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Viscosity and structure of molten halide glasses based on chlorides, bromides and iodides
106
Journal of Non-Crystalline Solids 112 (1989) 106-110 North-Holland, Amsterdam
VISCOSITY AND STRUCTURE OF MOLTEN HALIDE GLASSES BASED ON CHLORIDES, BROMIDES AND IODIDES M A Fuding and Z H A N G Meixin Shanghai Institute of Building Materials, Shanghai, PR China
The viscosity of a series of molten halide glasses has been measured as a function of temperature in a nitrogen atmosphere by the rotating viscometer method. The viscosity and the activation energy for viscous flow of molten halides are much less than those for the glass-forming oxides, but they are much higher than those for other ionic salts such as KBr and TII. The structures of molten halide glasses are discussed in terms of the experimental results.
1. Introduction There has been a great deal of both scientific and technological interest in halide glasses [1,2]. The reasons behind the scientific interest are mainly due to the u n c o m m o n structure of these glasses as c o m p a r e d to oxide glasses. Technological developments are primarily motivated towards the fabrication of fibers on account of the superior infrared transmission of halide glasses. Because viscous flow is governed b y melt structure and has a direct bearing on the glass forming tendency, m a n y valuable studies of the viscosity of fluorozirconate glasses have been reported [3], but to date the viscosity of halide glasses based on chlorides, bromides and iodides has not been reported. In this paper a systematic study of the viscosity of these molten halide glasses is represented. The structure and the glass-forming tendency of halide glasses are also discussed.
2. Experimental A series of halide glasses were chosen for study. Their exact batch compositions are given in table 1. All samples were prepared by using a mixture of anhydrous chlorides, bromides and iodides. Starting materials were high-purity monochlorides, m o n o b r o m i d e s and m o n o i o d i d e s a n d anhydrous zinc bromide of 99.5% purity (Cerac 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Inc.) The commercial zinc bromide was purified by distillation at 550 ° C in a glove box filled with high-purity nitrogen and the monohalides were dried in a v a c u u m oven overnight. The desired quantity of the appropriate mixture was melted in a platinum crucible at 4 0 0 - 5 0 0 ° C for about 30 min and then the melt was cast o n t o an a l u m i n u m block. All operations involving zinc bromide were carried out inside the glove box to avoid hydration of the zinc bromide. Melt viscosity was determined by the rotating m e t h o d using a L V T - m o d e l Brookfield SynchroLectric viscometer. Preliminary experiments indicated that the temperature variation inside the platinum crucible was within 2 ° C and that the volatilization of the sample was slight during the viscosity measurement. The viscometer was calibrated at r o o m temperature with Brookfield standard oils whose viscosoties were 47.9 cP and 485 cP at 25 ° C respectively.
Table 1 Compositions of the glasses for viscosity measurements (1) (2) (3) (4) (5) (6) (7)
ZnBr 2 55ZnBr2-45KC1 55ZnBr2-45KBr 55ZnBr2-45KI 55ZnBr2-45T1C1 55ZnBr2-45T1Br 55ZnBr2-45Tli
Ma Fuding, Zhang Meixin / Viscosity and structure of molten halide glasses
107
1000
The apparatus constant K was evaluated from the following equation: ~1 = K S / O , where ~/ is the viscosity of the standard oil, S is the viscometer scale reading and ~2 is the angular velocity of the spindle. The viscosity measurement was carried out under a flowing high-purity nitrogen atmosphere to avoid the hydration of the sample. On comparing the infrared absorption spectra of glass before and after the viscosity measurement, no detectable change was found. This indicated that none of the sample was oxidized or hydrolyzed during the measurement.
