Ultralow Thermal Conductivity Research Articles

Facet dependent ultralow thermal conductivity of zinc oxide coated silver fabric for thermoelectric devices

The conversion efficiency of a thermoelectric power generator depends on the dimensionless figure-of-merit (ZT) of the constituent thermoelectric materials, which is mainly determined by their Seebeck coefficient as well as the electrical and thermal conductivity. ZnO holds promise for thermoelectric applications, yet its use is currently limited by low electrical conductivity and high thermal conductivity. Herein, we demonstrate how thermal conductivity of ZnO can be significantly reduced by intelligently combining it with a cellulose-based Ag fabric using a one-step hydrothermal method, and how different ratios of zinc nitrate hexahydrate (ZNH) to hexamethylenetetramine (HMT) can be used to fine-tune the thermoelectric performance of the resulting composite. We show that as-prepared samples have a composite structure of Ag, Zn and O without any other impurity phases. We propose that the facet dependent crystal growth orientation, from the c-axis in (101) planes to the a-axis in (100) plane, amplify phonon scattering within the material, impeding effective heat transfer and thereby lowering overall thermal conductivity to 0.046 W/mK at room temperature for composites with a 1:1 ZNH to HMT ratio.

Temperature and Pressure Effects on Phase Transitions and Structural Stability in CsPb2Br5 and CsPb2Br4I Perovskite-Derived Halides.

All-inorganic perovskites exhibit outstanding light absorption properties in the visible range suitable for solar energy applications. We focus on the synthesis of CsPb2(Br,I)5 using mechanochemical procedures. Synchrotron X-ray diffraction (SXRD) data are essential for determining the crystallographic evolution in the 295-753 K temperature range. From room temperature to 573 K, the crystal structure is refined in a two-dimensional (2D) tetragonal framework (space group: I4/mcm) consisting of layers of face sharing [Pb(Br,I)8] polyhedra. Above a transition temperature of Tt = 630 and 621 K, identified from differential scanning calorimetry (DSC) curves for CsPb2Br5 and CsPb2Br4I, respectively, a cubic three-dimensional (3D) corner-sharing perovskite structure is identified, showing substantial Cs and (Br,I) deficiency. A more ionic character for Cs-halide versus Pb-halide is derived from the evolution of the anisotropic atomic displacements. An ultralow thermal conductivity of 0.35-0.18 W m-1 K-1 is related to the low Debye temperatures. From high-pressure SXRD studies, the change in B0 can be attributed to a basic expansion of the unit-cell volume caused by the chemical pressure from iodine within the CsPb2Br5 framework. UV-vis-NIR spectroscopy revealed optical gaps of approximately 2.93 and 2.50 eV, respectively, which agrees with ab initio calculations.

Ultralow Thermal Conductivity and Very High Seebeck Coefficient in Two-Dimensional TeSe2 Semiconductor.

Emerging chalcogenide-based two-dimensional (2D) materials possess various unique yet fully explored properties and are thus considered promising candidates for next-generation optoelectronic and energy conversion applications. Here, TeSe2 crystals with interesting thermoelectric features were synthesized using a simple solid-state reaction. High-resolution transmission electron microscopy reveals that TeSe2 stabilizes in a 2D atomic structure with helical chains, resembling 2D tellurene. The thermoelectric properties were measured from 2 to 390 K in a polycrystalline pellet, showing an ultralow thermal conductivity below 0.25 W m-1 K-1 and a very high positive Seebeck coefficient of up to 865 μV K-1. Particularly, the thermal conductivity shows a hysteresis effect upon temperature cycling, which is tentatively explained as cracks opening and partially closing. Optical measurements indicate that TeSe2 is a semiconductor with two bandgaps at 1.43 and 1.65 eV. These results highlight that TeSe2 is an intriguing 2D semiconductor with complex thermoelectric properties, which provides a platform to further study the interplay of emerging 2D structure, thermal, and electronic properties.

