DE3324368A1 - ELONGITAL PIEZOELECTRIC CONVERTER AND METHOD FOR THE PRODUCTION THEREOF - Google Patents

Description

Raytheon Company, 141 Spring Street, Lexington, MA 02173, United States of America

Elongated piezoelectric transducer and method of making it

The invention relates to piezoelectric transducers and methods for their manufacture.

The piezoelectric properties of polymers have been around since known for some time. Piezoelectricity relies on the resulting polar alignment of the repetitive molecular units of long molecular chains, each molecular unit forming a dipole. The resulting molecular Polarization can either appear by itself or the piezoelectricity of the polymer can be caused by polarization of long molecular chains can be increased. This is usually done by stretching the polymer to length the molecular chains and by applying a polarity, d. H. Apply a strong electric field to the material for the correct one Realignment of the molecular dipoles in the direction of the field so that a desired resulting polar alignment comes about. Known developments and applications with transducers containing a ■ polymer use the polymer material ' in sheet form or plate form. A difficult problem with j polarizing the material of known transducers of interest here: sizing type consists in the formation of discharges, which

generally when applying strong electric fields by means of electrodes on the opposite sides occur on the polymer sheet or film. It has been shown that the formation of discharges or sparks or arcs around the edges of the flat sheet material or plate material is difficult to prevent and that the spark or the The formation of discharges prevents an effective polarization of the polymer material, since in this case the electrical Field is short-circuited.

The invention aims to solve the problem, a to train and manufacture elongated piezoelectric transducer in such a way that a polarization of the Polymer material is possible without the formation of discharges. An elongated transducer of the ones given here Art is said to be able to be used to form a compliant transducer assembly which can be constructed by weaving or winding the elongated transducer is formed into a desired shape.

According to the invention, the basic idea for solving this problem is that the transducer is designed in the form of a wire wherein a piezoelectric coating surrounds a central conductor over at least part of its length.

A method for producing a wire-shaped piezoelectric transducer is also proposed, according to which the from a polymer existing coating of a central conductor or inner conductor is stretched and a high voltage between the central conductor or inner conductor and the outer surface of the covering made of the polymer is applied. In this way it arises an effective continuous method of polarizing the polymer of the transducer as opposed to batchwise Treatment of the transducer material in known methods. Preferably the converter polymer contains polyvinylidene fluoride.

Advantageous refinements and developments of the basic The solution to the above-mentioned object is also the subject matter of claim 1 which is subordinate to the appended claim Claims, the content of which is hereby expressly made part of the description, without being affected by this Ask to repeat the wording.

Exemplary embodiments are described in more detail below with reference to the accompanying drawings. Show it:

Fig. IA is a schematic representation of a

Embodiment of a piezoelectric Converter of the type specified here,

Fig. IB shows another embodiment of a

piezoelectric transducer of the type specified here in a schematic Representation,

2 shows a cross section through a device for producing a wire-shaped piezoelectric transducer,

Fig. 3 is a cross section through another

Embodiment of part of the device according to FIG. 2,

4 shows a schematic illustration of a transducer arrangement for use under water, wherein the transducer arrangement comprises a plurality of piezoelectric transducers approximately according to Fig. IA contains and

Fig. 5 is a section through a converter of the

type proposed here, with the arrows pointing in the direction of a polarizing field point and indicate the orientation of the alignment of the molecular dipoles.

