SF6 Molecules Research Articles

Sub-Ångström-level engineering of ultramicroporous carbons for enhanced sulfur hexafluoride capture

Ultramicroporous carbon materials are realized by exploiting nitrogen-doping and alkali ions activation strategies on classical carboxylic phenolic resins. Through tuning the nitrogen-doped content as well as activation strength and size of the alkali ions, carbon materials present pore sizes that are controlled at the sub-Ångström level from 5.8 to 7.7 Å with precision between 0.1 and 0.4 Å. The fine-tuning of the ultramicropore size using melamine as the sacrificial templating agent improves the practicality of the carbon materials for the recovery of SF6 which are larger than conventional molecules such as CO2. Resultantly, the optimized carbon material at 1 bar and 25 °C saw a >16-fold increase in SF6 adsorption from 0.39 to 6.55 mmol g−1 while the SF6/N2 selectivity reached as high as 78.8. We attribute this to the fine-tuning of ultramicropores, leading to optimizable pore sizes that allow easy access to the large surface areas of carbon materials by the SF6 molecules. Furthermore, owing to its high adsorption performance, a 1.5-fold higher breakthrough SF6 adsorption capacity alongside a 2-fold increase in SF6/N2 selectivity were realized under dynamic breakthrough conditions. The stability and recyclability of the carbon materials were also demonstrated with a long-term performance evaluation of up to 10 consecutive adsorption-desorption cycles.

Adsorption and decomposition of SF6 molecule on α-Al2O3 (0 0 0 1) surface: a DFT study

Based on the first principle, the interaction process between SF6 molecule and α-Al2O3 (0 0 0 1) surface was calculated. The results show that, under five adsorption sites, SF6 can form a relatively stable chemical adsorption at the O-3 site on α-Al2O3 (0 0 0 1) surface, while the other sites mainly showed physical adsorption processes. At O-3 site, 0.664 e electrons were transferred from α-Al2O3 to the SF6 molecule, and DOS analysis indicates that there was a strong electron orbital interaction between S and F atoms in SF6 and Al and O atoms on α-Al2O3 surface. After the adsorption, the molecule structure of SF6 changed significantly, the S–F bonds were elongated and partially broken, and the whole SF6 showed a tendency to be close to the surface of α-Al2O3. The energy barrier for SF6 decomposition was 0.147 eV. The results show the adsorption and decomposition process of SF6 molecules on the surface of α-Al2O3, which proves that α-Al2O3 has certain catalytic properties. The results indicate a simulation support for the efficient and harmless treatment of SF6 gas with Al2O3 catalysts.

Spectral energy and angular distributions of bremsstrahlung radiation produced by 3.5 keV electrons in scattering with a free SF6 molecule

Absolute double differential cross sections (DDCS) of bremsstrahlung (BS) radiation produced by 3.5 keV electrons in scattering with a free SF6 molecule were measured by using a Si-PIN photodiode detector in the angular detection range of 45°−110° relative to the direction of incident electrons. Angular distributions of the BS photons are found to be anisotropic. The experimental DDCS spectra are compared with the theoretical calculations obtained from atomic-field BS formulations of Kissel–Quarles–Pratt (KQP) using the “additivity hypothesis”. A fair agreement is observed between the measured results and those obtained from the KQP calculations for both the energy and the angular spectral distributions of BS radiation. The measured anisotropy of bremsstrahlung radiation is reproduced reasonably well by a modified Sommerfeld formula, if the relative velocity β is chosen to be half of the incident electron velocity β0. The measured anisotropy may be described by a parameter P, which is compared with the degree of polarisation obtained from theoretical calculations. Further, it is noted that the molecular weight, geometry and the number of constituent atoms present in the considered molecule do not seem to influence the usefulness of “additivity hypothesis”. In conclusion, the present results have once again corroborated the validity of “additivity hypothesis” for predicting the spectral distributions of molecular bremsstrahlung produced by keV-electrons.

