JP2001261435A - Piezoelectric ceramics and production process of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain piezoelectric ceramics which consist of BNT (Bi0.5Na0.5TiO3) or contains BNT as a terminal component and stably show high sintered body density and/or a high degree of orientation of 100} planes, and also to provide a production process of the piezoelectric ceramics. SOLUTION: This piezoelectric ceramics consist essentially of a perovskite- type compound represented by the formula x (Bi0.5Na0.5TiO3)-($1-x) ABO3 (wherein: 0.1<=x<=1) and further contain Bi in an excess sufficient to provide a Bi content ratio by at least 0.1% higher than the stoichiometric Bi content ratio, wherein desirably, the respective constituent crystal grains of the piezoelectric ceramics have oriented pseudo-cubic 100} planes. The production process comprises: adding a Bi-containing raw material having an excess Bi content by at least 0.5% to another raw material which consists of a stoichiometric blend containing platelike-powdery Bi4Ti3O12 to obtain a raw material mixture; forming the raw material mixture into a green body so that the platelike powder grains are oriented; and thereafter, subjecting the green body to heat treatment at a prescribed temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a piezoelectric ceramic and a method for manufacturing the same, and more particularly, to a piezoelectric ceramic used for an acceleration sensor, an ultrasonic sensor, a piezoelectric transformer, a piezoelectric actuator, an ultrasonic motor, a piezoelectric phone, a resonator, and the like. The present invention relates to a piezoelectric ceramic suitable as a material and a method for manufacturing the same.

[0002]

2. Description of the Related Art Piezoelectric materials are materials having a piezoelectric effect, and are classified into single crystals, ceramics, thin films, polymers, and composites. Among these piezoelectric materials, in particular, piezoelectric ceramics have high performance,
Because the degree of freedom of shape is large and material design is relatively easy,
It is widely applied in the fields of electronics and mechatronics.

[0003] Piezoelectric ceramics are obtained by applying a direct current to a ferroelectric ceramic and subjecting it to a so-called polarization process in which the direction of the domain of the ferroelectric is aligned in a fixed direction. In order to align spontaneous polarization in a certain direction by polarization processing,
Since a perovskite-type crystal structure in which spontaneous polarization can be three-dimensionally is advantageous, most of practically used piezoelectric ceramics are perovskite-type ferroelectric ceramics.

[0004] Perovskite ferroelectric ceramics include, for example, Pb (Zr.Ti) O ₃ (hereinafter referred to as “PZT”), and PZT obtained by adding a lead-based composite perovskite to PZT as a third component. Ternary system with relaxor added, BaTiO ₃ , Bi _0.5 Na
_0.5 TiO ₃ (hereinafter referred to as “BNT”) and the like are known.

[0005] Among these, lead-based piezoelectric ceramics represented by PZT have higher piezoelectric properties than other piezoelectric ceramics, and account for most of the piezoelectric ceramics currently in practical use. ing. However, since lead-based piezoelectric ceramics have a high vapor pressure and contain lead oxide (PbO) that is harmful to the human body, there is a problem that the load on the environment is large. Therefore, a piezoelectric ceramic having low lead or lead-free and having the same piezoelectric characteristics as PZT is required.

On the other hand, BNT or solid solution ceramics containing BNT as an end component has relatively high piezoelectric characteristics among lead-free piezoelectric materials, and is one of the leading candidate materials for lead-free piezoelectric ceramics instead of PZT. Is considered one. Therefore, various proposals have conventionally been made regarding the composition, manufacturing method, and the like of BNT-based piezoelectric ceramics.

For example, Japanese Patent Publication No. Hei 4-60073 discloses a chemical formula x (Bi _0.5 Na _0.5 TiO ₃ )-(1
-X) MTiO ₃ (where M is Ba or Bi _0.5 K
_0.5 , 0.80 ≦ x ≦ 0.99) as a main component, and if necessary, MnO ₂ , Fe ₂ O ₃ ,
A piezoelectric ceramic to which at least one selected from Cr ₂ O ₃ , NiO and the like is added as an additive and a method for producing the same are disclosed. According to the publication, a piezoelectric ceramic having a thickness mode electromechanical coupling coefficient Kt larger than a spread mode electromechanical coupling coefficient Kp can be obtained without using lead.

Further, for example, Japanese Patent Application Laid-Open No. 10-139552
Japanese Patent Application Laid-Open Publication No. H10-157, discloses an oxide containing at least one element of Bi, Sr, and Ca, which has a crystal orientation degree of 10% or more by a Lotgering method and a method for producing the same. Proposed. Further, the publication discloses that a plate-like Bi ₄ Ti ₃ O ₁₂
Orient the powder and use this as a template material in-
A process for producing a crystallographically-oriented ceramic in which a perovskite-type compound, BNT, or a solid solution containing BNT as an end component is produced by a situ reaction is described.

[0009]

[Problems to be Solved by the Invention]
As disclosed in Japanese Patent Application Laid-Open Publication No. H10-209, BNT-based piezoelectric ceramics are piezoelectric materials that do not contain lead, which is a harmful substance, and have relatively high characteristics. However, a conventional manufacturing process (hereinafter, referred to as "conventional method") in which raw material powders composed of fine-grained simple compounds are mixed in a stoichiometric ratio and calcined, molded, and sintered is performed. BNT
The performance of piezoelectric ceramics is greatly inferior to that of lead-containing piezoelectric materials such as PZT. This is because the BNT-based piezoelectric ceramic obtained by the conventional method becomes a non-oriented polycrystalline body, and there is a large disturbance in the polarization axis even after the polarization treatment.

