Magnetite particle size determinations

Comparison of Magnetite Particle Size Determinations for Silica-Supported Samples ... 

From this equation It Is clear that a plot of M/Mg versus MgpH/3kT will yield a line with slope v /v. The only unknown In this equation Is the value of Mg. Assuming that this does not change as a function of particle size (no dead layers) this can be determined for magnetite from the formula of Pauthenet (23) ... 

The confinement of two species in stoichiometric amounts within the nanodroplets also allows the synthesis of mixed species. A mixture of Fe2+ and Fe3+ salts leads to the formation of magnetite, Fe304. The final dispersion with a particle size of 200 nm is black and shows magnetic properties. As is seen in the TEM pictures (Fig. 23b), the superstructure composed of 10-nm nanoparticles as determined by WAXS is anisotropic (lemon shaped), and constituting needle shaped nanocrystals, can be identified inside the particles, arranged as bundles along the main axis of the lemons . 

When a series of silica supported-magnetite catalysts of varying iron oxide particle size were investigated, it was determined that Si substitutes into the magnetite lattice according to the following reaction(49) ... 

The effect of Si substitution on the turnover frequency for WGS is shown in Figure 11. The turnover frequencies plotted in this figure were based on the magnetite surface area as determined by the NO chemisorption technique. The turnover frequencies shown for unsupported Fe O indicate that the factor of 10 decline in activity for the silica-supported catalysts is not a particle size effect, but instead is a consequence of the substitution of Si into the lattice. However, when the adsorption of CO/COo at 663 K was used to titrate the surface sites instead of NO, the resulting turnover frequencies were essentially constant as shown in Figure 12. Accordingly, the CO/CO2 mixture apparently titrates the sites active for WGS. Clearly, the number of active sites is decreased markedly as the particle size decreases in the silica-substituted magnetite catalysts. 

Considerable effort has been made in the examination of the deposition of oxides of iron onto surfaces, since this has considerable relevance to the operation of boiler plant. Williamson [1990] has reviewed some experimental magnetite (FejOJ deposition data. He draws attention to the wide discrepancies in the results even for similar systems as may be seen from examination of Table 7.3. He attributes the discrepancies to inadequate control of one or more of the less obvious variables in the system such as the water chemistry (even deionised water is likely to "pick up" COj from the atmosphere with an effect on the pH). A fixed particle size distribution is extremely difficult to maintain in an experimental system due to potential agglomeration and is likely to be time dependent. Even particle concentration may not be uniform due to settlement in the parts of the equipment where velocities are low. Experimentally the consistency of these variables are difficult to determine. 

Table 2. Variation of the magnetite lattice cell with a determined particle size...

The influence of ionic species concentrations on the properties of magnetite particles was also followed. It was noted that the Fe2+/Fe3+ molar ratio was a determining factor in obtaining sub-micron sizes, while by increasing the ratio, the mean diameter of the magnetic particles increased, but unfortunately the yield decreased (Babes et al., 1999). 

It is impossible to describe the activation process with a simple kinetic model using only one rate-determining step such as the chemical reaction itself or hydrogen diffusion. Furthermore, it has been shown that gaseous diffusion did not influence the parameters. From the particle-size dependence of the parameters, it was found that diffusion within the particle plays a dominant role in contrast with the case of pure magnetite, it was not possible to distinguish between surface diffusion and bulk diffusion. 

Evidence for the presence of A1 dissolved in the magnetite phase has been found by X-ray powder diffraction [27,28, 38-42, 53] and by chemical analysis of powders of varying particle size [54]. The solubility of A1 in the (Fe,Al)3 04-phase has been determined to be 30 atom% A1 [55] from measurement of the Curie temperature and 50 atom % Al[55], or 67 atom % Al[39] from measurement of the X-ray powder diffraction lattice constant. Other studies have indicated homogeneous solution of A1 in magnetite, at least for small amounts of A1 [56] and not too high temperatures [57]. 

Ferrimagnetic nanoparticles of magnetite (Fc304) in diamagnetic matrices have been studied. Nanoparticles have been obtained by alkaline precipitation of the mixture of Fe(II) and F(III) salts in a water medium [10]. Concentration of nanoparticles was 50 mg/ml (1 vol.%). The particles were stabilized by phosphate-citrate buffer (pH = 4.0) (method of electrostatic stabilization). Nanoparticle sizes have been determined by photon correlation spectrometry. Measurements were carried out at real time correlator (Photocor-SP). The viscosity of ferrofluids was 1.01 cP, and average diffusion coefficient of nanoparticles was 2.5 10 cm /s. The size distribution of nanoparticles was found to be log-normal with mean diameter of nanoparticles 17 nm and standard deviation 11 nm. 

Adhesion to Plates, The number of dust particles adhering to plates placed in a flow is determined by the concentration in the flow, the properties of the dust and of the surface, and also the arrangement of the surface relative to the axis of the flow. In view of the absence of any general review on research on this subject, we shall confine attention to experimental data relating to the adhesion of magnetite dust [319] of diameter smaller than 10 p and a horizontal plate 40 cm in size situated in a horizontal, rectangular air conduit of cross section 40 x 40 cm (see Table VI.2). 

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