WO2018227122A1

WO2018227122A1 - Enhanced cymbal piezoelectric transducer

Info

Publication number: WO2018227122A1
Application number: PCT/US2018/036710
Authority: WO
Inventors: Ahmad Safari; Gregory YESNER; Max KUCIEJ; Ali Maher
Original assignee: Rutgers, The State University Of New Jersey
Priority date: 2017-06-09
Filing date: 2018-06-08
Publication date: 2018-12-13

Abstract

A piezoelectric cymbal transducer for use in piezoelectric power generators may include a piezoelectric element on which electrodes are disposed in a physically parallel configuration. The electrodes comprise a single electrode disposed on the top surface and a corresponding single electrode disposed on the bottom surface with electrical connections connecting alternating pairs of electrodes. The set of pairs of electrodes connected to the first electrical connection may be interleaved with the set of pairs of electrodes connected to the second electrical connection. The piezoelectric cymbal transducer also includes a top deformable cap connected at opposite ends of the top surface and a bottom deformable cap connected at opposite ends of the bottom surface.

Description

ENHANCED CYMBAL PIEZOELECTRIC TRANSDUCER

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/517,502, filed June 9, 2017, the disclosure of which is incorporated by reference herein.

STATEMENT OF GOVERNMENT INTEREST

[0002] This invention was made with government support under contracts DTRT12-G-UTC16 and DTRT13-G-UTC28, both awarded by the U.S. Department of Transportation, Office of the Assistant Secretary for Research and Technology. The government has certain rights in the invention.

FIELD OF THE INVENTION

[0003] This invention is related to the field of energy harvesting and, more specifically, to the harvesting of energy from piezoelectric transducers of novel design.

BACKGROUND OF THE INVENTION

[0004] As the power requirements of sensors, wireless links and other components fall, energy harvesting is becoming more significant as a way of easily powering devices in the home, on the factory floor and even in the body. This is stimulating researchers in companies and universities to look at different ways of generating power from the environment with some distinctly dramatic new approaches.

[0005] For example, thermoelectrics, or generating power from heat, is being investigated by organizations such as the U.S. Department of Energy, who are working in conjunction with BMW and GM to turn heat waste from engines and exhaust into power for the vehicle's electrical systems. NASA uses thermoelectrics to power Mars rovers where they work without light, unlike solar cells.

[0006] Another example of an emerging technology to address this problem is that of micro- windmills, (or "winMEMS"), which are windmills build using MEMS technology on a millimeter scale that can be used to generate power for small devices, such as smartphones. A single grain of rice could hold about 10 of the windmills, and hundreds of the windmills could be embedded in a sleeve for a smartphone which would generate power by waving the device in the air.

[0007] The generation of energy using piezoelectric transducers is well known in the art. Piezoelectric energy harvesters are of great interest due to their small form-factor and high efficiency. Cymbal-type piezoelectric transducers are also known. Cymbal-type transducers for energy harvesting are designed to support a load repeatedly without breaking. U.S. Patent 8,278,800, describes using a piezoelectric transducer for harvesting energy from vehicle induced stress in pavement, and uses "ceramic rods" as the active piezoelectric element.

SUMMARY OF THE INVENTION

[0008] This is an improvement to the "cymbal" flex-tensional transducer to be used for generating energy from mechanical loading that is more efficient and robust than prior art designs. The novel transducer design has a square or rectangular shaped instead of the circular shape traditionally used for the cymbal.

[0009] The disclosed design improves the ability of a piezoelectric transducer to capture energy compared to conventional transducer designs. When combined with a reliable mechanical input source, for example, vehicle induced stress in pavement of roads or bridges, it can be used to provide off-grid power for electronics and sensors. [0010] In an example embodiment, a piezoelectric cymbal transducer includes a piezoelectric element having a top surface on which a plurality of electrodes is disposed in a physically parallel configuration and a bottom surface on which a corresponding second plurality of electrodes is disposed in a physically parallel configuration. The electrodes form a plurality of physically parallel pairs of electrodes, each pair comprising a single electrode disposed on the top surface and a corresponding single electrode disposed on the bottom surface. A first electrical connection connects alternating pairs of electrodes and a second electrical connection connects opposite alternating pairs of electrodes. The set of pairs of electrodes connected to the first electrical connection are interleaved with the set of pairs of electrodes connected to the second electrical connection. The piezoelectric cymbal transducer also includes a top deformable cap connected at opposite ends of the top surface and a bottom deformable cap connected at opposite ends of the bottom surface.

