Properties Thermal Diffusivity Research Articles

Enhancements in thermal properties of binary alkali chloride salt by Al2O3 nanoparticles for thermal energy storage

The cost-effective molten chloride salts are promising heat transfer fluids for next-generation concentrating solar plants. However, their limited specific heat capacities and thermal conductivities hinder their applications. In this paper, molten salt-based nanofluids based on a binary chloride (50 mol.% NaCl, 50 mol.% KCl) with Al2O3 nanoparticles were proposed to enhance the thermal properties. Firstly, molecular dynamics simulations were employed to investigate the thermal properties of these nanofluids, focusing on various nanoparticle volume fractions within 1100∼1600 K. The findings reveal that as the nanoparticle fraction increases from 0% to 7%, the specific heat capacity and dynamic viscosity improve by≈14.9% and 34.8%–38.6%, respectively. Concurrently, the thermal conductivity increases by 10.0%–16.5%. Then, microstructure evolution was analyzed to elucidate the enhancing mechanism of the thermal conductivity. Specifically, the negatively charged nanoparticle surface selectively attracts Na+ and K+, resulting in the formation of a compressed layer around the nanoparticle where the short-range order of ions was intensified. As a result, the compressed layer serves as a thermal conduit between the Al2O3 nanoparticle and the surrounding salt, resulting in the enhancement of the thermal conductivity. Furthermore, additional analyses of thermal diffusion properties and energy density distributions provide indirect evidence for the existence of the compressed layer where ionic motion is restricted, further validating the enhancing mechanism.

Time-domain thermoreflectance measurement of the thermal diffusivity of Nb thin films

Nb-coated Cu superconducting radiofrequency (SRF) cavities are of interest as a possible alternative to bulk Nb cavities for particle accelerators due to the high thermal conductivity of Cu and its lower cost than Nb. Studying the thermal diffusion properties in Nb thin films on Cu is important for developing Nb-coated Cu SRF cavities. In the present study, the thermal diffusivity of 100‒800 nm Nb films on Cu is measured by time-domain thermoreflectance (TDTR). The thermal diffusivity is obtained from a curve fit of the TDTR signal to a one-dimensional Fourier heat diffusion model over a time extending from 25 to 985 ps after laser-surface interaction. The film was examined by atomic force microscopy and scanning electron microscopy. The grain size and thermal diffusivity increase with film thickness. The thermal diffusivity reaches a value similar to that reported for bulk Nb for the 400 and 800 nm films, for which the average grain size is 47 ± 13 nm and 68 ± 18 nm, respectively. TDTR measurement for the 400-nm film taken over the temperature range of 293 to 40 K showed that the thermal diffusivity increases as the temperature is reduced. The results indicate that grain boundaries reduce the thermal diffusivity of the Nb film and, therefore, must be considered in Nb-coated Cu SRF cavities.

Effect of Luffa Sponge Fibre Mats Embedded in the Plaster Matrix

Abstract The present experimental research work aims to elaborate a new composite material (CM) composed of plasters reinforced with mats of long unidirectional luffa sponge fibres, treated chemically by 1% NaOH solution during 90 min at a temperature of 50°C, in orderto improve its thermomechanical and physical properties. The influence of fibre mat and fibre napping numbers of luffa sponge on density, porosity, flexural strength, thermal conductivity, thermal diffusivity, thermal effusivity and specific heat capacity properties was investigatedto lower building energy consumption. As far as our case is concerned, we processed a composite using single, double and triple-layer luffa sponge fibre mats. In our study, we are basically confined our experiments to three-layer mats. The experimental results revealed that the networking structure of fibres increases the flexural strength and decreases the thermal conductivity for a two-layer fibre wire mesh imbedded in the plaster matrix as compared with the neat plaster. However, there is a decrease in strength for a triple-layer composite, which referred to poor wetting of the fiber with the matrix material.

