Nonsilica Glass Systems F. L. JONES Bausch & Lomb Optical Company, Rochester, New York
F
ROM the technological viewpoint, three systems other than silicate systems deserve intensive study. Fluoride, phosphate, and borate glasses resemble silicate glasses in general principles of constitution but differ from each other in important chemical properties. The common constitutional principle is the interlmkage of small strong cations with four surrounding highly polarized and relatively large anions. The total charge of these four anions is about twice that of the central cation. The arrangement is random, having been preserved in this randomness on cooling from the liquid state because the interlinkages are strong enough to prevent organization to the symmetry of the otherwise similarly constituted crystals of the family. FLUORIDE GLASSES
In fluoride glasses the anion is the monovalent fluorine ion, while in oxide glasses, silicates, phosphates, and borates, i t is the divalent oxygen ion. In the single compound glass, BeF2, beryllium is divalent.
Four monovalent fluorine ions surround i t to form groups interlinked a t each of the four fluorines. A polymer (BeF2), results which is entirely analogous to, although more weakly bound together than, the corresponding I
groups forming the polymer (SiOz)., well known in the Symmetrical modifications quartz, tridymite, and cristobalite and the randomly arranged silica glass. Because of the weaker bonds Be% is distinauished from silica by hinh solubility, low viscosity, l ~ ~ d e n s i tlow y, optical;efr&tion, and-absorption. 1t could be the base glass of X-ray windows and of low dispersion optics, but its extensive application is barred by its hygroscopic nature. The combination of fluoride and oxide structure-historically precedmg the discovery of the pure fluoride glass-is a compromise of high technological interest. It is limited by the volatility of fluorine from oxide-glass melts and the tendency of fluorides to crystallize from such melts. Not more than about 20 per cent can be,
DECEMBER, 1946
609
and not more than 10 per cent of oxygen usually is, relaced by fluorine. The presence of those alkaline earths and divalent ions whose fluorides precipitate easily from silicates is avoided. The so-called opal glasses thus formed by the precipitation of fluorides are not a subject of this paper. Potassium, aluminum, or both are used to combine fluorine in clear silicate glasses. The applications of fluoride-oxide glasses are low-viscosity glasses, enamels, vitreous colors, and glasses transparent to ultraviolet light. Glasses containing fluorine are also used where iron-bearing raw materials must be made into glass free from the green color that iron gives to regular silicate types. The iron ion loses its visible color if surrounding oxygens are replaced by F. Fluorine is also used in fluor-crowns-that is, optical glass of low index of refraction and relatively low dispersion. These glasses were formerly imported but they are now available in the U. S. PHOSPHATE GLASSES
In phosphate glasses the corresponding building unit is a PO4 group. However, PS+being pentavalent, more oxygen is available than required in a network where all oxygens are shared between PSS+. The polymer (PzOs). thus formed is lower than (SiOz). but resembles (NazSizOs),,the sodium disilicate glass with the same ratio of oxygen to silicon. The vitreous polymer (PzOs). is more stable than crystalline P206,which has a higher vapor pressure and is only slightly polymerized. Glasses derived from these simple glasses, however, are not at all similar in chemical respect, because the same anionic structure in a silicate glass requires electropositive ions like sodium. It is of high theoretical and practical interest that the combination (or alternation) of trivalent A l 3 + and pentavalent Ps+ leads to glassy AlP04 which is not only formally but also constitutionally equivalent to SiSi04(SiOZ). Glasses between PzOsand AlPO,, such as AIPsOs,the aluminum metaphosphate glass, consisf of PO4 groups only incompletely interlinked witbA104 groups. With a ratio of 0 to (Al P) equal to 9/(3 1) = 2.25 they resemble constitutionally the commercial soda lime glasses Na20.Ca0.6SiOz with 0:Si = 14/6 = 2.3. However, again this similar constitution is achieved in the absence of alkali. On this and other specific chemical differences are based the potential applications of multicomponent phosphate glasses such as the following: (1) enamels and "fluxes" of considerable resistance but low viscosity; (2) extremely resistant and dielectric glasses of normal viscosity; (3) alkalivapor-resistant tubes; (4) hydrofluoric acid-resistant glasses; (5) ultraviolet-transmittant glasses (to about 200 mp) ; (6) special colored glasses; and (7) as a specific case of (6), the neutral and highly transparent iron plasses of high heat absorption. The usual green color caused by a combination of Fez+ and Fea+ in a silicate surrounding is not found in phosphates.
+
a
+
BORATE GLASSES
BzOaforms an oxide glass quite different in constitution from sioz or pZos,rt is a po~ymer~consisting of ~0~groups, which may be represented as triangles in a plane, hi^ random network is more stable than any symmetrical arrangement conceivable. Bz08and borate glasses are of high interest because of their low ~scosity,high transparency to and ultraviolet light, and their relatively low dispersion. ' ~ h ~apir plication is limited by the water solubility of ~~0~ and many berates, H ~ the addition ~ of ~ B ~ ~O to ~silicate ~ glasses ~ causes the solubility and expansivity to decrease to certain minima, and glasses of extremeusefulness are obtained, ~t may to name porcelain enamels, vitrifiable colors on milk bottles, laboratory glasses, The constitutional cause of this agreeable contradiction is the fact that basic oxides, like NazO, permit boron to organize in BO1 groups
I
-0-B
I o I
O==X
0
I
resembling SiOa or No4 groups. Similar combinations exist between B,08 and other oxides. The desire for unusual optical properties has focussed interest on borates free of silica or of low silica content. The recent combination of rare element oxides with each other and BzOa has introduced lenses of very high refraction with.+elatively low dispersion. In the past, barium silicate glasses had been the best in this respect. The gradual substitution of BnOafor SizO4 in a barium crown produces a decrease in the relative dispersion (n,-n /n,-1) with rising refractive index n,. In almost any otherchemical substitution causing a rise in index, the relati~e'dis~ersion increases also. The high BzOa members of the series, however, are attacked by water acidulated with COX. The &responding lanthanum borates possess a very high index and a favorable ratio between index and relative dispersion. The acid resistance may be increased by permitting a certain silica content. Barium lanthanum borate glasses containing some silica and other oxides, such as zirconia, have a somewhat less favorable relative dispersion but a high refractive index and improved chemical resistance. Glasses of this type are the basis for more complex nonsilica glasses that have already proved their value in the photographic field. For general use in the field of optical instruments it is likely that glasses intermediate between the older silica glasses and the new nonsilica glasses will be valuable. The desire for new glasses of exceptional properties will lead to a more thorough study of uncommon glass systems in the future. The. fluoride, phosphate, and borate systems should be more thoroughly explored.
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