Title: SOL solution containing magnesium acetate tetrahydrate dehydrate colloidal particles and film formation method
Abstract: A method for forming a protective layer for a dielectric material in an alternating current type plasma display is provided. In this connection, a coating sol solution is provided which can form a protective layer on a large area substrate without the need to introduce any expensive equipment, the protective layer thus formed being excellent in properties such as strength, adhesion, transparency, and protective properties and capable of being formed by a sol-gel process without use of the conventional vacuum process. The sol solution comprises a dispersion of a precursor to magnesium oxide in a specific form. A method for film formation, using this solution is also provided.
Patent Number: 6,893,313 Issued on 05/17/2005 to Mitamura, et al.
| Inventors:
|
Mitamura; Satoshi (Tokyo-To, JP);
Yoshihara; Toshio (Tokyo-To, JP);
Takahashi; Nobuko (Tokyo-To, JP)
|
| Assignee:
|
Dai Nippon Printing Co., Ltd. (JP)
|
| Appl. No.:
|
164415 |
| Filed:
|
June 10, 2002 |
Foreign Application Priority Data
| Oct 09, 1995[JP] | 8-286326 |
| Oct 09, 1995[JP] | 8-286327 |
| May 23, 1996[JP] | 9-150485 |
| Current U.S. Class: |
445/59; 106/287.1; 427/66; 427/68; 427/372.2; 428/690 |
| Intern'l Class: |
H01J 017/49; B01F003/12 |
| Field of Search: |
445/24,25,50,51
313/582-587
427/66,68,380
106/287.1,287.162,287.17,287.19
|
References Cited [Referenced By]
U.S. Patent Documents
Other References
The Condensed Chemical Dictionary, 6th Edition, Edited by A. Rose
et al., Reinhold Publ. Corp., New York, p. 1187 [QD 5 C5 1961 C. 38].
|
Primary Examiner: Williams; Joseph
Attorney, Agent or Firm: Parkhurst & Wendel, L.L.P.
Parent Case Text
This is a Division of application Ser. No. 09/653,587 filed Aug. 31, 2000 now
U.S. Pat. No. 6,457,506, which in turn is a division of U.S. Ser. No. 08/726,173
filed Oct. 4, 1996, now U.S. Pat. No. 6,149,967.
Claims
1. A sol solution comprising a dispersion of a partial or complete dehydrate
of magnesium acetate tetrahydrate as colloidal particles in a medium composed mainly
of water.
2. The sol solution according to claim 1, wherein at least part of magnesium
acetate on the surface of the colloidal particle is in the form of magnesium hydroxide.
3. The sol solution according to claim 1, wherein the colloidal particles have
a diameter of 3 to 300 nm.
4. The sol solution according to claim 1, which further comprises a water-soluble
polymer dispersion stabilizer.
5. The sol solution according to claim 1, which further comprises an ethylene
glycol compound.
6. A method for film formation, comprising the steps of: coating the surface
of an object with the sol solution according to any one of claim 1; drying the
coating; and heating the dried coating to form a magnesium oxide-containing layer.
7. The method according to claim 6, wherein the object is a dielectric layer
in an alternating current type plasma display.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a sol solution and a method for film formation,
and more particularly to a sol solution useful for the formation of a protective
layer for a dielectric layer in an alternating current type plasma display and
a method for film formation using the sol solution.
For optical components, electronic and electrical components, magnetic material
components and the like, layers having various functions are formed on the substrate,
and, if necessary, a protective film is provided on the surface of these layers
for protection purposes. High film strength and large adhesion to the above functional
layers are general properties required of such protective films.
In recent years, research and development of flat panel displays as an alternative
to CRT have been energetically conducted. Among them, the so-called "plasma display"
which utilizes luminous phenomenon accompanying discharge in a display is classified,
according to the structure of the electrode (mainly ITO), roughly into a direct
current type, wherein metallic electrodes are exposed to a discharge space, and
an alternating current type wherein electrodes are covered with a dielectric layer.
In the latter alternating current type plasma display, those produced by both a
thin-film process using a vacuum system and a thick-film process using screen printing
have begun to be put to practical use.
When the use of the plasma display in a color television with a large screen
size is contemplated, the plasma display should have a memory function from the
viewpoint of increasing the brightness. In this respect, the alternating current
type plasma display inherently has a memory function by virtue of charges accumulated
in the protective layer provided on the dielectric layer and, hence, is considered
to be able to cope with a demand for an increase in screen size. Magnesium oxide
having high secondary electron emission efficiency and excellent sputtering property
has been used as a material for the protective layer. At the present time, a service
life of 15000 hr has been achieved for a full-color, alternating current type plasma
display, and a panel having a diagonal distance of 21 in. has been put on the market.
Methods for forming the protective layer include thin-film processes, such
as EB deposition, sputtering, and CVD (Japanese Patent Publication Nos. 42579/1985
and 59221/1988), and thick-film processes, such as one which comprises spray-coating
basic magnesium carbonate as a starting material for magnesium oxide on a substrate
to form a thick coating and firing the coating to convert the basic magnesium carbonate
to a metal oxide (Japanese Patent Publication No. 13983/1982) and one which comprises
dispersing a fine powder of magnesium oxide in a liquid binder which, upon firing,
can be converted to an oxide, thereby forming a magnesium oxide film (Japanese
Patent Publication No. 283020/1994).