On the other hand, as disclosed in JP-A-10-139552, a target perovskite compound is produced by an in-situ reaction using a plate-like powder oriented in a molded body as a template material. By using a method (hereinafter referred to as "RTGG method"), a BNT-based piezoelectric ceramic in which a specific crystal plane is oriented in a specific direction can be obtained. Further, the obtained piezoelectric ceramics exhibit higher piezoelectric characteristics than non-oriented ceramics having the same composition. However, when manufacturing a BNT-based piezoelectric ceramic in which a specific crystal plane is oriented in a specific direction by using the RTGG method, a high sintered body density and / or a high degree of crystal orientation can be stably obtained depending on the composition. In some cases, large variations occurred in the characteristics.

The problem to be solved by the present invention is that BNT
Another object of the present invention is to provide a piezoelectric ceramic having a high density of sintered body and / or a high degree of crystal orientation stably obtained in a piezoelectric ceramic having the end component, and a method of manufacturing the same.

[0012]

Means for Solving the Problems In order to solve the above problems, a piezoelectric ceramic according to the present invention contains a perovskite compound represented by the following formula 1 as a main component, and is further included in the perovskite compound. Stoichiometric Bi
The gist is that at least a 0.1% excess of Bi is contained.

[0013]

Embedded image x (Bi _0.5 Na _0.5 TiO ₃ )-(1-
x) ABO ₃ (provided that 0.1 ≦ x ≦ 1)

In this case, it is desirable that the pseudo-cubic {100} plane of each crystal grain constituting the piezoelectric ceramic is oriented.

The piezoelectric ceramic according to the present invention contains a perovskite compound represented by the following chemical formula 1 as a main component and a small amount of Bi in excess, so that a liquid phase is generated during sintering. Element diffusion is promoted. Therefore, it is easily densified regardless of its composition, and a high sintered body density can be stably obtained. Also, by adding a small amount of Bi in excess, BN in which the pseudo cubic {100} plane is oriented with a high degree of orientation is obtained.
T-based piezoelectric ceramics are obtained with good reproducibility, and exhibit a larger piezoelectric constant and electromechanical coupling coefficient than non-oriented sintered bodies having the same composition.

[0016] The manufacturing method of the piezoelectric ceramic according to the present invention, a plate-shaped Bi ₄ Ti ₃ O _{1 2} powder, by reacting with the plate-like powder, a perovskite type compound represented by the chemical formula 1 Mixing a perovskite-forming raw material to be produced with a Bi-containing raw material containing at least 0.5% of Bi in excess of Bi in the stoichiometric ratio contained in the perovskite-type compound, and mixing the mixture obtained in the mixing step. The gist of the present invention is to include a forming step of forming the plate-like powder so as to be oriented, and a heat treatment step of heating the formed body obtained in the forming step.

The plate-like Bi ₄ Ti ₃ O ₁₂ powder has a lattice matching with a pseudocubic {100} plane of a perovskite-type compound represented by the following formula (1). Therefore, a mixture containing the plate-like powder and the perovskite-forming raw material is formed so that the developed surface of the plate-like powder is oriented, and then heat-treated at a predetermined temperature, the oriented crystal nuclei of the perovskite-type compound are formed on the developed surface of the plate-like powder. Are generated, and the whole bulk sample becomes an oriented sintered body by grain growth of the oriented crystal nuclei. At this time, if a small amount of Bi is excessively contained in the raw material, the diffusion of elements is promoted, so that a piezoelectric ceramic having a high sintered body density and a high degree of {100} plane orientation can be stably obtained. Can be

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below in detail. The piezoelectric ceramic according to the present invention contains BNT or a perovskite compound having BNT as an end component. Its composition is represented by the following formula.

[0019]

## STR2 ## x (Bi _0.5 Na _0.5 TiO ₃ )-(1-
x) ABO ₃ (provided that 0.1 ≦ x ≦ 1)

In the formula, ABO₃Is BNT and solid solution
To form a perovskite compound represented by the formula:
Anything can be used as long as
ABO₃Specifically, Bi_0.5K_0.5T
iO₃, BaTiO₃, PbTiO₃, SrTiO₃,
CaTiO₃, NaNbO₃, KNbO₃, YMn
O ₃, CaMnO₃, LiNbO₃, LiTaO₃Etc.
Perovskite-type substances and LiNbO₃Type material, YMn
O₃A preferable example is a mold material. Also, A
BO₃Is a compound of two or more selected from these compounds
It may be a combination of compounds or a solid solution.

In the formula, the molar fraction of BNT is expressed as
The value of x may be in the range of 0.1 ≦ x ≦ 1. This
Here, the value of x was set to 0.1 or more because of a plate-like shape described later.
Bi ₄Ti₃O₁₂A place to use powder as a template
If the value of x is less than 0.1,
The amount of plate-like powder added as a plate is reduced,
Because a piezoelectric ceramic with a high degree of orientation cannot be obtained
It is.

Further, the piezoelectric ceramic according to the present invention comprises:
At least 0.1% excess of B in the stoichiometric ratio of Bi contained in the BNT-based perovskite compound represented by the formula (2) (hereinafter referred to as “BNT-based compound”).
i is included. The amount of Bi excessively contained in the piezoelectric ceramic (hereinafter referred to as “excess Bi”
Content ". Is less than 0.1%, it is not preferable because a piezoelectric ceramic having a high sintered body density and a high degree of orientation cannot be stably obtained.

