JP2692613B2 - Method for producing rare earth vanadate single crystal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a rare earth vanadate single crystal used for optics, and more particularly to a method for regenerating a raw material left in a crucible after crystal growth.

[0002]

2. Description of the Related Art Rare earth vanadate single crystals are useful as crystals for optical devices such as polarizers and solid-state lasers.
It is mainly grown by the CZ method (Czochralski method, pulling method). Rare earth oxide (R ₂ O
₃ , R: rare earth element) and vanadium pentoxide (V ₂ O ₅ ) are usually used, but both are usually mixed and fired to produce a rare earth vanadate (RVO ₄ ) by a solid phase reaction, which is then dissolved. It is used as a growing material.

Further, since the melting point is high, a crucible made exclusively of iridium is used, and from the viewpoint of protecting the crucible, crystal growth is carried out in a neutral or slightly oxidizing atmosphere. In the industrial crystal production, after the crystal growth is completed, the consumed weight is replenished with new raw material,
The so-called additional charge has been adopted to continue the training from the second time.

[0004]

In the rare earth vanadate crystal growth by the above-mentioned conventional CZ method, since the raw materials are gradually decomposed and scattered during the crystal growth, the composition as a whole is stoichiometry (stoichiometry ratio). Deviate from it. And due to the shift, the crystals that should be colorless and transparent originally become colored and cannot be used for optics,
Inclusion (inclusion) in the grown crystal in extreme cases
It was causing cracks.

Usually, when a high-purity raw material is used, such an inconvenience is unlikely to occur in the first growth, but when the additional charge growth is performed several times, the residual raw material becomes defective, which is the case. Problems will occur remarkably. Therefore, usually, a method of taking out the remaining raw material after growing the crystal several times and discarding it or regenerating it for reuse has been adopted. In the latter case, after cooling to room temperature, the remaining raw material is transferred from the crucible to another container, regenerated by heat treatment in an electric furnace in an oxygen gas atmosphere at about 1100 to 1200 ° C., and returned to the growing crucible to crystallize again. It has been used as a growing material.

However, since this raw material take-out operation requires a great deal of time and skill, it has been desired to develop a method for regenerating the residual raw material in the state of being kept in the crucible. However, regeneration requires high-temperature heat treatment in an oxidizing atmosphere, and the conventional method damages the iridium crucible, so it has been considered that regeneration in the crucible is impossible. The present invention has been made in view of this point, and an object thereof is to enable regeneration in a crucible without damaging the crucible and to greatly improve workability.

[0007]

In order to achieve the above object, according to the present invention, the composition remaining in the iridium crucible after the growth of a single crystal of rare earth vanadate (chemical formula RVO ₄ , R: rare earth element) by the CZ method. Misaligned ingredients,
Provided is a method for producing a rare earth vanadate single crystal, which comprises performing heat treatment at a temperature of 850 ° C. or higher and 1000 ° C. or lower for 1 to 40 hours in a crucible together with air to regenerate it and using it again as a growth raw material. Here, in the heat treatment for regenerating the residual material, after the crystal growth is finished, the temperature is gradually cooled to room temperature and then raised to the regenerative heat treatment temperature, or after the crystal growth is gradually cooled to the regenerative heat treatment temperature, the atmosphere gas is changed to air. This is performed by switching and directly entering the reproduction process.

[0008]

The present invention has found a condition that iridium is not damaged and that rare earth vanadate residual raw material is regenerated. Therefore, it is possible to perform regenerative heat treatment of the residual raw material together with the crucible in an oxidizing atmosphere, which has been conventionally difficult. Is provided. It has been conventionally considered that an iridium crucible cannot be used because it is damaged in an oxidizing atmosphere. Therefore, even in the case of growing an oxide single crystal, an atmosphere of an inert or neutral gas is mainly used, and even when oxygen is mixed, the content rate is at most 1 to 2%.

However, how iridium damage depends on temperature and oxygen concentration has not yet been understood in detail. On the other hand, the processing conditions necessary for regenerating the residual raw material after the growth of the rare earth vanadate crystal were not known in detail. Therefore, first of all, it was examined whether it is possible to effectively treat the residual raw material under the condition that it is advantageous for the crucible, that is, under the condition that the temperature is as low as possible and the oxygen concentration is as low as possible. It was found that good regeneration can be performed at 3 ° C for 3 hours or more, and at 1000 ° C for about 1 hour.

