Unveiling the Structural Properties, Optical Behavior, and Thermoelectric Performance of 2D CsSn2Br5 Halide Obtained by Mechanochemistry.

Metal halide perovskites with a two-dimensional structure are utilized in photovoltaics and optoelectronics. High-crystallinity CsSn2Br5 specimens have been synthesized via ball milling. Differential scanning calorimetry curves show melting at 553 K (endothermic) and recrystallization at 516 K (exothermic). Structural analysis using synchrotron X-ray diffraction data, collected from 100 to 373 K, allows for the determination of Debye model parameters. This analysis provides insights into the relative Cs-Br and Sn-Br chemical bonds within the tetragonal structure (space group: I4/mcm), which remains stable throughout the temperature range studied. Combined with neutron data, X-N techniques permit the identification of the Sn2+ lone electron pair (5s2) in the two-dimensional framework, occupying empty space opposite to the four Sn-Br bonds of the pyramidal [SnBr4] coordination polyhedra. Additionally, diffuse reflectance UV-vis spectroscopy unveils an indirect optical gap of approximately ∼3.3 eV, aligning with the calculated value from the B3LYP-DFT method (∼3.2 eV). The material exhibits a positive Seebeck coefficient as high as 6.5 × 104 µV K-1 at 350 K, which evolves down to negative values of -3.0 × 103 µV K-1 at 550 K, surpassing values reported for other halide perovskites. Notably, the thermal conductivity remains exceptionally low, between 0.32 and 0.25 W m-1 K-1.

Optimizing Thermoelectric Properties through Compositional Engineering in Ag-Deficient AgSbTe2 Synthesized by Arc Melting.

Thermoelectric materials offer a promising avenue for energy management, directly converting heat into electrical energy. Among them, AgSbTe2 has gained significant attention and continues to be a subject of research at further improving its thermoelectric performance and expanding its practical applications. This study focuses on Ag-deficient Ag0.7Sb1.12Te2 and Ag0.7Sb1.12Te1.95Se0.05 materials, examining the impact of compositional engineering within the AgSbTe2 thermoelectric system. These materials have been rapidly synthesized using an arc-melting technique, resulting in the production of dense nanostructured pellets. Detailed analysis through scanning electron microscopy (SEM) reveals the presence of a layered nanostructure, which significantly influences the thermoelectric properties of these materials. Synchrotron X-ray diffraction reveals significant changes in the lattice parameters and atomic displacement parameters (ADPs) that suggest a weakening of bond order in the structure. The thermoelectric characterization highlights the enhanced power factor of Ag-deficient materials that, combined with the low glass-like thermal conductivity, results in a significant improvement in the figure of merit, achieving zT values of 1.25 in Ag0.7Sb1.12Te2 and 1.01 in Ag0.7Sb1.12Te1.95Se0.05 at 750 K.

Multiscale Length Structural Investigation and Thermoelectric Performance of Double-Filled Sr0.2Yb0.2Co4Sb12: An Exceptional Thermal Conductivity Reduction by Filler Segregation to the Grain Boundaries.

Among thermoelectric materials, skutterudites are the most prominent candidates in the mid-temperature range applications. In the multiple-filled Sr0.2Yb0.2Co4Sb12 skutterudite, with Sr and Yb as fillers, we have enhanced the thermoelectric performance of CoSb3 through the reduction of lattice thermal conductivity and the optimization of carrier concentration and electrical conductivity. The high-pressure synthesis of the double-filled derivative promotes filling fraction fluctuation. This is observed by high angular resolution synchrotron X-ray diffraction, showing a phase segregation that corresponds to an inhomogeneous distribution of the filler atoms, located at the 2a positions of the cubic space group Im3̅. In addition, scanning transmission electron microscopy (STEM) combined with EELS spectroscopy clearly shows a segregation of Sr atoms from the surface of the grains, which is compatible with the synchrotron X-ray powder diffraction results. Mean square displacement parameters analysis results in Einstein temperatures of ∼94 and ∼67 K for Sr and Yb, respectively, and a Debye temperature of ∼250 K. The strong effect on resonant and disorder scattering yields a significantly lower lattice thermal conductivity of 2.5 W m-1 K-1 at 773 K. Still, good weighed-mobility values were obtained, with high filling fraction of the Yb and Sr elements. This drives a reduced electrical resistivity of 2.1 × 10-5 Ω m, which leads to a peak zT of 0.26 at 773 K. The analysis and results performed for the synthesized (Sr,Yb)-double filled CoSb3, shed light on skutterudites for potential waste-heat recovery applications.

