Effect of Fe2O3 doping on sintering of yttria-stabilized zirconia

This paper reports the effect of Fe₂O₃ doping on the densification and grain growth in yttria-stabilized zirconia (YSZ) during sintering at 1150 °C for 2 h. Fe₂O₃ doped 3 mol% YSZ (3YSZ) and 8 mol% YSZ (8YSZ) coatings were produced using electrophoretic deposition (EPD). For 0.5 mol% Fe₂O₃ doping, both 3YSZ and 8YSZ coatings during sintering at 1150 °C has similar densification. However, a significant grain growth occurred in 8YSZ during sintering, whereas grain size remains almost constant in 3YSZ. XRD results suggest that Fe₂O₃ addition substitutionally and interstitially dissolved into the lattice of 3YSZ and 8YSZ. In addition, colour of 3YSZ and 8YSZ changes differently with doping of Fe₂O₃. A Fe³⁺ ion interstitial diffusion mechanism is proposed to explain the densification and grain growth behaviour in the Fe₂O₃ doped 3YSZ and 8YSZ. A retard grain growth observed in the Fe₂O₃ doped 3YSZ is attributed to Fe³⁺ segregation at grain boundary.     

Fabrication and evaluation of NiO/Y2O3-stabilized-ZrO2 hollow fibers for anode-supported micro-tubular solid oxide fuel cells

In this work NiO/3 mol% Y₂O₃–ZrO₂ (3YSZ) and NiO/8 mol% Y₂O₃–ZrO₂ (8YSZ) hollow fibers were prepared by phase-inversion. The effect of different kinds of YSZ (3YSZ and 8YSZ) on the porosity, electrical conductivity, shrinkage and flexural strength of the hollow fibers were systematically evaluated. When compared with Ni–8YSZ the porosity and shrinkage of Ni–3YSZ hollow fibers increases while the electrical conductivity decreases, while at the same time also exhibiting enhanced flexural strength. Single cells with Ni–3YSZ and Ni–8YSZ hollow fibers as the supported anode were successfully fabricated showing maximum power densities of 0.53 and 0.67 W cm⁻² at 800 °C, respectively. Furthermore, in order to improve the cell performance, a Ni–8YSZ anode functional layer was added between the electrolyte and Ni–YSZ hollow fiber. Here enhanced peak power densities of 0.79 and 0.73 W cm⁻² were achieved at 800 °C for single cells with Ni–3YSZ and Ni–8YSZ hollow fibers, respectively.     

High performance BaCe0.8Y0.2O3−a (BCY) hollow fibre membranes for hydrogen permeation

In this work, BaCe_0.8Y_0.2O_3−α (BCY) perovskite hollow fibre membranes were fabricated by a phase inversion and sintering method. BCY powder was prepared by the sol–gel technique using ethylenediaminetetraacetic acid (EDTA) and citric acid as the complexing agents. Gel calcination was carried out at high temperature to form the desired crystal structure. The qualified BCY hollow fibre membranes could not be achieved even the sintering was carried out at temperatures up to 1550^oC due to the poor densification behavior of the BCY material. The addition of sintering aid (1 wt% Co₂O₃) inside BCY powder as the membrane starting material significantly improved the densification process, leading to the formation of gas-tight BCY hollow fibres. The optimum sintering temperature of BCY hollow fibre membrane was 1400 °C to achieve the best mechanical strength. H₂ permeation through the BCY hollow fibre membranes was carried out between 700 and 1050 °C using 25% H₂–He mixture as feed gas and N₂ as sweep gas, respectively. For comparison purpose, the disk-shaped BCY membrane with a thickness of 1 mm was also prepared. The measured H₂ permeation flux through the BCY hollow fibres reached up to 0.38 mL cm⁻² min⁻¹ at 1050 °C strikingly contrasting to the low values of less than 0.01 mL cm⁻² min⁻¹ from the disk-shaped membrane. After the permeation test, the microstructure of BCY hollow fibre membrane was still maintained well without signals of membrane disintegration or peeling off.     

