Metal working – Piezoelectric device making
Reexamination Certificate
1994-06-08
2003-07-01
Tugbang, A. Dexter (Department: 3729)
Metal working
Piezoelectric device making
C029S890100, C219S121690, C347S068000, C347S071000, C310S328000, C310S330000, C310S331000
Reexamination Certificate
active
06584660
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric device for converting an input mechanical quantity into an output electrical quantity, and vice versa.
2. Description of the Related Art
As generally known in the art, piezoelectric devices are typically used to convert an input mechanical stress into an output electric charge (voltage) and also to convert an input electric voltage into an output mechanical distortion. In the latter case, the device is sometimes referred to as an electrostrictive device. The term “piezoelectric device” as used herein is to be interpreted in its broadest sense, as encompassing an electrostrictive device as well. Similarly, the term “piezoelectric material” as used herein is to be interpreted in its broadest sense as encompassing an electrostrictive material as well. When the output of the piezoelectric device is a mechanical distortion, such distortion can be used to generate mechanical force, displacement or vibration.
Conventionally, piezoelectric devices have been used for various purposes. For example, in the field of optics or precision engineering, there is a progressive demand for actuators capable of adjusting lengths and/or positions of the optical path on a sub-micron order, or for sensors capable of detecting a fine mechanical displacement. There have been various proposals directed to such demands in conjunction with application of piezoelectric devices. Specifically, there is known unimorph- or bimorph-type piezoelectric device which undergoes bending or flexural deformation upon application of an input electrical voltage, and which can be used for ink jet printing heads, acoustic radiators (i.e., loudspeakers and the like), vibrators, etc. The unimorph- or bimorph-type piezoelectric device can also be used to convert a bending or flexural deformation into an electrical voltage, e.g., a microphone or sensor. With reference, for example, to ink jet printing heads including a unimorph- or bimorph-type piezoelectric device, in view of customers' or end users' requirement for high-quality and high-speed printing performance, it is highly desirable to realize a high density piezoelectric device having a minimized size, which operates at a low driving voltage and yet provides a satisfactory response characteristic and an operational reliability.
For realizing the above-mentioned desired properties of the piezoelectric device, because the substrate in the region of each piezoelectric transducer functions as a diaphragm, it has been generally considered necessary for the entire substrate to have a reduced thickness. In many instances, however, the reduced thickness of the substrate as a whole cannot be achieved without lowering the mechanical strength of the substrate. Thus, in order to simultaneously satisfy the seemingly incompatible requirements for the reduced thickness and a satisfactory mechanical strength of the substrate, U.S. Pat. No. 5,210,455 assigned to the assignee of this application discloses an improved piezoelectric device which includes a locally thin-walled substrate and piezoelectric transducers formed on, and integrated with the thin-walled regions of the substrate.
As particularly disclosed in U.S. Pat. No. 5,210,455, the locally thin-walled substrate may be formed by laminating a relatively thick green sheet of a first ceramic layer having window-like openings therein, and a relatively thin green sheet of a second ceramic layer without such openings. After integrating the two ceramic layers by firing to form the substrate, the openings in the first ceramic layer cooperate with the second ceramic layer to define cavities having thin-walled bottom walls which are constituted by the second ceramic layer. Furthermore, the piezoelectric transducers may each include a first electrode layer, a piezoelectric layer and a second electrode layer which are sequentially laminated with each other. The piezoelectric transducers are arranged on those outer surface regions of the substrate opposite to the relevant cavities, and are integrated with the substrate by firing.
The piezoelectric device according to the above-mentioned proposal proved to be highly advantageous in that the locally thin-walled regions of the substrate opposite to the respective piezoelectric transducers make it possible to achieve a satisfactory operation of the transducers, and further in that the remaining, relatively thick region of the substrate serves to realize a sufficient mechanical strength of the substrate as a whole.
Nevertheless, it would be highly desirable to provide a further improved piezoelectric device which is superior in the operational characteristics, and which yet retains the above-mentioned advantages. Having thus conducted thorough experiments and investigations based on the piezoelectric device disclosed in U.S. Pat. No. 5,210,455, the inventors reached a recognition that a significant improvement can be achieved by carefully taking into consideration the physical and/or chemical properties, structure and behavior of the substrate and the piezoelectric transducers at or near the thin walled regions of the substrate.
First of all, in the piezoelectric device according to the inventors' earlier proposal, the substrate and the piezoelectric transducer are generally composed of materials which are different in chemical composition and hence in the coefficient of thermal expansion. Thus, during manufacture of the piezoelectric device, in particular during cooling which follows the sintering for forming the piezoelectric transducer on the substrate or the firing for integrating the substrate and the piezoelectric transducer, undesirable residual stresses tend to be caused in the piezoelectric transducer, which may deteriorate the operational characteristics of the device as a whole. According to a recognition reached by the inventors, a highly improved piezoelectric device can be realized by effectively compensating for the difference in shrinkage of the materials for the substrate and the piezoelectric transducer.
Furthermore, in the piezoelectric device according to the inventors' earlier proposal wherein a common substrate is provided with an array comprising a plurality of piezoelectric transducers, when neighbouring transducers are simultaneously actuated, it is often difficult to achieve a sufficient amount of displacement of the thin-walled regions of the substrate in a direction perpendicular to the general surface of the substrate, and a satisfactory volumetric change of the cavity. Besides, the operation of the transducers may be affected by the operation of adjacent transducers, making it difficult to uniformly achieve a desired displacement amount of the thin-walled regions of the substrate. According to another recognition reached by the inventors, a highly improved piezoelectric device can be realized by eliminating or suppressing undesired interaction of the neighbouring transducers.
Specifically, it has been confirmed that when the piezoelectric device according to the inventors' earlier proposal is used, for example, as an actuator for an ink jet printing head, there may be instances wherein the ink filled within the cavity cannot be discharged with a sufficient amount or at a high speed, or wherein it is difficult to achieve a satisfactory printing quality. According to still another recognition reached by the inventors, significantly improved ink discharge characteristics can be achieved by realizing a sufficient displacement characteristic of the piezoelectric transducer and a satisfactory volumetric change of the cavity. It has also been recognized that a significantly improved printing quality can be realized by eliminating or minimizing fluctuation in the displacement characteristics of the piezoelectric transducers and hence in the discharge amount and speed of ink.
DISCLOSURE OF THE INVENTION
The present invention thus contemplates providing a highly improved piezoelectric device on the basis of the above-mentioned recognitions.
It is a p
Kimura Koji
Shimogawa Natsumi
Takeuchi Yukihisa
NGK Indulators, LTD
Parkhurst & Wendel L.L.P.
Tugbang A. Dexter
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