JP2010042969A - Manufacturing method of piezoelectric ceramic, piezoelectric ceramic, and piezoelectric element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a barium titanate piezoelectric ceramic which has higher electromechanical coupling coefficient using a two-step resistance heating sintering method of high practicality, taking notice of barium titanate powder having nanosize, and to provide the manufacturing method and various kinds of piezoelectric elements using the ceramic at a low cost. <P>SOLUTION: The 50-200 nm barium titanate powder having the property of a piezoelectric substance is solidified to have a given shape, and is formed into a sintered body by two-step sintering temperature. A microcrystalline structure of the sintered body has subparticle structure, which is in 0.5-2 μm within crystal grain of a single body, which has an unclear crystal interface, and in which subparticle structures having different molecular structure are present, and in which the maximum grain size of the crystal grain is suppressed to be ≤10 μm. The sintered body has ≥98.5% density to the theoretical density. The piezoelectric elements are formed by the polarization treatment of the sintered body of the ceramic. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a piezoelectric ceramic obtained by sintering barium titanate powder, a piezoelectric vibrator, an element for ultrasonic flaw detection, an actuator, a method for manufacturing piezoelectric ceramics used in many sensors, piezoelectric ceramics, The present invention also relates to a piezoelectric element.

Conventionally, PZT (PbTiO ₃ —PbZrO ₃ ) component systems containing lead have been used as piezoelectric ceramics. PZT has a large piezoelectricity and a high relative dielectric constant, and can easily produce materials having various characteristics used for various applications such as sensors, actuators, and filters. However, while piezoelectric ceramics made of PZT have excellent characteristics, their constituent elements contain lead, so that harmful lead may be eluted from industrial waste of products containing PZT, causing environmental pollution. was there. With the recent increase in awareness of environmental problems, the production and use of products that can cause environmental pollution such as PZT have been suppressed as much as possible.

  Therefore, development of high-performance piezoelectric ceramics containing no harmful lead is desired as an environmental problem. As shown in Patent Document 1, the inventors of the present invention pay attention to nano-sized barium titanate powder, Using a highly practical resistance heating two-step sintering method, a barium titanate piezoelectric ceramic having a higher dielectric constant and electromechanical coupling coefficient and a method for producing the same were proposed.

The piezoelectric ceramic disclosed in Patent Document 1 is a two-stage sintering with a two-step sintering temperature obtained by solidifying a barium titanate powder of 50 nm to 200 nm having a piezoelectric property into a predetermined shape and electrically resistance heating. A dense sintered body having an average particle diameter of 1 μm to 2 μm and a maximum particle diameter of 5 μm or less, and having a theoretical density (theoretical density of barium titanate is 6.01 g / cm ³ ) of 98% or more is obtained. After firing, this sintered body is subjected to polarization treatment at a voltage of 1 kV / mm at room temperature to 80 ° C. for about 30 minutes.

  The nano-sized barium titanate powder is prepared by hydrothermal synthesis method, coprecipitation method, heterogeneous precipitation method, alkoxide method, oxalate method, citrate method, sol-gel method. And a solid phase reaction method, and the powder is sintered by the two-stage sintering method by electrical resistance heating.

In the two-stage sintering method, the temperature is raised to a first sintering temperature of 1230 ° C. to 1340 ° C. by electrical resistance heating and then lowered to a second sintering temperature of 1150 ° C. to 1200 ° C. for a certain time. Maintain and sinter.
JP 2008-150247 A

  The piezoelectric ceramic obtained by the manufacturing method disclosed in Patent Document 1 has performance close to that of conventional PZT, but the electromechanical coupling coefficient does not reach that of PZT.

  The present invention has been made in view of the above-described background art, and focuses on nano-sized barium titanate powder and has a higher electromechanical coupling coefficient using a highly practical resistance heating two-stage sintering method. An object of the present invention is to provide barium titanate piezoelectric ceramics, a manufacturing method thereof, and various piezoelectric elements using the piezoelectric ceramics at low cost.

  In the present invention, 50 to 200 nm barium titanate powder having a piezoelectric property is solidified into a predetermined shape, heated to a first sintering temperature of 1350 ° C. to 1450 ° C., and then a second sintering temperature of 1190 ° C. Sintered by lowering to ˜1250 ° C. and maintaining for a certain period of time to form a density of 98.5% or more of the theoretical density, and crystal structures with different molecular arrangement directions are integrated into individual crystal grain boundaries. This is a piezoelectric ceramic manufacturing method in which crystal grains having a formed crystal structure (referred to as a sub-particle structure in this invention) are formed, and this sintered body is polarized.