Macro-porous dolomite hollow fibers sintered at different temperatures toward widened applications

Ceramic hollow fibers from natural dolomite with different pore structures have been designed. The unique hollow fibers were produced by the phase inversion method and applying different sintering temperatures. The hollow fiber precursor presented coagulated polymers through the fiber thickness due to the high granulometric size of the used dolomite material (11.3–47.2 µm). The fiber sintered at 400 °C presented mechanical strength of 4.5 MPa and water permeability of 84.7 L h⁻¹ m⁻² kPa⁻¹. The increase in the sintering temperature up to 1250 °C resulted in fragile hollow fibers due to dolomite transformations that resulted in gas release and a significant mass loss of 33.7%. At 1350 °C, the liquid phase sintering mechanism occurred and the dolomite hollow fiber sintered at 1350 °C presented mechanical strength of 5.5 MPa and water permeability of 2219 L h⁻¹ m⁻² kPa⁻¹. Doloma dissolution in water was investigated and calcium concentration was increased from 0.72 (pure water) to 2.905 ppm for a contact time from 4 h between the fiber sintered at 1250 °C and pure water. However, this dissolution did not decrease the mechanical resistance of the fiber. These results suggest the potential of applying natural dolomite for producing low cost membranes or substrates. The hollow fiber sintered at 400 °C is suggested to be used as a proper separation medium, while the hollow fiber sintered at 1350 °C may be used as a substrate for the deposition of a separation layer to be used in gas separations. The high porosity of the fiber sintered at 1250 °C suggests its application as a support for the impregnation of functional materials. Thus, depending on the applied sintering temperature the dolomite membrane can be used in different applications.     

Electrochemical performance of intermediate temperature micro-tubular solid oxide fuel cells using porous ceria barrier layers

We describe the manufacture and electrochemical characterization of micro-tubular anode supported solid oxide fuel cells (mT-SOFC) operating at intermediate temperatures (IT) using porous gadolinium-doped ceria (GDC: Ce_0.9Gd_0.1O_2−δ) barrier layers. Rheological studies were performed to determine the deposition conditions by dip coating of the GDC and cathode layers. Two cell configurations (anode/electrolyte/barrier layer/cathode): single-layer cathode (Ni–YSZ/YSZ/GDC/LSCF) and double-layer cathode (Ni–YSZ/YSZ/GDC/LSCF–GDC/LSCF) were fabricated (YSZ: Zr_0.92Y_0.16O_2.08; LSCF: La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ). Effect of sintering conditions and microstructure features for the GDC layer and cathode layer in cell performance was studied. Current density–voltage (j–V) curves and impedance spectroscopy measurements were performed between 650–800 °C, using wet H₂ as fuel and air as oxidant. The double-cathode cells using a GDC layer sintered at 1400 °C with porosity about 50% and pores and grain sizes about 1 μm, showed the best electrochemical response, achieving maximum power densities of up to 160 mW cm⁻² at 650 °C and about 700 mW cm⁻² at 800 °C. In this case GDC electrical bridges between cathode and electrolyte are preserved free of insulating phases. A preliminary test under operation at 800 °C shows no degradation at least during the first 100 h. These results demonstrated that these cells could compete with standard IT-SOFC, and the presented fabrication method is applicable for industrial-scale.     

Dispersion and stability of 8 mol.% yttria stabilized zirconia suspensions for dip-coating filtration membranes

The dispersion and the stability of the suspensions from commercially available 8 mol.% yttria stabilized zirconia (8YSZ) powder have been systematically investigated as a function of the system factors such as pH value, temperature, solid loadings, molecular weight and amounts of polyacrylic acid (PAA) dispersant. The interaction mechanisms between YSZ particles and organic additives in suspensions were analyzed and discussed in detail. The crack-free tubular membranes derived from the fully dispersed and well stabilized YSZ suspensions with appropriate viscosity exhibited an average pore diameter of 0.31 μm and membranes thickness of 10 μm. The pure water permeability of the membrane was up to 1.7 m³ h⁻¹ bar⁻¹ m⁻².     

