Space Materials Science in China: I. Experiment Studies under Microgravity*

ZHANG Xingwang YIN Zhigang YU Jianding YUAN Zhangfu ZHAO Jiuzhou LUO Xinghong PAN Mingxiang

Space Materials Science in China: I. Experiment Studies under Microgravity*

ZHANG Xingwang1,2YIN Zhigang1,2YU Jianding2,3YUAN Zhangfu4ZHAO Jiuzhou5,6LUO Xinghong5PAN Mingxiang7,8,9

1 (100083) 2 (100049) 3 (200050) 4 (100083) 5 (110016) 6 (110016) 7 (100191) 8 (100049) 9 (523808)

The virtual absence of gravity-dependent phenomena in microgravity allows an in-depth understanding of fundamental events that are normally obscured and therefore are difficult to study quantitatively on Earth. Of particular interest is that the low-gravity environment aboard space provides a unique platform to synthesize alloys of semiconductors with homogeneous composition distributions, on both the macroscopic and microscopic scales, due to the much reduced buoyancy-driven convection. On the other hand, the easy realization of detached solidification in microgravity suppresses the formation of defects such as dislocations and twins, and thereby the crystallographic perfection is greatly increased. Moreover, the microgravity condition offers the possibilities to elucidate the liquid/solid interfacial structures, as well as clarify the microstructure evolution path of the metal alloys (or composites) during the solidification process. Motivated by these facts, growths of compound semiconductors and metal alloys were carried out under microgravity by using the drop tube, or on the scientific platforms of Tiangong-2 and SJ-10. The following illustrates the main results.

Space materials, Microgravity, Bubble behavior, Microstructural evolution

1 Marangoni-convection-driven Bubble Behavior and Microstructural Evolution of Sn-based Alloy

The Marangoni convective effect gives rise to con-vection during the alloy solidification process. It plays a key role in the heat and mass transfers, and significantly affects the microstructure and elemental distribution of the alloys[1]. Microgravity condition provides a platform that favors a better understan-ding of this effect, due to the alleviation of gravity-induced buoyancy convection. Here we choose Sn- 3.5Ag/Sn-17Bi-0.5Cu as a model system to study the effects of Marangoni convection on the microstru-ctures of metal alloys under space microgravity con-dition. Cylindrical alloy bars of Sn-3.5Ag and Sn- 17Bi-0.5Cu with diameters of 5 mm were prepared using a Cu mold and were cast in a vacuum induction furnace. They were cut into cylindrical samples with a height of 5 mm and fixed to the Cu ring, and then the samples were packaged into a quartz tube under vacuum condition. A multi-function materials synthesis furnace was used as the heating device and the solidification was carried out on the recoverable sa-te-llite SJ-10. The total time from heating to cooling was 1664 min, with durations at 773 K for 52 min and 930 K for 165 min. For comparison, the Sn- 3.5Ag/Sn-17Bi-0.5Cu (wt. pct) alloy was also soli-dified under normal gravity conditions.

The comparative studies revealed that Marangoni convection significantly affects the solidification structure as it controls the bubble behavior and mass transfer in the melt under microgravity. The surface tension gradient induced by the Bi concentration difference leads to the formation of Maran-goni convection from the right to left of the melt. And in the left (Bi-scarce) part of the melt, Marangoni convection induced by the Cu concentration difference flows from the outside to the inside. Due to the bubble-agitation convection, Cu mainly migrates from the substrate to the right part of the melt. Therefore, a gradient distribution of dendrite-like CuSnis observed. While under the normal gravity condition, gravity-induced convection gives rise to an even distribution of Bi and Cu, which reduces the contact angle and the surface tension, thereby promoting the nucleation of the alloy. Therefore, fine dendrite-like CuSnwith high density is uniformly distributed in the melt.

2 Solidification of TC8 Alloy

Titanium alloy has amazing properties including high specific strength, good machining performance and strong corrosion resistance, and is widely used in the aerospace field[2]. Different from the commonly observed columnar structure, the solidification of duplex titanium alloy usually yields an equiaxed polycrystalline structure. However, few works have yet been reported on the solidification behavior of titanium alloy in the microgravity environment. For better understanding, the effect of microgravity on the structure of titanium alloy, the solidification of TC8 alloy was carried out under a microgravity environment with a drop tube. Rod samples with a diameter of 6 mm and a length of 28 mm were used in this study. The 50-m-high drop tube can supply a microgravity environment at an acceleration level down to 10–60for about 3.2 s.

The solidification microstructure of TC8 alloy is composed of fine equiaxed grains that appeared at an early stage and bigger elongated grains formed at later stage. Between these two kinds of grains, a flat transition interface was observed in thesample, while a curved one appeared in the 1sample. Gen-erally, the amounts and aspect ratios of the grains are smaller, and the grain sizes are larger in thesam-ple. Moreover, no visible element macro-segre-gation occurred in both theand 1samples. These observations indicate that the solidification velocities of the samples are rather rapid, and therefore the convection and solute transport driven by gravity only has limited influence on the solidification microstructure. To sum up, the solidification process is mainly controlled by the thermal diffusion, in which the hydrostatic pressure and wall effect plays a key role.

