velocities by decreasing the viscosity of the melt and increasing diffusion .... sodium silicate glasses (A a n d B) and the complete change of the redox transition.
Advances in Fusion & Processing of Glass - 6th Internat. Conf. 2 0 0 0
The influence of high melting temperatures on the behaviour of polyvalent ions in silicate glasses
M. L e i s t e r , D. Ehrt 1)
Otto-Schott-Institut für G l a s c h e m i e , Friedrich-Schiller-Universität J e n a 1)
n o w with S c h o t t Glas, M a i n z
Abstract The
influence
of
(thermodynamical
melting
temperature,
parameters)
as
well
atmosphere
and
as
holding
that
of
glass
composition
time
(kinetical
p a r a m e t e r s ) on t h e b e h a v i o r of the polyvalent ions iron, v a n a d i u m , c h r o m i u m a n d titanium w e r e
investigated.
As
host g l a s s e s three different
(33mol% N a O - 67 SiO , 15mol% N a O - 8 5 S i O 2
2
2
2
silicate
glasses
and 1 0 m o l % C a O - 1 0 B a O -
1 5 A I O - 6 5 S i O ) w e r e u s e d . T h e glasses w e r e melted in a t e m p e r a t u r e range of 2
3
2
1600 to 2 0 0 0 C at v a r i o u s (oxidizing, inert a n d reducing) melting conditions. 0
The
investigations
s h o w the
possibility of t h e r m a l
reduction
melting
at
high
t e m p e r a t u r e s u n d e r different a t m o s p h e r e s . Increasing melting t e m p e r a t u r e and/or d e c r e a s i n g o x y g e n partial p r e s s u r e shift the redox ratios to r e d u c e d states. For polyvalent ions having a single redox transition the r e a c h e d redox ratios in the melt could be f r o z e n . In t h e c a s e of polyvalent ions having m o r e t h a n o n e redox transition r e a c t i o n s during cooling m a y occur. A p p l y i n g high m e l t i n g t e m p e r a t u r e s lead to a n a c c e l e r a t i o n of redox reactions of polyvalent ions in glass melts d u e to t h e increasing o x y g e n diffusion coefficients a n d d e c r e a s i n g viscosity of t h e melt. C o n s t a n t redox ratios are r e a c h e d faster at high t e m p e r a t u r e s , too.
Introduction A
number
of different
physical
and
products, s u c h a s optical properties
chemical
properties
of m e l t s
( t r a n s m i s s i o n , refraction
and
index) but
glass also
crystallization a n d fining b e h a v i o r are influenced by c o n t a i n i n g polyvalent ions and their redox s t a t e s [1]. T h e r e f o r e redox states a n d redox ratios of polyvalent ions in g l a s s e s a n d m e l t s are a subject of g e n e r a l interest.
194
Glastech. Ber. Glass Sei. Technol. 7 3 C 2 (2000)
Advances in Fusion & Processing of Glass - 6th Internat. Conf. 2000 T h e t h e r m o d y n a m i c a l r e d o x equilibrium of a polyvalent ion in a glass melt with the physically d i s s o l v e d o x y g e n c a n be d e s c r i b e d by the following g e n e r a l e q u a t i o n [2]:
with the equilibrium c o n s t a n t :
a s s u m i n g a fixed O " activity in t h e melt. 2
T h e integrated f o r m of the v a n ' t Hoff e x p r e s s i o n gives the d e p e n d e n c y of redox ratio on t e m p e r a t u r e a n d o x y g e n partial p r e s s u r e :
T h e r e f o r e t h e redox e q u i l i b r i u m of polyvalent ions in a glass melt of a given c o m p o s i t i o n c a n be a d j u s t e d by melting t e m p e r a t u r e a n d o x y g e n partial p r e s s u r e . A s the redox reactions of polyvalent ions are e n d o t h e r m i c p r o c e s s e s , high melting t e m p e r a t u r e s a n d / o r low o x y g e n partial p r e s s u r e s during melting shift redox ratios to m o r e r e d u c e d s t a t e s a n d vice versa. High melting t e m p e r a t u r e s
additionally
s h o w t h e effect of s h o r t e n i n g equilibration t i m e s d u e to accelerating velocities
by
decreasing
the
viscosity
of
the
melt
and
increasing
reaction diffusion
coefficients. Especially in silicate m e l t s t h e s e p a r a m e t e r s a r e very i m p o r t a n t [3]. N o w the objective of this w o r k w a s to investigate the e x t e n d of shifting redox ratios of the p o l y v a l e n t ions: iron, v a n a d i u m , c h r o m i u m a n d t i t a n i u m in the
silicate
g l a s s e s : (A) 3 3 m o l % N a O - 67 S i O , (B) 1 5 m o l % N a O - 8 5 S i O a n d (C) 1 0 m o l % 2
2
C a O - 1OBaO - 1 5 А 1 0 з - 6 5 S i O 2
2
2
2
using high t e m p e r a t u r e s a n d low o x y g e n partial
pressures.
