Preparation,sintering and electrical properties of nano-grained multidoped ceria

Multiply doped ceria nanopowders were synthesized by applying MGNP (modified glycine/nitrate procedure). The overall concentration of dopants was kept constant (x = 0.2) whereby Gd ion as the main dopant was gradually substituted by Sm and by Sm + Y. The compositions of solid solutions were calculated by applying defect model introducing anion vacancy radius. Characterization of powders involved BET, TEM, XRD and chemical analyses. Densification was performed at 1500 °C, in an oxygen atmosphere for 1 h. The results showed that with increasing number of dopants, specific surface area of powders increased, followed by decrease of crystallite and grain sizes. Densification degree was also found to rise with increasing number of dopants. According to impedance measurements it was found that ionic conductivity was the highest 1.14 × 10^?3 S cm^?1 at 450 °C in sample doped with Gd, Sm and Y simultaneously.     

“Dark hole” cure in Ba4.2Nd9.2Ti18O54 microwave dielectric ceramics

Large size Ba_4.2Nd_9.2Ti₁₈O₅₄ (BNT) ceramics doped with MnCO₃, CuO and CoO were prepared by the conventional solid-state method. Only a single BaNd₂Ti₄O₁₂ phase was formed in all samples. No second phase was found in the XRD patterns. The bulk density increases slightly because of the dopants. The SEM results showed that the grain size of Mn²⁺and Cu²⁺-doped BNT ceramics became larger with the increasing amount of dopants. The permittivity of all samples stays the same. However, the Q×f value of BNT ceramics increases by doping, especially with Mn²⁺ ions. The conductivity of BNT ceramic doped with Mn²⁺(0.5 mol‰) under high temperature is lower than that without doping. There are fewer defects in Mn²⁺-doped BNT ceramics. The XPS results indicated that Ti reduction was suppressed in BNT ceramics doped with 0.5 mol‰ Mn²⁺. BNT ceramics doped with 0.5 mol‰ Mn²⁺ ions sintered at 1320 °C for 2 h exhibited good microwave dielectric properties, with ε_r=88.67, Q×f=7408 GHz and τf = 82.98 ppm/°C.     

Effect of dopants on synthesis of BaTi4O9 and Ba2Ti9O20 ceramics prepared by reaction-sintering process

The effect of dopants on BaTi₄O₉ (BT4) and Ba₂Ti₉O₂₀ (B2T9) ceramics by the reaction-sintering process was investigated. CuO addition is more effective in lowering the sintering temperature of BT4 and B2T9 ceramics. MnO₂ and CuO addition are effective to obtain temperature stable BT4 ceramics. With MnO₂ addition, Q × f of BT4 ceramics could be raised. ZrO₂ addition is effective to obtain B2T9 ceramics with higher dielectric constant. With CuO addition, τ_f of B2T9 ceramics shifted toward negative values and 0 ppm/°C could be obtained. Optimum properties in BT4 doped with MnO₂ of ɛ_r = 37.1, Q × f = 51,200 GHz (at 7 GHz) and τ_f = 0 ppm/°C and in B2T9 doped with ZrO₂ of ɛ_r = 37.9, Q × f = 39,700 GHz (at 7 GHz) and τ_f = 5.9 ppm/°C were obtained.     

Electrical characterization of multidoped ceria ceramics

Ceria ceramics was obtained from multi-doped nanosized ceria powders prepared by both modified glycine nitrate procedure (MGNP) and self-propagating reaction at room temperature (SPRT). Rare earth elements such as Nd, Sm, Gd, Dy, Y, Yb were used as dopants. The overall mole fraction of dopants was 0.2. One-hour long sintering of powder compacts was performed at 1500 °C in oxygen atmosphere. Phase composition, microstructure and ionic conductivity of sintered samples were analysed. Single-phase ceria was detected in all samples. In general, the increase in the number of dopants improved the ionic conductivity. The samples doped simultaneously with five dopants had the highest ionic conductivity, as evidenced by the impedance measurements. At 450 °C, the conductivity of sample obtained by MGNP was 3.94×10^?3 Ω^?1 cm^?1 whereas the conductivity of sample obtained by SPRT was 2.61×10^?3 Ω^?1 cm^?1. The conductivity activation energy for MGNP and SPRT samples was measured to be 0.348 and 0.385 eV, respectively. Finally, the conductivity decreased as the number of dopants increased to six.     

