Note: Descriptions are shown in the official language in which they were submitted.
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TRANSPARENT GLASS-CERAMICS BASED ON
ALPHA- AND BETA- WILLEMITE
CROSS-REFERENCE TO RELATED APPLICATIONS
An application entitled TRANSITION-METAL GLASS-CERAMIC GAIN
MEDIA, filed as a United States Provisional Application Serial Number
60/160,053, on October 18, 1999, in the names of George H. Beall et al., and
assigned to the same assignee as this application, is directed to transition-
metal doped, glass ceramic materials that exhibit properties that make them
suitable as gain media for use in optical amplifiers and/or laser pumps.
An application entitled TRANSPARENT (LITHIUM, ZINC, MAGNESIUM)
ORTHOSILICATE GLASS-CERAMICS, filed as a United States Provisional
Application Serial Number 60/159,967, on October 18, 1999, in the names of
George H. Beall and Linda R. Pinckney, and assigned to the same assignee as
this application, is directed to transition-metal-doped, glass-ceramic
materials
that exhibit properties that make them suitable as gain media in optical
amplifiers and/or laser pumps.
An application entitled GLASS-CERAMIC FIBER AND METHOD, filed
as United States Provisional Application Serial Number 60/160,052 on October
18, 1999 in the names of George H. Beall, Linda R. Pinckney, William Vockroth
and Ji Wang and assigned to the same assignee as this application, is directed
to glass-ceramic materials containing nanocrystals and being doped with a
transition metal, and to a method of producing such glass-ceramics in the form
of optical fibers.
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An application entitled TRANSPARENT AND TRANSLUCENT
FORSTERITE GLASS-CERAMICS, filed as a United States Provisional
Application Serial Number 60/160,093 filed on October 18, 1999, by George H.
Beall, and of United States Supplemental Provisional Application Serial
Number 60/174,012 having the same title and filed December 30, 1999 by
George H. Beall.
The present application claims the benefit of United States Provisional
Application Serial Number 60/160,138, entitled GLASS-CERAMICS BASED
ON ALPHA- AND BETA-WILLEMITE, filed on October 18, 1999, in the name of
Linda R. Pinckney, and of United States Provisional Application Serial No.
60/167,871 having the same title and filed November 29, 1999 by Linda R.
Pinckney.
FIELD OF INVENTION
The present invention relates to transparent glass ceramics, and in
particular to substantially transparent glass-ceramics based on crystals of
alpha- and beta-willemite.
BACKGROUND OF THE INVENTION
Glass-ceramics are polycrystalline materials formed by a controlled
crystallization of a precursor glass. The method for producing such glass-
ceramics customarily involves three fundamental steps: first, a glass-forming
batch is melted; second, the melt is simultaneously cooled to a temperature at
least below the transformation range thereof and a glass body of a desired
geometry shaped therefrom; and third, the glass body is heated to a
temperature above the transformation range of the glass in a controlled manner
to generate crystals in situ.
Frequently, the glass body is exposed to a two-stage treatment. Hence,
the glass will be heated initially to a temperature within, or somewhat above,
the transformation range for a period of time sufficient to cause the
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development of nuclei in the glass. Thereafter, the temperature will be raised
to levels approaching, or even exceeding, the softening point of the glass to
cause the growth of crystals on the previously-formed nuclei. The resultant
crystals are commonly more uniformly fine-grained, and the articles are
typically more highly crystalline. Internal nucleation allows glass-ceramics
to
possess such favorable qualities as a very narrow particle size distribution
and
highly uniform dispersion throughout the glass host.
Transparent glass-ceramics are well known to the art; the classic study
thereof being authored by G. H. Beall and D.A. Duke in "Transparent Glass-
Ceramics", Journal of Materials Science, 4, pp. 340-352 (1969). Glass-ceramic
bodies will display transparency to the human eye when the crystals present
therein are considerably smaller than the wavelength of visible light. More
specifically, transparency generally results from crystals less than 50 nm,
and
preferably as low as 10 nm in size.
Recently, much effort has been concentrated in the area of using
transparent glass-ceramics as hosts for transition metals which act as
optically
active dopants. Suitable glass-ceramic hosts must be tailored such that
transition elements will preferentially partition into the crystals. Co-
pending
application Serial No. 60/160,053, entitled "Transition Metal Glass-Ceramics"
by Beall et al. is co-assigned to the present assignee, and is herein
incorporated by reference in its entirety It is directed to transition-metal
doped
glass-ceramics suitable for formation of a telecommunications gain or pump
laser fiber.
Transparent glass-ceramics which contain relatively small numbers of
crystals can be of great use in cases where the parent glass provides an easy-
to-melt or an-easy-to-form vehicle for a crystal. The crystal, in itself, may
be
difficult or expensive to synthesize, but may provide highly desirable
features,
such as optical activity. The crystals in the glass-ceramic are generally
oriented randomly throughout the bulk of the glass, unlike a single crystal
which
has a specific orientation. Random orientation, and consequent anisotropy, are
advantageous for many applications, one example being that of optical
amplifiers, where polarization-independent gain is imperative.
