JPS6355245B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a piezoelectric vibrator, and particularly to a piezoelectric vibrator suitable as an oscillation element for electronic watches, radio equipment, etc., and an ultrasonic element for NE equipment, etc. A piezoelectric vibrator consists of a piezoelectric body and metal electrodes formed on both sides of the piezoelectric body, and utilizes mutual conversion between electrical vibration and mechanical vibration. Conventionally, the piezoelectric material used in piezoelectric vibrators is zircon lead titanate (Pb(Zr _{1-X Ti}
Ceramic piezoelectric materials such as so-called PZT-based porcelain whose main component is O ₃ ) and piezoelectric crystals such as quartz are known. Examples of piezoelectric vibrators using crystals are shown in Japanese Patent Application Laid-open Nos. 50-56889. However, in a vibrator using such a piezoelectric crystal,
The cutting orientation of the crystal becomes a problem, and precision cutting must be performed to reduce spurious response or the temperature dependence of the natural frequency. On the other hand, ceramic piezoelectric bodies have many advantages, such as being able to obtain a large amount of piezoelectric bodies with uniform characteristics, and a predetermined uniform polarization occurring through polarization treatment after cutting out a thin piece. However, since PZT-based porcelain is essentially a high-permittivity porcelain, the input and output impedance of the vibrator decreases when used at high frequencies, causing problems in impedance matching with external circuits. In particular, recently, high frequencies of piezoelectric vibrators have been advanced, and it is desired to develop piezoelectric vibrators with excellent temperature characteristics and vibration characteristics that can withstand use at high frequencies. PZT is used as a piezoelectric material for such high-frequency, high-temperature vibrators.
Lead titanate PbTiO ₃ -based porcelain, whose main components are lead oxide (PbO) and titanium oxide (TiO ₂ ), is considered promising because it exhibits a lower dielectric constant and a higher Curie temperature than porcelain. Recently, it is known that the temperature characteristics of PbTiO ₃ type porcelain, which had been regarded as a problem, can be greatly improved by adding neodymium oxide Nd ₂ O ₃ , manganese oxide MnO ₂ , etc. 54-41496, JP-A-54-41497, and JP-A-54-41498). This results in
PbTiO ₃ -based porcelain has the potential to further expand its range of applications. However, PbTiO ₃ -based porcelain has the disadvantage that, due to its small Poisson's ratio, it cannot trap frequency-lowering energy under the partial electrodes like PZT-based porcelain. Therefore, several MHz using thickness vibration
As a vibrator or filter in the range of ~ several tens of MHz, it generates unnecessary vibrations and has the drawback of not being able to increase the Q value. Recently, various methods have been proposed to realize frequency-increasing energy confinement in order to improve the vibration characteristics of PbTiO ₃ -based porcelain, but none have been put into practical use yet. The present invention has been made in view of the above points, and its purpose is to provide a multi-MHz signal with a simple configuration and excellent characteristics.
Our goal is to provide piezoelectric vibrators in the ~ tens of MHz band. In particular, the present invention provides a thickness vibrator that can prevent vibration characteristics from deteriorating due to unnecessary vibrations even when a piezoelectric material having a small Poisson's ratio, such as PbTiO ₃ -based porcelain, is used. The vibrator of the present invention makes use of the fact that by vibrating a piezoelectric plate in an antisymmetric mode with a phase shift of 180°, spurious waves caused by unnecessary vibrations are mutually canceled out. That is, the present invention provides a piezoelectric vibrator consisting of a piezoelectric plate polarized in the same direction in the thickness direction and electrodes formed on both surfaces of the piezoelectric plate, in which the electrode formed on one surface is divided into two pieces. At the same time, a voltage is supplied to these divided electrodes. Hereinafter, the present invention will be explained in detail with reference to the drawings. FIG. 1a is a diagram showing the configuration of an embodiment of the present invention. The piezoelectric vibrator 1 of the present invention consists of a rectangular piezoelectric ceramic plate 2 and electrodes 4, 6, and 8 formed on two opposing main surfaces of the piezoelectric plate 2, respectively.
