Structural and Electrical Analysis of Microwave Processed YSZ Electrolytes for SOFC Prepared by Co-precipitation Method

The aim of the present study is to investigate the effect of microwave energy on the structural and electrical properties of yttria-stabilized zirconia (YSZ) electrolyte for solid oxide fuel cells (SOFC). Five different compositions of Zr_1?x Y _x O_2?x/2 (x = 0.06–0.14) were prepared by a co-precipitation method and were then sintered by microwave as well as conventional heating at 1400°C for 20 min and 240 min, respectively. The structural and electrical properties of the samples sintered by both the methods were compared. The x-ray diffraction (XRD) results revealed that YSZ samples sintered by both methods had either a tetragonal crystal structure or a cubic structure depending on its composition. Almost the same degree of densification as well as conductivity of same order was found from impedance analysis for both microwave- and conventionally sintered YSZ products, which showed that microwave sintering is the better alternative for material processing in terms of saving energy and time without compromising the product quality.     

Microstructure characterization of a cobalt-oxide-doped cerium-gadolinium-oxide by analytical and high-resolution TEM

The microstructure and chemistry of 2 mol.% and 5 mol.% cobalt-oxide-doped Ce_0.8Gd_0.2O_1.9 sintered at different temperatures were examined by a combination of electron energy-loss spectroscopy and energy-filtering and high-resolution transmission electron microscopy. Co grain boundary excess was evaluated. It is found that Co solubility in Ce_0.8Gd_0.2O_1.9 is low at temperatures between 800 and 1150 °C, resulting in a large number of Co precipitates at grain boundaries. With increasing sintering temperature, precipitates grow, influencing the Co redistribution and further altering the segregation amount in the grain boundary. The Co grain boundary concentration is shown to increase with the increase of sintering temperature from 890 to 1050 °C, which is suggested to be due to grain growth. It is found that Co grain boundary segregation induces a detectable variation in the ELNES of Ce-M_4,5 and O-K absorption edges, indicating a reduction of Ce atoms in the grain boundary region. The phase of the precipitates was identified as CoO at temperatures between 890 °C and 1150 °C. HRTEM reveals that grain boundaries are less disordered after prolonged sintering time at higher temperature. At a dopant level of 5 mol.% Co oxide in Ce_0.8Gd_0.2O_1.9, the grain boundaries become more disordered, and exhibit a high amount of Co segregation.     

Electrochemical performance of microwave synthesized Nd1.8Ce0.2CuO4±δ cathode for intermediate temperature solid oxide fuel cell applications

The microwave combustion of reagents followed by microwave sintering resulted in superfine crystalline Nd_1.8Ce_0.2CuO_4±δ solid solution. The Nd_1.8Ce_0.2CuO_4±δ crystal lattice volume increases with the reduction of crystallite size. Increase in sintering temperature increases crystallite size and sintered density. A good interfacial contact between Ce_0.9Gd_0.1O₂ electrolyte and Nd_1.8Ce_0.2CuO_4±δ cathode forms to yield symmetric cells for electrochemical studies. The electrochemical impedance data obtained at various temperatures simulated using electrical equivalent circuit. The low-frequency response well fitted to Gerischer element. The sintering temperature 900 °C during the cathode preparation is optimized on the basis of least area-specific-resistance (ASR) = 1.19 Ω cm⁻² at 700 °C.     

Effect of Dy on the properties of Sm-doped ceria electrolyte for IT-SOFCs

Co-doped ceria-based electrolytes of Ce_0.8Sm_0.2−xDy_xO_2−δ (x = 0, 0.02, 0.06, 0.10, 0.14) were sintered from powders obtained by solid state reaction method. The phase identification, thermal expansion, ionic conductivities and microstructures of samples were studied by X-ray diffraction (XRD), dilatometry, AC impedance spectroscopy (IS) and scanning electron microscopy (SEM). The results showed that the addition of Dy led to higher ionic conductivity and lower activation energy in comparison with Sm singly doped ceria Ce_0.8Sm_0.2O_2−δ (SDC) in the temperature range of 300-800 °C. As the addition amount of Dy increased up to 2 mol% (Ce_0.8Sm_0.18Dy_0.02O_2−δ), the sample attained the highest ionic conductivity, about 50% higher than that of SDC at 500 °C. The effect of Dy on the grain boundary conductivity was more apparent than that of the bulk conductivity. XRD measurements indicated that all the samples were single phase. The thermal expansion was linear for all the samples. The addition of Dy did not change the thermal expansion coefficient (TEC) significantly.     

