US2836737A - Piezoelectric transducer - Google Patents

Description

y 1 1958 J. w. CROWNOVER 2,836,737

PIEZOELECTRIC TRANSDUCER Filed July 20, 1953 FIG. I. I0

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I INVENTOR 9 3O JOSEPH w. CROWNOVER 3| 32 BY- ATTORNEYS United States Patent t 35 2,836,737 PIEZOELECTRIC TRANSDUCER Joseph W. Crownover, Sherman Oaks, Calih, assignor, by mesne assignments, to Electric Machinery Mfg.

Company, a corporation of Minnesota Application July 20, 1953, Serial No. 369,123 11 Claims. (Cl. Mil-81) This invention relates to improved electromechanical transducers and more particularly to an improved bender type transducer characterized by its ability to produce under applied voltage a greater bending displacement or deflection per unit length of the bender element than has heretofore been obtainable.

Bender elements of the type disclosed herein have great utility in those applications wherein there is need for a mechanism to produce small rapid linear or substantially linear movements, in a reciprocal sense, with or without a high rate of repetition, or frequency of motion. For example, certain types of relays require actuator means for opening and closing electrical contacts which are spaced only a short distance apart. A motor element comprising joined, elongated strips of electrostrictive titanate material that is worked in bending by a voltage applied through one of the strips so as to actuate a relay unit is disclosed and claimed in my copcnding application, Serial No. 357,132, filed May 25, 1953, and entitled Piezoelectric Relay, now abandoned.

it is known that certain bonded titanate composi liOl'lS such as barium titanate, or strontium titanate, or mixtures of the two are electrostrictive in nature, that is to say, a proper voltage applied across a portion of such a material will bring about an expansion of the material in the direction of the applied voltage and a corresponding contraction of the material in a plane lying at right angles to the direction of the applied voltage. The change in dimension of the material in the direction of the applied voltage is roughly two and one-half times greater than the oppositely directed change in dimension of the material in the plane at. right angles to the voltage gradient. Furthermore, it is known that ceramic materials of this type when subjected to voltage gradients become polarized, or made relatively permanently piezoelectric, if the temperatures thereof are kept below the Curie point for the material. As the material is heated to the Curie temperature, rcrnanent polarization in the material diminishes to zero, and the piezoelectric properties thereof are lost. The Curie point for such a material occurs when the dielectric constant thereof reaches a maximum as the temperature is increased.

fit/hen ceramic materials of the type referred to are once positively polarized, they may be made to expand in the direction of the applied voltage gradient by applying a positive voltage across the material in the direction of the polarizing gradient; similarly, the material may be made to contract in the direction of the applied voltage by applying a negative voltage thereto. However, once the material has been polarized, the displacement from the polarized position resulting from the application of a positive voltage across the material equal to the polarizing voltage gradient, is much less than the maximum displacement from the unpolarized position obtained during initial polarization. Furthermore, a negative voltage gradient of not inconsiderable magnitude is required before the material can be caused to contract to the extent that it will again assume its unpolarized dimension. If a greater negative voltage is applied to the material after it has once contracted to its unpolarized dimension, the material will begin to expand again to become oppositely polarized And correspondingly, the material must be subjected to a considerable positive voltage to depolarizc it. Thus it is evident that for many applications, the displacements obtainable from working the material in its polarized state are too small for practical use.

it is also known that the material referred to becomes polarized with the application thereto of voltage of any magnitude; however, the time required to completely polarize the material will be increased if weaker voltages are applied across the material. For example, if barium titanate is subjected to a voltage of 60 volts per mil thickness of the material, the time required to completely piezoelcctrically sensitize the material will be approximately 4 minutes, whereas it the voltage is decreased to 30 volts per mil thickness of the material the time required is increased to approximately 40 minutes. If the voltage applied across the material is increased to volts per mil, which is roughly the dielectric breakdown value for barium titanate, the time required to piezoelectrically sensitize the material will be decreased to only a few seconds. Since many applications call for a frequency of reciprocal movement considerably in excess of several times per second, it is evident that even with the cyclic application of an actuating voltage of 100 volts per mil, the material could not attain its full piezoelectric sensitivity, or stated more simply, the material could not be made to move back and forth with the maximum amplitude obtainable unless the cyclic period were of the order of 4 times the time required to completely sensitize the material. Thus it is evident that there is :a need for a device that will make use of the small reciprocal movements that are obtainable as a result of applying voltages to the polarized, piezoelectric material, and to amplify these small movements to practical, usable values.

