Raghunandan Sharma

Magnetotransport in polycrystalline La2/3Sr1/3MnO3 thin films of controlled granularity

Polycrystalline La2/3Sr1/3MnO3 thin films were synthesized by pulsed laser ablation on single crystal (100) yttria-stabilized zirconia substrates to investigate the mechanism of magnetotransport in a granular manganite. Different degrees of granularity are achieved by using the deposition temperatures (TD) of 700 and 800 °C. Although no significant change in magnetic order temperature (TC) and saturation magnetization is seen for these two types of films, the temperature and magnetic field dependence of their resistivity (ρ(T,H)) is strikingly dissimilar. While the ρ(T,H) of the 800 °C film is comparable to that of epitaxial samples, the lower growth temperature leads to a material which undergoes insulator-to-metal transition at a temperature (TP≈170 K) much lower than TC. At T⪡TP, the resistivity is characterized by a minimum followed by ln T divergence at still lower temperatures. The high negative magnetoresistance (≈20%) and ln T dependence below the minimum are explained on the basis of Kondo-type s...

Effects of structural disorder and nitrogen content on the oxygen reduction activity of polyvinylpyrrolidone-derived multi-doped carbon

Multi-doped carbons, having high N-content (∼11 wt%, bulk) and the presence of C- and N-bound O/Co, were synthesized by pyrolysis of a polyvinylpyrrolidone (PVP) and cobalt nitrate (Co(NO3)2·6H2O) mixture at various temperatures (Tp) ranging from 400 to 800 °C. To study the effects of surface modification on the oxygen reduction reaction (ORR) activity, doped carbon (CNx) samples were further acid-treated with nitric acid (HNO3). Structural characterization by Raman spectroscopy and X-ray diffraction reveals an amorphous phase of carbon for a Tp of 400 to 700 °C, and a partially graphitic structure for a Tp of 800 °C. The electrocatalytic performance of the as-synthesized and acid-treated CNx determined by linear sweep as well as cyclic voltammetry in 0.1 mol L−1 KOH electrolyte reveals significant catalytic activities with an ORR onset potential of −144 mV (vs. Ag/AgCl), as compared to −210 mV for a Pt electrode under similar conditions. Correlations between the electrochemical parameters and structural parameters such as the degree of disorder and N-content suggest high electrocatalytic activity for CNx with low disorder and high N-content. In addition to their high ORR activity, the CNx samples synthesized at lower Tp also exhibit catalytic activity for methanol oxidation, which makes them suitable catalyst supports for direct methanol fuel cells.

Correlations between morphology, crystal structure, and magnetization of epitaxial cobalt-platinum films grown with pulsed laser ablation

The effects of growth rate (G_r), deposition temperature (T_d), film thickness (t_F), and substrate induced strain (epsilon) on morphological, crystallographic and magnetic characteristics of equiatomic CoPt epitaxial films synthesized with PLD are investigated. The (001) substrates of MgO, STO and LAO provide different degree of epitaxial strain for growth of the disordered face centered cubic (fcc) and ordered face centered tetragonal (L1_0) phases of CoPt. The films deposited at T_d~600 ^0C on all three substrates are fcc with in-plane magnetization and a narrow hysteresis loop of width~200 Oe. The L1_0 phase, stabilized only at T_d~700 ^0C becomes predominantly c-axis oriented as T_d is increased to 800 ^0C. While the crystallographic structure of the films depends solely on the T_d, their microstructure and magnetization characteristics are decided by the growth rate. At the higher G_r (~1A/sec) the L1_0 films have a maze-like structure which converts to a continuous film as the t_F is increased from 20 to 50 nm. The H_c of these films increases as the L1_0 phase fraction grows with T_d and its orientation becomes out of the film plane. The evolution of microstructure with T_d is remarkably different at lower growth rate (~0.4A /sec). Here the structure changes from a self-similar fractal pattern to an assembly of nano-dots as the T_d is raised from 700 to 800 ^0C, and is understood in terms of the imbalance between strain and interfacial energies. MFM of such films reveals no distinct domain walls within the nano-islands while a clear contrast is seen between the islands of reversed magnetization. The simple picture of coherent rotation of moment appears incompatible with the time dependence of the remanent magnetization in these films.

