Jorgen F. Rufner

In situ transmission electron microscopy study of dielectric breakdown of surface oxides during electric field-assisted sintering of nickel nanoparticles

The removal of ultra-thin oxide surface layers on nanometric nickel particles is investigated in the framework of electric field-induced dielectric breakdown. In situ transmission electron microscopy was used to directly apply electrical biasing to agglomerates of nanoparticles during simultaneous imaging of the contact area between two adjacent particles. The applied electrical field initiated dielectric breakdown of the surface layers through percolation of oxygen vacancies and the migration of oxygen away from the particle contact, which leads to the formation of metallic necks and their subsequent growth. The experimental results represent direct evidence for surface cleaning effects during electric field-assisted sintering.

Determination of Reliable Grain Boundary Orientation using Automated Crystallographic Orientation Mapping in the Transmission Electron Microscope

Xinming Zhang, Jorgen F. Rufner, Thomas LaGrange, , Ricardo H.R. Castro, Julie M. Schoenung, Geoffrey H. Campell, Klaus van Benthem Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616 USA Physical and Life Sciences Directory, Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550 USA 3 now at: Ecole Polytechnique Federale de Lausanne, Interdisciplinary Center of Electron Microscopy, 1015 Lausanne, Switzerland

Probing the Structure and Mechanical Properties of Individual MgAl 2 O 4 Porous Agglomerates and Their Effects on Densification

In powder processing agglomeration can be a severe problem limiting achievable final densities and causing irregular microstructures due to residual porosity. Agglomeration can spur from many process aspects from powder synthesis and overly high calcination temperatures to poor green body pressing. Commonly, high uniaxial or isostatic pressure is used to mechanically breakdown agglomerates in an attempt to alleviate the issues mentioned above. Unfortunately, agglomerate breakdown strength is often difficult to determine making it challenging to improve or even determine processing parameters. In this study, MgAl2O4 nanopowder was synthesized via the Pechini/polymeric precursor method [1] with an average crystallite size of 20nm. The powder was isothermally sintered at 1300 °C for 1, 5, 10, and 20 minutes. The maximum achievable density for the samples was very low, only 55%. SEM imaging of the sintered pellets revealed a microstructure with large amounts of pores, ranging in size from tens of nanometers to microns in diameter. Hydraulic uniaxial compression to nearly 1GPa was applied in an effort to mechanically remove the porosity in the green pellet by deforming/breaking the agglomerates but was unsuccessful. The forces on the pellet surpassed even that of the die causing failure of the die before the agglomerates [4].

Nanodiffraction Characterization of Grain Boundary Structures in Nanocrystalline MgAl2O4 prepared by Electric Field Assisted Sintering

Appling a contact or non-contact electric field during sintering, i.e., electric field assisted sintering (EFAS), can improve consolidation behavior and is commonly used in many sintering techniques, e.g., Spark Plasma Sintering (SPS)[1], Flash Sintering[2], and Pulsed Electric Current Activated Sintering (PECAS)[3][4]. Electric fields have been shown to densify ceramic powders within seconds as well as, depending on the experimental conditions, retarding or enhancing grain growth, however, the mechanisms which affect sample densification and grain growth remain unclear [1] [2][4]. To control and tailor microstructural aspects of sintered materials, better correlation is required of the applied non-contact electrical fields and current to the resultant microstructure. Due to the complexities of the sintering processes, we have chosen to isolate our investigation to the influence of noncontact electric fields on microstructural evolution.

Local Current-Activated Growth of Individual Nanostructures with High Aspect Ratios

The development of vertical nanoelectronics is hindered by limited control over the growth of nanostructures in specific directions. A scanning tunneling microscopy tip was used as a guide to locally consolidate individual nanoparticles and grow metallic nanopillars with high aspect ratios. Consolidation from random agglomerates occurs through electromigration and diffusion in a temperature gradient. The results of this study provide a new avenue for the controlled growth of complex metallic nanostructures for future three-dimensional architectures.