CN101829767B - Silicon electromagnetic casting device - Google Patents

Abstract

The invention discloses a silicon electromagnetic casting device which comprises a furnace body container, a crucible with conductivity arranged in the furnace body container and an induction coil arranged on the periphery of the crucible. The furnace body container is internally provided with specified gas with certain pressure, and after an induction coil in the silicon electromagnetic casting device is electrified, the silicon in the crucible is induced to generate heat, is melted and then is solidified. The induction coil is formed by vertically arranging more than two coils with different induction frequencies. The present invention uses two or more induction coils having different induction frequencies, and sets the terminal voltage of the induction coil of each induction frequency to a predetermined voltage or less, and generates a large total output power by combining the induction coils, according to the difference in the induction frequency and the setting of the induction output power loaded on each induction coil. By adopting the silicon electromagnetic casting device, the silicon ingot with high quality and larger volume can be produced by a simple and feasible method.

Description

Silicon electromagnetic casting device

technical field

The invention relates to a silicon electromagnetic casting device for manufacturing silicon ingots of solar cell silicon substrates.

Background technique

As one of the methods to improve global environmental problems, solar cells are gradually becoming popular. Silicon crystals with abundant resources and high photoelectric conversion efficiency are widely used in solar cells. Among them, the output of polycrystalline silicon substrate solar cells manufactured by electromagnetic casting method is also increasing.

As shown in Figure 5, the current silicon electromagnetic casting device is roughly composed of a copper crucible 200 placed in a furnace container 100 with a cooling water circulation system inside, and an induction coil 300' arranged on the periphery of the crucible. The silicon block S is placed in the crucible 200 , floats and melts due to the electromagnetic force, and as the height of the silicon block continues to drop, the silicon block crystallizes downward to produce a silicon ingot. Inside the water-cooled crucible 200, the silicon block S floats and melts due to the electromagnetic force, so the molten silicon S' has no contact with the inside of the crucible 200, and will not be polluted by impure substances from the crucible. At the same time, since contact is avoided, the crucible 200 will not be damaged, thereby greatly prolonging the service life of the crucible.

It can be seen from the above that the silicon electromagnetic casting method can continuously produce long and large-volume silicon ingots. The process efficiency is high and the casting conditions are stable. A widely used manufacturing method in industry.

However, the above-mentioned silicon electromagnetic casting method has the following problem: if the volume of the silicon block S is increased in order to improve the production efficiency, the required input power will also increase as the amount of the silicon block to be melted increases. At the same time, the volumes of the copper crucible 200 and the induction coil 300' will increase. As a result, the amount of energization of the induction coil 300' increases, and the terminal voltage of the induction coil 300' also increases in order to supply the increased required power while maintaining the same induction frequency. For example, when the silicon block S with a side length of 20 cm has 2 turns in the induction coil 300 ′, in order to maintain an induction frequency of 35 kHz for induction melting, the output power needs to reach 250 kW, and the terminal voltage of the induction coil 300 ′ needs to reach 550 V; However, when the silicon block S with a side length of 35 cm is also 2 turns in the induction coil 300 ′, in order to maintain an induction frequency of 35 kHz for induction melting, the output power needs to reach 450 kW, and the terminal voltage of the induction coil 300 ′ needs to reach 1000 V. .

As a result, corresponding problems have also arisen. In the silicon electromagnetic casting process, in order to prevent the molten silicon S' from being oxidized, the furnace container 100 is filled with an inert gas of 1 atmosphere pressure. Both argon and helium can be used as inert gas, but argon is usually used from an economic point of view. However, since the ionization voltage of argon gas is very low, when the electrical conductors inside the furnace container 100 filled with argon gas are energized, an arc discharge phenomenon is easily generated between the electrical conductors. Under normal circumstances, when there is argon gas below 1 atmospheric pressure inside the induction melting industrial equipment, an arc discharge will occur if a voltage of more than 600V is input between the closely facing conductors. As the voltage increases, the arcing phenomenon will become more intense, and the electrical conductors that generate the discharge will melt and vaporize.

