Low-temperature preparation of Si3N4/SiC porous ceramics via foam-gelcasting and microwave-assisted catalytic nitridation

Si₃N₄/SiC porous ceramics were fabricated by a novel foam-gelcasting and microwave-assisted catalytic nitridation method at a temperature as low as 1273?K for 60?min or after only 10?min at 1373?K utilizing commercial Si and SiC with trace of impurity Fe (0.33?wt%) as starting materials. The Si₃N₄/SiC porous ceramics containing porosity of 68.54?±?0.73% which were fabricated at 1373?K for 10?min had flexural and compressive strengths of 5.28?±?0.17?MPa and 12.86?±?1.55?MPa.     

One-pot foam-gelcasting/nitridation synthesis of high porosity nano-whiskers based 3D Si3N4 porous ceramics

Nano-whiskers based 3D Si₃N₄ porous ceramics (3D-NWSNPC) with high-porosity (about 91–93 %), low density (0.17–0.25 g/cm³), low thermal conductivity, and a certain degree of recoverability under cyclic compressive loading and reasonably strengths were prepared at 1423–1523 K via a one-pot foam-gelcasting/nitridation route using inexpensive commercial Si powders as starting materials and hexadecyl trimethyl ammonium bromide as foaming agent. After nitridation at 1523 K, the sample with an original solid loading of 12.5 wt% exhibited the highest compressive strength of 1.9 MPa, even though its density was lowered to 0.25 g/cm³. The sample nitrided at 1473 K had a relative density of 7.3 %. Its compressive and specific strength were respectively 1.1 MPa and 5.5 MPa·cm³ g^?1, and its thermal conductivity was as low as 0.074 W/(m K) (measured at 323 K). These outstanding properties would make the as-prepared 3D-NWSNPC a promising candidate for applications in catalysis, filtration, thermal insulation and many other important areas.     

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.     

Preparation of Si3N4 porous ceramics via foam-gelcasting and microwave-nitridation method

Si₃N₄ porous ceramics with improved mechanical strength were fabricated for the first time by a combined foam-gelcasting and microwave-nitridation method at 1273–1373?K. The Si₃N₄ porous samples prepared at 1373?K/20?min with the porosity of 68.9% had respectively flexural and compressive strength as high as 8.1 and 20.8?MPa, which values were comparable or even superior to those of Si₃N₄ porous ceramics prepared previously by the conventional heating technique at a much higher temperature of 1773–1973?K, indicating that present preparation strategy is feasible to prepare high quality Si₃N₄ porous ceramic at a much milder condition. Moreover, the thermal conductivity of as-prepared Si₃N₄ porous ceramics at 1073?K was as low as 1.697?W/(m?K), suggesting it could be a potentially good heat insulating material for aluminum electrolyte cells.     

Preparation of Si3N4-BCxN-TiN composite ceramic aerogels via foam-gelcasting

Recently, high-performance ceramic aerogels (CAs) have become a research hotspot because of their multi-functional properties. Compared to their oxide counterparts, non-oxide CAs show better thermal shock resistance and other high-temperature properties, making them suitable for applications in harsh environments. In this work, Si₃N₄-BC_xN-TiN ceramic aerogels (SBCNT CAs) with molar ratios of B/C/Si/Ti = 2:1:5:5 were synthesized at 1623 K for 3 h via foam-gelcasting, from Si, Ti, melamine and boron acid raw materials. The prepared SBCNT CAs containing 91.0% porosity exhibited the highest compressive strength up to 1.0 MPa, and thermal conductivity of 0.144 W/(m·K) at 298 K. They also exhibited excellent service performance at 1473 K in Ar, demonstrating their great potential for high-temperature applications.     

