Method of producing a multilayer piezoelectric element as well as multilayer piezoelectric element

Abstract: The invention relates to a method of producing a multilayer piezoelectric element. The multilayer comprises layers of piezoelectric synthetic resin material, more specifically PVDF and metal layers, more specifically Al. Under the influence of a heat treatment at increased pressure, the multilayer has been made form-retaining and mechanically stable. The connections provided on two facing sides of the multilayer preferably include at least one sprayed metal layer, which preferably consists of the same metal as the metal layers of the multilayer.

Description: The invention relates to a method of producing a piezoelectric element, in which electrically conducting electrode layers and piezoelectric synthetic material layers are stacked alternately to form a multilayer, which subsequently is provided, on two facing sides, with contact planes in such a manner that the consecutive electrode layers are alternately electrically connected to one or to the other contact plane. The invention also relates to a piezoelectric element produced by means of this method. Using such a piezoelectric element an electrical potential difference can on the one hand be converted into a mechanical deformation. On the other hand subjecting such an element to a mechanical force produces an electric voltage. Piezoelectric elements are inter alia used in audio equipment (loudspeakers, headphones, microphones, hydrophones, sonar equipment), in strain detectors and in energy transformers. A method of producing a multilayer piezoelectric element of the type described in the opening paragraph is inter alia disclosed in the PTC-application filed under the international application number WO 88-04475. Therein the production of a piezoelectric element via a co-extrusion process is described. In this process electrode layers of a synthetic resin material and piezoelectric layers are alternately stacked to form a multilayer. Polymethylacrylate is used as the electrode material. Carbon is added to this material to make it electrically conducting. Polyvenylidene fluoride (PVDF) is used as the piezoelectric material. Contact planes are subsequently deposited onto facing sides in the form of a silver strip. The electrode layers are stacked in the co-extrusion process in such a manner that the consecutive electrodes run alternately to the one or to the other abovementioned side of the multilayer. The prior art method has a number of disadvantages. For example it has appeared in practice that the electrode layers of the known piezoelectric element are rather thick as a direct result of the extrusion process used. This has the disadvantage that the quantity of active piezoelectric material at a given volume and at a given piezoelectric synthetic resin material (the "volume efficiency") is relatively small in the prior art piezoelectric element. It is not very well possible to produce, by means of co-extrusion, optimally functioning multilayer piezoelectric elements, with electrode layer thicknesses less than 25 µm and more particularly less than 10 µm. Applicants have furthermore found that the piezoelectric elements produced by means of the prior art method do not have optimal properties. In particular, the contact resistance between contact planes and the electrode layers is rather high. As a result thereof the prior art piezoelectric elements have an unacceptably high heat loss by dissipation, if these elements are subjected to an a.c. voltage. This head dissipation appears to increase versus time. The invention has inter alia for its object to provide a method which does not have the disadvantages of the prior art method mentioned in the foregoing. To that end, the invention provides in particular a method in which multilayer piezoelectric elements having a high "volume efficiency" can be produced. According to a further object of the invention, a method is provided by means of which a piezoelectric element can be produced which is mechanically stable and form-retaining and has a relatively low heat dissipation. These and other objects are achieved by a method of the type described in the opening paragraph, which according to the invention is characterized in that piezoelectric synthetic resin material layers onto which a metal layer is deposited on one side, are stacked to form a multilayer, and in that this stacked multilayer is subjected to a heat treatment under increased compression. By means of the method according to the invention multilayer piezoelectric elements can be manufactured, wherein the thickness of the separate layers is very small, so that layer thicknesses of 1 µm or less may be realised without any problems. If a piezoelectric synthetic layer is used onto which a metal layer is deposited by evaporation then electrode layer thicknesses of less than 0.1 µm and even of less than 0.05 µm can be applied in the multilayer piezoelectric element. The heat treatment of the multilayer under the influence of the increased pressure is an essential manufacturing step in the inventive method. This manufacturing step allows the multilayer to retain its form and makes it mechanically stable. The mechanical strength is obtained in that the free surface of the metal layer is firmly connected to the free surface of an adjacent piezoelectric synthetic layer during this manufacturing step. Therefore it may be obvious that preferably single-sided metallised synthetic layers can be used in the method of the invention, and no synthetic layers metallised on both sides, which must alternate with uncoated synthetic layers. In principle, all types of synthetic layers are eligible for use in the piezoelectric element. Examples are: polyvenylidene