Sintering evaluation of doped lanthanum gallate based on thermodilatometry

The sintering behavior of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ oxide-ion conductor was systematically investigated by thermodilatometry. The shrinkage data obtained with heating rates of 4, 7, 10 and 12?°C?min^?1 were analyzed by the constant rate of heating model and by construction of the master sintering curve. Validation of the master sintering curve was carried out by measurements of density in conventionally sintered specimens. Slight anisotropy of shrinkage data was found and changes to the basic equation of density was proposed to account for this effect. Plotting the data determined by the constant rate of heating model versus density allowed an easy identification of the density range of constant activation energy. The activation energy (865?kJ?mol^?1) obtained from the master sintering curve correlates quite well with that (874?kJ?mol^?1) obtained by the constant rate of heating model.     

Synthesis and electrical conductivity of La-Sr-X-Mg-O (X = Ti, Zr, Al) perovskite solid solution

Perovskite solid solutions of (La_0.6Sr_0.4)(X_1−yMg_y)O_3−δ (X = Ti, Zr, Al) were prepared by a coprecipitation method using corresponding aqueous solutions and ammonium carbonate solution. The freeze-dried powders were sintered in air at 1000-1500 °C for 1-36 h. Single phase solid solutions were produced in the compositions of (La_0.6Sr_0.4)(Zr_0.6Mg_0.4)O_3−δ and (La_0.6Sr_0.4)(Al_0.9Mg_0.1)O_3−δ where (3 − δ) < 3. For the compositions of X = Ti and Zr for y = 0.1 where (3 − δ) > 3, two phases including perovskite solid solution were produced at 1400-1500 °C. The stability of perovskite solid solution was closely related to the fraction of lattice oxygen atom (3 − δ). A relatively high conductivity was measured for (La_0.6Sr_0.4)(Al_0.9Mg_0.1)O_3−δ (σ = 4.15 × 10⁻⁴ S/cm at 600 °C, activation energy 113.4 kJ/mol). The influence of fraction of oxide ion vacancy on the activation energy was small for δ = 0.1-0.3 of perovskite solid solution.     

Proton conducting solid oxide fuel cells with chemically stable BaZr0.75Y0.2Pr0.05O3-δ electrolyte

A BaZrO₃-based electrolyte with low Pr-doping concentration is proposed as electrolyte for proton-conducting solid oxide fuel cells (SOFCs). The new material BaZr_0.75Y_0.2Pr_0.05O_3-δ (BZYP5) shows a good chemical stability against CO₂. In addition, the low doping concentration of Pr in BaZrO₃ improves the sinterability of BaZrO₃ and also allows its structure to remain stable even in the reducing atmosphere, which is critical for fuel cell applications. The cell with BZYP5 as electrolyte shows maximum power densities of 124, 70, and 43 mW cm⁻² at 600, 550, and 500 °C, respectively, which are larger than that for the cell with conventional high Pr-doping BaZrO₃ electrolyte reported previously. Electrochemical analysis indicates that the BZYP5 electrolyte shows a good ionic conductivity. These results suggest that the low Pr-doping strategy presented in this study promotes the densification for BaZrO₃ and the good electrolyte conductivity of BaZrO₃ is maintained which could be the reason for the improved cell performance, suggesting BZYP5 is a promising electrolyte for proton-conducting SOFCs.     

Electrical conduction behaviors of isovalent and acceptor dopants on B site of (La0.8Ca0.2)CrO3−δ perovskites

