JPH0557166A - Production of coated fine particle - Google Patents

Abstract

PURPOSE:To obtain coated fine particles by dissolving a 1st and 2nd solutes separately into corresponding 1st and 2nd solvents to form the 1st and 2nd supercritical systems respectively, forming fine particles of the 1st solute by the adiabatic expansion of the 1st system, pressurizing the expanded system to mix with the 2nd system, depositing the 2nd solute on the surface of fine particles of the 1st solute by the adiabatic expansion of the mixed system. CONSTITUTION:A 1st solute (e.g. SiO2) and a 2nd solute (e.g. La2O3) are dissolved separately into a 1st solvent (e.g. water and a 2nd solvent (e.g. water) contained in corresponding vessels 1, 3 to form 1st and 2nd systems in supercritical or subcritical state, respectively. The 1st system is introduced into a vessel 1 by adiabatic expansion to form fine particles of SiO2 through an oversaturated state. This system, after being pressurized again to the same pressure with the 2nd system, is mixed with the 2nd system; the mixed system is then introduced into the vessel 2 by adiabatic expansion to deposit La2O3 on the surface of of SiO2 fine particles through an oversaturated state to prepare coated fine particles. In result, the solute can be deposited from the solvent in a supercritical or subcritical state with good reproducibility to grow to fine particles.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing coated fine particles, and in particular, the fact that a solvent in a supercritical state or a subcritical state exhibits a high dissolving ability, and a solute dissolved in the solvent is utilized. The present invention relates to a method of depositing as fine particles by pressure control. The coated fine particles are applied to catalysts, fine ceramics molding materials, magnetic materials, photosensitive materials, sensor materials and the like.

[0002]

2. Description of the Related Art Fine particles have a remarkably large surface area per volume, and their surface energy also remarkably increases.
It plays an important role in various technical fields such as sintering, adsorption, and catalysts. Further, when the fine particles are amorphous solid, the energy of the fine particles themselves becomes higher than that of the crystalline particles, and the surface activity becomes very high. For these reasons, various means have been studied for producing fine particles, and various effects have been exhibited in each technical field.

In recent years, a solute is dissolved in a supercritical fluid formed under a high pressure of several hundreds of atmospheres or more, and the solute is jetted from an orifice onto a substrate toward an atmosphere under atmospheric pressure or reduced pressure, and instantaneously in a jet stream. A technique for forming a solid film or ultrafine particles on a substrate by utilizing the occurrence of a supersaturated state in the depressurizing process has been proposed by Smith Richard D. et al. (Published patent No. 61-500210). According to them
"The injection process is subject to the formation of solvent clusters during expansion and the phenomenon of solvent cluster" freak up "in Mach Disks resulting from free jet or ultrasonic expansion of the solution" from the orifice. It has been pointed out (the above-mentioned published patent publication, page 4, upper right column). That is, the agglomeration nuclei of the solvent are destroyed at once by the impact of the Mach disk to form a solid precipitate composed of solute ultrafine particles, which is deposited in a film form on the substrate. Therefore, it can be said that this technique provides one guide in the field of membrane production. It is generally known that the solubility of supercritical fluids becomes extremely high, and this property has been applied to the growth of single crystals of quartz and the extraction of specific solutes from a mixture. Can be considered to come from the idea of forming a solid film or ultrafine particles from a supercritical fluid.

Oxide coating on fine particles
As a technique, a CVD method is known as a vapor phase method, and an alkoxide method is known as a liquid phase method. Since the CVD method handles a gas that has both strong toxicity and flammability depending on the coating material, a complicated and large-scale gas facility is indispensable and the manufacturing cost becomes high. When processed at high temperature, thermal strain remains in the product. Since the alkoxide method itself is very expensive, the alkoxide method also raises the manufacturing cost. Furthermore, it is said that each of these methods has drawbacks such as many technically difficult problems in producing them on a large scale, and few of them have been put into practical use.

