High temperature humidity sensors based on sputtered Y-doped BaZrO3 thin films

Sputter deposited Y-doped BaZrO3 thin films (BaZrxY1-xO3-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 degrees 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 BaZrO3 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 degrees C, based on anionic proton conduction mechanism [W. Wang, AN. Virkar, Ionic and electron-hole conduction in BaZr0.93Y0.07O3-delta by 4-probe dc measurements, J. Power Sources 142 (2005) 1-9; W. Wang, AN. Virkar, A conductimetric humidity sensor based on proton conducting perovskite oxides. J. Sens. Actuators, B, Chem. 98 (2004) 282-290]. Thin films of BaZrO3: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 degrees C should be based on absorption of H2O, which dissolves into the BaZrO3: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 BaZrO3:Y films show that the material changed from amorphous to crystalline at annealing temperatures >500 degrees 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 degrees C to 400 degrees 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 degrees C. Sensitivity values ranging from 22 atm(-1) to 62 atm(-1) with a response time of tau(60) approximate to 6 s were achieved. We have demonstrated Y-doped BaZrO3 thin film humidity sensors that operate at high temperatures. (C) 2009 Elsevier B.V. All rights reserved.