Engineering topics

Marin, C.

Publications and source records attributed to Marin, C..

11 records

NASA NTRS2010

In-Situ TEM-STM Observations of SWCNT Ropes/Tubular Transformations

Single-walled carbon nanotubes (SWCNTs) prepared by the HiPco process were purified using a modified gas phase purification technique. A TEM-STM holder was used to study the morphological changes of SWCNT ropes as a function of applied voltage. Kink formation, buckling behavior, tubular transformation and eventual breakdown of the system were observed. The tubular formation was attributed to a transformation from SWCNT ropes to multi-walled carbon nanotube (MWCNT) structures. It is likely mediated by the patching and tearing mechanism which is promoted primarily by the mobile vacancies generated due to current-induced heating and, to some extent, by electron irradiation.

Sola, F.↗

NASA NTRS2009

Initial Transient in Zn-doped InSb Grown in Microgravity

Three Zn-doped InSb crystals were directionally solidified under microgravity conditions at the International Space Station (ISS) Alpha. The distribution of the Zn was measured using SIMS. A short diffusion-controlled transient, typical for systems with k greater than 1 was demonstrated. Static pressure of approximately 4000 N/m2 was imposed on the melt, to prevent bubble formation and dewetting. Still, partial de-wetting has occurred in one experiment, and apparently has disturbed the diffusive transport of Zn in the melt.

Ostrogorsky, A G.↗

NASA NTRS2009

Dewetting and Segregation of Zn-Doped InSb in Microgravity Experiments

In directional solidification, dewetting is characterized by the lack of contact between the crystal and the crucible walls, due to the existence of a liquid meniscus at the level of the solid-liquid interface. This creates a gap of a few tens of micrometers between the crystal and the crucible. One of the immediate consequences of this phenomenon is the dramatic improvement of the quality of the crystal. This improvement is partly due to the modification of the solid-liquid interface curvature and partly to the absence of sticking and spurious nucleation at the crystal-crucible interface. Dewetting has been, commonly observed during the growth of semiconductors in crucibles under microgravity conditions where it appears to be very stable: the gap between the crystal and the crucible remains constant along several centimetres of growth. The physical models of the phenomenon are well established and they predict that dewetting should not occur in microgravity, if sufficient static pressure is imposed on the melt, pushing it towards the crucible. We present the results of InSb(Zn) solidification experiments conducted at the International Space Station (ISS) where, in spite of a spring exerting a pressure on the liquid, partial dewetting did occur. This surprising result is discussed in terms of force exerted .by the spring on the liquid and of possibility that the spring did not work properly. Furthermore, it appears that the segregation of the Zn was not affected by the occurrence of the dewetting. The data suggest that there was no significant interference of convection with segregation of Zn in InSb.

Ostrogorsky, A. G.↗

NASA NTRS2005

Reproducible Crystal Growth Experiments in Microgravity Science Glovebox at the International Space Station (SUBSA Investigation)

Solidification Using a Baffle in Sealed Ampoules (SUBSA) is the first investigation conducted in the Microgravity Science Glovebox (MSG) Facility at the International Space Station (ISS) Alpha. 8 single crystals of InSb, doped with Te and Zn, were directionally solidified in microgravity. The experiments were conducted in a furnace with a transparent gradient section, and a video camera, sending images to the earth. The real time images (i) helped seeding, (ii) allowed a direct measurement of the solidification rate. The post-flight characterization of the crystals includes: computed x-ray tomography, Secondary Ion Mass Spectroscopy (SIMS), Hall measurements, Atomic Absorption (AA), and 4 point probe analysis. For the first time in microgravity, several crystals having nearly identical initial transients were grown. Reproducible initial transients were obtained with Te-doped InSb. Furthermore, the diffusion controlled end-transient was demonstrated experimentally (SUBSA 02). From the initial transients, the diffusivity of Te and Zn in InSb was determined.

Ostrogorsky, A.↗

NASA NTRS2004

Te-and Zn-Doped InSb Crystals Grown in Microgravity

In 2002, within the SUBSA (Solidification Using a Baffle in Sealed Ampoules) investigation, seven doped InSb crystals were grown in microgravity at the International Space Station. The key goals of the SUBSA investigation are: (a) to clarify the origin of the melt convection in space laboratories; (b) to reduce melt convection to the level which allows reproducible diffusion-controlled segregation; (e) to explore the submerged baffle process and liquid encapsulation in microgravity. 30 crystal growth experiments were conducted in the ground unit, to optimize the design of flight ampoules and to test the transparent SUBSA furnace developed by TecMasters Inc. The specially designed furnace, allowed observation of the crystal growth process (melting, seeding, motion of the solid-liquid interface, etc.). In the summer of 2002, eight crystal growth experiments were conducted in the Microgravity Science Glovebox (MSG) facility at the ISS. Four Te-doped (k = 0.5) and three Zn-doped (k2.9) crystals were grown on undoped seeds. In one experiment, we were not able to seed and grow. The seven grown crystals were sectioned and analyzed using SIMS. The design of the SUBSA ampoules, the segregation data and the video images obtained during the SUBSA flight experiments will be presented and discussed.