if the material referred to is operated at temperatures immediately above the Curie point, it is known that the remanent piezoelectric effect is lost, that is to say, the ability of the material to retain induced polarization is lost, which necessarily results in the loss of the ability of the material to be made to expand and to contract from its polarized state. However, the material still retains the ability to expand in the direction of the applied voltage, and the extent of the expansion is approximately the same as exists below the Curie point as long as the operating temperature is not increased too far beyond the Curie temperature. By way of explanation, it is found that at temperatures below the Curie point for the material the degree to which the electrostrictive titanate responds to an applied voltage, as measured by the coupling coeificient thereof, remains substantially constant; however, as the operating temperature is increased above the Curie point, the degree to which the electrostrictive material responds to an applied voltage diminishes slowly to zero.

With the removal of the applied voltage, the material will approximately resume its initial unexpanded dimension, at temperatures above the Curie point, so that negative voltages are not required to cause the material to resume its original dimension. Also, the material may be caused to be fully expanded with the application of a voltage in a much shorter time. For example, when a voltage of 100 volts per mil is applied to a polycrystalline barium titanate material at temperatures above the Curie point for the material, it will become fully expanded in approximately 10 milliseconds, and furthermore, when the voltage is removed, the material will resume its original dimension in approximately the same short interval of time.

As far as available electrostrictive materials themselves are concerned, the principal disadvantage accompanying the use of pure barium titanate above the Curie point consists in the relatively high temperature at which the Curie point exists, it being in the neighborhood of Patented May 27, 1958' to 130 degrees centigrade. Thus pure barium titanate ceramic can only be practically utilized in this way if the material is artificially heated. The principal disadvantage attending the use of pure strontium titanate stems from the fact that the Curie point temperature thereof is so low, being less than minus 100 degrees centigrade, that at ordinary temperatures the coupling coefficient or degree to which the material will respond electrostrictively to an applied voltage is considerably reduced, resulting in the production of relatively small displacements. However, the Curie point temperatures for mixtures of barium titanate and strontium titanate lie between the extreme temperatures for the pure materials. Therefore, an ideal mixture of the two materials will be such that the Curie point temperature thereof lies just below the minimum expected operating temperature so as to gain the benefits of the relatively large and rapid displacements of the material which are available above the Curie point temperature when the material is worked electrostrictively, and so as to minimize the adverse effect upon the coupling coefficient of the material resulting from operating the material at temperatures considerably above the Curie temperature.

For example, if we assume the minimum temperature of the operating range to be 22 centigrade, a material comprising, by weight, 73% barium titanate and 27% strontium titanate may be chosen since this has a Curie point of approximately 20 centigrade.

With these principles and limitations in mind, it is, accordingly, the principal object of my invention to produce an improved electrostrictive bending transducer wherein means is provided for causing the transducer to produce, under an applied voltage, a deflection per unit length of the transducer which is considerably greater than has heretofore been obtainable.

It is another object of my invention to provide an improved titanate transducer wherein there is provided a means for causing the transducer to produce, under applied positive or negative voltage, positive or negative deflections having magnitudes considerably greater than have heretofore been obtainable.

It is another object of my invention to provide an improved electrostrictive titanate bending transducer wherein there is provided a means for causing the transducer to produce, under rapid cyclic applications of voltage, amplitudes of deflection having magnitudes greater than have heretofore been producible.

It is a further object of my invention to provide an improved electrostrictive titanate bending transducer wherein means is provided for obtaining and combining high mechanical compliance, favorable bending strength, and a high degree of displacement per unit length of the bending elements.

