JP2006001786A - Crucible for growing calcium fluoride crystal, method for producing calcium fluoride crystal, and calcium fluoride crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a crucible for growing a crystal, with which the polycrystallization is prevented and a calcium fluoride single crystal can be surely grown, and to provide a method for producing the calcium fluoride crystal, and the calcium fluoride crystal. <P>SOLUTION: The crucible is used for growing the crystal along the crystal plane of a seed by melting calcium fluoride and cooling the molten calcium fluoride. The crucible has an inner surface formed from niobium carbide (NbC) and is characterized in that the contact angle between its inner surface and a water drop is ≤150°. The method for producing the calcium fluoride crystal, and the calcium fluoride crystal are also provided. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a crystal growth crucible grown by melting and cooling calcium fluoride, a method for producing the crystal, and a calcium fluoride crystal.

  Conventionally, as a crucible for growing calcium fluoride into a single crystal by cooling and cooling, one configured to grow calcium fluoride into a single crystal along the crystal plane of a seed (seed crystal) It is known (see, for example, Patent Document 1).

  On the inner surface of this type of crucible, there is a tapered cone having a large-diameter raw material container into which calcium fluoride raw material is charged and a small-diameter seed container into which calcium fluoride seeds (seed crystals) are contained. It is formed continuously through the surface.

Japanese Patent Laid-Open No. 10-265296

  By the way, in the conventional example of this type of crucible described above, when calcium fluoride melted in the crucible contracts by cooling, calcium fluoride adheres to the inner surface of the crucible and residual stress or distortion occurs in the crystal. Since this is the starting point and grain boundaries are likely to occur, polycrystals (heterogeneous phases) are likely to occur. As a result, there is a problem that a single crystal of calcium fluoride cannot be easily grown.

  Then, this invention makes it a subject to provide the crucible which can prevent the polycrystallization and can grow the single crystal of calcium fluoride easily.

  The inventors of the present application have confirmed the following phenomenon as a result of experiments for growing a calcium fluoride single crystal with a crucible. That is, it was confirmed that the higher the wettability between the inner surface of the crucible made of niobium carbide (NbC) and water suitability, the better the single crystal was grown. And, if the contact angle between the crucible inner surface and the water droplet is 150 ° or less, the present invention has been obtained with the knowledge that the wettability between the crucible inner surface and the calcium fluoride solution is reduced and a good single crystal can be grown. Was completed.

  That is, the present invention relates to a crucible for growing a calcium fluoride into a single crystal along a crystal plane of a seed by cooling and melting the inner surface of the crucible composed of niobium carbide (NbC) and water droplets. And a calcium fluoride crystal growing crucible characterized in that a contact angle with the crucible is 150 ° or less.

  In the crucible according to the present invention, the contact angle between the crucible inner surface and the water droplet is 150 ° or less and the wettability between the crucible inner surface and the calcium fluoride solution is low, so that the calcium fluoride melted in the crucible is cooled by cooling. When shrinking, the calcium fluoride is easily separated from the inner surface of the crucible, so that the occurrence of residual stress and distortion in the calcium fluoride crystal is suppressed. As a result, a single crystal of calcium fluoride is easily grown.

  In the crucible of the present invention, the contact angle between the crucible inner surface and the water droplet is preferably 120 ° or less, and more preferably 100 ° or less. Moreover, it is preferable that the inner surface of the crucible is made of niobium carbide and the portion excluding the inner surface of the crucible is made of carbon.

  Moreover, this invention relates to the manufacturing method of the calcium fluoride crystal produced using the above-mentioned calcium fluoride crystal growth crucible.

  Moreover, this invention relates to the calcium fluoride crystal produced using the manufacturing method as described above.

  In the crucible according to the present invention, the contact angle between the inner surface of the crucible made of niobium carbide (NbC) and the water droplet is 150 ° or less, and the wettability between the inner surface of the crucible and the calcium fluoride solution is low. When the calcium fluoride melted in (1) is shrunk by cooling, the calcium fluoride is easily separated from the inner surface of the crucible, so that the occurrence of residual stress and strain in the calcium fluoride crystal is suppressed. Therefore, according to the present invention, a single crystal of calcium fluoride can be easily grown.

  Hereinafter, embodiments of the crucible according to the present invention will be described with reference to the drawings. In the drawings to be referred to, FIG. 1 is a schematic diagram showing a schematic structure of a vacuum VB furnace provided with a crucible according to an embodiment, and FIG. 2 is a cross-sectional view showing a structure of the crucible according to the embodiment shown in FIG. .

