Glass ceramic

Details Glass ceramic Glass ceramic

2002-02-05

2004-01-06

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Devitrified glass-ceramics

C501S007000, C428S426000, C428S428000, C428S432000, C359S359000

Reexamination Certificate

active

06673729

ABSTRACT:

The invention relates to a glass and a glass-ceramic comprising beta-quartz and/or keatite solid solutions, and to a process for producing them, and to their use as a substrate material for coating.
It is known that glasses from the system Li
2
O—Al
2
O
3
—SiO
2
can be converted into glass-ceramics comprising beta-quartz solid solutions and/or keatite solid solutions as the main crystal phases. These glass-ceramics are produced in a plurality of stages. After melting and hot-shaping, the material is usually cooled at temperatures in the region of the transformation temperature (Tg), in order to eliminate thermal stress. The material is cooled further to room temperature. The specified quality features of the glass body are investigated.
A second controlled heat treatment is used to crystallize the starting glass and convert it into a glass-ceramic article. This ceramicization takes place in a multistage thermal process in which first of all, by nucleation at temperatures of 600° C. to 800° C., nuclei are produced from TiO
2
or ZrO
2
/TiO
2
solid solutions. SnO
2
may also be involved in the nucleation. During the subsequent temperature rise, beta-quartz solid solutions grow on these nuclei at the crystallization temperature of 700° C. to 900° C. As the temperature rises further, in the range from 800° C. to 1100° C., these beta-quartz solid solutions are further transformed into keatite solid solutions. Depending on the composition, the stability range of the glass-ceramic containing beta-quartz solid solution is extensive. With some compositions, the temperature of transition to the keatite solid-solution phase lies up to 150° C. higher than the crystallization temperature of the beta-quartz solid solution glass-ceramic. With other compositions, the beta-quartz solid solutions are converted into keatite solid solutions almost without any transition. The transition to keatite solid solution is associated with crystal growth, therefore with increasing crystallite size. This leads to increasing light scattering. The light transmission is reduced to an increasing extent. As a result, the glass-ceramic article appears increasingly translucent and ultimately opaque. The high light transmission of the glasses and glass-ceramics allows effective assessment of quality. Shaped bodies with defects which are relevant to safety or jeopardize the specified product properties can be sorted out prior to further process steps.
A key property of these glass-ceramics is that it is possible to produce materials which have an extremely low coefficient of thermal expansion in the range from 20° C. to 300° C. and above of <1.5•10
−6
/K. With glass-ceramics which contain beta-quartz solid solutions as the main crystal phase, even materials with virtually no expansion are obtained in this temperature range. For use as substrate material for reflectors used in astronomy, glass-ceramics are modified in such a way that their zero thermal expansion lies in the temperature range of −50° C. to +50° C. which is important for this application. A glass-ceramic material of this type is produced under the name ZERODUR at SCHOTT GLAS.
A recent development is for these glass-ceramics also to be used in illumination engineering as a material for reflectors in applications in which, on account of miniaturisation and high luminous powers, too high thermal loads occur. Compared to the widespread reflectors made from borosilicate or aluminosilicate glass, these glass-ceramics satisfy extremely high demands with regard to the ability to withstand thermal loads and temperature gradients. In the reflectors, light sources which allow a high luminous intensity to be produced within a small volume are used. The light sources are based on the technical principle of high-power halogen lamps, arc lamps or gas discharge lamps. The radiation maximum from these ultrahigh power lamps lies at wavelengths of 1 &mgr;m, i.e. in the near infrared.
