Poly(ethyleneglycol methacrylate phosphate) and heterocycle based proton conducting composite materials

Novel anhydrous proton conducting polymer electrolytes based on poly(ethyleneglycol methacrylate phosphate) (PEGMAP) and heterocycle have been investigated. The materials were synthesized via conventional radical bulk polymerization of ethylene glycol methacrylate phosphate in the presence of proton solvents such as imidazole (Im) or benzimidazole (BnIm). The poly(EGMAP–Im_x) or poly(EGMAP–BnIm_x) composites were produced where x is the molar ratio of heterocycle to monomer in the feed. The polymer–heterocycle electrolytes were characterized by elemental analysis (EA), FT-IR spectroscopy, thermogravimetry analysis (TG), differential scanning calorimetry (DSC) and impedance spectroscopy. Maximum proton conductivity of 2 × 10^− 4 S/cm has been obtained for the anhydrous composite electrolytes at 160 °C.     

Thermal diffusivity and electrical conductivity in fast ion conducting composites: A correlation

The thermal diffusivity and electrical conductivity have been measured for two ion conducting polyethylene oxide (PEO) based polymer–ceramic composites viz. (PEO:NH₄I) + xAl₂O₃, (PEO:LiBF₄) + xBa_0.70Sr_0.30TiO₃ and two solid–solid composites viz. AgI + xAl₂O₃, AgI +xBa_0.70Sr_0.30TiO₃. The thermal diffusivity has been measured by the novel photoacoustic technique while the electrical conductivity has been measured by impedance spectroscopy technique using complex impedance plots. The pattern of variation in the electrical conductivity (σ) vs. composition plot and that in the thermal diffusivity (α_s) vs. composition plot are similar. Interestingly, the correlation between α_s and σ is not only qualitative but is quantitative as well in the sense that the ratio (α_s / σ) remains constant for all the samples within the same system (though their conductivities are different) similar to Wiedmann–Franz law applicable to metallic conductors.     

Synthesis and electrical transport properties of Lu2+xTi2−xO7−x/2 oxide-ion conductors

The Lu_2+xTi_2−xO_7−x/2 (x = 0; 0.052; 0.096; 0.286; 0.44; 0.63; 33.3–49 mol% Lu₂O₃) nanoceramics with partly disordered pyrochlore-type structure are prepared by sintering freeze-dried powders obtained by a co-precipitation technique with 1600 °C annealing. Similar to pyrochlore-like compositions in the zirconate system, some of the new titanates are good oxide-ion conductors in air. The new solid-state electrolytes have oxide-ion conductivity in the interval of 1.0 × 10^− 3 – 2.5 × 10^− 3 S/cm at 740 °C in air. This value of conductivity is comparable with that of ZrO₂/Y₂O₃ ceramics. The conductivity of Lu_2+xTi_2−xO_7−x/2 depends on the chemical composition. The highest ionic conductivity is exhibited by nearly stoichiometric Lu_2+xTi_2−xO_7−x/2 (x = 0.096; 35.5 mol% Lu₂O₃) material containing ∼ 4.8 at.% Lu_Ti anti-site defects.     

Ionic and electronic conductivity of Yb2+xTi2−xO7−x/2 materials

Bulk and grain boundary conductivities of Yb_2+xTi_2−xO_7−x/2 (x = 0, 0.1, 0.18 and 0.29) materials were studied by impedance spectroscopy in the range 300–900 °C in air. Ionic and electronic conductivities were separated by both ion blocking Hebb–Wagner measurements and total conductivity measurements as a function of oxygen partial pressure in the temperature range 700–1000 °C. The oxygen partial pressure dependence of the total conductivity shows that these materials are nearly pure ionic conductors in air and that the ionic conductivity decreases for Yb-rich compositions. This was interpreted as a predominant effect of a decrease in mobility of ionic charge carriers, opposing the expected increase in concentration of oxygen vacancies with increasing Yb content. The studied materials become mixed conductors under typical fuel conditions, except possibly at temperatures below about 700 °C. Yb-excess slightly suppresses the electronic conductivity.     

Electric properties and structure of Agx(As0.33S0.335Se0.335)100−x bulk glasses

In this paper, results of investigation of the bulk glasses with composition of Ag_x(As_0.33S_0.335Se_0.335)_100−x (x=0–28 at%) are revealed. The amorphous structure of samples was confirmed by the X-ray diffraction analysis. The structure was deduced from the Raman spectra measured for all silver contents in As–S–Se matrix. From the point of their electrical properties, all glasses behave as ionic conductors. Their ac conductivity increases with increasing content of silver. As determined from the comparison of ac and dc conductivities, the contribution of electronic conductivity to the overall conductivity is very low and decreases from about 3% for the glass with 12 at% of Ag to about 1% for the glass with 22 at% of Ag.     

