JPH062586B2 - Method for producing lead titanate microcrystals - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention is useful as a precursor of perovskite-type lead titanate microcrystals, or as an additive for ferroelectric materials, piezoelectric materials, and pyroelectric materials, which is a pyrochlore type. The present invention relates to a method for producing lead titanate microcrystals.

[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 to 1000 ° C.
It is synthesized by the method of calcining at 1, pulverizing again until it becomes uniform, and then performing main firing.

By the way, when synthesizing fine particles of lead titanate by such a method, evaporation of PbO becomes a big 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, a considerable amount of PbO remains, and in order to suppress evaporation of this unreacted PbO, it is conceivable to lower the calcination temperature and then perform the main calcination. At the end of the process, a considerable amount of unreacted PbO remains, and this unreacted PbO may vaporize 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-phase reaction method utilizing the heat treatment as described above, Pb _1-δ
A in the perovskite structure, such as TiO _3.
Site defects are likely to occur, and this non-stoichiometry is likely to adversely affect the piezoelectric characteristics, pyroelectric characteristics and the like. Further, 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 raw material adjustment procedure is performed. This is because lead titanate has the highest crystal anisotropy among those having a perovskite type structure, that is, since the square strain is large, 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 it causes cracks. Further, in order to avoid this, various additives may be added 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 a portion that cannot be reached by other piezoelectric materials. It is fully conceivable that the characteristics will be deteriorated by adding. Therefore, for the above reasons, there are few examples of application of lead titanate in a pure form to dielectric porcelain by the solid-state reaction method, and one of the contradictory characteristics of piezoelectric property and sinterability is The reality is that they are put to practical use with an emphasis on them.

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 A ion and B ion in ABO ₃ having a perovskite structure to be synthesized is reacted in an alkaline aqueous solution under boiling. However, this method requires the condition of [A ion] / [B ion] ≧ 1.8, 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, 300
Heat treatment at ~ 400 ° C is required. Furthermore, 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, as another method, the oxalate method and the oxalate ethanol method are known, but in the former case, the solubility of the oxalate may differ depending on the type of metal ion, or the pH range in which precipitation occurs may differ. It is difficult to obtain a uniform composition. Further, 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 method has been proposed, there are many problems in terms of manufacturing cost and productivity. The obtained precipitate is highly pure but amorphous, and
It is necessary to perform heat treatment at about 00 ° C.

[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, it has been completely impossible to control the shape of lead titanate microcrystals depending on the application.

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 performing wet synthesis of crystalline lead titanate fine particles having high purity, uniform and less lattice distortion without heat treatment, and as a result of extensive research over a long period of time, pH and 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 setting the synthesis temperature to a predetermined value. Found.

The present invention has been completed based on such knowledge, and a soluble titanium compound or a hydrolysis product thereof and a lead compound are mixed in an aqueous solution at a pH of 12.1 or higher and a temperature of 100.
It is characterized in that the reaction is carried out at a temperature of from 90 ° C to 190 ° C, and in the pH and temperature conditions within the region where the pyrochlore type lead titanate microcrystals PY are mainly produced in the attached FIG.

In the present invention, in order to prepare 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 A water-soluble salt is 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 and then an alkali cation such as K ⁺ or Na ⁺ or an anion such as Cl ^−. Ions may be completely removed, filtered and dried.

Here, the pH and reaction temperature at the time of the above reaction are important, and the pyrochlore phase (hereinafter, P
Lead titanate microcrystals of Y phase), perovskite phases (hereinafter, PE phase) or acicular novel crystal phases of lead titanate microcrystals (hereinafter, PX phase) are formed. 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 FIG. 1, the PY phase is stable in the low alkaline high temperature range to the high alkaline low temperature range, and the PE phase is stable only in the high alkaline high temperature range.
It has been found that the X phase only forms within a certain range. In FIG. 1, parentheses represent those that are slightly produced as by-products, and AM represents lead titanate in an amorphous state.

That is, by setting the pH to 12.1 or higher and the reaction temperature to 100 to 190 ° C., preferably to set the pH to 13.0 or higher and the reaction temperature to 110 to 175 ° C., the PY phase is selectively produced. A reaction time of 1 hour or less is sufficient, but the longer the reaction time, the better the crystallinity of the PY phase. The PY-phase lead titanate microcrystals obtained here have extremely high stoichiometry, and their particle size is 0.2μ or less, which is an order of magnitude smaller than that obtained by the conventional solid-phase reaction method. Therefore, it has a high utility value as a base material or an additive material of a capacitor material or a raw material of a high-purity sintered body.

