AU2095800A - Agglomeration of silicon powders - Google Patents

Abstract

According to a method for agglomerating silicon powders, the silicon powder is heated to a temperature of at least 250 DEG C, preferably 1200 to 1500 DEG C, using the effect of microwave radiation in the wavelength range 0.5 kHz to 300 GHz. Fine silicon dust with a particle size of less than 100 mu m is preferably used as the starting material. Metal-containing and/or boron, phosphorus, arsenic or antimony-containing doping agents can be added to the silicon powder before or after the microwave treatment. The resulting granulate is optionally broken, milled and sifted. The invention also relates to a silicon granulate which can be produced by this method. Said granulate preferably has a porosity of 0 to 80 % and foreign phases or precipitates on the grain boundary of the monocrystals. The inventive silicon granulate can be used for producing silicon monocrystals or multicrystalline silicon blocks or for reacting with hydrogen chloride or halogenated hydrocarbons in fluidised beds or shaft furnaces.

Description

- 1 Aglomeration of silicon powders The present invention relates to a process for the agglomeration of silicon powders, 5 and to silicon granules which can be prepared by this process, and to the use thereof. The chemical conversion of metallic silicon into silanes or, vice versa, the conversion of silanes into silicon is often carried out in fluidized beds, for example the MUller Rochow reaction, the hydrochlorination, the decomposition of Sill 4 or the 10 decomposition of HSiCl 3 (cf. DE 36 38 931, DE 37 39 895, DE 43 27 308, US 5,382,412, US 4,883,687, US 4,784,840, US 3,963,838). In these processes, the particle size of useful powders are limited to certain particle diameters by the aerodynamic requirements of the fluidized bed reactors. Only these selected ranges allow optimum realization of the advantages of fluidized bed reactors (cf. Y.G.Yates, 15 "Fundamentals of Fluidized-bed Chemical Processes", Butterworths London 1983; E. Klar, "Copper Catalysts for the Direct Process", p.174, in Silicon for the Chemical Industry III, Sandefjord Norwegen 1996 - The Norwegian Univ. Sci.A.Tech., Trondheim p. 9-10). 20 During reaction of Si powders with the reactants, the initially charged particles are continously reduced in size and lose their optimum diameter of > 40 /m. On the other hand, another process is based on particle growth during SiH 4 decomposition for the preparation of compact Si powders so as to produce particle diameters of more than 600 tm. As this cannot be accomplished in an ideal manner by a gas phase 25 decomposition process, powder fractions below these limits have to be tolerated. However, these ultrafine powders impede the processing thereof in subsequent melt processes, or it is virtually impossible to process them. The conversion of metal powders into compact melts is known per se and can be 30 achieved by a sufficient supply of thermal energy by means of heat conduction. As heating of powders becomes more difficult with decreasing heat conductivity, -2 uniform penetration of the material is not possible. This results in a considerable temperature gradient. Thus, the heating rate which can be achieved is limited by the heat conduction. 5 Furthermore, a sintering process utilizing microwaves is known for heating large volumes and porous bodies. This process is disclosed e.g. in DE 195 15 342 in a particular process design. However, this document does not mention the application of this process to ultrafine Si powders containing catalytically relevant contaminants, the distribution of which can be crucial for the success of fluidized bed syntheses 10 using these powder mixtures. Nor does this document disclose conditioning of amorphous high-purity micropowders to form sintered bodies for subsequent melt and refining processes while maintaining the purity level in the manner according to the invention. 15 In existing processes, such as crucible pulling in quartz or graphite crucibles (Czochralski crystal pulling process) or various float zone processes, particulate granules of relatively high density are heated by electromagnetic induction to provide a compact melt bath for the zone purification. Suitable processes for such a metallurgical purification (segregation) are in principle any processes which 20 comprise heating the Si metal and controlled cooling in accordance with defined temperature/time conditions (EP 699 625, ERDA/JPIJ954559 70/2 Hunt, Dosaj, McCormick, Solar Silicon Via Improved a. Expanded Metallurgical Silicon Technology 1977; Amerlaan, C.A.J. Hrsg., Proc. Symos. Sci. a. Techn. of Defects in Silicon of the E-Mrs Meeting StraBbourg, May/June 1989; W. Zulehner, Elesevier 25 Sequoia 1989 Mater. Sci. A. Eng. Vol. B4 No. 1-4; Heimann, Huber, Seidensticker, Zulehner, in Silicon Chemical Etching, Ed. J. Grabmeier, Springer Berlin 1982; Sil bernagel, Chem. Ing. Tech. 50 (1978) 8, p. 611-617; Dietze, Miuhlbauer, Chemiker Zeitung 97 (1973) 3 p. 151-155; Dietl, Helmreich, Sirtl, "Solar Silicon", in Crystals: Growth Properties a. Appl. 5 Springer Verlag Berlin 1981, p. 43ff).

