ABSTRACT

Glasses of the compositions 20Li₂O-(50 − x)Li₂WO₄-xFe₂O₃-30P₂O₅ where (x = 0, 1, 5, 8, 10, 15 mol%) were elaborated by the melt-quenching route. Synthesized glasses are characterized using X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), differential scanning calorimetric (DSC), and density determination. The XRD patterns confirmed the amorphous nature of samples, and IR spectra showed the structural groups and highlight the depolymerization of phosphate network with the introduction of iron oxide. It is found that the structural unit Q² converts to Q¹ and Q⁰ as Fe₂O₃ replaces Li₂WO₄. Chemical durability tests on the glasses have shown that the compositions containing pyrophosphate (Q¹) and orthophosphate (Q⁰) units are more water-resist. The electrical conductivity measurements were performed by complex impedance spectroscopy in the frequency range of 20 - 10⁶ Hz at various temperatures from ambient to 400˚C. It is found that the conductivity is activated thermally and follows an Arrhenius law. The obtained electrical data were analyzed by the modulus formalism and dielectric formalism. The determined asymmetric nature of $M^{″}/{M^{″}}_{\max }$ spectra suggested that the relaxation behavior is non-Debye and characterized by the stretched exponent parameter β < 1.

Keywords:

Phosphate, Glasses, Thermal Analysis, Structure, Electrical Conductivity

1. Introduction

Phosphate glasses are one of the good hosts of several transition metal oxides (TMO) which show different valence states. The valence state of such metal depends on the content and the properties of the other components of the glasses [1] [2] [3]. The transition metal ion can locate in the network either in the substitution or interstitial positions. When the ions take the substitution position, it is able to crosslink between the glass former and modifier. But, in the case where the ions locate in the interstitial positions, it can link to non-bridging oxygen (NBO) [4]. Transition metals are used as activators in advanced technologies such as phosphors, lasers, solar energy converters and in a number of electronic devices. Recently, the transition metal oxides doped alkali phosphate glasses have been the subject of intensive investigation in view of their potential applications in various domains of modern technology [5]. The investigation of relationship between structure and properties is essential in material science. Alkali phosphates form good glasses over a wide range of composition. But, one of the major issues that limit the use of phosphate glasses is their poor chemical durability. The improvement of their chemical durability is often realized by the addition of some oxides such as Al₂O₃, SnO, Fe₂O₃ [6]. Also, the addition of iron oxide to lead phosphate improves the chemical durability of the glass without substantial change in the physical properties [7]. In addition, iron phosphate glasses have attracted much attention owing to their high chemical durability, wide compositional flexibility and low melting temperature [8] [9]. These glasses are known for their high chemical durability and for their application in vitrifying nuclear waste [10] [11]. Owing to the insertion of iron in the phosphate matrix, the easily hydrolysable P-O-P bridging links have been replaced by the water-resist Fe-O-P linkages thereby providing these glasses with excellent chemical durability [12] [13]. In this context, S. Nakata et al. have reported that the addition of divalent Fe²⁺ into NaPO₃ broke the P-O-P bonds and forms water-resist Fe-O-P linkages in xFeO-(100 − x)NaPO₃ glasses [14]. Also, Y. M. Moustafa et al. reported that the increasing Fe₂O₃ in the xFe₂O₃-(100 − x)P₂O₅ system leads to the formation of P-O-Fe²⁺ and P-O-Fe³⁺ bonds [15].

Recently, we have studied the effect of substitution of Li₂WO₄ by TiO₂ inside the Li₂O-Li₂WO₄-TiO₂-P₂O₅ glasses [16]. In the present work our interest is focused on the investigation of the influence of the substitution of Li₂WO₄ by Fe₂O₃ with the glasses 20Li₂O-(50 − x)Li₂WO₄-xFe₂O₃-30P₂O₅ where (x = 0, 1, 5, 8, 10, 15 mol%). Some physical properties such as density, molar volume, glass transition temperature are studied. The structural approach of the glasses is realized by Infrared spectroscopy. The temperature and composition dependencies of their electrical properties are also investigated.

