Philipp Keuter

Phase transformations and crystallographic defects are two essential tools to drive innovations in materials. Bulk materials design via tuning chemical compositions has been systematized using phase diagrams. We show here that the same thermodynamic concept can be applied to understand the chemistry at defects. We present a combined experimental and modelling approach to scope and build phase diagrams for defects. The discovery was enabled by triggering phase transformations of individual defects through local alloying, and sequentially imaging the structural and chemical changes using atomic-resolution scanning transmission electron microscopy. By observing atomic-scale phase transformations of a Mg grain boundary through Ga alloying, we exemplified the method to construct a grain boundary phase diagram using ab initio simulations and thermodynamic principles. The methodology enables a systematic development of defect phase diagrams to propel a new paradigm for materials design utilizing chemical complexity and phase transformations at defects.

The temperature-dependent composition and phase formation during the physical vapor deposition (PVD) of Mg-Ca thin films is modeled using a CALPHAD-based approach. Considering the Mg and Ca sublimation fluxes calculated based on the vapor pressure obtained by employing thermochemical equilibrium calculations, the experimentally observed synthesis-temperature trends in the thin-film composition and phase formation were reproduced. The model is a significant step towards understanding how synthesis parameters control composition and, therefore, phase formation in the PVD of metals with high vapor pressures.

Impurities can be incorporated during thin film deposition, but also can originate from atmosphere exposure. As impurities can strongly affect the composition—structure—property relations in magnetron sputter deposited thin films, it is important to distinguish between both incorporation channels. Therefore, the impurity incorporation by atmosphere exposure into sputtered Mg, Al, and Ca thin films is systematically studied by a variation of the deposition temperatures and atmosphere exposure times. Deposition temperature variation results in morphological modifications explained by considering surface and bulk diffusion as well as grain boundary motion and evaporation. The film morphologies exhibiting the lowest oxygen concentrations, as measured by energy dispersive X-ray spectroscopy, are obtained at a homologous temperature of 0.4 for both Mg and Al thin films. For Ca, preventing atmosphere exposure is essential to hinder impurity incorporation: By comparing the impurity concentration in Al-capped and uncapped thin films, it is demonstrated that Ca thin films are locally protected by Al-capping, while Mg (and Al) form native passivation layers. Furthermore, it can be learned that the capping (or self-passivation) efficiency in terms of hindering further oxidation of the films in atmosphere is strongly dependent on the underlying morphology, which in turn is defined by the growth temperature.

In a textured Mg thin film, two types of 〈0001〉 tilt grain boundaries are identified by electron microscopy and atomistic simulation. Coincidence site lattice and dislocation models are applied to study boundaries in hexagonal close-packed crystals.

The HfV₂–HfV₂O₇ composite is proposed as a material with potentially temperature-independent thermophysical properties due to the combination of anomalously increasing thermoelastic constants of HfV₂ with the negative thermal expansion of HfV₂O₇. Based on literature data, the coexistence of both a near-zero temperature coefficient of elasticity and a coefficient of thermal expansion is suggested for a composite with a phase fraction of approximately 30 vol.% HfV₂ and 70 vol.% HfV₂O₇. To produce HfV₂–HfV₂O₇ composites, two synthesis pathways were investigated: (1) annealing of sputtered HfV₂ films in air to form HfV₂O₇ oxide on the surface and (2) sputtering of HfV₂O₇/HfV₂ bilayers. The high oxygen mobility in HfV₂ is suggested to inhibit the formation of crystalline HfV₂–HfV₂O₇ composites by annealing HfV₂ in air due to oxygen-incorporation-induced amorphization of HfV₂. Reducing the formation temperature of crystalline HfV₂O₇ from 550 °C, as obtained upon annealing, to 300 °C using reactive sputtering enables the synthesis of crystalline bilayered HfV₂–HfV₂O₇.