Properties and consolidation of nanostructured Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>1.9</Subscript> by pulsed-current-activated sintering

Dense nanophase Ce_0.8Gd_0.2O_1.9 was sintered by a pulsed-current-activated sintering method within 10 min from Ce_0.8Gd_0.2O_1.9 nanopowder made using the co-precipitation method. Sintering was accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense Ce_0.8Gd_0.2O_1.9 with a relative density of up to 96.5% was produced under the simultaneous application of 80 MPa of pressure and a pulsed current. The ionic conductivities and mechanical properties of the Ce_0.8Gd_0.2O_1.9 were investigated.     

Efficient cerium-based sol-gel derived phosphors in different garnet matrices for light-emitting diodes

Efficiency of sol-gel derived Y_3−xAl₅O₁₂:Ce_x³⁺ (YAG:Ce) and Y_3−xMg₂AlSi₂O₁₂:Ce_x³⁺ (YMASG:Ce) phosphors, which are prospective for application in white light emitting diodes (LED), is studied. Sets of samples containing different cerium amount x from 0.015 to 0.06 and sintered at different temperatures (1400-1550 °C) were investigated. Importance of absorption peculiarities in agglomerates of phosphor nanocrystals is demonstrated by studying the excitation wavelength dependence of quantum efficiency and by applying PL measurements in confocal mode. Emission saturation is demonstrated to occur at higher excitation intensities than those typical for operating white LEDs.     

Fabrication and characterization of nano-Y2O3, Al2O3, La2O3 dispersed mechanically alloyed and liquid phase sintered WNi for structural application

The investigation involves the fabrication of various nano oxide (Y₂O₃, Al₂O₃, La₂O₃) dispersed WNi alloys by mechanical alloying for 10 h and sintering in Ar atmosphere at 1400 °C, 1500 °C for 2 h. The selected composition for the study is W₈₉Ni₁₀(Y₂O₃)₁ (alloy A), W₈₉Ni₁₀(Al₂O₃)₁ (alloy B) and W₈₉Ni₁₀(La₂O₃)₁ (alloy C) (in weight%). Alloy A exhibit least crystallite size and maximum lattice strain at 10 h of milling. The lattice parameter of W exhibits expansion and contraction behavior during the milling. Microstructure of sintered alloys reveals the presence of both faceted and nearly spherical shaped grains. Bimodal grain size distribution is higher in alloy A at 1500 °C as compared to other alloys and 1400 °C sintering temperature. Texture study and Young's Modulus value reveals that hardness of alloy A is higher against alloy B and alloy C. Maximum % relative sintered density, mean hardness, compressive strength of 89%, 5.53 GPa, 2.25 GPa respectively has been achieved in alloy A at 1500 °C.     

Microstructure and properties of Y2O3-doped steel-cemented WC prepared by microwave sintering

Steel-cemented WC was prepared by ball milling,cold compacting and microwave sintering with Fe powder as the matrix,WC as the hard phase and the addition of rare earth Y2O3.The results show that the interface of the WC particles and Fe matrix exhibits excellent wettability and liquidity when the microwave sintering temperature reaches 1,280°C.The density and mechanical properties of the steel bonded WC carbides could be greatly improved,the hard phases become finer and more uniform dispersed owing to the addition of Y2O3.With the increase of the Y2O3contents,the grain becomes uniform and fine first,and then gathers and grows up.The relative density,microhardness and bending strength all rise first,reaching the maximum values of 97.29%,HV1024 and 1,267.60 MPa at 0.5%Y2O3,respectively,and then decrease.Moreover,the relative density and mechanical properties of the steel-cemented WC with nano-Y2O3are higher than that with micron-Y2O3,which indicates that the effect of nano-Y2O3is better than that of the micron-Y2O3.     

Effect of partial substitution of calcium for yttrium on the structure and properties of the Y<Subscript>0.9</Subscript>Ca<Subscript>0.1</Subscript>Ba<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.8</Subscript> superconductor

The crystal structure of the high-temperature Y_1–xCa_xBa₂Cu₃O_6.8 superconductor has been studied in a temperature range of 80–300 K using low-temperature X-ray diffraction analysis; its microstructure has been studied by scanning and transmission electron microscopy. Changes of the bond length in the structure of principal phase and precipitation topology of impurity phases and their compositions have been analyzed. An addition of calcium was shown to increase the environmental tolerance of the principal Y123 phase and its microhardness and ensures the low unchanged coefficient of thermal expansion. All of the facts indicate that the material can be used to manufacture composite superconducting articles.     

