JP3382200B2 - Piezoelectric ceramics - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric ceramic which can be widely applied to fields such as a resonator and a pressure sensor.

[0002]

2. Description of the Related Art A piezoelectric material is a material having a piezoelectric effect in which electric polarization is changed by receiving a stress from the outside and an inverse piezoelectric effect in which strain is generated by applying an electric field. Piezoelectric is a sensor for measuring pressure and deformation,
It is applied to resonators and actuators.

Most of the piezoelectric materials currently in practical use are tetragonal or rhombohedral PZT (PbZrO _3).
-PbTiO ₃ solid solution) or tetragonal PT (PbT
Ferroelectric materials having a perovskite structure such as iO ₃ ) are generally used. By adding various subcomponents to these, various required characteristics can be met.

However, PZT-based and PT-based piezoelectric materials are
Most practical compositions have a Curie point of about 300 to 350 ° C. On the other hand, since the current processing temperature in the soldering process is usually 230 to 250 ° C., the piezoelectric material having a Curie point of about 300 to 350 ° C. is likely to cause characteristic deterioration in the soldering process. Moreover, when a solder containing no lead (lead-free solder) is put into practical use, the processing temperature in the soldering process becomes higher. Therefore, it is extremely important to raise the Curie point of the piezoelectric material.

Further, these lead-based piezoelectric materials contain a large amount (60 to 70) of lead oxide (PbO), which is extremely volatile even at low temperatures.
It is not preferable from the ecological point of view and from the standpoint of pollution prevention. Specifically, when manufacturing these lead-based piezoelectric materials as ceramics or single crystals, heat treatment such as firing and melting is inevitable, and when considered at an industrial level, lead oxide, which is a volatile component, in the atmosphere. The amount of volatilization and diffusion is extremely large. Also, lead oxide released in the manufacturing stage can be recovered, but most lead oxide contained in piezoelectric materials marketed as industrial products is currently unrecoverable, and these are widely used in the environment. If released to, it is unavoidable to cause pollution.

As a piezoelectric material containing no lead, for example, BaTi having a perovskite structure belonging to the tetragonal system is used.
O ₃ is well known, but it has a Curie point of 120.
Since it is as low as ℃, it is not practical. In addition, JP-A-9-10
Japanese Patent No. 0156 discloses that the perovskite structure is (1-x).
Although a (Bi _1/2 Na _1/2 ) TiO ₃ —xNaNbO ₃ solid solution is described, the publication does not describe a Curie point of more than 370 ° C.

Bismuth layered compounds, for example, are known as piezoelectric materials having a Curie point of 500 ° C. or higher.
Since the bismuth layered compound has a high Curie point, once it is polarized, it is thermally stable and has sufficient characteristics as a high temperature sensor. However, on the other hand, since polarization itself is difficult, it is difficult to obtain sufficient piezoelectric characteristics. In addition, the bismuth layered compound containing no lead is important when applied to a resonator.
There is a problem that max is small. Qmax is tan θmax when the maximum value of the phase angle is θmax. That is,
When X is the reactance and R is the resistance, Q (= | X | /) between the resonance frequency and the anti-resonance frequency.
It is the maximum value of R). The larger Qmax is, the more stable the oscillation becomes, and the lower voltage the oscillation becomes possible.

The 16th Ferroelectric Application Conference (1999.5.26-2)
The reports of the proceedings of 9), pages 97 to 98, describe a report for improving the Qmax of a bismuth layered compound containing no lead at all. In this report, as a bismuth layer compound containing no lead, (Sr _{1- x} Me _x ) Bi ₄ Ti ₄ O ₁₅
Is listed. Me = Ba, Ca, La, Sm, G
d and Ba and Ca in the range of x ≦ 0.1, Sm
And Gd are added in the range of x ≦ 0.4, and La is added in the range of x ≦ 0.5. In the above-mentioned lecture proceedings, Qmax in the thickness longitudinal fundamental vibration is measured.
Improvement of Qmax by addition of a and Ba or C
It has been shown that the addition of a reduces Qmax.