As shown in FIG. 1b, the rectangular piezoelectric ceramic plate 2 has a length direction, a width direction, and a thickness direction, respectively.
Direction, determined by direction. This piezoelectric ceramic plate 2
The polarization direction is the direction of arrow P.
The electrode 4 is an entire surface electrode formed on one of the two opposing main surfaces (the surface perpendicular to the polarization direction P) of the piezoelectric ceramic plate 2, and the two electrodes 6 and 8 are formed on the other surface of the piezoelectric ceramic plate 2. It is a polarized electrode divided into two parts with the center line in the length direction (direction) as the axis of symmetry. 10 and 12 are electrical terminals taken out from the divided electrodes 6 and 8, respectively. Moreover, the vibrator 1 of the present invention
The entire surface electrode 4 may be grounded if necessary. Next, the operation of the piezoelectric vibrator 1 of the present invention is shown in FIG.
This will be explained using d. Figure 2 is A- in Figure 1 a.
FIG. 3 is a sectional view taken along line A'. When a voltage is applied between the terminals 10 and 12, an electric field distribution 14 as shown in FIG. 2a is generated inside the porcelain plate 2 because the entire surface electrode 4 is on the other surface. This electric field distribution has a component 16 perpendicular to the polarization direction, as shown in Figure 2b and c.
It can be considered by decomposing it into components 18 and 18' parallel to . For example, as piezoelectric ceramic plate 2
When PbTiO ₃ -based porcelain is used, the electromechanical coupling coefficient in the polarization direction of the PbTiO ₃ -based porcelain plate is approximately 1 compared to the electromechanical coupling coefficient in the direction perpendicular to the polarization direction.
Because it is an order of magnitude larger, the electric field components in the polarization direction are 18, 18'
The piezoelectric coupling with becomes dominant. Moreover, electric field component 1
The directions of 8, 18' are always completely opposite (180° out of phase) on either side of the centerline 20 of the porcelain plate. Therefore, when an alternating current electric field is applied between the terminals 10 and 12, the thickness longitudinal vibration combined with the horizontal electric field components 18 and 18' is excited, and moreover, as shown in FIG.
6 and 8 are omitted), a thickness vibration in an antisymmetric mode is excited with respect to the center line 20. Therefore, spurious waves caused by unnecessary vibrations that appear in a single strip-like thick vibrator are canceled out. Therefore, even if a piezoelectric plate with a small Poisson's ratio, such as PbTiO ₃ -based porcelain, is used, unnecessary vibrations can be attenuated and eliminated, and the Q value can be increased. Next, a method for supporting the vibrator 1 of the present invention will be explained. As shown in FIG. 3, the length l of the rectangular plate 2 is made sufficiently larger than its thickness t and width w, and only one end in the length direction of the surface on which the entire surface electrode 4 is provided is fixed to the support base 22. . The divided electrodes 6 and 8 are formed so that the width w'' of the part facing the support part (support part) is narrower than the width w' of the other part (vibration part).The lead wire 34 is glued to these thin parts. By taking out the electric terminals 10 and 12, a vibrator with a sufficiently high Q value was obtained.In the figure, g indicates the gap width between the divided electrodes. Although the electrodes are provided on the entire surface of the other main surface of the plate 2, it is not necessary to provide the electrodes on the part that comes into contact with the support base 22.Hereinafter, experiments were conducted using the piezoelectric vibrator of the present invention using PbTiO ₃ ceramic. We sintered PbTiO ₃ -based porcelain containing neodymium Nd, indium In, and manganese Mn, which reduces the resonance frequency temperature coefficient against thickness vibration, and heated it in the thickness direction at 55 KV/cm for 10 minutes (150°C).
Vibrators of various sizes shown in Table 1 were formed from porcelain plates polarized under the following conditions. In the table, l and t indicate the length and thickness of the rectangular porcelain plate, respectively, and w' indicates the width of the vibrating part of the divided electrode. Note that the gap width g between the divided electrodes is 0.2 mm in each case. Therefore, the width of the porcelain plate w
is 2w′+0.2. The electrode pattern was formed by depositing electrode materials (chromium and gold) directly onto the porcelain plate through a metal mask. Table 1 shows the resonant frequency of each vibrator.
