Influence of Carrier Gas Composition on the Stress of Al2O3 Coatings Prepared by the Aerosol Deposition Method

Al2O3 films were prepared by the aerosol deposition method at room temperature using different carrier gas compositions. The layers were deposited on alumina substrates and the film stress of the layer was calculated by measuring the deformation of the substrate. It was shown that the film stress can be halved by using oxygen instead of nitrogen or helium as the carrier gas. The substrates were annealed at different temperature steps to gain information about the temperature dependence of the reduction of the implemented stress. Total relaxation of the stress can already be achieved at 300 °C. The XRD pattern shows crystallite growth and reduction of microstrain while annealing

Half-Cell Potential Analysis of an Ammonia Sensor with the Electrochemical Cell Au | YSZ | Au, V2O5-WO3-TiO2

Half-cell potentials of the electrochemical cell Au, VWT | YSZ | Au are analyzed in dependence on oxygen and ammonia concentration at 550 °C. One of the gold electrodes is covered with a porous SCR catalyst, vanadia-tungstenia-titania (VWT). The cell is utilized as a potentiometric ammonia gas sensor and provides a semi-logarithmic characteristic curve with a high NH(3) sensitivity and selectivity. The analyses of the Au | YSZ and Au, VWT | YSZ half-cells are conducted to describe the non-equilibrium behavior of the sensor device in light of mixed potential theory. Both electrode potentials provide a dependency on the NH(3) concentration, whereby VWT, Au | YSZ shows a stronger effect which increases with increasing VWT coverage. The potential shifts in the anodic direction confirm the formation of mixed potentials at both electrodes resulting from electrochemical reactions of O(2) and NH(3) at the three-phase boundary. Polarization curves indicate Butler-Volmer-type kinetics. Modified polarization curves of the VWT covered electrode show an enhanced anodic reaction and an almost unaltered cathodic reaction. The NH(3) dependency is dominated by the VWT coverage and it turns out that the catalytic properties of the VWT thick film are responsible for the electrode potential shift

A Laboratory Test Setup for in Situ Measurements of the Dielectric Properties of Catalyst Powder Samples under Reaction Conditions by Microwave Cavity Perturbation : Set up and Initial Tests

The catalytic behavior of zeolite catalysts for the ammonia-based selective catalytic reduction (SCR) of nitrogen oxides (NOX) depends strongly on the type of zeolite material. An essential precondition for SCR is a previous ammonia gas adsorption that occurs on acidic sites of the zeolite. In order to understand and develop SCR active materials, it is crucial to know the amount of sorbed ammonia under reaction conditions. To support classical temperature-programmed desorption (TPD) experiments, a correlation of the dielectric properties with the catalytic properties and the ammonia sorption under reaction conditions appears promising. In this work, a laboratory test setup, which enables direct measurements of the dielectric properties of catalytic powder samples under a defined gas atmosphere and temperature by microwave cavity perturbation, has been developed. Based on previous investigations and computational simulations, a resonator cavity and a heating system were designed, installed and characterized. The resonator cavity is designed to operate in its TM010 mode at 1.2 GHz. The first measurement of the ammonia loading of an H-ZSM-5 zeolite confirmed the operating performance of the test setup at constant temperatures of up to 300 °C. It showed how both real and imaginary parts of the relative complex permittivity are strongly correlated with the mass of stored ammonia

Thermopower and conductivity of aerosol deposited BaFe1-xTaxO3-δ films

The thermopower, also known as Seebeck coefficient, is an important parameter to determine constants for defect chemical models of semiconductor materials since it is a measure for the charge carrier concentration. The Seebeck coefficient does not depend on the geometry of the material, like the often investigated conductivity. BaFe0.7Ta0.3O3-δ is known as a temperature independent conductometric oxygen sensor material with perovskite crystal structure. The present work considers the thermopower as well as the electrical conductivity of BaFe1-xTaxO3-δ for x in the range of 0.1 to 0.45 (BFTx). Seebeck coefficient and conductivity were measured simultaneously between 400 and 850°C under varying oxygen partial pressures from 10-2 to 1 bar. BFTx fine powders have been prepared by conventional mixed-oxide route and were calcined at 1350°C. Crystal structure and phase purity were investigated by X-ray diffraction. BFTx thick-films have been successfully deposited by the novel Aerosol Deposition Method (ADM) at room temperature on a special transducer. This well designed transducer includes four platinum electrodes and two gold-platinum thermocouples. Both, thermopower and electrical conductivity, of samples with tantalum contents between x=0.2-0.3 show almost no temperature dependency but depend strongly on the oxygen partial pressure in the temperature range from 700 to 850°C. Results are shown for sample BFT20 in figure 1 and figure 2. All samples have a positive Seebeck coefficient and no n-p-type transition was observed in the investigated oxygen partial pressure range. In contrast to the electrical conductivity, the Seebeck coefficient of all samples decreases with increasing oxygen partial pressure. An initial defect chemical model of BFTx will be presented in this study as well. Please click Additional Files below to see the full abstract

Platform for a Hydrocarbon Exhaust Gas Sensor Utilizing a Pumping Cell and a Conductometric Sensor

Very often, high-temperature operated gas sensors are cross-sensitive to oxygen and/or they cannot be operated in oxygen-deficient (rich) atmospheres. For instance, some metal oxides like Ga2O3 or doped SrTiO3 are excellent materials for conductometric hydrocarbon detection in the rough atmosphere of automotive exhausts, but have to be operated preferably at a constant oxygen concentration. We propose a modular sensor platform that combines a conductometric two-sensor-setup with an electrochemical pumping cell made of YSZ to establish a constant oxygen concentration in the ambient of the conductometric sensor film. In this paper, the platform is introduced, the two-sensor-setup is integrated into this new design, and sensing performance is characterized. Such a platform can be used for other sensor principles as well

Effect of Oxygen Partial Pressure on the Phase Stability of Copper-Iron Delafossites at Elevated Temperatures

Oxide-based materials are promising candidates for use in high temperature thermoelectric generators. While their thermoelectric performance is inferior to commonly used thermoelectrics, oxides are environmentally friendly and cost-effective. In this study, Cu-based delafossites (CuFeO2), a material class with promising thermoelectric properties at high temperatures, were investigated. This work focuses on the phase stability of CuFeO2 with respect to the temperature and the oxygen partial pressure. For this reason, classical material characterization methods, such as scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction, were combined in order to elucidate the phase composition of delafossites at 900 °C at various oxygen partial pressures. The experimentally obtained results are supported by the theoretical calculation of the Ellingham diagram of the copper–oxygen system. In addition, hot-stage X-ray diffraction and long-term annealing tests of CuFeO2 were performed in order to obtain a holistic review of the phase stability of delafossites at high temperatures and varying oxygen partial pressure. The results support the thermoelectric measurements in previous publications and provide a process window for the use of CuFeO2 in thermoelectric generators