JPH066489B2 - Method for producing perovskite-type lead titanate microcrystals - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a perovskite-type lead titanate microcrystal useful as a piezoelectric material, particularly as a raw material for high-density firing.

[Conventional technology]

In the field of dielectric porcelain, development of a new synthesis method of dielectric oxide fine particles as a raw material has been advanced due to miniaturization of electronic parts and diversification of applications.

For example, in a multilayer ceramic capacitor, it is necessary to reduce the thickness of the ceramic layer in order to increase the capacity and reduce the size and weight, and it is an important issue to make the dielectric oxide as a raw material into fine particles. Further, from the viewpoint of the withstand voltage of the capacitor, abnormal grain growth and generation of non-uniform particles are not preferable at the sintering stage, and development of a method for synthesizing uniform fine particles is urgently needed.

On the other hand, as the dielectric oxide that is the raw material of the dielectric porcelain,
Lead titanate, which has a number of excellent properties, is widely used. This lead titanate (PbTiO ₃ ) is generally mixed with lead oxide (PbO) and titanium oxide (TiO ₂ ), pulverized and mixed in a ball mill, and then 800-100
Pre-fire at 0 ° C, crush again until uniform,
It is synthesized by the solid-phase reaction method of performing main firing.

By the way, when synthesizing lead titanate fine particles by such a solid-phase reaction method, a ball mill is used, so that impurities are easily mixed, and evaporation of PbO becomes a serious problem.
That is, the higher the temperature during the calcination, the more the amount of PbO evaporated exponentially increases, which may change the composition of the obtained lead titanate fine particles. Therefore, in order to avoid this, it is necessary to take considerable measures during the heat treatment, such as firing in a PbO atmosphere. Alternatively, Pb
In order to suppress the evaporation of O, it is conceivable to lower the temperature of calcination and then carry out the main calcination. In this case,
A considerable amount of unreacted PbO remains at the end of the preliminary calcination, and this unreacted PbO may be vaporized at the stage of the main calcination, and it is necessary to control the atmosphere here as well. Therefore, in the lead titanate fine particles obtained by the solid-state reaction method utilizing the heat treatment as described above, A site defects in the perovskite type structure, such as Pb _1-8 TiO _3, are likely to occur. There is a high possibility that the stoichiometry will adversely affect the piezoelectric characteristics and pyroelectric characteristics.
Even if it is assumed that a material having a high stoichiometry is obtained by the high temperature heat treatment, the sinterability is deteriorated as long as the above-mentioned raw material preparation procedure is performed. This is because lead titanate has the highest crystal anisotropy among the perovskite type structures, that is, since it has a large square strain, the coefficient of thermal expansion in a specific direction is significantly different, and when the temperature is lowered from a high temperature, This is because cracks may occur. Further, in order to avoid this, it is possible to add various additives to improve the sinterability, but in this case, a problem in piezoelectric characteristics arises. That is, lead titanate takes advantage of the fact that the electromechanical coupling coefficient of longitudinal waves is larger than that of transverse waves, and is effectively used in parts that cannot be reached by other piezoelectric materials. It is fully conceivable that the characteristics will be deteriorated by adding. Furthermore, it is difficult to produce uniform fine particles by the above solid-phase reaction, and it is particularly impossible to produce fine particles having a particle size of 1 μm or less.

On the other hand, in order to put it into practical use as a raw material for transparent ceramics, additives for capacitors, raw materials for low temperature sintering, etc., non-uniformity of particles in the solid-phase reaction method, lack of activity, mixing of impurities, etc. Attempts have been made to improve the above and obtain uniform fine particles.

For example, Japanese Patent Publication No. 51-2080 discloses a wet synthesis method in which a salt of an A ion and a B ion in an ABO having a perovskite structure to be synthesized is reacted in an alkaline aqueous solution under boiling. However, in this method, the condition of [A ion] / [B ion] ≧ 1.8 is required, and since the ion concentration ratio of synthesis is not 1, composition variation is likely to occur. It is a hydrate. Therefore, to obtain crystalline particles, 30
A heat treatment of 0 to 400 ° C is required. Further, decantation is performed to remove excess PbCl ₂ that becomes an impurity during synthesis, but it is difficult to completely remove this PbCl ₂ .

Alternatively, the oxalate method and the oxalic acid ethanol method are known as other methods. In the former case, the solubility of the oxalate may differ depending on the type of the metal ion, or the pH range for precipitation may differ. It is difficult to obtain a uniform composition. Moreover, since an organic compound called oxalate is used, there are many problems in terms of manufacturing cost and productivity. Further, in the latter, although the uniformity of the composition is improved to some extent, problems remain in terms of manufacturing cost and productivity.

