Gradient interface magnetic composites and methods therefor

Details Gradient interface magnetic composites and methods therefor Gradient interface magnetic composites and methods therefor

1999-12-10

2001-10-16

Reifsnyder, David A. (Department: 1723)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C429S010000, C429S047000, C429S127000, C210S222000, C428S692100

Reexamination Certificate

active

06303242

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 and separators including fuel cells, batteries, membrane sensors, flux switches, chromatographic separations, and nonelectrochemical separations resulting in enhanced and modified flux and performance in those systems. The invention also relates to apparatus, methods of making, and methods of using composite magnetic materials and interface gradients for the separation of transition metals, both light and heavy, from other species and from each other.
2. Background of the Related Art
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. Such interfacial gradients may also be exploited in separators involving chromatographic separations and nonelectrochemical separations including, but not limited to, separations of light and heavy transition metals and transition metal complexes. The heavy transition metals include the lanthanides and the actinides which have atomic numbers 58-71 and 90-103, respectively. 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. These bulk properties of molecules in magnetic fields are fairly well understood.
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, &khgr;
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, &khgr;
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. These dipoles 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 Telsa (T) (1 Tesla=10 kGauss) applied field, |&Dgr;G
m
|≦0.05 J/mole. Even in the strongest laboratory fields of 10T, 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 the magnetic/Nafion (DuPont) composites, 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
‡
; k=A exp[−&Dgr;G
‡
/RT]. An externally applied, homogeneous magnetic field will have little effect on &Dgr;G
‡
, 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 &Dgr;G
‡
); 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 at which 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- or f-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 7T 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 fie

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