Synthesis and densification of lanthanum silicate apatite electrolyte for intermediate temperature solid oxide fuel cell via co-precipitation method

Lanthanum silicate apatite (LSA, La_9.33+xSi₆O_26+1.5x, x = 0–0.67) has been widely investigated as a promising electrolyte material for intermediate temperature solid oxide fuel cell (SOFC). In this work, a facile and low-cost co-precipitation method is used to synthesize LSA precursor powders. The well dispersed nanopowders (ca. 70 nm) with pure hexagonal LSA phase are obtained by calcining the precursor at 900 °C. Impurity of La₂SiO₅, caused by the different precipitation productivities of La(NO₃)₃ and TEOS, can be eliminated through lowering the La/Si ratio in the starting mixtures. The dispersant (PEG200) plays a crucial role in co-precipitation processes, which can effectively mitigate the agglomeration and therefore significantly improve the sinterability of the nanoparticles. Dense LSA ceramic with relative density of 98% is obtained after sintering at 1550 °C, which exhibits a conductivity of 0.13 mS cm⁻¹ at 500 °C.     

Phase relationships and transport in Ti-, Ce- and Zr-substituted lanthanum silicate systems

The solubility of Ti⁴⁺ in the lattice of apatite-type La_9.83Si_6−xTi_xO_26.75 corresponds to approximately 28% of the Si-site density. The conductivity of La_9.83Si_6−xTi_xO_26.75 (x = 1–2) is predominantly oxygen-ionic and independent of the oxygen partial pressure in the p(O₂) range from 10⁻²⁰ to 0.3 atm. The electron transference numbers determined by the modified faradaic efficiency technique are lower than 0.006 at 900–950 °C in air. The open-circuit voltage of oxygen concentration cells with Ti-doped silicate electrolytes is close to the theoretical Nernst value both under oxygen/air and air/10%H₂–90%N₂ gradients at 700–950 °C, suggesting the stabilization of Ti⁴⁺ in the apatite structure. Titanium addition in La_9.83Si_6−xTi_xO_26.75 (x = 1–2) leads to decreasing ionic conductivity and increasing activation energies from 93 to 137 kJ/mol, and enhanced degradation in reducing atmospheres due to SiO volatilization. At p(O₂) = 10⁻²⁰ atm and 1223 K, the conductivity decrease after 100 h was about 5% for x = 1 and 17% for x = 2. The solubility of Zr⁴⁺ in the La_9.83Si_6−xZr_xO_26.75 system was found to be negligible, while the maximum concentration of Ce⁴⁺ in La_9.4−xCe_xSi₆O_27−δ is approximately 5% with respect to the number of lanthanum sites.     

Enhanced sinterability of mechanical alloyed La9.33Si2Ge4O26 oxyapatite powders for IT-SOFC electrolytes

Dense oxyapatite-based La_9.33Si₂Ge₄O₂₆ electrolytes have been successfully prepared by electrical sintering at 1400 °C in static air for 1 h from dry milling La₂O₃, SiO₂ and GeO₂ powders, in adequate atomic proportions, at 350 rpm for 15 h, under controlled environmental conditions, in a planetary ball mill. The densification behaviour of apatite-type phase La_9.33Si₂Ge₄O₂₆ powders synthesized by mechanical alloying was investigated through microstructural evolution with sintering temperature by means of XRD and SEM/EDS analyses. The content of germanium in the sintered samples remained almost constant, suggesting that its incorporation in the apatite phase hinders the high temperature (>1250 °C) volatilization process.     

Structural and electrical characterization of Sr- and Al- doped apatite type lanthanum silicates prepared by the pechini method

The subject of this work regards the synthesis of La_9.83−xSr_xSi_6−yAl_yO_26+δ (0≤x,y≤0.5) via a modified Pechini method. The compounds where characterized by XRD and SEM. Pure La_9.83Si₆O_26+δ, La_9.38Sr_0.45Si₆O_26+δ and La_9.38Sr_0.45Si_5.70Al_0.30O_26+δ were prepared after sintering at 1400 °C for 20 h while La_9.38Sr_0.45Si_5.55Al_0.45O_26+δ and La_9.38Sr_0.45Si_5.50Al_0.50O_26+δ contained traces (<8%) of La₂SiO₅ as secondary phase. Rietveld analysis showed that La_9.83−xSr_xSi_6−yAl_yO_26+δ (0≤x,y≤0.5) compounds crystallize in the P6₃/m space group. Al doping on Si site exhibits more pronounced effect upon structural parameters in comparison to Sr doping on La site. Interstitial oxygen accommodates a position at the periphery of the hexagonal channels in the vicinity of the SiO₄ groups. Ion conduction is close related with the size of the hexagonal channels and the interstitial oxygen content. The ion conductivity is promoted when an optimum balance between the aforementioned magnitudes is reached. The LS and LsSa4530 compounds exhibit the highest values of ionic conductivity at 700 °C with 11 and 14 mS/cm and activation energy of 0.47 and 0.46 eV, respectively.     

