US2480535A - Enclosure for vibratile elements - Google Patents

Description

Aug. 30, 1949;

F. J. ALOIS ET AL ENCLOSURE FOR'VIBRATILE ELEMENTS Filed March 15, 1947 hwventorsz Frank J Akvis, Richard W. Samsefl, by 7M4. 3 M

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Patented Aug. 30, 1949 ENCLOSURE FOR VIBRATILE ELEMENTS Frank J. Alois and Richard W. Samsel, Schenectady, N. Y., assignors to General Electric Company, a corporation of New York Application March 13, 1947, Serial No. 134,360

4 Claims. (Cl. 177-386) This invention is directed to an enclosure for vibratile elements, and is particularly related to the manufacture of piezo-electric crystal transducers for underwater sound transmission and reception. The invention also relates to improved completely enclosed unitary piezo-electric crystal transducers.

An object of the invention is to provide an improved casing for vibratile elements. Another object is to provide an improved method of making devices incorporating vibratile elements in a waterproof structure.

A more specific object of the invention is to provide a piezo-electric crystal transducer wherein the crystal elements are totally encased in a solid block of a plastic material, such as a polymerized unsaturated polyester resin.

The novel features which we believe to be characteristic'of our invention are set forth with particularity in the appended claims. Our invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which Fig. 1 is a partially cutaway view of a piezo-electric crystal transducer constructed-in accordance with the invention; Fig. 2; is a view showing a group of piezo-electric crystals arranged in accordance with the preferred embodiment of the invention showing the crystal elements, metal foil electrodes for the elements and resilient non-damping members, shown partially cutaway, arranged about and between the elements; and Figs. 3, 4 and 5 are partially cutaway side views illustrative of steps in the manufacture of a complete transducer of the type shown in Fig. 1.

The device shown in Fig. 1 is an eflicient receiving or transmitting transducer for operation at sonic or supersonic frequencies wherein the crystal elements are hermetically sealed within a block of solid plastic material. The transducer, as shown in Fig. 1, comprises a plurality of piezoelectric crystal elements I, which are preferably of ammonium dihydrogen orthophosphate, although Rochelle salt, quartz or other piezo-electric crystal material may be used. Electrodes and associated conductors in the form of thin conductive metal foil are provided for each crystal as best shown by the heavy lines II to 24 inclusive in Fig. 2 discussed below; Each of the crystal elements is isolated from the casing by covering all except an upper -operative face which comprises one of the minor faces, with blocks 2 of a soft material such as that known by the tradename Corprene, a granular cork material formed with a suitable binder. A suitable material is one that provides a minimum of mechanical loading of the crystal by absorbing only a small amount of the vibratory energy. Materials possessing this property are referred to herein as soft mate- Piezo-electric transducers have been heretofore used for transmitting and receiving sonic and supersonic energy in depth indicators and in underwater object locating equipments, and various means of protecting the crystal elements from water have been suggested. There has been a long felt need for an eflicient transducer comprising a completely enclosed crystal element, or group of elements, wherein the casing consists of a unitary structure permanently protecting the crystals from damage from liquids and other hazards. One of the principal diificulties heretofore encountered. in constructions of this type has been that vibration-transmitting materials either cannot be bonded to an operative vibratory face of the crystal element or will destroy the crystal element due to difference in expansion due to temperature changes, or due to shrinkage during manufacture.

It is, accordingly,,a general object to provide a transducer of the total enclosed or hermetically sealed type arranged to overcome these dimculties.

rials. Electric connections including conductors 3 and 4 are made to the metal foil electrodes and a coil or transformer 5 may be connected in series or in parallel with the foil electrodes if required to provide proper electrical impedance for the device. The electrical connections are shown as terminating in a concentric cable comprising an inner conductor 6 and an outer shielding conductor I with interposed insulating material 8, the cable being provided for completing connections to any apparatus which may be used in association with the transducer. All of the transducer elements mentioned above are enclosed in a solid block 9 of hard plastic material preferably comprising an unsaturated polyester resin as later discussed.

It has been found that the crystal elements I will vibrate efi'lciently when excited and that vibratory energy will be transmitted efiectively through the plastic layer to or from face III of the plastic block if a layer, film or thin coating Illa of suitablecement is provided across the top or operative ininor faces of the crystals separating the operative faces from the plastic material, and that the etficiency of transmission is increased if the layer or film is under compression.

Provision of this film or coating of cement has proved successful in bonding the faces of ammonium dihydrogen orthophosphate crystal elements to the plastic material of the block to permit passage of the vibrations therethrough to or from the face l0. Thus, sound waves which impinge on face ill will be transmitted through the plastic material and the film of cement Illa to the operative faces of the crystal elements, and electrical signals on the foil electrodes will be generated by the crystals in response to such vibrations. Similarly. mechanical vibration of the'crystal faces, generated by application of an alternating current voltage to the electrodes, are transmitted through the cement Illa and the plastic material to the face i0.

