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Redox reactions during temperature change in soda-lime–silicate melts doped with copper and iron or copper and manganese
Journal of Non-Crystalline Solids 352 (2006) 4062–4068 www.elsevier.com/locate/jnoncrysol
Redox reactions during temperature change in soda-lime–silicate melts doped with copper and iron or copper and manganese Ladislav Kido, Matthias Mu¨ller, Christian Ru¨ssel
*
Otto-Schott-Institut, Universita¨t Jena, Fraunhoferstrasse 6, 07743 Jena, Germany Received 21 June 2006 Available online 28 August 2006
Abstract Glasses with the base composition 16Na2O Æ 10CaO Æ 74SiO2 doped with copper and iron or copper and manganese were studied by high temperature UV–vis–NIR spectroscopy. The spectra exhibited distinct absorption bands attributed to the respective transition metal ions present (Cu2+, Fe2+, Fe3+, Mn3+). In glasses doped with only one polyvalent element, the absorption decreases linearly with increasing temperature, the absorption bands are shifted to smaller wave numbers and get broader. In glasses doped with two types of transition metals, the situation is the same up to a temperature of around 550 C. At larger temperature, the Cu2+-absorption in glasses also co-doped with iron increases again, while in glasses doped with both copper and manganese the absorption is approximately the same as in glasses solely doped with copper. It is shown that this is due to redox reactions between polyvalent species. These reactions are frozen in at temperatures <550 C. 2006 Elsevier B.V. All rights reserved. PACS: 81.05.Kf; 82.33.z Keywords: Oxidation reduction; Soda-lime–silica
1. Introduction Transition metals occur in glasses in different oxidation states. This strongly affects the physical, especially the optical properties. First of all, the optical transmission in the UV, visible and NIR-range is influenced by the type, concentration and redox ratio of polyvalent transition elements. At temperatures far above the glass transition temperature, polyvalent elements are in equilibrium with the physically dissolved oxygen of the melt [1–3]: K 0A ðT Þ z z AðnzÞþ þ O2 ¢ Anþ þ O2 : ð1Þ 4 2 The equilibrium constant K 0A can be defined as follows [4,5]:
*
Corresponding author. Tel.: +49 3641 948500; fax: +49 3641 948502. E-mail address: [email protected] (C. Ru¨ssel).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.06.027
z=2
K 0A ðT Þ
¼
aAnþ aO2 z=4
aAðnzÞþ aO2
ð2Þ
;
with ai: activities of the respective species. The equilibrium constant K 0A ðT Þ depends on temperature and furthermore on the glass composition. Since aO2 is usually much larger than the concentrations of the polyvalent ions; it can be considered as constant if Eq. (1) is shifted, and hence can be incorporated into the equilibrium constant. The redox ratios of polyvalent elements does not depend upon their total concentration in the cooled glass (for concentrations 61 mol%) [4]. Therefore, in a good approximation, the activity coefficients can be considered to be unity if referenced to an ideally diluted system. Then the activities in Eq. (2) can be replaced by the respective concentrations: K A ðT Þ ¼
½Anþ z=4 ½AðnzÞþ aO 2
:
ð3Þ
L. Kido et al. / Journal of Non-Crystalline Solids 352 (2006) 4062–4068
With increasing temperatures the equilibrium constant KA decreases, i.e. the reduced state is favored [3–5]. During cooling of a melt, maintaining the equilibrium with the surrounding atmosphere, the redox ratio hence should be shifted to the oxidized state. This, however, can only be achieved by diffusion of oxygen into the melt. In silicate melts, however, this is a slow process which requires long reaction times, additional convection and extremely small cooling rates. By contrast, using cooling rates suitable in glass technology or laboratory experiments, the diffusion of oxygen into the melt will be of minor importance and in a good approximation, the diffusion can be neglected if cooling a melt, in which concentration of the polyvalent element is larger than 0.1 mol% [4]. During cooling, the oxygen activity in the melt will hence decrease. Since, however, the absolute concentration of physically dissolved oxygen is very small [4] this will not lead to a detectable shift in the redox ratio. This means, the redox ratio in the melt at high temperatures will be the same as that in the quenched glass [1,4,6]. If more than one redox pair is present, this situation changes, because now a redox reaction between two polyvalent ions may occur [6–11]. This can e.g. be seen if two melts, one of them containing the polyvalent element A, the other one containing two polyvalent elements A and B, are equilibrated with the same atmosphere. After cooling, they will usually exhibit different redox ratios with respect to the polyvalent element A [6–11]. At the equilibration temperature, the redox ratio of a polyvalent element should not be affected by the presence of another type of polyvalent element, if their concentrations are small. This can also be seen in voltammetric studies where peak potentials (which are directly attributed to equilibrium constants) do not change if a second type of polyvalent element is present. Redox reactions during cooling of a melt can directly be observed using high temperature EPR [7] or UV–vis– NIR spectroscopy [8–11]. In these studies, it has been shown that the concentrations of respective redox species change with temperature and are frozen in below a certain temperature, typically in the range from 500 to 600 C for soda-lime–silica glasses [8–11]. Up to now, studies on melts doped with iron and arsenic [8], copper and tin, copper and antimony [9] as well as manganese and chromium [10,11] have been reported. The shift in the redox ratios was quantitatively explained by using high temperature thermodynamic data of the respective elements determined by voltammetry [1,5]. UV–vis–NIR spectroscopy carried out at constant temperatures in the range from 520 to 600 C also enabled the determination of redox relaxation times and related rate constants [11]. This paper provides a study on UV–vis–NIR high temperature spectroscopy on glass doped with copper and iron and copper and manganese.
