Search

Complex liquid–solid reactions occur during brazing. A deep understanding of brazing liquid evolution is therefore of important concern for the bonding between filler and substrates. The joining of gas‐pressure‐sintered silicon nitride (Si3N4) ceramic to oxygen‐free copper (OFC) foil was prepared using Ag–Cu–TiH2 filler at 875°C for 0.5 h. The effect of the Ag:Cu ratio on the interfacial microstructure and peeling strength of the joints was analyzed. The reaction of brazing liquid with Si3N4 ceramic includes reactions with both the β‐Si3N4 and the grain boundary phases (Y–Si–Mg–O–N). The resulting interfacial reaction products include TiN, Ti5Si3, and potential Cu3Ti3O. Based on the phase composition and three‐dimensional morphology of the interfacial reaction products in the Si3N4 ceramic/OFC foil joints, a model for the evolution of the brazing liquid as the reaction proceeds was proposed. Peeling test results indicated that the peeling strength of the joints rises unilaterally with the increase of the Ag:Cu ratio in the filler until reaching the peak value of 24.3 N/mm at the maximum Ag:Cu ratio of 90:10. The mechanisms of the joint peeling fracture were discussed in detail. [ABSTRACT FROM AUTHOR]

Solar-driven interfacial evaporation has emerged as a promising way for renewable energy-driven clean water production, due to its ability to concentrate heat on the evaporation surface and facilitate the evaporation process. Nevertheless, the weak interaction between solar-absorbing materials and porous support substrates has posed a significant challenge in achieving the necessary combination of light absorption, mechanical strength, and long-term stability required for practical solar-driven water evaporation. In this work, we prepared porous black ceramics with both high light absorption and strong structural support, exhibiting a remarkable light absorption of 94% in the solar spectrum. Afterward, inspired by the water transport and evaporation process of mushrooms, we developed an energy-efficient bionic system. The resulting system demonstrates an evaporation rate of 1.41 kg/m2/h under 1 sun illumination (1 kW/m2) and 25 °C, attributable to the heat localization effect. Furthermore, under natural sunlight with an average solar intensity of 0.44 kW/m2, the bionic system exhibits an evaporation rate of 0.7 kg/m2/h. This work not only developed a highly stable, regenerable photothermal material and an energy-efficient bionic system for practical solar-driven water evaporation, but also highlights the immense potential of porous ceramics in advancing the field of solar-driven interfacial evaporation. [ABSTRACT FROM AUTHOR]

Gas-pressure sintered silicon nitride (Si 3 N 4) ceramic was brazed to oxygen-free copper (OFC) foil using 20.22 ± 0.93 mg/cm2 of Ag-Cu-TiH 2 filler at 875 ◦C for 0.5 h. The effect of TiH 2 content on the 3D morphology, including the cross-section microstructure and etched surface morphology of Si 3 N 4 ceramic/OFC foil joints, was analyzed. An evolution model of the interfacial microstructure for joints was proposed based on 3D morphological analysis. The reaction between the brazing alloy and Si 3 N 4 ceramic formed the interfacial reaction layer and the inside reaction zones during brazing. The growth mechanism of the interfacial reaction layer was discussed in detail. The peeling test was used to evaluate the bonding strength of the Si 3 N 4 ceramic/OFC foil joints, and the optimum peeling strength of 25.1 N/mm was achieved at 5 wt% TiH 2 content. The interfacial microstructure of the joints changed with TiH 2 content, leading to different main fracture mechanisms and corresponding peeling strengths. [ABSTRACT FROM AUTHOR]

Lithium–Sulfur battery is deemed as one of the most promising next-generation energy storage systems. Identifying the reduction of soluble lithium polysulfides (LiPSs) is critical for rational material designs for Lithium–Sulfur batteries. In this work, a simple molten-salt method is applied to synthesize titanium nitride/silicon nitride (TiN–Si 3 N 4) core-shell nanoparticles to inhibit soluble polysulfides shuttling. This Ti-reacted strategy has outstanding advantages such as facile and recyclable preparation process, tunable TiN morphology, and good polysulfide adsorption. With the modified separator, Li–S cells exhibit an outstanding initial capacity of 1243 mA h g−1 at 0.5C, good rate capacity of 782 mA h g−1 at 2C, and high capacity retention of 49.2% after 500 cycles. This low-cost and eco-friendly Ti-reacted strategy provides a practical route for the actual application of separator modification in the field of lithium-sulfur battery. [ABSTRACT FROM AUTHOR]

The permeability performance of porous ceramics from particle-stabilized foams is closely associated with the connectivity between bubble-evolved pores. In order to regulate the connectivity of pore structure, the zirconia ceramic foams were fabricated by direct foaming with mixed surfactants of cetyltrimethyl ammonium bromide (CTAB) and sodium N-lauroyl sarcosinate (SLS). Different solid loadings and CTAB:SLS mixing ratios were used in this study. The results indicate that the pore structures of zirconia ceramic foams were interconnected by open windows on the cell walls, and the porosity and average size of cell and cell windows could be tailored by adjusting the solid loading and CTAB:SLS mixing ratio. The decrease in solid loading and CTAB:SLS mixing ratio caused larger porosity and size of cells and cell windows, and thus resulted in the obvious augment of Darcian (k 1) and non-Darcian (k 2) permeability constants. The ranges of k 1 and k 2 of the as-fabricated zirconia ceramic foams are 6.92 × 10−13-4.05 × 10−10 m 2 and 2.09 × 10−5-3.19 × 10−9 m respectively. [ABSTRACT FROM AUTHOR]

