- Email: [email protected]
Solubility of Ag2O into the Na2O–B2O3–Al2O3 system
Journal of Non-Crystalline Solids 210 Ž1997. 141–147
Solubility of Ag 2 O into the Na 2 O–B 2 O 3 –Al 2 O 3 system Takashi Wakasugi ) , Akio Hirota, Jiro Fukunaga, Rikuo Ota Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan Received 13 February 1996; revised 13 August 1996
Abstract The solubility of Ag 2 O was measured for the Na 2 O–B 2 O 3 and Na 2 O–B 2 O 3 –Al 2 O 3 system with the rotating crucible method and static method, respectively, under air atmosphere at temperatures ranging from 1273 to 1423 K. The contamination of melts from crucibles could be avoided by the rotating crucible method, with which it became possible to measure the solubility of Ag 2 O for the Na 2 O–B 2 O 3 system above the melting point of Ag for the first time. It was found that the addition of Na 2 O decreases the solubility of Ag 2 O while the addition of Al 2 O 3 had little effect on the solubility. The effect of Na 2 O and Al 2 O 3 on the solubility of Ag 2 O is expressed by interaction coefficients and is analyzed in terms of the basicity of melts. The solubility of Ag 2 O in Na 2 O–B 2 O 3 –Al 2 O 3 melts increased with increased temperature. This phenomena was explained by a small enthalpy change in oxidation of silver.
1. Introduction Since Ag 2 O in glasses shows the mixed cation effect in alkali containing systems, many investigators have been interested in the phenomena of mixed cation effects w1–7x. Silver halide containing glasses have attracted attention due to its photochromic property and high ionic conductivity w8–12x. Recently, glasses containing silver have been used as anti-bacteria agents because the Ag-ion prevents the increase of bacteria. Silver must exist in glasses as Agq-ion in order to exhibit the properties mentioned above. However, the form of Ag added to a glass batch, for example AgNO 3 , is easily reduced during the glass melting process because silver is a noble metal, so that the solubility of silver in glass melts is limited by thermodynamic factors; partial pressure of oxygen, temperature and glass composition. A )
Corresponding author. Tel.: q81-75 724 7575; fax: q81-75 724 7580; e-mail: [email protected].
method to produce glasses containing large concentrations of silver is required. Since it is difficult to change the oxygen partial pressure and temperature in the glass melting process, the glass composition has a large range. The ability of a glass melt to dissolve Ag 2 O or the solubility of Ag 2 O is expressed in term of the activity coefficient of Ag 2 O in the glass melt. It is known that the activity coefficient varies with composition. Therefore, there is a possibility to find a composition favorable for a glass with high silver content. It is important to understand the effect of each glass component on the solubility of Ag 2 O for this purpose. In order to measure the solubility of Ag 2 O, the crucible composition in which a glass melts affects solubility. Although metallic crucibles Že.g., platinum. reduce contamination of melts by crucibles, its use is almost impossible because of its oxidation or alloy formation with silver which is equilibrated with glass melts. An exception is the use of a silver
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 5 9 0 - X
142
T. Wakasugi et al.r Journal of Non-Crystalline Solids 210 (1997) 141–147
crucible. Willis and Hennessy w13x and Maekawa et al. w14x measured the solubility of Ag 2 O in B 2 O 3 and Ag 2 O–B 2 O 3 melts with silver crucible. However, the use of silver crucibles limits the experimental temperature to temperatures less than the melting point of silver. On the other hand, if refractory crucibles, for example Al 2 O 3 and so on, were used, contamination could not be avoided. In order to overcome the above difficulties, ‘the rotating crucible method’ was developed. With this method, it became possible to avoid crucible contamination. In this paper, the solubility of Ag 2 O measured for the Na 2 O–B 2 O 3 and Na 2 O–B 2 O 3 –Al 2 O 3 systems by the rotating crucible method and static method, respectively, is reported. Its temperature dependence and the effect of Na 2 O and Al 2 O 3 addition on the solubility of silver were evaluated. 2. Experimental Glass batches of given compositions were prepared from reagent grade H 3 BO 3 and Na 2 CO 3 , melted at 1473 K for 20 min in platinum crucible and quenched to glasses. The glass composition was in the range 0.600 F X B 2 O 3 F 0.743 in the Na 2 O– B 2 O 3 system, and 0.454 F X B 2 O 3 F 0.564 and 0 F X Na 2 O F 0.236 in the Na 2 O–B 2 O 3 –Al 2 O 3 system. These glasses were equilibrated with silver in mullite or alumina crucible between 1273 and 1423 K under air atmosphere. Since the crucible is attacked by the glass liquid during equilibration, the rotating crucible method was developed as shown in Fig. 1 in order to eliminate the contamination from the crucibles. A mullite crucible with about 50 g of Ag can be rotated at 500 rpm by a motor under the furnace. A hole was formed at the center of the silver melt by centrifugal force, and about 2 g of glass was put in it. In this way we could obtain glass melts with no contamination. Usually it was difficult to avoid the contamination from crucibles in the rotating crucible method. The solubility of Ag 2 O in the Na 2 O–B 2 O 3 –Al 2 O 3 system was also measured with the static method. In these experiments, alumina crucibles were used without rotation, so that the melt was saturated with Al 2 O 3 . Five grams of Ag and 2.5 g of glass, which is close to Al 2 O 3 saturation, were put in the crucible. The surface level of silver was higher than that of
Fig. 1. Schematic diagram of experimental apparatus.
the glass liquid although the surface of silver was covered by a thin layer of glass liquid. This configuration accelerated the oxidation of silver. If the top of silver was covered with a thick glass layer, a longer time to reach equilibrium was necessary. The initial glass contained no Ag 2 O. Addition of AgNO 3 to the glass liquid at the beginning of the experiment was tried in order to shorten the equilibration time, but little effect was observed. The experimental time to reach equilibrium was 24 h by the rotating crucible method and 48 h by the static method. After equilibration, glass samples for chemical analysis were collected by sticking a part of glass liquid on an iron rod. The content of Ag 2 O, B 2 O 3 , Na 2 O, and SiO 2 was analyzed by the Volhard method w15x, the titration method w16x, the flame spectrochemical method JIS, and the gravimetric method w16x, respectively.
