Reduction In Thermal Conductivity Research Articles

Enhanced thermoelectric performance of Bi2-xCuxS3 by hydrothermal synthesis and spark plasma sintering

Bi2S3 has been considered as a promising medium temperature thermoelectric material due to its low thermal conductivity and cost-effective features. However, the intrinsically poor electrical conductivity of Bi2S3 still limits its further thermoelectric applications. In this work, we optimize the thermoelectric performance of Bi2S3 using Cu doping via a facile hydrothermal method combined with spark plasma sintering. A well-tuned carrier concentration of 1.2 × 1019 cm−3 and electrical conductivity of 47 S cm−1 at 523 K was attained, which is attributed to the increased electrons by Cu doping. In addition, the appeared micro-nano pores in both Bi2S3 and Cu doped Bi2S3 samples after spark plasma sintering benefits the reduction of lattice thermal conductivity (κl). The Bi1.985Cu0.015S3 sample exhibited the lowest κl of 0.405 W m−1 K−1 at 573 K, which is the lowest value as far as we know in the reported Cu doped Bi2S3 materials at the same temperature. Finally, A maximum ZT value of 0.3 at 573 K was achieved in Bi1.985Cu0.015S3 sample. This study provides a new strategy to achieve low κl and high ZT value in Bi2S3-based thermoelectric materials.

Characterisation and thermal improvement of adobe walls from earth-straw lightweight panels

Soil as a building material has great potential in terms of low embodied energy, although the great diversity of soils means that more research is needed to characterise them with precise values. In addition, their high thermal transmittance is one of the main drawbacks of earths, so the incorporation of agrowastes could imply a reduction of thermal conductivity without any harm to the embodied energy. Three types of agrowastes have been evaluated: long and short wheat straw fibres and a sawdust. These three fibres were mixed with barbotine to reduce the amount of solid material in the mixture. These panels were compared with the performance of a 20-cm-thick adobe wall. The thermal transmittance of the materials was analysed and the results showed that panel 2 with short wheat straw fibres performed well as a thermal insulation board with a thermal conductivity of λ = 0.05 W/mK.Graphical abstract

Computational thermal analysis of a latent heat storage unit augmented with porous fins

Phase change material (PCM)-based thermal energy storage systems provide a large capacity to store energy; however, PCMs are impaired by relatively low thermal conductivity. Presently, the use of copper foam annular fins was numerically investigated in a PCM-based thermal storage unit. Six copper annular fin cases were studied: three cases with 10 fins and three cases with 20 fins. The 10 and 20 fin cases included configurations with solid fins and two porous fin cases including 40 PPI with porosities of 90.91% and 97.20%. Simulations were completed in ANSYS-Fluent to examine the effects of porous annular fins on the charging and discharging cycles of a vertical shell-tube unit with Rubitherm RT-55 PCM. The cases were compared based on required time to completely melt and solidify the PCM, along with the energy response of the system. Charging times for the solid, 90.91% porous, and 97.20% porous 10 fin cases were 11.89, 19.92, and 23.33 h. The same cases discharged the system in 11.39, 19.50, and 27.28 h. Equal cases with 20 annular fins charged the system in 10.00, 22.78, and 28.08 h, and discharged in 9.08, 18.14, and 26.22 h. Increased convection from porosity cannot overcome reduction of effective thermal conductivity.

The effect of in-situ TiB2 segregated network structure on the thermoelectric properties of spark plasma sintered B4C ceramics

Different amounts of TiC particles between 1 and 7.5 vol% were coated on spray-dried B4C granules with a simple dry particle coating method, then spark plasma sintered at 2200 °C under vacuum, forming 3D-segregated network structures (SNS) of in-situ TiB2 between 1 and 9 vol%, to improve the thermoelectric (TE) performance of B4C. With the in-situ TiB2-SNS, a maximum ZT of 8.02×10−2 was achieved with 2 vol% TiB2-SNS at 923 K by simultaneous improvement of electrical conductivity due to the electrically conductive network of TiB2 and reduction of thermal conductivity due to the increased phonon scattering, improving the ZT of monolithic B4C by an impressive ≈ 120%. Higher quantities of TiC addition proved inefficient in improving the TE performance; by forming 9 vol% TiB2, ≈ 945% increase in electrical conductivity was achieved; however, the Seebeck coefficient was affected negatively, causing a drastic decrease in the ZT value.