The results of measuring f ₀ , equivalent resistance R and Q value at resonance are shown. As is clear from the table, according to the present invention, a piezoelectric vibrator with a sufficiently high Q value can be obtained. As an example, Example 18 (w′=0.8mm, l=6mm, t=0.54
Figure 4 shows the resonance characteristics of In the figure, the horizontal axis indicates the frequency f (MHz), and the vertical axis indicates the absolute value of the impedance of the vibrator |Z〓|(Ω). Resonant frequency f ₀
is approximately 4.25MHz, and it is clear that a single resonance characteristic with almost no spurious has been obtained. In addition, the Q value determined from the admittance circle near the resonance point was approximately 2200, which was a sufficiently large value. Furthermore, as is clear from Table 1, the equivalent resistance R
And the Q value is l, when the thickness t is constant,
Although it changes depending on the value of w', the resonant frequency f ₀ is determined almost only by the thickness t and does not depend much on the value of w'. Therefore, as mentioned above, it can be considered that most of the thickness longitudinal vibrations are excited. This is also clear from FIG. Fifth

[Table] The figure shows the relationship between the thickness t of a rectangular ceramic plate and the resonant frequency f ₀ from Table 1. In the figure, the horizontal axis indicates the thickness t, the vertical axis indicates the resonance frequency f ₀ , and the number appended to each measurement point indicates the value of the ratio w'/t of the width w' to the thickness t of the divided electrode. It is clear that the resonant frequency f ₀ has little to do with the width w' of the divided electrode and is inversely proportional to the thickness t. Therefore, by changing the thickness t, the resonance frequency can be changed. Furthermore, there is a preferable size for the vibrator. That is, when the thickness t is determined according to the desired frequency, good characteristics are exhibited when the width w' of the divided electrode and the length l of the vibrator are in the range of 0.5t≦w'≦3t, l≧5w'. Ta. For example, as can be seen from Example 16 in Table 1, when w' became smaller than t/2, the Q value decreased rapidly. Furthermore, as can be seen from Example 22, when w' became larger than 3t, spurious signals began to occur, making it difficult to measure the Q value. Regarding the length l of the vibrator, as is clear from Examples 7 and 8, when it became smaller than 5w', the Q value decreased rapidly. When l was larger than 5w', good characteristics were exhibited within the range of the examples. However, if l becomes too large, it will be difficult to support it, so from the point of view of practical size, l should be limited to about 30 times w'. Also, the gap width g between the divided electrodes is g<
If the condition of w′ is satisfied, a product with good characteristics can be obtained, but from the viewpoint of increasing Q
Approximately 0.2w′ is desirable. Note that the piezoelectric vibrator of the present invention can obtain excellent resonance characteristics without spurious even in the third harmonic. FIG. 6 shows an example of application of the piezoelectric vibrator of the present invention. FIG. 6 shows a two-terminal oscillation circuit, in which a piezoelectric vibrator 1 according to the present invention is connected as shown. In the figure, 24 is a C-MOS amplifier, C ₁ ,
C ₂ is a capacitor. The capacitors C ₁ and C ₂ serve as equivalent load capacitance for the piezoelectric vibrator 1 . In addition, the dotted line in the figure indicates the entire surface electrode 4 as necessary.
This indicates that it may be grounded. Next, a method for adjusting the resonant frequency of a piezoelectric vibrator according to the present invention will be explained. Since the resonant frequency of a piezoelectric vibrator is determined by the geometric shape of the vibrator, extremely strict processing precision is required depending on the purpose if the frequency characteristics of the piezoelectric vibrator are to be made uniform during mass production. In reality, it is difficult to completely match a predetermined resonant frequency only at the element processing stage, and the current situation is that products are manufactured using some kind of frequency adjustment method. Therefore, it is desirable to solve such problems with the piezoelectric vibrator of the present invention as well. The piezoelectric vibrator of the present invention has an antisymmetric vibration mode as shown in FIG. 2d by dividing the electrode formed on one main surface into two parts and supplying an operating voltage to the divided electrodes. This is an excited thickness oscillator. Therefore, as shown in FIG.
