Abstract:
There is provided an optical glass that (1) has a high refractive index and high dispersion, (2) does not include any Pb compound that will tend to adversely impact the environment or human bodies, (3) is excellent in terms of economic efficiency, and (4) is suitable for mass production. This optical glass contains SiO 2 , B 2 O 3 , BaO, La 2 O 3 , TiO 2 , and, as necessary, one or more from Nb 2 O 5 , Ta 2 O 5 , WO 3 , ZrO 2 , MgO, CaO, SrO, ZnO, Li 2 O, Na 2 O, K 2 O, Y 2 O 3 , Gd 2 O 3 , and Al 2 O 3 —each in an amount within a predetermined range. This optical glass also has optical constant values such as a refractive index (nd) of 1.70-1.93 and an Abbe number (vd) of 28-45.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to an optical glass, and more particularly to a high-refractive-index and high-dispersion optical glass.  
           [0002]    Various types of high-refractive-index and high-dispersion optical glass containing lead compounds have been well-known and have been commercially manufactured. However, they have the problem that most of the glasses contain toxic lead in their composition.  
           [0003]    High-refractive-index and high-dispersion glass compositions that are lead-free have been proposed, but most of them have not been satisfactory in terms of economic efficiency or suitability for mass production.  
           [0004]    Japanese Unexamined Published Patent Application No. S58-69739 discloses a B 2 O 3 —SiO 2 —La 2 O 3 —ZrO 2 —(Nb 2 O 5 /Ta 2 O 5 )-based optical glass. The glass of this composition, however, is inferior in its meltability and devitrification resistance, because the composition contains a large amount of La 2 O 3  instead of BaO. It also is unsatisfactory in terms of cost efficiency, because it includes expensive Nb 2 O 5  and Ta 2 O 5 , which together account for 14% or more of the total weight of the composition.  
           [0005]    Japanese Unexamined Published Patent Application No. S58-125637 discloses an SiO 2 —B 2 O 3 —CaO—La 2 O 3 —ZrO 2 —TiO 2 —Nb 2 O 5 -alkaline metal-oxides-based optical glass. The glass of this composition, however, is poor in chemical durability, is volatile in a molten condition because alkaline metal oxides account for more than 2% of its weight, and is inferior in devitrification resistance and moldability because in a molten condition its viscosity is too low.  
           [0006]    Japanese Unexamined Published Patent Application No. S59-50048 discloses an SiO 2 —B 2 O 3 —La 2 O 3 —Nb 2 O 5 —ZrO 2 —TiO 2 -alkaline earth-metals-based optical glass. When the glass of this composition is in a molten condition, sometimes portions of it remain unmolten because it contains a larger amount of SiO 2  than of B 2 O 3 . Another shortcoming is that it requires the addition of expensive Nb 2 O 5  in order to secure devitrification resistance.  
           [0007]    Japanese Unexamined Published Patent Application No. S62-100449 discloses a B 2 O 3 —La 2 O 3 —ZnO—Li 2 O—Sb 2 O 3 -based optical glass. The glass of this composition easily becomes stained to an umber color due to its strong reductivity, which results because it contains too much Sb 2 O 3 , which ranges from 2% to 20% of the total weight of the composition (hereinafter “wt. %”).  
           [0008]    Japanese Unexamined Published Patent Application No. H07-41334 discloses an SiO 2 —B 2 O 3 —La 2 O 3 —TiO 2 —CaO-based optical glass. The glass of this composition also easily becomes stained, due to its high content of TiO 2 , which ranges from 19 wt % to 31 wt % in order to raise the refractive index.  
           [0009]    Japanese Unexamined Published Patent Application No. 2001-72432 discloses a SiO 2 —B 2 O 3 —La 2 O 3 —TiO 2 —CaO—BaO-based optical glass. The meltability of this composition is not very good, and it has difficulty in securing both a high refractive index and devitrification resistance.  
           [0010]    The glass of this composition is not good in meltability and difficult in securing both a high refractive index and devitrification by the reason of containing as high as 7 to 40% of CaO.  
           [0011]    Under these circumstances, there is a strong need for optical glass that has a high refractive index and high dispersion, and that does not contain toxic lead.  
         SUMMARY OF THE INVENTION  
         [0012]    The primary objectives of the present invention are to provide optical glass that has a high refractive index and high dispersion, that does not contain lead in its composition, and that maintains economic efficiency that makes mass-production feasible.  
