High-frequency ultrasonic generators

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March 18, 1969 t. _x scarpa 3,433,461

HIGH-FREQUENCY ULTRASONIC GENERATORS

Filed May 22, 1967 . , „

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ATTORNEY

March 18, 1969

T. J. SCARPA

3,433,461

HIGH-FREQUENCY ULTRASONIC GENERATORS

Filed May 22, 1967

Sheet 2 of 3

FfG. 6

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ATTORNEY

March 18, 1969 τ. j scarpa 3,433,461

HIGH-FREQUENCY ULTRASONIC GENERATORS

Filed May 22, 1967 _{Sheet 3 of}

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ATTORNEY

United States Patent Office , ,. _M^3433461

Patented Mar. 18, 1969

3,433,461 HIGH-FREQUENCY ULTRASONIC GENERATORS Thomas J. Scarpa, Metuchen, N.J., assignor to Edison Instruments, Inc., Rahway, N.J., a corporation of New Jersey

Continuation-in-part of application Ser. No. 395,475, Sept. 10,1964. This application May 22,1967, Ser. No. 640,247

U.S. Cl. 259—1 15 Claims

Int. Cl. BOlf 13/08; B05b 17/06, 3/14

ABSTRACT OF THE DISCLOSURE

A composite generator designed for a thickness of substantially one wavelength at a desired frequency of thickness mode vibration, which both generates and focuses the ultrasonic beam. One type includes a piezoelectric bowl or trough of ceramic to the interior of which is bonded a supporting element of matching curvature comprising metal or the like. A further variation of this type is a ceramic piezoelectric tube to the interior of which is bonded a supporting tube of metal or the like. An alternative focusing type includes a supporting disk of which the external face is formed to include slight variations in topography which serve to focus the generated beam by differential refraction. In an ultrasonic nebulizer system, the refracting element is included in the bottom of a vessel containing liquid to the nebulized, which is disposed in the path of the ultrasonic beam.

This is a continuation-in-part of my application Ser. No. 395,475, filed Sept. 10, 1964, which issued on May 23, 1967, as Patent No. 3,321,189. This relates in general to the generation of high-frequency mechanical vibrations; 35 and more particularly, to the mounting of high-frequency piezoelectric transducers.

Piezoelectric crystal wafers, whether homogeneous or of ceramic material, which are designed to vibrate in a principal thickness mode at frequencies of a megacycle 4° or more, are generally thin and frangible. In the prior art, it has been customary to mount such wafers on backing or loading elements, which, in turn, are mounted; in supporting frames for handling purposes. At high frequencies, it has been found that these supporting frames 45 or mounting devices operate to substantially damp or clamp the vibratory motion, so that the maximum power which can be generated with a piezoelectric generator of this type at frequencies of the order of a megacycle or more is of the order of 50 watts. 50

For many types of applications which make use of highfrequency compressional waves for stirring, mixing, or atomizing functions, a more powerful piezoelectric generator of rugged construction, operative to produce a concentrated beam in the megacycle range would be very desirable.

Accordingly, a principal object of the present invention is to provide a high-frequency ultrasonic generator of increased efficiency and power. A more particular object of the invention is to provide a holder for high-frequency piezoelectric driving elements which imposes minimum damping action on the vibratory motion. Another object of the invention is to provide a high-powered piezoelectric vibrator for frequencies in the megacycle range which is adapted to be grasped and applied manually for a variety of ultrasonic mixing, stirring, and atomizing operations. Still another object is to provide a powerful highfrequency generator of rugged construction in which the ultrasonic beam is focused or concentrated in a given area or along a given line.

These and other objects are attained in the combina2 tion of the present invention, which comprises a unique mounting device for piezoelectric crystal elements, designed to reduce to a minimum the clamping action of the holder assemblage on the generated vibrations, and g to generally increase the facility with which such a generator is used. In accordance with the present invention, a piezoelectric crystalline wafer, a half-wavelength thick in the vibrating frequency, and poled to vibrate in a thickness mode, is mounted concentrically on the underside ₁₀ of a backing or supporting disk, also a half-wavelength thick. On the upper and lower surfaces of the disk, slightly removed from the periphery of the piezoelectric wafer, is a pair of matching annular indentations, the flat inner surfaces of which form between them an annular ring a few thousandths of an inch thick, which makes annular contact with the central, wafer mounting portion, of the disk at its quarter-wave or nodal plane in a thickness direction, where the vibratory motion is at a minimum. The backing element, with the crystalline wafer rigidly 20 mounted on its underside, forms a vibratory unit substantially one wavelength thick. The disk 2 is mounted at its edges, beyond the annular indentations, on a cylindrical cup, so that the vibratory crystal element projects into the cavity. An elbow connector which is screwed to an 25 opening in the curved wall of the cylindrical cup, serves for connection to an elongated tube which acts both as a rigid handle and as a conduit for electrical connections to the piezoelectric wafer.

Inasmuch as the piezoelectric wafer and the supporting portion of the half-wave backing disk are suspended only at a nodal plane in the vibratory system, they are substantially mechanically isolated from the supporting frame, the vibratory motion generated by the unit being virtually undamped. It has been found, using a transducer assemblage in accordance with the present invention, that it is possible to generate vibrations in the megacycle range up to a maximum power of about 250 watts.