Preparation and thermal conductivities of Gd2Ce2O7 and (Gd0.9Ca0.1)2Ce2O6.9 ceramics for thermal barrier coatings

Two kinds of rare earth cerium oxides Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 were prepared by solid state reaction method at 1600 °C for 10 h. The phase compositions, microstructures, and their thermal conductivities of these materials were investigated. XRD results revealed that single phase Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 with fluorite structure were synthesized. Results of SEM and EDS showed that the microstructures of these materials were dense and no other phases existed among grains. Because of phonon scattering resulted by the oxygen vacancies and difference in atomic mass between substitutional atoms and host atoms, thermal conductivities of Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 are lower than that of 8YSZ at 800 °C, and thermal conductivity of (Gd_0.9Ca_0.1)₂Ce₂O_6.9 is lower than that of Gd₂Ce₂O₇. These results imply the Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 can be used as novel candidate materials for thermal barrier coatings in the future.     

Preparation and properties of Gd and Y co-doped ceria-based electrolytes for intermediate temperature solid oxide fuel cells

Co-doped ceria electrolytes of Ce_0.8Gd_0.2−xY_xO_1.9 (x = 0.0–0.20) fine powders were prepared with glycine–nitrate method. The results of X-ray diffraction showed that all powders crystallite calcined at 873 K were single phase with cubic fluorite structure. The average crystallite sizes calculated by the Scherrer formula were between 21 and 23 nm, which was in good agreement with the results of TEM and particle size distribution measurements. The thermal expansion curves of Ce_0.8Gd_0.2−xY_xO_1.9 were measured and the thermal expansion coefficients between 373 and 1123 K were calculated. The SEM results exhibited that electrolyte pellets sintered at 1523 K were dense, and the relative densities of these pellets were over 96%. The impedance spectra analysis of these electrolytes has been performed at 623–1023 K. The results showed that co-doped ceria exhibited higher ionic conductivity and lower activation energy than the singly doped ceria with the same dopant concentration at the temperature range of 773–1023 K, and the electrolytic domain boundary of co-doped ceria was smaller than that of singly doped ceria at 873–973 K. It suggested that co-doping with appropriate ratio gadolinium and yttrium could further improve the electrochemical performance of ceria-based electrolytes. These co-doped samples are ideal electrolyte materials of intermediate temperature solid oxide fuel cells.     

Oxidation behavior of LPS-SiC ceramics sintered with AlN/Y2O3 as additive

In this work, the oxidation behavior of SiC ceramics sintered with additives based on AlN–Y₂O₃ system was investigated. SiC ceramics doped with different AlN:Y₂O₃ contents of 8.4:11.6 wt.% or 2.2:17.8 wt.% were sintered at 2080 °C for 1 h under nitrogen atmosphere, obtaining ceramics with relative density near to 96% in both compositions. Samples were oxidized at 1200 °C, 1300 °C or 1400 °C in air for up to 120 h. Oxidation was monitored by the weight gain of the samples as function of exposition time and temperature. A parabolic growth of the oxidation layer has been observed and the coefficient of the growth rate has been determined by relating the weight gain and the surface area. In oxidation testing performed at 1200 °C, samples containing lower Y₂O₃ amounts showed greater oxidation resistance; however, by raising the temperature (to 1400 °C), the samples containing higher Y₂O₃ amounts showed greater oxidation resistance. The oxidized layer characterized by X-ray diffraction presented SiO₂ and Y₂Si₂O₇ as crystalline phases. Furthermore, the activation energy for oxidation of 780 kJ/mol and 405 kJ/mol for AlN:Y₂O₃ contents of 8.4:11.6 wt.% or 2.2:17.8 wt.%, respectively.     

