Dowsil 3140 rtv coating

Standard alumina (Al₂O₃, 96%) plates were chosen as the substrates for the pH electrodes due to their compatibility with thick films and high tolerance toward various environmental conditions. First, Ag/Pd thick-film paste (9695, Electro-Science Laboratories, King of Prussia, PA, USA) was screen-printed on the substrates, dried at 120 °C for 15 min, and fired at 860 °C for 30 min. Freshly prepared RuO₂ paste was then screen printed on the substrates in such a way that the RuO₂ layer slightly overlapped the Ag/Pd conducting layer. After drying at 120 °C for 15 min, electrodes were sintered at 800 °C (RuO₂-800), 850 °C (RuO₂-850), or 900 °C (RuO₂-900) for one hour. Three RuO₂ electrodes were prepared for each sintering temperature. Next, electrical contact was attached to an open end of the conducting layer by soldering a copper wire. To avoid any contact between the conducting layer and the electrolyte, the electrical contact and the conducting layer were covered with noncorrosive polydimethylsiloxane coating (DOWSIL™ 3140 RTV Coating, Dow Chemical Company, Midland, MI, USA), and the sensitive area was left uncovered. Finally, the silicone resin cover was hardened at room temperature for 48 h. The schematic representation of the various stages of fabrication of RuO₂ electrodes is presented in Figure 1.

The electrodes
were fabricated similarly to previously described methods.^{34 (link)} Briefly, two layers were deposited on an alumina
(Al₂O₃) substrate by the screen printing: a
conductive layer and a pH-sensitive layer. A conductive layer of Ag/Pd
thick film paste (9695, Electro-Science Laboratories, King of Prussia,
Pennsylvania) was printed first, and a pH-sensitive layer of commercially
available RuO₂ paste (10 kΩ/sq, 3914, Electro-Science
Laboratories, King of Prussia, Pennsylvania) was printed second. Furthermore,
the RuO₂ layer was printed in such a way that it would
partly overlap the conductive Ag/Pd layer. The substrates were dried
at 120 °C for 15 min and consequently sintered at 850 °C
for 1 h after the first printing step and at 900 °C for 1 h after
the second printing step. After cooling the substrate, a copper wire
was connected to the Ag/Pd layer by soldering with a Sn/Pb alloy.
Finally, a protective layer of silicone rubber (DOWSIL 3140 RTV Coating,
Dow Chemical Company, Midland, Michigan) was used to cover the conductive
layer and the electric contact. The dimensions of the fabricated RuO₂ electrodes are presented in Figure 6a,b. The screen-printed RuO₂ electrodes
were previously characterized by Manjakkal et al.^{35 (link)−37 (link)}