Ultralow Thermal Conductivity in Vacancy-Ordered Halide Perovskite Cs3Bi2Br9 with Strong Anharmonicity and Wave-Like Tunneling of Low-Energy Phonons.

Halide perovskites are of great interest due to their exceptional optical and optoelectronic properties. However, thermal conductivity of many halide perovskites remains unexplored. In this study, an ultralow lattice thermal conductivity κL (0.24 W m-1 K-1 at 300 K) is reported and its weak temperature dependence (≈T-0.27) in an all-inorganic vacancy-ordered halide perovskite, Cs3Bi2Br9. The intrinsically ultralow κL can be attributed to the soft low-lying phonon modes with strong anharmonicity, which have been revealed by combining experimental heat capacity and Raman spectroscopy measurements, and first-principles calculations. It is shown that the highly anharmonic phonons originate from the Bi 6s2 lone pair expression with antibonding states of Bi 6s and Br 4p orbitals driven by the dynamic BiBr6 octahedral distortion. Theoretical calculations reveal that these low-energy phonons are mostly contributed by large Br motions induced dynamic distortion of BiBr6 octahedra and large Cs rattling motions, verified by the synchrotron X-ray pair distribution function analysis. In addition, the weak temperature dependence of κL can be traced to the wave-like tunneling of phonons, induced by the low-lying phonon modes. This work reveals the strong anharmonicity and wave-like tunneling of low-energy phonons for designing efficient vacancy-ordered halide perovskites with intrinsically low κL.

High-Performance SnTe Thermoelectric Materials Enabled by the Synergy of Band Convergence and Phonon Scattering.

SnTe, an environmentally friendly thermoelectric material, has garnered widespread scholarly interest owing to its lead-free nature; however, its intrinsic thermoelectric performance is constrained by a relatively low Seebeck coefficient and an extremely high lattice thermal conductivity. In this investigation, we employ the alloying of Ge and AgSbTe2 to enhance the zT value of SnTe. The study found that Ge, Ag, and Sb can effectively enhance the Seebeck coefficient and power factor of SnTe by utilizing band convergence. At the same time, a multitude of point defects induce phonon scattering, consequently decreasing the lattice thermal conductivity of SnTe. Collectively, these synergistic effects result in Sn0.75Ge0.25Te-15% AgSbTe2 achieving its highest zT value of 1.28 at 823 K, with an average zT value of 0.77 between 400 and 823 K. Such high zT values of the SnTe-based thermoelectric material provide the potential for applications in high-performance solid-state thermoelectric devices.

New Lead-free Hybrid Layered Double Perovskite Halides: Synthesis, Structural Transition and Ultralow Thermal Conductivity.

Hybrid layered double perovskites (HLDPs), representing the two-dimensional manifestation of halide double perovskites, have elicited considerable interest owing to their intricate chemical bonding hierarchy and structural diversity. This intensified interest stems from the diverse options available for selecting alternating octahedral coordinated trivalent [M(III)] and monovalent metal centers [M(I)], along with the distinctive nature of the cationic organic amine located between the layers. Here, we have synthesized three new compounds with general formula (R'/R'')4/2M(III)M(I)Cl8; where R'=C3H7NH3 (i.e. 3N) and R''=NH3C4H8NH3 (i.e. 4N4); M(III)=In3+ or Ru3+; M(I)=Cu+ by simple solution-based acid precipitation method. The structural analysis reveals that (4N4)2CuInCl8 and (4N4)2CuRuCl8 adopt the layered Dion Jacobson (DJ) structure, whereas (3N)4CuInCl8 exhibits layered Ruddlesden Popper (RP) structure. The alternative octahedra within the inorganic layer display distortions and tilting. Three compounds show temperature-dependent structural phase transitions where changes in the staking of inorganic layer, extent of octahedral tilting and reorientation of organic spacers with temperature have been noticed. We have achieved ultralow lattice thermal conductivity (κL) in the HLDPs in the 2 to 300 K range, marking a distinctive feature within the realm of HLDP systems. The RP-HLDP compound, (3N)4CuInCl8, demonstrates anisotropy in κL while measured parallel and perpendicular to layer stacking, showcasing ultralow κL of 0.15 Wm-1K-1 at room temperature, which is one of the lowest values obtained among Pb-free metal halide perovskite. The observed ultralow κL in three new HLDPs is attributed to significant lattice anharmonicity arising from the chemical bonding heterogeneity and soft crystal structure, which resulted in low-energy localized optical phonon modes that suppress heat-carrying acoustic phonons.