In the electromagnetic casting device, when the induction output power increases, the terminal voltage of the induction coil 300' increases, the current increases, and the output power increases. The terminal load voltage of the induction coil 300' causes a current to be generated on the induction coil, and due to electromagnetic induction, the surface layer of the cooled crucible 200 generates voltage and current; also due to the electromagnetic induction, the silicon block placed inside the copper crucible 200 The surface of S generates a voltage.

That is to say, in the case of manufacturing the above-mentioned silicon ingot with a side length of 35 cm, the terminal voltage of the induction coil 300' will increase, and the surface voltage of the crucible 200 and the molten silicon S' will also increase due to electromagnetic induction. As shown in Figure 6, an arc discharge A will be generated between the surface layer of the opposite crucible 200 and the surface layer of the molten silicon S', and the crucible 200 after water cooling will be melted and vaporized due to the heat generated by the arc discharge A, thereby causing a deep hole on the crucible. Groove, over time, the erosion will become more and more serious.

Due to the arc discharge A, the surface layer of the crucible 200 is melted and vaporized, and the copper on the surface of the crucible is melted and mixed into the molten silicon S', which reduces the purity of the silicon ingot and the diffusivity of the minority carrier with semiconductor properties, thereby reducing the efficiency of the solar cell. Photoelectric conversion efficiency.

At the same time, the surface erosion of the crucible 200 caused by the arc discharge A becomes more and more serious. As the range of deep grooves on the flat surface becomes larger and larger, the normal electromagnetic action will be hindered in a small area of the surface, thus Causes abnormal phenomenon of melting and solidification in the process of electromagnetic casting. Over time, it may even cause the casting process to be interrupted.

Contents of the invention

Purpose of the invention: The purpose of the present invention is to overcome the deficiencies of the prior art and provide a silicon electromagnetic casting device capable of producing high-quality silicon ingots in a simple and practicable method.

Technical solution: In order to solve the above technical problems, the silicon electromagnetic casting device described in the present invention includes a furnace container, a conductive crucible disposed inside the furnace container, and an induction coil installed on the periphery of the crucible. The furnace container is provided with a specified gas at a certain pressure. After the induction coil in the silicon electromagnetic casting device is energized, the silicon in the crucible is induced to heat and melt, and then solidified in the silicon cell casting device. The induction coil is composed of more than two coils with different induction frequencies arranged up and down. Using more than two induction coils with different induction frequencies, according to the setting of the induction frequency and induction output power loaded on each induction coil, the terminal voltage of the induction coils with different induction frequencies is set below the specified voltage (for example, 900V less than 600V), at the same time, the combination of induction coils can also generate a large synthetic induction output power.

Wherein, preferably, among the induction coils with two or more different induction frequencies, the induction frequency of the lower coil is preferably higher. Therefore, the setting of the induction frequency provides a necessary condition for the manufacture of high-quality silicon ingots, that is to say, the induction frequency of the lower coil should be set to not only suppress the stirring of molten silicon, but also make the solid ingot heat The necessary high-frequency induction output power; because the upper coil is far away from the solidification interface, the effect of induction frequency is difficult to affect the solidification interface. While suppressing the arc discharge inside the crucible, it can effectively increase the combined output power of each induction coil.

Wherein, preferably, the induction frequency of the above-mentioned lower coil is preferably set above 25-30 kHz, thereby effectively increasing the necessary high-frequency induction output power of the lower high-frequency induction coil to suppress stirring of molten silicon and causing the solid ingot to heat up. Between the above-mentioned two or more induction coils of different frequencies, it is preferable to install a magnetic shielding plate, which can prevent unnecessary magnetic interaction between the coils.

Among them, preferably, the terminal load voltage of each induction coil is preferably set below 900V, so that due to the relationship of induction efficiency, the surface voltage of molten silicon can be controlled below 600V.

Wherein, it is further preferred that the terminal load voltage of each induction coil can be set below 600V, so that the surface voltage of the molten silicon must be below 600V.

Wherein, preferably, a plasma torch is arranged above the above-mentioned crucible for heating the molten silicon inside the crucible, which can effectively provide heat for melting the silicon block in the crucible.

