JP6347673B2 - Method for producing YAG single crystal - Google Patents

Description

  The present invention relates to a method for producing a YAG single crystal (R: YAG single crystal, where R is an additive such as Nd, Ce, etc.) containing a YAG single crystal and an additive by the CZ method or the EFG method. The present invention relates to a method for producing a YAG single crystal that enables growth of a single crystal using an inexpensive crucible such as a molybdenum crucible.

Among the single crystals for solid-state lasers that contain active ions, Nd: YAG (neodymium-doped yttrium, aluminum, garnet) single crystals are the most useful solid-state laser materials in the industry and are used in various laser oscillators. Has been. The Nd: YAG single crystal is usually manufactured using an expensive iridium (Ir) crucible by a high frequency induction heating type CZ (Czochralski) method. Further, since the melting point is as high as about 1950 ° C., high-melting high-purity alumina (Al ₂ O ₃ ) or zirconia is formed around the iridium crucible filled with the raw material melt and above the melt surface during crystal growth. It is necessary to cover and keep warm with a refractory or powder made of (Zr ₂ O ₃ ). In the growth of Nd: YAG single crystals, how to configure the refractories used as these heat insulating materials is an important growth condition.

  When these refractories are used in the vicinity of a high-temperature melt or iridium crucible, deterioration such as shrinkage cracks occurs. In particular, refractories placed close to the top of the melt are hot and are also exposed to infrared radiation from the melt. For this reason, the temperature environment tends to be in the vicinity of the maximum usable temperature of the refractory, and the heat retention is greatly deteriorated. This deterioration of the refractory causes a change in crystal growth conditions such as a change in temperature gradient. In order to prevent this, as described in Patent Documents 1 and 2, when growing a YAG single crystal, an iridium disk having a circular hole is generally used between a melt and a refractory, or instead of a refractory. To place. The iridium disk itself does not retain heat but does not undergo deformation deterioration, plays a role in blocking infrared radiation from the melt throughout its growth, increases the temperature gradient and smoothes crystal growth, and at the same time degrades the refractory To stabilize the growth conditions.

Further, by placing an iridium cylinder in contact with the iridium disk and separating the refractory and the like from the space where the raw material melt and crystal growth are performed, the contamination from the refractory and the like can be avoided. Although the purity of the single crystal raw material powder is about 99.999%, transition metals such as iron oxide (Fe ₂ O ₃ ), which are particularly problematic as impurities, are removed from the raw material to the extent that they cannot be detected even in the PPM unit. Has been. When the transition metal such as iron oxide is mixed into the crystal, the color center is generated, so that the light absorption is increased and the oscillation light is lost at the time of laser oscillation. However, the material purity of alumina (Al ₂ O ₃ ) or zirconia (Zr ₂ O ₃ ) refractories is about 99%, and silicon oxide (SiO ₂ ), iron oxide, calcium oxide (CaO), etc. are each about 0.000%. Contains several percent. Therefore, the measure using the iridium cylinder is an important means for preventing contamination of the refractory by impurities due to shrinkage cracking of the refractory and preventing deterioration of the characteristics of the laser material.

JP-A-6-183895 JP-A-6-135800

  In the conventional manufacturing method using the above-mentioned iridium crucible, there are problems that iridium is expensive and therefore the manufacturing cost is high and the usable limit of the refractory is as short as several times.

Although it is conceivable to change the iridium crucible to a cheaper molybdenum crucible, it is impossible to grow a YAG single crystal simply by replacing it. In the configuration using the refractory material such as alumina (Al ₂ O ₃ ) or zirconia (Zr ₂ O ₃ ) described above, the molybdenum crucible burns with oxygen generated from the refractory material. On the other hand, when the refractory is changed to a carbon heat insulating material, in the high frequency induction heating method, a part of the high frequency is cut off by carbon and the heating efficiency is deteriorated. Although it is conceivable to use carbon felt as a refractory material to prevent this high-frequency cutoff, the problem of carbon mixing into the crystal remains because carbon tends to evaporate.

  Conventionally, growth of a YAG single crystal in a molybdenum crucible by a resistance heating method has been attempted, but the molybdenum crucible has melted into the raw material and the melting point has increased, so that sufficient growth has not been possible. Even if the crystallization raw material was used to grow carefully so as not to react with the molybdenum crucible, it was difficult to control the temperature gradient in the resistance heating furnace, and only a black colored crystal was obtained and was not put into practical use. Only YAG single crystals could be grown using a tungsten heater, a molybdenum crucible, and a molybdenum heat insulating material, but the life of the tungsten heater and the molybdenum heat insulating material was short and impractical.

