literature. When CNTs are not well dispersed into the metal matrix, by incorporating the interfacial thermal resistance, CNT curvature and agglomeration, the predictions are in good agreement with the experiment of MMNC thermal conductivities. The results reveal that the CNT agglomeration can dramatically decrease the MMNC thermal properties. It is confirmed that the key issues to enhance the thermal conductivity of CNT-reinforced MMNCs are homogeneous distribution of CNTs and decrease of CNT/metal interfacial thermal resistance. Also, it is observed that the MMNCs show a significant improvement of thermal conductivity by alignment of CNTs into the metal matrix. The effects of CNT volume fraction, diameter and length on the MMNC thermal conductivities are examined. • Thermal conductivity of CNT-metal nanocomposites with agglomeration is predicted. • A novel hierarchical micromechanical method is developed. • Predictions agree well with experimental data. • Agglomeration and interfacial resistance can decrease the thermal conductivity. [ABSTRACT FROM AUTHOR]

Abstract In order to meet the increasing demand of high-power light-emitting-diode (LED) lighting, state-of-the-art white light-emitting diode technology needs phosphors with high thermal conductivity and high luminous efficacy as color converters. In this work, translucent Al 2 O 3 -Ce:GdYAG composite phosphors were prepared by solid-state reactive sintering. The microstructure shows that the Al 2 O 3 particles are uniformly dispersed in the Ce:GdYAG matrix. These particles can not only improve the thermal conductivity of the ceramics, but also promote the extraction efficacy. The luminous characteristics of the Ce:GdYAG and Al 2 O 3 -Ce:GdYAG ceramics were analyzed after being packaged with blue LED. When the molar ratio of Al 2 O 3 /Ce:GdYAG is 0.8, a high luminous efficacy value of 112.6 lm/W is achieved by the Al 2 O 3 -Ce:GdYAG composite ceramic phosphor with the thickness of 0.4 mm, as well as the highest CRI valve of 71.4. The appropriate photoelectric properties of this kind of ceramic phosphor make it a promising candidate for high-power LED device. [ABSTRACT FROM AUTHOR]

Depending on their structure and order (individual, films, bundled, buckypapers, etc.), carbon nanotubes (CNTs) demonstrate different values of thermal conductivity, from the level of thermal insulation with the thermal conductivity of 0.1 W/mK to such high values as 6600 W/mK. This review article concentrates on analyzing the articles on thermal conductivity of CNT networks. It describes various measurement methods, such as the 3-ω method, bolometric, steady-state method and their variations, hot-disk method, laser flash analysis, thermoreflectance method and Raman spectroscopy, and summarizes the results obtained using those techniques. The article provides the main factors affecting the value of thermal conductivity, such as CNT density, number of defects in their structure, CNT ordering within arrays, direction of measurement in relation to their length, temperature of measurement and type of CNTs. The practical methods of using CNT networks and the potential directions of future research in that scope were also described. [ABSTRACT FROM AUTHOR]

Abstract Herein, we have demonstrated a scalable synthesis route to develop nitrogen-doped graphene embellished carbonized cottons (NGCs) via ammonia-assistant thermal activation processes combining with freeze-drying technique. It is found that the nitrogen-doped graphene (NG) nanosheets are well dispersed and cross-linked with the carbonized cotton skeleton fibers. The assistance of ammonia in thermal treatments can not only benefit the carbonization of the cotton and the reduction of the graphene oxide, but also make the nitrogen elements dope into the whole carbon composites. Due to the synergistic effect between the doped N functional groups and the highly conductive cross-linked networks, the NGCs electrodes exhibit a high capacitance (291 F g−1 at 1.0 A g−1) and a high cycle stability (almost no attenuation after 10,000 cycles). Furthermore, a type of lightweight and flexible symmetric supercapacitors was assembled, delivering a high energy density (5 W h kg−1 at 0.2 A g−1) and an excellent bending stability, which demonstrates its potential application in wearable energy storage devices. Graphical abstract Image 1 Highlights • A facile process was developed for NG immobilized on N-doped carbonized cotton. • NH 3 activation achieved cotton carbonization, GO reduction, and overall N doping. • The composite demonstrates an 11% increase in capacitance after 10,000 cycles. • A fully foldable supercapacitor delivers high energy density of 5 Wh/kg at 0.2 A/g. [ABSTRACT FROM AUTHOR]

