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On the interdiffusion of Ag+ and Na+ ions in the glass Na2O·2SiO2 around the glass transition temperature
Journal of Non-Crystalline Solids 53 (1982) 143-153 North-Holland Publishing Company
143
O N T H E I N T E R D I F F U S I O N O F Ag + A N D N a + I O N S I N T H E GLASS Na20.2SiO 2 AROUND THE GLASS TRANSITION TEMPERATURE Ch. K A P S a n d G . V O L K S C H Department of Chemistry, Friedrich Schiller University, Jena, GDR
Received 26 June 1981 Revised manuscript received 18 June 1982
The Ag+/Na ÷ ion-exchange is studied for the glass Na20-2SiO 2 below and above the glass transition temperature using a eutectic melt (AgCl)0.42(AgI)0.5s. The interdiffusion coefficients /)Ag/Na are calculated from the Ag concentration profiles in the Glass measured by X-ray micro-analysis. The evaluation gives concentration-independent values DAg/Na corresponding to a low degree of exchange (1%). The Arrhenius diagram lg D = f ( 1 / T ) shows a distinct bending at the glass transition temperature. The observed change of the activation parameters Do and EA is discussed in relation to the generation of free volume for T > T~ and to chemical bonding of Ag and Na in the glass network. The results are compared with ion-exchange experiments using an AgCI(T > Tg) and AgNO3 melt (T < T~), respectively.
1. Introduction D i f f u s i o n - d e t e r m i n e d exchange processes of cations between glasses a n d salt melts offer a r e m a r k a b l e p o s s i b i l i t y for g e n e r a t i o n of d e f i n e d - i n h o m o g e n e o u s optical materials. In this process investigations are a i m e d at sufficiently large p e n e t r a t i o n d e p t h s in the glass or high diffusion coefficients, respectively. In glasses singly-charged cations show r e m a r k a b l y higher mobilities than m u l t i p l y - c h a r g e d ions. T h e diffusion coefficient of alkali ions in alkali silicate glasses increases with rising t e m p e r a t u r e a n d with increasing c o n t e n t of netw o r k modifiers [ 1]. F u r t h e r extension of the i o n - e x c h a n g e into the glass b u l k is limited on the o n e side b y the glass-forming b o u n d a r y to higher c o n t e n t s of alkali oxide a n d on the other side b y c o n s i d e r a b l e d e f o r m a t i o n s of the glass s a m p l e s d u r i n g the exchange process a b o v e the glass transition temperature. Therefore, the c a t i o n - e x c h a n g e investigated in this p a p e r was carried out on a b i n a r y alkali silicate glass with a high c o n t e n t of alkali oxide at t e m p e r a t u r e s up to the glass t r a n s i t i o n range. T h e glass used of the c o m p o s i t i o n N a 2 0 - 2SiO 2 shows a t r a n s i t i o n t e m p e r a t u r e of 465°C. Because of the possibility of a glassy melt u n d e r g o i n g a transition to its e q u i l i b r i u m state at T > Tg a change of the a c t i v a t i o n p a r a m e t e r s has to be expected in the range T > Tg [2]. Changes of the o p t i c a l p r o p e r t i e s of the m a t e r i a l should be i n d u c e d b y the 0022-3093/82/0000-0000/$02.75
© 1982 N o r t h - H o l l a n d
144
Ch. Kaps, G. Vi~lksch / Interdiffusion of .4g + and Na + ions
substitution of light ions by heavy ones. Melts of silver salts are a rich source of Ag + ions, but the temperature range of their applicability is small. The silver salts show in the molten state a sufficiently high mobility of Ag + ions. The employment of some melts is limited by their decomposition at higher temperatures. Therefore the ion-exchange in the glass N a 2 0 . 2 S i O 2 (Tg = 465°C) was carried out with a low-melting eutectic of the composition (AgC1)042(AgI)058(Fp = 257°C). In that way the diffusion behavior of the glass could be studied over a considerable temperature range (AT = 250 K) with one and the same salt melt. The A g + / N a + interdiffusion has not previously been investigated for the glass N a 2 0 - 2 S i O 2 over such an extended temperature range. Meistring et al. [3] previously carried out the A g + / N a + exchange with AgC1 melts at temperatures T > Tg (AT = 100 K). The surface treatment of glasses with Ag-containing salts or pastes by heating has already been known for some centuries as a staining process. The cation-exchange is followed by a partial reduction of the Ag + ions forming different aggregation states of silver atoms. These particles cause a yellow or red-brown colour by light scattering in the glass. However, the diffusion process is not influenced by the following nucleation and coloration [3], especially when the degree of exchange is low. Obviously, such conditions are maintained in the case of application of AgCl-AgI melts in the interdiffusion experiments, because the Ag + activity of the iodine-containing melt is relatively small.
2. Experimental procedure 2.1. Preparation of the glass and of the Ag-containing melts The glass of the composition N a 2 0 . 2 S i O 2 was melted from a mixture of soda and quartz sand of optical quality in a platinum crucible (65 l) at 1500°C. The glass samples were annealed at 475°C and then cooled very slowly (2.4 K h - ~). Because of the hygroscopic behavior of the glass the samples were kept in a desiccator containing CaC12. Dilatometric measurements of the glass (5 K min -1) give a linear thermal expansion coefficient 0t20/400 = 1.61 × 10 -5 K - l and a transition temperature Tg = 465°C. For the preparation of the Ag-containing melts chemicals of p.a. quality were used. The silver halogenides AgCI and AgI were precipitated from aqueous solution of AgNO 3 by distilled hydrochloric acid or NaI solution, respectively. After desiccation of the silver halides at 130°C the eutectic composition (AgCl)0.42(AgI)058(Fp = 257°C) was mixed and melted down at 500°C in air. The AgC1 melt (Fp = 456°C) was prepared in a corresponding way. For the experiments with the AgNO 3 melts (Fp = 207°C) a markstable p.a. silver nitrate was used.