Sol–gel synthesis of La0.6Sr0.4CoO3−x and Sm0.5Sr0.5CoO3−x cathode nanopowders for solid oxide fuel cells

Nano-powders of La_0.6Sr_0.4CoO_3?x (LSC) and Sm_0.5Sr_0.5CoO_3?x (SSC) compositions, which are being investigated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) with La(Sr)Ga(Mg)O_3?x (LSGM) as the electrolyte, were synthesized by low-temperature sol–gel method using metal nitrates and citric acid. Thermal decomposition of the citrate gels was followed by simultaneous DSC/TGA methods. Development of phases in the gels, on heat treatments at various temperatures, was monitored by X-ray diffraction. Sol–gel powders calcined at 550–1000 °C consisted of a number of phases. Single perovskite phase La_0.6Sr_0.4CoO_3?x or Sm_0.5Sr_0.5CoO_3?x powders were obtained at 1200 °C and 1300 °C, respectively. Morphological analysis of the powders calcined at various temperatures was done by scanning electron microscopy. The average crystallite size of the powders was ～15 nm after 700 °C calcinations and slowly increased to 70–100 nm after heat treatments at 1300–1400 °C.     

Study of polar relaxation processes in Sr(1−1.5x)LaxTiO3 ceramics by using field-induced thermally stimulated currents

Polar relaxation processes in Lanthanum doped SrTiO₃ (STO) ceramics, with general formulae Sr_(1−1.5x)La_xTiO₃, were studied by undertaking field-induced thermally stimulated currents measurements below room temperature.The experimental results obtained for doped ceramic (x = 0.0133) were analysed by using dipolar and space-charge relaxation thermally stimulated depolarization currents (TSDC) models in order to determine the nature of the relaxation processes involved.Our results reveal the existence of different relaxation processes in the temperature range 60–300 K. Whereas at low temperature, a relaxation mechanism of a dipolar type was disclosed within the temperature interval centred around 100 K, a space-charge relaxation process could be identified in the temperature range 120–300 K. The temperature dependence of the relaxation parameters will be also discussed in detail.     

Synthesis of La1−xSrxMO3 (M = Mn,Fe, Co,Ni) nanopowders by alanine-combustion technique

La_1?xSr_xMO₃ (M = Mn, Fe, Co, Ni, x = 0–0.3) powders were obtained by solution combustion technique using metal nitrates and α-alanine. The as-prepared powders, resulted by the combustion reaction, were annealed at different temperatures to investigate the evolution of crystalline phases. For the strontium-doped lanthanum-based perovskites, higher annealing temperatures than for the corresponding pure lanthanum-based perovskites are needed to obtain single-phase compounds depending on M-site metal and strontium content. The oxide powders were investigated by FT-IR spectra, X-ray diffraction (XRD), scanning electron microscopy (SEM) and specific area measurements. Based on our results we propose different mechanisms for La_1?xSr_xMO₃ (M = Mn, Fe, Ni, x = 0–0.3) obtaining, depending on the intermediary compounds formed in the combustion reaction or during the thermal treatment of the as-prepared powders.     

Synthesis,structure and oxygen thermally programmed desorption spectra of La1−xCaxAlyFe1−yO3−δ perovskites

Perovskites La_1−xCa_xAl_yFe_1−yO_3−δ (x, y = 0 to 1) were prepared by high-temperature solid-state synthesis based on mixtures of oxides produced by colloidal milling. The XRD analysis showed that perovskites La_0.5Ca_0.5Al_yFe_1−yO_3−δ with a high Fe content (1 − y = 0.8–1.0) were of orthorhombic structure, perovskites with a medium Fe content (1 − y = 0.8–0.5) were of rhombohedral structure, and perovskite with the lowest Fe content (1 − y = 0.2) were of cubic structure. Thermally programmed desorption (TPD) of oxygen revealed that chemical desorption of oxygen in the temperature range from 200 to 1000 °C had proceeded in the two desorption peaks. The low-temperature α-peak (in the 200–550 °C temperature range) was brought about by oxygen liberated from oxygen vacancies; the high-temperature β-peak (in the 550–1000 °C temperature range) corresponded to the reduction of Fe⁴⁺ to Fe³⁺. The chemidesorption oxygen capacity increased with increasing Ca content and decreased with increasing Al content in the perovskites. The Al³⁺ ions restricted, probably for kinetic reasons, the reduction of Fe⁴⁺ and the high-temperature oxygen desorption associated with it.     