Effects of LnAlO3 (Ln = La,Nd, Sm) additives on the properties of Ba4.2Nd9.2Ti18O54 ceramics

The effects of LnAlO₃ (Ln = La, Nd, Sm) additives on the microstructure and the dielectric properties of Ba_4.2Nd_9.2Ti₁₈O₅ (BNT) materials have been investigated. The microstructure of BNT doped with NdAlO₃ was analyzed using X-ray diffraction and scanning electron microscopy (SEM). Over 10 wt.% additions of NdAlO₃ diminished grain growth and the densifications process. In addition, typical columnar grain morphology observed in BNT ceramics was transformed to quasi-rectangular shapes accompanied by increasing porosity. XRD analysis did not reveal any second phases or modification of the BNT crystal structure. The addition of small amounts of LnAlO₃ (≤10 wt.%) increased the Q values remarkably and improved the TCf. The relative permittivity and the TCf values showed a linear decrease with increasing additions of LnAlO₃. It was observed that near zero TCf values can be achieved with addition of LnAlO₃ dopants to BNT ceramics. The Q values exhibited a non-linear behaviour with LnAlO₃ additions with maxima attained at approximately 10 wt.% LnAlO₃.     

Effect of CuO/MnO additives on microstructure and microwave dielectric properties of 3ZrO2-3TiO2-ZnNb2O6 ceramics

The effect of CuO/MnO additives on phase composition, microstructures, sintering behavior, and microwave dielectric properties of 3ZrO₂-3TiO₂-ZnNb₂O₆ (3Z-3T-ZN) ceramics prepared by conventional solid-state route were systematically investigated. CuO/MnO doped ceramics exhibited a main phase of α-PbO₂-structured ZrTi₂O₆ and a secondary phase of rutile TiO₂. SEM results showed that the grain size of MnO doped ceramics became larger with increasing amount of dopants. The presence of CuO/MnO additives effectively reduced the sintering temperature of 3Z-3T-ZN ceramics to 1220 °C. MnO doped into ceramics could enhance the Q×f values significantly. The 0.5 wt% CuO doped 3Z-3T-ZN ceramics with 0.5 wt% of MnO, sintered at 1220 °C for 4 h, was measured to show superior microwave dielectric properties, with an ε_r of 41.02, a Q×f value of 44,230 GHz (at 5.2 GHz), and τ_f value of +2.32 ppm/°C.     

Structure and dielectric properties of novel low temperature co-fired Bi2O3-RE2O3-MoO3 (RE=Pr,Nd, Sm,and Yb) based microwave ceramics

Novel monoclinic Bi₂O₃-xRE₂O₃-yMoO₃ (RE=Pr, Nd, Sm, and Yb) based low temperature co-fired ceramics (LTCC) systems with high sintering density and low microwave dielectric loss are synthesized by conventional solid state reaction technique. The structure and dielectric properties of Bi₂O₃-xRE₂O₃-yMoO₃ ceramics are investigated. Dense BiNdMoO₆ ceramics sintered at 900 °C for 8 h in air have a low dielectric constant ε_r=~7.5, a high quality factor Q×f=~ 24, 800 GHz at 7.0 GHz, and τ_f=~−16 ppm/̊C. Especially, good chemical compatibility of BiNdMoO₆ with Ag electrodes is represented as well. In contrast, BiSmMoO₆ ceramics sintered at 1000 °C for 8 h show enhanced Q×f=~43, 700 GHz at 7.8 GHz with ε_r=~8.5 and τ_f=~−27 ppm/°C. Bi₂O₃-xRE₂O₃-yMoO₃ (RE=Pr, Nd, Sm, and Yb) based ceramics could be considered as promising microwave ceramics for LTCC applications.     

Microwave dielectric properties of CeO2–0.5AO–0.5TiO2 (A = Ca,Mg, Zn,Mn, Co,Ni, W) ceramics

Microwave dielectric ceramic materials based on cerium [CeO₂–0.5AO–0.5TiO₂ (A = Mg, Zn, Ca, Mn, Co, Ni, W)] have been prepared by a conventional solid state ceramic route. The crystal structure was studied by X-ray diffraction, microstructure by scanning electron microscopy (SEM) techniques and the phase composition was studied using energy dispersive X-ray analysis (EDXA). The sintered ceramics had a relative dielectric constant (ɛ_r) in the range 17–65 and quality factor Q_uxf up to 50,000 GHz and a temperature variation of resonant frequency (τ_f) ranging from a negative value (−62 ppm/°C) to a high positive value (+399 ppm/°C). The majority of the synthesized ceramics were of a two phase composite consisting of a fluorite CeO₂ and perovskite ATiO₃ phase. The microwave dielectric properties were further tailored by adding various amounts of dopants of different valencies to the calcined powder. This made it possible to either tune τ_f to zero or improved the quality factor further.     

