U.S. patent application number 11/688104 was filed with the patent office on 2009-01-15 for lithium-aluminosilicate glass with short glazing times.
Invention is credited to Ulrich Schiffner, Wolfgang Schmidbauer, Klaus Schonberger, Friedrich Siebers.
Application Number | 20090018007 11/688104 |
Document ID | / |
Family ID | 37022863 |
Filed Date | 2009-01-15 |
United States Patent
Application |
20090018007 |
Kind Code |
A1 |
Siebers; Friedrich ; et
al. |
January 15, 2009 |
LITHIUM-ALUMINOSILICATE GLASS WITH SHORT GLAZING TIMES
Abstract
A lithium-aluminosilicate glass or a corresponding glass ceramic
that has a content of 0-0.4 SnO.sub.2, 1.3-2.7% by weight of
.SIGMA.SnO.sub.2+TiO.sub.2, 1.3-2.5% by weight of ZrO.sub.2,
3.65-4.3% by weight of .SIGMA.ZrO.sub.2+0.87 (TiO.sub.2+SnO.sub.2),
.ltoreq.0.04% by weight of Fe.sub.2O.sub.3, 50-4000 ppm of
Nd.sub.2O.sub.3 and 0-50 ppm of CoO is described. The glass or the
glass ceramic is color-neutral, has a turbidity of less than 1%
HAZE and a high light transmission. The glazing time for conversion
of the glass into glass ceramic is especially short with less than
2.5 hours.
Inventors: |
Siebers; Friedrich;
(Nierstein, DE) ; Schiffner; Ulrich; (Mainz,
DE) ; Schmidbauer; Wolfgang; (Mainz, DE) ;
Schonberger; Klaus; (Mainz, DE) |
Correspondence
Address: |
MILLEN, WHITE, ZELANO & BRANIGAN, P.C.
2200 CLARENDON BLVD., SUITE 1400
ARLINGTON
VA
22201
US
|
Family ID: |
37022863 |
Appl. No.: |
11/688104 |
Filed: |
March 19, 2007 |
Current U.S.
Class: |
501/63 ; 501/53;
501/64 |
Current CPC
Class: |
C03C 10/0054 20130101;
C03C 10/0045 20130101; C03C 10/0027 20130101; C03C 3/095
20130101 |
Class at
Publication: |
501/63 ; 501/53;
501/64 |
International
Class: |
C03C 3/097 20060101
C03C003/097; C03C 3/04 20060101 C03C003/04; C03C 3/095 20060101
C03C003/095 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 20, 2006 |
EP |
06 005 593.6 |
Claims
1. Lithium-aluminosilicate glass, which can be converted with
glazing times of under 2.5 hours into a transparent glass ceramic
with high-quartz mixed crystals as the prevailing crystal phase,
which has a turbidity of less than 1% according to ASTM D 1003 and
has a neutral color shade, characterized by a content in % by
weight based on oxide of: TABLE-US-00007 0-0.4 SnO.sub.2 1.3-2.7
.SIGMA.SnO.sub.2 + TiO.sub.2 1.3-2.5 ZrO.sub.2 3.65-4.3
.SIGMA.ZrO.sub.2 + 0.87 (TiO.sub.2+ SnO.sub.2) .ltoreq.0.04
Fe.sub.2O.sub.3 0.005-0.4 Nd.sub.2O.sub.3 0-0.005 CoO
2. Lithium-aluminosilicate glass according to claim 1,
characterized by a content (in % by weight based on oxide) of:
TABLE-US-00008 Li.sub.2O 3.0-4.5 Na.sub.2O 0-1.5 K.sub.2O 0-1.5
.SIGMA.Na.sub.2O + K.sub.2O 0.2-2.0 MgO 0-2.0 CaO 0-1.5 SrO 0-1.5
BaO 0-2.5 ZnO 0-2.5 B.sub.2O.sub.3 0-1.0 Al.sub.2O.sub.3 19-25
SiO.sub.2 55-69 TiO.sub.2 1.4-2.7 ZrO.sub.2 1.3-2.5 SnO.sub.2 0-0.4
.SIGMA.SnO.sub.2 + TiO.sub.2 <2.7 P.sub.2O.sub.5 0-3.0
Nd.sub.2O.sub.3 0.01-0.4 CoO 0-0.004
optionally with additions of chemical refining agents such as
As.sub.2O.sub.3, Sb.sub.2O.sub.3, and CeO.sub.2 and refining
additives such as sulfate, chloride and fluoride compounds in total
contents of up to 2.0% by weight.
3. Lithium-aluminosilicate glass according to claim 2,
characterized by a content (in % by weight based on oxide) of:
TABLE-US-00009 Li.sub.2O 3.2-4.3 Na.sub.2O 0.2-1.0 K.sub.2O 0-0.8
.SIGMA.Na.sub.2O + K.sub.2O 0.3-1.5 MgO 0.1-1.5 CaO 0-1.0 SrO 0-1.0
BaO 0-2.5 ZnO 0-2.0 Al.sub.2O.sub.3 19-24 SiO.sub.2 60-68 TiO.sub.2
1.4-2.7 ZrO.sub.2 1.3-2.2 SnO.sub.2 0-0.3 .SIGMA.SnO.sub.2 +
TiO.sub.2 <2.7 P.sub.2O.sub.5 0-1.5 Nd.sub.2O.sub.3 0.02-0.3 CoO
0-0.003
optionally with the additions of chemical refining agents such as
As.sub.2O.sub.3, Sb.sub.2O.sub.3, and CeO.sub.2 and refining
additives such as sulfate compounds, chloride compounds, and
fluoride compounds in total amounts of up to 1.5% by weight.
4. Lithium-aluminosilicate glass according to claim 1,
characterized by an MgO content of <0.8, preferably <0.6% by
weight.
5. Glass ceramic, converted from a lithium-aluminosilicate glass
with a composition according to claim 1.
6. Transparent glass ceramic converted from a
lithium-aluminosilicate glass with a composition according to claim
1, with a content of Nd.sub.2O.sub.3<2000 ppm, CoO<20 ppm and
a thermal expansion coefficient .alpha..sub.20/700 of
0.+-.0.510.sup.-6/K, preferably 0.+-.0.310.sup.-6/K, a light
transmission with a thickness of 4 mm of greater than 80% and a
variegation of colors in the CIELAB color system C*<3.5.
7. Use of the transparent glass ceramic, converted from a
lithium-aluminosilicate glass with a composition according to claim
1, as fire protection glass, fireplace door windows, oven door
windows, cooking surfaces with bottom coating and covers in the
lighting sector.
