Rachana Kumar

Improved microwave absorption in lightweight resin-based carbon foam by decorating with magnetic and dielectric nanoparticles

Carbon foams (CFoams) are sponge-like high performance lightweight engineering materials that possess excellent electrical and mechanical properties as well as thermal stability. CFoams possess bulk density in the range from 0.30 to 0.40 g cm−3 and open porosity of more than 70%. The CFoam consists of pore walls, i.e., ligaments, which are responsible for the conduction path and hence the electrical conductivity due to mobile charge carrier (delocalized π electron), are interconnected to each other. The high value of electrical conductivity causes the CFoam to act as an electromagnetic radiation reflector rather than an absorber; however, in certain applications, shielding materials must be able to absorb the maximum electromagnetic radiation. Therefore, to improve the absorptivity of electromagnetic radiation in lightweight CFoams, the CFoams are decorated by Fe3O4 and ZnO nanoparticles. It is observed that coating with Fe3O4 and Fe3O4–ZnO nanoparticles not only improved the absorption losses but also enhanced the compressive strength of CFoam by 100%. This modified CFoam demonstrated excellent shielding response in the frequency range from 8.2 to 12.4 GHz, in which the total shielding effectiveness (SE) was dominated by absorption losses. The total SE is −45.7 and −48.5 dB of Fe3O4 and Fe3O4–ZnO-coated CFoam, respectively, and it is governed by absorption losses of −34.3 dB and −41.5 dB, respectively. Moreover, the absorption losses increased by 236% in Fe3O4-coated CFoam and 281% in Fe3O4–ZnO-coated CFoam without much enhancement in the bulk density. This is due to the high level of magnetic and dielectric losses of nanoparticles with high surface area. Note that the absorption losses are 80% higher than any value reported for CFoam; thus, lightweight CFoam decorated with magnetic and dielectric nanoparticles is an excellent material for stealth technology.

Bulk synthesis of highly conducting graphene oxide with long range ordering

Graphene oxide with high conductivity is todays demand not only for high quality graphene synthesis but also for direct applications in electronic devices. Here we demonstrate a milder bulk synthesis approach for graphene oxide (mGO) from tattered graphite showing long range ordering and much higher conductivity (27 S m−1) compared to Hummers graphene oxide (H-GO) (0.8 S m−1). A two step mild oxidation process is adapted instead of excessive oxidation of graphite based on Hummers method which creates permanent defects in carbon sheets. This work demonstrates the mild oxidation process for highly conducting GO preparation without use of NaNO3 inhibiting the evolution of toxic gases and also possessing bulk synthesis possibilities.

Stable graphite exfoliation by fullerenol intercalation via aqueous route

Graphene is a wonder material possessing unique properties; however, graphene prepared by exfoliation of graphite has property to restack because of van der Waals interactions to form graphite. This restacking can be prevented by insertion of large molecules like fullerene, which not only exfoliates graphite layer but also prevents restacking of prepared graphene sheets. The present article also describes a mild method of graphite oxide synthesis (GO) for lower degree of oxidation resulting in less defected (ruptured carbon framework) graphene sheets. Exfoliation is performed by intercalation of large fullerene molecules by aqueous reaction of fullerene hydroxide (fullerenol) with the epoxy functionalities on graphite oxide to prepare fullerene intercalated graphite (G-Fol). Fullerene functionalization of GO to form G-Fol has been established by FTIR spectroscopy, UV-Vis spectroscopy, TGA and number of layers has been ascertained by Raman spectroscopy, XRD and HRTEM. Stable exfoliation of G-Fol has been confirmed by change in absorbance with time. Photoluminescence property of the material is also evaluated by fluorescence emission and excitation measurement at different excitation and emission wavelengths, respectively. The present article explains a new method of exfoliation of graphite to form stable functionalized graphene layers with fewer defects for future applications as buffer layer in electronic devices.

Synthesis and ultrafast spectroscopic study of new [6,6]methanofullerenes

Ultrafast transient absorption and terahertz spectroscopic studies have been performed on new [6,6]methanofullerenes synthesized by the reaction of diazomethane (generated in situ from their hydrazone precursor) with fullerene[60] via our eco-friendly methodology, i.e., amine-assisted 1,3 dipolar cycloaddition (AACA). Synthesized materials have been characterized by different spectroscopic techniques for their structure establishment, including terahertz spectroscopy to study changes in spectra on functionalization. Attachment of different types of functional groups on exohedral chains resulted in band gap tuning. Photoinduced charge generation and charge separation studies have been performed to understand the charge carrier dynamics in a methanofullerene : P3HT mixture. Efficient charge separation efficiency is observed in both the acceptors on mixing with P3HT, making them potential acceptor materials in organic solar cells. High photoconductivity calculated by terahertz time domain spectroscopy of these new fullerene derivatives can also be exploited in other organic electronic devices.

A cost effective and eco-friendly one-pot process for PC61BM synthesis under aerobic conditions

Here we demonstrate a cost effective and eco-friendly process for one-pot synthesis of PC61BM under aerobic conditions where, the key step of diazomethane intermediate preparation is modified. Instead of using pyridine and sodium methoxide under inert atmosphere, we used triethyl amine and dichloromethane under aerobic conditions. This process is envisaged as a green chemistry and will open channels for the large scale synthesis of PC61BM and its derivatives for solar cells applications without bothering for controlled environment conditions.

