PB 151362

^ecknical ^lote No. 3

DETWINNING QUARTZ CRYSTALS

by

F. P. Phelps

FORMERLY:

NBS Report 3326, issued May 28, 1954

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NATIONAL BUREAU OF STANDARDS

technical <^f\ote

3

April 1959

DETWINNING QUARTZ CRYSTALS

by

F. P. Phelps

The work described in this Note was

sponsored by the Signal Corps Engineering

Laboratories, Department of the Army,

Fort Monmouth, New Jersey

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

Contents were formerly

NBS Report 3326, issued May 28, 1954

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

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-1-

DETWINNING QUARTZ CRYSTALS

by

F. P. Phelps

ABSTRACT

Attempts were made to detwin quartz by cooling through the

inversion temperature while a d. c. potential was applied and later

with RF potential across the plate. BT, AT and X cut plates were

used. Air was largely removed to improve the insulation but this was

not wholly satisfactory because of the difficulty of adequately degassing

the sample. Some samples yielded untwinned plates, others did not.

It appeared that different samples of quartz behaved differently in

respect to ease of changing the twin pattern. The method used did not

appear commercially useful because of the erratic behavior of different

pieces of quartz.
I. INTRODUCTION

In the course of work which involved the temperature cycling

of crystalline quartz about the inversion temperature (573°C) twin

patterns were readily changed in an apparently haphazard manner; yet

the character of the twinning patterns indicated that recrystallization

started at one or more points and spread until it met the boundary of

another growing domain resulting from another crystallizing source.

In other words the quartz after passing back and forth through the

inversion was divided into several domains of considerable size and

not into a multitude of very small domains. The relatively large size

of these domains suggested that proper manipulation of conditions

might possibly cause one domain to extend over the entire crystal and

thereby result in an untwinned crystal.

Consideration of the foregoing and the characteristics of

the transformation curves, particularly the cc, a transformation, the

polarization effects of quartz in an electric field, known irregularities

in the conductivity and other effects, led to investigation of whether the

quartz undergoing or about to undergo the transformation could be so

polarized or otherwise acted upon by the application of a d. c. potential

across the plate, that the molecules would orient themselves in the

same direction and hence transform into an apparently untwinned

crystal either when the quartz plate was unrestrained or when it was

clamped to resist the bending or other distorting stresses set up by the

d. c. field.

The first experiments (in air) indicated that detwinning might

be accomplished only in that part of the plate directly beneath the

electrodes and that if the electrodes were as large or larger than the

plate the breakdown strength of air was reached and arcing occurred

before the required voltage could be attained across the plate. It was

planned to repeat the experiments in a high vacuum.

The Signal Corps became acquainted with this work and

realized that if the vacuum method would work the process might lend

itself to mass production. The work was carried out between January

1949 and November 1953.
II. EXPERIMENTAL RESEARCH

During initial phases of the work out-gassing difficulties

were encountered causing voltage breakdown before reaching the desired

field strength. Work immediately after that dealt largely with instrumentation and efforts to eliminate the undesired arcing and limitations

to high electric fields. After solution of these difficulties, data on the

first successful detwinning were obtained and it was also noted that

plates of about 0. 1 inch thickness sometimes became colored at least

one -fourth way through.

The importance of having very intimate contact (as by evaporated

electrodes) between the quartz and the electrode was soon noted. Several

plates were only partially detwinned. In all cases the detwinned portion

appeared to start from the positive electrode (in this case grounded).

Very severe strain (sometimes producing cracks outlining the electrodes)

was observed around the edges of the electrodes when these only partially

covered the plate.

Subsequent work dealt with the effect of prolonged heating

above the inversion point and the effect of repeated cycling through the

inversion point. Neither appeared to have an appreciable beneficial

effect. Studies were made upon the depth of detwinning in crystals. As

successive layers of detwinned material were removed from the grounded

side of the crystal, more and more of the original twin pattern was found.

Additional research was concerned with a sidewise (diagonal) gradient

in the field strength attained by plating wedge shaped crystals (opposite

corners, one thick and the other thin). No appreciable effect was found.

