JP4427723B2 - Piezoelectric composition - Google Patents

Description

  The present invention relates to a piezoelectric composition used for sensors, actuators, ultrasonic devices, and the like, and a method for producing the same.

  Piezoelectric materials that convert electrical energy into mechanical energy or conversely convert mechanical energy into electrical energy include various piezoelectric devices such as piezoelectric sensors, piezoelectric actuators, vibrators for ultrasonic cleaners, ultrasonic motors, and piezoelectric transformers. Is widely used.

Here, a piezoelectric ceramic material is generally used as the piezoelectric material, but most of the piezoelectric ceramic materials currently in practical use are tetragonal or rhombohedral PZT (PbZrO ₃ −) near room temperature. It is a ferroelectric having a perovskite structure such as PbTiO ₃ solid solution) or PT (PbTiO ₃ ). For these compositions, a third component such as Pb (Mg _1/3 Nb _2/3 ) O ₃ or Pb (Mn _1/3 Nb _2/3 ) O ₃ is substituted, or various subcomponents. By adding, it is possible to cope with a wide variety of required characteristics.

  However, these piezoelectric ceramic materials generally contain about 60% to 70% by weight of lead oxide (PbO), which is extremely volatile even at low temperatures. It is not preferable from the viewpoint of prevention. Specifically, if these lead-based piezoelectric ceramic materials are no longer needed after being put on the market as industrial products and disposed of in the environment, there is a risk that lead in the ceramics may be eluted by acid rain. There is concern about causing soil contamination.

As a piezoelectric material not containing lead at all, for example, BaTiO ₃ having a perovskite structure belonging to a tetragonal system is well known, but this is not practical because the Curie temperature is as low as 120 ° C. The Curie point is a phase transition point between a ferroelectric and a paraelectric, and is also a temperature at which the piezoelectric characteristics disappear.

As other lead-free piezoelectric ceramic materials, for example, bismuth layered compounds are known. Most of the bismuth layered compounds have a Curie point of 400 ° C. or higher (in the case of Na _0.5 LaBi _3.5 Ti ₄ O ₁₅ , the Curie point is 402 ° C.), and there is no problem like BaTiO ₃ having the perovskite structure. Since the electromechanical coupling coefficient is small, it is not suitable for an application such as an actuator that requires displacement. The electromechanical coupling coefficient represents the conversion efficiency between electric energy and mechanical energy, and is one of important characteristics in piezoelectric materials.

As a device having a relatively large electromechanical coupling coefficient, a niobic acid compound such as KNbO ₃ has been reported, and it has been proposed to use the single crystal for a surface acoustic wave substrate, a piezoelectric transducer, or the like (for example, (See Patent Document 1). This niobic acid compound has a high Curie point of 450 ° C., and has a very large electromechanical coupling coefficient as a single crystal.
JP-A-10-65488

  However, there is no example of using the niobic acid compound as a piezoelectric ceramic material, and there is no report on the piezoelectric characteristics of a piezoelectric ceramic material made of a niobic acid compound. This is because the niobic acid compound has deliquescent properties and, for example, if it is left in the air after sintering, it reacts with moisture and the ceramic material collapses, making it impossible to evaluate the characteristics.

  Therefore, it is difficult to use a niobic acid compound as a piezoelectric material despite the fact that it has excellent characteristics such as the Curie point and the electromechanical coupling coefficient as described above, and its use is limited. It is.

  The present invention has been proposed in view of such a conventional situation, and proposes a technique for eliminating the deliquescence of the niobic acid compound, thereby having a high Curie point and a large electromechanical coupling coefficient. And it aims at providing the piezoelectric composition excellent in practicality, and also aims at providing the manufacturing method.

  In order to achieve the above-mentioned object, the present inventors have made various studies over a long period of time. As a result, the inventors have found that the addition of Bi is extremely effective in eliminating the deliquescence of the niobic acid compound, and that the resistivity is also maintained at a high value by adding Mn together.

