US 2795709 A
Description (OCR text may contain errors)
June 11, 1957 L. w. CAMP ELECTROPLATED CERAMIC amcs Filed Dec. 21, 1953 INVENTOR. Leon 14 Camp ATTORNEY ELECTROPLATED CERAMIC RINGS Application December 21,1953, Serial No. 399,202
1 Claim. 01. 3109.6)
This invention relates to electroacoustic vibrators of ceramic materials, such as barium titanate, having the property of changing their dimensions in response to an applied potential, and vice versa, the potential being applied to or derived from conductive metallic electrodes on the surface of the ceramic. It is particularly adapted to vibrators of annular or ring shape intended to vibrate radially at their resonant frequency, which is a function of the dimensions of the ring and the mechanical characteristics of the material. Such vibrators are useful as transducers for producing compression waves in fluids in response to electrical oscillations, and vice versa, for producing electrical oscillations in response to compression waves in fluids. I
An object of the invention is to provide a practicable method of varying the natural resonant frequency of a ceramic vibrator. I
Another object is to increase the Q of a ceramic vibrator.
Another object is to make feasible the construction of large ceramic vibrators consisting of a plurality ofarcuate sections joined together.
Still another object is to increase the mechanical strength and corrosion resistance of ceramic vibrators.
Other more specific objects and features of the invention will appear from the description to follow.
Heretofore it has been the practice to form conductive electrodes on the inner and/or outer surfaces of ceramic vibrators by coating the ceramic surface with a silver paint and firing it. This produces a coating of metallic silver that is fused to the ceramic and has the necessary electrical conductivity, but is extremely thin and has substantially no effect on the mechanical properties of the vibrator.
In accordance with the present invention, 1 have discovered that in the case of radially vibratile ceramic rings, certain advantages are obtained by building up the thickness of the electrodes. This can be accomplishedby depositing additional metal on the silver electrodes by what I define as incremental microdeposition. -Su'ch deposition is most commonly effected by electroplating, although it can also be done in other ways, as by metal spraying. The essential characteristic of such deposition is that the metal is deposited in very small increments, whereby the total amount of metal added can be accurately controlled. At the same time, the process is such as to produce a strong, firmly adherent coating having essentially the strength attributes of the same metal as formed by other processes.
The chief advantages of electrodes produced in accordance with the invention are:
(l) The natural or resonant frequency of vibration of the ring is lowered in proportion to the amount of metal added, and, since it is easy to accurately control the amount, rings of a desired frequency can be easily and economically produced. As heretofore produced, a
are t T 'the invention.
batch of rings would vary considerably in frequency, and
there would be many rejects.
(2) A ceramic ring in accordance with the invention has a higher Q than those with thin silver electrodes.
(3) The invention enables construction of large ceramic rings consisting of a plurality of arcuate sections bonded together end to end because of the mechanical reinforcement supplied by the thick, tightly adherent electrodes. It is difficult and expensive to form large ceramic rings in one piece, and there are many rejects.
(4) The mechanical strength and'corrosio'n resistance of electrodes in accordance with the invention is greater than that of thin silver electrodes. This is particularly important in high power transducers 'where cavitation may result at the electrode surface.
A full understanding of the invention may be had from the following detailed description with reference to the drawing, in which: I I
Fig. 1 is a longitudinal sectional view of a simpletransducer incorporating a ceramic ring inaccordance with Fig. 2 is a cross section through 'the'ceramic ring of Fig. 1, drawn to a larger scale. p
Fig. 3 is a' sectional view in a plane perpendicular to the axis of a ring, showing a modified electrode structure.
Fig. 4 is a view similar to Fig. 3 but showing a multipiece ring in accordance with-the'invention.
may be backed by a solid member 16 of substantial size,
which may be made of some material, such as steel or aluminum, having good sound transmission characteristics and specific acoustic impedance much greater than that of water. The diameter of the backing member 16 may be substantially the same as that of the ring 13, and both are shown supported within the case 10 by a mass of some sound-insulating material 17, such as Corprene or air. cell rubber. The entire space between the front end of themember 16 and the rubber end cap or Window 11 is filled with some fluid, such as castor oil, which has substantially the same sound propagation characteristics as water. The purpose of providing the rubber window .and the filling of castor oil or the like is to protect the internal parts of the transducer from deleterious chemical action of the water in which the device is submerged.
'In some instances in which the internal parts are made resistant to the action of water, the window and the filling of castor oil may be omitted, acoustic connection between the transducer and the body of water in which it is submerged being then effected directly instead of through the castor oil and the rubber window.
The direct or primary mechanical motion produced in the ceramic ring 13 by potential between the electrodes 14 and 15 is radial, but there is a secondary resultant circumferential movement which determines the resonant frequency, it being approximately equal to the frequency corresponding to one wave length of sound in the material. The radial vibration of the outer face of the ring 13 is not utilized, it being insulated from the water by the material 17. The inner surface of the ring is the radiating and receiving face.