Experimental For h i g h - t e m p e r a t u r e melts at 1 6 0 0 - 2 0 0 0 C g l a s s e s w e r e m e l t e d in a special 0
induction-heated
furnace
(Linn,
"Platicast
lr-crucibles. A s a t m o s p h e r e inert A r (or N / H 2
600μΡ") 2
for
10 to
60
minutes
in
for reducing conditions) h a s to be
Glastech. Ber. Glass Sei. Technol. 73 C2 (2000)
195
Advances in Fusion & Processing of Glass - 6th Internat. Conf. 2 0 0 0 u s e d , b e c a u s e Ir oxidises at higher t e m p e r a t u r e s . T h e m e l t s w e r e q u e n c h e d by centrifugal
casting
into
a cold
graphite
mold. Afterwards
all
samples
were
a n n e a l e d . T o achieve longer melting t i m e s the s a m p l e s w e r e remelted several times. For c o m p a r i s o n with redox states r e a c h e d under air a t m o s p h e r e d o p e d s a m p l e s w e r e m e l t e d in a resistance h e a t e d f u r n a c e at 1 6 0 0 C in Pt-crucibles. T o freeze 0
the
redox
ratios, t h e
melts
were
quenched
by casting
on
a copper
block.
A f t e r w a r d s the glasses w e r e a n n e a l e d . R e d u c i n g conditions w e r e a c h i e v e d by a d d i n g 0.2 w t . % c a r b o n to the p r e m e l t e d g l a s s e s . T h e n the g l a s s e s w e r e m e l t e d in silica-crucibles in a resistance h e a t e d f u r n a c e at 1 6 0 0 C for 60 m i n u t e s . 0
Q u e n c h e d g l a s s s a m p l e s w e r e quantitatively investigated by optical a b s o r p t i o n s p e c t r o s c o p y ( O S ) [ 4 , 5 ] a n d e l e c t r o n s p i n r e s o n a n c e s p e c t r o s c o p y ( E S R ) [6].
Results and Discussion A) Iron In the c a s e of iron d o p e d g l a s s e s t h e melting conditions have a strong influence o n the redox ratio in all investigated g l a s s e s . T h e F e / F e - r a t i o is a d j u s t a b l e f r o m 3+
2+
8 3 : 1 7 (air, 1 5 0 0 C ) to 4 0 : 6 0 (Ar, 1 8 0 0 ° C ) (Glass B). T h e intensity of C - T - b a n d s 0
d u e to F e Thereby
3 +
d e c r e a s e s a n d t h e a b s o r p t i o n e d g e is shifted f r o m 2 9 0 to 2 5 0 n m .
it is possible to i m p r o v e t h e U V t r a n s m i s s i o n
behaviour
c o n t a i n i n g iron only as an impurity [7]. T h e intensity of the d-d-bands r a n g e [5] d u e to F e
2 +
in
glasses
in the NIR
increases.
