Essentially colorless silver-containing glasses through ion exchange

This invention relates to the production of an essentially colorless alkali meal oxide-Al.sub.2 O.sub.3 and/or B.sub.2 O.sub.3 -SiO.sub.2, wherein at least a portion thereof contains silver ions resulting from an ion exchange reaction wherein Ag.sup.+ ions from an external source are exchanged with alkali metal ions in the glass. The amount of exchange can be varied across a portion of the glass to impart a gradient in refractive index thereto. Such technology is especially suited to the optical engineering field, particularly the making of high performance fiber-optic components.

BACKGROUND OF THE INVENTION 
This invention has for a specific objective the production of gradients in 
the refractive index of glass articles through the use of ion exchange 
techniques, such technology being applicable to the optical engineering 
field, particularly the making of high performance fiber optic components. 
The current interest in making high performance fiber optic components has 
led to a resurgence of efforts to further pursue optical engineering 
technologies. One such technology being developed combines ion exchange 
techniques with photolithography for integrating optical waveguides in a 
glass substrate. Optical waveguides can be embedded in a glass substrate 
to create a wide variety of optical circuits and passive optical functions 
for devices such as splitters, stars, wavelength division multiplexers, 
and optical power taps. These functions are readily integrated into a 
single glass substrate to facilitate component miniaturization and 
controlled performance. 
Such research was presented by Kaps, Karthe, Muller, Possner, and Schreiler 
in "Glasses for Optoelectronics," ECO Proceedings, Paris, France, Vol. 
1128, Apr. 24-27, 1989, wherein a special glass type that is favorable for 
silver-sodium ion exchange is described. This special glass is used for 
fabricating channel waveguides and waveguide devices whereby the glass 
substrate is covered with a metal film. Patterns are generated by 
photolithography and a wet etching technique using electron beam written 
masks. 
Ion exchange, a technique for producing gradients in the refractive index 
of glass articles, has been in use since the early sixties. The essence of 
this method lies in the exchange of ions having different 
polarizabilities, viz., exchanging one alkali ion for another. For 
example, U.S. Pat. Nos. 3,524,737 and 3,615,322 describe glass 
strengthening techniques whereby the sodium ion in glass is replaced by 
potassium and copper ions, respectively. Similarly, U.S. Pat. No. 
3,615,323 describes a similar glass strengthening technique, yet with the 
sodium ion being replaced by a lithium ion. Modest changes in refractive 
index are achieved by such exchanges. 
Thallium has commonly been chosen over other elements as a doping ion to 
create regions with a higher refractive index. Large changes in the 
refractive index of glasses have been achieved by the ion exchange of 
thallium; however, the use of thallium is limited to some extent by its 
toxicity. Nevertheless, thallium is the ion most often used today in ion 
exchange processes in spite of its inherent toxicity problems. 
The instant invention discloses a promising alternative to thallium in 
creating large gradients in the refractive index of silica-based 
glasses--the exchange of Ag.sup.+. The exchange of silver for an alkali 
metal produces a change in refractive index comparable to that produced by 
the thallium exchange, yet without the inherent toxicity problems. The 
potential advantages of this exchange have not been previously realized 
because the introduction of more than minimal amounts of silver into a 
silicate glass by ion-exchange techniques has invariably led to extensive 
chemical reduction of silver, with attendant increase in attenuation in 
the optical path. Hence, the intense color which characterizes the 
formation of colloids when silver is reduced is unacceptable for optical 
waveguide applications and, indeed, for most optical applications where an 
essentially colorless, transparent glass is required. 
From the studies on silver dissolved in borosilicate glass, particularly in 
the photochromic glasses, it can be deduced that reduction of silver ions 
can result from the extraction of electrons intrinsic to the glass 
network. Furthermore, the relative ease with which this extraction occurs 
varies strongly with the composition of the glass. Indeed, glasses were 
found in which no reduction occurred. 
