RESUMEN
In the present study, we have investigated the stability of different Ba-Si clathrates with pressure and temperature using DFT calculations and studied the stability of type I Ba8Si46 and type IX Ba24Si100 clathrates using high pressureâhigh temperature synthesis technique, calorimetry, and diffraction experiments. When increasing pressure, the type I Ba8Si46 clathrate and BaSi6 become more stable. In good qualitative agreement with experiments, the type IX Ba24Si100 clathrate becomes stable at a pressure of 1-2 GPa thanks to the pressure and thermal effect of both electronic and vibrational contributions. One can notice that the presence of Ba in the cages of type IX clathrate increases significantly the stability and the mechanical properties of type IX clathrate. We have determined the P-T existence domain of type IX Ba24Si100 clathrate from ex situ experiments, which was confirmed by in situ synchrotron X-ray experiments. At room pressure and under an oxidizing atmosphere, the type I Ba8Si46 and the type IX Ba24Si100 clathrates are stable up to about 560 °C and up to about 600 °C, respectively. The thermoelectric properties of type IX Ba24Si100 are also reported.
RESUMEN
A new tridimensional metal-organic chalcogenolate, made of a 1,3-benzenedithiolate bridging ligand and Ag(I), [Ag2(1,3-BDT)]n, is reported. This coordination polymer has good thermal stability in air and displays both photoluminescence properties and a second harmonic generation response.
RESUMEN
We have explored the effect of high pressure post-treatment in optimizing the properties of carbon nanotube yarns and found that the application of dry hydrostatic pressure reduces porosity and enhances electrical properties. The CNT yarns were prepared by the dry-spinning method directly from CNT arrays made by the hot filament chemical vapour deposition (HF-CVD) process. Mechanical hydrostatic pressure up to 360 MPa induces a decrease in yarn resistivity between 3% and 35%, associated with the sample's permanent densification, with CNT yarn diameter reduction of 10%-25%. However, when increasing the pressure in the 1-3 GPa domain in non-hydrostatic conditions, the recovered samples show lower electrical conductivity. This might be due to concomitant macroscopic effects such as increased twists and damage to the yarn shown by SEM imaging (caused by strong shear stresses and friction) or by the collapse of the CNTs indicated byin situhigh pressure Raman spectroscopy data.
RESUMEN
Rutile is the most common and stable polymorph form of titanium oxide TiO2 at all temperatures. The doping of rutile TiO2 with a small amount of niobium is reknown for being responsible for a large increase of the electrical conductivity by several orders of magnitude, broadening its technological interest towards new emerging fields such as the thermoelectric conversion of waste heat. The electronic conduction has been found to be of a polaronic nature with strongly localized charges around the Ti3+ centers while, on the other side, the relatively high value of the thermal conductivity implies the existence of lattice heat carriers, i.e. phonons, with large mean free paths which makes the nanostructuration relevant for optimizing the thermoelectric efficiency. Here, the use of a high-pressure and high-temperature sintering technique has allowed to vary the grain size in rutile TiO2 pellets from 300 to 170 nm, leading to a significant reduction of the lattice thermal conductivity. The thermoelectric properties (electrical conductivity, Seebeck coefficient and thermal conductivity) of Nb-doped rutile nanostructured ceramics, namely NbxTi1-xO2 with x varying from 1 to 5%, are reported from room temperature to â¼900 K. With the incorporation of Nb, an optimum in the thermoelectric properties together with an anomaly on the tetragonal lattice constant c are observed for a concentration of â¼2.85%, which might be the fingerprint of the formation of short Nb dimers.
RESUMEN
High-quality thermoelectric La0.2Sr0.8TiO3 (LSTO) films, with thicknesses ranging from 20 nm to 0.7 µm, have been epitaxially grown on SrTiO3(001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 × 10-4 Ω cm at room temperature), one order of magnitude lower than standard commercial Nb-doped SrTiO3 single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of approximately -60 µV/K have been recorded for all films. These LSTO films can be integrated on Si for non-volatile memory structures or opto-microelectronic devices, functioning as transparent conductors or thermoelectric elements.
RESUMEN
We present a joint experimental and theoretical study of the superconducting phase of the layered binary silicide BaSi(2). Compared with the AlB(2) structure of graphite or diboridelike superconductors, in the hexagonal structure of binary silicides the sp(3) arrangement of silicon atoms leads to corrugated sheets. Through a high-pressure synthesis procedure we are able to modify the buckling of these sheets, enhancing the superconducting transition temperature from 6 to 8.9 K when the silicon planes flatten out. By performing ab initio calculations based on density-functional theory we explain how the electronic and phonon properties are strongly affected by changes in the buckling. This mechanism is likely present in other intercalated layered superconductors, opening the way to the tuning of superconductivity through the control of internal structural parameters.
