Method of production of high-refractive thin glass substrates

Abstract

A redrawing method for the production of thin glasses is provided that allows redraw of high refractive index optical glasses. The includes the steps of providing a vitreous preform with a mean width B, a mean thickness D, and a refractive index n.sub.D of at least 1.68 in a redrawing device, heating at least a part of the preform, redrawing of the preform to a thin glass with a mean width b and a mean thickness d. The heated part of the preform exhibits, for the duration of at most 30 minutes, a temperature above a lower limit of devitrification of the glass. The glass of the preform exhibits a dependence of the viscosity on the temperature, which is characterized by a mean decrease of the viscosity with increasing temperature in an viscosity range of 10.sup.8 to 10.sup.5 dPas of at least 3*10.sup.5 dPas/K.

Claims

1. A thin glass comprising a refractive index of at least 1.68, an average thickness (d) of less than 2 mm, and a liquidus viscosity of less than 10.sup.3 dPas, wherein the refractive index is at least 0.001 smaller than a theoretical refractive index.

2. The thin glass of claim 1, wherein the liquidus viscosity is less than 10.sup.2.5 dPas.

3. The thin glass of claim 1, wherein the thin glass further comprises at least one fire-polished surface having a roughness R.sub.a of at most 20 nm.

4. The thin glass of claim 1, wherein the thin glass has a density of more than 2.6 g/cm.sup.3.

5. The thin glass of claim 4, wherein the density is more than 2.85 g/cm.sup.3.

6. The thin glass of claim 1, wherein the thin glass has a mean coefficient of linear thermal expansion .sub.+20/+300 C. of more than 7*10.sup.6 K.sup.1.

7. The thin glass of claim 6, wherein the mean coefficient of linear thermal expansion .sub.+20/+300 C. is more than 8.2*10.sup.6 K.sup.1.

8. The thin glass of claim 1, wherein the thin glass has a warp of less than 1500 m.

9. The thin glass of claim 8, wherein the warp is less than 300 m.

10. The thin glass of claim 1, wherein the thin glass is configured for a use selected from the group consisting of an OLED display glass, a LCD 2D display glass, a LCD 3D display glass, a lighting device, an OLED, a wafer-level-optic device, and a filter glass.

11. A method for the production of the thin glass of claim 1, comprising the steps of: providing a vitreous preform with an average width (B), an average thickness (D), and a refractive index of at least 1.68 in a redrawing device; heating at least a part of the preform; and redrawing the preform to the thin glass, with an average width (b) and the average thickness (d), wherein the heated part of the preform exhibits, for a duration of at most 30 minutes, a temperature above a lower limit of devitrification of the glass, wherein the preform comprises glass that has a viscosity dependent on temperature, the dependence having a mean decrease of viscosity with increasing temperature in a viscosity range of 10.sup.8 to 10.sup.5 dPas of at least 3*10.sup.5 dPas/K, and wherein the heating step comprises heating to a temperature (T.sub.2) at which the glass of the preform exhibits a viscosity of at least 10.sup.4 dPas and at most 10.sup.8 dPas.

12. The method of claim 11, wherein the dependence is a mean decrease of the viscosity in a viscosity range of 10.sup.8 to 10.sup.5 dPas of at least 5*10.sup.5 dPas/K.

13. The method of claim 11, wherein the duration is at least 3 seconds.

14. The method of claim 11, wherein the duration is at least 30 seconds.

15. The method of claim 11, wherein the duration is at most 15 minutes.

16. The method of claim 11, wherein the duration is at most 6 minutes.

17. The method of claim 11, further comprising the step of cooling the glass of the preform from a temperature that corresponds to a viscosity of 10.sup.12 dPas to a temperature that corresponds to a viscosity of 10.sup.13 dPas at a mean cooling rate of at most 1000 K/min.

18. The method of claim 17, wherein the mean cooling rate is at most to 500 K/min.

19. The method of claim 11, wherein the glass of the preform, before the heating step, is preheated at least partially, to a temperature (T.sub.1) where the glass exhibits a viscosity of 10.sup.10 to 10.sup.14 dPas.

20. The method of claim 19, wherein, during the preheating step, the temperature is higher in border areas of the preform than in a middle of the preform.

21. A thin glass comprising a refractive index of at least 1.68, an average thickness (d) of less than 2 mm, and a liquidus viscosity of less than 10.sup.3 dPas, wherein the thin glass has a warp of less than 1500 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows the setup of an exemplary embodiment of the invention of a redrawing device in a side view,

(2) FIG. 2 schematically shows a preform,

(3) FIG. 3 schematically shows the setup with a laser,

(4) FIG. 4 schematically shows the mode of operation of a possible radiant heater as heating facility,

(5) FIG. 5 shows the influence of the height of the deformation zone during the redrawing,

(6) FIG. 6 shows a possible thickness distribution,

(7) FIG. 7 shows an exemplary mean width b (gross width) of a redrawn thin glass component as well as the necessary drawing force each in dependence on the viscosity of the glass of the preform,

(8) FIG. 8 shows an exemplary ratio of the mean width b (gross width) to the mean thickness d (net thickness) of the redrawn thin glass component as well as the necessary drawing force each in dependence on the viscosity of the glass of the preform in the deformation zone,

(9) FIG. 9 shows the viscosity profile during redrawing after example 1, and

(10) FIG. 10 shows the viscosity profile during redrawing after example 2.

