Crystal Growth In Microgravity Research Articles

Protein crystal growth on board Shenzhou 3: a concerted effort improves crystal diffraction quality and facilitates structure determination

The crystallization of 16 proteins was carried out using 60 wells on board Shenzhou 3 in 2002. Although the mission was only 7 days, careful and concerted planning at all stages made it possible to obtain crystals of improved quality compared to their ground controls for some of the proteins. Significantly improved resolutions were obtained from diffracted crystals of 4 proteins. A complete data set from a space crystal of the PEP carboxykinase yielded significantly higher resolution (1.46 Å vs. 1.87 Å), I/sigma (22.4 vs. 15.5), and a lower average temperature factor (29.2 Å 2 vs. 42.9 Å 2) than the best ground-based control crystal. The 3-D structure of the enzyme is well improved with significant ligand density. It has been postulated that the reduced convection and absence of macromolecule sedimentation under microgravity have advantages/benefits for protein crystal growth. Improvements in experimental design for protein crystal growth in microgravity are ongoing.

Dynamics of three-dimensional convection in microgravity crystal growth: g-jitter with steady magnetic fields

We present results from three-dimensional simulations of the flow induced by transient acceleration (g-jitter) in microgravity crystal growth. Transient accelerations in both axial and transverse directions are considered for a simple prototype of a vertical Bridgman crystal growth system. We also consider the effects of applying a steady magnetic field in axial or transverse directions to suppress the flow. In most cases, application of a magnetic field suppresses flow oscillations, but for transverse jitter at intermediate frequencies, flow oscillations are increased. This counter-intuitive effect is a dynamic one, in which boundary layer formation under the influence of a magnetic field shortens the time scale of momentum transfer, allowing the flow to respond more quickly to the time variation of acceleration. The effect of the magnetic field on an enclosed flow with electrically insulating boundary conditions is to preferentially suppress the velocity component tangential to the magnetic field. The ability to filter a single velocity component by application of a specified magnetic field could be useful for simultaneously improving both axial and radial segregation in semiconductor crystal growth.

Apocrustacyanin C(1) crystals grown in space and on earth using vapour-diffusion geometry: protein structure refinements and electron-density map comparisons.

Models of apocrustacyanin C(1) were refined against X-ray data recorded on Bending Magnet 14 at the ESRF to resolutions of 1.85 and 2 A from a space-grown and an earth-grown crystal, respectively, both using vapour-diffusion crystal-growth geometry. The space crystals were grown in the APCF on the NASA Space Shuttle. The microgravity crystal growth showed a cyclic nature attributed to Marangoni convection, thus reducing the benefits of the microgravity environment, as reported previously [Chayen et al. (1996), Q. Rev. Biophys. 29, 227-278]. A subsequent mosaicity evaluation, also reported previously, showed only a partial improvement in the space-grown crystals over the earth-grown crystals [Snell et al. (1997), Acta Cryst. D53, 231-239], contrary to the case for lysozyme crystals grown in space with liquid-liquid diffusion, i.e. without any major motion during growth [Snell et al. (1995), Acta Cryst. D52, 1099-1102]. In this paper, apocrustacyanin C(1) electron-density maps from the two refined models are now compared. It is concluded that the electron-density maps of the protein and the bound waters are found to be better overall for the structures of apocrustacyanin C(1) studied from the space-grown crystal compared with those from the earth-grown crystal, even though both crystals were grown using vapour-diffusion crystal-growth geometry. The improved residues are on the surface of the protein, with two involved in or nearby crystal lattice-forming interactions, thus linking an improved crystal-growth mechanism to the molecular level. The structural comparison procedures developed should themselves be valuable for evaluating crystal-growth procedures in the future.

Influence of Local Magnetic Fields on P-Doped Si Floating Zone Melting Crystal Growth in Microgravity

A two-dimensional axisymmetric numerical model is presented to study the influence of local magnetic fields on P-doped Si floating zone melting crystal growth in microgravity. The model is developed based on the finite difference method in a boundary-fitted curvilinear coordinate system. Extensive numerical simulations are carried out, and parameters studied include the curved growth interface shape and the magnetic field configurations. Computed results show that the local magnetic field is more effective in reducing the impurity concentration nonuniformity at the growth interface in comparison with the longitudinal magnetic field. Moreover, the curved growth interface causes more serious impurity concentration nonuniformity at the growth interface than the case with a planar growth interface.