2. Experimental Procedure

The phosphate 20Li₂O-(50 − x)Li₂WO₄-xFe₂O₃-30P₂O₅ (0 ≤ x ≤ 15 mol %) glasses were prepared from precursors purchased from Sigma-Aldrich. Powders of lithium carbonate (Na₂CO₃), iron trioxide (Fe₂O₃), lithium tungstate Li₂WO₄, and hydrogen ammonium phosphate (NH₄H₂PO₄) were weighed to get the required composition and ground in a mortar with pestle for 30 min to obtain homogenous compositions. Each mixture was heated in an electric furnace at 300˚C and 600˚C by placing them in an alumina crucible for several hours. Then, the temperature of the furnace is increased to attain the melt for about 30 min at 900˚C, the preparation temperature until a bubble free liquid was formed. The melt was then rapidly quenched by pouring it on to a stainless steel plate maintained at T_g-20˚C to avoid excess thermal shocks. The obtained glasses were annealed for 3 h at 300˚C to relieve the mechanical strains. The glasses were placed in a desiccator before uses.

X-ray patterns of powdered samples were recorded using a Phillips D5000 diffractometer equipped with a CuK_α X-ray source and a Ni filter (K_α = 1.54 Å) at room temperature. The XRD data were recorded in 2θ range (10˚ - 90˚) with 0.02 step width.

The density (ρ) of glasses was measured at room temperature by the Archimedes method using diethyl orthophthalate as the suspension medium. The uncertainty of the measurements is estimated to be ±0.01 g/cm³. Molar volume (V_m) of each glass is derived from the molar weight values (M) and the density (V_m = M/ρ). Oxygen packing density (OPD) of each glass was calculated from the density and composition using the formula OPD = 100 C (ρ/M), C is the number of oxygens per formula unit.

The glass transition temperature (T_g) and crystallization temperature (T_c) of the prepared glasses, they are determinated from DSC curves, which have been obtained by using DSC 131 Evo analyser. The estimated error on the temperature is ±4˚C. As type of, as the DSC curves depend of the particle size of ground glass batches which fixed about at 300 µm Sieve in this study for all glasses. The amount of ground glass batches is about 40 mg in nitrogen atmosphere at heating rate 10˚C/min.

FTIR absorption spectra of the prepared glasses were collected at room temperature in the range 1400 - 400 cm⁻¹ (wavenumber) using the Fourier Transform Infrared FTIR TENSOR27 Spectrometer. The measurement carried out by using the pellets. These pellets were prepared by mixing about 1 mg of glass powder with 100 mg of anhydrous KBr.

The chemical durability tests are realized on the glasses which were cut in parallelepiped shapes. The block of glass was rinsed with acetone and then dried. Each sample of glass was placed in a bottle containing deionized water (pH = 6.8). The bottles were then suspended in a thermostat bath maintained at a temperature of 30˚C for duration of 250 hours. All the experiments are done under the same conditions and the ratio S/V (S: glass surface, V: volume of the altering solution) is maintained at 0.04 cm⁻¹. The mass change (Δm) is determined as a function of time according to the equation: Δm = (m₀ − m_t)/A, where: m₀, m_t and A are the initial mass, mass at specific time point and surface area of the specimen (cm²), respectively. The pH values of the test solutions were measured by a pH-meter.

Electrical measurements of the prepared glasses were performed by Impedance Spectroscopy (IS) Hewlett Packard Model 4284 A precision LCR. The amplitude of the DC signal applied across the sample was 0.5 V. These glasses were ground to fine powder and made into pellets with 10 mm diameter and thickness of 1 mm. The pellets were dried and coated with silver paint on either faces. The real and imaginary parts of complex impedance measurements were measured as a function of frequency (20 Hz - 10⁶ Hz) and temperature (328 k - 627 k).

3. Results and Discussion

3.1. X-Ray Diffraction

To ascertain the vitreous state of the prepared glasses we have first of all tested these samples by the X-ray diffraction (XRD) analysis. Figure 1 shows the XRD patterns of the studied samples. These patterns are free from peaks suggesting the absence of any crystalline phases inside the samples thereby confirming their amorphous state. All the glasses are colored except the composition (x = 0) which is colorless suggesting that the transition metal is in its high valence state (W⁶⁺) in this composition.

