Size Of Secondary Phases Research Articles

Структура и фазовые превращения при горячей деформации высокотемпературного сверхпроводника Bi(Pb)2223:I. Вторичные фазы

The effect of hot deformation conditions by torsion under quasi-hydrostatic pressure on phase composition and basal plane texture of high-Tc superconductor Bi(Pb)2223 was systematically studied. A narrow temperature range Td = 863-867oC in which a strong texture with a factor of orientation of the basal planes at the level of F = 0,996-0,997 to form at relatively low twist angles (~ 10o) was found. By EDS and x-ray diffraction techniques phase ratio dependence on deformation temperature, twist angle and sample region have been determined. The correlation between volume fraction and the mean particle size of secondary phases with the degree of texture was found. <br />

Effect of secondary phases evolution in the first sintering process on the critical current density of Bi-2223/Ag tapes

Effect of the evolution of the secondary phases in the first sintering process on the microstructure and critical current density of Bi-2223/Ag tapes has been studied. The amount and grain size of secondary phases were characterized by XRD and SEM/EDS. It has been found that both the microstructure and critical current of the fully reacted tapes depended strongly on the secondary phases formed in the first sintering process. The (Ca,Sr) 2CuO 3 and (Ca,Sr) 14Cu 24O 41 phases were easily formed and to be stable at higher temperature, however, at lower temperature, the CuO particles appeared easily and keeping stable with dwelling time. The best conversion to Bi-2223, together with the lowest amount of the total secondary phases was achieved when samples treated at 830 °C in 8.5% O 2. Samples with the lowest amount and minimum size of secondary phase produced in the first sintering process will obtain the best performance of the fully reacted tapes. The optimum sintering parameters are obtained by controlling the evolution of the secondary phases during the first sintering process.

Phase transformation in Bi-2223/AgMg alloyed PIT tapes during different sintering processing and its influence on critical current density

The effect of various sintering conditions on the phase transformation and critical current density of multifilament Bi-2223 PIT tapes with AgMg sheaths has been investigated. A two stage sintering process was used comprising a first (HT1) and second (HT2) heat treatment with an intermediate roll pass between each stage. The sintering atmosphere during each stage was a mixture of nitrogen and oxygen with a N/O percentage ratio of either 80/20% (in N80/O20%) or 92.5/7.5% (in N92.5/O7.5%). It was found that the conversion of 2212 precursor phase to 2223 was significantly influenced by the sintering atmosphere of HT1. Sintering in N80/O20% tended to produce larger 2223 grains and a higher degree of texturing compared to sintering in N97.5/O7.5%. The lower partial pressure of O promoted faster and more complete conversion of 2212 and smaller 2223 grain size, however, retained much more 2201 phase. It was observed that conditions of HT2 had less of an impact on the conversion of 2212 phase but influenced the proportion and size of secondary phases, specially influenced that of the lead-rich secondary phase. A possible explanation for the differences in phase transformation and the effect on the critical current was discussed.

Spectral characterization of weathering products of elemental iron in a Martian atmosphere: Implications for Mars hyperspectral studies

Visible and near-infrared spectroscopic properties have been measured on elemental iron experimentally weathered in simulated Martian atmosphere and correlated to mineralogical compositions determined by X-ray diffraction. Two main features are observed in the reflectance spectra, corresponding to two deep bands located at 0.9 and 3.1 μm, respectively the iron band and the hydration band. In early weathering stages both Fe 2+ and Fe 3+ bands are identified. In addition, whereas the water band position does not change with time, the Fe 2+ band disappears, and the Fe 3+ band shifts towards longer wavelength (from 0.88 to 0.92 μm) because of transition from Fe 2+ phases (siderite) to Fe 3+ phases (ferrihydrite and goethite). Apart from these spectral signatures, other bands more specific of each phase are not clearly evidenced, especially for siderite. This is due to relatively low abundance of siderite (<20 wt%), but also to the very small grain size of secondary phase as well as surface coatings of iron (oxy)hydroxides. Therefore, our results suggest that carbonates, even if not detected, could be present in the form of very small grains in the surface of Mars.

Influence of Secondary Phase Particles on Zr-Alloy Plastic Flow Stability and Fracture

The influence of intermetallic particles and hydrides on the total and local ductility as well as on the fracture mechanism in Zr-2.5Nb and Zr-lNb-1.3Sn-0.4Fe alloys was studied by the methods of acoustic emission (AE). Based on measurements of maximum (peak) values of AE amplitudes depending on secondary phase size and distribution two alternative mechanisms were established of loss of flow stability in alloys under tension: geometrical loss of strength for uniform fine particle distribution, and microcracking at the stage of uniform strain for coarse particle aggregates or fine particle stringers. Ways of increasing the fracture toughness resistance of alloys were suggested by increasing the stability of local flow at a crack tip through optimization of matrix deformation ability, secondary phase size and distribution within matrix. Analysis is given to quantitative relationships between fracture toughness and microstructure characteristics of alloys. Original devices and techniques were developed to quantitatively analyze local processes of strain and fracture based on AE measurements which allows assessment of unstable flow zones and measurement of microcrack 5 μm and more in size.