Porous Yttria-stabilized Zirconia Ceramics Research Articles

Microstructure and permeability of porous YSZ ceramics with aligned and gradient pore channels fabricated by freeze casting

Porous yttria-stabilized zirconia (YSZ) ceramics with aligned and gradient pore channels were fabricated by the unidirectional freezing of aqueous suspensions with distinct solid loadings under different sintering temperatures. The results indicate that the microstructure and response properties could be tailored by adjusting the solid loading and sintering temperature. The pore channel size increased with the increase of solid loading or the decrease of sintering temperature, regardless of microstructure changes in an individual sample. With increasing the solid loading and sintering temperature, the compressive strength (7.18–30.76 MPa) increased owing to the lower porosity of individual samples. The fabricated samples had increasing Darcian and non-Darcian constants along the freezing direction due to the enhanced pore size and open porosity. The Darcian and non-Darcian constants in this paper were in the range of 7.06 × 10−13 to 5.92 × 10−12 m2 and 1.66 × 10−7 to 8.83 × 10−7 m, respectively. These results are of practical value for the fabrication and optimization of porous ceramics in the applications of filters and membrane supports.

Correction to: Porous yttria-stabilized zirconia ceramics with low thermal conductivity via a novel foam-gelcasting method

Correction to: Porous yttria-stabilized zirconia ceramics with low thermal conductivity via a novel foam-gelcasting method

Porous yttria-stabilized zirconia ceramics with low thermal conductivity via a novel foam-gelcasting method

In this work, the preparation of foam-gelcasting was simplified with a copolymer of isobutylene and maleic anhydride (Isobam), avoiding the traditional gelling system including many additives such as monomer, cross-linking agent, initiator and catalyst. Highly porous yttria-stabilized zirconia (YSZ) ceramics with 76.2–84.9% porosity were prepared via foam-gelcasting method using YSZ powder as the raw material, Isobam as the dispersant and gelling agent, triethanolamine lauryl sulfate as the foaming agent and sodium carboxymethyl cellulose as the foam stabilizing agent. The effects of solid loading, the content of foaming agent as well as sintering temperature were investigated. With the increase in solid loading of slurries, lower porosity was obtained, and the compressive strength and thermal conductivity showed a tendency of increase. Porous YSZ ceramics with compressive strength of 0.69–3.66 MPa showed low thermal conductivity in the range of 0.137–0.254 W/(m·K) at room temperature. The as-prepared sample showed good thermal insulating performance at high temperature. The km in GM model was during 0.4–0.7, which indicated the pore structure containing the internal and external porosities. This work provides a facile method for the fabrication of porous YSZ ceramics which can be potentially applied as thermal insulation materials at high temperature.

Microstructure and permeability of porous YSZ ceramics fabricated by freeze casting of oil-in-water suspension

Porous yttria-stabilized zirconia (YSZ) ceramics are fabricated through freeze casting of oil-in-water suspension followed by sintering at 1250−1550 °C. The pore structure, compressive strength and permeability of porous YSZ ceramics are tailored via altering the emulsion content and sintering temperature. The samples obtained using higher emulsion content or at lower sintering temperature show larger Darcian and non-Darcian constants due to their higher open porosity and larger pore size. Furthermore, the investigation on individual contributions of viscous and inertial resistances on the total pressure drop during permeation process indicates that the viscous resistance increases but the inertial resistance decreases with increasing the emulsion content or decreasing the sintering temperature for samples. Porous YSZ ceramics obtained in this work with a k1 range of 3.14 × 10−13–1.12 × 10−12 m2 are appropriate for applications in filters and membrane supports.