Further, a so-called metal alkoxide in which an organometallic compound represented by the general formula M (OR) _n is synthesized, and a composite alkoxide represented by the general formula M _I M _II (OR) _m is synthesized and then hydrolyzed Although a law has been proposed,
There are many problems in terms of manufacturing cost and productivity. Also,
The precipitate obtained is of high purity but is amorphous and
Since it is necessary to perform a heat treatment at about 00 ° C., there is a possibility that PbO may be vaporized as in the solid-phase reaction method.

[Problems to be solved by the invention]

As described above, it is difficult to synthesize the crystalline lead titanate fine particles from the liquid phase without heat treatment by the conventional synthesis method.
It was difficult to obtain lead titanate microcrystals with high uniformity and purity. Further, according to the conventional synthesis method, it is difficult to produce fine particles having a particle size of 1 μm or less, and it is difficult to supply lead titanate microcrystals for high-density firing.

Therefore, the present invention has been proposed in view of the actual situation of the technical field as described above, and is extremely fine, and it is possible to cope with diversification of applications such as raw materials such as transparent ceramics, and composition. It is an object of the present invention to provide a method for producing high-purity lead titanate microcrystals, which has high homogeneity.

[Means for solving problems]

The present inventors have developed a synthetic method capable of synthesizing perovskite-type lead titanate fine particles having high purity and a small particle size, and as a result of extensive research over a long period of time, the pH and the synthesis temperature were set to predetermined values. It is possible to synthesize perovskite-type lead titanate microcrystals, pyrochlore-type lead titanate microcrystals or acicular lead titanate microcrystals having a novel crystal phase by performing wet synthesis by setting to It was found that the obtained pyrochlore-type lead titanate microcrystals are extremely fine and easily undergo a phase change to a perovskite phase by heat treatment.

Soluble titanium compound or its hydrolysis product and lead compound in aqueous solution at pH 12.1 or higher at a temperature of 100 ° C
After reacting at 190 ° C. under the pH and temperature conditions within the region where the pyrochlore type lead titanate microcrystal PY is mainly produced in FIG.
It is characterized by heat treatment at 0 ° C. or higher.

In the present invention, to synthesize perovskite type lead titanate microcrystals, first, pyrochlore type lead titanate microcrystals are prepared by wet synthesis. To produce the pyrochlore type lead titanate microcrystals, for example, a soluble titanium compound such as titanium tetrachloride (TiCl ₄ ) or a hydrolysis product thereof and a hydrolysis product of a lead compound or a water-soluble salt thereof are used. Are mixed and reacted in an alkaline aqueous solution at a high temperature of 100 ° C. or higher, and the resulting precipitate is washed with water or warm water to completely remove alkali cations such as K ⁺ and Na ⁺ and anions such as Cl ^−. It may be removed, filtered and dried.

Here, the pH and reaction temperature during the reaction are important,
Depending on these pH and reaction temperature, lead titanate microcrystals in the pyrochlore phase (hereinafter referred to as PY phase), perovskite phase (hereinafter referred to as PE phase) or a new needle-like crystal phase (hereinafter referred to as PX phase) And lead) titanate microcrystals are produced. The present inventors repeated experiments and attempted to create a phase diagram of lead titanate microcrystals obtained by changing the pH and reaction temperature during the above reaction. The results are shown in Fig. 1. From this FIG. 1, P
The Y phase is stable from a low alkali high temperature range to a high alkali low temperature range, and the PE phase is stable only in a high alkali high temperature range.
It has been found that the PX phase only forms within a certain range.
In addition, in FIG. 1, the inside of the parentheses indicates that a small amount is generated as a by-product, and AM is amorphous.
Represents lead titanate in the state.

That is, pH 12.1 or higher, reaction temperature 100 to 190
C, preferably pH 13.0 or higher, reaction temperature 110-1
By setting it to 75 ° C., the PY phase is selectively generated. A reaction time of 1 hour or less is sufficient, but the longer the time, the better the crystallinity of the obtained PY phase. P obtained here
The Y-phase lead titanate microcrystals have extremely high stoichiometry, and their particle size is 0.2 μm or less, which is an order of magnitude smaller than that obtained by the conventional solid-phase reaction method.

In synthesizing the lead titanate microcrystals described above, in order to obtain a starting Ti compound or a hydrolysis product thereof, T
A salt such as iCl ₄ , Ti (SO ₄ ) ₂ is dissolved in water, or its aqueous solution is dissolved in KOH, NaOH, NH.
_It may be hydrolyzed with an alkaline aqueous solution such as ₄ OH or LiOH. However, when Ti (SO ₄ ) ₂ is used,
TiO ₂ · nH ₂ O hydrolyzed with these alkaline solutions
Create a (titanium oxide hydrate), by repeated decantation or filtration, may be removed ▲ SO ^2- ₄ ▼.

