Art of mounting piezoelectric crystals
United States Patent 2313129

551,954. Gramophone pick-ups. RADIO CORPORATION OF AMERICA. Sept. 19, 1941, No. 12165. Convention date, Jan. 31. [Class 40 (iv)] [Also in Group XL] A Rochelle salt crystal is mounted in a casing which is filled with a semisolid moisture proofing insulating materials such as lanolin, petroleum jelly, calcium stearate or aluminium stearate. Fig. 1 shows the invention applied to a conventional pick-up in which a needle 13 is connected by a yoke 9 to a bi-morph crystal 3 having electrode foils 5 and which is clamped at 7 inside a casing 1 filled with the moisture-proofing compound.

Dohan, William R.
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310/344, 369/144, 600/500
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My present invention relates to the art of mounting piezoelectric crystal elements, particularly those constituted of Rochelle salt (sodiumpotassium tartrate) and has for its principal object to provide improvements in moisture-proof mounts for such piezoelectric crystal elements.

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 sectional view, and Figure 2 a top plan view, with the cover removed, of a piezoelectric pick-up device embodying my invention.

At a temperature of 20* C. the water of crystallization in Rochelle salt is in equilibrium with air at a relative humidity of 35.9 percent. The saturated solution is in equilibrium with air at 87.2 percent relative humidity. At humidities lower than 35.9 percent such crystals will slowly dehydrate; at humidities higher than 87.2 percent they will slowly dissolve. In either event the useful piezoelectric properties of such crystal elements are destroyed.

Since the above described conditions of humidity may be encountered (particularly in tropical countries) in normal use, various means have heretofore been proposed for protecting Rochelle salt and similar crystal elements from excessive hydration and dehydration. By way of example, it has previously been proposed to enclose the crystals in hermetically sealed envelopes and/or to coat them with certain asphaltic or wax compounds. However the necessity of transmitting mechanical vibrations to the crystal or crystals (as in phonograph "pick-ups") renders it difficult to maintain a hermetic seal about the crystal and, as will hereinafter more fully appear, conventional rigid coatings provide but little real protection for the crystal. In this latter connection it may be observed that to increase the thickness of a wax-like or asphaltic coating with a view to increasing its protective qualities may result in undue damping of the crystal vibrations.

I have discovered that the foregoing objections to the "moisture-proof" coatings of the prior art are substantially obviated by the use of certain semisolid or semifluid embedments. Thus, I may surround, immerse or embed a Rochelle salt or similar crystal element or elements, and preferably the electrodes therefor, in petrolatum, or in lanolin, or in a mixture comprising petrolatum and lanolin or other water emulsifying agent, such for example as calcium stearate or aluminum stearate, Preferably these moisture-proofing substances or mixtures are applied in the form of a filling to the casing or cartridge in which the crystal assembly is mounted, in which case the crystal and its electrodes are all preferably immersed in and surrounded by the filling.

While a Rochelle salt crystal which is mounted and operated in accordance with my invention in petrolatum exhibits a substantially higher factor or merit than one provided with an asphaltic coating, I have achieved very much better results when the embedment material is constituted in whole or in part of lanolin or other emulsifying agent. By way of example: under similar extreme conditions (90 percent relative humidity) a Rochelle salt crystal element coated with an asphaltic compound exhibited a useful life of 24 hours, one embedded in petrolatum a useful life of 300 hours, while one embedded in lanolin was apparently unaffected at the expiration of 500 hours. The failure of the first mentioned crystal may have been due to minute voids, inherent in the granular structure of asphaltic coatings, through which moisture seeped and destroyed the crystal. The eventual failure of the crystals which were embedded in petrolatum may be attributed to the fact that moisture settling on the petrolatum eventually worked inwardly through the mass and was absorbed by ithe crystal.

I attribute the vastly superior protective action of the lanolin, and of the other water emulsifying agents, to the fact that these materials hold the moisture in the dispersed phase. That is to say, the fine particles or globules of water apparently become coated with the embedment material so that if or when these coated liquid particles reach the region of the crystal, the coatings (i. e., the continuous phase of the emulsion) on these discrete particles inhibit or prevent actual contact of the water with the crystal so that it can not be absorbed by the crystal.

Of the emulsifying agents, above mentioned, I prefer lanolin (lanum anhydrous U. S. P.), as this material has a melting point of 31° to 340 C. and has a viscosity such that it serves not only to damp-out resonance peaks (at temperature of the order of 20° to 25' C.) but also serves to compensate for the usual drop in the crystal output at temperatures above 20* C., by reason of its decreased viscosity at increasing temperatures.

Further, lanolin is capable of absorbing up to 80 percent of its weight of water although it is not water soluble. The advantages of my invention, however, may be achieved to a greater or less degree, in the above respects, with any of the aforementioned or equivalent semiliquid or semisolid materials, either alone or in combination.

In the accompanying drawing I have shown the invention as applied to a conventional phonograph pick-up comprising a casing I which encloses a piezoelectric unit comprising a pair of Rochelle salt crystals 3 with which are associated terminal foils 5 of conductive material in a well known manner, the crystal unit 3 being spaced from the casing I as by the clamps 7 in which it Is supported, or otherwise. As in standard practice the casing I also carries a torsion yoke 9 which extends through a suitable bearing II, and the outer or free-end of the yoke 9 being provided. with a needle socket 13. In accordance with my invention the casing is provided with a filling comprising a semisolid or semiliquid mass which is preferably constituted of one of the insulating materials or compounds above described and within which the crystal unit 3 is adapted to be vibrated as by the force transmitted thereto through the yoke 9.

Various modifications of my invention will sug. gest themselves to those skilled in the art. It ý' to be understood, therefore, that the foregoing should be interpreted as illustrative and not in a limiting sense except as required by the prior art and by the spirit of the appended claims. I claim: 1. A piezoelectric unit comprising a casing containing a piezoelectric crystal element and a semisolid mass of insulating material in which said crystal is embedded, and means extending from said crystal through said insulating material to the exterior of said casing for applying an operating force to said crystal.

2. The invention as set forth in claim 1 and wherein said insulating material comprises a water emulsifying agent.

3. The invention as set forth in claim 1 and wherein said insulating material comprises lanolin.

4. The invention as set forth in claim 1 and wherein said insulating material comprises a water-insoluble metallic soap.

5. The invention as set forth in claim 1 and wherein said insulating material comprises petrolatum.