Debye-Waller factors of caesium halides

The Debye-Waller factors of 133Cs in three caesium halides at 4.2°K and of 129I ion in caesium iodide at 80°K are calculated and compared with experimental results.

Temperature dependence of non-Debye disorder in doped manganites

Ca-doped manganite La1-xCaxMnO3 samples with x=0.2 and 0.4 were investigated by extended x-ray absorption fine structure (EXAFS) as a function of temperature and preparation method. The samples exhibit characteristic resistivity change across the metal-insulator (MI) transition temperature whose shape and position depend on Ca-doping concentration and sample thermal treatment. EXAFS results evidenced an increase of nonthermal disorder at the MI transition temperature which is significantly correlated with the resistivity behavior.

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 K-u-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.

Electromagnetic Interference Shielding Materials Derived from Gelation of Multiwall Carbon Nanotubes in Polystyrene/Poly(methyl methacrylate) Blends

Blends of polystyrene (PS) and poly(methyl methacrylate) (PMMA) with different surface-functionalized multiwall carbon nanotubes (MWNTs) were prepared by solution blending to design materials with tunable EMI (electromagnetic interference) shielding. Different MWNTs like pristine, amine (similar to NH2), and carboxyl acid (similar to COOH) functionalized were incorporated in the polymer by solution blending. The specific interaction driven localization of MWNTs in the blend during annealing was monitored using contact mode AFM (atomic force microscopy) on thin films. Surface composition of the phase separated blends was further evaluated using X-ray photoelectron spectroscopy (XPS). The localization of MWNTs in a given phase in the bulk was further supported by selective dissolution experiments. Solution-casted PS/PMMA (50/50, wt/wt) blend exhibited a cocontinuous morphology on annealing for 30 min, whereas on longer annealing times it coarsened into matrix-droplet type of morphology. Interestingly, both pristine MWNTs and NH2-MWNTs resulted in interconnected structures of PMMA in PS matrix upon annealing, whereas COOH-MWNTs were localized in the PMMA droplets. Room-temperature electrical conductivity and electromagnetic shielding effectiveness (SE) were measured in a broad range of frequency. It was observed that both electrical conductivity and SE were strongly contingent on the type of surface functional groups on the MWNTs. The thermal conductivity of the blends was measured with laser flash technique at different temperatures. Interestingly, the SE for blends with pristine and NH2-MWNTs was >-24 dB at room temperature, which is commercially important, and with very marginal variation in thermal conductivity in the temperature range of 303-343 K. The gelation of MWNTs in the blends resulted in a higher SE than those obtained using the composites.

Electromagnetic shielding materials and coatings derived from gelation of multiwall carbon nanotubes in an LCST mixture

Thermally induced demixing in an LCST mixture, polystyrene (PS)/polyvinyl methyl ether] (PVME), was used as a template to design materials with high electrical conductivity. This was facilitated by gelation of multiwall carbon nanotubes (MWNTs) in a given phase of the blends. The MWNTs were mixed in the miscible blends and the thermodynamic driven demixing further resulted in selective localization in the PVME phase of the blends. This was further confirmed by atomic force microscopy (AFM). The time dependent gelation of MWNTs at shallow quench depth, evaluated using isochronal temperature sweep by rheology, was studied by monitoring the melt electrical conductivity of the samples in situ by an LCR meter coupled to a rheometer. By varying the composition in the mixture, several intricate shapes like gaskets and also coatings capable of attenuating the EM radiation in the microwave frequency can be derived. For instance, the PVME rich mixtures can be molded in the form of a gasket, O-ring and other intricate shapes while the PS rich mixtures can be coated onto an insulating polymer to enhance the shielding effectiveness (SE) for EM radiation. The SE of the various materials was analyzed using a vector network analyzer in both the X-band (8.2 to 12 GHz) and the K-u-band (12 to 18 GHz) frequency. The improved SE upon gelation of MWNTs in the demixed blends is well evident by comparing the SE before and after demixing. A reflection loss of -35 dB was observed in the blends with 2 wt% MWNTs. Further, by coating a layer of ca. 0.15 mm of PS/PVME/MWNT, a SE of -15 dB at 18 GHz could be obtained.