Polyvalent impurities such as arsenic or tin can, of course, provide 
electrons leading to the reduction of a small amount of silver and 
consequently can cause a small degree of coloration. It is unrealistic, 
however, to believe that the small levels of impurities typically found in 
these glasses can be responsible for the considerable coloration observed 
in alkali silicate glasses upon the introduction of modest amounts of 
silver. Moreover, scrupulous efforts to exclude polyvalent ions from the 
glass failed to prevent extensive silver reduction. It is therefore an 
object of this invention to provide a prescription for making glasses in 
which the physical properties can be varied within moderately wide limits, 
but in which the amount of silver reduction does not exceed that caused by 
polyvalent ion impurities. 
DESCRIPTION OF THE INVENTION 
The basic product of the present invention is an essentially colorless, 
silver-containing, alkali metal oxide-Al.sub.2 O.sub.3 and/or B.sub.2 
O.sub.3 SiO.sub.2 glass produced through an ion exchange reaction wherein 
Ag.sup.+ ions from an external source are exchanged with alkali metal ions 
in a glass having a base composition wherein the alkali metal oxide and 
the Al.sub.2 O.sub.3 and/or B.sub.2 O.sub.3 are present in such 
concentrations that the glass possesses an atomic structure in which the 
fraction of non-bridging oxygen atoms is less than 0.03. 
The method of making that glass is comprised of the following steps: 
a. forming a glass article having a base composition in the alkali metal 
oxide-Al.sub.2 O.sub.3 and/or B.sub.2 O.sub.3 -SiO.sub.2 system wherein 
the alkali metal oxide and Al.sub.2 O.sub.3 and/or B.sub.2 O.sub.3 are 
present in such concentrations that the glass possesses an atomic 
structure in which the fraction of non-bridging oxygen atoms is less than 
0.03; and then 
b. contacting said article with an external source of Ag.sup.+ ions at a 
temperature of about 350.degree.-750.degree. C. for a sufficient period of 
time to replace at least a portion of the alkali metal ions with Ag.sup.+ 
ions on a one-for-one basis. 
In that region of the glass article subjected to the ion exchange reaction, 
the refractive index will be changed. Accordingly, by varying the area of 
the glass exposed and the time of exposure to the ion exchange reaction, 
it is possible to impart a gradient in refractive index to the glass. 
PRIOR ART 
A variety of ion exchange techniques have been documented in the art. The 
general methodology utilized in such techniques involves the substitution 
of alkali ions of one size for those of another. For example, U.S. Pat. 
Nos. 3,681,041 and 3,533,888 disclose a process for strengthening glass 
articles utilizing an ion exchange technique wherein smaller ions are 
substituted for larger ones. Conversely, U.S. Pat. No. 3,790,430 describes 
a process for strengthening glass articles through an ion exchange process 
wherein alkali metal ions in a surface of a glass article are replaced by 
larger monovalent metal ions. U.S. Pat. Nos. 3,687,649, 3,628,934, and 
3,615,320 disclose similar techniques wherein larger alkali ions are 
substituted for smaller alkali ions. A number of more specific variations 
of ion exchange is exhibited in U.S. Pat. No. 4,053,679 wherein a 
potassium ion is exchanged into opal glass, and U.S. Pat. Nos. 3,524,737, 
3,615,322, and 3,615,323 mentioned previously, whereby the sodium ion in 
soda-lime type glasses is replaced with potassium, copper, and lithium 
ions, respectively. 
None of these references, however, mentions ion exchange of silver. Also, 
no mention of the manipulation of the fraction of non-bridging oxygen is 
made. 
The focus of the instant invention is to manipulate the non-bridging oxygen 
atoms in a glass so that silver ions can be incorporated therein without 
coloring the glass. The use of silver in ion exchange is well documented, 
yet with little success in producing glass articles with little or no 
coloration. However, those references make no mention of manipulating the 
number of non-bridging oxygens to produce colorless glass articles when 
silver is ion exchanged. For example: 
The previously mentioned technical paper authored by Kaps et al. describes 
a special glass type that is favorable for silver-sodium ion exchange, 
this aluminoborosilicate glass being comprised of 25 mole % Na.sub.2 O, 25 
mole % Al.sub.2 O.sub.3, 37.5 mole % SiO.sub.2, and 12.5 mole % B.sub.2 
O.sub.3. The exchange experiment was carried out at a temperature of 
400.degree. C. for 1 hour in a pure silver nitrate salt melt as well as 
several other diluted salt melts. Manipulation of the fraction of 
non-bridging oxygen atoms is not mentioned. 