Description
[0001] It is generally known that glasses from the
L.sub.2O--Al.sub.2O.sub.3--SiO.sub.2 system can be converted into
glass ceramics with high-quartz mixed crystals or keatite mixed
crystals as main crystal phases.
[0002] The invention relates to a lithium-aluminosilicate glass
that can be converted with short glazing times of less than 2.5
hours, preferably less than 100 minutes, into a transparent glass
ceramic with high-quartz mixed crystals as a prevailing crystal
phase and in this case is without visually disruptive light
scattering (turbidity) or inherent color.
[0003] A key property of these glass ceramics is that in a
temperature range from room temperature up to about 700.degree. C.,
they have an extremely low thermal expansion coefficient
.alpha..sub.20/700<1.510.sup.-6/K. These glass ceramics are
therefore used in transparent form as, e.g., fire protection glass,
fireplace door windows, cookware, and cooking surfaces, as well as
substrate material for wafer stages or for mirror supports for
telescopes and for reflectors in beamers.
[0004] In numerous applications of transparent glass ceramics, the
latter are required in flat form, for example in the form of panes,
as windows for fireplaces, for fire protection glazings, cooking
surfaces with colored bottom coating and for display purposes. The
production of such flat glass from the glass melt, which is used as
a starting glass for the production of glass ceramics, is usually
carried out by rolling and recently also by floating. For an
economic production of these lithium-aluminosilicate glasses, on
the one hand, a low melting point and a low processing temperature
V.sub.A are desired; on the other hand, the glass must show no
devitrification during shaping, i.e., no disruptive crystals should
form that impair the resistance in the starting glasses and the
glass ceramics that are produced therefrom. During shaping via
rolling, the last contact of the glass melt is with the debiteuse
that is made of noble metal (usually Pr/Rh) before the glass is
formed by the rolling and is cooled.
[0005] In the industrial production of glass ceramic, first the
starting glass is produced via conventional glass manufacturing
processes. In the melt, typically arsenic oxide and/or antimony
oxide is (are) used as a refining agent. These refining agents are
compatible with the required glass ceramic properties and result in
good bubble qualities in the melt. Also, SnO.sub.2 is used as a
refining agent in particular in connection with high-temperature
refining above 1700.degree. C.
[0006] After the melting and refining, the glass usually undergoes
hot forming by rolling, pouring, pressing or, recently, floating.
Then, the starting glass is converted into the glass-ceramic
article by controlled crystallization. This glazing is carried out
in a two-stage temperature process, in which nuclei, usually from
ZrO.sub.2/TiO.sub.2 mixed crystals, are produced first by
nucleation at a temperature of between 600 and 800.degree. C. Also,
SnO.sub.2 can be involved in the nucleation. In the case of
subsequent temperature increase, the high-quartz mixed crystals in
these nuclei grow at the crystallization temperature of 800 to
900.degree. C. If desired, the high-quartz mixed crystals can then
also be converted into keatite mixed crystals. The conversion into
keatite mixed crystals is carried out with a temperature increase
in a temperature range of about 900 to 1200.degree. C. As a rule,
glass ceramics with keatite mixed crystals as a main phase are
translucent or opaque and have a white color shade, which can be
altered by the addition of colored oxides. With the transfer of
high-quartz mixed crystals to keatite mixed crystals, the thermal
expansion coefficient of the glass ceramic is increased, and the
transparency is reduced by the light scattering that is associated
with the enlargement of the crystals.
[0007] For the quality of transparent glass ceramics with
high-quartz mixed crystals as the main crystal phase, transparency
and low inherent color are decisive. Transparency means that the
glass ceramics are to have high light transmission in the visible
range as well as low light scattering (turbidity). The low light
scattering is achieved via a high nuclear density, which has the
effect that the sizes of the growing high-quartz mixed crystals lie
below the wavelength range of visible light. Typically, the mean
crystallite size of the high-quartz mixed crystals is in the range
of 20 to 80 nm. A high nuclear density requires sufficient contents
of nucleating agents as well as sufficient nucleation times during
the glazing.
[0008] It is disadvantageous in these glass ceramics that they
regularly have a weak inherent coloring, which is undesirable per
se.
[0009] The inherent color of transparent glass ceramic plates can
have various causes. In the raw materials of the batch for the
melts, the coloring element Fe is contained as a contaminant. Also,
the use of refining agents Sb.sub.2O.sub.3 and CeO.sub.2 results in
a low inherent color. The described brownish-yellow inherent
coloring of the transparent glass ceramics is based to a decisive
extent on electronic transfers to color complexes, which absorb in
the area of the visible light and in which the components that are
necessary for the nucleation, the Ti ion, is involved. The most
frequently absorbing color complex is the formation of adjacent Fe
and Ti ions between which electronic charge-transfer transitions
take place.
[0010] Sn/Ti complexes also produce an inherent color. The Fe/Ti
color complexes result in a red-brown discoloration, and the Sn/Ti
color complexes result in a yellow-brown discoloration. The
formation of these adjacent color complexes takes place as early as
during cooling of the starting glass and in particular during later
glazing of the glass ceramics. In the melt, the ions are still
dispersed uniformly, and during cooling, at high temperatures and
during glazing, they preferably accumulate in a clustered manner.
During glazing of the transparent glass ceramics, the inherent
color is thus quite considerably intensified compared to the
starting glass. By absorption in the short-wave portion of the
visible spectrum, the transparent flat glasses and in particular
the glass ceramics that are produced therefrom obtain a clear
inherent color, which greatly increases with the thickness.
[0011] The glazing of the lithium-aluminosilicate glasses that
consist of glass ceramics usually takes place in roller passage
kilns or tunnel kilns. To avoid rough spots of the plates during
the glazing, it is necessary that this kiln have a very good
temperature homogeneity such that the top and bottom of the plates
of the starting glasses crystallize simultaneously. Otherwise,
process-induced distortions result. Because of these high
requirements and the associated high acquisition costs of the
glazing kiln, it is economically advantageous if the kilns can be
run with high throughputs, i.e., that the total glazing time is
kept as short as possible. This results in the conflict of purpose,
however, that the starting glasses have to have a sufficient amount
of nucleating agents so that in the conversion into the glass
ceramic, no light scattering on large crystallites takes place
(Tyndall Effect).
[0012] To ensure sufficient nucleation, however, minimum contents
of the nucleating agent TiO.sub.2 are necessary, since this
nucleating agent can be replaced only disadvantageously in the case
of melts and devitrification by the alternative nucleating agent
ZrO.sub.2. This means that the desired quick glazing times and
short nucleation times with the TiO.sub.2 contents required for
this purpose result in an enhanced inherent color.
