Densification and grain growth kinetics of Ce0.9Gd0.1O1.95 in tape cast layers: The influence of porosity

The sintering behavior of Ce_0.9Gd_0.1O_1.95 (CGO) tape cast layers with different porosity was investigated by an extensive characterization of densification, microstructural evolution, and applying the constitutive laws of sintering. The densification of CGO tapes associates with grain coarsening process at the initial sintering stage at T < 1150 °C, which is mainly influenced by small pores and intrinsic characteristics of the starting powders. At the intermediate sintering stage, densification is remarkably influenced by large porosity. Moreover, the sintering constitutive laws indicate that increasing the initial porosity from 0.38 to 0.60, the densification at the late stage is thermally activated with typical activation energy values increasing from 367 to 578 kJ mol⁻¹. Similar effect of the porosity is observed for the thermally activated phenomena leading to grain growth in the CGO tapes. The analysis of sintering mechanisms reveals that the grain growth behavior at different porosity can be described using an identical master curve.     

Sintering process optimization for multi-layer CGO membranes by in situ techniques

The sintering of asymmetric CGO bi-layers (thin dense membrane on a porous support; Ce_0.9Gd_0.1O_1.95?δ = CGO) with Co₃O₄ as sintering additive has been optimized by combination of two in situ techniques. Optical dilatometry revealed that bi-layer shape and microstructure are dramatically changing in a narrow temperature range of less than 100 °C. Below 1030 °C, a higher densification rate in the dense membrane layer than in the porous support leads to concave shape, whereas the densification rate of the support is dominant above 1030 °C, leading to convex shape. A flat bi-layer could be prepared at 1030 °C, when shrinkage rates were similar. In situ van der Pauw measurements on tape cast layers during sintering allowed following the conductivity during sintering. A strong increase in conductivity and in activation energy E_a for conduction was observed between 900 and 1030 °C indicating an activation of the reactive sintering process and phase transformation of cobalt oxide.     

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.     

Texturation of model clay materials using tape casting and freezing

The present work aimed to investigate the processing of textural clay based materials using tape casting together with freezing. Two model raw materials were used, namely: BIP kaolin from France and ABM montmorillonite from Mediterranean region. The mixtures of both clays were studied, whereby, the amount of montmorillonite was 0, 5, 10, 20 or 50 mass%. After tape casting, the as-obtained green bands were frozen into liquid nitrogen, lyophilized and then fired at 1050 °C or 1200 °C.The amount of montmorillonite appeared as a critical parameter that controls the cohesion of the dry products. For montmorillonite content ≥20 mass%, the products exhibited multiple cracks after lyophilisation. With lower montmorillonite content, the cohesion of the dry products was satisfactorily and a macroscopic cross-linked surface texturation was observed. After calcination at 1050 °C or 1200 °C, the texturation appeared well defined. Moreover, calcination at 1200 °C increased the densification of products and the occurrence of a glassy phase was noted.The combination of both tape casting and freezing (freeze tape casting) is a promising way to develop various clay-based and composites materials exhibiting unique microstructure organization and characteristics with potential application in the field sustainable and environmentally friendly filtration, adsorption or catalysis.     

Enhanced water desalination performance through hierarchically-structured ceramic membranes

Developments of membrane water desalination are impeded by low water vapor flux across the membrane. We present an innovative membrane design to significantly enhance the water vapor flux. A bilayer zirconia-based membrane with a thick hierarchically-structured support and a thin functional layer is prepared using a combined freeze drying tape casting and screen printing method. The hierarchically-structured YSZ support has a porosity of 42.6%, pores of 4.5 μm or larger, and a relatively low tortuosity of 1.58 along the thickness direction. The bilayer membrane is then converted from naturally hydrophilic to hydrophobic via grafting with a fluoroalkylsilane. A water flux of 28.7 Lm⁻² h⁻¹ and a salt rejection of 99.5% are achieved by exposing the functional layer to 80 °C salt water of 2 wt.% NaCl and the support layer to 20 °C distilled water. These results are the best performing ones for ceramic membranes in direct contact membrane distillation operation.     

Tape-cast asymmetric membranes for hydrogen separation

Ceramic hydrogen separation membrane is a promising technology for obtaining pure hydrogen in a wide range of processes including power generation with pre-combustion CO₂ capture, water-gas shift, methane reforming, etc. This work presents for the first time the production of cer-cer asymmetrical composite membranes. BaCe_0.65Zr_0.20Y_0.15O_3-δ (BCZY) supported BCZY- Gd_0.2Ce_0.8O_2-δ (GDC) membranes were produced by tape casting. Three different sintering aid incorporation methods were investigated to enhance the final density of the BCZY-GDC layer. The optimization of the whole process leads to produce planar crack-free asymmetrical proton conductive membranes with Ø=12 mm, constituted by a porous 350 µm thick BCZY substrate with an open porosity of 48%, and a 20 µm thick gas tight BCZY-GDC layer.     

