Page 162 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 9
(4) The confinement of the plasma in micro spatial scale not only leads to very high energy density, but also ensures a relatively short residence time and uniform RTD for the precursors. Therefore, nanoparticles with smaller average size and narrower size distributions can be fabricated at the dissipated plasma power as low as ~1.0 W, which cannot be realized by the existing approaches.
(5) Due to the versatility of the process and the high flexibility of the plasma configuration, it is expected that this technique can be applied as “dry-process” for local production of well- defined nanostructures (printing-like technologies).
Despite the above advantages, some challenges remain. The involved plasma physics and reaction kinetics are still not fully understood, the rates of many relevant elementary processes remain unknown complicating the analysis of mechanisms. Multi-disciplinary collaborations from physics, chemistry as well as novel diagnostics approaches are required. In terms of application, it should be pointed out that the current throughput of this technique is not high, since most reported lab-scale processes use only a single microplasma unit to fabricate metallic nanoparticles as a prove of principle, and reactions were confined in micro reactors. However, the process can be scaled up by using the microplasma array design. 3–5 One possible solution is to arrange microplasma jets in an array structure. As estimated in the chapter 4, if a two-dimensional microplasma array (Figure 9.1(a)) with 100 jets in each dimension (100×100) was applied for the gas-phase fabrication of TiN nanoparticles, the throughput could be improved by four orders of magnitude.6 Meanwhile, due to the high flexibility of microplasma configurations, such an array structure could also be applied for the liquid-phase synthesis processes to increase the surface area for plasma-liquid interactions (Figure 9.1(b)). An alternative design of microplasma array is to perforate a matrix of micro holes in two separated planar metallic sheets. By connecting the two metal layers to a power supply, each hole functions as an independent microplasma source.7 In this manner metallic nanoparticles can be fabricated by delivering the precursors into the holes to be dissociated. On the other hand, owing to the extremely small dimension of the microplasma jet or the micro hole, the array structure is very compact, making them favored by industrial or portable applications. It is reported that a 2D microplasma array with 100 jets in each dimension occupying a spatial space of ~1.5 × 1.5 m2, while metallic sheets with 200 holes covering ~50 × 50 mm2.
Regarding the scale-up behavior, for the microjets-array, the sustaining voltage is reported to be the same for each jet while the total current increased by a factor equal to the number of jets. Meanwhile, since each microplasma jet is independent and behaves similarly, it is expected that the product properties of the microjets-array are similar to a single microplasma jet when sustained at the same voltage. More general scaling behavior for a single microplasma reactor as well as for the array of identical microplasmas is expected to be similar to Yasuda’s concept, initially proposed for plasma polymerization. Within this concept the plasma enhanced process is defined by the composite power parameter or process scaling factor W/FM, where W is the discharge wattage, F is the volume flow rate and M is the molecular weight of the precursor. It should be noted, that in the case of nanostructures growth, in addition to the specific power-driven plasma-chemistry scaling (Yasuda parameter), the residence time becomes the key parameter, because together with growth rate 150