Carbon Shieldings Research Articles

Recent advances in MXene nanomaterials: Fundamentals to applications in environment sector

AbstractMXenes are a new type of 2D transition metal carbon/nitride or carbonitride, which are composed of Mn+1AXn phase material (MAX phase) through single‐layer or thin‐layer nanosheets obtained by exfoliation. Owning to unique two‐dimensional layered structure, large specific surface area, excellent electrical conductivity and mechanical stability, the MXenes have quickly become a research hotspot due to their magnetic and other properties, and have been widely used in many fields such as electrochemical sensors, energy storage, catalysis, and adsorption. This article summarizes and introduces preparation methods of two‐dimensional materials MXenes, and focus on reviewing their application research progress in the electrochemical sensors and environmental field in recent years, including detection of biomarkers and environmental pollutants, adsorption of heavy metals, adsorption of radiation metals, adsorption of organic matter, selective adsorption of carbon dioxide, membrane separation, sensors, electrocatalysis, photocatalysis, electromagnetic absorption and shielding, etc. A summary and review were conducted, and finally the existing problems and future development at this stage were analyzed.

Cobalt-catalyzed soft-hard carbon composite anodes for enhanced sodium-ion storage

Hard carbon is expected to be a high-capacity anode material for sodium-ion batteries (SIBs). However, its Na+ storage performance, especially the low discharge capacity, remains a great challenge. Herein, the soft-hard carbon composite anodes with high degree of graphitization were synthesized via low-temperature pyrolysis (at only 900 °C) of Cobalt-catalyzed perylene tetracarboxylic dianhydride (PTCDA-Co) and cotton, with subsequent removal of cobalt. The composite resulting from a 2:3 mass ratio of soft to hard carbon exhibits a good discharge capacity of 355.81 mAh g−1 at a current density of 50 mA g−1 and an enhanced initial Coulombic efficiency (ICE) of 81.41%. The enhanced electrochemical storage performance of cotton is attributed to the introduction of PTCDA-Co, which creates low-defect graphitic layer, hierarchical porous channels, and carbon shielding coating on hard carbon.

Effect of sintering temperature on impedance and EMI shielding of carbon modified CoFe2O4 nanofibers via electrospinning

The electrospinning technique is employed for the synthesized cobalt ferrite/carbon nanofibers, utilizing iso-propanol as the solvent. The nanofibers prepared are subjected to sintering at three different temperatures. A comprehensive analysis is carried out on the structural, morphological, dielectric, impedance, and electromagnetic shielding effectiveness within the X-band frequency range (8.2–12.4 GHz). The application of the Debye-Scherer formula yielded an estimated crystallite size of 22 nm. The electrical response of cobalt ferrite/carbon nanofibers pellet is characterized by employing impedance spectroscopy. Variations in impedance plane plots are discussed based on frequency dependent relaxation time modulations. The impact of sintering temperature is observed in terms of increased impedance values and modifications in the conduction mechanism. These changes are contributed to distinct hopping carriers’ linkages between Fe3+-Fe2+ and Co3+-Co2+ ions. Dielectric studies are elucidated considering cation polarizability, colossal resistivity, and tangent loss. This study revealed notable characteristics of CoFe2O4/C nanofibers, including colossal resistivity (8.45 × 108 Ωcm), a dielectric constant of 22, and colossal tangent loss. The CoFe2O4/C nanofibers exhibit a characteristic electromagnetic interference (EMI) shielding performance of 30–35 dB. This remarkable shielding performance is observed in nanofibers with a 3 mm thickness, specifically in sample S1 that underwent sintering at a low temperature. The shielding effectiveness is demonstrated within the X-band frequency range.

