M.B. Koss

Velocity and radius transients during pressure mediated dendritic growth of succinonitrile

Abstract This study was part of a larger effort called the transient dendritic solidification experiment (TDSE), which uses the well known Clapeyron effect to study transient effects in dendritic solidification. The transient behaviour was studied between well defined steady states using pressure mediated changes since it is almost impossible to study the transient behaviour during growth of a dendrite from initial state to steady state. The time constants calculated for the velocity and radius transients are of the same order of magnitude. The velocity starts changing almost immediately after the pressure changes. The radius also changes rapidly but the change starts after an initial lag. This is attributed to its geometric memory and the fact that the change in velocity results from a change in the thermal field ahead of the tip, whereas a change in radius also entails a change in the lateral thermal field. These results affirm that pressure changes affect the growth behaviour and interfacial morphology of dendrites, which can be used for controlling solidification microstructure.

Capillary Mediated Melting of Ellipsoidal Needle Crystals

Measurements of video data on melting dendritic crystal fragments in reduced gravity show that a fragment’s ellipsoidal axial ratio, C/A, rises initially until it melts down to a pole-to-pole length of C ≈ 5 mm. At that point we observe a sudden fall in the C/A ratio with time, as the polar regions melt toward each other more rapidly than C/A times the melting speed, dA/dt, of the equatorial region. This accelerated melting allows the C/A ratio to fall from values around 10–20 (needle-like) towards values approaching unity (spheres) just before total extinction occurs. Analytical and numerical modeling will be presented that suggest that the cause of these sudden changes in kinetics and morphology during melting at small length scales is due to a crystallite’s extreme shape anisotropy. Shape anisotropy leads to steep gradients in the mean curvature of the solid-melt interface near the ellipsoid’s poles. These curvature gradients act through the Gibbs-Thomson effect to induce unusual thermo-capillary heat fluxes within the crystallite that account for the observed enhanced polar melting rates. Numerical evaluation of the thermo-capillary heat fluxes shows that they increase rapidly with the C/A ratio, and with decreasing length scale, as melting progresses toward total extinction.

Melting Kinetics of Prolate Spheroidal Crystals

The melting kinetics of a pivalic acid (PVA) dendritic mushy zone was observed for the first time under convection-free conditions. Video data show that PVA dendrites melt into fragments that shrink at accelerating rates to extinction. Individual fragments follow a characteristic time-dependence derived here for the diminishing length scales within a melting mushy zone. The melting kinetics against which the experimental observations are compared is based on the conduction-limited quasi-static process of melting under shape-preserving conditions. Agreement between analytic theory and experiment was found for the melting of a prolate spheroidal crystal fragment with an aspect ratio of C/A = 12.

Melting processes under microgravity conditions

The kinetics of melting pivalic acid (PVA) dendrites was observed under convection-free conditions on STS-87 as part of the United States Microgravity Payload Mission (USMP-4) flown on Columbia in 1997. Analysis of video data show that PVA dendrites melt without relative motion with respect to the quiescent melt phase. Dendritic fragments display shrinking to extinction, with fragmentation occurring at higher initial supercoblings. Individual fragments follow a characteristic time-dependence derived elsewhere. The microgravity melting kinetics against which the experimental observations are compared is based on conduction-limited quasi-static melting under shape-preserving conditions. Agreement between analytic theory and our experiments is found when the melting process occurs under shape-preserving conditions as measured using the CA ratio of individual needle-like crystal fragments.

Quasi-static Melting of Crystals: Experiments and Analysis

The kinetics of melting pivalic acid (PVA) dendrites was observed under convection-free conditions on STS-87 as part of the United States Microgravity Payload Mission (USMP-4) flown on Columbia in 1997 during STS-87. The analysis of video data show that PVA dendrites melt without relative motion with respect to the quiescent melt phase. Dendritic fragments display shrinking to extinction, with fragmentation occurring at higher initial supercoolings. Individual fragments follow a characteristic time-dependence derived elsewhere. The microgravity melting kinetics against which the experimental observations are compared is based on conduction-limited quasi-static melting under shape-preserving conditions. Agreement between the analytic theory and our experiments is found when the melting process occurs under shape-preserving conditions as measured using the C/A ratio of individual needle-like crystal fragments.

DENDRITIC CRYSTAL GROWTH DYNAMICS

With considerable NASA support, the scientific community has learned much in recent decades about the process of dendritic crystal formation, particularly as an example of steady-state pattern formation. Recently, efforts here and elsewhere have shifted towards the study of the timevarying aspects of the process. This paper will discuss the recent observations of the Transient Dendritic Solidification Experiment (TDSE). Data will be presented on the differences and similarities in the process by which two critical growth parameters, the growth rate and the tip radius of curvature, evolve as they approach steady state. This transient stage in the growth process is integral to many real-world industrial processes, and therefore a better understanding of it will serve as an important element of future industrial process models.