Marangoni Flow of Molten Silicon: Instability and the Effect of Oxygen Partial Pressure of an Ambient Atmosphere(<Special Issue>Crystal Growth in Micro-Gravity)

Marangoni Flow of Molten Silicon: Instability and the Effect of Oxygen Partial Pressure of an Ambient Atmosphere(<Special Issue>Crystal Growth in Micro-Gravity)

Three-dimensional analysis of flow and segregation control by slow rotation for Bridgman crystal growth in microgravity

Although the buoyancy convection in the melt during crystal growth is greatly reduced in microgravity, the residual gravity, or the so-called g-jitter effect, can lead to three-dimensional (3D) unsteady flow and severe radial dopant segregation. Using a centrifuge to rotate the system about the growth axis could be an effective way to suppress the 3D flow and improve dopant uniformity. Through fully time-dependent 3D simulation of Bridgman growth of gallium-doped germanium crystals, we investigate the feasibility of using a centrifuge at low rotation rate to improve the dopant uniformity in the grown crystal. In this numerical model, in addition to the heat flow and the moving interface, both radial and axial segregations are also computed simultaneously for a growth period. The effect of slightly eccentric rotation is also considered and they can be an issue in practical implementation.

Transient convection of Te doped GaSb induced by g-jitter

Gallium antimonide (GaSb) is commercially used for solar cells and thermophotovoltaic devices. It is also used as a substrate material for the epitaxial growth of heterostructures, which are used for optoelectronic applications. The semiconducting properties of the material are governed by an antisite defect that requires compensation by a donor dopant such as tellurium to provide n-type conductivity. The electronic and optical properties of Te doped GaSb are greatly influenced by the degree of Te segregation in the crystal. The latter is strongly affected by buoyancy and thermocapillary (Marangoni) driven convections. Both types of convections are associated with a temperature-induced density and surface tension gradients along the sample. During 1996 and 1997, nine microgravity experiments were conducted on the MIR space station to determine the segregation of dopants during microgravity crystal growth using the QUELD furnace on the microgravity isolation mount. The space-grown samples displayed fluctuations in [p] and hence in [Te] attributed to g-jitters events and not to Marangoni convection during crystal growth (J. Vac. Sci. Technol. A 18(2) 2000). On the numerical side, attempts were made to study the effect of convection and the g-jitter on the segregation of Te in terrestrial and in microgravity conditions. Since no data of the g-jitter were available during the experiment attempt is made to study the transient convection due to the two different types of g-jitters. In the first case, the time dependent gravity was introduced in the momentum equations. In the second case, the time dependent velocity was implemented at the boundary of the quartz sample. The full Navier–Stokes equation combined with heat and mass transfer equations were solved numerically using the finite element techniques. The results show that g-jitter and time dependent velocity applied at the quartz wall play an important role in segregation of tellurium in the melt. A detailed parametric study was performed in order to reveal the convection pattern under various conditions.

Vibration controlled convection — preparation and perspective of the Maxus 4 experiment

A comparatively new possibility to influence convection in crystal growth melts is the application of controlled interface vibration, especially in systems with free melt surfaces like the float-zone process. The present paper concerns the development and testing of a vibrational device which will be integrated into a microgravity crystal growth facility for the growth of silicon crystals. In the case of silicon grown by the float-zone technique, time-dependent thermocapillary convection is present even under mirogravity and leads to unfavourable variations of the crystal composition profile. The developed setup can operate in the range of approximately 0.1 to 50kHz producing maximum amplitudes of 0.25μm (non resonance case) and 3.5μm (resonance case) respectively. The power consumption is below 5W, the maximum operation temperature of the device is restricted to 200°C, limited by an epoxy-based connection between vibrator and sample. The first microgravity application will be during the European Maxus 4 campaign in April 2001.

Investigating the Effect of Impurities on Macromolecule Crystal Growth in Microgravity

Chicken egg-white lysozyme (CEWL) crystals were grown in microgravity and on the ground in the presence of various amounts of a naturally occurring lysozyme dimer impurity. No significant favorable differences in impurity incorporation between microgravity and ground crystal samples were observed. At low impurity concentration the microgravity crystals preferentially incorporated the dimer. The presence of the dimer in the crystallization solutions in microgravity reduced crystal size, increased mosaicity, and reduced the signal-to-noise ratio of the X-ray data. Microgravity samples proved more sensitive to impurity. Accurate indexing of the reflections proved critical to the X-ray analysis. The largest crystals with the best X-ray diffraction properties were grown from pure solution in microgravity.

Microgravity and Macromolecular Crystallography

Macromolecular crystal growth is seen as an ideal experiment to make use of the reduced acceleration environment provided by an orbiting spacecraft. The experiments are small, are simply operated, and have a high potential scientific and economic impact. In this review we examine the theoretical reasons why microgravity is a beneficial environment for crystal growth and survey the history of experiments on the Space Shuttle Orbiter, on unmanned spacecraft, and on the Mir space station. The results of microgravity crystal growth are considerable when one realizes that the comparisons are always between few microgravity-based experiments and a large number of earth-based experiments. Finally, we outline the direction for optimizing the future use of orbiting platforms.

