US2484428A - Piezoelectric crystal mounting - Google Patents

Description

Oct. 11, 1949. M. L. SMITH PIEZOELECTRIC CRYSTAL MOUNTING Filed July 15, 1947 2 Sheets-Sheet 1 IN VEN TOR.

M151. V/N L L 544/ TH A TTORNE Y 5 Oct. 11, 19.49. M. 1. SMITH 2,484,428

PIEZOELECTRIC CRYSTAL MOUNTING Filed July 15, 1947 2 Sheets-Sheet 2 IO A. C.VOLTAGE ACROSS CRYSTAL.

cavsm. FREQUENCY 5992.0 0 AMBIENT TEMPERATURE 23c 4" BT CUT CRYSTAL PLATED o o o 0 Q o o 1- l I. l GNODSS 83d 931910 Nl .ldlUCl X'DNBHO'JUA IN VEN TOR.

MEL w/v L. 5 4/ TH ATTORNEY-5 Patented Oct. 11, 1949 UNITED: STATES rarest oer-1 PIEZOELECTRIC CRYSTAL MOUNTING Melvin L. Smith, Kane, Pa.' I I Application July '15, 1947, Serial=N .761,082

3 Claims. (Cl. 171-1-327) Another objectis to provide a mounting adapted to' re'duc'e the frequency drift of the crystalduring use, which is due to heating, by providing a mounting having improved heat dissipation,

Another object is to provide improved means of attaching-the crystalitself to supports.

Afurther object is to provide a mounting of simple, rugged construction which is capable of being easily and economically assembled on a volume production basis.

Other objects and advantages will hereinafter appear;

The invention will best be understood from the follow-in-g-detaileddescription of the present preferred embodiment of the invention; taken in conjunction with the drawings in which:

Fig. 1 shows a mounted crystal, the size of the mounting, in theorigi-nal drawing being substantially full scale for a crystal one-quarter inch square;

Fig. 2 shows, on an enlarged scale, a crystal mounted on the conductors which are carried by the press, prior to its assembly with the envelope;

Fig. 3 is a longitudinal section ofthe envelope on the same scale as Fig. 2;

Fig. 4; is a longitudinal section through a completed unit, parts thereof being shown in section, taken on the line 4-4 of Fig. 2 and also on the scale .of Fig. 2;

Figs. 5. and6 are greatly enlarged detailed views, taken at right angles to each other, of the coiled end of a support wire;

Fig. '7 shows a modified mounting of a crystal which is supported at opposite corners instead of at adjacent corners as shown in Fig. 2;

Fig. 8 shows the mounting of a circular crystal;

Fig. 9 shows a modified form of mounting in which either a rectangular or round crystal is supportedv ina horizontal instead of a vertical positionas shown in :the other. figures; .and FigulO is. a graph:sh0Wing curves comparing;

the frequency drift with changein appliedvoltage of a crystal mounted in an envelope filledi with airs.

with the drift of thesamecrystalhin the same enev velope filled 'withhelium.

Theconstruction 1 of the crystal mounting and thesteps. in forming it will be described: together;

Referring to Fig-Vi a pair of conductors: l5 of a suitable material' are. sealed in aglass press 16 which: has a depending. skirt portion! terminal? ing in a circular flaredsealing edge 19. Theskirt" portionds of a substantial length so as to space the press lfi from the sealing edge l9. The skilt may, for example; have a length of approximately i one third the over-all length of the. finished mounting.

Supportingwires 20 of resilient material such as; for example; tungsten wire having' a diameter? of .006" are'provided with coiled ends having the configuration shown in Figs. 5 and -6." These" coils comprise a cross-bar 2 I formed from the end of the wire and a plurality of turns22 in the form of'a closelycoiled spring. Materials other than tungstenm'aybe used; for example hard nickel;

The crystal 25is provided on opposite faces with conductive coatings which may be applied thereto in various well known ways, for example bythe 1 evaporation process.- In Fig. 2 thecoating is shown at 26 ascircular in outline except for a tab portion 26a. which extends to one corner of the crystal. The opposite side of the crystal is provided with'a similar coating having a.tab portion 26b extendirigtoan adjacent corneru After the crystal has been thus coated andotherwise prepared for mounting, it is fixed in itssupport by forcing its 'coated corner portions be'-' tween the coils'zz of thesupport wires 20. Injorder to make .-the electrical bond between the support wires'and thecoatings 26 more perfect, a small'quantity ofa suitable conductive paste is applied which bridges the support wire and the conductive coating. Many such pastes are known; includingthose having a volatile base which may be set to solid form by evaporation," and thosewhich are fused by the application of heatz- The -use'of the right type of paste is im-" portant and it has been found that one composed of finely divided silver in an organic base, designed to be fused at a temperature of from 900 to 1000 F., is suitable. The high fusion point assures its stability during the subsequent sealing of the glass elements. Such a paste is available from Hanovia Chemical and Manufacturing Co. under the designation 122B. The amount of such paste is kept to a. minimum in order to avoid reduction of the crystal activity, and to this end a small quantity of the conductive paste is Placed within thecoils 22 in contact with the cross-bars 2!. The unit is then passed through an oven providing a temperature adequate to fuse the paste and adequate air circulation to remove volatile material.