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.

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.

Tailoring the dispersion of multiwall carbon nanotubes in co-continuous PVDF/ABS blends to design materials with enhanced electromagnetic interference shielding

Highly conducting composites were derived by selectively localizing multiwall carbon nanotubes (MWNTs) in co-continuous PVDF/ABS (50/50, wt/wt) blends. The electrical percolation threshold was obtained between 0.5 and 1 wt% MWNTs as manifested by a dramatic increase in the electrical conductivity by about six orders of magnitude with respect to the neat blends. In order to further enhance the electrical conductivity of the blends, the MWNTs were modified with amine terminated ionic liquid (IL), which, besides enhancing the interfacial interaction with PVDF, facilitated the formation of a network like structure of MWNTs. This high electrical conductivity of the blends, at a relatively low fraction (1 wt%), was further explored to design materials that can attenuate electromagnetic (EM) radiation. More specifically, to attenuate the EM radiation by absorption, a ferroelectric phase was introduced. To accomplish this, barium titanate (BT) nanoparticles chemically stitched onto graphene oxide (GO) sheets were synthesized and mixed along with MWNTs in the blends. Intriguingly, the total EM shielding effectiveness (SE) was enhanced by ca. 10 dB with respect to the blends with only MWNTs. In addition, the effect of introducing a ferromagnetic phase (Fe3O4) along with IL modified MWNTs was also investigated. This study opens new avenues in designing materials that can attenuate EM radiation by selecting either a ferroelectric (BT-GO) or a ferromagnetic phase (Fe3O4) along with intrinsically conducting nanoparticles (MWNTs).

Polyvinylidene fluoride based lightweight and corrosion resistant electromagnetic shielding materials

Various NixCo1-x alloys (with x varying from 0-60 wt%, Ni: nickel, Co: cobalt) were prepared by vacuum arc melting and mixed with polyvinylidene fluoride (PVDF) to design lightweight, flexible and corrosion resistant materials that can attenuate electromagnetic radiation. The saturation magnetization scaled with the fraction of Co in the alloy. Two key properties such as high-magnetic permeability and high-electrical conductivity were targeted. While the former was achieved using a Ni-Co alloy, multiwalled carbon nanotubes (CNTs) in the composites accomplished the latter. A unique approach was adopted to prepare the composites wherein PVDF powder along with CNTs and Ni-Co flakes were made into a paste, using a solvent, followed by hot pressing. Interestingly, CNTs facilitated in uniform dispersion of the Ni-Co alloy in PVDF, as manifested from synergistic improvement in the electrical conductivity. A significant improvement in the shielding effectiveness (41 dB, >99.99% attenuation) was achieved with the addition of 50 wt% of Ni40Co60 alloy and 3 wt% CNTs. Intriguingly, due to the unique processing technique adopted here, the flexibility of the composites was retained and more interestingly, the composites were resistant to corrosion as compared to only Ni-Co alloy.

Simultaneous enhancement in mechanical strength, electrical conductivity, and electromagnetic shielding properties in PVDF-ABS blends containing PMMA wrapped multiwall carbon nanotubes

A unique approach was adopted to drive the multiwall carbon nanotubes (MWNTs) to the interface of immiscible PVDF-ABS blends by wrapping the nanotubes with a mutually miscible homopolymer (PMMA). A tailor made interface with an improved stress transfer was achieved in the blends with PMMA wrapped MWNTs. This manifested in an impressive 108% increment in the tensile strength and 48% increment in the Young's modulus with 3 wt% PMMA wrapped MWNTs in striking contrast to the neat blends. As the PMMA wrapped MWNTs localized at the interface of PVDF-ABS blends, the electrical conductivity could be tuned with respect to only MWNTs, which were selectively localized in the PVDF phase, driven by thermodynamics. The electromagnetic shielding properties were assessed using a vector network analyser in a broad range of frequency, X-band (8-12 GHz) and Ku-band (12-18 GHz). Interestingly, enhanced EM shielding was achieved by this unique approach. The blends with only MWNTs shielded the EM waves mostly by reflection however, the blends with PMMA wrapped MWNTs (3 wt%) shielded mostly by absorption (62%). This study opens new avenues in designing materials, which show simultaneous improvement in mechanical, electrical conductivity and EM shielding properties.

