Method and apparatus for fracturing polycrystalline silicon

Abstract

The present invention provides a method and an apparatus for fracturing polycrystalline silicon, and the method includes steps of placing the polycrystalline silicon in a water tank containing water; applying an instant high voltage to the water tank so that high-voltage discharge occurs in the water of the water tank to fracture the polycrystalline silicon. The method and apparatus have advantages of simple process, uniform fragments and no metal contamination.

Claims

1. A method for fracturing polycrystalline silicon, comprising steps of placing the polycrystalline silicon in a water tank containing water; and applying an instant high voltage to the water tank so that high-voltage discharge occurs in the water of the water tank, to fracture the polycrystalline silicon; wherein the step of applying an instant high voltage to the water tank comprises steps of: charging a charging capacitor; and continuing charging the charging capacitor until voltage of the charging capacitor reaches a breakdown voltage of a disconnecting switch, so that the disconnecting switch is broken down and all voltage stored in the charging capacitor is applied to the water tank; wherein the breakdown voltage of the disconnecting switch is in a range of 30200 kV; wherein the polycrystalline silicon before being fractured is a cylindrical polycrystalline silicon rod with a diameter of 80200 mm, a length of 2002800 mm; and wherein the fractured polycrystalline silicon has irregular shapes, and the distribution range of sizes of the fractured polycrystalline silicon is specified as follows: the polycrystalline silicon with a linear dimension of 025 mm takes up 3-21% of total weight; the polycrystalline silicon with a linear dimension of 2550 mm takes up 4%36.5% of total weight; and the polycrystalline silicon with a linear dimension of 50100 mm takes up 43.5%91.5% of total weight.

2. The method according to claim 1, wherein a discharge gap of the disconnecting switch is in a range of 1050 mm, and a discharge gap of the water tank is in a range of 3080 mm.

3. The method according to claim 1, wherein in the step of charging a charging capacitor is specifically implemented by charging the charging capacitor with alternating current which has been converted by a high-voltage transformer.

4. The method according to claim 1, wherein the step of placing the polycrystalline silicon in a water tank containing water comprises: filling water in the water tank, then placing the polycrystalline silicon in the water such that the polycrystalline silicon is submerged in the water.

5. The method according to claim 1, wherein the water in the water tank takes up of the volume of the water tank.

6. The method according to claim 1, wherein intensity of electric field generated by the instant high voltage is greater than or equal to a critical electric field intensity of the water in the water tank.

7. The method according to claim 1, wherein pure water is adopted as the water in the water tank.

8. The method according to claim 7, wherein an electrical resistivity of the water in the water tank is no less than 16.2 M.Math.cm, content of SiO.sub.2 is no greater than 10 g/L, content of Fe is no greater than 1.0 g/L, content of Ca is no greater than 1.0 g/L, content of Na is no greater than 20 g/L, and content of Mg is no greater than 1.0 g/L.

9. The method according to claim 1, wherein the polycrystalline silicon rod has a smooth surface.

10. The method according to claim 1, wherein the polycrystalline silicon rod has a surface with nodules thereon.

11. A method for fracturing polycrystalline silicon, comprising steps of placing the polycrystalline silicon in a water tank containing water; and applying an instant high voltage to the water tank so that high-voltage discharge occurs in the water of the water tank, to fracture the polycrystalline silicon; wherein the step of applying an instant high voltage to the water tank comprises steps of: charging a charging capacitor; and continuing charging the charging capacitor until voltage of the charging capacitor reaches a breakdown voltage of a disconnecting switch, so that the disconnecting switch is broken down and all voltage stored in the charging capacitor is applied to the water tank; wherein the breakdown voltage of the disconnecting switch is in a range of 30200 kV; wherein the polycrystalline silicon before being fractured is a silicon lump with a linear dimension of 80300 mm; and wherein the fractured polycrystalline silicon has irregular shapes, and the distribution range of sizes of the fractured polycrystalline silicon is specified as follows: the polycrystalline silicon with a linear dimension of 025 mm takes up 3-21% of total weight; the polycrystalline silicon with a linear dimension of 2550 mm takes up 4%36.5% of total weight; and the polycrystalline silicon with a linear dimension of 50100 mm takes up 43.5%91.5% of total weight.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram illustrating a structure of an apparatus for fracturing polycrystalline silicon according to the present invention.

