Germanas Peleckis

Al-Doped Zinc Oxide Nanocomposites with Enhanced Thermoelectric Properties

ZnO is a promising high figure-of-merit (ZT) thermoelectric material for power harvesting from heat due to its high melting point, high electrical conductivity σ, and Seebeck coefficient α, but its practical use is limited by a high lattice thermal conductivity κ(L). Here, we report Al-containing ZnO nanocomposites with up to a factor of 20 lower κ(L) than non-nanostructured ZnO, while retaining bulklike α and σ. We show that enhanced phonon scattering promoted by Al-induced grain refinement and ZnAl(2)O(4) nanoprecipitates presages ultralow κ ∼ 2 Wm( -1) K(-1) at 1000 K. The high α∼ -300 μV K(-1) and high σ ∼ 1-10(4) Ω(-1 )m(-1) result from an offsetting of the nanostructuring-induced mobility decrease by high, and nondegenerate, carrier concentrations obtained via excitation from shallow Al donor states. The resultant ZT ∼ 0.44 at 1000 K is 50% higher than that for the best non-nanostructured counterpart material at the same temperature and holds promise for engineering advanced oxide-based high-ZT thermoelectrics for applications.

Phase formation and magnetotransport of alkali metal doped Na0.75CoO2 thermoelectric oxide

Synthesis and characterization of bulk NaxCoO2 samples substituted by K and Rb is reported. Phase formation studies revealed a narrow stable region for Na-alkali metal-Co system. Whisker and platelike single crystalline structures have been found to form on the surface of the pellets in case of K doping. All samples were metallic and no characteristic anomaly in R-T curves was observed for Rb doped sample. Magnetoresistance measured has a pronounced positive response only for K-doped and pure NaxCoO2 phases, reaching 11% and 7% at 5 K temperature, respectively.

Room-temperature ferromagnetism in Mn and Fe codoped In2O3

The synthesis and characterization of polycrystalline room-temperature ferromagnetic semiconductor (In0.9Fe0.1−xMnx)2O3 (x=0–0.1) oxide are reported. All of the samples with intermediate x values are ferromagnetic at room temperature. The highest saturation magnetization moment at 300K per total amount of magnetic ion is reached in the (In0.9Fe0.04Mn0.06)2O3 sample. The lattice constant a increases linearly with increasing Mn content. Fe-only doped samples were paramagnetic, while a Mn-only doped sample was found to be ferromagnetic below TC=46K.

High temperature ferromagnetism in Ni-doped In2O3 and indium-tin oxide

Observation of high temperature ferromagnetism in Ni-doped In2O3 and indium-tin-oxide (ITO) samples prepared by a solid state synthesis route is reported. Both Ni-doped compounds showed a clear ferromagnetism above 300K with the magnetic moments of 0.03–0.06μB∕Ni and 0.1μB∕Ni at 300 and 10K, respectively. Ni-doped In2O3 samples showed a typical semiconducting behavior with a room temperature resistivity of ρ∼2Ωcm, while Ni-doped ITO samples were metallic with ρ∼2×10−2Ωcm. Analysis of different conduction mechanisms suggested that variable range hopping model explains our ρ-T data for the Ni-doped In2O3 sample the best.

Very strong intrinsic flux pinning and vortex avalanches in (Ba,K)Fe2As2 superconducting single crystals

We report that the (Ba,K)Fe2As2 crystal with Tc =3 2 K shows a pinning potential, U0, as high as 104 K, with U0 showing very little field dependence. The (Ba,K)Fe2As2 single crystals become isotropic at low temperatures and high magnetic fields, resulting in a very rigid vortex lattice, even in fields very close to Hc2. The isotropic rigid vortices observed in the two-dimensional (2D) (Ba,K)Fe2As2 distinguish this compound from 2D high-Tc cuprate superconductors with 2D vortices. The vortex avalanches were also observed at low temperatures in the (Ba,K)Fe2As2 crystal. It is proposed that it is the K substitution that induces both almost isotropic superconductivity and the very strong intrinsic pinning in the (Ba,K)Fe2As2 crystal.

