US2329498A - Art of mounting piezoelectric crystals - Google Patents

Description

Sept. 14,1943 E. M. WASHBURN ART OF MOUNTING PIEZOELECTRJ IC CRYSTALS Filed Aug. 12, 1942 rlll Ottorneg Patented Sept. 14, 1943 ART OF MOUNTING PIEZOELICTMO CRYSTALS mm m. Waaliburn, Merehantville, N. 1., acaignor to Radio Corporation of America, a corporation of Delaware Application August 1:, 1942, Serial No. 454,505 401 m. (01. 171-321) This invention relates to the art of mounting piezoelectriccrystals and has special reference to the provision of an improved mount for socalled "plated" crystals.

U. 8. Patent 2,228,601 to H. W. N. Hawk discloses a crystal assembly wherein the electrodes comprise a pair of adherent film-like electrodes each partly covering an electrode face of the crystal. The non-covered areas of the electrode faces in the Hawk patent lie adjacent to .the opposite ends of the crystal and the necessary clamping force is applied to the crystal at these end zones. Thus in Hawk's holder, each clamp is arranged to engage one covered and one uncovered surface area on the crystal, so that a short circuit between the electrodes is avoided so long as the crystal is prevented from being moved in its own plane to a position whereat the jaws of a given clamp engage both electrodes. To prevent such movement Hawk provides his clamps each with a shoulder or end-piece (designated a: in the patent) which extend around the ends of the crystal and between which the crystal is snugly fitted.

The present invention is predicated upon an appreciation of the fact that optimum performance of a crystal assembly of the general character described is achieved only when the crystal is permitted at least a limited range of movement and the invention may be said to reside, at least in part, in the provision of a clamped crystal assembly wherein the crystal may shift its position (whether due to its natural vibration or to shocks and tremors of external origin) without causing 'a short circuit between its electrode faces 6: otherwise disturbing its normal mode of operation.

The present invention also provides a mounting arrangement wherein the crystal though free to move a limited distance is maintained in its own plane with the smallest practical clamping force against tilting, thus permitting the assembly tobe supported in a crowded compartment, or one of minimum width.

Other objects and advantages of the invention will be apparent upon reference to the following speciilcation and to the accompanying drawing, wherein:

Fig. 1 is a sectional plan view with the easing in section of a plated-crystal assembly constructed in accordance with the principle of the invention;

Fig. 2 is a fragmentary plan view on an enlarged scale showing an end portion of the crystal assembly;

Fig. 3 is a side elevational view, with the casing shown partly in section, of the clamped crystal assembly of Figs. 1 and 2; and

Fig. 4- is a sectional view of the holder taken on the line IV-IV of F18. 3.

In the drawing, wherein like reference characters are applied to the same or corresponding parts in all figures, l designates generally an hermetically sealed casing constituted of insulating material and comprising a base portion 3 and a covert having an inner compartment 1 containing a "plated" piezoelectric quartz element 9 which is supported in an upright position on or above the base 3 by means of two clamps II and it which are designed, positioned and arranged, in a manner later described. to permit the crystal to move or to be shifted a limited distance in its own plane, without causing a short circuit between its plated electrode sur- 5 faces 15 and H.

The pattern of the film-like electrodes i5 and H on the crystal 5 is similar to that shown in the Hawk patent; that is to say, each electrode covers all but a marginal edge or end-zone m, or m. of the electrode face of the crystal to which it is applied; the. said uncovered end-zones m, m being at diagonally opposite ends of the crystal. These separate electrodes i5 and i! may be formed in the desired pattern by mask- .ing the marginal edge portions m with Scotch tape, or the like, then immersing the masked crystal in a silver solution, precipitating the silver, thereafter removing the precipitated metal from the minor surfaces of the crystals as by means of a bufilng operation, and finally removing the masking tape. Alternatively, these electrodes may be applied to the crystal by thermal evaporation, in vacuo, of the silver or other electrode metal.

The separate clamps II and (3 which support the crystal 9 in an upright position on the base 3 of the casing l are formed of spring metal and are of duplicate substantially U-shape construction. One arm of each U has a flat section a and the other arm b is of a special curved construction later described. The free ends of the flat arms a are preferably inclined inwardly about 2 with respect to the vertical so that when the crystal is inserted in the clamps the said arms will engage the crystal throughout their length. One arm or jaw (say, the part d) of each of the clamps ii and I3 is provided with a finger or tongue 0 which projects laterally therefrom beyond and around the adjacent end of the crystal so that the crystal may move, or be moved,

a limited distance either to the left or to the right, as viewed in Figs. 1 and 3.