           [0013]    These objectives are attained by providing optical glass that includes: (a) SiO 2  in an amount ranging from 1 wt % to 10 wt %; (b) B 2 O 3  in an amount ranging from 10 wt % to 35 wt %; (c) BaO in an amount ranging from 13 wt % to 30 wt %; (d) La 2 O 3  in an amount ranging from 10 wt % to 40 wt %; and (e) TiO 2  in an amount ranging from 5 wt % to 15 wt %.  
           [0014]    Optical glass of this invention can further contain one or more compositions of the following components: Nb 2 O 5 , Ta 2 O 5 , WO 3 , ZrO 2 , MgO, CaO, SrO, ZnO, Li 2 O, Na 2 O, K 2 O, Y 2 O 3 , Gd 2 O 3 , and Al 2 O 3 , wherein Nb 2 O 5  is in an amount less than 20 wt %, Ta 2 O 5  is in an amount less than 10 wt %, WO 3  is in an amount less than 10 wt %, ZrO 2  is in an amount less than 10 wt %, MgO is in an amount less than 10 wt %, CaO is in an amount less than 7 wt %, SrO is in an amount less than 10 wt %, ZnO is in an amount less than 20 wt %, the sum of Li 2 O, Na 2 O and K 2 O is in an amount less than 2 wt %, Y 2 O 3  is in an amount less than 15 wt %, Gd 2 O 3  is in an amount less than 15 wt %, Yb 2 O 3  is in an amount less than 10 wt %, and Al 2 O 3  is in an amount less than 5 wt %.  
           [0015]    It is preferable that optical glass of this invention have a refractive index (nd) of 1.70-1.93 and an Abbe number (vd) of 28-45. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    Optical glass according to the present invention includes specific components as described above. Each of these components has different functions in optical glass. The specific combination of these components can bring about preferred properties, such as a high refractive index of 1.70-1.93 and a high-dispersion Abbe number of 28-45, which are suitable for optical glass without lead being added. Thus, the ratios of these components are determined based attaining on a subtle balance among the individual components used. In other words, satisfactory optical glass can be obtained only when all of the components that are used are well-balanced within the specific ranges prescribed for the preferred embodiments of this invention.  
         [0017]    SiO 2  is a network-forming oxide. For the preferred embodiments of this invention, the level of SiO 2  in optical glass will be 1 wt %-10 wt %. A content of less than 1 wt % will tend to easily devitrify the glass and make it difficult to mold the glass, due to the low viscosity of the composition in a molten condition. A content in excess of 10 wt % will tend to lower the refractive index of the glass and leave some portions of the composition unmolten when it is melted.  
         [0018]    B 2 O 3  also is a network-forming oxide. For the preferred embodiments of this invention, the level of B 2 O 3  will be 10 wt %-35 wt %. A content of less than 10 wt % will tend to deteriorate both the devitrification and the moldability of the glass. In contrast, a content in excess of 35 wt % will tend to make it difficult for the glass to have a high refractive index and will tend to lower the moldability of the glass, due to the low viscosity of the glass in a molten condition.  
         [0019]    BaO increases the refractive index of the glass without staining the glass, and it represses the devitrification that can occur in the glass. For the preferred embodiments of this invention, BaO will be 13 wt %-30 wt % of the optical glass in order to obtain a high refractive index. A content of less than 13 wt % will tend to be too little for the BaO to be effective, and a content in excess of 30 wt % will tend to lower the chemical durability of the glass.  
         [0020]    La 2 O 3  increases the refractive index without staining the glass, and it improves the chemical durability of the glass. For the preferred embodiments of this invention, the level of La 2 O 3  in the optical glass will be 10 wt %-40 wt %, in order to obtain optical glass having a high refractive index. A content of less than 10 wt % will tend to be too little for the La 2 O 3  to be effective, and a content in excess of 40 wt % will tend to deteriorate the meltability of the glass and cause it to easily become devitrified.  
         [0021]    TiO 2  increases both the refractive index and the dispersion of the glass. For the preferred embodiments of this invention, TiO 2  will be 5 wt %-15 wt % of the optical glass. A content of less than 5 wt % will tend to be too little for the TiO 2  to be effective, and a content in excess of 15 wt % will cause the optical glass to stain easily.  
         [0022]    Optical glass of this invention can optionally contain, in addition to the above-mentioned components, one or more of the following components: Nb 2 O 5 , Ta 2 O 5 , WO 3 , ZrO 2 , MgO, CaO, SrO, ZnO, Li 2 O, Na 2 O, K 2 O, Y 2 O 3 , Gd 2 O 3 , Yb 2 O 3 , and Al 2 O 3 —in an amount specified for each component.  