Porous β-SiAlON ceramics are fabricated by pressureless sintering with Y-α-SiAlON ceramic powders obtained by the combustion synthesis method as the main additives. The microstructure and composition of porous β-SiAlON ceramics are tailored by changing the addition of the synthesized Y-α-SiAlON ceramic powders. The formation mechanism of porous β-SiAlON ceramics illustrates that their bimodal grain size distributions are determined by the β-SiAlON seed crystals which are derived from the starting synthesized powders and the first precipitates in the liquid phase, respectively. A small addition of Y-α-SiAlON is conducive to the reduction of the thermal conductivity of porous β-SiAlON ceramics. Furthermore, the higher the addition of Y-α-SiAlON ceramics powders, the lower the open porosity, columnar grain size, and dielectric loss, but the larger the density, flexural strength, dielectric constant, and thermal conductivity. [ABSTRACT FROM AUTHOR]

The target of the present research is to fabricate single-phase β-Sialon porous ceramics by self-propagating high-temperature synthesis (SHS). Samples with composition of Si 6-z Al z O z N 8-z (z = 0.5) were prepared by the spontaneous reaction of initial mixture of Si, Si 3 N 4 , Al, and Al 2 O 3 at high-pressure N 2 gas atmosphere. The impacts of Si content on the porosity, phase composition, microstructural morphology, and flexural strength of the fabricated materials were studied in detail. Anticipative target was achieved by adjusting Si content of initial materials and resultant reaction temperature of SHS process. Sample prepared with 35 wt% Si presented single phase of β-Sialon, homogeneous and elongated morphology, and high flexural strength of 150 MPa. To the best of our knowledge, it is the first report to synthesize single-phase β-Sialon porous ceramics by such a rapid and cheap method. [ABSTRACT FROM AUTHOR]

Porous silicon nitride (Si 3 N 4) ceramics were fabricated by self-propagating high temperature synthesis (SHS) using Si, Si 3 N 4 and sintering additive as raw materials. Effects of different types of sintering additives with varied ionic radius (La 2 O 3 , Sm 2 O 3 , Y 2 O 3 , and Lu 2 O 3) on the phase compositions, development of Si 3 N 4 grains and flexural strength (especially high-temperature flexural strength) were researched. Si 3 N 4 ceramics doped with sintering additive of higher ionic radius had higher average aspect ratio, improved room-temperature flexural strength but degraded high-temperature flexural strength. Besides, post-heat treatment (PHT) was conducted to crystallize amorphous grain boundary phase thus improving the creep resistance and high-temperature flexural strength of SHS-fabricated Si 3 N 4 ceramics. Excellent high-temperature flexural strength of 140 MPa~159 MPa and improved strength retention were achieved after PHT at 1400 °C. [ABSTRACT FROM AUTHOR]

The aim of present work is to fabricate porous Si 3 N 4 ceramics with considerable dimensions and homogeneous microstructure by self-propagating high temperature synthesis (SHS) using Si, Si 3 N 4 diluent and Y 2 O 3 as raw materials. The results indicate that Si 3 N 4 diluent with coarse particle sizes and appropriate β-phase content is beneficial to obtain porous Si 3 N 4 ceramics with homogeneous microstructure and excellent mechanical property by controlling the shrinkage inside the sample. The produced Si 3 N 4 ceramics possessed excellent flexural strength of 168 MPa∼259 MPa, and high Weibull modulus of 11.0∼17.2. Additionally, BN and SiC are added as second phase to modify the properties of Si 3 N 4 -based ceramics. Optimum flexural strength of 170 MPa and 137 MPa were obtained with 10 wt.% addition of BN and SiC respectively. After oxidation at 1100 °C∼1300 °C, second phase-doped Si 3 N 4 ceramics also presented higher residual strength than pure Si 3 N 4 ceramics. [ABSTRACT FROM AUTHOR]

The highly porous Si3N4 ceramic foams were prepared by direct foaming with mixed surfactants. The surface tension and viscosity of slurries were tailored by different carbon chain length of surfactants and different mean Si3N4 particle size to achieve the pore size controlling. The nearly linear relationship between the pore size and the ratio of surface tension to viscosity was observed, which indicates that the pore size could be precisely tailored by the slurry properties. Si3N4 ceramic foams with porosity of nearly 94%, mean pore size of 110–230 µm, and compressive strength of 1.24–3.51 MPa were obtained. [ABSTRACT FROM AUTHOR]

The effect of the GdH 2 on the shrinkage behavior, microstructure, thermal conductivity, and flexural strength was investigated. The pre-sintering enables the elimination of native SiO 2 and the simultaneous formation of Gd 2 O 3. The N/O ratio in the liquid phase was increased because of the increasing Gd 2 O 3 /SiO 2 ratio, resulting in the restricted shrinkage during gas pressure sintering. The high N/O ratio in the liquid phase benefits not only the nucleation and growth of β-Si 3 N 4 but also the removal of lattice oxygen, leading to the increase of thermal conductivity. Although the replacement of Gd 2 O 3 by GdH 2 leads to enhanced flexural strength at low temperature, it tends to reduce the flexural strength at high temperature. This variation related largely to the exaggerated bimodal microstructure. This work signifies that GdH 2 is an effective sintering additive for obtaining Si 3 N 4 ceramics with both high flexural strength and high thermal conductivity. [ABSTRACT FROM AUTHOR]

