Note: Descriptions are shown in the official language in which they were submitted.
11 306~0
The present invention relates to a process for the
production of a quartz glass by the vacuum melting method
using silica as a raw material, by which process a
transparent or active, high-quality glass can be produced at
a low production cost.
In general, glasses as industrial products are produced
by heating raw material powders prepared in a prescribed
mixing proportion in a crucible or a tank furnace at
temperatures higher than the liquidus temperature to form a
homogeneous mixture in the melting state and then quenching
the mixture. In the production of glass, the glass is
usually made transparent by, for example, a means in which
the bubbles in the melt formed from adsorbed gas in the raw
materials and the gas generation during the reaction are
removed by thoroughly elevating the temperature of the melt
to reduce the viscosity of the melt and floating the gases or
air bubbles.
However, in the case of producing a glass from silica as
the raw material, because of its high melting point, the
temperature cannot be elevated to an extent effective for the
bubble-removing owing to the restrictions in refractories of
the crucible or furnace or other reasons and if the
temperature is elevated excessively, gases are generated by
the volatilization of the raw material per se and the
reaction between the raw material and the crucible to form
bubbles. Therefore, the above-described method cannot be
employed. For the reasons set forth above, a method for the
production of a transparent quartæ glass using silica as the
raw material is restricted to either one of a generally known
Verneuil method, a zone melting method, or a vacuum melting
method.
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The Verneuil method is a method in which a silica powder
is gradually fed into an argon-oxygen plasma flame or an
oxygen-hydrogen flame and melted for glass formation and the
resulting melt is deposited onto a stand, and the generated
gases are dissipated from the surface.
The zone melting method is a method in which a porous
body composed of a silica fine powder is prepared and melted
from one end thereof in a band-like state for glass
formation, and the generated gases leave through the unmelted
porous body.
The vacuum melting method is a method in which a rock
crystal powder prepared to have a particle diameter of about
100 ~m is placed in a crucible and melted in a vacuum heating
furnace for glass formation, and the generated gases are
removed by force.
However, with respect to the Verneuil method and the
zone melting method, it is well known that an extremely long
period of time is required for producing one glass block and
its productivity is poor, and especially in the case of the
Verneuil method, the yield is extremely low, 30% to 40%.
Further, when the argon-oxygen plasma flame is employed as a
heat source, though a glass having a small number of residual
OH groups and a relatively small number of bubbles can be
obtained, the energy cost is high, whereas when the oxygen-
hydrogen flame which is cheap in the energy cost is employed,a product having a large number of residual OH groups is
obtained. Still further, since the shape of ingots which can
be produced is restricted to a cylindrical and slender one,
there is a further disadvantage to the subsequent
processings.
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13Q6870
According to the vacuum melting method, though a
relatively large-sized ingot having a small number of
residual OH groups and a high viscosity at high temperatures
can be obtained, since the raw material powder filled in a
vessel such as a crucible is melted for glass formation, not
only is there a disadvantage to the
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~06~370
debubbling but also a reaction gas caused by the contact with the
vessel is generated and the resulting glass has a relatively
large number of bubbles. Therefore, those having a high quality
cannot be obtained. Further, because of the use of the rock
crystal powder, there is a disadvantage to the raw material
supply due to exhaustion of resources.
It is an object of the invention to provide an improved
process for the production of a high purity glass.
According to a first aspect of the invention there is
provided a process for the production of a high purity quartz
glass comprising the steps of: heating a silica powder having a
particle size of not greater than about 0.02 pm in the presence
of an accelerator for phase conversion containing one or more
alkali metal compounds to convert the silica powder into a porous
sintered body consisting of a porous ~ cristobalite phase;
transferring the porous sintered body to a vacuum melting furnace
while maintaining the porous sintered body at a temperature not
less than the inversion temperature from a ~ -cristobalite phase
to an -cristobalite phase; heating the porous sintered body in
the vacuum melting furnace while maintaining the temperature at
the inversion temperature or higher for degassing the accelator
and any other impurities contained in the porous sintered body by
elevating the temperature;
melting the sintered body by raising the temperature to a
temperature higher than the melting point; and causing glass
formation.