Isotope Selective Control over Clustering of SF6 Molecules and Dissociation of (SF6)mArn van der Waals Clusters Using an IR Laser

We present the results of an investigation into the interaction of SF6 molecules and clusters in a molecular beam with resonant IR laser radiation at different stages of the beam evolution along the axis of its propagation. The beam has been formed as a result of gas-dynamic expansion of a mixture of SF6 with argon carrier gas during expansion from a pulsed nozzle. The experimental setup and the investigation method are described. It has been shown that selective vibrational excitation of SF6 molecules with a specific sulfur isotope by a CO2 laser near the nozzle edge causes suppression of the clustering process of these isotopic molecules. Selective IR excitation of clusters under the conditions of the formed cluster beam leads to isotopically selective dissociation of clusters. Depending on the experimental conditions including different distances of the irradiation zone of particles from the nozzle edge, the results of measuring the efficiency and selectivity of molecular clustering suppression and cluster dissociation processes are presented. It has been shown that both of these processes make it possible to achieve high selectivity values for the 32S and 34S sulfur isotopes. In the case in which the clustering of SF6 molecules was selectively suppressed, selectivity values α ≥ 25–30 have been obtained. Upon selective dissociation of (SF6)2 dimers under similar expansion conditions of the gas mixture, selectivity values α ≥ 20–25 for 32SF632SF6 dimers with respect to 34SF632SF6 dimers have been obtained. Particular attention has been paid to measurements at a high dilution of SF6 in argon under conditions of predominant formation of (SF6)mArn mixed clusters. The potential of using studied processes as a basis for the technology of the laser isotope separation are discussed.

Theoretical study on the interaction between SF6 molecule and BaTiO3(0 0 1) surface: A DFT study

Based on the first principle, the adsorption and decomposition process of SF6 gas molecules on the surface of BaTiO3(0 0 1) was calculated. The results show that there is a strong interaction between SF6 molecule and BaTiO3 surface, and the adsorption energy reached 4.453–4.963 eV, which is likely to be a chemical adsorption process. According to Mulliken analysis, about 1.6 e of electrons transferred from BaTiO3 surface to the SF6 gas molecule, the SF6 molecule behaved as an electron acceptor. According to density of states calculation results, there were obvious electron orbital interactions between SF6 and BaTiO3 surface atoms, especially F and O atoms, and the charge transfer process was also confirmed by the differential charge density results. The SF6 molecules interacted strongly with the BaTiO3 surface, which led to the elongation of SF bonds and made SF6 tend to undergo a decomposition process. The results indicate that BaTiO3 may have a catalytic potential for SF6 abatement and provide a simulation support for the experimental study of efficient and harmless treatment of SF6 gas.

Understanding the effect of host flexibility on the adsorption of CH4, CO2 and SF6 in porous organic cages

Abstract Molecular simulations for gas adsorption in microporous materials with flexible host structures is challenging and, hence, relatively rare. To date, most gas adsorption simulations have been carried out using the grand-canonical Monte Carlo (GCMC) method, which fundamentally does not allow the structural flexibility of the host to be accounted for. As a result, GCMC simulations preclude investigation into the effect of host flexibility on gas adsorption. On the other hand, approaches such as molecular dynamics (MD) that simulate the dynamic evolution of a system almost always require a fixed number of particles in the simulation box. Here we use a hybrid GCMC/MD scheme to include host flexibility in gas adsorption simulations. We study the adsorption of three gases – CH4, CO2 and SF6 – in the crystal of a porous organic cage (POC) molecule, CC3-R, whose structural flexibility is known by experiment to play an important role in adsorption of large guest molecules [L. Chen, P. S. Reiss, S. Y. Chong, D. Holden, K. E. Jelfs, T. Hasell, M. A. Little, A. Kewley, M. E. Briggs, A. Stephenson, K. Mark Thomas, J. A. Armstrong, J. Bell, J. Busto, R. Noel, J. Liu, D. M. Strachan, P. K. Thallapally, A. I. Cooper, Separation of rare gases and chiral molecules by selective binding in porous organic cages. Nat. Mater. 2014, 13, 954, D. Holden, S. Y. Chong, L. Chen, K. E. Jelfs, T. Hasell, A. I. Cooper, Understanding static, dynamic and cooperative porosity in molecular materials. Chem. Sci. 2016, 7, 4875]. The results suggest that hybrid GCMC/MD simulations can reproduce experimental adsorption results, without the need to adjust the host–guest interactions in an ad hoc way. Negligible errors in adsorption capacity and isosteric heat are observed with the rigid-host assumption for small gas molecules such as CH4 and CO2 in CC3-R, but the adsorption capacity of the larger SF6 molecule in CC3-R is hugely underestimated if flexibility is ignored. By contrast, hybrid GCMC/MD adsorption simulations of SF6 in CC3-R can accurately reproduce experiment. This work also provides a molecular level understanding of the cooperative adsorption mechanism of SF6 in the CC3-R molecular crystal.