Even if the excess Bi content is increased, the sinterability does not decrease, and a high-density piezoelectric ceramic can be stably obtained. However, when a specific crystal plane is oriented using the RTGG method, excess Bi
When the content is excessive, the degree of orientation is reduced, and as a result, piezoelectric characteristics such as a piezoelectric constant and an electromechanical coupling coefficient are reduced. Therefore, in order to stably obtain a piezoelectric ceramic having a high density and a high degree of orientation, the excess Bi content must be 3%.
The following is desirable.

The excess Bi contained in the piezoelectric ceramic is considered to be present at the grain boundary in the form of an oxide, but the details are unknown. In short, it is only necessary that the piezoelectric ceramic contains at least 0.1% of Bi in excess of the stoichiometric ratio Bi obtained from the formula (2). Further, “excess Bi content” refers to a numerical value represented by the following equation (1).

[0025]

## EQU1 ## Excess Bi content = (B _x −B ₀ ) _× 100 / B
₀ (%)

However, B₀Is a stoichiometric BNT compound
BNT-based compounds determined based on Ti contained in materials
The number of moles of all Bi contained therein,_xIs a piezoelectric ceramic
Piezoelectric ceramic based on the Ti in the mix
This is the number of moles of all Bi contained in the mix. In addition, above
This value is the value of x (Bi_0.5Na
_0.5TiO₃)-(1-x) ABO₃From
ABO₃Is determined for the composition excluding the amount of

In the piezoelectric ceramic according to the present invention, in order to obtain high piezoelectric properties, it is desirable that the pseudo-cubic {100} plane of each crystal grain constituting the polycrystal is oriented. Here, “pseudo cubic {HKL}” generally means that a perovskite-type compound has a structure distorted from a cubic system such as tetragonal system, orthorhombic system, and trigonal system. It means that it is regarded as a crystal and the Miller index is indicated. The degree of orientation of each crystal grain is specifically determined by Lotgering (Lot) expressed by the following equation (2).
The degree of crystal orientation Q (HKL) by the gering method.

[0028]

(Equation 2)

However, in the equation (2), ΔI (hkl)
And Σ′I (HKL) are the sum of the X-ray diffraction intensities from all crystal planes (hkl) measured for the crystallographically-oriented ceramic and the crystallographically equivalent specific crystal plane (HKL), respectively. It is the sum of X-ray diffraction intensities. On the other hand, ΣI ₀ (hkl) and Σ′I ₀ (HKL) have the same composition as the crystallographically-oriented ceramic, respectively, and have X from all the crystal planes (hkl) measured for the non-oriented polycrystalline ceramic. The sum of the X-ray diffraction intensities and the sum of the X-ray diffraction intensities from a crystallographically equivalent specific crystal plane (HKL).

Therefore, in the equation (2), when each crystal grain is non-oriented, the degree of crystal orientation Q (HKL) is 0%, and the {HKL} planes of all the crystal grains are oriented in one direction. If there is, it becomes 100%.

In the piezoelectric ceramic according to the present invention,
In order to obtain high piezoelectric characteristics, at least the pseudo-cubic {100} plane crystal orientation degree Q (100)
Is preferably not zero. In order to obtain a piezoelectric ceramic having a small hysteresis of electric field-strain behavior or a piezoelectric ceramic exhibiting a high piezoelectric constant, in many cases,
The larger the degree of crystal orientation Q (100) of the pseudo cubic {100} plane, the better.

Next, the operation of the piezoelectric ceramic according to the present invention will be described. It is known that the density of a sintered body of a piezoelectric ceramic generally affects electrical characteristics such as a dielectric constant and a piezoelectric constant, and the higher the density of a sintered body, the better the piezoelectric properties. Piezoelectric ceramics have crystal anisotropy in their piezoelectric properties, and exhibit high piezoelectric properties as compared to non-oriented sintered bodies when a specific crystal plane of each crystal grain constituting the polycrystal is oriented. It is known.

However, some BNT compounds include
Although a sintered body having a high density can be obtained by the conventional method, RTG
When attempting to produce a sintered body in which a specific crystal plane is oriented using the G method, there is a composition in which a high-density sintered body cannot be stably obtained. In such a composition, the degree of orientation varies, and in some cases, a high degree of orientation cannot be stably obtained. Examination of a material having such an unstable sintered body density or degree of orientation revealed that the composition did not produce a liquid phase during sintering, or that the composition was unlikely to be produced.

On the other hand, in the piezoelectric ceramics according to the present invention, Bi is added to the BNT compound in excess of the stoichiometric ratio.
High sintered body density can be obtained stably. This is because if an excessive amount of Bi is added to a material system having an unstable sintered body density,
It is considered that a liquid phase is likely to be generated during sintering, which promotes the diffusion of elements and facilitates densification.

In the case where each crystal grain is oriented by the RTGG method, if an excessive amount of Bi is added more than the stoichiometric ratio, the high degree of orientation is stable even in a material system in which the degree of orientation tends to be unstable. Is obtained. Further, as a result, the electromechanical coupling coefficient, the piezoelectric constant, and the pyroelectric constant are higher than those of a non-oriented sintered body having the same composition. Therefore, it is suitable as a piezoelectric material used for the piezoelectric ceramics according to the present invention, a sensor having high sensitivity, a sonar having high characteristics, and the like.

Further, a small amount of ABO _{3 is} added to BNT or BNT.
Is a rhombohedral crystal and has a spontaneous polarization axis in the <111> direction. B having such a crystal structure
When the NT-based compound is oriented in the {100} plane and polarized in the <100> direction, although the reason is not clear, the electromechanical coupling coefficient increases and the dielectric loss shows a low value. Therefore, among the piezoelectric ceramics according to the present invention, those exhibiting such an oriented structure are particularly suitable as a piezoelectric material used for a piezoelectric actuator having a high energy efficiency and a small loss, or a sensor having a small noise. is there.

