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Influence of rare earth addition on the thermal and structural stability of CaOFe2O3P2O5 glasses
Accepted Manuscript Influence of rare earth addition on the thermal and structural stability of CaO– Fe2O3–P2O5 glasses Haijian Li, Xiaofeng Liang, Cuiling Wang, Huijun Yu, Zhen Li, Shiyuan Yang PII: DOI: Reference:
S0022-2860(14)00862-X http://dx.doi.org/10.1016/j.molstruc.2014.08.032 MOLSTR 20892
To appear in:
Journal of Molecular Structure
Received Date: Revised Date: Accepted Date:
1 July 2014 29 July 2014 18 August 2014
Please cite this article as: H. Li, X. Liang, C. Wang, H. Yu, Z. Li, S. Yang, Influence of rare earth addition on the thermal and structural stability of CaO–Fe2O3–P2O5 glasses, Journal of Molecular Structure (2014), doi: http:// dx.doi.org/10.1016/j.molstruc.2014.08.032
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Influence of rare earth addition on the thermal and structural stability of CaO–Fe2O3–P2O5 glasses Haijian Li a,b, Xiaofeng Liang a,b, *,1, Cuiling Wang a,b, Huijun Yu a,b, Zhen Li b, Shiyuan Yang b a
Analytical and Testing Center, Southwest University of Science and Technology,
Mianyang 621010, PR China b
State Key Laboratory Cultivation Base for Nonmetal Composites and Functional
Materials, Southwest University of Science and Technology, Mianyang 621010, PR China Abstract The thermal and structural stability of calcium iron phosphate glasses doped with rare earth oxides has been studied by investigating differential scanning calorimetry, the concentration of modified ions in leachate, Fourier transform infrared spectra, and Raman spectra. The results showed that thermal stability of the rare earth phosphate glasses increased and the release rate of rare earth ions in leachate decreased with the increase of cationic field strength. It related to the known structural units and characteristic bands of these glasses detected by infrared and Raman spectra. Metaphosphate chains by the cross-links of calcium ions and P–O–P linkages were easier to hydrolyze, which converted to orthophosphate units after corrosion. The formation of more chemically resistant RE–O–P bonds with the addition of rare earth leaded to an increased quantity of nonbridging oxygen, which caused consequent variations in the chemical durability of rare earth glasses. * Corresponding author. Tel.: +86 0816-6089507. E-mail addresses: [email protected] (X.F. Liang) 1 Co-first authors. 1
Keywords: Phosphate glass; Rare earth; Thermal stability; FTIR spectra; Raman spectra 1.
Introduction In radioactive nuclear glasses, the rare earths (RE) are present as waste forms in
the glass matrix. For example, gadolinium with low non-radiative decay rate and high thermal neutron capture cross-section may be added to the final actinide-containing glasses to minimize the likelihood of criticality during the storage period [1,2]. On the other hand, rare earth elements are also present as surrogates for the heaviest transuranic elements occurring in High Level Waste (HLW) such as Cm and Am [3,4]. They have cation radii r similar to the one of rare earth ions (for instance in sixfold coordination: r(Am3+) = 1.01 Å, r(Cm3+) = 0.98 Å and r(Nd3+) = 0.995 Å [5]). Wang et al. [6,7] have reported the corrosion behavior of soda lime silicate glasses containing about 1.0 mol% rare earths (Y2O3, La2O3, CeO2, Nd2O3 and Gd2O3), and they pointed out that the formation of a hydrated layer depleted the dissolution rate of rare earth ions. Phosphate glasses have been investigated for immobilization in high-level waste for over 30 years [8]. The glass has lower melting temperatures and higher waste loading capacities than borosilicate glass. However, phosphate glasses have relatively low chemical durability. Several studies have shown that the chemical durability of phosphate glasses can be improved by the addition of various oxides such as calcium oxide and, especially, ferric oxide [8-10]. It has been suggested that Fe3+ cations can enter in the glass network with four-fold coordination and form stable POFe 2
covalent bonds and divalent cations like Ca2+ can form POCa covalent bonds and serve as ionic crosslinks between the nonbridging oxygens of two different phosphate chains, and accordingly increase chemical durability [10-12]. Corrosion mechanism of glass has received attention for a long time [13,14]. It is known that an ionic exchange takes place between mobile ions such as alkali earth ions and H+, and the water molecules diffuse into the glass, react with P–O–P and P–O–M
(where M = metal cation), forming P–OH groups [10,15]. Hamilton et al.
[16] found that the aqueous corrosion behavior of glasses was related to the concentration of nonbridging oxygen (NBO) sites in the glass network structure. Vibrational spectroscopy has been employed to investigate the structure of glasses and specifically identification of the main structural units, which are connected through nonbridging oxygen (NBO) and bridging oxygen (BO) [17]. Therefore, vibrational spectroscopy is a valuable tool for studying the changes of glass structure before and after aqueous corrosion. Palavit et al. [18] examined the chemical reactions between P2O5–ZnO–H2O ultraphosphate glasses and water by
31
P nuclear magnetic resonance, and the
microstructure of the devitrified glass consisted of crystalline Zn2P4O12. Banerjee et al. [19] studied the structural aspects of caesium borosilicate glasses with varying amounts of CaO before and after leaching by using infrared spectroscopy. The reduction of the peak intensity at Si–OH and B–OH bonds showed that the chemical durability of the glasses was improved significantly as CaO was increased at the expense of B2O3 content in the glass. Molières et al. [20] showed that the role of 3
lanthanum in the alteration kinetics of borosilicate glass at different degrees of leaching progress was investigated by Raman spectroscopy. After leaching, the Raman spectrum of the glass exhibited the presence of highly depolymerized entities. The chemical durability of phosphate glasses was investigated by the dissolution rate of ions in previous studies [10,15]. However, little systematic work in such glasses with rare earth addition has been done to identify the structural units before and after corrosion, to follow the structural changes and to correlate these changes with the corrosion mechanisms of glass. Therefore, the aim of this paper is to focus on the effect of rare earth oxides on the structural stability of CaO–Fe2O3–P2O5 glasses before and after aqueous corrosion by FTIR spectra and Raman spectra, and the thermal stability of the glass was investigated by differential scanning calorimetry (DSC). 2. Experiment 2.1. Preparation of glass samples The glass system of xRE2O3–(100–x)(12CaO–20Fe2O3–68P2O5) (x = 10 mol%) with RE = Y, La, Nd, Sm and Gd were prepared using a standard melt-quench technique. After thorough mixing the powders were introduced in corundum crucibles, in order to prevent the excess boiling and consequent spillage, water and ammonia in ammonium phosphate monobasic were removed initially by preheating it at 220 °C for about 2h and then the electric furnace was raised to 1250 °C (heating rate was 10 °CÂPLQ-1), and samples were melted at 1250 °C for 3h. The melts were then poured into preheated steel molds, and moved quickly to an annealing furnace, annealed at 4
475 °C for 2h and cooled down to room temperature more than 12h. Samples for property measurements were ground to give fine powder. 2.2. Chemical durability tests The chemical durability of glasses was determined according to Product Consistency Test B (PCT–B) in ASTM C1285-02, using 3 g of glass powder (100 to +200 mesh) in 30 ml deionized water. Tests were carried out in duplicate at 90 °C for 7 days. The rare earth elements in the leachate were measured by inductively coupled plasma mass spectrometry (ICP-MS) using the Agilent 7700x (Agilent, U.S.A), and the calcium and iron elements were measured by inductively coupled plasma (ICP) emission spectroscopy using the iCAP 6500 (Thermo Fisher, U.S.A). Solution pH was measured with a calibrated KEDIDA CT-6023 pH meter. The normalized elemental mass release rate, ri, was calculated from the equation: r =
C f (AV)t
