Goran Branković

Mechanochemical synthesis of PZT powders

PZT ceramic powders were successfully prepared from the mixture of PbO, ZrO2 and TiO2 by mechanochemical synthesis in a planetary ball mill, under different milling conditions. Phase evolution during synthesis was monitored by X-ray diffraction analysis. Intensive milling resulted in formation of the nanocrystalline, perovskite PZT powders after 1 h of milling. This is a significant improvement in comparison to milling conditions reported by other authors. Depending on milling parameters the presence of some other phases, such as unreacted ZrO2, was also detected in some samples. The changes in powder size and morphology due to intensive milling, were determined by SEM and TEM, while BET analysis was used to determine specific surface area of the powders. Conclusions about processes taking place during mechanochemical synthesis of PZT powders were made based on the results of characterization. # 2002 Elsevier Science B.V. All rights reserved.

Structural and electrical properties of ZnO varistors containing different spinel phases

Abstract Structural and electrical properties of ZnO varistors were investigated as a function of spinel composition. Six varistor mixtures differing only in chemical composition of spinel, were prepared by mixing separately synthesized constituent phases (DSCP method). Compositions of constituent phases in sintered samples were investigated by changes of lattice parameters of the phases, as well as by EDS analysis of the constituent phases. It was found that compositions of ZnO, intergranular and spinel phases were partially changed during sintering due to redistribution of additives, that was controlled by starting spinel composition and its stability. Electrical characterization showed significant difference in electrical properties of investigated varistors: nonlinearity coefficients ranging from 22 to 55 and leakage currents differing by the order of magnitude. Activation energies of conduction were obtained from ac impedance spectroscopy measurements. Calculated values of activation energies were in the range 0.61–1.0 eV confirming difference in defect structure of ZnO grain boundaries in varistors containing different spinel phases.

Structural characterization of self-assembled ZnO nanoparticles obtained by the sol?gel method from Zn(CH3COO)2?2H2O

Zinc oxide nanopowders were synthesized by the sol-gel method from an ethanol solution of zinc acetate dihydrate. Detailed structural and microstructural investigations were carried out using x-ray diffraction, Raman spectroscopy, thermogravimetric and differential thermal analyses, as well as high-resolution transmission electron microscopy (TEM) and field-emission scanning electron microscopy. The intermediate compound of the reaction was layered zinc hydroxide acetate that further transforms into hexagonally shaped ZnO crystalline nanoplates (d(m) = 4 nm), which aggregate into larger spherical particles. According to the TEM analysis the ZnO nanoparticles were self-assembled into larger particles with the same orientation, i.e. aligned lattice planes of the particles. A further solvothermal treatment resulted in hexagonal, prismatic ZnO mesocrystals.

Formation of two-dimensional (2D) lead dendrites by application of different regimes of electrolysis

Electrodeposition of lead from nitrate electrolyte in constant regimes of electrolysis was analyzed and the obtained powder lead deposits were examined by scanning electron microscopy. Polarization curve for lead electrodeposition consisted of two parts separated by an inflection point. The first part of the polarization curve was characterized by a linear dependence of the current density on overpotential. The linear part of the polarization curve corresponded to ohmic-controlled electrodeposition and single lead crystals were formed in this range of overpotentials. A rapid increase in the current density with increasing overpotential was observed after the inflection point (the second part of the polarization curve). Two-dimensional dendrites were the dominant morphological forms obtained at overpotentials and current densities belonging to the second part of the polarization curve, indicating that the rapid increase of the current density with increasing overpotential corresponded to activation controlled electrodeposition at the tips of the formed dendrites. Comparing the morphologies of the obtained lead deposits with those belonging to the same group of metals (metals characterized by a high exchange current density), such as silver, cadmium, and tin, a strong dependence between the nucleation type and the shape of dendrites for the metals belonging to the same group was established.

Solid state synthesis of extra phase-pure Li4Ti5O12 spinel

Extra phase-pure Li4Ti5O12 spinel with particle sizes less than 500 nm was synthesized by solid state reaction of mechanochemicaly activated mixture of nano anatase and Li2CO3 for a very short annealing time, 4 h at 800 °C. Structural and microstructural properties, the mechanism of solid state reaction between anatase and Li2CO3 as well as thermal stability of prepared spinel were investigated using XRPD, SEM and TG/DSC analysis. The mechanism of reaction implies decomposition of Li2CO3 below 250 oC, formation of monoclinic Li2TiO3 as intermediate product between 400 and 600 °C and its transformation to Li4Ti5O12 between 600–800 oC. The spinel structure is stable up to 1000 oC when it is decomposed due to Li2O evaporation.