Microwave dielectric properties of BaxLa4Ti3 + xO12 + 3x (x = 0.0–1.0) ceramics

In the BaO–La₂O₃–TiO₂ system, the Ba_nLa₄Ti_3 + nO_12 + 3n homologous compounds exist on the tie line BaTiO₃–La₄Ti₃O₁₂ besides tungstenbronze-type like Ba_6 − 3xR_8 + 2xTi₁₈O₅₄ (R = rare earth) solid solutions. There are four kinds of compounds in the homologous series: n = 0, La₄Ti₃O₁₂; n = 1, BaLa₄Ti₄O₁₅; n = 2, Ba₂La₄Ti₅O₁₈; n = 4, Ba₄La₄Ti₇O₂₄. These compounds have the layered hexagonal perovskite-like structure, which has a common sub-structure in the crystal structure. These compounds have been investigated in our previous studies. In this study, we have investigated the phase relation and the microwave dielectric properties of Ba_xLa₄Ti_3 + xO_12 + 3x ceramics in the range of x between 0.2 and 1.0. With the increase in x, the dielectric constant ɛ_r locates around 45, the quality factor Q × f shows over 80,000 GHz at x = 0.2 and the minimum value of 30,000 GHz at x = 0.9, and the temperature coefficients of resonant frequency τ_f is improved from −17 to −12 ppm/°C. At x = 0.2, the ceramic composition obtained has dielectric constant ɛ_r = 42, the temperature coefficient of the resonant frequency τ_f  = −17 ppm/°C and a high Q × f of 86,000 GHz.     

Evaluation of the La0.75Sr0.25Mn0.8Co0.2O3−δ system as cathode material for ITSOFCs with La9Sr1Si6O26.5 apatite as electrolyte

In the past years, a major interest has been devoted to decrease the working temperature of solid oxide fuel cells (SOFCs) down to about 700 °C.Apatite materials (La_10?xSr_xSi₆O_27?x/2) are attractive candidates for solid electrolytes, with a high ionic conductivity at 700 °C, a chemical and a dimensional stability for a pO₂ ranging from 10^?25 to 0.2 atm. A perovskite oxide (La_0.75Sr_0.25Mn_0.8Co_0.2O_3?δ) has been used as a cathode material.Symmetrical cathode/electrolyte/cathode cells were fabricated by stacking layers obtained by tape casting of apatite and perovskite powders and co-sintering at 1400 °C for 2 h in air.Impedance spectroscopy measurements were performed on these cells in order to determine the electrode resistance. It has been shown that the latter decreases with the porosity content of the cathode and with the use of a composite material (apatite/perovskite) instead of a simple perovskite.     

Dielectric properties of cubic bismuth based pyrochlores containing lithium and fluorine

In this work dielectric properties of Bi_1.5Zn_1?xLi_xNb_1.5O_7?xF_x with x = 0.25 were investigated in a 20 Hz–12 GHz frequency and 120–500 K temperature range and compared to that of regular cubic BZN (when x = 0). Measurements showed that both ceramics have dipolar glass type dielectric dispersion with wide relaxation time distributions. Mean relaxation time follows Arrhenius law in the investigated frequency range, although Vogel–Fulcher law was anticipated.     

Characterization of Li–Zn–Fe crystalline phases in low temperature ceramic glaze

Ceramic glaze containing Li₂O and ZnO was prepared at a low firing temperature of 1100 °C. Addition of 0–30 wt.% iron oxide content developed brown color with a metallic sparkling effect from crystallization after soaking at 980–1080 °C. Using XRD, SEM/EDS and Raman microscopy the crystalline phases were determined as lithium zinc ferrite (Li_xZn_1?2xFe_2+xO₄ where x = 0.05–0.20), hematite (α-Fe₂O₃) and anorthite (CaAl₂Si₂O₈). The most preferable metallic sparkling effect was caused by the lithium zinc ferrite phase obtained from the glaze containing 10 wt.% of iron oxide. Thermal analysis by STA after heat treatment indicated that crystallization temperature of lithium zinc ferrite and the effective soaking temperature depended on the iron oxide content in the glaze. The influence of excessive iron oxide content on the crystallization behavior of lithium zinc ferrite, anorthite and hematite phases is discussed.     

