Synthesis of novel alkaline-earth silicon nitrides and evaluation from the viewpoint of materials science

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The synthesis conditions of alkaline-earth silicon oxynitrides, Ca-Si-O-N and Ba-Si-O-N systems, have been investigated in this research; properties of the resulting powders were examined chiefly from the viewpoints of their luminescence characteristics due to the addition of rare-earth ions. The oxynitride materials examined in this research were Ca_3Si_2O_4N_2, Ca_2Si_30_2N_4 and BaSi_6ON_8. The starting materials used in this research were calcium carbonate (CaCO_3), barium carbonate (BaCO_3), silicon oxide (SiO_2), silicon nitride (Si_3N_4) and rare-earth oxide (= activator; Eu_2O_3, Yb_2O_3, CeO_2, Tb_4O_7 and Sm_2O_3).Firstly, the optimum conditions (temperature and time) for the synthesis of Ca_3Si_2O_4N_2 (N/O ratio = 0.5) prepared using CaO, SiO_2 and Si_3N_4 were found to be 1450℃ and 2 h, respectively. The luminescence properties of (Ca_<1-x>REx)_3Si_2O_4N_2 (RE: Rare earth element) were examined. Among these rare-earth doped compounds, (Ca_<1-x>Smx)_3Si_2O_4N_2 and (Ca_<1-x>Tbx) 3Si_2O_4N_2 indicated red and green emitting, respectively. On the other hand, the single phase of Ca_3Si_2O_2N_4(N/O ratio = 2.0) was obtained heating the starting materials (CaO and Si_3N_4) at 1550℃ for 2 h. The (Ca_<1-x>Eux)3Si_2O_2N_4 emitted the reddish orange light whose peak wavelength was shifted from 606 to 620 nm with increasing x value from 0.01 to 0.1.Furthermore, the single phase of BaSi_6ON_8 was obtained heating the starting materials (BaCO_3, SiO_2 and Si_3N_4) 1750℃ for 2 h. The excitation and emission peaks of (Ba_<1-x>Ce_x) Si_6ON_8 were observed at around 363 and 480 nm, respectively; the relative intensity of (Ba_<0.95>Ce_<0.05>) Si_6ON_8, compared to the intensity of CaWO_4 as a standard substance, attained 0.82. The excitation and emission peaks of (Ba_<1-x>Eux) Si_6ON_8 were found at around 300 and 500 nm, respectively, showing the blue-green emission. The relative intensity of (Ba_<0.96>Eu_<0.04>) Si_6ON_8 attained 2.7.