Glass manufacturing – Processes – With shaping of particulate material and subsequent fusing...
Reexamination Certificate
1999-07-30
2003-07-08
Colaianni, Michael (Department: 1731)
Glass manufacturing
Processes
With shaping of particulate material and subsequent fusing...
C065S413000, C065S531000, C239S416000, C239S416100, C239S416400, C239S533200, C423S337000
Reexamination Certificate
active
06588230
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to fused silica glass and, in particular, to burners for producing silica soot from which such glass can be made. As used herein, the term “silica glass” includes glass which is pure or may contain one or more dopants, as does the term “silica soot”.
BACKGROUND OF THE INVENTION
An effective method for making fused silica glass comprises the steps of: (1) generating silica soot particles using soot producing burners, and (2) collecting and consolidating the particles on a rotating substrate to form a glass “boule”. Such boules can have diameters on the order of five feet (1.5 meters) and thicknesses on the order of 5-10 inches (13-25 cm). The process is typically carried out in a furnace which has a rotatable base, an outer ring wall, and a crown which carries the soot producing burners.
FIG. 1
shows the front face
8
of a soot producing burner
7
which has been used in the past to produce fused silica boules. This burner has five zones or regions
10
,
12
,
14
,
16
, and
18
through which gases of different compositions pass to (1) supply the raw material(s) from which the soot particles are produced, and (2) generate a flame of suitable size and temperature to (a) convert the raw material(s) into soot particles and (b) generate sufficient heat to consolidate the particles as they are collected at the surface of the boule.
For the burner of
FIG. 1
, region
10
is referred to as the “fume tube” and carries, for example, a mixture of nitrogen gas and a vaporized silicon-containing compound, regions
12
and
18
are known as the inner and outer shields, respectively, and carry oxygen, and regions
14
and
16
are referred to as the “premix rings” and carry a mixture of fuel (e.g., methane) and oxygen. The diameter of outer shield
18
is typically about 1.1 inches (2.8 cm), while the overall dimensions of front face
8
are typically about 3.4 inches by 3.4 inches (8.5 cm by 8.5 cm).
Historically, the vaporized silicon-containing compound supplied to fume tube
10
was silicon tetrachloride or a mixture of silicon tetrachloride and chlorides of other materials, e.g., titanium tetrachloride, when a doped glass was desired. As a result of environmental concerns, silicon tetrachloride has now been replaced with halide-free, silicon-containing compounds, of which octamethylcyclotetrasiloxane (OMCTS) is a particularly preferred example since in addition to providing silicon, it is also provides energy for the burner's flame. In the same manner, organometallic compounds have been substituted for chloride compounds in the production of doped glasses.
FIG. 2
shows the manner in which burners of the type shown in
FIG. 1
have been positioned relative to the furnace's crown
20
. With regard to the present invention, it is significant to note that burner
7
is spaced from the outer face
22
of the crown (the “cold” face of the crown) by gap
24
. This gap, which in practice is about a quarter inch in height, allows air to be inspirated into the furnace so as to cool burner hole
26
and prevent soot buildup on the walls of the hole. The entrained air also ensures that complete combustion of the fuel occurs in burner flame
38
.
In addition to illustrating the spatial relationship between burner
7
and crown
2
,
FIG. 2
also shows the connection of feed lines
28
,
30
, and
32
to the burner, as well as lines
34
and
36
which carry cooling water to and from the burner.
Although burners and burner/crown configurations of the type shown in
FIGS. 1 and 2
have worked successfully in practice, they have had some drawbacks. In particular these burners have suffered from the following problems:
(1) Maintenance Problem
Because of their relatively large frontal areas exposed to furnace conditions, the previously used burners tend to collect deposits on burner face
8
which must be removed to avoid variations in the burner's flame characteristics and/or the soot produced by the burner. In particular, large frontal areas make a burner subject to recirculation effects whereby soot which is not deposited on the boule recirculates back and fouls the face of the burner.
(2) Water Cooling Problem
The large frontal areas of the previously used burners also result in substantial heat transfer from the hot furnace to the burner, thus requiring water cooling of the burners. This is especially so in view of the fact that the burners are made out of aluminum. (It should be noted that the heat transfer occurs both through burner hole
26
and through the crown material itself since the crown is desirably made as thin as possible.) The need for water cooling makes the burners more complex to build and operate.
(3) Furnace Atmosphere Control Problem
The inspiration of air through the burner holes in the crown makes it more difficult to control the composition of the atmosphere within the furnace. Variations in the furnace atmosphere can result in variations in the properties (e.g., hydrogen content) of the glass boules produced by a furnace, both between different parts of a single boule and between different boules.
(4) Emissions Problem
The inspiration of air through the burner holes can also result in elevated levels of NO
x
in the exhaust gases exiting the furnace since N
2
is the major constituent of the inspirated air and furnace temperatures are high enough for NO
x
production, e.g., above 1600° C.
(5) Energy Consumption Problem
Inspiration of ambient air through the burner holes leads to an increase in the amount of energy which must be inputted to the furnace to keep it at its operating temperature.
(6) Potential Safety Problem
The feeding of a premix of fuel and oxygen to regions
14
and
16
makes these regions and the feed lines leading thereto susceptible to flame flashback.
As discussed below, the burners of the present invention address and provide solutions to each of these problems.
DESCRIPTION OF THE PRIOR ART
The use of halide-free, silicon-containing compounds to form fused silica glasses by soot deposition is discussed in Dobbins et al., U.S. Pat. No. 5,043,002, and Blackwell et al., U.S. Pat. No. 5,152,819. The incorporation of a dopant, specifically, titanium, in such glasses is discussed in Blackwell et al., U.S. Pat. No. 5,154,744. The contents of these prior patents are incorporated herein by reference.
PCT Patent Publication No. WO 97/22553, published on Jun. 26, 1997, discloses soot producing burners which can be used with halide-free, silicon-containing compounds such as octamethylcyclotetrasiloxane (OMCTS). The halide-free, silicon-containing compound is preferably provided to the burner as a liquid, atomized in the burner by an integral atomizer, and then directly converted into soot particles by the burner's flame. See also pending U.S. applications Ser. No. 08/767,653 and Ser. No. 08/903,501, filed Dec. 17, 1996 and Jul. 30, 1997, respectively, the contents of both of which are incorporated herein by reference.
Miller et al., U.S. Pat. No. 5,110,335 discloses a burner for producing soot from silicon tetrachloride which includes an ultrasonic nozzle which when operated at a frequency of 120 kilohertz converts liquid silicon tetrachloride into a fine mist.
Brown et al., U.S. Pat. No. 5,092,760 discloses an oxygen/fuel burner which atomizes liquid fuel by means of an integral atomizer. Brown et al., U.S. Pat. Nos. 5,405,082 and 5,560,758, disclose oxygen/fuel burners for use in glass conditioning. These burners employ a tube-in-tube construction and, during use, are sealed to the wall of a glass distribution channel. Brown et al., U.S. Pat. No. 4,986,748 discloses a further construction for an oxygen/fuel burner. Significantly, with regard to the present invention, the burners of these various Brown et al. patents are concerned with heat production, not with the production of silica soot. Among other things, such heat producing burners do not have to be concerned with soot build-up on the burner face or with the adverse effects of the burner's internal operating temperature on the heat
Adler Meryle D. M.
Brown John T.
Misra Mahendra K.
Colaianni Michael
Corning Incorporated
Klee Maurice K.
Schaeberle Timothy M.
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