TECHNICAL FIELD

This invention relates to improved piezoelectric ceramic switching devices and to novel electrical systems for the energization, control and utilization of such devices.

More particularly, the invention relates to improved piezoelectric ceramic switching devices, their fabrication, and to novel electrical circuits for the energization as well as use of such improved devices as switching elements in electrical systems, some parts of which may be physically mounted on and supported by the improved piezoelectric ceramic switching devices themselves.

BACKGROUND PRIOR ART PROBLEM

In conventional electrical circuits, electrical relays and switches are employed at points in such circuits where it is desired either to initiate or interrupt (or both) electric current flow through the circuit. In the past, electromagnetic solenoid operated switches and relays have been employed to either close or open the contacts of a power switch or relay in response to a small control signal (low voltage, low current) which initiates either closure or opening of the contact contacts of a larger power rated switch that thereafter controls current flow through the contacts to a circuit being supplied via the switch contacts.

Relays and switches which are piezoelectric drive elements have a number of advantages over their electromagnetic counterparts. For example, a piezoelectric driven relay or switch requires substantially lower current and dissipates very little power during operation to open or close a set of contacts in comparison to an electromagnetic driven device of the same rating. Additionally, piezoelectric driven switching devices have very low mass and therefore require less space and introduce less weight into circuit systems with which they are used. Additionally, piezoelectric driven switching devices possess very short actuation times. Thus, fast acting switching is possible with smaller and lower weight devices that dissipate less power and hence can operate with lower temperature rises if piezoelectric ceramic switching devices are used.

Piezoelectric plate elements may be fabricated from a number of different polycrystalline ceramic materials such as barium titanate, lead zirconate titanate, lead metaniobate and the like which are precast and fired in a desired shape, such as a rectangular-shaped plate. Electrically conducting surfaces in the form of metalized metallized electrodes usually are deposited on the surface of the plates which then are used to apply a polarizing voltage across the piezoceramic plate in order to make them piezoelectric in a chosen polar direction by a prepoling treatment which involves exposing the ceramic plates to a high electric field applied across the metalized electrode while the plates are held at a temperature not far below their Curie point. As a result of this prepolarizing treatment, the plate elongates in the same direction as the applied field. After cooling of the plates and removal of the prepoling field, the dipoles within the ceramic plate which were aligned as a result of the prepoling treatment, cannot easily be returned to their original position and therefore possess what is known as remanent remanent polarization. Thus, the ceramic plates are made permanently piezoelectric whereby the dipoles are permanently enhanced and can convert mechanical energy into electrical energy, and vice versa. The piezoelectric effect is described more fully in a booklet entitled "The Piezoelectric Effect in Ceramic Materials" edited by J. Van Randeraat & R. E. Setterington and published by Philips Gloeilampenfabrieken of Eindhoven, The Netherlands, second edition, dated January 1924.

In piezoelectric ceramic materials, the direction of the electrical and mechanical dipole axes depends upon the direction of the original unidirectional prepolarizing high voltage field. During the prepoling process the ceramic plate element experiences a permanent increase in dimension between the poling electrodes and a permanent decrease in dimension parallel to the electrodes. When a DC excitation voltage of the same polarity as the prepoling voltage, but of smaller magnitude, subsequently is applied between the poling electrodes, the element experiences further but temporary expansion in the poling direction and contraction parallel to the electrodes. Conversely, when a DC excitation voltage of opposite polarity is applied to the plate element electrodes, the plate contracts in the poling direction and expands parallel to the electrodes. In either case, the piezoelectric ceramic plate element returns to its original prepolarized dimensions when the later applied excitation voltage is removed from the electrodes.

