GB2123602A - Piezoelectric transducer and method of making same - Google Patents

Abstract

A polymer, e.g. polyvinylidene fluoride (PVDF), piezoelectric transducer is produced in a continuous process and comprises a center wire conductor 30 surrounded by a layer of molecularly polarized polyvinylidenefluoride 36. An outer conductive layer is provided if required. A transducer may be assembled in any required configuration by folding the coated wire transducer to the appropriate shape. A transducer array is assembled by connecting a plurality of PVDF coated wire transducer elements to a central multiconductor cable, and may be used in a soner system. The surrounding layer of PVDF is applied in a continuous wire coating process, polarization of the layer being achieved by drawing the coated wire 37 through a coating-reducing die 40 and applying a radially directed high voltage electric field to the coating 36, the field being established between a cylindrical electrode 42 surrounding the coated wire and the wire 30 itself. The stretching die 40 and poling electrode 42 may be a unitary structure, for simultaneous action. <IMAGE>

Description

SPECIFICATION Wire transducer and method of making same Background of the invention The piezoelectricity of polymeric materials has been known for some time. The piezoelectricity is due to the net polar alignment of the repeating molecule units of the long molecule chains, each unit forming a molecular dipole. This net molecular polarization may occur naturally, otherwise the piezoelectricity of the polymer may be increased by polarizing the long molecule chains. This is normally done by stretching the polymer to elongate the molecule chains and by poling, i.e. applying a large electric field to the material to properly reorient the molecular dipoles in the direction of the field to produce the desired net polar alignment. Known developments and applications of polymer transducers have used the polymer in sheet form.A severe problem in the poling of present transducer materials has been the arcing which normally results when large electric fields are applied by electrodes on the opposite sides of the polymer sheet. It has been found that it is difficult to prevent arcing around the edges of the flat sheet and that the arcing prevents the effective poling of the material, since the electric field is then shorted.

Summary of the invention The present invention provides for a transducer having a cylindrical configuration which affords two important advantages. First, it permits poling of the polymer material without the arcing problems of the prior art and, second, it permits the configuration of a conformal transducer by weaving the cylindrical transducer into any desired shape.

The present invention further provides for a method for producing such a wire transducer comprising the steps of stretching the polymer coating of an inner conductor and applying a high voltage between the outside surface 'of the coating and the inner conductor. This results in an efficient continuous method of polarizing the polymeric transducer as opposed to the batch processing as is known in the art. Preferably, the polymer comprises polyvinylidenefluoride.

Brief description of the drawings The present invention may be better understood as the description thereof progresses with reference to the accompanying drawings in which like numbers refer to like elements and wherein: Fig. 1 A shows a first embodiment of the transducer of the present invention; Fig. 1 B shows a second embodiment of the transducer; Fig. 2 shows in cross-section an apparatus for the manufacture of a wire transducer; Fig. 3 shows in cross-section a second embodiment of a portion of the apparatus of Fig.

2: Fig. 4 shows diagrammatically an underwater application of a transducer array using a plurality of transducer elements such as those shown in Fig. 1 A; and Fig. 5 shows a cross-section of the transducer with the arrows showing the direction of poling field and the direction of alignment of the molecular dipoles.

Description of the preferred embodiment Referring now to Figure 1 A, there is shown the wire transducer 10 of the present invention. It comprises center conductor 12 surrounded by a coating of polarized polymer material 14. Polymer material 14 is a polymer which exhibits piezoelectricity and in a preferred embodiment consists of polyvinylidenefluoride (herein referred to as PVDF). PVDF is a semi-crystalline polymer formed by long molecule chains of repeating CF2-CH2 molecule units. PVDF exhibits piezoelectricity when the repeating molecule units of its long molecule chains are oriented to produce a net polarization. In applications which require use of the transducer in a conductive medium, such as sea water, the transducer need not have an additional electrode on the outside surface of the coating 14 in order to utilize the piezoelectric signal.Figure 1 A shows such an application. Transducer 10 is formed by a predetermined length of central conductor 12 and polymer coating 14, with one end of cylindrical coating 14 being sealed with an electrically insulating cap 15. Typically, the length of wire transducer 10 is a fraction of the wavelength of the selected signal in the medium in which the transducer 10 is used. Cap 15 may be formed by applying heat to the polymer coating in the region of the end to be sealed and welding an insulating piece thereto. Figure 1 A also shows utilization device 17 being connected to central conductor 12 and by electrode 19, through conductive medium 18, here shown as sea water, to the outside surface of polymer coating 14.Utilization device 17 processes the signals generated by transducer 10 and may comprise a conventional sonar signal processor and display. For applications in non-conductive media or other applications, a second conductor 16, such as an outer conductive coating, is placed over the outer surface of polymer material 14, to form transducer 10', as shown in Fig. 1 B. Utilization device 17 in this case is connected directly to conductors 12 and 16 in order to process the signals generated by the piezoelectric layer 14 sandwiched therebetween.

