Li_(2)O-Si_(3)N_(4)-AlN系统相关系和Li-α′-Sialon的形成

Phase Relationships in the Li_(2)O-Si_(3)N_(4)-AlN System and the Formation of Li-α'-Sialon

研究了 Li_2O-Si_3N_4-AlN 系统1750℃的等温截面。在 Si_3N_4-Li_2O∶3AlN 连线上,测定了 Li-α′-Sialon 固溶体(Li_(?)Si_(?)Al_(3n)O_nN_(?),2n≤2)区,其稳定范围为2n=0.25～1.50。研究了 Li-α′-Sialon的形成反应和致密化过程。发现在1300～1600℃范围内先生成一个中间相:Li-O′-Sialon,从1600℃才开始并快速地形成 Li-α′-Sialon,1700℃可获得接近理论密度的纯 Li-α′-Silon 相。对 Li-α′-Sialon 的高温稳定性也作了讨论。

α′-Sialon structures based on the Si_(12)N_(16) unit cell of α-Si_3N_4 occur in the M-Si-Al-O-N systems where M includes Li,Ca,Y and many rare-earth elements.Theα-Si_3N_4 unit cell has two large interstitial sites that can accommodate additionalatoms M.α′-Sialon is derived from α-Si_3N_4 by partial replacement of Si^(4+) with Al^(3+)and valency compensation is affected by M cations occupying these interstices inthe Si-Al-N network.The phase relationships of α′-Sialon-containing systems M_xO_y-Si_3N_4-AlN(M=Ca,Y,Nd,Sm,Gd,Dy,Er and Yb) have been studied in our previous works.Li-α′-Sialon also occurs in the corresponding system Li_2O-Si_3N_4-A1N and the singlephase α′-Sialon lies on the line Si_3N_4-Li_2O:3AlN.In the present work,the isothermal section at 1750℃ of Li_2O-Si_3N_4-AlN sys-tem was studied.On the line Si_3N_4-Li_2O:3 AlN,the homogeneous range of Li-α′-Sialon(Li_(2n)Si_(12-3n)Al_(3n)O_nN_(16-n),2n≤2) was determined to be 2n=0.25～1.50.This value is larger than that of the corresponding Ca-α′-Sialon(Ca_nSi_(12-3n)Al_(3n)O_nN_(18-n),n=0.3～1.40)and Y-α′-Sialon Y_nSi_(12-4.5n)O_(1.5n),N_(16-1.5n),n=0.33～0.67).Anextensive liquid phase region starting from Li_2O-rich region Widely expands towardsA1N and Si_3N_4.The eutectic temperature of the liquid phase has not been accuratelydetermined,but the composition 10 Li_2O:Si_3N_4:AlN has the lowest melting point(～900℃) among the compositions explored.The reaction sequence of Li-α′-Sialonwas studied.Using Si_3N_4,Li_2O and AlN as starting materials,Li-O′-Sialon(LiSiAlN_2O)is a transitional phase (1300～1600℃) before the formation of Li-α′-Sialon,which starts to form at 1600℃.Single～phase Li-α-Sialon can be obtained by hot-pressing at 1700℃ for 2 h.The thermal stability of Li-α′-SiaIon was also testedand the results show that it is stable up to 17(?)0℃ under N_2 and the stability tem-perature can be raised to 1800℃ if the sample is embedded in A1N powder.