Influence of Microstructure and Surface Activation of Dual‐Phase Membrane Ce0.8Gd0.2O2−δ–FeCo2O4 on Oxygen Permeation

Dual‐phase oxygen transport membranes are fast‐growing research interest for application in oxyfuel combustion process. One such potential candidate is CGO‐FCO (60 wt% Ce_0.8Gd_0.2O_2?δ–40 wt% FeCo₂O₄) identified to provide good oxygen permeation flux with substantial stability in harsh atmosphere. Dense CGO‐FCO membranes of 1 mm thickness were fabricated by sintering dry pellets pressed from powders synthesized by one‐pot method (modified Pechini process) at 1200°C for 10 h. Microstructure analysis indicates presence of a third orthorhombic perovskite phase in the sintered composite. It was also identified that the spinel phase tends to form an oxygen deficient phase at the grain boundary of spinel and CGO phases. Surface exchange limitation of the membranes was overcome by La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) porous layer coating over the composite. The oxygen permeation flux of the CGO‐FCO screen printed with a porous layer of 10 μm thick LSCF is 0.11 mL/cm² per minute at 850°C with argon as sweep and air as feed gas at the rates of 50 and 250 mL/min.     

Experimental Phase Diagram Determination and Thermodynamic Assessment of the CeO2‐Gd2O3‐CoO System

New phase diagram data and a thermodynamic assessment of the CeO‐Gd₂O₃‐CoO system using the CALPHAD approach are presented. This information is needed to understand the surprisingly low sintering temperature (950°C–1050°C) of CeO₂‐based materials doped with small amounts of transition metal oxide (e.g., CoO). Experimental phase equilibria between 1100°C and 1300°C are reported based on the analysis of annealed and molten samples. No isolated compound exists in the ternary. At 1300°C the Co solubility in the ternary compounds Ce_1?x?yGd_xCo_yO_2?x/2?y (fluorite) is 2.7 mol% and is less than 1 mol% in the Gd_2?xCe_xO_3+x/2 (bixbyite). The Ce solubility in the perovskite GdCoO_3?δ was found to be 1 mol%. The lowest temperature eutectic melt in the ternary has a composition of 57.2 mol% Co and 41.1 mol% Gd melting at an onset temperature of 1303 ± 5°C, which is close to the binary eutectic in the Gd₂O₃‐CoO system at 60 ± 2 mol% Co and 1348 ± 1°C.     

Promoting Influence of Doping Indium into BaCe0.5Zr0.3Y0.2O3‐δ as Solid Proton Conductor

The influence of indium doping on chemical stability, sinterability, and electrical properties of BaCe_0.5Zr_0.3Y_0.2O_3‐δ was investigated. The phase purity and the chemical stability of the powders in humid pure CO₂ were evaluated by XRD. The dense electrolyte pellets were formed after the calcination at 1450°C for 8 h. SEM images and shrinkage plot showed that the sinterability of the samples was apparently improved by doping indium. The electrical conductivity was measured by impedance test through two‐point method, at both low (200–350°C) and high temperature ranges (450–850°C) in different atmospheres. BaCe_0.4Zr_0.3In_0.1Y_0.2O_3‐δ has been proved to be the optimal composition which simultaneously maximized the chemical stability, sinterability, and electrical conductivity which reached 1.1 × 10^?2 S/cm in wet hydrogen at 700°C, comparing with the 1.3 × 10^?2 S/cm for original BaCe_0.5Zr_0.3Y_0.2O_3‐δ. Anode support fuel cell with a thin BaCe_0.4Zr_0.3In_0.1Y_0.2O_3‐δ electrolyte (15 μm) was fabricated by spin coating method. Maximum power density of 0.651 W/cm² was obtained when operating at 700°C and fed by humid H₂ (containing H₂O 3 vol%). The obtained fuel cell could efficiently run at 650°C for more than 100 h without any attenuation.     

