Under microgravity conditions, interdiffusion experiments of molten Au-Ag alloy system were undertaken. The diffusion in the flight specimens was faster than that in the ground ones, and their concentration curves deviated from theoretical ones, extraordinarily. These results may be ascribed to Marangoni convection flow, which became dominant under microgravity conditions in place of the gravitational convection one. It may be caused by the existence of free surface and a very large concentration gradient at the interface of the diffusioncouple type of specimen. The suppression of free surface formation and consideration for the wettability between molten alloy and crucible material will be indispensable.
For material processing in space it is expected that the uniform distribution of dopant will be achieved because buoyancy convection will be suppressed under microgravity. However, it is hard to avoid the temperature gradient along free surface in crystal growth process accompanied with heating and cooling. And then surface tension changes along free surface. This unbalance generates convective motion (called Marangoni convection) which becomes to effect strongly upon crystal quality. Some examples of defects in crystals have been reported that was considered to be caused by Marangoni convection. Many fundamental researches have been conducting to clarify the characteristics of Marangoni convection. The present authors have also conducted a flow visualization experiment of Marangoni convection for a project Fuwatto '92 where crystal growth was modeled. In the present report, the flow characteristics of Marangoni convection, its effect on crystal growth and control methods of the convection are summarized as well as the result of Fuwatto '92.
Al-Pb-Bi monotectic alloys with three different compositions were melted and solidified under microgravity environment in the Space Shuttle. The (Pb, Bi) particles were dispersed uniformly in the Al matrix, while evident sedimentation was observed in the reference sample processed under 1 gravity (G). The alloys were cold-worked into wires and superconducting properties of the wires were investigated. During cold-working, the (Pb, Bi) particles were elongated into fine fibers in the Al matrix and the distance between the (Pb, Bi) fibers became so small that the wires showed complete zero resistance below 9K due to proximity effect.
To clarify the solidification mechanisms of an eutectic system alloy under microgravity conditions, Al-Cu alloys containing 32.4 mass%Cu and 33.5 mass%Cu have been remelted and resolidified on orbit aboard the Space Shuttle, Endeavour. Both primary crystals of α aluminum dendrite in the hypoeutectic and CuAl_2 in the hypereutectic alloys grew continously from the surface of the samples inwards. Neither primary crystals moved during solidification because of the absence of fluid flow. The matrix of the samples showed irregular eutectic lameller structures. Small gas bubbles, which could not move in the molten metal in space, appeared near the surface of the samples.
An In_<1-x>GaAs (x=0.03) ternary compound semiconductor was grown on Earth using the horizontal and vertical Bridgman (HB and VB) methods. An attempt was made to grow the same semiconductor in a microgravity environment in a space laboratory. The aim was to explore the molten liquid convection and segregation phenomena, and to investigate the growth of ternary compound crystals with a uniform composition profile. The InGaAs crystal grown by the HB method on Earth had a segregation coefficient of 3.2, whereas the crystal grown in microgravity had a coefficient of 2.6. This result endorses Camel's prediction, providing that uniform concentration of Ga is not attainable for InGaAs compound, even in microgravity. It was revealed, however, that an InGaAs crystal with uniform Ga concentration profile could be grown with the VB method. The mechanism by which uniform Ga concentration is obtained using the VB method cannot explained with Camel's prediction.
Pb_<1-x>Sn_xTe crystals were grown by the directional solidfication method under microgravity in the SL-J/FMPT mission on boad the space shuttle "Endeavour". A cylindrical crystal, 15 mm in diameter and 58 mm in length, was obtained. A constant SnTe mole fraction of about 0.16 (i.e., a constant Pb/Sn ratio) was achieved along the growth axis to a distance of about 10 mm, and the etch pit density is about one-tenth that of a terrestrially grown crystal. The space-grown crystal has also improved electrical properties. In addition, about 25 spherical crystals, ranging from 0.5 to 11 mm in diameter, were unintentionally formed on the graphite spring.Melt leakedfrom the reservoir into the spring enclosure and formed spherical melt drops in the hollow of the spring that solidified into spherical crystals during cooling. Some of the small crystals have low dislocation density, on the order of 10^4/cm^2, two orders smaller than terrestrially grown crystals.
A PbSnTe single crystal was grown by traveling the molten zone at the speed of 2 mm/hr for 4 hours under microgravity in space. Sn composition in the crystal grown without thermal convection in the molten zone was almost constant in the growth length above 5 mm from the seed crystal. Carrier mobility at 77 K in the crystal grown in space was 3 times as large as that in the crystal grown under the same growth condition on the earth, indicating the high quality of the crystal grown under microgravity.
A space experiment on floating-zone crystal growth was carried out in Spacelab-J mission aboard space shuttle Endeavour. Prior to the experiment, analysis on the stability of the floating zone was made. It was elucidated that melt zones of any size of diameter are stably sustained in microgravity by its surface tension without depending on its surface tension or its density in the floating zone method in microgravity. In the space experiment, the verification was made on large diameter crystal of semiconducting compound InSb that can not be processed in terrestrial condition by floating zone method, because of its low surface tension and of its high density. The crystal obtained by the space experiment was 20 mm in diameter and 100 mm in length, which is the largest one that has ever been grown in space and is the first compound semiconductor grown in space by floating zone method. The crystallographic quality was highly improved by the space processing. The etch pits density was reduced down to 8.2×10^6/m^2, and carrier density down to 4.2x10^<19>/m^3. The high quality is likely due to the thin oxide film formed on the floating liquid zone. The oxide film coating the liquid zone during all over the crystal growth process acted as a flexible container that prohibited the Marangoni convection flow in the melt without making stress and making chemical contaminations on the growing crystal.
