Mechanical properties of mixed conducting La0.5Sr0.5Fe1−x Co x O3−δ (0≤x≤1) materials

Young’s modulus, strain–stress behavior, fracture strength, and fracture toughness of (0≤×≤1) materials have been investigated in the temperature range 20–1,000°C. Young’s moduli of and , measured by resonant ultrasound spectroscopy, were 130±1 and 133±3 GPa, respectively. The nonlinear stress–strain relationship observed by four-point bending at room temperature was inferred as a signature of ferroelastic behavior of the materials. Above the ferroelastic to paraelastic transition temperature, the materials showed elastic behavior, but due to high-temperature creep, a nonelastic respond reappeared above ∼800°C. The room temperature fracture strength measured by four-point bending was in the range 107–128 MPa. The corresponding fracture toughness of , measured by single edge V-notch beam method, was 1.16±0.12 MPa·m^1/2. The measured fracture strength and fracture toughness were observed to increase with increasing temperature. The fracture mode changed from intragranular at low temperature to intergranular at high temperature. Tensile stress gradient at the surface of the materials caused by a frozen-in gradient in the oxygen content during cooling was proposed to explain the low ambient temperature fracture strength and toughness.     

Crystalline Al1 − x Ti x phases in the hydrogen cycled NaAlH4 + 0.02TiCl3 system

The hydrogen (H) cycled planetary milled (PM) NaAlH₄?+?0.02TiCl₃ system has been studied by high resolution synchrotron X-ray diffraction and transmission electron microscopy during the first 10?H cycles. After the first H absorption, we observe the formation of four nanoscopic crystalline (c-) Ti-containing phases embedded on the NaAlH₄ surface, i.e. Al₂Ti, Al₃Ti, Al₈₂Ti₁₈ and Al₈₉Ti₁₁, with 100% of the originally added Ti atoms accounted for. Al₂Ti and Al₃Ti are observed morphologically as a mechanical couple on the NaAlH₄ surface, with a moderately strained interface. Electron diffraction shows that the Al₈₂Ti₁₈ phase retains some ordering from the L1₂ structure type, with the observation of forbidden (100) ordering reflections in the fcc Al₈₂Ti₁₈ lattice. After 2?H cycles the NaAlH₄?+?0.02TiCl₃ system displays only two crystalline Ti-containing phases, Al₃Ti and Al₈₉Ti₁₁. After 10?H cycles, the Al₈₉Ti₁₁ is completely converted to Al₈₅Ti_15. Al₈₉Ti₁₁, Al₈₅Ti₁₅ and Al₃Ti do not display any ordering reflections, and they are modeled in the A1 structure type. Quantitative phase analysis indicates that the Al₃Ti proportion continues to increase with further H cycles. The formation of Ti-poor Al_{1??? x} Ti _x (x?<?0.25) phases in later H cycles is detrimental to hydrogenation kinetics, compared to the starting Ti-richer near-surface Al₂Ti/NaAlH₄ interface present during the first absorption of hydrogen.     

Observations of nanoscopic,face centered cubic Ti and TiH<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Transmission electron microscopy has been used to study ball milled and H cycled NaAlH₄ with 10 mol% TiCl₃. Isolated from the main phases in this hydrogen storage system, nanocrystalline aggregates of fcc TiH _x (0≤x<0.67) were found. The value of x was determined based on the assumption of a linear increase of the TiH _x lattice parameter by increasing H content. The size of the TiH _x crystallites was in the range 10 to 20 nm, and the lattice parameter decreased from 4.22 Å in TiH_0.67 to 4.10 Å in pure fcc Ti. Non-equilibrium ball milling and subsequent H cycling in combination with a small crystallite size are believed to make the TiH _x phase stable. The present results are the first observations of fcc TiH _x with low hydrogen content, and the measured fcc lattice parameter of Ti matches first-principles calculations.     

