JP7486743B2 - Method for producing FeGa alloy single crystal - Google Patents

Description

The present invention relates to a method for producing an iron-gallium alloy (FeGa alloy) single crystal, and in particular to a method for producing an FeGa alloy single crystal having giant magnetostrictive properties formed by a unidirectional solidification crystal growth method in which a melt, such as a vertical Bridgman method (hereinafter sometimes abbreviated as "VB method") or a vertical gradient freeze method (hereinafter sometimes abbreviated as "VGF method"), is solidified in a crucible.

FeGa alloys can be machined and exhibit large magnetostriction of about 100 to 350 ppm, making them suitable as materials for magnetostrictive vibration power generation and actuators, and have been attracting attention in recent years.

Furthermore, since FeGa alloys can exhibit large magnetostriction in a specific crystal orientation, it is believed that their best application is as a single crystal component in which the direction in which magnetostriction is required matches the direction in which the crystal magnetostriction is at its maximum.

Manufacturing methods for FeGa alloy polycrystals include powder metallurgy, rapid solidification (e.g., Patent Document 1), and a method of pressurizing and sintering a flake or powdered raw material produced by liquid rapid solidification (e.g., Patent Document 2). However, all of these manufacturing methods result in a polycrystalline material rather than a single crystal, and it is impossible to match all the crystal orientations in the material with the orientation that maximizes magnetostriction, resulting in inferior magnetostriction properties to single crystal materials.

On the other hand, there is a method for producing single crystals, the pulling method. For example, Patent Document 3 describes a method for growing single crystals using the pulling method (Czochralski method). However, this method requires high power costs because the raw material is melted using high-frequency induction heating. In addition, the equipment configuration is complex and the equipment costs are high, so the pulling method ultimately results in high manufacturing costs.

Japanese Patent No. 4053328 Japanese Patent No. 4814085 JP 2016-28831 A

As such, it is difficult to mass-produce single crystals of iron-gallium alloys at low cost using the conventional methods described in Patent Documents 1 to 3.

In comparison, FeGa alloy single crystals with giant magnetostrictive properties can be produced inexpensively using unidirectional solidification crystal growth methods, such as the VB method and VGF method, in which a melt is solidified in a crucible.

The magnetostriction of FeGa alloy single crystals depends on the crystal composition, so when manufacturing FeGa alloy single crystals, it is important to control the gallium content in the single crystal. For example, when the gallium content in the single crystal is 17-19% by atomic weight, the FeGa alloy single crystal will have a high magnetostriction of 300 ppm or more.

In the unidirectional solidification crystal growth method, a mixture of iron and gallium is placed on top of a seed crystal, and after the mixture is melted, the melt must be solidified from the seed crystal side while inheriting the crystal orientation of the seed crystal. FeGa alloys are incongruently meltable crystals, and the solidus and liquidus do not coincide in the phase diagram. When the gallium content is 17-19% by atomic weight, the temperature difference between the solidus and liquidus is approximately 70-80°C, and the melt melts the entire seed crystal, making it impossible to grow a single crystal in the desired crystal orientation. Furthermore, a common issue in crystal growth is that if the solidification speed during growth is increased, polycrystallization is more likely to occur.

In view of these circumstances, the present invention aims to provide a method for producing FeGa alloy single crystals using a unidirectional solidification crystal growth method, which prevents all of the seed crystals from melting during crystal growth and allows stable growth of the single crystal.

To solve the above problems, the method for producing FeGa alloy single crystals of the present invention is a method for producing FeGa alloy single crystals using a unidirectional solidification crystal growth method, in which the temperature in the crucible during seeding is adjusted so that it increases vertically from bottom to top, and the temperature gradient in the crucible is adjusted to be within the range of 2.5 to 5.5°C/mm.

The unidirectional solidification crystal growth method may be the vertical Bridgman method, and during the growth of the FeGa alloy single crystal, the lowering speed of the crucible may be 2 mm/hour or less, and the temperature gradient in the crucible may be 2.5°C/mm or more.

The unidirectional solidification crystal growth method may be the vertical Bridgman method, and during the growth of the FeGa alloy single crystal, the lowering speed of the crucible may be 5 mm/hour or less, and the temperature gradient in the crucible may be 3.5°C/mm or more.

The unidirectional solidification crystal growth method may be the vertical Bridgman method, and during the growth of the FeGa alloy single crystal, the lowering speed of the crucible may be 7.5 mm/hour or less, and the temperature gradient in the crucible may be 4.0°C/mm or more.

The unidirectional solidification crystal growth method may be the vertical Bridgman method, and during the growth of the FeGa alloy single crystal, the lowering speed of the crucible may be 10 mm/hour or less, and the temperature gradient in the crucible may be 4.5°C/mm or more.

