Thermal stability and transport studies of (100 − 2x)TeO2-xAg2O-xWO3 (7.5 ≤ x ≤ 30) glass system

Differential scanning calorimetry (DSC), infrared (IR) and direct current (DC) conductivity studies have been carried out on (100 − 2x)TeO₂-xAg₂O-xWO₃ (7.5 ≤ x ≤ 30) glass system. The IR studies show that the structure of glass network consists of [TeO₄], [TeO₃]/[TeO₃₊₁], [WO₄] units. Thermal properties such as the glass transition (T_g), onset crystallization (T_o), thermal stability (ΔT), glass transition width (ΔT_g), heat capacities in the glassy and liquid state (C_pg and C_pl), heat capacity change (ΔC_p) and ratios C_pl/C_pg of the glass systems were calculated. The highest thermal stability (237 °C) obtained in 55TeO₂-22.5Ag₂O-22.5WO₃ glass suggests that this new glass may be a potentially useful candidate material host for rare earth doped optical fibers. The DC conductivity of glasses was measured in temperature region 27-260 °C, the activation energy (E_act) values varied from 1.393 to 0.272 eV and for the temperature interval 170-260 °C, the values of conductivity (σ) of glasses varied from 8.79 × 10⁻⁹ to 1.47 × 10⁻⁶ S cm⁻¹.     

Structural study of (50 − x)Na2O-xCuO-10Bi2O3-40P2O5 glasses

(50−x)Na₂O-xCuO-10Bi₂O₃-40P₂O₅ glasses (0≤x≤25) were prepared by melting at 900-1100°C mixtures of Na₂CO₃, Bi₂O₃, CuO and (NH₄)₂HPO₄. DSC measurements give the variation of glass transition temperature T_g from 318 (x=0) to 378°C (x=25). FTIR spectroscopy shows the evolution of the phosphate skeleton: (PO₃)_∞ chains for 60Na₂O-40P₂O₅ to P₂O₇ groups in the glass containing Bi₂O₃ or both Bi₂O₃ and CuO. When bismuth and copper oxides replace Na₂O, phosphate chains are depolymerized by the incorporation of Bi₂O₃ and CuO through POBi and POCu bonds. P₂O₇ groups are predominant structural units in the richest CuO glass. The variation of T_g also supports these results.     

Influence of mixed alkali on the spectral properties of vanadyl ions doped xNa2O-(30 − x)K2O-60B2O3 glasses—an EPR and optical study

Electron paramagnetic resonance (EPR) and optical investigations have been performed in the mixed alkali borate xNa₂O-(30 − x)K₂O-60B₂O₃ (5 ≤ x ≤ 25) glasses doped with 10 mol% of vanadyl ions in order to look for the effect of ‘mixed alkalis’ on the spectral properties of the glasses. The observed EPR spectra have structures for x > 5 mol% which are characteristic of a hyperfine interaction arising from an unpaired electron with the ⁵¹V nucleus and it builds up in intensity as x increases. It is observed that the mixed alkali play a significant role in accommodating the vanadyl ions in these mixed alkali glasses and for x > 5 mol%, shows a well resolved hyperfine structure typical for isolated VO²⁺ ions. The spin-Hamiltonian parameters (g and A), the dipolar hyperfine coupling parameter (P) and Fermi contact interaction parameter (k) have been evaluated. It is observed that the spin-Hamiltonian parameters do not vary much with the change in composition. It is observed that with increase of x, an increase occurs in tetragonal distortion for VO²⁺. The number of spins (N) participating in resonance and the paramagnetic susceptibility (χ) have been calculated. It is observed that N and χ increase with x. The optical bandgap energies evaluated for these glasses slightly increase with x and reach a maximum around x = 20 and thereafter decrease showing the mixed alkali effect. Optical band gap energies obtained in the present work vary from 2.73 to 3.10 eV for both the direct and indirect transitions. The physical parameters of the glasses are also determined with respect to the composition.     

Properties of (0.5−x)Zn-xFe2O3-0.5P2O5 glasses

Experimental determination of the properties of less studied zinc-iron-phosphate glasses was investigated. Glasses of the general composition (50−x)ZnO-xFe₂O₃-50P₂O₅, mol%, with x=0, 10, 20, 30 and 40, was chosen for these investigations. These studies included, glass forming, glass density, thermal expansion coefficient, dilatometric softening temperature, an initial test of chemical durability and vibrational properties. It is shown that an Fe/P ratio of the compositions at about 0.6 and 0.8 and the O/P ratio at 3.4 and 3.8 could be considered as chemically durable phosphate candidates.     

