Synthesis of the MAX-phase Ti3SiC2 by the flux method

MAX phases combine metallic and ceramic properties, in particular electrical conductivity, thermal conductivity, resistance to high temperatures, oxidation and thermal shock, and compressive strength. Their widespread use is limited by the difficulty of obtaining single-phase samples. We have found that an alternative method for the synthesis of the MAX phase Ti 3 SiC 2 can be the flux method. Elemental Bi was used as flux medium because it only forms binary compounds with the Ti–Si–C system: TiBi 2 , Ti 8 Bi 9 , Ti 2 Bi. The melting point of Bi, T m = 271 °C, and the boiling point, T b = 1564 °C, give an appropriate temperature range for MAX phase formation. The whole experiment was carried out in a glove-box, in which a high-temperature furnace was installed, for working under a protective argon atmosphere. Samples of compositions Ti 50 Si 25 C 25 and Ti 50 Si 17 C 33 were synthesized. A mixture of elemental powders was placed in the corundum crucible containing pieces of Bi (2.2 g of bismuth for 1 g of powder). The crucible was then sealed in a tantalum ampoule, which was placed in a high-temperature furnace. The first step consisted in rapid heating to 1500 °C (220 °C /h). The sample was kept at 1500 °C for 50 h, slowly cooled to 1000 °C and kept at this temperature for 30 h, and then cooled to room temperature. The second step was the separation of the products from the Bi-flux using a centrifuge. Phase analysis was performed using X-ray powder diffraction (Cu K α 1 radiation). Both samples appeared to be multiphase and contained titanium silicon carbide, Ti 3 SiC 2 , as the main phase, with a maximum content of 84 mass%. For the sample Ti 50 Si 25 C 25 three phases were observed: ternary carbide Ti 3 SiC 2 (own structure type, Pearson symbol hP 12, space group P 6 3 / mmc , a = 3.06499(6), c = 17.6562(4) Å, 84 mass%) and two binary silicides, Ti 5 Si 3 (Mn 5 Si 3 , hP 16, P 6 3 / mcm , a = 7.4306(2), c = 5.1245(2) Å), 9 mass%) and TiSi 2 (own type, oF 24, Fddd , a = 4.7951(2), b = 8.2599(3), c = 8.5412(4) Å, 7 mass%). Three phases were also observed for the sample Ti 50 Si 17 C 33 : carbides Ti 3 SiC 2 (own structure type, Pearson symbol hP 12, space group P 6 3 / mmc , a = 3.06596(6), c = 17.6619(4) Å, 68 mass%) and TiC (NaCl, cF 8, Fm -3 m , a = 4.3183(1) Å, 11 mass%), and a phase corresponding to the crucible material, Al 2 O 3 (own type, hR 30, R -3 c , a = 4.7574(1), c = 12.9889(7) Å, 21 mass%). No evidence of elemental Bi or Bi-based byproducts was observed on the diffraction patterns. To increase the content of the main phase, it is necessary to add an excess of Si to the initial stoichiometric (3:1:2) mixture. Keywords : titanium silicon carbide, MAX phase, flux method, X-ray powder diffraction, crystal structure.