A polymeric material such as PVDF, exhibits substantially greater piezoelectric activity when its long molecule chains are elongated and the repeating molecule units are oriented to produce a net dipole moment. This may be achieved, respectively, by stretching the polymer and by poling, that is the application of an electric field in the direction along which the dipoles are to be aligned. Piezoelectric activity is further increased by forcing the semi-crystalline structure of PVDF into that of Form I, also referred to as B. Form I, has the dipoles of the repeat units pointing in the same direction. It is found that crystallization into Form I is effected by the stretching and poling of the polymer. A more complete description of the piezoelectricity of polymers and of the various forms for PVDF may be found in R. Hayakawa and Y.Wada, "Piezoelectricity and Related Properties of Polymer Films", Volume 11, in Advances in Polymer Science, Springer-Verlag 1 973.

Referring now to Figure 2, there is shown an apparatus which may be used to carry out the polarization of the polymer and the manufacture of the wire transducer. First, a source of polymer material 20, such as PVDF pellets is contained within hopper 22. PVDF pellets 20 are brought by a feed screw 24 to a melting chamber 26. PVDF pellets 20 in melting chamber 26 melt in the zone defined by the heating coils 28 which encircle the lower portion of melting chamber 26 and provide heat sufficiently to melt the polymer pellets. A conductor 30 is drawn from a supply reel, not shown, through the molten polymer 20' at the bottom of melting chamber 26 past an extrusion die 32, thus producing a wire 37 having a coating 36 of polymer material.The thickness of coating 36 is determined by the speed of conductor 30 going through the molten material 20' and the hydrostatic pressure of the molten material. A typical temperature for the extrusion process is in the 2500C range. Wire 37 is then passed through a region in which an air gun 34 directs a jet of air towards the wire 37 to quench, or reduce, the temperature of the coating 36 so that it solidifies on the conductor 30. Conductor 30 with its now solidified polymer coating 36 is then drawn through a stretching die 40 which has a tapered or stepped down bore so that the cross-section of the coating 36 is reduced. Wire 37 is pulled through stretching die 40 by rollers 48. Rollers 48 rotate in opposite direction, as shown by the arrows in Fig. 2, to grab wire 37 and pull it from die 40.The stretching of coating 36 occurs when coating 36 is pulled through die 40 at a predetermined rate and the tapered bore of the die keeps back a portion of coating 36. The crosssection of the coating 36 is reduced in the stretching die 40 by a factor of 5, also referred to as a draw ratio of 5:1. Other draw ratios may be used to achieve a predetermined level of molecular orientation. For a draw ratio of 5, coating 36 exits extrusion die 32 at 1/5 the speed of conductor 30, while coating 36 exits stretching die 40 at the same speed as that of conductor 30.

The stretching die is physically close through the extrusion die 32 because the coating is sliding on the wire between the two dies due to the stretching operation of the stretching die 40. The temperature of the stretching die, maintained by heating coils 41, is such as to facilitate the stretching of the coating 36 but not so hot as to permit the molecules to relax to their original position after the stretching. A preferred temperature for the stretching die is in the range of 1 10--1300C. Temperatures in excess of approximately 1 300C must be avoided, or the molecule alignment achieved by the stretching will be lost. Since the coating must slide on the wire between the extrusion die 32 and stretching die 40, a lubricant on the wire is preferred to facilitate the stretching operation.The air blast between the two dies is adjusted so that the temperature of the coating exiting the extrusion die 32 is within the temperature range of the stretching die 40.

As coated conductor 37 exits the stretching die 40 it goes through a poling electrode 42 and into a take up reel (not shown) where conductor 30 is electrically grounded. Poling electrode 42 is cylindrical in shape and is connected to a high voltage supply 44 which provides a voltage to produce in coating 36 an electric field which extends radially from conductor 30 and sufficiently polarizes the molecular dipoles of the polymer. Fig. 5 shows a cross-section of transducer wire 37 with arrows 39 representing the direction of the electric field applied by supply 44. The arrows also represent the direction along which the molecular dipoles are aligned. The voltage applied is selected to produce an electrical field of at least 100 volts per micrometer.The length of the poling electrode and the speed with which the wire 37 is drawn past electrode 42 are such that a predetermined amount of electric field is present for a period of time sufficient to complete the polarization.

