Effect Of Rare-Earth Chloride Additives On Sintering Behavior And Properties Of Silicon Nitride Ceramics

Influence on the fabrication of are-earth oxide (Re2O3) on silicon nitride (si3N4) ceramics additives had been very well researched, while rare-earch chloride (ReC13) additives were found to be scarely studied. Thus this paper focused on the action mechanism of ReC13on Si3N4ceramics, through observing the density, morphologies, phase, thermal conductivity and mechanical properties of Si3N4ceramcis, when changing the types of ReC13, the ratios of LaC13/MgO as well as La2O3/LaC13.This paper mainly studied the effect of ReC13types (Re:La, Nd, Gd, Yb) on the sintering behavior and properties of Si3N4ceramcis. Row materials with5wt%ReC13and3wt%MgO were sintered by SPS method at1600℃for5min, and following heat treated at1900℃for3hours to get Si3N4ceramics. Results demonstrated that most dissolve-precipitation of Si3N4particles during SPS process were between1300℃and1500℃. Futuremore, different kinds of ReC13additives showed different degree in helping to promote sintering process, namely, the smaller the Re3+was, the bigger positive influence of ReC13. Element Mg content decreased and element Cl disappeared after heat treatment stage, which leaded to the drop of density, aâ†'β phase transformation was finished and columnar Si3N4grains were formed after heat treatment. Residuary Re-Mg-Si-O liquid gathered between Si3N4grains and generated several intergranular phase. In general, adding ReC13additives could obtain Si3N4ceramics of excellent mechanical properties and good thermal conductivity properties.Then, keep the total amount of LaC13-MgO as8wt%when changing their ratios from0:8to8:0. Results showed that aâ†'β phase transformation temperature was lowered by adding merely LaC13. However, sintering rate of Si3N4ceramics was promoted only in the case of adding MgO at the same time. After heat treatment, density of samples decreased for vaporization of element Mg and Cl. Although it could not prevent the formation of intergranular phase or reduction of element by changing the ratios of LaC13/MgO, it played a big role in affecting thermal conductivity and mechanical properties of Si3N4ceramics. Si3N4ceramics obtained outstanding comprehensive performance when the ratio was4:4and5:3according to the study.Given the disappearance of element Cl in LaC13after heat treatment, La2O3-LaC13 compound additives were used to improve the properties of Si3N4ceramics. In the experiment to prepare Si3N4ceramics, MgO content was3wt%and La2O3-LaC13content was totally5wt%, while the ratios of La2O3and LaCh ranged from0:5to5:0. The research found that it could reduce the temperature of forming liquid by adding La2O3, LaC13and MgO collectively. Compared with LaC13, the density of La2O3-added samples was higher for the reason that La2O3not only promoted the sintering rate but it also could partly compensate for density reduction caused by element Mg’s ablation. What’s more, element La reacted with Si3N4and erosed the surface of columnar β-Si3N4grains, which resulted in the lower thermal conductivity and bending strength of LaC13-added Si3N4ceramics than La2O3-added Si3N4ceramics.