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Chemical durability and structure of zinc–iron phosphate glasses
Journal of Non-Crystalline Solids 292 (2001) 150±157
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Chemical durability and structure of zinc±iron phosphate glasses S.T. Reis a,*, M. Karabulut b, D.E. Day a a
Graduate Center for Materials Research, 301 Straumanis Hall, University of Missouri±Rolla, Rolla, MO 65409-1170, USA b Department of Physics, University of Kafkas, 36000 Kars, Turkey Received 11 June 2001
Abstract The chemical durability of zinc±iron phosphate glasses with the general composition 40 xZnO±xFe2 O3 ±60P2 O5 has been measured. The chemical durability and density of these glasses increase with increasing Fe2 O3 content. Glasses containing more than 30 mol% Fe2 O3 had an excellent chemical durability. The dissolution rate (DR), calculated from the weight loss in distilled water at 90 °C for up to 32 days, was 10 9 g=cm2 = min which is 100 times lower than that of window glass and 300 times lower than that of a barium ferro, aluminoborate glass. The structure and valence states of the iron ions in these glasses were investigated using M ossbauer spectroscopy, X-ray diraction, infrared spectroscopy and dierential thermal analysis. X-ray diraction indicates that the local structure of the zinc±iron phosphate is related to the short range structures of crystalline Zn2 P2 O7 , Fe3 P2 O7 2 and Fe PO3 3 . Both Fe(II) and Fe(III) ions are present in all of these glasses. The presence of an Fe±O±P related band in the infrared (IR) spectra of the glasses containing more than 30 mol% Fe2 O3 is consistent with their excellent chemical durability. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The practical application of phosphate glasses is often limited by their poor chemical durability. However, as chemically durable phosphate glasses become identi®ed, their potential application should expand accordingly. For example, iron phosphate glasses have been receiving increasing attention as a host matrix for the vitri®cation of radioactive waste [1±3]. Besides high waste loading and low processing temperature, one of the key properties of these glasses is their exceptionally
* Corresponding author. Tel.: +1-573 341 4359; fax: +1-573 341 2071. E-mail address: [email protected] (S.T. Reis).
good chemical durability: The DR in distilled water at 90 °C is typically 10 9 ±10 10 g=cm2 = min [3±5]. Some other important applications range from sealing and laser technologies to biological applications [6±8]. The physical and chemical properties of phosphate glasses can be optimized by controlling the melting conditions and chemical composition. The chemical durability of lead±phosphate glasses increases dramatically with the addition of Fe2 O3 [4,5]. The structure of a phosphate glass is based on corner-sharing PO4 tetrahedra which form chains, rings or isolated PO4 groups. With the addition of Fe2 O3 to a phosphate glass, the P±O±P bonds are replaced by more chemically durable P± O±Fe(II) and/or P±O±Fe(III) bonds [9]. In the structure of iron phosphate glass, the Fe(II) and
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 8 0 - 8
S.T. Reis et al. / Journal of Non-Crystalline Solids 292 (2001) 150±157
Fe(III) ions are believed to be bonded in FeO6 16 groups which form Fe3 O12 clusters that are 4 interconnected via P2 O7 groups throughout the network [1]. The purpose of the present work was to investigate the chemical and physical properties of zinc± iron phosphate glasses and to relate these properties to the structure of the glass using M ossbauer and IR spectroscopies, X-ray diraction, and DTA.
2. Experimental procedure Zinc±iron phosphate glasses with the composition 40 xZnO±xFe2 O3 ±60P2 O5 , x 10; 20; 30 and 40 (mol%), were prepared by melting homogeneous mixtures of reagent grade ZnO, Fe2 O3 and P2 O5 =NH4 H2 PO4 in dense alumina crucibles at 1100 °C in air for 2 h. The melt was quenched in air by pouring it into a steel mold to form bars of 1 1 5 cm3 . The bars were immediately transferred to a furnace and annealed at 450 °C for 3 h. Table 1 gives the batch composition and raw materials used. The density of each glass was measured by the Archimedes method using distilled water as the buoyancy liquid. The chemical durability of the bulk glasses was evaluated from the weight loss of samples 1 1 1 cm3 immersed in deionized water at 90 °C for 2±32 days. The samples were polished to 600 grit ®nish with SiC paper, cleaned with acetone and suspended in glass ¯asks containing 100 ml of deionized water at 90 °C. Duplicate measurements were made for each glass and the
151
average dissolution rate (DR), normalized to the glass surface area and the corrosion time, was calculated from the weight loss. Dierential thermal analysis (DTA) measurements were performed in ¯owing air at a heating rate of 10 °C/min. A small glass sample was crystallized by heat treating at 650 and 780 °C for 48 h in air. Room temperature X-ray powder diraction (XRD) patterns for both glass and crystalline counterparts were collected using an X-ray diffractometer. The infrared (IR) spectrum for each glass was measured between 450 and 1500 cm 1 using an FT-IR spectrometer. Samples were prepared as pellets by pressing a mixture of about 4 mg of glass powder and 150 mg of anhydrous KBr powder. The spectrometer was purged with dry nitrogen while 20 scans were collected for each glass. The spectrum for pure KBr was subtracted from each glass spectrum to correct the background. The M ossbauer spectra were obtained at room temperature on a spectrometer which utilized a room temperature 50 mCi cobalt-57 source embedded in a rhodium matrix. The spectrometer was calibrated at 23 °C with a-iron foil and the line width of the a-iron spectrum was 0.27 mm/s. M ossbauer absorbers of approximate thickness 140 mg=cm2 were prepared using 200 mesh powders. The M ossbauer spectra were ®t with broadened paramagnetic Lorentzian doublets. This ®tting method has been proved to give reliable hyper®ne parameters [10]. Details of this ®tting procedure have been discussed previously [11].