Ultra-Low Thermal Conductivity and Improved Thermoelectric Performance in Tungsten-Doped GeTe.

Compared to SnTe and PbTe base materials, the GeTe matrix exhibits a relatively high Seebeck coefficient and power factor but has garnered significant attention due to its poor thermal transport performance and environmental characteristics. As a typical p-type IV-VI group thermoelectric material, W-doped GeTe material can bring additional enhancement to thermoelectric performance. In this study, the introduction of W, Ge1-xWxTe (x = 0, 0.002, 0.005, 0.007, 0.01, 0.03) resulted in the presence of high-valence state atoms, providing additional charge carriers, thereby elevating the material's power factor to a maximum PFpeak of approximately 43 µW cm-1 K-2, while slightly optimizing the Seebeck coefficient of the solid solution. Moreover, W doping can induce defects and promote slight rhombohedral distortion in the crystal structure of GeTe, further reducing the lattice thermal conductivity κlat to as low as approximately 0.14 W m-1 K-1 (x = 0.002 at 673 K), optimizing it to approximately 85% compared to the GeTe matrix. This led to the formation of a p-type multicomponent composite thermoelectric material with ultra-low thermal conductivity. Ultimately, W doping achieves the comprehensive enhancement of the thermoelectric performance of GeTe base materials, with the peak ZT value of sample Ge0.995W0.005Te reaching approximately 0.99 at 673 K, and the average ZT optimized to 0.76 in the high-temperature range of 573-723 K, representing an increase of approximately 17% compared to pristine GeTe within the same temperature range.

Chemically Transformed Ag2 Te Nanowires on Polyvinylidene Fluoride Membrane For Flexible Thermoelectric Applications.

Flexible thermoelectric devices of nanomaterials have shown a great potential for applications in wearable to remotely located electronics with desired shapes and geometries. Continuous powering up the low power flexible electronics is a major challenge. We are reporting a flexible thermoelectric module prepared from silver telluride (Ag2 Te) nanowires (NWs), which are chemically transformed from uniquely synthesized and scalable tellurium (Te) NWs. Conducting Ag2 Te NWs composites have shown an ultralow total thermal conductivity ~0.22 W/mK surpassing the bulk melt-grown Ag2 Te ~1.23 W/mK at ~300 K, which is attributed to the nanostructuring of the material. Flexible thermoelectric device consisting of 4 legs (n-type) of Ag2 Te NWs on polyvinylidene fluoride membrane displays a significant output voltage (Voc ) ~2.3 mV upon human touch and Voc ~18 mV at temperature gradient, ΔT ~50 K, which shows the importance of NWs based flexible thermoelectric devices to power up the low power wearable electronics.

Design and process parametric investigations on acrylic-based single slope solar still to enhance daily energy efficiency and productivity of water: an application to desalination and dye removal.

Solar-driven water desalination is growing quickly, typically using other renewable energy sources. However, its efficiency is heavily reliant on design and process parameters. The aim of this study is to experimentally investigate the impact of various design and process parameters on the performance of single slope solar still. Thus, a homemade solar still has been fabricated using acrylic sheet with a basin area of 0.25 × 0.25 m2 to carry out the experiments in Vellore, India (latitude 12.9692° N and longitude 79.1559° E). Additionally, this solar still setup is investigated using different absorbing plates (copper plate and copper plate with black coating), various glass cover angles (15°, 30° and 45°) and changing the wind speed (3 m/s, 3.5 m/s and 4 m/s) with help of electric fan. Daily energy efficiency and productivity of water are compared for the same basin area with different design and process parameters. From the results, daily energy efficiency and water productivity are improved with the increase of glass cover angle and wind speed. It is found that the best combination is copper plate with black coating, glass cover angle of 45° and wind speed of 4 m/s. This exhibits 34.09% in daily energy efficiency and 2640 ml/m2 in productivity of water. After the desalination process, the primary ions (Na+, K+, Mg2+, and Ca2+) of seawater are significantly reduced and satisfy the requirement of WHO standards. Subsequentially, dye removal is effectively achieved in the proposed solar still.