Viscoelastic properties of coil carbon nanotube-coated carbon fiber-reinforced polymer nanocomposites

Strong interfacial bonding is necessary between carbon fiber and polymer matrix to take advantage of carbon fiber-reinforced polymer composites in structural applications. An attempt has been made to improve the interfacial bonding by coating coiled carbon nanotubes on carbon fiber surface through a single-step chemical vapor deposition process. Coiled structures were synthesized on nickel-coated carbon fiber by using thiophene as a sulphur impurity and acetylene as a carbon precursor. The coiled carbon nanotube-coated carbon fiber and epoxy were used, respectively, as the reinforcement and the matrix to form carbon fiber-reinforced polymer composites. The role of coiled carbon nanotubes on thermo-mechanical properties of carbon fiber-reinforced polymer composites was investigated using dynamic mechanical thermal analysis in three different modes of deformation. Unsized carbon fiber (carbon fiber heat-treated to remove sizing agents)/epoxy composites were used as the reference to evaluate the enhancement due to coiled carbon nanotubes. Substantial improvements in viscoelastic properties of coiled carbon nanotube/carbon fiber/polymer composites over unsized carbon fiber/polymer composites were observed in all of the deformation modes. Coiled Carbon nanotube composites in shear mode exhibit highest enhancements in both storage as well as loss moduli due to superior mechanical interlocking between coiled carbon nanotubes and polymer matrix.

Exploring the doping effects of copper on thermoelectric properties of lead selenide

In this work, we have explored the effect of dopant concentration (copper (Cu)) on the thermoelectric performance of Cu doped lead selenide (Pb1−x Cu x Se (0 ≤ x ≤ 0.1)). With increasing the dopant concentration, sign inversion of majority charge carriers takes place for x ≥ 0.04 due to the donor behaviour of Cu in the P-type pristine PbSe. The room temperature Seebeck coefficients of Pb1−x Cu x Se with x = 0.01, 0.02, 0.04, 0.06 and 0.08 are observed to be 233, 337, −473.7, −392.5 and −257.6 μV K−1, respectively as compared to that of 186.4 μV K−1 of the pristine PbSe. This increment in Seebeck coefficient is the result of low carrier concentration and is not related to the resonance states created by Cu dopant. At room temperature, the lattice thermal conductivity of pristine PbSe is 0.52 W m−1 K−1 while for Cu doped PbSe, it varies from 0.8 to 1.1 W m−1 K−1. Finally, with ZT of ~0.59 and power factor of ~700 at 500 K, Pb0.98Cu0.02Se exhibits the highest thermoelectric performance among the studied Pb1−x Cu x Se systems. Owing to the high ZT and power factor, a single thermoelement of Pb0.98Cu0.02Se exhibits thermovoltage of >100 mV at a temperature gradient of 200 °C.

Lattice-mismatch-induced granularity in CoPt-NbN and NbN-CoPt superconductor-ferromagnet heterostructures: Effect of strain

The effect of strain due to lattice mismatch and of ferromagnetic exchange field on superconductivity (SC) in NbN-CoPt bilayers is investigated. Two different bilayer systems with reversed deposition sequence are grown on MgO (001) single crystals. While robust superconductivity with high critical temperature (T c ≈15.3 k) and narrow transition width (ΔT c ≈0.4 k) is seen in two types of CoPt-NbN/MgO heterostructures where the magnetic anisotropy of CoPt is in plane in one case and out of plane in the other, the NbN-CoPt/MgO system shows markedly suppressed SC response. The reduced SC order parameter of this system, which manifests itself in T c , temperature dependence of critical current density J c (T), and angular variation of flux-flow resistivity ρ f , is shown to be a signature of the structure of NbN film and not a result of the exchange field of CoPt. The ρ f (H,T) data further suggest that the domain walls in the CoPt film are of the Neel type and hence do not cause any flux in the superconducting layer. A small but distinct increase in the low-field critical current of the CoPt-NbN couple is seen when the magnetic layer has perpendicular anisotropy.

Mangifera indica, Ficus religiosa and Polyalthia longifolia leaf extract-assisted green synthesis of graphene for transparent highly conductive film

A green approach to synthesize graphene nanosheets (Gns) by reduction of graphene oxide (GO) using Mangifera indica, Ficus religiosa or Polyalthia longifolia leaf extract as reducing agent has been demonstrated. Further, transparent conducting films (TCFs) have been fabricated by spray coating of the synthesized Gns and the optoelectrical properties of the TCFs have been investigated. Of the three leaf extracts, Mangifera indica offers the best alternative reducing agent for the green synthesis of Gns at large scale. Raman spectroscopy reveals that GO has been deoxygenated significantly, with a Raman D to G band intensity ratio of ∼1.21, while the attainment of a C/O ratio of ∼4.58 in the synthesized Gns has been confirmed by elemental analysis. The TCF fabricated after thermal graphitization (900 °C for 1 h) of the spray coated Gns film shows a sheet resistance of ∼2.08 kΩ □−1 and transmittance of ∼57.31% at 550 nm. The study suggests significant potential for the application of plant leaf extracts as non-toxic reducing agents in large-scale production of Gns. Apart from providing an alternative to hazardous reducing agents, this approach opens the possibility of preparation of Gns at large scale by using more accessible natural resources with mild synthesis conditions.

