U.S. patent application number 13/057428 was filed with the patent office on 2012-05-17 for process for obtaining glass and glass obtained.
This patent application is currently assigned to SAINT-GOBAIN GLASS FRANCE. Invention is credited to Junbo CHOI, Octavio CINTORA, Byoung-Ouk KIM, Kidong MOON, Thomas SCHUSTER, Pedro SILVA.
Application Number | 20120121915 13/057428 |
Document ID | / |
Family ID | 41456084 |
Filed Date | 2012-05-17 |
United States Patent
Application |
20120121915 |
Kind Code |
A2 |
CINTORA; Octavio ; et
al. |
May 17, 2012 |
PROCESS FOR OBTAINING GLASS AND GLASS OBTAINED
Abstract
The object of the invention is a continuous method for obtaining
glass, comprising steps consisting of: charging raw materials
upstream of a furnace, along which a plurality of burners is
disposed, obtaining a mass of molten glass, and then leading said
mass of molten glass to a zone of the furnace situated further
downstream, at least one burner disposed in the region of this zone
being fed with an over-stoichiometric quantity of oxidant, and
then, forming a glass sheet, said glass sheet having a chemical
composition that comprises the following constituents in an amount
varying within the weight limits defined below: TABLE-US-00001
SiO.sub.2 60-75% Al.sub.2O.sub.3 0-10% B.sub.2O.sub.3 0-5%,
preferably 0 CaO 5-15% MgO 0-10% Na.sub.2O 5-20% K.sub.2O 0-10% BaO
0-5%, preferably 0, SO.sub.3 0.1-0.4% Fe.sub.2O.sub.3 (total iron)
0 to 0.015%, Redox 0.1-0.3.
Inventors: |
CINTORA; Octavio; (Taverny,
FR) ; SCHUSTER; Thomas; (Wuerselen, DE) ; KIM;
Byoung-Ouk; (Dongjak-gu, KR) ; MOON; Kidong;
(Jeollabuk-do, KR) ; CHOI; Junbo; (Jeollabuk-do,
KR) ; SILVA; Pedro; (Douai, FR) |
Assignee: |
SAINT-GOBAIN GLASS FRANCE
18, Avenue d'Alsace
Courbevoie
FR
F-92400
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20110135938 A1 |
June 9, 2011 |
|
|
Family ID: |
41456084 |
Appl. No.: |
13/057428 |
Filed: |
September 1, 2009 |
PCT Filed: |
September 1, 2009 |
PCT NO: |
PCTFR09051655 |
371 Date: |
February 3, 2011 |
Current U.S.
Class: |
428/426; 501/65;
501/66; 501/70; 501/72; 65/83; 65/90; 65/99.2 |
Current CPC
Class: |
C03B 5/235 20130101;
C03C 3/078 20130101; Y02P 40/57 20151101 |
Class at
Publication: |
428/426; 065/090;
065/083; 065/099.2; 501/065; 501/066; 501/070; 501/072 |
International
Class: |
C03C 3/078 20060101
C03C003/078; C03B 18/02 20060101 C03B018/02; C03C 3/087 20060101
C03C003/087; C03C 3/089 20060101 C03C003/089; C03C 3/091 20060101
C03C003/091; C03B 5/235 20060101 C03B005/235; B32B 17/00 20060101
B32B017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 1, 2008 |
FR |
0855847 |
Feb 27, 2009 |
FR |
0951243 |
Jun 26, 2009 |
KR |
1020090057687 |
Claims
1. A glass production method, comprising: charging raw materials in
an upstream part of a furnace, wherein a plurality of burners are
disposed along the upstream part, obtaining a mass of molten glass,
and then leading said mass of molten glass to a zone of the furnace
situated further downstream, wherein at least one burner is
disposed in the region of this zone being fed with an
over-stoichiometric quantity of oxidant, and then, forming a glass
sheet, said glass sheet having a chemical composition that
comprises the following constituents in an amount varying within
the weight limits defined hereinafter: TABLE-US-00010 SiO.sub.2
60-75% Al.sub.2O.sub.3 0-10% B.sub.2O.sub.3 0-5%, CaO 5-15% MgO
0-10% Na.sub.2O 5-20% K.sub.2O 0-10% BaO 0-5%, SO.sub.3 0.1-0.4%
Fe.sub.2O.sub.3 (total iron) 0 to 0.015%, Redox 0.1-0.3.
2. The method as claimed in claim 1, wherein the furnace comprises
several overhead burners disposed in the region of the sidewalls of
the furnace, each of said burners being able to develop a flame
transversely to the axis of the furnace.
3. The method as claimed in claim 2, wherein the overhead burners
are disposed regularly upstream to downstream and are arranged in
pairs of burners facing each other, the burners of each pair
operating alternately so that at a given instant only burners
disposed in the region of one of the sidewalls develop a flame.
4. The method as claimed in claim 3, wherein the furnace comprises
between 6 and 8 pairs of burners and only the two or three pairs of
burners situated furthest downstream, or the last pair of burners
situated furthest downstream, are fed with an over-stoichiometric
quantity of oxidant.
5. The method as claimed in claim 1 wherein the furnace comprises,
from upstream to downstream, a first chamber delimiting a glass
melting zone and then a refining zone and then a second chamber
delimiting a cooling zone for molten glass, all the burners being
disposed in the region of the first chamber.