When the temperature gradient is Y and the rate of descent is X, Y ≥ 0.247X + 2.00 may be satisfied.

The method for producing FeGa alloy single crystals of the present invention prevents all of the seed crystals from melting during crystal growth, and allows for stable growth of single crystals.

FIG. 1 is a schematic cross-sectional view of a growth apparatus for growing a single crystal of an iron-gallium alloy. FIG. 1 is a schematic cross-sectional view of a growth apparatus when measuring a temperature gradient at the center of a seed crystal. FIG. 13 is a diagram showing the relationship between the temperature gradient and the crucible descending speed. 1 is a schematic cross-sectional view showing the arrangement of a single crystal growth crucible 10 and a resistance heater 12 in each step of a method for producing a FeGa alloy single crystal by the VB method.

The method for producing a FeGa alloy single crystal according to one embodiment of the present invention will be described in more detail below with reference to the single crystal growth apparatus shown in Figures 1 and 2. Note that the present invention is not limited to the following examples, and can be modified as desired without departing from the gist of the present invention.

[Single crystal growth equipment]
Fig. 1 is a schematic cross-sectional view of a single crystal growth apparatus for growing a FeGa alloy single crystal. Fig. 1 shows a schematic positional relationship between a crucible 10 for single crystal growth, a FeGa alloy seed crystal 16, and a mixture 17 of iron and gallium as raw materials in the single crystal growth apparatus 100.

The single crystal growth apparatus 100 includes a heat insulating material 11, an upper heater 12a, a middle heater 12b, a lower heater 12c, a movable rod 13, a crucible receiver 14, a thermocouple 15, a vacuum pump 18, and a chamber 19. The chamber 19 is configured to realize a temperature distribution in which the upper part is hot and the lower part is cold, and a FeGa alloy single crystal can be grown by solidifying a molten mixture of iron and gallium 17 in the crucible 10 using a unidirectional solidification crystal growth method such as the VB method or the VGF method.

As shown in FIG. 1, in the single crystal growth apparatus 100, a carbon resistance heater 12 is placed inside the insulating material 11. When growing an FeGa alloy single crystal, a hot zone is formed by the resistance heater 12. The resistance heater 12 is composed of an upper heater 12a, a middle heater 12b, and a lower heater 12c, and by adjusting the power input to these heaters 12a to 12c, it is possible to control the temperature gradient within the hot zone.

A single crystal growth crucible 10 is placed inside the resistance heater 12 and placed on a crucible holder 14 (support) equipped with a movable rod 13 that can move up and down. A FeGa alloy seed crystal 16 is filled in the lower part of the single crystal growth crucible 10, and a mixture 17 of iron and gallium is filled on top of this FeGa alloy seed crystal 16.

The growth furnace is equipped with a chamber 19 and a vacuum pump 18, allowing the raw material to be adjusted to a vacuum atmosphere to grow a single crystal. Furthermore, an inert gas such as argon or nitrogen can be introduced into the chamber 19, allowing the raw material to be adjusted to an inert atmosphere.

The material of the crucible 10 for growing single crystals is preferably alumina, which has low chemical reactivity with the FeGa alloy single crystal and a high melting point. Magnesia and pyrolitic boron nitride may also be used.

The single crystal growth crucible 10, which is open at the top, may be covered with a lid. As described above, the single crystal growth crucible 10 is placed on a crucible receiver 14 provided with a movable rod 13 in the single crystal growth apparatus 100, and the single crystal growth crucible 10 can be moved up and down in the growth furnace by moving the movable rod 13 up and down. In addition, a thermocouple 15 is attached to the single crystal growth crucible 10 to monitor the temperature of the crucible.

[Method for producing FeGa alloy single crystal]
Next, a method for producing an FeGa alloy single crystal according to one embodiment of the present invention will be described with reference to FIGS.

FeGa alloy single crystals with giant magnetostrictive properties can be grown, for example, by solidifying a melt of iron and gallium in a crucible, and in the present invention, they can be grown by a unidirectional solidification crystal growth method, such as the VB method or the VGF method.

In the VB method, first, an FeGa alloy seed crystal 16 with a principal plane orientation of <100> is placed at the bottom of a crucible 10 for growing single crystals. Then, a required amount of the raw material iron and gallium mixture 17 is placed on top of the FeGa alloy seed crystal 16 (Crucible placement step in FIG. 4(a)), and then the crucible 10 is appropriately covered with a lid.

