Patent Publication Number: US-3970463-A

Title: Glasses and glass-ceramics and products made therefrom

Description:
This is a division of application Ser. No. 229,959, now U.S. Pat. No. 3,841,950, filed Feb. 28, 1972 which is relied upon and the entire disclosure and specification of which is hereby incorporated by reference. Which in turn is a continuation-in-part of copending U.S. Pat. application Ser. No. 146,664, filed May 25, 1971 now abandoned. 
    
    
     In U.S. Pat. application Ser. No. 30,859 filed Apr. 22, 1970, and in U.S. Pat. application Ser. No. 146,665 filed May 25, 1971, both of which are assigned to Owens-Illinois, Inc., assignee of the present application (the disclosure of each of which is incorporated herein by reference and thus forms a part of this application), there are disclosed several embodiments of heat exchangers, namely recuperators and regenerators for use with gas turbine engines for various automotive vehicles, including automobiles, trucks, busses, and the like, together with a full disclosure as to how to make such heat exchangers. 
     In each embodiment of the disclosures, the regenerative heat exchanger comprises a matrix of integrally fused tubes forming a series of smooth longitudinal parallel passageways therethrough, wherein the walls defining the passageways are nonporous and consist essentially of an inorganic crystalline oxide ceramic material having an average lineal coefficient of thermal expansion of about -12 to +12 × 10 -   7  /°C. (0°-300°C.). The walls of the passageways also have a maximum thickness of about 0.03 to 0.002 inches through portions of such walls common to adjacent fused tubes. The ratio of the diameter of the passageways to the aforementioned thickness is at least 3 and the matrix has an open frontal area of at least 60%. Furthermore, the matrix walls have a thermal conductivity at 400°C. of less than 0.01 cal/cm/sec/cm 2  /°C. 
     In the aforementioned U.S. Pat. application Ser. No. 30,859 and in the later filed application Ser. No. 146,665 which is a continuation-in-part of Ser. No. 30.859, there is disclosed a recuperator comprising a matrix comprising an assembly of integrally fused tubes arranged in a plurality of layers of tubes superimposed one above the other in successive parallel planes, the tubes within each plane being essentially parallel to each other and transverse to the tubes in adjacent layers, the tubes in each layer forming a series of longitudinal parallel passageways through the matrix. The composition and size of the tubes is substantially the same as that of the aforementioned regenerator tubes. Because of the arrangement of the layers of tubes in the recuperator, the open frontal or cross-sectional area of each face of the matrix containing passageways is at least 32% of the cross-sectional area across each face. 
     In U.S. Pat. application Ser. No. 127,127 filed Mar. 23, 1971, in the names of Andrew W. Zmuda and Yu K. Pei and entitled &#34;Exhaust Reactor for Combustion Engine &#34; (also assigned to the common assignee) the disclosure of which is also incorporated herein and forms a part of this application, there is disclosed an integral monolithic exhaust reactor for a combustion engine including a first reaction zone comprising an elongated tube for receiving the flow of hot glass effluents from a combustion engine. A matrix comprising a plurality of parallel tubes extends longitudinally of and is disposed about the first tubular reaction zone and is bonded thereto. The parallel tubes are bonded together in the matrix and form a rigid support for the elongated tube. A plurality of the matrix tubes disposed about the elongated tube have open passageways which are in communication with the elongated tube and receive the flow of gas effluents therefrom. These matrix tubes define a second reaction zone for the gas effluents and connect with outlet means to permit passage of gas therefrom. 
     During their operation, the regenerators, recuperators, and exhaust reactors described in the aforementioned pending patent application are subjected to extremely high temperatures of about 1,500° F. and more for extended periods of time. When the various components of such devices are formed of an inorganic crystalline oxide ceramic material, such material must have thermal stability to withstand such high temperatures, i.e., its coefficient of thermal expansion, its modulus of rupture, its structural dimensions, and other properties must not change to any appreciable extent upon exposure to such high temperatures over extended periods of time. 
     The coefficient of thermal expansion must be low so that any rapid heating or cooling of the ceramic material components will not cause any build-up of thermal stresses and result in breakage of the component. 
     The modulus of rupture must be high so that such components can withstand the wear and tear inherent in normal operations of the gas turbines and automotive vehicles. Such strength must be maintained at the high temperatures of at least 1,500° F. to which such components are normally subjected. 
     Heat exchangers, such as regeneraters and recuperators used in gas turbine engines for trucks, operate at temperatures as high as 1,500° F. and must be able to withstand this temperature for at least 1,000 to 2,000 hours without shattering during the operation of the truck. Automobile turbine heat exchangers will operate at temperatures as high as 1,900° F., while catalytic and thermal reactors which ensure substantially total combustion of exhaust gas effluents of internal combustion engines will be subjected to temperatures of about 1,000° - 1,900° F. Thus, all components of these devices must retain their strengths while being subjected to these temperatures over extended periods of time and, most importantly, must retain their thermal stability during operation, particularly dimensional stability. Expansion of less than 250 parts per million when exposed to temperatures of at least 1,500° F. for a period of 2,000 hours should be a property of any glass-ceramic component utilized in these heat exchangers and reactors, and preferably such expansion should be less than 100 ppm with, of course, the ideal being an expansion of 50 ppm or less at these temperatures over these times. 
     It has now been found that certain thermally crystallizable glasses having certain narrow, critical composition limits within the broad area of Li 2  O-Al 2  O 3  -SiO 2  and Li 2  O-Al 2  O 3  -SiO 2  -ZnO systems, when shaped into specific structures and then thermally in situ crystallized to an at least partially crystalline ceramic (also referred to as glass-ceramic), impart to such structures excellent thermal stability at high temperatures of at least 1,500°F while simultaneously imparting a high modulus of rupture. Furthermore, such structures have a coefficient of thermal expansion within the range of -12 to +12×10 -   7  /°C.(0°-700°C). Many of the compositions have a coefficient of thermal expansion of less than -3 to +3×10 -   7  /°C. and some are within the range of -0.5 to +0.5×10 -   7  /°C.(0°-700°C). 
     Thermally crystallizable glass compositions of the invention coming within the SiO 2  -Al 2  O 3  -Li 2  O system consist essentially of the following ingredients present within the indicated ranges, expressed in weight percent, and also within the indicated molar ratios. 
     
         ______________________________________                                    
Ingredient      Weight Percent                                            
______________________________________                                    
SiO.sub.2       55 - 80                                                   
Al.sub.2 O.sub.3                                                          
                12 - 27                                                   
Li.sub.2 O      3.2 - 7.6                                                 
Nucleating Agent                                                          
                3 - 9                                                     
______________________________________                                    
 
