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BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a bimorph element
according to the present invention. 1.1 ? иииииии Piezoelectric element, 2.2 ?, 3.3 ? иииииииии
Electrodes, 4 ииииии Intermediate electrode plate, 5, 5 ? ииии? Adhesive layer.
DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a bimorph
element, and it is an object of the present invention to be obtained with a simple structure to
increase the reliability and reduce the cost. In general, bimorph elements are used for vibrating
bodies such as microphones, pickups, buzzers, and speakers. Using this element enables power
saving and thinning. The conversion element handled here is one in which electrodes are
provided on both sides, and two piezoelectric sheets are arranged to face each other, and a
conductive member is interposed between the electrodes to form a bimorph vibrator. The
bimorph oscillator is a pair of pressure / 7y-: '-"two current collector sheets which stretch in the
longitudinal direction, and when one is stretched, the other is shrunk to perform a bending
motion as a whole, It lowers the mechanical impedance so that a large output voltage can be
obtained. Usually, the piezoelectric element used for the bimorph element is in the form of a thin
disk, and the thin disk element will be described as an example in the following description. The
fundamental resonant frequency, which is an important characteristic in application of the
bimorph element, is substantially determined by the size and shape. Due to this, in order to make
the fundamental resonance frequency an audio frequency band, a thin piezoelectric element is
required. Moreover, in order to obtain high conversion efficiency, a high electrical conductivity
and a high electro-mechanical coupling coefficient are required as material properties. However,
thin ceramic elements are weak in mechanical strength, and there are problems with reliability
when they are used as electronic parts. Therefore, a conductive reinforcing plate is used. An
adhesive is used to integrate the reinforcing plate and the piezoelectric element). At the interface
between the piezoelectric element and the reinforcing plate, substantial acoustic reflection
occurs, so that a substantial change in acoustic energy occurs, so that the adhesive layer and the
electrode layer need to be sufficiently dulled. The mono-baked silver electrode in which baked
silver is mainly used as an electrode to be formed on a thin piezoelectric element is a silver paste
having thixotropy properties (a silver-palladium paste is printed with a screen of 200 mesh: + to
400 mesh, After drying it is baked at a temperature of SOO-1000 ░ C. The quality of the
electrode formation can be determined by measuring the capacitance at both ends of the
piezoelectric element after baking. The method of forming an electrode with baked silver is the
most mass-produced, but uses a large amount of silver paste for printing in sheet form, especially
in the age of high price of silver such as the silver material that accounts for the production cost
The expense rate will be high. On the other hand, in addition to the problem of such
manufacturing costs, there is another problem. That is the point that the reliability of the
bimorph element is not always at a sufficiently satisfactory level.
As a typical test method for examining the reliability of bimorph elements, a load life test in
which a voltage is applied to both ends of a bimorph element is often performed4 but
performance deterioration is observed when the test time is extended for a long time May be The
present invention is intended to solve the above-mentioned problems in the prior art, and to
provide a bimorph element having excellent basic characteristics represented by acoustic
conversion efficiency. Examples of the present invention will be described below. First, the
configuration of a bimorph element according to the present invention will be described using
the drawings. In the figure, 1 and 1 'are piezoelectric elements, and electrodes 2, 3 and i and 3'
are provided on both sides of each. These electrodes 2, 3. i, Go is made of a conductive material
containing graphite powder. Reference numeral 4 denotes a conductive member (hereinafter
referred to as an intermediate positive electrode plate), and piezoelectric elements 1, 1 'are
electrodes on both sides thereof on both sides thereof. The d side is adhered as an adhesive
surface. 5 and g are adhesive layers. Reference numerals 6 and 7.8 are lead wires respectively
connected to the electrodes 2 and 2 'and the intermediate electrode plate 4. The bendable
bimorph element as shown in the figure in which the acoustic conversion efficiency as a bimorph
element is determined by the electrostatic capacity value and the mechanical coupling coefficient
is a series type bimorph element and an interlocking type bimorph element depending on how to
take out the electric terminal 5 It is divided. In any case, the intermediate electrode plate 4 and
the electrode q3 bonded thereto are attached. It is necessary that the terminals are electrically
connected to a. When assembling the device, the adhesive is made a very thin film and both are
bonded over the entire surface. The conductive material used for the electrode and containing
graphite powder is usually used as a thick film circuit resistance material. The piezoelectric
element is usually subjected to polarization processing. Next, specific examples will be described.
