Co-doped ceria-based solid solution in the CeO2-M2O3-CaO, M = Sm, Gd system

The Pechinni method (A) as well as hydrothermal treatment (B) of co-precipitated CeO₂-based gels with NaOH solution were used to synthesise pure CeO₂, and CeO₂-based solid solutions with formula Ce_1−xM_xO₂, Ce_1−x(M_0.5Ca_0.5)_xO₂ M = Gd, Sm for 0.15 < x < 0.3 nanopowders. The thermal evolution of CeO₂-based precursors during heating them up to 1000 °C was monitored by thermal (TG, DTA) analysis and X-ray diffraction method. All nanopowders and samples sintered were found to be pure CeO₂ or ceria-based solutions with fluorite-type structure. The microstructure of CeO₂-based sintered samples at 1500 °C (A) or 1250 °C (B) was observed for 2 h under the scanning electron microscope. The electrical properties of singly Ce_1−xM_xO₂ or doubly doped CeO₂-based samples with formula Ce_1−x(M_0.5Ca_0.5)_xO₂, M = Gd, Sm, 0.15 < x < 0.30 were investigated by means of the ac impedance spectroscopy method throughout the temperature range of 600-800 °C. It has been stated that partial substitution of calcium by samarium or calcium by gadolinium in the Ce_1−x(M_0.5Ca_0.5)_xO₂, M = Gd, Sm solid solutions leads to ionic conductivity enhancement comparable with only samaria- or gadolina-doped ceria. The CeO₂-based samples with small-grained microstructures obtained from powders synthesised by hydrothermal method exhibited better ionic conductivity than samples with the same composition obtained from powders synthesised by the Pechinii method. The stability of the electrolytic properties of selected co-doped ceria sinters in fuel gases (H₂, CH₄) as well as exhaust gases from diesel engine was also investigated. The co-doped Ce_0.8(Sm_0.5Ca_0.5)_0.2O₂ or Ce_0.85(Gd_0.5Ca_0.5)_0.15O₂ dense samples would appear be to more adequate oxide electrolytes than Ce_1−xM_xO₂, M = Sm, Gd and x = 0.15 or 0.2 for electrochemical devices operating at temperatures ranging from 600 to 700 °C.     

Synergistic effect of RE2O3 (RE = Sm, Eu and Gd) on Pt/mesoporous carbon catalyst for methanol electro-oxidation

This work aims at developing the synergetic catalysis of rare earth oxides, which can serve as the anchoring sites for Pt and in turn as the active sites for the methanol electro-oxidation. Ordered mesoporous carbon has been modified with RE₂O₃ (RE = Sm, Eu and Gd) and subsequently deposited with Pt nanoparticles by a microwave heating process. The catalytic activities for methanol electro-oxidation are evaluated by cyclic voltammogram, chronoamperometry and electrochemical impedance spectroscopy. Pt/C-Sm₂O₃ exhibits an improvement on dispersion and electrocatalytic performance, with the highest mass activity (145.1 mA mg⁻¹) as well as good stability. A possible mechanism is proposed for the effect of RE₂O₃ on Pt nanoparticles. The adsorbed hydroxyl species supplied by RE₂O₃ in the support are contributed to the release of Pt active sites through CO removal and therefore promote the catalytic activity spontaneously.     

Thermal Conductivity of Monazite-Type REPO4 (RE=La, Ce, Nd, Sm, Eu, Gd)

Low-thermal conductivity ceramics in monazite-type REPO₄ (RE=La, Ce, Nd, Sm, Eu, Gd) ceramics are expected to have potential in structural (refractories, thermal insulator) and nuclear applications. To this end, the present study determines their thermal conductivities and examines how differences of the rare earth ions change their thermal conductivity at different temperatures. The results show that their conductivities are remarkably low from 25° to 1000°C. In addition, different conductivity variation mechanisms exist that change gradually upon altering from LaPO₄ to GdPO₄ at low and high temperatures. At relatively lower temperatures (≤400°C), the thermal conductivities of all the REPO₄ ceramics decrease nearly at first, reach a minimum value, and then rise with gradual altering from LaPO₄ to GdPO₄. It may be due to the combined effects of the increase of both the anharmonicities in lattice vibrations and the bond strength. As the temperature increases, the conductivity trends become obscure, and the conductivities of the monazite-type REPO₄ approach their minimum thermal conductivities when the temperature is above 800°C.     

