Simultaneous large enhancements in thermopower and electrical conductivity of bulk nanostructured half-Heusler alloys

Large reductions in the thermal conductivity of thermoelectrics using nanostructures have been widely demonstrated. Some enhancements in the thermopower through nanostructuring have also been reported. However, these improvements are generally offset by large drops in the electrical conductivity due to a drastic reduction in the mobility. Here, we show that large enhancements in the thermopower and electrical conductivity of half-Heusler (HH) phases can be achieved simultaneously at high temperatures through coherent insertion of nanometer scale full-Heusler (FH) inclusions within the matrix. The enhancements in the thermopower of the HH/FH nanocomposites arise from drastic reductions in the "effective" carrier concentration around 300 K. Surprisingly, the mobility increases drastically, which compensates for the decrease in the carrier concentration and minimizes the drop in the electrical conductivity. Interestingly, the carrier concentration in HH/FH nanocomposites increases rapidly with temperature, matching that of the HH matrix at high temperatures, whereas the temperature dependence of the mobility significantly deviates from the typical T(-α) law and slowly decreases (linearly) with rising temperature. This remarkable interplay between the temperature dependence of the carrier concentration and mobility in the nanocomposites results in large increases in the power factor at 775 K. In addition, the embedded FH nanostructures also induce moderate reductions in the thermal conductivity leading to drastic increases in the ZT of HH(1 - x)/FH(x) nanocomposites at 775 K. By combining transmission electron microscopy and charge transport data, we propose a possible charge carrier scattering mechanism at the HH/FH interfaces leading to the observed anomalous electronic transport in the synthesized HH(1 - x)/FH(x) nanocomposites.     

Fluorescent zinc(II) complex exhibiting "on-off-on" switching toward Cu2+ and Ag+ ions

Binuclear zinc(II) and copper(II) complexes based on a new Schiff base ligand N,N'-bis(2-hydroxybenzilidene)-2,4,6-trimethylbenzene-1,3-diamine (H(2)L) have been synthesized. The ligand H(2)L and complexes under investigation have been characterized by elemental analyses, spectral (FT-IR, (1)H, (13)C NMR, ESI-MS, electronic absorption, emission), and electrochemical studies. The structures of H(2)L and complexes [{Zn(C(23)H(18)N(2)O(2))}(2)] (1) and [{Cu(C(23)H(18)N(2)O(2))}(2)]·H(2)O (2) have been determined crystallographically. Selective "On-Off-On" switching behavior of the fluorescent complex 1 has been studied. The fluorescence intensity of 1 quenches (turns-off) upon addition of Cu(2+), while enhances (turns-on) in the presence of Ag(+) ions. The mechanisms of "On-Off-On" signaling have been supported by (1)H NMR, ESI-MS, electronic absorption, and emission spectral studies. Job's plot analysis supported 1:1 and 1:2 stoichiometries for Cu(2+) and Ag(+) ions, respectively. Association and quenching constants have been estimated by the Benesi-Hildebrand method and Stern-Volmer plot. Moreover, 1 mimics a molecular keypad lock that follows correct chemical input order to give maximum output signal.     

Impacts of TV and radio advertisements on the dynamics of an infectious disease: A modeling study

TV and radio advertisements are widely acknowledged as important interventions in raising issues of public health care and play promising role to control the infection through propagating awareness among the individuals. In this paper, a nonlinear susceptible‐infected‐susceptible (SIS) model is proposed and analyzed to see the impacts of TV and radio advertisements on the spread of influenza epidemic. In the model formulation, it is assumed that the susceptible individuals contract infection through the direct contact with infected individuals. The information regarding the protection against the disease is propagated via TV and radio advertisements, and their growth rates are assumed to be proportional to the fraction of infected individuals. However, the growth rate of TV advertisements decreases with the increase in number of aware individuals. The information broadcasted through TV and radio advertisements induces behavioral changes among the susceptible individuals, and they form an isolated aware class. The epidemiological feasible equilibria, their stability properties, and direction of bifurcation are discussed. The expression for modified basic reproduction number is obtained. The model analysis shows that the dissemination rate of awareness among susceptible individuals due to TV and radio advertisements and baseline number of TV and radio advertisements have potential to reduce the epidemic peak and, thus, control the spread of infection. Further, the analytical findings are well supported through numerical simulation.     