A wire-shaped piezoelectric transducer is shown in FIG. 1A. It contains an inner conductor 12 which is surrounded by a polarized polymer material 14. The polymeric material 14 is formed of a polymer has the piezoelec- j tric properties and consists according to a preferred embodiment of polyvinylidene fluoride, \ successor neighborhood abbreviated to PVDF designated. The PVDF is a semi-crystalline polymer, which is formed by long molecular chains ^ st ^ made up of repeating CF -CH -molecular units! are built. The PVDF exhibits piezoelectricity when the repeating molecular units with a long molecular chain are oriented so that there is a resulting polarization. In applications which provide for the use of a transducer in a conductive medium, for example in sea water, the transducer does not have to carry an additional electrode on the outer surface of the covering 14 in order to be able to derive the piezoelectric signal. Figure IA shows such an application. The transducer 10 is formed by a certain length of a central conductor or inner conductor 12 and a covering 14 made of polymer material, one end of the covering 14 having a cylindrical shape by a

■ electrically insulating cap 15 is tightly sealed.

j It is characteristic of the length of the wire-shaped transducer 10 a fraction of the wavelength of the selected signal

in the medium in which the transducer 10 is used. the

• electrically insulating cap 15 can be produced by

J that the covering consisting of the polymer in the area of the end, which is to be sealed, is heated and then a j The insulating piece is welded on. Figure IA also shows the Evaluation device 17, which on the one hand with the center conductor 12 and on the other hand via an electrode 19 and the conductive

Γ Medium 18, in this case sea water, with the outer surface of the

j polymer existing covering 14 is connected. The evaluation input

'Direction 17 processes the signals generated by the converter 10

nale and can use a conventional sonar signal processing unit.

j unit as well as display means included. For use in

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non-conductive media or for other applications, a second conductor 16 is, for example, an externally conductive one Covering, provided, which is provided over the outer surface of the polymer material 14 so that a transducer 10 'is formed, which is shown in Figure IB. In this case, the evaluation device 17 is directly connected to the center conductor 12 or the External conductor 16 connected in order to process and evaluate the signals which are transmitted by the piezoelectric layer 14 are generated, which is embedded between the said conductors.

A polymer material, for example PVDF, shows significantly stronger piezoelectric behavior when the long; Molecular chains are elongated and the repeating molecular units are oriented so that there is a resulting dipole moment. This can be achieved in that: on the one hand, the polymer is stretched and, on the other hand, a polarization is carried out, which is achieved by applying an electric field in a direction in which the desired alignment of the dipoles is to take place. The piezoelectric effect is further increased by forcing the semi-crystalline structure of the PVDF into shape I, which is also referred to as shape B. Exhibit in the form I j the dipoles of the repeating molecular units in \ the same direction. It has been shown that the crystallization in Form I can be achieved by stretching and polarizing the polymer material. For a detailed description of the piezoelectric properties of polymer materials and the various forms of PVDF, see the publication by R. Hayakawa and Y. Wada, "Piezoelectricity; and Related Properties of Polymer Films", Volume 11, Advances! in Polymer Science, Springer-Verlag 1973 to find. j

It will now be considered in more detail in Figure 2, in which a device ■ for the production of the wire-shaped transducer and for the polarization of the polymer material is shown. In a funnel or j

Storage trough 22 contains PVDF grains or balls to form a supply of polymer material 20. The PVDF grains 20 are introduced into a melting chamber 26 j by a screw conveyor 24. The PVDF-grains 20 in the melting chamber 26 are melted I in a zone which is determined by a heating coil 28, which surrounds the lower part of the melting chamber 26, and feeds sufficiently heat to melt the polymer grains. A conductor 30 is fed from a supply reel, \ which is not shown by the molten area 20 'of the polymer at the bottom of the melting chamber 26 to extrusion die 32 so that a wire is formed 37 having a lining 36 of polymeric material. The thickness of the coating 36 is determined by the rate of advance of the conductor 30 through the region 20 'of the molten polymer and by the hydrostatic pressure of the molten material. For example, the temperature for the extrusion process is on the order of 250 ° C. The wire 37 then runs through an area in which an air nozzle 34 directs an air jet at the wire 37 in order to quench it or to reduce the temperature of the covering 36 to such an extent that it can Solidified polymer on conductor 30. The conductor 30 with the solidified polymer coating located thereon is now passed through a drawing form 40 which has a conically tapering or stepped bore so that the cross section of the coating 36 is reduced. The wire 37 is drawn through the drawing die 40 by means of rollers 48. The rollers or cylinders 48 rotate in the opposite direction to one another in the manner that can be seen from FIG. The stretching of the covering 36 takes place when the covering 36 is pulled through the drawing die 40 at a certain speed and the tapering bore of the drawing die holds back a part of the covering material 36 in each case. The cross section of the covering 36 is reduced by a factor of 5 in the drawing mold 40, which can also be expressed by specifying the drawing ratio of 5: 1. Should certain values of the