100 t9
'-,,-I
m
10
o ,~.1
::>
oo
3. Results and discussion In figs. 1 and 2, the viscosities of the halide melts are plotted as a function of the inverse temperature. The addition of monovalent halides results in a reduction of the viscosity of ZnBr 2. In all cases, the temperature dependence of the viscosity is characterized by an obvious curvature,
I000
500
7o0
900
T~=peratare (K) Fig. 2. Viscosity-temperature curves for halide melts. (A) ZnBr2; (B) ZnBr2-TIC1 (55-45); (C) ZnBr2-TlBr (55-45); (D) ZnBr2-TII (55-45); (E) TIC1; (F) TlBr; (G) TII.
and its activation energy for viscous flow increases rapidly with falling temperature. This indicates that molten halide glasses based on chlorides, bromides and iodides resemble fluorozirconate melt in that the viscosity-temperature relations of both do not obey the c o m m o n Arrhenius equation, 71 = A exp( E / R T ) ,
100
where E is the activation energy for viscous flow at constant pressure, A is a constant, and 7/is the viscosity, but appear to follow the Fulcher equation,
tO O
= A exp( B / T -
to
0.1
i
;00'
60'0
800
Pem'~)erature
1000
t
1200
(K)
Fig. 1. Viscosity-temperature curves for halide melts. (A) ZnBr2; (B) ZnBr 2-KC1 (55-45); (C) ZnBr 2-KBr (55-45); (D) ZnBr 2 -KI (55-45); (E) KCl; (F) KBr; (G) KI.
To),
where A, B and To are constants for a particular liquid. A comparison of the viscosity-temperature behavior of a number of molten halides is presented in fig. 3. ZnBr 2 resembles ZnC12 in that the viscosities of both are much higher than those of the other halides. It is seen that although the viscosity of ZnBr 2 and ZnC12 is much less than those for the glass-forming oxides and BeF2, they are much higher than those for other ionic salts such as PbBr 2 and CdBr 2.
108
Ma Fudin~ Zhang Meixin / Viscosity and structure of molten halide glasses
/
1000
~tO0
is known to be dominated by a strong, polarized band at 230 cm-1. There are two crystal structures for ZnBr 2. One crystal structure resembles that of CdC12 in that the coordination number for Zn is 6. The other resembles that of y-ZnC12 in that ZnBr 4 tetrahedra are the basic building units. The latter contains Zn atoms tetrahedrally surrounded by Br atoms in a cubic close-packed lattice and is isomorphic with crystalline Z n I 2. It is known from R a m a n studies that the melt structure in the MC1-ZnC1 a system (M = Li, K, Rb or Cs) resembles that of the K B r - Z n B r 2 system in that Z n X 4 tetrahedra (X = C1 or Br) are the basic building units [8]. At low temperatures, the bromide melt is composed of various polymeric groupings of corner-linked ZrBn 4 tetrahedra, but, as the temperature increases of halides such as TII and KBr are added, these groupings are broken up. The similarity in the viscosity-temperature behavior of ZnBr 2 and ZnC12 is assumed to be due to the resemblance of the structure of molten ZnBr 2 to that of molten ZnC12, whereas other divalent cations such as Hg, Cd and Pb have higher coordination numbers. The R a m a n spectrum of 50ZnBr2-50KI glass is dominated by a very strong band at 139 cm -a [9]. This spectrum appears to be similar to that of
'4 O
to
1
~
0.8
I
i
~ .2
I
i
I
~ .6
2.2
~Ob,/,KK) Fig. 3. Viscosity-reciprocal temperature curves for halide melts. (A) ZnCIz; (B) ZnBr2; (C) PbC12; (D) PbBr2; (E) CdC12; (F) CdBr2; (G) BiC1; (H) HgI 2.
The interpretation of the present viscosity data is that molten ZnBr 2, like molten ZnC12, is somewhat polymerized due to c o m e r sharing of the constituent ZnBr 4 tetrahedra. In the case of ZnC12 glass the structure comprises a three-dimensional random network of corner-sharing ZnC14 tetrahedra [7]. Its reduced polarized R a m a n spectrum
1600
1200 1000
i
I
I
~oo
600
BOO i
I
I
I
3oo
zoo
I
/,/ / //
10
/
,/
/
/
O
I
0.1
6
I
[
8
I
I
]0
I
I
]2
I
I
1~
I
]6
18
~0
Z2
IO~/T(K) Fig. 4. Viscosity-reciprocal temperature curves for halide and oxide melts. (A) Li2SiO3 [12]; (B) NaPO 3 [16]; (C) LiBeF3 [19]; (D) 30BaF2-60ZrF4-10LaF 3 [17]; (E) BaZr2Fl0 [17]; (F) ZnBr2; (G) 55ZnBr 2-45KI.