• Native SiO 2 on the raw Si 3 N 4 powder was removed by Si through a silicothermic reduction reaction in pre-sintering. • SRT leads to a reduced amount of liquid phase and an increased N/O ratio in the liquid phase. • A faster phase transformation rate relative to densification rate was achieved during GPS. • Large β-Si 3 N 4 grains were well developed in a porous fine β-Si 3 N 4 matrix skeleton without grain impingement. • Ultimately, dense Si 3 N 4 ceramics with exaggerated bimodal microstructure was obtained after sintered at 1900°C for 4 h. The development of bimodal microstructure in Si 3 N 4 ceramics was crucial to the thermal conductivity and fracture toughness. Silicothermic reduction treatment (SRT) combined with gas-pressure sintering (GPS) were conducted to promote the development of bimodal microstructure. SRT leads to a reduced amount of liquid phase and an increased N/O ratio in the liquid phase, resulting in a faster phase transformation rate relative to densification and a promoted grain growth during GPS. Consequently, large β-Si 3 N 4 grains were well developed in a porous fine β-Si 3 N 4 matrix skeleton without grain impingement. Dense Si 3 N 4 ceramics with exaggerated bimodal microstructure was obtained after sintered at 1900 °C for 4 h. Summarily, the addition of Si leads to the improvement of thermal conductivity and fracture toughness simultaneously. [ABSTRACT FROM AUTHOR]

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 k 1 range of 3.14 × 10−13–1.12 × 10−12 m2 are appropriate for applications in filters and membrane supports. [ABSTRACT FROM AUTHOR]

The effect of YH 2 on densification, microstructure, and thermal conductivity of Si 3 N 4 ceramics were investigated by adjusting the amount of YH 2 in the range of 0–4 wt% using a two-step sintering method. Native SiO 2 was eliminated, and Y 2 O 3 was in situ formed by a metal hydride reduction reaction, resulting in various Y 2 O 3 /SiO 2 ratios. Full densification of YH 2 -doped samples could be achieved after sintering at 1900 °C for 4 h. The Y 2 O 3 /SiO 2 ratio had a significant influence on the composition of crystalline secondary phases. Besides, the increased Y 2 O 3 /SiO 2 ratio is conducive not only to the grain growth but also to the reduction of activity of SiO 2 in the liquid phase, resulting in enlarged purified grains, reduced volume fraction of intergranular phases and increased Si 3 N 4 -Si 3 N 4 contiguity. Ultimately, the thermal conductivity increased by 29 % from 95.3 to 123.0 W m−1 K−1 after sintering at 1900 ℃ for 12 h by the substitution of Y 2 O 3 with YH 2. [ABSTRACT FROM AUTHOR]

Porous Si3N4 ceramics with unidirectionally aligned channels were prepared via freezing ceramics suspension with distinct solid contents under different freezing temperatures. The samples obtained using lower solid content in ceramic suspension at higher freezing temperature exhibit larger Darcian and non‐Darcian constants due to their higher open porosity, larger pore size and lower tortuosity. Moreover the investigation on individual contributions of viscous energy losses and inertial energy losses on the total pressure drop during permeation process indicates that with decreasing the solid content or the freezing temperature the viscous energy losses increase but the inertial energy losses decrease for samples owing to the differences in their pore structures. It is worth mentioning that porous Si3N4 ceramics with unidirectionally aligned channels exhibit larger Darcian and non‐Darcian constants than those with similar pore size distributions and open porosity owing to their lower tortuosity, thus rendering them appropriate for filters and membrane supports. [ABSTRACT FROM AUTHOR]

Porous silicon nitride ceramics with high flexural strength and high porosity were directly fabricated by self-propagating high temperature synthesis (SHS). The effects of N 2 pressure and Si particle size on the phase composition, microstructure, and mechanical property were investigated. N 2 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 N 2 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-Si 3 N 4 grains and the formation mechanism of porous Si 3 N 4 ceramics. [ABSTRACT FROM AUTHOR]

Highly porous silica ceramics with unidirectional pores were prepared using the freeze casting method. By adjusting the solid content and freezing temperature, the porosity of the ceramics was tailored in the range of 78.20%‐84.59% and pore size in the range of 8.4‐71.4 μm, respectively. Sound absorption properties of porous silica ceramics was studied and the effect of structural factors was systematically investigated. The results showed that higher porosity and smaller pores of the porous ceramics favored the sound absorption in the entire sound wave frequency. By backing the sample with small pore size porous ceramics, the sound absorption property was enhanced, particularly in the low and medium frequency range, thus the sound absorption peak shifted towards lower frequency. The presence of air gap in the back would also favor sound absorption in low and medium frequency range. The as‐fabricated porous silica ceramics owed excellent sound absorption properties due to their unidirectional pores and low flow resistances. [ABSTRACT FROM AUTHOR]

Binary nonoxide additives ZrSi2‐MgSiN2 were applied to obtain Si3N4 ceramics with high thermal conductivity by gas pressure sintering. The shrinkage behaviors during sintering were investigated by in situ dilatometry. Although densification was retarded due to a higher N/O ratio in the liquid phase, the incorporation of oxygen was inhibited because of the lower oxygen content in the liquid phase. The substitution of oxide additives by nonoxide additives eliminated native SiO2 and induced a nitrogen‐rich liquid, resulting in purified Si3N4 grains, reduced glassy phase, and enhanced Si3N4‐Si3N4 contiguity. Ultimately, a thermal conductivity of 117.32 W m−1 K−1 was achieved using ZrSi2‐MgSiN2 as sintering additives, increased by 32% compared to those doped with ZrO2‐MgO. [ABSTRACT FROM AUTHOR]

A two‐step sintering process was conducted to produce β‐Si3N4 ceramics with high thermal conductivity. During the first step, native SiO2 was eliminated, and Y2O3 was in situ generated by a metal hydride reduction process, resulting in a high Y2O3/SiO2 ratio. The substitution YH2 for Y2O3 endow Si3N4 ceramics with an increase of 29% in thermal conductivity from 95.3 to 123 W m−1 K−1 after sintered at 1900°C for 12 hours despite an inferior sinterability. This was primarily attributed to the purified enlarged grains, devitrified grain boundary phase, and reduced lattice oxygen content in the YH2‐MgO‐doped material. [ABSTRACT FROM AUTHOR]