Ch. Kaps, G. V6lksch / lnterdiffusion of Ag + and Na + ions
145
2.2. Diffusion experiments The glass samples (18 x 18 x 20 mm 3) were shaped from the melt bars and roughly ground. The face chosen for the ion-exchange was ground (ethanol; Ax > 0.03 mm) and polished (glycol, CeO 2) in a water-free medium. Finally, the samples were cleaned with absolute methanol. A tube of silica glass with a polished frontage was put up on the face of the glass samples chosen for the ion-exchange. Then the Ag-containing melt (1.0 g) was introduced (see fig. l(a)). For a diffusion time of about 2 days exchange experiments were carried out up to temperatures of 528°C. Above 530°C a remarkable deformation of the glass samples was observed. An electrically heated tube furnace was used for heating the samples (see fig. l(b)). The furnace was equipped with a copper drum for maintaining a zone of high constancy of temperature. The temperature ( + 1 K for 50 h) was controlled by an electronic device (type LP 829, Mikki, Budapest).
tube of
~ A~C~Q42AglO'58
Na20.~ Si02
thermo-couple for
meas
silicaglass t [-~ J
/j
~Chndnr~ye
Na
thermo- couple for control
,'t
recorder
o o o
drum(rasothermglass)with sample for ion- exchange
Cu
drurr
o o / Io v
Ao ao ~o /o Ao ao
electronic controller of
temperature I power of
heating
wire of heating Fig. 1. (a) Experimental arrangement for ion-exchange. (b) Furnace device.
146
Ch. Kaps, G. Vi~lksch / lnterdiffusion of Ag + and Na + ions
The glass samples and the Ag-containing melt were put in the pre-heated furnace. The removal of the ion-exchange samples from the furnace was carried out after switching off the furnace and cooling down to 150°C (150-200 K h-~). Because of the small part of the heating and cooling time in comparison with the duration of the diffusion experiment (at most 4%), the time interval from the moment of the constant temperature to the switch-off of the furnace could be used for evaluation. After the A g ÷ / N a + interdiffusion a glass core with the dimensions 4 x 4 x 6 mm 3 adjacent to the exchange boundary was taken out from the glass sample. The Ag concentration profile of these specimens was determined.
2.3. Determination of the concentration gradients For investigation of the ion-exchange process a scanning electron microscope with a wavelength-dispersive X-ray spectrometer (type JSM-U3, JEOL, Japan) was available. The concentration profiles of silver were measured by Ag radiation (U~cc = 25 kV; Ipr = 2 × 10 8 A). The diameter of the analyzed zone was about 4 ~m. In the most unfavorable case, this value only runs up to 2% of the penetration for depth of silver X~o%(Xto~ is the depth for the decrease of the count rate to 10% of the maximum rate measured at the exchange boundary). The glass cores of the samples (4 x 4 x 6 mm 3) were supplied with two plane-parallel faces (normal to the exchange boundary). The surface for X-ray micro-analysis was treated with a water-free medium described in §2.2. Only in this way could a measurable leaching of the alkali content including the entered silver be prevented. After coating with carbon (20 nm) the samples were moved in the micro-probe with a constant velocity of 100 ~m min-~ in the direction of the silver diffusion. The average of at least three Ag concentration profiles parallel in a distance of 0.5 to 1.0 mm was used for the analysis of a sample. Under the measuring conditions a stable Ag signal could be proved, whereas the simultaneously recorded Na signal considerably decreased. The instability of the sodium signal is a consequence of the known evaporation effects of high-alkali glasses in micro-probe investigations [4,5]. The Ag count rate was calibrated by comparison with standard glasses or thin sections of ion-exchanged samples, respectively. Homogeneous samples of glasses of the composition (Na20)333 v(mg20)v(SiO2)667(0
--.
.
•
Ch. Kaps, G. Vi~lksch / Interdiffusion of Ag + and Na + ions
147
3. Results and discussion 3.1. A g concentration profiles a n d interdiffusion coefficients
Figs. 2(a) and 2(b) show the Ag concentration profiles in the glass N a 2 0 2SiO 2 after cation-exchange with the eutectic (AgC1)0.42(AgI)0.58. T h e Ag ÷ / N a ÷ exchange at temperatures between 292 and 497°C shows a m a x i m u m A g 2 0 content of about 0.3 mol% in the surface of the glass. According to the formula (Na20)33.3_y(Ag20)y(SiO2)66.7 this value yields a degree of exchange of 0.9%. At the lowest temperature (271°C) this A g 2 0 concentration could not be reached. For temperatures T > Tg a tendency for a decrease of the A g 2 0
L
T
15 ~,]~-. .,, Z "I\'\\]\ \'\. "".,.. .
[
\
"\
\
"-
0.301
0.15
L \ "\ ""'" "". 4240C 51", \ \',,, "'..382 "'~-, I ",. ". "',337'",,, "~'~. I '. \ 292 "-. "-. "'~. 250
500
750 X
1000
1250
(/,,I m)
"\
Z
15
T>Tg
-..."-"\ "\
466oC
t = 2 days
~'. "N
(~")
'497" >..-.... 10
0.30I
CA O (mole.%)
\
5 . . . . 527 ...........
0.15
"~'.'~... .2~."~.. ""'-721~:22:. I
f
500
I
I
1000
1500
I
2000
i
2500
x(.m) Fig. 2. Silver concentration profiles after ion-exchange with an (AgCl)o.a2(AgI)o.ss melt at T < Ts (a) and T > Tg (b).
148
Ch. Kaps, G. ViJlksch / lnterdiffusion of Ag + and Na + ions
concentration in the glass surface is found with rising temperature. The penetration depth x,0~ range from 200/~m at 271 °C to the remarkably depth of more than 6000/~m at 527°C. In general, the interdiffusion coefficient depends on concentration. However, the shape of the profiles in fig. 2(a) and 2(b) refers to concentration-independent diffusion coefficients. This results corresponds to the low Ag20 content. The model of a constant source allows the calculation of the interdiffusion coefficients. The coefficient DAg/Na was calculated from 20 to 30 values z ( x ) of the Ag concentration profiles (see figs. 2(a), 2(b)) using eq. (1): z=z
o
[
l-erf
2(/)t),/2
1 ,
(1)
where : is the count rate, erf is the error function, x is the depth, L) is the interdiffusion coefficient, t is the diffusion time and x is supposed that z ec CAg. The mean standard deviation of 13% for the /)Ag/Na values shows the usefulness of the procedure both for T < Tg and T > Tg. In order to prove the influence of the composition of the salt melt on the interdiffusion kinetics, exchange experiments with AgCI melts at T > Tg were carried out. The maximum concentration of silver oxide is found to be 5 to 15 times higher than that for the experiments with the AgC1-AgJ melt (see fig. 3). This is a consequence of the increase of Ag ÷ activity of the salt melt after removal of iodide. In spite of the higher degree of exchange (4-14%) these Ag concentration profiles could also be interpreted by means of eq. (1). Meistring et al. [3] also found concentration-independent interdiffusion coefficients /)Ag/Na for exchange experiments with the glass N a 2 0 - 2SiO 2 using AgC1 as a melt, but they did not measure the Ag20 content in the glass. For comparison, the A g + / N a + exchange was carried out for the glass
50 \ , \ Z
t = 2days •
(gl 10(1
\
468°C
\
5.0 l CAg20
(rno, )
"\
\ '\
528 50 . . . .
