Title: Perovskite titanium-type composite oxide particle and production process thereof
Abstract: A perovskite titanium-containing composite oxide particle having a composition represented by general formula (I), where the specific surface area is about 10 to about 200 m2/g, the specific surface area diameter D1 of primary particles defined by formula (II) is about 10 to about 100 nm, and a D2/D1 ratio of the average particle size D2 of secondary particles to D1 is about 1 to about 10:
- wherein M is at least one of Ca, Sr, Ba, Pb, or Mg,
- wherein ρ is the density of the particles, and S is the specific surface area of the particles is disclosed. The present invention has a small particle size and excellent dispersion properties, so that the particle is suitable for application to functional materials.
Patent Number: 6,893,623 Issued on 05/17/2005 to Ohmori, et al.
| Inventors:
|
Ohmori; Masahiro (Chiba, JP);
Kotera; Akihiko (Chiba, JP)
|
| Assignee:
|
Showa Denko Kabushiki Kaisha (Tokyo, JP)
|
| Appl. No.:
|
579708 |
| Filed:
|
May 26, 2000 |
| Current U.S. Class: |
423/598; 423/71 |
| Intern'l Class: |
C01G 023/00; C01G021/00; C01F011/00; C01F005/00 |
| Field of Search: |
423/598,71
501/108,123,134,136,137
|
References Cited [Referenced By]
U.S. Patent Documents
| 3292994 | Dec., 1966 | Kiss et al.
| |
| 3577487 | May., 1971 | Sanchez et al.
| |
| 3647364 | Mar., 1972 | Mazdiyasni et al.
| |
| 4764493 | Aug., 1988 | Lilley et al.
| |
| 4816072 | Mar., 1989 | Harley et al.
| |
| 4832939 | May., 1989 | Menashi et al.
| |
| 4898843 | Feb., 1990 | Matushita et al.
| |
| 4937213 | Jun., 1990 | Bernier et al.
| |
| 5204031 | Apr., 1993 | Watanabe et al.
| |
| 5219811 | Jun., 1993 | Enomoto et al.
| |
| 5242674 | Sep., 1993 | Bruno et al.
| |
| 5900223 | May., 1999 | Matijevic et al.
| |
| 6017504 | Jan., 2000 | Kaliaguine et al.
| |
| 6129903 | Oct., 2000 | Kerchner.
| |
| Foreign Patent Documents |
| 5-178617 | Jul., 1993 | JP.
| |
| 6-305729 | Jan., 1994 | JP.
| |
| 6-316414 | Nov., 1994 | JP.
| |
| 7-69635 | Mar., 1995 | JP.
| |
| 7-277710 | Oct., 1995 | JP.
| |
| 7-291607 | Nov., 1995 | JP.
| |
| 8-119745 | May., 1996 | JP.
| |
| 11-228139 | Aug., 1999 | JP.
| |
Other References
Search Report for WO00/35811 dated Mar. 7, 2000.
|
Primary Examiner: Bos; Steven
Attorney, Agent or Firm: Sughrue Mion, PLLC
Parent Case Text
This is an application for patent filed under 35 U.S.C. section 111(a) which
is entitled under 35 U.S.C. section 119(e)(1) to the benefit of the filing date
of provisional application Ser. No. 60/136,217 filed May 26, 1999 under 35 U.S.C.
section 111(b).
Claims
1. A perovskite titanium-containing composite oxide particle having a composition
represented by general formula (I), wherein the specific surface area is 28 to
about 200 m
2/g, the specific surface area diameter D
1 of
primary particles defined by formula (II) is about 10 to 50 nm, and a D
2/D
1
ratio of the average particle size of secondary particles to D
1 is
about 1 to about 10:
wherein M is at least one of Ca, Sr, Ba, Pb, or Mg,
wherein ρ is the density of the particles, and S is the specific surface
area of the particles.
2. The perovskite titanium-containing composite oxide particle as claimed in
claim 1, wherein the particle is obtained by removing a dispersion medium from
a sol in which the perovskite titanium-containing composite oxide particle is dispersed,
wherein said sol is obtained by a process comprising the step of reacting a titanium
oxide particle comprising brookite crystalline form with a metal salt comprising
at least one of Ca, Sr, Ba, Pb, or Mg in a liquid phase.
3. The perovskite titanium-containing composite oxide particle as claimed in
claim 1, wherein M represents Sr.