Ma Fuding, Zhang Meixin / Viscosity and structure of molten halide glasses Table 2 Viscous properties of halide melts, glass-forming liquids and molten salts at their liquidus temperature System
Structural type
T1 ( o C)
Viscosity (P)
Actiation energy
Ref.
(kcal/ mol) SiO 2 B203 BeF 2 ZnCI 2 Li z SiO3 NaPO 3 KBr TII BaZr2Flo ZnBr 2 ZnBr 2 - K B r (55-45) ZnBr 2 - K I (55-45)
polymeric polymeric polymeric polymeric chain and rings chain and rings ionic ionic chain polymeric
1710 450 540 318 1200
107 105 106 50 4
615
17
180 40 100 40 24 16.5
734 440 590 394
0.012 0.026 0.6 1.65
5.2 3.6 16.1 16.7
chain
187
0.5
8.8
chain
187
1.1
10.1
12 13 14 15 12 16
17
pure molten ZnBr 2 at 425 ° C [8] and also to that of vitreous ZnC12 [7]. Thus it appears that the 74 cm -1 and 139 cm -1 band of ZnBr2-KI mixed halide glass are due to - Z n - B r - Z n - skeletal vibrations, corresponding to a symmetric stretch of the bridging Br atoms. Assuming 4-fold coordination for Zn in the glass, one possible structure at the 1 : 1 stoichiometry would consists of covalent chains of distorted [ZnBr3I ] tetrahedra, containing two bridging Br atoms, one non-bridging Br atom and one non-bridging I atom, cross-linked by K - B r and K - I ionic bonds. With increasing (C1 + Br + I) : Zn ratio, the viscosities of the melts decrease because of a break-up of these linked tetrahedra. This is confirmed by the experimental results of the viscosity measurements. As shown in table 2, the viscosity of the 55ZnBr2-45KI melt at Tt is higher than that of molten 55ZnBr2-45KBr 2. Because viscous flow is governed by melt structure and has a direct beating on the glass-forming tendency, 55ZnBr2-45KBr glass is not easily formed. A rapid quench is required to obtain a transparent glass with a thickness of about 0.5 mm. On the other hand
109
55ZnBr2-45KI glass is the most stable and a thickness of more than 3 mm can be obtained without rapid quenching. The structural reason of the increase of viscosity and improvement of the glass-forming tendency for mixed halide glasses remains a fruitful area for fundamental research. In table 2 the viscosity and the activation energy for viscous flow of halide melts are compared with those of other inorganic melts, including major glass-forming liquids and salts, at the melting point or liquidus temperature. As shown in table 2, many oxide glass-forming systems exhibit a high viscosity and a high activation energy for viscous flow at the liquidus temperature. For B203 the viscosity is about 105 P and E , is 40 kcal/mol. Fluoride glass-forming systems, on the other hand, have a much lower viscosity and En. The viscosity at T l is seldom higher than 10 P and En is less than 10kcla/mol. There are viscosity results for chlorides, bromides and iodides. Recent measurements in our laboratory have confirmed that, as with the fluorides, other halide melts also exhibit low viscosity and low E,. For instance, the viscosity of ZnBr 2 at Tm is 1.7 P and E n is 17 kcal/mol. For ZnBr2-KBr and ZnBr2-KI, the respective values are 0.5 and 1.1 P and 8.8 and 10.1 kcal/mol. As mentioned above, the ZnBr 2 glass-forming melt consists of a three-dimensional network. The addition of alkali halides results in a break-up of these linked ZnBr 2 tetrahedra and can lead to a chain-like structure. The glass-forming ability of materials is a complex subject concerning many physico-chemical parameters. Many authors have pointed out that the most important factor affecting the ability of materials to form a glass is the viscosity at the liquidus temperature. Glass formation is therefore likely to be relatively difficult for low viscosity melts.
