Decrease In Thermal Conductivity Research Articles

Sandy soil based foam concrete with ultra-small pore structure through in-situ mechanical frothing

During the manufacturing of foam concrete, conventional chemical and physical foaming methods are still inadequate for regulating and stabilizing the bubble properties. In this work, a one-pot method, denoted as in-situ mechanical frothing, was proposed preparing foam concrete by vigorously stirring the mixture of water, cement, sandy soil, and foaming agent. Sandy soil (SS) was used in the content range of 25–75 % as cement replacement to decrease the binder consumption and reduce costs. The influence of in-situ mechanical frothing technology and SS dosage on the properties of paste rheology, pore structure, compressive strength, and thermal properties was investigated. The results showed that the average pore size of foam concrete prepared by this method is between 45.73 and 74.58 μm. The compressive strength of the formed foam concrete at dry density of 600–1000 kg/cm3 was 2.78–16.37 MPa, which was 85%–231 % higher than that the standard values. The incorporation of SS resulted in smaller and homogeneous pore structure in foam concrete. Moreover, foam concrete with 25–75 % SS dosage showed 7–40 % decrease in thermal conductivity. The overall analysis showed that the 25–50 % SS-incorporated foam concrete enabled achieving higher standard properties through in-situ mechanical frothing.

Effect of biaxial tensile strain on the thermoelectric properties of monolayer ZrTiCO2

Using ab-initio calculation principle calculations, we investigate the effect of biaxial tensile strain on the stability and electronic properties of the two-dimensional double transition metal MXenes ZrTiCO2. The monolayer ZrTiCO2 has stability and semiconducting properties under different strain conditions. The results of thermoelectric properties under other strain conditions calculated by Boltzmann transport theory and Slack model show that biaxial tensile strain can improve the electrical transport properties of monolayer ZrTiCO2 materials and lead to the decrease of lattice thermal conductivity. The thermoelectric efficiency of a material can be evaluated using the figure of merit ZT. The n-type ZrTiCO2 has a maximum ZT value of 1.49 at 900 K without adding biaxial strain and reaches a ZT value of 2.86 with 2% biaxial strain. The monolayer ZrTiCO2 material has the potential to become a thermoelectric material, and its thermoelectric properties can be improved by biaxial tensile strain.

Room Temperature Lattice Thermal Conductivity of GeSn Alloys.

CMOS-compatible materials for efficient energy harvesters at temperatures characteristic for on-chip operation and body temperature are the key ingredients for sustainable green computing and ultralow power Internet of Things applications. In this context, the lattice thermal conductivity (κ) of new group IV semiconductors, namely Ge1-xSnx alloys, are investigated. Layers featuring Sn contents up to 14 at.% are epitaxially grown by state-of-the-art chemical-vapor deposition on Ge buffered Si wafers. An abrupt decrease of the lattice thermal conductivity (κ) from 55 W/(m·K) for Ge to 4 W/(m·K) for Ge0.88Sn0.12 alloys is measured electrically by the differential 3ω-method. The thermal conductivity was verified to be independent of the layer thickness for strained relaxed alloys and confirms the Sn dependence observed by optical methods previously. The experimental κ values in conjunction with numerical estimations of the charge transport properties, able to capture the complex physics of this quasi-direct bandgap material system, are used to evaluate the thermoelectric figure of merit ZT for n- and p-type GeSn epitaxial layers. The results highlight the high potential of single-crystal GeSn alloys to achieve similar energy harvest capability as already present in SiGe alloys but in the 20 °C-100 °C temperature range where Si-compatible semiconductors are not available. This opens the possibility of monolithically integrated thermoelectric on the CMOS platform.

Hierarchically porous Ca3Co4O9 ceramics prepared by freeze casting: Emerging ultra‐low thermal conductivity and apparent anisotropy

AbstractThe freeze‐casting method shows great potential for environmentally friendly and industrial scale‐up application in thermoelectric materials due to low cost, low toxicity, and pore control. The porous Ca3Co4O9 ceramics with anisotropic thermoelectric performance were fabricated by the freeze‐casting method. High porosities, hierarchical pores, and three‐dimensional (3D) networks significantly contributed to the suppressed thermal conductivity. An ultralow total thermal conductivity of 0.21 W/(m·K) was obtained. The optimized freeze‐casting conditions gave rise to a Seebeck coefficient of 232.3 µV/K. In addition, the ZT value of 0.39 was obtained at 800°C due to the manipulation of hierarchically connected pores and small portions of isolated hexagonal pores. This work provides a new route to form the 3D pores network in oxide thermoelectric ceramics, leading to a decrease in thermal conductivity and optimized thermoelectric performance parallel to the freeze‐casting direction.