-3 A variant on sintering Si powders is diclosed by C.J. Santana, K.S. Jones in J. Mater. Sci. 31 (1996) p. 4985-90, which comprises sintering/compacting the Si powder to full density, i.e. to high density to produce a wafer, by hot-pressing. 5 In view of this prior art, the object of the present invention is to improve the suitability of fine Si powders which have diameters of 1 to 1000 pim and poor heat transfer characteristics, owing to their low bulk density, for various methods of further processing. It should be possible to vary selectively the particle size, chemical composition, reactivity and/or phase distribution of the contaminants in the Si matrix, 10 using as little energy as possible. Furthermore, the treated silicon should be readily convertible into a fluidizable powder, if desired. This object is achieved by heating the silicon powder to a temperature of at least 250*C, preferably from 1200 to 1500*C, by exposure to microwave radiation in the 15 wavelength range from 0.5 kHz to 300 GHz, thereby agglomerating the silicon powder. It has been found, surprisingly, that a silicon powder treated in this manner has many improved properties. Depending on the conditions, a sintered body of relatively high density and/or a powder of different reactivity are obtained. It is possible to vary selectively the particle size, chemical composition, reactivity or 20 phase distribution of the contaminants in the Si matrix. Furthermore, the sintered bodies/powders have a porosity after heating which makes it possible: a) to convert the Si body into a fluidizable powder with minimum additional effort (crushing instead of grinding), or 25 b) to deplete the contaminants by classifying after crushing/grinding or by chemical purification steps, or c) to provide advantageously the sintered body, which only contains minor gas 30 inclusions, in Si melts for further refining steps.

