JPS61158822A - Production of lead titanate crystallite - Google Patents

Abstract

PURPOSE:In wet synthesis in an alkali aqueous solution, to obtain the titled crystal of pyrochlore type, capable of corresponding diversification of uses, having high uniformity of composition and high purity, without requiring heat treatment, by specifying pH and the reaction temperature. CONSTITUTION:A soluble Ti compound or its hydrolyzate is reacted with a lead compound in an aqueous solution at >= 12.1pH at 100-190 deg.C. Consequently, lead titanate crystallite of pyrochlore phase can be selectively synthesized. The prepared crystallite has high purity and high stoichiometric properties, so characteristics for piezoelectric element, etc., can be improved. The crystallite has about one order smaller particle diameter than that of crystallite obtained in an existing solid reaction and has high activity. Consequently, it is useful as a parent material for condenser material, an additive, and, since it is uniform, it is useful as a raw material for high-density sintered material.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention provides a pyrochlore-type compound useful as a precursor of perovskite-type lead titanate microcrystals or as an additive for ferroelectric materials, piezoelectric materials, and pyroelectric materials. The present invention relates to a method for producing lead titanate microcrystals.

[Conventional technology]

In the field of dielectric ceramics, new synthesis methods for dielectric oxide fine particles, which serve as raw materials, are being developed in response to miniaturization of electronic components and diversification of applications.

For example, in multilayer ceramic capacitors, it is necessary to reduce the thickness of the ceramic layer in order to increase the capacitance as well as reduce the size and weight, and making the dielectric oxide, which is the raw material, finer particles is an important issue. Furthermore, from the viewpoint of the withstand voltage of the capacitor, abnormal grain growth and generation of non-uniform particles during the sintering stage are undesirable, and there is an urgent need to develop a method for synthesizing uniform fine particles.

On the other hand, the dielectric oxide that is the raw material for dielectric ceramics is
Lead titanate has many excellent properties and is widely used.
are generally lead oxide (PbO) and titanium oxide (TiOΩ).
After mixing and pulverizing with a ball mill, 800 to 1
It is synthesized by a method of pre-calcining at 1,000°C, pulverizing it again until it becomes uniform, and then performing main firing.

By the way, when lead titanate fine particles are synthesized by such a method, evaporation of pbo poses a major problem. That is, as the temperature during the above-mentioned pre-calcination is higher, the amount of evaporation of pbo increases exponentially, and there is a possibility that the composition of the obtained lead titanate fine particles may change. Therefore, in order to avoid this, it is necessary to take considerable measures during heat treatment, such as performing firing in a PBO atmosphere. Alternatively, in order to suppress the evaporation of Pbo, it is possible to lower the temperature of the pre-calcination and then perform the main firing, but in this case, a considerable amount of unreacted PbO remains at the end of the above-mentioned pre-calcination. ,
There is a risk that this unreacted pbo may vaporize during the main firing stage, so atmosphere control is also required here. For this reason, lead titanate fine particles obtained by the solid phase reaction method using heat treatment as described above,
A-site defects are likely to occur in perovskite structures such as Pb and 4T i Ox, and there is a high possibility that this non-stoichiometry will adversely affect piezoelectric properties, pyroelectric properties, and the like. Further, even if it is assumed that a product with high stoichiometry can be obtained by high-temperature heat treatment, the sinterability will deteriorate as long as the raw material cleavage procedure as described above is used.

This is because lead titanate has the highest crystal anisotropy among those with a perovskite structure, that is, it has a large tetragon, so the coefficient of thermal expansion in a specific direction is significantly different.
This is because cracks may occur when the temperature is lowered from a high temperature. Furthermore, in order to avoid this, it is conceivable to add various additives to improve the sinterability, but in this case, problems with piezoelectric properties arise. In other words, lead titanate has the advantage that the electromechanical coupling coefficient for longitudinal waves is larger than that for transverse waves, and is effectively used in areas that cannot be reached with other piezoelectric materials. It is quite conceivable that the addition of this material will degrade its properties.

Therefore, for the reasons mentioned above, there are almost no examples of application of lead titanate in its pure form to dielectric ceramics using the solid phase reaction method, and it is difficult to obtain either one of the contradictory properties of piezoelectric properties and sinterability. The reality is that we are putting a lot of emphasis on it and putting it into practical use.

