U.S. patent application number 13/951119 was filed with the patent office on 2013-11-28 for method for producing glass or glass ceramic and in particular glass or glass ceramic article.
This patent application is currently assigned to Schott AG. The applicant listed for this patent is Schott AG. Invention is credited to Markus Borrmann, Roland Dudek, Helga Goetz, Gerhard Hahn, Sybill Nuettgens, Wolfgang Schmidbauer, Friedrich Georg Schroeder.
Application Number | 20130316142 13/951119 |
Document ID | / |
Family ID | 36202532 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130316142 |
Kind Code |
A1 |
Nuettgens; Sybill ; et
al. |
November 28, 2013 |
Method for producing glass or glass ceramic and in particular glass
or glass ceramic article
Abstract
In order to obtain glass or glass ceramic materials having
increased strength, a method is provided for producing glass or
glass ceramic articles, which comprises: producing an initial glass
body, mounting the initial glass body on a gas cushion between a
levitation support and the initial glass body, and at least
partially ceramizing the initial glass body on the levitation
support. The levitation support comprises at least one continuous
surface region having at least one gas feed region where levitation
gas for the gas cushion is fed out from the levitation support, and
at least one gas discharge region where gas from the gas cushion is
at least partially discharged into the levitation support.
Inventors: |
Nuettgens; Sybill;
(Frankfurt, DE) ; Schmidbauer; Wolfgang; (Mainz,
DE) ; Dudek; Roland; (Bad Kreuznach, DE) ;
Schroeder; Friedrich Georg; (Ingelheim, DE) ; Hahn;
Gerhard; (Allenfeld, DE) ; Borrmann; Markus;
(Mainz, DE) ; Goetz; Helga; (Heidesheim,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Schott AG |
Mainz |
|
DE |
|
|
Assignee: |
Schott AG
Mainz
DE
|
Family ID: |
36202532 |
Appl. No.: |
13/951119 |
Filed: |
July 25, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11720915 |
Feb 4, 2009 |
8516851 |
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PCT/EP2005/013245 |
Dec 9, 2005 |
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13951119 |
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Current U.S.
Class: |
428/156 ;
428/174; 428/220; 428/410; 501/32; 65/25.2 |
Current CPC
Class: |
C03B 23/035 20130101;
C03C 10/0027 20130101; Y10T 428/24479 20150115; C03B 32/02
20130101; C03B 35/24 20130101; C03C 14/00 20130101; C03B 35/243
20130101; C03C 10/0009 20130101; C03C 3/097 20130101; C03C 3/085
20130101; C03B 29/12 20130101; Y10T 428/24628 20150115; C03C
10/0045 20130101; Y10T 428/315 20150115 |
Class at
Publication: |
428/156 ;
428/174; 65/25.2; 428/220; 428/410; 501/32 |
International
Class: |
C03C 14/00 20060101
C03C014/00; C03B 29/12 20060101 C03B029/12 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 11, 2004 |
DE |
10 2004 059 728.6 |
Claims
1. A glass or glass ceramic article produced in accordance with a
process comprising the steps of: producing an initial glass body;
mounting the initial glass body on a gas cushion between a
levitation support and the initial glass body; and at least
partially ceramizing the initial glass body on the levitation
support; wherein the levitation support has at least one continuous
surface region with at least one gas feed region where levitation
gas for the gas cushion is fed out from the levitation support, and
at least one gas discharge region where gas from the gas cushion is
at least partially discharged into the levitation support.
2. The glass or glass ceramic article as claimed in claim 1,
wherein the glass or glass ceramic article has an average breaking
fall height which is at least 15 cm per millimeter thickness of the
glass or glass ceramic article in a range of between 3 and 5
millimeters thickness with the format 10.times.10 cm and a steel
ball dropping test with a steel ball of mass 200 g.
3. The glass or glass ceramic article as claimed in claim 1,
wherein the glass or glass ceramic article has an average breaking
fall height which is at least 20 cm per millimeter thickness of the
glass or glass ceramic article in a range of between 3 and 5
millimeters thickness with the format 10.times.10 cm and a steel
ball dropping test with a steel ball of mass 200 g.
4. The glass or glass ceramic article as claimed in claim 1,
wherein the glass or glass ceramic article has an average breaking
fall height which is at least 30 cm per millimeter thickness of the
glass or glass ceramic article in a range of between 3 and 5
millimeters thickness with the format 10.times.10 cm and a steel
ball dropping test with a steel ball of mass 200 g.
5. The glass or glass ceramic article as claimed in claim 1,
comprising a material that is breakproof on average with a fall
height of more than 45 centimeters in the form of a 3 millimeter
thick plate plane on both sides with the format 10.times.10 cm and
in a steel ball dropping test with a steel ball of mass 200 g.
6. The glass or glass ceramic article as claimed in claim 1,
comprising a material that is breakproof on average with a fall
height of at least 60 centimeters in the form of a 3 millimeter
thick plate plane on both sides with the format 10.times.10 cm and
in a steel ball dropping test with a steel ball of mass 200 g.
7. The glass or glass ceramic article as claimed in claim 1,
comprising a material that is breakproof on average with a fall
height of at least 80 centimeters in the form of a 3 millimeter
thick plate plane on both sides with the format 10.times.10 cm and
in a steel ball dropping test with a steel ball of mass 200 g.
8. The glass or glass ceramic article as claimed in claim 1,
comprising a material that is breakproof on average with a fall
height of at least 140 centimeters in the form of a 5 millimeter
thick plate plane on both sides with the format 10.times.10 cm and
in a steel ball dropping test with a steel ball of mass 200 g.
9. The glass or glass ceramic article as claimed in claim 1,
comprising a material that is breakproof on average with a fall
height of at least x centimeters at least 60 centimeters in the
form of a thick plate plane on both sides with the format
10.times.10 cm and with a thickness in the range of from 3 to 5
millimeters in a steel ball dropping test with a steel ball of mass
200 g, where x is given by the interpolation relation: x=(140 cm-55
cm)/2 mm*(plate thickness in mm-3 mm)+55 cm.
10. The glass or glass ceramic article as claimed in claim 1,
comprising a plate which is smooth on both sides.
11. The glass or glass ceramic article as claimed in claim 1,
comprising a plate which is knopped on one side.
12. The glass or glass ceramic article as claimed in claim 1,
wherein the article has a fire-polished surface.
13. The glass or glass ceramic article as claimed in claim 1,
comprising a curved plate.
14. The glass or glass ceramic article as claimed in claim 13,
wherein the glass or glass ceramic article has uniaxial
curvature.
15. The glass or glass ceramic article as claimed in claim 1,
comprising a material of at least one of the systems:
SiO.sub.2--Al.sub.2O.sub.3--Li.sub.2O,
SiO.sub.2--Al.sub.2O.sub.3--MgO,
SiO.sub.2--Al.sub.2O.sub.3--BaO.