         [0023]    Nb 2 O 5  increases both the refractive index and the dispersion of the glass, and it improves the glass&#39;s devitrification resistance. For the preferred embodiments of this invention, Nb 2 O 5  will be less than 20 wt % of the optical glass. A content in excess of 20 wt % will lower the devitrification resistance of the glass and will stain the glass. However, because Nb 2 O 5  is expensive, its content is preferably less than 5 wt % in view of the need for economic efficiency. An Nb 2 O 5  content of less than 5 wt % also reduces tendency of Nb 2 O 5  to stain the glass.  
         [0024]    Ta 2 O 5  increases the refractive index and improve the chemical durability of the glass. For the preferred embodiments of this invention, Ta 2 O 5  will be less than 10 wt % of the optical glass. A content in excess of 10 wt % will tend to deteriorate the devitrification resistance. However, because Ta 2 O 5  is expensive, its content is preferably less than 2 wt %.  
         [0025]    WO 3  increases both the refractive index and the dispersion of the glass and improves its devitrification resistance. For the preferred embodiments of this invention, WO 3  will be less than 10 wt %. A content in excess of 10 wt % will tend to stain the glass.  
         [0026]    ZrO 2  increases the refractive index and the dispersion of the glass without staining the glass, and for the preferred embodiments of this invention, it will be limited to less than 10 wt % of the optical glass. A content in excess of 10 wt % will tend to reduce the meltability of the glass and will tend to lower the glass&#39;s devitrification resistance.  
         [0027]    MgO is useful for adjusting the optical properties of the glass, and for the preferred embodiments of this invention, it will be limited to less than 10 wt % of the optical glass. A content in excess of 10 wt % will deteriorate the meltability of the glass.  
         [0028]    CaO lowers the specific gravity of the glass, improves the chemical durability of the glass, and for the preferred embodiments of this invention, it will be limited to less than 7 wt % of the optical glass. A content in excess of 7 wt % will tend to reduce the meltability of the glass and will tend to lower the glass&#39;s devitrification resistance.  
         [0029]    SrO is useful for adjusting the optical properties of the glass, and for the preferred embodiments of this invention, it will be limited to less than 10 wt % of the optical glass. A content in excess of 10 wt % will tend to lower the glass&#39;s devitrification resistance.  
         [0030]    ZnO improves the meltability of the glass and lowers its press-molding temperature so as to prevent degradation of a press mold. For the preferred embodiments of this invention, ZnO will be less than 20 wt % of the optical glass. A content in excess of 20 wt % will tend to lower the glass&#39;s devitrification resistance.  
         [0031]    Any of Li 2 O, Na 2 O, or K 2 O improves the meltability of the glass and lowers its press-molding temperature so as to prevent the degradation of a press mold. For the preferred embodiments of this invention, the sum of Li 2 O, Na 2 O, and K 2 O will be less than 2 wt % of the optical glass. If their combined content is in excess of 2 wt % total of the optical glass, they will tend to lower its devitrification resistance, and will tend to make press-molding difficult by increasing the volatility and lowering the viscosity of the glass in a molten condition.  
         [0032]    Any of Y 2 O 3 , Gd 2 O 3 , or Yb 2 O 3  improves the chemical durability of the glass and increases its refractive index without staining the glass. For the preferred embodiments of this invention, Y 2 O 3  will be less than 15 wt %, Gd 2 O 3  will be less than 15 wt %, and Yb 2 O 3  will be less than 10 wt % of the optical glass. A content in excess of any of those limits will tend to lower the glass&#39;s devitrification resistance.  
         [0033]    Any of Y 2 O 3 , Gd 2 O 3 , or Yb 2 O 3  is preferably replaced with La 2 O 3 , because in terms of functioning La 2 O 3  acts nearly the same as each of them does.  
         [0034]    Al 2 O 3  increases the chemical durability of the glass, and for the preferred embodiments of this invention it will be less than 5 wt % of the optical glass. A content in excess of 5 wt % will tend to make it difficult for the glass to have a high refractive index and will tend to leave some portions of the composition unmolten when the glass is melted.  
         [0035]    As mentioned above, optical glass according to the present invention includes SiO 2 , B 2 O 3 , BaO, La 2 O 3 , and TiO 2  as essential components. In addition, the glass can optionally include one or more components from Nb 2 O 5 , Ta 2 O 5 , WO 3 , ZrO 2 , MgO, CaO, SrO, ZnO, Li 2 O, Na 2 O, K 2 O, Y 2 O 3 , Gd 2 O 3 , and Al 2 O 3 .  