Design of ceramic springs for use in semiconductor crystal growth in microgravity

Segregation studies can be done in microgravity to reduce buoyancy-driven convection and investigate diffusion-controlled growth during the growth of semiconductor crystals. During these experiments, it is necessary to prevent free surface formation in order to avoid surface tension driven convection (Marangoni convection). Semiconductor materials such as gallium arsenide and germanium shrink upon melting, so a spring is necessary to reduce the volume of the growth chamber and prevent the formation of a free surface when the sample melts. A spring used in this application must be able to withstand both the high temperature and the processing atmosphere. During the growth of gallium arsenide crystals during the GTE Labs/USAF/NASA GaAs GAS Program and during the CWRU GaAs programs aboard the First and Second United States Microgravity Laboratories, springs made of pyrolytic boron nitride (PBN) leaves were used. The mechanical properties of these PBN springs have been investigated and springs having spring constants ranging from 0.25 to 25 N/mm were measured. With this improved understanding comes the ability to design springs for more general applications, and guidelines are given for optimizing the design of PBN springs for crystal growth applications.

Universal Multi-Zone Crystallizator, the ISS Oriented Hungarian Apparatus

This is a description of a high tech space device for single crystal growth in microgravity. The particular specialty of the apparatus is a well-controlled temperature field with arbitrary gradients travelling operation mode under high stability without any mechanical motion The technical specification and references are included Bridgmann-Stockberger, travelling heater, physical vapout transport and single crystal casting experimental serials in order to grow semiconductor single crystals are demonstrated.

Effects of microgravity on the crystal quality of a collagen-like polypeptide.

(Pro-Pro-Gly)(10) is one of the most widely studied collagen polypeptide models. Microgravity crystal growth of (Pro-Pro-Gly)(10) was carried out in the Advanced Protein Crystallization Facility aboard the Space Shuttle Discovery during the STS-95 mission. Crystals were successfully grown in all experiments, using both dialysis and free-interface diffusion methods. The quality of the microgravity-grown crystals and of ground-grown counterparts was assessed by X-ray synchrotron diffraction. Microgravity-grown crystals exhibited a significant improvement in terms of dimensions and resolution limit. As previously reported, crystals were orthorhombic, space group P2(1)2(1)2(1). However, the diffraction pattern showed weak reflections, never previously measured, that were consistent with new unit-cell parameters a = 26.9, b = 26.4, c = 182.5 A. This allowed the derivation of a new model for the arrangement of the triple-helical molecules in the crystals.

Enhancement of mercuric iodide detector performance through increases in wafer uniformity by purification and crystal growth in microgravity

Wafers from mercuric iodide crystals grown in microgravity on two occasions have previously been found to be characterized by a higher hole mobility-lifetime product, which enables energy dispersive radiation detectors with superior resolution. In the present work, we have identified the specific structural modifications that are responsible for this enhanced performance. As a result of this study, the performance of terrestrial wafers also has been improved but not yet to the level of wafers grown in microgravity. High resolution synchrotron x-ray diffraction images of a series of wafers, including those grown both in microgravity and on the ground, reveal two principal types of structural changes that are interrelated. One of these, arrays of inclusions, affects performance far more strongly than the other, variation in lattice orientation. Inclusions can be formed either from residual impurities or in response to deviations from ideal stoichiometry. The formation of both types is facilitated by gravity-driven convection during growth. As the level of inclusions is reduced, through growth from material of higher purity, through the achievement of balanced stoichiometry, or by suppression of convection mixing during crystal growth, the hole mobility-lifetime product is enhanced in spite of an accompanying decreased uniformity in lattice orientation. Sixfold enhancement in the performance of x- and γ-ray detectors has been accomplished to date. Further augmentation in performance appears likely.

Measurements of thermal creep in binary gas mixtures

Thermal creep occurs in all nonisothermal gas–mixture systems. The effect is more pronounced in rarefied gases and can play a significant role in, for example, microgravity crystal growth experiments, where natural convection is suppressed. Experimental measurements of the thermal creep in gas mixtures are, however, simply not available. We report experimental measurements of thermal creep in three binary gas mixtures: He:Ar, He:CO2, and Ar:CO2 at different mole ratios (mixture fractions). A stainless steel two-bulb measurement system employing seven capillaries to connect the bulbs is used to measure the steady-state pressure differences that develop between the bulbs for each mixture at a selection of total system pressures. Values of the total system pressure that were used range from 0.12 to 10.00 Torr. For all of the data reported, the two bulbs were held at temperatures of 397 and 297 K for the hot and cold sides, respectively. From the measured pressure differences, experimental values are obtained for a newly identified creep factor from which the thermal creep is extracted. These experimental values agree well with those obtained from some recently reported theoretical expressions, provided that suitable values for the accommodation coefficients of the gases are chosen.

