Carbon Nanotube Dispersion in Solution and Freeze-cast

Chapter IV: Carbon Nanotube-reinforced Composites

4.3 Results and Discussion

4.3.1 Carbon Nanotube Dispersion in Solution and Freeze-cast

cooling ramp rate under argon. During pyrolysis, the Mylar decomposed, leaving a pre-crack in the center of the disk. The pre-crack length, thickness, and diameter were recorded for each sample. At least four samples for each CNT concentration were tested.

Two pieces of rubber were placed between the sample and the compression platens to avoid the crushing at contact points mentioned earlier. An aligner was used to set up the disk perpendicular to the compression platens. Once the disk was clamped between two compression platens with 1 N applied force, the aligner was removed before the compression test. The setup is shown in Fig. 4.1(b). Uniaxial compression tests were performed on an Instron 4204 universal testing machine in constant displacement mode at a displacement rate of 0.5 mm/min.

The toughness was calculated following [7, 28]

𝑁_𝐼 = 𝜋 𝑅𝑡 𝑃

√ 𝜋 𝑎

𝐾_𝐼

=𝑇₁𝐴₁(𝜃) +𝑇₂𝐴₂(𝜃) +𝑇₃𝐴₃(𝜃) +...

(4.3)

where 𝑁_𝐼 is the normalized stress intensity, 2𝑎 is the length of the pre-crack, 𝑅 is the radius and𝑡 is the thickness of the Brazilian disk, 𝑃 is the applied load, 𝐾_𝐼 is the stress intensity factor of mode I, and 𝐴_𝑖 and𝑇_𝑖 s are, respectively, angular and numerical constants. For𝜃 =0, 𝐴₁=1 and 𝐴_𝑖 =0 if𝑖 ≠ 1, the expression reduced to

𝐾_𝐼 =

√ 𝜋 𝑎 𝜋 𝑅𝑡

𝑃×𝑇₁ (4.4)

where 𝑇₁ is a function of 𝑎/𝑅 and was evaluated/interpolated from the values reported in [7] for each sample. The maximum load associated with crack extension is where K𝐼 = K𝐼 𝑐.

within the duration of the freezing process. Sonication broke up the CNT ag- glomerates and the dispersant attached to the surfaces of CNTs, preventing them from entangling with other CNTs again. Adding dispersants and sonicating are two essential steps in successfully dispersing carbon nanotubes and forming a stable dispersion to freeze.

Figure 4.2: Suspensions with 3 mg/ml CNTs and 300 mg/ml MK (a) without, and (b) with KD1. Representative SEM micrographs of the resulting freeze-cast structures with 1.3 wt.% CNTs (c) without and, (d) with KD1.

The effectiveness of the dispersion in suspension has a direct correlation with the uniformity of CNT distribution in freeze-cast structures. Shown in Fig. 4.2(c) is an SEM image of the freeze-cast structure made from the suspension without the dis- persant, which had massive agglomerates containing concentrated clusters of CNTs.

Macro-pores of more than 100 𝜇m in diameter were created by air bubbles trapped in the suspension due to the high viscosity from poorly-dispersed CNTs. Shown in Fig. 4.2(d) is an SEM image of the freeze-cast structure made from the suspension with the dispersant. It has no visible agglomerates and is similar to typical lamellar freeze-cast structures made with DMC [125]. The comparison demonstrates that the effectiveness of dispersion of CNTs is especially crucial for freeze casting to produce CNT-SiOC composites. In the case of poor CNT dispersion, not only will agglomerates be present, but equally important, the freezing process during freeze casting will be disrupted. With large CNT agglomerates present, the freezing front

Figure 4.3: Representative SEM micrographs of freeze-cast structures (a) pure SiOC, (b) 1.3, (c) 4.3, (d) 8.2 wt.% CNT composites.

Shown in Fig. 4.3 are SEM images of four freeze-cast samples prepared with both dispersant and sonication, with equal preceramic polymer contents, but with increasing CNTs. With an increasing concentration of CNTs, the agglomerates could not be prevented and protruded from the walls, making the surfaces of walls rougher. If agglomerates were sufficiently large, they formed bridges of CNTs and SiOC between walls as in Fig. 4.3(d). However, despite extra bridges seen in freeze-cast samples with 8.2 wt.% CNTs, overall, the structure did not change significantly compared to CNT-free compositions. Even the freeze-cast structure of the highest CNT concentration (8.2 wt.%) has few small (<20𝜇m) agglomerates, a stark contrast to that made without dispersants in Fig. 4.2(a), having a large number of >50 𝜇m agglomerates with just 1.3 wt.% CNTs.

A closer look at cross-sections of lamellar walls in Fig. 4.4 provides an added view of CNT dispersion—this time within the SiOC walls. We observed localized CNT-

dense regions of 20 to 500 nm, noted by circles in Fig. 4.4(b) through (d), indicative of sub-micron CNT agglomerates, which were reduced in size by several orders of magnitude compared to those of as-received CNTs. As CNT concentration increases, the number of agglomerates increases. No pores or delamination are observed between CNTs and SiOC.

Figure 4.4: Representative SEM micrographs of freeze-cast walls of (a) pure SiOC, (b) 1.3 (c) 4.3, (d) 8.2 wt.% CNT composites. The white dots, present in 1.3, 4.3, 8.2 wt.% samples, are carbon nanotubes. Sub-micron agglomerates are highlighted with green circles.

X-ray diffraction patterns in Fig. 4.5(a) shows that pure SiOC pyrolyzed at 1100℃is structurally amorphous, with a broad peak centered at 23.4°. No distinct crystalline peaks were observed, consistent with the literature [36, 72, 166]. As CNTs are added, new peaks begin to emerge. With peak fitting software, Fityk [187], the XRD pattern was resolved into several peaks, shown in Fig. 4.5(b). Peaks located at 26.0° and 43.6° are attributed to MWCNT. The ratio of the height of the two peaks (ratio∼3) suggest carbons are in the form of MWCNT (ratio∼5) rather than graphite (ratio 20) [148]. The peaks from MWCNT confirmed the preservation

reactions between Si-O and Si-C bonds and subsequent phase separation of SiOC glass usually at 1200-1400℃[22, 132, 165]. We hypothesize that the extra carbon introduced into the system might have facilitated the formation of SiC. Sujith et al. [171] added graphene nanoplatelets to polysiloxane and also saw peaks of SiC emerge at temperatures as low as 1000℃.

Figure 4.5: (a) XRD pattern of pure SiOC, 4.3, 8.2 wt.% CNT composites and MWCNTs. (b) Background subtraction and peak fitting performed on XRD pattern of 8.2 wt.% CNT composite to identify phases present.