Browsing by Author "Chandra Shekhar"

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Concentration and release patterns of nutrients (N, P and K) during decomposition of maize and wheat roots in a seasonally-dry tropical region
(1989) K.P. Singh; Chandra Shekhar
Maize and wheat roots, enclosed in fine- (3 μ m) and coarse- (1 mm pore) mesh bags, were buried in soil in their respective post-harvest seasons. In all instances the N and P concentrations within the bags increased with time except for an initial decrease during the first few months. Concentrations of N and P were always higher in fine-mesh bags than in coarse-mesh bags. The nutrient concentrations in maize were higher in case of roots buried at 12.5 cm than at 5 cm depth. However, the absolute amount of N, P and K in the residual root mass decreased with time in all instances. Nutrient release rates were always greater in roots confined in coarse-mesh bags, suggesting a significant role for the soil fauna. The total release time for the various nutrients was about twice as long for wheat roots compared to maize roots. © 1989.
Dependence of superconductivity and its weakly linked behavior in bulk LaO 1-xF xFeAs on F doping
(2012) Anurag Gupta; Hannu Huhtinen; Chandra Shekhar; Kim Schlesier; Pankaj Srivastava; Amit Srivastava; O.N. Srivastava; Reino Laiho; A.V. Narlikar
Samples of oxypnictide compound LaO1-xFx FeAs, with x = 0.15 and 0.2 corresponding to over- and highly over-doped compositions, respectively, were prepared by solid-state reaction. We present their characterization by XRD and HRTEM, as well as resistivity ρ(T), magnetization M(B) and microwave modulated absorption (MMA) response between 4.2-300 K and applied fields B = 0-8 T. With change in x, both the superconducting and magnetic behavior of the samples shows an interesting pattern. The "magnetic anomaly" at T ∼ 130 K, observed in M(T) for x = 0, instead of getting totally suppressed shows a tendency to reappear in x = 0.2 sample. Both samples typically show ρ(300 K) > 2.8 × 10- 3 Ω;2 cm and critical current density J c(5 K, 1 T) < 2 × 10 7 A/m 2. The superconducting transitions as measured by ρ(T) at B = 0 are found broad for both x = 0.15 and 0.2 samples with transition widths ∼2.5 and 6 K, respectively. The slope |dB c2/dT|(where B c2 is upper critical field) determined by resistive onsets, for the x = 0.15 and 0.2 samples, has values ∼7.5 and 3.5 T/K, respectively. The superconducting state characteristics as reflected by ρ(T, B), M(T), magnetic J c(B) and MMA response are typical of the presence of weakly linked inter-grain regions in both the samples. Our HRTEM images of the x = 0.15 sample show the presence of high angle (∼43°) grain boundaries, which are well known to limit the J c in cuprate-based high T c bulk materials. © 2012 Springer Science+Business Media, LLC.
Effect of film thickness on the transport properties of MgB2 synthesized by spray pyrolysis
(2011) Chandra Shekhar; O.N. Srivastava
Polycrystalline MgB2 films of different thickness have been prepared by employing spray pyrolysis technique on MgO (1 0 0) substrate. The MgB2 and other phases have been confirmed using X-ray diffraction technique and no trace of impurities phases have been found. The resistivity behavior shows that the superconducting transition temperature lies in the range of 37-39 K with narrow transition width. The transport critical current density vary with films thickness and achieved highest value ∼1.2 × 10 6 A/cm2 at 20 K for 2.0 μm thick film and its values increase as thickness increases. © 2011 Elsevier B.V. All rights reserved.
