R. V. Kumar

Oxide Ion Conduction in Indium-Oxide-Substituted Calcium Zirconate

Indium-oxide-substituted calcium zirconate of the nominal composition CaZr 0.9 In 0.1 O 3-δ is an important perovskite-type high-temperature proton conductor. In the present study, the transition from proton conduction to oxide ion conduction in this material has been investigated experimentally. A specially designed galvanic cell was employed, in which the oxygen chemical potential at the electrodes is determined by appropriate metal/metal-oxide mixtures while the hydrogen chemical potential is fixed by a gas atmosphere of known hydrogen partial pressure. Through cell voltage measurements at different temperatures proton and oxide ion conduction were discriminated, and conditions were identified under which virtually pure oxide ion conduction occurs despite the presence of a significant hydrogen partial pressure. Conductivity measurements allowed the determination of the activation energy of oxide ion conduction. The impact of a variable oxide ion vacancy concentration on the ion conducting properties of the solid electrolyte is discussed.

The detection of hydrogen in molten aluminium

The presence of hydrogen in aluminium poses problems to the foundry and casting industries, because high residual hydrogen contents in molten aluminium cause significant porosity in the solid aluminium after casting. This usually renders the product useless as it may fail mechanically. Therefore, fast, accurate and reliable techniques are required for monitoring the dissolved hydrogen content in molten aluminium, but this particularly harsh environment places considerable restrictions on the equipment that can be used. Several methods are available for the determination of hydrogen in aluminium melts, but they either suffer problems of accuracy, reliability and longevity or are not applicable to industrial environments. It is considered that the most appropriate device for the hydrogen analysis in aluminium melts should be an electrochemical sensor, which employs a proton conducting solid electrolyte in conjunction with a measuring electrode and a suitable reference electrode. The electromotive force of such a cell allows direct calculation of the hydrogen concentration in the melt. However, all the electrochemical sensors reported in the literature thus far exhibit distinct drawbacks. This article discusses the various techniques for the determination of hydrogen in molten aluminium with particular emphasis on the benefits and shortcomings of the existing electrochemical sensors.

Ionic conductivity in hydrogels for contact lens applications

Biphasic hydrogel polymers are in the forefront of new extended wear contact lens development. In the biphasic hydrogel the objective is to produce co-continuous domains of siloxane units for high oxygen permeability, coupled with hydrophilic units forming aqueous channels for hydraulic and ion mobility. These are distributed in phase separated nano-scale regions such that the material is optically clear while achieving the required properties to maintain corneal health and lens movement.This paper describes how Impedance Spectroscopy permits a rapid measurement of ion conductivity in a range of silicone and non-silicone hydrogel materials with water contents ranging from 18% to 75% equilibrium water content. For non-silicone hydrogels relative sodium ion conductivity follows a typical percolation curve. However, for silicone hydrogels ion mobility is three orders of magnitude higher than conventional hydrogels of the same equilibrium water content.The influence of electrolyte concentration, interfacial electrode sample contact pressure and temperature are also reported.

An all-solid-state potentiometric SO2 gas sensor with solid reference electrode

A SO2 sensor based on Na-β-Al2O3 interfaced with an auxiliary electrode of Na2SO4 formed in-situ and a solid mixture of Fe3O4, Fe2O3, 3Na2O·5Fe2O3 as the reference electrode was developed. The sensor displayed a good response to changes in the partial pressure of SO2 with a linear relationship between the measured emf and the logarithmic values of the partial pressure of SO2, as predicted by the Nernstian equation. Using an electron probe X-ray microanalyser, the formation of a Na2SO4 reaction layer was confirmed on the surface of Na-β-Al2O3 tube due to surface reaction with the SO2 gas.

Humidity sensing under aggressive conditions using nafion conductors

A sensor capable of detecting moisture in a chlorine-air environment has been assembled from Nafion and platinum. Nafion acts as the solid electrolyte while platinum chemically deposited on the Nafion is the electrode material. Thermogravimetric analysis suggests that Nafion is stable up to 473 K. A Frequency Response Analyser (FRA) was used to calibrate the sensor in terms of impedance as a function of relative humidity. The frequency range was 10−2 to 107 Hz and an amplitude of 100 mV was applied. Impedance measurements show that only a bulk process occurs and when relative humidity is high, diffusion of moisture into and out of the platinum/Nafion composite becomes significant. This is reflected in the form of a spike at about 45° to the real axis. The sensor detects chlorine in air without loosing its chemical and physical stability. Conductivity increases as a function of temperature and reaches its peak at 433 K. The response time of the sensor is governed by temperature, thickness of the Nafion film and the amount of platinum on the surface. In practice the sensor can be operated with a frequency controller.