We grew GaAs crystals using a gradual cooling method aboard the German Spacelab mission D-2. Surfaces of GaAs substrates attached with Ga dissolved into the Ga by heating to 850℃ using an isothermal heating furnace. After that the temperature was gradually decreased to grow GaAs on the undissolved substrate surfaces. By using our original sample setup which eliminated free surfaces from the Ga solution, diffusion controlled convection-free growth was achieved under microgravity. Surface roughness of space-grown crystals was much smaller than that of earth-grown reference samples. This is due to annihilation of macrosteps during growth in space. We discuss macrostep annihilation in an isothermal system.
The effct of gravity on Vapor Phase Epitaxy of compound semiconductor was studied. The VPE growth experiments of InP in closed ampoules with halide transport agent were conducted on the ground and in a microgravity environment by German Spacelab mission D-2. It was fooud that epitaxial layer thickness distribution have various patterns and they are highly influenced by the gravity on the ground. However, the ones in a microgravity are almost flat and are mainly governed by diffusion processes.
Two kinds of experiments are carried out in Spacelab-J. In the first experiment, spherical single crystal of Si was used as a starting material and this crystal was melted and regrown in a furnace with almost uniform temperature distribution. In the second experiment, the starting material was Si rod of single crystal and the rod was heated in a temperature gradient to form a molten sphere at high temperature end of the rod. Then, the growth was started from melt-solid interface with un-melted rod as a seed cystal. It was found that there was a loss of Si from spherical melt in the first experiment due to the eutectic reaction between Si melt and Ta cartridge through a small hole in quartz crucible. The shape of the regrown crystal was hemi-spherical and on the surface of spherical part several facets were observed. The grown crystal was cut and mechanically polished. The chemical etching showed the presence of strong impurity striation in the part grown on the ground while no striation was observed in the space grown part. This means that unsteady Marangoni flow was successfully suppressed as well as thermal convection because of a uniform temperature distribution in the growth cell. In the melting process of Si rod, the molten sphere with 19 mm diameter was formed at one end. However, this molten sphere had moved from the tip to the side of the rod and the sphere touched with quartz tube. Due to this event, the crystal was broken. It turned out that the growth was done from un-melted rod but the growth direction was perpendicular to its axis.
The boule grown under microgravity conditions (Boule S) was 2.7 mm in diameter and 8 mm in length. It was found that Boule S is not a single crystal and composed of 5 phases: (Fe, Ca, Y) Nb_2O_6, (Y, Ca, U) NbO_4, Fe_3Nb_5 O_<16>, Nb_2O_5 and (Y, Ca, Fe, U)_<1-x>NbO_4; x=0.02-0.1. No Samarskite was found in Boule S. These phases are identical with those found in the boule grown on the earth (Boule E). The distribution and the size of each phase was different from those in Boule E. The grains of Nb_2O_5 were found everywhere in Boule E In Boule S, Nb_2O_5 crystallites were found only in a solidified molten zone, not in the boule. On the earth, during the growth, the solution of the molten zone was caught by the crystallized body and became Nb_2O_5 inclusion in Boule E. But the solution was not trapped under microgravity conditions. On the earth, the solution migrated to the grown crystal and covered it. The surface area of the molten zone was enlarged. Accordingly it is difficult to keep the volume of the molten zone constant on the earth. On the other hand, under microgravity conditions, the volume could be easily kept constant and the boule with nearly uniform diameter was obtained. This result shows that the composition of the solution can be easily kept constant under microgravity conditions. To keep the composition constant is essential for the growth of a single crystal by the TSFZ method. It has been revealed that the microgravity conditions are suitable environment for the TSFZ method. A large bubble was observed in Boule S. Its diameter was larger than a half of the diameter of Boule S. On the earth, due to buoyancy, bubbes gather at the upper part of the molten zone. Accordingly, bubbles are seldom trapped in the crystal grown by the floating zone method. While under microgravity conditions, bubbles grew larger and spoiled quality of the grown crystal. The study of the formation and the movement of bubbles in oxide solution is necessary to grow a high quality oxide crystal under microgravity conditions.
Protein single crystals of diffraction quality should be prepared for determination of three dimensional structure of whole protein molecule by X-ray crystallography. Microgravity environment obtained by space flights using space shuttle and space station has been ascertained to be convenient for protein crystallization since the first experiment by Littke in 1983. In space microgravity, there are no sedimentation and no convection of prtein and solute molecules in the solution. Also, crystal growth slowly proceeds by diffusion control nearby where crystal nuclei grew. Several times of our space experiments regarding protein crystallization are explained, and the space-grown crystals are emphasized to be of better difrraction quality and of less mosaicity. It is, therefore, expected that these characteristics of the space-grown crystals will contribute to the establishment of the time-resolved X-ray crystallograpy by the Laue method.