Hollow oxide particle and process for producing the same

The present invention provides a hollow oxide particle including a shell wall constituting a hollow room, and the shell wall has a thickness of 20 nm or less. The shell wall may be mainly composed from at least one selected from the group consisting of alumina, spinel, iron oxides, yttrium oxides, and titanium oxides. A process for producing the hollow oxide particle comprises the steps of: forming a Water in Oil (W/O) type emulsion including aqueous microspheres having a each diameter of 100 nm or more, by adding an organic solvent to an aqueous solution dissolving and/or suspending at least one of metal salts and metal compounds; and forming the hollow oxide particle by atomizing the Water in Oil (W/O) type emulsion to burn. When the hollow oxide particle is brought into contact with a water-containing solution, a surface of the shell wall may have a minutely irregular surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a hollow oxide particle and a process for 
producing the same. The hollow oxide particle is, for example, applicable 
to catalytic carriers. 
2. Description of the Related Art 
There has been provided a hollow oxide particle whose specific surface area 
is generally 5-10 times or more times larger than that of a solid powder 
particle, a non-hollow particle. The hollow oxide particle is larger in 
diameter than the non-hollow powder particle, when the specific surface 
areas of both particles are same. Such particle having larger diameter is 
treated easily. So, the hollow oxide particle is applicable to catalytic 
carriers because of their large specific surface area and large diameter. 
It is also expectable as concealing agents and micro-capsules. 
Japanese Unexamined Patent Publication (KOKAI) 6-7,670 discloses a process 
for producing a porous particle or a hollow particles formed. In this 
publication technique, firstly, spherical polymer particles for forming 
cores are uniformly dispersed in a solution including a metal salt having 
a hydrolysis property, so that polymer-metal compound particles are formed 
in which each spherical polymer core is covered with a metal compound 
layer. The polymer-metal compound particle is in the range of 0.07-30 
.mu.m in diameter, and its ratio of inner-diameter to outer-diameter is in 
the range 0.40-0.95. In this publication technique, secondly, the 
polymer-metal compound particles are heated for decomposing the 
core-polymer into carbon or carbon dioxide, so that they are transformed 
into another spherical particles in which each spherical carbon is covered 
with a metal compound layer or a hollow particle. This publication 
technique exhibits complicated steps to increase costs. Also, this 
publication technique, using a precipitating method, can hardly produce a 
homogeneous complex oxide particle because the precipitation rates of the 
metal salts are usually different from each other. 
Japanese Unexamined Patent Publication (KOKAI) 63-258,642 discloses a 
process for producing a hollow particle. In this publication process, 
after an Oil in Water (O/W) type emulsion is composed by mixing an organic 
solvent with an aqueous solution including inorganic compounds, the Oil in 
Water (O/W) type emulsion is added into an organic solvent including a 
hydropholic surfactant to compose an Oil in Water in Oil (O/W/O) type 
emulsion. Next, the O/W/O type emulsion is mixed with a solution including 
compounds capable of precipitating a water-insoluble precursor in order to 
form a hollow particle. This publication process requires a calcination 
step of the precipitated precursor to cause complicated steps, the process 
exhibits a problem in homogeneity of the particle, and it can not produce 
a hollow particle having a shell wall thickness of 20 nm or less. 
Japanese Unexamined Patent Publication (KOKAI) 3-47,528 discloses a process 
for producing spherical a hollow particle of metal oxide. In this 
publication process, firstly, an emulsion is composed by a mix of a first 
oil and a second oil into a metal compound solution. Secondly, the 
spherical hollow particle is made by removing of the oils and water from 
the emulsion. This publication process causes complicated steps, it can 
hardly produce a homogeneous complex oxide particle because the 
solubilities of metal compounds are different from each other, and it can 
not produce the particle whose shell wall has a thickness of 20 nm or 
less. 
Japanese Unexamined Patent Publication (KOKAI) 60-122,779 discloses a 
technique for producing porous particles. This technique uses MgAl.sub.2 
O.sub.4 obtained by a spray pyrolisis as a raw material. The raw material 
is formed and sintered to produce the porous body in which the pore size 
and distribution are easily controlled. In this technique, each of 
atomized droplets, the diameter of which is in the range of tens .mu.m in 
size under efficient conditions, constitutes a reaction site. Namely, size 
of the reaction site of this technique is ten times or much larger than 
that of an emulsion-combustion method. This technique, therefore, may 
cause heterogeneous particles, and it has difficulty in producing the 
hollow particle having a shell wall thickness of 20 nm or less, judging 
from the aforementioned presupposition that each of the atomized droplets 
constitutes each of porous particles. 