2.5
~. \
1
1000
I
2000
I
3000
Fig. 3. Silver concentration profiles after ion-exchange with an AgC1 melt at T > Tg.
Ch. Kaps, G. V6lksch / Interdiffusion of .4g + and Na + ions
149
1.0
Z Zo
07
0.5G
\
",
t = 2days \
/.'\
\'\
368°C 'X
0.25
/
'\.\ 307
\ \ \
\. 261'\.
\
,'~.. \.
250
500 -
-
750
1000
1250
x(.m)
Fig. 4. Silver concentration profiles after ion-exchange with an A g N O 3 melt at T < Tg.
N a 2 0 . 2 S i O 2 with AgNO 3 melts at T < Tg, too (see fig. 4). These experiments resulted in maximum AgzO content in a wide range from 7-17 mol% in the glass surface. Therefore a normalized representation is shown in fig. 4. The Ag20 content of the glass surface corresponds to a high degree of exchange (25-50%). Nevertheless, the values reached are small compared with those of Peters et al. [6] and Permjakova et al. [8], whose values were also estimated from experiments with AgNO a melts in different alkali alumina silicate glasses. The experimental arrangement used cannot surely exclude an influence of the increase of the Na + concentration in the salt melt during the ion-exchange on the equilibrium a g + (melt) + Na ÷ (glass) ~ Na + (melt) + Ag + (glass). With regard to this possible influence, the above mentioned tendency for decrease of the maximum Ag20 concentration with rising exchange temperature is also understandable. The high degree of exchange (25-50%) causes a distinct concentration-dependence of the interdiffusion coefficient. The concave curves in fig. 4 refer to higher for the region of higher Ag20 content. Concentration-dependent interdiffusion coefficients has already been observed, when the Ag + / N a + ion-exchange is performed by contact with AgNO 3 melts. Peters et al. [8] found for an especially high degree of exchange (90%) the mixed alkali effect in a Na20-A1203-SiO2 glass. Most papers only varified that the interdiffusion coefficient is higher for the high Ag20 concentration than for the low Ag20 content, however [3,8,9]. The glass N a 2 0 . 2 S i O 2 after the ion-exchange with an AgC1-Agl or an AgC1 melt at T = Tg showed a yellow coloration. Glass samples which were in contact with an AgNO 3 melt have been coloured from yellow to red with
coefficients/~)Ag/Na
Ch. Kaps, G. ViJlksch / lnterdiffusion of Ag + and Na + ions
150
increasing exchange temperature. The highest Ag20 content (---15 mol%) caused cracks in the glass surface at T < T~.
3.2. Activation parameters of the Ag + / N a + interdiffusion In fig. 5 the interdiffusion coefficients of the cation-exchange with the eutectic melt (AgC1)o.42(AgI)0.58 are plotted in an Arrhenius diagram ( l g / ) =
500 450
350
300
ITg
-6
EA= 157KJ.mole "1
\\\ -7
400
"\
LgDAg/Na \0 \
MEISTRING -8
0
et. al. E31
e~.
EA= 77,1KJ.mole-I
-9
I
I
I
i
i
1.3
1.4
1.5
1.6
1.7
i
1.8
• AgCL(~42
Ag 10.58
o AgCL o Ag CL0.42 Ag 10.58 with annealed glass samples
Fig. 5. Arrhenius diagram of the interdiffusioncoefficients/)Ag/Nain the glass Na20-2SiO2.
Ch. Kaps, G. ViJlksch / Interdiffusion of Ag + and Na + ions
f(l/T),
151
corresponding to eq. (2):
13 = / 3 0 e x p ( - E A / R T ) ,
(2)
where b 0 is the pre-exponential factor and E A is the molar activation energy. The curve shows a straight line with constant activation parameters ( E A and /)0) for temperatures T < Tg. On the other hand, a distinct bending of the curve to higher diffusion coefficients is observed in the transition range of the glass. In the temperature range T > Tg the interdiffusion coefficients calculated from the exchange experiments with AgCI melts (D) also satisfy the activation curve. Since structural relaxation processes are possible at temperatures T > Tg, an influence from the change of the glass structure during the diffusion time of two days has to be taken into consideration. For that reason samples of the glass N a 2 0 . 2 S i O 2 were annealed in air above the transition temperature at 500°C for two days and than rapidly cooled (220 K h - l ) . This cooling was 100 times faster than the one of the glass samples used in the common diffusion runs (see §2.1). The annealing was followed by a cation-exchange of these samples at temperatures T < Tg. The interdiffusion coefficients calculated as concentration-independent values from the experiments with the annealed samples ( O ) also correspond to the activation curve. Consequently, the glass N a 2 0 . 2 S i O 2 obviously remains structurally unchanged during the diffusion at T > Tg. However, a relaxed structure and smaller viscosities than the ones for T < Tg are present (for 450 < v~ < 550°C is valid: 13 > lg ~(dPa s) > 8 [10]). In table 1 the activation energies and the pre-exponential factors deduced from fig. 5 are compared with the reference values for the A g + / N a ÷ interdiffusion in glasses of the composition N a 2 0 . 2 S i O 2 and for self-diffusion, respectively. Considering the low degree of A g / N a exchange (1%), the measured interdiffusion coefficient DAg/Na corresponds to the self-diffusion coeffi~
Table 1 Activation energies E A and pre-exponential factors D O of the A g ÷ / N a + interdiffusion and self-diffusion in the glass N a 2 0 . 2 S i O 2 D
T < Tg EA(kJ mol - I )
/)Ag/Na
T > Tg D0(cm 2 s i )
77.1
1.3 X 10 - 2
DNa
79.1 65.7 68.3 -- 95
3.2X 10 -2 2.5X 10 -3 6.7X 10 -3
DAg
100
1.3 × 10- I
EA(kJ m o l - I ) 157 113
-- 95
* 34.1 mol% N a 2 0 ; a small step-like change at Tg in lgD = f ( 1 / T ) . t 15-25 mol% N a 2 0 .
