Electrical Properties of Phosphate Glasses

Investigation of the electrical properties of phosphate glasses where transition metal oxide such as iron oxide is the network former and network modifier is presented. Phosphate glasses containing iron are electronically conducting glasses where the polaronic conduction is due to the electron hopping from low to high iron valence state. The identification of structural defects caused by ion/polaron migration, the analysis of dipolar states and electrical conductivity in iron phosphate glasses containing various alkali and mixed alkali ions was performed on the basis of the impedance spectroscopy (IS). The changes in electrical conductivity from as-quenched phosphate glass to fully crystallized glass (glass-ceramics) by IS are analyzed. A change in the characteristic features of IS follows the changes in glass and crystallized glass network. Using IS, the contribution of glass matrix, crystallized grains and grain boundary to the total electrical conductivity for iron phosphate glasses was analyzed. It was shown that decrease in conductivity is caused by discontinuities in the conduction pathways as a result of the disruption of crystalline network where two or more crystalline phases are formed. Also, phosphate-based glasses offer a unique range of biomaterials, as they form direct chemical bonding with hard/soft tissue. The surface charges of bioactive glasses are recognized to be the most important factors in determining biological responses. The improved bioactivity of the bioactive glasses as a result of the effects of the surface charges generated by electrical polarization is discussed

The Effects of Silica on the Properties of Vitreous Enamels

Ground coat enamels for low carbon steel that contain silica as a mill addition have been developed to study the changes of their properties. Acid-resistant commercial enamel where silica addition was varied from 0 to 10.0 wt % was used for this investigation. The effects of the addition on the corrosion resistance, thermal properties, electrical properties, and mechanical adherence of the enamel to low carbon steel were studied. The corrosion resistance of the steel enameled coupons was tested using a salt spray (fog) apparatus for time periods reaching 168 h at room temperature. It was found that, although the density was not affected, the adherence decreased with an increase in silica content. As expected, the silica addition decreased the coefficient of thermal expansion, which is directly related to the increasing stress between the glass and steel in accordance with the adherence results. A mill addition of 7.5 wt% of silica to the samples was sufficient to obtain adequate enamel adherence and good corrosion resistance. Furthermore, the addition of silica influenced the electrical conductivity and dielectric permittivity measurements at room temperature and the conductivity measured in a wide frequency range (1 Hz-1 MHz). The dielectric permittivity measured at 1 MHz showed decrease after the addition of up to 7.5 wt% of silica

Investigation of the Jahn-Teller Transition in TiF3 using Density Functional Theory

We use first principles density functional theory to calculate electronic and magnetic properties of TiF3 using the full potential linearized augmented plane wave method. The LDA approximation predicts a fully saturated ferromagnetic metal and finds degenerate energy minima for high and low symmetry structures. The experimentally observed Jahn-Teller phase transition at Tc=370K can not be driven by the electron-phonon interaction alone, which is usually described accurately by LDA. Electron correlations beyond LDA are essential to lift the degeneracy of the singly occupied Ti t2g orbital. Although the on-site Coulomb correlations are important, the direction of the t2g-level splitting is determined by the dipole-dipole interactions. The LDA+U functional predicts an aniferromagnetic insulator with an orbitally ordered ground state. The input parameters U=8.1 eV and J=0.9 eV for the Ti 3d orbital were found by varying the total charge on the TiF$_6^{2-}$ ion using the molecular NRLMOL code. We estimate the Heisenberg exchange constant for spin-1/2 on a cubic lattice to be approximately 24 K. The symmetry lowering energy in LDA+U is about 900 K per TiF3 formula unit.Comment: 7 pages, 9 figures, to appear in Phys. Rev.

Atomic structure of sodium iron phosphate glasses

The atomic structure of a series of sodium iron phosphate glasses is studied using different experimental techniques: X-ray and neutron diffraction (ND), infrared spectroscopy, extended X-ray absorption fine structure (EXAFS), and X-ray absorption near-edge structure (XANES). Detailed information about the atomic pair correlations is obtained. The high resolution of ND in real space resolves two P–O distances at 1.48 Ǻ and 1.59 Ǻ as expected. All the glasses are found to consist of a phosphate tetrahedral network with metaphosphate chains and pyrophosphate units, and every phosphate unit is found to have two or three nonbridging oxygen (NBO) links available to coordinate with Na and Fe cations. The Fe–O coordination number in these glasses is found to decrease from 5.7 to 4.8 with increasing the Fe content, whereas the Na coordination number of approximately 5 is detected for all the samples

