SP-401 Skylab, Classroom in Space

[162] Part III - Science Demonstrations
 

Chapter 16: Crystal Growth.

a picture of the Apollo/Skylab capsule in the ocean after a return from orbit

 

[163] The diamond in most engagement rings is an octahedral crystal of carbon, formed naturally by very high temperatures and great pressures deep within Earth. In addition to the valuable diamond, all metals and many other substances, such as relatively cheap salt and sugar, are made of crystals.

Crystals are important to industry because of their unusual properties. For example, certain electrically excited crystals oscillate at extremely stable rates. It is this property that makes the crystal-controlled wristwatch such an accurate time piece. Transistors are fabricated from crystalline material, and crystals are extensively used in precision electronic and optical equipment.

Most artificially produced crystals are formed or "grown" from solutions, melts, or by ion exchange. In order for them to form from a solution, it must be saturated with the substance to be crystallized.

The formation of perfect crystals of large size is not easy on Earth. Circulation or convection in a supersaturated solution poses a problem in the growth of imperfect crystals. Supersaturated solutions are not stable and will deposit their solute material on the walls of a container or on small seed crystals of the solution if the saturation level is high enough.

 

Rochelle Salt Growth

Growth of most crystals, particularly metallic crystals, requires high temperatures and very carefully controlled cooling rates, accompanied by a precisely regulated withdrawal of the developing crystal. To demonstrate the capability for producing crystals in orbit, I. Miyagawa of the University of Alabama suggested the use of Rochelle salt, a common crystal. The choice was dictated because such crystals can be formed from a solution in near-normal room temperatures, and they can so generate relatively large crystals that have good piezoelectric properties or the ability to generate an electric current when pressure is applied to them, enabling a simple measurement to assess the quality of the crystals.

A small Skylab food can was filled with saturated Rochelle salt solution, Rochelle salt powder, and a Rochelle salt-seeded crystal. The can was placed in a food-heating tray and warmed until three-fourths of the seed crystal dissolved at approximately 158°F. The can was then removed from the tray, wrapped in several towels for insulation, and stored. During storage in zero gravity, the seed crystal slowly regrew as the can cooled down to workshop temperature. The growth occurred because the saturated solution became supersaturated when the temperature dropped, and the solute crystallized out of solution until it became saturated at the lower temperature. This process was carried out slowly in the presence of a "seed" crystal, so that much of the solute deposited on the "seed," resulting in a large single crystal.

Approximately 2 weeks later, the can was opened to observe the results of crystal growth. It.....

 


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I. Miyagawa, of the University of Alabama, suggested that a meaningful science demonstration in the growth of crystals could be made aboard Skylab. A large crystal of Rochelle salt was grown in the wrightlessness environment, but it broke during reentry into Earth's atmosphere while it was being returned by the astronauts.

I. Miyagawa, of the University of Alabama, suggested that a meaningful science demonstration in the growth of crystals could be made aboard Skylab. A large crystal of Rochelle salt was grown in the wrightlessness environment, but it broke during reentry into Earth's atmosphere while it was being returned by the astronauts.

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I. Miyagawa, of the University of Alabama, suggested that a meaningful science demonstration in the growth of crystals could be made aboard Skylab. A large crystal of Rochelle salt was grown in the wrightlessness environment, but it broke during reentry into Earth's atmosphere while it was being returned by the astronauts.

 

[165] ....contained many small Rochelle salt crystals that had formed in the solution. The astronaut described the solution as "slushy," with crystals appearing "mica-like." The seed crystal, which was returned to Earth, had regrown in a plate about 0.5 inch on each side and about 0.2 inch thick.

The space-grown crystal contained at least five crystals. The corresponding crystal axes of these component crystals are parallel to each other. In Earth-grown crystals consisting of several component crystals, the orientation of any axis of a component crystal relative to any other component crystal is completely random. The unusual arrangement of Skylab's component single crystals suggested the presence of a long-range molecular force of attraction between crystals. It was assumed that this force oriented the crystals so that their axes were parallel when they came close to each other in the solution.

However, the Skylab crystals had several defects. Most appeared to be long, tubular cavities when viewed through a microscope. The average length of the cavities was about 0.16 inch. Almost all of the tubular cavities had their long axis parallel to the crystal axis. Such very regular tubular cavities were not observed in any Rochelle salt crystals prepared in a ground-based laboratory. However, Earth-grown crystals may have several types of irregular microscopic cavities. Although the crystal was not as good as expected (probably because the growth of the Skylab crystals was not precisely regulated), several interesting facts evolved. The demonstration provided the first experimental evidence of molecular forces on growing crystals in zero gravity. For reasons still unknown, the Skylab crystal was much more fragile than similar crystals grown on Earth. Perhaps future space experiments on the growth of crystals of this type will provide control over temperature, growth rate, dissolved air in solution, pressure, vibration, and other factors that may affect the growth of a crystal.

 

Deposition of Silver Crystals

A second crystal-growth demonstration, using an ion-exchange procedure, demonstrated the effect of zero gravity on such a crystal-growth technique. When a copper wire is placed in a silver nitrate solution, silver crystals will deposit on the wire. Many such crystals have been grown by this process on Earth. Results have shown that as the....

 


Upon examination on Earth, Rochelle salt crystals grown on Skylab were found to have parallel, tubular cavities (1). Similar crystals produced on Earth had irregular cavities (2).

Upon examination on Earth, Rochelle salt crystals grown on Skylab were found to have parallel, tubular cavities (1). Similar crystals produced on Earth had irregular cavities (2).


 

....gravity force increases, the crystals become much more compact and cohesive.

P. Grodzka of Lockheed Missiles & Space Co. and B. Facemire of the Marshall Space Flight Center provided a silver-crystal-growth demonstration to extend such studies to Skylab's zero gravity. It was expected that weightlessness would have no effect on the microscopic process of diffusion and chemical reaction but that it would affect the motion of silver ions due to the absence of convection. Extrapolating the results of Earth-based studies, the Skylab crystals-grown without convection because of zero gravity-would be less compact and less cohesive than those produced on....

 


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Crystals of metallic silver also were grown in Skylab.

As expected, silver crystals grown in Skylab center were much finer and more powdered than those grown on Earth.

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Crystals of metallic silver also were grown in Skylab.

As expected, silver crystals grown in Skylab center were much finer and more powdered than those grown on Earth.

 

....Earth, where similar crystals should be powderlike.

The Skylab experiment, performed by Commander Carr, consisted of inserting a scored, insulated copper wire into a 5-percent aqueous solution of silver nitrate. Silver crystals began to grow immediately at the exposed metal sites. Carr reported that the silver crystals were growing beautifully with a "classical lattice structure." Later, in a debriefing session, he described the crystals as long branches like trees. He also said that most growth occurred during the first 24 to 48 hours, being almost complete by 72 hours.

The vial containing the silver crystals was returned to Earth. Unfortunately, the crystals were knocked loose from their growth sites in return process and handling. But, significantly, they were powdered, as anticipated.

The fact that the Skylab-grown crystals were more powdered than Earth-grown crystals indicated a promising area for space processing: the electrolytic growth of powders for catalyst applications. Silver, for example, is the only known catalyst for converting ethylene to ethylene oxide, a necessary compound for the manufacturing of many petrochemical products.

 


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Photomicrographs at 700x and 6000x show a silver crystal grown in Skylab.

Photomicrographs at 700x and 6000x show a silver crystal grown in Skylab.


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