Further, as the lead compound, lead acetate Pb (CH ₃ COO) ₂
・ 3H ₂ O, lead nitrate Pb (NO ₃ ) ₂ , lead chloride PbCl _2, etc. can be used. However, when lead chloride is used, it is preferable to treat it with alkaline hot water in advance.

The molar ratio of these starting materials is not particularly limited, but they can be synthesized at a ratio of 1: 1. At this time, if Pb is excessive, it can be easily washed, but if Ti is excessive, a removing operation is necessary.

As described above, an apparatus called a so-called autoclave is used as an apparatus used when the reaction is carried out at a high temperature of 100 ° C. or higher.
It is preferable to use a material that can withstand high temperatures, such as polytetrafluoroethylene (so-called Teflon).

Next, the PY phase lead titanate microcrystals are heat-treated.

The heat treatment temperature is 520 in the case of long-time heat treatment such that the holding time at the predetermined heat treatment temperature is about 10 hours or more.
It may be higher than or equal to ℃, and is preferably 550 ℃.
Further, when there is no holding time at the final reached temperature, if the final reached temperature is 57 ° C. or higher, the phase change to the PE phase (perovskite phase) starts. Here, when the ferroelectric phase of the perovskite phase is completely used, it is preferable to set the temperature to 650 ° C. or higher. Further, when the activity is particularly required, it is preferable to lower the heat treatment temperature. For example, if heat treatment is performed at 570 to 610 ° C., the activity of the particles can be maintained to some extent and titanic acid, which is a promising raw material for high-density materials, can be obtained. Lead crystallites are obtained.

The temperature rising rate during the heat treatment is not particularly limited,
The heat treatment may be slow when the heat treatment is performed at a low temperature, and may be made fast when the heat treatment is performed at a high temperature. This selection depends on the productivity and the like.

Further, in the case of molding at the time of the heat treatment, it is possible to perform firing in a perovskite type in one step without calcination, and the present invention is also effective for this purpose.

[Action]

Thus, pH 12.1 or more, temperature 100 ℃ ~ 190
After the wet synthesis is performed at a temperature of ℃, the pyrochlore phase lead titanate microcrystals are selectively synthesized, and then the pyrochlore phase lead titanate microcrystals are heat-treated at 520 ° C. Very fine and uniform perovskite type lead titanate microcrystals of about 0.2 μm are produced.

〔Example〕

Hereinafter, the present invention will be described from specific experimental examples. Needless to say, the present invention is not limited to these experimental examples.

Experimental example 1. Ice water was prepared in a beaker, and titanium tetrachloride solution was gently added dropwise thereto. At this time, although it became cloudy in the initial stage, when stirring was continued for several hours, a completely transparent titanium tetrachloride aqueous solution was obtained. This was transferred to a 250 ml volumetric flask and used as a standard solution. From this standard solution, exactly 10 ml was taken and hydrolyzed with excess ammonia water, and TiO ₂
After nH ₂ O was filtered off, heat treatment was carried out at 1000 ° C. and the concentration was determined gravimetrically. Here, the concentration of titanium tetrachloride was 1.010 mol /.

On the other hand, lead acetate Pb (CH ₃ COO) ₂ .3H ₂ O of 22.
32 g was precisely weighed and dissolved in 100 ml of water.

Next, 58.26 ml of the above titanium tetrachloride standard solution was gradually added to this lead acetate solution so that Pb / Ti = 1.000. At this time, white precipitation of PbCl ₂ occurs, but this does not hinder the subsequent reaction.

Furthermore, a KOH solution was prepared in advance, and this was added to adjust the pH to pH = 13.1. At this time, the total amount of the solution was adjusted to 400 ml.

This is divided into 4 equal parts to make 100 ml of each sample solution, each sample solution is transferred to a Teflon autoclave container, and the autoclave is used to produce 100 to 2 by an electric furnace.
The synthesis was performed under the conditions of 30 ° C. and a reaction time of 1 hour.

The obtained precipitate was thoroughly washed with warm water, and decantation was repeated until the pH of the supernatant became around 7, and impurities were removed. Then, the precipitate was filtered and dried for 24 hours to obtain lead titanate microcrystals. .

The obtained lead titanate microcrystals were analyzed by X-ray diffraction, and the proportion of the contained crystal phase was investigated. Results are shown in FIG. Here, the amount of synthesis was represented by the height of the diffracted X-ray peak of (101) for the PE phase and (222) for the PY phase.