Remarkable Thermal Conductivity Reduction of Silicon Nanowires during the Bending Process.

The one-dimensional geometry of silicon nanowire helps to overcome the rigid and brittle nature of bulk silicon and enables it to withstand substantial bending stresses. This provides exciting opportunities for the development of flexible electronics. The bending strain introduces atomic displacement in the lattice structure, which inherently has a significant impact on the thermal conductivity. The strain-dependent thermal conductivity of silicon nanowire is crucial to the thermal management and performance of flexible electronic devices. However, in situ thermal conductivity measurement of bending silicon nanowires remains challenging and unreported due to the varying thermal contact resistances between the sample and sensor/heat sink. In this study, the Raman spectroscopy-assisted steady state thermal conductivity measurement method is coupled with a micromanipulation system to successively monitor the thermal conductivity variation of silicon nanowires during the bending process. The result shows that the thermal conductivity of silicon nanowires steeply decreases 55-78% owing to the strain-induced structural deformation during bending. Furthermore, the proposed in situ thermal conductivity measurement method can also be extended to other nanomaterials.

Preparation of eco-friendly functional lightweight gypsum: Effect of three different lightweight aggregates

Phosphogypsum (PG) is the main solid waste produced in the phosphorous chemical industry, and its resource utilization has become an international problem. Simultaneously, the research and development of lightweight building materials are the key means to achieving building energy conservation and reducing energy consumption. This paper uses hemihydrate phosphogypsum-based cementitious material as the matrix material, and porous structure lightweight aggregate (LWA) as the lightweight component to prepare lightweight gypsum. The properties of the composites were carefully investigated by the examination of mechanical strength, bulk density, drying shrinkage, thermal conductivity, sound absorption coefficient, pore structure, and microstructure. The results show that it can be achieved to obtain lightweight gypsum with a bulk density ranging from 300 kg/m3 to 1500 kg/m3. In terms of thermal behaviour and sound absorption, the lightweight gypsum with pottery sand has the smallest reduction in thermal conductivity, and EPS particles have the largest reduction. Through the introduction of a cavity resonance structure, expanded perlite significantly improved the sound-absorbing performance of the composites, while pottery sand had the least favorable effects. Pottery sand reduces the number of open pores, while expanded perlite increases the number of open pores in the composites. However, the addition of EPS particles can greatly increase the number of closed pores in the composites. This research offers a useful method for the preparation of eco-friendly and functional integrated lightweight gypsum.

Impact of nonstoichiometry on the mechanical properties and thermal conductivity of gadolinium zirconate ceramics

Low elastic properties, high hardness, and low thermal conductivity are desirable for a rare-earth zirconate thermal barrier coating material. Realizing nonstoichiometry is a significant way to improve the elastic properties, hardness and thermal conductivity of rare-earth zirconate to further meet the requirements of thermal barrier coating materials, nonstoichiometric Gd2-xZr2+xO7+0.5x (x = 0, ±0.125, and ±0.25) was studied by first-principles calculations first and then verified by the solid-state reaction methods. Calculations revealed that excess Gd3+ and Zr4+ decrease the elastic modulus, sound velocity, and Debye temperature of Gd2Zr2O7, and increase its ductility. The elastic properties of the material exhibit significant anisotropy, which further increases with increasing Gd3+ and Zr4+ contents, resulting from lattice distortion caused by excess Gd3+ and Zr4+ entering the lattice. The hardness of Gd2Zr2O7 increases with a moderate excess of Zr4+, while it decreases with an excess of Zr4+ and Gd3+. In addition, with the disordered occupation of lattice atoms, the anharmonicity of lattice waves is increased, and phonon scattering is enhanced as the concentrations of Gd3+ and Zr4+ increase, resulting in the reduction of the phonon thermal conductivity. Moreover, Zr4+ is more conducive to the decrease in the thermal conductivity. The experimental consequences match well with the calculation consequences. This work is expected to provide support for improving mechanical properties and thermal conductivity of rare-earth zirconate materials.

3D Printing of Bi2Te3-Based Thermoelectric Materials with High Performance and Shape Controllability.

