Improving NASICON Sinterability through Crystallization under High-Frequency Electrical Fields

The effect of high frequency (HF) electric fields on the crystallization and sintering rates of a lithium aluminum germanium phosphate (LAGP) ion conducting ceramic was investigated. LAGP with the nominal composition Li1.5Al0.5Ge1.5(PO4)3 was crystallized and sintered, both conventionally and under effect of electrical field. Electrical field application, of 300V/cm at 1MHz, produced up to a 40% improvement in sintering rate of LAGP that was crystallized and sintered under the HF field. Heat sink effect of the electrodes appears to arrest thermal runaway and subsequent flash behavior. Sintered pellets were characterized using XRD, SEM, TEM and EIS to compare conventionally and field sintered processes. The as-sintered structure appears largely unaffected by the field as the sintering curves tend to converge beyond initial stages of sintering. Differences in densities and microstructure after 1 hour of sintering were minor with measured sintering strains of 31% vs. 26% with and without field, respectively . Ionic conductivity of the sintered pellets was evaluated and no deterioration due to the use of HF field was noted, though capacitance of grain boundaries due to secondary phases was significantly increased

GALVANIC CELL FORMATION DURING MEMS RELEASE PROCESSES: IMPLICATIONS FOR SUB-MICRON DEVICE FABRICATION

ABSTRACT The addition of a noble metallization layer to doped polysilicon results in the formation of a galvanic cell when the composite is submerged in aqueous hydrofluoric acid. A corrosion current created by the galvanic cell promotes the electrochemical etching of silicon in contact with the acidic solution. Here, we demonstrate the galvanic corrosion of phosphorus-doped polysilicon when a gold metallization layer is used. As a consequence of galvanic corrosion, a number of significant changes to the polysilicon structural layers are observed including a finite polysilicon etch rate, an increase in electrical resistance (both ohmic and non-ohmic), a change in curvature (i.e. mechanical shape), and a decrease in mechanical resonant frequency. The observed change in electrical and mechanical performance on micromechanical structures necessitates more careful consideration of the post-processing procedures, as well as the choice of device metallization layer. The physical impact of corrosion becomes even more significant as device scale is decreased

Nanocrystalline Iron Monosulfides Near Stoichiometry

Abstract Solids composed of iron and sulfur are earth abundant and nontoxic, and can exhibit interesting and technologically important optical, electronic, and magnetic phenomena. However, the iron-sulfur (Fe-S) phase diagram is congested in regions of slight non-stoichiometric iron vacancies, and even when the iron atomic composition changes by even a few percent at standard temperature and pressure, there are myriad stable crystal phases that form with qualitatively different electronic properties. Here, we synthesized and characterized nanocrystals of the pyrrhotite-4M structure (Fe7S8) in an anhydrous oleylamine solvent. Upon heating from 140 °C to 180 °C, the solid sequentially transformed into two kinetically trapped FeS intermediate phases before reaching the pyrrhotite-4M final product. Finally, we assessed the effects of iron vacancies using the stoichiometric end-member, troilite, as a reference system. Density functional theory calculations show that iron vacancies in troilite shift the structure from hexagonal FeS to a monoclinic structure, similar to crystal structures of pyrrhotites, and suggest that this iron deficient troilite may be a stable intermediate between the two crystal structures. The calculations predict that defects also close the band gap in iron deficient troilite

Glass-ceramic Li2S-P2S5 electrolytes prepared by a single step ball billing process and their application for all-solid-state lithium-ion batteries

We report that glass–ceramic Li2S–P2S5 electrolytes can be prepared by a single step ball milling (SSBM) process. Mechanical ball milling of the xLi2S??(100 − x)P2S5 system at 55 ??C produced crystalline glass–ceramic materials exhibiting high Li-ion conductivity over 10−3 S cm−1 at room temperature with a wide electrochemical stability window of 5 V. Silicon nanoparticles were evaluated as anode material in a solid-state Li battery employing the glass–ceramic electrolyte produced by the SSBM process and showed outstanding cycling stability.close293