By the way, when the piezoelectric ceramic is applied to a resonator or a filter, it is required that the temperature dependence of the resonance frequency is small. However, no piezoelectric ceramic has been reported so far, which has a sufficiently large Qmax that can be practically used as a resonator and has a small temperature dependence of the resonance frequency.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a piezoelectric ceramic which does not contain lead, has a high Curie point, has excellent piezoelectric characteristics, and has a small temperature dependence of resonance frequency. That is.

[0011]

[Means for Solving the Problems] The above-mentioned object is as follows (1)
It is achieved by the present invention of (3). (1) When at least one element selected from Sr, Ba and Ca is represented by M ^II and at least one element selected from lanthanoids is represented by Ln, M ^II , B
A bismuth layered compound containing i, Ti, Ln and O, which contains M ^II Bi ₄ Ti ₄ O ₁₅ type crystals and has a molar ratio Ln.
Piezoelectric ceramics containing Y oxide, wherein / (Ln + M ^II ) is 0 <Ln / (Ln + M ^II ) <0.5. (2) The piezoelectric ceramic according to the above (1), which contains Y oxide in an amount of 0.5 mass% or less in terms of Y ₂ O ₃ . (3) The piezoelectric ceramic according to the above (1) or (2), which contains a Mn oxide and / or a Co oxide.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION When at least one element selected from Sr, Ba and Ca is represented by M ^II and at least one element selected from lanthanoids is represented by Ln, the piezoelectric ceramic of the present invention is , M ^II , Bi, Ti,
A bismuth layered compound containing Ln and O, M
^II A composite oxide containing Bi ₄ Ti ₄ O ₁₅ type crystals.

The piezoelectric ceramic of the present invention contains a lanthanoid oxide in order to improve Qmax. Lanthanoids are La, Ce, Pr, Nd, Pm, Sm, E
u, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, among which La, Nd, Sm, G
At least one of d, Dy, Ho, Er and Yb is preferable, and La is most preferable. The molar ratio Ln / (Ln + M ^II ) in the piezoelectric ceramic of the present invention is 0 <Ln / (Ln + M ^II ) <0.5, and preferably 0.03 ≦ Ln / (Ln + M ^II ) ≦ 0.3. . If Ln / (Ln + M ^II ) is too large, Qmax
On the contrary, it becomes low. It is considered that the improvement of Qmax due to the addition of the Ln oxide is due to the improvement of sinterability. Further, M ^II Bi ₄ Ti ₄ O ₁₅ series ceramics containing no Ln oxide, particularly CaBi ₄ Ti ₄ O ₁₅ series ceramics,
The polarization is difficult, but the addition of Ln oxide improves this.

The piezoelectric ceramic of the present invention contains Y oxide in addition to Ln oxide. By adding Y oxide together with Ln oxide, the temperature characteristic of the resonance frequency is improved, and a sufficiently large Qmax can be obtained. The content of Y oxide is preferably 0.5% by mass in terms of Y ₂ O _3.
Or less, more preferably 0.4% by mass or less. If the Y oxide content is too high, Qmax will be low.
On the other hand, in order to fully exert the effect of the addition of the Y oxide, the Y oxide is preferably contained in an amount of 0.1% by mass or more, particularly 0.2% by mass or more in terms of Y ₂ O _3. .

By including Mn oxide and / or Co oxide in the piezoelectric ceramic of the present invention, Qma
Further improvement of x is possible.

In order to fully bring out the effect of the addition of Mn oxide, the Mn oxide is converted to MnO in an amount of 0.0
The content is preferably 2% by mass or more, and when it is 0.03% by mass or more, the Qmax improving effect is particularly enhanced. However, when the content of Mn oxide is too large, the insulation resistance becomes low and the polarization treatment becomes difficult. Therefore, the content of Mn oxide is preferably less than 0.62 mass% in terms of MnO, more preferably Is 0.60 mass% or less, more preferably 0.43 mass% or less.

In order to fully bring out the effect of the addition of Co oxide, the content in terms of CoO is preferably 0.1% by mass or more. However, if the content of Co oxide is too large, the insulation resistance increases and polarization becomes difficult. Therefore, the content in terms of CoO is preferably 0.
It is less than 7% by mass, more preferably 0.5% by mass or less.

