The Project

 

Synopsis/Abstract

The International Space Station (ISS) will provide the ability to synthesize new materials and to manufacture new products in space. For example, semiconductor crystals and protein crystals, which have important applications to computer technology and biotechnology,  can be grown to a greater degree of perfection in microgravity than on  earth. Repeated experiments since Sky Lab and subsequent shuttle missions have demonstrated this.[5]  

The Space Station will provide a platform that can offer months or even years of microgravity conditions at a time. [6] Among the earliest commercial products to be manufactured on the ISS will be high-purity semiconductor crystals and large protein crystals with enhanced crystallinity. [5]

In order to support a crystal-manufacturing platform in space, it will be necessary to have on the ISS, certain instruments that can be used to characterize samples while still in orbit. [5] An ideal instrument for this application would be one that can resolve the structure of crystals down to the atomic scale, is small in size, has low power requirements, and is operationally clean. Such an instrument can be developed in the form of a Scanning Probe Microscope (SPM) for the microgravity environment, or a microgravity SPM system.

Scanning Probe Microscopy is a general term used to describe a type of microscopy in which a local probe is raster scanned over a sample of interest. As the probe is scanned across the sample, a local interaction between the probe and sample is measured (see Figure 1). Scanning Probe Microscopes (SPMs) include the Scanning Tunneling Microscope (STM), the Atomic Force Microscope (AFM) and many other types that have been developed from these earlier predecessors. [3, 4, 7]

 

 

 

 

 


Drawing not to scale

Interaction between probe and sample

Scanning Tube

Sample

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Probe

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           

                                                                                                                                                                                   

            Figure 1:    Probe/sample interaction in the AFM.

 

 

 

The International Space Station (ISS) is being assembled with various experimental apparatus and instrumentation in mind; however, a Scanning Probe Microscope (SPM) has not been intended.  It is an ideal instrument for materials processing in space where direct atomic resolution images can be obtained of crystalline materials as they form instead of returning them to Earth for analysis.

SPMs are generally very sensitive to mechanical and acoustic noise, especially when imaging at the atomic scale.  This usually requires some degree of vibration isolation. The instrument can be isolated from acoustic vibration by placing it under a vacuum.  The mechanical vibrations usually require either an elaborate passive vibration isolation system or a sophisticated electronic active vibration isolation system. 

Due to the scientific benefits of installing an SPM on the ISS, an experiment has been devised to determine the effectiveness of such an instrument in reduced gravity. 

In the experiment an AFM will be mounted to an active vibration isolation system, which will minimize mechanical resonance. A vacuum will be produced around the instrument, which will eliminate acoustic noise. Mounting a hollow acrylic cylinder directly to the vibration isolation unit and evacuating the air inside the chamber will accomplish this. 

The AFM requires minor adjustments during operation; therefore, radio-controlled servos will be used to adjust certain controls remotely. This will allow for the device to be adjusted without any direct contact from the operations team, eliminating outside vibration.  When reduced gravity is achieved, the operations team will attempt to image samples at the micrometer range  (1 micron gold grating and 0.10 micron polystyrene calibration spheres on mica) and at the atomic scale (a 24nm corrugation Highly Oriented Pyrolytic Graphite (HOPG) sample and a 2nm corrugation mica sample).  The force curves corresponding to the images will be recorded along with the accelerations. The force curves and images will be acquired according to preset configurations as described in the Test Procedures section. These microgravity data will then be compared to the results of similar experiments performed on the surface of the Earth.

 

            Before an SPM can be implemented for the ISS, it must be known how such an 

instrument will operate in the microgravity environment.  The two main requirements for this project include:

 

1)      A long duration microgravity environment or a repeatable short duration microgravity   environment.

2)      A recoverable platform that also allows for direct user operation.

 

NASA has developed many possible microgravity platforms. [6] A review of these platforms shows that only one is feasible for this project.

 

1)      Drop Facilities – Drop towers and drop tubes can offer from 2.2 seconds to 5.2 seconds of microgravity (free fall). [6] For our purposes, this platform offers too short a duration of microgravity and poses a definite hazard to the instrument. The Drop facilities are therefore not a suitable platform for this project.

2)      Sounding Rockets – These platforms can offer up to 6 minutes of microgravity, but are unable to allow direct user operation and present a definite hazard to the instrument. In addition, the payload in this case is usually not easily recoverable. [6] The Sounding Rockets are therefore not a suitable platform either.

3)      Orbiting Spacecraft – The Space Shuttle can offer up to 17 days of microgravity and the International Space Station will be able to carry experiments for months or years. [6] Although these would be ideal platforms,

they are cost prohibitive, and the ISS is not yet available.

4)      Parabolic Aircraft – NASA’s KC-135 can fly parabolic paths giving 15-25 seconds of microgravity, which is then repeated over 40-60 parabolas. [6] In this case, the apparatus can be secured to the aircraft and the experimenters can fly with their experiment. This facility offers repeatable short duration microgravity, allows for direct user interaction with the experiment, and can be considered as an acceptable risk to the instrument. The KC-135 will also serve as a good test environment for vibrational considerations.

 

From the above potential microgravity platform analysis, it can be seen that the KC-135 is the most feasible platform for this experiment when all options are considered.