U.S. patent application number 09/942779 was filed with the patent office on 2002-06-27 for opaque silica glass article having transparent portion and process for producing same.
This patent application is currently assigned to TOSOH CORPORATION. Invention is credited to Akiyama, Tomoyuki, Kikuchi, Yoshikazu, Kudo, Masayuki, Nagata, Hiroya, Tsukuma, Koji.
Application Number | 20020078709 09/942779 |
Document ID | / |
Family ID | 27280581 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020078709 |
Kind Code |
A1 |
Nagata, Hiroya ; et
al. |
June 27, 2002 |
OPAQUE SILICA GLASS ARTICLE HAVING TRANSPARENT PORTION AND PROCESS
FOR PRODUCING SAME
Abstract
An opaque silica glass article comprising a transparent portion
and an opaque portion, wherein the opaque portion has an apparent
density of 1.70-2.15 g/cm.sup.3 and contains
5.times.10.sup.4-5.times.10.sup.6 bubbles per cm.sup.3, said
bubbles having an average diameter of 10-100 .mu.m; and the
transparent portion has an apparent density of 2.19-2.21 g/cm.sup.3
and the amount of bubbles having a diameter of at least 100 .mu.m
in the transparent portion is not more than 1.times.10.sup.3 per
cm.sup.3. The opaque silica glass article is made by a process
wherein a mold is charged with a raw material for forming the
opaque portion, which is a mixture comprising a silica powder with
a small amount of a silicon nitride powder, and a raw material for
forming the transparent portion so that the two raw materials are
located in the positions corresponding to the opaque and the
transparent portions, respectively, of the silica glass article to
be produced; and the raw materials are heated in vacuo to be
thereby vitrified.
Inventors: |
Nagata, Hiroya; (Atsugi-shi,
JP) ; Kudo, Masayuki; (Machida-shi, JP) ;
Tsukuma, Koji; (Tsukuba-shi, JP) ; Kikuchi,
Yoshikazu; (Sagae-shi, JP) ; Akiyama, Tomoyuki;
(Yamagata-shi, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN,
MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3213
US
|
Assignee: |
TOSOH CORPORATION
|
Family ID: |
27280581 |
Appl. No.: |
09/942779 |
Filed: |
August 31, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09942779 |
Aug 31, 2001 |
|
|
|
09173685 |
Oct 16, 1998 |
|
|
|
Current U.S.
Class: |
65/17.5 |
Current CPC
Class: |
C03C 2201/24 20130101;
C03C 3/06 20130101; C03C 2201/80 20130101; C03C 11/007 20130101;
Y10S 65/08 20130101; Y10T 428/1317 20150115; C03B 19/09 20130101;
C03C 2203/10 20130101; Y10T 428/249969 20150401 |
Class at
Publication: |
65/17.5 |
International
Class: |
C03B 019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 16, 1997 |
JP |
09-283506 |
Jan 27, 1998 |
JP |
10-14248 |
Claims
What is claimed is:
1. An opaque silica glass article comprising a transparent portion
and an opaque portion, wherein the glass of the opaque portion has
an apparent density of 1.70 to 2.15 g/cm.sup.3 and contains
5.times.10.sup.4 to 5.times.10.sup.6 bubbles per cm.sup.3 of the
glass, said bubbles having an average bubble diameter of 10 to 100
.mu.m; and the glass of the transparent portion has an apparent
density of 2.19 to 2.21 g/cm.sup.3 and the amount of bubbles having
a diameter of at least 100 .mu.m in the transparent portion is not
more than 1.times.10.sup.3 per cm.sup.3 of the glass.
2. The opaque silica glass article according to claim 1, wherein
the linear transparency, as measured by irradiating glass article
with light having a wavelength of300 to 900 nm and expressed as the
value at a thickness of 1 mm, of the opaque portion is not larger
than 5% and that of the transparent portion is at least 90%.
3. The opaque silica glass article according to claim 1, wherein
the shape of the opaque silica glass article is flange-form,
ring-shaped, columnar, square pillar or hollow-square pillar.
4. A process for producing an opaque silica glass article as
claimed in any of claims 1 to 3, which comprises the steps of:
charging a heat-resistant mold with a raw material for forming the
opaque portion of the silica glass article, which is a uniform
mixture comprising a finely divided silica powder having an average
particle diameter of 10 to 500 .mu.m with 0.001 to 0.05 parts by
weight, based on 100 parts by weight of the silica powder, of a
finely divided silicon nitride powder, and a raw material for
forming the transparent portion of the silica glass article so that
the two raw materials are located in the positions corresponding to
the opaque portion and the transparent portion, respectively, of
the silica glass article to be produced; and heating the raw
materials in vacuo at a temperature in the range of the melting
temperature of the raw materials and 1,900.degree. C. whereby the
raw materials are vitrified.
5. The process for producing an opaque silica glass article
according to claim 4, wherein the transparent portion-forming raw
material is a finely divided silica powder having an average
particle diameter of 10 to 500 .mu.m.
6. The process for producing an opaque silica glass article
according to claim 4, wherein the raw material for forming portion
of the silica glass article is a transparent molded silica glass
article.
7. The process for producing an opaque silica glass article
according to claim 6, wherein the transparent molded silica glass
article is ring-shaped.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] This invention relates to an opaque silica glass article and
a process for producing the same. More particularly, it relates to
an opaque silica glass article comprising a transparent portion and
an opaque portion, and having good heat insulating property and
good surface smoothness, and to a process for producing the opaque
silica glass article by melt-forming together a raw material for
the opaque portion and a raw material for the transparent portion
into an article of an arbitrary shape.
[0003] (2) Description of the Related Art
[0004] An opaque silica glass article has good heat-insulating
property, i.e., is capable of cutting-off heat rays transferring as
radiant heat. In the case where the silica glass article contains a
salient amount of fine bubbles uniformly distributed therein, its
heat-insulating performance is superior.
[0005] One example of the opaque silica glass article is a flange
provided at the base of a furnace tube used as a furnace for
heating a silicon wafer, as illustrated in FIG. 1. A heating race
illustrated in FIG. 1 has heretofore used widely for heating a
silicon wafer, which comprises a heating element 1, a furnace tube
2, a boat 4 for supporting silicon wafers 3, an insulating cylinder
5 and a base 6. A flange 9 is provided at the base of the furnace
tube 2. The flange 9 is made of opaque silica glass and welded
together with the furnace tube 2 by an oxyhydrogen flame. The
flange 9 has a function of heat insulation for cutting off heat
transferring to the base 6 and a packing 7, which have a poor heat
resistance. A desired atmosphere can be kept within the furnace
tube 2 by the seal by means of packing 7 between the flange 9 and
the base 6. Opaque silica glass is widely used in many fields
including the flange of a heating furnace.
[0006] The opaque silica glass article is usually made by a method
for heating a powdery siliceous raw material to melt and vitrify
the raw material. The method for heating the raw material includes,
for example, Verneuil's method wherein the raw material is
subjected to flame fusion by using an argon-oxygen plasma flame or
an oxyhydrogen flame, and a vacuum melting method wherein a vessel
is charged with the raw material and the raw material is heated and
melted in vacuo.
[0007] As the raw material for the opaque silica glass article,
natural silica rock or stone, and rock crystal of a low quality
level have heretofore been widely used. These raw materials contain
a multiplicity of fine bubbles therein, and, when the raw materials
are melted for vitrification, the bubbles remain within he glass to
yield opaque silica glass articles.
[0008] In recent years, LSI is being highly integrated in the field
of a semiconductor, and thus a raw material with a high purity of
an opaque silica glass article is eagerly desired. A most typical
example of the silica glass article is the above-illustrated flange
of a furnace tube used in a furnace for heating a silicon wafer.
However, natural raw materials used for the production of an opaque
silica glass article contain a salient amount of impurities as well
as a salient amount of fine bubbles, and the bubbles are very
difficult to remove. Namely it is difficult to obtain a raw
material with a high purity by purification. On the other hands, a
rock crystal with a relatively high purity contains a minor amount
of fine bubbles therein in the crystal, and therefore, even when
the rock crystal is melted, the degree of opaqueness is not
enhanced and the resulting silica glass article is translucent.
[0009] To solve the above-mentioned problems of the prior art, many
proposals have been made. For example, a process has been proposed
wherein an amorphous silica with a high purity which contains
reduced amounts of an alkali metal, an alkaline earth metal, iron
and aluminum, and a salient amount of fine bubbles, and has a
silanol group as a vaporizable ingredient contained uniformly at a
specific concentration is subjected to flame fusion (Japanese
Unexamined Patent Publication (abbreviated to "JP-A") H6-24711).