Next, an inert gas such as argon or nitrogen is flowed into the chamber 19 to adjust the inside of the chamber 19 to an inert atmosphere. If there is a risk of gallium nitride or the like being generated, it is preferable to introduce argon gas. After the inside of the chamber 19 becomes an inert atmosphere, the upper heater 12a, the middle heater 12b, and the lower heater 12c arranged to surround the single crystal growth crucible 10 are operated to raise the temperature and start melting the mixture 17 of iron and gallium (Melting process in FIG. 4(b)).

When the iron and gallium mixture 17 has almost melted and become a molten material, the vacuum pump 18 is operated to reduce the pressure inside the chamber 19 and remove any air bubbles in the molten material (air bubble removal process).

After the bubble removal process, an inert gas such as argon or nitrogen is flowed into the chamber 19 to adjust the chamber 19 to an inert atmosphere again, and then an FeGa alloy single crystal is grown inside the crucible 10 for single crystal growth (growth process in FIG. 4(d)). Specifically, the crucible 10 for single crystal growth, which contains the FeGa alloy seed crystal 16 and the melt (iron and gallium mixture 17) and is covered with a lid, is heated using a resistance heater 12 so that the temperature distribution is high at the top in the height direction and low at the bottom. In this state, the temperature inside the chamber 19 is changed to a position where the FeGa alloy seed crystal 16 is melted to about the upper half in the height direction by moving the movable rod 13 to raise the crucible 10, and seeding is performed (seeding process in FIG. 4(c)).

The seeding position and the power input to the resistance heater 12 are adjusted so that the temperature inside the single crystal growth crucible 10 during seeding increases vertically from bottom to top, and the temperature gradient during seeding inside the single crystal growth crucible 10 is 2.5 to 5.5°C/mm.

After that, while maintaining the temperature gradient in the chamber 19, the descent speed of the single crystal growth crucible 10 is set in the range of 1 to 10 mm/hour, and the movable rod 13 is moved at a constant speed to lower the single crystal growth crucible 10 to grow an FeGa alloy single crystal 16 (growth step in Figure 4(d)), and all of the melt is solidified. It is possible to grow a single crystal even if the descent speed of the single crystal growth crucible 10 is set to 1 mm/hour or less, but the growth period will be longer and productivity will decrease. In order to suppress changes in the crystal composition, it is preferable to set the descent speed of the single crystal growth crucible 10 to 5 mm/hour or more.

After the lowering of the crucible 10 is completed and the growth of the single crystal is completed (Figure 4 (e)), the mixture is cooled at a predetermined rate to obtain a FeGa alloy single crystal (cooling process).

Next, after confirming that the temperature inside the chamber 19 has reached room temperature, the single crystal growth crucible 10 containing the grown single crystal is removed from the crucible holder 14, and the lid is removed to remove the grown single crystal from the single crystal growth crucible 10.

The above describes a method for producing FeGa alloy single crystals using the VB method with the single crystal growth apparatus 100. However, using the same single crystal growth apparatus 100, FeGa alloy single crystals can also be produced using the VGF method, in which the temperature is controlled by adjusting the resistance heater 12 instead of moving the single crystal growth crucible 10 up and down during single crystal growth.

[Temperature gradient in the crucible and the rate of descent of the crucible]
Next, a method for measuring the temperature gradient in the single crystal growth crucible 10 in the vertical Bridgman method using the single crystal growth apparatus 100 according to one embodiment of the present invention will be described with reference to FIG.

First, remove the thermocouple 15 used to monitor the temperature during single crystal growth from the single crystal growth apparatus 100. Next, set up the single crystal growth crucible 10 with a hole drilled in the side. Attach the thermocouple 50 so that its tip is positioned halfway up the height of the FeGa alloy seed crystal installation portion of the single crystal growth crucible 10 (at the solid-liquid interface during seeding). Do not fill with the FeGa alloy seed crystal or the mixture of iron and gallium raw materials.

Next, an inert gas such as argon or nitrogen is flowed into the chamber 19 to adjust the inside of the chamber 19 to an inert atmosphere. After the inside of the chamber 19 becomes an inert atmosphere, the upper heater 12a, the middle heater 12b, and the lower heater 12c arranged to surround the single crystal growth crucible 10 are operated to raise the temperature.

The temperature of the upper heater 12a, the middle heater 12b, and the lower heater 12c is kept constant, and the movable rod 13 is moved up and down at a constant speed to move the single crystal growth crucible 10 up and down, while measuring the temperature of the tip of the thermocouple 50 in the single crystal growth crucible 10 at each position. If the single crystal growth crucible 10 moves too fast, the temperature cannot be measured accurately, so the measurement is performed at a moving speed of, for example, 2 to 5 mm/hour. If the target FeGa composition is an FeGa alloy single crystal with a gallium content of 18% by atomic weight, the solidification temperature is about 1450°C, so the position of the single crystal growth crucible 10 when the measured temperature at the tip of the thermocouple 50 is 1450°C is the seeding position (Figure 4 (c)). The temperature gradient in the single crystal growth crucible 10 measured at the seeding position is taken as the temperature gradient at the time of seeding when growing the FeGa alloy single crystal.

