US 2229172 A
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11111.21, 1941, H W N HAWK `2,229,172
METHOD OF AND APPARATUS FOR MONTING PIEZOELECTRIC CRYSTALS Filed Nov. 28, 1938 Snnentor dnorneg Patented Jan. 21, 1.941
UNITED STATES METHOD OF AND APPARATUS FOR MOUNT- ING PIEZOELECTRIC CRYSTALS Henry W. N. Hawk, Merchantville, N. J., assignor to Radio Corporation of America, a'corporation of Delaware Application November 28, 1938, Serial No. 242,805
My present invention relates to the art of mounting piezoelectric crystals and has special reference to the provision of improved methods and means for mounting quartz crystal elements of the type capable of being vibrated at a predetermined thickness-mode frequency or frequencies.
The prior art dictates the mounting of thickness-mode (high frequency) piezoelectric crystals between two metal electrode plates which are provided at their four corners with risers or protruding portions through which sufficient clamping force is applied to the crystal element to prevent its displacement. While this type (air- 4 gap, pressure type) of mount is usually satisfactory when applied to crystals cut to respond to frequencies up to say, 10 megacycles, I have found that it becomes increasingly diiiicult and eventually practically impossible to use it in connection with crystal elements cut to respond to higher fundamental or harmonic frequencies. I attribute the failure of this type of mount, when applied to crystals of very high frequency, to the fact that the dimension of the air gap which lies adjacent the central area of the electrode faces of the crystal is extremely critical. Thus, I have observed that when the depth of the air gap is equal to substantially .5 or a multiple of .5 of the wave length of the crystal frequency, the oscillations of the crystal element are substantially completely damped apparently by the supersonic vibrations set up in the air gap. It would appear that satisfactory performance could be achieved by carefully machining the electrodes so that the air gap adjacent the center thereof will be less than .5 or more than .5 and less than 1 or more than 1 and less than 1.5 times the wave length of the oscillations of the particular crystal with which the electrodes are to be used. This, however, is a commercially impractical procedure not only because of the micromatic precision required but also because each crystal element requires the separate manufacture of the electrodes to be used therewith.
Accordingly, an object of my present invention is to provide a simple, inexpensive, interchangeable mounting for piezoelectric crystals and one capable of maintaining the crystal against displacement without undue damping.
Another object of my invention is to provide an improved pressure type, low-capacity mounting for high frequency piezoelectric crystals.
Other objects and advantages will be apparent and my invention itself'will be best understood by reference to the following specification and to the accompanying drawing wherein:
Figure 1 is a view in perspective of an electrode constructed in accordance with the principle of my invention;
Figure 2 is a. sectional view of a pair of electrades, similar to the one shown in Fig. 1, and showing a piezoelectric crystal interposed therebetween;
Figure 3 is a sectional view of an alternative embodiment of the electrode of Figures 1 and 2; and
Figure 4 is a sectional View of a holder containing the crystal assembly of Fig. 2 and in- 'corporation means for adjustablyv applying a clamping and spacing force thereto.
In carrying my invention into effect, I mount an` electrode surface adjacent the center of the crystal, apply a clamping force adjacent the edge of the crystal and adjust the air gap between the electrode and the crystal to that point required to ensure optimum performance. Usually, maximum amplitude of vibration is achieved when the electrode barely touches or, in some cases, barely clears the surface of the crystal. This is simply achieved, in accordance with my invention, my mounting the crystal between two electrodes of special construction, confining the clamping force to the edges of the crystal and permitting the air gap to be varied upon variation of the clamping force.
Referring now to Fig. 1 which shows an electrode constructed in accordance with the principle of my invention, here P designates generally a rectangular plate-like element preferably constituted in its entirety of a chrome-nickel alloy such for example as Monel metal, which is provided with a small upright fiat face or riser, R1 to R4, respectively, adjacent each of its four corners and a fifth riser which is confined to the center of the plate and is designated E. This central part E has a rise somewhat less than the corner portions R1 to R4, inclusive, and comprises the electrode surface of the plate P. The thickness of the plate P is small enough to permit it to flex upon the application of a clamping force to the central portion of the opposite surface of the plate. This requirement is satisfied in a Monel metal plate substantially one inch square when the thickness dimension of the plate as measured through the corner risers is .00625" and is substantially .00365 thick in the area surrounding the electrode surface E. The rise of the surfaces R is not critical. The rise of the surface E will depend to some extent upon the iiexibility of the plate but should ordinarily be from .001 to .003 of an inch less than the rise of the clamping faces R.