However, only silica glass articles having a simple shape such as
an IC (Integrated circuit)-sealing silica filler and a matrix ingot
for silica glass powder can be directly produced, and
after-treatments such as after-shaping by lathing are necessary for
the production of silica glass articles with a complicated shape
such as a flange-form, a ring-shape, column, square pillar or
hollow-square pillar. Utilization of the raw material is low in the
production of silica glass articles with a complicated shape, and
thus, the production cost is inevitably increased.
[0010] As another process for producing an opaque silica glass
article, a process has been proposed wherein a highly purified
crystalline silica powder is heated in an ammonia atmosphere and
then the thus-ammoniated silica powder is heated and melted in an
inert gas atmosphere to give an opaque silica glass article having
an increased number of very fine bubbles, i.e., having a large
total cross-sectional area of bubbles per unit volume of the opaque
silica glass, and thus exhibiting an enhanced heat insulation (JP-A
H7-61827 and JP-A H7-300341). However, this process has problems
such that the density of opaque silica glass, and the diameter and
amount of bubbles contained therein greatly varies depending upon
the particle diameter and particle diameter distribution of raw
material powder and the state of raw material powder charged in a
vessel for fusion, and thus, the diameter and amount of bubbles in
the surface portion and those in the central portion greatly differ
from each other, and an opaque silica glass article having bubbles
uniformly distributed therein is difficult to produce with good
reproducibility.
[0011] As still another process for producing an opaque silica
glass article, a process has been proposed wherein a finely divided
powder of a foaming agent such as carbon or silicon nitride is
incorporated in a siliceous raw material such as silica rock or
stone, a-quartz or cristobalite, and the mixture is subjected to a
flame fusion using an oxyhydrogen flame (JP-A H4-65328). The
abovementioned problems can be solved by this proposed process.
However, the use of oxyhydrogen flame invites introduction of a
hydroxyl group within silica glass which leads to reduction of the
viscosity of molten glass and results in an opaque silica glass
article not suitable as articles used for a long period of time at
a high temperature, such as a jig for the production of
semiconductor devices. Further, in this flame fusion step, the
residence time of finely divided particles in the flame is very
short, and the completion of reaction in the flame is difficult and
it is possible that the foaming agent incorporated remains in the
molten material as a foreign matter, and fiber that the siliceous
raw material reacts with the forming agent with the result of
undesirable coloration of the molten material.
[0012] It is said that, when a silica glass jig for the production
of a semiconductor is cleaned after the use thereof, the bubbles
exposed on the surface is removed, i.e., the surface is partly
scraped down. To solve this problem, a procedure has been adopted
for adhering a protective transparent silica glass film of a
predetermined shape on the surface by heating with oxyhydrogen
flame or in an electric furnace.
[0013] For the flange provided at the base of a furnace tube of a
heating furnace for a silicon wafer, a heat insulating property as
well as a sealing property are required to stably control the
atmosphere within the furnace tube. Conventional opaque silica
glass flanges have a rough surface due to the presence of bubbles
and thus, even where a packing is used, a complete seal cannot be
attained. For overcoming this defect, a flange having an opaque
portion with good heat insulating property and a transparent
portion with good sealing property is suitable.
[0014] Several processes have been proposed for producing the
flange having an opaque portion with good heat insulating property
and a transparent portion with good sealing property is suitable.
As examples of such processes, there can be mentioned (1) a process
for fusion-bonding a transparent silica glass article to an opaque
silica glass article, (2) a process wherein a powdery raw material
for an opaque silica glass is added to a taparent silica glass
article and the combination thereof is fusion-bonded, (3) a process
wherein a powdery raw material for an opaque silica glass and a
powdery raw material for a taparent silica glass are melted, and
(4) a process wherein a surface portion of an opaque silica glass
article containing bubbles therein is melted whereby bubbles within
the surface portion is removed and thus the surface portion is
rendered transparent.
[0015] The above-recited processes have the following problems.
Namely, in the process of (1), at the step of fusion-bonding,
bubbles are liable to occur at the interfacial boundary between the
transparent silica glass portion and the opaque silica glass
portion thereof In general the adhesion between the transparent
portion and the opaque portion thereof is not sufficient and the
adhered transparent portion and opaque portion are liable to be
separated. Further when the shape of the opaque silica glass
article is complicated, the transparent silica glass becomes very
difficult to fabricate and to fusion-bond to the opaque silica
glass.
[0016] In the process of (2), bubbles do not readily occur at the
interfacial boundary between the two silica glass portions, but the
powdery raw material for the opaque silica glass portion shrinks in
the course from the fusion bonding step to the completion of
vitrification, and thus the resulting silica glass article is
liable to warp. More specifically, JP-A H7-300326 discloses a
process wherein a transparent silica glass article is placed in a
heat-resistant mold, a powdery raw material for forming opaque
silica glass is superposed upon the transparent silica glass
article, and then the combined material is subjected to fusion
bonding in an inert gas atmosphere to give a silica glass article
having an opaque silica glass layer and a transparent silica glass
layer. In this process, when the superposed powdery raw material
containing an inert gas among the particles is melted and
vitrified, the inert gas contained among the particles is entrapped
within the molten material and becomes bubbles in the resulting
glass article. However, the amount of gas derived from the raw
material, the number and diameter of bubbles occasionally vary and
the bubbles are difficult to uniformly distribute within the glass,
and sometimes an inert gas introduced at the step of fusion bonding
becomes part of the bubbles within the glass. Therefore, the
bubbles within the opaque portion of the silica glass article are
difficult to control.
[0017] In the process of (3), the gas contained in the powdery raw
material for forming an opaque portion partly penetrates into the
powdery raw material for forming a transparent portion with the
result of occurrence of bubbles in the vicinity of the interfacial
boundary. Further the opaque silica glass portion and the
transparent silica glass portion, both of which shrink in the
course from fusion bonding to the completion of vitrification,
exhibit different shortages, and thus, the resulting silica glass
article tends to warp.
[0018] In the process of (4), it is difficult to melt uniformly in
thickness the surface portion of the bubble containing opaque
silica glass article, and further to desecrate the molten surface
portion to a satisfying extent.
SUMMARY OF THE INVENTION
[0019] In view of the foregoing, a primary object of the present
invention is to provide an opaque silica glass article having a
transparent portion and an opaque portion containing bubbles
uniformly distributed therein, characterized as exhibiting
excellent high-temperature viscosity and heat insulation, and
having a smooth surface, i.e., not having a roughness, which has
occurred due to bubbles contained in the glass article, over the
entire surf-ace or part of the surface.
[0020] Another object of the present invention is to provide a
process for producing the above-mentioned opaque silica glass
article industrially advantageously, whereby a silica glass article
of a complicated shape such as, for example, a flange-form,
ring-shaped, columnar, square pillar or hollow-square pillar can be
directly produced from raw materials.
[0021] In accordance with the present invention, there is provided
an opaque silica glass article comprising a transparent portion and
an opaque portion, wherein the glass of the opaque portion has an
apparent density of 1.70 to 2.15 g/cm.sup.3 and contains
5.times.10.sup.4 to 5.times.10.sup.6 bubbles per cm.sup.3 of the
glass, said bubbles having an average bubble diameter of 10 to 100
.mu.m; and the glass of the transparent portion has an apparent
density of 2.19 to 2.21 g/cm.sup.3 and the amount of bubbles having
a diameter of at least 100 .mu.m in the transparent portion is not
more than 1.times.10.sup.3 per cm.sup.3 of the glass.
[0022] In another aspect of the present invention, there is
provided a process for producing the above-mentioned opaque silica
glass article, which comprises the steps of:
[0023] charging a heat-resistant mold with a raw material for
forming the opaque portion of the silica glass article, which is a
uniform mixture comprising a finely divided silica powder having an
average particle diameter of 10 to 500 .mu.m with 0.001 to 0.05
parts by weight, based on 100 parts by weight of the silica powder,
of a finely divided silicon nitride powder, and a raw material for
forming the transparent portion of the silica glass article so that
the two starting materials are located in the positions
corresponding to the opaque portion and the transparent portion,
respectively, of the silica glass article to be produced; and
[0024] heating the raw materials in vacuo at a temperature in the
range of the melting temperature of the raw materials and
1,900.degree. C. whereby the raw materials are vitrified.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a vertical cross-sectional view illustrating a
furnace for heating silicon wafers;
[0026] FIG. 2 is a perspective view illustrating a heat-resistant
mold having a ring-form cavity, which is cut along a central
vertical plane;
[0027] FIG. 3 is a perspective view of a heat-resistant mold having
a flange-shaped cavity, which is cut along a central vertical
plane;
[0028] FIG. 4 is a perspective view of a flange-shaped opaque
silica glass article made by using the mold illustrated in FIG.