[0013] Many efforts have been made to reduce or to avoid these
disruptive inherent colors that are based on an Fe/Ti complex.
[0014] The reduction of the Fe content is a measure that is
economically usable only to a certain extent. A certain amount of
Fe.sub.2O.sub.3 in the batch always develops through the
industrially available raw materials of the batch for the
production of the glass and by wear and tear from system parts for
the production and homogenization of the batch. Based on the costs
for extremely pure raw materials and special plant design measures,
it is economically no longer justifiable to reduce the
Fe.sub.2O.sub.3 content below about 50 ppm in transparent glass
ceramics. The Fe.sub.2O.sub.3 content is usually on the order of
magnitude of about 100 to 500 ppm.
[0015] U.S. Pat. No. 4,438,210 describes attempts to reduce the
Fe/Ti color complex. Here, transparent glass ceramics with low
inherent color are obtained, which contain 2-6% by weight of
TiO.sub.2 and 0-2% by weight of ZrO.sub.2 as nucleating agents and
up to about 0.1% by weight of Fe.sub.2O.sub.3 as contaminants, by
the component MgO essentially being omitted.
[0016] JP 03-23237 A describes the production of transparent glass
ceramics without inherent color. These glass ceramics avoid the
addition of TiO.sub.2 as nucleating agents and are based on a mixed
nucleation by ZrO.sub.2/SnO.sub.2. The SnO.sub.2 contents that are
necessary for this purpose are more than 1% by weight. In the case
of these high SnO.sub.2 contents, however, the devitrification
resistance of the glass deteriorates. In the area of shaping, in
the case of viscosities around the processing temperature V.sub.A
of 10.sup.4 dPas, a disruptive Sn-containing crystal phase
crystallizes out. Thus, it results in an unreliable reduction of
the strength of the glasses and the glass ceramics produced
therefrom. Also, it has been shown that high SnO.sub.2 contents
result in a strong application of action of the glass melts on the
noble-metal components, such as stirrers and electrodes. As a
result, the service life of the noble-metal components is
shortened.
[0017] In the patent specification JP 05-193985, a glass ceramic
that is also TiO.sub.2-free is described preferably for
applications in fire protection glazing, which has the addition of
Nd.sub.2O.sub.3 in contents of 0.2-1% by weight to stain over the
inherent color produced by Fe ions. The glass ceramics are
germinated by 3-7% by weight of ZrO.sub.2. With these high
ZrO.sub.2 contents, however, the devitrification resistance of the
glass deteriorates in the area of the shaping at viscosities around
the processing temperature V.sub.A. It crystallizes out ZrO.sub.2
(baddeleyite) as a disruptive crystal phase.
[0018] U.S. Pat. No. 4,093,468 describes the use of Nd.sub.2O.sub.3
for the staining over of the color hue produced by the Fe/Ti color
complex. From this patent specification, transparent glass ceramics
almost without inherent color are known, which contain TiO.sub.2 as
a nucleating agent in contents of 0.5-6% by weight, a content of
Fe.sub.2O.sub.3 with contamination up to 500 pm, and which are
stained over by the addition of 0.03-0.75% by weight of
Nd.sub.2O.sub.3. This patent specification describes that, in
contrast to the conventional staining agents such as Co, Se, Cu,
Cr, Mn, Ni, and V, Nd.sub.2O.sub.3 is especially well suited to
neutralize the color hue produced by the Fe/Ti color complex.
[0019] However, the patent specification does not provide any
indication as to how to achieve a high transparency, i.e., low
turbidity and high light transmission, within short glazing times,
by optimizing the nucleating agents. The examples of this patent
contain disadvantageous nucleating agent combinations, thus high
TiO.sub.2 contents or nucleating agent concentrations that are too
low. Owing to the deficient use of the alkalis Na.sub.2O and
K.sub.2O, the glasses, moreover, have an insufficient
devitrification resistance. No assessment is made on the influence
of the refining agent on the formation of the Fe/Ti color
complex.
[0020] The principle of the staining over of an existing,
undesirable color hue by nature results in a stronger absorption of
light and thus reduces the light transmission. Since the existing
absorption bands are neutralized with complementary absorption
bands of the staining agent, overall a reduced light transmission
is produced.
[0021] Another problem develops during recycling of the glass
ceramic. It is a known fact that scrap glass, e.g., container
glass, but also flat glass, is collected to a large extent and
supplied to recycling in the form of cullets. Glass ceramic also
finds its way into these cullets, and this results in problems,
since the compositions that consist of
Li.sub.2O--Al.sub.2O.sub.3--SiO.sub.2 glass systems have higher
melting points and thus have a very disruptive effect on the
lime-sodium glasses and their shaping during remelting. In the area
of scrap cullet recycling, optical processes are increasingly used,
which separate the cullets based on their different absorption
bands.
[0022] The object of the invention is to find a
lithium-aluminosilicate glass, [0023] which can be converted with
glazing times of less than 2.5 hours into a glass ceramic with
high-quartz mixed crystals as the prevailing crystal phase, [0024]
which has a neutral color and a good transparency, [0025] which has
an extremely low turbidity, and [0026] which can be reliably
recognized in optical cullet sorting facilities owing to its
optical properties, and can be separated from used-glass
cullet.
[0027] This object is achieved by the lithium-aluminosilicate glass
that is described in Claim 1.
[0028] The contents of the nucleating components TiO.sub.2,
ZrO.sub.2, and SnO.sub.2 have to be kept within relatively narrow
limits. In this case, however, certain minimum contents of these
compounds are necessary to produce nuclei in high density in the
desired short glazing times of less than 2.5 hours, preferably less
than 100 minutes during the nucleation, so that after the
high-quartz mixed crystals are grown, transparent glass ceramics
can be produced without turbidity. With the high nuclear density,
the mean crystallite size of the high-quartz mixed crystals remains
limited to values of <80 nm, by which a disruptive light
scattering is avoided. For an effective nucleation, minimum
contents of TiO.sub.2 and ZrO.sub.2 are necessary.
[0029] The turbidity (English: haze) is to be less than 1%,
preferably less than 0.5% (measured for a 3.6 mm-thick plate with
polished surfaces on both sides). According to ASTM D 1003,
turbidity is the proportion, in percentage, of the transmitted
light, which deviates from the irradiated light beam on average by
more than 2.5.degree..
