arXiv subjects

O. N. Srivastava

Publications and source records attributed to O. N. Srivastava.

At least 19 recordsLinked to original sources

arXiv2021

Glassy nature of Ce65Al25Co10 alloy: A metallic glass

In the present investigation the glass forming ability of Ce65Al25Co10 metallic glass has been reported. It was successfully synthesised using melt spinning techniques. The existence of amorphous phase in Ce65Al25Co10 alloy have been proved using X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The thermal analysis of the synthesised sample using differential scanning calorimetry (DSC) shows a glass transition around 371 K, thus confirms existence of glassy phase at room temperature. The hardness of the synthesised sample at different load have been demonstrated. Moreover, the yield strength of the synthesised sample has also been calculated by means of hardness data and Meyer exponent.

cond-mat.mtrl-sci↗

arXiv2018

Unexplored photoluminescence from bulk and mechanically exfoliated few layers of Bi2Te3

We report the exotic photoluminescence (PL) behaviour of 3D topological insulator Bi2Te3 single crystals grown by customized self-flux method and mechanically exfoliated few layers (18 plus minus 2 nm)/thin flakes obtained by standard scotch tape method from as grown Bi2Te3 crystals.The experimental PL studies on bulk single crystal and mechanically exfoliated few layers of Bi2Te3 evidenced a broad red emission in the visible region. These findings are in good agreement with our theoretical results obtained using the ab initio density functional theory framework.

cond-mat.mtrl-sci↗

arXiv2014

Hydrogen storage in carbon derived from solid endosperm of coconut

Carbons are being widely investigated as hydrogen storage material owing to their light weight, fast hydrogen adsorption kinetics and cost effectiveness. However, these materials suffer from low hydrogen storage capacity, particularly at room temperature. The aim of the present study is to develop carbon-based material from natural bio-precursor which shows at least moderate hydrogen storage at room temperature. For this purpose, hydrogenation characteristics of carbon derived from solid endosperm of coconut is studied. Solid endosperm and its carbonized version have a native grown distribution of light elements like K (from KCl), Na, Mg, etc. The hydrogen storage measurements reveal that the as synthesized materials have good hydrogen adsorption and desorption capacity with fast kinetics. The synthesized material adsorbs 2.30 wt. % at room temperature and 8.00 wt.% at liquid nitrogen temperature under 70 atm pressure. The present work is likely to initiate research on native light metal bearing carbon derived from natural precursors.

cond-mat.mtrl-sci↗

arXiv2014

Hydrostatic pressure: A very effective approach to significantly enhance critical current density in granular Sr4V2O6Fe2As2 superconductor

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. In addition, we found that the transformation from surface pinning to point pinning induced by pressure was accompanied by a reduction of anisotropy at high temperatures. 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.

cond-mat.supr-con↗

arXiv2013

Synthesis of High-Quality Graphene through Electrochemical Exfoliation of Graphite in Alkaline Electrolyte

Owing to wide variety of applications of graphene, high-quality and economical way of synthesizing graphene is highly desirable. In this study, we report a cost effective and simple approach to production of high-quality graphene. Here the synthesis route is based on electrochemical exfoliation of graphite. Instead of using strong acids (which oxidise and damage the geometrical topology of graphene), we have used alkaline solution (KOH dissolved in water) as electrolyte. TEM analysis shows that as prepared graphene has 1-4 layers and large lateral size (up to ~18 micrometer). Raman analysis shows a unique feature. The D peak which is invariably present in graphene is absent here. This suggests that disorder in the graphene sheets synthesized by the present method, is nearly absent. A tentative mechanism of electrochemical exfoliation in alkaline electrolyte is discussed and described.

cond-mat.mtrl-sci↗

arXiv2013

Magnetotransport and thermal properties characterization of 55 K superconductor SmFeAsO0.85F0.15

