Title of Invention

METHOD FOR PRODUCING ULTRA-HIGH PURITY, OPTICAL QUALITY, GLASS ARTICLES

Abstract A method for producing ultra-high purity, optical quality, glass articles is disclosed which involves: 1. compacting metaloxide or metal loidoxide to granules having a mean particle size of less than about 1 millimeter; 2. optionally fully sintering the granules to produce high purity, artificial sand; 3. casting the granulesar artificial sand by conventional techniques, such as, slip casting, to form a high density, porous, green body; 4. optionally drying and partially sintering the green body; 5. optionally fully sintering the green body under vacuum, - and 6, optionally hot isostatic pressing the fully sintered green body whereby the metal oxide or metalloid oxides is a pyrogenic silicon dioxide powder with a BET surface area of 30 to 90 m2/g, a DBP index of 80 or less, a mean aggregate area of less than 25000 nm2 and a mean aggregate circumference of less than 1000 nm, wherein at least 70% of the aggregates have a circumference of less than 1300 nm or a high-purity pyrogenically prepared silicon dioxide having metal contents of less than 0.2 μg/g.
Full Text Method for producing ultra-high purity, optical quality,
glass articles
This invention relates to a method for producing ultra-high
purity, optical quality glass articles.
Numerous investigators have attempted to apply the sol-gel
technique to the production of optical quality glass
products.
For example, Matsuyama et al., UK patent application No. GB
2,041,913, describes a gel casting method for producing
"mother rods" from which optical waveguide fibers can be
prepared wherein a solution of a silicon alkoxide is
formed, allowed to gel so as to produce a porous preform,
dried, and then sintered at a temperature below its melting
temperature to produce the mother rod. The application
describes a three step sintering process in which an
atmosphere of oxygen and helium is used up to a temperature
of 700 °C, an atmosphere of chlorine and helium is used
between 700 °C and 1000 °C and an atmosphere of just helium
is used above 1000 °C.
As acknowledged in this application, drying the gel without
cracking is difficult and can take as long as 10 days.
In addition to the foregoing, sol-gel casting processes
have also been described in Hansen et al., U.S. Pat. No.
3,535,890, Shoup, U.S. Pat. No. 3,678,144, Blaszyk et al.,
U.S. Pat. No. 4,112,032, Bihuniak et al., U.S. Pat. Nos.
4,042,361, and 4,200,445, and Scherer, U.S. Pat. No.
4,574,063, European patent publication No. 84,438, and
Scherer et al., "Glasses from Colloids", Journal of Non-
Crystalline Solids, 63: 163-172 (1984).
In particular, the Hansen et al. patent relates to a
process in which an aqueous solution of colloidal silica
particles is formed, dried to produce a gel, and the gel is
sintered in a three step process, the first step comprising

heating the gel to around 600 °C in a vacuum, the second
step comprising flushing the gel with chlorine gas to
remove bound water, and the third step comprising sintering
the gel under vacuum by raising its temperature to 1200 °C
The patent acknowledges the gel's high sensitivity to
cracking during the drying process and states that drying
times on the order of many days or weeks are needed to
overcome this problem.
The Bihuniak et al. patents describe processes for
densifying fumed silica and other fumed metal oxides by
forming a sol, drying the sol to form fragments, and
densifying the fragments by calcining them at 1150-1500 °C.
Thereafter, the densified material can be milled, e.g., to
an 8 to 10 micron average particle size, suspended in a
casting medium, slip cast to form a porous preform, and
fired to produce the desired finished product.
Because it employs fumed silica, the Bihuniak et al.
process is more difficult to perform than the process of
the present invention. For example, it is relatively
difficult to form gels from fumed silica, and as
acknowledged in the Bihuniak et al. patents, once formed,
gels made from fumed silica tend to break up into large
chunks, rather than small particles, as is desired.
Further, extensive pollution abatement equipment is
required to produce fumed silica since such production
involves the creation of hydrochloric acid.
In addition, densified silica particles made from fumed
silica tend to have higher impurity levels than the
densified silica particles produced by the process of the
present invention. These higher impurity levels are due in
part to the fact that impurities, including trace amounts
of radioactive materials, are introduced into the silica
during the fuming process.

The higher impurity levels also arise from the fact that
densification of particles made from fumed silica gels
requires higher temperatures than densification of
particles formed from gels prepared in accordance with the
present invention, i.e., densification of particles made
from fumed silica gels require temperatures above, rather
than below, 1150 °C. Such higher temperatures generally
mean that metal-containing furnaces must be used to perform
the densification. The use of such furnaces, in turn, means
that the silica particles will be exposed to and thus will
pick up metal ions which are released from the walls of the
hot furnace. In addition to the purity problem, the need to
generate higher temperatures to achieve densification is in
general undesirable.
The use of hot isostatic pressing ("hipping"), as well as
other pressing techniques, to compress gas bubbles in
vitreous materials has been described in a number of
references. See Rhodes, U.S. Pat. No. 3,310,392, Bush, U.S.
Pat. No. 3,562,371, Okamoto et al., U.S. Pat. No.
4,358,306, and Bruning et al., U.S. Pat. No. 4,414,014 and
UK patent application No. 2,086,369. The Bush patent, in
particular, discloses forming a green body, sintering the
body in a vacuum, and then subjecting the consolidated body
to isostatic pressure at a temperature equal to or greater
than the sintering temperature.
In view of the foregoing state of the art, it is an object
of the present invention to provide an improved process for
producing optical quality, high purity, glass articles. In
particular, it is an object of the invention to provide a
process for producing such articles which involves the
sintering of a porous silica body but avoids the cracking,
shrinkage and purity problems encountered in prior art
processes of this type.
With regard to products, it is an object of the invention
to provide ultra-pure silica granules which can be used in