Reduced Thermal Conductivity in Nanostructured AgSbTe2 Thermoelectric Material, Obtained by Arc-Melting.

AgSbTe2 intermetallic compound is a promising thermoelectric material. It has also been described as necessary to obtain LAST and TAGS alloys, some of the best performing thermoelectrics of the last decades. Due to the random location of Ag and Sb atoms in the crystal structure, the electronic structure is highly influenced by the atomic ordering of these atoms and makes the accurate determination of the Ag/Sb occupancy of paramount importance. We report on the synthesis of polycrystalline AgSbTe2 by arc-melting, yielding nanostructured dense pellets. SEM images show a conspicuous layered nanostructuration, with a layer thickness of 25-30 nm. Neutron powder diffraction data show that AgSbTe2 crystalizes in the cubic Pm-3m space group, with a slight deficiency of Te, probably due to volatilization during the arc-melting process. The transport properties show some anomalies at ~600 K, which can be related to the onset temperature for atomic ordering. The average thermoelectric figure of merit remains around ~0.6 from ~550 up to ~680 K.

Mechanical, Electrical, and Thermal Characterization of Pure Copper Parts Manufactured via Material Extrusion Additive Manufacturing.

Material Extrusion Additive Manufacturing (MEAM) is a novel technology to produce polymeric, metallic, and ceramic complex components. Filaments composed of a high-volume content of metal powder and a suitable binder system are needed to obtain metallic parts. Thermal and energetic controversies do not affect MEAM technology, although common in other additive manufacturing (AM) techniques. High thermal conductivity and reflectivity of copper to high-energy beams are the most challenging properties. A material extrusion technique to produce high density and quality copper parts is deeply studied in this research. Characterization of the filament, printed parts, brown parts and final sintered parts is provided. The sintering stage is evaluated through density analysis of the sintered copper parts, as well as their dimensional accuracy after part shrinkage inherent to the sintering process. The mechanical behavior of sintered parts is assessed through tensile, hardness and impact toughness tests. In addition, the measured electrical and thermal conductivities are compared to those obtained by other AM technologies. High-density components, with 95% of relative density, were successfully manufactured using MEAM technology. Similar or even superior mechanical, thermal and electrical properties than those achieved by other 3D printing processes such as Electron Beam Melting, Selective Laser Melting and Binder Jetting were obtained.

Detailed Structural Features of the Perovskite-Related Halide RbPbI3 for Solar Cell Applications.

All-inorganic lead halide perovskites like CsPbBr3, CsPbI3, or RbPbI3 are good replacements for the classical hybrid organic-inorganic perovskites like CH3NH3PbI3, susceptible to fast degradation in the presence of humid air. They also exhibit outstanding light absorption properties suitable for solar energy applications. Here, we describe the synthesis of RbPbI3 by mechanochemical procedures with green credentials, avoiding toxic or expensive organic solvents; this specimen exhibits excellent crystallinity. We report neutron powder diffraction data, essential to revisit some subtle structural features around room temperature (200-400 K). In all these regimes, the orthorhombic Pnma crystal structure is characterized by the presence along the b direction of the crystal of double rows of edge-sharing PbI6 octahedra. The lone electron pairs of Pb2+ ions have a strong stereochemical effect on the PbI6 octahedral distortion. The relative covalency of Rb-I versus Pb-I bonds shows that the Pb-I-related motions are more rigid than Rb-I-related vibrations, as seen in the Debye temperatures from the evolution of the anisotropic displacements. The optical gap, measured by diffuse reflectance UV-vis spectroscopy, is ∼2.51 eV and agrees well with ab initio calculations. The thermoelectric Seebeck coefficient is 3 orders of magnitude larger than that of other halide perovskites, with a value of ∼117,000 µV·K-1 at 460 K.

