Magnetite/maghemite particles

The industrial dust deposited in the metropolitan area of Shanghai consisted of high-coercivity (hematite) and low-coercivity (magnetite/maghemite) particles, usually < 10 pm in size. Four different dust sources could be distinguished on the basis of magnetic properties (Shu et al. 2000). Given previously reported links between... 

Sun Y-K, Ma M, Zhang Y, Gu N (2004) Synthesis of nanometer-size maghemite particles from magnetite. Colloids Surf A 245 15-19... 

The maghemite particles are also synthesized according to a next recipe [352, 353]. Concentrated ammonium hydroxide (250 mL, 11 mol.dm ) is added to an acidic aqueous solution of iron (II) chloride and iron (III) chloride ([Fetotai] = 0.13 mol.dm with Vtotai=3.8 L and [Fe(II)]/[Fe(III)] = 0.5) containing various amounts of citric acid trisodium salt. The molar ratio of citrate to metallic species (Fe(II) -1- Fe(III), varies from 0 to 10%. The precipitate, consisting of anionic magnetite particles... 

Stearic acid CH3(CH2)ieCOOH] play a similar role. Fatty acids are used in the preparation of ferrofluids, in the dispersion of magnetite or maghemite particles in kerosene or various oils [94]. 

In acid media (pH 2) magnetite crystals ca. 10 nm across transform topotactically to maghemite via an adsorption reaction which traps mobile electrons from the bulk material and reduces interfacial Fe the Fe ions that form are selectively leached into solution (Jolivet Tronc, 1988). Electron delocalization also induces ferrihydrite in contact with small magnetite particles to transform into a spinel layer (Belleville etal., 1992). 

The seed particles are then grown to desired sizes in the presence of iron and ferrous ions. Second, the goethite particles are dehydrated by heating, then reduced by hydrogen into magnetite, and finally oxidized into maghemite through the reactions... 

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