Japanese Unexamined Patent Publication (KOKAI) 8-91,821 discloses a 
technique for producing a hollow silica particle by use of a sol-gel 
method of alkoxide. This technique uses alkoxide raw material exhibiting 
expensive costs. With this publication disclosing nothing about a shell 
wall thickness, judging from the fact that the hollow silica particle is 
as large as 2-8 .mu.m in diameter, it is hardly thought that the hollow 
silica particles has a shell wall thickness of 20 nm or less. 
Further, "Ceram. Inter." (vol.,14 (1988), 239-244) discloses a technique 
for producing a hollow alumina particle by use of an emulsion-evaporating 
technique. This reference technique requires a calcination step after 
producing a precursor. According to a Scanning Electron Microscopy (SEM) 
photograph in this reference, a shell wall of the particle is hundreds nm 
or more in thickness. 
SUMMARY OF THE INVENTION 
The present invention has been developed in a view of the aforementioned 
circumstances. It is therefore an object of the present invention to 
provide a hollow oxide particle whose shell wall is thin in thickness. It 
is a further object of the present invention to provide a process for 
simply producing a hollow oxide particle whose shell wall is thin in 
thickness. It is a still further object of the present invention to 
provide a process for simply producing a hollow oxide particle whose shell 
wall is thin in thickness and minutely irregular or porous. 
According to a first aspect of the present invention, a hollow oxide 
particle comprises a shell wall defining a hollow room, and the shell wall 
has a thickness of 20 nm or less. According to the first aspect of the 
present invention, the hollow oxide particle is 5-10 times or much larger 
than the spherical non-hollow particle in a specific surface area. Thus, 
such particle is to be treated easily and to be applicable to catalytic 
carriers or the like requiring a large specific surface area. Also, the 
hollow oxide particle has a small heat-conductivity to be applicable to 
heat-insulating material, etc. 
According to a second aspect of the present invention, a process for 
producing a hollow oxide particle comprises the steps of: (1) forming a 
Water in Oil (W/O) type emulsion including a plurality of aqueous 
microspheres by adding an organic solvent to an aqueous solution 
dissolving and/or suspending at least one of metal salts and metal 
compounds; and (2) forming a hollow oxide particle having a shell wall by 
atomizing and burning the Water in Oil (W/O) type emulsion. 
According to a third aspect of the present invention, a process for 
producing a hollow oxide particle comprises the steps of: (1) forming a 
Water in Oil (W/O) type emulsion including a plurality of aqueous 
microspheres by adding an organic solvent to an aqueous solution 
dissolving and/or suspending at least one of metal salts and metal 
compounds; (2) forming a hollow oxide particle having a shell wall by 
atomizing and burning the Water in Oil (W/O) type emulsion; and (3) 
bringing the hollow oxide particle into contact with an aqueous solution 
for finely roughening a surface of the shell wall. The step for bringing 
the hollow oxide powder into contact with the solution is otherwise called 
a water-treatment herein. 
The second aspect and the third aspect of the present invention exhibit an 
emulsion-combustion process, a short time process, which instantaneously 
carries out the heating of the aqueous microsphere containing the metal 
salts, evaporation of the water, and oxidation of the metal salts. 
In other words, it is assumed that the hollow oxide particle is synthesized 
by way of: (1) evaporation of water at the surface of the aqueous 
microsphere; (2) shrinkage of the aqueous microsphere and nucleation of 
crystalline at the surface of the aqueous microsphere; and (3) growth and 
sintering of the nucleated crystallines. 
The step for forming the particle in the second aspect and the third aspect 
is to atomize and to burn the Water in Oil (W/O) type emulsion in order to 
form the hollow oxide particle. When the Water in Oil (W/O) type emulsion 
is atomized into a heated reactor, the atomized aqueous microsphere 
including metal salts or metal compounds is covered with the organic 
solvent. Thus, the organic solvent is burned to oxidize the metal salts 
and to evaporate water, so that oxide is formed in a shell-shape to 
produce the hollow oxide particle. 
