Reduction In Thermal Conductivity Research Articles

Enhancing thermoelectric properties of higher manganese silicide through facilitated non-equilibrium reactions via the introduction of additional Carbon nano-dots

Higher manganese silicide (HMS) has gained significant attention as a compelling choice for thermoelectric (TE) applications at medium temperatures due to its abundance, environmental friendliness, and impact resistance properties. Recent achievement in HMS involves leveraging the synergistic effect of energy filtering, modulation doping, and boundary scattering to enhance the TE performance. However, excellent TE properties on HMS usually demands extreme preparation processes such as shock compression, melt spinning technique, or chemical vapor transport approach, as well as the use of rare and expensive elements or nano-dots (NDs), such as Re, Ge, Ag/Pt alloy, or CsPbBr3 quantum dots. In this study, a facile wet ball milling combing with spark plasma sintering is used to streamline the synthesis processes. Relatively green, cost-effective, and accessible C NDs are used to facilitated non-equilibrium reactions and enhance TE properties of HMS. Due to the C-doped effect, the PF increased from 1.42 to 1.62 mW m−1 K−2 at 723 K, representing an approximately 14 % improvement compared to the pristine HMS. The introduction of C, derived SiC and Mn NDs result in the enhancing phonon scattering, and leading to a reduction in lattice thermal conductivity from 2.21 to 1.81 W m−1 K−1. Consequently, the figure of merit (zT) increases from 0.42 to 0.55 at 823 K, representing an enhancement of approximately ∼ 31 %. Predictably, the incorporation of C NDs into semiconductors stands as one of the effective strategies for enhancing the TE properties.

Desalination setup coated by silicone epoxy nanocomposite: A study on hydrophobic characteristics and condensation mechanisms

The formation of tortuous filmwise condensation in desalination setup alongside with reduction in thermal conductivity is a challenge accompanied by efficiency loss. This issue is directly influenced by hydrophobicity, which is a function of surface roughness, as well as surface chemistry. In this sense, the hydrophobicity of variant silicone epoxy based nanocomposites was evaluated to identify the optimum proportion composite for surface condensation of desalination setup. To this end, three types of fumed silica with distinct surface chemistry were utilized in a matrix at the weight percentage of 0.5, 1, 1.5, and 2. To investigate the hydrophobicity, roughness, and distribution of reinforcement, a series of characterization techniques, including contact angle, AFM, and FESEM were used. It was found that, in comparison to pure matrix, the nanocomposite modified with 5 wt % of hydrophobic additive and 0.5 wt % of hydrophilic fumed silica nanoparticles (SE/5B/0.5R200) has the highest contact angle (about 35%) and roughness (13 times), due to the migration of nanoparticle onto the surface. Also, coating the condensation surface of the desalination setup with optimum thickness, i.e. 60 × 10−6 m, resulted in the efficiency improvement by about 33 % compared to the uncoated setup, even after 168 hrs. Indeed, dropwise condensation, straight path movement of droplets, besides thermal conductivity adjust the efficiency.

Construction of Polysiloxane Induced Multilevel Bowl-shaped Janus Particles for Fabrication of Photoluminescent, Enhanced, Heat-insulative Poly(styrene-co-butyl acrylate) Latex Coating

Silica-based nanoparticles (SNPs) are highly regarded as advantageous nanofillers for composite coating, owing to their high surface area, robust mechanical properties, chemical stability, and adaptability for surface modification. Recent studies have unveiled the potential photoluminescence of SNPs induced by intrinsic defect sites, eliminating the need for additional fluorescein molecules. However, the inadequate quantum yield and complex preparation processes for generating defect sites within SNPs have constrained their broader applications. Herein, we propose a facile method to prepare Janus silicon-based nanoparticles with a hierarchical bowl-shaped structure (JHSNPs) through the emulsification of methyltriethoxysilane hydrolysis and condensation products, coupled with the capillary force during nanoparticle formation. The distinctive structure enhances the formation of dioxasilyrane (=Si(O2)) and silylene (=Si:) defects, thereby exhibiting strong blue photoluminescence. The JHSNPs then serve as nanofillers for commonly used poly(styrene-co-butyl acrylate) (SA) waterborne coatings, significantly enhancing their mechanical properties, abrasion resistance, and UV radiation resistance compared to the original SA coatings. Moreover, with just a 1% addition of JHSNPs, the composite coating exhibits visible fluorescence intensity, conferring the self-detecting monitoring properties for external scratches. Due to the hierarchical air-entrapping void structure, JHSNPs composite coatings demonstrate excellent thermal isolation performance, with a 22% reduction in thermal conductivity. The design of JHSNPs opens new avenues to expand the versatility of silica-based nanoparticles in composite coatings, especially in the optical, safety, and energy-saving fields.

