A new process for fabricating CO2-sensing layers based on BaTiO3 and additives

Abstract We present a new process for fabricating CO2 gas sensors. The process was to introduce heat-treatment in vacuum into fired sensing layers. The powders used for paste materials were BaTiO3 and additives, which were all microcrystalline in the range of 2–5 μm. Thick films were fired in air and subsequently heat-treated in vacuum. Their impedance was measured during vacuum-treatment. When the paste materials contained the additive of LaCl3, changes of impedance were observed. A low sensor resistance was achieved by adding 10 wt.% LaCl3 or higher amount to an equimolar mixture of BaTiO3 and CuO. When added LaCl3 amounted to 10 wt.%, a good sensitivity was obtained. Conventionally fired and vacuum-treated layers showed a different response to a change in partial oxygen pressure. Sensor surfaces were analysed using scanning electron spectroscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and Auger electron spectroscopy (AES). No grain size effects were observed. A change of the chemistry in the sensing material was measured. The change indicated that LaOCl, being thermally transformed from LaCl3 during firing, is further oxidised to LaxOy during vacuum-treatment. The analysis gives a possible explanation of the changes in sensor resistance and sensitivity induced by the vacuum process.