Optical Glass

This invention relates to an optical glass and, more particularly, to a high refractive and low dispersive optical glass having optical constants of 1.85 - 1.96 for Nd and 43-28 for .nu.d and consisting of a B.sub.2 O.sub.3 --La.sub.2 O.sub.3 --Gd.sub.2 O.sub.3 --WO.sub.3 --ZrO.sub.2 and/or Ta.sub.2 O.sub.5 system which is free of poisonous oxides such as ThO.sub.2, CdO and BeO and sufficiently stable to devitrification.

Various high refractive and low dispersive optical glasses containing the poisonous ingredients such as ThO.sub.2, CdO and BeO have hitherto been reported and attempts have been made to improve these optical glasses and make them harmless to humans, as disclosed in British Pat. No. 1183996, Japanese Laid-open Patent Publication Nos. 37410/1973 and 55705/1974. These improved optical glasses, however, cannot achieve a high refractivity and a low dispersion observed in the optical glasses before the improvement was made. Further, optical glasses of higher optical properties have a greater tendency to devitrification and are not suitable for mass production.

It is, therefore, an object of the present invention to eliminate the above described disadvantages of the prior art optical glasses by providing a novel high refractive, low dispersive optical glass free of the poisonous oxides.

Concentrated studies and repeated experiments made by the inventors of the present invention have resulted in a finding that the above object can be attained and a high refractive and low dispersive optical glass of 1.85 - 1.96 for Nd and 43 - 28 for .nu.d can be obtained by using an optical glass of a B.sub.2 O.sub.3 -- La.sub.2 O.sub.3 -- Gd.sub.2 O.sub.3 -- WO.sub.3 -- ZrO.sub.2 and/or Ta.sub.2 O.sub.5 system whose research data has hitherto been quite unknown.

According to the present invention, there is provided an optical glass containing a relatively small amount of B.sub.2 O.sub.3, a relatively large amount of La.sub.2 O.sub.3, an amount of a relatively wide range of Gd.sub.2 O.sub.3 and WO.sub.3, a proper amount of ZrO.sub.2 and/or Ta.sub.2 O.sub.5, and having the desired optical properties and stability. The optical glass according to the invention consists of a composition in weight percent of 10 to less than 17% B.sub.2 O.sub.3, 20 to 55% La.sub.2 O.sub.3, 0.5 to 38% Gd.sub.2 O.sub.3, 0.5 to 32% WO.sub.3, 0 to 10% ZrO.sub.2, 0 to 30% Ta.sub.2 O.sub.5, the total of said ZrO.sub.2 and Ta.sub.2 O.sub.5 being 2 to 35%, 0 to 5.5% SiO.sub.2, 0 to 5% GeO.sub.2, 0 to 16% Y.sub.2 O.sub. 3, 0 to 5% Tb.sub.4 O.sub.7, 0 to 38% Yb.sub.2 O.sub.3, the total of said Y.sub.2 O.sub.3, Tb.sub.4 O.sub.7 and Yb.sub.2 O.sub.3 being 0 to 40%, 0 to less than 2% TiO.sub.2, 0 to 31% Nb.sub.2 O.sub.5, 0 to 3% SnO.sub.2, 0 to 5% Al.sub.2 O.sub.3 0 to 5% In.sub.2 O.sub.3, 0 to 5% Bi.sub.2 O.sub.3 and one or more oxides selected from the group consisting of MgO, CaO, SrO, BaO, ZnO and PbO, the amount of or the total amount of said MgO, CaO, SrO, BaO, ZnO and PbO being 0 to less than 2%.

In the optical glass made according to the present invention, the above mentioned composition is selected for reasons stated below.

If the content of B.sub.2 O.sub.3 is less than 10%, a devitrification tendency considerably increases and mass production of the optical glass becomes difficult whereas if the content of B.sub.2 O.sub.3 exceeds 17%, the desired optical properties cannot be obtained.

La.sub.2 O.sub.3 is an essential ingredient for producing a high refractive and low dispersive optical glass and at least 20% of the La.sub.2 O.sub.3 content is required for satisfying the optical properties of the optical glass according to the present invention. If, however, the content of La.sub.2 O.sub.3 exceeds 55%, the devitrification tendency increases and a stable glass cannot be produced.

Gd.sub.2 O.sub.3 which provides the glass with optical properties similar to those provided by La.sub.2 O.sub.3 is necessary for preventing devitrification and maintaining stability required for achieving mass production of the optical glass. If the content of Gd.sub.2 O.sub.3 is less than 0.5%, such effects are reduced whereas if the content of Gd.sub.2 O.sub.3 exceeds 38%, phase separation occurs with a resulting difficulty in obtaining a homogeneous optical glass.

WO.sub.3 is an important ingredient which has been found to be capable of being contained in a fairly wide content range for increasing the refractive index and preventing devitrification. If the content of WO.sub.3 is less than 0.5%, such effects can scarcely be obtained whereas increasing the content of WO.sub.3 above 32% increases tinting of the glass to an undesirable degree.

ZrO.sub.2 contributes to preventing devitrification and is also effective for increasing the refractive index as compared with La.sub.2 O.sub.3. If, however, the content of ZrO.sub.2 exceeds 10%, the devitrification tendency increases instead of decreases.