Abstract The transparent conducting un-doped, Cd 0.99 Mn 0.01 O (Mn: 1.0%) and Mn/Cu co-doped CdO [(Cu x Mn 0.01 Cd 0.99-x) (x: 0.005, 0.01 and 0.02 respectively)] films were prepared by successive ionic layer adsorption and reaction (SILAR) technique on soda lime glass substrates. The effect of Mn and Cu-dopant on structural, morphological and optical characteristics of the CdO films was analyzed by XRD, SEM, UV–Visible spectrophotometer and FT-IR spectroscopy. The XRD studies showed that the all-SILAR prepared films were polycrystalline and had preferential growth along the (111) directions. SEM analysis revealed that the Mn doping and Mn/Cu co-doping significantly influenced the surface morphologies of the CdO films. The EDX results confirmed that the dopant ions were incorporated properly into the CdO lattices. The optical band gap energy values of the all deposited films were determined by extrapolation method and observed to be in the range of 2.08–2.38 eV. Optical analysis results disclosed that doping alters the band gap facilitating the potential of transparent conductive films to be exploited in optoelectronic devices. Furthermore, FT-IR was used to confirm the existence of metal-doped CdO. The investigations showed that co-doping significantly affects the physical properties of SILAR-grown CdO films. Highlights • Attempt to research the change of physical properties of CdO nanostructures with Mn doping and Mn/Cu co-doping. • Mn and Cu were successfully incorporated into host CdO structure. • Double-doping with transition metals induces a drastic change in the band gap (E g). • Correlation between co-doping and physical properties was established. [ABSTRACT FROM AUTHOR]

Abstract La 2 Ce 2 O 7 (LC) is drawing increasing attention to be a new candidate material for thermal barrier coatings (TBCs) because of its higher phase stability, lower thermal conductivity and larger thermal expansion coefficient (TEC) than the traditional yttria partially-stabilized zirconia (YSZ). Unfortunately, the sudden TEC decrease at low temperatures greatly limits its application. In this study, Ta 2 O 5 was added into LC to produce lanthanum-cerium-tantalum-oxide (LCT) solid solution, and LCT coatings were also newly developed by atmospheric plasma spraying (APS) using La 2 Ce 1.7 Ta 0.3 O 7.15 powder. LCT coating exhibits excellent thermal stability between room temperature and 1573 K (up to 1000 h). The sudden TEC decrease is effectively suppressed due to the reduced oxygen vacancy concentration resulted from partial Ce4+ substitution by higher valence Ta5+, which is confirmed by Raman spectroscopy. Thermal cycling results showed that LCT TBC has relatively better thermal shock resistance than LC TBC, and spallation failure occurred within the ceramic top coat near the bond coat. It is considered that TEC mismatch stresses, low fracture toughness and coating composition deviation are primarily responsible for the poor thermal cycling performance. These findings provide valuable data for future technical optimization. Highlights • La 2 Ce 1.7 Ta 0.3 O 7.15 (LCT) coating was developed by atmospheric plasma spraying. • Ta 2 O 5 doping can suppress the thermal contraction of La 2 Ce 2 O 7 (LC) coating. • LCT coating exhibits excellent phase stability at least 1573 K up to 1000 h. • LCT coating shows relatively better thermal shock resistance than LC coating. [ABSTRACT FROM AUTHOR]

