Fabrication and electrochemical properties of SOFC single cells using porous yttria-stabilized zirconia ceramic support layer coated with Ni

A porous yttria-stabilized zirconia (YSZ) ceramic supported single cell with a configuration of porous YSZ support layer coated with Ni/Ni–Ce_0.8Sm_0.2O_1.9 (SDC) anode/YSZ/SDC bi-layer electrolyte/La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ cathode was fabricated. The porosity, mechanical strength, and microstructure of porous YSZ ceramics were investigated with respect to the amount of poly(methyl methacrylate) (PMMA) used as a pore former. Porous YSZ ceramics with 56 vol.% PMMA showed a mechanical strength of 24 ± 3 MPa and a porosity of 37 ± 1%. The electrochemical properties of the single cell employing the porous YSZ support layer were measured using hydrogen and methane fuels, respectively. The single cell exhibited maximum power densities of 421 mW/cm² in hydrogen and 399 mW/cm² in methane at 800 °C. Moreover, at a current density of 550 mA/cm², the cell maintained 91% of its initial voltage after operation in methane for 13 h at 700 °C.     

Foam-gelcasting preparation,microstructure and thermal insulation performance of porous diatomite ceramics with hierarchical pore structures

Hierarchically pore-structured porous diatomite ceramics containing 82.9∼84.5% porosity were successfully prepared for the first time via foam-gelcasting using diatomite powder as the main raw material. Sizes of mesopores derived from the raw material and macropores formed mainly from foaming were 0.02∼0.1 μm and 109.7∼130.5 μm, respectively. The effect of sintering temperature, additive content and solid loading of slurry on pore size and distribution, and mechanical and thermal properties of as-prepared porous ceramics were investigated. Compressive strength of as-prepared porous ceramics increased with sintering temperature, and the one containing 82.9% porosity showed the highest compressive strength of 2.1 ± 0.14 MPa. In addition, the one containing 84.5% porosity and having compressive strength of 1.1 ± 0.07 MPa showed the lowest thermal conductivity of 0.097 ± 0.001 W/(m·K) at a test temperature of 200 ̊C, suggesting that as-prepared porous ceramics could be potentially used as good thermal insulation materials.     

Investigating the effect of SPRM on mechanical strength and thermal conductivity of highly porous alumina ceramics

For recycling waste refractory materials in metallurgical industry, porous alumina ceramics were prepared via pore forming agent method from α-Al₂O₃ powder and slide plate renewable material. Effects of slide plate renewable material (SPRM) on densification, mechanical strength, thermal conductivity, phase composition and microstructure of the porous alumina ceramics were investigated. The results showed that SPRM effectively affected physical and thermal properties of the porous ceramics. With the increase of SPRM, apparent porosity of the ceramic materials firstly increased and then decreased, which brought an opposite change for the bulk density and thermal conductivity values, whereas the bending strength didn’t decrease obviously. The optimum sample A2 with 50 wt% SPRM introducing sintered at 1500 °C obtained the best properties. The water absorption, apparent porosity, bulk density, bending strength and thermal conductivity of the sample were 31.7%, 62.8%, 1.71 g/cm³, 47.1 ± 3.7 MPa and 1.73 W/m K, respectively. XRD analysis indicated that a small quantity of silicon carbide and graphite in SPRM have been oxidized to SiO₂ during the firing process, resulting in rising the porous microstructures. SEM micrographs illustrated that rod-like mullite grains combined with plate-like corundum grains to endow the samples with high bending strength. This study was intended to confirm the preparation of porous alumina ceramics with high porosity, good mechanical properties and low thermal conductivity by using SPRM as pore forming additive.     

Control of pore channel size during freeze casting of porous YSZ ceramics with unidirectionally aligned channels using different freezing temperatures

Porous yttria-stabilized zirconia (YSZ) ceramics with unidirectionally aligned pore channels were prepared by freezing YSZ/tert-butyl alcohol (TBA) slurry under different freezing temperatures of ?30, ?78 and ?196 °C, respectively. After removing the frozen TBA via freeze-drying in vacuum at ?50 °C, the green samples were sintered at 1450 °C for 2 h in air. The results showed that the freezing temperature significantly influenced microstructure and properties of the porous YSZ ceramics. Both microstructure observation and pore size distribution indicated that the pore channel size decreased significantly with decreasing freezing temperature, regardless of microstructure variations in the individual sample. Both porosity and room-temperature thermal conductivity of the porous YSZ ceramics varied under different freezing temperatures. Regardless of microstructure variations in the samples under different freezing temperatures, all samples had unidirectional pore channels with increasing pore channel size along the freezing direction. The fabricated samples had remarkably low thermal conductivities both in directions perpendicular and parallel to the channel direction, thus rendering them suitable for applications in thermal insulations.     

