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High-Temperature–High-Pressure NMR Probe for Self-Diffusion Measurements in Molten Salts
JOURNAL OF MAGNETIC RESONANCE, ARTICLE NO.
Series A 122, 72–75 (1996)
0177
High-Temperature–High-Pressure NMR Probe for Self-Diffusion Measurements in Molten Salts U. MATENAAR, J. RICHTER,
AND
M. D. ZEIDLER
Institut fu¨r Physikalische Chemie, RWTH Aachen, Templergraben 59, 52056 Aachen, Germany Received March 26, 1996; revised May 17, 1996
M(2t, G)
Among the several techniques for measuring self-diffusion coefficients, the NMR spin-echo method with pulsed field gradients (SE-PFG) provides a tool for investigating diffusion without disturbing the system under study. This experiment was first demonstrated by Stejskal and Tanner (1). In previous papers we reported new high-temperature probes for self-diffusion measurements at atmospheric pressure in molten sodium and lithium nitrates (2, 3). Since selfdiffusion depends on both temperature and density, it is desirable to separate the two effects in experiments. Thus, we decided to extend our NMR measurements with molten salts to high pressure. Like all high-temperature and highpressure equipment, an NMR probe must meet complex design requirements. All materials must be nonmagnetic; both temperature and pressure should be held constant within small limits; radiofrequency feed-throughs should not lower the quality factor of the tuning circuit measureably; the pressure must be transmitted to the sample without contaminating it; and the probes must fit into a wide-bore superconducting magnet with a field strength of 7.04 T (bore 89 mm, Oxford Instruments, Inc.). A new NMR probe for self-diffusion measurements up to 200 MPa and 673 K for 23Na, and optionally for 7Li ions, in molten salts is described. First, a brief description of the SE-PFG experiment is given. Second, we will focus on the construction of the high-pressure vessel, including closure plugs and feed-throughs, and then give a description of the different coil systems and sample tubes. In the NMR spin-echo method a sample is placed in a magnetic-field gradient ÌB/ Ìz, applied along the static magnetic field B0 . The pulse sequence used is based on the 907 — t —1807 sequence first reported by Hahn (4) with additional pulsed magnetic-field gradients (1). By this, a spin echo is generated at a time 2t. Diffusion of the nuclei in the field gradient causes irreversible loss of phase coherence so that the echo amplitude is attenuated. The spin-echo amplitude M(2t, G) in the presence of a magnetic-field gradient of strength G and duration d is given by
1064-1858/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AID
JMRA 0932
/
6j10$$$101
F
S DG
Å M(2t, 0)exp 0 g 2G 2 Dd 2 t 0
d 3
,
[1]
where M ( 2 t, 0 ) refers to the amplitude of the NMR signal including relaxation effects obtained in the absence of the field-gradient pulses, g is the magnetogyric ratio, and D is the diffusion coefficient of the observed nucleus. The time interval between the two gradient pulses is t. In an experiment, the echo amplitude is measured as a function of d. G must be determined by calibrating the gradient coil with a substance of known diffusion coefficient. As beryllium copper, which we used in former experiments, loses its strength at temperatures above 350 K, the high-pressure vessel for high temperatures is made of a titanium alloy ( IMI 834 ) with a much better mechanical strength at high temperatures. Figure 1 shows the schematic drawing of the cylinder. The thick-walled cylinder has an outer diameter ( do ) of 6 cm and an inner diameter ( di ) of 2.5 cm. Equation [ 2 ] was used to calculate the pressure p at which yielding of the bore will occur: pÅ
s0.2 ( v 2 0 1) q , v2 3
v Å do /di .