[0005]

The present inventors have been conducting various researches from the viewpoint of application of the supercritical state for some time (for example, Chemical Engineering 54th Annual Meeting, 1989), and the above-mentioned conventional techniques are also The effect was confirmed. However, according to the experiments by the present inventors, it was found that the method of forming ultrafine particles in a jet has some problems as follows. Solids produced in the sonic region have poor reproducibility of their properties (number of fine particles formed, size, shape, etc.). Not all of the solutes in the jet flow are the target thin film or fine particles, and there are some that scatter into the atmosphere without being collected, and thus the yield becomes poor. It is difficult to stably manufacture the nozzle with the same hole diameter and the same shape, and the jet flow changes even with a slight difference, so that the reproducibility of the manufacturing conditions and the recovery of the produced fine particles are poor. It is difficult to collect the solidified particles.

The present invention has been made in view of the above circumstances, and its purpose is to produce fine particles by reproducibly depositing and growing a solute from a solvent in a supercritical state or a subcritical state. It is an object of the present invention to provide a method capable of producing inexpensive coated fine particles with a simple device configuration that can be obtained, is easy to collect, is easy to operate, and can be made large in size.

[0007]

Means for Solving the Problems The present invention, which has achieved the above-mentioned object, means that the first solute and the second solute are separately dissolved in the corresponding first solvent and second solvent, respectively, and each of them is brought into a supercritical state or a sub-solvent. After making the first system and the second system in the critical state, the first system is adiabatically expanded to generate fine particles of the first solute through the supersaturated state, and this is boosted again to the same pressure as the second system before The gist of the present invention is to form a coated fine particle by mixing the second solute with the second system and then adiabatically expanding the mixed system to cause a supersaturated state to deposit the second solute on the surface of the fine particles of the first solute.

[0008]

The principle and operation of the present invention will now be described with reference to the drawings.
The effect will be described. Figure 1 shows SiO in high temperature and high pressure water.
_It is a graph (GC Kennedy, 1950) which shows the change of the solubility of ₂ . As is clear from FIG. 1, above the critical temperature, the solubility of SiO ₂ increases with increasing temperature and / or pressure.

For example, point A in FIG. 1 (550 ° C., 750 Bars)
In the case of adiabatically expanding supercritical water in which SiO ₂ is saturated and dissolved under an atmospheric pressure through an orifice, according to thermodynamic calculation, the temperature and pressure of the supercritical water follow the course as shown by line L _1. Decreases, and therefore the solubility of SiO ₂ also decreases. However, even if the solubility decreases, the solute solid does not immediately precipitate, but it precipitates after the supersaturated state is formed.

Now, assuming that the solubility under the initial conditions is Xo and the solubility corresponding to an arbitrary instantaneous pressure during depressurization is Xp, the supersaturation degree C is expressed by the following equation (I).

[0011]

[Equation 1]

As is apparent from FIG. 1, the solubility of SiO ₂ under atmospheric pressure is almost zero, and therefore when jetted from the orifice (that is, when adiabatic expansion occurs instantaneously).
The degree of supersaturation of is infinite.

Generally, if the degree of supersaturation is small, the solute does not precipitate, and if the degree of supersaturation exceeds a certain degree (critical supersaturation degree), the probability of nucleation begins to appear, and if the degree of supersaturation is further higher, the probability of nucleation increases. It will become. Therefore, when the pressure decreases and the degree of supersaturation approaches infinity, an infinite number of ultrafine nuclei are generated, and when they are deposited on the solid surface on the downstream side of the jet flow, a thin film is formed. From this, when it is desired to form fine particles instead of a thin film, it is necessary to suppress the degree of supersaturation to a certain degree and keep the number of nuclei generated within a certain range to grow the generated nuclei into fine particles.

Many examples of the relationship between the degree of supersaturation and the probability of nucleation as described above are recognized in the actual industry, but this is based on the method of statistical mechanics, McDonald (McDonald)
An example of the calculation is shown in Table 1 below (by Toshio Kuroda, Crystals are alive, published by Science Co., Ltd.). This Table 1 shows various supersaturation ratios P /, where Pe is the vapor pressure (4.58 Torr) of water vapor at 0 ° C and P is the vapor pressure of the environment to be handled.
The nuclear generation frequency Js for Pe, the radius r ^{* of} the critical nucleus, and the number of molecules i ^* contained in the critical nucleus are shown.