a variety of conventional ceramic forming processes, such
as, powder pressing, extrusion, slip casting, and the like,
to produce green bodies. It is an additional object of the
invention to produce glass articles of complex shapes which
have higher purities, more uniform transmittance
characteristics, and smaller index of refraction
variations, i.e., better homogeneity, than similar articles
produced by prior art techniques. It is a further object of
the invention to economically produce optical waveguide
fibers which have transmission characteristics equivalent
to optical waveguide fibers produced by more expensive
techniques.
Subject of the invention is a method for producing a fused
glass article comprising the steps of:
a) compacting metal oxide or metalloid oxide into granules
having a mean particle size less than about 1
millimeter;
b) optionally sintering the granules at a temperature less
than about 1.100°C, the density of the granules after
sintering being approximately equal to their maximum
theoretical density;
c) forming a green body from the granules or mixture or
mixture of the granules, according to step a) and/or b)
using uniaxial, cold isostatic and hot isostatic powder
pressing, slip casting, extrusion, moulding and
injection moulding;
d) optionally drying and partially sintering the green
body in a chamber by:
I) raising the temperature of the chamber optionally
to above about 1.000 °C, e.g., to 1.150 °C, and
optionally introducing chlorine gas into the
chamber and/or purging the chamber with an inert
gas and/or subjecting the chamber to a vacuum;

e) optionally fully sintering the green body in a chamber
within a temperature range from about 1.200 °C to a
temperature above about 1.720 °C while optionally
purging the chamber with helium or preferably applying
a vacuum to the chamber and
f) optionally hot isostatic pressing the fully sintered
green body in a chamber by raising the temperature of
the chamber to above about 1150 °C and introducing an
inert gas into the chamber at a pressure above about
100 psig (= 6,895 bar), preferably above 1,000 psig (=
68,95 bar) and more preferably above about 15,000 psig
(= 1.034,25 bar).
Particular process steps can also be omitted depending on
the specific conditions used and the purity requirements of
the final product. For example, chlorine treatment may not
be required in step (d) if the finished product does not
have to have a low water content. Other modifications of
this type are discussed below in connection with the
description of the preferred embodiments of the invention.
Unlike prior art techniques which have employed sol-gel
technology, the foregoing method provides a practical
procedure for commercially producing ultra high purity,
optical quality glass articles. The success of this
technique is due to a number of factors. In the first
place, the technique of the present invention does not use
sol-gel technology to form a green body.
In addition to using metal oxide or metalloid oxide
granules, the method of the invention also carries the high
purity level of the granules through to the final product
and, at the same time, produces a finished product having
excellent optical properties. In particular, the oxygen and
chlorine treatments during the drying of the green body
specifically reduce the level of water in the finished
product. In addition, the use of the preferred vacuum

sintering means that any bubbles or similar defects which
are created during sintering will in essence be empty
voids. These empty spaces can be easily closed during
hipping.
In a preferred subject of the invention the compacting of
the metal oxides or metalloid oxides can be prepared by
dispersing the metal oxides or metalloid oxides in water,
spray drying it and heating the granules obtained at a
temperature of from 150 to 1.100 °C for a period of 1 to
8 h.
In preferred subject of the invention the metaloxide or
metalloidoxide can be silica granules i.e.:
a) pyrogenically produced silicon dioxide, which has been
compacted to granules having
a tamped density of from 150 g/1 to 800 g/1,
a granule particle size of from 10 to 800 um and
a BET surface area of from 10 to 500 m2/g, or
b) pyrogenically produced silicon dioxide, which has
been compacted to granules, having the following
physico-chemical data:
mean particle diameter: from 25 to 120 um,
BET surface area: from 40 to 400 m2/g,
pore volume: from 0.5 to 2.5 ml/g,
pore distribution: no pores with a diameter nm, only raeso- and macro-pores are present,
pH value: from 3.6 to 8.5,
tamped density: from 220 to 700 g/1.
The compacting step can be made according to US Patent No.
5,776,240.
In a preferred embodiment of the invention, a pyrogenically
produced silicon dioxide, which has been granulated or
compacted in a known manner according to US Patent No.

5,776,240 can be used in the production of sintered
materials.
The silicon dioxide so compacted or granulated can be a
pyrogenically produced oxide having a BET surface area of
from 10 to 500 m2/g, a tamped density of from 150 to 800
g/1 and a granule particle size of from 10 to 800 urn.
According to the invention, mixtures of compacted and
uncompacted silicon dioxide can also be used.
Hereinbelow, the expressions ,,pyrogenically produced
silica", ,,pyrogenically produced silicon dioxide",
,,pyrogenic silica" and ,,pyrogenic silicon dioxide" are to
be understood as meaning very finely divided, nanoscale
powders produced by converting gaseous silicon chloride,
such as, for example, methyltrichlorosilane or silicon
tetrachloride in a high temperature flame, wherein the
flame is fed with hydrogen and oxygen and water vapor can
optionally be supplied thereto.
Hereinbelow, the term ,,granule" is to be understood as
meaning pyrogenically produced silicon dioxide powders
highly compacted by means of the compaction process
described in US Patent No. 5,776,240 or analogously to that
process.
c) For the method according to the invention, either
pyrogenically produced silicon dioxide, which has been
compacted to granules by means of a downstream
compacting step according to DE 196 01 415 Al is used,
which corresponds to US Patent 5,776,240, having a
tamped density of from 150 g/1 to 800 g/1, preferably
from 200 to 500 g/1, a granule particle size of from 10
to 800 urn and a BET surface area of from 10 to 500
m2/g, preferably 20 to 130 m2/g, or granules according
to US Patent No. 5,776,240, based on pyrogenically
produced silicon dioxide are used, having the following