-4 The invention allows the agglomeration of an ultrafine Si metal powder below the melting temperature of the silicon, with minimal energy consumption, or the controlled conduct or preparation of phase separations or refining processes at a temperature no higher than the melting temperature. In the case of powders which are 5 already in the desired particle size range, the process is solely aimed at conditioning at an elevated temperature. Adhesion to the wall material should be minimal. This form of heating makes it possible to let the sintering process only proceed up to the point at which the majority 10 of the ultrafine powder has multiplied in its particle size by a factor of 2 to 100. The starting material used for the agglomeration is preferably fine silcion dust having a particle size of less than 600 tm, particluarly preferably of less than 100 yIm. Powders of this type are widely available industrially as particle size fractions from 15 Si powder grinding for MUller-Rochow synthesis or hydrochlorination, and as dusts from ultrafine dust separation (e.g. in a cyclone) in these fluidized bed processes, such as MUller-Rochow, hydrochlorination, decomposition of silane to form ultrapure Si. These dusts can no longer be used advantageously in a fluidized bed process, in a shaft furnace or in subsequent processes owing to their aerodynamic or 20 chemical properties. In contrast, the agglomerate obtained preferably has an average particle size of at least 30 tm, rendering it reusable for the abovementioned fluidized bed processes. 25 The microwave heat treatment according to the invention can be carried out in a stirred moving bed, preferably in a fluidized bed. Any inhomogenities in the heat distribution which might still occur are eliminated in such a stirred bed. The microwaves are introduced by means of appropriate antennae through the external wall or into the interior of the agglomeration reactors. 30 The heat treatment is preferably carried out in a reactor made of microwave transparent and/or microwave-reflecting material, in particular non-oxide ceramic, -5 graphite, oxide ceramic or quartz. Such crucible materials allow efficient heating of the material and ensure low wall adhesion with respect to liquid Si. Prior to the microwave treatment, the silicon powder can be admixed with dopants 5 containing metals and/or boron, phosphorus, arsenic or antimony (Ag, Al, As, Cd, Cs, Cu, Ce, Co, Cr, Fe, In, Ir, Mo, Mn, Cr, Ni, P, Pt, Pd, Rh, Ru, Se, Sb, Sn, V, Zn). During subsequent further processing, substances of this type then act as catalysts, e.g. when a reaction with halides RX (R = Me, Et, Phe; X = Cl, Br) is carried out. 10 Alternatively or additionally, the granules obtained by the microwave treatment can first be admixed with dopants containing metals and/or boron, phosphorus, arsenic or antimony in accordance with e.g. the usual requirements for the Rochow synthesis or the hydrochlorination. 15 Furthermore, the granules obtained by the microwave treatment can be ground and optionally classified. Grinding of the granules is considerably facilitated owing to their porosity. In addition, the crushing or grinding procedure preferably produces surfaces of different chemical composition which generate a different reactivity with respect to reactants of the type RX and a different product composition compared to 20 untreated Si. Owing to their increased particle size, the powders which have been agglomerated and then classified are easier to handle in a fluidized bed than the precursor powders. Occasionally, these powders allow impurity depletion by classifying and/or chemical 25 purification steps. The invention further comprises silicon granules obtainable by a process according to the invention. The advantageous properties of such granules have been described hereinbefore. 30 The silicon granules preferably have a porosity of 0 to 80 %. High-porosity granules are particularly suitable for easier grinding (or crushing).

-6 The average crystal size of the individual crystals of the silicon granules is generally less than 50 pm. Metal-containing and/or nonmetal-containing foreign phases or precipitates are preferably present at the grain boundaries of the individual crystals. 5 The silicon granules according to the invention can preferably be used for the preparation of silicon single crystals, in particular by the Czochralski method or the float zone method. 10 However, it is likewise possible to prepare multicrystalline silicon blocks, preferably by oriented crystallization of melts of the silicon granules. An agglomerated ultrafine Si powder containing few impurities is utilized in a refining process by heating the ultrafine powder by means of electromagnetic waves to temperatures up to the melting point followed by introducing the resulting sintered body into another melt 15 furnace in which multicrystalline Si for semiconductor applications is prepared by a segregation process. Finally, it is likewise possible to use the silicon granules according to the invention for reaction with hydrogen chloride or halogenated hydrocarbons. In this case, the 20 particle size of the granules which is adapted to fluidized bed processes has a particularly beneficial effect. The silicon granules have preferably been admixed with catalysts suitable for processes such as the MUller-Rochow synthesis or the hydrochlorination, to promote the reaction with halides RX (R = Me, Et, Phe; X = Cl, Br). 25 The sintered powders which have been crushed in a subsequent step according to the invention can furthermore be admixed with catalysts from the group of those elements which are used in prior art fluidized bed synthesis of methylchlorosilanes, phenylchlorosilanes, hydridochlorosilanes or chlorosilanes (or hydrogenation thereof 30 in the presence of Si powders), i.e. in particular Ag, Al, As, Cd, Cs, Cu, Ce, Co, Cr, Fe, In, Ir, Mn, Mo, Ni, P, Pt, Pd, Rh, Ru, Se, Sb, Sn, V, Zn.

