Tailoring the structure and thermoelectric properties of BaTiO3via Eu2+ substitution

A series of Ba1_xEuxTiO3_d (0.1 < x < 0.9) phases with B40 nm particle size were synthesized via a Pechini method followed by annealing and sintering under a reducing atmosphere. The effects of Eu2+ substitution on the BaTiO3 crystal structure and the thermoelectric transport properties were systematically investigated. According to synchrotron X-ray diffraction data only cubic perovskite structures were observed. On the local scale below about 20 \uc5 (equal to B5 unit cells) deviations from the cubic structure model (Pm%3m) were detected by evaluation of the pair distribution function (PDF). These deviations cannot be explained by a simple symmetry breaking model like in EuTiO3_d. The best fit was achieved in the space group Amm2 allowing for a movement of Ti and Ba/Eu along h110i of the parent unit cell as observed for BaTiO3. Density functional calculations delivered an insight into the electronic structure of Ba1_xEuxTiO3_d. From the obtained density of states a significant reduction of the band gap by the presence of filled Eu2+ 4f states at the top of the valence band was observed. The physical property measurements revealed that barium\u2013europium titanates exhibit n-type semiconducting behavior and at high temperature the electrical conductivity strongly depended on the Eu2+ content. Activation energies calculated from the electrical conductivity and Seebeck coefficient data indicate that at high temperatures (800 K o T o 1123 K) the conduction mechanism of Ba1_xEuxTiO3_d (0.1 r x r 0.9) is a polaron hopping when 0 o x r 0.6 and is a thermally activated process when 0.6 o x o 1. Besides, the thermal conduc tivity increases with increasing Eu2+ concentration. Due to a remarkable improvement of the power factor, Ba0.1Eu0.9TiO3_d showed a ZT value of 0.24 at 1123 K

Effects of Cr Doping and Water Content on the Crystal Structure Transitions of Ba2In2O5

Temperature dependent crystal structure alterations in the brownmillerite type material Ba2In2O5 play a fundamental role in its applications i photocatalytic CO2 conversion; ii oxygen transport membranes; and iii proton conduction. This is connected to a reversible uptake of up an equimolar amount of water. In this study, in situ X ray and neutron diffraction were combined with Raman spectroscopy and solid state nuclear magnetic resonance experiments to unravel the effects of Cr doping and water content on the crystal structure transitions of Ba2In2O5 H2O x over a wide temperature range 10 K lt; T lt; 1573 K, x lt; 1 . A mixture of isolated and correlated protons was identified, leading to a highly dynamic situation for the protons. Hence, localisation of the protons by diffraction techniques was not possible. Cr doping led to an overall higher degree of disorder and stabilisation of the tetragonal polymorph, even at 10 K. In contrast, a further disordering at high temperatures, leading to a cubic polymorph, was found at 1123 K. Cr doping in Ba2In2O5 resulted in severe structural changes and provides a powerful way to adjust its physical properties to the respective applicatio

A self-forming nanocomposite concept for ZnO-based thermoelectrics

Zinc oxide (ZnO) has a very broad and versatile range of applications provided by its high abundance and optical and electrical properties, which can be further tuned by donor substitution. Al-doped ZnO is probably the most thoroughly investigated material with regard to thermoelectric properties. Fairly reasonable electrical properties of donor-doped zinc oxide are usually combined with high thermal conductivity limiting potential applications. Here we report a new self-forming nanocomposite concept for ZnO-based thermoelectrics, where a controllable interplay between the exsolution of the nanophases and modification of the host matrix suppresses the thermal transport while imparting enhanced electrical performance. The thermoelectric performance of the best-obtained composite, described by the dimensionless figure-of-merit ZT, at 920-1200 K is almost twice that of the pure matrix composition and reaches up to 0.11. The proposed approach invokes controlled interactions between composite components as a novel tool for decoupling the electrical and thermal transport parameters and shows clear prospects for an implementation in other thermoelectric oxide systems. The results indicate that the proposed concept may also constitute a promising pathway to achieve stable electrical performance at high temperatures, which currently represents one of the major challenges towards achieving ZnO-based thermoelectrics. © The Royal Society of Chemistry

Upcycling Waste Plastics into Multi-Walled Carbon Nanotube Composites via NiCo₂O₄ Catalytic Pyrolysis

In this work, multi-walled carbon nanotube composites (MWCNCs) were produced by catalytic pyrolysis of post-consumer plastics with aluminium oxide-supported nickel, cobalt, and their bimetallic (Ni/α–Al₂O₃, Co/α–Al₂O₃, and NiCo/α–Al₂O₃) oxide-based catalysts. The influence of catalyst composition and catalytic reaction temperature on the carbon yield and structure of CNCs were investigated. Different temperatures (800, 900, 950, and 1000°C) and catalyst compositions (Ni, Co, and Ni/Co) were explored to maximize the yield of carbon deposited on the catalyst. The obtained results showed that at the same catalytic temperature (900°C), a Ni/Co bimetallic catalyst exhibited higher carbon yield than the individual monometallic catalysts due to a better cracking capability on carbon-hydrogen bonds. With the increase of temperature, the carbon yield of the Ni/Co bimetallic catalyst increased first and then decreased. At a temperature of 950°C, the Ni/Co bimetallic catalyst achieved its largest carbon yield, which can reach 255 mg g⁻¹ plastic. The growth of CNCs followed a “particle-wire-tube” mechanism for all studied catalysts. This work finds the potential application of complex oxide composite material catalysts for the generation of CNCs in catalytic pyrolysis of wasted plastic

Cr-Substitution in Ba₂In₂O₅ · (H₂O)_x (x = 0.16, 0.74)

Cr-substituted and pristine Ba2In2O5·(H2O)x powders were synthesized by solid state reaction. The influence of Cr-substitution on the crystal structure, chemical composition, magnetic and optical properties were investigated. Powder X-ray diffraction (XRD), elemental analysis and TGA-MS reveal that with substitution of In for Cr, the unit cell volume and the unit cell parameter b increase together with the oxygen and hydrogen content. Magnetic property measurements indicate that Ba2In2O5·(H2O)x is diamagnetic in the temperature range of 2 K < T < 300 K becoming ferromagnetic upon Cr-substitution. In the UV–vis spectra of the Cr-substituted sample a distinctive shift of the absorption-edge energy from 430 to 690 nm was observed corresponding to a bandgap narrowing from 2.88 to 1.80 eV. The replacement of tetrahedral InO4 units by octahedral CrO6 units was found to be the main factor for the drastic change of the magnetic and optical properties