    
     wherein the nucleating agent is either TiO 2  or a mixture of TiO 2  with ZrO 2 . Usually, in order to obtain the desired properties, the compositions will consist of 98-100 weight percent SiO 2  +Al 2  O 3  +Li 2  O+ the specified nucleating agents. When the SiO 2  /Al 2  O 3  molar ratio is from 4 to less than 5, the Li 2  O/Al 2  O 3  molar ratio is from 0.75 to 0.97; when the SiO 2  /Al 2  O 3  molar ratio is from 5 to 7.5, the Li 2  O/Al 2  O 3  molar ratio is from 0.65 to 0.97; when the SiO 2  /Al 2  O 3  molar ratio is more than 7.5 and less than 9, then the Li 2  O/Al 2  O 3  molar ratio is from 0.8 to 0.97;  and when the SiO 2  /Al 2  O 3  molar ratio is more than 9 and up to 10, the Li 2  O/Al 2  O 3  molar ratio is from 0.87 to 0.97. More usually, the weight percent ranges of SiO 2  and Al 2  O 3  in these compositions are 62.8-80 SiO 2  and 12.7-26.7 Al 2  O 3 . 
     When the nucleating agent is a mixture of TiO 2  +ZrO 2  then the ZrO 2  should not exceed 3% and preferably should be less than 3% i.e. up to 2.8% by weight and preferably at least 0.5% by weight, and the total amount of TiO 2  plus ZrO 2  is at least 3 weight percent up to about 9 weight percent. 
     While more than about 9 weight percent TiO 2  or total nucleating agent may be used, there is no advantage in using such large amounts because the properties of the resulting glass-ceramics are not improved. Further, raising the TiO 2  levels has the result of raising the thermal expansion coefficient of the glass-ceramic. It is critical to have at least 3 weight percent nucleating agent in the composition. When less than 3 weight percent total nucleants are present, the nucleation rate is so slow that the heat treatment cycles are excessively long and the strength of the glass-ceramic which is formed is decreased. When ZrO 2  is present in an amount of 3 weight percent or more it raises the liquidus of the glass melt and makes more difficult the processing of the glass into products. 
     Refining agents, such as antimony tri-oxide, arsenic oxide, or the like, may be present in the composition in an amount up to about 0.5 weight percent if such ingredients are deemed necessary for fining the glasses. 
    
    
     In the preferred embodiment of the invention, the thermally crystallizable glass comes within the following composition ranges wherein the sole essential ingredients, expressed in weight percent, are as follows, and wherein the molar ratios are as indicated: 
     
         Ingredient      Weight Percent                                            
______________________________________                                    
SiO.sub.2        63 - 78.5                                                
Al.sub.2 O.sub.3                                                          
                13.8 - 23                                                 
Li.sub.2 O      3.5 - 6.5                                                 
Nucleating Agent                                                          
                3 - 9                                                     
______________________________________                                    
 
    
     wherein the SiO 2  /Al 2  O 3  molar ratio is from 5 to less than 9. The nucleating agent is as defined above and when the SiO 2  /Al 2  O 3  molar ratio is from 7.5 to less than 9, the Li 2  O/Al 2  O 3  molar ratio is from 0.87 to 0.97; and when the SiO 2  /Al 2  O 3  molar ratio is from 5 to less than 7.5, the Li 2  O/Al 2  O 3  molar ratio is from 0.75 to 0.97. 
     Representative compositions coming within the scope of the present invention are set forth in Table I. 
     
                                           TABLE I                                 
__________________________________________________________________________
Composition                                                               
No.     1    2    3    4    5    6    7    8    9    10   11              
__________________________________________________________________________
SiO.sub.2                                                                 
        78.3 78.90                                                        
                  75.85                                                   
                       76.45                                              
                            74.8 73.00                                    
                                      70.85                               
                                           70.85                          
                                                70.75                     
                                                     70.80                
                                                          70.85           
Al.sub.2 O.sub.3                                                          
        14.2 13.65                                                        
                  16.10                                                   
                       16.20                                              
                            17.15                                         
                                 18.45                                    
                                      20.05                               
                                           20.05                          
                                                19.75                     
                                                     20.05                
                                                          20.00           
Li.sub.2 O                                                                
        3.95 4.00 4.50 3.80 4.50 5.00 5.55 5.40 5.55 5.55 5.45            
TiO.sub.2                                                                 
        1.45 1.45 1.45 1.45 1.45 1.45 1.45 1.40 1.40 1.40 1.40            
ZrO.sub.2                                                                 
        1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60            
Sb.sub.2 O.sub.3                                                          
        0.30 0.30 0.30 0.32 0.30 0.30 0.30 0.30 0.30 0.30 0.30            
Na.sub.2 O                                                                
        0.10      0.10 0.10 0.10 0.10 0.10 0.30 0.10 0.10 0.10            
K.sub.2 O                                                 0.20            
Fe.sub.2 O.sub.3                                0.45 0.10                 
Molar Ratio                                                               
SiO.sub.2 /Al.sub.2 O.sub.3                                               
        10.00                                                             
             9.80 8.00 8.00 7.40 6.70 6.00 6.00 6.00 6.00 6.00            
Molar Ratio                                                               
Li.sub.2 O/Al.sub.2 O.sub.3                                               
        0.95 0.97 0.95 0.80 0.90 0.95 0.95 0.95 0.95 0.95 0.95            
__________________________________________________________________________
Composition                                                               
No.     12   13   14   15   16   17   18   19   20   21   22              
__________________________________________________________________________
SiO.sub.2                                                                 
        70.75                                                             
             68.00                                                        
                  71.05                                                   
                       71.30                                              
                            71.50                                         
                                 69.35                                    
                                      67.10                               
                                           72.00                          
                                                68.45                     
                                                     64.00                
                                                          64.85           
Al.sub.2 O.sub.3                                                          
        20.00                                                             
             22.25                                                        
                  20.10                                                   
                       20.15                                              
                            20.20                                         
                                 19.60                                    
                                      18.95                               
                                           20.25                          
                                                23.25                     
                                                     25.35                
                                                          25.60           
Li.sub.2 O                                                                
        5.35 6.20 5.30 5.00 4.75 4.55 4.45 4.20 4.75 7.10 6.00            
TiO.sub.2                                                                 
        1.40 1.45 1.45 1.45 1.45 6.00 9.00 1.45 1.45 1.45 1.45            
ZrO.sub.2                                                                 
        1.60 1.60 1.60 1.60 1.60           1.60 1.60 1.60 1.60            
Sb.sub.2 O.sub.3                                                          
        0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30            
Na.sub.2 O                                                                
        0.50 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10            
K.sub.2 O                                                                 
Fe.sub.2 O.sub.3                                                          
Molar Ratio                                                               
SiO.sub.2 /Al.sub.2 O.sub.3                                               
        6.00 5.20 6.00 6.00 6.00 6.00 6.00 6.00 5.00 4.30 4.30            
Molar Ratio                                                               
Li.sub.2 O/Al.sub.2 O.sub.3                                               
        0.95 0.95 0.90 0.85 0.80 0.80 0.80 0.70 0.70 0.95 0.80            
__________________________________________________________________________
 
    
     Whenever the glass-ceramic falls outside the range of ingredients set forth above, either the coefficient of thermal expansion is too high or the dimensional stability at high temperatures, and/or the strengths of such glass-ceramics during high temperature operations are poor. To show the criticality in the Li 2  O/Al 2  O 3  molar ratio reference is made to Table II wherein several compositions of the invention having different Li 2  O/Al 2  O 3  ratios were compared with two compositions which were outside the range of the invention because of lower ratios. All compositions were melted, drawn as 1/4 inch rods from the melt, and the glass rods heated to form glass-ceramic rods. 
     The glass rods were heated to their nucleating temperature at a rate of 300°F per hour, and held at that temperature for 8 hours. They were then heated to the finishing temperatures set forth in Table II and held there for two hours. All rods were then cooled at 25°F per hour to 1,500°F and then cooled to room temperature at the furnace rate of about 300°F per hour. All compositions had a nucleating temperature of 1,350°F except Compositions 16 and 26which had nucleating temperatures of 1,375° and 1,300°F, respectively. 
     The foregoing heat treatment schedule was used to form the glass-ceramic rods referred to in all subsequent tables of this application, and the nucleating temperatures of the particular compositions will be set forth in the discussions regarding each table. 
     The following batch materials were mixed and melted in a furnace to form the glass of Example 16: 
     
         Ingredients     Parts by Weight                                           
______________________________________                                    
Ottawa silica   3580                                                      
A-10 Alumina    1016                                                      
Titanox         73                                                        
Lithium Zirconate                                                         
                108.1                                                     
Sodium Antimonate                                                         
                20.5                                                      
Lithium Carbonate                                                         
                488                                                       
Lithium Chloride                                                          
                47.8                                                      
Lithium Nitrate 27.6                                                      
______________________________________                                    
 
    
     The sodium antimonate, lithium chloride and lithium nitrate are present as refining agents for the glass melt. The batch was melted at 2,970°F. for a period of 23 hours and mechanically stirred during this time. From the disclosure of the batch of the foregoing Example, those in the glass art can readily make the glasses coming within the present embodiment of the invention. 
     