4. Conclusions
The viscosity-temperature relations for halide glass melts do not obey the Arrhenius equation because their activation energy for viscous flow increases rapidly with falling temperature. However, the viscosity data do appear to follow the Fulcher equation.
110
Ma Fuding~ Zhang Meixin / Viscosity and structure of molten halide glasses
The viscosity and activation energy for viscous flow at the liquidus temperature and much less than those for the glass-forming oxides and BeF2. They are nevertheless much higher than those for other ionic salts. The structure of molten ZnBr2, similar to that of molten ZnC1 e, is extensively polymerized by corner-linking of the constituent ZnBr 4 tetrahedra. The addition of alkali halides results in a break-up of these linked ZnBr 4 tetrahedra and can lead to a chainlike structure. There is a remarkable mixed halide effect, for ZnBr2 based glasses, on the viscosity of the melt and on the glass-forming ability.
References [1] M. Poulain, M. Poulain and J. Lucas, Mat. Res. Bull. 10 (1975) 243. [2] C.A. Angell and D.C. Ziegler, Mat. Res. Bull. 16 (1981) 279. [3] Hu Hefang and J.D. Mackenzie, J. Non-Cryst. Solids 51 (1981) 269.
[4] Hu Hefang, Ma Fuding and J.D. Mackenzie, J. Non-Cryst. Solids 55 (1983) 169. [5] Hu Hefang, Ma Fuding and J.D. Mackenzie, J. Chinese Silicate Soc. 12 (3) (1984) 307. [6] S. Cantor, W.T. Ward and C.T. Moynihan, J. Chem. Phys. 50 (1969) 2874. [7] F.L. Galeener et al., J. Non-Cryst. Solids 42 (1980) 23. [8] R.B. Ellis, J. Electrochem. Soc. 113 (1966) 485. [9] R.M. Almeida, in: Proc. Third Int. Symp. on Halide Glasses, Rennes (July 1985), Part II, p. 30. [10] Ma Fuding, Mater. Sci. Forum 5 (1985) 95. [11] H. Rawson, Inorganic Glass-Forming Systems (Academic Press, New York, 1967) p. 95. [12] J.O. Bockris, J.D. Mackenzie and J.A. Kitchener, Trans. Faraday Soc. 51 (1955) 1734. [13] J.D. Mackenzie, Trans. Faraday Soc. 52 (1956) 1564. [14] J.D. Mackenzie, J. Chem. Phys. 32 (1960) 1150. [15] J.D. Mackenzie and W.K. Murphy, J. Chem. Phys. 33 (1960) 366. [16] C.F. Callis, J.R. Van Wazer and J.S. Metcalf, J. Amer. Chem. Soc. 177 (1955) 1471. [17] Hu Hefang and J.D. Mackenzie, J. Non-Cryst. Solids 54 (1983) 241. [18] Ma Fuding, J. Lau and J.D. Mackenzie, J. Non-Cryst. Solids 80 (1986) 538.
Journal of Non-Crystalline Solids 112 (1989) 106-110 North-Holland, Amsterdam
VISCOSITY AND STRUCTURE OF MOLTEN HALIDE GLASSES BASED ON CHLORIDES, BROMIDES AND IODIDES M A Fuding and Z H A N G Meixin Shanghai Institute of Building Materials, Shanghai, PR China
The viscosity of a series of molten halide glasses has been measured as a function of temperature in a nitrogen atmosphere by the rotating viscometer method. The viscosity and the activation energy for viscous flow of molten halides are much less than those for the glass-forming oxides, but they are much higher than those for other ionic salts such as KBr and TII. The structures of molten halide glasses are discussed in terms of the experimental results.
1. Introduction There has been a great deal of both scientific and technological interest in halide glasses [1,2]. The reasons behind the scientific interest are mainly due to the u n c o m m o n structure of these glasses as c o m p a r e d to oxide glasses. Technological developments are primarily motivated towards the fabrication of fibers on account of the superior infrared transmission of halide glasses. Because viscous flow is governed b y melt structure and has a direct bearing on the glass forming tendency, m a n y valuable studies of the viscosity of fluorozirconate glasses have been reported [3], but to date the viscosity of halide glasses based on chlorides, bromides and iodides has not been reported. In this paper a systematic study of the viscosity of these molten halide glasses is represented. The structure and the glass-forming tendency of halide glasses are also discussed.
