High-Durable, Radiative-Cooling, and Heat-Insulating Flexible Films Enabled by a Bioinspired Dictyophora-Like Structure

Radiative cooling, achieved by selectively emitting thermal radiation to outer space, holds great promise for addressing global energy challenges and mitigating the effects of climate change. However, most radiative cooling materials face limitations in effectively cooling in high-heat environments, and their performance deteriorates significantly with prolonged outdoor use. These shortcomings restrict their widespread application in various settings. To address this, we draw inspiration from the unique biostructure of dictyophora and propose a novel hollow@porous radiative cooling film by integrating hollow microparticles and porous polymer. The fabricated hollow@porous flexible film exhibits high sunlight reflection (93.7%), strong infrared emissivity (89.1%), as well as ultralow thermal conductivity (17.56 mW/m k). The daytime cooling performance of the prepared cooler is experimentally demonstrated with a marked temperature decrease to 17.4 °C under a peak solar intensity of 980 W/m2. Furthermore, the unique hollow@porous structure also strengthens the film's long-term durability by incorporating weather resistance and self-cleaning properties, which ensures stable and efficient radiative cooling performance even in harsh climatic conditions. This advancement in radiative cooling materials opens up new possibilities for thermal management, energy conservation, and cooling of solar panels, engine components, electronic equipment, new energy batteries, etc.

Synergistic Approach Toward a Reproducible High zT in n-Type and p-Type Superionic Thermoelectric Ag2Te

Recently, superionic thermoelectrics have attracted enormous attention due to their ultralow thermal conductivity and high figure-of-merit (zT). However, their high zT is generally obtained deep inside the superionic phase, e.g., near 1000 K in Cu₂X (X: chalcogen atom) family despite a relatively low superionic transition temperature of ∼400 K. At such high temperatures, the liquid-like flow of the metal ions results in material's degradation. Here, we present thermoelectric properties of superionic Ag₂Te synthesized by various methods. The sintered Ag₂Te samples are shown to exhibit an unpredictable behavior with respect to the sign of thermopower (S) in the superionic phase and the magnitude of electrical conductivity (σ). We overcome this issue using an all-room-temperature fabrication technique leading to an excellent reproducibility from one sample to another. To improve the zT of Ag₂Te beyond the phonon-liquid electron-crystal limit (∼0.64 at 575 K in the ingot samples), we adopted a heirarchical nanostructuring technique, which effectively suppressed the thermal conductivity, leading to a significant improvement in the zT values for both n-type and p-type samples. We obtained zT of 1.2 in the n-type and 0.64 in the p-type Ag₂Te at 570 K. These values supersede the zT of any Ag₂Te previously reported. At 570 K, for our ball-milled/cold-pressed samples, the critical current density for metal-ion migration exceeds 15 A cm^-2, which further confirms that Ag₂Te is a promising thermoelectric material. Our results are supported by first-principles density functional theory calculations of the electronic and thermal properties.

Self-Modulation-Guided Growth of 2D Tellurides with Ultralow Thermal Conductivity

Ultralow thermal conductivity materials have triggered much interest due to diverse applications in thermal insulation, thermal barrier coating, and especially thermoelectrics. Two dimensional (2D) indium tellurides with ultralow thermal conductivity provide a versatile platform for tailoring the heat transfer, exploring new candidates for thermoelectrics, and achieving miniature, lightweight, and highly integrated devices. Unfortunately, their nanostructure and structure-related heat transfer properties at a 2D scale are much less studied due to difficulties in material fabrication. The ionic character between interlayers and strong covalent bonds in 3D directions impede the anisotropic growth of indium telluride flakes; meanwhile, the low environmental stability and chemical reactivity of tellurium also limit the fabrication of high-quality tellurides, thus hindering the exploration of thermal transport properties. Here, a self-modulation-guided growth strategy to synthesize high-quality 2D In₄ Te₃ single crystals with ultralow thermal conductivity (0.47 W m^-1  K^-1 ) is developed. This strategy can also be extended to synthesize a series of highly crystallized metal tellurides, providing excellent candidates for further application in thermoelectrics.

An Efficient Strategy for Reinforcing Flexible Ceramic Membranes

Herein, we present a facile reinforcement method for the large-scale fabrication of highly flexible, mechanically stable, temperature-resistant ceramic lightweight membranes based on the cross-linked assembly of zirconia-silica (ZrO₂-SiO₂) nanofibrous and montmorillonite (MMT) nanosheets through electrospinning and a subsequent calcination process. The resulting MMT@ZrO₂-SiO₂ membranes exhibit high flexibility with a bending rigidity of 0.2 cN mm^-1, robust mechanical performance with a tensile strength of up to 1.83 MPa, robust fire resistance, and temperature-invariant mechanical stability from -196 to 1000 °C. The thermal superinsulation with a thermal conductivity as low as 0.026 W m^-1 K^-1 and the improved mechanical strength can be attributed to the cross-linked interfacial interaction between the ZrO₂-SiO₂ nanofibers and the MMT nanosheets. Additionally, a firefighter uniform with MMT@ZrO₂-SiO₂ membranes inside features a superior thermal protective property up to the A2 level (combined flame and radiant exposure) and an excellent fire resistance of up to 1000 °C, which is ideal for next-generation firefighter uniform manufacturing.

