JPH05163098A - Method for growing lanthanum aluminate crystal - Google Patents

Abstract

PURPOSE:To readily and efficiently take out a crystal without requiring troublesome operation to remove melt residues in a crucible after growing the crystal and causing the breakage or deterioration in the crucible in a method for growing a lanthanum aluminate crystal according to a method for pulling up the crystal. CONSTITUTION:Lanthanum oxide is added to a melt residue in a crucible after growing a crystal to solidify the residue into a polycrystal containing a phase rich in the lanthanum. Thereby, the residue is solidified in a state rich in the lanthanum. As a result, the residue can mechanically and extremely readily be removed. Self fine powdering of the residue considered due to moisture absorption occurs especially by exposing the solidified residue to steam or water under specific conditions. The residue can extremely easily be separated from the crucible and the life of the expensive crucible can remarkably be prolonged. Consequently, the crucible cost and further crystal growth cost can remarkably be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for growing a lanthanum aluminate crystal, and more particularly to a precious metal (iridium).
By immersing a seed crystal in the melt in the crucible and pulling it up, when growing a predetermined amount of a lanthanum aluminate single crystal in a predetermined direction, a crystal that terminates the growth with the melt residue left in the crucible. In the growing method, it relates to a method for efficiently removing the residue in the crucible.

[0002]

2. Description of the Related Art Lanthanum aluminate (LaAlO ₃ )
Crystals are substances that have received some attention as substrate materials for high Tc superconducting materials.

Conventionally, lanthanum aluminate is generally grown by the Bernoulli method, but some examples have been reported by the crystal pulling method called Czochralski method. Since lanthanum aluminate has a high melting point, how to grow crystals with high quality and at low cost is important. Therefore, it is desired to establish a technique for reducing the crucible cost by using the crystal pulling method.

In the crystal pulling method, lanthanum aluminate is melted in a crucible, and a seed crystal is immersed in this melt,
This is a method of pulling and growing a crystal.

In such a crystal pulling method, as the crucible material, it is necessary to consider that the melting point is higher than the melting temperature of the crystal composition to be grown and that it does not react with the melt. It is a very important requirement and a difficult problem. Conventionally, platinum and iridium have been generally used as crucible materials for growing complex oxide crystals. These materials are very expensive,
After repeated use for 10 to 30 times, it is first subjected to recasting.

By the way, in general, the crystal pulling method is 2
When 0 to 80% of the crystals have grown, the crystals are separated from the melt, and the crystals are taken out after cooling. At that time, a melt residue remains in the crucible. Therefore, when the crucible is reused, it is necessary to remove this residue, and conventionally, various removal methods have been adopted for removing this residue depending on the physical properties thereof. For example, a method of re-dissolving and taking out, or taking out in combination with machining or acid washing is adopted.

[0007]

Among the conventional methods for removing the residue of the melt, the re-melting method in the case of lanthanum aluminate has an extremely high melting temperature of about 2080 ° C., which causes melting damage of the crucible. There is a risk. Particularly, in the crucible made of iridium, the life of the crucible is shortened by (oxidation) evaporation of iridium. Further, the mechanical processing and the acid cleaning method also have the problems of risk of crucible breakage, crucible deterioration, and complicated process.

Therefore, in growing lanthanum aluminate crystals, a method of efficiently removing the melt residue without causing damage or deterioration of the crucible and reusing the crucible for a long period of time, particularly when the crucible is fragile. The advent of methods to significantly improve the life of expensive iridium crucibles is desired.

[0009]

A method for growing a lanthanum aluminate crystal according to claim 1 is a method for adding a lanthanum compound to a lanthanum aluminate crystal to grow the residue by a pulling method, wherein the crucible after the crystal is grown. By adding a lanthanum compound to the melt residue in the
The residue is solidified into a polycrystal containing a lanthanum-rich phase.

A method for growing a lanthanum aluminate crystal according to claim 2 is the method according to claim 1, wherein the solidified melt residue is converted into steam or water at a temperature of 0 to 100 ° C and a H ₂ O content of 10 to 100%. It is characterized by exposing for 10 minutes or more.

That is, the present inventor has conducted extensive studies on a method for removing a lanthanum aluminate residue from an iridium crucible, and utilizes the fact that a lanthanum aluminate polycrystal having a lanthanum-rich segregation phase is brittle and hygroscopic. As a result, they have found that the lanthanum aluminate residue can be easily taken out from the inside of the crucible, and have completed the present invention.

In the present invention, the lanthanum-rich phase means a phase in which the lanthanum is excessive with respect to the lanthanum aluminate phase in which the ratio of antan and aluminum is 1: 1.

The present invention will be described in detail below.

In the method of the present invention, the lanthanum aluminate crystal is grown by the crystal pulling method in which the lanthanum aluminate is melted in the crucible, and the seed crystal is immersed in the melt to pull up the crystal. A lanthanum compound is added to the liquid residue to make it a lanthanum-rich state, and the lanthanum-rich phase is solidified in a form including a segregation phase.

