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/cm³ 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/cm³ to 1.75 g/cm³, 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/cm³ 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/cm³ density ΔMA = 1.2° (S), 2.6° (M), and 2.4° (L)), 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_0.70 = 1.05, A_1.30 = 1.10, and A_1.75 = 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.

The physical and mechanical properties and structural condition of flexible graphite foils produced by processing natural graphite with nitric acid, hydrolysis, thermal expansion of graphite and subsequent rolling were studied. The processes of obtaining materials and changing their characteristics has been thoroughly described and demonstrated. The structural transformations of graphite in the manufacture of foils were studied by X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). A decrease in the average size of the coherent scattering regions (CSR) of nanocrystallites was revealed during the transition from natural graphite to thermally expanded graphite from 57.3 nm to 20.5 nm at a temperature of 900 °C. The rolling pressure ranged from 0.05 MPa to 72.5 MPa. The thickness of the flexible graphite foils varied from 0.11 mm to 0.75 mm, the density—from 0.70 to 1.75 g/cm³. It was shown that with an increase in density within these limits, the compressibility of the graphite foil decreased from 65% to 9%, the recoverability increased from 5% to 60%, and the resiliency decreased from 10% to 6%, which is explained by the structural features of nanocrystallites. The properties’ anisotropy of graphite foils was studied. The tensile strength increased with increasing density from 3.0 MPa (ρ = 0.7 g/cm³) to 14.0 MPa (ρ = 1.75 g/cm³) both in the rolling direction L and across T. At the same time, the anisotropy of physical and mechanical properties increased with an increase in density along L and T to 12% with absolute values of 14.0 MPa against 12.5 MPa at a thickness of 200 μm. Expressed anisotropy was observed along L and T when studying the misorientation angles of nanocrystallites: at ρ = 0.7 g/cm³, it was from 13.4° to 14.4° (up to 5% at the same thickness); at ρ = 1.3 g/cm³—from 11.0° to 12.8° (up to 7%); at ρ = 1.75 g/cm³—from 10.9° to 12.4° (up to 11%). It was found that in graphite foils, there was an increase in the coherent scattering regions in nanocrystallites with an increase in density from 24.8 nm to 49.6 nm. The observed effect can be explained by the coagulation of nanocrystallites by enhancing the Van der Waals interaction between the surface planes of coaxial nanocrystallites, which is accompanied by an increase in microstrains. The results obtained can help discover the mechanism of deformation of porous graphite foils. The obtained results can help discover the deformation mechanism of porous graphite foils. We assume that this will help predict the material behavior under industrial operating conditions of products based flexible graphite foils.

In this work, the possibilities of introducing nitric acid molecules with a solution concentration of 75–98% into graphite matrices in the form of synthetic quasi-monocrystal graphite and natural graphite of four different farcical compositions were determined in order to identify factors of the acid concentration and graphite size on the production process and properties of graphite foil. The actual stage of graphite intercalation in the resulting compound was determined by X-ray diffraction analysis (XRD). The differences in the temporal patterns of the intercalation process for different intercalation stages (from 2 to 5) are demonstrated. The obtained acid solutions were used in the manufacturing of flexible graphite foil from natural graphite of four different particle size distributions. The mass characteristics of the intermediate and final products were determined as the graphite was treated with these solutions. The actual difference in the characteristics of the raw materials and intermediate synthetic products was recorded by measuring the electrical conductivity of the final material, graphite foil. Analysis of the results has shown that a decrease in the acid concentration of a solution leads to an increase in the intercalation stage. Weight gains due to the formation of oxygen-containing groups and the introduction of water and acid were reduced by this effect, whereas the yield of the final product (thermally expanded graphite) increased. Foil made of thermally expanded graphite obtained from intercalated compounds of high stages had greater electrical conductivity. An improvement in the conductive properties of the material implies that there should be fewer defects in its structure.