A particular feature of the device of the present invention is the facility with which it can be grasped, transported, and applied manually. For example, the shallow cylindrical airtight cup in which the piezoelectric driving element is suspended may be of the order of two inches in outer diameter, and about three-quarters of an inch thick, with a connecting conduit or handle, say, six inches long. Such a device readily fits into an ordinary drinking glass or cocktail shaker where it is especially adapted to apply high-powered ultrasonic waves in a very highfrequency range to the mixing and stirring of beverages and the like; or, it readily fits into a beaker or other container for generating a beam for ultrasonic cleaning purposes or for emulsifying^ mixing, dissolving, or atomizing components for medical or chemical uses.

In a modified form of the invention, the high-frequency ultrasonic beam is focused to a substantially restricted area. This may be accomplished in a number of different ways. In accordance with one such modification, the piezoelectrically active element may take the form of a curved ceramic focusing bowl or trough which may be, for example, semispherical, parabolic, or even cylindrical in ⁶⁰ shape. The inner surface of the piezoelectric element is bonded to the outer surface of a rigid mounting or facing element of matching curvalinear shape. The bowl or trough-shaped assemblage, which is constructed to have a combined thickness of one or more wavelengths for ⁰⁵ vibration in a principal resonant-thickness mode, may, in preferred form, be mounted by means of a nodal contact member which is integrally connected in the quarterwave area of the mounting or facing element. In the case ₇₀ of a semispherical or paraboloidal embodiment, this may take the form of a thin-walled contacting ring; whereas in the case of the trough, the contacts may assume the

3,433.461 form of two thin linear members connected along the nodal line on each of the edges of the mounting member, or alternatively, protruding outwardly from the ends thereof. The composite vibratory assemblage may be suspended with the curved end protruding inwardly to an air chamber. In the case of the semisphere or paraboloid body, this may take the form of a rigid cylindrical chamber, whereas in the case of the semicylindrical generating element, the housing may be in the form of a parallepiped.

It is contemplated that the focusing generator of curved cross-section may assume a number of alternative forms. In one embodiment, the active element is a ceramic pipe, to the inner surface of which is bonded the outer surface of a metal pipe of slightly smaller cross-section. The active element is treated to vibrate in a principal resonantthickness mode, electrode contacts being applied to the inner and outer curved surfaces thereof. The composite ceramic and metal pipe may be mounted by means of a thin nodal collar protruding from the nodal area at each of the ends of the metal pipe. The nodal collar is formed into an outwardly directed flange which is fastened to the edge of an enclosing pipe of substantially larger diameter which may serve as the housing for the ceramic pipe. Liquid to be ultrasonically processed is passed in a stream through the inner pipe.

Although each of the preferred embodiments disclosed by way of illustration includes a nodal contacting ring or equivalent member, it will be apparent to those skilled in the art that in vibrating systems within the megacycle range of frequencies omission of this element impairs the efficiency of the system, but does not render it inoperative.

In accordance with another modified form of the invention, the high-frequency ultrasonic beam is focused by fashioning the mounting plate in the form of an ultrasonic lens. This may be accomplished in a number of different ways, such as, for example, chiseling discontinuities into the outer face of the mounting element in a Fresneltype pattern to cause refraction of the beam in accordance with Snell’s law; or changing the shape of the mounting element to make it slightly concave. It has been found that an angle of refraction of only a few degrees greatly increases the intensity of the vibrational activity in the focal area. Unlike applications for focusing light, it is unnecessary, in accordance with the present invention, to obtain a sharp focus, or even to provide a lens having rotational symmetry in order to produce a useful effect in the focal area, which in the present case results in a substantial increase in the intensity of the generated beam. In each case, in the preferred embodiments, a thin nodal ring integral with the mounting element, may serve as a contacting member for supporting the generator in a closed housing, in the manner of the previously described embodiments.

An application to which the ultrasonic generator of the present invention, and particularly the modified focally directed embodiments are adapted, is the ultrasonic generation of fog for thereapeutic and research applications.

It has been found that when an actively vibrating transducer of the form of the present invention is placed in a container of liquid, it generates a fog comprising substantially uniform droplets, the cross sectional dimension of which is a function of the frequency of the vibrations, the quantity of fog being a function of the power of the generator. The focally-modified transducer, as disclosed herein, has the particular feature of concentrating the generated ultrasonic energy in a focal area or along a focal line, in such a manner that the vibrational intensity of the generated beam is substantially increased. Hence, such a focally-modified form of the invention is substantially more effective than nonfocal embodiments for fog generation and most other applications. In accordance with one modified form of the invention, a refractive lens type element is incorporated in the base of a container of liquid to be nebulized, which is disposed in the path of the generated beam.

Many other objects, features, and advantages will occur to those skilled in the art after a study of the specifications g hereinafter, together with the attached drawings, in which:

FIG. 1 is a showing, in perspective of one embodiment of the piezoelectric driving unit of the present invention;

FIG. 2 is a cross sectional view of the assemblage of FIG. 1, taken along the line 2—2;

i₀ FIGS. 3 and 4 are slight modifications of the combination shown in FIGS. 1 and 2;

FIG. 5 is a perspective showing of the upper plate of FIG. 1, cut away to show a portion of the crystal wafer mounted on its underside;

FIG. 6 is a sectional showing of a modified spheroidal focusing embodiment of the ultrasonic generator of the present invention;

FIG. 7 shows in perspective, partially broken away, a further modification of the focusing high-frequency 20 ultrasonic transducer of FIG. 6 in which the active and mounting elements form a semicylinder;

FIG. 8 of the drawings shows in longitudinal section a further modification of the ultrasonic generator of the present invention in which the active element takes the 25 form of a ceramic pipe, to the inner surface of which is bonded a metal pipe to serve as a mounting element; the composite comprising the ceramic and inner metal pipe being mounted in a larger pipe which serves as a housing;

FIG. 9 is a modification of the generator assemblage of FIG. 1 in which the metal supporting element is slightly concave;

FIG. 10 shows a further modification of the supporting element of FIG. 9 in which the outer face includes an35 gular discontinuities in a Fresnel pattern;

FIG. 11 is an illustration of the theory relating to the angular indentations of FIG. 10;

FIG. 12 shows a nebulizing system wherein the generating element disposed at the base of a tank of water di⁴⁰ rects an ultrasonic beam into a second container of liquid, the bottom of which contains a Fresnel-type lens; and FIG. 13 shows a modification of FIG. 12 in which the second container, including the Fresnel-type lens, rests directly on the ultrasonic generator.