3. Theoretical consideration The dissolution of silver into a glass liquid proceeds by an oxidation reaction as given by Eq. Ž1.. Generally a stable phase at the experimental temperature is chosen as a reference state. However, Ag 2 O completely decomposes at 573–613 K w17x and is not stable at the experimental temperatures. Therefore, solid Ag 2 O was chosen as a reference state of Ag 2 O
T. Wakasugi et al.r Journal of Non-Crystalline Solids 210 (1997) 141–147 Table 1 Solubility and activity coefficient of Ag 2 O in the Na 2 O–B 2 O 3 system in air No.
Temp. ŽK.
XAg 2 O Ž"0.0001.
XB 2O 3 Ž"0.001.
X Na 2 O Ž"0.001.
ln gAg 2 O Ž"0.01.
1 2 3 4
1323 1323 1373 1423
0.0191 0.0345 0.0368 0.0488
0.600 0.743 0.706 0.730
0.381 0.223 0.220 0.221
y2.15 y2.74 y3.02 y3.42
and the reaction of Eq. Ž2. is considered here, where the reference state is shown in parenthesis. The equilibrium constant of Eq. Ž2., K 2 , calculated from its standard free energy change, DG 20 , is expressed as Eq. Ž4., where a i is the activity, gAg 2 O the activity coefficient, XAg 2 O mole fraction and PO 2 the partial pressure of oxygen w18x 2Ag Ž l . q 12 O 2 Ž g . s Ag 2 O Ž in melt . ,
Ž 1.
2Ag Ž l . q 12 O 2 Ž g . s Ag 2 O Ž s . ,
Ž 2.
DG 20 s y53 100 q 84.1T
Ž 3.
ž
K 2 s exp y
DG 20 RT
/
s
Ž Jrmol. ,
aAg 2 O 2 aAg
PO1r2 2
s
gAg 2 O XAg 2 O 2 aAg
PO1r2 2
143
but the activity coefficient of Ag 2 O is unique for a melt independent of PO 2 . Then it is convenient to express the thermodynamic behavior of Ag 2 O in glass melts in terms of gAg 2 O , which is a measure of the rate of dissolution of Ag 2 O. Since aAg s 1 and PO 2 s 0.21 atm when the glass melt is equilibrated with metallic silver under air atmosphere, the activity of Ag 2 O can be calculated at any temperature and the activity coefficient, gAg 2 O can be obtained. An activity coefficient in a multi component system is usually expressed by the following equation as a first order approximation w19x: ln g i s ln g i0 Ž in solvent. q Ý
j i
Xj ,
Ž 5.
j
where g i0 is the activity coefficient of the ith component in the infinite dilute solution, and ij is the first order Gibbs energy interaction coefficient. The effect of glass component on the solubility of Ag 2 O j is expressed in terms of Ag in this paper. 2O
4. Results
.
Ž 4. The solubility of Ag 2 O, XAg 2 O , in a melt varies with the oxygen partial pressure of atmosphere, i.e., PO 2 ,
The results of Ag 2 O solubility in the Na 2 O–B 2 O 3 system obtained by the rotating method is shown in Table 1 together with the activity coefficients of Ag 2 O calculated from Eq. Ž4.. Since it was difficult
Table 2 Solubility and activity coefficient of Ag 2 O in the Na 2 O–B 2 O 3 –Al 2 O 3 system in air No.
Temp. ŽK.
XAg 2 O Ž"0.001.
X B 2 O 3 Ž"0.001.
X Na 2 O Ž"0.001.
XAl 2 O 3 Ž"0.001.
ln gAg 2 O Ž"0.01.
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
1273 1273 1273 1273 1323 1323 1323 1323 1373 1373 1373 1373 1423 1423 1423 1423
0.035 0.091 0.159 0.229 0.039 0.091 0.150 0.229 0.045 0.098 0.165 0.239 0.059 0.101 0.165 0.232
0.475 0.518 0.539 0.506 0.488 0.526 0.564 0.527 0.478 0.506 0.496 0.478 0.434 0.514 0.454 0.505
0.236 0.156 0.069 0.000 0.232 0.138 0.061 0.000 0.223 0.148 0.076 0.000 0.198 0.140 0.079 0.000
0.254 0.234 0.233 0.265 0.241 0.245 0.226 0.244 0.254 0.247 0.263 0.284 0.309 0.245 0.302 0.262
y2.56 y3.52 y4.08 y4.44 y2.86 y3.71 y4.21 y4.63 y3.18 y3.96 y4.48 y4.85 y3.62 y4.15 y4.64 y4.98
144
T. Wakasugi et al.r Journal of Non-Crystalline Solids 210 (1997) 141–147
solubility of Al 2 O 3 because Al 2 O 3 crucibles were used. The solubility of Al 2 O 3 does not change greatly with composition and temperature studied here. Therefore, the variation of B 2 O 3 content is small.