Structural characterization of mechanically milled ZnO: influence of zirconia milling media

Zinc oxide nanoparticles were obtained by milling in a planetary ball mill with a zirconia milling assembly for up to 5 h in air. The samples were characterized by scanning electron microscopy, x-ray diffraction (XRD) and Raman spectroscopy methods. The deviation of the lattice parameters from single crystal values was related to defect creation and increase of strain inside the hexagonal lattice of milled ZnO nanoparticles. The observed redshift and peak broadening of the major first-order Raman modes were ascribed to the formation of intrinsic defects by mechanical milling combined with the effects of phonon confinement in nanosized powders. To investigate the type of intrinsic defects and impurities introduced during milling, it was necessary to analyze both milled and thermally treated ZnO. After thermal treatment, the intensity of the Raman spectra increased and the peak positions reverted to values similar to those in unmilled ZnO powder, pointing to defect annihilation. XRD patterns of sintered samples confirmed the existence of zirconia impurities and the Rietveld analysis revealed a small amount of zirconium introduced in the ZnO crystal lattice on the Zn sites or interstitial sites. The large influence of those impurities on the micro-Raman spectra of thermally treated samples was observed in this study.

Strontium titanate films prepared by spray pyrolysis

Strontium titanate thin films were prepared by spray pyrolysis technique. Deposition parameters, such as solution concentration, time and temperature of deposition, and flow rate of carrier gas were optimized to obtain dense films without cracks. Films with different thicknesses were prepared through the control of deposition time. Prepared thin films were homogeneous, well crystallized, with uniform grain size.

Solvothermal syntheses of nano- and micro-sized ZnO powders with a controllable morphology

Zinc oxide powders with different morphologies and grain sizes were synthesized using solvothermal methods from ethanolic zinc acetate solutions in the presence of lithium hydroxide. The influence of the temperature and the time of the reaction, as well as the pH value of the starting solution, on the ZnO particle size and morphology were examined. It was found that an increase in the pH value from 8 to 12 results in a significant decrease in the mean particle size. Also, the particles’ morphology can be changed from hexagonal plates and prisms to rods by controlling the reaction time and the temperature. The crystallization mechanism is discussed, based on established correlations such as the particle size/shape versus the reaction parameters. Dissolution/recrystallisation is the most probable growth mechanism responsible for the ZnO particles’ morphology obtained in the solvothermal (hydrothermal) reactions with a basic solution. The planar structure of the zinc-hydroxy-acetate molecule plays the main role in growing the structures during the sovothermal reactions with a slightly acid solution.

Morphological and crystallographic characteristics of electrodeposited lead from a concentrated electrolyte

Electrodeposition of lead from a concentrated nitrate solution was examined by scanning electron microscopy (SEM) and by X-ray diffraction (XRD) analysis of the obtained powder particles. Single crystals of the (111) preferred orientation were formed at a low overpotential by ohmic controlled electrodeposition. Irregular crystals, needle-like and fern-like dendrites, predominantly of the (111) preferred orientation, were formed at high overpotentials (the diffusion control of the electrodeposition). The ratio of Pb crystallites oriented in the (200), (220), (311) and (331) planes increased with increasing electrodeposition overpotential. The correlation between the morphologies and crystallographic structures of the lead deposits was discussed by the consideration of general characteristics of growth layers in electrodeposition processes.

Morphological and crystallographic characteristics of lead powder obtained by electrodeposition from an environmentally friendly electrolyte

Lead powder obtained by potentiostatic electrodeposition from alkaline electrolyte, based on hydroxide ions, was investigated. The shape of lead crystals strongly depends on overpotentials of electrodeposition. The regular crystals are formed in the ohmic control. The shape of dendrites formed in the control of diffusion has a function of overpotentials of the electrodeposition. Increasing overpotential leads to branching of dendrites from primary type to those with developed tertiary branches. Formation of the very branchy dendrites of the strong (111) preferred orientation is explained on the basis of the affiliation of this electrolyte to the group of the complex Pb electrolytes.