Andrew M. Thron

Field assisted sintering of nickel nanoparticles during in situ transmission electron microscopy

This study reports the in situ transmission electron microscopy (TEM) observation of pressure-less field-assisted sintering of agglomerated nanometric nickel particles. Scanning tunneling microscopy inside the TEM was used to apply an electrical current directly to the powder particles. Electrical testing during the experiment reveals that consolidation occurs in the absence of an external heat source. Neck formation between adjacent particles and attendant increase in local Joule heating causes rapid densification. The results represent a first stepping stone towards achieving a fundamental mechanistic understanding of the atomic-scale processes that enable field-enhanced sintering of conductive nanogranular materials.

Strong immobilization of charge carriers near the surface of a solid oxide electrolyte

A promise of nano-structured solid electrolytes (SEs) to present enhanced ionic conductivity necessary for low-temperature solid oxide fuel cell applications arises from the hypothesis that interfaces in these materials accelerate the ionic transport. However an in-depth knowledge of ionic dynamics in the interfacial regions, relative to the bulk, imperative to verify such a hypothesis is currently lacking. Here, we report the results of an atomistic study of oxygen-vacancy hopping dynamics in nano-crystalline Sc-doped ceria (SDC), one of the most studied oxygen-anion conducting SEs, using 45Sc magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Our results provide direct experimental evidence that oxygen vacancies in the surface region of SDC are nearly immobile even at temperatures as high as 600 °C. Such findings suggest that accelerated oxygen-anionic transport along interfaces in SDC is highly unlikely, and thus lead to the conclusion that nano-structuring of this SE, and possibly other oxide SEs, will unlikely benefit their anionic conductivities.

In-situ observation of equilibrium transitions in Ni films; agglomeration and impurity effects.

Dewetting of ultra-thin Ni films deposited on SiO2 layers was observed, in cross-section, by in situ scanning transmission electron microscopy. Holes were observed to nucleate by voids which formed at the Ni/SiO2 interface rather than at triple junctions at the free surface of the Ni film. Ni islands were observed to retract, in attempt to reach equilibrium on the SiO2 layer. SiO2 layers with 120 nm thickness were found to limit in situ heating experiments due to poor thermal conductivity of SiO2. The formation of graphite was observed during the agglomeration of ultra-thin Ni films. Graphite was observed to wet both the free surface and the Ni/SiO2 interface of the Ni islands. Cr forms surface oxide layers on the free surface of the SiO2 layer and the Ni islands. Cr does not prevent the dewetting of Ni, however it will likely alter the equilibrium shape of the Ni islands.

Temperature gradient and microstructure evolution in AC flash sintering of 3 mol% yttria-stabilized zirconia

ABSTRACT Systematic statistical analysis of the microstructural changes in 3 mol% yttria-stabilized zirconia was performed after flash sintering by alternating current (AC). The micrographs in the gauge section of the specimen were identical to those from DC flash sintered samples while no evident electrode effect was present for AC flash sintered samples. However, finite element modeling revealed a temperature gradient from the surface to the volume of the sintered body. Microstructure gradients, across the width of gauge section, were revealed for the AC flash sintered sample. Classical grain growth models due to Joule heating were insufficient in justifying the microstructural evolution under the simulated temperature distribution. Bimodal grain diameter distributions in flash sintered samples were observed. Therefore, it is proposed that faster grain growth mechanisms activated on a fraction of the grains by electric field/current occurred during flash sintering, and is responsible for the instantaneous grain growth.

Cross-sectional characterization of the dewetting of a Au/Ni bilayer film.