[0015]

[Table 1]

From Table 1, the following can be considered. P / Pe
It may be considered that when n is 3 or less, nuclei are not substantially generated.
When P / Pe is 4, the probability of nuclear generation per second is 5 /
It becomes cm ³ · sec, and when P / Pe is larger than that, it is a condition in which a nucleus can be practically generated. However, when P / Pe is increased to 8, the nuclear generation frequency Js is also increased to 4 × 10 ¹⁵ pieces / cm ³ · sec, which is actually expressed as an infinite number.

The gas molecules are aggregated around the thus-generated nuclei, and the nuclei that have been previously generated become larger in size and the number of gas molecules decreases. If it is done in a closed system, P / Pe will eventually decrease and the frequency of nucleation will decrease. This tendency is the same when P / Pe is intentionally reduced, and for example, when P / Pe is 3 or less, the probability of new nuclei is virtually zero, and by that time Grains based on the generated nuclei grow and consume gas molecules, while P / Pe further decreases and finally reaches 1. However, as is clear from Table 1, the radius r of the critical nucleus is
^* Is 10 Å or less, cannot be used as fine particles as it is, is unstable in energy, and tends to grow into large ones. I went.

Table 1 above is an example of calculation of steam (steam) at 0 ° C. of pure component (water), and relates to generation of droplets from steam. However, this tendency also holds in the case of gas-solid transition and liquid-solid transition. Further, it is also true for a component system in which one is a solute and the other is a solvent. However, in this case, as the degree of supersaturation, instead of (P-Pe) / Pe, C shown in the above formula (I) is used.
Is often used.

When the technique of Smith et al. Was examined in consideration of the above circumstances, the following findings were obtained. SiO under atmospheric pressure immediately after being injected from the orifice
Solubility Xp of ₂ is almost 0, and therefore supersaturation C
Is infinite, and at this time, innumerable nuclei are generated, and the supersaturation degree C instantly approaches 0 with it, so that the innumerable nuclei generated hardly occur. This is the basic principle of forming a thin film and ultrafine particles in the above technique. Moreover, in the above technique, when jetted from the orifice, the cluster (a group of molecules serving as the core of the nucleus) is crushed by the impact on the Mach disk, and further miniaturization is achieved. In addition, in the research by Smith et al.
O ₂ is also shown an example of polystyrene as a representative of organic matter in addition to, but has been further injected to a reduced pressure atmosphere from atmospheric pressure wherein this equilibrium concentration X in the formula (I)
The value of p is further reduced to increase the degree of supersaturation C, which results in accelerating the above tendency.

On the other hand, the present invention aims to produce fine particles that have grown to some extent, and the principle thereof will be described here. For example, in Table 1 above, P / Pe is 4
In this state, 5 × 10 = 50 nuclei are formed, and these nuclei are aggregates of at least 87 molecules as shown by i ^* in Table 1. Therefore, the nuclei grow by taking in molecules one after another by using this molecular aggregate as a nucleus, but when P / Pe grows into an aggregate of more than 695 aggregates.
If it is changed to = 2, the nucleus can continue to grow without being decomposed and disappeared because it is a molecular aggregate of 695 or more. That is, 50 particles will continue to grow as they are. The new nuclei that are further generated and increased while the number of 50 particles grows to more than 695 molecules is that the number of molecules is i
^{* Because} it is less than 695, when P / Pe is changed from 4 to 2, it decomposes and vaporizes again and disappears. Eventually, only particles having a relatively large particle size can achieve particle growth, and finally become fine particles having a uniform particle size. When P / Pe is changed from 4 to 3 → 2 → 1, a considerably large difference occurs in the supersaturated state in the vicinity of the particles. According to the calculation, when the vapor pressure P is 1, 2, 3, 4, the number of vapor molecules in 1 cm ³ is 1.6, 3.2, 4.8, 6.4 (× 1), respectively.
0 ^17), and this difference is considered to cause the watch and form a serious difference from the standpoint of growth occurs.

