Nd-Fe-B Permanent Magnet Electrodes. Theoretical Evaluation and Experimental Demonstration of the Paramagnetic Body Forces


Cyclic voltammetry with Nd-Fe-B disk magnet electrodes (3.2 mm diameter) at slow sweep rates (≤0.01 V s -1) in relatively concentrated solutions (e.g., 80 mM) of diamagnetic redox-active species (e.g., TMPD) is controlled by diffusion. Under similar conditions, cyclic voltammetry with conventional noble metal disk millielectrodes is characterized by the absence of diffusion waves and the presence of density gradient driven natural convection. Although the magnetic field in the vicinity of Nd-Fe-B electrodes is relatively strong (∼0.5 T at the surface of the magnet electrode), the absence of magnetohydrodynamic stirring effects is attributed to the fact that the i and B vectors are almost parallel, and therefore the magnetohydrodynamic force F B (=i × B) is very small. On the other hand, the absence of natural convection is attributed to the two possible paramagnetic body forces, F ΔB and F ΔC, exerted by the magnet electrode on the diffusion layer. Of those two forces, the former depends on field gradients (F ΔB ∼ B·ΔB), while the latter depends on concentration gradients (F ΔC ∼ ΔC j) and is directed toward areas with higher concentration of paramagnetic j. Through thorough analysis of the magnetic field and its gradients, it is found that the average F ΔC force acting upon the entire diffusion layer is ∼1.75 times stronger than F ΔB. Nevertheless, it is calculated that either force independently is strong enough and would have been able to hold the diffusion layer by itself. Further evidence suggests that, integrated over the entire solution, F ΔB is the dominant paramagnetic force when the redox-active species is paramagnetic, e.g., [Co(bipy) 3](ClO 4) 2 (bipy = 2,2′-bipyridine). Finally, convective behavior with diamagnetic redox-active species and magnet millielectrodes can be observed by holding closely (2-3 mm away) a repelling second magnet that bends the induction B to the point that the i × B product is not equal to 0.



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