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Gas solubilities in fluoride melts
Mat. R e s . B u l l . , Vol. 22, p p . 275-279, 1987. P r i n t e d in t h e USA. 0025-5408/87 $3.00 + .00 C o p y r i g h t ( c ) 1987 P e r g a m o n J o u r n a l s L t d .
GAS SOLUBILITIES IN FLUORIDE ~ELTS
R. C. P a s t o r Hughes R e s e a r c h L a b o r a t o r i e s Malibu, C a l i f o r n i a 90265 ( R e c e i v e d N o v e m b e r 6, 1986; R e f e r e e d ) ABSTRACT The solubility behavior of HF(g) in molten fluorides is shown to be chemical in nature with covalent and ionic components. The exothermic nature of the process accounts for solubility decreasing with increase in temperature. The solubility behavior of inert gases in molten fluorides is physical in nature. The endothermic change in enthalpy appears accountable by the energy required to overcome repulsion. In this respect, the choice of He as a carrier gas in processing is a poor one because of the small molecular size. MATERIALS INDEX: Melt Processing, Metal Fluoride Glasses
Solution of Gases, Metal Fluoride Crystals,
Introduction Two sources of controllable losses in optical transparency are extrinsic absorption and scattering. Both are affected by the atmosphere employed in melt processing. Consider the effect on optical absorption of the use of hydrogen-introducing agents in fluoride-melt processing. Fluorides are obtained from oxides by treatment with ]IF or N]]dKF2 . Hydrogen bonding occurs when these conversion agents are used in processing because ~luoride comes from the most electronegative element. Optical losses occur because of the hydrogen impurity; two features of the 3-#m-absorption band - broad and displaced to longer wavelengths - indicate hydrogen bonding (I). The absorption band is broad because the F-H distance in the hydrogen bond, F-H...F, varies with the fluoride-fluoride separation. The F-H stretching frequency variation could span the wavelength range of 2.8 to 7.I ~m (2).
I n e r t g a s e s used as c a r r i e r s in f l u o r i d e p r o c e s s i n g (He, At, e t c . ) do not contribute to optical absorption hut can affect optical transparency through exsolved bubbles in the fiber. The drive to exsolve the dissolved gas stems from the temperature dependence of Henry's law constant, kp(T). The unit for k. is mole cm- 3 arm- l . The thermal coefficient of k_ is negative for a gas in w~ich dissolution in the melt is exothermic from a c~emical reaction (e.g., H20 or HF) and positive for the endothermic (inert) case. For the latter case, exsolution is a threat because the glass is supersaturated with respect to the gas at the drawing temperature. 275
276
R.C.
PASTOR
Vol. 22, No. 2
Consider the glass melt equilibrated with an inert gas close to the fusion point (T~). The supercooled state is drawn into a fiber at a temperature just past the transition point (T.). Since kp(Tg) < kp(T~) the gas volume exsolved per unit volume of glass (f), is given by: f = RT0 [kp(T~)P~ - k p ( T 0 ) P 0 ] / P ' ,
(1)
where P' is the critical exsolutlon pressure at drawing. P~ and Pg are the gas partial pressures at equilibration and at drawing, respectively. When the gas is not provided in the drawing, f = RT0kp ( T ~ ) P ~ / P ' .