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

For example, Japanese Patent Publication No. 51-2080 discloses a wet synthesis method in which salts of A ions and B ions in A B O of the perovskite structure to be synthesized are reacted under boiling in an alkaline aqueous solution. . However, in this method, [A ion]/[B ion concentration ratio,
Since the ion concentration ratio during synthesis is not 1, the composition tends to fluctuate, and the synthesized product is amorphous and hydrated. Therefore, heat treatment at 300 to 400°C is necessary to obtain crystalline particles. Furthermore, excess p b c I! becomes an impurity during synthesis! , but this P b Cl
, is difficult to completely eliminate.

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

Furthermore, an organometallic compound represented by the general formula M(OR)n is synthesized, and a complex alkoxide represented by the general formula M, M, (OR>, . . . The alkoxide method has also been proposed, but it has many problems in terms of production cost and productivity, and although the resulting precipitate is of high purity, it is amorphous and requires heat treatment at about 400°C. It is necessary to apply

[Problem that the invention seeks to solve]

Thus, using conventional synthesis methods, it is difficult to synthesize crystalline lead titanate fine particles from the liquid phase without heat treatment.
It has been difficult to obtain lead titanate microcrystals with high uniformity and purity. Furthermore, 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 in the technical field as described above. An object of the present invention is to provide a method for producing highly pure lead titanate microcrystals with high uniformity.

[Means for solving problems]

The inventors of the present invention sought to develop a synthesis method capable of wet-synthesizing crystalline lead titanate fine particles of high purity, uniformity, and low lattice strain without heat treatment, and as a result of long-term research, they found that the pH and By setting the synthesis temperature to a predetermined value, it is possible to synthesize perovskite-type lead titanate microcrystals, pyrochlore-type lead titanate microcrystals, or acicular lead titanate microcrystals having a new crystal phase. I found out.

The present invention was completed based on such knowledge, and the present invention was developed by combining a soluble titanium compound or its hydrolysis product and a lead compound in an aqueous solution at a pH of 12, 1 or higher and a temperature of 10.
It is characterized in that the reaction is carried out at 0°C to 190°C.

In the present invention, in order to create pyrochlore type lead titanate microcrystals, a soluble titanium compound such as titanium tetrachloride (TiC) or its hydrolysis product and a lead compound hydrolysis product or its hydrolysis product are used. The mixture is mixed with a water-soluble salt 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 hot water to form alkali cations such as K'', Na+ and anions such as CI!-. All you have to do is completely remove the ions, filter and dry.

Here, the pH and reaction temperature during the above reaction are important,
These p) (and depending on the reaction temperature, lead titanate microcrystals of pyrochlore phase (hereinafter referred to as PY phase), perovskite phase (hereinafter referred to as PE phase), or new acicular crystalline phase (hereinafter referred to as PE phase) , PX phase) is formed.The inventors have repeatedly conducted experiments and determined that pl(
We attempted to create a phase diagram of lead titanate microcrystals obtained by varying the reaction temperature. The results are shown in Figure 1. From this figure 1,
It was found that the PY phase is stable in the low-alkali high temperature range to the high-alkali low temperature range, the PE phase is stable only in the high-alkali high temperature range, and furthermore, the PX phase is produced only within a specific range. In Fig. 1, the numbers in parentheses represent slightly generated by-products, and AM is amorphous.
Represents the state of lead titanate.

That is, pH 12.1 or more, reaction temperature 100-190
℃, preferably pH 13.0 or higher, reaction temperature 110~
By setting the temperature to 175°C, the PY phase is selectively generated. Although a reaction time of one hour or less is sufficient, the longer the reaction time, the better the crystallinity of the obtained PY phase. The PY phase lead titanate microcrystals obtained here have extremely high stoichiometry, and the particle size is less than 0.2μ, which is one order of magnitude larger than that obtained by conventional solid-phase reaction methods. Because of its small size, it has high utility value as a base material or additive for capacitor materials, or as a raw material for high-density sintered bodies.

In contrast, the pH is above 12.7, preferably 13.
When the reaction temperature is set to 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 generated. In this case as well, a reaction time of 1 hour or less is sufficient. The PE phase lead titanate microcrystals obtained here are crystalline precipitates that contain almost no water, and
They are extremely uniform dice-shaped crystal fine particles with a particle size of about 6 to 8 μm. In particular, the dice-shaped crystal plane is (h OO)
and (OOJ), and it is also possible to create a highly oriented sheet by mixing an appropriate resin binder and performing so-called casting. Moreover, it is also possible to create an a-plane disk by pressure molding 5, and it is expected to be used as a raw material powder that has been subjected to highly oriented firing. Furthermore, the particle size of the resulting PE phase lead titanate microcrystals can be changed by operations such as changing the stirring speed during the reaction or slightly changing the pH.
It also has high stoichiometry. Furthermore, by heat-treating the PE phase obtained here, the lattice distortion decreases as the temperature increases, and in particular, the C-axis lattice distortion is extremely small, which is advantageous in terms of piezoelectric and pyroelectric properties. It is.