Abstract Temperature dependence of the thermal conductivity of Mn 1.03 As 0.70 Sb 0.30 and Ru-doped or Ni-doped (MnFeRu) 2 (PSi) was studied. These compounds undergo a first-order magnetic transition (FOMT) near room temperature and exhibit a giant magnetocaloric effect at around the Curie temperature. The thermal conductivity of Ru-doped and Ni-doped compounds is abruptly reduced at the Curie temperature during the first cooling. This is due to the micro-cracks generated during the FOMT. On heating, the thermal conductivity shows positive temperature dependence. The Ru-doped and Ni-doped compounds show small humps in the temperature dependence of the thermal conductivity near T C. The thermal conductivity of ferromagnetic metals is given by the sum of the phonon component, the electronic component, and the magnetic component. The results are discussed by taking account of the temperature dependence of each component. Highlights • Ru-doped or Ni-doped (MnFeRu) 2 (PSi) undergoes a first-order magnetic transition. • The thermal conductivity λ is abruptly reduced at T C on the first cooling due to the micro-cracks. • On heating, λ smoothly increases with increasing temperature. • The thermal cycle has little impact on the λ − T curve, once the sample is cooled. [ABSTRACT FROM AUTHOR]

Abstract In this work, effect of processing temperature in electrochemical capacitors was investigated for g-C 3 N 4. The g-C 3 N 4 materials (CN) were synthesized by thermal condensation of melamine precursor at different temperature (450, 550 and 650 °C). Then, the g-C 3 N 4 materials were deposited on electrochemically reduced TiO 2 nanowires/nanotubes arrays (rTWTA)/Ti substrate. The g-C 3 N 4 /rTWTA/Ti electrodes were used as supercapacitor electrodes. Morphology, crystal structure and chemical composition of the g-C 3 N 4 /rTWTA/Ti electrodes were studied by FESEM, XRD, FTIR and elemental analysis. Based on galvanostatic charge/discharge measurements, the CN(450)/rTWTA/Ti electrode exhibited the highest specific capacitance up to 22 mF/cm2 at 0.3 mA/cm2 with long cyclic durability at 96.8% capacitance retention after 500 cycle. This effect was attributed to the existence of higher nitrogen content, more active sites, the improved hydrophilicity and three-dimensional morphology of the CN(450)/rTWTA/Ti electrode as compared to the other electrodes. This research introduces the optimum thermal condensation temperature of the melamine for synthesis of the g-C 3 N 4 with great potentials in supercapacitors. Highlights • g-C 3 N 4 is synthesized by thermal condensation of melamine at different temperature. • Reduced TiO 2 nanowires/nanotubes arrays/Ti (rTWTA/Ti) is used as substrate. • The g-C 3 N 4 /rTWTA/Ti electrodes are prepared as supercapacitor electrode. • The obtained g-C 3 N 4 at 450 °C (CN(450)) shows the highest specific capacitance. • Higher nitrogen content and three dimensional morphology lead to this result. [ABSTRACT FROM AUTHOR]

Abstract We report electrical and thermal transport properties of polycrystalline ZrTe 3. The polycrystalline sample shows semiconducting behavior in contrast to the established semi-metallic character of the compound. However the charge density wave (CDW) transition remains intact and its clear signatures are observed in thermal conductivity and Seebeck coefficient, in the wide temperature range 50–100 K. The thermal conductivity points to additional scattering from the low frequency phonons (phonon softening) in the vicinity of CDW transition. The transport in the polycrystalline compounds is governed by smaller size polarons in the variable range hopping (VRH) region. However, the increasing disorder in polycrystalline compounds suppresses the CDW transition. The VRH behavior is also observed in the Seebeck coefficient data in the similar temperature range. The Seebeck coefficient suggests a competition between the charge carriers (electrons and holes). Highlights • The polycrystalline ZrTe 3 prepared at low T show localization of charge carriers. • The ρ(T) and S(T) data show variable range hopping and formation of small polarons. • The CDW observed in T range 50–100 K is quenched with disorder. • The κ(T) shows fluctuations near the CDW transition and lattice entropy dominance. • The electrons and holes compete with each other near the CDW transition. [ABSTRACT FROM AUTHOR]