(2)
An o r d e r - o f - m a g n i t u d e e s t i m a t e of s lower bound to P ' may be based on one lO0-pm s gas b u b b l e per 1 mmS of preform, where f = 10 -7 i s an a r b i t r a r y upper
limit to scattering. In a 200-pm-diameter fiber, that limit amounts to one bubble per 3.2 cm length. The fiber-drawing temperature is ~600°K. An orderof-magnitude value for k.(T~) is 10 -z and ~1 atm for P~; hence when P < 5xlO 4 atm, exsolution could occur. The following study examines gas solubilities in fluoride melts with two objectives: to evaluate the merit of HF(g) in fluoride-melt processing since its efficiency in reactive atmosphere processing is limited (3), and to rate inert gas carriers with respect to exsolution. Results and Discussion HF(g) s o l u b i l i t y i n NaY-ZrF4 m e l t s w i t h mole~ Na~ v a r y i n g from 45 to 80.5 has been s t u d i e d i n t h e range 600 t o 800°C (4). At each c o m p o s i t i o n , the l i n e a r dependence,
(3)
i n kp(T) = CA/T) + B,
was s a t i s f i e d . E q u a t i o n (3) p a r a m e t e r s are g i v e n i n the second and t h i r d columns of Table 1; t h e f o u r t h column shows r , the c o r r e l a t i o n c o e f f i c i e n t o b t a i n e d by l i n e a r r e g r e s s i o n . The v a l u e s i n the second column (A) range from - 3 . 9 2 t o - 9 . 8 6 k c a l i n s o l u t i o n e n t h a l p y change (AR), i n agreement with the r e p o r t e d - 3 . 8 5 t o - 9 . 7 0 k c a l (4). TABLE 1 The V a r i a t i o n of kp a t 600 t o 800°C f o r HF(g) in NaF-ZrF 4 ~ e l t s v e r s u s Mole~ NaF NaF 45 53 60 65 80.5
At °K-X 1.973 2.441 2.993 3.355 4.966
x x x x x
lO s 10 s 10 s 10 s 10 s
B
r
-14.006 -14.100 -14.530 -14.583 -14.650
o.g90 1.000 0.997 1.000 1.000
x 0.1698 0.21gg 0.2727 0.3171 0.5079
AH*~ kcal -17.5 -22.2 -21.3 -20.8 -19.5
Vol. 22, No. 2
FLUORIDE MELTS
277
An exothermic process (AH < O) is indicative of a chemical type of solubility. The monotonic variation of An with mole% NaF leads one to suspect a chemical-solubility behavior containing two components, covalent and ionic. The covalent bond is measured by the fluoride fraction of the melt, an amount proportional to four times the concentration of ZrF4; the ionic bond is measured by x, an amount proportional to the concentration of NaF. The covalent component was attributed to 2nF(g) + ZrF4(c ) and the i o n i c component t o b i f l u o r i d e F-(c)
+ HF(g)
=
H2ZrFs(c),
(4)
formation, =
~2-(c),
(5)
where (c) refers to the condensed phase (melt). In Table 1, parameter A (column 2) depends linearly on x (column 5), A = 525.8 + 8,807.9x, with r = o . g g g f o r t h e f i t
(6)
and
x = (mole~ N a F ) / [ ( m o l e ~ NaF)+ 4(mole~ ZrF4) ] .
(7)
an = -AR,
(8)
Whereas before,
where R = 1.986 cal deg -I mol -I, An now depends on x as seen in Eq. (S). Thus, An ranges from -I.0 kcal of the covalent (x = 0), Eq. (4), to -18.5 kcal of the ionic (x = 1), Eq. (5). Normalizing to one mole fluoride in the melt, the corresponding solution enthalpy ranged from -0.25 kcal of the covalent to -18.5 kcal of the ionic. Because of this disparity, the solution enthalpy An* corresponding to one mole HF(g) dissolved in one mole ionic fluoride in the melt, an" = an/x,
i s more i n d e p e n d e n t of x t h a n AH.
From Eqs.
AH" = [(s25.S/x)
(g)
(6),
(8) and (g), (I0)
+ 8,807.011.085,
as shown in the sixth column of Table i. Note that the magnitude of AH by Eq. (7) steadily rose with x by a factor of 2.5 (cf. A, Table I) while 6H" fluctuated
within
i 0 ~ a t AH" = - 2 0 . 3 * 1.8 k c a l
(Table I,
column 6 ) .
Chemical solubility at x = 1 by gq. (5) is
NaF(c) + HF(g)
=
NaHF2(c),
(11)
f o r which the s o l u t i o n e n t h a l p y was - 1 8 . 5 k c a l . The s t a n d a r d thermodynamic c o o r d i n a t e s t o Eq. (11), which are d e r i v e d from the r e a c t a n t s and p r o d u c t s (5), are AH° = -18.05 k c a l , AS° = - 3 2 . 1 0 3 e . u . , and AC. = - 0 . 2 3 3 e . u . Since 5Cp i s s m a l l , n e g l e c t i t s t h e r m a l c o e f f i c i e n t . Use 1060°K as the m i d p o i n t of
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R.C. PASTOR
Vol. 22, No. 2
the 600 to 800°C range. It follows that the reaction enthalpy of Eq. (II) at IO00°K is -18.2 kcal, in agreement with -18.5 kcal for the solution enthalpy of Eq. (5). Hence, the reaction enthalpy for bifluoride formation accounts for the ionic component of the solution enthalpy change. This exothermic nature explains the decline of kp with temperature increase. (A negative entropy change accounts for a positive thermal coefficient of the free-energy change for chemical solubility.) Assuming the same held true of the covalent component, Eq. (4), the reaction enthalpy to form H2ZrF 6 at IO00°K follows. The reactant formation enthalpies at IO00°K are -64.65 kcal for HF(g) and -439.8 kcal for ZrF4(c ) (6). Hence, AH°(IOOO°K) = -570.1 kcal for H2ZrF e formation. Although H2ZrF s has not been isolated, its existence is inferred from the presence of the salts
(fluozirconates).