U.S. Pat. No. 3,425,816 describes a method for chemically strengthening 
alkali metal silicate glasses through an ion exchange reaction employing a 
bath of molten alkali metal salt to avoid coloration of the glass by 
silver ions present in said bath, wherein the silver ions are complexed by 
adding about 0.5-25 per cent by weight of a material capable of providing 
an ion selected from the group consisting of chloride, bromide, iodide, 
cyanide, phosphate, and chromate to the bath to inhibit migration. 
Manipulation of the number of non-bridging oxygen atoms is not mentioned. 
U.S. Pat. No. 3,484,224 discloses a method of strengthening glass, 
particularly soda-lime glass, without the development of any yellow color, 
by a process of etching and cation exchange in melt systems; the cation 
exchange, silver-for-sodium, taking place in systems containing silver 
ions and other cations in which at least 90 per cent of the total cations 
are sodium ions. Manipulation of the number of non-bridging oxygen atoms 
is not mentioned. 
U.S. Pat. No. 3,495,963 details a method of treating glass by simultaneous 
multiple exchange of ions from the same treating bath to color and 
strengthen a base glass which comprises contacting said glass with a 
molten mixture comprising from about 50 to about 99.5 per cent by weight 
of an alkali metal salt selected from the class consisting of sodium and 
potassium salts, and from about 0.5 to 50 per cent by weight of a silver 
metal salt. Manipulation of the fraction of non-bridging oxygen atoms is 
not mentioned. 
U.S. Pat. No. 3,287,201 describes a method of strengthening an alkali metal 
containing glass which comprises replacing the alkali metal ions in a 
surface of the glass by smaller electropositive metal ions selected from 
the group consisting of the ions of alkali metals, copper, silver, and 
hydrogen. Manipulation of the number of nonbridging oxygen atoms is not 
mentioned. 
U.S. Pat. No. 4,108,621 discloses a method of producing a soft aperture 
filter comprising heating a glass having a base composition of 55-72 mole 
% SiO.sub.2, 15-35 mole % Na.sub.2 O, 0-5 mole % divalent oxides other 
than ZnO, 4-15 mole % ZnO, 0-5 mole % Al.sub.2 O.sub.3 with Sb.sub.2 
O.sub.3 and/or As.sub.2 O.sub.3, in a fused bath containing 14-60 mole % 
silver salt, and optionally, NaNO.sub.3 and/or Na.sub.2 SO.sub.4. 
Manipulation of the number of non-bridging oxygen atoms is not mentioned. 
U.S. Pat. No. 4,022,628 details an ion exchange-strengthened silicate glass 
filter and a method for making such via incorporating cerium oxide into 
the glass followed by ion-exchange strengthening using a mixture of 
potassium salt and silver nitrate. Manipulation of the fraction of 
non-bridging oxygen atoms is not mentioned. 
U.S. Pat. No. 3,508,895 describes a method of strengthening a soda-lime 
glass body by producing a compressive layer on the order of at least 2 to 
3 microns in depth at the surface of the body to strengthen the same; the 
viscosity of said surface layer being altered by substituting fluorine 
ions for non-bridging oxygen ions and/or hydroxyl ions to a sufficient 
depth to produce a surface layer of more viscous glass of a thickness 
sufficient to provide a compressive stress layer. Nevertheless, 
manipulation of the number of non-bridging oxygen atoms for the purpose of 
avoiding silver reduction is not mentioned. 
