10 research outputs found
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 minâ1 with a decay factor of â0.773. 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
Design and Implementation of a Magnetic Press System for Creating Magnetically Oriented Barium Hexaferrite Pucks
Fabrication of Photoluminescent Quantum Dot Thiolâyne Nanocomposites via Thermal Curing or Photopolymerization
Strained Polymer Thermal Conductivity Enhancement Counteracted by Additional Off-Axis Strain
Thin-film (5-20 ÎŒm) 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 BaTiO3âBiMO3 ceramics
Often, addition of BiMO3 to BaTiO3 (BT) leads to improvement in resistivity with a simultaneous shift to nâtype conduction from pâtype for BT. In considering one specific BiMO3 composition, that is, Bi(Zn1/2Ti1/2)O3 (BZT), several prospective candidates for the origin of this nâtype behavior in BTâBZT were studiedâloss of volatile cations, oxygen vacancies, bismuth present in multiple valence states and precipitation of secondary phases. Combined xâray and neutron diffraction, prompt gamma neutron activation analysis and electron energy loss spectroscopy suggested much higher oxygen vacancy concentration in BTâBZT ceramics (>4%) as compared to BT alone. Xâray photoelectron spectroscopy and xâray 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 BTâBZT ceramics. Using experimentally determined thermodynamic parameters for BT and slopes of KrögerâVink 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 KrögerâVink plots to higher oxygen partial pressure values in BTâBZT ceramics as compared to BT, resulting in a significantly higher resistivity values in air atmosphere and nâtype behavior. This provides an important tool to tailor transport properties and defects in BTâBiMO3 ceramics, to make them better suited for dielectric or other applications
Below the HallâPetch Limit in Nanocrystalline Ceramics
Reducing
the grain size of metals and ceramics can significantly
increase strength and hardness, a phenomenon described by the HallâPetch
relationship. The many studies on the HallâPetch 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 HallâPetch 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)
HallâPetch 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 Thiolâyne 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 thiolâyne 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 thiolâyne prepolymer matrices. These matrices were subsequently
polymerized to form QD thiolâyne 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 thiolâyne polymers without significantly
altering QD expression. With the inclusion of QDs, thiolâyne
nanocomposite polymers are promising candidates for use in numerous
applications including as device display materials, optical lens materials,
and/or sensor materials