Highly efficient electromagnetic interference shielding and superior mechanical performance of carbon nanotube/polydimethylsiloxane composite with interface-reinforced segregated structure

The formation of segregated structure in conductive polymer composite is an effective strategy to improve electromagnetic interference (EMI) shielding performance. However, inferior mechanical properties always existed in the segregated composite due to weak interfacial adherence or defects. Herein, an interface-reinforced segregated structure was constructed in a carbon nanotube (CNT)/polydimethylsiloxane (PDMS) composite by creating chemical bonds between PDMS microcells and the continuous PDMS interfacial phase embedded with CNT. The interface-reinforced segregated composites achieved the integration of excellent EMI shielding and good mechanical properties. Specifically, the resultant composite exhibited a superior EMI shielding effectiveness (EMI SE) of 47.0 dB at only 2.2 vol% CNT content. The tensile strength and elongation at break of the composite were 3.6 MPa and 87.0%, increased by 35.0 and 7.0 times in comparison to the conventional segregated composite. The interface-reinforced segregated composite also demonstrated excellent reliability with high EMI SE retention of 80% even after suffering from 1000 stretching-releasing cycles at a terminal strain of 30%. This work provides an important guideline for the development of high-performance polymer composite with efficient EMI shielding and good mechanical performances.

Effect of surface conductivity on electromagnetic shielding of multi-walled carbon nanotubes/Poly(ε-caprolactone) composites

Herein, the effect of surface conductivity on microwave shielding effectiveness (SE) is discussed in the sandwiched multi-walled carbon nanotubes/poly(ε-caprolactone) (MWCNT/PCL) composites. The electrical conductivity on single or double-side surface of the composites is regulated by controlling different MWCNT loadings on the surface. The electromagnetic SE values of the MWCNT/PCL composites decrease with decreasing surface conductivity, and decline acutely when the surface becomes non-conductive. The shielding mechanism is investigated by a simulation and experimental study for the composites having different surface conductivity. The results indicate that the microwave reflection is very important to contribute the shielding performance of the composites. In addition, the effect of dielectric properties of surface layers on electromagnetic shielding performance was discussed for the composites with insulation surface. The composites with different polymers as insulting surface layers exhibit the close SE values, suggesting that the microwave shielding performance of the In/Con/In composites is irrelevant to the different polymer with different dielectric constant on the surface layers. Furthermore, the conductive network on surface exerts a great influence on microwave attenuation because of the enhancement on surface conduction loss and polarization loss. Therefore, the surface conductivity is a non-negligible factor for designing materials with excellent microwave shielding performance.

Anchoring Bimetal Single Atoms and Alloys on N-Doping-Carbon Nanofiber Networks for an Efficient Oxygen Reduction Reaction and Zinc–Air Batteries

Electrocatalysts for the oxygen reduction reaction (ORR) play a central role in fuel cells and zinc-air batteries. Bimetal single atoms and nanoparticle hybrids are emerging ORR electrocatalysts, superior to the most exploited unary metal single-atom catalysts (SACs). Here, we report bimetal SAC-based nanofiber networks of Co3Fe7@Co/Fe-SAC for efficient ORR electrocatalysis and zinc-air batteries. A facile and easy-to-scale-up process is developed, and the versatility is validated in three hybrids. Strong electronic interaction is revealed between bimetal single atoms and alloy nanoparticles, leading to improved catalytic performances for ORR. Specifically, the Co3Fe7@Co/Fe-SAC hybrids exhibit a half-wave potential of 0.841 V in a basic electrolyte, comparable to the Pt/C electrocatalyst. Assembled in a zinc-air battery, a Co3Fe7@Co/Fe-SAC hybrid-based cell demonstrates a power density 1.8 times higher than the benchmark Pt/C-IrO2-based one, and it is stable for 150 cycles galvanostatic charge/discharge. The superior device performance is attributed to the appealing intrinsic activity, the carbon shielding effect for anti-leaching, and the hierarchical porous networks for large accessibility of active sites and favorable mass transport. Theoretical calculations suggest that alloy nanoparticles significantly improved the intrinsic catalytic activity of Fe single-atom sites at the expense of slightly lowering the activity of Co single-atom sites. This work presents a versatile process for the mass production of efficient composite electrocatalysts and highlights the power of bimetal single-atom-based hybrids and hierarchically porous structures for ORR device performances.

Plasma Oscillation Behavior and Electromagnetic Interference Shielding of Carbon Nanofibers/Conductive Polymer Metacomposites at Radarwave Frequency.

