6,267 research outputs found
Structural, Magnetic and Magneto-caloric studies of Ni50Mn30Sn20Shape Memory Alloy
We have synthesized a nominal composition of Ni50Mn30Sn20 alloy using arc
melting technique. Rietveld refinement confirms the austenite L21 structure in
Fm-3m space group. Electrical resistivity has been found to clearly exhibiting
two different phenomena viz. a magnetic transition from paramagnetic to
ferromagnetic and a structural transition from austenite to martensitic phase.
Thermo-magnetization measurements M(T) confirms ferromagnetic transition
temperature TC at 222 K and martensitic transition starting at 127 K(MS).
Magnetization measurement M(H) at 10 K confirms the ferromagnetic state.
Frequency dependence of ac susceptibility \c{hi}' at low temperature suggests
spin glass behavior in the system. The isothermal magnetic entropy change
values have been found to be 1.14 J/Kg.K, 2.69 J/Kg.K and 3.9 J/Kg.K, with
refrigeration capacities of 19.6 J/kg, 37.8 J/kg and 54.6 J/kg for the field
change of 1, 2 and 3 Tesla respectively at 227 K.Comment: 16 pages text + Figs. Ni50Mn30Sn20 alloy: reasonable refrigeration
capacity tunable to Room
Neoarchean crustal shear zones and implications of shear indicators in tectonic evolution of Bundelkhand craton, central India
The gneisses and granitoids emplaced along E-W sub-vertical crustal shear zones are represented as important tectonic units in Bundelkhand craton of central India. The tonalite-trondhjemite-granodiorite (TTG) gneisses (3.5-3.2 Ga; oldest unit), and streaky to mafic gneisses structurally deformed in D 1 deformation. The metabasic, felsic, banded iron formation and metasedimentaries of greenstone complex exposed in central part, have characteristics of three sets of folding (F 1 -F 3 ). These gneisses associated with migmatite, amphibolite, quartzite, and schist were evolved in D 2 compressive phase, which are not occurring in northern part of craton. The K-rich Neoarchean granitoids (2.6-2.49 Ga) were intruded as granitic complex (D 3 magmatic phase) and the E-W strike-slip Raksa-Garhmau shear zone reported as important tectonic unit, were evolved in asyn-to post-tectonic D 3 phase. The dolerite dykes (ca. 2.0 Ga) were emplaced along NW-SE fractures in extension setting during D 4 magmatic event. The NE-SW riedel shears occupied by giant quartz veins (reefs) evolved in Paleoproterozoic during D 5 endogenic activity.
The relationship between macro and microstructural fabrics has been documented within mylonitic foliation, stretching lineation, S-C planes and rotated fabrics, reflect mesoscopic shear indicators, as noted in three types of mylonitic rocks. i) The rotated porphyroclasts of quartz, feldspars and asymmetric pressure shadows showing strong undulose extinction, deformation lamellae, and dynamic recrystallization are characteristic features of protomylonite where altered orthoclase and kinked plagioclase are noticed. ii) Mylonite, a distinct mylonitic foliation represented by parallel orientation of elongated quartz and feldspar with flakes of mica. iii) The ground matrix of recrystallized quartz with few protoliths of quartz and feldspar are observed, important features of ultramylonite. The asymmetric microstructures viz. σa and σb mantled porphyroclasts, othermicrostructures show progressively deformed by crystal plastic (non-coaxial) strain softening under low to moderate temperature conditions. The sinistral top- to- SW sense of shear movement was dominant. The microfractures/ microfaults, kinking and pull apart structures observed in K- feldspars and are indicative of overprinting of brittle deformation on ductile shearing
Star formation activity in the Galactic H II region Sh2-297
We present a multiwavelength study of the Galactic H II region Sh2-297,
located in Canis Major OB1 complex. Optical spectroscopic observations are used
to constrain the spectral type of ionizing star HD 53623 as B0V. The classical
nature of this H II region is affirmed by the low values of electron density
and emission measure, which are calculated to be 756 cm^-3 and 9.15 x 10^5
cm^-6 pc using the radio continuum observations at 610 and 1280 MHz, and VLA
archival data at 1420 MHz. To understand local star formation, we identified
the young stellar object (YSO) candidates in a region of area ~ 7.5' x 7.5'
centered on Sh2-297 using grism slitless spectroscopy (to identify the Halpha
emission line stars), and near infrared (NIR) observations. NIR YSO candidates
are further classified into various evolutionary stages using color-color (CC)
and color-magnitude (CM) diagrams, giving 50 red sources (H-K > 0.6) and 26
Class II-like sources. The mass and age range of the YSOs are estimated to be ~
0.1 - 2 Msolar and 0.5 - 2 Myr using optical (V/V-I) and NIR (J/J-H) CM
diagrams. The mean age of the YSOs is found to be ~ 1 Myr, which is of the
order of dynamical age of 1.07 Myr of the H II region. Using the estimated
range of visual extinction (1.1 - 25 mag) from literature and NIR data for the
region, spectral energy distribution (SED) models have been implemented for
selected YSOs which show masses and ages to be consistent with estimated
values. The spatial distribution of YSOs shows an evolutionary sequence,
suggesting triggered star formation in the region. The star formation seems to
have propagated from the ionizing star towards the cold dark cloud LDN1657A
located west of Sh2-297.Comment: 19 pages, 13 figures, 3 tables. Accepted for publication in The
Astrophysical Journa
Late Quaternary stratigraphic development in the lower Luni, Mahi and Sabarmati river basins, Western India
This study reviews the Quaternary alluvial stratigraphy in three semi-arid river basins of western India i.e., lower Luni (Rajasthan), and Mahi and Sabarmati (Gujarat alluvial plains). On the basis of OSL chronologies, it is shown that the existing intra-valley lithostratigraphic correlations require a revision. The sand, gravel and mud facies are present during various times in the three basins, however, the fluvial response to climate change, and the resulting facies associations, was different in the Thar desert as compared to that at the desert margin; this makes purely lithostratigraphic correlations unviable. It is further shown that the rivers in the Thar desert were more sensitive to climate change and had small response times and geomorphic thresholds as compared to the desert-margin rivers. This is illustrated during the early OIS 1, when the Luni river in the Thar desert was dynamic and showed frequent variations in fluvial styles such as gravel bedload braided streams, sand-bed ephemeral streams and meandering streams, all followed by incision during the early Holocene. The coeval deposits in Sabarmati, however, only show a meandering, floodplain-dominated river. Late Quaternary alluvial deposits in these basins unconformably overlie some older deposits that lack any absolute chronology. Based on the facies types and their associations, and the composition and architecture of the multistoried gravel sheets in the studied sections, it is suggested that older deposits are of pre-Quaternary age. This hypothesis implies the presence of a large hiatus incorporating much of the Quaternary period in the exposed sections
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