1, St. Petersburg State Chemical Pharmaceutical Academy, St. Petersburg, , Russian Federation
3, St Petersburg National Research Academic University of the Russian Academy of Sciences, St. Petersburg, , Russian Federation
We propose a model of a thermoelectric generator (TEG) based on composite material obtained by sintering of diamond nanoparticles . The effect of electrons drag by ballistic phonons is used to increase the useful conversion of heat into electric current. The effect of the thermal resistance of the boundaries between the graphite-like and diamond-like phases of the composite is used to reduce the ineffective heat spread. It is predicted, that such TEG can possess a record value of thermoelectric parameter, ZT, up to 3,5 
We calculate an optimal thickness of sp2 layers that occurs between diamond nanoparticles during the process of sintering (Fig. 1). The thicker are sp2 layers, the larger is conductivity of the composite and the smaller is thermal conductivity, which is good for thermoelectric conversion. However, if sp2 layers are thicker than a phonon mean free pass, the effect of electrons drag by ballistic phonons is reduced, the electrons drag by phonons comes to a diffusive regime and thermoelectric parameter drops drastically. We estimate the phonon mean free path in the sp2 region. It turns out to be approximately 5 nm. Then we deduced that optimal thickness of sp2 layers l is l = (L/2) cos(π/6), where L is an initial mean size of nanodiamonds. Hence, the optimal thickness of sp2 layers is l ≈ 1.1 nm.
We calculate thermal resistance of the composite with optimal structure taking into account thermal resistance of the boundaries between the graphite-like and diamond-like phases. The thermal resistance of such boundaries arises because electrons transferring heat in the metal do not transfer it through the interface, but are involved in the heat transfer only at a certain distance from it . The thermal resistance of the boundaries crucially restricts thermal conductivity of such composites thus increasing thermoelectric parameter.
An existence of an optimal volume ratio between graphite-like and diamond-like phases of the composite is predicted and obtained experimentally. The maximum value of the thermoelectric coefficient exceeds its minimum values of 5 μV/K for graphite by 20 times – but not by 1000 times as it is expected. Probably, this is explained by a failure in creating the TEG of the optimal design.
The authors are grateful to A.Ya. Vul’ for his attention to this work. A.P. Meilakhs and E.D. Eidelman are grateful to the RSF (Project 16-19-00075) for support. B.V. Semak is grateful to the Russian Foundation for Basic Research (Project 16-03-01084a).
 Eidelman E. D., Meilakhs A.P., Semak B.V., Shakhov F.M. Journal of Physics D: Applied Physics, In Press.
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