The present invention has been devised based on such knowledge, and comprises a niobic acid compound represented by the general formula KNbO ₃ as a main component, the niobic acid compound, and Bi and Mn. The content is 0.2% by weight or more and less than 1% by weight with respect to the niobic acid compound in terms of Bi ₂ O ₃ , and the content of Mn is 0.2% with respect to the niobic acid compound in terms of MnCO ₃ . It is characterized by being 2% by weight or more and 1.0% by weight or less.

The present invention is mainly composed of a niobic acid compound containing no lead (Pb). Niobic acid compounds such as KNbO ₃ are characterized by a high Curie point and a very large electromechanical coupling coefficient. However, since it has deliquescence, it is difficult to use it as a ceramic material, for example. Therefore, in the present invention, deliquescence is suppressed by adding Bi. Although the details are unknown for the reason, by adding Bi, the deliquescence property is surely suppressed, and the characteristics that the Curie point is high and the electromechanical coupling coefficient is very large are also maintained.

  Further, the present invention is characterized by further containing Mn in addition to the above configuration. In the piezoelectric composition, when the resistivity decreases, the probability of dielectric breakdown in the polarization process increases. When Mn is added to the niobic acid compound, the resistivity is maintained at a high level, and the dielectric breakdown in the polarization process is suppressed.

  According to the present invention, since Bi is added to the niobic acid compound, deliquescence can be eliminated, and for example, it can be used as a piezoelectric ceramic material. Ceramic materials (porcelain materials) can be manufactured much more easily and in a shorter time than single crystals. Therefore, the present invention is extremely useful in putting niobic acid compounds into practical use as piezoelectric materials. It is possible to realize a high-performance and general-purpose piezoelectric composition.

  In addition, in the piezoelectric composition of the present invention, by adding Mn, it is possible to increase the resistivity and to provide a piezoelectric composition that does not break down in the polarization process or the like. .

  Furthermore, the piezoelectric composition of the present invention is very useful from the viewpoint of biological and pollution prevention because it contains a niobic acid compound as a main component and contains substantially no Pb. That is, it is possible to provide a naturally-friendly piezoelectric composition that does not adversely affect the ecosystem and does not cause environmental destruction.

  Hereinafter, a piezoelectric composition to which the present invention is applied and a method for producing the same will be described in detail.

  The piezoelectric composition of the present invention is mainly composed of a niobic acid compound, and has the advantages as a piezoelectric material possessed by the niobic acid compound, such as a high Curie point and a very large electromechanical coupling coefficient. It is something to save.

The niobic acid compound is, for example, a compound represented by the general formula ANbO ₃ (wherein A is an alkali metal), and its single crystal is used for, for example, a surface acoustic wave substrate or an optical lens. The compound represented by the above general formula takes the form of a complex oxide of Nb and an A site element, where the A site element is an alkali metal element. Specifically, the alkali metal element is at least one selected from K, Na, and Li, but potassium (K) is preferable from the viewpoint of piezoelectric characteristics and the like. The niobic acid compound is KNbO ₃ .

In the general formula, a part of the Nb site may be substituted with Ta. In this case, the general formula is represented by ANb _(1-x) Ta _x O ₃ (where x is 0 <x <1). However, if the amount of substitution with Ta becomes too large, the niobic acid compound itself does not exhibit deliquescence, and thus is not an object of the present invention for the purpose of eliminating deliquescence. Further, since the piezoelectric characteristics are also deteriorated by the substitution with Ta, the substitution amount x of Ta is naturally limited. In view of these, it is preferable that the Nb site is not substituted with Ta.