THERMOPHYSICAL PROPERTIES OF HYPOEUTECTIC, EUTECTIC AND HYPEREUTECTIC Al-Si AUTOMOTIVE ALLOYS UNDER AGEING TREATMENT

Influence of different levels of silicon under a variety of thermal treatment is investigated on the thermophysical behaviour of Al-Si-Cu-Mg automotive alloys. Conventional cast alloys are subjected to T6 heat treatment for age hardening. Hardness values, thermal conductivity along with microstructural observation under different heat-treated conditions are well thought-out to comprehend the ageing performance and the precipitation behaviour of the alloys. From these investigational results it is indicated that two successive hardening peaks take place in the agedalloys. Ageing sequence consists of different phases like a supersaturated solid solution, GP zones, intermediate βʹʹ, intermetallic βʹ, equilibrium β and the Q phase but GP zones and the metastable phases are most likely responsible for these ageing peaks. Thermal conductivity decreases for these precipitates formation during ageing and increases for relieving the internal stress, dissolution of metastable phase and coarsening the precipitation of the alloys. Addingup of Si to the alloy showed earlier ageing peaks with higher intensities intended for its increasing properties of heterogeneous nucleation and diffusion kinetics. After ageing around at 200°C for four hours the alloys offer the height hardness. Microstructural study reveals that eutectic silicon makes the grain boundary coarsen and beyond the eutectic composition the star-shaped blocky primary Si is formed. Subsequent to ageing at 350oC for one hour the alloys reach completely recrystallized state.

Fabrication and Characterization of Functionally Graded Nanocomposites: Impact of Graphene and Vanadium Carbide on Aluminum Matrix

Abstract Functional graded nanocomposites (FGNCs) based on Al are artificially tailored heterogeneous materials intended to serve the demand for diverse and contradicting properties used in various industrial applications. FGNCs and hybrid FGNCs (HFGNCs) based on Al reinforced with graphene and vanadium carbide (VC) were prepared using powder metallurgy techniques and investigated. Both samples were designed with a gradient composition, where the bottom layer consisted of 100% pure Al, followed by three consecutive layers containing progressively increasing amounts of reinforcement. The incorporation of graphene and VC into layer powders resulted in a decrease in both particle and crystal dimensions compared to pure Al. Adding graphene has a negative effect on bulk density samples, while VC has a positive effect. Reinforcing materials led to a decrease in thermal conductivity that reached 26.7 % for samples reinforced with VC reinforcement, except for FGNCs reinforced with graphene, which increased by ~3.3 compared to Al. The samples’ CTE and electrical conductivity values decreased, although adding graphene alone led to a slight decrease in electrical conductivity. A significant improvement in all mechanical properties was noted with additional. The HFGCNs reinforced with the largest amount of hybrid reinforcement recorded an improvement in CTE value, Young's modulus, and compressive strength by about 38.1%, 22.2%, and 20.5%, respectively, compared to Al

Thermoelectric Properties of Layered CuCr0.99Ln0.01S2 (Ln = La…Lu) Disulfides: Effects of Lanthanide Doping

A comprehensive study of the thermoelectric properties of CuCr0.99Ln0.01S2 (Ln = La…Lu) disulfides was carried out in a temperature range of 300 to 740 K. The temperature dependencies of the Seebeck coefficient, electrical resistivity, and thermal conductivity were analyzed. It was found that the cationic substitution of chromium with lanthanides in the crystal structure of layered copper–chromium disulfide, CuCrS2 resulted in notable changes in the thermoelectric performance of CuCr0.99Ln0.01S2. The cationic substitution led to an increase in the Seebeck coefficient and electrical resistivity and a thermal conductivity decrease. The highest values of the thermoelectric figure of merit and power factor corresponded to the praseodymium-doped sample and an initial CuCrS2-matrix at 700–740 K. The cationic substitution with lanthanum, cerium, praseodymium, samarium, and terbium allowed for an enhancement of the thermoelectric performance of the initial matrix at a temperature range below 600 K. The cationic substitution of CuCrS2 with lanthanum and praseodymium ions appeared to be the most promising approach for increasing the thermoelectric performance of the initial matrix.

