JP2595710B2 - Manufacturing method of alumina sintered sheet - Google Patents

Description

The present invention relates to a method for producing an alumina sintered sheet from an alumina sol.

[Prior art] When calcining an alumina molded body formed by gelling an alumina sol, γ-alumina, δ-alumina, θ-alumina, and α-alumina are transformed in the order of increasing the calcining temperature, and finally stable when calcined. A sintered body of α-alumina.

Of the transition to the alumina polymorph, during the α-transition, alumina particles often grow abnormally, which makes it extremely difficult to obtain a high-density sintered body.

Conventionally, in order to solve this problem, a small amount of α-alumina powder is previously added to a boehmite sol (alumina sol) as a seed powder, and the α-alumina powder serves as an α crystal nucleus when the alumina gel is converted to α-alumina. Accordingly, a technique for producing a high-purity, high-density alumina sintered body has been disclosed (J. Am. Ceram. Soc., 68 [9] p500-p50).
5,1985). According to this technique, the addition of α-alumina powder significantly lowers the temperature at which the α-alumina powder is transformed than when no α-alumina powder is added, so that the transformed α crystal grains can be sintered without abnormal growth. Further, by adjusting the number of crystal nuclei of α-alumina powder to be added in advance, that is, by adding the amount, α-crystal grains of the sintered body can be controlled.

[Problems to be Solved by the Invention] However, the seeding effect of the α-alumina powder is
It is greatly affected by the purity and form of the alumina powder. That is, α is attached around the seeded α-alumina powder and α
Alumina that is transferred grows epitaxially (epitaxial glowth) along the crystal plane of α-alumina powder,
If the crystal plane orientation of the α-alumina powder grains in the alumina sol is random, the alumina compact shrinks in an unexpected direction during firing, resulting in deformation, cracks, warpage, voids, etc. of the alumina sintered compact. There was a problem that occurred. In particular, when the sintered body is a sheet, its deformation or the like becomes a serious problem.

An object of the present invention is to provide an alumina green sheet which shrinks and sinters in a certain direction without causing deformation, cracks, warpage, voids, etc. by firing, has high smoothness, high purity and high density alumina. It is to provide a method for manufacturing a sintered sheet.

[Means for Solving the Problems] In order to achieve the above object, the present invention relates to a method of adding and mixing at least 0.5% by weight of hexagonal plate-like α-alumina powder as a seed powder with respect to 100% by weight of alumina as an alumina sol. The slurry is prepared, and the slurry is formed into a film by drying the slurry so that the crystal plate surface of the hexagonal plate crystal is oriented in parallel with the sheet surface to form a green sheet. This is how to get a sheet.

The alumina sol of the present invention can be a sol prepared by placing commercially available fine alumina powder in water and uniformly dispersing it with ultrasonic waves or the like, but it produces a sintered sheet of higher purity and higher density. From the viewpoint, an alumina sol having fine colloid particles obtained by hydrolyzing and peptizing aluminum alkoxide is desirable.

Hexagonal plate α-alemina powder is added and mixed with at least 0.5% by weight based on 100% by weight of alumina sol. This α-alumina powder is a hot sodium hydroxide solution 10
From the viewpoint of densification at the time of calcination, it is desirable to add a calcium salt such as calcium chloride or calcium acetate to 0.5% by weight to 0% by weight and to manufacture by a Bayer method. The calcium salt is required to be at least 0.5% by weight in order to convert the α-alumina powder into hexagonal plate crystals, and the upper limit can be suppressed to 1.0% by weight from the viewpoint of densification during firing. When the amount of the α-alumina powder is less than 0.5% by weight, the temperature at which α-alumina is transformed does not decrease and α crystal grains may grow abnormally, so it is necessary to add 0.5% by weight or more. . About 2.0% by weight lowers the transition temperature to α-alumina most, and further does not lower the transition temperature. Therefore, about 2.0% by weight is economically desirable.

In addition to the α-alumina powder, an organic binder and a plasticizer may be added to the alumina sol of the present invention in accordance with the shape and use of the alumina sintered sheet. Examples of the organic binder include polyvinyl alcohol, polyvinyl butyral, polyvinyl acetate, methyl cellulose, polyacrylic acid, and polymethacrylic acid. The plasticizer is selected according to the organic binder, and examples thereof include glycerin, polyethylene glycol, dibutyl phthalate, dioctyl phthalate, and dioctyl adipate. Each amount of the organic binder and the plasticizer is 5 to 60% by weight from the viewpoint of green sheet moldability, processability and high density.
Is preferably within the range.