According to the second aspect and the third aspect of the present 
invention, each microsphere, ranging from 100 nm to 10 .mu.m in diameter 
in the Water in Oil (W/O) type emulsion, constitutes each reaction site, 
so that a scatter is suppressed in temperature-distribution, and the oxide 
particle is more homogeneous. 
The second aspect and the third aspect of the present invention can easily 
produce the hollow oxide particle whose shell wall is homogeneous and 20 
nm or less in thickness. Since the hollow oxide particle produced in the 
second aspect and the third aspect has a large specific surface area, the 
particle is 5-10 times or much larger than a non-hollow particle in 
specific surface area. Accordingly, the diameter of the hollow particle is 
larger than that of the non-hollow particle, when both specific areas are 
same; so, the hollow particle is to be treated easily. Since the second 
aspect and the third aspect do not require a drying, a calcining, and a 
milling, these aspects realize a simple step and inexpensive cost as 
compared with the conventional precipitation method. 
The third aspect of the present invention comprises the steps of: composing 
the Water in Oil (W/O) type emulsion by adding the organic solvent to the 
aqueous solution dissolving and/or suspending at least one of the metal 
salts and metal compounds; and forming the hollow oxide particle by 
atomizing and burning the Water in Oil (W/O) type emulsion. Such aspect 
can obtain the oxide particle which is not perfectly oxidized by selecting 
of atomizing conditions and burning conditions. These oxide powder 
particle synthesized in such a way is brought into contact with 
water-containing solution--the water treatment. This water treatment cuts 
bondings of hydroxides and/or bondings of the metal salts, so that the 
surface of the particle is minutely-irregular or porous and then its 
specific surface is considerably increased. A change of burning conditions 
and the types of the solutions can control the specific surface area of 
the particle. 
According to the third aspect of the present invention, the 
water-treatment, the step for bringing the particle into contact with the 
solution containing water, generates structural changes at the surface of 
the particle. Thus, non-hollow particle has a small effect caused by the 
water-treatment. On the other hand, the hollow particle has a large effect 
caused by the water-treatment since its surface area is considerably large 
with respect to its volume.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to the first aspect of the present invention, a hollow oxide 
particle is prefererably 50 nm-5 .mu.m in average diameter, and it may 
have an approximately spherical shape. The shell wall constituting a 
hollow room of the hollow oxide particle comprises at least one selected 
from the group consisting of alumina, spinel, iron oxides, yttrium oxides, 
and titanium oxides. The shell wall may be simple oxide or complex oxide. 
For example, the hollow oxide particle, having a shell wall whose 
thickness is 20 nm or less, is preferably made from a hollow aluminum 
oxide, or a complex oxide of aluminum. The shell wall may have a finely 
roughened structure or a porous structure. 
The second aspect of the present invention comprises the steps of forming a 
Water in Oil (W/O) type emulsion whose each of aqueous microspheres is 100 
nm or more by adding an organic solvent to an aqueous solution dissolving 
and/or suspending at least one of metal salts and metal compounds; and 
forming a hollow oxide particle by atomizing and burning the Water in Oil 
(W/O) type emulsion so as to form a hollow oxide particle. The 
emulsion-forming step is to compose a Water in Oil (W/O) emulsion 
including an organic solvent as a matrix, by mixing of the organic solvent 
and the aqueous solution in which metal ions or metal compounds are 
dissolved or suspended. For keeping the aqueous microsphere of the Water 
in Oil (W/O) emulsion 100 nm or more in diameter, it is preferable to 
adjust the diameter of the aqueous microsphere by use of surfactants on 
occasion. The diameter of the aqueous microsphere in the emulsion 
including at least one of metal salts and metal compounds, is kept stable 
at the atomizing and burning step, so that reaction site can be controlled 
during the synthesis. A diameter of the aqueous microsphere in the Water 
in Oil (W/O) emulsion implies a diameter of an aqueous phase dispersed in 
the organic solvent of the emulsion. 
