JOURNAL OF MAGNETIC RESONANCE, Series A 122, 72–75 (1996)ARTICLE NO. 0177

High-Temperature–High-Pressure NMR Probe for Self-DiffusionMeasurements in Molten Salts

U. MATENAAR, J. RICHTER, AND M. D. ZEIDLER

Institut fur 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-diffu-sion coefficients, the NMR spin-echo method with pulsed

Å M(2t, 0)expF0g 2G 2Dd 2St 0 d3DG , [1]field gradients (SE-PFG) provides a tool for investigatingdiffusion without disturbing the system under study. Thisexperiment was first demonstrated by Stejskal and Tanner(1) . In previous papers we reported new high-temperature where M (2t, 0 ) refers to the amplitude of the NMR signalprobes for self-diffusion measurements at atmospheric pres- including relaxation effects obtained in the absence ofsure in molten sodium and lithium nitrates (2, 3) . Since self- the field-gradient pulses, g is the magnetogyric ratio, anddiffusion depends on both temperature and density, it is D is the diffusion coefficient of the observed nucleus.desirable to separate the two effects in experiments. Thus, The time interval between the two gradient pulses is t.we decided to extend our NMR measurements with molten In an experiment, the echo amplitude is measured as asalts to high pressure. Like all high-temperature and high- function of d. G must be determined by calibrating thepressure equipment, an NMR probe must meet complex de- gradient coil with a substance of known diffusion coeffi-sign requirements. All materials must be nonmagnetic; both cient.temperature and pressure should be held constant within As beryllium copper, which we used in former experi-small limits; radiofrequency feed-throughs should not lower ments, loses its strength at temperatures above 350 K, thethe quality factor of the tuning circuit measureably; the pres- high-pressure vessel for high temperatures is made of asure must be transmitted to the sample without contaminat- titanium alloy ( IMI 834 ) with a much better mechanicaling it; and the probes must fit into a wide-bore superconduct- strength at high temperatures. Figure 1 shows the sche-ing magnet with a field strength of 7.04 T (bore 89 mm, matic drawing of the cylinder. The thick-walled cylinderOxford Instruments, Inc.) . has an outer diameter (do ) of 6 cm and an inner diameter

(di ) of 2.5 cm. Equation [2] was used to calculate theA new NMR probe for self-diffusion measurements up topressure p at which yielding of the bore will occur:200 MPa and 673 K for 23Na, and optionally for 7Li ions,

in molten salts is described. First, a brief description of theSE-PFG experiment is given. Second, we will focus on the

p Å s0.2√3

(v 2 0 1)v 2 , v Å do /di . [2]construction of the high-pressure vessel, including closure

plugs and feed-throughs, and then give a description of thedifferent coil systems and sample tubes. With a yield strength of s0.2 Å 875 MPa at room tempera-

In the NMR spin-echo method a sample is placed in a ture and s0.2 Å 475 MPa at 873 K, the pressure lies be-magnetic-field gradient ÌB /Ìz , applied along the static mag- tween 417 and 227 MPa, respectively. With a securitynetic field B0 . The pulse sequence used is based on the factor of 1.5, the probe covers a pressure range between907— t—1807 sequence first reported by Hahn (4) with 270 and 151 MPa. The pressure vessel is closed byadditional pulsed magnetic-field gradients (1) . By this, a Bridgeman-type plugs. These consist of a plug head withspin echo is generated at a time 2t. Diffusion of the nuclei a seal, an O-ring, and a driver. The seal ring is made ofin the field gradient causes irreversible loss of phase coher- copper. As shown in Fig. 1, the pressurizing oil as wellence so that the echo amplitude is attenuated. The spin-echo as the double thermocouple are fed through the top plugamplitude M(2t, G) in the presence of a magnetic-field into the vessel. The thermocouple is soldered into a beryl-

lium copper cone inserted into the plug and placed neargradient of strength G and duration d is given by

721064-1858/96 $18.00Copyright q 1996 by Academic Press, Inc.All rights of reproduction in any form reserved.

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73NOTES

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. InsetI: 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 con- A resistance wire (Phillips Thermocoax, 5.5 V /mA) isnoninductively wound on the vessel and fixed in slots sonected to the pressure tubing. The bottom plug contains

the electrical feed-throughs for the gradient and the RF that the vessel is heated from outside. The cylinder itselfis surrounded by a cooling jacket. Through the latter,coil. The high-pressure cylinder is filled with a high-tem-

perature-resistant oil which serves as pressurizing liquid water is circulated to ensure that the superconductingmagnet cannot be damaged by heat. A regulation unit( Ultra-Therm 33 SCB, Lauda ) .

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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 differentgradient pulse lengths. (b) Normalized intensity of spin echos versus [0g 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 In Fig. 1, inset I, one can see the double Maxwellgradient coil wound on a MACOR support and a holder.{0.5 K. As mentioned above, the top plug is connected

with the pressure tubing. Pressure is generated by a screw The holder fulfills two functions. First, it serves as insula-tion by separating the bottom plug from the hot oil. Sec-press and controlled by a wire strain gauge (Burster ) .

