Pore wall precipitation

Porous aluminum oxide can be used as a template for the production of nanowires and nanotubes. For example, metals can be deposited on the pore walls by the following procedures deposition from the gas phase, precipitation from solution by electrochemical reduction or with chemical reducing agents, or by pyrolysis of substances that have previously been introduced into the pores. Wires are obtained when the pore diameters are 25 nm, and tubes from larger pores the walls of the tubes can be as thin as 3 nm. For example, nanowires and nanotubes of nickel, cobalt, copper or silver can be made by electrochemical deposition. Finally, the aluminum oxide template can be removed by dissolution with a base. 

Pressure solution incorporates the three serial processes dissolution at grain contacts, diffusion along the interfacial water film, and precipitation on pore walls. 

Finally, precipitation of solute to the free faces of pore wall is defined in terms of the precipitation mass flux, dMp,t /dt, the rate of deposition of solute from the pore space onto the grain surfaces, is defined as (Yasuhara et al., 2003 modified from Canals and Meunier, 1995) ... 

Suib and coworkers demonstrated that a mesoporous mixed-valent manganese oxide can be successfully synthesized by the direct precipitation of manganese hydroxide precursors in basic conditions.The ordered mesoporous structure with crystalline pore walls was retained after calcination as confirmed by powder x-ray diffraction (XRD) and transmission electron microscope (TEM). Such mixed-valent mesoporous oxides are semiconducting and possess catalytic activity in the oxidation of alkanes. The mixed oxidation states of manganese (III/IV) in these materials were introduced by the direct oxidation of the metal precursor [Mn(II)j. Both hexagonal and cubic phases of such mesoporous materials can be synthesized. 

Salt damage is caused by precipitation of Na2S04 IOH2O crystals from a supersat-mated solution in liquid-filled pores (reaction equations (b) and (c) above). Dming their growth, the crystals are assumed to exert a mechanical pressure on the surrounding pore walls by a sufficient snpersaturation in the liquid phase this pressure can result in cracks and spalling of the material concerned. Such transformations can, for example, occur if the temperatme of the wet salt-saturated material fluctuates around 32.4 °C. 

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