Preparation, Characterization, and Microwave Properties of RETiNbO6 (RE = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, and Yb) Dielectric Ceramics

Microwave ceramic dielectric resonators (DRs) based on RETiNbO₆ (RE = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, and Yb) have been prepared using the conventional solid-state ceramic route. The DR samples are characterized using XRD and SEM methods. The microwave dielectric properties are measured using resonant methods and a net work analyzer. The ceramics based on Ce, Pr, Nd, and Sm have dielectric constants in the range 32–54 and positive coefficient of thermal variation of resonant frequency (τ_f). The ceramics based on Gd, Tb, Dy, Y, and Yb have dielectric constants in the range 19–22 and negative τ_f.     

Synthesis and thermophysical properties of RETa3O9 (RE = Ce,Nd, Sm,Eu, Gd,Dy, Er) as promising thermal barrier coatings

Thermal barrier coatings (TBCs) are one of the most important materials in gas turbine to protect the high temperature components. RETa₃O₉ compounds have a defect‐perovskite structure, indicating that they have low thermal conductivity, which is the critical property of TBCs. Herein, dense RETa₃O₉ bulk ceramics were fabricated via solid‐state reaction. The crystal structure was characterized by X‐ray diffraction (XRD) and Raman Spectroscope. Scanning electron microscope (SEM) was used to observe the microstructure. The thermophysical properties of RETa₃O₉ were studied systematically, including specific heat, thermal diffusivity, thermal conductivity, thermal expansion coefficients, and high‐temperature phase stability. The thermal conductivities of RETa₃O₉ are very low (1.33‐2.37 W/m·K, 373‐1073 K), which are much lower than YSZ and La₂Zr₂O₇; and the thermal expansion coefficients range from 4.0 × 10^?6 K^?1 to 10.2×10^?6 K^?1 (1273 K), which is close to La₂Zr₂O₇ and YSZ. According to the differential scanning calorimetry (DSC) curve there is not phase transition at the test temperature. Due to the high melting point and excellent high‐temperature phase stability with these oxides, RETa₃O₉ ceramics were promising candidate materials for TBCs.     

Co-precipitation synthesis of spherical Li1.05M0.05Mn1.9O4 (M = Ni, Mg, Al) spinel and its application for lithium secondary battery cathode

Metal-doped spinel lithium manganese oxides were successfully synthesized via co-precipitation. The synthesized Li_1.05M_0.05Mn_1.9O₄ (M = Ni, Mg, Al) had highly crystalline cubic spinel phase with the space group Fd3m, as confirmed by X-ray diffraction study. From scanning electron microscopic observation, it was found that the prepared materials had spherical morphology with high tap-density. The Li_1.05M_0.05Mn_1.9O₄ (M = Ni, Mg, Al) electrode exhibited improved cycling performance at elevated temperature. Especially, the Li_1.05Al_0.05Mn_1.9O₄ showed capacity retention of 91.5% during 100 cycles at elevated temperature (55 °C). Also the thermal stability of the Li_1.05M_0.05Mn_1.9O₄ (M = Ni, Mg, Al) was also significantly improved.     

Facile synthesis and luminescence properties of highly uniform YF3:Ln (Ln = Eu, Tb, Ce, Dy) nanocrystals in ionic liquids

Uniform YF₃ nanocrystals were prepared through a facile ethylene glycol (EG)/ionic liquid interfacial synthesis route in a imidazolium ionic liquids (1-octyl-3-methylimidazolium hexafluorophosphate) with the ionic liquids acting as both reagents and templates. The partial hydrolysis of PF₆⁻ was utilized to introduce a fluoride source. X-ray diffraction (XRD), field emission scanning microscopy (FE-SEM) and transmission electron microscopy (TEM) have been used to study the morphologies and crystal structure. The detailed growth mechanism of the YF₃ nanocrystals was researched. Furthermore, under ultraviolet exaltation, the phosphor of YF₃:Eu³⁺ and YF₃:Tb³⁺ shows red and green emission, corresponding to ⁵D₀-⁷F₁ transition of Eu³⁺ and ⁵D₄-⁷F₅ transition of Tb³⁺. The emission spectrum of YF₃:Ce³⁺ phosphor exhibits one dissymmetrical band extending from 350 to 500 nm with a maximum at about 383 nm. A bright fluorescent yellow emission at 574 nm and blue emission at 487 nm were observed in the YF₃:Dy³⁺. These novel nanocrystals could be potentially used in biolabels and light emitting diodes (LEDs).     