Study on Low Thermal-Conductivity of PVDF@SiAG/PET Membranes for Direct Contact Membrane Distillation Application.

In order to enhance the separation performance and reduce the heat loss of transmembrane for membrane distillation, the thermal efficiency and hydrophobicity of the membrane distillation need to be simultaneously enhanced. In this work, a polyvinylidene difluoride/polyethylene glycol terephthalate (PVDF/PET) hydrophobic/hydrophilic membrane has been prepared by non-solvent phase induction method. Nanosized silica aerogel (SiAG) with high porosity has been added to the composite membranes. The modifying effects and operating conditions on permeate flux and thermal efficiency in direct contact membrane distillation (DCMD) are investigated. Furthermore, the latent heat of vaporization and the heat transfer across the membranes have been compared for SiAG addition, which indicates that the composite PVDF@SiAG/PET membranes demonstrate a great potential for distillation-separation application due to their high heat efficiency.

First Experimental Evidence of Anti-Stokes Laser-Induced Fluorescence Emission in Microdroplets and Microfluidic Systems Driven by Low Thermal Conductivity of Fluorocarbon Carrier Oil.

With the advent of many optofluidic and droplet microfluidic applications using laser-induced fluorescence (LIF), the need for a better understanding of the heating effect induced by pump laser excitation sources and good monitoring of temperature inside such confined microsystems started to emerge. We developed a broadband highly sensitive optofluidic detection system, which enabled us to show for the first time that Rhodamine-B dye molecules can exhibit standard photoluminescence as well as blue-shifted photoluminescence. We demonstrate that this phenomenon originates from the interaction between the pump laser beam and dye molecules when surrounded by the low thermal conductive fluorocarbon oil, generally used as a carrier medium in droplet microfluidics. We also show that when the temperature is increased, both Stokes and anti-Stokes fluorescence intensities remain practically constant until a temperature transition is reached, above which the fluorescence intensity starts to decrease linearly with a thermal sensitivity of about -0.4%/°C for Stokes emission or -0.2%/°C for anti-Stokes emission. For an excitation power of 3.5 mW, the temperature transition was found to be about 25 °C, whereas for a smaller excitation power (0.5 mW), the transition temperature was found to be about 36 °C.

Thermoelectric Properties of n-Type Bi4O4SeX2 (X = Cl, Br).

The multiple anion superlattice Bi4O4SeCl2 has been reported to exhibit extremely low thermal conductivity along the stacking c-axis, making it a promising material for thermoelectric applications. In this study, we investigate the thermoelectric properties of Bi4O4SeX2 (X = Cl, Br) polycrystalline ceramics with different electron concentrations by adjusting the stoichiometry. Despite optimizing the electric transport, the thermal conductivity remained ultra-low and approached the Ioffe-Regel limit at high temperatures. Notably, our findings demonstrate that non-stoichiometric tuning is a promising approach for enhancing the thermoelectric performance of Bi4O4SeX2 by refining its electric transport, resulting in a figure of merit of up to 0.16 at 770 K.

Theoretical Prediction of Thermoelectric Performance for Layered LaAgOX (X = S, Se) Materials in Consideration of the Four-Phonon and Multiple Carrier Scattering Processes.

Inspired by the experimental achievement of layered LaCuOX (X = S, Se) with superior thermoelectric (TE) performance, the TE properties of Ag-based isomorphic LaAgOX are systemically investigated by the first-principles calculation. The LaAgOS and LaAgOSe are direct semiconductors with wide bandgaps of ≈2.50 and ≈2.35 eV. Essential four-phonon and multiple carrier scattering mechanisms are considered in phonon and electronic transport calculations to improve the accuracy of the figure-of-merit (ZT). The p-type LaAgOX (X = S, Se) shows excellent TE performance on account of the large Seebeck coefficient originated from the band convergency and low thermal conductivity caused by the strong phonon-phonon scattering. Consequently, the optimal ZTs along the out-of-plane direction decrease in the order of n-type LaAgOSe (≈2.88) > p-type LaAgOSe (≈2.50) > p-type LaAgOS (≈2.42) > n-type LaAgOS (≈2.27) at 700 K, and the optimal ZTs of ≈1.16 and ≈1.29 are achieved for p-type LaAgOS and LaAgOSe at the same temperature. The present work would provide a deep insight into the phonon and electronic transport properties of LaAgOX (X = S, Se), but also could shed light on the way for the rational design of state-of-the-art heteroanionic materials for TE application.

