Optical elements made from ThF.sub.4 --BeF.sub.2 glasses

BACKGROUND OF THE INVENTION
The present invention relates to glass compositions based on BeF.sub.2 and particularly to ThF.sub.4 --BeF.sub.2 glasses exhibiting improved melting properties and an extended melting range, rendering them particularly suitable for infrared or u.v.-transmitting optical devices.
In recent years there has been considerable interest in fluoroberyllate glasses, primarily for applications in laser technology. Very low refractive index, dispersion power, and non-linear coefficients of refractive index are among the properties that are unique to this family of glasses. In addition, fluoroberyllate glasses possess better ultraviolet transmission than fused silica and are good hosts for fluorescing elements such as the rare earths. The primary drawbacks to these glasses are the toxicity of beryllium and the poor water durability of BeF.sub.2 -based compositions.
Besides laser applications, the low dispersion characteristics and long range infrared transparency of these glasses make them good candidates for ultra-low-loss optical waveguides. In a cladded optical waveguide structure the importance of core durability is minimal if the cladding can be formed of a durable glass.
Many possible variations of BeF.sub.2 compositions appear in the literature, but nearly all of these require relatively large amounts of a second or third component to achieve the necessary softening of the glass. Rawson, "Inorganic Glass-Forming Systems", Academic, New York (1967) pages 236-247, provides a good background description of the known BeF.sub.2 glass-forming systems, while U.S. Pat. No. 3,650,779 describes alkali-fluoroberyllate glass used as a host matrix for xenon.
In U.S. Pat. No. 3,958,970, MgF.sub.2 --PbF.sub.2 --BeF.sub.2 glasses are used to make fluorescing materials via additions of ytterbium fluoride, erbium fluoride and thulium fluoride. U.S. Pat. No. 4,142,986 discloses NaF--AlF.sub.3 --BeF.sub.2 glasses containing NaPO.sub.3 and doped with NdF.sub.3 to provide an infrared laser glass, while similarly doped glasses based in the BeF.sub.2 --AlF.sub.3 composition system are described in published French application No. 2,445,820.
The list of additives to BeF.sub.2 -based glasses has included, in addition to the modifying alkali and alkaline earth metal fluorides, lanthanide series elements, cerium, and thorium. U.S. Pat. Nos. 2,466,507 and 2,511,224 disclose complex optical glasses of this type.
As noted by Rawson, supra, BeF.sub.2 forms a glass by itself and has a structure which is analogous to fused silica. However, the melting characteristics of pure BeF.sub.2 are not well suited to the preparation of homogeneous glasses containing minor additions of modifying constituents, because of the high viscosity and short working range of BeF.sub.2 glass. In order to effectively mix BeF.sub.2 melts, particularly crucible melts, viscosities below 100 poises are needed. To achieve these viscosities in pure BeF.sub.2 melts ordinarily requires heating to temperatures above 970.degree. C., and at these temperatures the vapor pressure of BeF.sub.2 becomes appreciable. It is this characteristic which has necessitated the development of the complex BeF.sub.2 -containing glass compositions known in the prior art.
Phase equilibrium studies of the systems BeF.sub.2 --ThF.sub.4 and LiF--BeF.sub.2 --ThF.sub.4 are reported by R. E. Thoma et al. in J. Phys. Chem., 64 (July 1960) 865-870. However, Thoma et al. characterized the BeF.sub.2 melts as high in viscosity. No information is provided as to the clarity of the melts prepared for the study, nor as to any of the other material properties of the system.
In any glass composition field, it is advantageous to be able to obtain desired property modifications with as small a composition excursion as possible. In BeF.sub.2 glasses, small additions of alkali can be tolerated without destroying clarity; however, other additions such as PbF.sub.2 have a strong destabilizing effect and do not produce useful glasses. Both systems have been described by D. M. Ray et al., J. Am. Cer. Soc., 37(7) 300-305 (1954).
This consideration is particularly critical for optical applications such as optical fibers wherein substantial modification of optical constants such as refractive index must be achieved with very small quantities of additives, in order to avoid large shifts in physical properties and, most importantly, phase separation of any kind. Optical fibers provide extremely large optical path lengths wherein even the slightest crystallization or phase separation destroys utility.