Highly tunable sulfur hexafluoride separation by interpenetration control in metal organic frameworks

Abstract The separation of fluid SF6/N2 mixtures using the recently synthesized metal-organic framework SIFSIX-2-Cu and its interpenetrated polymorph SIFSIX-2-Cu-i has been systematically studied by employing Monte Carlo and Molecular Dynamics simulations. These predictions revealed that SIFSIX-2-Cu shows a high thermodynamic adsorption selectivity for SF6 over N2 combined with one of the highest SF6 uptakes reported so far for an adsorbent. On the other hand, the smaller pore dimensions of the interpenetrated network cause a molecular sieving effect, allowing only the adsorption of N2. Furthermore, the self-diffusivity of the adsorbed SF6 molecules in the SIFSIX-2-Cu framework was predicted to be significantly lower than the one corresponding to N2, while the smaller pore channel network of SIFSIX-2-Cu-i decreases the self-diffusivity of N2 by a factor of about 6. This study revealed that controlling the interpenetration in MOFs is an efficient way to tune their separation performances for strategic gas mixtures.

Theoretical study of SF6 decomposition on the MoS2 monolayer doped with Ag, Ni, Au, Pt: a first-principles study

SF6 as a greenhouse gas, how to efficiently decompose it becomes a hotpot in the environmental field. Based on the first-principles calculations, this paper studied the interaction mechanism of SF6 gas molecules on transition metal (TM) doped MoS2 surface. The adsorption energy, energy barrier, charge transfer, density of states and electron density difference have been discussed. The results show that TM doping can enhance the interaction of MoS2 surface with SF6 molecules compared to undoped MoS2. Among the four (Au, Pt, Ag, Ni) doping conditions, the adsorption energies of SF6 molecules in the Au–MoS2 and Pt–MoS2 systems were 0.306 eV and 0.249 eV, the charge transfer process was weak, and the SF6 molecule did not change significantly. In the Ag–MoS2 and Ni–MoS2 systems, the adsorption energies reached 0.464 eV and 0.473 eV, and the DOS and differential charge analysis show that there were strong charge transfer process and electron orbital interaction between SF6 and MoS2. The decomposition energy barriers of SF6 on Ag–MoS2 and Ni–MoS2 surface were 0.696 eV and 0.432 eV, respectively. The S–F bonds were obviously elongated. The results show that Ag–MoS2 and Ni–MoS2 have catalytic potentials for the decomposition of SF6.

Fabrication of defect-free Matrimid® asymmetric membranes and the elevated temperature application for N2/SF6 separation

N2/SF6 mixtures have been widely used in gas-insulated equipment. Highly efficient separation approaches are urgently required to recycle SF6 from the mixtures during equipment maintenance. Gas separation with glassy polymeric membranes is regarded as a promising approach due to the great size difference between SF6 and N2 molecules. However, the common membranes are quite low permeable for N2/SF6 mixtures. In this research, the dry-wet phase inversion was customized with sectionalized air gap, i.e., dried air at upper section and humidified air at lower section, to manufacture Matrimid® asymmetric membranes in bulk with an ultrathin and defect-free selective layer. After optimization, the defect-free selective layer could be thinned to 80 nm. The pure gas test at 25 °C revealed that JN2 was increased to 2.58 GPU, and αN2/SF6 was approximately 41, very close to the value in the literature [J. Membr. Sci., 2014, 452, 311]. Without the restriction from the usual PDMS coating layer for defect-blocking, the elevated temperature operation was able to be employed to further enhance gas permeation. At 128 °C, the selectivity αN2/SF6 was maximized to 115, and the relevant JN2 is 9.0 GPU. At 200 °C, the temperature close to the allowable upper limit, JN2 was increased to 15.9 GPU, and the relevant αN2/SF6 was 97. Furthermore, the mixed-gas test demonstrated the favorable long-term stability of the customized membranes, even under the extreme running condition with pressure up to 2.0 MPaG and temperature up to 200 °C. With high efficiency and throughput under elevated temperature operation mode, the Matrimid® asymmetric membrane with ultrathin and defect-free selective layer is a promising approach for the separation and recycle of SF6 from the N2/SF6 mixtures.

