Microwave absorptive behavior of ZnCo-substituted W-type Ba hexaferrite nanocrystalline composite material

Nanoparticle-assisted microwave absorption by single-wall carbon nanotubes

“…It has recently been reported that Ba hexaferrite crystallites in a rubber matrix absorb significantly more microwave radiation when the crystallites are nanometer-size instead of micron size. 10 This effect is attributed to an increase in interfacial polarization as the crystallite size decreases due to an increase in the number of surface atoms with unsaturated bonds. 10 For unpurified HiPco SWNTs, interfacial dipole absorption would be greater for the Fe catalyst nanoparticles imbedded in carbon shells than for the coalesced crytallites shown in Fig.…”

“…10 This effect is attributed to an increase in interfacial polarization as the crystallite size decreases due to an increase in the number of surface atoms with unsaturated bonds. 10 For unpurified HiPco SWNTs, interfacial dipole absorption would be greater for the Fe catalyst nanoparticles imbedded in carbon shells than for the coalesced crytallites shown in Fig. 2͑b͒ since the former are smaller and have greater interfacial polarization effects.…”

“…In Ref. 10, it was also speculated that enhanced absorption in nanocrystals may be due to their quantum size which results in an increase in their energy level spacing. If the energy states lie in the range of the microwave energies, there could be microwave absorption due to electron transitions between the states.…”

“…Examples of such structures include Ba hexaferrite nanocrystals 10 and micron-size ferrite powders 11,12 in a rubber matrix, Fe-Si-Al alloy flakespolymer composites, 13 and CaCoTi ferrite-epoxy composites. 14 Microwave absorption in these materials is thought to be mainly due to magnetic resonance and interfacial electric polarization.…”

“…In addition, it is believed that since ferromagnetic nanoparticles have a single magnetic domain structure and higher coercive force, this may lead to hysteresis related losses. 10 In summary, we have studied the effects of microwaves on unpurified and purified HiPco SWNTs and carbon black in an UHV. We find that unpurified HiPco SWNTs heat up to significantly higher temperatures than purified SWNTs and carbon black.…”