Ionic conductivities of Nasicon-type glass-ceramic superionic conductors in the system Na2O–Y2O3–XO2–SiO2 (X = Ti,Ge, Te)

Glass-ceramics of the titanium-, germanium- or tellurium-containing Na₅RSi₄O₁₂-type (R = rare earth; Y) Na⁺-superionic conductors (N5YXS) were prepared by crystallization of glasses with the composition Na_3+3xY_1 ? xX_ySi_3 ? yO₉ (X = Ti; NYTiS, Ge; NYGeS, X = Te; NYTeS), and the effects of X elements on the separation of the phase and the microstructural effects on the conduction properties of glass-ceramics were discussed. The combination of x and y was most varied in N5YGeS and more limited in the order of N5YTeS > N5YTiS. Their conductivities and activation energies are of the order of 10^? 2 S/cm at 300 °C and of 15 to 24 kJ/mol, respectively. The conductivity of the glass-ceramic N5YXS decreases giving the order N5YGeS > N5YTeS > N5YTiS. It is considered that this order corresponds to the N5 single phase region. Large enhancement of electrical conductivity was observed in the glass-ceramics as the grain growth was promoted with increase of heating temperature and heating time for crystallization.     

Structural,vibrational study and superprotonic behavior of a new mixed dipotassium hydrogenselenate dihydrogenphosphate K2(HSeO4)1.5(H2PO4)0.5

Ongoing studies of the KHSeO₄–KH₂PO₄ system aiming at developing novel proton conducting solids resulted in the new compound K₂(HSeO₄)_1.5(H₂PO₄)_0.5 (dipotassium hydrogenselenate dihydrogenphosphate). The crystals were prepared by a slow evaporation of an aqueous solution at room temperature. The structural properties of the crystals were characterized by single-crystal X-ray analysis: K₂(HSeO₄)_1.5(H₂PO₄)_0.5 (denoted KHSeP) crystallizes in the space group P 1¯ with the lattice parameters: a = 7.417(3) Å, b = 7.668(2) Å, c = 7.744(5) Å, α = 71.59(3)°, β = 87.71(4)° and γ = 86.04(6)°. This structure is characterized by HSeO₄⁻ and disordered (H_xSe/P)O₄⁻ tetrahedra connected to dimers via hydrogen bridges. These dimers are linked and stabilized by additional hydrogen bonds (O–H–O) and hydrogen bridges (O–H…O) to build chains of dimers which are parallel to the [0, 1, 0] direction at the position x = 0.5.The differential scanning calorimetry diagram showed two anomalies at 493 and 563 K. These transitions were also characterized by optical birefringence, impedance and modulus spectroscopy techniques. The conductivity relaxation parameters of the proton conductors in this compound were determined in a wide temperature range. The transport properties in this material are assumed to be due to H⁺ protons hopping mechanism.     

Intermediate temperature proton conductors based on phosphonic acid functionalized oligosiloxanes

Fully immobilized phosphonic acid based proton conductors, where phosphonic acid groups are tethered to cyclic siloxanes via flexible alkane spacers, are synthesized. Unlike conventional hydrated ionomers containing sulfonic acid groups, which are commonly used as separator material in PEM fuel cells, the proton conductivity of these materials occurs within a dynamical hydrogen bond network formed by the protogenic groups (phosphonic acid), which are present at very high concentrations. Conductivities of up to 2 · 10^− 3 S cm^− 1 are obtained at T ≈ 130 °C and RH ≈ 37%. This is only slightly higher than the conductivity of similar imidazole based systems although neat phosphonic acid has a much higher proton conductivity compared to neat imidazole. The proton conductivity of phosphonic acid is more sensitive towards immobilization at cyclic siloxanes and the corresponding restrictions for hydrogen bond formation (aggregation).     

Impedance and Modulus studies of the solid electrolyte system 20CdI2–80[xAg2O–y(0.7V2O5–0.3B2O3)], where 1 ≤x/y ≤ 3

In the present work, an evaluation of the transport properties of super ion conducting quaternary system 20CdI₂–80[xAg₂O–y(0.7V₂O₅–0.3B₂O₃)], where 1 ≤ x/y ≤ 3, in steps of 0.25, to study the effect of changing the modifier to former ratio on the conduction phenomena has been undertaken. Electrical conductivity measurements were made using complex impedance method. The electrical conductivity and conductivity relaxation of the system were studied in the temperature range from 303 K to 333 K and in the frequency range from 100 Hz to 10 MHz. The highest conductivity at room temperature is obtained for the system with modifier to former ratio 1.75. Impedance and modulus analyses had indicated the temperature independent distribution of relaxation times and the non-Debye behavior in these materials. The co-operative motion due to strong coupling between the mobile Ag⁺ ions is assumed to give rise to non-Debye type of relaxation. The silver ionic transport number (t_Ag+) obtained by the emf technique suggested the occurrence of silver ion conduction in the CdI₂-doped Ag₂O–V₂O₅–B₂O₃ system.     