Ref. D0(cm 2 s t ) 1.1 X 104 2.4X 102
[3] [11] [12] [13] [14] * [15] J"
152
Ch. Kaps, G. ViJlksch / lnterdiffusion of Ag + and Na + ions
cient DAg of Ag + tracer-concentrations in the glass N a 2 0 - 2 S i O 2, as can be seen from eq. (3) [3]: ~
DAg DNa
(3)
DAg/Na -- NAgDAg "1- NNaDNa '
where NAg
CAg "1- C N
a '
NAg + NNa = 1.
For NAg -~ 0 ~
O~g/~ = O~g. ~
The activation parameters E A and D O calculated from DAg/N a = DAg = f ( 1 / T ) at T < Tg are comparable with the values for N a self-diffusion in the glass N a 2 0 . 2 S i O 2 (see table 1 [ 11-13]; a reference value for Ag + tracer-diffusion is only available for CNa2O < 33 mol% [15]). Consequently, the activation process of Ag ÷ ions in tracer-concentrations is fit to be compared with that of Na + ions in the glass N a 2 0 . 2 S I O 2 , although the mass and the radius of the Ag + ions are about 4.7 and 1.3, respectively, times those of the Na + ions. Kolitsch et al. [16] checked the dependence of the activation energy for the tracer-diffusion in the glass N a 2 0 . 2 S i O 2 on the radius of the entering alkali ions. According to this relation an activation energy of about 120 kJ mol - I is calculated for silver with rAg+ = 126 pm. The experimental value (77.1 kJ mol-1) is again noticeably lower. Obviously, the effective radius of the more polarizable silver ions is smaller than that of the corresponding alkali ions. In the range T > Tg the activation parameters can be compared with the results of Meistring et al. [3]. Although the activation energy and the pre-exponential factor exceed the corresponding values of Meistring (see table 1 [3]), ~ however, the diffusion coefficients D A g / N a s h o w a sufficient coincidence at T--- ~ (fig. 5). For diffusion processes in glasses at the transition temperature a more or less abrupt increase of the diffusion coefficient is often observed. Obviously, this change of Arrhinius behavior of the diffusion is the result of the generation of free volume or of an additional rise in the equilibrium defect concentration at T > Tg, respectively. According t o / ) = / ) 0 e x p [ - ( E o + E t ) / R T ] , E d is the energy for generation of defects and E t the activation energy E A for transport at T < Tg, corresponding to eq. (2) [2]. The increase of the values for E A and D O deduced from the A g + / N a + interdiffusion is considerable (factors of 2 and 10 6, respectively). The change of the Arrhenius behavior for diffusion at T ~ Tg should depend on the nature of the bonding of the migrating ions in the glass network. Therefore, the distinct change of the activation parameters of the A g + / N a + interdiffusion in comparison with the slight change for the N a + self-diffusion will be understandable (see remarks in table 1 [14]). The difference of electronegativity in an A g - O bond is smaller than that of an
Ch. Kaps, G. V6lksch / Interdiffusion of Ag + and Na + ions
153
N a - O b o n d i n v o l v i n g a h i g h e r c o v a l e n t p o r t i o n for the silver b o n d . C o n s e q u e n t l y , the A g - O b o n d is m o r e s t r o n g l y directed. T h e smallest r e l a x a t i o n processes i n the n e t w o r k yield a d i s t i n c t i n c r e a s e of the i n t e r d i f f u s i o n coefficient/). P e r m j a k o v a a n d M o i s e e v [8] c o n c l u d e d f r o m e x c h a n g e e x p e r i m e n t s o n a n a l u m i n a silicate glass a d i s t r i n c t s t r o n g e r i n t e r a c t i o n of A g ÷ ions t h a n alkali ions with the glass n e t w o r k . Likewise, D o r e m u s [9] d e r i v e d a s m a l l e r t r a n s i t i o n f r e q u e n c y for A g ÷ ions t h a n for N a ÷ ions in a m u l t i - c o m p o n e n t silicate glass. W e wish to express o u r g r a d i t u d e to Prof. Dr. A. F e l t z for m a n y s t i m u l a t i n g discussions, to Dr. M.-L. M a r t i n for the a n a l y s i s of silver a n d to Mr. R. F r i e d e l for s a m p l e processing.
References [1] G.H. Frischat, Ionic Diffusion in Oxide Glasses (Trans Tech Publications, Aedermannsdorf, Switzerland, 1975) p. 47. [2] H.A. Schaeffer, Habilitationsschrift (Erlangen, 1980) p. 22. [3] R. Meistring, G.H. Frischat and H.W. Hennicke, Glastechn. Ber. 49 (1976) 60. [4] G.H. Frischat, U. Eichhorn, R. Kirchmeyer and H. Salge, Glastechn. Ber. 47 (1974) 26. [5] G. V~lksch, H. Reiss and L. Horn, Silikattechnik 2 (1981) 52. [6] E. Peters, J.D. Dietrichs and G.H. Frischat, Glastechn. Ber. 53 (1980) 162. [7] M.-L. Martin, to be published. [8] T.V. Permjakova and V.V. Moiseev, Neorg. Mater. 6 (1970) 78. [9] R.H. Doremus, J. Phys. Chem. 68 (1964) 2212. [10] K. Hunold and R. Briickner, Glastechn. Ber. 53 (1980) 149. [I 1] A. Kolitsch, R. Kiichler and E. Richter, Silikattechnik 29 (1978) 302. [12] G.H. Firschat, Glastechn. Ber. 44 (1969) 146. [13] R. Terai, Phys. Chem. Glasses 10 (1969) 146. [14] J.R. Johnson, R.H. Bristow and H.H. Blau, J. Amer. Ceram. Soc. 34 (1951) 165. [15] V.R. Maltnin, K.K. Evstropev and V.A. Cechomskij, Zur. Priklad. Chem. 45 (1972) 184. [16] A. Kolitsch, R. KiJchler and E. Richter, Silikattechnik 29 (1978) 369.