Beneficial effects: compared with the prior art, since the present invention adopts more than two induction coils with different induction frequencies, according to the induction frequency loaded on each induction coil and the setting of induction output power, each induction frequency While the terminal voltage of the induction coil is set below the specified voltage (for example, below 900V, preferably below 600V), a large combined output power can be generated through the combination of each induction coil. Adopting the silicon electromagnetic casting device described in this invention makes it possible to produce silicon ingots with high quality and large volume by using a simple and practical method.

Description of drawings

Fig. 1 is a schematic diagram showing the configuration of the device according to the first embodiment.

Figure 2 is an enlarged view of the main body of the device.

Fig. 3 is a sectional view of part III-III of Fig. 2 .

Fig. 4 is an enlarged view of the main part of the device related to the second embodiment.

Fig. 5 is a schematic diagram of the structure of the device at the present stage.

Fig. 6 is a state diagram of the arc discharge phenomenon in the device at the present stage.

Symbol Description

1 This device

100 furnace container

200 Crucible Containers

300 induction coil

310 induction coil on the upper side

320 induction coil on the lower side

330 magnetic shielding plate

400 graphite table

500 up and down moving device

600 temperature control furnace

700 raw material feeders

Detailed ways

Next, a first embodiment of the present invention will be described with reference to FIGS. 1 to 3 .

[Embodiment 1]

<overall composition>

FIG. 1 is a schematic diagram showing the configuration of a silicon electromagnetic casting device (hereinafter referred to as the present device 1 ) according to a first embodiment of the present invention. FIG. 2 is an enlarged view of the main body of the device 1 . Fig. 3 is a sectional view of part III-III of Fig. 2 .

This device 1 consists of a furnace container 100, a crucible 200 installed inside the furnace container 100, an induction coil 300 arranged on the outer periphery of the crucible 200, a graphite table 400 for placing a silicon block S, and a vertical moving device 500 for moving the graphite table 400 up and down. , the temperature-controlled furnace 600 for controlling the solidification and crystallization of the molten silicon S′, and the raw material supplier 700 above the crucible 200 are composed of these parts. The materials constituting each part, except for the induction coil, are the same as those required for the construction of the device at this stage.

Regarding silicon, the state before heating is called "silicon ingot S", the state after heating and melting is called "melted silicon S'", and the solidified state after cooling is called "silicon ingot".

<Composition of Furnace Container>

The above-mentioned furnace container 100 refers to an airtight container including the crucible 200 , the induction coil 300 , and the like.

The upper part of the furnace body container 100 has an air supply port 110 and the lower part has an exhaust port 120 . During casting, use a vacuum pump (not shown) to reduce the pressure in the furnace container 100 to 0.1 torr, and then send a specified gas (for example, argon) through the gas inlet 110 to make it reach the level of atmospheric pressure.

In addition, the bottom wall 130 of the furnace body container 100 is pierced with an insertion hole 130a, and the above-mentioned up and down moving device 500 is inserted there. In order to make the furnace container 100 an airtight container, it is preferable to use a sealing material 140 made of rubber or the like for the insertion hole 130a.

<The composition of the crucible>

The above-mentioned crucible container 200 is a copper product, and cooling circulating water is provided inside the crucible to cool the side wall of the crucible.

In addition, as shown in FIG. 3, since the crucible 200 is electrically insulated in the circumferential direction, the circumference is divided into a plurality of regions, and it is preferable to insert an electrically insulating material such as mica between the divided regions of the circumference of the crucible 200.

<Structure of induction coil>

After the induction coil 300 is energized, the silicon block S placed inside the crucible 200 is induced to heat and melt. The induction coil 300 is composed of two induction coils 310 and 320 with different induction frequencies arranged up and down. Between the two induction coils 310 and 320 with different frequencies, a magnetic shielding plate 330 is provided to block the mutual magnetic interaction.

The terminal load voltage of each induction coil 310, 320 is preferably below 900V, more preferably below 600V, for the following reasons:

Normally, in an electromagnetic casting device using a water-cooled copper crucible 200, the power input into the induction coils 310 and 320 uses the crucible 200 as a medium to transmit 60 to 65% of the power to the molten silicon in the crucible. Among S', that is, the induction power is 60 to 65%. Accordingly, when the load voltage at the terminals of the induction coils 310 and 320 is 900V, the surface voltage of the molten silicon S' is about 600V. As mentioned above, when there is argon gas below 1 atmospheric pressure inside the electromagnetic casting device, an arc discharge will occur if a voltage of 600V or more is input between closely attached opposite conductors. Therefore, in order to completely suppress the arc discharge phenomenon during induction melting, the induction coil terminal voltage is preferably set below 600V; The coil terminal voltage can also be set at 900V.