Effects of firing temperature on the microstructures and properties of porous mullite ceramics prepared by foam-gelcasting

Porous mullite ceramics were prepared at 1300–1600°C for 2?h via a foam-gelcasting route using industrial-grade mullite powders as the main raw material, Isobam 104 as the dispersing and gelling agent, triethanolamine lauryl sulphate as the foaming agent and sodium carboxymethyl cellulose as the foam stabilising agent. The effects of firing temperature on the sintering behaviour of green samples as well as microstructures and properties of final porous mullite products were investigated. With increasing the temperature from 1300 to 1600°C, linear shrinkage and bulk density values of fired samples increased, whereas their porosity decreased. Mechanical strength and thermal conductivity values of fired samples decreased with increasing their porosities. Even at a porosity level as high as 79.4%, compressive and flexural strengths of fired samples (with average pore size of 314?μm) remained as high as 9.0 and 3.7?MPa, respectively, and their thermal conductivity (at 200°C) remained as low as 0.21?W?(m^?1?K^?1).     

Synthesis of Si3N4 whiskers by rapid nitridation of silicon droplets

Belt-like β-Si₃N whiskers were successfully synthesized by nitriding of liquid silicon without catalysts at 1500°C by using micron-sized silicon powders within 10 minutes. Silicon droplets formed by the melting of silicon particles greatly facilitates the diffusion of nitrogen. Several whiskers cling together to form a whisker-cluster. The whisker-clustermorphology results from nitriding of separate silicon droplets. The growth of the belt-like β-Si₃N₄ whisker was controlled by vapor-liquid-solid mechanism. The synthesis of silicon nitride whiskers can be effectively improved by nitriding liquid phase silicon.     

Foam gel-casting preparation of SiC bonded ZrB2 porous ceramics for high-performance thermal insulation

Lightweight SiC-ZrB₂ porous ceramics is of great potential as thermal insulation material used in aerospace, chemical and energy industries. In this work, a series of SiC bonded ZrB₂ (SiC_b-ZrB₂) porous ceramics with porosity high up to 86.9% were prepared by a simple foam gel-casting method. The SiC_b-ZrB₂ porous ceramic prepared at 1573 K exhibited a low thermal conductivity of 0.280 W/(m?K) and a reasonable compressive strength of 0.52 MPa. It could maintain the original geometric shape and microstructure after a secondary heat treatment at 1473 K in inert atmosphere. When heating the samples with thickness of 30 mm for 12 min with an alcohol spray lamp (～1273 K), the temperatures of the cold sides of SiC_b-ZrB₂ ceramics were all lower than 432 K, demonstrating their exceptional insulation capabilities. The present work provides a simple route to produce robust and thermally-insulating non-oxide porous ceramics for use under high temperature.     

Effects of N2 pressure and Si particle size on mechanical properties of porous Si3N4 ceramics prepared via SHS

Porous silicon nitride ceramics with high flexural strength and high porosity were directly fabricated by self-propagating high temperature synthesis (SHS). The effects of N₂ pressure and Si particle size on the phase composition, microstructure, and mechanical property were investigated. N₂ influences not only the thermodynamics but also the kinetics of the SHS as initial reactant. Flexural strength ranged between 67 MPa and 134 MPa with increasing N₂ pressure. On the other hand, flexural strength ranged from 213 MPa to 102 MPa with different Si particle sizes. This plays an important role on the final diameter and length of β-Si₃N₄ grains and the formation mechanism of porous Si₃N₄ ceramics.     

Synthesis of hierarchically porous mullite ceramics with improved thermal insulation via foam-gelcasting combined with pore former addition

ABSTRACT

To further improve the thermal insulation performance of porous mullite ceramics used in important industrial sectors, a combined foam-gelcasting and pore-former addition approach was investigated in this work, by which hierarchical porous mullite ceramics with excellent properties, in particular, thermal insulation property, were prepared. Both mesopores (2–50?nm) and macropores (117.8–202.7?μm) were formed in porous mullite ceramics resultant from 2?h firing at 1300°C with various amounts of submicron-sized CaCO₃ pore former. The former mainly arose from the decomposition of CaCO₃, and the latter from the foam-gelcasting process. The porous samples prepared with CaCO₃ addition had low linear shrinkage of 2.35–4.83%, high porosity of 72.98–79.07% and high compressive strength of 5.52–14.82?MPa. Most importantly, they also exhibited a very low thermal-conductivity, e.g. 0.114?W?m^?1?K^?1 at 200°C, which was much lower than in the cases of their counterparts prepared via the conventional foam-gelcasting route.     

