A. Ganguly

Thermal expansion of select Mn+1AXn (M=earlytransitionmetal, A=Agroupelement, X=C or N) phases measured by high temperature x-ray diffraction and dilatometry

Herein we report on a systematic investigation of the thermal expansion of select Mn+1AXn phases. The bulk dilatometric thermal expansion coefficient αdil was measured in the 25–1200 °C temperature range and the thermal expansion of more than 15 of these phases was studied by x-ray diffraction in the 25–800 °C temperature range. The coefficient of thermal expansion for the a axis αa ranged between (2.9±0.1)×10−6 °C−1 (Nb2AsC) and (12.9±0.1)×10−6 °C−1 (Cr2GeC) while the coefficient for the c axis (αc) ranged between (6.4±0.2)×10−6 °C−1 (Ta2AlC) and (17.6±0.2)×10−6 °C−1 (Cr2GeC). Weak anisotropy in the thermal expansion was seen in most phases, with the largest value of αc/αa belonging to Nb2AsC. The Gruneisen parameters along the a and c directions were calculated from ab initio values for the elastic compliances and were relatively isotropic. A good correlation was found between the thermal expansion anisotropy and the elastic constant c13 and we conclude that the anisotropy in thermal expansion is relate...

Synthesis and compressibility of Ti3(Al,Sn0.2)C2 and Ti3Al(C0.5,N0.5)2

In this paper we report on the synthesis of a composition, Ti3Al(C0.5,N0.5)2, belonging to the Mn+1AXn family of ternary layered carbides and nitrides. X-ray and selected area diffraction confirm that this compound is isostructural with Ti3SiC2; its a and c-lattice parameters are 3.0404(5) and 18.414(6) A, respectively. Chemical analysis performed by electron dispersive and electron energy loss spectroscopy confirmed the Ti3AlCN chemistry. Using a synchrotron radiation source and a diamond anvil cell, we also measured the pressure dependencies of the lattice parameters. Up to a pressure of ≈50 GPa, no phase transformations were observed. The bulk modulus is 219±4 GPa, with a pressure derivative, Ko′, of 3.7±0.3. We also fabricated the ternary Ti3AlC2, with some Sn [nominal composition Ti3(AlSn0.2)C2]. Its a and c-lattice parameters are 3.0804(7) and 18.5426(7) A, respectively. Its bulk modulus is 226±3 GPa, with a pressure derivative, Ko′, of ≈4. In both cases, the compressibility was greater along the c ...

Corrosion Behavior of Ti3GeC2 and Ti2AlN in 1 M NaOH

In this paper we report on the corrosion behavior of Ti 3 GeC 2 and Ti 2 AlN in a 1 M NaOH solution, investigated by electrochemical impedance spectroscopy (EIS), polarization, and (-potential measurements. The EIS results for Ti 3 GeC 2 and some results for Ti 2 AlN could be fitted to a complex equivalent circuit, where the double layer is represented by a constant phase element. The passive film is represented by a polarization resistance in series with a parallel connection of a resistance of the passive film (R pf ) and its capacitance (C pf ). In both cases passive layers were formed. Based on the ζ-potential measurements, we conclude that the passivating layer that forms on Ti 3 GeC 2 , over a wide pH range, is most likely GeO 2 -based, while the one formed on Ti 2 AlN is most likely Al 2 O 3 -based and/or TiO 2 -based.

High-Temperature Oxidation of Ti3GeC2 and Ti3Ge0.5Si0.5C2 in Air

In this paper, we report on the oxidation behavior of Ti 3 GeC 2 and Ti 3 (Ge 0.5 ,Si 0.5 )C 2 . The oxidation kinetics were studied thermogravimetrically in air in the 700-1000°C temperature range. The oxidation layers formed on Ti 3 GeC 2 at 700°C were protective; at 800°C and higher, they were not. The addition of Si to Ti 3 GeC 2 slightly enhanced its oxidation resistance, but at 800°C and above, the layers formed were again not protective. In both cases the oxidation occurred mostly by the inward diffusion of oxygen through a rutile-based (Ti 1-y ,Ge y )O 2 solid solution with y < 0.1. At higher temperatures and/or longer times, Ge accumulation was observed at the oxide/carbide interface. At 900°C and higher, however, the oxygen induces the peritectic decomposition of Ti 3 GeC 2 into Ge and, presumably, a titanium oxycarbide. At 1000°C, part of the Ge escapes to the surface and forms copious amounts of GeO 2 whiskers with hexagonal symmetry. In the temperature range explored there is no oxygen dissolution in the carbide matrices.