Patent Application
20090233782
The present invention relates to an optical glass of a high refractive index and low dispersion, and a lens using the same.
In recent years, with the spread of high-definition and compact digital cameras and camera-equipped mobile phones, demands of reduction in weight and size in optical system are rapidly increasing. In order to meet those demands, optical design using an aspheric lens made of a high performance glass becomes the mainstream. In particular, a large aperture aspheric lens using a glass showing a high refractive index and low dispersion is important on optical design.
Glasses comprising B2O3 and La2O3 as main components have conventionally been known as a glass showing a high refractive index and low dispersion. However, such glasses had the problem that because a molding temperature is generally high, life of a noble metal protection film formed on WC mold matrix is short, and the durability of a molding mold is short, and also had the problem that molding cycle is long, resulting in low productivity.
To solve the above problems, a glass comprising Li2O as a main component, besides B2O3 and La2O3 is known. However, there was the problem that because a rare earth element such as La2O3 is contained in a large amount, devitrification is liable to occur during high temperature molding process.
Furthermore, as a production method of an aspheric lens, a precision press molding method directly using a glass without polishing press surfaces becomes the mainstream from the points of productivity and production costs. In the precision press molding, a lower press molding temperature gives improved mold durability and shorter molding cycle to thereby increase the productivity. Therefore, an optical glass having a low molding temperature is desired.
When the content of an alkali metal or alkaline earth metal component as a glass component is increased to lower the molding temperature, thermal expansion coefficient of an optical glass becomes large. WC, ceramics and the like used as a mold has a thermal expansion coefficient far smaller than that of an optical glass. As a result, a thermal strain due to the difference in a thermal expansion coefficient between the mold and the optical glass is generated in an optical part as a molded product. By the molding strain, optical properties vary, and in the worst case, defects such as cracks are generated in a molded product. Therefore, an optical glass is required to have a lower molding temperature, and simultaneously a low thermal expansion coefficient.
To solve the above problems, Patent Document 1 proposes a glass comprising B2O3—SiO2—La2O3—Gd2O3—ZnO—Li2O—ZrO2 as main components. However, any composition of a high refractive index glass having a refractive index of 1.79 or more is not specifically described in the Examples, and additionally there is the problem that a molding temperature is high.
Patent Document 2 proposes an optical glass for mold press molding comprising B2O3—La2O3—ZnO—Ta2O5—WO3 as main components, wherein nd is from 1.75 to 1.85, νd is 35 or more and a softening point is 700° C. or lower. However, this glass is not sufficient in the aspect of the balance in optical properties, devitrification properties during high temperature process and low thermal expansion properties.
Patent Document 1: JP-A-2003-201143
Patent Document 2: JP-A-2005-15302
The present invention has an object to provide an optical glass which has optical properties of high refractive index and low dispersion, has a low molding temperature, is difficult to devitrify and has excellent moldability.
The present invention provides an optical glass comprising, in mass % on oxide basis;
B2O3: 10 to 25%
SiO2: 0.5 to 12%
La2O3: 17 to 38%
Gd2O3: 5 to 25%
ZnO: 8 to 20%
Li2O: 0.5 to 3%
Ta2O5: 5 to 15%, and
WO3: 3 to 15,
wherein the optical glass has a (SiO2+B2O3)/(ZnO+Li2O) value, which is a mass ratio of the total content of SiO2 and B2O3 to the total content of ZnO and Li2O, of from 1.35 to 1.90.
The optical glass of the present invention (hereinafter referred to as “the present glass”) has a high refractive index, preferably a refractive index nd to d line of from 1.79 to 1.83, and an Abbe number νd of from 38 to 45.
The present glass has a molding temperature as low as 650° C. or lower and a liquidus temperature, which is the maximum temperature at which devitrification does not occur, as low as 1,000° C. or lower. Therefore, the present glass has excellent formability in a high temperature process. Furthermore, a thermal expansion coefficient of the present glass is α=66 to 82 (×10−7 K−1) which is low as compared with an optical glass of the same system, and therefore the difference in a thermal expansion coefficient relative to a press mold such as WC system is small. As a result, rejection rate of molded products due to thermal strain can considerably be reduced. Moreover, owing to the above-mentioned reasons, optical products such as a lens can be produced with good productivity, and this also contributes to reduction in production costs.