Table 1 Batch composition, density, and calculated O/P and Fe/P ratios of the glasses studied
a
Sample
Compositiona (mol%)
Densityb g=cm3
O/Pc ratio
Fe/P ratio
DR d g=cm2 = min
S1 S2 S3 S4
30ZnO±10Fe2 O3 ±60P2 O5 20ZnO±20Fe2 O3 ±60P2 O5 10ZnO±30Fe2 O3 ±60P2 O5 40Fe2 O3 ±60P2 O5
2.94 2.97 3.01 3.03
3.00 3.15 3.29 3.42
0.17 0.33 0.50 0.67
1:4 10 2:1 10 1:5 10 1:3 10
Raw materials used were ZnO, Fe2 O3 , P2 O5 except for S4 glass where NH4 H2 PO4 was used. The error is 0.01 g=cm3 . c Calculated using the Fe(II) and Fe(III) concentrations from the M ossbauer spectra. d Samples were immersed in 100 ml distilled water at 90 °C for 8 days. b
5 7 7 9
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3. Results 3.1. Chemical durability The dissolution rates (DR ) of the zinc±iron phosphate glasses, measured from their weight loss in distilled water at 90 °C, are given in Table 1 and Fig. 1. Clearly, DR varies with Fe2 O3 content, decreasing from 1:4 10 5 g=cm2 = min for the glass containing 10 mol% Fe2 O3 to less then 1:3 10 9 g=cm2 = min for glasses containing more than 35 mol% Fe2 O3 . The glass with 10 mol% Fe2 O3 (S1) dissolved after 16 days in distilled water at 90 °C. Zinc±iron phosphate glasses whose Fe2 O3 content is 6 20 mol% clearly have a higher DR. Glasses containing 40 mol% Fe2 O3 had a DR 100 times less than the DR for window glass and 300 times less than the DR for a 27BaO2 ± 45B2 O3 ±18Al2 O3 ±10Fe2 O3 , mol%, glass which has been studied as an alternative to the conventional methods for waste immobilization [12], see Figs. 1 and 2, respectively. Fig. 3 shows that the DR (8 days) of these glasses tends to decrease with increasing O/P ratio. The O/P ratio was calculated from the ®nal glass composition which was calculated using the measured Fe(II) and Fe(III) concentrations from M ossbauer spectra. The original Fe/P ratio of the batch composition was assumed to remain constant in the glass. The Fe(II) ions in the glass, resulting from the reduction of Fe(III) ions during
Fig. 1. Dissolution rates (DR ) of zinc±iron phosphate glasses, as a function of the Fe2 O3 content after immersion in distilled water at 90 °C for 2, 4, 8, 16 and 32 days.
Fig. 2. Dissolution rate of the S4 glass and a barium aluminoborate glass in distilled water at 90 °C for 2, 4 and 8 days.
Fig. 3. Dependence of DR (8 days) of zinc±iron phosphate glasses on the oxygen to phosphorus molar ratio.
melting, were considered to form FeO in the calculations [13]. Fig. 4 shows the pH of the solution (water) after the corrosion tests. Because of the higher durability of the glasses containing more than 30 mol% Fe2 O3 , the pH decreased only slightly from its initial value. However, there was a much larger decrease in the pH for the solutions in which glasses containing less than 20 mol% Fe2 O3 were immersed. The reduction in the pH is attributed to the dissolution of phosphorus from the glass, which formed phosphoric acid. This decrease in pH is consistent with the larger DR values. There was no detectable dierence in the visual appearance of the glass samples immersed in water, but the glasses of lower Fe2 O3 content (S2) were more heavily corroded than the S3 and S4 glasses. The S1 glass dissolved completely after 16
S.T. Reis et al. / Journal of Non-Crystalline Solids 292 (2001) 150±157
Fig. 4. The pH values of the solution in which zinc±iron phosphate glasses were immersed at 90 °C for 2, 4, 8, 16 and 32 days. Initial pH of distilled water is shown by dotted line.
days in distilled water at 90 °C and the external surface of the S2 glass was rough and discolored. On the other hand, the surfaces of the S3 and S4 glasses were only slightly discolored and had their original sharpness. 3.2. X-ray diraction, DTA and density measurements All of the compositions listed in Table 1 formed glass as no crystalline phase was detected by XRD.
153
Fig. 5. Dierential thermal analysis (DTA) curve for S1 glass.
The DTA pattern in Fig. 5 is typical for iron phosphate glasses [11]. In the present study, the endothermic peak at 492 °C is related to the glass transition and the exothermic peaks at 640 °C and 770 °C are attributed to crystallization of the 30ZnO±10Fe2 O5 ±60P2 O5 mol% glass. Similar exothermic peaks were observed at 650 and 800 °C in the DTA pattern of a 40Fe2 O3 ±60P2 O5 , mol%, iron phosphate glass which were attributed to Fe3 P2 O7 2 and Fe4 P2 O7 3 , respectively [14]. Small portions of the S1 glass were crystallized by heat treating for 48 h in air at 650 and 780°C which corresponds to the temperature of the
Fig. 6. X-ray diraction pattern for S1 glass after thermal treatment at 650 °C (top) and 780 °C (bottom) for 48 h in air.
