JPS63260836A - Glass composition for lens of refractive index distribution type - Google Patents

Abstract

PURPOSE:To obtain a glass composition having excellent chemical durability, providing lens of refractive index distribution type with a large angle of aper ture, comprising Li2O, Na2O, K2O, Al2O3, B2O3, TiO2, SiO2, ZrO2, GeO2, MgO, SnO2 and PbO having given mol%, respectively. CONSTITUTION:The glass composition has the following composition of 3-22mol% Li2O, 0-20mol% Na2O, 0-20mol% K2O, 3-20mol% Na2O+K2O, 0-20mol% Al2O3, 0-25mol% B2O3, 2-25mol% TiO2, 40-62mol%SiO2, 3-10mol% ZrO2, 0-20mol% GeO2, 0-20mol% MgO, 0-10mol% SnO2 and 0-15mol% PbO. The main characteristics of this inventions are that the glass composition contains 2-35mol% TiO2 as a component for raising refractive index of prepared glass and contains 3-10mol% ZrO3. Consequently, the composi tion has improved chemical durability and its refractive index distribution close to an ideal refractive index distribution near to the circumferential part.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a glass composition, and more particularly to a glass composition suitable for manufacturing a gradient index lens by an ion exchange method.

[Conventional technology]

A transparent cylindrical body whose refractive index changes continuously in the radial direction from the central axis toward the outer circumferential surface with an approximately square law distribution has a lens effect and is well known as a gradient index lens.

Both end surfaces of this gradient index lens are polished into parallel planes perpendicular to the optical axis, and the single lens is mainly used as a material for optical communication components, and the lens array in which many such lenses are closely arranged is used for copying machines and It has been widely used as a facsimile coupling element.

The ion exchange method is well known as a method for industrially producing this gradient index lens.

This method uses T1. +1. s, Li, R
This glass is immersed in a molten salt such as sodium nitrate to perform ion exchange between the cations (for example, TI" and Na") to change the refractive index. The idea is to form a distribution.

Conventionally, many studies have been conducted on glasses containing Tl and Cs as glasses for gradient index lenses, but the former allows for lenses with a large aperture angle θ (θ = about 20°), but has very large chromatic aberration. , and the latter has a relatively small chromatic aberration, but has the disadvantage that the maximum aperture angle is only about 6 degrees. These drawbacks have become a major problem in response to the need for higher apertures and higher resolutions for the above-mentioned lenses as the performance of copying machines, facsimile machines, etc. has increased in recent years.

For this reason, Li-containing glasses have been increasingly studied in recent years. In the case of this glass, chromatic aberration is further reduced by ion exchange in sodium nitrate molten salt than in the case containing Cs, and the aperture angle is also 9° to 12°.
(Japanese Unexamined Patent Publication No. 58-125632)
．． Special Publication No. 59-41934).

However, the conventional 1. The aperture angle of lenses obtained with i-containing glasses is still not sufficient, and there is a need to develop glass compositions that can have a larger aperture angle, in other words, a larger difference in refractive index.

In addition, since conventional Li-containing glasses contain a large amount of alkali in their composition, they have poor chemical durability, are easily eroded by molten salt during ion exchange, and tend to cause discoloration on polished lens end surfaces.

[Problems that the invention attempts to solve]

An object of the present invention is to eliminate the drawbacks of the conventional Li-containing gradient index lens compositions, and to provide a glass composition for obtaining a gradient index lens with better chemical durability and a larger aperture angle. It is about providing.

Another object of the present invention is to provide a glass composition for obtaining a gradient index lens with a large effective field of view by making the refractive index distribution extremely close to the ideal distribution from the center to the periphery of a cylindrical lens.

[Means for solving problems]

That is, the present invention is a glass composition for a gradient index lens having the following composition expressed in mol%.

Li2O3~22 Na2O 0~20 2O 0~20 However, Na2O+K2O3~20 81203 0~20 8203 0~25 Tin22~25 Si02 40~62 Zr0. 3-10 GeO30-20 Mg0 O-20 Sn02 (1-10 pbo o-15
By containing 25 mol% of ZrO□ and 3 to 10 mol% of ZrO□, the chemical durability is improved and the refractive index distribution is made close to the ideal distribution up to the peripheral portion of the lens.