Ta.sub.2 O.sub.5 provides the glass with substantially the same refractive index as La.sub.2 O.sub.3 and, if the content of Ta.sub.2 O.sub.5 is up to 30%, it contributes to making an optical glass which is stable to devitrification. If the content of Ta.sub.2 O.sub.5 exceeds 30%, the quantity of insoluble matters in the melt increases resulting in difficulty in making a homogeneous optical glass.

In the optical glass made according to the present invention, introduction of ZrO.sub.2 and/or Ta.sub.2 O.sub.5 as an ingredient is essential for preventing the devitrification tendency as well as maintaining the desired optical properties. If the sum of ZrO.sub.2 and/or Ta.sub.2 O.sub.5 is less than 2%, sufficient effects of these ingredients cannot be expected whereas if the sum exceeds 35%, the glass becomes hard to melt and the tendency to devitrification increases.

Ingredients to be described below are not essential ingredients for the optical glass according to the invention but they are effective for correcting the optical properties of the glass and preventing the devitrification tendency further if they are used within their proper content ranges.

SiO.sub.2 and GeO.sub.2 are effective for increasing viscosity of the molten glass and thereby preventing devitrification. If the content of SiO.sub.2 exceeds 5.5%, the quantity of insoluble matters in the melt increases resulting in difficulty in obtaining a homogeneous optical glass. If the content of GeO.sub.2 increases above 5%, the devitrification tendency increases, instead of decreases so that a stable glass cannot be obtained.

Y.sub.2 O.sub.3, Tb.sub.4 O.sub.7 and Yb.sub.2 O.sub.3 are similar to La.sub.2 O.sub.3 in their effects on the optical glass made according to the present invention and are effective for preventing devitrification if the content of Y.sub.2 O.sub.3 is up to 16%, that of Tb.sub.4 O.sub.7 is up to 5% and that of Yb.sub.2 O.sub.3 is up to 38%, respectively. However, the devitrification tendency does not decrease, but on the contrary increases, if the contents of Y.sub.2 O.sub.3 and Yb.sub.2 O.sub.3 exceed the above mentioned amounts, so that a stable glass cannot be obtained. Increasing of the content of Tb.sub.4 O.sub.7 above the above mentioned amount is undesirable because the glass is tinted. Further, if the sum of the contents of Y.sub.2 O.sub.3, Tb.sub.4 O.sub.7 and Yb.sub.2 O.sub.3 exceeds 40%, the devitrification tendency increases and the glass is likely to become unstable.

TiO.sub.2 and Nb.sub.2 O.sub.5 are effective for increasing the refractive index and preventing devitrification. If, however, the content of TiO.sub.2 exceeds 2%, the glass is tinted to an undesirable degree. If the content of Nb.sub.2 O.sub.5 exceeds 31%, the devitrification tendency sharply increases.

SnO.sub.2 is effective for preventing devitrification but increasing the content thereof above 3% increases tinting of the glass to an undesirable degree.

Al.sub.2 O.sub.3 is effective for increasing viscosity of the molten glass, preventing devitrification and preventing phase separation which occurs due to Gd.sub.2 O.sub.3. Increasing the content of Al.sub.2 O.sub.3 above 5%, however, increases the devitrification tendency.

In.sub.2 O.sub.3 and Bi.sub.2 O.sub.3 are effective for increasing the refractive index and preventing devitrification. If, however, the content of each of these ingredients exceeds 5%, tinting of the glass increases to an undesirable degree.

MgO, CaO, SrO, BaO, ZnO and PbO may effectively be used as melting agents for melting the SiO.sub.2 material. If, however, the amount of or the total amount of one or more of these ingredients exceeds 2%, the devitrification tendency increases.

One or more of the above described oxide glass ingredients, e.g., Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 and MgO, may be replaced, in part or in whole, with fluorides such as YF.sub.3, AlF.sub.3 and MgF.sub.2 for further facilitating melting of the glass and preventing devitrification. The content of fluorine in the glass should preferably be held within a range not exceeding 2%.

Examples of the composition (in weight percent) of the optical glass according to the invention as well as optical properties (Nd, .nu.d) and liquidus temperatures (.degree. C) thereof are shown in Table 1. In these examples, the liquidus temperatures were obtained by maintaining a glass grain of about 1 mm in a temperature gradient furnace for 30 minutes and thereafter observing the glass sample.

As will be apparent from the Table, these examples of the optical glass all possess high refractive and low dispersive characteristics. Further, as will also be apparent from the liquidus temperatures in the Table, the optical glass is sufficiently stable to devitrification. Since these optical glasses contain a relatively small amount of B.sub.2 O.sub.3, they are higher in viscosity in melting than conventional optical glasses containing B.sub.2 O.sub.3 and La.sub.2 O.sub.3 and, accordingly, require a relatively high melting temperature (1340.degree.-1420.degree. C). These optical glasses, however, can be readily manufactured by melting the raw materials in a platinum crucible or the like, refining and stirring the melt to homogenize it and thereafter pouring it in a mould and annealing it.