                                           TABLE II                                
__________________________________________________________________________
Composition No.                                                           
           23    24                                                       
                   7      14     15     16     19    25   26              
__________________________________________________________________________
Ingredients                                                               
(Moles)                                                                   
 Li.sub.2 O                                                               
           1.05  1.0                                                      
                   0.95   0.9    0.85   0.8    0.7   0.6  0.5             
 Al.sub.2 O.sub.3                                                         
           1     1 1      1      1      1      1     1    1               
 SiO.sub.2 6     6 6      6      6      6      6     6    6               
Heat Treatment                                                            
Temp. °F(Time,Hrs)                                                 
           2220(2)                                                        
                 --*                                                      
                   2100(2)                                                
                          2200(2)                                         
                                 2200(2)                                  
                                        2200(2)                           
                                               2100(2)                    
                                                     2200(2)              
                                                          2200(2)         
Dimensional Sta-                                                          
bility, change in                                                         
ppm:                                                                      
at 1500°F                                                          
           +490(10)                                                       
                   +27(100)             +35(100)                          
                                               -13(250)                   
 (Time,Hrs)                                                               
at 1900°F   -196(2000)                                             
                          -104(2000)                                      
                                 -182(2000)                               
                                        +205(2000)                        
                                               -104(500)  +76(100)        
 (Time,Hrs)                                                               
Modulus of Rupture                                                        
(psi, Room Temp.)                                                         
           14,600  11,300 10,500 10,700 11,500 12,200     4300            
Coefficient of                                                            
Thermal Expansion                                                         
×10.sup.+.sup.7 (0-700°C.)                                   
           +3.8    +1.6   +1.0   +1.1   +1.6   +1.8       +8.8            
__________________________________________________________________________
 *Samples broke. At Li.sub.2 O/Al.sub.2 O.sub.3 molar ratios of 1, the    
 resulting glass could not be heat treated to a glass-ceramic since the   
 samples all broke during the heat treating steps.                        
 
    
     The Li 2  O/Al 2  O 3  molar ratios of 1.05 in Composition 23 resulted in the glass-ceramics having very poor dimensional stability at 1,500°F. The expansion of +490 parts per million after only 10 hours at 1,500° F. showed that with longer times the expansion would be considerably greater and the glass-ceramic would not be satisfactory where the product made therefrom would be subjected to temperatures of at least 1,500°F. for extended periods of time. Compositions 24 also was unacceptable since the glass-ceramic broke during the heat treatment. When the Li 2  O/Al 2  O 3  molar ratio was within the range of 0.95 to 0.70, the resulting glass ceramic had good high temperature properties including good dimensional stability at 1,900° F. However, when the molar ratio of Li 2  O/Al 2  O 3  was as low as 0.6 and lower as in Compositions 25 and 26, the resulting glass-ceramics had very low strengths. The samples of Example 25 were very weak at the intermediate heat treatment step of 1,800° to 2,000° F. and were very unsatisfactory. Compositions of the invention, however, had modulus of rupture values in excess of 10,000 psi and are suitable for use at high temperatures over long periods of time because of these strengths. The glass-ceramic of Composition 26 had a coefficient of thermal expansion of 18.2 × 10 -   7  (0°-300°C). Such compositions are unsuitable for the purposes of this invention. 
     Compositions 20, 22, and 29, which are representative of those coming within the scope of the present invention, were compared with other compositions falling outside of the invention because of too low a silica content and a resulting low SiO 2  /Al 2  O 3  ratio. Again, the compositions were melted, glass rods of one-fourth inch diameter were drawn out therefrom and then heat treated according to the schedule set forth in connection with the Table II compositions and the finishing temperatures given in Table III. The nucleating temperatures were 1,250° F. for Composition 28; 1,300° F. for Compositions 22 and 27; and 1,350° F. for Compositions 20 and 29. Various properties of the resulting glass-ceramics were then measured. 
     
                                           TABLE III                               
__________________________________________________________________________
Composition No.                                                           
              22     27     28    20    29                                
__________________________________________________________________________
Ingredients                                                               
 (Moles)                                                                  
  Li.sub.2 O 0.95   0.95   0.7   0.7   0.8                                
  Al.sub.2 O.sub.3                                                        
             1      1      1     1     1.0                                
  SiO.sub.2  4.3    3.2    4.3   5     4.3                                
Heat Treatment                                                            
Temp.°F(Time,Hrs.)                                                 
             2100(2)                                                      
                    2100(2)                                               
                           2100(2)                                        
                                 2200(2)                                  
                                       2100(2)                            
Dimensional Stability                                                     
Parts Per Million                                                         
(Time, Hrs.)                                                              
at 1500°F (hrs)                                                    
             -15(2000)                                                    
                    -298(100)                                             
                           -79(250)                                       
                                 -8(250)                                  
at 1900°F (hrs)           +37(250)                                 
Modulus of Rupture                                                        
(psi room temp)                                                           
             11,200        6,100 13,800                                   
                                       11,300                             
Coefficient of                                                            
Thermal Expansion                                                         
α ×10.sup.+.sup.7 (0-700°C)                            
             +6.8   +5.3   -5.9  +4.2  7.1                                
__________________________________________________________________________
 
    
     Composition 22 gives excellent high temperature and good strength properties. When the SiO 2  /Al 2  O 3  molar ratio is 3.2 as in composition 27, however, the forming properties of the glass are very poor because of the low silica content and the resulting glass-ceramic also has very poor dimensional stability at 1,500°F. The expansion after 100 hrs. indicates that the glass-ceramics will expand considerably more after longer times at this high temperature, thus making it very unsatisfactory for purposes of this invention. As seen from Composition 28, a Li 2  O/Al 2  O 3  molar ratio of 0.7 is too low when the SiO 2  /Al 2  O 3  ratio is 4.3 and results in a glass ceramic having very low strength values. However, with the same low 0.7 Li 2  O/Al 2  O 3  molar ratio, a good glass ceramic is obtained when the SiO 2  /Al 2  O 3  ratio is 5, as shown by Composition 20. 
     Composition 29, which has a lower Li 2  O/Al 2  O 3  molar ratio than Composition 22, still produced a glass ceramic having a high modulus of rupture and a low expansion coefficient, comparable to that of Composition 22. 
     The effect of Li 2  O on compositions having a high SiO 2  /Al 2  O 3  ratio was ascertained and the results are set forth in Table IV. Again, each composition had its batch ingredients melted, and a rod having a one-fourth inch diameter was drawn therefrom and heat treated according to the nucleating and crystallization rates set forth above, using the finishing temperature set forth in Table IV. The nucleating temperature was 1,350°F. for Compositions 3, 30 and 31 and 1,375°F for Compositions 1 and 4. 
     
