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Thermo-chemical effects in photochromic glasses
Journal of Non-Crystalline Solids 38 & 39 (1980) 205-210 Q North-Holland Publishing Company
THERMO-CHEMICAL IN P H O T O C H R O M I C George
B. Hares
Research
EFFECTS GLASSES
and Thomas
P. Seward,
III
and D e v e l o p m e n t L a b o r a t o r i e s C o r n i n g Glass Works Corning, N e w York U.S.A.
Glass c o m p o s i t i o n and thermal h i s t o r y have a c o m b i n e d i n f l u e n c e on p h o t o c h r o m i c properties. C e r t a i n heat treatments are shown to cause r e v e r s i b l e changes in these properties. The changes are a s c r i b e d to c o m p o s i t i o n a l r e d i s t r i b u tion b e t w e e n the p h o t o c h r o m i c p a r t i c l e phase and the glass matrix, and to structural rearrangem e n t s w i t h i n the particles.
INTRODUCTION P h o t o c h r o m i c glass p e r f o r m a n c e characteristics, such as e q u i l i b r i u m d a r k e n e d t r a n s m i t t a n c e and fading rate, depend on thermal history (heat t r e a t m e n t temperature, c o o l i n g rate, etc.) as well as on glass composition. S u b s e q u e n t heat treatments, even at or b e l o w the glass strain point temperature, often cause large changes in the photochromic c h a r a c t e r i s t i c s 1,2,3. In this paper we p r e s e n t e v i d e n c e for the i n t e r r e l a t e d effects of glasB c o m p o s i t i o n and thermal h i s t o r y on p h o t o c h r o m i c properties, and conclude that the changes after s e c o n d a r y heat t r e a t m e n t result from a c o m p o s i t i o n a l r e d i s t r i b u t i o n b e t w e e n the p h o t o c h r o m i c phase p a r t i c l e s and the glass matrix, and from structural r e a r r a n g e m e n t s w i t h i n the particles. As in Ref. 3, the o b s e r v e d effects were i n f l u enced by w h e t h e r the t r e a t m e n t t e m p e r a t u r e was above or b e l o w the m e l t i n g points of the p h o t o c h r o m i c phase particles. EXPET{IMENTAL The
CODE. SX02 AL203 B203 L~20 NA20 K20
glasses
discussed
in this
TABLE I -GLASSCOMPOSIT~ ~ ~ ~ ~I_ L (BATCHED) 50,5 9 22,5 23 23,5 24 24.5 6 6 5,5 5 4.5 4 6
WEI.C~IIT~ (ANALYZED)
CuO AG CL BR
0,01 0,19 0,12 0,12
0,01 0.19 0.15 0.Ii
0.01 0.01 0,01 0,19 0.18 0,18 0,11 0,12 0,11 0.i0
0.i0
0.i0
report consist of a series o[ experim e n t a l alkali a l u m i n o b o r o s i l i c a t e p h o t o c h r o m i c glasses of varied soda and b o r i c o x i d e content d o p e d with silver, copper, c h l o r i n e and bromine, as shown in Table I. (Base glass e x p r e s s e d in cation % as c a l c u l a t e d from the batch, dopants r e p o r t e d in wt. % as analyzed by x-ray e m i s s i o n techniques.) Similar effects to those reported in this paper were found when potash, instead of soda, was c o r r e s p o n d i n q l y varied. The e x p e r i m e n t a l glasses were batched, m e l t e d and formed into patties approx. 3/3" thick using standard p h o t o c h r o m i c glass p r e p a r a t i o n techniques. Initial
205
206
G.B. Hares, T.P. Seward / Thermo-Chemical Effects
heat treatments were performed on small s a m p l e s ( ~ I " x 2" x 3/8" cut f r o m the l a r g e r patty) +300°C/16HRS in a small, f r o n t - l o a d i n g Pt. +275°C/15HRS r e s i s t a n c e h e a t e d kiln. After +225°C/16HRS i n i t i a l h e a t t r e a t m e n t the +150Oc/16HRS s a m p l e s w e r e p o l i s h e d to 2 . 0 m m +I00Oc/64NRS thickness. Darkening and +400Oc/16HRS f a d i n g p e r f o r m a n c e at 80°F 550oc/87 HRS) (27°C) w a s then m e a s u r e d on a solar simulator built a c c o r d i n g to Ref. 4. Secondary h e a t t r e a t m e n t s w e r e p e r f o r m e d on t h e s e p o l i s h e d samples. The sequence of h e a t t r e a t m e n t s is l i s t e d in T a b l e II.
