JPH04114982A - Method for forming digestion-protecting layer on calcia ceramics - Google Patents

Abstract

PURPOSE:To obtain a sintered calcia ceramics useful as a material for a melting crucible free from problem of the lowering of the purity of active material by heating the sintered calcia ceramics in carbon dioxide gas atmosphere at a specific temperature to form a calcium carbonate layer on the surface. CONSTITUTION:(A) A sintered calcia ceramics having a porosity of <=20% and optionally containing <=2wt.% of a fluorine component selected from calcium fluoride, magnesium fluoride and aluminum fluoride is heated in (B) a carbon dioxide gas atmosphere at 400-800 deg.C to form a digestion-protecting layer having a thickness of 0.1-5mum on the surface of the component A.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention can be used as a material for a sintering setter or a melting crucible for stably producing high purity metals, high melting point metals, and even highly active metals. The present invention relates to suitable calcia ceramics.

[Conventional technology]

When high-purity metals or alloys are melted, it is necessary to use a ceramic lining material that has a high melting point and low reactivity with metals for the container.

BACKGROUND ART When melting metals and alloys in high-frequency induction heating furnaces, plasma arc furnaces, carbon resistance heating furnaces, etc., containers made of ceramics such as alumina, magnesia, and zirconia have traditionally been used.

However, when melting metals such as titanium-based alloys such as Ti-Ni, which have a high melting point, are oxidizable, and are highly reactive, using a container made of such ceramic lining material, the lining material Sooty ceramics are mixed into the alloy, significantly reducing the purity of the molten metal.

In addition, recently, impurities from the melting container may influence the properties of the melting material, such as rare earth-transition metal alloys used as materials for magneto-optical recording, and the melting container may be used to prevent reactions with the alloy. It is desirable that the

When these alloys are melted, calcia ceramics are known as jA K4, which has low reactivity with molten metal.

Calcia itself is thermodynamically stable, has characteristics such as being resistant to reduction by carbon, metals, etc., and having low vapor pressure under vacuum, and can also be expected to have desulfurization and dephosphorization effects. However, there is a problem with digestion as it reacts with moisture in the atmosphere to form calcium hydroxide, which expands and disintegrates.

Conventionally, as a means of solving these problems,
Fe20. as described in Gypsum and Lime J No. 191.1984. ,, Reducing the digestion rate by adding a flux component such as T i 02, ■ As disclosed in Japanese Patent Application Laid-Open No. 186457/1982, adding calcium chloride and hot pressing can reduce the rate of digestion. Forming a body, ■Unexamined Japanese Patent Publication No. 50-1
As disclosed in Japanese Patent No. 29611, there have been attempts to prevent digestion by coating the surface of the sintered body with calcium carbonate.

However, when adding the flux component in case (2), a reduction reaction occurs due to reaction with the metal when the added component is dissolved, and impurity components are mixed in. In addition, when adding calcium chloride (2) and hot pressing, the shape becomes larger and extra effort is required. In addition, in the case of coating (■), since it is a gas-in-phase reaction, a sufficient coating is not formed on the contact area with the sample firing table, and furthermore, when the sintered body reaches a temperature of 900 °C or higher, the coating does not form. There is a problem that it disassembles, making it difficult to put it into practical use.

[Problem to be solved by the invention]

The purpose of the present invention is to solve the problem of calcia digestion in calcia ceramics, and to be able to stably dissolve active materials such as titanium, zirconia, and rare earth metals without deteriorating their purity. Our goal is to provide ceramics.

[Means to solve the problem]

The method for forming a calcia ceramic digestible protective layer of the present invention involves maintaining a calcia ceramic sintered body having a porosity of 20% or less in a temperature range of 400 to 80 °C (1 °C) in a carbon dioxide atmosphere. It is characterized by forming a calcium carbonate layer with a thickness of 01 to 5 μm on the surface of the sintered body.

In addition, by making the calcia ceramic sintered body to be treated contain 2% by weight or less of fluorine compounds such as calcium fluoride, magnesium fluoride, and aluminum fluoride, the protective ability of the digestive protective layer of the calcium carbonate layer can be improved. It can be improved further.

[Effect]

A strong calcium carbonate layer is formed on the surface of calcia ceramics, which can reduce digestion due to reaction with atmospheric moisture.

The porosity of the calcia ceramic sintered body to be treated is 2.
It is desirable that it is 0% or less.

This is because the calcium carbonate coating layer has the effect of suppressing the surface reaction between calcia and water, and if it is porous, the specific surface area increases and at the same time the open pores exist extremely deep from the surface to the inside. Therefore, when the porosity increases, it becomes difficult for carbon dioxide gas to enter the pores, resulting in a situation where it is difficult to form a calcium carbonate film.

Regarding the calcium carbonate formation temperature, substantially 200
It can be formed over a wide range of temperatures around ~900°C. However, if the temperature is too low, the formation rate of the calcium carbonate film will be slow and the adhesion of the film will be weak, making it difficult to withstand practical strength.If the temperature is too high, even if the film is formed, the calcium carbonate will decompose. The apparent production rate of a chemical that involves a reaction is so slow that it is impossible to produce it industrially. For this reason, it is desirable that the carbon dioxide treatment temperature is 400 to 800°C.

Moreover, it is desirable that the thickness of the calcium carbonate layer is 0.1 to 5 μm.

If it is less than 0.1 μm, it is difficult to form a film with a uniform thickness and the effect on suppressing digestion is small;
The reason why it is not recommended is that if the film is too thick, cracks will occur due to the difference in thermal expansion between the calcia raw material and the calcium carbonate film, which will not be reflected in the suppression of digestion, and it will take time to form the film, resulting in high costs. It will be done. Furthermore, the most important thing is that if the film is too thick, the carbon dioxide gas generated by the decomposition of calcium carbonate during use will easily oxidize highly active metals and alloys, so the absolute amount should be reduced as much as possible. Therefore, the thickness of the coating is also limited.