Large scale fabrication of recyclable and multifunctional sandwich-structured electromagnetic interference shielding films based on waste Nylon-6 silk

Modern wearable electronics are being miniaturized with high integration and frequency, leading to excess electromagnetic radiation, heat storage, and fire risk. Besides overcoming these safety issues, realizing large-scale, low-cost and recyclable preparation of multifunctional films are urgently required for practical applications of electromagnetic interference (EMI) shielding. Herein, sandwich-structured films have been prepared using commercial nylon mesh (CNM) with a waste nylon-6 silk/ammonium polyphosphate (PA6/APP) porous flame-retardant outer layer and inner conductive ultra-long silver nanowires (AgNWs) network. Only 0.2 mg/cm2 of the conductive inner layer endows the films with excellent EMI shielding performance (53.7 dB), thermal diffusivity (5.206 ± 0.087 mm2/s) and antibacterial properties. In addition, the unique honeycomb structure of the PA6/APP outer layer produced during water bath treatment endows the films with superior printability. And such modified porous outer layer of the sandwich-structured films realizes a V0 rating during the UL-94 test. Even when these films are scrapped, they can still be recycled and reused. In summary, the methods described herein can be extended to realize the large-scale production of sandwich-structured films.

The Thermal Conductivity, Thermal Diffusion, Thermal Expansion, and Mechanical Properties of Mg-2Nd-4Zn Alloys Subjected to Aging Treatment

The Thermal Conductivity, Thermal Diffusion, Thermal Expansion, and Mechanical Properties of Mg-2Nd-4Zn Alloys Subjected to Aging Treatment

Effects of sintering temperature on the microstructure, mechanical, tribological and thermophysical properties of GNPs/IN738 composite

While IN738 Ni-based superalloy is a high strength alloy, it is feasible to improve its properties at the bulk level by reinforcing with graphene nanoplatelets (GNPs), taking advantage of the superior mechanical, tribological and thermal properties using the spark plasma sintering technique. In the present study, the influence of spark plasma sintering temperature range between 900 and 1100 °C on the microstructure, mechanical, tribological and thermophysical properties of GNPs/IN738 composite is assessed. The dispersion of GNPs reinforcement and alloying metals to form composite powder is conducted using a turbular mixer and low-frequency planetary ball milling, followed by spark plasma sintering. The relative density of the sintered samples assessed following Archimedes' method indicates increasing densification with the increasing sintering temperature from 94.7% (900 °C) to 98.5% (1100 °C). The microstructure assessed via SEM, XRD and Raman spectroscopy indicates the formation of precipitate gamma, intermetallic gamma prime, solid solution and GNPs strengthening phases. Thus, the mechanical (micro/nano hardness and Young's modulus), tribological (wear rate and coefficient of friction), and thermophysical (thermal diffusivity, thermal conductivity, and specific heat capacity) properties increased with the increasing sintering temperature. The microhardness increased from 354HV (900 °C) to 469HV (1100 °C), nanohardness from 8 GPa (900 °C) to 17 GPa (900 °C), and Young's modulus from 190 GPa (900 °C) to 291 GPa (1100 °C). The wear rate reduced with an increase in sintering temperature for the three loads of 5, 10 and 20N. The thermophysical properties assessed from 25 to 600 °C show the formation of few inflection points as the temperature increases, which is attributed to the dissolution and rearrangement of precipitate gamma prime and Cr in solid solution phases. Similarly, the small increment in the thermal diffusivity is equally associated with the smoother phonon transition at the GNPs/matrix interface.