Even if a minute force 26 is applied on the center line 20 of the vibrator 1, the Q value of the vibrator 1 will not be greatly affected. That is, distortion can be applied to the vibrator without significantly affecting the Q value. Moreover, the vibrator 1 of the present invention can be operated with only one end fixedly supported as shown in FIG.
By applying force to a portion near the other end (free end), a large strain can be applied to the vibrator 1 with a small amount of force. When strain is applied to the vibrator,
Since the resonant frequency changes depending on the amount of distortion,
The resonance frequency can be adjusted by controlling the amount of distortion. FIG. 8 is a diagram showing another embodiment of the present invention,
A frequency adjustment means is added. As shown in the figure, the piezoelectric vibrator 1 of the present invention is placed in a package 28 made of, for example, iron nickel via a support member 22. A minute hole 30 is formed at the bottom of the package cage 28, and an adjustment screw 32 is formed in the bottom of the package cage 28.
is inserted. The tip of the adjustment screw 33 is arranged so as to be located approximately on the center line near the free end of the vibrator. In addition, the two lead wires 34 of the vibrator
is connected to a pin 36 provided at the bottom of the package 28 via an insulator, so that voltage can be supplied to the divided electrodes from outside the package. In addition, there is a ground pin 3 on the bottom of the package cage.
8 is provided, and by grounding this earth pin, the entire surface electrode can be grounded. With this configuration, distortion can be applied to the vibrator by slightly moving the adjustment screw, and the resonant frequency can be adjusted without substantially lowering the Q value of the vibrator. Figure 9 shows the strain S applied to the vibrator 1 by the adjustment screw 32 (calculated from the amount of screw movement) and the ratio of the amount of change Δf in the resonant frequency to the resonant frequency _f0 when no strain is applied (change in frequency). It is a figure which shows an example of the relationship between (rate) Δf/f ₀ and Q value. As is clear from the figure, the resonant frequency can be adjusted by approximately 6×10 ⁻⁴ by applying a minute strain of 10 ⁻⁵ to 10 ⁻⁴ . Moreover,
The Q value of the vibrator did not change significantly.
In Fig. 9, the vibrator 1 is made of a PbTiO ₃ ceramic plate polarized in the thickness direction, with a length l of 6 mm, a width w of 2 mm, and a thickness t of 0.53 so that the resonance frequency f ₀ is approximately 4 MHz. In this case, a rectangular plate with a diameter of 1.5 mm is formed, a divided electrode with a width w′ of 0.9 mm is formed on one main surface of this rectangular plate, and an electrode is formed on the entire surface of the other main surface. . As explained above, according to this embodiment, by applying a minute force approximately on the center line of the vibrator,
The resonance frequency can be adjusted by approximately 6×10 ⁻⁴ without substantially reducing the Q value. Furthermore, the piezoelectric vibrator of the present invention can be used as a pressure (gas pressure) detection element. A gas pressure detection device using a piezoelectric vibrator applies distortion to the vibrator using gas pressure, and detects a change in the oscillation frequency of the vibrator caused by this distortion. Therefore, it is desirable for the pressure detection element to be a vibrator that can handle a large amount of strain against pressure. The vibrator of the present invention operates as a thickness longitudinal vibrator with extremely little spurious. Moreover, the vibrator of the present invention has
Since it can be operated with only one end fixedly supported, a large amount of strain can be obtained by applying pressure near the other end (free end). This is similar to the fact that conventional surface acoustic wave elements used as pressure detection elements can have a large amount of distortion. FIG. 10 shows the configuration of a pressure detection device using the piezoelectric vibrator of the present invention. The vibrator 1 fixedly supported by the support stand 22 is fixed to a stand 40 made of a conductive member and placed in a container 42. Container 42 is a cylindrical glass tube with a closed end at one end, and an open end at the other end joined to base 40 . Further, a gas introduction pipe 44 and a gas outlet pipe 46 are connected to the side wall of the container 42, and the gas is configured to flow from the gas introduction pipe 44 through the container 42 to the outlet pipe 46. The introduction pipe 44 is provided at a position where gas flows into the vicinity of the free end of the vibrator 1 placed in the container 42 . Further, if necessary, the diameter of the portion of the introduction tube 44 connected to the container 42 is made smaller. The two lead wires 34 of the vibrator 1 are connected to pins 36 that are electrically insulated and implanted in the stand 40.