U.S. Pat. No. 3,873,408 discloses a method of increasing the refractive 
index of a surface layer of predetermined depth of a glass body comprising 
the step of maintaining the body in contact with a molten source of ions 
for at least 1 hour at a temperature between 800.degree. and 1100.degree. 
F, the ions being selected from the group consisting of silver, thallium, 
and copper ions, and mixtures thereof. Manipulation of the fraction of 
non-bridging oxygen atoms is not mentioned.

DESCRIPTION OF PREFERRED EMBODIMENTS 
One specific embodiment of the present invention is the glass article and 
method of making such wherein (a) up to 7.5 cation per cent Al.sub.2 
O.sub.3 and/or B.sub.2 O.sub.3 is replaced with at least one metal oxide 
selected from the group consisting of BeO, CaO, MgO, ZnO, Ga.sub.2 
O.sub.3, La.sub.2 O.sub.3, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, Yb.sub.2 
O.sub.3, and ZrO.sub.2, while maintaining the fraction of non-bridging 
oxygen atoms at less than 0.03, and (b) said glass article is contacted 
with a source of Ag.sup.+ ions at a temperature between 400.degree. and 
600.degree. C. for a period of time between 2 and 24 hours. 
The nature of the invention will be more easily comprehended if a few facts 
about bonding in glass are understood. In amorphous silica each oxygen 
atom is covalently bonded to two silicon atoms and is therefore called a 
bridging oxygen atom. When ionic oxides such as alkali metal oxides are 
added to silica, some non-bridging oxygen atoms are formed. These are 
oxygen atoms that are covalently bonded to only one silicon atom and, 
hence, bear a negative charge (an electron) which must be compensated by a 
vicinal positively charged ion such as the alkali ion. The inventor has 
discovered that it is this electron responsible for the negative charge 
observed on non-bridging oxygen atoms which can be extracted by silver 
ions to produce silver colloids. 
Certain very small highly charged positive ions, the most notable of which 
is the aluminum ion, are most stable in a glass when they are surrounded 
by four oxygen atoms, thereby creating an environment which has 
tetrahedral symmetry. The bonds between the high field strength ion, i.e., 
the highly charged positive ion, and its four oxygen neighbors are highly 
covalent. If the formal charge, which has been designated in the 
scientific literature as Z, of the high field strength ion is less than 
four, then a negative charge of 4-Z must be distributed over the four 
neighboring oxygen atoms. Such a bonding arrangement, in effect, removes 
non-bridging oxygen atoms from the system. The removal of non-bridging 
oxygen atoms and the formation of tetrahedrally coordinated aluminum ions, 
when alumina is added to an alkali silicate glass, has been universally 
recognized for many years. The present inventor discovered that silver 
ions cannot extract an electron from the oxygen atoms bonded to aluminum. 
The reason for this, presumably, is the fact that the electron associated 
with a tetrahedrally bonded aluminum atom is much more delocalized than 
that associated with a non-bridging oxygen and, hence, is more stable. In 
addition to aluminum, trivalent gallium and ytterbium exhibit this 
behavior when introduced in modest amounts (.apprxeq.7.5 cation %) into 
silicate glasses containing large amounts of alkali. As shall be shown 
below, even trivalent lanthanum, in spite of its large size, seems to 
exhibit this behavior. 
Niobium and tantalum show analogous behavior. Even though these ions do not 
exist in an environment with tetrahedral symmetry in silicate glasses, 
their high field strength, in effect, removes non-bridging oxygen atoms 
from the system. Thus, the use of any of these ions or the use of several 
of them in combination will have the effect of diminishing the number of 
non-bridging oxygen atoms. 
