Pressureless sintering of SiC ceramics with improved specific stiffness

The effects of B₄C content on the specific stiffness and mechanical and thermal properties of pressureless-sintered SiC ceramics were investigated. SiC ceramics containing 2.5 wt% C and 0.7–20 wt% B₄C as sintering aids could be sintered to ≥ 99.4% of the theoretical density at 2150 °C for 1 h in Ar. The specific stiffness of SiC ceramics increased from 136.1 × 10⁶ to 144.4 × 10⁶ m²‧s⁻² when the B₄C content was increased from 0.7 to 20 wt%. The flexural strength and fracture toughness of the SiC ceramics were maximal with the incorporation of 10 wt% B₄C (558 MPa and 3.69 MPa‧m^1/2, respectively), while the thermal conductivity decreased from ∼154 to ∼83 W‧m⁻¹‧K⁻¹ when the B₄C content was increased from 0.7 to 30 wt%. The flexural strength and thermal conductivity of the developed SiC ceramic containing 20 wt% B₄C were ∼346 MPa and ∼105 W‧m⁻¹‧K⁻¹, respectively.     

Processing and properties of glass-bonded silicon carbide membrane supports

Porous SiC membrane supports were fabricated from SiC and glass frit at a temperature as low as 850 °C in air by a simple pressing and heat-treatment process. The effects of the initial SiC particle size and frit content on the porosity, flexural strength, and air permeation of the membrane supports were investigated. During heat-treatment, the glass frit transformed to a viscous glass phase, which acted as a bonding material between SiC particles and as a protecting layer for severe oxidation of SiC particles. The porosity of the porous SiC membrane supports could be controlled within a range of 37–46% with the present set of processing conditions. The typical flexural strength, permeability, and specific air flow rate of the porous membrane supports fabricated using 23 μm SiC particles with 15 wt% glass frit were 75 MPa, 4.2 × 10⁻¹³ m², and 32.4 L/min/cm², respectively.     

Mechanical properties of silicon carbide—in situ zirconium carbonitride composites

SiC–Zr₂CN composites were fabricated by conventional hot pressing from β-SiC and ZrN powders with 2 vol% equimolar Y₂O₃–Sc₂O₃ as a sintering additive. The effects of the ZrN addition on the room-temperature (RT) mechanical properties and high-temperature flexural strength of the SiC–Zr₂CN composites were investigated. The fracture toughness gradually increased from 4.2 ± 0.3 MPa·m^1/2 for monolithic SiC to 6.3 ± 0.2 MPa·m^1/2 for a SiC–20 vol% ZrN composite, whereas the RT flexural strength (546 ± 32 MPa for the monolithic SiC) reached its maximum of 644 ± 87 MPa for the SiC–10 vol% ZrN composite. The monolithic SiC had improved strength at 1200°C, whereas the SiC–Zr₂CN composites could not retain their RT strengths at 1200°C. The typical flexural strength values of the SiC–0, 10, and 20 vol% ZrN composites at 1200°C were 650 ± 53, 448 ± 31, and 386 ± 19 MPa, whereas their RT strength values were 546 ± 32, 644 ± 87, and 528 ± 117 MPa, respectively.     

Consolidation and high-temperature strength of monolithic lanthanum hexaboride

In this study we explored the densification, microstructure evolution, and high-temperature properties of bulk lanthanum hexaboride. LaB₆ bulks were consolidated using spark-plasma sintering only in the temperature range between 1400°C and 1700°C. We adopted flash spark plasma sintering (SPS) of LaB₆ using a direct current heating without a graphite die. We observed a peculiar grain-size gradient when coarse grains exceeding 300 μm were observed on the top side of the specimen, while the bottom side had a grain size of 15–20 μm. Such large grain was not observed using SPS at 2000°C, suggesting that these might originate from a local overheating. Based on the three-point flexural tests, it was observed that the toughness and strength of the LaB₆ were acceptable at room-temperature (3.1 ± 0.2 MPa m^1/2, 300 ± 20 MPa). However, at 1600°C, these parameters would decrease to 1.3 ± 0.1 MPa m^1/2 and 120 ± 40 MPa, respectively.     