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

Structure and Crystallization Study of Iron Phosphate Glasses

Phosphate glasses have been studied extensively for their potential applications as disposal of nuclear waste. Addition of iron oxide to phosphate glasses has a significant effect on the structure and properties of these glasses. Differential thermal analysis, x-ray diffraction analysis and Raman spectroscopy have been used in the investigation of structural changes in the phosphate network and crystalline products of xFe2O3(100-x)P2O5 (15 \u3c x \u3c 50) glasses. The addition of Fe2O3 in the phosphate glass results in the systematic changes from chain-like phosphate structure present in metaphosphate to the orthophosphate structure. With increasing iron content, there is a corresponding increase in the concentration of iron(III) ions in the glass and a decrease of the number of P-O-P bonds since these bonds are replaced by an increasing number of Fe-O-P bonds. The results of the crystallization process suggest that the glassy and its crystalline compositions both have the same basic phosphate structure

Electrical Conductivity and Structural Properties of Cesium Iron Phosphate Glasses: A Potential Host for Vitrifying Nuclear Waste

The thermally stimulated current (TSC) and dc conductivity for iron phosphate glasses containing up to 28 mol% Cs2O have been measured in a temperature range from 120 to 400 K. The dc conductivity and activation energy were constant and independent of Cs2O content. With increasing cesium concentration in cesium iron phosphate glasses the slowly moving cesium ions are more tightly bound to the non-bridging oxygen ions and make no measurable contribution to dc conductivity. The dc conduction in these glasses is totally electronic, controlled by electron hopping between iron ions. The ionic conduction is immeasurably small because of the low mobility of the cesium ions. This agreement is reinforced by the excellent chemical durability of the glasses, where the dissolution rate at 90°C changes little with increasing Cs2O content. Raman spectroscopy indicated that the structure of these glasses was composed of predominantly pyrophosphate (P2O7) groups, but the metaphosphate chains (PO3) also existed

Electrical transport in iron phosphate glass-ceramics

In this study, the effect of crystallization on electrical transport in iron phosphate glass with composition 40Fe(2)O(3)-60P(2)O(5) (mol%) was investigated. This type of glasses exhibits the electronic conduction which is described by polaronic conduction mechanism. Induced and controlled heat-treatment of 40Fe(2)O(3)-60P(2)O(5) glass leads to partially and almost fully crystallized glass-ceramics. The influence of heat-treatment temperature on structural changes, the appearance of crystalline phases, their interactions and influence on electrical transport were studied in details. Depending on heat-treatment temperature various phases, such as Fe-3(P2O7)(2), Fe-4(P2O2)(3) and Fe(PO3)(3), embedded in glass matrix, were confirmed by powder X-ray diffraction (XRPD) and Raman spectroscopy (RS), whereas thermally induced microstructural changes were evaluated by high-resolution transmission electron microscopy (HR-TEM). Impact of the structural changes on the electrical properties was explored by means of impedance spectroscopy (IS) in wide frequency and temperature range. DC conductivity decreases in the first stage of the crystallization process and achieves its minimum. With further increase of the heat -treatment temperature, the electrical conductivity rises as a result of an enhanced crystallinity. IS results show the evolution of crystallization and formation of crystalline phases in glassy matrix. Correlation between the shape of the spectra and the nature of the electrical transport was confirmed. Different electrical processes have been identified by detail analysis and contribution of each crystalline phase, glass matrix and grain boundaries to total resistance/conductivity is determined by complex impedance plots by means of equivalent circuit modeling

Structural Study of Iron Phosphate Glasses

The effect of iron content on the structure of xFe2O3(100-x)P2O5, (15≤x≤50), glasses was investigated by Raman spectroscopy and x-ray diffraction. The Raman spectra for these glasses indicate that the nature of the phosphate network changes from the chain like metaphosphate to the orthophosphate structure with increasing Fe2O3 content. As the Fe2O3 content increased from 15 to 38 wt% the average length of the polyphosphate chains became shorter as the iron ions were effective in bonding the ends of the chains to the surrounding glass structure. At higher Fe2O3 content, up to 50 wt%, the iron ions occupy tetrahedral sites as a result of iron substitution into phosphate chains and formation of FeO4 tetrahedra. The structure of the glass differs from that of the crystal but the local environment of the iron ions is retained in the glass and crystallised counterpart of these iron phosphate compositions