Page 103 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Solvent-Free Nickel Nanoparticles Synthesis and Engineering ‒Controllable Magnetic Properties
As described above, the microplasma-assisted process has shown its ability to synthesize Ni nanoparticles with adjustable magnetic properties in a simple, solvent-free, continuous, and one-step manner, which is still regarded as a challenge by the state of technology. This technique combines the advantages of micro reactors and the non-thermal plasma chemistry, resulting in a new and facile route for the gas phase fabrication of Ni nanoparticles. Compared to the existing methods (Table 2 33–45), the present study chooses Ni(cp)2 as the precursor to replace the commonly used Ni(CO)4 which is extremely toxic and dangerous.39 As a consequence, special safety precautions are not needed. The confinement of the plasma in micro spatial scale leads to very high energy density. Ni nanoparticles are produced at the dissipated plasma power as low as ~1.0 W (despite low quantity), which cannot be achieved by the existing approaches. Meanwhile, due to the short residence time (~10-4 s, derived from gas flow rate, tube size and electrode distance) and the uniform RTD of Ni(cp)2 vapors in the plasma, the obtained Ni nanoparticles are much smaller and have narrower size distributions compared with other gas phase processes. Possible side reactions and by-products are considerably suppressed since only the Ni(cp)2 being used as the precursor and dissociated in an inert atmosphere, obviating the use of complex procedures to purify the products. Another distinctive advantage is the demonstrated ability to tune the product properties in- flight by simply adjusting the controlling “knobs” such as the plasma power or the Ni(cp)2 concentration, without any pre/post treatments i.e. separation, drying or annealing. Thus the overall synthesis workflow is greatly simplified. Moreover, owing to the versatility of microplasma sources and the high degree of flexibility in processing parameters, this technique is expected to have many promising applications, such as surface modifications or coatings, on-site/direct-write deposition of well-defined nanostructures, fabrication of metal patterns on the polymer films and in situ formation of patterned electrical conductors.
On the other hand, it should be pointed out that the throughput of the current work is not high, since only a single microplasma unit has been applied to fabricate nanoparticles, and the synthesis takes place in a micro reactor. For the studied parametric range, the upper limit of the production rate of Ni particles in an assumption of 100% precursor conversion efficiency at 35 ppm Ni(cp)2 vapors by a single plasma jet is calculated as 4.65×10-3 g/h. However, the process has the potential to be scaled up by arranging microplasma units in an array structure. One feasible solution is the microjets-array, where a certain number of microplasma jets are arrayed to achieve the parallel operation. We have previously estimated that a 2D microjets- array with 100 microplasma jets in each dimension (100×100) can improve the throughput by four orders of magnitude.46 If such a array was applied to the present study, the production rate is estimated to be 1116 g/day, much higher than the claimed high-throughput synthesis of Ni nanoparticles by a continuous flow method (27 g/day).47 An alternative way for scaling- up this technique is the so-called planar microdischarges. In such a configuration a matrix of holes are perforated in two planar metallic sheets, which are separated by an insulator. Each hole acts as an independent plasma source after power coupling on the two metal layers48. Precursors can be delivered into these holes and being dissociated. Furthermore, the extremely small dimension of the capillaries or the holes results in rather compact reactors. A 2D microjets-array with 100 jets in each dimension is reported be ~1.5 m×1.5 m, while planar microdischarges with 200 holes only occupies a spatial space of 50×50 mm2, making
them attractive for industrial or portable applications.48
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