where ri is the normalized elemental mass release (g/m2/min) of element i, Ci the concentration of element i in solution (µg/ml or g/m3) , fi the mass fraction of element i in the glass (unitless) and t the time duration of test in minutes (min). A/V is the ratio of the sample surface area to volume of leachate (m-1), a value of 1800 m-1 was used. All tests were made in duplicate and the error in the average reported herein was estimated at ±15%. 2.3. Structural investigation X-ray diffraction (XRD) analysis was performed on samples employing a X-ray diffractometer (PANalytical X’Pert PRO, The Netherlands). The 2Q scans were made 5
between 5° and 80° with step width of 0.03° and utilized Cu KA radiation (Ȝ Å). Preparation of the samples used a simple top pack loading method for an acquired smooth surface [21]. The density, D, of each glass was measured at room temperature using the Archimedes method with water as an immersing liquid. The sample weights varied between 3 and 4 g, and the measured densities were reproducible within 0.01 g·cm-3. The molar volume (Vm) was calculated using the relation Vm £(xM)/D, where x is the molar fraction and M is the total molecular weight of the component. The glass transition temperature (Tg) and first crystallization temperature (Tr) were measured on DSC by utilizing a SDT Q600 instrument (TA, USA) in a flowing air atmosphere at a heating rate of 20 °CÂmin-1. The temperature scanned over a range from room temperature to 800 °C and the estimated error in Tg and Tr were ±2 °C. The infrared spectra of the samples were measured from 400 to 2000 cm-1 using a Spectrum One FTIR spectrometer (Perkin Elmer, USA) and the KBr standard pellet method. Glass pellets were prepared by mixing about 2 mg powder with 200 mg dried KBr powder and compressing the resulting mixture in an evacuated die. The accuracy of this technique is estimated to be ± 4 cm-1. Because the majority of the bands in the infrared spectra are broad and asymmetric, presenting also some shoulders, a deconvolution of the experimental spectra was necessary. This was made with ORIGIN 7.5 program (Originlab Corp.) using a Gaussian type function and allowed us a better identification of all the bands which appear in these spectra and their assignments. The proportion of particular 6
structures corresponding to different vibration modes, was calculated from the areas of the fitted Gaussian bands divided by the total area of all bands. The two parameters of each band (peak frequency and relative areas) were allowed to be variable during the iterations. Raman spectra at 400–1600 cm-1 were collected from glass powders using the InVia Raman Microscope (Renishaw, U.K.) at room temperature. The Raman spectra were measured by excitation light of 514.5 nm light from an argon ion laser. The spectral resolution was about 1~2 cm-1 and the wavenumber accuracy was 0.2 cm-1. Six multiple measurements per sample were done to check for the potential micron-range heterogeneity and the effects of sample orientation; both have not been found, as it would be expected for a nearly homogeneous glass sample. 3. Results and discussion 3.1. XRD, DSC and density measurements The crystallization tendency of phosphate glasses adding rare earth elements is related to the high amount of rare earth ions that act as nucleating agent in the calcium iron phosphate glass. In our previous paper, the investigation on structural aspects of iron phosphate glasses containing neodymium oxide prepared with traditional melt-quenching methods was carried out by XRD, infrared and Raman spectra [21]. It showed that excess neodymium oxide will induce the crystallization of the phosphate glass, which was disadvantageous for the vitrification of high-level radioactive wastes and was often assumed to reduce the chemical durability [22]. X-ray diffraction analysis indicated that base glass for undoped rare earths (RE0) and doped 10 mol% 7
rare earth glasses were amorphous before and after corrosion. Hence, these glasses could contain abundant the surrogates of transuranic elements and remained amorphous after corrosion. Table 1 showed that the density increase from 2.82 to 3.33 g/cm3 as the atomic number of the rare earth elements increase. This increase is primarily due to the increased atomic weights of the rare earth cation. The molar volume decrease slightly as the atomic weight of the rare earth elements increase. It is mainly attributed to that the higher molecular weight of rare earth decrease the total number of oxygen atoms [23]. This decrease in molar volume indicates that the phosphate glass network becomes more compact as the atomic weight of the rare earth elements increase. Cationic field strength (CFS) of the rare earth elements is defined as CFS = Z/r2, where Z is the valence of the corresponding elements and r is its ionic radius. Ionic radius at the given valence state depend upon the coordination number, which was reported by Shannon [5]. Table 1 shows the CFS values of the rare earth elements, which are inversely proportional to the ionic radii square of the elements. It causes consequent variations in the various properties, such as greater hardness, greater thermal characterization and elastic modulus, especially, higher chemical durability [24,25]. The thermal characteristics of all samples, which are the glass transition (Tg) and the first crystallization peak (Tr), are presented in Table 1. The Tg values are improved by rare earth addition, which is mainly due to the increase of cross-linking density and bonding strength of the structure owing to the incorporation of the mixed oxides. 8
Several studies have shown that the changes of Tg values are consistent with the density of the studied glasses [26,27]. However, Table 1 doesn’t show that the increase of Tg is attributed to the changes of density. On the other way, the difference between the glass transition temperature (Tg) and the onset crystallization temperature (Tr ǻ7 7r í Tg, has been frequently used as a rough estimate of glass formation ability or glass thermal stability [26]. The base glass demonstrates good glass-forming tendency or thermal stability with the temperature GLIIHUHQFHǻ7 § 127 °C, which is larger than the rare earth glass. The rare earth glass exhibits lower exothermic behavior than the base glass, which is consistent with lower thermal stability. Table 1 shows that the ǻ7 YDOXHV are seen to increase with the increase of CFS (except Nd-doped glass). Lofaj et al. [25] showed that Nd-doped silicon glass had higher Tg and thermal stability than other rare earth glasses. Therefore, it is then deduced that larger cation filed sterength increased structural rigidity, and so increasing thermal stability. 3.2. Leachate analyses The initial pH values of the deionized water before corrosion was 5.60. Table 2 shows that the final pH values of all samples decreases. This is attributed to the dissolution of phosphorus from the glass, which formed phosphoric acid [28]. In addition, the decrease of pH values for the base glass (from 5.60 to 1.85) is larger than the one for the rare earth glass. Yu et al. [27] thought that the change of pH was related to the chemical durability of the glass network. Therefore, rare earth elements addition can improve the chemical durability of glasses, and the reduction in pH 9
values is consistent with the concentration of rare earth ions in leachates. The effect of CFS on the concentration of different rare earth ions in leachate is shown in Fig. 1. The concentration of rare earth ions is seen to decrease with the increase of CFS. In other words, the smaller ionic radii of rare earth with the same coordination number, the larger cationic field strength (CFS), and so the better the chemical stability of the rare earth glass will be. Similar behavior was observed by Yin et al. [29]. The normalized elemental mass release rate of the studied glasses in deionized water at 90 °C for 7 days is shown in Fig. 2. After a 7-day test period at 90 °C, the normalized elemental mass release rate of lanthanum ions (rLa) with the highest concentration La-doped glass was as low as ~2.19 ɯ 10-7 g/m2/min for iron phosphate glass waste forms [30]. Moreover, the release rate of rare earth ions in leachate decrease with the increase of cationic field strength. Small amounts, usually <20 ȝJāP/-1 Ca and Fe were in leachate from the samples containing CaO and Fe2O3 (Table 2). Compared to the base glass, the normalized release rate of calcium ions by rare earth addition decrease obviously in Fig. 2 (except Nd-doped glass). Brow et al. [31] found that the Ca–O–P bond is weaker than the Fe–O–P bond. Leachate analyses show that the normalized release rate of calcium ions in leachate is higher than iron ions. In addition, divalent cations Ca2+ act as a glass modifier, which could serve as (P–O- Ca2+ -O–P) ionic cross-links, they are easier to hydrolyze [10,32]. The presence of rare earth (Y, La, Sm and Gd) reduces the release of Ca ions, but the introduction of Nd increases the release of Ca. It may be due to that Nd element in the phosphate glass melts exists the form of Nd4+ ions. 10
