US2410825A - Piezoelectric crystal apparatus - Google Patents

Description

Nov. 12, 1946.

C. E. LANE PIEZOELECTRI C CRYSTAL APPARATUS Filed March 4, 1943 (R.H.QuARrz) (L.

.3 f1. QUARTZ) 2 Sheets-Sheet 1 2 SAME mwen .3 QUARTZ /NVENTOR C E. LANE VLUSQCMAAM ATTORNEY Nov. 12, 194s. c. E. LANE 2,410,825

PIEZOELECTRIG CRYSTAL APPARATUS Filed March 4, 1943 2 Sheets-Sheet 2 PER SECONIJ PARS' P5P MILLION AT 4K6 FREQUENCY /N C Y 30 40 50 70 M .90 100 /0 /20 /JO TEMPERATURE IN DECE FAHRENHE/ T EVACUATED 0R GAS FILLED /N VEN TOR cfg/ AME A T7' ORNE V Patented Nov. 12, 1946 2,410,825 PIEzoELEC'rRIo CRYSTAL APPARATUS p Clarence E. Lane, Maplewood, N. J., assigner' toy Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March 4, 1943, Serial No. 477,915

21 claims.

This invention relates to piezoelectric crystal apparatus and particularly to low frequency flerure mode composite or duplex type quartz crystals, suitably bonded together and mounted for use as frequency control units in such systems as oscillation generator systems, electric wave filter systems, and in electromechanical.vibratory systems generally.

One of the objects of this invention is to provide a composite or duplex type flexure mode piezoelectric crystal body of low temperature coefficient of frequency.

Another object of this invention is to provide a duplex type exure mode piezoelectricV crystal ioody with such nodes of motion that the body may be there supported and electrically connected with minimum interference with its desired frequency of vibration.

Another object of this invention is to provide a low temperature-frequency coefficient piezo-f electric crystal body of'relatively low impedance, and of relatively small and economical size at low frequencies such as, for example, frequencies below or 10 kilocycles per second.

In such systems as low frequency electric Wave nlter systems and oscillation generator systems, for example, it is often desirable to utilize vibratory crystals which have a low temperature coelcient of frequency at a low frequency such as a frequency `oelov 5 to 10 kilocycies per second, and which for many applications may have a l relatively lower impedance than is usually attainlow frequency of low temperature coeicient is desired. A In accordance with this invention, relatively thin bonded piezoelectric quartz crystal plates of suitable handedness, orientation, electric poling,

and dimensions may be subjected to a thickness direction electric field or fields and vibrated at a resonance frequency thereof dependent both upon the longest or length dimension and also upon the thickness dimension of the bonded crystal plates in a mode of vibration which may degrees about its X axis thickness dimension,

2 be called a flexural mode bending in the thickness direction. To obtain a low temperature coefficient of frequency, the orientation of each of the bonded crystal plates may be that of an X- cut crystal element rotated in effect about +5 which may be called a +5degree X-cut crystal element. Examples of quartz crystal cuts, which may be utilized in the face-to-face bonded form of this invention to obtain a. low frequency of low temperature coeiilcient, are disclosed in W. P. Mason Patent 2,259,317, dated October 14, 1941, which discloses the |5degree X-cut crystal element, and in W. P. Mason Patent 2,268,413 dated December 30, i941, which discloses a +En-degree X-cut type of crystal element that is in addition rotated in effect about its length or longest dimension L.

l While the +5-degree X-cut crystal element gives a low temperature coenlcient of frequency for any ofthe smaller dimensional ratios of the width W with respect to the length L thereof, certain ratios such as, for' example, a width W to length i.. ratio of about from .20 to .35 may be utilized to obtain a low temperature-frequency coefficient over a quite wide temperature range. The composite or duplex type piezoelectric crystal body may consist of two 4-l--degree X-cut type quartz crystal plates, or of other suitable cut of crystal plates which preferably have a low temperature coefficient of frequency for their length longitudinal mode of vibration. The two crystal plates may be soldered or otherwise securely bonded together in face-to-face relation to form the composite or duplex type piezoelectric crystal body and with a suitable electric field applied thereto, the composite crystal cody is adapted for exure mode vibrations bending in the thickness `direction. along tivo nodal regions located about .224l of the length dimension from each end. thereof. The frequency oi vibration may be a low frequency of 'the order of i to lil kllocycles per second, more or less, dependent mainly upon the length and thickness dimensions selected for the composite body. With such a composite crystal body a low frequency may .be obtained with a relatively small amount of quartz material and where the individual crystal elements thereof are made of a suitable cut such as +5-degree X-cut type crystal plates of suitable handedness and poling. a. very low temperature coefficient of frequency of the order of 0.3 cycle per million per degree centigrade may be obtained for the composite ilexure mode vibrator.`

The two quartz crystalplates to be bonded 3 may be made from quartz of the same handedness or one of them may be of left-handed quartz and the other of right-handed quartz. When the two bonded quartz crystal plates are made of the proper handedness and electric poling with respect to each other, the resultant nodal lines of the composite lbody may; be perpendicular to the length dimension and the long edges thereof.

The crystal body may be conveniently 'mounted at points on or as near as possible to such nodal lines with a minimum of interference with the desired vibration of the crystal body. The crystal mounting may consist of pressure type clamping pins or alternatively of ne spring wires soldered to the crystal major surface electrodes or to side surface coatings at'any point or points on or as near as possible to such nodal lines.

Examples of crystal wire supporting systems that may be utilized Vare illustrated in A. W. Ziegler United States Patent 2,275,122, dated March 3, 1942. If desired, the crystal supporting wires may be provided with vibration clamping means, or with nodal reflectors as disclosed in I. E. Fair United States Patent 2,371,613, dated March 20, 1945, granted on application Serial No. 470,759, filed December 31, 1942, in order to remove the adverse effects of undesired wire vibrations on the crystal frequency and activity. When provided with such nodal reflectors, the crystal supporting wires attached to the crystal body may have a natural frequency that is substantially equal tothe frequency of the piezoelectric crystal body whereby -the crystal body and its supporting spring wires secured thereto operate as a composite vibrator at a common natural frequency with minimum interference with the crystal frequency and activity.

For operation at `the fundamental flexure mode frequency, the crystal electrodes may substantially wholly cover the two outside, major faces of the bonded crystal plates. For operation at any overtone or harmonic frequency of the fundamental exure mode, the crystal electrodes may consist of a plurality of pairs of interconnected platings to drive the bonded crystal at any selected overtone exure mode frequency, as illustrated, for example, in W. G. Cady United State Patent 1,860,529, dated May 31, 1932. The crystal electrodes may fully, or may partially cover the outer major surfaces of the bonded crystal plates leaving in the latter case, the end areas thereof uncovered and the central areas covered, in order to obtain a desired value of capacitance, or an improved driving efficiency that may result from such partial electrodes.