The electrical conduction behaviors of isovalent and acceptor dopants on B site of (La_0.8Ca_0.2)CrO_3−δ perovskites at high and low oxygen activities were investigated systematically. In this study, the concept of defect chemistry is used to explain the relationship between the concentration of electron hole with the electrical conductivity. The information of charge compensation mechanisms and defect formation may be valuable for a better understanding of the interconnect of (La_0.8Ca_0.2)CrO_3−δ-based ceramics used for solid oxide fuel cells (SOFCs). Since (La_0.8Ca_0.2)CrO_3−δ-based specimens belong to p-type conductors, their conductivities are proportional to the concentration of electron hole. In reducing atmosphere, the oxygen may be lost and ionic compensation may be take place through the formation of oxygen vacancies and the electrical compensation may arise by changing the valence of Cr from tri-valence to tetra-valence in reducing atmosphere. However the formation of oxygen vacancies has no contribution to electrical conductivity, the compensation mechanism is dominated by the electrical compensation, i.e. the take place a transition of Cr³⁺ → Cr⁴⁺ rather than that of ionic compensation, i.e. the formation of oxygen vacancies. Based on the defect chemical reactions and the results of electrical conductivity, the concentration of electron hole at high oxygen activity is larger than that at low oxygen activity. Therefore the electrical conductivity of (La_0.8Ca_0.2)CrO_3−δ-based ceramics at air is larger than that at 5% H₂–95% Ar forming gas. The compensation mechanisms contain ionic and electrical compensation and the ratios of electrical to ionic compensation varied with the kind of dopant which significantly effects the electrical conductivity. The results suggest that the (La_0.8Ca_0.2)Cr_0.9Co_0.1O_3−δ specimen shows high electrical conductivity in air (σ_{850 °C} = 59.59 S/cm) and 5% H₂–95% Ar forming gas (σ_{850 °C} = 47.98 S/cm) leading it a promising candidate as an interconnect material for SOFCs applications.     

Dielectric relaxation and electrical conductivity in Ca5Nb4TiO17 ceramics

The temperature dependence of dielectric properties and electrical conduction of Ca₅Nb₄TiO₁₇ ceramics were characterized in a broad temperature range. A dielectric anomaly with strong frequency dispersion was detected in the temperature range 700–1010 °C. This dielectric relaxation could be almost removed completely by annealing in an oxidizing atmosphere. Complex impedance analysis confirmed the electrical inhomogeneity of the ceramics with different contributions from the bulk and grain boundaries. This suggests that the main mechanism for the observed relaxation is the Maxwell–Wagner polarization. ac conductivity results revealed the variation of conduction mechanism with increasing temperatures from localized hopping to long-range motion of the doubly ionized oxygen vacancies.     

Effect of temperature on the porosity, microstructure, and properties of porous La0.8Sr0.2MnO3 cathode materials

Porous La_0.8Sr_0.2MnO₃ cathode materials were prepared by the gelcasting technology. Carbon was employed to produce pores. It is shown that the open porosity decreases with increasing temperature. The proper sintering temperature is 1100 °C and the median pore size of the sample obtained at this temperature is about 460 nm. The microstructure indicates that the grain grows as the sintering temperature increases, which leads to the decrease of the number of open pores. Generally, the pores are located at multi-grain boundaries. Some closed pores appear in the sample prepared at 1100 °C and more. Both the conductivity and the interface bonding between La_0.8Sr_0.2MnO₃ and YSZ get better as the temperature increases. When the sintering temperature is more than 1250 °C, La and Mn ions begin to diffuse into YSZ, and therefore interface reactions happen. According to ln(σT)  1/T curves, E_a was calculated to be 10.18 kJ/mol.     

Characterization of IT-SOFC non-symmetrical anode sintered through conventional furnace and microwave

This work investigated the mechanical and electrical properties of NiO–SDC/SDC anode sintered by two different methods: in a microwave at about 1200 °C for 1 h and in a conventional furnace at 1200 °C with a holding time of 1 h (total sintering time of 21 h). Nano-powders Sm_0.2Ce_0.8O_1.9 (SDC) and NiO were mixed using a high-energy ball mill, followed by the co-pressing technique at a compaction pressure of 400 MPa. No binder was used between the layers. The electrical behaviors of all sintered samples were studied using electrochemical impedance spectra in the frequency range of 0.01–10⁵ Hz under 97% H₂–3% H₂O, an amplitude of 10 mV, and at high temperature range of 600–800 °C. Results indicate that the non-symmetrical NiO–SDC/SDC anode achieved through microwave sintering has finer grain size and higher electrochemical performance. However, hardness and Young׳s modulus increased in the samples sintered through a conventional furnace.     

