Microstructure of ultrafine carbonyl iron powder
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I.
INTRODUCTION
IT has
been known that it is possible to improve the properties of metals and ceramics by controlling their microstructures through submicron powder processing.ltl Analytical electron microscopy is a useful tool to characterize these powders at the nanometer level, t2j Carbonyl iron powders have wide application in the electronic and magnetic industries. Conventional electron microscopy has been used to examine the microstructure of carbonyl iron powder having a diameter of microns, t31 Recently, ultrafine carbonyl iron powder (UFCIP) with sizes ranging from 5 to 50 nm is attracting increasing attention for its application in radar absorbent materials (RAM). These particles are particularly suitable for direct observation on the analytical electron microscope in their as-received condition. In this article, UFCIP having a diameter of about 20 nm was examined by conventional transmission electron microscopy (CTEM), electron energy loss spectroscopy (EELS), high-resolution electron microscopy (HREM), and microdiffraction in a JEOL-2000FX operating at 200 kV. II.
EXPERIMENTAL
The UFCIP was formed by thermal decomposition of iron pentacarbonyl vapor and then passivated in nitrogen. The powder was suspended in ultrasonically agitated alcohol before it was spread on slides. Conventional carbon replicating techniques were used to extract the particles from the surface of the slides for examination in the electron microscope. III.
RESULTS
Typical UFCIP particles are shown in Figure l(a). An individual particle is nearly spherical in shape with a rugged surface. The size distribution of the particles ranges from 5 to 30 nm in diameter. It is very difficult to disperse these particles completely, since they always link together into chains unless a dispersing agent is used. Electron energy loss spectra taken from typical UFCIP XIWEI LIN and LIMIN CHEN, Engineers, and JING ZHU, Professor, are with the Metal Physics Department, Central Iron and Steel Research Institute, Beijing 100081, People's Republic of China. Manuscript submitted September 18, 1990. METALLURGICAL TRANSACTIONS A
particles, as shown in Figure 2, indicate that the primary components of UFCIP are iron and oxygen. A considerable amount of oxygen suggests that some form of iron oxide may be present. A selected area electron diffraction pattern (SAEDP) from these particles is shown in Figure l(b). Table I summarizes the planar spacings of various iron oxides. From the measurements of the rings in Figure l(b) (the last two columns of Table I), the dominant phases in these particles were determined to be Fe304 and a-Fe. In order to examine the structure in detail, the UFCIP particles were observed at higher magnification. It was found that there are two kinds of the particles: one with a bright core and the other with a dark core. Figure 3(a) is an image of a collection of the particles with a bright core. Changes in the defocus and in the orientation of the particles had no effect on the contrast of the cores; i.e., they did not come from p
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