Vacuum Insulation Panel Production with Ultralow Thermal Conductivity—A Review

Vacuum Insulation Panel Production with Ultralow Thermal Conductivity—A Review

Spectrally selective and thermally insulating hybrid nanofiber aerogel coolers for building energy conservation

With the advancement of worldwide carbon neutralization, passive radiative cooling (PRC) has attracted tremendous interest in energy conservation by dissipating heat into the ultracold outer space without any electricity consumption. Despite some progress has been made in tailoring spectral properties for PRC, it still remains a crucial challenge in fabricating efficient and low-cost coolers for building energy saving. Herein, hierarchical cellulose-based aerogel coolers consisting of beads-on-string structural electrospun nanofibers are manufactured for all-day and all-region energy saving by combining radiative cooling and thermal insulation in one design. The hierarchically porous architectures and well-designed chemical compositions endow the aerogels with strong solar reflectance (∼0.974), high mid-infrared emittance (∼0.985), and ultra-low thermal conductivity (0.0285 W (m K)−1), achieving a sub-ambient cooling of ∼8.24 °C during the daytime and ∼7.41 °C during the nighttime. The aerogels also exhibit exceptional compressive resiliency, anti-aging, and self-cleaning properties, promising for durable cooling under harsh conditions. The building energy simulations show that about 23.1 kWh m−2 of the total energy consumption per year can be saved if the aerogel coolers are widely deployed as building envelopes in China. This work provides new perspectives for the development of advanced aerogel coolers for future energy saving applications.

Constructing high-performance bulk thermoelectric composites by incorporating uniformly dispersed fullerene sub-nanoclusters

Sub-nanomaterials possess unprecedented size-dependent properties compared to conventional nanomaterials, which endow them with great potential in catalysis, biomedicine, sensors, and so on. However, their applications in thermoelectrics are unknown due to poor thermal stability and low yields. Herein, we construct a series of thermoelectric composites by incorporating highly thermally stable and commercial fullerene sub-nanoclusters (C60 or C70). We find that sub-nanoclusters as the second phase can conduce to optimized carrier concentration through charge transfer at interfaces while the carrier mobility is significantly enhanced due to atom orbital hybridization and size-dependent electrical scattering mechanism. Furthermore, the ultra-low thermal conductivity of C60 due to its distorted chemical bonding and sub-nanometer pore, and the interfacial thermal resistance greatly suppress the phonon transport. Consequently, the 0.15 mol% C60/Bi0.4Sb1.6Te3 realizes an ultra-high ZT of ∼1.6 at 373 K, an excellent thermoelectric conversion efficiency of ∼7.4%, and a huge cooling performance of ∼73 K. This work demonstrates the application of sub-nanomaterials in thermoelectrics and may shed light on other fields such as electronic devices, thermal management, and fullerene chemistry.

Developing Lightweight Silanized Cellulose Ultramaterials as the Strongest Engineered Aerogel

AbstractSilica aerogel materials exhibit exceptional properties, including unique light, heat, fluid, and ion characteristics with vast potential. Despite similarities in composition to glass, silica aerogel lacks ductility and is prone to breakage upon impact. Therefore, the development of advanced engineering aerogel ultra materials that combine the mechanical strength of engineered materials with the ultralight superinsulation of aerogels remains a significant challenge. Herein, a multidimensional and multiscale gradient‐pore strategy is proposed for the scalable production of nanostructured silanized cellulose (SiCell) nanofibrous frameworks through colloidal self‐assembly using an active earth‐abundant biopolymer from the wood as a functional component. The configuration of the SiCell/polymer‐crosslinked silica‐aerogel‐powder (SiAP)‐locked structure creates a regional framework that forms SiCell/polymer‐crosslinked SiAP‐locked aerogels (SiCSiPA) with a 3D nanoporous architecture. With an ultralow thermal conductivity of 15.9 mW m−1 K−1, the lightweight super insulating SiCSiPA emerges as a prime candidate for thermal insulation, offering exceptional strength for practical applications. Moreover, SiCSiPA exhibits superior strength, stiffness, and toughness, making it the best insulating material. Owing to its outstanding strength‐to‐weight ratio and robust thermomechanical stability, SiCSiPA introduces a new technology for lightweight construction applications requiring superior thermal insulation.

