Study and suppression of the microstructural anisotropy generated during the consolidation of a carbonyl iron powder by field-assisted hot pressing

Published OnlineA spherical carbonyl iron powder was consolidated by the field-assisted hot pressing technique using graphite tools at two different temperatures, both above the austenitizing temperature. The microstructures obtained exhibited a compositional gradient in carbon along the consolidated material. Thus, the outer rim of the cylindrical samples was composed of cementite and pearlite that gradually turned to pearlite, leading to a fully ferritic microstructure at the core of the sample. The increase in the temperature has led to a higher introduction of carbon within the sample. The interposition of a thin tungsten foil between the graphite die/punches and the powders has significantly reduced the diffusion of the carbon through the iron matrix and has suppressed the microstructural anisotropy.Publicad

Local field strengths during early stage field assisted sintering (FAST) of dielectric materials.

The role of an applied elec. field during the densification of ionic ceramics is still being debated. Here we describe how the polarization of a dielec. material contributes to the field strengths at particle surfaces and interfaces during the initial stage of sintering. Using numerical models, it is shown that significant increases in local field strengths can be expected during initial neck formation and continue, with decreasing contribution, through approx. half of the first stage of sintering. The field strengths achievable in common com. and custom lab-scale elec. field assisted sintering systems are found to be comparable to those at which elec. fields have been shown to enhance the densification behavior of ionic ceramics. [on SciFinder(R)

Athermal and thermal mechanisms of sintering at high heating rates in the presence and absence of an externally applied field.

In order to establish the relative contributions of thermal and athermal mechanisms to densification in the absence of an extrinsic sintering pressure, nanometric powder compacts were sintered with and without applied fields using varied heating rates from 50 °C/min up to 800 °C/min. The relative contribution of the thermal and athermal mechanistic contributions to the densification behavior of two model dielec. ceramics, hydroxyapatite and zinc oxide, is evaluated in the context of the current leading theories of field-assisted sintering mechanisms. The effects of elevated heating rates in nanometric, dielec. ceramics are found to be minimal in the absence of a field. However, in the presence of an applied field there appears to be a synergistic effect with heating rate. [on SciFinder(R)

Multimodal grain size distribution and high hardness in fine grained tungsten fabricated by spark plasma sintering

Preparation of fine grained, hard and ductile pure tungsten for future fusion reactor applications was tested using the bottom-up approach via powder consolidation by spark plasma sintering (SPS) at different temperature (1300-1800 degrees C) and pressure (90-266 MPa) conditions. Pure tungsten powders with an average particle size of about 1 mu m were sintered to high density (about 94%) with almost no grain growth at a temperature below 1400 degrees C and an applied pressure up to 266 MPa. These samples had a multi-modal grain size distribution (resembling the size distribution of the initial powder) and a very high Vickers hardness (up to 530 kg/mm(2)). Above 1500 degrees C fast grain growth occurred and resulted in a drop in hardness. XRD on the surface of bulk samples showed a small amount of tungsten oxides; however, XPS and EDS indicated that these oxides were only surface contaminants and suggested a high purity for the bulk samples. The results demonstrate that SPS can lead to ultrafine and nanocrystalline tungsten if used to consolidate pure nano tungsten powders. (C) 2011 Elsevier B.V. All rights reserved

FeCr2S4 in magnetic fields possible evidence for a multiferroic ground state

We report on neutron diffraction, thermal expansion, magnetostriction, dielectric, and specific heat measurements on polycrystalline FeCr2S4 in external magnetic fields. The ferrimagnetic ordering temperatures T C approximate to 170 K and the transition at T OO approximate to 10 K, which has been associated with orbital ordering, are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the state below T OO. The magnetic moments of the Cr and Fe ions are reduced from the spin only values throughout the magnetically ordered regime, but approach the spin only values for fields gt;5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample below about 60 K. Below T OO this contraction is followed by a moderate expansion of the sample for fields larger than similar to 4.5 T. The transition at T OO is accompanied by an anomaly in the dielectric constant. The dielectric constant depends on both the strength and orientation of the external magnetic field with respect to the applied electric field for T lt; T OO. A linear correlation of the magnetic field induced change of the dielectric constant and the magnetic field dependent magnetization is observed. This behaviour is consistent with the existence of a ferroelectric polarization and a multiferroic ground state below 10