Among the above methods, those using a vacuum process, such as EB deposition,
sputtering, and CVD, are disadvantageous in that it is difficult to accommodate
a large panel substrate, like a plasma display, in a vacuum chamber, posing problems
of cost of equipment and productivity when an increased screen size is assumed.
Coating is a simple method and, hence, has been extensively and intensively
studied in the art. At the present time, however, no satisfactory performance could
have been attained yet. The reason for this is as follows. In the coating, a magnesium
oxide printing paste containing magnesium oxide particles is used for printing
a magnesium oxide protective layer for alternative current type PDP. In order that
the protective layer has sputtering resistance, magnesium oxide should be in the
form of homogeneous particles having a diameter of 30 to 300 nm and, at the same
time, the particles should be homogeneously dispersed in a binder. However, fine
particles of magnesium oxide are likely to agglomerate and very difficult to be
homogeneously dispersed in the binder. For this reason, in the conventional commercially
available printing paste, the diameter of incorporated magnesium oxide particles
per se is large and the viscosity of the paste per se is high, making it difficult
to reduce the thickness of the protective layer. Consequently, no highly even protective
film can be formed. Further, the conventional printing paste is disadvantageous
in that the conventional heat treatment process (600° C. or below) cannot
offer satisfactory film strength, adhesion and other properties and heat treatment
causes cracking of the film. The coatability of the paste is also unsatisfactory.
Furthermore, the magnesium oxide particles incorporated are large, and the viscosity
of the paste per se is high, making it difficult to reduce the layer thickness.
This in turn poses a problem that a demand for minimized firing voltage and drive
voltage cannot be met.
In order to solve the above problems involved in the coating method, the present
inventors have previously proposed a method, using the so-called sol-gel process,
which comprises the steps of: coating a magnesium hydroxide sol, prepared by hydrolyzing
a magnesium oxide compound, onto a substrate; drying and firing the coating to
form a magnesium oxide-containing thin film (Japanese Patent Laid-Open No. 111177/1996).
This method, however, is disadvantageous in that the adhesion of the magnesium
hydroxide sol to the substrate is so poor that a part of the resultant magnesium
oxide layer is easily separated from the substrate and the film thickness is uneven.
Therefore, the above method is substantially ineffective in improving the firing
voltage and the drive voltage (power consumption). Further, heat treatment of the
magnesium hydroxide sol at a conventional temperature (600° C. or below) does
not accelerate the crystallization of the resultant magnesium oxide. Therefore,
the film thus obtained has a low crystallinity and, hence, unsatisfactory film
strength, causing a problem that properties as a protective film cannot be satisfactorily exhibited.
On the other hand, the use of an organometallic compound, such as a magnesium
alkoxide, as a starting compound for the magnesium hydroxide sol can offer magnesium
oxide having relatively good adhesion to the substrate and uniformity in the film.
Magnesium alkoxides and the like involve a problem of cost, because they are very
expensive, and, in addition, a problem associated with handling because the reactivity
of the magnesium alkoxides is so high that the control of the reaction is difficult
and the life of the resultant magnesium hydroxide is short.
Under the above circumstances, the present invention has been made, and an
object of the present invention is to solve the above problems of the prior art
and to provide a sol solution which can form a film on a panel having a large area
without the need to use expensive equipment such as required in the vacuum process,
the formed film having excellent film strength, adhesion, transparent, protective
properties and other properties and being usable as a protective layer which can
lower the firing voltage and the drive voltage (power consumption), and to provide
a method for film formation using said sol solution.
DISCLOSURE OF INVENTION
The first invention relates to a sol solution comprising a dispersion of fine
particles of magnesium hydroxide bonded to a polyhydric alcohol or a derivative
thereof in an organic solvent containing an organic compound having at least one
hydroxyl group, and a method for film formation using said sol solution.
The second invention relates to a sol solution comprising a dispersion of an
agglomerate of fine particles of magnesium hydroxide in an organic solvent containing
at least one hydroxyl-containing organic compound, and a method for film formation
using said sol solution.
The third invention relates to a sol solution comprising a dispersion of a partial
or complete hydrate of magnesium acetate tetrahydrate as colloidal particles in
a medium composed mainly of water, and a method for film formation using said sol solution.
According to the present invention, the formation of a protective layer,
for example, in an alternating current type plasma display from the above specific
sol solution realizes the formation of a film, as the protective layer, on a panel
having a large area without the need to use expensive equipment such as required
in the vacuum process, the formed film having excellent film strength, adhesion,
transparent, protective properties and other film properties and being usable as
a protective layer to provide an alternating current type plasma display which
can lower the firing voltage and the drive voltage (power consumption).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross-sectional view of a plane discharge, alternating
current type plasma display to which the sol solution according to the present
invention has been applied;
FIG. 2 is a schematic cross-sectional view of an opposed discharge, alternating
current type plasma display to which the sol solution according to the present
invention has been applied;
FIG. 3 is a scanning electron photomicrograph (magnification: 50000 times) showing
the surface structure, in terms of particles, of a protective layer constituted
by a magnesium oxide film prepared in Example A1;
FIG. 4 is a scanning electron photomicrograph (magnification: 50000 times) showing
the surface structure, in terms of particles, of a protective layer constituted
by a magnesium oxide film formed by vacuum deposition in Comparative Examples A4
and B3; and
FIG. 5 is a scanning electron photomicrograph (magnification: 50000 times) showing
the surface structure, in terms of fine particles, of a protective layer constituted
by a magnesium oxide film prepared in Example B1.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention will be described in more detail with reference to the
following preferred embodiments.
