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6ISSN 2072-5981 doi: 10.26907/mrsej
aänetic Resonance in Solids
Electronic Journal
Volume 23 Issue 2 Article No 21201
1-6 pages 2021
doi: 10.26907/mrsej-21201
http://mrsej.kpfu.ru http://mrsej.ksu. ru
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"Magnetic Resonance in Solids. Electronic Journal" (MRSey) is a
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Editors
Vadim Atsarkin (Institute of Radio Engineering and Electronics, Moscow) Yurij Bunkov (CNRS, Grenoble) Mikhail Eremin (KFU, Kazan) David Fushman (University of Maryland, College Park) Hugo Keller (University of Zürich,
Zürich)
Yoshio Kitaoka (Osaka University,
Osaka)
Boris Malkin (KFU, Kazan) Alexander Shengelaya (Tbilisi State University, Tbilisi) Jörg Sichelschmidt (Max Planck Institute for Chemical Physics of Solids, Dresden) Haruhiko Suzuki (Kanazawa University, Kanazava) Murat Tagirov (KFU, Kazan) Dmitrii Tayurskii (KFU, Kazan) Valentine Zhikharev (KNRTU,
Kazan)
* Address: "Magnetic Resonance in Solids. Electronic Journal", Kazan Federal University; Kremlevskaya str., 18; Kazan 420008, Russia
f In Kazan University the Electron Paramagnetic Resonance (EPR) was discovered by Zavoisky E.K. in 1944.
Short cite this: Magn. Reson. Solids 2, 21201 (2021)
doi: 10.26907/mrsej-21201
Multipurpose Portable Q-Band Bridge
D.G. Zverev1, S.N. Lukin2, A.A. Rodionov1, D.V. Shurtakova1, M.R. Gafurov1*
1Kazan Federal University, Kremlevskaya 18, Kazan 420008, Russia
2R.E. Kavetsky Institute of Experimental Pathology, Oncology and Radiobiology NAS of Ukraine, Vasylkivska str. 42, Kyiv 03022, Ukraine
*E-mail: marat.gafurov@kpfu.ru
(Received October 1, 2021; revised October 24, 2021; accepted October 26, 2021; published October 29, 2021)
The paper describes the design, implementation, and technical characteristics of a portable Q-band microwave (mw) bridge based on the Gunn diode with the potential use for electron paramagnetic resonance (EPR) and dynamic nuclear polarization (DNP) spectroscopies. The mw frequency can be electronically adjusted in the range of 36-38 GHz with the maximal mw output of 120 mW and electronic attenuation of 0-60 dB. The value of the mw frequency can be stabilized and changed via automatic frequency control for direct and alternating current. A self-written Matlab-based program allows tuning and operating the bridge through the RS-485 interface. Examples of the EPR spectra implemented into the magnetic system of the Bruker ESP300 commercial spectrometer are shown.
PACS: 76.30.-v, 76.60.-k, 76.60.Lz, 07.57.Pt, 75.78.-n, 89.30.aj
Keywords: microwave; spectrometer; electron paramagnetic resonance; dynamic nuclear polarization
1. Introduction
High initial and maintenance costs, immobility, excess complexity of the commercial Q-band spectrometers for electron paramagnetic resonance (EPR) and microwave (mw) equipment operating at the mw frequency in the range of 34-42 GHz, as well as closed program code for device control, monopoly of manufacture(s) with various sanctions restrictions, forced us to construct portable Q-band bridge for potential use as (a) a mw source to study the effect of mw radiation on the properties of substances and materials in laboratory conditions [1-4]; (b) a mw source for dynamic nuclear polarization (DNP) [5-7]; (c) a necessary part of the EPR spectrometer. There are many reasons for choosing Q-band for the mentioned purposes. Firstly, dielectric losses of water are maximal at Q-band [8] favoring selectivity for the water heating in the study of microwave influence on the water-containing materials. Secondly, as concerning EPR and DNP applications, besides the higher spectral resolution comparing to the conventional for EPR X-band frequencies of 9-10 GHz, the most Q-band EPR measurements can be done by exploiting magnetic systems based on the standard electromagnets while for the higher allowed mw frequencies use of expensive superconducting systems is required [9,10].
The purpose of this work was to create a portable Q-Band bridge with the potential use as a part of EPR and DNP spectrometers as well as a tunable source of mw irradiation at various locations.