-7 The silicon granules according to the invention should be admixed with the abovementioned catalysts prior to heating in an amount which allows sufficient activation for the abovementioned syntheses. In this case, heating can then preferably remain limited to temperatures below the melting point of the elements. 5 In particular, the silicon granules according to the invention can be prepared by optionally purifying, agglomerating, crushing, grinding and classifying powders derived from the grinding or dust separation sections of fluidized bed reactors in which RX and Si are reacted (R = Me, Et, Phe; X = Cl, Br). Subsequently, 0 to 95 % 10 of fresh metallurgically prepared silicon of the same particle size suitable for fluidized beds can be added. The silicon granules according to the invention which are particularly suitable for use in reactions with RX can be prepared in such a manner that all or part of the 15 constituents contaminating the Si are present in separate phases after heating by means of electromagnetic waves, and can be depleted by crushing/grinding/evaporating and classifying and/or by washing, leaching or extracting. 20 The examples which follow illustrate the invention.

-8 1. Principles The ultrafine Si powders which are widely avaliable industrially are particle size fractions from Si powder grinding for MUller-Rochow synthesis or 5 hydrochlorination, and dusts from ultrafine dust separation (e.g. cyclone) in these fluidized bed processes (MUller-Rochow synthesis, hydrochlorination, decomposition of silane). These dusts can no longer be used advantageously in a fluidized bed process or in subsequent processes owing to their aerodynamic or chemical properties. 10 General chemical suitability for MCS synthesis can be evaluated e.g. using a laboratory-scale stirred bed which is relatively insensitive to the particle size distribution. The main characterizing physical parameter determining the suitability for use in a fluidized bed is known to be a defined average particle size of more than 15 30 pm, in addition to the chemical composition. Exemplary details and the desired particle size are given by R6sch, Kalchauer p.7 in "Silicon for the Chemical Industry III", Sandefjord Norwegen 1996 - Ed: Oye, Rong, and by Ceccaroli, Nygaard, Tuset, The Norwegian Univ. Sci. a. Tech., Trondheim 1996 p. 269 ff. , E. Klar ibid. p. 169, or in DE 195 32 315, p.3 1. 55-64. 20 2. Agglomeration 2.1. Induction furnace process 25 A vacuum induction melting and casting furnace IS 1 1II from LEYBOLD HERAEUS was used to prepare sintered bodies in an induction furnace. The features of this furnace are, interalia an operating frequency of 10 kHz, a power of 30 kW and a maximum vacuum of 1.3-10-' mbar. 30 The powder samples were mechnically precompacted at a pressure of about 0.5 bar. About 100-200 g of the precompacted sample were then placed in a quartz, SiC or corundum crucible, ideally, for reasons of easy demoulding and high heating rate, in -9 a graphite crucible which was superficially coated with Si 3

N

4 (BAYER CFI 06051, about 0.3-0.5 mm thick) or was made of Si 3

N

4 only. The furnace was evacuated twice at 0.5 - 8 mbar (using an oil diffusion pump to reach 1.3-10- mbar) and then repressurized with Ar or He at atmospheric pressure. 5 Heating to 700-1600'C took 15-40 min (depending on the crucible material and preliminary conventional preheating to about 200 0 C). A specific heating energy of about 28 kJ/g (for 100 g of sample) = 7.8 kWh/kg was 10 found which was inversely proportional to the sample weight. The samples were cooled down to about 300-150*C within about 2 h under He protective gas. 2.2. Microwave furnace process 15 It is likewise possible to use a microwave furnace instead of the induction furnace. This furnace was characterized by generation of energy by means of a magnetron at 2.45 GHz, 3 kW adjustable from 250 W by means of variable plates to limit the irradiated cavity. Energy supplied: 500-700 W, adjustable up to a maximum of 1700 W, rapid heating between 500 and 1200'C. 20 Crucible materials used for Si were A1 2 0 3 and Si 3

N

4 crucibles superficially coated with Si 3

N

4 (BAYER CFI 06051, about 0.3-0.5 mm thick). Sample sizes were 100 200 g. The samples were mechanically precompacted as described above. 25 The furnace cavity was evacuated twice at 0.1 mbar and repressurized with He or Ar. Depending on sample size, crucible insulation and preheating to 200'C, temperatures of 1200-1450*C (surface temperature determined by means of a pyrometer) and thus the melting point of Si (1423*C) were obtained after 15-60 min. 30 A specific heating energy of about 19 kJ/g (for 100 g of sample) = 5.3 kWh/kg was found which was likewise inversely proportional to the sample weight.

