Michael Holzinger

Fast potentiometric CO2 sensors with open reference electrodes

Abstract It is shown how a stable, simple and fast potentiometric EMF sensor for CO 2 can be constructed based on an early idea by one of us. Whilst the gas sensitive electrode consists of Na 2 CO 3 , the reference electrode uses mixtures of SnO 2 and Na 2 SnO 3 or TiO 2 and Na 2 Ti 6 O 13 . The activity of the elements is simply fixed by the ambient oxygen partial pressure. In this way the oxygen partial pressure cancels in the overall electrochemical reaction and the EMF measured does not depend on P O2 . Since the reaction free enthalpies of the systems chosen are such that CO 2 does not interfere with Na 2 SnO 3 or Na 2 Ti 6 O 13 , the reference side need not be sealed at all. Detailed results on the P CO2 - and T-dependence of the cell EMF and also results concerning the response time (∼ 1 s or even better) are presented.

Application of the van der Pauw Method to Conductivity Relaxation Experiments on YBa 2 Cu 3 O 6+ δ

The van der Pauw method has been applied to conductivity relaxation experiments on YBa2Cu3O6+δ at 600°C in order to determine the chemical diffusion coefficient as a function of the oxygen partial pressure in the surrounding atmosphere (10° > p O2/bar > 10-3). It is shown that the van der Pauw technique is suitable for monitoring the conductivity relaxation when the oxygen diffusion is perpendicular to the direct current flowing through the sample in accordance with the van der Pauw geometry using thin tablets as samples. The oxygen partial pressure is changed stepwise (generally ∆log/po2 ≤ 0.5) by employing appropriate gas mixtures as well as an electrochemical oxygen pump device. An evaluation formula is given for the determination of the chemical diffusion coefficient neglecting slow surface processes. In addition, the electronic conductivity of YBa2Cu3O6+δ has been measured at 600°C as a function of oxygen partial pressure of the ambient atmosphere (10° > po 2 /bar > 10–5) by means of the van der Pauw method applying the same experimental set-up. Typical values of the chemical diffusion coefficient are in the range of 10-6cm2s-1; the results of the conductivity measurements are interpreted in terms of an appropriate defect model.

(Ba,Ca)TiO3 PTCR Ceramics with LaNiO3 Thin-Film Electrodes: Preparation and Characterization of the Interface

Conductive LaNiO3 thin film electrodes were deposited by chemical solution deposition (CSD) from nitrate solutions onto polycrystalline Al2O3 and (Ba,Ca)TiO3 PTCR ceramic substrates. The electrical properties of the LaNiO3 thin film on Al2O3 and of the interface consisting of LaNiO3 and the semiconductive oxide ceramic were investigated. The deposited LaNiO3 films were about 250 nm thick and consisted of nanosized particles. The resistivity of the LaNiO3 film was about 3 × 10-3Ωcm at 20°C. The PTCR ceramic consisted of μm sized particles and exhibited an electronic resistivity of about 10 Ωcm at 20°C and a steep increase of the resistivity of a few orders of magnitude above the Curie point at about 120°C. The electrical properties of the LaNiO3/PTCR interface were dominated by the properties of a barrier layer between the PTCR ceramic and the LaNiO3 electrode. The potential dependence of the impedance indicated that the barrier layer consisted of a depletion layer within the PTCR ceramic, when the flat band potential of LaNiO3 on the PTCR ceramic at about −250 mV was exceeded. Additionally the formation of an insulating layer at the LaNiO3 electrode has to be taken into account.

Sensors for Acid-Base-Active Gases

The theoretical background for potentiometric and conductometric (bulk and surface) sensors is briefly discussed with regard to the usual application, viz. the detection of redox-active gases, and the importance of understanding chemical diffusion is stressed in this context. It is then shown how the analogous concept applied to acid-base interactions leads to powerful possibilities to measure partial pressures of acid-base active gases such as CO 2 , NH 3 and H 2 O. Two examples of novel sensor principles are discussed: i) The response of the surface ionic conductivity of AgCl or CaF 2 upon changes of partial pressures of (Lewis) basic or acid gases can be used to sense NH 3 and BF 3 . ii) In the major part of the paper it is shown how the deliberate use of overall acid-base cell reactions in potentiometric arrangements leads to a fast and precise CO 2 -sensor. For this purpose, open reference electrodes based on Na 2 Ti 6 O 13 or Na 2 SnO 3 have been constructed. Owing to the criteria developed in the text the whole cell can be exposed to a CO 2 and O 2 containing atmosphere without the necessity of sealing. Detailed results are given.