Silicon Atoms Research Articles

Influence of alkyl groups on the formation of softenable polysilsesquioxanes

Polysilsesquioxanes (PSQ) containing phenyl groups as substituents on the silicon atom can form transparent, hard and glassy materials at room temperature, which reversibly soften when heated above the glass transition temperature. Further increase in temperature leads to irreversible curing. With this property, polyphenylsilsesquioxanes can be assigned to the so-called melting gels. In contrast to the aromatic systems, polysilsesquioxanes with alkyl groups belonging to this class of materials are not known. To identify structural differences between aryl and alkyl systems, polyalkylsilsesquioxanes (RPSQs with R = methyl, ethyl, propyl, hexyl, octyl, decyl, dodecyl, hexadecyl, and octadecyl) were synthesised by a solvent-free, acid-catalysed hydrolysis and condensation reaction of alkyltrimethoxysilanes followed by thermal treatment at 300 °C under N2 atmosphere. The influence of the alkyl chain length on the structure built and the thermally initiated further condensation reactions were investigated via NMR, FTIR, TGA, DSC, SEC, and XRD. Depending on the alkyl chain length, the formation of highly crosslinked, insoluble systems (Me-PrPSQ), low molecular weight oligosilsesquioxanes in the form of cages (Hex-DecPSQ) to semicrystalline, lamellar layers (Dodec-OctadecPSQ) were detected. A low degree of condensation, inhibition of self-assembly and preferentially intermolecular condensation reactions were found to be crucial factors in the melting gel formation.Graphical So called melting gels are usually prepared by acid-catalysed hydrolysis and condensation of aromatic trialkoxysilanes. The final materials show a thermoplastic and a thermoset temperature regime based on the stabilisation of silanol groups in the material. In this study we transferred the typical synthetic conditions to alkyltrialkoxysilanes with different alkyl chain lengths and obtained completely changed structures of the synthesised polyalkylsilsesquioxanes compared to their aromatic counterparts. Depending on the chain length, densely cross-linked solids, viscous gels with cage-like structure and waxes with a lamellar composition were formed. Moreover, missing stabilisation of the OH groups and self-assembly phenomena, which favour the cage and semicrystalline layer formation were detected in the resulting materials, which leads to the loss of the typical melting gel properties.

Improved selectivity and anti-aging ability for 6FDA type carbon molecular sieve membranes by mixing silicon from macro-phase separation to molecule doping

Good gas separation performance and anti-aging ability are important for the applications of carbon molecular sieve membrane (CMSM). In this work, silicon atoms in the form of siloxane-Cl are introduced into 6FDA-DABA type polyimide (PI) precursor membrane, and benzimidazoles (BI) is copolymerized into PI to effectively improve the compatibility between PI and siloxane-Cl. By varying the content of siloxane-Cl, the composite membrane changes from macro-phase separation to molecule level miscibility, which leads to the formation of silicon-rich interface layer and silicon molecule doping after high temperature pyrolysis, respectively. The residual silicon content in CMSM could be as high as 1 wt%. The high content silicon doped CMSM exhibits much improved H2 permeability retention degree (3617 Barrer of H2 permeability, 62% higher) and H2/CH4 selectivity (138, 51% higher) as compared to control CMSM after 45 days of physical aging. This work proves that the incorporation of silicon in CMSMs in proper forms could significantly improve their molecular sieving and anti-aging performances; a higher content of residual silicon and molecule level doping are highly desired. The heteroatom doping is expected to pave a new way for finely tuning the micro-structures of CMSMs and improving gas separation performances.

Topological variations in aluminum-silicon alloys through inoculation with alkali earth elements: A first-principles study

Aluminum-silicon is a simple binary eutectic system with limited solid solubility of aluminum in silicon. Hypoeutectic compositions of Al-Si are the basis of several alloys for aerospace and automotive industries. The eutectic phase mixture contains coarse plate-like silicon. Typically, alkaline earth elements such as strontium, calcium, and barium are added in minute quantities that produce a fibrous network of silicon to improve the fracture toughness. Here, we use first-principles calculations to understand the origin of these morphological variations and explain these through interatomic interactions between aluminum, silicon, and inoculant atoms. Our results show that silicon atoms cluster together and arrange themselves in a planar (100) configuration within the aluminum matrix. When strontium, calcium, or barium atoms are added to the system, these atoms prefer to be near the silicon clusters and change the planar morphology to a three-dimensional configuration with facets on (111) planes.