Low cost foam-gelcasting preparation and characterization of porous magnesium aluminate spinel (MgAl2O4) ceramics

Porous MgAl₂O₄ ceramics were prepared via a low cost foam-gelcasting route using MgAl₂O₄ powders as the main raw material, ammonium polyacrylate as a dispersant, a small amount of modified carboxymethyl cellulose as a gelling agent, and TH-IV polymer as a foaming agent. The effects of additive's content, solid loading and gelling temperature on slurry's rheological behavior were investigated, and microstructures and properties of as-prepared porous MgAl₂O₄ ceramics examined. Based on the results, the roles played by the foaming agent in the cases of porosity, pore structure, pore size, mechanical properties and thermal conductivity were clarified. Porosity and pore sizes of as-prepared porous MgAl₂O₄ ceramics increased with increasing the foaming agent from 0.05 to 0.6 vol%. Porous MgAl₂O₄ ceramics with porosity of 75.1% and average pore size of 266 µm exhibited a compressive strength as high as 12.5±0.8 MPa and thermal conductivity as low as 0.24 W/(m K) (at 473 K).     

Preparation of calcium hexaluminate porous ceramics by gel-casting method with polymethyl methacrylate as pore-forming agent

Calcium hexaluminate (CA₆) porous ceramics were prepared by gel-casting method, with α-Al₂O₃ and CaCO₃ as raw materials and polymethyl methacrylate (PMMA) microspheres as pore-forming agent. The effects of the amount of pore-forming agent PMMA microspheres on the phase composition, bulk density, apparent porosity, flexural strength, microstructure, thermal shock stability and thermal conductivity of CA₆ porous ceramics were systematically studied. The pores of CA₆ porous ceramics are mainly formed by the burning loss of PMMA microspheres and the decomposition of organic matter. Adding an appropriate amount of PMMA microspheres as pore-forming agent has a positive effect on the thermal shock stability of CA₆ porous ceramics. When the amount of pore-forming agent is 15 wt%, the volume density of CA₆ porous ceramics is 1.33 g/cm³, the porosity is 63%, the flexural strength is 13.9 MPa, the thermal shock times can reach 9 times, and the thermal conductivity is 0.293 W/(m·K), which can meet the application in refractory, ceramics or high temperature cement industries.     

Preparation of porous Y2SiO5 ceramics with relatively high compressive strength and ultra-low thermal conductivity by a TBA-based gel-casting method

Porous Y₂SiO₅ ceramics with relative high compressive strength (as high as 24.45 MPa) and ultra-low thermal conductivity (～0.08 W/m K) were successfully fabricated by a tert-butyl alcohol based gel-casting method. The formation mechanism of the 3D interconnected pores and the properties of the green body are discussed. The porosity, pore size, compressive strength and thermal conductivity could be controlled by varying the initial solid loading and the sintering temperature. When regulating the initial solid loading (from 20 to 50 wt%) and sintering temperature (from 1200 to 1500 °C), the porosity can be controlled between 47.74% and 73.93%, and the compressive strength and the thermal conductivity of porous Y₂SiO₅ ceramics varied from 3.34 to 24.45 MPa and from 0.08 to 0.55 W/m K, respectively. It should be noted that the porous Y₂SiO₅ ceramics with 30 wt% solid loading and sintering at 1400 °C had an open porosity of 61.80%, a pore size of 2.24 μm, a low room-temperature thermal conductivity of 0.17 W/m K and a relatively high compressive strength of 13.91 MPa, which make this porous Y₂SiO₅ ceramics suitable for applications in high-temperature thermal insulators.     