First Invention
The sol solution according to the first invention comprises a dispersion of fine
particles of magnesium hydroxide bonded to a polyhydric alcohol or a derivative
thereof (fine particles of a double hydroxide of magnesium) in an organic compound
having at least one hydroxyl group or an organic solvent containing said organic
compound. The fine particles of the double hydroxide of magnesium may be produced
by hydrolyzing a magnesium compound, convertible to magnesium hydroxide in the
presence of water, in water and a polyhydric alcohol or a derivative thereof in
the presence of a suitable catalyst to produce magnesium hydroxide and separating
the particles from the reaction medium. The particles of the double hydroxide of
magnesium thus obtained is considered to have such a structure that magnesium hydroxide
and the polyhydric alcohol or derivative thereof are bonded to each other through
a covalent bond, ionic bond, coordinate bond, or a hydrogen bond. That is, it is
considered that a composite of magnesium hydroxide and the polyhydric alcohol or
derivative thereof having an indefinite ratio is formed.
An embodiment of preparation of fine particles of double hydroxide of magnesium
is as follows.
| TABLE A1 |
| |
| Example of preparation of fine particles |
| of double hydroxide of magnesium |
| |
| |
| |
Pure water |
50 parts by weight |
| |
Ethylene glycol |
150 parts by weight |
| |
Magnesium acetate (tetrahydrate) |
21 parts by weight |
| |
Aqueous ammonia (28 vol %) |
6 parts by weight |
| |
|
Stirring of a mixed solution containing a composition as specified in Table
A1 for one hr at room temperature results in the formation of a double hydroxide
of magnesium which is then separated from the medium by a suitable method to give
fine particles of a double hydroxide of magnesium.
This is merely one embodiment, and, in general, any magnesium compound is usable
so far as it can form magnesium hydroxide in the presence of water. The amount
of the polyhydric alcohol or derivative thereof used is about 50 to 950 parts by
weight based on 100 parts by weight of the magnesium compound, and the amount of
water used is about 25 to 1500 parts by weight based on 100 parts by weight of
the magnesium compound. The magnesium compound is hydrolyzed in the presence of
a suitable catalyst. The catalyst may be any one so far as it can accelerate the
hydrolysis of the magnesium compound. For example, when the magnesium compound
is a magnesium salt, a basic compound is used, as the catalyst, in an amount of
not less than one equivalent, preferably about 1 to 5 equivalents, based on one
equivalent of the magnesium compound. When the catalyst is in the form of an aqueous
solution, such as aqueous ammonia, the water contained in the aqueous ammonia solution
can be used as water specified in the above table.
Magnesium compounds usable instead of the magnesium compound given in the
table include, for example, magnesium salts of strong acids typified by magnesium
chloride, magnesium nitrate, and magnesium sulfate, magnesium salts of weak acids
typified by magnesium phosphate, magnesium hydrogenphosphate, magnesium dihydrogenphosphate,
magnesium carbonate, magnesium citrate, magnesium hydrogencitrate, and magnesium
formate, and magnesium salts of aliphatic carboxylic acids typified by magnesium
stearate and magnesium myristate.
In Table A1, ethylene glycol, which forms fine particles of a double hydroxide
of magnesium and serves also as a solvent, is one preferred example of the polyhydric
alcohol or derivative thereof, and, for example, diethylene glycol, dihydric alcohols
typified by 2-methoxyethanol, 2-ethoxyethanol, and triethylene glycol and derivatives
thereof, trihydric or higher polyhydric alcohols typified by glycerin, mixture
systems thereof, and organic solvents containing the above compounds, are usable
instead of ethylene glycol.
Water as the solvent is an indispensable substance for the formation of fine
particles of the double hydroxide of magnesium. However, it is needless to say
that, in some cases, water need not be intentionally added on the premise that
water contained in the aqueous ammonia is utilized.
Ammonia in the aqueous ammonia functions as a catalyst which accelerates
the formation of magnesium hydroxide. Various ammonium salts typified by ammonium
acetate, ammonium amidosulfate, ammonium carbonate, ammonium hydrogencarbonate,
ammonium borate, diammonium citrate, ammonium dihydrogenphosphate, diammonium hydrogenphosphate,
triammonium phosphate, ammonium formate, and ammonium tartrate, and amines typified
by hydroxylamine, ethanolamine, and methanolamine are usable instead of this ammonia.
The aqueous ammonia is particularly preferred from the viewpoint of attaining the
object of the present invention because it is less likely to cause inclusion of
impurities in the sol solution.
The double hydroxide of magnesium can be separated from the mixed solution containing
the double hydroxide of magnesium by any method, and examples of methods usable
herein include filtration, decantation, and centrifugation. A cooling/centrifuge
(Model 7930, manufactured by Kubota Product Company Limited) was used in the separation
of fine particles of the double hydroxide of magnesium produced from the composition
specified in Table A1.
