Thermal Plasma

In recent decades, nanoparticle synthesis by using thermal plasma has gained attention (Macwan et al. 2014; Kim and Kim 2019). In thermal plasma, a high temperature is achieved to evaporate all input materials. Then, the resulting vaporized materials can be condensed to form nanoparticles. These gas phase synthesis methods have some advantages over conventional sol-gel and other methods. Various experimental and modeling studies are reported to explain the effects of reactions parameters on synthesis of the nanoparticles by using a thermal plasma method (Girshick et al. 1993; Ishigaki et al. 2005; Shigeta et al. 2004; Suda et al. 2002). The reaction parameters mostly included are: pressure, feed rate, quench gas injection, and other reaction conditions. It is shown that a solid feed rate and quench gas injection majorly affect morphology and the size of nanoparticles. Sundstrom and DeMichiell have compared quenching methods by reviewing the literature and have shown that the direct quench gas mixing method is more effective (Sundstrom and DeMichiell 1971). However, it leads to a diluted product. Pratsini’s research group also studied a comparison of two mechanisms for the quench gas injection: cup mixing and no-cup mixing (Wegner et al. 2002). They employed an aerosol flow reactor in the study and demonstrated that appropriate cup mixing of the quench gas with Bi vapors leads to the production of small-sized homogenous nanoparticles.

The report has shown that primary particle size is increased with increasing the feed rate(Girshick et al. 1993). They applied quench gas injection along the length of the radiofrequency plasma reactor. Similarly, the effect of quench configuration of RF plasma reactors on particle morphology and size has beeninvestigated (Leparoux et al. 2005). They employed two gas jets and eight nozzle ring configurations and demonstrated that eight nozzle configurations are likely to synthesize uniform nanoparticles. Ishigaki et al. have analyzed the effect of quench gas injection by applying it to an RF plasma tail and shown that it leads to narrow sized nanoparticles (Ishigaki et al. 2005). In advances in the field, the systematic study of the effects of different quenching configurations on the production of silica nanoparticles was performed (Mendoza-Gonzalez et al. 2007). In addition, they evaluated aggregation level and sintering in the produced nanopowder and differentiated nanostructure as highly aggregated, sintered nanospheres, and spherical nanoparticles. This study also showed the effect of temperature and velocity distributions inside the reactor on the particle size and structure. In details of this RF plasma-based study, three reactors (radical-top quench reactor, radical-bottom quench reactor, and alumna-wall reactor) are designed with the same set-up of feed injection system, RF torch, and filtering system. In the process of particle synthesis, quartz micrometer-sized silica particles in methanol are injected into an RF torch. The particles suddenly start to evaporate and form silica vapors and then it condenses to form nuclei due to the cold wall and quench gas injection. The nuclei grow throughout the length of plasma reactors and are finally collected from the filter. In a comparative analysis of all three reactors, they showed that a radial-top quench reactor has synthesized very fine primary nuclei with a 20 nm diameter. But a high degree of aggregation and nano-micrometer-sintered particles are also formed. In the case of a radical-bottom quench reactor, a similar kind of results are obtained,except for the lower level of sintered particles. On the other hand, an alumina wall reactor has prepared complete spherical and monodispersed nanoparticles (diameter = 90 nm) with a lower level of aggregation. Hence, the proper position of quench inside the plasma reactors and reactor wall temperature are both important factors in controlling the size and morphology of silica nanoparticles.

Thermal Spraying

To fabricate or functionalize ceramic/polymer nanocomposites, a low viscosity is required for the dispersion and coating of nanoparticles. In this process, either a higher temperature or large amounts of solvent are needed. The difficultyof proceeding with melt mixing of polymer with nanoparticles is due to the dramatic increase in the viscosity of the melt. Hence, a higher temperature of melt is essential to lower its viscosity with the addition of nanoparticles. In addition, thermal treatment is used to coat the surface of substrate with ceramic/polymer mixture and for that; parts of substrate are kept in an oven. This method applies to perform coat on a field or large surface. On the other hand, the use of solvent at a high temperature for coating is problematic due to the volatile nature of the solvent. Therefore, thermal spraying is used to overcome these limitations. During the thermal spray process, particles are melted in a thermal jet and heated via plasma or combustion. The vaporized particles are accelerated against substrate. The viscosity of particles is reduced in flight and powder spread against substrate. No requirement of solvent is needed to spread coating. In the beginning, a thermal spray method is reported to make nanoparticulate-silica-reinforced nylon 11 coating (Schadler et al. 1997). They prepared silica/nylon nanocomposite coating, which is scratch- and wear-resistant. In addition, they showed that a hydrophobic silica surface (methylated) provided better mechanical properties to nanocomposite than an hydrophilic surface. Further, it is utilized to coat silica nanoparticles to create a suitable substrate (Siegmann et al. 2005).

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