3;
[0029] FIG. 5 is a perspective view of a heat-resistant mold having
a columnar cavity, which is cut along a central vertical plane;
[0030] FIG. 6 is a perspective view of a columnar opaque silica
glass article made by using the mold illustrated in FIG. 5;
[0031] FIG. 7 is a perspective view of a ring-form opaque silica
glass article made by using the mold illustrated in FIG. 2;
[0032] FIG. 8 is a perspective view of a heat-resistant mold having
a square pillar-form cavity, which is cut along a central vertical
plane;
[0033] FIG. 9 is a perspective view of a square pillar-form opaque
silica glass article made by using the mold illustrated in FIG.
8;
[0034] FIG. 10 is a perspective view of a heat-resistant mold
having a hollow square pillar-form cavity, which is cut along a
central vertical plane;
[0035] FIG. 11 is a perspective view of a hollow square pillar-form
opaque silica glass article made by using the mold illustrated in
FIG. 10;
[0036] FIG. 12 is a vertical cross-sectional view illusions a
heat-resistant mold in which a powdery raw material is charged;
[0037] FIG. 13 is a perspective view illustrating the powdery raw
material-charged heat-resistant mold illustrated in FIG. 12;
[0038] FIG. 14 is a side view of an opaque silica glass article
made by using the mold illustrated in FIG. 12 and FIG. 13;
[0039] FIG. 15 is a perspective view of the opaque silica glass
article illustrated in FIG. 14;
[0040] FIG. 16 is a cross-sectional view of a heat-resistant mold
charged with a raw material for forming a ring-form transparent
silica glass article;
[0041] FIG. 17 is a cross-sectional view of a heat-resistant mold
charged with a ring-form transparent silica glass article and a
powdery raw material for forming an opaque portion of a silica
glass article;
[0042] FIG. 18 is a perspective view of an opaque silica glass
article made by using the mold illustrated in FIG. 17, which
article is cut along a central vertical plane; and
[0043] FIG. 19 is a perspective view of an opaque silica glass
article made by using the mold illustrated in FIG. 17, which is a
comparative article and is cut along a central vertical plane.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] (1) Raw Materials
[0045] As the raw material for forming an opaque portion of the
silica glass article of the invention, a mixture of a finely
divided silica powder and a silicon nitride powder is preferably
used As the raw material for forming a transparent portion of the
silica glass article of the invention, a finely divided silica
powder or a shaped transparent silica glass article is used. More
specifically, in the opaque silica glass article having an opaque
portion and a transparent portion of the invention (hereinafter
abbreviated to "silica glass article of the invention"), the opaque
portion constituting the main part of the silica glass article of
the invention is made preferably by melting a mixture of a finely
divided silica powder and a finely divided silicon nitride powder.
The transparent portion of the silica glass article of the
invention, which forms the entirety or a part of a surface of the
silica glass article, is made from a finely divided silica powder
or a shaped transparent silica glass article. In the following
explanation of the transparent portion, either a finely divided
silica powder or a shaped silica glass article is used, but it
should be construed that any of the silica powder and the shaped
silica glass article can be used
[0046] (1-1) Silica Powder
[0047] As the finely divided silica powder used in the invention, a
finely divided crystalline or amorphous silica powder having a high
purity and containing metal impurities such as Na, K, Mg and Fe
each in an amount of 0 to 1 ppm is preferably used for the
following reason. When the silica glass article of the invention is
heated, for example, when a flange made thereof is attached to a
wafer-heating furnace and is exposed to a high temperature, the
vaporization of impurities exhibiting a high vapor pressure leading
to an environmental contamination, the partial crystallization of
the silica glass article of the invention occasionally leading to
rupture thereof, and the coloration of the silica glass article of
the invention can be avoided by the use of the high-purity silica
powder.
[0048] The high-purity silica powder is prepared by a synthesis
process or a purification of a natural raw material. For example,
an amorphous silica powder is synthesized by a process wherein an
aqueous alki metal silicate solution (water-glass) is reacted with
an acid to remove an alkali metal thereby yielding silica, a
process for hydrolyzing SiCl.sub.4 to give silica, and a process
wherein a silicon alkoxide is hydrolyzed to give silica. Of these,
the first process, especially a process wherein an aqueous alkali
metal silicate solution (water-glass) composed of an alkali metal
such as Na, K or Li and silicon dioxide is allowed to react with an
inorganic acid such as sulfuric acid, nitric acid or hydrochloric
acid, is preferable from a viewpoint of commercial production. A
crystalline silica powder can be obtained from natural raw material
by treating natural qua with hydrofluoric acid.
[0049] Preferably the finely divided silica powder has an average
particle diameter of 10 to 500 .mu.m in view of fluidity for
charging into a heat-resistant mold. If the average particle
diameter is smaller than 10 .mu.m, the silica powder has a poor
fluidity and is difficult to charge into the mold. In contrast, if
the average particle diameter is larger 500 .mu.m voids among the
particles are too large and large bubbles having a diameter of at
least 300 .mu.m tend to occur, and especially, in the case where a
transparent portion of the silica glass article is formed from the
silica powder, a salient amount of large bubbles having a diameter
of larger than 500 .mu.m occasionally occur.
[0050] The diameter of bubbles contained in the silica glass
article of the invention varies depending upon the particular
average diameter of the silica powder, and thus, the bubble
diameter can be varied by controlling the average diameter of the
silica powder. Namely, fine bubbles with a small diameter and
bubbles with a large diameter can be obtained from a silica powder
having a small average diameter and a silica powder having a large
average diameter, respectively.
[0051] (1-2) Silicon Nitride Powder
[0052] As the silicon nitride powder, a high-purity silicon nitride
prepared by nitriding a starting material such as silicon
tetrachloride, silicon or silica is preferably used.
[0053] By using the high-purity silicon nitride powder, when the
resulting opaque silica glass article of the invention is heated,
the vaporization of impurities exhibiting a high vapor pressure
leading to an environmental contamination, the partial
crystallization of the silica glass article of the invention
occasionally leading to rupture thereof, and the coloration of the
silica glass article of the invention can be avoided.
[0054] The amount of the silicon nitride powder is 0.001 to 0.05
parts by weight based on 100 parts by weight of the silica powder.
If the amount of the silicon nitride powder is smaller than 0.001
parts by weight, the amount of bubbles formed is too small, and the
opaque silica glass article has a poor heat insulation. In
contrast, if the amount of the silicon nitride powder is larger
than 0.05 parts by weight, the bubbles formed become too large and
the opaque silica glass article has a poor mechanical strength.
[0055] The silicon nitride powder preferably has an average
particle diameter of 0.1 to 1 .mu.m, more preferably 0.1 to 0.5
.mu.m. By using the silicon nitride powder having an average
particle diameter falling within this range, the amount and size of
bubbles formed become adequate, and the uniform mixing of the
silicon nitride powder and the silica powder can be effected
without agglomeration.
[0056] (2) Mixing
[0057] A finely divided silica powder and a finely divided silicon
nitride powder are mixed together to prepare a raw material for
forming the opaque portion of the silica glass article of the
invention. The extent and state of dispersion of the silicon
nitride powder in the mixture influences upon the diameter and
distribution of bubbles formed, the silicon nitride should be
uniformly dispersed in the powdery mixture. The mixing means is not
particularly limited provided that a uniform dispersion can be
obtained. For example, a mortar and a ball mill can be used. To
obtain a highly uniform dispersion of the silicon nitride powder in
the powdery mixture, a wet process using a dispersing medium is
preferably employed. As examples of the dispersing medium, there
can be mentioned water and alcohols such as ethanol and methanol.
To enhance the dispersibility of the silicon nitride powder in the
powdery mixture, an ultrasonic vibration can be applied by using an
ultrasonic generator.
[0058] (3) Charging of Raw Material in Mold
[0059] The raw material for forming the transparent portion of the
silica glass article and the raw material for forming the opaque
portion thereof are charged in a heat-resistant mold.
[0060] First, charging of a finely divided silica powder as the
transparent portion-forming raw material will be explained.
[0061] Namely, the silica powder as the transparent portion-forming
raw material and the silicalsilicon nitride powdery mixture as the
opaque portion-forming raw material are placed in a heat-resistant
mold. The material and shape of the heat-resistant mold are not
particularly limited provided that the mold exhibits a good
resistance and does not influence the raw material at the melt
fusion step. The heat-resistant mold may be either a single mold or
a split mold composed of two or more parts. The split mold is used
for molding a silica glass article having a complicated shape. The
shape and combination of two or more parts of the split mold can be
appropriately chosen depending upon the desired shape of the opaque
silica glass article. As the material of the mold, there can be
mentioned those which do not react with silica to any appreciable
extent, such as carbon, boron nitride and silicon carbide. To
impart a good sliding property between the inner wall of the mold
and the raw material, carbon felt or carbon paper can be inserted
between the inner wall of the mold and the raw material during
charging of the raw material and heating the raw material.
