US2436202A - Piezoelectric apparatus - Google Patents

Description

Feb. 1?}, 1948. J. J. CHESS 2,436,202

PIEZOELECTRIC APPARATUS Filed Feb. 13, 1945 illlliulil 5Q 55 g5 ANGLE an: @EGRPEES.

Jay d. Cress,

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Mventorw Patented Feb. 17, 1948 PIEZOELECTRIC APPARATUS Jay J. Cress, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application February 13, 1945, Serial No. 577,695

My invention relates to piezoelectric apparatus and particularly to piezoelectric crystals suitable for use as frequency determining elements in vacuum tube oscillation generator circuits and in electric wave filter systems or other networks.

More particularly, the invention relates to low frequency piezoelectric crystals in the range of 50 to 250 kilocycles per second, and has for a general object the provision of a new and improved crystal of this character having a very low temperature coefficient of frequency over a wide range of temperatures.

It is a further object of this invention to provide a low frequency contour oscillating piezoelectric crystal having a low temperature coefficient over a wide temperature range.

It is a still further object of my invention to provide a new and improved low frequency contour piezoelectric crystal having a low temperature coefficient and a small physical size.

In accordance with my invention, a piezoelectric crystal element may be cut to have such multiple orientation angles with respect to a set of orthogonal crystallographic axes X, Y and Z as to obtain a low or substantially zero temperature coefiicient of frequency for the crystal over a selected range of temperatures without necessitating the large surface area of existing low frequency crystal cuts having this characteristic.

In a particular embodiment of the invention, the crystal element may be a quartz plate which has a substantially rectangular major plane of a selected dimensional ratio of width to length and a double orientation with respect to the orthogonal X, Y and Z axes in order to obtain a low or substantially zero temperature coefilcient of frequency within a desired range of temperatures for a low frequency crystal having a relatively small major surface area.

For a more complete understandingof my invention and a further appreciation of the objects and advantages thereof, reference should now be had to the following detailed specification taken in conjunction with the accompanying drawings in which Fig. 1 is a perspective view of a piezoelectric quartz crystal oriented in accordance with my invention with respect to the major orthogonal axes of the crystal structure; Fig. 2 is a graphical representation of the principal axes of a crystallographic system; Fig. 3 is a graphical representation of the relation between a critical orientation angle and the major surface dimensional ratio of various crystals embodying my invention; and Fig. 4 is an end view of the crystal plate shown in perspective at Fig. 1

5 Claims (Cl. 171-327) This specification will follow the standard terminology as applied to quartz which employs orthogonal X, Y, and Z axes to designate the orthogonal crystallographic electric, mechanical and optic axes, respectively, of the piezoelectric quartz crystal material, and employs X, Y, and Z to designate the directions of axes or surfaces of a piezoelectric body angularly oriented with respect to the orthogonal X, Y, and Z crystallographic axes thereof. Where the orientation is obtained by multiple rotation about the X, Y, and Z axes of the piezoelectric body. as particularly illustrated in Fig. l, the orientation angles a, o and 0, respectively, designate the angular rotation of the plane of the crystal about the Z axis, the angular position of the plane of the crystal with respect to the Z axis, and the angular rotation of the crystal in its plane about an axis perpendicular to the plane, respectively,

Quartz crystals may occur in two forms,

namely, right-hand and left-hand crystals. A

crystal is designated as right-hand if it rotates the plane of polarization of plane polarized light traveling along the optic or Z axis in a clockwise direction when looking toward the light source,

and is designated as left-hand if it rotates the plane of polarization in the opposite or counterclockwise direction. If a, compressional (or positive) stress be applied to the ends of a mechanical axis of a quartz crystal body and not removed, a charge will be developed which is positive at the positive ends of the electric axis and negative at the negative ends of the electric axis for either right-hand or left-hand crystals. The magnitude and sign of the charge may be mea ured with a vacuum tube electrometer, for example.