Abstract High thermal conductive β-silicon nitride (β-Si 3 N 4) ceramics were fabricated from fine α-Si 3 N 4 powder as the raw material and coarse β-Si 3 N 4 particles as the nuclei through spark plasma sintering (SPS) at 1650 °C for 5 min and post-sintering heat treatment at 1900 °C for 4 h. The microstructures of the sintered β-Si 3 N 4 ceramics could be tailored with the nuclei. The mean sizes and frequencies of large grains increased and consequently decreased as the increased amount of coarse β-Si 3 N 4 particles increased. The thermal conductivity of the sample with 10 mol% of β-Si 3 N 4 nuclei reached a maximum value of 84.6 W m−1 K−1. These results revealed that the thermal conductivity of β-Si 3 N 4 ceramics was independent of the grain size and controlled by the amount of reprecipitated large grains. Graphical abstract Image 1 Highlights • Thermal conductivity of β-Si 3 N 4 ceramics can be tailored through controlled microstructure. • Microstructure of β-Si 3 N 4 ceramics is controlled by adding β-Si 3 N 4 seeding. • Thermal conductivity is related to frequencies of large grains. • The maximum thermal conductivity value reached 84.6 W m−1 K−1. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Sb 2 Te 3 hexagonal nanoplates were synthesized under various synthesis conditions. • The morphology, size, and uniformity of the nanoplates were investigated. • Sintered bulk pellet showed low thermal conductivity. Abstract Antimony telluride is a well-known p-type semiconductor material and considered to be one of the best candidates for thermoelectric applications. In this report, Sb 2 Te 3 hexagonal nanoplates were synthesized by a simple solvothermal process under various synthetic conditions, and their morphology, size and uniformity changes were investigated. Synthesized nanoplates were sintered and the resulting bulk pellet showed low thermal conductivity. These results suggest that controllable synthesis of Sb2Te3 nanoplates and thermoelectric properties is feasible, and this methodology can be applied to other materials with a layered structure. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The anisotropic thermoelectric properties of AgIn 5 Te 8 are identified. • The carrier concentration significantly impacts on the thermoelectric properties. • The lattice thermal conductivity is obtained and agrees to the experimental data. • The maximum ZT of 2.28 is achieved for p -type AgIn 5 Te 8 at 700 K along xx direction. Abstract The synthesis of AgIn 5 Te 8 is realized and the thermoelectric properties dependent on temperature is examined. However, the mechanism remains unclear and the synergistic effect of the temperature and the carrier concentration on thermoelectric properties is not explored. Here, the electronic transport properties are calculated by using the first-principles density functional theory combined with the semi-classical Boltzmann transport theory. The relaxation time is estimated by the deformation potential theory. The lattice thermal conductivity is evaluated by the Slack's model, and the result is in good agreement with the experimental value. High anisotropic behavior is identified for the thermal and electronic transport properties, which supports the experimental observation. By tuning the carrier concentration and temperature, a maximum thermoelectric figure of merit of 2.28 can be achieved for p -type AgIn 5 Te 8 in xx direction. The present results provide insight into the thermoelectric properties of AgIn 5 Te 8 and a guide to prepare thermoelectric materials with AgIn 5 Te 8. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Pre-Screening of effective dopant for ZT enhancement in Cu 3 SbSe 4. • A maximum ZT˜1 at 623 K (∼3 times higher than pristine Cu 3 SbSe 4) in Cu 3 Sb 1-x Sn x Se 4. • Density functional theory (DFT) based formation energy calculations. • Sn doping at the Sb site results in thermal conductivity reduction of Cu 3 SbSe 4. Abstract Here, we report a thermoelectric material, Sn-doped Cu 3 SbSe 4 , with a figure-of-merit (ZT) of unity, which is stable up to 623 K. The Cu 3 SbSe 4 samples, individually doped with Bi, Pb and Sn, were synthesized using conventional vacuum melting followed by the spark plasma sintering. The first-principle density functional theory (DFT) based calculations suggest that these doped Cu 3 SbSe 4 compounds have negative formation energy throughout the temperature range, suggesting their chemical/thermal stability. The current study is to support the synchronization of theoretical and experimental study and pre-screening of the dopant which ensures stability and the TE performance. Among all the dopants studied, we realized a ZT max ∼1 at 623 K with Sn doping for the optimized composition Cu 3 Sb 0.985 Sn 0.015 Se 4 owing to the favourable optimization of the electrical and thermal transport properties. This is also supported by the DFT calculations, which suggests that Sn-doping leads to a decrease in lattice thermal conductivity. [ABSTRACT FROM AUTHOR]