Fabrication and properties of Ba(Zr1−xCex)0.9Y0.1O2.95/NaCl composite electrolyte materials

Ba(Zr_1−xCe_x)_0.9Y_0.1O_2.95/NaCl (x = 0.1, 0.2 and 0.3) composite electrolyte materials were fabricated with ZnO as sintering aid. The effect of ZnO on the properties of Ba(Zr_1−xCe_x)_0.9Y_0.1O_2.95 matrix were investigated. The phase composition and microstructure of samples were characterized by XRD and SEM, respectively. The electrochemical performances were studied by three-probe conductivity measurement and AC impedance spectroscopy. XRD results showed that Ba(Zr_1−xCe_x)_0.9Y_0.1O_2.95 with 2 mol% of ZnO was perovskite structure. The relative density of this sample was above 95% when sintered at 1450 °C for 6 h. By adding 10 mol% of NaCl to Ba(Zr_1−xCe_x)_0.9Y_0.1O_2.95 with 2 mol% of ZnO that was sintered at 1400 °C for 6 h, the conductivity was increased. The electrical conductivity of 1.26 × 10⁻² S/cm and activation energy of 0.23 eV were obtained when tested at 800 °C in wet hydrogen.     

Densification and grain growth during early-stage sintering of Ce0.9Gd0.1O1.95−δ in a reducing atmosphere

The present work investigates the processes of densification and grain growth of Ce_0.9Gd_0.1O_1.95?δ (CGO10) during sintering under reduced oxygen partial pressure. Sintering variables were experimentally characterized and analyzed using defect chemistry and sintering constitutive laws. Based on the results achieved, the grain size–relative density relationship, the densification rate and the grain-growth rate were determined. The activation energies for densification and grain growth were evaluated, and the dominant densification mechanism was indicated. For comparison, the densification behavior of CGO10 sintered in air was also studied. Accelerated densification was observed in early-stage sintering of CGO10 in a reducing atmosphere. This might be attributed to the oxygen vacancies generated by the reduction of Ce⁴⁺ to Ce³⁺ in the reducing atmosphere, which facilitate the diffusion of ions through the lattice. The densification activation energy of CGO10 in the reducing atmosphere was evaluated to be 290 ± 20 kJ mol^?1 in the relative density range of 0.64–0.82, which was much smaller than that of CGO10 sintered in air (770 ± 40 kJ mol^?1). The grain-growth activation energy of CGO10 sintered in the reducing atmosphere was evaluated to be 280 ± 20 kJ mol^?1 in the grain size range of 0.34–0.70 μm. The present work describes a systematic investigation of sintering behavior of CGO10 under reduced oxygen partial pressure, which contributes to the first known determination of the fundamental parameters associated with densification and grain growth during early-stage sintering of CGO10 in a reducing atmosphere.     

Y2O3 and Yb2O3 Co-doped Strontium Hafnate as a New Thermal Barrier Coating Material

Y₂O₃ and Yb₂O₃ co-doped strontium hafnate powder with chemistry of Sr(Hf_0.9Y_0.05Yb_0.05)O_2.95 (SHYY) was synthesized by a solid-state reaction at 1450 °C. The SHYY showed good phase stability not only from 200 to 1400 °C but also at a high temperature of 1450 °C for a long period, analyzed by differential scanning calorimetry and x-ray diffraction, respectively. The coefficient of thermal expansion of the sintered bulk SHYY was recorded by a high-temperature dilatometer and revealed a positive influence on phase transitions of SrHfO₃ by co-doping with Y₂O₃ and Yb₂O₃. The thermal conductivity of the bulk SHYY was approximately 16% lower in contrast to that of SrHfO₃ at 1000 °C. Good chemical compatibility was observed for SHYY with 8YSZ or Al₂O₃ powders after a 24 h heat treatment at 1250 °C. The phase stability and the microstructure evolution of the as-sprayed SHYY coating during annealing at 1400 °C were also investigated.     

Phase Equilibria in the Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>–InSb System

The phase diagram of the (Sb₂Te₃)_100?x –InSb _x system was determined based on x-ray diffraction (XRD) analysis, differential thermal analysis (DTA), and microhardness and density measurements. An intermediate compound with composition Sb₂Te₃·2InSb was formed as a result of syntectic reaction, melting incongruently at 553 °C. This compound has tetragonal lattice with unit cell parameters of a = 4.3937 Å, b = 4.2035 Å, c = 3.5433 Å, α = 93.354°, and β = γ = 90°. Sb₂Te₃·(2 + δ)InSb (?1 ≤ δ ≤ +1) and (Sb₂Te₃)_100?x (InSb) _x (90 ≤ x ≤ 100) solid solutions exist in the investigated system, based on the intermediate compound Sb₂Te₃·2InSb and on InSb, respectively. Also, two invariant equilibria exist in the system, with eutectic point coordinates at compositions of x = 60 and x ≈ 85 mol% InSb and eutectic temperatures of T _E = 541 and T _E ≈ 501 °C, respectively.     

