Physical method solar cell polysilicon

What is magnetic field division?

admin on April 12, 2022 0 Comments

What is magnetic field division?

It is a method to separate impurities (mainly non-metallic) by using the action of electromagnetic field.

  1. Constant magnetic field method

Liquid silicon is placed in a uniform magnetic field. The electromagnetic field induction intensity B acts outward in the plane direction, and a current is applied in the direction perpendicular to the magnetic field. The current density is J. from left to right, the Lorentz force is the vector product of J and B. from top to bottom, a force opposite to the Lorentz force acts on the non-metallic inclusions in the liquid metal to make the inclusions move upward, which makes the smaller particles migrate and separate from the melt.

It must be pointed out that it is very difficult to generate a large separation force in a large melt, mainly because it is difficult to generate a strong uniform magnetic field. When the size of impurities is less than 50 µ m, the separation efficiency is quite low, which is the reason why electromagnetic purification has not been applied in practice. Using the magnetic field generated by modern superconductive technology can greatly improve the separation efficiency and separate inclusions with smaller size. The key problem is the uniformity of electromagnetic force distribution in large volume. If the force field is uneven, the electromagnetic force drives the melt to produce irregular movement and eddy current, resulting in uncontrollable conditions, the separation efficiency decreases, and even stirring.

Constant magnetic field method
Constant magnetic field method
  1. Alternating magnetic field method

When the silicon liquid is placed in the alternating magnetic field, the eddy current with the same frequency as the alternating magnetic field is induced in the metal. The eddy current interacts with the induced magnetic field to produce the electromagnetic force pointing to the center. Because the conductivity of the non-metallic inclusions in the silicon liquid is much lower than that in the metal liquid, the induced current in the inclusions is close to zero and is not affected by the electromagnetic force itself: the centripetal force on the metal liquid makes the inclusions subject to the reaction force in the opposite direction, It also becomes electromagnetic extrusion force. The inclusion moves outward in the direction of inverse electromagnetic force, which is biased near the outer container wall, separated from the liquid metal, and the metal in the heart is purified. Its biggest advantage is that there is no need to set another circuit to conduct the applied current or induced current, there is no electrode pollution, and the magnetic induction intensity is easy to adjust. The relationship between alternating current and induced magnetic field intensity is as follows:

Be=µNI/L

Where, be is the effective magnetic induction intensity; µ is the permeability of the melt; N is the number of coil turns; I is the applied alternating current and l is the coil length.

Due to the skin effect of Wison current, the density of induced current is different at different radial positions in the melt. The induced current density on the outside is large, the electromagnetic extrusion pressure is large, the outward movement speed of inclusions is fast, the electromagnetic extrusion pressure at the center is small, and the movement speed of inclusions is slow. The thickness of skin layer is related to frequency. The higher the frequency, the smaller the thickness of skin layer, the smaller the penetration depth of magnetic field, and the more uneven the electromagnetic extrusion pressure, Affect the purification effect; Moreover, the magnetic induction intensity is also different in the length direction of the induction coil. The magnetic induction intensity at both ends is weak and strong in the middle. The shorter the induction coil is, the more uneven the magnetic induction intensity is.

When the volume of the melt is large, the more uneven the electromagnetic extrusion force in the melt, and even lead to the irregular movement of the melt and the formation of stirring. At present, the melt volume of the purification device used in the research is very small, the diameter of the separator is only a few millimeters, the difference between the thickness of the skin layer and the diameter of the melt is small, and the skin effect is not obvious. The experiment shows that when the diameter of the separator is greater than three times the thickness of the skin layer, the separation efficiency decreases rapidly, and the large diameter of the separator increases the moving distance of inclusion particles, And it will affect the flow state of the melt: if the pipe diameter is too small, it is easy to cause the deposition of inclusions and reduce the separation efficiency.

AC / DC magnetic field detector
AC / DC magnetic field detector
  1. Traveling wave magnetic field method

The traveling wave magnetic field is similar to the expanded three-phase asynchronous motor stator. The traveling wave magnetic field induces induced current in the liquid metal and drives the liquid metal in the traveling wave direction. When the liquid metal flow pipe is perpendicular to the traveling wave direction, the inclusions in the liquid metal move to the pipe wall in the opposite direction of the electromagnetic force under the action of the electromagnetic force induced by the traveling wave magnetic field. The continuous purification of melt can be realized by using traveling wave magnetic field.