Both pressure vessel and cooling jacket are fixed on an ond, it carries plug contacts which adhere tightly on thefeed-throughs. Both the gradient and the RF coil arealuminum tube which also contains the capacitors of the

tank circuit and the coaxial transmission lines. brazed on the plugs. The RF pulses are produced by aone-turn saddle coil. The design of the gradient coil hasWhen self-diffusion coefficients are measured in liquid

salts, several problems arise because of the high tempera- been described elsewhere (8 ) . It produces a linear mag-netic-field gradient in the z direction (within 1% over atures. First, the salts are solid at room temperature and un-

dergo a volume expansion of 10 to 20% during melting. sample which is 5 mm in diameter and 10 mm in length ) .The calculated efficiency is 0.043 T/mA.Second, high pressure within the vessel should be transmit-

ted to the highly corrosive melt without contaminating it With regard to the quality of the RF signal, the electricalfeed-throughs are the most critical part of the probe. Sincewith the pressurizing liquid. Two different types of cells

were considered: first, glass cells with bellows, but we found sodium and lithium ions are much less sensitive nuclei thanprotons, and as high temperatures also lower the quality ofthat glass-to-metal junctions are penetrated by the aggressive

melt; and, second, piston-type cells which have been used the NMR signal, we use more stable feed-throughs designedin the Roger Adams Laboratory, Urbana, Illinois (9) , andpreviously by different authors (5–7) . Using the latter, seal-

ing is a problem since normal O-rings do not stand the high shown in Fig. 1, inset II. A copper wire is connected on oneside to the saddle coil and on the other side forms a smalltemperatures. Finally, high-precision ceramic tubes which

are also used as parts of MAS rotors closed with a piston cone before it is soldered to a coaxial transmission-typeconnection leading to the capacitors outside the vessel. Theof the same material fitting with an accuracy of a few mi-

crometers (Cerobear by Wemhoner & Popp) proved to sealing cone as well as the pusher is made out of Vespel(Du Pont de Nemours & Co.) , whereas the driver consistsmatch the various requirements best (Fig. 1) . In case of

contamination they are easily replaced without disassem- of titanium alloy. The feed-through imitates a coaxial trans-mission line with improved electronic properties. The tankbling the coils.

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Illinois, for his help with the radiofrequency parts. Financial support fromcircuit is tuned at 79.39 MHz which is the resonance fre-the Deutsche Forschungsgemeinschaft, Bonn, and the Fonds der Chem-quency of 23Na in a 7.04 T field.ischen Industrie, Frankfurt, is gratefully acknowledged.

Preliminary diffusion measurements with the high-tem-perature and high-pressure probe show that all components REFERENCESwork satisfactorily. Calibration of the magnetic-field gradi-

1. E. O. Stejskal and J. E. Tanner, Chem. Phys. 42, 288 (1965).ent with self-diffusion data of aqueous NaCl solution (10)2. C. Herdlicka, J. Richter, and M. D. Zeidler, Z. Naturforsch. A 43,gives a gradient strength of 0.047 T/mA. In addition, calibra-

1075 (1988).tion with NaNO3 melt is also possible as shown before (2) .3. C. Herdlicka, J. Richter, and M. D. Zeidler, Z. Naturforsch. A 47,Figure 2a shows a stacked plot of a spin-echo experiment

1047 (1992).with molten sodium nitrate at 613 K. The t value is 30 ms,

4. E. L. Hahn, Phys. Rev. 80, 580 (1950).and the duration of the gradient pulses lies between 0.5 and

5. J. Jonas, Rev. Sci. Instrum. 41, 1240 (1970).13 ms at 3.42 A. Fitting of the data to Eq. [1] (Fig. 2b) 6. J. Jonas, ‘‘NMR Basic Principles and Progress’’ (P. Diehl, E. Fluck,gave a self-diffusion coefficient D Å 2.10 1 1009 m2 s01 . H. Gunther, R. Kosfeld, and J. Seelig, Eds.) , Vol. 24, Chap. 3,

High-temperature–high-pressure probes have been re- Springer-Verlag, Berlin/Heidelberg, 1991.7. J. W. Akitt and A. E. Merbach, ‘‘NMR Basic Principles and Prog-ported before (11, 12) , but, as far as we know, the probe

ress’’ (P. Diehl, E. Fluck, H. Gunther, R. Kosfeld, and J. Seelig,head described here is the first NMR probe for self-diffusionEds.) , Vol. 24, Chap. 5, Springer-Verlag, Berlin/Heidelberg, 1991.measurements in molten salts at high temperatures and high

8. M. Buszko and G. E. Maciel, J. Magn. Reson. A 107, 151 (1994).pressures.9. J. Jonas, P. Koziol, X. Peng, C. Reiner, and D. M. Campbell, J.

Magn. Reson. B. 102, 299 (1993).ACKNOWLEDGMENTS 10. M. Holz and H. Weingartner, 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, 5218We especially thank Professor Jiri Jonas, Urbana, Illinois, for his gener-

ous support with regard to the feed-throughs and Carl Reiner, Urbana, ( 1995 ) .

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