M. M. Ristic

Electronic paramagnetic resonance investigation of the evolution of defects in zinc oxide during tribophysical activation

Zinc oxide powder was tribophysically activated by grinding in a vibromill in a continual regime in air. Investigations based on the application of the electronic paramagnetic resonance (EPR) method were performed, with the purpose of establishing laws for defect formation from the viewpoint of the evolution of the fine defect structure of dispersed systems. The main focus was on the influence of the duration of tribophysical activation (TA) on the process of defect formation. Six signals were seen on the EPR spectra obtained. These signals were identified, which enabled definition of the evolution scheme of the defect structure in polycrystalline samples of zinc oxide during tribophysical activation by grinding, based on the established dependence of the formation of different centres from the grinding time.

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.

Evolution of the microstructure of disperse Zinc-oxide during tribophysical activation

The process of macro- and microstructural transformations of zinc-oxide powders, which were tribophysically activated by grinding in a vibro-mill was investigated using methods of transmission electron microscopy, infrared spectroscopy and X-ray. It is shown that tribophysical activation contributes to a gradual modification of the fine defect structure of zinc-oxide powders. In the starting stage agglomerates and bigger, longer particles are destroyed first of all. As a result of the formation of both volume and surface defects and changes of the character of interparticles interactions the plate-like polycrystal particles are created. They actually present sets of coherent scattering region.

Synthesis and Characterization of Zinc Titanate Nano-crystal Powders Obtained by Mechanical Activation

Development of dielectric materials for microwave frequencies is increasing with rapid progress in mobile and satellite communications systems, where zinc titanates have found application due to their semi-conducting and dielectric properties. Mechanical activation by grinding is a well-known method and common part of the powder preparation route in the field of ceramics. The aim of this work is investigation of the influence of experimental conditions for mechanochemical synthesis of zinc orthotitanate. Starting powder mixtures of ZnO and TiO2, in the molar ratio that is in accordance with the stoichiometry of zinc titanate spinel type Zn2TiO4, were mechanically activated using a high-energy planetary ball mill. The process of mechanical activation was performed during different time intervals from 0 to 300 minutes. Microstructure characterization was determined by X-ray diffraction analysis and scanning electron microscopy. Also, the specific surface area (SSA) of powders samples was measured by a nitrogen gas sorption analyzer using the BET method. The very first traces of zinc titanate are detectable after only 5 minutes of activation. The most interesting occurrence during the mechanical method of activation is that we have an almost pure phase after 90 minutes.

The dependence of the work function of rare earth metals on their electron structure

Abstract The work function primarily depends on the electron structure of the analysed material. Having in mind the importance of rare earth metals in advanced electronics, in this paper the dependence of the work function of rare earth metals on their electron structure has been analysed. It is shown that dependencies of work functions on the number of electrons of the most stable f 7 and f 14 configurations can be established for the series CeSm and GdTm.

Sintering and characterization of Bi4Ti3O12 ceramics

Polycrystalline ferroelectric Bi4Ti3O12 ceramics have been prepared by the method of reactive liquid phase sintering. The sintering behaviour of the Bi2O3-TiO2 composite was examined by plotting the isothermal densification curves. The results indicate that the starting oxides are involved in the reaction even at temperatures lower than or equal to 800°C, but the reaction advances at a very slow rate. Above solidus, the liquid phase promotes an extended reaction. Saturation observed in two densification curves, at 875 and 1100°C demonstrate that the reaction proceeds by two steps. A completion of the Bi4Ti3O12 formation occurs after 60 min of sintering at 1100°C. Optical micrographs of sintered bismuth titanate ceramics show randomly oriented ferroelectric grains separated by a paraelectric intergranular layer. The Bi4Ti3O12 crystallites exhibit a platelike morphology, similar in the appearance to mica, as evidenced by scanning electron micrographs. Isothermal annealing (750 to 950°C) does not affect the microstructure and electric properties of sintered bismuth titanate. The considerable value of dielectric permittivity and the appearance of hysteresis have been correlated to the presence of oxygen vacancies within the pseudotetragonal structure of Bi4Ti3O12. The oxygen vacancies are preferentially sited in the vicinity of bismuth ions as evidenced by X-ray photoemission data. XPS and AES measurements confirm that the surface concentration of cations comprising the Bi4Ti3O12 ceramics does not deviate from the nominal bulk composition.

Influence of mechanical activation on microstructure and crystal structure of sintered MgO-TiO2 system

Mixtures of MgO-TiO2 were mechanically activated using high-energy planetary ball mill during 5, 10, 20, 40, 80 and 120 minutes. Sintering process was preformed in air at 1100 o -1400 o C for 2h. The decrease in powder’s particle size was noticed as the time of mechanical activation increased and confirmed by particle size analyzer. XRD analyses were performed in order to acquire the information about phase composition. Different ratio mixtures of MgTiO3 and Mg2TiO4 are present within all sintered samples. The effect of tribophysical activation on microstructure was investigated by scanning electron microscopy. The differential thermal gravimetric analysis has been performed in order to investigate thermal behaviour of the mixtures.

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.