In addition, it is better to have a higher frequency of the lower induction coil 320, and this high frequency is preferably above 25-30kHz for the following reasons:

Usually, when the molten silicon S' generated based on electromagnetic induction is heated, the surface layer of the molten silicon S' is within the range of the penetration depth of the magnetic force. Thrust, due to the influence of this thrust, the molten silicon S' is stirred while floating and melting. Relatively speaking, the stirring force around the low frequency is strong so that the molten silicon S' can be fully stirred, and the stirring effect around the high frequency gradually decreases until it maintains a relatively static melting state.

The surface layer of molten silicon S' is equivalent to the area of magnetic penetration depth. Around low frequency, the surface layer depth through which the current can pass increases, and a very large area from the surface to the inside will be heated. Around high frequencies, the depth of the surface through which the current can pass decreases, and the area in which the surface is heated also decreases. Therefore, in the case of applying the same amount of induction heating, the heating intensity per unit volume is relatively small for the heating area with a large surface area around the low frequency, and relatively small for the heating area with a small surface layer around the high frequency. , the heating intensity per unit volume is relatively large, so the surface layer can be forced to be heated by using high frequency.

As for the selection of the induction frequency of the induction coil, it should be bounded by 25 ~ 30kHz. The molten silicon S' around the low frequency has a strong stirring intensity, which promotes the convective heat transfer of the molten silicon S'. The bottom expands and deepens. With the deepening of the solidification interface, silicon solidification is also proceeding as usual, and a temperature difference occurs between the inside of the solidified silicon ingot and the surface layer, and stress is generated inside the ingot.

Because the heating intensity of the surface layer of the ingot around the low frequency is low, once the temperature of the surface layer of the ingot is not maintained sufficiently, it is easy to cool down, and the temperature difference between the surface layer and the interior increases. That is, around low frequencies, the solidification interface deepens due to the stronger stirring effect on the molten silicon S' and the weaker heating intensity on the surface layer of the solidified solid ingot. The expansion of the solidification interface below increases the internal stress of the solidified silicon ingot, resulting in crystallization, which weakens the diffusivity of minority carriers and reduces the quality of polycrystalline silicon semiconductors.

On the other hand, around the high frequency above 25-30kHz, the stirring strength of molten silicon S' is weak, and it can maintain a relatively static melting state, and the convective heat transfer with the solidification interface is small, and the solidification interface is difficult to expand downward, forming a relatively Shallow solidification interface. At the same time, due to the high frequency, the surface layer of the solidified silicon ingot has high heating intensity, the surface is not easy to cool, and the temperature difference between the inside and the surface of the ingot is very small. In this way, around the high frequency of the silicon ingot, the solidification interface will not expand downward, forming a shallow solidification interface, the temperature difference between the inside and the surface of the ingot is small, and the generation rate of internal stress is low. This makes it difficult to generate crystals in the ingot, enhances the diffusibility of minority carriers in polycrystalline silicon semiconductors, and improves the quality of solar cells.

Therefore, in the silicon electromagnetic casting method, in order to increase the volume of the silicon ingot and improve the production efficiency, it is necessary to increase the induction output power, especially to set the frequency of the lower induction coil to a high frequency. If the quality of the semiconductor used as a solar cell is to be outstanding, it is preferable to set the induction frequency to 25 to 30 kHz or higher.

<Configuration of other parts>

The temperature-controlled furnace 600 described above can slowly cool the molten silicon S' and make it solidify. In general, the molten silicon S' is finally slowly cooled at a specified temperature by maintaining a specified temperature difference from top to bottom.

The above-mentioned graphite stand 400 is a stand made of graphite. When casting silicon, after the graphite table 400 is moved to a height equivalent to that of the lower induction coil by the up and down moving device 500, the loaded silicon block S is placed on the graphite table. Then, the up and down moving device descends along the center line in the furnace container 100, and the dissolved silicon S' solidifies gradually while descending.

