Struktura krystaliczna i przewodnictwo elektryczne tlenków bizmutowo-niobowych domieszkowanych itrem

Marcin Hołdyński

Abstract

Oxide ion conductors are investigated because of potential application as solid electrolyte in solid oxide fuel cells - SOFC. Solid electrolyte exhibiting high ionic conductivity at temperature about 700℃ is needed for lowering of the working temperature of SOFC. Oxide ion conductors based on bismuth oxide are promising. Phase δ-Bi2O3 of cubic fluorite type structure found at temperature above 725℃ has the highest known oxide ion conductivity. In this work, triple oxides of the system Bi2O3 – Nb2O5 – Y2O3, exhibiting crystal structure of the δ-Bi2O3 type, were studied. The literature part, after description of physical bases of ionic conductivity of solids, contains review of research efforts aimed at stabilization of the δ-Bi2O3 type phase exhibiting high conductivity to lower temperature by partial replacement of bismuth atoms by atoms of other metals. Experimental methods are described next: the X-ray and neutron diffraction including the Rietveld method for crystal structure refinement, differential thermal analysis, modified method of concentration cell for evaluation of the transference number of oxide ions, impedance spectroscopy. Two families of oxides: Bi3.5Nb1−xYxO7.75−x and Bi4Nb1−xYxO8.5−x (0.0 ⩽ x ⩽ 1.0) were investigated in detail. Bi3NbO7 oxide was also investigated for comparison. Polycrystalline samples of oxides were prepared by reaction in solid state. Conditions of synthesis were established with the aid of differential thermal analysis and the X-ray diffraction patterns collected at different temperatures. Measurement of density served to optimize the sintering conditions. Oxides of the Bi3.5Nb1−xYxO7.75−x family, synthesized at temperature 740℃ and sintered at 900℃ exhibited cubic structure of fluorite type after slow cooling. Production of the Bi4Nb1−xYxO8.5−x family oxides as single phase of cubic structure required rapid quenching after two annealing steps at 740℃ and 800℃ as well as after sintering at 900℃. Crystal structure refinement, performed simultaneously on the X-ray and neutron diffraction patterns, allowed to define the crystallographic positions of oxide ions and estimate their occupancy. In the case of parent oxides Bi3.5NbO7.75 and Bi4NbO8.5 with structure described by the Fm3m space group, the lattice position 8c is not occupied while the oxide ions are mostly found at the 32f split position. Additional reflexes visible in the neutron diffraction patterns were ascribed to superstructure associated with a long range order in the oxygen sublattice, which did not undergo change upon heating to 800℃. Reflexes from superstructure were also visible in the case of oxides with small rate of yttrium substitution in place of niobium (x = 0.2). Diffraction patterns of oxides with higher rate of substitution indicate only local ordering in the oxygen sublattice. For x ⩾ 0.4 the 8c lattice position exhibits occupancy, which grows at the expense of the 32f position as x increases. At 800℃ occupancy of the 8c position is higher than at room temperature. The lattice constant increases with the increase of x. Its temperature dependence is nonlinear, the slope of the a(T) plot becoming higher at temperatures above about 450℃. The total electrical conductivity, determined based on the impedance spectra, is higher for oxides with higher molar ratio Bi:(Nb,Y ) and increases rapidly with the increasing rate of yttrium substitution for niobium. The temperature dependence of conductivity is characterized by two values of activation energy: EHT above 550℃ and ELT below 500℃. In the case of oxides with low content of yttrium x ⩽ 0.2, EHT > ELT , while in the case of oxides with higher content of yttrium x ⩾ 0.4 the relation is reversed EHT < ELT . These two forms of the Arrhenius plot of conductivity may be related with presence of superstructure in oxygen sublattice and lack of occupancy at the 8c position for oxides of low yttrium content x ⩽ 0.2, while the oxides with x ⩾ 0.4 have opposite structural features. During impedance measurements of the Bi3.5Nb1−xYxO7.75−x family oxides x ⩽ 0.6, reproducibility of the measured values of the conductivity was observed in the consecutive heating and cooling runs and no changes of conductivity with time at constant temperature were noticed. In contrary, oxides of the Bi4Nb1−xYxO8.5−x family with high rate of yttrium substitution, x = 0.6 and x = 0.8, revealed instability during impedance measurements and conductivity exhibited dependence on the thermal history, which appears to stem from „freezing” of the cubic structure by rapid cooling during the preparation of samples. The studied oxides exhibit high transference number of oxide ions, to > 0.85, Bi3NbO7 of tetragonal structure being an exception with to ∼= 0.75. While the