Preparation of Lutetium Nitride by Direct Nitridation
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Junichi Takahashi and Hisanori Yamane Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan (Received 11 November 2003; accepted 22 December 2003)
Lutetium nitride (LuN), an end member of a new quaternary system Si3N4–SiO2– Lu2O3–LuN, was synthesized by direct nitridation of a lutetium metal. The nitridation extent of the lutetium ingot (10 × 5 × 2 mm) reached about 97% by heating at 1600 °C for 8 h with an applied N2 pressure of 0.92 MPa; the initial shape of the bulk metal was maintained in the course of nitridation. The resulting nitrided lutetium possessed a moderately low oxygen content (∼0.7 wt%), which enables the preparation of uncharacterized high nitrogen-containing phases in the Lu–Si–O–N system.
I. INTRODUCTION
In the recent development of rare-earth (RE)-doped silicon nitride (Si3N4) ceramics, lutetium oxide (Lu2O3) has been found to be a good sintering additive for producing fully dense materials with excellent strength at temperatures up to 1500 °C.1 The high-temperature properties of Si3N4 materials are determined mainly by the grain boundary phase assemblage, and this in turn is determined by the phase relationships in the RE–Si–O–N system. Thus, numerous researches have been devoted to clarifying the phase relationships in the Si3N4–SiO2– RE2O3 (RE⳱Y,2–4 La,5 Ce,6 Yb7,8) pseudoternary systems, and the rare-earth silicon oxynitride phases of RE 2 Si 3 O 3 N 4 (M-phase), RESiO 2 N (K-phase), RE4Si2O7N2 (J-phase), and RE5Si3O12N (H-phase) have been reported. Hirosaki et al.9 investigated the phase relationships in the Si3N4–SiO2–Lu2O3 system and revealed the presence of a new nitrogen-rich compound, Lu3Si5ON9, which coexists with Si3N4 and Lu-J-phase10 at the composition corresponding to the Lu-M-phase. This result indicated the existence of the compatibility triangles across the Si3N4–Lu2O3 line, triggering the expanding research on the previously unexplored Si3N4–Lu2O3–LuN region of the Lu–Si–O–N quaternary system (Fig. 1). It is generally acknowledged that the phase relationships of the Si3N4–RE2O3–REN region of the quaternary system are difficult to assess because of the difficulty of
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 19, No. 3, Mar 2004
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synthesizing a constituent rare-earth nitride in a moderate processing condition; this subject is also rarely found among literature.11–13 Recently, the phase relationships in the Si 3 N 4 –YN 14 and Si 3 N 4 –SiO 2 –RE 2 O 3 –REN (RE⳱Y, La)15 systems were investigated without using the nitride starting material. In both studies, the phase relationships were deduced from samples synthesized mainly by the carbothermal reduction–nitridation of RE2O3–SiO2,14,15 RE2O3–Si,14 or RE2O3–Si3N415 mixtures with various RE/Si ratios. It should be noted, however, that this approach is applicable only if the reduction–nitridation of the constituent metal oxide is possible in practice, and precise
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