  The heating rate up to the first sintering temperature is 7 ° C. to 15 ° C. per minute, and the holding time at the first sintering temperature is 0.5 minute to 2 minutes. Further, the temperature is lowered from the first sintering temperature to the second sintering temperature at a rate of 20 ° C. to 40 ° C. for 1 minute, and the holding time at the second sintering temperature is 4 hours to 10 hours.

  The present invention is also a sintered body formed by solidifying 50 nm to 200 nm barium titanate powder having properties of a piezoelectric body into a predetermined shape at a two-step sintering temperature, and the fine crystal structure of the sintered body is piezoelectric sintering. Within the single crystal grain of the aggregate, there is a sub-particle structure with a different molecular arrangement direction although the crystal interface is not clear at 0.5 μm to 2 μm, and the crystal grain size is suppressed to 10 μm or less, and Piezoelectric ceramics formed by forming a sintered body having a density of 98.5% or more of the theoretical density and subjecting it to a polarization treatment.

The sintered body has a relative dielectric constant of 4000 or more and an electromechanical coupling coefficient kp of 0.44 to 0.50. Further, the sintered body, the piezoelectric constant d ₃₃ is 460~500pC / N.

  Further, the present invention is a piezoelectric element formed into a single plate or a laminated type, a rectangular shape, a disc shape, a dome shape, or a ring shape using the piezoelectric ceramic made of a sintered body of the barium titanate powder.

  Furthermore, the present invention is a piezoelectric vibrator using a piezoelectric ceramic made of a sintered body of the barium titanate powder, such as a vibration pickup used as a vibration detecting element or vibrator, an ultrasonic cleaner, or a buzzer.

  According to the present invention, there is provided a piezoelectric ceramic using a nano-sized barium titanate powder of 50 nm to 200 nm, a method for producing the same, and a piezoelectric vibrator using the same. A higher electromechanical coupling coefficient can be obtained as a piezoelectric element than the method or the microwave sintering method. In addition, there is also a dependency on the sintering temperature, and by selecting an appropriate temperature range, it is possible to provide a more effective high-density piezoelectric ceramic and its manufacturing method at low cost and without polluting the environment. it can.

Furthermore, by using a sintering method using nano-sized barium titanate powder and resistance heating two-stage sintering method, the sintering density of barium titanate ceramics also reaches 98.5% or more of the theoretical density, and the piezoelectric constant d ₃₃ is also A very large value was exhibited, and good characteristics as a piezoelectric element could be obtained.

The piezoelectric ceramic obtained by the present invention has higher performance than other lead-free piezoelectric ceramics and also those obtained by microwave sintering of nano-sized barium titanate powder. As shown in FIG. It has a performance comparable to ceramics. Accordingly, piezoelectric ceramics of the present invention is more suitable as various piezoelectric vibrator sought high d ₃₃ or higher d ₃₁ properties.

The present invention will be specifically described below. In the present invention, a high-performance piezoelectric ceramic can be obtained by using a resistance heating two-step sintering method using a barium titanate powder, which is a heating method using electric resistance and a sintering method using two-step temperatures. . For this purpose, a nano-sized ceramic powder of 50 nm to 200 nm is essential as a material having piezoelectric properties. By sintering nano-sized barium titanate powder to form piezoelectric ceramics, the crystal interface is not clear at 0.5 μm to 2 μm in a single crystal grain of the piezoelectric sintered body, but the molecular orientation direction A sub-particle structure having a different particle size, a maximum particle size of 10 μm, and a fine polarization domain structure due to the particle size effect of the sub-particle is formed, and a piezoelectric element having an extremely large electromechanical coupling coefficient kp and piezoelectric constant d ₃₃ is formed. Can be formed.

  This nano-sized barium titanate powder is obtained by a known hydrothermal synthesis method, coprecipitation method, partial coprecipitation method, alkoxide method, oxalate method, citrate method, sol-gel method, solid phase reaction method, etc. It is done. Particularly suitable for high-performance piezoelectric ceramics is a powder having a sharp and uniform particle size distribution. The particle size is preferably 50 nm to 200 nm. For example, the hydrothermal synthesis method is based on a method of synthesizing an inorganic compound or an organic compound using a high-temperature and high-pressure aqueous solution. By applying this method, nano-sized barium titanate powder can be produced.

  Next, a method for manufacturing a piezoelectric ceramic formed by sintering 50 nm to 200 nm nano-sized barium titanate powder will be described. Prior to firing, a molded body determined by the intended use, shape, etc. is prepared. Using an aqueous solution of polyvinyl alcohol as a binder, the mixture was uniformly mixed with barium titanate powder and then molded by molding. The molding pressure was 200 MPa, the dimensions of the molded body were 12 to 20 mm in diameter, and the thickness was 1 to 5 mm. Thereafter, the binder is removed at 600 ° C. for 2 hours, and then used for the main firing by the resistance heating two-stage sintering method. Even those molded by other molding methods such as extrusion molding and doctor blade molding can be sintered under optimum sintering conditions.