Chemical stability and dielectric properties of RO-La2O3-B2O3 (R = Ca, Mg, Zn)-based ceramics

New lanthanum borate (La₂O₃-B₂O₃) glasses modified with divalent oxides, such as CaO, MgO and ZnO were investigated as potential low temperature dielectrics by understanding compositional dependence of dielectric properties and chemical leaching resistance. Firing behavior, such as densification and crystallization, depended strongly on the glass composition and is found to influence the resultant dielectric performance. Specifically, the dielectric composition of 20ZnO-20La₂O₃-60B₂O₃ glass with 40 wt% Al₂O₃ as a filler showed distinct enhancements of dielectric properties, i.e., k ∼ 8.3 and Q ∼ 1091 at the resonant frequency of 17.1 GHz, as a result of 850 °C firing. The result was believed related to earlier densification and unexpected evolvements of ZnAl₂O₄ and La(BO₂)₃ phases during firing. The Mg-containing glass sample was most stable in strong acid solutions and did not show any significant changes in microstructure even after 300 min exposure. The Ca-containing glass sample was not regarded as a promising candidate for low temperature dielectrics from the observed low quality factor and weak chemical durability.     

Investigation of fast-ionic transport in the ternary system (Cu1−xAgxI)-(Ag2O)-(V2O5), (0.05≤x≤0.25)

The occurrence of fast-ionic conduction in the ternary system 40(Cu_1−xAg_xI)-40(Ag₂O)-20(V₂O₅), (0.05≤x≤0.25) has been described. The formation of composite solid electrolyte materials comprising glassy and crystalline phases has been identified by means of X-ray diffraction analysis. Fourier transform infrared spectroscopic studies have confirmed the presence of VO₄³⁻ and V₂O₇⁴⁻ groups in these new materials. Detailed thermal characterization of these materials carried out by differential scanning calorimetry has indicated the transition temperature of one of the reaction products viz., AgI. From the conductivity measurements carried out using the complex impedance analysis, the values of room temperature electrical conductivity (σ_RT) and activation energy for ionic migration in these materials are found to be of the order of 10⁻² to 10⁻⁴ S cm⁻¹ and 0.22-0.35 eV, respectively. The ionic transport number (t_i) measurements made using Wagner’s polarization method and evaluation of silver ionic transport number (t_Ag⁺) by galvanic cell method have been used to estimate the extent of contribution of ionic conductivity especially due to silver ionic transport to the total conductivity observed in these materials.     

Solution calorimetric study of mixing enthalpy in 0.8[xB2O3-(1 − x)SiO2]-0.2K2O glasses at 298 K

The mixing enthalpies in the 0.8[xB₂O₃-(1 − x)SiO₂]-0.2K₂O glassy system with 0 ≤ x ≤ 1 have been deduced from accurate calorimetric measurements of solution enthalpies in acidic solvent at 298 K. The substitution of SiO₂ by B₂O₃ corresponds to a negative enthalpic effect and suggests the absence of some glassy miscibility gap. The mixing enthalpies are of the same order of magnitude as mixing enthalpies usually found in molten salts.     

Oxide-ion conductivity in CuxCe1−xO2−δ (0 ≤ x ≤ 0.10)

Up to 10 at.% of copper readily substitutes for cerium in ceria. It is found that at oxygen partial pressures between 0.21 atm and 10⁻⁵ atm, Cu_xCe_1−xO_2−δ (0 ≤ x ≤ 0.10) solid solution behave as an oxide-ion electrolyte. Interestingly, Cu_0.10Ce_0.90O_2−δ exhibits the oxide-ion conductivity of ca. 10⁻⁴ Ω⁻¹ cm⁻¹ at 600 °C at an oxygen partial pressure of 10⁻⁵ atm.     

Synthesis and properties of new CdSe-AgI-As2Se3 chalcogenide glasses

The glass-forming region in the pseudo-ternary CdSe-AgI-As₂Se₃ system was determined. Measurements including differential scanning calorimetry (DSC), density, and X-ray diffraction were performed. The effect resulting from the addition of CdSe or AgI has been highlighted by examining three series of different base glasses. The characteristic temperatures of the glass samples, including glass transition (T_g), crystallisation (T_x), and melting (T_m) temperatures are reported and used to calculate their ΔT = T_x − T_g and their Hruby, H_r = (T_x − T_g)/(T_m − T_x), criteria. Evolution of the total electrical conductivity σ and the room temperature conductivity σ₂₉₈ was also studied. The terahertz transparency domain in the 50-600 cm⁻¹ region was pointed for different chalcogenide glasses (ChGs) and the potential of the THz spectroscopy was suggested to obtain structural information on ChGs.     