Metacomposites have attracted widespread attention due to their unique negative electromagnetic properties and stupendous applications. Although there are systems that realize metamaterial properties in low radio frequency bands, the research on the construction of polymer matrix metacomposites with negative performance in the pivotal GHz band is still undiscovered. Herein, carbon nanofiber/conductive polymer metacomposites with 3D overlapping network structures are innovatively constructed to achieve negative permittivity characteristics in the radarwave frequency range, and convenient methods for further adjusting the electromagnetic parameters is also proposed. The results show that the negative permittivity of CNFs/PANI metacomposites can be conveniently altered via adjusting PANI content. Furthermore, electromagnetic shielding has also been fully discussed as one of the most valuable applications of the metacomposites. The SET of CNFs/PANI-70 has an average value of 70dB at 4-18GHz and can reach a maximum of 80dB at 4GHz, which far exceeds the current commercial electromagnetic shielding standards. This work greatly broadens the promising application of metacomposites for perfect electromagnetic shielding, novel capacitance, and frequency selective surfaces.

Molecular models of three ω-3 fatty acids based on NMR and DFT calculations of 1H NMR chemical shifts

Structures of low-energy conformers of α-linolenic acid (ALA;18:3-cis-9,12,15 ω-3), eicosapentaenoic acid (EPA;20:5-cis 5,8,11,14,17 ω-3) and docosahexaenoic acid (DHA;22:6-cis 4,7,10,13,16,19 ω-3) in the liquid state, based on NMR and density functional theory calculations of 1H NMR chemical shifts, are provided. A critical discussion of the J. Mol. Liq. 314 (2020) 113,376 article is also presented on a number of issues related to: (i) the method of assignment of 1H and 13C chemical shifts of ALA, EPA and DHA; (ii) attribution of NMR signals of impurities to those of EPA and (iii) the conclusion of an unusually strong electron 13C and 1H shielding of HCCH2CH protons and carbons, due to an asymmetry in a twist between the two ends, which results in torque at the methylene site near the geometric center. The present data demonstrate that the folding effects of the chain as a function of the fragment number (CHCHCH2)n, were n = 3, 5 and 6 for ALA, EPA and DHA, do not result in unusually strong electron 1H and 13C shielding.

Off/on switchable smart electromagnetic interference shielding aerogel

Summary Smart electromagnetic interference (EMI) shielding materials with tunable EM wave response characteristics are attractive for future EM devices. Here, we report an off/on switchable EMI shielding material that can be tuned from EM wave transmission to shielding and vice versa by mechanical compression and decompression. This smart material is prepared by filling conductive carbon nanoparticles into wood-derived lamellar carbon aerogel, showing reversible compressibility and strain-sensitive conductivity. The original aerogel has good impedance matching and low dielectric loss, allowing the EM wave to pass through. Mechanical compression enables the carbon nanoparticles to establish highly conductive pathways in the aerogel, which significantly increases the conductivity of aerogel and thus activates its EMI shielding performance. This function switch is reversible by repeatedly compressing and decompressing the aerogel. The availability of such a never-before-realized off/on switchable EMI shielding material provides an opportunity for the development of advanced smart EM devices.

Impact of inclusion of graphene oxide nanosheets on polypropylene thermal characteristics