(2) Reference numerals: 1first electrode, 2second electrode, Bhigh-voltage transformer, Ghigh-voltage rectifier, Rcharging resistor, Ccharging capacitor, Kdisconnecting switch, Fwater tank.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(3) To make those skilled in the art better understand the technical solutions of the present invention, the present invention will be further described in detail in conjunction with the accompanying drawings and the specific implementations

(4) The present invention provides a method for fracturing polycrystalline silicon which comprises the steps of:

(5) placing polycrystalline silicon in a water tank containing water;

(6) applying an instant high voltage to the water tank so that high-voltage discharge occurs in the water of the water tank, to fracture the polycrystalline silicon.

(7) Here, the intensity of the electric field generated by the instant high voltage applied to the water tank is greater than or equal to the critical electric field intensity of the water in the water tank, wherein the critical electric field intensity is the lowest electric field intensity that deprives the medium (water) of insulating property.

(8) Preferably, as the water in the water tank, pure water is adopted.

(9) Here, in the pure water, the electrical resistivity of the water is no less than 16.2 M.Math.cm, the content of SiO.sub.2 is no greater than 10 g/L, the content of Fe is no greater than 1.0 g/L, the content of Ca is no greater than 1.0 g/L, the content of Na is no greater than 20 g/L, and the content of Mg is no greater than 1.0 g/L.

(10) This is because the quality index of polycrystalline silicon includes content of surface metal impurities, for example, the content of surface metal impurities of electronic-grade polycrystalline silicon is required to be less than 15 ppbw (ppbw stands for parts per billion by weight). In the method for fracturing polycrystalline silicon of the present invention, when fracturing polycrystalline silicon, the polycrystalline silicon needs to be placed in the water, therefore the metal impurities in the residual water, which generally remains on the surface of the fractured polycrystalline silicon taken out from the water in the water tank, remains on the surface of the polycrystalline silicon after drying. It is assumed that the thickness of water film is d, the linear dimension of the fractured polycrystalline silicon lump is D, and the concentration of metal impurities in the water is C, then the content of the surface metal impurities is about dC/D, that is, the content of the residual metal impurities on the surface of polycrystalline silicon (due to the residual water) is in direct proportion to the concentration of the metal impurities in the water. Therefore, the contamination of polycrystalline silicon due to the water can be reduced in the process of fracturing by using pure water with low content of metal ions.

(11) The present invention further provides an apparatus for fracturing polycrystalline silicon, which comprises a high-voltage transformer, a high-voltage rectifier, a charging capacitor, a disconnecting switch, a water tank containing water, and a first electrode and a second electrode which are submerged in the water, the first electrode and the second electrode being disposed with a certain distance therebetween, and the distance between the first and second electrodes being a discharge gap of the water tank, wherein

(12) a primary winding of the high-voltage transformer is connected to mains supply, a first terminal of a secondary winding of the high-voltage transformer is sequentially connected to the high-voltage rectifier, the disconnecting switch and the first electrode, a second terminal of the secondary winding is grounded and connected to the second electrode, and the charging capacitor is connected between a common terminal of the high-voltage rectifier and the disconnecting switch and a common terminal of the secondary winding and the second electrode.

(13) Embodiment 1:

(14) The present invention provides an apparatus for fracturing polycrystalline silicon, as shown in FIG. 1, the apparatus comprises a high-voltage transformer B, a charging resistor R, a high-voltage rectifier G, a charging capacitor C, a disconnecting switch K, a water tank F, and a first electrode 1 and a second electrode 2 which are submerged in the water, wherein the water tank F contains water, and the first electrode and the second electrode are disposed opposite to each other in the water tank.