[2]
With a yield strength of s0.2 Å 875 MPa at room temperature and s0.2 Å 475 MPa at 873 K, the pressure lies between 417 and 227 MPa, respectively. With a security factor of 1.5, the probe covers a pressure range between 270 and 151 MPa. The pressure vessel is closed by Bridgeman-type plugs. These consist of a plug head with a seal, an O-ring, and a driver. The seal ring is made of copper. As shown in Fig. 1, the pressurizing oil as well as the double thermocouple are fed through the top plug into the vessel. The thermocouple is soldered into a beryllium copper cone inserted into the plug and placed near 72
08-07-96 14:30:19
magas
AP: Mag Res, Series A
73
NOTES
FIG. 1. High-temperature–high-pressure probe. (1) Closure plug, (2) seal ring, (3) O-ring, (4) driver, (5) double thermocouple, (6) pressure tubing, (7) pressure vessel, (8) cooling jacket, (9) heating coil, (10) sample cell (shown above its site), (11) gradient coil support, (12) feed-throughs. Inset I: gradient coil. (a) Gradient coil support, (b) holder. Inset II: feed-through. (A) Copper wire, (B) driver, (C) pusher, (D) sealing cone.
the sample, whereas the plug itself can be directly connected to the pressure tubing. The bottom plug contains the electrical feed-throughs for the gradient and the RF coil. The high-pressure cylinder is filled with a high-temperature-resistant oil which serves as pressurizing liquid ( Ultra-Therm 33 SCB, Lauda ) .
AID
JMRA 0932
/
6j10$$$102
08-07-96 14:30:19
A resistance wire ( Phillips Thermocoax, 5.5 V /mA ) is noninductively wound on the vessel and fixed in slots so that the vessel is heated from outside. The cylinder itself is surrounded by a cooling jacket. Through the latter, water is circulated to ensure that the superconducting magnet cannot be damaged by heat. A regulation unit
magas
AP: Mag Res, Series A
74
NOTES
FIG. 2. Stacked plot and evaluation of D. (a) Stacked plot of Fourier-transformed spin echos of 23Na ions in molten sodium nitrate at different gradient pulse lengths. (b) Normalized intensity of spin echos versus [ 0 g 2d 2 ( t 0 d /3)] and fitted curve; d Å 0.5–13 ms, t Å 30 ms.
( West Instruments ) controls the temperature to within {0.5 K. As mentioned above, the top plug is connected with the pressure tubing. Pressure is generated by a screw press and controlled by a wire strain gauge ( Burster ) . Both pressure vessel and cooling jacket are fixed on an aluminum tube which also contains the capacitors of the tank circuit and the coaxial transmission lines. When self-diffusion coefficients are measured in liquid salts, several problems arise because of the high temperatures. First, the salts are solid at room temperature and undergo a volume expansion of 10 to 20% during melting. Second, high pressure within the vessel should be transmitted to the highly corrosive melt without contaminating it with the pressurizing liquid. Two different types of cells were considered: first, glass cells with bellows, but we found that glass-to-metal junctions are penetrated by the aggressive melt; and, second, piston-type cells which have been used previously by different authors (5–7). Using the latter, sealing is a problem since normal O-rings do not stand the high temperatures. Finally, high-precision ceramic tubes which are also used as parts of MAS rotors closed with a piston of the same material fitting with an accuracy of a few micrometers (Cerobear by Wemho¨ner & Popp) proved to match the various requirements best (Fig. 1). In case of contamination they are easily replaced without disassembling the coils.
