Chemical apparatus and process disinfecting – deodorizing – preser – Physical type apparatus
Patent
1999-08-04
2000-12-26
Powell, William
Chemical apparatus and process disinfecting, deodorizing, preser
Physical type apparatus
4222451, B01D 900
Patent
active
061654256
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to melting pots provided with silicon protective layers, a process for applying the silicon protective layer and the use thereof.
BACKGROUND OF THE INVENTION
In the course of processing high-purity silicon for applications in semiconductor technology and photovoltaic applications, dealing with silicon melts plays a central role. For example, single crystals for semiconductor technology and photovoltaic applications are pulled from the silicon melts using the Czochralski method. Furthermore, silicon melts can be processed by crystallization using the Bridgman method or by block casting with subsequent controlled crystallization in an economically favourable way to form block material with an advantageous polycrystalline lattice structure, which is subsequently sawed to form silicon wafers for the production of polycrystalline solar cells. Silicon melts also play an important role in various metallurgical processes which are used for cleaning the silicon, for example bubbling various reactive gases through them or slag extraction methods.
In these processes, the use of materials which withstand the attack by the corrosive hot silicon melts and, at the same time, do not release any unacceptable impurities into the ultrapure melt or the melt which is still to be purified is of essential importance.
Usually, for handling ultrapure silicon melts, quartz-glass or fused-silicon instruments are used. Sometimes, however, shaped ceramic articles made of silicon nitride or ceramic containing silicon nitride are also proposed.
On contact between quartz and liquid silicon, however, a chemical reaction takes place with the formation of SiO gas. Owing to this corrosive attack, not only is the material wetted by the melt gradually dissolved and therefore, in the case of prolonged contact times, destroyed, but also in the case of crystallization of the melt or of melt residues in melting pots or moulds, extensive adhesion and baking occurs between the shaped article and the solid silicon. Because of the different coefficients of thermal expansion of quartz and silicon, thermal stresses are created which regularly lead to cracks, flaws and flaking during the cooling of the crystallized silicon. In the case of silicon blocks resulting from controlled crystallization which are intended to be processed further for photovoltaic applications, this phenomenon leads to significant losses of material. In the extreme case, the thermomechanical stresses may be so great that owing to cracks, flaws or flaking, as well as quartz residues baked solid on the block walls, the entire silicon block becomes unusable.
Also in the case of contact between silicon melts and shaped articles made of silicon nitride or ceramic containing silicon nitride, solid baking between crystallized silicon and the ceramic shaped article occurs.
Baking not only leads to considerable losses in the yield of usable silicon, but also prevents reuse of the melting pots. Furthermore, crystallized silicon blocks which have become baked require considerable outlay in order to obtain material which can be used at all. Reliable prevention of such baking is therefore an important contribution for economic production of finished articles made of crystalline silicon.
In order to avoid contact between the silicon melt and the shaped quartz article, Conf. Rec. of 15th Photovolt. Spec. Conference 1981, pp. 576 et seq. and Solar Energy Materials 9, 337-345 (1983) have proposed coating the quartz articles with silicon nitride powder which exhibits a needle- or whisker-like particle morphology.
It has been found, however, that such coating cannot reliably prevent adhesion between quartz or ceramic components and silicon with the described negative consequences primarily when handling relatively large amounts of melt with prolonged contact times between the melt and the shaped article, over the entire contact area between the liquid silicon and the shaped article. Added to this, there are toxicology problems which allow processing of
REFERENCES:
patent: 5431869 (1995-07-01), Kumar et al.
Conf. Rec. of 15.sup.th Photovolt. Spec. Conference (month unavailable) 1981, pp. 576-580, Saito et al, A New Directional Solidification Technique for Polycrystalline Solar Grade Silicon.
Solar Energy Materials 9, pp. 337-345, (month unavailable) 1983, Saito et al, A Reusable Mold In Directional Solidification for Silicon Solar Cells.
Journal of Crystal Growth 94, (month unavailable) 1989, pp. 62-68, R.S. Ravishankar, Liquid Encapsulated Bridgman (LEB) Method for Directional Solidification of Silicon using calcium Chloride.
Gmelin Handbook of Inorganic and Organometallic Chemistry, Si Silicon vol. B 5d2, p. 214, Author, Raymond C. Sangster, Inorganic Salts (month unavailable) 1995, 8th Edition.
Gatzweiler Heinz
Krumbe Wolfgang
Lange Horst
Bayer Aktiengesellschaft
Gil Joseph C.
Powell William
Van Eyl Diderico
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