(15) Here, a primary winding of the high-voltage transformer B is connected to mains supply, a first terminal of a secondary winding of the high-voltage transformer is sequentially connected to the charging resistor R, the high-voltage rectifier G, the disconnecting switch K and the first electrode 1, a second terminal of the secondary winding is grounded and connected to the second electrode 2, and the charging capacitor C is connected between a common terminal of the high-voltage rectifier G and the disconnecting switch K and a common terminal of the secondary winding and the second electrode 2. In other words, one terminal of the charging capacitor is connected to the common end of the high-voltage rectifier G and the disconnecting switch K, and the other terminal of the charging capacitor is connected to the second terminal of the secondary winding.

(16) Here, the capacitance of the charging capacitor may be selected based on the energy required for breaking the polycrystalline silicon into fragments of desired sizes, which can be calculated according to the formula: discharge energy E=0.5 U.sup.2C. In the above formula, U denotes discharging voltage, and C denotes high-voltage pulse capacitance. In general, discharge energy varies in a range of 1100 kJ, and preferably in a range of 432 kJ, thereby according to the above formula, the capacitance of the charging capacitor may be selected based on an upper limit of the discharge energy and an upper limit of the discharging voltage. For example, when the upper limit of the discharge energy E is set as 20 kJ and the upper limit of the voltage-adjusting range is 200 kV (i.e. the breakdown voltage of the disconnecting switch is 200 kV), the capacitance of the charging capacitor C is C=2 E/U.sup.2=1 F. As another example, when the upper limit of the discharge energy E is set as 8 kJ and the upper limit of the voltage-adjusting range is 20 kV (i.e. the breakdown voltage of the disconnecting switch is 20 kV), the capacitance of the charging capacitor C is C=2 E/U.sup.2=40 g. In this embodiment, the capacitance of the charging capacitor is 0.5 F.

(17) Here, the discharge gap (i.e. auxiliary discharge gap) of the disconnecting switch is mainly used for isolation, and in the present invention, there are some requirements for the selection of the auxiliary discharge gap, since isolation effect cannot be achieved with a too small auxiliary discharge gap, and breakdown effect cannot be realized within a specified voltage range with a too large auxiliary discharge gap. Also, there are some requirements for the selection of the discharge gap of the water tank (i.e. main discharge gap), since a too small main discharge gap may give rise to electrode erosion, and a too large main discharge gap requires a greatly increased critical breakdown voltage of the main discharge gap, such that the voltage level and the insulation level of the entire electrical equipment are increased, thus pushing up the cost for fracturing eventually.

(18) Moreover, it is necessary to ensure that the critical breakdown voltage of the auxiliary discharge gap should be larger than that of the main discharge gap. In this way, the main discharge gap is broken down as soon as the auxiliary discharge gap is broken down, thus achieving an instant (on the order of s) discharging. If the main discharge gap cannot be broken down, relevant parameters need to be adjusted, for example, the auxiliary discharge gap is increased, or the main discharge gap is decreased, or both gaps are adjusted at the same time.

(19) Preferably, the discharge gap of the disconnecting switch (i.e. auxiliary discharge gap) is in a range of 1050 mm, the breakdown voltage of the disconnecting switch is in a range of 30200 kV and the discharge gap of the water tank (i.e. main discharge gap) is in a range of 3080 mm.

(20) As the water in the water tank F, pure water is adopted, in which the electrical resistivity of the water is no less than 18.2 M.Math.cm, the content of SiO.sub.2 is no greater than 10 g/L, the content of Fe is no greater than 1.0 g/L, the content of Ca is no greater than 1.0 g/L, the content of Na is no greater than 20 g/L, and the content of Mg is no greater than 1.0 g/L.

(21) Preferably, a screen mesh is provided at the bottom of the water tank, and the hole size of the screen mesh is in a range of 25100 mm. In this way, after one instant high-voltage discharging happens, qualified fractured polycrystalline silicon can be filtered out by the screen mesh, while the fractured polycrystalline silicon with a size larger than the hole size of the screen mesh remains in the water tank for the next fracturing.

(22) Embodiment 2

(23) This embodiment provides a method for fracturing polycrystalline silicon which can be implemented by using the apparatus in Embodiment 1.