All-Scale Hierarchical Structure Contributing to Ultralow Thermal Conductivity of Zintl Phase CaAg0.2Zn0.4Sb

TiNiSi-type Zintl phase CaAgSb can transform into LiGaGe-type Zintl phase CaAg x Zn(1- x )/2Sb when some of the Ag atoms are substituted by Zn atoms, leading to an ultralow thermal conductivity of ≈0.4 W m-1 K-1 in the whole measured temperature range of CaAg0.2Zn0.4Sb. The microstructure is then investigated by spherical aberration-corrected electron microscopy on an atomic scale, which reveals an all-scale hierarchical structure that can scatter the phonons in a wide frequency range. There exist a large quantity of CaAgSb nanometer precipitates as well as quite a lot of edge dislocations close to these nanometer precipitates, thus releasing the stress caused by the mismatch between the precipitates and the parent phase. Many twin boundaries also exist around the CaAgSb precipitates. High-density point defects contain the randomly dispersed Ag vacancies and Zn atoms substituted for the Ag atoms. All these widely distributed multidimensional defects contribute to the decrease of lattice thermal conductivity in a wide temperature range.

Ultralow Thermal Conductivity and High Thermoelectric Performance in AgCuTe1-xSex through Isoelectronic Substitution

In this paper, we report a series of x polycrystalline AgCuTe_1-xSe samples with high thermoelectric performance. X-ray photoelectron spectroscopy data suggest the observation of Ag⁺, Cu⁺, Te^2-, and Se^2- states of Ag, Cu, Te, and Se. Meanwhile, the carrier concentration of the obtained p-type samples changes from 9.12 × 10¹⁸ to 0.86 × 10¹⁸ cm^-3 as their carrier mobility varies from 698.55 to 410.12 cm²·V^-1·s^-1 at 300 K. Compared with undoped AgCuTe, an ultralow thermal conductivity is realized in AgCuTe_1-xSe_x due to the enhanced phonon scattering. Ultimately, a maximum figure of merit (ZT) of ∼1.45 at 573 K and a high average ZT above 1.0 at temperatures ranging from room temperature to 773 K can be achieved in AgCuTe_0.9Se_0.1, which increases by 186% compared to that of the undoped AgCuTe (0.82 at 573 K). This work provides a viable insight toward understanding the effect of the Se atom on the lattice structure and thermoelectric properties of AgCuTe and other transition-metal dichalcogenides.

Ultralow Thermal Conductivity and Large Figure of Merit in Low-Cost and Nontoxic Core-Shell Cu@Cu2O Nanocomposites

Identification of novel materials with enhanced thermoelectric (TE) performance is critical for advancing TE research. In this direction, this is the first report on TE properties of low-cost, nontoxic, and abundant core-shell Cu@Cu₂O nanocomposites synthesized using a facile and cheap solution-phase method. They show ultralow thermal conductivity of nearly 10^-3 of the copper bulk value, large thermopower of ∼373 μVK^-1, and, consequently, a TE figure of merit of 0.16 at 320 K which is larger than those of many of the potential TE materials such as PbTe, SnSe, and SiGe, showing its potential for TE applications. The ultralow thermal conductivity is mainly attributed to the multiscale phonon scattering from intrinsic defects in Cu₂O, grain boundaries, lattice-mismatched interface, as well as dissimilar vibrational properties. The large thermopower is associated with a sharp modulation in carrier density of states due to charge transfer between Cu and Cu₂O nanoparticles and carrier energy filtering. They are tuned by varying the trioctylphosphine concentration.

Lone-Electron-Pair Micelles Strengthen Bond Anharmonicity in MnPb16Sb14S38 Complex Sulfosalt Leading to Ultralow Thermal Conductivity

Designing crystalline solids in which intrinsically and extremely low lattice thermal conductivity mainly arises from their unique bonding nature rather than structure complexity and/or atomic disorder could promote thermal energy manipulation and utilization for applications ranging from thermoelectric energy conversion to thermal barrier coatings. Here, we report an extremely low lattice thermal conductivity of ∼0.34 W m^-1 K^-1 at 300 K in the new complex sulfosalt MnPb₁₆Sb₁₄S₃₈. We attribute the ultralow lattice thermal conductivity to a synergistic combination of scattering mechanisms involving (1) strong bond anharmonicity in various structural building units, owing to the presence of stereoactive lone-electron-pair (LEP) micelles and (2) phonon scattering at the interfaces between building units of increasing size and complexity. Remarkably, low-temperature heat capacity measurement revealed a C_p value of 0.206 J g^-1 K^-1 at T > 300 K, which is 22% lower than the Dulong-Petit value (0.274 J g^-1 K^-1). Further analysis of the C_p data and sound velocity (ν = 1834 m s^-1) measurement yielded Debye temperature values of 161 and 187 K, respectively. The resulting Grüneisen parameter, γ = 1.65, further supports strong bond anharmonicity as the dominant mechanism responsible for the observed extremely low lattice thermal conductivity.