Fe3O4 nanoparticles encapsulated in electrospun porous carbon fibers with a compact shell as high-performance anode for lithium ion batteries

Fe₃O₄ nanoparticles encapsulated in porous carbon fibers (Fe₃O₄@PCFs) as anode materials in lithium ion batteries are fabricated by a facile single-nozzle electrospinning technique followed by heat treatment. A mixed solution of polyacrylonitrile (PAN) and polystyrene (PS) containing Fe₃O₄ nanoparticles is utilized to prepare hybrid precursor fibers of Fe₃O₄@PS/PAN. The resulted porous Fe₃O₄/carbon hybrid fibers composed of compact carbon shell and Fe₃O₄-embeded honeycomb-like carbon core are formed due to the thermal decomposition of PS and PAN. The Fe₃O₄@PCF composite demonstrates an initial reversible capacity of 1015 mAh g⁻¹ with 84.4% capacity retention after 80 cycles at a current density of 0.2 A g⁻¹. This electrode also exhibits superior rate capability with current density increasing from 0.1 to 2.0 A g⁻¹, and capacity retention of 91% after 200 cycles at 2.0 A g⁻¹. The exceptionally high performances are attributed to the high electric conductivity and structural stability of the porous carbon fibers with unique structure, which not only buffers the volume change of Fe₃O₄ with the internal space, but also acts as high-efficient transport pathways for ions and electrons. Furthermore, the compact carbon shell can promote the formation of stable solid electrolyte interphase on the fiber surface.     

Preparation and characterization of BaCe0.95Tb0.05O3−α hollow fibre membranes for hydrogen permeation

BaCe_0.95Tb_0.05O_3?α (BCTb) perovskite hollow fibre membranes were fabricated by spinning the slurry mixture containing 66.67 wt% BCTb powder, 6.67 wt% polyethersulphone (PESf) and 26.67 wt% N-methyl-2-pyrrolidone (NMP) followed by sintering at elevated temperatures. The influence of sintering temperature on the membrane properties was investigated in terms of crystal phase, morphology, porosity and mechanical strength. In order to obtain gas-tight hollow fibres with sufficient mechanical strength, the sintering temperature should be controlled between 1350 and 1450 °C. Hydrogen permeation through the BCTb hollow fibre membranes was carried out between 700 and 1000 °C using 50% H₂–He mixture as feed on the shell side and N₂ as sweep gas in the fibre lumen. The measured hydrogen permeation flux through the BCTb hollow fibre membranes reached up to 0.422 μmol cm^?2 s^?1 at 1000 °C when the flow rates of the H₂–He feed and the nitrogen sweep were 40 mL min^?1 and 30 mL min^?1, respectively.     

Preparation,sintering and electrochemical performance of novel Fe2N-TiN nanocomposites

Transition metal nitrides (TMNs) hold great promises as electrode materials in energy devices like supercapacitors, lithium ion batteries and solar cells. However, the poor electrochemical stability severely limits their real-life applications. In this study, we prepared electrochemically stable Fe₂N-TiN nanocomposite with varied TiN contents by the nitridation of oxide precursors by ammonia. The formation mechanism consists of the solid reaction between TiO₂ and Fe₂O₃. A protective sintering (PS) technique has been developed for the first time to fabricate pure Fe₂N-TiN nanocomposite ceramics after comparing different sintering methods including spark plasma sintering (SPS). Porous microstructures formed by homogenous distribution of ultrafine TiN nanoparticles within large Fe₂N micro-grains skeleton were obtained after sintering. The electrochemical performances of the Fe₂N-TiN nanocomposites by PS have been investigated in different electrolytes. The mechanism of charge storage is mainly due to the double layer capacitance. The composites with 15 wt% TiN loading show the highest specific capacitance of 58.4 F g⁻¹ in 7.5 M KOH. Furthermore, excellent cycling stability with zero degradation after 1000 cycles was proved for such nanocomposite electrodes, confirming their potential for energy storage.     

