WO2011067275A1

WO2011067275A1 - Method for producing a crucible from silica glass

Info

Publication number: WO2011067275A1
Application number: PCT/EP2010/068613
Authority: WO
Inventors: Waltraud Werdecker; Burkhard Oberle; Johann Leist; Rolf Goebel; Paul Guenther
Original assignee: Heraeus Quarzglas Gmbh & Co. Kg
Priority date: 2009-12-04
Filing date: 2010-12-01
Publication date: 2011-06-09
Also published as: DE102009056751B4; CN102648165A; DE102009056751A1; CN102648165B

Abstract

A method for producing a crucible from silica glass with side walls (19) arranged in the shape of a polygon and connected to a planar base (18) is known, in which the side walls are produced by virtue of a grain layer (15) being formed from SiO2 grain around a molding die (13) in a rotating fusion mold (1), and regions of said grain layer (15) being melted by means of a heat source (5, 6, 12) after extraction of the molding die (13). To make it possible, taking this as a starting point, to produce cheap crucibles, in particular solar crucibles composed of silica glass, with high dimensional stability, it is proposed according to the invention that the base (18) is produced by virtue of a grain layer (11, 22) being formed from SiO2 grain, said grain layer being thermally or mechanically compacted to form a base plate (17, 27), and the compacted base plate (17, 27) being provided with the side walls (19) in the fusion mold (1).

Description

Method of making a quartz glass crucible

The invention relates to a method for producing a crucible made of quartz glass with disposed in a polygon and connected to a flat bottom side walls, which are formed by SiO ₂ granulation in a rotating about a longitudinal axis of the melt mold an annular granulation layer around a mold and this is partially melted after removal of the mold by means of a heating source.

In the production of blocks of polycrystalline solar silicon

Melting crucible made of quartz glass with a rectangular shape used, which are referred to here as a solar crucible.

Background Art Solar bars are generally made by ceramic slip casting from amorphous SiO ₂ grains. In this case, SiO ₂ particles are dispersed in water, poured into a mold with absorbent walls, or deposited on a porous plastic membrane, and the resulting SiO ₂ green body is dried and sintered into the solar crucible. The sintering of the SiO ₂ grain is typically carried out at a temperature below the cristobalite transformation temperature, which prevents the use of a crystalline, inexpensive quartz sand grain. Suitable amorphous grains must be produced consuming by melting crystalline raw materials and subsequent comminution. A porous, open-pored wall is obtained. In order to avoid that the crystallized silicon connects to the inner wall of the crucible, which makes the removal of the Sililziumblocks difficult and can lead to cracks, it is customary to provide the crucible inner wall with an acting as a separating layer Si ₃ N ₄ layer. - -

In order to achieve the highest possible efficiency of the solar silicon, an entry of metallic impurities from the crucible material during the crystallization process should be avoided as far as possible. Therefore, both the solar crucible and the Si ₃ N ₄ layer is made from as pure as possible starting materials.

In DE 101 14 484 A1 a Schlickergießverfahren for producing a solar pot made of a composite material is proposed, which is produced exclusively from synthetic amorphous starting materials, and having only closed pores without crystalline fractions. However, the synthetic starting materials for the production of the composite material are particularly expensive.

Further problems in slip casting arise in particular by the shrinkage of the green body during drying and sintering. Shrinkage cracks can occur and the dimensional accuracy of the components is often low. The dry shrinkage also complicates the production of solar crucibles by so-called core casting, in which the slurry is poured around a rectangular core, which is removed after drying. Due to the shrinkage during drying, the green body shrinks on the core and ruptures, or the core can not be pulled without damaging the green body of this. Solar bars produced by this process are therefore conical, so that the mold release is guaranteed. However, this results in a significant waste of polysilicon for the user since the melt blocks have to be straightened.