Microstructure of TiO2 porous ceramics by freeze casting of nanoparticle suspensions

During freeze casting of TiO₂ porous ceramics, the porous architecture is strongly influenced by TiO₂ particle size, solids loading, and cooling temperature. This work investigates the influences of particle size, freezing substrate, and cooling temperature on the TiO₂ green bodies prepared by freeze casting. The results show that the lamellar channel width with 100 nm particles is larger than that of 25 nm particles, yet the ceramic wall thickness is noticeably decreased. The lamellar structure is more ordered when using a copper sheet than glass as its freezing substrate. A finer microstructure results when frozen at − 50 ℃ than − 30 ℃. Such porous materials have application potentials in a wide range of areas such as photocatalysis, solar cells, and pollutant removal and should be further studied.     

Densification and grain growth during sintering of porous Ce0.9Gd0.1O1.95 tape cast layers: A comprehensive study on heuristic methods

The sintering behavior of porous Ce_0.9Gd_0.1O_1.95 (CGO10) tape cast layers was systematically investigated to establish fundamental kinetic parameters associated to densification and grain growth. Densification and grain growth were characterized by a set of different methods to determine the dominant sintering mechanisms and kinetics, both in isothermal and at constant heating rate (iso-rate) conditions. Densification of porous CGO10 tape is thermally activated with typical activation energy which was estimated around 440–470 kJ mol^?1. Grain growth showed similar thermal activation energy of ～427 ± 22 kJ mol^?1 in the temperature range of 1100–1250 °C. Grain-boundary diffusion was identified to be the dominant mechanism in porous CGO10 tapes. Grain growth and densification mechanism were found strictly related in the investigated temperature range. Porosity acts as a grain growth inhibitor and grain boundary mobility in the porous body was estimated around 10^?18–10^?16 m³ N^?1 s^?1 at the investigated temperature range.     

Effect of zirconia tape porosity on fluorapatite surface film formation by dip coating

Thin fluorapatite (FA) layers on porous 3 mol% yttria-partially stabilized zirconia (Y-PSZ) substrates have been fabricated by dipping porous zirconia tapes into aqueous 27.4 vol% fluorapatite slurries. Two porous Y-PSZ tapes with different volume fraction of porosity were developed using an acrylic latex binder: tapes with 31.4 vol% porosity were prepared using 16.6 vol% starch as fugitive additive and those with 12.7 vol% porosity were fabricated without starch. The influence of the porous structure of the tape surfaces, top and bottom, on the casting rate and consequently on the layer thickness formed on each surface was studied. Layers formed on the top and bottom surfaces of the tapes with 12.7 and 31.4 vol% porosity were compared. The formation of a thin layer on the surface of the tape was governed by both liquid entrainment and slip casting mechanisms. The data for the FA layer formation were in good agreement with the slip casting model for immersion times>0. The casting rate at the top surface of both tapes was greater than that at the bottom surface. This difference was attributed to a greater porosity of the top surface with respect to that of the bottom one and was more pronounced for the tapes prepared with starch. Layers formed on the top surface were found to be about 55 and 32% thicker than those formed on the bottom surface for the tapes fabricated with and without starch, respectively. For the tapes prepared with starch, the greater porosity and number of smaller pores in the matrix of the top surface increased the casting rate and produced the thickest dip coated layers     

Microstructure and mechanical properties of ZrB2–SiC porous ceramic by camphene-based freeze casting

Camphene-based freeze casting technique was adopted to fabricate ZrB₂–SiC porous ceramic with 3-dimensional (3D) pore network. ZrB₂–SiC/camphene slurries (initial solid loading: 20 vol%, 25 vol% and 30 vol%) were prepared for freeze casting. Regardless of initial solid loading, the fabricated sample had dense/porous dual microstructure. The thickness of dense layer was about 200–300 μm. The microstructures of ZrB₂–SiC porous ceramics were significantly influenced by the initial solid loading, which determines the pore size, porosity and mechanical properties of the final products.     