To illustrate the efficacy of the instant invention, silver was introduced 
into several glasses containing varying ratios of alumina to alkali by an 
ion exchange technique. According to the principles of glass structure 
outlined above, the fraction of oxygen atoms in an alkali alkaline earth 
aluminosilicate glass which are bonded into the structure as non-bridging 
oxygen (NBO) atoms is given by the equation, 
EQU NBO=(M.sub.2 O+2MO-Al.sub.2 O.sub.3) /(2SiO.sub.2 +1.5Al.sub.2 O.sub.3 
+MO+0.5M.sub.2 O), 
where all the concentrations are expressed in cation percent. (M.sub.2 O 
designates a monovalent metal oxide and MO represents a divalent metal 
oxide.) This equation accurately describes the fraction of non-bridging 
oxygen atoms in silicates, wherein lanthanum, ytterbium, zirconium, 
hafnium, niobium, or tantalum may be used to replace some of the alumina, 
and wherein a negative sign multiplies the coefficient of the alkaline 
earth when magnesium, calcium or zinc is used. 
However, in borosilicates the fraction of non-bridging oxygen atoms must be 
interpolated from other sources, such as the work of Y.H. Yun and P.J. 
Bray, in "Nuclear Magnetic Resonance Studies of Glasses in the System 
Na.sub.2 O-B.sub.2 O.sub.3 -SiO.sub.2," Journal of Non-Crystalline Solids, 
Vol. 27, pp. 363-380 (1978) and of R.J. Araujo and J.W.H. Schreurs in 
"Tetrahedral Boron in Sodium Aluminoborate Glasses," Physics and 
Chemistry of Glasses, Vol. 23, pp. 108-109 (1982). The structures of 
borates and borosilicates at room temperature have been well studied by 
the use of nuclear magnetic resonance and the number of non-bridging 
oxygen atoms in borosilicates is easily determined by reference to such 
studies. These workers found that the number of non-bridging oxygen atoms 
is well correlated to a special ratio, R, which is defined by the equation 
EQU R=(M.sub.2 O-Al.sub.2 O.sub.3)/B.sub.2 O.sub.3 
where all the concentrations are expressed in cation percent. The ratio of 
tetrahedral boron atoms to non-bridging oxygen atoms bonded to boron atoms 
depends on temperature; but at temperatures normally used for ion 
exchange, the dependence is very weak and usually may be ignored. 
All of the glasses listed in Table 1 were immersed in molten silver 
chloride at a temperature of 500.degree. C. for ten days. Diffusion of 
silver throughout the thickness of the sample is expected to be complete 
and the amount of silver introduced represents its equilibrium value. 
Since no concentration gradients are expected to result from such a long 
immersion, the change in refractive index as a function of the amount of 
silver introduced is easily determined. Furthermore, the coloration 
observed represents the worst possible case. In any application wherein a 
gradient in silver concentration is desired, the total silver introduced 
into the sample would be less than that found in the present samples. 
EXAMPLE 1 
Extensive reduction of the silver in glass #1 was indicated by a deep 
magenta color when the sample was viewed in transmitted light, and by a 
green color when the sample was viewed in reflected light. Microscopic 
examination indicated the existence of a plethora of gaseous inclusions. 
Mass spectrometry disclosed no evidence that the bubbles contained 
chlorine. Thus, one can rule out the possibility that chloride ions 
diffusing into the glass concurrently with the exchange of cations 
provided the electrons for the reduction of silver. Consistent with the 
thesis that the non-bridging oxygen atoms supply the electrons, was the 
observation that the bubbles contained primarily oxygen. Observation of 
small amounts of nitrogen in some of the bubbles has not been explained. 
EXAMPLE 2 
Glass #2 was melted to provide a sample with somewhat fewer, but 
nonetheless a plentiful supply of, non-bridging oxygen atoms. After it had 
been subjected to ion exchange, the sample was black and contained large 
shiny specks that appeared to be metallic silver. The sample appeared to 
have crystallized extensively. 
EXAMPLE 3 
Glass #3 was melted to provide a glass with still fewer non-bridging oxygen 
atoms than were possessed by the previous glasses. Nevertheless, extensive 
reduction of silver is observed. 
EXAMPLE 4 
In glass #4 only slightly more than 1% of the oxygen atoms are of the 
non-bridging type. Ion exchange of this glass resulted in several small 
areas which manifested a red color much paler than that seen in sample #1. 
