The Sintering and Densification Behavior of an Iron Nanopowder Characterized by Comparative Methods

Fe nanopowder, derived from microwave plasma synthesis (Materials Modifications Inc., Fairfax, VA), was obtained and characterized for particle size and size distribution. The methods used included static light scattering (SLS) and dynamic light scattering (DLS), surface area and size by Brunauer, Emmett, and Teller (BET) analysis, small angle neutron scattering (SANS), neutron diffraction (ND), x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Based on these methods, it was concluded that Fe powder was composed of nanosized particles, but in micrometer-sized aggregates. DLS indicated a mean agglomerate size with a single mode distribution of 70 plus or minus 6 nanometers. In contrast, SLS revealed a wide bimodal distribution ranging from 0.5 to 20 microns. The mean particle sizes that resulted from BET and XRD analyses were 60 and 20 nanometers, respectively. SANS, in combination with ND, determined that the powder had a bimodal distribution of mean size 24 and 64 nanometers. TEM and FESEM confirmed that the powder is composed of 50-to- 80 nanometer particles that are found in large, dendritic particle agglomerates that are on the order of micrometers. Subsequent to characterization, the sinterability and densification behavior of the Fe nanopowder was examined by further experimentation wherein powder compacts were pressureless sintered under hydrogen. Results showed that without pressure the powder could not be sintered to full density; however, the densification of the Fe powder strongly depended on temperature. There was little or no coarsening below 500 degrees C, but significant grain growth was observed in the samples above that temperature. In further experiments using a plasma pressure compaction (P2C) apparatus, the powder was consolidated to final densities near 80% of the theoretical full density. (5 tables, 18 figures, 20 refs.) Prepared with the assistance of Nissei Sangyo America. Supported in part by the research participation program at the U.S. Army Research Laboratory, which is administered by the Oak Ridge Inst. for Science and Education through an interagency agreement between the U.S. Dept. of Energy and ARL. The original document contains color images. All DTIC reproductions will be in black and white.