physico-chemical data:
mean particle diameter from 25 to 120 um;
BET surface area from 4 0 to 400 m2/g
pore volume from 0.5 to 2.5 ml/g
pore distribution: no pores with a diameter nm, only meso- and macro-pores are present,
pH value: from 3.6 to 8.5,
tamped density: from 22 0 to 700 g/1.
In the example according to the invention the following
presintering compositions can be used:
a) A pyrogenically produced silicon dioxide having a BET
surface area of 90 m2/g and a bulk density of 35 g/1
and a tamped density of 59 g/1 is compacted to a
granule according to US Patent No. 5,776,240.
The compacted silicon dioxide has a BET surface area
of 90 m2/g and a tamped density of 246 g/1.
b) A pyrogenically produced silicon dioxide having a BET
surface area of 50 m2/g and a tamped density of 130
g/1 is compacted to a granule according to US Patent
No. 5,776,240.
The compacted silicon dioxide has a BET surface area
of 50 m2/g and a tamped density of 365 g/1.
c) A pyrogenically produced silicon dioxide having a BET
surface area of 300 m2/g and a bulk density of 30 g/1
and a tamped density of 50 g/1 is compacted according
to US Patent No. 5,776,240.
The compacted silicon dioxide has a BET surface area
of 300 m2/g and a tamped density of 289 g/1.
d) A pyrogenically produced silicon dioxide having a BET
surface area of 200 m2/g and a bulk density of 35 g/1
and a tamped density of 50 g/1 is compacted according
to US Patent No. 5,776,240.

The compacted silicon dioxide has a BET surface area
of 200 m2/g and a tamped density of 219 g/1.
The chief process for the preparation of pyrogenic silicon
dioxide, starting from silicon tetrachloride which is
reacted in mixture with hydrogen and oxygen, is known from
Ullmanns Enzyklopadie der technischen Chemie, 4th edition,
Vol. 21, pp. 464 et seq. (1982) .
The metal oxide or metalloid oxide to be used according to
the invention can be granules based on pyrogenically
produced silicon dioxide powder with
a BET surface area of 30 to 90 m2/g,
a DBP index of 80 or less
a mean aggregate area of less than 25000 nm2,
a mean aggregate circumference of less than 1000 nm,
wherein at least 70% of the aggregates have a
circumference of less than 1300 nm. This pyrogenically
produced silicon dioxide is disclosed in WO
2004/054929.
The BET surface area may preferably be between 35 and
75 m2/g. Particularly preferably the values may be between
40 and 60 m2/g. The BET surface area is determined in
accordance with DIN 66131.
The DBP index may preferably be between 60 and 80. During
DBP absorption, the take-up of force, or the torque (in
Nm), of the rotating blades in the DBP measuring equipment
is measured while defined amounts of DBP are added,
comparable to a titration. A sharply defined maximum,
followed by a drop, at a specific added amount of DBP is
then produced for the powder according to the invention.

A silicon dioxide powder with a BET surface area of 40
to 60 m2/g and a DBP index of 60 to 80 may be particularly
preferred.
Furthermore, the silicon dioxide powder to be used
according to the invention may preferably have a mean
aggregate area of at most 20000 nm2. Particularly
preferably, the mean aggregate area may be between 15000
and 20000 nm2. The aggregate area can be determined, for
example, by image analysis of TEM images. An aggregate is
understood to consist of primary particles of similar
structure and size which have intergrown with each other,
the surface area of which is less than the sum of the
individual isolated primary particles. Primary particles
are understood to be the particles which are initially
formed in the reaction and which can grow together to form
aggregates as the reaction proceeds further.
A silicon dioxide powder with a BET surface area of 40
to 60 m2/g, a DBP index of 60 to 80 and a mean aggregate
area between 15000 and 20000 nm2 may be particularly
preferred.
In a preferred embodiment, the silicon dioxide powder to be
used according to the invention may have a mean aggregate
circumference of less than 1000 nm. Particularly
preferably, the mean aggregate circumference may be between
600 and 1000 nm. The aggregate circumference can also be
determined by image analysis of TEM images.
A silicon dioxide powder with a BET surface area of 40 to
60 m2/g, a DBP index of 60 to 80, a mean aggregate area
between 15000 and 20000 nm2 and a mean aggregate
circumference between 600 and 1000 nm may be particularly
preferred.