The dispersion medium of the alumina sol is not limited as long as it is water or an organic solvent that dissolves an organic binder and a plasticizer, but water, an alcohol or an ether ester of a polyhydric alcohol in consideration of dispersibility of the alumina sol and ease of drying. Systems are preferred.

Alumina sol to which α-alumina powder and an organic binder and a plasticizer are added as required is deconcentrated by removing the dispersion medium in consideration of the thickness of the sintered sheet, workability, and the like, to form a slurry.

As a method of forming this slurry so that the crystal plate surface of the α-alumina powder is oriented parallel to the sheet surface, there are a doctor blade method, an extrusion molding method, a roll rolling method, a slurry casting method, and the like. The doctor blade method, which has a high orientation effect, extremely good smoothness, and leaves no molding distortion, is preferred.

As for the orientation of the hexagonal plate-like α-alumina powder in the green sheet, the smaller the viscosity of the alumina sol and the smaller the thickness of the green sheet, the better.

The green sheet obtained by forming and drying the slurry is punched into a predetermined shape, calcined to completely remove the dispersion medium, and then calcined at a temperature (1250 to 1500 ° C) at which alumina is completely sintered. You. After firing, the alumina sintered sheet is obtained by cooling to room temperature with an appropriate cooling device.

[Operation] Since the crystal plate surface (c-plane) of the hexagonal plate-like crystal is oriented parallel to the sheet surface of the green sheet, the shrinkage direction during sintering of the green sheet is always constant, and no unexpected deformation occurs. Alumina sintered sheet can be made.

[Effects of the Invention] As described above, according to the present invention, when a small amount of hexagonal plate-like α-alumina powder is added to alumina sol as a starting material, particles in the sol are converted to α-alumina. The transition speed or the sintering speed can be controlled, whereby a high-density and high-purity sintered body can be manufactured.

In particular, if the green sheet is formed so that the hexagonal plate-like crystals are oriented parallel to the sheet surface, the alumina green sheet can be shrunk in a certain direction by firing and sintered into a predetermined shape.

[Example] Aluminum isopropoxide [A
l (C ₃ H ₇ O) ₃ ] was added to hydrolyze aluminum isopropoxide to produce boehmite [AlOOH]. By adding water adjusted to pH 2 to 4 to the boehmite and pulverizing it, a stable pseudo-boehmite sol was obtained.

2% by weight of hexagonal plate-like α-alumina powder as a seed powder, 20% by weight of polyvinyl alcohol as an organic binder, and 10% by weight of glycerin as a plasticizer are added to 100% of the weight of this pseudo-boehmite sol in terms of alumina. After mixing, the mixture was concentrated to a solid content of 10% by weight to prepare a slurry.

This slurry is coated on a high-density polyethylene tape as a moving carrier by a doctor blade method so as to have a thickness of 2 mm, and the polyethylene tape is dried to remove water as a dispersion medium of the slurry by a thickness of 0.2 mm. Green sheet was obtained.

When this green sheet was fired at 1400 ° C. for 1 hour under atmospheric pressure, an α-alumina sintered sheet having a very smooth sheet surface and a uniform thickness was obtained. The sintering rate of this α-alumina sintered sheet was 99.5% or more of the theoretical density, and was extremely dense. When the α-alumina sintered sheet was observed with a scanning electron microscope, it was confirmed that the α-alumina sintered sheet was composed of fine and uniform particles having an average particle diameter of 2 μm.

[Comparative Example] The same procedure as in the above example was conducted except that hexagonal plate-like α-alumina powder was not added to the pseudo-boehmite sol of the above example.
An alumina sintered sheet was manufactured.

Although the sintering rate and the particle structure of this α-alumina sintered sheet were as high as those of the above examples, the alumina sintered sheet was deformed with cracks, warpage, and voids.

Claims (3)

(57) [Claims] 1. A slurry is prepared by adding at least 0.5% by weight of hexagonal plate-like α-alumina powder as a seed powder to alumina sol to 100% of alumina equivalent weight and preparing a slurry. A method for producing an alumina sintered sheet, wherein a film is formed by drying the slurry so as to be oriented parallel to the sheet surface to form a green sheet, and the green sheet is fired to obtain an alumina sintered sheet. 2. The alumina sintered sheet according to claim 1, wherein the hexagonal plate-like α-alumina powder is produced by a Bayer method by adding 0.5 to 1.0% by weight of a calcium salt to 100% by weight of a high-temperature sodium hydroxide solution. Manufacturing method. 3. The method for producing an alumina sintered sheet according to claim 1, wherein the green sheet is formed by a doctor blade method.