Compositions: ceramic – Ceramic compositions – Aluminum compound containing
Reexamination Certificate
2000-08-28
2002-11-19
Koslow, C. Melissa (Department: 1755)
Compositions: ceramic
Ceramic compositions
Aluminum compound containing
C501S127000
Reexamination Certificate
active
06482761
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a translucent alumina sintered body wherein the content of an alkali metal element and an alkaline earth metal element is small (hereinafter, abbreviated as translucent alumina), and a method for producing the same.
BACKGROUND OF THE INVENTION
The durability of translucent alumina used in an arc tube of a high brilliance lamp has a relation to the whole life of the lamp and significantly influences commodity value. Recently, in particular, for enhancing the brightness and color performing property of a lamp, the pressure and temperature of a filler gas in an arc tube are raised, and higher corrosion resistance to an acid and alkali, halogen and ion plasma is required. Also in uses of a semiconductor production apparatus and the like, for example, in a plasma etching apparatus member, plasma energy in etching is raised for increasing pit depth accompanying high integration, and translucent alumina excellent in corrosion resistance is required.
Corrosion of an alumina sintered body by an acid solution or alkaline solution or halogen gas and plasma is caused mainly due to remaining pores and impurities. In the case of translucent alumina, remaining pores are small and the content thereof is small, therefore, corrosion resistance is influenced significantly by impurities. Though it is most general to use magnesium as a sintering agent in sintering translucent alumina, when magnesium oxide is present in large amount, the corrosion resistance of an alumina sintered body against ion plasma and halogen gas and acid solution or alkali solution lowers (U.S. Pat. No. 3,026,210, JP-A No. 8-245259, J.Illum.Engng.Inst.vol. 74 (1990), No. 9, p. 34).
In addition to magnesium added as a sintering agent, an alumina raw material powder, and alkali metal elements (Li, Na, K, Rb, Cs, Fr), particularly, Li, Na, K or alkaline earth metal elements (Be, Mg, Ca, Sr, Ba, Ra), particularly, Ca, mixed in an alumina sintered body production process, manifest a chemical reaction with ion plasma and halogen gas and acid solution or alkali solution and the like. Therefore, it is important, for enhancing the durability of translucent alumina, to control the amount of an alkali metal element and alkaline earth metal element in a sintered body to the lowest necessary level.
However, in sintering under reducing atmosphere from atmospheric pressure to vacuum so far, when magnesium oxide is not present or the presence amount thereof is small, translucent alumina is not obtained due to abnormal grain growth. Further, FC Report, vol. 13 (1995), No. 11, p. 308 wherein an effective sintering agent for suppressing grain growth other than magnesium oxide is not known, also discloses that magnesium oxide is inevitable in sintering translucent alumina and magnesium oxide having a concentration of at least 100 ppm or more is necessary.
On the other hand, a method for producing translucent alumina having extremely small magnesium content are described in JP-A Nos. 4-193760 and4-370643. However, there are problems that this method is not a constant pressure sintering method and a high pressure treating equipment such as a hot isostatic press and the like is necessary, leading to high cost, and a large scale sintered body can not be produced, and the like.
Further, a method for producing translucent alumina in which magnesium is not added at all is described in JP-A No. 8-301666. However, also this method has problems that (1) it is necessary to effect washing with a chelating agent since translucency is significantly influenced by a small amount of an impurity in a raw material or a prefired body, (2) an expensive oxide of a transition metal element is necessary as an additive, (3) a hot isostatic press apparatus is necessary in a production step, leading to complicated process and higher cost, as well as other problems.
OBJECT OF THE INVENTION
The present inventors have intensively studied for solving the above-mentioned problems, and resultantly found that the intended translucent alumina sintered body is obtained by using a mixed powder prepared by adding a sintering agent which is low content of a magnesium compound to an &agr; alumina powder comprises a polyhedral primary particle having substantially no fractured surface and sintering the mixed powder under specific condition, and have completed the present invention.
SUMMARY OF THE INVENTION
Namely, the present invention provides the following aspects (1) to (2).
(1) A translucent alumina sintered body wherein the total content of an alkali metal element and an alkaline earth metal element is 50 ppm or less, and the in-line transmittance of a light having a wavelength of 600 nm is 40% or more at a thickness of the sintered body of 0.85 mm.
(2) A method for producing a translucent alumina sintered body described in (1), wherein the sintered body is obtained by
molding a mixed powder prepared by adding a sintering agent to an &agr; alumina powder which comprises a polyhedral primary particle having substantially no fractured surface, has a BET specific surface area of 1 to 10 m
2
/g and has a purity of 99.99% or more, and
sintering the green body at a temperature of from 1700 to 1900° C. under reducing atmosphere of from atmospheric pressure to vacuum.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be illustrated in detail below.
First, the translucent alumina sintered body of the present invention will be described.
The translucent alumina sintered body of the present invention is a translucent alumina sintered body wherein total content of an alkali metal element and an alkaline earth metal element is as low as 50 ppm or less, and the in-line transmittance of a light having a wavelength of 600 nm is 40% or more at a thickness of the sintered body of 0.85 mm. Further, the average grain size of the alumina sintered body is preferably 5 &mgr;m or more to 50 &mgr;m or less, and the maximum grain size is preferably 1.5-fold or more of the average grain size and it is preferable that the microstructure is uniform.
Next, the method for producing an alumina sintered body of the present invention will be illustrated.
As the alumina used as a raw material in the production method of the present invention, namely, as the &agr; alumina powder having a BET specific surface area of 1 to 10 m
2
/g comprising a polyhedral primary particle having substantially no fractured surface, there is listed an a alumina powder obtained by sintering transition alumina or an alumina powder which is converted to transition alumina by heat treatment in a gas atmosphere containing hydrogen chloride , and said &agr; alumina is obtained according to a method for producing an alumina powder having an alumina purity of as high as 99.99% or more comprising a single crystal particle of &agr; alumina described in JP-A Nos. 6-191833, 6-191836 and the like.
As the above-mentioned &agr; alumina powder used as a raw material in the production method of the present invention, there are listed, for example, SUMICORUNDUM AA03 (primary particle diameter: 0.3 &mgr;m), AA04 (primary particle diameter: 0.4 &mgr;m), AA05 (primary particle diameter: 0.5 &mgr;m) and AA07 (primary particle diameter: 0.7 &mgr;m) manufactured by Sumitomo Chemical Co., Ltd. The purities thereof are all 99.99 wt % or more. The alkali metal element and alkaline earth metal element contained in AA04 are in the alkali metal element, Li=0.01 ppm, K<2 ppm, Na=1.7 ppm, and in the alkaline earth metal, Be<0.05 ppm, Mg=0.62 ppm, Ca<1 ppm, Sr<0.1 ppm, Ba<2 ppm, according to a glow discharge mass spectrometry (GD-MS).
&agr; alumina used in the present invention is a nearly monodispersed powder that a primary particle is uniform and a defect is not contained, the D/H ratio of the particle is 0.5 or more to 3.0 or less, and primary particles are not mutually agglomerated, in addition to a purity of 99.99% or more. If this powder is used as a raw material, a green body having uniform packing structure is obtained.
In a conventional high purity al
Uchida Yoshio
Watanabe Hisashi
Koslow C. Melissa
Sumitomo Chemical Company Limited
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