Laser-ablation-assisted SF6 decomposition for extensive and controlled fluorination of graphene

We present a safe, clean, convenient and easy-to-control method for extensive fluorination of graphene using laser-ablation-assisted decomposition of gaseous SF6 molecules. The decomposition process is based on irradiation of a silicon target using a Nd:YAG laser (532 nm) in an SF6 atmosphere. The reactive species produced in the plume above the silicon target are consequently responsible for fluorination of graphene placed in proximity to the plume. The applicability of the proposed method is demonstrated on fluorination of CVD-grown graphene on copper foil. Samples with different fluorination levels were evaluated using both X-ray photoelectron spectroscopy and Raman spectroscopy. The influence of the applied number of laser pulses on the nature of the CF bonds, fluorine and sulphur concentrations, as well as graphene damage, is discussed. The method enables control of fluorine content by simple counting of laser pulses. We show that a fluorination level corresponding to a stoichiometry up to C1.4F can be achieved. The fluorination does not lead to the formation of CuF, in contrast to previously published approaches.

Resonant Photoelectron Confinement in the SF6 Molecule

Recent thorough experimental activity aiming to generate high harmonics in the SF6 molecules requires the knowledge of, on one hand, accurate valence-shell photoionization cross sections and phases, from a threshold up to a few tens of eV, where resonances are likely to appear, and, on the other hand, the effect of the nuclear vibrational dynamics on the process. In this work, we have experimentally determined and theoretically evaluated vibrationally resolved photoionization cross sections of SF6 up to 80 eV photon energies, with an emphasis on the E2T1u channel, for which vibrational progressions are fully resolved in the experiment. Our results reveal the presence of shape resonances due to excitation to SF6 virtual states lying just above the ionization threshold, in agreement with previous synchrotron radiation work and theoretical calculations. More interestingly, our calculations also disclose resonance features at photoelectron energies as high as 40-50 eV, which are due to the transient confinement of the ejected electron in the octahedral cage formed by the peripheral F atoms. In the vicinity of all resonances, including those due to confinement, the calculated ionization phases experience an excursion of about π or π/2 and significantly depend on the final vibrational state of the remaining cation. Both effects should be taken into account to correctly interpret ongoing high-harmonic generation work in SF6. A similar behavior is expected for other symmetric molecules containing a central atom, such as BF3, CF4, and the like.

3D-printed scaffolds of biomineralized hydroxyapatite nanocomposite on silk fibroin for improving bone regeneration

In an attempt to fabricate biomimetic bone repair scaffolds and improve bone regeneration point of view, we have three dimensionally printed porous scaffolds with biomineralized hydroxyapatite/silk fibroin nanocomposites. SF/HA composite particles were firstly produced via an in-situ mineral precipitation process when SF molecules were served as templates.. Microscopy observations of SF/HA showed homogeneous morphology and narrowly distributed size. By using sodium alginate (SA) as paste binder, scaffolds with different contents of SF/HA were subsequently 3D-printed under proper conditions. All the scaffolds were porous with 3D interconnected large pores (size ～400 μm) and an overall porosity about 70%, combined with a relative high compressive strength (>6 MPa). The in vitro biological properties of degradation, apatite formation, human bone marrow-derived mesenchymal stem cells (hBMSCs) proliferation and differentiation were characterized on SF/HA-SA scaffolds. Results indicated that more apatite newly formed on the scaffold surface after soaking in SBF and original deposited HA minerals would recrystallize. In addition, the pH values of medium microenvironment remained steady in the range from 6.9 to 7.1 in one month. Cell attachment and penetration into scaffolds were supported by all the groups. Increased content of SF/HA led to better cell proliferation, and enhanced ALP activity as well. Bovine serum albumin (BSA) protein was selected as a model drug to be loaded into SF/HA-SA scaffolds in printing process, and the drugs released in a relative sustained manner in 5 days. These results suggest a promising application of SF/HA-SA scaffolds in bone regeneration.

