High-density YSZ tapes fabricated via the multi-folding lamination process

A new method of ceramic processing to obtain high green and fully sintered yttria-stabilized zirconia (YSZ) ceramic parts has been studied. The procedure involved slip casting, multi-folding lamination, and sintering. A rheological study revealed correlation between compositional parameters and densities. A particular method of folding and lamination we named multi-folding lamination was proved to be an appropriate route to obtain dense, homogeneous green bodies, reaching density values of ca. 61%. Further studies on the sintered parts were performed in this work, obtaining YSZ sintered tapes suitable for the use in high temperature solid-state devices. This tapes, sintered at 1550 °C, reached values of 98% of theoretical density and average particle sizes within 1.7–12 μm.     

Enhanced ionic conductivity of scandia-ceria-stabilized-zirconia (10Sc1CeSZ) electrolyte synthesized by the microwave-assisted glycine nitrate process

Scandia-stabilized-zirconia is a potential zirconia-based electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this study, the properties of zirconia co-doped with 10 mol% Sc and 1 mol% Ce (scandia-ceria-stabilized-zirconia, 10Sc1CeSZ) electrolyte synthesized by the microwave-assisted glycine nitrate process (MW-GNP) were determined. The effects of microwave heating on the sintering temperature, microstructure, densification and ionic conductivity of the 10Sc1CeSZ electrolyte were evaluated. The phase identification, microstructure and specific surface area of the prepared powder were investigated using X-ray diffraction, transmission electron microscopy and the Brunauer-Emmett-Teller technique, respectively. Using microwave heating, a single cubic-phase powder was produced with nanosized crystallites (19.2 nm) and a high specific surface area (16 m²/g). It was found that the relative density, porosity and total ionic conductivity of the 10Sc1CeSZ electrolyte are remarkably influenced by the powder processing method and the sintering temperature. The pellet sintered at 1400 °C exhibited a maximum ionic conductivity of 0.184 S/cm at 800 °C. This is the highest conductivity value of a scandia-stabilized-zirconia based electrolyte reported in the literature for this electrolyte type. The corresponding value of the activation energy of electrical conductivity was found to be 0.94 eV in the temperature range of 500–800 °C. Overall, the use of microwave heating has successfully improved the properties of the 10Sc1CeSZ electrolyte for application in an IT-SOFC.     

Hydrogen formation from a real biogas using electrochemical cell with gadolinium-doped ceria porous electrolyte

The electrochemical cell consisting of a gadolinium-doped ceria (GDC, Ce_0.9Gd_0.1O_1.95) porous electrolyte, Ni–GDC cathode and Ru–GDC anode was applied for the dry-reforming (CH₄+CO₂→2H₂+2CO) of a real biogas (CH₄ 60.0%, CO₂ 37.5%, N₂ 2.5%) produced from waste sweet potato. The composition of the supplied gas was adjusted to CH₄/CO₂=1/1 volume ratio. The supplied gas changed continuously into a H₂–CO mixed fuel with H₂/CO=1/0.949–1/1.312 vol ratios at 800 °C for 24 h under the applied voltage of 1–2 V. The yield of the mixed fuel was higher than 80%. This dry-reforming reaction was thermodynamically controlled at 800 °C. The application of external voltage assisted the reduction of NiO and the elimination of solid carbon deposited slightly in the cathode. The decrease of heating temperature to 700 °C reduced gradually the fraction of the H₂–CO fuel (61.3–18.6%) within 24 h. Because the Gibbs free energy change was calculated to be negative values at 700–600 °C, the above result at 700–600 °C originated from the gradual deposition of carbon over Ni catalyst through the competitive parallel reactions (CH₄→C+2H₂, 2CO→C+CO₂). The application of external voltage decreased the formation temperature of carbon by the disproportionation of CO gas. At 600 °C, the H₂–CO fuel based on the Faraday's law was produced continuously by the electrochemical reforming of the biogas.     

The fabrication and characterization of sintered diatomite for potential microfiltration applications

Porous ceramic membranes have lately become a subject of special interest due to their outstanding thermal and chemical stability. We investigated whether a sintered diatomite support layer could also serve as a separation layer to minimize any processing difficulties, and investigated whether the support layer and the separation layer could be made from the same material to avoid a thermal mismatch during a high-temperature sintering process. We prepared sintered diatomite as a porous ceramic membrane for microfiltration, as diatomite particles are inherently porous and irregular. The pore characteristics of the sintered diatomite specimens were studied by scanning electron micrography, mercury porosimetry, and capillary flow porosimetry.     