The above-mentioned up and down moving device 500 can move the graphite table 400 up and down along the center line of the furnace container 100 . Compared with other driving devices (not shown), this up and down movement is more suitable for casting conditions.

The function of the raw material supply container 700 is to put the silicon block S and the graphite block as raw materials into the crucible 200 from above. Firstly, a silicon block S of a predetermined weight is loaded, and then a graphite block is placed on top of the silicon block. Graphite block is used to assist raw material silicon to generate heat. After the induction coil is energized, the graphite block heats up first, and then the silicon block S below is affected by the radiant heat of the graphite to heat up. When the temperature of the silicon block S rises to a certain temperature, the resistance value drops, the induced current increases, and it starts to generate heat by itself. When the silicon block S starts to heat itself, the upper graphite block will be pulled out from the top of the crucible 200 .

The induction coil 300 in the present embodiment is constituted by two upper and lower coils 310 and 320, and may be constituted by using three or more coils.

Next, a second embodiment of the present invention will be described with reference to FIG. 4 .

[Embodiment 2]

Fig. 4 is an enlarged view of the main part of the device according to the present embodiment.

In this embodiment, a plasma torch 800 is installed inside the furnace container 100 and above the crucible 200 . The plasma torch 800 is used to accelerate heating when casting silicon. For example, for a cylindrical plasma torch with a diameter of 10 cm, the internal negative electrode and the torch as a whole can be cooled by water, and can move up, down, left, and right.

During casting, the top of the plasma torch 800 moves down to approach the silicon block S, and specified gas such as argon is sent into the plasma torch 800, and direct current plasma is ignited between the cathode of the plasma torch 800 and the anode of the molten silicon S'. Then, by gradually increasing the input power to the induction coils 310, 320, the heating of the silicon block can be accelerated.

The materials and symbols used in the remaining parts are the same as those in the first embodiment (Fig. 1 to Fig. 3), and will not be repeated here.

Example 1:

In this device 1, two induction coils 310, 320 with different induction frequencies are used, and the conductive crucible 200 which is insulated in the vertical direction and has a cooling water circulation inside interacts with the induction coil 300 arranged on the periphery of the crucible 200 to make silicon The block S melts and solidifies while falling. When using this device 1 to carry out silicon electromagnetic casting, the steps are as follows:

In this example, as shown in FIG. 1 , a crucible 200 and induction coils 310 , 320 mounted up and down on the periphery of the crucible 200 are provided in a furnace container 100 with controllable internal pressure. Right below the crucible, a temperature-controlled furnace 600 for controlling the temperature of the solidification of the silicon block S and a vertical moving device 500 for moving the graphite table 400 up and down are installed so that the silicon block S can continue to descend.

Above the furnace container 100, a raw material supplier 700 for supplying silicon lumps S, graphite lumps, and the like is installed. The graphite block is put in from above at the initial stage of the melting of the silicon block S and positioned at a position corresponding to the height of the induction coil 300 in the crucible 200 to generate heat by induction and assist the heating of the silicon block S.

In this example, the cross section of the silicon block S in the casting direction is a square with a side length of 35 cm; the crucible 200 with a square cross section in the horizontal direction has an inner side length of 35 cm and an outer side length of 41.6 cm. The number of insulating divisional areas in the vertical direction of the crucible 200 was set to 60, and the length of each area of the crucible 200 divided into 60 areas was 70 cm. During processing, there is cooling circulating water inside, and electrical insulating material mica is inserted between each area. The flow of cooling water inside the crucible 200 is 500 liters per minute in total.

In addition, the two induction coils 310 and 320 are arranged up and down. The upper induction coil 310 is a square with 2 turns, an inner diameter of 42.6cm, and a height of 15cm. It is connected to an induction power supply with a maximum output power of 350kw, and the induction frequency is set to 10KHz. The lower induction coil 320 has the same shape as the upper induction coil 310, and is connected to an induction power supply with a maximum output power of 150kw, and the induction frequency is set to 35KHz. The induction coils 310, 320 are arranged side by side at the center of the height direction of the crucible 200, a copper magnetic shielding plate 330 with a thickness of 3mm is arranged between the two induction coils 310, 320, and a water pipe is provided on the outer periphery of the magnetic shielding plate 330 to realize water cooling. .

