IRIDIUM OXIDE THIN-FILM STABILITY IN HIGH-TEMPERATURE CORROSIVE SOLUTIONS

The stability of sputtered iridium oxide films is investigated after exposure to pH 4, 7, and 10 solutions at 200-degrees-C (15 bar) and 245-degrees-C (40 bar). These reactively sputtered films are being considered as pH-sensing electrodes in high-temperature high-pressure saline solutions such as those found underground in potential nuclear repositories. The sputtered iridium oxide films (SIROF) have a linear response of approximately 53-58 mV per pH at room temperature under a wide range of solution conditions. Other advantages include their ruggedness, small size, high voltage/low impedance output, and the low cost of fabrication. To evaluate their stability the films are exposed in a Teflon-lined bomb for up to 70 h at high temperature. The pH testing of 50 samples includes cycling between pH 2 and pH 12 before and after exposure. Changes of the potential intercept and the slope are most severe after acidic conditions at 250-degrees-C. It is found that ion-assisted deposition helps to maintain film adherence and continuity on the alumina substrates during exposure to 200-degrees-C solutions.