4. The perovskite titanium-containing composite oxide particle as claimed in
claim 1, wherein the particle is obtained by removing a dispersion medium from
a sol in which the perovskite titanium-containing composite oxide particle is dispersed,
wherein said sol is obtained by a process comprising the step of reacting a titanium
oxide sol prepared by subjecting a titanium salt to hydrolysis in an acid solution
with a metal salt comprising at least one of Ca, Sr, Ba, Pb, or Mg in a liquid phase.
5. The perovskite titanium-containing composite oxide particle according to claim
1, consisting of a composition represented by general formula (I).
6. The perovskite titanium-containing composite oxide particle according to claim
2, consisting of a composition represented by general formula (I).
7. The perovskite titanium-containing composite oxide particle according to claim
4, consisting of a composition represented by general formula (I).
Description
FIELD OF THE INVENTION
The present invention relates to a titanium-containing composite oxide particle,
a sol of the above-mentioned composite oxide particle, a production process thereof,
and a thin film made therefrom. More specifically, the present invention is to
provide a perovskite titanium-containing composite oxide particle and a sol thereof,
with a small particle size and excellent dispersion properties.
BACKGROUND OF THE INVENTION
A perovskite titanium-containing composite oxide represented by barium titanate
is widely used for functional materials such as a dielectric material, a laminated
ceramic capacitor, a piezoelectric material, and a memory. In recent years, in
line with the trend toward small-size, light-weight electronic devices, it has
been desired to develop a method for obtaining a perovskite titanium-containing
composite oxide particle with a smaller particle size and more noticeable dispersion
properties at low cost. Further, such a titanium-containing composite oxide particle
that has the above-mentioned characteristics is expected to be applied to a photocatalyst.
The perovskite titanium-containing composite oxide is obtained by a solidus method
of mixing finely-divided particles of the raw materials such as an oxide and a
carbonate in a ball mill, and carrying out the reaction at a high temperature of
over about 800° C.; by an oxalate method of first preparing a composite salt
of oxalic acid, followed by thermal decomposition; by an alkoxide method of subjecting
a raw material such as a metal alkoxide to hydrolysis to obtain a precursor; or
by a hydrothermal synthesis method of allowing the raw materials to react at high
temperature under high pressure to obtain a precursor. In addition to the above,
the perovskite titanium-containing composite oxide can be also obtained by a method
of preparing titanium oxide or the precursor thereof, dispersing the titanium oxide
or precursor thereof in a solvent, and making a composite of titanium oxide or
precursor thereof in the solution by the addition of a predetermined element (Japanese
Laid-Open Patent Application 8-119633), and a method of employing titanium tetrachloride
or titanium sulfate as the raw material (Japanese Laid-Open Patent Application 59-39726).
However, the solidus method produces particles with a large particle size,
lacking uniformity, which are not suitable for the functional materials such as
the dielectric material and piezoelectric material although the low manufacturing
cost is industrially advantageous. The particle size obtained by the oxalate method
is 0.2 to 0.5 μm, which is smaller than that by the solidus method, but not
sufficiently small. The alkoxide method can produce particles with a particle size
of 20 nm to 30 nm, but the manufacturing cost is high because organic materials
are used as the raw materials. In addition, the hydrothermal synthesis method has
a problem in that the cost becomes high because special facilities are required
to carry out the synthesis under conditions of high temperature and high pressure.
Unless the dispersion properties of the particles are sufficient, the particles
will aggregate in a solvent even if they can be prepared with a small particle
size by any of the above-mentioned methods. The result is that when the particles
are molded and sintered to prepare a product of the functional material such as
a dielectric material or piezoelectric material, the particles cannot exhibit satisfactory
characteristics. Furthermore, of the perovskite titanium-containing composite oxide
particles, SrTiO
3 is particularly expected because of its photocatalytic
activation performance. However, to obtain small particles is not easy. Namely,
it is hard to inexpensively obtain particles with excellent photocatalytic activation performance.
The object of the present invention is to provide a perovskite titanium-containing
composite oxide particle with a small particle size and excellent dispersion properties,
and a sol thereof at low cost.