[0030] It was found that the required quick glazing and the short
glazing times that are required for this purpose can then be
achieved with the nucleating agents ZrO.sub.2, TiO.sub.2 and
SnO.sub.2 if certain conditions are maintained. The nucleation
action of SnO.sub.2 and TiO.sub.2 (in % by weight) is approximately
the same, and thus these two components can be considered together.
The nucleating action of ZrO.sub.2 (in % by weight) is clearly
greater. Therefore, combinations of the nucleating agents ZrO.sub.2
and (TiO.sub.2+SnO.sub.2), which lie in a straight line in a
corresponding diagram (FIG. 1), can be produced with identical
nucleating action. For the slight turbidity at short glazing times
according to the invention, a lower boundary line for the minimum
contents of the nucleating agents is produced:
ZrO.sub.2=-0.87(TiO.sub.2+SnO.sub.2)+3.65.
[0031] For the preferred minimum contents of the nucleating agents
(in % by weight), the following thus applies:
ZrO.sub.2+0.87(TiO.sub.2+SnO.sub.2).gtoreq.3.65
[0032] Excessive nucleating agent contents result, however, in a
deterioration of the devitrification behavior in shaping, as was
already indicated. To ensure that the upper devitrification
temperature (OEG) is below the processing temperature V.sub.A, the
upper limit for the nucleating agent contents (in % by weight) must
follow the condition:
ZrO.sub.2+0.87(TiO.sub.2+SnO.sub.2).ltoreq.4.3
[0033] The upper limit is also shown in FIG. 1.
[0034] In summary, the relationship reads:
3.65.ltoreq.ZrO.sub.2+=0.87(TiO.sub.2+SnO.sub.2).ltoreq.4.3
[0035] The sum of the TiO.sub.2 and SnO.sub.2 contents is not to
exceed 2.7% by weight, however, since these components are involved
in the formation of the Fe/Ti and Sn/Ti color complexes for the
inherent color.
[0036] Additional limits are produced from the requirement for a
good devitrification resistance
ZrO.sub.2<2.5% by weight
SnO.sub.2<0.4% by weight
[0037] The content of SnO.sub.2 is not to exceed 4% by weight,
since otherwise, in the case of shaping near the processing point
V.sub.A, it results in an undesirable devitrification in the form
of an Sn-containing crystal phase. The equivalent holds true for
the content of ZrO.sub.2, in which an upper limit of <2.5% by
weight is to be maintained, so that devitrification in the form of
a ZrO.sub.2-containing crystal phase (baddeleyite) does not result.
This is expressed by the upper devitrification limit that is to be
below the processing temperature V.sub.A. With these conditions,
the acceptable range of the nucleating agent contents can be
defined in a diagram (FIG. 1). For ZrO.sub.2, the minimum contents
of 1.3% by weight, and for .SIGMA.SnO.sub.2+TiO.sub.2, the minimum
contents of 1.3% by weight are necessary to ensure a low turbidity
(haze) of less than 1%.
[0038] In the lithium-aluminosilicate glasses according to the
invention and the transparent glass ceramics produced therefrom,
the disruptive inherent color that is based on Fe/Ti and/or Sn/Ti
color complexes is reduced by additions of Nd.sub.2O.sub.3 in
contents of 50-4000 ppm. Below 50 ppm of Nd.sub.2O.sub.3, the
action is no longer reliable, and above 4000 ppm, either the
transmission of the glasses is poor (with excessive Fe.sub.2O.sub.3
contents) or the inherent color of the Nd ion is disruptive.
Contents of 100 to 4000 ppm and in particular 200 to 3000 ppm of
Nd.sub.2O.sub.3 are preferred. An upper limit for the
N.sub.2O.sub.3 content of less than 2000 ppm is quite especially
preferred. The Nd.sub.2O.sub.3 contents are necessary to achieve
the goal, according to the invention, of a reduction of the
inherent color of the floated flat glasses and the transparent
glass ceramics that are produced therefrom by staining over.
[0039] In addition, they are used to clearly label the flat glasses
according to the invention and the glass ceramics that are produced
therefrom and to improve the recyclability.
[0040] Additions of Nd have the advantage that this element of the
red-brown coloring is especially readily counteracted by Fe/Ti or
this element of the yellow-brown coloring is especially readily
counteracted by Sn/Ti complexes. The color point that is measured
in the various color systems, such as, e.g., in the CIE color
system or the CIELAB (in short, lab) color system is readily
shifted in the direction of the achromatic point by Nd. In
addition, as a coloring ion from the 4f group of the periodic
table, the Nd has a great number of characteristic absorption
bands, which make possible a clear labeling. In the conversion into
the transparent glass ceramic, these absorption lines are changed
only slightly.
[0041] The labeling of the glass or the transparent glass ceramic
that is produced therefrom can therefore be detected very easily
with commercially available spectrometers. This allows the
manufacturer of the original product to recognize his product and
also ensures the simplified assignment of product liability in the
event of damage or loss. A differentiation of transparent glass
ceramics of various manufacturers is otherwise possible only via
expensive analytical measuring methods, as they are available only
in a few special laboratories.
[0042] The characteristic absorption lines for the Nd also make
possible a detection and separation in preparation processes during
recycling of old cullets that consist of lime-sodium glass. The
addition of Nd for labeling is especially advantageous because of
the characteristic absorption lines and the property thereof to
fluoresce into infrared. Because of the above-mentioned properties,
it is possible to keep cullets--originating from transparent glass
ceramic that, because of the low inherent color, can be easily
confused with, e.g., normal window panes from low-melting
lime-sodium glass--from being incorporated into their preparation
process and remelted. The danger exists in that unmelted remnants
are formed by the high melting points of the lithium
aluminosilicate glasses (and glass ceramics) compared to those of
lime-sodium glasses. In the most advantageous case, this results in
remnants that are still visually recognizable in the lime-sodium
glass products; in the most disadvantageous case, this can result
in clogging the channels or nozzles in the shaping process and thus
in the total failure in the production of the lime-sodium
glasses.
[0043] The lithium-aluminosilicate glasses according to the
invention are to contain less than 400 ppm, preferably less than
200 ppm, of Fe.sub.2O.sub.3. Higher contents have the effect that
even higher contents of the staining agent Nd are required to
neutralize the color hue Fe/Ti. This results in lower light
transmission and a visually observable gray hue. The contents of
Fe.sub.2O.sub.3 can be minimized to an economically justifiable
extent; smaller contents than about 50 ppm are no longer economical
because of the high costs of Fe-free raw materials of the
batch.