Next, a method for manufacturing a piezoelectric ceramic according to the present invention will be described. The method for manufacturing a piezoelectric ceramic according to the present invention includes a mixing step, a forming step, and a heat treatment step.

In the mixing step, plate-like Bi ₄ Ti ₃ O
₁₂ (hereinafter, referred to as “BIT”) powder, a perovskite-forming raw material that reacts with the plate-like BIT powder to generate a BNT-based compound, and at least 0.1% from Bi of the stoichiometric ratio contained in the BNT-based compound. B containing 5% excess Bi
This is a step of mixing with the i-containing raw material.

Here, the plate-like BIT powder serves as a template when synthesizing a BNT compound. The reason for using BIT as the template material is that BIT is a kind of bismuth layered perovskite-type compound, which facilitates the synthesis of plate-like powder, and that the plate-like BIT powder develops (the surface with the largest area). )) And good lattice matching between the {100} plane of the BNT compound. Na _{0 .} A plate-like powder of ₅ Bi _4.5 Ti ₄ O ₁₅ can also be used as a template, but a BIT powder with a simpler structure is preferred.

Further, in order to obtain a piezoelectric ceramic having a high degree of orientation, the plate-like BIT powder needs to have a shape that can be easily oriented in one direction during molding. For that purpose, the average aspect ratio of the plate-like BIT powder is desirably 3 or more. If the average aspect ratio is less than 3, it is difficult to orient the plate-like BIT powder in one direction during molding, which is not preferable. Plate BIT
The average aspect ratio of the powder is more preferably 5 or more.

The plate BIT having such a shape is used.
Powders can be easily synthesized in the presence of a liquid phase. Specifically, a method of heating a raw material having a BIT composition together with flux (flux method), a method of heating fine BIT powder together with an aqueous alkali solution in an autoclave (hydrothermal synthesis method), and a method of precipitating from a solution (precipitation) Method) etc., but the plate-like BI synthesized by any method
Even T powder can be used.

The perovskite forming raw material may be any as long as it reacts with the plate-like BIT powder in the heat treatment step described below to become a BNT-based compound. Therefore, the composition of the perovskite forming raw material to be used is determined according to the blending amount of the plate-like BIT powder and the composition of the BNT compound to be produced.

Specific examples of the perovskite producing raw material include BNT, Bi _0.5 K _0.5 TiO ₃ , and BaTi
O ₃ , PbTiO ₃ , SrTiO ₃ , CaTiO ₃ , N
Ceramic powders such as aNbO ₃ and KNbO ₃ , Bi
Oxide materials such as ₂ O ₃ , PbO, TiO ₂ , Nb ₂ O ₅ , Na ₂ CO ₃ , K ₂ CO ₃ , BaCO ₃ , CaCO
₃ , a carbonate raw material such as SrCO ₃ is mentioned as a preferred example. In addition, a precursor of an oxide such as a hydroxide, an organic acid salt, or an alkoxide can also be used.

The mixing ratio between the plate-like BIT powder and the perovskite forming raw material can be arbitrarily determined according to the required characteristics of the piezoelectric ceramic to be produced. In order to obtain a piezoelectric ceramic having a high degree of orientation, the amount of the plate-like BIT powder is 5% in terms of B site atoms of a BNT compound having a perovskite structure.
The above is desirable. Even if the compounding amount of the plate-like BIT powder is less than 5% in terms of the B site atoms of the perovskite structure, a piezoelectric ceramic having a high sintered body density can be obtained, but the degree of orientation is reduced and the piezoelectric characteristics are reduced. It is not preferable because it lowers.

The Bi-containing raw material may be any as long as it can supply Bi in excess of the stoichiometric ratio with respect to the raw material capable of producing a stoichiometric BNT compound. Specific examples of suitable Bi-containing raw materials include Bi-containing oxides, carbonates, hydroxides, organic acid salts, and alkoxides.

The amount of Bi excessively mixed in the raw material (hereinafter referred to as “excess Bi compounding amount”) is 0.1%.
It is desirable that it be 5% or more. Excess Bi content is 0.
If it is less than 5%, depending on the composition of the BNT compound,
It is not preferable because a high sintered body density and / or a high degree of orientation may not be stably obtained.

Even if the amount of excess Bi is increased, the sinterability does not decrease, and a high-density piezoelectric ceramic can be manufactured stably. However, in the case where a specific crystal plane is oriented using the RTGG method, if the amount of excess Bi is excessive, the degree of orientation is reduced, and as a result, piezoelectric characteristics such as a piezoelectric constant and an electromechanical coupling coefficient are reduced. . Therefore, in order to stably produce a piezoelectric ceramic having a high density and a high degree of orientation, the amount of excess Bi is desirably 10% or less.

A part of Bi added excessively in the raw material scatters outside the sample during the heat treatment. Therefore, excess Bi
The difference between the blending amount and the excess Bi content represents the amount of Bi evaporated in the heat treatment process. The “excess Bi content” refers to a numerical value represented by the following equation (3).