Thermal transport in metal halide perovskites and other third-generation photovoltaic materials

Third-generation photovoltaic materials, including metal halide perovskites (MHPs), colloidal quantum dots (QDs), copper zinc tin sulfide (CZTS), and organic semiconductors, among others, have become attractive in the past two decades. Unlike their first- and second-generation counterparts, these advanced materials boast properties beyond mere photovoltaic performance, such as mechanical flexibility, light weight, and cost-effectiveness. Meanwhile, these materials possess more intricate crystalline structures that aid in understanding and predicting their transport properties. In particular, the distinctive phonon dispersions in MHPs, the layered architecture in quasi-two-dimensional (2D) perovskites, the strong quantum confinement in QDs, and the complex crystal structures interspersed with abundant disorders in quaternary CZTS result in unique and sometimes anomalous thermal transport behaviors. Concurrently, the criticality of thermal management in applications such as photovoltaics, thermoelectrics, light emitting diodes, and photodetection devices has received increased recognition, considering that many of these third-generation photovoltaic materials are not good thermal conductors. Effective thermal management necessitates precise measurement, advanced modeling, and a profound understanding and interpretation of thermal transport properties in these novel materials. In this review, we provide a comprehensive summary of various techniques for measuring thermal transport properties of these materials and discuss the ultralow thermal conductivities of three-dimensional (3D) MHPs, superlattice-like thermal transport in 2D perovskites, and novel thermal transport characteristics inherent in QDs and CZTS. By collecting and comparing the literature-reported results, we offer a thorough discussion on the thermal transport phenomenon in these materials. The collective understanding from the literature in this area, as reviewed in this article, can provide guidance for improving thermal management across a wide spectrum of applications extending beyond photovoltaics.

Thermoelectric performance of Bi2Sn2Te6 monolayer with ultralow lattice thermal conductivity induced by hybrid bonding properties: A theoretical prediction

Thermoelectric performance of Bi2Sn2Te6 monolayer with ultralow lattice thermal conductivity induced by hybrid bonding properties: A theoretical prediction

Construction and Properties of Ultralow Thermal Conductivity and High Strength Zirconia Aerogel Composites by Freeze-Drying.

Zirconia aerogels possess significant applications, including their use catalyst carriers, thermal insulation materials, and thermal barrier coatings. This is due to their ultrahigh temperature resistance, high porosity, and low thermal conductivity. Nonetheless, the inherent challenges associated with ZrO2 aerogels, such as high brittleness, low compressive strength, and inadequate formability, restrict their potential applications. In this paper, with ultralow thermal conductivity and high strength zirconia aerogel composites with inorganic zirconium salt zirconium carbonate as the raw material, acetic acid as the solvent, polyvinylpyrrolidone (PVP) as the viscosity builder to stabilize the structure of the aerogel during the freeze-drying process. Additionally, yttrium nitrate hexahydrate (Y(NO3)3·6H2O) is employed as a phase stabilizer. The sol-gel method, in conjunction with the freeze-drying process, is utilized to fabricate ZrO2 aerogel composites with an optimized microstructure. The findings indicate that optimal process parameters are achieved with a PVP solution concentration of 2.0 wt % and a zirconium carbonate concentration of 20 wt %. The mechanical properties of the resulting composites reach up to 550 kPa, while the thermal insulation performance exhibits a temperature difference of 207 °C/cm and a thermal conductivity of 0.0504 W/(m·K). This advancement addresses the mechanical stability issues commonly associated with traditional ceramic aerogels and widely used elastic insulating materials, thereby enhancing their applicability as thermal insulation and heat preservation materials.

