Glass manufacturing – Processes – With shaping of particulate material and subsequent fusing...
Reexamination Certificate
2000-01-14
2002-05-07
Colaianni, Michael (Department: 1771)
Glass manufacturing
Processes
With shaping of particulate material and subsequent fusing...
C065S017300, C065S425000, C065S427000, C065S474000
Reexamination Certificate
active
06381987
ABSTRACT:
DESCRIPTION
The invention relates to a process for producing opaque quartz glass by providing a mixture comprising SiO
2
particles and an additive that is volatile at a melting temperature, forming of a preform from the mixture and vitrification of the preform by heating at a melting temperature, with an advancing melt front in the preform. Furthermore, the invention concerns an article of opaque porous quartz glass.
In addition, the invention concerns an article of opaque porous quartz glass with an opening enclosed by an inner wall.
Opaque quartz glass is used to manufacture preforms for thermal applications where good heat insulation and high temperature stability are important. Increasing demands are made regarding the purity of such quartz glass preforms. Application examples may include uses in the semiconductor industry where opaque quartz glass is employed for tubes, bells and flanges for diffusion tubes. Opacity is present in low-purity quartz glass due to impurities contained therein. By contrast, with pure quartz glass source materials, opacity of the preform is achieved by pores in the quartz glass. In this context opacity means low transmission (less than a percent) both in the visible (between approx. 350 nm and 800 nm) and the IR spectra (approx. from 750 nm to 4,800 nm). The subject of this invention is the production of opaque quartz glass from pure source materials.
A process of this kind for the production of opaque quartz glass from pure source materials is described in EP A1 816,297. It is proposed there to mix high purity amorphous SiO
2
particles of purified natural crystalline quartz granulate or of synthetically produced amorphous SiO
2
with pure powdered silicon nitride (Si
3
N
4
), to place the powder mix in a graphite mold lined with graphite felt and to heat it in an electric furnace at between 1,400° C. and 1,900° C., either in a vacuum or in an inert gas atmosphere.
The heating temperature and duration is chosen such that the SiO
2
particles are completely melted, forming the article. The lower temperature limit of 1,400° C. mentioned there is given by the melting temperature of the SiO
2
particles used, while melting at a temperature above 1,900° C. results in very large bubbles which reduce the mechanical strength of the quartz glass. During the melting, the boundary of the softening and melting quartz glass progresses as the ‘melting front’ from the graphite mold radially to the interior. At the same time gaseous components such as nitrogen are released due to thermal decomposition of the Si
3
N
4
powder. The gaseous components form bubbles in the softened quartz glass, producing the desired opacity of the preform.
A preform manufactured according to the known process is composed of opaque glass with a specific density between 1.7 and 2.1 g/cm
3
, and contains between 3×105 and 5×106 bubbles/cm
3
of closed bubbles with a diameter between 10 and 100 mm, with a total bubble surface between 10 and 40 cm
2
/cm
3
and a homogenous bubble distribution.
Devitrification of the opaque quartz glass results in brittleness and reduced resistance to temperature change. In order to avoid this the known process proposes use of high purity source materials. However, contamination of the preform takes place during the manufacturing process as well. In the known process, sources of contamination can be the mold, the graphite felt and the melting atmosphere. In addition, remnants of additives that were not completely, or not at all transformed, can affect the quality of the preform.
The object of the invention is to provide a process for the manufacture of pure opaque quartz glass where the risk of contamination during the manufacturing process is reduced, and an article of pure opaque quartz glass distinguished by high resistance to temperature change, strength and chemical durability.
As concerns the process, the object is achieved on the basis of the process described initially, in that a preform is formed having an inner bore and that the heating takes place in such a way that the melt front advances from the inner bore to the outside.
The preform is formed either from loose fill or from mechanically, chemically or thermally pre-compacted porous mixture of amorphous or crystalline SiO
2
particles and an additive. The additive is generally present as a powder or a liquid.
The melt front is an inexact boundary region between melted and partially melted material. Open pores and channels are present in the partially melted material, while closed pores are present in the melted material.
“Melting temperature” means the highest temperature measured during the melting at the wall of the inner bore of the preform.
The preform is heated from the inner bore so that the melt front advances from the inner bore through the wall of the preform to the outside. Shape and location of the inner bore are not decisive for the invention; in the simplest case the inner bore is a central through bore.
According to the invention the melt front advances from the inner wall of the inner bore through the preform to the outside. Sublimable impurities pass into the gas phase. The relevant impurities here are primarily those escaping from the SiO
2
particles or from the additive at the melting temperature, or originating in the heat source or the heating atmosphere. The impurities are driven by the melt front to the outside in the direction of the preform regions which are still porous.
The additive is volatile at the melting temperature and releases gases at the melting temperature. The gases are created by transformation (evaporation or decomposition) of the additive and lead—as is intended—to the formation of bubbles in the region of the softened quartz glass. As the melt front advances to the outside the gases reach the boundary region of the preform where they can escape or be suctioned off. Remnants of the additive can impair the devitrification resistance of the opaque quartz glass; however, since the central regions around the inner bore are exposed to the melting temperature the longest, the transformation of the additive there is complete or most advanced. Since the gas being created there is being driven to the outside, the porosity of the melted preform in this region is especially low and increases toward the outside. In this the process according to the invention differs from the known process described initially. In the latter, the melt front advances radially from the outside to the inside so that the contaminants—originating for example from the graphite mold or the graphite felt—preferably accumulate in the central region of the preform where they have in general the most. damaging effect, and cannot be removed.
Due to the effect of the melt front advancing from the inside to the outside as described above in more detail, the danger of contamination of the opaque quartz glass is reduced in the manufacturing process according to the invention. The heat source in the inner bore of the preform is surrounded by like material (SiO
2
particles) acting simultaneously as the outer thermal insulation of the heat source. Contamination by foreign insulating material is therefore prevented.
It is not necessary, and in view of maintaining as high a purity of the melted preform as possible also generally not desired that the melt front should advance through the entire wall thickness of the preform. A remaining layer of unmelted SiO
2
particles facilitates removal of the melted preform from the mold, contributes to the removal of gases during the melting and prevents diffusion of contaminants into the preform from the outside, for example from foreign mold material.
Advantageously, the preform is heated by regions (zones) along a longitudinal axis of the inner bore. This variant of the process permits a particularly high melting temperature. A high melting temperature (for example above 1,900° C.) leads to a complete transformation of the chosen additive. Remnants of the additive in the melted preform are thus avoided. Also, in this variant of the process, add
Fabian Heinz
Göbel Rolf
Leist Johann
Rosin Erich
Uebbing Bruno
Colaianni Michael
Heraeus Quarzglas GmbH & Co. KG
Tiajoloff Andrew L.
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