GB2207062A

GB2207062A - Reducing heat losses and thermal strain during zone refining

Info

Publication number: GB2207062A
Application number: GB08717053A
Authority: GB
Inventors: David Ainsworth Hukin
Original assignee: Elkem ASA
Current assignee: Elkem ASA
Priority date: 1987-07-20
Filing date: 1987-07-20
Publication date: 1989-01-25
Anticipated expiration: 2007-07-20
Also published as: GB2207062B; GB8717053D0

Abstract

A method for reducing heat losses and thermal strain during horizontal zone refining of silicon in a water cooled cold metal boat comprises including a layer of thermally insulating material between the silicon and the inner faces of the boat, the thermal insulating material having a lower melting point than silicon. Preferably the thermal insulating material is calcium silicate or compounds and eutectics formed between binary or ternary mixed alkaline earth fluorides and oxides. A boat having such a layer is also described.

Description

"REDUCING HEAT LOSSES AND THERMAL STRAIN DURING ZONE REFINING" The present invention relates to a method and apparatus for reducing heat losses and thermal strain during, for example, horizontal levitational zone refining and crystallization of silicon in a water cooled cold metal boat.

It is known to produce ingots of solar grade silicon by horizontal levitational zone refining and subsequent crystallization in a cold metal boat. The ingots produced by this method can later on be further processed into solar cells.

In this known method of producing solar grade silicon ingots, granular silicon is placed within a metal boat constructed from a plurality of water cooled copper tubes with electrical insulation inserted between adjacent faces of the tubes. The boat containing the granular silicon is then inserted into a steel chamber, which is evacuated whereafter an inert gas such as for example argon is backfilled into the chamber. A stationary coil having at least three turns is supplied by an oscillating radio frequency current.

The boat is then passed through the coil at a controlled speed. The oscillating electromagnetic field which is formed within the volume enclosed by the coil, will induce circulating electric currents in each of the copper tubes of the boat. The current induced in the copper tubes of the boat will then create a secondary induced current to flow in the charge contained in the water-cooled metal boat. The electrical energy losses in the copper tubes are low due to the low electrical resistivity of copper.

The electric currents which are flowing through the adjacent surfaces of the copper tubes and the charge are parallel and produce electromagnetic repulsion between the two conductors. Thereby the radio frequency current produces electric current which both melts the charge and at the same time causes the molten charge to be repelled from contact with the water cooled copper.

This technique makes it possible to totally levitate the molten zone in horizontal zone refining of silicon. The molten zone will thereby not be in contact with the metal of the boat and hence freedom from contamination of silicon by the material of the boat is guaranteed.

However, the above described process suffers from two main disadvantages. As the metal of the cold boat is kept at room temperature during the melting of silicon, the energy losses due to radiation and conduction are large. Thus a considerable part of the energy supplied by the induction coil will be transferred to the cooling water flowing in the water cooled metal boat. Further, due to the high temperature difference between the inner surface of the cold boat and the molten silicon, the solidifying silicon is subject to strain, and the outer layer of the produced silicon ingot will cool very rapidly and a layer of small grained silicon will grow in this outer part of the ingot. It will therefore be necessary to cut away this outer layer if the ingot is to be further processed into solar cells.

It is an object of the present invention to provide a method and apparatus in which the above mentioned disadvantages are overcome or substantially reduced.

Accordingly, the present invention relates to a method and apparatus for reducing heat losses and thermal strain during horizontal levitational zone refining of silicon in a water cooled cold metal boat and the method is characterised in that a layer of a thermally insulating material is included between the silicon and the boat, the thermally insulating material being made from a material having a lower melting point than silicon.

According to a first aspect of the invention, there is provided a cooled boat for use in horizontal zone refining of a conductive material, having a layer of thermally insulating material intermediate the inner surface of the boat and the material to be refined, the insulating material having a lower melting point than the material to be refined.

According to a second aspect of the invention, there is provided a method of reducing heat losses and thermal strain during horizontal zone refining of a conductive material in a cooled boat comprising positioning a layer of thermally insulating material intermediate the inner surface of the boat and the material being refined, the insulating material having a lowert melting point than the material being refined.