Preparation of Hierarchical Porous and Tunable Silicon Microenvironment SAPO-18 Sheets for Methanol-to-Olefins

In this study, hierarchically porous SAPO-18 catalysts with abundant intracrystalline mesopores were designed and fabricated with a cost-effective non-surfactant cationic surfactant (polydimethyldiallylammonium chloride, PDADMAC) as a pore-forming agent, morphology regulator, and silicon inducer by a one-step method. PDADMAC promoted the introduction of silicon atoms in the SAPO-18 framework, making the hierarchical porous SAPO-18 sheets present abundant optimized acidity. With the combination of porous structure and suitable acid density, the hierarchical porous SAPO-18 sheets exhibited a threefold improvement in catalytic lifetime and a 4% increase in ethylene and propylene selectivity during the methanol-to-olefin (MTO) reaction, compared with the conventional SAPO-18 zeolite.

Microstructural characterization and mechanical properties of 6061 aluminum alloy processed with short-time solid solution and aging treatment

In order to reduce the cost of hot stamping of aluminum alloys and develop high-strength aluminum alloy parts for light-weight car body, short-time solid solution (SS) followed by shot-time artificial aging process was implemented for 6061 aluminum alloy with salt bath furnace and silicone oil bath. X-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM-EDS) and high-resolution transmission electron microscope (HR-TEM) were used to characterize the microstructural evolution in details. The holding time for SS can be reduced by 80 % with short-time SS treatment because of the decreased vacancy generation energy and effective boundary areas. Peak aging can be obtained at 200 °C for only 60 min, and the microstructure was characterized by a lot amount of acicular nano-size β’’ phase in the coherent with matrix, while precipitate-free-zone (PFZ) also appeared along the grain boundaries. All tensile properties such as 327 MPa yield strength, 362 MPa tensile strength and 13.1 % elongation meet the 6061 T6-standard. The micro stress introduced by superheated degree along with short-time treatment should be attributed to the accelerated diffusion of magnesium and silicon atoms, which greatly affects the diffusion hermodynamics and kinetics, especially nearby the grain boundaries.

Efficient deep-blue luminescence based on dual-channel intra/intermolecular exciplexes

Highly efficient and stable blue organic light-emitting diodes (OLEDs) cannot be easily obtained simultaneously. In particular, the efficiency roll-off as a reference index to evaluate the lifetime of deep-blue OLED at high luminescence is still severe. A novel molecule (CzSiTrz) connected with carbazole and triazine fragments by a nonconjugated silicon atom is designed. An intramolecular charge transfer emission and intermolecular exciplex luminescence in the aggregated state are obtained, resulting in a dual-channel intra/intermolecular exciplex (DCIE) emission with fast and efficient reverse intersystem crossing (RISC). Deep-blue OLED with Commission Internationale de l'Eclairage (CIE) coordinates of (0.157, 0.076) and a record-high external quantum efficiency (EQE) of 20.35% at high luminance (5000 cd m-2) is accomplished. Simple molecular synthesis and device fabrication of this strategy give a unique approach to realizing high-performance deep-blue electroluminescence.

Surface activation of Si-based Schottky diodes by bacterial biosynthesized AgInSe2 trimetallic alloy nanoparticles with evidenced negative capacitance and enhanced electro-dielectric performance

Semiconductor surface activation aims to modify the surface characteristics of a semiconductor material, such as its work function, surface energy, and chemical composition to improve its electrical properties and enable efficient charge transport across the device. Here, AgInSe2 complex trimetallic alloy was utilized as an interfacial layer in Si-based Schottky diodes and served as a semiconductor surface activator. The alloy nanoparticles were produced by green bacterial biosynthesis for the first time in the literature. XRD and micro-Raman manifested a nanostructured polycrystalline nature of the alloy with rhombohedral AgInSe2 and cubic Ag crystal phases. From the electrical measurements, the diode with AgInSe2 interfacial layer showed superior electrical and dielectric characteristics compared to the pure diode at room temperature. The introduction of interlayer resulted in a smaller ideality factor, larger barrier height, higher rectification ratio, lower leakage current, and better dielectric properties compared to the pure diode. The diode also recorded a negative permittivity, up to a certain frequency, that could enhance its performance when integrated into electronic circuits. The evidenced negative capacitance and its possible benefits in Schottky diodes are elaborated. The electro-dielectric enhancement was attributed to AgInSe2-induced semiconductor surface activation through the stimulation of the unpaired reactive silicon atoms at the semiconductor surface.

