Browsing by Author "Ram Manohar Yadav"

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An experimental and theoretical investigation on structure-property correlation of Cu2Mn1Al1−xGax full-Heusler alloy
(Elsevier Ltd, 2022) Shashank Shekhar Mishra; Anurag Bajpai; Thakur Prasad Yadav; Ram Manohar Yadav; Anand B. Puthirath; Liangzi Deng; Moein Adnani; Ching-Wu Chu; Robert Vajtai; Pulickel M. Ajayan; Krishanu Biswas; Nilay Krishna Mukhopadhyay
The present study reports the evolution of microstructure and magnetic properties of Cu2MnAl1−xGax Heusler alloys. The L21 phase, which remained stable up to a Ga substitution of x = 0.3, transformed into mixed phases, i.e., hexagonal close packed (HCP) and complex cubic structure (CCS) on further addition of Ga. The non-monotonic increase in the lattice constant indicates that some amount of Ga retains its monovalent state. The magnetic phase showed a transition from ferromagnetic to paramagnetic state with increasing Ga concentration. The re-entrant temperature (TR) decreased from 8.2 K to 5.2 K as Ga content increased from 5 at% to 10 at%. The strength of magnetic exchange-coupling also reduced with an increase in Ga content. Gaussian process regression (GPR) was used to estimate the lattice parameter using the ionic radii and Pauling electronegativity of the constituents. The modeling approach showed high accuracy and stability, providing new insights into future alloy development. © 2021 Elsevier B.V.
Effect of ferrocene concentration on the synthesis of bamboo-shaped carbon-nitrogen nanotube bundles
(2005) Ram Manohar Yadav; T. Shripathi; Anchal Srivastava; O.N. Srivastava
We have investigated the effect of ferrocene concentration on the synthesis of carbon-nitrogen (C-N) nanotubes. The bamboo-shaped carbon-nitrogen nanotubes were synthesized by spray pyrolysis of Fe(C 5H 5) 2 and CH 3CN solution using argon as a carrier gas at the optimum temperature of ∼900°C. The effect of ferrocene concentration on the length and concentration of nitrogen in nanotubes was studied. Micro-structural features of the nanotubes were monitored employing scanning and transmission electron microscopic techniques. SEM studies reveal that with decreasing ferrocene concentration from 25 mg ml -1 to 5 mg ml -1, the length of the nanotubes vary from 80 μm to 430 μm. A feasible growth model has been described and discussed. X-ray photoelectron spectroscopic studies have confirmed the formation of nitrogen-doped carbon nanotubes. These studies reveal that the nitrogen concentration in the nanotubes decreases with the increase of ferrocene concentration. The present synthesis route also provides means of producing carbon nanotubes with different concentrations of nitrogen. Copyright © 2005 American Scientific Publishers All rights reserved.
Effect of Ga substitution on structural and magnetic properties of Fe50Mn25Al25-xGax Heusler alloys
(Elsevier Ltd, 2021) S.S. Mishra; T.P. Yadav; Ram Manohar Yadav; Anand B. Puthirath; Liangzi Deng; Moein Adnani; Ching-Wu Chu; Robert Vajtai; P.M. Ajayan; Krishanu Biswas; N.K. Mukhopadhyay; O.N. Srivastava
Heusler alloys possess the structure of an ordered compound while displaying most of the properties of metals. Numerous magnetic exchange mechanisms may be operating in such systems. Herein, the structural and magnetic properties have been investigated as a function of substitution of Al by Ga in Fe50Mn25Al25-xGax (x = 0, 2.5, 5.0, 7.5, 10.0) quaternary (pseudo-ternary) full Heusler alloys. The X-ray diffraction analysis confirms the substituting Al by Ga promoting the lattice expansion in the L21 crystal structure in Fe50Mn25Al25-xGax alloys. The line scanning profile using energy dispersive X-ray analysis investigations suggests the homogeneous distribution of Ga in all the alloys. The magnetic moment of the alloys was found to be affected by Ga as the antiferromagnetic counterpart increases with Ga. A decrease in the value of Curie temperature was found with increase in Ga content. A second-order magnetic phase transition was observed in the alloys above room temperature. © 2020 Elsevier B.V.
