Development of High Thermal Conductivity Silicon Nitride Ceramics Using Rare-Earth Metal Complexes as the Precursors for Sintering Aids

1.稀土金属配合物的合成及合适溶剂的寻找选择CN-作为不含氧原子的配体，考察了在任何条件下只生成氢氧化物的Y3+水溶液中氰基配合物的合成。以乙二胺(ED)为溶剂，合成了具有高吸湿性的[Y(ED)3]C13配合物，发现乙二胺是一种溶解YC13的非水溶剂。以它为溶剂，用化学计量的ED合成了[Y(Ed)3]C13溶液。将氮化硅粉末和助烧剂与溶液混合干燥，这是一种均匀加入络合物的新方法。氮化硅烧结体的制备及其评价添加[Y(Ed)3]C13作为“脱氧烧结助剂”，制备了氮化硅烧结体。然而，它们的烧结密度很低，约为60%。用麦克斯韦关系式估算的致密烧结体的导热系数比没有“脱氧助烧剂”的烧结体要高。X-射线衍射仪表明Y2Si3N6的存在，这是以前没有报道过的。在1800℃下热压致密烧结体，在1850℃下进行退火热处理，完成相变。这导致了烧结体的显著膨胀，这可能是由于复合体在完全分解之前的致密化所致。评价了添加第IV族元素的氮化物和碳化物对氮化硅烧结体的影响。虽然热力学计算表明，在MX(M=Ti，Zr或Hf，X=N或C)中，只有ZrN和HfN能起到“脱氧助烧剂”的作用，但所有烧结体的导热系数都低于没有MX的烧结体。结果发现，IV族元素氮化物显著提高了致密化程度。

1. Synthesis of rare-earth metal complexes, and search for appropriate solventsSelecting CN-as a ligand without oxygen atom, the synthesis of cyano complex was examined from an aqueous solution of Y3+, which produced only hydroxides in any conditions. Selecting ethylene diamine (ED), the [Y(ED)3]C13 complex was successfully synthesized using ED itself as a solvent, which was confirmed to be highly hygroscopic.It was found that a non-aqueous solvent CH3CN dissolved YC13. Using it as a solvent, a [Y(ED)3]C13 solution was synthesized using a stoichiometric amount of ED. Powders of Silicon nitride and sintering aids were mixed with the solution and were subsequently dried, which was a new method for homogeneously adding the complex.2. Fabrication of silicon nitride sintered bodies and their evaluationAdding [Y(ED)3]C13 as a "de-oxygen sintering aid", silicon nitride sintered bodies were fabricated. However, they showed very low sintered densities about 60 %. Estimated thermal conductivities of dense sintered bodies using the Maxwell relationship were found to be higher than those without "de-oxygen sintering aid". The XRD showed the existence of Y2Si3N6 which had never reported. The dense sintered body fabricated by hot-pressing at 1800℃ was annealed at 1850℃ to finish the phase transformation. This resulted in significant swelling of sintered body, which would be due to the densification before the complete decomposition of complex.3. Effect of the nitrides and carbides of the group IV elementsSilicon nitride sintered bodies fabricated by adding the nitrides and carbides of the group IV elements were evaluated. Although the thermodynamic calculations predicted that only the ZrN and HfN could act as the "de-oxygen sintering aid" among MX (M=Ti, Zr or Hf, X=N or C), all the sintered bodies showed lower thermal conductivities than those without MX. It was found that the group IV element nitrides significantly improved densification.