16. The glass or glass ceramic article as claimed in claim 1,
comprising a material that has at least one of the oxides
TiO.sub.2, ZrO.sub.2, P.sub.2O.sub.5.
17. The glass or glass ceramic article as claimed in claim 1,
comprising a material that has the following components in percent
by weight: TABLE-US-00004 Li.sub.2O 2.5-5.5%, Na.sub.2O 0-3.0%,
K.sub.2O 0-3.0%, .SIGMA.Na.sub.2O + K.sub.2O 0-4.0%, MgO 0-3.0%,
CaO 0-2.5%, SrO 0-2%, BaO 0-3.5%, ZnO 0-3.5%, Al.sub.2O.sub.3
18-27%, SiO.sub.2 52-75%, TiO.sub.2 1.0-5.5%, ZrO.sub.2 0-3.0%,
SnO.sub.2 <1.0%, .SIGMA.TiO.sub.2 + ZrO.sub.2 + SnO.sub.2
2.0-6.0%, P.sub.2O.sub.5 0-8.0%.
18. The glass or glass ceramic article as claimed in claim 1,
comprising a material that has the following components:
TABLE-US-00005 Component: Proportion: SiO.sub.2 63-67,
Al.sub.2O.sub.3 22 to 24, Li.sub.2O 2.5-4, Na.sub.2O 0-0.5, MgO
0.2-0.8, BaO 0-0.5, ZnO 0-0.5, ZrO.sub.2 2-2.5, TiO.sub.2 1.5-2.5,
As.sub.2O.sub.3 1.0-2.5, P.sub.2O.sub.5 0.5-1.5, K.sub.2O 0-0.5,
V.sub.2O.sub.3 0-0.1,
or the following components: TABLE-US-00006 Component: Proportion:
SiO.sub.2 65-69, Al.sub.2O.sub.3 19 to 21, Na.sub.2O 0-0.5, MgO
0.5-1.5, BaO 0.5-1.5, ZnO 1.5-2, ZrO.sub.2 1.5-2, TiO.sub.2 2-3,
As.sub.2O.sub.3 0.5-1, K.sub.2O 0-0.5,
in percent by weight.
19. The glass or glass ceramic article as claimed in claim 1,
comprising a material that has at least one of the fining agents
As.sub.2O.sub.3, Sb.sub.2O.sub.3, CeO.sub.2, SnO.sub.2.
20. The glass or glass ceramic article as claimed in claim 1,
comprising a material that has at least one coloring oxide.
21. The glass or glass ceramic article as claimed in claim 1,
comprising a material whose composition comprises from 0.02 to 0.6
percent by weight of V.sub.2O.sub.5.
22. The glass or glass ceramic article as claimed in claim 1,
comprising a material whose composition has at least one compound
from a group which comprises Cr, Mn, Fe, Co, Cu, Ni, Se, Cl
compounds.
23. The glass or glass ceramic article as claimed in claim 1,
wherein the glass ceramic article is ceramized surface-wide.
24. The glass or glass ceramic article as claimed in claim 1,
wherein the glass ceramic article is not chemically
prestressed.
25. A hotplate comprising a glass or glass ceramic article as
claimed in claim 1.
26. A flameproof pane or fireproof glazing comprising a glass or
glass ceramic article as claimed in claim 1.
27. A stove windowpane comprising a glass or glass ceramic article
as claimed in claim 1.
28. Security glazing, in particular armored glass, particularly
preferably bulletproof armored glass, comprising a glass or glass
ceramic article as claimed in claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Patent application U.S. Ser. No. 11/720,915, filed Jun. 5,
2007, and Patent application DE 10 2004 059 728.6, filed Dec. 11,
2004, are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates in general to the production of glass
or glass ceramics and in particular to the production of glass or
glass ceramics by mounting on a gas bed during ceramization, as
well as to glass or glass ceramic articles produced according to
the method.
BACKGROUND OF THE INVENTION
[0003] Glass ceramic plates are employed inter alia extensively as
cooking surfaces for modern hobs, as oven windowpanes or fireproof
glazing. With glass ceramics, the strength and surface condition
plays an essential role for their fields of use.
[0004] Glass ceramics used for hobs currently often have a knopped
underside structure, in order to increase the strength of the glass
ceramic plates for hob applications.
[0005] In particular, the knops provide protection of the underside
of the ceramic plate against strength-reducing damage, which is
incurred especially during the process of ceramizing the glass
ceramic.
[0006] The strength is finally achieved by the knops absorbing
damage to the lower sides.
[0007] A disadvantage with such knopped glass ceramic plates,
however, is inter alia the scattering of light which is sent
through the glass ceramic plate. It is therefore impossible, or
possible only with difficulty, for displays or structures below the
glass ceramic plate to be made visible without distortion.
[0008] In the past, glass ceramic hob surfaces which have smooth
surfaces on both sides have also been produced for hobs. With a
plate thickness of 4 mm, an average strength value of 36 cm fall
height was achieved in a test format of 10.times.10 cm.
[0009] In modern ceramizing methods, strengths of up to 50 cm fall
height are achieved for 4 mm thick glass ceramic plates smooth on
both sides. Such glass ceramic hob surfaces smooth on both sides
with a sufficient strength have previously been obtained by
re-polishing the glass ceramic plates. This process cannot be
carried out economically for industrial production.
[0010] For the application of glass ceramic plates for stove and
oven windowpanes, it is likewise necessary on the one hand to
achieve a certain strength. On the other hand, the surfaces of the
glass ceramic should be free of damage, since damage to the surface
impairs the optical transmission and the esthetic impression.
Damage furthermore constitutes potential sites where the glass can
easily break by impact.
[0011] The glass ceramic plates currently used primarily comprise
damage due to the ceramization process, which has a visually
perturbing effect and significantly reduces the strength.
[0012] The strength properties are determined by the quality of the
surface. In a known ceramization process, a green glass plate to be
ceramized is placed on a ceramic support plate, in which case
separating means may be used between the green glass plate and the
support plate. In this case, the glass ceramics which have been
ceramized on a support generally comprise damage on the side that
faces the support plate.
[0013] Another method uses suspended ceramization. In this method,
the green glass plate is mounted in suspension at one end. Although
the glass ceramic is mounted contact-free in suspended
ceramization, so that no damage can occur on a support, it is
nevertheless difficult to achieve a planarity of the ceramic glass
plates that meets requirements. Furthermore, the entire area of the
glass ceramics ceramized in suspension cannot be utilized, since
the plates become damaged in the region of the suspension
points.
[0014] Another way of producing high-strength glass ceramics is
provided by chemically prestressing the article after ceramization.
In this regard, see patent specification DE 1803540. A disadvantage
with this method, however, is that a further process step is
necessary in which the glass ceramic article additionally needs to
be heated. The chemical prestressing method can furthermore be
applied only, as indicated in the patent specification, to very
particular compositions.