These and other objects and advantages of the present invention will become apparent from a consideration of the following description and the appended claims in conjunction with the accompanying drawings wherein:

Fig. l is a cross sectional View of a bender type transducer of the present invention;

Fig. 2 is a View of the bender unit taken on line 2-2 of Fig. 1;

Fig. 3 is an enlarged cross sectional view of the bender unit taken on line 33 of Fig. 1;

Fig. 4 is an enlarged fragmentary sectional view of a portion of the bender unit and is taken on line 44 of Fig. 3;

Fig. 5 is a cross sectional view of a modified form of the bender unit of the present invention;

Fig. 6 is an enlarged fragmentary sectional view of a portion of the bender unit illustrated in Fig. 5;

Fig. 7 is a cross sectional view of another form of the bender unit of the present invention;

Fig. 8 is an enlarged cross sectional view of the bender unit illustrated in Fig. and is taken on line 38 thereof; and

Fig. 9 is a cross sectionalview of a portion of the bender unit illustrated in Fig. 7 and is taken on line 9-9 thereof.

Referring now to the cantilevered bending transducer unit illustrated in Fig. 1, it will be seen that a pair of elongated, thin strips or elements of ceramic material 11 and 12 are firmly joined together and insulated from one another by applying a cement 13 such as a nonconducting thermosetting resin between adjacent faces of the strips. The strips thus adjacent one another in face to face relatt n. Each of the strips is longer than it is wide, and wider than it is thick so as to give the bender unit the greatest degree of mechanical compliance in a plane defined by the length and thickness dimensions of the strips. The strips are composed of a polycrystalline aggregate such as a titanatc ceramic which is electrostrictive in nature. The preferred composition is barium titanate ceramic, or strontium titanate ceramic, or mixtures thereof.

Electrostrictive element 11 has a pair of electrodes 14 and 15 formed on opposite faces thereof, as by applying silver paint thereto. Electrostrictive element 12 has a plurality of transverse combor gridtype electrodes 18a and 18b formed on face 19 thereof, as by applying silver paint thereto. The transverse electrodes 18a and 18b extend spanwise across a portion of the face 19 of the element 12 and are closely spaced to one another. The transverse electrodes 18a are interconnected by means of an elongated conducting band 21 running lengthwise along one side of face 19, and the transverse electrodes 18b are interconnected by means of an elongated conducting band 22 running lengthwise along the opposite side of face 19. A source of electric potential 23 is connected across electrodes 14 and 15, and across conducting bands 21 and 22, so

as to apply a voltage between electrodes 14 and 15, and between adjacent transverse electrodes 18a and 18b. The major component of the voltage gradient between electrodes 14 and 15 will lie parallel to the thickness dimension of element 11, and the major component of the voltage gradient between any two adjacent transverse electrodes 18a and 1812 will lie parallel to the length dimension of strip 12. schematically shown voltage gradients 23 and 24 extending betweenadjacent pairs of transverse electrodes 18a and 18b will be oppositely directed, as illustrated in Fig. 4. It. is desirable to minimize the width of the segments between electrodes 18a and 1812 so as to maximize the longitudinal voltage gradients in strip 12. Thus there is provided a means for establishing a first voltage gradient or potential difference parallel to the thickness dimension of one of the elongated electrostrictive strips, and likewise a series of voltage gradients having major components directed parallel to the length dimension of the second elongated electrostrictive element.

In accordance with the principles outlined in a previous portion of this description, when voltage is applied through strip 11 parallel to the thickness dimension thereof, the thickness of the strip will increase and the length thereof decrease. Similarly, a voltage gradient applied through strip 12 parallel to the length dimension thereof will cause the length of the strip to increase and the thickness thereof to decrease. Stated another way, the result of the application of voltage to the various electrodes will be a contraction in the length mode of strip 11, and an elongation in the length mode of strip 12. Since the strips are firmy fastened together, the elongation of strip 12 will be resisted by the contraction of strip 11, so as to yield a bending of the free end of the cantilevered transducer unit upwardly, or toward the contracting side of the transducer unit. Furthermore, the degree of bending, or amount of deflection per unit length of the bending unit is considerably in excess of the deflection that would result were only one of the elongated strips caused to elongate or contract against the reaction provided by an adjacent unactivated strip. Thus the free end of the cantilevered bending unit may be caused to undergo a maximum deflection, and thereby to do useful work in many applications wherein electrostrictive bender type transducers were heretofore impractical due to the limited displacements obtainable therefrom.