Nonlinearity effects on thermal transport properties of a mass-spring chain

Using Green’s function technique, we present a self-consistent formalism to study the phonon transport properties of an extended nonlinear mass-spring chain. We calculate the phonon transmission coefficient, thermal conductivity, and specific heat for some chains with different configurations of masses feeling the nonlinearity potential. The numerical results show that in a critical value of the nonlinearity coefficient, a sharp decrease in thermal conductivity will be observed. The same scenario happens in a critical temperature proportional to the inverse of the nonlinearity coefficient for the specific heat. Indeed, thermal conductor-insulator transition can occur in the system depending on the strength and distribution of nonlinearity. The model can aid our understanding of the effect of lattice nonlinearity on the thermal properties of one-dimensional materials to design the thermal switches.

Enhancing Thermoelectric Performance of n-Type Bi2Te2.7Se0.3 through Incorporation of Amorphous Si3N4 Nanoparticles.

Bi2Te3-based thermoelectric (TE) materials are the state-of-the-art compounds for commercial applications near room temperature. Nevertheless, the application of the n-type Bi2Te2.7Se0.3 (BTS) is restricted by the comparatively low figure of merit (ZT) and intrinsic embrittlement. Here, we show that through dispersion of amorphous Si3N4 (a-Si3N4) nanoparticles both 14% increase in power factor (at 300 K) and 48% decrease in lattice thermal conductivity are simultaneously realized. The increased power factor comes from enhanced thermopower and reduced electrical resistivity while the reduced lattice thermal conductivity originates mainly from scattering of middle- and low-frequency phonons at the incorporated a-Si3N4 nanoparticles. As a result, a large ZTmax = 1.19 (at 373 K) and an average ZTave ∼ 1.12 (300-473 K) with better mechanical properties are achieved for the BTS/0.25 wt % Si3N4 sample. Present results demonstrate that the incorporation of a-Si3N4 is a promising way to improve TE performance.

An Ab Initio Investigation of Ultra‐Low Thermal Conductivity in Organically Functionalized TaS2${\rm TaS}_2$

AbstractAn ab initio characterization of the impact of tert‐Butyl isocyanate () functionalization on lattice thermal conductivity is presented. Such a system has been previously synthetized and experimentally charachterized, showing that the incorporation of covalently bonded on bulk leads to a dramatic in‐plane lattice thermal conductivity decrease by more than two orders of magnitudes. To elucidate these experimental findings, detailed calculations of the phonon dispersion relations and scattering rates in are performed. The analysis is addressed to discern the impact of inter‐layer covalently bonded molecules on these phonon properties, providing insights into the underlying mechanisms of the observed thermal conductivity decrease. Present calculations provide evidence that the observed lattice thermal conductivity reduction is attributed to two effects: the increase of inter‐layer separation and the presence of low‐frequency molecular optical modes. The first inhibits specific Van der Waals quasi‐acoustic inter‐layer vibrational modes contributing as much as in bulk . The second effect dramatically decreases the phonon group velocities as a consequence of phonon‐crossing phenomena among low‐frequency molecular modes and acoustic modes, eventually leading to a strong reduction of all phonon lifetimes.

