M. Kakazey

Hyper-Rapid thermal defect annealing during grinding of ZnO powders

We report on the changes in the defect structure of ZnO particles that take place during the grinding of pure ZnO powders and mixtures ZnO–SnO2 and ZnO–TiO2 powders. The qualitative differences in the electron paramagnetic resonance spectra for different specimens were discussed in the context of the hyper-rapid high-temperature spikes created in ZnO particles during mechanical treatment. The thermal spikes cause the defect structure to undergo annealing. The spike duration of the hyper-rapid thermal defects annealing was dependent on the heat conductivity of the ZnO particle environment.

Defect states and morphological evolution in mechanically processed ZnO + xC nanosystems as studied by EPR and photoluminescence spectroscopy

An evolution of electron paramagnetic resonance (EPR) and photoluminescence (PL) spectra of various active states (hydrogen donor DH EPR centers, Zn vacancy-related EPR centers , EPR DS centers from shallow donors (g = 1.9640) in ZnOW, Mn2+ ions in ZnOZB, C EPR centers in carbon nanoparticles, forming the near-band-edge (NBE) PL emission, PL emission typical for Zn-, O- and N-enriched ZnOW particles, as well as oxidized carbon nanodots (OCN)) was observed in the mixtures of ZnO + xC nanoparticles during prolonged high-energy mechanical processing (MP) in a hermetically sealed grinding chamber. The results of EPR and PL spectroscopy, X-ray diffraction analysis, as well as morphological analysis by means of atomic force microscopy (AFM) and laser particle sizer (LPS) measurements show a wide variety of interrelated series–parallel processes in the samples with increasing MP processing time (tMP). These processes include: (a) dramatic reduction in intensity of the DH EPR signal and PL bands at 3.14 (1.57), 2.53 and 2.3 eV during the first minutes of MP, which correlate with sample disaggregation; (b) grinding of ZnO particles and formation of Zn vacancy-related EPR centers in the area of destruction (AD); (c) an increase in sample temperature; (d) annealing of the Zn vacancy-related EPR centers formed; (e) initiation of carbon nanoparticle interaction with oxygen in the grinding chamber; (f) formation and growth of the EPR signal due to carbon nanoparticles; (g) formation of a reducing environment in the grinding chamber; (h) stabilization of donor DS-centers in AD of ZnO nanoparticles; (i) an increase in CO concentration in the grinding chamber; (j) inhibition of DS paramagnetic centers in ZnO; (k) initiation of the EPR signals due to Mn2+ ions in ZnOZB sphalerite phase in the sample with the lowest carbon content; (l) inhibition of nitrogen PL centers in ZnO; (m) the formation of oxidized carbon nanodots (OCN) showing a PL band at 2.8 eV. A detailed analysis for the localization of EPR and PL centers in the MP samples is presented.

Fine structure of EPR spectra of Fe3+ in α-Al2O3 crystal, powders, and textured ceramics

Abstract In this work, a comparison of the angular dependencies B r ( θ ) of the fine structure lines for Fe 3+ in crystal α-Al 2 O 3 with the location of EPR singularities B s in polycrystalline samples at the same frequency was made. It was shown that the number and location of the singularities are set by extreme points on curves B r ( θ ). An identification of singularities by the crystallographic orientation of the crystallites that are formed was carried out. The angular dependence of the intensities of the singularities intensity in ceramic specimens α-Al 2 O 3 :Fe 3+ was found for their revolution around of an axis, perpendicular to the direction of pressing of the initial preparations. The observed effect was explained by the texturing of disk-like particles of α-Al 2 O 3 :Fe 3+ during the technological operations of pressing and sintering. It was shown that the orientation distribution of crystallites over the volume of a ceramic plate represents an ellipsoid of revolution around an axis of the sample parallel to the direction of pressing. The anisotropy coefficient of the samples studied equals ≈1.7. A method for the choice of singularities, which can be used as EPR-probes for a correct determination of the anisotropy coefficient in ceramic samples is recommended.

Closed cycle of recycling of waste activated sludge

The recycling of waste activated sludge (WAS) formed in the process of biological purification of sewage is an urgent ecological problem. In the present work, two ways of recycling of WAS containing from 8 to 30% free water, namely, the synthesis of a carbon-containing component and synthesis of porous building ceramics (bricks) with the use of WAS and waste carbonizate, have been considered. For the preparation of a carbon adsorbent, the carbonization of WAS has been carried out in an argon atmosphere. For the synthesis of ceramics, clay-cullet-tezontle-WAS mixtures with different contents of the components have been used. Sintering has been performed in air. It has been established that, in treatment of WAS at 600 °C for 30 min, better adsorption properties are obtained due to the presence of free carbon bonds. The efficiency of water purification from dyes (methylene blue) depends on the standard conditions: the methylene blue concentration, cabonizate-to-solution ratio, and exposure time of the carbonizate in solution. The use of wet WAS makes it possible to exclude the addition of water from the traditional scheme of preparation of a plastic semiproduct, i.e., realize a water-saving technology. The introduction of low-melting cullet, basalt, and WAS powders into red clay makes enables us to reduce substantially the sintering time of porous bricks (down to 8 h) and vary their strength properties.

Laser Synthesis Features of Composite Ceramics Y3Al5O12-Y2Ti2O7-Al2O3-Al2TiO5

The effect of directed laser treatment on the compacted binary mixtures Al2O3–Y2O3, Y2O3–TiO2, Al2O3–TiO2 and ternary mixtures Al2O3−Y2O3−TiO2 was investigated. It was shown that phase formation occurs within the framework of binary mixtures Al2O3–Y2O3, Y2O3–TiO2, Al2O3–TiO2 and is accompanied by the formation of Y3Al5O12, Y2Ti2O7 and Al2TiO5 compounds. The set of phases and microstructures of generated ceramics depend on Al2O3 content and TiO2/Y2O3 ratio in initial mixtures. Basing in the obtained results the tentative phase diagram for the Al2O3–Y2O3−TiO2 system was constructed.