Low temperature elastic and electronic properties of MAX phases

Peter Finkel

doi:10.17918/etd-199

The M[n+1]AX[n] phases (where M is an early transition metal, A is an A-group element and, X is C and/or N and n = 1 to 3) represent a new class of carbides and nitrides and can be best described as polycrystalline nanolaminates. They combine some of the best properties of ceramics and metals. Their physical properties (stiffness, damage and thermal shock resistance, high thermal and electrical conductivity) along with the fact they are readily machinable, make them extremely attractive in terms of the potential technological applications. Knowledge of low-temperature behavior is vital because it can provide insight into M[n+1]AX[n]-phases' physical properties. This work entails the systematic study of the elastic, electrical, galvanomagnetic and thermal properties of these materials in the 4-300 K temperature range. The elastic constants of these compounds (Ti₃SiC₂, Ti₃AlC₂ and Ti₄AlN₃) were measured over the 20-300 K temperature range. Their Young's and shear modulii determined from ultrasonic velocities were in 300-335 and 124-140 GPa range, respectively; both moduli increase slowly with decreasing temperature and reaching a maximum at temperatures below 125 K; Poisson's ratio is 0.2. The Debye temperatures, [theta][D], of these compounds calculated from the mean ultrasonic velocity are in 650-780K range which is in agreement with data obtained from low- temperature heat capacity measurements. To characterize the electronic transport properties, the resistivity, magnetoresistance, Hall effect, Seebeck coefficient and magnetic susceptibility were measured in the 4-300K range, and in magnetic fields up to 9T. All MAX-phases exhibit metal-like temperature dependence of the resistivity [rho](T). [theta][D] for most of the MAX-phases determined by fitting [rho](T) with the Bloch T5 formula were in good agreement with the values determined from elastic and calorimetric measurements. The carrier density of electrons n (or holes, p) and their mobilities were calculated utilizing a semi-classical isotropic two-band model. It was shown that most of the M[n+1]AX[n] (n=2) phases (Ti₃SiC₂, Ti₃AlC₂, Ti₃GeC₂) are nearly compensated conductors with n ~ p and total density of electrons and holes of ~1028 m³. This result also was is in agreement with conclusions based on negligible thermopower of these solids. Extensive study of electronic and galvanomagnetic properties and analysis of the results suggest M[n+1]AX[n] (n=2) phases can be characterized by relatively high charge carrier mobilities.

Low temperature elastic and electronic properties of MAX phases

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Low temperature elastic and electronic properties of MAX phases

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