The effect of different temperatures was studied with a group

of crystals with voltage impressed for about two hours at three different

temperatures, 525°C, 450°C and 350°C. No satisfactory detwinning

was obtained and no change in the twin pattern except in the case of two

crystals which fractured.

Several erratic and sometimes excessive electrical conductivity

currents were observed. The question whether these were due to

spurious gaseous discharges (metal vapor or occluded gases) or were

inherent in the crystal was not satisfactorily answered. Test runs with

evaporated gold electrodes on fused quartz dummy crystals showed

that the volatility of gold was not a significant factor in accounting for

the heavy conductivity currents.

Silver electrodes caused high electric currents through the

system. In most cases, when nickel electrodes were used with high
voltage and high temperature, coloration of the quartz was noted,

which could not be removed by heating --the higher the voltage the

greater the coloration. Sixty cycle alternating current in place of

d. c. appeared to result in only random changes in the twin pattern.

Additional apparatus was set up for the application of RF

to the crystal to investigate the effect of driving the crystal at its

resonant frequency while near the inversion temperature and preliminary

measurements were made. At 415 to 500°C no change resulted in the

twin pattern with 30 to 50 volts RF applied. Crystals heated to 595 and

to 705°C likewise showed no changes due to the RF drive.

Work was continued on the use of RF drive but at a higher

level- -50 to 70 volts. It was apparently found that the greater the

amount of twinning in a plate the greater the RF voltage required to

detwin it. In those cases where detwinning was achieved the central

area (under the electrodes) was cleared and the marginal 25% of the

plate was twinned. For BT type crystals the effective drive level

appeared to lie between 40 and 75 volts, depending upon the amount of

miscut.

Eight plates of AT cut crystals were operated upon. Four

were apparently completely freed of twinning and when made into

oscillator plates performed with good activity; two were unaffected and

two were changed into miscuts with marginal twinning. On the basis of

the AT crystal experiments it was concluded that the nature or history

of the twinned crystal is an important factor in determining whether

or not it can be detwinned.

Only four X-cut plates were studied and no favorable results

were obtained. This, however, may have been due to faulty technique

since not enough work was done to permit the development of the best

technique for this type of crystal.

Considerable improvement in instrumentation was developed

by using an oscilloscope instead of the previously used RF voltmeters.

This gave a better way of indicating resonance, antiresonance, voltage

across the crystal and impedance (or conductivity) vs. temperature.

III. CONCLUSIONS

The method of detwinning quartz as carried out in the

experiments described was only partially successful. Some plates

(BT and AT) were detwinned, others were not. It appeared from this

work that the possibility of satisfactory detwinning depends both upon
the previous history and also upon the nature or condition of the

individual crystal of quartz.

The vacuum method of insulation was not highly successful.

At the high temperatures the quartz was not adequately out-gassed

and one could not be certain that the results obtained were not compli

cated by the presence of these gases.

IV. RECOMMENDATIONS FOR FUTURE WORK

The work done has pointed up the desirability of:

(1) Further study of conductivity at temperatures near the

inversion temperature and determination of the relative

makeup of the conductivity (ionic, electronic, etc.) with

due attention to what might be called "apparent polarization

effects. "

(2) Application of the detwinning method outlined (slow

cooling through the inversion with a large d. c or RF

voltage across the crystal) in an insulating atmosphere

(compressed gas, such as hydrogen or helium) or in a high temperature liquid dielectric.

(3) Attempting to adequately degas the crystal by heating

to 7 00 or 800°C in high vacuum before applying the

detwinning process to determine if some of the effects

already observed were actually due to residual gas or are

really inherent in the quartz itself.

(4) Repeating experiments already made, but with certain

changes in manipulative details, e.g. , much slower

cooling rate.

(5) Studying quartz as a semi-conductor at elevated temperature.

(6) Carrying out studies on the homogeneity or inhomogeneity

of natural quartz used in the above experiments by use of a powerful optical method developed at the NBS.

USCOW*-NeS-BL
NBS