Reactive Wetting Controlled by Very Small Vertical Temperature Gradients in a Chemical Transport Mini Reactor

Abstract

Abstract Reactive wetting of quartz by silicon is an ideal model system for the understanding and control of reactive wetting effects. Very slow and well controlled reactive spreading of a small silicon droplet on quartz can be achieved and observed in a new type of chemical transport mini reactor equipped with a video microscope setup operating in the melting point vicinity of silicon at Tm=1685 K. The spreading velocities are investigated in dependence of a very small temperature gradient that causes a slight chemical gradient and induces a slow oxygen transport through the system. The oxygen source is a SiO2 substrate at the temperature TA. From here the oxygen is transported upwards through the liquid silicon droplet and is released to the gas phase in form of SiO-molecules. The SiO-molecules are transported towards a cooler SiO2 substrate at TB where SiO2-nano whiskers are formed via a vapor–liquid–solid-growth mechanism. In this way the oxygen drain is located in the gas phase above the droplet. The source reaction drives the reactive spreading process. By reducing the temperature difference ΔT=TB−TA<0 a very small chemical potential difference ΔμO=μOB−μOA<0 of the oxygen between the sessile droplet at position A and the whiskers at position B can be created and adjusted. The velocity of the moving triple line is then measured in dependence of ΔμO and expressed by a power law. The results are explained in terms of an irreversible thermodynamic model that couples the dynamics of the triple line to an ongoing solid state reaction in the underlying substrate, where a thin solid silicon suboxide gradient layer forms between substrate and melt. By fitting the data to the model formula one can determine reactive contributions to the tensions at the triple line and investigate these quantities in the limit ΔμO→−0.