3.2. Density and Molar Volume

Figure 2 illustrates the variations of the density and molar volume as a function of iron content (Fe₂O₃ in mol %). The addition of Fe₂O₃ on behalf of tungsten oxide decreases the density and increases the molar volume. The increase of Fe₂O₃ ratio results in an open glass structure with lower compaction. The density decreases from 4.18 g/cm³ for (x = 0) to 3.61 g/cm³ for the glass (x = 15). The experimental data of these parameters as listed in Table 1. As in our previous

work [15] , the decrease in density can be explained by the large difference in molecular weight between Li₂WO₄ (261.72 g/mol) and Fe₂O₃ (159.69 g/mol). The introduction of the lighter molecule in the glass instead of a heavier one leads to decrease the density. On the other hand, the increase of molar volume from 42.92 cm³/mol (x = 0) to 45.26 cm³/mol (x = 15) when Fe₂O₃ replaced Li₂WO₄ reveals either an increase of the interatomic spacing or the decrease of the strength constant of the bonds inside the glassy network. The increase of volume with (x) put forward that the interatomic spacing among the atoms of glass network increases when Fe₂O₃ ratio increases. Specially, the increase of the average length of M-O (M = P, W, Fe) bonds can explain the observed increase of (V_m). This happens when some bridging oxygen (BO) convert to non-bridging oxygen (NBO) in the glassy-matrix.

3.3. DSC Analysis

The DSC curves of the prepared glasses are shown in Figure 3. The endothermic peaks in DSC traces represent glass transition phenomena and the onset temperature of endothermic peak corresponds glass transition temperature (T_g). An exothermic peak represents the crystallization occurring at the peak temperature (T_c). The DSC curves of the glasses (x = 0, 1 and 5) are characterized by an

endothermic peak (T_g) and an exothermic peak (T_c1). However, the glasses with the compositions (x = 8, 10 and 15) show two exothermic peaks (T_c1, T_c2). The values of T_g, T_c1 and T_c2 for the prepared glasses are summarized in Table 1. The substitution Li₂WO₄ with Fe₂O₃ increases the glass transition temperature (T_g) from T_g = 407˚C ± 2˚C to T_g = 432˚C ± 2˚C.

From the analysis of Figure 4 and data of Table 1, one can see that T_g increases with increasing Fe/P ratio. The increase of T_g reflects an increase of the glass viscosity as well as the formation of stronger bonding in the glass such as P-O-Fe bonds. The increase of T_g with iron content can also be associated to the reticulation effect of iron in the network. If such assumption is true one can expect an increase of the oxygen packing density (OPD) when the iron ratio increases. However, an opposite trend for (OPD) is obtained (Figure 4). Therefore, the most prominent reason for increasing T_g with iron oxide is the formation of strong bonds in the glass network.

4. Infrared Spectroscopy

The structure approach of the prepared glasses has been investigated by using FTIR spectroscopy in frequency range 400 - 1400 cm⁻¹. Such study allows us to check the phosphate units and other vibrational modes present in the network. The obtained IR spectra of the glasses are shown in Figure 5. The assignment of the different bands is realized according to previous literature data [17] [18]. The band positions and their assignments are gathered in Table 2.

The principal bands are observed in the range 950 to 1220 cm⁻¹. They correspond to symmetric and asymmetric stretching modes of non-bridging oxygen on different P-tetrahedra. The band ν_as(PO₂)⁻(Q²) in metaphosphate is located in the range 1220 - 1200 cm⁻¹, ν_as(PO₃)²⁻(Q¹) of pyrophosphate groups and the mode vibration of isolated orthophosphate groups ${\text{PO}}_4^{3-}$ (Q⁰) are situated in the range 1040 - 1000 cm⁻¹. The band at 970 - 930 cm⁻¹ is assigned to the asymmetric vibrations ν_as(P-O-P) and/or ν_as(P-O-M) (M = W, Fe). The band at 730 - 720 cm⁻¹ is attributed to the symmetric stretching modes ν_s(P-O-P). The broad band in the range 500 - 480 cm⁻¹ belongs to bending vibrations of basic structural units of phosphate glasses. The substitution of Li₂WO₄ by Fe₂O₃ leads to changing in frequency and intensity of some bands. With the increase of the iron content in the glasses, it is observed that the intensity of the band associated to Q¹ species increases. Also, the intensity of Q⁰ species (1040 - 1000 cm⁻¹) increases with increasing the iron content. It seems that the iron oxide depolymerizes the network and its content increasing induces the formation of pyrophosphate and orthophosphate structural units in the network.