When the unit has been thus far assembled, the crystal may be given such electrical tests as may be desired and its frequency brought to the desired value in known manner while it is still accessible.

When the crystal has been adequately adjusted and tested, the press upon which it is supported is inserted in atubular glass envelope 21, such as that shown in Fig. 3, equipped with an evacuation tube 28 and having anopen bottom 29. Thebottom 29 is of a size to fit the sealingedge 19 and, in a suitable sealing machine, these two parts are fused together into the form shown in Fig. 4. It will be observed that thecrystal 25 is supported at a point as far removed from the point of sealing .as possible, considering the size of theenvelope 21 so that the temperature rise of the crystal during sealing is kept to a minimum.

After the crystal assembly and envelope have been formed into an integral unit, the envelope is evacuated through tube 28 and filled with helium, preferably at a pressure slightly below atmospheric pressure so as to facilitate sealing off of the tube 28. The degree of partial vacuum may be varied and it has been found that a reduction in pressure below atmospheric corresponding to between 2" and 8 of mercury is satisfactory. The filling of the envelope with helium is an important aspect of the invention as it has been discovered that when the crystal is surrounded by an atmosphere of helium the heat, which is generated in the crystal during use, is conducted away from it With much greater rapidity than would be the case were the envelope-filled with air and therefore frequency drift of the crystal is substantially reduced and one is enabled to operate the crystal at much higher voltages without undue change in frequency. This phenomenon is illustrated in Fig. 10 where frequency drift is plotted against the voltage applied to the crystal. a crystal having a normal frequency of 5992 kc. at an ambient temperature of 23 C. suifered a drop in frequency at an applied voltage of 30 volts of nearly 400 cycles per second when contained in an air-filled envelope. The same crystal mounted in the same envelope when filled with helium suffered a drop in frequency at the same Voltage of only about 80 cycles per second. It has been determined that this difference is due to the lower operating temperature of the crystal in the helium-filled envelope, due to the fact that the helium more rapidly conducts heat away from the crystal.

While theconductive coating 25 applied to thecrystal 25 may be of various materials, it is preferred to use a coating of aluminum applied by the evaporation method. It has surprisingly been discovered that when aluminum is used, no fre- In the specific test illustrated by the chart,

quency change in the crystals occurs during the heating of the elements of the unit during fusion of the glass parts and the .annealing thereof. Coatings of silver and copper have been found to cause undesirable changes in frequency, probably due to oxidation, and while gold is reasonably satisfactory from this standpoint, it is lacking in abrasion resistance due to its softness, some abrasion occurring at the point where the crystal is forced into the coils of the supporting wires. The aluminum coating has Very good abrasion resistance.

Fig. 7 shows a mounting similar to that shown in Fig. 2 except that theportions 26a and 28b of theconductive coatings 26 are positioned over diametrically opposite corners of the crystal instead of over adjacent corners.

Fig. 8 illustrates the mounting of acircular crystal 30 which is clamped at any suitably spaced points on its periphery by thecoils 22 of thesupport wires 20.

Fig. 9 is a mount formed in the same manner as the others except that the crystal is turned to a position at right angles to the longitudinal axis of the mounting, permitting the use of a somewhat shorter envelope.

While various specific embodiments of the invention have been described above, it is to be understood that they are merely illustrative of the invention which is to be construed broadly within the purview of the claims.

What is claimed is:

1. A piezoelectric crystal holder comprising a hermetically sealed tube, and means for supporting the crystal in the tube, the space within the tube being filled with helium at a pressure reduced below atmospheric pressure by an amount corresponding to not over eight inches of mercury.

2. A piezoelectric crystal holder comprising a hermetically sealed tube, and means for supporting the crystal in .the tube, the space within the tube being filled substantially exclusively with helium at a pressure corresponding to between 2 inches and 8 inches of mercury.

3. A piezoelectric crystal holder comprising a hermetically sealed tube, and means for supporting the crystal in the tube, the space within the tube being filled substantially exclusively with helium at a pressure reduced below atmospheric pressure by an amount which is no more than suflicient to seal off the evacuating tube when the latter is softened by heating.

MELVIN L. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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