Electro-chemo-mechanical studies of perovskite-structured mixed ionic-electronic conducting SrSn<Subscript>1-x</Subscript>Fe<Subscript>x</Subscript>O<Subscript>3-x/2+δ</Subscript> Part III: Thermal and chemical expansion

The thermal and chemical expansion of a potential solid oxide fuel cell (SOFC) cathode material SrSn_0.65Fe_0.35O_{3–0.35/2+δ} (SSF35) were investigated to assess its thermo-chemo-mechanical stability at SOFC operating temperatures and to establish the correlation between defect concentrations (oxygen vacancies and electrons) and chemical expansion with the aid of the defect chemical model reported in part I of this study. Thermochemical expansion was measured as a function of temperature and oxygen partial pressure. The chemical expansion of SSF35 showed a strong correlation with changes in oxygen nonstoichiometry associated with changes in Fe valence state. Coefficients of both chemical (CCE) and thermal (CTE) expansion were calculated and found to be smaller than that of the closely related mixed conducting perovskite oxide SrTi_0.65Fe_0.35O_{3–0.35/2+δ} (STF35). The thermal expansion coefficient of SSF was found to be close to that of YSZ (most popular solid oxide electrolyte), which makes SSF35 more attractive in terms of overall thermo-chemical stability. The chemical expansion of SSF35 showed decreasing CCE with increasing temperature and decreasing CTE with increasing oxygen deficiency, both opposite to the trends observed for STF35. Distortion in symmetry from the cubic structure seems to be responsible for the smaller coefficients and increasing asymmetry with expansion seems accountable for opposite trends of CCE and CTE compared to the STF counterpart.     

Electro-chemo-mechanical studies of perovskite-structured mixed ionic-electronic conducting SrSn<Subscript>1-x</Subscript>Fe<Subscript>x</Subscript>O<Subscript>3-x/2+δ</Subscript>part I: Defect chemistry

Oxygen nonstoichiometry and the defect chemistry of the SrSn_1-xFe_xO_3-x/2+δ (SSF) system were examined by means of thermogravimetry as a function of oxygen partial pressure in the temperature range of 700–1000 °C and compared against the corresponding mixed ionic-electronic conducting titanate, SrTi_1-xFe_xO_3-x/2+δ (STF) system. The alternate B site host cation, Sn, was selected to replicate and extend the STF studies, given its distinct band structure and higher electron mobility associated with its 5s derived conduction band as compared to the 3d nature of the conduction band in the titanate. Though shifted slightly by the larger size of Sn, the defect equilibria – including the oxygen vacancy concentration – were found to be largely dominated by Fe oxidation state, and thus differed only in a limited way from those in STF. Key thermodynamic parameters for SrSn_0.65Fe_0.35O_2.825+δ (SSF35) were derived including the reduction enthalpy (4.137 ± 0.175 eV), the high temperature electronic band gap (1.755 ± 0.015 eV) and the anion Frenkel enthalpy (0.350 ± 0.350 eV). The implications these observations have for cathode behavior in solid oxide fuel cells are briefly discussed.     

Temperature Sensitive Properties of the La(Ti<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>)O<Subscript>3</Subscript> System

The electric mechanisms of perovskite-type LaMnO₃ was investigated with B-site substitution in this paper. Samples of La(Ti_xMn_{1 − x})O₃ (0.1 ≰ x ≰ 0.7) were sintered at different temperature. The voltage-temperature (V-T) curves of the samples were tested from room temperature (25^∘C) to 300^∘C, then the electric properties were measured and analyzed. The experimental results showed that the resistivity-temperature (ρ-T) curves of the samples matched NTC characteristic. The resistivity increased slightly with the increase of Ti amount as x was less than 0.5, however, it rose greatly after x exceeded 0.5; The sintering temperatures have a little influence on the resistivity, except for the sample with x = 0.7.     

