No abstract

We describe the results of two-dimensional finite difference analysis of the thermal profile, in both transient and steady state, of a symmetric U-shape designed high-sensitive nanocalorimeter. The thin film calorimeter, with a heat capacity of 100 nJ K−1 at room temperature, consists of a 180 nm thick freestanding silicon-rich nitride membrane on which thin film heaters and sensors are deposited. Simulated temperature profiles are in good agreement with in situ experimental data obtained at the heater and sensor locations. The first-order solid-to-liquid transition of indium films, from a few Å to hundreds of nm thick, was used as an experimental reference of the thermal profiles obtained from the 2D modeling. Temperature differences inside the sample region induced by the symmetric U-shape design of the Pt heaters limit the use of the nanocalorimeter to two different heating rate regimes. At low heating rates, β < 10 K s−1, especially with a thermal layer, the temperature profile is reasonably flat so that small samples can be characterized in power compensation mode. At heating rates faster than 4 × 104 K s−1 the nanocalorimeter works in adiabatic mode and measures transitions occurring in the sample directly loaded underneath the heater.

No abstract

In the present paper, the ionic conductivity and the dielectric relaxation properties on the poly(vinyl alcohol)-CF 3 COONH 4 polymer system have been investigated by means of impedance spectroscopy measurements over wide ranges of frequencies and temperatures. The electrolyte samples were prepared by solution casting technique. The temperature dependence of the sample's conductivity was modeled by Arrhenius and Vogel-Tammann-Fulcher (VTF) equations. The highest conductivity of the electrolyte of 3.41×10 −3 ( cm) −1 was obtained at 423 K. For these polymer system two relaxation processes are revealed in the frequency range and temperature interval of the measurements. One is the glass transition relaxation (α-relaxation) of the amorphous region at about 353 K and the other is the relaxation associated with the crystalline region at about 423 K. Dielectric relaxation has been studied using the complex electric modulus formalism. It has been observed that the conductivity relaxation in this polymer system is highly nonexponential. From the electric modulus formalism, it is concluded that the electrical relaxation mechanism is independent of temperature for the two relaxation processes, but is dependent on composition.

No abstract

No abstract

No abstract

We performed a detailed study of the structure and transport properties of Bi2212/22Y2/2212 planar-type tunnel junctions. Both high-temperature superconducting electrodes and semiconducting barriers are highly epitaxial thin films deposited onto SrTiO3 single-crystal (001) substrates. Deposition of the films was carried out by a high oxygen pressure dc-sputtering technique, which produces high-quality epitaxial thin films, as determined by x-ray diffraction, lattice resolution transmission electron microscopy, and Rutherford backscattering. Critical temperatures for the superconducting electrodes of 85 K were determined by transport measurements (ρ and χ versus T). A study of resistivity as a function of temperature of the semiconducting barriers was performed. Clear quasiparticle tunneling indicating a gap structure at about 30–35 mV, a zero-bias peak, as well as linear and flat background at high voltages have been observed. For junctions with very thin barriers weak-link-type behavior was observed. An analysis of the I–V curves for these junctions has been made based on the resistively shunted junction model.