The solution entropy change (AS) is derived from B (Table I, column 3). Allowing for the factor R, the values agree with the literature (4). Observe that B is nearly constant, giving an average of AS = -28.6 * 0.6 e.u. The near-constancy behavior gives a poorer linear fit to the dependence of B on x, B(x) = -1.87x - 13.82 with r = 0.82. Thus, for the covalent case (x = 0), AS c = -27.4 e.u., and from the model, that value is the entropy change for Eq. (4). For the ionic case (x = 1), &S; = -31.2 e.u., a value close to AS ° = -32.1 e.u. for bifluoride formation of Eq. (11). Inert gases over molten fluoride provide a good example of physical solubility. Fluoride-glass formulations are equivalent to an ionic-fluoride cc;~tent of 50 to 60 mole~ NaF in NaF-ZrF 4. The solubility of inert gases in 53 mole~ NaF at 600 to 800°C has been measured (7); curve-fit parameters to Eq. (3) are listed in Table 2. It can be shown from Eq. (3) that chemical solubility is one to two orders of magnitude larger than physical solubility, k. = 10 "s mole cm -s atm -I for HF and I0 "7 for Xe. Observe in Tables 1 and 2, teat A is opposed in sign: chemical solubility is exothermic, while physical solubility is endothermic. Hence, opposite signs in the dependence of kp on temperature. TABLE 2 The Variation of k. at 600 to 800°C for Inert Gases in NaF-ZrF4~Melt that is 53 Mole~ NaF Gas He Ne Ar Xe
B
A t °K-I -3.094 -3.881 -4.042 -5.517
x x x x
10 s lO s lO s 10 s
-11.824 -11.763 -12.173 -11.451
r -0.994 -0.997 -1.000 -0.999
The v a l u e of AH a p p e a r s t o be r e s p o n s i b l e f o r t h e energy r e q u i r e d t o overcome r e p u l s i o n . That o v e r l a p energy i s r e p r e s e n t e d as, C e x p [ D r ] , where r i s t h e a t o m i c r a d i u s of t h e i n e r t g a s . Thus, t h e l a r g e r t h e v a l u e of r , t h e lower t h e v a l u e of kp and t h e l a r g e r t h e v a l u e of AH. Now, &H ( u n i t : k c a l ) from A of Table 2 compared t o t h e o v e r l a p e n e r g y based on C = 4.475 k c a l and D = 0.673 A-1 , u s i n g r = 0 . 5 0 A f o r He, 0.65 f o r Ne, 0.95 f o r Ar, and 1.30 f o r Xe, i s as f o l l o w s : 6.1 v e r s u s 6.3 f o r He, 7 .3 v e r s u s 6.9 f o r Ne, 8 . 0 v e r s u s 8 . 5 f o r Ar and 11.0 v e r s u s 10.7 f o r Xe. The e n t r o p y change i n t h e s o l u t i o n p r o c e s s ( c f . B, Table 2) i s e s s e n t i a l l y c o n s t a n t a t AS = - 2 3 . 4 * 0 . 6 e . u .
