Andrea Moguš-Milanković

Iron Redox Equilibrium, Structure and Properties of Zinc Iron Phosphate Glasses

Abstract Iron redox equilibrium, structure and properties were investigated for 40Fe2O3–60P2O5 (mol%) glasses melted at different temperatures. The Fe2+/(Fe2++Fe3+) ratio increased from 17% to 50% as the melting temperature changed from 1150°C to 1400°C. The equilibrium constant, K, for the reaction of Fe3+ being reduced to Fe2+ varied with temperature as lnK=9.40–1.58×104/T. The Raman and infrared spectra indicated that the basic iron pyrophosphate structure of the 40Fe2O3–60P2O5 (mol%) glasses did not change as the Fe2+/(Fe2++Fe3+) ratio changed. All of the properties did not change to any major degree with increasing the melting temperature. The molar volume decreased while the density increased with increasing Fe2+/(Fe2++Fe3+) ratio. It was found by DTA and XRD that two phases, Fe3(P2O7)2 and Fe4(P2O7)3, crystallized from the glass when the glass was heated in nitrogen. The crystallization behavior suggested that the amount of the crystal, Fe3(P2O7)2, may increase with increasing Fe2+/(Fe2++Fe3+) ratio, which supported the opinion that there are some structural similarities between the iron phosphate glass and the crystalline Fe3(P2O7)2 in terms of the iron coordination number and bonding of the phosphate groups. The decrease in dc resistivity and increase in dielectric constant and dielectric loss tangent, which occurred with increasing the Fe2+/(Fe2++Fe3+) ratio, were attributed to the increase of the electronic hopping from Fe2+ ions to Fe3+ ions.

Chemically Durable Iron Phosphate Glasses for Vitrifying Sodium Bearing Waste (SBW) Using Conventional and Cold Crucible Induction Melting (CCIM) Techniques

A simulated sodium bearing waste (SBW) was successfully vitrified in iron phosphate glasses (IPG) at a maximum waste loading of 40 wt% using conventional and cold crucible induction melting (CCIM) techniques. No sulfate segregation or crystalline phases were detectable in the IPG when examined by SEM and XRD. The IPG wasteforms containing 40 wt% SBW satisfy current DOE requirements for aqueous chemical durability as evaluated from their bulk dissolution rate (DR), product consistency test, and vapor hydration test. The fluid IPG wasteforms can be melted at a relatively low temperature (1000 °C) and for short times (<6 h). These properties combined with a significantly higher waste loading, and the feasibility of CCIM melting offer considerable savings in time, energy, and cost for vitrifying the SBW stored at the Idaho National Engineering and Environmental Laboratory in iron phosphate glasses.

Structure of sodium phosphate glasses containing Al2O3 and/or Fe2O3. Part I

Abstract The relationship between the composition and structure for three series of glasses: Na2O–Al2O3–P2O5 (NAP); Na2O–Al2O3–Fe2O3–P2O5 (NAFP), and Na2O–Fe2O3–P2O5 (NFP) has been studied. The structural changes in the NAP, NAFP and NFP glasses have been investigated by Raman and by Raman difference spectroscopy (RDS). The Raman spectra show that the addition of Al2O3 content to sodium phosphate glasses has a different effect on the phosphate network than the addition of Fe2O3. With increasing Fe2O3 content up to 20 mol% the structure changes from the chain-like metaphosphate to the pyrophosphate structure. The iron ions play an important role in forming P–O–Fe bonds that strengthen the cross-bonding of shorter pyrophosphate chains. On the other hand, with increasing Al2O3 content up to 20 mol%, the sodium metaphosphate is replaced by aluminium metaphosphate where Al(OP)6 units cross-link phosphate chains. The addition of Fe2O3 to Al2O3 in phosphate glasses containing both Al2O3 and Fe2O3 (NAFP series), enhances the formation of pyrophosphate units because iron ions have stronger effect on the depolymerization of metaphosphate chains if compared to the aluminium ions.

Electrical Conductivity in Mixed-alkali Iron Phosphate Glasses

Abstract The electrical conductivity of mixed-alkali, sodium and potassium, iron phosphate glasses has been studied in the frequency range from 0.1 Hz to 10 kHz and over a temperature range from 303 to 423 K. The dc conductivity of the alkali-free iron phosphate glasses was 5–10 times higher than that of the single- or mixed-alkali iron phosphate glasses containing a total of 20 mol% alkali. The dc conductivity for the mixed-alkali, sodium and potassium, iron phosphate glasses is independent of the Na/K ratio and there is no evidence of any mixed-alkali effect. The sodium and potassium ions have such a low mobility in both single- and mixed-alkali iron phosphate glasses that they make no detectable contribution to the total conductivity that is electronic in origin. The Raman spectra for all the glasses are identical which indicates that the structure of the single- or mixed-alkali glasses are the same.

Thermally stimulated polarization and dc conduction in iron phosphate glasses

Thermally stimulated polarization (TSPC) and depolarization current (TSDC) techniques were used to study electrical polarization and conduction mechanisms in iron phosphate and sodium-iron phosphate glasses. TSDC measurements from 120 to 350 K show two current peaks, P1, attributed to the polarization caused by intrinsic dipolar defects, and P2, due to space-charge relaxation. The electrical conductivity was examined on the basis that the activation energy for electronic conduction is lower than that for ionic conduction. The dc conductivity depends upon iron oxide content and distance between iron ions, which suggests electronic conduction. The difference in activation energy between TSDC peaks and dc conductivity is discussed. Infrared absorption spectra indicate that iron ions can act as a network former and/or modifier depending upon the Fe(II)/Fe(III) ratio in the glass.