On the other hand, when the pH is set to 12.7 or higher, preferably 13.1 or higher, and the reaction temperature is set to 175 ° C. or higher, preferably 190 ° C. or higher, PE phase lead titanate microcrystals are selectively formed. Also in this case, the reaction time of 1 hour or less is sufficient. The PE phase lead titanate microcrystals obtained here are crystalline precipitates containing almost no water, and have a particle size of 6-8.
It is a very uniform dice-shaped crystal fine particle of about μ. Particularly, the dice-shaped crystal planes are the (h00) and (001) planes a and c, and a highly oriented sheet can be prepared by mixing a suitable resin binder and performing so-called casting. It is possible. It is also possible to produce an a-plane disk by pressure molding, and it is expected as a raw material powder suitable for highly oriented firing. further,
The particle size of the obtained lead titanate microcrystals of the PE phase can be changed by changing the stirring speed during the reaction, slightly shifting the pH, and the like, and the stoichiometry is high. Furthermore, the PE phase obtained here is advantageous in piezoelectric / pyroelectric properties because the strain of the lattice decreases as the temperature rises by heat treatment, and in particular, the lattice strain of the c-axis is extremely small. Is.

Further, by setting the pH to 11.2 to 13.0 and the reaction temperature to 145 ° C. or higher, lead titanate microcrystals in the PX phase are generated, and particularly, the pH is
11.5 ~ 12.5, by setting the reaction temperature of 180 ℃ or more,
The PX phase forms almost as a single phase. The reaction time of 1 hour or less is sufficient. The lead titanate microcrystal of the PX phase is a novel crystal phase which has not been known so far, and its particle shape is acicular. Therefore, this PX-phase lead titanate microcrystal is expected as a composite material.

Further, in a region other than the above, amorphous lead titanate having a Pb / Ti molar ratio of almost 1 is produced, and in this case as well, it is preferable to carry out the hydrothermal reaction on the alkaline side having a pH of 7 or more. Particularly, when only lead titanate in an amorphous state is required, a pH condition of 10 or lower is required, and a reaction temperature is preferably 110 ° C. or higher, more preferably 150 ° C. or higher. . The above-mentioned amorphous lead titanate undergoes a phase transition to the PE phase when it is heat-treated at 700 ° C. or higher. However, when amorphous lead titanate synthesized at a pH of 7 or less is heat-treated at 830 ° C., PbTi ₃ O ₇ was mainly generated. Especially at pH = 4, most of this PbTi ₃ O ₇ is present, and as the pH is raised, PbT ₃ O ₇ becomes PbT ₃ O ₇ .
It was found that iO ₃ was gradually mixed. From this, it can be said that pH ≧ 7 is preferable in order to use the amorphous lead titanate as an additive or the like.

On the other hand, in order to obtain the starting Ti compound or its hydrolysis product in synthesizing the lead titanate microcrystals, a salt such as TiCl ₄ , Ti (SO ₄ ) ₂ is dissolved in water. , Or its aqueous solution, KOH, NaOH, N
It may be hydrolyzed with an alkaline aqueous solution such as H ₄ OH or LiOH. However, when Ti (SO ₄ ) ₂ is used, it is hydrolyzed with these alkaline solutions to produce TiO ₂ · nH ₂ O (titanium oxide hydrate), and decantation and filtration are repeated to obtain SO ² ₄ Just remove it.

Further, as lead compounds, lead acetate Pb (CH ₃ COO) ₂ 3H ₂ O,
Lead nitrate Pb (NO ₃ ) ₂ and lead chloride PbCl ₂ can be used. However,
When lead chloride is used, it is preferably treated 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).

[Action]

Thus, in the wet synthesis method of lead titanate microcrystals,
By selecting pH and synthesis temperature, it is possible to synthesize three types of lead titanate microcrystals of PE phase (perovskite phase), PY phase (pyrochlore phase), and PX phase (new crystal phase). In particular, pH 12.1 or higher, temperature 100 ℃ ~ 1
By setting the temperature to 90 ° C., lead titanate microcrystals in the pyrochlore phase are selectively synthesized.

〔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 aqueous solution of titanium tetrachloride was obtained. This was transferred to a 250 ml volumetric flask and used as a standard solution. Accurately collect 10 ml from this standard solution, hydrolyze it with excess ammonia water, and remove TiO ₂ · nH ₂
After O was filtered off, heat treatment was performed at 1000 ° C. to determine the concentration by gravimetric method. Here, the concentration of titanium tetrachloride is 1.010 mol
It was / l.