5. Discussion 5.1. EÕaluation of interaction parameters
Fig. 2. Solubility of Ag 2 O into the Na 2 O–B 2 O 3 –Al 2 O 3 system saturated with Al 2 O 3 in air at 1273 K. The line is drawn as a guide for the eye.
to avoid contamination from crucibles, we could get only four samples even by using the rotating crucible method. Other samples contained considerable amounts of Al 2 O 3 and SiO 2 which came from the mullite crucible. Although sample No. 3 contained about 4 wt% ŽAl 2 O 3 q SiO 2 . by contamination, the effect of this contamination was neglected in the following discussion. The Ag 2 O solubility in the Na 2 O–B 2 O 3 –Al 2 O 3 system obtained by the static method is shown in Table 2 and that at 1273 K is illustrated in Fig. 2. The activity coefficients are also shown in Table 2. The solubility of Ag 2 O decreases with increase of Na 2 O content and with decrease of temperature. The dependence of the solubility of Ag 2 O on Na 2 O content and temperature is similar to that in the Na 2 O–B 2 O 3 systems ŽTable 1, Nos. 1, 2.. Maekawa et al. w14x reported the solubility of Ag 2 O in Na 2 O–B 2 O 3 melts under various oxygen partial pressure at 1123–1208 K. According to them, the interpolated solubility of Ag 2 O in B 2 O 3 melt under air atmosphere is about XAg 2 O s 0.26 in their temperature range. The solubility of Ag 2 O into B 2 O 3 –Al 2 O 3 melts obtained in this work is smaller than that. Taking the difference of temperature and composition into consideration, our value is reasonable. The content of Al 2 O 3 in Table 2 means the
Since the Ag 2 O solubilities are in a range of 0.019 F XAg 2 O F 0.239 and these are relatively large, Ag 2 O the effect of the self interaction coefficient, Ag , 2O can not be neglected in the evaluation of interaction parameters. However, it is impossible to evaluate Ag 2 O Ag 2 O from this experiment because aAg 2 O is fixed in this experiment and the dependence of gAg 2 O on Ag 2 O content cannot be obtained. Then, the activity 0 coefficient of Ag 2 O at infinite dilution, gAg was 2O evaluated from Eq. Ž6. which is derived by assuming a regular solution model w19x: 0 ln gAg s 2O
ln gAg 2 O
Ž 1 y XAg O .
2
.
Ž 6.
2
0 The variation of gAg in the Na 2 O–B 2 O 3 –Al 2 O 3 2O system is shown in Fig. 3 as a function of Na 2 O 0 content. gAg increases with increase of Na 2 O con2O
Fig. 3. Variation of activity coefficient of Ag 2 O at infinite dilution with Na 2 O content in the Na 2 O–B 2 O 3 –Al 2 O 3 system saturated with Al 2 O 3 . Lines are drawn by least squares method as a guide for the eye.
T. Wakasugi et al.r Journal of Non-Crystalline Solids 210 (1997) 141–147
tent almost linearly at each temperature, and we expect that it can be expressed by a linear function Na 2 O of the first order with interaction parameters Ag 2O Al 2 O 3 and Ag 2 O . In order to evaluate the two interaction 0 Žin B 2 O 3 . which is the activity parameters and gAg 2O coefficient of Ag 2 O at infinite dilution in pure B 2 O 3 , a least squares method was used according to the Ag 2 O following equation, which does not contain a Ag 2O 0 term because gAg 2 O is the activity coefficient at XAg 2 O ™ 0: 0 0 ln gAg s ln gAg Ž in B 2 O 3 . q 2O 2O
q
Na 2 O Ag 2 O X Na 2 O
Al 2 O 3 Ag 2 O XAl 2 O 3 .
Ž 7.
Data from solubility obtained by the rotating crucible method and from static conditions were used for the above calculation. The result is shown in Table 3. Since the data of solubility of the rotating crucible Al 2 O 3 was method at 1273 K are not available, Ag 2O assumed to be zero in this case to obtain the other parameters. This assumption would not cause a large Al 2 O 3 error because Ag at other temperatures were 2O close to zero. Using the data reported by Maekawa et al. w14x, the solubility of Ag 2 O in Na 2 O–B 2 O 3 melts under 0 air condition was interpolated and gAg was calcu2O lated in the same way, which is shown in Fig. 3. The 0 gAg increases almost linearly with the increase of 2O Na 2 O Na 2 O content and the Ag calculated from the 2O data of X Na 2 O - 0.2 was 27.7 " 2.2. This value is close to that obtained in present work and we considered it reasonable. The interaction parameter shows the degree of interaction of two components. The interaction parameter between two components is positive if they are repulsive. The magnitude of interaction has a close relationship with the basicity, so that the inter-
action parameter between basic oxides should be positive and that between basic oxide and an acidic one should be negative. From the positive value of Na 2 O Ag 2 O , Ag 2 O was found to behave as a basic oxide. Electronegativity is often used as a measure of basicity. According to Allred and Rochow w20x, the electronegativity of Ag is 1.42, which is less than that of Al Ž1.47.. Since Al 2 O 3 is known as amphoteric oxide, we consider Ag 2 O to be a weak base, less basic than Al 2 O 3 . The Pauling scale of electronegativity is based on bonding energy, so the electronegativity of a noble metal becomes unusually high. Thus it is not adequate to use the Pauling scale as a measure of basicity for noble metal oxide. 5.2. Temperature dependence of Ag 2 O solubility Since the oxidation reaction is generally exothermic, low temperature is thermodynamically favorable for an oxidation reaction to occur. So the solubility of Ag 2 O at low temperatures is expected to be greater than that at high temperatures. In fact, it was reported that the solubility of Ag 2 O in B 2 O 3 melts increases with decrease of temperature w14x. However, it has been shown that the solubility of Ag 2 O in this work increases with increasing temperature. This may be explained by the small value of enthalpy change in the oxidation of silver ŽEq. Ž2.. and low basicity of Ag 2 O. The decrease of Ag 2 O solubility with temperature decrease corresponds to a positive value of D H1 , which is the enthalpy change of Eq. Ž1.. This value can be obtained from the Arrhenius plot of temperature dependence of the Ag 2 O solubility ŽFig. 4. with Eq. Ž8.: y
Table 3 0 Na 2 O Al 2 O 3 Žin B 2 O 3 ., Ag ln gAg , and Ag in the Na 2 O–B 2 O 3 – 2O 2O 2O Al 2 O 3 satd melts calculated by the least squares method Temp. ŽK.
0 ln gAg 2O Žin B 2 O 3 .
1273 1323 1373 1423
y7.3"0.2 y7.1"0.8 y8.0"0.2 y8.6"0.1
Na 2 O Ag 2 O
19.6"1.0 15.1"2.6 21.7"0.8 22.1"0.7
Al 2 O 3 Ag 2 O
– 0.8"2.8 y0.9"0.6 0.9"0.5
145
D H1 R
s
E ln XAg 2 O E Ž 1rT .