Utilizing bio-based and industrial waste aggregates to improve mechanical properties and thermal insulation in lightweight foamed macro polypropylene fibre-reinforced concrete

This research intends to examine the potential of incorporating waste materials in place of coarse aggregates to augment the mechanical characteristics and thermal insulation of lightweight foamed concrete (LWFC), along with the inclusion of polypropylene (PP) fibres. The PP fibre exhibits excellent tensile strength and enhancement on thermal insulation properties of cementitious materials. In this study, eco-friendly bio-based oil palm shell (OPS) and robust solid polyurethane (SPU) aggregates derived from industrial waste were utilized as substitutes for conventional aggregates. Additionally, silica fume was partially incorporated as supplementary cementitious materials. The addition of PP fibre impacted the workability of the LWFC with the inverted slump value decreased by 1.7 %–17.5 %. The inclusion of PP fibre proportion increased from 0 % to 0.5 % revealed a positive result in mechanical properties and thermal insulation performance. The OPSSPU/0.5 showed the significantly increment of 40.4 % compressive strength, 102.9 % of splitting tensile strength, and 70.8 % flexural strength, respectively. The optimum result was obtained in the OPSSPU/0.5 mixture, which exhibited 3.74 MPa residual compressive strength. Furthermore, utilizing PP fibres brought about a significant reduction in thermal conductivity in the environmentally sustainable LWFC, leading to improvements ranging from 0.2 % to 16.3 % when compared to the control composition. Thus, these findings are essential for encouraging the adoption of such practices in the construction industry and contributing to the reduction of environmental impacts associated with sustainable concrete production.

Synergistic effect of fly ash and polyvinyl alcohol fibers in improving stability, rheology, and mechanical properties of 3D printable foam concrete

Foam concrete, a lightweight cellular cementitious mixture has gained significant attention globally due to its multifunctional properties. Employing such multifunctional material to additive manufacturing helps in reducing the material wastage, time for construction and cost of production. However, there are various challenges encountered in employing such material with conflicting attributes in 3D concrete printing technology. Therefore, it is important to address these challenging requirements (i.e. stability along with printability). In this study, fly ash and polyvinyl alcohol fibers are employed as sand replacement and reinforcing material respectively to improve stability, rheology and mechanical characteristics of 3D printable foam concrete. Initially, printability of the mixture is achieved through moderation of slump and slump flow. Further, these mixtures are studied for fresh density, rheological properties and buildability. Essential hardened properties like oven dry density, compressive strength, flexural strength, bond strength, and thermal conductivity are studied. Later, microstructural studies are conducted to analyze the variations in hardened properties. Results show that replacement of sand with fly ash has significant influence (more than 100%) in improving the printability (for both 1300 and 1000 kg/m3 densities), however it has negative impact on the stability of foam. Stability of foam concrete could be counteracted by adding the fibers which forms networks with the foam bubbles and reduced the breakage and coalescence. Nonetheless, addition of fly ash increased the mechanical performance and thermal conductivity, but the dry density is increased due to bubble breakage. Addition of fibers reduced dry density with significant reduction in thermal conductivity and minimal compromise in mechanical performance.

Effect of fission products on the thermal conductivity of ThO2-A molecular dynamics study

Thermal conductivity (k), as an important thermal property of nuclear fuels, would be deteriorated due to fission products. Therefore, to investigate the effect of fission products on the thermal conductivity of nuclear fuels is essential. Two typical fission products: Xe and Kr with 0–2 % concentration are considered in this work. The lattice constants (L) of ThO2 increase due to fission products at all testing temperatures. The extent of increase in L due to Xe interstitials is the maximum. The fission products significantly reduce the thermal conductivity of ThO2. The extent of reduction in thermal conductivity of ThO2 by the defects follows the trend Xe (interstitials) > Xe (substitutional defects) > Kr (substitutional defects) > Kr (interstitials). Finally, the full-filled Xe/Kr bubble has a nearly identical thermal conductivity as an empty void or half-filled Xe/Kr bubble. The underlying reason may be that the thorium atoms have a lower mobility than uranium atoms. These calculated values can be used to predict the thermal properties of the irradiated ThO2.