Thermal steadiness and flame-resistant characteristics of PP/GO nanocomposites were explored. Inclusion of graphene oxide nanosheets (GOs) increased thermal steadiness of the nanocomposites by at least 50 °C through nano-confinement of PP connections and restriction of release of gaseous molecules during degradation. Pyrolysis showed that process of the degradation of PP was not changed in the presence of nanofillers and alkenes consisting of carbon particles that were deterioration products. With addition of GOs to PP, cone calorimetry experiments exposed a substantial deviation in ignition under irradiation. In addition, inclusion of GOs reduced PP's rate of combustion owing to the creation of a carbon shielding sheet acting as the heat and mass transfer barrier. Uniformly dispersed GOs shifted the decomposing onset temperature (Tonset) of PP to around 40 °C higher which induced a great melt flow rate in the composites and dramatically changed their fire behavior. Delayed dripping resulted in considerably enhanced heat absorption and reduced time for ignition. As a result of development of a shielding film in combustion, under flaming conditions, the highest thermal discharge level of 78% was accomplished. The most prominent features that made PP to prefer carbon nanoparticles were demonstrated by the well-exfoliated GO nanosheets in the PP matrix which was evidenced by TEM images. Findings of TGA indicated that the GO's addition enhanced the thermal steadiness and PP's char yields. The nanocomposites of PP/GO 5.0 wt% attained UL-94 with a grade of no dripping and grade V-1. Cone calorimeter test results showed that the PP's burning performance, peak heat release rate (PHRR) and average specific extinction area decreased noticeably when the GOs' combination effects was intervened. PHRR showed a decrease of 46% compared to the neat PP for the PP/GO 5.0 wt% nanocomposite. This role offered a new modified approach to enhance the GO's flame-resistant effectiveness.

Population synthesis of exocometary gas around A stars

ABSTRACT The presence of CO gas around 10–50 Myr old A stars with debris discs has sparked debate on whether the gas is primordial or secondary. Since secondary gas released from planetesimals is poor in H2, it was thought that CO would quickly photodissociate never reaching the high levels observed around the majority of A stars with bright debris discs. Kral et al. showed that neutral carbon produced by CO photodissociation can effectively shield CO and potentially explain the high CO masses around 9 A stars with bright debris discs. Here, we present a new model that simulates the gas viscous evolution, accounting for carbon shielding and how the gas release rate decreases with time as the planetesimal disc loses mass. We find that the present gas mass in a system is highly dependant on its evolutionary path. Since gas is lost on long time-scales, it can retain a memory of the initial disc mass. Moreover, we find that gas levels can be out of equilibrium and quickly evolving from a shielded on to an unshielded state. With this model, we build the first population synthesis of gas around A stars, which we use to constrain the disc viscosity. We find a good match with a high viscosity (α ∼ 0.1), indicating that gas is lost on time-scales ∼1–10 Myr. Moreover, our model also shows that high CO masses are not expected around FGK stars since their planetesimal discs are born with lower masses, explaining why shielded discs are only found around A stars. Finally, we hypothesize that the observed carbon cavities could be due to radiation pressure or accreting planets.

Local aromaticity in polyacenes manifested by individual proton and carbon shieldings: DFT mapping of aromaticity.

Exponential dependencies between locally calculated geometric and magnetic indexes of aromaticity, harmonic oscillator model of aromaticity (HOMA) and nucleus independent chemical shifts (NICS)(0), NICS(1) and NICS(1)zz, and the number of conjugated benzene rings in linear acenes, from benzene to decacene were observed at B3LYP/6-311+G** level of theory. Correlations between HOMA and NICS indexes showed exponential dependencies and were fitted with simple three-parameter function. Similar correlations between both indexes of aromaticity and proton and carbon nuclear isotropic shieldings of individual acene rings were observed. Contrary to proton data, the predicted 13 C nuclear isotropic shieldings of carbon atoms belonging to inner rings in polyacenes were less shielded, indicating lower aromaticity and therefore, higher reactivity.

Preparation, Pharmaceutical Properties and Stability of Lesinurad Co-crystals and Solvate

