WO2011014077A1

WO2011014077A1 - Crucible

Info

Publication number: WO2011014077A1
Application number: PCT/NO2010/000295
Authority: WO
Inventors: Oddvar SKÅRDAL
Original assignee: Nordic Ceramics As
Priority date: 2009-07-31
Filing date: 2010-07-30
Publication date: 2011-02-03
Also published as: NO20092797A1

Abstract

Reusable crucible together with manufacturing and use of this are disclosed. Two material groups are disclosed, wherein the crucible is characterized by comprising a carrying layer and an inner layer, optionally also an outer layer. It can also be characterized by a crucible with one carrying core material which is covered completely or partly by the outer surfaces.

Description

Crucible

Background of the invention

Field of the invention

The invention regards crucibles in general and particularly crucibles for melting and solidification of the silicon crystals.

Background information

There is need for melting and solidification of silicon crystals in a cost effective method and without contamination of the silicon with impurities from the crucible.

Quartz is today the most used material for crucibles used for multi-crystalline silicon melting within the PV - industry (Photo Voltaic). This crucible material has, however, a number of weaknesses that broadly can be described as:

• Use of quartz crucibles causes contamination problem in the processing of solidified silicon for the PV-industry.

• It is only possible to use a quartz crucible ones since it cracks and becomes destroyed in the process of melting down and solidifying silicon.

From the known art one should refer to a number of patents and applications. The patent JP-59-162199 discloses a crucible with high density with correspondingly high strength and is manufactured in the material RBSN (reaction bound silicon nitride) using a dry pressing and then a sintering in two steps, first by heating in an inert atmosphere and then a sintering at a higher temperature. This crucible is wetted by the silicon melt which then causes the solidified silicon block to attach to the crucible. The result of this is that these crucibles can only be used for one cycle.

The patent NO 317 080 provides a solution for wetting of the material by use of a material and process in the manufacturing of a crucible for use of melting and solidification of silicon crystals. The manufacturing of crucible is performed by the use of a specific graining of the raw materials together with a certain production process followed by sintering of the crucible material. At the given boundary values the density and partly also the pore size for the material is strengthened. The defined boundaries for density is given to be between 40-60 % of theoretical value, and it is stated that at least 50 % of the surface pores must have a larger diameter than the mean value for the licon nitride powder). The crucible according to NO 317 080 is mαiiuiαciuicu tu> a complete unit, it is manufactured as a complete crucible by filling a mould with raw material/powder and that the powder then is compacted using vibration.

US application 2004-0211496 discloses the manufacture of an RBSN (reaction bound silicon nitride) material or an isostatically pressed silicon nitride crucible where both variance are stated with a limit size and with relatively large wall thickness.

The application PCT/NO2007/000220 discloses a recipe based on a small part Si (silicon) with correspondingly large part of Si3N4 powder with maximum 40 % Si and minimum 60 % Si3N4 respectively. In addition they have a method of manufacture where the crucible is provided with holes in the base plate together with pins in the four side plates. These plates are then nitrided and sintered to a NBSN (nitride bounded silicon nitride) material. Then the plates are assembled into a complete crucible using a silicon glue/paste. The assembled crucible is then heated in a nitrogen atmosphere until conversion of glue/paste is performed to a NBSN material. No limitation is stated for this method of manufacturing only and the patent also refers to the assembly of green crucible manufactured by different methods where the nitrating is performed by a complete crucible. Problems

Manufacturing large crucibles in the material NBSN is complicated and there is today no crucible of this material available for use within the PV industry. During use large temperature tensions arises, thermo-shock and mechanical loads on the crucibles, something that can lead to uncontrolled breakage and cracks in these. The coefficient of expansion between the crucible and finished product must be adapted to the process, the result of unequal expansion/contraction of crucible material will give rise to too large tensions in the material and thus cracks will appear in the crucibles during temperature changes. A difference in coefficient of expansion is possibly the most important cause for the cracking of today's quartz crucibles during the first time use. The quartz material has the property of transformation during cooling of the bind phase from a glass phase to a crystalline phase and this transformation causes formation of cracks in the quarts material.

Other causes for the damaging of the crucible or making it impossible to use for several cycles is that there is adhesion between the produced unit and crucible, and this is the case for all crucible materials including quartz crucibles wetted by the melt of the produced unit. Form stability during high temperature is an absolute requirement for a reusable crucible, for quartz crucible there is a need for additional material for maintaining form stability. aterials used by to manufacture pure silicon melt as of today is μi uuu^Gu uy α ₄1ml tz material which is not chemically inert with respect to the silicon melt, that is there is during the course of the temperature cycle having a maximum of about 1500 ⁰C, a migration of contaminations from the crucible to the produced unit, There is therefore a need for a product that overcomes the above mentioned problems.