[0015] It is known from GB 1383202 to mount a plate to be ceramized
on a gas cushion between a support and the plate. To this end, the
support comprises perforated tiles, through which the gas for the
gas cushion is delivered. The gas delivered through the perforated
tiles flows between the plate and the support to the edge of the
plate, where it finally escapes. The effect of this arrangement,
however, is that only comparatively small glass plates can be
ceramized. For large glass plates, the pressure which is created
under the glass plate is too great, so that curvature of the plate
takes place.
[0016] Furthermore, GB 1383202 proposes to use combustion gas as a
levitation gas for the gas cushion. This, however, is
disadvantageous for the properties of the glass ceramic, since the
combustion gases on the one hand contain particles which are
preferentially deposited on the glass ceramic and therefore lead to
a strength reduction. Likewise, readily scaling metals and
combustion products in the oven space likewise lead to the glass
plate being contaminated on its surface.
[0017] DE 10045479 describes a method for the contactless mounting
and transporting of flat glass, which likewise involves mounting on
a gas cushion. To this end the support has a segmented structure,
in which the gas is supplied through openings in the segments and
can escape again through the gaps between the segments.
[0018] U.S. Pat. No. 5,078,775 describes a gas cushion support with
a diaphragm, the upper side of which has slotted gas feed and gas
outlet openings. The gas outlet openings are in communication with
gas outlet chambers in the diaphragm. The gas feed openings are
connected via manifold shafts to the lower side of the diaphragm.
On the lower side of the diaphragm, compressed gas is supplied
which flows through the manifold shafts and the gas feed openings
to the upper side, where it generates a gas cushion on which a
glass plate can then be mounted. With such an arrangement in which
the feed gas is supplied perpendicularly to the upper side,
however, the gas comes only comparatively briefly in contact with
the walls of the manifold containers, so that full heat exchange
does not take place and the gas can therefore flow into the gas
cushion at a temperature which may differ from the temperature of
the diaphragm and in particular the temperature of the supported
glass.
SUMMARY OF THE INVENTION
[0019] It is an object of the invention to provide glass or glass
ceramic materials, which in particular have a smooth fire-polished
and/or knopped surface, the materials having a significantly
increased strength compared with known glass ceramics together with
less optically perturbing damage of the surface.
[0020] This object is directly achieved in a very surprisingly
simple way by a method for producing a glass or glass ceramic
article as well as a glass or glass ceramic article.
[0021] Accordingly, the method according to the invention comprises
[0022] producing an initial glass body, [0023] mounting the initial
glass body on a gas cushion between a levitation support and the
initial glass body, and [0024] at least partially ceramizing the
initial glass body on the levitation support. The levitation
support has at least one continuous surface region with at least
one gas feed region where levitation gas for the gas cushion is fed
out from the levitation support, and at least one gas discharge
region where gas from the gas cushion is at least partially
discharged into the levitation support.
[0025] In contrast to the device disclosed in U.S. Pat. No.
5,078,775, in which the gas is applied via manifold shafts through
the diaphragm, according to the invention the gas which is supplied
to the gas cushion is preferably also delivered through one or more
chambers arranged in the levitation support, before it emerges
therefrom. In this way, in conjunction with the gas discharge
chamber or chambers, not only a particularly homogeneous pressure
distribution but also good temperature equilibration is achieved.
Owing to the residence time of the gas in the gas feed, its
temperature is matched better to the temperature of the diaphragm
and of the mounted glass plate or glass ceramic plate. This
arrangement and procedure according to the invention for mounting
the glass or the glass ceramic helps to achieve a very uniform
temperature distribution along the mounted plate. This also leads
in a surprisingly simple way to an increased strength of glass
ceramic plates which are mounted according to the invention during
the ceramization.
[0026] For effective heat transfer between the levitation gas
flowing out of the diaphragm and that flowing into it, it is
advantageous for the at least one gas feed chamber and at least one
gas discharge chamber to comprise closed channels, which extend in
the direction along the bearing surface inside the diaphragm. The
term closed channel is accordingly intended to mean a channel which
is bounded by a wall in the manner of a tube. The channels may in
particular be configured so that they are closed or at least
partially closed from the bottom surface of the levitation support,
which lies opposite the bearing surface. A closed channel in the
context of the invention does not mean a fully sealed cavity,
however, since at least one inlet opening for the gas feed chamber
and one outlet opening for the gas discharge chamber are provided
for the feed and discharge from the chambers or respectively to the
chambers of this embodiment.
[0027] The gas feed chamber may advantageously have a gas inlet
opening and the gas discharge chamber a gas outlet opening, which
are arranged so that the gas flow direction inside the gas feed
chamber and the gas discharge chamber extends transversely to the
normal of the bearing surface, and in particular along the bearing
surface. If the levitation gas in the gas feed chamber and the gas
discharge chamber in the diaphragm flows transversely to the normal
of the bearing surface, particularly in the direction along the
bearing surface, then a long flow path of the gas in the chambers
and concomitantly also effective heat transfer to the chamber walls
can be achieved even in a thin diaphragm as the levitation
support.
[0028] According to yet another preferred embodiment of the
invention, the levitation gas is fed into the gas feed chamber
through at least one gas-permeable connection on the lower side of
the diaphragm, or the levitation support. An antechamber,
preferably with a ceramic wall, arranged below the gas feed and gas
discharge chambers, may then be provided which is connected via at
least one gas-permeable connection to the gas feed chamber in order
to feed gas into the gas feed chamber. It would also be conceivable
for the antechamber itself to be a component of the diaphragm or
levitation support, or to use an integral diaphragm having at least
one antechamber, gas feed chamber and gas discharge chamber. In
each case, the gas feed chamber is at least partially closed at the
bottom, the levitation gas being introduced into the gas feed
chamber by a downwardly directed gas-permeable connection in the
diaphragm into the gas feed chamber. Although the gas is also
supplied from below, for example in the gas cushion support known
from U.S. Pat. No. 5,078,775, this diaphragm does not however have
chambers or channels for the gas feed which are closed or at least
partially closed at the bottom. Rather, the channels are entirely
open at the bottom. In contrast to this, the effect achieved by the
gas feed chamber with a gas-permeable connection, particularly in
the form of channels with a small cross section as the gas feed
chamber, is that the gas resides for a longer time in the gas feed
chamber before it emerges from the bearing surface via the further
gas-permeable connection. In this embodiment of the invention as
well, improved heat exchange with the diaphragm and therefore
particularly good temperature homogeneity in the gas cushion are
therefore achieved. In particular, with a device according to the
invention, the temperature of the levitation cushion can be kept
with a temperature gradient of less than 10.degree. C., preferably
less than 5.degree. C. in the direction along the bearing surface
by means of the diaphragm with chambers for gas feed and gas
discharge. Likewise conceivable, albeit somewhat more elaborate, is
a reversed configuration in which it is not the gas feed chambers
but the gas discharge chambers that are correspondingly attached to
an antechamber by means of a downwardly directed gas-permeable
connection, so that the gas from the levitation cushion travels via
the gas discharge chambers into the antechamber and is discharged
there.