Design, Fabrication, and Characterization of Porous Yttria‐Stabilized Zirconia Ceramics for Hot Gas Filtration Applications

Hot gas filtration is a lucrative target for a number of high temperature industrial operations, but new materials must be developed to withstand intense high temperature conditions. Here, the authors put the emphasis on porous yttria‐stabilized zirconia (YSZ) ceramics for hot gas filtration, owing to their high thermodynamic stability and toughness. The authors design and fabricate porous YSZ ceramics using a pore‐forming inclusion technique, where a polymeric pore former is mixed and compacted with YSZ powder. After burning off, the polymer leaves behind a controlled amount of porosity before sintering commenced. The authors systematically investigate the sintering temperature, polymer particle size, and polymer‐to‐ceramic ratio to optimize the open porosity, pore size, gas permeability, and hardness of porous YSZ ceramics. The authors then evaluate the performance of porous YSZ ceramics under industrial conditions by cyclic heating/cooling testing and high temperature exposure testing. The authors show, for the first time, porous YSZ ceramics can perform in hot gas filtration applications at 1400 °C for extended thermal cycling or at 1700 °C for limited periods of time.

Preparation of Porous Yttria-Stabilized Zirconia Ceramics By Thermal Removal of Potassium Iodide

Near to full skeletal density ZrO2:8 mol% Y2O3 polycrystalline ceramics with different pore content were prepared by mixing different amounts of KI and thermally removing them during sintering at 1400oC. That removal of the alkali halide was evaluated by thermogravimetric and dilatometric measurements from room temperature to 1500oC. The sintered pellets were analyzed by X-ray diffraction and X-ray fluorescence (XRF) for structural phase identification and KI content, respectively. Images of the polished and thermally etched surfaces were obtained by scanning probe microscopy (SPM) and scanning electron microscopy (SEM) for evaluation of average pore content and distribution. Electrochemical impedance spectroscopy measurements were carried out in the 300-450oC range in the 5 Hz-10 MHz frequency range. The thermal removal of KI starts at its melting point, ~ 680oC and ends at ~ 900oC. No traces of KI are found by XRF analysis in sintered pellets. The final linear shrinkage value is higher for compositions without KI and the temperature of maximum shrinkage rate increases for increasing KI content. The total oxide ion electrical resistivity, extracted from the impedance plots, also increases for increasing KI content. The SPM images observed at polished and thermally etched surfaces of sintered pellets show that the larger is the amount of KI addition, the larger is the average pore size and the smaller is the average grain size. The effect of KI addition before sintering leads, upon sintering, dense porous ceramics with high skeletal density. Moreover, grain growth is inhibited. The impedance spectroscopy results allowed for the evaluation of the bulk and the grain boundary conductivities. The increase in both contributions with the increase in KI addition was correlated to the observation by scanning probe microscopy of the increase of intragranular as well as of intergranular porosity. Other parameters determined after the deconvolution of the impedance plots, namely, the blocking factor, the frequency factor and the descentralization angle showed that the impedance spectroscopy technique is an important tool for the characterization of porous ionic conducting solids.

YSZ fiber-reinforced porous YSZ ceramics with lowered thermal conductivity: Influence of the sintering temperature

Yttria-stabilized zirconia (YSZ) fiber was introduced to reinforce porous YSZ ceramics fabricated by tert-butyl alcohol (TBA)-based gel-casting process and pressureless sintering. The influence of sintering temperature on microstructure and properties was investigated systematically. Results showed that linear shrinkage of the green bodies during sintering increased and porosity decreased with increasing sintering temperature. Interfacial bonding can be adjusted easily through different sintering temperatures. Compressive strength of the porous YSZ ceramics was improved significantly with the introduction of fibers and the strengthening effect reached its optimum at the sintering temperature of 1500°C. Compared with the specimens without fiber addition, the compressive strength of fiber-reinforced specimens was almost doubled and the highest reached 45.0MPa with the porosity of 58.5%. In addition, the thermal conductivity was decreased with the introduction of fibers. The improved mechanical and thermal properties make porous YSZ ceramics more applicable as thermal insulating materials at high temperature.

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

Abstract A porous yttria-stabilized zirconia (YSZ) ceramic supported single cell with a configuration of porous YSZ support layer coated with Ni/Ni–Ce0.8Sm0.2O1.9 (SDC) anode/YSZ/SDC bi-layer electrolyte/La0.6Sr0.4Co0.2Fe0.8O3−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.