Thermal conductivity of NbO is studied experimentally and theoretically. Sputtered NbO samples, slightly overstoichiometric in oxygen, are x-ray amorphous with a combined NbO and NbO ₂ short-range order, which is consistent with the quantum mechanical data. Upon annealing, NbO ₂ crystallites form in the amorphous matrix due to an energetic preference of 44 meV atom ⁻¹ over NbO counterparts. The measured thermal conductivity at room temperature using time-domain thermoreflectance is 6.9 W m ⁻¹ K ⁻¹ , followed by a continuous increase due to electronic contributions, and it yields a total increase of 40 W m ⁻¹ K ⁻¹ at 650 K partly due to crystallisation. Phonon–phonon scattering can be induced by introducing Ni _0.75 Ta _0.25 O layers in NbO. These multilayers exhibit a thermal conductivity of 5.4 W m ⁻¹ K ⁻¹ at room temperature and a monotonic drop upon increasing temperature. It is thus feasible to modulate the high-temperature thermal conductivity of amorphous NbO by an order of magnitude. Hence, amorphous NbO may be of interest for thermoelectric devices, sensors and thermal insulation applications.

We investigated the elastic properties of the HfV₂O₇ high-temperature phase, exhibiting negative thermal expansion, in a synergetic strategy of first-principle calculations and nanoindentation experiments performed on sputtered films. Self-consistent results were obtained for the measured elastic modulus (73 ± 14 GPa) and dispersion-corrected density functional theory calculations. The elastic properties of HfV₂O₇ are affected by long-range dispersion interaction, which may be induced by severe modification in the second-nearest neighbor O-O bond distance as obtained upon compression. HfV₂O₇ is composed of HfO₆, VO₄, and V₂O₇ building blocks, whereby the latter is characterized by an increasing V-O(-V) bond length upon compression.

The synthesis–composition–structure relationship in the Mg–Ca–Al system is studied using combinatorial magnetron sputtering. With increasing deposition temperature, a drastic decrease in Mg concentration is obtained. This behavior can be understood based on density functional theory calculations yielding a desorption energy of 1.9 eV/atom for Mg from a hexagonal Mg nanocluster which is far below the desorption energy of Mg from a Mg₂Ca nanocluster (3.4 eV/atom) implying desorption of excess Mg during thin film growth at elevated temperatures. Correlative structural and chemical analysis of binary Mg–Ca thin films suggests the formation of hexagonal Mg₂Ca (C14 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 (Mg_1−x,Al_x)₂Ca thin films by additional co-sputtering of elemental Al, exhibiting a higher desorption energy (6.7 eV/atom) compared to Mg (3.4 eV/atom) from Mg₂Ca, which governs its preferred incorporation during synthesis. X-ray diffraction investigations along the chemical gradient suggest the formation of intermetallic C14 (Mg_1–x,Al_x)₂Ca with a critical aluminum concentration of up to 23 at.%. The introduced synthesis strategy, based on the thermally-induced desorption of weakly bonded species, and the preferential incorporation of strongly bonded species, may also be useful for solubility studies of other phases within this ternary system as well as for other intermetallics with weakly bonded alloying constituents.

Nb exhibits an anomalous temperature dependency of the elastic constant c₄₄, which increases from around 500 to 2500 K. This anomaly can be affected by alloying. To study the effect of atomic size and electronic structure on the thermoelastic behavior in bcc Nb-X (X = Zr, V, Mo) solid solutions, the shear thermoelastic constants c₄₄(T) and c′(T) are investigated theoretically using a density functional theory based model in which electronic and thermal expansion effects are treated separately. For all binary solid solutions, an anomalous thermoelastic behavior is predicted, which can be attributed to the electronic entropy induced by a high population of electronic states at the Fermi level (>0.8 states/eV atom). 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 with Zr or Mo, due to the valence electron concentration induced change in the density of states in the vicinity of the Fermi level, leading to a shift in the anomalous trend of c₄₄ to lower temperatures. An anomalous temperature behavior is also predicted for the shear elastic constant c′ for Nb-Mo solid solutions with Mo concentrations between 24 and 33 at. %. With increasing Mo concentrations, the anomaly in both elastic constants is suppressed due to the continuous reduction in electronic states at the Fermi level.