The present glass can be used as a glass substrate requiring a high refractive index. Specifically, examples thereof include a substrate for increasing a light extraction efficiency for organic LED. In general substrate glasses such as soda lime glass, borosilicate glass and non-alkali glass, a refractive index is less than 1.6. Therefore, the extraction efficiency of light generated in an organic layer by reflection at the interface with a transparent conductive film such as ITO having high refractive index (refractive index: about 1.9) is decreased, but when the present glass is used, it is possible to improve the light extraction efficiency. Furthermore, in the present glass, mold molding at a low temperature is possible while achieving a high refractive index. Therefore, imparting a texture to a surface can easily be carried out, and this makes it possible to further improve the light extraction efficiency.
The reasons of setting each component range of the present glass are described below.
In the present glass, B2O3 is a component to form a glass backbone and to lower a liquidus temperature TL, and is an essential component. In the present glass, the B2O3 content is from 10 to 25 mass % (hereinafter abbreviated as “%” for brevity). Where the 3203 content is less than 10%, vitrification is difficult or the liquidus temperature TL becomes high, which is not preferred. To lower the liquidus temperature TL, the B2O3 content is preferably 12% or more. The B2O3 content is more preferably 13% or more, and further preferably 14% or more. When the B2O3 content is 15% or more, the liquidus temperature is lowered, and additionally, the Abbe number can be increased, which is particularly preferred.
On the other hand, in the present glass, where the B2O3 content exceeds 25%, the refractive index nd may be possibly low, or the chemical durability such as resistance to water may possibly deteriorate. It is preferred in the present glass that the B2O3 content is 23% or less. Where the refractive index nd is desired to increase, the B2O3 content is preferably 21% or less, and more preferably 20% or less.
In the present glass, ZnO is a component to stabilize a glass and to lower a molding temperature Tp or a melting temperature, and is an essential component. In the present glass, the ZnO content is from 8 to 20%. Where the ZnO content is less than 8%, the glass may be possibly instable, or the molding temperature may be possibly high. The ZnO content is preferably 10% or more, and more preferably 11% or more. On the other hand, in the present glass, where the ZnO content exceeds 20%, the stability of the glass may be possibly poor, and the chemical durability may possibly deteriorate. The ZnO content is preferably 19% or less, and more preferably 18% or less.
In the present glass, La2O3 is a component to increase a refractive index nd and to improve chemical durability, and is an essential component. In the present glass, the La2O3 content is from 17 to 38%. Where the La2O3 content is less than 17%, the refractive index nd may be possibly too low. The La2O3 content is preferably 19% or more, and more preferably 21% or more. On the other hand, where the La2O3 content exceeds 38%, it may be possibly difficult to vitrify. As a result, the molding temperature may be possibly high or the liquidus temperature TL may be possibly high. The La2O3 content is preferably 35% or less, and more preferably 33% or less.
In the present glass, Gd2O3 is a component to increase a refractive index nd and to improve chemical durability, similar to La2O3, and is an essential component. In the present glass, the Gd2O3 content is from 5 to 25%. Where the Gd2O3 content is less than 5%, the refractive index nd is low. The Gd2O3 content is preferably 6% or more, and more preferably 7% or more. On the other hand, where the Gd2O3 content exceeds 25%, it may be possibly difficult to vitrify. As a result, the molding temperature may be possibly high, or the liquidus temperature TL may be possibly high. The Gd2O3 content is preferably 22% or less, and more preferably 20% or less.
In the present glass, the total amount of the La2O3 content and the Gd2O3 content is preferably from 33 to 50%, Where the total amount is less than 33%, the refractive index nd may be possibly low, or chemical durability may possibly deteriorate. The total amount is preferably 35% or more, and more preferably 37% or more. On the other hand, where the total amount exceeds 50%, it may be possibly difficult to vitrify. As a result, the molding temperature may be possibly high, or the liquidus temperature TL may be possibly high. The total amount is preferably 47% or less, and more preferably 45% or less.