6. The method as claimed in claim 5, wherein each burner fed with
an over-stoichiometric quantity of oxidant is situated in the
region of the glass refining zone.
7. The method as claimed in claim 1, wherein the burners are fed
with air and fuel.
8. The method as claimed in claim 7, wherein the fuel is chosen
from natural gas and fuel oil or any mixtures thereof.
9. The method as claimed in claim 1, wherein the
over-stoichiometric quantity of oxidant is such that the molar
ratio of oxygen to fuel is between 1.05 and 1.5.
10. The method as claimed in claim 1, wherein the glass sheet is
formed by floating on a bath of tin.
11. The method as claimed in claim 1, wherein the partial pressure
of oxygen above the glass bath is between 4 and 7%.
12. A glass sheet having a chemical composition that comprises the
following constituents in an amount varying within the weight
limits defined below: TABLE-US-00011 SiO.sub.2 60-75%
Al.sub.2O.sub.3 0-10% B.sub.2O.sub.3 0-5%, CaO 5-15% MgO 0-10%
Na.sub.2O 5-20% K.sub.2O 0-10% BaO 0-5%, SO.sub.3 0.1-0.4%
Fe.sub.2O.sub.3 (total iron) 0 to 0.015%, Redox 0.1-0.3.
13. The glass sheet as claimed in claim 12, wherein the iron oxide
content is less than 0.015%.
14. The glass sheet as claimed in claim 12, wherein the redox is
between 0.2 and 0.30.
15. The glass sheet as claimed in claim 12, wherein the SO.sub.3
content is greater than or equal to 0.2%.
16. The glass sheet as claimed in claim 12, comprising 0% of the
following oxides or metals Sb.sub.2O.sub.3, As.sub.2O.sub.3,
CeO.sub.2, CoO, CuO, NiO, Cr.sub.2O.sub.3, MnO.sub.2,
V.sub.2O.sub.5, La.sub.2O.sub.3, Nd.sub.2O.sub.3, Er.sub.2O.sub.3,
Se, Ag, Cu.
17. The glass sheet as claimed in claim 12, produced by a process
comprising floating on a tin bath.
18. The glass sheet as claimed in claim 12, wherein said glass
sheet is coated with at least one thin transparent
electroconducting layer and/or an anti-reflecting coating.
19. A photovoltaic cell, solar cell, flat or parabolic mirror for
concentrating solar energy, a diffuser for back-lighting display
screens of the LCD type, a screen or flat lamp based on organic
electroluminescent diodes, comprising at least one glass sheet as
claimed in claim 12.
20. The glass sheet as claimed in claim 12, wherein said glass
sheet is coated with a thin transparent electroconducting layer on
a first face and an antireflecting layer on a second face.
Description
[0001] The present invention relates to a method for obtaining
glass and to a glass composition capable of being obtained by this
method. It relates in particular to a soda-lime-silica glass
intended for the production of objects, in particular being in the
form of flat glass sheets, said composition giving these said
objects high transmission properties for visible and infrared
radiation. It also relates to the method enabling said composition
to be obtained.
[0002] Although not limited to such an application, the invention
will more particularly be described with reference to applications
in the field of flat glass, notably glass capable of being obtained
by the float method consisting of pouring molten glass onto a bath
of molten metal (in particular tin).
[0003] In some fields of the art, it is essential that the glasses
employed have extremely high transmission for visible and/or
infrared radiation, notably greater than 90%. This is the case for
example in applications where glass is used in the form of a
substrate that covers photovoltaic cells or solar cells. In point
of fact in this case, the quantum efficiency of the cells may be
largely affected even by a very small reduction in transmission of
visible or infrared radiation.
[0004] Transmission in the visible or infrared range is generally
expressed in the form of a transmission factor incorporating, over
a certain part of the spectrum, the transmission for each
wavelength taking account of a particular spectral distribution and
as appropriate the sensitivity of the human eye. In order to
quantify the transmission of the glass in the visible range, a
light transmission factor is thus defined, called light
transmission, often abbreviated to "T.sub.L", calculated between
380 and 780 nm and based on a glass thickness of 3.2 mm or 4 mm,
according to ISO standard 9050:2003, thus taking into consideration
the D65 illuminant as defined by ISO/CIE standard 10526 and
standard calorimetric observer C.I.E. 1931 as defined by ISO/CIE
standard 10527. In order to quantify transmission of the glass in
the range encompassing the solar visible and infrared (also called
"near infrared") an energy transmission factor is defined called
"energy transmission", abbreviated to "T.sub.E", calculated
according to ISO standard 9050 and reduced to a glass thickness of
3.2 mm or 4 mm. According to ISO standard 9050, the wavelength
range used for the calculation extends from 300 to 2500 nm.
However, some values will be given in the remainder of the text
while limiting the calculation to wavelengths extending from 400 to
1100 nm.
