JP5435732B2 - Piezoelectric ceramic composition - Google Patents

Description

  The present invention relates to a piezoelectric ceramic composition, and more particularly to a lead-free piezoelectric ceramic composition.

Conventionally composed of two components as the piezoelectric ceramic composition _PZT (PbTiO 3 -PbZrO ₃₎ ceramics and _PCM consists of three components _{(PbTiO 3 -PbZrO 3 -Pb (Mg} 0.5 Nb 0.5) TiO ₃ ) ceramics have been mainly used. The reason for this is that, of course, the above materials exhibit large piezoelectricity, but the following can also be mentioned. Piezoelectric materials have a wide variety of applications such as sensors, actuators, filters, etc., and various characteristics are required for each application. In the above materials, the characteristics can be met by adjusting the proportion of each component amount. This is because it can be appropriately operated.

  However, the main components of these materials are all composed mainly of lead, and in terms of the raw material ratio, 60 mass% or more is contained as lead oxide. Lead oxide has high volatility even at low temperatures, and enormous costs must be invested to install the equipment in order to take environmental measures such as calcination and sintering and measures against elution from industrial waste. Therefore, there is a demand for a lead-free material that exhibits large piezoelectric characteristics.

The required value of the Curie point at which piezoelectricity disappears or the phase transformation point is determined by the operating temperature, and when used as a drive element for an actuator, ultrasonic motor, etc., the piezoelectric disappearance temperature is generally 150 ° C. The piezoelectric ceramic composition described above is practical in many cases. On the other hand, the piezoelectric constant d ₃₃ is a characteristic representing the amount of displacement with respect to the electric field strength. Generally, the larger the piezoelectric constant d ₃₃ , the better. When applied to an actuator, an ultrasonic motor, etc., it is desirable that at least d ₃₃ = 200 pm / V or more.

In recent years, as disclosed in Non-Patent Document 1, by producing barium titanate represented by the chemical formula BaTiO ₃ by a special sintering method, the piezoelectric constant d ₃₃ which is a characteristic representing the displacement required for the actuator is 350 pC. / N (= 350 pm / V) is reported to be improved.

Patent Document 1 discloses rhombohedral perovskite structure compounds such as (Na _0.5 Bi _0.5 ) TiO ₃ and tetragonal crystals such as BaTiO ₃ and (K _0.5 Bi _0.5 ) TiO _3. A piezoelectric ceramic that can improve the piezoelectric characteristics including a perovskite structure compound and an orthorhombic perovskite structure compound such as NaNbO ₃ , KNbO ₃ , and CaTiO ₃ is described.

JP 2002-220280 A

Japan Journal of Applied Physics Vol. 45, no. 1,2006, ppL30

Since BaTiO ₃ described in Non-Patent Document 1 has a Curie point of 120 ° C., there is a problem that the usage environment is limited and the temperature change of the piezoelectric characteristics is large. A special sintering method is not suitable for mass production in terms of cost. Further, in the series of examples described in Patent Document 1, as the orthorhombic perovskite structure compound, only NaNbO ₃ is described, and there is no description about the Curie point and the phase transformation point. Further, there is no description regarding the piezoelectric constant, and further improvement in characteristics is desired.

  That is, an object of the present invention is to provide a piezoelectric ceramic composition having a high piezoelectric constant, without using lead, the temperature at which the piezoelectric characteristics disappear is not a practical problem.