Referring in detail to the drawings, FIG. 1 is substantially a showing, in perspective, of the device of the present invention, as actually constructed.

In the cross sectional showing of FIG. 2, the piezoelectric crystal element 1 in the present illustrative em50 bodiment may comprise any of the piezoelectric crystal elements well-known in the art which are cut or formed and poled to vibrate in a thickness direction such as, for example, a ceramic wafer of lead zirconate titanate or alternatively, a wafer of modified barium titanate with a 55 cobalt additive, known by the trade name “Channelite” which is manufactured by Channel Industries of California. Prior to mounting, the piezoelectric wafer 1 is treated in a manner well-known in the art by applying an electrical potential across the electrode contacts, through the 60 thickness thereof, while heating it up to above the Curie temperature and letting it cool again to room temperature, in order to pole the wafer for vibration in a principal thickness mode. It is then aged in a manner well-known in the art. The wafer 1 is designed to be substantially a 65 half-wavelength thick in the principal vibrating frequency. In the present illustrative embodiment, the wafer 1 has a diameter of one and one-half inches and a thickness of one-eighth of an inch for a resonant frequency of substantially one megacycle, although it will be under70 stood that this is varied in accordance with the desired resonant frequency.

Crystal element 1 is concentrically mounted on the underside of a backing disk 2 which may comprise any material having a good coefficient of conductivity for sonic waves such as, for example, aluminum, stainless steel,

3,433,461 brass, the alloy known by the trade name “Monel,” or alternatively any nonmetallic solid having similar acoustic properties (such as those known by the trade name “Bakelite”), and “nylon.” In the present illustrative embodiment, the disk 2 which is also a half-wavelength thick in _Jthe principal vibrating frequency of the piezoelectric wafer ⁵1, is aluminum having a two inch diameter and a thickness of three-sixteenths of an inch. To accommodate the crystalline wafer 1, disk 2 has a slight concentric recess of several thousandths of an inch on its underside. Crystal wafer 1 is fitted into this recess and bonded thereto by ¹⁰means of a conventional epoxy bonding material or other similar bonding material which is cured in a manner wellknown in the art. Prior to bonding, the mating surface of crystal element 1 is cleaned ultrasonically by exposing ^_git to ultrasonic action in a container of isopropyl alcohol or acetone, or any similar solvent characterized by rapid evaporation, at say, 40 kilocycles, for about ten minutes. Similarly, backing element 2, after machining, is exposed to ultrasonic cleaning in a bath of tepid water, at a fre- ₂₀quency within a similar frequency range. After ultrasonic cleaning the crystal mating surface of disk 2 may be etched for improved performance, in a ten to twenty percent solution of hydrochloric acid, or a similar etchant, until it is sufficiently free of grease and oil on the surface, _2gso that water under an open tap will wet the surface completely. The vibratory structure will be better understood by reference to the perspective showing in FIG. 5, the cutaway portion of which indicates the manner in which the ceramic wafer 1 is fitted into the underside of the disk 2. ₃₀Thus, it will be apparent that the combination of the wafer 1 with the backing element 2 forms a vibrating system substantially one wavelength thick, in the direction of propagation of the vibrations, which is the thickness direction of the wafer and backing disk. _3g

For the purposes of the present invention, the bonding agent which is applied may or may not be conductive. In the former case the conductive epoxy forms the upper electrode coating of the crystal wafer 1. Alternatively, there is a thin conducting layer evaporated on, or other- 40 wise applied to the upper face. For convenience, a contact 5a which is connected to the upper face may be brought around to the underside, where it is electrically insulated from the electrode contact 5b which is connected to an electrode coating of silver paste or the like, a few'thousandths of an inch thick which is evaporated or otherwise ⁴³applied to the under surface of crystalline wafer 1.

A principal feature of the present invention is the fact that between the edge of the mounting disk 2 and the periphery of the attached piezoelectric element 1 are machined or otherwise formed a pair of annular grooves 3a ⁵⁰and 3b of rectangular cross-section on the upper and lower faces, each substantially .0575 inch deep and threethirty-seconds of an inch wide. Grooves 3a and 3b are matched on the upper and lower surfaces so that they __ form between them a thin annular supporting ring 4 which ^&amp;Sis approximately .01 inch thick and three-thirty-seconds of an inch across. This is shown in perspective in the cutaway portion of FIG. 5. The annular ring 4 serves to contact the central 'portion of the disk 2 precisely at a nodal plane in the vibratory motion where the longitudinal ^Wdisplacement is practically at a minimum. Extending outwardly from the matching grooves 3a and 3b is a flanged projection or continuation of the disk 2 which is approximately one-eighth of an inch wide in the radial direction and which serves for mounting the vibrating system in a ^b&amp;holder.