Mechanical properties at the nanometer scale of GDC and YSZ used as electrolytes for solid oxide fuel cells

The Young’s modulus (E), hardness (H) and fracture toughness (K_IC) of various compositions of gadolinia doped-ceria (GDC, Gd_xCe_1?xO_2?x/2, 0.1 ? x ? 0.2) and yttria-stabilized zirconia (YSZ, Y_0.08Zr_0.92O_1.96) electrolytes were investigated by nanoindentation. All samples were produced by the sol–gel method, formed by uniaxial pressure and sintered at 1400 °C. In order to determine the mechanical properties, a Berkovich diamond tip was employed at applied loads of 5, 10, 30, 100 and 500 mN. The results were interpreted by the Oliver–Pharr method and values of K_IC were determined using the method of Palmqvist cracks. The residual imprints were observed by field emission scanning electron microscopy. The results obtained showed that the H, E and K_IC of GDC decreased with increasing gadolinia concentration, due to the oxygen vacancies generated by the dopant addition. As a result, the mechanical properties of GDC were significantly lower than those of YSZ electrolyte.     

Sinterability and electrical properties of ZnO-doped Ce0.8Y0.2O1.9 electrolytes prepared by an EDTA-citrate complexing method

The effect of ZnO addition on the sinterability and ionic conductivity of Ce_0.8Y_0.2O_1.9 is investigated. Ce_0.8Y_0.2O_1.9 is prepared using an EDTA-citrate complexing method in order to further improve its electrical properties. Using a ZnO content over 1 mol %, the sinterability of Ce_0.8Y_0.2O_1.9 is significantly improved by reducing the sintering temperature from 1500 to 1350 °C and a relative density of above 95% was achieved. The highest ionic conductivity of 0.0516 S cm⁻¹ was obtained at 750 °C for (YDC)_0.99(ZnO)_0.01 sintered at 1350 °C. Pure YDC sintered at 1500 °C, on the other hand, yielded 0.0289 S cm⁻¹.     

Integrated constitutive model for flow behavior of pure Titanium considering interstitial solute concentration

The aim of this work is to develop a constitutive model that can predict the flow behavior of pure Ti with different interstitial concentrations and grain sizes. To build a database required for identifying material constants, three different grades of Ti were subjected to tensile tests at temperatures of 223, 300, 473, 673 or 773 K and at a fixed strain rate of 10^?3 s^?1. In the modeling procedure, the mechanical threshold stress model was further modified to capture both the hardening effects attributed to the changes in equivalent oxygen concentration (O _eq ) and the softening effect caused by deformation heating at high strain rates. The developed model can reasonably predict the flow behavior of pure Ti having different O _eq (0.14–0.32 wt%), and grain size (14.5–90 μm) over a temperature range of 135 to 673 K, and a strain rate range of 2×10^?4 to 1400 s^?1.     

Development and Optimization of Tailored Composite TBC Design Architectures for Improved Erosion Durability

Rare-earth pyrochlores, RE₂Zr₂O₇, have been identified as potential thermal barrier coating (TBC) materials due to their attractive thermal properties and CMAS resistance. However, they possess a low fracture toughness which results in poor erosion durability/foreign object damage resistance. This research focuses on the development of tailored composite air plasma spray (APS) TBC design architectures utilizing a t′ Low-k secondary toughening phase (ZrO₂-2Y₂O₃-1Gd₂O₃-1Yb₂O₃; mol.%) to enhance the erosion durability of a hyper-stoichiometric pyrochlore, NZO (ZrO₂-25Nd₂O₃-5Y₂O₃-5Yb₂O₃; mol.%). In this study, composite coatings have been deposited with 30, 50, and 70% (wt.%) t′ Low-k toughening phase in a horizontally aligned lamellar morphology which enhances the toughening response of the coating. The coatings were characterized via SEM and XRD and were tested for erosion durability before and after isothermal heat treatment at 1100 °C. Analysis with mixing laws indicated improved erosion performance; however, a lack of long-term thermal stability was shown via isothermal heat treatments at 1316 °C. An impact stress analysis was performed using finite element analysis of a coating cross section, representing the first microstructurally realistic study of mechanical properties of TBCs with the results correlating well with observed behavior.