Portland, Ore. Sensors developed by Elizabethtown College researchers could pave the way for next-generation materials that would be equipped with 50-ohm coaxial connections that engineers could "ping" to read out the material's internal state. The embedded capacitive sensors could be the cure for advanced processing of materials from carbon nanotube-reinforced composites to plain old concrete, enabling functions ranging from elaborate process control monitoring to simple electronic pop-up alerts that would go the traditional turkey timer one better.
"The sensors we make are very small capacitors I'm talking picofarad parallel-plate capacitors with the material serving as the dielectric. At those frequencies, you don't need much loading to get an effective match to a 50-ohm line," said Nathaniel Hager III, an adjunct professor in the physics and engineering department at Elizabethtown (Pa.) College. Hager performed the work with materials chemist Roman Domszy. A patent was awarded just days ago covering sensing and instrumentation using time-domain reflectometry (TDR) dielectric spectroscopy.
The 50-ohm lines could be useful during the manufacture of advanced materials. Through them, the various stages of curing could be monitored and fine-tuned, resulting in optimized materials. After manufacturing, the 50-ohm coaxial connections could be used for all sorts of one-shot tests, from pinging for stress fractures in composite aircraft wings to looking for faults in concrete buildings after an earthquake.
"We put capacitive sensors in the material, so when we send a 13-ps step pulse down the 50-ohm line you can see the surrounding permittivity of the material by analyzing the reflection coming back with a Fourier transform giving you the complex permittivity as a function of frequency for a range of from about 10 kHz up to almost 20 GHz," said Hager.
By substituting the material being created for a capacitor's dielectric, materials manufacturers could get real-time feedback from the internal state of the new material as it cures. By subtly changing parameters like temperature and pressure, the curing material could be optimized.
"We can read out the signatures for the various stages of the curing process and follow them continuously," said Hager. "First you have molecular relaxations in the very high-frequency, 10-GHz range, then [below that] you have the signature of water molecules rotating. And if you get down to the 100-MHz to 1-MHz range, you see signatures of what we think is bound-water relaxation and, finally, ion conductivity and electrode polarization, which decrease for advanced stages of curing."
During curing, both frequency and transient analysis could monitor variations in hydration, particle size distribution, addition of retarders and accelerants, and other additives, the researchers said.
The original application for equipping a material with instrumentation via embedded sensors sprang from an Air Force contract, on which Hager and Domszy had worked, for aircraft wings made from advanced composites. While consulting with a local contractor, the pair decided concrete curing could profit from being similarly instrumented: By embedding cheap disposable sensors in new concrete structures, engineers could walk up and plug any number of instruments into the 50-ohm lines.
Many additives are used during the concrete-curing process to improve it. A sensor model that combined real-time feedback could be used to fine-tune such cures. That same 50-ohm line, which would remain in the cured structure throughout its lifetime, could also be used years later. For example, after an earthquake occurred, a technician could plug in a simple instrument that would ping if it found the specific signature of cracks caused by the earthquake.
"There are all kinds of ways the analysis step can be dumbed down for detecting a single signature this is a very versatile method," said Hager.
Even during the curing process, various sensors are needed that pop up electronically when a certain stage is reached. Like an pop-up thermometer in a turkey, they could be used to indicate when it's time to pour a second layer, for example, or when it's safe to apply moisture-sealing epoxies atop a floor.
Indeed, theoretically the turkey application might benefit. "This thermometer would be better than the pop-up kind, because it could tell you when the turkey is getting done too quickly or too slowly so you could take corrective action, turning the oven temperature up or down," said Hager.
While EEs usually work with capacitors whose dielectric constant does not vary, that is not the case with Hager and Domszy's capacitive sensors.
In all standard types of capacitors especially parallel-plate capacitors the whole point of the dielectric separating the two plates is that it does not "break down." The assumption is that the dielectric constant is unchanged over the frequency range of the capacitor; when that value ceases to be constant, the dielectric is said to break down.
But using a material whose dielectric constant is a function of frequency works in favor of equipping materials with instrumentation via embedded sensors.
'Monitor over time'
"All the interesting materials have changing dielectric constants," said Hager. "You get all these signatures because the dielectric constant is changing with frequency, so what we can do is monitor all these signatures and see how they change over time."
In the researchers' TDR dielectric spectroscopy approach, the readouts are fed into a mathematical relaxation model that tracks Cole-Davidson signatures near 1 MHz, which grow initially and then decrease with an advancing cure; the Debye signature near 100 MHz, which grows initially and then decreases with an advancing cure; the free-water signature near 10 GHz, which decreases with an advancing cure; and an ion conductivity and electrode polarization signature, which decreases with an advancing cure. Parameters for the signature amplitudes, frequencies and distribution are tracked as a function of cure time.
The sensor research that Hager and Domszy conducted was funded by the National Science Foundation's Small Business Innovation Research program.
Hager and Domszy were awarded a patent for concrete-cure monitoring using their TDR dielectric spectroscopy method on Nov. 21 just in time for Thanksgiving.