.
Ž 8.
Obtained values of D H1 are shown in Table 4. Eq. Ž1. consists of Eqs. Ž2., Ž9. and Ž10. and D H1 is equal to D H2 q D H9 q D H10 , where D H10 is equal 0 to the relative partial enthalpy of Ag 2 O, D HAg : 2O Ag 2 O Ž s . s Ag 2 O Ž l . , Ag 2 O Ž l . s Ag 2 O Ž in melt . .
Ž 9. Ž 10 .
146
T. Wakasugi et al.r Journal of Non-Crystalline Solids 210 (1997) 141–147
of temperature. For the Na 2 O–B 2 O 3 system, the relative partial molar enthalpy of Na 2 O does not vary significantly at low Na 2 O content and becomes less negative with the increase of Na 2 O content when X Na 2 O ) 0.05 w21x. The compositional depen0 0 dence of D H Na is consistent with that of D HAg . 2O 2O
6. Conclusion
Fig. 4. Temperature dependence of Ag 2 O solubility in the NA 2 O–B 2 O 3 –Al 2 O 3 system saturated with Al 2 O 3 . Lines are drawn by least squares method as a guide for the eye.
D H2 is y53.1 " 1.0 kJrmol according to Eq. Ž3.. Its magnitude is small compared with other metals. In case of copper which belongs to the same group as silver, the corresponding value is y168 kJrmol. Although the value of D H9 is unknown due to the decomposition of Ag 2 O at about 573 K, D H9 should be positive. The crystal structures of Cu 2 O and Ag 2 O are the same and D H9 would not be much different from the enthalpy change in melting of Cu 2 O Ž57.2 kJrmol.. Then, we assumed D H10 to be close to zero or slightly positive. The magnitude of D H10 depends on the basicity of Ag 2 O and glass melts. As noted in the previous section, Ag 2 O is a weak base. So, the positive value of D H10 is probable when Na 2 O content is relatively high. From the above discussion, it is clear that the variation of D H1 is the same as that of D H10 , which gradually becomes larger with the increase of Na 2 O content. Then, it is not unreasonable that the solubility of Ag 2 O into B 2 O 3 melt increases with decrease Table 4 Enthalpy changes, D H1 , of the dissolution of Ag 2 O into Na 2 O– B 2 O 3 –Al 2 O 3 satd melts X Na 2 O
D H1 ŽkJrmol.
0 0.069–0.078 0.138–0.156 0.223–0.236
2.5"2.8 6.1"5.5 11.6"3.0 36.4"3.7
The solubility of Ag 2 O into Na 2 O–B 2 O 3 –Al 2 O 3 melts increased with increase of temperature, which is opposite to the dependence of other elements. This phenomenon was explained by the small enthalpy change in oxidation of silver and weak basicity of Ag 2 O. The addition of Na 2 O to melts decreased the solubility of Ag 2 O, but the effect of Al 2 O 3 addition was quite small. The activity coefficient of Ag 2 O in pure B 2 O 3 melt was Ž1.8 " 0.2. = 10y4 , and interacNa 2 O Al 2 O 3 tion parameters, Ag and Ag , were 22.1 " 0.7 2O 2O and 0.9 " 0.5 at 1423 K, respectively.
References w1x E.F. Riebling, J. Am. Ceram. Soc. 56 Ž1973. 25. w2x H.M.J.M. van Ass and J.M. Stevels, J. Non-Cryst. Solids 15 Ž1974. 215. w3x W.J. Th. van Gemert, H.M.J.M. van Ass and J.M. Stevels, J. Non-Cryst. Solids 16 Ž1974. 281. w4x S. Sakka, K. Kamiya and B. Ozawa, J. Am. Ceram. Soc. 60 Ž1977. 285. w5x S. Sakka, K. Matusita and K. Kamiya, Phys. Chem. Glasses 20 Ž1979. 25. w6x C. Spurcaciu and T.P. Negreanu, J. Non-Cryst. Solids 84 Ž1986. 105. w7x J. Ruller and J.E. Shelby, Phys. Chem. Glasses 29 Ž1988. 209. w8x S.W. Martin, J. Am. Ceram. Soc. 74 Ž1991. 1767. w9x S. Rossignol, J.M. Reau, B. Tanguy, J.J. Videau and J. ´ Portier, J. Non-Cryst. Solids 155 Ž1993. 77. w10x T. Minami, H. Nambu and M. Tanaka, J. Am. Ceram. Soc. 60 Ž1977. 283. w11x T. Kawamoto, R. Kikuchi and Y. Kimura, Phys. Chem. Glasses 17 Ž1976. 23. w12x S. Morimoto and M. Mishima, J. Ceram. Soc. Jpn. 88 Ž1980. 453. w13x G. Will and F. Hennessy, Trans. AIME 197 Ž1953. 1367. w14x T. Maekawa, T. Yokokawa and K. Niwa, Bull. Chem. Soc. Jpn. 42 Ž1969. 677. w15x S. Ato, Bunseki Kagaku ŽBaitu-Kan, 1967. p. 251 Žin Japanese..
T. Wakasugi et al.r Journal of Non-Crystalline Solids 210 (1997) 141–147 w16x JIS R3105-1981. w17x R.E. Kirk and D.F. Othmer, eds., Encyclopedia of Chemical Technology, Vol. 12 ŽInterscience Encyclopedia, 1954. p. 441. w18x E.T. Turkdogan, Physical Chemistry of High Temperature Technology ŽAcademic Press, New York, 1980. p. 5.
147
w19x C.H.P. Lupis, Chemical Thermodynamics of Materials ŽNorth-Holland, Amsterdam, 1983. pp. 165, 252. w20x A.L. Allred and E.G. Rochow, J. Inorg. Nucl. Chem. 5 Ž1958. 264. w21x H. Ito, A. Sasahara, T. Maekawa and T. Yokokawa, J. Chem. Soc. Faraday Trans. I 80 Ž1984. 473.
