Collective effects on thermophoresis of aqueous particle suspensions are studied experimentally and theoretically. A microfluidic device is used to characterize thermophoretic transport of 100 nm, 500 nm and 1 mm particles of various concentrations in deionized (DI) water. Our experimental findings show two interesting collective effects on the Soret coefficient of colloids: (i) for smaller particles (e.g., 100 nm and 500 nm), a sign change of the Soret coefficient is observed when increasing the particle concentration; (ii) for larger particles (e.g., 1 mm), a negative Soret coefficient is always seen. A model is derived to account for the collective effect on the thermophoresis of colloids using the well-known Derjaguin-Landau-Verwey-Overbeek (DLVO) theory that combines the van der Waals (VDW) attraction and the electric double layer (EDL) repulsion. Such DLVO interactions in an inhomogeneous particle suspension can exert an additional force on particles and thus modify the mass transport of particles under both temperature and concentration gradients and also alter the corresponding Soret coefficient.It is found that the proposed theoretical model can favorably explain our experimental observations.
The R-Fe-B (R, rare earth) sintered magnets prepared with different ratio of alloys of MM-Fe-B (MM, misch-metal) and Nd-Fe-B by dual alloy method were investigated. As expected, the high ratio of MM-Fe-B alloy degrades the hard magnetic properties heavily with intrinsic coercivity lower than 5 kOe. When the atomic ratio MM/R ≤ 21.5% the magnetic properties can reach a practical level of Br ≥ 12.1 kGs, Hcj ≥ 10.7 kOe, and (BH)max ≥ 34.0 MGOe. And the effect of Hcj enhancement by the grain boundary diffusion process is obvious when MM/R ≤ 21.5%. It is revealed that the decrement of intrinsic magnetic properties of R2Fe14B matrix phase is not the main reason of the degradation of the magnets with high MM ratio. The change of deteriorated microstructure together with phase component plays fundamental roles in low Hcj. In high MM ratio magnets, (a) after annealing, Ce atoms inside main phase are inclined to be segregated in the outer layer of the main phase grains; (b) there is no thin layer of Ce-rich phase as an analogue of Nd-rich phase to separate main phase grains; (c) excessive Ce tends to form CeFe2 grains.
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