Mat. Res. Bul l . , Vol. 16, pp . 179-186, 1981. Pr inted in the USA. 0025-5408/81/020179-08502.00/0 Copyr ight (c) 1981 Pergamon Press Ltd.

FORMATION AND PROPERTIES OF GLASSES IN THE CaF2-AIF3-P205 SYSTEM

Binod Kumar University of Dayton Research Institute

Dayton, Ohio 45469

(Received December i , 1980; Communicated by W. White)

ABSTRACT Glasses from the system CaF2-AIF3-P205 have been syn- thesized. Glass formation characteristics, thermal expansion, and optical transmission of these glasses in the visible and infrared regions are described. Incorporation of P205 in the system greatly suppressed the spontaneous crystallization tendencies of the CaF2-AIF 3 binary system. The glasses obtained from the system transmit in the mid-infrared range. A strong absorption band centerd at 4.8 microns is a characteristic of these glasses. The absorption band is related to the presence of P205 in the glass.

Introduction

Recent technological advances in the areas of high energy lasers, mid-infrared and multispectral devices, and optical communications have motivated strong interest in the fluoride and fluorophosphate glasses. Optical components for high energy lasers such as the amplifying medium, lenses, windows, Faraday rotation media, and substrates for thin film polarizers and beam splitters require low-linear and non-linear refractive indices materials (i). Due to the low-linear and non-linear refractive indices, fluoroberyllates and fluorophosphate glasses provide great potential for these laser components. Another important characteristic of these glasses is their ability to transmit in the mid- and far-infrared range of the electromagnetic spectrum. This characteristic makes them attractive for IR optical lenses, windows, and other components. Existing high silica optical fibers operate in the 0.8 to 0.9 micron range. It is likely that future generations will operate in the mid-infrared range (2) where atmospheric windows exist at 3.5 to 4.0 microns, allowing the wave guide core to be made larger facilitating fiber

179

180 B. KUMAR Vol. 16, No. 2

splicing and alignment. The fluoride based glasses, obviously, are the potential candidates for future optical wave guide materials.

Fluoride glasses derived from BeF 2, generally referred to as fluoroberyllate glasses, have been known and studied for almost half a century. These fluoride glasses exhibit desirable low refractive indices and high Abb~ numbers. These character- istics of fluoroberyllates are illustrated in the Abb~ diagram for optical glasses shown in Figure 1. Adjacent to the fluoroberyllate region shown in Figure 1 is the region of fluorophosphates which also exhibit desirable low refractive indices and wide rangeof Abb~ numbers. Phosphate and silicate glasses have low Abb~ numbers and high refractive indices - an undesirable feature for their application in the high energy laser components. Superimposed in Figure 1 is a constant non- linear refractive index coefficient (shown by the dashed straight lines), illustrating that fluoroberyllates and fluorophosphates have lower non-linear refractive indices than phosphate and silicate glasses.

,-,,, k, , ,n-13=.., ~ ~ \ SILICATES /

1,8 " i ' " '~°"' ~ \ \ P H O S P H A T E S / .< .- . . f x ~ . F L U O R O P H O S P H A T E . ~ " \

= / . . . . I.o

• .. F L U O R O B E R Y L L A T E S ~ . ~ .

BeF2 , " ~ - .p ..:25 ~ . 5 1.2 I , I .,

I 0 0 8 0 6 0 4 0

ABBE NUMBER

FIG. 1 Abbe Diagram for optical glasses with lines of constant non-linear refractive index superposed. The labeled points indicate location of the various standard optical glass compositions in the diagram. Compositions of the present investigation would be expected to be in the region of fluorophosphates.

Vol. 16,No. 2 GLASSES 181

However, the favorable properties of these fluoroberyllate and fluorophosphate glasses are associated with serious problems in their processing and application. For example, beryllium fluoride based glasses are toxic, chemically unstable, and devitrify during forming. These difficulties are the primary reasons for the lack of commercial development of these glasses over the past five decades. Nevertheless, because of the motivating factors indicated earlier, several investigations have recently been reported in the literature (1,2,3). Although these studies cover a few of the fluoroberyllate and fluoro- phosphate glass compositional regions, a need exists to explore and understand the extensive area of fluoride glasses. Realizing the need, a program has been initiated at the University of Dayton Research Institute to synthesize and characterize glasses from the fluorophosphate system. Data obtained from the CaF2-AIF3 -P250 system are presented in this paper. The system CaF2-AI~3-P205 was chosen in view of its non-toxic behavior, ease of meltability, and desirable optical properties such as low-linear and non-linear refractive indices.