[0005] It is known, in order to reach values of T.sub.L and T.sub.E
greater than 90%, to reduce by a maximum the total iron oxide
content. Iron oxide, present as an impurity in most natural raw
materials used in glassmaking (sand, feldspar, limestone, dolomite,
etc), absorbs both in the visible and near ultraviolet range
(absorption due to the ferric ion Fe.sup.3+) and especially in the
visible and near infrared (absorption due to the ferrous ion
Fe.sup.2+). With ordinary natural raw materials, the iron oxide
content by weight is of the order of 0.1% (1000 ppm). Transmissions
of greater than 90% require however a reduction of the iron oxide
content to less than 0.02% or 200 ppm, or even less than 0.01% (100
ppm), which makes it necessary to choose particularly pure raw
materials and increase the cost of the final product.
[0006] In order to increase transmission of the glass still
further, it is also known to reduce the ferrous iron content to the
profit of the ferric iron content, and therefore to oxidize the
iron present in the glass. In this way, glasses are aimed at having
as small as possible "redox", ideally zero or virtually zero, the
redox being defined as the ratio between the FeO content by weight
(ferrous iron) and the total iron oxide content by weight
(expressed in the form of Fe.sub.2O.sub.3). This number may vary
between 0 and 0.9, zero redoxes corresponding to a totally oxidized
glass.
[0007] Glasses containing normal iron oxide contents, of the order
of 1000 ppm or more, naturally have redoxes of the order of 0.25.
On the other hand, glasses containing small quantities of iron
oxide, notably less than 200 ppm, or even less than 150 ppm, have a
natural tendency to have high redoxes, greater than 0.4, or even
than 0.5. This tendency is probably due to a displacement of the
oxydoreduction equilibrium of iron as a function of the iron oxide
content.
[0008] Various solutions have been proposed for oxidizing iron
oxide as much as possible, which contribute to obtaining very low
redoxes, less than 0.2. It is known for example from U.S. Pat. No.
6,844,280 to add cerium oxide (CeO.sub.2) to glass. Cerium oxide is
however expensive and capable of being the origin of a process
called "solarization", in which transmission of the glass falls
considerably following absorption of ultraviolet radiation. It is
also known to add antimony oxide (Sb.sub.2O.sub.3) or arsenic oxide
(As.sub.2O.sub.3), oxides traditionally used as glass refining
agents and that have the particular property of oxidizing iron. Use
of Sb.sub.2O.sub.3 is for example described in US application
2006/249199 or FR 2317242. These oxides prove however incompatible
with the float glass method. It would seem that under reducing
conditions necessary for non-oxidation of the bath of tin, part of
these oxides volatilize and then condense on the glass sheet as it
forms, generating an undesirable haze. Vanadium and manganese
oxides have also been proposed with the aim of oxidizing iron.
[0009] Oxidation of glass by chemical means involves a high cost
and/or is not compatible with the float glass method. Moreover,
production of very oxidized glasses has been revealed to reduce
considerably the life of furnaces. The very high radiative
conductivity of a very oxidized glass bath (and thus one with high
transmission in the infrared), generates very much higher hearth
temperatures. The result is increased corrosion of the refractories
constituting the hearth of the furnace and to a reduction in the
life of the furnace.
[0010] The object of the present invention is to provide a lower
cost method making it possible to obtain extra-clear glass with an
intermediate redox without using chemical oxidation means. The
object is also to provide a glass sheet having a low iron oxide
content and an intermediate redox.
[0011] To this end, the object of the invention is a continuous
method for obtaining glass, comprising steps consisting of: [0012]
charging raw materials upstream of a furnace, along which a
plurality of burners is disposed, [0013] obtaining a mass of molten
glass, and then [0014] leading said mass of molten glass to a zone
of the furnace situated further downstream, at least one burner
disposed in the region of this zone being fed with an
over-stoichiometric quantity of oxidant, and then,
[0015] forming a glass sheet, said glass sheet having a chemical
composition that comprises the following constituents in an amount
varying within the weight limits defined below: TABLE-US-00002
SiO.sub.2 60-75% Al.sub.2O.sub.3 0-10% B.sub.2O.sub.3 0-5%,
preferably 0 CaO 5-15% MgO 0-10% Na.sub.2O 5-20% K.sub.2O 0-10% BaO
0-5%, preferably 0, SO.sub.3 0.1-0.4% Fe.sub.2O.sub.3 (total iron)
0 a 0.015%, Redox 0.1-0.3.
[0016] In all the text, percentages are percentages by weight.
[0017] The fusion furnace generally consists of refractories,
generally ceramics such as silicon oxide, aluminum oxide, zirconium
oxide, chromium oxide, or solid solutions of aluminum, zirconium
and silicon oxides. The furnace generally has an arch supported by
uprights forming the sidewalls of the furnace, front and rear walls
and a hearth. In a continuous melting method, the upstream part of
the furnace may be distinguished corresponding to the zone for
charging in raw material, and then zones further downstream: the
fusion zone in which the raw materials are converted into molten
glass and then the refining zone in which any gaseous inclusion is
removed from the bath of molten glass, then the cooling-down zone,
in which glass is progressively cooled to the forming temperature,
and finally the thermal conditioning zone where the glass is held
at its forming temperature, before the forming zone. The forming
zone is not an integral part of the furnace.
[0018] A burner is understood to mean any association of at least
one injector of fuel (generally gaseous, such as natural gas or
propane, or liquid such as fuel oil) and at least one injector for
oxidant (generally air or oxygen) the association being disposed so
that is can develop a flame by combustion between fuel and
oxidant.