Ultrasound assisted synthesis of Gd and Nd doped ceria electrolyte for solid oxide fuel cells

In this study, the ceramic powders of Ce_1?xGd_xO_2?x/2 and Ce_1?xNd_xO_2?x/2 (x=0.05, 0.10, 0.15, 0.20 and 0.25) were synthesized by ultrasound assisted co-precipitation method. The ionic conductivity was studied as a function of dopant concentration over the temperature range of 300–800 °C in air, using the impedance spectroscopy. The maximum ionic conductivity, σ_800 °C=4.01×10^?2 Scm^?1 with the activation energy, E_a=0.828 kJmol^?1 and σ_800 °C=3.80×10^?2 Scm^?1 with the activation energy, E_a=0.838 kJmol^?1 were obtained for Ce_0.90Gd_0.10O_1.95 and Ce_0.85Nd_0.15O_1.925 electrolytes, respectively. The average grain size was found to be in the range of 0.3–0.6 μm for gadolinium doped ceria and 0.2–0.4 μm for neodymium doped ceria. The uniformly fine crystallite sizes (average 12–13 nm) of the ultrasound assisted prepared powders enabled sintering of the samples into highly dense (over 95%) ceramic pellets at 1200 °C (5 °C min^?1) for 6 h.     

Control of coring effect in BaTi4O9 microwave dielectric ceramics by doping with Mn4+

In the present work, we have attempted to reduce the effect of coring effect in the titanate ceramic system BaTi₄O₉ (BT4) by doping it with Mn⁴⁺. The microwave dielectric BaTi₄O₉ ceramics doped with 0, 0.5 and 1.0 mol% Mn⁴⁺ were synthesized by conventional ceramic processing route. The XRD studies confirmed a single phase crystalline structure for all the ceramic samples studied. The SEM micrographs of the ceramics reveal a microstructural change leading towards a more uniform grain size distribution as the Mn⁴⁺ content increases to 1.0 mol%. In the low frequency region (100 Hz to 1 MHz), the temperature stability of dielectric properties exhibits a marked improvement with the increasing amount of Mn⁴⁺ in the ceramic system. In the microwave frequency region (9.3 GHz), Q-factor increases from 11,625 GHz to 46,500 GHz for BaTi₄O₉ ceramic doped with 1.0 mol% Mn⁴⁺. The present paper reveals that the commonly observed degradation of dielectric properties due to coring effect in the BaTi₄O₉ ceramic system can be controlled by doping it with an appropriate quantity of Mn⁴⁺.     

Processing and characterisation of fine crystalline ceria gadolinia–yttria stabilized zirconia powders

For low temperature SOFCs the yttria stabilized zirconia (YSZ)-coated ceria is a promising candidate for replacing YSZ-electrolyte. An important requirement for the co-firing feasibility of such a configuration is the densification of ceria at low temperatures (<1400°C). Fine crystalline gadolinia doped ceria (CGO)-powder readily sinterable at 1250°C was synthesized by co-precipitation with oxalic acid of 0·05 M and crystallization in methanol at 200°C for 6 h. The fabrication and characterisation of solid solution phases with a graded composition (CGO)_x(YSZ)_1−x, to be used as an interlayer between YSZ and CGO, in order to avoid delamination, were also studied and discussed. CGO_xYSZ_1−x powders, prepared by the glycine combustion method, required higher sintering temperatures (1500°C) to densify, while they showed significantly lower ionic conductivity than YSZ and CGO, attributed to the large lattice deformation and scattering of oxygen ions.     

Synthesis,characterization, and dielectric properties of low loss LaBO3 ceramics

The preparation, sintering behaviour, and dielectric properties of low loss LaBO₃ ceramics have been investigated. Single-phase LaBO₃ powder was synthesized by the conventional solid-state ceramics route and dense ceramics (relative density >96%) with uniform microstructure (grain size ～30 μm) were obtained by sintering at 1300 °C in air. The electrical conductivity of LaBO₃ follows the Arrhenius law and the related activation energies for electrical conduction of bulk and grain boundary are 0.62 eV and 0.90 eV, respectively. The LaBO₃ ceramics sintered at 1300 °C exhibit excellent microwave dielectric properties with a relative permittivity, ?_r ～ 11.8, a quality factor, Q × f₀ value ～76,869 GHz (at ～15 GHz), and a negative temperature coefficient of resonant frequency τ_f ～ ?52 ppm/°C.     