SUMMARY OF THE INVENTION
The present invention is directed to a perovskite titanium-containing composite
oxide particle having a composition represented by general formula (I), wherein
the specific surface area is about 10 to about 200 m
2/g, the specific
surface area diameter D
1 of the primary particles as defined by formula
(II) is about 10 to about 100 nm, and the ratio D
2/D
1 of
an average particle size D
2 of the secondary particles to D
1 is
about 1 to about 10:
(wherein M is at least one of Ca, Sr, Ba, Pb, or Mg)
(wherein ρ is the density of the particles, and S is the specific
surface area of the particles.)
The perovskite titanium-containing composite oxide particle of the present invention
is most suitable for functional materials such as a dielectric material and a piezoelectric
material, a memory, and a photocatalyst because the particle size is very small
and the dispersion properties thereof are excellent.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a photomicrograph taken by a transmission electron microscope, which
shows titanium oxide particles in a titanium oxide sol obtained in Example 1.
FIG. 2 is a photomicrograph taken by a transmission electron microscope, which
shows titanium oxide particles in a titanium oxide sol obtained in Comparative
Example 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be explained in detail.
The perovskite titanium-containing composite oxide particle of the present invention
has a composition represented by general formula (I), with the specific surface
area being about 10 to about 200 m
2/g, the specific surface area diameter
D
1 of the primary particles being about 10 to about 100 nm, and the
ratio D
2/D
1 of an average particle size D
2 of
the secondary particle to D
1 being about 1 to about 10.
The specific surface area diameter D
1 of the primary particles is
obtained in accordance with formula (II), wherein ρ is the density of the
particles, and S is the specific surface area of the particles measured by the
BET method. The average particle size D
2 of the secondary particles
is a value measured with a particle size distribution analyzer after dispersing
titanium-containing composite oxide particles in a solvent. The particle size distribution
is usually measured by centrifugal sedimentation, electrozone (by use of the Coulter
Counter®), or a light scattering measurement method. The light scattering
measurement method is preferable because the sensitivity is high. The smaller the
calculated value of D
2/D
1, the better the dispersion properties
of the particles. Theoretically, the value of D
2/D
1 cannot
be less than 1 when the particles are spherical. On the other hand, a value of
more than about 10 is not preferable because the primary particles show poor dispersion
properties and tend to aggregate.
In the perovskite titanium-containing composite oxide particles of the present
invention, the value of D
2/D
1 is about 1 to about 10, which
means excellent dispersion properties of the primary particles. Further, since
these particles can show sufficient transparency when formed into a thin film,
the particles can be applied to a memory and a photocatalyst. In particular, SrTiO
3
of general formula (I) wherein M represents Sr is suitable for the photocatalyst.
The above-mentioned perovskite titanium-containing composite oxide is available
not only in the form of particles, but also in the form of a sol in which these
particles are dispersed.
(wherein M is at least one of Ca, Sr, Ba, Pb, or Mg.)
Next, the production process according to the present invention will be explained.
With respect to the titanium oxide particle with a brookite crystalline form
for use in the present invention, titanium oxide with a brookite crystalline form
may be used alone, or be used in combination with a rutile titanium oxide and an
anatase titanium oxide so long as a brookite crystalline form is contained. When
the rutile titanium oxide and the anatase titanium oxide are contained, the ratio
of the brookite titanium oxide in the entire titanium oxide is not particularly
limited, but preferably in the range of about 1 to about 100 wt. %, more preferably
about 10 to about 100 wt. %, and further preferably about 50 to about 100 wt. %.
This is because crystalline titanium oxide particles can be dispersed in the form
of separate single particles in a liquid phase more easily and can exhibit more
significant dispersion properties than amorphous titanium oxide particles. In particular,
the brookite titanium oxide is preferred to the titanium oxide of a rutile crystalline
form and anatase crystalline form in terms of dispersion properties. The reason
why the dispersion properties of the brookite titanium oxide are superior is not
clear, but is believed to be related to the fact that the ζ-potential (electrokinetic
potential) of the brookite crystal is higher than that of the rutile crystal or
anatase crystal.
There are methods of obtaining titanium oxide particles comprising a brookite
crystalline form, for example, a method of subjecting anatase titanium oxide particles
to thermal treatment in a vapor phase, and a manufacturing method in a liquid phase
whereby a titanium oxide sol of dispersed titanium oxide particles is prepared
by neutralizing a solution of titanium tetrachloride, titanium trichloride, titanium
alkoxide, or titanium sulfate or subjecting the above-mentioned solution to hydrolysis.
