This invention relates to the production of highly transparent glass-ceramic articles containing about 30-50% by volume gahnite having dimensions finer than about 300 .ANG. as essentially the sole crystal phase. The articles have compositions essentially free of Li.sub.2 O, Na.sub.2 O, CaO, SrO, and BaO and consist essentially, by weight, of about 1.5-5% R.sub.2 O, consisting of 0-3% K.sub.2 O and/or 0-3% Rb.sub.2 O and/or 0-3% Cs.sub.2 O, 13-22% ZnO, 16-28% Al.sub.2 O.sub.3, 45-65% SiO.sub.2, and 1.5-8% TiO.sub.2.

BACKGROUND OF THE INVENTION 
U.S. Pat. No. 3,681,102 disclosed the production of transparent 
glass-ceramic articles wherein the zinc spinel gahnite (ZnO.Al.sub.2 
O.sub.3) constituted the predominant crystal phase. That patent described 
the preparation of such articles as involving three general steps: first, 
a glass forming batch consisting essentially, expressed in terms of weight 
percent on the oxide basis, of about 2-20% ZnO, 8-25% Al.sub.2 O.sub.3, 
55-75% SiO.sub.2, and 2-12% ZrO.sub.2 as the nucleating agent was melted 
at 1650.degree.-1800.degree. C; second, that melt was cooled to a 
temperature below the transformation range thereof and simultaneously a 
glass body of a desired configuration was shaped therefrom; and, third, 
that glass body was exposed to a temperature between about 
975.degree.-1250.degree. C. for a sufficient length of time to cause the 
development of nuclei and the subsequent growth of gahnite crystals on 
those nuclei. (The transformation range has been defined as the 
temperature at which a liquid melt is deemed to have become a vitreous 
solid, that temperature generally being considered as residing in the 
vicinity of the annealing point of the glass.) 
The crystals were described as having diameters less than 1000.ANG. and as 
comprising between 20-50% by weight of the articles; the residual glassy 
matrix having a highly siliceous content. The final products were stated 
to exhibit linear coefficients of thermal expansion 
(25.degree.-300.degree. C.) ranging between about 25-40.times.10.sup.-7 
/.degree. C., and to maintain their transparency for extended periods of 
exposure at temperatures up to 1000.degree. C. and for brief intervals at 
1200.degree. C. 
The patent also disclosed the incorporation of 0.01-1.0% Cr.sub.2 O.sub.3 
into the base compositions to produce Cr.sub.2 O.sub.3 -doped gahnite 
crystals, thereby imparting red photoluminescence to the glass-ceramics. 
The inclusion of Cr.sub.2 O.sub.3 did not adversely affect the 
transparency or other physical properties of the products. 
As optional additional ingredients, the patent observed the use of up to 5% 
MgO, up to 10% total of BaO, CaO, SrO, K.sub.2 O, Rb.sub.2 O, and Cs.sub.2 
O, and up to 5% total of Li.sub.2 O, Na.sub.2 O, B.sub.2 O.sub.3, and 
TiO.sub.2. The working examples, however, contained only MgO as an 
optional addition. 
SUMMARY OF THE INVENTION 
The present invention had its basis in U.S. Pat. No. 3,681,102 and is an 
improvement upon the products of that disclosure. Thus, the present 
invention is founded in the discovery that transparent glass-ceramic 
articles containing gahnite as the predominant crystal phase, but 
exhibiting much improved physical properties, can be prepared from a 
narrow range of compositions within the R.sub.2 O-ZnO-Al.sub.2 O.sub.3 
-SiO.sub.2 system, wherein R.sub.2 O consists of K.sub.2 O and/or Rb.sub.2 
O and/or Cs.sub.2 O, which are nucleated with TiO.sub.2. To illustrate: 
The precursor glass forming batches for the present glass-ceramics can be 
melted at lower temperatures, viz., about 1600.degree. C. Such lower 
melting temperatures provide two significant advantages. First, 
conventional, large scale commercial glass melting tanks may be employed. 
Second, the energy cost for melting the batch is significantly reduced. 
The instant glass-ceramics also demonstrate greater refractoriness, as is 
evidenced by their retention of transparency after extended periods of 
exposure to temperatures up to 1100.degree. C. That increased 
refractoriness is believed to be due to two factors: first, the crystal 
content developed in the inventive glass-ceramics can exceed 50%, with a 
consequent decrease in the proportion of residual glassy phase; and, 
second, the crystals are exceedingly small, with diameters of less than 
about 300.ANG., normally only about 100-200.ANG., such that grain growth 
of those crystals must be quite considerable before haze resulting from 
light diffusion becomes substantial. 
The increased crystal content can also give rise to bodies exhibiting 
improved flexural strength. 
