Structural Evolution in Glassy Carbon Investigated Based on the Temperature Dependence of Young’s Modulus

Abstract

As a non-graphitized carbon material, possessing exceptional hardness and chemical inertness, glassy carbon (GC) is often synthesized through the pyrolysis method, which includes a compression procedure of powdered precursor materials, thus increasing the costs for production of glassy carbon at an industrial scale. Direct preparation of GC via pyrolysis of bulk precursors is a low-cost approach but encounters challenges arising from an insufficient knowledge of carbon structure formation. In order to solve this problem, a new analysis of the temperature-dependent variation in Young’s modulus of GC obtained by the pyrolysis of phenolic resin at 1000 °C, utilizing the impulse excitation technique (IET), was performed. Our findings demonstrate that there is a critical temperature range of 500–600 °C where pyrolysis leads to the most significant density change and GC is formed as a result. When GC samples are heated again, a significant structural reformation occurs in the same temperature range. It causes a decrease in stiffness, especially at heating rates >3 °C/min, and an interesting restorative effect–increase in stiffness when a GC sample is annealed at temperatures of 500–550 °C. These results bring important implications for the direct formation of large amounts of glassy carbon using bulk precursors.

Insights into molecular and bulk mechanical properties of glassy carbon through molecular dynamics simulations and mechanical tensile testing

“…In typical cases, a load applied to such a nanostructure on the axes tagged here as x-and y-axes produces additional responses such as shear, which obviously complicates the separation of single-axis stress-strain responses. In addition, from the modeling perspective, limitations in MD modeling in assigning a neutral axis complicate the evaluation of moduli in the x and y directions, as the model produces Kotlensky and Fischbach [22] Bare GC (dog-bone) Tension Uniaxial load 14.5-28.3 Zhang et al [36] Bare GC-Micro Pillars Tensile and compression Uniaxial load 16-26 Yang et al [38] Bare GC Flexure Flexural vibration (IET) 21.5 Fielda and Swain [39] Bare GC Compression Spherical indenters 20.8 Albiez and Schwaiger [40] Bare GC Compression Nanoindenter 27-47 Jenkins and Kawamura [41] Bare GC Tension Uniaxial load 30 Manoharan et al [42] Bare GC-Thin-film Tensile Uniaxial load 62 Diss et al [46] Bare GC Compression Nanoindenter 21.7 Sakai et al [47] Bare GC Compression Nanoindenter 28.0 ± 2 Garion [48] Bare GC Compression/tension Four-point-bending 32.4…”

“…Table 1 summarizes some of the modulus values for GC bulk materials reported in the literature that are in a range between a lower value of ∼15 GPa to a higher value of 55 and 62 GPa, demonstrating a rather significant variation among these values. In the majority of the reported cases, the moduli were determined through nanoindentation experiments and, therefore, reflect the mechanical properties of GCs under compression or flexure [5,21,[38][39][40]. Out of the remaining reported cases that were conducted through a direct tensile testing [22,41,42], the work of Kotlensky and Fischbach [22] is based on a GC material that was heated to almost 2600 • C and then annealed at still high temperatures, making its direct comparison with others rather difficult to justify.…”

“…In addition, the effect of maximum pyrolysis temperature on GC's mechanical property has been reported by several researchers in this area and is well documented. For example, for pyrolysis at 1000 • C, a modulus in the range of 21.5 GPa [38] and 21.7 GPa [46] have been reported while for a pyrolysis temperature of 2000 • C, modulus as high as 28 GPa was reported [22,47]. The effect of the testing methodology has also been mentioned as a source of discrepancy between the various modulus values reported.…”