Flexible Strain Sensor Using Vertically Aligned Carbon Nanotube Forest

There are many researches about producing strain sensors with carbon nanotubes (CNTs). But most of them are studying nanocomposite film of CNTs and polymer and using nanocomposite film to fabricate the strain sensor. There are few researches about fabricating the strain sensors with vertically aligned carbon nanotubes (VA-CNTs). In this research, we synthesize the different height of iron-filled CNTs on aluminum foil by controlling the synthesis temperature in furnace and using the chemical vapor deposition method (CVD). There are some characters for iron-filled CNTs in this research, such as uniformity, equal height and vertically aligned. In this research, we fabricate four different strain sensors by iron-filled VA-CNTs, such as sensor type 1, sensor type 2, sensor type 3 and sensor type 4. One of the most special is sensor type 3. We constrain the height of CNTs by using package method and exerting an external force to depress the initial height of CNTs. This method can enlarge the initial contact area between the CNTs. When the bending strain occurs in the CNTs, it will lead the change rate of contact area increase. That is why this result can enhance the gauge factor (GF). The experimental results show that the height of CNTs near 12.5μm has the best GF performance. We use 12.5μm single layer iron-filled VA-CNTs to fabricate the sensor type 3. We find out that this kind of sensor has the value 208 of GF and the value 0.989 of linearity. After that, we even extend this kind of CNTs to sensor type 4. The results show that the GF of sensor type 4 can reach to 367. Furthermore, previously exerting an external force to sensor type 3 is an important factor in our research. We find out that there is a mathematical relationship between the external force and the GF. So we can use the result of GF to derive the external force value which is exerted on the strain sensor. At the last, we also show that resistance still has the significant changes by the simple up-down-shaking dynamic measurement. We even successfully apply strain sensor to measure the human pulse. The strain sensors in this research have high linearity, good repeatability and big GF value.