Furthermore, no scattered green light was observed nor were any gas 
bubbles present. The largest fraction of the glass displayed only a pale 
yellow color. Microprobe examination revealed that about 18% of the alkali 
had been replaced by silver in either area. 
EXAMPLE 5 
Glass #5 was designed to contain an amount of alumina exactly equal to the 
total alkali in an attempt to completely eliminate the non-bridging atoms. 
Ion exchange produced areas that were pale yellow and areas that were 
completely colorless. Once again, microprobe examination failed to 
identify any difference in the degree of ion exchange in different areas. 
In this glass 23% of the sodium ions were replaced by silver ions as a 
result of the ion exchange. Considering the large amount of silver (5.1 
cation %) in this glass, the amount of coloration is small indeed. These 
observations are taken as strong support for the thesis that non-bridging 
oxygen atoms were instrumental in producing the extensive silver reduction 
always observed in previous experiments. The negligible yellow color 
observed in some areas might be attributable to reduction by traces of 
polyvalent ions. Spark spectra analysis indicated a level of iron between 
0.003 and 0.01 weight per cent. 
EXAMPLE 6 
In an attempt to demonstrate that silver would not be reduced when the 
fraction of non-bridging oxygen atoms was kept sufficiently low no matter 
which alkalis were present, glass #6 was melted. In spite of an alkali 
content very slightly in excess of the alumina, the glass was completely 
colorless after ion exchange. The silver content exceeded 17% by weight 
and produced a refractive index change of 0.042. The alkalis differed in 
their tendency to be replaced by silver. Five per cent of the lithium, 
eight percent of the sodium and thirty-eight per cent of the potassium 
ions were replaced. 
EXAMPLE 7 
A second sample of this glass was ion-exchanged at 650.degree. C. In this 
latter sample, the silver content after ion exchange exceeded 23% by 
weight. The refractive index change caused by the replacement of alkalis 
with silver was 0.06. About fifteen per cent of the sodium and lithium and 
about fifty-five per cent of the potassium ions were replaced in this 
experiment. The increased amount of exchange is, of course, not related to 
larger diffusion coefficients, since the samples were uniformly exchanged 
throughout in any case. Rather, it indicates that the chemical potential 
of the alkalis in the molten silver halide decreases as the temperature 
increases. 
EXAMPLE 8 
To test the influence of the anion in the salt bath, a third sample was 
immersed in a salt bath comprised of 70% by weight of silver chloride and 
30% of silver sulfate. In this case, approximately 90% of the alkalis was 
replaced. A refractive index change of 0.132 was produced. All of the 
exchange treatments of this glass produced samples that were virtually 
colorless. 
EXAMPLE 9 
A series of sodium aluminosilicates characterized by various concentrations 
of non-bridging oxygens was ion-exchanged at 400.degree. C. for two hours 
in a bath of molten AgCl. The amount of silver introduced into the glasses 
by this treatment is not uniform through the thickness of the glass and is 
much lower than the amounts introduced into the glasses cited in Table 1. 
This experiment involves levels of silver more typical of those required 
to produce index gradients for many applications. The experiment was 
performed primarily so that only slight amounts of coloration were 
produced, thus permitting the measurement of visible absorption spectra. 
The intensity of the spectra is indicated in the Table 2 by citing the 
wavelength at which fifty per cent of the light is absorbed. The longer 
this wavelength, the more intense is the absorption. The first two glasses 
show only very slight yellow coloration consistent with the short 
wavelength cutoff [.lambda.(0.5)]. Sample #10 is a noticeably darker 
yellow. Sample #11 is a very deep yellow, sample #12 is orange and sample 
#13 is dark brown. Thus, a very clear relationship is established between 
the degree of silver reduction and the number of non-bridging atoms in 
these simple glasses. 