Next, the target temperature gradient is determined. For example, if the temperature gradient in the single crystal growth crucible 10 during seeding is set to 3.5°C/mm, the power input to the upper heater 12a is changed and the movable rod 13 is moved up and down at a constant speed to measure the temperature. If the temperature gradient is to be increased, the power input to the upper heater 12a is increased and the seeding position is adjusted downward.

The above steps are repeated to set the seeding position and power input to the upper heater 12a such that the temperature gradient in the single crystal growth crucible 10 is between 2.5°C/mm and 5.5°C/mm. The temperature gradient is set to 5.5°C/mm or less to prevent an increase in power consumption due to an increase in heater output and to prevent the generation of gallium vapor from the top surface of the melt. An attempt was made to find a seeding position where the temperature gradient would be 6.5°C/mm, but the platinum-rhodium thermocouple 50 broke during temperature measurement due to the high temperature, so the experiment was discontinued.

The method for producing a FeGa alloy single crystal of the present invention can prevent the iron gallium alloy seed crystal 16 from melting completely. Since the FeGa alloy is a non-uniform melting crystal, when the gallium content in the FeGa alloy is 18% by atomic weight, the temperature difference between the solidus and liquidus on the phase diagram is about 75°C. Therefore, in order to prevent the iron gallium alloy seed crystal 16 from melting completely, a temperature difference of 75°C or more is required in the vertical direction of the iron gallium alloy seed crystal 16. Since the length of the iron gallium alloy seed crystal 16 in the vertical direction is usually 30 mm, in order to prevent the iron gallium alloy seed crystal 16 from melting completely, the temperature gradient in the single crystal growth crucible 10 needs to be 2.5°C/mm or more in order to provide a temperature difference of 75°C or more in the vertical direction of the iron gallium alloy seed crystal 16.

By increasing the descending speed of the crucible 10 for single crystal growth, it is possible to grow the FeGa alloy in a short time, but polycrystallization has occurred. When the FeGa alloy is grown under the condition of a descending speed of the crucible 10 for single crystal growth of 1 to 10 mm/hour, polycrystallization occurs from the surface of the grown crystal at the increased diameter part from the top of the iron gallium alloy seed crystal 16 to the start of the straight body. The polycrystallization is classified into two types: one in which polycrystallization is observed on the crystal surface but does not propagate to the inside of the crystal and becomes a single crystal, and one in which the grain boundary propagates to the inside of the crystal, inherits the grain boundary along the growth axis, and becomes polycrystallized up to the top of the crystal. When these two types were classified, it was found that a high temperature gradient in the crucible 10 for single crystal growth results in single crystals, and a low temperature gradient results in polycrystals. Furthermore, it was found that the faster the descending speed of the crucible 10 for single crystal growth is, the higher the temperature gradient must be set in order to prevent polycrystallization.

By setting the temperature gradient in the crucible 10 for single crystal growth at the time of seeding high, it becomes possible to grow a single crystal at a wider range of descending speed of the crucible 10 for single crystal growth. However, if the temperature gradient in the crucible 10 for single crystal growth at the time of seeding is set high, the heater output becomes high. As a result, the amount of evaporation of the molten material increases, and it adheres to the insulation material 11, the heater 12, the crucible receiver 14, etc., and cleaning becomes necessary. In order to suppress the amount of evaporation of the molten material, it is preferable to cover the crucible 10 for single crystal growth with a lid. When the temperature gradient in the crucible 10 for single crystal growth at the time of seeding is set to 6.0 ° C. / mm or more, the amount of evaporation of the molten material becomes intense due to the output of the heater. Therefore, it is preferable to set the temperature gradient in the crucible 10 for single crystal growth at the time of seeding to 5.5 ° C. / mm or less.

Figure 3 shows the relationship between the temperature gradient in the crucible 10 for growing single crystals during seeding and the descent speed of the crucible 10 for growing single crystals, based on the results obtained from Examples 1 to 5 and Comparative Examples 1 to 4 described below. As can be seen from Figure 3, there is a correlation between the temperature gradient and the crucible descent speed, and when the temperature gradient is Y and the crucible descent speed is X, it can be seen that good FeGa alloy single crystals were obtained in Examples 1 to 5, which satisfy Y ≥ 0.247X + 2.00.