Direct catalytic decomposition of nitrous oxide gas over rhodium supported on lanthanum silicate

The environmental impacts of nitrous oxide (N₂O) have received much attention, including contributions to the greenhouse effect and ozone depletion. Currently, the direct catalytic decomposition of N₂O is considered to be the simplest and most promising method for N₂O abatement. In this study, we focused on the high activity of rhodium and the oxide-ion conducting property of lanthanum silicate and prepared novel Rh/La₁₀Si_6 − xFe_xO_27 − δ catalysts. From the results of catalytic N₂O decomposition activities, Rh/La₁₀Si_6 − xFe_xO_27 − δ (x = 1.0) exhibited the highest catalytic activity and N₂O was completely decomposed at 600 °C.     

Polymorphism in the Lu2 − xYxSi2O7 system at high temperatures

Samples in the system Lu_2 − xY_xSi₂O₇ (1.25 ≤ × ≤ 2) have been synthesised following a sol–gel method and calcined to high temperatures (≥1400 °C). X-ray diffraction (XRD) has shown that all compositions crystallize as β-Lu_2 − xY_xSi₂O₇ at the low temperatures, while increasing calcination temperature produces the formation of the γ- and δ-polymorphs, the temperatures of formation of each polymorph depending on the Y/Lu ratio. Unit cell parameters of the samples crystallizing as γ-Lu_2 − xY_xSi₂O₇ have been calculated and plotted as a function of composition. They show a linear change with increasing Y content, indicating a degree of solid solubility of Lu₂Si₂O₇ in γ-Y₂Si₂O₇. Based on these data and on those reported in our previous studies [Becerro, A.I. and Escudero, A., XRD and ²⁹Si MAS NMR spectroscopy across the β-Lu₂Si₂O₇–β-Y₂Si₂O₇ solid solution. J. Solid State Chem., 2005, 178; Becerro, A.I. and Escudero, A., Phase transitions in Lu-doped Y₂Si₂O₇ at high temperatures. Chem. Mater., 2005, 17, 112] a temperature–composition diagram of the Lu₂Si₂O₇–Y₂Si₂O₇ system is given. Finally, the influence of Lu on the reversibility of the γ-Y₂Si₂O₇ → β-Y₂Si₂O₇ transition is studied by means of XRD and ²⁹Si MAS NMR spectroscopy.     

Microstructural,structural and electrical properties of La3+-modified Bi4Ti3O12 ferroelectric ceramics

Bi_4?xLa_xTi₃O₁₂ (BLT) ceramics were prepared and studied in this work in terms of La³⁺-modified microstructure and phase development as well as electrical response. According to the results processed from X-ray diffraction and electrical measurements, the solubility limit (x_L) of La³⁺ into the Bi₄Ti₃O₁₂ (BIT) matrix was here found to locate slightly above x = 1.5. Further, La³⁺ had the effect of reducing the material grain size, while changing its morphology from the plate-like form, typical of BIT ceramics, to a spherical-like one. The electrical results presented and discussed here also include the behavior of the temperature of the ferroelectric–paraelectric phase transition as well as the normal or diffuse and/or relaxor nature of this transition depending on the La³⁺ content.     

On the influence of silica type on the structural integrity of dense La9.33Si2Ge4O26 electrolytes for SOFCs

Apatite-type rare earth based oxides, such as R-doped lanthanum oxides of general formula La_9.33(RO₄)₆O₂ with R = Ge, Si, exhibit high ionic conductivity and low activation energy at moderate temperatures, when compared to the yttria-stabilized zirconia electrolyte making them potential materials to be used in the range 500–700 °C, for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this study, dense oxyapatite-based La_9.33Si₂Ge₄O₂₆ electrolytes have been successfully prepared either by electrical sintering at 1400 °C or microwave hybrid sintering at 1350 °C for 1 h from La₂O₃, SiO₂ and GeO₂ powders dry milled at 350 rpm for 15 h in a planetary ball mill. The densification behaviour of the apatite-type phase synthesized by mechanical alloying was found to be dependent on the grade of SiO₂ used: either pre-milled quartz powder or amorphous nanosized fumed silica. The influence of the silica type on the La_9.33Si₂Ge₄O₂₆ integrity was assessed by dynamic Young's modulus, microhardness and indentation fracture toughness measurements. A good correlation between the degree of densification (as observed by SEM/EDS) and the resulting mechanical properties could be established. Pre-milling of quartz powder has favoured higher densification rates to be attained suggesting that both Fe content, resulting from the dry milling (as determined by PIXE analyses) and crystallinity of SiO₂ do promote densification of these electrolytes thereby improving their structural integrity.     

