Processing of high entropy carbide based ceramics

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Deposition efficiency of barium hexaferrite by aerosol deposition

We present results of barium hexaferrite powder mass consumption across a wide range of starting powder quantities and deposition times. From these results we develop a transfer efficiency figure of merit to describe deposition efficiency and growth rates applicable to aerosol deposition and similar spray deposition techniques. We find that the transfer efficiency of barium hexaferrite was 0.082% and the transfer efficiency rate coefficient was 0.056 min−1 with a decay factor of −0.773. As a means to further understanding the deposition efficiency we present flow simulations of an aerosol deposition system using different particle sizes and standoff distances. We find that impact with the substrate strongly depends on the particle size, particle location within the nozzle, and standoff distance. We find that the values in the simulation are consistent with those used to produce films with the aerosol deposition system used at the Naval Research Laboratory and consistent with values found in the literature. We find that to improve the transfer efficiency nozzle design must be optimized, particle size must be carefully selected, standoff distance must be selected, and the powder in the aerosol chamber must be delivered at an optimal rate. These factors may be individually tuned to contribute to the final transfer efficiency figure of merit that can be used to assess the efficiency of the aerosol deposition process

Strained Polymer Thermal Conductivity Enhancement Counteracted by Additional Off-Axis Strain

Thin-film (5-20 μm) polymer dielectrics are a critical component in high energy density capacitors. Understanding thermal transport in these materials is critical to addressing thermally enabled dielectric breakdown, a primary failure mechanism. Here, we measure the anisotropy in thermal conductivity for strained polymer films using frequency-domain thermoreflectance (FDTR), which provides us with unique sensitivity to thermal conductivity in the cross-plane and radial directions using a wide range of imposed modulation frequencies. We find that the anisotropy in the thermal conductivity is significantly enhanced by in-plane polymer alignment with strain in polypropylene films. Interestingly, this enhancement is then reduced by the application of additional strain in the orthogonal direction. We use insights from molecular dynamics simulations and Raman spectroscopy to understand the physical mechanism for this reduction

Defect mechanisms in BaTiO3‐BiMO3 ceramics

Often, addition of BiMO3 to BaTiO3 (BT) leads to improvement in resistivity with a simultaneous shift to n‐type conduction from p‐type for BT. In considering one specific BiMO3 composition, that is, Bi(Zn1/2Ti1/2)O3 (BZT), several prospective candidates for the origin of this n‐type behavior in BT‐BZT were studied—loss of volatile cations, oxygen vacancies, bismuth present in multiple valence states and precipitation of secondary phases. Combined x‐ray and neutron diffraction, prompt gamma neutron activation analysis and electron energy loss spectroscopy suggested much higher oxygen vacancy concentration in BT‐BZT ceramics (>4%) as compared to BT alone. X‐ray photoelectron spectroscopy and x‐ray absorption spectroscopy did not suggest the presence of bismuth in multiple valence states. At the same time, using transmission electron microscopy, some minor secondary phases were observed, whose compositions were such that they could result in effective donor doping in BT‐BZT ceramics. Using experimentally determined thermodynamic parameters for BT and slopes of Kröger‐Vink plots, it has been suggested that an ionic compensation mechanism is prevalent in these ceramics instead of electronic compensation. These ionic defects have an effect of shifting the conductivity minimum in the Kröger‐Vink plots to higher oxygen partial pressure values in BT‐BZT ceramics as compared to BT, resulting in a significantly higher resistivity values in air atmosphere and n‐type behavior. This provides an important tool to tailor transport properties and defects in BT‐BiMO3 ceramics, to make them better suited for dielectric or other applications

Below the Hall–Petch Limit in Nanocrystalline Ceramics

Reducing the grain size of metals and ceramics can significantly increase strength and hardness, a phenomenon described by the Hall–Petch relationship. The many studies on the Hall–Petch relationship in metals reveal that when the grain size is reduced to tens of nanometers, this relationship breaks down. However, experimental data for nanocrystalline ceramics are scarce, and the existence of a breakdown is controversial. Here we show the Hall–Petch breakdown in nanocrystalline ceramics by performing indentation studies on fully dense nanocrystalline ceramics fabricated with grain sizes ranging from 3.6 to 37.5 nm. A maximum hardness occurs at a grain size of 18.4 nm, and a negative (or inverse) Hall–Petch relationship reduces the hardness as the grain size is decreased to around 5 nm. At the smallest grain sizes, the hardness plateaus and becomes insensitive to grain size change. Strain rate studies show that the primary mechanism behind the breakdown, negative, and plateau behavior is not diffusion-based. We find that a decrease in density and an increase in dissipative energy below the breakdown correlate with increasing grain boundary volume fraction as the grain size is reduced. The behavior below the breakdown is consistent with structural changes, such as increasing triple-junction volume fraction. Grain- and indent-size-dependent fracture behavior further supports local structural changes that corroborate current theories of nanocrack formation at triple junctions. The synergistic grain size dependencies of hardness, elasticity, energy dissipation, and nanostructure of nanocrystalline ceramics point to an opportunity to use the grain size to tune the strength and dissipative properties

Fabrication of Photoluminescent Quantum Dot Thiol–yne Nanocomposites via Thermal Curing or Photopolymerization

Strong, flexible, and transparent materials have garnered tremendous interest in recent years as materials and electronics manufacturers pursue devices that are bright, flexible, durable, tailorable, and lightweight. Depending on the starting components, polymers fabricated using thiol–yne chemistry have been shown to be exceptionally strong and/or flexible, while also being amenable to modification by the incorporation of nanoparticles. In the present work, novel ligands were synthesized and used to functionalize quantum dots (QDs) of various diameters. The functionalized QDs were then incorporated into thiol–yne prepolymer matrices. These matrices were subsequently polymerized to form QD thiol–yne nanocomposite polymers. To demonstrate the versatility of the fabrication process, the prepolymers were either thermally cured or photopolymerized. The resulting transparent nanocomposites expressed the size-specific color of the QDs within them when exposed to ultraviolet irradiation, demonstrating that QDs can be incorporated into thiol–yne polymers without significantly altering QD expression. With the inclusion of QDs, thiol–yne nanocomposite polymers are promising candidates for use in numerous applications including as device display materials, optical lens materials, and/or sensor materials