A generic method for producing rectangular quartz glass crucibles is known from JP 58-08829 A. It is proposed to insert into a mold a rectangular casting core having a bottom and four square side walls. The parts of the casting core are connected to sliding elements which are mounted so as to be bendable on a central axis. By moving the sliding elements of the casting core can be folded, similar to an umbrella. The central axis is connected to the bottom of the mold. in the , ,

In the unfolded state, a gap remains between the melt-mold inner wall, the side walls and the bottom of the casting core which is filled with quartz glass powder. The mold is then rotated with velvet of the casting core, so that the quartz glass powder is pressed due to the centrifugal force to the wall of the melt mold. With sufficiently high centrifugal force to stabilize the powder layers, the casting core is collapsed and the centrifugal stabilized quartz glass powder layer is sintered by introducing an arc or a gas flame. In this way, a rectangular quartz glass crucible with uniform wall thickness is obtained.

However, it has been found that the centrifugal force required to stabilize the quartz glass powder layer is so great that the granular layer forming the bottom deforms so that it is substantially thinner in the central region than in the central region

Outdoors.

Technical task

The invention is therefore based on the object to provide a method which allows the production of inexpensive crucibles made of quartz glass with a polygonal base, in particular the production of inexpensive solar bars, with high dimensional accuracy.

This object is achieved on the basis of the above-mentioned method according to the invention in that the soil is produced by forming a granulation layer from SiO ₂ granulation, solidifying it thermally or mechanically to a bottom plate, and providing the solidified bottom plate in the melt mold with the side walls ,

In the method according to the invention it is provided that first a bottom plate is produced, and is connected in a separate process step with the wall. As a result, the process parameters in the manufacture of the bottom plate and in the production of the side walls can be optimized individually. In particular, it is not necessary, for example, to ensure a shape rotation in the production of the base plate, as otherwise used for producing the base plate. Positioning the side walls to fix a vertical graining layer is required or helpful.

The preparation of the soil involves the formation of a granulation layer of SiO ₂ grain having a uniform or uneven thickness. This is solidified to the flat bottom plate, the solidification by applying pressure to the granulation layer and / or by heating the granulation layer takes place. Upon heating, the granulation layer either completely closes

sintered transparent or opaque quartz glass, or it is merely sintered into a porous molded part, for example by brief heating of the granular layer or by heating at low temperature, for example at less than 1000 ° C. It is essential that the bottom plate has a mechanical stability, which prevents their geometric shape during

Significantly changed subsequent connection with the side walls.

The base plate is produced in the molten form in which the side walls are also produced or in another form. Connecting from

Bottom plate and side walls take place in the molten form. In the melt mold SiO ₂ granular layers are poured on the bottom plate or on the bottom plate, wherein the granulation layers are connected to the bottom plate during the melting of the granulation, or the bottom plate is connected to pre-consolidated side walls. At the same time, only a precompressed bottom plate is melted into opaque or transparent quartz glass.

One advantage is that the side walls of the crucible thus produced do not require a conical design for removal from the mold, so that the otherwise usual material waste by grinding the conical silicon blocks is eliminated. In a preferred variant of the method it is provided that the production of the bottom plate takes place in the molten mold, wherein the bottom graining layer is produced in a bottom region of the molten mold and thermally consolidated by using a heating source to the bottom plate. , ,

In this case, first of all an SiO ₂ grain layer is produced at the bottom of the melt mold and then thermally either completely or at least solidified using an arc, a laser or a burner flame such that there is no appreciable change in the geometry of the solidified bottom plate in a subsequent process step, and although even if the production of the sidewalls takes place with rapid rotation of the mold. For this purpose, it is advantageous to previously thermally solidify or densify the bottom graining layer, which takes place using a heat source without rotation or with slow rotation of the molten mold. Optionally, the rotation is so slow that a significant mass transport of SiO ₂ grain outward under the effect of centrifugal force fails.

Subsequently, the graining layer is filled into the molten mold to produce the sidewalls, and mechanically solidified with rotation of the molten mold and centrifugal force on the wall of the molten mold, so that the mold can be removed without or with further rotation of the molten mold without the graining layer in itself

coincides. Subsequently, the granulation layer is melted by forming the side walls by means of the heating source and thereby simultaneously connected to the pre-consolidated bottom plate. In an alternative and equally preferred procedure it is provided that the production of the bottom plate takes place in a bottom plate forming device spatially separate from the mold, wherein the bottom granulation layer is produced in the bottom plate forming device and subsequently solidified mechanically or thermally to the bottom plate. In this case, an SiO ₂ grain layer is produced in the bottom plate forming device and then solidified by means of an arc, a burner flame or by laser to the bottom plate. The consolidated and compacted bottom plate is then placed in the mold for connection to the sidewalls. For this purpose, a solidification or compaction of the bottom plate, which ensures that a subsequent rapid , ,