Referring now to Fig. 2, the foil electrodes and foil conductors for carrying current to the electrodes are numbered ll through 24 and are arranged as indicated by the heavy lines denoting the edges of the foil members. By means of the foil, a connection is established from the foil end ll along the back face of one of the end crystals by an electrode l2, and through connecting foil sheet l3 along one face of each of four intermediate crystal-s by electrodes l4 and I5, as well as to the outer faces of two additional crystal elements by electrode l6, and the inner face of the remaining end element by electrode l1. Connections are also established from foil end l8 to electrode ill on the outer face of the last mentioned end element and through foil sheet 20 to electrodes 2|, 22, 23 and 24 providing contacts with the other faces of all of the remaining crystal elements in such manner that each crystal element has electrodes on each of two opposed major faces.

Separating adjacent major faces of each pair of crystal elements is a block or sheet of soft, vibration-isolating material 2 and completely enclosing the group of elements, except on the top operative face, are similar blocks 2. It will be noted that a relatively thick block 25 completely covers the under face of the group of crystals. All of these blocks are preferably of the soft material mentioned above. The interposition of the soft mass between adjacent crystal faces and between the faces and the plastic block serves to isolate the vibrating faces of the crystals, excepting of course the top face of each, from the casing and from the faces of adjacent crystals without imposing more than a small mechanical loading on the vibratory faces.

To form a group of crystals into a unit such as shown in Fig. 2, the preferred method is to apply the strips of metal foil to the desired crystal faces using any cement which is capable of bonding to the foil and to the crystal surfaces. A suitable cement may prove to be nonconductive, and if so, the foil may be pressed against the face of the crystal by rubbing the foil with the rounded tip of a suitable instrument to cause small areas of the foil to directly contact the crystal face. Types of cement found suitable include those commonly used for cementing foil electrodes to crystals, such as that identified as BC-6052 sold by the Bakelite Corporation.

.these sheets are conveniently folded over blocks of the soft material. The blocks of soft material 2 are inserted between abutting crystal faces so that the crystal elements are mechanically isolated from each other. Cement is applied to the blocks and foil to bind the whole into a unitary structure, a cement being selected which will bond to the foil and to the soft material. The cement identified above satisfies this requirement. Finally, the thick block 25 is cemented to the under side of the crystal group to complete the structure shown in Fig. 2. a

The structure of Fig. 2 accordingly comprises a group of eight crystal elements, each of the elements being provided with two electrodes on opposite major faces respectively connected to foil ends II and it. Each of the elements is enclosed on five sides or faces by the soft material,'leaving'only the-upper operative minor face of each element uncovered.

To complete the device as shown in Fig. 1, the crystal unit of Fig. 2 is encased in the solid plastic material by the remaining steps in the construction, which are best explained with reference to Figs. 3, 4 and 5. The unit of Fig. 2 is first coated completely across the upper exposed face with a layer of black neoprene-base cement to a thickness of between /61 to 5; inch. The cement layer is indicated as layer Illa in Fig. 1. This cement may be of the type supplied by the Union Bay State Company of Cambridge, Massachusetts, identified as N-521. which is a chlorobutadiene polymer with a suitable accelerator, although other types of cement with a rubber or neoprene base may be utilized. Materials of this nature are referred to herein as rubber-like materials. The cement layer, as discussed later, serves to complete a bond between the faces of the crystal elements and the solid plastic material of the encasing block 9. The foil ends are wrapped tightly around and soldered to short conductive wires 26.

A glass mold is now prepared in the shape shown in Fig. 3 such as by cementing a flat glass plate 21 to the periphery of one end of a hollow glass cylinder 28. Into the bottom of the vessel thus formed is poured a liquid plastic to a depth of about A inch which is allowed to solidify to form a block 29 in the bottom of the vessel.

The liquid plastic which is poured into the vessel may be that sold under the trade name Selectron" by the Pittsburgh Plate Glass Company identified as No. 5003 which comprises about 70% polystyrene with about 30% methacrylate resin. To this material is added a suitable quantity of catalyst such as tertiary butyl diperphthalate dissolved in styrene monomer. Such material when maintained at a temperature of approximately 60 C. for about /2 hour will be found to be polymerized into a hard amber colored plastic material, which will be electrically insulating and capable of transmitting sound or supersonic energy.

While the plastic is still liquid, it is desirable to place the mold under a bell jar, which is then partially evacuated to remove any bubbles of entrapped air which would interfere with supersonic transmission in the use of the completed device. After hardening of the plastic block 29, an additional inch of the same material in a liquid state is poured into the form and the structure shown in Fig. 2 is pressecbwith the rubber cement coated face downward, into the liquid plastic until it begins to make contact with the hardened block. Additional liquid plastic is poured in up to about the level indicated by dashed line 30 so as just to cover the structure shown in Fig. 2, and again entrapped air bubbles are removed by application of a vacuum to the surface of the liquid plastic.