Moreover, pH11.2-13. O, by setting the reaction temperature to 145°C or higher, lead titanate microcrystals of the PX phase are generated, especially when the pH is 11.5 to 12.5 and the reaction temperature is 180°C.
'C or more, the PX phase is almost generated as a single phase. Further, a reaction time of one hour or less is sufficient. This PX phase lead titanate microcrystal is of a new crystal phase that has not been previously known, and its particle shape is acicular. Therefore, this PX phase lead titanate microcrystal is expected to be used as a composite material.

Further, in regions other than the above, lead titanate in an amorphous state with a Pb/Ti molar ratio of almost 1 is produced, but in this case as well, it is preferable to carry out the hydrothermal reaction on the alkaline side with a pH of 7 or more. In particular, when only lead titanate in an amorphous state is required, a pH of 1O or less is required, and the reaction temperature is preferably 110°C or higher, more preferably 150°C or higher. . The above-mentioned amorphous lead titanate can be made into P by heat treatment at 700°C or higher.
Although it undergoes a phase transition to the E phase, when amorphous lead titanate synthesized at pH 7 or lower was heat-treated at 830°C, P b
T i,O, was mainly generated. In particular, it was found that at pH=4, most of the P b T 1j07 was present, and as the pH was raised, PbTi0a gradually mixed into the P b T i 707. From this point of view, it can be said that pH≧7 is preferable in order to utilize the amorphous lead titanate as an additive or the like.

On the other hand, in order to obtain a Ti compound or its hydrolysis product as a starting material for synthesizing the above-mentioned lead titanate microcrystals, a salt such as T i C m T i (S (%)) is added to water. Melt or the aqueous solution with K O
Hydrolysis may be carried out using an alkaline aqueous solution such as H, Na OHlN H, OH, Li OH.

However, when Ti(Soω) is used, it is hydrolyzed with these alkaline solutions to form TiO, L−nH(
5OS- can be removed by preparing a titanium Mide hydrate) and repeating decantation and filtration.

In addition, as a lead compound, lead acetate Pb (CH3CO2)
Yo 3H Yu O1 Lead nitrate Pb (No, Lead chloride Pbcβ
etc. can be used. However, when using lead chloride,
It is preferable to treat with alkaline hot water in advance.

Although the molar ratio of these starting materials is not particularly limited, they can be synthesized at a ratio of 1:1. Further, at this time, if Pb is in excess, it can be easily cleaned, but if Ti is in excess, a removal operation is required.

As mentioned above, an apparatus called an autoclave is used for the reaction at a high temperature of 100°C or higher, and its inner container contains high alkali,
It is preferable to use a material that can withstand high temperatures, such as polytetrafluoroethylene (so-called Teflon).

[Effect]

In this way, in the wet synthesis method of lead titanate microcrystals,
By selecting pH and synthesis temperature, PE phase (perovskite phase), PY phase (pyrochlore phase), PX phase (
It is possible to synthesize three types of lead titanate microcrystals (new crystalline phase), especially at a pH of 12.1 or higher and a temperature of 1.
By setting the temperature to 00°C to 190°C, pyrochlore phase lead titanate microcrystals are selectively synthesized.

〔Example〕

The present invention will be explained below using specific experimental examples.

It goes without saying that the present invention is not limited to these experimental examples.

Experimental Example 1 A glass of ice water was prepared, and a titanium tetrachloride solution was gently dropped little by little into it. At this time, the solution was cloudy at the beginning, but after stirring for several hours, a completely transparent titanium tetrachloride aqueous solution was obtained. This was transferred to a 250ml volumetric flask and used as a standard solution. Accurately take 10 ml of this standard solution, hydrolyze it with excess aqueous ammonia, and
After filtering off O>·n H,0, the mixture was heat-treated at 1000°C and the concentration was determined by gravimetric method. Here, the concentration of titanium tetrachloride is 1.010 mol/j! Met.

On the other hand, 22.32 g of lead acetate Pb (CH,Coo).3H,O was accurately weighed and dissolved in 100 ml of water.

Then, P b /T i = 1.0 was added to this lead acetate solution.
The above titanium tetrachloride standard solution was heated to 58°2 so that
6 mρ was gradually added. At this time, a white precipitate of PBCP is formed, but this does not pose any problem in the subsequent reaction.