[0049]

## EQU3 ## Excess Bi compounding amount = (B ′ _x −B ₀ ) _× 100 /
B ₀ (%)

Here, B ₀ is the total number of moles of Bi contained in the BNT compound determined based on Ti contained in the stoichiometric BNT compound, and B ′ _x is It is the total number of moles of Bi contained in the raw material determined based on the contained Ti. Note that the above value is x (Bi
_0.5 Na _0.5 TiO ₃₎ - from (1-x) ABO _3, it is determined for the composition excluding the amount of the advance ABO _3.

The mixing of the plate-like BIT powder, the perovskite forming raw material and the Bi-containing raw material may be performed in a dry manner, or may be performed in a wet manner by adding an appropriate solvent such as water or alcohol. At this time, a binder and / or a plasticizer may be added as necessary.

The molding step is a step of molding the mixture obtained in the mixing step so that the plate-like BIT powder is oriented.
The molding method is not particularly limited as long as it is a method capable of orienting the pseudo-tetragonal crystal display {001} plane, which is the development surface of the plate-like BIT powder, in a specific direction. Specifically, a uniaxial pressure molding method, a tape molding method, an extrusion molding method, a rolling method and the like are mentioned as preferable examples.

Further, the plate-like B obtained by these methods
In order to increase the thickness of the molded body in which the IT powder is oriented (hereinafter, referred to as “oriented molded body”) or to increase the degree of orientation, the oriented molded body is further subjected to processing such as lamination pressing, pressing, and rolling ( Hereinafter, this is referred to as “alignment treatment”). In this case, any one type of orientation treatment may be performed on the oriented molded body, or two or more types of orientation treatment may be performed. Also, for the oriented molded body,
One type of alignment treatment may be repeated a plurality of times, or two or more types of alignment treatments may be repeated a plurality of times.

The heat treatment step is a step of synthesizing a BNT-based compound and simultaneously sintering the generated BNT-based compound by heating the oriented formed body obtained in the forming step. As the heat treatment temperature, an optimum temperature may be selected according to the composition of the piezoelectric ceramic to be produced.

In the heat treatment step, the oriented molded product is directly
A one-step heat treatment of heating to the sintering temperature may be used, but in order to produce a piezoelectric ceramic having a high density and a high degree of orientation, it is desirable to perform the heat treatment in two steps.

In the case of performing the two-stage heat treatment, the first-stage heat treatment is performed to form the BNT-based compound on the surface of the plate-shaped BIT powder and at least near the surface of the plate-shaped BIT powder. Therefore, it is desirable that the first heat treatment temperature is higher than the temperature at which the synthesis reaction of the BNT-based compound starts and lower than the temperature at which the densification greatly proceeds. Specifically, the temperature of the first heat treatment is 100
0 ° C. or lower is preferred. More preferably, the temperature is 800 ° C. or lower. At this time, if the raw material contains a plasticizer or a binder, the degreasing is performed at the same time by burning and removing these.

When the degreasing is performed, the degree of orientation of the plate-like BIT powder in the oriented green body may decrease. Also,
When the BNT-based compound is synthesized from the plate-like BIT powder and the perovskite-forming raw material, the oriented molded body may swell. In order to suppress such a decrease in the degree of orientation, or a decrease in density due to swelling of the oriented molded article,
After performing the first-stage heat treatment and before performing the second-stage heat treatment, it is preferable to further perform hydrostatic pressure (CIP) treatment on the oriented formed body.

The second-stage heat treatment is performed to advance the densification by sintering and simultaneously grow grains of the oriented BNT-based compound. Thereby, a piezoelectric ceramic having a large degree of orientation of the pseudo cubic {100} plane can be manufactured. The temperature of the second heat treatment is slightly different depending on the composition of the BNT-based compound, but is usually preferably about 1050 ° C to 1200 ° C.

The second heat treatment may be performed in the air, but it is preferable to perform sintering in an oxygen atmosphere in order to obtain a high-density sintered body. This is because when sintering generally proceeds and isolated pores are formed inside the sintered body, sintering is hindered by the atmospheric gas remaining inside the pores,
This is because, when sintering is performed in an oxygen atmosphere, oxygen remaining in the pores is easily discharged to the outside through the grain boundaries, and does not hinder sintering.

After the heat treatment step, if necessary, the obtained sintered body is cut into a predetermined shape, the surface parallel to the orientation direction is polished, and electrodes are formed on the polished surface, so that various devices can be manufactured. The piezoelectric element used can be manufactured. When a tape-formed sample such as a doctor blade is wound around a cylinder and sintered, a pipe-shaped piezoelectric ceramic with a pseudo-cubic {100} plane oriented perpendicular to the radial direction must be produced. Can be. If the pipe-shaped sintered body is cut perpendicularly to the axis, an annular piezoelectric ceramic with a pseudo-cubic {100} plane oriented perpendicular to the radial direction is obtained, and is used as a piezoelectric element that is polarized in the radial direction. it can. Such a piezoelectric ceramic can also be manufactured by extrusion molding.

Next, the operation of the method for manufacturing a piezoelectric ceramic according to the present invention will be described. Perovskite-type compounds generally have an extremely small anisotropy in the crystal lattice.
It is extremely difficult to produce a polycrystal in which specific crystal planes are oriented in a specific direction by a normal sintering process.

On the other hand, in the present invention, the plate-like BI
Since the T powder is used as a starting material, if the molding is performed using a molding method in which a force acts on the plate BIT powder from one direction, an oriented molded body in which the developed surface of the plate BIT powder is oriented can be obtained. Can be easily obtained. Further, when the obtained oriented molded article is further subjected to an orientation treatment, the degree of orientation of the plate-like BIT powder in the oriented molded article can be further improved.