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The density of the zinc±iron phosphate glasses at room temperature increased linearly from 2.94 to 3:03 g=cm3 with increasing Fe2 O3 content (Fig. 7). This increase, although small, is believed to be due to the replacement of Zn2 ions with Fe3 ions which increases the cross-link density in the glass. This increase in cross-link density also strengthens the network and increases the chemical durability of the glass which is consistent with the decreasing DR values with increasing Fe2 O3 content. Fig. 7. Change in density of zinc±iron phosphate glasses with the Fe2 O3 content. Line is drawn as a guide to the eye.
3.3. Mossbauer and IR measurements
exothermic peaks in the DTA pattern. The resulting XRD patterns shown in Fig. 6 indicate that Zn2 P2 O7 [15], Fe3 P2 O7 2 [16] and Fe PO3 3 were present in both samples [17]. The partial conversion of Fe3 P2 O7 2 , which contains both Fe(II) and Fe(III) to Fe PO3 3 which contains only Fe(III), could be due to the oxidation of Fe(II) to Fe(III) during the heat treatment in air at 780 °C.
The room temperature M ossbauer spectra of the zinc±iron phosphate glasses are shown in Fig. 8. The iron valence and hyper®ne parameters, isomer shift and quadrupole splitting DEQ , calculated from the M ossbauer spectra, are given in Table 2. Some of the Fe(III) ions in the starting batch are reduced to Fe(II) ions during melting as the M ossbauer spectra indicate that all the glasses contain from 6% to 23% of Fe(II). The isomer shift
Fig. 8. M ossbauer spectra measured at 23 °C for 40
xZnO±xFe2 O3 ±60P2 O5 glasses.
S.T. Reis et al. / Journal of Non-Crystalline Solids 292 (2001) 150±157
155
Table 2 Room temperature M ossbauer isomer shifts hdi and quadrupole splitting hDEQ i parameters and fraction of Fe(II) for zinc±iron phosphate glasses Glass S1 S2 S3 S4
hDEQ i (mm/s)
hdi (mm/s)
Fraction
Fe(II)
Fe(III)
Fe(II)
Fe(III)
Fe(II)
1.12 1.26 1.26 1.26
0.43 0.40 0.39 0.39
2.37 2.19 2.12 2.08
0.75 0.86 0.88 0.90
6.0 12.0 17.0 23.0
a
of Fe(II) (%)
The estimated error in hdi and hDEQ i is 0.03 mm/s. a Calculated from the M ossbauer spectra.
for Fe(II) and Fe(III) ranges from 1.12 to 1.26 mm/s and from 0.39 to 0.43 mm/s, respectively, while the quadrupole splitting ranges from 2.08 to 2.37 mm/s and from 0.75 to 0.90, respectively. These values for the isomer shift correspond to octahedral or distorted octahedral coordination for both Fe(II) and Fe(III) ions in these glasses. The IR spectra of all four glasses are shown in Fig. 9. The spectra are very similar to those reported for other phosphate glasses [18±21]. The band around 500 cm 1 may be due to overlapping vibrations of iron oxygen polyhedra and P2 O7 groups [2,21]. The band at 760 cm 1 is assigned to the symmetric stretching vibration of the P±O±P bridge [21,22], while the band at 923 cm 1 is assigned to the asymmetric stretching vibration of P± O±P bridge [21,22]. The bands observed at 1080 and 1330 cm 1 are attributed to the symmetric and asymmetric vibrations of PO2 and PO3 2 terminal groups, respectively [21].
Fig. 9. The IR spectra of zinc±iron phosphate glasses.
4. Discussion The DR for the 40 xZnO±xFe2 O3 ±40P2 O5 glasses, with an iron content >35 mol%, is less than that of window glass and the barium aluminoborate glass (see Fig. 2). A DR between 10 7 and 10 9 g=cm2 min measured in deionized water at 90 °C for glasses containing <35 mol% Fe2 O3 is comparable to soda lime and borosilicate glasses. All of the properties measured for the zinc±iron phosphate glasses indicate that the iron±phosphorus±oxygen network becomes stronger with increasing Fe2 O3 content. The improvement in chemical durability and increase in density and pH values with increasing Fe2 O3 content are all consistent with stronger bonding in the glass. The improved durability of zinc±iron phosphate glasses is attributed to the replacement of the easily hydrated P±O±P bonds by corrosion resistant Fe±O±P bonds [18]. As the Fe2 O3 content in the glass increases, the number of Fe±O±P bonds also increases. It was found that the S4 glass, where the Fe/P is 0.67, has the highest chemical durability [19]. As reported previously [9,18], the O/P ratio in the zinc±iron phosphate glasses is also important to the aqueous chemical durability, see Fig. 3, where the chemical durability improved with increasing O/P ratio. Glasses with an O/P ratio less than 3.5 (S1, S2, and S3) have structures approaching the metaphosphate composition which consists of long chains of PO4 groups and contains more P±O±P bonds. These glasses have a higher DR compared to the glass with O/P ratio of 3.42 whose structure is closer to the pyrophosphate
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S.T. Reis et al. / Journal of Non-Crystalline Solids 292 (2001) 150±157