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2. Theory The redox equilibria of the redox couples A and B are described by Eqs. (1) and (3). The equilibrium constants KA(T) and KB(T) depend on temperature: RT ln K A ðT Þ ¼ DG0A ¼ DH 0A T DS 0A ;
ð4Þ
RT ln K A ðT Þ ¼ DG0B ¼ DH 0B T DS 0B ;
ð5Þ
where DG0 is the standard free enthalpy, DH0 and DS0 are the standard enthalpy and the standard entropy, respectively. If the redox couples A and B are simultaneously present, then the following redox reaction may take place: K AB ðT Þ
z BðmyÞþ þ y Anþ ¢ z Bmþ þ y AðnzÞ :
ð6Þ
The equilibrium constant KAB(T) can be calculated from KA(T) and KB(T): ðK A ðT ÞÞy ðK B ðT ÞÞz " # zDH 0B yDH 0A yDS 0A zDS 0B ¼ exp exp : ðR T Þ R
K AB ðT Þ ¼
ð7Þ The equilibrium constant KAB(T) depends on temperature, if y DH 0A 6¼ z DH 0B . Then, the redox ratios will change during cooling until the equilibrium is kinetically frozen in. The spectra recorded were illustrated by the absorptivity versus the wave number. Absorbance is defined by Lambert–Beer’s Law: log
I ¼ ecd; I0
ð8Þ
with e = molar absorptivity, c = concentration of the absorbing species, and d = sample thickness. Absorbance divided by sample thickness results in absorptivity (in cm1) [12]: logðI=I 0 Þ ¼ ec: d
ð9Þ
3. Experimental procedure Glasses with the mol% composition 16Na2O Æ 10CaO Æ 74SiO2 doped with different concentrations of CuO, MnO and Fe2O3 were melted from reagent grade raw materials Na2CO3, CaCO3, SiO2 (quartz), CuO, Fe2O3 and MnO at a maximum temperature of 1500 C in a platinum crucible using an induction furnace. The glasses were casted on a graphite mould preheated to 530 C and then slowly cooled to room temperature (cooling rate: 30 K/h). The glass compositions are summarized in Table 1.
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L. Kido et al. / Journal of Non-Crystalline Solids 352 (2006) 4062–4068
Table 1 Chemical compositions of the samples studied (in mol%) Sample
[CuO]
[FeO]
[MnO]
A B C D E F G H
0.3 0.6 – – 0.3 0.6 0.15 0.3
– – 1 – 0.3 0.6 – –
– – – 0.67 – – 0.15 0.3
UV–vis–NIR transmission spectra were recorded using a specially designed spectrometer in the temperature range from 25 to 1000 C. The samples were placed inside a microscope heating stage (TS 1500, LINCAM, Waterfield, Great Britain). The experimental equipment is schematically shown in Fig. 1. The light from a halogen lamp was reflected by a planar mirror, passed through a chopper (frequency 113 s1) and the sample. The transmitted light was focused and reflected by an off-axis parabolic mirror (focal distance: 45 cm) to the entrance slit of a monochromator (TRIAX 320, Jobin-Yvon, Edison, NY, USA). The signals were collected by an Si-detector and given to a lock-in amplifier (SR 830, Stanford Research Systems, Stanford, CA, USA) which was adjusted to the chopper frequency. 4. Results Fig. 2 shows UV–vis–NIR spectra of glasses doped with 0.3% Cu, 0.3 % MnO and 1% Fe2O3 recorded at room temperature. The spectrum of the copper containing glass exhibits a peak at a wave number of 12 700 cm1. According to literature, the peak is due to the eg (xy, yz) a1g tran-
Fig. 2. UV–vis–NIR spectra of glass containing 1: 0.3 mol% CuO, 2: 0.3 mol% MnO and 1 mol% Fe2O3, recorded at room temperature.
sition [13,14]. The spectrum of the MnO-doped glass exhibits a peak at 20 300 cm1 and a shoulder at 15 100 cm1 which both are due to Mn3+. The spectrum of the iron-doped glass show distinct absorptions at around 5500, 9000, 23 000 and 27 000 cm1. The first two absorptions are caused by Fe2+, while the latter two are due to Fe3+. In Fig. 3, room temperature UV–vis–NIR spectra of glasses doped with both CuO and Fe2O3 are shown. In these spectra, peaks at 12 700, 23 000 and 27 000 cm1 are observed. The first peak is due to Cu2+, while the other ones are caused by Fe3+. Absorptions attributed to Fe2+ are seen as shoulders at around 9000 cm1 [15]. In the glasses doped with 0.6 Fe2O3 and 0.15 or 0.3 CuO, also a small peak at around 5000 cm1, caused by Fe2+ is observed. The absorptions at 12 700 cm1 are smaller than
Fig. 1. Schematic drawing of the experimental arrangement used for UV–vis–NIR high temperature spectroscopy.
L. Kido et al. / Journal of Non-Crystalline Solids 352 (2006) 4062–4068
Fig. 3. UV–vis–NIR spectra of glasses containing 1: 0.6 mol% Fe2O3 and 0.6 mol% CuO, 2: 0.3 mol% Fe2O3 and 0.3 mol% CuO, 3: 0.6 mol% Fe2O3 and 0.3 mol% CuO, 4: 0.6 mol% Fe2O3 and 0.15 mol% CuO.
those of glasses doped with the same CuO concentration which, however, do not contain iron. The absorption at 23 000 and 27 000 are larger than those expected from a glass with the same iron concentration which does not contain copper. Fig. 4 presents UV–vis–NIR spectra of a glass doped with both 0.3 mol% CuO and 0.3 mol% MnO as well as of a glass doped with 0.15 mol% CuO and 0.15 mol% MnO. Here, predominantly the Cu2+ absorption at 12 700 cm1 and the Mn3+ absorption at 20 300 cm1 is seen ([16]). The absorption of the Cu2+ peak is larger than in the glass doped with the same concentration of CuO which does not contain MnO. The absorption at 20 300 cm1 due to Mn3+ is much smaller than in a solely MnO doped glass with the same MnO concentration. Fig. 5 shows high temperature UV–vis–NIR spectra of a glass doped with 0.6 mol% CuO. All spectra exhibit a distinct maximum which appears in the wavelength range from 11 500 to 12 700 cm1. With increasing temperature, the wave number attributed to the maximum absorption is continuously shifted to smaller wave numbers. The absorption is also affected by the temperature and slightly decreases while increasing the temperature.