Furthermore, a KOH7S solution was prepared in advance, and the pH was adjusted by adding this to pH=14. It was set as O.

Further, at this time, the total solution amount was set to 400 mff.

Divide this into four equal parts to obtain sample solutions of 100 ml each, transfer each sample solution to a Teflon autoclave container, use an autoclave, and heat in an electric furnace for 100 to 23
The synthesis was carried out for a reaction time of 1 hour while changing the temperature within a range of 0°C.

The obtained precipitate was thoroughly washed with warm water, and decantation was repeated until the pH of the supernatant reached around 7. After removing impurities, this was filtered and dried day and night to obtain lead titanate microcrystals. Ta.

The obtained lead titanate microcrystals were analyzed by X-ray diffraction to determine the proportion of crystal phases contained therein. The results are shown in Figure 2. Here, the synthesized amount is expressed by the height of the diffraction X-ray peak of (101) for the PE phase and (222) for the PY phase.

From this figure 2, the composite sum is PY
It can be seen that there is a sudden change from the phase to the PE phase.

In particular, by setting the reaction temperature at 110°C to 160°C, lead titanate microcrystals of the PY phase are generated as a substantially single phase. In Figure 3, pH = 13.1, reaction temperature 148
The diffraction X-ray spectrum (by Cu target and Ni filter) of a PY phase lead titanate microcrystal synthesized at 1° C. for 1 hour is shown.

This diffraction X-ray spectrum is shown on the JCPDS card [26-
142) and was confirmed to be a pyrochlore phase. Further, it was confirmed that the lattice constant was a cubic crystal of 10.37 people.

Furthermore, from the scanning electron micrograph (SEM) shown in FIG. 4, it was observed that the lead titanate microcrystals of the PY phase obtained in this experimental example were spherical particles with a particle size of 0.2 μm.

When the composition of the PY phase lead titanate microcrystals synthesized in this experimental example was analyzed, it was confirmed that the lead titanate microcrystals had extremely high stoichiometry.

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

On the other hand, lead titanate microcrystals obtained at a reaction temperature of 200°C or higher have a single PE phase, and their diffraction X-ray spectra (
Cu target, Ni filter) is as shown in Figure 6, and A37M card (6-0452)
This coincided with the peak position of 61, and it was confirmed that the perovskite phase was a single phase. Moreover, when the lattice constant of this PE phase lead titanate microcrystal is determined from the Nelson-Riley extrapolation function, it is a.

= 3.901 people, C, = 4.150 people was confirmed to be a tetragonal crystal. However, the relative intensity of each diffraction X-ray peak of the PE phase is significantly different from that of the A37M card, and compared with the diffraction X-ray spectrum of the one obtained by a normal solid-phase reaction at 1055°C shown in Figure 7, for example. (10'O), (001) and (200),
It was found that the diffraction X-ray peak of (002) was very large. From this, it can be seen that the lead titanate microcrystals of the PE phase are predominantly oriented in the pressing direction when filling the glass sample plate by the powder method.

Moreover, from the scanning electron micrograph (SEM), the lead titanate microcrystals of the PE phase obtained in this experimental example have − sides of 7 to 8.
It was observed that it had a cubic shape (+ icon shape) with an angular surface.

Experimental Example 2゜Similar to Experimental Example 1, the concentration was 0.9681 mol/j!
A standard solution of titanium tetrachloride was prepared.

On the other hand, 19.49 g of lead nitrate Pb (No,) 3Hρ
After accurately weighing and dissolving it in 100ml of water, Pb/
Ti=1. Add 60.79 m6 of the above titanium tetrachloride standard solution so that it becomes OOO, and then add p)(
While adjusting the total solution volume 400 mz, pH = 12.9
And so.

Next, this was divided into five equal parts, and in the same manner as in Experimental Example 1, 80 m6 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 subjected to X-ray diffraction (Cu target).
, Ni filter) to determine the proportion of the crystalline phase contained. The results are shown in Figure 2.

Here, the synthesized amount is expressed by the height of the diffraction X-ray peak of (101) for the PE phase, (222) for the PY phase, and (330) for the PX phase.

From this figure, it can be seen that when the reaction temperature is below 130°C, the state is amorphous, but when the reaction temperature exceeds 140°C, the PY phase begins to precipitate, and as the temperature is raised further, the PX phase begins to precipitate. It was confirmed that the PE phase precipitated gradually, and the PE phase suddenly precipitated at 195°C.