In the present invention, Bi is added to the aforementioned niobic acid compound to eliminate deliquescence. Here, the addition amount (content) of Bi is preferably 0.2% by weight or more and less than 1% by weight with respect to the niobic acid compound in terms of Bi ₂ O ₃ . If the amount of Bi added is less than 0.2% by weight in terms of Bi ₂ O ₃ , deliquescence cannot be sufficiently eliminated and piezoelectric characteristics may not be evaluated. If the amount of Bi added exceeds 1% by weight in terms of Bi ₂ O ₃ , a pyrochlore phase is generated, so that a uniform composition may not be obtained. The formation of the pyrochlore phase causes deterioration of piezoelectric characteristics and is not preferable.

In the piezoelectric composition of the present invention, it is also effective to add Mn in addition to the Bi. By adding Mn, the resistivity can be increased and dielectric breakdown in the polarization process can be avoided. However, the content of Mn is preferably less than 1% by weight with respect to the niobic acid compound in terms of MnCO ₃ . If the amount of Mn added exceeds 2% by weight, it will not be densely sintered.

  The piezoelectric composition of the present invention is a lead-free piezoelectric composition that does not substantially contain lead from the viewpoint of environmental protection, etc., but here, “substantially does not contain lead” means an impurity level. This means that it does not contain lead exceeding the unacceptable amount, and it may be contained as long as it is in an impurity level. Lead may be contained in a trace amount as an inevitable impurity.

  Since the deliquescence is eliminated as described above, the piezoelectric composition of the present invention can be used as a piezoelectric ceramic composition (piezoelectric ceramic). If it can be used as a piezoelectric ceramic, it can be easily manufactured, and its application can be greatly expanded.

In the case of a piezoelectric ceramic, for example, a raw material compound serving as a niobium source and an alkali metal source and a raw material compound serving as a Bi source may be mixed and fired. At this time, for example, niobium oxide (Nb ₂ O ₅ ) is used as a raw material compound that becomes a niobium source. For example, potassium carbonate (K ₂ CO ₃ ) is used as a raw material compound serving as an alkali metal source, and bismuth oxide (Bi ₂ O ₃ ) is used as a compound serving as a Bi source.

When Mn is used in combination, a compound serving as a Mn source may be mixed in addition to the above raw materials and fired in the same manner. For example, manganese carbonate (MnCO ₃ ) is used as a raw material compound that becomes a Mn source.

  The piezoelectric ceramic manufactured as described above is presumed to be in the form of a complex oxide containing Nb, alkali metal (K), Bi, and further Mn, but it is not always necessary that all elements are in solid solution. Alternatively, a part may constitute another phase.

  As described above, the piezoelectric composition of the present invention is useful as a piezoelectric ceramic. Such a piezoelectric composition of the present invention can be used for various piezoelectric devices such as a piezoelectric sensor, a piezoelectric actuator, a vibrator for an ultrasonic cleaning machine, an ultrasonic motor, a piezoelectric transformer, etc., and has a very high Curie point. Since it has an electromechanical coupling coefficient, it exhibits excellent piezoelectric performance in practical use.

Hereinafter, specific examples of the present invention (Experiment 1 is a reference example) will be described based on experimental results.

(Experiment 1)
In this experiment, the effect of adding Bi to the niobate compound was examined. As starting materials, powder materials of potassium carbonate (K ₂ CO ₃ ), niobium oxide (Nb ₂ O ₅ ), and bismuth oxide (Bi ₂ O ₃ ) were used. About potassium carbonate (K ₂ CO ₃ ) and niobium oxide (Nb ₂ O ₅ ), they are mixed at a ratio that makes the stoichiometric composition of potassium niobate (KNbO ₃ ), and bismuth oxide (Bi ₂ O ₃ ) is mixed there. A predetermined amount was added. These starting materials were put into an organic solvent such as acetone and mixed with a Zr ball by a ball mill for 5 to 20 hours. After fully drying the mixed slurry, it was press-molded and calcined at 800 ° C to 900 ° C. These were again finely pulverized with a ball mill, dried, and granulated with an appropriate amount of PVA (polyvinyl alcohol) added as a binder. The granulated powder was molded under a load of 196 MPa using a uniaxial press molding machine so as to form a disk-shaped pellet shape having a diameter of about 20 mm and a thickness of about 3 mm. The molded sample was heat treated to remove the binder, and then fired at 1070 ° C. for 2 hours to obtain a porcelain sample.