Fabrication of graphene nanosheet (GNS)–Fe3O4 hybrids and GNS–Fe3O4/syndiotactic polystyrene composites with high dielectric permittivity

Graphene nanosheet–Fe₃O₄ (GNS–Fe₃O₄) hybrids were obtained by a one-step solvothermal reduction of iron (III) acetylacetonate [Fe(acac)₃] and graphene oxide (GO) simultaneously, which had several advantages: (1) the Fe₃O₄ nanoparticles were firmly anchored on GNS surface even after mild ultrasonication; (2) the loading amount of Fe₃O₄ nanoparticles could be effectively controlled by changing the initial feeding weight ratio of Fe(acac)₃ to GO; (3) the Fe₃O₄ nanoparticles were homogeneously distributed on the GNS surface without much aggregation. Composites based on syndiotactic polystyrene (sPS) and GNS–Fe₃O₄ were prepared by a solution-blending method and the electric and dielectric properties of the resultant GNS–Fe₃O₄/sPS composites were investigated. The percolation threshold of GNS–Fe₃O₄ in the sPS matrix was determined to be 9.41 vol.%. Slightly above the percolation threshold with 9.59 vol.% of GNS–Fe₃O₄, the GNS–Fe₃O₄/sPS composite showed a high dielectric permittivity of 123 at 1000 Hz, which was 42 times higher than that of pure sPS. The AC electrical conductivity at 1000 Hz increased from 3.6 × 10⁻¹⁰ S/m for pure sPS to 2.82 × 10⁻⁴ S/m for GNS–Fe₃O₄/sPS composite containing 10.69 vol.% of GNS–Fe₃O₄, showing an obvious insulator-semiconductor transition.     

Preparation of dual-phase composite BaCe0.8Y0.2O3/Ce0.8Y0.2O2 and its application for hydrogen permeation

Dual-phase ceramic membranes composed of BaCe_0.8Y_0.2O₃ (BCY) and Ce_0.8Y_0.2O₂ (CYO) were successfully synthesized by solid state reaction method for hydrogen permeation. The influences of the BCY/CYO volume ratios on phase composition, microstructure, chemical stability and electrical property were investigated. The hydrogen permeation of the dual-phase composite was characterized as a function of temperature and feed side hydrogen partial pressure. The results showed that there was no reaction between the two constituent oxides observed under the preparation conditions. The dual-phase composite with different BCY/CYO volume ratios after sintering at 1550 °C exhibited dense structure, as well as good stability in 4% H₂/Ar, wet Ar and pure CO₂ atmosphere. The conductivity of the dual-phase composite increased with the content of CYO increasing and 30BCY–70CYO exhibited the highest total conductivity of 2.6×10⁻² S cm⁻¹ at 800 °C in 4% H₂/Ar. The hydrogen permeability of 30BCY–70CYO sample was improved as the temperature and the hydrogen partial pressure in feed gas increased. The hydrogen permeation flux of 1.7 μmol cm⁻² s⁻¹ was achieved at 850 °C.     

Influence of Fe2O3 addition on the densification and oxygen ion conductivity of the GdSmZr2O7 ceramic

The influence of Fe₂O₃ addition as a sintering aid on the microstructure and electrical properties of the GdSmZr₂O₇ ceramic has been studied. The GdSmZr₂O₇ ceramic with 1 wt.% Fe₂O₃ is composed of a pyrochlore-type phase and a small amount of Gd_0.5Sm_0.5FeO₃. Fe₂O₃ is found to be an effective sintering aid for the GdSmZr₂O₇ ceramic, and a reduction in the sintering temperature of about 200 K is achieved. The total conductivity of the GdSmZr₂O₇ ceramic incorporated with or without 1 wt.% Fe₂O₃ obeys the Arrhenius relation. At 1173 K, the highest total conductivity of the GdSmZr₂O₇ ceramic with 1 wt.% Fe₂O₃ is about 20% higher than that of the GdSmZr₂O₇ ceramic. The GdSmZr₂O₇ ceramic with or without 1 wt.% Fe₂O₃ is an oxide-ion conductor in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels.     

New morphological Ba0.5Sr0.5Co0.8Fe0.2O3−α hollow fibre membranes with high oxygen permeation fluxes

Perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?α (BSCF) hollow fibre membranes were fabricated by a combined phase inversion and sintering technique. The membranes were characterised by XRD, SEM and tested for air separation. The membrane possesses a novel morphology consisting of one dense layer and one porous layer. Oxygen permeation fluxes through the obtained hollow fibre membranes were measured in the temperature range 650–950 °C using helium sweep gas rates from 50 to 200 mL min^?1. Experimental results indicated the oxygen permeation flux through the BSCF hollow fibre membrane sintered at 1050 °C was approximately 11.46 mL min^?1 cm^?2 at 950 °C when the helium sweep rate was kept at 200 mL min^?1. The BSCF hollow fibre membrane showed a stable oxygen permeation flux of 8.60 mL min^?1 cm^?2 over the investigated period of 120 h at 900 °C.     