The decrease of the normalized release rate of Ca ions could be explained by the presence of rare earth which probably increases the compactness and connection of the glass structure. 3.3. Infrared spectroscopy studies FTIR spectroscopy of different rare earth phosphate glasses before and after corrosion is given in Fig. 3(a) and (b). In the base glass, the main features of FTIR spectra are eight bands at ~1618, ~1256, ~1061, ~920, ~777, ~698, ~553 and ~487 cm-1, respectively (Fig. 3(a)). The weak band at ~1618 cm-1 reflected the bending vibrations of P–OH bonds [33]. The new band at ~1734 cm-1 in the Y-doped glass may be assigned to water-bending mode [33,34]. The band at ~1256 cm-1 is assigned to the asymmetric stretching vibration of the P=O bonds, similar band at ~1235 cm-1 was observed in Nd-doped glass [34,35]. The band position has an obvious red shift with the addition of rare earth, the new band at ~1162 cm-1 is assigned to the terminal (PO2)- groups of Q2 units [36]. It is then deduced that the rare earth ions act as a glass modifier to the breaking of the P=O bonds (Q3) and may be to form RE–O–P bonds (Q1). The strong absorption bands at ~1061, ~920 and ~777 cm-1 can correspond to the symmetric stretching vibrations of (PO4)3- tetrahedra (PO- ionic group) in Q0 units [35], symmetric stretching modes of P–O–P bonds [37] and asymmetric modes of the P–O–P linkages in Q1 units [35,37]. The another weak absorption band at ~698 cm-1 is assigned to the asymmetric vibration of P–O–P linkages, similar bands was observed in Fe2O3–PbO–P2O5 glasses by Doweidar et al. [37]. The weak shoulder 11
peak at ~553 cm-1 is assigned to the bending mode of O–P–O in Q1 units, while the low-frequency absorption band at ~487 cm-1 may be assigned to harmonics of bending vibration of O=P–O linkages in Q3 units [35,37]. The two bands are merged into one band at ~527 cm-1 with the addition of rare earth, which is assigned to the symmetric stretching vibrations of (PO4)3- in Q0 units [37]. It suggests that rare earth addition strengthens the crosslinking of the glass network by creating more NBOs. Compared to the FTIR spectra of phosphate glasses before corrosion, Fig. 3(b) shows that the O=P–O linkages at ~484 cm-1 and the P–O–P linkages at ~698 cm-1 in the base glass disappears or decreases in intensity, the (PO4)3- of Q0 units at ~503 cm-1 appears after corrosion. Bunker et al. [10] showed that the corrosion process depended on chain hydrolysis, and hydroxyl ions attacked the long phosphate chains, to released orthophosphate anions. Brow et al. [31] studied alkaline corrosion resistant properties of calcium iron phosphate glasses by infrared spectroscopy. It showed that the samples formed (PO4)3- and (P2O7)4- surface species after corrosion. In the rare earth glasses, the bands at ~1162 cm-1 shifts to lower frequency after corrosion, which is due to that the Ca ions act as a glass modifier, which breaks up the P–O–P linkages creating NBOs by forming (P–O- Ca2+ -O–P) ionic cross-links [10,11,37]. It leads to that the Q2 units are easier to hydrolyze [10]. The special bands at ~1115 cm-1 in Sm-doped glass before corrosion may be is assigned to the (PO4)3- of Q0 units and (PO2)- of Q2 units, it shifts to the low frequency bands at ~1095 cm-1 ((PO4)3- of Q0 units) after corrosion. Compared to the base glass before and after corrosion, the spectrum shapes of rare earth glasses do not show a clear change. It is 12
attributed to the replacement of easy hydrated P–O–P bonds by more chemically resistant RE–O–P bonds with the addition of rare earth, in agreement with the chemical durability analyses. The quantitative analyses of FTIR spectra by using the deconvolution method can determine the relative concentration of structural units and thus, to analyze the modified role of rare earth oxides. The deconvoluted FTIR spectra of the base glass after corrosion are shown in Fig. 4. The new band at ~1375 cm-1 is assigned to the asymmetric stretching vibration of the P=O bonds [35]. The new bands at ~1033 and ~720 cm-1 can correspond to the symmetric stretching vibration of (PO3)2- and the symmetric stretching vibration of P–O–P rings in Q1 units [35-37]. While that at ~584 cm-1 may be assigned to the symmetric stretching vibration of M–O–P bonds [37]. Bingham et al. [38] showed that rings or chains linked P–O–P bonds were susceptible to hydrolyze in corrosion. Table 3 shows that P–O–P linkages decrease slightly in the base glass and the rare earth (La, Y) phosphate glasses after corrosion, and the P–O–P rings of the base glass were easier to hydrolyze than the rare earth (La, Y) phosphate glasses. It may be due to that rare earth ions breaks up the P–O–P rings by forming the RE–O–P bonds [31,37]. Too many polyphosphate chains are released into solution, which leads to the solution pH to drop, this hydrolysis continues, the glass continues to dissolve and a lower pH of the solution will accelerate the ion exchange process as well as the direct hydrolysis of P–O–P rings [37,39]. Therefore, the lower pH of the base glass in leachate analyses will release more phosphoric acid into solution. 13
Hydrolysis and leaching usually act simultaneously in glass corrosion. Hydrolysis according to the following reaction: P–O–P + H2O l 2P–OH, it can rupture directly the P–O–P to destroy the main network structure of glass [10,15]. Leaching according to the following reaction: M–O–P + H2O l P–OH + M–OH, the exchange between hydrogen and glass network modifiers weakens the structural stability of phosphate glass [15,40]. The decline of M–O–P bonds in the base glass is larger than the rare earth glasses after corrosion, and the P–OH bonds have relatively higher fraction. It is then deduced that chemical durability of phosphate glasses was improved with the addition of rare earth. Several studies have shown that RE3+ acts as network modifiers, producing more nonbridging oxygen as its cationic field strength increases, these NBOs are favorable sites for slowing down the corrosion mechanism [41,42]. Therefore, Y-doped phosphate glass could produce more NBOs by the formation of the RE–O–P bonds than La-doped glass, which leads to the best corrosion resistance with the Y-doped phosphate glass. 3.4. Raman spectroscopy Raman spectroscopy is able to reveal vibrational mechanisms inside materials in different phases. One advantage of this approach is non-destructive and non-contact, which assures the integrity of the material and permits long distance detection [43,44]. The Raman spectra, in the frequency region 400–1600 cm-1, of different rare earth phosphate glasses before corrosion (B) and after corrosion (A) are shown in Fig. 5. The Raman spectra of the base glass before corrosion, Fig. 5a, shows that the two 14
prominent bands at 1065 and 1205 cmí1 can correspond to the symmetric stretching mode of (PO3)2- bonds in Q1 units and symmetric stretching mode of ‘strained’ (PO2)(strained structural units, possibly three- or four-membered rings) in Q2 units, respectively [34,36]. The ‘strained’ (PO2)- bonds after corrosion shifts slightly to lower frequency and decrease obviously in intensity, in agreement with infrared spectroscopy studies. Silva et al. [45] showed that divalent cations Ca2+ acted as a network modifier, which could serve as (P–O- Ca2+ -O–P) ionic cross-links in the metaphosphate glass. Therefore, the metaphosphate chains exhibited lower resistance to aqueous corrosion. The higher frequency band at ~1295 cm-1 is assigned to the P=O symmetric stretching [34]. The band at ~1130 cm-1 is assigned to phosphorus-oxygen stretching modes in Q1 phosphate chain terminator units formed by the scission of phosphate chains [46]. It is observed that the bands at ~524 and ~706 cm-1 is diminished after corrosion tests, which can correspond to (P2O7)4- groups of Q1 units and P–O–P bonds of Q2 units, respectively [46,47]. The new bands at ~600, ~787, ~884 cm-1 and a weak band at ~961 cm-1 can correspond to the symmetric stretching mode of P–O–P bonds in Q2 units [45], the symmetric stretching mode of P–O–P bonds in Q1 units [26,45,47], the asymmetric stretching mode of P–O–P bonds and nonbridging (PO4)3- oxygen ions in Q0 units [48]. The appearance of these bands, especially, isolated (PO4)3- groups, exhibits the depolymerization of the base glass network after corrosion. In Fig. 5e, the most notable change with rare earth addition is the increase of (PO3)2- bonds in frequency, and ‘strained’ (PO2)- shifts markedly the lower-frequency 15