Nano-Ag sintering technology is a promising die attach method for power semiconductors in high reliability and high temperature (i.e. 300°C) applications. However, the present predictive models for thermal conductivity of multiphase materials are not suitable for the porous sintered Ag due to the model limitations of low porosity, i.e. < 10%, and simple pore geometry (sphere or ellipsoid). In this paper, an extension differential scheme (EDS) model based on the classical differential scheme (DS) approach has been developed. The thermal conductivity of the microporous Ag die attach layer on a SiC device was developed by measuring seven different sintering parameters that are fitted with the model. The finite element method (FEM) was also employed to analyze the influence of different factors. The results indicate that the EDS model has better adaptability and accuracy, which will be important for implementation of this new die attach material and technology. [ABSTRACT FROM AUTHOR]

Abstract Layered SnSe 2 have much attention due to their potential novel applications in electronic devices areas, such as photo-transistors, solar cells and memory devices. We investigate the thermal properties of few layer SnSe 2 with capping of Al 2 O 3 layer using the temperature and power-dependent Raman spectroscopy. The first order temperature coefficient of A 1g modes of SnSe 2 layers on a SiO 2 /Si substrate was found to be −0.01757 cm−1/K, which is much higher than the corresponding value −0.01270 cm−1/K of SnSe 2 with Al 2 O 3 layer. The difference in temperature coefficients is attributed to compressive stress introduced by the Al 2 O 3 stress layer. Meanwhile, the Al 2 O 3 layer significantly increases the thermal conductivity to 3.102 W/mK, which is larger than that of the SnSe 2 films without Al 2 O 3 (∼2.378 W/mK). The first principle calculation using CASTP demonstrated that this improvement of thermal conductivity is attributed to the compressive strain. This work suggests an effective way to improve the performances of SnSe 2 based devices, and also need to understand the vibrational properties and electron-phonon interactions at the interface of Al 2 O 3 and SnSe 2 layers. Highlights • A thin Al 2 O 3 layer (∼4 nm) was used as a passivation layer on exploited SnSe 2 films. • Temperature and power-dependent Raman study was done on these films. • Thermal conductivity of SnSe 2 with and without Al 2 O 3 stress layer was evaluated. • The enhancement in k of SnSe 2 is mainly attributed to compressive strain. • Effect of strain on SnSe 2 have been analyzed with first principle calculations. [ABSTRACT FROM AUTHOR]

Abstract Waste glass-based cellular geopolymers with total porosity about ~55 vol%, thermal conductivity of ~0.21 W/mK, and compression strength of ~7.3 MPa were successfully produced by a direct foaming technique using hydrogen peroxide (chemical pore-forming agent) in combination with Triton X-100 (stabilizing agent). Thermal treatment of these porous geopolymers at 700–900 °C resulted in a significant increase in volume (secondary foaming). Interconnected pores with a wide range of sizes, ranging typically from 50 to 3000 µm, formed through this foaming process. Glass and glass-ceramic foams (GFs), possessing high overall porosity (77 < P t < 88 vol%), low bulk density (0.27 < ρ b < 0.48 g/cm3), low thermal conductivity (0.11 < λ < 0.15 W/(m K)), and remarkable compression strength (1.2 < σ < 5.5 MPa), were successfully fabricated through the secondary foaming. The obtained results confirmed that this is a viable method for the waste-to-resource production of glass-based foams. [ABSTRACT FROM AUTHOR]