According to the second and third aspects of the present invention, a 
diameter of the aqueous microsphere in the emulsion is preferably 100 
nm-10 .mu.m. The metal salt is not limited in its type; it may be 
water-soluble metal salts, such as metal nitrate, metal acetate, metal 
sulphate, and metal chloride. The aqueous solution containing the metal 
salt is not limited in concentration of the metal salt. 
When the metal salt is used in a suspended state, the size of the metal 
salt is preferably 1 .mu.m or less, more preferably 0.1 .mu.m or less. The 
organic solvent preferably is a material capable of composing a Water in 
Oil (W/O) type emulsion and not dissolved into water. The organic solvent 
is not limited in its type; it is preferably organic solvents having 
hydrocarbon groups, such as hexane, octane, kerosine, and gasoline. 
According to the second and the third aspects of the present invention, for 
keeping the Water in Oil (W/O) type emulsion stable, as above-mentioned, 
it is preferable that a surfactant is added to the emulsion. The 
surfactant is not limited in its type and its amount. It can be used as 
follows: cationic surfactants, anionic surfactants, and nonionic 
surfactants. Its type and its amount may be selected depending upon the 
type of organic solvent and diameter of the aqueous microsphere. 
According to the second and third aspects of the present invention, a mix 
ratio is not limited in an aqueous component and an organic solvent 
component composing the emulsion. When a rate of water is more than 70% in 
the emulsion, a phase inversion is sometimes caused between a dispersed 
phase and dispersion medium. So, the rate of water is preferably 70% or 
less for stabilizing the Water in Oil (W/O) type emulsion. A diameter of 
the aqueous microsphere is controlled by selecting the types and the 
amount of surfactants. So, the diameter of microsphere is not limited in 
the emulsion; the diameter of the aqueous microsphere in the emulsion is 
preferably 100 nm or more for synthesizing hollow particles. When the 
diameter of the aqueous microsphere is more than 10 .mu.m, reaction sites 
are so large that a temperature distribution is easily generated and that 
a burning reaction is unstable in burning emulsion. Therefore, the 
diameter of the aqueous microsphere in the emulsion is preferably 10 .mu.m 
or less. 
According to the second and third aspects of the present invention, burning 
temperature is not limited; it is preferably 600-1,000.degree. C. When the 
burning temperature is 600.degree. C. or less, the organic solvent is hard 
to completely burn. When the burning temperature exceeds 1,000.degree. C., 
the synthesized particles may be aggregated, further in the third aspect 
of the present invention, the metal salt may completely be oxidized to 
cause a problem that effect of water-treatment disappears. Burning 
atmosphere is not limited. Insufficiency of oxygen causes a problem that 
carbon contained in the organic solvent sometimes remains because of 
incomplete combustion. Thus, it is preferable to supply oxygen, for 
burning the organic solvent of the emulsion completely. 
According to the third aspect of the present invention, it is preferable to 
control oxidation reactions of the metal salts and metal compounds. Change 
of burning temperature of the emulsion can control the oxidation reaction. 
On the other hand, even when burning temperature can not be changed 
because of powder composition, particle shape and the like, the oxidation 
reaction can be controlled by heat-treatment after the combustion of 
emulsion. In such a case, the heat-treatment is not limited in its 
temperature, its time, and its atmosphere. 
The size of the aqueous microsphere in the emulsion is preferably 100 nm or 
more in order to make the particle hollow. When the microsphere in the 
emulsion is less than 100 nm, the microsphere completely shrinks before 
the shell wall is formed at the surface of the microsphere, so that the 
oxide does not constitute a hollow particle but a non-hollow particle. 
When the microsphere in the emulsion exceeds 10 .mu.m, the reaction site 
is so large that oxidation reaction requires a long time, and the product 
may disadvantageously be heterogeneous in its composition. 
When the oxide is made from alumina or complex oxide whose main component 
is aluminum, the process of the present invention may provide a hollow 
oxide particle having an extremely thin shell wall. Such reason is not 
clear at the present time, but it is assumed that an aluminum ion tends to 
be precipitated, oxidized and coalesced to form the shell wall of the 
hollow oxide particle in this condition. 