Lesinurad is a BCS class 2, low solubility drug, indicated for hyperglycemia associated with gout in patients who have not achieved target serum uric acid levels with a xanthine oxidase inhibitor alone. Co-crystallization and solvate formation with pharmaceutically acceptable coformers and solvents are one of the best ways to improve solubility of BCS class 2 drugs. Lesinurad DMSO (dimethyl sulphoxide) solvate (1:1 mole ratio), urea and 4-Hydroxy benzoic acid co-crystals (1:1 mole ratio) prepared and scaled up to 100g and characterized using PXRD, FTIR, DSC, TGA, solid state 13C NMR and H1 NMR. Unique PXRD pattern, red shift in acid carbonyl stretching of Lesinurad in DMSO solvent, Urea co-crystal and parahydroxybenzoic acid co-crystals are the evidence of solvate and co-crystal formation. Urea co-crystal is further characterized using 13C solid state NMR. Deshielding of acid carbonyl carbon from δppm of 168.98 to 173.12 and shielding of urea carbonyl carbon from δppm of 169 to 164.56 indicates co-crystal formation between lesinurad and urea. DMSO solvate has shown 1.26 fold increase in Intrinsic dissolution rate measurement (IDR) over marketed Form-II and urea co-crystal. DMSO solvate, urea and 4-Hydroxy benzoic acid cocrystals of lesinurad are stable for six months at 40 °C and 75%RH.

Relativistic heavy atom effect on 13 C NMR chemical shifts initiated by adjacent multiple chalcogens.

In this paper, we have investigated the cumulative peculiarity of the "heavy atom on light atom" effect on the 13 C NMR chemical shifts, initiated by the adjacent chalcogens. For this purpose, the most accurate hybrid computational scheme for the calculation of chemical shifts of carbon nuclei, directly bonded with several heavy chalcogens, is introduced and attested on the representative series of molecules. The best hybrid scheme combines the nonrelativistic coupled cluster-based approach with the different types of corrections, including vibrational, solvent, and relativistic. The dependences of the total relativistic corrections to carbon shielding constants in 2 series of model compounds, namely, X═13 C═Y (X, Y=O, S, Se, Te) and C(XH)m (YH)n (ZH)p (QH)s H1-m H1-n H1-p H1-s (X, Y, Z, Q=S, Se, Te and m, n, p, s=0, 1), on the total atomic number of the adjacent chalcogens have been obtained.

Efficient electromagnetic interference shielding of lightweight carbon nanotube/polyethylene composites via compression molding plus salt-leaching.

Carbon nanotube/high density polyethylene (CNT/HDPE) foam composites with high electrical conductivity and electromagnetic interference (EMI) shielding performance were developed by means of compression molding plus salt-leaching. The uniform porous structure and interconnected CNT networks throughout the cell backbones endowed the as-prepared foam composites with a significantly lower electrical percolation threshold (0.22 vol%) than that of the solid composites (0.84 vol%). Owing to the multiple reflections and scattering between the cell–matrix interfaces, the foam composites presented a superior specific EMI shielding effectiveness (EMI SE) of 104.3 dB cm3 g−1, 2.2 times higher than that of their solid counterpart. Besides this, the pore sizes of the CNT/HDPE foam composites could be easily tuned by controlling the particle size of the porogen. Also, the electrical conductivity and specific EMI SE increased with an increase in the cell diameter, which was attributed to the formation of a more perfect conductive network in the cell backbones. Our approach provides a novel idea for fabricating new lightweight EMI shielding materials, especially for aircraft and spacecraft applications.

Enhanced electromagnetic interference shielding of carbon fiber/cement composites by adding ferroferric oxide nanoparticles

Ferroferric oxide (Fe3O4) nanoparticles were introduced into carbon fiber (CF)/cement composites to enhance the electromagnetic interference (EMI) shielding effectiveness (SE) in X-band (8.2–12.4GHz). The spherical Fe3O4 nanoparticles were synthesized through solvothermal method, whose average diameter was about 30–50nm. It was observed that incorporation of Fe3O4 nanoparticles (5wt%) along with 0.4wt% CF in the cement matrix led to a shielding effectiveness of 29.8dB, which had 34.4% increase than that of CF/cement composite. The excellent EMI shielding property was attributed to the synergistic effect between CF and Fe3O4 nanoparticles. SEM revealed the homogeneous dispersion of Fe3O4 and CF in the cement matrix. The CF/Fe3O4/cement composite is believed to be a promising material for high EMI shielding.