Thermal Expansion and Rattling Behavior of Gd-Filled Co4Sb12 Skutterudite Determined by High-Resolution Synchrotron X-ray Diffraction.

In this work, Gd-filled skutterudite GdxCo4Sb12 was prepared using one step method under high pressure in a piston-cylinder-based press at 3.5 GPa and moderate temperature of 800 °C. A detailed structural characterization was performed using synchrotron X-ray diffraction (SXRD), revealing a filling fraction of x = 0.033(2) and an average bond length of 3.3499(3) Å. The lattice thermal expansion accessed via temperature-dependent SXRD led to a precise determination of a Debye temperature of 322(3) K, from the fitting of the unit-cell volume expansion using the second order Grüneisen approximation. This parameter, when evaluated through the mean square displacements of Co and Sb, displayed a value of 265(2) K, meaning that the application of the harmonic Debye theory underestimates the Debye temperature in skutterudites. Regarding the Gd atom, its intrinsic disorder value was ~5× and ~25× higher than those of the Co and Sb, respectively, denoting that Gd has a strong rattling behavior with an Einstein temperature of θE = 67(2) K. As a result, an ultra-low thermal conductivity of 0.89 W/m·K at 773 K was obtained, leading to a thermoelectric efficiency zT of 0.5 at 673 K.

Structural Features, Anisotropic Thermal Expansion, and Thermoelectric Performance in Bulk Black Phosphorus Synthesized under High Pressure.

Black phosphorus (BP) allotrope has an orthorhombic crystal structure with a narrow bandgap of 0.35 eV. This material is promising for 2D technology since it can be exfoliated down to one single layer: the well-known phosphorene. In this work, bulk BP was synthesized under high-pressure conditions at high temperatures. A detailed structural investigation using neutron and synchrotron X-ray diffraction revealed the occurrence of anisotropic strain effects on the BP lattice; the combination of both sets of diffraction data allowed visualization of the lone electron pair 3s2. Temperature-dependent neutron diffraction data collected at low temperature showed that the a axis (zigzag) exhibits a quasi-temperature-independent thermal expansion in the temperature interval from 20 up to 150 K. These results may be a key to address the anomalous behavior in electrical resistivity near 150 K. Thermoelectric properties were also provided; low thermal conductivity from 14 down to 6 Wm-1K-1 in the range 323-673 K was recorded in our polycrystalline BP, which is below the reported values for single-crystals in literature.

Smart Hybrid Graphene Hydrogels: A Study of the Different Responses to Mechanical Stretching Stimulus.

Polymer-based hydrogels, in particular those containing nanoscale fillers, are currently regarded as promising candidates for a plethora of applications. With respect to graphene, the vast majority of publications concern chemical derivatives, and, as a consequence, knowledge of the potential of pristine graphene within these systems is lacking. In this study, novel graphene-based hydrogels containing nonoxidized graphene have been prepared by a mild aqueous process. The mechanical and electrical properties of these hybrid materials were characterized. In the compositions studied, maximum improvements of Young's modulus, ultimate tensile strength, and toughness of 30, 100, and 70% were obtained, respectively. In addition to obtaining an improved hybrid material in terms of mechanical and electrical properties, the response experienced by these systems on applying mechanical stretching was evaluated and stimuli-response behavior is generated by the presence of graphene. Two different kinds of responses were found. A significant change in electrical resistance was observed with a strain gauge effect and with an average gauge factor of ∼9 (for 30% strain). The electromechanical performance of these hybrid hydrogels was tested for a range of mechanical strains and graphene contents, and the stability of these materials was assessed with successive stretching cycles. It was also observed that upon stretching these hybrid hydrogels were able to release the internalized water more efficiently in the presence of graphene, and, as a result, a second possible stimulus response was studied in the form of controlled drug delivery as a proof of concept. The release of a loaded aqueous solution of ibuprofen stimulated by controlled stretching and aided by wet capillary was studied. Much more efficient delivery was achieved in the presence of graphene. These novel systems can be used in the future for sensing or drug-delivery applications.