[0019] The inventors have demonstrated that combustion that is
over-stoichiometric in oxidant in a downstream part of the furnace
made it possible to obtain glasses low in iron oxide and with an
intermediate redox, glasses that could not previously be obtained.
This result was particularly surprising since it was usually
thought that in a furnace, taking into account the large volume of
glass relative to a free surface area, oxidation of a glass bath by
maintaining a more oxygenated atmosphere in the region of the glass
surface was not possible.
[0020] The furnace preferably has several overhead burners disposed
in the region of the sidewalls of the furnace, each of said burners
being able to develop a flame transversely to the axis of the
furnace. An "overhead burner" is understood to mean a burner
developing a flame situated above the molten glass bath and capable
of heating the glass bath by radiation. It is also possible for the
furnace to have other types of burners, notably burners able to
heat the glass bath by conduction, for example burners situated in
the arch and/or in the front or back walls and of which the flame
impacts the glass bath, or moreover immersed burners, in the sense
that the flame is developed within the glass bath.
[0021] Overhead burners are preferably disposed regularly upstream
to downstream and/or are arranged in pairs of burners facing each
other, the burners of each pair operating alternately so that at a
given instant only burners disposed in the region of one of the
sidewalls develop a flame.
[0022] This type of furnace is sometimes called a "transverse
burner furnace". Alternating the operation of pairs of burners
makes it possible to use regenerators, through which the combustion
gas and oxidant are obliged to pass. Consisting of stacks of
refractory parts, regenerators make it possible to store heat
emitted by the combustion gas and to give this heat back to the
oxidant gas. In a first phase of the alternation, regenerators
situated in the region of the burners that are not in operation
(these burners are disposed in the region of the first wall) store
energy emitted by the flames developed by the burners situated in
the region of the second wall, facing the first wall. In a second
phase of the alternation, burners disposed in the region of the
second wall stop operating, while burners disposed in the region of
the first wall are put into operation. The combustion gas (in this
case generally air), which passes into the regenerators, is then
preheated, which makes substantial energy savings possible.
[0023] In order to optimize melting, the furnace preferably has
between 6 and 8 pairs of burners and only the two or three pairs of
burners situated furthest downstream, or the last pair of overhead
burners situated further downstream, are fed with an
over-stoichiometric quantity of oxidant. The other burners,
situated further upstream, are preferably fed by a stoichiometric
or sub-stoichiometric quantity of oxidant.
[0024] The furnace preferably has, from upstream to downstream, a
first chamber delimiting a glass melting zone and then a refining
zone and then a second chamber delimiting a cooling zone for molten
glass, all the burners being disposed in the region of the first
chamber. In general, a transition zone called a restriction and
being in the form of a chamber with a narrower cross section
separates the two previously described chambers.
[0025] Refining is understood to mean removal of gaseous inclusions
incorporated in the glass mass, in particular on account of
decarbonation reactions of some raw materials. In the
abovementioned type of furnace, the refining zone is situated
downstream of the first chamber of the furnace.
[0026] The, or each, burner fed with an over-stoichiometric
quantity of oxidant is then preferably situated in the region of
the glass refining zone. It is in point of fact in this refining
zone that oxidation of the glass is most effective.
[0027] The burners are preferably fed with air and a fuel. Oxygen
may also be used, as well as any type of oxygen-enriched air.
Oxygen is more costly to use but makes it possible not to use
regenerators.
[0028] The fuel is preferably chosen from natural gas or fuel oil
or any mixtures thereof. The use of fuel oil is preferred since it
makes it possible to obtain more useful redoxes.
[0029] The over-stoichiometric quantity of oxidant is preferably
such that the molar ratio of oxygen to fuel is greater than or
equal to 1.05, notably 1.1, and/or less than or equal to 1.5,
notably 1.3.
[0030] The partial pressure of oxygen above the glass bath is
preferably between 4 and 7%. Below 4% it is difficult to control
the redox, while above 7% energy consumption problems are
presented. Control of the redox by means of the partial pressure of
oxygen is achieved according to the following chemical reaction:
O.sub.2+4Fe.sup.2+.fwdarw.2O.sup.2-+4Fe.sup.3+
[0031] Heat convection phenomena inside the furnace create two
bands (or streams) of glass circulation, a first band in the region
of the fusion zone extending from the zone for introducing raw
materials to the hot point, in which the hot glass surface is
brought to the zone for introducing raw materials, and a second
circulation band from the hot point to the outlet from the furnace,
thus in the region of the refining zone and of the cooling zone, in
which part of the surface glass dives to the hearth in order to
return to the hot point. The existence of these bands contributes
widely to the chemical uniformity of the glass. Strict control of
the length of each of the bands is necessary in order to ensure
good yield. Generally, in the case where a low iron content glass
is fused, the hearth temperature is increased in comparison with
the case of melting glass with a normal iron content. For this
reason, the first band is shortened and the second band is
extended, which may cause bubbling problems associated with the
quantity of residual SO.sub.3 in the glass.
[0032] Surprisingly, holding a high oxygen partial pressure, higher
than in the case of melting glass with a normal iron content, makes
it possible not to extend the second circulation band, for
increased production stability and better yield.
