Dispersion and Lorentz Microscopy of Samarium Cobalt Nanoparticles in a Polymer Matrix
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Mat. Res. Soc. Symp. Proc. Vol. 501 ©1998 Materials Research Society
lengths of time. The results presented here are for particles gently milled for 15 days, but the times are very sensitive to the type of mill used. High energy milling can achieve the same average particle size in a few hours [6]. The average particle size was determined by removing small amounts of powder periodically and using Scherrer analysis on the SmCo5 (110) x-ray diffraction peak at 20 = 35.922'. To disperse the particles in a polymer matrix, the particles were first milled to the desired size, and then thermoplastic powder was added to the milling container, and the entire sample was milled for an additional day to embed the nanoparticles2 in the thermoplastic. The embedded powders were pressed into pellets at 150 'C at 2000 lbs/in. to form solid magnetic composites. The sectioning of TEM samples was done with an Reichert-Jung Ultracut E microtomy sectioning device using a DDK diamond knife. Thin slices less than 100 nm thick were collected on 300 mesh copper TEM grids. A thin layer of carbon was then evaporated over the sections to increase the radiation resistance to the electron beam. TEM observations were made in a JEOL 120 CX electron microscope. In the first series of measurements, conventional (bright field) imaging was used. Later a Lorentz pole piece was installed in the microscope, and the Foucault (shifted aperture) method [7] of Lorentz microscopy was used to image individual nanoparticles and determine their magnetization direction. In the Foucault method, the sample is first observed in focus (normal bright field conditions), and then the objective aperture is shifted to cut off part of the transmitted beam, forming an image preferentially from electrons which have passed through regions of the sample magnetized in a particular direction. A series of images for different aperture shift directions were recorded in order to determine the magnetization direction of the particles. For magnetic measurements a series of samples was prepared. The control samples contained 10 wt.% SmCo 5 nanoparticles thoroughly mixed in epoxy and hardened within the glove box to prevent oxidation. These were compared with samples of the nanoparticles dispersed in thermoplastic both before and after thermal compaction. RESULTS AND DISCUSSION TEM of the microtomed sections of dispersed and compacted nanoparticles showed a marked reduction in the degree of clumping, compared to samples where an equal weight fraction of powder was mixed with epoxy resin, as shown in Fig. 1. While the hand mixed samples also contained some dispersed particles, a substantial fraction of the nanoparticles existed in clumps typified by Fig. la. Similarly there remain some clumps in the dispersed and compacted samples, but Fig. l b is representative. In the first experiment, the commercial SmCo 5 powder and the thermoplastic powder were added to the milling container at the beginning of the milling process. As the micrometer-sized SmCo5 grains collide with the zircon
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