The solid state dewetting of Au/Ni bilayer films was investigated by cross-sectional transmission electron microscopy techniques, including energy-dispersive X-ray spectroscopy, electron energy-loss spectroscopy and precession electron diffraction. After annealing under high vacuum conditions the early stage of film agglomeration revealed significant changes in film morphology and chemical distribution. Both Au and Ni showed texturing. Despite the initial deposition sequence of the as-deposited Au/Ni/SiO2/Si interface structure, the majority of the metal/SiO2 interface was Au/SiO2 after annealing at 675°C for 1h. Void nucleation was predominantly observed at Au/Ni/SiO2 triple junctions, rather than grain boundary grooving at free surface of the metal film. Detailed cross-sectional characterization reveals that the Au/Ni interface in addition to small amounts of metal alloying strongly affects film break-up and agglomeration kinetics. The formation of Au/SiO2 interface sections is found to be energetically preferred over Ni/SiO2 due to compressive stress in the as-deposited Ni layer. Void nucleation is observed at the film/substrate interface, while the formation of voids at Ni/Au phase boundaries inside the metal film is caused by the Kirkendall effect.

Increased thermal conductivity polycrystalline diamond for low-dissipation micromechanical resonators

This paper reports an investigation of microcrystalline diamond (MCD) films deposited under different conditions to increase thermal conductivity and therefore mechanical quality factor (Q) in micromechanical resonators. Through a study of different deposition conditions, we demonstrate a three-fold increase in thermal conductivity and quality factor. Quality factor measurements were conducted on double ended tuning fork resonators, showing Q = 241,047 at fn = 246.86 kHz after annealing, the highest Q reported for polycrystalline diamond resonators. We further present a study of the unique microstructure of hot filament chemical vapor deposition (HFCVD) diamond films and relate growth conditions to observed microstructural defects.

Dewetting Transitions of Au/Ni Bilayer Films

Thin films deposited at low temperatures are often kinetically constrained and will dewet the underlying substrate when heat-treated. Dewetting can be a serious concern in microelectronics reliability [1], while it can also be utilized for engineering of nanostructures with potentials in storage [2], catalysis [3], or optical/magnetic applications [4]. Mechanisms for dewetting of single layer films have been studied extensively. However little work has been reported on the cross-sectional characterization of dewetting processes for multilayer or alloyed thin films.

PtSi dominated Schottky barrier heights of Ni(Pt)Si contacts due to Pt segregation

Temperature dependent current-voltage measurements show that the addition of only 10% Pt to NiSi causes an increase of Schottky barrier height (SBH) from 0.65 eV for NiSi to 0.78 eV for the 10% Pt alloy. Internal photoemission measurements resolve two SBHs in all alloyed samples with ≥5% Pt incorporation corresponding to NiSi and PtSi (∼0.68 eV and ∼0.80 eV), proving that each contributes independently to junction current. High angle annular dark field imaging with scanning transmission electron microscopy confirms Pt segregation to the Ni(Pt)Si/Si interface. The resulting increased SBH may therefore be detrimental to contact resistivity in future technology nodes.

Nanovoids in dense hydroxyapatite ceramics after electric field assisted sintering

ABSTRACT Nanoporous hydroxyapatite (HA) ceramics were consolidated using conventional or free sintering, spark plasma sintering (SPS) and flash sintering (FS). Microstructures formed during electric field assisted sintering, which includes SPS and FS revealed that nanovoids were retained within HA grains during processing. After free sintering, however, no nanovoids were detected by transmission electron microscopy (TEM). The observed nanovoids were confirmed not induced by electron beam damaging, and showed either a round or faceted shape with diameters ranging between 5 and 10 nm. In-situ TEM heating experiments demonstrated that nanovoids remained stable up to 900°C but disappeared at a temperature as high as 1100°C.

Investigation of Dielectric Breakdown on the Atomic Length-Scale Using In Situ STM-TEM

The insulating properties of dielectrics are based on their low density of free charge carriers in the conduction band. Dielectric materials are insulating and are technologically relevant due to their polarization properties, which modify the complex dielectric function of the vacuum, e.g. to increase the ability to store charges in capacitors. Dielectric breakdown describes the loss of insulating properties. A fundamental understanding of dielectric breakdown mechanisms is critical for many technological applications, such as field-effect transistors [1], storage devices and even field-assisted sintering [2].