Silicate hydration is prevalent in natural and technological processes, such as, mineral weathering, glass alteration, zeolite syntheses and cement hydration. Tricalcium silicate (Ca3SiO5), the main ...constituent of Portland cement, is amongst the most reactive silicates in water. Despite its widespread industrial use, the reaction of Ca3SiO5 with water to form calcium-silicate-hydrates (C-S-H) still hosts many open questions. Here, we show that solid-state nuclear magnetic resonance measurements of (29)Si-enriched triclinic Ca3SiO5 enable the quantitative monitoring of the hydration process in terms of transient local molecular composition, extent of silicate hydration and polymerization. This provides insights on the relative influence of surface hydroxylation and hydrate precipitation on the hydration rate. When the rate drops, the amount of hydroxylated Ca3SiO5 decreases, thus demonstrating the partial passivation of the surface during the deceleration stage. Moreover, the relative quantities of monomers, dimers, pentamers and octamers in the C-S-H structure are measured.
We used a combination of experimental and modelling techniques to study the effect of NaAlO2 on C3S hydration. pH sensitive inhibition of C3S hydration occurred at an early age of reaction, but was ...followed by an increased amount of hydrates formed later. Most results suggest that aluminates hinder C3S dissolution. It is hypothesised that this takes place in active dissolution areas, present with a higher density on finer particles. Annealing reduces their number and increases retardation for a given dosage of aluminates. The view that aluminates act by hindering dissolution is supported by molecular dynamics (MD) simulations. They establish that aluminates can adsorb on the hydroxylated C3S mainly through strong ionic interactions between aluminate and calcium ions on the surface of silicate. Upon progress of hydration and at higher pH values, the binding strength of aluminates to the hydroxylated C3S decreases so that its passivating effect, and retardation, are reduced.
A facile NMR approach based on dipolar filtering (DF) and spin echo (SE) is proposed to study commercially available dealuminated USY zeolites, such as H-CBV760, H-CBV720, and NH4-CBV712 of nominal ...Si/Al ratio of 30, 15, and 6, respectively. The proposed 1H DF-SE magic angle spinning (MAS) NMR approach provides the substantial suppression of water signal intensity in partly hydrated samples providing an opportunity to obtain the signal of other surface groups. 1H DF-SE MAS NMR technique has been demonstrated its usability for quantitative (semiquantitative) analysis of dried, i.e., partly hydrated, samples. It is essential to use this approach when calcination under vacuum used as a reference procedure leads to drastic surface changes. Moreover, the technique is applicable for qualitative analysis of fully hydrated samples. This method is found to be extremely sensitive to the residual ammonium content in zeolite structure even in transformed to H form by calcination. Finally, the framework stacking faults species are found to be more pronounced in 1H DF-SE MAS NMR spectra in hydrated state as ∼1 ppm peak that can be crucial for understanding of relationships of structure and performance. Additionally, the “standard” 27Al and 29Si MAS NMR approaches are also discussed in both hydrated and dried states of zeolites. 29Si MAS NMR spectra demonstrate that a dependence on hydration state and the highest quantity of crystalline part is achieved in dried samples, whereas the best resolution of 27Al MAS NMR spectra is obtained in a fully hydrated state. Finally, a local order of Si framework given by full width at half-maximum parameter of crystalline Q4 0 peak correlates with increasing relative Al content, which is responsible for the distortion of zeolite structure.
We present novel polyethylene (PE) composites for electromagnetic interference (EMI) shielding application. They are based on cobalt modified multi-walled carbon nanotubes (MWCNTs) produced via in ...situ polymerization of ethylene, with the Ti-Ziegler–Natta catalyst preliminarily immobilized on Co/MWCNT hybrids. The electromagnetic properties of the composites were tuned by varying the filler loading and Co:MWCNT ratio. The microstructure of the composites and electromagnetic absorption process were carefully characterized by transmission and scanning electron microscopy, X-ray diffraction, vibrating sample magnetometry, ferromagnetic resonance and vector network analysis. The electromagnetic wave absorbing properties of the nanocomposite were investigated in the 10 MHz−18 GHz frequency range revealing that the EMI absorption properties can be tuned by varying the Co:MWCNT weight ratio in the filler. Interestingly, the Co/MWCNT-PE composite with a total filler and Co loading of only 12 and 1.7 wt%, respectively, showed extremely high reflection loss (RL) of −55 dB. More importantly, an effective bandwidth of 12.8–17.8 GHz (RL below −10 dB) was achieved for a matching thickness of only 1.5 mm. The specific RL value (RL/filler loading) of the composite was superior in comparison with the previously reported nanostructured carbon materials. The highly effective absorbing properties of Co/MWCNT-PE composites are explained primarily by the unprecedented uniform filler distribution in the polyethylene as well as by the synergistic effect of MWCNTs and Co nanoparticles. This approach thus offered an effective strategy to design cost-effective, lightweight and flexible EMI shielding materials with tunable dielectric and magnetic performance.
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•New Co/MWCNT/polyethylene composites for EMI shielding applications were designed.•In situ polymerization ensured uniform distribution of Co/MWCNT filler in PE.•EMI shielding properties can be tuned by varying filler loading and Co:MWCNT ratio.•The effective bandwidth was 12.8–17.8 GHz at reflection loss below −10 dB.•Co/MWCNT/PE composite shows a minimum reflection loss value of −55 dB at 5.2 GHz.