According to a second aspect of the invention there is
provided a process for the production of a high purity quartz
~3~6~0
glass comprising the steps of: heating a silica powder having a
particle size of not greater than about 0.02 ,um in the presence
of an accelerator for phase conversioncontaining one or more
alkali metal compounds and one or more ingredients to convert
silica powder into a porous sintered body consisting of a porous
~ -cristobalite phase; transferring the porous sintered body to a
vacuum melting furnace while maintaining the porous ~intered body
at a temperature not less than the inversion temperature from a
-cristobalite phase to an a -cristobalite phase; heating the
porous sintered body in the vacuum melting furnace while
maintaining the temperature at the inversion temperature or
higher for degassing the accelerator and any other impurities
contained in the porous sintered body by elevating the
temperature melting the sintered body by raising the temperature
to a temperature higher than the melting point; and causing glass
formation.
According to a third aspect of the invention there is
provided a process for the production of a high-quality quartz
glass comprising the steps of: incorporating an accelerator for
pha~e conversion into silica powder having a particle size of not
greater than about 0.02 ~m to form accelerator-containing ~ilica
powder having a particle size of 50 pm to 500 ~m, said
accelerator for phase conversion containing at least one alkali
metal; placing the resulting powder in a vessel and heating the
powder to form a self-supporting porous sintered body consisting
essentially of ~ -cristobalite phase; transferring the porous
sintered body to a vacuum melting furnace while maintaining the
temperature of the porous sintered body at a temperature not
lower than the inversion temperature from ~ -cristobalite phase
to a -cristobalite phase; heating the porous sintered body in
thevacuum melting furnace while maintaining the ~ -cristobalite
phase to degas the accelerator and any other impurities contained
~ 306fi70
in the porous sintered body by elevating thetemperature to the
temperature rioht below the melting point of the porous sintered
body; and raising the temperature in the vacuum melting furnace
to a temperature higher than the melting point of the porous
sintered body to melt the sintered body and for~ quartz glass.
According to a fourth aspect of the invention there is
provided a process for the production of a high-purity quartz
glass comprising the steps of: incorporating an accelerator for
phase conversion into silica powder having a particle size of not
greater than about 0.02 ~m to form accelerator-containing powder
having a particle size of from S0 ~m to 500 ~m, said accelerator
containing at least one alkali metal and an active ingredient;
placing the resulting accelerator containing powder in a vessel
and heating the powder to form a self-supporting porous sint~red
body consisting essentially of ~ -cristobalite phase;
transferring the porous sintered body to a vacuum melting furnace
while maintaining the temperature of the porous sintered body at
a temperature not lower than the inversion temperature from ~ -
cristobalite phase to ~ - cristobalite phase; heating the porous
sintered body in the vacuum melting furnace while maintaining the
-cristobalite phase to degas the accelerator and any other
impurities contained in the porous sintered body by elevating the
temperature to the temperature right below the melting point of
the porous sintered body, the active ingredient remaining in the
porous sintered body: and raising the temperature in the vacuum
melting furnace to a temperature higher than the melting point of
the porous sintered body to melt the sintered body and form
quartz glass.
The present invention has the advantage that it provides a
process for obtaining a quartz glass by the vacuum melting method
using as a raw material a silica powder which is notrestricted in
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~ ;~06~70
terms of resources, by which process a large-sized ingot having a
high purity can be produced and which process is excellent in
productivity and economy.
The preferred embodiment of the present invention provides a
process for the production of a glass, comprising filling a
silica powder in a suitable vessel, heating it in the presence of
an accelerator for phase conversion, such as alkali metal
compounds to form a porous sintered body consisting of a
cristobalite phase, and heating and melting the sintered body in
vacuo for glass formation. That is, the preferred embodiment of
the present invention provides a process for the production of a
glass, comprising a calcination step for obtaining a sintered
body consisting of a cristobalite phase using a
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silica powder and a glass formation step for heating and
melting the porous, standing sintered body obtained by the
above calcination step in a vacuum for glass formation.