         [0036]    Other than above, Sb 2 O 3 , As 2 O 3  or another component also can be added as a defoamer, which is usually 1 wt % or less of optical glass. Refining agents, colorants, fluorides P 2 O 5  also can be added to an optical glass composition. This invention does not limit the use of those supplemental components insofar as they do not have any adverse effect on this invention.  
         [0037]    Optical glass according to this invention can be manufactured by any suitable method and manner known in the art. Typically, raw materials such as oxides, carbonates, and nitrates are blended to make the prescribed composition, which is then heated at 1100° C.-1400° C. so as to make it molten; the composition is then agitated so as to make it uniform, after which it is defoamed and then poured into a metallic die.  
       EXAMPLES  
       [0038]    The present invention will be discussed in further detail in the presentations of the following examples, but the present invention is not limited to these examples.  
         [0039]    Glass raw materials such as oxides, carbonates and nitrates were blended into the compositions shown in Table 1 and 2, mixed well, then put into a platinum pot and kept in an electrically heated furnace at 1200° C. to 1400° C. for 1 hour to 2 hours while being C with agitated. The mixture, after being clarified, was put into a pre-heated iron mold and cooled so as to give optical glass. The refractive index (nd) at the helium d-line and the Abbe number (vd) were measured by methods well-known to those in the art. Data regarding the refractive index and the Abbe number also are shown in Tables 1 and 2.  
                                                                                                     TABLE 1                           Examples of Glass Compositions and Properties Thereof (Nos. 1-8)                Examples of Glass Compositions           (wt % of each component)       Components   Examples→                          No. 1   No. 2   No. 3   No. 4   No. 5   No. 6   No. 7   No. 8                    SiO 2     8.7   8.8   7.5   1.8   9.9   9.9   7.8   7.3       B 2 O 3     17.4   10.5   21.3   29.7   10.6   34.9   15.5   15.8       BaO   29.5   25.9   25.5   13.7   29.5   29.5   13.3   13.1       La 2 O 3     30.3   39.9   39.8   39.9   35.1   10.8   29.0   11.1       TiO 2     14.1   14.9   5.9   14.9   14.9   14.9   13.0   12.8       Nb 2 O 5                             10.3   8.5       Ta 2 O 5         WO 3                                 2.9       ZrO 2                             8.2   3.0       MgO       CaO                               6.8       SrO       ZnO                           4.8   4.5       Li 2 O                           1.0       Na 2 O                           0.5       K 2 O                           0.3       Y 2 O 3                                 13.7       Gd 2 O 3         Yb 2 O 3         Al 2 O 3                             1.1   0.5       Properties       nd   1.82   1.87   1.77   1.82   1.86   1.72   1.88   1.86       νd   34.5   32.5   43.3   37.7   32.9   35.4   29.8   30.2                  
 
         [0040]    [0040]                                                                                                     TABLE 2                           Examples of Glass Compositions and Properties Thereof (Nos. 9-16)                Examples of Glass Compositions           (wt % of each component)       Components   Examples→                          No. 9   No. 10   No. 11   No. 12   No. 13   No. 14   No. 15   No. 16                    SiO 2     4.7   8.0   9.9   9.9   6.6   7.8   7.8   7.8       B 2 O 3     16.2   15.6   19.1   15.6   12.0   15.6   15.6   14.6       BaO   16.7   22.0   23.2   13.3   15.5   15.7   13.1   13.3       La 2 O 3     19.0   21.3   22.2   16.3   34.5   26.6   16.3   16.7       TiO 2     12.5   13.1   8.9   14.8   10.0   12.9   14.8   12.0       Nb 2 O 5     3.0               19.9           3.0       Ta 2 O 5             8.8   3.3           1.5       WO 3         4.5   7.6               7.8   2.9       ZrO 2                                 8.2       MgO               9.3           9.2       CaO       2.0       0.8               6.8       SrO               8.8           8.8       ZnO   8.4   3.9       2.1       19.9       Li 2 O       0.5                   1.0       Na 2 O                   0.5   0.5       0.5       K 2 O           0.3       1.0   1.0   0.5   0.5       Y 2 O 3         5.0       3.5               13.7       Gd 2 O 3     12.3           2.3           3.6       Yb 2 O 3     7.2       Al 2 O 3         4.1       Properties       nd   1.86   1.80   1.80   1.81   1.92   1.83   1.81   1.84       νd   32.7   34.1    34.5   33.2   32.0   33.9   33.4   33.3