It is possible to construct a bender type transducer having a pair of joined, elongated strips with a common electrode between the strips and a pair of electrodes on opposite faces of the strips, so that a voltage gradient may be applied across each strip between the common electrode and the outer electrodes. If the strips are operated above the Curie point temperature, there will be no bending produced, since both strips will expand in the direction of the voltage gradient and contract together in the longitudinal mode. However, if the strips have been polarized previously, the first in the direction of the presently applied voltage gradient, and the second in the direction opposite to the applied voltage gradient, and if they are operated below the Curie point temperature, then the first strip will expand in the longitudinal mode and the second strip will contract in the longitudinal mode to produce bending. However,

the amount of bending thereby producible is severely limited due to the fact that if the voltage gradients are increased above approximately volts per mil, the second strip will cease to contract in the longitudinal mode and will begin to expand along with the first strip so that further bending will be precluded. In contradistinction to this type of bender unit, one great advantage of the present invention is the fact that voltages up to the breakdown strength of the ceramic material may be applied to the material, and bending will increase so long as the voltage gradient is increased. Thus in the present invention, bending does not cease at the 10 volt per mil voltage gradient level, but continues as long as the applied voltage gradient is increased.

The bender unit illustrated in Figs. 5 and 6 is the same as that illustrated in Fig. l with the addition thereto of a second series of transverse electrodes a and 25b formed on the face of strip 12, opposite face 19 thereof. Electrodes 25a and 25b are spaced longitudinally of the element and alternately disposed, similar to electrodes 18a and 18b and furthermore, electrodes 25a are spaced opposite electrodes 18a and electrodes 25b are spaced opposite electrodes 18b. Electrodes 25a are interconnected by means of a conductive band, not shown, and are electrically connected to electrodes 18a. Similarly, electrodes 2% are interconnected by means of a conducting band, and are electrically connected to electrodes 1811. When a voltage is applied across electrodes 18a, 18b, 25b and 25a, a stronger voltage gradient component parallel to the length dimension of strip 12 is established, as schematically illustrated in Fig. 6, thereby increasing the tendency of the electrostrictive strip 12 to expand in the lengthy mode, as compared with the strip having transverse electrodes applied to'one side only thereof. Thus, a somewhat stronger moment of force is set up in strip 12, opposing the oppositely directed moment in strip 11, and yielding a somewhat increased deflection of the free end of the bending unit 10 in the upward direction.

The bender unit illustrated in Figs. 7, 8, and 9 is similar to the unit illustrated in Fig. l, with the addition thereto of an elongated thin strip 3% of a metal conductor such as a strip of brass. Also, the strips 11 and 12 are not insulated from one another. in shape, there being an elongated blanked-out portion 31 formed at the center thereof, which is perimetrically enclosed by longitudinal side strips 32 and transverse end strips 33. The strip is disposed between elongated ceramic strips 11 and 12' in alignment therewith,

Strip 30 is perimetrical 6 conductive cement or fastening means not shown. Strip 30 is only a few thousandths of an inch thick so as to admit of a high degree of bending compliance. The strip 30 is in electrical contact with electrode 15 on one face of strip 11, and with conducting band 21 on strip 12 to which the transverse electrodes 18a are joined.