A principal object of the present invention is to simplify the production of optical devices comprising BeF.sub.2 -based glasses through the use of glass compositions of improved stability and reduced viscosity.
It is a further object of the invention to adapt BeF.sub.2 -based glass compositions comprising only simple additions of a single viscosity-modifying constituent to the fabrication of BeF.sub.2 glass optical devices.
Other objects and advantages of the invention will become apparent from the following description.
SUMMARY OF THE INVENTION
The present invention comprises the manufacture of glass optical devices from ThF.sub.4 --BeF.sub.2 glass compositions. These compositions have been found to demonstrate excellent glass melting behavior, and can therefore can be used to produce clear ThF.sub.4 --BeF.sub.2 glass articles of good quality by casting, drawing or other conventional glass-forming techniques. Compositions according to the invention consist essentially, in mole percent, of about 3-30% ThF.sub.4 and 70-97% BeF.sub.2. Within this composition range clear, water-white glasses are typically formed, while compositions containing more than 30 mole percent ThF.sub.4 exhibit slight to moderate phase separation and/or devitrification, and can exhibit varying degrees of haze or translucence depending upon composition and the forming process which is used.
In addition to providing high quality base glasses, the ThF.sub.4 --BeF.sub.2 compositions are generally compatible with additives such as the transition metal and rare earth elements. Thus while the pure ThF.sub.4 --BeF.sub.2 glasses may be used for optical fiber elements or elemets for u.v. or infrared light transmission, glasses doped with additional elements are useful for applications wherein active or passive optical components such as optical filters, glass luminescent or fluorescent elements, or laser glasses elements are needed. Examples of elements which can be included in these glasses include any of the rare earth or Lanthanide Series elements of the period Table (atomic numbers 58-71), including for example Ce, Sm, Er, Tb, Pr, Gd and Dy. Transition metal additives may include elements of atomic numbers 21-30, such as iron and copper.

DESCRIPTION OF THE DRAWINGS
The invention may be further understood by reference to the drawings, wherein:
FIG. 1 is a plot of glass viscosity as a function of temperature for a BeF.sub.2 glass; also shown is an estimated viscosity for a representative ThF.sub.4 --BeF.sub.2 glass melt;
FIG. 2 includes specific absorbance plots of a BeF.sub.2 glass and of a ThF.sub.4 --BeF.sub.2 glass of the invention;
FIG. 3 is a specific absorbance plot of a ThF.sub.4 --BeF.sub.2 glass doped with erbium; and
FIG. 4 is a specific absorbance plot of a ThF.sub.4 --BeF.sub.2 glass doped with iron.

DETAILED DESCRIPTION
Because of the toxicity of beryllium and the fact that BeF.sub.2 glasses tend to be hygroscopic, an enclosed melting facility for preparing ThF.sub.4 --BeF.sub.2 glasses is desirable. A suitable apparatus for preparing glasses according to the invention is an atmospherically controlled glove box melter wherein crucible melts may be prepared and the resulting glasses handled and stored in a dry, inert environment.
Batches for the glasses may be compounded from fluorides or other compounds of the cationic constituents in proportions which will yield the desired glass compositions at the temperature utilized for melting the batch. The preferred beryllium-containing batch material is ammonium beryllium fluoride, (NH.sub.4).sub.2 BeF.sub.4, while ThF.sub.4 may be used as the thorium batch ingredient. Rare earth and transition metal additives, if included, may also be introduced as fluorides. For additives introduced in small proportions chlorides or other halides may be used; the presence of minor amounts of chloride does not adversely affect glass quality.
Glass batches compounded according to the invention may be readily melted at temperatures of about 900.degree.-1000.degree. C. Melt viscosities at 950.degree. C. are suitable for homogenizing the melt by swirling or stirring, and glass articles may be formed from the melt by drawing, rolling, casting or any other conventional method. The melts are typically of good quality, and clear, water-white glass articles can easily be formed by casting.
Table I below reports glass compositions useful for making optical devices within the scope of the invention, together with a comparative BeF.sub.2 glass. Included in Table I are compositions for each glass, in mole percent as calculated from the batch, and batch compositions in parts by weight. Also included in Table I are indications of the characteristics of the melt, whether fluid or viscous, and the characteristics of glass patties formed by slow cooling from the melt at thickness of 5-8 mm.