Fig. 4. UV–vis–NIR spectra of glasses containing 1: 0.3 mol% CuO and 0.3 mol% MnO, 2: 0.15 mol% CuO and 0.15 mol% MnO.
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Fig. 5. High temperature UV–vis–NIR spectra of a glass doped with 0.6 mol% CuO. -: 25 C, - -: 400 C, - - - -: 600 C, : 700 C, - Æ -: 800 C.
Fig. 6. High temperature UV–vis–NIR spectra of a glass doped with 0.3 mol% CuO and 0.3 mol% Fe. -: 25 C, - -: 400 C, - - - -: 600 C, : 700 C, - Æ -: 800 C.
In Fig. 6, high temperature UV–vis–NIR spectra of a glass doped with 0.3 Fe2O3 and 0.3 CuO are shown. In analogy to Fig. 5, they exhibit maxima at wave numbers in the range from 11 500 to 12 700 cm1. With increasing temperature, the maxima are shifted to smaller wave numbers. The maximum absorptions of the spectra recorded at 25 and at 400 C are approximately the same. While further increasing the temperature, the absorptions increase. While increasing the temperature, the UV-cut off is continuously shifted to smaller wave numbers. Fig. 7 presents high temperature UV–vis–NIR spectra of a glass doped with 0.15 mol% CuO and 0.15 mol% MnO. Here, the same shift in the wave numbers is observed as in Figs. 5 and 6. With increasing temperature, the absorption decreases slightly over the whole temperature range studied.The effect of temperature upon the Cu2+ peak absorption is shown in Figs. 8 and 9 for glasses doped with CuO and Fe2O3 as well as MnO, respectively. In Fig. 8 the absorption decreases linearly in the same way as in a glass solely doped with CuO up to a temperature of around 550 C.
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L. Kido et al. / Journal of Non-Crystalline Solids 352 (2006) 4062–4068
and 0.3 mol% Fe2O3 and that with 0.6 mol% CuO and 0.6 mol% Fe2O3. In Fig. 9 the effect of temperature on the Cu2+ peak is shown for glasses doped with 0.15 mol% CuO and 0.15 mol% MnO as well as with 0.3 mol% CuO and 0.3 mol% MnO. Here, the absorption decreases steadily in the whole temperature range studied in the same way as in a glass solely doped with CuO. 5. Discussion
Fig. 7. High temperature UV–vis–NIR spectra of a glass doped with 0.15 mol% CuO and 0.15 mol% MnO. -: 25 C, - -: 400 C, - - - -: 600 C, : 700 C, - Æ -: 800 C.
All spectra recorded exhibit a well pronounced absorption peak of Cu2+ which according to the literature [13,14] is attributed to the eg (xy, yz) ! a1g transition. The dependency of absorption upon wave number can fully be described by a fit to a curve of Gaussian shape. The wave numbers attributed to the peak are slightly shifted to smaller values with increasing temperature as already reported in Ref. [9]. Simultaneously the absorption bands get slightly broader and slightly decrease in intensity. It should be noted that all these effects are comparably small e.g. in comparison to those of the Cr6+ peak at around 10 000 cm1 [17] or the peak attributed to the amber chromophore [18]. Polyvalent elements regarded in these papers all exhibit one electron redox transitions: Cu2þ þ 1=2O2 ¡ Cuþ þ 1=4O2 ; Fe
3þ
Mn
þ 1=2O
3þ
2
þ 1=2O
¡ Fe
2
2þ
¡ Mn
þ 1=4O2 ; 2þ
þ 1=4O2 :
ð10Þ ð11Þ ð12Þ
The possible redox reactions in glasses under investigation doped with two types of polyvalent elements are as follows: Fig. 8. Absorptions of the Cu2+ peak of glasses doped with s: 0.3 mol% CuO and 0.3 mol% Fe2O3, d: 0.6 mol% CuO and 0.6 mol% Fe2O3 as a function of temperature.
Fig. 9. Absorptions of the Cu2+-peak of glasses doped with n 0.15 mol% CuO and 0.15 mol% MnO, m: 0.3 mol% CuO and 0.3 mol% MnO.