Synthesis and photophysical properties of cyclosiloxanes with substituted naphthyl groups

Cyclosiloxanes bearing 1-naphthyl and 5-substituted naphthyl groups have been prepared from the corresponding silanediols with bases such as triethylamine and 1,8-diazabicyclo[5.4.0]undec-7ene (DBU) in moderate yields. The selectivity for the synthesis of cyclotrisiloxanes [-SiAr2-O-]3 and cyclotetrasiloxanes [-SiAr2-O-]4 can be controlled by the substituents on the naphthyl rings and the strength of the base. A less reactive silanediol by introducing of electron-donating groups such as dimethylamino groups gave kinetically controlled cyclotrisiloxane. Di(1-naphthyl)silanediol was also found to give the corresponding cyclotrisiloxane with triethylamine. Meanwhile, reactive silanediol bearing electron-withdrawing groups afforded the corresponding thermodynamically stable cyclotetrasiloxane even in the presence of triethylamine. Strong base such as DBU promoted the rearrangement from cyclotrisiloxane to cyclotetrasiloxane by HPLC analysis, therefore the latter product was obtained as the main product. The photophysical properties of cyclotrisiloxanes 4b (Ar = 1-naphthyl) and 4c (Ar = 1-(5-N,N-dimethylaminonaphthlyl)), and cyclotetrasiloxanes 5a (Ar = 1-(5-cyanonaphthyl)) and 5b (Ar = 1-naphthyl) were carried out in different organic solvents. Cyclotrisiloxane 4b showed both monomer and excimer emission. Cyclotetrasiloxanes 5a and 5b showed characteristic excimer emission due to the fully overlapping naphthyl groups on the vicinal silicon atoms in the ground state from X-ray and theoretical calculations. Cyclotrisiloxane 4c showed broad emission and high quantum yield. The emission wavelength depended on the solvent polarity due to the ICT of the naphthyl groups. The solubility of the cyclosiloxanes was improved compared to previously reported cyclosiloxanes having 1-pyrenyl groups.

Influence of the Duration and Temperature of the Al-Fin Process for the Cast Iron Insert on the Microstructure of the Bimetallic Joint Obtained in the Piston Casting Process

The aim of this work was to determine the effect of the duration and temperature of the Al-Fin process for casting the bimetallic joint on the formation of the metallurgical bond between the AlSi9 aluminium alloy piston and the cast iron insert. Knowledge about the formation of individual bonding layers has a direct impact on the bonding strength between the aluminium alloy and the EN-GJLA-XNiCuCr15-6-2 austenitic cast iron ring. In the performed tests, the Al-Fin casting process was carried out for different temperatures of the liquid AlSi9 alloy. The temperature of the AlSi9 aluminium alloy varied in the range 700–950 °C in steps of 50 °C. The duration of the Al-Fin process ranged from 1 to 10 min. An optical microscope and a scanning electron microscope were used to study the microstructure of the bimetallic joints. The analysis of the microstructure of the bimetallic bond showed that characteristic layers are formed between the aluminium alloy piston and the cast iron insert: a transition layer, in which iron and aluminium atoms from both joined materials are mixed, and a diffusion layer, in which aluminium and silicon atoms penetrate the surface layer of the joined metals. The thickness of the intermetallic diffusion layer formed between the aluminium alloy and the cast iron insert is thinner and does not depend on the heating time of the aluminium insert in the bath. However, there is a significant effect of both the annealing time and the temperature of the AlSi9 aluminium alloy on the thickness of the transition zone.

