Highly refractive optical glass with refractive indices &gt;1.83 and Abbe numbers &lt;25, and with very good chemical stability
DESCRIPTION OF THE INVENTION 
     1. Technical Field of the Invention 
     The invention relates to a new highly refractive optical glass with very good chemical stability. 
     2. Background Art 
     The state of the art is reflected in numerous patent applications. JP-OS No. 80-126549 describes the use of F -  ions up to 5% in the SiO 2  -MO-M 2  O-TiO 2  -system with the addition of up to 12 w.% of Nb 2  O 5  (MO=MgO, CaO, SrO, BaO, ZnO; M 2  O=Li 2  O, Na 2  O, K 2  O). 
     The stabilizing effect of F -  ions in conjunction with SnO 2 , WO 3  and B 2  O 3  is explained in German OS 32 16 451. The glasses described in JP-OS No. 84-086 37 are mainly obtained by application of SiO 2 , BaO, K 2  O and TiO 2  up to 35 w.% and of P 2  O 5  up to 6.5 wt.%. Japanese specifications Nos. 77-25812 and 77-45612 describe glass composition which allegedly can comprise almost all know chemical elements in quantities from 0 to 60 wt.%. The given examples are within a much narrower range; nevertheless they do not produce optical glasses but only glassy-crystalline structures. Also, the stated optical position (n d  /v d ) is below the potentials of desirable glasses. 
    
    
     DISCLOSURE OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a composition range for an optical glass which allows the production of highly refractive optical glasses (n d  ≧1.83) with high dispersion (v d  ≦25), very good chemical stability at relatively low density (σ≦3.5 g/cm 3 ) and adequate crystallization stability so that even large castings can be produced with high yield and in accordance with the usual quality standards. 
     Aother object of the present invention is to provide such glasses which are characterized by a refractive index n d  of 1.83-1.88, and a dispersion index v d  of 22-25. 
     Another object of this invention is to provide such glasses which are simultaneously characterized by good stain resistance, acid resistance, and alkali resistance. 
     Another object of this invention is to provide such glasses having a density of 3.0-3.5 g/cm 3 . 
     A further object of this invention is to provide such glasses which are substantially free of lead oxide. 
     Other objects, features, and advantages of the present invention will become more fully apparent to those skilled in the art to which this invention pertains from the following detailed description. 
     DETAILED DESCRIPTION 
     Briefly, the above and other objects of this invention are attained in one aspect thereof by providing a glass composition having the above-identified properties characterized by the followng composition (in wt.%): 
     
         ______________________________________                                    
Oxide             Useful   Preferred                                      
______________________________________                                    
SiO.sub.2         19-25      19-23.5                                      
Na.sub.2 O         7-10     7-10                                          
K.sub.2 O         4-7        4-6.5                                        
Cs.sub.2 O         0-12    0.5-12                                         
Na.sub.2 O + K.sub.2 O + Cs.sub.2 O                                       
                  11.0-25.0                                               
                           11.5-25.0                                      
CaO               0.5-2.0  0.5-1.5                                        
BaO                9-15     9-15                                          
TiO.sub.2         25-30    25-28                                          
ZrO.sub.2         0-6      2-6                                            
Nb.sub.2 O.sub.5  16-20    16-19                                          
WO.sub.3          0-3      0                                              
Nb.sub.2 O.sub.5 :TiO.sub.2                                               
                   0.6-0.71                                               
                           0.64-0.68                                      
______________________________________                                    
 
    
     The glasses according to this invention achieve the high refractive index due to the balanced application of Nb 2  O 5 , TiO 2  and ZrO 2 . The ratio of Nb 2  O 5  :TiO 2  is preferably 0.60-0.71; optimum conditions are obtained at a ratio Nb 2  O 5  :TiO 2  =0.64 to 0.68. The crystallization stability of the glasses is also additionally achieved by additions of SiO 2 , alkaline earths and alkali metals. Cs 2  O has a particularly stabilizing effect on devitrification behavior and also, in contrast with the other alkali metals, increases the refractive index. 
     The glasses of this invention can further contain an effective amount of one or more of the following additional oxides in the amounts indicated: 
     
         ______________________________________                                    
       B.sub.2 O.sub.3                                                    
                     0.2-3%                                               
       Li.sub.2 O    0.1-5%                                               
       PbO           0.3-1%                                               
       SrO           0.2-1%                                               
       Y.sub.2 O.sub.3                                                    
                     0.2-4%                                               
       La.sub.2 O.sub.3                                                   
                     0.2-2.5%                                             
       Bi.sub.2 O.sub.3                                                   
                     0.2-2.5%                                             
       Ta.sub.2 O.sub.5                                                   
                     0.2-5%.                                              
______________________________________                                    
 