composition P2 O7 [23]. In fact, the S4 glass (O=P 3:42) had the best chemical durability with a DR of only 1:3 10 9 g=cm2 = min. A lead±iron± phosphate (LIP) glass whose O/P ratio was 3.7 [4], which contains a mixture of pyrophosphate P2 O7 and orthophosphate groups (isolated PO4 tetrahedra), had a chemical durability comparable to the 40Fe2 O3 ±60P2 O5 (S4) glass. The M ossbauer results indicate that even though there is no Fe(II) in the starting batch, the as-annealed glasses contain from 6% to 30% Fe(II) due to a reduction of a portion of the Fe(III) ions during melting in air. The isomer shift hdi values in Table 2 indicate that both Fe(II) and Fe(III) ions have oxygen neighbors in octahedral or distorted octahedral coordination. In a previous study [24], it was shown that glasses containing a mixture of Fe(II)±O±P and Fe(III)±O±P bonds had an equally good chemical durability. However, melts where more than 40% of the iron was present as Fe(II) ions had a higher tendency for crystallization. The IR spectra show that the band around 505 cm 1 (S4 glass) is due to overlapping vibrations of iron oxygen polyhedra and P2 O7 groups. This band is related to the larger number of Fe±O± P bonds present in glasses having an Fe2 O3 content >20 mol%, which is consistent with the excellent chemical durability observed for these glasses. The IR band at 760 cm 1 , which is due to symmetric stretching of P±O±P bonds [25,26], is broader for the S1 and S2 glasses than for the S3 and S4. This is a result of the larger number of Fe± O±P bonds, which replace the bridging P±O±P bonds in these glasses with increasing Fe/P ratio (Fe/P ratio is 0.17 and 0.33 for the S1 and S2 glasses, respectively) [18]. The general structure of the zinc±iron phosphate glasses can be visualized as consisting of PO4 tetrahedra joined together by oxygen polyhedra which contain Fe(II) and Fe(III) and Zn2 ions in interstitial positions which act as a modi®er in the glass network. The XRD patterns in Fig. 6 for the crystallized glasses suggest that these glasses contain structural groups like those presents in Zn2 P2 O7 , Fe3 P2 O7 and Fe PO3 3 . The structure of crystalline Fe3 P2 O7 2 consists of 16 Fe3 O12 clusters where the Fe(II) ions in tri-
gonal prism coordination are sandwiched between two Fe(III) ions in octahedral coordination which 4 are connected by P2 O7 groups [27]. In the structure of crystalline Fe PO3 3 , Fe(III) ions in octahedral coordination are connected by PO3 1 tetrahedra. Even though the structure of a glass may not be identical to its crystalline counterpart, it is reasonable to expect general similarities between the crystal and glass structures [14]. This is consistent with the M ossbauer results which indicated that both Fe(II) and Fe(III) ions are in octahedral coordination. However, in a previous M ossbauer study of crystalline Fe3 P2 O7 2 , the Fe(II) ions had an unusually large quadrupole splitting of 4.33 mm/s due to the trigonal prism coordination [11]. The quadrupole splitting measured for the glasses in the present study is in the range 2.08±2.37 mm/s which indicates that the Fe(II) environment in the glass is dierent from that in crystalline Fe3 P2 O7 2 , i.e., the Fe(II) ions are unlikely to be in well-de®ned trigonal prism coordination in the glass. 5. Conclusions The structure and properties of zinc±iron phosphate glasses have been investigated using various techniques. These glasses have a chemical durability comparable to soda lime and borosilicate glasses and the iron±phosphorus±oxygen network becomes stronger with increasing Fe2 O3 content. The improved chemical durability with increasing iron content is attributed to the easily hydrated P±O±P bonds being replaced by more chemically resistant Fe±O±P bonds.The O/P ratio is also an important factor to the aqueous chemical durability as indicated by the 40Fe2 O3 ±60P2 O5 glass which had the best chemical durability. Since this glass had an O/P ratio 3.42 closest to 3.5, it is expected to contain primarily P2 O7 groups. The IR and M ossbauer spectra indicate that these glasses 4 are dominated by P2 O7 dimer units and contain a large number of Fe(II)±O±P and Fe(III)±O± P bonds where both Fe(II) and Fe(III) ions have oxygen neighbors in either octahedral or distorted octahedral coordination.
S.T. Reis et al. / Journal of Non-Crystalline Solids 292 (2001) 150±157
Acknowledgements S.T.R. thanks Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP) for a postdoctoral fellowship as well as the support from the University of Missouri±Rolla which has made this work possible. References [1] G.K. Marasinghe, M. Karabulut, C.S. Ray, D.E. Day, J. Non-Cryst. Solids 263&264 (2000) 146. [2] M. Karabulut, G.K. Marasinghe, P.G. Allen, C.H. Booth, M. Grimsditch, J. Mater. Res. 15 (2000) 1972. [3] M.G. Mesko, D.E. Day, J. Non-Cryst. Solids 273 (1999) 27. [4] S.T. Reis, PhD thesis, Brazilian Nuclear Energy Commission ± Energy and Nuclear Research Institute, 1999. [5] B.C. Sales, L.A. Boatner, Mater. Lett. 2 (1984) 301. [6] I.W. Donald, B.L. Metcalfe, R.N.J. Taylor, J. Mater. Sci. 32 (1997) 5851. [7] W.S. Key, J.C. Miller, in: Advanced Photonics at ORNLORNL Rev. 27 (3) (1994) 3. [8] Y. Lin, Y. Zhang, W. Huang, K. Lu, Y. Zhao, J. NonCryst. Solids 112 (1993) 136. [9] X. Yu, D.E. Day, G.J. Long, R.K. Brow, J. Non-Cryst. Solids 215 (1997) 21. [10] M. Dyar, Am. Mineral 70 (1985) 304. [11] G.K. Marasinghe, M. Karabulut, C.S. Ray, D.E. Day, J. Non-Cryst. Solids 222 (1997) 144.