[0029] The pressure drop in the chambers is preferably at most 0.5
mbar. If the levitation gas experiences a pressure drop of at most
0.5 mbar when flowing through the gas feed chamber and/or the gas
discharge chamber, then a particularly homogeneous pressure
distribution can also be achieved in the gas cushion.
[0030] The initial glass body may be produced by a conventional
melting and shaping process, before the initial glass body is then
ceramized according to the invention for example in a levitation
oven.
[0031] The least partial ceramization may in particular also
comprise nucleation. It is important to mount the glass or the
glass ceramic on a levitation cushion on the one hand whenever the
glass or the glass ceramic becomes very soft and/or whenever the
glass or glass ceramic plate expands or contracts strongly. In the
conventional ceramization process, the latter leads to a relative
movement between the glass plate and the support plate, the effect
of which is that scratches are formed on the more sensitive glass
plate. For ceramization, the glass is initially brought to a
nucleation temperature. This nucleation temperature lies at a
temperature for which the glass plate reaches viscosities in the
region of from c=10.sup.10 dPas. It is therefore precisely during
this phase that mounting on a maximally homogeneous pressure
profile is favorable for the properties, in particular planarity
and strength, of the article finally produced.
[0032] It is furthermore advantageous for the glass to have no
contact with the support at said viscosities. If contact of the
initial glass body with a support takes place, then adhesion of the
glass to the support may occur.
[0033] During the ceramization process, this nucleation phase is
followed by a crystal growth phase.
[0034] By the method according to the invention, a particularly
homogeneous temperature profile can also be achieved both along the
surface of the initial glass body and between the upper and lower
sides of the body, since there is no longer any contact with the
support. The support otherwise represents a heat reservoir which
can adapt only slowly and inhomogeneously to the temperature
changes occurring during the ceramization.
[0035] Particularly during the crystal formation of phase, the
temperature homogeneity is important for the future quality and
strength of the ceramized glass, so that levitational mounting is
particularly advantageous here.
[0036] The crystal growth may furthermore take place very rapidly
with a particular temperature adjustment. Because of this, so much
heat is released inside the glass plate that the initial glass body
become substantially softer and viscosities of c=10.sup.8 dPas or
less are reached. Even at such low viscosities, damage-free
processing is possible with levitational mounting according to the
invention on a gas consumed.
[0037] The least partial ceramization also need not comprise full
ceramization. For example, the material of the initial glass may be
only semi-ceramized in order to obtain desired physical properties
of the article finally produced.
[0038] In the course of the ceramization process, the glass plate
may change in its geometrical dimensions. This effect often occurs,
for example, because of the phase transition during ceramization.
This may involve both shrinkage and expansion of the glass plate.
These changes often occur both in the nucleation range and in
various phases of the crystal growth. In the conventional
ceramization method, a strong relative movement of the glass plate
relative to the support plate occurs in these phases, which may
lead to scratches in the product. According to an advantageous
refinements of the method according to the invention, therefore,
the glass plate or the initial glass body is mounted levitating on
the glass cushion while it shrinks or expands.
[0039] By mounting on a levitation support with a continuous
surface, which comprises regions in which gas is supplied and
discharged, a previously unachieved homogeneous pressure
distribution is achieved under the initial glass body. This leads
to particularly little deformation or even no longer any
deformation of the initial glass, even though it generally becomes
very soft during the ceramization, in which case viscosities of
10.sup.8 dPas or even lower may be reached. Owing to the
homogeneous pressure distribution, furthermore, essentially no
tensile or compressive stresses occur any longer during the
ceramization. Concomitantly with a homogeneous pressure
distribution, a particularly homogeneous pressure distribution in
the initial glass can furthermore be achieved by the method
according to the invention during the ceramization. The
substantially larger pressure gradients otherwise occurring under
the material in known methods lead to gas flows extending laterally
in the direction of the gradients. If the temperature of the gas
does not correspond accurately to the temperature of the initial
glass, then heat will be dissipated or supplied via the locally
differing gas flows. A homogeneous temperature distribution under
and over the initial glass, however, is important for the
ceramization in the initial glass and the planarity. In particular,
temperature differences even in the range of a few degrees can lead
to curvature of the glass.
[0040] It is furthermore advantageous for the levitation gas to be
at least partially recirculated. In this way, a circuit of the
levitation gas is achieved. This is advantageous particularly when
the glass or the glass ceramic is mounted in the hot state on the
gas cushion, for example for ceramization. The recirculated gas is
thus already heated when it enters the gas feed chambers, so that
the levitation support is cooled only little by the supplied
levitation gas. This on the one hand saves energy, and on the other
hand the homogeneity of the temperature distribution is perturbed
little or not at all. For temperature differences which are as
small as possible, it is furthermore advantageous for the
levitation gas to be taken from the environment of the initial
glass body, for example the oven space of a ceramizing oven, in
which the levitation support is arranged.
[0041] Compared with an article produced by suspended separation, a
glass or glass ceramic article produced according to the invention
is furthermore distinguished by a planer surface. With suspended
ceramization, in the softened state of the initial glass body, flow
of the material can take place in the gravitational direction i.e.
along the surface of the initial glass body, which leads to a
significantly inhomogeneous thickness of the finally ceramized
article. Restoring forces in the event of curvatures, with a
ceramized initial glass body lying according to the invention on a
plane support, are also much higher than with an initial glass body
mounted in suspension. In the ceramization according to the
invention, the surface of the initial glass body matches the
surface of the support, so that undesired curvatures are
compensated for. This effect does not occur with a freely suspended
body, however, so that curvatures may remain.
[0042] According to one embodiment of the invention, surface-wide
ceramization of the initial glass body is carried out so that a
glass wall glass ceramic article ceramized surface-wide is
obtained. This is possible owing to the levitational support, since
no or only minimal holding or guiding is necessary in order to hold
or guide the initial glass body. Conversely, for example in
suspended ceramization, no ceramization can be carried out in the
holding region since the material there becomes too soft or
significant damage occurs.
[0043] Devices which are suitable for carrying out the method
according to the invention, i.e. for producing glass or glass
ceramic articles according to the invention, are also described in
the Applicant's German application filed on the same day as the
present invention with the title "Method and Device for the
Contactless Transport or Support of Glass or Glass Ceramic", the
disclosure of which is also fully incorporated in the
subject-matter of the present invention.
[0044] The production of the initial or preliminary glass body may
advantageously also comprise the separation of sections from an
initial glass web. The separated sections may then be ceramized
separately. This avoids subsequent coating of the ceramized
material, which may cause damage reducing strength in the glass
ceramic.
[0045] Conversely, the crystallization process in the initial glass
is not perturbed by the method according to the invention. This
correspondingly leads to a glass ceramic, or a glass or glass
ceramic article producible by the method according to the
invention, having novel and surprising properties. A glass or glass
ceramic article, which is producible by the method according to the
invention, is intended to mean an article having a material which
may comprise glass and/or in particular glass ceramic and/or
semi-ceramized glass.