Ionic conductivity in borate glasses with three types of mixed alkali cations

We have investigated the temperature- and frequency-dependent ionic conductivity in (Li_0.67 ? xNa_0.33 Rb_x)₂B₄O₇ (LNRBO) glasses with x = 0, 0.07, 0.2, 0.33, 0.47, and 0.6. The mixed alkali effect of the ternary mixed alkali system LNRBO is compared with that of the binary mixed alkali systems (Li_1 ? xNa_x)₂B₄O₇ (LNBO), (Li_1 ? xRb_x)₂B₄O₇ (LRBO) and the single alkali glass Rb₂B₄O₇ (RBO). From the results of the dc conductivity and dc activation energy, we observe that the LNRBO system exhibits the combined characteristic of binary mixed alkali systems LNBO and LRBO. It is found that the power-law exponent n for binary alkali glass is the same as that for ternary alkali glass but it is lower than that for single alkali glass. This indicates that the dimensionality of conducting pathway in the mixed alkali glasses of LNBO, LRBO and LNRBO is lower than that in the single alkali RBO. We discuss the concentration dependence of the dc conductivity and dc activation energy in the framework of the bond valence technique to reverse Monte Carlo produced structural model [Phys. Rev. Lett. 90, 155507 (2003)].     

Electrical properties and electronic structure of Cu1−xZnxInSe2 and Cu1−xZnxInS2 single crystals

Temperature dependence of the electrical conductivity of CuInS₂–ZnIn₂S₄ and CuInSe₂–ZnIn₂Se₄ solid solutions possessing n-type conductivity has been studied. It has been established that when the temperature decreases down to ~100 to 27 K, the hopping mechanism of electrical conductivity with a variable jumping length between localized states positioned in a narrow energy band near the Fermi level becomes dominant. The main parameters of the hopping conductivity have been determined. At higher temperatures (150–300 K), in the CuInSe₂–ZnIn₂Se₄ single crystals containing 15 and 20 mol% ZnIn₂Se₄ the thermally activated conductivity with activation energy of 0.018 and 0.04 eV, respectively, is detected. Among the CuInSe₂–ZnIn₂Se₄ single crystals, samples with 5 and 10 mol% ZnIn₂Se₄ were found to be close to degenerate semiconductors. Temperature dependences of the electrical conductivity of CuInS₂–ZnIn₂S₄ single crystals are described by a more complicated function that may indicate a competition of several conduction mechanisms in these compounds. For the CuInS₂–ZnIn₂S₄ solid solutions, X-ray photoelectron core-level and valence-band spectra have been measured for both pristine and Ar⁺ ion-bombarded surfaces. Our results indicate that the Cu_1−xZn_xInS₂ single-crystal surfaces are sensitive to Ar⁺ ion-bombardment. Additionally, for the Cu_1−xZn_xInS₂ crystal with the highest ZnIn₂S₄ content, namely 12 mol% ZnIn₂S₄, the X-ray emission bands representing the energy distribution of the Cu 3d, Zn 3d and S 3p states have been measured and compared on a common energy scale with the X-ray photoelectron valence-band spectrum.     

Ionic conductivities of various GeS2-based oxy-sulfide amorphous materials prepared by melt-quenching and mechanical milling methods

We have previously explored the Li₂S + GeS₂ + GeO₂ system to determine the specific effect of added GeO₂ to a base 0.5Li₂S + 0.5GeS₂ glass composition. In this new study, we report the conductivities of these Li₂S + GeS₂ + GeO₂ glasses over their full glass forming range to more fully optimize the ionic conductivity. In addition to this study of bulk glasses, we have also studied the effect of creating powders of the bulk glasses and compared the conductivities of the bulk glasses to those of compacted powders. This latter study is relevant due to the fact that in most battery applications powders are the form of choice in forming battery stacks. Since we used the mechanical milling technique to produce the powders, we further extended this study to determine the extent to which the amorphous range could be expanded using the mechanical milling technique. Having access to these extended compositional range materials, we were able to extend all examinations of the role of composition, powder size, and compaction of the sample on the conductivity and we compared these results to those of the bulk glass samples.     

