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Growing single crystals under pressure


The Materials Preparation Center has long had the reputation for producing some of the highest quality and largest single-crystal materials available. But with the addition of a new piece of equipment, those capabilities have increased dramatically.

A new Cyberstar crystal-growth furnace, installed about a year ago gives Ames Lab’s crystal-growth experts Tom Lograsso and Deborah Schlagel capabilities to grow both larger crystals using the Bridgman technique as well as the ability to work with a wider variety of materials, including those too volatile to tackle with the existing equipment.

“We’re able to greatly increase the pressure within the furnace so that we can work with and grow crystals from volatile materials,” Schlagel says. “The new furnace is capable of 15 bar, which is just over two times more pressure than our previous equipment allowed.  The increased pressure will allow us to better suppress the evolution of vapors from volatile alloy components.

(opposite page) Testing the resistance of a sample. (above) This single crystal of copper, approximately an inch in diameter and 11 inches long, shows off the capabilities of the furnace; (right) Deborah Schlagel with the crystal- growth furnace; (below) The induction coils in the furnace used to heat the furnace during crystal growth.

“For example, if we have an alloy of manganese and iron, we worry about manganese volatilizing off, which will change the composition of the remaining liquid, resulting in the first part of the ingot to solidify being richer in manganese than the last part to solidify,” Schlagel explains.  “Ideally we want the grown crystal to be the same composition as the material we started with and homogenous along its entire length.  Also the vapors can make a mess of the chamber walls.”

ImageThis feature has already paid dividends by allowing Schlagel to produce a sample of galfenol, a magnetostrictive — it changes shape in response to a magnetic field — alloy of iron and gallium, that for the first time also contains zinc. Over the past 15 years, Schlagel has tried unsuccessfully to introduce zinc into the alloy, but the volatile metal flashes off before the crystal forms.

“We’ve produced a polycrystalline sample for the Naval Surface Warfare Center (NSWC) Carderock division that appears to contain eight atomic percent,” Schlagel says. “The composition will vary along the length of the crystal but that’s the highest percentage we’ve been able to achieve so far.”

The equipment also allows MPC staff to grow much larger crystals. The furnace, which stands well above normal ceiling height, also extends below floor level. This provides added travel length for the crucible on which the crystals form.

“It gives us the ability to grow samples up to 2.5 times bigger in diameter and 5.25 times longer than our previous capabilities,” Schlagel says. That gives researchers more latitude in the types and range of characterization that can be done with a specific sample.”

The other benefit of the new equipment is that it uses induction heating as opposed to resistance heating in the existing furnace. Think of it in terms of the new style of stove top that uses magnetic field induction coils to heat the pan instead of the glowing red heating elements.

“Induction heating allows us to produce steeper temperature gradients, which are more ideal for crystal growth,” Schlagel says, adding that the new furnace’s maximum temperature is 2000 C, which is slightly lower than the existing Bridgman equipment.

“The higher temperature gradients allow faster growth which further minimizes evaporation and reactions with crucibles,” says Lograsso, who is also the Lab’s interim director. “This gives us a capability of working with reactive materials like the rare earths where crucible choices are limited.”

The French-made Cyberstar furnace was supported with funds from the Department of Energy’s Basic Energy Sciences Materials Sciences and Engineering Division.

“This equipment vastly improves our previous capabilities,” Schlagel says, “but the fact that we can develop techniques to work with volatile and reactive materials will help set us apart from other labs.”


~ by Kerry Gibson