Mixing of nano-particles by rapid expansion of high-pressure suspensions

The mixing of two different species of nano-particles using an environmentally benign technique called rapid expansion of high-pressure suspensions (REHPS) has been studied experimentally. Comparative experiments were also performed by mixing the nano-particles in an organic solvent under ultrasonic agitation and in a dry mechanical mixing process called magnetically assisted impaction mixing. Various characterization methods for evaluating the degree of mixing at length scales of about 1 μm and lower based on electron microscopy are also described. An analysis of the experimental results indicates that the REHPS mixing, which also includes supercritical conditions, provides results that are significantly better than those of the other two mixing methods considered. It appears that the sudden decrease in pressure in the REHPS process breaks up the nano-particle agglomerates and results in good mixing, especially when the two constituents do not vary significantly in density. The characterization results show that field emission scanning electron microscopy can be used for distinguishing mixtures at the nano-scale if a significant difference in size or shape exists. However, in general, electron energy loss spectrography is the most powerful method to characterize nano-particles mixtures as it maps elemental distribution at nanometer resolution. Energy dispersive X-ray spectroscopy can also be used as a cheap and simple semi-quantitative method to measure the degree of mixing.