A number of different piezoelectric ceramic switching devices have been offered for sale in the past having a variety of different configurations. One of the more popular, if not the prevailing structural approach employed in the past, is known as a bimorph bender-type piezoelectric ceramic switch which employs two adjacent piezoelectric plate elements mounted side by side having conductive electrodes , coating their outer surfaces and sharing a common conductive inner surface to form a bimorph bender-typ bender-type device. A known commercially available bimorph bender-type piezoelectric ceramic switch is described in an application note copyrighted in 1978 and published by the Piezo Products Division of Gulton Industries , Inc. located in Metuchen, N.J. and Fullerton, Calif. If one end of such a piezoelectric ceramic bimorph bender is clamped cantilever fashion, the bender can be made to bend in either direction from its central neutral unenergized condition by application of an energizing potential of either polarity but lower than the prepolarizing potential to one of its conductive outer electrodes. If a suitable value energizing potential of either polarity is applied across only one of the piezoelectric ceramic plate elements of the bender, it enhances dipole alignment of that particular plate element resulting in a shortening and thickening of the plate element. This in turn results in bending of the overall bimorph bender device due to the fact that the two piezoelectric plate elements are physically secured together. By suitable design, the bending action can result in the closing of two switch contacts or other similar effect.

Unfortunately, prior art attempts to provide piezoelectrically driven switch devices have resulted in devices having poor electrical and mechanical performance characteristics. In the case of prior art bimorph bender-type switching devices as described briefly above, they possess severe performance limitations which are founded in the trade-offs between contact force, contact separation, depolarization, retentivity and reliability in service and the uncertainity uncertainty of contact position due to creep and temperature effects which build up over a period of continued device usage. One such prior art switching device employing a piezoelectric bender-type drive member is described in U.S. Pat. No. 2,166,763 issued July 18, 1939 for a "Piezoelectric Apparatus and Circuits". The piezoelectric bender-type drive member described in U.S. Pat. No. 2,166,763 is comprised by two juxtaposed piezoelectric plate elements having electrodes as described briefly above, and is energized in such a manner that one of the piezoelectric plate elements has the energizing potential applied to it in the same direction as the direction of the prepoling electric field; however, the other piezoelectric plate element has an energizing signal applied thereto of opposite polarity from that of its prepolarizing electric field. As a consequence, the device of U.S. Pat. No. 2,166,763 undergoes long term depolarization of either one or both of the piezoelectric plate elements after a period of usage due to the depolarizing effect of the repeated application of a wrong polarity (out of phase anti-poling direction) energizing signal. The deleterious effect on dipole enhancement of operation in this mode greatly restricts the applied voltage stress and thus the useful work output obtainable with such devices. In addition, the device of this prior art patent possesses a number of other weaknesses sought to be overcome by the present invention. The same objectionable characteristics are present in a number of different prior art piezoelectric driven bender-type switches and/or relay devices such as the following: U.S. Pat. No. 2,1682,340 2,182,340 --issued Dec. 5, 1939 for "Signaling System"; U.S. Pat. No. 2,203,332--issued June 4, 1950 for "Piezoelectric Device"; U.S. Pat. No. 2,227,268--issued Dec. 31, 1940 for "Piezoelectric Apparatus"; U.S. Pat. No. 2,365,738--issued Dec. 26, 1944 for "Relay"; U.S. Pat. No. 2,714,642--issued Aug. 2, 1955 for "High Speed Relay of Electromechanical Transducer Material"; U.S. Pat. No. 4,093,883--issued June 6, 1978 for "Piezoelectric Multimorph Switches"; U.S. Pat. No. 4,395,651--issued July 26, 1983 for "Low Energy Relay Using Piezoelectric Bender Elements"; and U.S. Pat. No. 4,403,166--issued Sept. 6, 1983 for "Piezoelectric Relay with Oppositely Bending Bimorphs". In addition to the above prior art patented piezoelectric bender-type switching devices, the textbook "Manual of Electromechanical Devices" by Douglas C. Greenwood published by McGraw-Hill Book Company and copyrighted in 1965 discloses a somewhat similar piezo ceramic switching device on page 64 thereof.