Then the absorption increases again. The same effect is observed in both glasses, that doped with 0.3 mol% CuO
K Cu=Fe
Cu2þ þ Fe2þ ¢ Cuþ þ Fe3þ ; K Cu=Fe
Cu2þ þ Mn2þ ¢ Cuþ þ Mn3þ :
ð13Þ ð14Þ
The changes in the equilibrium constants KCu/Fe and KCu/Mn can be quantified by Eq. (7). The corresponding thermodynamic values, in the case of copper and iron have been reported in the literature from high temperature voltammetric studies [3,5] (DH 0Cu ¼ 92 kJ mol1 , DS 0Cu ¼ 32 J mol1 K1 , DH 0Fe ¼ 102 kJ mol1 and DS 0Fe ¼ 33 J mol1 K1 ). The thermodynamic values of manganese DH 0Mn ¼ 106 8 kJ mol1 and DS 0Mn ¼ 120 J mol1 K1 have been reported in Ref. [10]. According to Eq. (6), the equilibrium constants depend on temperature, if DH 0A 6¼ DH 0B ðif z ¼ yÞ. For the glasses with copper and iron, the difference in the standard enthalpies DH 0Cu=Fe is 10 kJ mol1, while it is 14.8 kJ mol1 for DH 0Cu=Mn . So, both equilibria according to Eqs. (13) and (14) should be shifted to the right during cooling. This should result in a decrease of the Cu2+-concentration during cooling. The glass solely doped with 0.3 mol% Cu2+ shows an absorption of 0.193 at room temperature, whereas the glass doped with 0.3 mol% Cu2+ and 0.3 mol% Fe2O3 possesses an absorption of 0.179 at room temperature. The glass with 0.6 mol% CuO and 0.6 mol% Fe2O3 shows approximately
L. Kido et al. / Journal of Non-Crystalline Solids 352 (2006) 4062–4068
the same behaviour; also in this glass, the absorption of the sample doped with both polyvalent elements is smaller than in that solely doped with CuO. This clearly shows that the redox reaction took place. In Figs. 6 and 8, the absorption as a function of the temperature is shown. Here, clearly the increase in absorption at temperatures larger than 550 C can be seen. Below 550 C, however, the temperature dependency of the absorption is approximately the same as for the glass solely doped with CuO. Hence, below 550 C, the Cu2+ concentration does no longer change, and the redox reaction can be considered to be frozen in. In the glass doped with 0.3 mol% CuO and 0.3 mol% MnO, the absorption of Cu2+ does not depend upon temperature within the limits of error, it is twice as large as in the glass doped with 0.15 mol% CuO and 0.3 mol% MnO. At room temperature, the absorption is 27% larger than that of the sample solely doped copper. In attributed redox reaction (see Eq. (14)), Cu2+ reacts with Mn2+ during cooling. Hence, at a first glance, it is surprising that the absorption does not significatly change during cooling. However, it should be noted that at temperatures in the range from 1300 to 1600 C, only trace quantities of Mn3+ occur. If now, the equilibrium is shifted to the right, the concentration of Mn3+ increases. From the thermodynamic data given above for the Mn3+/Mn2+ equilibrium, at the used melting temperature of 1500 C (see Section 3), a Mn3+/ Mn2+ ratio of 9.2 · 104 can be calculated. If equimolar copper concentrations are present, during cooling, the Mn3+/Mn2+-ratio increases and reaches a value of 8.9 · 103 at 550 C, i.e. around 10 times more Mn3+ occurs at 550 C (and at room temperature) than at 1500 C. This, however, corresponds to a negligible decrease in the Cu2+ concentration. Hence, it is not surprising that in the investigated temperature range from room temperature to 800 C, the absorption of Cu2+ does not change significantly. In Fig. 10, the room temperature absorptions of the Cu+-peak are plotted against the Cu2+-concentrations
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Table 2 Room temperature absorptivities of Cu2+ absorption peak Sample
Cu2+ concentration (in mol%)
Maximum absorptivity at room temperature (in cm1)
A B E F G H
0.193 0.386 0.179 0.359 0.096 0.192
1.81 3.61 1.16 2.91 1.13 2.31
calculated from the thermodynamic data presented above and the respective total concentrations of the polyvalent elements. Here also data from literature obtained from high temperature spectroscopic studies of glasses doped with copper and antimony or copper and tin are included (Ref. [9]). A fairly good linear correlation of the absorption and the calculated copper concentrations is observed (Table 2). For this calculation, an equilibrium with air of the respective melt composition at melting temperature was assumed. Furthermore, it was assumed that the thermodynamic data obtained from voltammetric experiments at temperatures in the range from 1000 to 1600 C do not change during cooling down to 550 C. Regarding the assumptions made, the linearity in Fig. 10 is even better than expected. 6. Conclusions Glasses with the base composition 16Na2O Æ 10CaO Æ 74SiO2 were either solely doped with copper or with copper and iron or with copper and manganese. UV–vis–NIR spectra were recorded as a function of temperature. Glasses solely doped with copper showed a slight decrease in the absorptions (at wavelengths attributed to the absorption peak) from room temperature to 800 C. In the samples doped with both copper and iron, approximately the same behaviour was observed up to a temperature of 550 C. At further increasing temperatures, the absorption increases again. In the glasses doped with both copper and manganese the absorption did not change significantly at temperatures above 550 C. The temperature dependency of the respective equilibria calculated using experimentally determined high temperature thermodynamic data fully explains the behaviour observed. In the case of the copper/manganese equilibrium, Mn3+ occurs at high temperature only as minor component; its concentration increases around one order of magnitude during cooling. Nevertheless, due to the small quantities of Mn3+, the Cu2+-concentration cannot decrease within the limits of error. Below 550 C, the redox equilibria are kinetically frozen in. References
Fig. 10. Absorption of the Cu2+ peak versus the calculated Cu2+concentrations. j: data from the Cu/Fe and Cu/Mn – equilibria d: data from the Cu/Sb and Cu/Sn – equilibria (see Ref. [9]).
[1] A. Paul, J. Non-Cryst. Solids 71 (1985) 269. [2] H.D. Schreiber, J. Am. Ceram. Soc. 70 (1987) C 106. [3] C. Ru¨ssel, Glastech. Ber. 66 (1993) 93.
4068 [4] [5] [6] [7] [8] [9] [10] [11] [12]
L. Kido et al. / Journal of Non-Crystalline Solids 352 (2006) 4062–4068
C. Ru¨ssel, R. Kohl, H.A. Schaeffer, Glastech. Ber. 61 (1988) 209. C. Ru¨ssel, E. Freude, Phys. Chem. Glasses 30 (1989) 62. C. Ru¨ssel, Glastech. Ber. 62 (1989) 199. G. Gravanis, C. Ru¨ssel, Glastech. Ber. 62 (1989) 345. H. Schirmer, Ma. Mu¨ller, C. Ru¨ssel, Glass Sci. Technol. 76 (2003) 49. L. Kido, Ma. Mu¨ller, C. Ru¨ssel, Phys. Chem. Glasses 45 (2004) 21. L. Kido, Ma. Mu¨ller, C. Ru¨ssel, J. Non-Cryst. Solids 351 (2005) 523. L. Kido, Ma. Mu¨ller, C. Ru¨ssel, Chem. Mater. 17 (2005) 3929. J. Wong, C.A. Angel, Glass – Structure by Spectroscopy, Marcel Dekker Inc., New York, 1976, p. 197. [13] Z.D. Xiang, M. Cable, Phys. Chem. Glasses 38 (1997) 167.