Determination of Abstract presentation of the point group of the symmetries of SF6 molecule

Determination of Abstract presentation of the point group of the symmetries of SF6 molecule

Isotope-selective control of the clusterization of SF6 molecules by an IR laser during gasdynamic expansion of an SF6/Ar mixture

We present the results of our investigation into the isotope-selective control of the clusterization of SF6 molecules during their gasdynamic expansion in a mixture with an argon carrier gas using IR laser radiation. The method is based on the selective vibrational excitation of molecules of a chosen isotopic composition by an IR laser near the nozzle edge, which leads to the suppression of their clusterization with each other and with argon atoms. The experimental setup and the method of investigation are described. Results are presented on determining the efficiency and the selectivity of the clusterization suppression of molecules at different gas compositions above the nozzle, as well as at different distances of the irradiation region of particles from the nozzle edge. Dependences of the clusterization suppression efficiency on the frequency and power of the exciting laser radiation are obtained. Experiments on the selective suppression of the clusterization of SF6 molecules in mixtures with other carrier gases have also been performed. It is found that controlling the clusterization process of SF6 molecules in a mixture with the argon carrier gas makes it possible to achieve relatively high selectivity values. Thus, in the case of using an SF6/Ar mixture at a 1/200 pressure ratio, the obtained selectivity of the clusterization suppression of 32SF6 molecules with respect to 34SF6 molecules under jet irradiation at the 10P(16) CO2 laser line (at a frequency of 947.74 cm−1) was . Conditions are found under which optimal values of the efficiency and selectivity of the clusterization suppression of molecules are realized.

Theoretical study of the interaction of SF6 molecule on Ag(1 1 1) surfaces: A DFT study

As a kind of gas insulation medium, SF6 is widely used in the power industry. Due to its high global warming potential value, the degradation and recycle of SF6 has become a research hotspot in recent years. At present, there have been some achievements in the experimental study on the degradation of SF6, but there is little research on the catalytic degradation of SF6 by metal catalysts. Based on the first-principle, this paper investigated the interaction between SF6 and Ag(1 1 1) and evaluated the possibility of using Ag as a catalyst in SF6 degradation. It is found that SF6 can interact with Ag(1 1 1) surface with eight initial configurations. After the interaction, the distance between the F atom in the SF6 molecule and the Ag surface is significantly shortened and the SF bond is elongated. There is a strong charge exchange process between the Ag metal interfaces and SF6 molecule. Ag (1 1 1) transfers electrons of about 1.1e− to the SF6 molecule and the dissociation of SF6 by Ag metal shows a catalytic effect. The related research results provide a theoretical reference for the silver-catalyzed degradation of SF6.

Low temperature plasmas induced in SF6 by extreme ultraviolet (EUV) pulses

In this work, a comparative study of extreme ultraviolet (EUV) induced low temperature SF6-based plasmas, created using two different irradiation systems, was performed. Both systems utilized laser-produced plasma (LPP) EUV sources. The essential difference between the systems concerned the formation of the driving EUV beam. The first one contained an efficient ellipsoidal EUV collector allowing for focusing of the EUV radiation at a large distance from the LPP source. The spectrum of focused radiation was limited to the long-wavelength part of the total LPP emission, λ > 8 nm, due to the reflective properties of the collector. The second system did not contain any EUV collector. The gas to be ionized was injected in the vicinity of the LPP, at a distance of the order of 10 mm. In both systems, energies of the driving photons were high enough for dissociative ionization of the SF6 molecules and ionization of atoms or even singly charged ions. Plasmas, created due to these processes, were investigated by spectral measurements in the EUV, ultraviolet (UV), and visible (VIS) spectral ranges. These low temperature plasmas were employed for preliminary experiments concerning surface treatment. The formation of pronounced nanostructures on the silicon surface after plasma treatment was demonstrated.

