Mat. Res. Bul l . , Vol. 18, pp . 1391-1398, 1983. Pr in ted in the USA. 0025-5408/83 $3.00 + .00 Copyr igh t (e) 1983 Pergamon Press Ltd.

THE EFFECT OF WATER ON ZnCI 2 AND ITS GLASSES* L.E. Gorre and R.C. Pastor

Hughes Research Laboratories Malibu, CA 90265

(Reeeived Augus t i , 1983; Refereed)

ABSTRACT

From the literature it has been shown that above 90°C anhydrous ZnCl 2 crystal at P(H20) = 10 mm is not deliquescent. It has also been shown experimentally that when H20 is incorporated in the ZnOl2-based glasses, the glass transition, devitrification, and fusion occur at a much lower temperature. The first step in the degradation of conventional ZnC1 2 glass upon exposure to low humidity is deliquescence, while that of water-free ZnCi 2 glass is surface devitrification.

Introduction

The vitreous (glass) state -- a liquid supercooled below the crystallization point -- is characterized by two non- equilibrium temperature parameters: the onset of glass transition (tq) and devitrification (tx). Such metastable liquids (glasSes) based on the heavy halogen anion are of interest because they exhibit good optical transmission beyond 10 pm (I). The halides which have recently been considered are ZnCI2, ZnBr2, and BiCI 3-based and ThOl%-based glasses.

As a practical window material, the glass piece should be rigid; i.e., the value of tq should be considerably above room temperature. While BiCi 3abased glasses offer the promise of a flat IR transmission up to 14 um, they suffer from the drawback that tg < 50°C (2).

*This study was supported by the U.S. Army Avionics Research and Development Command (Fort Monmouth, NJ), Contract No. DAAK80- 8 1 - C - 0 1 5 5 . 1391

1392 L . E . GORRE, et al . Vol. 18, No. 11

For the casting of large pieces and for optical fiber fabrication, it is further desired that t x- t u be large. The ThC 4-based glass shows promise of a flat-transmission up to 12 ~m, but t x- t a = 35'C (3). Similarly, ZnBr 2 glass transmits up to 50 um, with t x- t a = 39°C (4). The ZnCl 2 glass transmits up to 13 ,m (5); =t x- t a = 80"C (cf. Figure I). At the melting point (591°K~, the viscosity is 49 poise (6,7). Large pieces are easily cast without resorting to complicated methods of quenching.

12

8

4

8

_,=L ,=o I/=oo-==, J ZnCI 2

-10 0 40 80 120 160 200 240 280 320 360 400 440

TEMPERATURE (°C)

FIG. I DTA thermograms comparing RAP ZnCi 2 glass with non-RAP ZnCI 2 glass.

These halides are water soluble. The surface layer of their glasses are moisture sensitive. While solubility is a thermodynamic property, hygroscopic behavior and deliquescence are kinetic parameters. In the usual S-shaped curve of H20- uptake versus time, only the initial horizontal section -- the quiescent stage (induction period) -- is of interest in the present study. In that time interval, the sticking coefficient is low; i.e., the energy barrier is high. The object of processing is to condition the surface layer of the glass so that the time scale for effective handling falls within the induction period. Hygroscopic behavior, which progresses on to deliquescence, follows the quiescent stage. The final stage of the uptake is set by the thermodynamic limit to the solution concentration at the given temperature and humidity.

We report below our observation on the H20-uptake behavior of ZnCI 2, anhydrous crystal and glass, and of Zn~ 2-based glasses.

Vol . 18, No . 11 ZnCI 2 1393

Results and Discussion

The literature highlights deliquescence as the foremost problem in handling anhydrous ZnCi 2- However, above 28°C the anhydrous form is stable in contact with the saturated solution (8). The basis for this claim is seen in Figure 2 where the solubility curve of the anhydrous solid intersects that of the monohydrate at 28°C.