Silicon nitridation mechanism in reaction‐bonded Si3N4–SiC and Si3N4‐bonded ferrosilicon nitride

Reaction‐bonded Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride, with Si powder, SiC particles and Fe₃Si–Si₃N₄ particles as raw materials, respectively, are prepared in flame‐isolation nitridation shuttle kiln with flowing N₂ at 1723K. There is columnar β‐Si₃N₄ in both Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride. However, fibrous α‐Si₃N₄ is only observed in Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride contains much more Si₂N₂O than Si₃N₄–SiC. By analyzing the oxidation thermodynamics of Si and Si₃N₄, it is known that in the process of producing Si₃N₄–SiC, Si is oxidized first to gaseous SiO and fibrous α‐Si₃N₄ is generated with SiO and N₂. The existence of SiO is the reason of low silicon nitridation rate. But in the process of producing Si₃N₄‐bonded ferrosilicon nitride, Si₃N₄ is easier to be oxidized than Si and Si₂N₂O is generated on the surface of Si₃N₄ hexagonal prisms in ferrosilicon nitride particles. Meanwhile, Si in raw materials forms new ferrosilicon alloys with Fe₃Si, which decreases the temperature of liquid appearance and blocks some open pores in the samples, which stops the matter loss of nitridation. Liquid ferrosilicon alloys favors β‐Si₃N₄ generation from Si direct nitridation and fibrous α‐Si₃N₄ transformation, which used to exist in ferrosilicon nitride raw materials.     

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.     

The effect of fabrication parameters on the mechanical properties of sintered reaction bonded porous Si3N4 ceramics

Porous silicon nitride ceramics were prepared via sintered reaction bonded silicon nitride at 1680 °C. The grain size of nitrided Si₃N₄ and diameter of post-sintered β-Si₃N₄ are controlled by size of raw Si. Porosity of 42.14–46.54% and flexural strength from 141 MPa to 165 MPa were obtained. During post-sintering with nano Y₂O₃ as sintering additive, nano Y₂O₃ can promote the formation of small β-Si₃N₄ nuclei, but the large amount of β-Si₃N₄ (>20%) after nitridation also works as nuclei site for precipitation, in consequence the growth of fine β-Si₃N₄ grains is restrained, the length is shortened, and the improvement on flexural strength is minimized. The effect of nano SiC on the refinement of the β-Si₃N₄ grains is notable because of the pinning effect, while the effect of nano C on the refinement of the β-Si₃N₄ grains is not remarkable due to the carbothermal reaction and increase in viscosity of the liquid phase.     

The preparation and properties of high-strength porous mullite ceramics by a novel non-toxic gelcasting process

A novel approach to fabricate porous mullite ceramics with homogeneous pore size and high-strength using green non-toxic and cost-effective poly-γ-glutamic acid (γ-PGA) gelling system was reported for the first time. Effect of γ-PGA addition, additive amount and solid loading on rheological behavior of the slurries, and microstructure and properties of samples were investigated systematically. By optimizing the solid loading of mullite samples, we are able to get the sample with small pores (< 200 µm) dominating (93.3% of the total pores), and compressive strength of the sample reaches up to 26.62 MPa. In addition, the mullite ceramics exhibited high porosity of 75.7% with low thermal conductivity of 0.279 W/(m·K) at room temperature. This study not only provides a green and non-toxic gelling system but also offers porous mullite ceramics with low thermal conductivity and excellent mechanical strength as an energy-saving thermal insulation material.     

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).     

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.     