143
O N T H E I N T E R D I F F U S I O N O F Ag + A N D N a + I O N S I N T H E GLASS Na20.2SiO 2 AROUND THE GLASS TRANSITION TEMPERATURE Ch. K A P S a n d G . V O L K S C H Department of Chemistry, Friedrich Schiller University, Jena, GDR
Received 26 June 1981 Revised manuscript received 18 June 1982
The Ag+/Na ÷ ion-exchange is studied for the glass Na20-2SiO 2 below and above the glass transition temperature using a eutectic melt (AgCl)0.42(AgI)0.5s. The interdiffusion coefficients /)Ag/Na are calculated from the Ag concentration profiles in the Glass measured by X-ray micro-analysis. The evaluation gives concentration-independent values DAg/Na corresponding to a low degree of exchange (1%). The Arrhenius diagram lg D = f ( 1 / T ) shows a distinct bending at the glass transition temperature. The observed change of the activation parameters Do and EA is discussed in relation to the generation of free volume for T > T~ and to chemical bonding of Ag and Na in the glass network. The results are compared with ion-exchange experiments using an AgCI(T > Tg) and AgNO3 melt (T < T~), respectively.
1. Introduction D i f f u s i o n - d e t e r m i n e d exchange processes of cations between glasses a n d salt melts offer a r e m a r k a b l e p o s s i b i l i t y for g e n e r a t i o n of d e f i n e d - i n h o m o g e n e o u s optical materials. In this process investigations are a i m e d at sufficiently large p e n e t r a t i o n d e p t h s in the glass or high diffusion coefficients, respectively. In glasses singly-charged cations show r e m a r k a b l y higher mobilities than m u l t i p l y - c h a r g e d ions. T h e diffusion coefficient of alkali ions in alkali silicate glasses increases with rising t e m p e r a t u r e a n d with increasing c o n t e n t of netw o r k modifiers [ 1]. F u r t h e r extension of the i o n - e x c h a n g e into the glass b u l k is limited on the o n e side b y the glass-forming b o u n d a r y to higher c o n t e n t s of alkali oxide a n d on the other side b y c o n s i d e r a b l e d e f o r m a t i o n s of the glass s a m p l e s d u r i n g the exchange process a b o v e the glass transition temperature. Therefore, the c a t i o n - e x c h a n g e investigated in this p a p e r was carried out on a b i n a r y alkali silicate glass with a high c o n t e n t of alkali oxide at t e m p e r a t u r e s up to the glass t r a n s i t i o n range. T h e glass used of the c o m p o s i t i o n N a 2 0 - 2SiO 2 shows a t r a n s i t i o n t e m p e r a t u r e of 465°C. Because of the possibility of a glassy melt u n d e r g o i n g a transition to its e q u i l i b r i u m state at T > Tg a change of the a c t i v a t i o n p a r a m e t e r s has to be expected in the range T > Tg [2]. Changes of the o p t i c a l p r o p e r t i e s of the m a t e r i a l should be i n d u c e d b y the 0022-3093/82/0000-0000/$02.75
© 1982 N o r t h - H o l l a n d
144
Ch. Kaps, G. Vi~lksch / Interdiffusion of .4g + and Na + ions
substitution of light ions by heavy ones. Melts of silver salts are a rich source of Ag + ions, but the temperature range of their applicability is small. The silver salts show in the molten state a sufficiently high mobility of Ag + ions. The employment of some melts is limited by their decomposition at higher temperatures. Therefore the ion-exchange in the glass N a 2 0 . 2 S i O 2 (Tg = 465°C) was carried out with a low-melting eutectic of the composition (AgC1)042(AgI)058(Fp = 257°C). In that way the diffusion behavior of the glass could be studied over a considerable temperature range (AT = 250 K) with one and the same salt melt. The A g + / N a + interdiffusion has not previously been investigated for the glass N a 2 0 - 2 S i O 2 over such an extended temperature range. Meistring et al. [3] previously carried out the A g + / N a + exchange with AgC1 melts at temperatures T > Tg (AT = 100 K). The surface treatment of glasses with Ag-containing salts or pastes by heating has already been known for some centuries as a staining process. The cation-exchange is followed by a partial reduction of the Ag + ions forming different aggregation states of silver atoms. These particles cause a yellow or red-brown colour by light scattering in the glass. However, the diffusion process is not influenced by the following nucleation and coloration [3], especially when the degree of exchange is low. Obviously, such conditions are maintained in the case of application of AgCl-AgI melts in the interdiffusion experiments, because the Ag + activity of the iodine-containing melt is relatively small.
2. Experimental procedure 2.1. Preparation of the glass and of the Ag-containing melts The glass of the composition N a 2 0 . 2 S i O 2 was melted from a mixture of soda and quartz sand of optical quality in a platinum crucible (65 l) at 1500°C. The glass samples were annealed at 475°C and then cooled very slowly (2.4 K h - ~). Because of the hygroscopic behavior of the glass the samples were kept in a desiccator containing CaC12. Dilatometric measurements of the glass (5 K min -1) give a linear thermal expansion coefficient 0t20/400 = 1.61 × 10 -5 K - l and a transition temperature Tg = 465°C. For the preparation of the Ag-containing melts chemicals of p.a. quality were used. The silver halogenides AgCI and AgI were precipitated from aqueous solution of AgNO 3 by distilled hydrochloric acid or NaI solution, respectively. After desiccation of the silver halides at 130°C the eutectic composition (AgCl)0.42(AgI)058(Fp = 257°C) was mixed and melted down at 500°C in air. The AgC1 melt (Fp = 456°C) was prepared in a corresponding way. For the experiments with the AgNO 3 melts (Fp = 207°C) a markstable p.a. silver nitrate was used.