50

40

E 30

J <

0 2O

I I I

10

/

I I I I -40 -20 0 20

I t I I /

]

/ .

0 I I I -60 40 60 80 1 O0

t , °C

FIG. 2

Concentration (m) of saturated aqueous solution of ZnC 2.nH20. Legend for the solid phase: O, n = 4; A, n = 3; o, n = 5/2; ®, n = 3/2; w, n = I; e, n = 0. Data taken from International Critical Tables, Vol. IV, 221 (1928).

1394 L . E . GORRE, et a l . Vol. 18, No. 11

Our next object was to establish how much above 280C the handling of ZnCl2(c) should be conducted. The choice of operating temperature depends on P(H20), the ambient humidity. Table I gives the vapor pressure (P) of the concentrated solution as a function of temperature (t) and concentration (m). When t is constant, £n P varies linearly with m. The constants obtained by linear regression are given in Table 2; r is the correlation coefficient.

TABLE I.

Vapor Pressure of Concentrated Aqueous Solutions of Zinc Chloride as a Function of Temperature and

Concentration( a ) . H

t, °C m, mol kg -I P, mm

14.6

24.6

29.6

100.0

11.0 13.5 17.0

11.0 13.5 17.0

11.0 13.5 17.0

5.0 7.0 9.0

4.37 2.55 I .41

8.07 5.10 2.92

10.8 7.09 4.20

607. 513. 415.

(a)International Critical Tables, III, 294 (1928)

TABLE 2.

Functional Dependence of Vapor Pressure (P) on Concentration(m): £n P = -Am + B

t, °C A B -r

14.6

24 .6

29 .6

100.0

0 .1873

0 . 1 6 8 8

0.1569

0.0951

3 .5090

3.9311

4 .0950

6.8911

0 .998

0 .999

0 .999

0 .998

Vol. 18, No. i i ZnCI 2 1395

Table 3 gives the solubility or saturation concentration (m*) as a function of temperature (t) for anhydrous gnCl 2 at 30°C • t ~ 100°C. By linear regression, the functional dependence of m* on T(OK) is

-20,322 m* ffi T + 98.23 , (I)

with r = -0.975. Values for m* at 24.6°C, 29.60C and I000C were obtained using Eq. (I) as shown in Table 4. Since Eq. (I) applies to the solution where the equilibrated solid is anhydrous ZnCl2(c) , the data at 14.60C were not used (cf. Fig. I).

TABLE 3.

Solubility of Anhydrous Zinc Chloride as a Function of

Temperature (a)

t, "C m*, mol kg -I

30

40

60

80

100

32.2

33.2

35.8

39.8

45.1

(a)International Criti~a! . Tables, IV, 221 (1928).

TABLE 4.

Calculated Concentration and Vapor Pressure of Saturated Aqueous Solutions of Zinc Chloride as a Function of Tem

t, °C

24.6

29.6

100.0

m*, mol kg -I

29.9

31 .I

43.8

~erature

P*, m m

0.33

0.46

15.27

1396 L . E . GORRE, et a l . Vol. 18, No. 11

Assuming that the functional dependence of P on m held up to saturation, values for P* at 24.6°C, 29.6°C and 100°C were calculated and given in Table 4. Thus, the functional dependence of P* on T was established:

-5,644 £n P*(mm) - T + 17.859 , (2)

with r = -1.0000. From Eq. (2), ZnCl2(c) should remain anhydrous at temperatures above 90°C for an ambient humidity of P(H20) = 10 mm; i.e., 42% relative humidity at 25°C. However, the vapor pressure data at t > 90°C (cf. International Critical Tables, Vol. III, 366 (1928)) extrapolates to a value of t = 70°C for P* = 10 mm. The 20°C difference stems from the inaccuracy in the determination of m*. From Eq. (I), m* (90°C) = 42.2 and m* (70°C) = 39.0, yielding an average of 40.6 and a standard deviation (SD) of 5.7%, a value approximately equal to or less than the SD of m*s taken from a more recent compilation in Table 5. Thus, the inaccuracy in t resulting from the inaccuracy in m* gives a t ffi 80 ± I0°C at P(H20) ffi 10 mm. The upper limit of 90°C would be a more prudent choice to avoid deliquescence in the handling of ZnCl2(c ).