Functional Carbon Springs Enabled Dynamic Tunable Microwave Absorption and Thermal Insulation.

Electromagnetic (EM) wave pollution and thermal damage pose serious hazards to delicate instruments. Functional aerogels offer a promising solution by mitigating EM interference and isolating heat. However, most of these materials struggle to balance thermal protection with microwave absorption (MA) efficiency due to a previously unidentified conflict between the optimizing strategies of the two properties. Herein, this study reports a solution involving the design of a carbon-based aerogel called functional carbon spring (FCS). Its unique long-range lamellar multi-arch microstructure enables tunable MA performance and excellent thermal insulation capability. Adjusting compression strain from 0% to 50%, the adjustable effective absorption bandwidth (EAB) spans up to 13.4GHz, covering 84% of the measured frequency spectrum. Notably, at 75% strain, the EAB drops to 0GHz, demonstrating a novel "on-off" switchability for MA performance. Its ultralow vertical thermal conductivity (12.7mW m-1 K-1) and unique anisotropic heat transfer mechanism endow FCS with superior thermal protection effectiveness. Numerical simulations demonstrate that FCS outperforms common honeycomb structures and isotropic porous aerogels in thermal management. Furthermore, an "electromagnetic-thermal" dual-protection material database is established, which intuitively demonstrates the superiority of the solution. This work contributes to the advancement of multifunctional MA materials with significant potential for practical applications.

Optimally Suppressed Phonon Tunneling in van der Waals Graphene-WS2 Heterostructure with Ultralow Thermal Conductivity.

Van der Waals heterostructures have great potential for realizing ultimately low thermal conductivity because defectless interfaces can be constructed at a length scale smaller than the phonon wavelength, allowing modulation of coherent phonon transport. In this Letter, we demonstrate the mechanism for thermal conductivity reduction at a mode-resolved level. The graphene-WS2 heterostructure with the lowest cross-plane thermal conductivity of 0.048 W/(m·K) is identified from 16,384 candidates by combining Bayesian optimization and molecular dynamics simulations. Then, the angle-resolved phonon transmission is calculated using the mode-resolved atomistic Green's function. The results reveal that the optimal heterostructure nearly completely terminates phonon transport with finite incident angles, owing to the reduced critical incident angle and suppression of phonon tunneling.

Ultralow lattice thermal conductivity in K2AuBi and its thermoelectric properties

Thermoelectric materials are best known for their prowess to transform the environment’s waste heat into electricity. In an endeavor to explore new thermoelectric prospects, in the present study, we investigate K2AuBi using density functional theory-based first-principles simulations. From our simulations, we find an intrinsically low lattice thermal conductivity of 0.43 W m−1 K−1 at 300 K in K2AuBi. Based on our detailed analysis, we find the reasons for such a low value of lattice thermal conductivity as, low phonon group velocities, short phonon lifetimes, anharmonicity in the lattice vibrations, and significant mean square displacements of K and Au atoms. The large mean square displacements hint at weak bonding and anharmonicity in the lattice vibrations, favoring more phonons scattering. We also find that the vibrations of K-atoms can be related to rattlers, conducive to low lattice thermal conductivity. Our simulations predict a high value, ∼784 μV K−1, of Seebeck coefficient at 700 K on account of the large density of states in the vicinity of Fermi level. Combining our computed lattice thermal conductivity with electrical transport properties, we obtain a high figure of merit, ZT∼ 1.04, at 700 K in K2AuBi.

Electric Field-Induced Ordered-Structural Aerogels Enable Superinsulation and Multifunctionality.