The highly porous silica ceramics were fabricated by direct foaming with mixed surfactants and the influence of silicon nitride addition and solid content on the microstructures and properties were investigated. The results showed that silicon nitride can impede the formation of cristobalite and facilitates the sintering of silica ceramics. When the addition of silicon nitride powders reached 15 wt%, the highest compressive strength of silica ceramic foams could be obtained. The porosity of silica ceramic foams was tailored in the range of 84.61%–91.35% by adjusting the solid content, and the compressive strength of the obtained ceramic foams ranged from 5.89 MPa to 0.94 MPa. Sound absorption characteristics of silica ceramics foams were investigated. With the porosity of ceramic foams increased from 84.61% to 91.35%, the sound absorption coefficients in the entire sound wave frequency were enhanced due to the reduction of flow resistances, besides, the sound absorption peak varied from 4200 Hz to 2300 Hz, and became more intense and sharper. [ABSTRACT FROM AUTHOR]

A novel ZrSi2–MgO system was used as sintering additive for fabricating high thermal conductivity silicon nitride ceramics by gas pressure sintering at 1900°C for 12 hours. By keeping the total amount of additives at 7 mol% and adjusting the amount of ZrSi2 in the range of 0‐7 mol%, the effect of ZrSi2 addition on sintering behaviors and thermal conductivity of silicon nitride were investigated. It was found that binary additives ZrSi2–MgO were effective for the densification of Si3N4 ceramics. XRD observations demonstrated that ZrSi2 reacted with native silica on the Si3N4 surface to generate ZrO2 and β‐Si3N4 grains. TEM and in situ dilatometry confirmed that the as formed ZrO2 collaborated with MgO and Si3N4 to form Si–Zr–Mg–O–N liquid phase promoting the densification of Si3N4. Abnormal grain growth was promoted by in situ generated β‐Si3N4 grains. Consequently, compared to ZrO2‐doped materials, the addition of ZrSi2 led to enlarged grains, extremely thin grain boundary film and high contiguity of Si3N4–Si3N4 grains. Ultimately, the thermal conductivity increased by 34.6% from 84.58 to 113.91 W·(m·K)−1 when ZrO2 was substituted by ZrSi2. [ABSTRACT FROM AUTHOR]

The mixed surfactants were successfully applied to fabricate the highly porous Si3N4 ceramic foams by the direct foaming method. The oppositely charged surfactants mixed in slurries could combined into catanionic surfactant by the electrostatic attraction and facilitate the formation of ultra‐stable foams. The microstructure of the Si3N4 ceramic foams, including pore structure, mean pore size, pore size distribution and porosity were tailored by varying the mixing ratio of surfactant, mixed surfactants concentration and solid content of the initial slurries. Si3N4 ceramic foams with porosity of 92%‐97%, mean pore size of 140‐240 µm and compressive strength of 0.85‐5.38 MPa were obtained by adjusting mixed surfactants between 0.1 and 0.4 wt% and solid content between 22 and 30 vol%. The compressive strength of Si3N4 ceramic foams in current work was much higher than most reported results. [ABSTRACT FROM AUTHOR]

Porous silicon nitride ceramics were directly prepared by self-propagating high temperature synthesis (SHS). The effects of Si 3 N 4 diluent and Y 2 O 3 sintering additive on the phase composition and microstructure were investigated. The XRD patterns and SEM microstructures proved the sample with 63 wt% Si 3 N 4 diluent could provide appropriate temperature, reaction time and porosity for the fabrication of elongated β-Si 3 N 4 porous ceramics. A porosity of 40%–47% and a flexural strength of 118–216 MPa were obtained after SHS with different Y 2 O 3 content. Besides, the excellent thermal shock resistance of Si 3 N 4 assured that strength retention keeps stable in the temperature difference (Δ T) of 800 °C. [ABSTRACT FROM AUTHOR]

Porous Si 3 N 4 ceramics with monomodal and bimodal pore structure were prepared by cold isostatic pressing and freeze-casting, respectively. Both the pore structure and permeability behavior of the porous Si 3 N 4 ceramics were tailored by altering the pressure of cold isostatic pressing and the composition and content of solvent during freeze-casting. The specimens obtained by cold isostatic pressing exhibited smaller Darcian and non-Darcian permeability than those of freeze-casted samples due to their lower open porosity, smaller pore size and higher tortuosity. On the other hand, compared with the ice-templated specimens having the same solvent volume in the ceramic slurries as them during freeze-casting, the emulsion-ice templated samples showed smaller open porosity, macropore size and Dacian permeability, but the similar non-Darcian permeability because of their larger micropores and better pore interconnectivity. [ABSTRACT FROM AUTHOR]

β-Si 3 N 4 whiskers reinforced Al matrix composites were prepared by hot pressing. The effect of β-Si 3 N 4 whiskers addition on the microstructure, mechanical properties and tribological behaviors of composites were investigated. The dry sliding wear test was conducted with a sliding speed of 0.5 m/s, at different applied load of 10, 20 and 40 N, respectively. The Vickers hardness, ultimate compression strength (UCS) and the wear resistance of Al matrix were enhanced significantly with the addition of whiskers. As the increase in whisker content, there was a clear transfer of wear mechanism from the combination of serious delamination wear and abrasive wear to the mild abrasive wear at the 20 N applied load condition. The composites reinforced with 15 vol% whiskers exhibited the best wear resistance under higher applied load. The mechanically mixed layer (MML) formed during sliding had a protective effect on the worn surface. Higher mechanical properties of composites and lower applied load helped to keep the MML stable. The slighter the wear degree of composites is, the lower the coefficient of friction is. • The mechanical properties of composites were greatly improved with the β-Si 3 N 4 w addition. • There was a significant change in wear mechanism with the whisker content increasing. • The 15 vol% β-Si 3 N 4 w/Al composites showed the best wear resistance under higher applied load. • The mechanically mixed layer formed during sliding could protect the worn surface. • The β-Si 3 N 4 whisker addition helped to reduce the COF of the composites. [ABSTRACT FROM AUTHOR]

