KR20040015951A - Method of preparation of surface-coated fine powder - Google Patents

Abstract

PURPOSE: A method for coating fine powder is provided to minimize cohesion between particles, simultaneously perform synthesizing and coating of powder and separate the two processes of synthesizing and coating as occasion demands by forming various types of strong metal oxide films on the surface of fine powder. CONSTITUTION: The method for preparing surface coated fine powder comprises a step of forming a primary coating on the surface of powder by contacting matrix powder or precursor powder with sol phase coating solution containing coating material; and a step of milling and heat treating the coated powder, wherein the coating material is an organometallic compound, and the powder is ceramic powder, wherein the organometallic compound tetraethyl orthosilicate, isoproxy aluminum or tetrabutyl titanium, wherein the precursor powder is a metallic compound, wherein the primary coating forming step is performed under the blending, mixing or milling conditions, wherein the milling step is performed under the existence of diluent of metal salt, wherein the heat treating step is performed at a solvent boiling point or a melting point or less of the matrix powder or coating material, wherein the milling step is performed before, after or before and after the heat treating step, and wherein the method further comprises the step of separating a mixture of coating powder and diluent produced after the heat treating step.

Description

Surface-coated fine powder manufacturing method {METHOD OF PREPARATION OF SURFACE-COATED FINE POWDER}

The present invention relates to a method for producing a surface-coated fine powder.

Ceramic fine powder is used in many applications such as structural materials, catalysts, magnetic recording media, abrasives, and the like, and is used to change the color of pigment powders, to control the dispersion characteristics in solution, and / or to improve the sintering characteristics of ceramic moldings. In order to achieve the effect, the surface of the powder is imparted with unique material properties.

Powder coating is a typical method of imparting a function to the surface of the powder. The coating method is a method of using an organic surfactant, a method of adsorbing another small powder on a base powder, or a sol gel coating method. There is this.

Among them, the sol-gel coating method is a method of exposing the powder to a coating sol composed of an organometallic compound to form a gel on the surface, and then heat-treating it. The coating is even and the film is not removed even at high temperatures. You can find an example. However, when the method is applied to the microparticles, since the microparticles are agglomerated by the gel formed on the surface, there is a problem such that characteristics such as high surface area, low sintering temperature, and light transmittance peculiar to the microparticles are damaged.

In addition, all of the above methods are a method of forming a separate coating layer after the synthesis of the powder is completed, and the manufacturing cost is high because it has to go through several times of drying, heat treatment, cleaning, and the like.

Therefore, the present inventors can form various kinds of strong metal oxide films on the surface of the fine powder, minimize the aggregation between particles, and simultaneously achieve the synthesis and coating of the powder, and can be separated economically and flexibly as needed. In order to develop the powder coating method, research was conducted. As a result, all kinds of materials to which the sol-gel coating method such as SiO ₂ , Al ₂ O ₃ , TiO ₂ can be applied can be formed into a film. To provide a fine powder coating method having a wide range of material selection to be applied to a powder precursor that can be a base powder.

In addition, the present invention can control the size of the base metal powder in the milling and heat treatment step to freely generate from nano-sized particles to micron-sized particles, but the process configuration is very simple, the production equipment used in the present invention is already industrially Since they are widely used, they provide a coating method of fine powder, which is relatively inexpensive and easy to mass produce.

1 is a transmission electron microscope (TEM) image of silica-coated cerium oxide powder of Example 1. FIG.

2 is an enlarged photograph of silica-coated cerium oxide powder particles of Example 1. FIG.

Figure 3 is a graph showing the surface potential change with the pH of the silica-coated cerium oxide powder of Example 1.

4 is a transmission electron micrograph of the TiO ₂ -coated alumina powder of Example 2. FIG.

In the present invention, by contacting the base material powder or precursor powder to the sol-like coating solution containing the coating material to form a primary coating on the surface of the powder (pretreatment step), pulverizing the coated powder (grinding step) and heat treatment It provides a surface-coated fine powder manufacturing method comprising the step (heat treatment step).

Hereinafter, the present invention will be described in detail.

