JPH05111627A - Method for mixing fine fragments - Google Patents

Abstract

PURPOSE:To efficiently mix fine fragments without causing problems of crushing, etc. CONSTITUTION:Fine fragments 30 to be mixed are agitated together with a supercritical fluid dispersion medium by a propeller 32 in a mixing chamber 12. After agitation completed, a stop valve is opened, causing the supercritical fluid only to be discharged from the mixing chamber 12 through a filter 34 and a conduit 38.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for mixing fine particles such as powders, whiskers and fibers.

[0002]

2. Description of the Related Art For example, in the case of sintering ceramics or metals, in order to improve the homogeneity of the sintered product, raw material powder,
That is, since it is necessary to mix the ceramic powder or the metal powder and the powdery sintering aid as uniformly as possible, conventionally, generally, the raw material powder and the liquid dispersion medium such as water or alcohol are mixed, and the mixture thereof is used. Is wet-mixed by stirring with a ball mill, a sand mill, or other stirring devices.

[0003]

However, since a liquid dispersion medium such as water or alcohol has a high viscosity and a large surface tension, it is difficult to uniformly mix fine raw material powders having a high cohesive force. It takes a long time. Further, in order to achieve uniform mixing, the mixture of the raw material powder and the dispersion medium must be stirred for a long time, and if the mixing is carried out for a long time, the powder is easily crushed, and thus desired powder characteristics are obtained. It may not be possible or impurities may be mixed. Such problems are more prominent when the powders to be mixed have different sizes, and are more prominent in the case of whiskers and fibers than powders.

In view of the above-mentioned problems in the conventional powder mixing method, the present invention provides fine particles of different kinds having different constituent substances or fine particles of the same kind having different shapes and sizes without causing problems such as crushing. It is an object of the present invention to provide an improved method for mixing fine particles so that they can be mixed efficiently.

[0005]

According to the present invention, the above-mentioned objects are achieved by a method of mixing fine particles in which fine particles to be mixed are stirred together with a supercritical fluid as a dispersion medium.

[0006]

[Function] Since the supercritical fluid has lower viscosity and surface tension than liquids such as water and alcohol, it easily wets the fine particles to be mixed, and the supercritical fluid and the supercritical fluid are more attractive than the attractive force between the individual fine particles. The attractive force with the fluid is greater. Therefore, by using the supercritical fluid as the dispersion medium, the dispersibility of the fine pieces is improved, which makes it possible to uniformly mix the fine pieces in a short time.

[0007]

[Supplementary Explanation of Means for Solving the Problem] Generally, water or the like is attached to the surface of fine particles such as ceramic powder or metal powder, and mixed fine particles are molded and sintered according to the method of the present invention. In such a case, it is preferable to remove water and the like attached to the surface of the fine pieces before mixing the fine pieces.

Thus, according to one embodiment of the method of the present invention, the fine pieces are heated by heating the fine pieces prior to mixing the fine pieces, or by subjecting the fine pieces to a reduced pressure atmosphere, or both. Water and the like adhering to the surface of is removed.

[0009]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

FIG. 1 is a schematic diagram showing one embodiment of a mixing apparatus suitable for use in carrying out the method for mixing fine particles according to the present invention.

In FIG. 1, reference numeral 10 denotes a mixing tank having a mixing chamber 12 inside. The mixing chamber 12 is connected by a conduit 14 to a switching valve 16. The switching valve 16 is the conduit 1
4 is switched between a first position connecting the conduit 18 and a second position connecting the conduit 20. Conduit 18
A cylinder 22 for storing high-pressure CO ₂ is connected to the inside thereof, and a vacuum pump 24 is connected to the conduit 20.

The space inside the mixing chamber 12 is heated to a predetermined temperature by a heater 26, and the temperature and pressure inside the mixing chamber 12 are controlled by a thermometer and a pressure gauge 28, which are not shown in the drawing, respectively. It is supposed to be measured. Further, inside the mixing chamber 20, there is provided a propeller 32 for stirring the fine pieces 30 to be mixed, which are loaded into the mixing chamber, and the propeller is rotatably driven by a driving device (not shown). It is like this.

A filter 34 is provided at the lower end of the mixing tank 10 midway.
And a gas discharge conduit 38 having an on-off valve 36 is connected. The filter 34 substantially only allows the passage of gas,
As a result, when the opening / closing valve 36 is opened, the supercritical fluid in the mixing chamber 12 flows out into the atmosphere as it is or as a normal gas through the conduit 38.

Silicon nitride powder, yttrium oxide powder, and alumina powder were mixed according to the present invention using the mixing apparatus formed as described above.

First, 10 kg of silicon nitride powder (average particle size 0.5 μm, specific surface area 10 m ² / g) and 500 g of yttrium oxide powder (average particle size 0.1 μm, specific surface area 1
5 m ² / g) and 500 g of alumina powder (average particle size 0.1 μm, specific surface area 18 m ² / g) as raw material powder 30 in the mixing chamber 12 of the mixing tank 10 (volume: 15 liters).
Charged inside.

Next, the switching valve 16 is switched to the second position with the on-off valve 36 closed, the conduit 14 is connected to the conduit 20, and the inside of the mixing chamber 12 is 10 ^-2 t by the vacuum pump 24.
The pressure in the mixing chamber 20 was reduced to orr, and the inside of the mixing chamber 20 was heated to 200 ° C. for 1 hour by the heater 26, whereby the deposits on the surface of the raw material powder were evaporated and removed.