In-situ reaction synthesis of porous Si2N2O-Si3N4 multiphase ceramics with low dielectric constant via silica poly-hollow microspheres

In the present work, a novel and facile process has been proposed to fabricate porous Si₂N₂O-Si₃N₄ multiphase ceramics with low dielectric constant (ε_r＜4.0). Since silica poly-hollow microspheres could serve as the source of SiO₂ and the pore-forming agent, they have been introduced into Si₃N₄ slurry through the gelcasting technique. This process is benefited from the liquid phase sintering reaction between SiO₂ and Si₃N₄ with the aid of sintering additives, leading to in-situ synthesis of Si₂N₂O phase and porous structure. The content of silica poly-hollow microspheres has great influence on the properties of the final products. It indicates that Si₂N₂O phase would become the major phase when the content of silica poly-hollow microspheres was above 25 wt%. Furthermore, the micromorphology results reveal that the content of pores with many smaller aggregate microspheres increases as microspheres amount rises. As a result, along with the addition of silica poly-hollow microspheres, the bulk density decreases to 1.32±0.01 g/cm³, and open porosity ranges from 28.4±0.4% to 52.0±0.5%. Porous Si₂N₂O-Si₃N₄ multiphase ceramics prepared with 25 wt% silica poly-hollow microspheres addition possess flexural strength of 42.3±3.8 MPa, low dielectric constant of 3.31 and loss tangent of 1.93×10⁻³. It turns out to be an effective method to fabricate porous Si₂N₂O-Si₃N₄ composites with excellent mechanical and dielectric properties, which could be applied to radome materials.     

In situ synthesis of Si2N2O/Si3N4 composite ceramics using polysilyloxycarbodiimide precursors

In situ synthesis of Si₂N₂O/Si₃N₄ composite ceramics was conducted via thermolysis of novel polysilyloxycarbodiimide ([SiOSi(NCN)₃]_n) precursors between 1000 and 1500 °C in nitrogen atmosphere. The relative structures of Si₂N₂O/Si₃N₄ composite ceramics were explained by the structural evolution observed by electron energy-loss spectroscopy but also by Fourier transform infrared and ²⁹Si-NMR spectrometry. An amorphous single-phase Si₂N₂O ceramic with porous structure with pore size of 10–20 μm in diameter was obtained via a pyrolyzed process at 1000 °C. After heat-treatment at 1400 °C, a composite ceramic was obtained composed of 53.2 wt.% Si₂N₂O and 46.8 wt.% Si₃N₄ phases. The amount of Si₂N₂O phase in the composite ceramic decreased further after heat-treatment at 1500 °C and a crystalline product containing 12.8 wt.% Si₂N₂O and 87.2 wt.% Si₃N₄ phases was obtained. In addition, it is interesting that residual carbon in the ceramic composite nearly disappeared and no SiC phase was observed in the final Si₂N₂O/Si₃N₄ composite.     

Catalytic nitridation preparation of high-performance Si3N4(w)-SiC composite using Fe2O3 nano-particle catalyst: Experimental and DFT studies

The complete conversion from Si into Si₃N₄ was achieved after 2 h nitridation at 1400 °C by using in-situ formed Fe₂O₃ nano-particles (NPs) as a catalyst. Such a synthesis condition was remarkably milder than that (>1450 °C for many hours) required by the conventional Si nitridation method. Density functional theory (DFT) calculations suggest that Fe₂O₃ catalyst accelerates the Si nitridation via weakening the bond strength of absorbed N₂ molecule. Furthermore, Si₃N_4(w)-SiC composites prepared by the present catalytic nitridation method showed excellent high-temperature properties including modulus of rupture (MOR of 29.9 MPa at 1400 °C), thermal shock resistance (residual MOR percentage of 50% at ΔT = 1300 °C), as well as good oxidation resistance and cryolite corrosion resistance against molten cryolite. It can be concluded that, Fe₂O₃ NPs not only greatly accelerated the Si nitridation and Si₃N₄ formation, but also facilitated the epitaxial growth of reinforcement phase of Si₃N₄ whisker in the Si₃N_4(w)-SiC composites.