TABLE 1 
__________________________________________________________________________ 
(Amount in Cation Percent) 
Sample 
1 2 3 4 5 6 7 
__________________________________________________________________________ 
SiO.sub.2 
53.50 
40.50 
40.00 
37.50 
35.00 
37.50 
35.00 
Al.sub.2 O.sub.3 
7.00 
20.00 
27.50 
30.00 
32.25 
31.25 
32.25 
Li.sub.2 O 
-- -- -- -- -- -- 7.00 
Na.sub.2 O 
39.50 
39.50 
32.50 
32.50 
32.75 
31.25 
1.00 
K2O -- -- -- -- -- -- 24.75 
NBO 0.237 
0.149 
0.036 
0.013 
0.004 
0.000 
0.004 
Color 
magenta/ 
black 
black/ 
red/ 
clear 
pale 
clear 
green bronze 
yellow yellow 
__________________________________________________________________________ 
TABLE 2 
______________________________________ 
(Amount in Cation Percent) 
Sample 8 9 10 11 12 13 
______________________________________ 
SiO.sub.2 
35.00 37.50 55.00 45.00 55.00 45.00 
Al.sub.2 O.sub.3 
32.50 30.00 17.50 22.50 15.00 20.00 
Na.sub.2 O 
32.50 32.50 27.50 32.50 30.00 35.00 
NBO 0.000 0.018 0.067 0.071 0.102 0.109 
.lambda.(0.5) 
395 398 418 450 534 627 
______________________________________ 
The relationship between the amount of silver reduction and the fraction of 
non-bridging oxygen atoms is equally valid in borosilicate glasses, albeit 
the calculation of the number of non-bridging oxygen atoms is somewhat 
more complicated. Table 3 illustrates the relationship observed in 
borosilicate glasses utilizing an ion exchange conducted for two hours in 
a bath of molten AgCl at a temperature of 400.degree. C. Samples #14, #15, 
and #16 are examples of essentially colorless, silver-containing, alkali 
metal oxide-Al.sub.2 O.sub.3 and borosilicate glass whose properties have 
been improved by the ion exchange of silver into a borosilicate glass 
having an R value (as defined above) that is no higher than 1. More 
specifically, the properties of such glasses may be improved by the 
introduction of silver ions into a borosilicate glass containing at least 
45 cation percent silica, at most 15 cation per cent alumina, and having 
an R value that is not greater than 0.6. The first two glasses appear to 
the naked eye to be completely colorless while a very pale yellow color 
can be detected in sample #16. A deep orange color is observed in sample 
#17. It is not known whether the shift in .lambda.(0.5) in sample #15 is 
due to polyvalent ion impurities or to a slight error in the estimated 
value of NBO introduced by assuming the temperature of ion exchange to lie 
within the realm of the low temperature limit. 
TABLE 3 
______________________________________ 
(Amount in Cation Percent) 
Sample 14 15 16 17 
______________________________________ 
SiO.sub.2 50.00 50.00 50.00 50.00 
Al.sub.2 O.sub.3 
15.00 5.00 15.00 15.00 
B.sub.2 O.sub.3 
15.00 25.00 10.00 5.00 
Na.sub.2 O 20.00 20.00 25.00 30.00 
R value 0.330 0.600 1.000 3.000 
NBO 0.000 0.000 0.027 0.103 
.lambda.(0.5) 
318 340 380 525 
______________________________________ 
The effect of temperature upon the fraction of non-bridging oxygen atoms is 
discussed by R.J. Araujo in two articles published in the Journal of 
Non-Crystalline Solids entitled "Statistical Mechanics of Chemical 
Disorder: Application to Alkali Borate Glasses," 58, pp. 201-206 (1983) 
and "The Effect of Quenching on the Color of Glasses Containing Copper," 
71, pp. 227-230 (1985). 
