Lj. Karanović

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.

Cation distribution and magnetic properties of ternary spinels

The series of spinels was prepared by ceramic technology. The crystal structures were refined from the x-ray diffraction data using the Rietveld profile method. The high-temperature magnetic susceptibility was measured by the Faraday method in the temperature range 290 - 600 K and the data were fitted to the Curie - Weiss law. The cation distribution in these diluted magnetics is obtained from x-ray diffraction and high-temperature magnetic susceptibility measurements. It was found that, for , ions occupy exclusively the octahedral position in the spinel type of structure. In the low-concentration region a strong clustering of magnetic ions is observed. In the range , ions are distributed in both tetrahedral and octahedral cationic sites. The Curie - Weiss paramagnetic temperature is negative in the whole range of concentrations, as the consequence of a predominant antiferromagnetic interaction. The appearance of the ions in tetrahedral sites (around x = 1.33) leads to a drop in the absolute -value. The effective magnetic moments of ions are about for and depend on the concentration for x>1.33.

On the possibility of pyrochlore phase formation in zinc oxide varistor ceramic

Abstract Formation of a pyrochlore phase and its transformation to the spinel phase in some multicomponent oxide mixtures has been investigated by X-ray diffraction. The effect of reaction temperature and material composition on the formation of pyrochlore phase and its lattice constant is discussed. In the system Bi 2 O 3 ·Sb 2 O 3 ·0.5 Cr 2 O 3 ·0.5 MnO 2 ·0.15 Co 3 O 4 ·0.5 NiO, an almost pure pyrochlore phase with a =1.0339(5) nm was formed at 950°C. In the presence of ZnO the pyrochlore gradually transforms into spinel phase in the temperature range 690–1200°C. The complete pyrochlore → spinel transformation occurs below 1000°C in the system 10ZnO·Bi 2 O 3 ·Sb 2 O 3 ·0.5 Cr 2 O 3 ·0.5 MnO 2 ·0.15 Co 3 O 4 ·0.5 NiO.

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.

The change of crystal symmetry and cation ordering in Li–Mg ferrites

Abstract In this work we present results of synthesis and structural investigation of the spinels with general formula Li x Mg 1−2 x Fe 2+ x O 4 , where 0.00≤ x ≤0.50. Ultrafine powder of MgFe 2 O 4 ( x =0.00) was prepared by thermal decomposition of a mixture of complex compounds [Mg(AA) 2 ] and [Fe(AA) 3 ] at 500°C (AA=acetylacetone). The solid solutions of Li x Mg 1−2 x Fe 2+ x O 4 (0.10≤ x ≤0.45) were synthesized by conventional ceramic method. The samples with x ≥0.40 belong to ordered spinels with primitive cubic cell, space group P4 3 32 and the samples with x ≥0.35 are spinels with face-centered cubic cell, space group Fd-3m . Crystal structures of samples with x =0.00, 0.40 and 0.45 were refined by the Rietveld method using DBWS-9411 software package. In ordered spinels ( x =0.40, 0.45) Li + ions occupy both 4 b (preferentially) and 12 d octahedral sites. At tetrahedral 8 c positions Li + ions were not found (within the experimental error). Mg 2+ ions occupy octahedral 12 d and tetrahedral 8 c sites and Fe 3+ ions occupy all three different cationic sites. Cation ordering in 1:3 type ordered spinel (the occupancy ratio of the Wyckoff’s positions 4 b and 12 d ) of Li + and Fe 3+ at octahedral sites is not complete as in Li 0.5 Fe 2.5 O 4 . In MgFe 2 O 4 spinel, cation distribution in the tetrahedral sites was determined and given by occupation numbers N: N(Mg 2+ 8 a )=0.183(7) and N(Fe 3+ 8 a )=0.817(7).

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.

X-Ray Powder Data for Ca4Al6O12SO4

An indexed X-ray powder diffraction pattern is reported for Ca 4 A 6 O 12 SO 4 , prepared by solid state reaction. The compound crystallizes in the tetragonal system with a = 13.031(3) and c = 9.164(2) A, V = 1556.1 (8) A 3 , Z = 4.

A practical approach to Rietveld analysis. Comparison of some programs running on personal computers

Three computer programs for Rietveld analysis DBWS-9411 , HILL-93.06 , and FULLPROF-3.1 have been tested and compared using data for two samples of different complexity, spinel, and anglesite. The investigation shows that results are “program dependent.” The obtained R indices for spinel are in the ranges 10.60%–13.26%( R wp ) and 3.15%–5.25%( R B ). Similarly, the ranges for anglesite are 9.76%–14.06%( R wp ) and 2.15%–5.06%( R B ). Some atom and displacement parameters are significantly different, too. In attempt to define the standard procedure for Rietveld analysis, three parameters, n , BKPOS, and RLIM were examined. It was found that the most appropriate values for them are: n =8–10, BKPOS =90°, and RLIM =40° 2θ. Using Fourier filtering for background modeling, significantly lower R indices were obtained in comparison to polynomial and interpolated background. At the same time the great numbers of atom and cell parameters agree within ±3σ max and e.s.d.s were identical or lower than those achieved for polynomial and interpolated background. It was found that the function given by Berar and Baldinozzi (1993) (J. Appl. Crystallogr. 26 , 128–129) much better described asymmetric peak profiles at low 2θ angles. This function and Fourier filtering were implemented only in FULLPROF , which has more possibilities and some advantages over the other two programs. In addition, the peak shift parameters (sample displacement and transparency) were tested. It was shown that under present circumstances these parameters do not have much effect on atom parameters and R indices. However, differences in unit cell parameters were considerable greater, most probably because of the large correlation between zero-point, lattice, and peak shift parameters.