-10 The samples were cooled down to about 300*C within about 1-1.5 h under He protective gas. 5 3. Evaluation 3.1. Evaluation of the sintered Si powder in the reaction with methyl chloride or hydrocarbon 10 The powder was classified or screened, and then 40 g of powder of the reported selected particle size were filled into a quartz tube having a glass frit bottom and a stirrer inserted via a stirrer bearing. The preferred particle size was usually 71 160 ym, after crushing, grinding and classifying or screening the granules. 15 3.1.1 In the case of the reaction of Si with methyl chloride, the Si powder was usually admixed with 3.2 g of Cu/Cu 2 0/CuO catalyst (86% of Cu) and 0.04g ZnO. The methyl chloride flow was 1.8 1/h and 2 bar abs. The gas space was subsequently filled with nitrogen. The contents of the glass tube were heated to 370'C for a short period 20 of 1 h by means of an electrically-heated tube furnace from HERAEUS, and then a pressure of 2 bar abs was maintained during reaction by means of an inclined tube manometer at 300'C or 330'C, respectively, within 8 hours. The resulting products which vaporize readily at 300'C were withdrawn through a 25 descending glass condenser having a coolant temperature of 15*C, and condensed. The effluent gases which were not condensable under these conditions, consisting of MeCl, HCI etc., were passed into the exhaust air stream. The liquids of the various methylchlorosilanes or chlorosilanes which were condensable at 15-20'C were collected in a receiver underneath the condenser and analyzed in a gas 30 chromatograph. (For details, cf. DE 195 32 315).

- 11 3.2.1 In the case of the reaction Si+HCI instead of methyl chloride, the subsequent addition of catalytic additives was omitted. The pressure selected was likewise 1 bar gauge, but in this case the amount of HCl (4.2/9.6 I/h, via digital flow meters) and the 5 temperature (220-350'C) were varied. Preactivation at 370*C was omitted. 3.2. Suitability for MCS synthesis: Examples 3.2.1. Comparative example: Reaction using standard metal-grade Si in a 10 laboratory-scale stirred bed Metal grade silicon granules obtained from LILEBY were ground in an agate mill using an electromechanical pestle. A fraction having a particle diameter of 71-160 ytm was isolated from the laboratory-scale stirred bed by means of screening 15 using ASTM standard screens. Trace component contents were determined by ICP-AS. The reaction with methyl chloride as mentioned in 3.1.1 produces the methylchlorosilane amounts reported in the first column, "comparison", of Table 1. 20 3.2.2. Example la (Comparative example: Use of untreated grinding fines) Grinding fines obtained from the silicon granules employed in the Comparative Example during grinding in a hammer mill using ceramic balls under N 2 protective gas under industrial conditions were blown from the mill into a filter. These grinding 25 fines were determined to have an average particle diameter of 35 ytm and a distribution of 5-50 Im. The particle size distribution was determined by means of a MALVERN Master Sizer 2000 (measurement of scattered light) in this case and by means of ASTM standard screens in the other cases above 36 ym. 30 The above-described procedure 3.1.1. for reacting methyl chloride in accordance with the MUller-Rochow method in a laboratory-scale stirred bed reactor produced the methylchlorosilane mixture reported in Table 1 (column 2, heading "la").