According to the third aspect, the solution for water-treatment is not 
limited in its type, except including water. Such solution may be at least 
one of water, deionized water, acid solutions, alkaline solutions, and 
mixed solutions mixing alcohol with water. Such solution may be water; 
acid solutions such as nitric acids and hydrochloric acids; and alkaline 
solutions such ammonia solutions and sodium hydroxide solutions. Also, the 
solution for water-treatment may be a solution mixing the aforesaid 
solution and water-soluble organic solvent such as ethanol. The third 
aspect of the present invention is not limited in temperature and time for 
the water-treatment. Changing solution-types, temperature and time of the 
water-treatment can control a surface structure of the oxide particle. The 
third aspect of the present invention is not limited in a manner for 
bringing the particle brought into contact with an aqueous solution. For 
example, the synthesized oxide particle may be mixed with the aqueous 
solution, or, may be brought into contact with atomized aqueous solutions. 
When oxidation is sufficiently proceeded, nucleated crysallines of the 
oxide generated at the surface of the aqueous microsphere grow to have a 
diameter of tens nm or less, and the nucleated crysallines of the oxide 
are sintered to form one oxide particle. On the other hand, when oxidation 
is insufficiently proceeded, each of nucleated crysallines is 
insufficiently oxidized and insufficiently sintered; so, it is thought 
that hydroxide and metal salts sometimes remain partially at the surface 
of the nucleated crysallines. In such a case, it is assumed that the water 
cuts the bondings linked with the residual hydroxides and metal salts to 
make the surface of the particles minutely irregular or porous. 
EXAMPLE 
Examples will concretely be described hereinafter. 
Example 1 
(Emulsion-Composing Step) 
There was used an emulsion including an aqueous phase and an oil phase. The 
aqueous phase was formed with an aluminum nitrate aqueous solution in 
which commercial aluminum nitrate (Al(NO.sub.3).sub.3. 9H.sub.2 O) was 
dissolved in deionized water at a concentration of 0.1-2 mole/liter. An 
organic solvent was commercial kerosine. SUN SOFT No. 818H (TAIYOU KAGAKU 
Co., Ltd.) was used at 5-10 wt. % with respect to the kerosine as a 
surfactant. The kerosine including such surfactant was used as the oil 
phase in the emulsion. 
The aqueous phase and the oil phase were mixed so that a ratio of aqueous 
phase/oil phase might be (40-70)/(60-30) by volume %. This mixed solution 
was stirred by use of a homogenizer at 1,000-20,000 rpm for 5-30 minutes 
to compose a Water in Oil (W/O) emulsion. According to observations using 
an optical microscope, each aqueous microsphere was approximately 1-2 
.mu.m in diameter in the emulsion concerning Example 1. 
(Powder-Producing Step) 
The composed Water in Oil (W/O) emulsion was atomized by use of a 
emulsion-burning apparatus, developed by the present inventor and 
disclosed in Japanese Unexamined Publication Patent 7-81,905, so that the 
oil phase of the emulsion was burned and metal ion existing in the aqueous 
phase was oxidized to synthesize oxide particles. As for conditions for 
synthesizing the oxide particles, the emulsion-burning apparatus 
controlled an atomizing flow rate of the emulsion, a flow rate of air, and 
the like in such a manner that the atomized emulsion was completely burned 
and a temperature of flame was constant in the range of 700-1,000.degree. 
C. The produced particles were collected by a bag filter disposed at the 
rear side of a reaction tube of the apparatus 
The produced oxide particle concerning Example 1 was observed by use of a 
transmission electron microscopy (TEM). FIG. 1 shows a TEM picture 
photographed at a low magnification. According to FIG. 1, the particle was 
a hollow particle whose shell wall was extremely thin since the back side 
thereof was clearly observed. FIG. 2 shows another TEM picture 
photographed at a high magnification with respect to the particle 
concerning Example 1. According to FIG. 2, a thickness of the shell wall 
of the powder particle was approximately 10 nm. 
Example 2 
This example is to produce hollow spinel particles. 
In this example, weight was measured with respect to commercial aluminum 
nitrate (Al(NO.sub.3).sub.3.9H.sub.2 O), and magnesium nitrate 
(Mg(NO.sub.3).sub.2.6H.sub.2 O) in such a manner that a ratio of Al/Mg 
(mole ratio) is 2/1. They were dissolved in deionized water to obtain a 
solution mixing aluminum nitrate and magnesium nitrate at a concentration 
of 0.1-2 mole/liter so as to constitute an aqueous phase. The conditions 
in this example were the same as those of Example 1 in an oil phase, an 
emulsion-producing step, and a powder-producing step. 
