WO2008115072A2

WO2008115072A2 - Electrolyte and method for electrochemical refining of silicon

Info

Publication number: WO2008115072A2
Application number: PCT/NO2008/000105
Authority: WO
Inventors: Espen Olsen
Original assignee: Sinvent AS
Current assignee: Sinvent AS
Priority date: 2007-03-21
Filing date: 2008-03-17
Publication date: 2008-09-25
Anticipated expiration: 2009-09-21
Also published as: WO2008115072A3; NO20071496L; NO328263B1

Abstract

The invention discloses an electrolyte for electrochemical refining of a material containing silicon at a temperature above the melting point of silicon in a three layer electrochemical cell. The electrolyte comprises 50-100% by weight of at least one alkaline earth metal fluoride selected from BaF2, CaF2, SrF2 and MgF2. The invention also comprises the method and the electrochemical cell involved in the electrochemical refining of silicon.

Description

"Electrolyte and method for electrochemical refining of silicon".

The invention discloses an electrolyte for electrochemical refining of a material containing silicon at a temperature above the melting point of silicon in a three layer electrochemical cell. The invention also comprises the method and the electrochemical cell involved in the electrochemical refining of silicon.

The rapid growth of the solar cell-, or photovoltaic (PV) industry at an annual rate of more than 30% since the early 1980's has generated a shortage of silicon with the right quality and price for the continued growth of this industry. A number of processes have been proposed to yield such a material, but so far only a modification of the so called Siemens process has been shown to give material with high enough quality with regard to impurities. This technology is, however, associated with solid to gas to solid phase transitions, which unavoidably lead to high non-reversible thermodynamic losses. The quality of the material from this process is, however, of very high purity, better than what is actually needed for the PV industry. On the other hand, metallurgical (MG) methods are being developed, but these have not yet been shown to exhibit significantly lower production costs compared with the Siemens process, combined with acceptable purity. The continued growth of the PV industry is invariably dependent on further reductions in the price of solar electricity, and there is a need for a low-cost silicon production process which is capable of producing larger amounts of silicon with an acceptable quality and to a substantially lower price than can be achieved today. The quality requirements are in general considered to be 99.99% (metals base) and with a particular emphasis on impurities of boron (B) and phosphorous (P) which should be below 0.5 and 1 ppmw, respectively. These elements are characterized by distribution coefficients close to 1 which makes them difficult, if not impossible, to remove by zone refining. On the other hand, this particular trait is used during the doping process of silicon, yielding a low gradient in dopant concentration through the solidified ingot due to the small segregation effects.

Electrochemical refining of aluminium is carried out in the so called "3-layer process" invented by Hoopes in the 1920s, where metallurgical grade Al is alloyed with Cu to give a heavy alloy which is polarized anodically under a layer of a chloride and/or fluoride based electrolyte. On top of this, a layer of pure Al is deposited cathodically. The process runs at ~750°C and is shown schematically in figure 1. Three molten layers; - alloy at the bottom, an intermediate layer of fluoride electrolyte and a top layer of refined metal is maintained by differences in density (p).

The process yields a product with purity in the range of 99.999% by weight in industrial scale electrochemical cells (10-5OkA).

The present invention is based on this technical principle. Electrochemical refining of silicon (Si) has been performed with a successful result. The mechanism can be described by the following: A molten alloy of impure metallurgical grade silicon (MG-Si) and a heavy, noble metal (i.e. Cu) is placed at the bottom of a reactor with a layer of molten fluoride based electrolyte on top. On top of this, a layer of the molten, pure solar grade silicon is positioned. The heavy alloy in the bottom of the cell is polarized anodically. This causes the silicon in the alloy to dissolve anodically into the electrolyte as electrons are extracted from the system, as described by equation [1].

Si (I) → Si⁴⁺ (diss) + 4e^" . E_rev, i₇ooκ = +1.6V [1]

Along with Si, all the less noble metals will dissolve. To be more specific, all impurities with E_rev^>E_rev,sι will, according to thermodynamics, dissolve. These elements are Na, K, Ca, Ba among others. Elements with E_rev<E_re_V,sι more noble than Si, for example Cu, Fe, Ni, P and B will not dissolve, and thereby stay in the anodically polarized alloy until all Si has been consumed if not replenished during the process. Thus, the required low levels of B and P in the refined material can be achieved.

