JP2868204B2 - Equipment for producing lithium tetraborate single crystal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an apparatus for producing lithium tetraborate, and more particularly to an apparatus for producing lithium tetraborate useful as a substrate material for a surface acoustic wave device.

[0002]

2. Description of the Related Art Conventionally, as substrate materials for surface acoustic wave devices, lithium niobate, lithium tantalate, quartz,
Lithium tetraborate and the like have been put to practical use. Among these single crystals, lithium tetraborate is a material that is useful as a filter for mobile communication equipment because it has a relatively large electromechanical coupling coefficient and a very small delay time temperature coefficient at room temperature. It has been.

As a method of growing a lithium tetraborate single crystal, a method of growing from a melt by the Czochralski method or the vertical Bridgman method has been conventionally known.
Of these, the Czochralski method puts raw materials in a crucible,
It is heated and melted, the seed crystal is brought into contact with the melt, and the crystal is grown by gradually pulling it while rotating it, and there is an advantage that the growth rate can be increased compared to the vertical Bridgman method, Thermal distortion occurs in the crystal due to a steep temperature gradient, and cracks are easily generated during growth due to temperature fluctuation due to gas convection in the furnace,
There is a disadvantage that the yield of crystals is low.

On the other hand, the vertical Bridgman method moves a crucible containing a melt in a furnace having a temperature gradient, and solidifies the lithium tetraborate melt from a crucible tip into which a seed crystal is inserted. Since the gradient is relatively gentle and the temperature fluctuation is small, a single crystal of good quality can be obtained as compared with the Czochralski method. Crucibles used in the vertical Bridgman method are generally made of platinum from the viewpoint of heat resistance, thermal shock resistance, corrosion resistance, stability and the like (Japanese Patent Application Laid-Open No. 6-247796).

[0005]

However, when the growth of a single crystal of lithium tetraborate by the conventional Bridgman method using a platinum crucible is examined in more detail, it can be said that there are no problems as described below. Did not. That is, when a platinum crucible is used, the growth rate of the single crystal changes due to the effect of temperature fluctuation, and a lattice defect is likely to occur due to inclusion of bubble inclusions. Furthermore, platinum has good wettability with lithium tetraborate, and when the platinum crucible shrinks during cooling,
There is a problem that distortion occurs in the crystal due to the difference in thermal expansion, and it has been difficult to produce a high-quality crystal with high yield. Further, in the vertical Bridgman method, since the melt must be held at a high temperature for a long time, the crucible needs to have a wall thickness of at least 0.3 to 0.35 mm, and the purification cost at the time of crucible regeneration is high. There was a disadvantage.

In order to solve the above-mentioned problems of the platinum crucible, Japanese Patent Laid-Open No. 5-262596 proposes to use a graphite crucible. In this case, the temperature fluctuation hardly directly affects the melt, and the inclusion of bubble inclusions is eliminated.However, the mixing and concentration of impurities from the crucible causes the lithium tetraborate single crystal to partially fuse to the graphite and cool. There is a problem that a tensile stress acts on lithium tetraborate due to a difference in thermal expansion at the time and a crack is partially generated.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a manufacturing apparatus capable of growing a high quality lithium tetraborate single crystal with good reproducibility and high yield. Things.

[0008]

SUMMARY OF THE INVENTION The present inventors have made it indispensable to obtain a crystal without bubble inclusions by preventing temperature fluctuations from directly affecting the melt in the crucible. However, suppressing the deformation of the platinum crucible at high temperatures is an essential factor in obtaining crack-free crystals, and as a result of intensive research to improve this point,
The present inventors have found that it is useful to cover the outside of a platinum crucible with a predetermined heat-resistant material and to make the thickness of the platinum crucible thinner than before, and have completed the present invention. That is, the first invention of the present application is an apparatus for producing a lithium tetraborate single crystal from a melt by a vertical Bridgman method or a vertical temperature gradient solidification method, comprising alumina, cordierite, mullite, quartz glass, nitride glass, A single crystal of lithium tetraborate, comprising: an outer container made of a heat-resistant material containing any one selected from boron and graphite as a main component, and a platinum crucible to be placed in the outer container. Manufacturing apparatus. A second invention of the present application is the apparatus for producing a lithium tetraborate single crystal according to the first invention, wherein the thickness of the platinum crucible is set to 0.05 to 0.25 mm.