For example, point A in FIG. 1 (750 atm)
If it reaches a point A '(500 atm) after adiabatic expansion from, the solubility of SiO ₂ in the supercritical fluid changes from X ₇₅₀ = 0.165 to X ₅₀₀ = 0.07, at which time X ₇₅₀ / X ₅₀₀
(Hereinafter sometimes referred to as the solubility ratio) is 2.2. Assuming that a finite number of SiO ₂ agglomerated nuclei have started to be generated and grown, the value of the solubility X decreases and new nuclei do not occur. Therefore, when the pressure is gradually reduced from 500 atm to 300 atm, the solubility ratio (X / X ₃₀₀ ) at that time increases again, but no new nucleation occurs unless the solubility ratio above 2.2 is exceeded. Only existing particles grow. Thereafter, according to the same principle, the vapor pressure is reduced to the atmospheric pressure at a speed at which new nuclei are not generated, whereby fine particles having a uniform particle size can be obtained. In the vicinity of atmospheric pressure, the solubility is almost zero, and even if the pressure and temperature are drastically reduced, the properties of the particles are hardly affected. However, if the solubility before the sudden release of the residual pressure shows a substantial value that cannot be ignored, even if the absolute value of the solubility is low, the solubility ratio in that condition satisfies the nucleation condition and a critical nucleus is generated. By doing so, fine particles may adhere to or grow independently on the surface of large particles that have already grown to some extent.

In the above description, the supersaturation ratio X / Xp for critical nucleation was assumed to be 2.2 for convenience, but this value cannot be easily measured with the current technology. Even if it could be measured, it would be affected by all operating conditions such as temperature, pressure, solubility, type of solute and solvent, and acid or alkali added in a trace amount, or so-called entrainer. Clarifying the conditions has no general meaning. Regarding the operating conditions, how much decompression should be performed, or whether it should be interrupted once in the middle and gradually reduced in pressure, etc.
Adaptive conditions are determined based on the properties of the target particles. However, since the present invention is carried out in a closed system of a high-pressure container, it can be reproduced in a relatively wide range of conditions, and strict condition setting is required so as to cause confusion in industrial operation. is not.

In the above description, the example of adiabatic expansion from the point A in FIG. 1 has been described as the starting condition, but under this condition, the liquid enters the gas-liquid coexistence line through the pressure below the critical point, Drops start to form. At this point, if a considerable amount of solubility still remains, the water itself which aggregates as droplets has the tendency shown in Table 1, and water molecules and SiO 2
_{It is possible that two} molecules form a mixed or hydrogen-associated cluster or even a critical nucleus. Then, the water droplets increase in number or grow. Then, when water is finally removed, a solid is obtained. If this process is shown on the temperature-specific entropy diagram of the water vapor pressure in FIG. 2, it means that the point on the right side of the apex (critical point c) of the boundary line showing the gas-liquid coexistence region descends directly from the point A.

On the other hand, point B in FIGS. 1 and 2 is changed from the supercritical state to the subcritical state (liquid phase region lower than the critical temperature).
Or at the point B'starting from the subcritical state, the pressure is reduced and passes through the liquid side (left side) of the critical point c into the gas-liquid coexisting region.

Regarding the critical nucleus generation in the subcritical state, although the numerical values are different from those in Table 1, the relationship between the critical nucleus radius, the nuclear generation frequency and the supersaturation ratio shows the same tendency. In addition, when entering the gas-liquid coexistence region from the subcritical region, unlike the case where the point A is used as a starting point, the saturated solubility decreases while the state change progresses in the direction in which the proportion of water vapor increases.

Even when the fine particles are produced by depressurizing from or through the subcritical state, various forms of fine particles can be obtained by controlling the pressure lowering rate. Furthermore, by entering the gas-liquid coexistence region from the subcritical liquid, the change in the agglomeration mechanism accompanying the progress of vaporization is also utilized, and more diverse shapes and particle size distributions can be expected.

