Čedomir Jovalekić

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.

Mechanochemical synthesis of NiFe2O4 ferrite

A mixture of crystalline NiO and α-Fe2O3 powders in equimolar ratio was mechanochemically treated in a planetary ball mill up to 50 h of milling. The mechanochemical reaction leading to the formation of the NiFe2O4 spinel phase was followed by X-ray diffraction and magnetization measurements. The spinel phase was first observed after 10 h of milling and its formation was completed after 35 h of milling. The synthesized NiFe2O4 ferrite has a nanocrystalline structure with a crystallite size of about 6 nm. Powders obtained after 20, 35 and 50 h of milling, as well as the starting mixture without mechanochemical treatment were compacted by cold pressing and sintering. The specific electric resistivity of the compacts greatly depends on the milling time of the powder and decreases by four orders of magnitude as the milling time increases from 0 to 50 h.

Mechanochemical treatment of α-Fe2O3 powder in air atmosphere

Abstract Powder of α -Fe 2 O 3 was mechanochemically treated in a planetary ball mill in an air atmosphere. Structural changes were followed by X-ray diffraction analysis, magnetization measurements and differential scanning calorimetry after various milling times. It was found that complete transformation of α -Fe 2 O 3 to Fe 3 O 4 is possible during milling in an air atmosphere under appropriate milling conditions. Presumably, the decrease in the oxygen partial pressure during milling has a critical influence on promoting the dissociation of α -Fe 2 O 3 . Before nucleation of the Fe 3 O 4 phase, the crystallites of the α -Fe 2 O 3 phase are reduced to a minimal size accompanied by the introduction of atomic-level strain. Local modeling of a collision event, coupled with a classical thermodynamic assessment of the Fe 2 O 3 -Fe 3 O 4 system, were used to rationalize the experimental results. It is proposed that the mechanochemical reactions proceed at the moment of impact by a process of energization and freezing of highly localized sites of a short lifetime. Excitation on a time scale of ∼10 −5 s corresponds to a temperature rise of the order of (1–2)×10 3 K. Decay of the excited state occurs rapidly at a mean cooling rate higher than 10 6 K s −1 .

The ball milling induced transformation of α-Fe2O3 powder in air and oxygen atmosphere

Abstract The mechanochemical treatment of α -Fe 2 O 3 powder was done concurrently in air and oxygen atmospheres using a conventional planetary ball mill. The influence of the duration of milling and of the balls-to-powder mass ratio on the transformation of α -Fe 2 O 3 was investigated. Under appropriate milling conditions, α -Fe 2 O 3 completely transforms to Fe 3 O 4 , and for prolonged milling to the Fe 1− x O phase, either in air or oxygen atmosphere. Owing to the higher oxygen pressure, the start of the reaction in oxygen is delayed by ∼1 h in comparison with the reaction in air. The reverse mechanochemical reaction Fe 1− x O→Fe 3 O 4 → α -Fe 2 O 3 takes place under proper oxygen atmosphere. The oxygen partial pressure is the critical parameter responsible for the mechanochemical reactions. The balls-to-powder mass ratio has a considerable influence on the kinetics of mechanochemical reactions. Below the threshold value the reaction does not proceed or proceeds very slowly. Plausibly, three phenomena govern mechanochemical reactions: (i) the generation of highly energetic and localized sites of a short lifetime at the moment of impact; (ii) the adsorption of oxygen at atomically clean surfaces created by particle fracture; and (iii) the change of activities of the constituent phases arising from a very distorted (nanocrystalline) structure.

Ferroelectric properties of mechanically synthesized nanosized barium titanate

Barium titanate ceramics were prepared through mechanochemical synthesis starting from fresh prepared barium oxide and titanium oxide in rutile form. Mixture of oxides was milled in zirconia oxide jar in the planetary ball-mill during 30, 60, 120 and 240 min. Extended time of milling directed to formation of higher amount of barium titanate perovskite phase. Barium titanate with good crystallinity was formed after 240 min. Sintering without pre-calcinations step was performed at 1330°C for 2 hours with heating rate of 10°C/min. The XRD, DSC, IR and TEM analyses were performed. Electric and ferroelectric properties were studied. Very well defined hysteresis loop was obtained.