[0018] M molar ratio in ^II Sr _x Ba _y Ca _z (provided that, x + y + z = 1 ) when expressed in, 0 ≦ x ≦ 1, 0 ≦ y ≦ 0.9 that is a 0 ≦ z ≦ 1 Is preferred. If the ratio y of Ba in M ^II is too high, the piezoelectric ceramics are likely to melt during firing.

The piezoelectric ceramics of the present invention include an M ^II Bi ₄ Ti ₄ O ₁₅ type crystal which is a bismuth layered compound, and it is preferable that the piezoelectric ceramics be substantially composed of this crystal, but it is not absolutely homogeneous. , For example, may contain a heterogeneous phase.
In this piezoelectric ceramic, Ln is M ^II Bi ₄ T
It is considered that the M ^II site of the i ₄ O ₁₅ type crystal is mainly substituted, but a part of the i ₄ O ₁₅ type crystal may be replaced with another site, or a part thereof may be present at the crystal grain boundary. Good.

The overall composition of the piezoelectric ceramic of the present invention is
In general, (M ^II _1-x Ln _x ) Bi ₄ Ti ₄ O ₁₅ may be added with Y ₂ O _3, but it may be deviated from this. For example, the ratio of (M ^II + Ln) to Ti,
The ratio of Bi to Ti may deviate from the stoichiometric composition by about ± 5%. Further, the amount of oxygen can also change depending on the valence of the metal element, oxygen defects, and the like.

The piezoelectric ceramics of the present invention may contain Pb oxide, Cr oxide, Fe oxide and the like as impurities or trace additives, but the content of these oxides is PbO, Cr. It is preferably 0.5% by mass or less of each of the oxides having a stoichiometric composition such as ₂ O ₃ and Fe ₂ O _3, and the total amount of these oxides is not more than 0.
It is more preferably 5% by mass or less. If the content of these oxides is too high, the effects of the present invention may be impaired. It is most preferable that the piezoelectric ceramic of the present invention does not contain Pb, but if the content is in the above range, there is substantially no problem.

The crystal grains of the piezoelectric ceramics of the present invention are spindle-shaped or needle-shaped. The average crystal grain size is not particularly limited, but is preferably 1 to 10 μm in the major axis direction.
m, more preferably 3 to 5 μm.

The Curie point of the piezoelectric ceramics of the present invention can be at least 380 ° C. or higher and 430
It is also easy to set the temperature above ℃.

The piezoelectric ceramics of the present invention are suitable for resonators, high temperature sensors, actuators, etc.
Since the temperature characteristic of the resonance frequency is good, it is particularly suitable for a resonator.

The use mode of the piezoelectric ceramics of the present invention is not particularly limited, and any mode such as thickness longitudinal vibration or thickness shear vibration can be used. A relatively high Qmax is obtained with the thickness longitudinal fundamental vibration. However, there are more spurious vibrations, and as a result, the stability of oscillation is slightly lower. On the other hand, in the third harmonic mode of thickness longitudinal vibration,
Although Qmax decreases, spurious vibration decreases. On the other hand, in the thickness-shear fundamental vibration, spurious vibrations are small and a sufficiently large Qmax is obtained.

According to the research conducted by the present inventors, it was found that when the thickness shear vibration is used, the temperature characteristic of the resonance frequency becomes relatively steep, and the temperature dependence of the oscillation frequency becomes relatively large. . Therefore, as a result of further experiments, by setting the molar ratio in M ^II to x / 6 + 0.2 ≦ y ≦ 0.8, that is, as M ^II , at least B
a + and / or Ca is used, and Ba + C occupied in M ^II
It has been found that the temperature characteristic of the resonance frequency can be made fairly flat by setting the ratio of a within the predetermined range.

Manufacturing Method Next, an example of a method for manufacturing the piezoelectric ceramics of the present invention will be described.

First, as a starting material, an oxide or
Compounds that can be converted into oxides by firing, such as carbonates, hydroxides, oxalates, nitrates, specifically M ^II
CO ₃ , Bi ₂ O ₃ , TiO ₂ , La ₂ O ₃ , MnO ₂ , Mn
Powders such as CO ₃ and Y ₂ O ₃ are prepared, and these are wet mixed by a ball mill or the like.