It can be seen from FIG. 2 that the synthetic phase rapidly changes from the PY phase to the PE phase at around 175 ° C.

In particular, by setting the reaction temperature to 110 ° C. to 160 ° C., lead titanate microcrystals of PY phase are produced as a substantially single phase. The X-ray diffraction spectrum of the obtained lead titanate microcrystals was measured and found to be JCPDS card [26-142].
It was confirmed that it was a pyrochlore phase. In addition, it was confirmed that the cubic crystal had a lattice constant of 10.37Å.

A scanning electron micrograph (SEM) of this PY phase lead titanate microcrystal is shown in FIG. From FIG. 3, the PY phase lead titanate microcrystals obtained here have a grain size of about 0.2.
It was observed to be spherical particles of μ. The composition of the PY phase lead titanate microcrystals synthesized in this experimental example was analyzed, and Pb / Ti = 0.99, which was extremely stoichiometric, and K ⁺ was 0.01% by weight. % of, Na ⁺ and Cl ^-
Was below the measurement limit.

Next, the raw material powder of the lead titanate microcrystals of the PY phase was fired as it is in an electric furnace without molding. The heat treatment conditions were heating at a temperature rising rate of 100 ° C. per hour to a predetermined temperature and quenching without holding time. At this time, the PbO atmosphere, which is usually used in the solid-phase reaction, was not provided outside.

The state of phase change of the powder thus obtained at each heat treatment temperature is shown in FIG. Here, the relative amount of the lead titanate microcrystals of the PY phase is expressed as the diffraction X-ray peak height of (222), and the relative amount of the lead titanate microcrystals of the PE phase is represented by the diffraction X of (110). Expressed as a line peak height, a graph showing how the PY phase changes to the PE phase depending on the heat treatment temperature is plotted.

As a result, the transition to the PE phase begins at around 550 ° C,
It was found that the phase transition from the PY phase to the PE phase was almost completed at 650 ° C.

A scanning electron micrograph of the obtained lead phase titanate microcrystals of PE phase is shown in FIG. This PE phase lead titanate microcrystal was confirmed to match with PbTiO ₃ shown in ASTM card [6-0452].

It can be seen from FIG. 5 that although the particle shape becomes slightly spherical by heat treatment, the particle diameter and the like are hardly changed, and very fine perovskite type lead titanate microcrystals are obtained.

Experimental Example 2. PY phase lead titanate microcrystals were synthesized in the same manner as in Experimental Example 1 above.

Next, this PY phase lead titanate microcrystal was subjected to a long-time heat treatment for 10 to 13 hours while changing the temperature.

When the state of the phase change of each of the obtained lead titanate microcrystals at each heat treatment temperature was examined, it was found that the phase transition to the PE phase started at 520 ° C or higher and almost completely changed to the PE phase at 550 ° C or higher. found.

〔The invention's effect〕

As is apparent from the above description, according to the present invention, fine perovskite-type lead titanate microcrystals that are uniform and do not cause secondary aggregation can be produced. In particular, the obtained lead titanate microcrystals have an extremely fine grain size of about 0.2 μm, and are therefore effective not only as a base material for electronic parts but also as an additive to other substances. It is considered to be optimal as a density sintered material.

Further, in the present invention, Pb / Ti≈1.
Although the PY phase of 0 is wet-synthesized and the PY phase is heat-treated, it is not necessary to perform the heat treatment in the PbO vaporization temperature range, so that the composition variation of the obtained lead titanate microcrystals is very small, High stoichiometry.

Furthermore, according to the present invention, the activity of the obtained lead titanate microcrystals can be increased by controlling the heat treatment temperature, and the lead titanate microcrystals that can be used as a raw material for the high-density sintered material can be produced. can do.

[Brief description of drawings]

FIG. 1 is a phase diagram according to pH-temperature in the wet synthesis method. FIG. 2 is a characteristic diagram showing the temperature dependence of the PY phase, PE phase, and PX phase at pH 14.0. FIG. 3 is a scanning electron micrograph of lead titanate microcrystals of PY phase, and FIG. 4 is a characteristic diagram showing a phase transition state from PY phase to PE phase. FIG. 5 is a scanning electron micrograph of the lead phase titanate microcrystals obtained.

Claims (1)

[Claims] 1. A soluble titanium compound or a hydrolysis product thereof and a lead compound in an aqueous solution at a pH of 12.1 or higher, at a temperature of 100 ° C. to 190 ° C., and in the attached FIG. 1, pyrochlore-type lead titanate microcrystals PY. A method for producing perovskite-type lead titanate microcrystals, which comprises reacting the microcrystals obtained at a temperature of 520 ° C. or higher after reacting under the pH and temperature conditions in the region in which is mainly produced.