Piezoelectric stacks are composed of piezoceramic wafers that are stacked physically in series but are connected electrically in parallel. Piezoceramic wafers are elements that can be stressed electrically. When a voltage is applied, their dimensions change and a resulting force is exerted by the piezoceramic.
As shown in FIG. 1, a stacked piezoelectric device 10 includes a piezoelectric stack formed of alternating piezoelectric layers 12 and 14, internal electrode layers 16 and 18 alternately located between the piezoelectric layers 12 and 14 for applying positive and negative voltages to the piezoelectric layers 12 and 14, and a pair of side electrodes 24 formed on the sides 20 and 22 of the piezoelectric stack.
In the piezoelectric stack, the internal electrode layers 16 are arranged to be exposed to the side 20, while the internal electrode layers 18 are arranged to be exposed to the other side 22.
A side electrode 24 is located on each side 20 and 22 of the piezoelectric stack in such a manner as to be electrically coupled to each of the internal electrode layers 16 and 18 separately. The electrode on side 22 (not visible in FIG. 1.) is electrically connected to the ends of the internal electrode layers 18
When an electric field having the same polarity and orientation as the original polarization field of the piezoceramic material is placed across the thickness of a single layer of piezoceramic material, its thickness increases (i.e. along the axis of polarization). At the same time, the layer contracts in the transverse direction (i.e. perpendicular to the axis of polarization).
Because the various layers of the stack are expanding and contracting, both longitudinally and laterally, the prior art stacked piezoelectric device 10 of FIG. 1 has the problem that cracking of the layers and separation of the electrical connection between the side electrodes and the internal electrodes layers tend to occur at the junction where the side electrodes 24 contact the internal electrode layers 16, 18.
Though not shown, the end portion of the electrode layer 18 is exposed to the side 22 to make contact with the side electrode strip 24 on side 22, and the end of the electrode layer 16 does not extend to the side 22 of the piezoelectric stack.
Referring to FIG. 2, the piezoelectric layers 12 and 14 are divided into a portion M sandwiched between the electrode layer 16 and the electrode layer 18, and a portion N in contact with the internal electrode layer 16.
Upon application of a voltage from the electrode layers 16 and 18 to the piezoelectric layers 12 and 14, the portion M sandwiched between the internal electrode layers 16 and 18 can be displaced along the height of the stack. The end portion N, however, cannot be displaced, as it is in contact with only one of the internal electrode layers 16 and 18. As a result, stress develops in the portion indicated by dashed line L which is the boundary between portions M and N which is in contact with the portion displaced and the portion not displaced. Thus, the piezoelectric stack may be damaged by cracking occurring from the end portion toward the side.
This damage is most likely to occur after the stacked piezoelectric device is used for a long period of time or in a harsh operating environment. It can be a major cause of device deterioration.
To obviate this problem, it has been proposed to form each internal electrode layer over the entire surface of the corresponding piezoelectric layer.
In this configuration, the electrode layers and the piezoelectric layers have substantially the same area. Also, each side electrode is configured in such a manner that the ends of alternate ones of the internal electrode layers are covered with an insulating film, and the other ends are electrically connected to the side electrode 24, so that each piezoelectric layer is sandwiched between internal electrode layers of different polarities.
This configuration, however, still has the problem of durability of the piezoelectric device.
Specifically, in view of the fact that the stacked piezoelectric device is displaced along both the height and width of the stack as it is cycled, stresses occur at the electrical connections where the internal electrodes are connected to the side electrodes 24 along the height of the stack. Since the connections between the side electrodes and the internal electrodes only occur at a small contact area, the mechanical strength of this connection is so low that they can easily become separated.
As noted above, with the configuration having side contacts 24 of strips of conductive material, such as copper, as shown in FIG. 1, electrically connected to the internal electrode layers in the stack, it is difficult to produce a piezoelectric device high in durability.