In the present glass, Li2O is a component to stabilize a glass and to lower a molding temperature and a melting temperature, and is an essential component. In the present glass, the Li2O content is from 0.5 to 3%. Where the Li2O content is less than 0.5%, the molding temperature or the melting temperature may be possibly too high. The Li2O content is preferably 1.1% or more, and more preferably 1.3% or more. On the other hand, where the Li2O content exceeds 3%, it is liable to vitrify, and deterioration of chemical durability and volatilization of components during melting may be possibly vigorous. The Li2O content is preferably 2.5% or less, and more preferably 2.3% or less.
In the present glass, Ta2O5 is a component to stabilize a glass, to increase a refractive index nd and to suppress devitrification during high temperature forming, and is an essential component. In the present glass, the Ta2O5 content is from 5 to 15%. Where the Ta2O5 content is less than 5%, the refractive index nd may be possibly too low, or the liquidus temperature TL may be possibly too high. The Ta2O5 content is preferably 7% or more, and more preferably 8% or more. On the other hand, the Ta2O5 content exceeds 15%, the molding temperature may be possibly too high, or the Abbe number νd may be possibly too small. The Ta2O5 content is preferably 14% or less, and more preferably 13% or less.
In the present glass, WO3 is a component to stabilize a glass, to increase a refractive index nd and to suppress devitrification during high temperature forming, and is an essential component. In the present glass, the WO3 content is from 3 to 15%. Where the WO3 content is less than 3%, the refractive index nd may be possibly low, and the liquidus temperature TL may be possibly too high. The WO3 content is preferably 4% or more, and more preferably 5% or more. On the other hand, where the WO3 content exceeds 15%, the molding temperature may be possibly high, and the Abbe number νd may be possibly too small. The WO3 content is preferably 14% or less, and more preferably 13% or less.
In the present glass, SiO2 is a component to stabilize a glass, or to suppress devitrification during high temperature forming, and is an essential component. In the present glass, the SiO2 content is from 0.5 to 12%. Where the SiO2 content exceeds 12%, the molding temperature may be possibly high, and the refractive index nd may be possibly too low. The SiO2 content is preferably 10% or less, and more preferably 9% or less.
On the other hand, when it is desired to suppress devitrification during high temperature forming or to adjust a viscosity, the SiO2 content is 0.5% or more. The SiO2 content is preferably 2% or more, and more preferably 4% or more.
The present inventors have found that a low molding temperature, a low liquidus temperature and thermal expansion coefficient can be made compatible when the mass ratio of the total amount of the B2O3 content and the SiO2 content, which are network forming oxide components of a glass, relative to the total amount of the Li2O content and the ZnO content, which are a monovalent or divalent glass modifying oxide component, (SiO2+B2O3)/(ZnO+Li2O) (hereinafter referred to as a “network modification ratio”), is adjusted to a specific value.
In the present glass, the network modification ratio is from 1.35 to 1.90. Where the network modification ratio is less than 1.35 or exceeds 1.90, it is difficult to make compatible a low molding temperature and a low liquidus temperature. The lower limit of the network modification ratio is preferably 1.38 or more, and more preferably 1.40 or more. On the other hand, the upper limit of the network modification ratio is preferably 1.85 or less, and more preferably 1.80 or less.
In the present glass, ZrO2 is not an essential component, but may be contained in an amount of from 0 to 5% for stabilizing a glass, increasing a refractive index nd, suppressing devitrification during high temperature forming, and the like. Where the ZrO2 content exceeds 5%, the molding temperature may be possibly too high, or the Abbe number νd may be possibly too small. The ZrO2 content is preferably 4% or less, and more preferably 3% or less. On the other hand, to obtain the effect of addition, the ZrO2 content is more preferably 0.1% or more, and further preferably 0.2% or more.
In the present glass, TiO2 is not an essential component, but may be contained in an amount of from 0 to 5% for stabilizing a glass, increasing a refractive index nd, suppressing devitrification during high temperature forming, and the like. Where the TiO2 content exceeds 5%, the Abbe number νd may be possibly too small, or the transmittance may be possibly decreased. The TiO2 content is more preferably 3% or less.