3 Solidification of Al-Bi-Sn Immiscible Alloy in Space

Immiscible alloys are characterized by the occurrence of a miscibility gap in the liquid state, and have a strong industry application background. These alloys transform into two liquids enriched with different components when cooling into the miscibility gap on the ground, generating a phase-segregated microstructure. Microgravity is an excellent environment to inhibit the gravity-related convection, which is helpful for elucidating the roles of nucleation, growth, Ostwald ripening and motions of the minority phase droplets. Herein, the directional solidification experiment was performed with Al-Bi-Sn immiscible alloy under microgravity environment onboard the Tiangong-2 space laboratory. During the solidification, the hold temperature and the withdrawn velocity are about 950 K and 28 μm·s–1, respectively[3].

4 Detached Growth of InSb

Bridgman method is one of the mainstream techniques to grow semiconductor crystals. However, con-siderable thermal mismatch appears when the crystal adheres to the container, due to the difference in th-eir thermal expansion coefficients. Calculations show that, this mismatch can result in thermal stress orders of magnitude larger than that caused by the temper-ature gradient. Experimentally, such large thermal stress usually leads to an increased dislocation den-sity or even worse, macroscopic cracks, when cooling the crystals from the growth temperature. Detached growth, under which the crystal grows without con-tacting the container, is a possible way to alleviate the thermal stress and therefore improve the crystal quality. Herein, detached Bridgman growth of InSb, a typical narrow bandgapIII~V semiconductor, was achieved onboard the Tiangong-2 space laboratory[4].

It was found that in the region adjacent to the seed most of the space-based InSb crystal grew without touching the crucible wall. By contrast, the ground-grown InSb crystal has a uniform diameter and its outer-surface replicates the inner-surface of the crucible. As a result, the space-grown InSb cry-stal has a largely reduced defect density, as compared with its terrestrial counterpart. Moreover, room temperature electrical characterizations of the space InSb crystal within the detached region yield consi-derably improved electron mobility. The space-based InSb crystal was utilized to fabricate Corbino disk, a two-terminal magnetic sensor, and a considerably enhanced sensitivity was achieved. The observed ma-gnetoresistance increases by about 50% as compared with that of the terrestrial device. Our results have significant implications for the high-quality growth of InSb-related materials and their future applications.

5 Space-grown Homogeneous InxGa1–x Sb Crystal

The growth of high-quality homogeneous InGa1–xSb bulk crystals is a challenging task by conventional methods such as Czochralski and Bridgman techniques, since there exists a large separation between the solidus and liquidus lines in the InSb-GaSb binary phase diagram. The Vertical Gradient Freezing (VGF) method is very promising for addressing this issue. Herein, InGa1–xSb crystal growth was perfor-med on the recoverable satellite SJ-10 by using the VGF method. A GaSb(111)A/InSb/GaSb(111)Asan-dwich sample was used as the starting material, and the lengths of GaSb and InSb crystals were 23 mm and 4 mm, respectively. After holding for 3 h at the growth temperature of about 933 K, the temperature was decreased at a rate of 0.5 K·h–1to grow homogeneous crystals for 49 h. An experiment was also conducted on the ground using a 3-zone vertical gra-dient furnace to replicate the microgravity experiment[5].

In0.11Ga0.89Sb with uniform composition was ob-tained under microgravity environment on board the platform of SJ-10. The shapes of the initial and final growth interfaces, the dissolution tendency of the seed and feed crystals, and the growth kinetics of this experiment are similar to the long duration micro-gravity experiments performed at the international space station, suggesting the high repeatability and reproducibility of the microgravity experimental re-su-lts. As compared with the composition uniformity of space-grown InGa1–xSb, crystal growth under nor-mal gravity only yields an indium composition that is gradually increased along the growth direction. Our results show that normal gravity is helpful for achieving a steady state of equilibrium in the melt composition. However, the non-steady state equili-brium in the melt composition under microgravity favors for a higher growth rate and compositional ho-mogeneity at higher indium composition of InGa1–xSb solid solution.

[1] YUAN Zhangfu, WANG Rongyue, XIE Shanshan,. Wettability of high-temperature melts under microgravity and ground gravity conditions [J]., 2020, 50:047004

[2] LUO X H, WANG Y Y, LI Y. Role of hydrostatic pressure and wall effect in solidification of TC8 alloy [J]., 2019, 5:23

[3] LI Wang, JIANG Hongxiang, ZHANG Lili,. Solidification of Al-Bi-Sn immiscible alloy under microgravity conditions of space [J].., 2019, 162:426-431

[4] YIN Zhigang, ZHANG Xingwang, WU Jinliang. Growth of III-V semiconductor crystals under microgravity [J]., 2020, 50:047003

[5] YU J, INATOMI Y, KUMAR V N,. Homogeneous InGaSb crystal grown under microgravity using Chinese recovery satellite SJ-10 [J], 2019, 5(1):8

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ZHANG Xingwang, YIN Zhigang, YU Jianding, YUAN Zhangfu, ZHAO Jiuzhou, LUO Xinghong, PAN Mingxiang. Space Materials Science in China: I. Experiment Studies under Microgravity., 2020, 40(5): 946–949. DOI:10.11728/ cjss2020.05.946

* Supports by the National Natural Science Foundation of China (U1738114), the Strategic Priority Research Program on Space Science, the Chinese Academy of Sciences (XDA15051200), the China’s Manned Space Station Project (TGJZ800-2-RW024), and the Chinese manned space flight pre-research project (030302)

March 26, 2020

E-mail: xwzhang@semi.ac.cn, panmx@iphy.ac.cn