Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-03-04
2003-04-01
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S366000
Reexamination Certificate
active
06541895
ABSTRACT:
This application claims the benefit of Japanese Application 2001-59118, filed Mar. 2, 2001, the entirety of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
This application claims the benefit of Japanese Application Number 2001-59118 filed Mar. 2, 2001, the entirety of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to piezoelectric/electrostrictive film elements, and in particular to piezoelectric/electrostrictive elements employed as actuators utilizing flexural displacement and sensors for detecting fluid properties, sound pressures, minute weights, acceleration, and so on, as for example, in microphones or viscosity sensors.
2. Description of the Related Art
Piezoelectric/electrostrictive film elements have conventionally been used as actuators and various types of sensors. A piezoelectric/electrostrictive film element used as a sensor is used for measuring the properties of a fluid such as the density, concentration, and viscosity as disclosed in Japanese Patent publication No. 5-201265A. Such elements are used as sensors because there is a correlation between the amplitude of a piezoelectric oscillator and the viscosity resistance of a fluid in contact with the oscillator. A mode of vibration in a mechanical system such as the vibration of an oscillator can be converted to an equivalent circuit in an electrical system. A piezoelectric/electrostrictive oscillator vibrates in a fluid, and receives a mechanical resistance based on the viscosity resistance of the fluid.
The oscillator thereby senses the variation of an electrical constant of an equivalent electrical circuit of the piezoelectric/electrostrictive element of the oscillator. As a result, it becomes possible to measure various parameters, which include the viscosity, density, and concentration of the fluid.
Fluids that can be measured include liquids and gases and include not only liquids consisting of a single component such as water, alcohol, and oils but also fluids composed of slurries, and pastes obtained by dissolving mixing or suspending soluble or in soluble media. Electrical constants to be detected include loss factor, phase, resistance, reactance, conductance, susceptance, inductance, and capacitance. In particular, preferred electrical constants are loss factors and phase because they have a single maximum or minimum point of variation near a resonance frequency of an equivalent circuit. This makes it possible to measure not only the viscosity of a fluid but also the density and concentration of the same. For example, the concentration of sulfuric acid in an aqueous solution of sulfuric acid can be measured through the use of the above electrical constants. In addition to the use of electrical constants, the variation in resonance frequency may also be utilized as an index for sensing variations in the mode of vibration, if there are no specific problems from the standpoint of precision of measurement and durability.
A conventional piezoelectric/electrostrictive film element is also disclosed in Japanese Patent publication 6-267742A.
An auxiliary electrode
8
is formed at a position independent of a lower electrode
4
on a substrate
1
which is made of ceramic having a thin diaphragm portion
3
and thick portion
2
on the periphery thereof as shown in
FIGS. 3A
,
3
B, and
3
C. A portion of the auxiliary electrode is positioned beneath a piezoelectric/electrostrictive film
5
. As a result of this configuration, it is possible to improve the reliability of the connection of an upper electrode
6
through the continuous formation of the upper electrode on the face of the auxiliary electrode
8
and the piezoelectric/electrostrictive film
5
without any breakage. In
FIGS. 3A
,
3
B, and
3
C, a fluid to be measured is introduced into a cavity
10
via through hole
9
.
Further, a piezoelectric/electrostrictive film element is disclosed in Japanese Patent publication 6-260694A. As shown in
FIG. 3
, a piezoelectric/electrostrictive film
5
is on a lower electrode
4
and is of a size that the surrounding portion of the piezoelectric/electrostrictive film
5
extends beyond the electrode
4
such that the periphery of the piezoelectric/electrostrictive film
5
projects above the ceramic substrate
1
. As a consequence, it is not necessary to precisely position the lower electrode
4
and the piezoelectric/electrostrictive film
5
, and thus short circuits between the upper and lower electrodes are easily prevented. Further, an extending portion
11
of the piezoelectric/electrostrictive film
5
can manifest sufficient flexural displacement, generation and vibration because it is in an incompletely bonded state with the substrate
1
. The extended portion
11
is not bonded with the substrate due to the presence of incompletely bonded portions
7
A.
An incompletely bonded state means that a portion of the extended portion
11
is either partially bonded with the ceramic substrate or that a portion an unbonded region without any bonded portion is in existence. Specifically, it is defined to mean that the peeling (tear off) strength of the film
5
to substrate
1
is 0.5 kg/mm
2
or less.
With respect to the formation of in unbonded state as described above, there are instances when it is necessary to have a low reactivity between the materials selected for the substrate and the piezoelectric/electrostrictive film. In this regard, it is also possible to form a dummy layer between the piezoelectric/electrostrictive film and the substrate so as to prevent their direct contact. The dummy layer is formed by a stamping method, a screen printing method or an ink jet method. The incompletely bonded portion
7
A is formed when the dummy layer is subsequently dissolved. For example, the dummy layer is fabricated with combustible/removable materials, such as resin materials, which are dissolved away to form the incompletely bonded portions
7
A when the piezoelectric/electrostrictive film
5
is heat treated. In the case where the piezoelectric/electrostrictive film and the upper electrode are not heat treated, the dummy layer is formed with a resin material to be dissolved in a composition such as water or organic solvents, etc. Accordingly, after the formation of either the piezoelectric/electrostrictive film alone or in conjunction with the upper electrode
6
, the incompletely bonded portions
7
A is formed by dissolving or removing the dummy layer.
In the case of such sensor elements whose electrical properties during vibration are detected to perform sensing, it is desirable that the electrical properties do not vary. In a conventional structure of piezoelectric/electrostrictive film elements, the electrical constants between the individual sensor elements tend to vary in both the initial phase and with the subsequent passage of time. In such cases, a bothersome fine-tuning process is required to insure the proper performance of the oscillator.
In particular, significant changes in the properties have sometimes occurred under sever conditions of use, e.g., high temperatures and humidity.
Further, cracks have sometimes occurred on a piezoelectric/electrostrictive film depending on the type of the material of the piezoelectric/electrostrictive film at edges of the thin diaphragm portion. Because the stresses originating from vibration or displacement of the thin diaphragm are likely to concentrate at this portion, it has resulted in breakage of the upper electrode to disable the element.
In conventional piezoelectric/electrostrictive film elements, an incompletely bonded portion
7
B is formed over a thin diaphragm
3
and a thick portion of a substrate between a lower electrode
4
and an auxiliary electrode
8
as shown in FIG.
3
B. The incompletely bonded portion
7
B is in an incompletely bonded state similarly to the incompletely bonded portion
7
A of the extended portion
11
. Studying the conventional piezoelectric/electrostrictive film elements, we found that a time-passage variation or change in the incompletely bonde
Burr & Brown
Dougherty Thomas M.
NGK Industries, Ltd.
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