Design Considerations of a Dual Mode X-Band EPR Resonator for Rapid In-Situ Microwave Heating

Abstract

This paper describes the design considerations for a dual mode X-band continuous wave (CW) Electron Paramagnetic Resonance (EPR) cavity, for simultaneous EPR measurement and microwave heating of the same sample. An elliptical cavity geometry is chosen to split the degeneracy of the TM110 mode, allowing for a well resolved EPR signal with the TM110,a and TM110,b modes resonating at around 10 GHz and 9.5 GHz, respectively, the latter of which is used for EPR measurements. This geometry has the benefit that the TM010 mode used for microwave heating resonates at 6.1 GHz, below the cut off frequency of the X-band waveguide used for the EPR channel, providing effective isolation between the heating and EPR channels. The use of a pair of 9 µm thick copper clad laminates as the flat cavity walls allows for sufficient penetration of the modulation field (Bmod) into the cavity, as well as maintaining a high cavity Q factor (> 5700) for sensitive EPR measurements. Locating the heating port at an angle of 135° to the EPR port provides additional space for easier coupling adjustment and for larger sample access to be accommodated. The associated decrease of EPR signal strength is fully compensated for by using a 7.2 GHz low pass filter on the heating port. EPR spectra using 1.6 mm and 4.0 mm sample tubes are shown at room temperature (298 K) and 318 K for a standard Cu(acac)2 solution, demonstrating the effectiveness of this dual-mode EPR cavity for microwave heating during EPR detection.

An in-situ study of the thermal decomposition of AIBN radical chemistry using a dual mode EPR resonator

“…6.1 GHz (electric eld) for dielectric heating, making this an ideal resonator to monitor the 'real-time' in-situ EPR response to dielectric heating. [16,17] In this present study, we hereby demonstrate the utility and effectiveness of this resonator to monitor the radical formation via the thermal decomposition of 2,2′-azobis(2methylpropionitrile) (hereafter labelled AIBN).…”

“…In addition, rapid temperature increases under conventional methods conditions were achieved using the highest gas ow possible (0.08 L/min, see Supporting Information). The variable temperature (VT) heating and EPR studies were also carried out using microwaves in our custom designed and built dual-mode EPR resonator (details of resonator construction can be found in our earlier publications [16,17]); hereafter, this use of microwaves is referred to as the dielectric heating method. A bre-optic probe (LumaSense LUXTRON 812 Industrial Temperature Monitor equipped with a MicroProbe) was used for temperature measurement.…”

“…To this end, we have recently developed and tested two generations of a dual-mode EPR 'reactor-resonator' capable of simultaneous EPR detection and microwave (MW)-induced dielectric heating. [16,17] These resonators utilise two discrete microwave frequencies at ca. 9.5 GHz (magnetic eld) for EPR detection and at ca.…”