The operation sequence of this example is as follows: First, in the descending direction, in order to keep the top of the graphite platform 400 with a square cross-section and a side length of 35 cm at the same height as the lower end of the lower induction coil 320, the graphite platform 400 rides up and down. The moving device 500 rises and is inserted into the crucible 200 from below. A 50kg silicon block S is loaded onto the graphite platform 400, and a graphite block with a side length of 30 cm, a height of 7 cm, and a square cross section is inserted from the top of the crucible 200 at 2 cm above the silicon block S.

Then, after the air pressure in the furnace container 100 is decompressed to 0.1 Torr by a vacuum pump, argon gas is fed in to make the air pressure in the furnace container 100 reach atmospheric pressure. Then, the induction output power of the upper induction coil 310 with a frequency of 10KHz is gradually increased to 200kw, and the induction output power of the lower induction coil 320 with a frequency of 35KHz is gradually increased to 100kw. In the above situation, the terminal voltage of the upper induction coil 310 is 170V, and the terminal voltage of the lower induction coil 320 is 280V.

In this way, after the two induction coils 310, 320 are energized, the graphite block firstly inserted above the silicon block S will induce heat, heat up and turn red. Under the action of radiant heat from the reddened graphite block, the silicon block S begins to heat up. When the temperature of the silicon block S reaches 500°C, the resistance value will drop, the induced current will increase, and it will start to heat itself. Simultaneously with the silicon block S itself starting to generate heat, the graphite block is pulled out from above the cooling crucible 200 .

Then, increase the induction output power of the upper induction coil 310 to 350kw, and the induction output power of the lower induction coil 320 to 150kw, so as to accelerate the melting of raw silicon. The silicon block S that has started to generate heat by itself further heats up and completely melts in a short while. Under the influence of the electromagnetic force of the inner wall and side surfaces of the crucible 200, the dissolved silicon S' is in a non-contact state with the cooling crucible 200. In the above situation where the induction output power is increased, the maximum terminal voltage of the upper induction coil 310 is 280V, and the maximum terminal voltage of the lower induction coil 320 is 490V.

After the raw material silicon loaded for the first time is completely melted and kept stable, the temperature of the silicon ingot temperature control furnace 600 arranged directly below the crucible 200 is raised to keep the temperature decreasing at about 35°C/cm along the falling direction of the silicon ingot.

Afterwards, through the upper raw material supplier 700, raw silicon particles with a size of 1 mm to 20 mm are continuously loaded into the crucible 200, and the vertical moving device 500 carrying the dissolved silicon S' is lowered to start casting. After the up and down moving device 500 starts to descend, the electromagnetic force received by the dissolved silicon S' drops to the lower end of the lower induction coil 320 gradually decreases and begins to cool and solidify. But at this time, the surface layer of the solidified silicon ingot is still red and hot due to the induction effect of the lower coil 320 because it is relatively close to the lower induction coil 320 in distance, and will not be cooled immediately.

In this way, while the raw material silicon is continuously supplied, the ingot is continuously solidified, thereby achieving continuous casting. In this operation example, the casting speed was 2.0 mm per minute. Regarding the output power of the induction power supply during stable casting, the upper induction coil 310 is about 260kw, and the lower induction coil 320 is about 80kw. Regarding the terminal voltages of 310 and 320, the upper coil 310 is about 200V, and the lower coil 320 is about 250V. Casting was stopped when the total length of the ingot reached 200 cm.

After the ingot cast according to the above sequence is cooled to room temperature, it is taken out from the furnace, and the inner edge of the crucible 200 is inspected. Test result: there is no trace of arc discharge at all, and the inner edge of the crucible 200 is as flat as before.

In order to test the actual performance of the silicon ingot, the silicon ingot was made into a solar cell substrate, and the performance of the solar cell using the solar cell substrate was also tested. First cut off a silicon ingot with a cross-section of 15 cm in side length and 40 cm in length with a diamond cutting machine, and then process it into a polycrystalline silicon substrate with a thickness of 200 microns by using an electric saw slicing method. 100 of these polycrystalline silicon substrates were extracted and trial production of solar cells was carried out. Using hydrogen passivation technology in the trial production of solar cells, it can be known that the average conversion efficiency of 100 polycrystalline silicon cells is 15.1%. So far, it has been confirmed in this example that the silicon ingot manufactured based on the present invention can provide a high-quality solar cell substrate.