electronic contribution to the conductivity increases with the increase of the substitution of yttrium in placed of niobium, the ionic conductivity increases more rapidly with x, such that the ionic transference number growth with the increase of x in the oxide family Bi3.5Nb1−xYxO7.75−x. The oxides Bi3.5Nb0.4Y0.6O7.15 and Bi3.5Nb0.2Y0.8O6.95 exhibit the transference number to > 0.99, thus they are nearly purely ionic conductors. Among the investigated triple oxides, which did not show any signs of instability in air at temperature up to 800℃, the oxide of composition Bi3.5Nb0.4Y0.6O7.15 has the highest electrical conductivity (at temperature 700℃ σ = 0.29 S/cm) and the highest ionic transference number to ∼= 0.995. Analysis of impedance spectra measured in a broad range of frequencies from 0.013 Hz to 107 Hz allowed evaluation of the electric permittivity of the oxides Bi3.5Nb1−xYxO7.75−x (x = 0.0, 0.2, 0.4, 0.6) at low temperatures up to 400℃. The high frequency dielectric constant was found to decrease with increase of the substitution of Y in place of Nb from 90 for x = 0.0 to 44 for x = 0.6. The change of the dielectric constant associated with relaxation of ionic charge carriers was approximately equal 100 at 100℃ for the investigated oxides and at temperatures around 350℃ increased from 40 for x = 0.0 to 120 for x = 0.6. The effective jump frequency of ions, estimated by the onset frequency of the conductivity dispersion described by the power law, exhibited activation energy close to the activation energy of the conductivity. Concentration of charge carriers evaluated based on the conductivity dispersion was roughly in agreement with concentration of oxygen vacancies calculated based on the crystal structure. The changes of carrier concentration caused by substitution of Nb by Y are much smaller than the observed changes of the ionic conductivity. The strong increase of conductivity of Bi3.5Nb1−xYxO7.75−x oxides with x results from an increase of the effective jump frequency, which can be ascribed to the entropy factor that growth with x while the ordering of the oxygen sublattice diminishes.
Diploma typeDoctor of Philosophy
Author Marcin Hołdyński (FP)
Marcin Hołdyński,,
- Faculty of Physics
Title in PolishStruktura krystaliczna i przewodnictwo elektryczne tlenków bizmutowo-niobowych domieszkowanych itrem
Languagepl polski
Certifying UnitFaculty of Physics (FP)
Disciplinephysics / (physical sciences domain) / (physical sciences)
Defense Date28-10-2010
Supervisor Józef Dygas (FP / SSID)
Józef Dygas,,
- Solid State Ionics Division

Internal reviewers Marek Wasiucionek (FP / SSID)
Marek Wasiucionek,,
- Solid State Ionics Division
External reviewers Maria Gazda - [Gdańsk University of Technology (PG)]
Maria Gazda,,
-
- Politechnika Gdańska
Pages226
Keywords in Englishxxx
Abstract in EnglishOxide ion conductors are investigated because of potential application as solid electrolyte in solid oxide fuel cells - SOFC. Solid electrolyte exhibiting high ionic conductivity at temperature about 700℃ is needed for lowering of the working temperature of SOFC. Oxide ion conductors based on bismuth oxide are promising. Phase δ-Bi2O3 of cubic fluorite type structure found at temperature above 725℃ has the highest known oxide ion conductivity. In this work, triple oxides of the system Bi2O3 – Nb2O5 – Y2O3, exhibiting crystal structure of the δ-Bi2O3 type, were studied. The literature part, after description of physical bases of ionic conductivity of solids, contains review of research efforts aimed at stabilization of the δ-Bi2O3 type phase exhibiting high conductivity to lower temperature by partial replacement of bismuth atoms by atoms of other metals. Experimental methods are described next: the X-ray and neutron diffraction including the Rietveld method for crystal structure refinement, differential thermal analysis, modified method of concentration cell for evaluation of the transference number of oxide ions, impedance spectroscopy. Two families of oxides: Bi3.5Nb1−xYxO7.75−x and Bi4Nb1−xYxO8.5−x (0.0 ⩽ x ⩽ 1.0) were investigated in detail. Bi3NbO7 oxide was also investigated for comparison. Polycrystalline samples of oxides were prepared by reaction in solid state. Conditions of synthesis were established with the aid of differential thermal analysis and the X-ray diffraction patterns collected at different temperatures. Measurement of density served to optimize the sintering conditions. Oxides of the Bi3.5Nb1−xYxO7.75−x family, synthesized at temperature 740℃ and sintered at 900℃ exhibited cubic structure of fluorite type after slow cooling. Production of the Bi4Nb1−xYxO8.5−x family oxides as single phase of cubic structure required rapid quenching after two annealing steps at 740℃ and 800℃ as well as after sintering at 900℃. Crystal structure refinement, performed simultaneously on the X-ray and neutron diffraction