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[12] S.T. Reis, W.M. Pontuschka, C.S.M. Partitti, D.L.A. Faria, Properties and Structural Features of Basal Glass, Proc. 14th Brazilian Meeting on Materials Science (ENFMC), Suo Lourenco, MG, 2001. [13] C.S. Ray, X. Fang, M. Karabulut, G.K. Marasinghe, D.E. Day, J. Non-Cryst. Solids 249 (1999) 1. [14] G.K. Marasinghe, M. Karabulut, C.S. Ray, D.E. Day, C.H. Booth, P.G. Allen, D.K. Shuh, Ceram. Trans. 87 (1998) 261. [15] J. Mjiling et al., Calc. Powder Diract. Patt. Anhydrous Phos. 149 (1979) (PDF#34-1275). [16] Tennessee Valley Authority, Cryst. Prop. of Fertilizer Compds, Bull. Y-217, 11 (1991) (PDF#44-0772). [17] M. Ijjaali, G. Venturini, B. Malamany, J. Solid State Inorg. Chem. 28 (1991) 983. [18] X. Fang, C.S. Ray, G.K. Marasinghe, D.E. Day, J. NonCryst. Solids 263&264 (2000) 293. [19] G.J. Exarhos, P.J. Miller, W.M. Risen Jr., J. Chem. Phys. 60 (1974) 4145. [20] J.J. Hudgens, S.W. Martin, J. Am. Ceram. Soc. 76 (1993) 1691. [21] B. Samuneva, P. Tzvetkova, I. Gugov, V. Dimitrov, J. Mater. Sci. Lett. 15 (1996) 21. [22] A.M. E®mov, J. Non-Cryst. Solids 209 (1997) 209. [23] D.E. Day, Z. Wu, C.S. Ray, P. Hrma, J. Non-Cryst. Solids 241 (1998) 1. [24] M. Karabulut, G.K. Marasinghe, C.S. Ray, O. Ozturk, D.E. Day, G.D. Waddill, J. Non-Cryst. Solids 249 (1999) 106. [25] G. Wang, Y, Wang, B. Jin, SPIE 2287 (1994) 214. [26] J. Hudgens, R.K. Brow, D.R. Tallant, S.W. Martin, J. Non-Cryst. Solids 223 (1998) 21. [27] D. Virgo, B.O. Mysen, Phys. Chem. Miner. 12 (1985) 65.
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Chemical durability and structure of zinc±iron phosphate glasses S.T. Reis a,*, M. Karabulut b, D.E. Day a a
Graduate Center for Materials Research, 301 Straumanis Hall, University of Missouri±Rolla, Rolla, MO 65409-1170, USA b Department of Physics, University of Kafkas, 36000 Kars, Turkey Received 11 June 2001
Abstract The chemical durability of zinc±iron phosphate glasses with the general composition 40 xZnO±xFe2 O3 ±60P2 O5 has been measured. The chemical durability and density of these glasses increase with increasing Fe2 O3 content. Glasses containing more than 30 mol% Fe2 O3 had an excellent chemical durability. The dissolution rate (DR), calculated from the weight loss in distilled water at 90 °C for up to 32 days, was 10 9 g=cm2 = min which is 100 times lower than that of window glass and 300 times lower than that of a barium ferro, aluminoborate glass. The structure and valence states of the iron ions in these glasses were investigated using M ossbauer spectroscopy, X-ray diraction, infrared spectroscopy and dierential thermal analysis. X-ray diraction indicates that the local structure of the zinc±iron phosphate is related to the short range structures of crystalline Zn2 P2 O7 , Fe3 P2 O7 2 and Fe PO3 3 . Both Fe(II) and Fe(III) ions are present in all of these glasses. The presence of an Fe±O±P related band in the infrared (IR) spectra of the glasses containing more than 30 mol% Fe2 O3 is consistent with their excellent chemical durability. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The practical application of phosphate glasses is often limited by their poor chemical durability. However, as chemically durable phosphate glasses become identi®ed, their potential application should expand accordingly. For example, iron phosphate glasses have been receiving increasing attention as a host matrix for the vitri®cation of radioactive waste [1±3]. Besides high waste loading and low processing temperature, one of the key properties of these glasses is their exceptionally
* Corresponding author. Tel.: +1-573 341 4359; fax: +1-573 341 2071. E-mail address: [email protected] (S.T. Reis).
good chemical durability: The DR in distilled water at 90 °C is typically 10 9 ±10 10 g=cm2 = min [3±5]. Some other important applications range from sealing and laser technologies to biological applications [6±8]. The physical and chemical properties of phosphate glasses can be optimized by controlling the melting conditions and chemical composition. The chemical durability of lead±phosphate glasses increases dramatically with the addition of Fe2 O3 [4,5]. The structure of a phosphate glass is based on corner-sharing PO4 tetrahedra which form chains, rings or isolated PO4 groups. With the addition of Fe2 O3 to a phosphate glass, the P±O±P bonds are replaced by more chemically durable P± O±Fe(II) and/or P±O±Fe(III) bonds [9]. In the structure of iron phosphate glass, the Fe(II) and
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 8 0 - 8
S.T. Reis et al. / Journal of Non-Crystalline Solids 292 (2001) 150±157
Fe(III) ions are believed to be bonded in FeO6 16 groups which form Fe3 O12 clusters that are 4 interconnected via P2 O7 groups throughout the network [1]. The purpose of the present work was to investigate the chemical and physical properties of zinc± iron phosphate glasses and to relate these properties to the structure of the glass using M ossbauer and IR spectroscopies, X-ray diraction, and DTA.