Effect of growth temperature on bamboo-shaped carbon-nitrogen (C-N) nanotubes synthesized using ferrocene acetonitrile precursor
(2009) Ram Manohar Yadav; Pramod Singh Dobal; T. Shripathi; R.S. Katiyar; O.N. Srivastava
This investigation deals with the effect of growth temperature on the microstructure, nitrogen content, and crystallinity of C-N nanotubes. The X-ray photoelectron spectroscopic (XPS) study reveals that the atomic percentage of nitrogen content in nanotubes decreases with an increase in growth temperature. Transmission electron microscopic investigations indicate that the bamboo compartment distance increases with an increase in growth temperature. The diameter of the nanotubes also increases with increasing growth temperature. Raman modes sharpen while the normalized intensity of the defect mode decreases almost linearly with increasing growth temperature. These changes are attributed to the reduction of defect concentration due to an increase in crystal planar domain sizes in graphite sheets with increasing temperature. Both XPS and Raman spectral observations indicate that the C-N nanotubes grown at lower temperatures possess higher degree of disorder and higher N incorporation.
Effect of nitrogen variation on the synthesis of vertically aligned bamboo-shaped C-N nanotubes using sunflower oil
(2011) Rajesh Kumar; Ram Manohar Yadav; Kalpana Awasthi; R.S. Tiwari; O.N. Srivastava
To examine the role of NH3 on bundles of aligned bamboo-shaped carbonnitrogen (CN) nanotube were synthesized the pyrolysis of ferrocene (Fe(C5H5)2) and sunflower oil mixtures with NH3 being the source of nitrogen. The concentration of NH3 was varying in volume (vol). Optimized temperature and concentration of ferrocene were 825°C and 10 mg/ml, respectively. With the increase of nitrogen concentration the bundles are breaking in nearly equal parts. With nitrogen doping, the nanotubes have a bamboo-like structure and reveal degraded crystallinity of graphitic sheets. Nitrogen plays key role in generating equal compartments inside the carbon nanotube. The nanotubes were characterized by scanning electron microscopy and transmission electron microscopy that reveal the vertically aligned and hollow structural features of the nanotubes. FTIR shows the incorporation of N atom inside carbon framework and Raman spectrum indicates the enhancement of the defects inside CN nanotube due to the N atom in CN nanotube. © 2011 World Scientific Publishing Company.
Freestanding 3D Graphene-Nickel Encapsulated Nitrogen-Rich Aligned Bamboo Like Carbon Nanotubes for High-Performance Supercapacitors with Robust Cycle Stability
(Wiley-VCH Verlag, 2015) Rajesh Kumar; Rajesh Kumar Singh; Pawan Kumar Dubey; Dinesh Pratap Singh; Ram Manohar Yadav; Radhey Shyam Tiwari
3D hierarchical structures are reported based on graphene-nickel encapsulated nitrogen-rich aligned bamboo like carbon nanotubes, which show not only high-performance supercapacitance behavior but also a great robust cyclic stability. A facile synthesis route is developed of 2D nickel oxide decorated functionalized graphene nanosheets (2D-NiO-f:GNSs) hybrids and 3D nitrogen doped bamboo-shaped carbon nanotubes (NCNTs) vertically standing on the functionalized graphene nanosheets (3D-NCNT@f:GNSs) by using a thermal decomposition method. The chemical reduction and morphology-dependent electrochemical response are investigated. The enhanced specific capacitance of 3D-NCNT@f:GNSs as compared to that of 2D-NiO-f:GNSs suggests the synergistic effects and indicates the importance of energy storage and superior long-term cycling performance that are achieved. This 3D-NCNT@f:GNSs hybrid shows a remarkable cycling stability with a maximum power density of 12.32 kW kg-1 and maximum energy density of 109.13 Wh kg-1 due to the good connection of NCNT and f:GNSs. This unique 3D nano network architecture enables the availability of large surface areas of NCNT, thus endowing the nanohybrids with high specific capacitance and excellent reusability. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Highly zone-dependent synthesis of different carbon nanostructures using plasma-enhanced arc discharge technique
(Kluwer Academic Publishers, 2015) Rajesh Kumar; Rajesh Kumar Singh; Pawan Kumar Dubey; Ram Manohar Yadav; Dinesh Pratap Singh; R.S. Tiwari; O.N. Srivastava
Three kinds of carbon nanostructures, i.e., graphene nanoflakes (GNFs), multi walled carbon nanotubes (MWCNTs), and spherical carbon nanoparticles (SCNPs) were comparatively investigated in one run experiment. These carbon nanostructures are located at specific location inside the direct current plasma-assisted arc discharge chamber. These carbon nanomaterials have been successfully synthesized using graphite as arcing electrodes at 400 torr in helium (He) atmosphere. The SCNPs were found in the deposits formed on the cathode holder, in which highly curled graphitic structure are found in majority. The diameter varies from 20 to 60 nm and it also appears that these particles are self-assembled to each other. The MWCNTs with the diameter of 10–30 nm were obtained which were present inside the swelling portion of cathode deposited. These MWCNTs have 14–18 graphitic layers with 3.59 Å interlayer spacing. The GNFs have average lateral sizes of 1–5 μm and few of them are stacked layers and shows crumpled like structure. The GNFs are more stable at low temperature (low mass loss) but SCNPs have low mass loss at high temperature. © 2015, Springer Science+Business Media Dordrecht.