To reduce or adjust the frequency of the duplex type flexure mode crystal unit, it may be loaded as by the addition of metal onto the major surfaces thereof. With such loading, bonded crystal plates of given dimensions may be used at a somewhat lower frequency than the unloaded crystal unit, a feature which is of special inter` est at the very low frequencies where the unloaded crystal plates may become too long and too thin for convenient use.

The crystal plates may be bonded by spraying the major surfaces to be bonded with a solution of Hanovia silver` paste, baking the sprayed crytal plates at an elevated temperature in order to fix the silver paste coating firmly to the surface of the quartz, burnishing the baked silver layer and then tinning it, using a` stearin flux and a solder to which is added silver sufficient for saturation at the melting temperature, placing the quartz plates to be bonded with the aeiaeae pressure to force out the excess molten solder. The addition of silver to the solder discourages the molten solder from absorbing the thin film of silverAwhich has been baked on the quartz plates. 9.2 mil in thickness. i

Thebonding means between the two +5-degree X-cut type or other type crystal plates may include a thin metal plate secured between the two crystal plates and made of steel or other metal suitably proportioned with respect to the crystal plates in order to obtain a temperature 'coeicient of frequency of selected value for controlling the over-all temperature coeicient of frequency of the bonded crystal unit. If desired, one of the two bonded crystal plates could be made of non-piezoelectric material such as, for

example, a metal plate secured thereto and made Y to have a temperature-frequency coeiiicient to balance that of the piezoelectric crystal plate secured thereto, thereby to obtain a low over-all temperature-frequency coefficient for the bonded unit.

The duplex type iiexure mode crystal body supported at or as near as possible to its nodes by supporting wires or by other suitable supporting means may be mounted in an evacuated or sealed metal or glass tube or other suitable sealed container, as disclosed for example in the A. W. Ziegler Patent 2,275,122 hereinbefore referred to.

'Ihe sealed crystal container may -be evacuated or alternatively, it may contain dry air or other inert gas which may be heavier or lighter than air and of suitable density or pressure which may be greater or less thanatmospheric pressure, in order to suppress or damp out the weaker secondary resonances of the crystal body4 or to slightly damp the major or desired res` onance thereof in case of excessive vibration and Figs. 1 and 2 are enlarged views of a major face and a long side edge, respectively, of wire mounted, electroded and bonded fundamental flexure mode +5-degree X-cut quartz crystal plates which are constructed of the same handed quartz and poled oppositely;

Figs. 3 and 4 are, respectively, major face and long side edge views of bonded quartz crystal plates similar to those of Figs. 1 and 2 but constructed of opposite handed quartz;

Figs. 5 and 6 are, respectively, major face and long side edge views of bonded quartz crysta1 plates similar to those of Figs. 3 and 4 but provided additionally with an inner electrode connection;

i Figs. 7 and are, respectively, major face and long side edge views of bonded quartz crystal plates similar to those of Figs. 5 and 6 but constructed of the same handedA quartz;

Fig. 9 is a graph illustrating the temperature- 'Ihe baked silver film is of the order of frequency characteristics of bonded +5-degree X-cut quartz crystal plates;

Figs. l() and 11 are, respectively, enlarged views of a major face and the small end of a bonded crystal body that is provided with a longitudinally divided electrode coating and a wire supporting system;

Fig. 12 is a greatly enlarged view of details of the crystal unit, illustrated in Figs. and 11; and

Fig. 13 is a greatly enlarged view illustrating a modification of the device shown in Fig. 12.

This specication follows the conventional terminology as applied to crystalline quartz which employs three orthogonal or mutually perpendicular X, Y and Z axes, as shown in the drawings, to designate an electric, a mechanical and the optic axes, respectively, of piezoelectric quartz crystal material, and which employs three orthogonal a'xes X', Y and Z' to designate the directions of axes of a piezoelectric body angularly oriented with respect to such X, Y and Z axes thereof. Where the orientation is obtained by a single rotation of the quartz crystal element substantially about an electric axis X, as particularly illustrated in Figs. l to 8, the orientation angle 0 designates in degrees the effective angular position of the crystal plate as measured from the optic axis Z and from the orthogonal mechanical axis Y.

Quartz crystals may occur in two forms, namely, right-handed and left-handed. A rightl'ianded quartz crystal is one in which the plane of polarization of a plane polarized light ray traveling along the optic axis Z in the crystalis ,rotated in a right-hand direction, or clockwise to the ends of an electric axis X of a quartz body v 2 or 3 and not removed, a charge will be ldeveloped which is positive at the positive end I of the X axis and negative at the negative end of such electric axis X, for either righthanded or left-handed crystals. The magnitude and sign of the charge may be measured in a known manner with a vacuum tube electrorneter, for example. ln specifying ther orientation of a. right-handed crystal, the sense of the angle e which the new axis Y makes with respect to the axis Y as the crystal plate is rotated in effect about the X axis is deemed positive when, with the compression positive end of the X axis pointed toward the observer, the rotation is in a clockwise direction as illustrated in Fig. l. A counter-clockwise rotation of' such a righthanded crystal about the X axis gives rise to a negative orientation angle 0 with respect to the Z axis. Conversely, the orientation angle of a left-handed crystal is positive when, with the compression positive end of the electric axis X pointed toward the observer, the rotation is counter-clockwise, and is negative when the rotation is clockwise. 'I'he crystal material 2, illustrated in Figs. 1 to 8, is right-handed as the term is used herein. For eitherv right-handed or lefthanded quartz, a positive angle 0 rotation of the Y axis with respect to the Y axis, as illustrated in Fig. 1, is toward parallelismwith the plane of a minor apex face of the natural quartz crystal, and a negative 0 angle rotation of the Y' axis with respect to the Y axis is toward parallelism with the plane of a major apex face of the natural quartz crystal.

Referring to the drawings, Figs. 1 to 8 are major face and corresponding long side edge views of thin piezoelectric quartz crystal plates or elements 2 and 3 cut from crystal quartz free from twinning, veils or other inclusions and made into a bonded plate l of substantially rectangular parallelepipedl shape having a length or longest dimension L, a width dimension W which is perpendicular to the length dimension L, and a thickness or thin dimension T which is perpendicular to the other two dimensions L and W.