Of the above-mentioned production processes, the process is not particularly
limited
as long as titanium oxide particles comprising the brookite crystalline form can
be obtained. However, the method of obtaining a titanium oxide sol by subjecting
a titanium salt to hydrolysis in an acid solution, which was previously invented
by the inventors of the present invention, is preferable. This is because when
the titanium oxide particles obtained by the above-mentioned method are made into
a titanium-containing composite oxide, a perovskite titanium oxide particle with
a small particle size and excellent dispersion properties can be obtained. More
specifically, preferable methods include adding titanium tetrachloride to hot water
at 75 to 100° C. to carry out hydrolysis of the titanium tetrachloride at
a temperature which is more than or equal to 75° C., and less than or equal
to the boiling point of the solution, with the concentration of the chlorine ions
being controlled, thereby obtaining titanium oxide particles with a brookite crystalline
structure in the form of a titanium oxide sot (Japanese Patent Application 9-231172),
and adding titanium tetrachloride to hot water at 75 to 100° C. to carry out
hydrolysis of the titanium tetrachloride in the presence of nitrate ions and/or
sulfate ions at a temperature which is more than or equal to 75° C., and less
than or equal to the boiling point of the solution, with the total concentration
of chlorine ions, nitrate ions, and sulfate ions being controlled, thereby obtaining
titanium oxide particles with a brookite crystalline structure in the form of a
titanium oxide sot (Japanese Patent Application 10-132195).
The particle size of the thus obtained titanium oxide particles with a brookite
crystalline form is usually in the range of about 5 to about 50 nm when determined
from the specific surface area of the primary particles. When the specific surface
area diameter of the primary particles exceeds about 50 nm, the particle size of
the titanium-containing composite oxide particles made from the above-mentioned
raw material particles becomes so large that those composite oxide particles are
not suitable for functional materials such as a dielectric material and a piezoelectric
material, a memory, and a photocatalyst. When the particle size is less than about
5 nm, handling of the titanium oxide particles becomes difficult in the process
of manufacturing thereof.
To produce a sot in which the perovskite titanium-containing composite oxide
particles
of the present invention are dispersed, a titanium oxide sol obtained by subjecting
a titanium salt to hydrolysis in an acid solution may be used instead of the titanium
oxide particles with a brookite crystalline form. There is no limitation to the
crystalline form of titanium oxide particles in the titanium oxide sol as long
as the titanium oxide sol is obtained by carrying out the hydrolysis of the titanium
salt in an acid solution.
When the titanium salt such as titanium tetrachloride or titanium sulfate is
subjected to hydrolysis in an acid solution, the reaction rate is reduced as compared
with the case where the hydrolysis is carried out in a neutral or alkaline solution.
Therefore, the particles can be formed in separate single particles, thereby obtaining
a titanium oxide sol with excellent dispersion properties. Further, since anionic
ions such as chlorine ions and sulfate ions are not readily trapped in the inside
of the generated titanium oxide particles, it is possible to restrain the inclusion
of anionic ions in the particles in the course of production of the titanium-containing
composite oxide particles. In addition, when a titanium salt is subjected to hydrolysis
in a neutral or alkaline solution, the reaction rate is increased to cause considerable
nucleation in the initial stage. The result is that the obtained titanium oxide
sol shows poor dispersion properties although the particle size is small, and consequently
the particles tend to form a cloud-like aggregate. When such a titanium oxide sol
is made into a sol of titanium-containing composite oxide particles, the dispersion
properties become poor although the particle size of the particles in the sol is
small. In addition, the anionic ions are easily tapped in the inside of the titanium
oxide particles in the sol. The removal of these anionic ions will thus become
difficult in the subsequent processes.
The method of obtaining a titanium oxide sol by subjecting a titanium salt to
hydrolysis in an acid solution is not particularly limited as long as the solution
can be maintained acid. The method of subjecting titanium tetrachloride serving
as a raw material to hydrolysis in a reaction vessel equipped with a reflux condenser,
the solution being maintained acid by inhibiting the chlorine atom generated in
the course of hydrolysis from escaping, which method was previously invented by
the inventors of the present invention (Japanese Patent Application 8-230776) is preferable.