Abstract The existence of gradient density in lightweight corundum–spinel refractory is expected to reduce thermal conductivity, while ensuring good mechanical performance. In this study, the formation mechanism and characterization of density-gradient structure in refractory were revealed. The effects of the particular structure on physical, mechanical properties and thermal conductivity of the prepared lightweight refractory were also investigated. The results showed that the high density in external zone of the material could guarantee excellent physical properties and mechanical strength. Meanwhile, the high porosity in internal district was beneficial to reduce thermal conductivity and achieve energy saving. The unique structure and outstanding properties of the lightweight corundum–spinel refractory would endow it with a bright prospect in the application of industrial furnaces. [ABSTRACT FROM AUTHOR]

Abstract Owing to its thermal conductivity property, the strength of structural steel is highly susceptible to elevated temperature. When steel is used in main load-bearing members in buildings that are vulnerable to fire, great attention should be paid to the effect of the elevated temperature on the residual strength of those members. Hence, the design criteria of such members must incorporate temperature in their requirements. Current standards and codes of practice for steel structures have attempted to address the elevated temperature in their design procedures. However, the design procedures suggested by these standards and codes still lack accuracy and rationality compared to experimental results, especially at high values of elevated temperature. The main objective of the present study is to suggest a new and accurate analytical method to reasonably predict the residual axial and lateral strength of steel beam columns when subjected to elevated temperature. The suggested method uses a newly derived moment-curvature equation of steel beam columns at elevated temperature. The effect of elevated temperature on the material and mechanical characteristics of the steel was accounted for by utilizing reduction factors suggested by Eurocode 3 (EC3). Validation of the suggested method against experimental and numerical simulation results has demonstrated that the developed method can reasonably predict the residual axial and lateral resistance of steel beam-columns at high values of elevated temperature. The suggested method has many significant applications in the design of steel members at elevated temperature induced by fire. Highlights • An analytical method to predict the residual resistance of steel beam-columns at elevated temperature has been presented. • A new moment-curvature equation of axially compressed steel sections at elevated temperature was derived. • The suggested method was validated against the experimental and numerical results. • Validation results have indicated the reliability of the proposed method. [ABSTRACT FROM AUTHOR]

Abstract A detailed numerical analysis of flame propagation in nano-aluminum–water (nAl–H 2 O) mixture is performed. Emphasis is placed on investigating the role of particle thermal conductivity in the prediction of the burning properties of the mixture. Flame structure and burning characteristics are obtained by solving the energy equation using finite difference discretization and the Gauss–Seidel iteration method. Particle thermal conductivity is modeled using the temperature-dependent thermal conductivities of the aluminum core and oxide layer, as well as their interface resistance. The effective thermal conductivity of the mixture is modeled as a function of temperature, spatial coordinate, and local mixture composition, by means of the unified Maxwell–Eucken–Bruggeman model, accounting for random particle distribution and inter-particle interaction. Results indicate that the combined thermal resistance offered by the oxide layer and the interface constitute 95% of the total resistance of the particle. The calculated particle-size dependent linear burning rates show good agreement with experimental data, with only 5% error. Error in burning rate prediction increases, however, to 20% when interface resistance is excluded from the particle thermal conductivity model. It was also observed that burning rate varies as the inverse of particle size. Finally, an analysis of the sensitivity of burning rate to the individual components of the particle thermal conductivity model is also performed. Results suggest a 30% decrease in burning rate for two orders of magnitude reduction in both interface conductance and oxide thermal conductivity. The burning rate drops by only 15%, however, for a similar reduction in aluminum thermal conductivity. A heat conduction perspective on flame propagation in nanocomposites is presented, identifying the highest and the lowest conductive pathways for energy transport. [ABSTRACT FROM AUTHOR]