Highlights • Fine rod-like grains of α-SiAlON. • Maintained high porosity after pressureless sintering. • Uniform one dimensional aligned channel structure. • A combination of low dielectric constant, dielectric loss and thermal Conductivity. Abstract Ca-α-SiAlON ceramics with high porosity and fine rod-like grains were fabricated using unidirectional freezing and pressureless sintering. The high porosity of green body was maintained during sintering, which was mainly controlled by solid loading of slurry. Phase analysis showed that α-SiAlON was major phase while small amount of AlN' was inevitable. The anisotropic growth of α-SiAlON was controlled by sintering temperature, fine rod-like α-SiAlON grain morphology was obtained after sample was sintered at 1810℃ for 1.5 h. The aligned channel became more uniform with higher solid loading, gave rise to better mechanical properties. The fabricated samples demonstrated remarkably comprehensive properties including low thermal conductivities, low dielectric constant and dielectric loss. [ABSTRACT FROM AUTHOR]

Abstract Silicon nitride foams with porosity higher than 90% were fabricated by direct foaming method. The influence of the surfactant content, solid content and rotating speed on the foamability and stability of wet foams were discussed and the corresponding effect on the total porosity, pore size distribution and mechanical property of ceramic foams were systematically investigated. The results showed that the pore structure and porosity, the compressive strength of Si 3 N 4 ceramic foams could be controlled by tailoring surfactant and solid content. The results indicated that rotating speed had a great impact on the pore size distribution. Si 3 N 4 ceramic foams with porosity of 90.8–96.4% and compressive strength of 0.37–5.53 MPa were obtained. [ABSTRACT FROM AUTHOR]

Abstract In order to modify the interfacial bonding of β-Si 3 N 4 whiskers (β-Si 3 N 4 w) reinforced Al matrix composites, the Ag-coated β-Si 3 N 4 w with varying Ag contents were incorporated into the Al matrix by a hot pressing method. The interfacial structure showed that the β-Si 3 N 4 w was surrounded by an amorphous Ag enrichment layer and well bonded with Al matrix with no pores and reaction products near the interface. The relative density, fracture toughness and thermal conductivities of composites were distinctly improved by the modified interface because of improved wettability, reduced thermal mismatch stress and lowered interfacial thermal resistance between Al and β-Si 3 N 4 w. When the Ag content on whiskers is 1.8 vol%, the composites shows the maximum fracture toughness (23.9 MPa·m1/2). When the Ag content is 2.7 vol%, both the maximum ductility and the maximum thermal conductivity were obtained, corresponding to an elongation at fracture and a 25 °C thermal conductivity of 15.5% and 198.6 W/m·K, respectively. However, excessive Ag content on whiskers didn't bring about promotion in mechanical properties and thermal conductivities due to the evolution of microstructure and the limitation of the relative density of composites. Graphical abstract Unlabelled Image Highlights • The Ag-coated β-Si 3 N 4 whiskers were successfully obtained by electroless plating. • The β-Si 3 N 4 whiskers was surrounded by an amorphous Ag enrichment layer and well bonded with Al matrix. • Both the fracture toughness and the thermal conductivities of composites were effectively improved. • The maximum fracture toughness and 25°C thermal conductivity of composites were 23.9 MPa·m1/2 and 198.6 W/m·K, respectively. [ABSTRACT FROM AUTHOR]

The high temperature strength and fracture behavior of porous Si 3 N 4 ceramics prepared via reaction bonded Si 3 N 4 (RBSN) and sintered reaction bonded Si 3 N 4 (SRBSN) were investigated at 800–1400 °C. The weight gain after oxidation for 15 min and the microstructure of the edge and center of the fracture surface clearly show that the internal oxidation of porous SRBSN is unavoidable with porosity of ~ 50% and mean pore size of 700 nm. The oxidation of Si 3 N 4 and intergranular Y 2 Si 3 O 3 N 4 phase may responsible for the high temperature strength degradation of SRBSN. Porous Si 3 N 4 ceramics prepared with addition of 1 wt% C showed low strength degradation at temperature > 1200 °C. [ABSTRACT FROM AUTHOR]

Aluminum matrix composites (AMCs) reinforced with pre-oxidized β-Si 3 N 4 whiskers (β-Si 3 N 4 w) were fabricated by hot pressing method. An amorphous SiO 2 layer was formed on the surface of β-Si 3 N 4 w aiming to enhance the interfacial bonding strength between aluminum and β-Si 3 N 4 . The interfacial structure showed that β-Si 3 N 4 w was well bonded with aluminum and no reaction products were formed near the interface. Effect of pre-oxidized β-Si 3 N 4 w on the mechanical properties of composites were investigated in detail. The results indicated that both the relative density and the mechanical behaviors of AMCs were increased. There was an optimum oxidation time for β-Si 3 N 4 w. The maximum promotion of ultimate tensile strength (UTS) was 25 MPa, 11% higher than the composites reinforced with as-prepared whiskers, when the oxidation time was 1 h and the whisker content was 5 vol%. However, long time oxidation would deteriorate the mechanical properties of β-Si 3 N 4 w, and thereby weaken the strengthening effect. [ABSTRACT FROM AUTHOR]