The top layer of pure, molten Si is polarized cathodically. At this electrode, the opposite happens; electrons are injected into the cell and the reaction is described by Eq. [2]. Si⁴⁺ (diss) + 4e^" → Si (I) E_rev, 1700K = -1.6V [2]

Along with Si, the elements more noble than Si will be reduced and alloyed into the cathode metal, the less noble elements requiring more energy to be reduced out from the electrolyte. The total reaction taking place can be described by Eq [3]:

Sianode (I) → Si_cathode (0 E_rev, 1700K ⁼ OV [3]

The total reaction occurring is effectively that Si is transported from the heavy, impure alloy in the bottom of the cell, through the electrolyte as ions, for subsequent deposition as Si metal at the cathode at the top of the cell. The principle is depicted in Figure 2. The elements with E_rev, 1700K < E_rev, si (more noble than Si) are retained in the heavy alloy at the bottom of the cell, while the elements with E_rev^>Er_ev, si (less noble than Si) are retained in the electrolyte. In theory, the only energy required to run the process is that consumed by IR losses (heat) when current runs through the cell and that necessary to overcome the lower thermodynamic energy of the Si-Cu alloy compared with the pure substances. In practice, overvoltages due to chemical gradients will also be present. Compared to the IR losses of several volts, however, these are small (~0.1 V).

The two main differences when substituting aluminium with silicon will then be the temperature (>1412°C vs. 750⁰C) and the density of the product (2.6 g/cm³ vs. 2.2 g/cm³). The challenges to be met are related to the temperature tolerance and chemical inertness of the construction materials in the reactor and finding an electrolyte with the right density, viscosity and low electric and chemical losses.

The 3-layer principle has primarily been used for the refining of aluminium. Norwegian Patent NO 156 172 (T. Grong and J. K. Tuset, Ha og Lilleby smelteverk 1984) describes a method for refining silicon, by use of the three-layer process. The electrolyte is an oxide-based electrolyte containing SiO₂, CaO and BaO with small additions of CaF₂ and BaF₂ to enhance the low ionic electrical conductivity of the oxide based melt. Fluoride-based electrolyte is specifically mentioned not to be suitable due to formation of SiF₄. Refined silicon is produced and the 3-layer approach is demonstrated as feasible in the oxide-based electrochemical system. This electrolyte is however associated with a number of fundamental problems. One is related to the high viscosity of the glass-forming silica (SiO₂)-based melt. This makes it difficult to maintain three individual layers as the refined metal on the top of the cell tends to form "fingers" down towards the anode alloy which eventually makes contact and thereby shuts down the operation. The high viscosity, furthermore, leads to the formation of stagnant layers and high overvoltages due to chemical gradients. Heavy losses due to the formation of gaseous SiO have been noted. Severe material problems are also encountered as the furnace lining materials are attacked by the electrolyte, leading to cell failure.

Since then, new production methods for refractory materials have been developed. The present invention avoids/solves these problems by using reaction bonded silicon nitride as a construction material in areas of the cell where magnesia or chamotte does not provide enough chemical inertness.

Silicon nitride ceramic bodies of high purity and chemical inertness can be reaction sintered to ~20% porosity. This material exhibits properties which through the invention, has been shown to be well suited for the use as construction material for an electrochemical reactor (cell) based on the three-layer principle for refining of Si above 1400⁰C.

The object of the present invention is to produce refined silicon of high purity while avoiding problems with high electrolyte viscosity, chemical attack of the reactor construction materials and the formation of SiO (g). To avoid the formation and subsequent loss of gaseous SiO, a fluoride based electrolyte capable of dissolving SiF₄(g) in the form of stable SiF₆ ^2"-ions can be used. The electrolyte may contain a Si-bearing species. This may be added as SiO₂ in small amounts to lower the loss of SiO, or as M_xSiF₆, which will eliminate formation of SiO. (M is an alkali- (x=2) or alkaline earth (x=1) metal) The first aspect of this invention is an electrolyte for electrochemical refining of a material containing silicon at a temperature above the melting point of silicon, wherein the electrolyte comprises 50-100% by weight of at least one alkaline earth metal fluoride selected from BaF₂, CaF₂, SrF₂ and MgF₂.

One preferred embodiment of the invention is an electrolyte for electrochemical refining of a material containing silicon at a temperature above the melting point of silicon, wherein the electrolyte comprises 80-100% by weight of at least one alkaline earth metal fluoride selected from BaF₂, CaF_2, SrF₂ and MgF₂.

