Ferrite, agglomeration

The longitudinal relaxation rate inversely decreases with the residence time of water molecules inside the agglomerate. This effect was demonstrated thanks to a controlled and chemically induced process of agglomeration amongst ferrite nanomagnets coated by polyelectrolyte polymers. The NMRD profile becomes flatter on increasing agglomeration (Pig. 10). 

An alternative method that has been used to prepare ferrite particles in a single step is the evaporative decomposition of the solutions, or spray-roasting (Ruthner, 1977). The solutions were mixed and atomised, and the droplets fell through a reaction chamber at 900-1050 °C. The solvent evaporated and the salts decomposed to oxides. The process took 3-5 s. By using a roasting furnace for industrial production, agglomerates of 40-200 pm containing 1 pm ferrite particles were obtained. The furnace feedstock was an aqueous suspension of the oxides, carbonates or hydroxides of the desired composition. However, the residence time was insufficient and complete transformation to the desired ferrite was not achieved. 

The sequence of compaction steps has been observed in ferrite powders (Youshaw Halloran, 1982) the early stage of compaction additionally included plastic deformation of the agglomerates. Their ductility plays an important role in the process. The pressing rate was observed to be important compaction was retarded at faster pressing rates. 

The sol-gel method is used to produce clean, non-stoichiometric, dense, equidi-mensional, and monodisperse particles of simple oxides such as T1O2 and Si02 [128]. This observation unfortunately cannot be extended to the complex oxides and ferrites in particular. In most general case, the resulting particles obtained agglomerate. 

Jasienska, S., Investigation of Physico-Chemical Properties of Ferrite of Lime CaO-FeO-Fc203 Crystallising in Self-fluxing Agglomerates , (in French), Mem. Sci. Rev. Metall, 68(11), 809-814 (1971) (Experimental, 13)... 

Spray drying of solutions has been found to be useful for the preparation of ferrite powders (88). For Ni-Zn ferrite, the solution of sulfates was broken up into droplets (10-20 p.m) by a rotary atomizer. The powder obtained by spray drying was in the form of hollow spheres having the same size as the original droplets. Calcination at 800-1000°C produced a fully reacted powder consisting of agglomerates with a primary particle size of 0.2 xm. The ground powder (particle size < 1 p,m) was compacted and sintered to almost theoretical density. 

Figure 17.5 (a) Ferrite-BC composite synthesized without sonication and surfactant PEG shows agglomerated magnetite particles (b) sonication alone leads to relatively dispersed magnetite particles (c) with sonication and PEG the magnetite particles are well dispersed and also larger. Inset shows the flexibility and magnetic actuation of the composite. 

The results fi om the observation of SEM micrographs reveal that the morphology of the pigments with agglomerates such as flakes, in the case of holmium ferrite, the particles are approximately from 0.5 pm to 1.0 pm in size. Neodymium, samarium and erbium ferrites are agglomerates like globules of approximately 1.0 pm, and in the case of erbium ferrite approximately 2.0 pm. See Figure 2a. 

The chromium doped lanthanide aluminates micrographs of Pr, Nd, Dy, Ho, and Er by SEM show the same type of agglomerates like globules of 1.0 pm, to 2.0 pm in size. The particle size, particle shape and particle morphology are very near between ferrites and aluminates and the materials are appropriate as pigments used in ceramic materials. See figure... 

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