                                           TABLE IV                                
__________________________________________________________________________
Composition No.                                                           
              4     3     1     30    31                                  
__________________________________________________________________________
Ingredients                                                               
 (Moles)                                                                  
  Li.sub.2 O  0.8   0.95  0.95  0.6   0.8                                 
  Al.sub.2 O.sub.3                                                        
              1     1     1     1     1                                   
  SiO.sub.2   8     8     10    10    10                                  
Heat Treatment                                                            
Temp.°F (Time,Hrs.)                                                
             2100(2)                                                      
                   2100(2)                                                
                         2100(2)                                          
                               2100(2)                                    
                                     2100(2)                              
Dimensional Stability                                                     
Parts Per Million                                                         
(Time, Hrs.)                                                              
at 1500°F (hrs)                                                    
             +71(100)                                                     
                   +96(100)                                               
                         +48(100)    +14(250)                             
at 1900°F (hrs)         -76(250)                                   
Strengths    8,700 11,600                                                 
                         10,700                                           
                               5,100 5,100                                
(psi, room temp)                                                          
Coefficient of                                                            
Thermal Expansion                                                         
              -2.7  -2.8  -4.9 +12.7 +12.4                                
α × 10.sup.+.sup.7 (0-700°C)                           
__________________________________________________________________________
 
    
     Compositions 1, 3 and 4 exhibit acceptable high temperature properties. On the other hand, Compositions 30 and 31 are not suitable for the purpose of this invention because of their high thermal expansion properties and their low strengths. Thus, the 0.7 Li 2  O/Al 2  O 3  ratio found acceptable at other SiO 2  /Al 2  O 3  molar ratios is unacceptable at a SiO 2  /Al 2  O 3  ratio of 10. Further, the glass-ceramic of Composition 30 had a coefficient of thermal expansion of 200.4 × 10 -   7  (0°-300°C) while that of Composition 31 had a coefficient of 21.1 over the same 0°-300°C range. Both compositions are unsuitable for purposes of this invention because of these very poor expansion properties. 
     It should be emphasized that for purposes of this invention, the thermally crystallizable glass compositions must consist of the three essential ingredients, SiO 2 , Al 2  O 3  and Li 2  O plus the nucleating agent(s), all falling within the narrow critical ranges set forth above, both as to weight percent of each ingredient and to molar ratios of Li 2  O/Al 2  O 3  and SiO 2  /Al 2  O 3 . Ingredients and impurities such as Na 2  O, K 2  O, fluorides and the like which are present in substantial amounts in the prior art Li 2  O-SiO 2  -Al 2  O 3  compositions cannot be tolerated above certain very minor levels and preferably should be omitted altogether from the compositions of the present invention. The effect of Na 2  O, K 2  O and F on the dimensional stability of the glass-ceramic compositions is set forth in Table V. All of the glass ceramics were formed from glass rods which were then heat treated at 2,100°F for 2 hours. 
     
                                           TABLE V                                 
__________________________________________________________________________
Composition No.                                                           
             7   8   12  11  32  33  9   10  34  35                       
__________________________________________________________________________
Ingredients                                                               
 (Wt.%)                                                                   
  Na.sub.2 O 0.1 0.3 0.5 0.1 0.5 0.8 0.1 0.1 0.1 0.1                      
  K.sub.2 O  --  --  --  0.2 0.2 0.4 --  --  --  --                       
  Fe.sub.2 O.sub.3                                                        
             --  --  --  --  --  --  0.45                                 
                                         0.1 --  --                       
  F.sub.2    --  --  --  --  --  --  --  --  0.3 0.2                      
Dimensional Stability                                                     
Parts Per Million                                                         
(Time, Hrs.)                                                              
at 1500°F-100 hrs.                                                 
             +27 0   --  --   +613                                        
                                  +307                                    
                                     -19 -9   +750                        
                                                 *                        
at 1500°F-2000 hrs.                                                
              -130                                                        
                 -11 -6  -3  --  --  +5  -74 --  --                       
at 1900°F-2000 hrs.                                                
             -32  +330                                                    
                      +434                                                
                          +987                                            
                             --  --   +168                                
                                         -78 --  --                       
__________________________________________________________________________
 *Composition 35 was unsatisfactory since it crazed on its surface, no    
 matter what heat treatment was used.                                     
 
    
     As evident from the data in Table V, Na 2  O and K 2  O have a very detrimental effect on the dimensional stability at high temperatures of the compositions of the present invention. For example, compositions 32 and 33 have a 0.95:1:6 molar ratio of Li 2  O-Al 2  O 3  -SiO 2  and contain different levels of Na 2  O+K 2  O, namely 0.7 and 1,2 total weight percent, respectively. While compositions 8, 11 and 12 are dimensionally unstable at 1,900°F, they do have satisfactory stability at 1,500°F for 2,000 hours and can be used in those devices wherein a temperature substantially over 1,500°F will not be encountered during normal operation, such as in a regenerator for a gas turbine engine used for trucks. 
     To achieve good dimensional stability at 1,500°F, the Na 2  O should not be present in an amount more than 0.5 weight percent and the K 2  O should not be present in an amount of more than 0.5 weight percent, nor should the combined Na 2  O+K 2  O exceed 0.5 weight percent. To obtain good stability at temperatures of 1,900°F no more than about 0.25 percent Na 2  O or K 2  O can be present, either by themselves or as a mixture. 
     Fe 2  O 3  when present as an impurity in an amount of less than 0.5 weight percent has no detrimental effect on the compositions of the invention. However, fluoride ion, which is known by the art to increase the strength of glass-ceramic compositions when present in small amounts, adversely affects the properties of the compositions of the present invention. Composition 35 contained 0.2 weight percent F and could not be heat treated without the formation of surface scale because of the crazing which took place. Composition 34 had very poor dimensional stability, namely a change of 750 ppm after only 100 hours at 1,500°F. That the effect was due to the F, even though ZnO was present in the composition will become evident from the further disclosure of another embodiment of the invention where it will be demonstrated that ZnO does not have a detrimental effect on dimensional stability at 1,500°F, but, instead, makes the glass-ceramic stronger. Fluorine should be kept to less than 0.1 weight percent and preferably is absent. 
     It is critical in this first embodiment of the invention to have the crystallizable glass and the glass-ceramic consist essentially solely of the three essential ingredients plus the nucleating agent coming within the weight amounts and molar ratios defined above. It is imperative that the inpurities be kept to an absolute minimum. While no impurities should be present it is recognized that it is almost impossible to make glass without some impurities unless only pure materials were used in the batch. This is not possible on any commercial scale because of the costs. However, care should be maintained in selecting batch materials for making the glasses to insure that impurities in the resulting glasses are kept to as low a figure as possible. It has been found that when petalite is used as a batch ingredient, for instance, that the resulting glass-ceramics do not have the desired properties of the glass-ceramics of the present invention. Accordingly, petalite cannot be used as a batch ingredient in making the glasses and glass-ceramics of the present invention. 
     In such other embodiment of the invention it has been found that thermally crystallizable glass compositions consisting essentially of SiO 2 , Al 2  O 3 , Li 2  O and ZnO, wherein each ingredient is within a narrow critical range and also within certain critical molar ratios, can be formed into articles which when heat treated to thermally in situ crystallize them to an at least partially crystalline ceramic, also referred to as a glass-ceramic, exhibit good thermal stability, good flexural strengths and low coefficients of thermal expansion. The presence of the ZnO vastly improves the strengths of the glass-ceramics to almost double the strengths of the compositions of the first embodiment of the invention discussed above. Strengths within the range of about 13,000-20,000 psi and more can be readily obtained. 
     Such compositions coming within the scope of this second embodiment of the invention consist essentially of the following ingredients present within the indicated weight percent ranges and also within the specific molar ratios: 
     
