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82ISSN 2072-5981 doi: 10.26907/mrsej
aänetic Resonance in Solids
Electronic Journal
Volume 21 Issue 2 Paper No 19203
1-6 pages
2019
doi: 10.26907/mrsej-19203
http: //mrsej. kpfu. ru http: //mrsej. ksu. ru
Established and published by Kazan University Endorsed by International Society of Magnetic Resonance (ISMAR) Registered by Russian Federation Committee on Press (#015140),
August 2, 1996 First Issue appeared on July 25, 1997
© Kazan Federal University (KFU)*
"Magnetic Resonance in Solids. Electronic Journal" (MRSey) is a
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Editors
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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,
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* In Kazan University the Electron Paramagnetic Resonance (EPR) was discovered by Zavoisky E.K. in 1944.
Short cite this: Magn. Reson. Solids 21, 19203 (2019)
doi: 10.26907/mrsej-19203
Pulsed NMR spectrometer with dynamic nuclear polarization
for weak magnetic fields
A.S. Alexandrov1'*, A.A. Ivanov1, R.V. Archipov1, M.R. Gafurov1, M.S. Tagirov1'2
1Kazan Federal University, Kremlevskaya 18, 420008 Kazan, Russia
2Institute of Applied Research, Tatarstan Academy of Sciences, 420111 Kazan, Russia
* E-mail: Alexandrov.Artem.Sergeyevich@gmail.com
(Received March 26, 2019; revised April 5, 2019; accepted April 5, 2019; published April 10, 2019)
Pulsed nuclear magnetic resonance (NMR) spectrometer operating in weak magnetic field (~100 Oe) with the dynamic nuclear polarization (DNP) is described. The spectrometer is designed to study the Overhauser DNP effect in natural hydrocarbons for potential using DNP techniques to enhance the NMR signal in crude oil and petroleum products. The unit was tested on a sample of the ethylene glycol chromium complex at room temperature. A DNP enhancement of e = -50 for NMR signal from solvent protons by pumping 52Cr5+ electronic transitions with the optimized pumping parameters was achieved.
PACS: 76.70.Fz, 83.80.Nb.
Keywords: magnetic resonance, Overhauser effect, dynamic polarization of nuclei, NMR, DNP.
1. Introduction
Investigation of the reservoir characteristics such as porosity, permeability, fluid saturation, type of saturating fluid, etc. is an important task for the oil industry. One of the most effective, non-destructive and express method for measuring these characteristics is nuclear magnetic resonance (NMR) [1,2]. However, despite the rapid development, an important problem in the field of NMR logging still remains the low sensitivity of the method at near room temperatures (RT). This is primarily due to the low nuclei magnetization (thermal polarization) at RT in weak magnetic fields (~100 Oe) used by the borehole logging devices. In addition, the fluid saturation of reservoir usually does not exceed a ten percent which means a low filing factor of the receiving antenna. It leads to the weak signal-to-noise ratio or increases the acquisition time, which is very crucial for logging application.
One of the ways to increase the NMR signal is to enhance the nuclei magnetization by the methods of dynamic nuclear polarization (DNP) based on the effect of transfer the polarization from electronic spins S (paramagnetic centers) to a system of nuclear spins I (see for example [3-5]). In liquids the Overhauser mechanism (ODNP) makes the largest contribution to the polarization transfer [6]. The ODNP enhancement can be factorized as:
e = ^ = (/.YS, (1)
Po YI
where P0 is equilibrium polarization of I spins, Pz is enhanced polarization of I spins, Z is coupling factor, / is leakage factor, s is saturation factor for electronic spin S, ys and yi are gyromagnetic ratios of the electron and nuclear spin systems, respectively [7]. Such a transfer requires the presence of paramagnetic centers in the system under study, as well as the presence of a magnetic coupling between such centers and the nuclei subsystem. In the mid-1950s the paramagnetism of hydrocarbon containing systems mainly due to the presence of stable carbonaceous free radicals was discovered [8,9]. Already in 1958 a fifteen-fold amplification of the 1H NMR signal of an oil sample due to DNP in the magnetic field of 15 Oe was demonstrated [10].
However, in later reports on this topic not native hydrocarbons, but oil asphaltene / asphalt solutions, as containing the highest amount of free radicals, were investigated by ODNP [11-13] in the magnetic fields of 14-74 Oe. Probably it is connected with the great diversity of the qualitative and quantitative composition of oils meaning that only in some of them the magnetic coupling between paramagnetic centers and protons, as well as their mutual mobility, are sufficient to observe a perceptible ODNP effect.