In order to solve the above problems, the present invention provides a piezoelectric ceramic comprising three kinds of perovskite structure compounds of (Na _0.5 Bi _0.5 ) TiO ₃ , BaTiO ₃ , and (Na _0.5 K _0.5 ) NbO _3. As a result of investigating the composition of the composition, it has been found that a piezoelectric ceramic composition having a Curie point at which piezoelectric characteristics disappear and a phase transformation temperature point are values that do not cause a practical problem and a high piezoelectric constant can be obtained. . The Curie point and phase transformation point values required for a piezoelectric ceramic composition are usually determined by the operating temperature of the product using it. Here, when used as a driving element such as an actuator or an ultrasonic motor, which is the main application of the piezoelectric ceramic composition, it is generally necessary that the Curie point and the phase transformation temperature point be 120 ° C. or higher. It is. If a piezoelectric ceramic composition having a Curie point of 150 ° C. or higher can be provided in consideration of a margin of 30 ° C., there is no practical problem in many cases. Therefore, the value of the Curie point and the phase transformation temperature point being 150 ° C. or higher is a condition to be satisfied by the piezoelectric ceramic composition for solving the above-mentioned problems.

That is, the piezoelectric ceramic composition of the present invention is represented by the formula x (Bi _0.5 Na _0.5 ) TiO ₃ —yBaTiO ₃ —z (Na _0.5 K _0.5 ) NbO ₃ , and x, y, z Are in the ranges of 0.910 ≦ x ≦ 0.960, 0.035 ≦ y ≦ 0.085, 0.002 ≦ z ≦ 0.007, respectively, and x + y + z = 1.

  Further, the ratio [(Bi mole number + Na mole number + Ba mole number + K mole number) / (Ti mole number + Nb mole number)] of the total mole number of Bi, Na, Ba, K to the total mole number of Ti, Nb is 0. It is preferable that it is .98 or more and 1.01 or less.

According to the present invention, the formula x (Bi _0.5 Na _0.5 ) TiO ₃ —yBaTiO ₃ —z (Na _0.5 K _0.5 ) NbO ₃ is used. 910 ≦ x ≦ 0.960, 0.035 ≦ y ≦ 0.085, 0.002 ≦ z ≦ 0.007, x + y + z = 1, Curie point, phase transformation point Td even in a normal firing method is a high enough temperature that no problem in normal use, it is possible to obtain a piezoelectric ceramic composition of excellent lead-free piezoelectric constant d _33.

  Hereinafter, embodiments of the piezoelectric ceramic composition of the present invention will be described. The piezoelectric ceramic composition in the present invention has a composition range represented by the following formula.

x (Bi _0.5 Na _0.5 ) TiO ₃ —yBaTiO ₃ —z (Na _0.5 K _0.5 ) NbO ₃ , where x, y and z are 0.910 ≦ x ≦ 0.960, respectively. 0.035 ≦ y ≦ 0.085, 0.002 ≦ z ≦ 0.007, and x + y + z = 1.

In the piezoelectric ceramic composition of the previous formula, the main component is (Bi _0.5 Na _0.5 ) TiO ₃ , and BaTiO ₃ and (Na _0.5 K _0.5 ) NbO ₃ are subcomponents. (Bi _0.5 Na _0.5 ) TiO ₃ is known to have a relatively high Curie point, and BaTiO ₃ is a compound that exhibits an excellent value in piezoelectric properties although the Curie point is not high. Further, (Na _0.5 K _0.5 ) NbO ₃ alone does not exhibit particularly excellent characteristics in terms of Curie point and piezoelectric characteristics compared to the other two kinds of compounds in terms of Curie point and piezoelectric characteristics. However, the main component (Bi _0.5 Na _0.5 ) TiO ₃ is a rhombohedral system and the minor component BaTiO ₃ is a tetragonal perovskite structure compound, whereas (Na _0.5 K _0.5 ). NbO ₃ is an orthorhombic perovskite structure compound. The piezoelectric ceramic composition of the present invention has many boundary points between regions where the crystal structure is different in the structure can be in contact with each other, which is effective in improving the piezoelectric properties d _33.