Direct bonding of silicon carbide ceramics sintered with yttria

SiC ceramics sintered with yttria were successfully joined without an interlayer by conventional hot pressing at lower temperatures (2000–2050 °C) compared to those of the sintering temperatures (2050–2200 °C). The joined SiC ceramics sintered with 2000 ppm Y₂O₃ showed almost the same thermal conductivity (˜198 Wm⁻¹ K⁻¹), fracture toughness (3.7 ± 0.2 MPa m^1/2), and hardness (23.4 ± 0.8 GPa) as those of the base material, as well as excellent flexural strength (449 MPa). In contrast, the joined SiC ceramics sintered with 4 wt% Y₂O₃ showed very high thermal conductivity (˜204 Wm⁻¹ K⁻¹) and excellent flexural strength (˜505 MPa). Approximately 16–22% decreases in strength compared to those of the base SC materials were observed in both joined ceramics, due to the segregation of liquid phase at the interface. This issue might be overcome by preparing well-polished and highly flat surfaces before joining.     

Reaction-bonded robust SiC ceramic membranes sintered at low temperature with coal gangue

Herein, we report an in-situ reaction-bonded SiC membrane sintered at low temperature using a solid waste (i.e. coal gangue) as the sintering aid to form strong neck connections. The effects of sintering temperature and coal gangue proportion on their properties regarding pore size, open porosity, bending strength and pure water permeability were investigated. The single-channel tubular SiC membrane sintered at 1300 °C with a coal gangue proportion of 12 wt% was optimal, exhibiting an average pore size of 2.78 μm, a open porosity of 47.08%, a bending strength of 34.01 ± 1.3 MPa and a high water permeability of 83967 L m⁻² h⁻¹ bar⁻¹. The membrane could completely reject D₅₀ = 0.87 μm SiC solids and presented a steady-state water permeability of 458 L m⁻² h⁻¹·bar⁻¹. The SiC membrane could be regenerated through ultrasonication and its steady-state water permeability was almost unchanged for 3 cycles, proving its mechanical robustness. This work may appeal to the practical low-cost production of high-performance SiC membranes.     

Synthesis and thermophysical properties of ATa2O6 (A = Co,Ni, Mg,Ca) tantalates with robust CMAS resistance

A new family of ceramic environmental/thermal barrier coating (E/TBC) materials, that is, ATa₂O₆ (A = Co, Ni, Mg, Ca), for high-temperature applications, are investigated and reported in this study. We focus on the synthesis and features of crystal structures, and on the mechanical and high-temperature properties. ATa₂O₆ oxides have an extraordinary phase stability (up to 1300°C), and their thermal expansion coefficients (6.2–7.3 × 10⁻⁶ K⁻¹) match those of SiC fiber-enhanced SiC ceramic matrix composites (3–7 × 10⁻⁶ K⁻¹). Their low thermal conductivities (min: 1.15 W·m⁻¹·K⁻¹) root in the slow phonon spreading speed and fierce phonon-phonon scattering process, and they will provide exceptional thermal insulation. Moreover, their hardness (5.6–8.8 GPa), toughness (1.4–1.9 MPa·m^1/2), and moduli (100–210 GPa) have good comparability with current E/TBCs. We propose the 33CaO-9MgO-13AlO_1.5-45SiO₂ (CMAS) corrosion mechanisms of ATa₂O₆ ceramics, and their robust CMAS resistance relies on the phase stability of CaTa₂O₆ oxides. The excellent high-temperature properties ensure that ATa₂O₆ can be used as E/TBCs to provide thermal insulation and CMAS corrosion protection.     