FIG. 3 shows a TEM picture, showing the produced oxide particles, 
concerning Example 2, photographed at a high magnification. According to 
FIG. 3, the oxide powder particle was a hollow particle whose shell wall 
is extremely thin--a thickness of the shell wall of the particle 
exhibiting a little less than 20 nm. 
Example 3 
This example is to produce other types of oxide particles. An aqueous phase 
of an emulsion was formed with a metal aqueous solution in which 
commercial magnesium nitrate (Mg(NO.sub.3).sub.2.6H.sub.2 O) was dissolved 
in deionized water at a concentration of 0.1-2 mole/liter. Another aqueous 
phase of another emulsion was formed with a metal aqueous solution in 
which commercial iron nitrate (Fe(NO.sub.3).sub.3.9H.sub.2 O) was 
dissolved in deionized water at a concentration of 0.1-2 mole/liter. 
Another aqueous phase of the still another emulsion was formed with a 
metal aqueous solution in which yttrium nitrate 
(Y(NO.sub.3).sub.3.6H.sub.2 O) was dissolved in deionized water at a 
concentration of 0.1-2 mole/liter. The other aqueous phase of the other 
emulsion was formed with a metal aqueous solution in which commercial 
titanium tetrachloride (TiCl.sub.4) was diluted with deionized water at a 
concentration of 0.1-2 mole/liter. 
The conditions in this example were the same as those of Example 1 in an 
oil phase for constituting the emulsion, an emulsion-producing step, and a 
powder-producing step. 
The present inventor confirmed that such examples provided hollow particles 
made from magnesium oxide, hollow particles made from iron oxide, hollow 
particles made from yttrium oxide, and hollow particle titanium oxides, 
respectively. They confirmed that these particles had a shell wall 
thickness of 20 nm or less. 
Example 4 
The emulsion concerning Example 4 was composed with the aqueous phase 
formed in Example 1, the oil phase formed in Example 1, and NP6 
(polyoxyethylene-6-nonyl-phenyl-ether) as a surfactant. A mole ratio of 
water/NP6 was adjusted between 10 and 100; so, a emulsion was composed 
including aqueous microspheres having a each diameter of 100-400 nm. 
A powder-producing step was carried out on the same conditions as those of 
Example 1. In the same way as Example 1, the present inventor confirmed 
that this example provided hollow alumina particles in which each particle 
had a shell wall thickness of 20 nm or less. Therefore, when the aqueous 
microsphere of the emulsion was 100 nm or more in diameter, one aqueous 
microsphere resulted in one hollow oxide particle. 
Comparative Example 
This comparative example is to alumina particle by way of a step in which 
the an aqueous microsphere is below 100 nm in diameter. The surfactant was 
NP6 (polyoxyethylene-6-nonyl-phenyl-ether). A mole ratio of water/NP6 was 
adjusted between 10 and 100; so, an emulsion was composed including an 
aqueous microsphere having a diameter of 30-80 nm. A powder-producing step 
was carried out on the same conditions as those of Example 1. This 
comparative example, unlike Examples 1-4, was not able to produce hollow 
particles. 
Examples 5 and 6 
There was used an aqueous phase and an oil phase composing an emulsion 
concerning Examples 5,6. The aqueous phase was formed with an aluminum 
nitrate aqueous solution in which commercial aluminum nitrate was 
dissolved in deionized water at a concentration of 0.1-2 mole/liter. An 
organic solvent was commercial kerosine. SUN SOFT No. 818H (TAIYOU KAGAKU 
Co., Ltd.) was used at 5-10 wt. % So with respect to the kerosine as a 
surfactant. The kerosine including such surfactant constituted the oil 
phase in the emulsion. The aqueous phase and the oil phase were mixed so 
that a ratio of aqueous phase /oil phase was (50-70)/(50-30) by volume %. 
This mixed solution was stirred by use of the homogenizer at 1,000-20,000 
rpm for 5-30 minutes to form a Water in Oil (W/O) emulsion. According to 
observations of the optical microscope, the aqueous microsphere of the 
emulsion was approximately 1-2 .mu.m in diameter. 
