ABSTRACT

The regular melting-quenching method allowed isolating very large vitreous domains within the ternary system Li₂OP₂O₅-Fe₂O₃ at 1100˚C. The vitrification and crystallization effects are discussed in terms of phosphorus pentaoxide concentration (mol%). In the course of the present study, we analyzed chemical durability along the glass domain and many sample glasses were isolated. We noticed that our compounds demonstrated very high chemical resistance to attack, even with very highly concentrated mineral acid solutions. This behavior can be assigned to the presence of poorly crystalline phases in these glasses, which tended to increase as the Fe₂O₃ content increased. This property is a prerequisite for many interesting industrial applications. XRD, IR spectroscopy and SEM micrographs allowed an efficient investigation of the structural changes versus composition within ternary diagrams. The results were found to be consistent with the regular structural changes of phosphate glasses.

1. Introduction

The practical application of phosphate glasses is often limited by their poor chemical resistance. Recently, several phosphate glasses with high aqueous corrosion resistance have been reported [1]. Compared with conventional oxide glasses such as SiO₂, phosphate glasses based on P₂O₅ are technologically important materials, primarily due to their superior physical properties such as low glass transition temperatures, chemical durability, low optical dispersions and relatively high thermal expansion coefficients. These properties make these glasses as potential candidates for many technological applications such as sealing materials, bioglasses, and electronic and optical devices, as laser hosts, nuclear waste glasses, glass-to-metal seals and as solid state electrolytes, etc., [1-6]. With their unusually high chemical durability and low processing temperature, iron phosphate glasses have been considered better for the vitrifying of nuclear wastes than borosilicate glasses [7,8]. The aim of the present work firstly is to define the glass area in a ternary diagram and especially in the Li₂O-Fe₂O₃-P₂O₅. So we have explored that the introduction of iron oxide in the glass network can be up to 30 mol% according to the conditions in which we worked, either at atmospheric pressure or at 1080˚C - 1100˚C. Secondly it is to investtigate the chemical resistance and structural properties of lithium-iron phosphate glasses and to relate the improve of chemical resistance to the structural change using IR spectroscopies, XR diffraction and SEM. Hence with increasing Fe₂O₃ content in the phosphate glass network, the P-O-P bands are replaced by more resistant Fe-O-P bands.

2. Experimental

The composition of the glasses was xLi₂O-yFe₂O₃-χP₂O₅, with (χ = (100 − (x + y); mol%), obtained by the melting-quenching method at 1100˚C. An appropriate mixture of mixing compounds, i.e. Li₂CO₃, ferric oxides and (NH₄)₂HPO₄, was initially tempered at various temperatures between 100˚C - 600˚C to achieve a preparation before glass preparation. The melts were achieved in alumina crucibles for about 15 min at 1080˚C ± 10˚C. The isolated glasses had regular shapes and an approximate size of 10 mm in diameter and 2 mm in thickness. The vitreous state was first evidenced from the shiny aspect and then confirmed from XRD patterns. Annealing of these glasses was performed at increasing temperatures in intervals of 100˚C. The first structural approach was made using X-ray diffraction which allowed us to follow the crystallization of the vitreous domain of Li₂O-Fe₂O₃-P₂O₅. The microstructure of sample glasses was characterized by scanning electron microy (SEM), equipped with a microanalyzer full system (EDXEDAX). The chemical durability of the glasses with the composition x Li₂O-y Fe₂O₃-χP₂O₅; (χ- = (100 − (x + y)) were subjected to acid solutions using weak and strong concentrations of HCl and H₂SO₄ (0.1 N, 1 N and 10 N). The duration of attack for some glasses and solution concentration continued for up to 30 days. The infrared (IR) spectra for each glass were measured between 400 and 1400 cm⁻¹ using mX-1 and NIC-3600 FTIR spectrometers. Samples were prepared by pressing a mixture of about 2 mg of glass powder with 100 mg of anhydrous KBr powder.