In order to overcome the shortcomings of the known prior art piezoelectric ceramic driven relays and switches such as those listed above, the present invention was devised.

SUMMARY OF INVENTION

It is therefore a primary object of the present invention to provide new and improved piezoelectric ceramic switching devices of novel construction having better operating characteristics than those of comparable prior art devices of the same general nature.

Another object of the invention is to provide improved energization circuit designs for use with piezo ceramic switching devices which provide improved longevity and greater reliability in operation to such piezo ceramic switching devices over extended periods of service requiring substantial numbers of switching operations.

Still another object of the invention is to provide improved piezoelectric ceramic switching devices and circuits therefor having the above-listed characteristics wherein many of the components employed in either the energization and/or utilization circuits employing such devices are formed or otherwise supported on an inactive unpolarized portion of the piezoelectric ceramic switching device thereby reducing to a minimum stray inductance of the circuits and enhancing miniaturization and batch processing.

A still further object of the invention is to provide improved piezoelectric ceramic switching devices which themselves carry and selectively close or open power rated switch contacts for controlling current flow therethrough or, alternatively, provide a sufficient electric discharge current to the control gate of a gate turn-on/turn-off semiconductor power switch such as an SCR, triac or transistor to cause it to turn on and conduct current or to turn off and block current flow selectively.

In practicing the invention, a novel piezoelectric ceramic switching circuit and bender-type piezoelectric ceramic switching device is provided wherein the piezoelectric ceramic switching device comprises at least two prepolarized piezoelectric plate elements having respective outer conductive surfaces and disposed on opposite sides of at least one central conductive surface sandwich fashion to which they are physically and electrically bonded. The piezo ceramic switching device coacts with a set of make and break electrical contacts to close or open such contacts and thereby make or break an electrically conductive path extending through the contacts. Selectively operable electric excitation circuit means are connected to the bender-type piezoelectric ceramic switching device for selectively and respectively exciting each piezoelectric plate element thereof with a direct current excitation electric field which is polarized and applied always in the same direction as the prepolarizing electric field enhancing dipole alignment previously permanently induced in the piezoelectric plate element whereby no depolarization of the piezoelectric plate element occurs during successive operations of the switch in order to close or open the make and break contacts. Further, continuous energization is not deleterious with the contacts opening the instant that charge is reduced in the bender.

The selectively operable electric excitation circuit means comprises respective switch energization circuit means connected in circuit relationship across respective ones of prepolarized piezoelectric plate elements of the piezo ceramic bender-type switching device for selectively closing or opening respective ones of the set of coacting electrical switch contacts for controlling electric current supplied through a load by opening and closing the contacts. Each switch energization circuit means selectively connects the bender- type switching device across a source of bender energization potential, a normally open low power rated user operated electric switch, a current limiting resistor and diode rectifier circuit means poled to provide an electric energization potential having the same polarity as the polarity of the prepolarizing potential used to polarize the prepoled dipole enhanced piezoelectric plate element of the piezoelectric bender-type switching device. The series electric circuit thus comprised is connected in series circuit relationship across a respective one of the prepoled piezoelectric plate elements of the bender-type switch so that upon closure of the normally open low power rated user's switch, the respective prepolarized piezoelectric plate element of the bender-type piezoelectric switch selectively and respectively is excited with a direct current excitation field which always has the same polarity as the polarity of the prepoling electric field dipole enhanced alignment previously permanently induced in the respective piezoelectric plate element and no depolarization of the piezoelectric plate elements occur occurs during continued or successive operation of the piezoelectric bender-type switch device for closing and/or opening the load current controlling electric switch contacts.