Typically, 80% of the possible polarization is achieved within the first 5 minutes. If a transducer of the type shown in Fig. 1 B is needed, coated wire 30 is drawn past a region, not shown, located prior to the take-up reel in which a conductive layer 16 is deposited on coating 36.

It is found that polarization of the polymer is facilitated by the application of the poling electric field simultaneously with the stretching of the polymer. This may be explained by considering that a predetermined amount of energy is required to achieve a predetermined amount of polarization. In the prior art, a portion of this energy was supplied in a mechanical form by the drawing step by stretching the long molecule chains while the remaining portion of the required energy was supplied at a later time in electrical form by the poling step by orienting the dipole units of the molecule chains. However, more than the required polarization energy needs to be used due to the limited efficiency of the separate methods of application. Simultaneous use of mechanical and electrical energy results in a more efficient application of the required energy. This has the advantage then of requiring, for a given level of polarization, a lower electric field or a faster poling time.

Referring now to Figure 3, there is shown another embodiment of the apparatus which may be used to produce the wire transducer 37 of the present invention by the simultaneous use of the stretching and poling steps. The apparatus for melting the polymer and extruding the coated conductor 30 is the same as that for Figure 2 and thus it is not shown. Conductor 30 coming out of the extrusion die 32 is now fed into die 50. A first portion 52 of stretching die 50 has a similar tapered or step down bore as that for the stretching die 40. The temperature is also kept within the same predetermined range by thermal coils 54.Wire 37 is then pulled through a second portion 56 of stretching die 50 which is separated by the first portion by a neck down region 57 providing a reduced amount of thermal conductivity so that the temperature in the second region 56, maintained by heating coils 58, is lower than the stretching temperature. This is done to prevent the depolarization of polymer coating 36 following the stretching. This time a high voltage supply 60 is connected directly between ground and the conductive die. As before, center conductor 30 of the transducer wire is grounded and is placed in a take up reel also not shown in the drawings. Thus the stretching and poling are performed simultaneously.The combination of thermal coils 54 and a reduced thermal transfer to region 56 of the stretching die 50 is such that the temperature at the point where the wire exits is approximately 900C or less. The optimum temperature for simultaneous stretch and pole may be as low as 500C. A further quenching step, after stretching die 50, is used to lock in the polarization.

Referring now to Figure 4, there is shown the transducer of the present invention in a sonar application. A vehicle such as surface ship 100 tows a transducer array 110 at a predetermined depth. Transducer array 110 is formed by a plurality of PVDF wire transducer elements 112.

Each transducer element 112 dissimilar to wire transducer 10 shown in Figure 1A and comprises a conductor section 12 surrounded by a cylindrical section of polarized polymer 14 capped at one end to insulate the center conductor. The opposite end of conductor 12 is connected to one of the conductors of central multiconductor cable 120. Each transducer element 12 is connected to a unique conductor of cable 120. Thus, sonar controller 130 can control the signals for each transducer element 112 independently. Of course, all conductors are insulated from the conductive sea water in order not to short out the signal present between the inner surface of piezoelectric layer 14, which is in contact with conductor 12, and the outside surface of layer 14, which is in contact with the sea water.Each transducer element may comprise a length of transducer wire 10 woven to produce a transducer patch of predetermined geometry. For instance, by loosely zigzagging and securing a predetermined length of transducer wire 10, a square transducer element may be formed having a dimension on the order of a fraction of the desired wavelength in the sea water medium.

The sonar system of Figure 4 can operate in either an active or a passive mode. In the active mode, sonar controller 130 includes beamforming means to first transmit electrical signal pulses to transducer elements 112 in the proper phase relationship to cause transducer array 120 to generate a sonic beam in a predetermined direction. Sonar controller then enables transducer elements 11 2 in a predetermined phase relationship in order to detect signal returns from a predetermined direction. In the passive mode, only the latter function is enabled. In either case, the signal returns generated are processed by a suitable signal processor and the resulting signals are passed to an utilization device such as a display on board ship 100. The beamforming function for both transmitting and receiving signal is well known in the art.

Modification of the embodiments of the invention may be apparent to persons skilled in the art without departing from the spirit and scope of the invention. For instance, a much larger diameter wire may be used to produce a polarized polymer coating. The cylindrical polymer may be later cut and removed from the wire or a polymer sheath may be extruded directly to produce a narrow continuous sheet of polarized polymer. Accordingly, it is desired that this invention is not to be limited to the embodiments disclosed herein but is to be limited only as defined by the appended claims.

