Jesús Prado-Gonjal

Microwave-assisted synthesis, microstructure, and physical properties of rare-earth chromites.

The full rare-earth (RE) chromites series (RE)CrO(3) with an orthorhombic distorted (Pnma) perovskite structure and the isostructural compound YCrO(3) can be synthesized through a simple microwave-assisted technique, yielding high-quality materials. Magnetization measurements evidence that the Néel temperature for antiferromagnetic Cr(3+)-Cr(3+) ordering strongly depends on the RE(3+) ionic radius (IOR), and a rich variety of different magnetic spin interactions exists. Dielectric spectroscopy on sintered pellets indicates electronic inhomogeneity in all samples as manifested by the presence of at least two dielectric relaxation processes associated with grain boundary and grain interior bulk contributions. X-ray diffraction, Raman spectroscopy, and temperature-dependent dielectric permittivity data do not indicate potential noncentrosymmetry in the crystal or concomitant ferroelectricity. Strong correlations between the magnetic and dielectric properties were not encountered, and microwave-synthesized (RE)CrO(3) may not be classified as magnetoelectric or multiferroic materials.

Microwave-Assisted Synthesis, Microstructure, and Magnetic Properties of Rare-Earth Cobaltites

The series of perovskite rare-earth (RE) doped cobaltites (RE)CoO3 (RE = La-Dy) was prepared by microwave-assisted synthesis. The crystal structure undergoes a change of symmetry depending on the size of the RE cation. LaCoO3 is rhombohedral, S.G. R3̅c (No. 167), while, for the rest of the RE series (Pr-Dy), the symmetry is orthorhombic, S.G. Pnma (No. 62). The crystal structure obtained by X-ray diffraction was confirmed by high-resolution transmission electron microscopy, which yielded a good match between experimental and simulated images. It is further shown that the well-known magnetism in LaCoO3, which involves a thermally induced Co3+ (d6) low spin to intermediate or high spin state transition, is strongly modified by the RE cation, and a rich variety of magnetic order has been detected across the series.

Anti-site disorder and physical properties in microwave synthesized RE2Ti2O7 (RE = Gd, Ho) pyrochlores

In this work we report on the microwave assisted synthesis of nano-sized Gd2Ti2O7 (GTO) and Ho2Ti2O7 (HTO) powders from the RE2Ti2O7 pyrochlore family (RE = rare earth). Synchrotron X-ray powder diffraction was used to study RE–Ti cationic anti-site defects with concentrations that decrease in both samples with increasing temperature starting from 1100 °C, and the defects disappear at 1400 °C. SQUID magnetometry measurements revealed that GTO shows a predominantly anti-ferromagnetic structure, whereas HTO exhibits magnetic saturation and a ferromagnetic component at low temperature. Impedance spectroscopy data revealed strongly increased ionic oxygen vacancy conduction in HTO ceramic pellets as compared to GTO, which may be associated with a higher degree of oxygen vacancy disorder. This argument was supported by Raman spectroscopy data.

CHAPTER 2:Novel Synthetic Techniques for Nanomaterials

In this work, we describe the use of novel environmentally friendly industrial processing techniques to produce nano-sized crystallite powders and fabricate bulk ceramic pellets of functional materials. We present the application of a microwave (MW)-assisted synthesis process to fabricate the un-doped and doped ceria phases CeO2−δ, Ce0.8Gd0.1Sm0.1O1.9, Ce0.85Gd0.15O1.925, Ce0.85Sm0.15O1.925 and Ce0.8Sm0.18Ca0.02O1.9 for potential application in solid oxide fuel cells. We find that the composition Ce0.8Sm0.18Ca0.02O1.9 may be most beneficial owing to the highest ionic conductivities encountered in the corresponding sintered ceramics. As a next step, we tested a MW-assisted sintering process to fabricate dense Ce0.8Sm0.18Ca0.02O1.9 pellets. We find that both the synthesis and sintering process can be achieved by using MW heat sources and the entire fabrication process may be regarded a “green ceramic processing”.

Low lattice thermal conductivity in arc-melted GeTe with Ge-deficient crystal structure

GeTe is a well-known thermoelectric material, with transport properties strongly dependent on the composition and crystal structure. Phase-pure polycrystalline GeTe has been prepared by a straightforward arc-melting technique, and its structural and physical properties are studied by neutron powder diffraction (NPD), electron microscopy, calorimetry, and transport measurements. The structural analysis from NPD data reveals a conspicuous Ge deficiency in the bulk structure (∼7% atomic vacancies), confirmed by the Hall-carrier concentration. The analysis of the atomic displacement parameters shows strong anisotropy of Ge ellipsoids, revealing a considerable anharmonicity of the chemical bonds. Concerning the thermoelectric properties, the samples display high electrical conductivity and reduced lattice contribution to the total thermal conductivity, exhibiting record-low 0.8 W m−1 K−1 at 770 K, as a consequence of the highly defective crystal structure. Both are essential ingredients of useful thermoelectric materials, indicating the applicability of defective GeTe in polycrystalline form.GeTe is a well-known thermoelectric material, with transport properties strongly dependent on the composition and crystal structure. Phase-pure polycrystalline GeTe has been prepared by a straightforward arc-melting technique, and its structural and physical properties are studied by neutron powder diffraction (NPD), electron microscopy, calorimetry, and transport measurements. The structural analysis from NPD data reveals a conspicuous Ge deficiency in the bulk structure (∼7% atomic vacancies), confirmed by the Hall-carrier concentration. The analysis of the atomic displacement parameters shows strong anisotropy of Ge ellipsoids, revealing a considerable anharmonicity of the chemical bonds. Concerning the thermoelectric properties, the samples display high electrical conductivity and reduced lattice contribution to the total thermal conductivity, exhibiting record-low 0.8 W m−1 K−1 at 770 K, as a consequence of the highly defective crystal structure. Both are essential ingredients of useful thermoelectri...