Multi-layer piezoelectric elements mounted on the fuel injection apparatus of automobile engine or the like have been known. FIG. 5A is a perspective view showing the multi-layer piezoelectric element of the prior art. The multi-layer piezoelectric element 101 comprises a stack 109 made by stacking a plurality of piezoelectric layers 105 and a plurality of internal electrode layers (metal layers) 107 which are stacked alternately one on another, and a pair of external electrodes 111 formed on the side faces of the stack 109.
The internal electrode layers 107 are not formed over the entire principal surface of the piezoelectric layer 105, and have the so-called partial electrode structure. FIG. 5B is an exploded perspective view explanatory of the partial electrode structure showing a part of the multi-layer piezoelectric element 101 shown in FIG. 5A. As shown in FIGS. 5A and 5B, the internal electrode layers 107 are stacked so as to be exposed on either side face of the stack 109 alternately in every other layer. Accordingly, the plurality of internal electrode layers 107 are electrically connected to the pair of external electrodes 105 alternately. The stack 109 has inactive layers 113 stacked on both ends of the stack 109 in the stacking direction.
In the multi-layer piezoelectric element which has the partial electrode structure shown in FIGS. 5A and 5B, there are an active region A in which the internal electrode layers 107 of different polarities oppose each other via the piezoelectric layers 105, and an inactive region B in which the internal electrode layers 107 of different polarities do not oppose each other via the piezoelectric layers 105. Accordingly, when the multi-layer piezoelectric element is operated, since only the active region A undergoes displacement and the inactive region B does not undergo displacement, stress may be concentrated in the border between the active region A and the inactive region B and become the start point of crack.
As shown in FIG. 5A, the multi-layer piezoelectric element 101 has the inactive layers 113 stacked on both ends thereof in the stacking direction. When the multi-layer piezoelectric element is operated, since the inactive region B does not undergo displacement, stress may be concentrated in the border between the region which undergoes displacement and the inactive layers 113 and become the start point of crack.
Such a crack as described above may grow from the border toward the side faces of the stack 109 (the inactive region B side), but also may grow toward the inside of the stack 109 (the active region A side). When an electric field is applied between the opposing internal electrode layers 107, the active layer A expands in the direction of the electric field by reverse piezoelectric effect and shrinks in the direction perpendicular to the direction of the electric field. When the piezoelectric layers 105 expand in the direction of the electric field, the element 101 expands in the stacking direction as a whole. In case the element 101 is housed in a casing or a frame which restricts the expansion, the element 101 receives a compressive force as a reaction.
A crack which starts in the border and grows toward the active region A may run in the direction of thickness of the piezoelectric layers 105 while bending and branching in accordance to the state of stress. When a crack growing in the direction of thickness of the piezoelectric layers 105 is generated between the internal electrode layers 107 which oppose each other, there has been such a problem that short-circuiting occurs between the internal electrode layers 107, thus resulting in a decrease in the amount of displacement of the multi-layer piezoelectric element 101.
In recent years, it is a common practice to operate the multi-layer piezoelectric element continuously over a long period of time with a higher electric field applied, since it is required to achieve a large amount of displacement from a compact multi-layer piezoelectric element under a higher pressure. To meet these requirements, multi-layer piezoelectric elements having stress relieving layer provided inside thereof have been proposed (refer to, for example, DE10234787A1 and DE10307825A1). However, there have been demands for a multi-layer piezoelectric element which has higher durability under conditions of continuous operation over a long period of time with a high pressure.