[0011] The top deformable cap may be electrically connected to the first electrical connection and the bottom deformable cap is electrically connected to the second electrical connection. The deformation of the top and bottom deformable caps places both compressive and tensile stresses on the piezoelectric element. In an embodiment, the top and bottom caps are composed of brass. In an embodiment, the piezoelectric element is composed of lead zirconate titanate (PZT).

[0012] In another embodiment, a piezoelectric power generator may include a box shaped casing consisting of a body and a cap with one or more layers of piezoelectric transducers disposed within said casing body, each layer consisting of one or more piezoelectric transducers. One or more layers of a conductive material placed in between each of said layers of piezoelectric transducers, each of said layers of conductive material electrically connected in a parallel configuration. The cap may extend into the body of the casing such that a compressive force applied to the casing will compress one or more layers of piezoelectric transducers.

[0013] In one embodiment, the casing body and cap are composed of aluminum. In one embodiment the conductive material is copper. The compressive force that can be transferred from the cap to the one or more layers of piezoelectric transducers may be limited by the travel of the cap with respect to the body.

[0014] The piezoelectric power generator may also include an electrical circuit to convert high voltage AC pulses to a steady usable voltage, said electrical circuit being connected to said one or more layers of conductive material. Each layer of piezoelectric transducers in the piezoelectric power generator may contain a 4 x 4 array of piezoelectric transducers.

[0015] A method of manufacturing a piezoelectric cymbal transducer for use in a piezoelectric power generator may include forming a piezoelectric element having a top surface and a bottom surface and forming a plurality of physically parallel pairs of electrodes by forming a first plurality of electrodes disposed in a physically parallel configuration along said top surface and forming a second plurality of electrodes disposed in a physically parallel configuration along said bottom surface. The method may also include electrically connecting alternating electrodes such that all electrodes formed on the top surface are connected with a first electrical connection and all electrodes formed on the bottom surface are connected to form a second electrical connection, wherein the electrodes forming the first electrical connection are interleaved with the electrodes forming the second electrical connection. The method may also include attaching a top deformable cap connected at opposite ends of said top surface and a bottom deformable cap at opposite ends of said bottom surface. [0016] Isolating the electrodes formed on the top surface from the electrodes formed on the bottom surface may include covering an edge of the piezoelectric alternating electrodes with an insulating material. The method may also include applying an insulating material to opposite ends of the piezoelectric element prior to attaching the top and bottom deformable caps. One layer of the insulating material may be applied to a side of the top surface and one layer is applied to a side of the bottom surface opposite to the side of the top surface. Forming a plurality of physically parallel pairs of electrodes may include applying silver paste on the surface of the piezoelectric element. The plurality of physically parallel pairs of electrodes may be spaced using a mask.

DESCRIPTION OF THE DRAWINGS

[0017] The various aspects, features and embodiments of the disclosed cymbal piezoelectric transducer, its architectural structure, and its method of operation will be better understood when read in conjunction with the figures provided. Embodiments are provided in the figures for the purpose of illustrating aspects, features and/or various embodiments of the piezoelectric transducer, its architectural structure and method of operation, but the claims should not be limited to the precise arrangement, structures, features, aspects, embodiments or devices shown, and the arrangements, structures, subassemblies, features, aspects, embodiments, methods, and devices shown may be used singularly or in combination with other arrangements, structures, subassemblies, features, aspects, embodiments, methods and devices.

[0018] Fig. 1 shows an example method of fabrication of the disclosed piezoelectric transducer.

[0019] Fig. 2 shows example methods for depositing electrodes.