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molecular orientation are achieved, so are appropriate, J other drawing ratios to be provided if necessary. For a With a draw ratio of 5, the lining 36 emerges from the extrusion mold , 32 from a fifth of the speed of the conductor 30, while the covering 36 from the die 40 at the same speed as the conductor 30 exits. The drawing form 40 is body. borrowed close to the extrusion mold 32, as the facing material on the center conductor due to the stretching process in the drawing mold 40 in the area between the two molds 32 and 40 slides. The temperature of the drawing die 40 is kept at such a value by a heating coil 41 that the drawing of the Lining 36 is facilitated, but such a high temperature is avoided that the molecules after drawing or stretching fall back into their original position. A preferred one The temperature range for the drawing form is between 110 ° C and 130 ° C. Temperatures over approximately 130 ° C must be avoided otherwise the molecular alignment achieved by stretching would be lost again. Because the topping; on the center conductor or conductor wire between the extrusion j

Form 32 and the drawing form 40 must slide, a lubricant or lubricant is expediently provided on the conductor in order to to facilitate the stretching process. The blown air between the two molds is adjusted so that the temperature of the Covering, which leaves the extrusion mold 32, in the Tempe ■ temperature range of the drawing form 40 comes.

When the coated center conductor 30 leaves the drawing die 40, it runs through a polarizing electrode 42 to a take-up coil, not shown, on which the center conductor 30 is electrically grounded. The polarizing electrode 42 has a hollow cylindrical shape and is connected to a high voltage source 44, which a voltage generated, which brings an electric field into effect in the covering 36, which extends radially to the center conductor 30 and the molecular dipoles of the polymer | polarized. Figure 5 shows a cross section through the converter!

wire 37, the arrows 39 indicating the direction of the electric field due to the connection to the voltage source 44. The arrows also indicate the direction along which the molecular Dipoles are aligned. The applied voltage is I chosen so that an electric field strength of at least; 100 kV / mm arises. The length of the polarizing electrode; and the speed at which the wire 37 passes through the; Electrode 42 is passed through, are set so that j a certain strength of the electric field for a pre | A certain period of time acts, which is sufficient to carry out the 'polarization completely. Characteristically 80% of the possible polarization is reached within the first five minutes. If a converter of the type shown in FIG. IB is required, the coated center conductor 30 is prior to winding on the take-up reel in FIG : fed to an area (not shown) of a station in which a conductive coating 16 is placed on the piezoelectric Pad 36 is formed.

It has been shown that the polarization of the polymer then

is simplified and facilitated if that is the polarization j generating electric field is brought into action simultaneously with the stretching I- of the polymer. This can be done in

explain the way that it is taken into account that a certain amount of energy is necessary to achieve a certain degree of polarization to reach. So far, some of this energy has been supplied in mechanical form by stretching the long molecular chains were further elongated, during the ver; remaining part of the required energy at a later date

Time was supplied in electrical form by the j polarization formed by the orientation of the respective dipoles ! Molecular units of the molecular chains has been achieved. It is ever-

but to spend more than the required polarization energy, which in each case has a limited degree of efficiency separate process steps. The simultaneous use of mechanical and electrical energy results in a

Increased efficiency with regard to the total energy required. This in turn leads to the advantage of a lower field or a faster polarization time if a certain degree of polarization is desired.