Ultra-high creep resistant SiC ceramics prepared by rapid hot pressing

The high-temperature compression creep of additive-free β/α silicon carbide ceramics fabricated by rapid hot pressing (RHP) was investigated. The creep tests were accomplished in vacuum at temperature range 1500 °C–1750 °C and compressive loads of 200 MPa to 400 MPa. Under investigated condition the RHP ceramics possessed the lowest creep rate reported in the literature. The observed strain rates changed from 2.5 × 10^?9 s^?1 at 1500 °C and a lowest load of 275 MPa to 1.05 × 10^?7 s^?1 at 1750 °C and a highest load of 400 MPa. The average creep activation energy and the stress exponent remain essentially constant along the whole range of investigated parameters and were 315 ± 20 kJ?mol^?1, and 2.22 ± 0.17, respectively. The suggested creep mechanism involves GB sliding accommodated by GB diffusion and β?α SiC phase transformation.     

Mechanical and thermal properties of silicon carbide ceramics with yttria–scandia–magnesia

Two different SiC ceramics with a new additive composition (1.87 wt% Y₂O₃–Sc₂O₃–MgO) were developed as matrix materials for fully ceramic microencapsulated fuels. The mechanical and thermal properties of the newly developed SiC ceramics with the new additive system were investigated. Powder mixtures prepared from the additives were sintered at 1850 °C under an applied pressure of 30 MPa for 2 h in an argon or nitrogen atmosphere. We observed that both samples could be sintered to ≥99.9% of the theoretical density. The SiC ceramic sintered in argon exhibited higher toughness and thermal conductivity and lower flexural strength than the sample sintered in nitrogen. The flexural strength, fracture toughness, Vickers hardness, and thermal conductivity values of the SiC ceramics sintered in nitrogen were 1077 ± 46 MPa, 4.3 ± 0.3 MPa·m^1/2, 25.4 ± 1.2 GPa, and 99 Wm⁻¹ K⁻¹ at room temperature, respectively.     

Thermal and electrical properties of additive-free rapidly hot-pressed SiC ceramics

The thermal and electrical properties of newly developed additive free SiC ceramics processed at a temperature as low as 1850 °C (RHP0) and SiC ceramics with 0.79 vol.% Y₂O₃-Sc₂O₃ additives (RHP79) were investigated and compared with those of the chemically vapor-deposited SiC (CVD-SiC) reference material. The additive free RHP0 showed a very high thermal conductivity, as high as 164 Wm⁻¹ K⁻¹, and a low electrical resistivity of 1.2 × 10⁻¹ Ω cm at room temperature (RT), which are the highest thermal conductivity and the lowest electrical resistivity yet seen in sintered SiC ceramics processed at ≤1900 °C. The thermal conductivity and electrical resistivity values of RHP79 were 117 Wm⁻¹ K⁻¹ and 9.5 × 10⁻² Ω cm, respectively. The thermal and electrical conductivities of CVD-SiC parallel to the direction of growth were ∼324 Wm⁻¹ K⁻¹ and ∼5 × 10⁻⁴Ω⁻¹ cm⁻¹ at RT, respectively.     

Permeability behavior of silicon carbide-based membrane and performance study for oily wastewater treatment

Porous SiC ceramic is considered as a suitable material for hot gas filtration, microfiltration, and many others industrial applications. However, full utilizations of porous SiC ceramics have been limited by high-processing costs. In this study, mullite-bonded porous SiC ceramics membranes were prepared using commercial SiC powder, alumina, clay, and different sacrificial pore formers. The effect of different pore formers on the microstructure, mechanical strength, porosity and pore size distribution, air, and water permeability of porous SiC ceramics were investigated. The average pore diameter, porosities, and flexural strength of the final ceramics varied in the range 3.7-6.5 µm, 38-50 vol. %, and 28-38 MPa, respectively, depending on the characteristics of pore former. The Darcian (k₁) and non-Darcian (k₂) permeability evaluated from air permeation behavior at room temperature was found to vary from 1.48 × 10⁻¹³ to 4.64 × 10⁻¹³ m² and 1.46 × 10⁻⁸ to 6.51 × 10⁻⁸ m, respectively. All membranes showed high oil rejection rate (89%-93%) from feed wastewater with oil concentration of 1557 mg/L. The membrane with porosity ~48 vol% and mechanical strength 31.5 MPa showed and highest pure water permeability of 13 298 Lm⁻²h⁻¹bar⁻¹.     

