REPORT CCG 71-57, NBS 2210472

NBS IR 75-646

STABLE PRESSURE TRANSDUCER

Jaok H. Colwell

Pressure and Vaouim Section

Heat Division

NBS

Washington^ B.C. 20234

January 1975

Interim Report for period July 1974 - December 1974

Prepared for CCG-Army/Navy/AF
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Abstract

This report describes the recent work on the development of a capacitive

pressure transducer which uses a solid dielectric material. We are continuing the

search for a pair of dielectric materials which will produce capacitors having the

same temperature dependence but markedly different pressure dependences, so that

the overall temperature dependence of the device can be minimized.

We have concentrated on anisotropic materials whose properties can be tailored

within its range by using the appropriate cut from the crystal. This particular

effort has not been fruitful. We have been fortunate, however, in finding that the

perpendicular and parallel cuts of calcite (CaC03) come close to satisfying our

conditions and we are pursuing possible further development of the device utilizing

this pair.

Progress on the fabrication of the automatic capacitance bridge to be used with

this device is discussed.

IrfTRODUCTION

We are attempting to develop a capacitive pressure transducer in which the

' capacitor consists of metallic electrodes deposited directly on a solid dielectric

- sample. The principle objective in developing a device of this type is that it

should possess long term stability, in that one cannot envision irreproducible

changes occurring in the measured property under hydrostatic conditions. Initial

attempts at such a device using CaF2 as dielectric material proved this contention.

Iji

' The desired precision for the device of better than 1:10^ at 70 MPa (10,000 psi)

was approached with CaF2. Unfortunately, to achieve this resolution with CaF2 required

I that the device be thermostated to within 1 mK. To overcome this drawback a search was

made for a material with a smaller temperature coefficient of capacitance while

having as large or larger pressure coefficient. No dramatic improvements have

been found. In our report of July, 1974 and at the CCG meeting in August, 1974

we set forth an idea for circumventing the temperature dependence of the device.

If two materials could be found for capacitors, which would have widely differing

pressure dependences but nearly identical temperature dependences, they could both

be placed in the pressure vessel but in opposite arms of the measuring bridge so

|i that their individual temperature dependences would cancel. No known pair of

I maiterials met the requirement. It was decided that the search for an appropriate

pair should concentrate on the measurement of anisotropic materials. In this way,

if cuts of crystals taken parallel and perpendicular to the crystalline axis gave

a temperature dependence of the capacitance spanning that of another material, they

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could be matched by taking the appropriate cut from the anisotropic crystal. This

work is currently in progress but has not produced the desired results. We have been

fortunate, however, in finding that the parallel and perpendicular cuts from calcite

(CaC03) come close to meeting our requirements and may be considered an interim

solution to the problem.

EXPERIMENTAL DATA

In looking for materials with the appropriate properties to form a

capacitive pair for the pressure transducer, we surveyed all existing data

and found only one good possibility. The temperature dependence of a-cut

paratellurite (Te02) fell midway between that of the a- and c- cuts of sapphire

so that a match could be made with a 45° cut from sapphire. The predicted

pressure dependence of the pair would have been about the same as CaF2. Our new

measurements on sapphire show however, that the temperature dependences reported

earlier were much to high and the new values for sapphire do not overlap on

paratellurite at all. With this result, we were left with no combinations which

would fulfill our requirements so we have been forced to continue making measurements on all available materials. The current set of results are summarized in

Table I and are discussed below. In Table I the dielectric constants, e, were

determined from the capacitance using the effective area of the electrode and the

measured thickness of the sample, the value has an uncertainty of about 2%. The

Quality factor, Q.F., is the absolute value of the ratio of the pressure dependence

the temperature dependence values for CaF are included in the table for comparisons.

The temperature and pressure dependences for the quartz samples (Valpey-Fisher

Corp.) are both small giving good quality factors for the z and y cuts, but the

magnitude of the pressure dependence is too small to make the material useful. In

addition, as was noted in an earlier report, there is a long (as much as 24 hrs.)

relaxation phenomenon in quartz which occurs after pressure changes and makes this

material unsuitable for our purposes. The relaxation is particularly prevalent in

samples with a component of the z-axis parallel to the field and we believed it to

be associated with the piezoelectric properties of the crystals.
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The capacitance measurements with -the sapphire samples (Crystal Systems Inc.)

were well behaved: reproducible, no relaxation phenomenon and low loss. This

was not true of the ceramic AI2O3 sample, Lucalox (obtained from John Fontanella,

US Naval Academy), which was very lossy and kept drifting about.

The MgF2 samples (Optivac, Inc.) had a relaxation phenomenon that appeared

to be associated with impurities. The loss component of the capacitive measurement with the field parallel to the c-axis was initially a factor of 10 larger than

in the perpendicular direction, an effect that has been noted with impure MgF2

crystals by other investigators. This loss component (though not large on an

absolute basis, tan 0 = 0.0007) slowly diminished over the four weeks that the

sample was in the apparatus and was constant at the end of that time. The in phase

component also changed by a small but significant amount as the out of phase

component changed. It is thought that this effect may have been due to the dissolving

of precipitated impurities when the sample was wa:rmed as electrodes were deposited,

the impurities then slowly reprecipitated. It would appear that this material would

still be suitable for device use.