Furthermore, it may be preferable for at least 80%,
particularly preferably at least 90%, of the aggregates to
have a circumference o'f less than 1300 nm.
In a preferred embodiment, the silicon dioxide powder to be
used according to the invention may assume a degree of
filling in an aqueous dispersion of up to 90 wt.%. The
range between 60 and 80 wt.% may be particularly preferred.
Determination of the maximum degree of filling in an
aqueous dispersion is performed by the incorporation of
powder, in portions, into water using a dissolver, without
the addition of other additives. The maximum degree of
filling is achieved when either no further powder is taken
up into the dispersion, despite elevated stirring power,
i.e. the powder remains in dry form on the surface of the
dispersion, or the dispersion becomes solid or the
dispersion starts to form lumps.
Furthermore, the silicon dioxide powder to be used
according to the invention may have a viscosity at a
temperature of 23*C, with respect to a 30 wt.% aqueous
dispersion at a rate of shear of 5 rpm, of less than 100
mPas. In particularly preferred embodiments, the viscosity
may be less than 50 mPas.
The pH of the silicon dioxide powder to be used according
to the invention may be between 3.8 and 5, measured in a 4%
aqueous dispersion.

The process for preparing the silicon dioxide powder to be
used according to the invention, is characterised in that
at least one silicon compound in the vapour form, a free-
oxygen-containing gas and a combustible gas are mixed in a
burner of known construction, this gas mixture is ignited
at the mouth of the burner and is burnt in the flame tube
of the burner, the solid obtained is separated from the gas
mixture and optionally purified, wherein
- the oxygen content of the free-oxygen-containing gas is
adjusted so that the lambda value is greater than or
equal to 1,
the gamma-value is between 1.2 and 1.8,
- the throughput is between 0.1 and 0.3 kg SiO2/m3 of core
gas mixture,
the mean normalised rate of flow of gas in the flame
tube at the level of the mouth of the burner is at least
5 m/s.
The oxygen content of the free-oxygen-containing gas may
correspond to that of air. That is, in this case air is
used as a free-oxygen-containing gas. The oxygen content
may, however also take on higher values. In a preferred
manner, air enriched with oxygen should have an oxygen
content of not more than 40 vol.%.
Lambda describes the ratio of oxygen supplied in the core
to the stoichiometrically required amount of oxygen. In a
preferred embodiment, lambda lies within the range
1 Gamma describes the ratio of hydrogen supplied in the core
to the stoichiometrically required amount of hydrogen. In a
preferred embodiment, gamma lies within the range
1.6
The normalised rate of flow of gas refers to the rate of
flow at 273 K and 1 atm.
A burner of known construction is understood to be a burner
with concentric tubes. The core gases are passed through
the inner tube, the core. At the end of the tube, the mouth
of the burner, the gases are ignited. The inner tube is
surrounded by at least one other tube, the sleeve. The
reaction chamber, called the flame tube, starts at the
level of the mouth of the burner. This is generally a
conical tube, cooled with water, which may optionally be
supplied with other gases (sleeve gases) such as hydrogen
or air.
The mean, normalised rate of flow of the gas in the flame
tube at the level of the mouth of the burner of at least 5
m/s refers to the rate of flow immediately after the
reaction mixture leaves the burner. The rate of flow is
determined by means of the volume flow of the reaction
products in vapour form and the geometry of the flame tube.
The core gases are understood to be the gases and vapours
supplied to the burner, that is the free-oxygen-containing
gas, generally air or air enriched with oxygen, the
combustible gas, generally hydrogen, methane or natural
gas, and the silicon compound or compounds in vapour form.
An essential feature of the process is that the mean
normalised rate of flow of gas in the flame tube at the
level of the mouth of the burner is at least 5 m/s. In a
preferred embodiment, the mean normalised rate of flow of
the gas in the flame tube at the level of the mouth of the
burner assumes values of more than 8 m/s.
The mean rate of discharge of the gas mixture (feedstocks)
at the mouth of the burner is not limited. However, it has
proven to be advantageous when the rate of discharge at the
mouth of the burner is at least 30 m/s.

In a preferred embodiment, additional air (secondary air)
may be introduced into the reaction chamber, wherein the
rate of flow in the reaction chamber may be raised further.
In a preferred embodiment, the mean normalised rate of flow
of gas in the flame tube at the level of the mouth of the
burner may be 8 to 12 m/s.
The type of silicon compound used in the process is not
further restricted. Silicon tetrachloride and/or at least
one organochlorosilicon compound may preferably be used.
A particularly preferred embodiment of the process is one
in which
silicon tetrachloride is used,
- the lambda value is such that 1 - the gamma-value is between 1.6 and 1.8,
- the throughput is between 0.1 and 0.3 kg SiO2/m3 of core
gas mixture,
in addition at least double the amount of air, with
respect to the amount of free-oxygen-containing gas
introduced into the burner, is introduced into the flame
tube and
the rate of flow of the gas of feedstocks at the mouth
of the burner is 40 to 65 m/s (with respect to standard
conditions)
- and the mean normalised rate of flow of gas in the flame
tube at the level of the mouth of the burner is between
8 and 12 m/s.
In general during the preparation of pyrogenic oxides, the
rate of flow of gas in the water-cooled reaction chamber
(flame tube) and in the subsequent cooling unit (cooling
stretch) is adjusted in such a way that the best possible
cooling power, that is to say rapid cooling of the reaction

products, is ensured. In principle, it is true that the
cooling power increases with decreasing rate of flow of
gas. The lower limit is simply based on the requirement of
still being able to transport the product through the pipes
with the gas stream.
It was demonstrated that although a considerable increase
in the rate of flow of gas in the reaction chamber resulted
in a reduced cooling power, it led to a powder with
unexpected properties. Whereas physical characteristics
such as BET surface area and DBP absorption are
substantially unchanged as compared with powders according
to the prior art, the powder exhibits a much lower
structure.
Furtheron the metal oxide or metalloid oxide to be used
according to the invention can be granules based on
pyrogenically produced silicon dioxide which is
characterised by a metals content of less than 9 ppm.

