Optical glass and process for the production of optical...

Details Optical glass and process for the production of optical... Optical glass and process for the production of optical...

2001-05-24

2004-11-16

Sample, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S079000, C501S051000

Reexamination Certificate

active

06818578

ABSTRACT:

TECHNICAL BACKGROUND
1. Field of the Invention
The present invention relates to an optical glass, a glass preform made of it, an optical product and a process for the production of the optical product. More specifically, it relates to an optical glass which has high-refractivity and low-dispersion optical properties, has a low glass transition point and allows the stable operation of a heat-treating furnace for a long period of time, a glass preform which is made of the above optical glass and suitable for the formation of various optical products, an optical product made of the above optical glass, and a process for the production of the above optical product.
2. Explanation of Related Art
Conventionally, an optical glass having high-refractivity and low-dispersion optical properties contains a large amount of La
2
O
3
, Gd
2
O
3
, Y
2
O
3
, Ta
2
O
5
, ZrO
2
, etc., for attaining a high refractive index and a low dispersion property as is shown, for example, in JP-A-54-90218 and JP-B-54-6042, and glass-network-forming components such as B
2
O
3
and SiO
2
are contained in a small amount, so that the above optical glass is extremely highly liable to undergo crystallization. Since a glass that can be stably produced is limited in compositional range, commercially available optical glasses having a high refractive index and a low dispersion have a glass transition point Tg of over 720° C. Table 1 shows properties of high-refractivity low-dispersion optical glasses shown in brochures supplied by optical glass manufacturers.
TABLE 1
Refractive
Abbe's
Glass transi-
index
number
tion point
[nd]
[vd]
[Tg] (° C.)
A
1.88300
40.8
730
B
1.88067
41.01
758
C
1.88300
40.8
738
Conventional high-refractivity low-dispersion glasses have a very high temperature for viscous flows as is typically shown by glass transition points Tg, and for example, annealing treatment thereof has required a temperature of 710° C. or higher. Generally, most of furnaces for glass annealing are made of a stainless steel plate, and this material has a deformation temperature around 700° C. When the annealing is carried out at a temperature over 710° C., therefore, there is caused a problem that the above stainless steel plate undergoes deformation, so that it is difficult to operate the furnace for a long period of time.
Further, the production of a lens material by re-heat pressing also requires a very high temperature, which causes a heat-treating furnace to deteriorate sooner and hinders stable production.
Meanwhile, when a glass has a glass transition point Tg of 700° C. or lower, stable production has been achieved without causing any special load on the operation of facilities.
SUMMARY OF THE INVENTION
Under the circumstances, it is a first object of the present invention to provide an optical glass that has high-refractivity low-dispersion optical properties, has a low glass transition point and allows the stable operation of a heat-treating furnace for a long period of time.
It is a second object of the present invention to provide a glass preform that is made of the above optical glass and is suitable for forming various optical products, and an optical product made of the above optical glass.
It is a third object of the present invention to provide a process for efficiently producing an optical product made of the above optical glass.
For developing optical glasses having the above desirable properties, the present inventors have made diligent studies with regard to effects of compositions of components constituting glasses on optical properties, thermal properties and devitrification resistance. As a result, it has been found that an optical glass having high-refractivity and low-dispersion optical properties and having a glass transition point of 700° C. or lower can be obtained by controlling the ratio of the total content of La
2
O
3
, Gd
2
O
3
, Y
2
O
3
and Yb
2
O
3
, and the ratio of the total content of ZrO
2
, Ta
2
O
5
and Nb
2
O
5
, to the total content of SiO
2
and B
2
O
3
such that the above ratios come into specific ranges, or by forming a glass having a specific glass composition, and that the first object of the present invention can be accordingly achieved.
Further, it has been also found that the above second object of the present invention can be achieved by a glass preform and an optical product made of the above optical glass each.
It has been further found that a optical product made of the above optical glass can be efficiently produced by employing a specific step, and that the above third object of the present invention can be accordingly achieved.
The present invention has been completed on the basis of the above findings. That is, the present invention provides:
(1) an optical glass having a refractive index nd of at least 1.875, an Abbe's number &ngr;d of at least 39.5 and a glass transition point Tg of 700° C. or lower (to be referred to as “optical glass I of the present invention” hereinafter),
(2) an optical glass which is a borosilicate glass comprising at least one selected from La
2
O
3
, Gd
2
O
3
, Y
2
O
3
or Yb
2
O
3
and at least one selected from ZrO
2
, Ta
2
O
5
or Nb
2
O
5
, wherein the ratio (weight ratio) of the total content of La
2
O
3
, Gd
2
O
3
, Y
2
O
3
and Yb
2
O
3
to the total content of SiO
2
and B
2
O
3
is from 3.2 to 5 and the ratio (weight ratio) of the total content of ZrO
2
, Ta
2
O
5
and Nb
2
O
5
to the total content of SiO
2
and B
2
O
3
is from 1.1 to 1.5, and which has a refractive index nd of at least 1.875 and an Abbe's number &ngr;d of at least 39.5 (to be referred to as “optical glass II of the present invention” hereinafter),
(3) an optical glass which is a borosilicate glass comprising at least one selected from La
2
O
3
, Gd
2
O
3
, Y
2
O
3
or Yb
2
O
3
, at least one selected from ZrO
2
, Ta
2
O
5
or Nb
2
O
5
and ZnO, wherein the ratio (weight ratio) of the total content of La
2
O
3
, Gd
2
O
3
, Y
2
O
3
and Yb
2
O
3
to the total content of SiO
2
and B
2
O
3
is from 2 to 5, the ratio (weight ratio) of the total content of ZrO
2
, Ta
2
O
5
and Nb
2
O
5
to the total content of SiO
2
and B
2
O
3
is from 0.5 to 3 and the ratio (weight ratio) of ZnO to the total content of SiO
2
and B
2
O
3
is at least 0.14, and which has a refractive index nd of at least 1.875 and an Abbe's number &ngr;d of at least 39.5 (to be referred to as “optical glass III of the present invention” hereinafter),
(4) an optical glass having a glass composition comprising, by % by weight, 3 to 10% of SiO
2
, 7 to 15% of B
2
O
3
, 30 to 60% of La
2
O
3
, 2 to 8% of ZrO
2
and 13 to 19% of Ta
2
O
5
, wherein the total content of SiO
2
and B
2
O
3
is 14 to 20% and the total content of the above components is at least 95% (to be referred to as “optical glass IV of the present invention” hereinafter),
(5) a glass preform made of any one of the above glasses I to IV,
(6) an optical product made of any one of the above glasses I to IV,
(7) a process for the production of an optical product made of any one of the above optical glasses I to IV, which comprises the steps of melting raw materials for glass and directly press-molding a molten glass, and
(8) a process for the production of an optical product, which comprises the steps of softening a glass preform made of any one of the above optical glasses I to IV under heat and press-molding the glass preform softened under heat.


REFERENCES:
patent: 2150694 (1939-03-01), Morey
patent: 4120732 (1978-10-01), Komorita et al.
patent: 4166746 (1979-09-01), Ishibashi et al.
patent: 53-4023 (1978-01-01), None
patent: 54-6042 (1979-03-01), None
patent: 54-90218 (1979-07-01), None
Derwent Abstract of JP-54-90218.*
English Translation of Japanese Patetn 53-4023.*
English Translation of Japanese Patent 54-090218.

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