Solubility of Ag 2 O into the Na 2 O–B 2 O 3 –Al 2 O 3 system Takashi Wakasugi ) , Akio Hirota, Jiro Fukunaga, Rikuo Ota Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan Received 13 February 1996; revised 13 August 1996
Abstract The solubility of Ag 2 O was measured for the Na 2 O–B 2 O 3 and Na 2 O–B 2 O 3 –Al 2 O 3 system with the rotating crucible method and static method, respectively, under air atmosphere at temperatures ranging from 1273 to 1423 K. The contamination of melts from crucibles could be avoided by the rotating crucible method, with which it became possible to measure the solubility of Ag 2 O for the Na 2 O–B 2 O 3 system above the melting point of Ag for the first time. It was found that the addition of Na 2 O decreases the solubility of Ag 2 O while the addition of Al 2 O 3 had little effect on the solubility. The effect of Na 2 O and Al 2 O 3 on the solubility of Ag 2 O is expressed by interaction coefficients and is analyzed in terms of the basicity of melts. The solubility of Ag 2 O in Na 2 O–B 2 O 3 –Al 2 O 3 melts increased with increased temperature. This phenomena was explained by a small enthalpy change in oxidation of silver.
1. Introduction Since Ag 2 O in glasses shows the mixed cation effect in alkali containing systems, many investigators have been interested in the phenomena of mixed cation effects w1–7x. Silver halide containing glasses have attracted attention due to its photochromic property and high ionic conductivity w8–12x. Recently, glasses containing silver have been used as anti-bacteria agents because the Ag-ion prevents the increase of bacteria. Silver must exist in glasses as Agq-ion in order to exhibit the properties mentioned above. However, the form of Ag added to a glass batch, for example AgNO 3 , is easily reduced during the glass melting process because silver is a noble metal, so that the solubility of silver in glass melts is limited by thermodynamic factors; partial pressure of oxygen, temperature and glass composition. A )
Corresponding author. Tel.: q81-75 724 7575; fax: q81-75 724 7580; e-mail: [email protected].
method to produce glasses containing large concentrations of silver is required. Since it is difficult to change the oxygen partial pressure and temperature in the glass melting process, the glass composition has a large range. The ability of a glass melt to dissolve Ag 2 O or the solubility of Ag 2 O is expressed in term of the activity coefficient of Ag 2 O in the glass melt. It is known that the activity coefficient varies with composition. Therefore, there is a possibility to find a composition favorable for a glass with high silver content. It is important to understand the effect of each glass component on the solubility of Ag 2 O for this purpose. In order to measure the solubility of Ag 2 O, the crucible composition in which a glass melts affects solubility. Although metallic crucibles Že.g., platinum. reduce contamination of melts by crucibles, its use is almost impossible because of its oxidation or alloy formation with silver which is equilibrated with glass melts. An exception is the use of a silver
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 5 9 0 - X
142
T. Wakasugi et al.r Journal of Non-Crystalline Solids 210 (1997) 141–147
crucible. Willis and Hennessy w13x and Maekawa et al. w14x measured the solubility of Ag 2 O in B 2 O 3 and Ag 2 O–B 2 O 3 melts with silver crucible. However, the use of silver crucibles limits the experimental temperature to temperatures less than the melting point of silver. On the other hand, if refractory crucibles, for example Al 2 O 3 and so on, were used, contamination could not be avoided. In order to overcome the above difficulties, ‘the rotating crucible method’ was developed. With this method, it became possible to avoid crucible contamination. In this paper, the solubility of Ag 2 O measured for the Na 2 O–B 2 O 3 and Na 2 O–B 2 O 3 –Al 2 O 3 systems by the rotating crucible method and static method, respectively, is reported. Its temperature dependence and the effect of Na 2 O and Al 2 O 3 addition on the solubility of silver were evaluated. 2. Experimental Glass batches of given compositions were prepared from reagent grade H 3 BO 3 and Na 2 CO 3 , melted at 1473 K for 20 min in platinum crucible and quenched to glasses. The glass composition was in the range 0.600 F X B 2 O 3 F 0.743 in the Na 2 O– B 2 O 3 system, and 0.454 F X B 2 O 3 F 0.564 and 0 F X Na 2 O F 0.236 in the Na 2 O–B 2 O 3 –Al 2 O 3 system. These glasses were equilibrated with silver in mullite or alumina crucible between 1273 and 1423 K under air atmosphere. Since the crucible is attacked by the glass liquid during equilibration, the rotating crucible method was developed as shown in Fig. 1 in order to eliminate the contamination from the crucibles. A mullite crucible with about 50 g of Ag can be rotated at 500 rpm by a motor under the furnace. A hole was formed at the center of the silver melt by centrifugal force, and about 2 g of glass was put in it. In this way we could obtain glass melts with no contamination. Usually it was difficult to avoid the contamination from crucibles in the rotating crucible method. The solubility of Ag 2 O in the Na 2 O–B 2 O 3 –Al 2 O 3 system was also measured with the static method. In these experiments, alumina crucibles were used without rotation, so that the melt was saturated with Al 2 O 3 . Five grams of Ag and 2.5 g of glass, which is close to Al 2 O 3 saturation, were put in the crucible. The surface level of silver was higher than that of
Fig. 1. Schematic diagram of experimental apparatus.
the glass liquid although the surface of silver was covered by a thin layer of glass liquid. This configuration accelerated the oxidation of silver. If the top of silver was covered with a thick glass layer, a longer time to reach equilibrium was necessary. The initial glass contained no Ag 2 O. Addition of AgNO 3 to the glass liquid at the beginning of the experiment was tried in order to shorten the equilibration time, but little effect was observed. The experimental time to reach equilibrium was 24 h by the rotating crucible method and 48 h by the static method. After equilibration, glass samples for chemical analysis were collected by sticking a part of glass liquid on an iron rod. The content of Ag 2 O, B 2 O 3 , Na 2 O, and SiO 2 was analyzed by the Volhard method w15x, the titration method w16x, the flame spectrochemical method JIS, and the gravimetric method w16x, respectively.
