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1. Single-Step Synthesis Process for High-Entropy Transition Metal Boride Powders Using Microwave Plasma

   https://doi.org/10.3390/ceramics4020020  

   Storr, Bria; Kodali, Deepa; Chakrabarty, Kallol; Baker, Paul A.; Rangari, Vijaya; Catledge, Shane A. (June 2021, Ceramics)

   null (Ed.)

   A novel approach is demonstrated for the synthesis of the high entropy transition metal boride (Ta, Mo, Hf, Zr, Ti)B2 using a single heating step enabled by microwave-induced plasma. The argon-rich plasma allows rapid boro-carbothermal reduction of a consolidated powder mixture containing the five metal oxides, blended with graphite and boron carbide (B4C) as reducing agents. For plasma exposure as low as 1800 °C for 1 h, a single-phase hexagonal AlB2-type structure forms, with an average particle size of 165 nm and with uniform distribution of the five metal cations in the microstructure. In contrast to primarily convection-based (e.g., vacuum furnace) methods that typically require a thermal reduction step followed by conversion to the single high-entropy phase at elevated temperature, the microwave approach enables rapid heating rates and reduced processing time in a single heating step. The high-entropy phase purity improves significantly with the increasing of the ball milling time of the oxide precursors from two to eight hours. However, further improvement in phase purity was not observed as a result of increasing the microwave processing temperature from 1800 to 2000 °C (for fixed ball milling time). The benefits of microwave plasma heating, in terms of allowing the combination of boro-carbothermal reduction and high entropy single-phase formation in a single heating step, are expected to accelerate progress in the field of high entropy ceramic materials. 

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2. Properties of high entropy borides synthesized via microwave-induced plasma

   https://doi.org/10.1063/5.0098276  

   Storr, Bria; Moore, Luke; Chakrabarty, Kallol; Mohammed, Zaheeruddin; Rangari, Vijaya; Chen, Cheng-Chien; Catledge, Shane_A (June 2022, APL Materials)

   Microwave-induced plasma was used to anneal precursor powders containing five metal oxides with carbon and boron carbide as reducing agents, resulting in high entropy boride ceramics. Measurements of hardness, phase structure, and oxidation resistance were investigated. Plasma annealing for 45 min in the range of 1500–2000 °C led to the formation of predominantly single-phase (Hf, Zr, Ti, Ta, Mo)B2 or (Hf, Zr, Nb, Ta, Mo)B2 hexagonal structures characteristic of high entropy borides. Oxidation resistance for these borides was improved by as much as a factor of ten when compared to conventional commercial diborides. Vickers and nanoindentation hardness measurements show the indentation size effect and were found to be as much as 50% higher than that reported for the same high entropy boride configuration made by other methods, with average values reaching up to 38 GPa (for the highest Vickers load of 200 gf). Density functional theory calculations with a partial occupation method showed that (Hf, Zr, Ti, Ta, Mo)B2 has a higher hardness but a lower entropy forming ability compared to (Hf, Zr, Nb, Ta, Mo)B2, which agrees with the experiments. Overall, these results indicate the strong potential of using microwave-induced plasma as a novel approach for synthesizing high entropy borides. 

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3. Mechanical Characterization of Reduced Graphene Oxide Using AFM

   https://doi.org/10.1155/2019/8713965  

   Teklu, Alem; Barry, Canyon; Palumbo, Matthew; Weiwadel, Collin; Kuthirummal, Narayanan; Flagg, Jason (January 2019, Advances in Condensed Matter Physics)

   Nanoindentation coupled with Atomic Force Microscopy was used to study stiffness, hardness, and the reduced Young’s modulus of reduced graphene oxide. Oxygen reduction on the graphene oxide sample was performed via LightScribe DVD burner reduction, a cost-effective approach with potential for large scale graphene production. The reduction of oxygen in the graphene oxide sample was estimated to about 10 percent using FTIR spectroscopic analysis. Images of the various samples were captured after each reduction cycle using Atomic Force Microscopy. Elastic and spectroscopic analyses were performed on the samples after each oxygen reduction cycle in the LightScribe, thus allowing for a comparison of stiffness, hardness, and the reduced Young’s modulus based on the number of reduction cycles. The highest values obtained were after the fifth and final reduction cycle, yielding a stiffness of 22.4 N/m, a hardness of 0.55 GPa, and a reduced Young’s modulus of 1.62 GPa as compared to a stiffness of 22.8 N/m, a hardness of 0.58 GPa, and a reduced Young’s modulus of 1.84 GPa for a commercially purchased graphene film made by CVD. This data was then compared to the expected values of pristine single layer graphene. Furthermore, two RC circuits were built, one using a parallel plate capacitors made of light scribed graphene on a kapton substrate (LSGC) and a second one using a CVD deposited graphene on aluminum (CVDGC). Their RC time constants and surface charge densities were compared. 

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4. Processing and room temperature mechanical properties of a zirconium carbide ceramic

   https://doi.org/10.1111/jace.17442  

   Korklan, Nicole; Hilmas, Gregory E.; Fahrenholtz, William G. (September 2020, Journal of the American Ceramic Society)

   Abstract Zirconium carbide (ZrC) powder, batched to a ratio of 0.98 C/Zr, was prepared by carbothermal reduction of ZrO₂with carbon black. Nominally phase‐pure ZrC powder had a mean particle size of 2.4 μm. The synthesized powder was hot‐pressed at 2150°C to a relative density of > 95%. The mean grain size was 2.7 ± 1.4 μm with a maximum observed grain size of 17.5 μm. The final hot‐pressed billets had a C/Zr ratio of 0.92, and oxygen content of 0.5 wt%, as determined by gas fusion analysis. The mechanical properties of ZrC_0.92O_0.03were measured at room temperature. Vickers’ hardness decreased from 19.5 GPa at a load of 0.5 kgf to 17.0 GPa at a load of 1 kgf. Flexural strength was 362.3 ± 46 MPa, Young's modulus was 397 ± 13 MPa, and fracture toughness was 2.9 ± 0.1 MPa•m^1/2. Analysis of mechanical behavior revealed that the largest ZrC grains were the strength‐limiting flaw in these ceramics. 

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5. High entropy alloy MoNbTaVW synthesized by metal-oxide reduction in a microwave plasma

   https://doi.org/10.1063/5.0192076  

   Storr, Bria; Catledge, Shane_A (March 2024, Applied Physics Letters)

   A unique approach was used to synthesize the high entropy alloy MoNbTaVW via reduction of metal-oxide precursors in a microwave plasma. The metal-oxides underwent ball milling and consolidation before plasma annealing at 1800 °C for 1 h with hydrogen as feedgas. X-ray diffraction, scanning electron microscopy/energy dispersive x-ray analysis, and Vickers hardness testing reveal characteristics of the high-entropy alloy. This includes a predominantly single-phase body-centered cubic structure, homogeneous distribution of all five metals, and 6.8 ± 0.9 GPa hardness, comparable with other reports for the same five-metal high entropy alloy configuration. Localized microwave plasma particle sintering is evident from the microstructure. These results highlight the promising potential of microwave plasma as a fast, economical, and flexible processing tool for high entropy alloys. 

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