band at ~1160 cm-1, which is assigned to the symmetric stretching mode of (PO2)groups in Q2 units [45]. The change of frequency is attributed to structural rearrangements in the main phosphate network due to the replacing of P–O–P and P=O (Q3) by RE–O–P bonds (Q1) [34]. The red shift of the (PO3)2- bonds, resulting from the formation of the RE–O–P bonds, is related to an increased quantity of NBOs. Peak positions and assignments of Raman spectra of different rare earth phosphate glasses before and after corrosion are shown in Table 4. The (PO4)3- groups shift mostly to higher frequencies and increase in intensity after corrosion. It may be due to a penetration process of intact water molecular towards glass network, which leads to the breaking of the P–O–P bonds and the formation of (PO4)3- groups [39,40]. The (PO3)2- groups of Q1 units shift to lower frequencies, which may be attributed to that hydration of the glass network results in the breakage of P–O–P linkages and rings in Q1 units in aqueous solution. The (PO2)- groups shifts mostly to lower frequencies and decrease in intensity after corrosion. Moreover, the (PO2)- for La-doped phosphate glass after corrosion closes to phosphorus-oxygen bond for the RE0 glass in Fig. 5a. It indicates that the ions exchange in corrosion will destroy the cross-link structure, which causes the dissociation of phosphate chains. 4. Conclusions The thermal and structural stability of CaO–Fe2O3–P2O5 glasses doped with rare earth oxides (Y2O3, La2O3, Nd2O3, Sm2O3, Gd2O3) in deionized water was investigated. X-ray diffraction analysis indicated that the base glass and the glasses doped 10 mol% rare earth remained amorphous before and after corrosion. DSC 16
results showed that thermal stability of the rare earth phosphate glasses increased with the increase of CFS. Leachate analyses showed that the release rate of rare earth ions in leachate decreased with the increase of its CFS and rare earth addition reduced the release of Ca ions. The decrease of pH for the base glass was larger than the rare earth glasses by the formation of phosphoric acid. Both Raman and IR spectra indicated that metaphosphate chains were easier to hydrolyze, which converted to orthophosphate units after corrosion. It was attributed to that Ca2+ ions could serve as (P–O- Ca2+ -O–P) ionic cross-links. The decline of M–O–P bonds in the base glass is larger than the rare earth glasses after corrosion, and the P–OH bonds have relatively higher fraction. The formation of more chemically resistant RE–O–P bonds with the addition of rare earth leaded to an increased quantity of nonbridging oxygen, which causes consequent variations in the chemical durability of rare earth glasses. Acknowledgments This work was supported by the Science Foundation of Southwest University of Science and Technology (11zx7157), the Scientific Research Fund of SiChuan Provincial Education Department (14ZA0105, 14ZD1122) and Postgraduate Innovation Fund Project by Southwest University of Science and Technology (14ycxjj0018). References [1] Y.H. Zhang, A. Navrosky, H. Li, L.Y. Li, L.L. Davis, D.M. Strachan, J. Non-Cryst. Solids 296 (2001) 93–101. [2] G.M. Tao, H.T. Guo, L. Feng, M. Lu, W. Wei, B. Peng, J. Am. Ceram. Soc. 92 17
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[42] J.S. Wu, J.F. Stebbins, J. Non-Cryst. Solids 362 (2013) 73–81. [43] L. Lyon, C. Keating, A. Fox, B. Baker, L. He, S. Nicewarner, S. Mulvancy, M. Natan, Anal. Chem. 70 (1998) 341R–361R. [44] H.Y. Mi, S. Mikael, T. Allen, K. Sridharan, D. Butt, J.P. Blanchard, Z.Q. Ma, J. Nucl. Mater. 445 (2014) 7–11. [45] A.M.B. Silva, R.N. Correia, J.M.M. Oliveira, M.H.V. Fernandes, J. Eur. Ceram. Soc. 30 (2010) 1253–1258. [46] R.K. Brow, D.R. Tallant, S.T. Myers, C.C. Phifer, J. Non-Cryst. Solids 191 (1995) 45–55. [47] Y.M. Lai, X.F. Liang, S.Y. Yang, J.X. Wang, B.T. Zhang, J. Mol. Struct. 1013 (2012) 134–137. [48] A. Chahine, M. Et-tabirou, J.L. Pascal, Mater. Lett. 58 (2004) 2776– 2780.
21
Figure Item Fig. 1 The effect of cationic field strength (CFS) on the concentration of different rare earth ions in leachate. Fig. 2 The normalized elemental mass release rate of the studied glasses in deionized water at 90 °C for 7 days. Fig. 3 FTIR spectra of different rare earth phosphate glasses (a) before corrosion and (b) after corrosion. Fig. 4 Deconvoluted FTIR spectra of the base glass after corrosion using a Gaussian-type function. Fig. 5 Raman spectra of different rare earth phosphate glasses before corrosion (B) and after corrosion (A) from the samples : RE0 (a), La10 (b), Nd10 (c), Sm10 (d), Gd10 (e) and Y10 (f). Table Item Table 1 The density, mole volume and DSC parameters of the studied glasses and cationic field strength (CFS) of the corresponding rare-earth element. Table 2 The results of leachate analyses performed in deionized water at 90 °C for 7 days. Table 3 Deconvolution parameters (the band centers and the relative area) and the bands assignments of rare earth phosphate glasses before corrosion (B) and after corrosion (A). Table 4 Peak positions and assignments of Raman spectra of different rare earth phosphate glasses before corrosion (B) and after corrosion (A). 22
Table 1 The density, mole volume and DSC parameters of the studied glasses and cationic field strength (CFS) of the corresponding rare-earth element. Physical parameters
RE0
Y10
La10
Nd10
Sm10
Gd10
CFS (Å-2)
0
3.703
2.817
3.105
3.269
3.410
D (g/cm3)
2.82
3.01
3.22
3.26
3.29
3.33
Vm (cm3/mol)
47.94
47.92
47.90
47.64
47.58
47.42
Tg (°C)
570
574
576
584
578
574
Tr (°C)
697
669
663
681
666
666
Tr-Tg (°C)
127
95
87
97
88
92
Table 2 The results of leachate analyses performed in deionized water at 90 °C for 7 days.
Glass composition
Initial pH
Final pH
Concentration of ions in leachates ȝJāP/-1) [RE]
[Ca]
[Fe]
RE0
5.60
1.85
0
15.368
10.946
Y10
5.60
3.16
0.056
13.878
11.275
La10
5.60
2.03
0.715
8.120
6.240
Nd10
5.60
2.09
0.657
17.393
12.178
Sm10
5.60
2.12
0.551
14.748
13.802
Gd10
5.60
2.34
0.524
14.517
12.526
Table 3 Deconvolution parameters (the band centers and the relative area) and the bands assignments of rare earth phosphate glasses before corrosion (B) and after corrosion (A). Range frequency
RE0
Assignments
-1
(cm )
La10
Y10
B (%)
A (%)
B (%)
A (%)
B (%)
A (%)
3
0
462~475
O=P–O linkages (Q )
3.57
2.46
2.74
2.81
2.21
2.66
515~526
(PO4)3-sym stretch
5.81
6.42
5.60
7.96
6.53
6.25
579~592
M–O–P bonds
2.84
2.54
2.30
2.13
2.64
2.56
712~727
(P–O–P)sym stretch in rings
3.38
2.83
1.81
2.23
2.40
2.56
769~782
(P–O–P)asym stretch in linkages
2.96
2.07
3.34
3.01
2.72
2.56
(Q )
1
904~920
(P–O–P)sym stretch (Q )
15.00
13.10
17.18
15.94
14.79
13.97
1028~1041
(PO3)2-sym stretch (Q1)
16.83
16.83
17.62
17.03
15.01
17.76
1139~1152
(PO2)-asym
12.33
12.87
18.78
15.99
13.23
14.18
1256~1263
(P=O)sym stretch
15.00
17.64
13.68
12.24
10.11
15.72
1368~1396
(P=O)sym stretch
12.70
10.67
12.11
14.39
19.50
11.37
1581~1632
P–OH bonds
9.56
12.59
4.84
6.11
9.44
10.18
2
stretch (Q )
Table 4 Peak positions and assignments of Raman spectra of different rare earth phosphate glasses before corrosion (B) and after corrosion (A). P–O–P bonds
(PO4)3- groups
(PO3)2- groups
B
A
B
A
B
A
B
A
La10
716
726
944
960
1089
1077
1148
1140
Nd10
708
716
933
924
1085
1075
1163
1151
Sm10
736
715
943
944
1097
1076
1166
1152
Gd10
713
716
934
954
1098
1079
1160
1164
Y10
719
722
949
951
1090
1078
1167
1152
Glass composition
(PO2)- groups
Fig. 1. The effect of cationic field strength (CFS) on the concentration of different rare earth ions in leachate.
Fig. 2. The normalized elemental mass release rate of the studied glasses in deionized water at 90 °C for 7 days.
Fig. 3. FTIR spectra of different rare earth phosphate glasses (a) before corrosion and (b) after corrosion.
