P. K. Gupta

Electro-caloric effect in 0.45BaZr0.2Ti0.8O3-0.55Ba0.7Ca0.3TiO3 single crystal

Electro-caloric effect in 0.45BaZr0.2Ti0.8O3-0.55Ba0.7Ca0.3TiO3 single crystal has been investigated and electro-caloric coefficient of ∼0.38u2009K mm/kV has been obtained near the tetragonal-to-cubic phase transition. The value obtained is significantly higher than that for the lead-free ferroelectric materials reported so far.

Electro-caloric effect in (Ba1−xCax)(Zr0.05Ti0.95)O3: A lead-free ferroelectric material

Electro-caloric effect in lead-free (Ba1−xCax)(Zr0.05Ti0.95)O3 ferroelectric ceramics has been studied for xu2009=u20090.05, 0.08, and 0.10. The electro-caloric coefficient as high as 0.253u2009K mm/kV has been obtained near tetragonal-to-cubic phase transition for xu2009=u20090.08. This value is nearly the same as observed in lead-based ferroelectric materials. The higher value of electro-caloric coefficient at xu2009=u20090.08 is attributed to the higher polarization flexibility.

Thermal stability of piezoelectric coefficients in (Ba1−xCax)(Zr0.05Ti0.95)O3: A lead-free piezoelectric ceramic

Temperature dependence of piezoelectric properties of calcium doped Ba(Zr0.05Ti0.95)O3 ceramic has been investigated. The piezoelectric coefficients k31, kp, and d31 show anomaly near orthorhombic to tetragonal and rhombohedral to orthorhombic phase transition temperatures. Above orthorhombic to tetragonal transition temperature, all the coefficients are decreasing gradually but a sharp drop is observed near tetragonal to cubic transition. A percentage variation of kp was calculated between temperature range of −20 to 70u2009°C. It shows a variation of 10%–25% of its values for all the compositions.

Enhanced salt-adsorption capacity of ambient pressure dried carbon aerogel activated by CO2 for capacitive deionization application

AbstractCarbon aerogels (CAs) were prepared by polycondensation of resorcinol and formaldehyde using ambient drying technique and were further activated by CO2. These were investigated for capacitive deionization (CDI) application. The gel parameters were optimized to obtain mesoporous CA with pore size distribution in 8–10u2009nm. The CA was further subjected to CO2 activation at 950°C for different time durations and its effect on surface properties was studied. The surface area of the activated CA increased from 698 to 2057u2009m2/g with corresponding increase in mesopore area from 289 to 644u2009m2/g. The pore volume also increased from 0.79 to 1.72u2009cm3/g. Cyclic voltammetry was performed on the test electrodes and specific capacitance values of 108u2009F/g was measured at scan rate of 10u2009mV/s in 1u2009M NaCl. Electrodes of size 200u2009×u2009200u2009mm were synthesized with an active material loading of ~15u2009mg/cm2 and their CDI performance was evaluated in batch mode experiment. Salt-adsorption capacity as high as 8.4u2009mg/g for CO2 ...

Relaxor ferroelectric behavior of BaMnO3 (2H) at room temperature

Geometrically frustrated hexagonal BaMnO3 (2H) is an interesting material due to the presence of more than one ferroic order parameters. Although the material is well known, still there exists confusion on the crystal structure of this material at room temperature and its ferroelectric properties have not been explored. Here, we report noncentrosymmetric crystal structure and ferroelectric properties of BaMnO3 (2H) at room temperature. The dielectric diffusivity calculated from modified Curie-Weiss laws and non linear Vogel-Fulcher fittings implies weak relaxor characteristic of BaMnO3 (2H). The ferroelectric properties arise due to off centering of Mn4+ ion in unit cell and weak relaxor properties are attributed to the presence of a smaller amount of Mn3+ cations which creates disorder in 2H-BaMnO3.

Applicability of Langmuir equation to oxygen pressure dependent photoluminescence from β-Ga2O3 nanostructures

β-Ga2O3 nanostructures were synthesized via vapor transport method on gold coated Silicon substrate in N2 ambient. The as synthesized products were investigated by grazing incident X-ray diffraction, scanning electron microscopy and photoluminescence (PL) spectroscopy. It is shown that the intensity of photoluminescence from the ensemble of β-Ga2O3 nanostructures in oxygen gas ambience is correlated with the oxygen pressure through the Langmuir equation. This correlation is found to be reversible and reproducible. This phenomenon, which was not observed in the bulk β-Ga2O3 single crystal, is attributed to the oxygen related shallow trap surface states of the nanostructures with energies at about 4.2u2009eV above the valance band. Based on the changes in the PL intensity with the oxygen pressures, a possible mechanism for the observed photoluminescence is suggested. The present results provide a route for room-temperature response of oxygen in the gallium oxide nanostructures.