The electrolyte mixture must exhibit the following features:

i) Low vapour pressure at the temperature of operation

ii) Density between that of pure, refined silicon and the anode alloy

iii) Low viscosity and high ionic conductivity

iv) Ability to dissolve ionic species of Si without formation of gaseous SiF₄.

These criteria are met by electrolytes based primarily on alkaline earth flourides, either in pure state or as mixtures, also in mixtures with alkali metal fluorides. If mixtures are used, the mixtures must be designed in such a way that the vapour pressure is kept low in order to avoid electrolyte losses. The formation of SiF₄ will not create a problem, since SiF₄ will dissolve easily as the ion SiF₆ ^2" in fluorides exhibiting basic (non-acidic) properties. BaF₂ and SrF₂ both show higher densities in the molten state than silicon and may be used in their pure, molten state. MgF₂ and CaF₂ have lower densities than Si in their molten state and must, subsequently, be mixed with a heavier component to be used as the intermediate electrolyte layer in three-layer refining.

Different fluorides can be used, preferably the electrolyte comprises a mixture based on at least one of the following compounds; BaF₂, CaF_2, SrF₂ and MgF₂. The electrolyte will comprise at least one alkaline earth fluoride MF₂ (M=Mg, Ca,

Sr, Ba) in an amount of 50-80% , preferably 60-80% and with the possible addition of from 0 to 50 % by weight, preferably 0-20% of one or more of the other three.

The melting point of Si is 1412°C and thus, the temperature of the electrolyte should be in the range from 1412°C to the boiling point of the electrolyte.

At lower temperatures, the silicon will be in its solid state severely limiting the deposition kinetics. The boiling point of the electrolyte may be in the range from around 2000 to 2300⁰C.

A silicon bearing species may be added to the electrolyte to enhance the process kinetics from the start. This may also be omitted, in which case the product deposited from the start will be contaminated by impurities until sufficient amounts of Si-carrying ions have been transported from the bottom anode to the top cathode.

Thermodynamic calculations suggest that a silicon bearing species is not necessary, as silicon exhibits very high solubility in these electrolytes in the form of

SiF₆ ^2'-ions.

The silicon bearing species can be K₂SiF₆, Na₂SiF₆, CaSiF₆, BaSiF₆, SrSiF₆ or

SiO₂. The silicon bearing species is added in an amount from 0 to 20 % by weight of the electrolyte, preferably from 0.1 to 10% by weight.

Another aspect of the invention is a method for electrochemical refining of a material containing silicon wherein at least one fluoride from the alkaline earth metals is used.

The electrochemical refining is performed in a three-layer process comprising a molten bottom layer of silicon and a heavy, noble metal which is polarized anodically, a pure, molten layer of silicon polarized catiodically and an intermediate electrolyte layer.

A third aspect with the invention is an electrochemical cell (or reactor) for electrochemical refining of a material containing silicon at a temperature above the melting point of silicon, wherein the electrochemical cell possesses a lining comprised of reaction bonded SJsN₄. The porosity of the reaction bonded Si₃N₄ is in the range from 10 to 60% of the theoretical value, preferably within the range from 20 to 40% of the theoretical value where low cost methods may be employed to make the ceramic bodies.

Figures

Figure 1 shows a cell for three-layer refining

Figure 2 shows schematically the principle for three layer refining of silicon

Figure 3 is a schematic drawing of the electrochemical cell used in the experiments.

Experimental and results

The experiments were conducted in the electrochemical cell depicted in Figure 3.

Example 1

The amounts and specifics of the chemicals used can be found in Table 1.