Effect of la doping on microstructure and critical current density of MgB2
(2005) Chandra Shekhar; Rajiv Giri; R.S. Tiwari; D.S. Rana; S.K. Malik; O.N. Srivastava
In the present study, La-doped MgB2 superconductors with different doping levels (Mg1-xLaxB2; x ≤ 0.00, 0.01, 0.03 and 0.05) have been synthesized by the solid-state reaction route at ambient pressure. Effects of La doping have been investigated in relation to microstructural characteristics and superconducting properties, particularly intragrain critical current density (Jc). The microstructural characteristics of the as-synthesized Mg(La)B2 compounds were studied employing the transmission electron microscopic (TEM) technique. The TEM investigations reveal inclusion of LaB6 nanoparticles within the MgB2 grains, which provide effective flux pinning centres. The evaluation of intragrain Jc through magnetic measurements on the fine powdered version of the as-synthesized samples reveal that Jc values of the samples change significantly with the doping level. The optimum result on Jc is obtained for Mg 0.97La0.03B2 at 5 K; Jc reaches ∼1.4 × 107 A cm-2 in self-field, ∼2.1 × 106 A cm-2 at 1 T, ∼2.5 × 10 5 A cm-2 at 2.5 T and ∼1.8 × 104 A cm-2 at 4.5 T. The highest value of intragrain Jc in the Mg0.97La0.03B2 superconductor has been attributed to the inclusion of LaB6 nanoparticles, which are capable of providing effective flux pinning centres. © 2005 IOP Publishing Ltd.
Enhancement of flux pinning and high critical current density in graphite doped MgB2 superconductor
(2007) Chandra Shekhar; Rajiv Giri; R.S. Tiwari; O.N. Srivastava; S.K. Malik
We report the synthesis and characterization of graphite (C) doped MgB2-x Cx (x=0.0, 0.1, 0.2, and 0.3) samples. The crystal structure and microstructural characterization have been investigated by x-ray diffractometer and transmission electron microscopic (TEM) analysis. The superconducting properties especially Jc and Hc2 have been measured by employing physical property measurement system. We found that the graphite doping affects the lattice parameters as well as the microstructure of MgB2 superconductor. In case of optimally doped (x=0.1) sample, the critical current density at 5 K corresponds to 1.1× 106 and 5.3× 104 A cm2 for 3 and 5 T fields, respectively. The upper critical field has been enhanced nearly two times after doping. The flux pinning behavior has been investigated by flux pinning force density curve and it reveals that the flux pinning behavior has improved significantly by doping. TEM micrographs show the graphite nanoparticles of size ∼5-10 nm which are invariably present in MgB2 grains. These nanoparticles act as flux pinning center and are responsible for enhancement of superconducting properties of MgB2. © 2007 American Institute of Physics.
High critical current density and improved flux pinning in bulk MgB 2 synthesized by Ag addition
(2007) Chandra Shekhar; Rajiv Giri; R.S. Tiwari; O.N. Srivastava; S.K. Malik
In the present investigation, we report a systematic study of Ag admixing in Mg B2 prepared by solid-state reaction at ambient pressure. All the samples in the present investigation have been subjected to structural microstructural characterization employing x-ray diffraction and transmission electron microscopic (TEM) techniques. The magnetization measurements were performed by physical property measurement system. The TEM investigations reveal the formation of MgAg nanoparticles in Ag admixed samples. These nanoparticles may enhance critical current density due to their size (∼5-20 nm) which is compatible with the coherence length of Mg B2 (∼5-6 nm). In order to study the flux pinning effect of Ag admixing in Mg B2, the evaluation of intragrain critical current density (Jc) has been carried out through magnetic measurements on the fine powdered version of the as synthesized samples. The optimum result on intragrain Jc is obtained for 10 at. % Ag admixed sample at 5 K. This corresponds to ∼9.23× 107 A cm2 in self-field, ∼5.82× 107 A cm2 at 1 T, ∼4.24× 106 A cm2 at 3.6 T, and ∼1.52× 105 A cm2 at 5 T. However, intragrain Jc values for Mg B2 sample without Ag admixing are ∼2.59× 106, ∼1.09× 106, ∼4.53× 104, and 2.91× 103 A cm2 at 5 K in self-field, 1 T, 3.6 T, and 5 T, respectively. The high value of intragrain Jc for Ag admixed Mg B2 superconductor has been attributed to the inclusion of MgAg nanoparticles into the crystal matrix of Mg B2, which are capable of providing effective flux pinning centers. A feasible correlation between microstructural features and superconducting properties has been put forward. © 2007 American Institute of Physics.
