Page 104 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 5
In terms of the scaling-up behavior, according to the Yasuda’s theory, it is supposed that the products should be similar if the so-called Yasuda composite power parameter remains the same, which is defined as the ratio of plasma power to precursor mass-flow rate.50 For instance, if we want to increase throughput we need to rise proportionally both power and precursor mass flow. For example: at the 35 ppm of Ni(cp)2 concentration, the Ni(cp)2 mole flow rate is 7.91×10-5 mol/h. The molar mass of Ni(cp)2 is 188.88 g/mol. Converting to mass flow rate, it is 7.91×10-5 mol/h × 188.88 g/mol= 0.01494 g/h = 4.15×10-6 g/s. Thus the Yasuda composite power parameter at condition 1 = 1.3 W/ 4.15×10-6 g/s = 3.13×105 J/g. If we want to increase the Ni(cp)2 mole flow rate 104 times (149.4 g/h) to increase the throughput, the plasma power also should be increased accordingly (13 kW). It should be noted, some of the plasma processing techniques are highly system-dependent, and it is no sense to carry out laboratory experiments that cannot be upscaled to the industrial-level production. The product properties also cannot be simply estimated using the Yasuda’s concept. More detailed numerical or pilot-scale studies are need to investigate the scaled-up process.
5.4 Conclusions
In this chapter the gas phase Ni nanoparticle production was studied in microplasma reactor, with the focus on governing the magnetic characteristics. Systematic experiments are designed and carried out, for the first time, to establish the relationship between operational conditions and product properties. Application of novel microplama approach brings several key benefits. It allows reduction in costs as well as hazard level of reactants and results in product with excellent magnetic properties. The process itself is simple, continuous, pre/post treatment-free with possibility for fine-tuning “in-flight”. The adjusting of plasma power or precursor concentration allows tuning composition, size distribution, morphology, crystal structure and, ultimately, magnetic behavior of the obtained products in a wide range. At relatively low discharge power to precursor mass flow ratio the products contain Ni nanoparticles in both fcc and hcp phases as well as CNTs. It is remarkable that the plasma assisted process can simultaneously result in both metal catalyst nanoparticles and CNTs from one metalorganic precursor. The increase of plasma power led to the inhibition of CNTs growth and preferential formation of fcc Ni with high magnetic properties. Single phase fcc Ni nanoparticles with characteristic size of 20-27 nm and Ms value of 44.4 mAm2/g are obtained at the optimized condition. Furthermore, based on experimental results and information from literature, a model is proposed to illustrate the possible mechanisms of microplasma assisted nickelocene dissociation process. Finally, microplasma array design is demonstrated as a feasible solution to scale up the technique, and the scale-up behavior of the products is also discussed.
In a broader view, it can be expected that this approach has the potential for practical fabrication of other magnetic nanoparticles, such as cobalt, iron oxide, nickel oxide, etc., just by changing precursors and/or adding oxygen in the plasma. The high flexibility and versatility also renders it possible to open up new synthesis route for nanomaterial fabrication in a controlled and environmental friendly manner. In association with the demonstrated adjustable magnetic properties of the products, this can bring critical advantage for medical applications such as cancer hyperthermia treatment.51 With the advances in microplasma-
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