Search

In recent years, investigations of the phase transition behavior of semiconducting nanoparticles under high pressure has attracted increasing attention due to their potential applications in sensors, electronics, and optics. However, current understanding of how the size of nanoparticles influences this pressure-dependent property is somewhat lacking. In particular, phase behaviors of semiconducting CdS nanoparticles under high pressure have not been extensively reported. Therefore, in this work, CdS nanoparticles of different sizes are used as a model system to investigate particle size effects on high-pressure-induced phase transition behaviors. In particular, 7.5, 10.6, and 39.7 nm spherical CdS nanoparticles are synthesized and subjected to controlled high pressures up to 15 GPa in a diamond anvil cell. Analysis of all three nanoparticles using in-situ synchrotron wide-angle X-ray scattering (WAXS) data shows that phase transitions from wurtzite to rocksalt occur at higher pressures than for bulk material. Bulk modulus calculations not only show that the wurtzite CdS nanomaterial is more compressible than rocksalt, but also that the compressibility of CdS nanoparticles depends on their particle size. Furthermore, sintering of spherical nanoparticles into nanorods was observed for the 7.5 nm CdS nanoparticles. Our results provide new insights into the fundamental properties of nanoparticles under high pressure that will inform designs of new nanomaterial structures for emerging applications.

The degradation in the strength of napkin-ring (NR) joints bonded with an epoxy thermoset is evaluated in a humid environment. While adherend composition (stainless steel and aluminum) and surface preparation (polished, grit blasted, primed, coupling agent coated) do not affect virgin (time=0) joint strength, they can significantly affect the role of moisture on the strength of the joint. Adherend surface abrasion and corrosion processes are found to be key factors in determining the reliability of joint strength in humid environments. In cases where surface specific joint strength degradation processes are not active, decreases in joint strength can be accounted for by the glass transition temperature, T g , depression of the adhesive associated with water sorption. Under these conditions, joint strength can be rejuvenated to virgin strength by drying. In addition, the decrease in joint strength associated with water sorption can be predicted by the Simplified Potential Energy Clock (SPEC) model by shifting the adhesive reference temperature, T ref , by the same amount as the T g depression. When surface specific degradation mechanisms are active, they can reduce joint strength below that associated with adhesive T g depression, and joint strength is not recoverable by drying. A critical relative humidity (or, potentially, critical water sorption concentration), below which the surface specific degradation does not occur, appears to exist for the polished stainless steel joints.

Electrostatic breakdown (“flashover”) can limit the maximum delivered power in pulsed power systems. One problematic region is the solid-vacuum interface. The solid/vacuum interface surface is generally the component most likely to suffer undesired breakdown in a well-designed pulsed power system driving a load in vacuum. A relatively small (in total number) and localized amount of neutrals or plasma can be enough to allow a self-sustaining discharge. A discharge along the solid-vacuum interface can readily and quickly desorb ions and neutrals from the insulator material. The spurious discharge can carry almost unlimited current, generally constrained only by the current source or the inductance of the current path. Radial insulator assemblies, where power flows along the axis of the insulator (used in a coaxial power feed with concentric insulators of differing diameters, maintaining radial electric field direction) can be lower insertion inductance and smaller in total size than axial insulators. Radial insulators have additional electric field grading and mechanical stress issues compared to an axial insulator system. With an interest in quantifying and improving performance of a radial insulator system, we have built a prototype 60 cm diameter assembly with four series insulator rings and 0.23 m2 total stressed area. By driving the insulator with a ∼2 MV, 15 ns (effective time) negative pulse from a low energy system, we are able to study insulator performance of a multi-level stack with a reasonable data rate in a relevant regime (∼180 kV/cm). The prototype insulator assembly is designed to demonstrate key concepts important for a large system, including 1) improved electric field uniformity using equipotential redistribution, 2) mechanical assembly viability with reasonable machining tolerances, and 3) useful electric field hold-off. The system is scalable to larger size insulators with acceptable electric field uniformity and mechanical stress in the insulating plastic. Mated to a (generally optimal) 40Ω oil-insulated transmission line, the electric field uniformity on the plastic-vacuum interface tested here is better than ±1%. Without equipotential redistribution, the electric field non-uniformity would be of order ±46%. To facilitate future fabrication of larger insulator assemblies, the Rexolite® 1422 cross-linked polystyrene plastic rings are cut from sheet or individually cast, then machined; a large monolithic slab of insulator material is not needed. We will discuss basic properties of vacuum interfaces, discuss scaling formulae used to relate performance of different systems, and show results of testing.

Decisions on material selections for electronics packaging can be quite complicated by the need to balance the criteria to withstand severe impacts yet survive deep thermal cycles intact. Many times, material choices are based on historical precedence perhaps ignorant of whether those initial choices were carefully investigated or whether the requirements on the new component match those of previous units. The goal of this program focuses on developing both increased intuition for generic packaging guidelines and computational methodologies for optimizing packaging in specific components. Initial efforts centered on characterization of classes of materials common to packaging strategies and computational analyses of stresses generated during thermal cycling to identify strengths and weaknesses of various material choices. Future studies will analyze the same example problems incorporating the effects of curing stresses as needed and analyzing dynamic loadings to compare trends with the quasi-static conclusions.