    
     The glasses according to the invention satisfy not only the demand for a high refractive index and good crystallization stability, but also the demand for particularly good chemical resistance to acids, alkalis and staining. The corresponding testing methods are described in the Schott-catalogue &#34;Optical Glass&#34; No. 311 d, 1980 and summarized as follows. 
     The glasses of this invention are characterized by very good resistance to staining and belong to resistance Class FR 0-2, generally Class FR 0 or 1, indicating no blue-brown staining after contact with standard acetate (pH 4.6) for 100 h (Class FR 0); some color change after 100 h (Class FR 1); or some color change after 6 h (Class FR 2). 
     The glasses furthermore have good resistance to acids and belong to acid resistance Class SR 1 or 2, generally class SR 1, indicating dissolution of a 0.1 m thick layer by 0.5N nitric acid (pH 0.3) after &gt;100 h (Class SR 1) or 100-10 h (Class SR 2). 
     In addition, the glasses exhibit good alkali resistance and belong to class AR 1-3, generally class AR 1 or 2, indicating decompositions of 0.1 μm thick layer by NaOH (pH 10) at 90° C. in &gt;120 min. (Class AR 1), 120-30 min. (Class AR 2) or 30-7.5 min (Class AR 3). 
     A further advantage of the glasses according to this invention resides in that the environmentally harmful PbO, normally the main ingredient in glasses of the optical position here under consideration, has been practically wholly replaced by components which are hardly harmful to the environment. 
     Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative and not limitative of the remainder of the disclosure in any way whatsoever. In the following Examples, the temperatures are set forth uncorrected in degrees Celsius; unless otherwise indicated, all parts and percentages are by weight. 
     The glasses according to the invention are produced as follows: The raw materials (oxides, carbonates and nitrates, possibly chlorides) are weighed out. A refining agent such as As 2  O 3  or Sb 2  O 3  is added in amounts of 0.1 to 0.5 wt.%, and the whole is well mixed. The glass batch is melted down at approximately 1200°-1300° C. in a platinum or ceramic crucible, then refined and well homogenized with the aid of an agitator. At a casting temperature of 1150°-1190° C. the glass is then worked to the desired dimensions. 
     
         ______________________________________                                    
Sample Melt for 100 kg calculated glass.                                  
oxide   weight   raw material weighed amount (kg)                         
______________________________________                                    
SiO.sub.2                                                                 
        22.80    quartz powder                                            
                              22.83                                       
Na.sub.2 O                                                                
        9.5      Na.sub.2 CO.sub.3                                        
                              16.24                                       
K.sub.2 O                                                                 
        5.50     K.sub.2 CO.sub.3                                         
                              8.09                                        
Cs.sub.2 O                                                                
        0.50     Cs.sub.2 CO.sub.3                                        
                              6.58                                        
CaO     1.00     CaCO.sub.3   1.78                                        
BaO     9.50     BaCO.sub.3   12.41                                       
La.sub.2 O.sub.3                                                          
        2.50     La.sub.2 O.sub.3                                         
                              2.50                                        
TiO.sub.2                                                                 
        28.00    TiO.sub.2    28.06                                       
ZrO.sub.2                                                                 
        2.50     ZrO.sub.2    2.51                                        
Nb.sub.2 O.sub.5                                                          
        18.00    Nb.sub.2 O.sub.5                                         
                              18.00                                       
                              119.10                                      
As.sub.2 O.sub.3                                                          
        0.20     As.sub.2 O.sub.3                                         
                              0.20   ref. agent                           
                              119.30 kg Batch                             
______________________________________                                    
 
    
     Table 1 comprises 13 similarly prepared examples in the composition ranges specified. The properties of the Sample Melt glass are listed in Table 1, Example 6. 
     