Batches for the glasses reported in Table I were compounded using ThF.sub.4 of 99.9% purity and (NH.sub.4).sub.2 BeF.sub.2 of 99% purity, the latter being commercially obtainable from Atomergic Chemetals Corp. of Plainview, N.Y. The batches were hand-mixed, and the mixed batches were then melted in vitreous carbon crucibles under a dry nitrogen atmosphere in a platinum-wound electric furnace operating at 950.degree. C. The melts were hand-stirred once during melting.
After melting, the crucibles were removed from the furnace and the glass was allowed to cool in the crucibles to room temperature, after which the glass samples were easily removed. The glasses were ground and polished under kerosene, and were stored under kerosene to avoid moisture pick-up when not in use.
TABLE I__________________________________________________________________________Glass and Batch Compositions 1 2 3 4 5 6 7__________________________________________________________________________GlassCompositionBeF.sub.2 95 97 90 80 70 60 100ThF.sub.4 5 3 10 20 30 40 --BatchComposition(NH.sub.4).sub.2 BeF.sub.4 23.0 23.5 21.8 19.3 4.2 3.60 25.00ThF.sub.4 3.1 1.9 6.2 12.3 4.6 6.10 --PropertiesMelt Fluid Fluid Fluid Fluid Fluid Fluid ViscousGlass Clear Clear Clear Slight Hazy Slight ClearQuality Haze Devit.__________________________________________________________________________
While the pure BeF.sub.2 glass produced in accordance with the above procedure (Example 7) was quite viscous, as little as 3 mole percent added ThF.sub.4 (Example 2) reduced the viscosity of the melt such that effective mixing and casting were easily accomplished. FIG. 1 of the drawing is a viscosity temperature curve for a pure BeF.sub.2 glass such as Example 7 showing a viscosity at 950.degree. C. of about 150 poises. In contrast, the estimated 950.degree. C. viscosity for the 3% ThF.sub.4 -97% BeF.sub.2 glass of Table I, also shown in FIG. 1, is less than 50 poises.
No phase separation or ThF.sub.4 loss from these melts was observed at ThF.sub.4 concentrations up to 10 mole percent. Some ThF.sub.4 volatilization occured at concentrations above 10 mole percent, some phase separation was observed at ThF.sub.4 concentrations above 12 mole percent. However, good quality clear-to-slightly opal glasses are obtainable at ThF.sub.4 concentrations (batched) about 30 mole percent, and up to 40 mole percent ThF.sub.4 if rapid-quench forming techniques such as rolling, rather than slow-cooling forming methods, are used.
In addition to improving the melting characteristics of the glass, ThF.sub.4 additions appear to improve somewhat the water durability of the glass, to shift the ultraviolet absorption edge of the glass to somewhat longer wavelengths, and to shift the infrared absorption edge to somewhat longer wavelengths. FIG. 2 of the drawing sets forth composite specific absorbance plots for a BeF.sub.2 glass and for a ThF.sub.4 --BeF.sub.2 glass containing 12 mole percent ThF.sub.4 wherein these spectral effects are shown.
Referring to FIG. 2, the infrared spectrum from 2.5-5 microns (measured with a Perkin-Elmer Model 681 infrared spectrophotometer) shows a strong absorption peak at 2.7 microns attributed to the presence of OH groups in the glass. The presence of OH is also known to effect the position of the ultraviolet absorption edge of BeF.sub.2 -based glasses. Hence in the spectral region between 0.25 and 2.5 microns (measured with a Cary Model 17 spectrophotometer) wherein another OH peak is observed at 1.4 microns, the ultraviolet absorption edges of both glasses are positioned at longer wavelengths than would be found in OH-free glasses of the same fluoride composition. The decrease in absorbance for the ThF.sub.4 --BeF.sub.2 glass in the visible to near-infrared spectral region is attributed to improved glass surface quality, rather than to any real improvement in glass transmission.
Due to the reduced viscosity of the melts, the homogeneous inclusion of minor amounts of rare earth or transition metal dopants in these ThF.sub.4 --BeF.sub.2 glasses can be easily accomplished, and these additions impart clear characteristic spectral absorption features to the doped glasses. FIG. 3 of the drawing sets forth a specific absorbance plot for a ThF.sub.4 --BeF.sub.2 glass containing 3200 ppm by weight of erbium as a dopant. The base glass had the composition of Example 2 in Table I above, including 3 mole percent ThF.sub.4 ; the Er dopant was introduced to the glass batch as pure ErF.sub.3. The absorption spectrum of this glass, including well-defined absorption peaks characteristic of the particular rare earth dopant selected, is representative of the types of absorption behavior exhibited by rare earth-doped ThF.sub.4 --BeF.sub.2 glasses of the invention.
