Shital Patangrao Pawar

Electromagnetic interference shielding through MWNT grafted Fe3O4 nanoparticles in PC/SAN blends

In this study, multiwall carbon nanotubes (MWNTs) were chemically grafted onto dopamine anchored iron oxide (Fe3O4) nanoparticles via diazotization reaction to design electromagnetic (EM) shielding materials based on PC (polycarbonate)/SAN poly (styrene-co-acrylonitrile)] blends. A two step mixing protocol was adopted to selectively localize the nanoparticles in a given phase of the blends. In the first step, MWNT-g-Fe3O4 nanoparticles were solution blended with PC, followed by dilution with SAN during melt mixing in the subsequent step. This strategy, besides improving the quality of dispersion of MWNTs in the blends, facilitated enhanced EM interference shielding effectiveness (SE). Both, the MWNTs and the modified MWNTs, selectively localized in the PC phase and led to high electrical conductivity, in striking contrast to PC filled MWNT composites. The SE was measured on toroidal samples over a broad range of frequencies; X-band (8.2-12 GHz) and K-u-band (12-18 GHz). It was observed that the shielding mechanism mostly involved reflection in the blends with MWNTs, whereas absorption dominated in the case of blends with MWNT-g-Fe3O4. To realize the efficacy of this strategy, a few compositions were prepared by physical mixing MWNTs with Fe3O4 nanoparticles. Intriguingly, blends with MWNT-g-Fe3O4 nanoparticles manifested enhanced microwave absorption over physically mixed nanoparticles. An SE of -32.5 dB was observed (at 18 GHz) for MWNT (3 wt%)-g-Fe3O4 (3 vol%) in PC/SAN blends.

Assessing the critical concentration of NH2 terminal groups on the surface of MWNTs towards chain scission of PC in PC/SAN blends: effect on dispersion, electrical conductivity and EMI shielding

The localization and dispersion quality of as received NH2 terminated multiwall carbon nanotubes (MWNT-I) and ethylene diamine (EDA) functionalized MWNTs in melt mixed blends of polycarbonate (PC) and poly(styrene-co-acrylonitrile) (SAN) were assessed in this study using rheo-electrical and electromagnetic interference (EMI) shielding measurements. In order to improve the dispersion quality and also to selectively localize MWNTs in the PC phase of the blends, EDA was grafted onto MWNTs by two different strategies like diazonium reaction of the para-substituted benzene ring of MWNTs with EDA (referred to as MWNT-II) and acylation of carboxyl functionalized MWNTs with thionyl chloride (referred to as MWNT-III). By this approach we could systematically vary the concentration of NH2 functional groups on the surface of MWNTs at a fixed concentration (1 wt%) in PC/SAN blends. XPS was carried to evaluate the % concentration of N in different MWNTs and was observed to be highest for MWNT-III manifesting in a large surface coverage of EDA on the surface of MWNTs. Viscoelastic properties and melt electrical conductivities were measured to assess the dispersion quality of MWNTs using a rheo-electrical set-up both in the quiescent as well as under steady shear conditions. Rheological properties revealed chain scission of PC in the presence of MWNT-III which is due to specific interactions between EDA and PC leading to smaller PC grafts on the surface of MWNTs. The observed viscoelastic properties in the blends were further correlated with the phase morphologies under quiescent and annealed conditions. Electromagnetic interference (EMI) shielding effectiveness in X and Ku-band frequencies were measured to explore these composites for EMI shielding applications. Interestingly, MWNT-II showed the highest electrical conductivity and EMI shielding in the blends.