The nal major axis length dimension L of the bonded quartz crystal elements 2 and 3 of Figs. 1 to 8 is determined by and is made of a value according to the desired flexure mode resonant frequency. 'I'he thickness dimension T also is related to the desired ilexure mode frequency. The width dimension W may be of the order of one-fth or other suitable value relative to the length dimension L to suit the desired irequency of the bonded ilexure mode crystal elements 2 and 3.

The length dimension L of each of the individual crystal plates or elements 2 and 3 of Figs. 1 to 8 lies along a Y' axis in the plane of a mechanical axis Y and the optic axis Z of the quartz crystal material from which the elements V2 and 3 are cut, and is inclined at a positive 0 angle of degrees with respect to said Y' axis, the angle 9 being one ci the values between about +i and +6 degrees more or less, or substantially +5 degrees. The maior surfaces and the major planes of the crystal elements 2 and 3 are disposed substantially in the plane of the Y and .Z axes mentioned. The angle fbetween the width dimension W, which lies along the Z axis in the plane of the Y and Z axes mentioned, and the- Z axis is 45 also inclined at the angle 0 with respect to the optic axis Z. it will be noted that the individual crystal elements 2 and 3 of Figs. l to 2 are in effect X-cut crystals rotated e=substantially +5 degrees about the X axis. At this angle ci 0=+5 degrees, tests show that the ilrst or fundamental ilexural mode vibrational frequency has a low temperature coefficient of frequency. *While the individual crystal plates 2 and 3 are shown in Figs. l to 3 as having their opposite major races disposed perpendicular to the axis, will be understood that they may be positioned nearly perpendicular or within a few degrees o' or con.. siderably away from such perpendicular relationship with respect to the X axis.

ture-frequency' coefficient fundamental iexure mode crystal body i comprising two bonded crystal elements 2 and Si has two nodal line regions t each extending from one side face to the opposite side face and disposed midway between the outside major surfaces of the bonded body i. The nodal lines 6 intersect the center line length dimension L or Y' axis of the duplex crystal element I at points spaced about 0.224 or less of the length dimension L from each end thereof, as shown in Figs. 1 to 8. At any point or points on or near to the two nodal lines 6, the duplex crystal body l may be mounted and electrically connected as by means of a supporting wire system 1, or by rigidly clamping it between one or As illustrated in Figs. l to 3, the low tempera- `major surfaces or on the side surfaces of the duplex crystal body I. The nodal line regions 8 of the bonded exure mode crystals 2 and 3 are I shown in Figs. 14 to 8, and inaddition, the integral electrode coatings 4 and 5 therefor, the bonding means I and the conductive projections 'I and 8 that may be utilized for mounting and estalblishing. electrical connections with the exure mode crystal body I.

As illustrated in Figs. 1 to 8, suitable conductive electrodes, such as the two crystal electrodes 4 and 5, for example, may be placed on or adjacent to or formed integral with the opposite outside major surfaces of the bonded crystal plates 2 and 3 to apply electric field excitation to the duplex type quartz body I in the direction of the X axis thickness dimension T, and by means of suitable electrode interconnections and any suitable circuit, such as for example, a filter or an oscillator circuit, the quartz body I maybe vibrated in the desired iirst or fundamental ilexural mode of motion at a response frequency which varies inversely as squaraof the major axis length dimension L, and directly as the thickness T.

The fundamental iiexure mode frequency of the bonded quartz crystal plates 2 and 3 of Figs:

1 to 8 is given approximately by the relation f=a (1K) where -f=frequency in cycles per second;

L=length or longest dimension in millimeters of the bonded crystal unit;

T=thickness or thinnest dimension in millime- As an example, the dimensions for a fundamental exure mode 4-kilocycle per second bonded crystal body i constructed from two +5-degree X-cut quartz crystal plates 2 and'3 may be about 1 millimeter in over-all thickness T, about 23 millimeters in length L, and about 11.5 millimeters more or less in width W, the bonded crystal body vibrating in the manner of a free-free bar bending about its two nodal lines 5 in the direction of the thickness T.

As another example, a bonded crystal unit I constructed following the arrangement illustrated in Figs. 1 and 2 and utilizing two +5-degree X- cut quartz crystal plates 2 and 3 each about 65 millimeters long, 13 millimeters wide For the -l-5-de-v capacities r of about 175, and a Q of about 30,000 when operated in a vacuum. As another example, two bonded -l-5-degree X-cut quartz plates 2 and 3 constructed as illustrated in Figs. 1 and 2 and each having a length 'L of about 60 millimeters, a width W of millimeters and a thickness of about 0.390 millimeter give a first or fundamental exure-mode frequency of about 1250 cycles per second. A similar bonded crystal body and .832 millimeter thick has a fundamental ex- I of the same length but constructed with plates zand s each of 0.427` millimeter thickness gives a fundamental ilexure mode frequency of about 1400 cycles per second. j

Small adjustments in the resonant `frequency ofthe bonded crystal plates l2 and 3 `ofFigs. 1 to 8 may be made by grinding oi or otherwise removing small amounts of quartz from either or both of the small ends of the bonded crystal` coatings of,silver, or other suitable metallic or conductive material, 'deposited upon the bare quartz by evaporation in vacuum or by'other suitable process. If desired, the crystal electrode 4 located on one major surface ofthe crystal body I or the crystal electrode 5 located on the oppo- -site major surface thereof may be longitudinally shortened, leaving the end portions of the crystal major ksurfaces equally uncovered. Also, the electrodes 4 or 5 maybe centrally separated or split along the center line of the length dimension L, thereby forming two separate electrodes on each major` surface in order to provide the crystal body I with additional connections to suit the oscillator or other circuit with which it may be connected. Figs. 10 and 11 illustrate such splits or separations in the crystal electrode 4. Where the electrodes 4 and 5 are shortened lengthwise to less than the distance between the two nodal lines 6, they may be provided with small ears extending over the mounting points la adjacent the nodal lines 6 of the crystal body i in order to make'electrical contact with the ends of the conductive supporting wires 'I disposed at or near such nodal points. Where the electrodes 4 and 5 are split lengthwise, the lengthwise gap or separation of the electrode platings 4 and 5 on the outside major surfaces of the crystal body I may be about 0.365 millimeter, the center line of such splits in the platings on opposite sides of the bonded crystalv body I being aligned with respect to each other. To drive the crystal body I in the desired first or fundamental exure mode, the opposite outside electrodes 4 and 5, and in certain cases, the inner electrode I0 also, are utilized to apply a field or elds in the thickness direction T through the crystal body I in order to lengthen one crystal plate 2 or 3 and simultaneously shorten the other crystal plate, thus bending the composite crystal b0dy`I in the thickness direction about the two stationary nodal lines E in the desiredrst flexural mode of motion, as illustrated by the curved broken line'in Figs. 2 and 6. Examples of crystal and electrode arrangements that may be utilized for operating the composite crystal body I in the fundamental fiexure mode vibration are illustrated in Figs. 1 to 8 which show ,duplex type exure mode crystal bodies I constructed in four different ways.