Now, it is apparent that as the crystal is moved to the left or to the right the border (indicated by the line Z Figs. 2 and 3) between the plated and unplated portions of each electrode face will approach one or the other of the arms b, b. on the clamps II and i3, respectively, and that should one of the said arms actually contact a metallized area on the crystal a short circuit would occur since, in that event, both electrodes i5 and I! would be electrically connected to a single clamp, H or i3. At first glance it would appear that this difficulty could be obviated by enlarging the unplated areas m on the electrode faces of the crystal but, as a practical matter, this is not desirable since to sustain vigorous oscillations at a single ifrecuiencywith electrodes of the type described it is necessary that the electrodes overlap each other over so much of the crystal body as is practical. The above described possibility of a short-circuit is prevented in accordance with the invention and without any diminution in the size of the electrodes by the special construction of that arm (b) of each clamp ii and I! which contacts the unpiated end-zones m, m of the crystal. As shown in the drawing, these clamping arms b are so shaped that, they contact the crystal at a mere point or node p, that is to say, they are dished inwardly in the direction of the crystal so-that they contact the crystal only at a point or, alternatively, along an axis which is normal to the direction'in which the crystal is permitted to move. The advantage of this construction will be apparent upon inspection of Fig. 2 wherein it will be observed that the crystal may be inoved to the left a distance defined by the lines :c-z before a. short circuit can occur whereas if the said arm 12 were fiat or merely bent along an axis parallel for mounting purposes is limited either by the small size of the crystal compartment or by the presence of other elements, such for example, as the screws 2i and 23 which serve to clamp the cover 5 to the base 3. (In this case, if the crystal tilted into contact with the screws Ii, 23 a short circuit would occur.) In the above connection it may be noted that casing I isdesigned to meet the most rigid Government specifications for hermetically sealed crystal holders and, to this end, is constituted of mics. filled or other low-los phenolic moulding compound and contains moulded in metal-inserts or nuts 25, 21 for engagement with the screws 2|, 2!, respectively, so that the clamping force applied to the gasket 29 between the base 3 and cover 5 is suflicient to provide and maintain a hermetic seal therebetween. It will be observed that the inner walls of the compartment I are provided with oppositely located inwardly extending shoulders I0 which serve to limit any upward movement of the crystal such as might be occasioned by a severe mechanical shock. The base 8 of the eas- .ing l is preferably formed of the same low-loss are preferably filled with a high melting point to the line of movement of the crystal a short circuit would occur if the crystal were to be moved over the much more limited distance 11-22. It will be noted that the foregoing advantage is achieved without any decrease in the mechanical strength of the arms b since the said arms may be just as wide and just as strong as the arms a.

Another very real advantage of the above described constrfiction is that the flat clamping surfaces a serve to maintain the crystal against tilting movement. tain the crystal against the flat arms a is exerted upon the crystal by the counterbiasing force applied through the contact points on the arms b and is thus confined to a limited area on the crystal-surface, hence the damping effect upon the crystal when it is vibrating is minimized. It should be noted that the clamps H and I3 are so oriented with respect to the crystal 9 that the similarly shaped arms (a and a, b and b) are biased, by their spring action, into engagement with diflerent major faces of the crystal, that is to say, each of the curved arms b engage an uncovered end-zone m, while the flat arms a engage the metaliized surface area which lies adjacent to the said end-zones on the opposite face of the crystal.

The present invention is not especially concerned .with the particular type of casing used in housing the crystal since, obviously, the in-' vention may' be applied to a crystal assembly encased in any knowntype of holder. However, it-mayybe pointed out. that the advantages of the invention are more fully realized where, as in .therillustrated embodiment, the space available The force required to mainwax subsequent to the assembling operation to ensure a fluid tight seal and to prevent tampering. Another feature of this holder is the two shallow rectangular recesses I5, 31 in the top surface of the base 1 and within which the ends of the wires 43, 45 connect the clamps II and iii to the prongs II and 33 are seated to prevent any internal twisting movement and to facilitate assembly and alignment problems.

From the foregoing it is apparent that the present invention provides an improved clamped crystal assembly and one wherein the crystal is free to move a limited distance in its own 'planeand yet is maintained, with the smallest practical clamping force, against tilting, thus permitting the assembly to be supported in a crowded compartment, or one of minimum width.

What is claimed is: I

1. In combination, a plated piezoelectric crystal having an unplated mounting area on each of its opposite electrode surfaces, and a support for said crystal, said crystal being susceptable of movement in said support, said support comprising conductive means engaging each of said plated surface areas and providing an electrical connection therein, means engaging each of said unplated surface areas at a predetermined distance from the end of the adjacent plated surface areas, and means limiting the said movement of said crystal to a distance less than-said predetermined distance.

2. In combination, a plated piezoelectric crystal having an unplated mounting area on each of its opposite electrode faces, a pair of clamps each having jaws adapted to engage and to support said crystal at one plated and one of said unplated surface areas, said crystal being normally susceptible of displacement within and with respect to said jaws of said clamps, and means for limiting the extent of said dis placement to a region defined by the boundaries of said unplated mounting areas.

asaaees i 3 3. The invention as set forth in claim 2 and I wherein one jaw of each clamp comprises a flat surface to inhibit tilting of said crystal and wherein the other jaws comprise point contacts whereby the clamping force applied to said crystal by said other jaws is localized.

4. In combination, a plated piezoelectric crystal having unplated' mounting areas at diametrically opposite end-zones of its electrode faces, and a pair of spring metal clamps each 10 having jaws adapted to engage and to support said crystal at one plated and one or said unplated end-zones, said crystal being normally susceptable of displacement in its own plane and within and with respect to said clamps, and means comprising a protuberance on each of said clamps extending beyond and around the ends of said crystal for limiting the extent of said displacement to a region defined by the boundaries of said unplated mounting areas.

EDWARD M. WASHBURN.

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