Sistem Switching pada Automactic Transfer Switch (ATS) menggunakan Arduino UNO

Panel is a form of alternative energy that is widely used by humans today to be managed into electrical energy. Solar panels are made of semiconductor materials, especially silicon which is coated by a special additive. When sunlight approaches the cell, the electrons will be released from the silicon atoms and flow to form an electrical circuit so that electrical energy can be generated. The results of the study, where the transfer or switching carried out by the relay was orderead by the coding of the arduino uno. At the time before the transfer of switching, the arduino uno will delay a few seconds before switching the source. Where this ATS perfomance is controlled throught a magnetic force from an oen condition to a closed condition. As abreaker and a power connector, this tool has a material in the from of connecting contact point has a large size so that it can distribute large power loads. Then the contact point has a large size so that it can distribute large electric currents, one example of a type of breaker and connector ia an AC and DC relay as an automatic regulating device. From the results of the analysis it is concluded that the Switching Automatic Transfer Switch work process can be carried out by the relay on the order of the coding from Arduino UNO automatically

Si‐Tethered Bis‐ and Tris‐Malonates for the Regioselective Preparation of Fullerene Multi‐Adducts

AbstractBis‐ and tris‐malonates constructed around a silicon atom have been prepared by reaction of malonate derivatives bearing an alcohol function with di‐tert‐butylsilyl bis(trifluoromethanesulfonate) and tert‐butyl(trichloro)silane, respectively. These compounds have been used for the regioselective bis‐ and tris‐functionalization of C60 under Bingel conditions. By changing the nature of the linker between the central Si atom and the reactive malonate groups, the malonate precursors have been optimized to produce specific bis‐ and tris‐adducts with excellent regioselectivity. A complete understanding of the electronic and stereochemical factors governing the regioselectivity has been obtained by combining computational studies with a complete analysis of the by‐products formed during the reactions of the Si‐tethered tris‐malonates with C60. Finally, desilylation reactions of the resulting fullerene bis‐ and tris‐adducts have been carried out to generate the corresponding acyclic fullerene bis‐ and tris‐adducts bearing alcohol functions.

The addition of low-cost few layers graphene (FLG) to improve flexural strength of coal fly ash based-geopolymer

The flexural strength of geopolymer has been improved by the addition of few layers graphene (FLG). The composites were made by mixing coal fly ash, alkaline activator, fine aggregate and FLG. FLG was obtained from a simple, low-cost and environmentally friendly liquid shear exfoliation method using a kitchen blender. Flexural strength and morphological properties were characterized using hydraulic concrete beam testing, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. The interaction between the geopolymer matrix and the FLG were also discussed through the density functional theory (DFT) approach. The flexural strength of composites increased with the increasing FLG content, and reached a maximum increase of 14.9% when 12% of FLG was added. SEM micrographs showed that the FLG addition changed the surface profile of geopolymers, from a grain-containing surface to a grain-less surface. TEM figures confirmed the existence of FLG, where various sizes of FLG were distributed and embedded in the matrix. The optimized structure through the DFT calculations revealed the emergence of several types of secondary bonds between carbon atoms on the FLG surface and the nearest oxygen (O), sodium (Na) and silicon (Si) atoms that strengthen the composite. Unexpectedly, this geopolymer composite could maintain its high flexural strength after exposure to a temperature of 300 °C. Overall, low-cost FLG is a promising additive for manufacturing high flexural strength geopolymers for industrial scale and for expanding their catalog of contemporary applications.

Microkinetic based growth and property modeling of plasma enhanced atomic layer deposition silicon nitride thin film

We demonstrate a microkinetic modeling framework which is a first principle-based surface reaction thermodynamics modeling methodology to describe the plasma-enhanced atomic layer deposition process of silicon nitride thin film formation. The results illustrating the relationship between silicon nitride growth per cycle (GPC) and quasi self-limiting behavior on both dichlorosilane precursor dose amount and plasma nitridation time are consistent with the experiment. Ultimately, GPC is limited to the equivalent of a half monolayer of a Si3N4 crystalline structure. Importantly, we have observed a strong correlation between subsurface NH terminated Si group concentration and HF wet etch rate by an experiment, which varies with substrate temperature.