Correlation between structural/microstructural and magneto-thermal transport properties of FeAs-based SmFeAsO and SmFeAsO0.85F0.15 has been studied in detail. The Rietveld analysis of room temperature powder X-ray diffraction (XRD) data reveals that both the samples are single phase, with very small amount of rare earth impurity in the F doped compound. Electron microscopic investigations show that compounds have layered morphology and structure, with the individual grains being surrounded by amorphous layers. The average grain boundary thickness is ~5 nm. The F free material is found to be magnetic and shows the appearance of Fe spin density wave (SDW) like order at T = 150 K. The F doped compound (SmFeAsO0.85F0.15) shows the occurrence of superconductivity at Tc(R=0, H=0)- 55 K, which decreases to 42 K at magnetic field (H) of 13 kOe. The superconducting transition was also confirmed by DC magnetization and AC susceptibility measurements. The intra-grain critical current density (Jc) calculated using the Bean critical state model is found to be around 5.26 x 10^4 A/cm2 at 5 K in zero field (H=0). The dependence of thermally activated flux flow energy (U/KB) on the applied magnetic field has been observed. AC susceptibility measurements at different amplitude of applied AC drive field confirm the granular nature of the superconducting compound. is confirmed. Both Fe (SDW) at 150K for SmFeAsO and 55K superconductivity in case of SmFeAsO0.85F0.15 sample has confirmed by Specific heat [Cp(T)] measurement too. Further Sm orders anti-ferro-magnetically at 4.5K for non-superconducting and at 3.5K for superconducting samples, also the entropy change is reduced significantly for the later than the former. Summarily complete physical property characterization for both non-superconducting SmFeAsO and 55K superconductor SmFeAsO0.85F0.15 samples is provided and discussed in the current article.

cond-mat.supr-con↗

arXiv2013

Effect of Ambient Gas Pressure on the Thermal Exfoliation of Graphite Oxide: Tuning the Number of Graphene Sheets

We have investigated the effect of ambient gas pressure (Argon) used during thermal exfoliation of graphite oxide on the number of graphene sheets in the resulting graphene. The ambience gas Ar pressures used are 80, 25, 1 atmosphere and 10-3 as well as 10-5 torr. We have found curious result that the number of graphene sheets in exfoliated graphene stack is dependent on the applied pressure for exfoliation of graphite oxide (GO). By lowering the exfoliation pressure, the numbers of sheets get reduced. The numbers of sheets in exfoliated graphene stack have been determined by Lorentzian fit of their respective 002 peak of XRD profile. The numbers of sheets have also been confirmed with high resolution transmission electron microscopy (HRTEM). The numbers of sheets in exfoliated graphene stack samples have been found to be 28, 12, 8, 5 and 3 at 80 atm, 25 atm, 1 atm, 10-3 torr and 10-5 torr pressures respectively. The results of present investigation suggest a simple but effective method for synthesizing graphene with a specific number of sheets.

cond-mat.mtrl-sci↗

arXiv2012

Upper critical field, critical current density and thermally activated flux flow in CaFFe0.9Co0.1As superconductor

In this paper, we report the synthesis, structure, transition temperature, upper critical field, critical current density and thermally activated flux flow in CaFFe0.9Co0.1As superconductor. Superconductivity arises at 23 K by Co substitution at the site of Fe atoms and upper critical field is estimated 102 T by using the Werthamer-Helfand-Hohenberg formula. The flux-flow activation energy (U0/kB) varies from 3230 K and 4190 K in the field of 1 T and 9 T respectively. At 2K, the Jc is found to be approximately 4x10^3A/cm^2 and 0.3x10^3 A/cm&^2 in zero and 6T field, respectively. Transmission electron microscopy analysis shows an amorphous region surrounding around 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 miss aligned grains, amorphous grain boundaries and impurities, which are invariably found between the grains. Presence of the weakly linked granules and their weakly pinned intergranular Josephson vortices are responsible for both low Jc and the Arrhenius temperature dependence of resistivity

cond-mat.supr-con↗

arXiv2010

Investigations on synthesis and characterization of functionalized graphene sheets-polyacrylamide composites