Electron paramagnetic resonance and spin-lattice relaxation in two-dimensional systems

The anisotropies of the EPR linewidth andg-factor were investigated in two-dimensional molecular composites of the type [NH₃?R?NH₃]MX₄. Measurements were performed both in single crystals and powders over the temperature range 4.2–290 K. The spin-lattice relaxation timeT ₁ was measured using the modulation method, as a function of temperature. The samples exhibit different structures and coupling interactions, according to the nature of the halogen X, the metal M and the organic radical R. We have analysed the influence of these parameters on spin behavior by studying the samples [NH₃?(CH) _n ?NH₃]MX₄ with M=Mn, Cu; X=Cl, Br, andn=2, 3, 4, 5. When R is constituted by molecules with unsaturated bonds, these materials can be considered as excellent matrices for selective polymerisation reactions by irradiation or thermal processing. We have performed EPR measurements on the heated complex of propargylamine and cadmium chloride [HC∈C?CH₂?NH₃]₂CdCl₄. The obtained data are interpreted taking into account the strong exchange interaction and the various coupling interactions in the samples. The thermal dependences ofT ₁ are explained by means of the Bloembergen and Wang three-reservoir model. The data whow spin diffusion when the metal is manganese, and an antisymmetric exchange interaction modulated by phonons in the case of copper. The nature of the halogen plays an important role in spin dynamics and namely in spin-lattice relaxation. The results obtained on [HC∈C?CH₂?NH₃]₂CdCl₄ after heating under vaccum show the creation of many paramagnetic centers due to the vanishing of triple bonds and the occurrence of a strong exchange interaction.     

Superconductivity in non-stoichiometric and tin-substituted La2CuO4: preparation, characterization, Mössbauer and microwave-absorption studies

Two samples of non-stoichiometric La₂CuO₄ were synthesized, one with La/Cu<2, and the other with 10% Sn substituting Cu. They were investigated by X-ray diffraction, Mössbauer spectroscopy, and microwave-absorption techniques. The microwave-absorption data indicated that they were both superconducting, with the transition temperatures T_c of 40.5 and 41.5 K, the one doped with Sn possessing the higher T_c. The Mössbauer spectra revealed that there exist two kinds of Sn(IV) atoms disordered with Cu. Their isomer shift, δ=−0.244(4) mm/s, is in agreement with Sn(IV) coordinated by oxygen. One site was characterized by a single Mössbauer line, being associated with a weakly distorted environment, wherein the Sn, coordinated more symmetrically, is surrounded by four Cu²⁺ ions. On the other hand, the other site, characterized by a Mössbauer doublet exhibited a quadrupole splitting Δ=1.07(2) mm/s, being associated with a highly distorted coordination, explained to be due to Sn occupying two adjacent cationic sites. To our knowledge, such a substitution for copper ions not resulting in a decrease of T_c has not been reported previously.     

A bioeconomic model of a ratio-dependent predator-prey system and optimal harvesting

This paper deals with the problem of a ratio-dependent prey-predator model with combined harvesting. The existence of steady states and their stability are studied using eigenvalue analysis. Boundedness of the exploited system is examined. We derive conditions for persistence and global stability of the system. The possibility of existence of bionomic equilibria has been considered. The problem of optimal harvest policy is then solved by using Pontryagin’s maximal principle.     

Development of a novel high speed (electron-mobility) epi-n-ZnO thin films by L-MBE for III–V opto-electronic devices

Intrinsic epitaxial zinc oxide (epi-ZnO) thin films were grown by laser-molecular beam epitaxy (L-MBE), i.e., pulsed laser deposition (PLD) technique using Johnson Matthey “specpure”-grade ZnO pellets. The effects of substrate temperatures on ZnO thin film growth, electrical conductivity (σ), mobility (μ) and carrier concentration (n) were studied. As well as the feasibility of developing high quality conducting oxide thin films was also studied simultaneously. The highest conductivity was found for optimized epi-ZnO thin films is σ=0.06×10³ ohm⁻¹ cm⁻¹ (n-type) (which is almost at the edge of semiconductivity range), carrier density n=0.316×10¹⁹ cm⁻³ and mobility μ=98 cm²/V s. The electrical studies further confirmed the semiconductor characteristics of epi-n-ZnO thin films. The relationship between the optical and electrical properties were also graphically enumerated. The electrical parameter values for the films were calculated, graphically enumerated and tabulated. As a novelty point of view, we have concluded that without doping and annealing, we have obtained optimum electrical conductivity with high optical transparency (95%) for as deposited ZnO thin films using PLD. Also, this is the first time that we have applied PLD made ZnO thin films to iso-, hetero-semiconductor–insulator–semiconductor (SIS) type solar cells as transparent conducting oxide (TCO) window layer. We hope that surely these data be helpful either as a scientific or technical basis in the semiconductor processing.     

Jordanian Uh,sgl(2) and its Coloured Realization

A two-parametric nonstandard (Jordanian) deformation of the Lie algebra gl(2) is constructed and then, exploited to obtain a new, triangular R-matrix solution of the coloured Yang–Baxter equation. The corresponding coloured quantum group is presented explicitly.     

A note on the paramagnetic resonance of free radical

Paramagnetic resonance in a free radical, Diphenyl-Picryl-Hydrazyl, has been studied and the results are compared with those of other workers. Low temperature work indicated that the transition from paramagnetic to antiferromagnetic state should occur at 1° K.