Tatjana Srećković

The influence of mechanical activation on zinc stannate spinel formation

Abstract Mechanical activation of inorganic materials leads to specific changes of their chemical and physical properties. Grinding, as one way of mechanical activation, is a widely used method for obtaining highly dispersed systems, and it could be performed in various types of mills (planetary, centrifugal, vibro-mill, etc.). The development of advanced materials is, therefore, dependent not only on the investigated material, but on characteristics of a device as well. Polycrystalline zinc stannate spinel, Zn 2 SnO 4 , is a material used for combustible gases and humidity detection, photoelectrochemical applications, coatings, etc. The subject of this work is the influence of mechanical activation on solid state chemical reaction, e.g. the formation of porous zinc stannate ceramics during different thermal treatments of compacts obtained from ZnO and SnO 2 powder mixtures mechanically activated in a high energy vibro-mill. X-ray diffraction analysis, scanning electron microscopy and non-isothermal dilatometric measurements were performed in order to investigate spinel formation.

Defect induced variation in vibrational and optoelectronic properties of nanocrystalline ZnO powders

Structural disorder of ZnO nanopowders with mean crystallite size down to 15 nm, produced by mechanical activation in high energy mills, has been analyzed by x-ray diffraction and Raman spectroscopy. The influence of such disorder on optical and electronic properties of activated ZnO nanopowders has been investigated using photoluminescence spectroscopy and spectroscopic ellipsometry. A revised interpretation of the resonant enhancement of the first and second order Raman scattering by the E1(LO) phonons in highly disorder ZnO nanopowders has been proposed. Detailed analysis of resonant Raman effects in ZnO powders under sub band gap excitation has given valuable information about defect induced electronic states in the band gap of ZnO. Systematic trend in the electron–phonon coupling strength, with the correlation length which depends on lattice disorder in ZnO, has been also demonstrated.

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.

Microstructural characterization of mechanically activated ZnO powders

In this paper, changes of microstructural characteristics of disperse systems during mechanical activation of zinc oxide (ZnO) have been investigated. ZnO powder was activated by grinding in a planetary ball mill in a continuous regime in air during 300 min at the basic disc rotation speed of 320 rpm and rotation speed of bowls of 400 rpm but with various balls‐to‐powder mass ratios. During ball milling in a planetary ball mill, initial ZnO powder suffered high‐energy impacts. These impacts are very strong, and large amounts of microstructural and structural defects were introduced in the milled powders. The morphology and dispersivity of particles and agglomerates of all powders were investigated by scanning electron microscopy and scanning transmission electron microscopy. The specific surface area of initial ZnO powder was determined as 3.60 m2 g−1 and it increased to 4.42 m2 g−1 in mechanically activated powders. An increase of the ball‐to‐powder mass ratio led to a decrease of particle dimensions as well as increased the tendency for joining into quite compact agglomerates, that is aggregates, compared with the very loose, soft initial agglomerates. The obtained results pointed out that activation of ZnO powders produces a highly disperse, nano‐scaled mixture of small particles, that is crystallites with sizes in the range of 10–40 nm. Most of these particles are in the form of aggregates with dimensions of 0.3–0.1 μm. The crystallite and aggregate size strongly depend on milling conditions, that is ball‐to‐powder mass ratio, as shown in this investigation.

SEM investigation of domain structure in (Ba,Ca,Pb)TiO3

In the present paper the microstructure and domain structure in modified BaTiO3 with Pb and Ca as additives have been investigated using SEM technique. The (Ba,Pb)TiO3 and (Ba,Ca,Pb)TiO3 ceramics show a slight diAerence in grain size, being smaller in composites with Ca additives which acts as grain growth inhibitor. The domain configuration is almost the same. The small grain microstructure with tiny domains have been observed in specimen sintered at 1300C and the average grain size is in the range 1‐3mm. For those specimens sintered at 1320C the homogenous microstructure is also obtained with grain size around 2‐4mm. For both types of specimens, the single domain structure is associated with grain which size is lower than 2mm. The banded domain structure could be observed in grains with size bigger than 3mm. The bar shape grains and elongated grains together with some large region in microstructure are free of domain structure. The observed domain patterns reveal mainly the straight domain boundary lines with 90 domains walls. The wall thickness ranged from 0.03m mt o 0.15mm, while the domain width is in the range of 0.1mm‐1mm. # 1999 Elsevier Science Limited. All rights reserved