Study on the electrical conductivity and oxygen diffusion of oxide-ion conductors La2Mo2−xTxO9−δ (T = Al, Fe, Mn, Nb, V)

In this paper, the influence of cationic substitutions at Mo site with Al³⁺, Fe³⁺, Mn⁴⁺, Nb⁵⁺ and V⁵⁺ ions on the structure, oxygen ion diffusion and electrical properties in La₂Mo₂O₉ oxide-ion conductors have been investigated by X-ray diffraction method, dielectric relaxation technique and direct current conductivity measurement. Except for V⁵⁺ substitution all of these substitutions up to 5% cannot suppress the phase transition in La₂Mo₂O₉. In the dielectric measurement, one prominent relaxation peak is observed in temperature spectrum as well as in frequency spectrum, which is associated with the short-distance diffusion of oxygen vacancies. The activation energy for oxygen ion diffusion is deduced as in the range of 1-1.1 eV for Al, Fe, Mn and Nb doped samples and 1.4-1.5 eV for V doped samples. All substituted samples have a higher conductivity than the un-doped compound. In the Al, Fe, Mn and Nb substituted materials the phase transition is not suppressed; however, K substitution at the La site can completely suppress the transition and maintains high conductivity at low temperature.     

Structural and electrical properties of La2−xBaxMo2O9−x/2 (x = 0-0.18)

La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) have been prepared by solid state reaction method. The lattice parameter of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18-0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La₂Mo₂O₉ is defined in range of 0.18 < x < 0.2. As the replacement degree of La³⁺ by Ba²⁺ increases, the bulk conductivity of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) decreases initially and then increases, a minimum value at La_1.9Ba_0.1Mo₂O_8.95 exists. Hebb-Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173 K. The average thermal expansion coefficient of La_1.84Ba_0.16Mo₂O_8.92 determined by high-temperature XRD was deduced as great as 17.5 × 10⁻⁶ K⁻¹ between 298 and 1173 K.     

All-solid-state cells based on solid electrolyte systems Cu1−xAgxI-Ag2O-Y, where x = 0.05-0.25; Y = MoO3, B2O3, SeO2, V2O5 and CrO3

All-solid-state cells of the configuration (−)Ag + SE//SE//I₂-phenothiazine + C(+) using the best conducting compositions of the solid electrolyte systems, namely, Cu_1−xAg_xI-Ag₂O-Y where x = 0.05, 0.1, 0.15, 0.2 and 0.25, Y = MoO₃, B₂O₃, SeO₂, V₂O₅ and CrO₃, as the electrolytes were fabricated. Discharge, polarization and power characteristics of these cells were also evaluated. The open circuit voltage values of these cells were in the range 620-635 mV. The stability of these cells has been indicated by the constancy of their OCV over a period of 6 months. The polarization and discharge studies on these cells have shown that typical cells based on the electrolytes with Y = B₂O₃, SeO₂ and V₂O₅ would possess discharge capacities of 12.84, 3.76 and 5.05 mA h and specific energy of 6.55, 1.81 and 2.77 W h kg⁻¹, respectively. The solid electrolytes have good electrochemical stability and compatibility with the Ag/Phenothiazine-I₂ electrode couple thus offering their suitability of application in microwatt power sources.     

Synthesis, characterization and electrical conductivity studies on A3Bi2P3O12 (A = Na, K) materials

Crystalline Na₃Bi₂P₃O₁₂, K₃Bi₂P₃O₁₂ and glassy K₃Bi₂P₃O₁₂ compounds were prepared by solid-state reaction method. The prepared samples are characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy and differential scanning calorimetry. The crystalline materials are found to be orthorhombic. The electrical conductivity measurements on the crystalline and glassy samples show that at ∼373 K, the σ_DC for crystalline K₃Bi₂P₃O₁₂ (0.81 × 10⁻⁸ S/cm) is about two orders of magnitude higher than the corresponding glassy phase (1.25 × 10⁻¹⁰ S/cm). The scaling results show that the conductivity relaxation mechanism is independent of temperature.     

Low-temperature sintering characteristics of nano-sized BaNd2Ti5O14 and Bi2O3–B2O3–ZnO–SiO2 glass powders prepared by gas-phase reactions

Nano-sized BaNd₂Ti₅O₁₄ (BNT) powders were prepared by spray pyrolysis from solutions containing ethylenediaminetetraacetic acid and citric acid. Treatment at temperatures ≥900 °C and subsequent milling resulted in nanoparticle powders with orthorhombic crystal structures. The mean particle size of the powder post-treated at 1000 °C was 160 nm. Nano-sized Bi₂O₃–B₂O₃–ZnO–SiO₂ glass powder with 33 nm average particle size was prepared by flame spray pyrolysis and used as a sintering agent for the BNT. BNT pellets sintered at 1100 °C without the glass had porous structures and fine grain sizes. Those similarly sintered with the glass had denser structures and larger grains.     

