Investigation of laser limiters based on disperse and composite nanomaterials

Abstract

Work is devoted to the search new materials of laser irradiation limiter. Were created nanomaterials (dispersion and composites) for limiters based on multiwalled and singlewalled carbon nanotubes. Were carried out studies of the structure of nanomaterials by atomic force microscopy. Investigated the thermal and photo stability of the samples. Measured and calculated nonlinear optical properties of nanomaterials by direct scanning and Zscan. A comparison of theoretical and practical nonlinear optical characteristics. And also investigated the electrical conductivity of liquid and solid nanomaterials. Keywords—laser; limiter; nanomaterials, dispers; composites; carbon nanotubes; transmittance The search for new materials for laser limiters now is carrying out among of nanoparticles (NP) with strong nonlinear behavior. The NP could be either fullerenes and graphenes or carbon nanotubes (CNT). We have developed the PC program for modeling laser irradiation transferring through nanocomposites (NC) and nanodispersive (ND) media. We have investigated nanocomposites and nanodispersions based on metallic or semiconducting single-walled CNT (SWNT). As shows atomic force microscopy investigation the most of nanotubes are present in clusters of separated free fibrils. This is the evidence of high nanotube dispersion. In the case of ND with multi-walled CNT (MWNT) there is a presence of the same features on the surface with typical lateral sizes from tenth of micron up to microns and height from nanometers to hundreds of nanometers. Besides the NPs with height from 10 to 100 nm are present in the form of agglomerates and separate ball-like structures. The typical need-like structures was not observed. The average diameter of NP was in 10-30 nm range. The features exceeding this value were agglomerates of NP. The spectral characteristics of MWNT ND were stable under the heating up to 40 °C during about 85 hours. The room temperature stability was as long as 940 hours. The NDs are also stable under LED irradiation at the wave length 365, 630 and 810 nm and power 30-80 mW during 50-65 hours. We used solid state Nd:YAG laser LS-2132UTF (1-3 harmonics, single pulse energy – up to 200 mJ, single pulse length 7 ns, frequency of pulse repeating 15 Hz, power density of laser pulse up to 450 MW/sm 2 ). Nonlinear characteristics of nanomaterials was measured with making use of straight nonlinear transmittance method. The sample was placed in the convergent beam of laser irradiation with using positive spherical lens F = 500 mm. The laser irradiation intensity was measured with using polarized Glan’s lens. Linear transmittance T0 of ND based on tetrahydrofuran (THF) was 55 -70 %, while attenuation coefficient (KO) of laser irradiation was ~ 13 for carboxyl SWNT and ~ 9 for MWNT and 13-14 for SWNT functionalized by zinc phthalocyanine derivatives. According to the results of the Z-scan, the beam radius in the waist 20-25 nm. The nonlinear absorption coefficient β is equal to 30-35 cm/GW in CNT dispersions in DMF and 45-50 in THF dispersions. A role to play here can distinguish the boiling point of liquids: 153 C for DMF and 65,6 C for THF. Probability shock boiling determines the scattering intensity laser irradiation. So nonlinearity dispersion medium of THF increase. Optically uniform NC based on MWNT was prepared during polymerization and mechanical stirring and ultrasound treatment of ND in THF. After that the benzoyl peroxide promoter was added into the solution with keeping out at 45 С for 24 hours. To improve NC quality we added THF modifier. The irradiation wave length was 532 nm for NC based on polymethyl methacrylate with carboxyl SWNT and KO was ~ 8 at T0 ~ 60 %. The nonlinear absorption coefficient β is equal to 0.5-1.6 cm/GW in CNT dispersions in polymethyl methacrylate. The specific conductivity of dried NC layers based on MWNT is several orders higher then for initial NDs. The current-voltage characteristics for these layers remain almost linear while they are not linear for ND. The conductivity of NCs layers grows several times after annealing. The highest conductivity was for thin layers (less than 5 μm). There are number of reasons for high conductivity for CNT-based NCs: high aspect ratio of nanotubes and optimal composition of nanomaterials that consist of isolating matrix and conductive additive (CNT). Such composition of nanomaterials enables effective evaporation and burning out of polymer organics during annealing. The residue consisting of conductive graphite form between nanotubes increases the layer conductance. Finally annealing leads to good binding between matrix and CNT as well as between CNTs due to strong matrix carbonization. This work was provided by the Foundation of the President of the Russian Federation for state support of young Russian scientists (Grants SP-2477.2012.4).