5. Chemical Durability

Generally, the dissolution process of the glasses depends on several parameters such as surface of the glass, layer formation, and solution chemistry. The dissolution behavior of the glass is controlled by the reaction of the glass network and the type of ions existing in the solution [19].

5.1. Weight Loss

Figure 6 shows the weight loss as a function of time for the 20Li₂O-(50 − x)Li₂WO₄-xFe₂O₃-30P₂O₅ glasses (0 ≤ x ≤ 15). When the content of Fe₂O₃ is increased from x = 0 mol% to x = 15 mol%, we can differentiate two stages: 1) for t < 100 h, the dissolution is almost linear. The initial corrosion rate is also

determined; 2) for 100 h < t < 250 h, there is a marked slowdown in dissolution. Indeed, Li₂WO₄ substitution by Fe₂O₃ has observable effect on the dissolution behavior of synthesized glasses. The temporal evolution of the dissolution shows that the glass of composition (x = 0) is characterized by a faster initial dissolution than that of the other glasses (x = 1 or 5 mol %). The solubility of the glasses studied depends on the degree of substitution of Li₂WO₄ by Fe₂O₃. It can be said that Fe₂O₃ leads to the improvement of the chemical durability of the network by the formation of P-O-Fe type bonds, which have been identified by the use of infrared spectroscopy of Fe₂O₃-containing glasses. These results are consistent with thermal analysis which showed that the introduction of Fe₂O₃ into the phosphate glass network makes the glass matrix harder. The correlation between the results of the dissolution and the results of the thermal analysis showed that the introduction of Fe₂O₃ in the vitreous network leads to the reinforcement of the chains constituting the vitreous matrix of the glass prepared [20].

The enhancement of the chemical durability of the glasses with high iron content could be due to the low rate of glass surface hydration. Bunker et al. developed the general description for the dissolution reactions between phosphate glasses and water [21]. The first stage of dissolution is controlled by diffusion, including penetration of H₂O into the glass network and diffusion of ions through the developing hydrated layer back into solution. Water hydrates the phosphate anions by reacting with the metal (alkali, Fe)-O-P bonds that link neighboring P-anions [22]. At longer times, linear reaction kinetic dominates as the hydrated metals and phosphate anions are released into solution.

5.2. pH Variation

From the measurements of the pH of the leachate solution (Figure 7), it is observed that pH values decrease with time. For the glasses without (x = 0) or with lower iron content the measured pH corresponds to acidic solutions. From the analysis of Figure 7 we observed that the pH values of the leachates show two phases: 1) before 48 h there is a sudden alteration or the thermodynamic imbalance between the glass and the altering solution is great which creates a sudden decrease in pH; 2) after 48 h, a phase of slowing down of corrosion which is associated with the saturation of the altering solution which leads to the formation of a protective layer against the diffusion through the surface of the glass.

This pH variation depends on the chemical composition of the glass. We can say that the alkaline ions and the phosphate ions of the glass surface were released into the solution. This ion exchange plays a major role in defining the acid-base character of the solution. Indeed, the decrease of the pH can be attributed to the disintegration of the phosphatic entities towards the altering solution and the possibility of formation of phosphoric acid controlled by the ${\text{H}}_{\text{2}}{\text{PO}}_4^{-}$ entities. These entities are obtained from the dissociation of orthophosphoric acid in water [23] [24] [25] [26] :

${\text{Н}}_{\text{3}}{\text{РО}}_4+{\text{Н}}_{\text{2}}\text{О}\leftrightarrow {\text{Н}}_{\text{2}}{\text{PO}}_4^{-}+{\text{H}}_{\text{3}}{\text{O}}^{+}\left(K_1=7.1\times {10}^{-3}\right)$

