Nitridation Of Si Powder Research Articles

Effect of Cr2O3 additive on the nitridation of Si powder

The effect of Cr2O3 additive on nitridation of Si powder was studied at 1200 °C, 1300 °C and 1350 °C. At 1200 °C for 2 h, only Cr2N and CrSi2 peaks were found except Si in sample with 8 wt% Cr2O3. At 1300 °C, no visible Si3N4 peak was detected by XRD in pure Si sample, but in Cr2O3 added samples and with increasing of Cr2O3 additive the intensity of Si was reduced. At 1350 °C, Si peaks weakened in pure Si sample and Si powder was completely nitrided with Cr2O3 additive. The results showed that Cr2O3 additive improved the nitridation degree by the transformation between Cr2O3 and Cr2N and increasing the nitridation temperature promoted the nitridation of Si.

Effect of SiO2 addition on Si3N4 ceramics prepared by rapid nitridation and post-sintering route

Abstract Effects of SiO 2 content and Y 2 O 3 :Al 2 O 3 mass ratio on phase compositions, densification, microstructures and mechanical properties of sintered reaction-bonded Si 3 N 4 ceramics prepared by rapid nitridation of Si powders using TiO 2 as catalyst and subsequent gas pressure sintering were investigated. The low Y 2 O 3 :Al 2 O 3 ratio could reduce the adverse effect of high SiO 2 content on densification and microstructure by the formation of O′-Sialon phase. The bulk density of the ceramics with low Y 2 O 3 :Al 2 O 3 ratio was higher than that with high Y 2 O 3 :Al 2 O 3 ratio, especially at high SiO 2 content. Irrespective of Y 2 O 3 :Al 2 O 3 ratio, the increasing SiO 2 content led to microstructural coarsening, but the ceramics with high Y 2 O 3 :Al 2 O 3 ratio and SiO 2 content contained more large-elongated grains. The hardness and toughness of the ceramics with low Y 2 O 3 :Al 2 O 3 ratio slightly decreased with increasing SiO 2 content, and the strength basically had no change. Whereas the mechanical properties of the ceramics with high Y 2 O 3 :Al 2 O 3 ratio obviously decreased with increasing SiO 2 content due to the markedly decreased bulk density and increased grain size. The present results suggested the low Y 2 O 3 :Al 2 O 3 ratio would contribute to the improvement of mechanical properties of SRBSN ceramics prepared from low-grade Si powers containing high content of SiO 2 .

Ni-catalyzed synthesis of hexagonal plate-like alpha silicon nitride from nitridation of Si powder in molten salt media

The catalytic effects of Ni during the nitridation of Si from a molten salt and the morphologies of the nitridation products were investigated. The phase composition, particle size, and morphological structure of the α-Si3N4 powder were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and a Brunauer–Emmett–Teller (BET) specific-surface-area analysis. The results showed that Ni significantly accelerated Si nitridation and the formation of the hexagonal plate-like α-Si3N4 phase by the formation of Ni–Si in the eutectic salt at 1050–1350°C. Further, the nitridation rate of the Si powder was increased to 93% after 5h at 1250°C with 2-wt% Ni used as the catalyst. The specific surface area of the α-Si3N4 synthesized powder was 5.70–10m2/g, exceeding that of 1.70m2/g of the Si powder.

Effect of TiO2 additives on nitridation of Si powders

Effect of TiO2 additives on nitridation of Si powders at 1300°C and 1400°C was studied by combining X-ray diffraction, thermogravimetric analysis, and thermodynamic calculations. Weak Si3N4 phase was detected in pure Si powders after nitridation at 1300°C, whereas no Si3N4 phase was found in TiO2-doped Si powders and only secondary TiN phase was observed in addition to main Si phase. The TiO2 demonstrated an inhibiting effect on nitridation of Si powders at 1300°C. However, the TiO2 led to the rapid nitridation of Si powders at 1400°C by circulating reaction mechanism between TiO2 and TiN, and promoted α-to β-Si3N4 phase transformation. In addition to TiN phase, secondary Si2N2O phase was observed in the TiO2-doped Si powders. The present study suggested that TiO2 additives exhibited effectively nitridation enhancing effect on the formation of reaction-bonded Si3N4 at 1400°C.

Porous Si3N4/SiC Ceramics Prepared via Nitridation of Si Powder with SiC Addition

Porous Si3N4/SiC ceramics with high porosity were prepared via nitridation of Si powder, using SiC as the second phase and Y2O3 as sintering additive. With increasing SiC addition, porous Si3N4/SiC ceramics showed high porosity, low flexural strength, and decreased grain size. However, the sample with 20wt% SiC addition showed highest flexural strength and lowest porosity. Porous Si3N4/SiC ceramics with a porosity of 36–45% and a flexural strength of 107‐46MPa were obtained. The linear shrinkage of all porous Si3N4/SiC ceramics is below 0.42%. This study reveals that the nitridation route is a promising way to prepare porous Si3N4/SiC ceramics with favorable flexural strength, high porosity, and low linear shrinkage.

Porous Si3N4 Ceramics Prepared via Nitridation of Si Powder with Si3N4 Filler and Postsintering

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.

Laser assisted synthesis of ultrafine silicon powder

Ultrafine Si powder has been produced by means of laser assisted decomposition of SiH4. Optical on-line diagnostics and final product analysis have been employed in order to establish both gas-phase precursor dissociation and particle growth mechanism. Morphologic and crystallographic characterizations of the powder have been performed. Nitridation of Si powders and compacts have been accomplished. Results are compared with those obtained by direct laser induced photosynthesis of Si3N4 powder.