Thermoelectric (TE) energy conversion technology provides a promising way to improve the efficiency of fossil energy by generating electricity from low-grade waste heat. With regard to these applications, thermoelectric generators (TEGs) should be designed from system integration perspectives to simultaneously improve heat transfer efficiency and system simplification as well as the robust mechanical properties. However, typical TEGs fabricated by conventional methods barely accomplish such requirements. Herein, high-quality TEGs were assembled by combining the well-flowable spherical bismuth telluride (BT) powdered precursors and selective laser melting (SLM) technology. By optimizing the electronic and phonon transport properties through defect engineering driven by 3D printing, a high figure of merit was accomplished for 1.27 (p-type) and 1.13 (n-type) in BT. This achievement is primarily attributed to the nonequilibrium solidification mechanism, which leads to the formation of multiscale defects during the 3D printing process. The introduction of these multiscale defects enables the effective scattering of wide frequency phonons, leading to a substantial reduction in lattice thermal conductivity. Meanwhile, robust mechanical properties were obtained in the printed p-type/n-type BT TE materials parallel to the building direction (BD) with a compressive strength reaching 257/250 MPa by employing the fine grain structure and the high density of nanotwins introduced during the SLM process. A well shape-controllable and high-performance TEG was designed using 3D-printed BT half-rings, and an output power of 134 mW was achieved at a temperature gradient of 38.9 °C. Our study opens a new route for the great potential of TE materials based on standard commercial SLM 3D printing technology for low-grade waste heat emitted from structures with heterogeneous shapes.

First-Principles Investigation of Point Defects on the Thermal Conductivity and Mechanical Properties of Aluminum at Room Temperature

The effects of point defects on the mechanical and thermal conductivity of aluminum at room temperature have been investigated based on the first-principles calculations combined with the Boltzmann equation and the Debye model. The calculated results showed the equilibrium lattice constants a0 of all REAl are larger than that of Al, and the defective formation energy Ef of all REAl is lower than that of VAl. Both a0 and Ef increase from Sc to La and then decrease linearly to Lu. The effects of solute atoms on the mechanical properties of the Al matrix were further calculated, and compared with Al, it is found that the REAl defects decrease the elastic constant Cij, Cauchy pressure C12–C44, bulk modulus B, shear modulus G, Young’s modulus E, B/G and Poisson’s ratio ν of Al, except for C44 of REAl (RE = La-Nd). With the increase of atomic number, the C11 and E of Al-containing REAl decrease from Sc to La and then slowly increase to Lu, whereas C12, C44, B, and G have little change. Meanwhile, the values of C12–C44 and B/G of Al-containing REAl increase from Sc to Ce, and it slightly change after Ce, while ν is nearly unchanged. All defects containing Al present nonuniform and ductility. Finally, the effects of rare earth (RE) atoms on the thermal conductivity (TC) of Al alloys have been investigated based on the first-principles calculations. The reduction of TC of Al alloys by RE solute atoms REAl is much greater than that by the L12 Al3RE phase with the same concentration of RE, which is in good agreement with the experiments. With the RE atomic number increasing, the total TC κ of the Al-RE solid solution decreases from Sc to La firstly and then increases linearly to Lu. Moreover, the decrement of TC Δκ of the Al matrix by early REAl (RE = La-Sm) is larger than that by VAl, while the later REAl (RE = Gd-Lu) shows the opposite influence.

In-depth analysis of the influence of bio-silica filler (Didymosphenia geminata frustules) on the properties of Mg matrix composites

A novel metal matrix composites (MMC) with Mg matrix reinforced with natural filler in the form of Didymosphenia geminata frustules (algae with distinctive siliceous shells) are presented in this work. Pulse plasma sintering (PPS) was used to manufacture Mg-based composites with 1, 5 and 10 vol.% ceramic filler. As a reference, pure Mg was sintered. The results show that the addition of 1 vol.% Didymosphenia geminata frustules to the Mg matrix increases its corrosion resistance by supporting passivation reactions, and do not affect the morphology of L929 fibroblasts. Addition of 5 vol.% the filler does not cause cytotoxic effects, but it supports microgalvanic reactions leading to the greater corrosion rate. Higher content than 5 vol.% the filler causes significant microgalvanic corrosion, as well as increases cytotoxicity due to the greater micro-galvanic effect of the composites containing 10 and 15 vol.% diatoms. The results of contact angle measurements show the hydrophilic character of the investigated materials, with slightly increase in numerical values with addition of amount of ceramic reinforcement. The addition of Didymosphenia geminata frustules causes changes in a thermo-elastic properties such as mean apparent value of coefficient of thermal expansion (CTE) and thermal conductivity (λ). The addition of siliceous reinforcement resulted in a linear decrease of CTE and reduction in thermal conductivity over the entire temperature range. With the increasing addition of Didymosphenia geminata frustules, an increase in strength with a decrease in compressive strain is observed. In all composites an increase in microhardness was attained.The results clearly indicate that filler in the form of Didymosphenia geminata frustules may significantly change the most important properties of pure Mg, indicating its wide potential in the application of Mg-based composites with a special focus on biomedical use.