Next, the reason for limiting the range of the glass composition of the present invention will be described.

Li2O is a component that forms a refractive index distribution, and is contained in the glass in an amount of 3 to 22 mol%. If it is less than 3 mol%, a sufficient refractive index difference cannot be obtained by ion exchange, and 22
If the amount exceeds mol%, devitrification of the glass occurs. Numerical aperture is 9°
In order to obtain the above lens, it is desirable that the content be 8 mol % or more, and if it is 20 mol % or less, devitrification can be completely prevented, so the yield of glass molding is good and it is more preferable.

Na2O and K2O serve to widen the vitrification area and improve the solubility of the glass composition. In addition, the driving force of ion exchange between LX+ and Na'' or Ni1 is Na2O in the glass composition when the molten salt is Na salt, and K2O in the glass composition when the molten salt is crab salt. Therefore, the higher the number, the higher the ion exchange rate.

The amount of (Na20+K2O) is 3 mol% or more and 20 mol% or less. If (Na20◆K2O) is less than 3 mol%, the ion exchange rate will be significantly reduced, and if it exceeds 20 mol%, the chemical durability will be reduced. In addition, K2O in the glass composition when the molten salt is Na salt, and Na2O in the glass composition when the molten salt is crab salt, inhibit the ion exchange between Li+ and Na+ or Ni+, so they are not mixed together. However, they may be mixed as long as they do not adversely affect the formation of the refractive index distribution.

Al2O3 and B2O3 can be contained up to 20 mol% and 25 mol%, respectively, but if this is exceeded, the former increases viscosity and becomes difficult to melt and mold the glass, while the latter deteriorates chemical durability.

Tie is an essential component for increasing the difference in refractive index, and its effect is quite large in Li-containing glass. Li
Even if the amount is the same, the difference in refractive index is usually several times different between when Tie2 is included and when it is not. The effect of Tie2 is particularly large at 2 mol% or more, and when it exceeds 25 mol%, devitrification tends to occur.

The effect of increasing the refractive index difference of TfO gradually becomes saturated when the amount of TiO□ exceeds 8 mol%, so in order to further increase the refractive index difference, PbO and MgO were added by 15
It is very effective to add up to 20 mol%. If this upper limit is exceeded, in the case of PbO, the glass becomes easily deformed and the ion exchange rate decreases. In the case of MgO, devitrification tends to occur, making it difficult to mold glass. Furthermore, the addition of PbO and MgO also helps to improve chromatic aberration, realizing a lens with a high aperture and low chromatic aberration.

SiO□ is the main component of glass network formation, and if it is less than 400 mol%, devitrification and chemical durability will occur, and if it exceeds 62 mol%, the content of refractive index distribution forming oxides and other oxides will decrease. is limited, making it impossible to obtain a sufficient refractive index difference, and as the viscosity increases, high temperatures are required for glass melting, reducing workability.
ZrO2 is a characteristic component of the present invention and can be contained up to 10 mol%. If this value is exceeded, devitrification tends to occur.

ZrO significantly improves the chemical durability of glass, and is particularly effective in preventing erosion by molten salt during ion exchange.

For example, when a glass rod that does not contain ZrO and a glass rod that contains Z"02 with their side surfaces polished are immersed in molten sodium nitrate salt kept at 500°C for K2O hours and then taken out.
In the former case, the side circumferential surface becomes a matte surface with irregularities of several tens of micrometers, but in the latter case, there is almost no change. This erosion of the glass surface by the molten salt causes the refractive index distribution to be disturbed at the lens periphery, and the effective field diameter of the lens is considerably reduced.

Furthermore, if the glass surface is eroded, the symmetry of the refractive index distribution deteriorates, resulting in an increase in astigmatism and deteriorating the optical performance of the lens. Furthermore, this surface erosion causes fine racks and reduces the mechanical strength of the lens. The above-mentioned problems can be solved by containing z'02, and it becomes possible to manufacture lenses with a large effective field of view with a high yield.