Vol. 22, No. 2
FLUORIDE MELTS
279
In summary, the s o l u b i l i t y behavior of HF(g) in molten f l u o r i d e s i s chemical in n a t u r e with c o v a l e n t and i o n i c components. I t s exothermic n a t u r e accounts f o r kp d e c r e a s i n g with i n c r e a s e in t e m p e r a t u r e . At a m e l t - p r o c e s s i n g temperature of 800°C f o r 60 mole~ NaP in NaP-ZrF4, the h y d r o g e n - i m p u r i t y c o n t e n t r e s u l t i n g from chemical s o l u b i l i t y approximates ~ 10 I9 cm-3. The s o l u b i l i t y concept (H20 included) s u p p o r t s the o b s e r v a t i o n of a s i g n i f i c a n t H-content in HF-processed m a t e r i a l s (8). Consequently, the i n t r i n s i c b a s e l i n e t r a n s m i s s i o n in the 3 #m r e g i o n of f l u o r i d e g l a s s e s s t i l l remains to be e s t a b l i s h e d . The s o l u b i l i t y b e h a v i o r of i n e r t gases in molten f l u o r i d e s i s p h y s i c a l in n a t u r e . The endothermic n a t u r e causes kp to i n c r e a s e with t e m p e r a t u r e . The value of AS appears accountable by the energy r e q u i r e d to overcome r e p u l s i o n : the s m a l l e r the atom r a d i u s , the l a r g e r k^ and the s m a l l e r AH. Equation (2) shows the tendency to exsolve bubbles during drawing a t ~T. to be d i r e c t l y p r o p o r t i o n a l to kp(Wf). Among the i n e r t g a s e s , the r i s k o~ e x s o l u t i o n o c c u r r i n g i s g r e a t e s t with the use of He. In 53 mole~ NaP in NaP-ZrF 4 melt a t 800°C, kp = 4.2x10 -7 mole cm-3 atm -1 for He and 0.63x10 -7 f o r Xe (7). N2, C02, e t c . , are more e f f i c i e n t and l e s s expensive s u b s t i t u t e s f o r He as a c a r r i e r gas.
References 1.
M. Robinson, e t a l . ,
Mat. Res. Bull. 15,
735 (1980).
2.
W.C. Hamilton and J.A. I b e r s , Hydrogen Bonding in S o l i d s (W.A. Benjamin, I n c . , New York, 1968). See F i g . 3-3, p. 87.
3.
R.C. P a s t o r , J. C r y s t . Growth 75, 54 (1986).
4.
J.H. S h a f f e r , W.R. Grimes, and G.M. Watson, J. Phys. Chem. 63, 1999 (1959).
5.
D.D. Wagman~ e t a l . , "The NBS Tables of Chemical Thermodynamic P r o p e r t i e s , " J. Phys. and Chem. Ref. Data, Vol. 11, Supplement No. 2 (1982). P u b l i s h e d by the American Chemical S o c i e t y and the American I n s t i t u t e of P h y s i c s f o r the N a t i o n a l Bureau of Standards.
6.
C.E. Wicks and F.E. Block, "Thermodynamc Properties of 65 Elements - Their Oxides, Halides, Carbides, and Nitrides," Bull. 605, Bureau of Mines (U.S. Government Printing Office, 1963).
7.
G.J. Janz, Molten Salts Handbook (Academic Press, New York, 1967). See Table II.F.I., p. 176. The unit of the temperature is not °K but °C, and the tabulated numbers are not kp(103) but kp(lOe).
8.
L.E. Gorre and R.C. P a s t o r , Mat. Res. B u l l . 20, 1441 (1985).
GAS SOLUBILITIES IN FLUORIDE ~ELTS
R. C. P a s t o r Hughes R e s e a r c h L a b o r a t o r i e s Malibu, C a l i f o r n i a 90265 ( R e c e i v e d N o v e m b e r 6, 1986; R e f e r e e d ) ABSTRACT The solubility behavior of HF(g) in molten fluorides is shown to be chemical in nature with covalent and ionic components. The exothermic nature of the process accounts for solubility decreasing with increase in temperature. The solubility behavior of inert gases in molten fluorides is physical in nature. The endothermic change in enthalpy appears accountable by the energy required to overcome repulsion. In this respect, the choice of He as a carrier gas in processing is a poor one because of the small molecular size. MATERIALS INDEX: Melt Processing, Metal Fluoride Glasses
Solution of Gases, Metal Fluoride Crystals,
Introduction Two sources of controllable losses in optical transparency are extrinsic absorption and scattering. Both are affected by the atmosphere employed in melt processing. Consider the effect on optical absorption of the use of hydrogen-introducing agents in fluoride-melt processing. Fluorides are obtained from oxides by treatment with ]IF or N]]dKF2 . Hydrogen bonding occurs when these conversion agents are used in processing because ~luoride comes from the most electronegative element. Optical losses occur because of the hydrogen impurity; two features of the 3-#m-absorption band - broad and displaced to longer wavelengths - indicate hydrogen bonding (I). The absorption band is broad because the F-H distance in the hydrogen bond, F-H...F, varies with the fluoride-fluoride separation. The F-H stretching frequency variation could span the wavelength range of 2.8 to 7.I ~m (2).