[0046] The glass ceramic articles producible according to the
invention, have an increased strength without chemical
prestressing, so that prestressing is obviated according to one
embodiment of the invention chemical, i.e. the a glass ceramic
article according to the invention is not chemically prestressed
therein.
[0047] In order to characterize the strength of the glass or glass
ceramic articles produced according to the invention, standardized
dropping tests may be carried out as a strength measurement. In
this case the plate to be tested is cut into samples with a defined
format (100 mm.times.100 mm) or produced in this defined format,
and tested by means of a ball dropping test. The ball dropping test
is carried out by letting a steel ball with a defined mass (m=200
g) and a defined diameter (O 36 mm) for a freely onto the center of
the sample from an initial height. If the sample survives this fall
without breaking, then the fall is repeated with an increased fall
height. This iterative method, with a fall height respectively
increased in a defined way, is carried out until breaking of the
sample occurs. The fall height at which breaking occurs is taken as
a measure of the strength of this sample. The strength of the
entire plate to be tested, or a batch of produced articles, is
given by the average value of the individual strengths of the
samples cut from it.
[0048] Owing to the production method, and article producible
according to the invention thus in particular has a significantly
increased breaking strength compared with known glass ceramic
materials, which is manifested by a correspondingly increased
average fall height in the test described above. This increased
strength is already exhibited by glass ceramic articles which are
not additionally prestressed, in particular without chemical
prestressing. The glass or glass ceramic articles according to the
invention are thus distinguished by an average breaking fall height
which is at least 15 cm per millimeter thickness of the glass or
glass ceramic article with a format of 10.times.10 cm. An average
breaking fall height of 18 cm per millimeter thickness of the glass
or glass ceramic article is also readily achieved or exceeded.
These values were verified particularly in a thickness range of
between 3 and 5 millimeters of the material of the article, but
also apply for other material thicknesses above and below this
range since there is an essentially linear relation between
breaking strength and average breaking fall height.
[0049] According to initial discoveries, the increased strength of
the glass ceramic articles producible according to the invention is
due inter alia to a vitreous film forming on the surface of the
article.
[0050] In general 20 cm per millimeter thickness, or even at least
an 25 cm average breaking fall height per millimeter thickness of
the ceramic article are also achieved. Furthermore, and even
average breaking fall heights of 30 or more centimeters per
millimeter of plate thickness are achievable by optimizing the
production parameters.
[0051] These advantageous strength properties of the glass ceramic
articles producible according to the invention make it possible to
reduce the thickness for an equal strength compared with
conventionally produced articles, which inter alia lowers the
material costs and therefore the price of such articles.
[0052] The linear relation between plate thickness and breaking
fall height applies so long as the glass plate has sufficient
opportunity to absorb the impact of the ball by bending. In the
case of particularly thick glass plates, however, the plate may
break earlier. A departure from the linear relation may take place
for thick glass plates in the range beyond about 10 mm thickness,
in which case the gradient of the average value of the fall height
then generally decreases with an increasing plate thickness. The
linear relationship of the breaking fall height and the plate
thickness, which applies in wide ranges, is known inter alia from
J. L. Glathart, F. W. Preston: "The behaviour of glass under
impact"; in: Glass Technology, 1968.
[0053] A glass or glass ceramic article producible according to the
invention may in particular comprise a material that is breakproof
with a fall height of more than 45 centimeters in the form of a 3
millimeter thick plate plane on both sides with the format
10.times.10 cm. In general, breaking fall heights of at least 60 cm
are even achieved.
[0054] In particular, it is also possible to produce a glass or
glass ceramic article according to the invention so that its
material is breakproof on average with a fall height of at least 80
centimeters in the form of a 3 millimeter thick plate plane on both
sides.
[0055] According to another embodiment of the glass or glass
ceramic article according to the invention, it comprises a material
that is breakproof on average with a fall height of at least 140
centimeters in the form of a 5 millimeter thick plate plane on both
sides. This surpasses the strength of known glass ceramic articles
without chemical stressing, measured from the fall height, by more
than a factor of 2.
[0056] The values specified above relate to material with a
particular shape and thickness. This does not generally mean that
the glass or glass ceramic article per se must have a format of
10.times.10 centimeters, rather that a test body cut with the
respectively indicated thicknesses from the glass or glass ceramic
article has said average breaking fall heights. A glass or glass
ceramic article according to the invention may accordingly have
many other shapes and also other thicknesses.
[0057] The specifications regarding strength serve in particular to
characterize the material of the article, but not its shape and
thickness. If an article according to the invention has a shape
which is not plate-like or a different thickness, then, in order to
determine its mechanical properties by a dropping test, one or more
plates with a defined thickness may be produced and ceramized
according to the invention from the same initial glass in order to
carry out the dropping test. For example, strength values may be
obtained in a simple way for various plate thicknesses by
interpolating the values specified above. According to one
embodiment of the invention, the article according to the invention
therefore comprises a material that is breakproof on average with a
fall height of at least x centimeters at least 60 centimeters in
the form of a thick plate plane on both sides with the format
10.times.10 cm and with a thickness in the range of from 3 to 5
millimeters in a steel ball dropping test with a steel ball of mass
200 g, where x is given by the interpolation relation: x=(140 cm-45
cm)/2 mm*(plate thickness in mm-3 mm)+45 cm.
[0058] According to one embodiment of the method according to the
invention, a preliminary glass body is produced in the form of a
glass plate plane on both sides, for example with a thickness of 3
or 5 millimeters, and subsequently at least partially ceramized
levitationally, so that a corresponding glass or glass ceramic
articles move on both sides is obtained. Such articles are suitable
for example as a glass ceramic hotplate, a stop windowpane, as
flameproof or fireproof panes. The levitationally carried out
ceramization process according to the invention provides a
fire-polished surface of the article. Compared with a surface
subsequently polished mechanically, the fire-polished surface has
substantially less or even known surface damage, which also leads
to an increased strength of the article according to the invention
compared with such subsequently polished plates. Glass or glass
ceramic articles producible according to the invention may
therefore be used as security glazing. Such security glazing may in
particular be armored glass, or even bulletproof glass.
[0059] Knopped plates, as are often manufactured to date in order
to achieve sufficiently high breaking safety, may also be produced
and levitationally ceramized die the method according to the
invention. To this end, a corresponding preliminary or initial
glass body is produced particularly in the form of a glass plate
knopped on one side. For this embodiment of the method according to
the invention, a somewhat higher gas flow with a constant floating
height is generally needed when the knops lie on the side facing
the diaphragm. The knopped structure may, for example, be impressed
in a melting and shaping process via a knopped roller during
molding in one side of a preliminary glass web, in particular the
lower side of the glass web. The knop structure may have a regular
pattern of pimples, which are round or oval, or other
projections.
