Turgut M. Gür

Permselectivity of zeolite filled polysulfone gas separation membranes

Abstract Gas permeabilities of molecular sieve 13X filled polysulfone and unblended polysulfone membranes fabricated by a melt extrusion process were measured by a constant volume technique at room temperature. The relative permeation rates of the industrially important gases were found to be in the order H 2 ⪢He⪢CO 2 ⪢O 2 ⪢CH 4 ≈N 2 .

Three-Dimensional Nanostructured Bilayer Solid Oxide Fuel Cell with 1.3 W/cm 2 at 450 °C

Obtaining high power density at low operating temperatures has been an ongoing challenge in solid oxide fuel cells (SOFC), which are efficient engines to generate electrical energy from fuels. Here we report successful demonstration of a thin-film three-dimensional (3-D) SOFC architecture achieving a peak power density of 1.3 W/cm(2) obtained at 450 °C. This is made possible by nanostructuring of the ultrathin (60 nm) electrolyte interposed with a nanogranular catalytic interlayer at the cathode/electrolyte interface. We attribute the superior cell performance to significant reduction in both the ohmic and the polarization losses due to the combined effects of employing an ultrathin film electrolyte, enhancement of effective area by 3-D architecture, and superior catalytic activity by the ceria-based interlayer at the cathode. These insights will help design high-efficiency SOFCs that operate at low temperatures with power densities that are of practical significance.

Properties of (Ti1−xAlx)N coatings for cutting tools prepared by the cathodic arc ion plating method

(Ti,Al)N coatings with several Ti/Al ratios were deposited on Si and WC–Co substrates by the cathodic arc ion plating method. The crystal structure was found to be cubic B1 type, which is isostructural with the titanium‐based nitrides. Morphology of the (Ti0.4Al0.6)N and TiN films was investigated, using transmission and scanning electron microscopy. The films seemed to have a less columnar structure than expected. The (Ti,Al)N films containing less than 75 mol % AlN had higher hardness than for the pure TiN films. Similarly, (Ti,Al)N coatings displayed superior oxidation resistance at elevated temperatures. Overall, (Ti,Al)N coated cutting tools offer significantly better performance characteristics compared to the tools with TiN coatings.

Oxygen chemical diffusion in strontium doped lanthanum manganites

Abstract The chemical diffusion coefficient of oxygen in (La 0.79 Sr 0.20 )MnO 3−δ and (La 0.50 Sr 0.50 )MnO 3−δ was measured in a solid state electrochemical cell using the potentiostatic step method. In the temperature range between 700°C and 860°C, the chemical diffusion coefficient of oxygen in (La 0.79 Sr 0.20 )MnO 3−δ was found to be in the range of 10 −8 to 10 −6 cm 2 /s and also strongly dependent on the oxygen partial pressure between 0.21 atm and 10 −8 atm. For the case of the stoichiometric oxygen content, the activation energy for oxygen chemical diffusion in this material was determine to be about 70 kJ/mole. Similar measurements between 700°C and 840°C using samples of the composition (La 0.50 Sr 0.50 )MnO 3−δ yielded comparable diffusion coefficient values. Such rapid oxygen transport in these materials make them attractive for high temperature fuel cell electrode and gas separation membrane applications. They also exhibited unusually high values of the order of 10 3 to 10 4 for the thermodynamic enhancement factor indicative of narrow oxygen nonstoichiometry.

Conversion of solid carbonaceous fuels in a fluidized bed fuel cell

A fluidized bed direct carbon fuel cell was employed to achieve direct conversion of solid fuels into electricity. Power was generated from pulverized Lower Kittanning (bituminous) coal, synthetic carbon, and biomass in a single process step. Current-voltage characteristics exhibited typical fuel cell behavior. Fluidization in flowing CO{sub 2} overcomes the difficulty of attaining solid fuel-to-anode contact and generates CO in situ via the Boudouard reaction. A mechanistic reaction pathway is proposed for anodic oxidation of the solid fuel. Conversion was verified by gas analysis of oxidation products in the flue stream and by oxygen mass balance.

