Magnetite aggregates

Fig. 26 The co-sonication process, representatively shown for magnetite aggregates. A dispersion of magnetite is mixed with a preformed monomer miniemulsion middle). Magnetite aggregates are engulfed by the monomer droplets after sonicating the mixture of both dispersions upper right). After subsequent polymerization, the hybrid nanoparticles are obtained lower right)...

Step 2 the magnetite particles were transformed to magnetite aggregates of ca. 40-200 nm in water by using a miniemulsion process. 

Step 3 the magnetite aggregates were encapsulated with a monomer by... 

Figure 2.13 Formulation of polymer-coated magnetite particles with a high magnetite ratio. In the first step, hydrophobized magnetite particles are produced and, in a second step, transferred to magnetite aggregates in water by using the miniemulsion process. In a third step, the principle of co-miniemulsion...

After evaporation of the octane, a water-based ferrofluid consisting of oleic add-coated aggregated magnetite dispersed in a water phase is obtained (Fig. 2.15). In other words, the magnetite aggregates must have a surfactant double layer the first layer is oleic acid, which provides a hydrophobicity of the partides for later encapsulation the second layer, bdng SDS, promotes the stabihzation in water. 

In the third step of the synthesis route, a monomer miniemulsion with 20 wt.% dispersed phase is prepared and added to the water-based ferrofluid containing the magnetite aggregates, as obtained above. This mixture is then cosonicated, and the controlled fission/fusion process which is characteristic for miniemulsification is expected to destroy all aggregates and liquid droplets. As a consequence, only hybrid particles composed of magnetite and monomer should remain, presumably since this species shows the highest stability. Polymerization of the monomer is then started for all the samples presented here by adding an initiator. 

Figure Z15 Magnetite aggregates obtained after a miniemulsion process in water.

Sintering consists of heating a mixture of fine materials to an elevated temperature without complete fusion. Surface diffusion and some incipient fusion cause the soHd particles in contact with one another to adhere and form larger aggregates. In the processing of hematite, Fe202, or magnetite,... 

Keywords Electron Magnetic Resonance, Ferrofluids, Magnetite, Nanoparticles, Linear aggregates. 

It should be taken into account that only part of the particles collect into chain structure and others remain separated. Both types of particles are in thermal equilibrium. Using EMR spectra it is possible to calculate the fraction of particles, which belong to aggregate by spectra separation procedure. The fraction of magnetite involved in aggregates increases with concentration (Figure2). 

Table 1. Magnetization valnes M for linear aggregates of magnetite nanoparticles at different orientations in MF of spectrometer at 298 K (PVP is a matrix)...

Some amorphous regions in ALHA77307 contain rounded grains of kamacite with 4-5 %Ni and nickel-bearing sulfides, which are both <200 nm in size. Elsewhere, magnetite occurs as crystals as small as 50 nm and aggregates <10 p.m... 

Fe, common in surhcial oxic environments, forms hydroxides, complexes formerly called hmonite (a collection of mst-Uke precipitates, chemically designated as Fe(OH)3). The aggregate has now been shown to contain several oxyhydroxide species, FeOOH, and the oxides, hematite, Fe203, and magnetite, Fe304, and some new minerals have been identified (Schwertmann and Fitzpatrick, 1992, tables 1-3, pp. 10-11). 

For magnetite particles (M = 4.5 x 10 A/m), this gives a particle size of about d = 7 nm, at which aggregation becomes important. 

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