Study of thin films of yttria-stabilized zirconia (YSZ) for the oxidation of some volatile organic compounds

Thin films of YSZ and 1%Pt/YSZ were deposited onto stainless steel tubes by an electrophoretic deposition technique. O₂-TPD from r.t. to 600 °C using induction heating was used to characterize the two films considering (i) the amount of oxygen desorbed (5.1 and 1.4 × 10^− 2 μmol O₂·g^− 1 for 1%Pt/YSZ and YSZ respectively) and (ii) the apparent activation energy of desorption E_app. Finally, complete oxidation of some VOCs (isopropanol and toluene) in air was studied on both films. From r.t. to 400 °C, oxidation of isopropanol can be achieved with either YSZ or 1%Pt/YSZ but only this last catalyst can achieve the complete oxidation of toluene.     

Constrained sintering of 8 mol% Y2O3 stabilised zirconia films

Constrained sintering kinetics of 8 mol% Y₂O₃/92 mol% ZrO₂ (8YSZ) films approximately 10–15 μm thick screen-printed on dense YSZ substrates, and the resulting stress induced in the films, were measured in the temperature range 1100–1350 °C. The results are compared with those reported earlier for 3YSZ films.Both materials behave similarly, although there are differences in detail. The constrained densification rate was greatly retarded compared with the unconstrained densification rate due to the effect of the constraint on the developing anisotropic microstructure (3YSZ) and, in the case of 8YSZ, considerable grain growth. The stress generated during constrained sintering was typically a few MPa. The apparent activation energies for free sintering, constrained sintering, creep and grain growth are found to cover a wide range (135–670 kJ mol^?1) despite all probably being mainly controlled by grain boundary cation diffusion.     

Improvement on the phase stability,mechanical properties and thermal insulation of Y2O3-stabilized ZrO2 by Gd2O3 and Yb2O3 co-doping

Gd₂O₃ and Yb₂O₃ co-doped 3.5 mol% Y₂O₃–ZrO₂ and conventional 3.5 mol% Y₂O₃–ZrO₂ (YSZ) powders were synthesized by solid state reaction. The objective of this study was to improve the phase stability, mechanical properties and thermal insulation of YSZ. After heat treatment at 1500 °C for 10 h, 1 mol% Gd₂O₃–1 mol% Yb₂O₃ co-doped YSZ (1Gd1Yb-YSZ) had higher resistance to destabilization of metastable tetragonal phase than YSZ. The hardness of 5 mol% Gd₂O₃–1 mol% Yb₂O₃ co-doped YSZ (5Gd1Yb-YSZ) was higher than that of YSZ. Compared with YSZ, 1Gd1Yb-YSZ and 5Gd1Yb-YSZ exhibited lower thermal conductivity and shorter phonon mean free path. At 1300 °C, the thermal conductivity of 5Gd1Yb-YSZ was 1.23 W/m K, nearly 25% lower than that of YSZ (1.62 W/m K). Gd₂O₃ and Yb₂O₃ co-doped YSZ can be explored as a candidate material for thermal barrier coating applications.     

Application of BaO–CaO–Al2O3–B2O3–SiO2 glass–ceramic seals in large size planar IT-SOFC

