Diets of Atlantic Sharpnose Shark (Rhizoprionodon terraenovae) and Bonnethead (Sphyrna tiburo) in the northern Gulf of Mexico

Diets of two coastal sharks, Atlantic Sharpnose Shark (Rhizoprionodon terraenovae) and Bonnethead (Sphyrna tiburo), were examined along the Texas and Alabama coasts in the northern Gulf of Mexico (GOM). Atlantic Sharpnose Sharks were collected from the northwest (n= 209) and northcentral (n= 245) GOM regions while Bonnetheads were collected from two locations within the northwest GOM (Galveston, Texas, n= 164; Matagorda, Texas, n= 79). Dietary analysis was conducted using stomach contents identified to the lowest taxonomic level, which were quantified using the index of relative importance (IRI) and non-parametric statistical analyses. Atlantic Sharpnose Sharks were revealed to be primarily piscivorous, with an overall %IRI of 79.76% for teleost fishes. Bonnetheads were shown to prey primarily on crustaceans (90.94% IRI), mainly crabs (22.06% IRI). Diets for Atlantic Sharpnose Sharks and Bonnetheads were evaluated by region and ontogeny, where variations by ontogeny were examined based on length at 50% maturity (L50) values, delineating mature from immature individuals. Atlantic Sharpnose Sharks and Bonnetheads showed a decrease in dietary prey species richness from juveniles to adults using %IRI. Regional dietary differences existed with Atlantic Sharpnose Sharks from the northwest GOM consuming more crustaceans than conspecifics from the northcentral GOM. Bonnetheads collected from Galveston, TX consumed more crab than Bonnetheads from Matagorda, TX, while Bonnetheads from Matagorda, TX displayed a diet with higher prey species richness. Our results highlight differences in diets of two common shark species at both local and regional spatial scales

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

Discovery of high-entropy ceramics via machine learning

AbstractAlthough high-entropy materials are attracting considerable interest due to a combination of useful properties and promising applications, predicting their formation remains a hindrance for rational discovery of new systems. Experimental approaches are based on physical intuition and/or expensive trial and error strategies. Most computational methods rely on the availability of sufficient experimental data and computational power. Machine learning (ML) applied to materials science can accelerate development and reduce costs. In this study, we propose an ML method, leveraging thermodynamic and compositional attributes of a given material for predicting the synthesizability (i.e., entropy-forming ability) of disordered metal carbides. The relative importance of the thermodynamic and compositional features for the predictions are then explored. The approach’s suitability is demonstrated by comparing values calculated with density functional theory to ML predictions. Finally, the model is employed to predict the entropy-forming ability of 70 new compositions; several predictions are validated by additional density functional theory calculations and experimental synthesis, corroborating the effectiveness in exploring vast compositional spaces in a high-throughput manner. Importantly, seven compositions are selected specifically, because they contain all three of the Group VI elements (Cr, Mo, and W), which do not form room temperature-stable rock-salt monocarbides. Incorporating the Group VI elements into the rock-salt structure provides further opportunity for tuning the electronic structure and potentially material performance

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

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

Oxidation of high entropy ultra-high temperature ceramics

Please click Additional Files below to see the full abstract