Substitution Of Si Atoms Research Articles

Boosted Lithium-Ion Transport Kinetics in n-Type Siloxene Anodes Enabled by Selective Nucleophilic Substitution of Phosphorus

HighlightsExtrinsic doping is proposed as a novel route to enhance the intrinsic properties of the two-dimensional oxidized Si nanosheet (namely, siloxene) anode for Li-ion batteries.Fabrication of P-doped n-type siloxene is revealed to be possible through selective nucleophilic substitution of Si atoms in siloxene with P atoms.Due to boosted charge transport kinetics, n-type siloxene (6.7 × 1019 P atoms cm-3) exhibits the excellent storage performance (594 mAh g-1 after 500 cycles) even at 2000 mA g-1.

Influences of C, Si and Mn on the wear resistance of coiled tubing steel

Coiled tubing steels of different grades with close compositions and similar microstructures may have a large difference in wear performance. Although contents of alloying elements, e.g., C, Si and Mn, in the steels are low, slightly adjusting their concentrations could markedly modify the wear resistance of the coiled tubing steel. In this work, atomic simulations were carried out to understand influences of the minor elements on the wear resistance of coiled tubing steel. Molecular dynamics simulations were conducted to study the wear behavior of steels with different contents of the elements, and first-principles calculations were implemented to investigate influences of the elements on atomic bonding based on their electronic properties. It is demonstrated that interstitial C atoms increase the local lattice size, and their bonding to Fe atoms shows hybrid covalent and ionic characteristics. Substitutional Mn atoms weaken the overall metallic bond but enhance the surrounding electron localizations; and substitutional Si atoms decrease local lattice size and are covalently bonded to surrounding Fe atoms. Corresponding changes in the electronic states caused by the elements strongly affect the atomic bonding. The three alloying elements have different solid-solution strengthening mechanisms, and their synergy strongly affects the wear resistance of the steel. The revealed mechanisms would help selecting and adjusting alloying elements to enhance the wear resistance of coiled tubing steels.

Correlation of Brønsted acid strengths and species substituted Si in ZSM-5 and their effects on propylene polymerization in MTA reaction

Methanol to aromatics (MTA) is regarded as an efficient process to generate benzene, toluene, xylene (BTX), and other important chemical raw materials, and the polymerization of alkenes is an important reaction in MTA. In this work, the effect of different Brønsted acid strengths over ZSM-5 on propylene polymerization was studied by using the density functional theory (DFT) method. H-M-ZSM5 with different Brønsted acid strengths are formed by substituting Si atoms in ZSM-5 by Al, Ga, In, Fe, and B atoms, respectively. And three steps including the formation of propoxide, propylene dimerization, and the deprotonation of C6 hydrocarbon are investigated. The i-propoxide is most easily generated by the protonation of propylene. And the polymerization of i-propoxide with propylene is easier compared to n-propoxide in the dimerization step. Subsequently, hexoxy primary carbon dehydrogenation to generate C6 olefins is prone in the C6 hydrocarbon deprotonation step. Therefore, the optimal path is the protonation of propylene to produce i-propoxide, which in turn polymerizes with propylene to produce hexoxy (IM5), followed by a deprotonation step to produce the most favorable product 2-Methyl-pentene. Meanwhile, the enhanced strength of the Brønsted acid significantly promotes the activity of propylene protonation and C6 hydrocarbons deprotonation, while it has little effect on the propylene dimerization. Therefore, the Brønsted acid strength is proposed as an appropriate descriptor to reflect the catalytic activity of H-M-ZSM5 to the propylene protonation and C6 hydrocarbons deprotonation. And H–Al-ZSM5 and H-Ga-ZSM5 with high acid strength are found to have the best performance for the propylene polymerization to C6 olefins.

Atomistic investigation on the impact of substitutional Al and Si atoms on the carbon kinetics in ferrite

The pairwise interactions of substitutional solute atom X = Al, Si with interstitial carbon at stable (octahedral) and saddle-point (tetrahedral) positions in body-centered-cubic iron (α-Fe) are computed using density-functional theory. These pairwise interactions are used in atomistic kinetic Monte Carlo approach to simulate carbon internal friction and tracer diffusion measurements in Fe-Si, Fe-Al, and Fe-Al-Si ferritic alloys without any adjusting parameters. The good agreement between the simulated and experimental Snoek relaxation profiles validates the pair interaction model for kinetic simulations. The predicted effect of Al on slowing down carbon diffusion is consistent with previous studies. We highlight a super-cell size effect on the Si-carbon interactions obtained from first principles. Using a carefully tested database, it is shown that the introduction of Si into ferrite only decreases the carbon diffusivity below a critical temperature.