Engineering Nanostructures by Decorating Magnetic Nanoparticles onto Graphene Oxide Sheets to Shield Electromagnetic Radiations

In this study, a minimum reflection loss of -70 dB was achieved for a 6 mm thick shield (at 17.1 GHz frequency) employing a unique approach. This was accomplished by engineering nanostructures through decoration of magnetic nanoparticles (nickel, Ni) onto graphene oxide (GO) sheets. Enhanced electromagnetic (EM) shielding was derived by selectively localizing the nanoscopic particles in a specific phase of polyethylene (PE)/poly(ethylene oxide) (PEO) blends. By introduction of a conducting inclusion (like multiwall carbon nanotubes, MWNTs) together with the engineered nanostructures (nickel-decorated GO, GO-Ni), the shielding efficiency can be enhanced significantly in contrast to physically mixing the particles in the blends. For instance, the composites showed a shielding efficiency >25 dB for a combination of MWNTs (3 wt %) and Ni nanoparticles (52 wt %) in PE/PEO blends. However, similar shielding effectiveness could be achieved for a combination of MWNTs (3 wt %) and 10 vol % of GO-Ni where in the effective concentration of Ni was only 19 wt %. The GO-Ni sheets facilitated in an efficient charge transfer as manifested from high electrical conductivity in the blends besides enhancing the permeability in the blends. It is envisioned that GO is simultaneously reduced in the process of synthesizing GO-Ni, and this facilitated in efficient charge transfer between the neighboring CNTs. More interestingly, the blends with MWNTs/GO-Ni attenuated the incoming EM radiation mostly by absorption. This study opens new avenues in designing polyolefin-based lightweight shielding materials by engineering nanostructures for numerous applications.

Exceptional microwave absorption in soft polymeric nanocomposites facilitated by engineered nanostructures

In this study, we present soft nanocomposites consisting of polycarbonate (PC) and poly styrene-co-acrylonitrile (SAN) that exhibit exceptional microwave absorption (ca. 91.1%) with a high attenuation constant. The blends containing multiwalled carbon nanotubes (MWNTs) and nickel nanoparticles nucleated on partially reduced graphene sheets (rGO–Ni) showed a high shielding effectiveness of −48 dB at 18 GHz. The ultra-small nickel nanoparticles, with average diameter of 5–8 nm, were uniformly decorated on the surface of rGO, providing pathways to disperse ferromagnetic nanoparticles in soft nanocomposites, which otherwise often suffer secondary agglomeration during processing. While synthesizing rGO–Ni, graphene oxide (GO) sheets were effectively reduced, manifesting enhanced electrical conductivity and effective charge transport in the nanocomposites. This was also facilitated by the uneven distribution of nanoparticles in the bi-phasic blend, thereby offering heterogeneous dielectric media and more interfaces that result in multiple scattering within the nanostructures. The underlying mechanism of attenuation, with the help of complex microwave properties, total losses, skin depth and attenuation constant in a broad frequency range (8–18 GHz) is systematically discussed. In addition, blends containing engineered nanostructures showed 1.5-fold higher storage modulus, compared to neat blends, as inferred from the dynamic mechanical thermal analysis (DMTA). Therefore, this study demonstrates an effective way to develop lightweight, soft and high performance microwave absorbers at relatively smaller fractions of nanoparticles for applications such as EMI shielding enclosures.

High performance electromagnetic wave absorbers derived from PC/SAN blends containing multiwall carbon nanotubes and Fe3O4 decorated onto graphene oxide sheets

A high performance electromagnetic wave absorber with high surface resistivity (i.e. minimum surface reflection) and an enhanced attenuation constant (i.e. maximum absorption) was designed using uneven distribution of lossy materials like multiwall carbon nanotubes (MWNTs) and magnetic ferrite (Fe3O4) nanoparticles (of 2–5 nm) nucleated on reduced graphene oxide sheets (rGO–Fe3O4) in biphasic polymeric blends of polycarbonate (PC) and poly(styrene-co-acrylonitrile) (SAN). The uneven distribution of MWNTs and rGO–Fe3O4 in the blends, which acted as micro-absorbers resulted in outstanding electromagnetic wave absorption (93%). Various parameters like transmission coefficient, attenuation constant and skin depth were assessed for a clear understanding of the attenuation mechanism in each of the blends containing different nano-inclusions like MWNTs and Fe3O4, only rGO–Fe3O4 and a combination of MWNTs and rGO–Fe3O4. It is understood that the absorption was significantly enhanced due to selective localization of nano-inclusions in the PC phase that provided heterogeneous dielectric media with multiple interfaces. The penetration of the high frequency electromagnetic waves into the shield was facilitated by high surface resistivity and high volume electrical conductivity (5 S cm−1). Further, in combination with lossy nano-inclusions the incoming EM radiation was attenuated through multiple scattering. This is realized in the blends containing both MWNTs and rGO–Fe3O4 which manifested in a total shielding effectiveness of −50.7 dB (at 18 GHz), indicating >99.9% attenuation of the incoming microwave radiation, in striking contrast to blends containing either only MWNTs or only rGO–Fe3O4. Taken together, this study clearly demonstrates that lightweight polymeric nanocomposites can be designed for high frequency electromagnetic wave absorption through unique synergism of MWNTs and ultra-small magnetic ferrite nanoparticles nucleated on rGO sheets.