Next, the heating temperature in the mixing chamber 20 by the heater 26 is 50 ° C. which is 31.1 ° C. or higher which is the critical temperature of CO _2.
℃ set to connect conduit 14 to conduit 18 switches the switching valve 16 in the first position, the mixing chamber of CO ₂ by opening the on-off valve which is not shown in the figure thereafter bomb 22 Then, the pressure of CO _{2 in} the mixing chamber was increased to 73 atm, which is the critical pressure thereof, to 90 atm by adjusting the opening amount of the on-off valve to bring CO ₂ into a supercritical state.

Next, while maintaining the supercritical state of CO _2, the propeller 32 was rotated at 200 rpm for 3 hours to stir and mix the raw material powder 30 with CO _{2 in the} supercritical state. Next, the on-off valve of the cylinder 22 was closed, and the on-off valve 36 was opened to allow CO ₂ in the supercritical state in the mixing chamber 12 to flow into the atmosphere through the conduit 38 and the filter 34. The filter used was a porous ceramic filter having an opening size of 0.2 μm.

Next, the raw material powder 30 mixed as described above is taken out from the mixing tank 10, and the raw material powder is subjected to a surface pressure of 200.
5 × 4 × 50mm by press molding at kg / cm ²
The test piece of No. ₂ was formed and the state of dispersion of Y ₂ O ₃ was investigated by EPMA. This Y-Lα image is shown in FIG. In FIG. 2, the white portion is Y ₂ O ₃ (same in FIG. 3 described later).

[0020]

Comparative Example The same raw material powder as in the above-mentioned example was charged into the mixing tank 10, 40 kg of ethyl alcohol was then charged, and they were stirred and mixed by the propeller 32 for 50 hours. Then, the raw material powder thus mixed and processed is taken out from the mixing tank 10, alcohol is removed from the raw material powder by drying, and the raw material powder is press-molded at a surface pressure of 200 kg / cm ² to obtain a 5 × 4 × 50 mm test piece. After being formed, the state of dispersion of Y ₂ O ₃ was investigated by EPMA.
This Y-Lα image is shown in FIG.

From the comparison of FIG. 2 and FIG. 3, it can be seen that the raw material powder can be much more uniformly mixed by the method of the embodiment than by the method of the comparative example.

[0022]

[Folding Test] The test pieces formed in the above-mentioned Examples and Comparative Examples were subjected to CIP (hydrostatic pressure press) at a pressure of 3000 kg / cm ² , and were subjected to 17 in an atmosphere of N _{2 of} 9.5 atm.
Firing was performed by heating to 50 ° C. for 4 hours. Then, 40 pieces of bending test pieces were formed from the thus fired test pieces based on JIS standard R1601, and a 4-point bending bending test was performed on each test piece. The results of this test are shown in Table 1 below.

[0023]

Table 1 Examples Comparative Examples Average strength (MPa) 1090 870 Weibull coefficient 159 Minimum strength (MPa) 910 652 Maximum strength (MPa) 1290 1236

From Table 1, it can be seen from the results of the bending test that the raw material powder can be much more uniformly mixed according to the method of the example as compared with the method of the comparative example.

Although the present invention has been described above in detail with reference to specific embodiments, the present invention is not limited to such embodiments, and various other embodiments are possible within the scope of the present invention. It will be apparent to those skilled in the art that it is possible. For example, the fine particles to be mixed are not limited to powders as in the above-mentioned embodiment, but may be other forms such as whiskers and short fibers.

[0026]

As is apparent from the above description, the supercritical fluid has lower viscosity and surface tension than liquids such as water and alcohol, and easily wets fine particles to be mixed.
According to the method of the present invention in which a supercritical fluid is used as a dispersion medium, the dispersibility of fine particles is improved, which allows the fine particles to be uniformly mixed in a short time as compared with the conventional method. Therefore, it is possible to avoid crushing of the fine pieces and mixing of impurities due to stirring the fine pieces for a long time.

Further, since the supercritical fluid has a smaller latent heat of vaporization than a liquid and easily becomes an ordinary gas by adiabatic expansion,
As compared with the case where a liquid is used as the dispersion medium, the dispersion medium can be easily and efficiently removed from the mixed fine particles.

In general, since the supercritical fluid has no chemical effect on the fine particles, it is possible to mix fine particles made of any substance such as ceramics and metals.
Further, since the characteristics of the supercritical fluid can be easily changed by controlling the temperature and the pressure, it is possible to easily control the mixing condition of the fine pieces. Further, the substance constituting the supercritical fluid may be a substance such as CO ₂ , air and N ₂ , which are harmless and inexpensive and enter a supercritical state at a relatively low temperature and pressure. The cost increase due to the use of fluid can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment of a mixing apparatus suitable for use in carrying out a method for mixing fine particles according to the present invention.

2 is a photograph showing a Y-Lα image of a powder compact molded and mixed according to the method of the present invention using the mixing apparatus shown in FIG. 1 at a magnification of 2000. FIG.

FIG. 3 is a photograph showing a Y-Lα image of a powder compact molded and mixed according to a conventional method as a comparative example at a magnification of 2000 times.

[Explanation of symbols]

 10 ... Mixing tank cylinder 12 ... Mixing chamber 22 ... Cylinder 24 ... Vacuum pump 26 ... Heater 30 ... Raw powder 32 ... Propeller 34 ... Filter 36 ... Open / close valve

Claims (1)

[Claims] A method of mixing fine particles in which fine particles to be mixed are stirred together with a supercritical fluid as a dispersion medium.