Also, in like manner to the products of U.S. Pat. No. 3,681,102, the 
compositions of the present invention can be doped with Cr.sub.2 O.sub.3 
to yield glass-ceramics which absorb ultraviolet radiation and fluoresce 
in the red and near infrared regions of the spectrum. Such capability has 
suggested applications for the inventive glass-ceramics in tunable lasers 
and in solar collectors. 
Because of the high efficiency of TiO.sub.2 as a nucleating agent, 
compositions operable for preparing glass-ceramic bodies containing 
gahnite as virtually the sole crystal phase and exhibiting the 
transparency and the physical properties desired in the inventive products 
must be essentially free of Li.sub.2 O, Na.sub.2 O, CaO, SrO, and BaO in 
order to avoid the development of other crystal phases during heat 
treatment of the precursor glass body. With care, minor amounts of MgO, 
perhaps up to 2% by weight, may be included to enter the gahnite crystal 
structure so as to produce a zinc spinel solid solution (ZnO, MgO). 
Al.sub.2 O.sub.3 comprising up to 20 mole percent MgO, while still 
maintaining transparency in the crystallized bodies. Nevertheless, the 
inclusion of MgO does not appear to improve the physical properties 
demonstrated by the glass-ceramics, and its presence hazards the 
development of such unwanted silicate phases as 7/8-quartz solid solutions 
which impair transparency. Therefore, its essential absence is generally 
preferred. 
In summary, compositions operable in the present invention to produce 
transparent glass-ceramic articles containing gahnite as essentially the 
sole crystal phase consist essentially, expressed in terms of weight 
percent on the oxide basis, of about 1.5-5% R.sub.2 O, consisting of 0-3% 
K.sub.2 O+O-3% Rb.sub.2 O+O-3% Cs.sub.2 O, 13-22% ZnO, 16-28% Al.sub.2 
O.sub.3, 45-65% SiO.sub.2, and 1.5-8% TiO.sub.2 as the nucleating agent. 
The presence of K.sub.2 O and/or Rb.sub.2 O and/or Cs.sub.2 O is essential 
to obtaining transparency in the crystalline body. The use of Rb.sub.2 O 
and/or Cs.sub.2 O instead of K.sub.2 O yields products demonstrating 
better electrical properties, since their larger ionic size more 
effectively inhibits their migration through the residual glass. Minor 
amounts of ZrO.sub.2 may be included in the compositions without adversely 
affecting the properties of the crystallized products. Nonetheless, its 
inclusion is unnecessary as a nucleating agent inasmuch as TiO.sub.2 is a 
much more efficient agent. Moreover, ZrO.sub. 2 is much less soluble than 
TiO.sub.2 in the glass melt; hence, its presence hazards unmelted 
particles and/or devitrification in the glass and requires higher melting 
temperatures. Finally, ZrO.sub.2 frequently contains trace amounts of the 
radioactive element thorium. The presence of that element limits the 
applications in which the products can be employed. The inventive 
glass-ceramics can be greater than 50% by volume crystalline and exhibit 
linear coefficients of thermal expansion (0.degree.-300.degree. C.) 
between about 30-50.times.10.sup.-7 /.degree. C. Such a range of thermal 
expansion enables products to be prepared which match the thermal 
expansion of silicon, thereby recommending their utility as a substrate 
for the active matrix in liquid crystal displays and for integrated 
circuit packaging. 
The inventive articles are produced in accordance with the following three 
general steps: (1) a glass forming batch for a glass having a composition 
within the above-cited ranges of components is melted; (2) that melt is 
cooled to a temperature at least below the transformation range thereof 
and simultaneously a glass article of a desired configuration is shaped 
therefrom; and (3) that glass article is exposed to a temperature within 
the interval of 900.degree.-1100.degree. C. for a period of time 
sufficient to cause the growth of gahnite crystals in situ. To insure the 
development of very fine-grained crystals of uniform dimensions, i.e., 
having diameters of less than 300.ANG., a two-stage heat treatment of the 
glass article may be employed. Such practice contemplates initially 
subjecting the glass article to a temperature within or slightly above the 
transformation range (about 775.degree.-825.degree. C.) for a sufficient 
period of time to generate a myriad of nuclei therein. The nucleated 
article is then heated to 900.degree.-1100.degree. C. to grow gahnite 
crystals on the nuclei. Hence, the combination of a nucleation period of 
about 1-8 hours followed by a crystallization period of about 2-8 hours 
has yielded articles exhibiting very fine-grained crystals of highly 
uniform dimensions. Such articles can demonstrate haze-free transparency. 