Structure control of hydroxyapatite ceramics via an electric field assisted freeze casting method

Using H₂O₂ aqueous solution as pore-forming agent, hydroxyapatite (HA) porous scaffolds with both lamellar and spherical pores were fabricated by a freeze casting method. The highest porosity was obtained in HA scaffolds prepared using 5 vol% H₂O₂ aqueous solution. The relationship between the electric field intensity and the properties of HA scaffolds was investigated. Results showed that when the electric field intensity was increased from 0 to 90 kV/m, the average diameters of lamellar and spherical pores of HA scaffold were increased from 460 μm to 810 μm, and from 320 μm to 420 μm, respectively. Vitro cellular assay indicated that HA scaffold with both the lamellar and the spherical pores has a better biocompatibility, compared with that with single pores.     

Osseous differentiation on freeze casted 10CeTZP-Al2O3 structures

Three-dimensional structures with directionally oriented pore networks were fabricated from a 10 mol% ceria-stabilized zirconia and alumina composite (10CeTZP-Al₂O₃) via freeze casting. Ceramic suspensions of different concentrations (30, 40 and 50 wt% solids) were frozen at various rates (2, 5 and 10 °C/min) to obtain lamellar structures with aligned tubular pores of different characteristics: porosity (75–84%), pore dimensions (small diameter of the elliptical pores: 10–23 μm; large diameter of the elliptical pores: ∼200 ± 70 μm), lamella thickness (2.7–4 μm) and compression strength (1–12 MPa). In vitro assays confirmed the non-cytotoxic nature of the samples. Furthermore, specific osseous differentiation genes were quantified after incubating osteoblasts on different cross sections of the samples during 7 days in supplemented culture medium; results demonstrated that the freeze casted structures induce up to nine times more osseous gene expression than tissue culture polystyrene (TCPS), an advanced surface used for optimized in vitro cell growth.     

Si/SiC coated Cf/SiC composites via tape casting and reaction bonding: The effect of carbon content

In this paper, a novel surface modification method for C_f/SiC composites is proposed. Si/SiC coating on C_f/SiC composites is prepared by tape casting and reaction bonding method. The effects of carbon content on the rheological property of the slurries along with the microstructure of the sintered coatings are investigated. The best result has been obtained by infiltrating liquid silicon into a porous green tape with a carbon density of 0.84 g/cm³. In addition, the effect of sintering parameters on the phase composition of the coatings is studied. Dense Si/SiC coating with high density as well as strong bonding onto the substrate is obtained. This Si/SiC coating exhibits an excellent mechanical property with HV hardness of 16.29±0.53 GPa and fracture toughness of 3.01±0.32 MPa m^1/2. Fine surface with roughness (RMS) as low as 2.164 nm is achieved after precision grinding and polishing. This study inspires a novel and effective surface modification method for C_f/SiC composites.     

Improvement of translucency in Al2O3 ceramics by two-step sintering technique

Pure, 140, 500, and 2500 ppm MgO-doped Al₂O₃ specimens were sintered at 1700 °C in a vacuum with and without pre-sintering heat treatment (HT) at 800 °C for 50 h in air. Pre-sintering HT improved the transmittance in the visible (400–700 nm) range significantly. This enhanced transmittance was explained in terms of the removal of residual pores and the homogenization of the microstructure due to the lowering of the boundary mobility as a result of the MgO addition and the suppression of local densification through a pre-coarsening step.     

Facile fabrication and luminescence characteristics of Ce:YAG phosphor glass thick films coated on a glass substrate for white LEDs

A pivotal step in providing a better fluorescent material that has high luminous efficacy and excellent thermal stability is to utilize inexpensive phosphors for white light-emitting diodes (W-LEDs). Herein, we demonstrate a feasible tape-casting technique for creating phosphor thick films that consist of Ce: YAG phosphor embedded in relatively low melting point glass frits on an ultrathin glass substrate with controllable film thickness. The glass matrix has ideal densification and interfaces with the glass substrate at a relatively low temperature of 580 °C. Subsequently, the structure and optical properties of the phosphor layer are investigated. In addition, the effect of the phosphor concentration, thick film thickness and location (top or bottom) of the phosphor layer on the photoluminescence properties and chromaticity are also discussed with respect to use in W-LEDs. Significantly, this promising structure has excellent thermal stability and the potential to overcome current limitations of phosphors in high-power W-LEDs. Finally, a high-performance W-LED based on the planar phosphor glass exhibits a luminous efficiency of 108.45 lm W⁻¹, a correlated color temperature of 5408 K and a color rendering index of 76.     