Rotation of the melt mold in the production of the side walls no significant spatial deformation of the bottom plate can cause more. The

complete compaction of the bottom plate takes place in the bottom plate forming device or in the melt mold. A particular advantage of this procedure is that the solidification of the soil-graining layer can take place in a specially optimized and adapted for this purpose bottom plate molding apparatus. In particular, the bottom plate forming device has the smallest possible free surface, which is attacked by an oxidizing atmosphere when solidifying the soil graining layer.

As SiO ₂ grain amorphous or crystalline grain size is used, which may be in loose form or as a so-called ramming mass. Preferably, an SiO ₂ grain is used, which contains crystalline quartz raw materials, wherein the melting of the grain at a temperature of at least 1900 ° C. In this case, SiO ₂ grains of crystalline, naturally occurring quartz or of synthetically produced quartz crystal is used. As a result of the high melting temperatures, the crystals melt and the phase is converted into the amorphous form (quartz glass). A cristobalite formation is reliably prevented. Preferably, the SiO ₂ grain contains naturally occurring quartz sand. Such quartz sand is inexpensive, but has a relatively low purity. Due to the high temperature during melting of the grain, however, there is a certain purification by evaporation of impurities, such as alkali metals, in particular from the region of the inner wall of the crucible, where they would be particularly detrimental to the intended use. Therefore, the requirements for the purity of the SiO ₂ grain are lower than for only sintered crucibles, which leads to a further cost reduction in the manufacturing process. The melting process results in a crucible inner wall having a smooth, fire-glazed surface. Therefore, a complex coating of the inner surface with Si ₃ N ₄ , as in only sintered

Solar labels is required, is omitted. This also contributes to a reduction in costs.

It has proved favorable if the SiO ₂ grain comprises splintery grains.

Split-pitched grains contribute to a certain interlocking within the granulation layer and stabilize them early. In addition, they produce a relatively low shrinkage during melting.

In the case of compaction of the bottom graining layer using an arc in the manufacture of the bottom plate, it has been found to be useful to produce a bottom graining layer having a thickness greater in the center than at the edge. When melting the bottom graining layer by means of centrally applied arc it comes to evaporation of the granulation material in the region of the bottom center due to the high Abstrahlleistung; especially in the case of SiO ₂ grain size. In addition, the arc pressure can cause blistering of the grain. Since the arc is applied substantially centrally of the bottom plate, there is a mass loss in the center of the ground and a mass transport from the center to the edge, which is at least partially compensated by said measure. In this way, a bottom plate is obtained, which has in the middle substantially the same thickness as at the edge.

Furthermore, it has proven to be advantageous if at least when producing the side walls, a vacuum mold with porous walls is used.

As a result, a vacuum can be applied to the SiO ₂ granulation layer during the melting process, which causes a shortening of the melting time and leads to a further reduction in the cost of the production process and to - -

contributes to a higher reproducibility of the crucible. Also contributing to this is a vacuum in the melting of the soil-graining layer as part of the production of the bottom plate.

In this context, it has proven useful if the vacuum melt mold has a higher gas permeability in the area of edges than in the area of surfaces.

The SiO ₂ grain layer experiences a lower temperature in edges and corners of the melt mold during melting. The higher gas permeability of the melt mold in such regions remote from the arc enables the application of a stronger vacuum (a higher negative pressure) and thus contributes to an easier melting of the grain.

embodiment

The invention will be explained in more detail with reference to embodiments and a drawing. 1 shows, in a schematic representation, the production of a base plate in a vacuum melt mold as a first process step for producing a solar hotplate according to a first embodiment of the method according to the invention,

FIG. 2 shows the production of a base plate in a melt mold as the first process step for producing a solar pot according to a second embodiment of the method according to the invention,

3 shows the bottom plate after removal from the melt mold according to FIG. 2, FIG.

4 shows the production of side wall graining layers in the vacuum melt mold on the bottom plate according to FIG. 1 or FIG. 3, FIG.

