IV: Wireless Carbon Nanotube Gas Sensor

4.1 Sensor Design

The microwave circular disk resonator was designed on a Duroid board (RO4350B) that has a dielectric constant of 3.48 and a thickness of ~1.5 mm. The resonant frequency was first determined by an approximate formula for the resonant frequency at the dominant mode:

where f ₀ is the resonant frequency of the resonator, c is the speed of light in vacuum, a is the radius of the disk, ? _r is the relative dielectric constant of the substrate (Duroid board). The resonator can be scaled to higher frequency to achieve greater sensitivity and reduce device size. Capacitive gap was designed to couple RF electromagnetic energy in and out of the circular disk resonator. A 50- ? transmission line was used to feed RF signals into the resonator with a gap of ~200 ?m. The resonator has a radius of 7.5mm and the resonant frequency of sensor is ~5.5GHz. The dimensions of the gap were designed so that the return loss at resonant frequency is beyond 20 dB. The Sonnet electromagnetic tool was used to optimize the capacitive gap for optimum impedance at the resonant frequency. A 3.5 mm connector was soldered to the other end of the microstrip line to provide connection to a microwave vector network analyzer.

Figure 2: Model used in Sonnet simulation

Single wall carbon nanotubes (SWNTs) and multi-wall carbon nanotubes (MWNTs) can be assembled on top of the...