Ch. Kaps, G. V6lksch / lnterdiffusion of Ag + and Na + ions
145
2.2. Diffusion experiments The glass samples (18 x 18 x 20 mm 3) were shaped from the melt bars and roughly ground. The face chosen for the ion-exchange was ground (ethanol; Ax > 0.03 mm) and polished (glycol, CeO 2) in a water-free medium. Finally, the samples were cleaned with absolute methanol. A tube of silica glass with a polished frontage was put up on the face of the glass samples chosen for the ion-exchange. Then the Ag-containing melt (1.0 g) was introduced (see fig. l(a)). For a diffusion time of about 2 days exchange experiments were carried out up to temperatures of 528°C. Above 530°C a remarkable deformation of the glass samples was observed. An electrically heated tube furnace was used for heating the samples (see fig. l(b)). The furnace was equipped with a copper drum for maintaining a zone of high constancy of temperature. The temperature ( + 1 K for 50 h) was controlled by an electronic device (type LP 829, Mikki, Budapest).
tube of
~ A~C~Q42AglO'58
Na20.~ Si02
thermo-couple for
meas
silicaglass t [-~ J
/j
~Chndnr~ye
Na
thermo- couple for control
,'t
recorder
o o o
drum(rasothermglass)with sample for ion- exchange
Cu
drurr
o o / Io v
Ao ao ~o /o Ao ao
electronic controller of
temperature I power of
heating
wire of heating Fig. 1. (a) Experimental arrangement for ion-exchange. (b) Furnace device.
146
Ch. Kaps, G. Vi~lksch / lnterdiffusion of Ag + and Na + ions
The glass samples and the Ag-containing melt were put in the pre-heated furnace. The removal of the ion-exchange samples from the furnace was carried out after switching off the furnace and cooling down to 150°C (150-200 K h-~). Because of the small part of the heating and cooling time in comparison with the duration of the diffusion experiment (at most 4%), the time interval from the moment of the constant temperature to the switch-off of the furnace could be used for evaluation. After the A g ÷ / N a + interdiffusion a glass core with the dimensions 4 x 4 x 6 mm 3 adjacent to the exchange boundary was taken out from the glass sample. The Ag concentration profile of these specimens was determined.
2.3. Determination of the concentration gradients For investigation of the ion-exchange process a scanning electron microscope with a wavelength-dispersive X-ray spectrometer (type JSM-U3, JEOL, Japan) was available. The concentration profiles of silver were measured by Ag radiation (U~cc = 25 kV; Ipr = 2 × 10 8 A). The diameter of the analyzed zone was about 4 ~m. In the most unfavorable case, this value only runs up to 2% of the penetration for depth of silver X~o%(Xto~ is the depth for the decrease of the count rate to 10% of the maximum rate measured at the exchange boundary). The glass cores of the samples (4 x 4 x 6 mm 3) were supplied with two plane-parallel faces (normal to the exchange boundary). The surface for X-ray micro-analysis was treated with a water-free medium described in §2.2. Only in this way could a measurable leaching of the alkali content including the entered silver be prevented. After coating with carbon (20 nm) the samples were moved in the micro-probe with a constant velocity of 100 ~m min-~ in the direction of the silver diffusion. The average of at least three Ag concentration profiles parallel in a distance of 0.5 to 1.0 mm was used for the analysis of a sample. Under the measuring conditions a stable Ag signal could be proved, whereas the simultaneously recorded Na signal considerably decreased. The instability of the sodium signal is a consequence of the known evaporation effects of high-alkali glasses in micro-probe investigations [4,5]. The Ag count rate was calibrated by comparison with standard glasses or thin sections of ion-exchanged samples, respectively. Homogeneous samples of glasses of the composition (Na20)333 v(mg20)v(SiO2)667(0
--.
.
•
Ch. Kaps, G. Vi~lksch / Interdiffusion of Ag + and Na + ions
147
3. Results and discussion 3.1. A g concentration profiles a n d interdiffusion coefficients
Figs. 2(a) and 2(b) show the Ag concentration profiles in the glass N a 2 0 2SiO 2 after cation-exchange with the eutectic (AgC1)0.42(AgI)0.58. T h e Ag ÷ / N a ÷ exchange at temperatures between 292 and 497°C shows a m a x i m u m A g 2 0 content of about 0.3 mol% in the surface of the glass. According to the formula (Na20)33.3_y(Ag20)y(SiO2)66.7 this value yields a degree of exchange of 0.9%. At the lowest temperature (271°C) this A g 2 0 concentration could not be reached. For temperatures T > Tg a tendency for a decrease of the A g 2 0
L
T
15 ~,]~-. .,, Z "I\'\\]\ \'\. "".,.. .
[
\
"\
\
"-
0.301
0.15
L \ "\ ""'" "". 4240C 51", \ \',,, "'..382 "'~-, I ",. ". "',337'",,, "~'~. I '. \ 292 "-. "-. "'~. 250
500
750 X
1000
1250
(/,,I m)
"\
Z
15
T>Tg
-..."-"\ "\
466oC
t = 2 days
~'. "N
(~")
'497" >..-.... 10
0.30I
CA O (mole.%)
\
5 . . . . 527 ...........
0.15
"~'.'~... .2~."~.. ""'-721~:22:. I
f
500
I
I
1000
1500
I
2000
i
2500
x(.m) Fig. 2. Silver concentration profiles after ion-exchange with an (AgCl)o.a2(AgI)o.ss melt at T < Ts (a) and T > Tg (b).
148
Ch. Kaps, G. ViJlksch / lnterdiffusion of Ag + and Na + ions
concentration in the glass surface is found with rising temperature. The penetration depth x,0~ range from 200/~m at 271 °C to the remarkably depth of more than 6000/~m at 527°C. In general, the interdiffusion coefficient depends on concentration. However, the shape of the profiles in fig. 2(a) and 2(b) refers to concentration-independent diffusion coefficients. This results corresponds to the low Ag20 content. The model of a constant source allows the calculation of the interdiffusion coefficients. The coefficient DAg/Na was calculated from 20 to 30 values z ( x ) of the Ag concentration profiles (see figs. 2(a), 2(b)) using eq. (1): z=z
o
[
l-erf
2(/)t),/2
1 ,
(1)
where : is the count rate, erf is the error function, x is the depth, L) is the interdiffusion coefficient, t is the diffusion time and x is supposed that z ec CAg. The mean standard deviation of 13% for the /)Ag/Na values shows the usefulness of the procedure both for T < Tg and T > Tg. In order to prove the influence of the composition of the salt melt on the interdiffusion kinetics, exchange experiments with AgCI melts at T > Tg were carried out. The maximum concentration of silver oxide is found to be 5 to 15 times higher than that for the experiments with the AgC1-AgJ melt (see fig. 3). This is a consequence of the increase of Ag ÷ activity of the salt melt after removal of iodide. In spite of the higher degree of exchange (4-14%) these Ag concentration profiles could also be interpreted by means of eq. (1). Meistring et al. [3] also found concentration-independent interdiffusion coefficients /)Ag/Na for exchange experiments with the glass N a 2 0 - 2SiO 2 using AgC1 as a melt, but they did not measure the Ag20 content in the glass. For comparison, the A g + / N a + exchange was carried out for the glass
50 \ , \ Z
t = 2days •
(gl 10(1
\
468°C
\
5.0 l CAg20
(rno, )
"\
\ '\
528 50 . . . .