Engineering a bandgap-regulable intermediate-band material based on diamond

A diamond-based material C63-xSixV compound is engineered by using density functional theory calculations. In the C63-xSixV compound, (1 + x) C atoms in the 2 × 2 × 2 diamond supercell are replaced by x Si and one V atoms. In this way, a bandgap-regulable intermediate-band (IB) photovoltaic material would be gotten. The calculational results indicate that it can reduce the bandgap of the diamond without changing the character of its band structure to substitute Si atoms for several C atoms in the diamond. And the bandgap decreases along with the increase of Si content. When the atomic concentration of Si is lower than 6.25%, the implantation of Si has less effect on the width and electron filling of the IB for the C63-xSixV compound. With the increase of Si content, the IB in the bandgap shifts toward the VB and the sub-bandgap absorption of the C63-xSixV compound shifts to longer wavelength range. Results of partial density of states indicate that the IB formed by implanting V in the diamond is mainly composed of the d states of V, while the contribution of the Si is negligible.

Synthesis of aluminum and gallium-incorporated MFI zeotypes and their catalytic activity for ethane dehydroaromatization

Heteroatoms such as gallium (Ga) atoms can be incorporated into the zeolite framework and can influence on the catalytic properties of zeolite-based catalysts. In particularly, Ga species is active for ethane activation, so the incorporation of Ga atoms into MFI zeolite can achieve high activity for ethane dehydroaromatization. In this work, Ga and Al atoms are incorporated into the framework of MFI zeolite (Ga-Al-MFI) with various Ga/Al ratios for ethane dehydroaromatization. Interestingly, for the incorporation behavior of Ga atom, similar trend with that of Al atom is observed in Ga-Al-MFI zeotypes synthesis. That is, these atoms need tetra-n-propylammonium (TPA+) molecules as charge compensators when they isomorphously substitute Si atoms. For ethane dehydroaromatization, catalytic activity and stability of Ga-Al-MFI catalysts are affected by Ga/Al ratio. In the framework of Ga-Al-MFI catalysts, the increase of Ga atom fraction enhances the dehydrogenation ability but weaken the Brønsted acidity of catalysts due to the different electronegativity of Al and Ga atoms. These results imply that there is trade-off for the production of aromatic compounds, so that the optimal Ga/Al ratio for achieving high activity and stability can be suggested in the range of 1–4. Additionally, various active sites in Ga-modified MFI catalysts, including Al sites, Ga species, and dehydrogenation components, were examined via several series of reaction to elucidate their roles in ethane dehydroaromatization.

New Magnetic Materials Based on RNi Compounds for Cryogenic Technology

Magnetocaloric properties of Gd(Ni0.98Si0.02) and Dy(Ni0.95Si0.05) compounds and their hydrides Gd(Ni0.98Si0.02)H3 and Dy(Ni0.95Si0.05)H4 have been studied in a temperature interval of 2–100 K. It is established that the partial substitution of Si atoms for Ni atoms and subsequent hydrogenation can lead to significant changes in the Curie temperature, magnitude of the magnetocaloric effect, and temperature at which this effect is at maximum. In the hydrides, the characteristic temperatures shift to lower values by several dozen degrees while the magnetocaloric effect increases or remains the same, which can significantly expand the application of these materials in cryogenic technology.

Electronic and optical properties of HoNi5 and HoNi4Si: An LSDA + U study

We investigated the effect of Si substitution on structural, electronic and optical properties of [Formula: see text]-type [Formula: see text] alloys. The optimized lattice constants and internal cell parameters are in agreement with the available data. We found that the valence band is mainly dominated by Ni-3d states in the energy range 0–4 eV below the [Formula: see text], whereas conduction band is contributed by spin-down Ho-[Formula: see text] states and lies about 2 eV above the [Formula: see text]. Substitution of Si atoms for Ni decreases the total magnetic moment from 6.308 [Formula: see text]/f.u. [Formula: see text] to 4.052 [Formula: see text]/f.u. [Formula: see text], whereas magnetic moments on [Formula: see text]-ions increase from 3.92 [Formula: see text]/atom [Formula: see text] to 4 [Formula: see text]/atom [Formula: see text]. On the other hand, induced moment on Ni[Formula: see text]-ions decrease rapidly to a negligibly small value. By the use of charge density estimates, we found that Ho-5d, Ni-3d and Si-3p orbitals are mainly involved in bonding and there is a weak hybridization between Ni-3d and Si-3p orbitals which varies with substitution of Si for Ni. Furthermore, the complex role played by Ho-[Formula: see text] electrons is also investigated, they are found to be partly involved in metallic bondings as well as in the intra-atomic charge transfer from [Formula: see text] to [Formula: see text] states in [Formula: see text]-ions. The calculated interband optical conductivity spectra reproduce the main features of experimental spectra well and also reveal their metallic nature.

