Inquiry 2016, Issue 1

Inquir y Is sue

| 2016

Inquir y I s sue

| 2016

To then separate the liquid tin from the Ce-Sb crystal, the

tube assembly is removed from the furnace and immediately

placed in a centrifuge, which spins the remaining liquid

tin off into the catch crucible, leaving the crystal behind.

The centrifuge delivers up to 100 times the force of simple

gravitational decanting, resulting in “cleaner” crystals.

“When you develop new materials, you need to have

some familiarity with the ingredients and the techniques

at hand,” said Canfield. “With solution growth, we can go

from looking at superconductors and ferromagnets, to spin

glasses, to quasicrystals—go from one material to another to

another—just by changing elements or growth conditions.

Over the course of 20 years here, we are closing in on 10

thousand different growths.”

at temperatures up to 2,100 degrees Celsius.

The sample ingot itself starts in two pieces. The shorter

“seed” side is on the bottom and held in a base. The longer

“feed” side is suspended closely above the seed side. As the

two sides begin to melt, a small pool of liquid collects on each

surface and as they are brought closer together, the surface

tension of the pools connect to form an hour-glass-shaped

band of molten material between the seed and feed sides.

By twisting the two sides in opposite directions, the

liquid sample is effectively “stirred” to ensure a uniform

distribution of material in the melt zone. The sample is then

slowly lowered through the focused circle of light, allowing

the narrow melt zone to progressively melt, mix and solidify

its way up the feed side of the sample.

“For materials with low vapor pressure, we can grow

crystals at a rate of one millimeter per hour,” Liu said. “We

can use the technique on a variety of materials, but we

always start with the phase diagram (kind of a growth map)

to determine if it’s possible. We can’t grow crystals with high

vapor pressure or that may be toxic using this method.”

SOLUTION/FLUX GROWTH

While the other three methods work well for materials

where the crystalline outcome is known, researchers also

look to discover and grow single crystals of new binary,

ternary, quaternary or higher compounds. In many cases,

the materials in these compounds don’t melt congruently

meaning they do not melt at a single temperature.

“Solution growth is extremely versatile, and you can

often optimize and cycle through it rapidly,” said Ames

Laboratory physicist and Iowa State University

Distinguished Professor Paul Canfield. “In general, it

does not give you as large a crystal, but for basic physical

measurements, something between a millimeter and a

centimeter is more than adequate.”

In practice, the compounds for the target crystal are

combined with a material that will serve as the solution in

which the crystal compound

will dissolve. For example, to

grow a cerium-antimony crystal

from a tin solution, or flux, you

may start with four percent each

of Ce and Sb with the other 92

percent Sn.

The materials go into a

“growth” crucible that’s paired

with a “catch” crucible. These

are then sealed in a silica tube as shown (top right). The

tube assembly is placed in a furnace and heated so all the

elements melt. The temperature is then lowered closer to

the melting point of the solution element, allowing the target

crystal to form. In the Ce-Sb in Sn flux example, the initial

temperature is roughly 1,000 degrees Celsius, then lowered

to 600 degrees.

A flux-grown ytterbium-

silver-germanium crystal.

Once a crystal has formed in the growth crucible, this assem-

bly is placed in a centrifuge. Excess liquid is captured in the

catch crucible. The glass wool then traps the liquid, leaving

the crystal in the growth crucible.

GROWTH

CRUCIBLE

CATCH

CRUCIBLE

SEALED

SILICA

TUBE

Glass wool

cushions crucibles

during decanting

Glass wool in

catch crucible

captures excess

liquid during

decanting

Higher melting

element (red),

lower melting

element (green)

f the single crystal growth techniques described

in the previous article sit at one end of the materials

synthesis spectrum, then rapid solidification techniques

fall at the opposite end of the scale. The former promote

the growth of the material’s equilibrium crystalline structure.

Rapid solidification techniques promote the opposite effect,

cooling the material so quickly from liquid to solid, that the

crystals formed are small, or in some cases non-existent,

becoming amorphous or glass-like with no discernable

crystalline pattern to their overall molecular structure.

It’s also a way to form composite materials whose

constituents have widely varying “freezing” temperatures.

“If you take a molten metal and cool it, what wants to

form will vary depending on its chemistry,” said Ames

Laboratory scientist and Division of Materials Sciences and

Engineering Director Matt Kramer, “because what wants to

form is not always a homogeneous solid.”

For example, if you freeze a mixture of water and alcohol,

the water will solidify first—turning to ice—while the

alcohol remains liquid, leaving a slushy mixture until the

temperature is lowered to the alcohol’s freezing temperature.

“So when you cast a molten alloy, small crystals will form

quickly on the surface of the mold, you get segregation of

the materials and the remaining liquid becomes enriched,”

said Kramer, who is also an Iowa State University adjunct

professor of materials science and engineering, “which

results in a heterogeneous bulk material.”

Rapid solidification allows the material to cool extremely

quickly so as to suppress or even eliminate the segregation.

Techniques range from strip casting, which cools materials

at about 1,000 Kelvins per second to splat quenching which,

as the name implies, squashes a droplet of liquid material

between two plates. Splat quenching can cool the material

as high as 10

Kelvins per second.

“Why is that important? Because there’s an intimate

relationship between temperature and the time at which

materials cool,” Kramer said. “We call it TTT—Time-

Temperature Transformation.”

It takes a certain finite amount of time for the initial

crystals to form, a process called nucleation. The molten

material has to organize itself into crystals only a few 10s of

atoms across and then those crystals need to grow.

“There’s a very non-linear relationship between time

and temperature transformation,” Kramer continued.

“Solidification occurs over a broad range of temperatures.

At too high a temperature, it stays molten. At a temperature

just below the melting temperature, the material solidifies

slowly, and in cases where constituents have different

melting temperatures, significant segregation in the casting

can occur if cooled slowly

Rapid solidification techniques allow researchers to

bypass the time-temperature transformation so a molten

metal alloy forms without a crystalline order, creating a

metallic glass.

“Glassy metals have some very unusual properties,”

Kramer said. “On average, they tend to have very good

strength, but not much plasticity, so they are hard to mold

into shapes.”

However, by first forming a metallic glass, then heating

the material back up, researchers can achieve metastable

phases of the material that aren’t attainable by other methods,

such as casting. And these intermediate phases may have

desirable properties such as strength, ductility, resistivity, or

conductivity.

“Manipulating the phases, their sizes, the degree

at which we can control their growth, and even their

morphology, or shapes, are all buried in the details of the

classical time-temperature transformation,” Kramer said.

“A lot of the work we’re doing is trying to understand the

relative balance of cooling rates to phase selection process.

How can we predict and control those so we can move

beyond an Edisonian approach.”

Researchers at Ames Laboratory use several techniques

B Y K E R R Y G I B S O N

Frozen in a Flash

Ames Laboratory scientist Brandt Jensen loads a sample into

the melt spinner. The induction coil melts the sample, which

is then forced in a stream onto the spinning copper wheel.

RAPID SOLIDIFICATION KEEPS MATERIALS ON COOLING FAST TRACK