-12 In this procedure, the most important parameters to be optimized are a sufficiently high production rate and a high Me 2 SiCl 2 content, however, it is equally or even more important to have achieved a particle diameter greater than 40 pm and thus 5 suitability for the fluidized bed, a property which cannot be evaluated. For la, the production rate and the Me 2 SiCl 2 concentration are found to be at the lower end of the expected range. 3.2.3. Example lb (Use of grinding fines treated according to the invention) 10 200 g of the grinding fines collected in the grinding process of Comparative Example 3.2.2. were subjected to microwave agglomeration (microwave furnace of type II) 2.2. under Ar protective gas. Heating time: 30 min. Temperature, sintered density and particle size after grinding and classifyng are reported in Table 1 (column 3, heading 15 "1b"). Energy supplied: 500-1500 watts. The energy was switched off and the material was cooled down to about 200*C and then discharged. The material was then cooled down to room temperature (25*C) in air and the agglomerate was subjected to grinding and screening. A crushing process 20 using a hammer and an agate mill equipped with an electromechanically movable pestle yielded a powder having the properties reported in Table 1. A 71-160 pm fraction having a D50% value of 120 pm was isolated by screening for the comparative tests in the laboratory-scale reactor. 25 The high Fe content of the grinding fines from Example la is found to be reduced to a level in the vicinity of the standard material from the Comparative Example in the grinding and classifying process. The production rate is at the lower end of the expected range, the Me 2 SiCI 2 30 concentration was improved compared to the Comparative Example/Standard.

- 13 3.2.4. Example 2 (untreated Si dust from the fluidized bed of the Rochow synthesis) Unsintered fine dust from the fluidized bed of the Rochow synthesis (see also Examples 2b-2d) was evaluated as described above (Table 1, column 4, heading 5 "12"). This Si dust had an average particle diameter of 20 Am and an Si content of about 76 %. In addition, it contained 11.4 % of Cu, 3 % of Fe, 2.9 % of C, 0.3 % of Zn, 2 % of Cl and other metals prior to sintering. The properties in the laboratory-scale stirred bed with respect to production rate and 10 Me 2 SiCl 2 concentration are very similar to those of the Si from the Comparative Example. 3.2.5. Examples 2a to 2d (Si fluidized bed dust treated according to the invention) 15 Dust 2 from the fluidized bed was subjected to stepwise heating by microwave exposure and agglomerated to various degrees. The microwave furnace was charged with the Si dust described in 2. The temperature was increased to about 800-820 0 C within 40 min and then maintained at this level for about 30 min, with Cu salts evaporating in the form of a plasma cloud. The material 20 was heated further within 20 min to a preselected level of 4 different final temperatures, with a maximum of 1500'C (in each case surface temperatures determined by means of a pyrometer). The final temperatures were maintained for about 20 min. The irradiation source was then switched off and the material was allowed to cool. 25 The powder separated after crushing, grinding and classifying to obtain a 10-160 Am fraction (Cu content: 11.4 %) was evaluated in comparison with unsintered fine dust from Example 2 in 3.2.4. (Table 1, columns 4-7, headings "2a" to "2d) 30 The production rate for methylchlorosilanes and the Me 2 SiCl 2 concentration were found to drop above 800-900*C and then arrive at a lower intermediate maximum for -14 the main component. The agglomerated Si is less suitable for reaction with methyl chloride when this process sequence is employed. 3.2.6. Examples 3a and b (mixture of fluidized bed dust and grinding fines from 5 metal grade silicon) 85 parts of metal grade silicon grinding fines from Example la. are admixed with 15 parts of ultrafine dust from the cyclone of a fluidized bed for methylchlorosilane synthesis. 10 Example 3a illustrates the result for the unsintered product (Table 1, column 8, heading "3a"). In Example 3b, the powder was partially sintered according to the invention. The 15 production rate is found to drop while the Me 2 SiCl 2 content increases (Table 1, column 9, heading "3b"). 3.2.7. Examples 4a and 4b (activation of metal grade Si powder + catalyst/promoter mixture) 20 A standard metal grade Si powder from 3.2.1.-Comparative Example of particle size 71-170 Am was admixed with the amounts of catalyst (Cu/CuO, Zn, Sn) reported in the Table and subjected to microwave heating under Ar protection in Example 4b. The reported temperature below the melting point was maintained for about 30 min. 25 The material was then cooled down to 150*C under protective gas. Table 1 (columns 10,11, headings "4a", "4b") shows an increase in the production rate of the resulting methylchlorosilane mixture and a small increase in Me 2 SiCl 2 concentration for a defined composition.