Crystal and magnetic structure and cation distribution of Mn2−xV1+xO4 spinels (x = 0, 1/3 and 1)

We synthesized the spinel-type compounds belonging to the Mn_2−xV_1+xO₄ series with x = 0, 1/3 and 1 as polycrystalline powders. Crystal and magnetic structures were refined using synchrotron X-ray and neutron powder diffraction. At 300 K all members crystallize in the cubic system, space group , and show a structural transition at low temperature, changing to a tetragonal symmetry (space group I4₁/amd). Cations distributions between octahedral and tetrahedral sites were refined from neutrons diffraction (ND) data and explained based on crystal field stabilization energies (CFSE) and ionic radii. The magnetic unit cell is the same as the crystallographic one, having identical symmetry relations. The magnetic structure was refined as an arrangement of collinear spins, antiferromagnetically ordered, parallel to the c-axis of the unit cell. The refined site magnetic moments are smaller than those obtained from hysteresis cycles of the M vs. H measurements, indicating that some non-collinear disordered component coexists with the ordered component along the c-axis.     

Synthesis, low-temperature sintering and the dielectric properties of the ZnO-(1 − x)TiO2-xSnO2 (x = 0.04-0.2)

ZnO-(1 − x)TiO₂-xSnO₂ (x = 0.04-0.2) ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. Fine particle powders were prepared by chemical processing to activate the formation of compound and to improve the sinterability. One wt.% of V₂O₅ and B₂O₃ with the mole ratios of 3:1 were used to lower the sintering temperature of ceramics. The effect of Sn content on phase structure and dielectric properties were investigated. The results show that the substituting Sn for Ti accelerates the hexagonal phase transition to cubic phase, and an inverse spinel structure Zn₂(Ti_1−xSn_x)O₄ solid solution forms. The best dielectric properties obtained at x = 0.12. The ZnO-0.88TiO₂-0.12SnO₂ ceramics sintered at 900 °C exhibit a good dielectric property: ?_r = 29 and tan δ = 9.86 × 10⁻⁵. Due to their good dielectric properties, low firing characteristics, ZnO-(1 − x)TiO₂-xSnO₂ (x = 0.04-0.2) can serve as the promising microwave dielectric capacitor.     

SrSn1−xFexO3−δ (0≤x≤1) perovskites: a novel mixed oxide ion and electronic conductor

The electrical conductivity of SrSn_1−xFe_xO_3−δ increases with the Fe content and reaches a value of ∼10⁻¹ S/cm at 25°C at x=1. Compounds with low Fe content exhibit both ionic and electronic conductivity, while the higher Fe content perovskites are mainly electronic conductors with a conductivity independent of the oxygen partial pressure over a wide range from 0.21 to 10⁻²² atm.     

(5 − x)BaO-xMgO-2Nb2O5 ceramics prepared using a reaction-sintering process

(5 − x)BaO-xMgO-2Nb₂O₅ (x = 0.5 and 1; 5MBN and 10MBN) microwave ceramics prepared using a reaction-sintering process were investigated. Without any calcinations involved, the mixture of BaCO₃, MgO, and Nb₂O₅ was pressed and sintered directly. MBN ceramics were produced after 2-6 h of sintering at 1350-1500 °C. The formation of (BaMg)₅Nb₄O₁₅ was a major phase in producing 5MBN ceramics, and the formation of Ba(Mg_1/3Nb_2/3)O₃ was a major phase in producing 10MBN ceramics. As CuO (1 wt%) was added, the sintering temperature dropped by more than 150 °C. We produced 5MBN ceramics with these dielectric properties: ?_r = 36.69, Qf = 20,097 GHz, and τ_f = 61.1 ppm/°C, and 10MBN ceramics with these dielectric properties: ?_r = 39.2, Qf = 43,878 GHz, and τ_f = 37.6 ppm/°C. The reaction-sintering process is a simple and effective method for producing (5 − x)BaO-xMgO-2Nb₂O₅ ceramics for applications in microwave dielectric resonators.     

YAG laser-induced structural modification in transition metal ion containing 40K2O-40Nb2O5-20SiO2 glasses

Glasses with the compositions of 40K₂O-40Nb₂O₅-20SiO₂ (in mol%) containing different concentrations (0.01, 1 and 2 mol%) of NiO were prepared by a melt quenching technique. The glasses were irradiated with a continuous wave Nd:YAG laser with a wavelength of 1064 nm, and a metastable crystalline phase of KNbO₃ was obtained. In 2 mol% NiO-doped glass, lines with a width of ∼10 μm are successfully patterned by laser irradiations with a power of 0.9 W and a scanning speed of 15 μm/s. It is found from micro-Raman scattering spectra that the lines are composed of the metastable crystalline phase of KNbO₃. The crystallization mechanism in Nd:YAG laser-induced crystallization in the glasses is discussed in comparison with the usual crystallization in an electric furnace.