Hydrogen production through dry reforming of biogas using a porous electrochemical cell: Effects of a cobalt catalyst in the electrode and mixing of air with biogas

This paper reports the performance of porous Gd-doped ceria (GDC) electrochemical cells with Co metal in both electrodes (cell No. 1) and with Ni metal in the cathode and Co metal in the anode (cell No. 2) for CO₂ decomposition, CH₄ decomposition, and the dry reforming reaction of a biogas with CO₂ gas (CH₄ + CO₂ → 2H₂ + 2CO) or with O₂ gas in air (3CH₄ +?1.875CO₂ +?1.314O₂ → 6H₂ +?4.875CO +?0.7515O₂). GDC cell No. 1 produced H₂ gas at formation rates of 0.055 and 0.33?mL-H₂/(min?m²-electrode) per 1?mL-supplied gas/(min?m²-electrode) at 600?°C and 800?°C, respectively, by the reforming of the biogas with CO₂ gas. Similarly, cell No. 2 produced H₂ gas at formation rates of 0.40?mL-H₂/(min?m²) per 1?mL-supplied gas/(min?m²) at 800?°C from a mixture of biogas and CO₂ gas. The dry reforming of a real biogas with CO₂ or O₂ gas at 800?°C proceeded thermodynamically over the Co or Ni metal catalyst in the cathode of the porous GDC cell. Faraday's law controlled the dry reforming rate of the biogas at 600?°C in cell No. 2. This paper also clarifies the influence of carbon deposition, which originates from CH₄ pyrolysis (CH₄ → C + 2H₂) and disproportionation of CO gas (2CO → C + CO₂), on the cell performance during dry reforming. The dry reforming of a biogas with O₂ molecules from air exhibits high durability because of the oxidation of the deposited carbon by supplied air.     

Densification and grain growth of 8YSZ containing NiO

The effects of NiO addition on sintering yttria-stabilized zirconia were systematically studied to understand the role of the additive in the sintering process of the solid electrolyte. Specimens of 8 mol% yttria-stabilized zirconia with NiO contents up to 5.0 mol% were prepared using different Ni precursors and sintered at several dwell temperatures and holding times. Densification and microstructural features were studied by apparent density measurements and scanning electron microscopy observations, respectively. The sintering dynamic study was carried out by following the linear shrinkage of powder compacts containing 0-0.75 mol% NiO. Small (up to 1.0 mol%) NiO addition was found to improve the sinterability of yttria-stabilized zirconia. The activation energy for volume diffusion decreases with increasing NiO content, whereas the grain boundary diffusion seems to be independent on this additive. The grain growth of yttria-stabilized zirconia is found to be enhanced even for small NiO contents.     

Structural studies of porous Ni/YSZ cermets fabricated by the solid-state reaction method

Carbon black-added NiO/YSZ composites exhibited a highly porous structure due to the enhanced evolution of CO and CO₂ gases associated with exothermic reactions during sintering. The addition of carbon black resulted in a decrease in the density and grain size of NiO/YSZ composites. The porosity of NiO/YSZ composites increased with decreasing the sintering temperature and increasing the NiO content. Additionally, highly porous Ni/YSZ cermets were fabricated by reducing the NiO/YSZ composites in (Ar+6% H₂), which was attributed to the change of NiO to Ni and then the extraction of oxygen. The carbon black used as a pore-former was highly effective for preparing porous Ni/YSZ cermets.     