Electrolyte materials for solid oxide fuel cells derived from metal complexes: Gadolinia-doped ceria

Gadolinia doped ceria (GDC) powders with different gadolinium contents were successfully prepared by the thermal decomposition of ceria complexes. All the calcined powder samples were found to be ceria-based solid-solutions having a fluorite-type structure. The powders were cold-isostatically pressed and sintered in air at 1500 °C for 5 h to attain a sintered density of about 90% of its theoretical value. The electrical conductivity of the GDC pellets in air was studied as a function of temperature in the 225–700 °C range, by using two-probe electrochemical impedance spectroscopy measurements. The highest total conductivity (σ_600 °C = 0.025 S/cm) was found for the Ce_0.85Gd_0.15O_1.925 composition.     

Photocatalytic reduction of CO2 to energy products using Cu–TiO2/ZSM-5 and Co–TiO2/ZSM-5 under low energy irradiation

Copper or cobalt incorporated TiO₂ supported ZSM-5 catalysts were prepared by a sol–gel method, and then were characterized by XRD, BET, XPS and UV–vis diffuse reflectance spectroscopy. Ti^3 + was the main titanium specie in TiO₂/ZSM-5 and Cu–TiO₂/ZSM-5, which will be oxide to Ti^4 + after Co was doped. With the deposition of Cu or Co, the efficiency of the CO₂ conversion to CH₃OH was increased under low energy irradiation. The peak production rate of CH₃OH reached 50.05 and 35.12 μmol g^− 1 h^− 1, respectively. High photo energy efficiency (PEE) and quantum yield (φ) were also reached. The mechanism was discussed in our study.     

Preparation and characterization of neodymium-doped ceria electrolyte materials for solid oxide fuel cells

The microstructure, thermal expansion, microhardness, indentation fracture toughness, and ionic conductivity of neodymium-doped ceria (NDC) prepared by coprecipitation were investigated. The results revealed that the average particle size (D_BET) ranged from 20.1 to 25.8 nm, crystallite dimension (D_XRD) varied from 17.5 to 20.7 nm, and the specific surface area distribution was from 31.25 to 40.27 m²/g for neodymium-doped ceria stacking powders. Dependence of lattice parameter, a, versus dopant concentration, x, of Nd³⁺ ion shows that these solid solutions obey Vegard's rule as a(x) = 5.4069 + 0.1642x for Ce_1?xNd_xO_2?(1/2)x for x = 0.05–0.25. For neodymium-doped ceria ceramics sintered at 1500 °C for 5 h, the bulk density was over 95% of the theoretical density. The maximum ionic conductivity, σ_800°C = 4.615 × 10^?2 S/cm, with the minimum activation energy, E_a = 0.794 eV was found for the Ce_0.75Nd_0.25O_1.875 ceramic. Trivalent, neodymium-doped ceria ceramics revealed high fracture toughness, the fracture toughness distribution was in the range of 6.236 ± 0.021 to 6.846 ± 0.017 MPa m^1/2. The high indentation fracture toughness of neodymium-doped ceria was attributed to crack deflection. Moreover, the porosity may influence the mechanical properties such as microhardness and fracture toughness. It was observed that as the porosity reduced, the microhardness and fracture toughness increased.     

Selective oxidation of soot over Cu doped ceria/ceria–zirconia catalysts

Copper doped ceria and ceria–zirconia mixed oxides were prepared using the citric acid sol–gel method. The temperature-programmed oxidation (TPO) results showed that the Cu modification helped to improve the activity and selectivity of ceria and ceria–zirconia for soot catalytic oxidation. The CO-TPR results showed that Cu–Ce had a better reducibility than pure ceria at low temperatures. After ageing at 800 °C for 20 h in flow air, CuO–CeO₂ showed the maximum soot oxidation rate at 378 and 519 °C under tight and loose contact conditions, respectively, achieving a nearly 100% selectivity to CO₂ production. This effect may be attributed to the existence of well dispersed copper oxide species strongly interacting with the ceria surface, which may decrease the activation energy of soot oxidation. A conceivable mechanism of this synergetic effect was proposed.     

Micromechanisms of solid electrolyte interphase formation on electrochemically cycled graphite electrodes in lithium-ion cells

The microstructural and compositional changes that occurred in the solid electrolyte interphase (SEI) formed on graphite electrodes subjected to voltammetry tests (vs. Li/Li⁺) at different voltage scan rates were investigated. The microstructure of the SEI layer, characterized using high-resolution transmission electron microscopy, consisted of an amorphous structure incorporating crystalline domains of ～5–20 nm in size. Evidence of lithium compounds, namely Li₂CO₃ and Li₂O₂, and nano-sized graphite fragments was found within these crystalline domains. The morphology and thickness of the SEI depended on the applied voltage scan rate (dV/dt). The variations in the Li⁺ diffusion coefficient (D_Li⁺) at the electrode/electrolyte interface during the SEI formation process were measured and two regimes were identified depending on the scan rate; for dV/dt ≥ 3.00 mV s^?1, D_Li⁺ was 3.13 × 10^?8 cm² s^?1. At lower scan rates where D_Li⁺ was low, 0.57 × 10^?8 cm² s^?1, a uniform and continuous SEI layer with a tubular morphology was formed whereas at high dV/dt, the SEI formed had a columnar morphology and did not provide a uniform coverage.     

