Manipulating Dendritic Growth: An Undergraduate Laboratory Experience with the Interplay between Mass Transport, Supersaturated Solutions, and Dendrite Structure

摘要

Dendrite growth affects material systems across applications as diverse as lithium batteries, organic light emitting diodes, turbine blades, and biological sensors. Their unique crystal structure and ability to physically see growth make for a unique undergraduate laboratory experience. This experiment uses dendrite growth to explore the physical and chemical driving forces behind dendrite growth through a set of viscous, supersaturated solutions of varying ammonium chloride and gelatin concentrations. The degree of NH_4Cl supersaturation determines growth rate, which can be mediated by the gelatin limiting diffusional mass transfer. This exercise was designed for a material science course, though it could easily be adapted to an inorganic or general chemistry course. Through this experiment, students are introduced to optical microscopy for quantitative analysis, a common, inexpensive analytical research tool rarely seen in the undergraduate laboratory. When chemical driving forces are dominant (low gelatin, high salt concentrations), a more ordered dendrite structure forms, with primary branches at 90° angles. Conversely, as diffusion becomes more dominant, a more disordered, denser dendrite structure is observed and the growth rate is slower. Students use both qualitative and quantitative observations to make connections between a fundamental laboratory exercise and critical materials processing techniques that rely on physicochemical driving forces.