2. Experimental procedure Zinc±iron phosphate glasses with the composition 40 xZnO±xFe2 O3 ±60P2 O5 , x 10; 20; 30 and 40 (mol%), were prepared by melting homogeneous mixtures of reagent grade ZnO, Fe2 O3 and P2 O5 =NH4 H2 PO4 in dense alumina crucibles at 1100 °C in air for 2 h. The melt was quenched in air by pouring it into a steel mold to form bars of 1 1 5 cm3 . The bars were immediately transferred to a furnace and annealed at 450 °C for 3 h. Table 1 gives the batch composition and raw materials used. The density of each glass was measured by the Archimedes method using distilled water as the buoyancy liquid. The chemical durability of the bulk glasses was evaluated from the weight loss of samples 1 1 1 cm3 immersed in deionized water at 90 °C for 2±32 days. The samples were polished to 600 grit ®nish with SiC paper, cleaned with acetone and suspended in glass ¯asks containing 100 ml of deionized water at 90 °C. Duplicate measurements were made for each glass and the
151
average dissolution rate (DR), normalized to the glass surface area and the corrosion time, was calculated from the weight loss. Dierential thermal analysis (DTA) measurements were performed in ¯owing air at a heating rate of 10 °C/min. A small glass sample was crystallized by heat treating at 650 and 780 °C for 48 h in air. Room temperature X-ray powder diraction (XRD) patterns for both glass and crystalline counterparts were collected using an X-ray diffractometer. The infrared (IR) spectrum for each glass was measured between 450 and 1500 cm 1 using an FT-IR spectrometer. Samples were prepared as pellets by pressing a mixture of about 4 mg of glass powder and 150 mg of anhydrous KBr powder. The spectrometer was purged with dry nitrogen while 20 scans were collected for each glass. The spectrum for pure KBr was subtracted from each glass spectrum to correct the background. The M ossbauer spectra were obtained at room temperature on a spectrometer which utilized a room temperature 50 mCi cobalt-57 source embedded in a rhodium matrix. The spectrometer was calibrated at 23 °C with a-iron foil and the line width of the a-iron spectrum was 0.27 mm/s. M ossbauer absorbers of approximate thickness 140 mg=cm2 were prepared using 200 mesh powders. The M ossbauer spectra were ®t with broadened paramagnetic Lorentzian doublets. This ®tting method has been proved to give reliable hyper®ne parameters [10]. Details of this ®tting procedure have been discussed previously [11].
Table 1 Batch composition, density, and calculated O/P and Fe/P ratios of the glasses studied
a
Sample
Compositiona (mol%)
Densityb g=cm3
O/Pc ratio
Fe/P ratio
DR d g=cm2 = min
S1 S2 S3 S4
30ZnO±10Fe2 O3 ±60P2 O5 20ZnO±20Fe2 O3 ±60P2 O5 10ZnO±30Fe2 O3 ±60P2 O5 40Fe2 O3 ±60P2 O5
2.94 2.97 3.01 3.03
3.00 3.15 3.29 3.42
0.17 0.33 0.50 0.67
1:4 10 2:1 10 1:5 10 1:3 10
Raw materials used were ZnO, Fe2 O3 , P2 O5 except for S4 glass where NH4 H2 PO4 was used. The error is 0.01 g=cm3 . c Calculated using the Fe(II) and Fe(III) concentrations from the M ossbauer spectra. d Samples were immersed in 100 ml distilled water at 90 °C for 8 days. b
5 7 7 9
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3. Results 3.1. Chemical durability The dissolution rates (DR ) of the zinc±iron phosphate glasses, measured from their weight loss in distilled water at 90 °C, are given in Table 1 and Fig. 1. Clearly, DR varies with Fe2 O3 content, decreasing from 1:4 10 5 g=cm2 = min for the glass containing 10 mol% Fe2 O3 to less then 1:3 10 9 g=cm2 = min for glasses containing more than 35 mol% Fe2 O3 . The glass with 10 mol% Fe2 O3 (S1) dissolved after 16 days in distilled water at 90 °C. Zinc±iron phosphate glasses whose Fe2 O3 content is 6 20 mol% clearly have a higher DR. Glasses containing 40 mol% Fe2 O3 had a DR 100 times less than the DR for window glass and 300 times less than the DR for a 27BaO2 ± 45B2 O3 ±18Al2 O3 ±10Fe2 O3 , mol%, glass which has been studied as an alternative to the conventional methods for waste immobilization [12], see Figs. 1 and 2, respectively. Fig. 3 shows that the DR (8 days) of these glasses tends to decrease with increasing O/P ratio. The O/P ratio was calculated from the ®nal glass composition which was calculated using the measured Fe(II) and Fe(III) concentrations from M ossbauer spectra. The original Fe/P ratio of the batch composition was assumed to remain constant in the glass. The Fe(II) ions in the glass, resulting from the reduction of Fe(III) ions during
Fig. 1. Dissolution rates (DR ) of zinc±iron phosphate glasses, as a function of the Fe2 O3 content after immersion in distilled water at 90 °C for 2, 4, 8, 16 and 32 days.
Fig. 2. Dissolution rate of the S4 glass and a barium aluminoborate glass in distilled water at 90 °C for 2, 4 and 8 days.