2.5
~. \
1
1000
I
2000
I
3000
Fig. 3. Silver concentration profiles after ion-exchange with an AgC1 melt at T > Tg.
Ch. Kaps, G. V6lksch / Interdiffusion of .4g + and Na + ions
149
1.0
Z Zo
07
0.5G
\
",
t = 2days \
/.'\
\'\
368°C 'X
0.25
/
'\.\ 307
\ \ \
\. 261'\.
\
,'~.. \.
250
500 -
-
750
1000
1250
x(.m)
Fig. 4. Silver concentration profiles after ion-exchange with an A g N O 3 melt at T < Tg.
N a 2 0 . 2 S i O 2 with AgNO 3 melts at T < Tg, too (see fig. 4). These experiments resulted in maximum AgzO content in a wide range from 7-17 mol% in the glass surface. Therefore a normalized representation is shown in fig. 4. The Ag20 content of the glass surface corresponds to a high degree of exchange (25-50%). Nevertheless, the values reached are small compared with those of Peters et al. [6] and Permjakova et al. [8], whose values were also estimated from experiments with AgNO a melts in different alkali alumina silicate glasses. The experimental arrangement used cannot surely exclude an influence of the increase of the Na + concentration in the salt melt during the ion-exchange on the equilibrium a g + (melt) + Na ÷ (glass) ~ Na + (melt) + Ag + (glass). With regard to this possible influence, the above mentioned tendency for decrease of the maximum Ag20 concentration with rising exchange temperature is also understandable. The high degree of exchange (25-50%) causes a distinct concentration-dependence of the interdiffusion coefficient. The concave curves in fig. 4 refer to higher for the region of higher Ag20 content. Concentration-dependent interdiffusion coefficients has already been observed, when the Ag + / N a + ion-exchange is performed by contact with AgNO 3 melts. Peters et al. [8] found for an especially high degree of exchange (90%) the mixed alkali effect in a Na20-A1203-SiO2 glass. Most papers only varified that the interdiffusion coefficient is higher for the high Ag20 concentration than for the low Ag20 content, however [3,8,9]. The glass N a 2 0 . 2 S i O 2 after the ion-exchange with an AgC1-Agl or an AgC1 melt at T = Tg showed a yellow coloration. Glass samples which were in contact with an AgNO 3 melt have been coloured from yellow to red with
coefficients/~)Ag/Na
Ch. Kaps, G. ViJlksch / lnterdiffusion of Ag + and Na + ions
150
increasing exchange temperature. The highest Ag20 content (---15 mol%) caused cracks in the glass surface at T < T~.
3.2. Activation parameters of the Ag + / N a + interdiffusion In fig. 5 the interdiffusion coefficients of the cation-exchange with the eutectic melt (AgC1)o.42(AgI)0.58 are plotted in an Arrhenius diagram ( l g / ) =
500 450
350
300
ITg
-6
EA= 157KJ.mole "1
\\\ -7
400
"\
LgDAg/Na \0 \
MEISTRING -8
0
et. al. E31
e~.
EA= 77,1KJ.mole-I
-9
I
I
I
i
i
1.3
1.4
1.5
1.6
1.7
i
1.8
• AgCL(~42
Ag 10.58
o AgCL o Ag CL0.42 Ag 10.58 with annealed glass samples
Fig. 5. Arrhenius diagram of the interdiffusioncoefficients/)Ag/Nain the glass Na20-2SiO2.
Ch. Kaps, G. ViJlksch / Interdiffusion of Ag + and Na + ions
f(l/T),
151
corresponding to eq. (2):
13 = / 3 0 e x p ( - E A / R T ) ,
(2)
where b 0 is the pre-exponential factor and E A is the molar activation energy. The curve shows a straight line with constant activation parameters ( E A and /)0) for temperatures T < Tg. On the other hand, a distinct bending of the curve to higher diffusion coefficients is observed in the transition range of the glass. In the temperature range T > Tg the interdiffusion coefficients calculated from the exchange experiments with AgCI melts (D) also satisfy the activation curve. Since structural relaxation processes are possible at temperatures T > Tg, an influence from the change of the glass structure during the diffusion time of two days has to be taken into consideration. For that reason samples of the glass N a 2 0 . 2 S i O 2 were annealed in air above the transition temperature at 500°C for two days and than rapidly cooled (220 K h - l ) . This cooling was 100 times faster than the one of the glass samples used in the common diffusion runs (see §2.1). The annealing was followed by a cation-exchange of these samples at temperatures T < Tg. The interdiffusion coefficients calculated as concentration-independent values from the experiments with the annealed samples ( O ) also correspond to the activation curve. Consequently, the glass N a 2 0 . 2 S i O 2 obviously remains structurally unchanged during the diffusion at T > Tg. However, a relaxed structure and smaller viscosities than the ones for T < Tg are present (for 450 < v~ < 550°C is valid: 13 > lg ~(dPa s) > 8 [10]). In table 1 the activation energies and the pre-exponential factors deduced from fig. 5 are compared with the reference values for the A g + / N a ÷ interdiffusion in glasses of the composition N a 2 0 . 2 S i O 2 and for self-diffusion, respectively. Considering the low degree of A g / N a exchange (1%), the measured interdiffusion coefficient DAg/Na corresponds to the self-diffusion coeffi~
Table 1 Activation energies E A and pre-exponential factors D O of the A g ÷ / N a + interdiffusion and self-diffusion in the glass N a 2 0 . 2 S i O 2 D
T < Tg EA(kJ mol - I )
/)Ag/Na
T > Tg D0(cm 2 s i )
77.1
1.3 X 10 - 2
DNa
79.1 65.7 68.3 -- 95
3.2X 10 -2 2.5X 10 -3 6.7X 10 -3
DAg
100
1.3 × 10- I
EA(kJ m o l - I ) 157 113
-- 95
* 34.1 mol% N a 2 0 ; a small step-like change at Tg in lgD = f ( 1 / T ) . t 15-25 mol% N a 2 0 .
