Fabrication of a Low Density Carbon Fiber Foam and Its Characterization as a Strain Gauge

Abstract

Samples of carbon nano-fiber foam (CFF), essentially a 3D solid mat of intertwined nanofibers of pure carbon, were grown using the Constrained Formation of Fibrous Nanostructures (CoFFiN) process in a steel mold at 550 °C from a palladium particle catalysts exposed to fuel rich mixtures of ethylene and oxygen. The resulting material was studied using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDX), Surface area analysis (BET), and Thermogravimetric Analysis (TGA). Transient and dynamic mechanical tests clearly demonstrated that the material is viscoelastic. Concomitant mechanical and electrical testing of samples revealed the material to have electrical properties appropriate for application as the sensing element of a strain gauge. The sample resistance versus strain values stabilize after a few compression cycles to show a perfectly linear relationship. Study of microstructure, mechanical and electrical properties of the low density samples confirm the uniqueness of the material: It is formed entirely of independent fibers of diverse diameters that interlock forming a tridimensional body that can be grown into different shapes and sizes at moderate temperatures. It regains its shape after loads are removed, is light weight, presents viscoelastic behavior, thermal stability up to 550 °C, hydrophobicity, and is electrically conductive.

Keywords: carbon nanofiber; electrically conductive; hydrophobic; low weight; porous; strain gauge; viscoelastic.

Figure 1.

Microstructural analysis by SEM. (a) The sample consists of fibers which diameters vary between 30 and 400 nm and regions of empty space; (b) The plurality of fibers are between 60 and 90 nm in width; (c) During compression the empty spaces between fibers disappear with no evidence of fiber delamination or fracture.

Figure 2.

Stress vs. strain curves for the constrained sample. Cycles performed between 10 and 90 N loads using Plexiglas fixture to maintain constant area. The first loading cycle from 0 to 90 N has been removed. (a) Cycling behavior at a rate of 0.05 mm/s; (b) Cycling at frequencies of 0.01 mm/s; and (c) Values reach a reproducible and stable profile after the first conditioning 15–17 cycles.

Figure 3.

Sample relaxation. A constrained sample was maintained at a constant strain of 0.63 at room temperature. Initially 50 N were applied; the program was adjusted to maintain such strain level and stress over time recorded.

Figure 4.

Resistance vs. time cyclic behavior. Cycles performed between 10 and 90 N loads using Plexiglas fixture illustrate that after some initial the conditioning cycles the Carbon Fiber Foam electrical behavior stabilizes.

Figure 5.

Strain Gauge. The resistance vs. strain values taken from the final six segments of cycling experiment show the “aged material” displays a linear relationship between resistance and strain (a). The slope of the (∆R − R_o)/R_o vs. strain, taken from the 3rd to last cycle, has been used to calculate the strain gauge factor (b).

Figure 6.

Thermal stability determined by TPO analysis. The sample maintains its weight up to at least 550 °C under an oxygen containing atmosphere, after such the carbon starts to burn off until only the weight of the original palladium catalyst particles, now oxidized, remains.

Figure 7.

Hydrophobicity. (a) Drop of water suspended in the foam surface; (b) A drop of oil gets readily absorbed within the foam structure, showing no evidence of oil on the surface after just fractions of a second of contact.

Figure 8.

Steel mold and catalyst geometry. Arranging Pd catalyst particles in the mold as shown (a), was found to be a necessary part of the protocol required to create, based on visual inspection, homogenous CFF (b).

Figure 9.

Anvils and sample placement for simultaneous mechanical and electrical tests. Sections of the carbon fiber based material were cut to the same diameter than the anvils, placed inside a Plexiglas cavity to avoid changes in cross sectional area during measurements.