Porous anorthite ceramics with ultra-low thermal conductivity

Porous anorthite ceramics with an ultra-low thermal conductivity of 0.018 W/m K have been fabricated by hydrous foam-gelcasting process and pressureless sintering method using γ-alumina, calcium carbonate and silica powders as raw materials. Microstructure and phase composition were analyzed by SEM and XRD respectively. Properties such as porosity, pore size distribution and thermal conductivity were measured. High porosity (69–91%) and low thermal conductivity (0.018–0.13 W/m K) were obtained after sintering samples with different catalyst additions at 1300–1450 °C. Porosity, pore size, pore structure and grain size had obvious effect on heat conduction, resulting in the low thermal conductivity. The experimental thermal conductivity data of porous anorthite ceramics were found to be fit well with the computed values derived from a universal model.     

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.     

Preparation and properties of mullite-bonded porous fibrous mullite ceramics by an epoxy resin gel-casting process

Porous fibrous mullite ceramics with a narrow range of pore size distribution have been successfully prepared utilizing a near net-shape epoxy resin gel-casting process by using mullite fibers, Al₂O₃ and SiC as raw materials. The effects of sintering temperatures, different amounts of fibers and Y₂O₃ additive on the phase compositions, linear shrinkage, apparent porosity, bulk density, microstructure, compressive strength and thermal conductivity were investigated. The results indicated that mullite-bonded among fibers were formed in the porous fibrous mullite ceramics with a bird nest pore structure. After determining the sintering temperatures and the amount of fibers, the tailored porous fibrous mullite ceramics had a low linear shrinkage (1.36–3.08%), a high apparent porosity (61.1–71.7%), a relatively high compressive strength (4.4–7.6 MPa), a low thermal conductivity (0.378–0.467 W/m K) and a narrow range of pore size distribution (around 5 µm). The excellent properties will enable the porous ceramics as a promising candidate for the applications of hot gas filters, thermal insulation materials at high temperatures.     

Effect of starch addition on microstructure and properties of highly porous alumina ceramics

Porous alumina ceramics with ultra-high porosity were prepared through combining the gel-casting process with the pore-forming agent technique. Porosity and pore size distribution of the sintered bulks were evaluated with and without adding starch, respectively. In particular, the influences of starch addition on the properties, including thermal conductivity and compressive strength were studied. It was found that the incorporation of starch increased the nominal solid loading in the suspension and subsequently promoted the particle packing efficiency. The porosity is raised with increasing starch content from 0 to 30 vol%, which brings the decrease in thermal conductivity, whereas the compressive strength isn't seriously degraded. The further higher starch addition (40 vol%), however, would deteriorate the performance of the alumina porous ceramics. It is believed that the appropriate starch amount (lower than 30 vol%), working as a pore-forming agent, suppresses the driving force of densification without affecting the connections of neighboring grains while excessive starch amount would lead to the collapse of the porous structure.     

Preparation and characterization of porous mullite ceramics via foam-gelcasting

Porous mullite ceramics were prepared via foam-gelcasting using industrial grade mullite powder as the main raw materials, Isobam-104 as the dispersing and gelling agent, sodium carboxymethyl cellulose as the foam stabilizing agent, and triethanolamine lauryl sulfate as the foaming agent. The effects of processing parameters such as type and amount of additive, solid loading level and gelling temperature on rheological properties and gelling behaviors of the slurries were investigated. The green samples after drying at 100 °C for 24 h were fired at 1600 °C for 2 h, and the microstructures and properties of the resultant porous ceramic samples were characterized. Based on the results, the effects of foaming agent on the porosity level, pore structure and size and mechanical properties of the as-prepared porous mullite ceramics were examined. Porosity levels and pore sizes of the as-prepared samples increased with increasing the foaming agent content up to 1.0%, above which both porosity levels and pore sizes did not change. The compressive strength and flexural strength of the as-prepared sample with porosity of 76% and average pore size of 313 μm remained as high as 15.3±0.3 MPa and 3.7±0.2 MPa, respectively, and permeability increased exponentially with increasing the porosity.     