Improved Heat Insulation and Mechanical Properties of Highly Porous YSZ Ceramics After Silica Aerogels Impregnation

To further improve heat insulation and mechanical properties, silica aerogels were impregnated into highly porous yttria‐stabilized zirconia (YSZ) ceramics with well‐distributed pores fabricated by tert‐butyl alcohol ‐based gel‐casting process and pressureless sintering. Pore size distribution, room‐temperature thermal conductivities, and compressive strength of the YSZ ceramics before and after impregnation with silica aerogels were examined and compared, respectively. After impregnating porous YSZ ceramics with silica aerogels, the porosity displayed a little decrease, whereas the pore size significantly decreased by one order of magnitude. Based on this microstructure development, the room‐temperature thermal conductivities were significantly lowered and the compressive strength was also promoted. Therefore, the heat insulation and mechanical properties could be simultaneously improved by impregnating porous ceramics with silica aerogels.

Effects of mono-dispersed PMMA micro-balls as pore-forming agent on the properties of porous YSZ ceramics

Abstract Porous yttria-stabilized zirconia (YSZ) ceramics were successfully fabricated by the dry pressing method with different size (1.8–20 μm) and amount (2–60 vol.%) of mono-dispersed poly methyl methacrylate (PMMA) micro-balls. Different PMMA additions with different size and amount were investigated to achieve optimal thermal and mechanical properties. With increases of the amount of PMMA, the porosity of porous YSZ ceramics ranges from 7.29% to 51.6%, the flexural strength increases firstly and then decreases, and the thermal conductivity decreases continuously. With decreases of the diameter of PMMA micro-balls, the mean pore size and thermal conductivity of porous YSZ ceramics decrease, and the flexural strength of porous YSZ ceramics with same porosity increases firstly and then decreases. The porous YSZ ceramics with a higher porosity (18.44 ± 1.24%), the highest flexural strength (106.88 ± 3.2179 MPa) and low thermal conductivity (1.105 ± 0.15 W/m K) can be obtained when the particle diameter and the amount of PMMA are 5 μm and 20 vol.%, respectively.

High-Strength Porous YSZ Ceramics Fabricated by TBA-Based Gelcasting with High Solid Loading Slurry

Porous yttria-stabilized zirconia (YSZ) ceramics were fabricated by tert-butyl alcohol (TBA)-based gelcasting method using high solid loading slurry. Micron pores were generated by using low-cost carbon powder as the pore-forming agent. The TBA-based slurries of 50% solid loading appropriate for gelcasting were successfully prepared by using commercial dispersant BYK-163. The rheological behavior of the slurries were measured and discussed. The porosity, microstructure, compressive strength and thermal conductivity of the porous ceramics were tested and analyzed. The results show that this low-cost technology is significant for fabricating porous ceramics with both high compressive strength and low thermal conductivity.

Effect of YSZ fiber addition on microstructure and properties of porous YSZ ceramics

Yttria-stabilized zirconia (YSZ) fiber was introduced as the reinforcement for porous YSZ ceramics fabricated by tert-butyl alcohol-based gel-casting process and pressureless sintering. Effect of YSZ fiber addition on the microstructure and properties of porous YSZ ceramics was studied systematically. Results showed that YSZ fiber obviously obstructed densification during the sintering process and therefore higher porosity could be achieved with the same solid loading of the initial slurry. Mean pore size regularly increased with increasing fiber addition. The reinforcing effect reached its optimum with 10 wt% YSZ fiber addition, yet decreased with increasing porosity. Fiber addition significantly changed the fracture mode of the porous ceramics from brittle to quasi-ductile with increasing fiber additions; fiber pull-out and crack deflection play major roles in the process. Compared with the porous ceramics without fibers, the thermal conductivity decreased a little. With improved mechanical and thermal properties, YSZ fiber-reinforced porous YSZ ceramics are more applicable in thermal insulation materials.