3. Theoretical consideration The dissolution of silver into a glass liquid proceeds by an oxidation reaction as given by Eq. Ž1.. Generally a stable phase at the experimental temperature is chosen as a reference state. However, Ag 2 O completely decomposes at 573–613 K w17x and is not stable at the experimental temperatures. Therefore, solid Ag 2 O was chosen as a reference state of Ag 2 O
T. Wakasugi et al.r Journal of Non-Crystalline Solids 210 (1997) 141–147 Table 1 Solubility and activity coefficient of Ag 2 O in the Na 2 O–B 2 O 3 system in air No.
Temp. ŽK.
XAg 2 O Ž"0.0001.
XB 2O 3 Ž"0.001.
X Na 2 O Ž"0.001.
ln gAg 2 O Ž"0.01.
1 2 3 4
1323 1323 1373 1423
0.0191 0.0345 0.0368 0.0488
0.600 0.743 0.706 0.730
0.381 0.223 0.220 0.221
y2.15 y2.74 y3.02 y3.42
and the reaction of Eq. Ž2. is considered here, where the reference state is shown in parenthesis. The equilibrium constant of Eq. Ž2., K 2 , calculated from its standard free energy change, DG 20 , is expressed as Eq. Ž4., where a i is the activity, gAg 2 O the activity coefficient, XAg 2 O mole fraction and PO 2 the partial pressure of oxygen w18x 2Ag Ž l . q 12 O 2 Ž g . s Ag 2 O Ž in melt . ,
Ž 1.
2Ag Ž l . q 12 O 2 Ž g . s Ag 2 O Ž s . ,
Ž 2.
DG 20 s y53 100 q 84.1T
Ž 3.
ž
K 2 s exp y
DG 20 RT
/
s
Ž Jrmol. ,
aAg 2 O 2 aAg
PO1r2 2
s
gAg 2 O XAg 2 O 2 aAg
PO1r2 2
143
but the activity coefficient of Ag 2 O is unique for a melt independent of PO 2 . Then it is convenient to express the thermodynamic behavior of Ag 2 O in glass melts in terms of gAg 2 O , which is a measure of the rate of dissolution of Ag 2 O. Since aAg s 1 and PO 2 s 0.21 atm when the glass melt is equilibrated with metallic silver under air atmosphere, the activity of Ag 2 O can be calculated at any temperature and the activity coefficient, gAg 2 O can be obtained. An activity coefficient in a multi component system is usually expressed by the following equation as a first order approximation w19x: ln g i s ln g i0 Ž in solvent. q Ý
j i
Xj ,
Ž 5.
j
where g i0 is the activity coefficient of the ith component in the infinite dilute solution, and ij is the first order Gibbs energy interaction coefficient. The effect of glass component on the solubility of Ag 2 O j is expressed in terms of Ag in this paper. 2O
4. Results
.
Ž 4. The solubility of Ag 2 O, XAg 2 O , in a melt varies with the oxygen partial pressure of atmosphere, i.e., PO 2 ,
The results of Ag 2 O solubility in the Na 2 O–B 2 O 3 system obtained by the rotating method is shown in Table 1 together with the activity coefficients of Ag 2 O calculated from Eq. Ž4.. Since it was difficult
Table 2 Solubility and activity coefficient of Ag 2 O in the Na 2 O–B 2 O 3 –Al 2 O 3 system in air No.
Temp. ŽK.
XAg 2 O Ž"0.001.
X B 2 O 3 Ž"0.001.
X Na 2 O Ž"0.001.
XAl 2 O 3 Ž"0.001.
ln gAg 2 O Ž"0.01.
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
1273 1273 1273 1273 1323 1323 1323 1323 1373 1373 1373 1373 1423 1423 1423 1423
0.035 0.091 0.159 0.229 0.039 0.091 0.150 0.229 0.045 0.098 0.165 0.239 0.059 0.101 0.165 0.232
0.475 0.518 0.539 0.506 0.488 0.526 0.564 0.527 0.478 0.506 0.496 0.478 0.434 0.514 0.454 0.505
0.236 0.156 0.069 0.000 0.232 0.138 0.061 0.000 0.223 0.148 0.076 0.000 0.198 0.140 0.079 0.000
0.254 0.234 0.233 0.265 0.241 0.245 0.226 0.244 0.254 0.247 0.263 0.284 0.309 0.245 0.302 0.262
y2.56 y3.52 y4.08 y4.44 y2.86 y3.71 y4.21 y4.63 y3.18 y3.96 y4.48 y4.85 y3.62 y4.15 y4.64 y4.98
144
T. Wakasugi et al.r Journal of Non-Crystalline Solids 210 (1997) 141–147
solubility of Al 2 O 3 because Al 2 O 3 crucibles were used. The solubility of Al 2 O 3 does not change greatly with composition and temperature studied here. Therefore, the variation of B 2 O 3 content is small.
5. Discussion 5.1. EÕaluation of interaction parameters
Fig. 2. Solubility of Ag 2 O into the Na 2 O–B 2 O 3 –Al 2 O 3 system saturated with Al 2 O 3 in air at 1273 K. The line is drawn as a guide for the eye.