Hydrostatic pressure: A very effective approach to significantly enhance critical current density in granular iron pnictide superconductors
(Nature Publishing Group, 2015) Babar Shabbir; Xiaolin Wang; S.R. Ghorbani; Chandra Shekhar; Shixue Dou; O.N. Srivastava
Pressure is well known to significantly raise the superconducting transition temperature, Tc, in both iron pnictides and cuprate based superconductors. Little work has been done, however, on how pressure can affect the flux pinning and critical current density in the Fe-based superconductors. Here, we propose to use hydrostatic pressure to significantly enhance flux pinning and Tc in polycrystalline pnictide bulks. We have chosen Sr4V2O6Fe2As2 polycrystalline samples as a case study. We demonstrate that the hydrostatic pressure up to 1.2 GPa can not only significantly increase Tc from 15 K (underdoped) to 22 K, but also significantly enhance the irreversibility field, Hirr, by a factor of 4 at 7 K, as well as the critical current density, Jc, by up to 30 times at both low and high fields. It was found that pressure can induce more point defects, which are mainly responsible for the Jc enhancement. Our findings provide an effective method to significantly enhance Tc, Jc, Hirr, and the upper critical field, Hc2, for other families of Fe-based superconductors in the forms of wires/tapes, films, and single crystal and polycrystalline bulks.
Improved critical current density of MgB 2-carbon nanotubes composite
(2007) Chandra Shekhar; Rajiv Giri; S.K. Malik; O.N. Srivastava
In the present study, we report a systematic study of doping/admixing of carbon nanotubes (CNTs) in different concentrations in MgB 2. The composite material corresponding to MgB 2 - x at.% CNTs (35 at.% ≥ × ≥ 0 at.%) have been prepared by solid-state reaction at ambient pressure. All the samples in the present investigation have been subjected to structural/microstructural characterization employing XRD, Scanning electron microscopic (SEM), and Transmission electron microscopic (TEM) techniques. The magnetization measurements were performed by Physical property measurement system (PPMS) and electrical transport measurements have been done by the four-probe technique. The microstructural investigations reveal the formation of MgB 2-carbon nanotube composites. A CNT connecting the MgB 2 grains may enhance critical current density due to its size (∼5-20 nm diameter) compatible with coherence length of MgB 2 (∼5-6 nm) and ballistic transport current carrying capability along the tube axis. The transport critical current density (J ct) of MgB 2 samples with varying CNTs concentration have been found to vary significantly e.g., J ct of the MgB 2 sample with 10 at.% CNT addition is ∼2.3 × 10 3 A/cm 2 and its value for MgB2 sample without CNT addition is ∼7.2 × 102 A/cm2 at 20 K. In order to study the flux pinning effect of CNTs doping/ admixing in MgB2, the evaluation of intragrain critical current density (J c) has been carried out through magnetic measurements on the fine powdered version of the as synthesized samples. The optimum result on J c is obtained for 10 at.% CNTs admixed MgB 2 sample at 5 K, the J c reaches ∼5.2 × 106 A/cm 2 in self field, ∼1.6 × 106 A/cm 2 at 1 T, ∼2.9 × 105 A/cm 2 at 2.6 T, and ∼3.9 × 104 A/cm 2 at 4 T. The high value of intragrain J c in 10 at.% CNTs admixed MgB 2 superconductor has been attributed to the incorporation of CNTs into the crystal matrix of MgB 2, which are capable of providing effective flux pinning centres. A feasible correlation between microstructural features and superconducting properties has been put forward. Copyright © 2007 American Scientific Publishers All rights reserved.
Improved superconducting properties of MgB2 bulk materials prepared by sintering
(2003) Anjana Asthana; Chandra Shekhar; Rajiv Giri; O.N. Srivastava
We report here the improvement of superconducting properties of MgB2 bulk samples by sintering at ambient pressure, employing RF induction heating. It has been found that Tc (R = 0) transition temperature of the sintered sample gets enhanced to ∼40.00 K as compared to Tc (R = 0) ∼25 K for the unsintered MgB2 sample. A high critical current density of ∼5.45 × 104 A cm-2 (20 K) has been achieved for a sintered MgB2 sample whereas for an unsintered sample, the Jc is found to be ∼1.20 × 103 A cm2 (20 K). Detailed microstructural investigations by transmission electron microscope (TEM) reveal that most of the intergrain interfaces correspond to low angle grain boundaries. Based on TEM explorations, these boundaries have been recognized to be dominantly low angle tilt boundaries. Surface microstructural features of the MgB2 sample as investigated by scanning electron microscopy reveal a well-connected uniform grain structure for the sintered samples.