Temperature-induced structure and microstructure evolution of nanostructured Ni0.9Zn0.1O

The crystal structure and microstructure of as-prepared and annealed Ni0.9Zn0.1O were refined at room temperature in both the Fm{\overline 3}m and R{\overline 3}m space groups. It is shown that below the Neel point (458 K), where magnetic ordering triggers the presence of a trigonal strain, the common usage of a higher-symmetry non-admissible space group for crystal structure and microstructure analysis via the Rietveld method may result in both an incorrect structure description and incorrect microstructure parameters (size and strain). More realistic microstructure data can be obtained by whole powder pattern modelling of the powder diffraction data. Increasing the annealing temperature causes a reduction of the trigonal distortion as well as an increase in domain size. Simultaneously, the Raman spectra become less resolved, a clear indication of domain growth and structural evolution of the structure towards cubic symmetry (R{\overline 3}m → Fm{\overline 3}m).

Study of NiFe2O4 and ZnFe2O4 Spinel Ferrites Prepared by Soft Mechanochemical Synthesis

Two types of ferrites, NiFe2O4 and ZnFe2O4 were prepared by soft mechanochemical synthesis. XRD and Raman spectroscopy were used to characterize the ferrite samples. On the basis of magnetic measurements was confirmed that the degree of inversion changes after sintering. The conduction activation energy, ΔE was determined by fitting the DC conductivity data with the Arrhenius relation. The effect of temperature on impedance parameters was studied using an impedance analyzer in a wide frequency range (100 Hz - 10 MHz). It was observed that the impedance spectra of NiFe2O4 and ZnFe2O4 ferrites include both grain and grain boundary effects.

Dielectric and Ferroelectric Properties of BaTi1-xSnxO3 Multilayered Ceramics

Multilayered BaTi1-xSnxO3 (BTS) ceramics with different Ti/Sn ratios were produced by pressing and sintering at 1420 oC for 2 hours. X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy were used for structural, microstructural and elemental analysis, respectively. The dielectric and ferroelectric behavior of sintered samples was studied, too. It is found that in ingredient materials, with increasing Sn content, the tetragonality decreases; Curie temperature moves towards room temperature, while the maximum of the dielectric constant increases, and also, they becomes less hysteretic. It is noticed that multilayered BTS ceramics with different Ti/Sn contents have a broad transition temperature and show a relatively high dielectric constant in a wide temperature range. It is shown that dielectric properties of these materials may be modified by a combination of different BTS powders as well as layers number.

Soft mechanochemical synthesis and characterization of nanodimensional spinel ferrites

NiFe2O4 and ZnFe2O4 ferrites have been prepared by soft mechanochemical synthesis. The sintered samples were analyzed by XRD and Raman spectroscopy. Investigation of the magnetization as a function of magnetic field confirms an expected change of the degree of inversion in the spinel structure with the sintering. Impedance spectroscopy on the sintered pellets of ferrites was performed in the wide frequency range (100 Hz-10 MHz) at different temperatures using an Impedance/Gain-Phase Analyzer (HP-4194).

The influence of mechanochemical treatment of the Bi2O3–ZrO2 system on the structural and dielectric properties of the sintered ceramics

A powder mixture of α-Bi2O3 and ZrO2, both monoclinic, in the molar ratio 2 : 3, was mechanochemically treated in a planetary ball mill in an air atmosphere for up to 20 h, using steel vial and hardened-steel balls as the milling medium. Mechanochemical reaction leads to the gradual formation of an amorphous phase. After 5 h of milling the starting α-Bi2O3 and ZrO2 were transformed fully into a non-crystalline phase. After milling for various times the powders were compacted by pressing and isothermal sintering. The pressed and sintered densities depended on the milling time. Depending on the duration of the mechanochemical treatment and sintering temperature, the phases: γ-Bi12(ZrxFe1−x)O20; Bi(ZrxFe1−x)O3 and Bi2(ZrxFe1−x)4O9 were obtained by reactive sintering, whereby the Fe originates from vial and ball debris. The dielectric permittivity of the sintered samples significantly depends on the milling time. Samples milled for 10 and 15 h and subsequently sintered at 800 °C for 24 h exhibit a hysteresis dependence of the dielectric shift (in altering electric fields higher than 10 kV/cm at room temperature), confirming that the synthesized materials possess ferroelectric properties.