The thermally insulating material may be of any form suitable for lining the boat prior to loading the silicon into the boat such as for example, woven cloth, felt, web or powders which may be sprayed onto the walls of the boat or simply placed as a layer covering the base.

As the cold metal boat is always kept at room temperature, the insulating material in contact with the metal boat will always be in the solid state. On the other hand, the insulating material which is in contact with molten silicon, will be in the liquid state. Hence there will exist a solid/liquid interface within the layer of insulating material at a position determined by its thermal conductivity.

Since the portion of insulating layer adjacent to the molten silicon remains liquid whilst the silicon solidifies, crystallization of the silicon occurs with fewer nucleation sites and there is a resultant increase in crystal grain size of the zone refined silicon. The presence of the insulating layer between the water cooled metal boat and the solidified silicon also reduces the otherwise severe temperature gradients experienced by the ingot and facilitates stress removal by annealing.

The layer of thermal insulating material will substantially lower the heat losses due to radiation and conduction. The energy consumption for production of silicon ingots by horizontal levitational zone refining may, by the method of the present invention, be reduced by up to 30%.

Any thermal insulating material having a lower melting point than silicon, and which does not contain electronically active elements which may be dissolved by the silicon, are suitable for use in connection with the method of the present invention. In addition the thermal insulating material should display as many of the following preferable features as possible: (A) Non-reactive or non-contaminating with molten silicon, but capable of acting as a fluxing solvent for impurities in the molten silicon.

(B) Low vapour pressure at the melting point of silicon.

(C) Non-toxic and not industrially hazardous.

(D) Non-hygroscopic.

(E) Liquid density higher than silicon.

(F) Inexpensive and readily available.

Examples of materials which confirm with these criteria are based on low melting point silicates and the series of low melting point compounds and eutectics formed between binary or ternary mixed alkaline earth fluorides and oxides.

Examples of suitable materials include the following where the composition is given in mol %: CaF2 41.9 % CaO 38.1 % SiO2 12.2 % melting point 11040C CuF2 28.5 % CaO 54.2 % SiO2 12.2 % melting point 11060C CaF2 53 % CaSiO3 47 % melting point 11280C CaF2 49 % MgF2 51 % melting point 9450C CaF2 35 % MgF2 35 % BaF2 30 % melting point 7950C BaF2 16 % CaF2 17 % LiF2 67 % melting point 7100C CaF2 74.8 % CaO 25.5 % melting point 13600C CaF2 87 % MgO 18 % melting point 13500C The method and apparatus of the present invention will now be further described in connection with the accompanying drawing in which: Figure 1 shows a cross-sectional view through a water cooled cold boat containing molten silicon.

In Figure 1, there is shown a water cooled cold metal boat in which sides and base are built up from a plurality of copper tubes 1 of square cross-section.

Sheets 2 of electrical insulation material are inserted between adjacent faces of the tubes 1. The boat is, during zone refining, passed slowly through an induction coil 3 which is supplied by radio frequency power from a generator (not shown) through a lead 4.

The magnetic field induced by the coil 3 will couple to the silicon 5 contained in the boat and the silicon enclosed by the induction coil will melt and be totally levitated from the boat. A thermal insulating material 6, 7 having a melting point lower than the melting point of silicon is placed in the boat prior to loading the boat with granular silicon.

The thermal insulating material in contact with the silicon will melt when the silicon is heated above the melting point of the insulating material. On the other hand, the thermal insulating material in contact with the inner faces of the boat will always be kept at room temperature. Hence there will exist a solid/liquid interface within the layer of insulating material at a position determined by its thermal conductivity. The liquid and solid layer of the thermal insulating material are shown by reference number 6 and 7 in Figure 1.

When the boat now is further passed through the induction coil 3, the molten silicon will start to solidify while the thermal insulating material in contact with silicon still is in the liquid state.

Thus there will be no thermal strain on the solidifying silicon and the silicon will be free to expand during solidification. The layer of small grain at the outer parts of the produced silicon ingot will be substantially reduced due to the reduced number of nucleation sites. Finally, as described above, the heat loss from the silicon will be reduced by up to 30% and thereby the energy efficiency of the process is greatly improved.