Then, calcination is performed. It should be noted that, usually, temporary forming is performed before calcination. The calcination temperature is preferably 700 to 1000.
C., more preferably 750 to 850.degree. If the calcination temperature is too low, the chemical reaction will not be completed sufficiently and calcination will be insufficient. On the other hand, if the calcination temperature is too high, the tentative compact will start to sinter, making subsequent pulverization difficult. Although the calcination time is not particularly limited, it is usually preferably 1 to 3 hours.

The obtained calcined product is slurried and wet-milled using a ball mill or the like. The average particle size of the powder obtained by this pulverization is not particularly limited, but considering the ease of subsequent molding, it is preferably about 1 to 5 μm.

After wet pulverization, the powder of the calcined product was dried, and a small amount of water (about 4 to 10% by mass) was added to the dried product, and then 10
Press molding is performed at a pressure of about 0 to 400 MPa to obtain a molded body. At this time, a binder such as polyvinyl alcohol may be added.

Next, the molded body is fired to obtain piezoelectric ceramics. The firing temperature is preferably 1100 to 1300 ° C
The firing time is preferably about 1 to 5 hours. The firing may be performed in the air, an atmosphere having a lower oxygen partial pressure than that in the atmosphere, a high atmosphere, or a pure oxygen atmosphere.

After firing, polarization treatment is performed. The conditions of the polarization treatment may be appropriately determined according to the composition of the piezoelectric ceramics, but normally the polarization temperature is 150 to 300 ° C., the polarization time is 1 to 30 minutes, and the polarization electric field is 1.1 times or more of the coercive electric field. do it.

[0034]

EXAMPLES The piezoelectric ceramic samples shown in Table 1 were prepared by the following procedure.

As starting materials, SrCO ₃ , Bi ₂ O ₃ ,
Powders of TiO ₂ , La ₂ O ₃ , Gd ₂ O ₃ , MnCO ₃ , and Y ₂ O ₃ were used, and the final composition was Sr _1-x Ln _x Bi ₄ Ti ₄ O ₁₅ + Mn.
O + Y ₂ O ₃ (contents of MnO and Y ₂ O ₃ are values shown in Table 1) were mixed and wet-mixed in pure water by a ball mill using zirconia balls for 15 hours. Ln
And x are shown in Table 1.

Then, the mixture was thoroughly dried, preformed, and then calcined in air for 2 hours. Calcination temperature is 75
It was selected from the range of 0 to 900 ° C. The obtained calcined product was crushed with a ball mill and then dried. Next, after adding 10% by mass of a 10% polyvinyl alcohol solution as a binder and granulating, press molding was performed at a pressure of 300 MPa using a uniaxial press molding machine to obtain a compact having a plane dimension of 20 mm × 20 mm and a thickness of 1.5 mm. Obtained.

The molded body thus obtained was heat-treated to volatilize the binder, and then fired in air. The calcination was performed in a sealed MgO container to prevent evaporation of Bi. The firing temperature was selected from the range of 1150 to 1300 ° C., and the firing time was 2 to 4 hours.

The obtained sintered body was polished to a thickness of 0.5 mm and cut into a plane size of 15 mm × 15 mm to obtain a thin plate.
Cu electrodes were formed on the upper and lower surfaces of this thin plate by vapor deposition. Then, in a silicone oil bath at 150 to 300 ° C., an electric field of 5 to 15 MV / m was applied for 1 to 10 minutes for polarization treatment.

Then, the Cu electrode is removed by etching with a FeCl ₂ solution, and then the thickness of 0.44 is obtained.
The chip was obtained by polishing to 7 mm × 4.5 mm, and polishing so that the polarization direction was the thickness direction. Ag electrodes for evaluating thickness longitudinal vibration were formed on the upper and lower surfaces of this chip by vapor deposition. The dimensions of this Ag electrode were 1.5 mm in diameter and 1 μm in thickness.