Effect of four-phonon scattering on the intrinsic thermal conductivity of penta-graphene

Penta-graphene (PG), as a novel and stable pentagonal structure, has attracted considerable attention due to its large band gap, ultrahigh strength, negative Poisson's ratio, which indicates promising potential in nanoelectronics and nanomechanics. Thermal conductivity is a key physical quantity for the potential applications of PG based nano-devices. However, previous studies focused on the thermal conductivity of PG dominated by the well-known cubic anharmonic effect, and the quartic anharmonicity, although probably significant, was still neglected. In this work, we study the effect of four-phonon scattering on the thermal transport in PG through using first-principles calculations and solving the Boltzmann transport equation (BTE). We find that the intrinsic lattice thermal conductivity of PG is 182.1 W/mK at room temperature after including four-phonon scattering, which is reduced by 73.5 % as compared to the obtained results with only three-phonon scattering. Meanwhile, the relative contribution of the flexural acoustic (ZA) branch is reduced from 60.5 % to 32.5 % when four-phonon scattering is included. Thus, the large reduction in thermal conductivity can be ascribed to the large four-phonon scattering rates, which are completely comparable with those of three-phonon processes in the acoustic phonon frequency range even at 300 K. It is well believed that this finding is not a special case but should exist in other 2D materials with similar phonon features. Furthermore, the validity of the sum of three- and four-phonon scattering rates has been verified by the results based on the MD simulations. Our finding provides critical revisit and valuable insight to the exact thermal conductivity value of PG.

Thermal Conductivity of 3C/4H-SiC Nanowires by Molecular Dynamics Simulation.

Silicon carbide (SiC) is a promising material for thermoelectric power generation. The characterization of thermal transport properties is essential to understanding their applications in thermoelectric devices. The existence of stacking faults, which originate from the "wrong" stacking sequences of Si-C bilayers, is a general feature of SiC. However, the effects of stacking faults on the thermal properties of SiC are not well understood. In this study, we evaluated the accuracy of Tersoff, MEAM, and GW potentials in describing the thermal transport of SiC. Additionally, the thermal conductivity of 3C/4H-SiC nanowires was investigated using non-equilibrium molecular dynamics simulations (NEMD). Our results show that thermal conductivity exhibits an increase and then saturation as the total lengths of the 3C/4H-SiC nanowires vary from 23.9 nm to 95.6 nm, showing the size effect of molecular dynamics simulations of the thermal conductivity. There is a minimum thermal conductivity, as a function of uniform period length, of the 3C/4H-SiC nanowires. However, the thermal conductivities of nanowires weakly depend on the gradient period lengths and the ratio of 3C/4H. Additionally, the thermal conductivity of 3C/4H-SiC nanowires decreases continuously from compressive strain to tensile strain. The reduction in thermal conductivity suggests that 3C/4H-SiC nanowires have potential applications in advanced thermoelectric devices. Our study provides insights into the thermal transport properties of SiC nanowires and can guide the development of SiC-based thermoelectric materials.