Impact of synthesis temperature on morphology, rheology and electromagnetic interference shielding of CVD-grown carbon nanotube/polyvinylidene fluoride nanocomposites

Employing chemical vapor deposition technique, multi-walled carbon nanotubes (CNTs) were synthesized over Fe catalyst at a broad range of temperatures, i.e. 550°C to 950°C (at 100°C intervals). CNTs were melt-mixed into a polyvinylidene fluoride (PVDF) matrix at various loadings, and then compression molded. Surprisingly, despite the ascending trend of CNT powder conductivity with the synthesis temperature, the nanocomposites made with CNT synthesized at 650°C had significantly lower percolation threshold (around 0.4wt%) and higher electromagnetic interference shielding effectiveness (EMI SE) (20.3dB over the X-band for 3.5wt% CNT and 1.1mm thickness) than the other temperatures. Exhaustive characterization studies were conducted on both CNTs and composites to unveil their morphological and electrical characteristics. Superior EMI shielding of CNT650°C was attributed to a combination of high carbon purity, aspect ratio, crystallinity, and moderate powder conductivity along with decent state of dispersion within the PVDF matrix.

Effect of Curvature on Carbon Chemical Shielding in Extended Carbon Systems.

The effect of curvature on the chemical shielding of carbons in curved polycyclic aromatic hydrocarbons has been systematically studied by examining structures analogous to the circumcoronene molecule with different degrees of curvature. We attempt to eliminate effects from Knight shifts in carbon nanotubes, differing ring currents from five-membered rings, and edge effects in finite nanotube models in order to separate out the change in shielding that is due to curvature alone. Using curved structures derived from geometry-optimized structures of carbon nanotubes, we calculate the carbon chemical shielding tensor for carbons in the central aromatic ring as well as the Nucleus Independent Chemical Shift (NICS) on the convex and concave side of each structure. All three tensor components become less shielded with increasing curvature of the system, with the σ33 component radial to the curve experiencing the greatest change. The NICS values are influenced by both the decrease in aromaticity as the structure is curved as well as geometric effects that bring the outside rings closer to the central aromatic ring.

The experimental and theoretical study of tautomerism of 3-substituted 2-methyl-quinoline-4 (1h)-ones

4-Hydroxy-/4-oxo tautomerism in the series of 3-substituted 2-methyl-quinolin-4(1H)-ones has been studied by 13C NMR-spectroscopy and quantum-chemical methods in various approximations (restricted Hartree-Fock method, DFT and MP2) for the isolated molecules and for solutions using empirical correction of effects for solvents (PCM COSMO procedure). Substituents that are different in their nature have no significant influence on the value of the chemical shift of carbon in position C4 of the quinolone cycle. The only exception is the carbon shielding associated with the bromine atom in the molecule of 3-bromo-2-methyl-1,4-dihydroquinoline-4-one. Significant deshielding detected in all cases in 13C NMR-spectra of the carbon nuclei in position 4 of the ring is in favour of the existence of all derivatives studied as 4-oxo forms in DMSO-d6 solution. The experimental and calculated values for the chemical shift of carbon in position C4 of 4-oxo and 4-hydroxy isomers differ considerably and can be used as a criterion for assigning quinolin-4 (1H)-ones to a particular tautomeric form.

Enhancing NMR Prediction for Organic Compounds Using Molecular Dynamics.

NMR spectroscopy is a crucial tool in organic chemistry for the routine characterization of small molecules, structural elucidation of natural products, and study of reaction mechanisms. Although there is evidence that thermal motions strongly affect observed resonances, conventional predictions are performed only on stationary structures. Here we show that quasiclassical molecular dynamics provides a highly accurate and broadly applicable method for improving shielding predictions. Gas-phase values of the absolute shieldings of protons and carbons are predicted to nearly within experimental uncertainty, while the chemical shifts of large systems such as natural products are closely reproduced. Importantly, these results are obtained without the use of any empirical corrections. Our analysis suggests that the linear scaling factors currently employed are primarily a correction for vibrational effects. As a result, our method extends the reach of prediction methods to the study of molecules with unusual dynamics such as the iconic and controversial [18]annulene. Our predictions agree closely with experiment at both low and high temperatures and provide strong evidence that the equilibrium structure of [18]annulene is planar and aromatic.