Claims (27)

Claims

1. In combination: a center conductor; and a molecularly polarized polymer surrounding said conductor.

2. A transducer comprising a layer of an oriented, poled polymer disposed over an inner conductor.

3. The transducer of Claim 2 further comprising a second conductor disposed over said polymer layer.

4. A wire transducer comprising a first conductor having a homogeneous layer of piezoelectric material thereon.

5. A transducer element comprising a central conductor having a predetermined length and a piezoelectric coating disposed on said conductor, said conductor being spatially arranged in a predetermined geometry.

6. An article of manufacture comprising an elongated sheath of piezoelectric material.

7. In combination: an elongated first conductor; a layer of piezoelectric material disposed around a portion of the surface of such conductor; and a second conductor disposed over a surface of said piezoelectric layer opposite said first conductor.

8. A transducer system comprising: a multiconductor cable comprising a plurality of insulated conductors; and a plurality of transducer elements, each of said transducer elements being connected to a predetermined one of said insulated conductor, each of said transducer elements comprising a conductor wire coated with a homogeneous layer of piezoelectric material.

9. A system comprising: at least one transducer comprising a conductor and a homogeneous layer of piezoelectric material disposed to surround at least a portion of said conductor; and means for producing an electrical signal of predetermined characteristics coupled to said transducer.

10. A system comprising: at least one transducer comprising a predetermined conductor and a homogeneous layer of piezoelectric material disposed around a portion of the surface of said conductor for producing an electrical signal in response to mechanical energy applied to said piezoelectric layer; and means for processing said electrical signal coupled to said transducer.

11. A sonar system comprising: a transducer array comprising a cable having a plurality of conductors and a plurality of transducer elements selectively coupled to said conductors, at least one of said transducer elements comprising a wire having a coating of piezoelectric material; and means for receiving electrical signals from said plurality of transducer elements in a predetermined phase relationship, said electrical signals being generated by said transducer elements in response to applied mechanical energy.

12. A process comprising the steps of: providing a conductor with an insulating polymer disposed about a portion of the surface of such conductor; polarizing said polymer.

13. The process of manufacturing a wire transducer comprising the steps of: providing a wire having a central conductor surrounded by a polymer insulation; applying an electric field between the central conductor and the outside surface of said polymer insulation.

14. The process of Claim 13 further comprising the step of heating said polymer prior to applying the electric field.

15. The process of manufacturing a wire transducer comprising the steps of: coating a conductor with a layer of polymer insulation; stretching said polymer insulation.

16. The process of manufacturing a wire transducer comprising the steps of: coating a conductor with a layer of polymer; stretching said polymer on said conductor; applying an electric field between said conductor and the outside surface of said polymer.

17. The process of Claim 16 further comprising the step of applying heat to the polymer prior to the application of the electric field.

18. The process of manufacturing a transducer comprising the steps of: coating a conductor with a layer of polymer at a first predetermined temperature; drawing said coated wire through a reduceddiameter die for stretching said layer of polymer at a second predetermined temperature; applying a predetermined voltage to said coated wire for producing an electric field between the outside and inside surface of the polymer.

19. The process of Claim 18 wherein said voltage is applied in a region electrically insulated from said die.

20. The process of Claim 18 wherein said voltage is applied in a region which includes said die.

21. A process for the continuous manufacture of a piezoelectric transducer comprising the steps of: drawing a conductor past a source of polymer to coat said conductor with a layer of said polymer; stretching said polymer layer as it is drawn past a first region adjacent said source; and applying an electric field radially through said layer in a second region adjacent to said first region.

22. The process of Claim 21 wherein said first and second regions are electrically coupled.

23. A process for polarizing a polymer coating on a wire comprising the step of simultaneously stretching and poling said polymer.

24. A continuous process for the polarization of a polymer comprising the steps of: providing a continuous supply of polymer; guiding the polymer through a first region, stretching said polymer in said first region; and applying a poling field to said polymer in a second region adjacent said first region.

25. The process of Claim 24 wherein: said poling second region includes said stretching first region resulting in the simultaneous stretching and poling of said polymer.

26. A process according to claim 24 and substantially as described hereinbefore with reference to Fig. 2 or Fig. 3 of the accompanying drawings.

27. A piezoelectric transducer substantially as described hereinbefore with reference to Fig. 1A or Fig. 1 B of the accompanying drawings.