It is preferable that the acid solution of a titanium salt serving as the raw
material have a concentration of about 0.01 to about 5 mol/L. When the concentration
exceeds about 5 mol/L, the reaction rate of the hydrolysis is accelerated, and
a titanium oxide sol with a large particle size and poor dispersion properties
is obtained. When the concentration is less than about 0.01 mol/L, the density
of the titanium oxide particles in the obtained sol is decreased, which lowers
the productivity.
A metal salt for use in the present invention which comprises at least one of
Ca,
Sr, Ba, Pb, or Mg is not particularly limited as long as any of the above-mentioned
metals are contained. It is preferable that such a metal salt be water-soluble.
Usually, a nitrate, an acetate, or a chloride salt is usable. These metal salts
may be used alone, or two or more metal salts may be mixed in an arbitrary ratio.
More specifically, when the metal salt contains Ba, barium chloride, barium nitrate,
and barium acetate are usable; and when the metal salt contains Sr, strontium chloride,
strontium nitrate, and strontium acetate are usable.
The method for producing a sol in which the perovskite titanium-containing composite
oxide particles are dispersed according to the present invention comprises the
step of allowing the titanium oxide particles with a brookite crystalline form,
or the titanium oxide sol obtained by subjecting a titanium salt to hydrolysis
in an acid solution to react with a metal salt comprising at least one of Ca, Sr,
Ba, Pb, or Mg in a liquid phase. Although the reaction conditions are not particularly
limited, in general, it is preferable to carry out the reaction in an alkaline
solution by employing an alkaline liquid phase. It is preferable that the pH of
the solution be about 13.0 or more, and more preferably 14.0 or more. When the
pH is set to 14.0 or more, the particle size of the titanium-containing composite
oxide particles dispersed in the sol can be decreased.
To make the liquid phase alkaline, an alkaline compound is added to the liquid
phase. When hydroxides of alkali metals, such as lithium hydroxide, sodium hydroxide,
and potassium hydroxide are used as the alkaline compounds, the alkali metals may
remain in the titanium-containing composite oxide particles, and there is a risk
that when the titanium-containing composite oxide particles are molded and sintered
to produce functional materials such as a dielectric material and a piezoelectric
material, the characteristics of the obtained functional materials will deteriorate.
In light of the above, it is preferable to use an organic alkaline compound such
as tetramethylammonium hydroxide as the alkaline compound.
It is preferable that the reaction solution be controlled to have the titanium
oxide particle concentration in the range of about 0.1 to about 5 mol/L, and the
concentration of the metal salt having a metal represented by M in the range of
about 0.1 to about 5 mol/L when calculated in terms of the concentration of the
metallic oxide.
The thus prepared alkaline solution is usually heated to about 40 to about 120°
C., preferably about 80 to about 120° C., with stirring at atmospheric pressure
to carry out the reaction. The reaction time is usually about one hour or more,
preferably about 4 hours or more. Thereafter, impure ions are removed from the
slurry obtained after completion of the reaction by various methods such as electrodialysis,
ion exchange, water washing, and osmosis using a membrane, whereby the pH is controlled
to about 10 or less. Then, with the addition of water and a water-soluble organic
solvent to the solution, the solid concentration in the solution is controlled
to a predetermined concentration. At that time, a dispersant and a film-forming
assistant may be added to the solution. Polyphosphoric acid, hexametaphosphoric
acid, and dodecylbenzenesulfonic acid can be employed as the dispersant; and alcohols
such as butyl alcohol, and water-soluble polymeric materials such as poly(vinyl
alcohol) and methyl cellulose can be employed as the film-forming assistant.
The perovskite titanium-containing composite oxide particles can be obtained
by removing the dispersion medium from the sol obtained in the above-mentioned
manner. The dispersion medium is commonly removed from the sol by filtration, centrifugal
separation, or drying. In this case, the solid may be washed with water when necessary.
Further, the obtained perovskite titanium-containing composite oxide particles
may be calcined.
The drying operation is usually carried out at a temperature ranging from room
temperature to about 150° C. for about one to about 24 hours. The drying atmosphere
is not particularly limited, but the drying operation is carried out in air or
under reduced pressure. Calcining is generally performed at about 300 to about
1000° C. in order to improve the crystallizability of the titanium-containing
composite oxide, and, at the same time, to remove the remaining impurities, for
example, anionic ions such as chlorine ions, sulfate ions, and phosphate ions,
and alkaline compounds such as tetramethylammonium hydroxide. The calcining atmosphere
is not particularly limited, and the calcining is generally carried out in air.
