Geological sequestration, where the CO2 generated from power plants was collected and dumped into the mines and oil fields deep under ground, has attracted lots of attention. There is urgent need for developing a sophisticated system that can monitor the ground leakage in those remote sequestration sites. Surface acoustic wave sensor equipped with on-chip sensitive layer is the best choice for low cost wireless monitoring of sequestration sites leakage monitoring with minimum power consumption.
A passive wireless CO2 sensing system based on surface acoustic wave technology and carbon nanotube nanocomposite was developed. Surface acoustic wave device was studied to determine the optimum sensor operation parameters. Delay line structure was adopted. A surface acoustic wave flow sensor was developed and showed linear relationship between the applied pressure and the change of the acoustic wave transmission time.
Percolation studied showed that the CNT began to form a network at concentration over 1wt%. CNT polymer nanocomposite was then fabricated and tested under different temperature and strain condition for natural environment impact evaluation. Nanocomposite resistance increased for 5 times under pure strain with the thermal coefficient of nanocomposite at 75ppm/℃, while the temperature dependence of resistance for CNT solely was -0.1375%/℃. The overall effect of temperature on nanocomposite resistance was -0.1%/℃.
Test of polyimide based nanocomposite showed less than 0.4% resistance increase over pure CO2, which corresponded to about 0.001% frequency change. Polyethyleneimine instead of polyimide was used to construct the nanocomposite. The gas response for the alternative nanocomposite was about 10% resistance increase under pure CO2, which gave an estimated 0.1% frequency change. The fabricated sensor frequency change was around 0.03% for pure CO2. The lowest detection limit of the sensor is 1% gas concentration, with 0.0036% frequency change.
The sensor was tested at various humidity environments and showed over 0.1% frequency change under saturated humidity. With paralyne packaging, the sensor frequency change on humidity reduced to less than 0.01% while maintaining the same gas sensing performance. Wireless module was tested and showed over one foot transmission distance at preferred parallel orientation