Dynamic Lone Pair Expression as Chemical Bonding Origin of Giant Phonon Anharmonicity in Thermoelectric InTe.

Loosely bonded ("rattling") atoms with s2 lone pair electrons are usually associated with strong anharmonicity and unexpectedly low thermal conductivity, yet their detailed correlation remains largely unknown. Here we resolve this correlation in thermoelectric InTe by combining chemical bonding analysis, inelastic X-ray and neutron scattering, and first principles phonon calculations. We successfully probe soft low-lying transverse phonons dominated by large In1+ z-axis motions, and their giant anharmonicity. We show that the highly anharmonic phonons arise from the dynamic lone pair expression with unstable occupied antibonding states induced by the covalency between delocalized In1+ 5s2 lone pair electrons and Te 5p states. This work pinpoints the microscopic origin of strong anharmonicity driven by rattling atoms with stereochemical lone pair activity, important for designing efficient materials for thermoelectric energy conversion.

Carrier Mobility Modulation in Cu2Se Composites Using Coherent Cu4TiSe4 Inclusions Leads to Enhanced Thermoelectric Performance.

Carrier transport engineering in bulk semiconductors using inclusion phases often results in the deterioration of carrier mobility (µ) owing to enhanced carrier scattering at phase boundaries. Here, we show by leveraging the temperature-induced structural transition between the α-Cu2Se and ß-Cu2Se polymorphs that the incorporation of Cu4TiSe4 inclusions within the Cu2Se matrix results in a gradual large drop in the carrier mobility at temperatures below 400 K (α-Cu2Se), whereas the carrier mobility remains unchanged at higher temperatures, where the ß-Cu2Se polymorph dominates. The sharp discrepancy in the electronic transport within the α-Cu2Se and ß-Cu2Se matrices is associated with the formation of incoherent α-Cu2Se/Cu4TiSe4 interfaces, owing to the difference in their atomic structures and lattice parameters, which results in enhanced carrier scattering. In contrast, the similarity of the Se sublattices between ß-Cu2Se and Cu4TiSe4 gives rise to coherent phase boundaries and good band alignment, which promote carrier transport across the interfaces. Interestingly, the different cation arrangements in Cu4TiSe4 and ß-Cu2Se contribute to enhanced phonon scattering at the interfaces, which leads to a reduction in the lattice thermal conductivity. The large reduction in the total thermal conductivity while preserving the high power factor of ß-Cu2Se in the (1-x)Cu2Se/(x)Cu4TiSe4 composites results in an improved ZT of 1.2 at 850 K, with an average ZT of 0.84 (500-850 K) for the composite with x = 0.01. This work highlights the importance of structural similarity between the matrix and inclusions when designing thermoelectric materials with improved energy conversion efficiency.

Modular Nanostructures Facilitate Low Thermal Conductivity and Ultra-High Thermoelectric Performance in n-Type SnSe.

Single crystals of SnSe have gained considerable attention in thermoelectrics due to their unprecedented thermoelectric performance. However, polycrystalline SnSe is more favorable for practical applications due to its facile chemical synthesis procedure, processability, and scalability. Though the thermoelectric figure of merit (zT) of p-type bulk SnSe polycrystals has reached >2.5, zT of n-type counterpart is still lower and lies around ≈1.5. Herein, record high zT of 2.0 in n-type polycrystalline SnSe0.92  + x mol% MoCl5 (x = 0-3) samples is reported, when measured parallel to the spark plasma sintering pressing direction due to the simultaneous optimization of n-type carrier concentration and enhanced phonon scattering by incorporating modular nano-heterostructures in SnSe matrix. Modular nanostructures of layered intergrowth [(SnSe)1.05 ]m (MoSe2 )n like compounds embedded in SnSe matrix scatters the phonons significantly leading to an ultra-low lattice thermal conductivity (κlat ) of ≈0.26 W m-1 K-1 at 798 K in SnSe0.92  + 3 mol% MoCl5 . The 2D layered modular intergrowth compound resembles the nano-heterostructure and their periodicity of 1.2-2.6 nm in the SnSe matrix matches the phonon mean free path of SnSe, thereby blocking the heat carrying phonons, which result in low κlat and ultra-high thermoelectric performance in n-type SnSe.