Lanthanum-doped kaolinite for hierarchical bi-modal porous inorganic membrane

Porous inorganic membranes can function in corrosive or high-temperature environments for prolonged duration without significant degradation in performance. Their high fabrication and material cost, however, outweigh their advantages. Here, adding lanthanum (La) to kaolinite, a cheaper inorganic material, enables a stronger and more permeable membrane sintered at 1150 °C; A 11 wt% La-doped kaolinite displays strength of ∼10 MPa and Darcy’s permeability of 1.5 × 10⁻¹³ m²–2.5 and 3 times higher than the pristine sintered kaolinite matrix respectively. The La-doping technique disrupts the randomly oriented stack of kaolinite flakes and increases the particle-particle contacts during sintering. Consequently, “pot-holes” are created to give a bi-modal porous structure, which reduces the tortuosity and improves the pore connectivity. It is concluded that La interacts with Al of the kaolinite during sintering based on XRD and EDX analysis. Finally, a carbon nanotube network (CNTN) is incorporated into the porous channel of La-doped membrane and is used as a membrane to purify emulsified oily water with a 100% rejection.     

Multicomponent synergistically affected mechanical properties,microstructure, and oxidation resistance of Zr–Al(Si)–C based composites

Herein, in-situ Zr₃[Al(Si)]₄C₆-based composites with 10–40 vol% ZrB₂–SiC (2-to-1 molar ratio) were prepared by hot-pressing sintering at 1850 °C. The simultaneously incorporated ZrB₂–SiC constitute multicomponent reinforcements and has a synergistic effect on the matrix, which improves the sinterability, mechanical properties, and oxidation resistance of materials. It is found that both of the toughness and strength increase first and then decrease with the increasing content of ZrB₂–SiC, while the hardness increases near linearly. Zr₃[Al(Si)]₄C₆–ZrB₂–SiC shows high strength (623 MPa), toughness (7.59 MPa m^1/2), and hardness (18.6 GPa), which can be ascribed to the synergistic mechanisms of the binary ZrB₂–SiC including fine-grained strengthening, particle reinforcement, intragranular microstructure, grain's pull-out and crack bridging, etc. In addition, the oxidation kinetics of as-prepared materials follow the parabolic law, and the composite shows a low oxidation rate of 0.87 × 10⁻⁵ kg² m⁻⁴ s⁻¹ when oxidized at 1400 °C.     

Influence of sintering atmosphere and BN additives on microstructure and properties of porous SiC ceramics

The effects of the boron nitride (BN) content on the electrical, thermal, and mechanical properties of porous SiC ceramics were investigated in N₂ and Ar atmospheres. The electrical resistivity was predominantly controlled by the sintering atmosphere and secondarily by the BN concentration, whereas the thermal conductivity and flexural strength were more susceptible to changes in the porosity and necking area between the SiC grains. The electrical resistivities of argon-sintered porous SiC ceramics (6.3 × 10⁵ – 1.6 × 10⁶ Ω·cm) were seven orders of magnitude higher than those of nitrogen-sintered porous SiC ceramics (1.5 × 10⁻¹ – 6.0 × 10⁻¹ Ω·cm). The thermal conductivity and flexural strength of the argon-sintered porous SiC ceramics increased from 8.4–11.6 W·m⁻¹ K⁻¹ and from 9.3–28.2 MPa, respectively, with an increase in the BN content from 0 to 1.5 vol%, which was attributed to the increase in necking area and the decrease in porosity.     

