An Apparatus for Magnetic Measurements at High PressureR. W. Vaughan, C. F. Lai, and D. D. Elleman Citation: Review of Scientific Instruments 42, 626 (1971); doi: 10.1063/1.1685188 View online: http://dx.doi.org/10.1063/1.1685188 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/42/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Investigation of a strong titanium alloy KS15-5-3 and the application to a high pressure apparatus formagnetization measurements Rev. Sci. Instrum. 72, 1472 (2001); 10.1063/1.1337074 New type of highpressure apparatus and pressure measurement Rev. Sci. Instrum. 58, 666 (1987); 10.1063/1.1139236 Highpressure apparatus for dielectric measurements in high frequency Rev. Sci. Instrum. 50, 625 (1979); 10.1063/1.1135895 Apparatus for the Measurement of Optical Rotation of Solutions at High Pressure Rev. Sci. Instrum. 35, 1281 (1964); 10.1063/1.1718724 Thermoelectric Measurement of High Temperatures in Pressure Apparatus Rev. Sci. Instrum. 10, 137 (1939); 10.1063/1.1751499

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THE REVIEW OF SCIENTIFIC INSTRl;MENTS VOLUME 42. NUMBER 5 MAY 1971

An Apparatus for Magnetic Measurements at High Pressure*

R. W. VAUGHAN AND C. F. LA!

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91109

AND

D. D. ELLEMAN

Jet Propulsion Laboratory Pasadena, California 91101

(Received 11 December 1970; and in final form, 25 January 1971)

A nonmagnetic high pressure cell capable of being used for a variety of magnetic measurements to over 100 kilo­bars is described. A Bridgman anvil type of cell, it has a 3-5 mm3 sample volume and is suitable for both pulsed and wide line nuclear magnetic resonance (NMR) measurements. In addition to a detailed description of the appa­ratus used for wide line NMR measurements, the use of the high pressure cell for magnetoresistance and suscepti­bility measurements is described.

N UCLEAR magnetic resonance (NMR) has proven to be a powerful tool for studying chemical and

physical properties at atmospheric pressure, and has been widely used for high pressure studies in the 10 kilobar range.1- 7 NMR has not, however, been extensively used in the higher pressure range where optical, Mossbauer, and x-ray techniques are now commonly used. The purpose of this paper is to describe a nonmagnetic Bridgman anvil high pressure cell usable to over 100 kilobars and which has sufficient sample volume to allow a variety of mag­netic measurements including conventional wide line or pulsed NMR studies.

High pressure NMR measurements require subjecting a sample simultaneously to both large compressive forces and a large, uniform, and stable magnetic field. The in­compatibility of conventional high pressure equipment with the requirement for large uniform magnetic fields has been the main obstacle to these measurements. Lister and Benedek8 avoided this problem in their early study of nuclear resonance in iron to 65 kilobars by limiting their investigations to a ferromagnetic material which itself furnished the necessary field. Development of nonmagnetic high pressure cells has been reported by Cleron, Coston, and Drickamer9 and Gardner et at. lO The Gardner cell was u~ed for an EPR study of ruby to 80 kilobars while the Cleron, Coston, and Drickamer cell was used for NMR linewidth studies to 25 kilobars. More recently Gossard, McWhan, and Remeikall have used a split coil super­conducting s@lenoid to encase a girdle high pressure cell for low temperature NMR measurements of 51V to 65 kilo­bars. The apparatus described in this paper has been designed for use with conventional NMR electromagnets (gap width needed 4.45 cm) and the electronics are com­patible with standard spectrometers.

DESCRIPTION OF EQUIPMENT

The high pressure NMR apparatus has three unique components: a 40 ton nonmagnetic press small enough to fit within the pole gap of a conventional electromagnet,

626

the high pressure cell, and the special electronics necessary to allow use of the high pressure cell as an NMR probe. Figure 1 illustrates these components of the apparatus in an operational configuration.

