KR100432907B1 - Method for growing single crystal of magnesia - Google Patents

Abstract

The present invention relates to a method for producing a large single crystal of magnesia by a skull melting method. The present invention is to fill the crucible with magnesia in the powder state in the state of filling the conductive material in the center of the cold crucible, and to increase the temperature of the electrically conductive material located in the center of the crucible by the high frequency induction heating Melting the magnesia in the state, maintaining the molten state so that the melt of the molten magnesia becomes homogeneous, growing the crystal of the magnesia by slowly lowering the crucible, and lowering the high frequency power after the completion of the crystal growth. And cooling the grown crystals, and adjusting the diameter of the crucible so that the high frequency penetration depth of the molten magnesia is 1/5 to 1/20 of the crucible.

Description

METHOD FOR GROWING SINGLE CRYSTAL OF MAGNESIA}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for growing magnesia single crystals, and more particularly to a method for producing large single crystals of magnesia by a skull melting method.

Single crystals of magnesia (MgO) have very good physical properties and therefore have a wide range of applications. For example, it is used as a dielectric layer of an AC PDP, a window of a high temperature furnace, a coating material of an optical lens, a substrate material for thin film single crystal growth of a high temperature superconductor, and the like.

Since magnesia has a high melting temperature of 2850 ° C., it is difficult to melt by a general method. Up to now, arc melting has been used for single crystal growth of magnesia.

The arc melting method is a method of melting magnesia using a carbon electrode and slowly cooling it at room temperature to grow single crystals, and is currently mainly used to produce magnesia refractory materials. Since the arc melting method uses a carbon electrode, there is a problem that the pollutant is discharged, large single crystal growth is difficult, and the yield is very low. Therefore, there is a limit to growing large single crystals of good quality.

Meanwhile, there is a method of growing a single crystal by slowly cooling the powdered material through high frequency heating in a cold crucible provided with a cooling water pipe and then slowly cooling it. This is called the skull melting method. In the skull melting method, since the crucible is low temperature, numerous seeds are attached to the surface of the crucible so that a separate seed for crystal growth is not used. The skull melting method is known to be effective for melting high temperature materials and to grow large single crystals without contamination of the co-melting compound.

However, the Skull Melting method has only been applied to the manufacture of zirconia (Zirconia) so far, and not to the manufacture of magnesia. This is because the electrical conductivity of magnesia is very low. If the electrical conductivity is very low, such as magnesia, it is difficult to keep the melt in a stable state because the heating efficiency is very low, and thus control over crystal growth becomes very difficult. There is no known conductivity or conductivity data for magnesia.

In order to grow large-scale high-quality magnesia single crystals by using the skull melting method, it is necessary to first maintain the molten state. In view of the low electrical conductivity of magnesia, a very high frequency is required. Typically, small laboratory-scale high-frequency oscillators (50 kHz or less) are less likely to generate frequencies above 10 MHz, and when predicting the electrical conductivity of molten magnesia, the penetration depth of 10 cm or less can not be obtained at these frequencies. Small crucibles having a diameter of 50 cm or less have low heating efficiency and are difficult to maintain a molten state.

Therefore, an object of the present invention is to propose a method of effectively applying the skull melting method to magnesia melting.

It is another object of the present invention to propose a method for producing a large magnesia single crystal having excellent quality from molten magnesia.

1 is a view showing a growth process of magnesia crystals according to the present invention.

2 shows the relationship between crucible size and penetration depth and heating efficiency.

3 shows the relationship between the temperature of a typical oxide and its resistivity.

In order to achieve this object, the present invention proposes an improved skull melting method which raises the heating efficiency by precisely setting the crucible's diameter with respect to the available high frequency frequency.

The present invention provides a method of growing a crystal of magnesia, comprising filling a crucible with a powdery magnesia in a state in which an electrically conductive material is filled in the center of a cold crucible, and an electrically conductive material in the center of the crucible. Melting the magnesia by high frequency induction heating, maintaining the molten state so that the melt of the molten magnesia becomes homogeneous, growing the crystals of magnesia by slowly lowering the crucible, and growing the crystal And cooling down the grown crystals by lowering the power of the high frequency wave after completion of the crystal growth by the step, wherein the penetration depth of the crucible is 1/5 to 1/20 of the crucible diameter. It is characterized by setting the diameter.

