Compression behavior of natural graphite sheet

Abstract

The compression behavior of natural graphite sheet (NGS) is studied during the forming process and during the compression of finished sheets. The forming process, in which the low-density semi-finished NGS is compressed to higher densities, is quantified by the proposed relationship p f = 0.35e 2.6d where d is the free-standing density in g cm −3 and p f is the forming pressure in MPa. The maximum achievable free-standing density is in the range 1.73-1.89 g cm −3. The compression behavior of finished NGS is non-linear, hysteretic, and density-dependent with the maximum strain of 6% at 0.55 g cm −3. The tangent compression modulus increases with the density and ranges from 20 to 220 MPa. A compact relationship for evaluating the strain at a given pressure and density is provided. NGS deforms viscously during the periods of constant pressure; the deformation is significant during the forming process but small during the compression of finished sheets. Keywords Natural graphite sheet • Flexible graphite • Compressed exfoliated natural flake graphite • Forming • Uniaxial compression • Tangent modulus • Viscoelastic behavior • Barreling List of symbols v Viscous strain d Density p Pressure p f Forming pressure t f Thickness after the forming cycle t n Thickness after the n-th cycle t v,0 Thickness at the beginning of the constant pressure period t v,end Thickness at the end of the constant pressure period * Majid Bahrami,

Material properties and structure of natural graphite sheet

“…The forming process is not covered in this publication, however, a brief summary is provided in Appendix A. The reader is referred to other publication 15 for details. Alongside the detailed description of the measurements and results further in this section, a simplified graphical summary of the measured material properties with explicit formulas of best fits is provided in Appendix B.…”

“…FEI Nova NanoSEM microscope was used for imaging. The compression behavior measurements were reported elsewhere 15 . Details about the measurement methods, sample preparation, and uncertainty analysis were addressed at length in the supplementary document.…”

“…Our measurements show a very low and negative coefficient of thermal expansion (CTE) in the in–plane direction ( to ) while in the through-plane direction it is positive and ranges from to . Coupling the results with our study of the compression behaviour (outlined here and detailed elswhere 15 ) allows for a quantitative analysis of thermomechanical stresses in devices such as fuel cells.…”

“…During subsequent rapid heating, the vaporized compound forces the graphite layers apart, forming exfoliated natural graphite (ENG) particles also known as worms 13 . NGS at densities 0.5 to is formed by compressing ENG particles at pressures ranging from 1 to 31 MPa 6 , 14 , 15 .…”

“…Compressing NGS causes reversible and irreversible structural changes that reflect in the overall properties. The irreversible changes happen during the forming process, and the reversible ones during the low-pressure loading of a finished sheet 15 .…”