Piezoelectric Bending Transducer and the Use Thereof

The invention relates to a piezoelectric bending transducer which contains at least one ceramic layer which is metallized on both sides and has piezoelectric properties.

German Offenlegungsschrift 3,608,550 has disclosed a valve which can be actuated piezoelectrically and which, in a housing, contains an elongate piezocrystal arrangement which is clamped in on one side and which is connectable to an electric voltage source. The free (mobile) end of the piezocrystal arrangement is provided with a sealing body. On application of a sufficient voltage, the piezocrystal executes a lateral movement. In this way, the sealing body can close a valve channel.

The disadvantage of this design is that the fitting of sealing bodies to the piezocrystal requires an additional working step and increases the mass of the bending transducer, i.e. the natural resonance thereof. The bending transducer therefore loses speed. As a result, a smaller number of switching cycles can be run per unit time. Since the sealing bodies also require space, miniaturization of the valve is made more difficult.

It was therefore the object to indicate a bending element for use in valves, which is designed constructionally in such a way that the indicated disadvantages to the valve manufacturer are avoided.

The invention starts from the recognition of the fact that the piezoceramic layer itself is also suitable for sealing a valve channel if the roughness of the surface is less than 0.5 .mu.m. However, with a conventional metallization of the piezoceramic body, which is necessary for later generation of the field strength, higher roughness values are obtained.

A piezoelectric bending transducer has now been found which, as the essential constituents, comprises

a) at least one piezoelectric ceramic layer which is metallized on both sides and is polarized such that on application of a voltage it undergoes a change in length, and

b) a rigid molding which is joined to the whole surface thereof and has approximately the same surface dimensions and which on application of a voltage does not undergo an elongation or undergoes an opposite elongation.

In the bending transducer, the ceramic layer has an unmetallized area of roughness R.sub.a of less than 0.5 .mu.m on at least one side. This unmetallized area should be located in the zone of maximum deflection when the bending transducer is connected to a voltage source and is suitably clamped in. The unmetallized area should be slightly larger than the valve channel which is to be covered. The metallization of the piezo-layers is usually effected by screen printing. In order to avoid running-down of the printing paste on the edge of the layer or later sparking at the edge, a narrow, approximately 0.3 mm wide strip is usually left unprinted at the edge. Such a strip is insufficient for covering a valve channel. The unmetallized area should have minimum dimensions of 1.times.1 mm.

The smaller the proportion of the unmetallized area in the total mobile surface, the smaller is the loss of deflection of the bending transducer on application of a fixed voltage. The unmetallized area should therefore be as closely coherent as possible.

The rigid two-dimensional molding can be composed of any desired material, e.g. it can be a metal sheet (no piezoelectric properties) or likewise a ceramic layer which is metallized on both sides and has piezoelectric properties. By means of suitable polarization of the piezo-layers used and observation of the sign of the applied voltage, it is possible to accomplish an elongation of the two layers in opposite directions, which leads to a curvature of the bending transducer, i.e. to a movement approximately perpendicular to the direction of the surface.

The metallized ceramic layer has preferably the shape of an elongate, in most cases rectangular strip. The unmetallized area is then located on a narrow end of the strip. When used in a valve, the strip is clamped in on one side at the other end and supplied with voltage.

Such a strip has in general a length/width ratio from 1:2 to 1:4. The unmetallized area should be of such a size that it covers a rectangle whose length is at least equal to the width and whose width amounts to at least 1 mm or even better at least 2 mm. Preferably, the length/width ratio is in the range from 1:1 to 4:1. For example, a rectangular region of 2.3.times.7.4 mm or 2.times.10 mm or a circular area (diameter 1.5.times.2.5 mm) can be left without metallization on a strip of piezoceramic material having a surface area of 25.times.10 mm. Circular regions are better for covering round valve orifices. For reasons of production engineering, rectangular regions are easier to produce free of metallization.

It is also possible to provide the ceramic layer having piezoelectric properties with a circular shape and to arrange the unmetallized area approximately in the center of the circle. Such a bending transducer is clamped in at the edge and the region which deflects most strongly on application of a voltage is likewise located in the center of the circle. In a bending transducer which has the shape of an elongate strip, it is also possible to provide metallization-free zones on the uppermost and lowermost ceramic plates of the composite. This makes it possible for one or the other side in a valve to cover a valve channel, depending on the deflection of the strip. This permits the construction of a valve having three valve channels and only one bending transducer.

To produce piezoelectric ceramics (e.g. based on lead zirconate/titanate), the starting materials used are in most cases powders of metal oxides and metal carbonates. The product having the final chemical composition is already obtained by calcination, as the result of a solid-state reaction. Other processes such as, for example, coprecipitation of the dissolved starting raw materials are likewise applied.

After calcination, the powder is in most cases subjected to a second grind in order to reduce the grain diameter. During this grind, liquid and plasticizer are added to the powder, so that a slip is formed. As a rule, the slip is then dried by a spraying process and pressed dry into the desired shape.

To produce thin parts, organic plasticizers and binders are added to the slip. Thin flexible foils are obtained from the slip by foil casting. Parts of the desired shape are punched out of these foils and, after expelling of the organic substances present, are sintered. Afterwards, the parts are provided with metal electrodes.

Piezo-ceramic products of small thickness can also be produced by compressing a piezo-material in block form and sawing it up by means of suitable saws.

After the sinter firing, the ceramic part is in the form of a polycrystalline body with irregularly distributed electric dipoles, i.e. it does not show any piezoelectric activity. It is only by the step of a so-called polarization, i.e. application of a strong electric field, that the dipoles are almost completely oriented.