TABLE 5.

Comparison of Solubilities of Aqueous Zinc(c~lOridea) Solutions at Various Temperatures

t, °C m*, mol kg -I S.D., % Number of Determinations

0

10

20

25.1

23.6

28.0

1 8 . 3

1 4 . 4

5 . 2

(a)solubilities of Inor@anic and Or@anic Compounds, ed. Dy H. Stephen and T. Stephen, Vol. I, Part I, (Macmillan, 1963), 262.

Of course, a higher temperature is required in the case of the ZnCI~ glass. At such temperature, it is likely that another degradatlve mode of H20 attack becomes active; viz., hydrolysis. The result makes it evident that the practical handling of ZnCI 2 glass must be conducted at P(H20) << 10 mm.

Vol . 18, No. i i ZnCI 2 1397

Experimental Studies

Such a method of preparing and handling ZnCl 2 glass by a reactive atmosphere process (RAP) has been described (5). The material is prepared from the six-nines pure metal by coupling the reaction with CCI ~ at 650°C to the distillation of the product. At 650°C, the vapor pressure of znCl 2(liq) is 320 mm (cf. page 63 of Ref. 6). A 20-cm 3 volume of the glass is easily obtained by quenching the glass ampoule in water prior to tip off. Figure 1 compares the DTA thermograms (DuPont 1090 Thermal Analyzer) of ZnCI 2 glass made by RAP and one without the rigorous exclusion of H20 (non-RAP). We observed that the latter was softer, which must have resulted from the large displacement in the glass transition temperature. Devitrification occurred at 80°C lower. The more than I00°C lowering in the fusion point leads us to believe that the OH- pseudohalide impurity was part of the less-rigid glass network, presumably through hydrogen bond i ng.

The same method failed to produce ZnC12-based glasses, using ThCI ~, PbCI 2 or KCI as the second component. The melt readily crystallized with just a few mole percent KCI.* Yet, much higher concentration of KCl in ZnCl 2-based glass has been reported (9). We observed that when H20 was not rigorously excluded, ZnCI - based glasses could be made. Like the non-RAP ZnCl 2 glass,2the ZnC!2-based glasses were also softer. The temperature displacements in non-RAP ZnCl 2 are also seen in the ZnCI 2-based glasses (cf. Fig. 3).

__ 00 ~ I I I I I I I I I I P ,,, -0.4 ~. 85 °C Z 1 o m"' -0.8 ZnCI2 + ~ 5 tool % KCI - w ,u. -1.2

w -1.0

,,""<~-D ~ -2.0_2.4 - ~ mol % PbCI2_

LU

P" -2.8

0 40 80 120 160 200 240 280 3 0 0 400 TEMPERATURE (°C)

FIG 3.

DTA thermograms of Zn~ 2-based glasses.

*A distinction is made between crystallization and devitrification. The latter description is reserved for the case where the vitreous state was observed as an intermediary.

1398 L . E . GORRE, et al. Vol. 18, No. 11

We observed another difference in behavior caused by H20, i.e., ambient humidity, which we attribute to the significant participation of OH- in the network. All the ZnCl 2-based glasses and non-RAP ZnCI 2 glass degrade by deliquescence. In more than ten observations, not once did we see deliquescence in RAP ZnC 2 glass. Surface devitrification occurred first, which was then followed by deliquescence.

The results of the study show that ZnCl 2 should be handled under conditions of very low humidity. Higher temperature accommodates the presence of a low concentration of water vapor but hydrolysis complicates handling. Zinc chloride glass is" easily produced when H20 is rigorously excluded. Under these conditions, glass formation based on ZnCI 2 is severely restricted.

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