When a glass is produced using silica as a raw material,
5 as described above, the Verneuil method by which a relatively
high-quality product can be obtained has disadvantages in
productivity and the like. On the other hand, the vacuum
melting method by which a relatively large-sized ingot can be
obtained has a disadvantage in debubbling, and hence, a
product having a high quality cannot be obtained. The
present inventors have made extensive investigations on the
debubbling problem which the conventional methods cannot
solve as well as the difficult problem of the raw material
supply due to the use of a rock crystal powder, while
considering the characteristics of the vacuum melting method,
and found that a satisfactory degassing processing can be
achieved by converting the raw material silica into a
sintered body consisting of a cristobalite phase and then
melting the sintered body in a vacuum.
It is known that heating crystalline silica causes phase
conversion from a low-temperature quartz phase to a tridymite
phase and further to a cristobalite phase, depending upon the
heating temperature in the heating step. This phase
conversion hardly takes place when silica is used singly, but
it readily takes place when a certain metal compound is added
to silica or silica containing a metal compound is used. For
example, Li2o, Na2O, K2O, MgO, CaO, P2O5, and B2O3 are known
to be useful as an accelerator for phase conversion. On the
other hand, since amorphous silica is dissolved directly when
used alone, in order that it is crystallized into a
cristobalite phase, the addition of the above-described
~06~i70
additives is required. As will be understood from the
foregoing explanation concerning the prior art techniques, in
the conventional methods, if such a metal compound is
incorporated in the raw material, there is caused a factor to
reduce the purity of a final product as in the case of
impurities such as water, and hence, it is not desirable.
That is, since in the conventional methods, there is an
opposite relation between obtaining a high-purity quartz
glass and adding impurities to the raw material, it is
required to use a high-purity raw material for high purity
quartz glass productions. Accordingly, in the prior art
techniques, there is no concept to incorporate a metal
compound into the raw material or to use the raw material
containing a metal compound as in the present invention.
In view of the foregoing facts, it can be said that the
method of the present invention in which an accelerator for
phase conversion is added to silica or silica containing an
effective component for the phase conversion is selectively
used is peculiar, and the characteristics of a sintered body
consisting of a cristobalite phase give rise to a number of
effects coupled with the employment of the vacuum melting
method. In other words, as is well known, since the melting
temperature of the sintered body is uniquely determined by
the cristobalite phase, the sintered body can be heated at
the temperature right below the melting point and subjected
to the degassing processing. Further, since the sintered
body consisting of a cristobalite phase is a porous body
having open pores, it can be degassed thoroughly and readily.
Still further, since some accelerators for phase conversion
are readily vaporized at temperatures below the melting point
of the sintered body, if such an accelerator is selectively
used, a transparent quartz glass from which the impurities
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~306~;70
have almost completely been removed can be obtained. On the
other hand, if an accelerator which is not decomposed and
removed is selected, an active glass containing only the
accelerator can be obtained. In summary, in the present
invention, the metal compounds which were considered to be
impurities in the prior art techniques are indispensable and
effectively function in practicing the present invention.
Next, as an actual example, a process for producing a
transparent quartz glass ingot using an amorphous silica fine
powder as the raw material is explained.
For example, an accelerator for phase conversion is
added to and mixed with an amorphous silica fine powder which
is obtained by oxidizing silicon tetrachloride. As the
accelerator for phase conversion, one of the alkali metal
compounds can be selected, but according to the knowledge of
the present inventors, it is contemplated to obtain a
transparent quartz glass, an Na additive is effectively used
as one which can be most readily vaporized. An amount of the
Na additive to be added is an amount in which the sintered
body consisting of a cristobalite phase can be readily
obtained. According to experiments made by the present
inventors, the invention can be practiced within the range of
from 100 ppm to 2,000 ppm in terms of weight ratio to the raw
material powder. If the amount is below the lower limit,
there if caused a problem in the crystallization, whereas if
it exceeds the upper limit, there is caused a problem in the
debubbling processing. Accordingly, if operability is also
taken account, it is desirable to synthesize an amorphous
silica fine powder containing about 1,000 ppm, based on the
weight of the raw material powder, of an Na additive.