U) ~N ciJU 0 -Cl LO CU .0 Ud v m ) N, 0 v - o a e, o0-o oo c -o Lo .0 . . o o o- 0i 66opo o w o o C- C) O , o oo c CQ o0. 6. 6- - li c, , c;06v cq ~ ~ -)c P cl c *0 G oi U,00 co 0n a qq' L q7u I( N 0 Z f( Ac9 8 1 4A ol - 16 3.2.8. Examples 5a and 5b (Si grinding fines from Example la in hydrochlorination) Table 2 shows that both the grinding fines in their original state and the grinding 5 fines agglomerated according to the invention have roughly the same reactivity and product composition in the case of hydrochlorination in a laboratory-scale stirred bed (process as described in 3.1.2 except that HCI was used here instead of CH 3 Cl). 3.2.9. Examples 6a and 6b (fluidized bed dust from Example 2 in hydro 10 chlorination When the hydrochlorination is carried out using fine dust discharge from the fluidized bed (cyclone) for methylchlorosilane synthesis, the differences as illustrated in Table 2 are found between the original ultrafine fluidized bed dust and the dust 15 agglomerated and ground according to the invention. The agglomerated, ground and screened dust produces more SiCl 4 at a production rate which is mostly lower. However, when the amount of available HCI is increased from 4.2 lI/h to 9,6 1/h, the production rate increases to reach a level similar to that of the unagglomerated Si from 2 or 6a. 20 3.3.10. Example 7 (Si microdust from the decompostion of SiH 4 to produce ultrapure Si) A powder having an average particle size of 2 pm and a distribution of 0.05-5 Jim 25 was separated from the process for the preparation of ultrapure Si by decomposition of SiH 4 at 700-800 0 C in accordance with US 47 84 840 or US 48 83 687. 200 g of this powder were then placed in a microwave furnace as described above. The powder was heated to 1450 0 C within 60 min under He to give a melt cake 30 having a density of 1.9 g/cm 3 which was allowed to cool down to 200*C under protective gas.

- 17 When using an Si 3

N

4 crucible, a compact ingot/regulus was obtained which was placed in a melt furnace (ULLMANN ENCYCLOPEDIA of Ind Chem., 1993 A 23 p.739, BRIDGEMAN melt furnace (J.C. Brice: Crystal Growth Processes, Blackie 5 Glasgow & London (1986) p. 104). The sample was melted at 1450'C and then subjected to programmed solidification in defined steps to produce multicrystalline Si having crystallite sizes of usually 5 50 mm. As a result, a multicrystalline Si block was obtained from which the outer 10 regions were cut off to provide a multicrystalline core of the composition and structure desired for solar grade Si. After agglomeration according to the invention and conversion into multicrystalline Si using a BRIGDEMAN furnace, this material had a conductivity of about 2000 Ohm-cm. The core can be sawed into 350 pm Si slices which can be used as wafers in a solar module.