2. Experimental A series of halide glasses were chosen for study. Their exact batch compositions are given in table 1. All samples were prepared by using a mixture of anhydrous chlorides, bromides and iodides. Starting materials were high-purity monochlorides, m o n o b r o m i d e s and m o n o i o d i d e s a n d anhydrous zinc bromide of 99.5% purity (Cerac 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Inc.) The commercial zinc bromide was purified by distillation at 550 ° C in a glove box filled with high-purity nitrogen and the monohalides were dried in a v a c u u m oven overnight. The desired quantity of the appropriate mixture was melted in a platinum crucible at 4 0 0 - 5 0 0 ° C for about 30 min and then the melt was cast o n t o an a l u m i n u m block. All operations involving zinc bromide were carried out inside the glove box to avoid hydration of the zinc bromide. Melt viscosity was determined by the rotating m e t h o d using a L V T - m o d e l Brookfield SynchroLectric viscometer. Preliminary experiments indicated that the temperature variation inside the platinum crucible was within 2 ° C and that the volatilization of the sample was slight during the viscosity measurement. The viscometer was calibrated at r o o m temperature with Brookfield standard oils whose viscosoties were 47.9 cP and 485 cP at 25 ° C respectively.
Table 1 Compositions of the glasses for viscosity measurements (1) (2) (3) (4) (5) (6) (7)
ZnBr 2 55ZnBr2-45KC1 55ZnBr2-45KBr 55ZnBr2-45KI 55ZnBr2-45T1C1 55ZnBr2-45T1Br 55ZnBr2-45Tli
Ma Fuding, Zhang Meixin / Viscosity and structure of molten halide glasses
107
1000
The apparatus constant K was evaluated from the following equation: ~1 = K S / O , where ~/ is the viscosity of the standard oil, S is the viscometer scale reading and ~2 is the angular velocity of the spindle. The viscosity measurement was carried out under a flowing high-purity nitrogen atmosphere to avoid the hydration of the sample. On comparing the infrared absorption spectra of glass before and after the viscosity measurement, no detectable change was found. This indicated that none of the sample was oxidized or hydrolyzed during the measurement.
100 t9
'-,,-I
m
10
o ,~.1
::>
oo
3. Results and discussion In figs. 1 and 2, the viscosities of the halide melts are plotted as a function of the inverse temperature. The addition of monovalent halides results in a reduction of the viscosity of ZnBr 2. In all cases, the temperature dependence of the viscosity is characterized by an obvious curvature,
I000
500
7o0
900
T~=peratare (K) Fig. 2. Viscosity-temperature curves for halide melts. (A) ZnBr2; (B) ZnBr2-TIC1 (55-45); (C) ZnBr2-TlBr (55-45); (D) ZnBr2-TII (55-45); (E) TIC1; (F) TlBr; (G) TII.
and its activation energy for viscous flow increases rapidly with falling temperature. This indicates that molten halide glasses based on chlorides, bromides and iodides resemble fluorozirconate melt in that the viscosity-temperature relations of both do not obey the c o m m o n Arrhenius equation, 71 = A exp( E / R T ) ,
100
where E is the activation energy for viscous flow at constant pressure, A is a constant, and 7/is the viscosity, but appear to follow the Fulcher equation,
tO O
= A exp( B / T -
to
0.1
i
;00'
60'0
800
Pem'~)erature
1000
t
1200
(K)
Fig. 1. Viscosity-temperature curves for halide melts. (A) ZnBr2; (B) ZnBr 2-KC1 (55-45); (C) ZnBr 2-KBr (55-45); (D) ZnBr 2 -KI (55-45); (E) KCl; (F) KBr; (G) KI.