[14] F. Rodriduez, A.G. Brenosa, I.A. Aramburn, M. Moreno, I.M. Calleja, J. Phys. C: Solid State Phys. 20 (1987) L 641. [15] C. Ades, T. Toganidis, I.P. Traverse, J. On-Cryst. Solids 125 (1990) 272. [16] W.A. Weyl, Colored Glasses, Society of Glass Technology, Sheffield, UK, 1967. [17] D. Go¨deke, M. Mu¨ller, C. Ru¨ssel, Glastech. Ber. Glass. Sci. Technol. 74 (2001) 177. [18] M. Mu¨ller, C. Ru¨ssel, O. Claußen, Glastech. Ber. Glass Sci. Technol. 72 (1999) 362.
Redox reactions during temperature change in soda-lime–silicate melts doped with copper and iron or copper and manganese Ladislav Kido, Matthias Mu¨ller, Christian Ru¨ssel
*
Otto-Schott-Institut, Universita¨t Jena, Fraunhoferstrasse 6, 07743 Jena, Germany Received 21 June 2006 Available online 28 August 2006
Abstract Glasses with the base composition 16Na2O Æ 10CaO Æ 74SiO2 doped with copper and iron or copper and manganese were studied by high temperature UV–vis–NIR spectroscopy. The spectra exhibited distinct absorption bands attributed to the respective transition metal ions present (Cu2+, Fe2+, Fe3+, Mn3+). In glasses doped with only one polyvalent element, the absorption decreases linearly with increasing temperature, the absorption bands are shifted to smaller wave numbers and get broader. In glasses doped with two types of transition metals, the situation is the same up to a temperature of around 550 C. At larger temperature, the Cu2+-absorption in glasses also co-doped with iron increases again, while in glasses doped with both copper and manganese the absorption is approximately the same as in glasses solely doped with copper. It is shown that this is due to redox reactions between polyvalent species. These reactions are frozen in at temperatures <550 C. 2006 Elsevier B.V. All rights reserved. PACS: 81.05.Kf; 82.33.z Keywords: Oxidation reduction; Soda-lime–silica
1. Introduction Transition metals occur in glasses in different oxidation states. This strongly affects the physical, especially the optical properties. First of all, the optical transmission in the UV, visible and NIR-range is influenced by the type, concentration and redox ratio of polyvalent transition elements. At temperatures far above the glass transition temperature, polyvalent elements are in equilibrium with the physically dissolved oxygen of the melt [1–3]: K 0A ðT Þ z z AðnzÞþ þ O2 ¢ Anþ þ O2 : ð1Þ 4 2 The equilibrium constant K 0A can be defined as follows [4,5]:
*
Corresponding author. Tel.: +49 3641 948500; fax: +49 3641 948502. E-mail address: [email protected] (C. Ru¨ssel).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.06.027
z=2
K 0A ðT Þ
¼
aAnþ aO2 z=4
aAðnzÞþ aO2
ð2Þ
;
with ai: activities of the respective species. The equilibrium constant K 0A ðT Þ depends on temperature and furthermore on the glass composition. Since aO2 is usually much larger than the concentrations of the polyvalent ions; it can be considered as constant if Eq. (1) is shifted, and hence can be incorporated into the equilibrium constant. The redox ratios of polyvalent elements does not depend upon their total concentration in the cooled glass (for concentrations 61 mol%) [4]. Therefore, in a good approximation, the activity coefficients can be considered to be unity if referenced to an ideally diluted system. Then the activities in Eq. (2) can be replaced by the respective concentrations: K A ðT Þ ¼
½Anþ z=4 ½AðnzÞþ aO 2
:
ð3Þ
L. Kido et al. / Journal of Non-Crystalline Solids 352 (2006) 4062–4068
With increasing temperatures the equilibrium constant KA decreases, i.e. the reduced state is favored [3–5]. During cooling of a melt, maintaining the equilibrium with the surrounding atmosphere, the redox ratio hence should be shifted to the oxidized state. This, however, can only be achieved by diffusion of oxygen into the melt. In silicate melts, however, this is a slow process which requires long reaction times, additional convection and extremely small cooling rates. By contrast, using cooling rates suitable in glass technology or laboratory experiments, the diffusion of oxygen into the melt will be of minor importance and in a good approximation, the diffusion can be neglected if cooling a melt, in which concentration of the polyvalent element is larger than 0.1 mol% [4]. During cooling, the oxygen activity in the melt will hence decrease. Since, however, the absolute concentration of physically dissolved oxygen is very small [4] this will not lead to a detectable shift in the redox ratio. This means, the redox ratio in the melt at high temperatures will be the same as that in the quenched glass [1,4,6]. If more than one redox pair is present, this situation changes, because now a redox reaction between two polyvalent ions may occur [6–11]. This can e.g. be seen if two melts, one of them containing the polyvalent element A, the other one containing two polyvalent elements A and B, are equilibrated with the same atmosphere. After cooling, they will usually exhibit different redox ratios with respect to the polyvalent element A [6–11]. At the equilibration temperature, the redox ratio of a polyvalent element should not be affected by the presence of another type of polyvalent element, if their concentrations are small. This can also be seen in voltammetric studies where peak potentials (which are directly attributed to equilibrium constants) do not change if a second type of polyvalent element is present. Redox reactions during cooling of a melt can directly be observed using high temperature EPR [7] or UV–vis– NIR spectroscopy [8–11]. In these studies, it has been shown that the concentrations of respective redox species change with temperature and are frozen in below a certain temperature, typically in the range from 500 to 600 C for soda-lime–silica glasses [8–11]. Up to now, studies on melts doped with iron and arsenic [8], copper and tin, copper and antimony [9] as well as manganese and chromium [10,11] have been reported. The shift in the redox ratios was quantitatively explained by using high temperature thermodynamic data of the respective elements determined by voltammetry [1,5]. UV–vis–NIR spectroscopy carried out at constant temperatures in the range from 520 to 600 C also enabled the determination of redox relaxation times and related rate constants [11]. This paper provides a study on UV–vis–NIR high temperature spectroscopy on glass doped with copper and iron and copper and manganese.