A Comparative Study of Dielectric Relaxation in Sr0.61Ba0.39Nb2O6 and Sr0.75Ba0.25Nb2O6 Single Crystals

Dielectric behaviour of Sr0.61Ba0.39Nb2O6 (61SBN) and Sr0.75Ba0.25Nb2O6 (75SBN) single crystals has been studied. It is observed that 61SBN has relatively low diffuseness as well as smaller relaxation compared to 75SBN. Increase in the configurational entropy imposed by the change in defect structure is suggested to be the origin of this behaviour. Analysis using Vogel–Fulcher expression and modified Curie–Weiss law give a signature of long-range polar order in 61SBN. The value of γ for 61SBN is closer to unity (1.14), where as for 75SBN it is 1.52. The fluctuation (in the orientation) of dipoles near dielectric maxima in SBN system is mediated by a paraelectric state.

Effect of Co addition on the atomic ordering of FeCo-phase in nanocrystalline Fe81-xCoxNb7B12 alloys (x = 20.25, 27, 40.5, 54, 60.75): An anomalous diffraction and Mössbauer study

The influence of Co addition on the atomic ordering of bcc FeCo-phase in partially nanocrystalline Fe81-xCoxNb7B12 alloys (xu2009=u200920.25, 27, 40.5, 54, 60.75) was investigated using synchrotron radiation based x-ray diffraction (SR-XRD) and 57Fe Mossbauer spectroscopy (MS). The results indicate that as the Co content in the alloy increases, there is a decrease in the thermal stability against crystallization of the amorphous phase. In the pure amorphous state, topological ordering as well as compositional homogeneity of the amorphous phase increases with Co addition (xu2009≥u200940.5). In the partially nanocrystalline state (isothermally annealed at 823u2009K for 1u2009h), the anomalous diffraction measurements close to Fe K-edge provide unambiguous evidence of atomically ordered α′-FeCo phase (CsCl type) as the nanocrystalline ferromagnetic phase by the presence of (100) super-lattice reflection for the alloys with xu2009≥u200940.5. However, alloys with xu2009≤u200927 present a disordered structure as confirmed by the absence of superlatti...

Crystal growth of Ru-doped congruent LiNbO3 and investigation of crystalline perfection and optical properties

Single crystals of undoped and Ru-doped congruent LiNbO3 (LN) were successfully grown by the Czochralski method. The axial and radial gradient of the radio frequency furnace was controlled in order to obtain crack-free single crystals. Wafers were cut from the grown Ru-doped single crystal at different axial positions along the growth direction and subjected to various characterization analyses. Good optical homogeneity and low residual strain in the grown crystal is confirmed by the conoscopy patterns. Further, it is confirmed that Ru doping does not influence the optical sign of the crystal. From a high-resolution X-ray diffractometry analysis, it is evident that the as-grown undoped LN crystal exhibited better crystalline perfection with a single and sharp diffraction curve in comparison to the Ru-doped crystals, which are characterized by a broader diffraction curve. Absorption coefficient and band-gap analysis across the axial length revealed a concentration variation of Ru across the length. Refractive index measurements carried out using a prism coupler showed variation in the optical birefringence, also due to the variation of Ru concentration at different positions of the grown single crystal.

Preparation and characterization of hydrophobic platinum-doped carbon aerogel catalyst for hydrogen isotope separation

We report preparation of hydrophobic platinum-doped carbon aerogel (PtCA) catalyst and its characterization for catalytic exchange reactions between hydrogen isotopes. The PtCA powder was synthesized by sol-gel polymerization method, mixed with colloidal PTFE solution, and coated on Dixon rings to obtain hydrophobic catalyst. The Pt cluster size in PtCA powder was observed to vary from 3 to 5 nm for a change in resorcinol to alkali molar ratio in synthesis solution from 20 to 200. Transmission electron microscopy of powder showed that the Pt clusters were uniformally dispersed and Pt0 metallic content estimated by X-ray photoelectron spectroscopy (XPS) was found to be of ~70%. The catalytic activity was found to depend on Pt cluster size and was higher for smaller cluster size. For the smallest achieved Pt cluster size of 3 nm, catalytic activity of ~0.8 m3 (STP) s−1 m−3 was obtained for hydrogen isotope exchange in atmospheric pressure conditions.