According to the above structure, the double crucible structure in which the outside of the platinum crucible is covered with the outer container made of a heat-resistant material, the influence of the temperature fluctuation on the growth rate of the single crystal is reduced, and the inclusion of bubble inclusions is reduced. High quality lithium tetraborate single crystal can be obtained.

Further, since the platinum crucible is covered with the outer container in a double structure, the thickness of the platinum crucible can be made thinner than before, and the deformation of the platinum crucible at high temperature can be suppressed by the double crucible structure. Since a thinner platinum crucible is used than before, the stress of thermal strain applied to the crystal can be reduced, and an extremely high-quality lithium tetraborate single crystal having no bubble inclusions and no cracks can be obtained. Further, since the amount of platinum for producing the platinum crucible is smaller than that of the conventional crucible, the purification cost when regenerating the platinum crucible is reduced.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a manufacturing apparatus according to the present invention will be described below with reference to the drawings, taking a case where a lithium tetraborate single crystal is manufactured by a vertical Bridgman method as an example. In FIG. 1, reference numeral 1 denotes an electric furnace in which a heater 2 is disposed. In the electric furnace 1, an outer container 4 supported by a support 3 is accommodated. The outer container 4 is made of a heat-resistant material mainly containing any one selected from alumina, cordierite, mullite, quartz glass, boron nitride, and graphite;
It is installed on the upper surface of the support 3. Inside the outer container 4,
A concave portion 4a is formed in which a platinum crucible 5 to be described later is fitted and is removably accommodated. The upper opening 4b of the concave portion 4a is opened and closed by a lid 4c made of the same material as the outer container 4.

The platinum crucible 5 has a large-diameter body portion 5a and a small-diameter seed tube 5b provided below the body portion 5a, like the one used in this type of manufacturing method. Thus, the outer container 4 is accommodated in the concave portion 4a and the periphery thereof is covered with the outer container 4.

An up-down mechanism 6 is provided at the lower end of the support 3, and the operation of the up-down mechanism 6 causes the outer container 4 and the platinum crucible 5 to move up and down integrally with the support 3.

[0014]

EXAMPLES Next, the production of a single crystal of lithium tetraborate using the apparatus of the present invention and an evaluation test of the lithium tetraborate obtained thereby will be described. First, purity 4 prepared at a predetermined molar ratio in the atmosphere or in an inert atmosphere.
An N (99.99 wt%) lithium tetraborate single crystal raw material a is put into the body 5a of a platinum crucible manufactured to have a diameter of 80 mm and a length of 300 mm, and has a diameter of 5 mm and a length of 100 m.
The seed crystal b of lithium tetraborate machined in the <110> direction of m is inserted into the seed tube 5b, and the platinum crucible 5 is placed in the outer container 4 so that the seed crystal b is not melted.
Melted above 0 ° C. Next, the temperature gradient at the growth point was set to 20 ° C., the crucible descending speed was set to 0.3 mm / hour, the platinum crucible 5 was moved by 200 mm to grow a single crystal, and then cooled to room temperature. Further, the platinum crucible 5 was broken to take out lithium tetraborate, and the inclusion of air bubble inclusions and the presence or absence of cracks in this were examined.
Table 1 shows the relationship between the material of the outer container 4, the thickness of the platinum crucible 5 and the inclusions of bubbles, and the occurrence of cracks, together with comparative examples.
Table 2 shows an example of the yield when a 3-inch lithium tetraborate substrate is manufactured by the apparatus of the present invention, together with a comparative example. The comparative example is lithium tetraborate produced by a vertical Bridgman method using a conventional apparatus consisting of only a platinum crucible without an outer container.