Table 1: Chemicals used in the experiments

The CuSi alloy (80/20), which was to be anodically polarized, was made prior to the experiment by melting down 80Og of Cu and 20Og of Si in a graphite (AB10, Svensk spesialgrafit) crucible under argon atmosphere in an induction furnace. The furnace was evacuated before being filled with Ar (Ar 4.6, AGA). The molten alloy was quenched by pouring it into graphite crucibles kept at room temperature and with a diameter smaller than the inner diameter of the cell. The anode alloy was cut into the right weight before being transferred to the electrochemical cell container consisting of a graphite crucible with a high purity reaction sintered Si₃N₄ tubular lining. The electrolyte was pre-mixed and melted down under argon (Ar 5.0) before being crushed and filled on top of the anode alloy. 100 g Si was cut from an undoped multicrystalline ingot in the form of a cylinder and put on top of the crushed electrolyte. The aim of this large amount was to form a stable layer of Si covering the whole surface area of the underlying electrolyte to trap potential SiO being formed in the electrolyte. The cell was placed in a tubular furnace and melted down under Ar atmosphere (Ar 5.0). Mo current leads were screwed into the graphite crucible from the top and a graphite cathode which was lowered into the molten Si after melting. Current (4.0A) was passed through the cell for 22 hours. When a stable current was flowing, a cell voltage of 2.2 - 4.5V was recorded. Occasionally the current would stop flowing due to some kind of open curcuit or a passivating layer being formed. A slight movement of the cathode would restore the current to a stable value of around 2.2V. After termination of the experiments, the cathode would be left in the Si to cool. After cooling down, the cell was cut by a diamond saw to study the behaviour of the three layers. Three distinct layers were found. However, the cathodically polarized Si was found to be present as a sphere rather than a layer. This is probably due to the high surface tension of metals with high purity compared to that of the electrolyte. This is an artefact related to the small scale of the cell and will not be present in large scale cells. The anodically polarized alloy formed a stable layer at the bottom of the cell.

Example 2

Another experiment was performed in a larger cell, with an electrolyte having the following composition: 30% BaF₂ / 70% CaF₂.

The pure Si obtained was analyzed. The impurity level was surprisingly low. The results were: Al ~ 900 ppm, B ~ 11 ppm; Ca ~22ppm; Fe ~5ppm; P ~1ppm; Ti ~20ppm

Claims

1. An electrolyte for electrochemical refining of a material containing silicon at a temperature above the melting point of silicon, wherein the electrolyte comprises 50-100% by weight of at least one alkaline earth metal fluoride selected from BaF₂, CaF₂, SrF₂ and MgF₂.

2. The electrolyte according to claim 1 , wherein the electrolyte comprises 80- 100% by weight of at least one alkaline earth metal fluoride selected from BaF₂, CaF₂, SrF₂ and MgF₂.

3. The electrolyte according to claim 1 , wherein the alkaline earth fluoride electrolyte contains 50-80% by weight of one of said fluorides and 0-50 % by weight of one or more of the other three.

4. The electrolyte according to claims 1 to 3, wherein the electrolyte contains at least one silicon bearing species.

5. The electrolyte according to claim 4, wherein the silicon bearing species is selected from K₂SiF₆, Na₂SiF₆, CaSiF₆, BaSiF₆, SrSiF₆ or SiO₂.

6. The electrolyte according to claim 4 wherein the electrolyte contains the silicon bearing species in an amount from 0 to 20 % by weight.

7. The electrolyte according to claim 6 wherein the electrolyte contains the silicon bearing species in an amount from 0.1 to 10 % by weight.

8. A method for electrochemical refining of a material containing silicon wherein an electrolyte comprising 50-100% by weight of at least one alkaline earth metal fluoride selected from BaF₂, CaF₂, SrF₂ and MgF₂, is used.

9. The method according to claim 8, wherein the electrolyte contains 50-80% by weight of one of said fluorides and is added 0-50 % by weight of one or more of the other three.

10. The method according to claims 8 and 9, wherein the maximum temperature of the electrolyte is below the boiling point of the electrolyte.

11. The method according to claims 8 to 10, wherein at least one silicon bearing species is added to the electrolyte.

12. The method according to claim 11 , wherein the silicon bearing species is K₂SiF₆, Na₂SiF₆, CaSiF₆, BaSiF₆, SrSiF₆ or SiO₂.

13. The method according to claim 12 wherein the silicon bearing species is added in an amount from 0 to 20 % by weight of the electrolyte.

14. The method according to claim 12 wherein the silicon bearing species is added in an amount from 0.1 to 10 % by weight of the electrolyte.

15. The method according to claims 8 to 14, wherein the electrochemical refining is performed in a three-layer process comprising a molten bottom layer of silicon and a heavy, noble metal, which is polarized anodically, an intermediate electrolyte layer and a pure, molten layer of silicon polarized cathodically.

16. An electrochemical cell for electrochemical refining of a material containing silicon at a temperature above the melting point of silicon, wherein the cell has a lining comprising of reaction bonded Si₃N₄.

17. The electrochemical cell according to claim 16, wherein the reaction bonded Si₃N₄ has a porosity of in the range from 10 to 60 % of the theoretical value.

18. The electrochemical cell according to claim 17, wherein the reaction bonded Si₃N₄ has a porosity in the range from 20 to 40 % of the theoretical value.