TABLE]]__~EAT IREATMEN!SEQUENCE HT#
i 2 5 q 5 6
CUMULATIVETREATNEN]
550OC/]0MIN + 650oc/30 MIN +550°C/10MIN +400OC/16HRS +550°C/10MIN +400°C/10MIN +350Oc/16HRS
HT#
7 8 9 10 11 12
(ALTERNATIVE INITIAL TREATMENT, HT#1A -
FIG I. GLASS R 80~ . . . . 601WAVELENGTH[ = 425nm
TREATMENT
T h e m e l t i n g p o i n t (liquidus t e m p e r a ture) of the p a r t i c l e s for e a c h g l a s s composition, a f t e r the i n i t i a l h e a t t r e a t m e n t , w a s d e t e r m i n e d u s i n g the t h e r m o - o p t i c t e c h n i q u e d e s c r i b e d in z 2o Ref. 3. The technique involves light transmittance measurements, at a w a v e 200 250 :500 :550 400 450 l e n g t h a b s o r b e d by the p h o t o c h r o m i c TEMPERATURE °C p h a s e p a r t i c l e s , as a f u n c t i o n of temperature. An i l l u s t r a t i v e m e a s u r e m e n t c u r v e is s h o w n in Fig. 1. The abrupt transmittance c h a n g e s n e a r 4 0 0 ° C a n d 275°C i n d i c a t e the p a r t i c l e p h a s e changes which occur during heating and cooling, respectively. Following the i n t e r p r e t a t i o n of Ref. 3, the t e m p e r a t u r e of 395°C is cons i d e r e d to be the m e l t i n g p o i n t (liquidus t e m p e r a t u r e ) of the p a r t i cles m o s t s t a b l e a g a i n s t m e l t i n g . P r e s u m a b l y t h e s e are the l a r g e s t particles.
~ 40
~
The particle melting temperatures so d e t e r m i n e d for g l a s s e s R , S , T , U and V are 395,405,405,402 a n d 405 +5°C, r e s p e c t i v e l y . At t r e a t m e n t t e m p e r a t u r e s of 4 0 0 ° C or above, e s s e n t i a l l y all the p a r t i c l e s in the e x p e r i m e n t a l g l a s s e s are m e l t e d . PROPERTIES
AFTER
STANDARD
HEAT
TREATMENT
T h e s t a n d a r d i n i t i a l h e a t t r e a t m e n t g i v e n the g l a s s e s w a s #i of T a b l e II. Photochromic property measurements for this t r e a t m e n t are s h o w n in F i g u r e 2a. T h e s y m b o l s are i n t e r p r e t e d as follows: T0=luminous transmittance b e f o r e any e x p o s u r e , T D l 0 = l u m i n o u s transmittance after ten m i n u t e s e x p o s u r e to the a r t i f i c i a l s u n l i g h t source, a n d TF5 = luminous transmittance a f t e r five m i n u t e s of f a d i n g f r o m the d a r k e n e d state. As p l o t t e d , the t o t a l a l k a l i d e c r e a s e s f r o m left to right, with a corresponding i n c r e a s e in the b o r i c o x i d e content. The data illustrate that e l e m e n t s the f a d i n g rate The lower alkali glasses g l a s s e s ; the f a d i n g r a t e the g l a s s e s c a n ' t darken. ibrium between darkening
for a g i v e n c o n c e n t r a t i o n of p h o t o c h r o m i c i n c r e a s e s as the a l k a l i c o n t e n t d e c r e a s e s 5 , 6. do n o t d a r k e n as m u c h as the h i g h e r a l k a l i of the low a l k a l i g l a s s e s is so h i g h t h a t (TDI 0 is the r e s u l t of a d y n a m i c e q u i l a n d fading.)
A s e c o n d set of s a m p l e s w a s g i v e n an a l t e r n a t i v e t r e a t m e n t of 550°C for 87 h o u r s (HT#1a) to d e t e r m i n e w h e t h e r p h o t o c h r o m i s m c o u l d be d e v e l o p e d at t h a t lower t e m p e r a t u r e by h e a t i n g for s u f f i c i e n t time. The corresponding photochromic p r o p e r t i e s are s h o w n in F i g u r e 2b.
G.B. Hares, T.P. Seward / Thermo-Chemical Effects
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The high alkali g l a s s e s darken more and fade f a s t e d w h i l e the low alkali glasses darken less, than do the c o r r e s p o n d i n g glasses w i t h the 650°C/30 m i n u t e treatment as part of the schedule.
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ment
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for i0 m i n u t e s
SECONDARY
HEAT
TREATMENTS
The q u e s t i o n arises: Does the photochromic b e h a v i o r after the long 550°C heat t r e a t m e n t depend on the sample never having been h e a t e d to 650°C or is it p r i m a r i l y a function of the final t e m p e r a t u r e the glass saw before being q u i c k l y cooled? In an a t t e m p t to answer this q u e s t i o n the samples w h i c h had been h e a t - t r e a t e d at 5 5 0 ° C / 3 0 ' + 6 5 0 ° C / 3 0 ' (HT#1) were given an additional treat(HT#2) and then cooled quickly.
In Figures 2a and b the p h o t o c h r o m i c properties of the samples after this heat t r e a t m e n t are c o m p a r e d w i t h their previous values as well as w i t h the p r o p e r t i e s of the samples w h i c h w e r e held for 87 hours at 550°C. N o t i c e that after only i0 m i n u t e s at 550°C the photochromic p r o p e r t i e s of the samples are shifting towards the properties of the samples held for 87 hours at 550°C. C o m p a r e d to the samples after t r e a t m e n t #i, the high alkali glasses get darker and the low alkali glasses get lighter, p i v o t i n g on the glasses that contain 17 cation % total alkali. However, the samples still do not fade as rapidly as those with the long heat treatment. Ten m i n u t e s was p r o b a b l y not long enough to reach c o m p l e t e equilibrium. This e x p l a n a t i o n could also account for the fact that most of the glasses given t r e a t m e n t #2 darken more than those w i t h the long heat treatment. The samples w h i c h had already seen t r e a t m e n t #2 were then given an a d d i t i o n a l t r e a t m e n t of 400°C for 16 hours (HT#3). Their photochromic p r o p e r t i e s are shown in Figure 3 in c o m p a r i s o n w i t h those for the p r e v i o u s t r e a t m e n t (#2) as well as the o r i g i n a l t r e a t m e n t (#i). In all cases the TF5 is higher; the glasses fade faster! (The i n c r e a s e d fade rate is even m o r e obvious if c a l c u l a t e d as a fractional change in absorbanee.) Glass R, o r i g i n a l l y a very slow fadin~ glass, now darkens to about 34% and fades 33 points to about 67%. A f t e r the o r i g i n a l treatment (#i) it only d a r k e n e d to about 45% and faded FIG. 3o 3b 9 points to 54%. On the other hand the lower alkali glasses do not get as 90< , -'t 1 dark as on previous heat treatments.
