Date of Award
Doctor of Philosophy (PhD)
Nonthermal plasmas offer a unique nonequilibrium environment that has been leveraged in a wide variety of applications in the fields of material processing, lighting, and waste management to name a few. In all of these cases, the plasma serves as a source of high energy electrons, ions, reactive gas species, and radicals that interact in several ways with surfaces brought into contact with the plasma. Specifically, nonthermal plasmas have been shown to be very successful in achieving continuous, high-throughput, monodisperse nanocrystals of a wide variety of materials. The crystallinity of nanoparticles synthesized in nonthermal plasmas can be attributed to the selective heating of the particles immersed in the plasma to temperatures well in excess of 1000 K. As a result, nonthermal plasmas are a promising synthesis environment for nanocrystals of high melting point materials, such as many metal oxides. As the material library accessible via nonthermal plasma extends ever wider, understanding critical processing parameters (i.e. reactor geometry, pressure, gas composition, applied power, etc.) for designing new synthetic processes is necessary. In addition, many applications of nanocrystalline materials achievable through plasma synthesis require consolidation into dense morphologies. While a number of methods of consolidation have been attempted in the past, there is a need to develop intentional consolidation methods with the aim of retaining nanocrystalline grain size and the monodispersity in the size distribution achieved via the nonthermal plasma. The first aim of this dissertation is to couple external plasma synthesis parameters to desired nanoparticle properties. A nonthermal plasma synthesis process to produce Al2O3 nanocrystals was developed. An analytical model for particle heating during plasma synthesis was extended to incorporate only external plasma parameters as inputs. Crystallization behavior of the Al2O3 nanocrystals showed agreement with the predictions of particle temperature calculated by the model. In addition, the model was used to successfully predict the power required to synthesize Cr2O3 nanocrystals demonstrating the generalizability of the model. Ultimately, this work resulted in a predictive model that can aid in designing nonthermal plasma nanocrystal synthesis processes. The second aim of this dissertation is to develop a method for consolidation of these nanocrystals and to explore the properties of dense, structural ceramic nanocomposites. A unique approach to consolidation was taken in which nanocrystals were first deposited by established aerosol deposition techniques followed by complete densification via atomic layer deposition (ALD) infill. The method retains the initial nanocrystal morphology from the nonthermal plasma synthesis while allowing for complete removal of porosity in the nanocomposite by ALD. Subsequent thermal post treatments were conducted in which a seeding effect on the crystallization of the ALD matrix was observed. This behavior along with the mechanical properties of the resulting nanocomposite films were investigated in detail.
Richard Axelbaum, Peng Bai, Katharine Flores, Shankar Sastry,