We have proposed and developed a method for measuring the thermal conductivity of highly efficient thermal conductors. The measurement method was tested on pure metals with high thermal conductivity coefficients: aluminum (99.999 wt.% Al) and copper (99.990 wt.% Cu). It was demonstrated that their thermal conductivities at a temperature of T = 22 ± 1 °C were <λ_Al> = 243 ± 3 W/m·K and <λ_Cu> = 405 ± 4 W/m·K, which was in good agreement with values reported in the literature. Artificial graphite (${\rho }_{\mathrm{G}1}$ = 1.8 g/cm³) and natural graphite (${\rho }_{\mathrm{G}2}$ = 1.7 g/cm³) were used as reference carbon materials; the measured thermal conductivities were <λ_G1> = 87 ± 1 W/m·K and <λ_G2> = 145 ± 3 W/m·K, respectively. It is well established that measuring the thermal conductivity coefficient of thin flexible graphite foils is a complex metrological task. We have proposed to manufacture a solid rectangular sample formed by alternating layers of thin graphite foils connected by layers of ultra-thin polyethylene films. Computer modelling showed that, for equal thermal conductivities of solid products made of compacted thermally exfoliated graphite and products made of a composite material consisting of 100 layers of thin graphite foil and 99 layers of polyethylene, the differences in temperature fields did not exceed 1%. The obtained result substantiates our proposed approach to measuring thermal conductivity of flexible graphite foil by creating a multi-layer composite material. The thermal conductivity coefficient of such a composite at room temperature was <λ_GF> = 184 ± 6 W/m·K, which aligns well with measurements by the laser flash method.

This study investigates the tribological properties of graphite foils (GF) with densities of 1.0, 1.3, and 1.6 g/cm³, produced from purified natural graphite of different particle sizes (40–80 μm, 160–200 μm, >500 μm). Surface roughness was measured after cold rolling and friction testing at static (0.001 mm/s) and dynamic conditions (0.1 Hz and 1 Hz). Results showed that static friction tests yielded similar roughness values (S_a ≈ 0.5–0.7 μm, S_q ≈ 0.5–1.0 μm) across all densities and particle sizes. Dynamic friction tests revealed increased roughness (S_a from 0.7 to 3.5 μm, S_q from 1.0 to 6.0–7.0 μm). Friction coefficients (µ) decreased with higher sliding speeds, ranging from 0.22 to 0.13. GF with 40–80 μm particles had the lowest friction coefficient (µ = 0.13–0.15), while 160–200 μm particles had the highest (µ = 0.15–0.22). Density changes had minimal impact on friction for the 40–80 μm fraction but reduced friction for the 160–200 μm fraction. Young’s modulus increased with density and decreased with particle size, showing values from 127–274 MPa for 40–80 μm, 104–212 MPa for 160–200 μm, and 82–184 MPa for >500 μm. The stress–strain state in the graphite foil samples was simulated under normal and tangential loads. This makes it possible to investigate the effect of the anisotropy of the material on the stress concentration inside the sample, as well as to estimate the elasticity modulus under normal compression. Structural analyses indicated greater plastic deformation in GF with 40–80 μm particles, reducing coherent-scattering region size from 28 nm to 24 nm. GF samples from 160–200 μm and >500 μm fractions showed similar changes, expanding with density increase from 18 nm to 22 nm. Misorientation angles of GF nanocrystallites decreased from 30° to 27° along the rolling direction (RD). The coherent scattering regions of GF with 40–80 μm particles increased, but no significant changes in the coherent scattering regions were observed for the 160–200 μm and >500 μm fractions during dynamic friction tests. Microstrains and residual macrostresses in GF increased with density for all fractions, expanding under higher friction-induced loads. Higher values of both stresses indicate a higher level of accumulated deformation, which appears to be an additional factor affecting the samples during friction testing. This is reflected in the correlation of the results with the roughness and friction coefficient data of the tested samples.

An automated system for measuring the thermal conductivity of functional and structural materials was developed. The main building blocks of the setup are the following: heating unit and cooling unit creating a heat flux gradient in the test sample; thermal resistances for temperature registration and control; and thermal pads for better contact between parts of the setup and the sample. The effect of the thermal conductivity of thermal pads and thermal resistances on the distribution of thermal fields in the developed setup was studied by computer modelling. A control software for the measuring setup was developed based on the hardware implementation of the steady-state Fourier’s law-based method for the determination of thermal conductivity. The stopping criterion for the setup control software is the equality of heat fluxes in the heating and cooling units, as well as the stability of the thermal conductivity coefficient readings. The testing and calibration of the device were carried out using a sample of pure aluminum (99.999 wt.% Al). It was found that the experimental value of the thermal conductivity coefficient of the aluminum sample at room temperature (T = 22 °C) is <λ> = 243 ± 3 W/m·K. This value of the thermal conductivity coefficient is consistent with the literature data and experimental values obtained by the laser flash method, which ranges within λ = 210–260 W/m·K.