Ternary Compounds in the Systems Al-M-Si (M=Ni,Co,Fe)

Research project P 14762 Ternary Compounds in the Systems Al-M-Si (M = Ni, Co, Fe) Klaus RICHTER 09.10.2000 A systematic study of phase equilibria, structure and physical properties will be carried out in the technologically interesting ternary systems AI-M-Si with M = Ni, Co, Fe. Investigations will be started with a thorough survey of phase equilibria and ternary phase reactions in the three systems using X-ray powder diffraction, differential thermal analysis, metallographic methods and electron probe microanalysis as experimental techniques. Based on the results from the phase diagram studies the crystal structure of insufficiently characterized and new ternary phases will be investigated by means of single crystal X-ray diffraction. Solid solubility ranges and the effect of AI-Si substitution in the ternary compounds will be investigated. The results of structure determination will be used as a basis for a discussion of chemical bonding employing electronic band structure calculations. Thermodynamic properties of selected ternary, alloys, especiany in the composition range of the technologically important transition-metal rich alloys like Fe3AI-Fe3Si and Ni3AI(Si), will be studied by means of high temperature solution calorimetry and emf-methods.

In the course of the project, a detailed investigation of the ternary phase diagrams Al-Ni-Si and Al-Co-Si was carried out. Phase equilibria were studied at several isothermal sections and these data were combined with differential thermal analysis within vertical sections in order to work out the reaction temperatures. The investigations yielded a complete description of the systems, including a list of invariant reactions, liquidus surface projections, reaction schemes (Scheil Diagrams) and a number of isothermal and vertical sections. The resulting phase diagrams provide a consistent and reliable basis for application related investigations in these technologically interesting systems. A number of previously unknown compounds could be identified and their structures were investigated in detail. In the Al-Ni-Si system this includes the phase Ni13+-x Aly Si 9-y (T`; hP66, P31 21) which adopts a partially filled superstructure of the NiAs-type and the low temperature compound Ni16Si 9 Al forming a new orthorhombic structure type related to Ni3 Si 2 (oC104, Cmcm). In the system Al-Co-Si four new compounds could be prepared which adopt new and unique structure types with interesting structural features: Co6 Al11Si 6 (oC184, Cmc2 1 ), Co4 Si 1+x Al8-x (mC26, C2/m), Co19+xAl45ySi 10y (mC296+, C2/c), Co20Al47xSi 17x (oP168, Pnma). Two additional phases with the approximate compositions Co32Al45Si 23 and Co42Al32Si 26 were synthesized but could not be structurally characterized up to this point. A highlight among the project results was the preparation and characterization of two new materials based on cubic NiSi 2 (CaF2 -type, cF12, Fm3m) that show no lattice mismatch to Si and are thus very promising materials for perfect epitaxial films grown on silicon. The perfectly lattice-matched materials are special compositions within the extended solid solution phases NiSi 2-x Alx and NiSi 2-x Gax with x = 0.26 for Al and x = 0.17 for Ga, respectively. Both phases are thermally stable up to temperatures around 1000C, are in thermodynamic equilibrium with pure silicon and show metallic electrical conductivity. First results with thin film preparation techniques confirm the high potential for the application in metal-silicon epitaxy. Potential applications include devices like metal- and permeable base transistors as well as sophisticated three dimensional device structures using buried interconnections.