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Link to original content: http://pubmed.ncbi.nlm.nih.gov/37299915/
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. 2023 May 30;23(11):5190.
doi: 10.3390/s23115190.

Investigating the Electromechanical Sensitivity of Carbon-Nanotube-Coated Microfibers

Elizabeth Bellott  1 , Yushan Li  1 , Connor Gunter  2 , Scott Kovaleski  2 , Matthew R Maschmann  1   3
Affiliations

Investigating the Electromechanical Sensitivity of Carbon-Nanotube-Coated Microfibers

Elizabeth Bellott et al. Sensors (Basel). .

Abstract

The piezoresistance of carbon nanotube (CNT)-coated microfibers is examined using diametric compression. Diverse CNT forest morphologies were studied by changing the CNT length, diameter, and areal density via synthesis time and fiber surface treatment prior to CNT synthesis. Large-diameter (30-60 nm) and relatively low-density CNTs were synthesized on as-received glass fibers. Small-diameter (5-30 nm) and-high density CNTs were synthesized on glass fibers coated with 10 nm of alumina. The CNT length was controlled by adjusting synthesis time. Electromechanical compression was performed by measuring the electrical resistance in the axial direction during diametric compression. Gauge factors exceeding three were measured for small-diameter (<25 μm) coated fibers, corresponding to as much as 35% resistance change per micrometer of compression. The gauge factor for high-density, small-diameter CNT forests was generally greater than those for low-density, large-diameter forests. A finite element simulation shows that the piezoresistive response originates from both the contact resistance and intrinsic resistance of the forest itself. The change in contact and intrinsic resistance are balanced for relatively short CNT forests, while the response is dominated by CNT electrode contact resistance for taller CNT forests. These results are expected to guide the design of piezoresistive flow and tactile sensors.

Keywords: carbon nanotube; microfiber; piezoresistance.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Floating catalyst chemical vapor deposition of CNT forests. (a) Schematic of floating catalyst CVD setup. (b) CNT forest synthesis conditions.
Figure 2
Figure 2
Schematic of the electromechanical test setup. The interdigitated electrode substrate resided with a nanoindenter. A 100 μm flat tip diametrically compressed a CNT-forest-coated fiber onto the patterned substrate while the electrical resistance was measured using a digital multimeter.
Figure 3
Figure 3
SEM micrographs of CNT-forest-coated glass fibers. (a,b) Alumina-coated glass fibers. (c,d) As-received glass fiber substrates. Inset images in (b,d) show histograms of CNT outer diameters measured by SEM.
Figure 4
Figure 4
SEM images of CNT-forest-coated microfibers. (a) Large-diameter CNT forests exhibit CNT banding instead of a continuous conformal coating. While most fibers exhibit two or more preferred bands, (b) some CNT forests collapse into a single band. The schematic in (b) demonstrates the position of the fiber (blue) relative to the CNT forest. (c) High magnification image shows catalyst nanoparticles on the glass fiber substrate and CNTs separated from the catalyst.
Figure 5
Figure 5
Electromechanical indentation data from diametrically compressed CNT-forest-coated glass microfibers. (a) Optical image of CNT-coated fiber tested in (b,c). (b) Mechanical and (c) simultaneous electrical data from a 12 μm diameter CNT forest grown from an alumina-coated glass fiber. (d) Optical image of CNT-coated fiber tested in (e,f). (e) Mechanical and (f) simultaneous electrical data from a 60 μm diameter CNT forest grown from an alumina-coated glass fiber.
Figure 6
Figure 6
Sensitivity of CNT-forest-coated glass fibers. (a) Gauge factor and (b) sensitivity of CNT forests grown on alumina-coated and as-received glass fibers. The gauge factor is computed with respect to diametric compression. (c) The initial resistance of CNT-coated fibers is inversely related to the fiber diameter.
Figure 7
Figure 7
Simulated CNT morphology as a function of synthesis attributes after 600 growth time steps. The attribute combinations include (a) 3 × 109 CNT/cm2, 40 nm O.D., 34 nm I.D. CNT diameter, 0.1 nN force to break CNT–CNT bonds, (b) 3 × 109 CNT/cm2, 40 nm O.D., 20 nm I.D. CNT diameter, 1 nN force to break CNT–CNT bonds, (c) 3 × 1010 CNT/cm2, 5 nm CNT diameter, 10 nN force to break CNT–CNT bonds. (d) A time evolution shows the formation of a single band using the attributes of 3 × 1010 CNT/cm2, 5 nm CNT diameter, 1 nN force to break CNT–CNT bonds. Note that the formation of different band structures is stochastic in nature. One set of synthesis attributes will not ensure one specific type of band structure.
Figure 8
Figure 8
Schematic of a simulated CNT forest for compression (left) and the electromechanical compression of the same forest (right). The colors within the compressed forest represent voltage.
Figure 9
Figure 9
Simulated electromechanical response of compressed CNT forests as a function of compressive displacement. Panels (a–c) represent simulations with 10 nm outer diameters, and areal density 3 × 1010 CNT/cm2, to simulate the CNT forests synthesized from alumina-coated fibers. Panels (d–f) represent CNT forests in which the CNT outer diameter is 40 nm and CNT areal density is 3 × 109 CNT/cm2 to simulate the CNT forests synthesized from as-received glass fibers. The legend represents the height of the CNT forest before compression. Panels (a,d) display the total electrical resistance through the CNT forests between the measurement electrodes. Panels (b,e) display the electrical contact resistance established between CNTs and the electrode surfaces. Panels (c,f) display the intrinsic internal resistance within the CNT forest obtained by setting the voltage of nodes in contact with an electrode equal to the voltage of the electrode.
Figure 10
Figure 10
Simulated CNT forest conductance as a function of the quantity of (a,b) CNT–electrode contacts, and (c,d) CNT–CNT contacts. Panels (a,c) represent CNT forests in which the CNT outer diameter is 10 nm and CNT areal density is 3 × 1010 CNT/cm2 to simulate the CNT forests synthesized from alumina-coated fibers. Panels (b,d) represent CNT forests in which the CNT outer diameter is 40 nm and CNT areal density is 3 × 109 CNT/cm2 to simulate the CNT forests synthesized from as-received glass fibers. The legend represents the height of the CNT forest before compression.
Figure 11
Figure 11
The quantity of CNT–electrode and CNT–CNT contacts as a function of compressive displacement for (a) 40 nm O.D., 3 × 109 CNT/cm2 CNT forest and (b) 10 nm O.D., 3 × 1010 CNT/cm2 CNT forest. The discretely plotted points represent CNT–electrode contacts, while the continuous lines represent CNT–CNT contacts.
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