  Therefore, the present invention has been made to solve such problems, and enables the growth of YAG single crystals having good crystallinity and YAG single crystals containing additives without using expensive iridium crucibles. An object of the present invention is to provide a practical method for producing a YAG single crystal.

  In a first aspect, the method for producing a YAG single crystal according to the present invention is a method for producing an yttrium aluminum garnet single crystal in which a raw material of yttrium aluminum garnet single crystal is put in a crucible and melted to grow the single crystal. In the above, a heat insulating material made of carbon whose side facing the region is covered with a molybdenum plate is used for at least a part of the periphery of the region where the single crystal is grown.

  As described above, the structure of the refractory is an important growth condition in growing a YAG single crystal. The resistance heating method has better heat exchange efficiency than the high frequency induction heating method, and the manufacturing apparatus can generally be constructed at a low cost. In this case, the normally used carbon heater is excellent in heat resistance and durability. Carbon is said to have high thermal conductivity and is not suitable as a heat insulating material, but recently, a felt or molded heat insulating material with excellent heat insulation properties is available, and it is possible to realize a resistance heating furnace with low cost and excellent durability. It has become. However, the resistance heating method has a drawback that it is difficult to create a temperature gradient as compared with the high frequency induction heating method in which the crucible is directly heated. Therefore, in the present invention, it is possible to eliminate the above-mentioned drawbacks and obtain a desired temperature gradient by using a felt made of carbon or a molded heat insulating material in combination with a molybdenum plate having high thermal conductivity.

In addition, carbon starts sublimation at a temperature of 1850 ° C. or higher. When the sublimated carbon reacts with the YAG single crystal, carbon dioxide (CO ₂ ) is generated and oxygen in the crystal is taken away, and the crystal is colored black. That is, the crystal is blackened by oxygen defects. In the present invention, the presence of the molybdenum plate prevents the raw material and the crystals from coming into contact with the evaporated carbon, thereby suppressing the above phenomenon.

  As described above, in the present invention, by using a heat insulating material combining a molybdenum plate having high thermal conductivity and carbon having high heat resistance and durability as a refractory, etc., in a high temperature region necessary for growing a YAG single crystal. The temperature gradient required in the resistance heating furnace can be controlled, and further, the side facing the crystal growth region of carbon is covered with a molybdenum plate, so that carbon can be prevented from evaporating and entering the crystal growth region. . This makes it possible to grow a YAG single crystal with good crystallinity without using an iridium crucible.

  In a second aspect, the present invention is characterized in that in the method for producing a YAG single crystal of the first aspect, a molybdenum crucible is used as the crucible. By using a molybdenum crucible that is generally used in single crystal growth and has a proven record in industry, a practical crystal manufacturing apparatus can be configured. For example, when configuring a resistance heating type apparatus, a carbon heater capable of heating a molybdenum crucible disposed in the center to a melting temperature of YAG single crystal of 1950 ° C. or higher is used. A carbon heat insulating material covered with a molybdenum plate is placed inside, a molybdenum cylinder is installed above the molybdenum crucible, and the upper end is covered with a molybdenum lid having an opening larger than the crystal diameter to be produced at the center. To do. In addition, it is also possible to apply this invention to the apparatus of a high frequency induction heating system, and a carbon heater becomes unnecessary in that case.

  In a third aspect, the present invention is the method for producing a YAG single crystal according to the first or second aspect, wherein the raw material is melted by heating the crucible using a resistance heating method. . In the present invention, it is possible to control a temperature gradient required in a resistance heating furnace in a high temperature region, and a single crystal manufacturing apparatus is configured by using a resistance heating method used for a general oxide single crystal. Therefore, it is possible to apply a matured conventional technique and to easily realize a manufacturing apparatus. In the high-frequency induction heating method, the above-described features of the present invention, such as ease of temperature distribution control and suppression of the influence of carbon on the grown crystal, can be effectively utilized.

In a fourth aspect, the present invention is characterized in that, in the YAG single crystal manufacturing method according to the first to third aspects, argon gas is introduced into the region in the step before melting the raw material. A lot of carbon compounds are contained in the gas released from the heat insulating material such as refractory. Therefore, argon gas is introduced into the region where the single crystal is grown, and the refractory is outgassed more effectively by heat-treating the refractory in a reduced pressure argon atmosphere at a temperature lower than 1850 ° C. where carbon evaporates, for example. be able to. At the same time, the impurity gas contained in the raw material is released. Thus, before the raw material is melted, an inert argon gas flows into the crystal growth region as an atmospheric gas, and is maintained at a high temperature in a reduced pressure argon atmosphere, so that the impurity gas, oxygen, and carbon contained in the raw material and the heat insulating material are sufficient. After being released outside the furnace, the raw material is melted, so that impurities such as carbon can be reduced in the crystal, and combustion of the molybdenum crucible by carbon can be prevented. When nitrogen gas is used instead of argon gas, NO _x or cyan toxic gas is generated at around 2000 ° C. In order to prevent the generation of toxic gas, it is desirable to use argon gas which does not generate harmful gas as the atmospheric gas in the furnace.