Electrochemical and microstructural characteristics of nanoperovskite oxides Ba0.2Sr0.8Co0.8Fe0.2O3−δ (BSCF) for solid oxide fuel cells

Nanoperovskite oxides, Ba_0.2Sr_0.8Co_0.8Fe_0.2O_3?δ (BSCF), were synthesized via the co-precipitation method using Ba, Sr, Co, and Fe nitrates as precursors. Next, half cells were fabricated by painting BSCF thin film on Sm_0.2Ce_0.8O_x (samarium doped ceria, SDC) electrolyte pellets. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS) measurements were carried out on the BSCF powders and pellets obtained after sintering at 900 °C. Investigations revealed that single-phase perovskites with cubic structure was obtained in this study. The impedance spectra for BSCF/SDC/BSCF cells were measured to obtain the interfacial area specific resistances (ASR) at several operating temperatures. The lowest values of ASR were found to be 0.19 Ω cm², 0.14 Ω cm² 0.10 cm², 0.09 Ω cm² and 0.07 Ω cm² at operating temperatures of 600 °C, 650 °C, 700 °C, 750 °C and 800 °C, respectively. The highest conductivity was found for cells sintered at 900 °C with an electrical conductivity of 153 S cm^?1 in air at operating temperature of 700 °C.     

Kinetic demixing/decomposition of Ba0.5Sr0.5CoxFe1−xO3−δ (x = 0.2 and 0.8)

Microstructural changes due to kinetic demixing within sintered BSCF ceramics (Ba_0.5Sr_0.5Co_xFe_1?xO_3?δ, x = 0.2 and 0.8: BSCF5528 and BSCF5582, respectively) have been investigated. When the specimens were subjected to 2 A/cm² at 1000 °C and pO₂ = 10^?5 atm, there was a significant enhancement of grain growth as well as 2nd phase formation observed in BSCF5528. At the anode, cobalt deficient aggregates within the grains; and, at the cathode, cobalt rich 2nd phase particles were observed on the grain surfaces of the microstructure. Such phenomena were not observed in BSCF5582, even under higher current density (7 A/cm²) and longer delay time. These results were explained by the kinetic demixing/decomposition.     

Phase-inversion tape casting and synchrotron-radiation computed tomography analysis of porous alumina

A variant of tape casting based on the phase inversion phenomenon was adopted for fabrication of porous ceramic wafer. A slurry was prepared by dispersing alumina powder in an N-methyl-2-pyrrolidone (NMP) solution of the polymers polyethersulfone (PES) and polyvinylpyrrolidone (PVP). The slurry was cast using a doctor blade, and immersed in water to solidify the polymer solution via phase inversion. The green tape was dried and sintered at 1500 °C. The as-prepared ceramic wafer was characterized using synchrotron-radiation computed tomography (SR-CT). It was revealed that the ceramic wafer contained typical finger-like macrovoids, and the porosity resulting from these macrovoids was ~30%. The overall porosity of the wafer was 59%, as derived from the density data measured by Archimedes method in mercury. It is concluded that the phase inversion tape casting is a simple and effective method for preparation of porous ceramics.     

A new approach for the net-shape fabrication of porous Si3N4 bonded SiC ceramics with high strength

In this study, Si₃N₄ bonded porous SiC ceramics with high strength had been net-shapely fabricated by a new approach. In this approach, we proposed a two-step processing route composed of freeze casting and carbothermal reduction reactions in which carbon aerogels, derived from sol infiltration and pyrolysis, involved. The phase components, microstructures and properties of the prepared ceramics were investigated. The results showed that carbon aerogels with high apparent surface area had been completely reacted and new SiC and Si₃N₄ grains had been produced. The porous ceramics with flexural strength of 164.3 MPa at 33% porosity and 80.5 MPa at 46% porosity were obtained, whose linear shrinkages were only 1.06% and 1.94% during the whole processing respectively.     

Oxygen permeation properties of BSCF5582 tubular membrane fabricated by the slip casting method

BSCF5582 tubular oxygen separation membranes were prepared using the most cost effective slip casting techniques. The optimum slurry composition was identified and a dense, and crack free 60 mm long BSCF5582 tubular membrane being successfully prepared after the programmed sintering process. The effects of the feed flow rate and the sweeping flow rate on the oxygen permeation flux of the tubular BSCF5582 membrane were investigated. The oxygen permeation flux increased with an increase of the oxygen chemical potential gradient to a maximum of 42.5 cm³/min in an O₂/N₂ condition at 1223 K for the 1.5 mm thick, 60 mm long BSCF tube, a value which corresponds to 1.42 cm³/min cm². The ionic conductivity of the oxygen was successfully calculated in the dominant electron conducting regime. The ionic conductivity was found to increase with an increase of the temperature to 900 °C, indicating that it is a thermally activated process with an activation energy of 0.70 ± 0.1 eV in an air environment.