JP2952349B2 - Monoclinic zirconia dense sintered body and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a dense monoclinic zirconia sintered body and a method for producing the same. More specifically, the invention of this application relates to a dense monoclinic zirconia dense sintered body having a nano-sized (<100 nm) microstructure, which is useful as a highly functional material, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, high-performance ceramic materials having various advanced characteristics have attracted attention.
As one of such ceramic materials, zirconia is known to have excellent strength and toughness, and to be excellent in chemical stability such as thermal conductivity and corrosion resistance, wear resistance and the like.

In order to realize a more sophisticated ceramic material from zirconia, it has been recognized that it is necessary to control the structure of the zirconia to a fine structure of submicron or less. Have been advanced from various viewpoints. As shown in Table 1 below, the crystal structure of pure zirconia is considered to be monoclinic at low temperatures and to undergo phase transformation to tetragonal and cubic at higher temperatures. .

[0004]

[Table 1]

However, in practice, zirconia ceramics generally used in the past are all tetragonal or cubic stabilized zirconia to which stabilizing elements such as yttria and ceria are added, and are monoclinic and tetragonal. Is accompanied by a large change in volume, and cracks occur during cooling, making it difficult to obtain a dense monoclinic zirconia sintered body.

Under such circumstances, a method for obtaining a tetragonal or cubic zirconia dense sintered body has been found, for example, according to the manufacturing process illustrated in FIG. 3 and has been put to practical use. The major feature of this method is that it uses high-purity and fine zirconia powder, is compacted into a compact state with less aggregation, and is sintered at a low temperature.

However, in this method for producing a dense monoclinic zirconia sintered body, it is theoretically sufficient to densify the material at a temperature (1170 ° C.) lower than the temperature of transformation from monoclinic to tetragonal. However, as a practical matter, a dense sintered body of monoclinic zirconia must be sintered at a temperature of 1300 ° C. or higher, which is higher than its transformation point, in order to obtain a dense sintered body of monoclinic zirconia. There was a problem.

[0008] This means that the size of the powder that can be dispersed using the colloid process technique is about 0.1 µm,
Fine powders of this size or less are extremely easy to agglomerate, leaving large bridges inside the compact before sintering, and it is virtually impossible to re-disperse nano-sized powder once aggregated. Was. For this reason, at a temperature lower than the transformation point, relatively large pores due to agglomeration exist inside the molded body, and in order to eliminate these pores, increase the sintering temperature and lengthen the sintering time. Was needed.

In order to obtain a dense sintered body of monoclinic zirconia at a temperature lower than the transformation point from monoclinic to tetragonal, a method of preventing agglomeration of zirconia powder as a method of preventing fine particles of primary particles is used. Although a method of packing at a high density while controlling the behavior of the device at high density has been considered, the control method and the device thereof are too complicated to be realistic.

Further, when sintering the monoclinic zirconia powder as a raw material, monoclinic zirconia sintering is carried out by mixing 3 to 20 wt% of a metal oxide powder having a relatively low melting point as a sintering aid and sintering. Although a method for producing a sintered body is known, addition of an auxiliary is indispensable in this method, and a pure monoclinic zirconia sintered body containing no impurities cannot be produced.

Further, as a method of obtaining a dense sintered body of monoclinic zirconia, a method of pressure-sintering fine particles produced by a method such as a gas evaporation method has been proposed. Since sintering was performed, the apparatus became large, and a dense sintered body of monoclinic zirconia could not be easily obtained. The invention of this application has been made in order to solve the above-mentioned drawbacks of the prior art, and even at a temperature below the transformation point from monoclinic to tetragonal, powder inside the molded body A new monoclinic zirconia dense sintered body, which does not have pores caused by aggregation of porcelain and enables easy production of pure monoclinic zirconia dense sintered body containing no impurities. It is intended to provide a manufacturing method.

[0012]

SUMMARY OF THE INVENTION The present invention provides a dense monoclinic zirconia sintered body having a relative density of at least 98% to solve the above-mentioned problems.
Further, the invention of this application is to remove chlorine ions from hydrated zirconia sol obtained by hydrolysis reaction of hydrochloric acid acidic hydrated zirconia sol, for example, aqueous solution of zirconium oxychloride or zirconium chloride, and press-mold in an acidic suspension state. The present invention provides a method for producing a dense monoclinic zirconia sintered body, characterized by sintering a pressureless sintered body at a temperature equal to or lower than a transformation point from monoclinic to tetragonal.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The invention of this application has the features as described above, and the embodiments will be described in detail below. First, the present invention is characterized by using a continuous process of producing and solidifying a monodispersed fine particle suspension without using the powder found in the conventional method as a starting material. And it can be solidified into a high-density compact.

