Gregory Jerman

Effect of residual accelerations during microgravity directional solidification of mercury cadmium telluride on the USMP-2 mission

Abstract Directional solidification of mercury cadmium telluride (MCT) requires that the temperature gradient to growth rate ratio be high to avoid constitutional supercooling. With the optimum gradient condition for solidifying MCT in NASAs advanced automated directional solidification furnace (AADSF), it is necessary to use translation rates as low as 0.2 μm/s. The result is that any fluid flow with a velocity comparable to or higher than this will dominate the solidification characteristics, particularly the compositional distribution in an alloy such as this which has a large solidus-liquidus separation. In an effort to reduce fluid flow velocities a space experiment was performed. On the second United States Microgravity Payload Mission (USMP-2), the AADSF made its maiden flight and successfully completed growth of a MCT boule 16 cm long. The furnace was located approximately 3 m away from the center of gravity of the space shuttle, and this combined with the drag component of residual acceleration present during flight, resulted in quasisteady residual accelerations of the order of 1 μg0 where g0 is the earths natural gravity. Of more importance is that different orbiter attitudes during the mission produced significant differences in the resultant residual acceleration vector, in both magnitude and direction and that these differences caused large compositional variations both across the radii of the boule and along the surfaces of the boule. Comparison will be made with examples grown on the ground and in magnetic fields.

Growth of cadmium-zinc telluride crystals by controlled seeding contactless physical vapor transport

Bulk crystals of cadmium-zinc telluride, 23 mm in diameter and up to 45 grams in weight were grown. Controlled seed formation procedure was used to limit the number of grains in the crystal. Most uniform distribution of ZnTe in the crystals was obtained using excess (Cd + Zn) pressure in the ampoule.

Simulation and characterization of a miniaturized Scanning Electron Microscope

A miniaturized Scanning Electron Microscope (mini-SEM) for in-situ lunar investigations is being developed at NASA Marshall Space Flight Center with colleagues from the University of Alabama in Huntsville (UAH), Advanced Research Systems (ARS), and the University of Tennessee in Knoxville (UTK). Scanning Electron Microscopes (SEMs) can provide information on the size, shape, morphology and chemical composition of lunar regolith. Understanding these basic properties will allow us to better estimate the challenges associated with In-Situ Resource Utilization and to improve our basic science knowledge of the lunar surface (either precluding the need for sample return or allowing differentiation of unique samples to be returned to Earth.) Miniaturization (and power reduction) of an SEM appropriate for in-situ planetary investigations has warranted several novel re-designs of traditional SEM components. As such, this research has been centered on these principle elements and includes: an electron gun, beam defining and focusing system, and deflection/scanning/imaging system. Of these, the electron gun development, which is the focus of this paper, is of particular importance as it dictates the design and operation of the remaining components.1,2

Miniature Variable Pressure Scanning Electron Microscope for in-situ imaging & chemical analysis

NASA Marshall Space Flight Center (MSFC) is leading an effort to develop a Miniaturized Variable Pressure Scanning Electron Microscope (MVP-SEM) for in-situ imaging and chemical analysis of uncoated samples. This instrument development will be geared towards operation on Mars and builds on a previous MSFC design of a mini-SEM for the moon (funded through the NASA Planetary Instrument Definition and Development Program). Because Mars has a dramatically different environment than the moon, modifications to the MSFC lunar mini-SEM are necessary. Mainly, the higher atmospheric pressure calls for the use of an electron gun that can operate at High Vacuum, rather than Ultra-High Vacuum. The presence of a CO2-rich atmosphere also allows for the incorporation of a variable pressure system that enables the in-situ analysis of nonconductive geological specimens. Preliminary testing of Mars meteorites in a commercial Environmental SEM™ (FEI) confirms the usefulness of low-current/low-accelerating voltage imaging and highlights the advantages of using the Mars atmosphere for environmental imaging. The unique capabilities of the MVP-SEM make it an ideal tool for pursuing key scientific goals of NASAs Flagship Mission Max-C; to perform in-situ science and collect and cache samples in preparation for sample return from Mars.

Miniaturized Scanning Electron Microscope for In-Situ Planetary Studies

The exploration of remote planetary surfaces calls for the advancement of low-power, low-mass, highly-miniaturized instrumentation. Multi-functional instruments of this nature will prove to be particularly useful in preparation for human return to the moon, and in exploring increasingly remote locations in the Solar System. To this end, our group has been developing a miniaturized Scanning Electron Microscope (mSEM) capable of remote investigations of mineralogical samples through in-situ topographical and chemical analysis on a fine scale. Specifically, the fabrication and testing of a proof-of-concept assembly has begun, and consists of a cold-fieldemission electron gun and custom high-voltage power supply, electrostatic electronbeam focusing column, and scanning-imaging electronics plus backscatter electron detector. The functioning of an SEM is well known: an electron beam is focused down to nanometer- scale onto a given sample resulting in emissions such as backscattered and secondary electrons, x rays, and visible light. Raster-scanning the primary electron beam across the sample results in a fine-scale image of the surface topography (texture), crystalline structure and orientation, with accompanying elemental composition. The flexibility in the types of measurements the mSEM is capable of makes it ideally suited for a variety of applications. The mSEM is appropriate for use on multiple planetary surfaces, and for a variety of mission goals (from science to non-destructive analysis to in-situ resource utilization). The current status of the development and potential mSEM applications for planetary exploration are summarized here.

Development of a Miniature Scanning Electron Microscope to Facilitate In-situ Lunar Science and Engineering

Returning humans to the Moon for prolonged periods will require extensive information on the properties and compositions of the lunar regolith, its resource potential, and in-situ characterization of the effects of space weathering of the soil and equipment. To address these concerns, we are developing a miniature Scanning Electron Microscope (mSEM), a project led by the NASA Marshall Space Flight Center (MSFC), for use on the lunar surface. Working within the lunar environment and at low-power provides opportunities to simplify the instrument, as well as sample preparation. The natural lack of an atmosphere eliminates the need and complexity of a vacuum chamber on the instrument and introduces the possibility of a “point-andshoot” configuration for the mSEM. Imaging with low-power does not require sample coatings of carbon or gold, or the use of a gas, to mitigate surface charging on the sample; this low-power greatly simplifies sample preparation. Engineering and science activities on the lunar surface will benefit from the presence of an mSEM and its ability to determine particle size distribution (PSD), mineral chemistry, and the general characteristics of the regolith.

Directional solidification of mercury cadmium telluride during the second United States Microgravity Payload Mission (USMP-2)

As a solid solution semiconductor having a large separation between liquidus and solidus, mercury cadmium telluride (MCT) presents a formidable challenge to crystal growers desiring an alloy of high compositional uniformity. To avoid constitutional supercooling during Bridgman crystal growth it is necessary to solidify slowly in a high temperature gradient region. The necessary translation rate of less than 1mm/hr results in a situation where fluid flow induced by gravity on earth is a significant factor in material transport. The advanced automated directional solidification furnace (AADSF) is equipped to provide the stable thermal environment with a high gradient, and the required slow translation rate needed. Ground based experiments in AADSF show clearly the dominance of flow driven transport. The first flight of AADSF in low gravity on USMP-2 provided an opportunity to test theories of fluid flow in MCT and showed several solidification regimes which are very different from those observed on earth. Residual acceleration vectors in the orbiter during the mission were measured by the orbital acceleration research experiment, and correlated well with observed compositional differences in the samples.