Abstract First-principles calculations were performed to study the structure, elastic properties and thermal conductivity of Cr 3 AlB 4 under pressures. Cr 3 AlB 4 is mechanically stable from the calculation of the elastic constants. The elastic properties such as bulk modulus, Young's modulus, shear modulus, shear anisotropic factors, and the Poisson ratio are calculated under different pressures. The 3D profiles of bulk modulus and Young's modulus are also investigated in order to study the elastic anisotropies of Cr 3 AlB 4. The longitudinal acoustic velocities ([ 100 ] v l , [ 010 ] v l , [ 001 ] v l ) are much larger than the transverse modes at 0 GPa, and the longitudinal waves of Cr 3 AlB 4 are the fastest along [1 0 0] direction. Debye temperature gradually increases with the increasing pressure. In addition, the thermal conductivity along [0 0 1] propagation direction is always smaller than that along [1 0 0] and [0 1 0] for Cr 3 AlB 4 under pressure, especially at higher temperature. Highlights • Cr 3 AlB 4 has the stronger resistance to deformation along the a-direction. • Cr 3 AlB 4 changes from ductile material to brittle material with the increasing pressure. • Cr 3 AlB 4 possesses ultra-low thermal conductivities below 400 GPa. [ABSTRACT FROM AUTHOR]

Abstract A novel and practical transient-type technique named as "Jet Flush" is proposed for measuring temperature-dependent thermal conductivity of solid materials with low thermal conductivity. In order to estimate thermal conductivity of the sample, its surface is exposed to an impinging air jet flow with higher (or lower) temperature than the sample for a short period of time, while an infrared thermometer records the heated (or cooled) surface temperature history. A nonlinear 1D inverse heat conduction problem with constant convective heat transfer coefficient on the boundary condition is then employed for determining the temperature-dependent thermal conductivity using the measured temperatures. As a requirement for solving the inverse problem, the convective coefficient on the surface of the sample is measured in a separate experiment with the same flow condition using a standard sample. Sensitivity and error analyses are performed to discuss the effects of the estimation points (the temperatures in which the thermal conductivity is to be estimated), and the simplification assumption of constant convective heat transfer coefficient (with respect to time) on the solution of the inverse problem and the accuracy of the estimated results. In order to demonstrate the applicability of the proposed method, the thermal conductivities of three polymer-type materials (HDPE, PMMA and PA6) are measured over the temperature range of ~22–62 °C, and the results are found to be in good agreement with literature data. Moreover, the uncertainty analysis performed in this study showed that the accuracy of the proposed method can be better than 5% for an appreciable temperature range. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract We present a guide to compute the absolute free energies of classical fluids using nonequilibrium free-energy techniques within the LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) code. The main approach is based on the construction of a thermodynamic path connecting the fluid of interest to either atomic or molecular variants of the Uhlenbeck-Ford (UF) model as reference systems. We describe these reference systems in detail, discuss their implementation in the LAMMPS package and make available source code, scripts as well as auxiliary files. As an illustration we detail a number of distinct applications, involving systems characterized by fundamentally different interactions. In addition to two different atomic models (mW water and the MEAM-2NN CuZr liquid binary alloy), we consider three molecular models for water, two of them rigid (TIP4P and SPC/E) and one flexible (q-SPC/Fw). For the molecular models we develop UF-based reference systems for which the free energies are given by a sum of two contributions: an intermolecular part described by the known UF free energy and an intramolecular contribution that can be determined analytically. The tools described in this paper provide a platform on which fluid-phase free energies can be easily and efficiently computed using the LAMMPS code. In addition to being useful for the development of new models for liquid phases, the tools may also find applications in the construction of community databases containing thermodynamic properties of existing models. [ABSTRACT FROM AUTHOR]