In Figure 3, another embodiment of a device is shown, which can be used to produce a wire-shaped transducer 37 of the type proposed here and a simultaneous one Provides stretching and polarizing in the course of manufacture. The device parts for melting the polymer and

: for extrusion molding of the coating for the center conductor 30 are designed exactly as in the embodiment according to Figure 2, so that a representation is not necessary. The one from the strand; Coated center conductor 30 emerging from compression mold 32 joins of the embodiment now described in a drawing mold 50. A first section 52 of the drawing die 50 has a similarly conical or stepped bore as the drawing die 40. The temperature is also within an appropriate range

, held a predetermined temperature range, which happens by means of the heating coil 54. The wire 37 is then through a second portion 56 of the drawing die 50 drawn, which of the

■ the first-mentioned section is separated by a recess 57, so that there is a reduced thermal conductivity in this area and the temperature in the second drawing die

j cut 56 maintained by heating coil 58. is lower than the drawing temperature can be. In this 'example, a depolarization of the polymer coating 36 in purchase' ■ circuit to avoid the drag operation. In the present ι

. Trap is a high voltage source 60 directly between earth and the form made of conductive material switched.Wie ι previously described, the center conductor 30 of the transducer is grounded

ί and leads to a take-up roll, which is not in the drawing; is shown. The stretching and polarizing is thus at the same time i

i

. tig carried out. The combination of the heating coil 54 and the ' reduced heat-conducting cross-section for generating a lower heat transfer to section 56 of drawing die 50

a · ΐ

Λ (

• *

causes the temperature at the exit point of the wire to be approximately 90 ° C or less. The optimal temperature for simultaneous stretching and polarizing can go down to 50 ° C. A subsequent deterrent step is used behind the drawing mold 50 to set or freeze the polarization.

In FIG. 4, transducers of the type specified here are shown as parts of sonar systems. A vehicle, for example a ship 100 tows a transducer assembly 110 at a predetermined depth. The transducer assembly 110 is off a number of PVDF transducer elements 112 wire-shaped or formed cylindrical shape. Each transducer element 112 is designed like the wire-shaped transducer element 10 according to FIG Figure IA and includes a center conductor section 12 extending from surrounded by a cylindrical section of polarized polymer 14 terminating at one end with an insulating seal or an insulation cap is provided to insulate the center conductor. The opposite end of the center conductor 12 is connected to one end of one of the conductors of a multi-core cable 120. The system sonar controller 130 can therefore control or process the signals for the individual transducer elements 112 independently of one another. Of course are all conductors are insulated from the conductive seawater in order to avoid that between the inner surface of the piezoelectric layer 14, which is in contact with the center conductor 12, and the outer surface of the layer 14 which is in contact with the sea water not to short-circuit occurring signals. Each transducer element can be a certain length of wire-shaped Transducer 10 included, which is interwoven in order to obtain a flat piece of certain geometry as a transducer organ. For example can by loosely zigzagging and fastening a certain length of the wire-shaped transducer 10 a square transducer element can be realized, the dimensions on the order of a fraction of the desired wavelength in seawater.

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AH

The sonar system according to FIG. 4 can be operated either actively or passively. In the active mode of operation, the ^; Sonar control unit 130 beam shaping devices, on the one hand, to supply electrical signal pulses to the individual transducer elements 112 in the correct phase relationship, so that the wall. Ler assembly 112 emits an acoustic beam in a certain direction. Then the control unit activates the converter

elements 112 in a predetermined mutual phase relationship, to pick up signals that each return from a specific direction.

In passive operation, only the last function described is implemented. In either case, the generated signal echoes processed by a suitable signal processing device and the resulting signals are sent to an evaluation device, for example a display device on the ship 100. The formation of a directional characteristic with respect to the transmit signals and the received signals is well known in the art.