${\text{Н}}_{\text{2}}{\text{РО}}_4^{-}+{\text{Н}}_{\text{2}}\text{О}\leftrightarrow {\text{НPO}}_4^{2-}+{\text{H}}_{\text{3}}{\text{O}}^{+}\left(K_2=6.3\times {10}^{-8}\right)$

${\text{НРО}}_4^{2-}+{\text{Н}}_{\text{2}}\text{О}\leftrightarrow {\text{PO}}_4^{3-}+{\text{H}}_{\text{3}}{\text{O}}^{+}\left(K_3=4.4\times {10}^{-13}\right)$

The dissociation constants allowed us to conclude that the dihydrogen phosphate ions ${\text{H}}_{\text{2}}{\text{PO}}_4^{-}$ in the water are much more abundant compared to the monohydrogen phosphate ions ${\text{НРО}}_4^{2-}$ and phosphate ions ${\text{PO}}_4^{3-}$ .

5.3. Electrical Properties

The conductivity for the prepared glasses is determined according to Equation (1):

$\sigma =\left(t/a\right)\ast \left(1/Z^{\prime }\right)$ (1)

where “σ” is the conductivity, “t” is the thickness, “a” is the cross-section area of the pellet and “ $Z^{\prime }$ ” is the bulk resistance.

Figure 8 shows the temperature dependence of the conductivity for the studied glasses. It increases with increasing temperature and follows an Arrhenius law (2):

$\sigma .T={\sigma }_0\exp \left(-E_a/k_{B}T\right)$ (2)

where, E_a is the activation energy, ${\sigma }_0$ is the pre-exponential term and k_B is the Boltzmann constant.

These electrical parameters are determined (Table 3). The conductivity of the glasses depends on the composition of iron oxide. It increases with Fe₂O₃ content up to (x = 10), then it decreases. The variation of the activation energy follows an opposite trend to that of the conductivity. It seems that up to (x = 10) the mobility is the more influencing parameter since the concentration of lithium ion decreases with Fe₂O₃ content. However, when the ratio of iron reaches x = 15 mol% the decrease of the conductivity may be due to the low lithium content in the glasses.

The obtained data of electrical measurements are analyzed by the modulus formalism ( $M=M^{\prime }+i{M}^{″}$ ). The real and imaginary parts of (M) are calculated from the measured impedance data using Equations (3) and (4):

$M^{\prime }=\omega C_0{Z}^{″}$ (3)

$M^{″}=\omega C_0{Z}^{\prime }$ (4)

where, $Z^{\prime }$ and $Z^{″}$ are the real and imaginary parts of the impedance, respectively. ω is the angular frequency and C₀ is the vacuum capacitance of the cell.

Figure 9 shows the variation of $M^{″}/{M^{″}}_{\max }$ as a function of frequency in the range (20 Hz - 10⁶ Hz) at different temperatures for the prepared glasses (x = 0 and x = 10). These $M^{″}/{M^{″}}_{\max }$ spectra are slightly asymmetric suggesting that the relaxation present in the material was non-Debye. The maximum in the $M^{″}/{M^{″}}_{\max }$ peak shifts to higher frequency with increasing temperature. The shift in $M^{″}/{M^{″}}_{\max }$ peak maximum corresponds to the conductivity relaxation. This behavior suggests that the dielectric relaxation is thermally activated. The relaxation frequency f_p of the charge carriers is determined from the maximum of $M^{″}/{M^{″}}_{\max }$ [27]. The variation of the determined peak frequency (f_p) as a function of temperature is shown in Figure 10. From the analysis of this latter figure, it is observed that the frequency relaxation is thermally activated and

follows an Arrhenius behavior with activation energy (E_f). In order to determine the mechanism of conduction, we plotted in the same figure the temperature dependence of the conductivity as shown in Figure 10 for the glasses (x = 5, and x = 15). We observe that both lines are quasi-parallel and the activation energies issued from impedance (E_a) and modulus (E_f) spectra are very similar (Table 3). This result suggests that the electrical conductivity in the glasses under study is probably due to hopping mechanism [28].