This paper reports the effect of the freeze-thaw on the suspension stability, particle size distribution, and thermal conductivity of EG/water-based nanofluids containing Al2O3 nanoparticles that can be used as improved working fluid for cooling systems. The EG/water-based Al2O3 nanofluids were prepared using a two-step method with a nanodisperser and decanting processes. To investigate the effect of freeze-thaw on the suspension stability and thermal conductivity of nanofluids, the prepared nanofluids were frozen at -32°C for 24 hours using a refrigerating chamber, and then they were completely thawed at room temperature for 24 hours. The suspension stability of the thawed nanofluids was quantitatively analysed for over a day using a Turbiscan. In addition, the particle size distributions and deformation of nanoparticles dispersed in the nanofluids were measured using a particle size analyzer (PSA) and TEM. Also, the thermal conductivity of the nanofluids was measured using a transient hot wire (THW) method in temperature from -10 to 70°C. Based on the results, we show that the suspension stability, thermal conductivity, and particle size of EG/water-based Al2O3 nanofluids were not affected by low temperature. [ABSTRACT FROM AUTHOR]

Abstract Nanofluids are a new group of materials with potentially broad applications in the industry, especially in the process of heat transfer. Recently, a new class of nanofluids called hybrid nanofluids are being used to enhance the heat transfer rate. In the present study, an experimental investigation on heat transfer characteristics of aqueous Al 2 O 3 /Ag hybrid nanofluids is presented. The main idea in producing hybrid nanofluids was to obtain better heat transfer rate compared to nanofluids dispersed with either of the dispersoids. The detailed surface properties of Ag and Al 2 O 3 nanoparticles were performed by X-ray diffraction and transmission electron microscopy. Polyvinylpyrrolidone used to stabilize the nanoparticles (Al 2 O 3 /Ag) in water. Dynamic light scattering and zetapotential measurements measured the particle size distribution of the nanoparticles in the nanofluid and the surface charge formed on the surface of the nanoparticle by the polyvinylpyrrolidone. The hybrid nanofluid was prepared by dispersing Al 2 O 3 /Ag (50,50) in distilled water, and the thermal conductivity measurements were conducted for particle loading varying from 0.005 vol% to 0.1 vol% using transient hot-wire method. The results indicate that Al 2 O 3 /Ag hybrid nanofluids exhibit higher thermal conductivity enhancement compared to Al 2 O 3 nanofluids alone. Based on the experimental data, a new correlation for predicting the thermal conductivity of aqueous Al 2 O 3 /Ag has been proposed. Graphical abstract Unlabelled Image Highlights • Two-step method has been employed to prepare aqueous Al2O3/Ag hybrid nanofluid. • Al2O3/Ag nanofluids exhibited higher thermal conductivity compared to Al2O3 nanofluid. • Proposed new correlation to predict the thermal conductivity of Al2O3/Ag nanofluids. [ABSTRACT FROM AUTHOR]

Abstract We investigated the effect of porosity on the thermal conductivity of armchair and zigzag nanoporous graphene (NPG) by using a nonequilibrium molecular dynamics approach. The thermal conductivity values of the NPG were calculated for porosities of P = 0%, 3.6%, 6.6%, 14.7%, and 24.7% at different temperatures. The simulation revealed that the thermal conductivity of armchair graphene with P = 0% is larger than that of zigzag graphene at 100 K. However, the trend is reversed for the other cases of graphene with pores. In addition, a steep decrease in the thermal conductivity of both types of NPG with a lower porosity is evident at 300 K. Thermal conductivity decreases slowly for a higher porosity. The results are important for understanding nanopore scattering in two-dimensional systems and for practical applications of NPGs in thermal management. Highlights • We study the thermal conductivity of nanoporous graphene by nonequilibrium MD. • The thermal conductivity decreased with increasing porosity. • The thermal conductivity of nanoporous graphene is sensitive to temperature. • The thermal conductivity decreases as the temperature increases. [ABSTRACT FROM AUTHOR]