Formation of C54TiSi(2): Effects of niobium additions on the apparent activation energy

The formation of C54 TiSi2 using Ti-Nb alloys deposited on polycrystalline Si substrates was studied by means of in situ x-ray diffraction and resista nce measurements during temperature ramping. Alloys with Nb contents rangin g from 0 to 13.6 at. % were used. The formation temperature of C54 TiSi2 wa s reduced in the presence of Nb. However, the addition of Nb in Ti did not cause fundamental changes in the evolution of resistance versus temperature . This latter observation suggests that the mechanism for the formation of C54 TiSi2 remained the same in spite of the enhancement effect. For alloys with up to 8 at. % of Nb, the C49 TiSi2 phase formed first, as with pure Ti . When annealing the alloy with 13.6 at. % Nb, neither C49 TiSi2 nor C54 we re found in the usual temperature ranges, instead, C40 (Nb,Ti)Si-2 was obse rved. This phase transformed to C54 (Nb,Ti)Si-2 above 950 degreesC. The app arent activation energy associated with the formation of C54 TiSi2 was obta ined by annealing the samples at four different ramp rates from 3 to 27 K/s ; it decreased continuously from 3.8 to 2.5 eV with increasing Nb content f rom 0 to 8 at. %. The apparent activation energy for the formation of C40 ( Nb,Ti)Si-2 was found to be 2.6 eV. The possible physical meaning, or lack t hereof, of the high activation energies derived from experimental measureme nts is extensively discussed. A qualitative model is proposed whereby nucle ation would be rate controlling in pure TiSi2, and interface motion in samp les with 8 at. % Nb. (C) 2001 American Institute of Physics.