Electrical, dielectric and spectroscopic studies on MnO doped LiI–AgI–B2O3 glasses

LiI–AgI–B2O3 glasses doped with different concentrations of MnO (ranging from 0 to 0.8 mol%) were prepared. Electrical and dielectric properties have been studied over a wide frequency range of 10−2 – 106 Hz and in the temperature range from 173 to 523 K. The valence states of manganese ions and their coordination in the glass network have been investigated using optical absorption, luminescence, and ESR spectroscopy. The analysis of the spectroscopic results has indicated that the manganese ions exist in both Mn2+ and Mn3+ states and occupy octahedral and tetrahedral positions. With increasing MnO concentration there is a gradual increase in the tetrahedral occupancy of Mn2+ ions at the expense of octahedral occupancy in the glass network. The results of dc conductivity have indicated that when T > θD/2, the small polaron hopping model is appropriate and the conduction is adiabatic in the nature. Further, the analysis of experimental data indicates that there is a mixed, ionic and electronic, conduction....

Iron valence and coordination in phosphate glasses as studied by optical spectroscopy

Abstract Optical spectroscopy was employed to study iron phosphate glasses of the general composition x Fe 2 O 3 (100− x )P 2 O 5 (14≤ x ≤47 mol%). Cation vibrational properties showed that iron is predominantly present as iron(II) in glasses of lower total iron content and as iron(III) in glasses of higher total iron content. In glasses containing 40 and 47 mol% Fe 2 O 3 some of iron(III) ions changed from octahedral to tetrahedral coordination. Increasing iron content increased network disorder to the point where the structural changes occur, i.e., where iron cations changed their charge or coordination.

Supramolecular Ionic-Liquid Gels with High Ionic Conductivity.

Supramolecular ionogels were prepared by the gelation of room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4 ]) with (S,S)-bis(leucinol)oxalamide. Remarkably, the ionic conductivity of solutions and ionogels with low gelator concentrations is higher than that of neat [BMIm][BF4 ]. On the basis of molecular dynamics simulations and quantum mechanical calculations, the origin of this phenomenon is attributed to the higher affinity of gelator molecules towards [BF4 ](-) ions, which reduces the electrostatic attraction between [BMIm](+) and [BF4 ](-) and thus increases their mobility. With increasing gelator concentration, the ionic conductivity decreases due to the formation of a denser gelator matrix, which hinders the pathways for ionic transport. However, even for very dense ionogels, this decrease is less than one order of magnitude relative to neat [BMIm][BF4 ], and thus they can be classified as highly conductive materials with strong potential for application as functional electrolytes.

Electrical properties of sodium phosphate glasses containing Al2O3 and/or Fe2O3. Part II☆

Abstract The electrical properties for three series of glasses: xNa2O–(40−x)Al2O3–60P2O5, (20⩽x⩽35), (NAP); 20Na2O–xAl2O3–(20−x)Fe2O3–60P2O5, (5⩽x⩽15), (NAFP), and xNa2O–(40−x)Fe2O3–60P2O5, (20⩽x⩽35), (NFP) glasses were measured by impedance spectroscopy in the frequency range from 1 Hz to 1 MHz and over the temperature range from 303 to 473 K. It was shown (in Part I) that the addition of Fe2O3 has significant effects on the structure of these glasses and Fe ions play a different structural role in phosphate network than that of Al ions. Such effects reflect changes in the origin of electrical conduction. With increasing Fe2O3 content in NAFP and NFP glasses the dc conductivity depends upon distance between iron ions and the activation energy of 53.1 kJ mol −1 indicates electronic conduction. On the other hand, the decrease in dc conductivity and activation energy for glasses in NAP series is attributed to the decrease in Na2O content from 35 to 20 mol%. The activation energy varies from 80.1 to 72.4 kJ mol −1 for NAP glasses suggesting ionic conduction. The impedance analysis for these glasses shows that the changes in the electrical conduction mechanisms coincide with the changes in the structure.

Electrical mobility of silver ion in Ag2O-B2O3-P2O5-TeO2 glasses.

The effect of adding TeO(2) into (100 - x)[0.5Ag(2)O - 0.1B(2)O(3) - 0.4P(2)O(5)] - xTeO(2), with 0-80 mol % TeO(2) glass, on the structural changes and electrical properties has been investigated. DSC and thermodilatomery were used to study their thermal behavior, structure was studied by Raman spectroscopy, and electrical properties have been studied by impedance spectroscopy over a wide temperature and frequency range. The introduction of TeO(2) as a third glass former to the glass network causes the structural transformation from TeO(3) (tp) to TeO(4) (tbp) which contributes to the changes in conductivity. The glasses with low TeO(2) content show only a slow decrease in dc conductivity with addition of TeO(2) due to the increase of the number of nonbridging oxygens, which increases the mobility of Ag(+) ions. The steep decrease in conductivity for glasses containing more than 40 mol % TeO(2) is a result of decrease of the Ag(2)O content and stronger cross-linkage in glass network through the formation of more Te-(eq)O(ax)-Te bonds in TeO(4) tbp units. The glasses obey ac conductivity scaling with respect to temperature, implying that the dynamic process is not temperature dependent. On the other hand, the scaling of the spectra for different glass compositions showed the deviations from the Summerfield scaling because of the local structural disorder which occurs as a result of the structural modifications in the tellurite glass network.