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.     

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     

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.     

Interpenetrating Al2O3-TiAl3 alloys produced by reactive infiltration

Dense alumina-TiAl₃ composites with interpenetrating networks have been fabricated by reactive gas-pressure infiltration and squeeze casting of Al into sintered porous preforms containing 30 vol% TiO₂ and 70 vol% Al₂O₃. Strength of up to 543±21 MPa with corresponding fracture toughness of 8·6±0·4 MPa√m and hardness of H_V10=565±27 have been obtained. The present paper discusses processing parameters such as particle size of oxide precursor and preform porosity which control microstructural development and mechanical properties of the composites.     

Polymer template fabrication of porous hydroxyapatite scaffolds with interconnected spherical pores

Porous hydroxyapatite (HA) scaffolds with interconnected spherical pores were fabricated by slip casting using a polymer template. Templates were produced using polymer beads, NaCl, and adhesive (N100). Effects of the preparation process on the pore structures and mechanical properties of the porous HA scaffolds were investigated. Pore interconnectivity was improved by adding NaCl particles with appropriate diameters to the polymer template. The size of the adhesive area could be controlled by adjusting the concentration of N100. The pore size could be controlled between 200 ± 42 and 400 ± 81 μm, and the porosity between 50.2 and 73.1%, by changing the size of the polymer beads and the volume of the NaCl particles. The compressive strength decreased as the porosity or pore size increased.     

Calcium phosphate ceramics with continuously gradient macrochannels using three-dimensional extrusion of bilayered ceramic-camphene mixture/pure camphene feedrod

We herein demonstrate a novel, versatile approach to produce calcium phosphate (CaP) ceramics with continuously gradient macrochannels using three-dimensional extrusion of a bilayered ceramic-camphene mixture/pure camphene feedrod. In this technique, the pure camphene used as the upper part could be preferentially extruded because of the wall slip phenomenon. This enabled the formation of green filaments comprised of a camphene core surrounded by a ceramic/camphene shell, where the core/shell thickness ratio increased gradually as extrusion proceeded. CaP ceramics with continuously gradient macrochannels could be successfully produced by three-dimensionally depositing the extruded filaments layer-by-layer. With increasing the distance from the dense bottom layer, macrochannels created after the removal of the camphene cores via freeze-drying became larger, while the CaP walls became thinner. The local porosity could increase gradually and continuously from the dense bottom and reach up to ~72 vol%.     

Structure and mechanical properties of porous silicon oxycarbide ceramics derived from silicone resin with different filler content

Porous silicon oxycarbide ceramics were obtained through pyrolysis of a new silicone resin filled with its pyrolyzed SiOC powders via a simple self-blowing process. The effects of filler content on the porosity, compressive strength and microstructure of the porous ceramics were investigated. The porosity (total and open) increased firstly and then decreased with the filler content increasing. It was possible to control the total and open porosity of porous ceramics within a range of 66.1–88.2% and 42.7–72.5% respectively, by adjusting the filler content from 0 vol% to 30 vol% while keeping the heating rate fixed at 0.5 °C/min. The compressive strength decreased firstly and then increased with the increasing filler content, and the average compressive strength of the porous ceramics was in the range of 1.1–3.4 MPa. Micrographs indicated that the porous ceramics with the filler content less than 20 vol% had a well-defined open-cell and regular pore structure.     

Processing and properties of low cost macroporous alumina ceramics with tailored porosity and pore size fabricated using rice husk and sucrose

The present study demonstrates a cost effective way to fabricate porous ceramics with tailored microstructures using rice husk (RH) of various range of particle sizes as a pore former and sucrose as a binder as well as a pore former. Sample microstructures reveal randomly oriented elongated coarse pores and fine pores (avg. size 4 μm) created due to burnout of RH and sucrose, respectively. Porous alumina ceramics with 20–66 vol% porosity and 50–516 μm avg. pore size (length), having isolated and/or interconnected pores, were fabricated using this process. Mechanical properties of the porous samples were determined as a function of porosity and pore microstructure. The obtained porous ceramics exhibited flexural strength of 207.6–22.3 MPa, compressive strength of 180–9.18 MPa, elastic modulus of 250–18 GPa and hardness of 149–18 HRD. Suggested application area includes thermal, filtration, gas purging etc.     