As shown in FIG. 1, a crucible 1 according to an embodiment is disposed inside a heater 2A in a vacuum VB furnace 2 as a single crystal growth apparatus by a vertical Bridgman (hereinafter abbreviated as VB) method, The calcium fluoride (CaF ₂ ) raw material M is melted and cooled by being moved up and down at a very low speed through the shaft 2B, and this is a seed (seed crystal) made of a single crystal of calcium fluoride (CaF ₂ ). For example, S is grown into a single crystal along a crystal plane of (1,1,1) orientation.

The inside of the vacuum VB furnace 2 is depressurized to 10 ⁻⁴ Pa or less by a vacuum pump 2C, and heated to, for example, about 1400 to 1500 ° C. by a heater 2A. In order to prevent the seed S from melting by the heating of the heater 2A, the shaft 2B of the vacuum VB furnace 2 is configured to constitute a cooling water circulation path.

  That is, the shaft 2B is formed of a double tube in which the upper end of the inner tube 2B1 is retracted from the upper end of the outer tube 2B2, and a cap-like heat transfer member 2D is fitted and fixed to the upper end portion. And this heat-transfer member 2D is comprised so that the lower part of the seed S may be forcedly cooled by connecting to the center part of the bottom member 1C of the crucible 1 mentioned later.

Here, as shown in FIG. 2, the crucible 1 of one embodiment includes a crucible body 1A, a lid member 1B that covers the opening of the crucible body 1A, and a bottom member 1C that is fixed to the lower portion of the crucible body 1B. It is prepared for. The crucible body 1A is made of a high-purity carbon material (C) as a material that has heat resistance and can improve the smoothness of the inner surface. The inner surface of the crucible body 1A is coated with niobium carbide (NbC) that can reduce wettability with a calcium fluoride (CaF ₂ ) solution. Made of niobium carbide (NbC).

The crucible body 1A is formed with a large-diameter raw material storage portion 1D in which a raw material M (see FIG. 1) of calcium fluoride (CaF ₂ ) is stored. A small-diameter seed accommodating portion 1E that accommodates, for example, a cylindrical seed S (see FIG. 1) is formed as a straight circular hole at the center of the crucible main body 1A to the bottom member 1C. And between the raw material accommodating part 1D and the seed accommodating part 1E, the taper-shaped (funnel-shaped) cone surface 1F which comprises the bottom of the raw material accommodating part 1D is formed.

  On the other hand, the lid member 1B and the bottom member 1C are also made of a heat-resistant high-purity carbon material. A connecting cylinder portion 1C1 for fitting and fixing a heat transfer member 2D (see FIG. 1) fixed to the upper end portion of the shaft 2B of the vacuum VB furnace 2 projects from the center portion of the bottom surface of the bottom member 1C. ing.

  Here, a concave curved surface 1J is formed at the boundary between the wall surface 1H of the raw material storage portion 1D and the cone surface 1F, and the wall surface 1H of the raw material storage portion 1D and the cone surface 1F are smoothly continuous via the concave curved surface 1J. is doing. Further, a convex curved surface 1L is formed at the boundary portion between the cone surface 1F and the wall surface 1K of the seed accommodating portion 1E, and the cone surface 1F and the wall surface 1K of the seed accommodating portion 1E are smoothly continuous via the convex curved surface 1L. ing.

  The wall surface 1H of the raw material container 1D is formed straight, and the inner diameter between the wall surfaces 1H is set to 250 mm, for example. Moreover, the internal diameter of the seed accommodating part 1E is set to 20 mm, for example.

Here, if the cone angle θ of the cone surface 1F is too small, residual stress and strain are generated in the crystal of calcium fluoride (CaF ₂ ) grown in the raw material container 1D. (Different phase) is likely to occur. On the other hand, if the cone angle θ of the cone surface 1F is too large, the growth of a single crystal of calcium fluoride (CaF ₂ ) is likely to be hindered. Therefore, the cone angle θ of the cone surface 1F is set to the range of 120 ° to 130 ° as the most preferable range among the range of 95 ° to 150 °.

Further, when the concave curved surface 1J and the convex curved surface 1L are angular because the radius of curvature is too small, when the calcium fluoride (CaF ₂ ) melted in the raw material container 1D is crystallized by cooling, the concave concave curved surface is formed. Polycrystals (heterogeneous phases) are likely to occur with 1J and the convex curved surface 1L as nuclei. In addition, when calcium fluoride (CaF ₂ ) contracts by cooling, calcium fluoride (CaF ₂ ) adheres to these angular concave curved surface 1J and convex curved surface 1L, and residual stress and strain are generated in the crystal. Due to this, polycrystal (heterogeneous phase) is likely to occur.

  Therefore, the curvature radii of the concave curved surface 1J and the convex curved surface 1L are set to a large curvature radius of 1/10 or more of the inner diameter (for example, 250 mm) between the wall surfaces 1H of the raw material container 1D. For example, the radius of curvature of the concave curved surface 1J is set to about 60 mm, and the radius of curvature of the convex curved surface 1L is set to about 50 mm.