[0044] Additions of CoO in a total amount of up to 50 ppm to the Nd
additive are advantageous to adjust the color point of the
transparent glass ceramic more precisely in the direction of the
achromatic point. The Nd additive by itself does not shift the
color point exactly in the direction of the achromatic point, such
that this slight correction may be advantageous. An amount of CoO
from 1 to 50 ppm is preferred. Preferably, however, an upper limit
for the CoO of 40 ppm, in particular 30 ppm, cannot be exceeded.
For fine corrections of the color sites, other staining agents,
such as, e.g., Cr, Ni, V, Cu, Mn, and Ce can also be added. The
total amount should not exceed 100 ppm.
[0045] The oxides Li.sub.2O, Al.sub.2O.sub.3 and SiO.sub.2 are
components of the high-quartz and/or keatite mixed crystal phases
that are necessary within the preferred limits indicated in the
claims. A minimum content of Li.sub.2O of 3% by weight is in
general necessary, but Li.sub.2O contents of more than 4.5% by
weight during the production process often result in an undesirable
devitrification. A content of 3.2 to 4.3% by weight of Li.sub.2O
leads to especially good results.
[0046] To avoid high viscosities of glasses and to suppress the
tendency toward undesirable devitrification of mullite during
shaping, the Al.sub.2O.sub.3 content is limited in the case of a
preferred minimum content of 19% by weight to preferably a maximum
25% by weight, in particular 24% by weight. The SiO.sub.2 content
is preferably to be at most 69% by weight, in particular 68% by
weight, since this component greatly increases the viscosity of the
glass. Thus, higher contents of SiO.sub.2 are disadvantageous for
the melting of the glasses and with respect to the temperature
stress during shaping. The minimum content of SiO.sub.2 is
preferably to be 55% by weight, in particular 60% by weight.
[0047] MgO, ZnO and P.sub.2O.sub.5 can be incorporated in the
crystal phases as additional components. Because of the problem of
forming undesirable crystal phases such as Zn spinel in the
glazing, the ZnO content is limited to values of at most 2.5% by
weight, preferably at most 2.0% by weight. The MgO content is
limited to at most 2.0% by weight, preferably up to 1.5% by weight,
since it otherwise unreliably increases the expansion coefficients
of the glass ceramic. For low inherent color, MgO contents of less
than 0.8% by weight and especially of less than 0.6% by weight are
advantageous. A minimum MgO content of 0.1% by weight is generally
required, so that the thermal expansion of the glass ceramic does
not drop to negative values of below 0.3.times.10.sup.-6/K.
[0048] The addition of alkalis Na.sub.2O and K.sub.2O to
alkaline-earths CaO, SrO, and BaO as well as B.sub.2O.sub.3 improve
the meltability and the devitrification behavior of the glass
during shaping. The contents have to be limited, however, since
these components essentially remain in the residual glass phase of
the glass ceramic and increase the thermal expansion in an
unreliable way. Also, higher contents can impair the
crystallization behavior in the conversion of the glass into the
glass ceramic and have a disadvantageous effect on the temperature
resistance of the glass ceramic. The sum of the alkalis
Na.sub.2O+K.sub.2O is to be at least 0.2% by weight, preferably at
least 0.3% by weight. The addition of P.sub.2O.sub.5 can be up to
3% by weight and is preferably limited to 1.5% by weight. The
addition of P.sub.2O.sub.5 is advantageous for the devitrification
resistance, but higher contents have a disadvantageous effect on
the acid resistance.
[0049] The glasses according to the invention are refined with use
of the refining agents arsenic oxide and/or antimony oxide that are
conventional for glasses from the
Li.sub.2O--Al.sub.2O.sub.3--SiO.sub.2 system. Also, SnO.sub.2 can
be added in particular in connection with a high-temperature
refining >1700.degree. C. as an alternative and/or in
combination in amounts of up to 0.3% by weight. Other refining
additives, such as, e.g., CeO.sub.2, sulfate compounds, chloride
compounds, and fluoride compounds can be added to the glass melts.
The total content of the refining agent and additives is not to
exceed 2% by weight.
[0050] The water content of the glasses according to the invention
is usually between 0.015 and 0.06 mol/l, depending on the selection
of the raw materials of the batch and the process conditions in the
melts. This corresponds to .beta..sub.OH values of 0.16 to 0.64
mm.sup.-1.
[0051] The glass according to the invention preferably has a
composition (in % by weight based on oxide) of:
TABLE-US-00001 Li.sub.2O 3.0-4.5 Na.sub.2O 0-1.5 K.sub.2O 0-1.5
.SIGMA.Na.sub.2O + K.sub.2O 0.2-2.0 MgO 0-2.0 CaO 0-1.5 SrO 0-1.5
BaO 0-2.5 ZnO 0-2.5 B.sub.2O.sub.3 0-1.0 Al.sub.2O.sub.3 19-25
SiO.sub.2 55-69 TiO.sub.2 1.4-2.7 ZrO.sub.2 1.3-2.5 SnO.sub.2 0-0.4
.SIGMA.SnO.sub.2 + TiO.sub.2 <2.7 P.sub.2O.sub.5 0-3.0
Nd.sub.2O.sub.3 100-4000 ppm CoO 0-40 ppm
optionally with the additions of chemical refining agents such as
As.sub.2O.sub.3, Sb.sub.2O.sub.3, and CeO.sub.2 and optionally
additional refining additives, such as, for example, sulfate
compounds, chloride compounds, and fluoride compounds in total
amounts of up to 2% by weight. In this information, the Nd content
is converted into an oxide base (Nd.sub.2O.sub.3), whereby the type
of Nd additive in the batch is not limited to the indicated oxide,
but rather any Nd compounds can be added.
[0052] In an especially preferred embodiment of the invention, the
glass has a composition, in % by weight based on oxide, of:
TABLE-US-00002 Li.sub.2O 3.2-4.3 Na.sub.2O 0.2-1.0 K.sub.2O 0-0.8
.SIGMA.Na.sub.2O + K.sub.2O 0.3-1.5 MgO 0.1-1.5 CaO 0-1.0 SrO 0-1.0
BaO 0-2.5 ZnO 0-2.0 Al.sub.2O.sub.3 19-24 SiO.sub.2 60-68 TiO.sub.2
1.4-2.7 ZrO.sub.2 1.3-2.2 SnO.sub.2 0-0.3 .SIGMA.SnO.sub.2 +
TiO.sub.2 <2.7 P.sub.2O.sub.5 0-1.5 Nd.sub.2O.sub.3 200-3000 ppm
CoO 0-30 ppm
optionally with the additions of chemical refining agents such as
As.sub.2O.sub.3, Sb.sub.2O.sub.3, and CeO.sub.2 and refining
additives such as sulfate compounds, chloride compounds, and
fluoride compounds in total amounts of up to 1.5% by weight.