Gold catalysts on Co-doped ceria for complete benzene oxidation: Relationship between reducibility and catalytic activity

Very high catalytic activity in complete benzene oxidation (CBO) was observed over gold catalyst on prepared by mechanochemical mixing CeO₂–Co₃O₄ (10 wt.% of dopant). It was significantly higher compared with gold catalysts supported on ceria doped with 5 wt.% or 15 wt.% Co₃O₄. The presence of Co₃O₄ phase and Co-modified ceria was observed by XRD data. The HRTEM/HAADF results revealed that the doping with 10 wt.% Co₃O₄ was favorable for the higher gold dispersion. The highest reducibility, i. e. ability of oxygen supplying of gold catalyst on ceria doped with 10 wt.% Co₃O₄ correlates with the highest oxidation activity in CBO.     

On the structure dependence of CO oxidation over CeO2 nanocrystals with well-defined surface planes

CO oxidation is a model reaction for probing the redox property of ceria-based catalysts. In this study, CO oxidation was investigated over ceria nanocrystals with defined surface planes (nanoshapes) including rods ({1 1 0} + {1 0 0}), cubes ({1 0 0}), and octahedra ({1 1 1}). To understand the strong dependence of CO oxidation observed on these different ceria nanoshapes, in situ techniques including infrared and Raman spectroscopy coupled with online mass spectrometer, and temperature-programmed reduction (TPR) were employed to reveal how CO interacts with the different ceria surfaces, while the mobility of ceria lattice oxygen was investigated via oxygen isotopic exchange experiment. CO adsorption at room temperature leads to strongly bonded carbonate species on the more reactive surfaces of rods and cubes but weakly bonded ones on the rather inert octahedra surface. CO-TPR, proceeding via several channels including CO removal of lattice oxygen, surface water–gas shift reaction, and CO disproportionation reaction, reveals that the reducibility of these ceria nanoshapes is in line with their CO oxidation activity, i.e., rods > cubes > octahedra. The mobility of lattice oxygen also shows similar dependence. It is suggested that surface oxygen vacancy formation energy, defect sites, and coordinatively unsaturated sites on ceria play a direct role in facilitating both CO interaction with ceria surface and the reactivity and mobility of lattice oxygen. The oxygen vacancy formation energy, nature and amount of the defect and low coordination sites are intrinsically affected by the surface planes of the ceria nanoshapes. Several reaction pathways for CO oxidation over the ceria nanoshapes are proposed, and certain types of carbonates, especially those associated with reduced ceria surface, are considered among the reaction intermediates to form CO₂, while the majority of carbonate species observed under CO oxidation condition are believed to be spectators.     

Microwave dielectric properties of Re3Ga5O12 (Re: Nd,Sm, Eu,Dy and Yb) ceramics and effect of TiO2 on the microwave dielectric properties of Sm3Ga5O12 ceramics

Re₃Ga₅O₁₂ (Re: Nd, Sm, Eu, Dy and Yb) garnet ceramics sintered at 1350–1500 °C had a high quality factor (Q × f) ranging from 40,000 to 192,173 GHz and a low dielectric constant (ɛ_r) of between 11.5 and 12.5. They also exhibited a relatively stable temperature coefficient of resonant frequency (τ_f) in the range of −33.7 to −12.4 ppm/°C. In order to tailor the τ_f value, TiO₂ was added to the Sm₃Ga₅O₁₂ ceramics, which exhibited good microwave dielectric properties. The relative density and grain size increased with addition of TiO₂, resulting in the enhancement of Q × f value. The τ_f increased with the addition of TiO₂. Excellent microwave dielectric properties of ɛ_r = 12.4, Q × f = 240,000 GHz and τ_f = −16.1 ppm/°C were obtained from the Sm₃Ga₅O₁₂ ceramics sintered at 1450 °C for 6 h with 1.0 mol% TiO₂. Therefore, Re₃Ga₅O₁₂ ceramics, especially TiO₂-added Sm₃Ga₅O₁₂ ceramics are good candidates for advanced substrate materials in microwave integrated circuits (MICs) applications.