New furnace boosts MPC crystal growth capabilities

ImageThe 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 just increased dramatically.

A new Cyberstar crystal growth furnace, installed earlier this summer, 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 said. “The new furnace is capable of 15 bar maximum, which is just over 2 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.

“For example, if we have an alloy of manganese and tin, 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 explained.  “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.”

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 said, “so we’re able to grow substantially larger samples. 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 is uses induction heating as opposed to resistance heating in the old 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 said, adding that the new furnace’s maximum temperature is 2000°C which is slightly lower than the old Bridgman equipment because of differences in the insulation being used.

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

The French-made Cyberstar furnace cost roughly $673,000 and was paid for with funds from the Department of Energy’s Basic Energy Sciences Materials Sciences and Engineering Division. As part of acquisition, Schlagel and Lograsso traveled to Grenoble, France to work with the company to customize the equipment and receive training in its operation.

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

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This is the lower portion of the furnace chamber. The 24 bolts in the
custom-built rack in the foreground are a testiment to the high
pressures achieveable inside the furnace. The photo above right shows
Deborah Schlagel standing beside the opened furnace, a portion of
which extends below floor level.
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The induction coil used to heat the materials used to create the
crystals.
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The black tube in the center is a graphite crucible on which crystals are
grown. As the crystal grows, the crucible holder retracts, allowing very
long crystals to form.