HIGH PRESSURE CELL

Figure 2 (a) is an enlarged view showing the high pressure cell located within the press while Fig. 2 (b) is a further en­largement illustrating the high pressure sample area with­in the high pressure cell. A Bridgman anvil type of cell, the anvils are of a 84% Ab03-14% Ti02 ceramic12 and the sample containing ring is normally of pyrophylliteY The anvils are 2.54 cm o.d. with a 7° taper to a 0.636 cm flat and the pyrophyllite ring has an inner diameter of 0.317 cm and an outer diameter of 0.635 cm. The outer diameter of the pyrophyllite ring has been reduced to 0.559 cm in some work without detrimental effects. The thickness of

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-BRIDGE CIRCUIT

_____ HyDRAULiC ,- PRESS

FIG. 1. Illustration of the operational configuration of the high pressure cell, bridge circuit, and hydrau­lic pressure.

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MAGNETIC MEASUREMENTS 627

the pyrophyllite containing ring varies as discussed below, but is normally in the range 0.038-0.056 cm. This results is a sample volume of 3-5 mma. The sample pellets were formed by precompacting a fine powder of the desired material to 3.5 kilobars in a separate die.

The high pressure cell has been calibrated by observing electrical resistance discontinuities accompanying pressure induced phase transitions in a number of metallic materials: bismuth, thallium, barium, and tin. The electrical calibra­tions were performed by positioning two small silver tabs (0.001 cm thick) under the pyrophyllite ring and con­necting them with a strip of the metal (typically 0.32 XO.02 XO.002 cm) being used for calibration. The metal strip

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FIG. 2. (a) Nuclear magnetic resonance high pressure cell within the hydraulic press; (b) the high pressure sample area within the high pressure cell.

thus extended across the sample region and could be used to estimate the size of any inhomogenities in pressure across the sample region as well as correlate the applied force with the internal pressure.

A considerable variation in the pressure calibration can be obtained depending upon the properties of the material used for the sample pellet, and, a more serious problem, the properties of the sample material also affected the relative dimensions of pellet and ring which are required to reduce internal pressure gradients to a few percent (as indicated by a tendency for the electrical transitions to smear). It is thus necessary each time a new material is to be examined to make a number of calibration runs, first optimizing the dimensions of the pellet and ring for pressure homogeneity and then determining a calibration curve for that geometry. In the materials so far studied the calibration curve is essentially linear, and the pressure intensification factor (the actual measured internal pressure per the average pressure on the flat) has run from 1.5 to 3. The repro­ducibility of the calibration for a specific geometry and specific sample material is within 5%. Occasionally a pressure run differs widely from this, but in all cases in which large deviations occurred, examination of the sample pellet after release of pressure revealed extrusion of the sample pellet.

The General Electric AbOr Ti02 ceramic appears to be well suited for use in this type of pressure cell when a nonconducting and nonmagnetic material is required. Some breakage of anvils has occurred at low pressures but this has almost always been on the first pressure run with that set of anvils and is presumed due to internal flaws in the material purchased. No significant effort has been ex­pended to determine the upper pressure limit of the cell; two sets of anvils were taken up in pressure until they broke. Both sets shattered violently into many small pieces at an applied force equivalent to an average pressure near 50 kilobars over the complete flat, sample, and containing ring. The actual pressure obtained in the sample area at the point of fracture can only be estimated from the upper bismuth transition which was seen at approximately one­half the applied load at which the anvils shattered. It is not normally possible, however, to extrapolate the bismuth calibration far above the upper bismuth point because large nonlinearities often occur above 100 kilobars in the calibration of Bridgman anvil devices.

