Process for the production of polarizing glass

Details Process for the production of polarizing glass Process for the production of polarizing glass

1998-08-19

2001-01-02

Vincent, Sean (Department: 1731)

Glass manufacturing

Processes

With chemically reactive treatment of glass preform

C065S032300, C065S032500, C065S033300, C065S102000

Utility Patent

active

06167727

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a process for producing polarizing glass articles. More specifically, the present invention relates to a process for producing infrared polarizing glass articles containing fine metallic particles having an anisotropic shape and being excellent in polarizing properties (with an extinction ratio of 40 dB or more). The articles can be used as polarizers utilized in miniature optical isolators adapted for optical communication, optical switches comprising a liquid crystal, an electro-optical crystal, a Faraday rotator etc., as well as electromagnetic sensors.
Glass articles containing shape-anisotropic fine metallic particles, such as silver or copper, dispersed in an oriented state therein have been known to behave as optical polarizers. In such glasses, the wavelength of the resonance absorption peak of the metal particles can vary depending upon the direction of incident polarized light. It has also been known that such polarizing glasses could be produced by reducing glasses containing elongated copper halide or silver halide particles.
Silver-free polarizing photochromic glasses and a production process thereof have been disclosed in U.S. Pat. No. 3,954,485. The disclosed glasses show polarizing properties in the darkened state of photochromic. The polarizing properties, however, are obtainable only in the darkened state and the resulting extinction ratio is as low as 10 dB, which is too low to apply them as optical isolators. According to the process described in the patent, starting glasses are stretched in a temperature region that imparts a viscosity of 1×10
7
to 1×10
9
poises to 100 to 1000 Å particles in the secondary phase (separated borate-rich phase) containing 20 to 50 Å copper halide/cadmium phase, so that the secondary phase can be elongated to have an aspect ratio of 2:1 to 5:1 whereby polarizing photochromic glasses are produced. However, when the intended polarizing glasses are to be produced by starting with a glass containing metallic halide particles, elongation of the particles is difficult and the elongated particles easily return to a spherical shape. This is because the interfacial energy generated between metallic halide particles and glass is higher than that generated between the secondary phase and glass on one hand and an aspect ratio of 10:1 or more is required on the other hand. Thus, it is impossible to elongate metallic halide particles at a viscosity of less than 1×10
8
poises.
A process for production of polarizing glass by starting with a glass containing copper halide particles is disclosed in the Japanese Patent Application Laid Open (JP-A-) No. 5-208844. The disclosed process comprises the steps of elongating copper halide particles in the glass by pulling or extruding the glass containing copper halide particles at a temperature at which the glass exhibits a viscosity of 10
8
to 10
11
poises and then heat treating the elongated glass under a reducing atmosphere to reduce the copper halide particles whereby a polarizing glass containing elongated shape-anisotropic metallic copper particles is produced.
A process for production of polarizing glass by starting with a glass containing silver halide particles is disclosed in the Japanese Patent Application Laid Open (JP-A-) No. 59-83951. Also in the process, the elongation of silver halide particles is effected by pulling or extruding the glass at a temperature at which the glass exhibits a viscosity of 10
8
to 10
13
poises, in a manner substantially similar to that of the glass containing copper halide particles.
In these glass stretching processes cited above, proper working temperature and cooling procedure should be applied so as to prevent the elongated particles from returning to a spherical shape. This is because, when the glass is elongated at a temperature at which the glass exhibits a viscosity lower than 10
8
poises, the elongated particles are likely to return to a spherical shape and thus it is substantially impossible to obtain a polarizing glass.
Moreover, in the processes mentioned above, a high stress is required so as to achieve the elongation of silver halide particles. Such high stress, however, may exceed the practical maximum breaking stress of the glass when applied to the glass in the above-defined viscosity range. Hence, the glass would frequently break or fracture during the elongation step irrespective of the nature of the particles contained therein. The same problem occurs in the process of the Japanese Patent Application Laid Open (JP-A-) No. 5-208844 comprising the elongation of copper halide particles to obtain a polarizing glass. The break or fracture of the glass during the elongation step may remarkably decrease the factory production efficiency of the polarizing glass, thus should not be neglected from the viewpoint of practical use.
Under these circumstances, the above-cited Japanese Patent Application Laid Open (JP-A-) No. 59-83951 has disclosed a process for drawing a composite (laminated) glass, according to which the glass particles may be elongated without breaking the glass.
The glass stretching step of the drawing process described in this document is schematically shown in FIG.
2
. In
FIG. 2
, a glass to be redrawn is composed of a potential polarizing core glass
9
and a surface glass
10
. A blank
8
is passed through a heater part of a redrawing furnace indicated by the arrow
11
, where it is heated and elongated under tension applied by pulling rolls
12
, whereby the blank can be converted to a laminated polarizing glass sheet or strip
13
. This document reports that if a silver halide-containing glass is subjected to redrawing or stretching under the conditions under which the glass exhibits a viscosity of about
101
poises, a polarizing glass could be produced without giving rise to glass break or fracture during the stretching step. According to this process, in order to avoid breaking of the stretched article, the potential polarizing core glass
9
is coated with glass
10
which exhibits a considerably low viscosity so that there remains scarcely any tensile stress on the surface of the glass.
However, when a laminated polarizing glass is to be produced according to the process mentioned above, the first problem is that there would still remain the glass having a low viscosity in the form of a surface layer even after the completion of the glass elongation. Hence, it would take a lengthy period of time to reduce the metallic halide particles contained in the core glass. In fact, the reduction of the metallic halide particles in the core glass can be started only after a gaseous reducing agent has passed through the surface layer.
The second problem is that, even if the glass could be prevented from breaking, since a glass containing metallic halide particles is coated with a superficial glass having a low viscosity, the elongated glass would be difficult to be cooled efficiently and hence the elongated particles will still retain the tendency of returning to a spherical shape.
That is, there has been as yet unknown a process for efficiently producing a polarizing glass while avoiding glass breakage or fracture during the elongation step and preventing the elongated particles from returning to a spherical shape as well.
In laboratory scale, a polarizing glass containing shape-anisotropic metallic particles is produced by pulling a glass containing metallic halide particles in the form of rod or plate from both ends. With such a process, however, it is only in the central part of the elongated sample that particles elongated to the desired state (aspect ratio) can be obtained and thus this process is unable to be practically used as such from the viewpoint of production efficiency.
To be practical, a starting glass containing metallic halide particles should be converted into a glass containing elongated metallic halide particles while minimizing glass loss.
Moreover, in order to avoid the insertion loss and the deformation of transmi

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