SILICON METAL SHEETS

PRIORITY CLAIM

This application claims priority from copending U.S. Provisional Patent Application Serial No. 61/226,371, filed 17 July 2009, the disclosure of which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to the manufacture of silicon metal sheets. BACKGROUND OF THE INVENTION

Silicon metal has numerous applications in many industries, from aluminum refining to photovoltaic panels and semiconductors. The metal is typically produced through furnace processes, and purification can involve chemical treatment, for example, for the production of photovoltaic and semiconductor grade silicon metal. In these two latter instances, it is the norm to take the chemically purified silicon and melt it into an ingot, which can then be cut into wafers. The size of the wafer is thus limited by the dimensions of the ingot.

In this invention, applications of silicon metal powders are described that use silicon metal particles coated in salt (NaCl). The salt coating prevents oxidation of the silicon (and also exposure of the silicon metal to other gases) and permits the manufacture of silicon powder metallurgical products such as sheets, in which the absence of gases is advantageous to the formation and performance of the powder metallurgical products.

SUMMARY OF THE INVENTION

The invention relates to a process for the manufacture of silicon metal sheets, suitable for a wide range of uses in which a salt-coated metal powder is particularly well-suited and also economically advantageous.

The invention relates to the manufacture of silicon metal sheets. First, silicon metal is produced in the form of a metal powder encapsulated in a salt or salts. The salt-coated particles are then applied to a substrate, and then the salt coating can be removed to leave a coating of silicon metal powder (which can be sintered to form a sheet or coating), or a coating of silicon metal if the salt removal is accomplished at high enough temperatures to sinter the silicon metal within the salt coating. Typical salts are NaCl and other alkali or alkaline earth salts.

One embodiment of the invention is a silicon sheet produced from salt- coated silicon particles in which the thickness of the sheet is less than 10% of the lesser of the width and length of the sheet. Preferably, the purity of the silicon is at least 99.99% by weight silicon. More preferably, the purity of the silicon is at least 99.999% by weight silicon. Most preferably, the purity of the silicon is at least 99.9999% by weight silicon.

Another embodiment of the invention is a silicon metal sheet or wafer with a surface area greater than 2500 square centimeters and a thickness of from at least about 10 microns and up to about 1000 microns. Preferably, the thickness will be in the range of from about 25 microns to about 500 microns. A most preferred thickness is from about 50 microns to about 250 microns. Preferably, the silicon metal sheet or wafer has surface area greater than 5000 square centimeters. More preferably, the silicon metal sheet or wafer has a surface area greater than 10000 square centimeters. Preferably, the purity of the silicon is at least 99.99% by weight silicon. More preferably, the purity of the silicon is at least 99.999% by weight silicon. Most preferably, the purity of the silicon is at least 99.9999% by weight silicon.

Another embodiment of the invention is a photovoltaic device produced using any of the silicon sheets or wafers described herein.

Another embodiment of the invention is a process for manufacturing silicon metal sheets in which the sheet is manufactured starting from salt-coated silicon particles.

DETAILED DESCRIPTION OF THE INVENTION

Silicon metal can be produced in the form of a metal powder encapsulated in a salt or salts. Depending on the application in mind, the metal powder can be separated from the salt/salts, or else can be further processed while encapsulated in the salt/salts. The preliminary steps of the process described herein are more fully described in PCT Publication No. WO 2009/018425, the disclosure of which is hereby incorporated herein by reference.

The particle size produced by this process is controlled by a number of factors, including the reaction temperature and the flow rates of the reagents, in a fashion that will be well understood by those skilled in the art. The ability to select particle size is an important and attractive aspect of the present invention.

The silicon metal produced by this methodology is well suited to the formation of silicon metal coatings and sheets. The salt-coated particles can be applied to a substrate, and then the salt coating can be removed to leave a coating of silicon metal powder (which can be sintered to form a sheet or coating), or a coating of silicon metal if the salt removal is accomplished at high enough temperatures to sinter the silicon metal within the salt coating. The salt coating of the silicon metal particles can be removed by various processing steps. These include, for example: by water washing; by the application of heat to melt or to boil the salt and thereby filter or evaporate away the salt; and by applying pressure (under which the salt will flow and can thereby be separated from the silicon metal). These and other common methods of salt removal can be applied individually and in combinations. The precise choice of method(s) to remove the salt depends on the choice of end product and desired metallic purity.

Binders can also be added to the silicon metal powder to enhance processing characteristics, for example to improve the ability to cast thick films of the silicon metal. The binder should be chosen such that it can be removed from the silicon metal by heating or other means in such a way as to avoid undesirable oxidation (or other chemical contamination) of the silicon metal surface.

Silicon metal particles coated in salt can be produced using the process described in the above referenced PCT Application. The particles comprise primary particles clustered into aggregates. Operating the reaction in the range 200 ⁰ C to 800 ⁰ C produces primary silicon particles in the size range 0.5 microns to 10 microns, and aggregates in the size range 1 micron to 50 microns.