Fig. 4.Deconvoluted FTIR spectra of the base glass after corrosion using a Gaussian-type function.
Fig. 5. Raman spectra of different rare earth phosphate glasses before corrosion (B) and after corrosion (A) from the samples : RE0 (a), La10 (b), Nd10 (c), Sm10 (d), Gd10 (e) and Y10 (f).
Highlights ڼThe thermal and structural stability of RE2O3–CaO–Fe2O3–P2O5 were studied. ڼ
Thermal stability of glass increased with the increase of cationic field strength.
ڼ
Release rate of rare earth ions decreased with the increase of its field strength.
ڼ
Metaphosphate chains by the cross-links of calcium ions were easier to hydrolyze.
ڼ
The RE–O–P bonds have more chemical resistance by producing non-bridging oxygen.
S0022-2860(14)00862-X http://dx.doi.org/10.1016/j.molstruc.2014.08.032 MOLSTR 20892
To appear in:
Journal of Molecular Structure
Received Date: Revised Date: Accepted Date:
1 July 2014 29 July 2014 18 August 2014
Please cite this article as: H. Li, X. Liang, C. Wang, H. Yu, Z. Li, S. Yang, Influence of rare earth addition on the thermal and structural stability of CaO–Fe2O3–P2O5 glasses, Journal of Molecular Structure (2014), doi: http:// dx.doi.org/10.1016/j.molstruc.2014.08.032
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of rare earth addition on the thermal and structural stability of CaO–Fe2O3–P2O5 glasses Haijian Li a,b, Xiaofeng Liang a,b, *,1, Cuiling Wang a,b, Huijun Yu a,b, Zhen Li b, Shiyuan Yang b a
Analytical and Testing Center, Southwest University of Science and Technology,
Mianyang 621010, PR China b
State Key Laboratory Cultivation Base for Nonmetal Composites and Functional
Materials, Southwest University of Science and Technology, Mianyang 621010, PR China Abstract The thermal and structural stability of calcium iron phosphate glasses doped with rare earth oxides has been studied by investigating differential scanning calorimetry, the concentration of modified ions in leachate, Fourier transform infrared spectra, and Raman spectra. The results showed that thermal stability of the rare earth phosphate glasses increased and the release rate of rare earth ions in leachate decreased with the increase of cationic field strength. It related to the known structural units and characteristic bands of these glasses detected by infrared and Raman spectra. Metaphosphate chains by the cross-links of calcium ions and P–O–P linkages were easier to hydrolyze, which converted to orthophosphate units after corrosion. The formation of more chemically resistant RE–O–P bonds with the addition of rare earth leaded to an increased quantity of nonbridging oxygen, which caused consequent variations in the chemical durability of rare earth glasses. * Corresponding author. Tel.: +86 0816-6089507. E-mail addresses: [email protected] (X.F. Liang) 1 Co-first authors. 1
Keywords: Phosphate glass; Rare earth; Thermal stability; FTIR spectra; Raman spectra 1.
Introduction In radioactive nuclear glasses, the rare earths (RE) are present as waste forms in
the glass matrix. For example, gadolinium with low non-radiative decay rate and high thermal neutron capture cross-section may be added to the final actinide-containing glasses to minimize the likelihood of criticality during the storage period [1,2]. On the other hand, rare earth elements are also present as surrogates for the heaviest transuranic elements occurring in High Level Waste (HLW) such as Cm and Am [3,4]. They have cation radii r similar to the one of rare earth ions (for instance in sixfold coordination: r(Am3+) = 1.01 Å, r(Cm3+) = 0.98 Å and r(Nd3+) = 0.995 Å [5]). Wang et al. [6,7] have reported the corrosion behavior of soda lime silicate glasses containing about 1.0 mol% rare earths (Y2O3, La2O3, CeO2, Nd2O3 and Gd2O3), and they pointed out that the formation of a hydrated layer depleted the dissolution rate of rare earth ions. Phosphate glasses have been investigated for immobilization in high-level waste for over 30 years [8]. The glass has lower melting temperatures and higher waste loading capacities than borosilicate glass. However, phosphate glasses have relatively low chemical durability. Several studies have shown that the chemical durability of phosphate glasses can be improved by the addition of various oxides such as calcium oxide and, especially, ferric oxide [8-10]. It has been suggested that Fe3+ cations can enter in the glass network with four-fold coordination and form stable POFe 2
covalent bonds and divalent cations like Ca2+ can form POCa covalent bonds and serve as ionic crosslinks between the nonbridging oxygens of two different phosphate chains, and accordingly increase chemical durability [10-12]. Corrosion mechanism of glass has received attention for a long time [13,14]. It is known that an ionic exchange takes place between mobile ions such as alkali earth ions and H+, and the water molecules diffuse into the glass, react with P–O–P and P–O–M
(where M = metal cation), forming P–OH groups [10,15]. Hamilton et al.
[16] found that the aqueous corrosion behavior of glasses was related to the concentration of nonbridging oxygen (NBO) sites in the glass network structure. Vibrational spectroscopy has been employed to investigate the structure of glasses and specifically identification of the main structural units, which are connected through nonbridging oxygen (NBO) and bridging oxygen (BO) [17]. Therefore, vibrational spectroscopy is a valuable tool for studying the changes of glass structure before and after aqueous corrosion. Palavit et al. [18] examined the chemical reactions between P2O5–ZnO–H2O ultraphosphate glasses and water by
31
P nuclear magnetic resonance, and the
microstructure of the devitrified glass consisted of crystalline Zn2P4O12. Banerjee et al. [19] studied the structural aspects of caesium borosilicate glasses with varying amounts of CaO before and after leaching by using infrared spectroscopy. The reduction of the peak intensity at Si–OH and B–OH bonds showed that the chemical durability of the glasses was improved significantly as CaO was increased at the expense of B2O3 content in the glass. Molières et al. [20] showed that the role of 3
lanthanum in the alteration kinetics of borosilicate glass at different degrees of leaching progress was investigated by Raman spectroscopy. After leaching, the Raman spectrum of the glass exhibited the presence of highly depolymerized entities. The chemical durability of phosphate glasses was investigated by the dissolution rate of ions in previous studies [10,15]. However, little systematic work in such glasses with rare earth addition has been done to identify the structural units before and after corrosion, to follow the structural changes and to correlate these changes with the corrosion mechanisms of glass. Therefore, the aim of this paper is to focus on the effect of rare earth oxides on the structural stability of CaO–Fe2O3–P2O5 glasses before and after aqueous corrosion by FTIR spectra and Raman spectra, and the thermal stability of the glass was investigated by differential scanning calorimetry (DSC). 2. Experiment 2.1. Preparation of glass samples The glass system of xRE2O3–(100–x)(12CaO–20Fe2O3–68P2O5) (x = 10 mol%) with RE = Y, La, Nd, Sm and Gd were prepared using a standard melt-quench technique. After thorough mixing the powders were introduced in corundum crucibles, in order to prevent the excess boiling and consequent spillage, water and ammonia in ammonium phosphate monobasic were removed initially by preheating it at 220 °C for about 2h and then the electric furnace was raised to 1250 °C (heating rate was 10 °CÂPLQ-1), and samples were melted at 1250 °C for 3h. The melts were then poured into preheated steel molds, and moved quickly to an annealing furnace, annealed at 4
475 °C for 2h and cooled down to room temperature more than 12h. Samples for property measurements were ground to give fine powder. 2.2. Chemical durability tests The chemical durability of glasses was determined according to Product Consistency Test B (PCT–B) in ASTM C1285-02, using 3 g of glass powder (100 to +200 mesh) in 30 ml deionized water. Tests were carried out in duplicate at 90 °C for 7 days. The rare earth elements in the leachate were measured by inductively coupled plasma mass spectrometry (ICP-MS) using the Agilent 7700x (Agilent, U.S.A), and the calcium and iron elements were measured by inductively coupled plasma (ICP) emission spectroscopy using the iCAP 6500 (Thermo Fisher, U.S.A). Solution pH was measured with a calibrated KEDIDA CT-6023 pH meter. The normalized elemental mass release rate, ri, was calculated from the equation: r =
