Fabrication of Novel Ceramics in the Rare-Earth Oxide and Silicon Nitride System and Their Evaluation for the Application to the Materials

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The author fabricated four kinds of ceramics in the Si_3N_4-SiO_2-Y_2O_3 system, i.e., melilite-type structure (Y_2Si_3O_3N_4), wollastonite-type structure (YSiO_2N), cuspidine-type structure (Y_4Si_2O_7N_2) and apatite-type structure (Y_5(SiO_4)_3N), and evaluated the thermal and mechanical properties. Moreover, synthesis conditions of RE_2Si_3O_3N_4 (RE = La, Ce, Nd, Sm, Gd, Dy, Er and Yb) were examined using Si_3N_4 and RE_2O_3. The high-density ceramics in the Si_3N_4-SiO_2-Y_2O_3 system were fabricated from the powders by hot pressing technique. Relative densities of the hot-pressed ceramics in the Si_3N_4-SiO_2-Y_2O_3 system were 98% or higher. The magnitude of the flexural strength of these ceramics could be arranged as follows; YSiO_2N(249 MPa) > Y_2Si_3O_3N_4(193 MPa) > Y_4Si_2O_7N_2(177 MPa) > Y_5(SiO_4)_3N(144 MPa). Oxidations of Y_2Si_3O_3N_4 and YSiO_2N ceramics started at around 1100℃ and 1050℃, respectively ; oxidation of Y_4Si_2O_7N_2 ceramic started at around 800℃, whereas that of Y_5(SiO_4)_3N ceramic started at around 850℃. Synthesis conditions of RE_2Si_3O_3N_4 (RE = rare-earth metal) were examined using Si_3N_4 and RE_2O_3. Single phases of RE_2Si_3O_3N_4 were formed in the case of RE = Nd, Sm, Gd, Dy and Er. Among these compounds, only Gd_2Si_3O_3N_4 emitted the light by the irradiation of near-ultraviolet beam. The activators (RE'_2O_3) were doped to the Gd_2Si_3O_3N_4. In the case of Sm-doped Gd_2Si_3O_3N4, the excitation peak appeared at 300 nm, whereas the emission peak appeared at 601 nm. This phosphor emitted the red light. With increasing amount of Sm dopant, the emission intensity of Gd_2Si_3O_3N4 with 1 mol% of Sm dopant achieved 1.8. It was concluded that among four kinds of the composites in the Si_3N_4-SiO_2-Y_2O_3 system, the flexural strengths of YSiO_2N ceramics fabricated by hot pressing-techniques showed maximum (249 MPa), whereas 1 mol% of Sm-doped Gd_2Si_3O_3N_4 intensity showed maximum (relative intensity:1.8).