3. Results

In the system Li₂O-P₂O₅, transparent glasses were prepared for 0 ≤ x ≤ 62 (Figure 1). These values are in agreement with published results [9,10], but they are superior to those given by other authors [11,12]. This was probably caused by the different experimental conditions (temperature of melting, speed of tempering, etc.)

[13]. In the binary system Fe₂O₃-P₂O₅, the substitution of P₂O₅with Fe₂O₃ led to black glasses that were not hygroscopic for ironoxide contents between 0 and 30 mol% (Figure 1). Ternary glasses rich in Fe₂O₃ content had a dark brown color which became more and more brown clear as the lithium oxide content increased and the Fe₂O₃ content decreased. The demarcation of the glassy zone within the ternary diagram Li₂O-Fe₂O₃-P₂O₅ is given by the following limits: 0 ≤ x ≤ 62; 0 ≤ y ≤ 30; 36 ≤ χ ≤ 1; (mol%) (Figure 1).

3.1. Annealing Temperature

The annealing of the ternary glasses was performed by increasing the temperature in intervals of 100˚C. An increase in the lithium oxide modifier led to a slight increase in the crystallization temperature. For the glasses with the composition xLi₂O-5Fe₂O₃-χP₂O₅, the crystallization temperature increased from 390˚C to 480˚C for x = 30 to x = 50, respectively (Figure 2 [14]. On the other hand, Figure 3 shows that an increase in the Fe₂O₃ content in the glasses xLi₂O-yFe₂O₃-χP₂O₅(with x/χ = 0.54) caused a quick increase in the value of the crystallization temperature from 120˚C for a binary glass Li₂O-P₂O₅ (y = 0) to 540˚C for y = 18 [6,15]. Figure 4 shows for the binary glasses Fe₂O₃-P₂O₅ that the variation in the annealing temperature (Tc) according to the Fe₂O₃ contenthad a maximum at 700˚C for the glass composition 16 Fe₂O₃-84P₂O₅ and was constant when the Fe₂O₃ content was more than y = 25 [16,17].

3.2. X-Ray Diffraction

The X-ray diffraction spectra show that the crystallized glasses seemed to be similar to the closest crystalline

phases within the ternary diagram of Li₂O-Fe₂O₃-P₂O₅. The X-ray diffraction spectra of crystallized glasses with the composition x Li₂O-5Fe₂O₃-χP₂O₅ were similar to those of the LiPO₃ glasses in the neighborhood of χ = 50 (Figure 5(a)) and those of Li₄P₂O₇ for χ ≤ 45 (Figure 5(b)). Figure 6 shows the structural evolution for the ternary glasses x.Li₂O-y.Fe₂O₃-χP₂O₅ localized along the line attributed to the ratio Li₂O/P₂O₅ = 0.54. We noted a mixture of metaphosphate phases for an Fe₂O₃ content ≤ 5, and pyrophosphate phases with an Fe₂O₃ content ≥ 15 [1,18,19]. The x-ray diffraction spectra for crystallized glasses of the binary system y Fe₂O₃-(100-y) P₂O₅ (Figure 7) seemed to favor the form of phases similar to Fe(PO₃)₃ which become more obvious when the Fe₂O₃ content increased to y = 25.

3.3. Chemical Stability

Binary glasses of the composition yFe₂O₃-(100-y) P₂O₅ had excellent chemical resistance toward chemical aggressors when the iron oxide content was increased in the glassy network. These glasses were immersed in weak and strong acid solutions of HCl (0.1 N, 1 N and 10 N) and H₂SO₄ (0.1 N, 1 N and 10 N) for 30 days. Their weights before and after attack were practically the same. Figure 1 shows that the iron ternary glasses had relatively weak chemical resistance to chemical aggressors when the Li₂O content was increased in the glass network. However, when this value decreased, the iron ternary glasses became resistant, even with a low Fe₂O₃ content [1,5,20-22].