` 4 and 5 but have no inner electrode connection to the bonding means I0. As illustrated by the plus -and minus/1 signs in Fig. 2, the two bonded quartz crystal plates 2 and 3 are poled in opposite ways so that when voltage is applied to the outer electrodes 4 and 5, the electric field produced thereby transverses the thickness dimension T of both 4of the crystal plates 2 and 3 in the same direction with the result that one crystal plate will expand along its length L, while the other crystal plate simultaneously contracts along its length L, thereby causing the bonded plates 2 and 3 to curve or bend slightly as shown ln exaggerated form by the curved dotted line in Fig. 2. The bending occurs in the thickness direction T about the two nodal lines 6 which are located at a region about .224 ci the length dimension L from each vend therect'and midway between thev outside major suraces. The quartz crystal plates 2 and 3 in Figs. 1 and. 2 are both made of the same handed quartz, that is, both may be constructed of right-hand quartz or both may be constructed of left-hand cuarta and the resultant two nodal lines t then occur at right angles or perpendicular to the side edge or length dimension L ofthe bonded crystals and 3, as illustrated in Figs. :lr and 2. Such per= pendicular nodal vlines 5 are obtainsdY in the bonded crystal body I of Figs. 1 and 2, although the individual +5-degree X-cut crystal plates 2 and 3 do not have such perpendicular nodal lines in themselves. The perpendicular arrangement of the nodal lines 5 resulting in the bonded crysu tal plates 2 and 3 of Figs. 1 and 2 is somewhat more convenient and easier to use in mounting and establishing electrical connections with the bonded crystal I by means of conductive clamping pins or supporting wires 1 that may be attached or soldered thereto at points on or as near as possible to the nodal lines 6, as illustrated in Figs. 1 and 2. In the individual length-mode +5-degree X-cut crystal plates 2 and 3, the nodal lines are inclined about 11 degrees to the perpendicular to the length dimension L. 'I'he nodal lines 6 in Figs. 1 and 2 illustrate the result ci" lche 11-degree inclined nodal lines of the individual crystal plates 2 and 3 which become the perpendicular nodal lines 6 when the two Y lo Y n the two crystal plates 2 and 3 may be placed in major face'tomajor face position one on top oi the other in unbonded condition and driven at the frequency at which each individual plate would resonate longitudinally. If the two plates 2 and 3are poled in the lsame direction, the two crystals will resonate longitudinally together and give approximately as good a Q or ratio of reactance to resistance as though each were driven individually; and if theyv are poled oppositely, no resonance will be observed. In this manner, the poling of the crystal plates 2 and 3 may be determined before bonding them together.

Secured togetherand suitably poled, one of the crystal plates 2 or 3 under the action of an electric eld, will lengthen in the length direction L and the other will simultaneously shorten, thus causing the bonded plates 2 and 3 to curve slightly into a cylindrical major surface form as shown in greatly exaggerated form by the curved broken line in Fig. 2. In an alternating field, the bonded plates 2 and 3 will curve rst in one direction and then in the other or opposite direction, producing flexural vibrations by bending in the thickness direction T about the lines t.

The tlexurai vibrations are ci' considerable amplitude and their frequencyl is much lower than that of the longitudinal or lengthwise vl-= bration of one' of the single crystal plates 2 er t thereo. A wide range of frequencies may be obtained bythe proper choice of the length L thickness T ci the bonded crystal plates 2 The :width dimension W is ci little eect the fiexure mode frequency ii not made toc large and, as an example, may conveniently ha abcut cne-iifth ci the length dimension or other able value.

Figs. 3 and e are, respectively, maior .face side views, the latter being a view taken on the line 'J-Q of 3, illustrate a second way in which a duplex or composite fundamental lenure mode crystal body i may be made from two bonded :-degree- Z-cut quarta crystal plates 2 and 3. As illustrated by the plus i-l-l and minus signs in Fig. e, 'the two bonded crystal plates t? and 3 are poled in opposite ways like the crystal plates 2 and s ci Fig. 2 so when the electric eld produced by the electrodes d I and 5 transverses both crystal plates 2 and in the same direction, one crystal plate expands along the length L, while the other simultaneously contracts along its length L, thereby slight= +5-degree X-cut crystal plates 2 and 3 of Figs. 1

and 2 are bonded and operated in the exure -mod.e. It will be noted that no inner electrode connection is used for the inner plating or bonding means I0 in the arrangement illustrated in Figs, 1 and 2, and that the electric ileld supplied by the outside electrodes 4 and `5 transverses the thickness dimension T ci both of the bonded crystals 2 and 3 resulting in a duplex crystal body of somewhat higher impedance level than that ly bending the bonded crystal plates and 3 in the thickness direction T about the'two nodal lines t, the inner major ,surface centers of which are located about .224 of the length L from each end thereof. It will be noted that the two crystal plates 2 and 3 of Figs. 3 and 4, unlike those of Figs. 1 and 2, are made of opposite handed quartz instead of the same handed quartz. By opposite handedness, it is meant that one crystal plate is constructed of right-handed quartz and the other crystal plate is constructed of lefthanded quartz, as illustrated in Fig. 4. Being of opposite handedness, the bonded crystal plates 2 and 3 of Figs. 3 and 4 have resultant nodal lines 6 which may beinclined at an angle to the perpendicular to the length dimension L and which in the case of the +5-degree X-cut plates 2 and 3 particularly illustrated are inclined about 11 degrees, as shown in Fig. 3. As illustrated in Figs. 3 and 4, the 11degree nodal lines 6 are both in one direction with reference to the perer Adirection of rotation may be located -by test for minimum motion. Y

In Figs. 3 and 4, as in Figs. 1 and 2, no inner electrode connection is'used and the electric eld that is supplied by the outer electrode coatings 4 and 5 traverses the thickness dimension T of both crystal plates 2 and 3 therebetween, giving a duplex crystal unit that may have a relatively higher impedance level than that obtained from the two types of duplex crystal body I of Figs. to 8, which utilize an inner electrode connection 8 that may be made by soldering to the bonding means I0. Y l

For high impedance level bonded crystal plates 2 and 3, the construction illustrated in Figs. 3 and 4 using one crystal plate taken from righthanded quartz and the other crystal platetaken from left-handed quartz represents a desirable arrangement for +5-degree -X-cut typeI quartz plates 2 and 3 from the standpoint of very low temperature coefficient of frequency, as illustrated by the curves of Fig. 9. It will be understood, however, that duplex ilexure mode crystals made from bonded `+5degree X-cut type quartz plates generally as shown in Figs. 1 to 8, display very low frequency-temperature coeilicients of the order of one part or less part per million per degree centigrade, a value which is less than that displayed by the individual plates when operated singly in unbonded condition.