The goal of this work was to create a pulsed NMR spectrometer with DNP for magnetic field of 100 Oe to study the ODNP effect in natural hydrocarbons and to increase the NMR signal of hydrocarbons for NMR logging applications.
2. Apparatus
The block diagram of the designed setup is shown in Fig. 1. The setup includes an NMR subsystem and a system of pulsed saturation of the electron paramagnetic resonance (EPR) line of paramagnetic centers. An NMR logging device magnetic system (MS) was used to generate the external magnetic field H0. The MS consists of two cylindrical permanent SmCo magnets 120 mm in diameter and 500 mm in height, directed by the analogous poles to each other. The distance between the poles of the magnets was 500 mm. This configuration creates an axially symmetric relatively uniform magnetic field of 99.4 Oe directed along the radial axis (see Fig. 1) at 180 mm distance from the center (O) of the magnetic system. Magnetic field gradients at this area was estimated as dt ~ dHT ~ 2 Oe/cm. The 1H NMR frequency in this field is 423 kHz.
The NMR subsystem consists of an NMR sensor, a receiver amplifier RA, a power amplifier PA, a pulse sequence programmer PP, and an RTO1012 oscilloscope. The NMR sensor radiofre-quency (RF) circuit is built according to the Fig. 2. The circuit was developed to match to the preamplifier constructed earlier in our group, which has an input impedance of 50 Q and a low noise factor K ~ 1 dB. The sensor consists of a transceiver parallel oscillating circuit L0C0, matched to 50 Q by C1, a quarter-wave line equivalent circuit C2-L1 -C3, and a cross-
m
<D
c S
O
sample tube
fT\
MR
Lo^D
Vi
H
t
H
1MW
H
SC —> RA
t
PA RTO
1012
\ t
BT0 PP
0500
t t
SML01 PC
Figure 1. Scheme of the designed setup: resonator MR, sensor circuit SC, receiver amplifier RA, RF pulse power amplifier PA, pulse sequence programmer PP, personal computer PC. More details are given in the text.
z
x
diode switches VD1-VD2, VD3-VD4. The antenna L0 is a solenoid 20 mm in diameter and 80 mm in height, made of copper wire 0.35 mm in diameter. The inductance of the antenna is L0 = 126 ^H, the quality factor Q = 45. By choosing the capacitance C0, the NMR sensor is tuned to the resonance frequency. By choosing the capacitance C1 the output impedance of the sensor is matched to 50 Q. The quarter-wave line circuit is tuned by trimming the inductance L1.
The electrical circuit of the receiver amplifier RA is shown in the Fig. 3. The RA is a selective amplifier, it has 50 Q input and output impedance, 67 dB total gain, 20 kHz bandwidth (-3 dB) and 60 dB dynamic range. The RMS noise voltage at the output of the RA with the input loaded by 50 Q is 60 ^V. The RA consists of a two-stage preamplifier assembled on MRF571 transistors according to a common emitter circuit, and a two-stage end amplifier assembled on
VD1
|__matched circuit____||_ quarter-wave line___|
Figure 2. The electrical circuit of the NMR sensor.
( P_OUT >
Figure 3. The electrical circuit of the receiver amplifier RA.
AD8021 ICs according to an inverting amplifier circuit. Selectivity is provided by a distributed filter consisting of LC circuits in each amplifying stage.
The electrical circuit of the power amplifier PA is shown at Fig. 4. The output stage of the amplifier was built on IRFPS3810 MOSFETs using a push-pull scheme. IR2113 IC is applied to drive IRFPS3810. The power supply voltage of the amplifier is up to 35 V. In the output stage 3:1 transformer T made on the ferrite ring M1500HM 35x20x10 is applied. The output impedance of the PA is 50 Q, the maximum pulse power is 100 W at nominal load.
The NMR signal was detected by spin echo spectrum using an RT01012 oscilloscope with an integrated Fourier transform function. The control and synchronization of all devices was carried out by the pulse sequence programmer PP, designed on the SPARTAN6 FPGA IC. The programmer generates radio-frequency n/2- and n-pulses, a trigger pulse for RT01012 oscilloscope, and a gating pulse for EPR line saturation. The programmer is controlled from a conventional computer via the TCP/IP interface and homemade software.