         Ingredient         Weight Percent                                         
______________________________________                                    
SiO.sub.2          52 - 78.4                                              
Al.sub.2 O.sub.3   12.8 - 27.5                                            
Li.sub.2 O         2.4 - 7                                                
ZnO                0.25 - 6.7                                             
Nucleating Agent   3 - 9                                                  
______________________________________                                    
 
    
     wherein the (ZnO + Li 2  O/Al 2  O 3 ) molar ratio is ≦1 and 
     when the (SiO 2  /Al 2  O 3 ) molar ratio is from 4 to less than 5 
     then the (Li 2  O/Al 2  O 3 ) molar ratio is from 0.55-0.91 
     and the (ZnO/Li 2  O) molar ratio is from 0.06 to 0.5; 
     when the (SiO 2  /Al 2  O 3 ) molar ratio is from 5 to 8; 
     then the (Li 2  O/Al 2  O 3 ) molar ratio is from 0.55 to 0.93 
     and the (ZnO/Li 2  O) molar ratio is from 0.04 to 0.5; and 
     when the (SiO 2  /Al 2  O 3 ) molar ratio is from more than 8 and up to 9, 
     then the (Li 2  O/Al 2  O 3 ) molar ratio is from 0.8 to 0.93 
     and the (ZnO/Li 2  O) molar ratio is from 0.04 to 0.5. Usually, in order to obtain the desired properties, the compositions will consist of 98-100 weight percent SiO 2  + Al 2  O 3  + Li 2  O + ZnO + the specified nucleating agents. 
     The nucleating agent can be TiO 2 , ZrO 2  or a mixture of TiO 2  and ZrO 2 . If TiO 2  alone is used, it should be in an amount of at least 3.2 weight percent. If ZrO 2  is used with TiO 2 , it should not exceed 3% and preferably should be less than 3%, i.e. from about 0.5 to about 2.8 weight percent. In the latter event, sufficient TiO 2  must be present to at least give a total of 3 weight percent nucleating agent mixture in the composition. 
     Representative compositions coming within the scope of the second embodiment of the invention are set forth in Table VI. 
     
                                           TABLE VI                                
__________________________________________________________________________
Composition No.                                                           
              36   37   38   39   40   41   42   43                       
__________________________________________________________________________
Ingredients                                                               
(Weight Percent)                                                          
SiO.sub.2     70.25                                                       
                   73.00                                                  
                        72.00                                             
                             72.60                                        
                                  70.2 71.15                              
                                            72.05                         
                                                 70.25                    
Al.sub.2 O.sub.3                                                          
              19.85                                                       
                   17.65                                                  
                        18.80                                             
                             18.85                                        
                                  18.4 18.50                              
                                            18.85                         
                                                 19.85                    
ZnO           1.70 1.70 1.80 1.70 3.7  2.50 1.0  1.70                     
Li.sub.2 O    4.65 4.15 3.85 3.30 4.3  4.30 4.60 4.65                     
TiO.sub.2     1.45 1.40 1.45 1.45 1.45 1.45 1.40 2.40                     
ZrO.sub.2     1.60 1.60 1.60 1.60 1.60 1.60 1.60 0.60                     
Sb.sub.2 O.sub.3                                                          
              0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30                     
Na.sub.2 O    0.10 0.10 0.10 0.10 0.1  0.10 0.10 0.10                     
K.sub.2 O     --   --   --   --   --   --   --   --                       
Fe.sub.2 O.sub.3                                                          
              --   --   --   --   --   --   --   --                       
Molar Ratios                                                              
SiO.sub.2 Al.sub.2 O.sub.3                                                
              6.0  7.0  6.5  6.5  --   6.5  6.5  6.0                      
Li.sub.2 O/Al.sub.2 O.sub.3                                               
              0.8  0.8  0.7  0.6  --   0.8  0.76 0.8                      
ZnO/Li.sub.2 O                                                            
              0.14 0.14 0.16 0.18 --   0.21  .086                         
                                                 0.14                     
ZnO + Li.sub.2 O/Al.sub.2 O.sub.3                                         
              0.91 0.91 0.81 0.71 --   0.97 0.84 0.91                     
__________________________________________________________________________
Composition No.                                                           
              44   45   45   47   48   49   50   51                       
__________________________________________________________________________
Ingredients                                                               
(Weight Percent)                                                          
SiO.sub.2     72.95                                                       
                   72.85                                                  
                        70.75                                             
                             70.75                                        
                                  73.25                                   
                                       72.70                              
                                            68.50                         
                                                 61.65                    
Al.sub.2 O.sub.3                                                          
              17.30                                                       
                   17.60                                                  
                        17.10                                             
                             17.10                                        
                                  19.00                                   
                                       18.85                              
                                            16.55                         
                                                 25.50                    
ZnO           1.70 1.70 1.65 1.65 0.85 1.70 1.60 1.80                     
Li.sub.2 O    4.10 4.15 4.00 4.00 3.35 3.30 3.85 4.50                     
TiO.sub.2     1.40 1.40 6.0  3.00 1.45 1.45 9.0  1.45                     
ZrO.sub.2     1.60 1.60 --   3.00 1.60 1.60 --   1.60                     
Sb.sub.2 O.sub. 3                                                         
              0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30                     
Na.sub.2 O    0.10 0.10 0.10 0.10 0.10 --   0.10 0.10                     
K.sub. 2 O    --   0.20 --   --   --   --   --   --                       
Fe.sub.2 O.sub.3                                                          
              0.45 --   --   --   --   --   --   --                       
Molar Ratios                                                              
SiO.sub.2 /Al.sub.2 O.sub.3                                               
              7.0  7.0  7.0  7.0  6.5  6.5  7.0  4.3                      
Li.sub.2 O/Al.sub.2 O.sub.3                                               
              0.8  0.8  0.8  0.8  0.6  0.6  0.8  0.6                      
ZnO/Li.sub.2 O                                                            
              0.14 0.14 0.14 0.14 0.08 0.18 0.14 0.13                     
ZnO + Li.sub.2 O/Al.sub.2 O.sub.3                                         
              0.91 0.91 0.91 0.91 0.65 0.71 0.91 0.68                     
__________________________________________________________________________
Composition No.                                                           
              52   53   54   55   56   57   58   59A                      
__________________________________________________________________________
Ingredients                                                               
(Weight Percent)                                                          
SiO.sub.2     70.42                                                       
                   73.35                                                  
                        59.80                                             
                             75.30                                        
                                  73.55                                   
                                       75.85                              
                                            72.30                         
                                                 72.55                    
Al.sub.2 O.sub.3                                                          
              18.40                                                       
                   19.15                                                  
                        29.00                                             
                             16.00                                        
                                  19.20                                   
                                       16.10                              
                                            18.85                         
                                                 18.90                    
ZnO           4.40 0.60 2.55 1.40 0.35 0.70 0.85 0.60                     
Li.sub.2 O    3.23 3.35 5.10 3.75 3.35 3.80 4.45 4.40                     
TiO.sub.2     1.45 1.45 1.45 1.45 1.45 1.45 1.45 1.45                     
ZrO.sub.2     1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60                     
Sb.sub.2 O.sub.3                                                          
              0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30                     
Na.sub.2 O    0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10                     
K.sub.2 O     --   --   --   --   --   --   --   --                       
Fe.sub.2 O.sub.3                                                          
              --   --   --   --   --   --   --   --                       
Molar Ratios                                                              
SiO.sub.2 /Al.sub.2 O.sub.3                                               
              6.5  6.5  3.5  8    6.5  8    6.5  6.5                      
Li.sub.2 O/Al.sub.2 O.sub.3                                               
              0.6  0.6  0.6  0.8  0.6  0.8  0.8  0.8                      
ZnO/Li.sub. 2 O                                                           
              0.5  0.06 0.18 0.14 0.04 0.07 0.07 0.05                     
ZnO + Li.sub.2 O/Al.sub.2 O.sub.3                                         
              0.90 0.64 0.71 0.91 0.62 0.86 0.86 0.84                     
__________________________________________________________________________
 