Effect of doping on cohesive and thermophysical properties of MgB2

The effects of doping Al and Mn on the cohesive and thermophysical properties of MgB₂ have been investigated using a Rigid Ion Model (RIM). The interatomic potential of this model includes contributions from the long-range Coulomb attraction and the short-range overlap repulsion and the van der Waals attraction. This model has been applied to describe the temperature dependence of the specific heat of MgB₂, Mg_1−xAl_xB₂ (x = 0.1–0.9) and Mg_1−xMn_xB₂ (x = 0.01–0.04) in the temperature range 5 K ⩽ T ⩽ 1000 K. The calculated results on cohesive energy (ϕ), Bulk modulus (B_T), molecular force constant (f), Restrahalen frequency (ν₀), Debye temperature (Θ_D) and Gruneisen parameter (γ) are also reported for these materials. Our results on Bulk modulus, Restrahalen frequency and Debye temperature are closer to the available experimental data. The comparison between our calculated and available experimental results on the specific heat at constant volume for MgB₂ and Mg_1−xAl_xB₂ (x = 0.1–0.4), particularly, at lower temperatures has shown almost an excellent agreement. The trend of variation of the specific heat with temperature is more or less similar in pure and doped MgB₂.     

Proton diffusion in the superprotonic phase of [(NH4)1 − xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation

Proton diffusion in [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 < x < 1) has been studied by means of ¹H spin-lattice relaxation times, T₁. The relaxation times were measured at 200.13 MHz in the range of 296–490 K and at 19.65 MHz in the range of 300–470 K. In the high-temperature phase (phase I), translational diffusion of the acidic protons relaxes both the acidic protons and the ammonium protons. Spin diffusion averages the relaxation rate of the two kinds of protons, whereas proton exchange between them are slow. The spin-lattice relaxation times in phase I were analyzed theoretically, and parameters of proton diffusion were obtained. The mean residence time of the acidic protons increases with increase in x for [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 ≤ x ≤ 0.54). Rb₃H(SO₄)₂ does not obey this trend. The results of NMR well explain the macroscopic proton conductivity.     

Structure and conductivity investigations of alkaline earth substituted uranium oxide,U1 − xMxO2 ± δ (M = Mg,Ca, Sr) for solid oxide fuel cell applications

Alkaline earth substituted UO₂ (U_1 − xM_xO_2 ± δ; M = Mg, Ca, Sr; 0.1 ≤ x ≤ 0.525) with fluorite structure was synthesized in reducing atmosphere. Structure and conductivity properties of U_1 − xM_xO_2 ± δ fluorites were investigated for possible application in solid oxide fuel cells (SOFC). At room temperature and ambient atmosphere the materials are stable; however they decompose at an oxygen partial pressure p_O2 > 10^− 4 atm and temperatures higher than 600 °C. The total conductivity measured for the best conducting U_1 − xM_xO_2 ± δ material with M = Ca and x = 0.177 is as high as 3 S/cm at p_O2 < 10^− 4 atm at 600 °C. The relatively low ionic transference number (t_i∼0.02) is disadvantageous for potential use as electrolyte material for SOFC applications. The high conductivity and possible depolarization effects suggest potential use as anode materials in SOFC.     

Tuneability of Sm(1 − x)CexFeO3 ± λ perovskites: Thermal stability and electrical conductivity

Trimetallic perovskite oxides, Sm_(1 ? x)Ce_xFeO_3 ± λ (x = 0–0.05), were prepared by thermal decomposition of amorphous citrate precursors followed by calcinations. The material properties of the substituted perovskites were characterized by X-ray diffraction (XRD), X-ray florescence spectroscopy (XRF), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The doped materials exhibited a single perovskite phase in air up to 1350 °C and have specific surface areas in the range of 2.696–8.665 m²/g. In reducing atmosphere (5%v/vH₂/N₂), the unsubstituted perovskite (x = 0) decomposed into two phases while the ceria stabilized materials (x = 0.01, x = 0.03, x = 0.05) remained in a single phase as revealed by XRD analysis. Their conductivities were measured by the four point probe method in air and in dilute hydrogen (5%v/vH₂/N₂) separately. The ceria substituted materials show increased stability versus reduction and phase separation for a wide temperature range (up to 1000 °C). Although undoped SmFeO₃ has higher conductivity under oxidizing conditions than ceria doped SmFeO₃ due its p-type nature, the situation is reversed under reducing conditions. The ceria substituted perovskites (Sm_(1 ? x)Ce_xFeO_3 ± λ, x = 0–0.05) showed higher conductivity in reducing than in oxidizing conditions, suggesting that ceria doping at the A-site has changed the SmFeO₃ from p-type to n-type semi-conducting behavior.     

