High temperature humidity sensors based on sputtered Y-doped BaZrO 3 thin films

Abstract

Sputter deposited Y-doped BaZrO 3 thin films (BaZr x Y 1− x O 3− y/2 , x = 0.2, y > 0), were investigated as to their viability for reliable, long-term stable humidity or water vapor microsensors operating at high operating temperatures ( T > 400 °C). Reliable exhaust gas composition measurements at these temperatures could allow optimization of power plant efficiency and reduce emissions. Electrical conductivity and gas sensitivity tests on bulk Y-doped BaZrO 3 in environments with varying humidity suggest that this material could be suitable as a highly selective humidity sensor with sensitivities between 2 atm −1 and 26 atm −1 in the temperature range of 500–700 °C, based on an ionic proton conduction mechanism [W. Wang, A.V. Virkar, Ionic and electron–hole conduction in BaZr 0.93Y 0.07O 3− δ by 4-probe dc measurements, J. Power Sources 142 (2005) 1–9; W. Wang, A.V. Virkar, A conductimetric humidity sensor based on proton conducting perovskite oxides, J. Sens. Actuators, B, Chem. 98 (2004) 282–290]. Thin films of BaZrO 3:Y were investigated for their potential to significantly reduce sensor response time, increase sensitivity, and allow integration in microsensor platforms. Unlike other sensors that are based on adsorption, the sensing mechanism presented here at temperatures >400 ̊C should be based on absorption of H 2O, which dissolves into the BaZrO 3:Y lattice, “fills in” the oxygen vacancies, and releases protons. Since protons have a higher mobility than oxygen ions, they would dominate the electric conductivity. XRD spectra from all as-deposited and annealed BaZrO 3:Y films show that the material changed from amorphous to crystalline at annealing temperatures >500 °C. After annealing, the samples exhibit an increasing particle size from 70 nm to 167 nm and an increase of root mean squared (RMS) roughness (from 1.3 nm to 4.2 nm) with increasing film thickness (200–500 nm). The film conductivity increases as a function of temperature (from 100 °C to 400 °C) and upon exposure to a humid atmosphere, supporting our hypothesis of a proton conduction based conduction and sensing mechanism. The water vapor sensitivity was measured using 0.058 atm partial pressure of water at 400 °C. Sensitivity values ranging from 22 atm −1 to 62 atm −1 with a response time of τ 60 ≈ 6 s were achieved. We have demonstrated Y-doped BaZrO 3 thin film humidity sensors that operate at high temperatures.