4063
2. Theory The redox equilibria of the redox couples A and B are described by Eqs. (1) and (3). The equilibrium constants KA(T) and KB(T) depend on temperature: RT ln K A ðT Þ ¼ DG0A ¼ DH 0A T DS 0A ;
ð4Þ
RT ln K A ðT Þ ¼ DG0B ¼ DH 0B T DS 0B ;
ð5Þ
where DG0 is the standard free enthalpy, DH0 and DS0 are the standard enthalpy and the standard entropy, respectively. If the redox couples A and B are simultaneously present, then the following redox reaction may take place: K AB ðT Þ
z BðmyÞþ þ y Anþ ¢ z Bmþ þ y AðnzÞ :
ð6Þ
The equilibrium constant KAB(T) can be calculated from KA(T) and KB(T): ðK A ðT ÞÞy ðK B ðT ÞÞz " # zDH 0B yDH 0A yDS 0A zDS 0B ¼ exp exp : ðR T Þ R
K AB ðT Þ ¼
ð7Þ The equilibrium constant KAB(T) depends on temperature, if y DH 0A 6¼ z DH 0B . Then, the redox ratios will change during cooling until the equilibrium is kinetically frozen in. The spectra recorded were illustrated by the absorptivity versus the wave number. Absorbance is defined by Lambert–Beer’s Law: log
I ¼ ecd; I0
ð8Þ
with e = molar absorptivity, c = concentration of the absorbing species, and d = sample thickness. Absorbance divided by sample thickness results in absorptivity (in cm1) [12]: logðI=I 0 Þ ¼ ec: d
ð9Þ
3. Experimental procedure Glasses with the mol% composition 16Na2O Æ 10CaO Æ 74SiO2 doped with different concentrations of CuO, MnO and Fe2O3 were melted from reagent grade raw materials Na2CO3, CaCO3, SiO2 (quartz), CuO, Fe2O3 and MnO at a maximum temperature of 1500 C in a platinum crucible using an induction furnace. The glasses were casted on a graphite mould preheated to 530 C and then slowly cooled to room temperature (cooling rate: 30 K/h). The glass compositions are summarized in Table 1.
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Table 1 Chemical compositions of the samples studied (in mol%) Sample
[CuO]
[FeO]
[MnO]
A B C D E F G H
0.3 0.6 – – 0.3 0.6 0.15 0.3
– – 1 – 0.3 0.6 – –
– – – 0.67 – – 0.15 0.3
UV–vis–NIR transmission spectra were recorded using a specially designed spectrometer in the temperature range from 25 to 1000 C. The samples were placed inside a microscope heating stage (TS 1500, LINCAM, Waterfield, Great Britain). The experimental equipment is schematically shown in Fig. 1. The light from a halogen lamp was reflected by a planar mirror, passed through a chopper (frequency 113 s1) and the sample. The transmitted light was focused and reflected by an off-axis parabolic mirror (focal distance: 45 cm) to the entrance slit of a monochromator (TRIAX 320, Jobin-Yvon, Edison, NY, USA). The signals were collected by an Si-detector and given to a lock-in amplifier (SR 830, Stanford Research Systems, Stanford, CA, USA) which was adjusted to the chopper frequency. 4. Results Fig. 2 shows UV–vis–NIR spectra of glasses doped with 0.3% Cu, 0.3 % MnO and 1% Fe2O3 recorded at room temperature. The spectrum of the copper containing glass exhibits a peak at a wave number of 12 700 cm1. According to literature, the peak is due to the eg (xy, yz) a1g tran-
Fig. 2. UV–vis–NIR spectra of glass containing 1: 0.3 mol% CuO, 2: 0.3 mol% MnO and 1 mol% Fe2O3, recorded at room temperature.
sition [13,14]. The spectrum of the MnO-doped glass exhibits a peak at 20 300 cm1 and a shoulder at 15 100 cm1 which both are due to Mn3+. The spectrum of the iron-doped glass show distinct absorptions at around 5500, 9000, 23 000 and 27 000 cm1. The first two absorptions are caused by Fe2+, while the latter two are due to Fe3+. In Fig. 3, room temperature UV–vis–NIR spectra of glasses doped with both CuO and Fe2O3 are shown. In these spectra, peaks at 12 700, 23 000 and 27 000 cm1 are observed. The first peak is due to Cu2+, while the other ones are caused by Fe3+. Absorptions attributed to Fe2+ are seen as shoulders at around 9000 cm1 [15]. In the glasses doped with 0.6 Fe2O3 and 0.15 or 0.3 CuO, also a small peak at around 5000 cm1, caused by Fe2+ is observed. The absorptions at 12 700 cm1 are smaller than
Fig. 1. Schematic drawing of the experimental arrangement used for UV–vis–NIR high temperature spectroscopy.
