Electrical leads for low conductivity measurements under hydrostatic pressureBui Ai, P. Destruel, Hoang The Giam, and R. Loussier Citation: Review of Scientific Instruments 45, 1180 (1974); doi: 10.1063/1.1686844 View online: http://dx.doi.org/10.1063/1.1686844 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/45/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Separation of electric charge flow mechanisms in conducting polymer networks under hydrostatic pressure Appl. Phys. Lett. 89, 222905 (2006); 10.1063/1.2397010 Electrical breakdown of polyethylene terephthalate under hydrostatic pressure J. Appl. Phys. 76, 8218 (1994); 10.1063/1.357885 Pressure Cell with Ten Electrical Leads for Liquid Hydrostatic Pressures to 60 Kilobars Rev. Sci. Instrum. 43, 132 (1972); 10.1063/1.1685411 MEASUREMENT OF ULTRASONIC VELOCITIES IN SOLIDS UNDER HYDROSTATIC PRESSURE J. Acoust. Soc. Am. 48, 1324 (1970); 10.1121/1.1912281 Apparatus for the Measurement of Internal Friction Under Hydrostatic Pressure Rev. Sci. Instrum. 41, 1588 (1970); 10.1063/1.1684349

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1180 Notes 1180

Electrical leads for low conductivity measurements under hydrostatic pressure

Sui Ai, P. Destruel, Hoang The Giam, and R. Loussier

Laboratoire de Genie Electrique. Equipe "Materiaux Dielectriques" Associee au CNRS. 2 rue Camichel 31000. Toulouse. France

(Received 27 March 1974; and in final form, 22 May 1974)

An electrode system using sapphire for the low conductivity measurement under hydrostatic pressure below 5000 bar is described. The leakage resistance of this design is 1.3 X 10160 at 1000 bar, and 6 X 10140 at 4000 bar.

The fundamental problem associated with the measurement of low electrical conductivity resides in the choice of a suit­able dielectric material for the measuring-electrode seal. The materials mostly used, so far, have been either a mixture of Araldite with powdered quartz or alumina,1 a beryllium­copper mixture2- 3 ; or else, a mixture of "portland cement and epoxy adhesive."4 Most of them, in particular the Araldite, have a leakage resistance of about 1011 n5. Hence it becomes difficult to observe electrical conductivity variations in the solid insulators with these electrode seals.

Taking into consideration this fact, we have designed a new type of electrode seal using sapphire. This material is particularly advantageous from the view point of its high leakage resistance (exceeding 1016 n at 20°C) and excellent mechanical properties (compressive strength around 21000 kg/cm2, tensile strength 1900 kg/cm2

).

Figure 1 shows the entire experimental set up including the measuring electrode system. The sapphire is in the form of a cone having an included angle of about 30°. We find, on the basis of experimental results, that the reliability of sapphire depends on this angle. A 1.5 mm hole is drilled at the center of the cone permitting the passage of a steel lead serving as the electrical connection. This latter is glued to the sapphire by an appropriate epoxy resin casting. We found the adherence to be quite adequate: a 100 kg tensile force

FIG. 1. Closure assembly showing in detail the electrical measuring electrode and its insulator system. I-Pressure vessel, inner and outer cylinders; 2-coiled tube for cryogenic fluid circulation; 3-closure seal; 4-ou ter closure plug; 5-inner closure plug; 6--electrical lead wire; 7-epoxy resin casting; 8-Teflon layer; 9-heat shrinkable sheath; lO-sapphire cone insulator; ll-atmosphere; 12-pressurized argon gas.

on this wire neither pulled it out nor led to any cracks in the sapphire. The outer part of the sapphire is sheathed with a heat shrinkable element and a thin Teflon layer.

The thickness of the entire outer covering does not exceed 0.5 mm. This reduces considerably the stresses which can appear on the sapphire-steel interface under high pressure because of the impossibility of having a perfectly polished surface in spite of all the precautions taken during its reali­zation. This latter is particularly true in case of the cone housing.