After polymerization of the plastic to the depth 30, conductors 3 and l are soldered or otherwise connected to the ends of conductors 28 which protrude from the now solid block of plastic encasing the crystal element structure. More of the liquid plastic may now be poured in to the level 3|, shown in Fig. 4, and allowed to polymerize.

the level of the top of the mold to fill the remaining space in the mold completely encasing not only the crystal element structure but conductors 3 and 4, coil 5, and the end of the concentric cable.

It may be found desirable to omit the layer of plastic between levels 30 and 3| shown in Fig. 4, placing the coil 5 on the top of the layer at level 30 and completely filling the mold with the next layer covering the coil and cable end.

After the plastic has hardened up to the level of the top of the mold, the mold may be brokenaway yielding the complete transducer shown in Fig. 1.

The addition of the liquid plastic in layers'as described has been found necessary since the polymerization is an exothermic chemical reaction and the heat thus generated may melt or damage through thermal expansion the ammonium di-hydrogen orthophosphate crystals if too great av volume of the material is allowed to polymerize at one time. A lower temperature is necessary in the case of Rochelle salt crystals, and a greater number of thinner layers will probably be found desirable.

The plastic material selected for encasing the crystal unit should be one which, like that described above, contracts slightly on hardening so that the crystal elements are tightly bound with the block, and so that the layer or film of rubber, or rubber-like, cementitious material l0a across the operative faces of the crystals is compressed between the plastic casing and the crystalfaces. This compression improves the vibration transmission through the layer of cement and tends to maintain the bonds between the cement and plastic and between the cement and crystal faces. It is desirable to use a plastic material which will not adhere to glass to permit removal of the finished transducer from the glass mold without destroying the mold. If it is found that the plastic material polymerizes too rapidly, generating too much heat, an inhibitor such as a saturated solution of quinone dissolved in benzene may be added to the liquid.

It has been found that best results are obtained if the solid plastic material is cured for several hours at about 60 C. to somewhat compensate for the slow polymerization which may be necessary to prevent heat damage to the crystal elements.

It may be desirable in an underwater transducer to form the block with a face Ill other than flat as shown, such as convex or concave, to utilize the plastic material as a lens between the crystal faces and the outside medium. The hard plastic material is adapted for such use as a lens because the velocity of sound in the plastic differs from the velocity in water.

It will be apparent that vibration of the elements of a transmitting transducer may be produced by electrostatic or electromagnetic coupling from devices outside the plastic block, rather than by electrical connections extending into the block to electrodes attached to the eleagainst ments as shown. In a receiver transducer, pickup of the energy produced by the vibrations imposed on the elements may be similarly arranged.

Thus, various modifications falling within the scope of the invention will occur to those skilled in the art, and it will be apparent that the methods of construction described as applicable to a particular type of transducer device may be applied to other devices wherein the total enclosure of a vibratile element is desired.

While we have shown only certain preferred embodiments of our invention by way of illustration, many modifications will occur to those skilled in the art and we therefore wish to have it understood that we intend, in the appended claims, to cover all such modifications as fall within the true spirit and scope of our invention;

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

l. A transducer comprising a vibratile element having an operative vibratory face, a thin layer of rubber-like cement bonded to said face, a solid plastic block completely embedding said element and in contact with said layer, and a soft mass interposed between said element and said plastic of sufficient quantity to minimize damping of said element.

2. A moisture-proof underwater transducer comprising a vibratile element with an operative face, a thin coating of a cement,,of rubber-like base on said face, a soft material mass enclosing said element except for said face, and a solid plastic block completely enclosing said element and said members and arranged to permit vibratory energy transmission through said coating and through said plastic block between said face and the medium in external contact with said block.

3. A transducer comprising a plurality of crystal elements each having an operative face, said operative faces being disposed in a plane, a coating of rubber-like cementitious material across said faces. spacers of soft material arranged all of the remaining faces of each of said elements, a solid hard plastic block completely encasing said elements and compressing said cementitious coating against the faces of said elements whereby said elements are hermetically sealed within said block, and conducting means sealed into said block comprising electrodes for said crystal elements.

4. A transducer comprising a vibratile element with at least two vibratory faces, a layer of rubher-like material bonded to one of said faces, a

casing for said element compressing said layer against said one of said faces and forming a vibration-transmitting path from said one of said faces, and isolating means interposed between the other of said faces and said casing.

FRANK J. ALOIS. RICHARD W. SAMSEL.

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

UNITED STATES PATENTS

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