1) Pretreatment Step

According to the present invention is first subjected to a step of pretreatment of the base material powder. Pretreatment of the base material powder refers to a process of uniformly fixing the sol coating material on the fine powder. In the present invention, the powder to be used as the base material is not limited in size, form and composition, and may be used as necessary for the application field. It does not have to be deformed into a form unsuitable for use, such as being dissolved by the coating solution or melted during heat treatment. In addition, not only a powder-like material as the desired base material can be used, but also a powder precursor that changes to a desired base material during heat treatment can be used. In the case of preparing the fine powder of the present invention using the precursor, since the hardness of the precursor is usually lower than that of the final powder which has been calcined, the time taken for the grinding process is shortened, and powder synthesis and coating are performed by a series of steps. It is advantageous in terms of manufacturing cost because it can combine the required calcination process and the heat treatment process of coating into one process. As long as a precursor is a compound suitable for the said objective, it does not matter in a kind. Metal compounds such as metal salts (carbonates, sulfates, halogen salts, etc.), hydroxides, and the like are preferred because of their good grinding efficiency and can be converted into metal oxides by calcination during heat treatment.

As the coating material, a material which is converted into a metal oxide by heat treatment may be used, and an organometallic compound is representative. For example, in the case of coating SiO ₂ , tetraethyl orthosilicate (TEOS) can be added to ethanol and a small amount of water and a trace amount of nitric acid as a catalyst can be added to prepare the coating solution. It is a widely used method for coating SiO ₂ film on glass or the like. In the case of Al ₂ O _{3 It} is possible to prepare a coating solution using a triisopropoxy aluminum (triisopropoxy aluminum) and the like in the same manner as described above, TiO ₂ may be used tetrabutyltitanium (tetrabutyltitanium).

In the present invention, in the pretreatment step, it is preferable to uniformly fix the coating solution to the base metal powder using a mixing method such as blending, mixing or milling. Powders in contact with the coating solution are separated by filtering or the solvent is evaporated to dryness to complete the primary coating. At this time, the coating film has an organic component (such as a solvent and an organic metal component of the organic metal) remaining inside the film, and thus the bonding is rather weak. In addition, the particles having a small particle diameter may be in a state in which the particles are aggregated in the process of gelling. Therefore, it is preferable to perform a drying process to give an appropriate film adhesion strength to the weak coating film formed in the coating step so as not to be destroyed in a subsequent grinding process. If the drying temperature is high, the cohesive strength between particles is also strong, which is disadvantageous for pulverization, and should be performed at an appropriate temperature. In general, the drying temperature may be in the range of 0 to 500 ° C. Preferably it is the range of 50-300 degreeC.

2) grinding step

The grinding step refers to a process of grinding the first coated powder into fine powder of a desired size through the pretreatment step. The grinding process can be carried out by known grinding means such as, for example, a ball mill or attrition mill. Grinding may be carried out alone or in the presence of a diluent. In the grinding process, the coating powder is crushed and combined. The base metal powder is pulverized to form a fine powder, and may be pulverized by coating a precursor material having good pulverization efficiency as described above in order to increase the miniaturization efficiency. Grinding may be carried out for several minutes to several days until particles of the desired size are formed.

In the grinding step, a diluent may be used if necessary. The diluent may vary in amount depending on the following purpose, but is generally used in the range of about 0.1 to 50 times by volume ratio of the coated powder. The use of a diluent prevents the fine powders from agglomerating with each other during the grinding and heat treatment processes, and thus makes it possible to easily obtain fine powders of several nm size. Such a diluent component is well ground, there is no problem such as reaction with the base material or coating component during the heat treatment, it is preferable that the material is easy to remove. As the diluent material, various metal salts are typical, and in particular, salt is easily dissolved in water after heat treatment of the powder, and is suitable for the present invention because it is inexpensive and does not melt at a temperature of 800 ° C. or lower. Potassium sulfate has a similar effect to salt, but the melting temperature is 1100 ℃ or more, it is a suitable diluent when a high temperature heat treatment of 800 ℃ or more is required.

As described above, the coating powder and the diluent are added to the grinding means in an appropriate ratio so as to be mechanically crushed and mixed. In the milling step of preventing contact between the milled particles by the action of the diluent, the particle size of the powder can be easily controlled from the individual particles to the aggregated particles by adjusting the ratio of the diluent and the coating powder. Depending on the grinding properties of the substrate, it is possible to produce particles down to several nanometers at this stage.

3) heat treatment step

The heat treatment step is to raise the pulverized coating powder to remove organic components in the coating film and to completely fix the coating on the powder surface. In addition to the above purpose, chemical change of the base material can be simultaneously achieved through heat treatment, and the size of the particles is grown. It is also possible. In particular, since the fine powder by pulverization can be grown to the desired particle size (tens of nm to 100 ㎛) in the heat treatment process, the coating powder manufacturing method of the present invention is completed the synthesis, fine grains and coating of the fine powder at once This can be a very productive way.