Strong Anharmonicity-Induced Low Thermal Conductivity and High n-type Mobility in the Topological Insulator Bi1.1 Sb0.9 Te2 S.

Intrinsically low lattice thermal conductivity (κlat ) while maintaining the high carrier mobility (µ) is of the utmost importance for thermoelectrics. Topological insulators (TI) can possess high µ due to the metallic surface states. TIs with heavy constituents and layered structure can give rise to high anharmonicity and are expected to show low κlat . Here, we demonstrate that Bi1.1 Sb0.9 Te2 S (BSTS), which is a 3D bulk TI, exhibits ultra-low κlat of 0.46 Wm-1 K-1 along with high µ of ≈401 cm2  V-1 s-1 . Sound velocity measurements and theoretical calculations suggest that chemical bonding hierarchy and high anharmonicity play a crucial role behind such ultra-low κlat . BSTS possesses low energy optical phonons which strongly couple with the heat carrying acoustic phonons leading to ultra-low κlat . Further, Cl has been doped at the S site of BSTS which increases the electron concentration and reduces the κlat resulting in a promising n-type thermoelectric figure of merit (zT) of ≈0.6 at 573 K.

Solid-State Janus Nanoprecipitation Enables Amorphous-Like Heat Conduction in Crystalline Mg3 Sb2 -Based Thermoelectric Materials.

Solid-state precipitation can be used to tailor material properties, ranging from ferromagnets and catalysts to mechanical strengthening and energy storage. Thermoelectric properties can be modified by precipitation to enhance phonon scattering while retaining charge-carrier transmission. Here, unconventional Janus-type nanoprecipitates are uncovered in Mg3 Sb1.5 Bi0.5 formed by side-by-side Bi- and Ge-rich appendages, in contrast to separate nanoprecipitate formation. These Janus nanoprecipitates result from local comelting of Bi and Ge during sintering, enabling an amorphous-like lattice thermal conductivity. A precipitate size effect on phonon scattering is observed due to the balance between alloy-disorder and nanoprecipitate scattering. The thermoelectric figure-of-merit ZT reaches 0.6 near room temperature and 1.6 at 773 K. The Janus nanoprecipitation can be introduced into other materials and may act as a general property-tailoring mechanism.

Cadmium-Rich Plant Powder/PAN/PU Foams with Low Thermal Conductivity.

Treating and utilizing heavy metal enriched plants have become growing problems. In this work, a series of composite foams were made from the powder of Cadmium-rich plant, polyacrylonitrile (PAN) and polyurethane (PU). Test results indicated that the addition of plant powder can not only increase the specific surface area, but also improve the apparent density and thermal stability of the foams. Besides, compared with the foam without plant powder, the powder-added foams exhibited a decreasing trend for thermal conductivity, and the minimum was 0.048 w/(m·k), which indicated that the addition of plant powder can help to enhance the thermal insulation of composite foam. More importantly, the results of leaching experiment showed that the leaching rate of heavy metal cadmium in the composite foam with 50% plant powder content was as low as 0.14% after being immersed in the acidic (pH = 3) solution for 5 days, which implies that the foam materials are very safe. This study provides a new way to realize high value-added resource utilization of heavy metal-enriched plants.

Thermoelectric Zintl Compound In1-x Gax Te: Pure Acoustic Phonon Scattering and Dopant-Induced Deformation Potential Reduction and Lattice Shrink.

We report a Zintl phase thermoelectric material, coarse grain-In0.99 Ga0.01 Te, achieving a ZT peak of 1.2 at 648 K and an average ZT=0.8 in 300-650 K, which outperforms all the known InTe-based materials to date. The synergistic optimization of electronic property and phonon transport are achieved by the purification of grain boundary scattering, together with the Ga-doping-induced weak phonon-electron coupling, which enhances the carrier mobility and carrier concentration simultaneously and consequently gives a remarkably increased power factor of 8.9 µW cm-1 K-2 . The DFT phonon calculations indicate the dopant reduces the deformation potential coefficient and induces the lattice shrink, which reduces significantly the acoustic cutoff frequency, and enhances the scattering phase space. Moreover, the bonding hierarchy leads to the dense intragranular dislocation arrays, which suppresses the lattice thermal conductivity further and induces an ultralow lattice thermal conductivity (0.21 Wm-1 K-1 ).