The above describes a method for growing single crystals of an iron-gallium alloy by the VB method using the single crystal growth apparatus 100. However, the same single crystal growth apparatus 100 can also be used to grow single crystals of an iron-gallium alloy by the VGF method, in which the temperature is controlled by adjusting the resistance heater 12 instead of moving the single crystal growth crucible 10 up and down during single crystal growth.

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

[Example 1]
First, in an environment of room temperature 20 ° C., a granular iron raw material (purity: 99.9%) and a gallium raw material (purity: 99.99%) having a median diameter of about 1 mm were weighed so that the ratio of iron to gallium was 82:18 in the stoichiometric ratio, that is, the gallium content was 18.0% in atomic weight %. The weighed gallium raw material was put into a Teflon (registered trademark) container and melted in a hot water bath. Furthermore, the iron raw material was put into the molten gallium raw material, stirred in the container, and then cooled to room temperature to prepare a mixture 17 of iron and gallium, which is a mixed raw material.

Then, a FeGa alloy seed crystal 16 (gallium content 17% by atomic weight, cylindrical shape with diameter 5 mm and length 30 mm) with a pre-adjusted gallium content was filled in the lower part of a single crystal growth crucible 10 made of dense alumina with a thickness of 4 mm, inner diameter 52 mm and height 200 mm, and a mixture of iron and gallium 17 was filled on top of the FeGa alloy seed crystal 16. At this time, a crystal with a main surface orientation of <100> was used for the FeGa alloy seed crystal 16.

Next, a lid made of dense sintered alumina was placed over the opening 10a of the single crystal growth crucible 10. Then, the single crystal growth crucible 10 filled with the FeGa alloy seed crystal 16 and the mixture of iron and gallium 17 was placed on a porous alumina crucible holder 14 as shown in FIG. 1, and the tip of the thermocouple 15 was brought into contact with the side of the single crystal growth crucible 10.

Next, the movable rod 13 was driven to set the crucible receiver 14 at the bottom of the chamber 19. After that, argon gas was introduced into the chamber 19, and the inside of the chamber 19 was adjusted to an argon atmosphere at atmospheric pressure. In addition, the upper heater 12a, middle heater 12b, and lower heater 12c, which were made of carbon resistance heaters, were independently controllable.

Then, the temperature of the upper heater 12a was set to 1500°C, the temperature of the middle heater 12b to 1400°C, and the temperature of the lower heater 12c to 1300°C, and the temperature inside the chamber 19 was raised. After the temperature rise was completed and the temperature inside the chamber 19 stabilized, the movable rod 13 was driven to raise the crucible receiver 14, and the single crystal growth crucible 10 was raised at a gentle speed. Since a temperature gradient was created inside the chamber 19, with a high temperature at the top and a low temperature at the bottom, the temperature inside the single crystal growth crucible 10 rose as it moved to the top of the chamber 19, and the mixture 17 of iron and gallium melted to form a molten material.

The temperature of the upper heater 12a was set to 1500°C by the method described above so that the temperature gradient of the single crystal growth crucible 10 was 2.5°C/mm. At this time, the temperature of the middle heater 12b was set to 1400°C, the temperature of the lower heater 12c was set to 1300°C, and the measurement was performed at a moving speed of the single crystal growth crucible 10 of 2 mm/hour. The FeGa composition is such that the gallium content in the FeGa alloy single crystal is 18% in atomic weight percent, and the solidification temperature of the FeGa alloy in this case is approximately 1450°C. Therefore, the position of the single crystal growth crucible 10 when the measured temperature at the tip of the thermocouple 50 is 1450°C was set as the seeding position. As a result, it was found that the temperature of the upper heater 12a should be set to 1500°C.

When the mixed raw materials were almost melted to form a molten mass, the introduction of argon gas into chamber 19 was stopped, and the pressure inside chamber 19 was reduced to 350 Pa using a vacuum pump, and this was maintained for approximately 30 minutes. Next, the pressure was gradually reduced at a gradient of 2.5 Pa/min over 60 minutes until the pressure reached 200 Pa or less, and air bubbles in the molten mass were removed (air bubble removal process). After the air bubble removal process, the introduction of argon gas was resumed, and the inside of chamber 19 was adjusted to an inert atmosphere of 1 atmosphere.