Enhancing thermoelectric performance of solution-processed polycrystalline SnSe with PbSe nanocrystals

There is a growing interest in cost-effective polycrystalline SnSe-based thermoelectric (TE) materials, which are able to replace the high performance but mechanically fragile and costly single-crystalline SnSe. In this study, we present a low-temperature solution-based approach to produce SnSe-PbSe nanocomposites with outstanding TE performance. Our method involves combining surfactant-free SnSe particles with oleate-capped PbSe nanocrystals in specific ratios, followed by thermal annealing and consolidation using spark plasma sintering. These nanocomposites are characterized by distinct compositional and structural properties that significantly impact their transport properties. In particular, the addition of oleate-capped PbSe nanocrystals results in: i) a reduction in the electrostatically adsorbed Na at the surface of the SnSe particles; ii) a reduction of Sn vacancies due to alloying with Pb; iii) an increase in grain boundary density; and iv) the formation of PbSnSe secondary phases. Notably, the SnSe-2.5 %PbSe nanocomposites demonstrate a 30 % decrease in thermal conductivity compared to that of the SnSe matrix. This reduction contributes to a maximum figure of merit (zT) of 1.75 at 788 K with a high average zT value of ca. 1.2 in the medium temperature range of 573–773 K. These values represent one of the highest reported in polycrystalline SnSe materials, showcasing the potential of our fabricated SnSe-PbSe nanocomposites for cost-effective TE applications.

Analyzing recycled waste-infused mortars: Preparation and Examination of thermal, mechanical, and chemical characteristics

In response to the urgent need for innovative and sustainable solutions in the construction industry, this study provides an in-depth investigation and experimental evaluation of the thermal, mechanical, and chemical properties associated with the incorporation of three different categories of waste into mortar. These categories include expanded polystyrene waste (EPSW), expanded perlite waste (EPW), human hair waste (HHW), textile fiber (TFW), and paraffin waste (PW). Various samples were prepared by adding different proportions of these waste materials as sand substitutes in cement mortar. To enable a thorough assessment of the material properties, a comprehensive series of tests were carried out, including measurements of thermal conductivity, flexural strength, compressive strength, chemical composition and crystalline structures. The results revealed that, for a replacement level of 50%, there was a decrease in thermal conductivity by 28%, 55%, 44% and 67% for EPSW, EPW, TFW, HHW and PW mortars, respectively, which resulted in a decrease in compressive strength by 75%, 71%, 50%, 79% and 81%, respectively. Furthermore, the chemical characterization showed that the chemical composition of the mortar remained consistent, even when a substantial proportion of waste materials was introduced. The study emphases the sustainable construction key role in reducing building life cycle emissions.

Comparing mechanism response and thermal conductivity of Ti3C2 and Ti3C2O2

This study uses molecular dynamics to investigate the effect of various temperatures and sample sizes on the mechanical mechanism and thermal conductivity of Ti3C2 and Ti3C2O2 Mxenes. The size of the Mxenes decides the severity of the crack and the von Mises stress clustering. The elastic phase trend of Ti3C2 materials in different sizes follows Hooke’s law, while the complex elastic trend is for the Ti3C2O2 models. The material toughness of Ti3C2 is relatively high, and the material’s response to the force is relatively stable and linear during the process of being subjected to pressure. The Ti3C2O2 Mxene presents a low toughness, low stability, and easier breakage during stress due to the complex structure and the formation of anatase and rutile TiO2 phases. The thermal conductivity decreases when the temperature increases or the material sizes decrease for both materials. Notably, Ti3C2 shows superior thermal conductivity in comparison to the Ti3C2O2 Mxene.

Disodium hydrogen phosphate dodecahydrate/fumed silica composites with high latent heat and suitable phase change temperature for use in building roof

Introducing phase change materials (PCM) with suitable phase change temperature and large latent heat into the application of building roofs is conducive to building energy efficiency, which can reduce indoor temperature fluctuation and improve thermal comfort. In this paper, the optimized disodium hydrogen phosphate dodecahydrate (DHPD) was combined with fumed silica (FS) to prepare a new type of DHPD/FS composites, with a phase change temperature of 41.5 °C, a latent heat of change of 103.46 J/g, and an optimal content of 22.5 % of FS. The leakage experiments proved that there was no leakage of the PCM at this ratio, and after 200 cycles of thermal cycling, the phase change temperature of DHPD/FS composites basically remains unchanged, with good thermal reliability, but the addition of FS will make the material decrease in thermal conductivity, and the thermal conductivity is reduced to 0.19 W·m−1·K−1. Furthermore, the study investigated the application of the encapsulated DHPD/FS composites as a phase change board on the roof. The results indicated that the inclusion of the phase change board improved indoor thermal comfort and demonstrated good thermal performance. When the phase change board was positioned in the top layer, the enhancement of thermal comfort was most optimal.