In a preferred embodiment the high-purity pyrogenically
prepared silicon dioxide to be used according to the
invention, can be characterised by the following metal
contents:

The total metal content can then be 3252 ppb (-3.2 ppm) or
less.

In an embodiment of the invention, which is further
preferred, the high-purity pyrogenically prepared silicon
dioxide can be characterised by the following metal
contents:

The total metal content can then be 1033 ppb (~ 1.03 ppm)
or less.
The process for the preparation of the high-purity
pyrogenically prepared silicon dioxide is characterised in
that silicon tetrachloride is in known manner reacted in a
flame by means of high-temperature hydrolysis to give
silicon dioxide, and a silicon tetrachloride is used here
which has a metal content of less than 30 ppb.

In a preferred embodiment of the invention a silicon
tetrachloride can be used which has the following metal
contents in addition to silicon tetrachloride:
Al less than 1 ppb
B less than 3 ppb
Ca less than 5 ppb
Co less than 0.1 ppb
Cr less than 0.2 ppb
Cu less than 0.1 ppb
Fe less than 0.5 ppb
K less than 1 ppb
Mg less than 1 ppb
Mn less than 0.1 ppb
Mo less than 0.2 ppb
Na less than 1 ppb
Ni less than 0.2 ppb
Ti less than 0.5 ppb
Zn less than 1 ppb
Zr less than 0.5 ppb
Silicon tetrachloride having this low metal content can be
prepared according to DE 100 30 251 or according to DE 100
30 252.
The metal content of the silicon dioxide according to the
invention is within the ppm range and below (ppb range).
It has been found that by means of the invention, finished
products of complex shapes, such as, optical domes, antenna
windows, sight glasses, aerospace viewports, lenses,
prisms, mirrors, etc., can be readily produced which have
equivalent or better optical properties than similar
products produced by other techniques. In particular, the

products have been found to have higher purities, smaller
index of refraction variations (better homogeneities), and
more uniform transmittance characteristics from the
ultraviolet through the infrared than similar commercial
products which have heretofore been available. The method
of the invention can be used to produce low loss, optical
waveguide fibers. Significantly, in accordance with the
invention, production costs for such fibers can be reduced.
The optional sintering of the granules is conducted at a
temperature of less than about 1.100°C. This low sintering
temperature allows the sintering to be conducted in the
quartz reactor. The use of such reactor, as opposed to a
metal furnace, helps maintain the purity of the granules
through the sintering procedure.
The sintering can be performed in a variety of atmospheres.
For example, helium, helium/oxygen, and argon/oxygen
atmospheres can be used. In some cases, a helium atmosphere
has been found preferable to an argon/oxygen atmosphere.
The sintering can also be performed in air.
The granules can be used as a filler for potting sensitive
electronic components, such as, semiconductor memory
devices. In comparison with prior art silica fillers, the
granules contain lower amounts of such radioactive
materials as uranium and thorium, and thus produce less
alpha particles which can interfere with the operation of
state-of-the-art electronic components.
In accordance with the present invention, the granules are
used to form high density green bodies. In particular, the
granules are used as the starting material for such
conventional processes as slip casting, injection molding,
extrusion molding, cold isopressing, and the like. A
description of these and other processes in which the
granules of the present invention can be used can be found
in such texts as Introduction to Ceramics, by W. D.

Kingery, John Wiley and Sons, Inc., New York, 1960, and
Ceramic Processing Before Firing, G. Y. Onoda, Jr., and L.
L. Hench, editors, John Wiley and Sons, Inc., New York,
1978, the pertinent portions of which are incorporated
herein by reference.
With regard to slip casting in particular, descriptions of
this technique can be found in U.S. Pat. No. 2,942,991 and
in Whiteway, et al., "Slip Casting Magnesia," Ceramic
Bulletin, 40: 432-435 (1961), the pertinent portions of
which are also incorporated herein by reference.
Such a slurry can be conveniently produced using a
urethane-lined vibra-mill to which the granules, silica
media, and water are added. Using a slurry of this type,
high density green bodies, e.g., green bodies having a
porosity on the order of 20%, are readily prepared.
For various of the other casting methods, e.g, the
injection, extrusion, and pressing techniques, it is
generally preferred to employ a binder in the slurry. Such
a binder can be conveniently formed by in situ
hydrolyzation of TEOS. By way of illustration, a slurry of
the granules of the present invention was successfully cast
in a plastic mold, as opposed to a plaster of Paris mold,
by adding 5 milliliters of an acid-catalyzed TEOS/water
mixture (4 moles water to each mole of TEOS) to 132
milliliters of slurry. After molding, 2-7 milliliters of a
basic solution (1.2% ammonium carbonate) was added to the
slurry. The basic solution shifted the pH causing the TEOS
to gel within a period of from about 2 to about 30 minutes,
thus binding the granules together to form a strong green
body, well-suited for further processing. Alternatively,
commercial binders, such as those sold by the Stauffer
Chemical Company under the SILBOND trademark, can be used.
Once formed, the green body can be purified purified and
consolidated by a two-step process. In the first step, the