Experimental

Glasses in quantities of 50 gms were obtained by melting pre- mixed batches of raw materials in a 250 cc platinum crucible. Reagent grade CaF 2, AIF3-H20, and P205 were used as the raw material sources. Due to the extreme hygroscopic nature of the P205, it was weighed and mixed with other materials in a dry box. The glass batches were melted in the range of 900-1000°C. Low P205 glasses melted rapidly and a good homogeneous liquid was obtained in about 10-15 minutes of melting time. High P205 glasses were relatively difficult to melt and required melting times as long as 45 minutes. Volatization of the fluorine or the fluorides, evidenced by fumes coming from the melt, were also related to the amounts of P205 . Increased P2.O5 level sub- stantially decreased the fuming rate of volatlles.

After a clear and homogeneous liquid was obtained, it was poured and pressed on a steel plate. Both wedge and flat type specimens, ranging in thickness from 0.i to 3 mm were obtained. The wedge type specimens revealed relative thickness of the glasses that could be obtained without crystallization. Cracking of the pressed specimens was a problem; this was minimized by allowing the specimen to free-form on the steel plate (no pressing) and immediately transferring to a hot plate at 500°C. Specimens as large as 8 x 8 x 0.3 cm were obtained by this technique. The glasses were then characterized for thermal expansion and optical transmission.

Results and Discussion

The glass formation region. . in. the. AIF3-CaF2-P205 system is shown by an enclosed solad lane an Fagure 2. As the glass formation in the CaF2-AIF 3 binary system can be obtained by a rapid quench technique (5~. Therefore, attempts were made to produce glasses from the binary system by quenching the molten liquid between two steel plates. The attempts always resulted in spontaneous crystallization. Addition of P205 to the binary

182 B. KUMAR Vol. 16, No. 2

s},stem in the amounts as low as 2%, greatly reduced the crystallization rate. The area enclosed by the dashed line in Figure 2 roughly approximates the composition range from which glass specimens of thickness 0.i mm to 3 mm could he obtained by the sample preparation technique described earlier. The formulated melt compositions are shown by the lettered points in Figure 2. The thickness of the glass sample strongly depended on the location of the composition in the enclosed area. Compositions within the enclosed area and high in either CaF 2 or AIF 3 resulted in rapid crystallization leading to a dense opaque crystalline mass (identified to be a mixture of CaF 2 and P205 ) in sample thickness greater than 0.5 mm. Compositions high in P205 led to a crystallization product of dull gray appearance. Compositions with extremely good glass formation tendencies were found to be centered around the point "I" in Figure 2. Compositions in any direction away from the Composition I exhibited greater degree of crystalliza- tion. The relative thickness of the glass specimens (as noted from the wedge type specimens) diminished approximately proportional to the relative distance from the Composition I. Compositions outside the boundary exhibited spontaneous crystallization and no visible evidence of glass formation even in extremely thin section (<0.i mm) was observed.

car

A,F, P,%

FIG. 2 Glass formation region shown by enclosed area in the AIF3-CaF 2- P205 system. Compositions in wt. %.

Vol. 16, No. 2 GLASSES 183

Samples as thick as 3 mm were obtained from melts of composition "I" as shown in Figure 3. In general, it was noted that increased P205 led to an increase in the viscosity of the melt and also reduced volatilization from the melt.

FIG. 3 A fluorophosphate glass specimen.

Thermal expansion characteristics of some glass compositions in the vicinity of point I are shown in Table i. As expected, an increase in the P205 led to a decrease in the coefficient of the thermal expansion. It is believed that the P205 partici- pates in network formation. The increased viscosity at the melting temperature and the decreased thermal expansion are the rationale for proposing the P205 participation in network forma- tion. Increase in the CaF 2 increases the thermal expansion and therefore the role of CaF 2 in the system can be visualized as that of a glass modifier.

It is also interesting to note that the glass transition temperatures of these glasses are fairly high and close to those of the stable silicate glasses. Modest heat treatment (30 min.) of these glasses at the glass transition temperatures did not result in crystallization. The compositions were also observed to be chemically durable as exposure to normal atmospheric conditions for several weeks have not produced any noticeable surface attack.