~306~370
The following method can be employed as a means for
adding the Na additive. That is, since when the amorphous
silica fine powder is added to the water purified through
ion-exchange resin and the mixture is stirred, there is
obtained a dispersion in which the solid can be hardly
separated from the liquid (i.e., a sol), the Na additive is
added in the NaOH state to the sol. Alternatively, the
amorphous silica fine powder is added to the water purified
through ion-exchange resin to which the Na additive has been
previously added in the NaOH state, and the mixture is
stirred, whereby Na ions is uniformly deposited onto the fine
powder. A practical useful amount of the Na additive to be
deposited is about 1,000 ppm in terms of weight ratio, and in
order to achieve this, the solution may be prepared such that
it contains about 2,300 ppm of the Na additive in terms of
weight ratio. A solution containing the Na additive-
deposited amorphous silica fine powder is dehydrated and
dried for repowdering by a suitable means. A commercially
available amorphous silica fine powder is usually a fine
powder having a size of about 0.02 ~m or less, and hence, if
it is crystallized by heating, the sintering rapidly proceeds
whereby a dense sintered body can be obtained. Thus, it is
important to avoid such matter. That is, it is desirable
that the pore of the resulting sintered body consisting of a
cristobalite phase is so large as to leave the residual gas
and so small as to have a suitable rigidity and correspond to
the shrinkage at the melting in the glass forming process.
In order to meet this requirement, the solution containing
the Na additive-deposited amorphous silica is, for example,
dispensed in a suitable vessel and frozen in a freezer, or
frozen by means of a known ice making apparatus. The
resulting solution is defrosted spontaneously or by an
optional means, su~h as heating. The sol consisting of
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~306~0
amorphous silica and water is thus separated into two phases,
i.e., solid and liquid. The supernatant is disposed, and the
aggregate of amorphous silica retained in the bottom of the
vessel is dehydrated and dried. The dried and repowdered
amorphous silica fine powder is controlled with respect to
granularity by simple fragmentation, whereby the fine powders
are aggregated to obtain a powder having a particle diameter
of from about 50 to 500 ~m, the original diameter of which
was about 0.02 ~m or less.
The thus obtained amorphous silica fine powder is filled
in, for example, a mullite container having a high-
temperature strength and is heated at l,000C or higher by an
optional heating means to convert it into a sintered body
consisting of a cristobalite phase. At this time, it is
desirable that the temperature-elevating rate is as low as
possible. The thus obtained sintered body is a porous body
which has a shape corresponding to that of the filling
container, has open pores, and which has a rigidity free from
any problem in standing and transportation. This sintered
body consists essentially of a ~-cristobalite phase and is a
high temperature-type cristobalite crystal. When the
sintered body is quenched to invert it into a low
temperature-type ~-cristobalite phase, fine cracks generated
by a volume reduction of about 6%. If the thus cracked
crystal is melted for glass formation, the formation of
cracks is further accelerated, and therefore, a desired
product is hardly obtainable. It is known that the inversion
from the ~-cristobalite phase to the ~-cristobalite phase
takes place at a temperature between 220C and 275C.
Accordingly, it is desirable that the sintered body obtained
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~3~6~70
by heating to about 1,000 D C is transferred to the glass
formation process while maintaining the temperature at the
inversion temperature or higher.
The glass formation process step is carried out in a
method similar to the known vacuum melting method, and the
sintered body consisting of a cristobalite phase, which is
obtained by the calcination process, is heated and melted in
vacuo for glass formation. At this time, for the reasons as
described above, the glass formation process is carried out
by placing the sintered body in a vacuum heating furnace and
putting it on a shallow tray without charging it in a
crucible, while maintaining the ~-cristobalite crystal state
(i.e., keeping the temperature at about 300C or higher).
This is because not only can the sintered body have a
satisfactory rigidity against transportation and stand by
itself, but also the degassing is rendered easy and
contamination caused by the contact with the crucible can be
prevented.