The nuclei described above are mainly agglomerates at the molecular level, but the particles when they grow do not necessarily mean that they are agglomerates of ultrafine particles. The grown particles are amorphous solids, and depending on the growth process and the temperature elapse until extraction,
Some or all of them may have a crystalline structure, and they may aggregate.

It will be explained below that the fine particles obtained by the present invention have a wide variety of morphology. When first prepared under relatively mild supersaturated conditions, molecules or very small particles (generally below the critical core radius) also grow to adhere to the surface of large particles. Further, when the condition that new nucleation is possible after growth with a small X / Xp is given, the newly generated nuclei should be attached to the large particles that are present before the nuclei become large. There is also. No further nucleation,
When produced under conditions where the degree of supersaturation is considerably high, whiskers grow rapidly on the surface of the spherical fine particles. As shown in the critical radius in Table 1, if the whisker-like particles grow, if the whiskers have a whisker of a certain size or more when the supersaturation ratio becomes small, they will exist under the condition of X / Xp = 1 in terms of energy. It is not allowed, and when X / Xp = 1 suddenly, the whiskers that have already occurred may disappear.

In the above description, the substance to be aggregated is described as a solute. However, it is naturally possible that a solute and a solvent associate with each other to generate a cluster or aggregate in a state of being adsorbed. In such a case, the particles agglomerated under high pressure, the solvent is vaporized again in the depressurization process and the particles become porous, or "air deflated balls" depending on the conditions.
After being deformed as described above, it may be extracted as hard particles.

Since the present invention uses the driving force to generate supersaturation due to the pressure drop, it is preferable from the standpoint of efficient use of pressure that the solute is dissolved close to the saturation solubility under the pressure drop start condition, if possible. It is preferable, but depending on the purpose, it is possible that the unsaturated state is reduced to depressurize and then saturated to reach supersaturation.

The present invention was obtained by considering the theory of critical nucleus generation based on statistical mechanics such as gas molecule kinetic theory, and is extremely general. Therefore SiO ₂
Not limited to supercritical water, it is applicable to all systems that increase solubility under high temperature and high pressure, and when water is used as a solvent, various oxides, some hydroxides, or some sulfuric acid are used. Fine particles of various compounds such as salts and phosphates are obtained. The water used at this time may be adjusted to increase the solubility by adding an appropriate amount of acid, alkali or halogen. Fine particles such as organic substances can also be obtained by using a solvent such as carbon dioxide, methane, propane, and methanol which has been used in recent years for supercritical extraction. It is also possible to dissolve two or more solutes in a supercritical fluid at an appropriate molar ratio and react them to obtain fine particles.

The driving force for producing fine particles in the present invention is adiabatic expansion. As a result, the inside of the high-pressure container can be uniformly controlled within a very wide expansion velocity range. Since the adiabatic expansion here is performed in the closed system of the high-pressure container, the temperature drop of the fluid during adiabatic expansion is unavoidable, and therefore the inflow of heat into the fluid from the container held at the expansion start temperature is avoided. I can't. On the contrary, when the fluid is transferred to another high-pressure container kept at a relatively low temperature to be adiabatically expanded, heat is conversely discharged. However, these are heat transfer necessary for operation, and the driving force for aggregation and precipitation is adiabatic expansion accompanying pressure change.

The method of coating fine particles of the present invention basically applies the above-described method of producing fine particles using a supercritical fluid, and the two solutes are separately supercritical fluid or subcritical fluid. After being dissolved in the fluid, the temperature and pressure of the first solvent are lowered to prepare solute fine particles, and then the second solvent is added.
The solvent and the solvent are mixed together, and then the pressure and temperature are lowered to precipitate the second solute on the surface of the first fine particles to prepare coated fine particles. The important point in this method is that after the formation of the first fine particles, the remaining solution does not have a solute concentration enough to generate new nuclei or grow the already formed particles. The three points are that the fine particles do not dissolve, or that there is a very small amount even if they dissolve, and that the second solute does not generate nuclei and grow independent particles with respect to the first solute during mixing. Is.

