JPH02243556A - Production of calcined alumina sheet - Google Patents

Abstract

PURPOSE:To produce a calcined high-purity and high-density alumina sheet having high smoothness by adding a specific amount of alpha-alumina powder which is hexagonal platy crystals as a seed powder to an alumina sol, forming the resultant mixture into a green sheet and calcining the formed green sheet. CONSTITUTION:alpha-Alumina powder which is hexagonal platy crystals in an amount of at least 0.5wt.% based on 100wt.% expressed in terms of alumina weight as seed powder is added to an alumina sol to prepare a slurry. The above-mentioned alpha-alumina powder which is the hexagonal platy crystals can be produced by adding 0.5-1.0wt.% calcium salt to 100wt.% NaOH solution at a high temperature by the Bayer process. The resultant slurry is then formed into a film and dried to form a green sheet. The forming is carried out by a doctor blade method, etc., and the crystal surface of the aforementioned hexagonal platy crystals is oriented parallel to the sheet surface. The green sheet is subsequently calcined. In the above-mentioned calcining, the green sheet is shrunk and calcined in a prescribed direction without causing deforming, cracking, warping, holes, etc., to afford the calcined high-purity and high-density alumina sheet having a prescribed shape.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing an alumina sintered sheet from an alumina sol.

[Prior art] When an alumina molded body made by gelling alumina sol is calcined, as the calcining temperature increases, γ-alumina, δ-alumina, α-alumina, and α-alumina are transformed in this order, and when this is fired, it is finally stabilized. The result is an α-alumina sintered body.

During the α transition during the transition to the alumina polymorph, abnormal grain growth of alumina particles often occurs, making it extremely difficult to obtain a high-density sintered body.

Conventionally, in order to solve this problem, a small amount of α-alumina powder was added as a sheet powder to the boehmite sol (alumina sol) in advance, and this α-alumina powder served as the α-crystal nucleus when the alumina sol turned into α-alumina. disclosed a technique for producing a high-purity, high-density alumina sintered body (J, Am, Ceram, Soc, 6
8 [9copsoo-psos, 1985). According to this technique, by adding α-alumina powder, the temperature at which it transforms into α-alumina is significantly lower than when α-alumina powder is not added, and the transformed α crystal grains can be sintered without abnormal growth. Further, by adjusting the number of crystal nuclei of the α-alumina powder added in advance, that is, the amount added, the 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 greatly influenced by the purity and form of the α-alumina powder. In other words, the alumina that adheres to the periphery of this sheet of α-alumina powder and undergoes α-transformation undergoes epitaxial growth along the crystal plane of the α-alumina powder.
h) If the crystal plane orientation of the α-alumina powder grains in the alumina sol is random, the alumina compact will shrink in an unexpected direction during firing, resulting in deformation, cracks, etc. in the alumina sintered compact. There were problems such as warping and holes. Especially when the sintered body is a sheet,
The deformation etc. become a big problem.

The object of the present invention is to produce an alumina green sheet that shrinks and sinters in a certain direction without causing deformation, cracks, warpage, holes, etc. when fired, has high smoothness, and has high purity and high density alumina. An object of the present invention 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 includes adding at least 0.5% by weight of hexagonal plate-like α-alumina powder as a sheet powder to the alumina sol based on 100% alumina equivalent weight. A slurry is prepared by mixing, the slurry is formed into a film and dried to form a green sheet so that the crystal plate planes of the hexagonal plate crystals are oriented parallel to the sheet surface, and this green sheet is fired to form an alumina sinter. This is a method of obtaining a binding sheet.

The alumina sol of the present invention can be prepared by adding commercially available fine alumina powder to water and uniformly dispersing it using ultrasonic waves, etc., but from the viewpoint of producing a sintered sheet with higher purity and higher density. An alumina sol with fine colloidal particles, which is obtained by hydrolyzing aluminum alkoxide and peptizing it, is desirable.

The hexagonal plate-like α-alumina powder is mixed in an amount of at least 0.5% by weight based on 100% by weight of the alumina sol. This α-alumina powder is prepared using a high-temperature sodium hydroxide solution.
From the viewpoint of densification during firing, it is preferable to use the Bayer process by adding 0.5 to 1.0 weight % of a calcium salt such as calcium chloride or calcium acetate to 0 weight %.

The minimum amount of calcium salt is 0.5% by weight in order to make the α-alumina powder into hexagonal plate crystals, and the upper limit is limited to 1.0% by weight from the viewpoint of densification during firing. Also, if the amount of α-alumina powder added is less than 0.5% by weight, the temperature at which it transforms into α-alumina may not decrease and the α crystal grains may grow abnormally, so add 0.5% by weight or more. It is necessary.

The transition temperature to α-alumina decreases the most at around 240% by weight, and the transition temperature does not decrease further, so 2.0
About % by weight is economically desirable.