Influence of Mechanical Activation on Synthesis of Zinc Metatitanate

Investigations of a ZnO-TiO2 binary oxide mixture during mechanical treatment were mainly focused on obtaining orthotitanate Zn2TiO4 with a spinel structure. Due to the specific way of energy transfer during mechanical treatment using a high-energy ball mill, the system passes through low temperature ZnTiO3 metatitanate phase formation. Mechanical activation was performed on an equimolar ratio mixture of ZnO and TiO2. The anatase phase was previously submitted to heat treatment for achieving а starting mixture rich in a rutile phase. Milling conditions were preset for observing the formation of a low temperature ZnTiO3 phase with a perovskite structure. The powder microstructure was characterized using scanning electron microscopy. A nitrogen gas sorption analyzer with the BET method was used to determine the specific surface area and porosity, indicating changes of powder sample properties during mechanical activation. Also, X ray powder diffractometry was applied to obtain the phase composition. Powders were then pressed into pellets and their compressibility was observed through density changes. According to microstructures obtained by scanning electron microscopy analysis, the system underwent a primary and secondary agglomeration process. Specific surface area measurements supported that conclusion. Compressibility investigations established the difference between compressibility of the non-activated mixture and activated powders. X-ray diffraction analysis revealed that a perovskite structure forms simultaneously with a spinel phase during the process of

Evolution of the Defect Structure of Zinc-Oxide as a Consequence of Tribophysical Activation

Zinc-oxide powder was tribophysically activated in a high-energy vibro mill in a continual regime in air for 3, 30 and 300 minutes with the purpose of modifying the powders physico-chemical properties. By analyzing of data obtained by X-ray powder diffraction, electron diffraction and transmission electron microscopy, the values of distances between corresponding crystallographic planes, average domain sizes of coherent scattering, i.e. crystallites, width of diffraction lines due to the existence of microstrains, and microstrain values, minimal dislocation densities, dislocation density due to microstrain and real dislocation density, and also average distances between dislocations were determined. The dependence of these values on the activation time was established, which enabled analysis of the evolution of the defect structure of zinc-oxide powders during tribophysical activation by grinding in the described regime.

Investigation of Zinc-Stannate Synthesis Using Photoacoustic Spectroscopy

Mixtures of ZnO and SnO2 powders, with molar ratio of 2:1, were mechanically activated for 40, 80 and 160 minutes in a planetary ball mill. The resulting powders were compacted into pellets and non-isothermally sintered up to 1200˚C with a heating rate of 5˚C/min. X-ray diffraction analysis of obtained powders and sintered samples was performed in order to investigate changes of the phase composition. The microstructure of sintered samples was examined by scanning electron microscopy. The photoacoustic phase and amplitude spectra of sintered samples were measured as a function of the laser beam modulating frequency using a transmission detection configuration. Fitting of experimental data enabled determination of photoacoustic properties including thermal diffusivity. Based on the results obtained a correlation between thermal diffusivity and experimental conditions as well the samples microstructure characteristics was discussed.

The Influence of Tribophysical Activation on Zn2TiO4 Synthesis

The influence of tribophysical activation on Zn2TiO4 synthesis along with the changes in powders during tribophysical treatment was observed. Mixtures of ZnO and TiO2 powders were mechanically activated using a high-energy ball mill at different time intervals from 0 to 300 minutes. XRD was performed in order to obtain information about phase composition variations. Microstructure parameters were revealed from an approximation method. Particle size distribution along with scanning electron microscopy gave very useful information about powder morphology.

Mechanochemical synthesis of CaTiO3 from CaCO3 - TiO2 mixture

The synthesis of calcium titanate, CaTiO3, was performed by mechanical activation and thermal treatment. Milling for up to 360 minutes in a planetary ball mill mechanically activated an equimolar mixture of CaCO3 and TiO2 powders. A small amount of mechanically activated mixtures was pressed into briquettes and calcined at 850°C for two hours. The effect of mechanical activation on the solid-state reaction was studied using X-ray powder diffraction and differential thermal analysis. The change of morphology and size of powder particles due to milling, were determined by SEM, while BET analysis was used to determine the specific surface area of the powder. The sintering process was followed by a dilatometer during thermal treatment up to 1300C. The main conclusion of the analysis of conducted investigations is that CaTiO3 ceramics can be obtained from an activated mixture at a much lower temperature than reported in the literature owing to acceleration of the chemical reaction and sintering.