Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-12-28
2004-05-11
Budd, Mark (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S328000
Reexamination Certificate
active
06734607
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an integrally fired, laminated electromechanical transducing element employing a piezoelectric or electrostrictive material.
2. Description of the Related Art
The integrally fired, laminated electromechanical transducing element used for an actuator, a piezoelectric transducer or an ultrasonic motor is fabricated by firing a plurality of ceramic layers, composed of piezoelectric ceramic or electrostrictive ceramic, integrally with internal electrode layers interposed between the ceramic layers.
This integrally fired, laminated electromechanical transducing element produces an inverse piezoelectric effect of generating a displacement upon application of a voltage thereto, and therefore a stress is generated between the internal electrodes and the ceramic layers. Further, another basic characteristic of this element is generating heat upon repeated application of a voltage thereto.
Thus, the following characteristics are required of the internal electrode layers:
1. low electric resistance and small loss of the electrical charge,
2. high heat conductivity and superior heat radiation characteristics,
3. superior antimigration characteristics,
4. a low rigidity and a low internal stress so as not to generate cracks or the like,
5. a high a bonding strength with ceramic, without separating in use, and
6. low cost.
With the conventionally integrally fired, laminated electromechanical transducing element, an Ag—Pd alloy is widely used as an electrode material. Although high in conductivity and comparatively inexpensive, the reliability of Ag itself is low, as it has a low melting point of 960° C. and easily migrates. On the other hand, Pd, though expensive, has a high melting point. Therefore, an Ag—Pd alloy material can produce an electrode material having a high melting point with the migration suppressed (Japanese Unexamined Patent Publication No. 5-304043). Thus, the Ag—Pd alloy material finds wide applications.
Although the migration is suppressed by adding Pd, the bonding between the electrode material and the ceramic material is not sufficient and, to cope with this problem, various measures have been taken as disclosed in Japanese Unexamined Patent Publications Nos. 5-304043 and 8-255509. Also, in spite of the fact that the migration is suppressed by adding Pd, the resultant higher cost poses a problem for industrial applications.
For these reasons, an integrally fired, laminated electromechanical transducing element is required which uses an inexpensive electrode material having the electrode characteristic at least equivalent to those of the Ag—Pd alloy material.
SUMMARY OF THE INVENTION
(I) The present invention has been achieved in view of the problems of the prior art described above, and the object of the invention is to provide an integrally fired, laminated electromechanical transducing element employing an inexpensive electrode material having an electrode characteristic at least equivalent to an Ag—Pd electrode or, in particular, an integrally fired, laminated electromechanical transducing element having a low rigidity of the internal electrode layers with only a small internal stress generated at the time of elongation or contraction of the ceramic layers (subject A).
According to a first aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element comprising an integrally fired laminate member including a plurality of ceramic layers of piezoelectric ceramics or electrostrictive ceramics and internal electrode layers interposed between the ceramic layers, wherein the main component of the internal electrode layers is a base metal having a rigidity not more than 160 GPa.
What should be noted in this invention is the use of a base metal having a specific property, i.e. a rigidity not more than 160 GPa as a main component of the internal electrode layers. As a result, the rigidity of the internal electrode layers, as a whole, of the integrally fired, laminated electromechanical transducing element can be reduced, which in turn can reduce the internal stress generated by the elongation or contraction of the ceramic layers when driving the integrally fired, laminated electromechanical transducing element. Further, the fact that the main component of the internal electrode layers is a base metal can reduce the cost as compared with the conventional Ag—Pd material.
In the case where the Vickers hardness Hv exceeds 50 or the rigidity exceeds 160 GPa, the rigidity of the internal electrode layers as a whole increases and so does the internal stress at the time of elongation or contraction. This is liable to induce cracking or the like.
Specifically, a laminated actuator has such a characteristic that an assumed distortion X longitudinal to the direction of lamination is accompanied by a simultaneous transverse distortion of about one third, thereby generating a shearing stress between the internal electrode layers and the ceramic layers. Thus, the electrode material is required to have a low rigidity. The rigidity, though not definitely known, of the pure alloy composed of Ag and 30% Pd (hereinafter referred to as “Ag—Pd 30%”) used as a conventional electrode material is estimated at about 160 GPa, a figure obtained as the product of 100.5 GPa (see Table 3), i.e. the rigidity of the pure metal Ag and 1.6, i.e. the hardness ratio of the Ag—Pd 30% alloy to the Ag annealed at 500° C. For preventing the adverse effect on the displacement performance and reliability of the actuator, the rigidity of the pure metal is required to be not more than 160 GPa, in which case the internal stress is estimated to be not higher than that of the Ag—Pd 30% alloy.
In the case where the rigidity of the pure metal is not more than 160 GPa, the internal stress is estimated at not more than equivalent to that of the pure metal Ag—Pd 30%. The base metals corresponding to this pure metal include Al, Cu, etc. Especially, Cu is desirable, as described later, as it has a volume resistivity as small as 1.7 &mgr;&OHgr;cm and a melting point of 1,083° C. which is higher than and approximate to the sintering temperature 900 to 1,050° C. of ceramics (refer to the ninth embodiment, described later).
According to a second aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element of which the displacement can be set to 0.06 to 0.15% when driven.
Specifically, the larger the displacement, the greater the internal stress of the electromechanical transducing element, resulting in a higher tendency to develop cracking.
Even in the case where the displacement is as high as 0.06% or more, however, the use of the specific base metal described above can suppress the internal stress and thus prevent the cracking.
The displacement of 0.15% or more reduces the strength of the ceramic layers themselves due to the repeated fatigue without regard to the electrodes, thereby undesirably shortening the service life thereof.
According to a third aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the average thickness of the electrode layers is desirably 1 to 8 &mgr;m.
In the case where the average thickness of the electrode layers exceeds 8 &mgr;m, the rigidity of the internal electrode layers increases for a greater internal stress when the element is driven. The average thickness of less than 1 &mgr;m, on the other hand, undesirably both increases the resistance value of the electrodes and causes greater variations in the fabrication process.
According to a fourth aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the electrode forming ratio, i.e. the ratio which the portion formed with the electrodes represents of the total length of the internal electrodes exposed to the cutting section along the direction of lamination of the laminated member can be set to not less than 75%. The higher the electr
Iwamoto Youta
Nagaya Toshiatsu
Shindo Hitoshi
Sumiya Atsuhiro
Yasuda Eturo
Budd Mark
Denso Corporation
Nixon & Vanderhye P.C.
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