patterns, allowed to define the crystallographic positions of oxide ions and estimate their occupancy. In the case of parent oxides Bi3.5NbO7.75 and Bi4NbO8.5 with structure described by the Fm3m space group, the lattice position 8c is not occupied while the oxide ions are mostly found at the 32f split position. Additional reflexes visible in the neutron diffraction patterns were ascribed to superstructure associated with a long range order in the oxygen sublattice, which did not undergo change upon heating to 800℃. Reflexes from superstructure were also visible in the case of oxides with small rate of yttrium substitution in place of niobium (x = 0.2). Diffraction patterns of oxides with higher rate of substitution indicate only local ordering in the oxygen sublattice. For x ⩾ 0.4 the 8c lattice position exhibits occupancy, which grows at the expense of the 32f position as x increases. At 800℃ occupancy of the 8c position is higher than at room temperature. The lattice constant increases with the increase of x. Its temperature dependence is nonlinear, the slope of the a(T) plot becoming higher at temperatures above about 450℃. The total electrical conductivity, determined based on the impedance spectra, is higher for oxides with higher molar ratio Bi:(Nb,Y ) and increases rapidly with the increasing rate of yttrium substitution for niobium. The temperature dependence of conductivity is characterized by two values of activation energy: EHT above 550℃ and ELT below 500℃. In the case of oxides with low content of yttrium x ⩽ 0.2, EHT > ELT , while in the case of oxides with higher content of yttrium x ⩾ 0.4 the relation is reversed EHT < ELT . These two forms of the Arrhenius plot of conductivity may be related with presence of superstructure in oxygen sublattice and lack of occupancy at the 8c position for oxides of low yttrium content x ⩽ 0.2, while the oxides with x ⩾ 0.4 have opposite structural features. During impedance measurements of the Bi3.5Nb1−xYxO7.75−x family oxides x ⩽ 0.6, reproducibility of the measured values of the conductivity was observed in the consecutive heating and cooling runs and no changes of conductivity with time at constant temperature were noticed. In contrary, oxides of the Bi4Nb1−xYxO8.5−x family with high rate of yttrium substitution, x = 0.6 and x = 0.8, revealed instability during impedance measurements and conductivity exhibited dependence on the thermal history, which appears to stem from „freezing” of the cubic structure by rapid cooling during the preparation of samples. The studied oxides exhibit high transference number of oxide ions, to > 0.85, Bi3NbO7 of tetragonal structure being an exception with to ∼= 0.75. While the electronic contribution to the conductivity increases with the increase of the substitution of yttrium in placed of niobium, the ionic conductivity increases more rapidly with x, such that the ionic transference number growth with the increase of x in the oxide family Bi3.5Nb1−xYxO7.75−x. The oxides Bi3.5Nb0.4Y0.6O7.15 and Bi3.5Nb0.2Y0.8O6.95 exhibit the transference number to > 0.99, thus they are nearly purely ionic conductors. Among the investigated triple oxides, which did not show any signs of instability in air at temperature up to 800℃, the oxide of composition Bi3.5Nb0.4Y0.6O7.15 has the highest electrical conductivity (at temperature 700℃ σ = 0.29 S/cm) and the highest ionic transference number to ∼= 0.995. Analysis of impedance spectra measured in a broad range of frequencies from 0.013 Hz to 107 Hz allowed evaluation of the electric permittivity of the oxides Bi3.5Nb1−xYxO7.75−x (x = 0.0, 0.2, 0.4, 0.6) at low temperatures up to 400℃. The high frequency dielectric constant was found to decrease with increase of the substitution of Y in place of Nb from 90 for x = 0.0 to 44 for x = 0.6. The change of the dielectric constant associated with relaxation of ionic charge carriers was approximately equal 100 at 100℃ for the investigated oxides and at temperatures around 350℃ increased from 40 for x = 0.0 to 120 for x = 0.6. The effective jump frequency of ions, estimated by the onset frequency of the conductivity dispersion described by the power law, exhibited activation energy close to the activation energy of the conductivity. Concentration of charge carriers evaluated based on the conductivity dispersion was roughly in agreement with concentration of oxygen vacancies calculated based on the crystal structure. The changes of carrier concentration caused by substitution of Nb by Y are much smaller than the observed changes of the ionic conductivity. The strong increase of conductivity of Bi3.5Nb1−xYxO7.75−x oxides with x results from an increase of the effective jump frequency, which can be ascribed to the entropy factor that growth with x while the ordering of the oxygen sublattice diminishes.
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