Example 2:

In this device 1, not only two induction coils 310, 320 with different induction frequencies are used, but also a plasma torch 800 is provided, and a conductive crucible 200 with a cooling water circulation inside is insulated in the vertical direction and is arranged on the periphery of the crucible 200. The induction coil 300 on the top interacts to melt and solidify the silicon block S while falling. When using this device 1 to carry out silicon electromagnetic casting, the steps are as follows:

In this example, as shown in FIG. 4 , a crucible 200 and induction coils 310 , 320 mounted up and down on the periphery of the crucible 200 are provided in a furnace container 100 with controllable internal pressure. Right below the crucible, a temperature-controlled furnace 600 for controlling the temperature of the solidification of the silicon block S and a vertical moving device 500 for moving the graphite table 400 up and down are installed so that the silicon block S can continue to descend.

Above the furnace container 100, a raw material supplier 700 for supplying silicon lumps S, graphite lumps, and the like is provided. The graphite block is put in from above at the initial stage of the melting of the silicon block S and positioned at a position corresponding to the height of the induction coil 300 in the crucible 200 to generate heat by induction and assist the heating of the silicon block S.

The plasma torch 800 is installed directly above the crucible 200 for additional heating of the molten silicon S' from above.

In this example, the cross-section of the silicon ingot S in the casting direction is a square with a side length of 51 cm. The crucible 200 with a square cross section in the horizontal direction has an inner side length of 51 cm and an outer side length of 57 cm. The number of insulating divisional areas in the vertical direction of the crucible 200 was set to 84, and the length of each area of the crucible 200 divided into 84 areas was 80 cm. During processing, there is cooling circulating water inside, and electrical insulating material mica is inserted between each area. The cooling water inside the crucible 200 has a total flow rate of 700 liters per minute.

Two induction coils 310, 320 are arranged up and down. The upper induction coil 310 is a square with 2 turns, an inner diameter of 58 cm, and a height of 15 cm. It is connected to an induction power supply with a maximum output power of 550 kw, and the induction frequency is set to 10 KHz. The lower induction coil 320 has the same shape as the upper induction coil 310, and is connected to an induction power supply with a maximum output power of 200kw, and the induction frequency is set to 35KHz. The induction coils 310, 320 are arranged side by side at the center of the height direction of the crucible 200, a copper magnetic shielding plate 330 with a thickness of 3mm is arranged between the two induction coils 310, 320, and a water pipe is provided on the outer periphery of the magnetic shielding plate 330 to realize water cooling. .

In order to provide additional plasma jet heating to the molten silicon S' from above, the plasma torch 800 is connected to a 100 kW direct current with the molten silicon S' as an anode. The plasma torch 800 is cylindrical with a diameter of 10 cm, and the internal negative electrode and the torch as a whole can be cooled by water, and the plasma torch 800 can move up, down, left, and right.

The operation sequence of this example is as follows: First, in the descending direction, in order to keep the top of the graphite platform 400 with a square cross-section and a side length of 51 cm at the same height as the lower end of the lower induction coil 320, the graphite platform 400 rides up and down. The moving device 500 rises and is inserted into the crucible 200 from below. A 110 kg silicon block S is placed on the graphite table 400 .

Descend the plasma torch 800 so that its top is close to the silicon block S placed on the graphite platform, and then send argon gas into the plasma torch at a speed of 250 liters per minute, and the plasma will flow between the cathode of the plasma torch 800 and the anode of the silicon block S lit between. After confirming that the plasma is ignited, the induction coils 310, 320 are energized.

Starting with the ignition of the plasma torch and the energization of the induction coil, the input power is gradually increased to accelerate the melting of the silicon block. The plasma torch has an output current of 700 amps and the output voltage goes all the way up to 125V. The induction frequency of the upper coil 310 is 10kHz, the induction output power is 550kW, and the terminal voltage is 380V; the induction frequency of the lower coil 320 is 35kHz, the induction output power is 200kW, and the maximum terminal load voltage is 560V.

