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The Forefront of Space Science

Crystal Growth Experiment in Space
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Major features of outer space environment are various gravities, atmosphere, cosmic rays, and bundles space. “Space environment utilization scienceEis a collective name of scientific researches where researchers focus on these features and actively to utilize i) the space environment features such as no-convection or levitation on the ground; ii) the space environment itself; and iii) the features in space environment.

In this article, I would like to discuss the space experiment history in Japan for crystal growth, one of materials science fields, and its future research themes.

Crystal growth experiments before “Kibo”

As crystals grow, temperature and concentration distribution at the environment phase around them become inhomogeneous to some degree. Under the terrestrial condition, these heterogeneities act as a drive force of buoyancy convection. Convection in crystal growth has various impacts including: i) deforming the symmetry of the growing crystal shape; ii) causing heterogeneity of constituents in the obtained crystal; and iii) impeding elucidation of the phenomena occurring in the growing interface. Therefore, the advantage of the microgravity environment was noted from the aspect of constraining as much as possible the impacts caused by the gravity such as buoyancy convection and sedimentation. With the recognition of the benefit, the crystal growth experiments have been performed, which includes: “Fuwatto E2EFirst Material Processing Test (FMPT) in 1992; the Second International Microgravity Laboratory mission (IML-2) in 1994; Space Flyer Unit (SFU) in 1996; the first Microgravity Science Laboratory (MSL-1) in 1997; and Unmanned Space Experiment Recovery Systems (USERS) in 2003.

During the period of those experiments, Japanese researchers and engineers used small rockets such TR-1A, aircrafts, dropping test facilities, etc. to perform R&D and preparatory experiments towards the future space experiments. Let me introduce an in-situ observation, one of measuring technologies that Japan boasts (Fig. 1). In the early stage of the space experiments, analysis of test samples was almost made after collecting them. However, in terms of effectively making use of limited equipment weight, dimension, electric power, etc. due to the space experiment and limited opportunities, the “in-situEobservation and measurement of phenomena are promising methods. If crystals or environment phase surrounding them are transparent to observation light at the time of crystal growing, we can visualizing the solute concentration and temperature distribution in the environment phase as well as change in shape of the crystals.

Figure 1
Figure 1. Example of the in-situ observation instrument for crystal growth
This was developed by researchers mainly from Tohoku University in around 1985.

After the above series of experiments, no full-scale, long duration experiment using the microgravity environment was not conducted until the utilization of the Japanese Experiment Module “KiboEon International Space Station (ISS) started in 2008. Nevertheless, during the period, the world's most advanced research methodology for measurement and control of flow, levitation of test samples, and so on was devised in Japan in addition to the in-situ observation. Those methods are effectively applied to the materials science and research on the ground.

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