While monitoring the temperature at the contact point of the thermocouple 15 near the location of the single crystal growth crucible 10 where the melt was formed, the movable rod 13 was driven to maintain a temperature gradient of 2.5°C/mm in the single crystal growth crucible 10, and the single crystal growth crucible 10 was raised several mm to the seeding position to stabilize the temperature. This process was repeated to raise the single crystal growth crucible 10 so that the temperature of the thermocouple 15 was stable and in the range of 1350 to 1400°C. Once the position to hold the single crystal growth crucible 10 was determined, it was held there for 3 hours to perform seeding. After seeding, the upper heater 12a was kept at a temperature of 1500°C, the middle heater 12b at a temperature of 1400°C, and the lower heater 12c at a temperature of 1300°C, while the movable rod 13 was driven to lower the crucible 10 for single crystal growth at a descending speed of 2 mm/hour, and the growth of the FeGa alloy single crystal was started. After the descending distance of the crucible 10 for single crystal growth reached 150 mm, the growth was terminated.

After the growth of the single crystal was completed, the grown FeGa alloy single crystal ingot was removed from the single crystal growth crucible 10, and a single crystal of FeGa alloy with a diameter of 52 mm and a length of the straight body 40 of 100 mm was obtained. No melting was observed at the bottom end of the FeGa alloy seed crystal 16, and it was confirmed that the entire FeGa alloy seed crystal 16 had not melted. Furthermore, the grown FeGa alloy single crystal was cut and the inside of the crystal was observed, but no defects such as voids that could be seen with the naked eye were confirmed.

The above process was repeated 10 times to produce 10 ingots of FeGa alloy single crystal. The results were the same for each ingot, and it was possible to stably produce ingots of FeGa alloy single crystal.

[Example 2]
A mixture 17 of iron and gallium was prepared and a single crystal was grown in the same manner as in Example 1, except that the temperature of the upper heater 12a was set to 1600°C and the temperature gradient in the crucible 10 for growing a single crystal during seeding was set to 3.5°C/mm.

After the growth of the single crystal was completed, the grown FeGa alloy single crystal ingot was removed from the single crystal growth crucible 10, and a single crystal of FeGa alloy with a diameter of 52 mm and a length of the straight body 40 of 100 mm was obtained. No melting was observed at the bottom end of the FeGa alloy seed crystal 16, and it was confirmed that the entire FeGa alloy seed crystal 16 had not melted. Furthermore, the grown FeGa alloy single crystal was cut and the inside of the crystal was observed, but no defects such as voids that could be seen with the naked eye were confirmed.

The above process was repeated 10 times to produce 10 ingots of FeGa alloy single crystal. The results were the same for each ingot, and it was possible to stably produce ingots of FeGa alloy single crystal.

[Example 3]
A mixture 17 of iron and gallium was prepared and a single crystal was grown in the same manner as in Example 1, except that the temperature of the upper heater 12a was set to 1600°C, the temperature gradient in the crucible 10 for single crystal growth during seeding was set to 3.5°C/mm, and the descending speed of the crucible 10 for single crystal growth during growth was set to 5 mm/hour.

After the growth of the single crystal was completed, the grown FeGa alloy single crystal ingot was removed from the single crystal growth crucible 10, and a single crystal of FeGa alloy with a diameter of 52 mm and a length of the straight body 40 of 100 mm was obtained. No melting was observed at the bottom end of the FeGa alloy seed crystal 16, and it was confirmed that the entire FeGa alloy seed crystal 16 had not melted. Furthermore, the grown FeGa alloy single crystal was cut and the inside of the crystal was observed, but no defects such as voids that could be seen with the naked eye were confirmed.

The above process was repeated 10 times to produce 10 ingots of FeGa alloy single crystal. The results were the same for each ingot, and it was possible to stably produce ingots of FeGa alloy single crystal.

[Example 4]
A mixture 17 of iron and gallium was prepared and a single crystal was grown in the same manner as in Example 1, except that the temperature of the upper heater 12a was set to 1625°C, the temperature gradient in the crucible 10 for single crystal growth during seeding was set to 4.0°C/mm, and the descending speed of the crucible 10 for single crystal growth during growth was set to 7.5 mm/hour.

After the growth of the single crystal was completed, the grown FeGa alloy single crystal ingot was removed from the single crystal growth crucible 10, and a single crystal of FeGa alloy with a diameter of 52 mm and a length of the straight body 40 of 100 mm was obtained. No melting was observed at the bottom end of the FeGa alloy seed crystal 16, and it was confirmed that the entire FeGa alloy seed crystal 16 had not melted. Furthermore, the grown FeGa alloy single crystal was cut and the inside of the crystal was observed, but no defects such as voids that could be seen with the naked eye were confirmed.

The above process was repeated three times to produce three ingots of FeGa alloy single crystal. The results were the same for each ingot, and it was possible to stably produce ingots of FeGa alloy single crystal.