Object of the invention

The object of the invention is to provide a reusable crucible and a method for manufacturing of this, where the crucible is stable with respect to the form and minimizes migration of contaminations from the crucible to the produced unit.

Summary of the invention

The invention provides a method for the manufacture of the reusable crucible characterized in comprising the steps construction of a carrying layer of material group

2, and application of an inner layer of material group 1 and the crucible manufactured by this method.

Beneficial and preferential embodiments of the invention are disclosed in the dependent claims.

The crucible according to the present invention is manufactures in a material which to a small degree contaminates the product and maintains its construction and material embodiment the stresses and strains that arise during use and a method for manufacture of the reusable crucible characterized in that the crucible is manufactured in a material configured as a sandwich with interior material of the crucible is a NBSN material which during its manufacturing process is sintered together with an external crucible material in NBSiC. Alternative embodiment is an NBSN-material with or without a combination of NBSiC-material.

Beneficial an preferential embodiments of the invention is disclosed in the dependent claims.

The crucible during use will provide a minimum of contaminations to the manufactured ingot. It will be reusable, also it will be easier to handle and to split the crucible and ingot from each other.

Brief description of the drawings

Fig. 1 shows a typical embodiment of the invention with two layers.

Fig. 2 shows a typical embodiment of the invention with three layers.

The following reference numbers and figures refer to the drawings:

Terminology

In the description the following terminology is used:

Detailed description of the invention

The invention will in the following be described in detail with reference to the drawings showing several embodiments and where fig. 1 schematically shows a typical embodiment of a reusable crucible.

The choice of material as well as manufacturing process is important and one has arrived to 2 material groups.

Details material group 1

Crucible manufactured in the material NBSN using mixture of Si, SiC and SN- powder to a licker with suitable properties for use for casting of crucible elements in gypsum moulds. The property of the licker is not limited only for use for moulding gypsum moulds but also for pressure moulding in resin forms, freeze casting or by use for any other adapted porous material for evacuation of water from the casted material.

The standard mixture for licker is >40 % Si and <60 % SN-powder where SiC can replace up to 100 % of the SN-powder. cker can be used for casting of a complete crucible or parts elements that can men oe assemoied into a crucible where the number of elements various with design and has in manufacturing few limitations. By use of licker casting where the numbers of elements are two or more for a product the assembly of elements will be necessary. After the individual elements are dried and thus have achieved a necessary handling strength an assembly can be made using a specially suited glue having the same base properties and is based on the same raw materials as can be found in the manufactured part elements.

During pressure casting, freeze casting or similar possible processes for the manufacturing of a complete product through use of a licker it will be possible to manufacture a crucible in one piece and an assembly of part elements will not be necessary.

After assembly or during manufacturing of a complete crucible the next process step will be nitrating of the crucible. This is performed in a closed furnace where one has an atmosphere comprising a gas mixture of nitrogen and Argon, also one has a controlled temperature curve with a maximum temperature of about 1500 ⁰C. The nitrogen which is a part of the gas mixture will react with the silicon and form the product NBSN and surround and form bindings between/of the additive material SN-powder, the mixture between SN and SiC powder or a 100 % addition of SiC-powder.

Detail material group 2

Crucible manufactured in the material NBSN by use of a mixture of Si, SiC and SN- powder combined to independent combinations which are then formed to a layered construction of a crucible. The standard mixture for inner layer is >40 % Si and <60 % SN- powder where SiC can replace up to 100 % of SN-powder. The core material which is the carrying part of the construction is based on a NBSiC material with a part SiC > 60 % and maximum amount of Si of 40 %. The product, the finished two-layer sandwich can also be combined with a third layer which can cover the entire or part of said two-layer sandwich. Application can be made after the first two layers are processed together and layer No. 3 can have the same material structure as given for the standard mixture of the inner layer 14 or it can be based on further more pure raw materials and thus provide an absolutely pure surface material.

The structure of the sandwich with the carrying construction based on a material of NBSiC, and where outer surfaces are based on the material NBSN, is formed in a combined vibration/strike and pressure press. The individual layers for the sandwich can be constructed separately but in the same form, and the assembly itself into the compacted can be made by use of the specially adapted press equipment.

