MgO-Si_3N_4-AlN 系统的相关系

Phase Relationships in the MgO-Si_3N_4-AlN System

本文提出了 MgO-Si_3N_4-AlN 系统亚固相图和1750℃等温截面相关系。系统中发现存在一个“Mg-12H′-Sialon”相,它于1400℃开始形成,并稳定存在至熔融(～1730℃)。实验得出12H′相的组成约为Mg_(2·3)Al_(0·7)Si_3O_(2·3)N_(4·7),结构为六方晶系(a=3.07(?),c=32.9(?)),它比以前所报道的12H 含有更高的 Mg和 Si。研究表明,Mg 可进入α-Si_3N_4晶格形成 Mg-α′-Sialon。此外,发现下面两个反应的存在,它们只有在液相存在下,才能进行反应:4MgO+Si_3N_4→Mg_2SiO_4+2MgSiN_2MgSiN_2+AlN→MgAlSiN_(?)

MgO has widely been used as sintering additive for Si_3N_4-based ceramics.The strength of Mg-containing ceramics is very high at room temperature,butdegrades at elevated temperatures because of the lower softening point of residualmagnesium silicate glassy phase at the grain boundaries.It has been known that thecombination of AIN and some big cations called “modifying cations”(Li,Ca,Yand many rare-earth elements) in nitride or oxide form can be accommodated in α-Si_3N_4 structure forming α′-sialon.The formation of α′-sialon provides the advant-age of incorporating “modifying cations” both as an impurity and a sintering addi-tive into α-Si_3N_4 structure thus cleaning up the grain boundary and improving thehigh temperature strength.However,the existence of Mg-α′-Sialon is still uncertain.In addition,there exist many A1N polytypoid phases in the Mg-Si-Al-O-N system.The morphology of the fibrous AlN polytypoid would be beneficial to the streng-thening and toughening of sialon ceramics.Therefore,in the present work,both thesub-solidus phase relationships and the isothermal section of 1750℃ in the MgO-Si_3N_4-AlN system were determined.A “Mg-12 H′-Sialon” phase was identified,which starts to form at 1400℃ andis stable upto melting(1730℃).The structure of 12H′ was determined to be hexa-gonal(a=3.07(?) and c=32.9(?))with a composition Mg_(2.3)Al_(0.7)Si_3O_(2.3)N_(4.7),whichcontains more Mg and Si than the Mg-12 H reported before.In the present work,the existence of Mg-α′-Sialon was confirmed.In addition,the following reactionswere identified to occur in the presence of liquid phase:4MgO+Si_3N_4→Mg_2SiO_4+2MgSiN_2MgSiN_2+A1N→MgAlSiN_3