Lithium‐Filled Skutterudites by Intercalation at Ambient‐Temperature

AbstractThe incorporation of lithium as a filler species in Co1‐2xFexNixSb3 skutterudites was accomplished by intercalation at 60 °C, using n‐BuLi as a reducing agent. Solid state 7Li NMR and ICP‐MS analysis confirm the presence of lithium in the product phases and provide an estimate of the lithium content. The maximum uptake of lithium increases as cobalt is progressively substituted by an equimolar mixture of iron and nickel. Difference Fourier maps, calculated during Rietveld structure refinement using powder neutron diffraction data, locate the lithium cations at the 2a (0,0,0) sites within the cavities of the skutterudite framework. The intercalation of lithium results in reductions in thermal conductivity of up to 47 %, indicative of phonon glass electron crystal (PGEC) type behaviour. Charge transfer from lithium to the framework that accompanies intercalation results in a substantial decrease in electrical resistivity in lithiated phases and a more metal‐like temperature dependence. The increased carrier concentration also decreases the Seebeck coefficient, with the consequence that modest increases in the figure of merit occur, despite the reduced thermal conductivity.

Structure and thermal properties of SrCe1-xSnxO3 (x=0.1 … 0.5) as a promising thermal barrier coating

Gas turbines efficiency growth is primarily associated with an increase in the operating temperature of the combustion chamber, which places new stringent requirements on the materials of thermal barrier coatings. Strontium cerate doped with tin SrCe1- xSnxO3 where x = 0.1 … 0.5, was proposed as a promising material. The research has shown that the lightly doped solid solution SrCe1-xSnxO3 has an orthorhombic Pnma structure at x < 0.3, whereas at a high content of Sn4+ the monoclinic structure P21/m becomes more favorable. Thermogravimetric analysis (TGA) in reducing atmosphere (5%H2 in Ar) shows no mass lost as a result of unchangeable charge of Ce4+ and Sn4+. An increase in the distortion of the crystal lattice, due to the large difference in the ionic radii of Ce4+ and Sn4+, leads to a deterioration in the symmetry of the crystal lattice, a reduction of thermal conductivity (from 1.9 to 1.4 W m−1 K−1 at 1000 °C) and at the same time, growth of hardness and porosity. The increase in porosity, along with an increase in the required temperature of solid-state synthesis, indicates an enhancement in the melting point of the obtained materials. For the compounds with an orthorhombic structure, the thermal expansion coefficient increases with a growth in the Sn content, achieving a highest point 12.47·10−6 K−1 at 1100 °C for x = 0.3. The combination of the revealed properties and their comparison with advanced refractories makes the solid solution, primarily SrCe0.5Sn0.5O3, a promising material for application as thermal barrier coatings.

Directional enhancement and potential reduction of thermal conductivity induced by one-dimensional nanoparticle addition in pure PCMs

To increase the thermal conductivity of organic phase change material (PCM), One-dimensional materials are often used as an additive due to their excellent thermal performance. Many experiments have shown that micro effects have a great influence on the thermal conductivity of composite materials. To clarify the micro effect, numerous molecular dynamics simulations (MD) have been carried out. Previous research has entirely overlooked the thermal conductivity changes of pure PCM in composite systems. Neglecting the thermal conductivity changes of pure PCM is unreasonable, as the mass fraction of nanoparticles is often small. We took paraffin wax (PW) with carbon nanotube (CNT) as an example, and successfully reproduced the phase transition of composite PCM. Then we optimized the details of simulating thermal conductivity, and two processes of replication and movement were taken to get a stable thermal conductivity. A disassembly of thermal conductivity based on heat flux was actualized to investigate the thermal conductivity of pure PCM in composite systems. We investigated the difference in thermal conductivities between solids and liquids and analyzed the underlying mechanism through vibrational changes. The change of thermal conductivity in solids is contributed by CNT itself and the structure of PW; the change of thermal conductivity in liquids is contributed by CNT itself and the interaction of CNT. Among these factors, structure change wouldn't always be beneficial. CNT always induces an axial enhancement of thermal conductivity in pure PCMs. At last, verification was carried out to prove the importance of structure. Our study brought a new view to understanding the behavior of thermal conductivity in composite PCM.