Within the scope of the design ideas given here, the person skilled in the art has a number of training and development options Modification options. For example, one can be significant larger diameter of the wire can be selected on which a coating of a polarized polymer is to be applied. The cylindrical Polymer can later be cut and removed from the wire, or it can be made of polymeric material Tube can be directly extruded to form a narrow continuous ribbon of polarized polymer produce.

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Claims (1)

Claims Converter, characterized in that it has elongated brm and carries a homogeneous layer (14) of a piezoelectric material on a conductor (12 or 30). 2. Transducer according to claim 1, characterized in that it has a cylindrical shape , that the conductor (12 or 30) forms a central conductor and that the homogeneous layer of piezoelectric material of a polymer with piezoelectric properties surrounding the central conductor at least over a certain length is formed. 3. Converter according to claim 1 or 2, characterized in that the homogeneous layer (14) made of piezoelectric material of a molecularly polarized polymer or of a polymer is formed with oriented electrical polarization. 4. Converter according to one of claims 1 to 3, characterized in that that the homogeneous layer (14) made of piezoelectric material is surrounded by an outer conductor (16). 5. converter system with a plurality of converters (112) according to one of claims 1 to 4, characterized in that each of the conductors (12 or 30) of the transducers coated with a homogeneous layer (14) of piezoelectric material with an insulated conductor of a multicore cable containing a corresponding number of such insulated conductors (120] is connected (Figure 4). 6. converter or converter system according to one of claims 1 to 5, characterized in that with which the homogeneous layer made of piezoelectric material carrying conductors or those carrying the homogeneous layer of piezoelectric material Conductors can be connected to a signal generating device for generating one or in each case a signal of predetermined properties is. - 1 - J7. Converter or converter system according to one of Claims 1 ibis 6, characterized in that with the or each supporting the j homogeneous layer of piezoelectric material Head of a signal processing device for processing the electrical signals can be connected, which as a function are generated by mechanical energy acting on the transducer or each transducer. <8. Sonar system with transducers or transducer systems according to one of Claims 1 to 7, characterized in that a plurality of transducer elements form a transducer arrangement for the controllable shaping of a transmission directional characteristic or reception directional characteristic. 9. A method for producing the piezoelectric body of a piezoelectric transducer, in particular a transducer according to one of claims 1 to 7, characterized in that the piezoelectric body (14) in elongated form, in particular as an elongated sleeve made of piezoelectric material. ; 10. The method according to claim 9, characterized in that the | piezoelectric body as a homogeneous layer on at least one length of a central conductor (12 or 30) surrounding is generated. 11. The method according to claim 9 or 10, characterized in that the material for producing the piezoelectric body (14) a polymer with piezoelectric properties is used, which is used to generate or increase the molecular Polarization is stretched and / or exposed to an electric field. ; 12. The method according to claim 11, characterized in that the stretching of the polymer prior to treatment by means of a electric field takes place. • * 13. The method according to claim 11, characterized in that the stretching of the polymer occurs essentially simultaneously with the treatment by means of an electric field. 14. The method according to any one of claims 11 to 13, characterized in that before the action of an electrical Field on the polymer this is heated. 15. The method according to any one of claims 11 to 13, characterized in that the stretching of the polymer directly is carried out on the conductor encased by him, in particular in a drawing mold (40) through which the with the Polymer in an extrusion mold (32) is drawn overmolded conductor. 16. The method according to any one of claims 11 to 15, characterized in that the coating of the conductor with the polymer at a first, elevated temperature and stretching the polymer on the conductor in a die (40) at a certain lower temperature takes place in such a way that the molecular orientation produced by the stretching of the polymer is not lost due to excessive material temperature. 17. The method according to any one of claims 11 to 16, characterized in that the action of an electrical Field is made in an area which is kept separate from the area in which the stretching of the polymer takes place will. 18. The method according to any one of claims 11 to 16, characterized in that the action of an electrical Field is made on the polymer in an area which is located in the area in which the stretching of the J Polymers is made.