Devices based on interfacing two or more solid electrolytes

It is conceivable that sophisticated sensing and other electrochemical devices can be formed by interfacing more than one solid electrolyte in an electrochemical chain involving more complicated sequences of electrodes, interfaces, in-situ catalysts and semi-permeable membranes. As sensor systems become more elaborate, the need for optimising several interfaces and reaction equilibria becomes critical.In the example of YSZ/Nasicon based sensors, the emf response can be adequately explained by the interfacial reactions. In the use of doped perovskite interfaced with an oxygen or a chloride ion conductor, although useful sensors can be devised with Nernstian type of response, the interfacial phenomena or the electrode reactions are more complicated and can not be explained by simple interfacial reactions. A brief review of the relevant concepts is presented.

A novel humidity sensor using yb-doped SrCeO3 ionic conductor with a au-pd filter

A novel humidity sensor operating at T<580K using Yb-doped SrCeO3 ionic conductor with a Au-Pd chemical filter has been developed. In this unique design, both the electrodes are exposed to the same test gas and therefore do not require separation of working and reference compartments. A thin layer of impervious Au-Pd alloy on one surface of the perovskite electrolyte allows selective access to oxygen gas, while the other surface coated with porous Pt is exposed to both H2O and oxygen. Sensor emfs are found to be directly proportional to log pH2O and independent of pO2. The design aspects of the sensor, the experimental set-up, some typical results and a postulated mechanism of sensor operation are described in this paper.

Method for making minor dopant additions to porous ceramics

Abstract An infiltration technique for doping porous ceramic bodies before sintering is evaluated for calcia additions to yttria stabilised zirconia. The method involves impregnating the porous body with a solution containing the required dopant nitrate, followed by drying and sintering to decompose the nitrate and densify the ceramic body. The resulting dopant distributions show equivalent homogeneity to the popular ball milling approach for dopant contents below 3 mol.-%CaO, while giving a product with a higher relative density.

Hydroxyl ion conduction in ytterbium-doped strontium cerate at T<580 K in H2O/air atmosphere

Open circuit voltage (OCV) measurements in H2O/air concentration cells at T<580 K using Yb-doped SrCeO3 electrolyte indicate that under these conditions, protons are transported through the electrolyte as -ve ions, possibly as hydroxyl (OH−) ions. The H+ ionic transport, which is generally reported, becomes the dominant mode for H2O/air concentration cells at temperatures greater than 750 K or when H2O/air electrodes are replaced by H2/Ar, and the anomalous OCV sign disappears. The combination of low temperature and the presence of hydrogen and oxygen as provided by the H2O/air system appears to be necessary for the postulated hydroxyl ion electrode reactions to take place.In addition to OCV measurements, results from impedance spectroscopy are used to provide evidence in support of the suggested hydroxyl ion mode of protonic transport under the specified conditions. These findings are directly relevant in the development of novel humidity sensors in the temperature range 450–580K and is reported in a separate paper in this conference.

Some innovative technologies using solid electrolytes in measuring gas compositions

Solid electrolytes have received considerable scientific attention and assumed important technological significance in recent years, due to potential practical applications in fuel cells, batteries, electrochromics, process control, sensors and environmental protection. Use of solid electrolytes for developing electrochemical gas sensors is a well established field with successful commercial applications related mainly to oxygen measurements. Discovery of new solid electrolytes have led to new areas of development and applications such as hydrogen monitoring. Applicability of solid electrolytes have also been extended to species which are not ionically mobile in the electrolyte by use of auxiliary phases. More recently, significant progress in sensing of molecular species such as CO2, SO2, SO3, NO, NO2, HCl, H2O and HF have been achieved by interfacing 2 or more electrolytes in electrochemical chains and/or by using auxiliary phases, semi-permeable coatings or in-situ catalysts to promote the desired electrode reactions.