Silicon Bond Research Articles

High temperature performances of tri-layer environmental barrier coating with Si bond layer modified by Yb2O3

Environmental barrier coatings (EBCs) have been expected to be applied on the surface of ceramic matrix composites (CMCs). However, the oxidation and propagation cracking of the silicon bond layer are the most direct causes to induce the failure of EBCs under high temperature service environment. The modification of silicon bond layer has become an important method to prolong the service life of EBCs. In this work, the Yb2O3 have been introduced to the silicon bond layer, and three kinds of tri-layer Yb2SiO5/Yb2Si2O7/(Si-xYb2O3) EBCs with modified Si bond layer by different contents of Yb2O3 (x = 0, 10 vol%, 15 vol%) were prepared by vacuum plasma spray technique. The thermal shock performance and long-term oxidation resistance of the EBCs at 1350 °C were investigated. The results showed that the addition of appropriate amount of Yb2O3 (10 vol%) can improve the structural stability and reduce the cracks of the mixed thermal growth oxide (mTGO) layer by forming the oxidation product of Yb2Si2O7 during long-term oxidation. The excessive addition of Yb2O3 increased the stress during thermal shock as well as accelerated the oxygen diffusion during long-term oxidation, leading to the failure of EBCs. Moreover, the distribution uniformity of Yb2O3 deserves further consideration and improvement.

Rhodium‐catalyzed Decarbonylation of Acylsilanes

AbstractThe rhodium‐catalyzed decarbonylation of acylsilanes is reported. This reaction proceeds through the cleavage of a carbon–silicon bond in acylsilanes. When two different acylsilanes are reacted in one batch, crossover products are formed. These results indicate that two mechanisms are operating in this catalytic reaction.

Mesoscale Modeling of Phase Separation Controlled by Hydrosilylation in Polyhydromethylsiloxane (PHMS)-Containing Blends.

Controlling morphology of polysiloxane blends crosslinked by the hydrosilylation reaction followed by pyrolysis constitutes a robust strategy to fabricate polymer-derived ceramics (PDCs) for a number of applications, from water purification to hydrogen storage. Herein, we introduce a dissipative particle dynamics (DPD) approach that captures the phase separation in binary and ternary polymer blends undergoing hydrosilylation. Linear polyhydromethylsiloxane (PHMS) chains are chosen as preceramic precursors and linear vinyl-terminated polydimethylsiloxane (v-PDMS) chains constitute the reactive sacrificial component. Hydrosilylation of carbon–carbon unsaturated double bonds results in the formation of carbon–silicon bonds and is widely utilized in the synthesis of organosilicons. We characterize the dynamics of binary PHMS/v-PDMS blends undergoing hydrosilylation and ternary blends in which a fraction of the reactive sacrificial component (v-PDMS) is replaced with the non-reactive sacrificial component (methyl-terminated PDMS (m-PDMS), polyacrylonitrile (PAN), or poly(methyl methacrylate) (PMMA)). Our results clearly demonstrate that the morphology of the sacrificial domains in the nanostructured polymer network formed can be tailored by tunning the composition, chemical nature, and the degree of polymerization of the sacrificial component. We also show that the addition of a non-reactive sacrificial component introduces facile means to control the self-assembly and morphology of these nanostructured materials by varying the fraction, degree of polymerization, or the chemical nature of this component.

The excellent protection effects of silicon/ytterbium silicate bilayer environmental barrier coatings on SiCf/SiC composites

AbstractThe protection effects of an environmental barrier coating (EBC) consisting of silicon bond coat and mixed ytterbium monosilicate and ytterbium disilicate composite topcoat are directly evaluated by measuring the strength retention rate of SiCf/SiC composites completely wrapped up by the previous EBCs after soaking in a mixed oxygen and water vapor environment at 1300°C for up to 200 h. The results show that the mixed topcoat exhibits not only extremely excellent phase stability but also fantastic protection effects toward composites. After 200 h of corrosion, the fully protected composites are unveiled to present not only dramatically reduced weight gain ratios, less than .6%, compared to ∼10% for those unprotected ones, but also extremely higher strength retention rates, more than 90%, compared to only 10%–15% for those unprotected ones. Further, the fully protected composites show a quasi‐ductile load versus displacement curve, suggesting the retention of the oxidation‐prone pyrolytic carbon interphase of current composites.