Microstructure evolution, growth kinetics and formation mechanisms of silicon-rich NbSi2 coatings on Nb substrate

A Si-rich NbSi2 coating was prepared on Nb substrate by hot dip silicon plating process, which mainly consists of Nb5Si3 interfacial layer, NbSi2 intermediate layer and silicon-rich NbSi2 outer layer. The Si-rich NbSi2 coating has flat surface and dense structure, and the surface roughness is only 0.218–0.548 μm. Raising the deposition temperature can also significantly increase the growth rate of the Si-rich NbSi2 coating. The hot-dip silicon plating process has an extremely low reaction activation energy (249.99 kJ mol−1), which indicates that there are a large number of activated molecules during hot dip silicon plating. The silicon atoms in the molten bath deposited on the Nb surface and maintained a high chemical potential gradient continuously. Therefore, the solid diffusion reaction of Nb–Si was continuous, and the Nb5Si3 phase was generated and consumed continuously, and transformed into NbSi2 phase, which led to a higher growth rate of coating (1.7 × 10−13 to 4.3 × 10−13 m2 s).

Isothiourea-Catalyzed Acylative Desymmetrization of Silicon-Centered Bisphenols.

The preparation of optically pure organosilicon compounds bearing a stereogenic center at the silicon atom is an attractive but challenging enterprise. Herein we disclose an isothiourea (ITU)-catalyzed monoacylation reaction of silicon-centered bisphenols with 2,2-diphenylacetic pivalic anhydride, delivering tetrasubstituted organosilanes in moderate to excellent yields (36-91%) with moderate to excellent enantiomeric ratios (68:32-97.5:2.5). This organocatalytic desymmetrization approach can be performed on gram scale, and the products can be converted to other valuable compounds.

Spatial distribution characteristics of plumes induced by femtosecond laser ablation of silicon in vacuum

The spatial distribution characteristics of plumes induced by femtosecond laser ablation of silicon in vacuum are studied by using spectroscopy. The plume spatial distribution clearly shows two zones with different characteristics. The center of the first zone is at a distance of approximately 0.5 mm from the target. Silicon ionic radiation, recombination radiation, and bremsstrahlung mainly occur in this zone, causing an exponential decay with a decay constant of approximately 0.151–0.163 mm. The second zone with a greater area, whose center is at a distance of approximately 1.5 mm from the target, follows the first zone. In this zone, the radiation from silicon atoms and electron-atom collisions dominates, leading to an allometric decay with an allometric exponent of approximately − 1.475 to − 1.376. In the second zone, the electron density spatial distribution is approximately arrowhead-shaped, which is potentially induced by collisions between ambient molecules and the particles in front of the plume. These results indicate that both the recombination effect and expansion effect play important roles and compete with each other in plumes. The recombination effect is dominant near the silicon surface, causing exponential decay. As the distance increases, the electron density decreases exponentially by recombination, causing a more intense expansion effect.

Vapor-phase grafting of functional silanes on atomic layer deposited Al2O3

Fundamental studies are needed to advance our understanding of selective adsorption in aqueous environments and develop more effective sorbents and filters for water treatment. Vapor-phase grafting of functional silanes is an effective method to prepare well-defined surfaces to study selective adsorption. In this investigation, we perform vapor phase grafting of five different silane compounds on aluminum oxide (Al2O3) surfaces prepared by atomic layer deposition. These silane compounds have the general formula L3Si–C3H6–X where the ligand, L, controls the reactivity with the hydroxylated Al2O3 surface and the functional moiety, X, dictates the surface properties of the grafted layer. We study the grafting process using in situ Fourier transform infrared spectroscopy and ex situ x-ray photoelectron spectroscopy measurements, and we characterize the surfaces using scanning electron microscopy, atomic force microscopy, and water contact angle measurements. We found that the structure and density of grafted aminosilanes are influenced by their chemical reactivity and steric constraints around the silicon atom as well as by the nature of the anchoring functional groups. Methyl substituted aminosilanes yielded more hydrophobic surfaces with a higher surface density at higher grafting temperatures. Thiol and nitrile terminated silanes were also studied and compared to the aminosilane terminated surfaces. Uniform monolayer coatings were observed for ethoxy-based silanes, but chlorosilanes exhibited nonuniform coatings as verified by atomic force microscopy measurements.