Experimental example 3゜Similar to the previous experimental example 1, the concentration was 0.9681 mol/j!
A standard solution of titanium tetrachloride was prepared.

On the other hand, lead acetate Pb (CHjCOO and 3Hyu0 is 22,
After accurately weighing 32 g and dissolving it in water at 100 mA, 60.79 m It of the above titanium tetrachloride standard solution was added so that Pb/Ti = 1.000, and then KOH
While adjusting the pH with an aqueous solution, the total solution volume is 400 mff1.
．． The pH was set to 12.0.

Next, this was 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) to determine the proportion of crystal phases contained. The results are shown in Figure 2.

Here, the synthesized amount is expressed by the height of the diffraction X-ray peak of (222) for the PY phase and (330) for the PX phase.

From this diagram, it can be seen that when the reaction temperature is below 140°C, it is in an amorphous state, but as the reaction temperature is increased, the PX phase gradually increases, and the PY phase also increases slightly. Understood.

Furthermore, scanning electron micrographs (SEM) revealed that the PX phase lead titanate microcrystals obtained were needle-like particles with a thickness of 0.1 to 0.2 μm and a length of 10 μm or more.

Experimental Example 4 According to the same method as in the previous experimental example, PY phase lead titanate microcrystals were synthesized under the conditions of pH = 14.0°, reaction temperature 118°C, and reaction time 1 hour, and thermal analysis measurements were performed. Ta.

The thermal analysis conditions were a temperature increase rate of 20° C./sin, and the reference standard sample was P b7. whose heat capacity is considered to be close to P b T i O*. L a7′; r O, was used. This is commercially available PbO, La, O,, T to
, are mixed at a predetermined molar ratio and subjected to solid phase reaction, and since the Curie point is below room temperature, there is no problem.

TG indicates the result of thermogravimetric analysis, and DTA indicates the result of differential thermal analysis. Furthermore, in differential thermal analysis, Exo represents exotherm and Endo represents endotherm.

From this convolution diagram, it can be seen that in the lead titanate microcrystals of the PY phase, adsorbed water is desorbed in the early stage, and pyrochlore is released at around 625°C.
Heat generation due to perovskite phase transition was observed.

〔Effect of the invention〕

As is clear from the above explanation, by selecting the pH and reaction temperature in wet synthesis in an alkaline aqueous solution, perovskite type, pyrochlore type, and acicular type with a new crystalline phase can be synthesized. However, in particular,
By setting the pH to 12.1 or more and the reaction temperature to 100°C to 190°C, pyrochlore phase lead titanate microcrystals are selectively synthesized.

The resulting pyrochlore phase lead titanate microcrystals have high purity and high stoichiometry, so they are less affected by impurities.
It is possible to improve the characteristics of piezoelectric elements, etc.

In addition, this lead titanate microcrystal has a particle size of 0.2μ or less, which is about an order of magnitude smaller than that obtained by conventional solid-phase reaction, and its activity is high, so it can be used as a base material for capacitor materials or as an additive. It is useful as a raw material for high-density sintered bodies because of its uniformity.

[Brief explanation of drawings]

FIG. 1 is a phase diagram according to pH and temperature in a wet synthesis method. Figure 2 is a characteristic diagram showing the temperature dependence of the PE phase and PY phase at pH 14.0, and Figure 3 is a graph of the lead titanate microcrystals of the PY phase synthesized at pH 13 and l2 reaction rate of 148°C. This is a diffraction X-ray spectrum, and No. 471 is a scanning electron micrograph of a lead titanate microcrystal in a PY phase. FIG. 5 is a characteristic diagram showing a phase transition from a PY phase to a PE phase. FIG. 6 shows the diffraction X-ray spectrum of the PE phase lead titanate microcrystals obtained, and FIG. 7 shows the diffraction X-ray spectrum of the perovskite-type lead titanate microcrystals obtained by the usual surface phase reaction method. Figure 7 shows the PE phase and PY phase at pH 12.9. FIG. 3 is a characteristic diagram showing the temperature dependence of the px phase.
It is a characteristic diagram showing the temperature dependence of the PX phase and the PY phase in H12,0. FIG. 5 is a characteristic diagram showing the results of thermal analysis of PY phase lead titanate microcrystals.

Claims (1)

[Claims] A soluble titanium compound or its hydrolysis product and a lead compound are mixed in an aqueous solution at a pH of 12.1 or higher and a temperature of 100°C.
A method for producing lead titanate microcrystals, characterized by carrying out the reaction at ~190°C.