[0020] Fig. 3 shows a method of operation of the disclosed piezoelectric transducer. [0021] Fig. 4 shows an example configuration of electrodes.

[0022] Fig. 5 shows the application of the end caps and a method to prevent the end caps from causing a short circuit.

[0023] Fig. 6 is a schematic representation of a piezoelectric generator comprising a plurality of piezoelectric tranducer devices within a modular design and casing.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The following description is made for illustrating the general principles of the invention and is not meant to limit the inventive concepts claimed herein. In the following detailed description, numerous details are set forth in order to provide an understanding of the piezoelectric transducer, its architectural structure and method of operation, however, it will be understood by those skilled in the art that different and numerous embodiments of the piezoelectric transducer, its architectural structure and method of operation may be practiced without those specific details, and the claims and invention should not be limited to the embodiments, subassemblies, features, processes, methods, aspects, features of details specifically described and shown herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations.

[0025] Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc. It must also be noted that, as used in the specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless otherwise specified. [0026] In an advantageous design for a cymbal piezoelectric transducer, a piezoelectric ceramic may be polarized in the lateral direction instead of vertically. This configuration utilizes the d₃₃ piezoelectric coefficient rather than the d₃i coefficient. The d₃₃ coefficient is at least two times greater than the d₃i coefficient, which enhances the energy output of the transducer. Polarization in the lateral direction may be achieved using repeating and alternating electrodes that are applied to the surface of the ceramic element. This electrically separates the single ceramic into multiple electrical/piezoelectric sections. When the sections are electrically connected in parallel, the piezoelectric coefficient of each section can be enhanced considerably. The effective piezoelectric coefficient, d₃₃, of a single ceramic is a product of the number of sections. This novel electrode configuration with a very large effective piezoelectric coefficient increases energy output. The metal end caps attached to the top and bottom of the ceramic amplify applied force and transfers it laterally as tensile stress to the ceramic element.

[0027] There are two advantagous components to this improved "cymbal" transducer design. First, the piezoelectric ceramic element is polarized in the lateral direction in order to utilize the d₃₃ piezoelectric coefficient, which is at least two times greater than the d₃i coefficient. This novel polarization is achieved by the second novel component of the design, a series of surface electrodes connected in a parallel configuration. In this configuration, the internal electric field is highest at the surface and passes through the body of the ceramic during polarization. Connecting the electrodes in parallel divides the ceramic electrically into individual electrical/piezoelectric sections. When the sections are electrically connected in parallel, the piezoelectric coefficient of each section is additive. The effective piezoelectric coefficient is the d₃₃ of a single ceramic multiplied by the number of sections. Second, the sections are connected in parallel using conductive silver epoxy on the sides of the ceramic, which is applied on top of an insulating layer of epoxy. No external wires are needed to connect the electrodes in a parallel configuration and the connection is completely contained to the surface of the ceramic.

[0028] Many cymbal transducers reported have been fabricated for sensor or actuator applications and require different properties than devices for energy harvesting. Cymbals for actuators are designed to maximize strain when an electric field is applied. Some of the ways that strain is maximized is using thin metal for the end caps and choosing the least stiff (softest) metal which is usually brass. The preferred embodiment of the invention uses soft PZT as the piezoelectric element and 0.3mm brass for the top and bottom caps to get an effective d₃₃ of 15,000 pC/N. Such cymbal transducers used for sensor or actuator applications also typically exhibit low capacitance and yield high voltages, on the order of 100 V.

[0029] Cymbal-type transducers for energy harvesting, in contrast, are designed to support a load repeatedly without breaking. These use steel because it is stronger (higher yield strength) than brass, which would deform permanently under the same stresses. The increased stiffness results in less strain in the end caps under deformation. A cymbal with stiffer end caps has less strain when electric field is applied, so the effective d₃₃ is lower. The d₃₃ coefficient also decreases as the metal thickness increases. Energy harvesting cymbal transducers typically exhibit high capacitance and yield low voltages, on the order of 10-15 V.