[0062] The raw material for forming the opaque portion of the
silica glass article (namely, a powdery silica/silicon nitride
mixture) and the raw material for forming the transparent portion
thereof (namely a silica powder) are placed in the mold so that the
raw materials are located in the positions corresponding to the
opaque portion and the transparent portion, respectively, of the
silica glass article to be shaped.
[0063] The shape and size of the heat-resistant mold is determined
depending upon the desired shape and size of the opaque silica
glass article. For example, when a heat-resistant mold having a
flange-shaped cavity as illustrated in FIG. 3 is used, a
flange-shaped opaque silica glass article as illustrated in FIG. 4
is produced. When a heat-resistant mold having a columnar cavity as
illustrated in FIG. 5 is used, a columnar opaque silica glass
article as illustrated in FIG. 6 is produced. When a heat-resistant
mold having a ring-form cavity as illustrated in FIG. 2 is used, a
ring-form opaque silica glass article as illustrated in FIG. 7 is
produced. When a heat-resistant mold having a polyhedron cavity
such as square pillar-form cavity as illustrated in FIG. 8 is used,
a polyhedron (such as square pillar-form) opaque silica glass
article as illustrated in FIG. 9 is produced. When a heat-resistant
mold having a hollow square pillar-form cavity as illustrated in
FIG. 10 is used, a hollow square pillar-form opaque silica glass
article as illustrated in FIG. 11 is produced. As a modified form
of the ring-form article of FIG. 7, a ring-form silica glass
article, one end of which is closed, can be produced. Similarly, as
a modified form of the hollow square pillar-form article of FIG.
11, a hollow square-form silica glass article, one end of which is
closed, can be produced.
[0064] In a specific example of charging the heat-resistant mold
with the raw materials, a silica powder is laid on the bottom of a
heat-resistant mold having a columnar cavity, a powdery
silica/silicon nitride mixture is laid thereon and further a silica
powder is laid thereon. By heating the thus-filled powdery raw
materials, a columnar opaque silica glass article having a
transparent top layer, an opaque central body and a transparent
bottom layer is produced. In another specific example of charging
the heat-resistant with the raw materials, a silica powder is laid
on the bottom of a heat-resistant mold having a columnar cavity, a
cylindrical auxiliary fame having a diameter slightly smaller than
the diameter of the columnar cavity is placed on the laid silica
powder, a powdery silica/silicon nitride mixture is charged within
the cylindrical auxiliary frame, a silica powder is filled in a
cylindrical space between the cylindrical auxiliary frame and the
inner wall of the mold, the cylindrical auxiliary frame is drawn
out gently; and finally, a silica powder is laid on the top of the
charged raw materials. By heating the thus-filled raw materials, a
columnar opaque silica glass article having a transparent layer
covering the entire surface of the glass article can be
produced.
[0065] The packing density of the powdery raw materials within the
mold is preferably in the range of 0.7 to 1.8 g/cm.sup.3. The
packing density of the raw material for forming the opaque portion
should preferably be as uniform as possible so as to form the
opaque portion having bubbles uniformly dispersed in the opaque
portion.
[0066] Secondly, placing of a transparent silica glass article as
the transparent portion-forming raw material within the mold will
be explained.
[0067] In one specific example, the powdery silica/silicon nitride
mixture for forming the opaque portion and a transparent ring-form
silica glass article for forming the transparent portion are
charged in the mold having a ring-form cavity as illustrated in
FIG. 2. The transparent ring-form silica glass article is
previously fabricated so that it is capable of being placed within
the mold. The transparent ring-form silica glass article preferably
has an apparent density of 2.19 to 2.21 g/cm.sup.3 and contains not
more than 1.times.10.sup.3 bubbles per cm.sup.3, said bubbles
having a diameter of at least 100 .mu.m. An opaque silica glass
article made by using the transparent ring-form silica glass
article is characterized in that the exposed surface of the
transparent portion does not have bubbles to any appreciable extent
and thus, when the surface is subjected to cleaning, surface
roughness due to development of bubbles does not occur, and thus,
good sealing property can be obtained.
[0068] The shape and size of the transparent ring-form silica glass
article are not particularly limited provided that it can be placed
within the mold and it forms a sealing surface of the resulting
opaque silica glass article. Usually the transparent ring-form
silica glass article has a shape and size corresponding to those of
the resulting opaque silica glass article.
[0069] The transparent ring-form silica glass article can be
prepared by a process wherein the silica powder is melted by
applying an oxyhydrogen flame or melted in an electric furnace in
vacuo to give a transparent silica glass block, followed by
grinding the block into the desired shape and size. In the process
employing an electric furnace, preferably a heat-resistant mold
having a ring-form cavity having a size substantially the same as
that of the transparent portion of the opaque silica glass article
is used. By using this mold, a transparent ring-form silica glass
article having a size similar to that of the transparent portion
can be produced, and thus, the after-processing of the transparent
ring-form silica glass article is easy and simple, and man hour and
material loss are minimized.
[0070] The material of the heat-resistant mold used is not
particularly limited provided that it is resistant to heat and does
not influence the raw material to any appreciable extent at the
heating step. For example, the heat-resistant mold can be made of
materials which do not easily react with silica, such as carbon,
boron nitride and silicon carbide.
[0071] To enhance the sliding property of the raw materials on the
inner wall of the mold, preferably carbon felt or carbon paper is
placed between the inner wall of the mold and the raw materials at
the step of charging and heat-melting.
[0072] The order of charging of the transparent silica glass
article and the powdery silica/silicon nitride mixture for forming
the opaque portion is not particularly limited, but, it is
preferable that the transparent silica glass article is first
placed on the bottom of the mold, and then the powdery mixture is
charged on the silica glass article because undesirably large
compaction of the powdery mixture can be avoided and gases evolved
at the heat-melting step can be effectively removed. The packing
density of the powdery mixture is preferably in the range of 0.7 to
1.8 g/cm.sup.3 for uniformly charging it in the mold.
[0073] (4) Vitrification and Bubble Formation
[0074] In order to completely decompose silicon nitride in the
powdery silica/silicon nitride mixture to form bubbles and to
convert the powdery silica/silicon nitride mixture into an opaque
silica glass, and fiber, to convert a silica powder, if used, as a
raw material for forming the transparent portion into a transparent
glass, the raw materials charged within the mold are heated to be
thereby melted. The heating apparatus used for heating the mold is
not particularly limited provided that it is capable of converting
the raw material into a glass state, and, for example, an electric
furnace is used.
[0075] The raw materials are heated to a temperature between the
temperature at which the raw materials are melted, and
1,900.degree. C. When an amorphous silica powder is used as a raw
material, it is melted via cristobalite, and thus, the temperature
at which the raw materials are melted is 1,713.degree. C. at normal
pressure. When a crystalline silica powder other than cristobalite
is used as a raw material, it is melted substantially without
through cristobalite and, thus, the temperature at which the raw
materials are melted is lower than the above melting temperature
for the amorphous silica powder. It should be noted that, when a
crystalline silica powder other than cristobalite is heated to a
temperature lower than the melting temperature, at least part of
this raw material is not melted, the resulting silica glass is very
fragile. When an amorphous silica powder is used and a part or the
entirety thereof is transferred to crystalline cristobalite, the
cristobalite is not melted at the heating step, and the resulting
glass is very fragile. If the raw material is heated to a
temperature higher than 1,900.degree. C., the opaque portion of the
opaque silica glass article have bubbles with a large size, and
consequently, the density of glass becomes low, and the mechanical
strength is too low to machine the glass article into a desired
shape and size. The heating time varies depending upon the
particular heating temperature, and is not particularly limited
provided that the entire amount of the raw material is melted and
vitrified. Usually the heating time is about one hour or
shorter.
[0076] In the course of heating the raw material, it is preferable
that a vacuum atmosphere is kept during a period spanning from the
state wherein pores among the particles of the powdery raw material
are open to the state wherein said voids are closed. The degree of
vacuum is preferably such that the pressure is not higher than 50
mmHg, more preferably not higher than 10 mmHg. By conducting the
heating in vacuo, gases eluted from nitrogen in the solid solution
produced by the reaction of silicon nitride with silica in the
powdery silica/silicon nitride mixture, and gases generated by
decomposition of the raw material form bubbles uniformly
distributed in the silica glass article. Further, when a
transparent silica glass article is used as a transparent
portion-forming raw material, the residual fine bubbles within the
transparent portion can be removed.
