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

A miniaturized capacitively coupled contactless conductivity detection (C4D) sensor based on high-k perovskite oxide of barium strontium titanate (BST) has been implemented for the first time. The BST films (∼120nm thick) of Ba0.25Sr0.75TiO3 composition were prepared on a p-Si-SiO2-Pt structure by pulsed laser deposition technique using BST targets fabricated by the self-propagating high-temperature synthesis method. The Pt electrodes were buried into the SiO2 layer 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 solutions with various conductivities using two- and four-electrode operation modes. The impedance spectra were recorded in a frequency range from 1Hz to 1MHz. An equivalent circuit of the C4D sensor is discussed as well. Both, the EC and C4D sensor, demonstrate nearly identical sensor characteristics. The obtained results clearly show the benefits of the use of the BST-based C4D sensor in a four-electrode configuration for contactless conductivity measurements. A linear dependence between the measured conductance and the electrolyte conductivity is obtained in a wide range of electrolyte conductivity from 0.3mS/cm to 50mS/cm. Moreover, typical problems associated with contact-mode EC detection such as the effect of possible redox processes, contamination and fouling of electrodes during continuous measurements can be minimized, thus, enhancing the life-time of conductivity sensors considerably.