Nanostructured Thick-Film Spinel Ceramic Materials for Sensor Device Applications
Nanostructured spinel ceramics based on mixed transition-metal manganites and/or magnesium aluminates are known to be widely used for temperature measurements, in-rush current limiting, liquid and gas sensing, flow rate monitoring and indication, etc. But their sensing functionality is sufficiently restricted because of bulk performance allowing, as a rule, no more than one kind of application. To fabricate the integrated temperature-humidity thick-film sensors, only two principal approaches have been utilized, they being grounded on temperature dependence of electrical resistance for humidity-sensitive thick films and/or on humidity dependence of electrical resistance for temperature-sensitive thick films.
Thick-film form of mixed spinel manganites restricted by NiMn2O4-CuMn2O4-MnCo2O4 concentration triangle has a number of essential advantages, non-available for other ceramic composites. Within the above system, one can prepare fine-grained semiconductor materials with p +-type (Cu0.1Ni0.1Co1.6Mn1.2O4) and p-type of conductivity (Cu0.1Ni0.8Co0.2Mn1.9O4). This opens a real possibility of preparing multilayer thick-film spinel-type structures for new device application. In addition, the prepared multilayer thick-film structures involving semiconductor NiMn2O4-CuMn2O4-MnCo2O4 and insulating (i-type) MgAl2O4 spinels can be used as simultaneous thermistors and integrated temperature-humidity sensors.
Temperature sensitive Cu0.1Ni0.1Co1.6Mn1.2O4/Cu0.1Ni0.8Co0.2Mn1.9O4-based and humidity sensitive MgAl2O4-based pastes were prepared by mixing powders of basic with ecological glass powders (without PbO), inorganic binder Bi2O3 and organic vehicle. Prepared paste were printed on alumina substrates (Rubalit 708 S) with Ag-Pt electrodes using a using a manual screen-printing device equipped with the steel screen. Then, thick films were annealed in the PEO-601-084 furnace.
All obtained separate temperature sensitive thick-film elements based on spinel-type NiMn2O4-CuMn2O4-MnCo2O4 ceramics have excellent electrophysical characteristics. Both thick films exhibit good temperature sensitivity in the region from 298 to 368 K. The studied thick-film elements based on i-type MgAl2O4 ceramics possess linear dependence of electrical resistance from relative humidity (RH) in semilogarithmic scale without hysteresis effects in desorption cycle in the range of RH ∼40–99%.