[0033] The glass sheet is preferably formed by floating on a bath
of tin. Other types of forming method may be employed, such as
drawing methods, a draw-down method, a rolling method, a Fourcault
method, etc.
[0034] The raw materials charged into a furnace are preferably
powdered solid materials. Reference may particularly be made to
sand, sodium carbonate, limestone, dolomite and feldspars. However,
dolomite frequently contains iron oxide as an impurity so that it
is preferably not employed within the context of the invention.
[0035] Sulfur (SO.sub.3) is preferably added as sodium sulfate or
calcium sulfate (called gypsum). In order to accelerate fusion, it
is preferable to add a reducer such as coke jointly with sulfate.
The quantity of sulfate added is preferably between 0.2 and 0.6%,
notably between 0.3 and 0.5%, or even between 0.4 and 0.5%,
expressed as percentages of SO.sub.3 by weight. The quantity of
coke is advantageously between 0 and 1000 ppm, or even between 50
and 120 ppm (1 ppm=0.0001%), notably between 60 and 80 ppm. It is
also possible, in order to promote oxidation of iron, to introduce
a nitrate, such as sodium nitrate.
[0036] Preferably, the glass sheet has a chemical composition that
comprises the following constituents in an amount varying within
the weight limits defined below: TABLE-US-00003 SiO.sub.2 60-75%
Al.sub.2O.sub.3 0-10% B.sub.2O.sub.3 0-5%, preferably 0 CaO 5-15%
MgO 0-10% Na.sub.2O 5-20% K.sub.2O 0-10% BaO 0-5%, preferably 0,
SO.sub.3 >0.2-0.4% Fe.sub.2O.sub.3 (total iron) 0 to 0.015%,
Redox 0.2-0.30.
[0037] The object of the invention is also a glass sheet having a
chemical composition that comprises the following constituents in
an amount varying within the weight limits defined below:
TABLE-US-00004 SiO.sub.2 60-75% Al.sub.2O.sub.3 0-10%
B.sub.2O.sub.3 0-5%, preferably 0 CaO 5-15% MgO 0-10% Na.sub.2O
5-20% K.sub.2O 0-10% BaO 0-5%, preferably 0, SO.sub.3 0.1-0.4%
Fe.sub.2O.sub.3 (total iron) 0 to 0.02%, Redox 0.15-0.3.
[0038] The method according to the invention is in point of fact
specially adapted to obtaining such a glass sheet and, to the
knowledge of the inventors, known methods do not enable such a
product to be obtained.
[0039] This redox range makes it possible to obtain very
satisfactory optical properties, while preserving a high furnace
life.
[0040] The presence of iron in a glass composition may result from
the raw materials, as impurities, or as an addition made
deliberately to color the glass. It is known that iron exists in
the structure of the glass in the form of ferric ions (Fe.sup.3+)
and ferrous ions (Fe.sup.2+). The presence of Fe.sup.3+ ions gives
the glass a very light yellow coloration and enables ultraviolet
radiations to be absorbed. The presence of Fe.sup.2+ ions gives
glass a more pronounced bluish green coloration and induces
absorption of infrared radiation. Increasing the iron content in
its two forms accentuates the absorption of radiation at the ends
of the visible spectrum, this effect being made to the detriment of
light transmission.
[0041] In the present invention, the Fe.sub.2O.sub.3 content (total
iron) is preferably less than 0.015%, or even less than or equal to
0.012%, notably 0.010%, this in order to increase the optical
transmission of the glass. The Fe.sub.2O.sub.3 content is
preferably greater than or equal 0.005%, notably 0.008% so as not
to increase the cost of the glass.
[0042] The redox is preferably greater than or equal to 0.15, and
notably between 0.2 and 0.30, notably between 0.25 and 0.30. Very
low redoxes contribute in point of fact to a reduction in the life
of furnaces.
[0043] The presence of sulfur in the composition, of which the
content is expressed as SO.sub.3 whatever its actual form,
generally results from the use of sulfates as refining agents.
Sulfates, notably of sodium or calcium (gypsum) are added with the
raw materials, generally jointly with a reducing agent such as
coke. Decomposition of these sulfates within the glass bath makes
it possible to refine the glass, that is to say to remove gaseous
inclusions. It has also been observed that adding sulfate makes it
possible to accelerate melting considerably, that is to say to
reduce the time necessary for the more refractory materials
(generally sand) to be perfectly dissolved in the glass bath. In
order to obtain glass at least cost, with a very high melting rate,
SO.sub.3 contents are thus preferably greater than 0.2%. Above 0.4%
there is a risk on the other hand of the appearance of sulfides,
which have a significant coloring effect, and a risk of the
appearance of foam or even of bubbling. The SO.sub.3 content in the
glass is preferably greater than or equal to 0.25% and/or less than
or equal to 0.35%, notably 0.30%.
[0044] In glasses according to the invention, silica SiO.sub.2 is
generally kept within narrow limits for the following reasons.
Above 75%, the viscosity of glass and its ability to devitrify
increase strongly which makes it difficult to melt and flow onto
the bath of molten tin. Below 60%, notably 64%, the hydrolytic
resistance of glass decreases rapidly. The preferred content lies
between 65 and 75%, notably between 71 and 73%.
