Development of low-firing lead-free thick-film materials on steel alloys for piezoresistive sensor applications

Piezoresistive sensors based on steel and other metallic substrates provide higher strain response than on standard ceramic substrates and are more easily packaged. But exposing high-strength steels to the standard high- temperature 850°C thick-film firing cycle affects their mechanical properties. In previous studies, we have developed a range of low-firing thick-film materials based on lead borosilicate glass, which allows processing at low temperatures. However, it is desirable to develop alternatives to potentially toxic lead-based glasses that to not include alkali metals, which degrade high-temperature insulation characteristics of dielectrics. To this end, this work concerns investigations in essentially substituting lead for bismuth, and presents a series of low-melting Bi-B-Zn-Si-Al oxide glasses having good stability against devitrification. However, these glasses, when formulated as thick-film pastes using standard vehicles based on ethylcellulose binders, were found to be quite sensitive to incomplete binder burnout, with strong bubble generation within the layer. Therefore, a novel organic binder based on polypropylene carbonate, featuring clean low temperature burnout, had to be introduced. On this basis, thick-film dielectric compositions have then been developed and tested, aiming to optimise the mechanical strength and their expansion matching with the steel substrates. In the goal of a complete materials system, first tests on compatible conductors and resistors, using the same glasses, are presented as well.