Thermoelectric properties and stability of the Re0.2Sr0.8CoO3-δ Re = Gd,Dy) complex cobalt oxides in the temperature range of 300‒800 K

Temperature dependences of the electrical resistivity and Seebeck coefficient of the Re_0.2Sr_0.8CoO_3?δ Re =?Gd, Dy) complex cobalt oxides in the temperature range of 300 – 800?K have been investigated. The effects of ordering in the cation and anion sublattices on the thermoelectric properties has been examined and their comparative analysis has been made. It was found that, in the investigated temperature range, the thermoelectric power factor of the ordered compounds significantly exceeds the analogous parameter of the disordered samples. The temperature dependence of the electrical resistivity was shown to obey the activation law. As the temperature increases, the samples undergo a semiconductor-semiconductor electronic transition with a decrease in the activation energy. The thermoelectric properties of all the samples are shown to be stable in the investigated temperature range. The maximum thermoelectric power factor of the ordered Dy_0.2Sr_0.8CoO_2.67 cobaltite at a temperature of 360?K has been obtained; it amounts to 0.23?µW/(cm·K²), which is a good parameter for this class of materials.     

Nanostructured co-precipitated Ce0.9Ln0.1O2 (Ln = La,Pr, Sm,Nd, Gd,Tb, Dy,or Er) for thermochemical conversion of CO2

The influence of lanthanide metal cations doped into the CeO₂ crystal structure (to form Ce_0.9Ln_0.1O₂; Ln = La, Pr, Nd, Sm, Gd, Tb, Dy, or Er) on thermochemical reduction and the CO₂ splitting ability of Ce_0.9Ln_0.1O₂ is scrutinized using thermogravimetric analysis. Ce_0.9Ln_0.1O₂ redox materials are effectively synthesized by co-precipitation of hydroxides. As-synthesized Ce_0.9Ln_0.1O₂ redox materials are further characterized based on their phase composition, crystallite size, surface area, and morphology using powder X-ray diffraction, Brunauer-Emmett-Teller surface area analysis, and scanning electron microscopy. The thermal reduction and CO₂ splitting aptitude of Ce_0.9Ln_0.1O₂ redox materials are examined by performing 10 consecutive thermochemical cycles. The results imply that insertion of Sm³⁺, Er³⁺, Tb³⁺, Dy³⁺, and La⁺³ in place of Ce⁴⁺ in the fluorite crystal structure of CeO₂ (forming Ce_0.9Ln_0.1O₂) enhances the O₂ liberation by 22.5, 14.6, 12.6, 5.85, and 2.96 μmol O₂/g·cycle, respectively. Besides, CeLa is observed to be more active towards the CO₂ splitting reaction than CeO₂ and the other Ce_0.9Ln_0.1O₂ redox materials investigated in this study.     

Electrochemical behavior of Ln0.6Sr0.4Co0.2Fe0.8O3−δ (Ln=Ce, Gd, Sm, Dy) materials used as cathode of IT-SOFC

The perovskite-type compounds Ln_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (Ln=Ce, Sm, Gd, Dy) used as the cathodes of intermediate temperature solid oxide fuel cell (IT-SOFC) were studied. The cells consisted of anode supported Sm-doped-ceria electrolyte bi-layer and cathode with 0.65 cm² effective area. Open-circuit voltage (OCV), V–I and P–I curves of the cells were measured over a temperature range from 400 to 800 °C, using H₂–3%H₂O as fuel and air as oxidant. Polarization potential of electrodes were measured with asymmetry three-electrode method during cell discharging. The results indicated that, Dy-SCF material cathode behaved with high catalytic activity for oxygen dissociation at low temperatures. For each cell with a particular cathode, there was a transition temperature, at which OCV of the cell reached the highest value. When temperature was higher than the transition temperature, OCV of the cell increases with decreasing temperature, whereas as temperature was lower than that, OCV decreased with lowering temperature.     