green body is dried and partially sintered. In the secon
step, the green body is fully sintered.
The drying and partial sintering step, among other things,
serves to remove water from the green body which cou
bubbles in the final product during full sintering- To
minimize contamination, this step is preferably performe
in a quartz tube furnace, although other types of furnaces
can be used, if desired. When a quartz tube furnace is
used, the temperatures employed are preferably kept be ow
about 1150 °C.
Drying and partial sintering are achieved by raisin? t e
temperature of the furnace to above about 1000 °Cr w e
introducing chlorine into the furnace and/or apply111? a
vacuum to the furnace and/or purging the furnace with one
or more inert gases, e.g., with argon and/or heH-VXRm e
chlorine treatment, vacuum stripping and/or inert purging reduces the chances that the water content of e
green body will cause bubbles to form during full
sintering. In addition to removing water, the chlorine
treatment has also been found to reduce the green body s
iron, copper, and calcium levels. When the green body is
formed by slip casting, the chlorine treatment's ability to
strip calcium is of particular value since the green body
tends to pick up calcium from the plaster of Paris mold.
Optionally, the drying and partial sintering step can
include subjecting the green body to an oxygen-containing
atmosphere to reduce its content of organic materials.
The oxygen treatment can be omitted if the green body
includes only minor levels of organic material
contamination. The chlorine treatment can be omitted m
cases where the final product can have a relatively high
water content, e.g., in cases where the absorption
characteristics of the final product in the infrared region
are not critical. When the chlorine treatment is omitted,

either vacuum stripping or inert gas purging should be
performed. If desired, both vacuum stripping and gas
purging can be used sequentially. Either or both the vacuum
stripping and the inert purging can be omitted when the
chlorine treatment is used.
After the green body has been dried and optionally partial
sintered, it is fully sintered at a temperature range, from
about 1.200 °C to above about 1.720. °C. Full sintering is
preferably performed in a vacuum of, for example, lxlO"5
torr. Alternatively, helium purging can be used, although
this is less preferred since any bubbles which form in the
glass during sintering will be filled with helium, rather
than being empty, as occurs during vacuum sintering.
The full sintering of the cast granules can be performed
in, for example, a tungsten-molybdenum furnace or a helium-
filled graphite furnace. To minimize contamination, the
green body is preferably supported on quartz cloth and
monoclinic unstabilized zirconia A grain.
In general, full sintering, as well as cooling of the
sintered product, can be completed in about 3 hours.
Thereafter, if desired, the surfaces of the consolidated
green body can be cleaned with hydrofluoric acid. Also,
areas of the green body which may have become deformed
during sintering, e.g., areas in contact with the quartz
cloth, can be removed by grinding.
For certain applications, e.g., the production of
consolidated preforms for optical waveguide fibers, the
fully sintered green body may be ready for use without
further processing. In most cases, however, it is preferred
to hip the sintered green body to collapse any bubbles
which may have formed in the body during the sintering
process.

The hipping is performed in the pressure chamber of a
hipping furnace (see, for example, U.S. Pat. No. 4,349,333)
by heating the chamber to a temperature greater that the
annealing point of the consolidated green body and less
than about 1800 °C, while introducing an inert gas, such
as, argon, helium, or nitrogen, into the chamber at a
pressure in the range of 100-45,000 psi (6,895 to 3.102,75
bar). In practice, temperatures in the range of 1150-1740
°C and pressures in the range of 1,000-30,000 psig (68,95
to 2.068,5 bar) have been found suitable for collapsing
bubbles and other voids in consolidated green bodies
produced in accordance with the present invention. Lower
pressures, e.g., pressures in the 100-1000 psig (6,895 to
68,95 bar) range, can also be used.
To avoid contamination of the consolidated green body
during hipping, it is preferred to wrap the body in glass
wool and steel foil before it is placed in the hipping
furnace. These precautions, however, can be omitted in the
case of a "clean" furnace which has only been used to hip
high purity silica materials.
After hipping has been completed, various conventional
glass treatment procedures, such as, annealing, grinding,
polishing, drawing, pressing, etc., can be applied to the
fully sintered and hipped green body. The resulting
finished product is then ready for use by the consumer.

Examples
The BET surface area is determined in accordance with DIN
66131.
The dibutyl phthalate absorption is measured with a
RHEOCORD 90 instrument made by Haake, Karlsruhe. For this
purpose, 16 g of the silicon dioxide powder, weighed out to
an accuracy of 0.001 g, is placed in a mixing chamber, this
is sealed with a lid and dibutyl phthalate is added at a
pre-set rate of addition of 0.0667 ml/s via a hole in the
lid. The mixer is operated with a motor speed of 125 revs
per minute. After reaching maximum torque, the mixer and
DBP addition are automatically switched off. The DBP
absorption is calculated from the amount of DBP consumed
and the amount of particles weighed out in accordance with:
DBP index (g/100 g) = (DBP consumed in g / initial weight
of particles in g) x 100.
A programmable rheometer for testing complex flow
behaviour, equipped with a standard rotation spindle, was
available for determining the viscosity.
Rate of shear: 5 to 100 rpm
Temperature of measurement: room temperature (23°C)
Concentration of dispersion: 30 wt.%
Procedure: 500 ml of dispersion are placed in a 600 ml
glass beaker and tested at room temperature (statistical
recording of temperature via a measuring sensor) under
different rates of shear.
Determination of the compacted bulk density is based on DIN
ISO 787/XI K 5101/18 (not sieved).
Determination of the pH is based on DIN ISO 787/IX, ASTM D
1280, JIS K 5101/24.

