JP2000128632A - Piezoelectric ceramics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a high Curie point in piezoelectric ceramics containing (Bi1-aNaa)bTiO3 as a principal component and BicFeO3 as an accessory component and further containing a perovskite type crystal by regulating the molar ratio of the accessory component to the principal component within a specific range. SOLUTION: The piezoelectric ceramics contain (Bi1-aNaa)bTiO3 [(a) is 0.4-0.6; (b) is 0.95-1.05] as a principal component and BicFeO3 [(c) is 0.95-1.05] as an accessory component. The molar ratio of the accessory component to the principal component is >0 and <=0.1. The piezoelectric ceramics are preferably substantially composed of a perovskite type crystal, but need not be completely homogeneous or may contain, e.g. a heterogeneous phase. The piezoelectric ceramics most preferably contain no Pb. The weight ratio thereof may actually be <=1,000 ppm expressed in terms of PbO. Thereby, the Curie point of the piezoelectric ceramics can be regulated to >=350 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric ceramic widely applicable to fields such as a resonator and a high-temperature 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 an external stress and an inverse piezoelectric effect in which distortion is generated by applying an electric field. Piezoelectric materials are sensors for measuring pressure and deformation,
It is applied to resonators and actuators.

Piezoelectric materials currently put to practical use, tetragonal or rhombohedral system PZT (PbZrO ₃ -PbTi
A ferroelectric material having a perovskite structure such as an O ₃ solid solution) or a tetragonal PT (PbTiO ₃ ) is generally used. By adding various sub-components to these, various requirements are met. For example, actuators for position adjustment that require a large amount of displacement in a DC manner have a large piezoelectric constant (d ₃₃ ) instead of a small mechanical quality factor (Q _m ). For applications where alternating current is used, such as an ultrasonic generator used for
₃₃ ) instead of having a small mechanical quality factor (Q _m ).

However, PZT and PT piezoelectric materials are
Many practical compositions have a Curie point of about 200 to 400 ° C. At higher temperatures, they become paraelectric and lose their piezoelectricity. Is not applicable. Further, since these lead-based piezoelectric materials contain a large amount (approximately 60 to 70% by weight) of lead oxide (PbO), which is extremely volatile even at a low temperature, it is also preferable from an ecological point of view and pollution prevention. Absent. Specifically, when producing these lead-based piezoelectric materials as ceramics or single crystals, heat treatment such as firing and melting is inevitable, and when considered on an industrial level, lead oxide, a volatile component, is contained in the atmosphere. The amount of volatilization and diffusion to the water becomes extremely large. In addition, lead oxide released during the manufacturing stage can be recovered, but most of the lead oxide contained in piezoelectric materials put on the market as industrial products cannot be recovered at present, and these are widely used in the environment. If released to
It is inevitable that it causes pollution.

As a piezoelectric material containing no lead, for example, BaTi having a perovskite structure belonging to a tetragonal system is used.
O ₃ is well known and has a Curie point of 120
It is not practical because of low temperature.

[0006] Also, SILICATES INDUSTRIELS 1993 / 7-8, p.
On p.136-142, as a lead-free piezoelectric ceramic, (Bi _1/2 Na _1/2 ) TiO ₃ having a perovskite structure is described.
And KNbO ₃ added thereto. However, as shown in FIG. 6 of this document (Bi _1/2 N
a _1/2 ) The Curie point of TiO ₃ is only about 335 ° C, and when KNbO ₃ is added thereto, the Curie point is further lowered.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to achieve a high Curie point in lead-free piezoelectric ceramics.

[0008]

The above objects are achieved by the present invention described below. (1) As the main component (Bi _1-a
Na _a ) _b TiO ₃ (a = 0.4-0.6, b = 0.95)
The ~1.05), Bi _c FeO ₃ as an auxiliary component (c = 0.
95-1.05), including perovskite-type crystals, and Bi _{c with} respect to (Bi _1-a Na _a ) _b TiO ₃ .
A piezoelectric ceramic in which the molar ratio of FeO ₃ is more than 0 and 0.1 or less.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The piezoelectric ceramic of the present invention contains (Bi _1-a Na _a ) _b TiO ₃ as _a main component and Bi _c FeO ₃ as a subcomponent. The main component (Bi _1-a N
The molar ratio of a _a) a minor component to _b TiO ₃ Bi _c FeO ₃ is greater than 0 0.1. If not included Bi _c FeO _3, it can not increase the Curie point. On the other hand, if the content of B _c FeO ₃ is too high, the insulation resistance is reduced, and the polarization treatment becomes impossible. The preferred range of this molar ratio is 0.1.
001 to 0.07.

In the main component (Bi _1-a Na _a ) _b TiO ₃ , a = 0.4 to 0.6, b = 0.95 to 1.05, and preferably a = 0.45 to 0.55, b = 0.98 to 1.02. If a is too small, satisfactory piezoelectric properties cannot be obtained,
If a is too large, the Curie point decreases, which is not practically preferable. Also, if b is too small or too large, a single perovskite structure will not be formed.

In Bi _c FeO ₃ as a subcomponent, c = 0.95 to 1.05, preferably c = 0.98 to 1.02. If c is too small or too large, a single perovskite structure will not be formed.

The perovskite crystal contained in the piezoelectric ceramic of the present invention is of the (Bi _1/2 Na _1/2 ) TiO ₃ type. The piezoelectric ceramic of the present invention is preferably substantially composed of this crystal, but may not be completely homogeneous and may contain, for example, a different phase. In this piezoelectric ceramic, the sub-component constituting element is considered to replace a part of the main component constituting element, but a part may be present at the crystal grain boundary.

