Quartz glass crucible and process for the production thereof

Details Quartz glass crucible and process for the production thereof Quartz glass crucible and process for the production thereof

2001-08-16

2004-01-06

Colaianni, Michael (Department: 1731)

Glass manufacturing

Processes

With shaping of particulate material and subsequent fusing...

C065S017300, C065S032100, C065S134900, C065S136100, C065S135600, C065S144000, C065S157000, C065S302000, C065S356000, C065SDIG004

Reexamination Certificate

active

06672107

ABSTRACT:

The invention concerns a quartz glass crucible having a crucible body of opaque quartz glass which is symmetrical relative to its rotational axis and has an outer zone of opaque quartz glass which transitions toward the inside into an inner zone of transparent quartz glass with a density of at least 2.15 g/cm
3
.
Furthermore, the invention concerns a process for the production of a quartz glass crucible by providing a mold rotatable about a rotational axis and having an inner wall, adding SiO
2
granulate into the mold to form a granulate layer on the inner wall of the mold, and heating the granulate layer from the inside toward the outside while rotating the mold and forming a vitrified crucible body with an opaque outer zone.
A quartz glass crucible of this kind is described in DE A1 4,440,104. The known quartz glass crucible comprises a crucible base body with an opaque outer zone which transitions toward the inside into a smooth, wear resistant, dense inner zone. The thickness of the inner zone is between 1 and 2 mm and the density is at least 2.15 g/cm
3
. The quartz glass crucible is produced in a slip casting process. The quartz glass is reduced to a powdered material having a particle size of less than 70 &mgr;m while water is being added The resulting slip is poured into a negative plaster mold for the crucible and the crucible blank obtained after drying is sintered at a temperature between 1350° C. and 1450° C. After the sintering, selected surface zones of the opaque and porous crucible blank are subjected to further heat treatment at between 1650° C. and 2200° C. in order to transform the opaque and porous base material into transparent quartz glass with a density of at least 2.15 g/cm
3
. As a result, the crucible base body obtains the above-said opaque outer zone which transitions toward the center into the smooth, wear resistant and dense inner zone.
A process of the kind mentioned above is known from DE C1 9,710,672, which describes the production of a quartz glass crucible for the pulling of a silicon monocrystal according to the Czochralski method, by means of the so-called pouring-in process. In the said process, a granulate layer of natural quartz glass granulate is formed at first on the inner wall of a rotating smelting mold and is vitrified, forming an opaque base body. Thereupon, in order to create a transparent, smooth, inner layer, synthetic quartz glass powder is poured in and deposits on the inner wall of the base body where it is melted into a dense, transparent inner layer by means of an electric arc. The quartz glass crucible produced in this manner consists of an opaque base body and a transparent, dense inner layer that forms the inner surface of the quartz glass crucible. The starting material of the inner layer differs from that of the base body. The inner layer is produced in a separate process step and primarily serves to prevent migration of impurities from the base body to the inner surface.
Due to its impurity content, the quartz glass crucible described initially is unsuitable for applications requiring high purity. The production of the transparent inner zone requires costly additional heat treatment. The opaque outer zone substantially blocks light in the visible spectrum, but is largely transparent in the infrared (hereinafter referred to as IR) spectrum. Radiation loss in the IR spectrum causes a considerable temperature gradients across the wall of the crucible. However, compensating for the radiation loss by raising the temperature of the melt inside the crucible can lead to softening, deformation and sagging of the crucible wall, which can result in reduced service life. This problem is most noticeable in large crucibles which are generally used for longer periods than smaller ones.
The aforementioned process for the manufacture of a quartz glass crucible using the pouring-in method also requires an additional process step for the manufacture of the inner layer and it too is therefore costly.
The object of the invention is to provide a quartz glass crucible distinguished by high purity and high opacity, i.e., low transmission in the IR spectrum, and a simple, cost efficient process for the production thereof.
The object to provide a quartz glass crucible is achieved on the basis of the crucible mentioned initially in that the crucible body is produced from synthetic SiO
2
granulate having a specific BET density ranging from 0.5 m
2
/g to 40 m
2
/g, a tamped volume of at least 0.8 g/cm
3
, and formed from at least partially porous agglomerates of primary SiO
2
particles.
The quartz glass crucible according to the invention is composed entirely of synthetically produced SiO
2
. Due to this highly pure starting material the quartz glass crucible is distinguished by high purity throughout. No measures are required to prevent migration of impurities from the quartz glass crucible into the melt contained therein.
The crucible body comprises an outer zone of opaque quartz glass and an inner zone of transparent quartz glass. The outer and inner zones are integrally joined regions. This means that there is no precise, defined boundary area.
The quartz glass crucible according to the invention is produced from synthetic SiO
2
granulate. An opaque outer zone and a transparent inner zone are obtained by vitrification of the appropriate granulate fill. During the vitrification process the vitrification front advances from the interior outward. Open pores and pore channels are closed during the process and gases are displaced in the direction of the inner wall of the form. Due to the greater effect of the temperature in the inner zone region (higher temperature and longer heating period) the inner zone is free of pores, or has very few pores, so that its density is at least 2.15 g/cm
3
. This density is nearly that of transparent quartz glass. Therefore the mechanical and chemical properties of the inner zone correspond to those of dense transparent quartz glass with respect to mechanical strength, hardness and chemical stability.
The outer zone is distinguished by high opacity in the IR spectrum. Opacity in the sense of the invention is low transparency (less than 1%) both in the visible (between about 350 nm and 800 nm) and in the IR spectra. In the IR spectrum, between 750 nm and 4800 nm, the transmissibility in a 3 mm thick disk is less than 1%. The high opacity in the IR spectrum is substantially achieved in that the outer zone is produced from SiO
2
granulate formed from partially porous agglomerates of primary SiO
2
particles and has a specific BET surface between 0.5 m
2
/g and 40 m
2
/g. Vitrification of such SiO
2
granulate results in opaque quartz glass with a homogenous pore distribution as well as with high pore density and high specific density. By contrast, opaque quartz glass produced from natural or synthetic quartz glass granulate with a low specific surface has, first of all, large bubbles and the bubble distribution density is relatively lower. This causes primarily opacity in the visible spectrum. When used as intended, the quartz glass crucible according to the invention reduces the temperature gradient across the crucible wall due to high opacity in the IR spectrum so that no compensation is required such as, for example, by the overheating of the melt or by the installation of a thermal screen (heat shield). A quartz glass crucible obtained by the vitrification of this type of granulate is therefore distinguished by good heat insulation and long service life.
The fineness of the pores in the outer zone required therefor is achieved by using a SiO
2
granulate which is present in form of at least partially porous agglomerates of primary SiO
2
particles. Such primary particles are obtained by, for example, flame hydrolysis or oxidation of silicon compounds, by hydrolysis of organic silicon compounds using the so-called sol-gel process, or by hydrolysis of inorganic silicon compounds in a fluid. Even though such primary particles are generally distinguished by high purity, they are difficult to handle due to their

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