[Example 5]
A mixture 17 of iron and gallium was prepared and a single crystal was grown in the same manner as in Example 1, except that the temperature of the upper heater 12a was set to 1650°C, the temperature gradient in the crucible 10 for single crystal growth during seeding was set to 4.5°C/mm, and the crucible descent speed of the crucible 10 for single crystal growth during growth was set to 10 mm/hour.

After the growth of the single crystal was completed, the grown FeGa alloy single crystal ingot was removed from the single crystal growth crucible 10, and a single crystal of FeGa alloy with a diameter of 52 mm and a length of the straight body 40 of 100 mm was obtained. No melting was observed at the bottom end of the FeGa alloy seed crystal 16, and it was confirmed that the entire FeGa alloy seed crystal 16 had not melted. Furthermore, the grown FeGa alloy single crystal was cut and the inside of the crystal was observed, but no defects such as voids that could be seen with the naked eye were confirmed.

The above process was repeated three times to produce three ingots of FeGa alloy single crystal. The results were the same for each ingot, and it was possible to stably produce ingots of FeGa alloy single crystal.

[Comparative Example 1]
A mixture 17 of iron and gallium was prepared and a single crystal was grown in the same manner as in Example 1, except that the temperature of the upper heater 12a was set to 1475°C and the temperature gradient in the crucible 10 for growing a single crystal during seeding was set to 2.0°C/mm.

After the above single crystal growth was completed, the grown FeGa alloy single crystal ingot was removed from the single crystal growth crucible 10, and an FeGa alloy with a diameter of 52 mm and a length of the straight body portion 40 of 100 mm was obtained. The seed crystal portion of the ingot had an uneven surface following the inner shape of the single crystal growth crucible 10. The seed crystal portion was cut and its orientation was confirmed, but it was significantly deviated from the <100> orientation, and the seed crystal was completely melted. The grown FeGa alloy ingot was further cut and the inside of the crystal was observed, but no defects such as voids that could be confirmed by the naked eye were confirmed, so it was a single crystal. The grown FeGa alloy ingot was further cut and its orientation was confirmed, but it was significantly deviated from the <100> orientation.

The above growth process was repeated once more to grow an FeGa alloy ingot, but all of the seed crystals had melted in the same way. The grown FeGa alloy ingot was cut and the inside of the crystal was observed, revealing grain boundaries and polycrystallization. The grown FeGa alloy was also cut and its orientation was confirmed, but it was significantly deviated from the <100> orientation.

[Comparative Example 2]
A mixture 17 of iron and gallium was prepared and a single crystal was grown in the same manner as in Example 1, except that the lowering speed of the crucible 10 for growing the single crystal during growth was set to 5 mm/hour.

After the above single crystal growth was completed, the grown FeGa alloy ingot was removed from the single crystal growth crucible 10, yielding an FeGa alloy with a diameter of 52 mm and a length of 100 mm at the body portion 40. No melting was observed at the tip of the seed crystal portion of the ingot. Furthermore, the grown FeGa alloy ingot was cut and the inside of the crystal was observed, revealing grain boundaries and polycrystallization. The above growth process was repeated three times, and all three pieces were polycrystallized.

[Comparative Example 3]
A mixture 17 of iron and gallium was prepared and a single crystal was grown in the same manner as in Example 1, except that the temperature of the upper heater 12a was set to 1575°C, the temperature gradient in the crucible 10 for single crystal growth during seeding was set to 3.0°C/mm, and the descending speed of the crucible 10 for single crystal growth during growth was set to 7.5 mm/hour.

After the growth of the single crystal was completed, the grown FeGa alloy single crystal ingot was removed from the single crystal growth crucible 10, and a single crystal of FeGa alloy with a diameter of 52 mm and a length of the straight body 40 of 100 mm was obtained. No melting was observed at the tip of the seed crystal portion of the ingot. Furthermore, the grown FeGa alloy ingot was cut and the inside of the crystal was observed, but no defects such as voids that could be seen with the naked eye were confirmed.

The above growth process was repeated two more times. No melting was observed at the tip of the seed crystal in either of the two crystals. Furthermore, the grown FeGa alloy ingots were cut and the inside of the crystal was observed, and grain boundaries were observed in both crystals, indicating that the crystals had become polycrystallized.

[Comparative Example 4]
A mixture 17 of iron and gallium was prepared and a single crystal was grown in the same manner as in Example 1, except that the temperature of the upper heater 12a was set to 1625°C, the temperature gradient in the crucible 10 for single crystal growth during seeding was set to 4.0°C/mm, and the descending speed of the crucible 10 for single crystal growth during growth was set to 10 mm/hour.