3.4. Infrared Spectra and SEM Analysis

The IR spectra of the binary yFe₂O₃-(1-y) P₂O₅ are shown in Figure 8. With an increase in the Fe₂O₃ content, both the bands observed at 1290 and 785 cm⁻¹ and the band at 940 cm⁻¹ shifted to lower frequencies and the intensities decreased by changing the Fe₂O₃ content from 10 to 28 mol%. The bands at 1290 cm⁻¹ assigned to υ_as PO₂ disappeared from the spectrum, while the band at 725 cm⁻¹ assigned to υ_s POP became a simple shoulder. The intensity of the IR band at 1070 cm⁻¹ assigned to υ_s PO₂ increased broadly [1,23-25]. The IR spectra observed with the composition xLi₂O-5Fe₂O₃-χP₂O₅ are shown in Figure 9. When lithium oxide was added to the glass network, this last one is depolymerized to the shorter group units, in agreement with results published by Ouchetto and al. [1,11,20-24]. The IR spectra show that the band at 1280 cm⁻¹ assigned to υ_as PO₃ decreased and shifted to lower frequencies when we shift away from the P₂O₅ point. On the other hand, the intensity of the band at 1100 - 1120 cm⁻¹ was assigned to increased υ_s PO₂ when

the lithium oxide content increased. This increase in the Li₂O content breaks P-O-P bonds and increases the number of non-oxygen bridges. The IR spectra observed with these glasses were similar to the spectra of the xLi₂O- χP₂O₅ family, which can be explained by a similar structural evolution [1,10,12]. Figures 10 and 11 show the IR spectra of glasses with the compositionsx Li₂O-yFe₂O₃- χP₂O₅ with x/χ = 0.54 and xLi₂O-yFe₂O₃-60P₂O₅, respectively. The analysis of these spectra showed that the band at 1290 cm⁻¹ assigned to υ_as PO₂ shifted toward lower frequencies and decreased when the iron oxides content increased. It disappeared completely when the Fe₂O₃ content was >25. On the other hand, the band at 1070 - 1100 cm⁻¹ assigned to υ_sPO₂ with an iron oxide content of 10 ≤ y ≤ 30 seemed to form an envelope of the spectra of the compound LiFeP₂O₇ (Figure 10), [1,19,23, 25-27]. These results suggest that these glasses were

formed, in general, by unit groups of pyrophosphate when the Fe₂O₃ content increased. Figure 12 shows SEM micrographs of sample S1 with the composition 30Li₂O·10Fe₂O₃·60P₂O₅ as well as micrographs of S2 and S3 with the composition 20Li₂O·20Fe₂O₃·60P₂O₅ and 10Li₂O·30Fe₂O₃·60P₂O₅, respectively. The SEM micrograph of S1 (Figure 12(a)) shows an approximately homogenous glassy phase. However, SEM analysis

indicated the presence of crystalline phases in samples S2 and S3 which had an Fe₂O₃ content ≥20 (mol%),

shown in Figures 12(b) and (c), respectively. Hence, it seems that with an increasing Fe₂O₃ content, the crystallization tendency is enhanced; the LiFeP₂O₇ and Fe(PO₃)₃ phases almost certainly crystallized in these glasses.

4. Discussion

The regular melting-quenching method allowed for isolating a very large vitreous domain within the ternary system Li₂O-P₂O₅-Fe₂O₃. An increase in the lithium oxide content in phosphate glasses caused a small increase