The invention encompasses the following clauses: 1) A method for reducing heat losses and thermal strain during horizontal levitational zone refining of silicon in a water cooled cold metal boat, characterised in that a layer of thermally insulating material is included between the silicon and the inner faces of the boat, the thermally insulating material having a lowert melting point than silicon.

2) A method according to clause 1, characterised in that the thermally insulating material is in the form of a woven cloth, felt or web.

3) A method according to clause 1, characterised in that the insulating material is in the form of powder which is sprayed onto the walls of the boat before granular silicon is loaded into the boat.

4) A method according to clause 1-3, characterised in that a calcium silicate having a melting point lower than the melting point of silicon is used as thermally insulating material.

5) A method according to clause 1-3, characterised in that compounds or eutectics formed between binary or ternary mixed alkaline earth fluorides and oxides is used as thermally insulating material.

6) A method according to clause 5, characterised in that a mixture of 41.9 mol % CaF2, 38.1 mol % CaO and 12.2 mol % SiO2 is used as thermally insulating material.

7) A method according to clause 5, characterised in that a mixture of 28.5 mol % CaF2, 59.2 mol % CaCo and 12.2 mol % SiO2 is used as thermally insulating material.

8) A method according to clause 5, characterised in that a mixture of 53 mol % CaF2 and 47 mol % CaSiO3 is used as thermally insulating material.

9) A method according to clause 5, characterised in that a mixture of 35 mol % CaF2, 35 mol % MgF2 and 30 mol % BaF2 is used as a thermally insulating material.

10) A method according to clause 5, characterised in that a mixture of 16 mol % BaF2, 17 mol % CaF2 and 67 mol % LiF2 is used as a thermally insulating material.

11) A method according to clause 5, characterised in that a mixture of 82 mol % CaF2 and 18 mol % MgO is used as a thermally insulating material.

Claims

1. A cooled boat for use in horizontal zone refining of a conductive material, having a layer of thermally insulating material intermediate the inner surface of the boat and the material to be refined, the insulating material having a lower melting point than the material to be refined.

2. A boat as claimed in Claim 1 in which the insulating material is in the form.of a woven cloth, felt or web.

3. A boat as claimed in Claim 1 in which the insulating material comprises a sprayed-on powder.

4. A boat as claimed in any one of Claims 1 to 3 in which the insulating material in calcium silicate.

5. A boat as claimed in any one of Claims 1 to 3 in which the insulating material comprises compounds or eutectics formed between binary or ternary mixed alkaline earth fluorides and oxides.

6. A boat as claimed in Claim 5 in which the insulating material comprises a mixture of 41.9 mol % CaF2, 38.1 mol % CaO and 12.2 mol % SiO2.

7. A boat as claimed in Claim 5 in which the insulating material comprises a mixture of 28.5 mol % CaF2, 59.2 mol % CaO and 12.2 mol % SiO2.

8. A boat as claimed in Claim 5 in which the insulating material comprises a mixture of 53 mol % CaF2 and 47 mol % CaSiO3.

9. A boat as claimed in Claim 5 in which the insulating material comprises a mixture of 35 mol % CaF2, 35 mol % MgF2 and 30 mol % BaF2.

10. A boat as claimed in Claim 5 in which the insulating material comprises a mixture of 16 mol % BaF2, 17 mol % CaF2 and 67 mol % LiF2.

11. A boat as claimed in Claim 5 in which the insulating material comprises a mixture of 82 mol % CaF2 and 18 mol % MgO.

12. A method of reducing heat losses and thermal strain during horizontal zone refining of a conductive material in a cooled boat comprising positioning a layer of thermally insulating material intermediate the inner surface of the boat and the material being refined, the insulating material having a lower melting point than the material being refined.

13. A method as claimed in Claim 12 in which the insulating material has an internal solid/liquid interface intermediate the material being refined and the boat.

14. A method as claimed in any one of Claims 12 to 14 in which the material being refined is silicon.

15. A coold boat substantially as specifically described.

16. A method of reducing heat losses and thermal strain during horizontal zone refining substantially as specifically described.