5 shows the melting of the side wall graining layers for the production of the side walls and their fusion connection with the bottom plate according to FIG. 4. - -

Production of a base plate Example 1

The vacuum melting device according to FIG. 1 comprises a melting mold 1 with a rectangular cross-section which is rotatable about a rotation axis 2. The walls fertil 10 and the bottom 9 of the mold 1 are made of porous graphite, as indicated by the openings 16. The interior of the mold 1 is thus evacuated and the mold 1 is connected to a (not shown in the figure) vacuum device. The porous bottom and side walls (the graphite melt mold 1 exhibit higher gas permeability in the regions of the edges and corners than in the central surface regions.) The edge length of the square bottom 9 of the melt mold 1 is 30 cm in the interior and the square sidewalls 10 have a height of 50 cm.

In the interior of the mold 1 protrude electrodes 5, 6 of graphite, which are movable in all directions in space and between which an arc 12 can be ignited. The open top of the mold 1 is covered by a heat shield 7 in the form of a water-cooled metal plate, which above the

Melt mold 1 is horizontally movable, and having a through hole through which the electrodes 5, 6 protrude into the melt mold 1. The heat shield 7 is provided with a closable gas inlet 8 for a protective gas (nitrogen). Between the mold 1 and the heat shield 7 remains a vent gap.

In the following, the production of a bottom plate for a solar crucible made of quartz glass according to the invention using the melt mold 1 shown in Fig. 1 will be explained in more detail.

In a first process step, crystalline grain size of naturally occurring quartz sand purified by means of hot chlorination, having a grain size in the range from 90 μm to 315 μm, is introduced into the slowly rotating melt mold 1. - -

crowded. These are splintered grains which have been obtained by grinding and which are characterized by a high bulk density.

The bottom granulation layer 11 deposited on the bottom 9 of the melt mold 1 has an average thickness of approximately 5 cm and drops somewhat from the center to the edge.

The electrodes 5, 6 are lowered downward in the direction of the granulation layer 1 1 and between them the arc 12 is ignited in a protective atmosphere of nitrogen and the melt mold 1 is rotated about its longitudinal axis 2 at a low speed of 5 rpm. The bottom-graining layer 1 1 is heated to a temperature of more than 1900 ° C and melted so far that the quartz grains are fixed in the region of the surface so that they can not move outward at an increased rotational speed in the subsequent process step.

The nitrogen atmosphere prevents oxidation of the side walls 10 of the melt mold 1, which are not covered by quartz sand and therefore completely unprotected. The middle higher layer thickness of the granulation layer 1 1 serves to balance the mass transport of grain to the outside due to the arc pressure.

In this way, a flat, approximately uniformly thick bottom plate 17 (see Figure 2) is obtained with a thickness of about 1 cm, which consists of a partially glazed and contiguous surface layer on the underside still largely loose quartz sand or partially glazed grain adheres. A part of the original granulation layer 1 1 remains at the bottom of the mold 1.

Example 2 In the procedure for producing the bottom plate of the solar pot illustrated in FIGS. 2 and 3, a separate bottom plate forming device 21 is used. - -

A granular layer of quartz sand, as described in Example 1, is introduced into the receptacle of a graphite molding apparatus 21 having the same edge length as the bottom 9 of the mold of FIG. In contrast, however, the side walls of the molding apparatus 21 are so low that they are almost completely covered by the graining layer 22. The quartz sand grain layer 22 has a thickness of 5 cm and is surface-glazed with slow rotation (5 rpm) of the shaping device 21 about its rotation axis 23 using a CO ₂ laser 24.

The surface-glazed, square base plate 27 is removed from the shaping device 21 and is shown schematically in FIG. It has a thickness of 8 mm and an edge length of 30 cm. It consists of a partially glazed and coherent surface layer 28, on the underside of which largely loose quartz sand or partly glazed grain 29 adheres. A part of the original quartz sand grain layer 22 remains at the bottom of the melt mold 1.

This procedure has the advantage that the side walls of the melt mold 21 show little free surface that can be corroded when heated to produce the bottom plate 27.

The bottom plate 27 is finally introduced into the mold 1 and provided with side walls, as will be explained in more detail below.

