Growth of ammonium chloride dendrites

The ammonium chloride-water system has been used extensively as a transparent metal analog to model solidification in binary metal alloys. In this work, the growth rate and morphology of NH₄Cl dendrites grown from aqueous solutions were studied. Since an accurate knowledge of the materials parameters is essential to predicting the growth behavior, the equilibrium segregation coefficient was measured and a detailed analysis of the other NH₄Cl-H₂O materials properties cited in the literature was conducted. Isothermal experiments on bulk NH₄Cl-H₂O samples confirmed that the previously reported discontinuity in the growth rate as function of undercooling and associated transition from oriented slowly growing dendrites to rapidly growing dendrites are not artifacts of the sample geometry. Directional solidification experiments conducted to study the dendrite growth morphology revealed oscillations in both the growth rate and orientation. Results from these studies show that both the undercooling at which the to transition occurs and the peak velocity vary with composition. However, the observed shifts toward smaller apparent undercoolings and the narrowing of the oscillations at higher drive velocities result from changes in the local composition caused by the velocity and orientation dependencies of the partition coefficient. The oscillatory behavior of the dendrites can be predicted using the residual compositional field and the applied temperature gradient. By using an anisotropic segregation coefficient, the slow and fast growth rates can be separately modeled as a function of undercooling using the standard dendrite growth equations. While the transition to the morphology can be attributed to the anisotropy in the k-value, several modifications need to be made to the existing dendritic growth models in order to describe the critical transition. Due to the complex relationships between the non-equilibrium segregation coefficient, composition, and growth rate, some of these modeling efforts have been left to future researchers. In addition to the inclusion of the overall anisotropy, our experiments indicate that the long-range compositional and thermal field effects must be incorporated into the dendrite growth models to explain the difference in growth rates of Primary branches when or side-branches are present