C f (AV)t
where ri is the normalized elemental mass release (g/m2/min) of element i, Ci the concentration of element i in solution (µg/ml or g/m3) , fi the mass fraction of element i in the glass (unitless) and t the time duration of test in minutes (min). A/V is the ratio of the sample surface area to volume of leachate (m-1), a value of 1800 m-1 was used. All tests were made in duplicate and the error in the average reported herein was estimated at ±15%. 2.3. Structural investigation X-ray diffraction (XRD) analysis was performed on samples employing a X-ray diffractometer (PANalytical X’Pert PRO, The Netherlands). The 2Q scans were made 5
between 5° and 80° with step width of 0.03° and utilized Cu KA radiation (Ȝ Å). Preparation of the samples used a simple top pack loading method for an acquired smooth surface [21]. The density, D, of each glass was measured at room temperature using the Archimedes method with water as an immersing liquid. The sample weights varied between 3 and 4 g, and the measured densities were reproducible within 0.01 g·cm-3. The molar volume (Vm) was calculated using the relation Vm £(xM)/D, where x is the molar fraction and M is the total molecular weight of the component. The glass transition temperature (Tg) and first crystallization temperature (Tr) were measured on DSC by utilizing a SDT Q600 instrument (TA, USA) in a flowing air atmosphere at a heating rate of 20 °CÂmin-1. The temperature scanned over a range from room temperature to 800 °C and the estimated error in Tg and Tr were ±2 °C. The infrared spectra of the samples were measured from 400 to 2000 cm-1 using a Spectrum One FTIR spectrometer (Perkin Elmer, USA) and the KBr standard pellet method. Glass pellets were prepared by mixing about 2 mg powder with 200 mg dried KBr powder and compressing the resulting mixture in an evacuated die. The accuracy of this technique is estimated to be ± 4 cm-1. Because the majority of the bands in the infrared spectra are broad and asymmetric, presenting also some shoulders, a deconvolution of the experimental spectra was necessary. This was made with ORIGIN 7.5 program (Originlab Corp.) using a Gaussian type function and allowed us a better identification of all the bands which appear in these spectra and their assignments. The proportion of particular 6
structures corresponding to different vibration modes, was calculated from the areas of the fitted Gaussian bands divided by the total area of all bands. The two parameters of each band (peak frequency and relative areas) were allowed to be variable during the iterations. Raman spectra at 400–1600 cm-1 were collected from glass powders using the InVia Raman Microscope (Renishaw, U.K.) at room temperature. The Raman spectra were measured by excitation light of 514.5 nm light from an argon ion laser. The spectral resolution was about 1~2 cm-1 and the wavenumber accuracy was 0.2 cm-1. Six multiple measurements per sample were done to check for the potential micron-range heterogeneity and the effects of sample orientation; both have not been found, as it would be expected for a nearly homogeneous glass sample. 3. Results and discussion 3.1. XRD, DSC and density measurements The crystallization tendency of phosphate glasses adding rare earth elements is related to the high amount of rare earth ions that act as nucleating agent in the calcium iron phosphate glass. In our previous paper, the investigation on structural aspects of iron phosphate glasses containing neodymium oxide prepared with traditional melt-quenching methods was carried out by XRD, infrared and Raman spectra [21]. It showed that excess neodymium oxide will induce the crystallization of the phosphate glass, which was disadvantageous for the vitrification of high-level radioactive wastes and was often assumed to reduce the chemical durability [22]. X-ray diffraction analysis indicated that base glass for undoped rare earths (RE0) and doped 10 mol% 7
rare earth glasses were amorphous before and after corrosion. Hence, these glasses could contain abundant the surrogates of transuranic elements and remained amorphous after corrosion. Table 1 showed that the density increase from 2.82 to 3.33 g/cm3 as the atomic number of the rare earth elements increase. This increase is primarily due to the increased atomic weights of the rare earth cation. The molar volume decrease slightly as the atomic weight of the rare earth elements increase. It is mainly attributed to that the higher molecular weight of rare earth decrease the total number of oxygen atoms [23]. This decrease in molar volume indicates that the phosphate glass network becomes more compact as the atomic weight of the rare earth elements increase. Cationic field strength (CFS) of the rare earth elements is defined as CFS = Z/r2, where Z is the valence of the corresponding elements and r is its ionic radius. Ionic radius at the given valence state depend upon the coordination number, which was reported by Shannon [5]. Table 1 shows the CFS values of the rare earth elements, which are inversely proportional to the ionic radii square of the elements. It causes consequent variations in the various properties, such as greater hardness, greater thermal characterization and elastic modulus, especially, higher chemical durability [24,25]. The thermal characteristics of all samples, which are the glass transition (Tg) and the first crystallization peak (Tr), are presented in Table 1. The Tg values are improved by rare earth addition, which is mainly due to the increase of cross-linking density and bonding strength of the structure owing to the incorporation of the mixed oxides. 8
Several studies have shown that the changes of Tg values are consistent with the density of the studied glasses [26,27]. However, Table 1 doesn’t show that the increase of Tg is attributed to the changes of density. On the other way, the difference between the glass transition temperature (Tg) and the onset crystallization temperature (Tr ǻ7 7r í Tg, has been frequently used as a rough estimate of glass formation ability or glass thermal stability [26]. The base glass demonstrates good glass-forming tendency or thermal stability with the temperature GLIIHUHQFHǻ7 § 127 °C, which is larger than the rare earth glass. The rare earth glass exhibits lower exothermic behavior than the base glass, which is consistent with lower thermal stability. Table 1 shows that the ǻ7 YDOXHV are seen to increase with the increase of CFS (except Nd-doped glass). Lofaj et al. [25] showed that Nd-doped silicon glass had higher Tg and thermal stability than other rare earth glasses. Therefore, it is then deduced that larger cation filed sterength increased structural rigidity, and so increasing thermal stability. 3.2. Leachate analyses The initial pH values of the deionized water before corrosion was 5.60. Table 2 shows that the final pH values of all samples decreases. This is attributed to the dissolution of phosphorus from the glass, which formed phosphoric acid [28]. In addition, the decrease of pH values for the base glass (from 5.60 to 1.85) is larger than the one for the rare earth glass. Yu et al. [27] thought that the change of pH was related to the chemical durability of the glass network. Therefore, rare earth elements addition can improve the chemical durability of glasses, and the reduction in pH 9
values is consistent with the concentration of rare earth ions in leachates. The effect of CFS on the concentration of different rare earth ions in leachate is shown in Fig. 1. The concentration of rare earth ions is seen to decrease with the increase of CFS. In other words, the smaller ionic radii of rare earth with the same coordination number, the larger cationic field strength (CFS), and so the better the chemical stability of the rare earth glass will be. Similar behavior was observed by Yin et al. [29]. The normalized elemental mass release rate of the studied glasses in deionized water at 90 °C for 7 days is shown in Fig. 2. After a 7-day test period at 90 °C, the normalized elemental mass release rate of lanthanum ions (rLa) with the highest concentration La-doped glass was as low as ~2.19 ɯ 10-7 g/m2/min for iron phosphate glass waste forms [30]. Moreover, the release rate of rare earth ions in leachate decrease with the increase of cationic field strength. Small amounts, usually <20 ȝJāP/-1 Ca and Fe were in leachate from the samples containing CaO and Fe2O3 (Table 2). Compared to the base glass, the normalized release rate of calcium ions by rare earth addition decrease obviously in Fig. 2 (except Nd-doped glass). Brow et al. [31] found that the Ca–O–P bond is weaker than the Fe–O–P bond. Leachate analyses show that the normalized release rate of calcium ions in leachate is higher than iron ions. In addition, divalent cations Ca2+ act as a glass modifier, which could serve as (P–O- Ca2+ -O–P) ionic cross-links, they are easier to hydrolyze [10,32]. The presence of rare earth (Y, La, Sm and Gd) reduces the release of Ca ions, but the introduction of Nd increases the release of Ca. It may be due to that Nd element in the phosphate glass melts exists the form of Nd4+ ions. 10