We describe a low-cost and simple method to prepare porous silicon nitride (Si 3 N 4 ) ceramics with unidirectional oriented pores. Using Si powders as raw material, porous ceramics were fabricated via solidification of the aqueous slurries followed by the nitridation and post-sintering. The formation mechanism is similar to the ordered porous metals that are fabricated under high temperature and high pressure. Differently, the green bodies with oriented pores were obtained at room temperature and atmospheric pressure. The nucleation and growth of the H 2 bubbles generated from hydrolyzed Si powders contributed to the highly oriented pores along the solidification direction of the aqueous slurries. Additionally, the pore microstructure was closely associated with the viscosity of the slurries, which can be well controlled by varying the concentration of organic additive and solid loading. [ABSTRACT FROM AUTHOR]

In order to enhance the interfacial bonding between aluminum and β-Si3N4 whiskers in Al matrix composites (AMCs), Ni coating was deposited on the surface of β-Si3N4 whiskers using an electroless plating method. The interfacial test results showed that Ni coating was successfully deposited on the surface of β-Si3N4 whiskers and the whiskers were well bonded with Al matrix without pores. In addition, the whiskers were homogeneously dispersed in the Al matrix. The effect of Ni-coated β-Si3N4 whiskers on the mechanical properties of composites was investigated in detail. Densification and the mechanical performance of AMCs reinforced by Ni coated β-Si3N4 whiskers (NSAMCs) were improved comparing to that of AMCs reinforced by uncoated β-Si3N4 whiskers (USAMCs). The maximum value of tensile strength and bending strength reached up to 290 MPa and 487 MPa, when the Ni content was 0.9 vol.% and the β-Si3N4 whiskers content was 20 wt.%, respectively. However, excessive Ni content weakened the strengthening effect. Medium Ni coating on β-Si3N4 whiskers significantly optimized the interface and improved the performance of the composites. [ABSTRACT FROM AUTHOR]

Porous SiC ceramics were prepared by freeze-casting process. In order to enhance the mechanical properties of the porous SiC, poly(vinyl alcohol) (PVA) was added as binder and pore morphology controller in this work. The results indicated that high porosity (>60%) SiC ceramics was obtained although the sintering temperature was over 2000 °C. The pore structure could be divided into two kinds: macropores generated by sublimation of large ice crystals, and micropores in the ceramic matrix caused by sublimating of small ice crystals, stacking of SiC particles, and burning out of PVA. With the increase of the sintering temperature, the specimens exhibited higher density, thus resulted in higher strength. Porous SiC ceramics sintered at 2100 °C showed a good flexural strength of 11.25 MPa with an open porosity as high as 66.46%. [ABSTRACT FROM AUTHOR]

A novel aqueous gelcasting technique was employed to prepare porous silicon nitride ceramics using water-soluble copolymer of isobutylene and maleic anhydride (Isobam) as both dispersant and gelling agent. The effects of fabrication parameters, including sintering additives, the second phase ( BN), and pore-forming agent (corn starch), on the microstructure and mechanical properties were separately investigated. The sample with Yb2O3 as sintering additive, having a porosity of ~48.6%, showed a high flexural strength of 233 MPa and a high fracture toughness of 2.72 MPa·m1/2. With addition of 2.5wt% BN, the sample exhibited a high flexural strength of 211 MPa due to the uniform microstructure with β-Si3N4 grains. A high porosity of ~60% and a flexural strength of 110 MPa were obtained by adding 20wt% starch into the sample. [ABSTRACT FROM AUTHOR]

The poor wettability between the ceramic reinforcements and the metal matrix makes it difficult to realize the densification process of composites, which is severely detrimental to their mechanical properties. To improve the mechanical properties of Al matrix composites (AMCs) reinforced by β-Si3N4 whiskers (β-Si3N4w), we prepare TiN coating on the surface of β-Si3N4w via molten salt process, remarkably strengthening the interface bonding between Al matrix and β-Si3N4w. By optimizing the coating layer thickness, the bending strength of AMCs reinforced by TiN-coated β-Si3N4w reaches 439 MPa at 850°C, exhibiting remarkable enhancement relative to the composites without TiN coating (335MPa). Meanwhile, the interfacial test shows that TiN-coated β-Si3N4w is well bonded with the Al matrix, and no interfacial defects were detected. Moreover, the TiN coating with appropriate thickness can efficiently alleviate the agglomeration of β-Si3N4w, promote the densification of AMCs, and modify the Al/β-Si3N4w interface. [ABSTRACT FROM AUTHOR]

Ag-coated β-Si 3 N 4 whiskers reinforced Cu matrix composites (ASCMMCs) were prepared by powder metallurgy method. A quite thin Ag layer was deposited on the surface of β-Si 3 N 4 whiskers with the aim of improving the interfacial bonding between β-Si 3 N 4 whiskers and Cu matrix. The results indicated that ASCMMCs had both higher bending strength and higher hardness than the uncoated β-Si 3 N 4 whiskers reinforced Cu matrix composites (USCMMCs). The enhanced mechanical property was attributed to the promoted densification of composites, the reduced aggregation of β-Si 3 N 4 whiskers and the enhanced interfacial bonding between β-Si 3 N 4 whiskers and Cu matrix. The Ag coating was helpful to the formation of a transition layer between β-Si 3 N 4 whiskers and Cu matrix to overcome the poor interfacial wetting, thus enhanced the mechanical property. But excessive Ag coating did not bring about further improved mechanical performance due to the increased possibility of interfacial sliding. [ABSTRACT FROM AUTHOR]

Highlights: [•] The Cu/Si3N4 interfacial structure was modified by Ag coating. [•] The highest thermal conductivity was 273Wm−1 K−1 at 25°C. [•] Enhanced thermal conductivity was benefit from high densification, uniform whisker distribution. [ABSTRACT FROM AUTHOR]

