Anomalous heat capacity and X-ray photoelectron spectroscopy of Superconducting FeSe1/2Te1/2

The bulk polycrystalline sample FeSe1/2Te1/2 is synthesized by solid state reaction route in an evacuated sealed quartz tube at 750 oC. The presence of superconductivity is confirmed through magnetization/thermoelectric/resistivity studies. It is found that the superconducting transition temperature (Tc) is around 12 K. Heat capacity (Cp) of superconducting FeSe1-xTex exhibited a hump near Tc, instead of well defined Lambda transition. X-ray Photo electron spectroscopy (XPS) studies revealed well defined positions for divalent Fe, Se and Te but with sufficient hybridization of Fe (2p) and Se/Te (3d) core levels. In particular divalent Fe is shifted to higher BE (binding energy) and Se and Te to lower. The situation is similar to that as observed earlier for famous Cu based HTSc (High Tc superconductors), where Cu (3d) orbital hybridizes with O (2p). We also found the satellite peak of Fe at 712.00 eV, which is attributed to charge carrier localization induced by Fe at 2c site.Comment: 12 pages text + Figs contact Author-VPS Awana (www.freewebs.com/vpsawana

Electrical conduction and thermal properties of Bi-doped Pr0.7Sr0.3MnO3 manganite

Electrical and thermal properties of Bi-doped Pr0.7Sr0.3MnO3 (PSMO) compounds are reported here. It is observed that Bi-doped PSMO compounds follow variable range conduction mechanism. Specific heat, thermal conductivity and thermo-power measurements show that larger Bi-ion, in place of smaller Pr ion, enhances their transition temperatures (T-MI, T-C, T-CP). Thermo-power measurements further strengthen our conclusion for the presence of magnetic polaron, generated due to hybridization of Bi3+-6s(2) and O2--2p orbital and polaronic conduction mechanism

Thermoelectric properties of SnSe nanoribbons: a theoretical aspect

Bulk SnSe is reported to be an excellent thermoelectric material at high temperatures. We now present the results on thermoelectric properties of nanoribbons of SnSe of variable widths obtained using density functional theory coupled with semi-classical Boltzmann theory. The calculated results find armchair nanoribbons of width <= 47 angstrom to be semiconducting and zigzag nanoribbons of width <= 52 angstrom to be metallic. A relatively high Seebeck coefficient (approximate to 1720 mu V K-1) and low thermal conductivity was calculated for the armchair nanoribbon of 6 angstrom width, while a large relaxation time and small effective mass was obtained for the armchair nanoribbon of 47 angstrom width. The calculated results suggest that patterning SnSe into nanoribbons may provide thermoelectric performance that is similar to the monolayer and low-temperature bulk phases of SnSe

Emerging cool white light emission from Dy3+ doped single phase alkaline earth niobate phosphors for indoor lighting applications

Single-phase cool white-light emitting BaNb2O6:Dy3+ phosphors have been synthesized via a conventional solid-state reaction method and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) observations and spectrofluorophotometric measurements. XRD and Rietveld structural refinement studies confirm that all the samples exhibit pure orthorhombic structure [space group - C222(1)(20)]. SEM observations reveal the dense particle packaging with irregular morphology in a micron range. The as-prepared phosphors exhibit blue (482 nm) and yellow (574 nm) emissions under 349, 364, 386 and 399 nm excitations corresponding to F-4(9/2) -> H-6(J) (J = 15/2, 13/2) transitions of Dy3+ ions. The energy transfer mechanism between Dy3+ ions has been studied in detail and the luminescence decay lifetime for the F-4(9/2) level was found to be around 146.07 mu s for the optimized phosphor composition. The calculated Commission Internationale de L'Eclairage (CIE) chromaticity coordinates for the optimized phosphor are (x = 0.322, y = 0.339), which are close to the National Television Standard Committee (NTSC) (x = 0.310, y = 0.316) coordinates. The values of CIE chromaticity coordinates and correlated color temperature (CCT) of 5907 K endorse cool white-light emission from the phosphor. The study reveals that BaNb2O6: Dy3+ phosphor could be a potential candidate for near ultra-violet (NUV) excited white-LED application

The effect of doping on thermoelectric performance of p-type SnSe: Promising thermoelectric material

Tin selenide (SnSe) based thermoelectric materials are being explored for making inexpensive and efficient thermoelectric devices with improved thermoelectric efficiency. As both Sn and Se are earth abundant and relatively inexpensive and these alloys do not involve toxic materials, such as lead and expensive tellurium. Hence, in the present study, we have synthesized SnSe doped with 2 at% of aluminium (Al), lead (Pb), indium (In) and copper (Cu) individually, which is not reported in literature. Out of these, Cu doped SnSe resulted in enhancement of figure-of-merit (zT) of similar to 0.7 +/- 0.02 at 773 K, synthesized employing conventional fusion method followed by spark plasma sintering. This enhancement in zT is similar to 16% over the existing state-of-the-art value for p-type SnSe alloy doped with expensive Ag. This enhancement in ZT is primarily due to the presence of Cu2Se second phase associated with intrinsic nanostructure formation of SnSe. This enhancement has been corroborated with the microstructural characterization using field emission scanning electron microscopy and X-ray diffraction studies. Also, Cu doped SnSe exhibited a higher value of carrier concentration in comparison to other samples doped with Al, Pb and In. Further, the compatibility factor of Cu doped SnSe alloys exhibited value of 1.62 V-1 at 773 K and it is suitable to segment with most of the novel TE materials for obtaining the higher thermoelectric efficiencies