[0077] In the course where the raw materials charged in a mold are
melted in vacuo whereby they are vitrified and bubbles are formed,
a cover made of; for example, carbon or the like can be placed on
the charged raw materials so that a uniform pressure is applied
onto the entire raw materials, or the bubbles formed are confined
within the molten material or controlled so as not to escape to the
outside.
[0078] At the time when the molten material maintained at a high
temperature is converted to a glass state, an inert gas is
introduced into a mold. The inert gas used is not particularly
limited provided that it does not react substantially with the
mold, the raw material and the product, and includes, for example,
nitrogen, argon and helium. Of these, nitrogen and argon are
preferable in view of the cost and air tightness. The pressure of
the inert gas is usually normal pressure so that, when the
resulting glass is reheated, for example, subjected to flaming
treatment, bubbles within the glass are neither greatly expanded
nor shrunk. A slightly higher or lower pressure may be
employed.
[0079] After the heating for vitrification, the molten material is
cooled to room temperature. Usually the molten material is cooled
by allowing it to stand or by a cooling apparatus to about
1,000.degree. C. The rate of cooling is usually about 1,000.degree.
C./hour. Finally the material is cooled to room temperature. It
should be noted that, the molten material tends to crystallize in
the course of cooling, especially at a high temperature. Therefore,
the molten material should be cooled relatively rapidly in a high
temperature region to avoid the undesirable crystallization. To
enhance the rate of cooling, the same inert gas as that used at the
heat-melting step can be introduced. In a low temperature region in
the course of cooling, there is no problem of crystallization, and
thus, the material is usually left to stand for cooling.
[0080] (5) Opaque Silica Glass Article
[0081] The silica glass article of the invention has an opaque
portion having an apparent density of 1.70 to 2.15 g/cm.sup.3,
preferably 1.80 to 2.12 g/cm.sup.3 and containing 5.times.10.sup.4
to 5.times.10.sup.6 bubbles per cm.sup.3 which bubbles have an
average particle diameter of 10 to 100 .mu.m. These characteristics
are important for imparting good mechanical strength and
processability to the glass article.
[0082] The diameter and amount of independent bubbles contained in
the opaque portion vary depending upon the amount of silicon
nitride powder added, the particle diameter and distribution of
silica powder, the melting temperature and the pressure of gas
introduced. For example, an opaque portion with good heat
insulating property, which has an apparent density of 1.95 to 2.05
g/m.sup.3, and contains 7.times.10.sup.5 to 8.times.10.sup.5
bubbles having an average bubble diameter of 50 to 70 .mu.m, is
obtained by selecting the following conditions: amount of silicon
nitride powder added=0.01 to 0.02 part by weight based on 100 parts
by weight of silica powder, average particle diameter of silica
powder=100 to 200 .mu.m (particle diameter distribution range=10 to
600 .mu.m), melting temperature of 1,800 to 1,850.degree. C.,
pressure of introduced gas of 1.0 to 2.0 kgf/cm.sup.2. An opaque
portion with high heat insulating property, which has an apparent
density of 2.05 to 2.12 g/cm.sup.3, and contains 1.times.10.sup.6
to 2.times.10.sup.6 bubbles having an average bubble diameter of 30
to 50 .mu.m, is obtained by selecting the following conditions:
amount of silicon nitride powder added=0.005 to 0.02 part by weight
based on 100 parts by weight of silica powder, average particle
diameter of silica powder=50 to 100 .mu.m (particle diameter
distribution range=10 to 200 .mu.m), melting temperature of 1,750
to 1,850.degree. C., pressure of introduced gas of 1.0 to 2.0
kgf/cm.sup.2. The amount of bubbles greatly varies depending upon
the particle diameter of silica powder. More specifically an opaque
silica glass article having an excellent heat insulating property,
which contains a large amount of bubbles having a small average
diameter, is obtained by using a finer silica powder.
[0083] The opaque portion of the glass article of the invention
contains bubbles uniformly distributed therein and has a white
appearance. The white opaque portion is characterized as possessing
preferably a linear transparency of not larger than 5% as measured
by irradiating the opaque portion with light having a wavelength of
300 to 900 nm and expressed as the value at a thickness of 1 mm. By
the reduced linear transparency, heat rays are readily scattered,
and thus, the silica glass article of the invention exhibits
excellent heat insulating property as well as a reduced thermal
conductivity.
[0084] The transparent portion of the opaque silica glass article,
which has a function of protecting the surface of the opaque
portion, is characterized as having an apparent density of 2.19 to
2.21 g/m.sup.3. The amount of bubbles having a diameter of at least
100 .mu.m in the transparent portion is not more than
1.times.10.sup.3 per cm.sup.3 of the glass. If the amount of
bubbles with a diameter of at least 100 .mu.m is more than
1.times.10.sup.3 per cm.sup.3, a salient amount of bubbles are
exposed on the surface of the transparent portion, and good sealing
property cannot be obtained. Further, the transparent portion
preferably has a linear transparency of at least 90% as measured by
irradiating said portion with the transparent portion with light
having a wavelength of 300 to 900 nm and expressed as the value at
a thickness of 1 mm. When the linear transparency is at least 90%,
the sealing property is more enhanced.
[0085] According to the process of the invention, a hydroxyl group
is not introduced in the glass at the step of heat-fusion, but is
rather expected to be volatilized from the molten material. The
opaque silica glass article containing the thus-reduced amount of a
hydroxyl group exhibits a high viscosity at a high temperature,
i.e., excellent high-temperature viscosity.
[0086] The shape of the opaque silica glass article of the
invention is not limited and is suitably chosen depending upon the
particular use thereof. For example, the shape thereof is
flange-form, ring-shaped, columnar, square pillar or hollow-square
pillar.
[0087] Especially, when a ring-form opaque silica glass article is
used for a flange attached to a furnace tube, the glass article
preferably has a wall thickness of not larger than 150 mm and a
height (i.e., a length along the axis of the ring) of 30 to 250 mm
in view of the uniformity in density of the opaque portion thereof
and the heat insulation thereof.
[0088] The ratio of the opaque portion to the transparent portion
varies depending upon the particular use, but the amount of the
transparent portion in the opaque silica glass is preferably in the
range of 2 to 30% based on the sum of the transparent portion and
the opaque portion.
[0089] The invention will now be specifically described by the
following examples that by no means limit the scope of the
invention.
[0090] The characteristics of raw materials and opaque silica glass
articles were determined by the following methods.
[0091] (1) Impurity
[0092] The impurities contained in a silica powder were analyzed by
ICP (inductively coupled plasma) atomic emission spectrochemical
analysis.
[0093] (2) Glass State
[0094] The glass state of the transparent portion and opaque
portion of an opaque silica glass article was examined by X-ray
diffraction as follows.
[0095] A specimen having a size of 20 mm.times.10 mm.times.2 mm
(thickness) was cut by a cutter from each of the opaque portion and
the transparent portion. Each specimen was examined by an X-ray
diffraction analyzer (supplied by MAC Science Co., type MXP3), and
the glass state was confirmed by the presence of diffraction peak
occurring due to crystals such as quartz and cristobalite in the
obtained diffraction pattern.
[0096] (3) Apparent Density
[0097] A specimen having a size of 30 mm.times.30 mm.times.10 mm
(thickness) was cut by a cutter from each of the opaque portion and
the transparent portion. Density of each specimen was measured by
using an electronic force balance (supplied by Mettler Instrument
Co., type AT261) according to the Archimedean method.
[0098] (4) Diameter and Amount of Bubbles
[0099] A specimen having a size of 30 mm.times.10 mm.times.0.3 mm
(thickness) was cut by a cutter from each of the opaque portion and
the transparent portion. The diameter and amount of bubbles in each
specimen were measured by using a polarization microscope having a
lens with graduation(supplied by Olympus Optical Co., Ape BH-2) The
average diameter of bubbles in the opaque portion was determined by
counting number of bubbles, calculating the total volume of the
bubbles provided that the bubbles are regarded as having a
spherical form, dividing the total volume of bubbles by the number
of bubbles to determine the average bubble volume, and then,
calculating the average diameter, i.e., the average bubble
diameter. The amount of bubbles in the transparent portion was
determined by counting the number of bubbles having a diameter of
at least 100 .mu.m in a view field of 10 mm.times.10 mm.times.0.3
mm (depth) and calculating the number of bubbles per cm.sup.3.
[0100] (5) Particle Diameter
[0101] Distribution of particle diameter and average particle
diameter of a powdery raw material were measured by the laser
diffraction scattering method using Coulter LS-130 (supplied by
Coulter Electronics Co.)