The fine particles of the double hydroxide of magnesium thus obtained is dispersed
in an organic compound having at least one hydroxyl group or an organic solvent
containing the above organic compound to prepare the sol solution of the present
invention. The amount of the organic solvent used for the double hydroxide of magnesium
may be suitably set. Since, however, the coating thickness is regulated by the
set value of this parameter, this parameter should be carefully determined. For
example, when the coatability of the resultant sol solution is taken into consideration,
the organic solvent as the dispersing medium is preferably 100 to 500 parts by
weight based on 100 parts by weight of the double hydroxide of magnesium. When
the solid content is excessively low, it is difficult to form a dense, continuous
protective layer, while when the solid content is excessively high, agglomeration
precipitation of the fine particles of the double hydroxide of magnesium and a
deterioration in uniformity of the protective layer are likely to occur. An embodiment
of the sol solution is given in the following Table A2.
| TABLE A2 |
| |
| Example of preparation of sol solution |
| |
| |
| |
Double hydroxide of |
5 parts by weight |
| |
magnesium |
| |
Ethanol |
10 parts by weight |
| |
|
The double hydroxide can be dispersed by conventional dispersing means, such
as simple stirring, forced stirring, ball mill, sand mill, and ultrasonic dispersion,
and a homogeneous sol solution can be easily prepared by the above dispersing means.
An ultrasonic device (MODEL US-300T, manufactured by NIHON SEIKi CO) was used in
the dispersion of the composition specified in Table A2.
Ethanol is one example of the dispersing medium in the sol solution, and
any compound may be used as the dispersing medium so far as it has at least one
hydroxyl group. Dispersing media usable instead of ethanol include, for example,
monohydric alcohols typified by methanol, n-propyl alcohol, i-propyl alcohol, 1-butanol,
and 2-butanol, dihydric alcohols and derivatives thereof typified by ethylene glycol,
diethylene glycol, 2-methoxyethanol, 2-ethoxyethanol, and triethylene glycol, trihydric
or higher polyhydric alcohols typified by glycerin, and mixed solvents of these
compounds or organic solvents containing at least one of the above organic solvents
having at least one hydroxyl group.
In the present invention, the reason why the fine particles of the double hydroxide
of magnesium can be easily dispersed in the above organic solvent by simple dispersing
means is considered as follows.
Specifically, since the preparation of the fine particles of the double
hydroxide of magnesium is carried out in a polyhydric alcohol or a derivative thereof,
such as ethylene glycol, the polyhydric alcohol or derivative thereof is bonded
or coordinated to each magnesium/aquo complex produced by hydrolysis, for example,
by hydrogen bond to form a kind of chelate complex. As the hydrolysis proceeds
through the action of the catalyst to increase the complex concentration, the chelate
complexes associate with one another to form fine particles of the double hydroxide
of magnesium. Since the formation of the fine particles (association of complexes)
is carried out from a chelate complex of a polyhydric alcohol or a derivative thereof
rather than from the magnesium/aquo complex, agglomeration of fine particles (agglomeration
of complexes), which is likely to occur accompanying an increase in complex concentration,
is reduced.
The fine particles, which are associated chelate complexes, when dispersed in
a hydroxyl-containing organic solvent, cause the hydroxyl-containing organic solvent
to adsorb to the fine particles through the hydroxyl group, so that a kind of protective
film formed of the hydroxyl-containing organic solvent is formed on the surface
of individual fine particles of the double hydroxide of magnesium. The presence
of this protective film results in improved affinity of the fine particles for
the organic solvent as a liquid medium for the sol solution and, at the same time,
inhibits the agglomeration of fine particles in the organic solvent, permitting
the fine particles to be easily and stably dispersed in the organic solvent. When
this effect alone is taken into consideration, all organic compounds having a hydroxyl
group are applicable. However, in order that, in the step of drying and firing
after coating of the sol solution, the organic solvent is removed to prepare a
film of pure magnesium oxide, it is preferred to use a polyhydric alcohol having
a relatively low boiling point or a derivative thereof and to use an organic solvent
having a relatively low boiling point as the solvent for the sol solution.
The surface of a magnesium oxide film formed from a sol solution prepared without
use of the polyhydric alcohol or derivative thereof in the preparation of magnesium
hydroxide has irregularities having a size of about 200 nm, that is, large roughness,
whereas, as shown in FIG. 3, the surface of a magnesium oxide film formed from
a sol solution prepared using a polyhydric alcohol or a derivative thereof in the
preparation of magnesium hydroxide has very small irregularities and is relatively even.
The reason why this phenomenon occurs is considered as follows. Specifically,
for example, ethylene glycol is a solvent having a boiling point of 196° C.,
and, hence, an agglomerate of a magnesium hydroxide/ethylene glycol complex after
centrifugation is viscous and creamy. On the other hand, magnesium hydroxide formed
without using any polyhydric alcohol or derivative thereof is fundamentally an
agglomerate of magnesium hydroxide alone, and, hence, the agglomerate after centrifugation
is not very viscous. That is, in the case of the sol solution with ethylene glycol
added thereto, it is considered that, upon coating of the sol solution, the effect
of leveling by the agglomerate of fine particles is developed, rendering the surface
of magnesium oxide after firing even as shown in FIG.
3.
Further, the fine particles of a double hydroxide of magnesium in the sol
solution according to the present invention are a complex and, hence, less likely
to associate with one another and less likely to agglomerate, as compared with
fine particles of magnesium hydroxide prepared without using any polyhydric alcohol
or derivative thereof. Therefore, the sol solution is stable even when the concentration
of the sol solution is increased, which is advantageous in storage stability, coatability,
and the steps of drying and firing.