In the heat treatment step, the heat treatment temperature and time are generally stronger and denser as the temperature is higher when the metal oxide coating material is fixed. In general, the heat treatment temperature is preferably the boiling point of the coating solution or less than the melting point of the base material powder or coating agent, and in the case of using a diluent, it is preferably less than the melting point of the base powder, coating agent or diluent. Heat treatment temperatures of 300 to about 800 ° C. are preferred. This is because when the heat treatment temperature is increased above the melting point of the diluent, the collision between the particles increases, the particles re-aggregate actively, and thus the particle size increases rapidly, which makes it difficult to control the particle size. The heat treatment step may be carried out before the grinding step as needed, or may be performed twice before and after grinding.

If the grinding and heat treatment steps are carried out using a diluent, the resulting mixture may be subjected to a separation step if necessary. The separation step refers to the process of removing the diluent from the resulting mixture of coating powder / diluent, which may be omitted if the presence of the diluent is not a problem depending on the application. When a metal salt is used as a diluent, it is the easiest and inexpensive method to dissolve and remove the diluent by adding water to the coating powder / diluent mixture. If one side of the resultant mixture is magnetic particles, it is also possible to use magnetic separation.

The powder produced through the above steps is provided with a coating layer on the surface is given a necessary function.

Hereinafter, the present invention will be described in more detail with reference to Examples, but these are not limitative of the present invention.

Example 1

Preparation of Silica Coating Solution

TEOS 2.56g, water 3.90g, and nitric acid 0.13g were sequentially added to 93.41g of ethanol, and the mixture was stirred for 10 minutes. The prepared solution was sealed in a glass bottle and aged for two days at a temperature of 80 ° C. to form a silica sol.

Pretreatment

80 g of the coating solution and 80 g of cerium hydroxide powder were put together with a 15 mm diameter alumina ball into a 370 ml mill pot and milled for 90 minutes. The resulting slurry was filtered through a mesh to separate the balls and then dried at 150 ° C. for 15 hours to remove the solvent.

smash

50 g of the dry powder and 50 g of salt were placed in a 600 ml ZrO ₂ pot with 1 kg of a 6 mm diameter steel ball at 300 rpm for 2 hours for planetary milling (product of Fritsch).

Heat treatment

10 g of the powder was removed and calcined in a 50 ml alumina crucible. The temperature increase rate was cooled to 10 ℃ per minute, 750 ℃ after 4 hours.

washing

The powder was placed in a 50 ml tube, followed by pure water, dispersed for 5 minutes in an ultrasonic cleaner, and centrifuged to discard the solution at the top five times. As a result, cerium oxide powder coated with silica was obtained.

result

The apparent particle size of the prepared powder was ultrasonically dispersed at pH 9 and measured by an optical particle size analyzer (Microtec, UPA), and its average value was 0.32 μm, and 95% of the weight of the powder was 0.74 μm or less.

The prepared silica-coated cerium oxide powder was observed with a transmission electron microscope (TEM), and the results are shown in FIG. 1. From Figure 1, it can be seen that the particles produced according to the present invention are crystal clear and relatively well separated.

A magnified photograph of FIG. 1 is shown in FIG. 2, and a regular cerium oxide lattice can be seen from FIG. 2, and it can be clearly seen that an amorphous silica film is formed thereon.

The coating of the particles according to the invention can also be confirmed by a change in the surface potential of the particles. The isoelectric point of the cerium oxide microparticles which have been subjected to the same process as above without the pretreatment step is pH 8.5, according to the present invention. The coated cerium oxide powder had an isoelectric point at pH 5.5. In order to confirm whether the change of the equipotential point is due to the coating, 10 wt% of the silica powder was mixed with the uncoated powder in the crushing step and the same process resulted in the fine potential of the fine powder at pH 8.1. That is, the produced fine powder was confirmed that the silica powder is coated on the surface of the base material powder, not a simple mixture of the base material powder and the silica powder. Figure 3 is a graph showing the surface potential of the pH change of the coated fine powder shows that the equipotential point is shown at pH 5.5.

Example 2

TiO ₂ Preparation of Coating Solution

2.56 g of tetraisopropoxy titanium and 3.90 g of water were sequentially added to 93.41 g of ethanol, and the mixture was stirred for 10 minutes. The prepared solution has a high gelation rate, so it was immediately added to the pretreatment without a separate aging process.

