Zorka Z. Vasiljevic

Analysis of electronic properties of pseudobrookite thick films with possible application for NO gas sensing

Two pseudobrookite (PSB) containing pastes were composed of a mixture of starting nanopowders of hematite (a-Fe2O3) and anatase (TiO2) in the molar ratios 1:1 (referred to as PSB-1) and 1:1.5 (referred to as PSB-1.5), respectively, organic vehicle and glass frit. The pastes were screen printed on alumina (Al2O3) substrates and sintered in a hybrid conveyor furnace at 850 °C/10 min in air. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis showed that the resulting thick film samples were composed of pseudobrookite (Fe2TiO5) and also excess rutile (TiO2) in the case of PSB-1.5 samples with a small grained and relatively homogenous microstructure. An electric resistivity with negative temperature coefficient (NTC) was observed. An interdigitated electrode geometry was designed and different electrode spacing (0.2 and 0.25 mm) was analyzed. The electrode structure was printed of PdAg paste. The resistivity of pseudobrookite was measured and analyzed in view of possible applications as a sensor of environmental gases with sufficiently high sensitivity at operating temperatures lower than conventional gas sensor systems.

Impedance response of pseudobrookite thick films with a sandwich configuration

We have investigated the impedance response of pseudobrookite thick films screen printed on alumina substrate in the temperature interval from room temperature to 350 °C in the frequency range 100 Hz–1 MHz. Two thick film pastes were made containing starting hematite and anatase nanopowders (in the 1:1 and 1:1.33 molar ratio — containing excess anatase), glass fritt and organic vehicle. A sandwich PdAg electrode structure was obtained by first printing a PdAg electrode, followed by 3 layers of PSB paste and a PdAg electrode on top with a mesh structure. Each layer was sintered at 850 °C in a hybrid conveyor furnace. XRD analysis confirmed the formation of pseudobrookite with an orthorhombic crystal structure. SEM analysis of thick film surfaces and cross sections showed the formation of a porous structure with rod-shaped grains. The impedance response of pseudobrookite thick films, showed a decrease with the rise in sample temperature. It was analyzed using an equivalent circuit reflecting the dominant influence of grain boundaries.

Pseudobrookite thick films for potential application as low-temperature sensitive material in NO gas sensors

Pseudobrookite thick films were obtained by screen printing paste composed of a 1:1.5 molar ratio mixture of starting nanopowders of hematite and anatase, glass frit and organic binder. The films were deposited on alumina substrates with interdigitated electrode geometry. Sintering at 850 °C resulted in the formation of pseudobrookite. X-Ray Diffraction (XRD) analysis showed pseudobrookite with a mixed orthorhombic and monoclinic structure and a small amount of excess rutile. Gas sensing measurements with low NO gas concentrations in air were performed using an in-house gas sensor testing setup in the temperature range 100–300 °C. On the example of one particular gas concentration, results showed that pseudobrookite exhibited a distinct response to NO already at 150 °C that further improved with increased sample temperature.

Nanostructured SnO2 thick films for gas sensor application: analysis of structural and electronic properties

This research is focused on structural and electrical characterisation of tin oxide (SnO2) applied as a thick film and investigation of its properties as gas sensitive material. Micron sized SnO2 powder was milled in an agate mill for six hours to fabricate SnO2 nanopowder, which was afterwards sieved by 325 mesh sieve and characterized by XRD and SEM. This powder was used as functional part in the production of thick film tin oxide paste containing a resin vehicle with 4 wt. % nanosize glass frits acting as permanent binder. The glass frits where additionally milled for twelve hours in the agate mills to nanosized powder and sieved by a 325 mesh sieve as well. The achieved thick film paste was screen printed on alumina and fired at 850oC peak temperature for 10 minutes in air. After the sintering process, thick film samples where characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The reflectivity was measured on the same samples by UV-VIS spectrophotometer: the band gap was determined from the slope of reflectance. After that a matrix of different interdigitated electrode structure of PdAg paste was printed and sintered using the mentioned sintering conditions. The tin oxide thick film was printed over the interdigitated electrodes as a top layer and sintered again under the same conditions. The total electrical resistance was measured as a function of the electrode spacing and temperature. A negative temperature coefficient (NTC) was identified and measured in the range from room temperature (27°C) to 180°C in a climate chamber. Finally the samples were placed into a gas reactor with NOx and CO gas and the resistance was measured in the same temperature range (27°C-200°C).

Lead Telluride Doped with Au as a Very Promising Material for Thermoelectric Applications

PbTe single crystals doped with monovalent Au or Cu were grown using the Bridgman method. Far infrared reflectivity spectra were measured at room temperature for all samples and plasma minima were registered. These experimental spectra were numerically analyzed and optical parameters were calculated. All the samples of PbTe doped with Au or Cu were of the “” type. The properties of these compositions were analyzed and compared with PbTe containing other dopants. The samples of PbTe doped with only 3.3 at% Au were the best among the PbTe