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  • 1
    ISSN: 1432-1130
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Basic investigations on the electrochemical behavior of alumosilicate zeolites were conduced in gas atmosphere and aqueous solution using impedance spectroscopy under in situ conditions where “zeolitically” bound water molecules are present in the channel and cage system. Natural stilbite (STI) and heulandite (HEU), zeolites of structure types with the same 4 – 4 = 1 building units, were used for these investigations. At a given temperature, well defined water partial pressures were applied in the gas atmosphere (in situ conditions) and the effect of polar organic molecules on conductivity was investigated. Furthermore, the effect of complete water saturation of the channel system on the Arrhenius type activation energy of conductivity was investigated in aqueous solution in comparison. The ion-exchange behavior of zeolite materials could be monitored under these conditions. These results show that, dependent on the different parameters affecting the zeolite conductivity behavior, they are applicable in the field of electrochemical analysis if these basic results on single crystals are transferable to polycrystalline materials.
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  • 2
    ISSN: 1433-0768
    Keywords: Key words Lithium nickelate ; Solid state electrochemisty ; Proton intercalation ; Voltammetry ; Immobilized microparticles
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Lithium nickelate was synthesized by self-propagating high-temperature combustion. The electrochemical behaviour of the product was studied by cyclic voltammetry of microparticles immobilized on the surface of graphite electrodes. Whereas numerous previous studies have dealt with non-aqueous electrolyte solutions, here the behaviour of lithium nickelate in contact with aqueous electrolyte solutions was investigated. It could be shown that protons are intercalating upon reduction of the Ni(III) to Ni(II) and deintercalating upon oxidation. This insertion electrochemistry is chemically reversible. Within 1600 oxidation-reduction cycles, the response diminished only by about 10%. Scanning electron microscopy reveals a considerable recrystallization during the electrochemical cycles. Energy dispersive X-ray detection proved that no metal cations are intercalating. The electrochemical system is accessible only in very alkaline solutions as it shifts to more positive values with decreasing pH.
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  • 3
    ISSN: 1862-0760
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The conductivity behavior of the natural single crystal zeolites ANA, BRE, CHA, HEU, PHI and STI was investigated in the temperature range between 60 °C and 110 °C. The effect of combustible gas components on the conductivity was also investigated using constant water vapor pressure conditions. In the temperature range studied, the conductivity is assumed to be mainly ionic and zeolitic water remains within the channel and cage system of the structures. All zeolites do have different frameworks, channel systems, channel sizes, channel cations and water contents. Dependent on the zeolite type, water partial pressures and combustible gas components cause different effects on the conductivity which are correlated with the geometry, charge distribution and occupation of the channels and cages of the respective zeolite single crystal.
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  • 4
    ISSN: 1862-0760
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Natural clays were characterized by XRD, TGA and SEM/EDX, then pressed to pellets and analyzed by impedance measurements under various defined gas compositions and temperatures. From the Nyquist plot, conductivity values were calculated for the different clay samples. A strong dependence of the conductivity on the water vapour concentration at low temperatures was found for all investigated clay samples. Partial substitution of water by methanol led to a decrease in conductivity at low temperatures, confirming the essential role water plays in the conductivity mechanism at these temperatures. Increasing the temperature resulted in a decrease in conductivity, until a turn-over point was reached. Beyond this temperature an Arrhenius behaviour was observed, suggesting a change in the conductivity mechanism.
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  • 5
    ISSN: 1862-0760
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Sensors with solid electrolytes provide the possibility of correct and fast measurements of partial pressures of various gases. By modification of the solid electrolyte, sensors with specific performances may be manufactured. Layers of Na+-, Li+-, Ca2+- and Sr2+-β“-alumina on top of polycrystalline α-alumina substrates were produced by an in-situ formation process and were used as solid electrolytes for CO2 and SOx sensors. Carbonates and sulphates were applied as measuring electrodes and oxidic mixtures of SiO2 and silicates were used as reference electrodes. The different performance of these sensors was investigated over a wide temperature range and the results were compared with theoretical data. Different solid electrolyte / electrode combinations were applied, which all showed different characteristic cross sensitivities against water and organic components.