For each sample, Qm of the third harmonic mode of thickness longitudinal vibration at about 16 MHz was measured using an impedance analyzer HP4194A manufactured by Hewlett Packard.
The ax was measured. The results are shown in Table 1.

The resonance frequency was measured in the temperature range of 20 to 85 ° C. using an impedance analyzer and a thermostat. Then, by using the resonance frequency fr ₈₅ at the resonance frequency fr ₂₀ and 85 ° C. at 20 ° C., by the equation _{△ fr = 100 × (fr 85} -fr 20) / fr 20 [%], resonance relative to the 20 ° C. Frequency change rate Δfr
I asked. The results are shown in Table 1.

[0042]

[Table 1]

From Table 1, in the piezoelectric ceramics of the present invention, Qmax sufficiently large for practical use as a resonator,
That is, it can be seen that Qmax of 5 or more is obtained and the temperature dependence of the resonance frequency is small.

The Curie temperatures of the samples of the present invention shown in Table 1 were all 450 ° C. or higher.

When the samples of the present invention shown in Table 1 were crushed and analyzed by the powder X-ray diffraction method, M ^II Bi ₄ Ti ₄ O _{15 was obtained.}
It was confirmed that the crystals were almost a single phase. FIG. 1 shows X-ray diffraction charts of Samples No. 2 to No. 5. In FIG. 1, since segregation of Y is not observed in any of the samples, it is considered that Y exists in the crystal lattice. For these samples, (20
From the diffraction peak of the (0) plane, the lattice constant of the a-axis is
When the c-axis lattice constants were obtained from the surface diffraction peaks, virtually no change in lattice constant due to the addition of Y was observed. Therefore, it is considered that Y exists in the crystal in a substitutional solid solution state.

Samples No. 2 to No. 5 in Table 1 were mirror-finished, chemically etched under the same conditions, and then photographed by SEM (scanning electron microscope). Sample No.2
2 to 5 are SEM photographs of No. 5 to No. 5, respectively. In these photographs, the larger the amount of Y added, the more the dissolution of the crystal grain boundaries. Therefore, it is possible that Y is present in large amounts in the grain boundary portions.

[0047]

According to the present invention, it is possible to realize a piezoelectric ceramic which does not contain lead, has a high Curie point, has excellent piezoelectric properties, and has a small temperature dependence of resonance frequency.

[Brief description of drawings]

FIG. 1 is an X-ray diffraction chart of piezoelectric ceramics.

FIG. 2 is a drawing-substituting photograph showing a particle structure, which is a scanning electron microscope photograph of Sample No. 2 shown in Table 1.

FIG. 3 is a drawing-substituting photograph showing a particle structure, which is a scanning electron microscope photograph of Sample No. 3 shown in Table 1.

FIG. 4 is a drawing-substituting photograph showing a particle structure, which is a scanning electron microscope photograph of Sample No. 4 shown in Table 1.

5 is a drawing-substitute photograph showing a particle structure, which is a scanning electron micrograph of sample No. 5 shown in Table 1. FIG.

─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP 2001-172078 (JP, A) JP 2001-220226 (JP, A) JP 2000-143340 (JP, A) JP 2001-192267 (JP, A) JP 2000-313662 (JP, A) JP 2000-72542 (JP, A) JP 2001-192268 (JP, A) JP 2001-181030 (JP, A) (58) Fields investigated (Int .Cl. ⁷ , DB name) C04B 35/46-35/50 CA (STN) REGISTRY (STN)

Claims (3)

(57) [Claims] 1. When at least one element selected from Sr, Ba and Ca is represented by M ^II , and at least one element selected from lanthanoids is represented by Ln, M
A bismuth layered compound containing ^II , Bi, Ti, Ln and O, containing M ^II Bi ₄ Ti ₄ O ₁₅ type crystal, and having a molar ratio Ln / (Ln + M ^II ) of 0 <Ln / (Ln + M ^II ) <0. Piezoelectric ceramics containing Y oxide. 2. The piezoelectric ceramic according to claim 1, which contains Y oxide in an amount of 0.5 mass% or less in terms of Y ₂ O ₃ . 3. The piezoelectric ceramic according to claim 1, which contains a Mn oxide and / or a Co oxide.