Prediction of elastic properties of nanofibrillated cellulose from micromechanical modeling and nano-structure characterization by transmission electron microscopy

Cellulose-based materials have a great potential in terms of mechanical performance, since crystalline cellulose is known to have excellent stiffness along the main axis. This potential is not completely fulfilled in structural wood materials and in composite materials, due to structural inhomogeneities, misalignment, voids etc. on several length scales. This study investigates the difference in stiffness of nanofibrillated cellulose (NFC) compared to that of cellulose crystallites, based on nanostructural characterization, image analysis and micromechanical modeling. Nanofibrillated cellulose is believed to be composed of a distribution of crystallites in an amorphous matrix, and it is assumed to represent the distribution of the crystalline allomorph I_β. To predict the elastic properties of NFC, a micromechanical model based on a Mori–Tanaka approach and self-consistent scheme was used. The input data, i.e. orientation distribution, aspect ratio and volume fraction of these crystalline regions, were estimated from image analysis of transmission electron micrographs. The model predicts a ca. 56 % loss of stiffness of NFC compared to that of cellulose crystals along the main axis.     

Solid solubility and phase transitions in the system LaNb1−xTaxo4

The solid solubility between LaNbO₄ and LaTaO₄ was investigated by X-ray diffraction, and a two-phase region was observed in the composition region LaNb_1−xTa_xO₄ where 0.4?x?0.8. Single-phase LaNb_1−xTa_xO₄ (0?x?0.4) with the monoclinic Fergusonite structure at ambient temperature, was observed to transform to a tetragonal Scheelite structure by in-situ high-temperature X-ray diffraction, and the phase transition temperature was shown to increase with increasing Ta-content. This ferroelastic to paraelastic second-order phase transition was described by Landau theory using spontaneous strain as an order parameter. The thermal expansion of LaNb_1−xTa_xO₄ (0?x0.4) was shown to be significantly higher below the phase transition than above. Single-phase LaNb_1−xTa_xO₄ (0.8?x?1) with another monoclinic crystal structure at ambient temperature was shown to transform to an orthorhombic crystal structure by X-ray diffraction and differential scanning calorimetry. The phase transition temperature was observed to decrease with decreasing Ta-content. Finally, orthorhombic LaTaO₄ could also be transformed to monoclinic LaTaO₄ at ambient temperature by applying a uniaxial pressure of 150-170 MPa, reflecting the lower molar volume of monoclinic LaTaO₄.     

TEM observations of rhombohedral and monoclinic domains in LaCoO3-based ceramics

Twin structures in rhombohedrally distorted polycrystalline LaCoO₃, La_0.8Ca_0.2CoO_3-δ and La_0.5Sr_0.5Co_0.5Fe_0.5O_3-δ have been studied by transmission electron microscopy. Normal {100} and {110} twinning was predominant in LaCoO₃ and La_0.8Ca_0.2CoO_3-δ. In addition, ferroelastic domains with a one-dimensional superstructure corresponding to a tripling of the pseudocubic lattice parameter, a _c, were found in some grains. The monoclinic superstructure vanished over time and was sensitive to external mechanical stress. In La_0.5Sr_0.5Co_0.5Fe_0.5O_3-δ no normal twinning was observed, but domains with a one-dimensional superstructure of two times a _c were observed. The superstructures have been interpreted as atomic scale twinning, which reduce the crystal symmetry from rhombohedral to monoclinic.     

HARVESTING ECONOMIC MODELS AND CATCH‐TO‐BIOMASS DEPENDENCE: THE CASE OF SMALL PELAGIC FISH

Abstract In the case of small pelagic fish, it seems reasonable to consider harvest functions depending nonlinearly on fishing effort and fish stock. Indeed, empirical evidence about these fish species suggests that marginal catch does not necessarily react in a linear way neither to changes in fishing effort nor in fish stock levels. This is in contradiction with traditional fishery economic models where catch‐to‐input marginal productivities are normally assumed to be constant. While allowing for nonlinearities in both catch‐to‐effort and catch‐to‐stock parameters, this paper extends the traditional single‐stock harvesting economic model by focusing on the dependence of the stationary solutions upon the nonlinear catch‐to‐stock parameter. Thus, we analyze equilibrium responses to changes in this parameter, which in turn may be triggered either by climatic or technological change. Given the focus in this study on the case of small pelagic fish, the analysis considers positive but small values for the catch‐to‐stock parameter.