Margarita Milanova

Glass Formation in the MoO3-La2O3-Nd2O3 System

Novel low melting glasses in the MoO3-La2O3-Nd2O3 system were obtained at different cooling rates (102 K/s and 104-105 K/s). Characterization of the amorphous samples was made by differential thermal analysis (DTA) and X-ray absorption fine structure (XAFS) method. According to DTA data of the glass samples, the glass transition temperatures are at 325-330 0C, the crystallization started above 410 0C and the melting temperatures are at 660-720 0C. A structural model of glasses was suggested on the basis of XAFS and IR investigations. It was shown that the predominant structural units in the amorphous network of glasses containing 90 -80 mol% MoO3 are MoO6 groups. The appearance of MoO4 groups deteriorates the glass formation ability.

Crystallization of Glasses in the MoO3-Bi2O3 System

α-Bi2Mo3O12 and β-Bi2Mo2O9 crystal phases have been synthesized by crystallization of a glass for a short time. Amorphous samples with the compositions corresponding to the alpha and beta bismuth molybdates have been obtained at high cooling rates (104-105 K/s). XRD, DTA and SEM studies were performed to examine the phase formation, thermal behavior of the glasses and the microstructure of obtained crystalline products. By scanning electron microscopy it was shown that the obtained bismuth molybdates consist of dense agglomerates, containing uniform distributed crystals with submicron size.

Nano- and Microsized Phases in the WO3-ZnO-Nd2O3-Al2O3 System for Applications

WO3 and WO3-based materials in the form of thin films, either as bulk or as nanostructures, have been widely used for the detection of a variety of gases, such as NO2, H2, NH3 and Cl2. The purpose of this study was to prepare materials from the WO3-ZnO-Nd2O3-Al2O3 system containing high amount of WO3 for applications in environmental monitoring by applying the melt quenching method. Homogenized batches of the starting oxides were melted for 20 min at 1,240 °C in platinum crucibles in air atmosphere. The melts were quenched by pouring and pressing between two copper plates (cooling rates 101–102 K/s). Glassy-crystalline samples containing the monophase AlW12O37.5 was produced by quenching a melt with the composition of 80WO3⋅10Nd2O3⋅10Al2O3. Glassy-crystalline sample containing nanosized W18O49 (~60 nm) was obtained after cooling of a melt with the composition 76WO3⋅9.5ZnO⋅9.5Nd2O3-5Al2O3. A mixture of WO3 and AlW12O37.5 in the amorphous matrix was prepared by crystallization of a quenched sample with the composition 75WO3⋅10Nd2O3⋅5ZnO⋅10Al2O3. Samples were prepared by the screen printing technique as gas sensor s; thereafter dc resistance measurements were performed in the presence of relative humidity and ammonia. It was established that the glassy-crystalline sample containing the monophase AlW12O37.5 exhibits a significant sensitivity towards ammonia and relative humidity, while other phases show a good sensitivity only towards relative humidity.

Humidity Sensing Properties of Tungsten Based Glass Crystalline Materials in the WO 3 -ZnO-La 2 O 3 -Al 2 O 3 System

Glass crystalline materials from the WO3-ZnO-La2O3-Al2O3 system containing high WO3 concentrations (60–76 mol%) were prepared by controlling the glass crystallization and were then employed as humidity based sensors. According toX-ray analysis, WO3 separates as a crystalline phase from the amorphous structures with the nominal compositions 76WO3·9.5ZnO·9.5La2O3·5Al2O3 and 60WO3·7.5ZnO·7.5La2O3·25Al2O3 after the heat treatment. Samples were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) for elemental analysis. Using the screen printing technique, the synthesized crystalline glass samples were deposited onto interdigidated Pt electrodes; then the sensors were tested in the range from 0.0% to 96% relative humidity (RH) at room temperature. It was observed that the increase in the content of WO3 leads to improve sensor sensitivity towards RH.