Simultaneous realization of high power factor and low thermal conductivity in medium-entropy La-doped (Sr1/3Ba1/3Ca1/3)1-xLaxTiO3 oxides with porous structure

Some studies have proven the effectiveness of high-entropy alloying in reducing lattice thermal conductivity; however, the synchronous degradation in carrier mobility limits the thermoelectric properties of high-entropy materials. Herein, we present a series of medium-entropy oxides (Sr1/3Ba1/3Ca1/3)1-xLaxTiO3 (x = 0.05, 0.1, 0.15) with different levels of La doping, breaking the trade-off between carrier and phonon transport. The medium-entropy design helps keep the carrier mobility at a moderate level while modulating the carrier concentration through La3+ doping, thereby yielding a peak power factor value of 1842 μW/(m⋅K2) at 773 K. Meanwhile, the incorporation of nano-micro-porous structure and medium entropy effect leads to a significant reduction in lattice thermal conductivity, reaching 2.3 W/(m⋅K) at 1073 K. Consequently, a ZTmax value of 0.67 is obtained for the MEP-10La sample at 1073 K, which is one of the state-of-the-art ZT values reported for the SrTiO3-based materials. The present work provides a viable approach for the development of thermoelectric materials with enhanced performance.

Nanocrystal-based thermoelectric SnTe-NaSbSe2 alloys with strengthened band convergence and reduced thermal conductivity

Ternary I-V-VI2 colloidal NaSbSe2 nanocrystals are herein used to improve the performance of lead-free SnTe thermoelectric materials. We showcase a versatile bottom-up engineering approach to produce nanocrystal-based SnTe-NaSbSe2 alloys from the rapid hot press of colloidal nanocrystal building blocks. The incorporation of NaSbSe2 nanocrystals significantly enhances the Seebeck coefficient of SnTe. The band convergence and simultaneous increasing band gap of SnTe-NaSbSe2 alloys are certified by the first-principles density functional theory calculations. Besides, defect engineering generated by the incorporation of NaSbSe2 nanocrystals such as Sn vacancies, substitution point defects, dense dislocations, and strains generated by the NaSbSe2 nanoparticles incorporation result in a dramatic reduction of the lattice thermal conductivity below the amorphous limit of pure SnTe, down to 0.38 W m−1 K−1. As a consequence, power factors enhance up to 1.77 mW m−1 K−2, which is ∼193 % higher than that of the pristine SnTe, and thermoelectric figures of merit up to 1.15 at 823 K for (SnTe)0.85(NaSbSe2)0.15 are achieved.

Efficient rhombohedral GeTe thermoelectrics for low-grade heat recovery

The existence of Ge vacancy induces an inherently hole concentration up to 1021 cm−3 in pristine thermoelectric GeTe, which results in a need of ∼10% aliovalent dopants for optimization. However, the extra carrier scattering by the dopants would notably deteriorate the carrier mobility. In this work, CuSbTe2-alloying is revealed to increase the formation energy of Ge-vacancy, leading to a rapid reduction in hole concentration down to 1.3×1020 cm-3 but with a mobility above 100 cm2/V-s. Along with a further Pb-substitution at Ge sites for lattice thermal conductivity reduction and band alignment, a peak figure of merit zT of ∼2.4 at 625 K is eventually realized in the rhombohedral structure, leading to a specific power density of as high as 70 W/m and a conversion efficiency of ∼10% under a temperature difference of 302 K. This illustrates r-GeTe as truly efficient thermoelectrics particularly for low-grade heat recovery applications.