Fire Resistance of Geopolymer Foams Layered on Polystyrene Boards.

Geopolymer foams are excellent materials in terms of mechanical loads and fire resistance applications. This study investigated the foaming process of geopolymers and foam stability, with a focus on the fire resistance performance when using polystyrene as the base layer. The main purpose is to define the influence of porosity on the physical properties and consequently to find applications and effectiveness of geopolymers. In this study, lightweight materials are obtained through a process called geopolymerization. Foaming was done by adding aluminum powder at the end of the geopolymer mortar preparation. The interaction between the aluminum powder and the alkaline solution (used for the binder during the mixing process) at room temperature is reactive enough to develop hydrogen-rich bubbles that increase the viscosity and promote the consolidation of geopolymers. The basic principle of thermodynamic reactions responsible for the formation of foams is characterized by hydrogen-rich gas generation, which is then trapped in the molecular structure of geopolymers. The geopolymer foams in this study are highly porous and robust materials. Moreover, the porosity distribution is very homogeneous. Experimental assessments were performed on four specimens to determine the density, porosity, mechanical strength, and thermal conductivity. The results showed that our geopolymer foams layered on polystyrene boards (with optimal thickness) have the highest fire resistance performance among others. This combination could withstand temperatures of up to 800 °C for more than 15 min without the temperature rising on the insulated side. Results of the best-performing geopolymer foam underline the technical characteristics of the material, with an average apparent density of 1 g/cm3, a volume porosity of 55%, a thermal conductivity of 0.25 W/mK, and excellent fire resistance.

Optimized Electronic Bands and Ultralow Lattice Thermal Conductivity in Ag and Y Codoped SnTe.

As a lead-free thermoelectric material, SnTe is inhibited by its inherent high carrier concentration and high thermal conductivity. This work describes the synergistic effect on the modulation of band structure and microstructural defects of SnTe by Ag and Y codoping, which gives rise to band convergence and multiple microstructural defects (secondary phases, dislocations, and boundaries) in the matrix and endows Sn0.94Ag0.09Y0.05Te with an increased power factor of ∼2485 µW m-1 K-2, an extremely low lattice thermal conductivity of ∼0.61 W m-1 K-1, and a peak zT as high as ∼1.2 at 873 K. This work reveals that the combination of Ag and Y could play a role in the synergistic optimization of electronic and phonon transport properties of SnTe by modifying the band structure and microstructures, providing guidance for enhancing the thermoelectric performance of the relevant materials.

Enhanced Band Convergence and Ultra-Low Thermal Conductivity Lead to High Thermoelectric Performance in SnTe.

SnTe, a structural analogue of champion thermoelectric (TE) material PbTe, has recently attracted wide attention for TE energy conversion. Herein, we demonstrate a co-doping strategy to improve the TE performance of SnTe via simultaneous modulation of electronic structure and phonon transport. The electrical transport is optimized by 3 mol % Ag doping in self-compensated SnTe (i.e., Sn1.03 Te). Further, Mg doping in SnAg0.03 Te resulted in highly converged valence bands, which enhanced the Seebeck coefficient markedly. The energy gap between two uppermost valence bands (ΔEv ) decreases to 0.10 eV in Sn0.92 Ag0.03 Mg0.08 Te compared to 0.35 eV in pristine SnTe. The optimized p-type carrier concentration and highly converged valence bands gave a high power factor of ca. 27 µW cm-1 K-2 at 865 K in Sn0.92 Ag0.03 Mg0.08 Te. The lattice thermal conductivity of Sn0.92 Ag0.03 Mg0.08 Te reached to an ultra-low value of ≈0.23 W m-1 K-1 at 865 K due to the formation of MgTe nanoprecipitates in SnTe matrix. These combined effects resulted in a high TE figure of merit, zT≈1.55 at 865 K in Sn0.92 Ag0.03 Mg0.08 Te.