[0030] Transducers for actuator/sensor applications cannot be directly compared to transducers for energy harvesting. The closest comparison of the disclosed transducer design is to circular vertically polled cymbal transducers that are similar in and are made with steel end caps, such as those described in Kim et al., IEEE Transactions On Ultrasonics, Ferroelectrics, And Frequency Control, Vol. 54, No. 9 (2007). Table 1 shows data describing metal thickness and ceramic thickness affects the d₃₃ value of prior art transducers. The effective d₃₃ of an example design of the disclosed transducer (19,000 pC/N) has been added into the Table 1 for comparison, identified with an asterisk (*).

[0031] Measurements of piezoelectric coefficient have confirmed that the PZT ceramic is polarized laterally with d₃₃ of 600pC/N per section and the effective piezoelectric coefficients, d₃₃, is (N) (600) where N is the number of polarized sections.

Table 1 - Effective piezoelectric coefficient (pC/N) for different end cap thickness and ceramic thickness

[0032] The disclosed transducer may have the same trend of increased d₃₃ if thinner metal is used. Thinner stainless steel is preferred since it would give higher d₃₃ but must be strong enough to withstand repeated loading.

[0033] Fig. 1 shows the steps of the fabrication of the device. The fabrication process starts with the piezoelectric element 100 as shown in step (a). Piezoelectric element 100 may be, in the preferred embodiment, PZT (lead zirconate titanate) or any other known material with piezoelectric properties. In step (b), electrodes 102 are added. This process is shown in more detail in Fig. 2. A first method of placing the electrodes on piezoelectric element 100 is shown in Fig. 2(a) and uses aligned strips of painters tape 202, aligned for example with frame 201, or similar adhesive tape on the piezoelectric element 100, and thereafter painting on the piezoelectric element using a silver paste between the strips of painters tape. Alternatively, a brass mask 204, shown in Fig. 2(c), may be used to outline the electrodes. The brass mask is placed over piezoelectric element 100 and electrodes 102 are painted on using the silver paste. In addition, piezoelectric element 100 may be heated to increase to the viscosity of the silver paste. The quality of electrodes 102 can be controlled using resistivity measurements. The resulting piezoelectric element 100 with electrodes 102 in place is shown in Fig. 1(b) and in Fig. 2(b). Another method to form the electrodes is screen-printing where a conductive paste is squeezed through a mesh screen to create the desired electrode pattern. Other methods of forming the electrodes are physical vapor deposition techniques such as sputtering or thermal evaporation, as well as chemical vapor deposition.

[0034] Additionally, an insulating layer 104 is applied to the sides of the device, as shown in Fig. 1(c), covering alternating "(+)" electrodes 102(a) to isolate the "(+)" set of electrodes 102(a) from the "(-)" set of electrodes 102(b). The insulating layer may be any suitable insulating material, such as an insulating epoxy, for example. A conductive layer 106 is applied, as shown in Fig. 1(d), over insulating layer 104 to electrically connect the "(-)" electrodes 102(b) to each other. A similar insulating layer and conductive layer is applied to the opposite edge of the device to electrically connect the "+" electrodes to each other and to electrically isolate them from the electrodes (not shown). Preferably, the conductive layer 106 applied over insulating layer 104 on the edges of the device is silver, but any conductive material may be used. In Fig. 1(e), insulating layer 108 is applied to opposite ends of piezoelectric element 100, one layer being applied on the top surface and one layer being applied on the bottom surface of the opposite end of piezoelectric element 100. In step (f) of Fig. 1, the end caps 110 and 112 are attached to piezoelectric element 100, completing the construction of the device. It should be noted that in this design end caps 110 and 112 are part of the electrical circuit and cannot contact oppositely charged electrodes. In the example shown, top cap 110 is electrically connected to the inner sets of electrodes 102(a) labeled "(+)" in Fig. 1(e) and bottom cap 112 is electrically connected to the sets of electrodes 102(b) labeled "(-)" in Fig. 1(e). Top cap 110 and bottom cap 112 may include a deformable section, for example, deformable section 114 of top cap 110 and deformable section 116 of bottom cap 112.

