Nanocomposites and nanomaterials
Metal-carbon nanocomposites for electrodes of low-temperature thermionic energy converters
Ye. Len, Ye. Tsapko, I. Sydorchenko, V. Koda, I. Galstian, O. Gerasymov
G. V. Kurdyumov Institute for Metal Physics of the N.A.S. of Ukraine
36 Academician Vernadsky Boulevard, UA-03142 Kyiv, Ukraine 
E-mail: stenforti@ukr.net

The new composite of Ti and thermally expanded graphite (Ti–TEG) was obtained. Two unique features of this Ti-TEG composite were observed at 3–5 times lower temperatures (170–350°С) in comparison with traditional refractory metal thermionic energy converters (TEC): voltage and direct current. Of special note is observation of direct current without application of additional external potential difference. Actually this fact is a confirmation of possibility to create “cold” cathodes for TEC.
TEC working forevacuum chamber of the original design was created. It was shown that the working medium contains not only a sample of Ti–TEG composite, but also a residual gas. Moreover the voltage and emission current are very sensitive to this gas pressure.
It was revealed the change in the surface morphology of the composite sample under the action of concentrated solar radiation and the considerable temperatures caused by it. Temperatures of 270–310°C are critical for initiating the structural rearrangement of the surface of composite sample. After the second treatment by sun exposure, the sample’s surface became more developed due to both the TEG release from some small gaps between Ti grains and the formation of new carbon nanostructures on the grains. These nanostructures have the form of separate cylindrical buildups with diameters of 20–80 nm, which can provide (during the further operation of the cathode) a significant increase in the contribution from field emission based on the quantum-mechanical tunneling.
Electron microscopic studies of obtained Te–TEG samples showed (Fig. 1) that near nanostructured carbon plates with linear dimensions of about 20 nm are present metal particles with sizes from 100 nm to 1 µm. These plates fill the pores and cracks in the metal matrix and are interconnected either directly or through metal bridges.

Fig. 1. Different-scale SEM images of the central part of the Ti–TEG composite sample, which was most exposed to concentrated solar radiation.