The temperature of the silicon block S is raised, and the melting is accelerated, and soon, the silicon block S is completely melted. Continue to feed raw silicon through the raw material supplier 700 until the amount of molten silicon S' reaches 180 kg. Because it is irradiated by the plasma torch 800 and the melting state of the molten silicon S' induced by induction melting in the crucible 200 is stable, under the influence of the electromagnetic force on the inner wall and the side surface of the crucible 200, the molten silicon S' and the cooling crucible 200 are in an incompatible state. contact status.

After the silicon block S is initially melted and the melted silicon S' remains stable, the temperature of the silicon ingot temperature control furnace 600 set directly below the crucible 200 is raised to keep the temperature decreasing at about 35°C/cm along the falling direction of the silicon ingot .

Then, raw silicon particles with a size of 1 mm to 20 mm are continuously loaded into the crucible 200 from the upper raw material supplier 700, and the vertical moving device 500 carrying the dissolved silicon S' is lowered to start casting. The up and down moving device 500 starts to descend, and the dissolved silicon S' falls to the position of the lower end of the lower induction coil 320, and the received electromagnetic force gradually decreases, and begins to cool and solidify. But at this time, the surface layer of the solidified silicon ingot is still red and hot due to the induction effect of the lower coil 320 because it is relatively close to the lower induction coil 320 in distance, and will not be cooled immediately.

In this way, while the raw material silicon is continuously supplied, the ingot is continuously solidified, thereby realizing continuous casting. In this operation example, the casting speed is 1.7mm per minute. Regarding the output power of the induction power supply during stable casting, the plasma torch is about 80kw, the upper induction coil 310 is about 350kw, and the lower induction coil 320 is about 150kw. Regarding the terminal voltages of 310 and 320, the upper coil 310 is about 250V, and the lower coil 320 is about 470V. Casting was stopped when the total length of the ingot reached 200 cm.

After the ingot casted according to the above sequence is cooled to room temperature, the inner edge of the crucible 200 is inspected. Test result: there is no trace of arc discharge at all, and the inner edge of the crucible 200 is as flat as before.

In order to test the actual performance of the silicon ingot, the silicon ingot was made into a solar cell substrate, and the performance of the solar cell using the solar cell substrate was also tested. Silicon ingots were processed into polycrystalline silicon substrates with a cross-section of 15 cm square and a thickness of 200 microns, which were used for trial production of solar cells. Trial production of solar cells was carried out using 100 such substrates, and it was known that the average conversion efficiency of 100 polycrystalline silicon cells was 15.2%. So far, it has been confirmed in this example that the silicon ingot manufactured based on the present invention can provide a high-quality solar cell substrate.

The invention can manufacture high-quality silicon ingots used for solar battery silicon substrates, and is suitable for silicon electromagnetic casting in which silicon ingots are produced by melting raw silicon inductively.

The above is only a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications are also possible. It should be regarded as the protection scope of the present invention.

Claims (5)

1. a silicon electromagnetic casting device comprises body of heater container (100) and is arranged on the crucible (200) of the inner tool electric conductivity of body of heater container (100) and the induction coil (300) that is installed in this crucible (200) periphery; Be provided with the designated gas of certain pressure in the described body of heater container (100), after the induction coil in this silicon electromagnetic casting device (300) energising, make silicon induction heating, fusion in the crucible (200), solidify then; It is characterized in that: described induction coil (300) is disposed up and down by the coil that has different induction frequencies more than two and forms, and wherein, the induction frequencies of lower coil is higher, and the induction frequencies of lower coil is arranged on more than the 25kHz.

2. silicon electromagnetic casting device according to claim 1 is characterized in that: be provided with magnetic shield panel (330) between the described last lower coil that has different induction frequencies more than two.

3. silicon electromagnetic casting device according to claim 1 and 2, it is characterized in that: the terminal load voltage of described each induction coil is arranged on below the 900V.

4. silicon electromagnetic casting device according to claim 3, it is characterized in that: the terminal load voltage of described each induction coil is arranged on below the 600V.

5. silicon electromagnetic casting device according to claim 1 is characterized in that: be provided with plasmatorch above the crucible (200), be used for the heating to the inner fusion of crucible (200) silicon.

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