[0060] Such a glass or glass ceramic article producible according
to the invention, which corresponds in its external shape to a
conventionally produced knopped plate, also has a breaking safety
increased by at least 20% compared with such a conventionally
produced knopped plate.
[0061] A glass or glass ceramic article according to the invention
advantageously comprises a material of at least one of the systems
SiO.sub.2--Al.sub.2O.sub.2--Li.sub.2O,
SiO.sub.2--Al.sub.2O.sub.2--MgO,
SiO.sub.2--Al.sub.2O.sub.2--BaO.
[0062] The material may furthermore comprise at least one of the
oxides TiO.sub.2, ZrO.sub.2, P.sub.2O.sub.5 in a conventional
concentration, in order to influence the mechanical and optical
properties and the viscosity of the initial glass.
[0063] The production of an initial glass body may advantageously
comprise fining of the initial glass. An essentially bubble-free
initial glass is obtained by the fining, which makes a considerable
contribution to the strength of the articles according to the
invention. To this end the initial glass may be supplemented with
fining agents such as As.sub.2O.sub.2, Sb.sub.2O.sub.2, CeO.sub.2
or SnO.sub.2, which are therefore also found in the material of the
article finally produced.
[0064] Particularly in order to influence the optical properties,
for instant in order to impart the desired coloration to the
article according to the invention, at least one coloring oxide may
be added to the material of the initial glass.
[0065] According to one embodiment of the invention, the material
of the finished article or of the initial glass has the following
components:
TABLE-US-00001 Li.sub.2O 2.5-5.5%, Na.sub.2O 0-3.0%, K.sub.2O
0-3.0%, .SIGMA.Na.sub.2O + K.sub.2O 0-4.0%, MgO 0-3.0%, CaO 0-2.5%,
SrO 0-2%, BaO 0-3.5%, ZnO 0-3.5%, Al.sub.2O.sub.3 18-27%, SiO.sub.2
52-75%, TiO.sub.2 1.0-5.5%, ZrO.sub.2 0-3.0%, SnO.sub.2 <1.0%,
.SIGMA.TiO.sub.2 + ZrO.sub.2 + SnO.sub.2 2.0-6.0%, P.sub.2O.sub.5
0-8.0%.
[0066] The quantity specifications are given as weight proportions
in percent by weight. The summation sign ".SIGMA." denotes the sum
of the substance quantities of the components listed after the
summation sign.
[0067] According to a second embodiment of the glass or glass
ceramic article, its material has one of the following
compositions:
TABLE-US-00002 Glass 1 Glass 2 Component: Proportion: Component:
Proportion SiO.sub.2 63-67, SiO.sub.2 65-69, preferably 65.5
preferably 67.5 Al.sub.2O.sub.3 22 to 24, l .sub.he.sub.at19 19 to
21, preferably 23.2 preferably 20 Li.sub.2O 2.5-4, Na.sub.2O 0-0.5,
preferably 3.3 preferably 0.1 Na.sub.2O 0-0.5, MgO 0.5-1.5,
preferably 0.4 preferably 1.1 MgO 0.2-0.8, BaO 0.5-1.5, preferably
0.5 preferably 0.9 BaO 0-0.5, ZnO 1.5-2, preferably 0.05 preferably
1.6 ZnO 0-0.5, ZrO.sub.2 1.5-2, preferably 0.05 preferably 1.8
ZrO.sub.2 2-2.5, TiO.sub.2 2-3, preferably 2.2 preferably 2.7
TiO.sub.2 1.5-2.5, As.sub.2O.sub.3 0.5-1, preferably 2.09
preferably 0.81 As.sub.2O.sub.3 1.0-2.5, K.sub.2O 0-0.5, preferably
1.13 preferably 0.2 P.sub.2O.sub.5 0.5-1.5, preferably 1.3 K.sub.2O
0-0.5, preferably 0.3 V.sub.2O.sub.3 0-0.1, preferably 0.03
[0068] The proportions in the table above are likewise given in
percent by weight.
[0069] A dark coloration, as is often desired for hobs, may
advantageously be achieved by a composition of the material of the
article or the initial glass whose composition comprises from 0.02
to 0.6% by weight of V.sub.2O.sub.5. For a transparent article,
correspondingly, a composition which is essentially free of
V.sub.2O.sub.5 may advantageously be selected.
[0070] The composition of the initial glass wall the finally
ceramized material may furthermore advantageously comprise at least
one compound from a group which comprises Cr, Mn, Fe, Co, Cu, Ni,
Se, Cl compounds. Such compounds are suitable in particular to
support the coloration and adjust particular color loci.
[0071] According to one embodiment of the method according to the
invention, the ceramization of the initial glass body comprises
particularly clean electrical heating of the initial glass body,
preferably in a levitation oven.
[0072] The levitation gas may also be purified, for example by
means of a purifying filter, and thus kept as clean as possible in
order to prevent the precipitation of extraneous substances and
avoid a reduction in the strength due to this.
[0073] The levitation support may advantageously comprise at least
one diaphragm with a continuous surface region and at least one gas
feed chamber and at least one gas discharge chamber. These chambers
are preferably arranged respectively below the gas feed and gas
discharge regions. In this way, gas is supplied to the gas feed
region via a gas feed chamber and forms a gas cushion between the
continuous surface region and the initial glass body. On the other
hand, excess gas can pass through the gas discharge region into a
gas discharge chamber arranged underneath. The gas feed and gas
discharge can be adjusted by setting up a suitable pressure
gradient between the gas discharge chamber and the gas discharge
region, are respectively between the gas feed chamber and the gas
feed region. In order to achieve this, a pressure gradient may be
set up in a straightforward way between the gas feed and gas
discharge chambers. A suitably shaped diaphragm with chambers may,
in particular, be produced by extrusion.
[0074] According to a preferred embodiment of the invention, the
gas is fed to a perforated surface of the levitation support and
discharged via feed and discharge channels. The surface of the
levitation support may advantageously also comprise a porous
material, through which gas for the gas cushion is fed or
discharged. The levitation gas is transported by the air supply
system through the diaphragm into the gas cushion, which is formed
between the diaphragm and the glass plate. The pressure under the
initial glass is stabilized by the air discharge system, which
comprises in particular the gas discharge region and the gas
discharge chamber. Owing to the gas film stabilized in this way,
the initial glass body then flows over the diaphragm and is thus
mounted contact-free.
[0075] The extruded diaphragm is perforated, so that a maximally
homogeneous pressure profile is formed. This homogeneous pressure
profile is advantageous in order to achieve a high planarity of the
glass or glass ceramic articles produced according to the
invention.
[0076] The desired temperature profile for the ceramization with as
small as possible a temperature difference between the upper and
lower sides of the glass, can advantageously be produced by
temperature homogenization of the levitation gas taking place in
the gas feed chambers of the extruded diaphragm. The initial glass
body may be partially or fully mounted on the glass film during the
ceramization process.