AID
JMRA 0932
/
6j10$$$102
08-07-96 14:30:19
In Fig. 1, inset I, one can see the double Maxwell gradient coil wound on a MACOR support and a holder. The holder fulfills two functions. First, it serves as insulation by separating the bottom plug from the hot oil. Second, it carries plug contacts which adhere tightly on the feed-throughs. Both the gradient and the RF coil are brazed on the plugs. The RF pulses are produced by a one-turn saddle coil. The design of the gradient coil has been described elsewhere ( 8 ) . It produces a linear magnetic-field gradient in the z direction ( within 1% over a sample which is 5 mm in diameter and 10 mm in length ) . The calculated efficiency is 0.043 T /mA. With regard to the quality of the RF signal, the electrical feed-throughs are the most critical part of the probe. Since sodium and lithium ions are much less sensitive nuclei than protons, and as high temperatures also lower the quality of the NMR signal, we use more stable feed-throughs designed in the Roger Adams Laboratory, Urbana, Illinois (9), and shown in Fig. 1, inset II. A copper wire is connected on one side to the saddle coil and on the other side forms a small cone before it is soldered to a coaxial transmission-type connection leading to the capacitors outside the vessel. The sealing cone as well as the pusher is made out of Vespel (Du Pont de Nemours & Co.), whereas the driver consists of titanium alloy. The feed-through imitates a coaxial transmission line with improved electronic properties. The tank
magas
AP: Mag Res, Series A
75
NOTES
circuit is tuned at 79.39 MHz which is the resonance frequency of 23Na in a 7.04 T field. Preliminary diffusion measurements with the high-temperature and high-pressure probe show that all components work satisfactorily. Calibration of the magnetic-field gradient with self-diffusion data of aqueous NaCl solution (10) gives a gradient strength of 0.047 T/mA. In addition, calibration with NaNO3 melt is also possible as shown before (2). Figure 2a shows a stacked plot of a spin-echo experiment with molten sodium nitrate at 613 K. The t value is 30 ms, and the duration of the gradient pulses lies between 0.5 and 13 ms at 3.42 A. Fitting of the data to Eq. [1] (Fig. 2b) gave a self-diffusion coefficient D Å 2.10 1 10 09 m2 s 01 . High-temperature–high-pressure probes have been reported before (11, 12), but, as far as we know, the probe head described here is the first NMR probe for self-diffusion measurements in molten salts at high temperatures and high pressures. ACKNOWLEDGMENTS We especially thank Professor Jiri Jonas, Urbana, Illinois, for his generous support with regard to the feed-throughs and Carl Reiner, Urbana,
AID
JMRA 0932
/
6j10$$$102
08-07-96 14:30:19
Illinois, for his help with the radiofrequency parts. Financial support from the Deutsche Forschungsgemeinschaft, Bonn, and the Fonds der Chemischen Industrie, Frankfurt, is gratefully acknowledged.
REFERENCES 1. E. O. Stejskal and J. E. Tanner, Chem. Phys. 42, 288 (1965). 2. C. Herdlicka, J. Richter, and M. D. Zeidler, Z. Naturforsch. A 43, 1075 (1988). 3. C. Herdlicka, J. Richter, and M. D. Zeidler, Z. Naturforsch. A 47, 1047 (1992). 4. E. L. Hahn, Phys. Rev. 80, 580 (1950). 5. J. Jonas, Rev. Sci. Instrum. 41, 1240 (1970). 6. J. Jonas, ‘‘NMR Basic Principles and Progress’’ (P. Diehl, E. Fluck, H. Gu¨nther, R. Kosfeld, and J. Seelig, Eds.), Vol. 24, Chap. 3, Springer-Verlag, Berlin/Heidelberg, 1991. 7. J. W. Akitt and A. E. Merbach, ‘‘NMR Basic Principles and Progress’’ (P. Diehl, E. Fluck, H. Gu¨nther, R. Kosfeld, and J. Seelig, Eds.), Vol. 24, Chap. 5, Springer-Verlag, Berlin/Heidelberg, 1991. 8. M. Buszko and G. E. Maciel, J. Magn. Reson. A 107, 151 (1994). 9. J. Jonas, P. Koziol, X. Peng, C. Reiner, and D. M. Campbell, J. Magn. Reson. B. 102, 299 (1993). 10. M. Holz and H. Weinga¨rtner, J. Magn. Reson. 92, 115 (1991). 11. DeFries and J. Jonas, J. Magn. Reson. 35, 111 (1979). 12. M. de Langen and K. O. Prins, Rev. Sci. Instrum. 66, 5218 ( 1995 ) .
