Mechanosynthesis of lanthanum manganite

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72 A.M. Bolarín et al. / Materials Science and Engineering A 454–455 (2007) 69–74 Fig. 4. Micrographs of LaMnO3 powders milled during different times: (a) 4.5 h, (b) 7 h, (c) 9 h and (d) 20 h. It is supposed in MCP that mechanical energy will activate thermally the atoms in the material in such a way that they take new positions as thermodynamical feasibility allows. High pressure could be locally achieved and even change the electronic properties of the product [30]. Just as a side comment, this feature could be usefully even for modifying the properties of a material that was already synthesized. The mechanism were the raw materials are kept vibrating until the atoms find their location has not actually been seen, however there are several successfully demonstrations [17–19]. Similarities of the lattices among the compounds as well as coherent interfaces are a factor that most be considered. Lattice of the obtained LaMnO3 was affected by iron presence, which could substitute any of the cations due to its double valence. Therefore, a conclusion regarding the kind of structure that is being produced assigned to MCP only is not easy to address, especially because low iron contamination was achieved at short milling times, which were not enough to produce LaMnO3. However, the synthesis was carried out completely meaning that this procedure is suitable for conducting the synthesis. Another aspect is that kinetic energy that was being applied to the powder became less effective as the tests between 6 and 20 h confirm with changes on the size of the agglomerates only. 3.2. Particle size and morphology Fig. 4 shows micrographs of powder samples milled at different times. The sample powder from 4.5 h of milling time consisted of at least two types of particles: small, roughly spherical particles about 200 nm in diameter and other large, also rounded about 2 �m in diameter. The powder becomes rounder as milling time increases, close to roughly spherical particles down to 1000 nm in diameter. Particle size distribution of sample milled during 20 h seems narrow with agglomeration particle of about 400 nm as shown in Fig. 5. Particle size of powders was determined quantitatively using a potential zeta analyzer. The results are shown in Fig. 6, confirming what it is presented in Figs. 4 and 5, as the milling time Fig. 5. Micrographs of agglomerates of LaMnO3 powders milled during 20 h.
Fig. 6. Effect of milling time on particle size. increases, particle size decreases exponentially up to 480 nm for 6 h of milling time. Agglomerates of fine grain with irregular shapes can be observed after 9 h of milling time. The agglomeration of powders increases with the milling time due to the refinement of the particle size. 3.3. Chemical analysis The MCP might generate iron impurities from the vials and balls of the system, this is inevitable during milling process [15], and it affects magnetic and electric properties of the materials. Therefore, iron contamination was measured in this work. A.M. Bolarín et al. / Materials Science and Engineering A 454–455 (2007) 69–74 73 Fig. 7. Effect of milling time on Fe contamination level. In Fig. 7 is presented the relation between the amount of iron (wt.%) against milling time. It can be observed that iron contamination increases continuously from 0.5 wt.% for 1 h to 1.2 wt.% for 7 h of milling time. A prolonged milling time (>20 h) produces a higher contamination, around 3.4 wt.% of the powder mixture, due to the friction between oxides and metallic parts of the miller, during the milling process. In order to confirm whether the iron impurities in milled powders existed as an atomic form substituted for manganese in the particles or as isolated metal form, EDS elemental analysis were carried out. The results are presented in Fig. 8, which shows the distribution of iron atoms in a cross-sectioned particle milled during 20 h. It can be seen that iron was well distributed in a par- Fig. 8. SEM micrograph of cross-sectioned powder milled for 20 h and EDS dot mapping of La, Mn and Fe atoms.
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