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

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.     

Effects of solid content on the phase assemblages,mechanical and dielectric properties of porous α-SiAlON ceramics fabricated by freeze casting

Porous Y-α-SiAlON ceramics were prepared by freezing camphene-based suspensions at 0 °C and subsequently sintering at 1900 °C for 1 h. The effects of solid loading content in the suspensions on porosities and formation of α-SiAlON as well as mechanical and dielectric properties of the porous ceramics were investigated. An XRD analysis performed on sintered samples indicated that the α-SiAlON did not fully form in the sample with initial solid loading content of 10 vol%, due to the high porosity of 90 vol% and interconnected pore of the green body. With the increase of initial solid loading content from 10 vol% to 30 vol%, the porosity decreased from 62.3% to 23.1% and the average pore size decreased from 19 μm to 8 μm. As a result, the flexural strength increased significantly from 72.4 MPa to 190.2 MPa, fracture toughness increased from 1.20 MPa m^1/2 to 3.48 MPa m^1/2, as well as the dielectric constant increased from 3.3 to 6.3. The dielectric loss (tan δ) of obtained material varied between 1.4×10^?2 and 2.8×10^?2, which did not depend on the porosity of samples.     

Processing and properties of cordierite-silica bonded porous SiC ceramics

Cordierite-silica bonded porous SiC ceramics were fabricated by infiltrating a porous powder compact of SiC with cordierite sol followed by sintering at 1300–1400 °C in air. The porosity, average pore diameter and flexural strength of the ceramics varied 30–36 vol%, ~ 4–22 µm and ~ 13–38 MPa respectively with variation of sintering temperature and SiC particle sizes. In the final ceramics SiC particles were bonded by the oxidation-derived SiO₂ and sol-gel derived cordierite. The corrosion behaviour of sintered SiC ceramics was studied in acidic and alkaline medium. The porous SiC ceramics were observed to exhibit better corrosion resistance in acid solution.     

Graphene oxide/hydroxyapatite composite coatings fabricated by electrophoretic nanotechnology for biological applications

Graphene oxide (GO) was firstly employed as nanoscale reinforcement fillers in hydroxyapatite (HA) coatings by a cathodic electrophoretic deposition process, and GO/HA coatings were fabricated on pure Ti substrate. The transmission electron microscopy observation and particle size analysis of the suspensions indicated that HA nanoparticles were uniformly decorated on GO sheets, forming a large GO/HA particle group. The addition of GO into HA coatings could reduce the surface cracks and increase the coating adhesion strength from 1.55 ± 0.39 MPa (pure HA) to 2.75 ± 0.38 MPa (2 wt.% GO/HA) and 3.3 ± 0.25 MPa (5 wt.% GO/HA), respectively. Potentiodynamic polarization and electrochemical impedance spectroscopy studies indicated that the GO/HA composite coatings exhibited higher corrosion resistance in comparison with pure HA coatings in simulated body fluid. In addition, superior (around 95% cell viability for 2 wt.% GO/HA) or comparable (80–90% cell viability for 5 wt.% GO/HA) in vitro biocompatibility were observed in comparison with HA coated and uncoated Ti substrate.     

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.     

Preparation and characterization of porous calcium-phosphate microspheres

Porous calcium-phosphate bioceramics are very important materials in bone tissue engineering. Recently, microsphere systems have been widely utilized in the treatment of defective tissues, including bone, cartilage and muscle. In this study, porous calcium-phosphate microspheres were prepared from calcium-deficient hydroxyapatite (d-HA) powders through a water-in-oil emulsion technique using camphene as the porogen and subsequently sintered at 700, 1100, 1200, or 1400 °C for 6 h. The microspheres produced in this study were characterized according to their morphology, properties, and biodegradation. The results indicated an interconnected porous structure with pore sizes ranging between several microns to as large as 250 μm. Approximately 35–50% of the pores were larger than 100 μm. In the microspheres sintered at 700 °C (Sample H), only the hydroxyapatite (HA) phase was present; when heated to 1100 °C (Sample BH), β-TCP was observed with HA; at 1200 °C (Sample ABH), the phase compositions included β-TCP and α-TCP, as well as a small quantity of HA; and at 1400 °C (Sample AH), the phases of samples included mainly α-TCP and HA. The degradation of the scaffolds was evaluated after immersion in distilled water for up to 28 days. Obvious dissolution and precipitation behavior was seen in the samples ABH and AH. The precipitates formed on the surface of ABH and AH could be carbonate-containing calcium-deficient HA (carbonated-CDHA) after immersion in distilled water for 28 days.     