Regarding the solvent for the sol solution, the use of water or a solvent
not having any hydroxyl group, instead of the above alcoholic organic solvent,
such as toluene or hexane, makes it difficult to stably disperse the fine particles
of the double hydroxide of magnesium in the solvent, causing agglomeration and
precipitation. For this reason, the organic solvent having a hydroxyl group, or
an organic solvent containing at least one organic solvent of the above type is
preferably used as the solvent for the sol solution.
Properties of sol solutions used in the present invention and Comparative
Examples A1 and A2 described below are summarized, as experimental results supporting
the above observation, in Table A3.
| TABLE A3 |
| |
| Properties of various sol solutions |
| |
|
Average |
|
|
| |
|
particle |
Solid |
Stability |
| |
Sample |
size |
content |
with time |
| |
|
| |
Ex. A1 |
836 nm |
18.5 wt % |
Good |
| |
Comp. Ex. A1 |
Immeasurable |
2.0 wt % |
Poor |
| |
Comp. Ex. A2 |
Immeasurable |
17.9 wt % |
Poor |
| |
|
The sol solution of Comparative Example A1 is one prepared by dispersing fine
particles of magnesium hydroxide, prepared without adding ethylene glycol, in pure
water instead of ethanol. The sol solution of Comparative Example A2 is one prepared
by dispersing fine particles of a double hydroxide of magnesium in pure water instead
of ethanol at the time of preparation of the sol solution.
Regarding the evaluation of the properties, the average particle size of
the fine particles in the sol solution was measured and evaluated with a laser
particle analyzer (PAR-III, manufactured by Otsuka Denshi K.K.) under conditions
of pinhole ø 0.2. The solid content was determined by placing a given weight
of each sol solution in a sample tube, drying the sample at 120° C. or 3 hr
and measuring the weight proportion of the residue. Regarding the stability with
the time, upon the preparation of the sol solution, the sol solution was allowed
to stand for one day to evaluate whether or not a precipitate occurred.
DETAILED DESCRIPTION OF THE DRAWINGS
A method for film formation using the sol solution according to the present invention
will be described with reference to the production of an alternating current type
plasma display shown in the drawing by way of example.
FIG. 1 is a schematic diagram showing the structure of a plane discharge, alternating
current type plasma display according to a preferred embodiment of the present invention.
In FIG. 1, numerals 1 and 2 respectively designate a front substrate
and a back substrate disposed parallel and opposite to each other with a gas discharge
space 3 sandwiched therebetween. These front substrate 1 and back
substrate 2 each are constituted by a glass sheet having a predetermined thickness.
A pair of electrodes, an electrode X 4
a and an electrode Y 4
b,
are formed on the front substrate 1 in its surface facing the back substrate
2. These pair of electrodes are covered with a dielectric layer 5
made of glass, and the dielectric layer 5 is covered with a protective layer
6 constituted by a magnesium oxide film formed by coating the sol solution
of the present invention and drying and firing the coating.
An address electrode 7, a barrier 8, and a phosphor layer 9
are formed on the back substrate 2 in its side facing the front substrate
1. Further, if necessary, for example, a titanium dioxide film (a high refractive
index layer) 10 and a silicon dioxide film (a low refractive index layer)
11 are formed as a reflection preventive layer on the front substrate 1.
FIG. 2 is a schematic diagram showing the structure of an opposed discharge,
alternating current type plasma display. In FIG. 2, an electrode X 4
a
is formed on the surface of a front substrate 1 in its surface facing
a back substrate 2, the electrode X 4
a is covered with a dielectric
layer 5 made of glass, and the dielectric layer 5 is covered with
a protective layer 6 formed of magnesium oxide prepared from a sol solution
described below.
An electrode Y 4
b, a dielectric layer 5, a protective layer
6 formed in the same manner as described above, and a barrier 8 and
a phosphor 9 are provided on the back substrate 2 in its surface
facing the front substrate 1.
Further, if necessary, for example, a titanium dioxide film (a high refractive
index layer) 10 and a silicon dioxide film (a low refractive index layer)
11 are formed as a reflection preventive layer on the front substrate 1.
A process for producing an alternating current type plasma display according
to
the present invention is characterized by coating the above sol solution on the
above dielectric layer and drying and firing the coating to form the above protective layer.
The sol solution may be coated on the dielectric layer by any coating method,
and various coating methods, for example, spin coating, dip coating, spray coating,
roll coating, meniscus coating, bar coating, curtain flow coating, bead coating,
and casting, are applicable.
Drying and firing of a wet coating formed by the coating method result in
the formation of a transparent magnesium oxide film as the protective layer, and
the magnesium oxide film has high adhesion to the dielectric layer 5. The
drying may be performed under conditions of such a temperature and a period of
time that the organic solvent component in the wet coating is substantially removed
by evaporation. For example, drying at a temperature of about 200 to 300°
C. for about 1 to 3 hr suffices for satisfactory results. Preferably, the firing
can be performed at a temperature of about 350 to 550° C. for about 1 to 5
hr. Excessively severe firing conditions pose problems attributable to softening
of the dielectric layer, such as delamination or cracking of the protective layer,
On the other hand, when the firing is unsatisfactory, no protective layer having
desired properties can be formed. Preferably, the drying and the firing are continuously
performed. However, they may be separately performed. In a working example described
below, the drying and the firing were continuously performed, that is, at 300°
C. for one hr and 400° C. for one hr.
In the present invention, a scanning type electron photomicrograph (magnification:
50000 times, device used: S-800, manufactured by Japan Electric Optical Laboratory)
showing the surface structure, in terms of particles, of the protective layer constituted
by a magnesium oxide film formed as described above is shown in FIG. 3.