After the growth of the single crystal was completed, the grown FeGa alloy single crystal ingot was removed from the single crystal growth crucible 10, and a single crystal of FeGa alloy with a diameter of 52 mm and a length of the straight body 40 of 100 mm was obtained. No melting was observed at the tip of the seed crystal portion of the ingot. Furthermore, the grown FeGa alloy ingot was cut and the inside of the crystal was observed, but no defects such as voids that could be seen with the naked eye were confirmed.

The above growth process was repeated two more times. No melting was observed at the tip of the seed crystal in either of the two crystals. Furthermore, the grown FeGa alloy ingots were cut and the inside of the crystal was observed, and grain boundaries were observed in both crystals, indicating that the crystals had become polycrystallized.

The growth conditions and growth results of the FeGa alloy ingots in Examples 1 to 5 and Comparative Examples 1 to 4 are shown in Table 1. For the temperature gradient in Table 1, the calculated value is the value of temperature gradient Y when the crucible descent speed is X, based on the formula Y = 0.247X + 2.01 shown in Figure 3.

As a result, by setting the temperature gradient in the single crystal growth crucible 10 during seeding to 2.5°C/mm or more, it was possible to prevent melting of the lower part of the seed crystal (Examples 1 to 5, Comparative Examples 2 to 4).

On the other hand, when the temperature gradient in the single crystal growth crucible 10 during seeding was set to 2.0°C/mm, the seed crystals all melted, and single crystals could not be grown in the <100> orientation (Comparative Example 1).

When the temperature gradient in the single crystal growth crucible 10 during seeding was 2.5°C/mm, a single crystal could be grown when the lowering speed of the single crystal growth crucible 10 was 2 mm/hour (Example 1), but polycrystallization occurred when the lowering speed of the single crystal growth crucible 10 was 5 mm/hour (Comparative Example 2). It was also found that the temperature gradient at which polycrystallization occurred increased when the lowering speed of the single crystal growth crucible 10 was increased (Comparative Examples 2 to 4).

[summary]
From the results of the above Examples 1 to 5 and Comparative Examples 1 to 4, it is understood that melting of the entire seed crystal and polycrystallization of the FeGa alloy can be prevented by adjusting the temperature gradient in the crucible during seeding in accordance with the descending speed of the crucible during growth. In other words, it is clear that the present invention can mass-produce FeGa alloy single crystals of stable quality at low cost.

10 Single crystal growth crucible, 10a Opening, 11 Insulation material, 12 Resistance heater, 12a Upper heater, 12b Middle heater, 12c Lower heater, 13 Movable rod, 14 Crucible holder, 15 Thermocouple, 16 Iron-gallium alloy seed crystal, 17 Iron-gallium mixture, 18 Vacuum pump, 19 Chamber, 50 Thermocouple, 100 Single crystal growth device

Claims (4)

A method for producing an FeGa alloy single crystal using a directional solidification crystal growth method, comprising the steps of:
The temperature in the crucible during seeding is adjusted so that it increases vertically from bottom to top, and the temperature gradient in the crucible is adjusted to be within the range of 2.5 to 5.5 ° C. / mm;
The unidirectional solidification crystal growth method is a vertical Bridgman method,
During the growth of the FeGa alloy single crystal, the rate of descent of the crucible is set to 5 mm/hour or less, and the temperature gradient within the crucible is set to 3.5° C./mm or more . A method for producing an FeGa alloy single crystal using a directional solidification crystal growth method, comprising the steps of:
The temperature in the crucible during seeding is adjusted so that it increases vertically from bottom to top, and the temperature gradient in the crucible is adjusted to be within the range of 2.5 to 5.5 ° C. / mm;
The unidirectional solidification crystal growth method is a vertical Bridgman method,
During the growth of the FeGa alloy single crystal, the lowering speed of the crucible is set to 7.5 mm/hour or less, and the temperature gradient in the crucible is set to 4.0° C./mm or more . A method for producing an FeGa alloy single crystal using a directional solidification crystal growth method, comprising the steps of:
The temperature in the crucible during seeding is adjusted so that it increases vertically from bottom to top, and the temperature gradient in the crucible is adjusted to be within the range of 2.5 to 5.5 ° C. / mm;
The unidirectional solidification crystal growth method is a vertical Bridgman method,
During the growth of the FeGa alloy single crystal, the rate of descent of the crucible is set to 10 mm/hour or less, and the temperature gradient within the crucible is set to 4.5° C./mm or more . When the temperature gradient is Y and the falling speed is X,
The method for producing an FeGa alloy single crystal according to any one of claims 1 to 3 , wherein Y≧0.247X+2.00 is satisfied.