Injection molding of ultra-fine Si_3N_4 powder for gas-pressure sintering

The ceramic injection molding technique was used in the gas-pressure sintering of ultra-fine Si3N4 powder. The feedstock＇s flowability, debinding rate, defect evolution, and microstructural evolution during production were explored. The results show that the solid volume loading of less than 50vol% and the surfactant mass fraction of 6wt% result in a perfect flowability of feedstock; this feedstock is suitable for injection molding. When the debinding time is 8 h at 40°C, approximately 50% of the wax can be solvent debinded. Defects detected during the preparation are traced to improper injection parameters, mold design, debinding parameters, residual stress, or inhomogeneous composition distribution in the green body. The bulk density, Vickers hardness, and fracture toughness of the gas-pressure-sintered Si3N4 ceramic reach 3.2 g/cm^3, 16.5 GPa, and 7.2 MPa·m^1/2, respectively.

Influence of Yb_2O_3-MgO on Mechanical Properties and Thermal Conductivity of Silicon Nitride Ceramics via Gas Pressure Sintering

In this work,Yb2O3 and Mg O were used as sintering aids in preparing silicon nitride ceramics by gas pressure sintering（ 0. 6 MPa N2atmosphere） to investigate how the amounts of Yb2O3- Mg O influence the mechanical properties and thermal conductivity of silicon nitride ceramics. The total contents of Yb2O3- Mg O added were 1 mol%,2 mol%,4 mol%,6 mol%,8 mol%,10 mol%,12 mol%,14 mol%,keeping the Yb2O3-Mg O molar ratio of 1 ∶ 1 steadily. Curves of the relative density,thermal conductivity and bending strength plotted against the aids content present a ‘mountain＇shape with a maximum at nearly 10 mol% aids. The fracture toughness increased with the amounts of additives up to10 mol% and decreased slightly thereafter. The mechanical properties and thermal conductivity were almost proportional to the amount of the additives before10 mol%. When the content of aids exceeded 10 mol%,it would weaken the mechanical properties and thermal conductivity of the ceramics. The optimum content of Yb2O3- Mg O was 10 mol% by gas pressure sintering（ 0. 6MPa） at 1 850 ℃ for 4 h,which led to a relative density of 98. 9%,a flexural strength of（ 966 ± 38）MPa as well as a fracture toughness of（ 6. 29 ± 0. 29）MPa·m1 /2and thermal conductivity of 82 W /（ m·K）.

Effect of Pretreatment Temperature on Properties of Silicon Powder

In order to reduce the hydrolysis rate of Si powder and inhibit the swelling and cracking of cast samples,Si powder was treated at different temperatures for 5 h,and then slurries and SiC-Si3N4 materials were prepared.The effect of the preheating temperature on the composition of the Si powder,the hydrolysis rate as well as the nitridation of the SiC-Si3N4 materials was studied.The results show that with the increase of the pretreatment temperature of Si powder,the oxidation degree of Si powders increases,the viscosity of the slurry with PAAS dispersant decreases gradually,and the hydrolysis reaction of Si powder decreases gradually;the Si powder pretreated at 700℃ or lower has little effect on the nitriding process,while the Si powder pretreated at 800 t has a great effect on the nitriding process,which is not conducive to the nitriding sintering of the samples.Therefore,the Si powder pretreated at 700 t for 5 h has the best properties,which not only has lower hydrolysis rate and good slurry fluidity,but also has mild effect on the nitriding reaction.

Reaction Mechanism and Microstructure Evolution of Reaction Sintered h-BN

Hexagonal boron nitride ceramic（h-BN） based on the nitridation of B powders was obtained by reaction sintering method. The effects of sintering temperature on the mechanical properties and microstructure of the resultant products were investigated and the reaction mechanism was discussed. Results showed that the reaction between B and N2 occurred vigorously at temperatures ranging from 1 000 ℃ to 1 300 ℃, which resulted in the generation of t-BN. When the temperature exceeded 1 450 ℃, transformation from t-BN to h-BN began to occur. As the sintering temperature increased, the spherical particles of t-BN gradually transformed into fine sheet particles of h-BN. These particles subsequently displayed a compact arrangement to achieve a more uniform microstructure, thereby increasing the strength.

Effect of CeO_2 on low temperature pressureless sintering of porous Si_3N_4 ceramics

Porous silicon nitride（Si_3N_4） ceramics were fabricated by low temperature pressureless sintering, using carbonized rice husk and α-Si_3N_4 powders as raw materials, and MgO-CeO_2 as sintering additives. The effects of CeO_2 concentration and sintering temperature on phase composition, microstructure, porosity and flexural strength of the sintered products were investigated by X-ray diffraction（XRD）, scanning electronic microscopy（SEM）, Archimedes＇ displacement method and three-point bending strength, respectively. The results suggested that MgO-CeO_2 was a much more effective sintering additive for Si_3N_4 than MgO alone. When CeO_2 concentration was 4 wt.%–5 wt.% and sintering temperatures were in a range of 1500 to 1550 oC, the obtained porous silicon nitride ceramics had the porosity of 45.78%–42.81% and flexural strength of 49.44–84.09 MPa. Moreover, when sintering temperature was 1550 oC and CeO_2 concentration was 5%, large elongated β-Si_3N_4 grains were well developed.