1D flexible fibers assembled 3D porous networked ceramic fiber aerogels (CFAs) are developed to overcome the brittleness of traditional ceramic particle aerogels. However, existing CFAs with disordered and quasi-ordered structures fail to balance the relationship between flexibility, robustness, and thermal insulation. Creating novel architectural CFAs with an excellent combination of performances has proven extremely challenging. In this paper, a novel strategy is adopted to fabricate porous mullite fibrous aerogels (MFAs) with ordered structures by combining fiber sedimentation and electric field-induced fiber alignment techniques. For the first time, electric field-induced alignment of ceramic fibers is utilized to prepare bulk aerogels on a large scale. The resulting MFAs exhibit ultra-low high-temperature thermal conductivity of 0.0830 W m-1 K-1 at 1000°C, anisotropic mechanical and sound absorption performances, and multifunctionality in terms of the combination of thermal insulation, sound absorption, and hydrophobicity. The successful synthesis of such fascinating materials may provide new insights into the design and development of multifunctional CFAs for various applications.

Boosting the Thermoelectric Properties of Ge0.94Sb0.06Te via Trojan Doping for High Output Power.

GeTe stands as a promising lead-free medium-temperature thermoelectric material that has garnered considerable attention in recent years. Suppressing carrier concentration by aliovalent doping in GeTe-based thermoelectrics is the most common optimization strategy due to the intrinsically high Ge vacancy concentration. However, it inevitably results in a significant deterioration of carrier mobility, which limits further improvement of the zT value. Thus, an effective Trojan doping strategy via CuScTe2 alloying is utilized to optimize carrier concentration without intensifying charge carrier scattering by increasing the solubility of Sc in the GeTe system. Because of the high doping efficiency of the Trojan doping strategy, optimized hole concentration and high mobility are obtained. Furthermore, CuScTe2 alloying leads to band convergence in GeTe, increasing the effective mass m* in (Ge0.84Sb0.06Te0.9)(CuScTe2)0.05 and thus significantly improving the Seebeck coefficient throughout the measured temperature range. Meanwhile, the achievement of the ultralow lattice thermal conductivity (κL ∼ 0.34 W m-1 K-1) at 623 K is attributed to dense point defects with mass/strain-field fluctuations. Ultimately, the (Ge0.84Sb0.06Te0.9)(CuScTe2)0.05 sample exhibits a desirable thermoelectric performance of zTmax ∼ 1.81 at 623 K and zTave ∼ 1.01 between 300 and 723 K. This study showcases an effective doping strategy for enhancing the thermoelectric properties of GeTe-based materials by decoupling phonon and carrier scattering.

Understanding the Ultralow Thermal Conductivity and Strong Anharmonicity of a Lanthanum-Based Germanium Halide Monolayer for Possible Thermoelectric Applications

Understanding the Ultralow Thermal Conductivity and Strong Anharmonicity of a Lanthanum-Based Germanium Halide Monolayer for Possible Thermoelectric Applications

Ultralow thermal conductivity and excellent thermoelectric performance in Janus Bi2X2Y (X=Se, Te; Y=S, Se, Te) monolayers induced by suppressed membrane effect

We use density functional theory (DFT) implemented in the Vienna Ab initio Simulation Package (VASP) combined with Boltzmann transport equations to investigate the relationships between the structure, phonon hybridization, transport and thermoelectric properties of two-dimensional (2D) Janus Bi2X2Y (X=Se, Te; Y=S, Se, Te) monolayers. Among them, Janus Bi2Te2S monolayer shows a unique ultralow lattice thermal conductivity (1.09 W·m−1·K−1 at 300 K) and excellent thermoelectric performance. We find that, by suppressing the membrane effect of 2D materials to enable the negative-positive conversion of the Grüneisen parameter of the out-of-plane acoustic (ZA) phonon modes, the three-phonon scattering of the material can be effectively enhanced thereby reducing their lattice thermal conductivity. Besides, we also find the suppression of the membrane effect is highly correlated to the non-orthogonal bond angles. Our results indicate a new path to reduce the lattice thermal conductivity of 2D materials and are meaningful for further theoretical and experimental research on the development of high-performance thermoelectric materials.