The image analyses were performed using a TEM instrument
H 7500 made by Hitachi and a CCD camera MegaView II, made
by SIS. Image magnification for evaluation purposes was
30000 : 1 at a pixel density of 3.2 nm. The number of
particles evaluated was greater than 1000. Preparation was
in accordance with ASTM 384 9-89. The lower threshold limit
for detection was 50 pixels.
Determining the maximum degree of filling in an aqueous
dispersion: 200 g of fully deionised water were initially
placed in a 1 1 vessel (diameter about 11 cm). A dissolver
from VMA-Getzmann, model Dispermat® CA-40-C with a
dissolver disc, diameter about 65 mm, was used as the
dispersing unit.
At the start, the dissolver is operated at about 650 rpm.
The powder is added in portions of about 5 g. After each
addition, there is a waiting period until the powder has
been completely incorporated into the suspension. Then the
next portion is added. As soon as incorporation of an added
amount of powder takes longer than about 10 s, the speed of
the dissolver disc is increased to 1100 rpm. Then further
stepwise addition is performed. As soon as incorporation of
an added amount of powder takes longer than about 10 s, the
speed of the dissolver disc is increased to 1700 rpm.
The maximum degree of filling is achieved when either no
further powder is taken up by the dispersion, despite
increased stirring power, i.e. the powder remains in dry
form on the surface of the dispersion, or the dispersion
becomes solid or the dispersion starts to form lumps.
The amount of powder added can be determined by difference
weighing (preferably difference weighing of the powder
stock). The maximum degree of filling is calculated as:

Maximum degree of filling = amount of powder added
[g]/(amount of powder added [g] + amount of water initially
introduced [g]) x 100%
Example 1 (comparison example):
500 kg/h SiCl4 are vaporised at about 90°C and transferred
to the central tube of a burner of known construction. 145
Nm3/h of hydrogen and 207 Nm3/h of air with an oxygen
content of 35 vol.% are also introduced into this tube.
This gas mixture is ignited and burnt in the flame tube of
the water-cooled burner. The mean normalised rate of flow
of gas in the flame tube at the level of the mouth of the
burner is 0.7 m/s. After cooling the reaction gases, the
pyrogenic silicon dioxide powder is separated from the
hydrochloric acid-containing gases using a filter and/or a
cyclone. The pyrogenic silicon dioxide powder is treated
with water vapour and air in a deacidification unit.
Examples 2 to 4 (comparison examples) are performed in the
same way as example 1. The parameters which are altered
each time are given in Table 1.
Example 5 (working example):
400 kg/h SiCl4 are vaporised at about 90°C and transferred
to the central tube of a burner of known construction.
195 Nm3/h of hydrogen and 303 Nm3/h of air with an oxygen
content of 30 vol.% are also introduced into this tube.
This gas mixture is ignited and burnt in the flame tube of
the water-cooled burner. The mean normalised rate of flow
of gas in the flame tube at the level of the mouth of the
burner is 10 m/s. After cooling the reaction gases, the
pyrogenic silicon dioxide powder is separated from the
hydrochloric acid-containing gases using a filter and/or a

cyclone. The pyrogenic silicon dioxide powder is treated
with water vapour and air in a deacidification unit.
Examples 6 to 8 are performed in the same way as described
in example 1. The parameters which are altered each time
are given in Table 1.
The analytical data for powders 1 to 8 are given in
Table 2.
The powders in examples 5 to 8 exhibit much lower values
for mean aggregate area, mean aggregate circumference and
maximum and minimum aggregate diameter and thus much less
structure than the powders in comparison examples 1 to 4.
The powders have a much higher maximum degree of filling
and a much lower viscosity in an aqueous dispersion.

Table 1: Experimental conditions and the flame parameters calculated therefrom

(a) kg SiC>2/m3 of primary air + hydrogen + SiCl4 (feedstocks) ;
(b) air with 21 vol.% O2;
(c) with reference to primary air;
(d) VB = mean rate of discharge at the mouth of the burner (normalised);
(e) VF = mean rate of flow in the reaction chamber at the level of the mouth of the burner
(normalised).



Example 9 (Comparison Example)
500 kg/h SiCl4 having a composition in accordance with
Table 3 are evaporated at approx. 90°C and transferred into
the central tube of a burner of known design. 190 Nm3/h
hydrogen as well as 326 Nm3/h air having a 35 vol.% oxygen
content are introduced additionally into this tube. This
gas mixture is ignited and burns in the flame tube of the
water-cooled burner. 15 Nm3/h hydrogen are introduced
additionally into a jacket nozzle surrounding the central
nozzle, in order to prevent baking-on. 250 Nm3/h air of
normal composition are moreover introduced additionally
into the flame tube. After cooling of the reaction gases
the pyrogenic silicon dioxide powder is separated by means
of a filter and/or a cyclone from the hydrochloric acid-
containing gases. The pyrogenic silicon dioxide powder is
treated with water vapour and air in a deacidifying unit in
order to remove adherent hydrochloric acid. The metal
contents are reproduced in Table 5.
Example 10 (Embodiment Example)
500 kg/h SiCl4 having a composition in accordance with
Table 4 are evaporated at approx. 90°C and transferred into
the central tube of a burner of known design. 190 Nm3/h
hydrogen as well as 326 Nm3/h air having a 35 vol.% oxygen
content are introduced additionally into this tube. This
gas mixture is ignited and burns in the flame tube of the
water-cooled burner. 15 Nm3/h hydrogen are introduced
additionally into a jacket nozzle surrounding the central
nozzle, in order to prevent baking-on. 250 Nm3/h air of
normal composition are moreover introduced additionally
into the flame tube. After cooling of the reaction gases
the pyrogenic silicon dioxide powder is separated by means
of a filter and/or a cyclone from the hydrochloric acid-
containing gases. The pyrogenic silicon dioxide powder is