Fig. 2 shows a piezoelectric crystal C interposed between two plate-like electrodes P1 and P2 similar to the one shown in Fig. 1. The arrows indicate symbolically the direction and points of application of a clamping to the outer surfaces of the plates. The application, as at the center of the plates P, of suicient force to produce a clamping action adjacent the corners of the crystal C will cause the plates to flex whereby the electrode faces E are urged toward the crystal. The air gap between the electrode faces E and the surfaces of the crystal C may thus be adjusted, over a limited range, by varying the force applied to the plates P1 and P2. Optimum performance is usually achieved when the force applied to the plates P1 and P2 is sufficient to maintain the electrode faces E in light contact with the surfaces of the crystal C, although a small air gap can be tolerated providing its depth is not .5, or a multiple of .5, of the wave length of the crystal frequency.
Fig. 3 shows an alternative form of electrode designed for use with crystals of such high frequency that the interelectrode capacitance becomes a damping factor. Here the interelectrode capacitance is minimized by forming the body of the plate P3 of an insulating material, such for example, as micalax, which is a commercial product formed by the fusion of silica and mica. The central electrode portion, here designated E3, comprises a circular metal insert, the periphery of which may be spun over the insulating material as indicated at e to provide a tight bond therebetween.
Fig. Li shows the crystal assembly of Fig. 2 cradled on a pair of rocker arms A1 and A2 within a plug type holder H, in a manner similar to that described by El. J. Schrader et al. in U. S. Patent No. 2,078,284. As shown in this figure the clamping force need not be applied to the electrode plates P at their exact center (as is suggested by the arrows in Fig. 2) but may be applied, as by the rocher arms to that flexible area on each plate which is between the risers R and the periphery of the electrode portion E. When thus mounted the clamping force applied through the plates P by the rocker arms to the crystal may be regulated by turning the screw S which terminates adjacent the outer surface of the cover T. As previously set forthja change in this adjustment likewise changes the depth of the air gap between the electrode surface of each plate P and the crystal C. A rubber gasket G interposed between the cover T and the body H of the holder provides a dust and moisture proof seal. Bolts B serve to clamp the cover to the holder.
When the gasket G is employed, there may be some tendency for the crystal to change frequency over long periods of use. I have traced this diliiculty to the variation in the clamping force applied` to the crystal when the rubber loses its resiliency. To obviate this objection to the use of a sealing gasket, I embed non-resilient metal studs or grommets D in the gasket about the orifices which accommodate the bolts B. Thus, when the bolts are tightened, the spacing of the cover from the body of the holder and hence the force applied to the crystal through screw S is controlled by the non-yielding studs or grommets D and is not altered by the presence or condition of the gasket.
While the invention has been illustrated and described as applied to a high frequency piezoelectric crystal having duplicate lia-t electrode surfaces, it may be applied to both high and low frequency elements irrespective of the exact contour of their crystal surfaces. Accordingly, the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the prior art and the spirit of the appended claims.
What is claimed is:
l. Method of mounting a piezoelectric crystal which comprises mounting an electrode surface adjacent the center of a surface of the crystal, applying a clamping force adjacent the edges of said crystal surface and Varying the spacing between said electrode surface and said crystal surface. 1
2. Method of mounting a rectangular piezoelectric crystal which comprises mounting an electrode surface adjacent the center of a surface of the crystal, adjustably applying a clamping force adjacent the corners of said crystal surface and concomitantly varying the spacing between said electrode surface and said crystal surface. l
3. An electrode for piezoelectric crystals comprising a plate-like element having a plurality of risers thereon, one of said risers constituting an electrode surface and another riser constituting a portion of the element through which a clamping force may be applied to Ithe crystal.
4. The invention as set forth in claim 3 wherein said electrode riser is constituted of metal and said clamping riser is constituted of insulating material.
5. An electrode for piezoelectric crystals comprising a plate-like element having a plurality of risers thereon positioned to contact the crystal at spaced points about the periphery thereof, and another riser projecting from said plate a distance less than that of said other risers and comprising an electrode surface of said plate.
6. An electrode for piezoelectric crystals comprising a rectangular plate having a central riser constituting an electrode surface and a riser at each of the corners of the rectangle and through which a clamping force may be applied to the crystal.
'7. In combination, a piezoelectric crystal, a plate-like element having a plurality of risers thereon presented to said crystal, one of said risers constituting an electrode surface of said element, and means for exerting a clamping force upon said crystal through the other of said risers.
8. In combination, a piezoelectric crystal, an electrode having a plurality of risers thereon presented to the crystal, means for exerting a clamping force upon said crystal through certain of said` risers and for varying the spacing between the other of said risers and said crystal.
9. In combination, an electrode for piezoelectric crystals comprising a plate-like element having a central electrode portion and an edge portion, means for applying a clamping force to the crystal through said edge portion, and means responsive to said clamping force for moving said central electrode portion with respect to said edge portion.
l0. An electrode for piezoelectric crystals comprising a plate-like element having a central electrode portion and a surrounding portion adapted to be flexed upon the application of a clamping force thereto, and means for applying a clamping force to said surrounding flexible electrode portion of said plate-like element.
HENRY W. N. HAWK.