| The use of Si3N4 ceramics for practical applications such as national defense, aviation, metallurgy, electronics, petroleum etc, has attracted much attention due to their excellent multi-performance. In this paper, porous silicon nitride was prepared by reaction-bonded technology using Si as starting material. The functions of additives and preparation process were mainly studied in this experiment. The nitridation sintering mechanism and model were also investigated.The porous RBSN ceramic was obtained successfully by reaction-bonded technology. The influence of additives (including sintering aids, whiskers and pore-forming agents) on properties of porous RBSN was studied, The microscopic morphology and phase were characterized by SEM and XRD,respectively, the relations between microstructure and macroscopic properties were discussed. The results showed that the effects of Y2O3+Al(OH)3 as sintering aids were better than Y2O3+Mg(OH)2?4MgCO3?6H2O due to better properties of sample. Appropriate content ofα-Si3N4 whisker were added as seeds and diluting agents, which could promote the growth ofα-Si3N4 whiskers, avoid the phenomenon of melting Si and optimize the microstructure. Carbamide and starch were investigated as pore-forming agents. Results indicated that the former is better in pore-forming and the latter is better in keeping intensity. However, carbamide is the better pore-forming agent considering porosity and intensity synthetically. The porous RBSN was prepared by adding pore-forming agents, with the range of porosity from 31.04% to 51.80%, and the intensity from 175.61MPa to 42.09MPa correspondingly, the dielectric constant of samples was reduced to 4.01 from 5.56, and the loss tangent of samples was improved to 13.25×10-3 from 5.02×10-3 respectively.The influences of preparing process (such as adding ways of sintering aids, particle gradation and nitridation sintering schedule) on properties of porous RBSN were investigated by laser particle size analysis and SEM. The influence was very obvious by studying relations of microstructure and sample properties. The results showed that the Si was coated with sintering aids by the liquid phase precipitation-coating method. RBSN ceramics with higher porosity were prepared and the nitridation rate was close to that obtained by solid technique. Porous ceramics were formed by utilizing the reasonable gradation of ceramic particles. When the mass proportion of coarse Si powders and fine Si powders was 7:3, the porosity could be promoted; nitridation rate and intensity could be improved too. The cyclic nitridation sintering schedule was applied, which was in favor of diffusion N2, and could promote the nitridation and intensity. The model and mechanism of Si nitridation were studied. Theα-Si3N4 was formed mainly on the mechanism of vapor or vapor-solid reaction, andβ-Si3N4 on the mechanism of liquid or vapor-solid reaction. The diffusion of N2 to Si through Si3N4 should be resolved for improving nitridation rate.
|
PDF Full Text Request