In addition to α-alumina powder, an organic binder and a plasticizer may be added to the alumina sol of the present invention depending on the shape and use of the sintered alumina sheet. As an organic binder,
Examples include polyvinyl alcohol, polyvinyl butyral, polyvinyl acetate, methylcellulose, polyacrylic acid, polymethacrylic acid, and the like. The plasticizer is selected depending on the organic binder, and includes, for example, glycerin, polyethylene glycol, dibutyl phthalate, dioctyl phthalate, dioctyl adipate, and the like.

The amount of each of the organic binder and plasticizer added is 5 to 60% from the viewpoint of moldability, processability, and densification of the green sheet.
It is preferably within the range of % by weight.

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

The alumina sol to which α-alumina powder and an organic binder and plasticizer are added as necessary is desorbed from the dispersion medium and concentrated into a slurry, taking into consideration the thickness of the sintered sheet, workability, etc.

Methods for forming this slurry into a film such that the crystal plate planes of the α-alumina powder are oriented parallel to the sheet surface include the doctor blade method, extrusion molding method, roll rolling method, slurry casting method, etc. The doctor blade method is preferred because it has a high orientation effect, extremely good smoothness, and leaves no molding distortion.

Regarding the orientation of the hexagonal plate-like α-alumina powder in the green sheet, it is preferable that the viscosity of the alumina sol and the thickness of the green sheet are as small as possible.

The green sheet obtained by forming and drying the slurry is punched into a predetermined shape, calcined to completely remove the dispersion medium, and heated to a temperature of 1250 to 1500 at which the alumina is completely sintered.
℃). After firing, the alumina sintered sheet is obtained by cooling to room temperature using a suitable cooling device.

[Function] Since the crystal plate plane (C plane) of the hexagonal plate crystal is oriented parallel to the sheet plane of the green sheet, the direction of contraction of the green sheet during sintering is always constant, and unexpected deformation does not occur. sintered alumina sheets can be made.

[Effects of the Invention] As described above, according to the present invention, by adding a small amount of hexagonal plate-like α-alumina powder to the alumina sol, which is the starting material, the particles in the sol become α-aluminated. The transition rate or sintering rate can be controlled, thereby making it possible to produce a sintered body with high density and high purity.

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

[Example] 1 mole of aluminum isopropoxide [AI (C!
100H] was generated. A stable pseudo-boehmite sol was obtained by adding water adjusted to pH 2 to 4 to peptize the boehmite.

Based on the 100% alumina equivalent weight of this pseudo-boehmite sol, 2% by weight of hexagonal plate-like α-alumina powder as sheet powder and 20% by weight of polyvinyl alcohol as organic binder.
After adding and mixing 10% by weight of glycerin as a plasticizer, the mixture was concentrated to a solid content of 10% by weight to prepare a slurry.

This slurry is coated onto a high-density polyethylene tape, which is a moving carrier, to a thickness of 2 mm using a doctor blade method, and the polyethylene tape is dried to remove water, which is a dispersion medium of the slurry, to a thickness of 0.2 mm. 2mm
I got a green sheet.

When this green sheet was fired at 1400'C for 11 hours under atmospheric pressure, an α-alumina sintered sheet with an extremely smooth sheet surface and a uniform thickness was obtained.

The sintering rate of this α-alumina sintered sheet is 99% of the theoretical density.
It was 5% or more and extremely dense. Further, when this α-alumina sintered sheet was observed using a scanning electronic microscope, it was confirmed that it was composed of fine and uniform particles with an average particle diameter of 2 μm.

[Comparative Example] An α-alumina sintered sheet was produced in the same manner as in the above Example, except that hexagonal plate-like α-alumina powder was not added to the pseudo-boehmite sol of the above Example.

Although the sintering rate and particle structure of this α-alumina sintered sheet showed high density and purity comparable to those of the previous example, cracks, warpage, and pores were observed, and the alumina sintered sheet was deformed.

Claims (1)

[Claims] 1) A slurry is prepared by adding and mixing at least 0.5% by weight of hexagonal plate-like alpha alumina powder as a sheet powder based on 100% alumina equivalent weight to alumina sol, A method for producing an alumina sintered sheet, which comprises forming a film of the slurry and drying it to form a green sheet so that the crystal plate surface of the crystal plate is oriented parallel to the sheet surface, and then firing the green sheet to obtain an alumina sintered sheet. 2) The alumina sintered product according to claim 1, wherein the hexagonal plate-like α-alumina powder is produced by the Bayer process by adding 0.5 to 1.0% by weight of calcium salt to 100% by weight of high-temperature sodium hydroxide solution. Method of manufacturing sheets. 3) The method for producing a sintered alumina sheet according to claim 1, wherein the green sheet is formed by a doctor blade method.