[0045] Alumina Al.sub.2O.sub.3 plays a particularly important role
in the hydrolytic resistance of glass. Its content preferably lies
between 0 and 5%, notably between 0 and 3%. When glass according to
the invention is intended to be used in hot humid environments, the
alumina content is preferably greater than or equal to 1% or even
2%. A content between 0.5 and 1.5% is optimal.
[0046] The alkali metal oxides Na.sub.2O and K.sub.2O facilitate
fusion of the glass and make it possible to adjust its viscosity at
high temperatures in order to keep it close to that of a standard
glass. K.sub.2O may be used up to 10% since beyond this a problem
is presented of the high cost of the composition. In addition,
increasing the percentage of K.sub.2O may essentially only be made
to the detriment of Na.sub.2O, which contributes to an increase in
viscosity. The sum of the Na.sub.2O and K.sub.2O contents,
expressed in weight percentages, is preferably equal to or greater
than 10% and advantageously less than 20%. If the sum of these
contents is greater than 20% or if the Na.sub.2O content is greater
than 18% the hydrolytic resistance is strongly reduced. Glasses
according to the invention are preferably free from lithium oxide
Li.sub.2O on account of its high cost. An Na.sub.2O content between
10 and 15%, notably between 13.5 and 14.5% is preferred. The
K.sub.2O content normally lies between 0 and 5%, preferably less
than 1%, or even less than 0.5%.
[0047] Alkaline earth oxides make it possible to adapt the
viscosity of the glass to the processing conditions.
[0048] A CaO content between 7 and 12%, notably between 7 and 10%,
or even 8 and 9% is preferred.
[0049] MgO may be used up to approximately 10% and its elimination
may be compensated for, at least partly, by an increase in the
Na.sub.2O and/or SiO.sub.2 content. Preferably, the MgO content is
less than 5%. Low MgO contents make it possible moreover to reduce
the number of raw materials necessary for melting the glass. The
MgO content is preferably between 1 and 5%, notably between 2 and
5%. Surprisingly, the best results for energy transmission have
been obtained for MgO contents between 1 and 5%, notably between
2.5 and 4.5%. The inventors have been able to demonstrate a
surprising effect of the MgO content on the redox of glass,
progressive substitution of CaO by MgO having the effect of
reducing said redox, and therefore of increasing energy
transmission. A reduction in CaO content moreover makes it possible
to reduce the risk of the glass devitrifying and to widen the
forming margin, enabling forming to be more stable.
[0050] BaO has a much smaller influence than CaO and MgO on the
viscosity of glass and its content is essentially made to the
detriment of alkali metal oxide, of MgO and especially of CaO. Any
increase in BaO contributes to an increase in the viscosity of the
glass at low temperatures. Preferably, glasses according to the
invention are free from BaO and also strontium oxide (SrO), these
elements having a high cost.
[0051] The glass according to the invention preferably has a
TiO.sub.2 content between 0 and 0.1%, notably between 0.01% and
0.05%.
[0052] Preferred compositions according to the invention are
reproduced below: TABLE-US-00005 SiO.sub.2 60-75% Al.sub.2O.sub.3
0-10% B.sub.2O.sub.3 0-5%, preferably 0 CaO 5-15% MgO 0-10%
Na.sub.2O 5-20% K.sub.2O 0-10% BaO 0-5%, preferably 0, SO.sub.3
>0.2-0.4% Fe.sub.2O.sub.3 (total iron) 0 to 0.015%, Redox
0.2-0.30.
[0053] Other preferred compositions according to the invention are
reproduced below: TABLE-US-00006 SiO.sub.2 65-75% Al.sub.2O.sub.3
0-3% CaO 7-12% MgO 2-5% Na.sub.2O 10-15% K.sub.2O 0-5% SO.sub.3
0.1-0.3% Fe.sub.2O.sub.3 (total iron) 0 to 0.015%, Redox
0.1-0.3.
[0054] Preferably, the glass sheet having such a composition has,
for a thickness of 4 mm, a light transmission greater than or equal
to 91%, an energy transmission greater than or equal to 90.2% over
a wavelength range extending from 300 to 2500 nm, and an energy
transmission greater than or equal to 90.5% over a wavelength range
extending from 400 to 1100 nm, which corresponds to the range where
the quantum efficiency of solar cells is at a maximum.
[0055] Other preferred compositions according to the invention are
reproduced below: TABLE-US-00007 SiO.sub.2 65-75% Al.sub.2O.sub.3
0-5% CaO 7-12% MgO 1-5% Na.sub.2O 10-15% K.sub.2O 0-5% SO.sub.3
0.2-0.4% Fe.sub.2O.sub.3 (total iron) 0 to to less than 0.015%,
Redox 0.1-0.3.
[0056] Preferably, the glass sheet having such a composition has,
for a thickness of 4 mm, a light transmission greater than or equal
to 91.2%, an energy transmission greater than or equal to 90.0%
over a wavelength range extending from 300 to 2500 nm, and an
energy transmission greater than or equal to 90.5% over a
wavelength range extending from 400 to 1100 nm.
