Metal-oxide films with magnetically-modulated nanoporous architectures
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Elizabeth C. Dickey and Oomman K. Varghese Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 13 November 2000; accepted 22 March 2001)
A magnetically-driven method for controlling nanodimensional porosity in sol-gel-derived metal–oxide films, including TiO2, Al2O3, and SnO2, coated onto ferromagnetic amorphous substrates, such as the magnetically-soft Metglas1 alloys, is described. On the basis of the porous structures observed dependence on external magnetic field, a model is suggested to explain the phenomena. Under well-defined conditions it appears that the sol particles coming out of solution, and undergoing Brownian motion, follow the magnetic field lines oriented perpendicularly to the substrate surface associated with the magnetic domain walls of the substrate; hence the porosity developed during solvent evaporation correlates with the magnetic domain size.
I. INTRODUCTION
Metal–oxide films of controlled nanoporosity offer an exciting opportunity for developing a new class of materials with unique physical, electrical, and magnetic properties. In recent years porous metal oxide films have attracted considerable attention for application in photovoltaic cells,2,3 catalysis,4 –9 gas sensors,10 –12 biotemplates,13–16 and electrochromic displays.17 Sol-gel self-assembly deposition methods are of great interest due to their inherent flexibility and low cost. Considerable effort has focused on the ordered alignment of a template around which the material of interest is assembled. Depending upon the desired pore size, organic polymers,18,19 block copolymers,20,21 latex22–24 and polystyrene25 spheres, water-in-oil emulsions,26 and water droplets27 have been used as templates. A templatefree method for fabrication of micrometer-sized honeycomb structures by self-assembly of block copolymers has been described in Refs. 28–30. We have recently discovered that within certain processing windows TiO2, Al2O3, and SnO2 metal–oxide films dip-coated onto magnetically-soft ferromagnetic substrates exhibit unique nanodimensional porous architectures; see Fig. 1. We have not been able to replicate this porous structure on nonferromagnetic substrates, prompting us to consider the influence of a magnetic field on the charged sol particles. The thick-film Metglas
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J. Mater. Res., Vol. 16, No. 6, Jun 2001 Downloaded: 15 Mar 2015
FIG. 1. SEM image of sol-gel-derived 400-nm-thick TiO2 nanoporous coating made while exposed to a 0.0-Oe static magnetic field. The average pore size is approximately 60 nm.
2826MB1 substrate used in this work, Fe40Ni38Mo4B18, is a magnetically soft alloy made by rapid meltquenching, and while it maintains no long-range order, it does have short-range order over several atom lengths;31–34 hence, the as-cast material has magnetic ordering on the scale of tens of nanometers. It appears that as the sol layer upon the 2826MB substrate evaporates the moving cha
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