Electrical transport and crystallization in Cu+ ion substituted AgI–Ag2O–V2O5 glassy superionic system

A new glassy solid electrolyte system Cu_xAg_1 − xI–Ag₂O–V₂O₅ has been synthesized. The structural, thermal and electrical properties of the samples have been investigated. The glassy nature of the samples is confirmed by X-Ray diffraction and Differential Scanning Calorimetry studies. The electrical conductivity of these samples increases with CuI content and approaches a maximum value of ∼ 10⁻² Ω^− 1 cm^− 1 for x = 0.35 at room temperature. Ionic mobility measurements suggest that enhancement in the conductivity with Cu⁺ ion substitution may be attributed to increase in the mobility of Ag⁺ ions. The electrical conductivity versus temperature cycles carried out at well-controlled heating rate above T_g and T_c reveal interesting thermal properties. For lower CuI content samples conductivity exhibits anomalous rise above T_g and subsequent fall at T_c. It is also found that CuI addition into AgI–Ag₂O–V₂O₅ matrix reduces the extent of crystallization.     

Elementary excitations,electronic correlation functions and sum rule in noble-metal chalcogenides

Noble-metal chalcogenides are known as both electronic and ionic conductors. Physics in superionic conductors is investigated on the basis of the idea of elementary excitations. First, the semiconducting properties of noble-metal chalcogenides are investigated by preparing the full Hamiltonian for conduction electrons and phonons. The influence of electron interactions on the longitudinal acoustic wave frequencies and the matrix element for the electron–phonon interaction are investigated. Three cases of ω >> kυ_F, ω < kυ_F and ω = 0 are investigated for polar semiconductors like noble-metal chalcogenides. Next, the structure factors, and the f-sum rule of conductivity are investigated in silver chalcogenides by making use of a continuum model. The structure factors S_ee, S_Ae and S_Be with which electrons are connected are expressed symmetrically in terms of the structure factors S_AA, S_BB and S_AB of ions in the long-wavelength limit using the fluctuation–dissipation theorem and the Kramers–Kronig relation. The obtained conductivity satisfies the f-sum rule.     

Oxygen non-stoichiometry and electrical conductivity of Pr2−xSrxNiO4±δ with x = 0–0.5

Oxygen non-stoichiometry and electrical conductivity of the Pr_2−xSr_xNiO_4±δ series with x = 0.0–0.5 were investigated in Ar/O₂ (pO₂ = 2.5 to 21 000 Pa) within a temperature range of 20–1000 °C. The equilibrium values of oxygen non-stoichiometry and electrical conductivity of these nickelates were determined as functions of temperature and oxygen partial pressure (pO₂). The nickelates with x = 0–0.5 appear to be p-type semiconductors in the investigated temperature and pO₂ ranges. The nickelates with x = 0.3–0.5 show very feebly marked pO₂ dependencies of the conductivity. Pr_1.7Sr_0.3NiO_4±δ shows the anomalies of the conductivity versus oxygen partial pressure which can be related to the orthorhombic–tetragonal crystal structure transformations. The conductivity of the Pr_2−xSr_xNiO_4±δ samples correlates with the average oxidation state of the nickel cations. The samples with x = 0.5 have the highest nickel oxidation state (≈ 2.5+), the highest [Ni³⁺]/[Ni²⁺] ratio close to 1 and show the highest conductivity (≈ 120 S/cm) in the whole pO₂ and temperature ranges investigated.     

Proton conductivity and low temperature structure of In-doped BaZrO3

The proton conductivity and structural features of In³⁺ substituted BaZrO₃ samples, i.e., BaZr_1−xIn_xO_3−δ, were investigated. Rietveld analysis of low temperature (10 K) neutron powder diffraction data collected on as-prepared and deuterated samples confirmed cubic symmetry (space group Pm-3m) for all compositions. The level of oxygen vacancies refined in the as-prepared samples were in good agreement with the values expected to conserve charge neutrality, whilst an increase in oxygen occupancy, reflecting the incorporation of OD⁻ species, was obtained for the deuterated materials. An expansion of the unit cell parameter, a, was observed as a function of In³⁺ doping as well as after the deuteration reaction. The conductivity of pre-hydrated and dry samples was measured using impedance methods. For 25% In-doped BaZrO₃, the low T (300 °C) conductivity of the heating cycle of the dried sample was greater than that of the cooling cycle of the pre-hydrated sample indicating a greater number of protons in the nominally dry sample. In contrast, the conductivity values were similar at higher temperatures e.g. T > 500 °C where proton conduction is not dominant.