For comparison, a photomicrograph of the surface of a protective layer prepared
by vacuum deposition in Comparative Example A4 described below is shown in FIG.
4. The measurement was conducted under conditions of accelerated voltage
5 kV, working distance 5 mm, beam monitor aperture No. 2, and objective movable
aperture No. 3. As is apparent from FIG. 3, the protective layer constituted by
the magnesium oxide film according to the present invention is such a dense, continuous
film that the surface of the protective layer is constituted by particles, in a
particulate form, having an average particle diameter of 30 nm. The form of the
surface of the above film is clearly different from that of a magnesium oxide film,
formed by vacuum deposition, constituted by flaky particles having an average particle
diameter of 200 nm (FIG. 4).
In the present invention, the surface area of fine particles of magnesium oxide
constituting the protective layer should be increased from the viewpoint of increasing
the secondary electron emission ratio in the alternating current type plasma display,
and, for this purpose, desirably, the diameter of fine particles of magnesium oxide
is in the range of from 5 to 100 nm, preferably in the range of from 5 to 30 nm.
The particle diameter is reduced to not more than 100 nm to reduce the gap between
the particles and to enable an magnesium oxide film to be efficiently formed even
by a conventional heat treatment process. Although the thickness of the magnesium
oxide film is not particularly limited, it is preferably not more than 10 μm
from the viewpoint of transparency, particularly preferably not more than 1 μm.
In the protective layer in the alternating current type plasma display thus obtained,
a thin film having a thickness of not more than 1 μm can be realized, and
such a film thickness cannot be realized by a paste using the conventional binder.
The sol solution according to the present invention can be used as a protective
layer provided on the dielectric layer as a substrate in an alternating current
type plasma display. Further, a magnesium oxide film can be formed on other substrates
according to purposes.
Second Invention
The sol solution according to the second invention comprises a dispersion of
an agglomerate of fine particles of magnesium hydroxide in an organic solvent containing
at least one hydroxyl-containing organic compound. The agglomerate of fine particles
of magnesium hydroxide may be produced by hydrolyzing a magnesium compound, convertible
to magnesium hydroxide in the presence of water, in the presence of a suitable
catalyst to produce magnesium hydroxide and separating the agglomerate from the
reaction medium.
An embodiment of preparation of the agglomerate of fine particles of magnesium
hydroxide is as follows.
| TABLE B1 |
| Example of preparation of fine particles |
| of magnesium hydroxide |
| |
| |
Pure water |
200 parts by weight |
| |
Magnesium acetate (tetrahydrate) |
21 parts by weight |
| |
Aqueous ammonia (28 vol %) |
6 parts by weight |
| |
Stirring of a mixed solution containing a composition as specified in Table
B1 for one hr at room temperature results in the formation of a precipitate of
an agglomerate of fine particles of magnesium hydroxide which is then separated
from water as the reaction medium by a suitable method to give an agglomerate of
fine particles of magnesium hydroxide.
This is merely one embodiment, and, in general, any magnesium compound is usable
so far as it can form magnesium hydroxide in the presence of water. The catalyst
may be any one so far as it can accelerate the hydrolysis of the magnesium compound.
For example, when the magnesium compound is a magnesium salt, a basic compound
is used, as the catalyst, in an amount of not less than one equivalent, preferably
about 1 to 5 equivalents, based on one equivalent of the magnesium compound. When
the catalyst is in the form of an aqueous solution, such as aqueous ammonia, the
water contained in the aqueous ammonia solution can be used as water specified
in the above table.
Magnesium compounds usable herein include those exemplified above in connection
with the first invention.
Ammonia in the aqueous ammonia functions as a catalyst which accelerates
the formation of magnesium hydroxide. Various ammonium salts typified by ammonium
acetate, ammonium amidosulfate, ammonium carbonate, ammonium hydrogencarbonate,
ammonium borate, diammonium citrate, ammonium dihydrogenphosphate, diammonium hydrogenphosphate,
triammonium phosphate, ammonium formate, and ammonium tartrate, and amines typified
by hydroxylamine, ethanolamine, and methanolamine are usable instead of this ammonia.
The aqueous ammonia is particularly preferred from the viewpoint of attaining the
object of the present invention because it is less likely to cause inclusion of
impurities in the sol solution.
The agglomerate of fine particles of magnesium hydroxide can be separated from
the mixed solution containing the precipitate of an agglomerate of fine particle
of magnesium hydroxide by any method, and examples of methods usable herein include
filtration, decantation, and centrifugation. A cooling/centrifuge (Model 7930,
manufactured by Kubota Product Company Limited) was used in the separation of the
agglomerate of fine particles of magnesium hydroxide produced from the composition
specified in Table B1.
The fine particles of the agglomerate of magnesium hydroxide thus obtained is
dispersed in an organic solvent containing at least one hydroxyl-containing organic
compound to prepare the sol solution of the present invention. The amount of the
organic solvent used for dispersing the agglomerate of fine particle of magnesium
hydroxide may be suitably set. Since, however, the coating thickness is regulated
by the set value of this parameter, this parameter should be carefully determined.
For example, when the coatability of the resultant sol solution is taken into consideration,
the organic solvent as the dispersing medium is preferably 100 to 1500 parts by
weight based on 100 parts by weight of the fine particles of magnesium hydroxide.