Record-Low and Anisotropic Thermal Conductivity of a Quasi-One-Dimensional Bulk ZrTe5 Single Crystal

Zirconium pentatelluride (ZrTe₅) has recently attracted renewed interest owing to many of its newly discovered extraordinary physical properties, such as 2D and 3D topological-insulator behavior, pressure-induced superconductivity, Weyl semimetal behavior, Zeeman splitting, and resistivity anomaly. The quasi-one-dimensional structure of single-crystal ZrTe₅ also promises large anisotropy in its thermal properties, which have not yet been studied. In this work, via time-domain thermoreflectance measurements, ZrTe₅ single crystals are discovered to possess a record-low thermal conductivity along the b-axis (through-plane), as small as 0.33 ± 0.03 W m^-1 K^-1 at room temperature. This ultralow b-axis thermal conductivity is 12 times smaller than its a-axis thermal conductivity (4 ± 1 W m^-1 K^-1) owing to the material's asymmetrical crystalline structure. First-principles calculations are further conducted to reveal the physical origins of the ultralow b-axis thermal conductivity, which can be attributed to: (1) the resonant bonding and strong lattice anharmonicity induced by electron lone pairs, (2) the weak interlayer van der Waals interactions, and (3) the heavy mass of Te atoms, which results in low phonon group velocity. This work sheds light on the design and engineering of high-efficiency thermal insulators for applications such as thermal barrier coatings, thermoelectrics, thermal energy storage, and thermal management.

Stabilizing n-Type Cubic GeSe by Entropy-Driven Alloying of AgBiSe2 : Ultralow Thermal Conductivity and Promising Thermoelectric Performance

The realization of n-type Ge chalcogenides is elusive owing to intrinsic Ge vacancies that make them p-type semiconductors. GeSe crystallizes into a layered orthorhombic structure similar to SnSe at ambient conditions. The high-symmetry cubic phase of GeSe is predicted to be stabilized by applying 7 GPa external pressure or by enhancing the entropy by increasing to temperature to 920 K. Stabilization of the n-type cubic phase of GeSe at ambient conditions was achieved by alloying with AgBiSe₂ (30-50 mol %), enhancing the entropy through solid solution mixing. The interplay of positive and negative chemical pressure anomalously changes the band gap of GeSe with increasing the AgBiSe₂ concentration. The band gap of n-type cubic (GeSe)_1-x (AgBiSe₂ )_x (0.30≤x≤0.50) has a value in the 0.3-0.4 eV range, which is significantly lower than orthorhombic GeSe (1.1 eV). Cubic (GeSe)_1-x (AgBiSe₂ )_x exhibits an ultralow lattice thermal conductivity (κ_L ≈0.43 W m^-1  K^-1 ) in the 300-723 K range. The low κ_L is attributed to significant phonon scattering by entropy-driven enhanced solid-solution point defects.

Soft Phonon Modes Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance in AgCuTe

Crystalline solids with intrinsically low lattice thermal conductivity (κ_L ) are crucial to realizing high-performance thermoelectric (TE) materials. Herein, we show an ultralow κ_L of 0.35 Wm^-1  K^-1 in AgCuTe, which has a remarkable TE figure-of-merit, zT of 1.6 at 670 K when alloyed with 10 mol % Se. First-principles DFT calculation reveals several soft phonon modes in its room-temperature hexagonal phase, which are also evident from low-temperature heat-capacity measurement. These phonon modes, dominated by Ag vibrations, soften further with temperature giving a dynamic cation disorder and driving the superionic transition. Intrinsic factors cause an ultralow κ_L in the room-temperature hexagonal phase, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high-temperature rocksalt phase. Despite the cation disorder at elevated temperatures, the crystalline conduits of the rigid anion sublattice give a high power factor.

Concerted Rattling in CsAg5 Te3 Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance

Thermoelectric (TE) materials convert heat energy directly into electricity, and introducing new materials with high conversion efficiency is a great challenge because of the rare combination of interdependent electrical and thermal transport properties required to be present in a single material. The TE efficiency is defined by the figure of merit ZT=(S(2) σ) T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. A new p-type thermoelectric material, CsAg5 Te3 , is presented that exhibits ultralow lattice thermal conductivity (ca. 0.18 Wm(-1)  K(-1) ) and a high figure of merit of about 1.5 at 727 K. The lattice thermal conductivity is the lowest among state-of-the-art thermoelectrics; it is attributed to a previously unrecognized phonon scattering mechanism that involves the concerted rattling of a group of Ag ions that strongly raises the Grüneisen parameters of the material.