    
     The Li 2  O-ZnO-Al 2  O 3  -SiO 2  compositions of this embodiment of the invention have better heat treatment characteristics than the aforementioned Li 2  O-Al 2  O 3  -SiO 2  compositions of the invention. At heat treatment temperatures of 2,000°-2,200°F, the latter compositions undergo a dimensional expansion which is caused by grain growth and the development of voids or pores at the grain boundaries. The change in length during heat treatment of Compositions 16 and 37 is shown in FIG. 1. Both compositions undergo an initial shrinkage and this shrinkage accompanies the crystallization of the high quartz solid solution phases in the temperature range of 1,350°F to 1,600°F. At temperatures between 1,600°F and 2,000°F, the high quartz phase undergoes a solid state transformation to the keatite solid solution phase. This phase change in Composition 37 is accompanied by a 0.3% lineal expansion at about 1,725°F. At temperatures above 2,000°F, Composition 16 expands by about 2.5% but Composition 37,  which contains ZnO, does not change. 
     The behavior of these compositions at high temperatures gives the ZnO-containing compositions of the invention an advantage over the compositions of the invention which do not contain ZnO. With the latter compositions, the heating rates in the heat treatment cycles must be slowed down in temperature ranges where dimensional changes occur, particularly when an article such as a matrix or assembly for a regenerator, recuperator, thermal exhaust reactor and the like, is being heat treated to thermally in situ crystallize it to form the integral monolithic glass ceramic structure. If the product being heat treated has a temperature gradient, parts of it will tend to change dimensions before other of its parts and mechanical stress resulting in fracture will occur. Thus it takes about a week to heat treat and thus crystallize a product having a substantial thickness of several inches or more made from Composition 16 and less than two days to heat treat and crystallize the same product made from Composition 37. 
     Furthermore, it is easier to predict and control the dimensions of a product made from a glass composition which undergoes very small dimensional changes in the final stage of the heat treatment cycle as it becomes a glass-ceramic product. 
     Another important advantage of the ZnO-containing compositions of the present invention is that they are significantly stronger than the compositions of the invention which do not contain ZnO. The room temperature modulus of rupture of the former is at least 13,000 and is often 20,000 psi or higher as compared to approximately 10,000 psi for the latter. 
     Na 2  O, K 2  O, F 2  and other impurities, when present in the ZnO-containing compositions of the present embodiment of the invention also adversely affect the dimensional stability of the glass-ceramic formed therefrom. Thus, as little as 0.3 weight percent Na 2  O causes poor dimensional stability of the glass-ceramic, namely an expansion of +631 ppm when held at 1,500°F for 250 hours for Composition 59A. Na 2  O+K 2  O in an amount of about 0.3 weight percent also adversely affects the stability as does K 2  O per se when present in such an amount. Thus, an important and essential feature of these compositions is that the weight percent of Na 2  O and K 2  O, either alone or combined be no more than 0.2 weight percent. Fe 2  O 3 , when present as an impurity in an amount less than 0.5 weight percent, has substantially no effect on the stability properties of the glass-ceramic. F, on the other hand, even when present in an amount as low as 0.2 weight percent, imparted a 750 ppm change in Composition 34 after 100 hours at 1,500°F. See Table V, supra. Thus, fluorine should be present in amounts less than 0.1 percent, and preferably not at all. 
     To show the effect of the Li 2  O/Al 2  O 3  molar ratio on the properties of the compositions, five glasses were melted, drawn as 1/4 inch rods and heat treated in accordance with the schedule set forth above with respect to the Table II compositions and where the finishing temperatures are set forth in the following Table VII. All compositions had a nucleating temperature of 1,350°F except for Composition 60, which was 1,300°F. Composition 37 was prepared from the following batch:Ingredient          Parts by Weight______________________________________Ottawa silica       3612A-10 Alumina        888Titanox             70.5Sodium Antimonate   20.5Lithium Carbonate   487Lithium Chloride    48Lithium Nitrate     28Zinc Oxide          70.6Zinc Zirconium Silicate                    176.5______________________________________ 
     The batch was melted at 2,980°F for twenty-two hours while being mechanically stirred. Other compositions coming within the scope of this embodiment of the invention were similarly made, except that the amounts of the ingredients varied accordingly. Those in the art will know how to make all of the compositions of the invention from the foregoing disclosure. 
     Dimensional stability and strengths of each glass-ceramic were then measured. 
     
                                           TABLE VII                               
__________________________________________________________________________
Composition No.                                                           
             37      38      39      60                                   
__________________________________________________________________________
Ingredients                                                               
 (Moles)                                                                  
 Li.sub.2 O  0.8     0.7     0.6     0.4                                  
 Al.sub.2 O.sub.3                                                         
             1       1       1       1                                    
 SiO.sub.2   7.0     6.5     6.5     6.5                                  
 ZnO         .11     .11     .11     .11                                  
Heat Treatment                                                            
Temp.°F(Time,Hrs.)                                                 
             2100(2) 2200(2) 2200(2) 2200(2)                              
Dimensional Stability                                                     
Parts Per Million                                                         
(Time, Hrs.)                                                              
at 1500°F (hrs)                                                    
             -157(2000)                                                   
                     -13.1(25)                                            
                             -164(1000)                                   
                                     48(500)                              
at 1900°F (hrs)                                                    
             --      --      -170(250 hr)                                 
                                     -174(250)                            
                                     -261(500)                            
Modulus of Rupture                                                        
             16,100  16,700  16,600  14,100                               
 (psi room temp)                                                          
Coefficient of                                                            
Thermal Expansion                                                         
             -2.7    -1.9    -1.5    35.8                                 
α × 10.sup.+.sup.7 (0-700°C)                           
__________________________________________________________________________
 
    
     Compositions 37, 38 and 39 have excellent stability after being subjected to the high temperatures for extended periods of time and also have high strengths. Composition 60 which has a low Li 2  O/Al 2  O 3  molar ratio has a very high coefficient of expansion which makes it completely unsatisfactory for purposes of the invention. 
     To show the effect of ZnO to the basic properties of the glass-ceramic, the following glass-ceramics were prepared from corresponding crystallizable glasses, drawn into 1/4 inch rods and heat treated in accordance with the schedule set forth above with respect to the Table II compositions. The finishing temperatures are set forth in Table VIII. The nucleation temperatures are as follows: 1,250°F for Compositions 40 and 41; 1,300°F for Composition 42 and 1,350°F for the remaining four compositions. 
     