Figs. 5 and 6 are, respectively, major face and side views illustrating a third way in which a duplex fundamental fiexure mode crystal body I may be made from two |5degree X-cut type quartz crystal plates 2 and 3 secured together by conductive bonding means I0. In Figs-5 and 6, the inner plating or bonding means I0 is used as one electrode for the crystal body l, the connection thereto being made by means of a fine lead wire 8 connected or soldered thereto at the node 5 or otherwise, and the two outer platings or coatings 4 and 5 being connected together by any suitable means'such as a connector 9, for example, and used as a second or outer electrode for the two crystal plates 2 and 3 connected in parallel. The arrangement shown in Figs. 5 and 6 provides a duplex crystal unit I which has about one-fourth of the impedance-level provided by the connections used in the two arrangements shown in Figs. 1 to 4 where no outside connection to the inner electrode is utilized. The lower impedance level provided by the inner electrode connection 8 of Figs. 5 and 6 may be of advantage in certain applications. When using the inner electrode connection 8 of Figs. 5 and 6, the quartz crystal plates 2 and 3 are poled in the same way, as illustrated by the plus ,and minus (-V) signs in Fig. 6, in order to obtain an expansion of one plate along its length L and simultaneously a contraction of the other plate lalong its length L, thereby bending the bonded . crystal plates 2 and 3 in the thickness direction T about the two nodal lines 6, in the manner described hereinbefore in connection with Figs. 1 and 2. In Figs. 5 and 6, the bonded crystal plates 2l and 3 are of opposite handedness, that is, one plate is constructed from right-handed quartz, while the other plate is constructed of lefthanded quartz, as illustrated in Fig. 6. The crystal- plates 2 and 3 of Figs. 5 and 6 being of opposite handedness, poled in the same way and operated with iields in opposite` directions, the two nodal lines 6 thereof are substantially at right angles to the length dimension L, as illustrated in Figs. 5 and 6. Accordingly, the bonded l2 crystal plates 2 and 3 -of Figs. 1, 2 and Figs. 5, 6 provide the same type of nodal lines 6 although constructed with dierent connections, poling and handedness. Also they have in general the same temperature coeilicients of frequency.

Figs. 7 and 8 are, respectively, majorface and side face views illustrating a fourth method by which a duplex fundamental exure mode composite crystal unit I may be made from two +5- degree X-cut type quartz crystal plates 2 and 3. In Figs. 7 and 8, the in-between plating or bonding means I8 is used as one externally connected electrode 8 and the two outer coatings 4 and 5 are connected together and used as a second electrode, as in the case of Figs. 5 and 6; and also, the crystal plates 2 and 3 are poled in the same way as illustrated by the plus and minus signs-in Fig. 8. ' I'he crystal plates 2 and 3 of Figs. 7 and 8 are made however of the same handedness, that is, both of the crystal plates 2 and 3 are constructed either of right-handed quartz or of lefthanded quarts, and the resulting nodal lines 8 are inclined at an angle of about 1l degrees with respect to the perpendicular to the length dimension L, as shown in Fig. 7, where the quartz plates are +5-degree X-cut type crystal plates 2 and 3. The duplex crystal unit I of Figs. 7 and 8, like that of Figs. 5 and 6, has an impedance level about one-fourth of that given by the duplex crystals I of Figs. 1 to 4. The characteristics of the duplex crystals of Figs. 7 and 8 and Figs. 3 and 4 are similaneach having an 11-degree nodal line 6, the same dimensions fora given frequency, and about the same temperature coeillcients of frequency.

While the connections required to form the bonded crystal units of Figs. 5 to 8 require an inner electrode connection that is not required in those shown in Figs. l. to 4, the inner electrode connection of Figs. 5 to 8 has the advantage that for the same crystal dimensions the impedance obtained is about one-fourth that obtained by the method used in Figs. 1 to 4 where no inner electrode connection is utilized. In Figs. 5 to 8, the

vand 5.

vIi thereof on the side surface thereof. It will be understood that the composite crystal unit I of Figs. 1 to 8 may be mounted and electrically connected if desired entirely at the side surface node ends 6 by means of four line conductive spring wires 8 soldered to .baked silver paste spots' I2 placed at the four side surface nodes6 or by pressure type conductive clamping pins, for example, the pins or wires 8 being, individually connected to the electrodes 4 and 5 by integral crystal coatings that are separated from each other and from the inner coating Ill, the inner coating being removed at the ends only of the nodal lines 6 Where connections are made to the outside coatings 4 It will be noted that in the ilexure mode of motion, one ofthe bonded crystal plates 2 or 3 becomes shorter while the other crystal plate simultaneously becomes longer, thusl throwing the bonded crystal plates 2 and 3 into the flexure mode vibration in the direction of their thinnest dimension T. To produce this vibration, the bonded crystal plates 2 and 3 are poled in opposite directions when voltage is applied only to the two outer maj or surfaces of the crystalplates as shown in Figs. l to 4, and are poled in the same direction when the electric field goes through them in opposite directions as shown in Figs. 5 to 8. To'

obtain the nodal lines 6 that run through the 2 and 3 may be made of the same handedness.

either right or left, as in Figs. 1 and 2, or of opposite handedness as in Figs. and 6. To obtain the ll-degree nodal lines 6, the poling and while in rigs. 1 tc a, the ts-degree X-cut type crystal plates are particularly illustrated, it will be understood that other low temperature cog efficient longitudinal mode crystal plates may also be used in the same manner of fabrication to obtain a low temperature coefiicient of 4frequency for the flexure mode vibration of the bonded crystal body.

