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

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 fur paste materials were BaTiO3 and additives, which were all microcrystalline in the range of 2-5 mu m. 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. (C) 2000 Elsevier Science S.A. All rights reserved.