Delong Cai

Microstructures, mechanical properties and oxidation resistance of SiBCN ceramics with the addition of MgO, ZrO2 and SiO2 (MZS) as sintering additives

Nano-crystalline SiBCN ceramics were prepared by mechanical alloying (MA) plus hot pressing (HP) with the addition of MZS1 (MgO, ZrO2 and SiO2) and MZS2 (ZrSiO4 and SiO2) as the sintering additives. The effects of the two additives on the microstructure, mechanical properties and oxidation resistance of SiBCN ceramics were carefully evaluated. The addition of MZS1 and MZS2 additives in the SiBCN matrix can boost elemental diffusion and matrix densification. Owing to the effective densification caused by the sintering additives, the mechanical properties of SM1 and SM2 samples are far superior to monolithic SiBCN. The oxide layers of SM1 and SM2 samples remain relatively dense and continuous, and have a strong binding capacity with the matrix after oxidation testing at 1500 °C for 20 h. The residual excess carbon in the SiBCN matrix should be responsible for the formation of ZrC and the outermost oxide layer structure is comprised of SiO2 and ZrC after the oxidation test. The flexural strength, Young’s modulus, fracture toughness and Vicker’s hardness of the SM2 sample are much higher than monolithic SiBCN ceramics, reaching to 394.2 ± 41.7 MPa, 152.9 ± 16.0 GPa, 5.86 ± 0.86 MPa m1/2 and 8.3 ± 0.6 GPa, respectively.

Facile synthesis, microstructure and photophysical properties of core-shell nanostructured (SiCN)/BN nanocomposites

Increasing structural complexity at nanoscale can permit superior control over photophysical properties in the precursor-derived semiconductors. We demonstrate here the synthesis of silicon carbonitride (SiCN)/boron nitride (BN) nanocomposites via a polymer precursor route wherein the cobalt polyamine complexes used as the catalyst, exhibiting novel composite structures and photophysical properties. High Resolution Transmission Electron Microscopy (HRTEM) analysis shows that the diameters of SiCN−BN core−shell nanocomposites and BN shells are 50‒400 nm and 5‒25 nm, respectively. BN nanosheets (BNNSs) are also observed with an average sheet size of 5‒15 nm. The photophysical properties of these nanocomposites are characterized using the UV-Vis and photoluminescence (PL) analyses. The as-produced composites have emission behavior including an emission lifetime of 2.5 ns (±20 ps) longer observed in BN doped SiCN than that seen for SiC nanoparticles. Our results suggest that the SiCN/BN nanocomposites act as semiconductor displaying superior width photoluminescence at wavelengths spanning the visible to near-infrared (NIR) spectral range (400‒700 nm), owing to the heterojunction of the interface between the SiC(N) nanowire core and the BN nanosheet shell.

Effect of the BN content on the thermal shock resistance and properties of BN/SiO2 composites fabricated from mechanically alloyed SiBON powders

Wave-transparent composites of BN/SiO2 were prepared via hot pressure sintering at 1650 °C of mechanically alloyed amorphous SiBON powders. The mechanical, dielectric and thermal properties and thermal shock resistance of the composites were carefully investigated with different BN contents. With increasing BN content, the flexural strength and fracture toughness of the composites increased first and then decreased. The sample with the composition of SiO2–5BN exhibited the highest flexural strength of 256.3 MPa and a fracture toughness of 3.15 MPa m1/2. The relative density decreased with the increase of BN content, which would influence the thermal properties and the thermal shock resistance of the composites. The formation of borosilicate glass and crystallization of fused silica in the surface at the high temperature greatly improved thermal shock resistance of the samples. The SiO2–6BN sample, possessing low relative density, exhibited the best thermal shock resistance, and the retained strength after a thermal shock of 1100 °C was 98.6% of the original strength. Besides excellent mechanical and thermal properties and thermal shock resistance, the as-sintered composites exhibited low dielectric constant (e < 4.62) and loss tangent (tan δ < 0.002), meeting the required values for high temperature wave-transparent materials.

The preparation, microstructure and mechanical properties of a dense MgO–Al2O3–SiO2 based glass-ceramic coating on porous BN/Si2N2O ceramics

A dense MgO–Al2O3–SiO2 based glass-ceramic coating was prepared by a doctor blade process on a porous BN/Si2N2O ceramic surface followed by heat treatment at 1050 °C under nitrogen flow. The phase composition, microstructure, mechanical properties and water absorption of the coating were studied. The coating consisted of α-cordierite phase with a small amount of glass phase. The dense coating without pores and cracks was favorable to seal and densify the porous ceramic surface due to part of the molten glass infiltrating the surface pores. The coating was defect-free and tightly bonded to the substrate because of a larger bonding area between the coating and the substrate. The elastic modulus and bending strength of the glass-ceramic coating were 37.9 GPa and 67.1 MPa, respectively. Moreover, the coated samples had a high Vickers hardness and low water absorption.