  In a fifth aspect, the present invention provides the method for producing a YAG single crystal according to the fourth aspect, wherein the raw material is degassed in a reduced-pressure atmosphere in a state where the temperature of the crucible is 1550 ° C to 1850 ° C. Thereafter, the raw material is melted. As described above, an inert argon gas is allowed to flow into the crystal growth region as an atmospheric gas, and the crucible temperature is kept at 1550 ° C. to 1850 ° C., for example, for 30 minutes or longer in a reduced pressure atmosphere, After sufficiently discharging the residual gas, the raw material is melted, whereby the impurity gas can be efficiently discharged from the raw material and discharged out of the furnace, and a high-quality single crystal can be obtained.

  In a sixth aspect, the present invention provides the method for producing a YAG single crystal according to the first to fourth aspects, wherein after the single crystal is grown, the single crystal is heat-treated in an oxygen atmosphere at 1100 ° C. to 1850 ° C. It is characterized by that. By heat-treating the grown single crystal in an oxygen atmosphere at 1100 ° C. to 1850 ° C., the light transmittance of the crystal can be improved, and a higher quality single crystal can be obtained.

  As described above, according to the YAG single crystal manufacturing method of the present invention, it is possible to grow a YAG single crystal having good crystallinity and a YAG single crystal containing an additive without using an expensive iridium crucible. A practical method for producing a YAG single crystal is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectional view illustrating an example of a configuration of a YAG single crystal manufacturing apparatus using a resistance heating method, illustrating a method for manufacturing a YAG single crystal according to Example 1; It is a figure which shows an example of the result of having measured the relationship between the heat processing temperature and light transmittance of a YAG single crystal, and shows the light transmittance in wavelength 370nm. The external appearance photograph of the produced YAG single crystal is shown, (a) is the single crystal produced by Example 1, (b) is the single crystal produced in the manufacturing method of the comparative example. The figure which shows the result of having measured the temperature distribution of the crystal pulling-axis direction in the manufacturing method of Example 1 and a comparative example.

  Hereinafter, the method for producing a YAG single crystal of the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description thereof is omitted.

  FIG. 1 is a diagram illustrating a method for manufacturing a YAG single crystal according to Example 1, and is a schematic cross-sectional view illustrating an example of the configuration of a YAG single crystal manufacturing apparatus using a resistance heating method. In FIG. 1, the carbon heat insulating material 3 is installed on the heat insulating material base 2, and the cylindrical carbon heat insulating material 6 surrounding the area | region where a YAG single crystal is grown is installed on it. The inner surface of the carbon heat insulating material 6, that is, the side facing the growth region of the YAG single crystal is covered with a molybdenum plate 15. A molybdenum crucible 8 is installed in the YAG single crystal growth region on the molybdenum crucible base 5, and a resistance heating carbon heater 7 is installed around the molybdenum crucible 8. The carbon heater 7 is connected to the carbon electrode 4 attached to the electrode base 1. Further, a molybdenum cylinder 13 is installed above the molybdenum crucible 8, and its upper end is covered with a molybdenum lid 14 having a hole in the center.

  In this embodiment, the die 10 is installed in the center of the molybdenum crucible 8 in order to grow by applying the EFG method. The die 10 was composed of two plates having a width of 40 mm, a height of 40 mm, and a thickness of 1 mm. The die 10 was configured such that the gap between the plates was 0.6 mm, and was arranged at the center of the molybdenum crucible 8.

  Here, the carbon heat insulating material 6 has an inner diameter of 80 mm, the carbon heater 7 has an inner diameter of 60 mm, the molybdenum cylinder 13 has an inner diameter of 55 mm and a height of 100 mm, and the crucible side further includes a cylinder having an inner diameter of 50 mm and a height of 50 mm. It has become. The molybdenum lid 14 has an inner diameter of 20 mm, an outer diameter of 80 mm, and a thickness of 0.5 mm. The molybdenum crucible 8 has a diameter of 50 mm, a depth of 30 mm, and a thickness of 2 mm.