The application of the sol of perovskite titanium-containing composite oxide
particles according to the present invention is not particularly limited. Since
the titanium-containing composite oxide particles dispersed in this sol show a
small particle size and excellent dispersion properties, the sol is favorably used
for the formation of a thin film of a titanium-containing composite oxide. For
the formation of a thin film using the sol, the solid content in the sol is first
adjusted by the addition of water and a water-soluble organic solvent, if necessary,
to the sol in which the perovskite titanium-containing composite oxide particles
are dispersed. The sol for which the solid content has been thus adjusted is then
coated on a base such as ceramic, metal, glass, plastic, paper, or wood. The sol
on the base is dried to eliminate the dispersion medium from the sol, and is subjected
to sintering when necessary, thereby forming a thin film of the titanium-containing
composite oxide. Thus, a thin-film laminated product in which a thin film is overlaid
on the base can be obtained. The transparency of the thin film thus obtained is
particularly so excellent that the thin film is most applicable to the functional
materials such as dielectric materials and piezoelectric materials, memory, and
photocatalyst. In particular, the thin film of SrTiO
3 is suitable for
the photocatalyst.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention will now be explained more specifically with reference
to examples.
EXAMPLE 1
An aqueous solution of titanium tetrachloride (with a purity of 99.9%) at a concentration
of 0.25 mol/L was placed in a reaction vessel equipped with a reflux condenser
and heated to a temperature close to the boiling point, with the acid solution
being maintained by restraining chlorine ions from escaping. The titanium tetrachloride
was subjected to hydrolysis with maintained at the above-mentioned temperature
for 60 minutes, thereby obtaining a titanium oxide sol. A photomicrograph of the
obtained titanium oxide sol, taken by a transmission electron microscope, is shown
in FIG. 1. FIG. 1 indicates a monodisperse sol containing particles with
a particle size of about 15 nm. This sol was thickened by sedimentation. In 320
g of a sol thus thickened to have a concentration of titanium oxide of 10 wt. %,
97.7 g of barium chloride•2 hydrate (made by Kokusan Chemical Works, Ltd.)
was dissolved. With the addition of 600 g of an aqueous solution containing 20
wt. % of tetramethylammonium hydroxide, the resultant mixture was adjusted to pH
14, followed by stirring for one hour. Thereafter, the slurry was heated at 110°
C. to carry out the reaction for 4 hours with the temperature being maintained.
The sol thus obtained was washed with water, filtered, and dried at 150° C.
for 12 hours, thereby obtaining finely-divided particles.
The above prepared particles were examined by X-ray diffraction using an X-ray
diffraction measuring instrument ("D-MAX-RB", made by Rigaku Corporation) to determine
that the obtained particles were BaTiO
3 of a perovskite cubic system.
The specific surface area S obtained by the BET method was 34 m
2/g,
and the specific surface area diameter D
1, which was calculated in accordance
with formula (II), was 0.03 μm. The average particle size D
2 was
0.21 μm when measured using a light scattering particle size distribution
analyzer ("ELS-8000", made by OTSUKA ELECTRONICS Co., Ltd.) under such conditions
that these particles were dispersed in water. The D
2/D
1 ratio
was 7.0.
EXAMPLE 2
A monodisperse titanium oxide sol comprising particles with a particle size of
about 8 nm was prepared by the same method as in Example 1. Using the above-mentioned
sol, finely-divided particles of BaTiO
3 with a perovskite cubic system
were obtained in the same manner as in Example 1. When the finely-divided particles
thus obtained were examined in the same manner as in Example 1, the specific surface
area S was 46 m
2/g, the specific surface area diameter D
1 was
0.02 μm, the average particle size D
2 was 0.19 μm, and the
D
2/D
1 ratio was 9.5.
EXAMPLE 3
A monodisperse titanium oxide sol comprising particles with a particle size of
about 10 nm was prepared by the same method as in Example 1 except that the titanium
tetrachloride was replaced by titanium sulfate, and therefore sulfate ions, not
chlorine ions, were restrained from escaping. Using the above-mentioned sol, finely-divided
particles of BaTiO
3 with a perovskite cubic system were obtained in
the same manner as in Example 1. When the finely-divided particles thus obtained
were examined in the same manner as in Example 1, the specific surface area S was
40 m
2/g, the specific surface area diameter D
1 was 0.03 μm,
the average particle size D
2 was 0.22 μm, and the D
2/D
1
ratio was 7.3.