To),
where A, B and To are constants for a particular liquid. A comparison of the viscosity-temperature behavior of a number of molten halides is presented in fig. 3. ZnBr 2 resembles ZnC12 in that the viscosities of both are much higher than those of the other halides. It is seen that although the viscosity of ZnBr 2 and ZnC12 is much less than those for the glass-forming oxides and BeF2, they are much higher than those for other ionic salts such as PbBr 2 and CdBr 2.
108
Ma Fudin~ Zhang Meixin / Viscosity and structure of molten halide glasses
/
1000
~tO0
is known to be dominated by a strong, polarized band at 230 cm-1. There are two crystal structures for ZnBr 2. One crystal structure resembles that of CdC12 in that the coordination number for Zn is 6. The other resembles that of y-ZnC12 in that ZnBr 4 tetrahedra are the basic building units. The latter contains Zn atoms tetrahedrally surrounded by Br atoms in a cubic close-packed lattice and is isomorphic with crystalline Z n I 2. It is known from R a m a n studies that the melt structure in the MC1-ZnC1 a system (M = Li, K, Rb or Cs) resembles that of the K B r - Z n B r 2 system in that Z n X 4 tetrahedra (X = C1 or Br) are the basic building units [8]. At low temperatures, the bromide melt is composed of various polymeric groupings of corner-linked ZrBn 4 tetrahedra, but, as the temperature increases of halides such as TII and KBr are added, these groupings are broken up. The similarity in the viscosity-temperature behavior of ZnBr 2 and ZnC12 is assumed to be due to the resemblance of the structure of molten ZnBr 2 to that of molten ZnC12, whereas other divalent cations such as Hg, Cd and Pb have higher coordination numbers. The R a m a n spectrum of 50ZnBr2-50KI glass is dominated by a very strong band at 139 cm -a [9]. This spectrum appears to be similar to that of
'4 O
to
1
~
0.8
I
i
~ .2
I
i
I
~ .6
2.2
~Ob,/,KK) Fig. 3. Viscosity-reciprocal temperature curves for halide melts. (A) ZnCIz; (B) ZnBr2; (C) PbC12; (D) PbBr2; (E) CdC12; (F) CdBr2; (G) BiC1; (H) HgI 2.
The interpretation of the present viscosity data is that molten ZnBr 2, like molten ZnC12, is somewhat polymerized due to c o m e r sharing of the constituent ZnBr 4 tetrahedra. In the case of ZnC12 glass the structure comprises a three-dimensional random network of corner-sharing ZnC14 tetrahedra [7]. Its reduced polarized R a m a n spectrum
1600
1200 1000
i
I
I
~oo
600
BOO i
I
I
I
3oo
zoo
I
/,/ / //
10
/
,/
/
/
O
I
0.1
6
I
[
8
I
I
]0
I
I
]2
I
I
1~
I
]6
18
~0
Z2
IO~/T(K) Fig. 4. Viscosity-reciprocal temperature curves for halide and oxide melts. (A) Li2SiO3 [12]; (B) NaPO 3 [16]; (C) LiBeF3 [19]; (D) 30BaF2-60ZrF4-10LaF 3 [17]; (E) BaZr2Fl0 [17]; (F) ZnBr2; (G) 55ZnBr 2-45KI.
Ma Fuding, Zhang Meixin / Viscosity and structure of molten halide glasses Table 2 Viscous properties of halide melts, glass-forming liquids and molten salts at their liquidus temperature System
Structural type
T1 ( o C)
Viscosity (P)
Actiation energy
Ref.