On the other hand, regarding the behavior of iridium in an oxidizing atmosphere, when the atmosphere was restricted to air and the relationship with temperature was examined, the surface treated with 900 ° C. was oxidized to a black color and its weight was reduced. Although it increases, the reaction does not proceed to a certain extent, and if it is subsequently placed under high temperature in an atmosphere close to neutrality, for example, it will return to its original state under the conditions of crystal growth, and the weight and metallic color will be restored. I understand. That is, it was found that this redox reaction occurs reversibly, so that there is no problem for the iridium crucible.

Next, when the iridium crucible was reversibly restored to a high temperature, it was found in the air that the behavior was the same as that at 900 ° C. up to 960 ° C. When the temperature exceeds 960 ° C, the oxide film formed on the surface gradually becomes rough, but reduction is possible up to around 1000 ° C. However, when the temperature exceeded 1000 ° C., the oxide formed on the surface became scattered during the treatment, and as a result, the weight was reduced from that before the treatment. It was found that the amount of weight loss depends on the treatment time and the scattering becomes steady. The surface state of the iridium after the heat treatment at 1000 ° C. or higher was not smooth as in the case of the treatment at 960 ° C. or lower, and the black powdery oxide powder was peeled off in the clogged state. Even if the crucible in such a state was heat-treated under the temperature and atmosphere during crystal growth, the surface state and weight could not be restored.

That is, it has been found that there is a change that leaves the reversible oxidation / reduction reaction range in an atmosphere in the air exceeding 1000 ° C. From the above experimental results, it was found that the regenerating treatment of the remaining raw material after growing the rare earth vanadate crystal in the crucible in air can be performed at 1000 ° C or lower, and more preferably at 960 ° C or lower.

On the other hand, since the oxidation of the iridium crucible is slight in the regenerative heat treatment at 900 ° C. or lower, it can be adopted as the regenerative heat treatment condition. However, if the temperature is 850 ° C. or lower, the heat treatment time for regeneration becomes too long, which is not preferable industrially. The required heat treatment time depends on the heat treatment temperature. At 850 ° C., heat treatment for about 40 hours is required, and at 1000 ° C., about 1 hour is sufficient.

[0014]

Next, embodiments of the present invention will be described with reference to the drawings. [First Embodiment] FIG. 1 is a sectional view of a high frequency induction heating type oxide single crystal pulling apparatus for carrying out an embodiment of the present invention. In the figure, 1 is a crystal growth furnace housing, 2 is a high-frequency coil for induction heating, 3 is an iridium crucible, 6 is a heat-resistant refractory, 7 is a gas inlet, and 8 is a gas outlet. Here, the crucible 3 used had a diameter of 50 mm, a depth of 50 mm, and was manufactured using a raw material having a purity of 99.99%.

YVO ₄ was introduced into the iridium crucible as a crystal growth raw material to grow a vanadate single crystal. The growth rate was 1 mm / h, and the growth atmosphere was nitrogen containing 0.1% oxygen. Using c-axis seed, diameter about 220mm,
A grown crystal 4 having a length of about 30 mm was obtained. The same weight of raw material as the grown crystal was added to the residual raw material left in the crucible, and the same crystal growth was performed four times.

After the fourth crystal growth was completed, the crucible was cooled to room temperature, taken out of the growth furnace, and the remaining raw material was heat-treated in a separate electric furnace together with the crucible in air at 940 ° C. for 15 hours. After the treatment, it was confirmed that the black-gray residual raw material changed to yellow-green and was regenerated. Although the iridium crucible was black, it showed that the surface was smooth and reversible. When a new raw material for the grown crystal was further added to this crucible and the same crystal growth was carried out, a good quality crystal similar to the first time could be obtained.

[Second Embodiment] Using the high-frequency induction heating type oxide single crystal pulling apparatus shown in FIG. 1, crystal growth of YVO ₄ and additional charge are carried out in the same manner as in the first embodiment. It carried out 4 times continuously, sandwiching it. After the completion of the fourth crystal growth, the temperature was lowered to room temperature, additional charge for the weight lost by the crystal growth was performed, the temperature was raised to 880 ° C., and then heat treatment was performed in the air at that temperature for 25 hours. After that, when crystal growth was performed using this growth material, it was possible to obtain a good-quality vanadate single crystal similar to the first time.