The functionalized graphene sheets (FGS)-polyacrylamide (PAM) composite films have been prepared by solution cast technique. The FGS have been synthesized by thermal exfoliation of graphite oxide. Several composites with different weight % (between 0 to 25 wt %) of FGS loading in PAM have been prepared and characterized by XRD, SEM, TEM and FTIR spectroscopy. FTIR analysis revealed the existence of the amide linkage (-NHC= O) between the polymer chain and oxygenated graphene sheets. The electrical and mechanical properties of FGS-PAM composite films have been studied by employing low temperature dc electrical conductivity measurement and nanoindentation technique. A significant increase in both electrical conductivity and mechanical properties (hardness and elastic modulus) is observed at 15 wt% loading of FGS in PAM. As the FGS loading increases from 0 to 25 wt% the conductivity of composite film increases by seven orders of magnitude (~5.55x10-7 to 1.42 S/cm). The hardness and elastic modulus of FGS (~15 wt%)PAM film increased four and five fold, respectively as compared to that of PAM film. To our knowledge studies on FGS-PAM composites and also improvement of both the electrical and mechanical properties have not yet been made.

cond-mat.mes-hall↗

arXiv2008

Synthesis of nanocrystalline Spinel phase by Mechanical Milling of Al-Cu-Fe and Al-Cu-Cr-Fe Quasicrystalline alloys

In the present study, low-temperature synthesis of (Cu Fe)Al2O4 and (Cu Cr Fe)Al2O4 spinels from the quasicrystalline phases was investigated with the variation of process parameters during milling and annealing. The milling of the quasicrystalline materials was carried out in an attritor mill at 400 rpm for 40 hours with ball to powder ratio of 40:1 in hexane medium. Subsequently, annealing was performed in an air ambience for 10, 20, and 40 h at 400, 500, 600 and 700 C in side a furnace in order to oxidize the mechanically milled quasicrystalline phase for the possible formation of the spinel phase. It was found that after annealing at higher temperature (500 C), mechanically milled quasicrystalline alloy transformed to spinel phase whereas annealing at lower temperature (500 C) it led to the formation of B2 phase as a major one along with minor amount of oxide phase. The X-ray diffraction and transmission electron microscopy of the annealed samples confirmed the formation of spinel phase with an average grain size of 20-40 nm. It is interesting to note that the nanospinel phases showed the different colors during various annealing time and temperature. The optical properties of nanospinel materials, investigated employing UV visible spectrometer exhibited absorption characteristics.

cond-mat.mtrl-sci↗

arXiv2008

Formation and stability of icosahedral phase in Al65Ga5Pd17Mn13 alloy

In this work, we present the formation and characterization of a quaternary (pseudo ternary) icosahedral quasicrystal in Al65Ga5Pd17Mn13 alloy. The X ray diffraction and transmission electron microscopy confirmed the formation of icosahedral B2 type and O crystalline (orthorhombic structure) phases in as cast alloy. The icosahedral phase gets formed after annealing at 800 C for 60 hours. The formation of icosahedral phase in AlGaPdMn quaternary alloy by present technique has been studied for the first time. The Energy dispersive X-ray analysis investigations suggest the presence of Ga (5 at) in the alloy. It is interesting to note that pseudo twelve fold pattern in the as cast alloy has been observed. Icosahedral AlGaPdMn provides a new opportunity to investigate the various characteristics including surface characteristics. Attempts will be made to discuss the micromechanisms for the formation of quasicrystalline phase in Al-Ga-Pd-Mn alloys.

cond-mat.mtrl-sci↗

arXiv2007

Enhancement of flux pinning and high critical current density in graphite doped MgB2 superconductor

We report the synthesis and characterization of graphite (C) doped MgB2-xCx (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 5K corresponds to 1.1 x 10^6 and 5.3 x 10^4 A/cm^2 for 3T and 5T 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 behaviour 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 centre and are responsible for enhancement of superconducting properties of MgB2.