Band 21 is connected to metal strip 30 by means of a conducting band 36 of silver paint applied to the side wall 35 of ceramic strip 12. A potential difference may be applied between electrodes 14 and 15, and between transverse electrodes 18a and 18b by applying the potential between conducting strip 30 and electrode 14, and similarly between strip 30 and electrodes 18b, since strip 30 is electrically joined to electrodes 15 and 18a. As a result, strip 11 will contract in its longitudinal mode and strip 12 will expand in its longitudinal mode so that the free end 38 of the transducer unit will be displaced upwardly toward the contracting side of the transducer unit. By limiting the aerial extent of metallic conducting strip 30 to two narrow transverse end strips 33 and two narrow side strips 32, interference with the voltage gradients between adjacent transverse electrodes 18a and 1812 caused by the proximity thereto of the charged side portions 33 of metal strip 30 will be minimized. Thus there is provided a bending transducer having greatly increased bending strength and yet having a high degree of mechanical compliance in the plane of bending.

It is pointed out that if the ceramic strips 11 and 12 are composed of barium titanate, they may be polarized by means of an applied voltage and operated piezoelectrically at ordinary temperatures to yield positive and negative bending deflections with respect to the polarized position of the transducer. Even though the piezoelectrically induced deflection in each strip is relatively small by itself, the particular combination of bending elements as disclosed will result in relatively large bending deflections per unit length of the transducer unit 10, due to the novel combination of ceramic strips 11 and 12 and electrodes 18a and 18b.

Also, if the ceramic strips 11 and 12 are worked at a temperature slightly above the Curie point, which can readily be done for ordinary applications by utilizing ceramic strips composed of barium titanate and strontium titanate in such relative proportions that the Curie point will be just below the lower end of the operating temperature range, the bending deflections obtained per unit length of the transducer unit will be to all intents and purposes maximized, and relative rapidity of deflection will also be realized, to the end that the bending transducer may be advantageously utilized as an actuator or motor unit in many applications requiring small, rapid actuating displacements.

While I have described my invention with particular reference to bender type transducers having a pair of elements formed of bonded titanate compositions such as barium titanate, or strontium titanate, or mixtures of the two, having electrostrictive or piezoelectric characteristics, because such compositions are eminently suited for commercial applications, in its broader aspects the invention is also applicable to a pair of elements formed of some other electromechanically sensitive dielectric material, such as a portion of a single crystal of a piezoelectric substance or an aggregate or composition of crystals other than titanates, such as certain barium zirconates. 1' therefore contemplate that various changes and modifications can be made without departing from the invention as indicated by the claims which follow.

I claim:

1. In a bender type transducer: a pair of elongated electrostrictive dielectric strips, each of said strips having a pair of major faces defined by the length and width dimensions thereof, said strips being rigidly joined together and insulated from one another; a pair of spaced electrodes each one of which is disposed longitudinally and is secured in position by means of an electrically in intimate physical contactwith one of the major faces "Z of one of said elongated strips; means for establishing a potential difference between said electrodes; a plurality of electrodes disposed in intimate physical contact with one of the major faces of the other of said strips, said electrodes lying transversely to the length axis of said strip and being spaced from one another; and means for establishing a potential difference between adjacent transversely lying electrodes.

2. in a bender type transducer: a pair of elongated electrostrictive dielectric elements, said elements being joined together and insulated from one another; first and second electrodes disposed in intimate physical contact with an opposite pair of faces of one of said elements; a plurality of electrodes disposed in intimate physical contact with at least one of the faces of the other of said elements, said electrodes lying transversely to the length axis of said element and being spaced from one another; and means for establishing a potential difference between said first and second electrodes and between'adjacent transverse electrodes.

3. In a bender type transducer: a pair of electrostrictive dielectric elements, said elements being joinedtogether and insulated from one another; electrically chargeable electrode means for establishing a voltage gradient through a portion of one of said elements having a component extending parallel to the length dimension of said element; the other of said elements having a pair of major faces, first and second electrodes disposed respectively in physical contact with said major faces; and means for establishing a voltage gradient between said first and second electrodes.

4. in a bender type transducer: :1 pair of electrostrictive dielectric elements, each of said elements having a pair of major faces, said elements being joined together in face to face relation and insulated from one another; means for establishing a first voltage gradient through one of said elements, having its major component directed parallel to the thickness dimension of said element, said means including a pair of electrodes formed on opposite faces of said one of said elements; and electrically chargeable electrode means for simultaneously establishing a second voltage gradient extending through a portion of the other of said elements, having its major component extending parallel to the length dimension of said other element.