Carbene‐Catalyzed Activation of C−Si Bonds for Chemo‐ and Enantioselective Cross Brook–Benzoin Reaction

AbstractThe first carbene‐catalyzed asymmetric chemoselective cross silyl benzoin (Brook–Benzoin) reaction has been developed. Key steps of this reaction involve activation of the carbon–silicon bond of an acylsilane by a chiral N‐heterocyclic carbene (NHC) catalyst to form a silyl acyl anion intermediate. These acyl anions then undergo an addition reaction with indole aldehydes in a highly chemo‐ and enantioselective manner to afford α‐silyloxy ketones with excellent optical purities. The reaction mechanism of this cross Brook–Benzoin reaction was investigated through both experimental and computational methods. The chiral α‐hydroxy ketone derivatives obtained by this approach show promising, agrochemically interesting activity against harmful plant bacteria.

Carbene-Catalyzed Activation of C-Si Bonds for Chemo- and Enantioselective Cross Brook-Benzoin Reaction.

The first carbene-catalyzed asymmetric chemoselective cross silyl benzoin (Brook-Benzoin) reaction has been developed. Key steps of this reaction involve activation of the carbon-silicon bond of an acylsilane by a chiral N-heterocyclic carbene (NHC) catalyst to form a silyl acyl anion intermediate. These acyl anions then undergo an addition reaction with indole aldehydes in a highly chemo- and enantioselective manner to afford α-silyloxy ketones with excellent optical purities. The reaction mechanism of this cross Brook-Benzoin reaction was investigated through both experimental and computational methods. The chiral α-hydroxy ketone derivatives obtained by this approach show promising, agrochemically interesting activity against harmful plant bacteria.

Process optimization for Hafnia‐doped silicon bond coats fabricated by plasma spraying for SiC‐CMC

HfO2-doped Si bond layer is expected to work at higher temperature with better oxidation resistance of environmental barrier coatings for SiC-based ceramic matrix composites. Establishing the relationship between process parameters and powder feeders with the property of coatings could help to further optimizing their performance. Three sets of plasma spray processes and two kinds of HfO2 powders were used to fabricate Si-HfO2 bond coats with different Si/HfO2 ratio. Particle size of HfO2 greatly affect the composition of coat, where smaller one induced uncontrollable Si/HfO2 ratio inside coatings. Larger HfO2 powder greatly improved the composition uniformity with a Si/HfO2 ratio similar to the starting feeder. However, phase segregation occurred for both cases, indicating the limit of mechanical mixing method to make composite feedstock. More strategies should be explored for precise control the phase and composition of composite coatings from spray technique. This article is protected by copyright. All rights reserved

Oxidative Addition of Silicon–Chloride Bonds to a Zerovalent Ruthenium Center and Direct Generation of an Ethylene Insertion Complex

The oxidative addition of a silicon-chloride (Si-Cl) bond to a metal center can be a key reaction step in coordinative silicon chemistry, but this reaction is seldom observed. Herein, we report direct oxidative addition of the Si-Cl bonds of dimethyldichlorosilane (Me2SiCl2) and cyclotrimethylenedichlorosilane [(CH2)3SiCl2] to low-valent ruthenium complexes, yielding the 16e- chloro(organosilyl)ruthenium complexes [N3]Ru(Cl)(SiMe2Cl) (4a) and [N3]Ru(Cl)(SiCl(CH2)3) (4b) ([N3] = 2,6-(MesN═CMe)2C5H3N; Mes = 1,3,5-trimethylphenyl; Me = methyl). The reversible reaction of 4a with ethylene yields an 18e- ethylene adduct, in which an ethylene is subsequently inserted into a ruthenium-silicon (Ru-Si) bond to produce the 16e- complex [N3]Ru(Cl)(CH2CH2SiMe2Cl) (7). This study provides a good example of the direct generation of an ethylene insertion product, which is an important intermediate in the catalytic reduction of unsaturated molecules.

Si-HfO2 composite powders fabricated by freeze drying for bond layer of environmental barrier coatings

Hafnia-doping is expected to improve the performance of silicon bond layer of EBC system for SiC-based CMC. However, obtaining a compositional uniform Si-HfO2 composite coating is still a challenge due to the big difference of their melting point, density and fluidity of powders. Fabrication of evenly distributed Si-HfO2 composite powders with good fluidity and activity is a promising strategy. Freeze drying, a cost-effective and green method, was applied to make Si-HfO2 composite powders for the first time. Highly active nano-sized Si and HfO2 powders were mixed uniformly into a slurry, which was frozen and dried into solid lamellar structure. After crushing down and sieving, composite powders with controllable particle size and good fluidity were achieved, which inherited the uniformity of the slurry. Thickness of the lamellar structure determines the size and shape of the final composite powders, which can be controlled by simply varying the freezing temperatures. Compact and smooth bond coatings with evenly distributed Si and HfO2 were successfully fabricated from these composite powders by plasma spraying. We hope this work could excite the whole society to focus more on the structural design of powders and coatings as well as material selection to further boost the performance of EBCs.

