1,721,077 research outputs found
Alloys and intermetallic compounds: From modeling to engineering
No full textThis book focuses on the role of modeling in the design of alloys and intermetallic compounds. It includes an introduction to the most important and most used modeling techniques, such as CALPHAD and ab-initio methods, as well as a section devoted to the latest developments in applications of alloys. The book emphasizes the correlation between modeling and technological developments while discussing topics such as wettability of Ultra High Temperature Ceramics by metals, active brazing of diamonds to metals in cutting tools, surface issues in medicine, novel Fe-based superconductors, metallic glasses, high entropy alloys, and thermoelectric materials
Crystal chemistry, stability and properties of interlanthanide perovskites: A review
No full textIn this review structural features, stability issues and physical properties of the eleven interlanthanide perovskites prepared at atmospheric pressure are surveyed and discussed. Due to the reduced size difference between cations, the structure of these oxides is strongly distorted with respect to the ideal cubic case. Therefore, in structure maps they are located close to the boundary of the perovskitic stability field; an interesting correlation between stability and the Goldschmidt tolerance factor t shows that the perovskitic temperature range narrows with decreasing t. Magnetic and optical properties are strictly related to the presence of 4f electrons, that determine the existence of many discrete energy levels. Acceptor-doped LaYO3 and LaYbO3 are comparable in terms of protonic conductivity to the well known cerates, and thanks to their better chemical stability against CO2, they are a good alternative to the latter in solid oxide fuel cells. The high dielectric constant of some interlanthanide perovskites makes them interesting candidates as gate oxides in MOSFETs
Synthesis and phase equilibria of Nd-Gd mixed oxycarbonates
No full textThe synthesis of mixed Gd-Nd oxycarbonates was performed via a solid state route involving the decomposition of mixed oxalates. Depending on the reaction atmosphere, the compounds can form through two different reaction paths: by partial decomposition of the corresponding mixed oxalates or by complete decomposition of the oxalates followed by partial carbonation. All the oxycarbonates, when prepared at 600 °C in static air or CO2, form a solid solution, crystallize in the hexagonal structure and follow Vegard's law. If treated at lower temperature, Nd oxalate gives rise to the monoclinic structure, so that the monoclinic → hexagonal phase transition can be observed. The decomposition of the oxycarbonates leads to the cubic form of the corresponding oxide for each composition. © 2008 Elsevier B.V. All rights reserved
Rare-Earth-Doped Ceria Systems and Their Performance as Solid Electrolytes: A Puzzling Tangle of Structural Issues at the Average and Local Scale
No full textRare-earth (RE)-doped ceria systems, in particular when RE ≡ Nd, Sm, or Gd, are well-known to be characterized by high values of ionic conductivity in the intermediate temperature range, which, in principle, makes them ideal solid electrolytes in solid oxide fuel and electrolysis cells. Defect chemistry turns out to be a pivotal issue in this framework because ionic conductivity is driven by the ability of oxygen vacancies to move through the lattice, and any form of defect clustering tends to depress the efficiency of oxygen transport. In this viewpoint, not only are factors at the average scale assessed, such as the compositional extent of the CeO2-like solid solution, but also the occurrence of local inhomogeneities due to vacancy-dopant association is discussed in correlation with its central role in hindering the migration of vacancies. The relationship between the stability of the hybrid phase and the RE3+ ionic size is presented, and the highly complementary role of Raman spectroscopy toward X-ray diffraction is described in detail. The key points of the whole discussion are finally used to identify the most relevant structure-related parameters affecting ionic conductivity in the studied material
New insights into the magnetic properties of LaErO3, (La0.5Er0.5)2O3 and (La0.5Dy0.5)2O3 oxides
