Design of materials with anomalous thermophysical properties and desorption-assisted phase formation of intermetallic thin films

  • Design von Materialien mit anormalen thermophysikalischen Eigenschaften und desorptionsunterstützte Phasenbildung von intermetallischen Dünnschichten

Keuter, Philipp; Schneider, Jochen M. (Thesis advisor); Raabe, Dierk (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020, Kumulative Dissertation

Abstract

Part I of this thesis is dedicated to the design of materials exhibiting anomalous thermophysical properties. To study the anomalous thermoelastic behavior of bcc V, Nb, Ta, as well as fcc Pd and Pt, a density functional theory (DFT) based model is used, which allows for the calculation of the elastic constants c' and c44 as a function of temperature. The combination of high electronic density of states (DOS) at the Fermi level and electronic reallocation upon lattice distortion, causing an increase in bond strength, is identified as the physical origin for anomalous thermoelastic behavior in these metals. Tailoring of the thermoelastic properties, crucial to adjust the thermoelastic behavior for specific applications, is studied theoretically for Nb-X (X=Zr, V, Mo) solid solutions. The onset of the increase in c44 remains unchanged for isoelectronic Nb-V indicating the absence of a size effect on the anomalous thermoelastic behavior. In contrast, the anomalous thermoelastic behavior can be tuned by alloying Nb with Zr or Mo, due to the valence electron concentration induced change in the DOS in the vicinity of the Fermi level, leading to a shift in the anomalous trend of c44 to lower temperatures. However, with increasing Mo concentrations, the anomaly in both elastic constants is suppressed due to the continuous reduction in electronic states at the Fermi level. After establishing that the computational strategy employed in this work is sound for anomalous elements and solid solutions, the stability of the cubic HfV2 Laves phase, exhibiting an increasing elastic modulus with temperature, is studied theoretically. The results suggest that the stabilization of cubic HfV2 is achieved by a complex interplay of H and O impurities, potentially present in real compounds, lattice vibrations, which contribute to the energetic stabilization, as well as the electronic entropy, supporting the mechanical stabilization. Besides anomalous thermoelastic behavior, some materials anomalously contract upon heating which is commonly referred to as negative thermal expansion (NTE). The elastic behavior of cubic HfV2O7, exhibiting isotropic NTE above 370 K, is investigated in a synergetic strategy of ab initio calculations as well as nanoindentation experiments performed on sputtered thin films due to its relevance for the applicability in zero thermal expansion composites. The elastic properties of HfV2O7 are affected by long range dispersion interaction, which may be induced by severe modification in second-nearest neighbor O-O bond distance as obtained upon compression. Self consistent results are obtained for the measured elastic modulus and dispersion corrected DFT calculations. In part II of this thesis, the synthesis-composition-structure relationship in the Mg Ca Al system, which is characterized by volatile constituents, is studied using combinatorial magnetron sputtering. The obtained decrease in Mg concentration with increasing temperature is explained by DFT calculations mimicking desorption processes during thin film growth. Correlative structural and chemical analysis of binary Mg-Ca thin films suggests the formation of hexagonal Mg2Ca Laves phase in a wide Mg/Ca range from 1.7 to 2.2, expanding the to date reported stoichiometry range. Pronounced thermally-induced desorption of Mg is utilized to synthesize stoichiometric (Mg1-x,Alx)2Ca Laves phase thin films by additional co-sputtering of elemental Al, which governs its preferred incorporation during synthesis. X-ray diffraction investigations along the chemical gradient suggest the formation of hexagonal (Mg1-x,Alx)2Ca with a critical Al concentration of up to 23 at%. The results of this thesis pave the way for the discovery and tailoring of materials with anomalous thermophysical properties (part I) and promote thermally-induced desorption of weakly bonded species, and the preferential incorporation of strongly bonded species during magnetron sputtering as a promising synthesis strategy to study intermetallics (part II).

Identifier

Downloads