2. Materials and methods
Q-band microwave bridge
The microwave bridge is built according to the classical scheme to obtain EPR spectra (Figure 1). The manufactured according to this block diagram microwave bridge (a box with the size of 60 x 40 x 20 cm3 with a mass of about 3 kg) is shown in Figure 2. 8mW microwave radiation generated by Gunn diode (1) is amplified to the 120 mW power at solid-state microwave
Multipurpose Portable Q-Band Bridge
Figure 1. Block diagram of the microwave bridge. For an explanation of abbreviations, see the text
Figure 2. Microwave Q-band bridge with open top covert
amplifier (2). Part of the radiation branched at precise directional coupler (3) then passes two electrically controlled attenuators (4, 30 dB each) and via circulator (5) goes to the resonator. The reflected microwave radiation again via circulator detects first by the path-trough detector (6) and then follows to the balance detector (7). The reference signal branched at directional coupler (3) passes through a phase shifter (8) again to the balance detector (7). Apart from the mw part, the microwave bridge consists of the power supply (9) which together with the specialized power supply of the Gunn (13) and solid-state microwave amplifier (14) provide power for all the necessary parts of the microwave bridge. Attenuation of the microwave attenuators (4) is controlled by the precision current source (10) which provides a direct current (DC) 0-100 mA to the attenuators. The microwave frequency of the Gunn source is stabilized to the frequency of the resonator using a digital automatic frequency control (AFC) board (11). There are three
Table 1. Microwave bridge characteristics
Parameter Value
operating frequency (average) 37 GHz
microwave frequency band 36-38 GHz
maximal output power (defined by solid state microwave amplifier) 120 mW
power adjustment range 0-60dB
modes of frequency stabilization without AFC, DC mode, two AC modes
frequency band of the IF signal 500 MHz
control interface RS-485 with MODBUS RTU protocol
possible modes of mw frequency stabilization: no stabilization, DC stabilization, and alternating current (AC) stabilization. Besides frequency stabilization, this board controls all the parts of the microwave bridge and provides a connection with a personal computer using MODBUS-RTU protocol with an RS-485 physical layer interface. The signal reflected from the resonator after balance detection amplifies at signal preamplifier (12). The resulting low-frequency signal is directed to the Bruker ESP300 console for final synchronous detection and digitizing. The mw bridge characteristics are gathered in Table 1.
Figure 3. Program menu item for tuning the microwave bridge with the resonator
The microwave bridge is controlled using control commands from a personal computer via the RS-485 interface using the MODBUS RTU protocol. The control software (written in Mat-lab) is integrated into the modified program of the Bruker ESP300 spectrometer. In it, you can select the operating mode of the microwave bridge, adjust the resonant frequency, select the operating mode of the autotuning of the resonant frequency (Figure 3). In addition, for
convenient tuning, one can select the frequency scan range. The dialog box of the developed program for measuring EPR spectra using the example of the spectrum of manganese in MgO powder is shown in Figure 4.
3. EPR tests
To test the microwave bridge, a cylindrical resonator with main modes TE0\x was produced, where x = 1,2,3 (Figure 5). Its properties are gathered in Table 2. Obtained characteristics are worse than the properties of the modern Q-band resonators and should be significantly optimized. However, test measurements of the several samples allowed us to estimate some of the microwave bridge characteristics.
Figure 5. Microwave resonator with modulation coils for observing EPR signals
The EPR spectrum of the manganese doped MgO obtained using above mentioned bundle (bridge and resonator) is shown in Figure 6. The detected six-line pattern represents standard hyperfine interaction of the manganese paramagnetic center with its nucleus (55Mn isotope with the nuclear spin of I =5/2 [11]). The signal-to-noise ratio (SNR) in the spectrum is relatively small and mostly related to the low quality factor of the test resonator. It is known that increase of the magnetic field amplitude modulation leads to increase SNR for the broad lines. We estimated maximum magnetic field modulation amplitude of the test resonator using ultranarrow signal from sealed in the tube lithium phthalocyanine (LiPc) samples as the reference (Fig. 7). LiPc is widely known material for the EPR oximetry; its linewidth (which can be as narrow as 10 ^T in a vacuum) nearly linear depends on the partial pressure of oxygen [12,13].
Table 2. Microwave resonator characteristics
Parameter Value
operating frequency band of TE01x modes, where x=1,2,3 35-41 GHz
the possibility of tuning the resonant frequency yes
the ability to change the matching of the resonator and the microwave bridge yes
Q-factor of the resonator 100-150 (depends on the mode)
magnetic field modulation on the sample 0-2 mT
maximal sample size up to 3 mm in diameter
Figure 6. Central part of the EPR spectrum of Mn in MgO acquired at Pmw = 10 ¡W and modulation amplitude of 0.1 mT
1327
B ( mT)
Figure 7. EPR of LiPc acquired at Pr and 2mT (right)
B ( mT)
= 25 ¡W and modulation amplitudes (MA) of 0.1 mT (left)
The left part of the figure 7 represents EPR spectrum of LiPc with modulation amplitude 0.1 mT and the right part corresponds to the highest possible modulation of 2 mT. The nonideal lineshape of the EPR line is due to the saturation of the LiPc signals. Since test resonator does not have the ability of the coupling change, the working point of the detector is tuned by the microwave power level.
Multipurpose Portable Q-Band Bridge
4. Conclusion
As it was stated above, main goal of the current work is developing of the low cost microwave bridge for EPR and DNP investigations. Technical characteristics of the bridge are quite promising and similar to those of the commercially available cw bridge. Unfortunately, absence of the proper resonator did not allow us to estimate real parameters of the bridge itself and we restrict ourselves to the rough experiments with measurements of the relatively strong signals. Thus, the next stage will be the development of the low cost resonator cavity, which will be a supplement for the developed Q-band microwave bridge.
Acknowledgments
The work is financially supported by Russian Science Foundation (grant #19-12-00332) References
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