Fig. 3. Dependence of DR (8 days) of zinc±iron phosphate glasses on the oxygen to phosphorus molar ratio.
melting, were considered to form FeO in the calculations [13]. Fig. 4 shows the pH of the solution (water) after the corrosion tests. Because of the higher durability of the glasses containing more than 30 mol% Fe2 O3 , the pH decreased only slightly from its initial value. However, there was a much larger decrease in the pH for the solutions in which glasses containing less than 20 mol% Fe2 O3 were immersed. The reduction in the pH is attributed to the dissolution of phosphorus from the glass, which formed phosphoric acid. This decrease in pH is consistent with the larger DR values. There was no detectable dierence in the visual appearance of the glass samples immersed in water, but the glasses of lower Fe2 O3 content (S2) were more heavily corroded than the S3 and S4 glasses. The S1 glass dissolved completely after 16
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Fig. 4. The pH values of the solution in which zinc±iron phosphate glasses were immersed at 90 °C for 2, 4, 8, 16 and 32 days. Initial pH of distilled water is shown by dotted line.
days in distilled water at 90 °C and the external surface of the S2 glass was rough and discolored. On the other hand, the surfaces of the S3 and S4 glasses were only slightly discolored and had their original sharpness. 3.2. X-ray diraction, DTA and density measurements All of the compositions listed in Table 1 formed glass as no crystalline phase was detected by XRD.
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Fig. 5. Dierential thermal analysis (DTA) curve for S1 glass.
The DTA pattern in Fig. 5 is typical for iron phosphate glasses [11]. In the present study, the endothermic peak at 492 °C is related to the glass transition and the exothermic peaks at 640 °C and 770 °C are attributed to crystallization of the 30ZnO±10Fe2 O5 ±60P2 O5 mol% glass. Similar exothermic peaks were observed at 650 and 800 °C in the DTA pattern of a 40Fe2 O3 ±60P2 O5 , mol%, iron phosphate glass which were attributed to Fe3 P2 O7 2 and Fe4 P2 O7 3 , respectively [14]. Small portions of the S1 glass were crystallized by heat treating for 48 h in air at 650 and 780°C which corresponds to the temperature of the
Fig. 6. X-ray diraction pattern for S1 glass after thermal treatment at 650 °C (top) and 780 °C (bottom) for 48 h in air.
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The density of the zinc±iron phosphate glasses at room temperature increased linearly from 2.94 to 3:03 g=cm3 with increasing Fe2 O3 content (Fig. 7). This increase, although small, is believed to be due to the replacement of Zn2 ions with Fe3 ions which increases the cross-link density in the glass. This increase in cross-link density also strengthens the network and increases the chemical durability of the glass which is consistent with the decreasing DR values with increasing Fe2 O3 content. Fig. 7. Change in density of zinc±iron phosphate glasses with the Fe2 O3 content. Line is drawn as a guide to the eye.
3.3. Mossbauer and IR measurements
exothermic peaks in the DTA pattern. The resulting XRD patterns shown in Fig. 6 indicate that Zn2 P2 O7 [15], Fe3 P2 O7 2 [16] and Fe PO3 3 were present in both samples [17]. The partial conversion of Fe3 P2 O7 2 , which contains both Fe(II) and Fe(III) to Fe PO3 3 which contains only Fe(III), could be due to the oxidation of Fe(II) to Fe(III) during the heat treatment in air at 780 °C.
The room temperature M ossbauer spectra of the zinc±iron phosphate glasses are shown in Fig. 8. The iron valence and hyper®ne parameters, isomer shift and quadrupole splitting DEQ , calculated from the M ossbauer spectra, are given in Table 2. Some of the Fe(III) ions in the starting batch are reduced to Fe(II) ions during melting as the M ossbauer spectra indicate that all the glasses contain from 6% to 23% of Fe(II). The isomer shift
Fig. 8. M ossbauer spectra measured at 23 °C for 40
xZnO±xFe2 O3 ±60P2 O5 glasses.
S.T. Reis et al. / Journal of Non-Crystalline Solids 292 (2001) 150±157
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Table 2 Room temperature M ossbauer isomer shifts hdi and quadrupole splitting hDEQ i parameters and fraction of Fe(II) for zinc±iron phosphate glasses Glass S1 S2 S3 S4
hDEQ i (mm/s)
hdi (mm/s)
Fraction
Fe(II)
Fe(III)
Fe(II)
Fe(III)
Fe(II)
1.12 1.26 1.26 1.26
0.43 0.40 0.39 0.39
2.37 2.19 2.12 2.08
0.75 0.86 0.88 0.90
6.0 12.0 17.0 23.0
a
of Fe(II) (%)
The estimated error in hdi and hDEQ i is 0.03 mm/s. a Calculated from the M ossbauer spectra.
for Fe(II) and Fe(III) ranges from 1.12 to 1.26 mm/s and from 0.39 to 0.43 mm/s, respectively, while the quadrupole splitting ranges from 2.08 to 2.37 mm/s and from 0.75 to 0.90, respectively. These values for the isomer shift correspond to octahedral or distorted octahedral coordination for both Fe(II) and Fe(III) ions in these glasses. The IR spectra of all four glasses are shown in Fig. 9. The spectra are very similar to those reported for other phosphate glasses [18±21]. The band around 500 cm 1 may be due to overlapping vibrations of iron oxygen polyhedra and P2 O7 groups [2,21]. The band at 760 cm 1 is assigned to the symmetric stretching vibration of the P±O±P bridge [21,22], while the band at 923 cm 1 is assigned to the asymmetric stretching vibration of P± O±P bridge [21,22]. The bands observed at 1080 and 1330 cm 1 are attributed to the symmetric and asymmetric vibrations of PO2 and PO3 2 terminal groups, respectively [21].
Fig. 9. The IR spectra of zinc±iron phosphate glasses.