Processing and Properties of Tungsten-Steel Composites and FGMs Prepared by Spark Plasma Sintering

Tungsten is the prime candidate material for the plasma-facing components of fusion reactors. For the joining of tungsten armor to the cooling system or support structure, composites or graded interlayers can be used to reduce the stress concentration at the interface. These interlayers can be produced by several technologies. Among these, spark plasma sintering appears advantageous because of its ability to fabricate fully dense parts at lower temperatures and in a shorter time than traditional powder metallurgy techniques, thanks to the concurrent application of temperature, pressure, and electrical current. In this work, spark plasma sintering of tungsten-steel composites and functionally graded layers (FGMs) was investigated. As a first step, pure tungsten and steel powders of different sizes were sintered at a range of temperatures to find a suitable temperature window for fully dense compacts. Characterization of the sintered compacts included structure (by SEM); porosity (by the Archimedean method and image analysis); thermal diffusivity (by the flash method) and mechanical properties (microhardness and flexural strength). Compacts with practically full density and fine grains were obtained; while the temperature needed to achieve full sintering decreased with decreasing powder size (down to about 1500 °C for the 0.4 μm powder). For fully sintered compacts, the hardness and thermal diffusivity increased with decreasing powder size. Composites with selected tungsten/steel ratios were produced at several conditions and characterized. At temperatures of 1100 °C or above, intermetallic formation was observed in the composites; nevertheless, without a detrimental effect on the mechanical strength. Finally, the formation of graded layers and tungsten-steel joints in various configurations was demonstrated.

Effects of boron oxide on the structure, properties and bioactivities of bioactive glasses: A review

Borate and boron oxide containing bioactive glasses have drawn attention because of their unique properties and potential biomedical applications. In this paper, we review recent advances of understanding of boron oxide addition on the structures and properties of bioactive glasses from both experimental and simulation studies. After reviewing the synthesis, characterization and applications of these novel bioactive glasses, recent developments of simulation methodologies including the interatomic potentials to model boron oxide containing bioactive glasses are reviewed. Effect of boron oxide to silica substitution on a range of properties such as density, glass transition temperature, coefficient of thermal expansion, diffusion and mechanical properties, as well as crystallization behavior, both from our own work and those from literature, are summarized. Furthermore, influence of boron oxide on in vitro bioactivity in several bioactive glass series are summarized and discussed in detail, emphasizing the importance of in vitro testing conditions on the bioactivity evaluation. Lastly, outlook and future directions of the field have been discussed. • A comprehensive review of borate and boron oxide containing bioactive glasses. • Synthesis, characterization, and bioactive testing of these glasses. • Atomistic and related simulations of boron containing bioactive glasses. • Effect of boron oxide addition on glass structure and properties. • Impact of boron oxide on bioactivity and applications.

Corrosion resistance, thermal diffusivity and mechanical properties of Ni–SiO2 nanocomposite coatings on a 316 stainless steel for heat exchanger applications

Abstract In this study, the effect of pure Ni and Ni–SiO2 nanocomposites coatings on corrosion, wear resistance and thermal conductivity of 316 stainless steel substrates was investigated with the purpose of extending the service life of 316 stainless steel plate heat exchangers. The nanocomposite coatings were developed by electroplating process in a Watts bath in different concentration values of SiO2 nanoparticles (10, 20 and 30 g l−1). Electrochemical corrosion was run to examine the corrosive performance of the coatings. The results showed that the Ni–SiO2 nanocomposite with concentration of 30 g l−1 had a higher corrosion resistance. A pin on disk wear test demonstrated that, in comparison to 316 stainless steel, the wear resistance of the Ni–SiO2 nanocomposite (30 g l−1) was up to 25% lower while its friction coefficient was almost the same. In addition, as measured via the laser flash method and differential scanning calorimetry, the thermal diffusivity and specific heat capacity of the sample respectively were found to be 32 and 43% lower in comparison to 316 stainless steel. Microhardness measurement via a Vickers microindenter showed that the microhardness of the Ni–SiO2 nanocomposite coating was more than three times higher than that of 316 stainless steel for all the reinforcement concentrations.