- 18 Table 2 Composition of sintered Si powders and reaction products with HCl 5a 5b 6a 6b 6b 7 prior to sintering g/mI 0.8 0.8 0.8 0.8 0.8 0.3 particle size D 90% Am 35 35 35 35 35 10 Si % 97 97 77 77 77 99.99999 C % 3 3 3 Cu % 11 11 11 Fe % 1 1 3 3 3 Al % 0.38 0.38 0.6 0.6 0.6 Ca % 0.13 0.12 Zn % 0.1 0.1 0.3 0.3 0.3 Sn % 0.018 0.018 0.018 sintering temp. ' - 1050 1300 1300 sintered density g/mI 0.5 1.6 0.5 1.6 1.6 2.6 particle size used D50% ym 10-50 40-160 10-50 40-160 40-160 additions after sintering % 0 0 0 0 0 0 resistance of Si Ohm-cm n.b. n.b. n.b. n.b. n.b. 2000 Hydrochlorination after 8h after 8h after 8h after 8h after 16h 4.2 I of HI/h H2SiCl2 % 0.7 0.1 HSiC3 % 87 58 SiCl4 % 12 42 Prod. rate @4.21 HC/h g/h)* 3.5 1.8 9.61 of HCVh 240*C H2SiCl2 % 0.1 0.5 0.1 0.3 HSiCl3 % 85 96 75 65 SiCl4 % 15 4 25 35 Prod. rate @9.61 HCVh g/h)* 7.2 9 8.5 10.4 4.21 of HCI/h 300*C H2SiCl2 % 0.01 0.06 0.3 0.1 HSiCl3 % 90.4 92.2 85 55 SiC4 % 9.6 7.8 15 45 Prod. rate @4.21 HCI/h g/h)* 2.4 3.2 3.7 1.8 9.61 of HCI/h 300*C H2SiCl2 % 0.1 0.1 HSiCl3 % 86 89.5 SiC4 % 14 10.5 Prod. rate @9.61 HC/h g/h)* 10 10 4.2 I of HCIh 350*C H2SiCl2 % 0.03 0.01 0.2 0.1 0.04 HSiCl3 % 81.9 79.3 84 50.5 25 SiCi4 % 17.5 20.6 15 49.5 74.9 Prod. rate @4.21 HCI/h gh)* 3.2 3.5 4 3.2 3.8 | g/h ) in g SiCl4/HSiC13

Claims (14)

1. Process for the agglomeration of silicon powders, characterized in that the silicon powder is heated to a temperature of at least 300'C, preferably from 5 1200 to 1500 0 C, by exposure to microwave radiation in the wavelength range from 0.5 kHz to 300 GHz, and compacted to form sintered bodies of variable porosity.

2. Process according to Claim 1, characterized in that the starting material used 10 is fine silicon dust having a particle size of less than 600 pm, preferably less than 100 pm.

3. Process according to Claim 1 or 2, characterized in that the agglomerate has a particle size of at least 30 pm. 15

4. Process according to any of Claims 1 to 3, characterized in that the heta treatment is carried out in a stirred moving bed, a tumble mixer, a rotary tube or a fluidized bed provided with internal or external means for introducing electromagnetic radiation. 20

5. Process according to any of Claims 1 to 4, characterized in that the heat treatment is carried out in a reactor made of microwave-transparent and/or microwave-reflecting material, preferably non-oxide ceramic, graphite, oxide ceramic or quartz. 25

6. Process according to any of Claims 1 to 5, characterized in that the silicon powder has been admixed with dopants containing metals, especially e.g. Cu, Ag, Al, Ca, Sn, Zn and/or boron, phosphorus, arsenic or antimony, prior to the microwave treatment. -20

7. Process according to any of Claims 1 to 6, characterized in that the agglomerate obtained by the microwave treatment has been admixed with dopants containing metals, e.g. Cu, Ag, Al, Ca, Sn, Zn and/or boron, phosphorus, arsenic oder antimony. 5

8. Process according to any of Claims 1 to 7, characterized in that the agglomerate obtained by the microwave treatment is ground and optionally classified, and the fine fraction is returned to the agglomeration zone. 10

9. Silicon granules, characterized in that they are obtainable by a process according to any of Claims 1 to 8.

10. Silicon granules according to Claim 9, characterized in that they have a porosity of 0 to 80 %. 15

11. Silicon granules according to Claim 9 or 10, characterized in that metal containing and/or nonmetal-containing foreign phases or precipitates are present at the grain boundaries of the individual crystals. 20

12. Use of the silicon granules according to any of Claims 9 to 12 for the preparation of silicon single crystals, preferably by the Czochralski method or the float zone method.

13. Use of the silicon granules according to any of Claims 9 to 10 for the 25 preparation of multicrystalline silicon blocks, preferably by oriented crystallization of melts of the silicon granules to form multicrystalline Si.

14. Use of the silicon granules according to any of Claims 9 to 10, optionally after grinding and classifying, for reaction with hydrogen chloride or 30 halogenated hydrocarbons RX in the temperature range 150-700*C in fluidized beds or shaft furnaces.