The supporting disk 2 is mounted in the manner of a closure on a shallow cylindrical cup which is formed from an aluminum tube 7 which has an outer diameter of two γ·θ and one-quarter inches, an inner diameter of one and three-quarter inches, and a depth of five-eighths of an inch. The upper portion of the tube 7 includes an inner shoulder of flanged portion 6 formed by stepping back the inner radius approximately one-eighth of an inch, to a 7g depth of three-sixteenths of an inch from the top. This flange or shoulder 6 serves as a peripheral support for the disk 2, where the latter may be sealed in place with any of the well-known bonding agents having good acoustic conducting properties, such as an epoxy known by the trade name “Epon VIII” manufactured by the Shell Chemical Company.

The base plate 9 is formed, for example, of aluminum, has an over-all diameter of two and one-quarter inches and is one-fourth of an inch thick. In the particular embodiment under description, base plate 9 is bolted or screwed onto the end of the tube 7 by means of the screws 11. In order to form a liquid-tight seal, a gasket 10 may be interposed between the contacting surfaces of the tube 7 and the base plate 9. The gasket 10 is cemented or bonded in a liquid-tight seal with the contacting surfaces by any of the cements well-known for such purposes.

FIGS. 3 and 4 illustrate combinations in which alternative methods are used for assembling the base plate 9 and the tube portion 7.

In the embodiment of FIG. 3, for example, the base plate 9', which has an outer diameter of two and one-half inches , as in the previous embodiment, is three-sixteenths of an inch thick, except for a slightly raised circular portion one and three-quarter inches in diameter, and from .0.1 to .015 inch thick, on its upper face. This provides a slight shoulder against which is mounted the lower annular surface of the tube 7. The latter is bonded with an adhesive of good acoustic conducting properties, such as an epoxy resin, which may, for example, be the bonding agent known by the trade name “Epon VIII” described above. A liquid-tight seal is thus formed. It will be noted that no screws are used externally in this embodiment.

In the embodiment of FIG. 4, the base plate 9 is formed so that the raised central portion is raised substantially higher with respect to the annular end portion than that of FIG. 3, and is screw-threaded, fitting into matching screw threads on the lower inside surface of the tube 7. Aside from the modifications described, these embodiments are substantially similar to the embodiment of FIG. 2.

The cup formed from tube 7 in the manner previously described, has an inner air-filled chamber 8 which in the embodiment of FIG. 2 has an inner diameter of one and three-quarter inches and a depth of seven-sixteenths of an inch within which chamber the inner face of the crystal wafer 1 is disposed to vibrate freely. A screw opening 13 in one of the curved walls of the tube 7 accommodates a hollow screw connector feeding into an elbow connector 15 of steel or the like which has an outer diameter of three-fourths of an inch and is approximately one inch from top to bottom. In addition to being screwed into place, elbow connector 15 may also be sealed with a liquidtight seal of epoxy resin of the type previously described or some other of the sealing compounds well-known in the art. The upper end of the elbow 15 has a hollow screw fitting 14 approximately five-sixteenths of an inch in diameter, the lower end of which connects with the inner passage from chamber 8. A steel tube 16, which has an outer diameter of three-eighths of an inch, an inner diameter of three-sixteenths of an inch, and in the present illustrative embodiment is six inches long, is screwed into the hollow fitting 14 of elbow 15. The tube 16 terminates in a shielded coaxial cable connector 18, such as one of the types manufactured by the American Phenolic Corporation, and known by the trade name “Amphenol” connectors. This is sealed to the outer surface of the rod 16 by means of an epoxy or other bonding agent of one of the types heretofore described.

As to the electrical connections for driving the device of the present invention to vibrate, the contact 5a to the uper electrode coating of wafer 1 is grounded by connection to a screw 12 on the inner surface of base plate

9. Contact to electrode 5b on the lower surface of the crystal element 1 is made by means of a lead wire 20

3,433,461 which passes through the hollow fitting 14 in the elbow connector 15, sealed in a liquid-tight seal with a bonding compound such as epoxy resin or the like. It then passes through the conduit 17 comprising the metal tube 16 and the coaxial connector 18, having a flange 19, to an outer terminal which may be connected to a source (not shown) of alternating current high-frequency oscillations, of any of the types well-known in the art.

To make the device liquid-tight, and to improve its appearance and wearing qualities, the top may be painted with an epoxy paint. The lower portions may be similarly painted with epoxy paint to seal up the openings.

It will be seen from the foregoing description that the device of the present invention includes a piezoelectric element in which the damping action of the supporting frame is substantially minimized. This permits the system, including the driving element and the supporting frame, to vibrate at frequencies exceeding a megacycle, generating vibrations at an output power of up to, for example, 250 watts, a level of power heretofore unobtainable with devices of this type.

It will be apparent to those skilled in the art that certain improvements in the operation of an ultrasonic generator in accordance with the present invention may be obtained by providing means for more specifically focusing the generated high-frequency ultrasonic beam in a manner which is peculiar to the megacycle range of frequencies because of the short wavelengths.

Referring to FIG. 6 of the drawings, there is shown in section a focusing embodiment of the high-frequency ultrasonic generator of the present invention comprising a semispherical bowl 21. The latter may comprise any of the well-known piezoelectric materials formed and poled to vibrate in a principal thickness mode of vibration. A suitable material for present purposes is a ceramic material known as lead zirconate titanate which has been formed into a bowl 0.078 inch thick, dimensioned externally three-quarters of an inch deep and one and one-half inches across the top. Prior to mounting, the piezoelectric ceramic bowl 21 has been poled to vibrate in a principal thickness mode of vibration by treatment in a manner well-known in the art, and described in detail with reference to the ceramic disk 1 of FIGS. 1, et seq., of the drawings, in which electrical potential is applied through the thickness of the element while it is heated up to the Curie temperature and allowed to cool to room temperature, and to properly age. Bowl 21 has been designed to have a thickness of approximately onehalf wavelength in the principal vibrating frequency desired for the generator, which for the present application is approximately one megacycle.