Referring now to the drawing I have shown in Fig. 1 a set of orthogonal crystallographic axes X, Y. and Z of a piezoelectric crystal element. The X axes have positive and negative directions chosen in accordance with the foregoing convention. At Fig. 2 I have shown all the crystallographic X and Y axes of a, quartz crystal, viewed from, the end of the Z axis and marked in polarity in accordance with the assumed convention,

Thus, since compression along the Y axis is regarded as a positive strain, the positive end of the electric axis +X will evidence a positive charge for both right-hand and left-hand crystal elements. The Z axis, perpendicular to the X, Y

plane, is the optical axis of the crystal.

Fig. 1 shows also the orientation with respect to the axes X, Y, and Z of a crystal plate of substantially rectangular parallelepiped shape. The crystal plate I comprises a pair of parallel major faces 2 and 3, one of which lies in a plane I disposed at an angle o with the Z axis,-measured in a plane perpendicular to the plane I and including the Z axis, and also disposed so that its intersection with the X. Y plane forms an angle a with the +X axis, measured in the x. Y plane. The plate I is so disposed in the plane I that its transverse edge along its width dimension W forms an angle a with the line determined by the intersection of theplane l and the X, Y plane, the angle a being measured in the plane 4.

I have found that, when the angle a is substantially 30 and the angle is substantially 45, 9. crystal plate I of small major surface area and having a very low temperature coei'ilcient frequency may be cut from the mother crystal at an angle a within a range of approximately 45' to 60'.

It will now b observed that the intersection of the p ane 4 and the X. Y plane lies along the crysta loe'rap ic Y axis shown as a brok n line Y: at F g. l and angularly s ac d by from the electric ax s 4-K. The axis Y1 is spaced by 90 from a cr tal o ra hic elec ric +x shown in Fig, l as a broken line x1. so that a cry tal plate cut from the mother crystal in such' a manner that one maior face lies in a plane in ludin the o tical axis Z and the Y1 axis would be a s'mole x-cut crystal p ate. If now such an X-cut crvstal plate is rota ed about the cr stal o raphic X1 axis b the angle 0 e ual to substantial y there is obtained a low frequency contour crystal plate which will os illate strongly at a frequency determined b the dimensions or ar a of its major faces. Such a crystal plate is called a contour crystal by reason of the fact that its natural frequency is de ermined prlmarilv by the dimensions or area of the major faces. and its principal mode of vibrat on is believed to be a face shear.

According to my invention such a 445 X-cut crystal lying in the plane Z. Y1 is modified to reduce its temperature coemcient by rotating it throu h the angle as about the Y1 axis. I have found that when the angle a lies between 45 and the temperature coeillcient of frequency is very low for a low frequency crystal of small maior surface area. It is believed that the principal mode of vibration of the crystal plate is a face shear vibration. for powder patterns taken on the major faces indicate a. nodal point at the center of the major face. This is the condition which occurs in crystals having a face shear mode of vibration. The orthogonal electric and mechanical axes of the crystal plate I itself are indicated as X and Y.

I have found that, with a crystal plate 1 cut in th manner described above, substantially zero temperature coefficient may be obtained with a substantially rectangular major surface configuration and a surface area small in respect to the area of low frequency low temperature coeincient cuts heretofore known. For example, in one out according to my invention, the major surface area is approximately 1% the area of a prior cut of equivalent frequency and temperatur coefficient of frequency.

By way of specific illustration,- I have found that crystal plates cut in the manner shown in Fig. 1 with a width to length ratio of .75 to 1.0 exhibit an average temperature coefficient of frequency of the order of .1 to 1.0 part per million per degree centigrade in the range 30 C. to '70 C. as the angle is varied between 45 and 57". These plates exhibited natural frequencies between 82 and 127 kilocycles per second. At Fig. 3,

the major surfaces 2 and 3, respectively.

I have shown the relation of the width to length ratio W/L of such crystal plates to the angle a necessary to maintain the temperature cceillcient of frequency within the frequency range.

It will be of course understood by those skilled in the art that my invention is not limited in its scope to those precise angles of out set forth by way of example in the foregoing paragraph. The dimensional ratios, as well as the exemplary values of the angles a, 0, and a used hereinbefore, are approximate only. These values will necessarily be varied slightly in accordance with the desired resonant frequency of the crystal plate and in accordance with the thickness of the plate, as well as by reason of the non-uniformity of var. ious samples of quartz. It will also be appreciated that my invention is not limited to plates within the narrow frequency range of the example, but by proper control of the dimensional ratio and orientation other low frequency plates may be obtained. Moreover, I wish to have it understood that, with the angle fixed, the angle 0 may be varied slightl without objectionably increasing the temperature coeillcient of frequency by properly adjusting the W/L ratio of the crystal plate.