Electrical terminals 10 and 12 are taken out of the container 42. The base 40 is provided with a grounding pin 38, and by grounding this grounding pin, the entire surface of the electrode of the vibrator 1 can be grounded, as in the embodiment shown in FIG. With this configuration, when the gas 48 flows into the container 42 from the introduction pipe 44, pressure is exerted on the free end of the vibrator 1 , so that the vibrator
Distortion occurs according to the pressure, and the resonant frequency changes (the amount of change in the resonant frequency depends on the gas pressure). Therefore, as shown in the figure, the terminals 10,
12 are respectively connected to the terminals 52 of the C-MOS amplifier 50,
54 to form an oscillation circuit, the oscillation frequency will change depending on the gas pressure. In other words, fluctuations in gas pressure can be detected as changes in oscillation frequency. FIG. 11 is a diagram showing the relationship between the gas flow rate F (and therefore the pressure) and the rate of change Δf/f ₀ of the oscillation frequency. As the gas flow rate F increased, the rate of change Δf/f ₀ of the oscillation frequency increased, and the oscillation frequency changed by about 4×10 ⁻⁵ (Δf ~ 160 Hz) for a flow rate of 1/min. Figure 11 shows a rectangular vibrator made of a PbTiO ₃ ceramic plate polarized in the thickness direction, with a length l of 8 mm, a width w of 3 mm, and a thickness t of 0.53 mm so that the resonance frequency is about 4 MHz. This is a case in which a rectangular plate is formed, a divided electrode with w' of 1.4 mm is formed on one main surface of this rectangular plate, and an electrode is formed on the entire surface of the other main surface. As described above, according to this embodiment, a highly sensitive gas pressure detection device can be realized by configuring the vibrator to receive pressure at its free end.

[Brief explanation of drawings]

FIG. 1a is a diagram showing the configuration of the piezoelectric vibrator of the present invention, FIG. 1b is a diagram for explaining the length, width, and thickness direction of the vibrator, and FIGS. 2a, b, c, and d is
FIG. 3 is a diagram showing the operating principle of the vibrator of the present invention.
FIG. 4 is a diagram showing the method of supporting the vibrator of the present invention.
FIG. 5 is a diagram showing the resonance characteristics of the vibrator of the present invention.
A diagram showing the relationship between the thickness t of the vibrator and the resonant frequency f ₀ ,
FIG. 6 is a diagram showing the configuration of an oscillation circuit using the vibrator of the present invention, FIGS. 7 and 8 are diagrams explaining the frequency adjustment method of the piezoelectric vibrator of the present invention, and FIG.
A diagram showing the relationship between the amount of distortion S, the frequency change rate Δf/f ₀ and the Q value, FIG. 10 is a diagram showing the configuration of a gas pressure detection device using the vibrator of the present invention, and FIG.
FIG. 3 is a diagram showing the relationship between the gas flow rate F and the frequency change rate Δf/f ₀ .

Claims (1)

[Claims] 1. In a piezoelectric vibrator consisting of a rectangular piezoelectric plate polarized in the same direction in the thickness direction and electrodes formed on two opposing main surfaces of the piezoelectric plate, one of the electrodes is connected to the piezoelectric plate. A divided electrode is divided into two with the longitudinal center as an axis of symmetry, and the other electrode is a uniform electrode facing both of the divided electrodes, and a voltage is supplied to the divided electrode to The piezoelectric vibrator is characterized in that it produces thickness vibration in an antisymmetric mode with respect to the center line.