A ceramic–carbon hybrid as a high-temperature structural monolith and reinforcing filler and binder for carbon/carbon composites

A new ceramic–carbon nanostructured hybrid (86 vol.% ceramics, 14 vol.% carbon) formed from organoclay during pyrolysis is reported. It functions as a reinforcing filler and a binder for carbon/carbon (C/C) composites. Alone, it can also serve as a high-temperature structural monolith. During pyrolysis, the ordered montmorillonite clay (d₀₀₁ 31.5 Å) is transformed to mullite, cristobalite and disordered clay, allowing the clay part of the organoclay to serve as both binder and reinforcement. The organic part serves as a binder. Thus, a unidirectional C/C composite (50 vol.% fibers, 33 vol.% carbon matrix, 5 vol.% hybrid and 12% porosity) exhibiting flexural strength 290 MPa, modulus 55 GPa and toughness 2.9 MPa is obtained by 1000 °C 21-MPa hot-press pyrolysis in the presence of mesophase pitch powder, which serves as an additional binder, without densification after the pyrolysis. With the hybrid incorporation, the fiber content decreases from 53 to 50 vol.%, but the flexural strength and modulus are increased by 46% and 14% respectively, relative to the composite without the hybrid but with densification. Hot pressing the organoclay alone forms a black monolithic sheet with high thermal stability, electrical resistivity 6 × 10⁶ Ω cm, flexural strength 180 MPa, modulus 69 GPa, but low ductility.     

Low thermal conductivity of atomic layer deposition yttria-stabilized zirconia (YSZ) thin films for thermal insulation applications

Thermal insulation applications have long required materials with low thermal conductivity, and one example is yttria (Y₂O₃)-stabilized zirconia (ZrO₂) (YSZ) as thermal barrier coatings used in gas turbine engines. Although porosity has been a route to the low thermal conductivity of YSZ coatings, nonporous and conformal coating of YSZ thin films with low thermal conductivity may find a great impact on various thermal insulation applications in nanostructured materials and nanoscale devices. Here, we report on measurements of the thermal conductivity of atomic layer deposition-grown, nonporous YSZ thin films of thickness down to 35 nm using time-domain thermoreflectance. We find that the measured thermal conductivities are 1.35–1.5 W m⁻¹ K⁻¹ and do not strongly vary with film thickness. Without any reduction in thermal conductivity associated with porosity, the conductivities we report approach the minimum, amorphous limit, 1.25 W m⁻¹ K⁻¹, predicted by the minimum thermal conductivity model.     

Processing of porous yttria-stabilized zirconia tapes: Influence of starch content and sintering temperature

The tape-casting process was used to produce porous yttria-stabilized zirconia (YSZ) substrates with volume fractions of porosity ranging from 28.9 to 53 vol.% by using starch as a fugitive additive. Concentrated aqueous YSZ slips with different amounts of starch and an acrylic latex binder were prepared. The influence of the volume fraction of starch and sintering temperature on the sintering behavior and final microstructure were investigated. The microstructure consisted of large pores created by the starch particles with lengths between 15 and 80 μm and smaller pores in the matrix with lengths between 0.6 and 3.8 μm. The pores in the matrix reduced the sinterability of the YSZ leading to the retention of closed porosity in the sintered tapes. The porosities were above those predicted for each of the starch contents. However, larger deviations from the predicted porosity were found as more starch was added. The open to total porosity ratio in the sintered tapes could be controlled by the volume fraction of added starch as well as by the sintering temperature. As the volume fraction of starch increased from 17.6 to 37.8 vol.% there was a gradual increase in the interconnectivity of the pore structure. The sintering shrinkage of the tapes at a given temperature could be directly related to the YSZ packing density in the matrix.     

Porosity control of porous silicon carbide ceramics

Porous SiC ceramics were fabricated by the carbothermal reduction of polysiloxane-derived SiOC containing polymer microbeads followed by sintering. The effect of the SiC powder:polysiloxane-derived SiC (SiC:PDSiC) ratio on the porosity and flexural strength of the porous SiC ceramics were investigated. The porosity generally increased with decreasing SiC:PDSiC ratio when sintered at the same temperature. It was possible to control the porosity of porous SiC ceramics within a range of 32–64% by adjusting the sintering temperature and SiC:PDSiC ratio while keeping the sacrificial template content to 50 vol%.The flexural strengths generally decreased with increasing porosity at the same SiC:PDSiC ratio. However, a SiC:PDSiC ratio of 9:1 and a sintering temperature of 1750 °C resulted in excellent strength of 57 MPa at 50% porosity. Judicious selection of the sintering temperature and SiC:PDSiC ratio is an efficient way of controlling the porosity and strength of porous SiC ceramics.     