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•Co/MWCNT hybrids with controllable localization of Co nanoparticles are obtained.•Localization of Co nanoparticles depends on the nature and structure of ...MWCNTs.•Hydrophobic/hydrophilic properties of MWCNT surface strongly affect Co deposition.•Oxidized MWCNTs with thin walls stabilize Co nanoparticles mainly inside their channels.•Oxidation of MWCNTs leads to the decrease of Co nanoparticles average size.
Decoration of one-dimensional multi-walled carbon nanotubes (MWCNTs) with zero-dimensional Co nanoparticles leads to hybrid structures with chemical and electromagnetic features that are not available to the individual components. This work addresses the influence of the nature and structure of MWCNTs on the localization of Co nanoparticles. Depending on synthesis conditions, Co can be deposited on the external or in inner surfaces of the nanotubes. Co/MWCNTs hybrids have been characterized by in situ X-ray powder diffraction, high-resolution transmission electron microscopy and 59Co internal field nuclear magnetic resonance. It has been shown that the average diameter (7.2, 9.4 and 18.6 nm), number of walls (5–7, 12–15, 15–20), and functional composition of the MWCNTs have a remarkable effect on the size of Co nanoparticles and their distribution in the structure of MWCNTs. The observed phenomenon has been rationalized in terms of nanotubes surface properties. Parent MWCNTs being hydrophobic and having limited porosity do not stabilize Co nanoparticles and, therefore, they are localized on the outside surface with relatively large average size and broad size distribution. On the other hand, the oxidation of the MWCNTs resulted in the penetration of Co nanoparticles inside of the nanotubes, presumably because of pore opening as well as increased hydrophilicity of the nanotubes.
Obtaining stable metal nanoparticles is of high interest for various applications such as catalysis, batteries, supercapacitors and electro-magnetic devices. Cobalt/multi-walled carbon nanotubes ...(MWCNT) hybrids with an original set of magnetic and electric properties were formed by casting Co nanoparticles (3 – 5 nm) of high aspect ratios within the internal space of MWCNTs. The Co particles localization and size were analyzed by transmission electron microscopy and synchrotron x-ray diffraction. The magnetism of the cobalt nanoparticles was probed by 59Co internal field nuclear magnetic resonance (IF NMR) and their electrical behavior by dielectric spectroscopy. The majority of Co particles were fully metallic. They resisted sintering up to 550 °C. Below 7.5 wt%, the Co was exclusively embedded inside the MWCNT. At higher loading, they coexisted with larger Co outside particles. While nanometer size particles are normally superparamagnetic at room temperature, the confinement of Co within MWCNTs resulted in a ferromagnetism revealed by 59Co IF NMR. This spectroscopy provided original information about the structure, size, and shape anisotropy of the nanoparticles. Finally, the MWCNT modification by Co metal nanoparticles improved the electrical conductivity of polyethylene based composite thus extending the useful frequency band of Co/MWCNT/PE composites for applications requiring light-weight conduction or energy absorption.
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The Si/Al ratio is a key parameter of acid-base, structural, textural and, consequently, catalytic properties of amorphous crystalline (micro- and mesoporous) aluminosilicates. The changes of ...structural and textural characteristics of Al-Si aerogels with gradual increase of aluminum content are investigated. Aerogels were prepared via sol-gel method using prehydrolysed tetraethoxysilane and aluminium isopropoxide stabilized by acetylacetone. The gelation of the obtained sols took place in the presence of ammonia with the following drying in supercritical isopropanol. It was shown all aluminum reacts with prehydrolyzed tetraethoxysilane forming spherical particles in case the content of Al in the samples is less than 20 mol %. Aluminum drives the increase of interparticle coupling leading to the particle agglomeration, which is associated with the increase of the particle size and decrease of specific surface area and pore volume. For the samples with the aluminum content of >50 mol % the formation of pseudoboehmite plate-like particles is observed. The pseudoboehmite particles prevent the sintering of SiO2 particles that leads to the increase of the aerogel specific surface area and pore volume. In case the high aluminum content (>80 mol %) the silica particles serve as a connector between boehmite plates. The ratio between Brønsted and Lewis acid sites decreases gradually with the increase of aluminum content of the aluminosilicate aerogels.
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•Al-Si aerogels has been prepared by two-stage sol-gel method using ammonia for sol gelation.•The first shows the changes in the morphology and texture of A-Si aerogels on the mole ratio of Si/Al from 49 to 0,11.•The aerogel particle vary from amorphous spheres to boehmite plates.•The pseudoboehmite layers (or plates) play the role of antiblocking agent for amorphous spherical particles.
Carbon hydrogasification is the slowest reaction among all carbon‐involved small‐molecule transformations. Here, we demonstrate a mechanochemical method that results in both a faster reaction rate ...and a new synthesis route. The reaction rate was dramatically enhanced by up to 4 orders of magnitude compared to the traditional thermal method. Simultaneously, the reaction exhibited very high selectivity (99.8 % CH4, versus 80 % under thermal conditions) with a cobalt catalyst. Our study demonstrated that this extreme increase in reaction rate originates from the continuous activation of reactive carbon species via mechanochemistry. The high selectivity is intimately related to the activation at low temperature, at which higher hydrocarbons are difficult to form. This work is expected to advance studies of carbon hydrogasification, and other solid‐gas reactions.
Plants capture CO2 from the air and fix it with H2O into carbohydrate molecules via photosynthesis. The main photosynthesized biomass, wood, can be converted into charcoal and chemicals via pyrolysis. The charcoal can then be hydrogenated into hydrocarbons using a mechanochemical method. The produced CH4 can be used directly as fuel, or as a feedstock for the synthesis of other chemicals, thereby creating a neutral or negative carbon cycle.
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