Characterization of cordierite based h‐BN doped ceramics as an electronic substrate/circuit board material

AbstractIn this study, h‐BN was added to the cordierite composition obtained from zeolite, which was prepared by the determined stoichiometry to facilitate machinability and increase thermal conductivity. Sintering behavior, hardness, machinability, and thermal/electrical properties of the samples obtained by sintering the compounds at different times were investigated. Thanks to these features, it is aimed to use cordierite as an alternative material to integrated circuit substrates and electronic packaging materials. The produced samples were analyzed by X‐ray diffraction analysis, examined by scanning electron microscopy, and field emission scanning electron microscopy. Afterward, thermal properties such as thermal diffusivity, thermal conductivity, and electrical properties such as electrical conductivity and dielectric permittivity were measured. The hardness and machinability of samples were investigated. Cordierite, spinel, glassy phase, and h‐BN phase were detected, and it was observed that the blocky cordierite grains turned into equiaxed grains with the increase of the h‐BN. According to the results obtained from the thermal conductivity test, it was seen that the h‐BN additive increased the thermal conductivity value in general. In addition, it was determined that with the increase of h‐BN, the hardness decreased, and the machinability properties of the samples improved.

Enhanced thermal energy storage performance of molten salt for the next generation concentrated solar power plants by SiO2 nanoparticles: A molecular dynamics study

Chloride molten salt is the most promising thermal energy storage materials for the next generation concentrated solar power (CSP) plants. In this work, to enhance the thermal performance of KNaCl2 molten salts, composited thermal energy storage (CTES) materials based on amorphous SiO2 nanoparticles and KNaCl2 were proposed and designed under the guidance of the material composition design strategy. The molecular dynamics simulation method has been conducted to investigate the thermal storage properties and analyze the mechanism of heat transfer improvement from the perspective of microstructure evolution, thermal diffusion properties and energy changes. Thermal conductivity, viscosity, and specific heat capacity of CTES materials at high temperatures with different volume fractions of nanoparticles were predicted to provide reference data for the design of heat transfer systems in CSP plants. The study discovered that increasing the volume fraction of nanoparticles increases the thermal conductivity and specific heat capacity of the systems significantly, with the maximum increase of 25.28% and 7.87%, respectively. Moreover, the enhancement of heat transfer characteristics is the result of the formation of a compressed interface layer with a thickness 5 Å on the outer surface of SiO2 nanoparticles. This work has important guiding significance for the material selection and composition design of molten salt-nanoparticle composite materials used for next-generation CSP plants.

Study of the Thermal Diffusivity and Optical Properties of Lead Oxide Nanoparticles Annealed at Different Temperatures

Study of the Thermal Diffusivity and Optical Properties of Lead Oxide Nanoparticles Annealed at Different Temperatures

Heat transfer in binary and ternary bulk heterojunction solar cells

Ternary strategy is one of the most commonly used methods to boost the performance of organic solar cells (OSCs) from a binary blend of donor and acceptor. Fullerene derivatives are popular choices for the ternary component as they could benefit the electrical property. However, the ternary component could also affect other physical properties of the bulk-heterojunction (BHJ). Among these properties, heat transfer has rarely been reported, despite its relevance for thermal durability of OSCs. Here, we employ scanning photothermal deflection technique to study thermal diffusion properties of binary PM6:Y7 and ternary PM6:Y7:X BHJs, where X = PC71BM, ICBA, and N2200. It is found that fullerene derivatives deteriorate the thermal diffusivity (D) of blend films and the device thermal durability, despite enhancing the electrical and device performance. In contrast, when an n-type conjugated polymer N2200 is used as the ternary component, both the electrical and thermal properties are enhanced, with improved power conversion efficiency and prolonged device thermal durability. These results offer a perspective on how to choose a favorable third component. Fullerene derivatives are not necessarily the optimal choice for ternary component for BHJ cells because of the inferior thermal properties.