The improved piezoelectric ceramic switching device comprises at least one piezoelectric bender-type switching device having two planar piezoelectric plate elements secured in opposed parallel relationship sandwich fashion on opposite sides of at least one central conductive surface and having respective outer conductive surfaces that are insulated from each other and the central conductive surface by the respective intervening piezoelectric plate element material thicknesses. The bender-type piezoelectric switching device further includes at least one set of coacting electrical switch contacts which are opened or closed by a prepolarized movable bender-portion of the piezoelectric ceramic switching device. The improved device further includes clamping means secured to a different portion of the bender-type piezoelectric ceramic switching device adjacent to and mechanically supporting the prepoled movable bender portion of the device cantilever fashion with the different portion of the piezoelectric ceramic plate elements comprising the bender-type device disposed under the clamping means being unpoled and electrically neutral.

In addition to being unpoled and electrically neutral, the different portion of the piezoelectric ceramic plate elements disposed under the clamping means have the outer conductive surfaces thereof removed from that portion which is disposed under the clamping means. In addition, a conformal electrically insulating protective coating covers at least some of the outer surfaces of the prepolarized movable portion of the bender-type piezoelectric device with the conformal electrically insulating protective coating comprising a polyimide siloxane copolymer.

In preferred embodiments of the invention, the conformal electrically insulating coating extends over and covers the outer planar conductive surfaces and the edges of the prepoled planar piezoelectric ceramic plate elements, and further extends over and covers the side edges of the piezoelectric ceramic plate element elements and their outer conductive surfaces and the outer edges of the central conductive surface sandwiched therebetween at least over the prepolarized portions of the device. Further, the conformal insulating coating covering the outer planar conductive surfaces of the prepolarized portions of the piezoelectric ceramic plate element elements also extends down to and covers the portions of the piezoelectric ceramic plate elements exposed by any removal of the outer conductive surfaces thereon as well as the edge portions of such outer conductive surfaces exposed by such removal.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects, features and many of the attendant advantages of this invention will be appreciated more readily as the same becomes better understood by a reading of the following detailed description, when considered in connection with the accompanying drawings, wherein like parts in each of the several figures are identified by the same reference char character and wherein:

FIG. 1 is a top elevational view of a new and improved piezoelectric ceramic bender-type switching device constructed according to the invention;

FIG. 1A is cross sectional view of the device shown in FIG. 1 taken through plane 1A--1A;

FIG. 1B is a schematic circuit diagram of a novel energization circuit employed in operating the switching device of FIG. 1;

FIG. 1C is a cross sectional view of the device shown in FIG. 1 taken through plane 1C--1C;

FIG. 1D is a top planar view of the movable bender end of the switching device shown in FIG. 1 in an unfinished condition during the manufacture thereof, and illustrates the manner of forming electric load current carrying contacts at the movable end of the bender-type piezo ceramic switching device;

FIG. 1E is a perspective end view of the same portion of the device shown in FIG. 1D at the a moment later in time during the manufacture thereof following the stage shown in FIG. 1D;

FIG. 1F is a partial side end view of the finished device showing the manner of fabrication of the end contacts when viewed in conjunction with FIG. 1D and FIG. 1E;

FIG. 2 is a longitudinal sectional view of a different embodiment of improved piezoelectric ceramic switching device constructed in accordance with the invention and illustrates the device mounted on a separate insulating base member;

FIG. 2A is a schematic circuit diagram illustrating a utilization circuit controlled by the device of FIG. 2 and which is fabricated on the device;

FIG. 3 is a partial top planar view of an unpolarized, electrically neutral end of the bender-type piezoelectric ceramic switching device illustrating an electric fuse element deposited on a portion of unpolarized piezoelectric ceramic;

FIG. 4 is a longitudinal sectional view of still a different embodiment of the invention showing active circuit components mounted on a non-polarized portion of a bender-type switch switching device constructed according to the invention with the active components comprising diode rectifier elements interconnected with discrete wired connectors to effect a desired excitation circuit design for the device shown in FIG. 4;

FIGS. 4A, 4B and 4C comprise schematic circuit diagrams of three different embodiments of a diode rectifier doubler circuit configuration suitable for use as an excitation circuit with the piezoelectric ceramic switching device shown in FIG. 4, with the circuit arrangement of FIG. 4B corresponding to the physical illustration of the circuit elements depicted in FIG. 4 and physically supported on unpoled portions of the bender-type switching device;

FIG. 5 is a top-side perspective view of a different form or of piezoelectric ceramic switching device constructed in accordance with the invention showing how the device would be fabricated for use with a voltage tripler energization circuit shown schematically in FIG. 5A of the drawings.