The decrease of the normalized release rate of Ca ions could be explained by the presence of rare earth which probably increases the compactness and connection of the glass structure. 3.3. Infrared spectroscopy studies FTIR spectroscopy of different rare earth phosphate glasses before and after corrosion is given in Fig. 3(a) and (b). In the base glass, the main features of FTIR spectra are eight bands at ~1618, ~1256, ~1061, ~920, ~777, ~698, ~553 and ~487 cm-1, respectively (Fig. 3(a)). The weak band at ~1618 cm-1 reflected the bending vibrations of P–OH bonds [33]. The new band at ~1734 cm-1 in the Y-doped glass may be assigned to water-bending mode [33,34]. The band at ~1256 cm-1 is assigned to the asymmetric stretching vibration of the P=O bonds, similar band at ~1235 cm-1 was observed in Nd-doped glass [34,35]. The band position has an obvious red shift with the addition of rare earth, the new band at ~1162 cm-1 is assigned to the terminal (PO2)- groups of Q2 units [36]. It is then deduced that the rare earth ions act as a glass modifier to the breaking of the P=O bonds (Q3) and may be to form RE–O–P bonds (Q1). The strong absorption bands at ~1061, ~920 and ~777 cm-1 can correspond to the symmetric stretching vibrations of (PO4)3- tetrahedra (PO- ionic group) in Q0 units [35], symmetric stretching modes of P–O–P bonds [37] and asymmetric modes of the P–O–P linkages in Q1 units [35,37]. The another weak absorption band at ~698 cm-1 is assigned to the asymmetric vibration of P–O–P linkages, similar bands was observed in Fe2O3–PbO–P2O5 glasses by Doweidar et al. [37]. The weak shoulder 11
peak at ~553 cm-1 is assigned to the bending mode of O–P–O in Q1 units, while the low-frequency absorption band at ~487 cm-1 may be assigned to harmonics of bending vibration of O=P–O linkages in Q3 units [35,37]. The two bands are merged into one band at ~527 cm-1 with the addition of rare earth, which is assigned to the symmetric stretching vibrations of (PO4)3- in Q0 units [37]. It suggests that rare earth addition strengthens the crosslinking of the glass network by creating more NBOs. Compared to the FTIR spectra of phosphate glasses before corrosion, Fig. 3(b) shows that the O=P–O linkages at ~484 cm-1 and the P–O–P linkages at ~698 cm-1 in the base glass disappears or decreases in intensity, the (PO4)3- of Q0 units at ~503 cm-1 appears after corrosion. Bunker et al. [10] showed that the corrosion process depended on chain hydrolysis, and hydroxyl ions attacked the long phosphate chains, to released orthophosphate anions. Brow et al. [31] studied alkaline corrosion resistant properties of calcium iron phosphate glasses by infrared spectroscopy. It showed that the samples formed (PO4)3- and (P2O7)4- surface species after corrosion. In the rare earth glasses, the bands at ~1162 cm-1 shifts to lower frequency after corrosion, which is due to that the Ca ions act as a glass modifier, which breaks up the P–O–P linkages creating NBOs by forming (P–O- Ca2+ -O–P) ionic cross-links [10,11,37]. It leads to that the Q2 units are easier to hydrolyze [10]. The special bands at ~1115 cm-1 in Sm-doped glass before corrosion may be is assigned to the (PO4)3- of Q0 units and (PO2)- of Q2 units, it shifts to the low frequency bands at ~1095 cm-1 ((PO4)3- of Q0 units) after corrosion. Compared to the base glass before and after corrosion, the spectrum shapes of rare earth glasses do not show a clear change. It is 12
attributed to the replacement of easy hydrated P–O–P bonds by more chemically resistant RE–O–P bonds with the addition of rare earth, in agreement with the chemical durability analyses. The quantitative analyses of FTIR spectra by using the deconvolution method can determine the relative concentration of structural units and thus, to analyze the modified role of rare earth oxides. The deconvoluted FTIR spectra of the base glass after corrosion are shown in Fig. 4. The new band at ~1375 cm-1 is assigned to the asymmetric stretching vibration of the P=O bonds [35]. The new bands at ~1033 and ~720 cm-1 can correspond to the symmetric stretching vibration of (PO3)2- and the symmetric stretching vibration of P–O–P rings in Q1 units [35-37]. While that at ~584 cm-1 may be assigned to the symmetric stretching vibration of M–O–P bonds [37]. Bingham et al. [38] showed that rings or chains linked P–O–P bonds were susceptible to hydrolyze in corrosion. Table 3 shows that P–O–P linkages decrease slightly in the base glass and the rare earth (La, Y) phosphate glasses after corrosion, and the P–O–P rings of the base glass were easier to hydrolyze than the rare earth (La, Y) phosphate glasses. It may be due to that rare earth ions breaks up the P–O–P rings by forming the RE–O–P bonds [31,37]. Too many polyphosphate chains are released into solution, which leads to the solution pH to drop, this hydrolysis continues, the glass continues to dissolve and a lower pH of the solution will accelerate the ion exchange process as well as the direct hydrolysis of P–O–P rings [37,39]. Therefore, the lower pH of the base glass in leachate analyses will release more phosphoric acid into solution. 13
Hydrolysis and leaching usually act simultaneously in glass corrosion. Hydrolysis according to the following reaction: P–O–P + H2O l 2P–OH, it can rupture directly the P–O–P to destroy the main network structure of glass [10,15]. Leaching according to the following reaction: M–O–P + H2O l P–OH + M–OH, the exchange between hydrogen and glass network modifiers weakens the structural stability of phosphate glass [15,40]. The decline of M–O–P bonds in the base glass is larger than the rare earth glasses after corrosion, and the P–OH bonds have relatively higher fraction. It is then deduced that chemical durability of phosphate glasses was improved with the addition of rare earth. Several studies have shown that RE3+ acts as network modifiers, producing more nonbridging oxygen as its cationic field strength increases, these NBOs are favorable sites for slowing down the corrosion mechanism [41,42]. Therefore, Y-doped phosphate glass could produce more NBOs by the formation of the RE–O–P bonds than La-doped glass, which leads to the best corrosion resistance with the Y-doped phosphate glass. 3.4. Raman spectroscopy Raman spectroscopy is able to reveal vibrational mechanisms inside materials in different phases. One advantage of this approach is non-destructive and non-contact, which assures the integrity of the material and permits long distance detection [43,44]. The Raman spectra, in the frequency region 400–1600 cm-1, of different rare earth phosphate glasses before corrosion (B) and after corrosion (A) are shown in Fig. 5. The Raman spectra of the base glass before corrosion, Fig. 5a, shows that the two 14
prominent bands at 1065 and 1205 cmí1 can correspond to the symmetric stretching mode of (PO3)2- bonds in Q1 units and symmetric stretching mode of ‘strained’ (PO2)(strained structural units, possibly three- or four-membered rings) in Q2 units, respectively [34,36]. The ‘strained’ (PO2)- bonds after corrosion shifts slightly to lower frequency and decrease obviously in intensity, in agreement with infrared spectroscopy studies. Silva et al. [45] showed that divalent cations Ca2+ acted as a network modifier, which could serve as (P–O- Ca2+ -O–P) ionic cross-links in the metaphosphate glass. Therefore, the metaphosphate chains exhibited lower resistance to aqueous corrosion. The higher frequency band at ~1295 cm-1 is assigned to the P=O symmetric stretching [34]. The band at ~1130 cm-1 is assigned to phosphorus-oxygen stretching modes in Q1 phosphate chain terminator units formed by the scission of phosphate chains [46]. It is observed that the bands at ~524 and ~706 cm-1 is diminished after corrosion tests, which can correspond to (P2O7)4- groups of Q1 units and P–O–P bonds of Q2 units, respectively [46,47]. The new bands at ~600, ~787, ~884 cm-1 and a weak band at ~961 cm-1 can correspond to the symmetric stretching mode of P–O–P bonds in Q2 units [45], the symmetric stretching mode of P–O–P bonds in Q1 units [26,45,47], the asymmetric stretching mode of P–O–P bonds and nonbridging (PO4)3- oxygen ions in Q0 units [48]. The appearance of these bands, especially, isolated (PO4)3- groups, exhibits the depolymerization of the base glass network after corrosion. In Fig. 5e, the most notable change with rare earth addition is the increase of (PO3)2- bonds in frequency, and ‘strained’ (PO2)- shifts markedly the lower-frequency 15