184 B. KUMAR Vol. 16, No. 2

TABLE 1

Tg, Thermal Expansion, and Dilatometric Softening Points of Some Glasses

Composition Thermal Expansion Tg Dilatometric Softening °C (25-300°C) Point

I 175 x 10 -7 444°C 463

N 177 x 10 -7 445°C 469

P 160 x 10 -7 457°C 475

Figure 4 compares the transmission characteristics of fused silica (6) and fluorophosphate glass composition I in the visible and near IR range. The fluorophosphate glass shows an UV absorption edge at .30 micrometers, and transmission remains constant up to 2.4 micrometers. Figure 5 shows IR transmission spectra of fluorophosphate glass I and a ZrF4-ThF4-BaF 2 glass. The fluorophosphate glass shows the characteristic water absorption band at 2.8 micrometers. It is believed that the water associated with AIF 3 is the primary contributor to this absorption band. Although no attempt has been made to produce water-free glass, it is believed that with more careful selection and control of raw materials and melting atmosphere, intensity of the absorption band can be eliminated. Another absorption

I00 ~f s,0, Gu~s

8oVf ,(3552,

g'oFi

0

0.3 0.4 0.5 0 ~ 0 ~ 0 . 8 1.0 1.2 1.4 I~ 1.8 2D Z 2 2.4

WAVELENGTH I N M I C R O ~

FIG. 4 Transmission spectra of the fused silica and fluorophosphate composition I.

Vol. 16, No. 2 GLASSES 185

band is centered around 4.8 micrometers. Fluoride glasses based on CaF2-AIF 3 do not exhibit this absorption band. Also other fluoride glasses such as 0.7 ZrF 4 • 0.07 ThF 4 • 0.33 BaF 2 whose transmission spectra is shown in Figure 5 do not exhibit the absorption band (7). Therefore, the absorption band is attributed to the presence of P205 in the glass.

WAVELET~GTH IN MICROMETERS

2.5 5 4 5 6 7 8 9 I0 I00 -

. _ ~ . .,,,,.0,6 ZrF 4 "0.07 ThF4" 0.33 BoF= "' /--~'-------~ ~"~ PREPARED IN ~ 8 0 - ' ~ " , C Cl. ATMOSPHIERE

60 ',, ~- 40 b- Z h, o 20

0 - 4C00 3500 3000 2500 2000 1800 1600 1400 1200 IOOO

WAVENUMBER (CM "1)

FIG. 5 IR transmission spectra of fluorophosphate composition I and 0.6 ZrF 4 • 0.07 ThF 4 . 0.33 BaF 2 glass (7).

Summary and Conclusions

There exists a large glass formation region in the CaF2-AIF 3. P205 system. About 2.5% of P205 is essential in order to obtain glass from the system by pressing on a steel plate. These glasses have thermal expansion coefficients in the range of

I O 160 to 180 x 10- / C, and a glass transition temperature in the range of 440 to 460°C. The glasses transmit in the mid IR range A strong absorption band centered around 4.8 micrometers is believed to be related to the presence of P205 in the glass.

Acknowledgments

The author expresses his sincere appreciation to the WPAFB Materials Laboratory personnel and Mr. George A. Graves, Jr., for their interest, support, encouragement, and reviewing the manuscript. Assistance of Mr. Edward Shoemaker for melting glasses and specimen preparation is gratefully acknowledged. Special thanks are extended to Mr. John Detrio for the optical transmission measurement.

186 B. KUMAR Vol. 16, No. 2

References

i. M. J. Weber, Energy and Technology Review, Lawrence Livermore Laboratory, Sept. 1977.

2. C. H. L. Goodman, Solid State and Electron Devices 5 (2), 129-37 (1978).

3. K. H. Sun, Glass Technology 1 (20) (1979).

4. J. T. Wenzel, D. H. Blackburn, W. K. Haller, S. Stokonski, and M. J. Weber, SPIE 204 -- Physical Property for Optical Materials (1979).

5. S. Takahashi, presented at the annual meeting of the American Ceramic Society 1980.

6. J. R. Hutchins, III and R. V. Harrington, Encyclopedia of Chemical Technology, John Wiley and Sons, Inc., 2rid Edition, Vol. i0, 533-604 (1964).

7. M. Robinson, R. C. Pastor, R. R. Turk, D. P. Deror, M. Braunstein, and R. Braunstein, Mat. Res. Bull., Vol. 15, pp. 735-742 (1980).