Electro-chemo-mechanical studies of perovskite-structured mixed ionic-electronic conducting SrSn<Subscript>1-x</Subscript>Fe<Subscript>x</Subscript>O<Subscript>3-x/2+δ</Subscript> part II: Electrical conductivity and cathode performance

The bulk electrical conductivity of the mixed ionic-electronic conducting perovskite-structured SrSn_1-xFe_xO_3-x/2+δ (SSF) was measured to examine how changes in defect chemistry and electronic band structure associated with the substitution of Ti by Sn impact defect charge carrier density and ultimately electrode performance. These results complement a defect chemical model for SSF investigated and reported in Part I of this study. The electrical properties of SSF were found not to differ significantly from the corresponding composition in SrTi_1-xFe_xO_3-x/2+δ (STF). It is believed that Fe dominates the character of the valence and conduction bands and thus governs the electronic properties in SSF. Though slightly shifted in energy due to the larger size of Sn, the defect equilibria and therefore the electrical conductivity of SSF were found to be largely dominated by Fe and thus differed only in a limited way from that in STF. Key kinetic parameters obtained include the migration enthalpy of oxygen vacancies (0.772 ± 0.204 eV), the activation energy of area-specific-resistance for oxygen exchange (1.65 ± 0.03 eV) and the magnitudes of electron (0.0002 ± 0.00005 cm²/V?s) and hole (0.0037 ± 0.0015 cm²/V?s) mobilities.     

Electrical and mechanical properties of MgO added 0.5Ba(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript>–0.5(Ba<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>)TiO<Subscript>3</Subscript> (BZT–0.5BCT) composite ceramics

MgO (0–2.0 vol%) added Ba(Zr_0.2Ti_0.8)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ (BZT-0.5BCT) ceramics have been prepared by the conventional solid-state reaction method. The effects of MgO powder on the phase formation, densification, dielectric, piezoelectric and mechanical properties (flexural strength, hardness) of the BZT-0.5BCT ceramics have been studied systematically. The synthesized powder could be densified to 97 % of true density at a temperature of 1350 °C. The MgO addition also provided materials with better mechanical properties. The most interesting aspect of MgO added samples is their relative permittivity vs. temperature response. MgO additions effectively suppress the relative permittivity around phase transition temperature. The aging rate of d₃₃ observed for BZT-0.5BCT is 14 %/decade. MgO addition reduces the ageing rate and for 1 vol% MgO added, BZT-0.5BCT shows aging rate of 3 %/decade. BZT-0.5BCT/MgO ceramics possesses good mechanical properties viz., flexural strength 93 MPa, which is almost 25 % higher than that of monolithic BZT-0.5BCT (73 MPa).     

Effect of mechanical loading on the tuning of acoustic resonances in Ba<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> thin films

The effect of mechanical loading on the tuning performance of a tunable Thin Film Bulk Acoustic Wave Resonator (TFBAR) based on a Ba_0.3Sr_0.7TiO₃ (BST) thin film has been investigated experimentally and theoretically. A membrane-type TFBAR was fabricated by means of micromachining. The mechanical load on the device was increased stepwise by evaporating SiO₂ on the backside of the membrane. The device was electrically characterized after each evaporation step and the results were compared to those obtained from modeling. The device with the smallest mechanical load exhibited a tuning of − 2.4% and − 0.6% for the resonance and antiresonance frequencies at a dc electric field of 615 kV/cm, respectively. With increasing mechanical load a decrease in the tuning performance was observed. This decrease was rather weak if the thickness of the mechanical load was smaller or comparable to the thickness of the active BST film. If the thickness of the mechanical load was larger than the thickness of the active BST layer, a significant reduction in the tuning performance was observed. The weaker tuning of the antiresonance frequency was due to a reduced tuning of the sound velocity of the BST layer with increasing dc bias. The resonance frequency showed a reduced tuning due to a decrease in the effective electromechanical coupling factor of the device with increasing mechanical load. With the help of the modeling we could de-embed the intrinsic tuning performance of a single, non-loaded BST thin film. We show that the tuning performance of the device with the smallest mechanical load we fabricated is close to the intrinsic tuning characteristics of the BST layer.     

