Nanoparticle Synthesis in a Plasma Downstream Reactor – From Plasma Parameters to Nanoparticle Properties

Abstract

In the presented study a tubular plasma reactor is investigated, which is normally used for the continuous plasma surface modification of fine-grained powders. The plasma reactor basically consists of a 1.5 m long glass tube with a gas and precursor feed unit at its top and a particle-gas separation unit at the lower end. The power is coupled inductively into the plasma via a coil which is wrapped around the reactor tube.

Substrate powders normally pass the discharge tube with high velocity and are functionalized on their way through the plasma in approximately 0.1 s. Possible plasma surface functionalization processes for powders are illustrated in Figure 1.1. The wettability of powders is increased by the formation of polar groups on the surface. Films are deposited on particle surfaces to protect the substrate from harsh environments or for catalytic applications. In recent years, also a new plasma process, which increases the flowability of fine-grained powders, gained increasing attention. Nanostructured SiOx is formed in the plasma and directly deposited on the substrate particle surface. These nanoparticle structures increase the surface roughness of the substrate particles. Thus, the interparticle van der Waals forces are reduced, which leads to a major improvement of the powder flowability. This process shows promise for companies dealing with cohesive granular materials.

The feasibility of this process was shown in the past, but at the same time the need for fundamental research in this field was recognized. Which ion density is required to yield in an effective surface modification? What is the thermal load of a substrate particle during the treatment? Which precursor should be used for a maximum improvement of the flowability?

In order to answer such questions, we measured axial profiles of plasma parameters in this continuous reactor and studied the nanoparticle synthesis in detail. No substrate powder was fed during these investigations to facilitate probe measurements and to focus on the produced nanoparticles.

Silica-like nanoparticles were produced from the four organosilicon monomers hexamethyldisiloxane (HMDSO), tetramethyldisiloxane (TMDSO), tetraethyl orthosilicate (TEOS), and tetramethyl orthosilicate (TMOS) in argon-oxygen gas mixtures. The chemical composition and morphology of the emerging particles and its production rate were studied as a function of process pressure (100 – 400 Pa), plasma power (200 – 350 W), gas velocity (5 – 16 m/s) and gas composition. Langmuir double probe and calorimetric probe measurements allowed determining the axial profiles of electron temperature, positive ion density, and energy influx along the vertical axis of this tubular reactor.