High-entropy high-hardness metal carbides discovered by entropy descriptors

High-entropy materials have attracted considerable interest due to the combination of useful properties and promising applications. Predicting their formation remains the major hindrance to the discovery of new systems. Here we propose a descriptor - entropy forming ability - for addressing synthesizability from first principles. The formalism, based on the energy distribution spectrum of randomized calculations, captures the accessibility of equally-sampled states near the ground state and quantifies configurational disorder capable of stabilizing high-entropy homogeneous phases. The methodology is applied to disordered refractory 5-metal carbides - promising candidates for high-hardness applications. The descriptor correctly predicts the ease with which compositions can be experimentally synthesized as rock-salt high-entropy homogeneous phases, validating the ansatz, and in some cases, going beyond intuition. Several of these materials exhibit hardness up to 50% higher than rule of mixtures estimations. The entropy descriptor method has the potential to accelerate the search for high-entropy systems by rationally combining first principles with experimental synthesis and characterization.Comment: 12 pages, 2 figure

Modelling and synthesis of high-entropy refractory carbides

Bulk samples of equiatomic, hexanery (5-metal), high-entropy refractory carbides were fabricated using a combination of high-energy ball milling (HEBM), spark plasma sintering (SPS), and hot pressing (HP) annealing. To select candidate composition that are likely to form single phase high-entropy materials at lower processing temperatures (\u3c2500°C), a novel, first-principles materials design method was developed. The theory follows that for low temperature single phase formation, the different configurations should have similar energies to increase the number of thermodynamically accessible states. A partial occupation method was implemented within AFLOW to automate the generation and calculation of the different configurations. The energy distributions were then used to construct a descriptor of Entropy Forming Ability (EFA) to predict the formation of high-entropy materials. CALPHAD results were found to agree with the configuration energy range descriptor for each composition, and these carbides exhibited broad, single-phase solubility across each system, making processing possible at reasonable temperatures. Many of the complex carbide compositions, including (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)C, (Hf0.2Nb0.2Ta0.2Ti0.2V0.2)C, (Hf0.2Nb0.2Ta0.2Ti0.2W0.2)C, and (Nb0.2Ta0.2Ti0.2V0.2W0.2)C demonstrated virtually single-phase, solid-solution compounds with the NaCl crystal structure as determined by x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS), while some compositions, including (Hf0.2Mo 0.2Ta0.2W0.2Zr0.2)C and (Hf0.2Mo0.2V0.2W0.2Zr0.2)C, exhibited multiple phases. Results were found to be in good agreement with the ab initio based formulation of entropic stability, where the compositions with the highest EFA values were found to form a single rocksalt structure and compositions with the lower EFA values were found to exhibit multiple phases. Further, among the systems that were found to form single phase materials at 2500°C, artificial segregation was introduced via lower processing temperatures. In these artificially segregated samples, the extent of mixing was analyzed via peak broadening in XRD according to the formulation of Williamson and Hall [1] and compositional mapping in EDS. Results of artificially segregated samples provide continued support for the viability of the EFA formulation, where broadening was found to be more pronounced (i.e. more chemical segregation) in samples that were determined to have a lower EFA value. This work demonstrates the extension of entropic-stabilization into refractory interstitial carbides, paving the way for development of an entirely new class of UHTCs. This work is supported by the U.S. Office of Naval Research MURI program (Grant No. N00014-15- 1-2863). [1] G.. Williamson, W.. Hall, X-ray line broadening from filed aluminium and wolfram, Acta Metall. 1 (1953) 22–31. doi:10.1016/0001-6160(53)90006-6

Interfacial Engineering of Inorganic Materials for Energy Storage and Conversion Applications

Since the micrometer-sized bulk materials have reached their inherent limits, development of new materials with high performance is essential for low cost and environmentally friendly electrochemical energy storage and conversion devices. One approach is to take advantage of interfacial engineering in order to modify currently developed materials, thus improving their properties for specific applications. The advantage of interfacial engineering is that it can also be applied to newly developed materials to further improve their properties for the specific applications. In first part of this dissertation, a systematic study is performed to investigate the effect of annealing in reducing atmospheres with different oxygen partial pressures and presence of other species (Ar, H2, N2, vacuum or hydrocarbon) on visible-light photocatalytic activity of TiO2. In second part, a facile nitridation method is used to improve the rate capability of TiO2 as anode material for Li ion batteries. The enhanced high-rate capacities are attributed to moderate surface nitridation with less-disordered nitridated regions, which may enhance the surface electronic conductivity without forming discrete, nanoscale, and surface amorphous films to block the lithium transport. In third part, pseudocapacitive properties of V2O5-based adsorbates supported on TiO2 nanoparticles is systematically measured. Surface amorphous films (SAFs), which form naturally at thermodynamic equilibria at 550-600 C with self-regulating or “equilibrium” thicknesses on the order of 1 nm, exhibit superior electrochemical performance at moderate and high scan rates (20-500 mV/s) that are of prime importance for supercapacitor applications, as compared with submonolayer and monolayer adsorbates formed at lower equilibration temperatures. In fourth part, we perform a combined experimental and computational investigation into the effects of aliovalent doping in NASICON on both bulk and grain boundary ionic conductivity. Our results show that the dopants with low solid solubility limits in NASICON solid solution lead to the formation of a conducting secondary phase at grain boundaries, thereby improving effective grain boundary conductivity that is otherwise hindered by the poorly-conducting Na3PO4 and ZrO2 secondary phases in undoped NASICON. In fifth part, inline electron holography technique is used to directly observe and investigate the space charge layers at grain boundaries of Y-doped BaZrO3