Grain Boundary Space Charge Effect and Proton Dynamics in Chemically Stable Perovskite‐Type Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3−δ (BSCZGY): A Case Study on Effect of Sintering Temperature

The present study reports the effect of sintering temperature on the proton dynamics of perovskite‐type Ba_0.5Sr_0.5Ce_0.6Zr_0.2Gd_0.1Y_0.1O_3?δ (BSCZGY) by establishing a co‐relation between the grain‐boundary (GB) space charge effect, electrical conductivity and dielectric loss of the BSCZGY samples sintered at 1300°C, 1400°C, and 1550°C for 20 h in air. Although, the GBs are the main source of resistance in BaZrO₃ based ceramic proton conductors, we show that the GB impedance disappeared above 450°C and 300°C, respectively, for BSCZGY samples sintered at 1300°C and 1400°C. Interestingly, the BSCZGY sample sintered at 1550°C showed absence of GB contribution to total conductivity even at 200°C. The GB electrostatic potential [?(0)] was found to vary between 0.35–0.38 V and 0.4–0.45 V, respectively, for the samples sintered at 1300°C and 1400°C at 200°C–300°C. The migration energy (E_m) of the protons was found to be 0.71, 0.65 and 0.58 eV for the sample sintered at 1300°C, 1400°C and 1550°C, respectively.     

Zr‐ and Tb‐doped barium cerate‐based cermet membrane for hydrogen separation application

Ba_0.8Ce_0.35Zr_0.5Tb_0.15O_3‐δ (BCZT) perovskite has been synthesized by glycine‐assisted solution combustion method. The Ni‐Ba_0.8Ce_0.35Zr_0.5Tb_0.15O_3‐δ‐based cermet membrane is obtained by cosintering NiO and BCZT powder mixture at 1550°C in reducing atmosphere. The X‐ray diffraction pattern of sintered pellet shows the characteristic peaks of both Ni and BCZT phases. FESEM image and elemental mapping confirm the presence of randomly distributed metallic nickel in the BCZT matrix. An electrical conductivity of ~14 S/cm at 700°C is achieved in Ni‐BCZT membrane, which reduced further with increase in temperature (>700°C). The cermet membrane (1.5 mm thick) shows a highest hydrogen permeation flux of ~0.07 mL/min/cm² at 900°C. Chemical stability of Ni‐BCZT membrane has also been examined under humid and carbon dioxide containing atmosphere. The membrane shows good structural stability without any significant change in hydrogen permeation flux.     

An investigation of metal ion diffusion in ceria‐based solid oxide fuel cell with barium‐containing anode

Metal ion diffusion is an effective strategy to suppress the internal electronic short circuit in ceria‐based solid oxide fuel cells (SOFCs). This could be achieved by fabricating an electron‐blocking layer between the barium‐containing anode and ceria‐based electrolyte. In this paper, a 0.6NiO‐0.4BaZr_0.1Ce_0.7Y_0.2O_3‐δ (NiO‐BZCY) anode‐supported cell based on Gd_0.1Ce_0.9O_2‐δ (GDC) electrolyte was employed to evaluate the internal metal ion diffusion behavior. The high open circuit voltages of about 1 V obtained at 550‐700°C can be attributed to in situ formation of an electron‐blocking interlayer between NiO‐BZCY and GDC. Microstructural analyses of the interlayer grains obtained by traditional solid‐state reaction were carried out. Phase identification demonstrated that the electron‐blocking interlayer had a perovskite structure. SEM and TEM analyses indicated formation of a new compound in the interlayer, of which the composition was determined as Zr, Y, and Ni co‐doped BaCe_0.9Gd_0.1O₃ with orthorhombic structure.     

New Sintered Li2O–Al2O3–SiO2 Ultra‐Low Expansion Glass‐Ceramic

We developed a new Li₂O–Al₂O₃–SiO₂ (LAS) ultra‐low expansion glass‐ceramic by nonisothermal sintering with concurrent crystallization. The optimum sintering conditions were 30°C/min with a maximum temperature of 1000°C. The best sintered material reached 98% of the theoretical density of the parent glass and has an extremely low linear thermal expansion coefficient (0.02 × 10^?6/°C) in the temperature range of 40°C–500°C, which is even lower than that of the commercial glass‐ceramic Ceran^® that is produced by the traditional ceramization method. The sintered glass‐ceramic presents a four‐point bending strength of 92 ± 15 MPa, which is similar to that of Ceran^® (98 ± 6 MPa), in spite of the 2% porosity. It is white opaque and does not have significant infrared transmission. The maximum use temperature is 600°C. It could thus be used on modern inductively heated cooktops.     

