Gerhard Tomandl

Synthesis, structure and electrochemical properties of In-doped BaCeO3

Abstract Samples in the system BaCe 1− x In x O 3− α were prepared in a concentration range 0.05 mole≤ x ≤0.25 mole InO 1.5 and the physical–chemical properties determined. The maximum doping level is 0.20 mole InO 1.5 in BaCeO 3 for which almost single phase products were obtained. The doped barium cerates correspond with the structure of BaCeO 3 with a small amount of CeO 2 . The compounds are stable in reducing and oxidizing atmospheres and have a high solubility of oxygen. The In-doped BaCeO 3 shows a point defect behaviour in the oxygen pressure range from 1 bar to 10 −6 bar. The highest total electrical conductivity was observed for a content of 0.20 mole InO 1.5 in BaCeO 3 . It is influenced by the water vapour partial pressure in reducing and inert gases in the temperature range from 700°C to 1000°C. The electrical conductivity of In-doped BaCeO 3 does not exceed the conductivity of yttria-stabilized zirconia and not even that of the brownmillerite-structure oxides Ba 2 In 2− x Ce x O 5 . The mean ionic transport number is t ion =1 in hydrogen between 630°C and about 800°C whereas above 800°C a deviation from 1 is detected. In argon larger deviations from 1 are observed in the whole temperature range.

Studies on oxidative coupling of methane using high-temperature proton-conducting membranes

Abstract A solid oxide proton conducting SrCe0.95Yb0.05O3−x (SCYO) membrane has been applied to methane oxidative coupling in a laboratory membrane reactor using disc-shaped membrane specimen equipped with porous silver electrodes. The investigations were carried out at 750°C and atmospheric pressure and covered variations of feed gases (methane versus helium or air), water vapor pressure, and electrical current. Under the conditions tested, the SCYO-membrane was found to exhibit both substantial oxide ion as well as proton conductivity and electronic conductivity as well. The addition of small amounts of water on the air side, up to 25 mbar partial pressure, led to a significant decrease of conversion at virtually constant C2-selectivity whereas the addition of small amounts of water on the hydrocarbon side caused methane steam reforming, producing a three- to fourfold increase of conversion while sharply decreasing C2-selectivity. By applying up to 10 mA current to the cell, the rate of methane consumption could be enhanced to about three times the open circuit rate, however, associated with a small reduction of C2-selectivity.

Mineralized scaffolds for hard tissue engineering by ionotropic gelation of alginate

Alginates form gels with tube-like pores when covered with solutions of di- or trivalent cations. This phenomenon also referred to as ionotropic gelation has been known for more than 30 years. By mixing a calcium phosphate powder and an alginate as the starting material, the mineral phase of bone is incorporated. Such porous structures can be used for scaffolds in hard tissue engineering. The starting materials and stabilizing additives are dispersed in an aqueous solution. Then a solution of Ca-ions is deposited onto the surface of the slurry. The slurry can be gelled by ion exchange of Na-ions in the alginate with Ca-ions. A primary thin gel layer with the function of a membrane is immediately formed. By diffusional control of cation transport through the membrane, the slurry gradually transforms to the gel forming tube-like pores in direction of cation diffusion. Like the gelation of pure alginate the concentration of electrolyte and the kind of cations and anions influence the size (diameter and length) and size growth of the pores, but the tolerance in the preparation conditions is much smaller. The diameters of the pores can be adjusted between 50 and 500 m which fits the optimum size for cell seeding and blood capillary ingrowth very well. By selecting the proper drying method the inherent shrinkage can be controlled. Hydroxyapatite sintered at high temperatures loses the ability to be resorbed by osteoclasts in vivo. Therefore, we have developed scaffolds with channel-like pores from alginate/calcium phosphate composites without the necessity for heating them to higher temperatures.

Simulation and investigation of pore size graded titania layers during sintering

The topic of this work is the investigation of the sintering process of thin self-supporting ceramic layers with a gradient in the particle size and pore size distribution. These graded titania layers were produced by zentrifugal deposition from mixed sol powder mixtures with wide particle size distributions. The graded structure causes strong deformations of the layers during the drying and the sintering step. Finite element simulations were used to describe the deformation behaviour during sintering. The results were compared with the parpage of real layers. Furthermore, a possibility to suppress a sintering deformation is shown with the help of experimental and simulated data. The sintered layers were charakterized by SEM, AFM, gas adsorption and roughness measurements. Quantitative image analysis of polished cross-sections and AFM investigations of ion beam cutted slopes were used for detecting the run of the pore size gradient within the layers.