Because this type of violent failure could possibly damage electromagnetic pole caps, all NMR studies have been kept well below this value. In future work it is planned to use anvils with tapered force fit jackets of beryllium copper to reduce the possibility of damaging the apparatus when failure occurs. If comparison with similar cells using tungsten carbide anvils is relevant the , use of force fit jackets could also allow an extension of the available pressure range by severalfold. In addition, the

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628 VAUGHAN, LAI, AND ELLEMAN

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FIG. 3. Bismuth calibrations of the NMR high pressure cell. Curve A: fired pyrophyllite ring 0.046 cm in thickness, and sample SQ.-50 wt% mixture of boron and lithium hydride 0.042 cm in thickness. Curve B: unfired pyrophyllite ring 0.046 cm thick, sample NaBF4 0.0405 cm thick. Curve C: unfired pyrophyllite ring 0.051 cm thick, sample FeSiF 6' 6H20 0.042 cm in thickness. Curve D: unfired pyro­phyllite ring 0.0405 cm thick, sample FeSiF6 ·6H20 0.0355 cm in thickness. Curve E: unfired pyrophyllite ring 0.043 cm in thickness, sample SQ.-50 wt% mixture of boron and lithium hydride 0.038 cm in thickness.

use of jacketed anvils is more attractive for the pulsed NMR studies presently being undertaken as the removal of the modulation coils [(Fig. 2(a)] furnishes enough additional space within the magnet to allow use of jacketed anvils, without having to reduce the size of the ceramic anvils below that presently being used.

Although a number of materials (BN, B, B-LiH mix­tures, silica, etc.) were investigated for use as containing rings, pyrophyllite was the only material found to be satisfactory. Both natural and fired pyrophyllite have been successfully used. The firing of the pyrophyllite is de­sirable for proton NMR because it removes the protons contained within the pyrophyllite which would otherwise interfere with observation of sample protons.

Figure 3 is a collection of bismuth calibrations which indicate variation in the pressure calibration that can be obtained by only minor modifications of either the ring or sample pellet. Calibrations A and E differ mainly in that unfired pyrophyllite was used for E and fired pyro­phyllite for A, while calibrations B, D, and E show the effect of changing the composition of the sample pellet and slight alterations in geometry. Calibrations C and D are both done with unfired pyrophyllite and the same sample material and illustrate the effects of changing the dimensions of the ring and sample pellet. In every case except A the calibrations were performed on a ring and

sample geometry which had been optimized to some extent to reduce internal pressure gradients. Calibration A demonstrates the effects of internal pressure inhomo­genity. It has been included in Fig. 5, however, to empha­size the size of the variation in the calibration that can be obtained; the pressure intensification factor varies from 0.9 (curve A) to slightly over 3 (curve E) in this figure.

PRESSURE GENERATION SYSTEM

The pressure generation system consists of a 2800 kg· cm-2 hand pump (Enerpac p-228), a Heise Bourdon tube gauge, and a hydraulic press capable of generating approxi­mately 40 tons force. The hydraulic press was constructed entirely of a beryllium copper alloy (Berylco 25) and utilized a double O-ring seal. It is 40.7 cm in over-all length and has a neck 25.9 cm long that is 4.07 cm in width and thus fits easily t.he 4.44 cm gap between pole faces in the standard Varian electromagnet. The high pressure cell is located near the end of the neck in a cavity (4.07X3.49X7.62 cm) and is compressed between the base of the neck and the press ram (3.17 em in diam­eter). This is partially illustrated in Fig. 2.

ELECTRONICS FOR NMR MEASUREMENTS

Figure 4 is a schematic diagram of the electronics used in the wide line NMR studies. A standard double modula­tion technique was used with a Princeton Applied Re­search phase sensitive detector (PAR-HR8) being used in conjunction with a Varian 4311 rf unit. An asymmetrical Anderson bridge14 was constructed to allow performance of single coil measurements as the geometry of the high pressure cell is not adaptable to double coil techniques. The Anderson type bridge can have a large effect on the

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FIG. 4. Schematic dia­gram of electronics used for high pressure NMR studies.

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MAGNETIC MEASUREMENTS 629

signal-to-noise (SIN) ratio of the apparatus and the com­ponent values given for the bridge circuit in Fig. 4 gave the best performance of the numerous combinations tried for operation at 56.4 and 60 mHz. As a further measure to improve the (SIN) ratio, the bridge was housed in a milled out block of aluminum which was attached to the end of the hydraulic press (Fig. 1). The four control rods extending out of the bridge housing (Fig. 1) are to allow rebalancing of the bridge while it remains in the magnet. This is necessary because the application of force to the high pressure cell can perturb the geometry of the NMR coil slightly.