Between the magnesia melting step and the molten state maintaining step, it is preferable to further supply the crucible with a new powdery magnesia, and to further melt the magnesia in the molten state by high frequency heating. The particle size of the magnesia in the powder state and the filling method of the magnesia in the powder state are constantly controlled so that the aspect of the skull formed by the magnesia melting step is constant. The frequency of the high frequency is specifically 1 to 10 MHz, and the diameter of the crucible is preferably 40 to 100 cm.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of the present invention; Like reference numerals in the drawings indicate the same or similar components.

1 is a diagram illustrating a growth process of magnesia crystals according to the present invention. First, the powder magnesia 101 is filled in the crucible 103 (FIG. 1A). An electrically conductive material 105 is filled in the center of the crucible for initial heat generation. The surface of the crucible is equipped with a cooling water pipe so that the surface of the crucible is kept at a low temperature. Depending on the particle size and filling method of the raw material to be filled (ie, magnesia powder), the shape of the scull formed after melting may vary, so that the particle size and the filling method of the raw material are controlled to maintain the reproducibility. It is preferable. Next, high frequency heating is performed to dissolve the magnesia (FIG. 1B). When the high frequency heating is performed, the temperature of the conductive material 105 first rises, followed by the temperature of the magnesia powder 101. As the temperature of the magnesia powder increases, the electrical conductivity increases, so the magnesia powder absorbs high frequency energy and melts at a high speed. Next, the raw materials are supplied again (FIG. 1C). When the raw material melts, the volume is reduced. However, in the skull melting method, since the temperature of the crucible is cold, the outer circumference of the magnesia in the molten state is surrounded by the magnesia crystal 107. Therefore, it drills a hole in the upper part and supplies raw materials. However, when the crucible is large, the upper skull comes down near the surface of the melt, and thus feeds the raw material on the upper skull. The molten state is maintained so that the solution is homogeneous after the re-feed of the raw material is melted (FIG. 1D). Next, the crucible is gradually lowered with respect to the high frequency heating device 109 to grow the crystal 111 (FIG. 1E). Since the rate of falling of the crucible has a direct influence on the rate of crystal growth, an appropriate rate and uniform rate of falling are required. After the crystal growth is completed in this way, the power of high frequency is lowered to cool the grown crystal (FIG. 1F).

FIG. 2 shows the relationship between crucible size and penetration depth and heating efficiency. Penetration depth is the depth of penetration when the electromagnetic energy at any depth becomes (1 / e) of the electromagnetic energy on the surface of the heating element. If the penetration depth is too deep, it means that energy is not absorbed by the heated object. On the other hand, if the depth of penetration is too shallow, only the surface is heated. As the surface is cooled by the cooling water flowing in the crucible as described above, high frequency energy is not effectively used for heating the material.

Equation 1 below shows the relationship between the penetration depth and the electrical conductivity and frequency of the heating element. Where δ is the penetration depth, sigma is the electrical conductivity of the heating element, μ is the magnetic permeability of the heating element, and ω is the respective frequency (2π f ).

δ = (2 / σμω) ^1/2

As can be seen in Equation 1, the conductivity and frequency are inversely proportional to a given penetration depth. Therefore, in the case of a material having very low electrical conductivity, such as high purity magnesia, a high frequency should be used to obtain a predetermined penetration depth. In reality, the industrial high frequency oscillator is limited to about 5 MHz, so that the penetration depth is large in the case of magnesia having a very low electrical conductivity. Therefore, magnesia is difficult to melt by a small crucible. However, large crucibles are not practical because they require a high-powered high-frequency heating device to melt magnesia.

Referring to FIG. 2, it can be seen that δ / R should be about 0.2 to 1 in order for the heating efficiency to be relatively good. Where δ is the penetration depth and R is the radius of the heating element (ie crucible). Therefore, considering the heating efficiency, the appropriate size of the crucible is about 2 to 10 times the penetration depth. That is, the heating efficiency is excellent when the penetration depth by a given high frequency becomes about 1/5 to 1/20 of the crucible diameter.

