Z. 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.

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.

Microstructural investigation of the PZT films prepared from the suspension of nanocrystalline powders

PZT thin films of composition Pb(Zr0.52Ti0.48)O3 were prepared by a novel method from the suspension of nanocrystalline PZT powders. The powders were obtained by mechanochemical synthesis. Films were deposited on silicon (1 0 0) and platinum covered silicon substrates (Pt (1 1 1) /Ti/SiO2/Si) using spin-on technique. Substrate type has influence on films crystallinity, orientation and can react with the films changing its phase composition. Films microstructure strongly depends on thermal treatment conditions due to phase and compositional changes of the films. # 2003 Elsevier Ltd. All rights reserved.

Novel simple methods for the synthesis of single-phase valentinite Sb2O3

Several methods with solid and dissolved reactants were investigated as possible routes for synthesis of single-phase valentinite Sb2O3. The methods are based on simple chemical reaction between SbCl3 and NaOH. The method with solid state reactants was established on self-propagating room temperature reaction (SPRT), while wet syntheses were based on the same chemical reaction, and performed in either distilled water or absolute ethanol. The prepared powders were characterized by X-ray powder diffraction, scanning electron microscopy and field emission scanning electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction (SAED) and UV/vis diffuse reflectance spectroscopy. SPRT and aqueous solution syntheses resulted in single-phase valentinite Sb2O3, but with significantly different morphologies. In the case of SPRT method the obtained powder contains well crystallized prismatic shaped submicronic particles, with hexagonal or lozenge basis typical for valentinite crystal structure, while aqueous solution synthesis resulted in powder containing micronic agglomerates. The ethanolic solution synthesis product was Sb2O3 with cubic senarmontite as predominant phase and traces of orthorhombic valentinite. It was confirmed that not only the aggregate state, but also the choice of solvent has a great influence on the structural and optical characteristics of synthesized Sb2O3 powders.

Synthesis of Pure and Doped LaMnO3 Powders from Citrate Precursors

In this work pure and doped LMO were prepared using modified Pechini method from lanthanum and manganese citrates. Lanthanum citrate was prepared starting from La2O3, while manganese citrate was prepared from Mn(NO3)2. Valence state of the manganese was controlled by adjusting pH value of the solution and confirmed by EPR and UV/VIS analysis. Thermal treatment conditions for preparation of LMO powders were determined from TG/DTA of dried precursors. XRD results confirmed that pure perovskite phase was successfully prepared in single LMO and Cadoped LMO. SEM and measurements of specific surface area of the powders showed the existence of large agglomerates consisting of submicronic primary particles.

Spectroscopic studies of SnO2 doped with Cr, Co or Nb

SnO 2 ceramics doped with different amounts of Co, Cr or Nb were investigated using visible and infrared spectroscopy at room temperature. Based on the observed d-d transitions the valence states of incorporated dopants were determined. Values of the optical band-gap were calculated in all samples. The infrared spectra of the samples displayed variations in the position, relative intensity and width of the bands, which were attributed to the presence of dopants.