Novel Three‐Dimensional Extrusion of Multilayered Ceramic/Camphene Mixture for Gradient Porous Ceramics

This study proposes an innovative way of creating porous ceramics with a unique gradient porous structure using three‐dimensional extrusion of a multilayered ceramic/camphene feed rod, denoted as “3D‐Ex_m”. This 3D‐Ex_m technique utilizes the wall slip phenomenon during the extrusion process, which can create a gradient core/shell structure with a gradual change in the core/shell thickness ratio. In addition, the microstructure of ceramic filaments can be tuned through the use of the camphene as a pore‐forming agent. Porous alumina ceramics produced using a bilayered feed rod comprised of the alumina/camphene mixtures with the relatively high (?_H = 40 vol%) and low ceramic contents (?_L = 10 vol%) showed a gradual change in porosity in the intermediate region between the relatively dense (porosity = ~3 vol%) and highly porous regions (porosity = ~85 vol%).     

Influence of sodium on the properties of sol-gel derived hydroxyapatite powder and porous scaffolds

This study investigates the properties of sol-gel derived sodium (Na)-doped hydroxyapatite (HA) powder. Different amounts of Na (1, 5, 10 and 15 mol%) were prepared and the sintered bodies were characterized to determine the current phases, microstructural evolution and mechanical properties. X-ray diffraction analysis reveals that a phase pure HA of crystallite sizes, which varied from 35 nm to 65 nm, was obtained in the synthesized powder after calcining from 500 °C to 1000 °C. Scanning electron microscopy examination shows evidence of larger particle sizes, particularly in samples that contain higher amounts of Na concentration. The resultant powders were subsequently used to prepare porous NA-doped HA bodies through a polymeric sponge method. The addition of 5% Na resulted in a porous body with 27% porosity and was beneficial in enhancing the compressive strength of HA 17-fold compared with undoped HA. The prepared scaffold also shows suitable pore interconnectivity with pore sizes that vary between 100 and 300 µm which is suitable for use as porous bone substitutes.     

Mechanical properties of porous ceramics in compression: On the transition between elastic,brittle, and cellular behavior

This paper deals with the uniaxial compression behavior of porous ceramics within a wide range of porosity, varying from 30 to 75 vol%. The load–displacement curves recorded on porous alumina samples showed a transition between a typical brittle behavior at porosity fractions below 60 vol% and a damageable, cellular-like behavior, at higher porosity fractions. This transition in fracture mode was confirmed by in situ compression tests in an X-ray tomograph. Based on a simple model taking into account the competition between the crack length initiating from spherical pores and the mean distance between pores, the porosity at which the transition took place was estimated. The influence of the pore size also depended on the volume fraction of pores: no size effect was noted at the lowest porosity whereas a statistical effect on the size of the solid walls was observed at higher porosity, with an increase in fracture strength with small pores.     

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.     

Fabrication of porous silicon carbide ceramics with high porosity and high strength

Unique porous SiC ceramics with a honeycomb structure were fabricated by a sintering-decarburization process. In this new process, first a SiC ceramic bonded carbon (SiC/CBC) is sintered in vacuum by spark plasma sintering, and then carbon particles in SiC/CBC are volatized by heating in air at 1000 °C without shrinkage. The honeycomb structure has at least two different sizes of pores; ∼20 μm in size resulting from carbon removal; and smaller open pores of 2.1 μm remaining in the sintered SiC shell. The total porosity is around 70% and the bulk density is 0.93 mg/m³. The bending and compressive strengths are 26 MPa, and 105 MPa, respectively.     

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.