Thermoelectric performance enhancement of Pb-doped α -MgAgSb near room temperature

α-MgAgSb is taken as the p-type leg material for recently focused Mg-based thermoelectric devices because of the high thermoelectric performance near room temperature. However, the thermoelectric performance of α-MgAgSb is inhibited by the existence of the Ag-rich second phase. The ordinary methods like carrier concentration optimization and minimizing lattice thermal conductivity were nearly invalid because of the extremely low doping level for heteroatoms and intrinsically low lattice thermal conductivity. The crystal structure of α-MgAgSb can be viewed as Ag atom filled in half distorted hexahedron in the distorted rock salt skeleton formed by the Mg–Sb sublattice. In this work, by replacing the smaller Mg in the sublattice with Pb, the volume of the distorted hexahedron is effectively expanded to accommodate Ag atoms and then lead to the re-dissolution of Ag-rich second phase in the matrix. In addition, as Ag is the main source of low-frequency phonons, the enhanced lattice anharmonicity by Pb doping leads to stronger scattering of phonons in the distorted hexahedron and results in 20% reduction of lattice thermal conductivity in the temperature range of 300–500 K. Finally, the figure of merit zT is enhanced by ∼40% in the whole temperature range, demonstrating that lattice management is a promising method for the optimization of α-MgAgSb materials.

Thermal conductivity reduction due to phonon geometrical scattering in nano-engineered epitaxial germanium

Nano-engineering crystalline materials can be used to tailor their thermal properties. By adding new nanoscale phonon scattering centers and controlling their size, one can effectively decrease the phonon mean free path, hence the thermal conductivity of a fully crystalline material. In this Letter, we use the 3ω method in the temperature range of 100–300 K to experimentally report on the more than threefold reduction of the thermal conductivity of an epitaxially grown crystalline germanium thin film with embedded polydispersed crystalline Ge3Mn5 nano-inclusions with diameters ranging from 5 to 25 nm. A detailed analysis of the structure of the thin film coupled with Monte Carlo simulations of phonon transport highlights the role of the nano-inclusions volume fraction in the reduction of the phononic contribution to the thermal conductivity, in particular its temperature dependence, leading to a phonon mean free path that is set by geometrical constraints.

Thermophysical Properties of Compressed Earth Blocks Incorporating Natural Materials

Building materials are responsible for significant CO2 emissions and energy consumption, both during production and operational phases. Earth as a building material offers a valuable alternative to conventional materials, as it naturally provides high hygrothermal comfort and air quality even with passive conditioning systems. However, disadvantages related to high density, conductivity, and wall thickness prevent its effective inclusion in the mainstream. This research explores enhancing the thermophysical properties of compressed earth blocks (CEBs) by using locally sourced natural materials. The study is framed in the Portuguese context and the natural materials involved are wheat straw (WS) as a by-product of wheat harvesting, cork granules (CGs) from bottle caps, and ground olive stone (GOSs) residues from olive oil production. Blocks were produced with different mixtures of these materials and the thermal response was examined in a hot box apparatus. Best results include a 20 and 26% reduction in thermal conductivity for mixtures with 5v.% CG and 10v.% GOS, respectively, and an associated reduction in bulk density of 3.8 and 5.4%. The proposed approach therefore proves to be effective in improving the key thermophysical characteristics of CEBs. The article includes a comparative analysis of the experimental data from this study with those from the literature. The study contributes to the growing knowledge of sustainable materials, providing insights for researchers and practitioners looking for innovative solutions for low-carbon and energy-efficient materials.

Probing the origin of abnormally strong electron-phonon interaction in phonon transport of semiconductor C3B monolayer

Recently, the two-dimensional semiconductor C3B monolayer has attracted much attention owing to its excellent physical, optical, and electronic properties. In this work, the origin of electron-phonon interaction (EPI) on the thermal properties of C3B by n-type and p-type doping is systematically investigated via first-principles calculations to provide fundamental knowledge for the thermal management the C3B-based electronic devices. The carrier concentrations for the largest reduction of the lattice thermal conductivity (κ) appear at 4 × 1014 cm−2 for n-type and 9 × 1014 cm−2 for p-type, which is closely related to the electron density of states (DOS). The boron (B) atoms break the structural symmetry and induce mass disorder scattering, which renders C3B more prone to the influence of EPI. Moreover, the electronic band structure in the C3B monolayer exhibits multivalley characteristics, which leads to an intervalley scattering. It is worth noting that the anisotropy in the C3B monolayer can be significantly enhanced by EPI. Additionally, an abnormal phenomenon of strong electron-phonon scattering but low electron-phonon coupling strength is found in C3B monolayer, which indicates that large electron-phonon coupling strength is sufficient but not necessary for strong electron-phonon scattering.