EXAMPLE 4
A monodisperse titanium oxide sol comprising particles with a particle size of
about 15 nm was prepared by the same method as in Example 1. Using the above-mentioned
sol, finely-divided particles of SrTiO
3 with a perovskite cubic system
were obtained in the same manner as in Example 1 except that 106.7 g of strontium
chloride•6 hydrate was used instead of the barium chloride. When the finely-divided
particles thus obtained were examined in the same manner as in Example 1, the specific
surface area S was 28 m
2/g, the specific surface area diameter
D
1 was 0.05 μm, the average particle size D
2 was 0.10
μm, and the D
2/D
1 ratio was 2.
EXAMPLE 5
A monodisperse titanium oxide sol comprising particles with a particle size of
about 15 nm was prepared by the same method as in Example 1. Using the above-mentioned
sol, a strontium titanate sol with pH 14 was obtained through the same reaction
as mentioned in Example 1 except that 106.7 g of strontium chloride•6 hydrate
was used instead of the barium chloride. The concentration of strontium titanate
in the sol was 7 wt. %.
The thus obtained sol was cooled, and adjusted to pH 8 in such a manner that
the remaining ammonium salt and chlorine were removed from the sol by electrodialysis.
In the electrodialysis, a commercially available membrane "Selemion ME-0", made
by Asahi Glass Co., Ltd. was adapted.
Part of the above prepared sol was dried in a vacuum dryer so that finely-divided
particles of SrTiO
3 with a perovskite cubic system were obtained. When
the finely-divided particles thus obtained were examined in the same manner as
in Example 1, the specific surface area S was 29 m
2/g, the specific
surface area diameter D
1 was 0.05 μm, the average particle size
D
2 was 0.08 μm, and the D
2/D
1 ratio was 1.6.
Ethyl alcohol was added to the rest of the strontium titanate sol containing
7 wt. % of strontium titanate to a strontium titanate concentration of 5 wt. %.
Thereafter, poly(vinyl alcohol) serving as a film-forming assistant was added to
the sol in an amount of 500 ppm of the total weight of the sol.
The thus obtained sol for film formation was coated on a glass plate by dip coating
and dried, and further calcined at 500° C. in air for one hour, thereby forming
a thin film of strontium titanate on the glass base. Thus, a thin-film laminated
product was obtained. The thickness of the thin film on the glass base was 0.3
μm. When this thin film was observed under a scanning electron microscope
(SEM), the particle size of the strontium titanate in the thin film was found to
be 0.043 μm.
Furthermore, the transparency and the photocatalytic power of the resulting
thin-film laminated product were evaluated by the following methods. The results
are shown in TABLE 1. The transparency was measured in accordance with the Japanese
Industrial Standard JIS K6718 using a commercially available hazemeter made by
Tokyo Denshoku Gijutsu Center, and evaluated on three levels. With respect to the
photocatalytic power, several drops of red ink were applied to the strontium titanate
thin film of the thin-film laminated product, and the red-ink applied portion was
exposed to ultraviolet light for 30 minutes, using black light with an ultraviolet
intensity of 2.1 mW/cm
2 at a wavelength of 365 nm. The degree of color
fading was visually inspected, and evaluated on three levels.
| |
TABLE 1 |
| |
| |
Transparency |
Photocatalytic Power |
| |
| |
| |
Example 5 |
⊚ |
⊚ |
| |
Comparative |
X |
X |
| |
Example 4 |
| |
| |
Symbols in TABLE 1 denote the following: |
| |
Transparency |
| |
⊚—haze of less than 2.0%. |
| |
◯—haze of 2.0% or more, and less than 5.0%. |
| |
X—haze of 5.0% or more. |
| |
Photocatalytic power |
| |
⊚—sufficient color fading. |
| |
◯—locally no color fading. |
| |
X—no color fading. |
COMPARATIVE EXAMPLE 1
A titanium oxide sol was prepared by the same method as in Example 1 except that
the aqueous solution of titanium tetrachloride (with a purity of 99.9%) at a concentration
of 2.5 mol/L was adjusted to pH 7 by the addition of ammonium hydroxide, and the
resultant mixture was placed in a reaction vessel equipped with a reflux condenser.