Atomic arrangement of Si adatom on the Silicene/Ag(111) surface

Silicene is a two-dimensional material with a honeycomb structure of silicon atoms that exhibits exotic electronic properties. Exploring Si adatom on monolayer silicene opens up new possibilities for studying tunable all-silicon-based devices at an atomic scale. Here we use non-contact atomic force microscopy and scanning tunneling microscopy to visualize and control the Si adatom on monolayer silicene at atomic resolution. We demonstrate that Si adatom forms a two-dimensional island on monolayer silicene. Si adatom can be manipulated on monolayer silicene, thereby delivering a tunable low-dimensional Si structure at an atomic scale. Local reconstruction of monolayer silicene with Si adatom has been confirmed by density functional theory calculations. Our results suggest that controlling the buckling structure of monolayer silicene by positioning Si adatom will locally tune the band structures and functionalities of monolayer silicene which is important for applications.

Features of wear of DLC-Si coating under microcontact conditions during the formation of secondary structures

Diamond-Like Carbon (DLC) coatings have high wear resistance, hardness, biocompatibility and chemical inertness. Secondary structures, which formed at the track in dry friction conditions, provide friction reduction. Doping DLC with both metallic and non-metallic elements leads to a rearrangement of the structure of graphite clusters, a change in their properties, and facilitates the formation of secondary structures. In the present, the silicon atoms were used to create the compositional structure of DLC surface layers at the level of the crystal lattice. DLC-Si coatings with the thickness of 2 µm were deposited on a (Cr, Al, Si)N layer with the thickness of 3 µm formed on a steel substrate. DLC-Si layer was deposited by plasma-enhanced chemical vapor deposition using acetylene and tetramethylsilane. Nanoindentation in the wear mode was used to study the nanomechanical properties of the surface. Atomic force microscopy was used to visualize the surface morphology and wear traces. In this study, secondary structures were created in a microfriction process using a NanoScratch mode. The surface structures after microfriction was observed by atomic force microscopy, and the specific surface energy was measured. The specific volumetric wear rate of the coatings after microfriction was estimated. During microtribological testing with double squares the specific volumetric wear rate of DLC coatings was 5.39·10-14 m3/N·m for 0.8 %Si and 12.39·10-14 m3/N·m for 10 % Si. When tested in the form of scratches, the specific volumetric wear rate for the coating was for the DLC-0.8 % Si (1.22 – 10.71)·10-14 m3/N·m and for the DLC-10 % (6.17 – 49.55) ·10-14 m3/N·m.

Graph theory-based structural analysis on density anomaly of silica glass

Analyzing the atomic structure of glassy materials is a tremendous challenge both experimentally and computationally, and the lack of direct, detailed insights into glass structure hinders our ability to navigate structure–property relationships. For instance, the structural origin of the density anomaly in silica glasses – the negative thermal expansion coefficient – is still poorly understood. Simulations based on molecular dynamics (MD) produce atomically resolved structures, but quantifying the role of disorder in the density anomaly is challenging. Here, we propose to use a graph-theoretical approach to assess topological differences between disordered structural arrangements from MD trajectories of silica glasses. A graph similarity metric quantifies the similarity between the covalent networks and can characterize the nature of the disordered solid, by comparing to reference crystalline solids, or with glasses in different thermodynamic states. This approach involves casting all-atom glass configurations as networks, and subsequently applying a graph-similarity metric (D-measure). Calculated D-measure values are then taken as the topological distances between two configurations. By measuring the topological distances of silica glass configurations across a range of temperatures, distinct topological features could be observed at temperatures higher than the fictive temperature. In addition, we compared topological distances between local atomic environments in the glass and crystalline silica phases. This approach suggests that more coesite-like and quartz-like local topologies emerge in silica glasses when the density is at a minimum during the heating process. Using graph-based analysis, we then found that the anomalous density behavior of silica glass could be attributed to the moderate increased formation of oxygen triclusters (oxygen coordinated with 3 silicon atoms), and possibly reduced number of larger sized cycles at the density minimum temperature.