Electrochemical reforming of CH4−CO2 mixed gas using porous Gd-doped ceria electrolyte with Cu electrode

This paper reports on the composition and flow rate of outlet gas and current density during the reforming of CH₄ with CO₂ using three different electrochemical cells: cell A, with Ni−GDC (Gd-doped ceria: Ce_0.8Gd_0.2O_1.9) cathode/porous GDC electrolyte/Cu−GDC anode, cell B, with Cu−GDC cathode/ porous GDC electrolyte/Cu−GDC anode and cell C, with Ru−GDC cathode/ porous GDC electrolyte/ Cu−GDC anode. In the cathode, CO₂ reacts with supplied electrons to form CO fuel and O²⁻ ions (CO₂+2e⁻→CO+O²⁻). Too low affinity of Cu cathode to CO₂ in cell B reduced the reactivity of the CO₂ with electrons. The CO fuel, O²⁻ ions and CH₄ gas were transported to the anode through the porous GDC mixed conductor of O²⁻ ions and electrons. In the anode, CH₄ reacts with O²⁻ ions to produce CO and H₂ fuels (CH₄+O²⁻→2 H₂+CO+2e⁻). The reforming efficiency at 700−800 °C was lowest in cell B and highest in cell A. The Cu anode in cells A and C worked well to oxidize CH₄ with O²⁻ ions (2Cu+O²⁻→Cu₂O+2e⁻, Cu₂O+CH₄→2Cu+CO+2H₂). However, a blockage of the outlet gas occurred in all the cells at 700−800 °C. The gas flow is inhibited due to a reduction in pore size in the cermet cathode, as well as sintering and grain growth of Cu metal in the anode during the reforming.     

Characterization of IT-SOFC non-symmetrical anode sintered through conventional furnace and microwave

This work investigated the mechanical and electrical properties of NiO–SDC/SDC anode sintered by two different methods: in a microwave at about 1200 °C for 1 h and in a conventional furnace at 1200 °C with a holding time of 1 h (total sintering time of 21 h). Nano-powders Sm_0.2Ce_0.8O_1.9 (SDC) and NiO were mixed using a high-energy ball mill, followed by the co-pressing technique at a compaction pressure of 400 MPa. No binder was used between the layers. The electrical behaviors of all sintered samples were studied using electrochemical impedance spectra in the frequency range of 0.01–10⁵ Hz under 97% H₂–3% H₂O, an amplitude of 10 mV, and at high temperature range of 600–800 °C. Results indicate that the non-symmetrical NiO–SDC/SDC anode achieved through microwave sintering has finer grain size and higher electrochemical performance. However, hardness and Young׳s modulus increased in the samples sintered through a conventional furnace.     

Temperature-dependent morphology changes of noble metal tricalcium phosphate-nanocomposites

Calcium phosphates, functionalized with nano-sized metal particles, are a promising material class for the treatment of bone defects. However, a sintering process is required in principle to achieve sufficient strength of calcium phosphate scaffolds. In this work laser-generated nano-sized silver, gold and platinum particles were adsorbed on micro-sized β-tricalcium phosphate particles and further heat treated at temperatures between 600 and 1200 °C. Gold and platinum nanoparticles underwent exponential growth starting at about 600 °C, while sintering of β-tricalcium phosphate started at 800 °C. We hypothesise that this phenomenon is caused by a heat-induced evaporation and growth process where the decrease of the particle number is directly correlated with the size increase. The silver nanoparticles on the other hand formed a new phase with the calcium phosphate (AgCa₁₀(PO₄)₇) during the heat treatments and could not be observed within the ceramic scaffold anymore. Addressing the lack of information in nanoparticle-combined calcium phosphate scaffolds, this study contributes to the further modification of bone replacement materials with biologically relevant functions and molecules.     

Microstructural evolution during fabrication of alumina via laser stereolithography technique

Application of additive manufacturing (AM) technology in production of ceramic parts is considered as a state-of-the-art technique which has been recently introduced to industry. In the current study the imperative microstructural characteristics of the alumina manufactured via laser stereolithography (SLA) has been investigated. The microstructural characteristics of the developed ceramic parts and components are still unknown and require detailed investigation. A combination of optical microscopy and scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), image analysis, X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and micro-computed tomography (micro-CT) scans was used to evaluate the microstructural features of the alumina samples after each step of the manufacturing process (i.e. printing, debinding and sintering). In addition, the apparent density of each sample was measured using water displacement method. Results indicated that the porosity of printed alumina samples was significantly reduced after sintering process. EDS analysis confirmed elimination of binder material after debinding and sintering processes. XRD analysis detected existence of α-Al₂O₃ in initial printed samples which was not changed during debinding and sintering processes. Due to detection of identical peaks for all samples, only one set of lattice parameters ($\mathrm{a}$ and $\mathrm{c}$) was calculated from XRD patterns of all samples which was close to the ones reported in literature for alumina. TEM micrographs and corresponding diffraction patterns confirmed polycrystalline structure from different layers of the samples. High resolution transmission electron microscopy (HRTEM) and diffraction patterns from single layers were used to calculate lattice parameters for each sample. A slight increase was noticed in unit cell and grain size after sintering process. The obtained results help for better understanding of the properties through microstructural characteristics of the 3D printed ceramic parts.     