L. Kido et al. / Journal of Non-Crystalline Solids 352 (2006) 4062–4068
Fig. 3. UV–vis–NIR spectra of glasses containing 1: 0.6 mol% Fe2O3 and 0.6 mol% CuO, 2: 0.3 mol% Fe2O3 and 0.3 mol% CuO, 3: 0.6 mol% Fe2O3 and 0.3 mol% CuO, 4: 0.6 mol% Fe2O3 and 0.15 mol% CuO.
those of glasses doped with the same CuO concentration which, however, do not contain iron. The absorption at 23 000 and 27 000 are larger than those expected from a glass with the same iron concentration which does not contain copper. Fig. 4 presents UV–vis–NIR spectra of a glass doped with both 0.3 mol% CuO and 0.3 mol% MnO as well as of a glass doped with 0.15 mol% CuO and 0.15 mol% MnO. Here, predominantly the Cu2+ absorption at 12 700 cm1 and the Mn3+ absorption at 20 300 cm1 is seen ([16]). The absorption of the Cu2+ peak is larger than in the glass doped with the same concentration of CuO which does not contain MnO. The absorption at 20 300 cm1 due to Mn3+ is much smaller than in a solely MnO doped glass with the same MnO concentration. Fig. 5 shows high temperature UV–vis–NIR spectra of a glass doped with 0.6 mol% CuO. All spectra exhibit a distinct maximum which appears in the wavelength range from 11 500 to 12 700 cm1. With increasing temperature, the wave number attributed to the maximum absorption is continuously shifted to smaller wave numbers. The absorption is also affected by the temperature and slightly decreases while increasing the temperature.
Fig. 4. UV–vis–NIR spectra of glasses containing 1: 0.3 mol% CuO and 0.3 mol% MnO, 2: 0.15 mol% CuO and 0.15 mol% MnO.
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Fig. 5. High temperature UV–vis–NIR spectra of a glass doped with 0.6 mol% CuO. -: 25 C, - -: 400 C, - - - -: 600 C, : 700 C, - Æ -: 800 C.
Fig. 6. High temperature UV–vis–NIR spectra of a glass doped with 0.3 mol% CuO and 0.3 mol% Fe. -: 25 C, - -: 400 C, - - - -: 600 C, : 700 C, - Æ -: 800 C.
In Fig. 6, high temperature UV–vis–NIR spectra of a glass doped with 0.3 Fe2O3 and 0.3 CuO are shown. In analogy to Fig. 5, they exhibit maxima at wave numbers in the range from 11 500 to 12 700 cm1. With increasing temperature, the maxima are shifted to smaller wave numbers. The maximum absorptions of the spectra recorded at 25 and at 400 C are approximately the same. While further increasing the temperature, the absorptions increase. While increasing the temperature, the UV-cut off is continuously shifted to smaller wave numbers. Fig. 7 presents high temperature UV–vis–NIR spectra of a glass doped with 0.15 mol% CuO and 0.15 mol% MnO. Here, the same shift in the wave numbers is observed as in Figs. 5 and 6. With increasing temperature, the absorption decreases slightly over the whole temperature range studied.The effect of temperature upon the Cu2+ peak absorption is shown in Figs. 8 and 9 for glasses doped with CuO and Fe2O3 as well as MnO, respectively. In Fig. 8 the absorption decreases linearly in the same way as in a glass solely doped with CuO up to a temperature of around 550 C.
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and 0.3 mol% Fe2O3 and that with 0.6 mol% CuO and 0.6 mol% Fe2O3. In Fig. 9 the effect of temperature on the Cu2+ peak is shown for glasses doped with 0.15 mol% CuO and 0.15 mol% MnO as well as with 0.3 mol% CuO and 0.3 mol% MnO. Here, the absorption decreases steadily in the whole temperature range studied in the same way as in a glass solely doped with CuO. 5. Discussion
Fig. 7. High temperature UV–vis–NIR spectra of a glass doped with 0.15 mol% CuO and 0.15 mol% MnO. -: 25 C, - -: 400 C, - - - -: 600 C, : 700 C, - Æ -: 800 C.
All spectra recorded exhibit a well pronounced absorption peak of Cu2+ which according to the literature [13,14] is attributed to the eg (xy, yz) ! a1g transition. The dependency of absorption upon wave number can fully be described by a fit to a curve of Gaussian shape. The wave numbers attributed to the peak are slightly shifted to smaller values with increasing temperature as already reported in Ref. [9]. Simultaneously the absorption bands get slightly broader and slightly decrease in intensity. It should be noted that all these effects are comparably small e.g. in comparison to those of the Cr6+ peak at around 10 000 cm1 [17] or the peak attributed to the amber chromophore [18]. Polyvalent elements regarded in these papers all exhibit one electron redox transitions: Cu2þ þ 1=2O2 ¡ Cuþ þ 1=4O2 ; Fe
3þ
Mn
þ 1=2O
3þ
2
þ 1=2O
¡ Fe
2
2þ
¡ Mn
þ 1=4O2 ; 2þ
þ 1=4O2 :
ð10Þ ð11Þ ð12Þ
The possible redox reactions in glasses under investigation doped with two types of polyvalent elements are as follows: Fig. 8. Absorptions of the Cu2+ peak of glasses doped with s: 0.3 mol% CuO and 0.3 mol% Fe2O3, d: 0.6 mol% CuO and 0.6 mol% Fe2O3 as a function of temperature.
Fig. 9. Absorptions of the Cu2+-peak of glasses doped with n 0.15 mol% CuO and 0.15 mol% MnO, m: 0.3 mol% CuO and 0.3 mol% MnO.