cond-mat.supr-con↗

arXiv2007

High critical current density and improved flux pinning in bulk MgB2 synthesized by Ag addition

In the present investigation, we report a systematic study of Ag admixing in MgB2 prepared by solid-state reaction at ambient pressure. All the samples in the present investigation have been subjected to structural/ microstructural characterization employing x-rays diffraction (XRD) and transmission electron microscopic (TEM) techniques. The magnetization measurements were performed by physical property measurement system (PPMS). 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) compatible with coherence length of MgB2 (~5-6 nm). In order to study the flux pinning effect of Ag admixing in MgB2, 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 5K. This corresponds to 9.23x10^7 A/cm^2 in self-field,5.82x10^7 A/cm^2 at 1T,4.24 x10^6 A/cm^2 at 3.6T and 1.52x10^5 A/cm^2 at 5T. However, intragrain Jc values for MgB2 sample without Ag admixing are 2.59x10^6 A/cm^2,1.09x10^6A/cm^2,4.53x10^4 A/cm^2 and 2.91x10^3A/cm^2 at 5 K in self field, 1T, 3.6T and 5T respectively.. The high value of intragrain Jc for Ag admixed MgB2 superconductor has been attributed to the inclusion of MgAg nanoparticles into the crystal matrix of MgB2, which are capable of providing effective flux pinning centres. A feasible correlation between microstructural features and superconducting properties has been put forward.

cond-mat.supr-con↗

arXiv2006

Effect of the Fe substitution in Ti-Ni shape memory alloys

Shape memory Ti-Ni alloys attracted much attention in the recent years, since they are shape memory, intelligent as well as functional materials. In the present investigation Ti51Ni49 and Ti51Ni45Fe4 alloys were synthesized through radio frequency (RF) induction melting. The alloy was characterized through x-ray diffraction (XRD), scanning electron microscopy (SEM), Mossbauer spectroscopy (MS) and positron annihilation techniques (PAT).The Fe substitution stabilized the TiNi type cubic (a=2.998 A) phase. The surface microstructure and presence of the oxide layer in Ti51Ni45Fe4 alloy have been investigated by SEM. The Positron annihilation measurements indicated a similar bulk electron density in both the as-cast and annealed (1000 0 C for 30 hrs) alloys, typically like that of bulk Ti. Mossbauer spectroscopy studies of as-cast and annealed iron substituted samples showed regions in the samples where nuclear Zeeman splitting of Fe levels occurred and an oxide phase was found to be present in as cast Ti51Ni45Fe4 alloy, while annealed sample indicated the presence of bcc iron phase .

cond-mat.mtrl-sci↗

arXiv2006

On the Evolution of Quasicrystalline and Crystalline Phases in Rapidly Quenched Al-Co-Cu-Ni Alloy

The occurrence of stable decagonal quasicrystalline phase in Al-Co-Ni and Al-Cu-Co alloys through conventional solidification is well established. Earlier, we have studied the effect of Cu substitution in place of Co in the Al70 Co15Ni15 alloy. Here we report the structural/micro-structural changes with substitution of Cu for Ni in rapidly solidified Al-Co-Ni alloys. The melt-spun ribbons have been characterized using X-ray diffractometry (XRD), Scanning and transmission electron microscopy (SEM & TEM). With an increase in Cu content in the melt spun Al70 Co15Cux Ni15-x (x=0 to 15) alloys, the relative amount of the decagonal phase decreased up to 10 at% of Cu. At this composition the quaternary alloy showed the coexistence of decagonal quasicrystal and superstructure of tau 3 vacancy ordered crystalline phases. The decagonal phase containing Cu showed more disordering compared to Al-Co-Ni alloys. The implication of the structural / microstructural changes due to Cu substitution in stable decagonal quasicrystal will be discussed