The same base glass (97% BeF.sub.2 --3% ThF.sub.4) was doped with a number of other rare earth elements to evaluate the characteristics of these glasses as suitable host materials. Table II below reports optical absorption data for six rare earth dopants at selected wavelengths where relatively strong characteristic absorption peaks appeared in the absorption spectra of the doped glasses. Included in Table II for each dopant are the doping concentration level, in weight percent, the wavelengths (nm) of the absorption peaks measured, the energy transition giving rise to each peak, and the absorption coefficient at the peak wavelength due to the presence of the dopant in the specified concentration.
TABLE II______________________________________Optical Absorption inDoped ThF.sub.4 - BeF.sub.2 Glass Doping Absorption Level Absorption Electron Coeff.Dopant (% Wt.) Peak (nm) Transition (cm.sup.-1)______________________________________Dysprosium 0.39 348 .sup.4 G.sub.11/12 0.1105 1070 .sup.9/2 H.sub.7/2 0.0829 1265 .sup.11/12 H.sub.9/2 0.2648Gadolinium 0.18 272 .sup.6 I.sub.13/2 0.023 275 .sup.6 I.sub.11/12 0.033Samarium 0.27 397 4K.sub.11/12 0.1221 1070 .sup.9/2 H.sub.15/2 0.1382 1270 .sup.7/2 H.sub.15/2 0.1681Erbium 0.32 376 .sup.4 G.sub.11/12 0.4353 517 .sup.3 H.sub.11/12 0.2764 647 .sup.4 F.sub.9/2 0.0668Terbium 0.435 269 .sup.3 D.sub.2 0.0921 1910 .sup.7 F.sub.11/12 0.0852Praseodymium 0.214 438 .sup.3 P.sub.2 0.1819 1515 .sup.3 F.sub.4 0.1382______________________________________
For all of the above dopants, absorption data similar to that reported above was used to establish that Beer's Law is obeyed in these glasses, ie., a linear dependance between absorption and the concentration of the absorber is observed. A cerium dopant was also tried but, unlike the above dopants, it did not produce strong absorption peaks in these glasses. However, both the infrared and the ultraviolet absorption peaks were shifted to longer wavelengths in the cerium-doped glass.
Transition metal doping of these glasses suggests that BeF.sub.2 creates a strongly reducing environment. Additions of copper (II) chloride (CuCl.sub.2) produces broad absorption at 700 nm, attributed to divalent Cu; chemical analysis indicates the Cl is preferentially volatilized from the melt. However, the intensity of the 700 nm absorption peak is nearly independent of copper concentration over the range from 0.1-0.36% by weight, suggesting that most of the copper is reduced to the monovalent state.
Similarly, iron dopants in these glasses produce an absorption characteristic of Fe.sup.+2 regardless of whether ferrous iron or ferric iron is introduced. FIG. 4 of the drawing is a specific absorbance plot for a glass having the composition of Example 2 of Table I, containing 97 mole percent BeF.sub.2 and 3 mole percent ThF.sub.4, to which was added 1130 ppm (weight) of iron as FeF.sub.3. The broad absorption peak centered at 1080 nm is attributed to Fe.sup.+2 absorption, and the absorption of iron follows Beer's law.
From the foregoing it is concluded that ThF.sub.4 --BeF.sub.2 glasses provide a suitable host material not only for rare earth dopants, but also for transition metal dopants. In either case, utilizing a preferred range of base glass compositions consisting essentially, in mole percent, of 3-12% ThF.sub.4 and 88-97% BeF.sub.2, the homogeneous addition of up to 1% by weight total of rare earth and/or transition metal dopants can easily be accomplished for optical device applications requiring such dopants.
