GB2106497A - ThiO2- and CdO-free, optical glass with refractive indices 1.73- 1.88 and Abbe numbers 35-52 - Google Patents

Abstract

The glass has the following composition: <IMAGE> in which from 1 to 4% of the total quantity of O<2>-ions are replaced by F- ions.

Description

SPECIFICATION ThO2- and CdO-free, optical glass with refractive indices 1.73-1.88 and Abbe numbers of 35- 52 The present invention relates to optical glasses with refractive indices of 1.73-1.88 and Abbe numbers of 35-52, which are free from the toxic oxides of CdO and ThO2, and which can be produced to an excellent degree of devitrification-stability, and which furthermore involve relatively low batchcomposition costs by comparison with known optical glasses of similar rating because expensive Ta2O5 can be dispensed with.An essential characteristic of these glasses resides in the simultaneous presence of W03 and fluoride-ions on which depends the said high devitrification stability, and the absence of alkaline earth- and alkali-oxides which have an opposite effect; alkaline earth- and alkalioxides notoriously form ionic fluorides with a high tendency to crystallisation The possibility of introducing inter alia fluoride ions in the glass was described in German OS 27 56 1 60. However, glasses according to this German OS have the drawback that they also contain SiO2 which readily leads to clouding of the glass; moreover, refractive indices of no higher value than 1.80 can be achieved, besides which they are rather dear because they contain in excess of 1 7 wt% of Gd2O3. Lastly they are not sufficiently stable in regard to devitrification to the extent which must be demanded for industrial production because they contain amounts of the rare-earth-oxides La2O3+Gd2O3+Y2O3 in excess of 58 wt%.

German AS 26 33 946 describes glasses containing fluorides on the basis of B2O3-La2O3-Y2O3; however, these glasses necessarily contain the expensive Tea205 in order to attain adequate stability in respect of devitrification since evidently the excellent stabilising effect of WO3 in co-operation with fluoride failed to be understood at the time.

The same applies to the very broadly drawn German AS 26 52 747; it is true that these glasses may optionally contain WO3 but there is no provision made for a partial substitution of oxygen by fluoride ions.

Glasses according to German OS 27 54 255 fall partially within and overlap the position rating in the nd-vd-diagram of the present application, but these glasses lack and addition of WO3 and fluoride ions by way of crystallisation-inhibitor component.

The glasses according to the invention contain from 1 9 to 37 wt% of B203 as chief vitrifer and from 1 to 20 wt% of WO3 as auxiliary vitrifer, with 1-4% of the 02-ions of the glass being substituted by 2 F- ions. Further substantive components in these glasses are La2O3 at the rate of 39 49 wt% and Y203 at the rate of 5-10 wt% for the purpose of increasing refractive index in order to achieve the desired optical values. The total percentage of rare earth oxides is between 44 and 55 wt%. For improved refractive index it is also possible to add from 3 to 9 wt% of ZrO2.

14% of the oxygen ions are substituted by fluoride ions. Less than 1% fails to show the desired clearly perceptible lowering of crystallisation tendency but has a negative effect on flow behaviour during casting (too much lowering of viscosity).

Further potentially present components are ZnO (0--14 wt%) Nb2O5 (0--9 wt%) TiO2 (0--8 wt%) PbO (0--12 wt%) and Ta2O5 (0--4 wt%), but in view of its high price Ta2O5 should be wholly dispensed with. It is known that all of the above mentioned components form fluorides of covalent character or complex fluorides, in other words, that the addition of these components has no adverse effect on devitrification stability. They serve to achieve higher refractive indices or to pre-set specifically desired Abbe values.

Besides having an inhibiting effect on crystallisation tendencies the fluorions make it easier to melt down the frit. LaF3 or YF3 wiil be preferred as fluoride-introducing components.

Glasses according to the invention are produced as follows: The oxides (carbonates, nitrates, or fluorides) are weighed to the specified percentage quantities, a smaller percentage of refining agents, e.g. As2O3 is added and the whole thoroughly mixed. This mixture is then melted down in a platinum crucible at about 1200--13000, refined homogenized and stirred down to the pouring temperature of 1 030-1 0700C. Finally it is poured into a ferrous mould.

The following table illustrates some examples of compositions according to this invention. Table

1 2 3 4 5 6 B203 31,35 21,05 36,50 20,30 27,10 29,45 La2O3 45,65 41,10 39,25 41,50 48,35 44,95 Y2O3 8,80 5,25 5,20 5,10 6,65 6,75 WO3 1,15 8,10 2,60 18,55 3,80 1,65 ZrO2 7,20 5,35 4,05 5,30 6,45 6,45 ZnO 2,60 - 12,40 - 1,60 0,85 Nb2O5 3,25 7,45 - 2,80 1,10 3,00 TiO3 - - - - - 6,90 PbO - 11,70 - 6,45 - Ta2O5 - - - - 2,95 - nd 1,7880 1,8700 1,7360 1,8610 1,8110 1,8281 vd 47,40 36,00 51,00 36,20 44,80 38,90 F-% 1,5 2,3 3,5 3,2 2,0 1,9 F-%=number of 02- ions in% which have been replaced by 2 F-.

Claims (7)

Claims

1. A ThO2- and CdO-free, optical glass with a refractive indexes of 1.73-1.88 and an Abbe number of 35-52, having the following composition: B203 19-37 wt% La2O3 39-49 wt% Y203 5-10 wt% La203+Y203 44-55 wt% WO3 1-20 wt% ZrO2 3-9 wt% in which from 1 to 4% of the total quantity of 02- ions are replaced by F- ions.

2. A glass according to Claim 1 further containing: 0-14 wt% ZnO.

3. A glass according to Claim 1 or 2 further containing: 0-9 wt% Nb2O5.

4. A glass according to Claim 1, 2 or 3, further containing: 0-8 wt% TiO2.

5. A glass according to any one of Claims 1 to 4, further containing: 0-12 wt% PbO.

6. A glass according to any one of Claims 1 to 5, further containing 0-8 wt% Ta2O5.

7. A glass according to Claim 1, having any one of compositions 1 to 6 in the accompanying table.