BaO–CaO–Al₂O₃–B₂O₃–SiO₂ (BCAS) glass–ceramics can be used as sealant for large size planar anode-supported solid oxide fuel cells (SOFCs). BCAS glass–ceramics after heat treatment for different times were characterized by means of thermal dilatometer, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the coefficients of thermal expansion (CTE) of BCAS glass–ceramics are 11.4×10⁻⁶ K⁻¹, 11.3×10⁻⁶ K⁻¹ and 11.2×10⁻⁶ K⁻¹ after heated at 750 °C for 0 h, 50 h, and 100 h, respectively. The CTE of BCAS matches that of YSZ, Ni–YSZ and the interconnection of SOFC. Needle-like barium silicate, barium calcium silicate and hexacelsian are crystallized in the BCAS glass after heat-treatment for above 50 h at 750 °C. The glass–ceramics green tape prepared by aqueous tape casting can be directly applied in sealing the cell of SOFCs with 10 cm×10 cm. The open circuit voltage (OCV) of the cell keeps 1.19 V after running for 280 h at 750 °C and thermal cycling 10 times from 750 °C to room temperature. The maximum power density is 0.42 W/cm² using pure H₂ as fuel and air as oxidation gas. SEM images show no cracks or pores exist in the interface of BCAS glass–ceramics and the cell.     

Diffusion of niobium in yttria-stabilized zirconia and in titania-doped yttria-stabilized zirconia polycrystalline materials

Bulk and grain boundary diffusion of Nb⁵⁺ cations in yttria-stabilized zirconia (YSZ, 8 mol% Y₂O₃–92 mol% ZrO₂) and in titania-doped yttria-stabilized zirconia (Ti–YSZ, 5 mol% TiO₂–8 mol% Y₂O₃–87 mol% ZrO₂) was studied in air in the temperature range from 900 to 1300 °C. Experiments were performed in the B-type kinetic region. Diffusion profiles were determined using the secondary ion mass spectrometry (SIMS). The temperature dependencies of the bulk diffusion coefficient D and the grain boundary diffusion parameter D′δs for both the materials were calculated. The activation energies of these transport processes in YSZ amounts to 258 and 226 kJ mol⁻¹, respectively, and 232 and 114 kJ mol⁻¹ in Ti–YSZ. The results were compared to the diffusion data of other cations previously obtained for the same material.     

Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method

Mn²⁺-doped Sn_1−xMn_xP₂O₇ (x = 0–0.2) are synthesized by a new co-precipitation method using tin(II)oxalate as tin(IV) precursor, which gives pure tin pyrophosphate at 300 °C, as all the reaction by-products are vaporizable at <150 °C. The dopant Mn²⁺ acts as a sintering aid and leads to dense Sn_1−xMn_xP₂O₇ samples on sintering at 1100 °C. Though conductivity of Sn_1−xMn_xP₂O₇ samples in the ambient atmosphere is 10⁻⁹–10⁻⁶ S cm⁻¹ in 300–550 °C range, it increases significantly in humidified (water vapor pressure, pH₂O = 0.12 atm) atmosphere and reaches >10⁻³ S cm⁻¹ in 100–200 °C range. The maximum conductivity is shown by Sn_0.88Mn_0.12P₂O₇ with 9.79 × 10⁻⁶ S cm⁻¹ at 550 °C in ambient air and 2.29 × 10⁻³ S cm⁻¹ at 190 °C in humidified air. It is observed that the humidification of Sn_1−xMn_xP₂O₇ samples is a slow process and its rate increases at higher temperature. The stability of Sn_1−xMn_xP₂O₇ samples is analyzed.     

Effects of manganese oxide addition and reductive atmosphere annealing on the phase stability and microstructure of yttria stabilized zirconia

The effects of Mn₃O₄ addition and reductive atmosphere (N₂:H₂ = 97:3) annealing on the microstructure and phase stability of yttria stabilized zirconia (YSZ) ceramics during sintering at 1500 °C for 3 h in air and subsequent annealing in a reductive atmosphere were investigated. Mn₃O₄ added 6 mol% YSZ (6YSZ) and 10 mol% YSZ (10YSZ) ceramics were prepared via the conventional solid-state reaction processes. The X-ray diffraction results showed that a single cubic phase of ZrO₂ was obtained in 1 mol% Mn₃O₄ added 6YSZ ceramic at a sintering temperature of 1500 °C for 3 h. A trace amount of monoclinic ZrO₂ phases were observed for 1 mol% Mn₃O₄ added 6YSZ ceramics after annealing at 1300 °C for 60 cycles in a reductive atmosphere by transmission electron microscopy. Furthermore, a single cubic ZrO₂ phase existed stably as Mn₃O₄ added 10YSZ ceramics was annealed at 1300 °C for 60 cycles in reductive atmosphere.