Influence of small amounts of gallium oxide addition on ionic conductivity of La0.9Sr0.1Ga0.8Mg0.2O3-δ solid electrolyte

The effects of small amounts of gallium oxide on intragrain and intergrain conductivity of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ are investigated by impedance spectroscopy in the 280–420 °C range. Bulk specimens with 0.5, 1.0 and 1.5 mol% gallium oxide are prepared by solid state reaction at 1350 °C. All specimens achieved relative density values higher than 95%. The additive promotes grain growth indicating solid solution formation. A small fraction of the additive remains at grain boundaries and increases the fraction of the gallium-rich, LaSrGa₃O₇, impurity phase. The intragrain conductivity of gallium oxide containing specimens is higher than that of the parent solid electrolyte. Similar effect is found for the intergrain conductivity, which is maximum for 1 mol% gallium oxide addition.     

Liquid-phase synthesis of SrCo0.9Nb0.1O3-δ cathode material for proton-conducting solid oxide fuel cells

A novel liquid-phase synthesis strategy is demonstrated for the preparation of the Nb-containing ceramic oxide SrCo_0.9Nb_0.1O_3-δ (SCN). In comparison with the traditional solid-state reaction (SSR) method, the liquid-phase synthesis route offers a couple of advantages, including a lower phase formation temperature and a smaller particle size of the SCN materials that are beneficial for applications as proton-conducting fuel cell cathode. With BaCe_0.4Zr_0.4Y_0.2O_3-δ (BCZY442) as the electrolyte and the SCN synthesized in this work as the cathode, a proton-conducting solid oxide fuel cell (SOFC) shows a peak power density of 348 mW cm^?2 at 700 °C, significantly higher than that of a SOFC fabricated with SCN cathode prepared using the SSR method, which can only deliver 204 mW cm^?2 at the same temperature. Additionally, this new synthesis strategy allows impregnation of Sr²⁺, Co³⁺and Nb⁵⁺ on the solid backbone in aqueous solution, further improving cell performance to reach a peak power density of 488 mW cm^?2 at 700 °C.     

Structural anomaly and electrical relaxation in (Na2/3Pb1/3)(Mn1/2Nb1/2)O3 ceramics

The X-ray diffraction patterns of (Na_2/3Pb_1/3)(Mn_1/2Nb_1/2)O₃ ceramics were measured within 15–850 K temperature range. The anomaly in the thermal expansion temperature dependence occurred in 250–365 K range. The generalised Cole–Cole model was proposed to describe the measured effective electric permittivity influenced by high electric conduction and the coexistence of two contributions ?*(T,f) = ?*_lattice + ?*_carriers was considered. The analysis of the electric permittivity and conduction exhibited two relaxation processes. The electric conduction relaxation characteristic time values indicated the small polaron mechanism with τ₀ ≈ 10⁻¹³ s occurring in 240–345 K range and the ionic mechanism with τ₀ ≈ 10⁻¹¹ s involved in the other relaxation occurring in the 320–510 K range. The ionic relaxation process was ascribed to a subsystem of defects, which was weakly interrelated to the anomaly in thermal expansion of the (Na_2/3Pb_1/3)(Mn_1/2Nb_1/2)O₃ ceramics. The Gate model was proposed to describe the ionic relaxation mechanism.     