Porcelain samples 1 to 6 were produced by changing the amount of bismuth oxide (Bi ₂ O ₃ ) as shown in Table 1. These porcelain samples were examined for deliquescence. The deliquescence was evaluated by placing the obtained porcelain sample in water and performing ultrasonic cleaning for 5 minutes. When the porcelain sample collapses due to the ultrasonic cleaning, it is deliquescent (×), and when it does not collapse, it is not deliquescent (◯). The results are shown in Table 1.

As shown in Table 1, deliquescence was observed in Sample 1 in which bismuth oxide (Bi ₂ O ₃ ) was not added to the starting material. Further, even in the sample in which bismuth oxide (Bi ₂ O ₃ ) is added to the starting material, the deliquescence is still not resolved in the sample 2 having a small amount of bismuth oxide (Bi ₂ O ₃ ) of 0.1 wt% . In contrast, Samples 3 to 6 to which 0.2% by weight or more of bismuth oxide (Bi ₂ O ₃ ) was added eliminated the deliquescence and did not collapse even when ultrasonic cleaning was performed in water. .

(Experiment 2)
In this experiment, the effect of adding Mn was examined. The starting material was the same as in Experiment 1 above, but manganese carbonate (MnCO ₃ ) was further added. The amount of bismuth oxide (Bi ₂ O ₃ ) added is 0.5% by weight. Porous samples 7 to 12 were prepared in the same manner as in Experiment 1 except that the amount of manganese carbonate (MnCO ₃ ) added was changed as shown in Table 2. With respect to these porcelain samples, insulation resistance (resistivity) and piezoelectric characteristics (electromechanical coupling coefficient k ₃₃ ) were measured. The measuring method is as follows.

(1) Measurement of insulation resistance An Ag paste was applied to both surfaces of a porcelain sample and baked at 550 ° C to obtain a test piece for measuring insulation resistance. The measurement was performed at room temperature, the insulation resistance was measured using a High Resistance Meter (HP4339B), and the resistivity was calculated.

(2) Measurement of piezoelectric characteristics A 2 mm × 2 mm × 4 mm prismatic sample was cut out from a porcelain sample to obtain a test piece. Ag paste was applied to both ends (4.7 mm direction) of the test piece, and the electrode was baked and formed at 550 ° C. Thereafter, an electric field of 5 kV / mm was applied for 10 minutes in a silicon oil bath at a temperature of 100 ° C. to 250 ° C. to perform polarization treatment. The polarized sample is measured by an impedance analyzer HP4294A the resonant frequency fr and the antiresonant frequency fa at room temperature, was calculated electromechanical coupling coefficient k ₃₃ according to the equation shown in the following equation (1).

  Table 2 shows the measurement results for the porcelain samples 7-12.

As apparent from Table 2, the porcelain sample 7 to which Mn was not added had a low resistivity and could not be polarized. In contrast, as in the porcelain sample 8 porcelain sample 10, the addition of Mn, the resistivity becomes large more than one order, it showed good values also electromechanical coupling coefficient k _33. However, as in the porcelain sample 12, when the amount of Mn added is too large, it was found that the ceramic sample was not densely sintered.

Claims (2)

The main component is a niobic acid compound represented by the general formula KNbO ₃ , and the niobic acid compound is composed of Bi and Mn.
The content of Bi is calculated as _Bi 2 _{O 3,} 0.2 wt% or more with respect niobate compound is less than 1 wt%,
The content of Mn is a MnCO ₃ basis, 0.2 wt% or more with respect niobate compound, the piezoelectric composition characterized in that 1.0 wt% or less.   The piezoelectric composition according to claim 1, wherein the piezoelectric composition is a piezoelectric ceramic composition.