The sandwich can also be structured in that the core material NBSiC undergoes a compaction and that one at a later stage applies the outer surfaces with an NBSiC material given for the inner layer 14 or corresponding licker based on additionally purer raw materials. Method for applying outer materials can then be use of glaze technology, (ref. porcelain cal application (brush, roll, spackling etc.) or an application by spraying wim iayer iυimauυii to the correct thickness. What is common for these methods and that characterize this method of manufacture of the sandwich is that the product is prepared in a green condition prior to sintering by backing of the product. Due to the construction and the composition of the material a controlled reaction in the backing process will at a correct addition of the gasses nitrogen and argon give a sintered sandwich with a continuous binding phase between the individual layers.

An unambiguous baking cycle does not exist, it is to a large degree dependent on the furnace which is available for nitrating. A standard procedure can be that the material is heated carefully up to 1250 ⁰C in a nitrogen or combination of nitrogen and argon atmosphere. During further increase of temperature it is necessary with a control of the temperature and with control of the reaction that takes place. The nitrogen generates reaction and the argon reduces or limits the reaction. With a combination of these gases and with a careful increase of temperature up to 1400-1500 ⁰C a NBSN-material can be produced. Depending of the type of furnace and the material which is produced a baking cycle can last from 2-3 days up to 8-10 days.

On using a pressed sandwich element as part elements for a crucible an assembly of the elements will be necessary for thereby create the complete crucible. Such an assembly can be made in that the elements are put together using specially adapted glue, where the glue is based on the same raw materials as can be found in the surface material of the one or more manufactures sandwiches.

The number of elements that can be used for the manufacturing of a crucible where the material is based on a sandwich solution is not limited in number, a complete crucible is not manufactured as a single element can be assembled from two to n-elements.

Naturally this sandwich material is not only suitable for use for crucibles or other baking facility in the solar industry, but can also be used within other type of industry where a high temperature ceramic material is necessary or as a ceramic construction for machine components or wear components with applications such as coating in chutes, containers, mills, cyclones, as tubes and tube bends, pumping housing and impellers, fan houses and fans, nozzles and more. This with materials within the industries such as energy productions using gas/coal, chemical industry, metallurgical industry, processing of minerals, paper industry and oil and gas industry, but certainly not limited to just these which are only meant as examples of application areas.

Figure 1 shows a typical embodiment of the invention 10 in cross section, comprising a carrying layer 16 of the material group 2. On the inside of the carrying layer an inner layer 14 is applied of the material group 1. This means that impurities typically in the form of carbon compounds will not come into contact with the melt inserted into the cavity of the crucible 12. iment

r igui c z. snows a typical embodiment of the invention 20 in cross section, comprising a carrying layer 16 of material group 2. On the inside of the carrying layer an inner layer 14 is applied of the material group 1. In addition an outer layer 22 is applied of the material group 1 with the advantage that impurities, typically in the form of carbon compounds in gas form, do not come into contact with the melt inserted into the cavity of the crucible 12.

Industrial applicability

The invention is applicable for the manufacturing of reusable crucibles and u se of these for melting and solidification of silicon crystals. There will also be applications within other types of industry where high temperature ceramic materials are necessary and can also be used as a ceramic construction such as machine component or wear component in abrasive processes. Industries such as energy productions using gas/coal, chemical industry, metallurgical industry, processing of minerals, paper industry and oil and gas industry are examples of industrial areas of use.

During typical use of a crucible according to the invention the crucible 10, 20 are filled with Si followed by melting of Si in the cavity of the crucible 12.

Claims

P a t e n t C l a i m s L A method for manufacture a reusable crucible (10, 20), characterized in comprising the steps:

construction of a carrying layer (16) of material group 2, and application of an inner layer (14) of material group 1.

2. The method according to claim 1, characterized in further comprising the application of an outer layer (22) of material group 1.

3. The method according to claim 1, characterized by further comprising sintering.

4. A reusable crucible (10, 20), characterized in comprising:

a carrying layer (16) of material group 2, and

an inner layer (14) of material group 1.

5. The reusable crucible according to claim 4, characterized by further comprising an outer layer (22) of material group 1.

6. A method for melting of Si, characterized in comprising the steps:

filling of a crucible (10, 20), comprising a carrying layer (16) of material group 2 and an inner layer (14) of material group 1, with Si, and

melting of Si in the cavity (12) of the crucible.