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  • 6
    ISSN: 1862-0760
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Investigations on lanthanum gallium chromium mixed oxides of the compositions La1.0Ga0.2Cr0.8O3−d and La0.9Ga0.2Cr0.8O3-d are presented regarding their structure, redox stability, conductivity and catalytic activity for the oxidation of propene. The mixed oxide has perovskite-type structure, high redox stability, an electronic p-type conductivity and low catalytic activity. If the perovskite-type compound has a deficiency of lanthanum, the electronic conductivity, surface area and catalytic activity are significantly higher. The catalytic activity is likely comparable to that of gold. Similar to gold electrodes of solid electrolytes, oxygen electrodes formed with gallium doped lanthanum chromite show a relatively high sensitivity to hydrocarbons such as propene at temperatures of about 700 °C. The mixed oxide is possibly suitable as electrode material for exhaust gas sensors using oxide-ion conducting solid electrolytes.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Ionics 2 (1996), S. 266-273 
    ISSN: 1862-0760
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Alkali-ion conducting glasses/glass ceramics of the system Me2O-A12O3-SiO2 (Me=Li, Na) were applied as solid electrolytes in potentiometric gas sensors to detect CO2 in the presence of O2 at increased temperatures. The corresponding Me-Carbonates were utilized as auxiliary electrodes. Sensors using the direct Au-glass contact as a kind of reference electrode (type I), as well as symmetrical sensors with carbonate phase at the reference and measuring electrode (type II - for comparative measurements) were manufactured. By applying Au as electrode metal, the theoretically expected EMF difference and the observed EMF difference of both sensor types agree quite well with the expected values according to the Nernst equation between 500 and 600 °C (over four orders of magnitude of CO2 partial pressure (10−5 – 10−1 bar) at constant O2 partial pressure (2.1×10−1 bar)). A long time stability of 120 days for sensors of type I with Li glasses has been observed, although evaporation of carbonate phase (Li2CO3) was detected under the conditions of sensor application. Sensors of type I (with Li2CO3) show thermodynamically unexpected cross-sensitivities to H2O.
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  • 8
    ISSN: 1862-0760
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Potentiometric CO2 gas sensors with Li conducting glasses/glass ceramics of the system Li2O-Al2O3-SiO2 (different nominal composition) as solid electrolytes have been investigated. Li2CO3 was used as CO2 and O2 sensitive auxiliary electrode. During the sensor test measurements, the CO2 partial pressure was varied between 1×10−3 and 1×10−1 bar at a constant O2 partial pressure of 2.1×10−1 bar whereas N2 was used as carrier gas. Comparative measurements were accomplished with sensors comprising Na and K ion conducting glasses. A metastable reference electrode was formed at the contact zone between the Au metal electrode and the former Li glasses of definite nominal composition by crystallization processes taking place, which lead to stable, reproducible CO2 dependent EMF signals for more than 90d. The thermodynamically expected EMF difference and the observed EMF difference agree quite well between 500 and 600 °C. At 600 °C, the drift of sensors with glasses as solid electrolytes and direct Au glass/glass ceramics contact as reference electrode amounts typically 0.32 mV/d (p(CO2)=1×10−3 bar, p(O2)=2.1×10−1 bar at the measuring electrode), if a metastable multiphase equilibrium is formed. At identical partial pressures of CO2 and O2, the signal reproducibility of these sensors with different solid electrolyte glasses of the same nominal composition lies within 30 mV at 600 °C.
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  • 9
    ISSN: 1862-0760
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The in-situ formation of layers consisting of β′'-alumina on high purity polycrystalline α-alumina substrates was investigated. The direct synthesis of sodium and potassium β′'-alumina layers was possible in a mixture of uncalcined powders of stoichiometries close to the β′'-alumina phase in a temperature-time window between 1200 and 1400 °C and 1 h to 12 h. These materials were employed for ion exchange processes in order to obtain Li+, H3O+ and Ca2+-β′'-alumina modifications, which could not be synthesized by the direct method. The resulting materials were characterized by SEM/EDX, XRD, DTA and IS. The obtained β′'-alumina layers show the same properties as polycrystalline monolithic ceramics but are available as thin films of thicknesses between 4 and 25 µm on insulating, well defined substrate materials.
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Ionics 3 (1997), S. 282-288 
    ISSN: 1862-0760
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Single crystals of Heulandite- (HEU) type zeolites were characterized for phase homogeneity (XRD), morphology, chemical composition (SEM/EDX) and thermal stability (DTA). Afterwards the crystals were ion-exchanged against Ag and Cu in order to obtain defined starting materials for sensor applications. AC conductivity type sensors were constructed using ion-exchanged stilbite (HEU-type zeolite) as gas sensitive solid electrolyte with sputtered Au as electrode material. Temperature dependent conductivity measurements showed activation energies for the ionic motion between 0.54 and 0.76 eV. Ag-ion exchanged stilbites were applied as gas sensing materials in the temperature interval between 60 °C and 105 °C under defined H2O concentrations and revealed stable signals with regard to different gaseous aliphatic components (mixtures with pure nitrogen or synthetic air). It could be shown that the sensor behaviour was affected by the polarity of the aliphatic molecules and their specific interaction with the ions and molecules of the channels within the zeolites lattice.
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