[0057] The glass composition may contain, apart from the inevitable
impurities contained notably in the raw materials, a small
proportion (up to 1%) of other constituents, for example agents
assisting melting or refining of the glass (Cl etc), or furthermore
elements coming from dissolution of the refractories serving for
the construction of furnaces (for example ZrO.sub.2). For the
reasons already stated, the composition according to the invention
preferably does not contain oxides such as Sb.sub.2O.sub.3,
As.sub.2O.sub.3 or CeO.sub.2.
[0058] The composition according to the invention preferably does
not contain any agent absorbing visible or infrared radiation
(notably for a wavelength between 380 and 1000 nm) other than those
already mentioned. In particular, the composition according to the
invention does not contain any of the following agents: oxides of
the transition elements such as CoO, CuO, Cr.sub.2O.sub.3, NiO,
MnO.sub.2 and V.sub.2O.sub.5, oxides of the rare earths such as
CeO.sub.2, La.sub.2O.sub.3, Nd.sub.2O.sub.3 or Er.sub.2O.sub.3, or
furthermore coloring agents in the elemental state such as Se, Ag
and Cu. Among other agents preferably excluded are the oxides of
the following elements: Sc, Y, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb
and Lu. These agents very often have an undesirable and very
powerful coloring effect, manifesting itself at very low
concentrations, sometimes of the order of a few ppm or less (1
ppm=0.0001%). Their presence thus very strongly reduces the
transmission of the glass.
[0059] The glass sheet according to the invention preferably has,
for a thickness of 3.2 mm, a light transmission T.sub.L at least
90%, notably 90.5% or even 91.0%. The light transmission, for a
thickness of 4 mm, is preferably greater than or equal to 91%,
notably 91.2%. Advantageously, the glass sheet according to the
invention has, still for a thickness of 3.2 mm, an energy
transmission T.sub.E of at most 91%. For a thickness of 4 mm, the
energy transmission is preferably greater than or equal to 90.2%.
For the same thickness, the energy transmission calculated for a
wavelength range extending from 400 to 1100 nm, is preferably
greater than or equal to 90.5%.
[0060] The object of the invention is finally the use of the glass
sheet according to the invention in photovoltaic cells, solar
cells, flat or parabolic mirrors for the concentration of solar
energy, or furthermore for diffusers for back-lighting display
screens of the LCD (liquid crystal screens) type. The glass sheet
according to the invention may also be employed for interior
applications (partitions, furnishings etc) or in domestic
electrical goods (refrigerator storage shelves etc) or for glazing
in the building construction or automobile fields. They may also be
employed in screen or flat lamps based on organic
electroluminescent dyes.
[0061] Generally, the object of the invention is also a
photovoltaic cell, a solar cell, a flat or parabolic mirror for the
concentration of solar energy, a diffuser for back-lighting display
screens of the LCD type, a screen or flat lamp based on organic
electroluminescent diodes, comprising at least one glass sheet
according to the invention.
[0062] The glass sheet according to the invention may
advantageously be covered by at least one thin transparent
electroconducting layer and/or an anti-reflecting coating,
preferably a thin transparent electroconducting layer on a first
face and an anti-reflecting coating on a second face. According to
the applications, other layers or multilayers may be deposited on
one of other faces of the glass sheet. There may be a
photocatalytic, self-cleaning or anti-soiling layer. There may also
be layers or multilayers with a thermal function, notably
anti-solar or low-emissive layers, for example multilayers
comprising a silver layer protected by dielectric layers. There may
moreover be a mirror layer, notably silver-based, or of a
decorative layer such as a lacquer or enamel.
[0063] The glass sheet according to the invention may be
incorporated in single or multiple glazing (notably double or
triple glazing), in the sense where it may comprise several glass
sheets providing a space filled with gas. The glazing may also be
laminated and/or toughened and/or hardened and/or bowed.
[0064] In the case of applications in the photovoltaic field, and
in order to maximize the energy yield of the cell, several
improvements may be provided, cumulatively or alternatively: [0065]
the substrate may advantageously be coated with at least one thin
transparent electroconducting layer, for example based on
SnO.sub.2:F, SnO.sub.2:Sb, ZnO:Al or ZnO:Ga. These layers may be
deposited on the substrate by various deposition methods, such as
chemical vapor deposition (CVD) or cathode spray deposition,
notably assisted by a magnetic field (magnetron method). In the CVD
method, halide or organometallic precursors are vaporized and
transported by a carrier gas to the surface of the hot glass, where
they are decomposed under the effect of heat to form the thin
layer. The advantage of the CVD method is that it is possible to
put it into operation within the method for forming the glass
sheet, notably when this consists of a float method. It is thus
possible to deposit a layer when the glass sheet is on the bath of
tin, on leaving the bath of tin, or in the lehr, that is to say the
moment when the glass sheet is annealed in order to eliminate
mechanical stresses. It is coated with a transparent
electroconducting layer which may in its turn be coated with a
semiconductor based on amorphous or polycrystalline silicon, with
chalcopyrites (notably of the CIS--CuInSe.sub.2 type or
CIGS--CuInGaSe.sub.2 type) or with CdTe to form a photovoltaic
cell. This may notably consist of a thin second coat based on
amorphous silicon, CIS or CdTe. In this case, another advantage of
the CVD method lies in that a greater roughness is obtained, which
generates a light-trapping phenomenon, which increases the quantity
of photons absorbed by the semiconductor. [0066] the substrate may
be coated on at least one of its faces with an anti-reflecting
coating. This coating may comprise a layer (for example based on
porous silica with a low refractive index) or several layers. In
the latter case a multilayer is preferred based on a dielectric
material alternating with layers with high and low refractive
indices and ending with a layer with a low refractive index. It may
notably consist of a multilayer described in application WO
01/94989 or WO 2007/077373. The anti-reflecting layer may also
include as the last layer a self-cleaning and anti-soiling layer
based on photocatalytic titanium oxide, as taught in application WO
2005/110937. A low reflection may also be obtained that is durable
with time. In applications in the photovoltaic field, the
anti-reflecting layer is deposited on the outer face, namely the
face in contact with the atmosphere, while any electroconducting
transparent layer is deposited on the inner face, on the
semiconductor side. [0067] the surface of the substrate may be
textured, for example having patterns (notably pyramidal), as
described in applications WO 03/046617, WO 2006/134300, WO
2006/134301 or WO 2007/015017. These texturing effects are
generally obtained by means of roll-forming the glass.