[0102] (6) Packing Density
[0103] Packing density of a powdery raw material was determined by
packing a predetermined amount of the powdery raw material in a
heat-resistant mold, and dividing the amount by weight of the
packed material by the volume occupied by the packed material.
[0104] (7) Presence of Pore
[0105] A glass article was cut by a cutter and the presence of
pores in the cut surface was checked by visual examination.
[0106] (8) Light Transmission (Linear Transparency)
[0107] Each of the opaque portion and the transparent portion was
cut into a rectangular plate, and both major surfaces of the paste
were polished by an alumna abrasive grain of #1200 to prepare a
specimen having a size of 30 mm.times.10 mm.times.1 mm (thickness).
The linear transparency was measured by irradiating the specimen
with light having a wavelength of 300, 500, 700 or 900 nm,
projected perpendicularly to the major surfaces of the specimen
(band-pass 2 nm) by using a spectrophotometer (supplied by Hitachi
Ltd., double-beam spectrophotometer type 220).
[0108] (9) Total Cross-Sectional Area of Bubbles
[0109] Bubbles are regarded as having a spherical form, and the
total cross-sectional area of bubbles is defined as the sum of
circles each including the diameter of bubble. The total
cross-sectional area of bubbles was determined by calculating the
average cross-sectional area of bubbles from the average bubble
diameter, and multiplying the average cross-sectional area of
bubbles by the amount of bubbles.
EXAMPLE 1
[0110] Powdery natural quartz having an average particle diameter
of 300 .mu.m and a particle diameter distribution in the range of
30 to 500 .mu.m was treated with hydrofluoric acid to prepare a
high-purity powdery silica (hereinafter referred to as "powdery
quartz"). Silicon tetrachloride was treated with ammonia to prepare
a powdery silicon nitride having an average particle diameter of
0.5 .mu.m. A powdery nature of powdery quartz with the powdery
silicon nitride was prepared as follows. 0.01 part by weight of the
powdery silicon nitride was put into 50 parts by weight of ethanol,
and the mixture was stirred while an ultrasonic vibration was
applied. To the thus-prepared silicon nitride dispersion, 100 parts
by weight of powdery quartz was incorporated and the mixture was
thoroughly stirred. Then ethanol was removed from the mixture by
using a vacuum evaporator and the mixture was dried to obtain a
powdery quartz/silicon nitride mixture (hereinfer referred to
"mixed powder") as a raw material for forming an opaque portion of
an opaque silica glass article.
[0111] The above-mentioned powdery quartz was also used as a raw
material for forming a transparent portion of the opaque silica
glass article. Namely, as illustrated in FIG. 12 and FIG. 13, 300 g
of powdery quartz 12 as the raw material for forming the
transparent portion was charged in a cylindrical carbon crucible 14
having an outer diameter 130 mm, an inner diameter of 100 mm and a
depth of 200 mm and having carbon felt 13 with a thickness of 2 mm
adhered on the inner wall of the crucible. 900 g of the mixed
powder 11 was placed on the charged powdery quartz 12. The charged
powdery quartz 12 and the charged mixed powder 11 had a packing
density of 1.4 g/cm.sup.3.
[0112] The state of the charged powdery quartz 12 and the charged
mixed powder 11 is illustrated in FIG. 12 and FIG. 13. The crucible
14 was placed in an electric furnace, and the inner atmosphere was
vacuumed to a pressure of 1.times.10.sup.-3 mmHg. Then the
temperature was elevated from room temperature to 1,800.degree. C.
at a rate of 300.degree. C./hour. The crucible was maintained at
1,800.degree. C. for 10 minutes, and then, a nitrogen gas was
introduced into the electric furnace until the inner pressure
reached normal pressure (1 kgf/cm.sup.2) and the heating was
ceased. Thereafter the power switch of the electric surface was
turned out and the crucible was allowed to stand. The inner
temperature of the electric furnace reached 1,000.degree. C. about
50 minutes later, and gradually fell to room temperature.
[0113] The thus-made glass article was a columnar opaque silica
glass article having a structure composed of an opaque portion 15
having a multiplicity of bubbles distributed therein, and a
transparent portion 16 firmly bonded to the opaque portion 15, as
illustrated in FIG. 14 and FIG. 15.
EXAMPLE 2
[0114] The same powdery quartz as that used in Example 1 was
pulverized by using a dry ball mill and further sieved to obtain a
powdery quartz having an average particle diameter of 500 .mu.m and
a particle diameter distribution in the range of 10 to 200 .mu.m.
100 parts by weight of the powdery quart and 0.03 part by weight of
silicon nitride powder was mixed together to obtain a powdery
mixture. By substantially the same procedure as that employed in
Example 1,300 g of the powdery quartz was charged in a carbon
crucible and then 900 g of the powdery mixture was charged on the
powdery quartz. The charged powdery quartz and the charged powdery
mixture had a packing density of 1.4 g/cm.sup.3. The charged raw
materials were heated and then cooled by the same procedure as that
in Example 1 to obtain a columnar opaque silica glass article
composed of an opaque portion 15 and a transparent portion 16
firmly bonded to the opaque portion 15, as illustrated in FIG. 14
and FIG. 15.
EXAMPLE 3
[0115] The same powdery quartz as that used in Example 1 was
pulverized by using a dry ball mill and further sieved to obtain a
powdery quartz having an average particle diameter of 50 .mu.m and
a particle diameter distribution in the range of 10 to 200 .mu.m. A
powdery mixture of the powdery quartz and a silicon nitride powder
was prepared by the same procedure as that employed in Example 1.
By substantially the same procedure as that employed in Example
1,300 g of the powdery quartz was charged in a carbon crucible and
then 900 g of the powdery mixture was charged on the powdery
quartz. The charged powdery quartz and the charged powdery mixture
had a packing density of 1.4 g/cm.sup.3. The charged raw materials
were heated and then cooled by the same procedure as that in
Example 1 to obtain a columnar opaque silica glass article composed
of an opaque portion 15 and a transparent portion 16 firmly bonded
to the opaque portion 15, as illustrated in FIG. 14 and FIG.
15.
EXAMPLE 4
[0116] The procedures described in Example 1 were repeated to
obtain a columnar opaque silica glass article composed of an opaque
portion 15 and a transparent portion 16 firmly bonded to the opaque
portion 15, as illustrated in FIG. 14 and FIG. 15, wherein the
crucible charged with the powdery quartz and the mixed powder was
maintained at 1,850.degree. C. instead of 1,800.degree. C. in the
electric furnace with all other conditions remaining the same. The
charged powdery quartz and the charged mixed powder had a packing
density of 1.4 g/cm.sup.3 as measured before the charged powdery
quartz and the charged mixed powder were heated to 1,850.degree.
C.
EXAMPLE 5
[0117] The procedures described in Example 1 were repeated to
obtain a columnar opaque silica glass article composed of an opaque
portion 15 and a transparent portion 16 firmly bonded to the opaque
portion 15, as illustrated in FIG. 14 and FIG. 15, wherein, after
the crucible charged with the powdery quartz and the mixed powder
was maintained at 1,800.degree. C. for 10 minutes in the electric
furnace, a nitrogen gas was introduced into the electric furnace
until the inner pressure reached 2.0 kgf/cm.sup.2 and the heating
was ceased. All other conditions remained the same. The charged
powdery quartz and the charged mixed powder had a packing density
of 1.4 g/cm.sup.3 as measured before the charged powdery quartz and
the charged mixed powder were heated to 1,800.degree. C.
EXAMPLE 6
[0118] Powdery amorphous silica having an average particle diameter
of 300 .mu.m and a particle diameter distribution in the range of
50 to 1,000 .mu.m was prepared by a process wherein sodium silicate
was reacted with an acid and then the reaction product was heated.
The powdery amorphous silica was pulverized by using a dry ball
mill and further sieved to obtain powdery amorphous silica having
an average particle diameter of 180 .mu.m and a particle diameter
distribution in the range of 10 to 600 .mu.m powdery amorphous
silica/silicon nitride mixture was prepared by the same procedure
as that employed in Example 1, from 100 parts by weight of the
powdery amorphous silica and 0.01 part by weight of the same
powdery silicon nitride as that used in Example 1 as follows.
Namely, 300 g of the powdery amorphous silica as a raw material for
forming the transparent portion was charged in the same carbon
crucible as that used in Example 1, and 900 g of the powdery
amorphous silica/silicon nitride mixture as a raw material for
forming the opaque portion was placed on the charged powdery
amorphous silica. The charged powdery amorphous silica and the
charged amorphous silica/silicon nitride mixture had a packing
density of 0.81 g/cm.sup.3. The charged materials were heated and
then cooled under the same conditions as those employed in Example
1 to obtain a columnar opaque silica glass article composed of an
opaque portion 15 and a transparent portion 16 firmly bonded to the
opaque portion 15, as illustrated in FIG. 14 and FIG. 15.