I n e r t g a s e s used as c a r r i e r s in f l u o r i d e p r o c e s s i n g (He, At, e t c . ) do not contribute to optical absorption hut can affect optical transparency through exsolved bubbles in the fiber. The drive to exsolve the dissolved gas stems from the temperature dependence of Henry's law constant, kp(T). The unit for k. is mole cm- 3 arm- l . The thermal coefficient of k_ is negative for a gas in w~ich dissolution in the melt is exothermic from a c~emical reaction (e.g., H20 or HF) and positive for the endothermic (inert) case. For the latter case, exsolution is a threat because the glass is supersaturated with respect to the gas at the drawing temperature. 275
276
R.C.
PASTOR
Vol. 22, No. 2
Consider the glass melt equilibrated with an inert gas close to the fusion point (T~). The supercooled state is drawn into a fiber at a temperature just past the transition point (T.). Since kp(Tg) < kp(T~) the gas volume exsolved per unit volume of glass (f), is given by: f = RT0 [kp(T~)P~ - k p ( T 0 ) P 0 ] / P ' ,
(1)
where P' is the critical exsolutlon pressure at drawing. P~ and Pg are the gas partial pressures at equilibration and at drawing, respectively. When the gas is not provided in the drawing, f = RT0kp ( T ~ ) P ~ / P ' .
(2)
An o r d e r - o f - m a g n i t u d e e s t i m a t e of s lower bound to P ' may be based on one lO0-pm s gas b u b b l e per 1 mmS of preform, where f = 10 -7 i s an a r b i t r a r y upper
limit to scattering. In a 200-pm-diameter fiber, that limit amounts to one bubble per 3.2 cm length. The fiber-drawing temperature is ~600°K. An orderof-magnitude value for k.(T~) is 10 -z and ~1 atm for P~; hence when P < 5xlO 4 atm, exsolution could occur. The following study examines gas solubilities in fluoride melts with two objectives: to evaluate the merit of HF(g) in fluoride-melt processing since its efficiency in reactive atmosphere processing is limited (3), and to rate inert gas carriers with respect to exsolution. Results and Discussion HF(g) s o l u b i l i t y i n NaY-ZrF4 m e l t s w i t h mole~ Na~ v a r y i n g from 45 to 80.5 has been s t u d i e d i n t h e range 600 t o 800°C (4). At each c o m p o s i t i o n , the l i n e a r dependence,
(3)
i n kp(T) = CA/T) + B,
was s a t i s f i e d . E q u a t i o n (3) p a r a m e t e r s are g i v e n i n the second and t h i r d columns of Table 1; t h e f o u r t h column shows r , the c o r r e l a t i o n c o e f f i c i e n t o b t a i n e d by l i n e a r r e g r e s s i o n . The v a l u e s i n the second column (A) range from - 3 . 9 2 t o - 9 . 8 6 k c a l i n s o l u t i o n e n t h a l p y change (AR), i n agreement with the r e p o r t e d - 3 . 8 5 t o - 9 . 7 0 k c a l (4). TABLE 1 The V a r i a t i o n of kp a t 600 t o 800°C f o r HF(g) in NaF-ZrF 4 ~ e l t s v e r s u s Mole~ NaF NaF 45 53 60 65 80.5
At °K-X 1.973 2.441 2.993 3.355 4.966
x x x x x
lO s 10 s 10 s 10 s 10 s
B
r
-14.006 -14.100 -14.530 -14.583 -14.650
o.g90 1.000 0.997 1.000 1.000
x 0.1698 0.21gg 0.2727 0.3171 0.5079
AH*~ kcal -17.5 -22.2 -21.3 -20.8 -19.5
Vol. 22, No. 2
FLUORIDE MELTS
277
An exothermic process (AH < O) is indicative of a chemical type of solubility. The monotonic variation of An with mole% NaF leads one to suspect a chemical-solubility behavior containing two components, covalent and ionic. The covalent bond is measured by the fluoride fraction of the melt, an amount proportional to four times the concentration of ZrF4; the ionic bond is measured by x, an amount proportional to the concentration of NaF. The covalent component was attributed to 2nF(g) + ZrF4(c ) and the i o n i c component t o b i f l u o r i d e F-(c)
+ HF(g)
=
H2ZrFs(c),
(4)
formation, =
~2-(c),
(5)
where (c) refers to the condensed phase (melt). In Table 1, parameter A (column 2) depends linearly on x (column 5), A = 525.8 + 8,807.9x, with r = o . g g g f o r t h e f i t
(6)
and
x = (mole~ N a F ) / [ ( m o l e ~ NaF)+ 4(mole~ ZrF4) ] .
