Anisotropy of Electrical and Thermal Conductivity in High-Density Graphite Foils

• Published in: Nanomaterials (Basel, Switzerland) Switzerland), 2024-07, Vol.14 (13), p.1162
• Main Authors: Shulyak, Vladimir A, Morozov, Nikolai S, Gracheva, Alexandra V, Gritskevich, Maria D, Chebotarev, Sergei N, Avdeev, Viktor V
• Format: Article
• Language: English
• Subjects: Anisotropy; Batteries; Coherent scattering; Conductivity; Crystal lattices; Crystal structure; Density; Electric contacts; Electrical conductivity; Electrical resistivity; Electron microscopy; Elongated structure; flexible graphite foil; Foils; Graphene; Graphite; Heat conductivity; Heat transfer; Kearns parameters; Mechanical properties; Misalignment; Nitrates; Plastic deformation; Rolling direction; Scanning electron microscopy; Specific gravity; structural parameters; Thermal conductivity; Transmission electron microscopy; X ray analysis
• ISSN: 2079-4991; 2079-4991
• DOI: 10.3390/nano14131162

Flexible graphite foils with varying thicknesses (S = 282 ± 5 μm, M = 494 ± 7 μm, L = 746 ± 8 μm) and an initial density of 0.70 g/cm were obtained using the nitrate method. The specific electrical and thermal conductivity of these foils were investigated. As the density increased from 0.70 g/cm to 1.75 g/cm , the specific electrical conductivity increased from 69 to 192 kS/m and the thermal conductivity increased from 109 to 326 W/(m·K) due to the rolling of graphite foils. The study showed that conductivity and anisotropy depend on the shape, orientation, and contact area of thermally expanded graphite (TEG) mesoparticles (mesostructural factor), and the crystal structure of nanocrystallites (nanostructural factor). A proposed mesostructural model explained these increases, with denser foils showing elongated, narrowed TEG particles and larger contact areas, confirmed by electron microscopy results. For graphite foils 200 and 750 μm thick, increased density led to a larger coherent scattering region, likely due to the rotation of graphite mesoparticles under mechanical action, while thinner foils ( 1.7 g/cm showed increased plastic deformation, indicated by a sharp reduction in the coherent scattering region size. This was also evident from the decrease in misorientation angles with increasing density. Rolling reduced nanocrystallite misorientation angles along the rolling direction compared to the transverse direction (TD) (for 1.75 g/cm density ΔMA = 1.2° ( ), 2.6° ( ), and 2.4° ( )), explaining the observed anisotropy in the electrical and mechanical properties of the rolled graphite foils. X-ray analysis confirmed the preferred nanocrystallite orientation and anisotropy coefficients (A) using Kearns parameters, which aligned well with experimental measurements (for L series foils calculated as: A = 1.05, A = 1.10, and A = 1.16). These calculated values corresponded well with the experimental measurements of specific electrical conductivity, where the anisotropy coefficient changed from 1.00 to 1.16 and mechanical properties varied from 0.98 to 1.13.