The preferred base compositions will contain at least 2.5% R.sub.2 O, at 
least 14% ZnO, at least 18% Al.sub.2 O.sub.3, less than 60% SiO.sub.2, at 
least 2.5% TiO.sub.2, and be essentially free from MgO. 
PRIOR ART 
U.S. Pat. No. 3,460,987 discloses the fabrication of composite articles 
consisting of a ceramic-carbon body coated with a glass-ceramic, said 
glass-ceramic coating consisting, by weight, of 
SiO.sub.2 : 16-32 
Al.sub.2 O.sub.3 : 12-16 
ZnO: 43-47 
B.sub.2 O.sub.3 : 10-25 
Li.sub.2 O: 0-6 
Na.sub.2 O: 0-6 
K.sub.2 O: 0-6 
MgO: 0-10 
CaO: 0-10 
SrO: 0-10 
BaO: 0-10 
The identity of the crystal phase(s) present is not provided, but the 
composition intervals are self-evidently remote from those of the present 
inventive glass-ceramics. No mention is made of transparency. 
U.S. Pat. No. 3,681,097 describes the preparat of glass-ceramic articles 
containing zinc petalite solid solution or .beta.-quartz solid solution as 
the predominant crystal phase. Operable compositions therefor are 
essentially free of alkali metal oxides and MgO, CaO, and SrO, and consist 
essentially, by weight, of 13-40% ZnO, 9-26% Al.sub.2 O.sub.3, 40-75% 
SiO.sub.2, and 3-10% ZrO.sub.2. Up to 6% BaO may advantageously be 
present. Up to 2% TiO.sub.2 may be tolerated, but its inclusion hazards 
the growth of gahnite which is expressly stated to be unwanted. No mention 
is made of transparency. 
U.S. Pat. No. 3,839,053 is directed to highly opaque glass-ceramic articles 
containing zinc petalite solid solution or .beta.-quartz solid solution 
consisting essentially, by weight, of about 10-20% ZnO, 12-20% Al.sub.2 
O.sub.3, 1-10% Ta.sub.2 O.sub.5, 50-65% SiO.sub.2, and 2-8% ZrO.sub.2. The 
inclusion of TiO.sub.2 is cautioned against as tending to lead to the 
development of gahnite, an unwanted crystal phase. 
U.S. Pat. No. 3,854,963 is drawn to densely opaque glass-ceramic articles 
containing zinc petalite solid solution or .beta.-quartz solid solution 
consisting essentially, by weight, or about 10-20% ZnO, 10-20% Al.sub.2 
O.sub.3, 50-75% SiO.sub.2, and at least 0.1% AgCl. The inclusion of 
TiO.sub.2 is discouraged as resulting in the formation of gahnite, an 
unwanted crystal phase. 
U.S. Pat. No. 3,951,669 is concerned with the use of a glass-ceramic 
containing zinc .beta.-quartz solid solution as the predominant crystal 
phase as a filler in a solder sealing glass. No nucleating agent as such 
was utilized; the precursor glass was powdered and nucleation was derived 
from the surfaces of the fine particles during firing thereof. The 
glass-ceramics consisted essentially, in weight percent, of 45-65% 
SiO.sub.2 and at least 35% ZnO+Al.sub.2 O.sub.3, wherein ZnO and Al.sub.2 
O.sub.3 are present in molar ratios ranging between 0.9-1.1. The presence 
of zinc spinel is explicitly warned against. No mention is made of 
transparency. 
U.S. Pat. No. 4,199,340 relates to glass-ceramics useful in 
glass-ceramic-to-metal seals, the glass-ceramics consisting essentially, 
by weight, of 25-32% ZnO, 2.5-10% Al.sub.2 O.sub.3, and 30-60% SiO.sub.2. 
ZrO.sub.2 in amounts up to 12.5% is cited as one of several operable 
nucleating agents. No identification of crystal phase(s) present is 
provided. TiO.sub.2 is nowhere referred to and transparency is not 
mentioned. 
