|Hydrogen is a major cause of gas porosity in aluminum and is frequently removed from the melt prior to casting. The degassing process can be better controlled if the hydrogen content in the melt is known. Thus, gas sensors which can make continuous in situ measurements in molten aluminum are needed. Current online hydrogen sensing systems are complex designs which are prohibitively expensive. Solid electrolyte based potentiometric sensors have been developed as an attractive alternate. These sensors have traditionally used a gas phase as the reference electrode. The present design has a condensed-phase reference electrode to avoid the need for transport of the reference gas into and out of the melt. The use of an oxide rather than a hydride phase reference is expected to considerably lower device cost and improve shelf life and reliability.
The sensor element consists of a solid electrolyte tube based on 10 mol% In-doped CaZrO3, which was synthesized using both solid oxide and oxalate co-precipitation techniques. Precursor oxalate powders prepared using polymeric surfactants (PEG) were characterized using SEM, XRD, FTIR and particle size analysis. PEG was found to reduce particle size and also influence the process of perovskite formation. The oxalate co-precipitation technique enabled powder synthesis at reduced processing time and temperature.
Closed-one-end tubes were slip cast and densified for use as solid electrolytes. Impedance spectroscopy and D.C. resistance measurements were made at temperatures between 650 and 900°C. Undoped CaZrO3 was found to be a p-type conductor in air. In-doped CaZrO3 acted as a proton conductor in air and argon+H2O, whereas the material was found to be a p-type conductor in pure argon. While bulk conduction was found to be homogenous with activation energies matching those from D.C. measurements, conduction across the grain boundary was found to be heterogeneous.
Potentiometric sensors using In-doped CaZrO3 as the electrolyte, and metal/metal oxides, metal/metal hydrides as the reference electrodes were fabricated. Sensors with Mg-MgO reference electrodes, on exposure to argon and aluminum environments, generated a potential matching the Nernstian voltage. Sensors with Ca-CaH2 reference electrodes measured the Nernstian potential when exposed to 5%H2. Laboratory tests conducted using gas mixtures also showed sensors with Mg-MgO reference electrodes to measure changes in hydrogen concentration. Pilot plant testing of sensor prototypes was carried out in commercial aluminum alloy melts and the sensors were found to respond to hydrogen degassing. The response of these sensors to changes in the gas content of the melt correlated reasonably well with that of a commercially available system for measuring hydrogen content in molten aluminum (AlScan).