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Transparent glass-ceramics doped with transition elements can combine
the optical efficiency of crystals with the forming flexibility of glass. For
example, both bulk (planar) and fiber forms can be fabricated from these glass-
ceramics.
Therefore, there exists a need for transparent glass-ceramic materials
which contain small tetrahedral and interstitial sites, and hence are suitable
as
potentially valuable hosts for small, optically active transition elements.
Such
elements include, but are not limited to, Cr4+, Cr3+, Co3+, Co2+, Cu2+, Mn2+,
Cu2+, and Ni2+. These elements impart luminescence and fluorescence to such
doped, glass-ceramic materials, thereby rendering them suitable for
application
in the optical field industry.
The crystal structures of both alpha- and beta-willemite (i.e., zinc
orthosilicate (Zn2Si04)) consist of frameworks of Si04 and Zn04 tetrahedra.
The alpha-willemite structure was determined in 1930. It is isostructural
with phenacite (Be2Si04), with rhombohedral space group R 3, and consists of
linked Si04 and Zn04 tetrahedra. All Zn2+ ions occur in tetrahedral
coordination. Each oxygen atom is linked to one silicon and two zinc atoms.
The beta-willemite phase has a crystal structure related to those of the
silica polymorphs tridymite and cristobalite. Half of the zinc ions are in
tetrahedral coordination while the remaining half lie in interstitial
positions.
Phase equilibrium studies confirm that the alpha-willemite form is the sole
thermodynamically stable binary compound in the Zn0-Si02 system. However,
the metastable beta-willemite is obtained quite readily as a devitrification
product in glasses. When held at temperatures above 850°C, beta-
willemite
ultimately transforms to the stable alpha polymorph.
The beta-willemite phase offers several potentially useful properties.
Unlike alpha-willemite, beta-willemite can have a widely variable composition,
ranging from 33 to 67 mole % ZnO. This wide range of solid solution allows the
phase to be obtained in glass-ceramics of widely varying composition.
Glass-ceramics containing the alpha-willemite form of Zn2Si04 are
known, particularly as materials for electronic applications. United States
Patent No. 4,714,687 is directed to glass-ceramic materials containing
willemite
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as a predominant crystal phase and especially designed for substrates in
integrated circuit packaging. The glass-ceramic consists essentially, in terms
of weight percent, of 30-55 Si02, 10-30 AI203, 15-45 ZnO, and 3-15 MgO.
However, what the prior art has failed to disclose, and what this
5 invention teaches, is a willemite glass-ceramic material that is transparent
and
is suitable for employment in the fiber optic industry.
Accordingly, the primary object of the present invention is to provide
glass-ceramic materials which are substantially and desirably totally
transparent, and which contain a predominant willemite crystal phase.
Another object of the present invention is to provide such willemite
glass-ceramics which are capable of being doped with ingredients that confer
luminescence and/or fluorescence thereto.
An important advantage of the present glass-ceramic family is that it
provides a material containing a willemite crystalline phase which can be
tetrahedrally-coordinated with transition metal ions including, but not
limited to,
Cr4+, Cr3+, Co3+, Co2+, Cu2+, Mn2+, Cu2+, and Ni2+. Further, the material is
glass-based thus providing the important flexibility of allowing for
fabrication of
both bulk (such as planar substrates) and fiber (such as optical fiber) forms.
Other objects and advantages of the present invention will be apparent
from the following description.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a transparent
glass-ceramic containing a predominant crystal phase of alpha- and/or beta-
willemite and having a composition consisting essentially, in weight percent
on
an oxide basis, of 25-60 Si02, 4-20 AI203, 20-55 ZnO, 0-12 MgO, 0-18 K20, 0-
12 Na20, 0-30 Ge02, with the condition that E K20+ Na20 >_ 5.
To obtain the greatest transparency in the final glass-ceramic article, the
most preferred composition will consist essentially, expressed in terms of
weight percent on the oxide basis, of 35-50 Si02, 8-15 AI203, 30-42 ZnO, 0-5
MgO, 3-10 K20, 0-6 Na20, 0-5 Ge02.
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To obtain optical activity in the present inventive willemite glass-ceramic
materials, i.e., fluorescence, over the communications transmission wavelength
range of 1100 to 1700 nm, up to 1 wt. % Cr203 may be added to the parent
glass.
A method of making is also provided comprising the steps of:
a.) melting a batch for a glass having a composition consisting
essentially, in weight percent on an oxide basis, of 25-60 Si02, 4-20 AI203,
20
55 ZnO, 0-12 MgO, 0-18 K20, 0-12 Na20, 0-30 Ge02, with the condition that E
K20+ Na20 >_ 5;
b.) cooling the glass to a temperature at least below the
transformation range of the glass;
c.) exposing the glass to a temperature between about 550-950°C
for a period of time sufficient to cause the generation of a glass-ceramic
which
is substantially transparent and which contains a predominant willemite
crystal
phase; and,
d.) cooling the glass-ceramic to room temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a powder X-ray diffraction spectra of a glass-ceramic that has
the composition of Example 2, that has been produced by heat treating at
975°C for 2 hours and that shows a predominant crystal phase of a,-
willemite.