Hydrothermal synthesis of a uniformly dispersed hybrid graphene-TiO2 nanostructure for optical and enhanced electrochemical applications
(Royal Society of Chemistry, 2015) Rajesh Kumar; Rajesh Kumar Singh; Pawan Kumar Dubey; Dinesh Pratap Singh; Ram Manohar Yadav; Radhey Shyam Tiwari
Highly dispersed TiO2 nanoparticles on graphene nanosheets were achieved by hydrothermal treatment of graphene nanosheets obtained by modified Hummer's method followed by thermal exfoliation. The hybrid graphene TiO2 nanostructure composite (H-GTN) showed enhanced optical and electrochemical properties for future application as a supercapacitor. The structural, optical and electrochemical properties of the composite are systematically investigated. The as-prepared H-GTN showed a quenching phenomenon of its photoluminescence properties, which was attributed to the specific properties of graphene. Remarkably, the CV test obtained for H-GTN showed a very high specific capacitance value up to 530 F g-1 at a scan rate of 3 mV s-1, and nearly stable capacitance of 400 F g-1 above 20 mV s-1. The cyclic stability test shows stable behavior after some initial cycles and the stability was then retained without obvious aging or performance degradation, showing long cyclic stability. This is attributed to the excellent electrochemical performance of the H-GTN electrode material for practical application in energy storage devices. © The Royal Society of Chemistry 2015.
Preparation of carbon-nitrogen nanotubes (CNNTs)-poly ethylene oxide (PEO) composites films and their electrical conductivity measurement
(2011) Ram Manohar Yadav; Rajesh Kumar; Kalpana Awasthi; O.N. Srivastava
The present work describes the preparation characterization and electrical conductivity measurement of carbonnitrogen (C-N) nanotubesPEO composites films. CNNTs-PEO composite films have been prepared by using the solution cast technique and characterized by scanning electron microscope (SEM PHILIPS XL-20). The D-C electrical conductivity measurements of the composite films revealed that for PEO film conductivity has been found to be ∼ 7.5 × 10 -8 Scm-1, and for C-N nanotubes (∼ 20 wt.%)PEO film it was found to be ∼6.2 Scm-1 at room temperature. Thus, compared to the PEO film, the conductivity of the C-N nanotubes (∼ 20 wt.%)PEO composite film is eight orders of magnitude higher. The same conductivity of ∼ 6.2 Scm-1 for the carbon nanotubes (CNT)-PEO composites comes out at 50 wt.% of CNT in PEO as reported earlier by our group. The conductivity increases with the increase of temperature, confirming the semiconducting nature of the C-N nanotubesPEO composites. © 2011 World Scientific Publishing Company.