Occurrence of superconductivity and magnetism in nominally undoped LaOFeAs
(2010) Anurag Gupta; Hannu Huhtinen; Chandra Shekhar; Kim Schlesier; Pankaj Srivastava; Amit Srivastava; O.N. Srivastava; Reino Laiho; A.V. Narlikar
Superconductivity in a LaOFeAs system is known to get introduced by F-doping (LaO 1-x F x FeAs) even under ambient conditions and oxygen deficiency (LaO 1-x FeAs) under high pressure conditions. Hitherto unreported, superconductivity in F-free undoped LaOFeAs samples is observed and confirmed for the first time by various characterization tools-resistive ρ(T,B), magnetic M(T,B) and modulated microwave absorption (MMA) measurements. The ρ(T) at B=0 shows a clear superconducting transition with an onset at T con∼17 K and a tail-like behavior when R goes to zero at T c0∼8 K. In the presence of B, the superconducting transition shifts to lower T with a rate ∼-5.5 and -1.65 T/K, depending on whether the ρ(T) has dropped to 90% or 5% of its normal state value, respectively. M(T) in zero field cooling at B=10 mT shows diamagnetic downturn below at ∼12 K. At TT c0, indicating the presence of weak link superconducting networks in the sample. The sample shows a complex electrical and magnetic behavior in the normal state. For instance, ρ(T) reveals a weak SDW-like anomaly at T∼132 K along with a resistivity minimum at T min∈∼78 K. M(T) also shows the presence of a magnetic anomaly at T∼130 K. Both below and above T con, presence of an additional ferromagnetic component is observed in the isothermal M(B) loop measurements. The superconducting and normal state features of our sample are compared with other undoped and doped LaOFeAs systems reported in the literature. © 2010 Springer Science+Business Media, LLC.
Remedial merits of Piper longum Linn with astonishing antidiabetic potential
(BRNSS Publication Hub, 2017) Komal Bansal; Chandra Shekhar; K.R.C. Reddy
Over the past three decades, the number of people with diabetes mellitus (DM) has doubled globally, making it one of the most important public health challenges to all nations. This has been stimulating the development of novel and potentially more effective therapeutic approaches to treat the disease. Even after the availability of a massive amount of hypoglycemic drugs in conventional system, herbals are gaining much attention because of its spectacular therapeutic potential and also help to sustain the healthy condition of the body. In this review article, a highly valuable drug Pippali (Piper longum) has been evaluated on the basis of its pharmacognostical, pharmacological, and therapeutical background. The plant possesses significant antidiabetic activity. The systemic dysfunction associated with DM can also be cured by Pippali. Plant through exhibiting synergistic action gives a valuable response in hepatic, digestive, cardiac, and respiratory disorders. The purpose of this review is to enlighten all the remedial aspects of P. longum with a special focus on its antidiabetic potential. © 2014 International Journal of Green Pharmacy.
Response of soil salinity on leaf nutrient status on different varieties of African marigold (Tagetes erecta)
(Enviro Media, 2015) Neerja Singh; Himanshu Trivedi; Chandra Shekhar; Pankaj Mishra; H.K. Singh
Six different varieties of African marigold (Tagetes erecta) viz. Local Selection, Pusa Basanti Gainda, Pusa Narangi Gainda, African Tall Orange, Sunset Giant and Inca Hybrid were grown in artificially prepared saline soils having 2, 4, 6 and 8 dSm-1 EC levels apart from control (0.98), to evaluate the response of salinity on nutrient content in leaves and result thus obtained was expressed in % dry weight basis. The experiment was laid out in Factorial CRD design with a total of thirty treatments and three replications for the two consecutive years i.e. 2010-11 and 2011-12. It was found that nutrients like nitrogen, phosphorus and potassium showed a gradual decrease with enhanced salinity level. The decrease was more pronounced at 6 and 8 dSnv-1 level while calcium and magnesium showed increase till 2dSnr-1 and further gradual decrease was also observed in this case also. Only sodium was found to increase with enhanced levels of salinity. © Copyright EM International.