Influence of different reactor types on Nannochloropsis oculata microalgae culture for lipids and fatty acid production

Greenhouse gases emitted into the atmosphere by burning of fossil fuels cause global warming. One option is obtaining biodiesel. Nannochloropsis oculata was cultured under different light intensities and reactors at 25°C for 21 days with f/2 medium to assess their effects on cell density, lipid, and fatty acids (FAs). N. oculata improved cell density on fed-batch glass tubular reactor (7 L) at 200 μmol E m⁻² s⁻¹, yielding 3.5 × 10⁸ cells ml⁻¹, followed by fed-batch Erlenmeyer flask (1 L) at 650 μmol E m⁻² s⁻¹ with 1.7 × 10⁸ cells ml⁻¹. The highest total lipid contents (% g lipid × g dry biomass⁻¹) were 44.4 ± 0.8% for the reactor (1 L) at 650 μmol E m⁻² s⁻¹ and 35.2 ± 0.2% for the tubular reactor (7 L) at 200 μmol E m⁻² s⁻¹, until twice as high compared with the control culture (Erlenmeyer flask 1 L, 80 μmol E m⁻² s⁻¹) with 21.2 ± 1%. Comparing the total lipid content at 200 μmol E m⁻² s⁻¹, tubular reactor (7 L) and reactor 1 L achieved 35.2 ± 0.2% and 28.3 ± 1%, respectively, indicating the effect of shape reactor. The FAs were affected by high light intensity, decreasing SFAs to 2.5%, and increased monounsaturated fatty acids + polyunsaturated fatty acids to 2.5%. PUFAs (20:5n-3) and (20:4n-3) were affected by reactor shape, decreasing by half in the tubular reactor. In the best culture, fed-batch tubular reactor (7 L) at 200 μmol E m⁻² s⁻¹ contains major FAs (16:0; 38.06 ± 0.16%), (16:1n-7; 30.74 ± 0.58%), and (18:1n-9; 17.15 ± 0.91%).     

Preparation,microstructure and ion-irradiation damage behavior of Al4SiC4-added SiC ceramics

Single-phase Al₄SiC₄ powder with a low neutron absorption cross section was synthesized and mixed with SiC powder to fabricate highly densified SiC ceramics by hot pressing. The densification of SiC ceramics was greatly improved by the decomposition of Al₄SiC₄ and the formation of aluminosilicate liquid phase during the sintering process. The resulting SiC ceramics were composed of fine equiaxed grains with an average grain size of 2.0 μm and exhibited excellent mechanical properties in terms of a high flexure strength of 593 ± 55 MPa and a fracture toughness of 6.9 ± 0.2 MPa m^1/2. Furthermore, the ion-irradiation damage in SiC ceramics was investigated by irradiating with 1.2 MeV Si⁵⁺ ions at 650 °C using a fluence of 1.1 × 10¹⁶ ions/cm², which corresponds to 6.3 displacements per atom (dpa). The evolution of the microstructure was investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The breaking of Si–C bonds and the segregation of C elements on the irradiated surface was revealed by XPS, whereas the formation of Si–Si and C–C homonuclear bonds within the Si–C network of SiC grains was detected by Raman spectroscopy.     

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.     

Fabrication and properties of macro-porous SiC using Al2O3–Y2O3–SiO2 as bonding additives