                                           TABLE 1                                 
__________________________________________________________________________
Oxide   1   2   3   4   5   6   7   8   9   10  11  12                    
__________________________________________________________________________
SiO.sub.2                                                                 
        19.57                                                             
            22.02                                                         
                19.80                                                     
                    22.30                                                 
                        22.30                                             
                            22.80                                         
                                23.10                                     
                                    20.40                                 
                                        19.40                             
                                            23.02                         
                                                23.25                     
                                                    22.90                 
B.sub.2 O.sub.3                                                           
        2.59                                                              
            2.38                                                          
                2.50            .50         2.37                          
Na.sub.2 O                                                                
        7.49                                                              
            7.84                                                          
                7.50                                                      
                    9.25                                                  
                        9.50                                              
                            9.50                                          
                                7.50                                      
                                    8.25                                  
                                        8.25                              
                                            7.84                          
                                                8.50                      
                                                    9.20                  
K.sub.2 O                                                                 
        6.48                                                              
            4.92                                                          
                6.50                                                      
                    5.25                                                  
                        .50 5.50                                          
                                5.00                                      
                                    4.25                                  
                                        4.25                              
                                            4.92                          
                                                5.00                      
                                                    5.10                  
Cs.sub.2 O              .50 .50 .50 .70 .50     1.45                      
                                                    .60                   
CaO     1.49                                                              
            1.47                                                          
                1.50                                                      
                    .80 .80 1.00                                          
                                .70 .50 .70 1.47                          
                                                .80 .80                   
BaO     14.20                                                             
            10.34                                                         
                13.70                                                     
                    13.70                                                 
                        10.70                                             
                            9.50                                          
                                13.50                                     
                                    14.70                                 
                                        14.00                             
                                            10.35                         
                                                12.60                     
                                                    13.00                 
PbO     .80                     .40                                       
La.sub.2 O.sub.3            2.50                                          
Y.sub.2 O.sub.3                                                           
            3.35                    3.50                                  
                                        3.50                              
                                            3.35                          
TiO.sub.2                                                                 
        26.46                                                             
            26.49                                                         
                27.00                                                     
                    27.50                                                 
                        27.00                                             
                            28.00                                         
                                27.70                                     
                                    27.00                                 
                                        27.50                             
                                            25.99                         
                                                27.30                     
                                                    27.30                 
ZrO.sub.2   3.00    2.50                                                  
                        5.50                                              
                            2.50                                          
                                2.50                                      
                                    2.00                                  
                                        2.70                              
                                            2.99                          
                                                2.50                      
                                                    2.50                  
Nb.sub.2 O.sub.5                                                          
        18.48                                                             
            6.99                                                          
                18.50                                                     
                    18.50                                                 
                        18.00                                             
                            18.00                                         
                                18.50                                     
                                    18.50                                 
                                        19.00                             
                                            16.49                         
                                                18.40                     
                                                    18.40                 
WO.sub.3                                                                  
        2.29    2.80                                                      
As.sub.2 O.sub.3                                                          
        .20 .21 .20 .20 .20 .20 .10 .20 .20 .21 .20 .20                   
SrO         .93                             .93                           
Nb.sub.2 O.sub.5 :TiO.sub.2                                               
        .70 .64 .69 .67 .67 .64 .67 .69 .69 .63 .67 .67                   
n.sub.d 1.8436                                                            
            1.8441                                                        
                1.8467                                                    
                    1.8456                                                
                        1.8454                                            
                            1.8456                                        
                                1.8562                                    
                                    1.8649                                
                                        1.8743                            
                                            1.8372                        
                                                1.8381                    
                                                    1.8420                
v.sub.d 23.80                                                             
            24.20                                                         
                23.60                                                     
                    23.80                                                 
                        23.90                                             
                            23.70                                         
                                23.30                                     
                                    23.70                                 
                                        23.30                             
                                            24.50                         
                                                24.01                     
                                                    23.90                 
SR-Class                                                                  
        1.00                                                              
            ≦2.0                                                   
                1.00                                                      
                    1.00                                                  
                        1.00                                              
                            1.00                                          
                                ≦2.0                               
                                    ≦2.0                           
                                        1.00                              
                                            ≦2.0                   
                                                ≦2.0               
                                                    1.00                  
FR-Class                                                                  
        0.00                                                              
            0.00                                                          
                0.00                                                      
                    0.00                                                  
                        0.00                                              
                            0.00                                          
                                1.00                                      
                                    0.00                                  
                                        0.00                              
                                            1.00                          
                                                1.00                      
                                                    0.00                  
AR-Class            1.00                            1.00                  
Density, g/cm.sup.3 3.477                           3.425                 
__________________________________________________________________________
 
    
     The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those specifically used in the examples. From the foregoing description, one skilled in the art to which this invention pertains can easily ascertain the essential characteristics thereof and, without departing from the spirit and scope of the present invention, can make various changes and modifications to adapt it to various usages and conditions.