Melting by the skull melting method basically performs high frequency induction heating, and accordingly, the size and the oscillation frequency of the crucible may be appropriately adjusted according to the electrical conductivity of the heated object to obtain a predetermined heating efficiency. As described above, when the appropriate crucible size is about 2 to 10 times the penetration depth, the heating efficiency is good, and when it is out of this, very large power is required by the low heating efficiency. Therefore, if the appropriate crucible size is not met for a given frequency, the molten state cannot be maintained and gradually cools down.

3 shows the relationship between the temperature of a typical oxide and the resistivity. Electrical conductivity at high temperatures of magnesia has not been published, but can be predicted to some extent based on data measured in a relatively low temperature region, as shown in FIG. 3. The graph of resistivity versus temperature of magnesia is approximately parallel to the graph of resistivity of alumina. As shown by reference numeral 301 using this, when the specific resistance of magnesia at 2850 degreeC which is the melting temperature of magnesia is estimated, it becomes about 0.2 [ohm / cm]. Since zirconia is approximately 10 ⁻³ [Ω / cm], the electrical conductivity of magnesia is about 1/100 of that of zirconia. Therefore, according to Equation 1, when melting in the same crucible, it can be seen that the frequency of the high frequency for magnesia should be 50 times that of the high frequency for zirconia. According to the experiment, when the zirconia is melted using a crucible having a diameter of 20 cm, an appropriate frequency is about 100 to 1000 kHz, so when melting magnesia into a crucible of the same size, it may be about 5 to 50 MHz. Estimation of the appropriate frequency according to the crucible size is shown in Table 1 below.

Crucible Size (cm) 10 20 40 60 100 Appropriate frequency (MHz) 20 to 200 5-50 1 to 10 0.5 to 5 0.2 to 2

Although the frequency of the industrially manufactured oscillator varies depending on the output, it is limited to about 5 MHz for current production technology reasons, and considering the table 1, the diameter of the crucible is preferably about 40 to 100 cm. The frequency of the oscillator is preferably about 0.5 to 5 MHz. If the diameter of the crucible is smaller than 40 cm, the appropriate frequency becomes too large as described above, and if the crucible is larger than 100 cm, the crucible is too large and practically not necessary because very large power is required. As the inductance of the coil installed in the oscillator for high frequency heating increases, the oscillation frequency is lowered, so it is also necessary to properly adjust the size and frequency of the crucible.

The description so far is directed to embodiments embodying the present invention, and is not intended to limit the scope of the present invention. Accordingly, those skilled in the art should note that various modifications or variations can be made to the configurations described in connection with the above embodiments. The scope of the invention is defined in principle by the claims that follow.

According to the present invention as described above, it is possible to produce a large-scale magnesia single crystal of excellent quality, and when applied to the production of molten magnesia refractory, the power source unit can be reduced by about 1/3 compared to the arc melting method.

Claims (5)

In the method of growing the crystal of magnesia, Filling the crucible with magnesia in a powder state while an electrically conductive material is filled in the center of a cold crucible; Melting the powdered magnesia into the molten magnesia by heating a high frequency to an electrically conductive material in the center of the crucible; Slowly lowering the crucible or raising working coils to grow magnesia crystals; Cooling the grown crystals by lowering the power of the high frequency wave after completion of the crystal growth by the crystal growth step; Equipped, The diameter of the crucible is set so that the penetration depth by the high frequency becomes 1/5 to 1/20 of the crucible diameter. The method of claim 1, Between the magnesia melting step and the molten state maintaining step, further supplying the crucible with a new powdery magnesia, and dissolving the magnesia in the molten state by high-frequency heating further comprising the growth of magnesia crystals Way. The method of claim 1, The method of growing magnesia crystals, characterized in that the particle size of the magnesia in the powder state and the filling method of the magnesia in the powder state are constant so that the pattern of the skull formed by the magnesia dissolving step is constant. The method of claim 1, The frequency of the high frequency is 0.5 to 5 MHz, characterized in that the growth method of magnesia crystals. The method of claim 1, The crucible has a diameter of 40 to 100 cm, characterized in that the growth of magnesia crystals.