Nanoporous Silicon Combustion: Observation of Shock Wave and Flame Synthesis of Nanoparticle Silica

The persistent hydrogen termination present in nanoporous silicon (nPS) is unique compared to other forms of nanoscale silicon (Si) which typically readily form a silicon dioxide passivation layer. The hydrogen terminated surface combined with the extremely high surface area of nPS yields a material capable of powerful exothermic reactions when combined with strong oxidizers. Here, a galvanic etching mechanism is used to produce nPS both in bulk Si wafers as well as in patterned regions of Si wafers with microfabricated ignition wires. An explosive composite is generated by filling the pores with sodium perchlorate (NaClO₄). Using high-speed video including Schlieren photography, a shock wave is observed to propagate through air at 1127 ± 116 m/s. Additionally, a fireball is observed above the region of nPS combustion which persists for nearly 3× as long when reacted in air compared to N₂, indicating that highly reactive species are generated that can further combust with excess oxygen. Finally, reaction products from either nPS–NaClO₄ composites or nPS alone combusted with only high pressure O₂ (400 psig) gas as an oxidizer are captured in a calorimeter bomb. The products in both cases are similar and verified by transmission electron microscopy (TEM) to include nano- to micrometer scale SiO_<i>x</i> particles. This work highlights the complex oxidation mechanism of nPS composites and demonstrates the ability to use a solid state reaction to create a secondary gas phase combustion

Ab Initio Thermodynamics and Kinetics for Coalescence of Two-Dimensional Nanoislands and Nanopits on Metal (100) Surfaces

Postdeposition coalescence or sintering of pairs of low-strain two-dimensional nanoislands and nanopits on unreconstructed metal (100) surfaces is typically mediated by diffusion along step edges, and is highly sensitive to the associated kinetics. Thus, for selected systems, we provide an ab initio density functional theory (DFT) level description of both system thermodynamics and kinetics. Specifically, we assess lateral pair and trio interactions both conventionally with adatoms at 4-fold hollow adsorption sites, and unconventionally with one adatom at the bridge-site transition state for hopping. Rather than use standard cluster expansion algorithms, these interactions are determined subject to the constraint that key step-edge properties are recovered exactly. Together, both classes of interactions determine barriers for edge diffusion processes for any local step configuration, including diffusion along close-packed ⟨110⟩ edges, kink rounding, meandering processes at kinked ⟨100⟩ steps, and extraction processes at pit corners. Our formalism applies for homoepitaxial systems, and also for several lattice-matched heteroepitaxial systems. The barriers provide input for stochastic models of nanocluster evolution, which are analyzed by kinetic Monte Carlo simulation. Such modeling with DFT energetics from the PBEsol functional recovers extensive experimental observations of both the time scale and the island-size dependence for sintering of Ag islands on Ag(100). Description of pit sintering on Ag(100) is more delicate, being sensitive to specific unconventional trio interactions

Multi-Scale Mechanical Behavior of the Li₃PS₄ Solid-Phase Electrolyte

The need for smaller, lighter, and longer lasting rechargeable batteries is projected to increase rapidly in the coming years because of high demand for portable electronics and electric vehicles. While traditional Li-ion batteries use liquid-phase electrolytes, these suffer from safety risks and low energy density. Solid-phase electrolytes can avoid these issues by enabling a Li metal anode, but tend to fail during cycling due to Li metal dendrite growth between the electrodes. Because Li dendrite nucleation and growth can be viewed in terms of the mechanical behavior of the battery components, it is critical to understand the mechanical response of candidate electrolyte materials. In this work, we use nanoindentation and bulk acoustic techniques to characterize the mechanical properties of β-Li₃PS₄, a promising Li-ion conducting ceramic. We find that the bulk and shear moduli of an 80% dense bulk LPS sample are 10–12 GPa and 5–6 GPa, respectively. Although this value of shear modulus may be too low to prevent Li dendrite propagation, it is likely that there are many other mechanical properties that must be taken into account to fully understand Li dendrite nucleation and growth. Ultimately, this work represents a first step in understanding the relationship between Li₃PS₄ separator manufacture and its mechanical properties