Internal exchange interaction of Tb+3 ions and influence of Si substitution on structural, electronic, magnetic and optical properties of YNi4Si -type TbNi5−xSix (x = 0, 1, 2) compounds

We report the FPLAPW + lo study on new YNi4Si-type TbNi5−xSix (x = 0, 1, 2) compounds. The YNi4Si structure is a new structure type, which is an orthorhombic derivative of CaCu5 structure. The optimized lattice constants and internal cell parameters are in agreement with the available data. The calculated band structures show the valence band (VB) is mainly dominated by Ni–3d states in the energy range 0–4 eV below the EF, whereas conduction band (CB) by Tb–4f states about 2 eV above the EF. With the substitution of Si atoms for Ni, total magnetic moments decreases from 8.308 μB/f.u. (x = 0) to 6.052 μB/f.u. (x = 1), whereas magnetic moments on Tb+3ions increase from 5.92 μB/atom (x = 0) to 6 μB/atom (x = 1, 2) and the induced moments on Ni+2(x = 1, 2) ions decrease rapidly to a negligibly small value. Charge density estimates show that Tb-5d, Ni-3d and Si-3porbitals are mainly involved in bondings and there is a weak hybridization between Ni-3d and Si-3porbitals which varies with Si substitution. Furthermore, the complex role played by the Tb-5d1 electrons is also investigated, and found to be partly involved in metallic bondings as well as in intra-atomic charge transfer from 5d1 to 4f8 states. The calculated interband optical conductivity spectra reproduce the main features of the experimental spectra well and also reveal their metallic nature. Frequency-dependent refractive index, η(ω), the reflectivity, R(ω) and the extinction coefficient, κ(ω) for (x = 0, 1, 2) are also calculated for the incident radiation up to 12 eV.

Effect of point defects on electronic and magnetic properties of single-layer SiO

ABSTRACTBased on spin-polarised density functional theory calculations, we investigated the effect of point defects on electronic and magnetic properties of the single-layer (SL) asymmetric washboard silicon oxide (aw-SiO). The SL-aw-SiO is a counterpart of black phosphorene, and a new candidate of two-dimensional material family. This structure is dynamically and thermally stable and is a nonmagnetic semiconductor with a direct band gap. We found that single vacancy and divacancy give rise to significant change in the electronic and magnetic properties of SL-aw-SiO. The band gap of aw-SiO can be tuned by the substitution of Si atom instead of O atom, the antisite defect, the O atom vacancy and two atom vacancies. In addition, impurity states due to the defects can occur in the band continua and hence the band gap of aw-SiO is reduced. Having an integer magnetic moment, SL-aw-SiO upon Si vacancy and by substitution of O atom instead of Si atom may display half-metallic features.

Магнитные свойства гидридов соединений RNi-=SUB=-1-x-=/SUB=-Si-=SUB=-x-=/SUB=- (R --- Dy, Gd, x=0.05, 0.02)

AbstractThe magnetic properties of intermetallic compounds GdNi_0.98Si_0.02 and DyNi_0.95Si_0.05 and hydrides based on them have been studied. It is found that a partial substitution of Si atoms for Ni atoms does not cause significant changes in the magnetic characteristics such as the Curie temperature. At the same time, incorporation of hydrogen into the crystal lattice of the GdNi_0.98Si_0.02 and DyNi_0.95Si_0.05 compounds leads to significant decrease in the Curie temperature, attenuation of exchange interactions due to significant increase in the unit cell volume (more than 20%), and an increase in the distances between magnetoactive ions. The magnetism of the initial and also hydrogenated compositions are mainly determined by the contribution from the subsystem of the rare-earth ions.

Magnetic Properties of Hydrides of RNi1 – xSix Compounds (R = Dy, Gd, x = 0.05, 0.02)

The magnetic properties of intermetallic compounds GdNi0.98Si0.02 and DyNi0.95Si0.05 and hydrides based on them have been studied. It is found that a partial substitution of Si atoms for Ni atoms does not cause significant changes in the magnetic characteristics such as the Curie temperature. At the same time, incorporation of hydrogen into the crystal lattice of the GdNi0.98Si0.02 and DyNi0.95Si0.05 compounds leads to significant decrease in the Curie temperature, attenuation of exchange interactions due to significant increase in the unit cell volume (more than 20%), and an increase in the distances between magnetoactive ions. The magnetism of the initial and also hydrogenated compositions are mainly determined by the contribution from the subsystem of the rare-earth ions.