Prior to mounting, the ceramic bowl 21 is carefully machined to the precise desired shape and ultrasonically cleaned and treated in the manner described in detail with reference to the ceramic disk 1 of FIG. 1. A suitable thin conductive coating, such as, for example, silver paste, is then applied by evaporation or in any manner wellknown in the art, to form electrode layers 22 and 23, a few mils thick, on the inner and outer curved surfaces of the bowl 21.

To the inner surface of bowl 21 is then bonded a second bowl 24 to serve as a mounting or facing element for the vibrations generator. Bowl 24 may comprise any material having a good coefficient of conductivity for sonic waves. Preferred for this purpose are the metals titanium, aluminum, or the alloy know by the trade name “Monel” (International Nickel Company). Other suitable materials are stainless steel, brass or certain nonmettalic materials having similar acoustic properties, such as, for example, polystyrene. In the present embodiment the bowl 24, which is dimensioned to closely fit to the inner surface of the bowl 21, comprises aluminum, 0.100 inch thick, having over-all dimensions of eleven-sixteenths of an inch deep and one and five-sixteenths of an inch across the top.

The inner surface of the bowl 21 is bonded to the outer surface of the bowl 24 by any conventional bonding material, such as one of the epoxies well-known in the art, which will be applied and cured in a manner well-known in the art to form a rigid bond of good acoustic conductance. Prior to bonding, the mating surface of bowl 24 is ultrasonically cleaned and treated in the manner described with reference to disk 2 of the structure of FIG. 1, to assure that it is free from grease and other contaminants. The composite structure including the bowls 21 and 24 bonded together is designed to have a combined thickness of approximately one wavelength in the desired thickness mode frequency of the system, the direction of propagation being inward in a substantially radial direction from any point along the inner surface of the bowl 24.

Formed integrally with and protruding directly Dpward from the outer edge of the inner mounting bowl 24, along a circle which is formed by the loci of quarterwave nodal points thereon, is a thin-walled cylinder 26 whose walls in the present embodiment are about onethirty-second of an inch thick. This serves as a nodal isolating ring for supporting the composite vibration generator 21-24 in a supporting frame. In the presently described embodiment of the nodal ring extends oneeighth of an inch upward. However, it need not be so limited in vertical extent, nor need it necessarily be th© particular shape shown in the illustrative embodiments; but can extend to any height desired and even be flanged at its outer end if need be, for mounting purposes. In accordance with a further alternative, nodal ring 26 can be omitted altogether, with some reduction in the efficiency of the generator.

In the present embodiment the nodal isolating ring 26 is integrally connected at its upper end to the downwardly directed flange 27α of an annular closure 27 of L-shaped cross-section, which is one-eighth of an inch thick. The flange 27α is one-quarter of an inch in vertical extent and fifteen-thirty-seconds of an inch in horizontal extent. Preferably the annular closure 27, the nodal insulating ring 26, and the mounting bowl 24 are formed from a single piece of material, which in the present illustrative embodiment is aluminum. However, it will be understood that this element can be formed out of any of the materials having good coefficients of acoustic conductivity, including those previously mentioned with reference to bowl 24.

The annular closure 27 is mounted in a cup-shaped supporting chamber. In the present embodiment, such a chamber may be formed from a hollow cylinder 28, which is two and one-quarter inches in outer diameter and one and seven-eighths inches in inner diameter. The inner edge of cylinder 28 is cut back one-sixteenth of an inch in from its inner edge, to a depth of one-eighth of an inch below the top and one-eighth of an inch above the bottom, to form a pair of annular upper and lower recesses. The annular closure 27 is fitted into the upper recess; and a bottom disk 29, one-eighth of an inch thick and two inches in diameter, is fitted into the lower recess. Whereas in the present illustrative embodiment the enclosing chamber is aluminum, it will be appreciated that it may be formed of any other rigid material, metal or nonmetal.

In order to form liquid-tight seals at each of the junctions, the gaskets 31 and 32 are respectively interposed between the closure 27 and the upper recess of cylinder 28; and between bottom disk 29 and the lower recess thereof. Each of the gaskets 31 and 32 is bonded to the contacting surfaces in a liquid-tight seal by any of the cements well-known for such purposes, so as to form a completely leak proof chamber.

A contact 34 fastens an electrically conducting lead 35 to the outer electrode coating 22. Lead 35 is passed through the cylindrical wall 28 by means of a liquid-tight seal 36 to a conventional generator of high-frequency electrical oscillations (not shown), which is adapted to

3,433 9 apply up to about 250 watts to drive the composite transducer comprising ceramic bowl 21 and mounting element 24. At the maximum applied wattage the voltage across electrodes 22 and 23 will be between 100 and 200 volts. It will be appreciated that the metal cladding provided „ by element 24 permits the use of much higher driving ^δpowers than in prior art devices, since it provides more heat dissipation. The housing cylinder 28 is connected to ground through contact 33, thereby grounding electrode 23, which is connected internally to housing 28 through contact 25α.

