Miodrag Zdujić

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.

Mechanochemical treatment of ZnO and Al2O3 powders by ball milling

Abstract Mechanochemical treatment of ZnO-Al 2 O 3 powder mixtures of nominal compositions 0, 10, 50 and 100 mol% Al 2 O 3 was carried out in a planetary ball mill up to 10 h of milling. Structural changes as a consequence of milling were followed by X-ray diffraction. Two possibilities of intensive ball milling were demonstrated: one is the synthesis of the ZnAl 2 O 4 spinel by mechanochemical reaction between ZnO and Al 2 O 3 , and the other is the preparation of nanocrystalline ZnO powders.

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.

Kinetics of heterogeneous methanolysis of sunflower oil with CaO∙ZnO catalyst: Influence of different hydrodynamic conditions

The kinetics of heterogeneous methanolysis of sunflower oil was studied at 60°C using mechanochemically synthesized CaO∙ZnO as catalyst. Influence of agitation speed, catalyst amount and methanol to oil molar ratio on the rate of reaction was analyzed. The rate of the process depends on the two resistances - mass transfer of triglycerides to the catalyst surface and chemical reaction on the catalyst surface, which are defined as the values of the overall triglyceride volumetric mass transfer coefficient, kmt,TG, and the effective pseudo first-order reaction rate constant, k, respectively. These kinetic parameters actually determine the value of the apparent reaction rate constant, kapp, whose change with time is defined with the change of triglyceride (TG) conversion. The kinetic model was proposed and the model parameters determined. [Projekat Ministarstva nauke Republike Srbije, br. 45001]

Intermetallic phases produced by the heat treatment of mechanically alloyed AlMo powders

Abstract The thermal behaviour of Al- x at.%Mo ( sx =, 10, 17, 20, 27, 50 and 75 powders mechanically alloyed in a conventional ball mill was investigated by differential scanning calorimetry, differential thermal analysis and X-ray diffraction. The temperature and heat of the solid state reactions leading to the formation of intermetallic phases markedly varied with milling time owing to significant microstructural changes occurring during milling. The intermetallic phases Al 2 Mo, Al 5 Mo, Al 4 Mo, Al 8 Mo 3 and AlMo 3 were identified as reaction products. The solid state reactions were discussed in terms of nanocrystalline structures created by mechanical alloying. After 1000 h of mechanical alloying, powders of various nominal compositions exhibited remarkably different microstructural parameters, i.e. crystallite size, intercrystalline (amorphous phase) volume fraction and intercrystalline thickness as well as intercrystalline composition. The most important factors affecting the change in thermal behaviour with milling time were suggested to be the reduction in crystalline size and increase in the intercrystalline volume fraction.

Synthesis of ZnO and ZrO2 Powders by Mechanochemical Processing

The ZnO and ZrO2 powders were prepared by mechanochemical processing and subsequent heat treatment of the starting powder of precursors mixture of ZnCl2 and Ca(OH)2, and ZrOCl2·8H2O and NaOH, respectively. Inert salt matrix, ether CaCl2 or NaCl, which prevents particle agglomeration was formed during mechanochemical solid state reaction. After mechanochemical treatment, samples were calcined at various temperatures. Selective removal of the matrix phase by washing the resulting powder with appropriate solvent yields almost pure ZnO and ZrO2 powders. Characterization of the powders was performed by X-ray diffraction (XRD), differential thermal and thermo gravimetric analysis (DTA−TG) and scanning electron microscopy (SEM).

Mechanochemical Activation in Synthesis of LaTi0.5Mg0.5O3 Perovskite-Type Oxides

Perovskite-type oxide of LaTi0.5Mg0.5O3 composition was synthesized from constituent oxide precursors by mechanochemical activation method in a high power planetary ball-mill, followed by thermal treatment of the product in air. The phase composition after mechanochemical activation of the binary and ternary mixtures was studied by XRPD, while the surface state of the constituent elements was characterized by XPS. The phase composition and surface properties of binary and ternary mixtures obtained by mechanochemical treatment show an affinity of the constituent oxides for the formation of perovskite-type compounds. Using thermally stable phase LaTi0.5Mg0.5O3 (perovskite-type) obtained by the applied procedure, a total oxidation of metane at 750 800 °C was achieved.

Microstructure and mechanical properties of hot-pressed Ni3Al-based intermetallic powder produced by rotating electrode process

Abstract Rapidly solidified powder of Ni3Al intermetallic compound modified with Fe, Ti and B was produced by a rotating electrode process (REP). The obtained powder has a mean particle size of 400 urn, spherical shape and very narrow size distribution. The powder was compacted by hot pressing to non-porous, homogeneous compacts with an average grain size of 50 μm. The yield strength shows positive temperature dependence with the maximum at 600°C. The ductility decreases moderately with temperature to the minimum at 600°C, and then abruptly increases. The results of this work show that the mechanical properties of the hot-pressed Ni3Al-based powder produced by REP are better than or at least comparable to nickelaluminides obtained by advanced consolidation techniques (hot isostatic pressing, hot extrusion).