The first results with this electrode device have been very encouraging. It is entirely leak proof as was evidenced by subjecting it to a pressure of 4 kilobar for a minimum period of 100 h using gaseous argon as the pressure medium. From the electrical point of view, it presents a leakage resistance varying from 1.3 X 1016 n at 1 kilobar to 6 X 1014 n at 4 kilobar at 20°C (Fig. 2).

From these results it can be concluded that in spite of the pressure dependent sapphire leakage resistance varia­tions the present electrode system permits low conductivity measurements with organic insulators such as Polyamide 6, PMMA. In order to work at pressures exceeding 5 kilobar, the maximum so far utilized, it would be necessary to study systematically the different stresses on the sapphire cone arising, particularly, out of its included angle as well as the hole diameter and the electrical lead. Nevertheless, when a liquid is used as a transmitting fluid for pressures not ex-

HAl

·13 10 +-~~-+---

__ ~_...j-_-.,;P =4000 bars

I 1~4r ____ ~_~~~~===P~=~20~0~0~b~ar~s

-.- I

FIG. 2. Leakage current vs time at different pressures. The leakage resistance is the ratio of the dc voltage to the steady current recorded about 3000 sec after the voltage is applied. Vdc = 15 V, cell tempera­ture=20°C.

Rev. Sci. Instrum., Vol. 45, No.9, September 1974 Copyright © 1974 by the American Institute of Physics

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1181 Letter to the Editor

ceeding 8000 bar, the design published by Mayburg6 seems to be available.

We are grateful to the Ministere de l'Industrie, du Com­merce et de I' Artisanat and the Centre National de la Recherche Scientifique for the financial aid.

ERRATUM

1181

lG. Mesnard, E. Souquet, and G. Noyel, Rev. Phys. AppJ. 6, 365 (1971).

!J. E. Schirber and D. W. Shanfeldt, Rev. Sci. lnstrum. 39, 270 (1968). 3B. E. Hammons, Rev. Sci. lnstrum. 42, 1889 (1971). 4R. H. Cornish and Arthur L. Ruoff, Rev. Sci. lnstrum. 32,639 (1961). 5D. Gugan, J. Sci. lnstrum. 33,160 (1956). 6S. Mayburg, Phys. Rev. 79, 375 (1950).

Erratum: Cylindrical flow reactor for the study of heterogeneous reactions of possible importance in polluted atmospheres [Rev. Sci. Instrum. 45, 344 (1974)]

H. Hedgpeth, S. Siegel. T. Stewart. and H. S. Judeikis

The Aerospace Corporation. EI Segundo. California 90245

(Received 20 May 1974)

References cited in paragraphs 2 and 3 on p. 347 should state "See Ref. 9" instead of "See Ref. 8." Also the 6th sentence of paragraph 3 should read "given in Ref. 9" instead of "given in Ref. 6."

LETTER TO THE EDITOR

Reference 9 should be added to the list of references.

9H. S. Judeikis and Seymour Siegel, Efficiency of Gas-Wall Reactions in a Cylindrical Flow Reactor, ATR-73(7256)-2, The Aerospace Corporation, EI Segundo, California (January 1974).

Sensitivity of vibrating sample magnetometers-and how to increase sensitivity if neededt

S. Foner

Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology. Cambridge. Massachusetts 02139

(Received 13 March 1974; and in final form, 26 Apri11974)

The purpose of this letter is to correct some common mis­understandings and errors which have appeared in the liter­ature concerning sensitivity limits of conventional vibrating sample magnetometers (VSM). In order to make a mean­ingful comparison between a VSM and other devices, the principles of operation and general discussion given earlier1

should be examined. Often VSM sensitivity is discussed in the literature in the following context:

(i) When a comparison is made with Faraday (force) methods, it is erroneously claimed that the force method is much more sensitive. In such cases the (limited) sensitivity of a VSM is given as a reason for choosing (or modifying) the Faraday method.

(ii) When a discussion of another magnetic measure­ment device is presented, the sensitivity of the VSM is used as a standard.

Rev. Sci. Instrum., Vol. 45. No.9. September 1974 Copyright © 1974 by the American Institute of Physics

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