Molecular Mechanism of Loading Sulfur Hexafluoride in γ-Cyclodextrin Metal-Organic Framework.

γ-Cyclodextrin metal-organic framework (γ-CD-MOF) is a new type of highly porous carrier for potential loading of therapeutic or diagnostic gas like sulfur hexafluoride (SF6). Here, loading of SF6 into γ-CD-MOF was investigated for its mechanism by molecular simulation and quantitative determination of SF6 using quantitative nuclear magnetic resonance (qNMR). For the SF6 loading, γ-CD-MOF was first degassed to remove the air without thermal decomposition or loss of framework crystallinity, then placed in the copper tube, and sealed to adsorb SF6 under 1.2 MPa and 25 °C for 12 h. The qNMR was employed for the determination of SF6 loaded in γ-CD-MOF using Span 80 as suspending agent and trifluoroacetic acid as internal standard. Then, the thermodynamic parameters had been estimated. Finally, molecular modeling combining with 19F NMR spectra was conducted to reveal the status of SF6 molecules in γ-CD-MOF. The results demonstrated that the content of SF6 loaded in γ-CD-MOF was 2.67 ± 0.46 wt %. After exposing to the environment of free SF6 at 0.1 MPa for 10 days, the relative content was 74.7%. It was confirmed that SF6 preferred to stay in the cavity of γ-CD-MOF cubes rather than in the γ-CD molecular pairs, which was a nonchemical adsorptive process. In conclusion, this research has established qNMR method and molecular simulation to demonstrate SF6 molecules in γ-CD-MOF and its loading mechanism.

Synthesis and properties of CSxFy thin films deposited by reactive magnetron sputtering in an Ar/SF6 discharge

A theoretical and experimental study on the growth and properties of a ternary carbon-based material, CSxFy, synthesized from SF6 and C as primary precursors is reported. The synthetic growth concept was applied to model the possible species resulting from the fragmentation of SF6 molecules and the recombination of S–F fragments with atomic C. The possible species were further evaluated for their contribution to the film growth. Corresponding solid CSxFy thin films were deposited by reactive direct current magnetron sputtering from a C target in a mixed Ar/SF6 discharge with different SF6 partial pressures (). Properties of the films were determined by x-ray photoelectron spectroscopy, x-ray reflectivity, and nanoindentation. A reduced mass density in the CSxFy films is predicted due to incorporation of precursor species with a more pronounced steric effect, which also agrees with the low density values observed for the films. Increased leads to decreasing deposition rate and increasing density, as explained by enhanced fluorination and etching on the deposited surface by a larger concentration of F/F2 species during the growth, as supported by an increment of the F relative content in the films. Mechanical properties indicating superelasticity were obtained from the film with lowest F content, implying a fullerene-like structure in CSxFy compounds.

Electrostatic Neutralization Characteristics of an AC‐Corona‐Discharge‐Type Ionizer in Mixed N2‐O2, N2‐SF6 Atmosphere

SUMMARYWe are developing an electrostatic neutralizer operating in an atmosphere without oxygen for the organic electronics fabrication industry. As a part of the study, we investigated ion generation and neutralization characteristics of an ac‐corona‐discharge‐type ionizer in different atmospheric gases such as ambient air, pure nitrogen, and mixed N2‐O2 and N2‐SF6 gases, with changing the partial pressure of O2 and SF6, respectively. The I–V characteristic of the emitter shows that the negative current in N2 gas is significantly greater than the positive one; thus, the product final voltage VPF gives a significant deviation to a negative value (about –2 kV). However, by introducing SF6 gas by 0.01%, VPF reduces to about 0 V as that in ambient air. The amount of SF6 needed to improve the ion balance is three orders of magnitude smaller than that of O2, due to higher electronic affinity that a SF6 molecule has.

High order harmonic generation from SF6: Deconvolution of macroscopic effects.

We measure high order harmonics from the molecule SF6 over a large range of phase matching conditions and observe several features in the harmonics that are largely independent of such macroscopic conditions. The experimental data are then compared to the quantitative rescattering theory for the generation of harmonics from three orbitals. With this comparison, we are able to assign spectroscopic features in the harmonics to contributions from 1t1g (HOMO) and 5t1u (HOMO-1) orbitals.