Highly Regio‐ and Enantioselective Hydrosilylation of gem‐Difluoroalkenes by Nickel Catalysis

AbstractThe synthesis of small organic molecules with a difluoromethylated stereocenter is particularly attractive in drug discovery. Herein, we have developed an efficient method for the direct generation of difluoromethylated stereocenters through Ni0‐catalyzed regio‐ and enantioselective hydrosilylation of gem‐difluoroalkenes. The reaction also represents the enantioselective construction of carbon(sp3)−silicon bonds with nickel catalysis, which provides an atom‐ and step‐economical synthesis route of high‐value optically active α‐difluoromethylsilanes. This protocol features readily available starting materials and commercial chiral catalysis, broad substrates spanning a range of functional groups with high yield (up to 99 % yield) and excellent enantioselectivity (up to 96 % ee). The enantioenriched products undergo a variety of stereospecific transformations. Preliminary mechanistic studies were performed.

Highly Regio- and Enantioselective Hydrosilylation of gem-Difluoroalkenes by Nickel Catalysis.

The synthesis of small organic molecules with a difluoromethylated stereocenter is particularly attractive in drug discovery. Herein, we have developed an efficient method for the direct generation of difluoromethylated stereocenters through Ni0 -catalyzed regio- and enantioselective hydrosilylation of gem-difluoroalkenes. The reaction also represents the enantioselective construction of carbon(sp3 )-silicon bonds with nickel catalysis, which provides an atom- and step-economical synthesis route of high-value optically active α-difluoromethylsilanes. This protocol features readily available starting materials and commercial chiral catalysis, broad substrates spanning a range of functional groups with high yield (up to 99 % yield) and excellent enantioselectivity (up to 96 % ee). The enantioenriched products undergo a variety of stereospecific transformations. Preliminary mechanistic studies were performed.

Optimization of SiO2 with GHA and basin hopping

In this paper we continue to develop our structural optimization algorithm built earlier on a numerical platform, the Genetic Hybrid Algorithm (GHA). Our goal now is to extend our algorithm to oxides, find an effective way to search for the known global minimum, alpha-quartz as a test case, and report our results and findings for this system. We studied unit cells of different sizes: 18, 36 and 72 atoms, but most of the presented results are for cases with 18 and 36 atoms. The algorithm makes heavy use of the basin hopping method for searching for the global minimum of the system. We show how we were able to apply basin hopping most effectively in this case and which variables were of importance.We identify three other low energy structures near the global minimum structure, that trap the search. We show that the energy guided basin hopping can be detrimental to the search and structure-based guiding works more reliably. Two different structure based guides were used, one that tries to maximize the shortest silicon–silicon bond in the cell, while the other tries to maximize the calculated order parameter. The guiding was implemented by generating multiple different options for the basin hopping jumps, and doing weighted choosing on those options based on their properties.

Healing of water tree aged cables using rejuvenation nanofluid

The insulation of high voltage power cables is aged due to the impact of water treeing. The voids created due to water treeing can be filled with the injection of nanofluids and restore the insulation of cables. In this work, cross-linked polyethylene (XLPE) cable insulation is restored with the help of tetraethyl orthosilicate (TEOS) rejuvenation nanofluid. The morphological changes in aged and rejuvenated samples are analyzed with the help of scanning electron microscopy (SEM). Moreover, the molecular changes in the aged and rejuvenated samples are detected using Fourier transform infrared (FTIR) spectroscopy. Both the samples are also tested with the help of electrical breakdown tests. The results indicate a positive recovery in the insulation of cables with the injection of TEOS nanofluid. The tanδ results showed a decrease in values after the injection of nanofluid in the aged sample. Moreover, the tanδ values of rejuvenated samples are even better than the unaged cables. SEM results also showed a recovery in the voids of the aged sample due to the injection of nanofluid. FTIR results confirmed that the nanofluid penetrated in the insulation of the cable and the peaks of silicone and alumina bonds are seen in the spectrographs.

