Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2002-01-02
2004-08-17
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S013000, C501S066000
Reexamination Certificate
active
06777359
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to polarizing glasses employed in optical products such as optical isolators, and more particularly, to high performance polarizing glasses comprising geometrically anisotropic metallic silver particles. The present invention further relates to methods of manufacturing these polarizing glasses.
2. Description of Related Art
Polarizing glasses comprising geometrically anisotropic metallic silver particles can be manufactured by the methods described in Japanese Patent Application Examined Publication No. Hei 2-40619 (Referenced publication 1) and Japanese Patent Application Un-examined Publication No. Sho 56-169140 (Referenced publication 2).
In these methods, glass comprising a silver halide is heat-treated to deposit out the silver halide and the glass is elongated to lengthen the silver halide particles. The glass is then heat treated in a reducing environment to reduce the silver halide particles to silver, thereby manufacturing polarizing glass comprising geometrically anisotropic silver particles.
For example, the following method is specifically described in Referenced publication 1.
A method of manufacturing glass articles exhibiting excellent polarization in the infrared region of the spectrum from glasses containing silver halide particles therein selected from the group of AgCl, AgBr, and AgI, characterized by comprising steps in which (a) a batch for a glass containing silver and at least one halide selected from the group of chloride, bromide, and iodide is melted and the melt shaped into a glass body of a desired geometry; (b) the glass body is subjected to a heat treatment at least above the strain point but not in excess of 50° C. above the softening point of the glass for a period of time adequate to cause the generation of AgCl and/or AgBr and/or AgI particles therein, said particles ranging in size between 200-5000 Å; (c) the glass body is elongated under stress at a temperature above the annealing point but below that where said glass exhibits a viscosity of about 10
8
poises, such that the particles are elongated to an aspect ratio of at least 5:1; and (d) the elongated glass body is exposed to a reducing environment at a temperature between 250° C. and about 25° C. above the annealing point of the glass to reduce at least a portion of the silver halide particles in the glass to silver particles which is deposited in or on the elongated particles.
Referenced publications 1 and 2 disclose glasses employed in polarizing glass, for example, exhibiting photochromic characteristics and having a composition consisting essentially, expressed in terms of weight percent on the oxide basis, of 6-20 percent R
2
O (where R
2
O consists of 0-2.5 percent Li
2
O, 0-9 percent Na
2
O, 0-17 percent K
2
O, and 0-6 percent Cs
2
O), 14-23 percent B
2
O
3
, 5-25 percent Al
2
O
3
, 0-25 percent P
2
O
5
, 20-65 percent SiO
2
, 0.004-0.02 percent CuO, 0.15-1.3 percent Ag, 0.1-0.25 percent Cl, and 0.1-0.2 percent Br, the molar ratio R
2
O:B
2
O
3
ranging between about 0.55-0.85 when the composition is essentially free from divalent metal oxides other than CuO, and the weight ratio Ag:(Cl+Br) ranging between about 0.65-0.95.
In such types of polarizing glasses, the silver halide that is reduced in the silver halide crystal reducing step is just the outer layer portion, with silver halide crystals being present in the glass matrix in large quantity. When the silver halide exhibits photochromic characteristics, exposure to ultraviolet or visible light causes darkening and absorption of near infrared light, compromising the polarization characteristics of the polarizing glass, and in particular, causing a significant transmission loss.
Thus, Referenced publication 1 discloses a molar ratio of (R
2
O—Al
2
O
3
):B
2
O
3
<0.25 and essentially the absence of CuO in the above-recited composition exhibiting photochromic characteristics as a composition rendering polarizing glass non-photochromic.
Japanese Patent No. 2628014 (Referenced publication 3) discloses another type of non-photochromic polarizing glass. Referenced publication 3 points out the problem in Referenced publication 1 that, in the glass batch melt or during the heat treatment generating silver halide crystals, silver is reduced to a metallic state and silver halide crystals are not generated in the heat treatment the purpose of which is to generate silver halide crystals, and describes a non-photochromic, silver halide-comprising, polarizing glass composition in the form of a composition comprising essentially no silver and a quantity of CeO
2
adequate to effectively maintain the silver in the glass in an oxidized state. CeO
2
oxidizes silver, and is employed as an oxidizing agent for the silver in place of CuO, which is thought to cause photochromism, thereby preventing the development of photochromic characteristics.
In such polarizing glasses, it is extremely important to stabilize the glass because of the use of a heat treatment step to deposit silver halide. However, the above-recited composition of polarizing glass has drawbacks in that the glass is thermally unstable and the glass loses transparency during the course of the heat treatment, that is, a haze is generated in the glass as the result of the deposition of crystals other than silver halide crystals. As a result, light entering the polarizing glass is scattered and transmission loss increases. In recent years in particular, since higher extinction ratios and lower losses have been demanded of the polarizing glass employed in optical components in the field of optical communications and the like, the increase in transmission loss is a major problem.
Further, in the manufacturing of non-photochromic polarizing glass, the CeO
2
employed in Referenced publication 3 has the same oxidizing effect as CuO and effectively prevents the reduction of Ag, as indicated by the formulas given below. However, in methods adding an oxidizing agent such as CuO and CeO
2
, the quantities added to prevent reduction of Ag in the course of melting the glass must be changed based on the melt environment and melt conditions. Further, Cu
2+
, Cu
+
, Ce
4+
, and Ce
3+
ions coexist in the glass. Since the chemical equilibrium of these ions tends to vary with temperature, there is a risk that silver will be reduced to a metallic state in the subsequent heat treatment step used to form silver halides.
2CuO<=>Cu
2
O+O 2CeO
2
<=>Ce
2
O
3
+O
Even the further addition of CeO
2
does not fully prevent photochromism, but causes nucleation promoting the growth of deposits of undesirable crystals other than Ag halide crystals. This is problematic in that it increases transmission loss.
The present invention, devised in light of the above-described problems, has for its object to provide polarizing glass with low transmission loss and a high extinction ratio. A further object of the present invention is to provide a polarizing glass permitting the reduction of silver without the deposition of metallic silver in the heat treatment step for generating a glass melt and silver halide crystals essentially without the addition of oxidizing agents such as CuO and CeO
2
.
SUMMARY OF THE INVENTION
The composition of photochromic glass is similar to that of the base glass employed in such polarizing glasses. The present inventors are the inventors of record of Japanese Patent Application Examined Publication No. Sho 56-51143 (Referenced Publication 4) disclosing the composition of photochromic glass comprising silver halide crystals for use in eyeglasses.
In glass for use in eyeglass lenses, there is the technical problem of conforming to the standard refractive index (Nd 1.523). Japanese Patent Application Examined Publication No. Sho 56-51143 describes the effectiveness, when incorporating refractive index raising components in the form of TiO
2
and ZrO
2
, of keeping the quantity of TiO
2
low and incorporating ZrO
2
into a composition with little
Yamashita Toshiharu
Yoneda Yoshitaka
Bolden Elizabeth A.
Hoya Corporation
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