The orientation is largely retained also after the polarization field has been switched off, and it is associated with a permanent change in the length of the body.

Further details on the production of ceramic piezocrystalline materials are known to those skilled in the art, for example, from Ullman, 3rd edition, Enzyclopadie der Technischen Chemie [Encyclopedia of Industrial Chemistry], volume 11, page 421, and Bauer, Buhling et al., "Technologie und Anwendung von Ferroelektrika [Technology and Application of Ferroelectrics]", Akademische Verlagsgesellschaft, Leipzig.

Since no additional sealing elements are necessary for a valve, the working steps for the involved assembly of the sealing elements are no longer necessary and the constructional shape of the valve can turn out to be very small.

For operation also in the low-voltage range (24 volt), ceramic layer thicknesses of less than 200 .mu.m are chosen. In this case, when a voltage of 24 volt is applied and after the transducer has been clamped in at the metallized end of the strip, such a deflection should take place in the unmetallized area that a valve channel is reliably closed.

When an interlayer of any desired material, preferably a metal sheet, is used, the rigidity of the bender is increased at unchanged ceramic layer thicknesses, since the distance of the ceramic plates from the central neutral zone is enlarged.

As a result, the natural resonance and hence the speed of the transducer are increased, which meets the demand of the user for increasingly fast switching cycles.

For efficient production, both ceramic foils are in general polarized in the same direction in a bender which consists of only two ceramic layers. To achieve the maximum deflection, the bender is activated in such a way that one piezoceramic layer is shortened (the field in the direction of polarization), whereas the other piezoceramic layer is elongated (the field opposite to the direction of polarization). For this purpose, the two outer electrodes of the upper and lower ceramic plates are in actual operation electrically connected to one another and form, for example, the minus terminal, while the central electrode forms the plus pole.

For very low voltages (for example 6 volt), it is also possible adhesively to bond a plurality (x), preferably 2 to 8, of very thin (100 .mu.m and thinner) piezoceramic layers metallized on both sides to one another, using up to (x-1) interlayers of a conductive material, for example interlayers of sheet metal, carbon or a conductive ceramic material. The ceramic bodies are then polarized and electrically conducted in such a way that, in a bender having, for example, 6 active ceramic layers, the upper three layers are elongated, while the lower three layers are shortened, when the supply voltage is applied.

Instead of adhesively bonding a metal sheet to a metallized piezoceramic body or two sintered metallized ceramic foils to one another, it is also possible to provide unsintered piezoceramic raw foils with a suitable electrode material, to laminate them together and then to subject them to sinter firing, a monolithic piezoceramic bender being formed. Since the electrode material must withstand sinter temperatures from 900.degree. C. to 1200.degree. C., without reacting with the metal oxides of the piezoceramic material, only a few materials such as, for example, platinum are suitable for this purpose.

Adhesive bonding of sintered metallized piezoceramic plates by means of an organic adhesive, for example a one-component methacrylate adhesive, is cheaper and simpler with regard to the production process.

FIG. 1a shows a cross-section of a piezoelectric bending transducer in strip form;

FIG. 1b shows the bending transducer of FIG. 1a in plan view;

FIG. 2a shows a cross-section of a bending transducer in strip form;

FIG. 2b shows the bending transducer of FIG. 2a in plan view;

FIG. 3a shows a cross-section of an approximately circular bending element and a valve channel;

FIG. 3b shows the bending transducer (without clamping) of FIG. 3a in plan view.

The invention is explained in more detail by the figures: FIG. 1a shows a cross-section of a piezoelectric bending transducer in strip form, composed of two ceramic layers (1, 2) metallized on both sides and a conductive interlayer, for example a metal sheet (3). The layers 1 and 2 are each coated on both sides with the thin metallization layer (4). In the figure, a small area (5) is free of metallization on both sides at one end of each strip. This area serves as a sealing face in contact with a valve channel (11). Bending transducers which have an unmetallized area on only one side can also be used (for certain valves). In the layers (1) and (2), the polarization vector P points in the same direction. The two layers (1) and (2) are adhesively bonded to the interlayer (3). The thin adhesive layer is not shown in the drawing. Current is supplied through the wires (6), (7) and (8). At the end carrying the current supply, the bending transducer is later clamped mechanically in the holder (9) and at the same time contacted. The length of the two piezo-layers is not exactly equal, so that it is easier to solder a current lead to the interlayer. It would equally be possible to make the two ceramic layers of equal length and to make the metal sheet (3) somewhat longer. FIG. (1b) shows the same bending transducer in plan view.

FIG. 2a shows a cross-section of a bending transducer in strip form, which is composed of two piezoceramic layers (1, 2) which each carry the metallization layer (4). There is no conductive interlayer. At one end, the metallization layers (4) are connected to the current leads (6, 7). At the opposite end of the strip, a small area (5) of the layer remains without metallization on at least one side. The polarization vector P of the two layers (1, 2) points in opposite directions. FIG. 2b shows the same bending transducer in plan view.

FIG. (3a) shows a cross-section of an approximately circular bending element and a valve channel which is to be covered by the bending element. It contains an active piezoceramic layer (1) which has metallization zones (4) on both sides and is adhesively bonded (adhesive not shown in the drawing) to a circular metal sheet (3). At its edge, the metal sheet (3) is mechanically clamped into a conductive circular holder (9). When the metallization zones (4) are electrically activated via the current leads (6) and (7), the composite transducer is deflected. The deflection is greatest in the center of the circular composite transducer. The center is also the location of the metallization-free zone (5) which comes to bear against the orifice (10) of the valve channel (11) and ensures sealing thereof. FIG. 3b shows the same bending transducer (without clamping) in plan view.