Fig. Q is a graph showing the measured temperatureufrequency coeidcients o sin duplex type fundamental nexure inode i--kilocycle per second crystalsl each composed of two bonded -l--degrec X-cut crystal plates 2 and 3 made in accordance with the method illustrated in Figs. 3 and 4. The curves of ii illustrate 'that maximum ireduel/icy stability with temperature change occurs in the region of lo" for bonded +5-deree X-cut type crystal plates 2 and 3 made accordance with the method as illustrated in Figs. 3 and d. Similar measurements made on bonded -l--degrec X-cut type e-lrilocycle per second ilexure inode crystal plates 2 and 3 but arranged in accordance with the method as illustrated in Figs. l and show that maximum frequency stability occurs in the region oi about 3G F.. While either arrangement of the bonded crystals may be used at ordinary temperatures to obtain a good temperature -coefilcient oi frequency, the curves of Fig. 9 chour that between 64 and 91 F., ior example, the ireduency or the bonded crystal plates t and 3 made by the method oi Figs. 3 and Fl shows a Variation of only about nine parts per million at 4 lillocycles per second, whereas the same cut of composite crystal plates made by the method of Figs. i and 2 vary about eighteen parts per million. These figures correspond to about two parts per million per degree Fahrenheit and four parts per million per degree Fahrenheit, respectively, and

represent a fairly high degree of frequency stabil.

ity. In accordance with the foregoing illustration, and as illustrated by the curves of Fig. 9, the temperature at which the zero temperature coefficient of frequency occurs for a composite exure mode crystal l may be varied by a suitable selection and arrangement of the proper crystal plates.

Figs. lo and ll are, respectively, major :face and small end views of a duplex fundamental iiexure mode crystal body I provided with longitudinally divided electrode coatings 4a and 4b on one outside major face thereof, a non-divided electrodecoating 5 on the other outside major face thereof, and a wire support system comprising fine phosphor bronze spring wires 1 soldered by means of small solder cones 1a to thecrystal coatings 4a, 4b and 5 at points over the two nodal lines S'of the flexure mode bonded crystal plates Cn f 2 and 3 held securely together by the bonding 14 construction of the type illustrated in Figs. 1 and 2, it will be understood that these features may be applied also to the other types of bonded l crystal plates illustrated in Figs. 3 to 8. While in 1 Figs. 10 and l1 the longitudinally divided system of electrodes 4a and 4b is shown as being applied only to the electrode 4 of Figs. 1 to 8, it may also be applied similarly to the crystal electrode 5. The longitudinally divided electrode, suchk as the electrodes 4a and 4b, may be utilized for the purpose of providing connections to suit the particular circuit such as an oscillator circuit with which the duplex crystal unit may be connected.

As shown in Figs. 10 and l1, the crystal supporting ne spring wires 1 may entendra short distance from the solder dots or cones 1a in a direction perpendicular to the major faces of the bonded crystal body i, may then be bent at right angles and extend outwardly in the direction shown in Figs. l0 and ll or in any direction, and may then be bent again at roughly right angles and attached to four larger supporting spring wires Il as illustrated in Figs. ll and l2, .Alternatively, instead of being provided with multiple l.-shaped'bends as illustrated in Figs. lo, ll and 12, the iine supporting spring wires l attached to the crystal body may extend directly7 to the support Wires li, as illustrated in Fig. i3. The sup port wires l i illustrated in Figs. il, l2 and i3 may be, for example, four upright parallel Wires extending through the press or an evacuated metal or glass tube ld illustrated in il and may be of the type disclosed in A. W. Ziegler Patent 2,275,'i22, dated March 3, i942. It will be understood that the crystal wire supporting system may be oi any suitable form that is adapted to support and establish electrical connections with the bonded crystal body l, and that the Wire supported crystal unit may be mounted in any suitable container such as a vacuum tube lil oi the type disclosed iny they A. W. Ziegler Patent 2,275,122 mentioned, for example.

The sealed crystal container lil, illustrated in cross-section in Fig. ll, may be evacuated or alternatively, it may contain dry air or other .inert gas which may be heavier or lighter than air and oi suitable density or pressure 'which may be greater or less than atmospheric pressure, in order to suppress or damp out the weaker secondary resonances ci the crystal body or to slightly damp the major or desired resonance thereof in case of excessive vibration and for other purposes such as to control or adjust the frequency of the desired resonance or resonances. Examples of gases which may be used to provide an inert atmosphere for control of the 'crystal resonances are helium, neon, hydrocarbons, carbon dioxide, argon, krypton, Xenon.

Fig. 12 is an enlarged detail view illustrating a crystal supporting wire 1 provided with multiple bends which may function to dampen or dissipate undesired wire vibrations and to absorb externally applied mechanical shock. Alternatively, as shown in Fig. 13, a straightwire 1 may be used extending perpendicularly from the major surface of the crystal body l to the slightly heavier support spring wire li. The iine crystal lead wire 1 may be attached to the support wire H by solder or other suitable means. The extreme end of the lead wire 1 that is adjacent theV crystal body l may be bent at right angles as illustrated in Figs. 12 and r13 or maybe bent in hook form or otherwise in order to retain it more iirmly in the solder cone 1a in which it is embedded. The lead wire 1 may be rmly attached to the crystal 4Magasins used for the bonding Vmeans III to be described.

The small silver spots I2 on the outside major surfaces and on the nodes I of the side surfaces of the bonded crystal plates 2 and 3 may be formed there by applying to the bare quartz, spots `I2 of silver paste and then baking in an oven at an elevated temperature.

As to the conductive crystal bonding means Ill, the inside major ysurfaces of the quartz crystal plates 2 and 3 may be firmly bonded together by applying to one major face of each of the unbonded bare quartz plates 2 and 3 a coating Illa of silver paste covering substantially the whole surface, of each of the inside major surfaces which after baking thereon may be soldered together by a layer`of solder IUb, as-illustrated in Figs. 12 and 13. The silver paste coating Illa may be applied to each of the inside major surfaces of the unbonded crystal plates 2 and 3 by spraying it thereon withV an air brush, Vfor example, using a mixture of one part by volume of silver paste such as Hanovia silver paste and two parts by volume of distilled turpentine and an air pressure of approximately 25 pounds per square inch. The