Study of bulk and grain-boundary conductivity of Ln<Subscript>2+x</Subscript>Hf<Subscript>2−x</Subscript>O<Subscript>7−δ</Subscript> (Ln = Sm-Gd; x = 0, 0.096) pyrochlores

The electrical conductivity of new solid electrolytes Eu_2.096Hf_1.904O_6.952 and Gd₂Hf₂O₇ have been compared with those for different pyrochlores including titanates and zirconates Ln_2+xМ_2−xO_7−δ (Ln = Sm-Lu; M = Ti, Zr; x = 0−0.81). Impedance spectroscopy data demonstrate that Eu_2.096Hf_1.904O_6.952 and Gd₂Hf₂O₇ synthesized from mechanically activated oxides have high ionic conductivity, comparable to that of their zirconate analogues. The bulk and grain-boundary components of conductivity in Sm_2.096Hf_1.904O_6.952 (Т_synth = 1600oС), Eu_2.096Hf_1.904O_6.952 and Gd₂Hf₂O₇ (Т_synth = 1670oС) have been determined. The highest bulk conductivity is offered by the disordered pyrochlores prepared at 1600oC and 1670oC: ~1.5 × 10⁻⁴ S/cm for Sm_2.096Hf_1.904O_6.952, 5 × 10⁻³ S/cm for Eu_2.096Hf_1.904O_6.952 and 3 × 10⁻³ S/cm for Gd₂Hf₂O₇ at 780oС, respectively. The conductivity of the fluorite-like phases at the phase boundaries of the Ln_2+xМ_2−xO_7−δ (Ln = Eu, Gd; M = Zr, Hf; x ~ 0.286) solid solutions, as well as that of the high-temperature fluorite-like phases Ln_2+xМ_2−xO_7−δ (Ln = Eu, Gd; M = Zr, Hf; x = 0−0.286), is lower than the conductivity of the disordered pyrochlores Ln_2+xМ_2−xO_7−δ (Ln = Eu, Gd; M = Zr, Hf; x = 0−0.096).     

Microwave dielectric properties of (1-<Emphasis Type="Italic">x</Emphasis>) BiVO<Subscript>4</Subscript>–<Emphasis Type="Italic">x</Emphasis>Ln<Subscript>2/3</Subscript>MoO<Subscript>4</Subscript> (Ln=Er,Sm, Nd,la) ceramics with low sintering temperatures

A series of microwave dielectric ceramics of (1-x) BiVO₄ -xLn_2/3MoO₄ (Ln = Er, Sm, Nd and La; x = 0.06, 0.08, 0.10) sintered below 900 °C were prepared via solid-state reaction. As the x values increase, the monoclinic scheelite continuously changes to a tetragonal structure at x = 0.10. The incorporation of Ln_2/3MoO₄ into the BiVO₄ matrix increases the product (Q × f) of quality factor (Q) and resonance frequency (f), and temperature coefficient (τ _f ), but lowers the dielectric constant (ε _r). Microwave dielectric ceramics with low sintering temperatures (<900 °C) are obtained: ε _r of ~71.1, 81.6, 75.6 and ~75.3; Q × f values of ~8292, 5508, 8695 and 9043 GHz; τ _f of ~ ?51, 134, 149 and 158 ppm/°C, for 0.94BiVO₄–0.06Er_2/3MoO₄, 0.92BiVO₄–0.08Sm_2/3MoO₄, 0.9BiVO₄–0.1Nd_2/3MoO₄ and 0.9BiVO₄–0.1La_2/3MoO₄ ceramics, respectively. Moreover, (1-x) BiVO₄ -xLn_2/3MoO₄ (Ln = Er, Sm, Nd and La; x = 0.06, 0.08 and 0.10) ceramics are chemically compatible with both Ag and Cu powders at their sintering temperatures. The series of microwave dielectric ceramics might be potential candidates for low temperature co-fired ceramics (LTCC) technology applications.