Jens Zosel

Recent developments in electrochemical sensor application and technology—a review

The review describes different developments of electrochemical sensors, which are used for measurements in a broad range of temperatures and concentrations of gases and dissolved ions in situ. The current approaches in sensor technology are targeted on miniaturization of electrochemical cells, application of multi-sensor setups, extensions of sensor application respecting pressure, temperature and aggressive media and on the use of dynamic methods.

Investigation of the dynamic response behaviour of ISFET pH sensors by means of laser Doppler velocimetry (LDV)

Abstract The dynamic response behaviour of ISFET pH sensors with different forms of encapsulation was investigated in flow cells by laser Doppler velocimetry (LDV). At a mean flow velocity of 8 cm/s, response times t(5–95%) varied between 0.3 and 2.0 s, depending on the geometry of the ISFET sensor, the direction of flow relative to the chemically sensitive region and the direction in which the pH value changed. The effect of the cavity containing the sensitive region on the response times of fully encapsulated sensor chips during measurements in low viscosity fluids is smaller than generally assumed.

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.

Au–oxide composites as HC-sensitive electrode material for mixed potential gas sensors

In potentiometric zirconia-based gas sensors, gold electrodes show a high sensitivity for hydrocarbons (HC) when the measurements take place in nonequilibrated oxygen containing gas mixtures at temperatures <700 °C. That behaviour explained by mixed potential theory is not stable and depends strongly on preparation and particularly on measuring conditions. To clarify the correlation between the potential behaviour of electrodes and their catalytic activity for the hydrocarbon combustion, we investigated composite electrodes Au–oxide/YSZ with Ga2O3, Y2O3, Al2O3, ZrO2, In2O3 and Nb2O5 as oxides and 70 mass percent gold in C3H6–O2 gas mixtures using a platinum air reference electrode. Gold composite electrodes using Nb2O5, Ga2O3, In2O3 as oxides show, as compared with pure gold, an enhanced propylene sensitivity and a time-independent characteristic at high concentrations of C3H6. Gold composite electrodes with Y2O3, Al2O3, ZrO2 tend to the equilibrium behaviour. The catalytic activity of the Au-composites for the HC combustion correlates very well with the cell voltage response.

The measurement of dissolved and gaseous carbon dioxide concentration

In this review the basic principles of carbon dioxide sensors and their manifold applications in environmental control, biotechnology, biology, medicine and food industry are reported. Electrochemical CO2 sensors based on the Severinghaus principle and solid electrolyte sensors operating at high temperatures have been manufactured and widely applied already for a long time. Besides these, nowadays infrared, non-dispersive infrared and acoustic CO2 sensors, which use physical measuring methods, are being increasingly used in some fields of application. The advantages and drawbacks of the different sensor technologies are outlined. Electrochemical sensors for the CO2 measurement in aqueous media are pointed out in more detail because of their simple setup and the resulting low costs. A detailed knowledge of the basic detection principles and the windows for their applications is necessary to find an appropriate decision on the technology to be applied for measuring dissolved CO2. In particular the pH value and the composition of the analyte matrix exert important influence on the results of the measurements.

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.

Investigations on selected gallium and strontium doped lanthanum chromites as electrode materials for HC detection

Perovskite-type mixed oxides with the formula LaCr1−yGayO3−δ and La0.95Sr0.02Cr1−yGayO3−δ with y=0.1; 0.2; 0.3 were investigated in view of their usability as electrode materials in exhaust gas sensors for hydrocarbon (HC) detection. The electrode materials were characterized regarding their catalytic activity for propylene and propane oxidation and their potentiometric behavior in oxygen containing gaseous mixtures of these combustibles. The potentiometric measurements with propane indicate higher sensitivities of the strontium doped materials than of the materials without strontium. The comparison of these results with those of the catalytic investigations using propane show, that a lower catalytic activity comes along with a higher sensitivity. The electrodes show non-Nernstian behavior.

Investigations on selected gallium doped lanthanum chromites as electrode materials for HC detection

Perovskite-type mixed oxides with the formula La1−xGa0.3Cr0.7O3−δ (x=0.2; 0.1; 0.05; 0) were investigated in view of their usability as electrode materials in exhaust gas sensors for HC detection. The electrode materials were characterised regarding their structure, catalytic activity for propene oxidation and potentiometric behaviour in combustible gaseous mixtures. XRD-measurements indicate, that the solid oxide method does not permit lanthanum deficiencies x ≥ 0.05. The electrodes show non Nernstian behaviour.

Die Bedeutung der Materialentwicklung für elektrochemische SensorenMaterials Research and Development for Electrochemical Sensors

Zusammenfassung Elektrochemische Sensoren werden in verschiedenen Ausführungsformen in einem weiten Temperaturbereich von ca. -30°C bis über 1500°C in festen, flüssigen und gasförmigen Medien eingesetzt. Die Erschließung neuer Anwendungsgebiete sowie die Erweiterung des Temperatureinsatzbereiches, die Erhöhung von Sensitivität, Selektivität und Stabilität und die Optimierung von Fertigungstechnologien der Sensoren beruhen fast immer auf Neu- und Weiter-entwicklungen von Sensormaterialien und Verfahren zu deren Herstellung. Der Übersichtsartikel zeigt die Vielfalt dieser Bemühungen anhand von Beispielen zu Materialentwicklungen für elektrochemische Referenzelektroden, Metalloxidelektroden, Metalloxid-Gassensoren sowie YSZ-basierten Festelektrolytsensoren.

pH-Messung in der Tierphysiologie, besonders im Pansen von Milchkühen

Zusammenfassung Die pH-Messung nimmt in tierphysiologischen Untersuchungen eine wesentliche Rolle ein. Sie wird für Messungen an der Haut, am Auge, im Blutkreislauf, in der Speiseröhre und vor allem im Magen von Tieren herangezogen. Nach einer kurzen Literaturübersicht über die genannten Anwendungen wird über die Entwicklung einer nicht kabelgebundenen Sonde für die pH-Bestimmung im Pansen von Milchkühen eingegangen. Diese Sonde ist zum In-vivo-Einsatz vorgesehen, um eine der wichtigsten Erkrankungen von Milchkühen, die subakute Pansenazidose, zu diagnostizieren und das Fütterungsregime entsprechend zu gestalten. Abstract The pH measurement plays an important role in animal physiological investigations. It is used for measurements on the skin, the eye, in the bloodstream, in the esophagus and particularly in the stomach of animals. After a brief literature review of these applications the development of a non-wired probe for the determination of pH in the rumen of dairy cows is presented. This probe is provided for in vivo applications to diagnose one of the most important diseases of dairy cattle, the subacute ruminal acidosis, and for optimizing the feeding regime accordingly.