Carbon nanotube/graphene oxide‐added CaO‐B2O3‐SiO2 glass/Al2O3 composite as substrate for chip‐type supercapacitor

A CaO‐B₂O₃‐SiO₂ (CBS) glass/40 wt% Al₂O₃ composite sintered at 900°C exhibited a dense microstructure with a low porosity of 0.21%. This composite contained Al₂O₃ and anorthite phases, but pure glass sintered at 900°C has small quantities of wollastonite and diopside phases. This composite was measured to have a high bending strength of 323 MPa and thermal conductivity of 3.75 W/(mK). The thermal conductivity increased when the composite was annealed at 850°C after sintering at 900°C, because of the increase in the amount of the anorthite phase. 0.25 wt% graphene oxide and 0.75 wt% multi‐wall carbon nanotubes were added to the CBS/40 wt% Al₂O₃ composite to further enhance the thermal conductivity and bending strength. The specimen sintered at 900°C and subsequently annealed at 850°C exhibited a large bending strength of 420 MPa and thermal conductivity of 5.51 W/(mK), indicating that it would be a highly effective substrate for a chip‐type supercapacitor.     

Low‐Cost Silicon Nitride from β‐Silicon Nitride Powder and by Low‐Temperature Sintering

Aiming to manufacture low‐cost silicon nitride components, a low‐cost β powder was chosen as a raw powder and low‐temperature sintering at 1550–1600°C under atmospheric pressure nitrogen was carried out. The silicon nitride from β powder with 5 wt% Y₂O₃ and 5 wt% MgAl₂O₄ additives and sintered at 1600°C for 8 h was successfully densified, and it exhibited moderate strength and toughness of 553 MPa ± 22 and 3.5 MPa m^1/2, respectively. The results indicate that the low‐temperature sintering of the low‐cost β powder has a potential to reduce cost of components.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Understanding the Flash Sintering of Rare‐Earth‐Doped Ceria for Solid Oxide Fuel Cell

A novel electrical current applied technique known as flash sintering has been applied to rapidly (within 10 min) densify electrolytes including Ce_0.8Gd_0.2O_1.9 (GDC20), Ce_0.9Gd_0.1O_1.95 (GDC10), and Ce_0.8Sm_0.2O_1.9 (SDC20) for application in Solid Oxide Fuel Cells (SOFCs). The densification temperature for the three electrolytes was 554°C, 635°C, and 667°C, respectively, which is far below conventional sintering temperatures. All specimens after flash sintering maintained the pure fluorite structure and exhibited a well‐densified microstructure. To investigate the flash‐sintering mechanism, we have applied Joule heating effect with blackbody radiation theory, and found that this theory could reasonably interpret the flash‐sintering phenomenon by matching theoretically calculated temperature with the real temperature. More importantly, one of the materials inherent properties, the electronic conductivity, has been found correlated with the onset of flash sintering, which indicates that the electrons and holes are the primary current carriers during the start of flash‐sintering process. As a result, potential densification mechanisms have been discussed in terms of spark plasma discharge.     

Fabrication of Gd2O3‐MgO nanocomposite optical ceramics with varied crystallographic modifications of Gd2O3 constituent

The fabrication of Gd₂O₃‐MgO nanocomposite optical ceramics via hot‐pressing using sol‐gel derived cubic‐Gd₂O₃ and MgO nanopowders was investigated. The precursor powder calcined at 600°C had an average particle size of 12 nm. The effects of hot‐pressing temperature on constituent phases, microstructure, mid‐infrared transmittance, and microhardness were studied. The crystallographic modifications of Gd₂O₃ phase varied with the increase in sintering temperature from 1250 to 1350°C. The monoclinic‐Gd₂O₃ phase was retained for the composite sintered at 1350°C and the sample had an average grain size of 90 nm, excellent transmission (80.4%‐84.8%) over 3‐6 μm wavelength range, and enhanced hardness value of 14.1 GPa.     

La0.6Sr0.4Co0.2Fe0.8O3-δ oxygen electrodes for solid oxide cells prepared by polymer precursor and nitrates solution infiltration into gadolinium doped ceria backbone

Infiltration is a method, which can be applied for the electrode preparation. In this paper oxygen electrode is prepared solely by the infiltration of La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ (LSCF) into Ce_0.8Gd_0.2O_2-δ (CGO) backbone. The use a polymer precursor as an infiltrating medium, instead of an aqueous nitrate salts solution is presented. It is shown that the polymer forms the single-phase perovskite at 600 °C, contrary to the nitrates solution. As a result, obtained area specific resistance (ASR) is lowered from 0.21 Ω cm² to 0.16 Ω cm² at 600 °C. More than 35% of LSCF in the oxygen electrode decreases the performance.     