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To c o n f i r m the r e v e r s i b i l i t y of the secondary heat t r e a t m e n t effects the samples w e r e given an additional heat t r e a t m e n t at 550°C for 10 minutes (HT#4). The p h o t o c h r o m i c p r o p e r t i e s after this t r e a t m e n t were almost identical to those o b s e r v e d after HT#2 even though the e q u i l i b r i u m was a p p r o a c h e d from a high
208
G.B. Hares, T.P. Seward / Thermo-Chemical Effects
t e m p e r a t u r e state the first the second time (HT#4).
time
(HT#2)
but
a low t e m p e r a t u r e
state
The rate of a p p r o a c h to e q u i l i b r i u m is also a function of temperature. As an illustration, the samples w e r e given a n o t h e r a d d i t i o n a l heat treatment, this time at 400~C for 10 m i n u t e s (HT#5). It was o b s e r v e d that, w h i l e the p h o t o c h r o m i c p r o p e r t i e s (both d a r k e n i n g and fading) s h i f t e d towards the values found after the 400°C/16 hours cycle (HT#3), they did not s u p e r i m p o s e as they did for the samples after the two d i f f e r e n t 550°C for i0 m i n u t e cycles. Ten minutes at 400°C is not long enough to reach equilibrium. F o l l o w i n g a n u m b e r of thermal cycles at t e m p e r a t u r e s lower than 400°C (to be d e s c r i b e d in the next section), the samples were again h e a t e d at 400°C for 16 hours (HT#12). The p h o t o c h r o m i c p r o p e r t i e s after this second 400°C/16 hrs. cycle, as will be shown in Figure 5, did s u p e r i m p o s e p o i n t for point on those of the first, showing that even at 400°C e q u i l i b r i u m can be r e a c h e d if s u f f i c i e n t time is allowed. LOWER TEMPERATURE
TREATMENTS
A series of successive low t e m p e r a t u r e (<400°C) treatments, #6-#11 of T a b l e II, was given these same glass samples. A dramatic change in p h o t o c h r o m i c p r o p e r t i e s was o b s e r v e d following the 300°C heat treatm e n t (Fig. 4). In fact, the trend of higher t e m p e r a t u r e treatments is r e v e r s e d for the low and high alkali glasses, i.e. the high alkali glasses d a r k e n less and the low alkali glasses darken more.
FIG. 4.
The d e t e r i o r a t i o n of photochromic p r o p e r t i e s for the high alkali glasses continues for additional heat t r e a t m e n t s as low as 225°C. On the other hand there is no change in p h o t o c h r o m i s m for the lowest alkali glasses b e y o n d the 300°C heat treatment. Little if any change in p h o t o c h r o m i c properties occurs for any of the glasses as a result of heat treatments at 150°C and 100°C. The p h o t o c h r o m i c p r o p e r t i e s of the first 400°C/16 hr. treatm e n t (#3) were i d e n t i c a l l y r e g e n e r a t e d by the final 400°C/16 hr. t r e a t m e n t (#12), as shown in Fig. 5.
FIG. 5
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500 580 460 540 620 TREATMENT TEMPERATURE °C
The d a r k e n i n g and fading data for high, m e d i u m and low alkali glasses (R,T, and V) are p l o t t e d in Fig. 6 for the entire "equilibrium" t e m p e r a t u r e range. The d r a m a t i c change in properties after the 300°C/16 hr. t r e a t m e n t can be easily seen in this plot.