Enhanced ionic conductivity of scandia-ceria-stabilized-zirconia (10Sc1CeSZ) electrolyte synthesized by the microwave-assisted glycine nitrate process

Scandia-stabilized-zirconia is a potential zirconia-based electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this study, the properties of zirconia co-doped with 10 mol% Sc and 1 mol% Ce (scandia-ceria-stabilized-zirconia, 10Sc1CeSZ) electrolyte synthesized by the microwave-assisted glycine nitrate process (MW-GNP) were determined. The effects of microwave heating on the sintering temperature, microstructure, densification and ionic conductivity of the 10Sc1CeSZ electrolyte were evaluated. The phase identification, microstructure and specific surface area of the prepared powder were investigated using X-ray diffraction, transmission electron microscopy and the Brunauer-Emmett-Teller technique, respectively. Using microwave heating, a single cubic-phase powder was produced with nanosized crystallites (19.2 nm) and a high specific surface area (16 m²/g). It was found that the relative density, porosity and total ionic conductivity of the 10Sc1CeSZ electrolyte are remarkably influenced by the powder processing method and the sintering temperature. The pellet sintered at 1400 °C exhibited a maximum ionic conductivity of 0.184 S/cm at 800 °C. This is the highest conductivity value of a scandia-stabilized-zirconia based electrolyte reported in the literature for this electrolyte type. The corresponding value of the activation energy of electrical conductivity was found to be 0.94 eV in the temperature range of 500–800 °C. Overall, the use of microwave heating has successfully improved the properties of the 10Sc1CeSZ electrolyte for application in an IT-SOFC.     

Hydrogen production through dry reforming of biogas using a porous electrochemical cell: Effects of a cobalt catalyst in the electrode and mixing of air with biogas

This paper reports the performance of porous Gd-doped ceria (GDC) electrochemical cells with Co metal in both electrodes (cell No. 1) and with Ni metal in the cathode and Co metal in the anode (cell No. 2) for CO₂ decomposition, CH₄ decomposition, and the dry reforming reaction of a biogas with CO₂ gas (CH₄ + CO₂ → 2H₂ + 2CO) or with O₂ gas in air (3CH₄ +?1.875CO₂ +?1.314O₂ → 6H₂ +?4.875CO +?0.7515O₂). GDC cell No. 1 produced H₂ gas at formation rates of 0.055 and 0.33?mL-H₂/(min?m²-electrode) per 1?mL-supplied gas/(min?m²-electrode) at 600?°C and 800?°C, respectively, by the reforming of the biogas with CO₂ gas. Similarly, cell No. 2 produced H₂ gas at formation rates of 0.40?mL-H₂/(min?m²) per 1?mL-supplied gas/(min?m²) at 800?°C from a mixture of biogas and CO₂ gas. The dry reforming of a real biogas with CO₂ or O₂ gas at 800?°C proceeded thermodynamically over the Co or Ni metal catalyst in the cathode of the porous GDC cell. Faraday's law controlled the dry reforming rate of the biogas at 600?°C in cell No. 2. This paper also clarifies the influence of carbon deposition, which originates from CH₄ pyrolysis (CH₄ → C + 2H₂) and disproportionation of CO gas (2CO → C + CO₂), on the cell performance during dry reforming. The dry reforming of a biogas with O₂ molecules from air exhibits high durability because of the oxidation of the deposited carbon by supplied air.     

Phase change and electrical resistivity of Zn–Mn–Ni–O-based NTC thermistors produced using IZC powder recycled from used dry batteries

Zn–Mn–Ni–Oxide-based NTC thermistors with variable Ni/Mn ratios were fabricated from powder mixtures of recycled IZC, and commercial MnCO₃ and NiCO₃. Solid phases and electrical resistivity of each sintered sample were studied as a function of Ni/Mn ratio, sintering temperature and sintering time. At 1200 °C for 2 h, samples with the Ni/Mn ratios of 0.38 and higher were found to consist of cubic spinel as a major phase. After sintering at 1250 °C for 10 h, densification proceeded with a phase change from cubic spinel to tetragonal one. The electrical resistivity of the samples obtained at 1200 °C for 2 h progressively decreased with an increasing Ni/Mn ratio up to 0.38, at which the value became the lowest (4.2 × 10³ Ω cm at room temperature) of all the samples fabricated.     