Pretreatment

80 g of the coating solution and 80 g of alumina (Sumitomo ALM-41, core diameter 1.8 um) powder were put together with a 15 mm diameter alumina ball in a 370 ml mill pot and milled for 90 minutes. The slurry was filtered through a mesh to separate balls, and then dried at 150 ° C. for 15 hours to remove solvent.

smash

300 g of the dry powder and 70 g of salt were put together with 1 kg of 15 mm diameter ZrO ₂ balls in a 600 ml ZrO ₂ pot and subjected to planetary milling (Fritsch) at 300 rpm for 1 hour.

Heat treatment

10 g of the powder was calcined in a 50 ml alumina crucible. The rate of temperature increase was 10 ° C. per minute, and the mixture was cooled at 4 ° C. after 4 hours.

washing

The powder was placed in a 50 ml tube, followed by pure water, dispersed for 5 minutes in an ultrasonic cleaner, and centrifuged to discard the solution at the top five times. As a result, TiO ₂ -coated alumina powder was obtained.

The sample which performed the remaining steps without performing the pretreatment is called Sample 1, and the sample which performs the remaining steps without performing the grinding step after performing the pretreatment is called Sample 2, and the sample that has performed all the above steps is referred to as Sample 3 The results of these experiments are as follows.

result

After dispersing the raw alumina and the samples 1, 2 and 3 in pure water, the apparent particle size was measured by an optical particle size analyzer (Microtec, UPA), and the results are shown in Table 1 below.

division Pretreatment smash Heat treatment detach Average particle size (㎛) Raw material X X X X 1.8 Sample 1 X O O O 1.4 Sample 2 O X O O 2.2 Sample 3 O O O O 1.3

In the above results, only pretreatment was performed and the sample 2, which was not pulverized, exhibited an increase in particle size compared to the raw material due to coagulation between particles. The pretreatment of the sample 3, which had been subjected to all processes, was omitted. It can be seen that the agglomeration did not occur even though the coating 1 had a particle size almost the same as that of the sample 1 which occurred in the coating. In addition, the sample 1 and the sample 3, which have been pulverized regardless of pretreatment, have a somewhat smaller particle size than the raw material.

In the above experiments it can be seen that the surface treatment according to the present invention does not cause particle agglomeration and thereby increase the particle size.

On the other hand, the transmission electron microscope (TEM) observation results of the sample 3 is shown in FIG. As in Example 1, the presence of an amorphous TiO ₂ film formed on the lattice structure of the alumina particles can be confirmed.

From the above, the surface coating method of the powder according to the present invention is generally applicable irrespective of the particle size, type, and thermochemical change of the base metal powder, and the coating material can be used for most metal oxides capable of preparing a sol-gel coating solution. It was confirmed.

Coating method of the fine powder according to the present invention to form a variety of strong coating film, to minimize the agglomeration between particles to maintain the nature of the fine powder, synthesis of the base metal powder, particle size control, coating can be achieved at once In addition to being very high, the range of material choices is very wide. In addition, the process configuration is very simple, the equipment required for production is already widely used industrially, it is relatively inexpensive, and it is very easy to enlarge.

Claims (14)

Forming a primary coating on the surface of the powder by contacting the base metal powder or precursor powder to the sol-like coating solution containing a coating material, pulverizing and heat-treating the coated powder, the surface-coated fine powder manufacturing method. The method of claim 1, The coating material is an organometallic compound, and the powder is a ceramic component. The method of claim 2, Wherein the organometallic compound is tetraethyl orthosilicate (TEOS), isopropoxy aluminum or tetrabutyl titanium. The method of claim 1, Characterized in that the powder precursor is a metal compound. The method of claim 1, Wherein the primary coating forming step is performed under blending, mixing or milling conditions. The method of claim 1, The grinding step is carried out in the presence of a diluent. The method of claim 6, The diluent is a metal salt. The method of claim 1, The diluent is used in an amount ranging from 0.1 to 50 times by volume relative to the powder to be coated. The method of claim 1, The heat treatment step is characterized in that the boiling point of the solvent of the coating solution to the base material powder or the melting point of the coating material. The method of claim 1, Wherein the grinding step is performed before, after or before and after the heat treatment step. The method according to any one of claims 6 to 8, And separating the resulting coating powder / diluent mixture after the heat treatment step. Surface-coated fine powder prepared by the method of any one of claims 1 to 11. The method of claim 12, Fine powder characterized in that the ceramic powder coated with a metal oxide. The method of claim 13, The fine powder, characterized in that the metal oxide is SiO ₂ , Al ₂ O ₃ or TiO _{2 .}