Characterization of Ln4Al2O9 (Ln=Y,Sm, Eu,Gd, Tb) rare-earth aluminates as novel high-temperature barrier materials

A family of cuspidine-type rare-earth (RE) aluminates with the general formula Ln₄Al₂O₉ (Ln= Y, Sm, Eu, Gd, Tb) was prepared for potential use as thermal-barrier coating (TBC) materials with appropriate properties. Various trivalent lanthanides were applied to tailor the properties of the oxides for use as ceramic top coat (TC) materials intended for high-temperature applications. Following various heat treatments, the X-ray diffraction (XRD) results obtained demonstrated that Eu₄Al₂O₉ (EuAM) possessed the greatest structural stability of all the samples at 1200 and 1300?°C. Moreover, Y₄Al₂O₉ (YAM) had a long lifetime at 1000?°C, and was stable at 1100?°C. At 1200?°C, Sm₄Al₂O₉ (SmAM) and Gd₄Al₂O₉ (GdAM) were more stable than Tb₄Al₂O₉ (TbAM). However, at 300–1000?°C, the TbAM exhibited the highest thermal expansion coefficient (TEC) of all the samples. At 600?°C, the thermal diffusivity values of the five compositions were favourable, and were lower than that of yttria-stabilised zirconia (YSZ) oxides.     

Creep behavior and related structural defects in Al2O3-Ln2O3 (ZrO2) directionally solidified eutectics (Ln = Gd, Er, Y)

The eutectic architecture of “in situ” composites prepared by solidification from the melt in the Al₂O₃-Ln₂O₃ (ZrO₂) systems gives rise to materials with a high creep resistance. With the objective to elucidate the high temperature deformation micro-mechanisms, microstructural features are investigated on crept specimens. Compressive creep experiments have been carried out between 1400 and 1550 °C for various eutectic compositions. Different deformation regimes depending on considered systems and conditions of stress and temperature are revealed. Transmission electron microscopy studies emphasize the activation of different slip systems in the alumina phase and the deformation by dislocation climb processes controlled by bulk diffusion.     

Structural and luminescence properties of RE (RE = Eu, Tb):(MgCa)2Bi4Ti5O20 ceramics

Rare-earth ions (Eu³⁺, Tb³⁺) activated magnesium calcium bismuth titanate [(MgCa)₂Bi₄Ti₅O₂₀] ceramics were prepared by conventional solid state reaction method for their structural and luminescence properties. By using XRD patterns, the structural properties of ceramic powders have been analyzed. Emission spectrum of Eu³⁺:(MgCa)₂Bi₄Ti₅O₂₀ ceramic powder has shown strong red emission at 615 nm (⁵D₀ → ⁷F₂) with an excitation wavelength λ_exci = 393 nm and Tb³⁺:(MgCa)₂Bi₄Ti₅O₂₀ ceramic powder has shown green emission at 542 nm (⁵D₄ → ⁷F₅) with an excitation wavelength λ_exci = 376 nm. In addition, from the measurements of scanning electron microscopy (SEM), Fourier transform-infrared (FTIR) and energy dispersive X-ray analysis (EDAX) results the morphology, structure and elemental analysis of these powder ceramics have been studied.     

Combustion synthesis of Ca1−xCuxAl2O4 (x = 0.0, 0.4 and 0.8) copper doped calcium aluminate

We report the effect of Cu²⁺ ion on CaAl₂O₄ with different molar concentrations of 0.0, 0.4 and 0.8 M prepared by simple combustion method. The materials have been characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR) and scanning electron microscopy (SEM). DC electrical conductivity has also been measured to study the electrical behavior of the materials. The XRD patterns confirm the formation of single-phase CaAl₂O₄ along with some impurity phases like CaAl₄O₇, CaAl₁₂O₁₉ and Ca₁₂Al₁₄O₃₃. The FT-IR spectra show the stretching and bending vibrations of the synthesized compounds. DC electrical conductivity of the Ca_1−xCu_xAl₂O₄ is found to vary from 26.46 × 10⁻⁴ to 515.68 × 10⁻⁴ S cm⁻¹ for x = 0.0 to x = 0.8 at the measuring temperature of 1000 °C. SEM images show the morphological features of the compounds.     

Microwave Dielectric Properties of Sr(B'1/2Nb1/2)O3 [B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, and In] Ceramics

Sr(B'_1/2Nb_1/2)O₃ [B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, and In] ceramics were prepared by the conventional solid-state ceramic route. The crystal structure and microstructure of the ceramics were characterized by X-ray diffraction and scanning electron microscopic methods, and the dielectric properties were measured in the microwave range. Addition of a small amount of CeO₂ as a sintering aid improved the densification and dielectric properties of the ceramics. The effects of cation substitution and glass addition on the microwave dielectric properties of the ceramics were also investigated. Glass addition resulted in the lowering of the processing temperature of the materials without much affecting the dielectric properties, whereas cation substitution resulted in the variation of the temperature coefficient of resonant frequencies. A correlation of dielectric properties with the tolerance factor and ionic radii of B'-site elements of the ceramics has been observed.     

