Capacitively coupled electrolyte-conductivity sensor based on high-k material of barium strontium titanate

Abstract

A miniaturized capacitively coupled contactless conductivity detection (C4D) sensor based on high-kperovskite oxide of barium strontium titanate (BST) has been implemented for the first time. The BST films(∼120 nm thick) of Ba0.25Sr0.75TiO3composition were prepared on a p-Si-SiO2-Pt structure by pulsed laserdeposition technique using BST targets fabricated by the self-propagating high-temperature synthesismethod. The Pt electrodes were buried into the SiO2layer to obtain a planar structure. For comparison,contact-mode electrolyte-conductivity (EC) sensors without the protective BST layer were also fabricated.To study the influence of the protective BST layer, both sensors were characterized in electrolyte solutionswith various conductivities using two- and four-electrode operation modes. The impedance spectra wererecorded in a frequency range from 1 Hz to 1 MHz. An equivalent circuit of the C4D sensor is discussed aswell.Both, the EC and C4D sensor, demonstrate nearly identical sensor characteristics. The obtained resultsclearly show the benefits of the use of the BST-based C4D sensor in a four-electrode configuration forcontactless conductivity measurements. A linear dependence between the measured conductance andthe electrolyte conductivity is obtained in a wide range of electrolyte conductivity from 0.3 mS/cm to50 mS/cm. Moreover, typical problems associated with contact-mode EC detection such as the effect ofpossible redox processes, contamination and fouling of electrodes during continuous measurements canbe minimized, thus, enhancing the life-time of conductivity sensors considerably.