band at ~1160 cm-1, which is assigned to the symmetric stretching mode of (PO2)groups in Q2 units [45]. The change of frequency is attributed to structural rearrangements in the main phosphate network due to the replacing of P–O–P and P=O (Q3) by RE–O–P bonds (Q1) [34]. The red shift of the (PO3)2- bonds, resulting from the formation of the RE–O–P bonds, is related to an increased quantity of NBOs. Peak positions and assignments of Raman spectra of different rare earth phosphate glasses before and after corrosion are shown in Table 4. The (PO4)3- groups shift mostly to higher frequencies and increase in intensity after corrosion. It may be due to a penetration process of intact water molecular towards glass network, which leads to the breaking of the P–O–P bonds and the formation of (PO4)3- groups [39,40]. The (PO3)2- groups of Q1 units shift to lower frequencies, which may be attributed to that hydration of the glass network results in the breakage of P–O–P linkages and rings in Q1 units in aqueous solution. The (PO2)- groups shifts mostly to lower frequencies and decrease in intensity after corrosion. Moreover, the (PO2)- for La-doped phosphate glass after corrosion closes to phosphorus-oxygen bond for the RE0 glass in Fig. 5a. It indicates that the ions exchange in corrosion will destroy the cross-link structure, which causes the dissociation of phosphate chains. 4. Conclusions The thermal and structural stability of CaO–Fe2O3–P2O5 glasses doped with rare earth oxides (Y2O3, La2O3, Nd2O3, Sm2O3, Gd2O3) in deionized water was investigated. X-ray diffraction analysis indicated that the base glass and the glasses doped 10 mol% rare earth remained amorphous before and after corrosion. DSC 16
results showed that thermal stability of the rare earth phosphate glasses increased with the increase of CFS. Leachate analyses showed that the release rate of rare earth ions in leachate decreased with the increase of its CFS and rare earth addition reduced the release of Ca ions. The decrease of pH for the base glass was larger than the rare earth glasses by the formation of phosphoric acid. Both Raman and IR spectra indicated that metaphosphate chains were easier to hydrolyze, which converted to orthophosphate units after corrosion. It was attributed to that Ca2+ ions could serve as (P–O- Ca2+ -O–P) ionic cross-links. The decline of M–O–P bonds in the base glass is larger than the rare earth glasses after corrosion, and the P–OH bonds have relatively higher fraction. The formation of more chemically resistant RE–O–P bonds with the addition of rare earth leaded to an increased quantity of nonbridging oxygen, which causes consequent variations in the chemical durability of rare earth glasses. Acknowledgments This work was supported by the Science Foundation of Southwest University of Science and Technology (11zx7157), the Scientific Research Fund of SiChuan Provincial Education Department (14ZA0105, 14ZD1122) and Postgraduate Innovation Fund Project by Southwest University of Science and Technology (14ycxjj0018). References [1] Y.H. Zhang, A. Navrosky, H. Li, L.Y. Li, L.L. Davis, D.M. Strachan, J. Non-Cryst. Solids 296 (2001) 93–101. [2] G.M. Tao, H.T. Guo, L. Feng, M. Lu, W. Wei, B. Peng, J. Am. Ceram. Soc. 92 17
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21
Figure Item Fig. 1 The effect of cationic field strength (CFS) on the concentration of different rare earth ions in leachate. Fig. 2 The normalized elemental mass release rate of the studied glasses in deionized water at 90 °C for 7 days. Fig. 3 FTIR spectra of different rare earth phosphate glasses (a) before corrosion and (b) after corrosion. Fig. 4 Deconvoluted FTIR spectra of the base glass after corrosion using a Gaussian-type function. Fig. 5 Raman spectra of different rare earth phosphate glasses before corrosion (B) and after corrosion (A) from the samples : RE0 (a), La10 (b), Nd10 (c), Sm10 (d), Gd10 (e) and Y10 (f). Table Item Table 1 The density, mole volume and DSC parameters of the studied glasses and cationic field strength (CFS) of the corresponding rare-earth element. Table 2 The results of leachate analyses performed in deionized water at 90 °C for 7 days. Table 3 Deconvolution parameters (the band centers and the relative area) and the bands assignments of rare earth phosphate glasses before corrosion (B) and after corrosion (A). Table 4 Peak positions and assignments of Raman spectra of different rare earth phosphate glasses before corrosion (B) and after corrosion (A). 22
Table 1 The density, mole volume and DSC parameters of the studied glasses and cationic field strength (CFS) of the corresponding rare-earth element. Physical parameters
RE0
Y10
La10
Nd10
Sm10
Gd10
CFS (Å-2)
0
3.703
2.817
3.105
3.269
3.410
D (g/cm3)
2.82
3.01
3.22
3.26
3.29
3.33
Vm (cm3/mol)
47.94
47.92
47.90
47.64
47.58
47.42
Tg (°C)
570
574
576
584
578
574
Tr (°C)
697
669
663
681
666
666
Tr-Tg (°C)
127
95
87
97
88
92
Table 2 The results of leachate analyses performed in deionized water at 90 °C for 7 days.
Glass composition
Initial pH
Final pH
Concentration of ions in leachates ȝJāP/-1) [RE]
[Ca]
[Fe]
RE0
5.60
1.85
0
15.368
10.946
Y10
5.60
3.16
0.056
13.878
11.275
La10
5.60
2.03
0.715
8.120
6.240
Nd10
5.60
2.09
0.657
17.393
12.178
Sm10
5.60
2.12
0.551
14.748
13.802
Gd10
5.60
2.34
0.524
14.517
12.526
Table 3 Deconvolution parameters (the band centers and the relative area) and the bands assignments of rare earth phosphate glasses before corrosion (B) and after corrosion (A). Range frequency
RE0
Assignments
-1
(cm )
La10
Y10
B (%)
A (%)
B (%)
A (%)
B (%)
A (%)
3
0
462~475
O=P–O linkages (Q )
3.57
2.46
2.74
2.81
2.21
2.66
515~526
(PO4)3-sym stretch
5.81
6.42
5.60
7.96
6.53
6.25
579~592
M–O–P bonds
2.84
2.54
2.30
2.13
2.64
2.56
712~727
(P–O–P)sym stretch in rings
3.38
2.83
1.81
2.23
2.40
2.56
769~782
(P–O–P)asym stretch in linkages
2.96
2.07
3.34
3.01
2.72
2.56
(Q )
1
904~920
(P–O–P)sym stretch (Q )
15.00
13.10
17.18
15.94
14.79
13.97
1028~1041
(PO3)2-sym stretch (Q1)
16.83
16.83
17.62
17.03
15.01
17.76
1139~1152
(PO2)-asym
12.33
12.87
18.78
15.99
13.23
14.18
1256~1263
(P=O)sym stretch
15.00
17.64
13.68
12.24
10.11
15.72
1368~1396
(P=O)sym stretch
12.70
10.67
12.11
14.39
19.50
11.37
1581~1632
P–OH bonds
9.56
12.59
4.84
6.11
9.44
10.18
2
stretch (Q )
Table 4 Peak positions and assignments of Raman spectra of different rare earth phosphate glasses before corrosion (B) and after corrosion (A). P–O–P bonds
(PO4)3- groups
(PO3)2- groups
B
A
B
A
B
A
B
A
La10
716
726
944
960
1089
1077
1148
1140
Nd10
708
716
933
924
1085
1075
1163
1151
Sm10
736
715
943
944
1097
1076
1166
1152
Gd10
713
716
934
954
1098
1079
1160
1164
Y10
719
722
949
951
1090
1078
1167
1152
Glass composition
(PO2)- groups
Fig. 1. The effect of cationic field strength (CFS) on the concentration of different rare earth ions in leachate.
Fig. 2. The normalized elemental mass release rate of the studied glasses in deionized water at 90 °C for 7 days.
Fig. 3. FTIR spectra of different rare earth phosphate glasses (a) before corrosion and (b) after corrosion.
Fig. 4.Deconvoluted FTIR spectra of the base glass after corrosion using a Gaussian-type function.
Fig. 5. Raman spectra of different rare earth phosphate glasses before corrosion (B) and after corrosion (A) from the samples : RE0 (a), La10 (b), Nd10 (c), Sm10 (d), Gd10 (e) and Y10 (f).
Highlights ڼThe thermal and structural stability of RE2O3–CaO–Fe2O3–P2O5 were studied. ڼ
Thermal stability of glass increased with the increase of cationic field strength.
ڼ
Release rate of rare earth ions decreased with the increase of its field strength.
ڼ
Metaphosphate chains by the cross-links of calcium ions were easier to hydrolyze.
ڼ
The RE–O–P bonds have more chemical resistance by producing non-bridging oxygen.