The salt-coated particles can be processed into silicon sheets using a range of approaches. The following examples are illustrative of the methods available to produce silicon sheets from the salt-coated silicon particles.

Example 1

First, the salt-coated silicon particles can be treated to remove all or substantially all of the salt coating, by heating in an inert atmosphere to above the melting point of the salt and then filtering or otherwise removing the molten salt from the silicon metal particles. Operating temperatures in the range 850 ⁰ C to 1450 ⁰ C are sufficient to melt the salt without also melting the silicon metal. Second, the silicon metal thus produced can be prepared as a sheet by calendaring the metal powder, or by layering the metal onto a suitably chosen substrate, in either case in an inert atmosphere. In the latter case the silicon powder can be compressed on the substrate to increase the adhesion of the silicon particles to each other. The application of pressure in the formation of the sheet has the additional beneficial effect of further separating any remaining salt from the silicon. The substrate can be chosen to produce minimal chemical changes to the silicon metal, or to effect a desired change in its physical properties, for example by using aluminum as the substrate to permit the formation of a p-type semiconductor layer at the junction of the silicon and aluminum.

Sample sheet dimensions include sheets with linear dimensions in the range of 10 cm to 20 cm, 20 cm to 50 cm, 50 cm to 100 cm, and also linear dimensions greater than 100 cm. There is no requirement to produce sheets with both linear dimensions in the same range as described here, and combinations of these ranges to produce more or less rectangular sheets are also possible.

Third, the silicon sheet can be heated to a temperature sufficient to cause the silicon particles to fuse and compact. Temperatures in the range 1250 ⁰ C to 1750° C are effective in accomplishing this process step, with the upper limit set in part by the choice of substrate. This step also further reduces any residual salt present in the silicon and causes the salt and the silicon to separate.

Fourth, any remaining salt can be removed, either by washing with a solvent such as water of suitable purity, or by heating the sheet under partial or full vacuum to above 825°C to evaporate the remaining salt. The higher the temperature and the harder the vacuum, the greater will be the rate of salt removal.

Finally, the silicon sheet can be annealed as desired in order to change the grain structure of the sheet, depending on the application in mind. Example 2

First, the salt coated silicon particles are washed to remove the salt coating. The water used to wash the salt away must be sufficiently pure not to produce undesirable contamination of the silicon particles. Electrical resistivity measurements can be used to determine when the salt levels have been reduced to a sufficiently low level. Typically, to reduce the salt levels to below 10 ppm requires five or more quantities of wash water per quantity of silicon, measured as Kg per Kg.

Second, the silicon powder, now with a surface coat of silicon oxides, is heated to above 900 ⁰ C in an inert atmosphere, to remove thermally the surface silicon oxide. The silicon powder thus produced can then be processed according to Example 1, starting at the second step.

Example 3

First, the salt-coated silicon particles are processed into a sheet, for example by layering the salt-coated particles onto a suitable chosen substrate. This process step is preferentially conducted in an inert atmosphere.

Second, the sheet is heated to above the melting point of the salt, in order to cause the salt to separate from the silicon metal. More preferentially the temperature is raised to the point at which the silicon particles coalesce and density while separated from the salt, typically at temperatures above 1250 ⁰ C.

Third, the salt is removed from the silicon sheet, for example by decanting the molten salt using an inclined belt.

The silicon sheet can be further processed using the process of example 1 , starting at the fourth step.

In any of the examples above, and in other methods comparable to the above examples, it is possible to choose the linear dimensions of the silicon sheet to suit desired applications. Silicon sheets can be produced with thicknesses in the range of 10 microns to 100 microns by these and comparable methods, and also with thicknesses greater than 100 microns. Sheets can be produced with a wide range of widths and lengths. Widths in the range of 5 cm to 15 cm, 15 cm to 50 cm, and greater than 50 cm are all possible. Lengths in the range 5 cm to 100 cm and also greater than 100 cm are also possible.

Moreover, it is possible to produce silicon sheets in a continuous or near- continuous manner provided the supply of salt-coated silicon particles is sufficient.

The purity of these silicon sheets is dependent on the initial purity of the silicon metal, the choice of processing materials used to produce the sheets

(substrates, salt solvents etc) and the degree to which the salt coating is removed during processing. Purities of 99.99% by weight silicon, 99.999% by weight silicon, and 99.9999% by weight silicon are all achievable. Sheets of high purity silicon metal, especially at purities of at least 99.999% by weight silicon and preferentially 99.9999% by weight silicon, are especially useful for producing photovoltaic devices based on silicon.

Large wafers of silicon offer much reduced handling and processing costs as compared to the ingot-to-wafer pathway currently in use in the production of silicon photovoltaic panels. In particular, the ability to produce large sheets of silicon permits the application of processing techniques such as those used in the production of flat panel displays, and offers a path to power cost silicon wafers than is currently possible.

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