in the crystallization temperature. However, an increase in the Fe₂O₃ content in phosphate glasses initiated an important increase in the crystallization temperature. This was more significant for the binary glasses Fe₂O₃- P₂O₅ where the crystallization temperature reached 700˚C for the sample with the composition 16Fe₂O₃·84P₂O₅. The X-ray diffraction spectra indicate the structural evolution of crystallized glasses inthe ternary diagram of Li₂O-Fe₂O₃-P₂O₅. The resultsshow that when the composition of the vitreous zone was moved away from the P₂O₅ point, the structure evolved toward one with a known crystallized composition neighborhood, LiPO₃, Fe(PO₃)₃, Li₄P₂O₇, LiFeP₂O₇. When the iron oxide content increased in the glass network, both binary and ternary glasses showed excellent chemical resistance toward chemical aggressors. However, an increase in the Li₂O content led to poor chemical resistance. The IR spectra for glasses with the composition xLi₂O-5Fe₂O₃- cP₂O₅ show that an increase in the lithium oxide content in the glass network induced the birth of small units,more than in the binary system Li₂O-P₂O₅ [1,9,11].Hence, the introduction of a low Fe₂O₃ content (y < 10) in the glass had no effect on the structural evolution. The IR spectra in the region between 1060 and 1100 cm⁻¹ for the glasses with the composition x Li₂O-yFe₂O₃-χP₂O₅ with x/χ = 0.54 and (40-y) Li₂O-yFe₂O₃-60P₂O₅ confirmed the formation hypothesis of more small groups of P₂O₇⁴⁻ [28,29] when the Fe₂O₃ content increased in the glass-network. IR spectra of the binary glasses Fe₂O₃-P₂O₅ show that the band at 1290 cm⁻¹ assigned to υ_a(PO₃) [1,2,7,9] disappeared when the Fe₂O₃ content increased from y = 10 to y = 28, which assigns the band at 1070 cm⁻¹ to υ_s (PO₂) [1,9]. These results agree with those obtained by X-ray diffraction. Hence, these results indicate that iron phosphate glasses which have a high Fe₂O₃ content are dominated by P₂O₇⁴⁻ and (PO₃)₃³⁻ dimer units and contain a large number of Fe-O-P bonds which are responsible for theirexcellent chemical durability [16, 30-31]. The X-ray diffraction and IR spectra for the iron phosphate glasses depend considerably on the composition and glass structure. The changes in the characteristic features of these spectra follow the changes in the glass network and glass stability in addition to crystallization.

In the model of the samples glasses S1, S2 and S3 we noted a clear evolution in chemical resistance, probably caused by grain boundary resistance as a result of partial glass crystallization [26,32]. It was found that the introduction of a small amount of Fe₂O₃ (y < 10) in the glass don’t changes the glass structure (Figure 9) and no significant effect on the chemical resistance. Looks like, it dissolves in the glass to give a homogeneous glass phase. When the iron oxide content reach 10 mol%, there are beginning of crystallites having a glassy phase dominant. Beyond 10% mole Fe₂O₃ content in the glass, the number of crystallites becomes increasingly important, and size becoming larger when approaching the boundary between the glass and the crystal (border).

5. Conclusion

The regular melting-quenching method allowed isolating a very large vitreous domain within the ternary system Li₂O-P₂O₅-Fe₂O₃. The structure and properties of lithium iron phosphate glasses were investigated using various techniques. Both X-ray diffraction and IR spectra indicate a structural evolution of glasses with composition in the ternary diagram of Li₂O-Fe₂O₃-P₂O₅. The results show that when the composition of the vitreous zone was moved away from the P₂O₅ point, the structure evolved toward one with a known crystallized composition neighborhood, LiPO₃, Fe(PO₃)₃, Li₄P₂O₇, LiFeP₂O₇. These glasses showed good chemical resistance for compositions with a high Fe₂O₃ content. The iron phosphorrus-oxygen network became stronger with an increase in the Fe₂O₃ content. The improved chemical resistance of iron phosphate glasses was attributed to the replacement of the easily hydrated P-O-P and Li-O-P bonds by corrosion-resistant Fe-O-P bonds. As the Fe₂O₃ content in the glass increased, the number of Fe-O-P bands also increased. The IR spectra indicates that iron phosphate glasses are dominated by, dimer units and contain a large number of Fe-O-P bonds. The structural change caused an important tendency for crystallization which is the origin of the high chemical durability.

REFERENCES

NOTES

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