Then the absorption increases again. The same effect is observed in both glasses, that doped with 0.3 mol% CuO
K Cu=Fe
Cu2þ þ Fe2þ ¢ Cuþ þ Fe3þ ; K Cu=Fe
Cu2þ þ Mn2þ ¢ Cuþ þ Mn3þ :
ð13Þ ð14Þ
The changes in the equilibrium constants KCu/Fe and KCu/Mn can be quantified by Eq. (7). The corresponding thermodynamic values, in the case of copper and iron have been reported in the literature from high temperature voltammetric studies [3,5] (DH 0Cu ¼ 92 kJ mol1 , DS 0Cu ¼ 32 J mol1 K1 , DH 0Fe ¼ 102 kJ mol1 and DS 0Fe ¼ 33 J mol1 K1 ). The thermodynamic values of manganese DH 0Mn ¼ 106 8 kJ mol1 and DS 0Mn ¼ 120 J mol1 K1 have been reported in Ref. [10]. According to Eq. (6), the equilibrium constants depend on temperature, if DH 0A 6¼ DH 0B ðif z ¼ yÞ. For the glasses with copper and iron, the difference in the standard enthalpies DH 0Cu=Fe is 10 kJ mol1, while it is 14.8 kJ mol1 for DH 0Cu=Mn . So, both equilibria according to Eqs. (13) and (14) should be shifted to the right during cooling. This should result in a decrease of the Cu2+-concentration during cooling. The glass solely doped with 0.3 mol% Cu2+ shows an absorption of 0.193 at room temperature, whereas the glass doped with 0.3 mol% Cu2+ and 0.3 mol% Fe2O3 possesses an absorption of 0.179 at room temperature. The glass with 0.6 mol% CuO and 0.6 mol% Fe2O3 shows approximately
L. Kido et al. / Journal of Non-Crystalline Solids 352 (2006) 4062–4068
the same behaviour; also in this glass, the absorption of the sample doped with both polyvalent elements is smaller than in that solely doped with CuO. This clearly shows that the redox reaction took place. In Figs. 6 and 8, the absorption as a function of the temperature is shown. Here, clearly the increase in absorption at temperatures larger than 550 C can be seen. Below 550 C, however, the temperature dependency of the absorption is approximately the same as for the glass solely doped with CuO. Hence, below 550 C, the Cu2+ concentration does no longer change, and the redox reaction can be considered to be frozen in. In the glass doped with 0.3 mol% CuO and 0.3 mol% MnO, the absorption of Cu2+ does not depend upon temperature within the limits of error, it is twice as large as in the glass doped with 0.15 mol% CuO and 0.3 mol% MnO. At room temperature, the absorption is 27% larger than that of the sample solely doped copper. In attributed redox reaction (see Eq. (14)), Cu2+ reacts with Mn2+ during cooling. Hence, at a first glance, it is surprising that the absorption does not significatly change during cooling. However, it should be noted that at temperatures in the range from 1300 to 1600 C, only trace quantities of Mn3+ occur. If now, the equilibrium is shifted to the right, the concentration of Mn3+ increases. From the thermodynamic data given above for the Mn3+/Mn2+ equilibrium, at the used melting temperature of 1500 C (see Section 3), a Mn3+/ Mn2+ ratio of 9.2 · 104 can be calculated. If equimolar copper concentrations are present, during cooling, the Mn3+/Mn2+-ratio increases and reaches a value of 8.9 · 103 at 550 C, i.e. around 10 times more Mn3+ occurs at 550 C (and at room temperature) than at 1500 C. This, however, corresponds to a negligible decrease in the Cu2+ concentration. Hence, it is not surprising that in the investigated temperature range from room temperature to 800 C, the absorption of Cu2+ does not change significantly. In Fig. 10, the room temperature absorptions of the Cu+-peak are plotted against the Cu2+-concentrations
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Table 2 Room temperature absorptivities of Cu2+ absorption peak Sample
Cu2+ concentration (in mol%)
Maximum absorptivity at room temperature (in cm1)
A B E F G H
0.193 0.386 0.179 0.359 0.096 0.192
1.81 3.61 1.16 2.91 1.13 2.31
calculated from the thermodynamic data presented above and the respective total concentrations of the polyvalent elements. Here also data from literature obtained from high temperature spectroscopic studies of glasses doped with copper and antimony or copper and tin are included (Ref. [9]). A fairly good linear correlation of the absorption and the calculated copper concentrations is observed (Table 2). For this calculation, an equilibrium with air of the respective melt composition at melting temperature was assumed. Furthermore, it was assumed that the thermodynamic data obtained from voltammetric experiments at temperatures in the range from 1000 to 1600 C do not change during cooling down to 550 C. Regarding the assumptions made, the linearity in Fig. 10 is even better than expected. 6. Conclusions Glasses with the base composition 16Na2O Æ 10CaO Æ 74SiO2 were either solely doped with copper or with copper and iron or with copper and manganese. UV–vis–NIR spectra were recorded as a function of temperature. Glasses solely doped with copper showed a slight decrease in the absorptions (at wavelengths attributed to the absorption peak) from room temperature to 800 C. In the samples doped with both copper and iron, approximately the same behaviour was observed up to a temperature of 550 C. At further increasing temperatures, the absorption increases again. In the glasses doped with both copper and manganese the absorption did not change significantly at temperatures above 550 C. The temperature dependency of the respective equilibria calculated using experimentally determined high temperature thermodynamic data fully explains the behaviour observed. In the case of the copper/manganese equilibrium, Mn3+ occurs at high temperature only as minor component; its concentration increases around one order of magnitude during cooling. Nevertheless, due to the small quantities of Mn3+, the Cu2+-concentration cannot decrease within the limits of error. Below 550 C, the redox equilibria are kinetically frozen in. References
Fig. 10. Absorption of the Cu2+ peak versus the calculated Cu2+concentrations. j: data from the Cu/Fe and Cu/Mn – equilibria d: data from the Cu/Sb and Cu/Sn – equilibria (see Ref. [9]).
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