(24) The method comprises the steps of:

(25) step 1: filling the water tank with water taking up approximately of the volume of the water tank, then placing the polycrystalline silicon in the water such that the polycrystalline silicon is submerged in the water;

(26) step 2: applying an instant high voltage to the water tank, the intensity of the electric field generated by the instant high voltage being greater than or equal to the critical electric field intensity of the water in the water tank, wherein the specific steps are as follows:

(27) a. The charging capacitor C is charged by the mains supply which has been converted by the high-voltage transformer B and then been rectified by the high-voltage rectifier G;

(28) b. Once the voltage of the charging capacitor reaches the breakdown voltage of the disconnecting switch K, the disconnecting switch K is broken down, and at this point, all of the energy stored in the capacitor C is applied between the first electrode 1 and the second electrode 2 in the water tank;

(29) c. When the intensity of electric field between the first electrode 1 and the second electrode 2 is greater than or equal to the critical electric field intensity of the water in the water tank, the strong shock wave generated by the high-voltage electrostatic discharge occurring drastically in the water tank F can fracture the polycrystalline silicon instantly; and

(30) d. Steps ac are repeated until all of the polycrystalline silicon is fractured; and

(31) Step 3: taking out the fractured polycrystalline silicon and drying the same.

(32) In the embodiment, the discharge gap of the disconnecting switch (i.e. auxiliary discharge gap) is 20 mm, the discharge gap of the water tank F (i.e. main discharge gap) is 50 mm, and the breakdown voltage of the disconnecting switch varies in the range of 30200 kV. The resulting fracturing effect of polycrystalline silicon by using the method is illustrated in table 2.

(33) TABLE-US-00002 TABLE 2 Average particle size Breakdown of polycrystalline Distribution of voltage of Main Auxiliary silicon (mm) fractured disconnecting discharge discharge Before After polycrystalline switch (kV) gap (mm) gap (mm) fracturing fracturing silicon 30 50 20 130 0-98 0-25 mm: 3%; 25-50 mm: 4%; 50-100 mm: 77%; above 100 mm: 16% 80 50 20 130 0-84 0-25 mm: 3.5%; 25-50 mm: 5%; 50-100 mm: 91.5% 130 50 20 130 0-68 0-25 mm: 12.5%; 25-50 mm: 20%; 50-100 mm: 67.5% 180 50 20 130 0-60 0-25 mm: 16%; 25-50 mm: 25%; 50-100 mm: 59% 200 50 20 130 0-48 0-25 mm: 21%; 25-50 mm: 36.5%; 50-100 mm: 43.5%;

(34) Table 2 illustrates the fracturing effect of polycrystalline silicon in the case that the main discharge gap and the auxiliary discharge gap remain unchanged and the breakdown voltage of the disconnecting switch is increased gradually. It can be inferred from Table 2 that the linear dimension of the fractured polycrystalline silicon decreases as the breakdown voltage of the disconnecting switch increases. It is thus obvious that the breakdown voltage of the disconnecting switch is a key factor that affects the fracturing effect of polycrystalline silicon.

(35) It should be noted that the method in the embodiment can also be implemented by using other apparatuses, not limited to the apparatus illustrated in the embodiment.

(36) Embodiment 3

(37) This embodiment provides a method for fracturing polycrystalline silicon which can be implemented by using the apparatus in Embodiment 1.

(38) The steps in the method of the embodiment are basically the same as those in Embodiment 2, except that in the embodiment, the breakdown voltage of the disconnecting switch is 80 kV, the discharge gap of the water tank F (i.e. main discharge gap) is 50 mm, and the discharge gap of the disconnecting switch (i.e. auxiliary discharge gap) varies in the range of 1050 mm. The resulting fracturing effect of polycrystalline silicon by using the method is illustrated in Table 3.