Making the sidewalls and connecting to the bottom plate

A centrally suspended, rotatable square forming tool 13 is centrally inserted into the melt mold 1 and on the bottom plate 17; 27 attached. Between the melt mold 1 and the mold 13 remains an annular gap with a rectangular shape. As shown in FIG. 4, the gap 14 becomes the same

Filled quartz sand grain, which is also used for the preparation of the bottom plate 17; 27 has been used. - -

The on the bottom plate 17; 27 heaped, vertical graining layers 15 fill the annular gap almost completely and thus protect the inner wall of the crucible 1 during the subsequent melting process.

After filling the gap, the melt mold 1 together with the mold 13 is set in rotation and the mold 13 upwards

pulled out. The rotational speed is 90 U / min and is chosen so that the vertical graining layers 15 remain stationary due to the centrifugal force.

FIG. 5 shows schematically that, after removal of the mold 13, the electrodes 5, 6 are reintroduced into the interior of the mold 1.

Between them, the arc 12 is ignited and the graining layers 15 for the side walls are heated starting from the crucible inside to a high temperature of more than 2000 ° C and melted to quartz glass. At the same time a vacuum is applied to the outside of the mold, which allows a bubble-free or low-bubble vitrification of the side walls and which contributes to a significant reduction in the duration of the process. At the same time there is a fusion of side walls and bottom plate 17; 27th

The high temperature of the arc 12 causes an evaporation of alkalis, so that there is a certain purification of the side walls in the near-surface region.

There is obtained a solar crucible with a square bottom 18 and vertical, squared side walls 19. The edge length of the floor (interior) is 30 cm, the height of the side walls 45 cm, the final thickness of the floor 15 mm and the average wall thickness of the side walls is 12 mm.

The solar crucible is characterized by an arc-sealed crucible inner wall with a smooth, sealed, fire-glazed surface, so that a costly Si ₃ N ₄ coating as in conventional solar crucible omitted - -

can. As a result of the mentioned evaporation of impurities shows the crucible inner wall, despite the use of natural quartz sand grains a relatively high purity.

Claims

claims

Method for producing a crucible made of quartz glass with side walls (19) arranged in a polygon and connected to a flat bottom (18), which are produced by forming a SiO ₂ grain in a melt mold (1) rotating about a longitudinal axis (2) Granulation layer (15) around a molding tool (13) is formed and these after removal of the mold (13) by means of a heating source (5; 6; 12) partially melted, characterized in that the bottom (18) is generated by SiO ₂ Formed a bottom graining layer (1 1; 22), this thermally or mechanically solidified to a bottom plate (17; 27), and the solidified bottom plate (17; 27) in the mold (1) provided with the side walls (19) becomes.

A method according to claim 1, characterized in that the manufacture of the bottom plate (17) in the melt mold (1), wherein the bottom graining layer (1 1) in a bottom region (9) of the melt mold (1) and using the heat source (5; 6; 12) is thermally consolidated to the bottom plate (17).

A method according to claim 1, characterized in that the production of the bottom plate (27) takes place in a bottom plate forming device (21) spatially separate from the melt mold (1), the bottom granulation layer (22) in the bottom plate forming device (21). is generated and then mechanically or thermally solidified to the bottom plate (27).

Method according to one of the preceding claims, characterized in that an SiO ₂ grain used, containing crystalline quartz raw materials, wherein the melting of the SiO ₂ grain at a temperature of at least 1900 ° C. Method according to one of the preceding claims, characterized in that the SiO ₂ grain contains naturally occurring quartz sand.

Method according to one of the preceding claims, characterized in that the SiO ₂ grain comprises splintered particles.

Method according to one of the preceding claims, characterized in that a bottom graining layer (1 1) is produced, which has a thickness in the middle which is greater than at the edge, and that the compaction of the bottom graining layer (1 1) using a Arc (12) takes place.

Method according to one of the preceding claims, characterized in that at least when generating the side walls (19) a

Vacuum-melt mold (1) with porous walls (9; 10) is used.

A method according to claim 8, characterized in that the vacuum melt mold (1) has regions remote from the arc with a higher gas permeability and regions near the arc with a lower gas permeability.