Complex microstructure induced high thermoelectric performances of p-type Bi–Sb–Te alloys

In this research, we have fabricated the complex microstructure features along with the high density of twin boundaries using the combination of magnetic pulse compaction (MPC) and spark plasma sintering (SPS), which are exclusively improved the thermoelectric transport properties of p-type Bi0.5Sb1.5Te3 alloys via effective scattering of carriers and phonons at the grain boundary interfaces. The texture analysis confirmed the formation of bimodal (mixed grains) microstructural features in MPC + SPS specimens sintered at 350 °C, and 400 °C respectively due to the adhesion of fine grains (grain growth) and initiation of recrystallization at high temperatures. In addition to high density of twin structure, high-angle grain boundaries have also been identified. The power factor of 3.53 mW/m.K2 was obtained for the MPC + SPS at 400 °C due to increased Seebeck coefficient and larger electrical conductivity values. The maximum reduced thermal conductivity of 1.14W/mK was recorded due to a significant reduction in lattice thermal conductivity, which is mainly owing to strong scattering of phonons at the formation of mixed grain microstructures with high-density dislocations. As a result, maximum figure of merit, ZT = 1.17was achieved for MPC + SPS at 400 °C temperature.

Superior thermoelectric performance in non-stoichiometric Cu3SbSe4 system: Towards synergistic optimization of carrier and phonon transport

To investigate the effect of cation disorders to modulate thermoelectric performance of Cu3SbSe4 system, we attempted to tune copper content in Cu3+xSbSe4 (x = -0.06, -0.04, 0, 0.04, 0.06, and 0.08) system synthesized via solid-state reaction route. Considering the asymmetry in charge and phonon transport properties, intentional deviations from the proper stoichiometry successfully enhance the electrical transport and reduce the phonon transport simultaneously. The self-doping effect induced by the off stoichiometry in Cu3SbSe4 provides acceptor levels, thereby elevating the electrical conductivity. Modulating the Fermi level within the valence band, we could realize a power factor to the highest value of ∼232 µW/mK2 for the sample with x = -0.06 at 210 K. Considerable reduction in thermal conductivity is the key factor in enhancing the figure of merit to a maximum value of ∼0.033 (at 350 K) for the sample with x = 0.06, which about three times higher than that of the pristine sample. The present study demonstrates that non-stoichiometry plays a substantial role in modulating the thermoelectric transport of the Cu3SbSe4 system.

Machine learning accelerated design of 2D covalent organic frame materials for thermoelectrics

Both thermodynamic stability and high thermoelectric figure of merit are required for organic thermoelectric materials for practical application. However, it is extremely time-consuming to design advanced organic thermoelectric materials via conventional theoretical calculation. In this work, combining first-principles calculations, machine learning fitted potential and solving phonon Boltzmann transport equation, we propose a new group of two-dimensional covalent organic frame semiconductors 2AL-PR-X (X = 2H, Ni, Pt, Zn) as promising organic thermoelectric materials. We found that the embedding metal atoms in the center can enhance remarkably the stability of porphyrin ring, demonstrated by evaluating the system total energy and phonon dispersion relationship. Moreover, the interaction between the metal atoms and the porphyrin ring will affect the phonon modes in the porphyrin ring, result in reduction in thermal conductivity. We found that 2AL-PR-Pt is the best thermoelectric candidate material among the studied 2AL-PR-X, which has a peak ZT of 0.32 at room temperature and can reach around 0.9 at 800 K. Our work provides strategy for the thermal transport regulation of two-dimensional covalent organic frame materials and demonstrates the large potential of 2AL-PR-X materials for thermoelectric applications.

Polymer-mixed Sb2Te3/Te nanocomposites exhibiting p-type to n-type conduction reversal and thermal conductivity reduction

Sb2Te3-based materials are potential room-temperature thermoelectric materials. In the present work, we choose polycrystalline Sb2Te3/Te nanocomposites and utilize Poly Methyl Methacrylate (PMMA) to reduce the thermal conductivity of Sb2Te3 samples. PMMA and polycrystalline Sb2Te3/Te were well mixed using ball milling. Pellets have been made by the cold press method. Thermoelectric transport properties of Sb2Te3/Te nanocomposites: composition, microstructure, and analysis are found to be influenced by PMMA. With increasing PMMA concentration a p-type to n-type transition has been observed because there are fewer charge carriers or the composites have a higher resistance. It is also observed that the thermal conductivity of Sb2Te3/Te nanocomposites decreases as the PMMA increases. This research paves the way for making the best thermoelectric materials by reducing thermal conductivity through the use of polymers.

Alloy-induced reduction and anisotropy change of lattice thermal conductivity in Ruddlesden–Popper phase halide perovskites

Alloy-induced reduction and anisotropy change of lattice thermal conductivity in Ruddlesden–Popper phase halide perovskites