  Next, the resistance heating two-stage sintering method will be described. FIG. 2 shows a sintering schedule when barium titanate powder is sintered to produce piezoelectric ceramics. In the figure, the vertical axis represents the sintering temperature and the horizontal axis represents the sintering time. T1 is the first sintering temperature and T2 is the second sintering temperature. Although the temperature increase rate from room temperature to 1000 ° C. does not affect the performance of the piezoelectric ceramic, it was performed at 10 ° C. for 1 minute as an example. The heating rate from 1000 ° C. to the first sintering temperature of 1350 ° C. to 1450 ° C. is 7 ° C. to 15 ° C. per minute, for example, 10 ° C. per minute. The holding time at the first sintering temperature is 0.5 minutes to 2 minutes, for example, 1 minute, and the temperature decreasing rate from the first sintering temperature to the second sintering temperature of 1190 ° C. to 1250 ° C. is 1 minute. 20 ° C. to 40 ° C., for example, 30 ° C. for 1 minute. The holding time at the second sintering temperature is 4 to 10 hours, and the cooling rate from the second sintering temperature to room temperature is 4 ° C. to 5 ° C. for 1 minute.

  The holding time at the first sintering temperature is 1 minute, but this is the time required until the temperature in the sintering furnace becomes uniform by resistance heating due to the control program, and the furnace structure and the amount of sintering It may be extended by The holding time at the second sintering temperature depends on the temperature, and a holding time until it reaches 98.5% or more of the theoretical density is necessary.

  The piezoelectric ceramic according to the present invention is a sintered body formed by solidifying a barium titanate powder of 50 nm to 200 nm in a predetermined shape at a sintering temperature in the above-described stage. The piezoelectric ceramic has a fine crystal structure of 0.5 μm to 2 μm in a single crystal grain, and there are sub-particle structures with different molecular arrangement directions although the crystal interface is not clear. FIG. 3 is an electron micrograph of the surface of a piezoelectric ceramic surface according to an embodiment of the present invention etched for 30 seconds. As shown in FIG. 3, in a single crystal grain boundary, arrangements having different texture directions are observed, and the different individual arrangements have a sub-grain structure, and the grain boundary can be said to be a sub-grain boundary. FIG. 4 shows an enlarged electron micrograph showing the sub-grain boundary s in the sub-particle structure. It can be seen that the arrangement directions of the crystal structures of the sub-grains at the upper left and lower right of the figure are different at the sub-grain boundary s. The arrangement direction of each subparticle structure is as shown in the schematic diagrams on the upper left and lower right of the figure. The ceramic of this example constitutes a sintered body having a maximum crystal grain size of 10 μm or less and having a density of 98.5% or more of the theoretical density.

This sintered body has a relative dielectric constant of 4000 or more and an electromechanical coupling coefficient kp of 0.44 to 0.50. The piezoelectric constant d ₃₃ is 460 to 500 pC / N.

  The piezoelectric ceramic comprising the sintered body of barium titanate powder of the present invention can be used to form a piezoelectric element formed in a single plate or laminated type, rectangular shape, disc shape, dome shape, or ring shape. .

The piezoelectric ceramic according to the present invention is used as a vibration detecting element or a vibrator. In particular, as a lead-free piezoelectric vibrator using an excellent piezoelectric ceramic with high density, high dielectric constant, high d ₃₃ , and high d ₃₁ , for example, a high sensitivity vibrator pickup, a piezoelectric buzzer, an ultrasonic cleaner, a flaw detector It is expected as a very high performance vibrator. These vibrators can be manufactured by known means, and can be manufactured at low cost and without polluting the environment.

  Next, embodiments of the present invention will be described below. Here, the grain size of the crystal grains of the formed piezoelectric ceramic mainly depends on the first sintering temperature. FIG. 5 is an electron micrograph after holding the first sintering temperature at 1350 ° C. for 1 minute and then holding the second sintering temperature at 1190 ° C. for 4 hours.

  FIG. 6 is an electron micrograph after holding the first sintering temperature at 1410 ° C. for 1 minute and then holding the second sintering temperature at 1190 ° C. for 4 hours.

  FIG. 7 is an electron micrograph after holding the first sintering temperature at 1410 ° C. for 1 minute and then holding the second sintering temperature at 1250 ° C. for 4 hours.