(kcal/ mol) SiO 2 B203 BeF 2 ZnCI 2 Li z SiO3 NaPO 3 KBr TII BaZr2Flo ZnBr 2 ZnBr 2 - K B r (55-45) ZnBr 2 - K I (55-45)
polymeric polymeric polymeric polymeric chain and rings chain and rings ionic ionic chain polymeric
1710 450 540 318 1200
107 105 106 50 4
615
17
180 40 100 40 24 16.5
734 440 590 394
0.012 0.026 0.6 1.65
5.2 3.6 16.1 16.7
chain
187
0.5
8.8
chain
187
1.1
10.1
12 13 14 15 12 16
17
pure molten ZnBr 2 at 425 ° C [8] and also to that of vitreous ZnC12 [7]. Thus it appears that the 74 cm -1 and 139 cm -1 band of ZnBr2-KI mixed halide glass are due to - Z n - B r - Z n - skeletal vibrations, corresponding to a symmetric stretch of the bridging Br atoms. Assuming 4-fold coordination for Zn in the glass, one possible structure at the 1 : 1 stoichiometry would consists of covalent chains of distorted [ZnBr3I ] tetrahedra, containing two bridging Br atoms, one non-bridging Br atom and one non-bridging I atom, cross-linked by K - B r and K - I ionic bonds. With increasing (C1 + Br + I) : Zn ratio, the viscosities of the melts decrease because of a break-up of these linked tetrahedra. This is confirmed by the experimental results of the viscosity measurements. As shown in table 2, the viscosity of the 55ZnBr2-45KI melt at Tt is higher than that of molten 55ZnBr2-45KBr 2. Because viscous flow is governed by melt structure and has a direct beating on the glass-forming tendency, 55ZnBr2-45KBr glass is not easily formed. A rapid quench is required to obtain a transparent glass with a thickness of about 0.5 mm. On the other hand
109
55ZnBr2-45KI glass is the most stable and a thickness of more than 3 mm can be obtained without rapid quenching. The structural reason of the increase of viscosity and improvement of the glass-forming tendency for mixed halide glasses remains a fruitful area for fundamental research. In table 2 the viscosity and the activation energy for viscous flow of halide melts are compared with those of other inorganic melts, including major glass-forming liquids and salts, at the melting point or liquidus temperature. As shown in table 2, many oxide glass-forming systems exhibit a high viscosity and a high activation energy for viscous flow at the liquidus temperature. For B203 the viscosity is about 105 P and E , is 40 kcal/mol. Fluoride glass-forming systems, on the other hand, have a much lower viscosity and En. The viscosity at T l is seldom higher than 10 P and En is less than 10kcla/mol. There are viscosity results for chlorides, bromides and iodides. Recent measurements in our laboratory have confirmed that, as with the fluorides, other halide melts also exhibit low viscosity and low E,. For instance, the viscosity of ZnBr 2 at Tm is 1.7 P and E n is 17 kcal/mol. For ZnBr2-KBr and ZnBr2-KI, the respective values are 0.5 and 1.1 P and 8.8 and 10.1 kcal/mol. As mentioned above, the ZnBr 2 glass-forming melt consists of a three-dimensional network. The addition of alkali halides results in a break-up of these linked ZnBr 2 tetrahedra and can lead to a chain-like structure. The glass-forming ability of materials is a complex subject concerning many physico-chemical parameters. Many authors have pointed out that the most important factor affecting the ability of materials to form a glass is the viscosity at the liquidus temperature. Glass formation is therefore likely to be relatively difficult for low viscosity melts.
4. Conclusions
The viscosity-temperature relations for halide glass melts do not obey the Arrhenius equation because their activation energy for viscous flow increases rapidly with falling temperature. However, the viscosity data do appear to follow the Fulcher equation.
110
Ma Fuding~ Zhang Meixin / Viscosity and structure of molten halide glasses
The viscosity and activation energy for viscous flow at the liquidus temperature and much less than those for the glass-forming oxides and BeF2. They are nevertheless much higher than those for other ionic salts. The structure of molten ZnBr2, similar to that of molten ZnC1 e, is extensively polymerized by corner-linking of the constituent ZnBr 4 tetrahedra. The addition of alkali halides results in a break-up of these linked ZnBr 4 tetrahedra and can lead to a chainlike structure. There is a remarkable mixed halide effect, for ZnBr2 based glasses, on the viscosity of the melt and on the glass-forming ability.
References [1] M. Poulain, M. Poulain and J. Lucas, Mat. Res. Bull. 10 (1975) 243. [2] C.A. Angell and D.C. Ziegler, Mat. Res. Bull. 16 (1981) 279. [3] Hu Hefang and J.D. Mackenzie, J. Non-Cryst. Solids 51 (1981) 269.
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