Synthesis and negative thermal expansion properties of solid solutions Yb2−xLaxW3O12 (0 ≤ x ≤ 2)

A new series of rare earth solid solutions Yb_2?xLa_xW₃O₁₂ were successfully synthesized by the solid-state method. Effects of substituted ion lanthanum on the microstructures and thermal expansion properties in the resulting Yb_2?xLa_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analyzer (TGA), field emission scanning electron microscope (FESEM) and thermal mechanical analyzer (TMA). Results indicate that the structural phase transition of the Yb_2?xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phases can form in the composition range of 0 ≤ x < 0.5 with orthorhombic structure and 1.5 < x ≤ 2 with monoclinic one. High lanthanum content leads to a low hygroscopicity of Yb_2?xLa_xW₃O₁₂. Negative thermal coefficients of the Yb_2?xLa_xW₃O₁₂ (0 ≤ x ≤ 2) also vary from ?7.78 × 10^?6 K^?1 to 2.06 × 10^?6 K^?1 with increasing substituted content of lanthanum.     

Solid-state synthesis and spark plasma sintering of submicron BaYxZr1 − xO3 − x/2 (x = 0, 0.08 and 0.16) ceramics

Fine powders (particle size of 100–200 nm) of BaY_xZr_1 − xO_3 − x/2 (x = 0, 0.08, 0.16) were produced by solid-state reaction at 1000–1050 °C using nanocrystalline ZrO₂ and BaCO₃ raw materials. The powders were densified by means of the spark plasma sintering process resulting in dense and homogeneous submicron microstructures. Near full density ceramics with grain size < 300 nm were obtained by sintering at 1600 °C for 1–5 min.     

High-temperature conductivity,stability and redox properties of Fe3−xAlxO4 spinel-type materials

Iron-based oxides are considered as promising consumable anode materials for high temperature pyroelectrolysis. Phase relationships, redox stability and electrical conductivity of Fe_3?xAl_xO₄ spinels were studied at 300–1773 K and p(O₂) from 10^?5 to 0.21 atm. Thermogravimetry/XRD analysis revealed metastability of the sintered ceramics at 300–1300 K. Low tolerance against oxidation leads to dimensional changes of ceramics upon thermal cycling. Activation energies of the total conductivity corresponded to the range of 16–26 kJ/mol at 1450–1773 K in Ar atmosphere. At 1573–1773 K and p(O₂) ranging from 10^?5 to 0.03 atm, the total conductivity of Fe_3?xAl_xO₄ is nearly independent of the oxygen partial pressure. The conductivity values of Fe_3?xAl_xO₄ (0.1 ≤ x ≤ 0.4) at 1773 K and p(O₂) ～10^?5 to 10^?4 atm were found to be only 1.1–1.5 times lower than for Fe₃O₄, showing high potential of moderate aluminium additions as a strategy for improvement of refractoriness for magnetite without significant deterioration of electronic transport.     

Nanocrystaline solid solution CeO2–Bi2O3

Nanocrystalline powders of solid solution CeO₂–Bi₂O₃ were synthesized by self-propagating room temperature reaction (SPRT) procedure with composition (Ce_1?xBi_xO_2?δ where the x = 0.1–0.5). X-ray diffraction analyses show that for x < 0.50 a solid solution with fluorite structure is formed. Rietveld's structure refinement method was applied to characterize prepared powders and its microstructure (size–strain). The lattice parameters increase according to Vegard's rule with increasing of Bi concentration. The average crystallite size is about 2–3 nm. Spectroscopic ellipsometry and Raman scattering measurements were used to characterize the samples at room temperature. The Raman measurements demonstrated electron molecular vibrational coupling and increase of oxygen vacancy concentration whereas increase of Bi content provokes a small decrease of optical absorption edge in comparison with pure ceria. Specific surface area of obtained powders was measured by Brunauer–Emmet–Teller (BET) method.