Preparation of Al2O3, TiO2, and hydroxyapatite ceramics with pores similar to a honeycomb structure

Alginate gelation is one of few methods that can produce porous ceramics with oriented tubular pores. This process allows the production of structured alumina, titania, and hydroxyapatite ceramics with an approximately honeycomb structure as demonstrated by the authors. The sizes of the capillaries can be adjusted by changing the material, the electrolyte, and the sintering temperature. Serial sectioning and automatic image analysis are suitable methods for investigating the capillary structure of the samples.

Drying of Ceramic Layers with a Graded Pore Structure

Thin self-supporting ceramic TiO2-layers with a graded pore structure were prepared by using centrifugal deposition of powders and sols with different particle size distributions from mixed, diluted suspensions. During the evaporation drying step the layers have a strong tendency to warping and crack-formation because of the resulting difference in the capillary pressure in the upper and the bottom side pores. Four drying methods were investigated concerning their suitability for diminishing or eliminating capillary forces and for the production of planar, crack-free dried specimens. The drying techniques used in the experiments are briefly introduced. It should be emphasized that the most successful drying method for the layers mentioned above is a combination of microwave drying and subsequent critical point drying.

Electrical conductivity of polycrystalline (CeO2)0.80(LaO1.5)0.20 with a small amount of (LaF3)0.95(SrF2)0.05

Abstract Adding 1 wt.% of the fluorine-ion conducting (LaF 3 ) 0.95 (SrF 2 ) 0.05 to polycrystalline oxygen-ion conducting (CeO 2 ) 0.80 (LaO 1.5 ) 0.20 solid solution, the effect on the electrical conductivity is studied as a function of sintering temperature. Highly sinterable (CeO 2 ) 0.80 (LaO 1.5 ) 0.20 starting powder was fabricated by co-precipitation using oxalate method and followed by a hydrothermal treatment. The oxide conductivity was measured by DC and AC four-probe techniques at high oxygen partial pressures between 0.21 and 1 bar. The polycrystalline microstructure of the sintered specimens was investigated by transmission electron microscopy (TEM). The oxide conductivity is slightly increased by the addition of 1 wt.% (LaF 3 ) 0.95 (SrF 2 ) 0.05 to polycrystalline (CeO 2 ) 0.80 (LaO 1.5 ) 0.20 , provided the specimens were sintered above 1500 °C. At sintering temperatures above 1700 °C, (LaF 3 ) 0.95 (SrF 2 ) 0.05 was detected at the grain boundaries of the polycrystalline (CeO 2 ) 0.80 (LaO 1.5 ) 0.20 . Obviously, the (LaF 3 ) 0.95 (SrF 2 ) 0.05 generates ion-conducting pathways between the (CeO 2 ) 0.80 (LaO 1.5 ) 0.20 grains causing a higher grain boundary conductivity.

Oxygen exchange behaviour and electrical conductivity of polycrystalline oxide-ion conductors based on Ce1-nLanO2-0.5n

Abstract Oxide-ion conductors are used in sensors, oxygen pumps and solid oxide fuel cells. However, their range of application is limited by a low oxide-ion conductivity in the solid electrolyte presently commercially available. The electrical conductivity and the oxygen exchange behaviour of polycrystalline ceria–lanthania specimens with different grain boundary areas have been studied in an attempt to improve the grain boundary conductivity. The results show that a large increase in the grain boundary conductivity was obtained. The thermodynamic properties of the oxygen nonstoichiometric defect reaction of Ce 1− n La n O 2−0.5 n − x solid solutions were measured by coulometric oxygen titration.

Characterization of Thin Ceramic Layers with a Graded Pore Structure

Thin ceramic layers with a gradient in the grain size and the pore size distribution were prepared by centrifugal deposition fron diluted suspensions with a very low solid content and a multimodal particle size distribution. Besides drying and sintering of the sediments, a main problem is the detection of the gradient which can be expected to appear owing to the settling behaviour of the particles during centrifugation. For this reason, the layers were characterized by quantitative image analysis, atomic force microscopy (AFM) and grazing incidence small angle x-ray scattering (GISAXS).

Development of Non-Silicate Microporous Membranes with High Chemical Resistance

TiO2-ZrO2 mixed-oxide membranes were prepared via polymeric sol-gel technique. The final composition is already predefined during the preparation of the sols by using the corresponding quantities of the starting alkoxides. The mixed-oxide membranes were characterized by Xray diffraction, nitrogen sorption, and field emission scanning electron microscopy as well as filtration and corrosion tests. The observed results prove the positive influence on the investigated properties by addition of a second oxide. Depending on the composition of the mixed-oxide membranes, a remarkable increase in the crystallization temperatures is demonstrated. Using mixed-oxide systems, the retardation of grain growth is confirmed.