G.B. Hares, T.P. Seward / Thermo-Chemical
Effects
209
DISCUSSION Changes several
in p h o t o c h r o m i c sources:
p r o p e r t i e s w i t h heat treatment may arise from
1. Droplet Growth. Obviously, if the glass is held at t e m p e r a t u r e s near the initial heat treatment temperature (and for c o m p a r a b l e times) the molten droplet phase can grow in size. 2. C__omposition Changes in the P h o t o c h r o m i c Phase. Since all photochromic qlasses are m u l t i - c o m p o n e n t systems and since copper (+i) and alkali ions are soluble in the p h o t o c h r o m i c phase, the chemical equilibrium w h i c h is a p p r o a c h e d at the initial heat treatment temperature, is in general different from that w h i c h w o u l d hold at a lower temperature. If the glass is held at these t e m p e r a t u r e s for an appreciable time, the composition of the particles may be expected to change. Even at t e m p e r a t u r e s b e l o w the strain point, silver, alkali and copper (+i) m o b i l i t y in the glass is sufficient to allow i o n - e x c h a n g e between the glass and the particle. 3. A n n e a l i n g of Defects. After the p h o t o c h r o m i c phase droplets have crystallized, they may contain a number of defects such as dislocations and grain boundaries. These defects, w h i c h may affect both d a r k e n i n g and fading of the glass, may be annealed out to some extent by heat treating the glass at a temperature b e l o w the m e l t i n g point of the particles, w h i c h is usually near 400°C. 4. Change in Crystal Structure and C o m p o s i t i o n Segregation. If the p h o t o c h r o m i c p h a s e - ~ n s i s t s o f silver halide plus other cation components (alkali, copper, lead, etc.) as is currently b e l i e v e d 7, the stable c r y s t a l l i n e form may be a solid solution or mixed crystal system. The phases formed and their compositions (or c o m p o s i t i o n gradients) may depend on the cooling rates and the degree of supercoolinq attained. C r y s t a l l i n e structural changes in response to lowtemperature secondary treatments have been reported I. 5. H y s t e r e s i s of M e l t i n g and Freezing. These effects are complic a t e d by the hysteresis in m e l t i n g and freezing. On cooling, the droplets can be supercooled, often by more than i00°C, before crys t a l l i z a t i o n occurs 3. On reheating, the crystals must be raised above the droplet liquidus temperature before complete m e l t i n g can occur. B e t w e e n these two temperatures, structural r e a r r a n g e m e n t is possible during reheating, but cannot occur during droplet cooling. Our data indicate that for heat treatments at temperatures above 400°C the p h o t o c h r o m i c p r o p e r t i e s are e s t a b l i s h e d by the final heat treatment temperature. These p r o p e r t i e s can be regained by reheating the glass to that temperature despite intervening l o w - t e m p e r a t u r e thermal history. A possible e x p l a n a t i o n involves a reversible change in the e q u i l i b r i u m d i s t r i b u t i o n of Cu + and/or Na + ions between the glass m a t r i x and the p h o t o c h r o m i c particles. C o n s i d e r i n g the important role of copper in d e t e r m i n i n g p h o t o c h r o m i c properties, we b e l i e v e it is the relative copper c o n c e n t r a t i o n which changes with temperature and is r e s p o n s i b l e for the property changes. As discussed in Refs. 6 and 7, the effect of increasing copper from very low levels is first to improve the darkening ability of the glass (by p r o v i d i n g more hole traps). B e y o n d some copper c o n c e n t r a t i o n (which depends on other c o m p o s i t i o n a l factors) the d a r k e n a b i l i t y decreases again, p r i m a r i l y as a c o n s e q u e n c e of an increased tendency for fading. Thus, if the relative solubility of Cu + in the photo-
210
G.B. Hares, T.P. Seward / Thermo-Chemical
chromic phase is greater at lower temperatures, could explain the observed effects.
Effects
this change alone
Below 400°C there is evidence for a reversed trend in the e q u i l i b r i u m properties. We suggest that this reversal is either because the e q u i l i b r i u m d i s t r i b u t i o n of Cu + and/or Na + (between particles and matrix) differs for the crystalline, as compared to the liquid, state of the particle, or because the particles crystallize to a p o l y - p h a s e system for w h i c h the internal c o m p o s i t i o n d i s t r i b u t i o n and structure changes as a result of annealing. In Ref. 3 evidence was p r e s e n t e d that cooling rate through the particle c r y s t a l l i z a t i o n range a f f e c t e d p h o t o c h r o m i c properties. It was suggested that different resulting degrees of crystalline perfection were r e s p o n s i b l e for property changes. The evidence reported here does not refute the role of c r y s t a l l i n e perfection, but argues that since the effect of heating varies so m u c h with composition, c o m p o s i t i o n a l changes w i t h i n the particle phase are p r o b a b l y a major factor. REFERENCES [i] Sakka, S., Matusita, K., Kamiya, K., Properties of silver halide p h o t o c h r o m i c glasses, in: The Xth I n t e r n a t i o n a l Congress on Glass, Kyoto, 1974, No. 5 (The Ceramic Society of Japan, 1974). [2] Besen, H., Einfluss der w a r m e b e h a n d l u n g auf das t r a n s m i s s i o n s v e r h a l t e n fotochromer glases, in: P r o c e e d i n g s of the XIth International Congress on Glass, Prague, 1977, Vol. III (CVTS, Dum T e c h n i k y Praha, 1977). [3] Tick, P. A., Seward, detected transitions Phys. Chem. Glasses,
T. P. III, and Butler, B. L., Optically in p h o t o c h r o m i c glass, scheduled pub., J. N o v e m b e r - D e c e m b e r (1979).
[4] Chodak, J. B., Solar simulator, 14, 1978).
U. S. Patent
#4,125,775
[5] Kerko, D. J., and Seward, T. P. III, P h o t o c h r o m i c positions and articles, U. S. Patent #4,018,965.
(November
sheet glass com(April 19, 1977).
[6] Hares, G. B., Morse, D. L., Seward, T. P. III, and Smith, D. W., P h o t o c h r o m i c glasses, p e n d i n g U. S. Patent a p p l i c a t i o n S.N. 14,981 (February 28, 1979). [7] Araujo, R. J., P h o t o c h r o m i c glass, in: Doremus, R. H. and Tomozawa, M. (eds.), Treatise on Materials Science and Technology, Vol. 12 (Academic Press, N.Y., 1977).