treated with water vapour and air in a deacidifying unit in
order to remove adhering hydrochloric acid.
The metal contents are reproduced in Table 5.

Table 4: Composition of SiCl4, Example 10





Measuring method
The pyrogenically prepared silicon dioxides which are
obtained are analysed as to their metal content. The
samples are dissolved in an acid solution which comprises
predominantly HF.
The SiO2 reacts with the HF, forming SiF4 + H20. The SiF4
evaporates, leaving behind completely in the acid the
metals which are to be determined. The individual samples
are diluted with distilled water and analysed against an
internal standard by inductively coupled plasma-atomic
emission spectroscopy (ICP-AES) in a Perkin Elmer Optima
3000 DV. The imprecision of the values is the result of
sample variations, spectral interferences and the
limitations of the measuring method. Larger elements have a
relative imprecision of _+ 5%, while the smaller elements
have a relative imprecision of + 15%.

WE CLAIM;
1. A method for producing a fused glass article comprising the steps of:
a) compacting metal oxide or metalloid oxide to granules having a mean
particle size less than about one millimeter;
b) optionally sintering the granules at a temperature less than about
1.100 °C, the density of the granules after sintering being approximately
equal to their maximum theoretical density;
c) forming a green body from the granules or mixture of the granules
according to step a) and/or step b), wherein theses granules can be
sintered granules using uniaxial, cold isostatic and hot isostatic powder
pressing, slip casting, extrusion, moulding and injection moulding;
d) optionally drying and partial sintering the green body in a chamber by
(i) raising the temperature of the chamber optionally to above about 1000
°C, and (ii) optionally introducing chlorine gas into the chamber and/or
subjecting the chamber to a vacuum and/or purging the chamber with
an inert gas; and
e) optionally fully sintering the green body in a chamber by raising the
temperature of the chamber within a temperature range from about
1.200°C to a temperature above about 1.720 °C while optionally purging
the chamber with helium or applying a vacuum to the chamber;
characterised in that the metal oxide or metalloid oxide is a pyrogenic
silicon dioxide powder with

- a BET surface area of 30 to 90 m2/g,
- a DBP index of 80 or less
- a mean aggregate area of less than 25000 nm2,
- a mean aggregate circumference of less than 1000 nm,
wherein at least 70% of the aggregates have a circumference of less
than 1300 nm
and in that the metal oxides or metalloid oxides is a high-purity
pyrogenically prepared silicon dioxide, characterised by a metal content
of less than 9 ppm with the following metal contents:





2. The method as claimed in claim 1 including the additional step after
step (e) of hot isostatic pressing the fully sintered green body in a
chamber by raising the temperature of the chamber to above about 1.150
°C and introducing an inert gas into the chamber at a pressure above
about 100 psig (6,895 bar).


A method for producing ultra-high purity, optical quality, glass articles is disclosed
which involves: 1. compacting metaloxide or metal loidoxide to granules having a mean
particle size of less than about 1 millimeter; 2. optionally fully sintering the granules to
produce high purity, artificial sand; 3. casting the granulesar artificial sand by
conventional techniques, such as, slip casting, to form a high density, porous, green
body; 4. optionally drying and partially sintering the green body; 5. optionally fully
sintering the green body under vacuum, - and 6, optionally hot isostatic pressing the
fully sintered green body whereby the metal oxide or metalloid oxides is a pyrogenic
silicon dioxide powder with a BET surface area of 30 to 90 m2/g, a DBP index of 80 or
less, a mean aggregate area of less than 25000 nm2 and a mean aggregate
circumference of less than 1000 nm, wherein at least 70% of the aggregates have a
circumference of less than 1300 nm or a high-purity pyrogenically prepared silicon
dioxide having metal contents of less than 0.2 μg/g.

Documents:


Patent Number 247590
Indian Patent Application Number 3337/KOLNP/2007
PG Journal Number 17/2011
Publication Date 29-Apr-2011
Grant Date 26-Apr-2011
Date of Filing 07-Sep-2007
Name of Patentee EVONIK DEGUSSA GMBH
Applicant Address RELLINGHAUSER STRAβE 1-11, 45128 ESSEN
Inventors:
# Inventor's Name Inventor's Address
1 DR. MONIKA OSWALD BURGALLEE 6C, 63454 HANAU
2 DR. KLAUS DELLER FRIEDHOFSTR. 47, 63512 HAINBURG
3 DR. JURGEN MEYER GROSSOSTHEIMER STR. 51, 63811 STOCKSTADT
PCT International Classification Number C03B 19/06
PCT International Application Number PCT/EP2006/050971
PCT International Filing date 2006-02-15
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 05005091.3 2005-03-09 EUROPEAN UNION