That is, first, after completion of crystal growth, immediately after separating the grown crystal, a lanthanum compound such as lanthanum oxide is added to the residue and dissolved. In this case, the lanthanum-rich ratio is not particularly limited, but if it is the residue of the melt in which lanthanum oxide and aluminum oxide are mixed in an equimolar ratio,
76.31 to 83.86 g for 100 g of this residue
It is preferable to dissolve about a lanthanum oxide so that the amount of lanthanum is 1.001 to 1.100 mol times that of aluminum in the residue.

The lanthanum compound to be added is
Lanthanum oxide is most advantageous in terms of recycling of the residue.

The lanthanum-rich residue thus obtained is cooled and solidified according to a conventional method to obtain a polycrystal containing a lanthanum-segregated phase.

This polycrystal has a very good peeling property from the inner wall of the crucible and can be easily taken out from the crucible by applying a light impact.

In particular, the polycrystalline residue in this crucible is removed at a temperature of 0.
By exposing it to steam or water having a H ₂ O content of 10 to 100 ° C. and a H ₂ O content of 10 to 10% for 10 minutes or more, the polycrystalline residue is efficiently separated from the inner wall of the crucible by self-pulverization and can be taken out more easily. Is possible. In order to expose the polycrystalline residue in the crucible to the steam or water under the above-mentioned specific conditions, for example, water enough for the polycrystalline residue to be immersed in the crucible is left for several hours after heating. The separation of the residue due to the steam or water easily occurs as the temperature of the steam or water to be treated increases. The water may be an aqueous solution.

The method of the present invention as described above is extremely effective particularly in a method for growing a lanthanum aluminate crystal using a brittle and expensive iridium crucible.

[0021]

Function: By solidifying the melt residue after the lanthanum aluminate crystal growth in a lanthanum-rich state, the residue can be removed very easily mechanically.
In particular, by exposing the solidified residue to steam or water under specific conditions, self-micronization of the residue, which is presumed to be caused by moisture absorption, occurs, and the residue deviates from the crucible very easily.

Although the details of the mechanism of such residue separation or dissociation according to the present invention are not clear, it is believed that impurities in the lanthanum-rich residue or the heterogeneous phase of the La-Al-O system are involved. .. By the way, in the grown crystal that does not contain voids (lanthanum-rich segregation phase, etc.), the phenomenon of self-pulverization due to moisture absorption is not recognized.

[0023]

EXAMPLES The present invention will be described in more detail below with reference to comparative examples and examples, but the present invention is not limited to the following examples unless it exceeds the gist.

Comparative Example 1 A crucible made of iridium (Ir) having a diameter of 50 mm, a height of 50 mm and a thickness of 1.5 mm was placed in a high frequency heating furnace of 110 KHz, and 50 mol of lanthanum oxide having a purity of 99.99% was placed in the crucible. %, Aluminum oxide 50 mol% 3
20 g was put and melted in a nitrogen gas atmosphere. The temperature of the melt is lowered to near the melting point, and a seed crystal of LaAlO ₃ is immersed in the melt,
While rotating at pm, 180 g of crystals were grown at a pulling rate of about 2 mm / hr and a crystal diameter of 30 mm. Then, the crystal was separated, and the residue was solidified in a crucible which was cooled by applying about 24 hours.

The grown crystal was a highly oriented substance containing a transparent, light brown-colored twin, and did not contain a void resulting from segregation.

On the other hand, the residue was solidified as a lump in a polycrystalline form while being attached to the crucible, and could not be removed by a simple artificial light mechanical shock or acid cleaning as it was. Then, water was immersed in the crucible containing the residue, heated to 80 ° C. and left for 1 day. No separation due to self-pulverization of the residue occurred at all, and it remained as a very hard lump before treatment, and it was difficult to remove the residue.

Example 1 In Comparative Example 1, 3 g of lanthanum was added immediately after the crystal was cut off and dissolved in the residue. Then, the residue was solidified by applying about 24 hours. As a result, the residue was polycrystalline and solidified, and was peeled from the inner wall of the crucible by a light mechanical impact caused by human power. Then, immerse the water in the crucible containing the residue at 80 ° C.
When heated for 5 hours and left for 5 hours, the residue was dissociated due to self-pulverization, and the residue could be easily removed.

[0028]

As described in detail above, according to the method for growing a lanthanum aluminate crystal of the present invention, in the method for growing a lanthanum aluminate crystal by the crystal pulling method, the melt residue in the crucible after the crystal growth is removed. It is possible to take out easily and efficiently without requiring a complicated residue removing operation and without causing damage or deterioration of the crucible.

Therefore, the life of an expensive crucible can be significantly extended, and as a result, the cost of the crucible and the crystal growth cost can be significantly reduced, and its industrial utility is extremely large.

According to the lanthanum aluminate crystal growing method of the second aspect, the residue can be removed more easily.

Claims (2)

[Claims] 1. A method of growing a lanthanum aluminate crystal by a pulling method, which comprises adding a lanthanum compound to a melt residue in a crucible after crystal growth to solidify the residue into a polycrystal containing a lanthanum-rich phase. And a method for growing a lanthanum aluminate crystal. 2. The solidified melt residue is treated at a temperature of 0-100.
℃, H ₂ O content 10 to 100% steam or water 10
The method for growing a lanthanum aluminate crystal according to claim 1, wherein the lanthanum aluminate crystal is grown for at least a minute.