cond-mat.mtrl-sci↗

arXiv2006

Study on the Formation of Nano tau 3 Phase by Mechanical Alloying

In the present investigation the pure elemental powder mixture of Al (70 at%) Ni (15 at %), Cu (15 at %) was mechanically milled in an attritor ball mill for 10, 20, 40, 60, 80 and 100 hours in hexane medium at 400 rpm. The isothermal annealing of 100 h mechanically milled powder has been done at 700 0C for 20, 40 and 60 hours. The mechanically alloyed powders are characterized using X-ray diffraction, differential thermal analysis and transmission electron microscopy techniques. It was observed that mechanical alloying led to the formation of nano tau 3 phases in Al70 Cu15Ni15 after 80 h of milling. In the case of 100 h MM and subsequent annealing at 700 0 C for 20, 40 and 60 h, powder exhibited the formation of tau 3 phases with bigger grain sizes. The phase formation and transformations in the above systems have been discussed based.

cond-mat.mtrl-sci↗

arXiv2006

Synthesis of nanocrystalline (Co,Ni)Al2O4 spinel powder by mechanical milling of quasicrystalline material

The (Co,Ni)Al2O4 is an aluminum transition metals oxide which falls into the category of aluminate spinels. The synthesis of nano-crystalline spinels has been investigated extensively due to their potential applications such as a high-density magnetic recording, microwave devices magnetic fluids, ceramic pigment as well as a heterogeneous catalytic material. In the present study, attempts have been made to synthesise the nano-crystalline (Co,Ni)Al2O4 spinel powders by ball milling and subsequent annealing. An alloy of Al70 Ni15 Co15, exhibiting the formation of a complex intermetallic compound known as decagonal quasicrystals in the as-solidified condition, is selected as the starting material for mechanical milling. It is interesting to note that this alloy is close to the stoichiometry of aluminum and transition metal atoms required to form of the aluminate spinel. The milling was carried out in an attritor mill at 400 rpm for 40 hours with ball to powder ratio of 20:1 in hexane medium and the annealing was performed for 10, 20 and 40 h at 600 0 C in air in side the furnace in order to oxidize the decagonal phase and finally to form the spinel structures. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the formation of nano-sized decagonal phase after milling and then (Co,Ni)Al2O4 spinel type structure from the nano-size decagonal phase during annealing. The average crystallite size has been found to be ~40 nm by XRD and TEM studies, the lattice strain is 0.6 %, and the lattice parameter is 8.0756 A . The ferromagnetic behaviour is also observed in the milled and the annealed samples.

cond-mat.mtrl-sci↗

arXiv2006

Influence of simultaneous doping of Sb & Pb on phase formation, superconducting and microstructural characteristics of HgBa2Ca2Cu3O8+δ

We report systematic studies of structural, microstructural and transport properties of (Hg_0.80 Sb_0.2-x Pb_x)Ba_2 Ca_2 Cu_3O_8+δ(where x = 0.0, 0.05, 0.1, 0.15, 0.2) compounds. Bulk polycrystalline samples have been prepared by two-step solid-state reaction route at ambient pressure. It has been observed that simultaneous substitution of Sb and Pb at Hg site in oxygen deficient HgO_δlayer of HgBa2Ca2Cu3O8+δcuprate high-Tc superconductor leads to the formation of Hg-1223 as the dominant phase. Microstructural investigations of the as grown samples employing scanning electron microscopy reveal single crystal like large grains embodying spiral like features. Superconducting properties particularly transport current density (Jct) have been found to be sensitive to these microstructural features. As for example (Hg0.80Sb0.05Pb0.15)Ba2Ca2Cu3O8+δcompound which exhibits single crystal like large grains (~ 50 micrometer) and appears to result through spiral growth mechanism, shows highest Jct (~ 1.85 x 103 A/cm2) at 77K. A possible mechanism for the generation of spiral like features and correlation between microstructural features and superconducting properties have been put forward.

cond-mat.supr-con↗