ECN - August 2015

Carbon nanotubes (CNTs) are tubular cylinders of carbon atoms that possess incredible mechanical, electrical, thermal, and chemical properties. In fact, the structures are 200 times stronger than steel and experience very little physical corrosion typical of other metals.

CNTs come
in three main
forms and are
distinguished by
how many walls
of carbon exist
within each
tube. A single-
wall CNT has
a single tube; a
multi-wall CNT
typically has
10 to 20 walls
nestled inside
each other; and
a few-wall CNT has two or three walls.

Multi-wall CNTs are easier to produce than single-wall CNTs, while few-wall CNTs are still a relatively new category. South West Nano Technologies (SWeNT) distinguishes itself as one of the only companies that produces all three types at a commercial scale. Recently, the company demonstrated the use of carbon nanotube inks as a material for specific touch capacitance and membrane switch replacement applications.

Last November, SWeNT had the opportunity to showcase the technologies at the Printed Electronics USA Show in Santa Clara, CA. The company exhibited a transparent conductive ink material, which is commercially available, and a semi-conducting ink material, which is currently in the R&D stage. The former is used for touch sensor applications, transparent electrodes for display products, and for smart windows. The latter is typically used for printing transistors and various types of sensors.

Printability

What makes the company’s transparent conductive inks
unique is that they’re printable. Manufacturers must
usually etch away at a transparent conductive film to
form patterned circuits, but by using this approach,
manufacturers can print the transparent conductive ink
exactly where it is desired on the circuit. “There is no
waste of material,” explains Dave Arthur, SWeNT CEO.
“You don’t have the associated cost with etching away
the material and dealing with waste.”
SWeNT’s design philosophy has been to formulate
the inks so they are printable with standard industrial
printing equipment. For example, the company has
developed a screen-printing ink that can be printed on
any screen-printing press and a slot die coating ink that
can be coated in any slot die coater.

Stretchability

Aside from the transparent conductive ink’s printability,
the ink is also stretchable. “This capability of nanotubes
to stretch and still maintain their electrical properties
is very exciting from a design standpoint,” says Arthur.
“It’s consistent with wearable electronics, flexible
electronics, and 3D-formed electronics.”
The ability to form a three-dimensional shape after
it’s been printed would allow touchscreens to have
curved touch buttons. For instance, a 3D button on a
car’s display screen could guide a driver’s finger where to
touch. Taking one’s eyes off the road would no longer be
an issue.

V2V technology

When first put on a screen, inks must have a consistency like paste so they don’t drip through. Then when the squeegee passes over the ink to force it through the screen, it must become thinner to flow. After it flows through the screen and the ink is printed onto the surface of the desired substrate, the ink must stop immediately so it doesn’t lose precision of the image that was just printed.