In the present glass, Nb2O5 is not an essential component, but may be contained in an amount of from 0 to 5% for stabilizing a glass, increasing a refractive index nd, suppressing devitrification during high temperature molding, and the like. Where the Nb2O5 content exceeds 5%, the Abbe number νd may be possibly too small, or the transmittance may be possibly decreased. The Nb2O5 content is preferably 3% or less.
In the present glass, Y2O3 and Yb2O3 each are not essential components, but may be contained in an amount of from 0 to 10% for increasing a refractive index nd, suppressing devitritication during high temperature forming, and the like. Where the total amount of those exceeds 10%, the glass may rather be possibly unstable, or the molding temperature may be possibly too high. The total amount of Y2O3 and Yb2O3 is preferably 7% or less.
In the present glass, Al2O3, Ga2O3, GeO2 and P2O5 each are not essential components, but may be contained in an amount of from 0 to 10% for the purpose of stabilizing a glass, adjusting a refractive index nd, and the like. Where the total amount of Al2O3, Ga2O3, GeO2 and P2O5 exceeds 10%, the Abbe number νd may be possibly too low. The total amount of Al2O3, Ga2O3, GeO2 and P2O5 is more preferably 8% or less, and further preferably 6% or less.
In the present glass, where Al2O3, Ga2O3 and GeO2 are contained, the total amount of the respective contents of Al2O3, Ga2O3, GeO2 and B2O3 is preferably from 15 to 35%. Where the total amount is less than 15%, vitrification may be possibly difficult, or the liquidus temperature TL may be possibly high. The total amount is more preferably 18% or more, and further preferably 22% or more.
On the other hand, the total amount of the respective contents of Al2O3, Ga2O3, GeO2 and B2O3 exceeds 35%, the refractive index nd may be possibly low, or the molding temperature may be possibly high. The total amount is more preferably 32% or less, and further preferably 29% or less.
In the present glass, BaO, SrO, CaO and MgO each are not essential components, but each may be contained in an amount of from 0 to 15% for stabilizing a glass, increasing an Abbe number νd, lowering a molding temperature, decreasing specific gravity and the like. Where the content of each of BaO, SrO, CaO and MgO exceeds 15%, the glass may be possibly unstable, or the refractive index nd may be possibly low.
Where BaO, SrO, CaO and MgO are contained, the total amount of the respective contents of BaO, SrO, CaO, MgO and ZnO is desirably from 8 to 25%. Where the total amount is less than 8%, the glass may be possibly unstable, or the molding temperature may be possibly too high. The total amount is more preferably 10% or more, and further preferably 11% or more. On the other hand, where the total amount exceeds 20%, the glass may be possibly rather unstable, the refractive index nd may be possibly low, or the chemical durability may possibly deteriorate. The total amount is more preferably 19% or less, and further preferably 18% or less.
In the present glass, where it is desired, for example, to further suppress devitrification during high temperature forming, the glass preferably comprises B2O3; 15 to 20%, SiO2: 3 to 10%, La2O3: 21 to 33%, Gd2O3: 7 to 19%, ZnO: 8 to 19%, Li2O: 1.2 to 2.4%, Ta2O5: 8 to 14%, and WO3: 5 to 13%, wherein the network modification ratio is from 1.38 to 1.82. When ZrO2 and/or TiO2 are further added to this composition in an amount of from 0.2 to 4%, the devitrification suppression effect is further secured, which is hence preferred.
The present glass consists essentially of the above-described components, but may contain other components to the extent not impairing the objects of the present invention. Where such other components are contained, the total amount of the contents of the other components is preferably 10% or less, more preferably 8% or less, and further preferably 6% or less or 5% or less.
The present glass may contain Sb2O3 in an amount of, for example, from 0 to 1% for the purpose of refining and the like. Furthermore, each component of Na2O, K2O, Rb2O or Cs2O may be contained in a total amount of from 0 to 5% for the purpose of further stabilizing a glass, adjusting a refractive index nd, adjusting specific gravity, lowering a melting temperature, and the like. Where the total amount of each component of Na2O, K2O, Rb2O or Cs2O exceeds 5%, the glass may be possibly unstable, the refractive index nd may be possibly low, the hardness may be possibly small, or the chemical durability may possibly deteriorate. Where importance is attached to the hardness or chemical durability, it is preferred that none of each component of Na2O, K2O, Rb2O and Cs2O is contained.