THERMO-CHEMICAL IN P H O T O C H R O M I C George
B. Hares
Research
EFFECTS GLASSES
and Thomas
P. Seward,
III
and D e v e l o p m e n t L a b o r a t o r i e s C o r n i n g Glass Works Corning, N e w York U.S.A.
Glass c o m p o s i t i o n and thermal h i s t o r y have a c o m b i n e d i n f l u e n c e on p h o t o c h r o m i c properties. C e r t a i n heat treatments are shown to cause r e v e r s i b l e changes in these properties. The changes are a s c r i b e d to c o m p o s i t i o n a l r e d i s t r i b u tion b e t w e e n the p h o t o c h r o m i c p a r t i c l e phase and the glass matrix, and to structural rearrangem e n t s w i t h i n the particles.
INTRODUCTION P h o t o c h r o m i c glass p e r f o r m a n c e characteristics, such as e q u i l i b r i u m d a r k e n e d t r a n s m i t t a n c e and fading rate, depend on thermal history (heat t r e a t m e n t temperature, c o o l i n g rate, etc.) as well as on glass composition. S u b s e q u e n t heat treatments, even at or b e l o w the glass strain point temperature, often cause large changes in the photochromic c h a r a c t e r i s t i c s 1,2,3. In this paper we p r e s e n t e v i d e n c e for the i n t e r r e l a t e d effects of glasB c o m p o s i t i o n and thermal h i s t o r y on p h o t o c h r o m i c properties, and conclude that the changes after s e c o n d a r y heat t r e a t m e n t result from a c o m p o s i t i o n a l r e d i s t r i b u t i o n b e t w e e n the p h o t o c h r o m i c phase p a r t i c l e s and the glass matrix, and from structural r e a r r a n g e m e n t s w i t h i n the particles. As in Ref. 3, the o b s e r v e d effects were i n f l u enced by w h e t h e r the t r e a t m e n t t e m p e r a t u r e was above or b e l o w the m e l t i n g points of the p h o t o c h r o m i c phase particles. EXPET{IMENTAL The
CODE. SX02 AL203 B203 L~20 NA20 K20
glasses
discussed
in this
TABLE I -GLASSCOMPOSIT~ ~ ~ ~ ~I_ L (BATCHED) 50,5 9 22,5 23 23,5 24 24.5 6 6 5,5 5 4.5 4 6
WEI.C~IIT~ (ANALYZED)
CuO AG CL BR
0,01 0,19 0,12 0,12
0,01 0.19 0.15 0.Ii
0.01 0.01 0,01 0,19 0.18 0,18 0,11 0,12 0,11 0.i0
0.i0
0.i0
report consist of a series o[ experim e n t a l alkali a l u m i n o b o r o s i l i c a t e p h o t o c h r o m i c glasses of varied soda and b o r i c o x i d e content d o p e d with silver, copper, c h l o r i n e and bromine, as shown in Table I. (Base glass e x p r e s s e d in cation % as c a l c u l a t e d from the batch, dopants r e p o r t e d in wt. % as analyzed by x-ray e m i s s i o n techniques.) Similar effects to those reported in this paper were found when potash, instead of soda, was c o r r e s p o n d i n q l y varied. The e x p e r i m e n t a l glasses were batched, m e l t e d and formed into patties approx. 3/3" thick using standard p h o t o c h r o m i c glass p r e p a r a t i o n techniques. Initial
205
206
G.B. Hares, T.P. Seward / Thermo-Chemical Effects
heat treatments were performed on small s a m p l e s ( ~ I " x 2" x 3/8" cut f r o m the l a r g e r patty) +300°C/16HRS in a small, f r o n t - l o a d i n g Pt. +275°C/15HRS r e s i s t a n c e h e a t e d kiln. After +225°C/16HRS i n i t i a l h e a t t r e a t m e n t the +150Oc/16HRS s a m p l e s w e r e p o l i s h e d to 2 . 0 m m +I00Oc/64NRS thickness. Darkening and +400Oc/16HRS f a d i n g p e r f o r m a n c e at 80°F 550oc/87 HRS) (27°C) w a s then m e a s u r e d on a solar simulator built a c c o r d i n g to Ref. 4. Secondary h e a t t r e a t m e n t s w e r e p e r f o r m e d on t h e s e p o l i s h e d samples. The sequence of h e a t t r e a t m e n t s is l i s t e d in T a b l e II.
TABLE]]__~EAT IREATMEN!SEQUENCE HT#
i 2 5 q 5 6
CUMULATIVETREATNEN]
550OC/]0MIN + 650oc/30 MIN +550°C/10MIN +400OC/16HRS +550°C/10MIN +400°C/10MIN +350Oc/16HRS
HT#
7 8 9 10 11 12
(ALTERNATIVE INITIAL TREATMENT, HT#1A -
FIG I. GLASS R 80~ . . . . 601WAVELENGTH[ = 425nm
TREATMENT
T h e m e l t i n g p o i n t (liquidus t e m p e r a ture) of the p a r t i c l e s for e a c h g l a s s composition, a f t e r the i n i t i a l h e a t t r e a t m e n t , w a s d e t e r m i n e d u s i n g the t h e r m o - o p t i c t e c h n i q u e d e s c r i b e d in z 2o Ref. 3. The technique involves light transmittance measurements, at a w a v e 200 250 :500 :550 400 450 l e n g t h a b s o r b e d by the p h o t o c h r o m i c TEMPERATURE °C p h a s e p a r t i c l e s , as a f u n c t i o n of temperature. An i l l u s t r a t i v e m e a s u r e m e n t c u r v e is s h o w n in Fig. 1. The abrupt transmittance c h a n g e s n e a r 4 0 0 ° C a n d 275°C i n d i c a t e the p a r t i c l e p h a s e changes which occur during heating and cooling, respectively. Following the i n t e r p r e t a t i o n of Ref. 3, the t e m p e r a t u r e of 395°C is cons i d e r e d to be the m e l t i n g p o i n t (liquidus t e m p e r a t u r e ) of the p a r t i cles m o s t s t a b l e a g a i n s t m e l t i n g . P r e s u m a b l y t h e s e are the l a r g e s t particles.