The crystal plate I of Fig. 1 has been described with respect to quartz of the right-hand type. From this description, the manner of cutting a crystal plate in accordance with my invention from a mother crystal of the left-hand type will be readily understood by those skilled in the art. and I have shown at Fig, 1 in the dotted lines a crystal plate I oriented in accordance with my invention for quartz of the left-hand type. The angle 0' at Fig. 1 for left-hand crystals corresponds to the angle 0 for right-hand crystals. As is well known by those skilled in the art, left-hand quartz may be out according to the same specifications as right-hand quartz if the polarity of the x axis is arbitrarily regarded as reversed.

It has been found that the crystal plates i and I may be rotated in either direction around the axis Y1, so that the angle a may be either positive or negative.

The crystal plates i or i may be provided on their opposite major surfaces with metallic electrodes B and 8 as shown at Fig. 3. The electrodes 5 and 8 may be formed integral with, or may be otherwise operatively disposed with respect to,

Preferably, the electrodes 5 and 6 consist of a thin coating of chemically deposited silver or aluminum, or other suitable electric conducting material deposited by sputtering or evaporation and afterwards annealed to relieve strains therein. The electrodes 5 and 6 may be used, when connected in circuit with a. suitable system such as for example an oscillation generator system or an electric wave filter system, for exciting the crystal plate I at a vibration frequency determined mainly by the dimensions or area of the major surfaces 2 and 3 thereof to obtain substantially a constant vibration frequency substantially independent of temperature within a desired range of temperatures.

While I have shown and described only a preferred embodiment of my invention by way of illustration, modifications thereof will occur to those skilled in the attend I therefore wish to have it understood that I intend in the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

l. A piezoelectric crystal element having a rectangular major electrode face lying in a plane including a crystallographic mechanical axis of the mother crystal and disposed at an angle of approximately 45 to 60 with respect to the optical axis 01' said crystal and having a longitudinal edge disposed at approximately 45 from said mechanical axis in the plane of said face.

2. A quartz piezoelectric element comprising a substantially rectangular plate having a major electrode face lying in a plane including a crystallographic mechanical axis of the mother crystal and having its longitudinal edge disposed at an angle oi approximately 45 with respect to said mechanical axis in the plane of said face said plane being inclined at an angle of approximately 45 with respect to the optical axis of said mother crystal.

it. A quartz piezoelectric element cut from a mother crystal having orthogonal optical, me= chemical, and electric axes 2;, Y, and X, respectively, and comprising a substantially rectangular plate having a major electrode face disposed in a plane including a crystallographic mechanical axis displaced by approximately 30 from said X axis in the X, Y plane and disposed at an angle of approximately 45 with respect to said optical axis and having a longitudinal edge disposed at approximately 45 with respect to said crystallographic mechanical axis in the plane of said face.

4. A quartz piezoelectric element cut from a mother crystal having orthogonal optical, me-

chanicai. and electric axes X. Y, and X, respectively, and comprising a substantially rectangular plate having a major electrode face substantially coincident with a plane intersecting the K. Y plane in a time at an angle of 30 from said X axis and disposed at an angle of to with respect to said 2 axis, said plate having an edge disposed at approximately 45 with respect to said line.

5. A quartz piezoelectric element cut from a mother crystal having orthogonal optical. mechanical, and electric axes, Z, Y, and X, respectively, and comprising a plate having a substantially rectangular major electrode face, said face being substantially coincident with a plane intersecting the X, Y plane in a line at 30 from said X axis and disposed at an angle of 45 with respect to said Z axis, a longitudinal edge of said major face being at an angle of approximately 45 with respect to said line.

JAY J. CRESS.

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

UNITED STATES PATENTS Number Name Date 2,173,589 Mason et a1 Sept. 19, 1939 2,212,139 Baldwin et a1 Aug. 20, 1940 Mason Mar. 24, 1942

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