Sintering evaluation of doped lanthanum gallate based on thermodilatometry

The sintering behavior of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ oxide-ion conductor was systematically investigated by thermodilatometry. The shrinkage data obtained with heating rates of 4, 7, 10 and 12?°C?min^?1 were analyzed by the constant rate of heating model and by construction of the master sintering curve. Validation of the master sintering curve was carried out by measurements of density in conventionally sintered specimens. Slight anisotropy of shrinkage data was found and changes to the basic equation of density was proposed to account for this effect. Plotting the data determined by the constant rate of heating model versus density allowed an easy identification of the density range of constant activation energy. The activation energy (865?kJ?mol^?1) obtained from the master sintering curve correlates quite well with that (874?kJ?mol^?1) obtained by the constant rate of heating model.     

Proton conducting solid oxide fuel cells with chemically stable BaZr0.75Y0.2Pr0.05O3-δ electrolyte

A BaZrO₃-based electrolyte with low Pr-doping concentration is proposed as electrolyte for proton-conducting solid oxide fuel cells (SOFCs). The new material BaZr_0.75Y_0.2Pr_0.05O_3-δ (BZYP5) shows a good chemical stability against CO₂. In addition, the low doping concentration of Pr in BaZrO₃ improves the sinterability of BaZrO₃ and also allows its structure to remain stable even in the reducing atmosphere, which is critical for fuel cell applications. The cell with BZYP5 as electrolyte shows maximum power densities of 124, 70, and 43 mW cm⁻² at 600, 550, and 500 °C, respectively, which are larger than that for the cell with conventional high Pr-doping BaZrO₃ electrolyte reported previously. Electrochemical analysis indicates that the BZYP5 electrolyte shows a good ionic conductivity. These results suggest that the low Pr-doping strategy presented in this study promotes the densification for BaZrO₃ and the good electrolyte conductivity of BaZrO₃ is maintained which could be the reason for the improved cell performance, suggesting BZYP5 is a promising electrolyte for proton-conducting SOFCs.     

Liquid-phase synthesis of SrCo0.9Nb0.1O3-δ cathode material for proton-conducting solid oxide fuel cells

A novel liquid-phase synthesis strategy is demonstrated for the preparation of the Nb-containing ceramic oxide SrCo_0.9Nb_0.1O_3-δ (SCN). In comparison with the traditional solid-state reaction (SSR) method, the liquid-phase synthesis route offers a couple of advantages, including a lower phase formation temperature and a smaller particle size of the SCN materials that are beneficial for applications as proton-conducting fuel cell cathode. With BaCe_0.4Zr_0.4Y_0.2O_3-δ (BCZY442) as the electrolyte and the SCN synthesized in this work as the cathode, a proton-conducting solid oxide fuel cell (SOFC) shows a peak power density of 348 mW cm^?2 at 700 °C, significantly higher than that of a SOFC fabricated with SCN cathode prepared using the SSR method, which can only deliver 204 mW cm^?2 at the same temperature. Additionally, this new synthesis strategy allows impregnation of Sr²⁺, Co³⁺and Nb⁵⁺ on the solid backbone in aqueous solution, further improving cell performance to reach a peak power density of 488 mW cm^?2 at 700 °C.     

Transport properties of BaZr0.5Ce0.3Y0.2O3−δ proton conductor prepared by spark plasma sintering

Dense BaZr_0.5Ce_0.3Y_0.2O_3−δ (BZCY532) proton conductors were prepared by a spark plasma sintering method. Their conductivities were determined in different atmospheres: dry air, wet N₂ and wet H₂. Moreover, the potential electronic conductivity contribution to the total conductivity was also identified by testing their total conductivities at different oxygen partial pressures (1–10⁻²⁴ atm) in combination with an XPS analysis. It is found that the prepared dense BZCY532 ceramics are good proton conductors at 600 °C. In addition, the Ce³⁺ concentration in the dense BZCY532 ceramics is around 3.5 atm% of the total Ce element, and the electronic contribution to the total conductivity can be neglected after a postheat treatment.     