[0035] Fig. 3 shows the difference between a device according to the present disclosure (Fig. 3(c)) and the prior art (Fig. 3(b)). In Fig. 3(a), the direction of the applied mechanical force 301 on device 300 is shown. When a compressive mechanical force 301 is applied on steel end cap 302 in the direction of the arrows (labeled "Applied force 301"), a tensile stress 304 is created in the middle of piezoelectric element 302 and compressive stresses 305 are created towards the ends of piezoelectric element 302. Fig. 3(b) shows the conventional cymbal type transducer 310 including piezoelectric element 312 and steel end cap 313, where the piezoelectric element 312 is a single electrical/piezoelectric section. Polarization direction 311 shows vertical polarization between electrodes covering either side of the piezoelectric element 312. Fig. 3(c), on the other hand, shows an a transducer having a piezoelectric element 322 and a steel endcap 323, where piezoelectric element 322 is broken down by the positive and negative electrodes (e.g. 102(a) and 102(b) in Figure 4) into multiple electrical/piezoelectric sections having alternating poling directions 321 and which are poled horizontally 324.

[0036] Fig. 4 is a schematic representation of the electrical topology of both the prior art design and of an example embodiment of the present disclosure. Fig. 4(a) shows an example electrode pattern 400 of the present disclosure, having electrodes to break piezoelectric element into multiple electrical/piezoelectric sections. Electrodes 102(a) (labelled "+") are electrically connected together in parallel. Electrodes 102(b) (labelled "-") are electrically connected together in parallel. The combination of the two sets of electrodes form alternating opposing poles in an interleaved design, generating the alternating poling directions 321 shown in FIG. 3(c). In Fig. 4(b), a prior art electrode patter is shown in contrast, having only one electrical/piezoelectric section where the top surface 412 represents one pole and the bottom surface 414 represents the opposite pole. Fig. 5 shows the poles of the device of the present invention, showing cap 110 being electrically connected to the positive electrodes 102(a) and cap 112 being electrically connected to the negative electrodes 102(b).

[0037] Fig 6 shows an example piezoelectric generator comprising a plurality of the devices described in this disclosure. In a preferred embodiment, casing 700 may be composed of half- inch aluminum with the top and bottom being coated with an insulating epoxy and the sidewalls lined with nylon. In the preferred embodiment, the module may, for example, contain four layers, each layer may comprise of a 4x4 array of transducers, for a total of 64 transducers 702. Conductive plates 704 may be disposed between each layer of transducers and serve as current collectors. In a preferred embodiment, conductive plates 704 may be composed of copper. Wires connect each plate 704 in parallel. The gap 706 between cover 701 of the modular casing and the body 700 of the modular casing provides the means by which transducers 702 may be compressed and serves to limit the maximum vertical displacement of transducers 702, to avoid damage via over-compression. Excessive loading to the modular casing causes cover 701 to bottom out on the casing body 700, thereby preventing damage to transducers 702.

[0038] A circuit may be used to convert the harvested energy from the modular configuration of transducers shown in Fig. 6 (or any other configuration of transducers). The circuit converts high voltage AC pulses to a steady usable voltage. The AC voltage is rectified, with negative voltage becoming positive. A circuit such as an efficient switch mode DC-DC step down voltage "buck" converter lowers the required impedance of the load. This type of circuit may be combined with a rechargeable battery to be self sufficient and completely isolated from external power sources to provide power to devices.

[0039] The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

We Claim:

1. A piezoelectric cymbal transducer comprising:

a piezoelectric element having a top surface and a bottom surface

a first plurality of electrodes disposed in a physically parallel configuration along said top surface and a corresponding second plurality of electrodes disposed in a physically parallel configuration along said bottom surface, thereby forming a plurality of physically parallel pairs of electrodes, each pair comprising a single electrode disposed on said top surface and a corresponding single electrode disposed on said bottom surface;

a first electrical connection which connects alternating pairs of electrodes;

a second electrical connection which connects opposite alternating pairs of electrodes, such that the set of pairs of electrodes connected to said first electrical connection are interleaved with the set of pairs of electrodes connected to said second electrical connection; and

a top deformable cap connected at opposite ends of said top surface; and a bottom deformable cap connected at opposite ends of said bottom surface.