The aforesaid Water in Oil (W/O) emulsion was atomized and burned by use of 
a emulsion-burning apparatus to synthesize the oxide powders. As for 
conditions for synthesizing the oxide powders, the emulsion-burning 
apparatus controlled a flow rate of the emulsion, a flow rate of air 
(oxygen), and the like in such a manner that the atomized emulsion was 
completely burned and temperature of flame was constant in the range of 
600-800.degree. C. The produced particles were collected by use of the bag 
filter disposed at the rear side of the reaction tube of the apparatus. 
The produced oxide powder particles were observed by use of the 
transmission electron microscope (TEM). According to the TEM observation, 
the alumina powder particle was a hollow particle whose shell wall was 
10-20 nm in thickness. This particles displayed Example 5. 
The synthesized oxide particles (1-10 g) concerning Example 5 were mixed 
deionized water (10-1000 cc) to form a suspension. This suspension was 
stirred by use of a magnetic stirrer at room temperature for 1-240 
minutes. Thereafter, the suspension was filtrated, and then the filtrated 
product was washed with deionized water several times. The product was 
dried, crushed. After that, the crushed powder product was estimated in 
observation of shape (SEM) and in measurement of the specific surface area 
(BET). 
In Example 6, a water-treatment was carried out with respect to the oxide 
particles produced in Example 5. The SEM photograph exhibited in FIG. 4 
shows a surface structure of the oxide particles after the water-treatment 
in Example 6, whereas the SEM photograph exhibited in FIG. 5 shows a 
surface structure of the oxide particles before the water-treatment in 
Example 5. 
FIG. 5 shows that the surface of the particle before the water-treatment 
was extremely smooth. As a result of measurement, the specific surface 
area before the water-treatment was 46 m.sup.2 /g. This measured result 
approximately corresponded to a specific surface area calculated from an 
amount of aluminium ion, a diameter of synthesized particle, thickness of 
shell wall, and its density, provided that the shell wall is smooth and 
nitrogen is absorbed at the inner surface and the outer surface of the 
shell wall. Thus, it was also thought that the surface of the particle was 
smooth, judging from correspondence of both specific surface areas. 
Meanwhile, according to the oxide particle after the water-treatment 
concerning Example 6, as shown in FIG. 4, it was found that a surface 
structure was collapsed to exhibit an irregular structure having the order 
of tens nm. 
Examples 7-9 
In Example 7, the oxide particles produced in Example 5 was water-treated 
and its specific surface area was measured in the same method as Example 
6, except that nitric acid having a concentration of 1 mol/liter was used 
instead of the deionized water in a water-treatment. 
In Example 8, the oxide particles produced in Example 5 was water-treated 
and then its specific surface area was measured in the same method as 
Example 6, except that an aqueous ammonia solution having a concentration 
of 1 mol/liter was used instead of the deionized water in a 
water-treatment. 
In Example 9, the oxide powder particles produced in Example 5 was 
water-treated and then its specific surface area was measured in the same 
method as Example 6, except that a mixed solution of deionized water and 
ethanol, having a volume rato of 1:1, was used instead of the deionized 
water in a water-treatment. 
Examples 10-13 
These examples changed the aqueous phase of the emulsion which is the mixed 
aqueous solution of aluminum nitrate and sodium nitrate at a Na/Al mole 
ratio of 1/99 and which have a concentration of 0.1-2 mol/liter, instead 
of the aluminum nitrate having a concentration of 0.1-2 mole/liter. These 
examples produced alumina powder particles containing Na (sodium) in the 
same way as Example 5. Such particles was hollow and their shell wall were 
extremely thin like Example 5. 
In Example 10, one group particles of such particles did not experience a 
water-treatment, and was measured about a specific surface area. 
In Examples 11-13, another group particles of such particles experienced a 
water-treatment in the following methods, and they were measured about a 
specific surface area. The oxide particles concerning Example 11 was 
water-treated with deionized water like Example 6. The oxide particles 
concerning Example 12 were water-treated with a nitric acid solution 
having a concentration of 1 mol/liter like Example 7. The oxide particles 
concerning Example 13 were water-treated with an aqueous ammonia solution 
having a concentration of 1 mol/liter like Example 8. 