Innovative fabrication of PZT pillar arrays by a colloidal approach

An innovative approach for fabricating pillar arrays for ultrasonic transducer applications is disclosed. It involves the preparation of concentrated piezoelectric lead zirconate titanate (PZT) suspensions in aqueous solutions of epoxy resin and its polymerization upon adding a polyamine based hardener. Zeta potential and rheological measurements revealed that 1 wt.% dispersant, 20 wt.% of epoxy resin and a hardener/epoxy resin ratio of 0.275 mL g⁻¹, were the optimized contents to obtain strong PZT samples with high green strength (35.21 ± 0.39 MPa). Excellent ellipsoidal and semi-circle shaped pillar arrays presenting lateral dimensions lower than 10 μm and 100 μm height were successfully achieved. The organics burning off was conducted at 500 °C for 2 h at a heating rate of 1 °C min⁻¹. Sintering was then carried out in the same heating cycle at 1200 °C for 1 h. The microstructures of the green and sintered ceramics were homogeneous and no large defects could be detected.     

Investigation of some niobate-based dielectrics in view of base metal co-sintering

Bearing in mind the excellent dielectric properties at high frequency of some niobates like ZnNb₂O₆, Zn₃Nb₂O₈, BaNb₂O₆, Ba₅Nb₄O₁₅ (_r 20–45, tan δ < 10 × 10⁻⁴ and ρ_i > 10¹⁰ Ω cm), synthesis, sintering and properties of these oxides are reported. The lowering of their sintering temperature has been investigated for these four ceramics using sintering aids. Using appropriate additive, it is possible to densify these ceramics at a temperature for which base metal electrodes, e.g. Cu and Ag can be employed. Two formulations were found to be sinterable at 1000 °C (lower than the copper melting point) whereas a third formulation is co-sinterable with silver electrodes. For this later, the dielectric properties are globally maintained in comparison with the pure compound sintered at 1200 °C. This result makes this formulation suitable for silver based passive components devices manufacturing.     

Characterization of Ce0.9Gd0.1O1.95 powders synthesized by spray drying

Ce_0.9Gd_0.1O_1.95 powders were synthesized by spray drying and successive calcinations. The phase purity, BET surface area, and particle morphology of as-sprayed and calcined powders were characterized. After calcination above 300 °C, the powders were single phase and showed a BET surface area of 68 m²/g when calcined at 300 °C. The conductivity, in air, of sintered pellets was measured by electrochemical impedance spectroscopy (EIS) and it was found to be comparable with literature values. The activation energy for the total conductivity was around 0.83 eV. The powder calcined at lower temperature showed better sinterability and higher total conductivity due to an increased bulk conductivity.     

Enhanced performance of Fe2O3 doped yttria stabilized zirconia hollow fiber membranes for water treatment

Fe₂O₃ powders were introduced as sintering aid to fabricate yttria-stabilized zirconia (YSZ) hollow fiber membranes using a combined wet-spinning and post-sintering method. The obtained Fe₂O₃-YSZ hollow fiber membranes show enhanced performance for water treatment with fine crystal structure in terms of bending strength and pure water permeability. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and thermogravimetric analysis (TGA) along with mechanical tests were employed to investigate the structural evolution in the sintering process and the effect of Fe₂O₃. It is suggested that the Fe₂O₃ dopants dissolve into YSZ at elevated temperatures, providing defect sites and vacancies for fast ion migration, favoring for densification and grain growth of the YSZ, which yields dense microstructures of fine crystallites at relatively low sintering temperature. The Fe₂O₃-YSZ hollow fiber membranes sintered at 1150 °C show a 3-fold increase of the permeate flux of pure water (F) (743 L m⁻² h⁻¹) along with comparable bending strength (152 MPa) compared to pure YSZ membranes. This modified method can reduce sintering costs and therefore fabrication costs which should pave the way for scale-up production for ceramic hollow fiber membranes.