In the present glass, optional components other than above can be selected according to the respective required properties. For example, where importance is attached to the high refractive index nd and the low glass transition point Tg, SnO may be contained in an amount up to 4%. Similarly, where importance is attached to the high refractive index, TeO2 and/or Bi2O3 may be contained in an amount of form 0 to 6% singly or as the total amount. Where the content of TeO2 and/or Bi2O3 exceeds 6%, the glass may be possibly unstable, or the transmittance may be possibly markedly decreased. However, where the Abbe number νd is desired to increase, it is preferred that none of TeO2 and Bi2O3 is contained.
To reduce an environmental load, it is preferred that the present glass does not substantially contain any of lead (PbO), arsenic (As2O3) and thallium (Tl2O) as components. Where fluorine is contained, it increases a thermal expansion coefficient, it adversely affects releasability and moldability, and the component is liable to vaporize. Therefore, there are the problems that the composition of the optical glass is liable to be heterogeneous, the durability of a mold such as a release film deteriorates, and the like. As a result, it is preferred that the present glass does not substantially contain fluorine.
It is preferred that the present glass does not contain Fe2O3 for the reasons of prevention of coloration and the like, but in general, Fe2O3 is unavoidably incorporated from raw materials. Even in this case, it is preferred in the present glass that Fe2O3 content is 0.0001% or less.
As the optical properties of the present glass, the refractive index nd is preferably from 1.79 to 1.83. When the refractive index nd is 1.79 or more, such a glass is suitable to downsize a lens, which is hence preferred. The refractive index nd is more preferably 1.80 or more. On the other hand, where the refractive index nd exceeds 1.83, the Abbe number becomes too small, which is not preferred. The refractive index nd of the present glass is more preferably 1.82 or less. The Abbe number νd is preferably from 38 to 45 when the refractive index nd is from 1.79 to 1.83, and more preferably from 39 to 44 when the refractive index nd is from 1.80 to 1.82.
In the present specification, the molding temperature Tp means a value calculated from a glass transition temperature Tg and a yield point At by the equation of
T
p
−At+(At−Tg)/2.
When the molding temperature Tp of the present glass is 650° C. or lower, it facilitates precision press molding, which is hence preferred. Where the molding temperature Tp exceeds 650° C., there are the possibilities that part of components of a preform which is a product to be molded in press molding evaporates to induce a damage of mold members or release film, so that the durability of a mold deteriorates, and additionally that the press molding productivity itself deteriorates. The molding temperature Tp of the present glass is more preferably 645° C. or lower, and further preferably 640° C. or lower.
Regarding the thermal expansion coefficient α of the optical glass, it is preferred that the difference from the thermal expansion coefficient of a mold, which is, for example, 40 to 50×10−7 K−1 in WC system, is not so large. In the present glass, the thermal expansion coefficient α is preferably 82×10−7 K−1 or less. Where the thermal expansion coefficient α exceeds 82×10−7 K−1, defects such as cracks are liable to occur during press molding, and where pressure conditions are made mild to avoid cracks and the like, shape transferability deteriorates by molding sink. In the present glass, the thermal expansion coefficient α is further preferably 80×10−7 K−1 or less.
On the other hand, where the thermal expansion coefficient α of the present glass becomes too small, a mold and an optical part are difficult to be released in a cooling process of press molding, and in the worst case, the optical part may be possibly fixed to the mold, resulting in molding defect. Therefore, in the present glass, the thermal expansion coefficient α is preferably 66×10−7 K−1 or more, and more preferably 67×10−7 K−1 or more. In the present description, the thermal expansion coefficient α means a value in a temperature range of from 50 to 350° C.
