arXiv subjects

R. S. Tiwari

Publications and source records attributed to R. S. Tiwari.

14 recordsLinked to original sources

arXiv2021

Micro-structural characteristics and indentation behavior of Ce-Al-Ga alloys

The first report of phase separation in metallic glass has attracted significant attention due to their unique micro-structural variations of amorphous phases at different domain size. In the continuation of this the view is to understanding the genesis of phase separation in Ce based metallic glass. In this paper, we present extensive investigations with particular reference to low concentration of Ga in Ce-Al-Ga metallic glass. Micro-structural characteristics and indentation behavior of melt spun Ce75Al25-xGax metallic glass has been investigated by X-ray diffraction (XRD) and Transmission electron microscopy.

cond-mat.mtrl-sci↗

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↗

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

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↗

arXiv2005

Superstructure in nano-crystalline Al50Cu28Fe22 alloy

This work reports the formation of nano- crystalline Al50Cu28Fe22 by high-energy milling. For obtaining the nano-crystalline material, the Al50Cu28Fe22 alloy synthesized through slow cooling of the molten alloy was subjected to ball milling, which was carried out in attritor mill at 400 rpm for 5 h, 10 h, 20 h, 40 h and 80 h with a ball to powder ratio 40 : 1 in hexane medium. The x-ray diffraction observation of ball-milled samples revealed that the milling duration of $5h$ to $40 hrs$ has led to the formation of nano-phase. The average crystallite size comprising the nano-phase has been found to be $\sim 17 nm$. When the nano-crystalline alloy, Al50Cu28Fe22 was vacuum annealed at a temperature of 500$^0C$ for 5 to 20 hrs, new structural phases representing superstructures of the parent nano-crystalline phase were found . The superstructure have been found to correspond to simple cubic with $a = \sqrt 2a_p$ and face central cubic with a = 2a_p (a_p = lattce parameter of parent nano-crystalline alloy). It has been proposed that the formation of different type of superstructure resulting due to different duration of ball milling followed by annealing is possibly governed by minimization of free energy of the disordered B2 phase.

cond-mat.mtrl-sci↗

arXiv2005

Nucleation and growth of catalyst-free ZnO nanostructures

This paper deals with the investigations on the nucleation and growth of ZnO nanostructures in a catalyst free synthesis. The ZnO nanostructures have been formed by evaporation of Zn (99.99%) in O_2 and Ar atmosphere in single zone furnace under two temperature regions, region A (~1173-1073K) and region B (~873-773K). Through application of XRD and TEM techniques, it has been shown that first ZnO is formed which changes to ZnOx through creation of oxygen vacancies. The ZnOx acts as self-catalyst and leads to formation of various nanostructures. Those observed in the present investigation are nanotetrapods (1D, diameter ~ 70-450nm, length ~ 2-4.5mm) nanorods (1D, diameter ~ 45-95nm, length ~ 2.5-4.5mm), nanoflowers(2D,central core diameter ~ 90-185nm, length of petals/nanorod ~ 1.0-3.5mm) and nanoparticles (3D, size ~ 0.85-2.5mm). These nanostructures have been revealed by SEM explorations. Attempts have been made to explain the formation of the various nanostructures in terms of the creation and distribution of the ZnOx, the temperature as well as oxygenation conditions.

cond-mat.mtrl-sci↗

arXiv2005

Colossal Magnetoresistance Study in Nanophasic La_0.7Ca_0.3MnO_3 Manganite

In this study we report the effect of sintering temperature on the low field magnetotransport properties of La_0.7Ca_0.3Mn_O3 manganite synthesized through polymeric precursor route. The La_0.7Ca_0.3MnO_3 has been sintered at 600^oC (S6), 700^oC (S7), 800^oC (S8), 900^oC (S9) and 1000^oC (S10). XRD confirms that phase formation starts at 600^oC. All the samples are single phasic having orthorhombic unit cell. The lattice parameters decrease on lowering the sintering temperature (T_S). The crystallite as well as particle size also show strong dependence on the sintering temperature. All the samples possess insulator-metal (T_IM) as well as paramagnetic-ferromagnetic (T_C) transitions. The T_C shows a small variation [274K (S10) to 256K (S6)] as a function of sintering temperature whereas the T_IM goes down from 267K (S10) to 138K (S6), a strong decrease of 129K. This T_C-T_IM discrepancy is due to the fact that whereas the former is an intrinsic characteristic, the latter depends strongly on the extrinsic factors e.g. synthesis conditions grain boundaries and associated disorders. For all the samples MR shows strong dependence on T_S. The MR increases on lowering the temperature as well as on increasing the field with the occurrence of an intrinsic contribution around T_C. These variations of MR for all the samples have been explained in terms of the microstructural variations and spin-polarized tunneling at low temperatures.

cond-mat.mtrl-sci↗