to avoid contamination from crucibles, we could get only four samples even by using the rotating crucible method. Other samples contained considerable amounts of Al 2 O 3 and SiO 2 which came from the mullite crucible. Although sample No. 3 contained about 4 wt% ŽAl 2 O 3 q SiO 2 . by contamination, the effect of this contamination was neglected in the following discussion. The Ag 2 O solubility in the Na 2 O–B 2 O 3 –Al 2 O 3 system obtained by the static method is shown in Table 2 and that at 1273 K is illustrated in Fig. 2. The activity coefficients are also shown in Table 2. The solubility of Ag 2 O decreases with increase of Na 2 O content and with decrease of temperature. The dependence of the solubility of Ag 2 O on Na 2 O content and temperature is similar to that in the Na 2 O–B 2 O 3 systems ŽTable 1, Nos. 1, 2.. Maekawa et al. w14x reported the solubility of Ag 2 O in Na 2 O–B 2 O 3 melts under various oxygen partial pressure at 1123–1208 K. According to them, the interpolated solubility of Ag 2 O in B 2 O 3 melt under air atmosphere is about XAg 2 O s 0.26 in their temperature range. The solubility of Ag 2 O into B 2 O 3 –Al 2 O 3 melts obtained in this work is smaller than that. Taking the difference of temperature and composition into consideration, our value is reasonable. The content of Al 2 O 3 in Table 2 means the
Since the Ag 2 O solubilities are in a range of 0.019 F XAg 2 O F 0.239 and these are relatively large, Ag 2 O the effect of the self interaction coefficient, Ag , 2O can not be neglected in the evaluation of interaction parameters. However, it is impossible to evaluate Ag 2 O Ag 2 O from this experiment because aAg 2 O is fixed in this experiment and the dependence of gAg 2 O on Ag 2 O content cannot be obtained. Then, the activity 0 coefficient of Ag 2 O at infinite dilution, gAg was 2O evaluated from Eq. Ž6. which is derived by assuming a regular solution model w19x: 0 ln gAg s 2O
ln gAg 2 O
Ž 1 y XAg O .
2
.
Ž 6.
2
0 The variation of gAg in the Na 2 O–B 2 O 3 –Al 2 O 3 2O system is shown in Fig. 3 as a function of Na 2 O 0 content. gAg increases with increase of Na 2 O con2O
Fig. 3. Variation of activity coefficient of Ag 2 O at infinite dilution with Na 2 O content in the Na 2 O–B 2 O 3 –Al 2 O 3 system saturated with Al 2 O 3 . Lines are drawn by least squares method as a guide for the eye.
T. Wakasugi et al.r Journal of Non-Crystalline Solids 210 (1997) 141–147
tent almost linearly at each temperature, and we expect that it can be expressed by a linear function Na 2 O of the first order with interaction parameters Ag 2O Al 2 O 3 and Ag 2 O . In order to evaluate the two interaction 0 Žin B 2 O 3 . which is the activity parameters and gAg 2O coefficient of Ag 2 O at infinite dilution in pure B 2 O 3 , a least squares method was used according to the Ag 2 O following equation, which does not contain a Ag 2O 0 term because gAg 2 O is the activity coefficient at XAg 2 O ™ 0: 0 0 ln gAg s ln gAg Ž in B 2 O 3 . q 2O 2O
q
Na 2 O Ag 2 O X Na 2 O
Al 2 O 3 Ag 2 O XAl 2 O 3 .
Ž 7.
Data from solubility obtained by the rotating crucible method and from static conditions were used for the above calculation. The result is shown in Table 3. Since the data of solubility of the rotating crucible Al 2 O 3 was method at 1273 K are not available, Ag 2O assumed to be zero in this case to obtain the other parameters. This assumption would not cause a large Al 2 O 3 error because Ag at other temperatures were 2O close to zero. Using the data reported by Maekawa et al. w14x, the solubility of Ag 2 O in Na 2 O–B 2 O 3 melts under 0 air condition was interpolated and gAg was calcu2O lated in the same way, which is shown in Fig. 3. The 0 gAg increases almost linearly with the increase of 2O Na 2 O Na 2 O content and the Ag calculated from the 2O data of X Na 2 O - 0.2 was 27.7 " 2.2. This value is close to that obtained in present work and we considered it reasonable. The interaction parameter shows the degree of interaction of two components. The interaction parameter between two components is positive if they are repulsive. The magnitude of interaction has a close relationship with the basicity, so that the inter-
action parameter between basic oxides should be positive and that between basic oxide and an acidic one should be negative. From the positive value of Na 2 O Ag 2 O , Ag 2 O was found to behave as a basic oxide. Electronegativity is often used as a measure of basicity. According to Allred and Rochow w20x, the electronegativity of Ag is 1.42, which is less than that of Al Ž1.47.. Since Al 2 O 3 is known as amphoteric oxide, we consider Ag 2 O to be a weak base, less basic than Al 2 O 3 . The Pauling scale of electronegativity is based on bonding energy, so the electronegativity of a noble metal becomes unusually high. Thus it is not adequate to use the Pauling scale as a measure of basicity for noble metal oxide. 5.2. Temperature dependence of Ag 2 O solubility Since the oxidation reaction is generally exothermic, low temperature is thermodynamically favorable for an oxidation reaction to occur. So the solubility of Ag 2 O at low temperatures is expected to be greater than that at high temperatures. In fact, it was reported that the solubility of Ag 2 O in B 2 O 3 melts increases with decrease of temperature w14x. However, it has been shown that the solubility of Ag 2 O in this work increases with increasing temperature. This may be explained by the small value of enthalpy change in the oxidation of silver ŽEq. Ž2.. and low basicity of Ag 2 O. The decrease of Ag 2 O solubility with temperature decrease corresponds to a positive value of D H1 , which is the enthalpy change of Eq. Ž1.. This value can be obtained from the Arrhenius plot of temperature dependence of the Ag 2 O solubility ŽFig. 4. with Eq. Ž8.: y
Table 3 0 Na 2 O Al 2 O 3 Žin B 2 O 3 ., Ag ln gAg , and Ag in the Na 2 O–B 2 O 3 – 2O 2O 2O Al 2 O 3 satd melts calculated by the least squares method Temp. ŽK.
0 ln gAg 2O Žin B 2 O 3 .
1273 1323 1373 1423
y7.3"0.2 y7.1"0.8 y8.0"0.2 y8.6"0.1
Na 2 O Ag 2 O
19.6"1.0 15.1"2.6 21.7"0.8 22.1"0.7
Al 2 O 3 Ag 2 O
– 0.8"2.8 y0.9"0.6 0.9"0.5
145
D H1 R
s
E ln XAg 2 O E Ž 1rT .