In order to achieve the important point, the fine particles of the first solute are prepared at the lowest possible temperature and low pressure. For example, in FIG. 1, the solubility decreases as the temperature and pressure are lowered. If the temperature is sufficiently lowered and the pressure is set to 0, the solubility decreases and the solvent around the generated fine particles has no ability to generate or grow new nuclei. Next, in this state, the pressure is increased to the same pressure as the second solvent. Depending on the temperature, the dissolution capacity increases at this time, but if the temperature is sufficiently low and the time is short, this concern does not occur, and the generated fine particles do not dissolve and disappear. Next, when the second solvent is mixed in the same pressure state, the temperature becomes an intermediate temperature between the first solvent and the second solvent immediately after mixing. That is, the temperature of the first solvent will rise. At this time, there is a problem when the solubility increases, but in reality it can be seen that the solubility takes a maximum value with respect to temperature changes at a constant pressure, as can be seen from the data up to 600 Bars in FIG. It is said that this tendency is due to an increase in solvent ability and a decrease in density due to temperature increase (the density and the solubility are almost proportional to each other), and is valid for any solvent and solute. Figure 3 shows the solubility of lanthanum oxide in water. Therefore, even if the temperature of the first solvent containing the solute particles is increased under the same pressure as that of the second solvent, the dissolving ability of the first solvent is rather decreased, and the particles are not dissolved and disappeared.

As to the important point of the above, since the second solvent is kept at a high temperature before the mixing and the second solute is saturated and dissolved, and the temperature immediately after the mixing is lowered in the first solvent, the maximum value of the above-mentioned solubility is obtained. Since the solubility changes from the high temperature side toward the maximum value, the solvent dissolution capacity increases, and therefore the second solute does not precipitate during mixing. In the mixed state of the first solvent and the second solvent, the second solute is completely dissolved in the second solvent slightly diluted with the first solvent, and the fine particles of the first solute are suspended in the second solvent. A condition arises. Next, when the mixed fluid is expanded under reduced pressure to cause supersaturation of the solubility of the second solute in the second solvent, the fine particles of the first solute serve as a seed, and the second solute is deposited on the surface of the fine particles. And grow. In this step, the first solute is hardly dissolved, so that no new nucleation occurs from the solution. Further, by controlling the degree of supersaturation, that is, controlling the pressure and temperature, new nuclei of the second solute are not generated, and the first solute fine particles can be coated with the second solute.

As described above, the characteristics of the temperature dependence of the solubility (having the maximum value) are utilized, and the particles are separately dissolved on both sides of the maximum value of the solubility of each substance, and then mixed to form the coated fine particles. Create.

FIG. 4 shows an apparatus configuration for realizing the above manufacturing method. The first, second, and third high-pressure high-temperature container three cylinders into which the piston rods are inserted respectively communicate with each other via valves as shown in the figure. The first container, the second container, and the third container have different temperatures. The first solute is dissolved in the first container and the second solute is dissolved in the third container. The second container is empty, the piston rod is on the upper end side, and the temperature is low. The first valve is opened, the solvent in the first container is expanded by pulling down the piston rod in the second container, and the solvent is delivered by the piston rod in the first container. In the second container, the temperature and pressure are lowered and fine particles are deposited. Next, the first valve is closed and the second container is pressurized to the same pressure as the third container. The second valve is opened, and the solvent in the second container is transferred into the third container with a constant pressure by the piston rod of the second container. (Or you may transfer from a 3rd container to a 2nd container.)
After that, the piston rod of the third container is retracted and expanded under reduced pressure, and the second solute in the third container is deposited on the surface of the fine particles sent from the second container and coated.
After that, the third valve is opened and the piston rod of the third container is advanced to collect the coated fine particles.