weight of the silver coatings a may be about 35 milligrams per square inch after final heat treatment. 'I'he silver paste coatings loa may be baked firmly onto the quartz by baking the silvercoated crystal plates in separated form in an oven burnished with a glass brush or other suitable means until a bright metallic lustre is obtained. The individual crystal'plates 2 and 3 may then be placed on a hot platen with the burnished side up and heated to a temperature of about 315 F. At this point stearine soldering'ilux may be applied to the heated silvered surfaces and solder Hlb evenly applied over these surfaces to be bonded. As an example, the solder Illb may be composed of about 32 per cent lead, 50 per cent tin, 18 per cent cadmium and a small quantity or suiiicient silver for saturation at the melting point of the solder which is about 300 F. The purpose of using the silver in the solder composition Illb is to prevent the solder 10b from absorbing the silver from the silver coatings 10a on the crystal plates 2 and 3. The molten solder 10b may be distributed with a suitable spreader such as a piece of tinned copper wire. After the solder is molten and has been evenly distributed over the entire upper major surfaces of the crystal plates 2 and 3, one of the two crystal plates 2 and 3 to be bonded may be. picked up and placed evenly on the other crystal platewith the major surfaces having the molten solder coating 10b facing each other. A pressure of about 4 poimds per square inch may be applied and the excess solder which is forced out from between the two crystal plates 2 and 3 may be removed. The pressure may then be released and the crystal plates 2 and 3 re- 16 moved from the hot platen. crystal plates 2 and 3 have cooled below the melting point of the ux, the flux may be removed by wiping with a clean lintless cloth or other suitable means. The bonded crystal plates 2 and 3 may be cleaned by immersing and brushing in carbon tetrachloride and drying with clean warm air. The baked silver paste coatings 10a adhere firmly to the quartz andwhen soldered together at Ib form a strong bond I0 between the two crystal plates 2 and 3. y

If desired, the bonding means I0 as illustrated in Figs. 2, 4, 6 and 8 for example, may comprise a thin metal plate I0 secured between the two crystal plates 2 and 3 and made of steel or other metal suitably proportioned with respect to the crystal plates 2 and 3 in order to obtain a temperature coefficient of frequency of selected value for controlling the over-all temperature coemcient of frequency of the bonded crystal unit 2,v 3 and Il). If desired, one of the two bonded crystal plates 2 or 3, such as the plate 3 illustrated in Fig. 12 for example, may be made of non-piezoelectric material and made to have a temperature-frequency coefficient to balance that of the crystal plate 2 secured thereto, thereby to obtain a low over-all temperature coeflicient of frequency for the bonded unit.

Although this invention has been described and illustrated in relation to specific arrangements, it is to be understood that it is capable of application in other' organizations and is therefore not to be hunted to the particular embodiments disclosed, but only by the scope of the appended claims and the state of the prior art.

What is claimed is:

l. A duplex type thickness flexure mode crystal body comprising two length-mode +5-degree X-cut type quartz crystal plates bonded together in major face to major face relation toobtain a low temperature coeicient for said flexure mode frequency of said body, the length and thickness dimensions of said crystal plates being made of values in accordance with the value of said flexure mode frequency, means for driving said crystal body in said thickness flexure mode comprising electrodes formed integral with the outside major faces of 'said body, and means comprising four pairs of conductive bent spring wires soldered to said electrodes substantially at the nodes of motion of said body for supporting and establishing electrical connections with said body substantially at the nodes of motion thereof.

2. A duplex type thickness iiexure mode crystal body comprising two length-mode +5-degree X-cut type quartz crystal plates bonded together in major face to major face relation to obtain a low temperature coefficient for said flexure mode frequency of said body, the length and thickness dimensions of said crystal plates being made of values in accordance with the value of said ilexure mode frequency, electrodes on the outside major faces of said body, and means for supporting and establishing electrical connections with said body substantially at the nodes of motion thereof, said means comprising conductive spring wires soldered to said electrodes substantially at said nodes of motion of said body.

3. A duplex type thickness flexure mode crystal body comprising two length-mode +5-degree X-cut type quartz crystal plates bonded together in major face to major face relation to obtain a low temperature coefficient for said flexure mode frequency of said body, the length and thickness dimensions of said crystal plates being made of Before the bonded "Y face to major face relation, the length and thickness dimensions of said crystal plates being made of Values in accordance with the value of said ilexure mode frequency, means for driving said body in said thickness exure mode comprising electrodes formed integral with the outside major faces of said body and a plurality of pairs of wire-like support members, said members having ends disposed in contact with said outside electrodes at a plurality of spaced points thereon, said points being substantially at each of the plurality of nodal lines extending midway between said outsidemajor faces of said body, one of said crystal plates being right-handed quartz and the other of said crystal plates being lefthanded quartz, said crystal plates being +-degree X-cut type quartz crystal plates poled in opposite ways.

11. A composite thickness ilexure mode piezoelectric crystal body comprising two length-mode quartz crystal plates bonded together in major face to major face relation, the length and thickness dimensions of said crystal plates being made of values in accordance with the value of said fiexure mode frequency, means for driving said body in said thickness exure mode comprising electrodes formed integral with the outside major faces of said body and a plurality of pairs of wire-like support members, said members having ends disposed in contact with said outside electrodes' at a plurality of spaced points thereon, said points being substantially at each of the plurality of nodal lines extending midway between said outside major faces of said body, one of said crystal plates being right-handed quartz and the other of said crystal plates being lefthanded quartz, said crystal lplates being +5-degree X-cut type quartz crystal plates poled in the same way.

12. A vlow temperature-frequency coefficient composite piezoelectric crystal body adapted to vibrate ilexurally by bending in its thickness dimension direction about its nodes of motion comprising two +5degree X-cut type piezoelectric ,quartz crystal elements soldered together in major face to major face relation, the length and thickness dimensions of said crystal elements being made of values in accordance with the Value of said lexure mode frequency, the dimensional ratio of the width of said major faces with respect to said length thereof being one of the values substantially` from 0.20 to 0.35, electrodes formed integral with the outside major faces of said crystal body, and means comprising conductive spring' wires secured to said electrodes substantially at said nodes of motion for supporting and establishing electrical connections with said composite body.

13. A' low temperature-frequency coefficient composite piezoelectric crystal body adapted to vibrate flexurally by bending in its thickness dimension direction comprising two +5-degree ,n X-cut type piezoelectric quartz crystal elements and means for bonding said crystal elements together in major face to major face relation, said bonding means comprising coatings of baked metallic paste formed integral with each of the inside or inner major faces of said crystal elements and a layer of solder disposed between and formed integral with said inner metallic coatings, one of said crystal elements being made from right-handed quartz and thel other of said elements being made from left-handed quartz.

14. A low temperature-frequency coemcient composite piezoelectric crystal body adapted to 20 vibrate exurally by bending in its thickness dimension direction comprising two piezoelectric quartz crystal elements and means for bonding said crystal elements together in major face to major face relation, said bonding means comprising coatings of baked metallic paste formed integral with each of the inside or inner major faces of said crystal elements and a layer of solder disposed between and formedintegral with said inner metallic coatings, one of said crystal elements being made from right-handed quartz and the other of said elements being made from lefthanded quartz.