When the solid content is excessively low, it is difficult to form a dense, continuous
protective layer, while when the solid content is excessively high, agglomeration
precipitation of the fine particles and a deterioration in uniformity of the protective
layer are likely to occur. An embodiment of the sol solution is given in the following
Table B2.
| TABLE B2 |
| Example of preparation of sol solution |
| |
| |
Agglomerate of fine particles |
1 part by weight |
| |
of magnesium hydroxide |
| |
Ethanol |
10 parts by weight |
| |
The agglomerate can be dispersed by conventional dispersing means, such as simple
stirring, forced stirring, ball mill, sand mill, and ultrasonic dispersion, and
a homogeneous sol solution can be easily prepared by the above dispersing means.
An ultrasonic device (MODEL US-300T, manufactured by NIHON SEIKi CO) was used in
the dispersion of the composition specified in Table B2.
Ethanol is one example of the dispersing medium in the sol solution, and
any organic compound having at least one hydroxyl group or any organic solvent
containing at least one compound of the above type may be used as the dispersing
medium. Dispersing media usable instead of ethanol include, for example, monohydric
alcohols typified by methanol, n-propyl alcohol, i-propyl alcohol, 1-butanol, and
2-butanol, dihydric alcohols and derivatives thereof typified by ethylene glycol,
diethylene glycol, 2-methoxyethanol, 2-ethoxyethanol, and triethylene glycol, trihydric
or higher polyhydric alcohols typified by glycerin, aromatic compounds, such as
phenol and cresol, and mixed solvents of these compounds or organic solvents containing
at least one of the above organic solvents having at least one hydroxyl group.
In the present invention, the reason why the agglomerate of fine particles of
magnesium hydroxide can be easily dispersed in the above organic solvent by simple
dispersing means is considered as follows.
Specifically, when an agglomerate of fine particles of magnesium hydroxide
is dispersed in a hydroxyl-containing organic solvent, the hydroxyl-containing
organic solvent is bonded, coordinated, or adsorbed to an agglomerate (secondary
agglomerate) of individual fine particles of magnesium hydroxide by taking advantage
of the hydroxyl group through a hydrogen bond or the like to form fine particles
covered with a kind of a protective film of the hydroxyl-containing organic solvent
formed on the surface of the particles. It is considered that the presence of this
protective film results in improved affinity of the fine particles for the organic
solvent as a dispersing medium for the sol solution and, at the same time, inhibits
the agglomeration of fine particles per se in the organic solvent, permitting the
fine particles to be easily and stably dispersed in the organic solvent. When this
effect alone is taken into consideration, all organic compounds having a hydroxyl
group are applicable. However, in order that, in the step of drying and firing
after coating of the sol solution, the organic solvent is removed to prepare a
film of pure magnesium oxide, it is preferred to use a hydroxyl-containing organic
solvent having a relatively low boiling point as the solvent for the sol solution.
Regarding the solvent for the sol solution, the use of water or a solvent
not having any hydroxyl group, such as toluene or hexane, instead of the above
alcoholic organic solvent, makes it difficult to stably disperse the fine particles
of magnesium hydroxide in the solvent, causing agglomeration and precipitation.
For this reason, the organic solvent having a hydroxyl group, or an organic solvent
containing at least one organic solvent of the above type is preferably used as
the solvent for the sol solution.
Properties of sol solutions used in the present invention and Comparative
Example B1 described below are summarized, as experimental results supporting the
above observation, in Table B3.
| TABLE B3 |
| Properties of various sol solutions |
| |
|
Average |
|
|
| |
|
particle |
Solid |
Stability |
| |
Sample |
size |
content |
with time |
| |
| |
Ex. B1 |
873 nm |
0.9 wt % |
Good |
| |
Comp. Ex. B1 |
Immeasurable |
2.0 wt % |
Poor |
| |
The sol solution of Comparative Example B1 is one prepared by dispersing an agglomerate
of fine particles of magnesium hydroxide specified in Table B1, prepared without
adding ethylene glycol, in pure water instead of ethanol.
Properties of the sol solutions were evaluated in the same manner as in
the first invention.
A film can be formed using the sol solution according to the second invention
in
the same manner as described above in connection with an embodiment of the production
of alternating current type plasma displays, according to the present invention,
shown in FIGS. 1 and 2.
In the present invention, a scanning type electron photomicrograph (magnification:
50000 times, device used: S-800, manufactured by Japan Electric Optical Laboratory)
showing the surface structure, in terms of particles, of the protective layer constituted
by a magnesium oxide film formed as described above is shown in FIG. 5.
For comparison, a photomicrograph of the surface of a protective layer prepared
by vacuum deposition in Comparative Example B3 described below is shown in FIG.
4. The measurement was conducted under conditions of accelerated voltage
5 kV, working distance 5 mm, beam monitor aperture No. 2, and objective movable
aperture No. 3. As is apparent from FIG. 5, the protective layer constituted by
the magnesium oxide film according to the present invention is such a dense, continuous
film that the surface of the protective layer is constituted by particles, in a
particulate form, having an average particle diameter of 30 nm. The form of the
surface of the above film is clearly different from that of a magnesium oxide film,
formed by vacuum deposition, constituted by flaky particles having an average particle
diameter of 200 nm (FIG. 4).