EXAMPLE 7
[0119] Powdery amorphous silica having the same average particle
diameter and particle diameter distribution as those mentioned in
Example 6 was prepared by a process wherein a silicon alkoxide was
reacted with water and then the reaction product was heated. The
powdery amorphous silica was pulverized by using a dry ball mill
and farther sieved to obtain a powdery amorphous silica having an
average particle diameter of 180 .mu.m and a particle diameter
distribution in the range of 10 to 600 .mu.m. A powdery amorphous
silica/silicon nitride mixture was prepared by the same procedure
as that employed in Example 1, from 100 parts by weight of the
powdery amorphous silica and 0.01 part by weight of the same
powdery silicon nitride as that used in Example 1 as follows.
Namely, 300 g of the powdery amorphous silica as a raw material for
forming the transparent portion was charged in the same carbon
crucible as that used in Example 1, and 900 g of the powdery
amorphous silica/silicon nitride mixture as a raw material for
forming the opaque portion was placed on the charged powdery
amorphous silica. The charged powdery amorphous silica and the
charged amorphous silica/silicon nitride mixture had a packing
density of 0.81 g/cm.sup.3. The charged materials were heated and
then cooled under the same conditions as those employed in Example
1 to obtain a columnar opaque silica glass article composed of an
opaque portion and a transparent portion firmly bonded to the
opaque portion.
[0120] The X ray diffraction analysis of the opaque silica glass
articles made in Examples 1 to 7 revealed that the opaque portion
and the transparent portion of each of the opaque silica glass
articles were in glass state.
[0121] The properties of the opaque silica glass articles made in
Examples 1 to 7 were evaluated. Namely, the apparent density,
average bubble diameter and bubble amount of the opaque portion of
each glass article are shown in Table 1.
[0122] The total cross-sectional area of bubbles and light
transmittance of the opaque portion of each glass article are shown
in Table 2. The apparent density, amount of bubbles with a diameter
of at least 100 .mu.m, and light transmittance of the transparent
portion are shown in Table 3.
1TABLE 1 Example Apparent density Average bubble Number of No.
(g/cm.sup.3) diameter (.mu.m) bubbles per cm.sup.3 1 2.01 74 4
.times. 10.sup.5 2 1.82 88 5 .times. 10.sup.5 3 2.10 34 2 .times.
10.sup.6 4 1.86 90 4 .times. 10.sup.5 5 1.97 63 8 .times. 10.sup.5
6 1.96 63 8 .times. 10.sup.5 7 2.05 66 5 .times. 10.sup.5 8 2.01 74
4 .times. 10.sup.5
[0123]
2TABLE 2 Total cross- Example sectional area Light transmittance
(%) No. of bubbles (cm.sup.2/cm.sup.3) 300 nm 500 nm 700 nm 900 nm
1 18 0.7 1.3 2.0 2.7 2 30 0.2 0.4 0.5 0.6 3 20 0.5 1.0 1.6 2.0 4 26
0.3 0.5 0.6 0.8 5 25 0.4 0.7 1.0 1.4 6 26 0.2 0.3 0.4 0.4 7 16 0.8
1.4 2.3 2.9 8 18 0.7 1.3 2.0 2.7
[0124]
3TABLE 3 Example Apparent Number of Light transmittance (%) No.
density(g/cm.sup.3) bubbles per cm.sup.3 300 nm 500 nm 700 nm 900
nm 1 2.20 50 92 95 95 95 2 2.20 50 92 95 95 95 3 2.20 50 92 95 95
95 4 2.20 50 92 95 95 95 5 2.20 50 92 95 95 95 6 2.20 50 92 95 95
95 7 2.20 50 92 95 95 95 8 2.20 50 92 95 95 95
COMPARATIVE EXAMPLE 1
[0125] The same powdery quartz as that used in Example 1 was
pulverized by using a dry ball mill, and for dispersed in ethanol
to be sedimented Thus, a powdery quartz having an average particle
diameter of 5 .mu.m and a particle diameter distribution in the
range of 1 to 10 .mu.m was obtained depending upon the difference
in sedimentation rate. A powdery silica/silicon nitride mixture was
prepared from the thus-prepared powdery silica and the same silicon
nitride powder as that used in Example 1 by the same procedure as
described in Example 1. By substantially the same procedure as that
employed in Example 1, 300 g of the powdery quart was charged in a
carbon crucible and then 900 g of the powdery silica/silicon
nitride mixture was charged on the powdery quartz. The charged
powdery quartz and the charged powdery mixture had a packing
density of 0.90 /cm.sup.3. The charged raw materials were heated
and then cooled by the same procedure as that in Example 1 to
obtain a columnar opaque silica glass article composed of an opaque
portion and a transparent portion firmly bonded to the opaque
portion.
[0126] The X ray diffraction analysis of the columnar opaque silica
glass article revealed that both the opaque portion and transparent
portion thereof were in glass state. However, the opaque portion
had a low apparent density, i.e., 1.2 g/cm.sup.3, and, when the
glass article was cut and the cross-section was visually examined,
the glass article proved to have pores having a diameter of about 2
to 5 mm. The transparent portion also has a low apparent density,
i.e., 2.15 g/cm.sup.3.sub.1 and proved to have pores having a
diameter of about 2 mm.
COMPARATIVE EXAMPLE 2
[0127] The procedures described in Example 1 were repeated to
obtain a columnar opaque silica glass article composed of an opaque
portion 15 and a transparent portion 16 firmly bonded to the opaque
portion 15, as illustrated in FIG. 14 and FIG. 15, wherein the
crucible charged with the powdery quartz and the mixed powder was
maintained at 1,950.degree. C. instead of 1,800.degree. C. and the
inner pressure of the electric furnace was changed to 1.0
kg/cm.sup.2 with all other conditions remaining the same. The
charged powdery quartz and the charged mixed powder had a packing
density of 1.4 g/cm.sup.3 as measured before the charged powdery
quartz and the charged mixed powder were heated to 1,950.degree.
C.
[0128] The X ray diffraction analysis of the columnar opaque silica
glass article revealed that both the opaque portion and transparent
portion thereof were in glass state. However, the opaque portion
has a low apparent density, i.e., 1.5 g/cm.sup.3. The average
bubble diameter was 200 .mu.m, and the opaque silica glass article
was very brittle.
EXAMPLE 8
[0129] A powdery quartz/silicon nitride mixture was prepared by the
same procedure as mentioned in Example 1 wherein the amount of the
powdery silicon nitride was changed to 0.03 part by weight based on
100 parts by weight of the powdery quartz with all other conditions
remaining the same.
[0130] As illustrated in FIG. 16, 5 kg of the same powdery quartz
19 as that used in Example 1 was charged in a carbon mold 10 with a
ring-form cavity having an outer diameter of 440 mm, an inner
diameter of 270 mm and a depth of 100 mm and having a carbon felt
18 with a thickness of 5 mm adhered on the inner wall of the mold.
The state of the charged powdery quartz 19 was illustrated in FIG.
16. The mold was placed in an electric furnace and the inner
atmosphere was vacuumed to a pressure of 1.times.10.sup.=3 mmHg.
Then the temperature was elevated from room temperature to
1,800.degree. C. at a rate of 300.degree. C./hour. The mold was
maintained at 1,800.degree. C. for 10 minutes, and then, the power
switch of the electric furnace was turned out and the mold was
allowed to stand. The inner temperature of the electric furnace
reached 1,000.degree. C. about 50 minutes later, and gradually fell
to room temperature. The thus-prepared transparent ring-form silica
glass article was cut to obtain specimens, and their properties
were evaluated. The apparent density, amount of bubbles with a
diameter of at least 100 .mu.m, and light transmittance as
irradiated with light of wavelength of 300 to 900 nm of the
specimens were 2.20 g/cm.sup.3, 50 bubbles per cm.sup.3, and 92 to
95%, respectively. The transparent ring-form silica glass article
was machined to obtain a transparent ring-form silica glass article
having an outer diameter of 440 mm, an inner diameter of 270 mm and
a thickness (height) of 10 mm, used as a raw material for forming
the transparent portion.
[0131] As illustrated in FIG. 17, the above-mentioned transparent
ring-form silica glass article 20 for forming the transparent
portion was placed on the bottom of the same mold 10 as the
above-mentioned carbon mold, which had a carbon felt 18 adhered on
the inner wall thereof, and 5 kg of the above-mentioned powdery
silica/silicon nitride mixture 21 was charged on the transparent
ring-form silica glass article 20. The charged powdery
silica/silicon nitride mixture 21 had packing density of 1.4
g/cm.sup.3.