magas
AP: Mag Res, Series A
Series A 122, 72–75 (1996)
0177
High-Temperature–High-Pressure NMR Probe for Self-Diffusion Measurements in Molten Salts U. MATENAAR, J. RICHTER,
AND
M. D. ZEIDLER
Institut fu¨r Physikalische Chemie, RWTH Aachen, Templergraben 59, 52056 Aachen, Germany Received March 26, 1996; revised May 17, 1996
M(2t, G)
Among the several techniques for measuring self-diffusion coefficients, the NMR spin-echo method with pulsed field gradients (SE-PFG) provides a tool for investigating diffusion without disturbing the system under study. This experiment was first demonstrated by Stejskal and Tanner (1). In previous papers we reported new high-temperature probes for self-diffusion measurements at atmospheric pressure in molten sodium and lithium nitrates (2, 3). Since selfdiffusion depends on both temperature and density, it is desirable to separate the two effects in experiments. Thus, we decided to extend our NMR measurements with molten salts to high pressure. Like all high-temperature and highpressure equipment, an NMR probe must meet complex design requirements. All materials must be nonmagnetic; both temperature and pressure should be held constant within small limits; radiofrequency feed-throughs should not lower the quality factor of the tuning circuit measureably; the pressure must be transmitted to the sample without contaminating it; and the probes must fit into a wide-bore superconducting magnet with a field strength of 7.04 T (bore 89 mm, Oxford Instruments, Inc.). A new NMR probe for self-diffusion measurements up to 200 MPa and 673 K for 23Na, and optionally for 7Li ions, in molten salts is described. First, a brief description of the SE-PFG experiment is given. Second, we will focus on the construction of the high-pressure vessel, including closure plugs and feed-throughs, and then give a description of the different coil systems and sample tubes. In the NMR spin-echo method a sample is placed in a magnetic-field gradient ÌB/ Ìz, applied along the static magnetic field B0 . The pulse sequence used is based on the 907 — t —1807 sequence first reported by Hahn (4) with additional pulsed magnetic-field gradients (1). By this, a spin echo is generated at a time 2t. Diffusion of the nuclei in the field gradient causes irreversible loss of phase coherence so that the echo amplitude is attenuated. The spin-echo amplitude M(2t, G) in the presence of a magnetic-field gradient of strength G and duration d is given by
1064-1858/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AID
JMRA 0932
/
6j10$$$101
F
S DG
Å M(2t, 0)exp 0 g 2G 2 Dd 2 t 0
d 3
,
[1]
where M ( 2 t, 0 ) refers to the amplitude of the NMR signal including relaxation effects obtained in the absence of the field-gradient pulses, g is the magnetogyric ratio, and D is the diffusion coefficient of the observed nucleus. The time interval between the two gradient pulses is t. In an experiment, the echo amplitude is measured as a function of d. G must be determined by calibrating the gradient coil with a substance of known diffusion coefficient. As beryllium copper, which we used in former experiments, loses its strength at temperatures above 350 K, the high-pressure vessel for high temperatures is made of a titanium alloy ( IMI 834 ) with a much better mechanical strength at high temperatures. Figure 1 shows the schematic drawing of the cylinder. The thick-walled cylinder has an outer diameter ( do ) of 6 cm and an inner diameter ( di ) of 2.5 cm. Equation [ 2 ] was used to calculate the pressure p at which yielding of the bore will occur: pÅ
s0.2 ( v 2 0 1) q , v2 3
v Å do /di .