That is, in the method of the present invention, fine zirconia particles immediately after generation are solidified without agglomeration,
As a result, it is possible to perform sintering at a temperature lower than the temperature of the monoclinic and tetragonal transformations, and at the same time, it is possible to control the structure of the obtained sintered body to a fine structure of a nano-order size. An outline of the manufacturing process can be shown in FIG. Note that the numbers described in FIG. 1 are examples of operation conditions, and are not particularly limited to these.

First, zirconia as a raw material is produced by subjecting a zirconium salt such as zirconium oxychloride to a wet method such as a hydrolysis method. Since this reaction involves the generation of hydrochloric acid, the resulting reaction solution is a sol (zirconia slurry) in which hydrated zirconia microcrystalline particles immediately after production are monodispersed in a strongly hydrochloric acid aqueous solution.

At this time, the pH value is about 1 to 5, and
It is appropriate that: At a lower pH value, the positive surface potential is large, so that the particles can be prevented from approaching each other due to electrostatic repulsion, and the dispersed state can be favorably maintained. The surface of the microcrystalline particles dispersed in the strong acid has a high positive zeta potential and an electrostatic repulsion potential acting between the particles, so that this sol exhibits extremely sharp particle size distribution and good particle dispersibility.

Then, the zirconia slurry is passed through an anion exchange resin column to remove chloride ions while keeping the solution acidic. In other words, while maintaining the pH of the aqueous solution on the acidic side, that is, while maintaining the zeta potential of the surface of the zirconia microcrystalline particles positive, without causing the particles to agglomerate, chloride ions that inhibit sintering of zirconia are eliminated. Remove.

The pH of the aqueous solution for removing chloride ions is as follows:
It is appropriate to keep it at about 1 to 5, more preferably 4 or less. At a lower pH value, the positive surface potential is large, so that the particles can be prevented from approaching each other due to electrostatic repulsion, and the dispersed state can be favorably maintained. The zirconia slurry from which chloride ions have been removed is concentrated by a rotary evaporator or the like to increase the solid phase ratio, and then solidified by a pressure casting method or the like.

The solid phase ratio by concentration is usually 40 to 80.
It is appropriate to set it in the range of about% by weight. This range is
The weight ratio with water is equivalent to about 10 to 40%. If the proportion of the solid phase is too small, a large-scale apparatus is required to obtain a molded article having a desired size. On the other hand, if the proportion is too large, the viscosity increases and dispersion becomes difficult. Also, solidification molding by pressure casting or the like, as a guide of the pressure level, 1 to
It is preferably 50 MPa.

Then, the compact is subjected to, for example, cold isostatic compression (CIP). The pressure at this time is preferably about 200 to 600 MPa, more preferably 400 MPa.
The pressure is about MPa (about 350 to 450 MPa). 20
At 0 MPa or more, a remarkable increase in density is observed. When the density is low, high temperatures are required to densify, usually with many holes. On the other hand, if the pressing force is applied too much, the particles will be pressed firmly only partially, resulting in an uneven density distribution. Sintering in such a state results in a sintered body having a non-uniform grain size and density.
By this treatment, the relative density is preferably 40 to 70%
And a solidified molded article of a certain degree. The relative density at this stage is 40
% May not always give a dense sintered body in the next pressureless sintering. On the other hand, if it exceeds 70%, the burden of the CIP treatment is large, and good sintering may not occur.

Next, in the method of the present invention, sintering is performed under no pressure. The temperature may be below the transformation point from monoclinic to tetragonal (approximately 1170 ° C.). More preferably, the range of 1050-1150 degreeC is suitable. If the temperature exceeds the phase transformation temperature (1170 ° C.), cracks or cracks occur due to the volume change due to the phase transformation when the temperature is lowered, and a dense sintered body cannot be obtained. The lower the temperature, the better the dense sintered body can be obtained. However, it is generally difficult to obtain a monoclinic zirconia dense sintered body at a temperature lower than 1000 ° C. For example, specifically, by sintering at 1100 ° C. for 6 hours under no pressure in the air, the relative density is 98% or more and the average crystal grain size is 1
A dense sintered body of not more than 00 nm can be obtained.

In the above methods, there is no known example of producing monoclinic zirconia ceramics having a relative density of 98% or more by pressureless sintering. Moreover, the monoclinic zirconia ceramics produced by the method of the present invention also has a microstructure having an average crystal grain size of 100 nm or less. Further, in the method of the present invention, in order to produce a suspension composed of monodispersed fine particles, so that the fine particles generated by the reaction for producing the hydrated zirconia sol as described above are immediately in a dispersed state, It is desirable to select raw material reagents and manufacturing conditions.