In the vacuum heating furnace, the process is carried
out in a reduced pressure of 0.5 mb or less at a temperature
of 1,730C or higher. Since the sintered body is a porous
body having open pores, the impurities in the sintered body
and the Na additive for crystallization and sintering are
readily vaporized upon heating at the respective vaporization
temperatures. Further, since the sintered body consists of a
cristobalite phase of the temperature right below the melting
point and the melting point of the cristobalite phase is
unique, the degassing processing can be extremely effectively
carried out. That is, when the melting takes place step by
step, the porous state is partly broken and the route for
degassing is clogged, whereby the degassing cannot be
-- 10 --
~ 3~G~O
satisfactorily carried out. However, as described above,
because the melting point of the sintered body is uniquely
determined by the cristobalite phase, there are not found
such troubles. Still further, unless the decomposition
reaction occurs, the higher the temperature, the more
effective the removal of adsorbed, reaction residual gases.
In the present invention, the degassing can be carried out by
elevating the temperature to the temperature right below the
melting point. Thus according to the vacuum heating
processing of the present invention, the interior portion of
the sintered body is made substantially in vacuo until it has
been melted, and the Na additive of 1,000 ppm added for
crystallization is reduced to an order of several ppm or
less, whereby a transparent quartz glass having less
impurities and free from bubbles and cracks can be obtained.
While the invention has been described with reference to
a specific embodiment in which an amorphous silica fine
powder is added with an Na additive as an accelerator for
phase conversion, those containing an Na additive, such as
colloidal silica, can also be used as the raw material.
Further, in the case of obtaining a transparent quartz glass,
accelerators for phase conversion which are decomposed at the
temperature right below the melting point of the sintered
body and are readily vaporized are selected. The present
inventors have experimentally confirmed that alkali metal
compounds are useful as such an accelerator for phase
conversion and that among them, Na is an effective additive
for most shortening the glass formation time.
As is clear from the foregoing description, the present
invention is characterized by converting a silica fine powder
into a sintered body consisting of a cristobalite phase and
13~6~0
subjecting the sintered body to the vacuum melting method.
Accordingly, in addition to the accelerator for phase
conversion, known additives for activation can be added to
obtain an active glass in which only the active additives are
retained. Further, it is possible to selectively use an
accelerator for phase conversion which functions for
activation such that it is not positively removed from the
final product. For example, a silica fine powder containing
0.3 mol% of Nd203 and 3 mol% of P205 is granulated into a
secondary particle by the above-described pre-treatment and
then calcined at about l,300C to convert into a sintered
body consisting of a cristobalite phase. The sintered body
is then heated and melted at 1,700C in a vacuum furnace,
whereby a laser glass can be effectively produced.
Accordingly, this method can be employed for the production
of various active glasses, such as photochromic glass, filter
glass, heat-absorbing glass and the like, in addition to the
laser glass, by doping active ions.
The process for the production of a glass according to
the present invention is a glass production process of an
organic combination of a calcination process for converting a
silica fine powder into a sintered body consisting of a
cristobalite phase and a glass formation process for heating
and melting the sintered body in vacuo, and has a number of
characteristics and effects which have not been found in the
conventional methods. In the case of producing a high-
viscosity glass such as a quartz glass, the present invention
cannot only solve the poor yield and poor productivity (i.e.,
an extremely long operation time for glass formation is
required) which have been unable to be avoided by the
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~ ~G~;70
conventional methods but also provide a product at an
inexpensive cost, coupled with no necessity of a particularly
expensive heat source.
In addition, while the present invention employs the
vacuum melting method, it can solve all of the problems which
cannot be solved by the conventional methods, such as the
generation of bubbles because of insufficient degassing and
the contamination caused by the contact with a vessel, a
glass ingot having a high purity can be produced.
The present invention is described in further detail
with reference to the following examples, but it is not to be
construed that the present invention is limited thereto.
