V. Vashook

Oxides of the AMO3 and A2MO4-type: structural stability, electrical conductivity and thermal expansion

Abstract The structural and chemical stabilities, electrical conductivity, and thermal expansion of the A 2− a A a ′MO 4− x oxides (A=La; A′=Sr; M=Mn, Fe, Co, Ni) with the perovskite-related K 2 NiF 4 -type structure were investigated and compared with the characteristics of perovskite-type oxides AMO 3− x containing the same cations. The K 2 NiF 4 -type manganites, ferrites, cobaltites and nickelates are assumed to be reduction products of the corresponding perovskite-type oxides. The thermodynamic stabilities, in terms of reversible oxygen desorption, were higher than those of the corresponding perovskite-type oxides. Within the range of oxygen partial pressure ( p O 2 ) from air to argon/H 2 /H 2 O, the oxidation states of the M cations were determined. The comparison of the oxidation states of M in AMO 3− x and (AMO 3− x )·AO gives evidence on the stabilizing influence of the AO interlayer on the perovskite layer. The electrical conductivity of the A 2 MO 4 oxides was of p-type and reached values close to 100 S cm −1 at high oxygen partial pressures and 800 °C for nickelates and cobaltites. The thermal expansion of K 2 NiF 4 -type oxides is generally lower than that of the comparable perovskite-type oxides.

Selectivity of HC-sensitive electrode materials for mixed potential gas sensors

Abstract In potentiometric zirconia-based gas sensors, electrodes with medium catalytic activity show a high sensitivity for hydrocarbons (HCs) when the measurements take place in non-equilibrated oxygen containing gas mixtures at temperatures ≤800 °C. This behaviour, explained by mixed potential theory, depends strongly on preparation and particularly on measuring conditions. To learn more about the processes at the electrode surface and their influence on the potential behaviour of the electrodes, we investigated composite electrodes Au-oxide/YSZ with Ga2O3, In2O3 and Nb2O5 as oxides and 80 mass% Au and perovskite electrodes made of La1−xSrxCr1−yGayO3−δ in N2-diluted gas mixtures containing O2 and different combustibles like C3H6, C3H8, CO and C7H8. As compared with perovskite electrodes, gold composite electrodes using Nb2O5 as oxide show enhanced sensitivities especially to C7H8. The temperature dependence of the catalytic activity for the HC combustion correlates very well with the temperature/voltage response.

Oxygen non-stoichiometry and electrical conductivity of the binary strontium cobalt oxide SrCoOx

Abstract Strontium cobaltite was investigated using solid-electrolyte coulometry and resistivity measurements in the temperature range 20–1050°C and oxygen partial pressures 0.5–400 Pa. Two observed oxygen desorption/sorption maxima within the temperature range 500–950°C correlate with phase transitions of this compound as reported in the literature. An additional oxygen desorption/sorption maximum was found at a temperature of 965–1000°C, which is explained an order-disorder transition of the cubic high-temperature phase. The dependencies of equilibrium values of oxygen content as well as specific resistivity on temperature and oxygen partial pressure were founded for the cubic phase.

Oxygen ion diffusion in perovskite-type oxides determined by permeation and by relaxation measurements

Oxygen ion diffusion in oxides of the perovskite-type structure was determined by two methods of measurement: i) stepwise change of temperature or oxygen partial pressure of the gaseous environment of a ceramic sample, and measurement of the relaxation of electrical conductivity or oxygen content, ii) oxygen permeation through a gas dense oxide ceramic pellet from air to argon. The experimental problems and sources of errors, calculation methods of chemical diffusion coefficients and oxide ionic conductivities from these two modes of investigation are discussed. The comparison of relaxation and permeation measurements on 10 oxides of the type A1−aA′aM1−bM′bO3−σ (A=La, Pr, Ce; A′=Sr, Ca; M=Mn, Fe; M′=Co, Ni) show advantages of the relaxation method in relation to the experimental performance and possible errors.

Synthesis, crystal structure, oxygen stoichiometry, and electrical conductivity of La1−aCaaCr0.2Ti0.8O3−δ

Abstract A series of perovskite-type compounds La 1− a Ca a Cr 0.2 Ti 0.8 O 3− δ ( a =0.6–1.0) was synthesized by a ceramic technique in air (final heating 1350 °C). For 0.4 a R- 3 c , and for a >0.6 as orthorhombic in space group Pbnm . The lattice constants for a >0.8 decrease noticeably with increasing Ca content and for a 2 at 20–1400 °C, and in 5% H 2 /Ar ( p H 2 O/ p H 2 =0.01) at 850–1000 °C. Oxygen stoichiometries and electrical conductivities of the solid solutions with a =0.6 to 1.0 were investigated. Increasing Ca-contents decrease the stability of the oxides in respect to the thermal dissociation with oxygen release. At 900 °C within the oxygen partial pressure region 1×10 −15 to 0.21×10 5 Pa the compounds with a >0.8 are p-type and with a 0.2 Ca 0.8 Cr 0.2 Ti 0.8 O 3− δ changes its conductivity type from n-type at p O 2 −10 Pa to p-type in more oxidizing atmosphere. For the compound with a =0.6 in 5% H 2 /Ar flow at 1000 °C, a maximum conductivity of about 50 S/cm has been observed. In air the composition with a =0.95 has the highest conductivity of about 1 S/cm. In an oxidizing environment the maximal conductivities correspond to the ratio [Cr 3+ ]/[Cr 4+ ]=1 (corresponding to an oxygen stoichiometry near 3.0), and in reducing environment to the ratio [Ti 3+ ]/[Ti 4+ ]=1 (corresponding to an oxygen stoichiometry near 2.95).