A novel way to fabricate highly porous fibrous YSZ ceramics with improved thermal and mechanical properties

Abstract Inspired by nest structure, highly porous fibrous yttria-stabilized zirconia (YSZ) ceramics were fabricated through tert-butyl alcohol (TBA)-based gel-casting process and pressureless sintering by using YSZ fibers as raw material and adding K 2 SO 4 as removable sintering aid. Different sintering temperature and soaking time were investigated to achieve optimal thermal and mechanical properties. The results show that all specimens consist of crystallized t-YSZ phase. Fibers interconnect with good interfacial bonding on junctions. Under higher sintering temperature, porosity drops gradually while compressive strength increases significantly. With prolonged soaking time, there is no obvious change in porosity and compressive strength increases gradually. All specimens have uniformly distributed pores with average size of 30.2 μm and show good structural stability at high temperature. Ultra-low thermal conductivity is achieved and ductile fracture mode with high elongation makes it more applicable in high-temperature thermal insulating applications.

Porous YSZ ceramics with unidirectionally aligned pore channel structure: Lowering thermal conductivity by silica aerogels impregnation

The previous report of this work has demonstrated the fabrication and properties of porous yttria-stabilized zirconia (YSZ) ceramics with unidirectionally aligned pore channels. As a follow-up study, the present work aims at lowering the thermal conductivity of the porous YSZ ceramics by silica aerogels impregnation. The porous YSZ ceramics were immersed in an about-to-gel silica sol. Both the unidirectionally aligned pore channels and the inter-grain pores by grain stacking in the channel-pore wall of the porous YSZ ceramics were impregnated with the silica sol. After aging and supercritical drying, silica aerogels formed in the macroporous network of the porous YSZ ceramics with unidirectionally aligned pore channels. The influences of silica aerogel impregnation on the microstructure and properties of porous YSZ ceramics with unidirectional aligned pore channels were investigated. The porosity decreased after impregnation with silica aerogels. Both microstructure observation and pore size distribution indicated that both channel-pore size and inter-grain pore-size decreased significantly after impregnation with silica aerogels. Impregnating porous YSZ ceramics with silica aerogels remarkably lowered the room-temperature thermal conductivity and enhanced the compressive strength. The as-fabricated materials are thus suitable for applications in bulk thermal isolators.

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.

Porous yttria-stabilized zirconia ceramics with ultra-low thermal conductivity. Part II: temperature dependence of thermophysical properties

This study describes the experimental results of thermal diffusivity, specific heat at constant pressure, and thermal conductivity of porous 8 mol% yttria-stabilized zirconia (YSZ) ceramics in a temperature range from room temperature to 1,400 °C. It is a follow-up study of the earlier report titled by “Porous YSZ ceramics with ultra-low thermal conductivity”, which focused on the room-temperature thermal conductivity. The thermal diffusivity of porous YSZ ceramics decreased with the increase of the measurement temperature up to 600–1,000 °C, followed by an increasing trend with increasing temperature. The specific heat did not exhibit any significant dependence on sintering temperature and agreed with literature data. The thermal conductivity of the porous YSZ ceramics showed an insensitive tendency of change with measurement temperature. The thermal conductivity fell in groups by the sintering temperature level. This investigation also discussed an appropriate sintering temperature of porous YSZ ceramics, which had both low thermal conductivity and high strength required by the practical service.

Effect of sintering temperature on compressive strength of porous yttria-stabilized zirconia ceramics

Porous yttria-stabilized zirconia (YSZ) ceramics were fabricated by tert-butyl alcohol (TBA)-based gel-casting method for potential applications in heat-insulation materials. The effect of sintering temperature on compressive strength of porous YSZ ceramics was investigated on the basis of measurements linear shrinkage, porosity and pore size. As the sintering temperature increased from 1350 to 1550 °C, a decrease of porosity from 77 to 65%, a decrease of average pore size from and an increase of linear shrinkage from 15.4 to 31.8% were observed. The compressive strength increased remarkably from 3 to 27 MPa with increasing sintering temperature from 1350 to 1550 °C, which was related to the corresponding change of linear shrinkage, porosity, pore size and microstructure. A remarkable decrease of compressive strength with increasing porosity was observed. The compressive strength decreased also with increasing pore size.