EXPERIMENTAL RESULTS

Figure 5 illustrates the type of results obtained when the high pressure cell is used for wide line NMR measure­ments. The curves are essentially raw data which have been numerically integrated and scaled to a common area, and they thus still exhibit instrumental effects, such as a slight modulation broadening. Although the data for these spectra were recorded digitally in a Fabri-Tek signal averager, no extensive signal averaging was required; each curve represents 4 min of data acquisition time. The increased width of these lines as the pressure is increased is due to a slowing of the rate of rotation of octohedral SiF 6 groups within this solid, and quantitative analysis of the spectra can furnish detailed information on the nature of the motion.15 Such a study of internal motion is only one example of a number of different types of studies the ability to take NMR data at these pressures allows.

In addition to the NMR measurements the high pressure cell has also been used for high pressure studies of mag­netoresistance in the semimetals16 and studies of magnetic susceptibility in ferri- and ferromagnetic materials,16 The standard four contact technique was used for the mag­netoresistance measurements. By using a magnet gap of 5.73 cm it was possible to obtain both transverse and longitudinal components of the magnetoresistance simul­taneously because it is then possible to rotate the electro­magnet relative to the pressure cell. Measurements of mag­netic susceptibility have been used to determine the effect of pressure on the Curie temperature of FeNi alloys of the Invar variety. For these measurements a modification of the bridge circuits of Daybell17 was used.

ACKNOWLEDGMENTS

The authors wish to acknowledge the aid of Caltech students G. L. Nicolaides and Raymond Pong, and the

FIG. 5. Wide line NMR spectra for 19F in FeSiF6 ·6H20 at several pressures: (A) 1 atm, (B) 34kilobars, and (C), 70kilobars.

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excellent experimental assistance of E. Olli and G. Griffith. C. F. Lai also wishes to acknowledge financial support from the National Science Foundation Grant to Caltech for Undergraduate Research.

* This work was supported in part by the National Science F ounda­tion and the National Aeronautics and Space Administration (NAS 7-100).

1 G. B. Benedek and E. M. Purcell, J. Chern. Phys. 22, 2003 (1954). 2 G. B. Benedek, Magnetic Resonance at High Pressures (Inter­

science, New York, 1963), and references therein. 3 R. Baron, J. Chern. Phys. 38, 173 (1963). 4 J. E. Anderson and W. P. Slichter, J. Chern. Phys. 44,1797,3647

(1965). 5 T. Koshimoto and L. Rimai, Phys. Rev. 143, 157 (1966); 148

593 (1966). ' 6 G. A. Matzkanm and T. A. Scott, Phys. Rev. 151,360 (1966). 7 T. E. Ball and J. Jonas, J. Chern. Phys. 53, 3315 (1970). 8 J. D. Litster and G. B. Benedek, J. App!. Phys. 34, 688 (1962). 9 V. Cleron, C. J. Coston, and H. G. Drickamer Rev. Sci. Instrum.

37, 68 (1966). ' 10 H. M. Nelson, I. B. Larson, and J. H. Gardner, J. Chern. Phys.

47, 1994 (1967). Als<? see J. H. Gardner, M. W. Hill, C. Johansen, D. Larsen, W. Mum, and M. Nelson, Rev. Sci. Instrum. 34 1043 (1963). '

11 A. C. Gossard, D. B. McWhan, and J. P. Remeika, J. App!. Phys. 41, 864 (1970); Phys. Rev. B 2, 3762 (1970).

12 Obtained from the General Electric Co. as Carboloy grade 030. 13 Obtained from American Lava Corp. (subsidiary of 3M) as

grade A lava. 14 H. L. Anderson, Phys. Rev. 76, 1460 (1949). 15 G. L. Nicolaides, R. W. Vaughan, and D. P. Elleman, Bull.

Amer. Phys. Soc. 15, 1603 (December 1970) and to be published. 16 R. Pong and R. W. Vaughan (to be published). 17 M. D. Daybell, Rev. Sci. Instrum. 38, 1412 (1967).

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