Boost in room temperature thermoelectric performance of PbSe:Alx through band modification and low densification

Optimization of the transport properties of PbSe to maximize its thermoelectric performance at room temperature has been achieved through a combination of elemental doping and low densification. Al doped PbSe (PbSe:Alx; 0 ≤ x ≤ 0.06) with both lattice substitutional (Pb site) and interstitial occupation has been synthesized through solid state reaction. High Seebeck coefficient of ∼300 to 400 μV/K is noticed at 300 to 500 K. This, combined with the lower thermal conductivity of ∼1.20 W/m K, provides an improved ZT value as high as ∼0.67 at 300 K to the PbSe:Alx Also, by substituting Al in PbSe, maximum power factors of ∼20 to 26.6 μW/cm K2 at 310 K are produced. The high room temperature thermoelectric performance of PbSe:Alx has been attributed to the mix contribution of the Al impurity states and the low densification. The strategy may be utilized to cost effective development of the low working temperature thermoelectric devices.

Functionally graded elastomeric composites as microwave shielding media

Magnetic particle-filled elastomeric composites are of significant importance in microwave shielding applications. An attempt has been made to enhance the microwave reflection properties of such composites by introduction of a magnetic filler concentration gradient along the thickness. Iron powder-filled functionally graded elastomeric composites (FGECs) are prepared by a construction-based method, wherein the volume fraction of iron powder is varied along the sheet thickness. The reflection losses of the uniformly dispersed elastomeric composites (UDECs) and of the FGECs, both employing same average amount of filler, has been measured through a reflection method with a vector network analyser, using the complex scattering parameter S11. FGECs with equal sample thickness and filler volume fraction exhibit 2.5 times higher microwave reflection over a broader frequency range, as compared to that of the UDECs. For a volume fraction of 0.39, the reflection losses of FGECs and UDECs at 12.3 GHz are observed to be −50 dB and −19 dB, respectively. The corresponding frequency bandwidth for reflection loss ≤ −15 dB is observed to be 9.4 GHz for FGECs compared to that of 3.5 GHz for UDECs.

Hierarchically structured catalyst layer for the oxygen reduction reaction fabricated by electrodeposition of platinum on carbon nanotube coated carbon fiber

The cost-efficient fabrication of cathode catalyst layers with low Pt-loadings and high oxygen reduction reaction (ORR) performances is of prime importance towards the commercialization of low temperature fuel cells. Here, an attempt has been made to fabricate a hierarchically structured cathode catalyst layer consisting of Pt-nanoparticle clusters supported on defective carbon nanotube (CNT) coated carbon fiber (CNTCF). CNTs were grown on carbon fiber (CF) using chemical vapor deposition (CVD), while electrodeposition was employed to deposit Pt-nanoparticle clusters onto the CNTCF. The effect of Pt-loading on the oxygen reduction reaction (ORR) performance of the Pt-coated CNTCF (Pt-CNTCF) electrocatalysts was studied by varying the electrodeposition time (ted) between 5 and 30 min, which resulted in a variation of the Pt weight fraction in Pt-CNTCF from almost zero to ∼0.3. Linear sweep voltammetry using a Pt-CNTCF modified glassy carbon (GC) rotating disc electrode (RDE) was employed to study the ORR performance of the samples. The CNTCF support, due to the defective structure of the CNTs, itself exhibits significant ORR activity with an electron transfer number (n) of ∼2.6, which leads to a synergistic enhancement of the overall electrocatalytic performance of Pt-CNTCF. For low Pt-loading on the GC (∼5 μg cm−2), the contribution of the CNTCF support dominated, while the Pt-nanoclusters governed the ORR performance at higher Pt-loading (>10 μg cm−2), with n > 3.5. Hence, a very low Pt-loading (∼5 μg cm−2) may not be suitable for polymer electrolyte membrane fuel cells as the production of substantial amounts of H2O2 on the catalyst supports may accelerate membrane degradation.