In growing a YAG single crystal, 45 g of a raw material prepared by weighing and mixing high-purity yttrium oxide (Y ₂ O ₃ ) and aluminum oxide (Al ₂ O ₃ ) powders to a required ratio is put in a molybdenum crucible 8, and the following It was melted in the following procedure. When an electric current is passed through the carbon heater 7 to melt the raw material in the molybdenum crucible 8, the raw material melt 9 is formed, and the raw material rises from the gap of the die 10. In the apparatus of this example, argon gas is allowed to flow into the crystal growth region, and the optimum condition for efficiently degassing the raw material at the upper limit of the argon gas pressure and temperature region where carbon does not evaporate is 1550 ° C to 1850 ° C. C. and an argon gas pressure of 0.02 to 0.06 MPa were selected. After leaving it for 30 minutes or more in the optimum high-temperature reduced pressure argon atmosphere, the raw material was dissolved over 3 hours. At this time, the melt temperature did not rise, and seeding was possible without melting the seed crystal 11 at the tip of the crystal pulling shaft 12. At this time, the current flowing through the carbon heater 7 was 390A, and the voltage was 31.5V. The solidified melt was in a transparent crystalline state, and after seeding, when the seed crystal was pulled up at 4 mm / h, a high-quality crystal having a width of 45 mm × thickness of 27 mm and a length of 100 mm without crystal defects and distortion was obtained. . In the experiment, depending on the structure and shape of each part of the apparatus, the target crystal, and the growth conditions, it was confirmed that the pressure was effective in a reduced pressure atmosphere.

  After growing the YAG single crystal, it was further heat-treated in an oxygen atmosphere at 1200 to 1850 ° C. FIG. 2 is a diagram showing an example of the result of measuring the relationship between the heat treatment temperature and the light transmittance of the YAG single crystal in this case, and shows the light transmittance at a wavelength of 370 nm. It was confirmed that absorption at a wavelength of around 370 nm disappeared at a temperature of 1200 ° C. or higher, and light transmittance characteristics having no practical problem were obtained. When the heat treatment temperature is set to 1850 ° C. or higher, carbon sublimation occurs as described above. Therefore, the heat treatment temperature is preferably 1850 ° C. or lower. In an atmosphere with a higher oxygen partial pressure, it was confirmed by experiments that the heat treatment temperature was 1100 ° C. or higher. Thus, a higher quality single crystal can be obtained by heat treatment in an oxygen atmosphere at 1100 ° C. to 1850 ° C.

  In the crystal growth, when the gap between the dies 10 is 0.1 mm, the supply amount of the raw material is small, so that the high-speed growth cannot be performed, and when the gap is 1.5 mm or more, the raw material melt 9 does not rise on the surface of the die 10. For this reason, the gap of the die 10 was set to 0.6 mm as described above. This is the optimum dimension in the present embodiment for efficiently raising the raw material melt 9 to the die surface by capillary action.

  Further, as a comparative condition, when using the apparatus of this example, the current of the carbon heater 6 is set to 400 A, the voltage is set to 32 V, and the seed crystal is melted in one hour without gassing and then seeded. 11 has melted. This is because the molybdenum crucible 8 is melted out by oxygen contained in the raw material, and the melting point of the raw material melt 9 mixed with molybdenum is increased.

  In the case of a single crystal grown under the optimum conditions of the above-described apparatus of the present embodiment, the solidified melt was in a transparent crystalline state, but the melt when grown under the comparative conditions described below has a surface with an opaque metallic luster. there were. From this, it is clear that in the case of growth under comparative conditions, molybdenum melts out of the molybdenum crucible 8 and the melt temperature rises.

  When grown under the optimum conditions of the apparatus of this example, high-quality plate crystals without crystal defects and distortion were obtained. When an orthogonal crossed Nicol photograph of the plate crystal was taken, no distortion was observed.

Comparative example

  In order to confirm the effectiveness of the present invention, YAG crystals were grown by a conventional manufacturing method. The manufacturing apparatus used here does not have the molybdenum plate 15 on the inner surface of the carbon heat insulating material 6, uses a carbon cylinder instead of the molybdenum cylinder 13, and uses a carbon lid instead of the molybdenum lid 14. Unlike the apparatus of FIG. 1 used in FIG. 1, other parts and shapes are the same as those of the apparatus of FIG. The growth atmosphere and temperature were also controlled in the same manner as in Example 1.