Transient Simulation of Double Diffusive Convection of Crystal Growth in Microgravity

Transient Simulation of Double Diffusive Convection of Crystal Growth in Microgravity

Magnetic damping of buoyant convection during semiconductor crystal growth in microgravity. Steady transverse residual acceleration

In this paper we treat the steady, three-dimensional buoyant convection in a circular cylinder with an axisymmetric temperature, with a uniform axial magnetic field, and with a steady transverse acceleration or gravitational force. The magnetic field is sufficiently strong that inertial effects and convective heat transfer are negligible for the typical residual accelerations on an earth-orbiting vehicle. Since the governing equations are linear, the solution for any steady acceleration or gravitational vector is given by the superposition of the solutions for an axial acceleration and for a transverse acceleration. The solution for an axial acceleration is given by a rescaling of the well-known solution for a terrestrial vertical Bridgman crystal-growth process, and in this paper we present the solution for a transverse acceleration. With an axial magnetic field, the melt motion consists of two circulations in opposite directions around two axial lines on opposite sides of the diameter plane which is parallel to the transverse acceleration vector a: both circulations involve flows in the direction opposite to a near the diameter plane and flows in the same direction as a near the cylinder wall. The small electrical conductivity of the crystal is important because the crystal provides a path for a major part of the electric current responsible for the magnetic damping.

Magnetic damping of buoyant convection during semiconductor crystal growth in microgravity: Spikes of residual acceleration

This paper treats the decay of the buoyant convection in a circular cylinder after a brief spike of large acceleration. There is a steady, uniform, axial magnetic field which is sufficiently strong that nonlinear inertial effects and convective heat transfer are negligible. Since the governing equations are linear, the solution for a spike at any angle to the cylinder’s axis is given by the superposition of two solutions for an axial spike and a transverse spike. The flows are axisymmetric and three-dimensional for the axial and transverse spikes, respectively. With the transverse spike, the axial temperature gradient drives a single cell of circulation with transverse vorticity, and the radial temperature gradient drives two opposite cells of circulation with axial vorticity on opposite sides of the diameter parallel to the acceleration. Although the axial temperature gradient is larger than the radial one in the Bridgman crystal-growth process, the damping of transverse vorticity with an axial magnetic field is much stronger than the magnetic damping of the axial vorticity, so that the two-cell axial-vorticity circulation dominates. For a typical Bridgman process with a 0.2 T magnetic field and with silicon, the buoyant convections driven by the axial and transverse spikes of acceleration decay to one percent of their magnitudes immediately after the spikes in 3 and 14 s, respectively.

Enhancement of Mercuric Iodide Detector Performance Through Crystal Growth in Microgravity: the Roles of Lattice Order

AbstractThe hole-mobility•carrier-lifetime product of α mercuric iodide high energy radiation detectors has been enhanced through vapor crystal growth in microgravity. This improvement is closely correlated with specific characteristics of the crystal lattice, which have been identified by high resolution synchrotron x-ray diffraction imaging. These structural features and the associated performance are now being approached in terrestrial growth of α mercuric iodide.Gravity may affect the uniformity of this crystal lattice in two distinct ways: 1) directly through deformation that it imposes on the lattice during growth and 2) indirectly through convection, which mixes any extraneous material. Inclusions associated with these processes harden the lattice and facilitate lattice folding. These changes affect the electronic parameters of detectors made from the crystals. As purification procedures are optimized, the incorporation of extraneous material is curtailed, enhancing electronic properties in spite of lattice flexing through loss of precipitation hardening.These studies provide insight into the contribution of various aspects of crystalline order in α-mercuric iodide crystals to property improvement. This knowledge has led to modification of requirements for starting materials, adjustment of physical vapor growth procedures, and change in crystal handling procedures. As a result, the electronic performance of terrestrially grown radiation detectors has been improved, and we provide evidence that further enhancement is still possible.

圧力制御の高速性を利用した結晶成長の微小重力実験

This article reviews works by the author and his colleagues where a novel technique for quick change of supersaturation by dynamic pressure-control (DPC) method is used for studies of crystal growth. Since the DPC method can change supersaturation much faster than the ordinary temperature-control method, it is very suitable for quick experiments in microgravity. Here we report an in situ observation experiment of a dendritic crystal growth in microgravity by the DPC method.