[0053] In the case of low refining agent contents of
As.sub.2O.sub.3, Sb.sub.2O.sub.3 and/or also SnO.sub.2, it may be
necessary to combine the chemical refining with a high-temperature
refining above 1700.degree. C., if good bubble qualities with
numbers of bubbles of below 5 bubbles/kg of glass (relative to
bubble sizes of greater than 0.1 mm) are desired.
[0054] For the inherent color, it is especially advantageous if the
glass ceramic plate contains As.sub.2O.sub.3 as a refining agent,
optionally with additional refining additives such as sulfate,
chloride and fluoride compounds in total contents of up to 1% by
weight and is plained without the refining agents Sb.sub.2O.sub.3
and SnO.sub.2.
[0055] By using 0.1-0.4% by weight of SnO.sub.2 as a refining agent
in combination with a high-temperature refining above 1700.degree.
C., it is possible to eliminate the refining agents As.sub.2O.sub.3
and Sb.sub.2O.sub.3 that are disadvantageous from environmental
standpoints and to obtain devitrification-stable starting glasses
(OEG<V.sub.A) with good bubble qualities. Since, however, Sn
forms a colored Sn/Ti complex with Ti, an amount of 0.1 to 0.3% by
weight of SnO.sub.2 is preferred for refining.
[0056] The lithium-aluminosilicate glass according to the invention
is typically characterized by a processing temperature V.sub.A
under 1350.degree. C. to promote the meltability of the glass and
to limit the thermal stress on the system components during
shaping. In this case, the upper devitrification temperature OEG is
below the processing temperature V.sub.A.
[0057] After conversion into the transparent glass ceramic with
high-quartz mixed crystals as the main crystal phase, the thermal
expansion coefficient .alpha..sub.20/700 is to deviate no more than
0.510.sup.-6/K from the zero expansion. The deviation is preferably
to be less than 0.310.sup.-6/K. With the low thermal expansion
coefficients, a high temperature difference resistance of the glass
ceramic is achieved. A preferred embodiment of the invention
consists in that, in addition to the low turbidity, a low inherent
color and a high light transmission are also to be provided. The
transparent glass ceramic then has a turbidity with a haze-value of
<1%, preferably <0.5% (with a 3.6 mm thickness), in
transmission at a 4 mm thickness via a variegation of colors in the
CIELAB color system of C*<3.5 and a light transmission
(brightness) Y of >80%, preferably >85%. This combination of
properties is possible with the low Fe.sub.2O.sub.3 contents that
are matched to one another according to the invention, the
limitation of the contents of the staining agent Nd.sub.2O.sub.3 to
less than 2000 ppm and CoO to less than 20 ppm in combination with
the specifically defined contents of the nucleating agents
TiO.sub.2, SnO.sub.2 and ZrO.sub.2.
[0058] The lithium-aluminosilicate glasses according to the
invention can be converted by an additional temperature treatment
at temperatures of between about 900 and 1200.degree. C. into a
keatite mixed crystal-containing glass ceramic. Glass ceramics of
this type have a higher temperature resistance, but at the expense
of an increase in the thermal expansion coefficient, which is to be
less than 1.510.sup.-6/K between room temperature and 700.degree.
C. Because of the crystal growth that accompanies the conversion,
they have a translucent to opaque-white appearance. The turbidity
is generally >50% in haze values.
[0059] The transparent, colorless glass ceramic with high-quartz
mixed crystals that is produced from the lithium-aluminosilicate
glass by glazing according to the invention is used as fire
protection glass, fireplace door windows, oven door windows, in
particular for a pyrolysis oven, and covers for high-power lights.
By applying a light-absorbing bottom coating, a colored cooking
surface with the required light transmission can be produced from
the transparent glass ceramic. After conversion into a glass
ceramic with keatite mixed crystals as a main crystal phase, the
latter in translucent or opaque form is preferably used as a
cooking surface or as a cover panel in microwave ovens.
[0060] This invention becomes clearer with the aid of the following
examples.
[0061] For some embodiments, compositions and properties of the
lithium-aluminosilicate glasses are cited in Table 1. In this case,
glasses 1 to 8 are glasses according to the invention and glasses 9
and 10 are comparison glasses that are outside of this invention.
The comparison glass No. 10 was taken from U.S. Pat. No. 4,093,468
(Example B).
[0062] Table 1 shows the compositions of the starting glasses Nos.
1 to 8 according to the invention and the starting glasses 9 and 10
for the comparison examples.
[0063] In Table 1, the properties in the vitreous state, such as,
e.g.: transformation temperature Tg, processing temperature
V.sub.A, upper devitrification limits OEG, thermal expansion
between room temperature and 300.degree. C., as well as the
density, are also cited. Based on the composition, in particular
the nucleating agent contents, the upper devitrification limit is
below the processing temperature V.sub.A. In Table 1, the
Fe.sub.2O.sub.3 contents produced by the raw material contaminants
are cited in the compositions. The water content of the glasses is
0.03-0.05 mol/l, corresponding to .beta..sub.OH values of 0.32 to
0.53 mm.sup.-1.
[0064] The starting glasses of Table 1 were melted and plained from
the raw materials, common in the glass industry, at temperatures of
about 1620.degree. C. After melting in crucibles that consist of
sintered silica glass, the melts were poured into the Pt/Rh
crucible with an inside crucible made of silica glass and
homogenized at temperatures of 1550.degree. C. for 30 minutes while
being stirred. After standing at 1640.degree. C. for 2 hours,
castings of about 140.times.100.times.30 mm in size were poured and
cooled in an annealing furnace, beginning from about 660.degree. C.
to room temperature. The castings were divided into the sizes
required for the tests and for the glazing.