The electronic transition saturation system consists of an R&S SML01 generator used as a reference high frequency generator, a TOMCO BT00500 power amplifier and a MR resonator. The Himw pulse is formed by applying a logic level pulse from PP to the gating input of the TOMCO BT00500 amplifier. The MR resonator (Fig. 5) was calculated and assembled according to the detailed description given by Diodato [14]. The MR is a double loop-gap resonator with
VD1 XW4
1
VD2
Figure 4. The electrical circuit of the power amplifier PA.
Al
Section A
Section B
" Coupling loop
Figure 5. The scheme of the resonator MR by Diodato [14].
l
10 mm internal diameter and 40 mm section height. The resonator was tuned by trimming the capacitance CS. The output impedance of the resonator was matched to 50 Q by a broadband coupling loop. Precise matching could be easily achieved by varying the distance Al from both sections to the coupling loop (Al was varied between 5-10 mm). The quality factor of the resonator is 557 at 280 MHz. The alternating magnetic field H1MW is directed along the Oz axis and equals 2.1 Oe at PMW = 16 W supplied to the resonator. According to Poindexter [10] that is sufficient to observe a significant DNP effect on oil protons.
3. Results
The designed setup was tested on an ethylene glycol Cr5+ complex sample. The parameters of this paramagnetic center in organic solvents are rather well studied [7,15] with the estimated e = -50 and the corresponding coupling factor Z = 0.077. Concentration of the prepared ethylene glycol Cr5+ complex was 0.01 mol/L, the volume was 8 mL. EPR spectrum of the sample is due to the presence of chromium 52Cr isotope (I = 0), giving a narrow (4 Oe) intense EPR line with a g-factor equal to 1.981. Odd 53Cr isotope (I = 3/2) with natural content of 9.5%, gives 4 lines of low intensity in EPR spectrum caused by the hyperfine interaction [16]. Intensive 52Cr5+ line was saturated to provide DNP, in the field of 99.4 Oe this line has Larmor frequency fMW = 275.39 MHz.
The tests were performed using the pulse sequence "TMW — n/2NMR — t — nNMR". It is started with the pulse H1MW with duration TMW and power PMW for EPR line saturation and then followed by NMR detection sequence. The duration of n/2- and n-pulses were 10 and 20 ^s, respectively, the time t = 600 ^s
The results for the ethylene glycol Cr5+ complex are presented in the Fig. 6. Figure 6 shows the absolute values of the experimentally measured ODNP enhancements e as functions of PMW and TMW. Changes in the NMR signal begin to appear already at TMW = 10 ms and PMW = 0.1 W. With an increase of PMW, the NMR signal first decreases to zero, then changes its sign and further growths as described in literature for the predominant dipole electron-nuclear coupling [3]. The maximum enhancement e = —50 was achieved that agrees well with Fedotov [15]. For TMW > 150 ms and PMW > 2 W, the ODNP enhancement is found to be
100
300 ms
10 ms 40 ms 80 ms 150 ms
0.01 0.1 1 10 100
10
100
independent of the applied power. It can be assumed, that in this case the EPR line is completely saturated, and therefore the saturation factor s « 1. The leakage factor was estimated as f = 0.7 by the measurements of nuclear spin-lattice relaxation time T1S of ethylene glycol Cr5+ complex protons and nuclear spin-lattice relaxation time T1 of pure ethylene glycol protons (f = 1 — T1S/T1 [7]). Using these parameters, we have estimated the coupling factor from Eq. 1 as Z = 0.11, which also agrees well with Fedotov [15].
4. Summary
NMR / DNP spectrometer has been designed and produced for studying DNP on samples up to 10 mm in diameter with the continuous and pulsed saturation of electronic transitions with a maximum value of the alternating magnetic field of about 2 Oe in a permanent magnetic field of about 100 Oe. Test experiments were performed to measure the effect of ODNP on protons of the ethylene glycol chromium complex. The designed setup can be exploited for DNP investigation of native hydrocarbons, their components and solutions as well as for the estimation of optimal parameters necessary for the effective NMR enhancement in NMR logging applications.
Acknowledgments
The authors are grateful to prof. V.D. Skirda and O.I. Gnezdilov for comprehensive project support. EPR measurements were performed on the equipment of the center of Shared Facilities for Physical and Chemical Research of Substances and Materials of Kazan Federal University.
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