Page 16 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 1
with “chosen” properties such as the crystallinity and morphology,33 in some cases plasma- enhanced chemistries enable reactions which are hardly realizable in mild ways.34 The non- thermal plasmas offer certain benefits for metal nanoparticle synthesis. On the one hand they provide a highly reactive condition for metal atoms nucleation, in which precursors are dissociated rapidly via impact with electrons, excited heavy particles and radicals generated in plasma. At the same time gas (and substrate) temperature is still low, allowing the use of temperature sensitive precursors and limiting the aggregation of nanomaterials.34,35 It should be noted here, that plasmas which are closer to thermal equilibrium such as arc or microwave discharges can offer certain advantages for bulk production of nanoparticles at the cost of somewhat less precise process control. In such systems thermal decomposition of precursor will likely to take place. Moreover the high concentration of low energy electrons can contribute to the enhancement of plasma-chemistry stimulated by vibrational excitation, this, in turn, can reduce production energy cost.36
Series of plasma-based technological platforms and methods were developed over decades. And a wide range of nanomaterials were synthesized under the different conditions, for example, carbides nanosized powders such as WC,37 TiC,38 TiCN,39 SiC,40,41 nitrides nanomaterials such as TiN,42,43 AlN,44 Mg3N2,45 GaN,46 BN,47 oxides nanomaterials such as Al2O3,48,49 SnO2,50 V2O5,51 ZnO,52 TiO2,53 and metal nanoparticles such as Ag, Cu, Fe.54–56 Plasma was also applied for carbon materials manufacturing. Two typical examples were carbon nanotubes 57–60 and carbon black.61–63
Although in past years significant progress has been achieved in plasma-assisted nanomaterials fabrication, several challenges still need to be solved. Currently most of the reported processes operate at low pressures, requiring expensive vacuum equipment and are not ready for industrialization. Process-relevant microscopic and macroscopic parameters in plasma such as like electron density, electron energy, temperature, current density and reduced electric field often have non-uniform spatial distribution, leading to the difficulty to provide homogenous conditions for particle nucleation and growth. As a result, the obtained products are commonly characterized by wide size distribution and partial agglomeration.64 Additionally, safety concerns are involved due to the high voltages as well as the high reactivity of utilized precursors, plasma species and nano-toxicity.
In addition to the synthesis of abundant nanomaterials using the plasma technology, there are also several rsearches conducted to study the process of particle nucleation and growth, aiming to apply the plasma technology effectively. Currently two typical numerical models are used to describe this process: “nucleation-coupled model” and “discrete model”.142 In the first model, a nucleation rate is calculated based on an analytical expression derived from nucleation theory, and this rate is coupled to a model for the time rate of change in the stable aerosol population. In the study of P. H. McMurry, et al., they proposed an expression for the calculation of the nucleation rate based on experimental and numerical work, as shown below:
s
J = bn2S Q exp[Q- 4Q3 ] (1.1)
12 2p
27(InS)2
4