FIG. 2 is a powder X-ray diffraction spectra of a glass-ceramic that has
the composition of Example 2, that has been produced by heat treating at
850°C for 2 hours and that shows a predominant crystal phase of ~3-
willemite.
FIG. 3 shows the fluorescence spectra for the glass-ceramics of
Examples 2 and 13 when doped with 0.08 wt. % Cr203.
DETAILED DESCRIPTION OF THE INVENTION
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The present inventive, substantially transparent, willemite glass-
ceramics have compositions consisting essentially, in weight percent on an
oxide basis, of
Si02 25-60
A1203 4-20
Zn0 20-55
Mg0 0-12
K20 0-18
Na20 0-12
E K20+ Na20 >_ 5
Ge02 0-30.
To obtain the greatest degree of transparency in the final glass-ceramic
article, the most preferred composition range consists essentially, in weight
percent on an oxide basis, of
Si02 35-50
AI203 8-15
Zn0 30-42
Mg0 0-5
K20 3-10
Na20 0-6
Ge02 0-5.
The following Table sets forth a number of glass compositions,
expressed in terms of parts by weight on the oxide basis, illustrating the
parameters of the present invention. The Table also presents the ceramming
schedule in °C and hours, as well as the crystal phases observed in the
final
glass-ceramics.
Inasmuch as the sum of the individual components in each recited glass
approximates 100, for all practical purposes the tabulated values may be
deemed to reflect weight percent. The batch ingredients for preparing glasses
falling within the inventive composition ranges may comprise any materials,
either the oxides or other compounds, which, upon being melted together, will
be converted into the desired oxide in the proper proportions.
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The exemplary glasses were produced in the following manner. The
batch materials were compounded, mixed together to assist in securing a
homogeneous melt, and then placed into platinum crucibles. The crucibles
were introduced into a furnace operating at temperatures of 1400-
1600°C, and
the batches were melted for 4-16 hours. The melts were poured as free
"patties" and transferred to an annealer operating at about 550-600°C.
The glass patties were subjected to the ceramming cycle by placing
them into a furnace and heat treating according to the following schedule:
300°C/hour to a crystallization temperature T°C, hold at
T°C for 1-2 hours, and
cool at furnace rate. The crystallization temperature T varied from 650-
900°C,
such that a substantially transparent, willemite glass-ceramic was obtained.
The inventive compositions are self-nucleating due to liquid-liquid phase
separation and therefore require no added nucleating agents. More
specifically, nucleation is promoted by amorphous phase separation. Even
though nucleating agents are not required, in many cases the addition of
nucleating agents, such as Ti02 (4 wt. %), results in a finer crystal size and
improved transparency: Care must be taken to avoid spontaneous
crystallization in the annealer, however.
Up to 2% Li20, or up to 5% CaO, BaO, SrC~, or Ga203, can be added.
The addition of germania tends to stabilize the alpha-willemite polymorph over
the beta-willemite polymorph.
The crystalline phases of the resulting glass-ceramic materials were
identified using X-ray powder diffraction. Representative diffraction patterns
are shown in FIG. 1 for a glass having the composition of Example 2 that has
been heat treated at 975°C for 2 hours, and in FIG. 2 for a glass
having the
composition of Example 2 that has been heat treated at 850°C for 2
hours.
The structure of the inventive glass-ceramics contains microcrystals (10-
50 nm in size) of alpha- and/or beta-willemite in a stable alkali
aluminosilicate
glass, with total crystallinity ranging from about 10% to 50% by volume
depending on the individual composition. The microcrystals are internally
grown
in the base glass during the ceramming cycle. Transparency in the inventive
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glass ceramics is a function of microstructure which in turn is a function of
the
composition.
The crystal structure in the present inventive glass-ceramic material
provides only small tetrahedral and interstitial sites. This feature renders
the
5 crystals potentially valuable hosts for small, optically active transition
elements
including, but not limited to, Cr4+, Cr3+, Co3+, Co2+, Cu2+, Mn2+, Cu2+, and
Ni2+.
These transition elements will fluoresce and luminesce at various wavelengths.
While larger amounts of some of these elements may be incorporated in the
precursor glasses, the amount employed in the present glasses will normally
10 not exceed about 1 % by weight.
As known in the optics and laser art, crystals with tetrahedrally
coordinated Cr4+ ions provide unique optical characteristics. Therefore, in
one
possible application, the present inventive, transparent, willemite glass-
ceramics, doped with transition metal ions, are suitable for employment in the
optics and laser industries. Specific applications include, but are not
limited to,
optical amplifiers and pump lasers.
In laboratory experiments, Examples 2 and 3 were doped with 0.08 wt.
chromium oxide and fluorescence measurements were taken. As shown in
FIG. 2, strong Cr4+ emission was observed, over the communications
transmission wavelength range between 1100-1700 nm, in both glass-
ceramics.
Although the present invention has been fully described by way of
examples, it is to be noted that various changes and modifications will be
apparent to those skilled in the art. Therefore, unless otherwise such changes
and modifications depart from the scope of the present invention, they should
be construed as included therein.