Examples 14-17 
These examples changed the aqueous phase which is the mixed solution of 
aluminum nitrate and magnesium nitrate at a Mg/Al mole ratio of 1/99 and 
which have a concentration of 0.1-2 mol/liter, instead of the aluminum 
nitrate having a concentration of 0.1-2 mole/liter. These examples 
produced alumina particle containing Mg (magnesium) in the same way as 
Example 5. Such particles were hollow and their shell walls were extremely 
thin like Example 5. 
In Example 14, one group particles of such particles did not experience a 
water-treatment, and was measured about a specific surface area. 
In Examples 15-17, another group particles of such particles experienced a 
water-treatment in the following methods, and were measured about a 
specific surface area. 
The oxide particles concerning Example 15 were water-treated with deionized 
water like Example 6. The oxide particles concerning Example 16 were 
water-treated with a nitric acid solution having a concentration of 1 
mol/liter like Example 7. The oxide particle concerning Example 17 was 
water-treated with an aqueous ammonia solution having a concentration of 1 
mol/liter like Example 8. Table 1 lists the specific surface area of the 
aforedescribed oxide particles concerning Examples 5-17. 
TABLE 1 
______________________________________ 
Item Specific Surface Area, m.sup.2 /g 
______________________________________ 
Example 5 46 
Example 6 201 
Example 7 185 
Example 8 212 
Example 9 80 
Example 10 45 
Example 11 253 
Example 12 193 
Example 13 230 
Example 14 50 
Example 15 148 
Example 16 130 
Example 17 162 
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As appreciated in Table 1, the specific surface area of particle after 
water-treatment concerning Examples 6-9, Examples 11-13, and Examples 
15-17 are considerably increased as compared with that of the particle 
before water-treatment concerning Examples 5,10,14. According to SEM 
observations concerning Examples 6-9, Examples 11-13, and Examples 15-17, 
with a difference depending upon compositions of the particle and 
conditions of the water-treatment, a collapse was observed in the surface 
structure of the particle like Example 6. Example 9, using the mixed 
solution of water and alcohol, reduced a chance for water to be brought 
into contact with the particle. 
Thus, it was found that collapse was small at the surface structure of the 
particle and increase of the specific surface was small in Example 9 as 
compared with the case using only water. This implies that change of 
water/alcohol ratio controls a surface structure and a specific surface 
area. According to the results of the above-examples, the specific surface 
area was as follows: 
particles treated with ammonia-solution&gt;particles treated with deionized 
water&gt;particles treated with nitric acid solution 
This implies that a pH value of the solution used in the water-treatment 
can control a specific surface area of particles 
Examples 18-21 
The alumina particles produced in Example 5 were heat-treated by use of an 
electrical furnace in the range 700-1,000.degree. C. for 4 hours. The 
heat-treated particles were water-treated with deionized water, and their 
specific surface areas were measured Table 2 lists the measured specific 
surface areas. As for temperature of the heat-treatment, Example 18 was 
carried out at 700.degree. C., Example 19 was carried out at 800.degree. 
C., Example 20 was carried out at 900.degree. C., and Example 21 was 
carried out at 1,000.degree. C. 
TABLE 2 
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Item Specific Surface Area, m.sup.2 /g 
______________________________________ 
Example 18 198 
Example 19 185 
Example 20 158 
Example 21 109 
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As appreciated from Table 2, it was founded that the particles after 
heat-treatment increased their specific surface area. Also, it was founded 
that the particles after heat-treatment decreased their specific surface 
area with increasing temperature in the heat-treatment. 
According to SEM observations with respect to the particles concerning 
Examples 18-21, with difference depending upon conditions of 
heat-treatment, a collapse was observed at the surface structure of the 
particle like Example 6. The higher the temperature of heat-treatment, the 
smaller the collapse at the surface structure. Namely, it was confirmed 
that the surface structure of the particles tended to approach a state 
displayed in FIG. 5 exhibiting Example 5. This may be based on the reason 
why oxidation is partially proceeded by heat-treatment to decrease 
hydroxide and undecomposed metal salts remaining at the surface of the 
crystallines., so that collapse is suppressed at the surface structure. 
This implies that heat-treatment carried out after synthesizing the oxide 
particle can control a surface structure and a specific surface area 
between water-treated state and not water-treated state.