Furthermore, this calcium carbonate layer exhibits a remarkable effect in preventing digestion before use of the ceramic, but when this sintered body is used at a temperature of 900° C. or higher, the coating decomposes. Substantially, the temperature range in which calcia ceramics are used is often 1000° C. or higher, and if they are used under conditions where they are cooled to around room temperature during use, it is necessary to take measures to prevent re-digestion.

Addition of a fluorine compound to the sintered body to be treated will react with the calcia raw material during sintering and will not promote microstructural grain growth of calcia ceramics. Compounds - Calunia-based compounds are precipitated to form a dense layer, which suppresses hydration reactions from grain boundaries and protects the surface of calcia ceramics.

If the amount of fluorine compound added is large, 1300 to 140
Although it has a melting point around 0°C, there is no particular problem when it is added in an amount of 2% by weight or less, and it can sufficiently withstand the temperature range in which calcia ceramics are used.

Fluorine compounds added to calcia ceramics are used in vacuum or inert gas environments and in high-temperature regions, and it is necessary to avoid contamination of molten metals, alloys, etc. From this point of view, it is assumed that the fluorine compound to be added is stable, that is, the bond energy between the cation and fluorine is large and it is difficult to dissociate, and that evaporation etc. do not occur in the operating temperature range. Therefore, examples of fluorine compounds to be added include calcium fluoride, magnesium fluoride, and aluminum fluoride. Among these, calcium fluoride is the thermodynamically most stable fluorine compound and is the best as an additive.

In order to suppress the digestion of calcia ceramics, fluorine compounds are added to produce fluorine compounds and calcia compounds on the grain boundaries and some of the ceramic surfaces, and then a calcium carbonate layer is formed on the ceramic surface. Digestion from the surface and grain boundaries hardly occurs.

Although the formed calcium carbonate layer exhibits a remarkable inhibitory effect on digestion before use, it is expected that it will decompose during use, resulting in poor digestion resistance during use. However, the effect of fluorine compounds is seen to be uniformly dispersed in the ceramic as it undergoes thermal history, and it is predicted that the effect will be maintained intermittently because it gradually coats the ceramic surface after the calcium carbonate layer disappears. be done. Basically, the calcium carbonate layer has a strong effect on suppressing digestion before use, and the fluorine compound is thought to gradually exert its effect before and during use, and these are combined to have a pronounced effect. bring about an effect.

〔Example〕

Using calcia powder of 2000μm or less, 1700℃
A calcium carbonate film was formed by treating the sample sintered at a calcination temperature of 100 ml in carbon dioxide gas. Additionally, calcium fluoride, aluminum fluoride, and magnesium fluoride were added to some samples. Table 1 shows the manufacturing conditions and the test piece results for digestion resistance.

In addition, Table 2 shows examples of the production of conventional calcia ceramics without carbon dioxide treatment and addition of fluorine compounds, and comparative examples of digestion tests.

Table 2 Comparative Examples In Table 1, ceramics having a calcia purity of 95.2 to 99.9% by weight and a porosity of 4.0 to 18.6% were prepared and heated at 400 to 800°C. The film thickness of calcium carbonate is controlled by changing the treatment time of carbon dioxide gas within the temperature range of .

As shown in the examples, the formation of the calcium carbonate film of the present invention can greatly reduce reactions with water and steam before use, and the addition of a fluorine compound can greatly reduce digestion during use. That is, by utilizing both the formation of a calcium carbonate layer and the addition of a fluorine compound, calcia ceramics can be kept stable in the atmosphere before and during use. Furthermore, in the examples of the present invention, the calcium carbonate coating and the fluorine compound did not affect the dissolved metals, alloys, etc., and did not inhibit the characteristics of calcia purity.

When Ti-Ni, Fe-Tb-Co, and FeN1 were melted in the calcia ceramic container shown in the example, the oxygen content in the alloy was only in the range of 100 to 5QQppm.

The oxygen content of alloys melted with alumina and zirconia ceramics under the same conditions was 3 to 10 times that value, indicating that the calcia ceramics of the present invention functioned satisfactorily.

〔Effect of the invention〕

The calcia ceramics on which the anti-digestion coating is formed according to the present invention have a uniform and dense calcium carbonate coating, and as such have sufficient anti-digestion properties during primary use.

Furthermore, by adding a fluorine compound to the sintered body, it is possible to create calcia ceramics that have unprecedented resistance to digestion even when reused.

Moreover, by making full use of the thermodynamic stability of calcia, it is possible to solve the problem of impurities being mixed into metals, alloys, etc. from ceramic materials.

The calcia ceramics created in this way are not only highly active titanium alloys, but also high-quality permalloy alloys and Tb-Fe-C.

It becomes possible to stably produce a high-quality rare earth-transition metal-based master alloy for magneto-optical recording, which is typified by the following.

Patent applicant: Kurosaki Ceramics Co., Ltd.

Claims (1)

[Claims] 1. A calcia ceramic sintered body having a porosity of 20% or less is maintained at a temperature range of 400 to 800°C in a carbon dioxide gas atmosphere to form a surface of the calcia ceramic sintered body with zero porosity.
．． A method for forming a calcia ceramic digestive protective layer forming a calcium carbonate layer with a thickness of 1 to 5 μm. 2. A method for forming a calcia ceramic digestible protective layer according to claim 1, wherein the calcia ceramic sintered body contains 2% by weight or less of a fluorine compound. 3. The method for forming a calcia ceramic digestible protective layer according to claim 2, wherein the fluorine compound is any one of calcium fluoride, magnesium fluoride, and aluminum fluoride.