Rapidly quenched Ni45Fe5Mn40Sn10 Heusler alloys
(Universidade Federal de Sao Carlos, 2015) Shashank Shekhar Mishra; Thakur Prasad Yadava; Semanti Mukhopadhyay; Ram Manohar Yadav; Vajapeyajula Srinivasa Subrahmanyam; Nilay Krishna Mukhopadhyay; Onkar Nath Srivastava
The present work describes the synthesis and characterization of Ni45Fe5Mn40Sn10 Heusler alloys. The constituent pure metals were melted in radio frequency induction furnace to form the alloy and then cooled it quickly. The as-cast alloy was annealed at 850 °C for 24 h in vacuum and cooled in two different conditions i.e. self-cooling at room temperature in air and sudden cooling at liquid nitrogen temperature (quenched). The X-ray diffraction, scanning and transmission electron microscopic techniques have been used for structural/microstructural characterization as well as chemical analysis of the material. The effect of annealing and subsequent cooling in two different conditions has been described and discussed in the context of evolution of B2 and orthorhombic martensite with lattice parameter a= 0.65 nm, b=0.59 nm and c=0.56 nm.
Synthesis and characterization of Cu nanotubes and nanothreads by electrical arc evaporation
(2003) Ram Manohar Yadav; A.K. Singh; O.N. Srivastava
We report the formation and characterization of copper nanostructures, nanotubules and nanothreads, which were obtained by electrical arc evaporation of Cu electrodes under varied conditions of He ambience. Electrical arc evaporation was done with ∼10 V and (∼50-100 A) DC current. The current was used in a pulse mode. The evaporated material was condensed on a formvarcoated Cu grid mounted on a liquid N 2-cooled specimen holder. Transmission electron microscopy was employed to characterize the condensed materials. These investigations revealed that the condensed materials consisted of the mentioned nanostructures. Nanotubes and nanothreads are formed for a He pressure in the chamber corresponding to ∼140 and ∼500 torr, respectively. Extensive electron microscopic investigations showed that the diameter of the nanotubes varied from ∼5 nm to ∼50 nm and their length from 2 μm to 3 μm.
Synthesis of bamboo-shaped carbon-nitrogen nanotubes using acetonitrile-ferrocene precursor
(2004) Ram Manohar Yadav; Anchal Srivastava; O.N. Srivastava
We report the formation of bamboo-shaped carbon-nitrogen nanotubes by employing a simple, one-step and economically viable spray pyrolysis technique using new precursor; acetonitrile and ferrocene solution. By varying the concentration of ferrocene with respect to acetonitrile, it has been found that the optimum concentration of ferrocene in acetonitrile is 5 mg/ml. The special feature of the as-synthesized bamboo-shaped carbon-nitrogen nanotubes bundles is that they are produced in a high yield (1.25 gms/run). They also have long linear extents (∼430 μm) and are very clean. The average composition of carbon-nitrogen nanotubes comes out to be C 26N.
Synthesis of C-N nanotube blocks and Y-junctions in bamboo-like C-N nanotubes
(2008) Ram Manohar Yadav; Dinesh Pratap Singh; T. Shripathi; O.N. Srivastava
We report the observations made on the synthesis and characterization of C-N nanotube blocks and Y-junctions in bamboo-like C-N nanotubes. The C-N nanotube Blocks have been synthesized by pyrolyzing the mixture of silver nitrate acetonitrile solution and ferrocene benzene solution. The structural/microstructural characterization of the as-synthesized material has been done using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopic (XPS) analysis has been carried out to confirm the presence of nitrogen in nanotubes. These investigations reveal the formation of blocks of bamboo-like nanotubes having the dimension 300 × 200 × 30 μm and the diameter is 20-50 nm. We also observe the formation of Y-junctions in bamboo-like nanotubes as we spray the acetonitrile ferrocene and AgNO3 mixture. The length of the synthesized Y-junction nanotube bundles is ∼2 μm. Some more complex Ψ-shaped junctions are also found to be present. The diameters of the Y-junction nanotubes is ∼80 nm at the junction and 25-50 nm at the branches. © 2008 Springer Science+Business Media B.V.
Synthesis of nanostructured silicon carbide films through spray pyrolysis of ball-milled silicon
(2005) Dinesh Pratap Singh; Ram Manohar Yadav; Onkar Nath Srivastava
The structural and luminescent properties of nanocrystalline silicon produced by high-energy ball-milling of silicon were investigated. The as-synthesized, ball-milled silicon in hexane medium was characterized for material identification by X-ray diffraction analysis. It was found that the free-standing nanostructured SiC film can be synthesize by employing a new route of spray pyrolysis of ball-milled silicon in hexane at 1000-1100°C. The results show that the micrometer-sized protrusion structures were obtained under high gas pressure by increasing the flow rate of argon gas inside the silica tube.