The piezoelectric ceramic composition of the present invention can use BaCO ₃ , TiO ₂ , Na ₂ CO ₃ , Bi ₂ O ₃ , K ₂ CO ₃ , and Nb ₂ O ₅ with a purity of 99.9% or more as a raw material. These are mixed at a predetermined ratio, and then mixed for 20 hours by a ball mill. The obtained powdery mixture is calcined by holding at 800 to 1100 ° C. for 1 hour, and then pulverized for 20 hours. It is granulated using polyvinyl alcohol as a binder, and pressure-molded into a disk shape having a diameter of 20 mm and a thickness of 1 mm, for example, at a pressure of 1 ton / cm ² . Firing is performed at a temperature of 1100 to 1400 ° C. for 2 hours. Silver electrodes are provided on both surfaces of the sintered body, and a DC voltage of 4 kV / mm is applied between the electrodes in 100 ° C. silicon oil to polarize in the thickness direction.

As an index for evaluating the piezoelectric characteristics, an index indicating the displacement in the polarization axis direction when a DC electric field is applied in the same direction as the polarization axis, considering the use of a piezoelectric ceramic composition such as a drive element such as an actuator or an ultrasonic motor. is, it is appropriate to focus on the value of the piezoelectric constant d _33. The piezoelectric constant d ₃₃ is a quantity representing the magnitude of the piezoelectric displacement per unit length with respect to the applied DC electric field, and the direction of the polarization axis at this time is when various piezoelectric devices are produced using the piezoelectric ceramic composition. In many cases, the displacement is taken out. Therefore, the value of the piezoelectric constant d ₃₃ is an index representing the piezoelectric properties of this orientation is an important indicator when considering the characteristics of the products produced using the piezoelectric ceramic composition. In the evaluation of the product of the present invention, not the Curie point but the phase transformation point at which the phase changed to a pseudo cubic phase was measured. This is because the phase transformation point exists at a temperature lower than the Curie point in the product of the present invention. Based on the above, in the present invention, as an evaluation method of the produced piezoelectric ceramic composition, two indices, that is, the value of the phase transformation point and the value of the piezoelectric constant d ₃₃ are evaluated.

The values that the piezoelectric ceramic composition of the present invention should satisfy for these indices are a phase transformation point of 150 ° C. and a piezoelectric constant d ₃₃ of 200 pC / N (= 200 pm / V). The reason why the value to be satisfied of the phase transformation point is 150 ° C. is for the reason described above. On the other hand, the value that the piezoelectric constant d ₃₃ should satisfy is 200 pC / N because the value of the piezoelectric constant d ₃₃ of the single BaTiO ₃ was 200 pC / N as a result of measurement.

Example 1
The piezoelectric ceramic composition of Example 1 of the present invention uses BaCO ₃ , TiO ₂ , Na ₂ CO ₃ , Bi ₂ O ₃ , K ₂ CO ₃ , and Nb ₂ O ₅ with a purity of 99.9% or more as a raw material. It was. These were mixed so that it might become a predetermined ratio. Specifically, the composition of the final piezoelectric ceramic composition is 0.910 (Bi _0.5 Na _0.5 ) TiO ₃ −0.085 BaTiO ₃ −0.005 (Na _0.5 K _0.5 ) NbO. _The raw materials were blended stoichiometrically to be ₃ .

These mixtures were then mixed for 20 hours by a ball mill. The obtained powdery mixture was calcined by holding at 900 ° C. for 1 hour, and then pulverized by a ball mill for 20 hours. The mixture was granulated using polyvinyl alcohol as a binder, and pressed into a disk shape having a diameter of 20 mm and a thickness of 1 mm at a pressure of 1 ton / cm ² . The disc-shaped shaped body was fired by holding at a temperature of 1200 ° C. for 2 hours. Silver electrodes were provided on both sides of the sintered body obtained in this manner, and a DC voltage of 4 kV / mm was applied between the electrodes in silicon oil at 100 ° C. and polarized in the thickness direction to obtain Example 1. In addition, the number of samples of the piezoelectric ceramic composition produced by this method is five.

  Here, the calcining and firing temperature at the time of producing the piezoelectric ceramic composition are optimum values obtained by experiments by changing the temperature raising conditions. The fact that the obtained sintered body is a three-phase compound has been confirmed by analysis using an X-ray diffractometer.