Preparation and properties of yttrium-doped SrTiO3 anode materials

Direct electrochemical oxidation of hydrocarbon fuels is a current development trend of solid oxide fuel cells (SOFCs) and finding new anode materials for this application is a key issue. In this study, promising candidates, Y₂O₃-doped SrTiO₃ perovskite compounds Sr_1−1.5xY_xTiO₃ (x = 0.02, 0.04, 0.06, 0.08, 0.10), were synthesized by solid-state reaction. The structure of the calcined powders was examined by X-ray diffraction (XRD). The sinterability and high temperature conductivity were measured by the Archimedes principle and a dc four-probe method, respectively. The effect of sintering temperature on the electrical conductivity was studied. The results indicated that the optimal sintering temperature is around 1400 °C. From 400 °C to 1000 °C, the conductivity decreased with increasing temperature. At 800 °C the highest conductivity (26.8 S/cm) was observed for x = 0.08.     

Hydrogen formation from a real biogas using electrochemical cell with gadolinium-doped ceria porous electrolyte

The electrochemical cell consisting of a gadolinium-doped ceria (GDC, Ce_0.9Gd_0.1O_1.95) porous electrolyte, Ni–GDC cathode and Ru–GDC anode was applied for the dry-reforming (CH₄+CO₂→2H₂+2CO) of a real biogas (CH₄ 60.0%, CO₂ 37.5%, N₂ 2.5%) produced from waste sweet potato. The composition of the supplied gas was adjusted to CH₄/CO₂=1/1 volume ratio. The supplied gas changed continuously into a H₂–CO mixed fuel with H₂/CO=1/0.949–1/1.312 vol ratios at 800 °C for 24 h under the applied voltage of 1–2 V. The yield of the mixed fuel was higher than 80%. This dry-reforming reaction was thermodynamically controlled at 800 °C. The application of external voltage assisted the reduction of NiO and the elimination of solid carbon deposited slightly in the cathode. The decrease of heating temperature to 700 °C reduced gradually the fraction of the H₂–CO fuel (61.3–18.6%) within 24 h. Because the Gibbs free energy change was calculated to be negative values at 700–600 °C, the above result at 700–600 °C originated from the gradual deposition of carbon over Ni catalyst through the competitive parallel reactions (CH₄→C+2H₂, 2CO→C+CO₂). The application of external voltage decreased the formation temperature of carbon by the disproportionation of CO gas. At 600 °C, the H₂–CO fuel based on the Faraday's law was produced continuously by the electrochemical reforming of the biogas.     

Improvement of electrical conductivity of silicon nitride/carbon nano-fibers composite using magnesium silicon nitride and ytterbium oxide as sintering additives

In order to find out the influence of sintering additives on the electrical conductivity of Si₃N₄-based ceramics composites with dispersed carbon nano-fibers (CNFs) two different mixtures of sintering additives were tested – Al₂O₃/Yb₂O₃ and MgSiN₂/Yb₂O₃, respectively. Optimization of hot-pressing conditions was performed for each mixture. The results show that the electrical conductivity can be effectively increased up to 1315 S/m by replacement of traditional sintering aid – alumina, with magnesium silicon nitride, while the mechanical properties remained on the same level. Other advantages of using MgSiN₂ instead of alumina are the preservation of higher amounts of CNFs in the ceramic composite and lower densification temperature (1500 °C) compared to samples sintered with alumina-based sintering aids (1550 °C).     

Structure, microstructure and electrical conductivity of 8YSZ containing NiO

The effects of NiO addition on the structure and microstructure of yttria-stabilized zirconia were investigated to clarify the role of the additive in the microstructure-related electrical conductivity of the solid electrolyte. Specimens of 8 mol% yttria-stabilized zirconia with NiO contents up to 5.0 mol% were prepared using nickel oxide and trihydroxi nickel carbonate as precursors. The specimens were sintered at 1350 °C for several holding times. The evolution of the lattice parameter with NiO content was evaluated by X-ray diffraction and the microstructural features by scanning electron microscopy. Electrical conductivity was evaluated by impedance spectroscopy measurements. The solubility limit of NiO at 1350 °C was found to be 1.5 mol% by X-ray diffraction. Energy dispersive spectroscopy results revealed Ni segregation for large holding times at 1350 °C. The grain boundary conductivity was found to be influenced by Ni segregation and to decrease with increasing holding times at high temperature.     