Melting at 1 8 0 0 C u n d e r reducing H / N a t m o s p h e r e leads to a further shift of t h e 0
2
2
F e / F e - r a t i o (10:90), but it is not possible to shift all of the iron into the ferrous 3 +
2 +
state by using reducing a g e n t s (graphite or N / H ) . At a r o u n d 10:90 the F e V F e 2
2
3
2 +
ratio r e m a i n s c o n s t a n t a n d the f o r m a t i o n of metallic iron begins [8] T h e a c c e l e r a t i o n of t h e redox reaction d u e to high melting t e m p e r a t u r e s is s h o w n in Figure 2. High melting t e m p e r a t u r e s i n c r e a s e the reaction velocity a n d s h o r t e n equilibration t i m e s by d e c r e a s i n g the viscosity of the melt a n d increasing o x y g e n diffusion coefficients [9]. A n o v e r v i e w of all iron redox ratios in t h e investigated g l a s s e s is given in T a b l e 1.
196
Glastech. Ber. Glass Sei. Technol. 7 3 C2 (2000)
Advances in Fusion & Processing of Glass - 6th Internat. Conf. 2 0 0 0
Fig.1
Optical a b s o r p t i o n s p e c t r a of glass B + 5 0 p p m Fe, m e l t e d under oxidizing, inert a n d r e d u c i n g conditions
Fig.2
D e p e n d e n c y of the F e / F e - r a t i o on holding time 3 +
2 +
Glastech. Ber. Glass Sei. Technol. 7 3 C2 (2000)
197
Advances in Fusion & Processing of Glass - 6th Internat. Conf. 2 0 0 0
V a l e n c e distribution of iron
Tab. 1
Temperature
Atmosphere
Glass A Fe VFe
( O
3
0
2 +
Glass C
Glass B Fe VFe 3
2 +
Fe VFe 3
(%)
(%)
(%)
1600
Air
80/20
76/24
87/13
1600
Air
71/29
55/45
81/19
1800
Ar
40/60
40/60
78/22
2000
Ar
-
25/75
75/25
1800
H /N
-
9/91
10/90
2
2
2 +
B) Vanadium The
V /V -ratio 5+
+
is adjustable f r o m 95:5 (air,1600°C) t o - 2 2 : 7 8
(Ar,
2000 C) 0
(glasses A a n d B). A p p l y i n g reducing conditions (graphite or N 2 / H 2 ) lead to a further i n c r e a s e of [ V ] , but no f o r m a t i o n of V 4 +
3 +
is o b s e r v e d in glass A (Fig. 3).
W h e r e a s in glass B the redox situation c h a n g e s completely. T h e V ^ / V ^ - t r a n s i t i o n is d o m i n a t i n g a n d [ V ] is less t h a n 1 % (Fig. 4). 5 +
Fig. 3
198
O p t i c a l a b s o r p t i o n spectra of glass A + 5 0 0 0 p p m V
Glastech. Ber. Glass Sei. Technol. 7 3 C2 (2000)
Advances.in Fusion & Processing of Glass - 6th Internat. Conf. 2000
Fig. 4
The
Optical a b s o r p t i o n s p e c t r a of glass B + 5 0 0 0 p p m V
non-simultanous
o c c u r r e n c e o f a l l v a n a d i u m redox states in t h e
binary
s o d i u m silicate g l a s s e s (A a n d B) a n d the c o m p l e t e c h a n g e of the redox transition in g l a s s B c a n be e x p l a i n e d by the s y n p r o p o r t i o n of V
5 +
and V
3 +
into V
4 +
during
cooling of t h e melt [10]: V In glass C the
X^/Watio
5 +
+ V
3 +
* 2 V
4 +
is d o m i n a t i n g under oxidizing melting conditions, but
(4)
all
t h r e e v a n a d i u m redox states coexist in the c o o l e d g l a s s e s m e l t e d a b o v e 1 6 0 0 ° C u n d e r inert c o n d i t i o n s . A n o v e r v i e w of all v a n a d i u m redox ratios in t h e investigated g l a s s e s is g i v e n in T a b l e 2.