~ 40
~
The particle melting temperatures so d e t e r m i n e d for g l a s s e s R , S , T , U and V are 395,405,405,402 a n d 405 +5°C, r e s p e c t i v e l y . At t r e a t m e n t t e m p e r a t u r e s of 4 0 0 ° C or above, e s s e n t i a l l y all the p a r t i c l e s in the e x p e r i m e n t a l g l a s s e s are m e l t e d . PROPERTIES
AFTER
STANDARD
HEAT
TREATMENT
T h e s t a n d a r d i n i t i a l h e a t t r e a t m e n t g i v e n the g l a s s e s w a s #i of T a b l e II. Photochromic property measurements for this t r e a t m e n t are s h o w n in F i g u r e 2a. T h e s y m b o l s are i n t e r p r e t e d as follows: T0=luminous transmittance b e f o r e any e x p o s u r e , T D l 0 = l u m i n o u s transmittance after ten m i n u t e s e x p o s u r e to the a r t i f i c i a l s u n l i g h t source, a n d TF5 = luminous transmittance a f t e r five m i n u t e s of f a d i n g f r o m the d a r k e n e d state. As p l o t t e d , the t o t a l a l k a l i d e c r e a s e s f r o m left to right, with a corresponding i n c r e a s e in the b o r i c o x i d e content. The data illustrate that e l e m e n t s the f a d i n g rate The lower alkali glasses g l a s s e s ; the f a d i n g r a t e the g l a s s e s c a n ' t darken. ibrium between darkening
for a g i v e n c o n c e n t r a t i o n of p h o t o c h r o m i c i n c r e a s e s as the a l k a l i c o n t e n t d e c r e a s e s 5 , 6. do n o t d a r k e n as m u c h as the h i g h e r a l k a l i of the low a l k a l i g l a s s e s is so h i g h t h a t (TDI 0 is the r e s u l t of a d y n a m i c e q u i l a n d fading.)
A s e c o n d set of s a m p l e s w a s g i v e n an a l t e r n a t i v e t r e a t m e n t of 550°C for 87 h o u r s (HT#1a) to d e t e r m i n e w h e t h e r p h o t o c h r o m i s m c o u l d be d e v e l o p e d at t h a t lower t e m p e r a t u r e by h e a t i n g for s u f f i c i e n t time. The corresponding photochromic p r o p e r t i e s are s h o w n in F i g u r e 2b.
G.B. Hares, T.P. Seward / Thermo-Chemical Effects
FIG 2a 90
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The high alkali g l a s s e s darken more and fade f a s t e d w h i l e the low alkali glasses darken less, than do the c o r r e s p o n d i n g glasses w i t h the 650°C/30 m i n u t e treatment as part of the schedule.
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ment
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207
for i0 m i n u t e s
SECONDARY
HEAT
TREATMENTS
The q u e s t i o n arises: Does the photochromic b e h a v i o r after the long 550°C heat t r e a t m e n t depend on the sample never having been h e a t e d to 650°C or is it p r i m a r i l y a function of the final t e m p e r a t u r e the glass saw before being q u i c k l y cooled? In an a t t e m p t to answer this q u e s t i o n the samples w h i c h had been h e a t - t r e a t e d at 5 5 0 ° C / 3 0 ' + 6 5 0 ° C / 3 0 ' (HT#1) were given an additional treat(HT#2) and then cooled quickly.
In Figures 2a and b the p h o t o c h r o m i c properties of the samples after this heat t r e a t m e n t are c o m p a r e d w i t h their previous values as well as w i t h the p r o p e r t i e s of the samples w h i c h w e r e held for 87 hours at 550°C. N o t i c e that after only i0 m i n u t e s at 550°C the photochromic p r o p e r t i e s of the samples are shifting towards the properties of the samples held for 87 hours at 550°C. C o m p a r e d to the samples after t r e a t m e n t #i, the high alkali glasses get darker and the low alkali glasses get lighter, p i v o t i n g on the glasses that contain 17 cation % total alkali. However, the samples still do not fade as rapidly as those with the long heat treatment. Ten m i n u t e s was p r o b a b l y not long enough to reach c o m p l e t e equilibrium. This e x p l a n a t i o n could also account for the fact that most of the glasses given t r e a t m e n t #2 darken more than those w i t h the long heat treatment. The samples w h i c h had already seen t r e a t m e n t #2 were then given an a d d i t i o n a l t r e a t m e n t of 400°C for 16 hours (HT#3). Their photochromic p r o p e r t i e s are shown in Figure 3 in c o m p a r i s o n w i t h those for the p r e v i o u s t r e a t m e n t (#2) as well as the o r i g i n a l t r e a t m e n t (#i). In all cases the TF5 is higher; the glasses fade faster! (The i n c r e a s e d fade rate is even m o r e obvious if c a l c u l a t e d as a fractional change in absorbanee.) Glass R, o r i g i n a l l y a very slow fadin~ glass, now darkens to about 34% and fades 33 points to about 67%. A f t e r the o r i g i n a l treatment (#i) it only d a r k e n e d to about 45% and faded FIG. 3o 3b 9 points to 54%. On the other hand the lower alkali glasses do not get as 90< , -'t 1 dark as on previous heat treatments.