Example 1 Ink prepared by mixing graphite powder, phenol resin and high boiling point organic
solvent (Calpitol) in a composition as shown in Table 1 as a 1% electrode material on one side of
a piezoelectric element having a thickness of 100 ?m and a diameter of 23 [ Were printed with
a 300 mesh, emulsion thickness 10 .mu.m nylon screen with a diameter of 22.5 r :, 20 disks. The
single-sided printed electrode is dried at a temperature of 80 ░ C. for 10 minutes, and then
printed on the other side under the same conditions. Thus, the piezoelectric 6 element printed
and dried on both sides was left at a temperature of 150 ░ C. for 30 minutes to cure the
electrode material. And the electrostatic capacitance and dielectric loss as a piezoelectric element
were measured. Next, two piezoelectric elements were assembled as a bimorph element as shown
in the drawing, and the electromechanical coupling coefficient was measured.
On the other hand, in order to investigate the load life of the pimorph element, a DC voltage 9v is
continuously applied between the electrode 2 ░ i outside the bimorph element shown in the
figure and the intermediate electrode plate 4 under normal temperature and normal humidity,
The number of samples that resulted in dielectric breakdown was examined. For the life test, six
samples were provided for each type of electrode. The lifespan was examined when one year and
two months had passed. Table 1 shows the resistance (area resistance) of the electrodes, the
capacitance of the piezoelectric element assembled as a bimorph element, the dielectric loss, and
the electromechanical coupling coefficient of the bimorph element corresponding to the
composition of each type of electrode. The number of samples that had reached dielectric
breakdown during the load life test was shown. Table 1 also shows the lifting table using baked
silver as in the prior art for comparison \. 7 Table 1 Sample 12 Conventional Product
Characteristics Graphite 90 weight 95 weight% pole incorporated no grease 10 weight 56
weight% silver acid 100 weight% 100 weight% electrode area resistance 10 port 0.6 ? port 5
m? port capacitance 885000 pF dielectric loss 35% 3.5% s, o% electric pressure supply threat
number 60% 66% 66% dielectric breakdown sample number 001 As shown in Table 1, the
bimorph element according to the present invention has an electromechanical coupling
coefficient, Silver was used for the electrode, and the value was almost close to that of the
conventional one. -Looking at the results of the blade life, if one year and two months have
passed, the present invention does not have dielectric breakdown in the one according to the
present invention. In the case of the silver electrode, one out of six is broken down. In general,
silver is easily ionized under conditions where humidity, temperature, electric field, etc. are
applied, and therefore, in the case of using silver for the electrode as in the conventional element,
the electric field by the life test and the voltage applied during the test The ionized silver in the
electrode is considered to move and diffuse in the piezoelectric element. On the other hand, when
graphite is used for the electrode as in the present invention, it is considered that the above
phenomenon does not occur. From these facts, it is expected that when the life test time is
further extended, a larger difference from the conventional one will occur. Example 2 The
piezoelectric element used in Example 1 was provided with a graphite electrode having a
composition as shown in Table 2, and 9 characteristics obtained by measuring each
characteristic as in Example 1 are shown in Table 2 below. ? ? ? 9 Table 2 sample, 2
characteristics electric graphite 9011j% by weight 95% by weight pole assembly epoxy resin 10%
by weight 5% by weight 100% by weight 100% by weight Amount% electrode area resistance 1?
port 0.5? B Capacitance 87000pF 84000pF dielectric loss 3.4% 3.3% electromechanical
coupling coefficient 63% 65% breakdown sample number o.
As shown in Table 2, similar results were obtained in this case also with Example 1> 2). That is,
the electromechanical coupling coefficient of the bimorph element ? ? was equal to or close to
that of the conventional one, while no load breakdown test resulted in breakdown. As described
above, according to the present invention, in the electromechanical coupling coefficient which is
the basic characteristic of the bimorph element according to the present invention, it is possible
to obtain a bimorph element comparable to that of the conventional electrode using silver.
Graphite used in the present invention can be obtained at a much lower cost than silver, and
therefore can produce a bimorph element at low cost. Moreover, according to the present
invention, a highly reliable bimorph element having excellent load life characteristics can be
easily obtained.
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