The liquidus temperature TL of the present glass is preferably 1,000° C. Or lower. Where the liquidus temperature TL exceeds 1,000° C., a product to be molded is liable to devitrify during high temperature forming, and carbon or heat-resistant alloy used as a receiver mold of high temperature forming deteriorates, which is not preferred. The liquidus temperature TL of the present glass is more preferably 990° C. or lower, and further preferably 980° C. or lower. The liquidus temperature TL is defined as the maximum temperature at which a crystal solidified product is not formed from a glass melt when held at a certain temperature.
The present glass has the above-described properties. Therefore, the glass is easily optically designed, and is suitable for optical parts, particularly an aspheric lens used in digital cameras or the like.
Specific embodiments of the present invention are described by the Examples (Runs 1 to 68) and the Comparative Examples (Runs 69 to 72), but the invention is not limited to those.
A raw material preparation method was as follows. Raw materials shown below were mixed such that glasses having compositions shown in the Tables are obtained, placed in a platinum crucible, and melted at 1,100 to 1,300° C. for 1 hour. In this case, a molten glass was stirred with a platinum-made stirrer for 0.5 hour to homogenize the same. The homogenized molten glass was flown out the crucible and molded into a plate. The plate was held at a temperature of Tg+10° C. for 4 hours, and then annealed to room temperature at a cooling rate of −1° C./min.
As raw materials, guaranteed reagents manufactured by Kanto Chemical Co., Inc. were used as boron oxide, aluminum oxide, lithium carbonate, sodium carbonate, zirconium dioxide, zinc oxide, magnesium oxide, calcium carbonate and barium carbonate. Reagents having a purity of 99.9% manufactured by Shin-Etsu Chemical Co., Ltd. were used as lanthanum oxide and gadolinium oxide. Reagents having a purity of 99.9% manufactured by Kojundo Chemical Lab. Co., Ltd. were used as tantalum oxide, silicon dioxide, tungsten oxide and niobium oxide.
A glass transition point T9, a yield point At (unit: ° C.), an average linear expansion coefficient α at 50 to 300° C. (unit: 10−7 K−1), a refractive index nd at a wavelength of 587.6 nm (d line), an Abbe number νd, a liquidus temperature TL (unit: ° C.) and a specific gravity d were measured on the glasses obtained. Those measurement methods are described below.
Thermal properties (Tg, At and α): A sample processed into a columnar shape having a diameter of 5 mm and a length of 20 mm was measured with a thermo-mechanical analyzer (a product of MAC Science Co., Ltd., trade name: DIALTOMETER 5000) at a temperature rising rate of 5° C./min
Optical properties (nd and νd): A sample processed into a rectangular solid shape having a side of 20 mm and a thickness of 10 mm was measured with a precision refractometer (a product of Kalnew Optical Industry Co., Ltd., trade name: KPR-2). The measurement value was obtained up to five places of decimals. The refractive index (nd) was described by rounding to two decimal places, and the Abbe number (νd) was described by rounding to one decimal place.
Liquidus temperature TL: A sample processed into a cube shape having one side of 10 mm was placed on a platinum pan, and held in an electric furnace set to a certain temperature for 1 hour. The sample was taken out of the furnace, and was observed with an optical microscope of 10 magnifications. The maximum temperature at which precipitation of crystal was not observed was considered as a liquidus temperature TL. When the liquidus temperature TL exceeds 1,000° C., it was expressed as “Exceeding 1000”.
As devitrification properties, a satisfactory glass in which devitrification (precipitation of crystal) was not observed at a liquidus temperature of 1,000° C. is indicated by “o”, and a glass in which devitrification (precipitation of crystal) was observed is indicated by “x”.
Although the present invention has been described in detail and by reference to the specific embodiments, it is apparent to one skilled in the art that various modifications or changes can be made without departing the spirit and scope of the present invention.
This application is based on Japanese Patent Application No. 2006-249552 filed Sep. 14, 2006, the disclosure of which is incorporated herein by reference in its entity.
An optical glass suitable as optical parts of digital cameras and the like, and additionally, glass substrates requiring high refractive index, such as substrates for increasing light extraction efficiency for organic LED can be provided.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-249552 | Sep 2006 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP07/67746 | Sep 2007 | US |
| Child | 12404039 | US |