15. A duplex type flexure mode crystal body comprising two length-mode +5-degree X-cut type quartz crystal plates Vbonded together in major face to major face relation to ,obtain a low temperature coeiiicient for said flexure mode frequency of said body, the length and thickness dimensions of seid crystal plates being made of values in accordance with the value of said flexure mode frequency, electrodes on the outside major faces of said body, and means for supporting and establishing electrical connections with said body. substantially at the nodes of motion thereof, said bonding means comprising coatings of baked silver paste formed integral with each l of the inside major surfaces of said crystal plates and a layer of solder disposed between and formed integral with said inside crystal coatings, said solder comprising silver as an element of its composition. A

16. Piezoelectric crystal apparatus comprising a composite or duplex type crystal body adapted to bend in thickness exure mode vibrationsat a relatively low frequency determined mainly by the length and the thickness dimensions of said crystal body, said length and thickness dimensions of said crystal body being of values corresponding tothe valu'e of said thickness flexure mode frequency, conductive electrodes disposed on the outside major faces of said crystal body, and means for supporting and establishing electrical connections with saidy electroded crystal body substantially adjacent' the nodes of motion thereof, said crystal body comprising two lengthmode quartz crystal plates and means for bond- @9 a relatively low frequency determined mainly by the length and the thickness dimensions of said crystal body. said length and thickness dimensions of said crystal body being 0f Values correspending to the value of said thickness iiexure mode frequency, conductive electrodes disposed on the outside major faces of said crystal body, and means for supporting and establishing electrical connections with said electroded crystal body substantially adjacent the nodes of motion thereof, said crystal body comprising two lengthmode quartz crystal plates and means'for bonding said crystal plates together in major face to maior face relation, said crystal plates being +5 degree X-cut type quartz crystal plates constructed from crystal quartz of opposite handedthe length and the thickness dimensions of said crystal body, said length and thickness dimensions of said crystal body being of values corresponding to the value of said thickness exure` mode frequency, conductive electrodes disposed on the outside major faces of said crystal body, and means for supporting and establishing electrical connections with said electroded crystal body substantially adjacent the nodes of motion i thereof, said crystal body comprising two lengthmode quartz crystal plates, and means including solder for bonding said crystal plates together in maior face to major face relation, said crystal plates being degree X-cut type quartz crystal plates constructed from crystal quartz of oppositehandedness one of said crystal plates being right-handed quartz and the other of said crystal plates being left-handed quartz whereby a very low temperature coemcient is obtained for said thickness ilexure mode frequency.

19. Piezoelectric crystal apparatus comprising a composite or duplex type crystal body adapted to bend in thickness ilexure mode vibrations at a relatively low frequency determined mainly by the length and the thickness dimensions of said crystal body, said length and thickness dimensions of said crystal body being of values corresponding to the value of said thickness exure mode frequency, conductive electrodes disposed on the outside maior faces of said crystal body, l

and means for supporting and establishing electrical connections with said electroded crystal body substantially adjacent the nodes of motion thereof, said crystal body comprising two lengthmode quartz crystal plates and means for bonding said crystal plates togetherin maj or face to major face relation, said crystal plates being +5 degree X-cut type quartz crystal plates constructed from crystal quartz of opposite handedness one of said crystal plates being right-handed quartz and the other of said crystal plates being left-handed quartz whereby a very low temperature coefilcient is obtained for said thickness ilexure. mode frequency, said crystal plates being electrically poled in opposite ways and subjected to a thickness direction electric field produced by said outside electrodes, and said nodes being lines disposed midway between said outside maior faces and ex tending from side edse to side edge of said body in a direction which is inclined substantially 1l degrees with respect to the perpendicular to said length dimension of said body.

20. Piezoelectriccrystal apparatus comprising a composite or duplex type crystal body adapted to bend in thickness ilexure mode vibrations at a relatively low frequency determined mainly by thelensthsndthethicknnsdimensionsofssid crystal body, said length and thickness dimensions of said crystal body" being of values cor responding to the value of said thickness fiexure mode frequency, conductive electrodes disposed on the outside major faces of said crystal body, and means for supporting and establishing electrical connections with said electroded crystal body substantially adjacent the nodes of motion thereof, said crystal body comprising two lengthnode quartz crystal plates and means for bond ing said crystal plates together in maior face to major face relation, said crystal plates being +5 degree X-cut type quartz crystal plates constructed from crystal quartz of opposite handedness one of said crystal plates being right-handed quartz and the other of said crystal plates being left-handed quartz whereby a very low temperature coefcient is obtained for said thickness flexure mode frequency, said crystal plates being electrically poled in opposite ways and subjected to a thickness direction electric field produced by said outside electrodes, and said nodes being lines ,disposed midway between said outside major faces and extending from side edge to side edge of said body in a direction which is inclined substantially 11 degrees with respect to the perpendicular to said length dimension of said body, and the dimensional ratio of the width of said major faces with respect to said length thereof being one of the values substantially from 0.20 to 0.35.

21. Piezoelectric crystal apparatus comprising a composite or duplex type crystal body adapted to bend in thickness'ilexure'mode vibrations at a relatively low frequency determined mainly by the length and the thickness dimensions of said crystal body, said length and thickness dimengw sions of said crystal body being of values crresponding to the value of said thickness ilexure mode frequency, conductive electrodes disposed on the outside major faces of said crystal body, and means for supporting and establishing elec-v trical connections with said electroded crystal body substantially adjacent the nodes of motion thereof, said crystal body comprising two lengthmode quartz crystal plates and means including solder for bonding said crystal plates together in maior face to major face relation, said crystal plates being +5 degree X-cut type quarts crystal plates constructed from crystal quartz of opposite handedness one of said crystal plates being right-handed quartz and the other of said crystal plates being left-handed -quartz whereby a very low temperature coeillcient is obtained for said thickness flexure mode frequency, said crystal 'plates being electrically poled in opposite ways and subjected to a thickness direction electric field produced by said outside electrodes, and said nodes beins lines disposed midway beo tween said outside maior faces and extending from side edge to side edge of `said body in a direction which is inclined substantially 11 degrecs with respect to the perpendicular to said length dimension of said body, and the dimen- 05 sional ratio of the width of said maior faces with respect to said length thereof being one of the values substantially from 0.20 to 0.35.

CLARENCEELANE.

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