Meanwhile, lead acetate _{Pb (CH 3 COO) 2 ·} 3H 2 O 2
2.32 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, a white precipitate 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 = 14.0. 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 temperature was changed within the range of 30 ° C., and the synthesis was carried out for 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, to remove impurities, which was then filtered off and dried for a day and night 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 diffraction 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. Fig. 3 shows diffraction X of lead titanate microcrystals of PY phase synthesized at pH = 13.1, reaction temperature 148 ° C, reaction time 1 hour.
The line spectrum (Cu target, by Ni filter) is shown. This diffracted X-ray spectrum was in agreement with JCPDS card [26-142] and was confirmed to be a pyrochlore phase. It was also confirmed that the lattice constant was 10.37Å cubic.

Further, from the scanning electron micrograph (SEM) shown in FIG. 4, it was observed that the PY phase lead titanate microcrystals obtained in this experimental example were spherical particles having a particle size of about 0.2 μm.
The composition analysis of the lead titanate microcrystals of the PY phase synthesized in this Experimental Example was confirmed to be extremely stoichiometric.

The lead titanate microcrystals of the PY phase undergo a phase transition to the PE phase by heat treatment. The obtained PY phase lead titanate microcrystals were subjected to a heat treatment to examine the phase transition state. As shown in the figure, it was found that the phase transition to the PE phase started at around 550 ° C., and the phase transition to the PE phase started almost completely at 650 ° C. or higher. Here, the ratio of the PY phase to the PE phase is defined by the (222) diffraction X-ray peak height of the PY phase and the PE phase.
It was determined by the height of the (110) diffraction X-ray peak of the phase.

On the other hand, the lead titanate microcrystals obtained at a reaction temperature of 200 ° C. or higher have almost a single PE phase, and the diffraction X-ray spectrum (by Cu target and Ni filter) is as shown in FIG. , ASTM card [6-045
It coincided with the peak position of 2], and it was confirmed that the perovskite phase was a single phase. Further, when the lattice constant of the lead titanate microcrystal of the PE phase was calculated from the Nelson-Riley extrapolation function, it was confirmed to be a tetragonal crystal with a _D = 3.901Å and c _D = 4.150Å. However, the relative intensity of each diffracted X-ray peak of the PE phase is significantly different from that of the ASTM card, and compared with the diffracted X-ray spectrum of that obtained by the usual solid phase reaction at 1055 ° C. shown in FIG. 7, for example. , (10
Diffraction X of 0), (001) and (200), (002)
The line peak was found to be very large. From this, it is understood that the lead phase titanate microcrystals of the PE phase are predominantly oriented in the pressing direction at the time of filling the glass sample plate by the powder method.

In addition, from the scanning electron micrograph (SEM), the PE phase lead titanate microcrystals obtained in this experimental example had a side of 7 to 8
It was observed that it had a cube shape (dice shape) of μ and had an angular surface.

Experimental example 2. A standard solution of titanium tetrachloride having a concentration of 0.9681 mol / l was prepared in the same manner as in Experimental Example 1 above.

On the other hand, 19.49 g of lead nitrate Pb (NO ₃ ) ₂ .3H ₂ O was precisely weighed, dissolved in 100 ml of water, and then Pb / Ti = 1.00.
The above titanium tetrachloride standard solution (60.79 ml) was added so that the total amount became 0, and the total amount of solution was adjusted to 400 ml and pH = 12.9 while adjusting the pH with an aqueous KOH solution.

Next, this was equally divided into 5 equal parts, and by the same method as in Experimental Example 1, 80 ml of each sample solution was reacted in the range of 130 to 230 ° C. using an autoclave to obtain lead titanate microcrystals.

The obtained lead titanate microcrystals were analyzed by X-ray diffraction (Cu target, Ni filter), and the proportion of contained crystal phases was investigated. The results are shown in Fig. 8. PE here
(101) for phase, (222) for PY phase
In the PX phase, the amount of each compound was represented by the height of the diffraction X-ray peak of (330).

From FIG. 8, it can be seen that when the reaction temperature is lower than 130 ° C., it is in an amorphous state, whereas when the reaction temperature exceeds 140 ° C., the PY phase begins to precipitate rapidly, and the PX phase gradually increases as the temperature becomes higher. It was confirmed that the PE phase suddenly began to precipitate at 195 ° C.

Experimental example 3. A titanium tetrachloride standard solution having a concentration of 0.9681 mol / l was prepared in the same manner as in Experimental Example 1 above.

On the other hand, 22.32 g of lead acetate Pb (CH ₃ COO) ₂ .3H ₂ O was precisely weighed, dissolved in 100 ml of water, and then Pb / Ti = 1.000.
60.79 ml of the above-mentioned titanium tetrachloride standard solution
Was added and the pH was adjusted with a KOH aqueous solution to adjust the total solution amount to 400 ml and pH = 12.0.

