Physical Characteristics and Morphology of Platinum Nanocrystals on Single Crystal Strontium Titanate

Eight nanometer platinum films were fabricated via electron beam deposition upon (001) strontium titanate (STO) substrates at ambient temperature then annealed at a range of temperatures in a reducing, oxidizing, or neutral environment. The evolution of platinum film dewetting and forming distinct nanocrystals was examined with XRD, AFM, and SEM. The initial deposition showed no crystallinity for the film. Upon annealing, a (001) cube on cube epitaxy was expected due to similar lattice parameters. However, an initial weak polycrystalline Pt (111) peak formed, which then transformed into (111) texture and eventually epitaxy. This (111) epitaxial relationship was observed in all annealing environments. The oxidizing environment suffered from significant platinum loss due to the sublimation of PtO2 at high temperatures, which as not seen in other environments. Secondary crystal growth from the STO substrate was seen in oxidizing and reducing environments and possibly indicated the in-plane epitaxy of the particles. Surface terminations of SrOx were found via AES despite etching to ensure a TiO2 termination, indicating migration of Ti atoms, most likely into the bulk platinum. The formation of (111) platinum epitaxy along with an SrOx termination in various environments indicates the substrate-platinum interactions are the predominant factor in determining platinum crystallinity and other features of the system

Oxidation of high entropy ultra-high temperature ceramics

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Influence of chemical disorder on atomic structure in high-entropy diborides

Density functional theory (DFT) calculations were performed on a set of high-entropy metal diborides composed of five equimolar transition metals in the layered hexagonal AlB2 structure. Atomic structure data was explored and related to that of experimentally synthesized bulk samples of this new class of ultra-high temperature ceramics. Charge disorder and lattice distortions of the relaxed structures were measured and compared between compositions. Interactions between near-neighbor atom pairs were analyzed to explore the effects of constituent elements on the local atomic structure. The high-entropy compositions allow for the incorporation of Mo into the AlB2 structure where it is typically not stable, as well as allowing for Cr concentrations well above the low solubility limit in conventional early transition metal diborides. The presence of these group six elements in certain compositions creates large lattice distortions within a stable single phase structure. Atom pair interactions were further explored by the introduction of vacancies in the structure. Vacancy formation energies were calculated by DFT methods for lattice sites with varying chemical coordination. Preferential vacancy configurations were examined as well as possible effects of atom pair interactions on short-range ordering of elements. Unexpected diffusion behavior observed in high temperature oxidation experiments was explored as it relates to vacancy configurations and vacancy mediated self-diffusion in high-entropy diborides. This work is supported by the U.S. Office of Naval Research MURI program (grant No. N00014-15- 1-2863)

Oxidation resistance of multi-component carbide and boride UHTCS

Bulk samples of high entropy ultra-high temperature ceramics (UHTCs) of the composition (HfNbTaTiZr)C and (HfNbTaTiZr)B2 were fabricated via high energy ball milling and spark plasma sintering. Oxidation behavior of this new class of UHTCs was tested at 1500⁰C and 1700⁰C using a resistive heating apparatus in 1 atmosphere reduced PO2 oxygen/argon gas mixtures for times between 5 minutes and 1 hour. Oxidation kinetics were determined from the variation of oxide thickness vs. time. Oxide composition and morphology were characterized using XRD, SEM, and EDS. A nearly continuous layer of complex oxides was observed on the surface, and a subsurface layer showed evidence of selective grain boundary oxidation. Rapid oxidation rates were observed for both carbide and boride at 1500⁰C, even in 1% O2/balance Ar. This work serves to further elucidate the oxidation behavior of a new class of ceramics that are proposed for ultra-high temperature applications where oxidation properties are of key importance

Synthesis of high entropy metal diborides

In our recent work, several five-component metal diborides, including (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2, (Hf0.2Zr0.2Ta0.2Mo0.2Ti0.2)B2, (Hf0.2Zr0.2Mo0.2Nb0.2Ti0.2)B2, (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2, (Mo0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2, and (Hf0.2Zr0.2Ti0.2Cr0.2Ta0.2)B2, were synthesized [Scientific Reports 6:37946 (2016)]. Here, we critically compare several different synthesis routes to fabricate these refractory high-entropy diborides via spark plasma sintering and conventional sintering, with or without sintering aids. While the majority of the compositions formed single phase AlB2 structures via spark plasma sintering, minor secondary oxide phases (mostly (Zr, Hf)O2), as well as porosity, remained. The utilization of multi-step conventional sintering along with appropriate sintering aids, e.g., boron carbide and carbon, allowed for the removal of secondary oxide phases as well as increasing the densification. Furthermore, conventional sintering led to improved homogenization of the different metal elements within the samples, which were verified by EDS mapping. Results on the process optimization for both spark plasma sintering and conventional sintering of the materials, as well as initial measurements of mechanical properties, will be presented and discussed. Please click Additional Files below to see the full abstract

Characterization of the thermal properties of entropy stabilized oxides and high entropy diborides

Entropy stabilized oxides and high entropy diborides are promising new materials capable of withstanding extreme environments consisting of high temperatures and pressures. In these novel materials, thermal characterization is essential for understanding and predicting performance at elevated temperatures. Moreover, these systems provide a unique opportunity to study the nature of thermal transport and phonon scattering in multicomponent, high-entropy materials. Please click on the file below for full content

Modelling and synthesis of high-entropy refractory carbides, nitrides and carbonitrides

It has been well demonstrated that, through entropic stabilization, many equiatomic multicomponent metallic compositions will form single-phase, complex solid solutions, often called high-entropy alloys. It is known for metallic systems that one can take advantage of the inherent favorable properties of these materials, including increased thermal stability and solid solution strengthening. In order to extend the field of high-entropy alloys into the ultra-high temperature realm, we investigate novel equiatomic, hexanery (5-metal + anion), high-entropy refractory carbides, nitrides, and carbonitrides of group IV, V, and VI transition metals via modeling and experimental synthesis routes. The CALPHAD technique enabled rapid screening of a vast number of material systems to find likely candidates for formation of truly single-phase high-entropy ultra-high temperature ceramics (UHTCs). Compositions that exhibited broad, single-phase solubility across a large temperature region were selected, making processing possible at reasonable temperatures (≤2500°C). For further screening of compositions, 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 to predict the formation of high-entropy materials. Following model predictions, bulk samples were synthesized using a combination of high-energy ball milling (HEBM), spark plasma sintering (SPS) at 2200°C, and hot press (HP) annealing at 2500°C. Phase determination was done via x-ray diffraction techniques as well as TEM microscopy, while chemistry was evaluated via energy dispersive x-ray spectroscopy and STEM-EDS. Many of the 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 and were sintered to greater than 95% theoretical density. Figure 1 shows the experimental X-ray diffraction patterns for a sample of composition (Hf0.2Nb0.2Ta0.2Ti0.2V0.2)C following each processing step. The material progresses into the desired single cubic NaCl structure following complete processing. Work on single-phase determination in nitride and carbonitride systems is ongoing. This work demonstrates the extension of entropic-stabilization principles into refractory interstitial ceramics and development of new classes of high-entropy ceramic materials for high-temperature applications Please click Additional Files below to see the full abstract