Synthesis and microstructural studies ofiron oxypnictide LaO 1-xFxFeAs superconductors
(2009) Chandra Shekhar; Sonal Singh; P.K. Siwach; H.K. Singh; O.N. Srivastava
We report on the synthesis and structural/microstructural studies of iron-based fluorine doped LaOFeAs superconductors. We have successfully synthesized fluorine doped superconducting LaO1-xFxFeAs materials by choosing lower temperature (∼1150 °C) and longer synthesis duration (∼60h) as compared to the standard values of these employed in the pioneering first contribution (Kamihara et al 2008 J.Am.Chem.Soc. 130 3296). A decrease of lattice parameters, as determined by x-ray diffraction, confirms the substitution of fluorine. The superconducting transition temperature is 27.5K which is observed at a doping level of x = 0.2. This superconducting material LaO1-xFxFeAs exhibits interesting microstructural characteristics. These relate to the existence of another structural phase, besides the standard phase, having c parameters of ∼12.67. This suggests the existence of a modulated structure, similar to the cuprates, in these new oxypnictides. This phase may have new impact on this new high-TC family. © 2009 IOP Publishing Ltd.
Temperature dependence of conduction noise of MgB2 superconductor
(2008) D.P. Singh; Neeraj Khare; Chandra Shekhar; O.N. Srivastava
Temperature dependent behavior of conduction noise of MgB2 superconductor has been studied. The frequency spectrum of conduction noise shows 1/fα type of behavior, with α = 0.7-0.8. Temperature dependence of conduction noise is found to exhibit two peaks at ∼26 K and ∼38 K respectively. The peak at ∼38 K is attributed to movement of vortices and due to fluctuation in resistance from change of state from superconducting to normal state. The peak in the conduction noise at ∼26 K is ascribed to the movement of vortices due to enhanced thermally activated vortex hopping. The analysis of enhanced conduction noise in the low temperature region indicates the presence large density of fluctuators with activation energies around 0.048 eV. © 2008 Elsevier B.V. All rights reserved.
Upper critical field, critical current density and thermally activated flux flow in CaFFe 0.9Co 0.1As superconductor
(2012) Chandra Shekhar; Amit Srivastava; Pramod Kumar; Pankaj Srivastava; O.N. Srivastava
In this paper, we report the synthesis, structure, transition temperature, upper critical field, critical current density and thermally activated flux flow in the CaFFe 0.9Co 0.1As superconductor. Superconductivity arises at 23K by Co substitution at the site of Fe atoms and the upper critical field is estimated as 102T using the WerthamerHelfandHohenberg formula. The flux-flow activation energy (U 0/k B) varies from 3230K and 4190K in a field of 9T and 1T, respectively. At 2K, the J c is found to be approximately 4×10 3Acm 2 and 0.3×10 3Acm 2 in zero and 6T field, respectively. Transmission electron microscopy analysis shows an amorphous region surrounding most of the grains which is likely to be present in the form of amorphous and weak link grain boundaries in this compound. It seems that most of the current is hindered by mis-aligned grains, amorphous grain boundaries and impurities, which are invariably found between the grains. The presence of the weakly linked granules and their weakly pinned intergranular Josephson vortices are responsible for both low J c and the Arrhenius temperature dependence of resistivity. © 2012 IOP Publishing Ltd.
Weight loss in relation to environmental factors during the decomposition of maize and wheat roots in a seasonally-dry tropical region
(1989) K.P. Singh; Chandra Shekhar
Wheat and maize root materials showed differences in the magnitude of weight loss from bags with coarse (1 mm) or fine-mesh (3 μm pores) buried in soil. For maize, weight loss differed in these two types of bag at two soil depths (5 and 12.5 cm). The total weight loss from root bags were: maize roots (during 214 days), at 5cm depth, 79 and 50% in coarse- and fine-mesh bags, respectively, and for wheat roots (during 275 days), at 5cm depth, 72 and 45% in coarse- and fine-mesh bags, respectively. The CO2 evolution rates from decomposing roots showed a significant positive relationship with weight loss. In all batches of the maize and wheat roots the relative decomposition rates (RDR) rapidly increased initially and subsequently declined. During the first 4 months the maize root RDR at 12.5 cm depth exceeded that at 5cm depth. Coarse-mesh RDR was always higher than that of fine-mesh bags. The two major environmental variables, temperature and soil moisture were positively related to the rate of wheat root decomposition, but negatively correlated with the decomposition rates of maize roots. © 1989.