Conductivity of CGO and CSO ceramics obtained from freeze-dried precursors

Ce_0.8Gd_0.2O_1.9 (CGO) and Ce_0.8Sm_0.2O_1.9 (CSO) have been prepared as polycrystalline materials using a freeze-dried precursor. This method yields amorphous nanometric powders. Crystallization of the fluorite phase occurred on heating at 600 °C or higher temperatures. The grain size of freeze-dried powders increases to about 100 nm after calcination at 800 °C, or about 200 nm after firing at 1000 °C. Freeze-dried powders were used to prepare dense ceramic disks by sintering at 1400 °C. Some disks were sintered at 1000 °C by adding small amounts of cobalt nitrate solution to assist the densification. The electrical conductivity results obtained for these gadolinia-doped ceria and samaria-doped ceria ceramics are similar to those obtained for CGO pellets obtained from commercial nanopowders (Rhodia). Though the bulk conductivity of CSO is probably higher than that of CGO, its grain boundary conductivity is inferior, and tends to control the overall behaviour, at least at relatively low temperatures.     

Influence of the preparation methods on the microstructure and oxygen permeability of a CO2‐stable dual phase membrane

A CO₂‐stable dual phase membrane of the composition 40 wt % NiFe₂O₄‐60 wt % Ce_0.9Gd_0.1O_2‐δ (40NFO‐60CGO) was synthesized in three different ways: mixing of the starting powders (1) in a mortar and (2) in a ball‐mill as well as by (3) direct in situ one‐pot sol–gel powder synthesis. Backscattered scanning electron microscopy revealed that the direct one‐pot synthesis of 40NFO‐60CGO gives the smallest grains in a homogeneous distribution, compared with powder homogenization in the mortar or the ball‐mill. The smaller is the grains, the higher is the oxygen permeability. The permeation of the membrane can be improved by coating a porous La_0.6Sr_0.4CoO_3‐δ (LSC) layer on the surface of the air side. The dual phase membrane of 40NFO‐60CGO prepared by in situ synthesis shows a steady oxygen flux of 0.30 ml/(min cm²) over more than 100 h when pure CO₂ was used as sweep gas, which indicated that the dual phases membrane is CO₂‐resistant at least over this 5 days testing period. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Ascendable method for the fabrication of micro-tubular solid oxide fuel cells by ram-extrusion technique

We elaborated a new ceramic paste formulation as a pre-step to ram extrusion process for obtaining porous and mechanically sustainable microtubular anodes. The methodology reported here can establish a scalable fabrication pathway for anode-supported microtubular solid oxide fuel cells (MT-SOFCs). We succeeded in the fabrication of NiO–Ce_0.9Gd_0.1O_2-δ (NiO-CGO) based MT-anodes from a ceramic paste which comprised of α-cellulose and ethyl cellulose as pore former and binder, respectively. The quantity of the pore former was optimized and 15 wt % of pore former was elected as the superior, based on the microstructure and mechanical behavior. The sintered (at 1450 °C) microtubular (MT-anodes) exhibited an excellent mechanical strength of ~60 MPa as flexural strength (modulus of rupture) with 0.23% of deformation resistance. The obtained MT-anodes were used as a support for the fabrication of MT-SOFCs prototype with configuration such as Ni-CGO/CGO/LSFC-CGO for anode/electrolyte/cathode, respectively. The ceramic nanocomposites NiO–Ce_0.9Gd_0.1O_2-δ (Ni-CGO), and La_0.6Sr_0.4Fe_0.8Co_0.2O₃–Ce_0.9Gd_0.1O_2-δ (LSFC-CGO) were synthetized by one-step solution combustion method and characterized by X-ray diffraction (XRD) and scanning electronic microscopy (SEM) analysis. The fabricated MT-SOFC prototype generated a maximum power density of 0.595 W/cm² at 600 °C signified the worth of the proposed paste formulation.     

Low‐Temperature Sintering and Electric Properties of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 Ferroelectric Ceramics with CuO Additive

The low‐temperature sintering and electric properties of Pb_0.99(Zr_0.95Ti_0.05)_0.98Nb_0.02O₃ (PZTN 95/5) ferroelectric ceramics with CuO addition was investigated. The CuO addition significantly promoted the densification and reduced the sintering temperature of PZTN 95/5 ceramics by more than 200°C. The 0.2 wt% CuO‐added sample sintered at 1150°C exhibited the optimum relative density of 96.7% and excellent electric properties with values of P_r = 37.80 μC/cm², T_C = 223°C, ε_r = 329, and tan δ = 0.016, which were superior to that of PZTN 95/5 ceramics sintered at 1350°C.     