[0132] The raw materials-charged mold was placed in an electrical
furnace, and the inner atmosphere of the furnace was vacuumed to a
pressure of 1.times.10.sup.-3 mmHg. Then the temperature was
elevated from room temperature to 1,800.degree. C. at a rate of
300.degree. C./hour. The mold was maintained at 1,800.degree. C.
for 10 minutes, and then, a nitrogen gas was introduced into the
electric furnace until the inner pressure reached normal pressure
(1 kgf/cm.sup.2) and the heating was ceased. Thereafter the power
switch of the electric furnace was turned out and the crucible was
allowed to stand. The inner temperature of the electric furnace
reached 1,000.degree. C. about 50 minutes later, and gradually fell
to room temperature.
[0133] As illustrated in FIG. 18, the thus-made glass article was a
ring-form opaque silica glass article having a structure composed
of an opaque ring-form portion 23 and a transparent ring-form
portion 22 firmly bonded to the opaque portion 23.
[0134] The X ray diffraction analysis of the opaque ring-form
silica glass article revealed that the opaque portion and the
transparent portion were in glass state. The properties of the
opaque ring-form silica glass article. Namely, the apparent
density, average bubble diameter and bubble amount of the opaque
portion of glass article are shown in Table 1. The total
cross-sectional area of bubbles and light transmittance of the
opaque portion of glass article are shown in Table 2. The apparent
density, amount of bubbles with a diameter of at least 100 .mu.m,
and light transmittance of the transparent portion are shown in
Table 3.
COMPARATIVE EXAMPLE 3
[0135] By the same procedures as employed in Example 8, a powdery
quartz/silicon nitride mixture as a raw material for forming the
opaque portion of the opaque silica glass article was prepared
wherein powdery quartz having an average particle diameter of 700
.mu.m and a particle diameter distribution in the range of 500 to
1,000 .mu.m was used with all other conditions are the same, and
fewer, a transparent ring-form silica glass article as a raw
material for forming the transparent portion of the opaque silica
glass article was prepared.
[0136] The transparent ring-form silica glass article was placed on
the bottom of the same carbon mold as used in Example 8, and 5 kg
of the powdery quartz/silicon nitride mixture was charged on the
transparent ring-form silica glass article. The charged transparent
ring-form silica glass article had a packing density of 0.78
g/cm.sup.3. The raw materials-charged mold was placed in an
electric furnace, and heated and cooled under the same conditions
as employed in Example 8 to obtain an opaque ring-form silica glass
article having a structure composed of an opaque portion 23 and a
transparent portion 22 firmly bonded to the opaque portion 23, as
illustrated in FIG. 19.
[0137] The X ray diffraction analysis of the opaque ring-form
silica glass article revealed that the opaque portion 23 and the
transparent portion 22 were in glass state. However, the opaque
portion 23 has a low apparent density, i.e., 1.4 g/cm.sup.3, and,
when the glass article was cut and the cross-section was visually
examined, the glass article proved to have pores having a diameter
of about 0.5 to 1 mm. The transparent portion 22 also has a low
apparent density, i.e., 2.17 g/cm.sup.3, and proved to have pores
having a diameter of about 1 mm.
[0138] The same powdery quartz/silicon nitride mixture and the same
transparent ring-form silica glass article as those prepared in
Example 8 were prepared. Further, as illustrated in FIG. 19, a
multiplicity of pores 24 having a diameter of about 2 to 3 mm were
present in the boundary between the opaque portion 23 and the
transparent portion 22.
COMPARATIVE EXAMPLE 4
[0139] The transparent ring-form silica glass article was placed on
the bottom of the same carbon mold as used in Example 8, and the
powdery quartz/silicon nitride mixture was charged on the
transparent ring-form silica glass article. The charged transparent
ring-form silica glass article had a packing density of 1.4
g/cm.sup.3. The raw materials-charged mold was placed in an
electric furnace, and the inner atmosphere was vacuumed to a
pressure of 1.times.10.sup.-3 mmHg. Then a nitrogen gas was
introduced into the electric furnace until the inner pressure
reached normal pressure (1 kgf/cm.sup.2), and then, the temperature
was elevated from room temperature to 1,800.degree. C. at a rate of
300.degree. C./hour. The mold was maintained at 1,800.degree. C.
for 10 minutes, and then, the heating was ceased. Thereafter the
power switch of the electric furnace was turned out and the mold
was allowed to stand. The inner temperature of the electric furnace
reached 1,000.degree. C. about 50 minutes later, and gradually fell
to room temperature.
[0140] As illustrated in FIG. 19, the thus-made glass article was a
ring-form opaque silica glass article having a structure composed
of an opaque portion 23 and a transparent portion 22 firmly bonded
to the opaque portion 23.
[0141] The X ray diffraction analysis of the opaque ring-form
silica glass article revealed that the opaque portion 23 and the
transparent portion 22 were in glass state. However, the opaque
portion 23 has a low apparent density, i.e., 1.2 g/cm.sup.3, and
when the glass article was cut and the cross-section was visually
examined, it was found that bubbles were distributed non-uniformly
in the glass article, i.e., the amount of bubbles was increased
radially outwardly toward the surface portion. Further, as
illustrated in FIG. 19, a multiplicity of pores 24 having a
diameter of about 2 to 3 mm were present in the boundary between
the opaque portion 23 and the transparent portion 22.
COMPARATIVE EXAMPLE 5
[0142] By the same procedures as employed in Comparative Example 4,
an opaque ring-form silica glass article having an opaque portion
and a transparent portion was made wherein the powdery silicon
nitride was not used as the raw material for forming the opaque
portion with all other conditions remaining the same. The powdery
quartz charged within the mold as the raw material for forming the
opaque portion had an apparent density of 1.4 g/cm.sup.3.
[0143] As illustrated in FIG. 19, the X ray diffraction analysis of
the opaque ring-form silica glass article revealed that the opaque
portion 23 and the transparent portion 22 were in glass state.
However, the opaque portion 23 has a low apparent density, i.e.,
1.5 g/cm.sup.3, and when the glass article was cut and the
cross-section was visually examined, it was found that i.e., the
amount of bubbles was increased radially outwardly toward the
surface portion. Further, as illustrated in FIG. 19, a multiplicity
of pores 24 having a diameter of about 2 to 3 mm were present in
the boundary between the opaque portion 23 and the transparent
portion 22.
[0144] The advantages of the opaque silica glass article of the
invention and the process for producing the same of the invention
are summarized as follows.
[0145] (1) The opaque portion of the opaque silica glass article is
composed of powdery silica having uniformly dispersed therein a
predetermined amount of a powdery silicon nitride. The amount and
diameter of bubbles, and apparent density of the silica glass
article are controlled by the amount of the powdery silicon
nitride, the particle diameter of the powdery silica and the
melting temperature, and thus, the opaque silica glass article
exhibiting excellent heat insulating property can be obtained.
[0146] (2) The bubbles are formed in the molten material by
vitrification of powdery silica and decomposition of powdery
silicon nitride, and thus, impurities such as alkali metals are not
incorporated in the glass article. Further, when the raw material
is melted, a hydroxyl group is not entrapped therein, but is
volatilized therefrom. Therefore, the content of a hydroxyl group
is minimized and the undesirable reduction of viscosity at a high
temperature of the silica glass article can be avoided.
[0147] Further, even when a transparent shaped silica glass article
is used as a raw material for forming the transparent portion of
the opaque silica glass article, the resulting opaque silica glass
article has a good resistance to distortion.
[0148] (3) Bubbles are not formed or formed only to a negligible
extent at the boundary between the opaque portion and the
transparent portion, and therefore, these two portions are firmly
bonded together. When the silica glass article is cleaned, the
surface portion is not readily cut out. The glass article has a
smooth surface, and the surface exhibits a good seal ability.
Therefore, the silica glass article is especially useful as a
flange member attached to a furnace tube for heating wafers.
[0149] (4) The opaque silica glass article can be made by a
heat-resistant mold of any desired shape, and thus, it can be of a
desired shape such as flange-shape, ring-form, column, square
pillar and hollow square pillar, or any other complicated shape.
The shaping is not complicated.
[0150] The distortion in the production process is very minor, and
a silica glass article having the finally intended size and shape
can be obtained. An after-treatment such as machine finishing can
be omitted or
[0151] (5) The powdery raw material for forming the opaque portion
is capable of being melted at a relatively low temperature, and
thus, when a transparent silica glass article is used as a raw
material for forming the transparent portion, the opaque silica
glass article can be made without substantial melting of the
transparent silica glass article.
* * * * *