In the conventional method, zirconia fine powder which is usually commercially available is usually washed with water in order to remove chloride ions and then re-milled. Must be carried out with the suspension still acidic. The ion-exchange resin column method used for removing chloride ions from the zirconia suspension is characteristic of the method of the present invention, and has been proposed so far to remove inorganic oxides or water from which ionic impurities have been removed. It is essentially different from the method for producing oxides.

That is, it is proposed as a method for producing a ceramic raw material powder for electronic parts requiring high electrical insulation from an inorganic oxide or hydroxide such as alumina or aluminum hydroxide containing sodium ions or other ionic impurities. Unlike the above method using an ion exchange resin, the object of the present invention is to remove chlorine ions without impairing the dispersibility of the suspension of zirconia ceramics, This is not to dry the slurry to obtain a powder, as is characteristic of the method described above.

In the present invention, in the method for producing zirconia as a raw material, for example, not only a hydrolysis method but also a neutralization coprecipitation method and an alkoxide method can be used. Hereinafter, the present invention will be described in more detail with reference to Examples.

[0026]

EXAMPLE 1 A hydrated zirconia sol produced by a hydrolysis reaction of an aqueous solution of zirconium oxychloride (solid fraction 11 wt%, pH
= 1.7) as the starting material. This sol is a sol in which hydrated zirconia microcrystal particles (particle diameter: about 80 nm) immediately after production are monodispersed in a strong hydrochloric acid aqueous solution because HCl is released in large amounts during the hydrolysis reaction.

The sol was passed through an anion exchange resin column (CA08P, manufactured by Mitsubishi Chemical Corporation) to remove chloride ions while keeping the solution acidic. The pH of the slurry after removal is 3.
By this method, it was possible to remove chlorine ions which would inhibit sintering of zirconia without aggregating the particles.

The particle size distribution before the removal of chlorine ions is illustrated in FIG. FIG. 2B shows the particle size distribution after removing the chloride ions. This FIG. 2 (A)
From (B), it can be seen that flocculation hardly occurred even after the chloride ions were removed. Next, the sol from which the chloride ions had been removed was concentrated using a rotary evaporator to increase the ratio of the solid phase to about 5 times. Then, the sol was solidified by a pressure casting method using a pressure of 10 MPa using a Teflon membrane filter having a pore diameter of 0.1 μm.

The solidified material is further subjected to cold isostatic compression (CIP) treatment at a pressure of 400 MPa to obtain a relative density of 4%.
An 8.6% solidified compact was obtained. Then, the solidified compact is sintered at 1100 ° C. for 6 hours under no pressure in the air to obtain a dense sintered body having a relative density of 98% or more and an average crystal grain size of 100 nm or less. Was.

The dense sintered body was confirmed by X-ray diffraction to consist of only monoclinic crystals.

[0031]

As described above in detail, according to the present invention, monoclinic dense sintering of zirconia, which is one of the most promising materials as a highly functional or highly structured ceramic, without containing other stabilizing elements. The compact can be produced by sintering at a temperature below the transformation point of the monoclinic and tetragonal crystals. It is possible to easily produce a monoclinic dense sintered body without pores caused by agglomeration of powder inside the sintered body, and as a result, a monoclinic zirconia dense sintered body can be manufactured at a low cost. In addition, it can be manufactured in large quantities.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a manufacturing process of the method of the present invention.

FIGS. 2A and 2B are diagrams showing changes in the particle size distribution of a monoclinic zirconia suspension before and after chloride ion removal, respectively.

FIG. 3 is a diagram showing steps of a conventional method.

────────────────────────────────────────────────── (5) References JP-A-4-349171 (JP, A) JP-A-61-97134 (JP, A) JP-A-60-226456 (JP, A) (58) Investigation Field (Int.Cl. ⁶ , DB name) C04B 35/42-35/49 C01G 25/00-25/06 C04B 35/00

Claims (4)

(57) [Claims] 1. A dense monoclinic zirconia sintered body having a relative density of 98% or more. 2. The sintered body according to claim 1, having fine crystals having an average crystal grain size of 100 nm or less. 3. A pressureless sintering method at a temperature lower than a transformation point from monoclinic to tetragonal after removing chlorine ions from a hydrochloric acid-acid hydrated zirconia sol and press-forming in a state of an acidic suspension. The method for producing a sintered body according to claim 1, wherein: 4. Transformation from monoclinic to tetragonal, which is obtained by removing chloride ions from a hydrated zirconia sol obtained by a hydrolysis reaction of zirconium oxychloride or zirconium chloride aqueous solution and press-molding in an acidic suspension state. The method for producing a sintered body according to claim 3, wherein the sintering is performed at a temperature equal to or lower than a pressure without pressure.