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“The active component in a laser is a single crystal,”
said Lograsso, who is also an Iowa State University adjunct
professor of materials science and engineering, “because the
crystal grain boundaries would scatter the light.”
From a research viewpoint, especially when creating a
new material, scientists want to remove as many variables
as possible to best understand a material’s properties. A
primary way to do this is to begin with raw materials that
are as pure as possible and to produce the material as a
single crystal.
“You don’t want defects in the crystal structure and
you don’t want impurities, which can be a source of extra
nucleation,” Lograsso said. “New materials can have new
physics, and we can determine what those are if we make
measurements on a clean, pristine sample (i.e. single crystal).
And if we do that consistently, we can make comparisons to
other materials and see how it fits into our understanding of
particular behaviors.”
Ames Laboratory scientists employ a number of
techniques to grow single crystals, with each suited to
producing crystals from different types of materials.
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However, the basic premise is the same—oversaturate a
solution, then precipitate out the crystal.
“As kids, we’re familiar with adding rock salt or sugar to
hot water until you supersaturate the liquid,” Lograsso said.
“Then, as the water cools and eventually starts to evaporate,
crystals of salt or sugar start to form and then grow.”
“You can do the same with about any two materials, using
one as the solvent and then using heat or high temperatures
to supersaturate the solvent,” he continued. “The tricky part
is to get a single crystal to first form and then grow.”
This “practitioner’s art” requires patience and skill,
though the various techniques described here provide
some assistance as well. Generally, a high temperature
gradient also helps promote a stable growth transition from
liquid to solid.
BRIDGMAN TECHNIQUE
One of the better known methods, the Bridgman
technique—named for Harvard physicist Percy Williams
Bridgman—uses a crucible with a pointed, conical end.
This fine point promotes the growth of a single crystal as
the crucible exits the heated portion of the furnace. Heat is
provided through a heating element similar to the one in a
home oven (resistance) or via a magnetic field (induction).
“Crucibles age over time and become better at producing
single crystals,” Lograsso said. “Unfortunately, you
sometimes break the crucible removing the crystal. Because
they grow inside a crucible, crystals formed in this manner
may also develop stresses such as cracks or voids.”
Ames Laboratory also has a special Bridgman furnace
that allows crystal growth at higher pressures—up to 15
Bar. This allows growth of crystals from alloys that contain
volatile components. The high pressure prevents these
components, which have a lower boiling point than alloy’s
other components, from flashing off as a vapor before the
crystal can form.
This furnace utilizes induction heating, which provides
a steeper temperature gradient, allowing faster crystal
growth rates to further minimize evaporation and reaction
with the crucible.
CZOCHRALSKI TECHNIQUE
This method also heats the material in a crucible,
but here, the crystal is actually drawn from the molten
solution. Lograsso likens it to dipping a candle “except
you only dip once.”
A seed crystal of the material is attached to the end of a
rod. The rod is lowered until the seed crystal just touches
the surface of the molten material in the crucible. The rod
is then rotated and withdrawn very slowly, pulling the newly
formed crystal from the liquid.
“Because the crystal is freestanding, it doesn’t have the
stresses that you sometimes get with the Bridgman method,”
Lograsso said. “Depending on the material, crystals can also
be 60 cm in diameter, or larger, and several feet in length. This
is a very common method for producing large silicon crystals
which are sliced into wafers for use in semiconductors.”
FLOAT-ZONE TECHNIQUE
Optical float-zone technique uses focused, high-intensity
light to create single crystals, particularly those containing
metal oxides. According to associate scientist Yong Liu, the
technique offers a couple of advantages for growing many
types of crystals.
“It’s container-free—you don’t need or use a crucible
to grow the crystal so it eliminates any potential reaction
between the sample and the container,” Liu said.
“Because the melt zone is very focused and narrow, we’re
able to achieve a very large temperature gradient between
the solid and liquid phases, which results in high-quality
crystal growth.”
A typical optical float zone furnace consists of four
high-powered halogen bulbs arranged in a ring around the
sample. Semi-spherical reflectors around each bulb focus
the intense light energy in a narrow band around the sample
hen it comes to creating new materials,
single crystals play an important role in
presenting a clearer picture of a material’s
intrinsic properties. A typical material will be
comprised of lots of smaller crystals and the grain boundaries
between these crystals can act as impediments, affecting
properties such as electrical or thermal resistance.
“Those boundaries can have profound effects, both
good and bad,” said Ames Laboratory materials scientist
and deputy director Tom Lograsso. “Generally, a material
that has smaller and smaller crystals actually has improved
mechanical properties.”
An exception to this rule is that at high temperature,
relative to the melting point, small crystals can have a
tendency to slide past one another, a property called creep.
It’s for this reason that turbine blades in some jet engines or
generators are actually formed from single crystals of nickel-
based alloy.
A few other everyday applications using single crystals
are semi-conductors, detectors, such as infrared or radiation
sensors, and lasers.
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B Y K E R R Y G I B S O N
Single crystals provide clarity
Ames Laboratory scientist Paul Canfield removes a sample from a flux-growth furnace.
Ames Laboratory scientist Deborah Schlagel holds a graphite
crucible (left) and a Bridgman-grown copper crystal (right).
Four semi-spherical reflectors focus light energy from high-
powered halogen bulbs onto the material, which is suspended
over the port in the center.