U.S. Pat. No. 4,526,873 presents transparent glass-ceramic articles 
containing mullite as the primary crystal phase consisting essentially, by 
weight, of 10-70% SiO.sub.2, 7-40% B.sub.2 O.sub.3, 14-50% Al.sub.2 
O.sub.3, 2-15% ZnO, 0-35% RO, wherein RO consists of 0-15% MgO, 0-20% CaO, 
0-30% SrO, 0-30% BaO, and 0-30% PbO, and 0-30% R.sub.2 O, wherein R.sub.2 
O consists of 0-10% Li.sub.2 O, 0-15% Na.sub.2 O, 0-25% K.sub.2 O, 0-30% 
Rb.sub.2 O, and 0-30% Cs.sub.2 O, and wherein the mole ratio Al.sub.2 
O.sub.3 :RO+R.sub.2 O&gt;1.3. The inclusion of 0.01-1% Cr.sub.2 O.sub.3 
provides a body which absorbs strongly in the 400-600 nm wavelength region 
of the radiation spectrum and fluoresces strongly in the red and near 
infrared portions of the spectrum when activated by ultraviolet and/or 
visible light.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Table I records a number of parent glass compositions prepared in the 
laboratory, expressed in terms of parts by weight on the oxide basis, 
illustrating the parameters of the present invention. Because the sum of 
the individual components totals or closely approximates 100, for all 
practical purposes the tabulated values can be deemed to reflect weight 
percent. The actual batch ingredients may comprise any materials, either 
oxides or other compounds, which, when melted together, will be converted 
into the desired oxide in the proper proportions. 
The constituents for each of the reported glasses were compounded, 
ballmilled together to aid in obtaining a homogeneous melt, and then 
charged into platinum crucibles. After placing lids thereon, the crucibles 
were introduced into a furnace operating at about 1600.degree. the batches 
melted for 16 hours. The melts were poured into steel molds to form glass 
slabs having dimensions of about 6".times.6".times.0.5"and those slabs 
were immediately transferred to an annealer operating at about 750.degree. 
C. 
Whereas the above description reflects laboratory glassmaking, it will be 
appreciated that the recited compositions could be melted in large scale, 
commercial glassmaking facilities. To improve glass quality, a fining 
agent such as As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3 may be added to the 
batch. The amount remaining in the glass after the batch is melted is too 
small to substantively affect the character and properties of the glass. 
TABLE I 
______________________________________ 
1 2 3 4 5 6 
______________________________________ 
SiO.sub.2 
61.1 59.9 50.6 46.0 49.4 50.9 
Al.sub.2 O.sub.3 
18.2 17.9 23.0 25.0 22.4 23.2 
ZnO 14.6 14.3 18.4 20.5 18.0 18.5 
TiO.sub.2 
3.0 5.0 5.0 5.0 3.6 5.0 
Rb.sub.2 O 
1.5 1.5 1.5 1.5 1.5 -- 
Cs.sub.2 O 
1.5 1.5 1.5 1.5 1.5 -- 
K.sub.2 O 
-- -- -- -- -- 2.4 
ZrO.sub.2 
-- -- -- -- 3.6 -- 
______________________________________ 
Specimens having the proper dimensions for use in the tests referred to 
below were cut from the glass slabs and introduced into an 
electrically-heated furnace. The furnace was heated at a rate of 
300.degree. C./hour to 800.degree. C., maintained at that temperature for 
two hours to promote good nucleation and then again raised at a rate of 
300.degree. C./hour to 1000.degree. C. and held at that temperature for 
four hours to insure extensive crystallization. Such rate was merely a 
matter of convenience. More rapid or slower increases are possible; 
however, too fast a rate may lead to thermal deformation or even breakage 
of the precursor glass body. The crystallized articles were cooled to room 
temperature by merely cutting off the electric current to the furnace and 
allowing the furnace to cool with the articles therewithin. The practice 
is, again, simply a matter of convenience and is referred to as "cooling 
at furnace rate"; the rate averages about 3.degree.-5.degree. C./minute. 
Table II reports a visual description of the crystallized samples, and 
values of linear coefficient of thermal expansion over the range 
0.degree.-300.degree. C. (Coef.Exp.), expressed in terms of 
.times.10.sup.-7 /.degree. C., electrical resistivity (Log P) at 
500.degree. C., dielectric constant (D.C.) from room temperature to 
300.degree. C. at 100 KHz, and dissipation factor (D.F.) over the range of 
100.degree.-200.degree. C. at 100 KHz where determined, utilizing 
measuring techniques conventional in the art. In all samples gahnite 
constituted essentially the sole crystal phase. 
TABLE II 
______________________________________ 
Visual Coef. 
Example 
Description Exp. Log .rho. 
D.C. D.F. 
______________________________________ 
1 Gray, transparent 
34 -- -- -- 
2 Gray, transparent 
33 7.90 6.0 0.007 
3 Gray, transparent 
41 7.91 6.6 0.007 
4 Gray, transparent 
44 7.93 6.7 0.007 
slight haze 
5 Dark red-to-violet, 
50 7.95 6.75 
0.005 
transparent 
6 Gray, transparent 
-- -- -- -- 
______________________________________ 
Example 3 appears to constitute an optimum compromise of glass melting and 
forming characteristics along with desirable physical properties in the 
final glass-ceramic. Accordingly, the most preferred compositions will 
contain less than 55% SiO.sub.2 and at least 20% Al.sub.2 O.sub.3.