.
Ž 8.
Obtained values of D H1 are shown in Table 4. Eq. Ž1. consists of Eqs. Ž2., Ž9. and Ž10. and D H1 is equal to D H2 q D H9 q D H10 , where D H10 is equal 0 to the relative partial enthalpy of Ag 2 O, D HAg : 2O Ag 2 O Ž s . s Ag 2 O Ž l . , Ag 2 O Ž l . s Ag 2 O Ž in melt . .
Ž 9. Ž 10 .
146
T. Wakasugi et al.r Journal of Non-Crystalline Solids 210 (1997) 141–147
of temperature. For the Na 2 O–B 2 O 3 system, the relative partial molar enthalpy of Na 2 O does not vary significantly at low Na 2 O content and becomes less negative with the increase of Na 2 O content when X Na 2 O ) 0.05 w21x. The compositional depen0 0 dence of D H Na is consistent with that of D HAg . 2O 2O
6. Conclusion
Fig. 4. Temperature dependence of Ag 2 O solubility in the NA 2 O–B 2 O 3 –Al 2 O 3 system saturated with Al 2 O 3 . Lines are drawn by least squares method as a guide for the eye.
D H2 is y53.1 " 1.0 kJrmol according to Eq. Ž3.. Its magnitude is small compared with other metals. In case of copper which belongs to the same group as silver, the corresponding value is y168 kJrmol. Although the value of D H9 is unknown due to the decomposition of Ag 2 O at about 573 K, D H9 should be positive. The crystal structures of Cu 2 O and Ag 2 O are the same and D H9 would not be much different from the enthalpy change in melting of Cu 2 O Ž57.2 kJrmol.. Then, we assumed D H10 to be close to zero or slightly positive. The magnitude of D H10 depends on the basicity of Ag 2 O and glass melts. As noted in the previous section, Ag 2 O is a weak base. So, the positive value of D H10 is probable when Na 2 O content is relatively high. From the above discussion, it is clear that the variation of D H1 is the same as that of D H10 , which gradually becomes larger with the increase of Na 2 O content. Then, it is not unreasonable that the solubility of Ag 2 O into B 2 O 3 melt increases with decrease Table 4 Enthalpy changes, D H1 , of the dissolution of Ag 2 O into Na 2 O– B 2 O 3 –Al 2 O 3 satd melts X Na 2 O
D H1 ŽkJrmol.
0 0.069–0.078 0.138–0.156 0.223–0.236
2.5"2.8 6.1"5.5 11.6"3.0 36.4"3.7
The solubility of Ag 2 O into Na 2 O–B 2 O 3 –Al 2 O 3 melts increased with increase of temperature, which is opposite to the dependence of other elements. This phenomenon was explained by the small enthalpy change in oxidation of silver and weak basicity of Ag 2 O. The addition of Na 2 O to melts decreased the solubility of Ag 2 O, but the effect of Al 2 O 3 addition was quite small. The activity coefficient of Ag 2 O in pure B 2 O 3 melt was Ž1.8 " 0.2. = 10y4 , and interacNa 2 O Al 2 O 3 tion parameters, Ag and Ag , were 22.1 " 0.7 2O 2O and 0.9 " 0.5 at 1423 K, respectively.
References w1x E.F. Riebling, J. Am. Ceram. Soc. 56 Ž1973. 25. w2x H.M.J.M. van Ass and J.M. Stevels, J. Non-Cryst. Solids 15 Ž1974. 215. w3x W.J. Th. van Gemert, H.M.J.M. van Ass and J.M. Stevels, J. Non-Cryst. Solids 16 Ž1974. 281. w4x S. Sakka, K. Kamiya and B. Ozawa, J. Am. Ceram. Soc. 60 Ž1977. 285. w5x S. Sakka, K. Matusita and K. Kamiya, Phys. Chem. Glasses 20 Ž1979. 25. w6x C. Spurcaciu and T.P. Negreanu, J. Non-Cryst. Solids 84 Ž1986. 105. w7x J. Ruller and J.E. Shelby, Phys. Chem. Glasses 29 Ž1988. 209. w8x S.W. Martin, J. Am. Ceram. Soc. 74 Ž1991. 1767. w9x S. Rossignol, J.M. Reau, B. Tanguy, J.J. Videau and J. ´ Portier, J. Non-Cryst. Solids 155 Ž1993. 77. w10x T. Minami, H. Nambu and M. Tanaka, J. Am. Ceram. Soc. 60 Ž1977. 283. w11x T. Kawamoto, R. Kikuchi and Y. Kimura, Phys. Chem. Glasses 17 Ž1976. 23. w12x S. Morimoto and M. Mishima, J. Ceram. Soc. Jpn. 88 Ž1980. 453. w13x G. Will and F. Hennessy, Trans. AIME 197 Ž1953. 1367. w14x T. Maekawa, T. Yokokawa and K. Niwa, Bull. Chem. Soc. Jpn. 42 Ž1969. 677. w15x S. Ato, Bunseki Kagaku ŽBaitu-Kan, 1967. p. 251 Žin Japanese..
T. Wakasugi et al.r Journal of Non-Crystalline Solids 210 (1997) 141–147 w16x JIS R3105-1981. w17x R.E. Kirk and D.F. Othmer, eds., Encyclopedia of Chemical Technology, Vol. 12 ŽInterscience Encyclopedia, 1954. p. 441. w18x E.T. Turkdogan, Physical Chemistry of High Temperature Technology ŽAcademic Press, New York, 1980. p. 5.
147
w19x C.H.P. Lupis, Chemical Thermodynamics of Materials ŽNorth-Holland, Amsterdam, 1983. pp. 165, 252. w20x A.L. Allred and E.G. Rochow, J. Inorg. Nucl. Chem. 5 Ž1958. 264. w21x H. Ito, A. Sasahara, T. Maekawa and T. Yokokawa, J. Chem. Soc. Faraday Trans. I 80 Ž1984. 473.