Ab-initio study of electronic properties of Si(C) honeycomb structures

In this work, the electronic structures of pure and concentrated graphene and Silicene have been studied by performing first-principles pseudo potential plane-wave calculations. The concentrated structures have been obtained by the substitution of Si(C) atoms in the graphene (silicene), respectively. Firstly, the calculations are performed for pure graphene and continued for its concentrations. The concentrated graphene is obtained by substitution of Si atoms (with: 12.5, 25, 37.5 and 50 mol percentage) at different positions in the unit cell of graphene. Similar to graphene, the same calculations are performed for pure silicene as well as for silicene after substitution of C atoms. We have modeled the lattice constant, the band structure and its directivity, while the position and mole fractions of the substituted atoms are changed in the unit cell of the studied compound. Our results showed that: the total energy, the density of States (DOS), the charge density (CD), the opening of the band gap and its directivity are strongly dependent both on the position and mole fraction of the substituted Si(C) atoms. As an interesting result, we found an indirect open band gap, as large as 2.53 eV for silicon doped graphene. Also, it was found that both the elemental concentration and unit cell geometry could offer remarkable advantages for band splitting and band gap opening in these graphene like structures, which have known as ideal structures with many promising potential applications in the electronic, optoelectronic and spintronic.

Boron doping induced thermal conductivity enhancement of water-based 3C-Si(B)C nanofluids

In this paper, the fabrication and thermal conductivity (TC) of water-based nanofluids using boron (B)-doped SiC as dispersions are reported. Doping B into the β-SiC phase leads to the shrinkage of the SiC lattice due to the substitution of Si atoms (0.134 nm radius) by smaller B atoms (0.095 nm radius). The presence of B in the SiC phase also promotes crystallization and grain growth of obtained particles. The tailored crystal structure and morphology of B-doped SiC nanoparticles are beneficial for the TC improvement of the nanofluids by using them as dispersions. Using B-doped SiC nanoparticles as dispersions for nanofluids, a remarkable improvement in stability was achieved in SiC-B6 nanofluid at pH 11 by means of the Zeta potential measurement. By dispersing B-doped SiC nanoparticles in water-based fluids, the TC of the as-prepared nanofluids containing only 0.3 vol.% SiC-B6 nanoparticles is remarkably raised to 39.3% at 30 °C compared to the base fluids, and is further enhanced with the increased temperature. The main reasons for the improvement in TC of SiC-B6 nanofluids are more stable dispersion and intensive charge ions vibration around the surface of nanoparticles as well as the enhanced TC of the SiC-B dispersions.

Stress‐Relieved Nanowires by Silicon Substitution for High‐Capacity and Stable Lithium Storage

AbstractSilicon is promising as a high energy anode for next‐generation lithium‐ion batteries. However, severe capacity fading upon cycling associated with huge volume change is still an obstacle for silicon toward practical applications. Herein, the authors report that Si‐substituted Zn2(GeO4)0.8(SiO4)0.2 nanowires can effectively suppress volume expansion effect, exhibiting high specific capacity (1274 mA h g−1 at 0.2 A g−1 after 700 cycles) and ultralong cycling stability (2000 cycles at 5 A g−1 with a capacity decay rate of 0.008% per cycle), which represents outstanding comprehensive performance. The superior performance is ascribed to the substitution of Si atom that imparts to the nanowires not only high reactivity and reversibility, but also the unique stress‐relieved property upon lithiation which is further confirmed by detailed density‐functional theory computation. This work provides a new guideline for designing high‐performance Si‐based materials toward practical energy storage applications.

Structure and Surface Properties of Al<sup>3+</sup>- Modified Sepiolite

Al3+-modified sepiolite was synthesized through hydrothermal treatment of acidified sepiolite using sodium aluminate as a modifier. The structure and surface properties of the modified sepiolite were characterized by XRD, FTIR, NMR, and Zeta potentials. The XRD and FTIR results demonstrated that Al3+ions were introduced into the sepiolite lattice.29SiNMR of the sepiolite ore showed that Si atoms had three configurations with chemical shifts of -98.4, -95.2, and -92.5 ppm with the atomic ratio of 1.39: 1.43: 1, which was assigned to the center silicon, edge silicon, and near-edged silicon atoms, respectively. For the as-prepared Al3+-modified sepiolite, the signal intensity of the edge silicon atoms decreased and shifted from -95.2 ppm to -94.5 ppm. Moreover, a new chemical shift of - 87.4ppm belonging to Q1structure appeared. The ratio of four configurations of silicon atoms was 1.43: 1: 1.53: 1.04.27AlNMR of Al3+-modified sepiolite showed that Al atoms have two configurations corresponding to the tetra-coordination and octahedral coordination, respectively. The results of NMR revealed that the sepiolite was modified by substitution of Si atoms in the framework and Mg atoms in the interlayer, resulting in a less negative charge on the surface of Al3+-modified sepiolite.