                                           TABLE VIII                              
__________________________________________________________________________
Composition No.                                                           
             40    41    37     42     61     62     63                   
__________________________________________________________________________
Ingredients                                                               
 (Moles)                                                                  
 Li.sub.2 O  0.8   0.8   0.8    0.76   0.8    0.8    0.8                  
 Al.sub.2 O.sub.3                                                         
             1     1     1      1      1      1      1                    
 SiO.sub.2   6.5   6.5   7.0    6.5    6.5    6.5    6.5                  
 ZnO         0.25  0.17  0.11   0.07   0.04   0.02   0.01                 
 Molar Ratio                                                              
 ZnO/Li.sub.2 O                                                           
             0.31  0.21  0.14   0.09   0.05   .025   .0125                
Heat Treatment                                                            
             2100(2)                                                      
                   2100(2)                                                
                         2100(2)                                          
                                2200(2)                                   
                                       2100(2)                            
                                              2100(2)                     
                                                     2100(2)              
Temp.°F (Time,Hrs.)                                                
Dimensional Stability                                                     
Parts Per Million                                                         
(Time, Hrs.)                                                              
at 1500°F (hrs)                                                    
             -95(500)                                                     
                   -32(100)                                               
                         -157(2000)                                       
                                -74(2000)                                 
                                       -147(500)                          
                                              +4(250)                     
                                                     -9(250)              
at 1900°F (hrs)                                                    
             --    --    --     --     -119(1000)                         
                                              +156(500)                   
                                                     -147(250)            
Modulus of Rupture                                                        
             20,100                                                       
                   13,800                                                 
                         16,100 15,100 13,400 10,100 9,900                
 (psi room temp)                                                          
Coefficient of                                                            
Thermal Expansion                                                         
             -1    -.2   -2.7   -1.0   +6.0   -0.1   -0.6                 
α × 10.sup.+.sup.7 (0-700°C)                           
__________________________________________________________________________
 
    
     From the above table it can be seen that compositions 37, 40, 41, 42 and 61, which have a SiO 2  /Al 2  O 3  ratio of 0.8 and less have a molar ratio of ZnO/Li 2  O within the range of 0.04 to 0.5 and thus the presence of the ZnO affects and increases the modulus of rupture properties of the glass-ceramics. However, when the ZnO/Li 2  O molar ratio is outside of the range of 0.04 to 0.5, the resulting glass ceramics, while they have good high temperature properties, are no better than glass-ceramic compositions having no ZnO. Thus, in order to achieve the increase in modulus of rupture, the amount of ZnO must come within the critical ranges set forth above, both as to weight percent and as to molar ratios. 
     The most usual compositions of this embodiment of the invention are as set forth hereinbefore but with the weight percent ranges of SiO 2  Al 2  O 3 , Li 2  O and ZnO in the following narrow ranges: 
     
         Ingredient      Weight Percent                                            
______________________________________                                    
SiO.sub.2       58.2 - 78.4                                               
Al.sub.2 O.sub.3                                                          
                13.7 - 26.5                                               
Li.sub.2 O       3.2 -  6.9                                               
ZnO              0.3 -  6.5                                               
______________________________________                                    
 
    
     The effect resulting from too high a SiO 2  level in the compositions is seen from Table IX. The nucleation temperature for Compositions 55, 57 and 65 were 1,350°F and the finishing temperatures and times are set forth in Table IX. Again, the heat treating schedule was the same as that given for the Table II compositions. 
     
                       TABLE IX                                                    
______________________________________                                    
Composition No.                                                           
             64     65         55     57                                  
______________________________________                                    
Ingredients                                                               
 (Moles)                                                                  
 Li.sub.2 O  0.6    0.8        0.8    0.8                                 
 Al.sub.2 O.sub.3                                                         
             1      1          1      1                                   
 SiO.sub.2   10     10         8      8                                   
 ZnO         0.11   0.11       0.11   0.056                               
Heat Treatment                                                            
Temp.°F(Time,Hrs.)                                                 
             --     2100(2)    2200(2)                                    
                                      2200(2)                             
Dimensional Stability                                                     
Parts Per Million                                                         
(Time,Hrs.)                                                               
at 1500°F                                                          
             --     +49(25)    20(10) --                                  
at 1900°F                                                          
             --     155(10)    +23(10)                                    
                                      --                                  
                    low strength                                          
Modulus of Rupture                                                        
 (psi room temp)                                                          
             --     7,000      21,000 20,800                              
Coefficient of                                                            
Thermal Expansion                                                         
             --     +11        -4.6   -4.0                                
α × 10.sup.+.sup.7 (0-700°C)                           
Coefficient of                                                            
Thermal Expansion   +31        --     --                                  
α × 10.sup.+.sup.7 (0-300°C)                           
______________________________________                                    
 
    
     Composition 64 could not be properly crystallized and was a very weak product having a modulus of rupture of 2,000°-3,000 psi. Composition 65 has a very low modulus of rupture and a high coefficient of thermal expansion, both over the 0°-700°C and the 0°-300°C ranges. When the SiO 2  /Al 2  O 3  molar ratio was 8, however, as in Compositions 55 and 57, the strengths of the glass-ceramics tripled over that of composition 65 the expansion coefficients became very low. 
     The effect resulting from too low a SiO 2  level in the compositions is seen from Table X. While Compositions 51 and 54 have good high temperature properties, where the Li 2  O/Al 2  O 3  ratio is as low as 0.6 with low SiO 2  levels, the SiO 2  /Al 2  O 3  should not be lower than 4, because of the very poor forming properties of the glass. Composition 66, on the other hand, did not crystallize and broke during the heat treatment. The nucleation temperature for Compositions 51 and 54 was 1,350°F. 
     
                       TABLE X                                                     
______________________________________                                    
Composition No.                                                           
               51        54        66                                     
______________________________________                                    
Ingredients                                                               
 (Moles)                                                                  
 Li.sub.2 O    0.6       0.6       0.6                                    
 Al.sub.2 O.sub.3                                                         
               1.0       1.0       1.0                                    
 SiO.sub.2     4.3       3.5       4.3                                    
 ZnO           0.08      0.11      0.024                                  
Heat Treatment 2200(2)   2200      --                                     
Temp.°F(Time,Hrs.)                                                 
Dimensional Stability                                                     
Parts Per Million                                                         
(Time, Hrs.)                                                              
at 1500°F                                                          
               -39(250)  -31(250   --                                     
at 1900°F                                                          
               --        -88(250)  --                                     
Modulus of Rupture                                                        
               13,400    16,000    --                                     
 (psi room temp)                                                          
Coefficient of +5.8      +10.8     --                                     
Thermal Expansion                                                         
α × 10.sup.+.sup.7 (0-700°C)                           
______________________________________                                    
 
    
     In all of the work set forth in the foregoing tables, five rods were drawn from each melt and the dimensional stability measurements are based on the average stability of the five rods. For the modulus of rupture tests, 10 rods were drawn from each melt and the strengths set forth in the tables is the average strength of the ten rods. The modulus of rupture tests were performed on a Dillon Universal Tester, Low Range Head, having a 4 point loading with a support span of 4 inches and a loading span of three-fourths inch. The rods were each 51/2 inches in length and the loading rate adjusted so that the samples broke between 36 and 48 seconds. The 10 samples were abraded by tumbling for 15 minutes in a ball mill jar rotating at 75 rpm and containing 250 grams of 240 grit SiC. 
     Compositions 23-35 and 59-66, inclusive, referred to in the above tables have the following compositions expressed in weight percents: 
     
