ABSTRACT
Two bismuth oxalates, namely, Bi2(C2O4)3·7H2O and Bi(C2O4)OH, were studied in terms of synthesis, structural characterization, particle morphology, and thermal behavior under several atmospheres. The oxalate powders were produced by chemical precipitation from bismuth nitrate and oxalic acid solutions under controlled pH, then characterized by X-ray diffraction (XRD), temperature-dependent XRD, IR spectroscopy, scanning electron microscopy, and thermogravimetric differential thermal analyses. New results on the thermal decomposition of bismuth oxalates under inert or reducing atmospheres are provided. On heating in nitrogen, both studied compounds decompose into small bismuth particles. Thermal properties of the metallic products were investigated. The Bi(C2O4)OH decomposition leads to a Bi-Bi2O3 metal-oxide composite product in which bismuth is confined in a nanometric size, due to surface oxidation. The melting point of such bismuth particles is strongly related to their crystallite size. The nanometric bismuth melting has thus been evidenced â¼40 °C lower than for bulk bismuth. These results should contribute to the development of the oxalate precursor route for low-temperature soldering applications.
ABSTRACT
P-type semiconducting copper oxide (CuO) thin films deposited by radio-frequency (RF) sputtering were integrated onto microsensors using classical photolithography technologies. The integration of the 50-nm-thick layer could be successfully carried out using the lift-off process. The microsensors were tested with variable thermal sequences under carbon monoxide (CO), ammonia (NH3), acetaldehyde (C2H4O), and nitrogen dioxide (NO2) which are among the main pollutant gases measured by metal-oxide (MOS) gas sensors for air quality control systems in automotive cabins. Because the microheaters were designed on a membrane, it was then possible to generate very rapid temperature variations (from room temperature to 550 °C in only 50 ms) and a rapid temperature cycling mode could be applied. This measurement mode allowed a significant improvement of the sensor response under 2 and 5 ppm of acetaldehyde.
ABSTRACT
The integration of a 50-nm-thick layer of an innovative sensitive material on microsensors has been developed based on silicon micro-hotplates. In this study, integration of ZnO:Ga via radio-frequency (RF) sputtering has been successfully combined with a low cost and reliable stencil mask technique to obtain repeatable sensing layers on top of interdigitated electrodes. The variation of the resistance of this n-type Ga-doped ZnO has been measured under sub-ppm traces (500 ppb) of acetaldehyde (C2H4O). Thanks to the microheater designed into a thin membrane, the generation of very rapid temperature variations (from room temperature to 550 °C in 25 ms) is possible, and a rapid cycled pulsed-temperature operating mode can be applied to the sensor. This approach reveals a strong improvement of sensing performances with a huge sensitivity between 10 and 1000, depending on the working pulsed-temperature level.