[2]
With a yield strength of s0.2 Å 875 MPa at room temperature and s0.2 Å 475 MPa at 873 K, the pressure lies between 417 and 227 MPa, respectively. With a security factor of 1.5, the probe covers a pressure range between 270 and 151 MPa. The pressure vessel is closed by Bridgeman-type plugs. These consist of a plug head with a seal, an O-ring, and a driver. The seal ring is made of copper. As shown in Fig. 1, the pressurizing oil as well as the double thermocouple are fed through the top plug into the vessel. The thermocouple is soldered into a beryllium copper cone inserted into the plug and placed near 72
08-07-96 14:30:19
magas
AP: Mag Res, Series A
73
NOTES
FIG. 1. High-temperature–high-pressure probe. (1) Closure plug, (2) seal ring, (3) O-ring, (4) driver, (5) double thermocouple, (6) pressure tubing, (7) pressure vessel, (8) cooling jacket, (9) heating coil, (10) sample cell (shown above its site), (11) gradient coil support, (12) feed-throughs. Inset I: gradient coil. (a) Gradient coil support, (b) holder. Inset II: feed-through. (A) Copper wire, (B) driver, (C) pusher, (D) sealing cone.
the sample, whereas the plug itself can be directly connected to the pressure tubing. The bottom plug contains the electrical feed-throughs for the gradient and the RF coil. The high-pressure cylinder is filled with a high-temperature-resistant oil which serves as pressurizing liquid ( Ultra-Therm 33 SCB, Lauda ) .
AID
JMRA 0932
/
6j10$$$102
08-07-96 14:30:19
A resistance wire ( Phillips Thermocoax, 5.5 V /mA ) is noninductively wound on the vessel and fixed in slots so that the vessel is heated from outside. The cylinder itself is surrounded by a cooling jacket. Through the latter, water is circulated to ensure that the superconducting magnet cannot be damaged by heat. A regulation unit
magas
AP: Mag Res, Series A
74
NOTES
FIG. 2. Stacked plot and evaluation of D. (a) Stacked plot of Fourier-transformed spin echos of 23Na ions in molten sodium nitrate at different gradient pulse lengths. (b) Normalized intensity of spin echos versus [ 0 g 2d 2 ( t 0 d /3)] and fitted curve; d Å 0.5–13 ms, t Å 30 ms.
( West Instruments ) controls the temperature to within {0.5 K. As mentioned above, the top plug is connected with the pressure tubing. Pressure is generated by a screw press and controlled by a wire strain gauge ( Burster ) . Both pressure vessel and cooling jacket are fixed on an aluminum tube which also contains the capacitors of the tank circuit and the coaxial transmission lines. When self-diffusion coefficients are measured in liquid salts, several problems arise because of the high temperatures. First, the salts are solid at room temperature and undergo a volume expansion of 10 to 20% during melting. Second, high pressure within the vessel should be transmitted to the highly corrosive melt without contaminating it with the pressurizing liquid. Two different types of cells were considered: first, glass cells with bellows, but we found that glass-to-metal junctions are penetrated by the aggressive melt; and, second, piston-type cells which have been used previously by different authors (5–7). Using the latter, sealing is a problem since normal O-rings do not stand the high temperatures. Finally, high-precision ceramic tubes which are also used as parts of MAS rotors closed with a piston of the same material fitting with an accuracy of a few micrometers (Cerobear by Wemho¨ner & Popp) proved to match the various requirements best (Fig. 1). In case of contamination they are easily replaced without disassembling the coils.