When the thus obtained oriented green body is heated, the oriented c-plane of the plate-shaped BIT powder becomes a pseudo-cubic {100} plane of the BNT-based compound.
An oriented crystal nucleus of the NT compound is generated, and the oriented crystal nucleus grows to make the whole bulk sample an oriented sintered body. By such a method, it is possible to obtain an oriented bulk ceramic having a thickness of millimeter order with good reproducibility.

In the raw material, an excess of B
When i is blended, a liquid phase is generated during sintering, and the diffusion of elements is promoted. Therefore, even in a system that is originally difficult to be densified, densification proceeds relatively easily, and a piezoelectric ceramic having a high sintered body density can be stably obtained. In addition, a high degree of orientation can be obtained stably, and as a result, a piezoelectric ceramic exhibiting high piezoelectric characteristics can be obtained with good reproducibility.

[0065]

Example 1 A plate-like BIT powder (average particle size: about 5 μm, thickness: 0.5 μm or less) synthesized by a flux method was used as a template to produce a piezoelectric ceramic by an RTGG method. That is, as shown in the chemical formula 3, the amount of the plate-like BIT powder is 20% in terms of the amount of B-site atoms (Ti), and the plate-like BIT powder and Bi are mixed so that the final composition is BNT. ₂ O ₃ , NaCO ₃ and T
iO ₂ was blended and used as a base material.

[0066]

Embedded image (1/15) Bi ₄ Ti ₃ O ₁₂ + (7/60) Bi ₂ O ₃
+ (1/4) Na ₂ CO ₃ + (4/5) TiO ₂ → Bi _0.5 Na
_0.5 TiO ₃ + (1/4) CO ₂ ↑

Next, excess Bi was added as Bi ₂ O ₃ with respect to the base material to prepare four kinds of raw material lots having different amounts of excess Bi. In addition, the excess Bi compounding amount is
0.5%, 2.0%, 8.2% and 14.0, respectively.
%.

Next, each raw material lot containing excess Bi is
Each is about 20 in a mixed solvent of ethanol and toluene.
After mixing for 1 hour, a binder (polyvinyl butyral) and a plasticizer (dibutyl phthalate) were added, and the mixture was further mixed for 1 hour, and then formed into a tape having a thickness of about 100 μm by a doctor blade device. Next, after stacking 20 of the obtained tapes and pressing them at 80 ° C., they were further subjected to a rolling treatment to produce a plate-shaped oriented molded body having a thickness of about 2 mm. The obtained oriented molded body was subjected to a first-stage heat treatment (calcination) at 700 ° C., and then subjected to a 300 MPa CIP treatment. Further, this is placed in an oxygen atmosphere at 1150 ° C. for 10 hours or 12 hours.
The second stage heat treatment (sintering) was performed under the conditions of 00 ° C. × 10 hours.

Comparative Example 1 The base material prepared in Example 1
That is, a piezoelectric ceramic having a final composition of BNT was produced according to the same procedure as in Example 1 except that a raw material lot having a stoichiometric composition without excessive addition of Bi was used.

(Comparative Example 2) B without using template
An NT sintered body was produced as follows. That is, B
Powder of i ₂ O ₃ , Na ₂ CO ₃ and TiO ₂
After weighing to a ratio of i: Na: Ti = 1: 1: 2 and mixing them, a heat treatment at 850 ° C. × 2 hours was performed to synthesize a stoichiometric BNT powder. Next, the synthesized B
After the NT powder was pulverized with a ball mill, it was dried, and a compact was produced by uniaxial pressure molding and hydrostatic molding. This compact was sintered in oxygen at 1100 ° C. for 10 hours to obtain a non-oriented sintered body according to a conventional method.

(Sintering Property and Degree of Orientation) For each of the BNT sintered bodies obtained in Example 1 and Comparative Examples 1 and 2, the density of the sintered bodies was measured. For each of the BNT sintered bodies obtained in Example 1 and Comparative Example 1, the surface of the sample (the surface parallel to the tape surface) was ground and removed, and then the Lotgering {100}.
The degree of orientation was measured. In Comparative Example 1, since a large variation occurred in the sintered body density and the degree of orientation, three most intermediate values were taken from among the sintered body densities and the degree of orientation measured for the prepared sample, and the average was taken. The values were taken as characteristic values.

FIG. 1 shows the density of the sintered body of the BNT sintered body (sintering temperature: 1200 ° C.) and the
Indicates 0 ° orientation. The BNT sintered body (Comparative Example 2) manufactured under the conditions of 1100 ° C. × 10 hours using the conventional method is a non-oriented sintered body, but has a density of 5.99 g / cm.
³ (relative density 99%).

On the other hand, when a BNT sintered body containing no excess Bi was produced by the RTGG method (Comparative Example 1), the relative density was reduced to about 90%. Some of the prepared samples have a high degree of {100} plane orientation of 0.8, and
Although a dense sample was included, it was not obtained stably, and many samples with a low degree of orientation were included.

On the other hand, when producing a BNT sintered body by the RTGG method, the excess Bi
% And a sintering temperature of 1200 ° C., the relative density is 9%.
8%, the average degree of orientation improved to 0.65. Excess B
Assuming that the i content is 2%, the relative density reaches the theoretical density,
The average degree of orientation reached 0.78. Moreover, these values could be obtained stably.

Even when the excess Bi content was further increased, the sinterability did not decrease, and the sintered body density exceeded the theoretical density of BNT. On the other hand, when the amount of excess Bi was 8.2% and 14%, the average degree of orientation was reduced to 0.51 and 0.43, respectively, but higher than Comparative Example 1 containing no excess Bi. The average degree of orientation was obtained stably.