Synthesis, crystal structure, and transport properties of La1−xCaxCr0.5Ti0.5O3−δ

A series of perovskite-type compounds La1−xCaxCr0.5Ti0.5O3−δ (x=0…1.0) was synthesized by a ceramic technique in air (final heating 1350 °C). The crystal structures of the compounds were characterised at room temperature for 0.0 0.2 as orthorhombic in space group Pbnm. The lattice constants with increasing Ca contents for x>0.5 decrease noticeably and for x<0.5 remain nearly constant. All compositions were stable at pO2=21,000 to 6×10−16 Pa at 850…1000 °C. Oxygen stoichiometries and electrical conductivities of the solid solutions were investigated. The stability of the oxides against thermal dissociation evaluated by the oxygen release decreases with increasing Ca-contents. The compounds with x≥0.6 are p-type and with x<0.4 n-type semiconductors at 900 °C and oxygen partial pressures between 1×10−15 and 0.21×105 Pa. La0.5Ca0.5Cr0.5Ti0.5O3−δ and La0.6Ca0.4Cr0.5Ti0.5O3−δ change their conductivities from n-type in the range pO2<10−10 Pa and pO2<10−5 Pa, respectively, to p-type in more oxidizing atmosphere. For the compound with x=0.5 in Ar/H2/H2O gas flow (pH2O/pH2=0.53) at 900 °C a maximum conductivity of about 1 S/cm has been observed. In air the composition with x=0.75 has highest conductivity of about 40 S/cm. Partial substitution of calcium by lanthanum leads to a decrease of the average thermal expansion coefficients (TEC) from 13.1×10−6 K−1 (CaCr0.5Ti0.5O3) to 10.5×10−6 K−1 (La0.25Ca0.75Cr0.5Ti0.5O3) and 9.5×10−6 K−1 (La0.5Ca0.5Cr0.5Ti0.5O3). No simple correlations between catalytic activity, electrical conductivity, and crystal structure parameters were found.

Catalytic and Electrical Properties of SOFC Anode Material Based on Lanthan‐Chromite‐Titanate

Series of perovskite-type compounds La 1-a Ca a Cr b-1 Ti b O 3-δ (a = 0.0-1.0, b = 0.0, 0.2, 0.5, 0.8, 1.0) were synthesized by a ceramic technique in air (final heating 1350°C). All compositions were stable in air and in Ar/1 Pa O 2 at 20 - 1400°C, and in 5% H 2 /Ar (p H2O /p H2 = 0.01) at 850-1000°C. The compounds with a = b change their conductivity type from n-type at p(O 2 ) <10 -10 Pa to p-type in more oxidizing atmosphere. Catalytic activity of compounds concerning the partial oxidation of hydrocarbons depends on cationic and anionic composition.

Structural stability of conducting oxide CaRuO3 at high temperatures

The structure of conducting oxide CaRuO3 has been studied as a function of temperature up to 1300°C by neutron powder diffraction. The structure is found to remain orthorhombic, in space group Pbnm, over the entire temperature range. The structural rigidity is evidenced by the fact that the octahedral tilt angles change by only ∼3° over the entire temperature range. The absence of any structural phase transition in this temperature range and retention of its conducting properties make CaRuO3 more useful compared to its counterpart SrRuO3 with regard to its application as metallic-interface/electrode/substrate material in epitaxial thin film devices to be used at high temperatures.

New Developments in Electrode Materials for Electrochemical Sensors

Electrode materials are the key components for electrochemical sensors which can be used for determination of gaseous and dissolved species. The sensitivity as well as the selectivity are mainly influenced by the kind and the structure of sensitive electrode material. In this paper two kinds of materials are described. Screen-printed carbon electrodes (SPCE) can be modified by thin layers of conducting polymers like Poly(3,4-ethylendioxythiophene) (PEDOT) and gold nano-particles. By means of differential pulse (DPV) and square wave voltammetry (SWV) it is possible to determine biogene amine like dopamine in liquids of human bodies and explosives in ground water in the nM und ppb level, respectively. Polyaniline (PANI) can be used not only in normal temperature sensors but also in high temperature sensors. For the first time we could show that zirconia based sensors with PANI electrodes in which Nb2O5 or FeCl3 and Co(NO3)2 are embedded are suitable to measure hydrogen and hydrocarbons in oxygen containing gases at 450 °C. The sensitivities of such electrodes are much higher than those of the usual applied oxide systems like Nb2O5 or La0.75Ca0.25Mn0.5Ni0.5O3 − δ . Due to the availability and compactness of electronic devices electrochemical sensors with modified electrodes can be applied in stationary (potentiometric) and non-stationary (SWV or DPV) mode in field application.

Chemical diffusion and oxygen exchange of LaNi 0.4 Fe 0.6 O 3- δ ceramics

Oxygen surface exchange and oxygen chemical diffusion coefficients of LaNi0.4Fe0.6O3−δ ceramics are determined via conductivity relaxation method after stepwise change of temperature in the range of 700–950 °C in air and Ar/O2 gas flow at oxygen partial pressures (