Nanocomposites and nanomaterials

Influence of MWCNT in polytetrafluoroethylene on the parameters of electronic structure and absorption of UHF radiation
Galstian I.Ye.1, Tsapko Ye.A.1, Makarenko O.V.2, Yampolskiy A.L.2, Len Ye.G.1
1 G. V. Kurdyumov Institute for Metal Physics, N.A.S. of Ukraine, 
 36 Academician Vernadsky Boulevard, UA-03142 Kyiv, Ukraine. 
E-mail: stenforti@ukr.net
2 Taras Shevchenko National University of Kyiv, 
 60 Volodymyrska Str., UA-01033 Kyiv, Ukraine

Using the methods of angular correlation of annihilation radiation (ACAR), attenuation of electromagnetic radiation (ER) in 1.5–2 2 GHz frequency range, and optical ellipsometry, it was shown that a 2% decrease in the probability of annihilation of positrons in pores of free volume leads to changes in the parameters of electronic structure of composites by 15–25% in composites of polytetrafluoroethylene+ multiwalled carbon nanotubes (PTFE+MWCNT). PTFE is transparent to electromagnetic radiation, but after the addition of 10 wt% MWCNTs, the composite demonstrates a more than 400-fold decrease in the ER intensity when the radiation passes through a specimen with a thickness of ≈2 mm.
It was found that the average radius of the free volume pores and the probability of annihilation of positrons in this area are determined by the electronic structure parameters of the polymer matrix only. The Fermi angle and the probability of positrons annihilation with free electrons are determined by the electronic structure parameters of MWCNTs only. Since the electronic structure of the cores and defects of the polymer matrix (at least outside the interphase) does not change, changes in these characteristics are mainly due to changes in the MWCNTs electronic structure. It was found that in a specimen with 10 wt% MWCNTs, the highest density of free electrons is observed, and the highest electron density is observed on defects. The average of free volume pore radius reaches its maximum value at 10 wt% MWCNTs in the composite.
The absorption of ER increases with an increase in the free electrons density in MWCNTs. The maximum absorption of ER is observed at a concentration of MWCNTs equal to about 10 wt%. When the composition of the composite approaches pure MWCNTs, the absorption again significantly decreases (by more than an order of magnitude). The colossal ER absorption effect (>400 times) is due to the fine structure of the π-electrons density of states in MWCNTs near the Fermi level. In MWCNTs there is a rather high concentration of itinerant π-electrons and, at the same time, almost zero density of their states at the Fermi level. There are almost no free states above the Fermi level, and π-electrons hardly participate in the processes of small excitations. Due to the transfer of charges from the matrix to MWCNTs (or vice versa), the position of the Fermi level in composites changes and allowed states appear in the density of states immediately above it. As evidenced by the spectral dependences of the refractive indices and absorption coefficient, this leads to an almost linear increase in absorption in the entire investigated wavelength range. Moreover, the same almost 100% absorption should presumably be observed in the infrared range of radiation.