Abstract: Cu matrix composites reinforced with β-Si3N4 whiskers (β-Si3N4w/Cu composites) were prepared by the powder metallurgy method. The results indicated that β-Si3N4 whisker addition and sintering program had significant influence on the densification and mechanical properties of composites. Hardness and bending strength of β-Si3N4w/Cu composites were both improved by a certain amount of β-Si3N4 whisker, although the densification was weakened. The highest hardness and bending strength of β-Si3N4w/Cu composites respectively reached up to 80.9HRF and 290MPa. By modifying the crack propagation, the rod-like β-Si3N4 whiskers could improve mechanical properties of composites. Moreover, dislocation strengthening and grain refinement also contributed to the improvement. The high mechanical performance was promoted by the appropriate amount of β-Si3N4 whiskers, while excessive whisker content (>10vol% for bending strength; >15vol% for hardness) caused a side effect. [Copyright &y& Elsevier]

Abstract: Porous Si3N4 ceramics were prepared via partial nitridation and self-propagating high temperature synthesis (SHS) process. Raw Si and additive Y2O3 were mixed and molded under 10MPa into a compact, the compact was partial nitridation at 1300°C to form a porous Si/Si3N4, and then it was buried in a Si/Si3N4 bed for SHS to obtain porous Si3N4 with rod-like β-Si3N4 morphology. The processing combined the advantages of the nitridation of Si and SHS with low cost, low shrinkage and time saving. Porous Si3N4 with a porosity of 47%, a strength of 143MPa were obtained by this method. [Copyright &y& Elsevier]

Sintered reaction‐bonded silicon nitride (SRBSN) with high thermal conductivity was obtained using (Y0.96Eu0.04)2O3 and MgO as sintering additives. Green compacts were nitrided at 1400°C for 4 h. Post‐sintering was carried out at 1850 and 1900°C for 4 h, respectively. In reaction‐bonded silicon nitride (RBSN) doped with Y2O3 and MgO, the β‐Si3N4 content and nitridation degree were 51.1% and 93.8%, respectively. However, the β‐Si3N4 content and nitridation degree were 72.6% and 96.7% in a nitrided compact doped with (Y0.96Eu0.04)2O3 and MgO. After post‐sintering, the phase composition, microstructure, mechanical properties, and thermal conductivity were investigated. After sintering at 1900°C for 4 h, the thermal conductivity of SRBSN doped with (Y0.96Eu0.04)2O3 and MgO was increased by 16.5% compared to that of the samples doped with Y2O3 and MgO. The highest hardness of 1639 HV and the good flexural strength of 776.4 MPa were also achieved in the sample doped with 2‐mol.% (Y0.96Eu0.04)2O3 and 5‐mol.% MgO. [ABSTRACT FROM AUTHOR]

Porous silicon nitride (Si3N4) ceramics with high porosity were prepared via nitridation of Si powder, using Si3N4 as filler and Y2O3 as sintering additive. After nitridation, samples with high Si3N4 filler content showed high porosity, low flexural strength, and low grain size. A porosity of 50-62% a flexural strength of 16-49 MPa were obtained. After postsintering at 1750°C, sample with Si3N4 filler content of 50% exhibited a bimodal grain size distribution of elongated β-Si3N4 grains with high aspect ratio embedded in fine-grained matrix. Porous Si3N4 ceramics with a porosity of 48-60% and a flexural strength of 100-145 MPa were obtained. [ABSTRACT FROM AUTHOR]

Abstract: Slip casting process combined with reaction bonded silicon nitride (RBSN) was used to prepare porous Si3N4 ceramic with near-net and complex shape. A butyl stearate (BS) coated process was introduced to restrain the hydrolysis of Si, and ammonium polyacrylate (NH4PAA) was used to enhance the dispersion of coated Si. The measured oxygen content showed that the hydrolysis of Si was strongly prohibited by BS coating, and relatively low viscosity was obtained with the addition of 0.25–1.5wt% NH4PAA to the 60wt% solid load slurry. 40–60wt% solid load slurries were used for slip casting in the experiment. After vacuum degassing, slip casting, debindering and nitridation, a density of 1.57–1.92g/cm3 (porosity 50.9–40%) and a flexural strength of 47–108MPa were obtained. The samples without vacuum degassing showed a large number of nanowires grown in the large pores. [Copyright &y& Elsevier]

Porous Al 2 O 3 with high porosity (~45%), remarkable flexural strength (~70 MPa), and low dimensional change (~1–2%), was produced by indirect 3D printing and reaction bonding. Coarse Al with good flowability was carried out printing green body. The green body was heat treated to get fully oxidized, volume expansion during oxidation simultaneously formed strong neck bonding, not only brought good strength, but also restricted shrinkage. Porous Al 2 O 3 with architecture of macro pores designed by 3D printing and micro pores in the strut formed by packing of particles was obtained. The near zero shrinkage can facilitate precise design of product with complex shape. [ABSTRACT FROM AUTHOR]

Gradient porous silicon nitride (Si 3 N 4 ) was fabricated by a novel vacuum foaming and freeze drying process. Aqueous Si 3 N 4 slurries were foamed at vacuum pressure of 50–90 kPa, the green body was obtained by the freeze drying process, a gradient pore structure with porosities of 72–90% was achieved after pressureless sintering at 1680 °C. The porosity was increased with decreasing vacuum pressure. The pore structure consists of large pores (~100 μm) on top, medium pores (~45 μm) on the wall of the large pores, and small pores (~0.7 μm) in the matrix. Such gradient porous Si 3 N 4 with macro- and micro-pores has potential application as high temperature filters. [ABSTRACT FROM AUTHOR]