New physical insights into the electromagnetic shielding efficiency in PVDF nanocomposites containing multiwall carbon nanotubes and magnetic nanoparticles

This work attempts to bring critical insights into the electromagnetic shielding efficiency in polymeric nanocomposites with respect to the particle size of magnetic nanoparticles added along with or without a conductive inclusion. To gain insight, various Ni-Fe (NixFe1-x; x = 10, 20, 40; Ni: nickel, Fe: iron) alloys were prepared by a vacuum arc melting process and different particle sizes were then achieved by a controlled grinding process for different time scales. Poly(vinylidene fluoride), PVDF based composites involving different particle sizes of the Ni-Fe alloy were prepared with or without multiwall carbon nanotubes (MWNTs) by a wet grinding approach. The Ni-Fe particles were thoroughly characterized with respect to their microstructure and magnetization; and the electromagnetic (EM) shielding efficiency (SE) of the resulting composites was obtained from the scattering parameters using a vector network analyzer in a broad range of frequencies. The saturation magnetization of Ni-Fe nanoparticles and the bulk electrical conductivity of PVDF/Ni-Fe composites scaled with increasing particle size of NiFe. Interestingly, the PVDF/Ni-Fe/MWNT composites showed a different trend where the bulk electrical conductivity and SE scaled with decreasing particle size of the Ni-Fe alloy. A total SE of similar to 35 dB was achieved with 50 wt% of Ni60Fe40 and 3 wt% MWNTs. More interestingly, the PVDF/Ni-Fe composites shielded the EM waves mostly by reflection whereas, the PVDF/Ni-Fe/MWNT shielded mostly by absorption. A minimum reflection loss of similar to 58 dB was achieved in the PVDF/Ni-Fe/MWNT composites in the X-band (8-12 GHz) for a particular size of Ni-Fe alloy nanoparticles. This study brings new insights into the EM shielding efficiency in PVDF/magnetic nanoparticle based composites in the presence and absence of conducting inclusion.

An efficient strategy to develop microwave shielding materials with enhanced attenuation constant

A mutually miscible homopolymer (here polymethyl methacrylate; PMMA) was employed to tailor the interfacial properties of immiscible polycarbonate/styrene acrylonitrile (PC/SAN) blends. In order to design materials that can shield microwave radiation, one of the key properties i.e. electrical conductivity was targeted here using a conducting inclusion; multiwall carbon nanotubes (MWNTs). Owing to higher polarity, MWNTs prefer PC over SAN which though enhance the electrical conductivity of the blends, they don't improve the interfacial properties and results in poor mechanical properties. Hence, an efficient strategy has been adopted here to simultaneously enhance the mechanical, electrical and microwave attenuation properties. Herein, the MWNTs were wrapped by PMMA via in situ polymerization of MMA (methyl methacrylate). This strategy resulted in the migration of PMMA modified MWNTs towards the blend's interface and resulted in an effective stress transfer across the interface leading to improved mechanical and dynamic mechanical properties. Interestingly, the bulk electrical conductivity of the blends was also enhanced, manifesting the improved dispersion of the MWNTs. The state of dispersion of the MWNTs and the phase morphology were assessed using scanning electron microscopy. The microwave attenuation properties were evaluated using a vector network analyzer (VNA) in the X and K-u-band frequencies. The blends with PMMA wrapped MWNTs manifested a -21 dB of shielding effectiveness which suggests attenuation of more than 99% of the incoming microwave radiation. More interestingly, the attenuation constant could be tuned here employing this unique strategy. This study clearly opens a new tool box in designing materials that show improved mechanical, dynamic mechanical, electrical conductivity and microwave shielding properties.