Size effects in oxide mechanochemistry
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MATERIALS Dedicated to the memory of B.K. Vainshtein
Size Effects in Oxide Mechanochemistry V. M. Kanevskii Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia email: [email protected] Received February 22, 2011
Abstract—A new size effect has been experimentally found: the mechanochemical synthesis of previously mechanically activated powder mixtures of MgO–Al2O3 oxides with a block size of about 80 nm leads to the formation of MgAl2O4 spinel at room temperature. A model is proposed according to which the mechanism of mechanochemical solidphase synthesis changes from dislocation transgranular embrittlement to grain boundary sliding, which is accompanied by the generation of dynamic vacancies and lowtemperature phase formation via the diffusioninduced migration of grain boundaries. DOI: 10.1134/S1063774511040080
INTRODUCTION Mechanochemical reactions in oxide systems and the relationship of these processes with the real struc ture, the features of lattice defects, the presence of impurities on the particle surface, and the surface structure of the particles involved in mechanochemi cal reactions was studied in detail in [1, 2]. It was established that the specific energy and load intensity, environmental composition, external effects (for example, magnetic field), and other factors are very important in these processes. Stable interest in the mechanochemical reactions in nanomaterials has been formed over the last few years, not only due to the practical importance of nanomaterials, but also due to the fact that the first published experimental results and effects observed could not be adequately explained in the framework of classical theoretical mechanochemistry. In this study we show that the mechanochemical reactions in a system of nanosized MgО–Al2O3 oxide particles occur according to a new mechanism in which the effects of mutual particle sliding and, there fore, the development of shear deformation play a key role. The experimental results on the mechanochemical synthesis of MgAl2O4 spinels from MgО–Al2O3 nanooxides will be presented in Section 1, a theoreti cal model of the observed effects is described in Sec tion 2, and the results obtained are discussed in the last section.
tial commercial Al2O3 powders (Fig. 1a) and 15 µm in MgO powders (Fig. 1b). In all experiments the molar ratio of the components in the mixture was 1 : 1. Mechanical activation and mechanochemical synthe
1. EXPERIMENTAL RESULTS The mechanochemical synthesis of MgAl2O4 spinel from a mixture of MgО–Al2O3 oxides was per formed at room temperature in air. The geometric mean (median) size of particles was 45 µm in the ini 662
(a)
50 μm
(b)
50 μm Fig. 1. Scanning electron micrographs of the initial (a) Al2O3 and (b) MgO powders.
SIZE EFFECTS IN OXIDE MECHANOCHEMISTRY
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I 80 h
5h
2θ, deg Fig. 2. Xray diffraction patterns of the initial mixture of Al2O3 and MgO powders mechanically activated for 80 and 5 h: (䉱) γ Al2O3, (䊉) MgO, and (䊏) MgAl2O4.
5 μm Fig
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