Rational design of high‐performance epoxy/expandable microsphere foam with outstanding mechanical, thermal, and dielectric properties

AbstractPrevious studies on epoxy foams were subjected to the specific property, and improvements in thermal insulation or dielectric property were usually accompanied by the sacrifice of mechanical strength or thermal stability. The current research reported a high‐performance epoxy foam by the pre‐curing process using E‐51/DDS and expandable microsphere (EMP) as polymer matrix and physical blowing agent, respectively. The effect of EMP loading on the structure & property of epoxy foam was reported. The mean diameter of E‐51/EMP foam was in the range of 19–21 μm. The compression property of E‐51/EMP foam was fit for the power‐law model and its integrity was well maintained during compressive test. Compared with bulky epoxy, a significant decrease of thermal conductivity of 62.93% was observed in 10 wt% EMP‐filled epoxy foam. E‐51/EMP foam showed superior thermal stabilities, with a T5% of over 300°C. The dielectric constant was decreased by 54.5% from 4.92 in bulky epoxy to 2.24 in E‐51/EMP foam (10 wt% EMP). Notably, there was a percolation phenomenon of EMP (6 wt%) regarding on the structure & property of E‐51/EMP foam. This study provides lightweight, robust, high‐temperature‐resistant, and low‐dielectric‐constant epoxy foams with the wide application prospects in buoyant, electronic packaging, and energy‐saving industries.

Vacancies tailoring lattice anharmonicity of Zintl-type thermoelectrics

While phonon anharmonicity affects lattice thermal conductivity intrinsically and is difficult to be modified, controllable lattice defects routinely function only by scattering phonons extrinsically. Here, through a comprehensive study of crystal structure and lattice dynamics of Zintl-type Sr(Cu,Ag,Zn)Sb thermoelectric compounds using neutron scattering techniques and theoretical simulations, we show that the role of vacancies in suppressing lattice thermal conductivity could extend beyond defect scattering. The vacancies in Sr2ZnSb2 significantly enhance lattice anharmonicity, causing a giant softening and broadening of the entire phonon spectrum and, together with defect scattering, leading to a ~ 86% decrease in the maximum lattice thermal conductivity compared to SrCuSb. We show that this huge lattice change arises from charge density reconstruction, which undermines both interlayer and intralayer atomic bonding strength in the hierarchical structure. These microscopic insights demonstrate a promise of artificially tailoring phonon anharmonicity through lattice defect engineering to manipulate lattice thermal conductivity in the design of energy conversion materials.

Interply hybridization of hollow glass fiber reinforced vascular composites

AbstractThe use of hybrid reinforcement in polymer composites has been getting attention due to its potential for weight and cost reduction while achieving improved design flexibility for mechanical and thermal properties. In this study, an interply hybridization of epoxy laminates was performed using layers of solid and hollow glass fibers. The structural and thermal performance of the resulting hybrid vascular composites was investigated. All composite parts were manufactured using vacuum‐assisted resin transfer molding (VARTM), following an interply hybridization plan for the stacking sequence of hollow and solid fiber layers. An expected decrease in flexural properties and thermal conductivity was observed for the laminates with completely hollow fibers. On the other hand, hybridization had a positive effect on the laminate properties. It was shown that hybrid vascular composites could be manufactured without a significant reduction in mechanical and thermal performance by a judicious hybridization plan. The optimum hybridization of the laminate achieved a 3.1% increase in the specific flexural strength and a 5.3% decrease in thermal conductivity while containing a vascular network that can be used for active thermal control.Highlights A vascular interply composite structure consisting of hollow and solid fibers was manufactured and characterized for the first time. The I4 composite, vascularized through the middle layers of the laminate, has almost as much flexural strength as solid composite and much more specific flexural strength. The thermal conductivity of the I4 composite is almost as much as that of the solid composite. Functional composites for thermal management and smart applications can be manufactured using hollow fibers without sacrificing flexural strength and thermal conductivity with interply vascularization.