Glastech. Ber. Glass Sei. Technol. 73 C2 (2000)
199
Advances in Fusion & Processing of Glass - 6th Internat. Conf. 2 0 0 0 V a l e n c e distribution of v a n a d i u m
Tab. 2
Atmosphere
Temperature
Glass A V ZV ZV
( C)
5+
0
4+
Glass C
Glass B 3+
V ZV ZV 5 +
4 +
3 +
V
5 +
ZV
4 +
ZV
(%)
(%)
(%)
3 +
1600
Air
95/5/0
84/16/0
93Z7/0
1600
Ar
89/11/0
54/46/0
73/22/5
1800
Ar
81/19/0
29/71/0
66Z28Z6
2000
Ar
63/27/0
22/78/0
64/29/7
1600
red.
52/48/0
< 0.5/28/72
90%, Cr
melting conditions.
3 +
is t h e main redox state in all investigated g l a s s e s u n d e r all Solely glass A stabilizes 4 0 % of the c h r o m i u m as C r
under
6 +
oxidizing conditions at 1600°C. A p p l y i n g reducing conditions nearly 1 0 0 % of t h e c h r o m i u m is shifted into the redox state C r . At t e m p e r a t u r e s > 1 8 0 0 ° C under inert c o n d i t i o n s C r 3+
glass C a n d C r
Fig.5
200
6 +
2 +
o c c u r s in
is not longer d e t e c t a b l e (Fig.5).
Fit of the optical a b s o r p t i o n s p e c t r u m (glass C +1 OOOppm Cr (2QOO C Ar) 0
Glastech. Ber. Glass Sei. Technol. 7 3 C 2 (2000)
1
Advances in Fusion & Processing of Glass - 6th Internat. Conf. 2000 A n overview of all c h r o m i u m redox ratios in t h e investigated g l a s s e s is g i v e n in T a b l e 3. V a l e n c e distribution of c h r o m i u m
Tab. 3
Temperature
Atmosphere
( C)
Glass A
Glass B
Cr /Cr
Cr /Cr
6 +
0
3 +
6 +
3 +
Glass C Cr /Cr /Cr 6 +
3 +
(%)
(%)
(%)
1600 (1700, C)
Air
40/60
6/94
3/97/0
1600
Ar
10/90
4/96
99/0
1800
Ar
7/93
99
0/97/3
2000
Ar
5/95
99
0/92/8
1800
H /N
0/100
0/100
0/62/38
2
2
2 +
D) Titanium In t h e investigated silicate g l a s s e s the T i / T i - r a t i o is a l m o s t completely shifted to 4+
3+
t h e T i . T h e influence of the melting conditions is nearly negligible (Fig. 6). A t 4 +
t e m p e r a t u r e s > 1 8 0 0 ° C (glass C) a n d > 2 0 0 0 ° C (glass B) respectively, the color of 1
the g l a s s e s b e c o m e s light pink and the relative a m o u n t of T i increase of the T i
3 +
is 1-5%.
3 +
A further
c o n t e n t is achieved melting u n d e r reducing conditions at high
temperatures. A n o v e r v i e w of all t i t a n i u m redox ratios in t h e investigated g l a s s e s is given in T a b l e 4. Tab. 4
V a l e n c e distribution of t i t a n i u m
Temperature
Atmosphere
( C) 0
Glass A
Glass B
Glass C
Ti ZTi
Ti ZTi
Ti ZTi
4 +
3 +
4+
3+
4+
(%)
(%)
(%)
3+
1600
Air
100/0
100Z0
100/0
1600
Ar
100/0
100Z0
100/0
1800
Ar
100/0
100Z0
>99/99Z