(39) TABLE-US-00003 TABLE 3 Average particle size Breakdown of polycrystalline Distribution of voltage of Main Auxiliary silicon (mm) fractured disconnecting discharge discharge Before After polycrystalline switch (kV) gap (mm) gap (mm) fracturing fracturing silicon 80 50 10 130 0-90 0-25 mm: 3%; 25-50 mm: 5%; 50-100 mm: 84%; above100 mm: 8% 80 50 20 130 0-84 0-25 mm: 3.5%; 25-50 mm: 5%; 50-100 mm: 91.5% 80 50 30 130 0-81 0-25 mm: 4.5%; 25-50 mm: 8%; 50-100 mm: 87.5% 80 50 40 130 0-78 0-25 mm: 8%; 25-50 mm: 11%; 50-100 mm: 81% 80 50 50 130 0-73 0-25 mm: 15%; 25-50 mm: 13.5%; 50-100 mm: 71.5%;

(40) Table 3 illustrates the fracturing effect of polycrystalline silicon in the case that the main discharge gap and the breakdown voltage of the disconnecting switch remain unchanged and the auxiliary discharge gap is increased gradually. It can be inferred from Table 3 that the linear dimension of the fractured polycrystalline silicon decreases as the auxiliary discharge gap increases. It is thus obvious that the auxiliary discharge gap is a key factor that affects the fracturing effect of polycrystalline silicon.

(41) Embodiment 4

(42) This embodiment provides a method for fracturing polycrystalline silicon which can be implemented by using the apparatus in Embodiment 1.

(43) The steps in the method of the embodiment are basically the same as those in Embodiment 2, except that, in the embodiment, the discharge gap of the disconnecting switch (i.e. auxiliary discharge gap) remains 20 mm, the breakdown voltage of the disconnecting switch varies in the range of 30200 mm, and meanwhile the discharge gap of the water tank F (i.e. main discharge gap) varies in the range of 3080 mm. The resulting fracturing effect of polycrystalline silicon by using the present method is illustrated in Table 4.

(44) TABLE-US-00004 TABLE 4 Average particle size Breakdown of polycrystalline Distribution of voltage of Main Auxiliary silicon (mm) fractured disconnecting discharge discharge Before After polycrystalline switch (kV) gap (mm) gap (mm) fracturing fracturing silicon 30 30 20 130 0-92 0-25 mm: 3%; 25-50 mm: 5.5%; 50-100 mm: 76.5%; above 100 mm: 15% 80 50 20 130 0-84 0-25 mm: 3.5%; 25-50 mm: 5%; 50-100 mm: 91.5% 130 60 20 130 0-80 0-25 mm: 8%; 25-50 mm: 10%; 50-100 mm: 82% 180 70 20 130 0-78 0-25 mm: 11%; 25-50 mm: 12%; 50-100 mm: 77% 200 80 20 130 0-76 0-25 mm: 13%; 25-50 mm: 14%; 50-100 mm: 73%;

(45) Table 4 illustrates the fracturing effect of polycrystalline silicon in the case that the auxiliary discharge gap maintains unchanged and both the main discharge gap and the breakdown voltage of the disconnecting switch are increased gradually. It can be inferred from Table 3 that the linear dimension of the fractured polycrystalline silicon decreases gradually.

(46) In addition, it can be seen from a comparison between the fracturing effects in Table 4 and Table 2 that, the linear dimension of the fractured polycrystalline silicon in Table 2 is smaller than that in Table 4 under the condition of the same breakdown voltage of the disconnecting switch, the same auxiliary discharge gap, and different main discharge gap. Therefore, it can be concluded that the linear dimension of the fractured polycrystalline silicon decreases as the breakdown voltage of the disconnecting switch increases; the linear dimension of the fractured polycrystalline silicon increases as the main discharge gap increases; and the breakdown voltage of the disconnecting switch has a greater impact on the fracturing effect of polycrystalline silicon than the main discharge gap in a state with experimental parameters in Table 4.

(47) It should be understood that above implementations are merely exemplary implementations used to explain the principle of the present invention, however, the present invention are not limited thereto. Various modifications and improvements may be made by those skilled in the art without departing from the spirit and substance of the present invention, and such modifications and improvements are also deemed as the protection scope of the present invention.