Unification of intensive and extensive properties of the passive cooling system under a single envelope for the thermal management of Li-ion batteries

Vehicle electrification can pave the way towards zero emission transport sector, provided renewable sources fulfil the major stakes of the electricity demand. While pushing this technology for a global scale implementation, several performance hurdles of the electric vehicle (EV) power unit are to be overcome. Many of the predicaments arise due to the negative sensitivity of Lithium-ion battery performance to elevated working temperatures. For the seamless operation of EVs in a hot climate, we examine a novel passive cooling system in this study. This cooling system features two distinct types of heat extraction media: a) PCM based isothermal storage-based heat sink and b) fin based augmented thermal transport-based heat sink. These two sub-components are investigated in mutually competitive and complementing scenarios to understand the relative contributions of each component in heat extraction and realize the maximum cooling potential achievable from a combined implementation. We analyse on a case-by-case basis the variations in the different heat extraction modes arising due to intensive material properties such as thermal diffusivity and extensive properties such as retrofit dimensions. Comparison of heat rates for the two cooling mechanisms shows that PCM and fin sub-components share a mass ratio (MR) and length ratio (LR) of 0.8 and 1, respectively, in their combined operation to achieve maximum cooling of 142.1 W. A maximum temperature drop of 29.3 K could be realized with the proposed passive cooling system, which is sufficient to maintain a favourable operating temperature for the EV power unit in a hot climate. A new index, VSR has been introduced based on non-dimensional input parameters. Finally, this study develops a set of two correlations that can be used to estimate the thermal performance of a battery pack with different retrofit materials and design variables.

Relationship between Thermal Diffusivity and Mechanical Properties of Wood

This paper describes an experimental study of the relationships between thermal diffusivity and mechanical characteristics including Brinell hardness, microhardness, and Young’s modulus of common pine (Pinus sylvestris L.), pedunculate oak (Quercus robur L.), and small-leaf lime (Tilia cordata Mill.) wood. A dependence of Brinell hardness and thermal diffusivity tensor components upon humidity for common pine wood is found. The results of the measurement of Brinell hardness, microhardness, Young’s modulus, and main components of thermal diffusivity tensor for three perpendicular cuts are found to be correlated. It is shown that the mechanical properties correlate better with the ratio of longitude to transversal thermal diffusivity coefficients than with the respective individual absolute values. The mechanical characteristics with the highest correlation with the abovementioned ratio are found to be the ratio of Young’s moduli in longitude and transversal directions. Our technique allows a comparative express assessment of wood mechanical properties by means of a contactless non-destructive measurement of its thermal properties using dynamic thermal imaging instead of laborious and material-consuming destructive mechanical tests.

Use of visible pulsed photoacoustic technique for the non-destructive measurements of absorption coefficients, thermal diffusion and viscosity properties of natural and clay-blended rubber nanocomposites

Use of visible pulsed photoacoustic technique for the non-destructive measurements of absorption coefficients, thermal diffusion and viscosity properties of natural and clay-blended rubber nanocomposites

Analysis of the Mechanical Properties of Concrete Mix Added with Water Lily Ash (Eichornnia Crassipes)

The pollution associated with construction materials production has led to searching for new or existing sustainable and efficient replacements. Some materials are not directly defined as construction materials, but they have properties applied in the field. The pursuit of sustainable materials to be used in the construction industry has been focused on organic materials such as extracts or fibers directly used in concrete mixtures and other construction elements like CEB (Compressed Earth Blocks). In this work, water lily ash (Eichornnia Crassipes) from Tamaulipas southern is proposed to replace cement in concrete mixtures and evaluate its effect on mechanical, thermal, and sound diffusion properties. The consumption of lily ash controls its exponential growth in aquatic environments and is presented as a solution to the adverse effects on aquatic flora and fauna developed by this invasive species. The water lily specimens were taken from the southern Tamaulipas and calcinated in a ground furnace to take advantage of traditional regional techniques with low energy consumption, obtaining 2,45 kg of ashes from approximately 18 kg of raw material (13,6% yield). Concrete mixtures were obtained with Portland cement, water, coarse gravel (19 mm, 2.7 ton/m³), and local river sand for 15,2; 8,8; 42,2; and 33,8% w/w, respectively. Lily ashes were dosed as cement replacement for 0,5; 1, 2, and 3% w/w. It was found that 1% of ashes substituting cement equals the compressive strength and reduces the thermal transmissive by 12.53%. Besides, sound diffusion velocity is increased, which indicates that ash promotes a better particle distribution without compromising mechanical properties. Therefore, lily ash is a good candidate as a cement substituent, and it would be double troubleshooting: reduction of cement consumption and elimination of lily ash as invasive species.