Tuning Thermal Conductivity of Hybrid Perovskites through Halide Alloying.

Tuning the thermal transport properties of hybrid halide perovskites is critical for their applications in optoelectronics, thermoelectrics, and photovoltaics. Here, an effective strategy is demonstrated to modulate the thermal transport property of hybrid perovskites by halide alloying. A highly tunable thermal conductivity of mixed-halide hybrid perovskites is achieved due to halide-alloying and structural distortion. The experimental measurements show that the room temperature thermal conductivity of MAPb(BrxI1- x)3 (x = 0─1) can be largely modulated from 0.27 ± 0.07W m-1 K-1 (x = 0.5) to 0.47 ± 0.09W m-1 K-1 (x = 1). Molecular dynamics simulations further demonstrate that the thermal conductivity reduction of hybrid halide perovskites results from the suppression of the mean free paths of the low-frequency acoustic and optical phonons. It is found that halide alloying and the induced structural distortion can largely increase the scatterings of optical and acoustic phonons, respectively. The confined diffusion of MA+ cations in the octahedra cage is found to act as an additional thermal transport channel in hybrid perovskites and can contribute around 10-20% of the total thermal conductivity. The findings provide a strategy for tailoring the thermal transport in hybrid halide perovskites, which may largely benefit their related applications.

Enhancement of thermoelectric performance in Mg3(Sb,Bi)2 through engineered lattice strain and controlled carrier scattering

Mg3(Sb,Bi)2 emerges as a promising thermoelectric (TE) material due to its rich elemental composition and high TE performance, offering potential for industrial applications. In this study, we found out that the elevated sintering temperatures can introduce dislocations, vacancies, and nanoscale precipitates in the lattice, inducing desirable lattice strain. Higher lattice strain broadens the phonon dispersion, effectively reducing lattice thermal conductivity. Simultaneously, high-temperature sintering increases grain size, significantly mitigating grain boundary carrier scattering and enhancing carrier mobility. As a result, we observe a 31.4 % increase in lattice strain, a 30 % reduction in lattice thermal conductivity, a 200 % rise in room temperature mobility, ultimately achieving an outstanding figure of merit (zT) of 1.7 at 750 K for Mg3.22Ho0.03Sb1.5Bi0.5. The synergistic effect of engineering lattice strain to minimize lattice thermal conductivity and controlling carrier scattering mechanisms to enhance mobility significantly provides a novel strategy to improve TE performance of Mg3(Sb,Bi)2-based materials.

Hydrogen evolution and thermal treatments for removal of plating induced impurities from Ni to extend life of solder joints

As operating temperatures and times continue to increase for solder joints with Ni pads, large voids inevitably form at the intermetallic compound (IMC) – solder interface when the IMC thickness nears 5 μm. This leads to a significant reduction of strength, thermal conductivity, and fatigue resistance, and in small joints, also to electromigration life degradation. We have identified the voiding root cause as the breakdown and subsequent outgassing of deposition-related Ni-oxo-hydroxide impurities, impractical to avoid during Ni commercial electroplating. This work takes as the starting point a successful post-deposition voiding-mitigation treatment with 24-hour annealing at 450 °C. It then explores the possibility of rendering this routine more practical by lowering the annealing temperature down to 250 °C. While that approach shows relatively limited success, an alternative strategy, involving treatment with hydrogen evolution reaction for one hour at room temperature, alone or in combination with thermal, leads to a substantial reduction in the voiding propensity.

First-Principles Study of Doped CdX(X = Te, Se) Compounds: Enhancing Thermoelectric Properties.