In this way, an orthogonal crossed Nicols photograph of the crystal grown by the conventional method was taken, and crystal defects and distortions appearing as black dots or bright stripes were confirmed. Further, a white layer was formed on the surface of the crystal, and when this layer was subjected to fluorescent X-ray analysis, it was confirmed to be a layer of yttrium oxide (Y ₂ O ₃ ). This is because evaporated carbon is mixed in the melt, YAG raw material is decomposed and evaporated, and yttrium (Y) or yttrium carbide (YC ₂ ) adheres to the crystal surface and then oxidizes to yttrium oxide (Y ₂ O ₃ ). The following reactions are considered to occur as this reaction.
Y ₂ O ₃ + 3C = 2Y + 3CO ↑
Or, Y ₂ O ₃ + 3C = Y ₂ C + 2CO ↑ + O ↑
After crystal deposition, 2Y + 3O → Y ₂ O ₃
Al ₂ O ₃ + 9C = Al ₄ C ₃ (residual in the melt) + 6CO ↑

  FIG. 3 shows an external appearance photograph of the produced YAG single crystal, (a) is a single crystal produced according to Example 1, and (b) is a single crystal produced by the production method of the comparative example. From this, it can be seen that in the manufacturing method of Example 1, a high-quality YAG crystal having an excellent appearance shape can be obtained as compared with the appearance shape having a large distortion according to the comparative example of the conventional manufacturing method.

  FIG. 4 is a diagram showing the results of measuring the temperature distribution in the crystal pulling axis direction in the manufacturing methods of Example 1 and Comparative Example. In the case of Example 1, a temperature distribution suitable for crystal growth is obtained due to the presence of the molybdenum plate, whereas the conventional apparatus has a smaller temperature gradient than that, and it is understood that sufficient temperature control is not possible. .

  As described above, it was confirmed that the production method of Example 1 according to the present invention can provide a high quality YAG crystal as compared with the comparative example obtained by the conventional production method.

Further, in Example 1, the crystal was grown by changing the pulling speed from 1 to 10 mm / h, and the crystal quality was confirmed. As shown in Table 1, a crystal with good quality was obtained at a pulling speed of 8 mm / h or less. It was confirmed. In each evaluation item in the table, ◯ indicates a good state and Δ indicates an insufficient state.

  Needless to say, the present invention is not limited to the above-described embodiments, and can be changed according to the characteristics and shape of the target single crystal. For example, tungsten or tantalum can be used as the crucible material. The structure, shape, material and the like of each part of the manufacturing apparatus can be changed according to the characteristics and shape of the target YAG crystal. It is also desirable to optimize the growth conditions such as the temperature of each part depending on the structure and shape of each part of the manufacturing apparatus to be used. Additives such as Nd and Ce may be added to the YAG single crystal according to the purpose of use.

  The present invention is also applied to a method for producing a YAG single crystal used as a window material for a display unit of a portable terminal device such as a mobile phone, a smart phone, or a tablet information terminal, or a watch, which requires an inexpensive plate-like crystal. Can be applied.

DESCRIPTION OF SYMBOLS 1 Electrode base 2 Heat insulating material base 3 Carbon heat insulating material 4 Carbon electrode 5 Molybdenum crucible stand 6 Carbon heat insulating material 7 Carbon heater 8 Molybdenum crucible 9 Raw material melt 10 Die 11 Seed crystal 12 Crystal pulling shaft 13 Molybdenum cylinder 14 Molybdenum lid 15 Molybdenum plate

Claims (6)

In the method for producing yttrium aluminum garnet single crystal, the raw material of yttrium aluminum garnet single crystal is put in a crucible and melted, and the single crystal is grown.
Wherein at least part of the periphery of a region where the single crystal is grown, have use a heat insulating material made of carbon side facing the area covered by molybdenum plate,
Argon gas was introduced into the region, and the raw material and the heat insulating material were outgassed in a state where the argon gas pressure in the region was 0.02 MPa to 0.06 MPa and the temperature of the crucible was 1550 ° C. to 1850 ° C. ,
Then, melting the raw material,
A method for producing an yttrium / aluminum / garnet single crystal.   The method for producing an yttrium-aluminum-garnet single crystal according to claim 1, wherein a molybdenum crucible is used as the crucible.   The method for producing a yttrium aluminum garnet single crystal according to claim 1 or 2, wherein the raw material is melted by heating the crucible using a resistance heating method. The method for producing a yttrium-aluminum-garnet single crystal according to any one of claims 1 to 3, wherein the single crystal is pulled at a pulling rate of 1 mm / hour to 8 mm / hour. The method for producing an yttrium / aluminum / garnet single crystal according to claim 4, wherein the single crystal is pulled at a pulling rate of 2 mm / hour to 8 mm / hour. After growing the single crystal, the production of yttrium-aluminum-garnet single crystal according to any one of claims 1 to 5, characterized in that heat treatment of the single crystal in an oxygen atmosphere at 1100 ℃ ~1850 ℃ Method.