[Third Embodiment] Using the high frequency induction heating type oxide single crystal pulling apparatus shown in FIG.
YVO ₄ single crystal growth in a nitrogen atmosphere containing oxygen,
It was done five times in a row with an additional charge in between. After completion of the fifth growth, the grown crystal 4 was separated from the raw material melt as shown in FIG. 1, and in that state, as shown in FIG. Slowly cool with 9
When the temperature reached 00 ° C., the temperature decrease was interrupted and the atmosphere gas was gradually changed to air over 1 hour, left as it was for 20 hours, and then decreased again at 200 ° C./hour and gradually cooled to room temperature.

By the above heat treatment, the residual raw material was regenerated and the annealing shown in FIG. 2 was performed on the grown crystal for the fifth time. When the obtained crystal was observed, cracks and inclusions were partially observed. This was the fifth training, so it was a natural result. However, the color tone of the crystal was yellow brown rather than black brown like the crystal grown after the usual fifth charge, and it was shown that the optical characteristics were improved by the effect of the heat treatment in the oxygen atmosphere according to the present invention.

Next, when the remaining raw material in the crucible was recharged and the sixth crystal growth was conducted in the same manner, no cracks or inclusions were observed in the obtained crystal, and the fifth crystal growth was performed. Exceeded the quality. This confirmed the effect of regenerating the residual raw material according to the present invention.

In the above embodiments, the case of growing a YVO ₄ single crystal was explained, but the present invention is not limited to this, and the same applies to the case of growing a rare earth vanadate crystal other than YVO _4. It is applicable and the same effect can be obtained.

[0022]

As described above, according to the present invention, since the operation of regenerating the residual raw material in the crucible after the growth of the rare earth vanadate by the CZ method is performed by the heat treatment at 1000 ° C. or less in the air, the residual raw material is removed. It became possible to heat-treat the crucible, the raw material take-out operation, which conventionally required a lot of time and skill, became unnecessary, and it became possible to omit another electric furnace which was required in the past.

Further, according to the embodiment in which the regenerated heat treatment of the remaining raw material is carried out during the slow cooling after the crystal growth in the growth furnace, the grown crystal can be annealed at the same time, and its crystallinity is improved to improve its optical properties. For example, it becomes possible to improve the light absorption characteristics. Moreover, since the residual raw material regenerated in this way can be effectively utilized again for growth, the material cost can be reduced and the crystal production cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a high frequency induction heating type oxide single crystal pulling apparatus for explaining an embodiment of the present invention.

FIG. 2 is a heat treatment program diagram for explaining a third embodiment of the present invention.

[Explanation of symbols]

 1 Crystal growth furnace housing 2 Induction heating high-frequency coil 3 Iridium crucible 4 Growth crystal 5 Remaining raw material 6 Insulating refractory 7 Gas inlet 8 Gas outlet

Claims (5)

(57) [Claims] 1. A raw material having a different composition remaining in an iridium crucible after growing a single crystal of a rare earth vanadate (chemical formula RVO ₄ , R: rare earth element) by a CZ method in an air at 850 ° C. or more and 1000 ° C. 1-4 at the following temperatures
A method for producing a rare earth vanadate single crystal, which comprises performing a heat treatment for 0 hour to regenerate it, and using it again as a growing material. 2. After the single crystal growth is completed and the grown crystal is separated from the raw material melt, it is gradually cooled to the raw material regeneration temperature, and the atmosphere gas is changed from the growth gas to air and the heat treatment for the raw material regeneration is performed as it is. The method for producing a rare earth vanadate single crystal according to claim 1, wherein 3. The single crystal growth is completed, the grown crystal is separated from the raw material melt, then gradually cooled to room temperature, and then heated to the raw material regeneration temperature to perform the heat treatment for the raw material regeneration. The method for producing a rare earth vanadate single crystal according to claim 1. 4. The method for producing a rare earth vanadate single crystal according to claim 3, wherein the crucible is gradually cooled to room temperature and then the crucible is transferred to another heating furnace to perform heat treatment for regenerating the raw material. 5. The method for producing a rare earth vanadate single crystal according to claim 3, wherein after the material is gradually cooled to room temperature, a new raw material is added to the remaining raw material and then a heat treatment for regeneration is performed.