this value depends a lot on the structure of the graphite, and there is no simple answer for it.  In graphite composites with graphite fibers, the lateral k can easily be several times copper.

you can look at the following paper and references herein:

A. A. Balandin "Thermal properties of graphene and nanostrctured carbon materials, Nature Materials" 10, 569 (2011) DOI: 10.1038/NMAT3064

Theoretical thermal conductivity of graphite crystal at 25 ºC; for ab directions: 398 W/m·K; for the c direction 2.2 W/m·K. Ref.: Hugh O. Pierson, Handbook of Carbon, Graphite, Diamond and Fullerenes ─ Properties, Processing and Applications, Noyes Publications, New Jersey, 1994, p. 54.

I found the value of 168 W/mK for thermal conductivity of Carbon.

The thermal conductivity of graphite depends on the anisotropy of the layers but generally it varies from 25-470 W/Mk.

Dear Kanishka Kobbekaduwa,

The thermal conductivity of graphite depends on the anisotropy of the layers but generally it varies from 25-470 W/Mk.Generally it is “GraphiteTIM(Compressible Type) with thermal conductivity : X-Y direction 390-400W/m∙K, and in Z direction (28W/m∙K).

Why this is different?

However, controlling the orientation of GFs still remains a challenge. The corresponding thermal conductivity is as high as the thermal expansion will be less.

Reason Property

Because of the weak binding and the large lattice spacing in the c-direction, the phonon spectrum of graphite is approximately two-dimensional for frequencies above f c = 4 THz. Thus one can model the a-plane thermal conductivity by a two-dimensional phonon gas. That’s why in XY plane conductivity value is high and it is anisotropic when we compare with Z plane.

Hope it is helpful for you.

Ashish

you can look at the following paper:

A numerical and experimental investigation on the performance of a low-flux direct absorption solar collector (DASC) using graphite, magnetite and silver nanofluids

TB Gorji, AA Ranjbar - Solar Energy, 2016 - Elsevier