GGG, gadolinium gallium garnet (Allied Chemical Corp.) was well behaved in all

respects. The two AS2S3 glasses (both obtained from Fontanella at different times)

were well behaved. The loss component was larger than most crystalline samples

but was reproducible. The same comment applies to the other two glasses (obtained

from Fontanella and Roy Waxier, NBS) although the loss component in these was

considerably larger. The zirconate ceramic (General Electric Co., Ltd.) has an

excellent quality factor but this material was very lossy and measurements tended

to drift considerably.

The capacitance measurements with the calcite, CaCOs, samples (Karl Lambrect Corp.)

were well behaved with very small loss components. One must use care in depositing

electrodes on this material, however, as it was noted that small splatters of the

aluminum coating caused pieces of the crytal to spall out.

In Fig. 1 we have plotted the pressure dependence versus the temperature

dependence of the capacitance of each material together with values from earlier

work on other materials. Values for anisotropic materials are joined by a line

indicating the locus of values which could be spanned by the different crystal

cuts of that material. What we want to find on this plot are two materials which

lie on a vertical line but are widely separated from each other on that line.

As can be seen in the figure the only overlaps that have significant vertical

displacement occur with paratellurite, Te02. The new sapphire data no longer overlap on this material and MgF2X is the only material that does gives a sizable

vertical displacement (28 TPa This combination would give considerably less

pressure resolution than is obtainable with a CaF2 capacitor.

The best immediate prospect is with the two cuts of calcite. They have very

nearly the same temperature dependence and their pressure dependences are widely

separated. The relevant quantities for comparison of a calcite pair with CaF2 is

the differences in the temperature and pressure dependences of the two cuts. These

are shown in the last line of Table 1. The calcite pair gives an improvement

over CaF2 of about 60% in the pressure dependence and more than 2 orders of

magnitude in the temperature dependence so that the quality factor is up by a factor

of about 230. With a device using a calcite pair it should be possible to resolve

700 Pa (0.1 psi) while thermostating the device with an accuracy of only ±0.05 K.

A drawback with using this particular pair is the large values of their individual

temperature dependences. As a result, small temperature differences between the

two capacitors from any thermal gradients will produce a correspondingly larger

apparent pressure change.

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CONTINUING DEVELOPMENT

We have finally acquired some Te02 samples so that we may measure this

material in our laboratory to check on its values. Unfortunately, the samples

are an odd size (0.6") and will require special handing. We have samples of

several other materials on hand for future measurement and are continually looking

for more in the hope of finding a better combination.

We are beginning design considerations of a pressure vessel and thermostat

arrangement which could be used with a calcite-pair pressure transducer.

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AUTOMATIC CAPACITANCE BRIDGE

Progress is continuing on the limited-range automatic capacitance bridge

being constructed for use with the pressure transducer. The analog portion of

the bridge circuitry is now essentially complete. After a considerable time was

spent tracing done noise problems originating from faulty components we have

a bridge capable of resolution in the range of 1:10^. Although some small

improvements are still contemplated on this portion of the bridge, all efforts

are now being directed toward the digital portion. After some recent design

changes , new drawings are complete and work has begun on assembly of the digital

counter and digital control parts of the bridge.

Table I

Dielectric d_An_c ^ ^^12,^^ din^ ^q6/k

Material e dP dT [ QF

CaF2 6.8 -37.6 262.7 0.143

Quartz

X-cut 4.5 7.7 -2.4 3.2

Y-cut 4.5 7.7 -3.5 2.2

Z-cut 4.7 -1.9 33.4 0.056

XLZ 45° 4.6 7.7 10.4 0.74

YLX 45" 4.6 2.8 20.4 0.14

Sapphire

(0001), 11.5 -11.6 136.3 0.085

(1010) 9.4 -10.6 95.8 0.111

(1120) 9.5 -10.7 95.9 0.112

(1102) 10.0 -10.6 , 107.8 0.098

Lucalox 10.2 -52.8 146 0.36

MgF2 X 5.5 -13.26 172.3 0.077

MgF2 f| 4.8 -21.13 199.4 0.106

GGG 12.6 -17.0 127 0.134

AS2S3(A) 7.4 114 63.4 1.80

(B) 7.V 112 78.2 1.43

Glass (F) 6.5 -34.3 139 0.162

Zirconate

Ceramic 34.4 -51 16.8 3.0

Calcite J. 8.0 12.2 327.2 0.037

Calcite II 7.7 72.0 329.0 0.219

Calcite pair - 59.8 1.8 33.
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FIGURE CAPTION

Fig. 1. The pressure dependence as a function the temperature dependence of

capacitors of various materials. Values for anisotropic materials

with the electric field parallel and perpendicular to the crystalline

c-axis are plotted and the line connecting them represents the values

attainable with crystals cut at intermediate angles.