No full textOrthorhombic LaErO3 and cubic (La0.5 Ln 0.5)2O3 oxides (Ln: Er, Dy) were examined by neutron powder diffraction between 1.5 K and 15 K in order to investigate their crystallographic and magnetic structures. At 1.5 K both LaErO3 and (La0.5Er0.5)2O3 display a magnetic moments ordering, whereas for (La0.5Dy0.5)2O3 only short range magnetic correlations can be argued, suggesting a possible magnetic moments ordering at lower temperature. LaErO3 is characterized by a magnetic wavevector k = (0, 0, 0) and forms an antiferromagnetic G x C y A z-type structure belonging to the Shubnikov group with a total magnetic moment of 6.78(3) μ B. The antiferromagnetic structure of (La0.5Er0.5)2O3 is similar to the one typical of Er2O3, with a total magnetic moment of 4.28(2) μ B at both different magnetic sites; it is characterized by k = (0, 0, 0) and belongs to the magnetic Shubnikov group
Study of the formation temperature of mixed LaREO3 (RE ≡ Dy, Ho, Er, Tm, Yb, Lu) and NdGdO3 oxides
No full textMixed LaREO3 (RE ≡ Dy, Ho, Er, Tm, Yb, Lu) and NdGdO 3 oxides were prepared by thermal decomposition of the corresponding co-precipitated mixed oxalates. The decomposition of La/RE and Nd/Gd oxalates was studied by means of differential thermal analysis and thermogravimetric (DTA-TG) measurements; in particular the last step, consisting of the loss of a CO2 molecule from the corresponding oxycarbonate, has been thoroughly investigated, as it is particularly interesting for the study of the formation temperature of mixed rare earth oxides. After the release of CO2, the oxides crystallize in a distorted perovskitic cell or one of the structures typical of rare earth sesquioxides, depending on the cationic size difference and on the average cationic radius. The mixed rare earth oxycarbonate decomposition has been studied in comparison to the decomposition of single rare earth oxycarbonates. A trend of the mixed oxides formation temperature as a function both of the average cationic size and of the cationic sizes difference has been observed and compared to the behaviour of single rare earth oxides. © 2010 Akadémiai Kiadó, Budapest, Hungary
Influence of Composition and Thermal Treatments on Microhardness of the Filled Skutterudite Sm<sub>y</sub> (Fe<sub>x</sub> Ni<sub>1−x</sub>)<sub>4</sub>Sb<sub>12</sub>
No full textSamples belonging to the series Smy(FexNi1-x)4Sb12 were heated in Ar atmosphere up to 400°C and cooled down to room temperature several times, with the aim to evaluate the effect of thermal cycles on microhardness. The treatment temperature was chosen in correspondence of the maximum ZT value, in order to simulate the operating conditions of the material in a thermoelectric device. Vickers measurements allowed to detect the effect of both composition and thermal treatments on the microhardness properties of the material. A decrease in the microhardness value was observed prior to thermal treatments with increasing Fe amount, due to the substitution of Ni by the larger Fe atom. Moreover, almost all compositions show an increase in the hardness of the skutterudite phase as a consequence of thermal cycles, accounting for the Sb enrichment of skutterudite. This evidence suggests also a possible improvement of the preparation procedure in order to obtain a stoichiometric Sb amount within the skutterudite. Samples were prepared by direct reaction of pure elements at 950°C followed by thermal treatment at 620°C. The composition and microstructure of the obtained samples were investigated by X-ray powder diffraction, and by optical and electronic microscopy
Stability fields and structural properties of intra rare earths perovskites.
No full textA stability study of perovskitic LaREO3 oxides (RE = Dy, Ho, Er, Tm, Yb, Lu) as a function of temperature was undertaken. A correlation between the Goldschmidt t value and the perovskitic stability field amplitude was found: the latter widens as t increases. Magnetic measurements, performed on all the perovskitic samples, showed that t is also related to the exchange interactions.
LaREO3 oxides were synthesized by thermal decomposition of the corresponding coprecipitated oxalates at temperatures ranging between 600 and 1800 ◦C. Simultaneous differential thermal and thermogravimetric analyses showed that all the La–RE mixed oxalates decompose similarly. All the oxides, except LaDyO3, crystallize in the perovskitic form in a temperature range that depends on the ionic size difference between La and the smaller rare earth; above and below the perovskitic stability field, the B or C form, typical of rare earth sesquioxides, is present. Rietveld refinements, carried out on all the LaREO3 samples synthesized at 1200 ◦C, showed the occurrence of an orthorhombic distorted perovskitic structure belonging to the Pnma space group
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