Enzymatically degradable EMI shielding materials derived from PCL based nanocomposites

In this study, two different types of multiwall carbon nanotubes (MWNTs) namely pristine (p-MWNTs) and amine functionalized (a-MWNTs) were melt-mixed with polycaprolactone (PCL) to develop biodegradable electromagnetic interference (EMI) shielding materials. The bulk electrical conductivity of the nanocomposites was assessed using broadband dielectric spectroscopy and the structural properties were evaluated using dynamic mechanical thermal analysis (DMTA). Both the electrical conductivity and the structural properties improved after the addition of MWNTs and were observed to be proportional to the increasing fractions in the nanocomposites. The shielding effectiveness of the nanocomposites was studied using a vector network analyzer (VNA) in a broad range of frequencies, X-band (8 to 12 GHz) and K-u-band (12 to 18 GHz) on toroidal samples. The shielding effectiveness significantly improved on addition of MWNTs, more in the case of p-MWNTs than in a-MWNTs. For instance, at a given fraction of MWNTs (3 wt%), PCL with p-MWNTs and a-MWNTs showed a shielding effectiveness of -32 dB and -29 dB, respectively. Moreover, it was observed that reflection was the primary mechanism of shielding at lower fractions of MWNTs, while absorption dominated at higher fractions in the composites. As one of the rationales of this work was to develop biodegradable EMI shielding materials to address the challenges concerning electronic waste, the effect of different MWNTs on the biodegradability of PCL composites was assessed through enzymatic degradation. The enzymatic degradation of the samples cut from the hot pressed films by bacterial lipase was investigated. It was noted that a-MWNTs exhibited almost similar degradation rate as the control PCL sample; however, p-MWNTs showed a slower degradation rate. This study demonstrates the potential use of PCL-MWNT composites as flexible, light weight and eco-friendly EMI shielding materials.

Synergistic interactions between silver decorated graphene and carbon nanotubes yield flexible composites to attenuate electromagnetic radiation

The need of todays highly integrated electronic devices, especially working in the GHz frequencies, is to protect them from unwanted interference from neighbouring devices. Hence, lightweight, flexible, easy to process microwave absorbers were designed here by dispersing conductive multiwall carbon nanotubes (MWNTs) and silver nanoparticles decorated onto two-dimensional graphene sheets (rGO@Ag) in poly(ε-caprolactone) (PCL). In this study, we have shown how dielectric losses can be tuned in the nanocomposites by rGO@Ag nano-hybrid; an essential criterion for energy dissipation within a material resulting in effective shielding of the incoming electromagnetic (EM) radiation. Herein, the conducting pathway for nomadic charge transfer in the PCL matrix was established by MWNTs and the attenuation was tuned by multiple scattering due to the large specific surface area of rGO@Ag. The latter was possible because of the fine dispersion state of the Ag nanoparticles which otherwise often agglomerate if mixed separately. The effect of individual nanoparticles on microwave attenuation was systematically assessed here. It was observed that this strategy resulted in strikingly enhanced microwave attenuation in PCL nanocomposites in contrast to addition of individual particles. For instance, PCL nanocomposites containing both MWNTs and rGO@Ag manifested in a SET of -37 dB and, interestingly, at arelatively smaller fraction. The SE shown by this particular composite makes it a potential candidate for many commercial applications as reflected by its exceptional absorption capability (91.3%).

