Herein we report on a systematic investigation of the thermal expansion of select M n+1 AX n phases. The bulk dilatometric thermal expansion coefficient ␣ dil 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 ␣ a ranged between ͑2.9Ϯ 0.1͒ ϫ 10 −6°C−1 ͑Nb 2 AsC͒ and ͑12.9Ϯ 0.1͒ ϫ 10 −6°C−1 ͑Cr 2 GeC͒ while the coefficient for the c axis ͑␣ c ͒ ranged between ͑6.4Ϯ 0.2͒ ϫ 10 −6°C−1 ͑Ta 2 AlC͒ and ͑17.6Ϯ 0.2͒ ϫ 10 −6°C−1 ͑Cr 2 GeC͒. Weak anisotropy in the thermal expansion was seen in most phases, with the largest value of ␣ c / ␣ a belonging to Nb 2 AsC. The Grüneisen 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 c 13 and we conclude that the anisotropy in thermal expansion is related to the bonding between the M − A elements.
In this paper we report on the electronic, magnetotransport, thermoelectric, and thermal properties of Ti3Al(C0.5,N0.5)2, Ti2Al(C0.5,N0.5), and Ti2AlN. The electrical conductivities, Hall coefficients, and magnetoresistances are analyzed within a two-band framework and compared with the end members, Ti2AlC and Ti3AlC2. The analysis shows that all compounds are compensated conductors with hole and electron carrier densities of about 1.5×1027 m−3. The room temperature thermal conductivities of the carbonitrides are both over ≈50 W/mK, with the phonon contribution of Ti3Al(C0.5,N0.5)2 particularly large at over 35 W/mK. The low-temperature specific heat of Ti3Al(C0.5,N0.5)2 and Ti2Al(C0.5,N0.5) yield Debye temperatures of 685 and 724 K, respectively, comparable to those of the end members.
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