W. Weppner

Low and high temperature TiO2 oxygen sensors

We have studied two types of TiO2-based oxygen sensors operating at different temperatures with different detection principles. At high temperatures, TiO2 devices can be used as thermodynamically controlled bulk defect sensors to determine oxygen over a large range of partial pressures. Their intrinsic behaviour can be controlled by carefully directed doping with tri- or pentavalent cations. At low temperatures, we find that Pt/TiO2 Schottky diodes make extremely sensitive oxygen detection possible. The latter show reversible shifts of current-voltage curves, which are determined by interface states formed by chemisorbed oxygen.

Solid-state electrochemical gas sensors

Abstract The basic principles of operation of gas sensors based on solid-state galvanic cells are described. The generation of potentials and currents as a function of the partial gas pressures is considered for three types of solid electrochemical sensors: (i) direct measurement of the mobile species; (ii) indirect measurement of immobile components; and (iii) analysis of other species by employing auxiliary solid phases. Recent experimental advances are presented for illustration. The limitations and areas of necessary future research are indicated.

Application of fast ionic conductors in solid state galvanic cells for gas sensors

Abstract Solid state galvanic cells are investigated in view of their application for selective and reversible determination of partial pressures of gaseous species which may not be transferred in the solid electrolyte. The activities of the mobile component in the electrolyte and the gas component under measurement are correlated by an auxiliary gas sensitive layer. Solid silver ion conductors which show high ionic conductivity at room temperature are preferably employed. The measurement of Cl 2 and NO 2 partial pressures is illustrated as examples of the solid state electrochemical technique. The devices may be miniaturized (“microionics”) and operated over a wide temperature regime.

Beta″-alumina solid electrolytes for solid state electrochemical CO2 gas sensors

Abstract The application of surface modified Na-β″-alumina solid ionic conductors for the potentiometric detection of CO 2 partial pressures at ambient and moderately increased temperatures has been investigated. The galvanic cell voltage is decreased compared to the thermodynamically expected value for the formation of Na 2 CO 3 . This indicates kinetically more favorable cell reactions that produce metastable compounds. The redox process involves 4 electrons for each CO 2 and 2 electrons for each O 2 molecule at the applied temperature (150°C). The investigations show that Na-β″-alumina allows to build up potentials which depend on the CO 2 partial pressure.

Tetragonal zirconia polycrystals — a high performance solid oxygen ion conductor

Abstract The ionic conductivity of sintered pellets of tetragonal zirconia polycrystals (TZP) was found to be higher than cubic stabilized zirconia (CSZ) below about 700°C in spite of the lower concentration of defects. At 400°C the bulk conductivity is 1.2 X 10 -4 Ω -1 cm -1 with an activation enthalpy of 0.92eV. Thisshows that the mobility of the defects in TZP is about one order of magnitude higher than in CSZ. TZP pellets show high thermo-mechanical stability and may be readily sealed into glass tubes to provide gastight separate electrode compartments. Such an arrangement shows a fast response to variations of the oxygen partial pressure even at temperatures as low as 150°C. The correct Nernstian voltage is observed within about 1 s under reducing conditions and within a few minutes under oxidizing conditions. It may be assumed that the fast response even at low temperatures is related to the electronic properties of TZP. Investigations of the electronic behavior of the surface by work function measurements and of the minority charge carrier conductivities are reported as a function of the temperature and oxygen partial pressure. The evaluation of Hebb-Wagner polarization measurements has to be modified by considering the formation of internal electrical fields. It appears that TZP is very useful for fuel cells in view of both its thermo-mechanical stability and its favorable electrical properties at lower temperatures. Limiting-current oxygen sensors have been developed on the basis of this material which show superior performance compared with CSZ. Replacement of the gas diffusion barrier by a solid oxide allows high oxygen partial pressure to be measured.

Potentiometric gas sensors based on fast solid electrolytes

Abstract The application of small all-solid-state galvanic cells for monitoring NO 2 , CO 2 and O 2 partial pressures at ambient or slightly increased temperatures is demonstrated. The approach makes use of a gas-sensitive mixed conducting thin layer, which rapidly relates the activity of the gaseous species to the activity of the mobile component according to the Gibbs-Duhem relationship. A reversible change of the potential as a function of the partial gas pressure is observed for the different galvanic cells, which are preferably based on Na + -β/β″-alumina and α-AgI solid electrolytes.

Proton conduction in zeolites

Abstract Protonic conduction in zeolites is optimized. The approach is based on a vehicle-mechanism for the transport process. The parameters controlling protonic conduction are discussed. Fully hydrated “NH 4 -zeolite A” has a room temperature conductivity of 2×10 −3 Ω −1 cm −1 . The material was successfully used in oxygen concentration cells (e.g. MnO 2 /NH 4 -zeolite A/Zn).

Fast ionic lithium conduction in solid lithium nitride chloride

Lithium nitride chloride (Li/sub 1/ /sub 8/N/sub 0/ /sub 4/Cl/sub 0/ /sub 6/) crystallizes in a defect anti-fluorite structure with 10% of the lithium sites being vacant. Its electrical conductivity and thermodynamic stability have been investigated in the temperature range from 25 to 400/sup 0/C. Lithium ions are the predominant charge carriers, yielding a conductivity temperature product of sigma T = 7.456 x 10/sup 4/ exp(-0.49/sub 5/ eV/kT) ..cap omega../sup -1/ cm/sup -1/K. The electronic contribution to the total conductivity is smaller by a factor of less than 10/sup -4/. The material is thermodynamically stable against pure metallic lithium and has a decomposition voltage larger than 2.5 V.

Investigation of proton-conducting solids

Abstract One-dimensional bulk proton conduction parallel to the c axis was observed in solid lithium hydrazinium sulfate, LiN 2 H 5 SO 4 . The conductivity in this direction is 2 × 10 −8 ω −1 cm −1 at 25°C and shows an activation enthalpy of 0.75 ± 0.07 eV. The two-dimensional conductors HUO 2 AsO 4 ·4H 2 O and HUO 2 PO 4 ·4H 2 O were studied as a function of their water content. The conductivities are 8 × 10 −6 and 3 × 10 −5 ω −1 cm −1 in the orthorhombic phase at −10°C, with activation enthalpies of 0.70 ± 0.05 and 0.57 ± 0.07 eV, respectively. Indications of peritectic transitions to the tetragonal phases were observed in the temperature ranges 15 to 47 and −5 to 10°C, respectively. The transition depends on the water content which appears to control the increase in conductivity in this material. The dependence upon various sample parameters is discussed. Fast proton transport in solids is proposed to occur by a “vehicle mechanism”, i.e. the motion of N 2 H + 5 , H 3 O + - or other proton-containing groups.

Low Temperature Limiting‐Current Oxygen Sensors Based on Tetragonal Zirconia Polycrystals

This paper reports that yttria doped tetragonal zirconia polycrystals can overcome the phase transition into the monoclinic phase at about 500{degrees} C and show higher ionic conductivities than cubic stabilized zirconia in spite of the lower defect concentration. This material is applied in oxygen gas sensors under limiting current conditions at intermediate and ambient temperatures. AC and dc conductivities, Tafel behavior, minority charge carrier diffusivity, and the i-V characteristics are reported. The detection limit of the oxygen partial pressure and the response time depend on the thickness of the electrolyte and are related to the oxygen ion conductance and the electronic mobilities, respectively. The sensors may be optimized by the application of thin film electrolytes and of modified configurations with solid oxide bulk conducting diffusion barriers.