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application. Thus, the coating process can be influenced by several properties of the particle. After the covering process, the external surface of the particle can show special properties, such as a catalytic property becoming a more efficient catalytic material, being the internal material protected against environmental aggressiveness5 . The heterocoagulation process is similar to those used for obtaining the uniform particles, as described by Matijevic 6,7 and others 8,9. However, the heterocoagulation process requires rigorous controls of the nucleation conditions and growth of the involved phases. It is known that magnetic properties depend strongly on the size, as well as on the dispersion medium where the particles are formed. Specifically, the preparation of magnetic powders with defined properties and a protecting layer by heterocoagulation is, therefore, of great interest for the increase of magnetic material performance which is drastically affected by environment1"'°. An alternative way to eliminate or to minimize these problems is the heterocoagulation of these particles with protecting layer, for instance, with silica. In this study, a procedure is described for building up of silica layers on yttrium iron garnet (YIG) particles. YIG, especially that of interest as magnetic pigment, microwave absorption and magnetic fluids have been coated with silica in order to protect its magnetic property from environment enlarging its technological applications.

21 Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society

EXPERIMENTAL YIG Cores Yttrium chloride, iron chloride, yttrium nitrate, iron nitrate, urea, ammonium hydroxide, and poly (vinylpyrrolidone) were all reagents used without further purification. YIG amorphous particles were obtained by hydrolysis of metal chloride or nitrate solutions"' 12. The starting solution was a mixture of iron chloride (1.10-3 mol L-1) and yttrium chloride in chloridic acid 1.10.2 mol L-1 or iron nitrate (1.10-3 mol L-) and yttrium nitrate in nitric acid 1.10.2 mol L-1. These solutions were mixed, keeping the ratio of Fe to Y at 5:3. Urea was added in different concentrations in order to increase the pH and improve the hydrolysis process. In some cases, poly(vinylpyrrolidone) and ammonium iron (111) sulfate [NH 4Fe(SO 4) 2 12 H 2 0 in (w/w %) were added to avoid aggregation13. After

keeping the solution at the temperature of 900 C for 3 hours, the powder was then dried in a desiccator under vacuum. Precipitates were calcinated at 1100 0C for one hour, yielding cubic YIG particles. Table 1 describes the precipitation conditions of yttrium iron garnet. Table 1: Identification and precipitation conditions of the yttrium iron garnet obtained from different anions by using urea as the precipitation agent in presence of PVP and/or iron ammonium sulfate. Different concentrations of urea was used as precipitation agent. Samples Urea agent Concentration of iron Addition of Addition of (moLL-1)

P16

0.5

chloride or nitrate (mol.L"1)

1.I0 3

P24

0.5

1.10.3

N3 N4