These glass-ceramics may be coated with metallic layers, such as aluminium, or with alternating layer systems of oxide substances. The multiple oxide layers use the interference principle and enable the visible light to be reflected while the incident infrared radiation is transmitted to the rear. The intention is for the substrate material to have a high IR transmission, so that it transmits the IR radiation to the rear without being heated to an unacceptable extent. Reflectors of this type are known as cold-light reflectors. Digital projection equipment and DVD or video recorder projection equipment are increasingly being equipped with glass-ceramic cold-light reflectors.
Glass-ceramics which are used as substrates for mirrors used in astronomy are described in DE-A-1902432 and U.S. Pat. No. 4,285,728. The shaping is produced by casting the molten glass into a refractory die. Prior to the mirror-coating, the glass-ceramics comprising beta-quartz solid solution as the predominant crystal phase which are obtained after the crystallization are initially ground and then polished. This process leads to the desired geometric contour and a low surface roughness. However, it is time-consuming and expensive.
JP-B-95037324 describes glass-ceramics made from beta-quartz or keatite solid solutions for use as reflective mirror substrate materials which, after the ceramicization, have a low surface roughness Ra of at most 0.03 &mgr;m even without polishing and have a composition in % by weight which comprises 50-65 SiO
2
, 18 -30 Al
2
O
3
, 3-8 Li
2
O, 3-5 TiO
2
+ZrO
2
, 0.3-7 RO(R=Mg, Ca, Zn, Pb or V) and up to 3 R
2
O (R=K, Na).
U.S. Pat. No. 4,438,210 describes transparent glass-ceramics comprising beta-quartz solid solution as the predominant crystal phase, which glass-ceramics, despite having relatively high contents of Fe
2
O
3
of up to 1000 ppm, are substantially colourless. The composition of the glass-ceramics, in % by weight, comprises 65-75 SiO
2
, 1-4 Li
2
O, 15-25 Al
2
O
3
, 0.5-2 ZnO, 0-2 Na
2
O and/or K
2
O, 2-6 TiO
2
, 0-2 ZrO
2
, 0-2.5 BaO, 0-1.2 F and 100-1000 ppm of Fe
2
O
3
.
It is an object of the invention to provide a glass and a glass-ceramic comprising beta-quartz and/or keatite solid solutions which are suitable for coating with a mirror coating, and to provide an economic and environmentally friendly process for producing the glass and glass-ceramic.
The object is achieved by a glass-ceramic having a composition in % by weight, based on the total composition, of:
Li
2
O
3.0-5.5  
Na
2
O
0-2.5
K
2
O
0-2.0
&Sgr; Na
2
O + K
2
O
0.5-3.0  
&Sgr; MgO + ZnO
<0.3
SrO
0-2.0
BaO
0-3.5
B
2
O
3
0-4.0
Al
2
O
3
19.0-27.0  
SiO
2
55.0-66.0  
TiO
2
1.0-5.5  
ZrO
2
0-2.5
&Sgr; TiO
2
+ ZrO
2
3.0-6.0
P
2
O
5
0-8.0
Fe
2
O
3
<200 ppm
F
0-0.6
as substitute for O
and, if appropriate, at least one refining agent, such as As
2
O
3
, Sb
2
O
3
, SnO
2
, CeO
2
, sulphate and chloride compounds.
The glass-ceramic according to the invention has
a low viscosity, which is advantageous for shaping by pressing, with a working point V
A
of <1300° C.
a good devitrification stability with an upper devitrification temperature which lies at most 50° C. above the working point V
A
a surface roughness of the glass and glass-ceramic without polishing of Ra<50 nm, preferably <20 nm
a thermal expansion of the glass-ceramic in the temperature range between room temperature and 300° C. of <1.2•10
−6
/K
a high transmission on the part of the glass and the glass-ceramic in the near infrared region at 1050 nm of >85% for a thickness of 4 mm.
For shaping by pressing or blowing, the glass is to have a low working point V
A
of <1300° C. As a result, the thermal loads in the region of the feeder, the outlet and for the press tools are reduced, so that the service lives are increased. The low viscosity also has a beneficial effect on the melting of the glass in the melting end and on the blowing quality of the glass obtained.
To avoid undesired devitrification of the mol

Affiliated with

Also associated with

Rating

Glass ceramic does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Glass ceramic, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Glass ceramic will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3198772