The asymmetric nature of $M^{″}/{M^{″}}_{\max }$ spectra suggested that the relaxation behavior was non-Debye and could be characterized by the stretched exponent parameter β < 1 [29]. For an ideal dielectric material, for which the dipole-dipole interaction is negligible, the value of β is equal to unity (β = 1). The stretched exponent parameters, β, can be obtained from the peak relaxation frequency (β = 1.14/FWHM), where (FWHM) is the width at half-maximum of M"(f) curves. For each glass, it is found that the value of β is independent of temperature and

the obtained values of β for all the compositions are regrouped in Table 3. The values of β are found to be less than unity. This demonstrated the non-Debye nature of the dielectric relaxation inside the glasses.

The electrical impedance data are also analyzed by the dielectric formalism ( ${\epsilon }^{\ast }={\epsilon }^{\prime }-j{\epsilon }^{″}$ ) by using the following Equation (5) [30] :

${\epsilon }^{\ast }\left(\omega \right)={\epsilon }^{\prime }\left(\omega \right)-j{\epsilon }^{″}\left(\omega \right)=1/\left(j{C}_0\left(Z^{\prime }+j{Z}^{″}\right)\right)$ (6)

where, ${\epsilon }^{\prime }$ is the real part of the permittivity (resistive) and ${\epsilon }^{″}$ is the imaginary part of the permittivity (reactive), C₀ the capacitance of the empty cell. These parameters are determinate according to Equations (7) and (8) respectively:

${\epsilon }^{\prime }=-Z^{″}/\omega C_0\left({Z^{\prime }}^2+{Z^{″}}^2\right)$ (7)

${\epsilon }^{″}=Z^{\prime }/\omega C_0\left({Z^{\prime }}^2+{Z^{″}}^2\right)$ (8)

The dielectric constant values measured from the impedance data are plotted as a function of frequency at different temperatures in Figure 11. It is observed

that the glasses showed high values of the permittivity at low frequency and high temperature. This can be explained on the basis of the polarization existing in the sample. The increased dielectric constant value at lower frequencies region is related to the contribution from space charge/interfacial polarization. Indeed, the higher ${\epsilon }^{\prime }\left(\omega \right)$ values of these amorphous glasses at lower frequencies were explained by the presence of bulk interfacial. Interfacial contains defects such as dangling bonds, vacancies, and vacancy clusters. Thus, the space charges can move under the application of an external field and when they are trapped by the defects, lots of dipole moments (space charge polarization) are formed [31]. The increase in ${\epsilon }^{\prime }\left(\omega \right)$ with temperature is due to the weakening of binding force between molecules/atoms with the increase in temperature, permitting the molecules/atoms to vibrate more and more which in turn increase the polarization, hence increase in the dielectric constant ${\epsilon }^{\prime }\left(\omega \right)$ [32]. In addition, the decrease in ${\epsilon }^{\prime }\left(\omega \right)$ with the frequency is explained by the decreasing number of dipoles which contribute to polarization. As a matter fact, the application of the electrical field with low frequency assists electron hopping between two different sites in the glasses. Whereas, at high frequency, the charge carriers will be unable to rotate with enough speed, so their oscillation is going to vanish [33].

6. Conclusion

The glasses inside the 20Li₂O-(50 − x)Li₂WO₄-xFe₂O₃-30P₂O₅ system were prepared by melt-quenching method. The substitution of Li₂WO₄ by Fe₂O₃ increases the density and the glass transition temperature T_g. The analysis of the glasses by IR spectroscopy has revealed that the introduction of Fe₂O₃ content in the glasses transforms some metaphosphate groups (Q²) to pyrophosphate (Q¹) and orthophosphate (Q⁰) structural units. The formation of the Fe-O-P water-resist bonds in the glass network is highlighted and this iron bond-type improved their chemical durability. The electrical conductivity of the glasses is thermally activated and follows an Arrhenius behavior. The dielectric relaxation behavior inside the glasses is non-Debye.

Acknowledgements

The authors would like to thank CNRST and OCP for their financial support of this work (PPR project).

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

Cite this paper

Es-Soufi, H., Bih, L. and Benzineb, M. (2019) Study of Tungsten Phosphate Glasses Containing Fe₂O₃. New Journal of Glass and Ceramics, 9, 33-49. https://doi.org/10.4236/njgc.2019.93004

References