It will be apparent that whereas the connecting lead 35 of FIG. 6 is passed through a liquid-tight seal in the lower part of the case, in an alternative embodiment, lead 35 could be designed to pass out of the enclosing cham- _lgber through a handle, such as the handle 16 of FIG. 1. Moreover, the handle could be shaped in accordance with a desired application. For example, it could be made in the form of a right angle to enable the driving assemblage to be more readily inserted in a beaker or container for ₂q stirring and cavitation operations.

FIG. 7 of the drawings shows another modification of a focusing high-frequency transducer in accordance with the present invention, which is similar to the device disclosed in FIG. 6, except for the fact that the active ele- ₂g ment is semicylindrical instead of semispherical, forming a trough-shaped generator producing a high intensity ultrasonic beam which is focused along a plane which bisects the trough along its longitudinal axis.

This comprises a piezoelectrically active element 41, ₃θ similar in composition and treatment to the element 21 of FIG. 6, having a thickness which is substantially a half-wavelength in the resonant-thickness mode in which it is poled to vibrate. Assuming this to be a ceramic composition of lead zirconate titanate, as in the case of the 35 previously described embodiment, element 41 is 0.078 inch thick, has a radius of curvature of five inches, and may measure from one to twelve inches along its outer circumference. It will be understood, however, that the radius of curvature of element 41 is not critical, and de- 40 pends only on the degree of focusing desired. la fact, a useful combination could be formed of the type shown in FIG. 7, in which the generating element 41 is flat. Active element 41 has electrode coatings 42 and 43 applied to the opposing curved surfaces in the manner _4gpreviously described.

To the inner surface of the element 41, a mounting or facing element 44, of a material similar to that described with reference to element 24 in a previous embodiment, is treated and bonded in a manner previously described ₅θ by means of a bond 45 of epoxy of the like. In the present illustration, the mounting or facing element 44 is aluminum, 0.100 inch thick and shaped to coincide on its outer curved surface with the inner curved surface of element 41 with which it is substantially coextensive. The outer _ggends of the mounting or facing element 44 have integrally formed, at a central or nodal area of each, a thin rib or contacting member 46, which is one-thirty-second of an inch thick, and which may, for example, extend oneeighth inch in a circumferential direction, and which may extend longitudinally to the full extent of the generator element 41. However, the width and shaping of the nodal ribs 46 may be varied, depending on the requirements of the specific embodiment. In fact, assuming operation to be in the megacycle range of frequencies, ribs 46 may be g_gomitted altogether, with an impairment in the efficiency of the generator, which would, nevertheless, be operative.

Assuming the ribs 46 are retained, as shown in FIG. 7, they are integrally connected on their upper long edges to a pair of substantially rectangular longitudinally ex- tending parallel members 47α and 47b, which, with elements 44 and 46, form the upper face or closure of the liquid-tight generator chamber. The rest of the chamber may, for example, have the shape of a parallelpiped, including side walls 48α, 48b, and end walls 50α, 50b, and 75 ,461 a bottom wall 49, all of these elements being fitted together with the upper gasket members 51α, 51b, and the lower gasket members 52α, 52b, to form a liquid-tight leak proof chamber. Lead wire 55 from electrode contact 54 passes out through the liquid-tight seal 56 and is connected to a driving source of high-frequency electromagnetic oscillations which may be of any of the types well-known in the art, delivering power of several hundred watts to the ultrasonic vibrator. It will be understood with any of these devices that the driving power is limited by the heat dissipation in the device, which is a function, in each case, of the total area of the generator. Thus, a generator of substantially large area, which is liquid cooled can accommodate a larger power input than a device of small area which is not liquid cooled. The metal housing is connected to ground through contact 53. This grounds the upper electrode 43, assuming the latter has been connected to the metal chamber wall 48b through a contacting wire 43α.

It will be apparent to those skilled in the art that when the structure disclosed in FIG. 7 is driven to vibrate in a resonant-thickness mode at frequencies in the megacycle range, the generated sonic energy will be concentrated along an imaginary longitudinal plane passing down the center of the trough and normal to the surface.

• A.focusing transducer of modified form is disclosed in ¹ longitudinal section in FIG. 8, in which the active element 61 is formed from a piezoelectrically active ceramic tube, in_: this illustration having an outer diameter of one and one-half inches, and walls, for example, 0.078 inch thick. The ceramic tube 61, which may comprise any piezoelectric ceramic material known in the art, such as, for example, barium titanate zirconate, has inner and outer electrode coatings 62 and 63 applied to its curved surfaces to a thickness of a few thousandths of an inch. The piezoelectric generating tube 61 is processed and treated to vibrate in a principal resonant-thickness mode, substantially in the manner described with reference to the piezoelectric element 1 of FIG. 1. The length of the generating element 61 is four inches in the present embodiment; but, the length is not critical and may vary from a length which, for practical purposes, is infinite, to a narrow annular ring.

A mounting element 64 may take the form of a metal tube substantially coextensive with ceramic tube 61 and of material such as aluminum having walls 0.100 inch thick. Tube 64 has an outer diameter which closely fits into the inner diameter of tube 61, and is bonded to the inner electrode-coated surface of ceramic tube 61 by a conventional epoxy bond 65. The thickness of each of the elements of the composite tube is such that the combined wall thickness is approximately one wavelength in the frequency of the resonant-thickness mode in which it is designed to vibrate.