DETAILED DESCRIPTION

(11) In the following detailed description of preferred embodiments for clarity reasons same reference signs denote essentially same parts in or on these embodiments.

(12) FIG. 1 shows the schematic setup of an exemplary embodiment of the invention of a redrawing device in a side view. In the redrawing device a preform 1 is moved from top to bottom through the device. The redrawing device exhibits two heating facility 2, which are placed in a middle region of the device. In this embodiment the heating facilities are shadowed with blinds 3 in such a way, that a deformation region 4 is generated. A section of the preform 1, which is inside the deformation region 4, is heated in such a way, that it reaches the temperature T.sub.2. Also the deformation zone 5 with the height H is shown. The preform 1 is drawn down by a drawing facility 6, which is realized here in form of two driven rolls. Due to the fact, that the feeding facility 7, here also realized in form of rolls, pushes the preform 1 slower that the drawing facility 6 draws, the preform 1 deforms in the deformation region 4. The preform 1 therewith becomes thinner, the thickness after deformation d is smaller than that thickness before the deformation D.

(13) Before the preform 1 is led in the deformation region 4, it is pre-heated to the temperature T.sub.1 by means of a pre-heating facility 8, symbolized here by a burner flame. After passing the deformation region 4 the preform 1 is led in a cooling facility 9, which is symbolized here by an ice crystal.

(14) FIG. 2 shows schematically a preform with a length L, a thickness D and a width B. Also the border areas R are shown, which extend from the border of the preform in direction of the middle. Preferably the border areas R take up a part of at least 1% and at most 50% of the width of the preform, so that to each border area accounts for at least 0.5% and at most 25% of the width of the preform. Particularly the border areas R extent over a part of at least 2% and at most 30%, preferably at least 5% at most 20% and particularly preferably at least 7% at most 15% of the width of the preform. In the border areas the temperature is during the pre-heating preferably higher than in the middle of the preform, in particular at least 5 C. or at least 20 C. higher.

(15) FIG. 3 shows schematically the setup of a heating facility with a laser 10. The beam of the laser is guided on the glass by means of a scanning mirror 11. By means of the movement of the scanning mirror the deformation zone is heated equally. Not shown is an optional beam-shaping optics.

(16) FIG. 4 shows schematically the mode of operation of a possible radiant heater, which could be applied as heating facility 2. Depending on its distance to the preform 1 the height of the deformation zone 5 is different. In the figure also is shown, how by means of shadowing, respectively, a blind 3 the deformation zone 5 can be limited, to obtain a deformation zone 5 as low as possible. Thus, both the distance and the design of the heating facility can serve to the adjustment of the height of the deformation zone 5.

(17) FIG. 5 shows, how the widths of a glass product depend on the height of the deformation zone during the redrawing. It is recognizable, that a low deformation zone has the effect, that the decrease of the width of the preform is reduced.

(18) FIG. 6 shows, how the thickness d of a flat glass product is distributed over the width b of the product. It can be perceived, that the edges at the border area of the glass product are relatively narrow. The part, which exhibits a homogeneous low thickness, can be used for the application of the glass product, the edge generally has to be removed. With the method according to the invention, the yield is especially high.

(19) FIG. 7 shows exemplary the mean width b (gross width) of the redrawn thin glass component and the drawing force needed for the drawing each in dependence on the viscosity of the glass of the preform in the deformation zone for the case of a 4 mm thick and 400 mm wide preform, which is pulled into a 40 mm high muffle with 5 mm/min. The glass is pulled of with 200 mm/min. It is clearly recognizable, that the drawing force needed grows increasingly which increasing viscosity. Furthermore it is apparent, that the mean width b of the product obtained decreases increasingly with increasing viscosity.

(20) FIG. 8 shows exemplary the ratio of the mean width b (gross width) to the mean thickness d (net width) of the drawn glass component and the drawing force needed for the pulling out each in dependence on the viscosity of the glass of the preform in the deformation zone for the case of a 4 mm thick and 400 mm wide preform, which is pulled into a 40 mm high muffle with 5 mm/min. The glass is pulled off with 200 mm/min. It is apparent, that the width-thickness-ratio b/d of the product obtained decreases increasingly with increasing viscosity. Compared to the decrease of the mean width b with increasing viscosity shown in FIG. 6 the ratio b/d decreases relatively still more strongly with increasing viscosity.

LIST OF THE REFERENCE SIGNS

(21) 1 Preform 2 Heating facility 3 Blind 4 Deformation region 5 Deformation zone 6 Drawing facility 7 Feeding facility 8 Pre-heating facility 9 Cooling facility 10 Laser 11 Scanning mirror