Development of a compensated capacitive pressure and temperature sensor using adhesive bonding and chemical-resistant coating for multiphase chemical reactors

We have developed an integrated capacitive pressure and temperature sensor for kraft pulp digesters (pH 13.5, temperatures from 25 to 175 °C, reaching a local maximum of 180 °C and pressures up to 2 MPa). The gauge capacitive pressure sensor was made by bonding silicon and Pyrex chips using a polymeric gap-controlling layer and a high temperature, low viscosity bonding adhesive. A simple chip bonding technique, involving insertion of the adhesive into the gap between two chips was developed. A platinum thin-film wire was patterned on top of a silicon chip to form a resistance temperature detector (RTD) with a nominal resistance of 1500 Ω. A silicon dioxide layer and a thin layer of Parylene were deposited to passivate the pressure sensor diaphragm and the sensors were embedded into epoxy for protection against the caustic environment in kraft digesters. The sensors were tested up to 2 MPa and 170 °C in an environment chamber. The maximum thermal error of ±1.74% (absolute value of ±36 kPa) full-scale output (FSO) and an average sensitivity of 0.257 fF/kPa were measured. Parylene-coated silicon chips were tested for a full kraft pulping cycle with no signs of corrosion.