Macro-porous SiC was fabricated without using pore-forming agents by an in situ reaction bonding process. A bonding additive, Al₂O₃–Y₂O₃–SiO₂, with a low melting temperature was mixed with SiC particles and sintered at 1500 °C and 1600 °C for 1 h in Ar. Macro-porous SiC with a porosity of 32.7–45.9%, a pore size of 3.4–4.2 μm, and a relatively narrow and uniform pore size distribution was fabricated by varying the amount of bonding additive. The flexural strength of macro-porous SiC prepared at 1500 °C increased from 47.2 MPa to 71.2 MPa while the porosity decreased from 45.9% to 42.8%, respectively. When the sintering temperature of the macro-porous SiC was increased to 1600 °C, the flexural strengths were significantly reduced to 32.6–35.6 MPa, along with a reduction in porosity and pore size. The permeability of macro-porous SiC prepared at 1500 °C varied from 1.59 × 10^?12 m² to 1.26 × 10^?12 m², depending on the porosity. As the sintering temperature increased from 1500 °C to 1600 °C, the permeability decreased to less than 1.00 × 10^?12 m² because of the reduced porosity and average pore size. The electrical resistivity of macro-porous SiC prepared at 1500 °C and 1600 °C varied from 2.7 × 10⁸ Ω-cm to 1.4 × 10⁹ Ω-cm and from 1.3 × 10⁸ Ω-cm to 1.7 × 10⁹ Ω-cm, respectively, with increasing volume percent of bonding additives. The relatively high electrical resistivity was apparently due to neck bonding phase between SiC particles formed by phases consisting of Y₂Si₂O₇, YAG, and residual Al₂O₃.     

Preparation of ZnO-glass composite hollow fiber and its application in formation of ZIF-8 membrane

The sintering temperature of ZnO ceramic hollow fibers (HFs) is generally up to 1400°C and presents a major challenge to obtain HFs with high permeability and mechanical strength at lower sintering temperature. This work proposed a glass powder-assisted method to reduce the sintering temperature by using their adhesive property. ZnO-glass composite HFs with longer finger-like channels, high permeability (3.12 × 10⁻⁵–9.1 × 10⁻⁶ mol·m⁻²·s⁻¹·Pa⁻¹) and good mechanical strength (42.12–52.75 MPa) were obtained at sintering temperature of 1150°C. More glass powders can generate stronger bonding effect during the ZnO particles, resulting in a decrease in porosity and an increase in the mechanical strength of ZnO-HF. These ZnO-HFs were further applied for inducing ZIF-8 membranes by one-step solvothermal growth. ZnO not only provides the growth and nucleation centers but also acts as transitional bridge to make the ZIF embed into support to improve the bonding force between membrane and support. Therefore, HF-supported-ZIF-8 membrane exhibited both mechanical and thermal robustness by maintaining their gas separation performance during the 30-min sonication treatment and 50-h operation testing at 25–200°C. Furthermore, this membrane provided good reproducibility. This work opens prospects for preparing ceramic HFs at lower sintering temperature and their functional applications as well as the preparation of MOF membranes.     

Nanowire-decorated SiC foam from tissue paper and silicon powder by filter-pressing

A facile and scalable method for the preparation of low-density SiC foam from tissue paper and silicon powder is reported. The co-dispersion of pulp-containing tissue paper micro-ribbons and silicon powder is coagulated by adjusting the pH to 3.5. The silicon particles adhere to the tissue paper micro-ribbons during the coagulation. The coagulated co-dispersion on filter-pressing consolidates the silicon particle-decorated tissue paper micro-ribbons and orients them perpendicular to the filter-pressing direction. The filter-pressed body on drying followed by heat treatment at 1600°C in inert atmosphere produces SiC foam. The tissue paper micro-ribbons retain their morphology during carbonization as well as high-temperature reaction with the silicon. The enormous growth of carbon-rich SiC nanowires is observed on the SiC micro-ribbons. The random orientation of SiC micro-ribbons in the X-Y plane with the hairy nanowires on the surface and their stacking in the Z-direction produces a porosity of ~94 vol.% with pore sizes in the range of 0.08 to 20 µm. The SiC foam shows a compressive strength and Young's modulus of 0.22 and 5.5 MPa, respectively. The thermal conductivity decreases from 0.11 to 0.07 W m⁻¹ K⁻¹ when temperature increases from 25°C to 350°C.