FIG. 6 is top-side elevational-partial perspective view of still another form of switching device according to the invention which employs a diode rectifier quadrupling circuit illustrated schematically in FIG. 6A of the drawings:

FIG. 7 is a side elevational view of still another form of improved piezoelectric ceramic bender-type switching device according to the invention to provide H-type double acting switching operations on each of the opposite sides of the neutral position of the bender element of the device;

FIG. 7A is a schematic circuit diagram of one embodiment of a utilization load circuit which could be operated w h with the device of FIG. 7;

FIG. 7B is a schematic circuit diagram of a second type of utilization load circuit which could be controlled by the bidirectional acting piezoelectric ceramic switching device of FIG. 7 wherein the device is employed to directly apply gating current to the gates of higher power rated, gated power semiconductor switches triggered by the device;

FIG. 7C is a schematic circuit diagram of a mirror image of the circuit shown in FIG. 7B and illustrates how inverse polarity voltages can be obtained to provide negative polarity gating currents for use with gated power semiconductor switches of the turn-off type;

FIG. 8 is a longitudinal sectional view of a preferred embodiment of piezoelectric ceramic bender-type switching device according to the invention wherein a conformal coating is provided over the active polarized movable bender portions of the device;

FIG. 8A is a cross sectional view of the device shown in FIG. 8 taken through plane 8A--8A;

FIG. 8B illustrates a cross sectional view taken through a device such as FIG. 8 but which has been provided with an alternative coating arrangement which covers the entire planar exterior surfaces of the polarized active movable bender portions of the device;

FIG. 8C is a partial cross sectional view of the device of FIG. 8 taken through that part of the device under the clamping means in order to better illustrate how the conformal coating is caused to cover any exposed parts or edges of the active, polarized portions of the piezoelectric ceramic plate elements;

FIG. 8D shows a number of characteristic curves plotting bender force versus time and illustrates the operating characteristics of a number of different piezoelectric ceramic bender-type switching devices constructed according to the prior art with or without some form of protective coating as well as the operating characteristics of preferred forms of the invention illustrating their force versus time operating characteristics over a period of time.

FIG. 9 is a longitudinal sectional view of an embodiment of the invention similar to that shown in FIG. 8 and illustrates the manner in which load current carrying contacts can be formed on the free movable bender portion of the device; and

FIG. 10 and 11 are perspective views of different techniques employed in order to obtain terminal tabs for application of energization potential to or providing electric load current flow through the electrically conductive surfaces formed on the piezoelectric plate elements of the devices shown in FIGS. 8 and 9.

BEST MODE OF PRACTICING INVENTION

FIG. 1 illustrates a piezoelectric ceramic switching device constructed according to the invention and comprises at least one piezoelectric bender-type switching device 11 having at least two planar piezoelectric plate elements formed by an upper plate 12 and a lower plate 13 best better seen in FIG. FIGS. 1A -1F of the drawings. The piezoelectric ceramic plate elements 12 and 13 are secured in opposed parallel relationship sandwich fashion on opposite sides of at least one central conductive surface 14 and have respective outer conductive surfaces 15 and 16 that are insulated from each other and the central conductive surface 14 by the respective intervening piezoelectric ceramic plate element thicknesses. The piezoelectric ceramic plates 12 and 13 may be formed from lead zirconate titanate, lead metaniobate, barium titanate or other known piezoelectric ceramic materials and, if desired, could even comprise naturally occuring occurring piezoelectric materials such as quartz. The conductive surfaces 14, 14A, 14B, 15 and 16 may be formed by nickel, silver or other like conductor conductors deposited or otherwise secured to the plate elements 12 and 13.