Synthesis and charge transport properties of new methanofullerenes

New methanofullerene derivatives have been synthesized and characterized for their electron transport properties. Two new methanofullerenes (PC61BP & HC61NP) have been synthesized by using our reported amine assisted cycloaddition reaction (AACA) method in good yield. PC61BM is also synthesized by using the AACA method for comparison of properties. PC61BP has phenyl group and butyric acid pentyl ester substituents on the C(61) bridging carbon of the cyclopropane ring on the fullerene. HC61NP has a completely different structure where the C(61) bridging carbon is bonded to a proton and a 2-nitro cinnamyl group. Both the products have been well characterized by FTIR, UV-vis absorption spectroscopy and NMR for their structures. Their electrochemical properties have been evaluated by cyclic voltammetry. Finally, the electron mobilities of the materials have been calculated at room temperature by preparing electron-only devices. PC61BP shows the highest mobility compared to HC61NP and PC61BM and proves to be a promising acceptor material for organic solar cells.

Synthesis and comparative charge transfer studies in porphyrin–fullerene dyads: mode of attachment effect

Fullerene–porphyrin dyad system has gained increasing interest because of not only their interesting optoelectronic properties but also their applications in single-material organic solar cells. In the present work, two non-metallated fullerene–porphyrin dyads are prepared, viz, H2P1-C60 (dyad I) and H2P2-C60 (dyad II), where the modes of attachment on fullerene are different, i.e. via 1,3-dipolar cyclopropanation reaction in dyad I and Prato reaction in dyad II. We compare the photophysical and thermally activated conducting properties of these two non-metallated dyads in solution as well as in film to establish the better mode of attachment for dyad preparation with exotic properties. Both the dyads are structurally characterized by FT-IR, NMR and HRMS. Photophysical studies reveal that for dyad II, the reaction rate of charge separation is much higher than for dyad I (0.62 × 109 s−1vs. 0.32 × 109 s−1) in nonpolar solvent. Moreover, dyad II shows higher quenching of porphyrin emission than dyad I (90% vs. 70%) due to faster deactivation of singlet excited state by photoinduced charge transfer between porphyrin and fullerene. The reaction rate for charge transfer (kET(CS)) for dyad II is found to be maximum with lowest activation energy value as compared to dyad I. Excited-state electronic interactions in solution as well as in film have been studied by transient absorption spectroscopy to identify charge separated states. Finally, both the dyads are tested for their thermally activated electrical conductivity where dyad II shows an increase in conductivity with increase in temperature, which is reversible. Thus, dyad II is found to be an excellent material with interesting photophysical and electronic properties that can be beneficial in various applications such as organic solar cells and sensors.

Magnetocaloric effect and refrigeration cooling power in amorphous Gd7Ru3 alloys

In this paper, we report the magnetic, heat capacity and magneto-caloric effect (MCE) of amorphous Gd7Ru3 compound. Both, temperature dependent magnetization and heat capacity data reveals that two transitions at 58 K and 34 K. MCE has been calculated in terms of isothermal entropy change (ΔSM) and adiabatic temperature change (ΔTad) using the heat capacity data in different fields. The maximum values of ΔSM and ΔTad are 21 Jmol−1K−1 and 5 K respectively, for field change of 50 kOe whereas relative cooling power (RCP) is ∼735 J/kg for the same field change.

Unusual Magnetism In TbRu2Ge2 Compound

Magnetic properties of TbRu2Ge2 were studied. TbRu2Ge2 shows unusual magnetism, i.e. at low field it shows frustration (like spin glass) and at high field, this frustration starts to disappear. It has been found that TbRu2Ge2 has a TN of 37K. To confirm the frustration in TbRu2Ge2, AC susceptibility and normalized magnetization calculations were also performed. Copyright

Pressure dependent magnetic, AC susceptibility and electrical properties of Nd7Pd3

The effects of pressure on the magnetization and isothermal entropy change, AC susceptibility, and resistivity for Nd7Pd3 have been studied. Nd7Pd3 shows three magnetic transitions Tt (=15 K), TC (=34 K), and TN (=38 K) at a pressure of zero bar. TC and TN are very sensitive to the pressure and rates of change of TC and TN with respect to pressure, i.e., dTC/dp and dTN/dp are −1.906 K kbar−1 and −0.313 K kbar−1 respectively, whereas Tt is insensitive to pressure change. It has also been observed that the isothermal entropy change is also sensitive to pressure. Below 15 K, an indication of spin glass nature in the AC susceptibility data is seen. The time dependent magnetization at low temperatures also supports the spin glass nature, but at high temperatures it indicates a complex nature. The electrical resistivity data has been fitted to the relation ρ = ρ0 + AT2 in the temperature range 12–30 K and ρ = ρ0 + BT − CT3 in the temperature range 40–220 K. The M–H isotherm confirms the AFM nature of Nd7Pd3, which is confirmed further by the Arrott’s plot.