[0065] The glazings of the starting glasses were carried out with
the following temperature/time programs:
[0066] Glazing Program 1, (Total Time 147 Minutes): [0067] Heating
within 50 minutes from room temperature to 790.degree. C. [0068] 30
minutes of holding time at 790.degree. C. [0069] Heating within 30
minutes from 790 to 900.degree. C. [0070] 7 minutes of holding time
at 900.degree. C. [0071] Cooling within 30 minutes from 900 to
750.degree. C. [0072] Quick cooling to room temperature
[0073] Glazing Program 2, (Total Time 96 Minutes): [0074] Heating
within 38 minutes from room temperature to 790.degree. C. [0075] 14
minutes of holding time at 790.degree. C. [0076] In 24 minutes from
790 to 910.degree. C. [0077] 10 minutes of holding time at
910.degree. C. [0078] Cooling within 10 minutes from 910 to
800.degree. C. [0079] Quick cooling to room temperature
[0080] Glazing Program 3, (Production of Keatite Mixed Crystal
Glass Ceramic): [0081] Heating within 33 minutes from room
temperature to 790.degree. C. [0082] 30 minutes of holding time at
790.degree. C. [0083] Additional heating within 32 minutes from
790.degree. C. to a maximum temperature T.sub.max [0084] 7 minutes
of holding time at T.sub.max [0085] Quick cooling to room
temperature
[0086] Tables 2 and 3 show the properties of the transparent glass
ceramics with high-quartz mixed crystals as the prevailing crystal
phase, which were produced with the glazing programs 1 or 2.
Examples 9 and 10 or 19 and 20 are comparison ceramics outside of
the invention. The transmission measurements were made on polished
plates with a thickness of 4 mm and with standard illuminant C,
2.degree.. In addition to the color coordinates L*, a*, b* in the
CIELAB system, the color coordinates x and y in the CIE system are
cited. The glass ceramics according to the invention confirm the
advantageous action of the Nd feedstock and optionally in addition
Co for reducing the disruptive inherent color. Also, the Yellowness
Index according to Standard ASTM 1925/70 (77, 85) is, like the
variegation of colors C*, a measurement of the inherent color. High
values of the light transmission (brightness) Y are also achieved.
The turbidity was measured with standard illuminant C on 3.6
mm-thick plates that are polished on both sides and with a
commercial "haze-guard plus" measuring device of the BYK-Gardner
Company and characterized by the haze value.
[0087] For the glazing program with total times of 147 minutes and
96 minutes, low degrees of turbidity (haze values) are achieved by
the selection of the nucleating agents.
[0088] For the glazing program 1, additional property values of the
glass ceramic, such as infrared transmission at 1600 nm, thermal
expansion between 20 and 700.degree. c., density and the phase
content of the main crystal phase that is measured by means of
x-ray diffraction, consisting of high-quartz mixed crystals, as
well as the mean crystallite size, are shown in Table 2.
[0089] In addition, some examples with the glazing program 3 were
converted into translucent glass ceramics with keatite mixed
crystals as the prevailing crystal phase, and their properties as
well as phase contents and crystal sizes were determined (Table 4).
The maximum temperatures T.sub.max in the production are indicated
in the table. The light transmission Y and the IR transmission at
1600 nm were measured on 3.6 mm-thick plates. The color values L*,
a* and b* were determined in remission (incident light) on 3.6
mm-thick polished plates with the Mercury 2000 measuring device
(Datacolor Company, Lawrenceville, USA), standard illuminant C,
2.degree.. The haze values of the examples (polished plates, 3.6 mm
thick) are more than 90%.
[0090] FIG. 1 shows a plot of the nucleating agent concentration of
the glass ceramics according to the invention and the comparison
ceramics (starting glasses 9, 10) within the indicated limits.
[0091] FIG. 2 shows the transmission spectra of the glass ceramic
of Example 8 according to the invention and the comparison glass
ceramic of Example 9. The comparison example shows the disruptive
coloring associated with a high Yellowness Index and the
variegation of colors C*. The glass ceramic according to the
invention shows the characteristic absorption bands of the Nd ion,
which are also extremely well suited for labeling the glass ceramic
plates according to the invention. Moreover, the Nd.sub.2O.sub.3
addition also simplifies the recyclability of the glass ceramic by
optical cullet-separating processes based on the absorption bands
and the infrared fluorescence of the Nd ion.
[0092] FIG. 3 shows the color coordinates of the glass ceramics 11
to 18 according to the invention and the comparison glass ceramic
of Example 19 in the CIELAB system. The comparison glass ceramic of
Example 20 cannot be usefully categorized owing to its high
turbidity (milky appearance).
TABLE-US-00003 TABLE 1 Compositions and Properties of Starting
Glasses According to the Invention and Comparison Glasses 9 and 10
Compositions in % by Weight Based Glass No. on Oxide 1 2 3 4 5 6 7
8 9 10 Al2O3 21.3 21.3 21.8 21.65 20.0 19.95 20.1 20.0 19.9 21.3
BaO -- -- 1.98 1.95 0.80 0.84 0.82 1.21 0.9 1.3 K2O 0.13 0.11 -- --
0.20 0.20 0.20 -- 0.22 -- Li2O 3.75 3.70 3.69 3.64 3.54 3.75 3.65
3.63 3.6 2.7 MgO 0.85 1.05 0.58 0.59 1.15 1.06 1.10 0.77 1.2 1.0
Na2O 0.35 0.36 0.52 0.50 0.15 0.16 0.15 0.45 0.20 -- P2O5 -- -- --
-- -- -- -- -- -- 0.7 SiO2 67.55 67.35 65.3 65.2 68.1 67.35 67.4
67.22 67.2 62.0 ZnO 1.57 2.0 1.70 1.57 1.48 1.60 1.54 1.57 1.55 6.0
CaO 0.50 SnO2 -- 0.11 -- -- -- -- -- -- -- -- TiO2 2.23 2.19 2.29
2.38 2.26 2.35 2.35 2.33 2.59 1.8 ZrO2 1.76 1.80 1.76 1.98 1.76
1.80 1.76 1.77 1.75 2.0 As2O3 0.40 -- 0.28 0.43 0.40 0.86 0.85 0.86
0.86 0.7 Sb2O3 -- -- -- -- -- -- -- -- -- -- Nd2O3 0.09 0.02 0.08
0.09 0.14 0.06 0.06 0.17 -- 0.15 Fe2O3 ppm 200 100 180 200 200 140
150 160 290 100 CoO ppm -- -- -- -- -- -- -- 5 -- -- NiO ppm -- --
-- -- -- -- 23 -- -- -- Tg .degree. C. 681 685 6.76 687 692 669 682
672 673 673 V.sub.A .degree. C. 1320 1320 1306 -- 1335 1315 -- 1325
1304 1281 OEG .degree. C. 1240 1270 1250 -- 1255 1280 -- 1265 1265
1340 .alpha..sub.20/300 10.sup.-6/K 3.87 3.88 4.08 4.05 3.79 3.90
3.90 3.91 3.87 3.56 Density g/cm.sup.3 2.436 2.446 2.472 2.479
2.444 2.451 2.450 2.447 2.454 2.562
TABLE-US-00004 TABLE 2 Properties of Glass Ceramics According to
the Invention and Comparison Glass Ceramics of Examples 9 and 10
(Glazing Program 1) Example No. 1 2 3 4 5 6 7 8 9 10 Glass No. 1 2
3 4 5 6 7 8 9 10 Transmission Standard 4 mm Illuminant C, 2.degree.
