Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2000-01-14
2002-04-30
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C065S032100
Reexamination Certificate
active
06380110
ABSTRACT:
DESCRIPTION
The invention relates to a process for the manufacture of opaque quartz glass by forming an initial body from synthetic SiO
2
granules and heating said body at a vitrification temperature to obtain a preform of opaque quartz glass.
Furthermore, the invention relates to a synthetic granulate suitable for the process, said granulate being prepared from at least partially porous agglomerates of SiO
2
primary particles.
Opaque quartz glass is primarily utilized in heat-technology applications where good thermal insulation and high temperature stability are of importance. Increasingly higher demands are made in these fields regarding the purity of such quartz glass bodies. The following examples of applications in the semiconductor industry may be mentioned: reactors, diffusion tubes, heat shields, bells or flanges. Primarily, opacity in the IR spectrum is required in these applications. Opacity means in this context low transmittance (less than one percent) both in the visible (approximately form 350 to 800 nm) and the IR spectra (approximately from 750 to 4,800 nm). In quartz glass of low purity the desired opacity occurs by itself due to the contaminants contained therein. On the other hand, when pure source materials are used, transparent quartz glass is obtained so that the opacity must be created by artificially introduced pores. The subject of the present 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 create opacity in the quartz glass by making and melting a powder mixture of synthetically produced SiO
2
particles having an average particle size of 300 &mgr;m and an additive material in form of powdered silicon nitride. Gaseous components such as nitrogen are released during the melting of the powder due to the thermal decomposition of the Si
3
N
4
powder. The gaseous components cause bubbles to be formed in the softened quartz glass thus creating the desired opacity of the body. A graphite mold lined with graphite felt is filled with the powder mixture and heated in an electric furnace in a vacuum at a temperature of 1,800° C. During the melting the frontal region of the softening and melting quartz glass advances as a “melt front” radially outward from the walls of the mold.
Devitrification of the quartz glass can take place due to contaminants, leading to brittleness and reduced resistance to temperature change. Remnants of the additive material can also diminish the quality of the quartz glass in this respect. Pore growth can also take place during vitrification, with large pores growing at the expense of smaller ones. However, large pores contribute little to opacity, lead to a lesser density of the opaque quartz glass and reduce the mechanical strength of the quartz glass body.
A SiO
2
granulate of this kind is known from DE A1 44 24 044. It is proposed therein that an aqueous suspension of silicic powder manufactured in a pyrogenic process be treated in a mixing container by rotating agitators. The rotational velocity in a first mixing phase is between 15 and 30 m/s, and 30 m/s or more in a second mixing phase. The solids content of the suspension is at least 75% by weight during the first mixing. A granular mass with an average granule diameter of less than 4 mm results from the first mixing phase. The granular mass is further consolidated by addition of amorphous silicic dust and the granular mass is reduced in size in a second mixing phase by intensive mixing and beating. Water emerges from the surface of the granular mass and is absorbed by addition of more silicic dust in order to prevent lumping of the granulate. The known process provides a pourable SiO
2
granulate of high powder density, suitable for applications as filler in dentistry or as catalyst support.
The object of the invention is to provide a process for the manufacture of pure opaque quartz glass with homogenous pore distribution and also with high density, high viscosity and low tendency to devitrify. The object of the invention is also to provide a SiO
2
granulate suitable to carrying out the process.
As concerns the process, the object is achieved according to the invention on the basis of the process cited initially in that the employed SiO
2
granulate is formed from at least partially porous agglomerates of SiO
2
primary particles, has a specific BET surface ranging from 1.5 m
2
/g to 40 m
2
/g and an apparent density of at least 0.8 g/cm
3
.
Vitrification of a starting body formed from at least partially porous agglomerates of SiO
2
primary particles with a specific BET surface ranging from 1.5 m
2
/g to 40 m
2
/g to an apparent density of at least 0.8 g/cm
3
, produces opaque pure quartz glass exhibiting homogenous pore distribution and high density, high viscosity and low tendency to devitrify. An article produced from the opaque quartz glass is distinguished by good heat insulation and long service life at high temperatures.
The SiO
2
granulate is present in form of at least partially porous agglomerates of SiO
2
primary particles. Such primary particles are obtained by for example flame hydrolysis or oxidation of silica compounds, by hydrolysis of organic silica compounds in a so-called sol-gel process or by hydrolysis of inorganic silica compounds in a liquid. Even though such primary particles stand out due to their high purity they are difficult to handle due to their low powder density. Therefore compacting by granulation processes is conventionally used. Agglomerates with greater diameters are formed due to the lumping together of fine primary particles. These agglomerates have a number of open pore channels, forming a correspondingly large volume of pores. The individual granules of the SiO
2
granulate used in the process according to the invention are formed from such agglomerates. Due to the large pore volume the granulate is distinguished by a specific BET surface ranging from 1.5 m
2
/g to 40 m
2
/g. This surface therefore does not appear as outer surface but predominantly as inner surface in form of pore channels. During vitrification of the starting body the larger part of the pore volume closes due to sintering and collapsing. However, a large number of fine closed pores remains from the pore channels that were previously open. Incident light is diffused by the closed pores, resulting in opacity or low transmittance. The large surface is favorable for the formation of gaseous silicon oxide (SiO) during vitrification which counters the collapse of small pores since gases trapped in closed pores can no longer escape.
Therefore no additive material volatile at vitrification is needed to achieve opacity as is the case in the known process described initially. As a result contaminants which go hand in hand with the use of such additives can be avoided.
According to the invention, the synthetically manufactured SiO
2
granulate used is distinguished by a specific surface ranging from 1.5 to 40 m
2
/g and at the same time by high apparent density. The density of at least 0.8 g/cm
3
primarily assures that the starting body can be formed from the granulate, while the opacity of the quartz glass is substantially the result of the large specific surface, as explained above.
The specific surface of the SiO
2
granulate is determined according to the BET process (DIN 66132), and the apparent density according to DIN/ISO 787, Part 11.
From the SiO
2
granulate the starting body is formed as loose fill or as a thermally or mechanically pre-compacted body.
It has been shown that a particularly suitable SiO
2
granulate for use in the process according to the invention has a specific BET surface ranging from 10 m
2
/g to 30 m
2
/g. Good results are achieved regarding opacity or low transmittance of the quartz glass, especially in the IR spectrum, with high density and low devitrification tendency. Especially good results regarding pourability and ease of handling of the granulate were achie
Fabian Heinz
Gertig Udo
Göbel Rolf
Leist Johann
Werdecker Waltraud
Heraeus Quarzglas GmbH & Co. KG
Sample David
Tiajoloff Andrew L.
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