fluoride, polyvenyl fluoride, polycarbonate, polyamides, and polyvenylidene fluoride - trifluoro ethylene. An interesting embodiment of the method according to the invention, is characterized in that the heat treatment is effected at a temperature which is 5-15°C below the melting point of the piezoelectric synthetic layer. It has been found that the capability of the multilayer to retain its form may be obtained in a very fast and efficient manner in the said temperature range. The use of a lower temperature has the disadvantage that the heat treatment at an increased pressure requires relatively much time. If the difference in temperature between the melting point and the temperature of the heat treatment is less than 5°C, there is a risk of flashover occurring in the finished product. An other advantageous embodiment of the method according to the invention is characterized in that the contact planes are provided in the form of a sprayed metallised layer. The use of this measure results in a great mechanical strength between the contact planes and the electrode layers. Furthermore, the invention also relates to a multilayer piezoelectric element manufactured in accordance with the method described in the foregoing. According to the invention, this element is characterized in that the material composition of the electrode layers and the contact planes is identical. The use of identical materials in the electrode layers and the contact planes is advantageous for obtaining a great mechanical strength of the connection between these two parts. Moreover, a good electric conduction between the two parts is obtained in this case, and the contact resistance in particular is very low. In the case in which the electrode layers and the contact planes are made of Al, leads can directly be secured to the sprayed metallised contact planes, for example by means of percussion welding. However, of particular interest is that embodiment of the piezoelectric element that is characterized, according to the invention, in that the contact planes include also a second sprayed metallised layer, which mainly contains copper, zinc, or an alloy of zinc and tin. It has been found that the second sprayed metallised layer is firmly attached to the first sprayed metallised layer. A lead wire may be attached to this second sprayed metallised layer in a simple manner, for example by soldering, when the second sprayed metallised layer is made of copper or by means of resistance welding when a second sprayed metallised layer of a zinc or an alloy of zinc and tin is used. FR-A-2 365 886 describes a method for simultaneously poling a number of piezoelectric synthetic material foils, the foils being uncoated or coated on one or both sides with an electroconductive coating. In this method, a number of such foils are stacked, maintained at an elevated temperature and polarized by means of a direct current voltage, which is applied via two electrodes, which are situated above and beneath said stack. Embodiments of the invention will now be described in greater detail by way of example with reference to the accompanying drawings, wherein Figs. 1a and 1b show schematically in a cross-sectional view how the multilayer is stacked by means of the method according to the invention, and Fig. 2 is a perspective view of a multilayer piezoelectric element according to the invention. It should be noted that the relative and absolute dimensions of the different parts of the drawings are not shown to scale for the sake of clarity. Embodiment. A thin layer of piezoelectric synthetic resin material (polyvenylidene fluoride; PVDF) was metallised on one side with a layer of Al (layer thickness 0.03 µm), which was deposited by means of thermal evaporation. The layer thickness of PVDF foil can range from 3 to 40 µm. It was found that the layer thickness of the metal layer must be greater than 0.01 µm to obtain an adequate conductance. Layer thicknesses ranging from 0.02 to 0.05 µm provide good results. The metal layer can alternatively be deposited by means of a sputtering process, but thermal evaporation is preferred from an economical point of view. After metallisation, the synthetic resin material layers were cut to strips having a length of, for example, 50 cms, and with approximately the width of the piezoelectric multilayer elements to be produced, for example 0.42 cms. A number of the metallised foil strips (20-2000) were subsequently stacked to form a multilayer. Figs. 1a and 1b are schematical cross-sectional views of such a multilayer, wherein for the sake of clarity only six of these stacked strips are shown. Herein the PVDF foils 1 are shown which are provided with a layer 2 of Al. The foils are stacked such that they are shifted relative to each other so that the successive metal layers alternately project to the one or to the other side. By covering strips of the foil during the Al-evaporation process, an edge 3 of the foil strips is not coated with an Al layer. (Fig. 1-a). It is alternatively possible to deposite metalless conductors 4 after metallisation of the complete foil by means of a laser (Fig. 1-b). The presence of these uncoated portions or conductors prevent shortcircuiting between the contact planes to be deposited on the side edges of the multilayer. The stacked multilayer is provided with a covering layer 5 at the upper and the lower side. Preferably, this layer is of the same material as the piezoelectric synthetic material layers. It is alternatively possible to use for this purpose a plurality of non-metallised synthetic material layers or a single, relatively thick cover layer. The stacked piezoelectric multilayer is subsequently compressed at a pressure of 2-3 MPa. Portions of both sides are covered by means of a mask, so