Ostrogorsky, A. G.↗

NASA NTRS2003

Solidification Using a Baffle in Sealed Ampoules (SUBSA)

Solidification Using a Baffle in Sealed Ampoules (SUBSA) will be the first materials science experiment conducted in the Microgravity Science Glovebox (MSG) Facility at the International Space Station (ISS) Alpha. The launch is schedule for May 31, 2002. Using the specially developed furnace, 10 Te and Zn-doped single crystals of InSb will be directionally solidified in microgravity. A key goal of the SUBSA investigation is to (i) clarify the origin of the melt motion in space laboratories and (ii) to reduce the magnitude of the melt motion to the point that it does not interfere with the transport phenomena. These goals will be accomplished through a special ampoule and furnace design. A disk-shaped baffle, positioned close to the freezing front, is used to reduce melt motion. Furthermore, the solidification will be visualized by using a transparent furnace, with a video camera, continuously sending images to the earth. This allows detection of bubbles and melt de-wetting that could cause surface tension driven convection. In preparation for the space experiments, 30 ground-based experiments were conducted. The results of ground based tests and numerical modeling will be presented. Based on numerical modeling, 12 mm 1D silica ampoules were selected. The small diameter ampoule favors closer placement of the baffle to the interface, without excessive radial segregation caused by forced convection while providing more damping of natural convection. The parts in the silica ampoule include 2 carbon springs made by Energy Science Laboratories, Inc., a pyrocarbon-coated graphite cylinder, pyrocarbon-coated graphite a baffle with the shaft and the InSb charge with the seed crystal grown by W.A. Bonner of Crystallod Inc.

Marin, C.↗

NASA NTRS2003

Solidification Using the Baffle in Sealed Ampoules

Ostrogorsky, A.↗

NASA NTRS2001

Space- and Ground-Based Crystal Growth Using a Baffle (CGB)

The composition of semiconductor crystals produced in space by conventional melt-growth processes (directional solidification and zone melting) is affected by minute levels of residual micro-acceleration, which causes natural convection. The residual acceleration has random magnitude, direction and frequency. Therefore, the velocity field in the melt is apriori unpredictable. As a result, the composition of the crystals grown in space can not be predicted and reproduced. The method for directional solidification with a submerged heater or a baffle was developed under NASA sponsorship. The disk-shaped baffle acts as a partition, creating a small melt zone at the solid-liquid interface. As a result, in ground based experiment the level of buoyancy-driven convection at the interface is significantly reduced. In several experiments with Te-doped GaSb, nearly diffusion controlled segregation was achieved.

Ostrogorsky, A. G.↗

NASA NTRS2001

Investigation of Vibrational Control of the Bridgman Crystal Growth Technique

The character of natural buoyant convection in rigidly contained inhomogeneous fluids can be drastically altered by vibrating the container. Vibrations are expected to play a crucial influence on heat and mass transfer onboard the International Space Station (ISS). It is becoming evident that substantial vibrations will exist on the ISS in the wide frequency spectrum. In general, vibrational flows are very complex and governed by many parameters. In many terrestrial crystal growth situations, convective transport of heat and constituent components is dominated by buoyancy driven convection arising from compositional and thermal gradients. Thus, it may be concluded that vibro-convective flow can potentially be used to influence and even control transport in some crystal growth situations.

Fedoseyev, Alexandre I.↗

NASA NTRS2001

Microgravity Science Glovebox Investigations SUBSA

Solidification Using a Baffle in Sealed Ampoules (SUBSA) is a Microgravity Science Glovebox Investigation manifested for the UF2 flight, on the U.S. Orbiter 111, to the International Space Station (ISS). SUBSA complements the "parent" flight investigation CG13 (Space-and Groundbased Crystal Growth Using a Baffle). During directional solidification, the disk-shaped baffle acts as a partition, creating a small melt zone at the solid-liquid interface. As a result, the level of buoyancy-driven convection at the interface is significantly reduced. In space, the baffle will reduce convection driven by residual micro acceleration. The baffle reduces the Rayleigh number (Ra) of the melt by a factor of 103. The combined effect of the baffle and microgravity will yield a reduction in Ra by a factor of 107 to 109 approaching effectively the acceleration conditions in "nanogravity". The results of ground based tests and numerical modeling will be presented. The furnace for directional solidification (flight hardware and the ground unit) was developed by Tec-Masters Inc. The flight ampoules were produced jointly at Rensselaer, Tec-Masters Inc. and Crystallod Inc.

Ostrogorsky, A.↗

NASA NTRS2000

Magnetic Field Suppression of Flow in Semiconductor Melt

One of the most promising approaches for the reduction of convection during the crystal growth of conductive melts (semiconductor crystals) is the application of magnetic fields. Current technology allows the experimentation with very intense static fields (up to 80 KGauss) for which nearly convection free results are expected from simple scaling analysis in stabilized systems (vertical Bridgman method with axial magnetic field). However, controversial experimental results were obtained. The computational methods are, therefore, a fundamental tool in the understanding of the phenomena accounting during the solidification of semiconductor materials. Moreover, effects like the bending of the isomagnetic lines, different aspect ratios and misalignments between the direction of the gravity and magnetic field vectors can not be analyzed with analytical methods. The earliest numerical results showed controversial conclusions and are not able to explain the experimental results. Although the generated flows are extremely low, the computational task is a complicated because of the thin boundary layers. That is one of the reasons for the discrepancy in the results that numerical studies reported. Modeling of these magnetically damped crystal growth experiments requires advanced numerical methods. We used, for comparison, three different approaches to obtain the solution of the problem of thermal convection flows: (1) Spectral method in spectral superelement implementation, (2) Finite element method with regularization for boundary layers, (3) Multiquadric method, a novel method with global radial basis functions, that is proven to have exponential convergence. The results obtained by these three methods are presented for a wide region of Rayleigh and Hartman numbers. Comparison and discussion of accuracy, efficiency, reliability and agreement with experimental results will be presented as well.

Fedoseyev, A. I.↗