Stability and magnetic properties of isomorphous substituted Si7-xMnx+

The optimized geometries, stability, and magnetic properties of cationic clusters Si7+, Si6Mn+, and Si5Mn2+ have been determined by the method of density functional theory using the B3P86/6-311+G(d) functional/basis set. Their electronic configurations have been analyzed to understand the influence of substituting Si atoms by Mn atoms on the structural and magnetic aspects of Si7+. It is shown that the manganese dopant does not alter the structure of the silicon host but significantly changes its stability and magnetism. In particular, while the magnetic moment of Si7+ is 1 mB, Si5Mn2+ exhibits a strong magnetic moment of 9 mB and that of Si6Mn+ takes a relatively high value of 4 mB. Among studied clusters, the pentagonal bipyramid Si5Mn2+ is assigned as the most stable one.

Electron Transport Properties of Two-Dimensional Monolayer Films from Au-P-Au to Au-Si-Au Molecular Junctions**Supported by the National Natural Science Foundation of China under Grant Nos 11374033, 11774030, 51735001 and 61775016, and the Fundamental Research Funds for the Central Universities under Grant No 2017CX10007.

We investigate the electronic-transport properties of two-dimensional monolayer films from Au-P-Au molecular junction to Au-Si-Au molecular junction using elastic scattering Green’s function theory. In the process of replacing the P atoms with Si atoms every other line from the middle of monolayer blue phosphorus molecular structure, the substitution of Si atoms changes the properties of Au-P-Au molecular junction significantly. Interestingly, the current value has a symmetric change as a parabolic curve with the peak appearing in Au-Si1P1-Au molecular junction, which provides the most stable current of 15.00 nA in a wide voltage range of 0.70–2.70 V. Moreover, the current–voltage characteristics of the structures indicate that the steps tend to disappear revealing the property similar to metal when the Si atoms dominate the molecular junction.

IR spectra of carbon-vacancy clusters in the topochemical transformation of silicon into silicon carbide

Using infrared (IR) spectroscopy and spectral ellipsometry, we experimentally confirmed the previously predicted mechanochemical effect of the stoichiometric composition disorder leading to the formation of carbon-vacancy structures in silicon carbide (SiC) films grown on silicon substrates by the atom substitution method. It was found that a band at 960 cm–1 in the IR spectra of SiC films on silicon, corresponding to “carbon-vacancy clusters” is always present in SiC films grown under pure carbon monoxide (CO) or in a mixture of CO with silane (SiH4) on Si substrates of different orientation and doping level and type. There is no absorption band in the region of 960 cm–1 in the IR spectra of SiC films synthesized at the optimum ratio of the CO and trichlorosilane (SiHCl3) gas pressures. The previously predicted mechanism of the chemical reaction of substitution of Si atoms for carbon by the interaction of gases CO and SiHCl3 on the surface of the silicon substrate, which leads to the formation of epitaxial layers of single-crystal SiC, is experimentally confirmed.

Substitution behavior of Si atoms in the Ti2AlC ceramics

The substitution of Si atoms in the Ti2AlC ceramics has been observed in the Ti2AlC brazing joints using AlSi eutectic filler alloy, contributing to the formation of Ti2(Al, Si)C solid solution in the interaction area according to the SEM analysis. Afterwards, a detailed characterization was performed using FIB/TEM technique. A remarkable decrease in the lattice constants has been observed in the modified Ti2AlC ceramics, and the presence of Si element in the corresponding area has been further proved by STEM. The formation and decomposition mechanisms of the Ti2(Al, Si)C solid solution have been studied via ab initio calculations. The results suggest that all the Ti2(Al1-x, Six)C structures with different Si contents are stable in the vibration aspect. However, due to the lower Gibbs energy of the competing phases, the structures with high Si content would decompose. Finally, the bulk Ti2(Al0.9, Si0.1)C has been successfully synthesized using SPS method.