Fabrication of thermoelectric Bi2Te2·5Se0.5 with adjustable porosity

Porous structures have attracted considerable interest for their impact on the transport and mechanical characteristics of materials. The fabrication of porous materials, however, often involves intricate preprocessing steps and typically lacks the ability to tailor porosity with ease. In the present study, we present a straightforward method to prepare porous Bi2Te2·5Se0.5 samples by employing low-pressure spark plasma sintering, with the porosity regulated by preset mass density using a modified graphite mold. This approach resulted in a substantial decrease in thermal conductivity, attributed to the introduction of pores that reduce mass density and disrupt phonon transmission. An accompanying decrease in electrical conductivity was also noted, arising from reduced carrier concentration and mobility. Despite these variations, all porous samples maintained similar levels of thermoelectric performance, with a peak zT of ca. 0.9 at 373 K. The average zT within the temperature range of 298–500 K remained slightly above 0.8 for all samples. Furthermore, the porous samples exhibited greatly enhanced mechanical properties. This work demonstrates a versatile and adaptable method to produce porous materials with controllable porosity, potentially applicable to other porous thermoelectric systems.

Preparing gypsum-based self-levelling energy storage mortar via fly ash cenospheres/paraffin used for floor radiant heating

In the underlayment of a floor radiant heating system (FRHS), using gypsum-based self-levelling mortar (GSM) with high fluidity and early strength properties could save labor force and material costs. Combining phase change materials (PCMs) with FRHS is a promising way to improve energy efficiency and provide a comfortable thermal environment. This paper investigates the feasibility of preparing gypsum-based self-levelling energy storage mortar (GSEM) by incorporating fly ash cenospheres/paraffin (FACP) into GSM. The selection of fly ash cenospheres (FAC) particle size, the pre-treatment of FACP, the additional amount of FACP, and the thermal performance of FRHS containing GSEM are investigated. Results show that the selection of FAC should consider the influence of strength, fluidity, thermal property, and possible economic and environmental impacts. FAC 2 with a particle size (D50) of 110.5 µm is considered to be a suitable choice. FACP pre-treated with 52.5 cement could decrease paraffin leakage and increase the mechanical strength and fluidity of GSEM. With the sand replacement ratio (Vf) increase, the mechanical strength of GSEM first increases and then decreases. When the Vf is 100%, the compressive strength and flexural strength of GSEM are 18.8 MPa and 4.3Mpa; the thermal conductivity decreases by about 15.8% and the specific heat capacity increases by about 53.5%. For FRHS, the matching between the supply water temperature and the plate thickness should be considered in the design. Compared with the GSM plate, the GSEM plate could reduce about 2.4℃ temperature fluctuation, and the time at a comfortable temperature is extended by 60.8%. This finding indicates that the prepared GSEM could be applied to FRHS and has good potential to provide a comfortable thermal environment for buildings.

Investigation of thermoelectric properties of nanocrystalline copper chalcogenides

Modern research efforts are aimed at developing fuel cells characterized by high efficiency, low cost and environmental friendliness, which largely depend on the properties of the corresponding catalyst materials ― the most important components of the fuel cell. Catalysts based on metal chalcogenides, predominantly S based, have activity in accelerating the oxygen reduction reaction comparable to the activity of Pt in H2SO4. The work uses the technique of compacting powder materials and obtained volumetric samples. Nanodisperse powder fractions with an average particle size of (50–100) nm were obtained. The values of the thermo-emf coefficient (about 0.08 mV/K) were obtained for the studied alloy with low defects in the cation sublattice of the Сu2S0.5Te0.5 type. It was found that a decrease in grain size leads to a significant decrease in electronic conductivity for all studied samples. The paper presents the results of a study of the thermoelectric properties of the Cu2S0.5Te0.5 triple alloy. For the studied composition, a decrease in thermal conductivity by (25‒30)% and a slight increase in the thermal emf coefficient compared with large–crystal samples were obtained. Low thermal conductivity was found in the range (0.3–1.1) W m-1 K-1 with a conductivity above 1000 ohms-1cm-1. For the studied sample Cu2S0.5Te0.5 ― thermoelectric efficiency (ZT = 0.25) at 400 °C, which allows us to hope for the possibility of improving the characteristics of samples of this composition to acceptable values for practical thermoelectric devices by selecting the optimal alloying.