  1. Rotating magnetic field separation method

There is no essential difference between separating impurities by rotating magnetic field and rotating magnetic field electromagnetic stirring. A rotating electromagnetic stirrer is installed on the outside of the barrel container. The rotating magnetic field electromagnetic force causes the liquid to rotate and generate centrifugal force, which is separated by the density difference between impurities and metal. Rotating magnetic field separation does not rely on the extrusion force generated by electromagnetic induction, but mainly on the centrifugal force generated by electromagnetic stirring. Its separation efficiency depends on the particle size of inclusions and the density difference between inclusions and metal melt. This method has the advantages of no contact pollution, continuous purification treatment and easy removal of impurities. It has been used in continuous cast steel with good effect. It is one of the most practical purification methods. Electromagnetic induction separation requires that the conductivity of inclusions is much lower than that of liquid metal itself, while electromagnetic centrifugal separation requires that the density of inclusions is much lower than that of liquid metal itself.

  1. Light quantum Division

The light quantum electric separation method is a method that uses the quantum action of light to change the state of impurities and separate them.

Impurity x, which is difficult to remove, mainly forms silicon compounds with silicon in silicon solution. The main process of the light quantum ionization removal method is: under the irradiation of special wave band light, the silicon x bond splits. The light acting on these chemical bonds is a microscopic quantum effect with selectivity, and there is a quantum state phenomenon. According to the equal energy driving principle of the quantum state model, only when the energy of photons is exactly equal to the energy required for the vibration and rotation of silicon x-bond splitting, resonance absorption is more likely to occur, resulting in energy state transfer.

We first analyze the internal molecules of silicon liquid, and then analyze the specific process under light. According to the equal energy driving principle, when the driving energy provided by light is just equal to the binding energy between Si: X bonds, it is conducive to the splitting of Si: X bonds. Let’s see which wavelength of light corresponds to this energy. We use the corresponding band light source to split the Si: X bond, and the cost of generating high-power variable band light in practical application is relatively expensive. When we use the commonly used tungsten halogen lamp, the light source of tungsten halogen lamp is a blackbody radiation, similar to sunlight. The radiation energy density is only related to the frequency and the absolute temperature of the object, and has nothing to do with the shape of the blackbody and the substances that make up the blackbody: the higher the temperature is, the greater the radiation energy density is, the more the energy peak moves towards the short wave direction, and its radiation energy h is Planck constant; V is the frequency of radiation; Δ V is the frequency band occupied by radiation; T is the thermodynamic temperature; K is the Boltzmann constant. The first term in braces is called Bose Einstein term, and the latter is called “zero point energy”.

In this way, the required wave band can be controlled according to the different temperatures of the tungsten halogen lamp, so that the impurities can be separated from the silicon in an ionized state, and then assisted by electromagnetic action to separate and remove the impurities

The goal of the above methods is to remove the impurities in silicon that affect the efficiency of solar cells. Of course, there are other methods, such as moving the point impurities in a certain direction under the action of external electric field, and then separating them. In addition, the acid washing method used in hydrometallurgy is to grind the metal silicon into powder and remove the metal impurities with acid washing. This method is actually a chemical method and has pollution, so it should be used with caution. If it is used, pay attention to the problem of pollution.

Each of the above methods has its own characteristics. As for the application in production, the specific selection of production cost and index requirements should be considered. The current process is generally: Mr. produces metal silicon with low impurities (especially P and b), and then uses physical methods such as blowing method, slagging method, vacuum smelting purification method and directional solidification method to produce 5N ~ 6N silicon, in which P and B impurities should be strictly controlled. Finally, after ingot casting, it can enter the next process such as slice cutting. The key is the relationship between purification cost and purity: the higher the purity requirement, the greater the purification cost; The purity requirement is not high, and the smaller the purification cost is. This is the key criterion for process selection.

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