The bender-type piezoelectric switching device further includes at least one set of coacting fixed electrical switch contacts 17 and 18 mounted on relatively rigid support arms which may be sufficiently flexible to absorb impact and which are opened and closed by movement of a prepolarized movable bender portion comprised by the piezoelectric ceramic plate elements 12A and 13A of the bender-type switching device. The contacts 17 and 18 coact respectively with contacts 19 and 21 formed on the movable end of the bender device 12A, 13A in a manner to be described more fully hereafter with relation to FIG. 1D, 1E and 1F.

The movable bender portions 12A, 13A of the piezoelectric ceramic switching device 11 are physically supported in a cantilever manner by clamping means shown at 22 and 23 which both serve to physically hold and clamp together the piezoelectric ceramic plates 12 and 13 with the central conductive surface 14 sandwiched therebetween. The clamping means 22 and 23 is illustrated better in FIG. 1C of the drawings where it can be seen that it is comprised by two elongated substantially rigid electrically insulating bars 22 and 23 whose ends extend beyond the side edges of the piezoelectric ceramic plate elements 12 and 13. Threaded set screws shown at 24 serve to clamp the two insulating bar members 22 and 23 together along with the interposed ceramic plate elements 12 and 13 and central conductive surface 14. Other forms of suitably clamping and holding the piezoelectric ceramic plate members 12 and 13 together in assembled relation will be suggested to those skilled in the art.

As best shown in FIG. 1A, the clamping means 22 and 23 are disposed over portions 12B and 13B of the piezoelectric ceramic plate elements 12 and 13 which have not been prepolarized and therefore are unpoled and electrically neutral as opposed to the prepolarized active movable bender portions 12A and 13A of the plate elements on which the contacts 19 and 21 are formed. Preferably, the clamping means 22 and 23 are disposed over the ends of the non-polarized or unpoled portions 12B and 13B which are immediately adjacent to and physically integrated with the end of the prepolarized active movable bender portion comprised by plate element portions 12A and 13A which have been prepolarized and therefore are indicated as poled. It has been discovered that by mounting the piezoelectric ceramic plate elements in this manner, the number of failures due to fracturing of the piezoelectric plates at their support points is greatly reduced.

With the bender-type piezoelectric ceramic switching device shown in FIGS. 1 and 1A, it is possible to prepolarize the plate portions 12A and 13A in-situ after fabrication of the device in the manner shown in these drawings. This is achieved by applying suitable value prepolarizing potentials of the same polarity to the terminals T3 and T4 respectively, while concurrently holding the common terminal Tc at the opposite polarity. Simultaneously the temperature of the device may be elevated in an oven or otherwise to a temperature just under the Curie temperature of the piezoelectric ceramic plate elements 12 and 13. The temperature to which the devices will be elevated and the value of the prepolarizing potentials will vary dependent upon the particular piezoelectric ceramic material employed to form the plate elements 12 and 13 as is known to those skilled in the art of peizoceramic piezoceramic fabrication. Ambient temperature polarization also is possible if the polarizing potential is sufficiently high. During the prepolarization operation, and in order to separate the piezoelectric ceramic plate elements 12 and 13 into the two separate poled portions 12A and 13A and the unpoled portions 12B and 13B, it is necessary to electrically isolate the two portions so that the prepolarizing potential is not applied across the unpoled portions 12B and 13B and the common conductive surface 14. For this purpose, suitable gaps shown at 15A and 16A are deliberately formed across the width of the exterior conductive surfaces 15 and 16, respectively, whereby an electric potential applied between either of the terminals T3 or T4 and the common terminal Tc connected to the central conductive surface 14, will not appear across the piezoceramic plate portions 12B and 13B which are to remain unpoled. It sould should be noted that the portions of the piezoelectric ceramic plate elements disposed under the clamping bars 22 and 23 have their outer conductive surfaces removed so that the portions 12B and 13B under the clamping means and immediately adjacent and physically integrated with the prepolarized plate portions 12A and 13A remain unpoled and electrically neutral.