ζ‐Ta4C3−x: A High Fracture Toughness Carbide with Rising‐Crack‐Growth‐Resistance (R‐Curve) Behavior

The microstructures and mechanical properties of tantalum carbides containing predominantly the ζ‐Ta₄C_3?x phase are compared with the properties of the monocarbide (γ‐TaC) and the hemicarbide (α‐Ta₂C) and two‐phase composites. It is shown that a Ta and γ‐TaC powder mixture corresponding to a C/Ta at. ratio of 0.66 can be hot‐pressed (1800°C, 2 h) to obtain ~95 wt% of ζ‐Ta₄C_3?x with a density of 98% of theoretical. This material has an attractive combination of high fracture toughness (13.8 ± 0.2 MPa√m) and fracture strength (759 ± 24 MPa) with modest hardness (5.6 ± 0.5 GPa). The fracture toughness and strength measured for this material were the highest among all the materials with C/Ta ratio ranging from 0.5 (hemicarbide) to 1.0 (monocarbide). It is also shown that a material containing 86 wt% ζ‐Ta₄C_3?x can be consolidated by pressureless sintering of a hydrogenated Ta and γ‐TaC powder mixture without significant drop in density (97% of theoretical) or mechanical properties (13.4 ± 0.2 MPa√m, 700 ± 20 MPa, 6.0 ± 0.4 GPa). Materials containing high weight fraction of the ζ‐Ta₄C_3?x phase exhibited rising crack‐growth‐resistance (R‐curve) behavior. Optical and scanning electron microscope observations suggested crack‐face bridging was the dominant toughening mechanism. The crack‐bridging ligaments were lamellae of the basal planes of the ζ‐Ta₄C_3?x phase produced by their easy cleavage. The thickness of the lamellae ranged from 40 to 2000 nm, significantly less than the grain size.     

Chemical compatibility of chromium-based interconnect related materials with doped cerium oxide electrolyte

X-ray diffraction and scanning electron microscopy were used to study the chemical compatibility of (La, Sr)CrO₃, SrCrO₄, CaCrO₄ and Cr₂O₃ (materials associated with the interconnect of a solid oxide fuel cell) with gadolinia-doped cerium oxide electrolyte (CGO). Powder mixtures and multilayer pellets of the interconnect related materials were annealed with CGO in air at temperatures ranging from 650 to 1600°C for durations of up to 400 h. No reaction was observed between (La,Sr)CrO₃ and Ce_0.8Gd_0.2O_1.9 after annealing at 1600°C for 10 h. However, SrCrO₄, CaCrO₄ and Cr₂O₃ reacted with CGO, forming an unidentified phase.     

Flexural strength and flaw distributions of SrFe0.2Co0.4Mo0.4O3−δ ceramic-supports for SOFCs at operating conditions

Solid-oxide fuel cells (SOFCs) have the potential to increase electricity generation efficiency, but traditional SOFCs supported by nickel cermets suffer from reliability challenges due to weaker mechanical strength caused by cracking after redox cycling. To solve this problem, a new ceramic anode material, SrFe_0.2Co_0.4Mo_0.4O_3−δ (SFCM) combined with Ce_0.9Gd_0.1O₂ (GDC), was evaluated for conductivity and mechanical strength at SOFC operating conditions and after redox cycling. Fracture toughness of SFCM was determined to be (0.124 ± 0.023) MPa√m at room temperature in air, increasing to (0.286 ± 0.038) MPa√m at 600°C. A mixture of SFCM:GDC showed fracture toughness between the two materials, following SFCM's trend with temperature. The SFCM-GDC anode supported half-cell strength increases by 31% from room temperature to 600°C as intrinsic stresses remaining from sintering are relaxed and thermal expansion pushes existing cracks closed. Exposure to reducing gasses decreases strength by 29% compared to ambient, due to oxygen vacancy formation and microstructural flaw changes. It is found that SFCM-GDC based cells tolerate cycling well because of phase stability but weaken from 34.3 to 22.4 MPa due to uniform growth of critical microstructural flaws.