The fine particles obtained by the present invention can be used for various purposes by making effective use of the surface area size, high activity, and other morphological characteristics of the fine particles. Since it is considered that the development of industrial use technology of fine particles can progress to many advanced technical fields in the future, the application range of the fine particles obtained by the present invention is expected to be further expanded in the future.

The present invention will be described in more detail with reference to the following examples. However, the following examples are not intended to limit the present invention, and any design changes may be made in view of the spirit of the preceding and the following. It is included in the technical scope of the present invention.

[0040]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 4 is a schematic explanatory view of an example of the configuration of an apparatus configured to carry out the present invention. Fine particles of silica (SiO ₂ ) were coated with lanthanum oxide (La ₂ O ₃ ) as described below.

Approximately 20 g of crystal (silica single crystal, SiO ₂ ) particles (about 2-3 mm rice grain size) are wrapped in a stainless steel net and inserted into the externally heated first high-pressure container 1 having an internal volume of 140 ml, and the lid is closed. After sealing, the temperature was raised to 550 ° C. After that, water was supplied into the first container 1 by a pump communicating with the first container 1 to pressurize it to 600 kgf / cm ² , and then the valve on the supply side was closed and kept for 5 hours. Next, open the communication valve 4 for the empty externally heated high-pressure vessel 2 with an internal volume of 140 ml that was kept at 200 ° C, advance the piston of the first vessel 1 in which silica was dissolved, and move the piston of the empty second vessel 2 It was retracted and expanded under reduced pressure into the empty second container 2. At this time, the temperature in the second container 2 (measured by CA thermocouple) is about 300.
It was ℃. After confirming that the pressure was 0, a powder (-80 mesh) of lanthanum oxide (La ₂ O ₃ ) was separately dissolved in water in the external heating type third container 3 in the same manner as in the case of silica, and 550 ° C. , 600 kgf / cm ² for 5 hours. Third container 3
The communication valve 5 was opened, and the piston of the third container 3 containing lanthanum oxide was pushed forward while the pressure of the piston of the second container 2 containing silica was kept constant to move the entire internal fluid. The temperature inside the second container 2 after transfer is about 400.
It was ℃. Next, the piston of the second container 2 was retracted to gradually reduce the pressure inside the container to zero. Then open valve 6
Open the piston and move the piston of the second container forward, discharge and collect the fluid inside, filter with a membrane filter, and remove the fine particles remaining on the membrane filter with an electron microscope (SEM).
As a result of the elemental analysis by EPMA at the same time, fine particles were confirmed and Si and La elements were detected.

[0042]

EFFECT OF THE INVENTION Since the present invention is constituted as described above, fine particles can be produced by reproducibly depositing and growing a solute from a solvent in a supercritical state or a subcritical state. Thus, it is possible to provide a method capable of producing inexpensive coated fine particles with a simple device configuration that is easy to recover, easy to operate, and can be upsized.

[Brief description of drawings]

FIG. 1 is a graph showing changes in the solubility of SiO _{2 in} high temperature and high pressure water.

FIG. 2 is a temperature-entropy diagram of water vapor pressure.

FIG. 3 is a graph showing changes in the solubility of La ₂ O _{3 in} high temperature and high pressure water.

FIG. 4 is a schematic explanatory diagram showing a configuration example of various devices for carrying out the present invention.

Claims (2)

[Claims] 1. A first solute corresponding to each of the first solute and the second solute.
Dissolve in a solvent and a second solvent separately to form a first system and a second system in a supercritical state or a subcritical state respectively, and then adiabatically expand the first system to generate fine particles of a first solute through a supersaturated state. Then
This is again pressurized to the same pressure as the second system, mixed with the second system, and then the mixed system is subjected to adiabatic expansion to cause a supersaturated state and then the second solute is deposited on the surface of the fine particles of the first solute and coated. A method for producing coated fine particles, which comprises forming fine particles. 2. The same operation as in claim 1 after forming coated fine particles by the method of claim 1 and further between the n-th (n is a natural number of 3 or more) solute and the n-th system consisting of the n-th solvent. (N
-1) A method for producing coated fine particles, which comprises forming coated fine particles with a coating layer of layer 1.