Colloidal Ag2SbBiSe4 nanocrystals as n‑type thermoelectric materials

Materials with low intrinsic thermal conductivity are essential for the development of high-performance thermoelectric devices. At the same time, the solution processing of these materials may enable the cost-effective production of the devices. Herein, we detail a high-yield and scalable colloidal synthesis route to produce Ag2SbBiSe4 nanocrystals (NCs) using amine-thiol-Se chemistry. The quaternary chalcogenide material is consolidated by a rapid hot-press maintaining the cubic crystalline structure. Transport measurements confirm that n-type Ag2SbBiSe4 exhibits an inherently ultralow lattice thermal conductivity of ca. 0.34 W m−1K−1 at 760 K. Moreover, a modulation doping strategy based on the blending of semiconductor Ag2SbBiSe4 and metallic Sn NCs is demonstrated to control the charge carrier concentration in the final composite material. The introduction of Sn nanodomains additionally blocks phonon propagation thus contributing to reducing the thermal conductivity of the final material. Ultimately, a peak thermoelectric figure of merit value of 0.64 at 760 K is achieved for n-type Ag2SbBiSe4-Sn nanocomposites that also demonstrate a notable Vickers hardness of 185 HV.

Electronic, optical, and thermoelectric behaviour of KCuX (X = S, Se, Te) monolayers.

In the past few decades, two-dimensional (2-D) materials gained huge deliberation due to their outstanding electronic and heat transport properties. These materials have effective applications in many areas such as photodetectors, battery electrodes, thermoelectrics, etc. In this work, we have calculated structural, electronic, optical, and thermoelectric properties of KCuX (X = S, Se, Te) monolayers (MLs) with the help of first-principles-based calculations and semi-classical Boltz- mann transport equation (BTE). The phonon dispersion calculations demonstrate the dynamical stability of the KCuX (X = S, Se, Te) MLs. Our results show that the monolayers of KCuX (X = S, Se, Te) are semiconductors with band gaps of 0.193 eV, 0.26 eV, and 1.001 eV respectively, and therefore they are suitable for photovoltaic applications. The optical analysis illustrates that the maximum absorption peaks of the KCuX (X = S, Se, Te) MLs are located in visible and ultraviolet (UV) regions, which may serve as a promising candidate for designing advanced optoelectronic devices. Furthermore, thermoelectric properties of the KCuS and KCuSe MLs, including See- beck coefficient, electrical conductivity, electronic thermal conductivity, power factor, and figure of merit, are calculated at different temperatures 300 K, 600 K, and 800 K. Additionally, we also focus on the analysis of Gru ̈neisen parameter and various scattering rates to further explain their ultra-low thermal conductivity. Our results show that KCuS and KCuSe possess ultra-low lattice thermal conductivity value of 0.15 Wm-1K-1 and 0.06 Wm-1K-1 respectively, which is lower than those of recently reported KAgSe (0.26 Wm-1K-1 at 300 K) and TlCuSe (0.44 Wm-1K-1 at 300 K), indicating towards the large value of ZT. These materials are found to possess desirable thermoelectric and optical properties, making them suitable candidates for efficient thermoelectric and optoelectronic device applications.

Cu-Atom Locations in Rocksalt SnTe Thermoelectric Alloy.

The development of functional thermoelectric materials requires direct evidence of dopants' locations to rationally design the electronic and phononic structure of the host matrix. In this study, Cs-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy is employed at the atomic scale to identify Cu atoms' locations in a Cu-doped SnTe thermoelectric alloy. It is revealed that Cu atoms in the rocksalt SnTe form solid solutions at both Sn and Te sites, contrary to their electronegativity order and the intentional Cu doping at Sn sites. Cu atoms are also located at the tetrahedral and crowdion sites of the face-centred cubic structure, with varying degrees of correlations. Such high flexibility of Cu atoms in the rocksalt SnTe offers diverse phonon-scattering mechanisms conducive to the ultra-low lattice thermal conductivity of singly Cu-doped SnTe. This study offers atomic-scale insights for achieving more precise dopant engineering, leading to the accelerated development of functional thermoelectric materials.