The structure so far described is old, the present invention relating primarily to the nature of the electrodes 14 and 15. As previously indicated, it has been the prior practice to form these electrodes by painting the inner per or other metal deposited thereon.
then firing the ring to leave a thin film of silver fused firmly to the ceramic. In accordance with present invention, this thin silver film is supplemented by an additional layer of metal added by incremental microdeposition, as previously indicated, the preferred, method of deposition being by electroplating. A suitable metal is copper, although any metal having desirable electrical and mechanical properties that lends itself to electroplating could be used.
Referring to Fig. 2, the electrodes 14 and 15, in accordance with the present invention, consist of the very thin film 22 of silver and the relatively thick layer 23 of cop- The section of Fig.2 is of. greatly exaggerated thickness to show the relative magnitudes of the silver film and the copper coating thereon. Actually,'the total thickness of the layers 14 and 15 is usually small compared to the total thickness of the ceramic ring 13. I
The process described may beused to produce ceramic rings of uniform resonant frequency by so choosing the size of the rings that, despite the warpageor shrinkage resulting from the firing operation, they will (before the addition of the electrodes) have a resonant frequency at least as high as the desired frequency. In actual manufacture some of the rings will be very slightly above the desired frequency, and othersmay be substantiallyabove it.- The resonant frequency of each ring is thenbrought down to the desired value by electroplating the proper amount of metal thereon. In practice, this is quite easy to do, because the amount of rnetal deposited by electroplating is a direct function of the time and the current, and by suitablyregulating the time and maintaining the current constant, or vice versa, any predetermined thickness of metal can be added. It is feasible to first measure the naturalfrequencies of a quantity of rings before plating and calculate the time that each ring must be placed in the plating bath to secure a deposit sufiiciently thick to bring thefr'equency down to the desired value.
' Where: the sole object is to vary the resonant frequency .of the ring, the deposition may be on either the inner or the outer electrode, or on both. It is usually simpler to plate both. This has the added advantage that. the additional thickness or metal on each electrode makes it mechanically stronger and more corrosion-resistant. This latter feature is particularly useful when the element is .to be exposed directly to the sea water or other liquid in which it is submerged.
It is not necessary that the deposited metal be continuous circumferentially. As shown in Fig. 3, it may be added in discontinuous sections to produce lumped loading, which is disclosed in my prior application, Serial No. 228,956, filed May 29, 1951. Lumped loading has the advantage of reducing the natural frequency to a greater extent than continuous loading. Obviously, the selective plating of certain areas of the silver electrodes can be readily accomplished by coating with a protective paint those portions that are not to be plated.
'Although the increase in mechanical strength afforded by the thicker electrodes produced in accordance with the invention may be useful on integral rings, it is particularly useful on rings that are formed from a plurality of arcuate sections bonded together at their ends, as shown in Fig. 4. There the ceramic ring 13 consists of a plurality of arcuate sections 30 bonded together at their ends, as indicated, at 31. Although methods are known of quite firmly bonding ceramic sections together, the bonds are usually not as strong as the material itself, and as previously constructed a multi-section ring is more fragile and subject to disruption than a single piece ring. However, when such a ring is provided with thick electrodes 14 and 15 in accordance with the present invention, the additional strength of the electrodes reinforces the ring and makes it amply strong for its intended purpose. As has been previously indicated, multisection construction of ceramic rings is often desirable when the rings are of large size, because of the difficulty of firing large pieces and the excessive distortion that often results.
The invention is also useful in connection with ceramic vibrators of the type shown in Fig. 5 in which the ring 32 has separate, circumferentially spaced electrodes alternate ones 40 of which are connected together and to one side of the circuit and the intervening electrodes 41 of which are connected together and to the other side of the circuit. This produces circumferentially extending electric fields in the core and is highly efiective. The electrodes can be on only the outside as shown, or they can be only on the inside, or on both the inside and outside.
' Although for the purpose of explaining the invention, a particular embodiment thereof has been shown and described, obvious modifications will occur to a person skilled in the art, and I do not desire to be limited to the exact details shown and described.
I claim: 7 A device of the type described comprising: a hollow cylindrical element consisting of a plurality of arcuate sections bonded together at their ends and having metalment and comprising a first thin layer of metal fused to said element and a second thicker layer of metal bonded to said first layer, and being'of sufficient thickness to have substantial mechanical tensile strength and constitute a hoop strongly opposing separation of said sections from each other in response to said peripheral tensile stresses.
References Cited in the file of this patent UNITED STATES PATENTS 2,497,666 Gravley Feb. 14, 1950 2,505,370 Sykes Apr. 25, 1950 2,540,412 Adler Feb. 6, 1951 2,618,698 Ianssen Nov. 18, 1952 2,645,727 Willard July 14, 1953 k I ll