Thickness % Light Transmission Y 87.0 87.3 87.7 86.3 85.6 87.2 85.6
83.8 84.8 72.2 Yellowness Index 3.3 5.4 3.5 4.0 2.8 5.8 6.3 2.1
15.0 25.0 x 0.3126 0.3148 0.3128 0.3133 0.3121 0.3150 0.3156 0.3117
0.3239 0.3338 y 0.3197 0.3215 0.3199 0.3206 0.3195 0.3222 0.3223
0.3183 0.3319 0.3443 L* 94.6 94.8 94.9 94.3 94.0 94.7 94.1 93.3
93.6 87.7 a* -0.5 -0.3 -0.4 -0.6 -0.6 -0.5 -0.3 -0.2 -0.7 -1.2 b*
1.8 2.8 1.9 2.2 1.6 3.1 3.2 1.1 8.1 13.4 C* 1.8 2.8 1.9 2.2 1.7 3.1
3.2 1.1 8.1 13.6 IR Transmission 4 mm 87.7 89.5 89.1 87.9 87.7 89.0
87.9 88.3 85.7 89.6 1600 nm Thickness % .alpha..sub.20/700
10.sup.-6/K -0.26 -0.05 -0.10 +0.03 +0.11 -0.07 +0.10 -0.05 +0.14
-0.07 Density g/cm.sup.3 2.519 2.526 2.545 2.550 2.534 2.543 2.538
2.544 2.546 2.628 X-Ray Diffraction: High-Quartz % 73 72 71 68 71
76 72 71 75 71 Phase Content nm 33 32 40 37 28 40 30 33 41 73
Crystallite Size Turbidity Haze 3.6 mm 0.28 0.28 0.50 0.42 0.27
0.23 0.25 0.32 0.20 2.7 Thickness % (milky)
TABLE-US-00005 TABLE 3 Properties of Glass Ceramics According to
the Invention and Comparison Glass Ceramics of Examples 19 and 20
(Glazing Program 2) Example No. 11 12 13 14 15 16 17 18 19 20 Glass
No. 1 2 3 4 5 6 7 8 9 10 Transmission Standard 4 mm Illuminant C,
2.degree. Thickness % Light Transmission Y 86.5 86.6 86.8 86.1 85.6
87.6 85.1 84.0 85.4 69.9 Yellowness Index 4.1 5.8 4.9 3.9 2.8 5.4
6.1 1.6 13.8 26.0 x 0.3133 0.3152 0.3141 0.3131 0.3121 0.3146
0.3154 0.3112 0.3228 0.3351 y 0.3207 0.3220 0.3214 0.3204 0.3195
0.3220 0.3222 0.3178 0.3307 0.3447 L* 94.4 94.5 94.5 94.2 94.0 94.8
93.9 93.4 93.9 86.6 a* -0.6 -0.3 -0.6 -0.6 -0.6 -0.6 -0.3 -0.2 -0.7
-1.3 b* 2.2 3.0 2.6 2.1 1.6 2.9 3.1 0.8 7.5 13.7 C* 2.3 3.0 2.7 2.2
1.7 3.0 3.1 0.8 7.5 13.8 Turbidity Haze 3.6 mm 0.31 0.34 0.65 0.52
0.27 0.26 0.39 0.29 0.27 5.96 Thickness % (milky)
TABLE-US-00006 TABLE 4 Properties After Conversion into Keatite
Glass Ceramic (Glazing Program 3) Example No. 21 22 23 24 25 Glass
No. 1 2 3 4 5 Maximum Temperature .degree. C. 1110 1120 1100 1100
1110 T.sub.max Transmission Standard 3.6 mm Illuminant C, 2.degree.
Thickness Light Transmission Y % 6.0 9.0 5.0 6.4 8.7 IR
Transmission % 80.9 79.9 67.1 68.3 68.6 1600 nm Color (Remission)
3.6 mm Thickness % L* 84.95 84.51 89.57 87.79 84.33 a* -3.80 -3.29
-2.04 -2.73 -2.91 b* -7.43 -6.23 -4.80 -6.30 -8.96 c* 8.34 7.04
5.22 6.87 9.44 .alpha..sub.20/700 10.sup.-6/K +0.86 +0.91 +1.2
+1.26 +0.95 Density g/cm.sup.3 2.515 2.515 2.538 2.544 2.512 X-Ray
Diffraction: Keatite Phase % -- 88 -- 83 -- Content Keatite
Crystallite nm -- >120 Size Example No. 26 27 28 29 30 Glass No.
6 7 8 9 10 Maximum Temperature .degree. C. 1090 1100 1090 1080 1080
T.sub.max Transmission Standard 3.6 mm Illuminant C, 2.degree.
Thickness Light Transmission Y % 4.5 5.1 5.0 3.9 0.7 IR
Transmission % 49.8 54.5 58.6 56.1 4.9 1600 nm Color (Remission)
3.6 mm Thickness % L* 90.91 88.75 87.22 87.32 96.08 a* -1.71 -1.14
-1.52 -1.02 -0.01 b* -2.98 -5.17 -7.56 -2.36 -1.27 c* 3.44 5.30
7.71 2.57 1.27 .alpha..sub.20/700 10.sup.-6/K +1.00 +1.00 +1.02
+1.05 +1.33 Density g/cm.sup.3 2.513 2.513 2.522 2.516 2.593 X-Ray
Diffraction: Keatite Phase % 86 -- 85 89 70-75 Content Keatite
Crystallite nm 99 >120 -- -- Size
[0093] Without further elaboration, it is believed that one skilled
in the art can, using the preceding description, utilize the
present invention to its fullest extent. The preceding preferred
specific embodiments are, therefore, to be construed as merely
illustrative, and not limitative of the remainder of the disclosure
in any way whatsoever.
[0094] In the foregoing and in the examples, all temperatures are
set forth uncorrected in degrees Celsius and, all parts and
percentages are by weight, unless otherwise indicated.
[0095] The entire disclosures of all applications, patents and
publications, cited herein and of corresponding European
application No. 06 005 593.6, filed Mar. 20, 2006, is incorporated
by reference herein.
[0096] The preceding examples can be repeated with similar success
by substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples.
[0097] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention
and, without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
* * * * *