2. The piezoelectric cymbal transducer of claim 1

wherein said top deformable cap is electrically connected to said first electrical connection; and

wherein said bottom deformable cap is electrically connected to said second electrical connection.

3. The piezoelectric cymbal transducer of claim 1 wherein deformation of said top and bottom deformable caps places both a compressive and a tensile stresses on said piezoelectric element.

4. The piezoelectric cymbal transducer of claim 1 wherein said top and bottom caps are composed of brass.

5. The piezoelectric cymbal transducer of claim 1 wherein said piezoelectric element is composed of PZT.

6. A piezoelectric power generator comprising:

a box shaped casing consisting of a body and a cap;

one or more layers of piezoelectric transducers disposed within said casing body, each layer consisting of one or more piezoelectric transducers; and

one or more layers of a conductive material placed in between each of said layers of piezoelectric transducers, each of said layers of conductive material electrically connected in a parallel configuration; and

wherein said cap extends into said body of said casing such that a compressive force applied to said casing will compress said one or more layers of piezoelectric transducers.

7. The piezoelectric power generator of claim 6 wherein said casing body and cap are composed of aluminum.

8. The piezoelectric power generator of claim 6 wherein said conductive material is copper.

9. The piezoelectric power generator of claim 6 wherein said compressive force that can be transferred from said cap to said one or more layers of piezoelectric transducers is limited by the travel of said cap with respect to said body of said casing.

10. The piezoelectric power generator of claim 6 further comprising:

an electrical circuit to convert high voltage AC pulses to a steady usable voltage, said electrical circuit being connected to said one or more layers of conductive material.

11. The piezoelectric power generator of claim 6 wherein each layer of piezoelectric transducers contains a 4 x 4 array of piezoelectric transducers.

12. The piezoelectric power generator of claim 6 wherein said one or more piezoelectric transducers comprise:

a piezoelectric element having a top surface and a bottom surface

a first electrical connection which connects alternating pairs of electrodes; a second electrical connection which connects opposite alternating pairs of electrodes, such that the set of pairs of electrodes connected to said first electrical connection are interleaved with the set of pairs of electrodes connected to said second electrical connection; and

13. A method of manufacturing a piezoelectric cymbal transducer for use in a piezoelectric power generator, the method comprising:

forming a piezoelectric element having a top surface and a bottom surface;

forming a plurality of physically parallel pairs of electrodes by forming a first plurality of electrodes disposed in a physically parallel configuration along said top surface and forming a second plurality of electrodes disposed in a physically parallel configuration along said bottom surface;

electrically connecting alternating electrodes such that all electrodes formed on the top surface are connected with a first electrical connection and all electrodes formed on the bottom surface are connected to form a second electrical connection, wherein the electrodes forming the first electrical connection are interleaved with the electrodes forming the second electrical connection;

attaching a top deformable cap connected at opposite ends of said top surface and a bottom deformable cap at opposite ends of said bottom surface.

14. The method according to claim 13, further comprising isolating the electrodes formed on the top surface from the electrodes formed on the bottom surface by covering an edge of the piezoelectric alternating electrodes with an insulating material.

15. The method according to claim 13, further comprising applying an insulating material to opposite ends of the piezoelectric element prior to attaching the top and bottom deformable caps.

16. The method according to claim 15 wherein one layer of the insulating material is applied to a side of the top surface and one layer is applied to a side of the bottom surface opposite to the side of the top surface.

17. The method according to claim 13 wherein the step of forming a plurality of physically parallel pairs of electrodes includes applying silver paste on the surface of the piezoelectric element.

18. The method according to claim 17, wherein the plurality of physically parallel pairs of electrodes are spaced using a mask.

19. The method according to claim 13, further comprising packaging a plurality of piezoelectric transducers into a piezoelectric power generator, wherein the piezoelectric generator comprises:

a box shaped casing consisting of a body and a cap; one or more layers of piezoelectric transducers disposed within said casing body, each layer consisting of one or more piezoelectric transducers; and