Electrochemical characterization of BaCe0·7Zr0·1Y0·16Zn0·04O3-δ electrolyte synthesized by combustion spray pyrolysis

BaCe_0·7Zr_0·1Y_0·16Zn_0·04O_3-δ perovskite has been investigated due to its potential as an electrolyte in industrial steam electrolysis applications. The lowest area specific resistance (ASR) is achieved as 4.0 Ω cm² at 711 °C under 3% wet Ar atmosphere. The conductivity is calculated as 2.93 × 10^?2 S cm^?1 and kept stable for a ~70 h testing period. ASR increased at lower temperature (511 °C) under the same atmosphere and a new impedance arc (with 4.5 Ω cm² ASR and 2 × 10^?8 F equivalent capacitance) is formed, indicating second phase formation. No second phase formation is observed at the same temperature under dry 5% H₂ in Ar. The second phase formation/degradation of the electrolyte is attributed to Ba(OH)₂ and CeO₂ formations around 500 °C under wet atmospheres. At elevated temperatures, ~700 °C, BaCe_0·7Zr_0·1Y_0·16Zn_0·04O_3-δ exhibits both excellent protonic conductivity and stability which makes it a great candidate for both industrial fuel cells and steam electrolysers.     

Electronic structure and electrical conduction by polaron hopping mechanism in A2LuTaO6 (A= Ba,Sr, Ca) double perovskite oxides

Impedance spectroscopy has been applied to explore electrical properties of polycrystalline double perovskite oxides A₂LuTaO₆ (A = Ba, Sr, Ca; ALT). The phase purity and microstructural analysis of the samples are obtained from XRD and SEM. Ba₂LuTaO₆ (BLT) crystallizes in a single phase whereas in Sr₂LuTaO₆ (SLT) and Ca₂LuTaO₆ (CLT) a small secondary phase of Lu₂O₃ was detected. The Nyquist plots for ALT reveals the rising dominance of the grain boundary contribution to the conduction process with increasing temperature. The complex modulus plots confirm the presence of both the grain and grain boundary contributions to the relaxation process at low temperatures. The relaxation mechanism is seen to be shifted from its ideal character as observed in the Nyquist plots. The frequency dependent conductivity spectra follow the power law behavior. Small polaron hopping is responsible for the electrical conduction in these materials in the entire temperature range. Density functional theory calculations are performed to understand the electronic structure of the materials. The density of states (DOS) for BLT, SLT and CLT establishes the semiconducting ground state of the materials. The conductivity mechanism is analysed on the basis of the calculated DOS.     

Composite cathodes of La0.9Ca0.1Ni0.5Co0.5O3-Ce0.8Sm0.2O1.9 for solid oxide fuel cells

The mixed ionic and electronic conductors of La_0.9Ca_0.1Ni_0.5Co_0.5O₃-Ce_0.8Sm_0.2O_1.9 (LCNC-SDC) are investigated systematically for potential application as a cathode for solid oxide fuel cells based on a Ce_0.8Sm_0.2O_1.9 (SDC) electrolyte. The electrochemical impedance spectroscopy (EIS) measurements are performed in air over the temperature range of 600-850 °C to determine the cathode polarization resistance. The exchange current densities for oxygen reduction reaction (ORR), determined from the low-field cyclic voltammetry, high-field cyclic voltammetry, and EIS data are systematically investigated. The activation energies (E_a) for ORR determined from the slope of Arrhenius plots are in the range of 102.33-150.73 kJ mol⁻¹ for LCNC-SDC composite cathodes. The experimental results found that LCNC-SDC (70:30) composite cathode has a maximum exchange current density and a minimum polarization resistance of 0.30 Ω cm² for 850 °C among LCNC-SDC composite cathodes.