Furthermore, when the surface roughness of the wall surface 1H and the cone surface 1F of the raw material container 1D is rough, when the calcium fluoride (CaF ₂ ) melted in the raw material container 1D is crystallized by cooling, the wall surface 1H and the cone Polycrystals (heterogeneous phases) are likely to be generated with minute irregularities such as the surface 1F serving as nuclei. In addition, when calcium fluoride (CaF ₂ ) contracts due to cooling, calcium fluoride (CaF ₂ ) adheres to the wall surface 1H and the cone surface 1F, resulting in residual stress and strain in the crystal. Therefore, polycrystal (heterogeneous phase) is easily generated.

  Therefore, the inner surface of the crucible extending from the wall surface 1H of the raw material container 1D of the crucible body 1A to the wall surface 1K of the seed container 1E through the concave curved surface 1J, the cone surface 1F, and the convex curved surface 1L has a surface roughness of at least Rmax6 by the maximum height method. .4 s or less, for example, about Rmax 3.2 s.

Here, if the wettability between the inner surface of the crucible such as the wall surface 1H and the cone surface 1F of the raw material container 1D and the solution of calcium fluoride (CaF ₂ ) is high, the calcium fluoride (CaF) melted in the raw material container 1D ₂ ) When shrinking due to cooling, calcium fluoride (CaF ₂ ) adheres to the wall surface 1H and the cone surface 1F, causing residual stress and distortion in the crystal, resulting in polycrystal (heterogeneous phase) Easy to do.

Therefore, in the crucible 1 according to the embodiment, the inner surface of the crucible coated with niobium carbide (NbC), that is, the concave curved surface 1J, the cone surface 1F, the convex curved surface 1L from the wall surface 1H of the raw material container 1D of the crucible body 1A. In order to reduce the wettability between the inner surface of the crucible over the wall surface 1K of the seed container 1E and the calcium fluoride (CaF ₂ ) solution, the contact angle between the inner surface of the crucible and the water droplet is, for example, 100 ° which is 150 ° or less. The inner surface of the crucible is finished.

The crucible 1 of one embodiment configured as described above is a vacuum VB furnace 2 (FIG. 1) that is decompressed to 10 ⁻⁴ Pa or less in order to melt the raw material M of calcium fluoride (CaF ₂ ) shown in FIG. In the reference), the inside of the heater 2A heated to around 1400 to 1500 ° C. is raised by the shaft 2B at a slow speed of about 10 mm / h and held at the raised position for about 10 hours. At that time, the lower part of the seed S is forcibly cooled through the heat transfer member 2D by the cooling water circulating in the shaft 2B from the inner pipe 2B1 to the outer pipe 2B2, thereby preventing melting of the portion other than the upper part of the seed S. Is done.

The crucible 1 cools the molten calcium fluoride (CaF ₂ ) raw material M and grows it into a single crystal along the crystal plane of the seed (seed crystal) S, for example, in the (1,1,1) orientation. Therefore, the shaft 2B is lowered at a very low speed of, for example, about 1.0 mm / h, which is 1.5 mm / h or less, and is held at the lowered position in the vacuum VB furnace 2 for about 5 hours.

Thereafter, the molten calcium fluoride (CaF ₂ ) in the crucible 1 is controlled to be 70 ° C./h or less by controlling on / off of the heater 2A of the vacuum VB furnace 2 in order to prevent quenching (cracking due to thermal shock). For example, it is cooled at a cooling rate of about 30 ° C./h.

Here, in the crucible 1 of one embodiment, the inner surface of the crucible body 1 coated with niobium carbide (NbC), that is, the concave curved surface 1J, the cone surface 1F, from the wall surface 1H of the raw material container 1D of the crucible body 1A, The crucible inner surface is finished so that the contact angle between the inner surface of the crucible and the water droplet over the wall surface 1K of the seed container 1E through the convex curved surface 1L is, for example, 150 ° or less. The inner surface of the crucible and the calcium fluoride (CaF ₂ ) solution The wettability of is extremely low.

For this reason, when calcium fluoride (CaF ₂ ) melted in the crucible contracts by cooling, the calcium fluoride (CaF ₂ ) is easily separated from the inner surface of the crucible. As a result, the occurrence of residual stress and strain in the calcium fluoride (CaF ₂ ) crystal is suppressed, and a single crystal of calcium fluoride (CaF ₂ ) is easily grown.

Moreover, since the inner surface of the crucible body 1 is finished to a smooth surface of, for example, about Rmax 3.2 s, the calcium fluoride (CaF ₂ ) melted in the raw material container 1D is cooled by the (1, 1, 1) When crystallization is performed along the crystal plane of orientation, the generation of nuclei that cause polycrystals on the inner surface of the crucible is suppressed. As a result, a calcium fluoride (CaF ₂ ) single crystal is easily grown.