[0077] According to one embodiment of the method according to the
invention, a particular chronological temperature profile is
executed during the ceramization process. For this temperature
profile, the initial glass body is first heated to a temperature
T1. This temperature lies for example in a range of from 650 to
800.degree. C. The body may be kept at this temperature for up to 4
hours, depending on the oven unit and the shape of the initial
glass body. The body is subsequently heated further to a
temperature T2 in a range of from 850 to 950.degree. C. The body
may then be kept at this temperature T2 for up to about 50 minutes.
The body is subsequently cooled again to room temperature.
[0078] As well as flat glass ceramic plates, for example, glass or
glass ceramic articles with a curved plate-like plate may also be
produced according to the invention. To this end, the levitation
support may have a curved surface. The initial glass body in the
heated state can then be curved by gravitational sinking over the
levitation support. It is advantageous in this case for levitation
gas to be supplied during the curvature process, so that the
curvature can take place contact-free. According to a refinement of
the invention, the article has curvature along one direction, i.e.
uniaxial curvature. Such an article may for example comprise a
uniform curvature, so that it has the shape of a cylinder lateral
surface segment. Nevertheless, the radius of curvature may also
change along the surface. The production of such a shaped article
may be carried out as described above, by gravitational sinking
over a diaphragm which in this case is uniaxially curved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0079] The invention will be described in more detail below with
reference to the appended figures. Parts which are the same and
similar are provided with the same references in the figures.
[0080] FIG. 1 shows a cross-sectional view through a levitation
support of a device for carrying out the method according to the
invention,
[0081] FIG. 2 shows a plan view of an embodiment of a levitation
support,
[0082] FIG. 3 shows a refinement of the embodiment shown in FIGS. 1
and 2,
[0083] FIGS. 4A and 4B show, with the aid of views of an embodiment
of a device for carrying out the method according to the invention,
the method steps for producing a curved glass or glass ceramic
article,
[0084] FIGS. 4C and 4D show with the aid of schematic views of
another embodiment of a device for carrying out the method
according to the invention, the method steps for producing a curved
article, and
[0085] FIG. 5 shows the dependency of the average breaking fall
height on the thickness of glass ceramic articles.
DETAILED DESCRIPTION
[0086] FIG. 1 represents a cross-sectional view of a levitation
support in the form of a diaphragm denoted overall by 1. The
diaphragm is preferably produced by extruding a suitable material,
such as a refractory ceramic. The diaphragm 1 has a number of
chambers 51, 52, 53, 71, 72, 73, which are arranged in the
diaphragm below the continuous bearing surface 3 arranged on one
side of the diaphragm. The chambers 51, 52, 53 are as gas feed
chambers and the chambers 71, 72, 73 are used as gas discharge
chambers for the levitation gas of the gas cushion, which is formed
between an initial glass body to be ceramized and the continuous
bearing surface 3 of the diaphragm by supplying the levitation gas
via the chambers 51, 52, 53. For illustration, FIG. 1 shows an
initial glass body 11, lying in levitation above the surface region
3 on a gas cushion or gas film 13, in the form of a plate which is
plane on both sides.
[0087] The surface region 3 is perforated and has gas feed channels
91, 92, 93 and gas discharge channels 101, 102, 103 as
perforations. The levitation gas is supplied and discharged via the
gas feed channels 91, 92, 93 and gas discharge channels 101, 102,
103 which are in communication with the surface region 1 and the
chambers 51, 52, 53 and 71, 72, 73. The gas flow direction is
represented by arrows in FIG. 1. To this end a pressure difference
is generated between the chambers 51, 52, 53 and 71, 72, 73, a high
pressure being set up in the gas feed chambers 51, 52, 53 than in
the gas discharge chambers 71, 72, 73. For the ceramization, the
initial glass body may then for example be heated electrically in
order to avoid precipitation. By a filter (not shown), the
levitation gas may be purified before entering the gas feed
chambers so that, for example, it is essentially free of suspended
particles that may accumulate on the surface of the initial glass
body in its softened state.
[0088] The levitation gas may advantageously be recycled by a
corresponding instrument. To this end the gas is taken from the
environment of the initial glass body 11, for instance an oven
space in which the diaphragm is arranged, and fed back to the
diaphragm 1 so as to achieve good temperature equilibration between
the gas cushion and the environment of the levitationally mounted
initial glass body 11.
[0089] The initial glass body 11 is mounted on the gas cushion 13
over the diaphragm 1 in particular whenever it shrinks or expands
and/or has low viscosities, at which the initial glass body could
otherwise adhere to the support. Shrinkage and growth processes
take place particularly during the crystal growth. Levitational
mounting during the crystal growth phase is also generally
favorable for the quality of the articles produced according to the
invention, since by mounting on the gas cushion it is possible to
achieve a particularly homogeneous temperature distribution along
the surface minimal temperature differences between the upper and
lower sides of the glass body.
[0090] During the ceramization, the temperature of the levitation
cushion is kept with a temperature gradient of less than 10.degree.
C., preferably less than 5.degree. C., in the direction along the
bearing surface of the levitation support. Particularly uniform
ceramization is achieved by this uniform temperature
distribution.
[0091] FIG. 2 shows a plan view of the continuous surface of one
embodiment of a levitation support 1 in the form of a diaphragm.
The surface region 3 of the levitation support 1 has gas feed
regions 151, 152, 153 at which levitation gas for the gas cushion
is supplied, and gas discharge regions 171, 172, 173 at which gas
from the gas cushion is at least partially discharged. The regions
151, 152, 153, 171, 172, 173 are marked by the dashed lines in FIG.
2. The gas is supplied via gas feed channels 95 arranged in the gas
feed regions 151, 152, 153, and the gas is discharged via gas
discharge channels 105 arranged in the gas discharge regions. For
the sake of clarity, only a few of the channels 95 and 105 are
labeled in FIG. 2. As represented in FIG. 1, the channels 95 and
105 are connected to chambers arranged below the surface region 3.
In particular, the chambers 51-53, 71--are formed as closed
channels which extend in the direction along the bearing surface on
the inside of the diaphragm 3 below the gas feed and the gas
discharge regions. The channels are also closed off at least
partially from the bottom surface of the diaphragm 1, which lies
opposite the bearing surface for the initial gas 11, so that there
is a thermal barrier to the lower side of the diaphragm 1.
[0092] FIG. 3 represents a refinement of the invention, in which
the gas is introduced into the gas feed chambers via an antechamber
6 arranged below the diaphragm 1. In this refinement of the
invention as well, the gas feed chambers 51, 52, 53 are bounded at
least partially at the bottom by a wall of the diaphragm 1. The gas
feed chambers 51, 52, 53 respectively have downwardly directed
gas-permeable connections in the form of channels 96 on the lower
side or bottom surface 4, for supplying gas from the antechamber 6.