The effect of carbon nanotubes (CNT) on thermoelectric properties of lead telluride (PbTe) nanocubes

We report the thermoelectric properties of different weight percentage of carbon nano tubes (CNT) dispersed (0.025, 0.05 and 0.1%) in PbTe nanocubes matrix. The sample with 0.05% CNT distribution in the PbTe matrix significantly enhanced the electrical conductivity, and Seebeck coefficient above 450 K. In this system CNT's acting as a low energy filter at potential barrier and good passage for high energy electron which may cause enhance the electrical conductivity. Mixing of CNT into PbTe nanocubes created numerous additional interfaces which are acted as scattering centers for phonons and thus resulted in ultra low thermal conductivity of 0.24 and 032 W/m K at 525 K for 0.025 and 0.05% CNT dispersed PbTe matrix, respectively. An optimum quantity of CNT addition to PbTe enhanced the zT similar to 0.11 at 525 K for both 0.025 and 0.05% CNT dispersed PbTe matrix, respectively, which is 90% higher in comparison to PbTe nanocubes. Finally, CNT's dispersed PbTe nanocomposite exhibited good thermoelectric compatibility factor and these materials are good candidate materials for segmentation to get maximum power output during power generation applications

Modulating the lattice dynamics of n-type Heusler compounds via tuning Ni concentration

Reducing the lattice thermal conductivity (kappa(L)) comprises one of the crucial aspects of thermoelectric research. Ternary intermetallic half Heusler compounds have revealed properties promising for thermoelectric applications. Studies have shown that self doping with Ni in Ni based half Heuslers leads to unprecedented lowering in the kappa(L). Although the underlying physical mechanisms have not been explored in detail, with ZrNiSn as a case study, we experimentally investigate the change in kappa(L) with increase in the Ni concentration in Ni based n-type half Heusler alloys. We observe that at excess Ni doping of 3% in the half Heusler lattice, the thermal conductivity reduces by more than 60%. Our density functional theory based analysis on the ongoing phenomena reveals that at ultralow Ni doping, the localized modes of the antisite Ni defect hybridize with the acoustic modes and this plays the most dominant role in scattering of the thermal phonons leading to significant lowering in kappa(L). Our theoretical analysis can be employed for predicting a suitable dopants that may reduce the kappa L prior to the synthesis of the compound in the laboratory

Studies on phase stability, mechanical, optical and electronic properties of a new Gd2CaZnO5 phosphor system for LEDs

A new ternary oxide Gd2CaZnO5 having interesting structural, mechanical, electronic and optical properties is synthesized and is studied in detail using density functional theory. The analysis revealed two polymorphs: orthorhombic and tetragonal; the orthorhombic phase was found to be the most stable structure under ambient conditions. A high-pressure (hydrostatic) phase transition to the tetragonal phase is predicted at about 4 GPa. This is one of very few reports that depict the phase transition of oxide materials under pressure. The calculated results are in agreement with the X-ray diffraction studies supported by Rietveld analysis. Analysis of the optical properties revealed both polymorphs to be direct-gap semiconductors with low dielectric constants. The calculated elastic constants of both phases satisfy the mechanical stability criteria. It is also identified that the half-filled 4f orbital of Gd induces a strong magnetic spin polarization in the host oxide lattice indicating that the material could be effectively used in versatile applications ranging from biomedical devices to light emitting diodes

Enhanced hard magnetic properties in partially-doped Mn3-xGdxGa (x <= 0.03)

The effect of partial (1-3%) rare-earth Gadolinium doping in D0(22 )Mn(3)Ga hard magnet has been investigated in this work. The Mn 3 Ga undergoes magnetic transition at T-c similar to 745 K, which decreases down to T-c similar to 715 K upon 1% Gd-doping with introduction of mixed Mn valence states, as revealed from XPS study. The hard magnetic behavior of Mn3Ga has been observed in a wide temperature range (up to T-c ). More importantly, the partial Gd-doping significantly enhances the room temperature hard magnetic properties i.e. squareness ratio (m(r)/m(s)), coercivity (H-c) and energy product (BHmax ) from 0.43, 2.73 kOe and 0.39 MGOe (Mn3Ga) to 0.57, 4.25 kOe and 0.5 MGOe (Mn2.97Gd0.03Ga), respectively. The modified Stoner-Wohlfarth model and low-field minor loop analysis reveal the nucleation hardening mechanism for the magnetization reversal. The study proposes the partial rare-earth doping as a new approach to enhance the hard magnetic properties of rare-earth free hard magnets

Double-Doping Approach to Enhancing the Thermoelectric Figure-of-Merit of n-Type Mg2Si Synthesized by Use of Spark Plasma Sintering

We report significant enhancement of the thermoelectric figure-of-merit of Mg2Si by double-doping with a combination of Bi, Pb, and Sb as doping elements. Addition of any two of these three elements to Mg2Si increases the electrical conductivity by more than three orders of magnitude at 323 K, irrespective of the doping elements used. However, a corresponding decrease in the Seebeck coefficient is observed in comparison with undoped Mg2Si. Irrespective of the combination of the three elements used for double doping, a figure-of-merit of approximately 0.7 at 873 K is obtained for Mg2Si; this is primarily because of enhancement of the electrical conductivit