                                           TABLE XI                                
__________________________________________________________________________
Composition Nos.                                                          
          24   23   25   26   27   28   29                                
__________________________________________________________________________
Ingredients                                                               
SiO.sub.2 70.7 70.5 72.4 72.9 61   65.4 64.9                              
Al.sub.2 O.sub.3                                                          
          20   19.9 20.5 20.6 27.8 25.8 25.6                              
ZnO       --   --   --   --   --   --   --                                
Li.sub.2 O                                                                
          5.9  6.1  3.6  3    7.7  5.3  6                                 
TiO.sub.2 1.45 1.45 1.45 1.45 1.45 1.45 1.45                              
ZrO.sub.2 1.6  1.6  1.6  1.6  1.6  1.6  1.6                               
Sb.sub.2 O.sub.3                                                          
          0.3  0.3  0.3  0.3  0.3  0.3  0.3                               
Na.sub.2 O                                                                
          0.1  0.1  0.1  0.1  0.1  0.1  0.1                               
K.sub.2 O --   --   --   --   --   --   --                                
Fe.sub.2 O.sub.3                                                          
          --   --   --   --   --   --   --                                
__________________________________________________________________________
Composition Nos.                                                          
          30   31   32   33   34   35   59                                
__________________________________________________________________________
Ingredients                                                               
SiO.sub.2 80.5 79.8 70.6 70.2 70.6 71.3 72.8                              
Al.sub.2 O.sub.3                                                          
          13.6 13.5 20   19.8 20   20.2 17.6                              
ZnO       --   --   --   --   1.7  --   1.7                               
Li.sub.2 O                                                                
          2.4  3.15 5.3  5.3  3.8  4.8  4.2                               
TiO.sub.2 1.45 1.45 1.45 1.45 1.45 1.45 1.4                               
ZrO.sub.2 1.6  1.60 1.6  1.6  1.6  1.60 1.6                               
Sb.sub.2 O.sub.3                                                          
          0.3  0.3  0.3  0.3  0.3  0.3  0.3                               
Na.sub.2 O                                                                
          0.1  0.1  0.5  0.8  0.1  0.1  0.3                               
K.sub.2 O --   --   0.2  0.4  --   --   --                                
Fe.sub.2 O.sub.3                                                          
          --   --   --   --   --   --   --                                
F.sub.2   --   --   --   --   0.3  0.2  --                                
__________________________________________________________________________
Composition Nos.                                                          
          60   61   62   63   64   65   66                                
__________________________________________________________________________
Ingredients                                                               
SiO.sub.2 73.5 72.5 72.7 72.9 79.3 78.8 65.6                              
Al.sub.2 O.sub.3                                                          
          19   18.9 19   19   13.5 13.4 25.9                              
ZnO       1.7  0.6  0.3  0.1  1.3  1.2  0.5                               
Li.sub.2 O                                                                
          2.25 4.4  4.4  4.4  2.4  3.2  4.6                               
TiO.sub.2 1.45 1.45 1.45 1.45 1.45 1.45 1.45                              
ZrO.sub.2 1.6  1.6  1.6  1.6  1.6  1.6  1.6                               
Sb.sub.2 O.sub.3                                                          
          0.3  0.3  0.3  0.3  0.3  0.3  0.3                               
Na.sub.2 O                                                                
          0.1  0.1  0.1  0.1  0.1  0.1  0.1                               
__________________________________________________________________________
 
    
     While glass-ceramics having high stability when exposed to temperatures as high as 1,900°F over extended periods of time are made from the SiO 2  -Al 2  O 3  -Li 2  O compositions of the first embodiment of the present invention, and while glass-ceramics having greater strengths are produced by the SiO 2  -Al 2  O 3  -Li 2  O-ZnO compositions of the second embodiment of the invention, many of the latter compositions do not produce glass-ceramics which are thermally stable at the 1,900°F temperatures. However, a small group of compositions containing ZnO and coming within the second embodiment of the invention, do have the requisite thermal stability at 1,900°F over a long period of time. Such compositions can be defined as having: 
     A. 1. a SiO 2  /Al 2  O 3  molar ratio of 5-8 
     2. a Li 2  O/Al 2  O 3  molar ratio of 0.55-0.75 
     3. a ZnO/Li 2  O molar ratio of 0.04-0.5, and 
     4. a ZnO + Li 2  O/Al 2  O 3  molar ratio: ≦1, or 
     B. 1. a SiO 2  /Al 2  O 3  molar ratio of 4 to less than 5 
     2. a Li 2  O/Al 2  O 3  molar ratio of 0.55-0.75 
     3. a ZnO/Li 2  O molar ratio of 0.06-0.5, and 
     4. a ZnO + Li 2  O/Al 2  O 3  molar ratio ≦1. 
     Compositions coming within these critical ratios and having good thermal stability at 1,900°F are Nos.: 39, 48, 49, 51, 53-56. 
     A matrix for a regenerative heat exchanger as disclosed in the aforementioned U.S. Pat. application Ser. No. 30,859 was made utilizing composition 37 of the present invention. Such a matrix, broadly speaking, comprises integrally fused tubes forming a series of smooth longitudinal parallel passageways therethrough wherein the walls defining the passageways have (1) essentially zero porosity, (2) consist essentially of an inorganic crystallinne oxide material, (3) a maximum diameter of 0.1 inch, (4) a maximum thickness of about 0.03 to 0.002 inch through portions of the walls common to adjacent fused tubes, with the ratio of the diameter to such thickness being at least 3. The matrix has an open frontal area of at least 60% and the walls of the matrix have a thermal conductivity of less than 0.01 cal/cm/sec/cm 2  /°C. at 400°C. 
     Glass tubing made from Composition 37 above and having an inner diameter of .030 inches and a wall thickness of .0015 inches was cut into lengths 31/2 inches long and each end was sealed, trapping air therein. 
     The sealed tubing was tightly packed parallel to the length of a mold lines with a layer of heat-resistant alumina-silica paper. The tubes within the mold were in a closely-packed parallel relationship with each other. The assembly was then heated in a kiln on the following schedule:Temperature            Time or Rate______________________________________Ambient to 1250°F                       300°F/hr. Hold at 1250°F  8 hrs.1250°F to 2100°F                       100°F/hr. Hold at 2100°F  2 hrs.2100°F to Room Temperature                       300°F/hr.______________________________________ 
     After this heat treatment, the resulting glass- ceramic matrix was removed from the mold and the ends of the tubes opened by grinding. The foregoing heat treatment thermally in situ crystallized the glass and produced a glass-ceramic matrix. The heat treatment had fusion bonded the walls of each tube to those of adjacent tubes and reformed each tube to substantially hexagonal shape. The matrix was suitable as a heat regenerator for a gas turbine engine, able to withstand the operating temperature of at least 1,500°F for an extended period of time without any deleterious thermal instability occurring. 
     The above glass tubing of Composition 37 is also suitable for making the glass-ceramic recuperatore matrix disclosed in U.S. Pat. application Ser. No. 30,859 and in the U.S. Pat. application Ser. No. 146,665 filed May 25, 1971, as a divisional and continuation-in-part thereof. There, a plurality of layers of tubes are superimposed one above the other in successive parallel planes, the tubes within each plane being essentially parallel to each other and transverse to tubes in adjacent layers. When subjected to the same heat schedule described above with respect to the regenerator matrix, each tube becomes integrally fused to each adjacent parallel tube and each adjacent transverse tube. When, as preferred, the tubes are essentially fully expanded, each tube wall is a common wall with each tube adjacent thereto, including those in the same plane and those in adjacent parallel planes. 
     Moreover, when fully expanded, the passageways are essentially in the shape of a parallelogram, usually a square or a rectangle. After the heat treatment and the subsequent opening of the sealed ends, such as by grinding, an integral glass-ceramic recuperator matrix is obtained. 
     Using glass tubing formed from the compositions of the present invention, such as Composition 37, a core assembly for a thermal reactor for receiving the flow of hot gas effluents from a combustion engine, such as that disclosed in the aforementioned Zmudz et al U.S. Pat. application filed Mar. 23, 1971 can be made. 
     Such glass tubing is tightly packed together about a tubular member in parallel relationship, with each end of the tubing being sealed. The outer periphery of the assembly is then constrained from outward movement in a direction perpendicular to the longitudinal passageways formed by the tubing and the tubular member. The assembly is then subjected to a heat schedule such that the tubing becomes softened and the air entrapped therein expands and urges the tubes into tight contact with adjacent tubes. A glass-ceramic matrix comprising a plurality of expanded tubes fusion-bonded together and to the tubular member is thus formed. The sealed ends of the tubing are opened and the matrix can then be utilized in the manner disclosed in the aforesaid Zmuda et al application to make the thermal reactor for a combustion engine.