(7)
an = -AR,
(8)
Whereas before,
where R = 1.986 cal deg -I mol -I, An now depends on x as seen in Eq. (S). Thus, An ranges from -I.0 kcal of the covalent (x = 0), Eq. (4), to -18.5 kcal of the ionic (x = 1), Eq. (5). Normalizing to one mole fluoride in the melt, the corresponding solution enthalpy ranged from -0.25 kcal of the covalent to -18.5 kcal of the ionic. Because of this disparity, the solution enthalpy An* corresponding to one mole HF(g) dissolved in one mole ionic fluoride in the melt, an" = an/x,
i s more i n d e p e n d e n t of x t h a n AH.
From Eqs.
AH" = [(s25.S/x)
(g)
(6),
(8) and (g), (I0)
+ 8,807.011.085,
as shown in the sixth column of Table i. Note that the magnitude of AH by Eq. (7) steadily rose with x by a factor of 2.5 (cf. A, Table I) while 6H" fluctuated
within
i 0 ~ a t AH" = - 2 0 . 3 * 1.8 k c a l
(Table I,
column 6 ) .
Chemical solubility at x = 1 by gq. (5) is
NaF(c) + HF(g)
=
NaHF2(c),
(11)
f o r which the s o l u t i o n e n t h a l p y was - 1 8 . 5 k c a l . The s t a n d a r d thermodynamic c o o r d i n a t e s t o Eq. (11), which are d e r i v e d from the r e a c t a n t s and p r o d u c t s (5), are AH° = -18.05 k c a l , AS° = - 3 2 . 1 0 3 e . u . , and AC. = - 0 . 2 3 3 e . u . Since 5Cp i s s m a l l , n e g l e c t i t s t h e r m a l c o e f f i c i e n t . Use 1060°K as the m i d p o i n t of
278
R.C. PASTOR
Vol. 22, No. 2
the 600 to 800°C range. It follows that the reaction enthalpy of Eq. (II) at IO00°K is -18.2 kcal, in agreement with -18.5 kcal for the solution enthalpy of Eq. (5). Hence, the reaction enthalpy for bifluoride formation accounts for the ionic component of the solution enthalpy change. This exothermic nature explains the decline of kp with temperature increase. (A negative entropy change accounts for a positive thermal coefficient of the free-energy change for chemical solubility.) Assuming the same held true of the covalent component, Eq. (4), the reaction enthalpy to form H2ZrF 6 at IO00°K follows. The reactant formation enthalpies at IO00°K are -64.65 kcal for HF(g) and -439.8 kcal for ZrF4(c ) (6). Hence, AH°(IOOO°K) = -570.1 kcal for H2ZrF e formation. Although H2ZrF s has not been isolated, its existence is inferred from the presence of the salts
(fluozirconates).