The catalytic activity and surface characterization of Ln2B2O7 (Ln=Sm, Eu, Gd and Tb; B=Ti or Zr) with pyrochlore structure as novel CH4 combustion catalyst

Ln₂B₂O₇ (Ln=Sm, Eu, Gd and Tb; B=Zr or Ti) with pyrochlore structure was prepared by sol–gel method for the high-temperature catalytic combustion. The crystal structure of Ln₂B₂O₇ was identified by XRD and their surface area was about 4 m²/g after calcinations at 1200 °C. Catalytic activity of methane combustion was observed for Ln₂Zr₂O₇ series and the best catalyst was Sm₂Zr₂O₇. Its relative reaction rate per unit surface area at 600 °C was 2 cm³/m² min, which was twice higher than that of Mn-substituted Sr hexaaluminate. From surface analysis by XPS, the low binding energy of each Ln element of Ln₂Zr₂O₇ compared to that of Ln₂Ti₂O₇, gave the catalytic activity of methane combustion.     

The Effect of Dopants on the Dielectric Properties of Ba(B'1/2Ta1/2)O3 (B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Yb, and In) Microwave Ceramics

Low-loss dielectric ceramics based on Ba(B'_1/2Ta_1/2)O₃ (B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Yb, and In) complex perovskites have been prepared by the solid-state ceramic route. The dielectric properties (ɛ_r, Q _u, and τ_f) of the ceramics have been measured in the frequency range 4–6 GHz by the resonance method. The resonators have a relatively high dielectric constant and high quality factor. Most of the compounds have a low coefficient of temperature variation of the resonant frequencies. The microwave dielectric properties have been improved by the addition of dopants and by solid solution formation. The solid solution Ba[(Y_{1− x} Pr _x )_1/2Ta_1/2]O₃ has x =0.15, with ɛ_r=33.2, Q _u× f =51,500 GHz, and τ_f≈0. The microwave dielectric properties of Ba(B'_1/2Ta_1/2)O₃ ceramics are found to depend on the tolerance factor ( t ), ionic radius, and ionization energy.     

Effects of ionic radius on phase evolution in Ln-Al co-doped Ca1-xLnxZrTi2-xAlxO7 (Ln = La,Nd, Gd,Ho, Yb) solid solutions

Zirconolite ceramic has been considered as a promising matrix to dispose high-level radioactive waste due to its excellent performance in immobilizing radionuclides. In this work, a series of zirconate solid solutions with stoichiometric Ca_1-xLn_xZrTi_2-xAl_xO₇ (Ln = La, Nd, Gd, Ho, Yb; x?=?0.1–1) were systematically studied to investigate the radius effect on their phase evolution. Powder X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-Ray spectrometry (SEM-EDX) were used to characterize the products. XRD and SEM results show that complete solid solutions of Ln and Al in zirconolite phase for Ca_1-xLn_xZrTi_2-xAl_xO₇ cannot found. In the Ca_1-xLa_xZrTi_2-xAl_xO₇ ceramics, single zirconolite phase cannot form, instead of multiple phases, such as zirconolite-2M, zirconia, perovskite and LaTi₂Al₉O₁₉. In the Nd-Al co-doping ceramics, nearly single zirconolite-2M and zirconolite-3O were found at x?≤?0.6 and 0.8?≤?x?≤?0.9, respectively. The miscibility gap between zirconolite-2M and 3O was found at x?=?0.7. Single zirconolite-2M formed in the Gd-Al, Ho-Al and Yb-Al co-doped ceramics can only be detected in a compositional range of 0.1?≤?x?≤?0.8. Higher incorporation contents in these three series can form an additional phase cubic zirconia which is usually a ceramic waste form for radionuclides. Based on the XRD data, lattice parameters of zirconolite-2M and zirconolite-3O were calculated by Pawley refinement method. The evolution of lattice parameters of zirconolite-2M shows great difference between different lanthanide ions, indicating different substitution mechanisms in the Ln-Al co-doped zirconolite-2M.