Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Reexamination Certificate
2001-06-08
2003-02-04
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
C427S058000, C427S123000, C427S126600, C427S127000, C427S128000, C427S129000, C427S130000, C427S132000, C427S190000, C427S191000, C427S195000, C427S215000, C427S216000, C427S221000, C427S222000, C427S385500, C427S388100, C427S393500, C427S443200, C427S598000
Reexamination Certificate
active
06514575
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a method for forming and exploiting gradients at the interfaces between components of a composite material as well as the composite material itself and devices which incorporate the material. In particular, the invention relates to a method for forming and exploiting magnetic gradients at the interfaces between components of a composite magnetic material and the magnetic composite material itself as well as devices which incorporate the composite material such as electrochemical systems including fuel cells, batteries, membrane sensors, and flux switches resulting in enhanced and modified flux and performance in those systems.
2. Background of the Related Art
Bulk properties of molecules in magnetic fields are fairly well understood. In the detailed description of preferred embodiments, it will be shown that interfacial gradients in properly prepared composite materials can be exploited to enhance flux in many types of electrochemical systems such as fuel cells, batteries, membrane sensors, filters and flux switches. First, however, the following discussion provides a brief overview of the current understanding of magnetic properties in composites. In particular, the discussion below summarizes the thermodynamic, kinetic and mass transport properties of bulk magnetic materials.
Rudimentary Magnetic Concepts
Paramagnetic molecules have unpaired electrons and are attracted into a magnetic field; diamagnetic species, with all electrons paired, are slightly repelled by the field. Radicals and oxygen are paramagnetic; most organic molecules are diamagnetic; and most metal ions and transition metal complexes are either para- or diamagnetic. How strongly a molecule or species in a solution or fluid responds to a magnetic field is parameterized by the molar magnetic susceptibility, P
m
(cm
3
/mole). For diamagnetic species, &khgr;
m
is between (−1 to −500)@10
−6
cm
3
/mole, and temperature independent. For paramagnetic species, P
m
ranges from 0 to +0.01 cm
3
/mole, and, once corrected for its usually small diamagnetic component, varies inversely with temperature (Curie's Law). While ions are monopoles and will either move with or against an electric field, depending on the sign of the ion, paramagnetic species are dipoles and will always be drawn into (aligned in) a magnetic field, independent of the direction of the magnetic vector. The dipole will experience a net magnetic force if a field gradient exists. Because electrochemistry tends to involve single electron transfer events, the majority of electrochemical reactions should result in a net change in the magnetic susceptibility of species near the electrode.
Magnetic field effects on chemical systems can be broken down into three types: thermodynamic, kinetic and mass transport. Macroscopic, thermodynamic effects are negligible, although local magnetic field effects may not be. Kinetically, both reaction rates and product distributions can be altered. Transport effects can lead to flux enhancements of several-fold. Quantum mechanical effects are also possible, especially on very short length scales, below 10 nm. The following summarizes what has been done with homogeneous fields applied to solutions and cells with external laboratory magnets.
Thermodynamics
A magnetic field applied homogeneously by placing a solution between the poles of a laboratory magnet will have a negligible nonexponential effect on the free energy of reaction. &Dgr;G
m
=−0.5&Dgr;&khgr;
m
B
2
J/mole, where &Dgr;G
m
is the change of the free energy of reaction due to the magnetic field, &Dgr;&khgr;
m
is the difference in magnetic susceptibility of the products and reactants, and B is the magnetic induction in gauss. For the conversion of a diamagnetic species into a paramagnetic species, &Dgr;&khgr;
m
≦0.01 cm
3
/mole. In a 1 T (1 Tesla=10 kGauss) applied field, |&Dgr;G
m
|≦0.05 J/mole. Even in the strongest laboratory fields of 10 T, the effect is negligible compared to typical free energies of reaction (≃kJ/mole). These are macroscopic arguments for systems where the magnet is placed external to the cell and a uniform field is applied to the solution. Microscopically, it may be possible to argue that local fields in composites are substantial, and molecules in composites within a short distance of the source of the magnetic field experience strong local fields. For example, for a magnetic wire or cylinder, the magnetic field falls off over a distance, x, as x
−3
. The field experienced by a molecule 1 nm from the magnet is roughly 10
21
times larger than the field experienced at 1 cm. This argument is crude, but qualitatively illustrates the potential advantage of a microstructural magnetic composite. (As an example, in composites comprising magnetic material and “Nafion” (DuPont) , a larger fraction of the redox species are probably transported through the 1.5 nm zone at the interface between the Nafion and the magnetic particles.) These redox species must therefore experience large magnetic fields in close proximity to the interface.
Kinetics
Reaction rates, k, are parameterized by a pre-exponential factor, A, and a free energy of activation, &Dgr;G
1
; k=A exp[−&Dgr;G
1
/RT]. An externally applied, homogeneous magnetic field will have little effect on &Dgr;G
1
, but can alter A. Nonadiabatic systems are susceptible to field effects. Magnetic fields alter the rate of free radical singlet-triplet interconversions by lifting the degeneracy of triplet states (affecting)G
1
); rates can be altered by a factor of three in simple solvents. Because magnetic coupling occurs through both electronic nuclear hyperfine interactions and spin-orbit interactions, rates can be nonmonotonic functions of the applied field strength. Photochemical and electrochemical luminescent rates can be altered by applied fields. For singlet-triplet interconversions, magnetic fields alter product distributions when they cause the rate of interconversion to be comparable to the rate free radicals escape solvent cages. These effects are largest in highly viscous media, such as polymer films and micellar environments. Larger effects should be observed as the dimensionality of the system decreases. For coordination complexes, photochemical and homogeneous electron transfer rates are altered by magnetic fields. Spin-orbit coupling is higher in transition metal complexes than organic radicals because of higher nuclear charge and partially unquenched orbital angular momentum of the d-shell electrons. The rate of homogeneous electron transfer between Co(NH
3
)
6
3+
and Ru(NH
3
)
6
2+
is below that expected for diffusion controlled reactions; in a 7 T magnetic field, the rate is suppressed two to three-fold. It has been argued that &Dgr;&khgr;
m
(and &Dgr;G
m
) is set by the magnetic susceptibility of the products, reactants, and activated complex, and a highly paramagnetic activated complex accounts for the field effect. For reversible electron transfer at electrodes in magnetic fields, no significant effect is expected. For quasireversible electron transfer with paramagnetic and diamagnetic species, electron transfer rates and transfer coefficients (&agr;) are unchanged by magnetic fields applied parallel to electrodes. Magnetic fields applied perpendicular to electrodes in flow cells generate potential differences, which just superimpose on the applied electrode potentials. Potentials of 0.25V have been reported. Reversing the applied magnetic field reverses the sign of the potential difference. This effect does not change standard rate constants, only the applied potential.
Mass Transport
Magnetically driven mass transport effects have been studied in electrochemical cells placed between the poles of large magnets. Effects vary depending on the orientation of the electrode, the relative orientation of the magnetic field and the electrode, forced or natural convection, and th
Amarasinghe Sudath
Leddy Johna
Pianalto Bernard
University of Iowa Research Foundation
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