Tuning the Band Gap in Silicene by Oxidation

Silicene monolayers grown on Ag(111) surfaces demonstrate a band gap that is tunable by oxygen adatoms from semimetallic to semiconducting type. With the use of low-temperature scanning tunneling microscopy, we find that the adsorption configurations and amounts of oxygen adatoms on the silicene surface are critical for band gap engineering, which is dominated by different buckled structures in √13 × √13, 4 × 4, and 2√3 × 2√3 silicene layers. The Si-O-Si bonds are the most energy-favored species formed on √13 × √13, 4 × 4, and 2√3 × 2√3 structures under oxidation, which is verified by in situ Raman spectroscopy as well as first-principles calculations. The silicene monolayers retain their structures when fully covered by oxygen adatoms. Our work demonstrates the feasibility of tuning the band gap of silicene with oxygen adatoms, which, in turn, expands the base of available two-dimensional electronic materials for devices with properties that is hardly achieved with graphene oxide.

Preparation and characterization of hybrid conducting polymer-carbon nanotube yarn

Hybrid polypyrrole (PPy)-multi walled carbon nanotube (MWNT) yarns were obtained by chemical and electrochemical polymerization of pyrrole on the surface and within the porous interior of twisted MWNT yarns. The material was characterized by scanning electron microscopy, electrochemical, mechanical and electrical measurements. It was found that the hybrid PPy-MWNT yarns possessed significantly higher mechanical strength (over 740 MPa) and Youngs modulus (over 54 GPa) than the pristine MWNT yarn. The hybrid yarns also exhibited substantially higher electrical conductivity (over 235 S cm(-1)) and their specific capacitance was found to be in excess of 60 F g(-1). Measurements of temperature dependence of electrical conductivity revealed semiconducting behaviour, with a large increase of band gap near 100 K. The collected low temperature data are in good agreement with a three-dimensional variable range hopping model (3D-VRH). The improved durability of the yarns is important for electrical applications. The composite yarns can be produced in commercial quantities and used for applications where the electrical conductivity and good mechanical properties are of primary importance.

Silicene: A Promising Anode for Lithium-Ion Batteries

Silicene, a single-layer-thick silicon nanosheet with a honeycomb structure, is successfully fabricated by the molecular-beam-epitaxy (MBE) deposition method on metallic substrates and by the solid-state reaction method. Here, recent progress on the features of silicene that make it a prospective anode for lithium-ion batteries (LIBs) are discussed, including its charge-carrier mobility, chemical stability, and metal-silicene interactions. The electrochemical performance of silicene is reviewed in terms of both theoretical predictions and experimental measurements, and finally, its challenges and outlook are considered.

The mechanical and the electrical properties of conducting polypyrrole fibers

The mechanical and the electrical properties of polypyrrole (PPy) fibers and electrochemically deposited PPy films were studied. It was found that the PPy fibers showed a significantly higher strength than the PPy films due to better orientation of the molecular structure. The electrochemically prepared PPy films had a higher electrical conductivity than that of the fibers at high temperature. At low temperature, the PPy fibers showed the higher conductivity. The conductivity results were analyzed in the frame of the three-dimensional variable range hopping model. The results showed that at room temperature the average hopping distance for the fibers was about 4 A while for the films it increases to about 5.7 A. This corresponds to about 1 and 2 monomer units in length for the fiber and film samples, respectively.

Magnetic and Transport Properties of Transition Metal Doped Polycrystalline In

Synthesis and characterization of transition metal (TM)-doped In ₂O₃ oxide is reported. Most of the samples were found to be paramagnetic, and only Fe and Cr co-doped sample showed as light trace of ferromagnetism at 300 K with saturation magnetization M _s= 0.35 emug/g. Measured transport properties revealed significant differences in transport among the samples. The absolute value of electrical resistivity for the In_1.8Fe_0.1Mn_0.1O₃ sample at 300 K was rho=9.4times10³ Omega middot cm, while In _1.8Fe_0.1Cr_0.1O₃ had rho=62 Omega middot cm at the same temperature. Furthermore, magnetoresistance (MR) studies of In_1.8Fe_0.1Cr_0.1O₃ showed that the sign of MR changes from negative (200 K), with MR_(200K)=-0.2%, to positive T< 50 K reaching maximum absolute value at 10 K, i.e., MR_(10K)=5.2%