Thermal conductivity of porous alumina ceramics prepared using starch as a pore-forming agent

The thermal conductivity of porous alumina ceramics prepared using different types of starch (potato, wheat, corn, and rice starch) as pore-forming agents is investigated from room temperature up to 500 °C. The temperature dependence measured for alumina ceramics of different porosity (in the range 6–47%) is fitted with second-order polynomials and 1/T-type relations, and compared to available literature data for dense alumina. It is found that the porosity dependence of the relative thermal conductivity k_r = k/k₀ is well described by a modified exponential relation of the form k_r = exp(?1.5?/(1 ? ?)), where ? is the porosity. This finding is in agreement with other literature data and seems to indicate a common feature of all porous materials with microstructures resulting from fugitive pore-forming agents.     

Recycling of harmful waste lead-zinc mine tailings and fly ash for preparation of inorganic porous ceramics

The porous ceramics were prepared by directly sintering of lead-zinc mine tailings and fly ash as the raw materials without any additional sintering and foaming agent. The effects of fly ash addition on the crystalline phases, pore structure, physical–chemical porosities and mechanical strength were investigated. The results showed that the bulk density decreased firstly and then increased while the porosity and water absorption presented the opposite tendency with the increase of fly ash content. Meanwhile, the chemical stability improved and the flexural strength had the same variation tendency of the bulk density. The phase evolution of sample with 60 wt% fly ash addition indicated that anorthite phase was formed at low temperature (1000 °C). The thermal behavior illustrated that the foaming process was initiated by the reaction of internal constituents in the lead-zinc mine tailings. Different pore structures indicated different foaming mechanisms that probably occurred at different temperatures. The porous ceramics with 60 wt% fly ash addition exhibited excellent properties, including bulk density of 0.93 g/cm³, porosity of 65.6%, and flexural strength of 11.9 MPa.     

Porosity analysis and oxidation behavior of plasma sprayed YSZ and YSZ/LaPO4 abradable thermal barrier coatings

In this research, the high temperature oxidation behavior, porosity, and microstructure of four abradable thermal barrier coatings (ATBCs) consisting of micro- and nanostructured YSZ, YSZ-10%LaPO₄, and YSZ-20%LaPO₄ coatings produced by atmospheric (APS) method were evaluated. Results show that the volume percentage of porosity in the coatings containing LaPO₄ was higher than the monolithic YSZ sample. It was probably due to less thermal conductivity of LaPO₄ phases. Furthermore, the results showed that the amount of the remaining porosity in the composite coatings was higher than the monolithic YSZ at 1000 °C for 120 h. After 120 h isothermal oxidation, the thickness of thermally growth oxide (TGO) layer in composite coatings was higher than that of YSZ coating due to higher porosity and sintering resistance of composite coatings. Finally, the isothermal oxidation resistance of conventional YSZ and nanostructured YSZ coating was investigated.     

Preparation and properties of bimodal porous apatite ceramics through slip casting using different hydroxyapatite powders

A bimodal porous hydroxyapatite (HAp) body with high flexural strength was prepared through slip casting. The effect of different particle sizes on the flexural strength and microstructure of three different types of hydroxyapatite (HAp) powders was studied. The powder characteristic of laboratory-synthesized HAp powder (L-HAp) was obtained through a wet-milling method, drying and heating of a mixture of calcium hydrogen phosphate di-hydrate and calcium carbonate. The median particle size of L-HAp was 0.34 μm, and the specific surface area was 38.01 m²/g. The commercial HAp had median particle sizes for the K-HAp (Kishida chemical Co. Ltd., K-HAp) and T-HAp (Taihei chemical Co. Ltd., T-HAp) of 1.13 and 3.65 μm, and specific surface areas of 11.62 and 6.23 m²/g, respectively. The different powder characteristics affected the slip characteristics, and the flexural strength and microstructure of the sintered porous HAp bodies were also different. The flexural strengths of the porous HAp ceramics prepared by heating at 1200 °C for 3 h in air were 17.59 MPa for L-HAp with a porosity of 60.48%, 3.92 MPa for commercial K-HAp with a porosity of 79.37%, and 4.55 MPa for commercial T-HAp with a porosity of 76.46%.