AID
JMRA 0932
/
6j10$$$102
08-07-96 14:30:19
In Fig. 1, inset I, one can see the double Maxwell gradient coil wound on a MACOR support and a holder. The holder fulfills two functions. First, it serves as insulation by separating the bottom plug from the hot oil. Second, it carries plug contacts which adhere tightly on the feed-throughs. Both the gradient and the RF coil are brazed on the plugs. The RF pulses are produced by a one-turn saddle coil. The design of the gradient coil has been described elsewhere ( 8 ) . It produces a linear magnetic-field gradient in the z direction ( within 1% over a sample which is 5 mm in diameter and 10 mm in length ) . The calculated efficiency is 0.043 T /mA. With regard to the quality of the RF signal, the electrical feed-throughs are the most critical part of the probe. Since sodium and lithium ions are much less sensitive nuclei than protons, and as high temperatures also lower the quality of the NMR signal, we use more stable feed-throughs designed in the Roger Adams Laboratory, Urbana, Illinois (9), and shown in Fig. 1, inset II. A copper wire is connected on one side to the saddle coil and on the other side forms a small cone before it is soldered to a coaxial transmission-type connection leading to the capacitors outside the vessel. The sealing cone as well as the pusher is made out of Vespel (Du Pont de Nemours & Co.), whereas the driver consists of titanium alloy. The feed-through imitates a coaxial transmission line with improved electronic properties. The tank
magas
AP: Mag Res, Series A
75
NOTES
circuit is tuned at 79.39 MHz which is the resonance frequency of 23Na in a 7.04 T field. Preliminary diffusion measurements with the high-temperature and high-pressure probe show that all components work satisfactorily. Calibration of the magnetic-field gradient with self-diffusion data of aqueous NaCl solution (10) gives a gradient strength of 0.047 T/mA. In addition, calibration with NaNO3 melt is also possible as shown before (2). Figure 2a shows a stacked plot of a spin-echo experiment with molten sodium nitrate at 613 K. The t value is 30 ms, and the duration of the gradient pulses lies between 0.5 and 13 ms at 3.42 A. Fitting of the data to Eq. [1] (Fig. 2b) gave a self-diffusion coefficient D Å 2.10 1 10 09 m2 s 01 . High-temperature–high-pressure probes have been reported before (11, 12), but, as far as we know, the probe head described here is the first NMR probe for self-diffusion measurements in molten salts at high temperatures and high pressures. ACKNOWLEDGMENTS We especially thank Professor Jiri Jonas, Urbana, Illinois, for his generous support with regard to the feed-throughs and Carl Reiner, Urbana,
AID
JMRA 0932
/
6j10$$$102
08-07-96 14:30:19
Illinois, for his help with the radiofrequency parts. Financial support from the Deutsche Forschungsgemeinschaft, Bonn, and the Fonds der Chemischen Industrie, Frankfurt, is gratefully acknowledged.
REFERENCES 1. E. O. Stejskal and J. E. Tanner, Chem. Phys. 42, 288 (1965). 2. C. Herdlicka, J. Richter, and M. D. Zeidler, Z. Naturforsch. A 43, 1075 (1988). 3. C. Herdlicka, J. Richter, and M. D. Zeidler, Z. Naturforsch. A 47, 1047 (1992). 4. E. L. Hahn, Phys. Rev. 80, 580 (1950). 5. J. Jonas, Rev. Sci. Instrum. 41, 1240 (1970). 6. J. Jonas, ‘‘NMR Basic Principles and Progress’’ (P. Diehl, E. Fluck, H. Gu¨nther, R. Kosfeld, and J. Seelig, Eds.), Vol. 24, Chap. 3, Springer-Verlag, Berlin/Heidelberg, 1991. 7. J. W. Akitt and A. E. Merbach, ‘‘NMR Basic Principles and Progress’’ (P. Diehl, E. Fluck, H. Gu¨nther, R. Kosfeld, and J. Seelig, Eds.), Vol. 24, Chap. 5, Springer-Verlag, Berlin/Heidelberg, 1991. 8. M. Buszko and G. E. Maciel, J. Magn. Reson. A 107, 151 (1994). 9. J. Jonas, P. Koziol, X. Peng, C. Reiner, and D. M. Campbell, J. Magn. Reson. B. 102, 299 (1993). 10. M. Holz and H. Weinga¨rtner, J. Magn. Reson. 92, 115 (1991). 11. DeFries and J. Jonas, J. Magn. Reson. 35, 111 (1979). 12. M. de Langen and K. O. Prins, Rev. Sci. Instrum. 66, 5218 ( 1995 ) .
magas
AP: Mag Res, Series A