that only those portions on which the connection must be deposited remain bare. In the present case these connections are deposited by means of a metal spraying procedure, whereby a first layer of Al is provided. Subsequently, the multilayer provided with the connection is exposed to temperatures of 165-175°C for approximately 30 minutes at the pressure mentioned in the foregoing. The multilayer retains its form and is mechanically stable because of the heat treatment at an increased pressure. The air is squeezed from between the successive metallised PVDF layers, while the Al layers are simultaneously attached to the adjacent PVDF layers. After cooling of the strips of the multilayer a second sprayed metallised layer of Cu is deposited on the first sprayed metallised layer. It is important that the first sprayed metallised layer (Al) is deposited before, and the second sprayed metallised layer (Cu) after the heat treatment. It was found that the heat treatment may distort the sides of the multilayer to such an extent, that the mechanical strength and the electric connection of a first sprayed metallised layer deposited later is relatively low. This may cause the electric connection between the electrode layers and the contact planes to be broken during protracted usage. The second sprayed metallised layer (Cu) is deposited after the heat treatment, since Cu easily oxidizes at higher temperatures. Finally, the multilayer piezoelectric element thus produced is polarised under the influence of a high temperature (100-140°C) and a high electric field intensity (about 80 MV/m. Fig. 2 shows a finished multilayer piezoelectric element according to the invention, its dimensions being 2.0 x 0.42 x 0.12 cm³. It consists of a multilayer 11 which is assembled from more than 100 piezoelectric PVDF synthetic material layers 12, having a thickness of 9 µm, upon which an electrically conducting Al electrode layer 13, having a thickness of 0.03 µm, is deposited on one side by thermal evaporation. For the sake of clarity, only six piezoelectric layers and 6 electrode layers are shown. Covering layers 14 are deposited on the top and bottom sides of the multilayer. Furthermore, the element has two connections 15, which partly cover two opposite sides. These connections comprise a first sprayed metallised layer 16 of Al and a second sprayed metallised layer 17 of Cu. The choice of Al as a first sprayed metallised layer provides a great mechanical rigidity as well as a low contact resistance between the connections and the multilayers, because the material of the electrode planes and the first sprayed metallised layer is identical. As this mechanical rigidity is directly proportional to the surface area between the sprayed metallised layer and the electrode plane, it is advantageous to allow the electrode planes to project slightly from the multilayer (in this case for 0.02 cms). This is achieved by stacking the layers such that they are shifted relative to each other. In that case the first sprayed metallised layer still contacts a portion 18 of the surface area of the electrode layers. The creepage paths 19 were deposited on the PVDF foil by covering them partly during metallisation. This prevents short-circuits from occurring between the opposite connections 15 in the finished product. As the thickness of the creepage paths is relatively small (about equal to the thickness of the electrode planes 13, i.e. about 0.03 µm) with respect to the dimensions of the sprayed metallised Al-particles, these particles cannot penetrate into the creepage paths. This prevents short-circuits from occurring, as mentioned in the foregoing. The second Cu sprayed metallised layer forms a mechanically firm connection with the first Al sprayed metallised layer. A lead wire can be secured to the second sprayed metallised layer in a simple manner by means of soldering. The product described has good piezoelectric and mechanical properties. The insulation resistance in particular is relatively high, of the order of 10⁴ MOhm. The contact resistance amounts to only a few mOhm. The piezoelectric element also has a very low heat loss under the influence of an a.c. voltage.

Claims: A method of producing a piezoelectric element, in which electrically conducting electrode layers and piezoelectric synthetic material layers are stacked alternately to form a multilayer, which subsequently is provided on two facing sides with contact planes in such a manner that the consecutive electrode layers are alternately electrically connected to one or to the other contact plane, characterized in that piezoelectric synthetic resin material layers onto which a metal layer is deposited on one side, are stacked to form a multilayer, and in that this stacked multilayer is subjected to a heat treatment under increased compression. A method as claimed in Claim 1, characterized in that the heat treatment is performed at a temperature which is 5-15°C lower than the melting temperature of the piezoelectric synthetic resin material layers. A method as claimed in Claim 2, characterized in that the contact planes are provided in the form of a sprayed metallised layer. A multilayer piezoelectric element, produced in accordance with the method as claimed in Claim 1, 2 or 3, characterized in that the composition of the material of the electrode layers and the contact planes is identical. A multilayer piezoelectric element as claimed in Claim 4, characterized in that the contact planes also include a second sprayed metallised layer which predominantly contains copper, zinc or an alloy of zinc and tin.

Inventors: PRONT HARRY WILLEM, WEEKAMP JOHANNUS WILHELMUS, PRONT, HARRY WILLEM, WEEKAMP, JOHANNUS WILHELMUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: H01L41
IPC codes: H01L41