A photomicrograph of the obtained titanium oxide sol, taken by a transmission electron
microscope, is shown in FIG. 2. FIG. 2 indicates that the primary particles
in the sol are aggregated particles with a particle size of about 5 nm. Using the
above-mentioned sol, finely-divided particles of BaTiO
3 with a perovskite
cubic system were obtained in the same manner as in Example 1. When the finely-divided
particles thus obtained were examined in the same manner as in Example 1, the specific
surface area S was 58 m
2/g, the specific surface area diameter D
1
was 0.02 μm, the average particle size D
2 was 0.25 μm,
and the D
2/D
1 ratio was 12.5.
COMPARATIVE EXAMPLE 2
Finely-divided particles of BaTiO
3 with a perovskite cubic
system were obtained in the same manner as in Example 1 except that 320 g of an
aqueous solution containing 10 wt. % of titanium oxide was employed, which solution
was prepared by sufficiently dispersing a commercially available titanium oxide
sol ("F-4", made by Showa Titanium Co., Ltd., of which the specific surface area
diameter was 28 nm) by the application of ultrasonic vibration. When the finely-divided
particles thus obtained were examined in the same manner as in Example 1, the specific
surface area S was 28 m
2/g, the specific surface area diameter D
1
was 0.04 μm, the average particle size D
2 was 0.44 μm,
and the D
2/D
1 ratio was 11.0.
COMPARATIVE EXAMPLE 3
To an aqueous solution of titanium tetrachloride (with a purity of 99.9%) at a
concentration of 2.5 mol/L, barium nitrate was added in an amount equimolar with
the titanium in the aqueous solution, and potassium hydroxide was further added
so that the solution was adjusted to pH 13.5. The solution was heated to a temperature
close to the boiling point with stirring, and thereafter maintained at the above-mentioned
temperature for 4 hours to carry out the reaction. A slurry thus obtained was washed
with water, filtered, and dried at 150° C. for 12 hours, whereby finely-divided
particles of BaTiO
3 with a perovskite cubic system were obtained. When
the finely-divided particles thus obtained were examined in the same manner as
in Example 1, the specific surface area S was 28 m
2/g, the specific
surface area diameter D
1 was 0.04 μm, the average particle size
D
2 was 0.45 μm, and the D
2/D
1 ratio was 11.3.
COMPARATIVE EXAMPLE 4
Water and ethyl alcohol were added to a commercially available strontium titanate
("ST-HP-1", made by Kyoritsu Ceramic Materials Co., Ltd., of which the specific
surface area was 20 m
2/g, D
1 was 0.1 μm, the average
particle size D
2 was 1.5 μm, and the D
1/D
2 ratio
was 15) so that the concentration of strontium titanate was adjusted to 5 wt. %,
and poly(vinyl alcohol) serving as a film-forming assistant was added to the sol
in an amount of 500 ppm with respect to the total weight of the sol in a similar
manner to that in Example 5.
Using the thus obtained sol for film formation, a thin film of strontium titanate
was formed on the glass base by the same method as in Example 5. Thus, a thin-film
laminated product was obtained. The thickness of the thin film on the glass base
was 3 μm. When this thin film was observed under a scanning electron microscope
(SEM), the particle size of the strontium titanate in the thin film was found to
be 1.5 μm.
Furthermore, the transparency and the photocatalytic power of the resulting
thin-film laminated product were examined in the same manner as in Example 5. The
results are shown in TABLE 1.
As previously mentioned, the perovskite titanium-containing composite oxide particle
and the sol thereof according to the present invention showed a small particle
size and excellent dispersion properties. It was possible to produce such a particle
and a sol using an inexpensive raw material such as titanium tetrachloride or titanium
sulfate. Further, when the titanium-containing composite oxide was strontium titanate,
the titanium-containing composite oxide was provided with high photocatalytic power.
INDUSTRIAL APPLICABILITY
As previously explained, the perovskite titanium-containing composite oxide particle
according to the present invention has a specific surface area of about 10 to about
200 m
2/g, a specific surface area diameter D
1 of the primary
particles of about 10 to about 100 nm, and a D
2/D
1 ratio
of the average particle size D
2 of the secondary particles to D
1
of about 1 to about 10. The particle size is small and the dispersion properties
are excellent, so that the perovskite titanium-containing composite oxide particle
is very suitable for the application to functional materials such as a dielectric
material and a piezoelectric material, a memory, and a photocatalyst.
*