• Porous Si3N4 ceramics with monomodal and bimodal pore structures were developed for novel wicks of loop heat pipes. • Exponential fitting of capillary data. • The balance between the permeability and capillary force of wicks • The comparison of experimental and calculated effective thermal conductivity. Porous Si 3 N 4 ceramics with monomodal and bimodal pore size distribution are selected as novel wick materials for loop heat pipe. The capillary and thermal performance of porous Si 3 N 4 wicks is investigated by tailoring their pore structures. The wicks with larger permeability affected by their pore structures exhibit lower filling coefficient representing better capillary performance, due to lower frictional resistance of the fluid flowing through them during the liquid uptake process. It is essential for wicks with excellent capillary performance to establish the balance between the improved permeability derived from the increased pore size and the reduced capillary force caused by that. Moreover, the wicks using working fluid with lower viscosity, larger surface tension and higher density show better capillary performance according to the comparison of three different working fluids. The effective thermal conductivity of wicks is determined by their total porosity. The comparison of experimental values and calculated values of effective thermal conductivity indicates that the most suitable calculated curve is provided by Alexander model based on limited data in the test. [ABSTRACT FROM AUTHOR]

The separator plays a pivotal role in the safe operation of rechargeable batteries. In this work, a novel separator fabricated by incorporating Poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) with silicon nitride (Si 3 N 4) whiskers was reported for the first time. Compared with introducing nano powders as inorganic fillers, incorporating inorganic whiskers into polymetric substrates not only suppresses the agglomeration of fillers, but also maintains high structural stability at high temperatures by forming a concrete composite structure. Systematical comparison of "powder-in-polymer" and "whiskers-in-polymer" separators demonstrated that the whisker filler can suppress unexpected dimensional shrinkage at high temperatures and improve electrochemical stability. The composite separator with 60 wt% silicon nitride whiskers exhibits enhanced ionic conductivity of 0.884 mS/cm at 25 °C, stable coulombic efficiency, higher capacity retention rate of 97.4% after 100 cycles and high specific capacity of 103.9 mAh·g−1 at 5 C. Image 1 • Silicon nitride whiskers were introduced into the PVDF-HFP polymetric matrices as inorganic fillers. • Inorganic whiskers were easy to disperse homogenously and embedded in the substrate firmly. • The ionic transport properties and high-temperature performances were improved. • The LiFePO 4 /Li batteries exhibited excellent rate and cycling performance. [ABSTRACT FROM AUTHOR]

This work reports the two-step sintering method and thermal conductivity of Si 3 N 4 doped with 1.5 wt% MgO and varied contents of ZrH 2 in the range of 0-5 wt%. The metal hydride reduction reaction in the first step sintering allows the in situ generations of ZrO 2 and the elimination of native SiO 2. A smaller amount of liquid phase with reduced oxygen content was produced during the second step sintering as a result of the replacement of ZrO 2 by ZrH 2. With the aid of ZrH 2 as an oxygen getter, the native oxygen in those ZrH 2 -doped samples was removed via a route of SiO 2 →ZrO 2 →SiO(g). Consequently, the densified Si 3 N 4 ceramics with larger volume percent of elongated purified β-Si 3 N 4 grains and less glassy phase were obtained. Ultimately, the optimal thermal conductivity of 116.4 W m−1 K−1 was achieved after sintered at 1900 °C for 12 h by using 3 wt% ZrH 2 and 1.5 wt% MgO as additives. • Native SiO 2 was removed and ZrO 2 was in situ generated via metal hydride reduction reaction during the first step sintering. • Higher N/O ratio in the liquid phase due to the replacement of ZrO 2 by ZrH 2 benefit the nucleation and growth of β-Si 3 N 4. • The native oxygen in those ZrH 2 -doped samples was removed via a route of SiO 2 →ZrO 2 →SiO(g). [ABSTRACT FROM AUTHOR]

• A new route of combining freeze-drying and combustion synthesis. • Porous Si 3 N 4 ceramics with high strength and low shrinkage. • Controllably unidirectional aligned channels. TBA-based freeze-drying and combustion synthesis (CS) are firstly combined to fabricate porous Si 3 N 4 ceramics with controllably unidirectional channels. Uniformly unidirectional channels with variable pore diameters were successfully formed through unidirectional growth of ice. In addition, the residual α-Si 3 N 4 content, development of grain morphology and mechanical property of sintered specimens were significantly influenced by the solid content of slurry. Porous Si 3 N 4 ceramics with porosity of 62.5–78.8%, axial compressive strength of 7.3–61.6Mpa, and transverse compressive strength of 3.3–41.6 MPa were obtained with increasing solid content. [ABSTRACT FROM AUTHOR]

The inspiration from biomass resources allows an efficient route to produce novel carbon materials for sustainable development of energy storage. The carbon monoliths are prepared by directly pyrolyzing cotton rose wood in an inert atmosphere. Their physicochemical properties and pore structure are controlled by altering the pyrolysis temperature. The obtained wood biochar monoliths (WBMs) inherit the hierarchically porous morphology of pristine wood, with unidirectional honeycomb channels with thin channel wall (0.7 μm) and unique interconnected macro-mesoporous pathway on the channel wall, as well as meso-micropores on the carbon sheets. The ultra-thick 1 mm WBM electrode that is pyrolyzed at 800°C with a specific surface area of 486 m2 g−1 shows the optimal electrochemical performance of 133 F g−1 (27 F cm−3) at 10 mA cm−2, which is higher than other reported wood biochar monoliths. Even the 10 mm thickness electrode retains 86% of the capacitance above. What's more, the electrode in a symmetric electrochemical capacitor delivers a competitive energy density of 9.4 W h kg−1 and an outstanding stability of 111% after 10,000 cycles. It is demonstrated that cotton rose wood is a promising carbon precursor for the production of ultra-thick electrodes. [ABSTRACT FROM AUTHOR]