The generated phase of each sintered body obtained in Example 1 was identified using an X-ray diffraction method. As a result, the sintered body of BNT having an excess Bi content of 0.5 to 8.2% was a perovskite single phase, but the excess Bi content was 14%.
% Of the BNT sintered body is BI
It was found that the T phase was contained.

FIG. 2A shows an X-ray diffraction pattern measured on a ground surface parallel to the tape surface of a BNT sintered body (sintering temperature: 1200 ° C.) having an excess Bi content of 2%. FIG. 2B shows an X-ray diffraction pattern measured for a sample having a low degree of orientation among the BNT sintered bodies (sintering temperature: 1200 ° C.) obtained in Comparative Example 1. As shown in FIG. 2, the BNT sintered body having an excess Bi content of 2% has a relative diffraction peak intensity from the (100) plane and the (200) plane in pseudo cubic display as compared with Comparative Example 1 having a low degree of orientation. It can be seen that it shows a high and strong {100} plane orientation.

(Electrical Characteristics) Next, Example 1 and Comparative Example
The following procedure was followed for the BNT sintered body obtained in Step 2.
And electrical characteristics were evaluated. That is, the obtained B
From an NT sintered compact, a disk-shaped
And a silver paste (Shoei Chemical H451)
0, 700 ° C x 10 mim)
Polarization treatment at 4kV / mmx30min
Was. Next, the piezoelectric characteristics (Kp, d₃₁, G ₃₁, Qm)
Is measured by the resonance anti-resonance method, and the dielectric properties (relative permittivity εr,
Dielectric loss tan δ) is measured under the conditions of 1 kHz and 1 V
did.

The excess B produced by the RTGG method
Regarding the BNT sintered body not containing i (Comparative Example 1), the sample product having an intermediate value had a low density and a low degree of orientation, and the polarization treatment could not be performed due to leakage. The evaluation of the electrical characteristics was not performed.

FIG. 3 shows the effect of the excess Bi content on the electromechanical coupling coefficient Kp. FIG. 4 shows the piezoelectric g _31.
The effect of excess Bi content on the constant is shown. Note that FIG.
In FIG. 4 and FIG. 4, the value of the stoichiometric BNT sintered body (the excess Bi compounding amount is 0%) is a value measured for the BNT sintered body obtained by the conventional method (Comparative Example 2).

The piezoelectric properties of the {100} plane oriented BNT sintered body produced by RTGG method, which contains Bi in an excessive amount, are all the same as the non-oriented BNT sintered body produced by the conventional method (Comparative Example 2). High values were also obtained. In particular, when 2% excess Bi amount, the electromechanical coupling factor Kp and piezoelectric _{g 31} constant becomes maximum, as compared with Comparative Example 2, 72% in the electromechanical coupling factor Kp, 83% higher in the piezoelectric _{g 31} constant Value. Although not shown, the piezoelectric d constant showed almost the same tendency as that of FIG.

The relative dielectric constant εr of the {100} -oriented BNT sintered body produced by the RTGG method (Example 1) and the non-oriented BNT sintered body produced by the conventional method (Comparative Example 2) were Comparing the dielectric loss tan δ, the relative dielectric constant ε
r increased with increasing excess Bi loading. The dielectric loss tan δ was 20 to 30% lower than that of the non-oriented BNT sintered body according to the conventional method (Comparative Example 2).

(Composition Analysis) Next, Example 1 and Comparative Example 1
In order to confirm the amount of Bi remaining in the sintered body obtained in the above, the sintered body was pulverized and subjected to elemental analysis (B
i, Na, Ti). Next, the excess Bi content was determined based on the obtained elemental analysis results. Table 1 shows the results.

[0084]

[Table 1]

In Table 1, “BNT + B0” corresponds to the raw material lot (base material) having an excess Bi content of 0.0% used in Comparative Example 1, and “BNT + B0.5”, “BN
T + B2 "," BNT + B8.2 "and" BNT + B1 "
"4" corresponds to the raw material lots in which the excess Bi content used in Example 1 is 0.5%, 2.0%, 8.2% and 14.0%, respectively.

In the sample containing excessive Bi, Bi remained excessively in the sintered body. However, excess B
The i content was smaller than the excess Bi content, and it was found that part of Bi was lost outside the system during the heat treatment process. In addition, it was found that, even when the amount of excess Bi was the same, the heat treatment at a high temperature reduced the excess Bi content in the sintered body.

Example 2 In the same manner as in Example 1, the compounding amount of the plate-like BIT powder synthesized by the flux method (average particle size: about 5 μm, thickness: 0.5 μm or less) was converted to B-site atom (Ti) equivalent. And a raw material lot having a final composition represented by the following empirical formula was prepared. (1) 0.95 BNT + 0.05 BaTiO ₃ (2) 0.90 BNT + 0.05 Bi _0.5 K _0.5 TiO ₃
+ 0.05BaTiO ₃ (3) 0.70BNT + 0.30BaTiO ₃ (4) 0.85BNT + 0.15KNbO ₃

Next, these raw material lots and the total Bi required for these raw material lots in further stoichiometric ratios
A raw material lot in which 2% of the amount was excessively mixed as Bi ₂ O ₃ was prepared, and a sintered body was produced under the same conditions as in Example 1. Further, the obtained sintered body was evaluated under the same conditions as in Example 1 for the density, orientation degree, and electrical characteristics of the sintered body.