  As can be seen from FIG. 5 to FIG. 7, in the single crystal grain boundary of the ceramic sintered body, an array having different texture directions is seen, and the different individual arrays have a sub-particle structure, and the grain boundary is a sub-grain boundary. It is a grain boundary. In particular, the irregularities appearing in the grain boundaries on the surface of the sintered body indicate the presence of the sub-particle structure, and the size of each concave or convex region indicates the size of the sub-particles in the grain boundary. .

Table 1 below shows the density of the sintered body when the first sintering temperature and the second sintering temperature are changed. Table 2 shows the electromechanical coupling coefficient of the sintered body when the first sintering temperature and the second sintering temperature are changed.

  The obtained sintered body was applied with a voltage of 1 kV / mm at 80 to 100 ° C., subjected to polarization treatment for 30 minutes, and then measured for dielectric and piezoelectric properties after 24 hours. However, as the electrode, a silver paint applied to a sample and baked at 600 ° C. for 30 minutes was used.

As can be seen from Tables 1 and 2, when the density of the sintered body is 5.93 g / cm ³ or more, the electromechanical coupling coefficient kp is 0.44 or more. For example, a sample having a first sintering temperature of 1350 ° C., a second sintering temperature of 1190 ° C., a holding time of 48.5 hours and a theoretical density of 98.5% reaches kp = 0.50. The constant d ₃₃ was 500 pC / N.

The figure which shows the relationship between the electrical coupling coefficient kp of various piezoelectric ceramics, and a dielectric constant. The sintering schedule figure of the resistance heating two-step sintering method of this invention. It is the electron micrograph of the surface after etching the piezoelectric ceramic surface of the Example of this invention for 30 seconds. It is the electron micrograph which expanded the photograph of FIG. 3 further. It is the electron micrograph after hold | maintaining at the 1st sintering temperature of 1350 degreeC by the manufacturing method of the piezoelectric ceramic of the Example of this invention for 1 minute, and hold | maintaining at the 2nd sintering temperature of 1190 degreeC for 4 hours. It is the electron micrograph after hold | maintaining for 4 hours at the 1st sintering temperature of 1190 degreeC after hold | maintaining at the 1st sintering temperature of 1410 degreeC by the manufacturing method of the piezoelectric ceramic of the Example of this invention. FIG. 4 is an electron micrograph after holding for 1 minute at a first sintering temperature of 1410 ° C. and holding for 4 hours at a second sintering temperature of 1250 ° C. by a method for manufacturing a piezoelectric ceramic according to an embodiment of the present invention.

Claims (7)

  A barium titanate powder of 50 nm to 200 nm having piezoelectric properties is solidified into a predetermined shape, heated to a first sintering temperature of 1350 ° C. to 1450 ° C., and then increased to a second sintering temperature of 1190 ° C. to 1250 ° C. Lowering and maintaining for a certain period of time, forming to a density of 98.5% or more of the theoretical density, forming crystal grains having sub-grain structures with different molecular arrangement directions in individual grain boundaries, A method for producing a piezoelectric ceramic, comprising subjecting the sintered body to polarization treatment.   The heating rate up to the first sintering temperature is 7 ° C. to 15 ° C. per minute, the holding time at the first sintering temperature is 0.5 minutes to 2 minutes, and from the first sintering temperature, The method for producing a piezoelectric ceramic according to claim 1, wherein the temperature is lowered to the second sintering temperature at a rate of 20 ° C to 40 ° C for 1 minute, and the holding time at the second sintering temperature is 4 hours to 10 hours.   A sintered body obtained by solidifying a barium titanate powder of 50 nm to 200 nm having the properties of a piezoelectric body into a predetermined shape and formed by a two-step sintering temperature. Within the crystal grains, there are sub-particle structures with different molecular arrangement directions, although the crystal interface is not clear at 0.5 μm to 2 μm, the maximum grain size of the crystal grains is suppressed to 10 μm or less, and the theoretical density is 98. A piezoelectric ceramic characterized in that a sintered body having a density of 5% or more is formed and subjected to polarization treatment.   The piezoelectric ceramic according to claim 3, wherein the sintered body has a relative dielectric constant of 4000 or more.   The piezoelectric ceramic according to claim 3, wherein the sintered body has an electromechanical coupling coefficient kp of 0.44 to 0.50. The sintered body according to claim 3, wherein the piezoelectric ceramic, characterized in that the piezoelectric constant d ₃₃ is 460~500pC / N. The piezoelectric element made of a piezoelectric ceramic according to claim 3, 4, 5, or 6, wherein the piezoelectric ceramic made of a sintered body of the barium titanate powder is used as a vibration detecting element or a vibrator.