As in previously described cases, in a preferred form, nodal contact is made with the mounting element 64. In the present case, contact is made along a circle halfway through the thickness of the mounting element 64, which represents the loci of the quarter-wave points in the resonant-thickness vibrational pattern, by means of a thin protruding collar 66α, 66b at each of the ends, which in the present embodiment is one-thirty second of an inch thick. Although in the present embodiment the collars 66α, 66b protrude only about one-eighth of an inch beyond the respective pipe ends, it will be understood that they are not so limited in extent. At the ends the collars 66α, 66b are respectively integrally connected to and concentric with the respective tubular elements 67α, 67b, also of aluminum having wall thickness, of say, oneeighth of an inch and an inner and outer diameter substantially the same as those of mounting element 64. These respectively extend several inches outward, in a longitudinal direction from the collars 66α, 66b.

The whole tubular generating assemblage, including composite tubes 61, 64, is housed in a much larger tubu·.

3,433,461 ¹¹ lar enclosure, comprising the cylinder 68 which can be of metal or any other rigid material suitable for a support. In the present embodiment this has an outer diameter of three inches and an inner diameter of two and one-half inches, and a length of five inches, extending one-half inch beyond each of the ends of the ceramic tube 61, terminating at each of its ends in inwardly directed annular flanges 69a and 69b of substantially the same thickness as tube wall 68. The flanges 69a and 69b are constructed to fit snugly around the terminating cylinders 67a and 67b against the respective gaskets 71« and 71b, to form liquid-tight, leak proof seals at each of the ends.

The high voltage terminal 74 to the outer electrode coating 62 of the ceramic tube 61 is connected to lead 75 which passes through the wall of the housing tube 68 through a liquid-tight seal 76. The low voltage electrode coating 63 on the inner face of the ceramic tube 61 is connected to ground through a lead 63«, fastened to the inner wall of the annular housing flange 696, which is grounded through contact 73. The high voltage terminal is connected to a high-frequency electrical oscillator (not shown) which is constructed to drive the transducer at several hundred watts, as described previously.

The liquid to be ultrasonically treated may be passed in a stream through the center of the pipe 64, the vibrational energy being focused along the central axis of the assemblage. Where the liquid to be ultrasonically treated is of a corrosive nature, it has been found, in accordance with the present invention, that the inner tube 64 should be formed preferably of titanium, which is highly corrosion resistant.

In addition to the focusing means disclosed in FIGS. 6, 7 and 8, it has been found that focusing can also be achieved by shaping the outer face of the mounting or facing element to thereby cause refraction of the generated ultrasonic beam in accordance with Snell’s law.

FIG. 9 shows a modification of the generator element of FIGS. 1-5, in which the outer face of the metal mounting disk 2 has been slightly modified so that instead of being flat as shown in FIG. 1, it is slightly concave.

In the embodiment under discussion, the piezoelectrically active element 1' is a ceramic wafer one and onehalf inches in diameter and 0.078 inch thick, bonded concentrically on the under surface of an aluminum disk 2' which is two inches in diameter and has a thickness of substantially 0.100 inch.

I have found that when the outer face of element 2' is slightly modified in a manner shown in greatly exaggerated form in FIG. 9 by machining into its face a conical hollow having a nadir, the depth of which is about 10 mils below the plane formed by the upper edge of disk 2', the conical hollow configuration acts as a lens slightly concentrating the generated beam in the direction of propagation. The focusing action of the lens is determined by Snell’s law, sin f—Vg sin r V₂ (I) where z—angle of incidence of the beam, r=angle of refraction of the beam, Vi and V₂ are the respective velocities of sound in the materials at the interface, at the specified frequency, which in the case under consideration are aluminum and water, at a frequency of one megacycle.

It has been found, however, that in order to retain the form of the mounting element 2' at a thickness approximating one-half wavelength in the desired resonant-thickness frequency, the depth of the nadir of the conical hollow machined on its face should not exceed about onetenth of the initial thickness of the element. Otherwise, the. essential resonant character of the vibrator is destroyed.

I have further discovered that at ultrasonic frequencies within the megacycle range, certain optical principles relating to refracting lenses, such as the principles taught by Fresnel, can be adapted to focusing and increasing the intensity of the generated sound beam in the direction of propagation (see, Encyclopaedia Britannica, 1954 edition, volume 14, pages 90 and 91; and A. Fresnel, Memoire sur un nouveau systeme d’eclairage des phares (1822), and Oeuvres Completes (1870)).

FIG. 10 shows, in exaggerated form, a mounting element 2, the outer face of which has been machined to include a plurality of discontinuities 30' which take the form of concentric circles comprising inwardly directed beveled edges, the angles of inclination decreasing toward the center. These serve to refract the generated ultrasonicwaves to produce a focusing effect at a point along the axis of symmetry, approximately one inch above the center. Assuming, as in the examples previously disclosed in FIGS. 1-5 and 9, that the piezoelectric element 1 is 0.078 inch thick, and that the initial thickness of the element 2 (which in the illustration is aluminum) is 0.100 inch thick, the discontinuities 30 will protrude 10 mils beyond the plane defined by the edge of the outer face of element 2. In the illustrative example, which is designed to have a rough focal length of about one inch, the concentric bevels 30 number six. Reading from left to right in the figure, the angles of inclination formed by each inclined face with the x axis, may be roughly computed as follows:

Distance along x axis from Angles: center line (inches) a (45 degrees) 0.745 b (41° 20') 0.6 c (28° 15') 0.4

The central cavity d is about 10 mils deep. The angles may be computed, for example, using the following wellknown physical and geometrical relationships:

sin z'=N sin r(2) z=90°— a-}-/·(3) which may be solved simultaneously to produce the following formulae:

. , sin a sin r— ± , —-=

V1V²+2W cos a-f-1(4) . . , N sin a sm i= ± =

7N²+21Vcos a+1(5) where:

&lt;=the angle of incidence of the generated beam with reference to the normal to the inclined face formed by the beveled discontinuity; also z=the angle of inclination of the inclined face to the x axis;

r=the angle between the emergent refracted beam and the normal to the inclined face;

a=the angle between the emergent beam and the x axis; and

N=the index of refraction, which is the ratio of the velocity of sound in the first medium to that in the second, at the specified frequency.