5. In a bender type transducer: a pair of elongated elements, said elements being joined together in face to face relation; electrically chargeable electrode means for establishing a first voltage gradient through one of said elements, having its major component directed parallel to the thickness dimension of said element; and means for establishing a second voltage gradient through a portion of the other of said elements, having its major component parallel to the length dimension of said other element.

6. ln abender type transducer: a plurality of elongated electrostrictive elements, said elements being joined together in face to face relation, said elements being insulated from one another; electrically chargeable electrode means for establishing a first voltage gradient through one of said elements, having its major component directed parallel to the thickness dimension of said element; and means for establishing a second voltage gradient tirough a portion of the other of said elements, having its major component directed parallel to the length dimension of said second element.

7. In a bender type transducer: a'plurality of elongated strips joined together in face to face relation, said strips including electrically conductive metallic strip, a first electrostrictive titanate strip joined to one face of said metallic strip, and a second electrostrictive titanate strip joined to the opposite face of said metallic strip; a pair of spaced electrodes each of which is disposed in intimate physical contact with one of the opposite major faces of the first of said titanate strips; means for establishing a potential difference between said electrodes; a plurality of electrodes disposedin physical contact with one of the major faces of the second of said titanate strips, said electrodes lying transversely to the length axis of said strip and being spaced from one another; andmeans for establishingfa potential difference between said transversely lying electrodes.

8. In a bender type transducer: a plurality of elongated strips joined together in face to face relation, said strips including anelectrically conductive metallic strip, a first electrostrictive titanate strip joined to one face of said metallic strip, and a second electrostrictive titanate strip joined to the opposite face of said metallic strip; means for establishing a voltage gradient through the first of said titanate strips, having a component directed parallel to the thickness dimension of said strip; a plurality of electrodes disposed in'physical contact with one of the faces of the second of said titanate strips, said electrodes lying transversely tothe length axis of said strip and being spaced from one another; and electrical means for establishing a potential difference between said transversely lying electrodes.

9. In a bender type transducer: a bending unit including a pair of elongated elements of electromechanically sensitive dielectric material connected to act conjointly, one of said elements having spaced electrodes formed longitudinally on two opposite faces thereof; means for establishing a potential difference between said electrodes; a plurality of electrodes disposed in physical contact with one of the faces of the second of said elongated elements, said electrodes lying transversely to the length axis of said element and being spaced from one another; and a source of electrical potential for establishing a potential difference between said transversely lying electrodes.

l0. In a bender type transducer: a bending unit including a pair of elongated elements of electromechanically sensitive dielectric material connected to act cenjointly, one of said elements having spaced electrodes formed longitudinally on two opposite faces thereof; means for establishing a potential difference between said electrodes; a plurality of electrodes disposed in physical contact with both of the faces of the second of said elongated elements, said latter electrodes lying transversely to the length axis of said element and being spaced from one another; and a source of electrical potential for establishing a potential difference between adjacent transversely lying electrodes.

11. In a bender type transducer: a plurality of elonf gated strips joined together in face to face relation, said strips including an electrically conductive spacer member between them, said strips including a first electrostrictive strip joined to one face of the conductive memher, a second electrostrictive strip joined to the opposite face of the member; a pair of spaced electrodes, each of which is disposed in intimate physical contact with one of the oppposite major faces of the first of said electrostrictive strips; means for establishing a potential difference between said electrodes; a plurality of electrodes disposed in physical contact with one of the major faces of the second of said electrostrictive strips, said electrodes lying transversely to the length axis of the strip and being spaced from one another; and. means for establishing a potential difference between said transversely lying electrodes.

References Cited in the file of this patent I UNITED STATES PATENTS 2,185,966 Pfanstiehl Jan. 2, 1940 2,497,108 Williams Feb. 14, 1950 2,540,187 Cherry Feb. 6, 1951 2,640,165 Howatt May 26, 1953 2,640,889 Cherry June 2, 1953 FOREIGN PATENTS 678,825 Great Britain Sept. 10, 1952

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