Next, this was equally divided into four equal parts, and 100 ml of each sample solution was reacted in the range of 150 to 220 ° C. using an autoclave in the same manner as in Experimental Example 1 to obtain lead titanate microcrystals.

The obtained lead titanate microcrystals were analyzed by X-ray diffraction (Cu target, Ni filter), and the proportion of contained crystal phases was investigated. The results are shown in Fig. 9. In addition, here PY
(222) for phase and (33 for PX phase
The height of the diffracted X-ray peak of 0) represents the amount of each synthesis.

From FIG. 9, it can be seen that while the reaction temperature is 140 ° C. or lower, it is in an amorphous state, whereas as the reaction temperature rises, the PX phase gradually increases and the PY phase also slightly increases. all right.

Further, from the scanning electron micrograph (SEM), it was found that the obtained PX-phase lead titanate microcrystals were acicular particles having a thickness of 0.1 to 0.2 μ and a length of 10 μ or more.

Experimental example 4. According to the same method as the above experimental example, pH = 14.0, reaction temperature 1
Under the conditions of 18 ° C. and a reaction time of 1 hour, PY phase lead titanate microcrystals were synthesized and subjected to thermal analysis measurement.

Further, the thermal analysis conditions are a heating rate of 20 ° C./min, and Pb which is considered to have a heat capacity close to that of PbTiO ₃ of the reference standard sample.
_1/2 L _1/3 TiO ₃ was used. This is commercially available PbO, La
₂ O ₃ and TiO ₂ are mixed at a predetermined molar ratio and subjected to solid phase reaction, and there is no problem because the Curie point is below room temperature.

The results are shown in Fig. 10. In addition, in FIG.
TG shows the result of thermogravimetric analysis, and DTA shows the result of differential thermal analysis. Furthermore, in differential thermal analysis, Exo is exothermic, and E
ndo represents endotherm.

From FIG. 10, it was confirmed that the PY phase lead titanate microcrystals had desorption of adsorbed water at the initial stage and generated heat at about 625 ° C. due to the pyrochlore-perovskite phase transition.

〔The invention's effect〕

As is clear from the above description, in wet synthesis in an alkaline aqueous solution, by selecting pH and reaction temperature, perovskite type, pyrochlore type, needle-like ones having a novel crystal phase, respectively, are synthesized. But especially the pH
By setting the reaction temperature to 12.1 or higher and the reaction temperature to 100 ° C to 190 ° C, the pyrochlore phase lead titanate microcrystals are selectively synthesized.

The obtained pyrochlore phase lead titanate microcrystals have high purity and high stoichiometry, and thus are less affected by impurities,
It is possible to improve the characteristics of the piezoelectric element and the like. In addition, since this lead titanate microcrystal has a particle size of 0.2 μm or less, which is about one digit smaller than that obtained by the conventional solid-phase reaction, and its activity is high, it is used as a base material of a capacitor material or an additive material. Or, since it is uniform, it is useful as a raw material for a high-density sintered body.

[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 PE phase and PY phase at pH 14.0, and Fig. 3 is pH 13.1 and reaction temperature 148 ° C.
FIG. 4 is a diffraction X-ray spectrum of the PY phase lead titanate microcrystals synthesized in FIG. 4, FIG. 4 is a scanning electron micrograph of the PY phase lead titanate microcrystals, and FIG. 5 is a phase from the PY phase to the PE phase. It is a characteristic view which shows a transition state. FIG. 6 is a diffraction X-ray spectrum of the obtained lead phase titanate microcrystal of PE phase, and FIG. 7 is a diffraction X-ray spectrum of the perovskite-type lead titanate microcrystal obtained by a usual solid-phase reaction method. FIG. 8 is a characteristic diagram showing the temperature dependence of PE phase, PY phase, and PX phase at pH 12.9, and FIG. 9 is P at pH 12.0.
It is a characteristic view which shows the temperature dependence of X phase and PY phase. FIG. 10 is a characteristic diagram showing the results of thermal analysis of PY phase lead titanate microcrystals.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hidemasa Tamura Inventor Hidemasa Tamura 6-735 Kita-Shinagawa, Shinagawa-ku, Tokyo Inside Sony Corporation (56) Reference JP-A-57-106524 (JP, A) JP 57-191232 (JP, A)

Claims (1)

[Claims] 1. A soluble titanium compound or a hydrolysis product thereof and a lead compound in an aqueous solution at pH 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 lead titanate microcrystals, which is characterized in that the reaction is carried out at pH and temperature conditions in the region where strontium is mainly produced.