Very high temperatures were required to melt the silicate glasses in which 
the number of non-bridging oxygen atoms was very low. Lower melting 
glasses having few or no non-bridging oxygen atoms at the temperature of 
ion exchange can be obtained by the use of ions which are known to behave 
like alumina, in that they remove non-bridging oxygen atoms from alkali 
silicates. These ions include hafnium, tantalum, niobium, and zirconium 
which, because of their large positive charge, have a high field strength 
and remove non-bridging atoms from the system in spite of their large 
size. Also included are the small ions such as zinc, beryllium, magnesium, 
or calcium, which remove non-bridging oxygen atoms in the process of 
attaining tetrahedral coordination. Although hafnium and tantalum are not 
likely to be used extensively because of their cost, the efficacy of 
tantalum is illustrated in Table 4, along with the efficacy of the small 
ions. The temperature (Temp.) at which each ion exchange was conducted for 
two hours in a bath of molten AgCl is recorded in the table. 
TABLE 4 
______________________________________ 
(Amount in Cation Percent) 
Sample 
18 19 20 21 22 
______________________________________ 
SiO.sub.2 
35.00 35.00 35.00 35.00 35.00 
Al.sub.2 O.sub.3 
27.50 30.00 30.00 27.00 25.00 
Ta.sub.2 O.sub.5 
-- -- -- 5.00 7.50 
ZnO 5.00 2.25 -- -- -- 
MgO -- -- 2.50 -- -- 
Na.sub.2 O 
16.25 10.00 16.25 8.00 8.00 
K.sub.2 O 
16.25 22.75 16.25 25.00 24.50 
Temp. 500 675 675 750 750 
NBO 0.000 0.054 0.056 0.007 0.000 
Color yellow black black yellow 
yellow 
______________________________________ 
The efficacy of magnesium, gallium, yttrium, lanthanum, hafnium, and 
niobium, in glasses ion exchanged in the low temperature regime is 
documented in Table 5. Magnesium in the low temperature regime, in 
contrast to the high temperature regime illustrated in Table 4, is quite 
effective in removing non-bridging oxygen atoms. Surprisingly, even 
strontium, which is almost as large as barium, while being associated with 
more coloration than observed in glass #23, produces no more color than 
observed in glass #9 in Table 2. Thus, while it does not remove 
non-bridging oxygens from the system as effectively as MgO, it does not 
introduce non-bridging oxygens in like manner to BaO. Each of the ion 
exchange reactions listed in Table 5 was conducted for two hours at 
400.degree. C. in a bath of molten AgCl. 
TABLE 5 
__________________________________________________________________________ 
(Amount in Cation Percent) 
Sample 
23 24 25 26 27 28 29 
__________________________________________________________________________ 
SiO.sub.2 
35.00 
35.00 
35.00 
35.00 
35.00 
35.00 
35.00 
Al.sub.2 O.sub.3 
30.00 
30.00 
30.00 
30.00 
30.00 
30.00 
30.00 
Na.sub.2 O 
32.50 
32.50 
32.50 
32.50 
32.50 
32.50 
32.50 
MgO 2.50 
0.000 
0.000 
0.000 
0.000 
0.000 
0.000 
SrO 0.000 
2.50 0.000 
0.000 
0.000 
0.000 
0.000 
Y.sub.2 O.sub.3 
0.000 
0.000 
2.50 
0.000 
0.000 
0.000 
0.000 
Ga.sub.2 O.sub.3 
0.000 
0.000 
0.000 
2.50 0.000 
0.000 
0.000 
Nb.sub.2 O.sub.5 
0.000 
0.000 
0.000 
0.000 
2.50 
0.000 
0.000 
HfO.sub.2 
0.000 
0.000 
0.000 
0.000 
0.000 
2.50 0.000 
La.sub.2 O.sub.3 
0.000 
0.000 
0.000 
0.000 
0.000 
0.000 
2.50 
NBO 0.000 
0.060 
0.000 
0.000 
0.000 
0.000 
0.000 
.lambda.(0.5) 
365 380 360 360 365 390 360 
Color 
clear 
yellow 
clear 
clear 
clear 
yellow 
clear 
__________________________________________________________________________ 
Whereas in the above examples a bath of a molten silver salt(s) was 
employed as an external source of Ag.sup.+ ions, it will be appreciated 
that other means, such as ion implantation, may be used to provide the 
external source of Ag.sup.+ ions.