Magnetically Assisted Impaction Coating Process to Synthesize Engineered Particulates with Controlled Surface Characteri
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particles creating a strong fluidized environment. A schematic of the process is shown in figure 1.
Magnetic Particles
Electromagnet
Fig.l: Schematic diagram of MAIC system.
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Fig. 2: A120 3 secondary particles in unmixed state. The secondary particles are typically in the sub-micron range. The small size results in agglomeration of particles. This is shown in figure 2. Although the size is approximately 0.5 gim, aggregates of 2-20 gim are observed. Thus, it is crucial to de-agglomerate the particles for efficient and uniform coating on core particles surface. The effect of particle size was studied by using PMMA (200 gim), alumina (20 jim) as primary particles and alumina as secondary particles. The secondary particle size was varied from 50 mn to I jim. The surface coverage values were obtained from detailed image analysis of the SEM.micrographs of primary particle surfaces. The MAIC process can be controlled by varying the frequency of the collisions or adjusting the processing time. Collision frequency depends on the ratio of primary, secondary and magnetic particles, average size of particles, and the velocity of magnetic particles. Since the total number of collisions depends on the processing time, process time was varied from 1 to 5 minutes. Longer times were avoided in order to prevent contamination coming from magnetic particles. Magnetic particles were separated from the coated powder using conventional magnetic separation techniques. 334
RESULTS & DISCUSSION Particle Size Effect Figure 3 shows the surface coverage of alumina secondary particles on PMMA and alumina core particles as a function of secondary particle size. The figures show a high degree of surface coverage when the size of secondary particles is < 0.4 [jm. However the surface coverage on alumina core particle is quite low for all secondary particle sizes. This was attributed to the difference in hardness of alumina and PMMA. Figure 3 shows poor surface coverage (< 10%) when the secondary particle size is >1 pjm. Although Van der Waals (VdW) adhesion forces increase linearly with size (F-R), the removal forces show a higher rate increase (F-R 3). As a result, tangential removal forces due to impacting magnetic and primary particles dominate resulting in poor surface coverage. 100 9080-
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Fig. 3: The effect of the particle size on surface coverage of alumina particle on PMMA and alumina core particle. Frequency Effect The collision frequency was found to play an important role in the coating process. The collision frequency between magnetic particles and primary particles was calculated using the equation:
f = N 1 .N 2.,r.(RI + R2)2.V where N, and N 2 are the number, R , and R2 are the radius of magnetic and primary particles respectively, and "V" is the average velocity of particles which was measured to be approximately
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- 10 cm/sec with a CCD camera attached to the chamber. The frequency of collisions
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