Isovalent doping offers a method to enhance the thermoelectric properties of semiconductors, yet its influence on the phonon structure and propagation is often overlooked. Here, we take CdX (X=Te, Se) compounds as an example to study the role of isovalent doping in thermoelectrics by first-principles calculations in combination with the Boltzmann transport theory. The electronic and phononic properties of Cd8Se8, Cd8Se7Te, Cd8Te8, and Cd8Te7Se are compared. The results suggest that isovalent doping with CdX significantly improves the thermoelectric performance. Due to the similar properties of Se and Te atoms, the electronic properties remain unaffected. Moreover, doping enhances anharmonic phonon scattering, leading to a reduction in lattice thermal conductivity. Our results show that optimized p-type(n-type) ZT values can reach 3.13 (1.33) and 2.51 (1.21) for Cd8Te7Se and Cd8Se7Te at 900 K, respectively. This research illuminates the potential benefits of strategically employing isovalent doping to enhance the thermoelectric properties of CdX compounds.

Unified deep learning network for enhanced accuracy in predicting thermal conductivity of bilayer graphene, hexagonal boron nitride, and their heterostructures

In this research, we utilized density functional theory (DFT) computations to perform ab initio molecular dynamics simulations and static calculations on graphene, hexagonal boron nitride, and their heterostructures, subjecting them to strains, perturbations, twist angles, and defects. The gathered energy, force, and virial information informed the creation of a training set comprising 1253 structures. Employing the Neural Evolutionary Potential framework integrated into Graphics Processing Units Molecular Dynamics, we fitted a machine learning potential (MLP) that closely mirrored the DFT potential energy surface. Rigorous validation of lattice constants and phonon dispersion relations confirmed the precision and dependability of the MLP, establishing a solid foundation for subsequent thermal transport investigations. A further analysis of the impact of twist angles uncovered a significant reduction in thermal conductivity, particularly notable in heterostructures with a decline exceeding 35%. The reduction in thermal conductivity primarily stems from the twist angle-induced softening of phonon modes and the accompanying increase in phonon scattering rates, which intensifies anharmonic interactions among phonons. Our study underscores the efficacy of the MLP in delineating the thermal transport attributes of two-dimensional materials and their heterostructures, while also elucidating the micro-mechanisms behind the influence of the twist angle on thermal conductivity, offering fresh perspectives for the design of advanced thermal management materials.

Realizing high conversion efficiency in all-ZrCoSb-based half-Heusler thermoelectric power generators

ZrCoSb-based thermoelectric materials exhibit promising potential for power generation applications due to their high performance in both p- and n-type constituents. In this work, a newly developed all-ZrCoSb-based module achieves a maximum conversion efficiency of ∼9.3 % at a temperature difference of 668 K. This high conversion efficiency is enabled by the enhanced average thermoelectric properties for both types of ZrCoSb-based materials via Bi alloying and Sn/Ni doping. The hole and electron carrier concentrations were tuned to match the theoretically predicted optimized values, contributing to high weight mobility and power factor. Additionally, the increased mass and strain field fluctuations enhance point defect scattering, leading to a significant reduction in the lattice thermal conductivity of ZrCoSb. Consequently, peak zT values of ∼1.2 and ∼1.0 at 973 K were respectively obtained in p-type ZrCo(Sb0.5Bi0.5)0.85Sn0.15 and n-type ZrCo0.85Ni0.15Sb0.5Bi0.5, resulting in high average zT values of ∼0.75 and ∼0.51 in the temperature of 300 to 973 K. This work underscores the significant potential of all-ZrCoSb-based half-Heuslers in waste heat recovery.

Understanding the anomalously low thermal properties of Zr3Ni3-xCoxSb4 thermoelectric material

By controlling the SPS temperature, we prepared Zintl Zr3Ni2.9Co0.1Sb4 samples, namely Co doping in Ni sites of Zr3Ni3Sb4, and investigated the corresponding thermoelectric properties. It was found that a significant reduction of the lattice thermal conductivity occurred due to Co-doping while tuning the SPS temperature slightly affected ZT values. The influence of aliovalent element Co doping on the phonon structure and transport properties of Zr3Ni3Sb4 was conducted through first-principles calculations. Co-doping enhanced phonon anharmonicity and induced a completely avoided crossing between acoustic and optical phonons, which resulted in the suppressed phonon velocity and lifetime, leading to a remarkable reduction in lattice thermal conductivity. Our finding enriches the understanding of aliovalent doping in thermal transport theories which should be applicable to other thermoelectric systems.