Regardless of the final composition of the piezoelectric ceramic, B
The sintered body obtained from the raw material lot containing i in excess is
The pseudo cubic {100} orientation degree was higher than that of the sintered body obtained from the raw material lot having the stoichiometric composition. As a result, a high electromechanical coupling coefficient Kp and a high piezoelectric constant were exhibited.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

For example, in the above-described embodiment, the normal pressure sintering method is used as the sintering method. However, after the normal pressure sintering, HIP processing may be performed to further densify the sintered body. Further, sintering may be performed by using a hot press method or a hot forging method instead of the normal pressure sintering method.

In the above embodiment, tapes of the same composition produced by the doctor blade method are laminated to produce an oriented molded article. Tapes of different compositions are laminated to form an oriented molded article. May be sintered. In addition, when the extrusion method is used as the molding method, it is possible to obtain an oriented molded body in which the {100} planes are not parallel to each other, but the {100} planes are parallel to the extrusion axis. You can also.

Further, the present invention is characterized in that a liquid phase is generated during sintering by adding Bi in excess of the stoichiometric ratio, thereby improving the sinterability of the BNT compound. However, this method is applicable not only to a system in which a plate-shaped BIT powder is used as a template to produce an oriented sintered body by the RTGG method, but also to other systems.

For example, a material other than BIT (for example, one of bismuth layered perovskite type compounds) which can easily synthesize a plate-like powder and whose developed surface has lattice matching with the pseudocubic {100} plane of the BNT-based compound The seed BaBi
₄ Ti ₄ O ₁₅ ) as a template and R
The method according to the present invention can be similarly applied to a case where a BNT-based piezoelectric ceramic is manufactured by the TGG method or a case where a non-oriented BNT-based piezoelectric ceramic is manufactured by a conventional method.

[0095]

The piezoelectric ceramic according to the present invention has x
(Bi _0.5 Na _0.5 TiO ₃ )-(1-x) ABO
₃ (provided that a perovskite compound represented by 0.1 ≦ x ≦ 1) is a main component, and the stoichiometric ratio Bi contained in the perovskite compound is at least 0.1%.
Since Bi is contained in excess of%, there is an effect that a piezoelectric ceramic having a high sintered body density can be stably obtained regardless of its composition or manufacturing method. Also, by adding a small amount of Bi in excess, pseudo-cubic
There is an effect that a BNT-based piezoelectric ceramic in which the 0 ° plane is oriented at a high degree of orientation can be stably obtained.

[0096] The manufacturing method of the piezoelectric ceramic according to the present invention, a plate-shaped _Bi 4 _Ti 3 _{O 1 2} powder, by reacting with the plate-like _{powder, x (Bi 0.5 Na 0.5 TiO} 3 )-
(1-x) A perovskite-forming material for producing a perovskite-type compound represented by ABO ₃ (where 0.1 ≦ x ≦ 1), and a stoichiometric ratio Bi contained in the perovskite-type compound of at least 0. A step of mixing a Bi-containing raw material containing a 5% excess of Bi, a molding step of molding the mixture obtained in the mixing step so that the plate-like powder is oriented, and a molded article obtained in the molding step Is provided with a heat treatment step of heating the piezoelectric ceramics, so that piezoelectric ceramics having a high sintered body density and a high degree of {100} plane orientation can be stably obtained regardless of the composition.

[0097]

[Brief description of the drawings]

FIG. 1 is a view showing the effect of excess Bi content on the sintered body density and the degree of {100} orientation of a BNT sintered body produced by an RTGG method.

FIG. 2 (a) shows a BN produced by a RTGG method using a raw material lot having an excess Bi content of 2%.
FIG. 2B is an X-ray diffraction pattern of the T sintered body. FIG. 2B shows a low orientation BNT sintered body among the BNT sintered bodies produced by the RTGG method using the raw material lots having the stoichiometric composition. X
It is a line diffraction pattern.

FIG. 3 is a view showing the effect of the excess Bi content on the electromechanical coupling coefficient Kp of a BNT sintered body manufactured by the RTGG method.

FIG. 4 is a graph showing the effect of excess Bi content on the piezoelectric g constant of a BNT sintered body manufactured by the RTGG method.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Toshio Kimura 2-1-20 Shimoda-cho, Kohoku-ku, Yokohama-shi, Kanagawa F-term (reference) 4G031 AA01 AA04 AA05 AA06 AA08 AA11 AA14 AA15 AA19 AA32 AA35 BA10 CA02 GA02

Claims (3)

[Claims] 1. x (Bi _0.5 Na _0.5 TiO ₃ ) —
(1-x) A perovskite compound represented by ABO ₃ (where 0.1 ≦ x ≦ 1) as a main component, and at least 0.1% of Bi of a stoichiometric ratio contained in the perovskite compound. Piezoelectric ceramics containing excess Bi. 2. The piezoelectric ceramic according to claim 1, wherein the pseudo cubic {100} plane of each crystal grain constituting the piezoelectric ceramic is oriented. 3. A plate-like Bi ₄ Ti ₃ O ₁₂ powder reacts with the plate-like powder to form x (Bi _0.5 Na _0.5 Ti
O ₃ )-(1-x) ABO ₃ (provided that 0.1 ≦ x ≦ 1)
A perovskite-forming raw material for producing a perovskite-type compound represented by the formula: and a B in excess of at least 0.5% of the stoichiometric ratio Bi contained in the perovskite-type compound.
a step of mixing the Bi-containing raw material containing i, a forming step of forming the mixture obtained in the mixing step so that the plate-like powder is oriented, and a heat treatment of heating the formed body obtained in the forming step And a process for producing a piezoelectric ceramic.