A microwave-heating technique provides for batch processing of multiple, identically sized and shaped samples of the same material. The technique involves (1) excitation of a symmetrical electromagnetic mode or modes in a symmetrical microwave cavity and (2) positioning the samples symmetrically in the cavity so that all samples are exposed to the same electromagnetic-field conditions and thus the same heating conditions. Typically, the electromagnetic mode(s) and the pattern for mounting the samples are chosen to maximize the heating effect and make it as nearly spatially uniform as possible.
For example, the figure illustrates an application of the technique to microwave heating of N (in this case, N = 8) rod samples in a circular cylindrical cavity of radius r0. In this case, the microwave excitation is supplied in the TM020 mode, in which the electromagnetic field depends on radial position r but is independent of azimuthal angle qand of axial position z. Then for identical treatment, the samples should be positioned in the cavity at the same radius at equal angular intervals Δθ = 2π/N.
When the cavity is empty, the magnitude of the electric field attains maxima at r = 0 and at the cylindrical surface r/r0≈ 0.6941, while the magnitude of the magnetic field attains maxima at the cylindrical surfaces r/r0≈ 0.3336 and r/r0≈ 0.9658. To a first approximation, assuming that the samples perturb the electromagnetic field minimally, the samples should be positioned on one of these surfaces to maximize heating: If the samples are electrically resistive or nonconductive, then they should be positioned at r/r0 ≈0.6941 for maximum coupling to the electric field; if the samples are highly electrically conductive, then they should be positioned at r/r0≈ 0.3336 or r/r0≈ 0.9658 for maximum coupling to the magnetic field.
Of course, the samples can be expected to perturb the electromagnetic field, the degree of perturbation increasing with the size of the samples. In the absence of an exact theory for the effect of the samples on the electromagnetic field, it could be necessary to conduct experiments to determine the radial position for maximum and/or most nearly uniform heating.
The same principle can be applied to microwave heating of multiple spherical or disk-shaped samples. In this case, the samples should not only be mounted at the same radius and at equal angular intervals but should also be mounted at the axial-mid-length plane to minimize nonuniformity of heating by enforcing symmetry with respect to z.
This work was done by Martin Barmatz and Henry W. Jackson of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category.
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Microwave-heating technique for batch processing
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