The antechamber 6 is formed by an antechamber housing 5 attached to
the bottom surface 4. The antechamber housing 5 is preferably made
from ceramic material like the diaphragm 1, in order to avoid
contamination of the levitation gas.
[0093] By means of the diaphragm according to the invention, as has
been described by way of example with the aid of FIGS. 2 to 3, the
glass or the glass ceramic 11 is kept by the levitation cushion at
a height of at least 750 micrometers, preferably up to at most 2
millimeters, above the bearing surface.
[0094] The method steps for producing a curved glass or glass
ceramic article are represented in FIGS. 4A and 4B as well as 4C
and 4D by way of example with the aid of a schematic view of two
embodiments of a device for carrying out the method according to
the invention. First, a plate-like initial glass body 11 is formed
in the conventional way by molding. This is subsequently put into a
levitation of an 19 with a levitation support arranged therein in
the form of a diaphragm 1, so that the initial glass body 11 lies
above the surface region 3 as represented in FIG. 4A. The diaphragm
1 is constructed similarly as represented in FIGS. 1 to 3. In
contrast to the embodiments shown in FIGS. 1 to 3, the embodiment
of the diaphragm 1 shown in FIGS. 4A to 4D has a curved surface
region 3. This is represented a convexly curved surface region by
way of example in FIGS. 4A and 4B. Concave curvature or a
combination of convexly and concavely curved regions, for example
as in a corrugated surface, are however likewise possible. FIGS. 4C
and 4D show an embodiment with a concavely curved surface
region.
[0095] In order to keep the initial glass body floating above the
diaphragm 1, so as to avoid contact with the support 1, a gas
cushion between the support and the initial glass body 11 is
generated by gas supply via the gas feed regions of the surface
region 3. Advantageously, the initial glass body may also be
slightly laterally held or guided, in order to prevent the body
from drifting away. Such holding or guiding requires only minimal
holding forces during the levitational support. The points of
contact with the holding or guiding instrument can therefore be
kept very small, so that surface-wide ceramization of the initial
glass body is achieved.
[0096] Subsequently, by means of an electrical heating device 21
arranged in the levitation oven, the initial glass body is heated
until it softens. Owing to the force of gravity acting on the
initial glass body, it likewise becomes curved, the regions of the
initial glass body lying further away from the levitation support 1
sinking until an essentially homogeneous pressure distribution is
produced by the gas cushion. This situation is shown by FIGS. 4B
and 4D, respectively. The ceramization on the support 1 may be
incorporated with the curving process or carried out subsequently.
Particularly during the nucleation process, the glass generally
becomes very soft and can easily be curved by gravitational sinking
during this phase.
[0097] According to one embodiment of the invention, the diaphragm
comprises a surface curved uniaxially, i.e. along one direction, so
that glass ceramic articles correspondingly curved uniaxially are
obtained.
[0098] The properties of glasses or glass ceramic articles produced
according to the invention will be explained further by way of
example below with the aid of application examples.
[0099] For a first plate-like article produced according to the
invention with a thickness of 5 millimeters, the strength values
achieved were on average with a breaking strength at a fall height
of more than 140 centimeters. The composition of the glass ceramic
material of this article corresponded to glass type 2. This value
for a 5 mm thick glass plate exceeds by more than two times the
values of 60 cm fall height, which are otherwise conventionally
measured for non-prestressed glass ceramic plates of this
thickness.
[0100] For another 3 millimeter thick plate-like glass ceramic
article with a composition of the glass ceramic corresponding to
glass type 1, an average breaking fall height of more than 80 cm
was found. An average breaking fall height of at least 55
centimeter or 18 cm per millimeter thickness of the glass ceramic
is readily achieved or even greatly exceeded, as shown by the
example above, for plate-like glass ceramic articles produced
according to the invention.
[0101] Here as well, the fall height achievable for the average
breaking strength is about two times as great as the fall height of
about 40 cm otherwise achievable with known glass ceramic
articles.
[0102] The measured values are listed again more accurately in the
following table.
TABLE-US-00003 Fall height of Fall height of the average the
average breaking strength breaking strength [cm] for glass [cm] for
ceramic produced conventionally Glass Thickness according to the
produced glass system [mm] invention ceramic Glass 1 3 mm 87 40
Glass 2 5 mm 142 63
[0103] With the aid of this table, it is clear that the plates
produced according to the invention achieve a significantly higher
strength owing to the novel method of ceramization on a levitation
support with a continuous surface and gas feed and gas discharge
regions and the high pressure homogeneity thereby achieved in the
gas cushion and the uniform temperature distribution in the initial
glass body. The measurement values indicated were achieved without
additional chemical prestressing.
[0104] Reference will be made below to FIG. 5, which shows the
dependency of the average breaking fall height of the thick glass
ceramic articles with the aid of measured values. Conventionally
produced glass ceramic plates and glass ceramic articles according
to the invention are respectively represented. With the aid of this
diagram, it is again clear that in terms of strength a glass
ceramic article produced according to the invention far surpasses a
glass ceramic article ceramized conventionally while resting on a
support and in contact with the support.
[0105] With the aid of the measurement values of the conventionally
produced plates and FIG. 5, the linear correlation of the plate
thickness with the average breaking fall height can also be seen.
Thus, according to this linear relation, the conventionally
produced glass ceramic plates tested have an average breaking fall
height of about 12.5 cm per millimeter of plate thickness,
corresponding to the straight line denoted by "A". Conversely, the
measurement values shown in the FIG. 5 for the tested glass ceramic
plates according to the invention showed an average breaking fall
height of about 28.5 cm per millimeter of plate thickness. These
achieved strength values lie above the minimum values achievable by
their production method according to the invention, namely 15, 18,
20 or 25 centimeters average breaking fall height per millimeter of
plate thickness. The plates for the dropping test were ceramized in
an experimental arrangement. The temperature homogeneity and the
homogeneity of the pressure profile can be improved further in an
industrial application, so that average breaking fall heights of 30
cm per millimeter of plate thickness or even more are possible.
[0106] The considerably increased strength of plates produced
according to the invention was demonstrated in the exemplary
embodiments with the plate thicknesses 3 mm and 5 mm. It is clear
to the person skilled in the art that plates produced according to
the invention with a different plate thickness, for example between
3 mm and 5 mm, also have a correspondingly increased strength. The
increased to strength of plates produced according to the invention
with thicknesses of between 3 mm and 5 mm can be interpolated with
the aid of relevant literature. (See for example: J. L. Glathart,
F. W. Preston: "The behaviour of glass under impact"; in: Glass
Technology, 1968). Accordingly, for plate thicknesses of between 3
mm and 5 mm, increased strengths are obtained which can be
interpolated linearly from the strength values of the exemplary
embodiments.
[0107] It is clear to the person skilled in the art that the
invention is not restricted to the exemplary embodiments described
above, but may be modified in a variety of ways. In particular, the
features of the individual exemplary embodiments may also be
combined with one another.
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