The molar ratio of oxygen in the above-described composite oxides as the main component and the subcomponent may vary depending on the valence of the metal element, oxygen deficiency, and the like.

The piezoelectric ceramic of the present invention may contain Ba, Ca or the like as an impurity or a trace additive, and the total content thereof is converted into oxides such as BaO and CaO. It is preferably at most 0.5% by weight. If the content of these elements is too large, the Curie point decreases. It is most preferable that Pb is not contained in the piezoelectric ceramics of the present invention, and it is possible to actually reduce the Pb content to below the detection limit. Does not substantially cause the problem described above.

The Curie point of the piezoelectric ceramic of the present invention can be set to 350 ° C. or higher, and it is easy to set the Curie point to 400 ° C. or higher by controlling the addition amount of the subcomponent. Also, may improve the piezoelectric properties by addition and piezoelectric properties of _{(Bi 1-a Na a)} b TiO 3, is compared with the piezoelectric properties upon addition of Bi _c FeO ₃ thereto, Bi _c FeO ₃ Further, even when the piezoelectric characteristics are deteriorated, the deterioration amount is small.

Next, an example of a method for producing the piezoelectric ceramic of the present invention will be described.

First, as a starting material, an oxide or
Compounds that can be converted to oxides by firing, for example, carbonates, hydroxides, oxalates, and powders of nitrates are prepared,
These are wet-mixed by a ball mill or the like.

Next, at about 800 to 1000 ° C., 1 to 3
Calcination is performed for about an hour, the obtained calcination product is converted into a slurry, and wet crushed using a ball mill or the like.

After the wet pulverization, the powder of the calcined product is dried, and a small amount of water (about 4 to 8% by weight) is added to the dried product,
Press molding is performed under a pressure of about 4000 kgf / cm ² to obtain a molded body. At this time, a binder such as polyvinyl alcohol may be added.

Next, the formed body is fired to obtain a piezoelectric ceramic. The firing temperature is preferably 900 to 1350 ° C.
And the firing time is preferably about 1 to 5 hours. The baking may be performed in the air, or may be performed in an atmosphere having a lower or higher oxygen partial pressure than in the air, or in a pure oxygen atmosphere.

[0021]

EXAMPLE A piezoelectric ceramic sample shown in Table 1 was produced by the following procedure.

As starting materials, Bi ₂ O ₃ , Na ₂ CO ₃ ,
Each powder of TiO ₂ and Fe ₂ O ₃ was converted to a final composition of x (BiFeO ₃ )-(1-x) [(Bi _1/2 Na _1/2 )
TiO ₃ ] and wet-mixed in an acetone solvent by a ball mill using zirconia balls for 10 hours.
For each sample, the molar ratio of the subcomponent to the main component x
Is shown in Table 1.

Next, the mixture was sufficiently dried, pressed, and calcined at 800 ° C. for 2 hours. The obtained calcined product was pulverized by a ball mill, dried, and granulated by adding a binder (polyvinyl alcohol). The obtained granulated powder was formed into a disk having a diameter of 20 mm and a thickness of about 1.5 mm by applying a load of 2000 kgf / cm ² using a uniaxial press forming machine.
The obtained molded body was subjected to a heat treatment at 500 ° C. for 3 hours to volatilize the binder, and then fired at 1150 ° C. for 4 hours. The rate of temperature rise during firing was 100 ° C./minute. After polishing the obtained sintered body to a thickness of about 1.0 mm, silver paste was baked on both surfaces at 550 ° C. to form electrodes.

Next, an LCR meter (YHP4275)
And a computer (HP9825B), the Curie point and the dielectric loss (tan δ) at room temperature and 1 MHz were automatically measured. Table 1 shows the results.

The remanent polarization (Pr) was determined from the DE hysteresis loop at room temperature and 50 Hz. Table 1 shows the results.

The above sintered body was polished to a thickness of about 1.0 mm, and then cut out to a width of 1 mm and a length of 5 mm to obtain a piece.
After baking at 50 ° C. to form electrodes, a polarization treatment was performed by applying an electric field of 7 to 12 kV / mm in a silicone oil bath at 50 ° C. for 20 minutes to obtain a piezoelectric property measurement sample. For these samples, an impedance analyzer (YHP4194A) and a computer (HP981
6S), the longitudinal electromechanical coupling coefficient (k ₃₃ ) was determined. Table 1 shows the results. Incidentally, k ₃₃ was calculated by the following equation.

[0027]

(Equation 1)

[0028]

[Table 1]

From Table 1, it can be seen that the Curie point is increased by adding BiFeO ₃ as a subcomponent to (Bi _1/2 Na _1/2 ) TiO ₃ . In addition, it is understood that the remanent polarization is improved by the addition of the subcomponent. Further, adverse effect on the dielectric loss and k ₃₃ by the sub-component added is not substantially observed, it can be seen that they are improved by the addition amount.

As a result of analysis by X-ray diffraction, it was found that all the samples shown in Table 1 were a single phase of (Bi _1/2 Na _1/2 ) TiO ₃ type crystal.

[0031]

According to the present invention, a piezoelectric ceramic containing no lead and having a high Curie point is realized.

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Claims (1)

[Claims] (1) As a main component, (Bi _1-a Na _a ) _b TiO ₃
(A = 0.4 to 0.6, b = 0.95 to 1.05)
Bi _c FeO ₃ (c = 0.95 to 1.05) as an accessory component
, Each containing a perovskite crystal, (B
i _1-a Na a) _b molar ratio of Bi _c FeO ₃ for TiO ₃ is greater than 0 0.1 or less is a piezoelectric ceramic.