EXAMPLE 1
In a water tank stored with the water purified through
ion-exchange resin was charged 15 kg of an amorphous silica
powder having a particle diameter of about 0.02 ~m or less,
and an aqueous solution of 60 g of NaOH dissolved in about
500 ml of the water purified through ion-exchange resin was
further charged thereinto, followed by stirring the mixture
for about one hour. The resulting amorphous silica-water sol
was dispensed in a 10~ stainless steel vessel and then stored
in a freezer for freezing. The frozen sol was taken out and
spontaneously defrosted. The resulting sol was separated
into two phases: a solid and a liquid. The sol was poured
out above a 200 mesh net plate and dehydrated. The residue
was brought in a dryer and dried at about 130C. Since the
resulting powder was in the agglomerated state, it was simply
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fragmented by means of a pulverizer to granulate into a
secondary particle having a particle diameter of from 50 ~m
to 500 ~m.
The powder was filled in a mullite cylindrical container
having an inside diameter of 270 mm and a height of 600 mm
and heated to 1,100C by an electric furnace. The heating
was carried out in such a pattern that it took about 40 hours
to elevate the temperature to 1,100C and it tooX about 4
hours to maintain the temperature at 1,100C. There was thus
obtained a cylindrical sintered body consisting of a
cristobalite phase having an outer diameter of 160 mm and a
height of 350 mm. This sintered body could stand by itself
and had a rigidity to such an extent that it did not
completely deform during the transportation, and further, it
was rich in porous properties. The sintered body was brought
into a vacuum furnace while keeping it at 500C or higher and
heated and melted for glass formation under a reduced
pressure of 0.5 mb or less at a temperature of 1,750C. The
program for heating was in such a manner that it took about 6
hours to elevate the temperature to 1,600C and it was kept
about one hour at 1,750C. After completion of the heating,
the vitrified body was cooled for about 2 hours and then
taken out. There was thus obtained about 10 kg of an ingot
of a high-purity, crack-free transparent quartz glass having
an outer diameter of 150 mm and a length of 260 mm.
EXAMPLE 2
200 g of an amorphous silica fine powder was mixed with
2,500 g of the water purified through ion-exchange resin, and
the mixture was stirred for about one hour. Thereafter, 200
g of an aqueous solution containing 7.2 g of NdC~3-6H20 was
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~3C~6~70
further added thereto, and the mixture was stirred for about
one hour. The resulting sol was dispensed in a 5 ~ stainless
steel vessel, and the same procedures as in Example 1 were
followed to obtain a powder having secondary particles of
from 50 ~m to 500~m in particle diameter.
The silica fine powder was filled in a mullite
cylindrical container having an inside diameter of 30 mm and
a height of 100 mm and heated to 1,100C by an electric
furnace. The heating was carried out in such a pattern that
it took about 20 hours to elevate the temperature to 1,100C
and it took about 5 hours to maintain the temperature at
1,100C. There was thus obtained a cylindrical sintered body
consisting of a cristobalite phase having an outer diameter
of 18 mm and a length of 60 mm. This sintered body was then
brought into a vacuum furnace while keeping it at 500C or
higher and heated for glass formation under a reduced
pressure of 0.5 mb or less at a temperature of 1,750C. The
program for heating was in such a manner that it took about 3
hours to elevate the temperature to l,600C and it was kept
about one hour at 1,750C. There was thus obtained about 20
g of a bluish purple, transparent glass having an outer
diameter of 15 mm and a height of 51 mm.
EXAMPLE 3
200 g of an amorphous silica fine powder was mixed with
2,500 g of the water purified through ion-exchange resin, and
the mixture was stirred for about one hour. Thereafter, 100
g of an aqueous solution containing 0.8 g of NaOH was further
added thereto, and the mixture was stirred for about one
hour. 0.5 g of TiCQ4 was gradually added thereto, and the
- 30 stirring was continued for an additional period of about one
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~3~6~0
hour. The resulting sol was processed in the same manner as
in Example 1 tc obtain a powder having secondary particles of
from 50 ~m to 500~m in particle diameter. This powder was
subjected to glass formation processing under the same
condition as in Example 2 to obtain about 20 g of a bluish
purple homogeneous glass having an Na additive of about 2 ppm
and a Ti additive of about 300 ppm and having an outer
diameter of 15 mm and a height of 51 mm.
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