While other binary systems such as PbF.sub.2 --BeF.sub.2 and BaF.sub.2 --BeF.sub.2 encompass low-melting eutectics as does the ThF.sub.4 --BeF.sub.2 system, such eutectics do not guarantee good glass forming behavior. To the contrary, additions of ThF.sub.4 appear uniquely effective to modify the high temperature viscosity of BeF.sub.2 melts without destabilizing the composition or promoting the development of phase separation or crystallization. This behavior is in marked contrast to other common fluoride modifiers which have been used in BeF.sub.2 -based glasses. Table III below reports data resulting from glass melting studies concerning the effects of various known modifiers on melt viscosity and melt and glass quality in BeF.sub.2 -based glasses. Included in Table III for each of a number of glass melts are the composition of each melt, in mole percent, the appearance of the melt at 950.degree. C., and the optical quality of glass patties formed by casting and cooling the melts.
TABLE III______________________________________Modified BeF.sub.2 Glass MeltsMelt Composition(mole %) Melt Appearance Glass Quality______________________________________BeF.sub.2 very stiff, grey grey glass, wouldn't pour98.5BeF.sub.2 --1.5AlF.sub.3 stiff, grey grey glass79% BeF.sub.2 --21ZrF.sub.4 very stiff devitrified90BeF.sub.2 --10AlF.sub.3 wouldn't pour opaque glass90BeF.sub.2 --10PbF.sub.2 clear, fluid crystallized92BeF.sub.2 --8PbF.sub.2 clear, stiff crystallized85BeF.sub.2 --15PbF.sub.2 clear, very fluid crystallized very rapidly90BeF.sub.2 --5PbF.sub.2 --5AlF.sub.3 clear, fluid hazy glass90BeF.sub.2 --7AlF.sub.3 --3PbF.sub.2 clear, fluid clear glass, some crystallization90BeF.sub.2 --10BaF.sub.2 clear, fluid hazy glass90BeF.sub.2 --10LaF.sub.3 clear, fluid devitrified90BeF.sub.2 --5LiF--5PbF.sub.2 clear, fluid devitrified90BeF.sub.2 --10HfF.sub.4 clear, fluid hazy glass90BeF.sub.2 --10GaF.sub.3 didn't melt --90BeF.sub.2 --10ZnF.sub.2 clear, fluid opal glass97BeF.sub.2 --3ThF.sub.4 clear, fluid clear glass95BeF.sub.2 --5ThF.sub.4 clear, very fluid clear glass______________________________________
As indicated by the data in Table III above, while some heavy metal fluorides or combinations thereof such as PbF.sub.2, BaF.sub.2 and the like are effective to reduce the high temperature viscosity of BeF.sub.2 -based melts, (e.g., the viscosity at 950.degree. C. which is of particular interest for melting these glasses), each of these commonly used fluorides exhibits a destabilizing effect on the glass, and thus hinders the production of products of optical quality. It is only ThF.sub.2 which appears capable of reducing melt viscosity without adversely affecting the stability of BeF.sub.2 as a glass former. Further, these reductions in melt viscosity are obtained without major changes in other glass properties, such as refractive index, which changes are necessary consequences of using the relatively large quantities of conventional modifiers which are required to obtain stable BeF.sub.2 melts of reduced viscosity. This makes the use of ThF.sub.2 --BeF.sub.2 glasses in accordance with the invention to fabricate optical devices, and particularly devices incorporating homogeneously dispersed additions of rare earth elements such as herein described especially valuable.
The fabrication of optical devices from ThF.sub.4 --BeF.sub.2 glasses according to the invention may be accomplished by any known method. Optical elements such as prisms or lenses may be formed by molding, with subsequent polishing to provide surfaces of optical quality. Plates for optical filters may be provided by casting, drawing, or rolling. Rod preforms for laser rods or optical fiber preforms may be formed by casting or drawing, with optical fiber formed by redrawing from the preform rod. Alternatively, optical fiber may be formed by direct drawing from crucible melts. Of course, these procedures are merely illustrative of the various known methods by which optical devices could be formed from ThF.sub.4 --BeF.sub.2 glasses according to the invention.

Number	Name	Date
2466507	Sun	Apr 1949
2511224	Sun et al.	Jun 1950
3650779	Riebling et al.	Mar 1972
3958970	Auzel	May 1976
4141741	Lucas et al.	Feb 1979
4142986	Rapp	Mar 1979

Optical elements made from ThF.sub.4 --BeF.sub.2 glasses

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