Regarding the content of Z'02, if it is less than 3 mol%, the above effects cannot be sufficiently obtained, so it is necessary to have a content higher than that, and a content of 4 mol% or more is desirable.SnO also has the same effect as Z'02. The above effect is further enhanced by containing 10 mol% as an upper limit.

If this value is exceeded, devitrification tends to occur. Since SnO is a component that is easily exposed to continuous light, it is usually desirable to melt the glass in an oxidizing atmosphere when it is contained in an amount of several mol % or more. The content of SnO2 may be adjusted depending on the balance with other oxides. 1 Usually, the effect is sufficiently improved by containing it in an amount of about 1 to 5 mol %. Further, the amount of (ZrO, ◆SnO□) is desirably 5 to 12 mol %, and if it exceeds this amount, devitrification tends to occur.

The effects of ZrO2 and SnO2 improve the chemical durability of the lens after ion exchange, and also contribute to improving the weather resistance of optical elements using this lens.

Gem2 is contained up to 20 mol% in order to increase the refractive index of the glass. If this value is exceeded, stall is likely to occur.

In the present invention, in addition to the above-mentioned components, the following oxides can be contained as glass stabilizers in the following ranges.

CaOQ~6 SrOO~[i Ba0
0~5zn0 0~5 Rb2O0~5 La2
O30~5YtOs O~3 Gd20s
O~3 Ga2O30~31r+, O,, o~:l
Ta2(160~5 Nb2O50~5Ce
O, G~3 WO30~3 ^s20. (
1 to 2 sb2o, o to 2 (Example) The glasses of Examples and Comparative Examples shown in Table 1 each have approximately 8
Batch preparations are made using various raw materials so as to obtain 10 g, and after thorough mixing, 1350 g is prepared using a 31 platinum crucible.
After melting at 1300°C for 2 hours, casting and quenching to create a cullet, the next cullet was placed in the same 3IL platinum crucible, melted again at 1300°C for 4 hours, and then cast into a block, which was homogeneous with no bubbles or veins. Obtained a glass block that makes no sense. From this glass block, the diameter is 20a+m and the length is 200mm.
After cutting out m-sized glass round rods, the round rods were drawn using a glass drawing furnace to obtain glass rod samples with a diameter of 1 mm.

This glass rod was immersed in molten salt under the treatment conditions shown in Table 1 to perform ion exchange, resulting in Comparative Example No. 1.

Gradient index lenses No. 2 and Example No. 1N No. 6 were obtained. The center refractive index and refractive index distribution of these lenses were measured, and the aperture angle was calculated using the values.

Regarding durability, the water resistance of the lens was measured by a surface method. Specifically, a lens with a polished end face was immersed in warm water at 90°C, and the time required for precipitation of alkaline components and the like to begin to occur on the end face was measured.

As is clear from Table 1, the durability of the lenses of Examples containing 3 mol % or more of ZrO2 was significantly improved compared to the lenses of Comparative Examples. In particular, even when the amount of TiO2 was increased in order to further increase the aperture angle, no decrease in durability was observed, and a lens with an aperture angle exceeding 13° C. and good chemical durability could be obtained.

Furthermore, the lens of the example has a larger effective field diameter than that of the comparative example, exhibits good imaging performance over 90% of the diameter, and has a symmetrical refractive index distribution with almost no astigmatism. Ta.

〔Effect of the invention〕

As detailed above, the glass composition of the present invention makes it possible to obtain a gradient index lens with a large aperture angle, excellent chemical durability, and a large effective field diameter, and its industrial value is extremely large.

Claims (2)

[Claims] (1) A glass composition for a gradient index lens having the following composition expressed in mol%. Li_2O 3-22 Na_2O 0-20 K_2O 0-20 However, Na_2O+K_2O 3-20 Al_2O_3 0-20 B_2O_3 0-25 TiO_2 2-25 SiO_2 40-62 ZrO_2 3-10 GeO_2 0- 20 MgO 0~20 SnO_2 0~10 PbO 0-15 (2) Li_2O is 8-20, ZrO_3 is 4-10, SnO in mol%
_2 is 1 to 5, and ZrO_2+SnO_2 is 5 to 1
2. The glass composition according to claim 1, which has a composition of 2.