Combustion synthesis technology can be advantageously applied to high temperature experiments under microgravity environments (MGE) not only as a direct synthesizing method but also as a chemical oven, because it is possible to get high temperatures more rapidly compared to convention al electric furnaces through its self-propagating reaction without any external energy. Combustion synthesis consists of the processes of reaction propagation and product crystallization. Since the former proceeds rapidly , a MGE for about 10 seconds should be long enough for attaining the synthesizing process. Under such a MGE, the combustion products should be expected to keep the position where they react because any fluctuation caused by heat convection and mass disturbance would not occur, resulting in finely dispersed combustion products.
In this paper , the potential and characteristics of combustion synthesis under MGEs is briefly explained by taking the formation of T iB2- AI- B and Zr02-Al203-Fe composite systems carried out in a free fall (～10 s) and a parabolic flight (～20 s) as an example.
Growth and dissolution of a Cdli2 crystal were investigated under microgravity by means of high resolution in situ observation using commonpath interferometry and modulation microscopy, which were employed for the observation of the concentration field and of the crystal surface, respectively. The microgravity was obtained for 6 minutes in the TR-lA rocket, which was flown from Tanegashima Island on September 16, 1991. Although the original scientific objective could not be fulfilled completely because of an accidentally introduced small gas bubble near the seed crystal, the development of Marangoni convection followed by the instability of the convection during dissolution and growth of the seed crystal was, for the first time, clearly observed. The development and the initiation of the instability of the Marangoni convection was respectively interpreted due to the coupling effects of the temperature gradient and the concentration gradient at the interface between the solution and the bubble during dissolution, and to the mutual interactions during growth of the crystal. It was also observed that the growth rate anisotropy of the crystal was considerably influenced by the Marangoni convection. The growth rate was calculated assuming the process was con trolled solely by diffusion, which was compared with the observed values.
Diamonds synthesis by a hot filament CVD (Chemical Vapor Deposition) method in a closed system was carried out as a pre-study under micro gravity. A gas mixture of 1.5 vol % ethanol and H2 was used for the starting gas. Temperature measurement and gas analysis by gas chromatograph in the chamber during the synthesis were carried out. Temperature profile and flow velocity in the chamber under micro gravity (5 X 10- 3 g) and normal one (1 g) were simulated with a numerical code. In the chamber used in this study (100 mm in diameter and 100 mm in height) quality of the synthesized diamonds became lower after 15 minutes in reaction, because oxygen radicals/ atoms might be consumed at that time. It was suggested by the calculation that the amount of atomic hydrogen near the substrate (the distance was 1 to 5 mm from the filament) under micro gravity was smaller by 1/100 than that under the normal one.