Moreover, since the molten calcium fluoride (CaF ₂ ) in the crucible 1 is cooled at a cooling rate of 70 ° C./h or less, for example, about 30 ° C./h, quenching (cracking due to thermal shock) is prevented and good. Grown into a single crystal.

In addition, in order to cool the molten calcium fluoride (CaF ₂ ) and grow it into a single crystal, the speed at which the crucible 1 is lowered at a very low speed, that is, the growth speed is 1.5 mm / h or less, for example 1.0 mm / h. Therefore, the crystal orientation of the grown single crystal is stable as shown in FIG. It was found that when the growth rate was 1.5 mm / h or more and 2 mm / h, the crystal orientation was dispersed and not stable as shown in FIG.

As Examples 1 to 3 and Comparative Examples 1 and 2, a crucible 1 having a different contact angle between the inner surface of the crucible body 1A and water droplets and a different niobium carbide (NbC) coating thickness is used. Then, a single crystal of calcium fluoride (CaF ₂ ) was grown in the vacuum VB furnace 2, and the rate of occurrence of polycrystals generated in the obtained crystal was measured and evaluated.

  The surface roughness was measured by the maximum height method by three-dimensional shape measurement of a scanning confocal laser microscope OLS1100 manufactured by Shimadzu Corporation. In addition, the incidence of polycrystals is such that the film faces are parallel using two Polarer Films (color: gray, area: 15 inches x 8.5 inches, thickness 0.29 mm) manufactured by Edmund Industrial Optics. Install and place crystals between the films, observe the crystal from one side of the film, the light source from the other side of the film, and the opposite side of the light source, change the angle and position of the crystal and measure the portion that is not a single crystal as a polycrystal did. Finally, the volume of the polycrystalline portion was calculated, and the ratio of the total crystal volume to the polycrystalline volume was defined as the occurrence rate of the polycrystalline body. And the thing of 30% or less of the generation rate of the polycrystalline body was evaluated as (circle), and the thing of 70% or more was evaluated as x.

The evaluation results are as shown in Table 1. Like Example 1 to Example 3, niobium carbide (NbC) was coated on the inner surface of the crucible with a thickness of 1.5 mm or more, and water droplets on the surface When the contact angle is 150 ° or less, it has been found that the occurrence rate of polycrystals is 30% or less and a single crystal of calcium fluoride (CaF ₂ ) is easily grown.

On the other hand, as in Comparative Example 1 and Comparative Example 2, when the thickness of the niobium carbide (NbC) coating is 1 mm or less and the contact angle with water droplets on the surface exceeds 150 °, It has been found that it is difficult to grow a single crystal of calcium fluoride (CaF ₂ ) with an occurrence rate of polycrystals of 70% or more.

It is a mimetic diagram showing a schematic structure of a vacuum VB furnace provided with a crucible concerning one embodiment of the present invention. It is sectional drawing which shows the structure of the crucible which concerns on one Embodiment shown in FIG. It is a figure which shows the distribution condition of the crystal orientation in the crystal | crystallization obtained when the growth speed | rate which lowers the crucible shown in FIG. 1 in the vacuum VB furnace 2 at very low speed was 1.0 mm / h. It is a figure which shows the distribution condition of the crystal orientation in the crystal | crystallization obtained when the growth speed | rate which lowers the crucible shown in FIG. 1 in the vacuum VB furnace 2 at very low speed was 2.0 mm / h.

Explanation of symbols

1: Crucible 1A: Crucible body 1B: Lid member 1C: Bottom member 1D: Raw material container 1E: Seed container 1F: Cone surface, water jacket 1G: Water jacket 1H: Wall surface 1J of raw material container 1: Concave surface 1K: Seed Wall surface 1L of accommodating portion: convex curved surface 2: vacuum VB furnace 2A: heater 2B: shaft 2C: vacuum pump 2D: cooling water passage M: raw material of calcium fluoride (CaF ₂ ) S: seed of calcium fluoride (CaF ₂ ) (Seed crystal)

Claims (4)

It is a crucible for growing calcium fluoride into crystals along the crystal plane of the seed by cooling and cooling. The inner surface of the crucible is made of niobium carbide (NbC), and the crucible inner surface and water droplets A crucible for growing calcium fluoride crystals, wherein the contact angle is 150 ° or less. The crucible according to claim 1, wherein a portion excluding the inner surface of the crucible is made of carbon as a raw material. The manufacturing method of the calcium fluoride crystal produced using the calcium fluoride crystal growth crucible of Claims 1-2. A calcium fluoride crystal produced by using the production method according to claim 3.

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