P
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To c o n f i r m the r e v e r s i b i l i t y of the secondary heat t r e a t m e n t effects the samples w e r e given an additional heat t r e a t m e n t at 550°C for 10 minutes (HT#4). The p h o t o c h r o m i c p r o p e r t i e s after this t r e a t m e n t were almost identical to those o b s e r v e d after HT#2 even though the e q u i l i b r i u m was a p p r o a c h e d from a high
208
G.B. Hares, T.P. Seward / Thermo-Chemical Effects
t e m p e r a t u r e state the first the second time (HT#4).
time
(HT#2)
but
a low t e m p e r a t u r e
state
The rate of a p p r o a c h to e q u i l i b r i u m is also a function of temperature. As an illustration, the samples w e r e given a n o t h e r a d d i t i o n a l heat treatment, this time at 400~C for 10 m i n u t e s (HT#5). It was o b s e r v e d that, w h i l e the p h o t o c h r o m i c p r o p e r t i e s (both d a r k e n i n g and fading) s h i f t e d towards the values found after the 400°C/16 hours cycle (HT#3), they did not s u p e r i m p o s e as they did for the samples after the two d i f f e r e n t 550°C for i0 m i n u t e cycles. Ten minutes at 400°C is not long enough to reach equilibrium. F o l l o w i n g a n u m b e r of thermal cycles at t e m p e r a t u r e s lower than 400°C (to be d e s c r i b e d in the next section), the samples were again h e a t e d at 400°C for 16 hours (HT#12). The p h o t o c h r o m i c p r o p e r t i e s after this second 400°C/16 hrs. cycle, as will be shown in Figure 5, did s u p e r i m p o s e p o i n t for point on those of the first, showing that even at 400°C e q u i l i b r i u m can be r e a c h e d if s u f f i c i e n t time is allowed. LOWER TEMPERATURE
TREATMENTS
A series of successive low t e m p e r a t u r e (<400°C) treatments, #6-#11 of T a b l e II, was given these same glass samples. A dramatic change in p h o t o c h r o m i c p r o p e r t i e s was o b s e r v e d following the 300°C heat treatm e n t (Fig. 4). In fact, the trend of higher t e m p e r a t u r e treatments is r e v e r s e d for the low and high alkali glasses, i.e. the high alkali glasses d a r k e n less and the low alkali glasses darken more.
FIG. 4.
The d e t e r i o r a t i o n of photochromic p r o p e r t i e s for the high alkali glasses continues for additional heat t r e a t m e n t s as low as 225°C. On the other hand there is no change in p h o t o c h r o m i s m for the lowest alkali glasses b e y o n d the 300°C heat treatment. Little if any change in p h o t o c h r o m i c properties occurs for any of the glasses as a result of heat treatments at 150°C and 100°C. The p h o t o c h r o m i c p r o p e r t i e s of the first 400°C/16 hr. treatm e n t (#3) were i d e n t i c a l l y r e g e n e r a t e d by the final 400°C/16 hr. t r e a t m e n t (#12), as shown in Fig. 5.
FIG. 5
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9O
/o.. o
b.I
-.4D
O"
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30
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V
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T • o L
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500 580 460 540 620 TREATMENT TEMPERATURE °C
The d a r k e n i n g and fading data for high, m e d i u m and low alkali glasses (R,T, and V) are p l o t t e d in Fig. 6 for the entire "equilibrium" t e m p e r a t u r e range. The d r a m a t i c change in properties after the 300°C/16 hr. t r e a t m e n t can be easily seen in this plot.