The measurement of the phase transformation point of the phase change in the pseudo-cubic crystal phase for each sample of Example 1 and the piezoelectric constant d ₃₃ was carried out. The reason why the above-mentioned phase transformation point was measured instead of the Curie point is that this phase transformation point exists at a temperature lower than the Curie point in the product of the present invention. The phase transformation point was determined from the profile of the temperature characteristic of the capacitance measured with an LCR meter. The piezoelectric constant d ₃₃ representing the piezoelectric displacement of the unit length with respect to the electric field was calculated by measuring the length change in the thickness direction when an electric field of 1 kV / mm was applied with a laser displacement meter. The measured values thus obtained are shown in Table 1 as Example 1. In addition, the value of Example 1 shown in Table 1 is an average value of five pieces.

(Examples 2-7, Comparative Examples 1-7)
The piezoelectric ceramic compositions of Examples 2 to 7 and Comparative Examples 1 to 7 were produced in the same manner as in Example 1 except that the mixing ratio of the raw materials was changed. However, the calcination and firing temperatures were different from those in Example 1, and optimum values were obtained by experiments at the respective composition ratios. Specifically, the calcination temperature is an optimum value in the range of 800 to 1100 ° C., and the firing temperature is in the range of 1100 to 1400 ° C. In addition, the number of samples of the produced piezoelectric ceramic composition is five as in the case of Example 1 in each Example and Comparative Example.

In each of these Examples and Comparative Examples, the phase transformation point and the piezoelectric constant d ₃₃ were measured in the same manner as in Example 1. The measurement results are shown in Table 1. Here, Comparative Example 1 is a measured value of BaTiO ₃ . The phase transformation point is higher than 150 ° C. in any sample except for BaTiO ₃ . In addition, d ₃₃ is larger than 200 pm / V in the examples (in the ranges of 0.910 ≦ x ≦ 0.960, 0.035 ≦ y ≦ 0.085, 0.002 ≦ z ≦ 0.007), and the maximum 230 pm / V shown in Example 2 was obtained. In the comparative example (outside the range of 0.910 ≦ x ≦ 0.960, 0.035 ≦ y ≦ 0.085, 0.002 ≦ z ≦ 0.007), d ₃₃ decreases and results below 200 pm / V. became. When z in Comparative Example 7 increased, d ₃₃ decreased extremely because the crystal system approached a cubic crystal having no piezoelectric characteristics, and the piezoelectric characteristics deteriorated.

As a result, the product of the present invention had a phase transformation point higher than 150 ° C. and d ₃₃ higher than 200 pm / V. As described above, it is represented by the formula x (Bi _0.5 Na _0.5 ) TiO ₃ —yBaTiO ₃ —z (Na _0.5 K _0.5 ) NbO ₃ , and x, y, and z are each 0.910. By providing a piezoelectric ceramic composition in the range of ≦ x ≦ 0.960, 0.035 ≦ y ≦ 0.085, 0.002 ≦ z ≦ 0.007, and x + y + z = 1, with some having a high Curie point or phase transformation point and the piezoelectric constant d _33, the actuator, in which the effect of allowing the application to the ultrasonic motor applications can be expected.

Claims (2)

It is represented by the formula x (Bi _0.5 Na _0.5 ) TiO ₃ —yBaTiO ₃ —z (Na _0.5 K _0.5 ) NbO ₃ , and x, y, and z are 0.910 ≦ x ≦ 0. The piezoelectric ceramic composition is in the range of 960, 0.035 ≦ y ≦ 0.085, 0.002 ≦ z ≦ 0.007, and x + y + z = 1.   The ratio of the total number of moles of Bi, Na, Ba and K to the total number of moles of Ti and Nb in the piezoelectric ceramic composition of claim 1 [(Bi mole number + Na mole number + Ba mole number + K mole number) / (Ti mole number). + Nb mole number)] is 0.98 or more and 1.01 or less.