Chemistry Reference
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research should work in parallel to develop a sensor substrate that would mini-
mize these potential issues in a real-world application.
We believe a carbonized polymer is a candidate for an ideal substrate. Kapton is
an example of an insulating polymer that can be laser-carbonized to form conduct-
ing filaments. This substrate is flexible, durable, inexpensive and easy to manufac-
ture. While the Kapton polyimide is a good insulator, research has shown that the
laser-carbonized filaments are fair conductors [18, 19, 24]. Additional opportunities
to increase the effectiveness of some sensor designs lie in the fact that the carbon-
ized filaments are porous and their resistivity can be manipulated easily during
processing. Researchers discovered that lines as narrow as 10
m in width and spe-
µ
cific resistance as low as 0.01
cm could be achieved using cw lasers in the 350-
380 nm UV region [22]. Raman spectra of the filaments have shown that the mate-
rial is principally a “glassy” carbon which is composed of small crystallites [18].
During the last year, our laboratory has developed a capacitive-type humidity
sensor prepared on a Kapton substrate using filaments carbonized with UV or
visible laser light as electrodes and a coating of 2-hydroxy-3-methacryloxypropyl-
trimethylammonium chloride (HMPTAC) as the hygrosensitive dielectric. Re-
search into current humidity sensors has led to the adoption of the general proce-
dure put forth by the Sakai Group [25]. Their procedure used silicon / gold circuit
design and coated it with the HMPTAC film. Using a variation of this procedure,
our experiment focused on developing a sensor using Kapton as the substrate to
see if a viable all-polymer sensor could be developed. Kapton circuits were pro-
duced through carbonization of a Kapton film via an argon ion laser operating in
the visible and UV regions. The variables involved in making the carbonized cir-
cuits were tested to find the lowest resistivity. Substrates were then coated with
HMPTAC under a nitrogen atmosphere. The Kapton sensor was then tested using
impedance measurements under varying conditions to assess its ability to measure
humidity changes. The Kapton-based circuit was able to determine humidity
changes in a controlled atmosphere between 5-95% relative humidity. The sensor
showed a smooth response to water vapor and displayed no deterioration over
four months of use. While research continues with the humidity sensor to opti-
mize the design and sensitivity parameters, we have demonstrated that an all-
polymer sensor is feasible.
Ω⋅
2. EXPERIMENTAL
2.1. Pyrolysis procedure
A Coherent Innova 200 argon ion laser operating at 514 nm was used to pyrolyze
Kapton samples (Figure 1). Pyrolysis took place in a custom-made chamber con-
sisting of a 10
8 cm black aluminum box with a removable lid sealed with a
rubber gasket. The chamber had a 0.635 cm (1/4") Swagelok inlet and exhaust
port to deliver the argon gas flow at around 2 L/min throughout pyrolysis. The
front side of the chamber contained a 1 cm thick quartz window. The chamber
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