High ionic conductivity in ultrathin nanocrystalline gadolinia-doped ceria films

High ionic conductivities were observed in ultrathin nanocrystalline gadolinia-doped ceria (GDC) films with thicknesses comparable to the grain size (20–50nm). Conductivities were determined to be effectively three to four orders of magnitude higher relative to those of thicker films (>500nm). The distinct properties in ultrathin GDC films were attributed to the reduction of cross grain boundary resistance and the segregation of the Gd dopants in the vicinity of grain boundaries.

Steady‐State D‐C Polarization Characteristics of the O 2, Pt/Stabilized Zirconia Interface

Four‐probe d‐c polarization studies were carried out on the Pt/stabilized zirconia interface between 600° and 900°C in the presence of 10−6–1 atm of oxygen partial pressure. Three types of polarization effects were observed. At high currents, the oxygen flux density is limited by diffusion of molecular oxygen in the gas phase through a boundary layer near the interface. At intermediate currents, porous Pt electrodes are almost completely non‐blocking to oxygen, and the primary impedance present is due to ionic resistivity in the electrolyte. At low overvoltages, a limiting current plateau is observed at 600° and 700°C, which disappeared completely at 800°C and above. Two alternative mechanisms involving slow diffusion of either atomic oxygen or an electronic species are proposed to account for the observed behavior.

Structure and properties of (Ti1−xAlx)N films prepared by reactive sputtering

(Ti1−xAlx)N films with several different Ti-to-Al ratios were deposited on silicon substrates by reactive cosputtering from Ti and Al targets in an ArN2 discharge. The crystal structure of the films was found to be cubic B1 type up to 50 mol.% A1N content. With further increase in the A1 content, the films of (Ti0.31Al0.69)N and (Ti0.29Al0.71)N were found to have mixed phases of hexagonal and cubic structures. The (Ti1−xAlx)N films had a well-developed cubic columnar structure with open voids which rendered lower hardness values than for stoichiometric single-crystal TiN.

A new class of oxygen selective chemically driven nonporous ceramic membranes. Part I. A-site doped perovskites

The transport properties of a new class of inorganic nonporous membranes based on mixed-conducting perovskites were measured by a solid state electrochemical technique. Since oxygen is the only ionic species that is transported, these membranes provide absolute selectivity for oxygen. The chemical diffusion coefficient of oxygen in (La0.79Sr0.20)MnO3−δ and (La0.50Sr0.50)MnO3−δ was found to be of the order of 10−8 to 10−6 cm2/sec between 700°C and 860°C. Such high transport rates open up exciting opportunities for chemically driven membrane applications for the selective separation or purification of oxygen. The oxygen permeation rates calculated for modest oxygen pressure differential values across 1 mm thick (La0.79Sr0.20)MnO3−δ membranes are about 10−11 mol/sec-cm2 at 700°C and close to 10−9 mol/sec-cm2 at 860°C. These values provide an attractive alternative to polymeric and porous ceramic membranes, all of which have only limited oxygen selectivity.

Electrochemical nanopatterning of Ag on solid-state ionic conductor RbAg4I5 using atomic force microscopy

This report introduces an electrochemical nanopatterning technique performed under ambient conditions without involving a liquid vessel or probe-to-sample material transfer. Patterning is accomplished by solid-state electrochemical nanodeposition of Ag clusters on the surface of the solid ionic conductor RbAg4I5 using an atomic force microscopy probe. Application of negative voltage pulses on the probe relative to an Ag film counter electrode on an RbAg4I5 sample induces nanometer-sized Ag deposition on the ion conductor around the probe. The patterned Ag particles are 0.5–70nm high and 20–700nm in diameter. The effect of the amplitude and duration of bias voltage on the size and shape of deposited Ag clusters is also shown.