Sustainable composite material based on surface-modified rape straw and environment-friendly adhesive

An energy efficient and sustainable composite building material based on rape straw and environment-friendly adhesive is designed. Bone glue with sodium lignosulfonate is used as the binder. Rape straw particles are modified using water and sodium hydroxide treatment. A wide range of functional properties is analyzed, and a detailed characterization of microstructure is performed. The experimental results show that the surface treatment by both water and sodium hydroxide improves the interface adhesion between rape straw and bone glue. The primary consequence is then an increase of both bulk- and matrix density, as compared with the reference material. The best mechanical properties are achieved for the water-treated samples. The sodium hydroxide treatment is found to influence negatively sorption isotherms and to cause greater swelling in a comparison with other composites. The effect of straw surface treatment on thermal and water vapor diffusion properties is not evidenced. The significant swelling of all designed composites at the relative humidities higher than 75% limits their practical applications. They are supposed to be used in dry environments, typically for cladding and insulation in construction, or for packaging purposes.

A New Method for Evaluating Li-Ion Battery Anode Materials Based on Surface Compositional and Structural Characterization

There is a global effort to develop safer lithium-ion batteries (LIB) with a high energy density and long lifetime. Material properties, their mutual interactions, and active interfaces are key factors in battery performance. Popular methods involve the mixing of different ingredients and development of a suitable fabrication process. Significant effort has been spent to understand the reaction mechanism of battery materials, but this is very challenging due to high system complexity. Different techniques have been developed to follow these reactions, and most significantly, the reactions between Li and electrode materials. Our team has developed a new method to characterize Li thermal diffusion and structural interaction on the surfaces of potential anode and cathode materials. This method is based on surface crystallographic and compositional characterization of deposited ultra-thin film of Li on potential single crystal or polycrystalline anode material. Surface crystallography characterization is done with w Low Energy Electron Diffraction (LEED) and surface composition with Auger Electron Spectroscopy (AES). This characterization monitors Li thermal diffusion into the electrode surface at room temperature and changes in surface crystalline structure and composition. The data provides a strong link between Lithium thermal diffusion and lithiation processes in LIB. The dynamic interaction of Li is monitored at the nanoscale level and gives the direct response how Li atoms interact with the substrate. The diffusion of evaporated Li on single crystals of HOPG, Si(111)-(211)-(100), SiC-6H, CVD-Diamond and LiNbO3 have been performed. The results shows that HOPG is the excellent reference substrate for this method as Li diffuses at room temperature and surface crystallography of HOPG remains unchanged. The structural reaction of Li with Si is very strong causing LEED pattern to disappear and there is no Li diffusion at room temperature as indicated from AES data. The data for SiC-6H and LiNbO3 shows better Li diffusional behavior than on Si but not as good as on HOPG. This method of using a single crystal substrate and basic surface characterization techniques is providing simple nanoscale access into the chemical reactions between Li and the substrate material that will result in better understanding of Lithium diffusion in the fabricated battery electrodes. The initial material classification for “native” Lithium thermal diffusion properties is proposed.

Influence of the Thermophysical Model on the CFD Analysis of Oil-Cooled Transformer Windings

A disc-type winding of an oil-immersed power transformer is modeled with Computational Fluid Dynamics. Different approaches are implemented to evaluate the feasibility of the Boussinesq approximation: (i) constant fluid properties, (ii) variable viscosity and thermal diffusivity and (iii) temperature-dependent fluid properties. Temperature and flow distributions are reconstructed and put into relation with physical phenomena and model assumptions. Their comparison suggests that numerical results are fairly sensitive to the thermophysical model as long as the buoyancy force is a relevant component of the flow. Nonetheless, all the cases converge to very close predictions of the hot-spot value and location, with possibly positive implications for the use of reference parameters when deriving flow and heat transfer correlations for this topic.