[0015]

[Table 1]

[0016]

[Table 2]

From the above results, an outer container made of a heat-resistant material containing any one of alumina, cordierite, mullite, quartz glass, boron nitride, and graphite as a main component, Lithium tetraborate manufactured by the vertical Bridgman method using the manufacturing apparatus of the present invention comprising a platinum crucible to be put into is a high-quality product without bubble inclusions, and can be manufactured with high reproducibility and high yield. It was confirmed that there was. Among them, it was confirmed that when the thickness of the platinum crucible was 0.05 to 0.25 mm, the crucible was of very high quality with no partial cracks, and an extremely high yield was obtained.

On the other hand, it was confirmed that the lithium tetraborate of the comparative example manufactured by the vertical Bridgman method using a conventional apparatus consisting of only a platinum crucible without an outer container had bubble inclusions and cracks. did it.

In this embodiment, an example of manufacturing a lithium tetraborate single crystal by the vertical Bridgman method has been described. However, the dimensions of a platinum crucible, the orientation of a seed crystal, the melting temperature, the temperature gradient at the growing point, Various conditions such as the crucible lowering speed and the crucible movement amount are not limited to those described above, and the outer container 4 and the platinum crucible 5 are fixed, and the electric furnace 1 is configured to be supported by a vertical mechanism. Changes within the scope are optional. Further, the present invention is not limited to the one used for the vertical Bridgman method, and appropriately controls the temperature of the heater 2 in the vertical direction of the electric furnace 1 without using the vertical mechanism 6 (the upper heater is set to a high temperature, Set the heater to low temperature)
It is needless to say that if it is configured to be able to perform this, it can be applied to the case of producing a lithium tetraborate single crystal by the vertical temperature gradient solidification method.

[0020]

As described above, the apparatus for producing a lithium tetraborate single crystal of the present invention mainly uses any one selected from alumina, cordierite, mullite, quartz glass, boron nitride, and graphite. With a new structure that covers the platinum crucible with an outer container made of a heat-resistant material as a component, the change in growth rate due to temperature fluctuations is reduced,
A high quality lithium tetraborate single crystal without bubble inclusions can be obtained.

Further, since the double crucible structure including the outer container and the platinum crucible is used, the thickness of the platinum crucible can be made smaller than that of the conventional crucible. Of extremely high quality lithium tetraborate single crystal, which can suppress the occurrence of
It is possible to grow with high yield with good reproducibility, and the above-mentioned effects can be made more effective. In addition, the refining cost for regenerating a platinum crucible can be significantly reduced as compared with the conventional one, and the above-mentioned effects can be obtained at a lower cost than before, so that there are many advantages.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional front view showing one embodiment of an apparatus for producing a lithium tetraborate single crystal according to the present invention.

[Explanation of symbols]

 1: electric furnace 2: heater 3: support base 4: outer container 5: platinum crucible 5a: body 5b: seed tube a: lithium tetraborate single crystal raw material b: seed crystal

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C30B 28/00-35/00 C30B 11/00 CA (STN)

Claims (2)

(57) [Claims] 1. An apparatus for producing a lithium tetraborate single crystal from a melt by a vertical Bridgman method or a vertical temperature gradient solidification method, comprising alumina, cordierite, mullite, quartz glass, boron nitride, and graphite. An apparatus for producing a single crystal of lithium tetraborate, comprising: an outer container made of a heat-resistant material containing at least one selected from the group consisting of: and a platinum crucible to be placed in the outer container. 2. The thickness of the platinum crucible is 0.05 to
The apparatus for producing a lithium tetraborate single crystal according to claim 1, wherein the thickness is 0.25 mm.