In the present invention, the surface area of fine particles of magnesium oxide
constituting the protective layer should be increased from the viewpoint of increasing
the secondary electron emission ratio in the alternating current type plasma display,
and, for this purpose, desirably, the diameter of fine particles of magnesium oxide
is in the range of from 5 to 100 nm, preferably in the range of from 5 to 30 nm.
The particle diameter is reduced to not more than 100 nm to reduce the gap between
the particles, thereby increasing the surface area, and to enable an magnesium
oxide film to be efficiently formed even by a conventional heat treatment process.
Although the thickness of the magnesium oxide film is not particularly limited,
it is preferably not more than 10 μm from the viewpoint of transparency,
particularly preferably not more than 1 μm.
In the protective layer in the alternating current type plasma display thus obtained,
a thin film having a thickness of not more than 1 μm can be realized, and
such a film thickness cannot be realized by a paste using the conventional binder.
The sol solution according to the present invention can be used as a protective
layer provided on the dielectric layer as a substrate in an alternating current
type plasma display. Further, a magnesium oxide film can be formed on other substrates
according to purposes.
Third Invention
The use of an organometallic compound, such as a magnesium alkoxide, as a starting
compound for the magnesium hydroxide sol can offer magnesium oxide having relatively
good adhesion to the substrate and uniformity in the film. Magnesium alkoxides
and the like, however, involve a problem of cost because they are relatively expensive.
Further, since the reactivity of the magnesium alkoxides is relatively high, the
reaction should be carefully controlled. Furthermore, the life of the resultant
magnesium hydroxide is so short that careful handling is necessary.
For the above reason, in general, an organometallic salt, such as magnesium acetate,
or an inorganic metal salt, such as magnesium chloride or magnesium sulfate, is
used instead of the magnesium alkoxide and the like. However, the use of a large
amount of an alkali, such as ammonia, is necessary to prepare a magnesium hydroxide
sol, requiring the step of removal of the alkali and the like. This often renders
the production process complicate. Bringing the particle diameter to not more than
1 μm is preferred from the viewpoints of improving the transparency of the
magnesium oxide layer and the adhesion of magnesium hydroxide to the substrate.
For attaining this purpose, the addition of an additive, such as ethylene glycol,
to the magnesium hydroxide sol production system is recommended from the viewpoint
of inhibiting the growth of the particles. The addition of the above additive often
inhibits the production of the magnesium hydroxide sol per se, resulting in lowered
yield of the sol.
According to the third invention, when a sol solution of colloidal particles
of magnesium acetate is used, for example, in the formation of a protective layer
in an alternating current type PDP, a film can be formed on a panel having a large
area without the need to use expensive equipment such as required in the vacuum
process. The film thus formed has film strength, adhesion, transparent, protective
properties and other film properties, and the of such a film enables the production
of an alternating current type PDP which can lower the firing voltage and the drive
voltage (power consumption).
The third invention is based on the present inventor's finding, that is, such
finding that although a commercially available magnesium acetate is generally in
tetrahydrate form and is completely soluble in water, partially dehydrated hydrous
magnesium acetate and anhydrous magnesium acetate produced by partially or completely
removing the water of hydration is insoluble in water and that the addition of
the above magnesium acetate to water causes a part of the surface of particles
to undergo hydrolysis to form hydrophilic magnesium hydroxide which is dispersed
as stable colloidal particles in water.
The starting compound for forming the magnesium acetate sol according to the
present invention is magnesium acetate tetrahydrate, and the removal of 20% by
weight of the water of hydration is preferred from the viewpoint of forming stable
colloidal particles in a medium composed mainly of water. Particularly preferably,
the magnesium acetate is completely dehydrated to form anhydrous magnesium acetate.
The dehydration may be performed by any method without limitation. For example,
heating under reduced pressure is simple and, hence, is preferred.
The medium for dispersing the anhydrous or partially dehydrated magnesium acetate
is composed mainly of water, that is, 100% water or a mixed solution of water and
a water-soluble solvent. In particular, the use of alcohols, glycols, esters, ethers,
or organic solvents having a high dielectric constant, such as propylene carbonate
and γ-butyrolactone, are preferred from the viewpoint of allowing the hydrolysis
of at least part of magnesium acetate on the surface of the particles to homogeneously
and efficiently proceed, thereby forming a stable sol solution. The amount of the
above organic solvent added to water is not particularly limited so far as the
proportion of water in the mixed solution is not 50% by weight or less. The content
(solid basis) of the anhydrous or partially hydrated magnesium acetate in the medium
composed mainly of water is usually 0.1 to 90% by weight. However, it is preferably
in the range of from 0.1 to 60% by weight from the viewpoint of forming a homogeneous
sol solution.
A dispersion stabilizer may be incorporated into the sol solution in order to
enhance
the stability of the sol solution. Examples of preferred dispersion stabilizers
include water-soluble polymers, for example, celluloses, such as hydroxypropyl
methylcellulose and hydroxypropylmethyl cellulose, acrylic polymers, such as polyacrylic
acid and polyacrylamide, and vinyl polymers, such as polyvinyl alcohol and a partially
saponified product thereof. They may be used alone or a mixture of two or more,
and the content of the dispersion stabilizer in the sol solution is usually in
the range of from 0.01 to 10% by weight.
The sol solution containing magnesium acetate particles dispersed in a colloidal
form can be prepared by adding anhydrous or partially hydrated magnesium acetate
to medium composed m