The solution entropy change (AS) is derived from B (Table I, column 3). Allowing for the factor R, the values agree with the literature (4). Observe that B is nearly constant, giving an average of AS = -28.6 * 0.6 e.u. The near-constancy behavior gives a poorer linear fit to the dependence of B on x, B(x) = -1.87x - 13.82 with r = 0.82. Thus, for the covalent case (x = 0), AS c = -27.4 e.u., and from the model, that value is the entropy change for Eq. (4). For the ionic case (x = 1), &S; = -31.2 e.u., a value close to AS ° = -32.1 e.u. for bifluoride formation of Eq. (11). Inert gases over molten fluoride provide a good example of physical solubility. Fluoride-glass formulations are equivalent to an ionic-fluoride cc;~tent of 50 to 60 mole~ NaF in NaF-ZrF 4. The solubility of inert gases in 53 mole~ NaF at 600 to 800°C has been measured (7); curve-fit parameters to Eq. (3) are listed in Table 2. It can be shown from Eq. (3) that chemical solubility is one to two orders of magnitude larger than physical solubility, k. = 10 "s mole cm -s atm -I for HF and I0 "7 for Xe. Observe in Tables 1 and 2, teat A is opposed in sign: chemical solubility is exothermic, while physical solubility is endothermic. Hence, opposite signs in the dependence of kp on temperature. TABLE 2 The Variation of k. at 600 to 800°C for Inert Gases in NaF-ZrF4~Melt that is 53 Mole~ NaF Gas He Ne Ar Xe
B
A t °K-I -3.094 -3.881 -4.042 -5.517
x x x x
10 s lO s lO s 10 s
-11.824 -11.763 -12.173 -11.451
r -0.994 -0.997 -1.000 -0.999
The v a l u e of AH a p p e a r s t o be r e s p o n s i b l e f o r t h e energy r e q u i r e d t o overcome r e p u l s i o n . That o v e r l a p energy i s r e p r e s e n t e d as, C e x p [ D r ] , where r i s t h e a t o m i c r a d i u s of t h e i n e r t g a s . Thus, t h e l a r g e r t h e v a l u e of r , t h e lower t h e v a l u e of kp and t h e l a r g e r t h e v a l u e of AH. Now, &H ( u n i t : k c a l ) from A of Table 2 compared t o t h e o v e r l a p e n e r g y based on C = 4.475 k c a l and D = 0.673 A-1 , u s i n g r = 0 . 5 0 A f o r He, 0.65 f o r Ne, 0.95 f o r Ar, and 1.30 f o r Xe, i s as f o l l o w s : 6.1 v e r s u s 6.3 f o r He, 7 .3 v e r s u s 6.9 f o r Ne, 8 . 0 v e r s u s 8 . 5 f o r Ar and 11.0 v e r s u s 10.7 f o r Xe. The e n t r o p y change i n t h e s o l u t i o n p r o c e s s ( c f . B, Table 2) i s e s s e n t i a l l y c o n s t a n t a t AS = - 2 3 . 4 * 0 . 6 e . u .
Vol. 22, No. 2
FLUORIDE MELTS
279
In summary, the s o l u b i l i t y behavior of HF(g) in molten f l u o r i d e s i s chemical in n a t u r e with c o v a l e n t and i o n i c components. I t s exothermic n a t u r e accounts f o r kp d e c r e a s i n g with i n c r e a s e in t e m p e r a t u r e . At a m e l t - p r o c e s s i n g temperature of 800°C f o r 60 mole~ NaP in NaP-ZrF4, the h y d r o g e n - i m p u r i t y c o n t e n t r e s u l t i n g from chemical s o l u b i l i t y approximates ~ 10 I9 cm-3. The s o l u b i l i t y concept (H20 included) s u p p o r t s the o b s e r v a t i o n of a s i g n i f i c a n t H-content in HF-processed m a t e r i a l s (8). Consequently, the i n t r i n s i c b a s e l i n e t r a n s m i s s i o n in the 3 #m r e g i o n of f l u o r i d e g l a s s e s s t i l l remains to be e s t a b l i s h e d . The s o l u b i l i t y b e h a v i o r of i n e r t gases in molten f l u o r i d e s i s p h y s i c a l in n a t u r e . The endothermic n a t u r e causes kp to i n c r e a s e with t e m p e r a t u r e . The value of AS appears accountable by the energy r e q u i r e d to overcome r e p u l s i o n : the s m a l l e r the atom r a d i u s , the l a r g e r k^ and the s m a l l e r AH. Equation (2) shows the tendency to exsolve bubbles during drawing a t ~T. to be d i r e c t l y p r o p o r t i o n a l to kp(Wf). Among the i n e r t g a s e s , the r i s k o~ e x s o l u t i o n o c c u r r i n g i s g r e a t e s t with the use of He. In 53 mole~ NaP in NaP-ZrF 4 melt a t 800°C, kp = 4.2x10 -7 mole cm-3 atm -1 for He and 0.63x10 -7 f o r Xe (7). N2, C02, e t c . , are more e f f i c i e n t and l e s s expensive s u b s t i t u t e s f o r He as a c a r r i e r gas.
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