Ref. D0(cm 2 s t ) 1.1 X 104 2.4X 102
[3] [11] [12] [13] [14] * [15] J"
152
Ch. Kaps, G. ViJlksch / lnterdiffusion of Ag + and Na + ions
cient DAg of Ag + tracer-concentrations in the glass N a 2 0 - 2 S i O 2, as can be seen from eq. (3) [3]: ~
DAg DNa
(3)
DAg/Na -- NAgDAg "1- NNaDNa '
where NAg
CAg "1- C N
a '
NAg + NNa = 1.
For NAg -~ 0 ~
O~g/~ = O~g. ~
The activation parameters E A and D O calculated from DAg/N a = DAg = f ( 1 / T ) at T < Tg are comparable with the values for N a self-diffusion in the glass N a 2 0 . 2 S i O 2 (see table 1 [ 11-13]; a reference value for Ag + tracer-diffusion is only available for CNa2O < 33 mol% [15]). Consequently, the activation process of Ag ÷ ions in tracer-concentrations is fit to be compared with that of Na + ions in the glass N a 2 0 . 2 S I O 2 , although the mass and the radius of the Ag + ions are about 4.7 and 1.3, respectively, times those of the Na + ions. Kolitsch et al. [16] checked the dependence of the activation energy for the tracer-diffusion in the glass N a 2 0 . 2 S i O 2 on the radius of the entering alkali ions. According to this relation an activation energy of about 120 kJ mol - I is calculated for silver with rAg+ = 126 pm. The experimental value (77.1 kJ mol-1) is again noticeably lower. Obviously, the effective radius of the more polarizable silver ions is smaller than that of the corresponding alkali ions. In the range T > Tg the activation parameters can be compared with the results of Meistring et al. [3]. Although the activation energy and the pre-exponential factor exceed the corresponding values of Meistring (see table 1 [3]), ~ however, the diffusion coefficients D A g / N a s h o w a sufficient coincidence at T--- ~ (fig. 5). For diffusion processes in glasses at the transition temperature a more or less abrupt increase of the diffusion coefficient is often observed. Obviously, this change of Arrhinius behavior of the diffusion is the result of the generation of free volume or of an additional rise in the equilibrium defect concentration at T > Tg, respectively. According t o / ) = / ) 0 e x p [ - ( E o + E t ) / R T ] , E d is the energy for generation of defects and E t the activation energy E A for transport at T < Tg, corresponding to eq. (2) [2]. The increase of the values for E A and D O deduced from the A g + / N a + interdiffusion is considerable (factors of 2 and 10 6, respectively). The change of the Arrhenius behavior for diffusion at T ~ Tg should depend on the nature of the bonding of the migrating ions in the glass network. Therefore, the distinct change of the activation parameters of the A g + / N a + interdiffusion in comparison with the slight change for the N a + self-diffusion will be understandable (see remarks in table 1 [14]). The difference of electronegativity in an A g - O bond is smaller than that of an
Ch. Kaps, G. V6lksch / Interdiffusion of Ag + and Na + ions
153
N a - O b o n d i n v o l v i n g a h i g h e r c o v a l e n t p o r t i o n for the silver b o n d . C o n s e q u e n t l y , the A g - O b o n d is m o r e s t r o n g l y directed. T h e smallest r e l a x a t i o n processes i n the n e t w o r k yield a d i s t i n c t i n c r e a s e of the i n t e r d i f f u s i o n coefficient/). P e r m j a k o v a a n d M o i s e e v [8] c o n c l u d e d f r o m e x c h a n g e e x p e r i m e n t s o n a n a l u m i n a silicate glass a d i s t r i n c t s t r o n g e r i n t e r a c t i o n of A g ÷ ions t h a n alkali ions with the glass n e t w o r k . Likewise, D o r e m u s [9] d e r i v e d a s m a l l e r t r a n s i t i o n f r e q u e n c y for A g ÷ ions t h a n for N a ÷ ions in a m u l t i - c o m p o n e n t silicate glass. W e wish to express o u r g r a d i t u d e to Prof. Dr. A. F e l t z for m a n y s t i m u l a t i n g discussions, to Dr. M.-L. M a r t i n for the a n a l y s i s of silver a n d to Mr. R. F r i e d e l for s a m p l e processing.
References [1] G.H. Frischat, Ionic Diffusion in Oxide Glasses (Trans Tech Publications, Aedermannsdorf, Switzerland, 1975) p. 47. [2] H.A. Schaeffer, Habilitationsschrift (Erlangen, 1980) p. 22. [3] R. Meistring, G.H. Frischat and H.W. Hennicke, Glastechn. Ber. 49 (1976) 60. [4] G.H. Frischat, U. Eichhorn, R. Kirchmeyer and H. Salge, Glastechn. Ber. 47 (1974) 26. [5] G. V~lksch, H. Reiss and L. Horn, Silikattechnik 2 (1981) 52. [6] E. Peters, J.D. Dietrichs and G.H. Frischat, Glastechn. Ber. 53 (1980) 162. [7] M.-L. Martin, to be published. [8] T.V. Permjakova and V.V. Moiseev, Neorg. Mater. 6 (1970) 78. [9] R.H. Doremus, J. Phys. Chem. 68 (1964) 2212. [10] K. Hunold and R. Briickner, Glastechn. Ber. 53 (1980) 149. [I 1] A. Kolitsch, R. Kiichler and E. Richter, Silikattechnik 29 (1978) 302. [12] G.H. Firschat, Glastechn. Ber. 44 (1969) 146. [13] R. Terai, Phys. Chem. Glasses 10 (1969) 146. [14] J.R. Johnson, R.H. Bristow and H.H. Blau, J. Amer. Ceram. Soc. 34 (1951) 165. [15] V.R. Maltnin, K.K. Evstropev and V.A. Cechomskij, Zur. Priklad. Chem. 45 (1972) 184. [16] A. Kolitsch, R. KiJchler and E. Richter, Silikattechnik 29 (1978) 369.