Magnetite shell

A profile of oxygen and pH concentration is shown in Fig. 3.5. A cross section of the outer crust, inner magnetite shell, and core material is shown in Fig. 3.6. 

Core. Friable core material is present beneath the magnetite shell (Fig. 3.6). The core consists mostly of ferrous hydroxide formed by Reaction 3.3 ... 

Figure 3.5 Schematic pH eind oxygen concentration profiles in an active tubercle. Below the magnetite shell oxygen concentration decreased sharply. pH rises above the magnetite shell due to cathodic hydroxyl-ion generation emd falls below the shell due to concentration of acidic anion, (Courtesy of National Association of Corrosion Engineers, Corrosion 91 Paper No. 84 by H. M. Herro.)...

Multiple magnetite shells may form by successive fracture. Ferrous species spew out of the fractured shell and are quickly oxidized to form a new ferric hydroxide crust. Beneath the new crust, another mag-... 

Figure 3.18 Multiple magnetite shells in a small tubercle. Multiple shells form due to successive fracture during growth. (Magnification 2x.)...

Tubercles consisted of hard, hlack oxide shells overlaid with friable carbonate-containing deposits. In places, several laminate black magnetite shells existed. The outer crust could be crushed by gentle pressure with a finger. Tubercles were riddled with white crystalline fibers. Other detritus was incorporated into the tubercle core and crust. Metal loss was less than 0.030 in. (0.076 cm) below each tubercle. Wall thickness was almost 0.25 in. (0.64 cm). 

Internal surfaces were moderately tuberculated (Fig. 3.14). Extremely thick, hard magnetite shells capped large internal cavities (Fig. 3.9). Pipe cross-sectional area was reduced by at least 30% in some places. Tubercles were aligned with flow, indicating that growth occurred during service. No failure occurred, and deepest metal loss was only 0.093 in. (0.033 cm) from the nominal pipe wall thickness of 0.225 in. (0.572 cm). 

A new class of PS core/magnetite shell particles was produced by the sequential adsorption of magnetite nanoparticles and polyelectrolyte, a process that allowed the shell thickness and composition to be controlled with nanometer precision. The poly electrolytes were poly(diallyldimethylammonium chloride) or poly(allylamine hydrochloride). The influence of an external magnetic field on ordering of the composite PS core/magnetic shell particles was examined. 22 refs. 

Sarkar, A., Biswas, S.K. and Pramanik, P. (2010) Design of a new nanostructure comprising mesoporous Zr02 shell and magnetite core (Fe304 mZr02) and study of its phosphate ion separation effidency. Journal of Materials Chemistry, 20 (21), 4417—4424. 

Under conditions leading to a porous shell of magnetite, the kinetic curve displayed an induction period corresponding to formation of nuclei and the subsequent reaction followed the cube root law. Diffusion of the reducing gas to the reactant/ product interface took place readily with a porous product. Whether chemical or diffusion control predominated depended on reaction conditions. With small crystals... 

Aliev, F.G. Correa Duarte, M.A. Mamedov, A. Ostrander, J.W. Giersig, M. Liz-Marjan, L.M. Kotov, N.A. (1999) Layer-by-layer assembly of core shell magnetite nanoparticles Effect of silica coating on interparticle interactions and magnetic properties. Adv. Mater. 11 1006-1010... 

Carty, P. White, S. Price, D. Lu, L. (1999) Smoke suppression in plasticised chlorinated poly(vinyl) chloride (CPVC). Polymer Degradation Stability 63 465-468 Caruso, F. Susha, A.S. Giersig, M. Moh-wald, H. (1999) Magnetic core-shell particles Preparation of magnetite multilayers in polymer latex microspheres. Adv. Mater. 11 950-952... 

Often it is necessary to treat diffusion between different layers as three dimensional diffusion. For isotropic minerals such as garnet and spinel (including magnetite), diffusion across different layers may be considered as between spherical shells, here referred to as "spherical diffusion couple." Oxygen diffusion in zircon may also be treated as isotropic because diffusivity c and that Tc are roughly the same (Watson and Cherniak, 1997). If each shell can be treated as a semi-infinite diffusion medium, the problem can be solved (Zhang and Chen, 2007) as follows ... 

Composite core-shell type microspheres were prepared by in situ heterogeneous polymerization on monodispersed seed latex particles suspended in an aqueous magnetite dispersion stabilized with sodium oleate (58). 

As summarized in Table 14.1, teeth, bones, shells, etc. are indispensable components, consisting of inorganic mineral crystals and protein film, with sizes, morphologies, and textures suitable to fulfil the function of the particular organs involved. In this section we will look at hydroxyapatite, aragonite and calcite (two polymorphs of CaCO ), and magnetite in greater detail. 

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