G.B. Hares, T.P. Seward / Thermo-Chemical
Effects
209
DISCUSSION Changes several
in p h o t o c h r o m i c sources:
p r o p e r t i e s w i t h heat treatment may arise from
1. Droplet Growth. Obviously, if the glass is held at t e m p e r a t u r e s near the initial heat treatment temperature (and for c o m p a r a b l e times) the molten droplet phase can grow in size. 2. C__omposition Changes in the P h o t o c h r o m i c Phase. Since all photochromic qlasses are m u l t i - c o m p o n e n t systems and since copper (+i) and alkali ions are soluble in the p h o t o c h r o m i c phase, the chemical equilibrium w h i c h is a p p r o a c h e d at the initial heat treatment temperature, is in general different from that w h i c h w o u l d hold at a lower temperature. If the glass is held at these t e m p e r a t u r e s for an appreciable time, the composition of the particles may be expected to change. Even at t e m p e r a t u r e s b e l o w the strain point, silver, alkali and copper (+i) m o b i l i t y in the glass is sufficient to allow i o n - e x c h a n g e between the glass and the particle. 3. A n n e a l i n g of Defects. After the p h o t o c h r o m i c phase droplets have crystallized, they may contain a number of defects such as dislocations and grain boundaries. These defects, w h i c h may affect both d a r k e n i n g and fading of the glass, may be annealed out to some extent by heat treating the glass at a temperature b e l o w the m e l t i n g point of the particles, w h i c h is usually near 400°C. 4. Change in Crystal Structure and C o m p o s i t i o n Segregation. If the p h o t o c h r o m i c p h a s e - ~ n s i s t s o f silver halide plus other cation components (alkali, copper, lead, etc.) as is currently b e l i e v e d 7, the stable c r y s t a l l i n e form may be a solid solution or mixed crystal system. The phases formed and their compositions (or c o m p o s i t i o n gradients) may depend on the cooling rates and the degree of supercoolinq attained. C r y s t a l l i n e structural changes in response to lowtemperature secondary treatments have been reported I. 5. H y s t e r e s i s of M e l t i n g and Freezing. These effects are complic a t e d by the hysteresis in m e l t i n g and freezing. On cooling, the droplets can be supercooled, often by more than i00°C, before crys t a l l i z a t i o n occurs 3. On reheating, the crystals must be raised above the droplet liquidus temperature before complete m e l t i n g can occur. B e t w e e n these two temperatures, structural r e a r r a n g e m e n t is possible during reheating, but cannot occur during droplet cooling. Our data indicate that for heat treatments at temperatures above 400°C the p h o t o c h r o m i c p r o p e r t i e s are e s t a b l i s h e d by the final heat treatment temperature. These p r o p e r t i e s can be regained by reheating the glass to that temperature despite intervening l o w - t e m p e r a t u r e thermal history. A possible e x p l a n a t i o n involves a reversible change in the e q u i l i b r i u m d i s t r i b u t i o n of Cu + and/or Na + ions between the glass m a t r i x and the p h o t o c h r o m i c particles. C o n s i d e r i n g the important role of copper in d e t e r m i n i n g p h o t o c h r o m i c properties, we b e l i e v e it is the relative copper c o n c e n t r a t i o n which changes with temperature and is r e s p o n s i b l e for the property changes. As discussed in Refs. 6 and 7, the effect of increasing copper from very low levels is first to improve the darkening ability of the glass (by p r o v i d i n g more hole traps). B e y o n d some copper c o n c e n t r a t i o n (which depends on other c o m p o s i t i o n a l factors) the d a r k e n a b i l i t y decreases again, p r i m a r i l y as a c o n s e q u e n c e of an increased tendency for fading. Thus, if the relative solubility of Cu + in the photo-
210
G.B. Hares, T.P. Seward / Thermo-Chemical
chromic phase is greater at lower temperatures, could explain the observed effects.
Effects
this change alone
Below 400°C there is evidence for a reversed trend in the e q u i l i b r i u m properties. We suggest that this reversal is either because the e q u i l i b r i u m d i s t r i b u t i o n of Cu + and/or Na + (between particles and matrix) differs for the crystalline, as compared to the liquid, state of the particle, or because the particles crystallize to a p o l y - p h a s e system for w h i c h the internal c o m p o s i t i o n d i s t r i b u t i o n and structure changes as a result of annealing. In Ref. 3 evidence was p r e s e n t e d that cooling rate through the particle c r y s t a l l i z a t i o n range a f f e c t e d p h o t o c h r o m i c properties. It was suggested that different resulting degrees of crystalline perfection were r e s p o n s i b l e for property changes. The evidence reported here does not refute the role of c r y s t a l l i n e perfection, but argues that since the effect of heating varies so m u c h with composition, c o m p o s i t i o n a l changes w i t h i n the particle phase are p r o b a b l y a major factor. REFERENCES [i] Sakka, S., Matusita, K., Kamiya, K., Properties of silver halide p h o t o c h r o m i c glasses, in: The Xth I n t e r n a t i o n a l Congress on Glass, Kyoto, 1974, No. 5 (The Ceramic Society of Japan, 1974). [2] Besen, H., Einfluss der w a r m e b e h a n d l u n g auf das t r a n s m i s s i o n s v e r h a l t e n fotochromer glases, in: P r o c e e d i n g s of the XIth International Congress on Glass, Prague, 1977, Vol. III (CVTS, Dum T e c h n i k y Praha, 1977). [3] Tick, P. A., Seward, detected transitions Phys. Chem. Glasses,
T. P. III, and Butler, B. L., Optically in p h o t o c h r o m i c glass, scheduled pub., J. N o v e m b e r - D e c e m b e r (1979).
[4] Chodak, J. B., Solar simulator, 14, 1978).
U. S. Patent
#4,125,775
[5] Kerko, D. J., and Seward, T. P. III, P h o t o c h r o m i c positions and articles, U. S. Patent #4,018,965.
(November
sheet glass com(April 19, 1977).
[6] Hares, G. B., Morse, D. L., Seward, T. P. III, and Smith, D. W., P h o t o c h r o m i c glasses, p e n d i n g U. S. Patent a p p l i c a t i o n S.N. 14,981 (February 28, 1979). [7] Araujo, R. J., P h o t o c h r o m i c glass, in: Doremus, R. H. and Tomozawa, M. (eds.), Treatise on Materials Science and Technology, Vol. 12 (Academic Press, N.Y., 1977).