Carbon Nanotube Sheet Synthesis in Nitrogen Environment and Flexible CNT Sheet Strain Sensor

This thesis focuses on the potential of mass-production of spin-capable carbon nanotubes (SCNTs), which produce carbon nanotube sheets (CNTSs). A CNT forest (CNTF) that satisfies the specific conditions, such as CNT diameter, distances between CNTs, and shapes, are available to produce CNTS via Van der Waals interaction between each CNT. To understand and be able to control those factors, the morphology of the metal catalyst (Fe) was studied. The CNTs were synthesized with the chemical vapor deposition (CVD) method at 780 °C. First, the thickness of the Fe film was controlled, using the electron beam deposition method. Samples were deposited with 1.5, 2.0, 2.5, 3.0, and 3.5 nm of Fe, then the samples were annealed in H2. After the annealing process, the Fe layer formed nanoparticles (NPs) with diameters of 16.91±1.88, 19.79±1.18, 23.28±5.42, 24.47±4.54, and 26.78±2.27 nm, respectively. The samples with 1.5, 2.0, and 2.5 nm produced SCNTs while others produced CNTF that were not spincapable. The catalyst morphology and the synthesized CNTs are examined with atomic force microscopy (AFM) and scanning electron microscopy (SEM). Secondly, the H2 exposure time was controlled to change the NP sizes at the annealing process. After annealing 20, 40, and 60 s of H2 exposure time, the annealed NPs with diameters of 16.45±2.2, 16.88±3.99, and 34.41±5.79 nm. The samples that were exposed to H2 for 20 and 40 s produced SCNT, but the one under 60 s produced non spin-capable CNTs. The results conclude that both initial Fe thickness and the H2 annealing duration affect the growth process of the catalyst NP. Simply by controlling the initial deposition or the annealing process, it is possible to produce SCNT and CNTS in a repeatable fashion. The production cost needs to be reduced for the mass-production to be possible. The conventional CVD system requires inert environment. However, relatively expensive gases, such as He and Ar are used in many studies. To solve this problem, we have replaced the inert gas with the N2 gas. The catalysts annealed in N2/H2 and He/H2 are examined thoroughly via AFM. The CNTs from each environment were taken under SEM and TEM to study their physical properties. The average densities of the particles and the CNTs from N2 and He exhibited very similar results. In addition, the TEM images show that the thickness and the number of the walls from either environment do not show much difference from each other. The IV characteristic also suggests that the CNTs from either environment can be interchangeable. With the experimental results, using N2 as the carrier gas for the spin-capability of CNTF is a notable discovery for mass-production and commercialization. Finally, CNTSs grown in an N2 environment were used as a sensing material for strain sensors. The resistances were measured for the devices. To find the relationship between the number of the CNTSs and the resistance and sensitivity, 2, 4, 6, and 8 layers of CNTSs were deposited on a flexible polymeric slab. The eight layers of CNTSs under 20% strain resulted in a sensitivity of 15.4%. After applying strain for 50 cycles, the resistance changes and sensitivity were very consistent after the first few cycles. The sensitivities were 4.00, 8.44, 11.14, and 15.40% for 2, 4, 6, and 8 layers of the CNTSs in the device, which depicts that the relationship between the sensitivity and the number of the CNTS is linear.