4. Discussion The DR for the 40 xZnO±xFe2 O3 ±40P2 O5 glasses, with an iron content >35 mol%, is less than that of window glass and the barium aluminoborate glass (see Fig. 2). A DR between 10 7 and 10 9 g=cm2 min measured in deionized water at 90 °C for glasses containing <35 mol% Fe2 O3 is comparable to soda lime and borosilicate glasses. All of the properties measured for the zinc±iron phosphate glasses indicate that the iron±phosphorus±oxygen network becomes stronger with increasing Fe2 O3 content. The improvement in chemical durability and increase in density and pH values with increasing Fe2 O3 content are all consistent with stronger bonding in the glass. The improved durability of zinc±iron phosphate glasses is attributed to the replacement of the easily hydrated P±O±P bonds by corrosion resistant Fe±O±P bonds [18]. As the Fe2 O3 content in the glass increases, the number of Fe±O±P bonds also increases. It was found that the S4 glass, where the Fe/P is 0.67, has the highest chemical durability [19]. As reported previously [9,18], the O/P ratio in the zinc±iron phosphate glasses is also important to the aqueous chemical durability, see Fig. 3, where the chemical durability improved with increasing O/P ratio. Glasses with an O/P ratio less than 3.5 (S1, S2, and S3) have structures approaching the metaphosphate composition which consists of long chains of PO4 groups and contains more P±O±P bonds. These glasses have a higher DR compared to the glass with O/P ratio of 3.42 whose structure is closer to the pyrophosphate
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composition P2 O7 [23]. In fact, the S4 glass (O=P 3:42) had the best chemical durability with a DR of only 1:3 10 9 g=cm2 = min. A lead±iron± phosphate (LIP) glass whose O/P ratio was 3.7 [4], which contains a mixture of pyrophosphate P2 O7 and orthophosphate groups (isolated PO4 tetrahedra), had a chemical durability comparable to the 40Fe2 O3 ±60P2 O5 (S4) glass. The M ossbauer results indicate that even though there is no Fe(II) in the starting batch, the as-annealed glasses contain from 6% to 30% Fe(II) due to a reduction of a portion of the Fe(III) ions during melting in air. The isomer shift hdi values in Table 2 indicate that both Fe(II) and Fe(III) ions have oxygen neighbors in octahedral or distorted octahedral coordination. In a previous study [24], it was shown that glasses containing a mixture of Fe(II)±O±P and Fe(III)±O±P bonds had an equally good chemical durability. However, melts where more than 40% of the iron was present as Fe(II) ions had a higher tendency for crystallization. The IR spectra show that the band around 505 cm 1 (S4 glass) is due to overlapping vibrations of iron oxygen polyhedra and P2 O7 groups. This band is related to the larger number of Fe±O± P bonds present in glasses having an Fe2 O3 content >20 mol%, which is consistent with the excellent chemical durability observed for these glasses. The IR band at 760 cm 1 , which is due to symmetric stretching of P±O±P bonds [25,26], is broader for the S1 and S2 glasses than for the S3 and S4. This is a result of the larger number of Fe± O±P bonds, which replace the bridging P±O±P bonds in these glasses with increasing Fe/P ratio (Fe/P ratio is 0.17 and 0.33 for the S1 and S2 glasses, respectively) [18]. The general structure of the zinc±iron phosphate glasses can be visualized as consisting of PO4 tetrahedra joined together by oxygen polyhedra which contain Fe(II) and Fe(III) and Zn2 ions in interstitial positions which act as a modi®er in the glass network. The XRD patterns in Fig. 6 for the crystallized glasses suggest that these glasses contain structural groups like those presents in Zn2 P2 O7 , Fe3 P2 O7 and Fe PO3 3 . The structure of crystalline Fe3 P2 O7 2 consists of 16 Fe3 O12 clusters where the Fe(II) ions in tri-
gonal prism coordination are sandwiched between two Fe(III) ions in octahedral coordination which 4 are connected by P2 O7 groups [27]. In the structure of crystalline Fe PO3 3 , Fe(III) ions in octahedral coordination are connected by PO3 1 tetrahedra. Even though the structure of a glass may not be identical to its crystalline counterpart, it is reasonable to expect general similarities between the crystal and glass structures [14]. This is consistent with the M ossbauer results which indicated that both Fe(II) and Fe(III) ions are in octahedral coordination. However, in a previous M ossbauer study of crystalline Fe3 P2 O7 2 , the Fe(II) ions had an unusually large quadrupole splitting of 4.33 mm/s due to the trigonal prism coordination [11]. The quadrupole splitting measured for the glasses in the present study is in the range 2.08±2.37 mm/s which indicates that the Fe(II) environment in the glass is dierent from that in crystalline Fe3 P2 O7 2 , i.e., the Fe(II) ions are unlikely to be in well-de®ned trigonal prism coordination in the glass. 5. Conclusions The structure and properties of zinc±iron phosphate glasses have been investigated using various techniques. These glasses have a chemical durability comparable to soda lime and borosilicate glasses and the iron±phosphorus±oxygen network becomes stronger with increasing Fe2 O3 content. The improved chemical durability with increasing iron content is attributed to the easily hydrated P±O±P bonds being replaced by more chemically resistant Fe±O±P bonds.The O/P ratio is also an important factor to the aqueous chemical durability as indicated by the 40Fe2 O3 ±60P2 O5 glass which had the best chemical durability. Since this glass had an O/P ratio 3.42 closest to 3.5, it is expected to contain primarily P2 O7 groups. The IR and M ossbauer spectra indicate that these glasses 4 are dominated by P2 O7 dimer units and contain a large number of Fe(II)±O±P and Fe(III)±O± P bonds where both Fe(II) and Fe(III) ions have oxygen neighbors in either octahedral or distorted octahedral coordination.
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