In the present case: (at one megacycle) „ _ V (aluminum) _ 5.1 X10³ m./sec. _ _

V (water) 1.45X10³ m./sec. ’ (6)

Assuming that the operating frequency for which the unit is designed is one megacycle, the resultant beam will be concentrated along the central axis of the element 2, exhibiting substantial focal activity in an area about one inch beyond the face.

It will be understood, however, that my invention is not specifically restricted to discontinuities which are formed on the basis of the foregoing relationship, since crude, focusing can be achieved by any means for shaping the

3,433,461 mounting element 2 so as to produce a differential refraction of the emergent beam in a manner to bend it inwardly toward the principal axis of symmetry.

Furthermore, whereas in the case of optical applications it is necessary to have a sharp focus, this is not necessary or desirable for many ultrasonic applications, inasmuch as even a partial concentration of energy in the desired direction may improve the efficiency of the operation.

A further modification of my invention is the system sho.wh in FIG. 12 of the drawings, which is useful for the ultrasonic nebulization of liquid and for other applications.

Assume that a high-frequency ultrasonic generator, substantially of the form shown in FIGS. 1-5, is placed in'a container 80^! of water, say six inches deep. Immediately above and centered with respect to the outer face of element 2 of the high-frequency generator is a smaller container 81, which may be, for example, from one to three inches in diameter, and may be formed of clear polystyrene or any other suitable plastic material. This is shown in FIG. 12 of the drawings. Container 81 is filled to a depth of three inches, in the present illustration, with a quantity of liquid 83 to be nebulized. The sides of container 81 may be of any thickness sufficient to give it the desired rigidity. The bottom of container 81 is formed by molding or casting the polystyrene (or like plastic) into a Fresnel-type lens similar in form to the lens shown and described with reference to FIG. 10, or any other suitable focusing modification, and has, in the present embodiment, a total thickness of 10 to 20 mils.

Container 81 may be supported in any usual manner, such as by a pair of supporting arms or a conventional spider arrangement 82, fastened to the inner wall of container 80, so that the bottom of container 81 may, for convenience of operation, be vertically separated a distance up to several inches from the upper face of the mounting or facing element 2 of the ultrasonic generator. Alternatively, the bottom of container 81 may be set directly on the outer face of element 2 of the ultrasonic generator, as shown in FIG. 13.

When the composite generator 1, 2 is connected to an external source of high-frequency alternating current, which is powered to drive the generator at several hundred watts, the generated ultrasonic beam passes up through the bottom of the container 81 and is concentrated or focused along the central longitudinal axis of the container 81, causing a column of activated vapor to rise from the surface of the liquid^: in container 81.

Although aluminum has been cited as an example of material which may be· used for the half-wave piezoelectric elements 2, 2, and 2, it will be understood that any other materials having similar acoustic properties, such as, for example, stainless steel, brass, and material known by the trade name “Monel” (International Nickel Company) may also be used. For medical applications in which the liquid to be treated, or nebulized, is corrosive, titanium has been found to be especially suitable for use as a backing or mounting element.

While each of the disclosed embodiments has been shown to include a quarter-wave isolating or decoupling ring, or equivalent member, it will be apparent, as previously stated, that for certain applications wherereduced efficiency is tolerable, the decoupling ring may be omitted altogether. Such a decoupling ring or member is not absolutely necessary to the operativeness of ultrasonic generators of the type described where the width of the radiating face of the generator is many orders of magnitude larger than the wavelength of the frequency of operation, such as is the case when the operating frequency is of the order of one megacycle. Although at such frequencies, operation without the isolating ring causes some loss of efficiency due to clamping at the edges, the central portion of the radiating face has been found to maintain effective activity.

Because of the unique design and the facility with which the devices of the present invention can be supported and grasped and moved from place to place for the application of ultrasonic waves, it will be obvious to those skilled in the art that such devices have utility for many different types of applications. For example, such units form highly mobile devices for various types of ultrasonic cleaning operations where the unit is simply thrust into a beaker or vat of the liquid in which it is desired to generate vibrations, Morevover, such units also have high utility in the field of materials testing, by means of cavitation, inasmuch as they are designed to generate highly intense ultrasonic disturbances. Other applications include medical and chemical uses for emulsifying and dissolving purposes and many other. commercial and household applications, such as stirring beverages and various types of whipping, beating, and spraying operations.

A. further novel use for high power high-frequency ultrasonic generators of the present invention, particularly of the elongated form indicated in FIG. 7, is for atomizing fuel in a petroleum combustion system. For this purpose, the device would be mounted in a reservoir of the petroleum .fuel, in a chamber designed to communicate with the carburetor of the system.

It will be apparent to those skilled in the art that in addition to-the treated piezoelecric ceramics disclosed by way of illustration, the use of natural piezoelectric crystalline materials and other synthetic piezoelectric crystalline materials for generating elements is within the contemplation of the invention. Although the invention has been described with reference to specific illustrative embodiments, in terms of specified forms, dimensions, and materials, it will be understood that the invention is limited only by the scope of the appended claims.