The systematic control of crystal growth from seeds in microgravity gives insight into the physical laws by which matter organizes itself. In the Binary Colloid Alloy Test-6 (BCAT-6), Seeded Growth, scientists observe crystal growth mechanisms and crystal structures. The results from this experiment have application to a wide variety of products based on nanoscale materials.Principal Investigator(s)
ZIN Technologies Incorporated, Cleveland, OH, United States
National Aeronautics and Space Administration (NASA)Sponsoring Organization
Human Exploration and Operations Mission Directorate (HEOMD)ISS Expedition Duration:
September 2010 - March 2013
25/26,27/28,29/30,31/32,33/34Previous ISS Missions
The BCAT predecessors, BCAT-3 operated on ISS, and BCAT, operated on Mir in 1997 and 1998. BCAT-4 launched March 11, 2008 on 1J/A, and BCAT-5 launched June 13, 2009 on 2J/A.
Previous work has shown that the rate of crystal growth initiation is accelerated when spherical seed particles of a certain size ratio are introduced into the colloidal suspension at or above a particular particle number density. Consequently, the number of small microscopic crystals (crystallites) should exceed the number of seeds resulting in smaller crystallites and a greatly increased nucleation rate. These experiments are therefore relevant to many industrial processes where the control of crystal size and crystallization rate are important.
Binary Colloidal Alloy Test-6, Seeded Growth, samples 6 and 7 (BCAT-6, Seeded Growth) builds upon the breakthrough NASA PHaSE experiment which showed that glasses in concentrated systems (as seen on Earth) tend to order into crystals when gravitational effects are absent. Particles which are of a single size have been created for this sample. The size of the particles is selected to be the size of a wavelength of visible light. It is necessary to have the particles this size so that they can be large enough to be observed with visible light but small enough for their movements to be a result of a temperature dependent but seemingly random motion known as Brownian motion. White light from a flashlight and the camera flash are used to detect the colorful reflections produced by colloidal crystals formed following the sample mixing. The idea of applying spherical seed particles to the wall of the sample cell is new and significant. According to theory and simulations, this allows for the formation of an array of crystallites rather than one large crystal as would be produced in a homogeneous solution. In this experiment, half of the surface seen by the camera is covered by seeds, while the other half of the container is not. Two volume fractions are prepared, one at the minimum theoretical volume fraction causing complete crystallization (volume fraction = 0.53) and one in the glassy regime (volume fraction = 0.60).
The ‘crystallization catalysts’ allow for the completion of several experiments at once in order to see the effects of placing seed particles on the cell walls, and to determine how the particle volume fraction can affect the rate of crystal nucleation. If this strategy is successful, this will guide the choice of the optimal particle volume fractions to be used in the seeded growth experiments studied over a wide range in greater detail in the Advanced Colloids Experiment (ACE). While predicted in theory, this experiment has not been possible on earth because of particle sedimentation. Microgravity allows for both index matching (for clarity) and density matching (to avoid sedimentation in 1g). Performing these experiments on the International Space Station (ISS) allows excellent index of refraction matching without the requirement of simultaneous density matching.
This experiment addresses basic physics questions that may impact crystal growth in space.Earth Applications
Seeding samples should allow crystal growth to be controlled so that it is either homogeneous (significant individual crystals) or heterogeneous (many little crystals). Knowing when either type of growth will occur and even more importantly, how to control the growth, is relevant to many industrial processes.
The BCAT-6 experiment consists of a set of ten small samples of colloidal particles. The BCAT-6 set of samples are each contained within a small case the size of a school textbook. The experiment requires a crew member to set up the experiment using a handrail/seat track configuration, ISS Laptop and the Kodak 760 or Nikon D2Sx camera to take digital photographs of the samples at close range. The pictures are down-linked to investigators on the ground for analysis.
The current plan for this experiment is to conduct it over a 7-day session, each of which can be run incrementally and will each require about 7 hours of crew-time; a third session to mix and photograph all 10 samples (about 4.6 hours of crew time) and then a fourth session at six months to photograph all ten samples which is slotted to take about four hours of crew time.
BCAT-6 typical operations consists of:
Session 1: Set up hardware, take baseline photos of all ten samples; homogenize samples 6-10 then samples 9 and 10, then automatically photograph sample 1 (using EarthKAM software on laptop) every hour for 7 days. Perform sample 1 daily status check each day. After seven-day run, perform crystal search/photography on 6-10. Homogenize sample 2, automatically photograph sample 2 (using EarthKAM software on laptop) every hour for 7 days. Perform sample 1 daily status check each day. After seven-day run, perform crystal search/photography on 6-10. If necessary, tear down after operations are complete but keeping setup intact is preferred to save crew time.
Session 2: Set up hardware, homogenize samples 3, 4 and 5 one at a time then automatically photograph each sample (using EarthKAM software on laptop) every hour for 14 days each. Perform Crystal Check and Photography procedures on 6-10 if crystals not found/photographed in Session 1. If necessary, tear down after operations are complete but keeping setup intact is preferred to save crew time. .
Session 3: Homogenize and photograph samples 1-10 (using EarthKAM software on laptop) and stow sample module for six months. The experiment is torn down after operations are complete. .
Session 4: At six months after homogenization, manually photograph Ssamples 1 through 10. Re-stow sample module and tear down after operations are complete.