Critical insights into understanding the effects of synthesis temperature and nitrogen doping towards charge storage capability and microwave shielding in nitrogen-doped carbon nanotube/polymer nanocomposites

In this study, various nitrogen-doped (N-doped) multiwall carbon nanotubes (MWNTs) were synthesized by varying the synthesis temperature (650 °C, 750 °C and 850 °C), and their charge storage capability and electromagnetic (EM) shielding effectiveness (SE) were assessed by incorporating them into a PVDF (polyvinylidene fluoride) matrix. Nitrogen doping was adopted to generate numerous polarizable centers in MWNTs. The concentration of nitrogen and polarizing centers was optimized by varying the synthesis temperature. The nitrogen doping had a significant impact on the structural, thermal, and electrical properties of MWNTs. Dielectric spectroscopy of the nanocomposites containing self-polarizable MWNTs showed significantly low loss tangent, exhibiting good charge storage ability at a given concentration of MWNTs. The electrical conductivity of N-doped nanocomposites decreased as the synthesis temperature increased from 650 °C to 850 °C. This phenomenon was observed to be significantly different to the bulk powder. The electrical conductivity of the nanocomposites was also reflected in the EM shielding results where the nanocomposites containing N-doped MWNTs showed lower shielding effectiveness than the un-doped MWNTs. Moreover, the SE decreased with increasing synthesis temperature for N-doped MWNTs. Taken together, this study demonstrates critical insights about the impact of nitrogen doping and synthesis temperature on electrical conductivity, charge storage ability, and EM shielding of MWNT polymer-based nanocomposites.

Does the Processing Method Resulting in Different States of an Interconnected Network of Multiwalled Carbon Nanotubes in Polymeric Blend Nanocomposites Affect EMI Shielding Properties

Electromagnetic interference (EMI), an unwanted phenomenon, often affects the reliability of precise electronic circuitry. To prevent this, an effective shielding is prerequisite to protect the electronic devices. In this study, an attempt was made to understand how processing of polymeric blend nanocomposites involving multiwalled carbon nanotubes (MWCNTs) affects the evolving interconnected network structure of MWCNTs and eventually their EMI shielding properties. Thereby, the overall blend morphology and especially the connectivity of the polycarbonate (PC) component, in which the MWCNTs tend to migrate, as well as the perfectness of their migration, and the state of nanotube dispersion are considered. For this purpose, blends of varying composition of PC and poly(methyl methacrylate) were chosen as a model system as they show a phase diagram with lower critical solution temperature type of characteristic. Such blends were processed in two different ways: solution mixing (from the homogeneous state) and melt mixing (in the biphasic state). In both the processes, MWCNTs (3 wt %) were mixed into the blends, and the evolved structures (after phase separation induced by annealing in solution-mixed blends) and the quenched structures (as the blends exit the extruder) were systematically studied using transmission electron microscopy (TEM). Both the set of blends were subjected to the same thermal history, however, under different conditions such as under quiescent conditions (in the case of solution mixing) and under shear (in the case of melt mixing). The electrical volume conductivity and the evolved morphologies of these blend nanocomposites were evaluated and correlated with the measured EMI shielding behavior. The results indicated that irrespective of the type of processing, the MWCNTs localized in the PC component; driven by thermodynamic factors and depending on the blend composition, sea-island, cocontinuous, and phase-inverted structures evolved. Interestingly, the better interconnected network structures of MWCNTs observed using TEM in the solution-mixed samples together with larger nanotube lengths resulted in higher EMI shielding properties (−27 dB at 18 GHz) even if slightly higher electrical volume conductivities were observed in melt-mixed samples. Moreover, the shielding was absorption-driven, facilitated by the dense network of MWCNTs in the PC component of the blends, at any given concentration of nanotubes. Taken together, this study highlights the effects of different blend nanocomposite preparation methods (solution and melt) and the developed morphology and nanotube network structure in MWCNT filled blend nanocomposites on the EMI shielding behavior.