As a result of fabrication in this manner, during operation of the bender-type switching device, energizing potentials may be selectively and respectively applied either to terminal T3 or terminal T4 relative to Tc to cause the polarized active movable bender plate portions 12A or 13A to bend and close their respective contacts 19 or 21 on either of the coacting contacts 17 or 18, respectively. As noted in the brief discussion earlier in the specification, prepolarization of the active movable portions 12A and 13A of the piezoelectric ceramic plate elements will leave these portions permanently altered in physical dimensions relative to what they were prior to prepolarization and relative to the unpoled portions 12B and 13B of the piezoelectric ceramic plates 12 and 13. This alteration will be in the form of a permanent increase in dimension of the plate portions 12A and 13A between the poling electrodes 15-14 and 16-14 and also will induce a permanent decrease in dimension parallel to the electrodes (i.e. along the longitudinal dimension of the device as shown in FIG. 1A). When a DC voltage of the same polarity as the prepolarizing voltage, but of smaller magnitude, subsequently is applied as an energizing potential between the poling electrodes, the plate element portions 12A and 13A experience a further temporary expansion in the poling direction and contraction parallel to the electrodes. When the energizing DC potential is removed, this temporary expansion in the poling direction and contraction parallel to the electrode is released, and the plate element portions 12A and 13A return to their normal, at rest unenergized condition established by the prepolarization, voltage effects only. Thus, it will be appreciated that the movable bender plate element portions 12A and 13A automatically return to their original prepolarized dimensions so that the bender moves back to its central, at rest, unenergized condition with contacts 19 and 21 being opened when the DC energizing voltage is removed from across the electrodes T3-Tc or T4-Tc.

A key feature of the present invention is the provision of piezoelectric ceramic bender-type switch energization and/or utilization circuit means which are built directly onto an unused portion of the piezoceramic plate elements 12 and 13 of the bender-type piezoelectric ceramic switching device 11 as will be described hereinafter. When thus constructed, circuit stray inductance is reduced to an absolute minimum since circuit interconnecting conductor runs formed on such unused piezoceramic plate portions require only minimum lengths. The energization circuits thus formed serve to supply a direct current energizing potential selectively and respectively to each piezoelectric plate element portion 12A or 13A which energization potential always is poled in the same direction as the prepolarizing electric field previously permanently induced in the piezoelectric plate element portions 12A and 13A whereby no depolarization of the piezoelectric plate element portions occurs during continued or successive operations of the switch to close or open the make and break contacts 17, 19 or 18, 21. It will be appreciated therefore than an improved piezoceramic switching device according to the invention such as that shown in FIGS. 1 and 1A can be operated as either a normally-open or a normally-closed switch without detriment to the long term stability and reliability characteristics of the switch. This is explained as follows.

Assume that the outer conductive surfaces 15 and 16 over peizoelectric piezoelectric ceramic plate portions 12A and 13A are maintained positive while the central conductive surface 14 is maintained negative during the prepolarization of the plate portions 12A and 13A as described briefly above. The prepolarization of these plate elements then will cause a permanent increase in dimension between the poling electrodes and a permanent decrease in dimensions parallel to the electrodes (i.e. the plate portions 12A and 13A will become thicker and shorter). Since both plate element portions 12A and 13A are prepolarized substantially simultaneously, this permanent change in dimension will not effect the centering position of the active movable bender comprised by plate portions 12A, 13A relative to the coacting contacts 17 and 18. However, in the event th