[0068] In the field of photovoltaic or solar cells, the glass sheet
according to the invention preferably constitutes the protective
cover for said cells. The glass sheet may be employed in all types
of technologies: mono- or poly-crystalline silicon wafers, thin
layers of amorphous silicon, CdTe, or CIS (copper indium selenide,
CuInSe.sub.2) or CIGS (CuInGaSe.sub.2).
[0069] The invention is illustrated by the following non-limiting
example.
[0070] Powdered raw materials (mainly sand, sodium carbonate,
limestone and dolomite) were charged into a furnace with transverse
burners and regenerators comprising 7 pairs of burners. The purity
of the raw materials was such that the iron oxide content
(Fe.sub.2O.sub.3) was only 0.0115%. The refining system employed
was the sodium sulfate/coke couple. The burners employed fuel oil
as the fuel and air as the oxidant. A glass bath was obtained that
was then poured onto a bath of molten tin in order to obtain a 3.85
mm glass sheet according to the method usually known under the name
"float method".
[0071] According to a comparative example, the 7 pairs of overhead
burners were fed by a stoichiometric mixture. The redox obtained
was 0.42, and the sulfate content was 0.25% SO.sub.3. The energy
transmission (T.sub.E) calculated according to ISO standard 9050
was, for a thickness of 3.85 mm, 90.0%.
[0072] In the example according to the invention, the 3 pairs of
burners situated most downstream were fed with an
over-stoichiometric quantity of oxidant, so that the molar ratio
O2/fuel was 1.1. The redox of the glass sheet obtained fell to
0.27, which was accompanied by an increase in energy transmission
to 90.7%, and even to 0.25 for an energy transmission of 90.9%. The
hearth temperature in the region of the hot spot remained below
1350.degree. C., which did not affect the life of the furnace.
[0073] The compositions tested are reproduced in table 1 below.
Concentrations are indicated in percentages by weight. The optical
properties are as follows, for a thickness of 3.85 mm: [0074] the
energy transmission (TE) calculated according to ISO standard
9050:2003
[0075] the overall light transmission factor (TL), calculated
between 380 and 780 mm, within the meaning of ISO standard
9050:2003, then taking into consideration the D65 illuminant as
defined in ISO/CIE standard 10526 and the standard calorimetric
observer C.I.E. 1931 as defined by ISO/CIE standard 10527.
TABLE-US-00008 TABLE 1 C1 1 2 SiO.sub.2 71.86 71.86 71.86
Al.sub.2O.sub.3 0.53 0.53 0.53 TiO.sub.2 0.01 0.01 0.01 CaO 9.4 9.4
9.4 MgO 4.0 4.0 4.0 Na.sub.2O 14.0 14.0 14.0 K.sub.2O 0.01 0.01
0.01 SO.sub.3 0.25 0.25 0.25 Fe.sub.2O.sub.3 0.0090 0.0090 0.0090
Redox 0.42 0.27 0.25 TL (%) 91.2 91.5 91.5 TE (%) 90.0 90.7
90.9
[0076] Example C1 is a comparative example, obtained by traditional
production methods, thus with burners not operating with an oxygen
over-stoichiometry.
[0077] Table 2 below illustrates the influence of the MgO content
on the redox. TABLE-US-00009 TABLE 2 3 4 5 6 7 8 SiO.sub.2 72.7
72.5 72.4 72.4 72.3 72.3 A1.sub.2O.sub.3 1.04 1.04 1.02 1.02 1.02
1.02 TiO.sub.2 0.03 0.03 0.03 0.03 0.03 0.03 CaO 12.3 11.4 10.4 9.4
8.4 7.4 MgO -- 1.0 2.0 3.0 4.0 5.0 Na.sub.2O 13.4 13.5 13.8 13.7
13.8 13.8 K.sub.2O 0.02 0.02 0.02 0.02 0.02 0.02 SO.sub.3 0.36 0.33
0.35 0.30 0.32 0.30 Fe.sub.2O.sub.3 0.0090 0.0096 0.0104 0.0124
0.0116 0.0125 Redox 0.19 0.14 0.15 0.13 0.12 0.11
* * * * *