Surface functionalization with nonalternant aromatic compounds: a computational study of azulene and naphthalene on Si(001)

Nonalternant aromatic π-electron systems show promises for surface functionalization due to their unusual electronic structure. Based on our previous experiences for metal surfaces, we investigate the adsorption structures, adsorption dynamics and bonding characteristics of azulene and its alternant aromatic isomer naphthalene on the Si(001) surface. Using a combination of density functional theory, ab initio molecular dynamics, reaction path sampling and bonding analysis with the energy decomposition analysis for extended systems, we show that azulene shows direct adsorption paths into several, strongly bonded chemisorbed final structures with up to four covalent carbon–silicon bonds which can be described in a donor–acceptor and a shared-electron bonding picture nearly equivalently. Naphthalene also shows these tetra-σ-type bonding structures in accordance with an earlier study. But the adsorption path is pseudo-direct here with a precursor intermediate bonded via one aromatic ring and strong indications for a narrow adsorption funnel. The four surface-adsorbate bonds formed lead for both adsorbates to a strong corrugation and a loss of aromaticity.

Palladium-Catalyzed Enantioselective Carbene Insertion into Carbon-Silicon Bonds of Silacyclobutanes.

We report herein a highly efficient palladium-catalyzed carbene insertion into strained Si-C bonds with excellent enantioselectivity, which provides a rapid and distinct method to access silacyclopentanes with a three- or four-substituted stereocenter asymmetrically. Mechanistic studies using hybrid density functional theory suggest a catalytic cycle involving oxidative addition, carbene migratory insertion, and reductive elimination. In addition, roles of the chiral ligands in controlling the reaction enantioselectivity are also elucidated.

Correlation of Process Conditions, Porosity Levels and Crystallinity in Atmospherically Plasma Sprayed Yb2Si2O7 Environmental Barrier Coatings

Environmental barrier coatings are necessary to protect fibre reinforced ceramics from high recession rates in fast and hot water vapor-containing gases as they typically are found in the hot gas sections of gas turbines. A standard material to protect SiC/SiC composites is atmospherically plasma sprayed (APS) Yb2Si2O7. For this material, it is difficult to obtain at reasonable substrate temperatures both low porosity and high crystallinity levels during APS. In this paper results of coatings prepared by a so-called high velocity APS process and also more conventional processes are presented. All coatings have been prepared by a single layer deposition method which avoids inter passage porosity bands. Furthermore, the samples were heat-treated in air at 1300 °C for 100 h and the influence of the topcoat density on the growth of the silica scale on the used silicon bond coat was studied.

Molecular recycling: A key approach to tailor the waste recycling for high-value nano silicon carbide

This study proposes the transformation of complex waste otherwise destined for landfill into valuable products and resources through molecular recycling. Specifically, it aims to leverage high-temperature reactions to thermally alter the atomic bonds of carbon and silicon within the molecules from the waste source in situ, to produce valuable silicon carbide (SiC). As such, it promises to deliver a cost-effective, sustainable solution to a high volume of waste for value added applications. The key process used in this study to tailor the molecular recycling process was selective high temperature transformation. This was achieved with two waste sources: waste coffee grounds as a source of carbon, and mixed automotive waste glasses as a source of silicon. These two sources were used to produce high-value SiC nanowires of 50–100 nm diameter with surface area of ~64.21 m2/g. The as-synthesised material produced by molecular recycling retains superior photocatalytic property compared to the commercially available SiC nanoparticles with excellent potential for reuse for photodegradation application.

A duplex bond coat approach to environmental barrier coating systems

SiC composites, when used in water vapor rich combustion environments, are vulnerable to volatilization of their protective SiO2 surface oxide by reaction with water vapor, and are typically protected by a silicon bond coat and ytterbium disilicate (Yb2Si2O7: YbDS) environmental barrier coating (EBC) system. Oxidation of the silicon bond coat at temperatures above 1200 °C results in formation of a ß-cristobalite SiO2 layer at the silicon-YbDS interface. Upon cooling through 240 °C, the ß→α-cristobalite phase change results in a 4.9% SiO2 volume decrease, increasing the probability of delamination of the coating system as the oxide layer thickness increases. The use of a duplex silicon-HfO2 bond coat is investigated here in which m-HfO2 dynamically reacts with SiO2 to form HfSiO4 (hafnon), which is phase stable and thermomechanically compatible with silicon and YbDS. Thermal cycling of the duplex bond coat system between 110 and 1316 °C in a high-temperature steam environment for up to 1000 1 h cycles confirmed that growth of a ß-cristobalite SiO2 layer on the silicon outer surface was accompanied by simultaneous reaction with m-HfO2 to form hafnon, reducing the average SiO2 thickness by approximately 50% compared to a conventional silicon-YbDS coating system. A parallel reaction between the YbDS and m-HfO2 layers transformed the m-HfO2 to Yb2O3-stabilized HfO2 with a high coefficient of thermal expansion. Thermomechanical modelling indicated that the energy release rates remained insufficient to cause delamination.

29Si NMR Spectroscopy as a Probe of s- and f-Block Metal(II)-Silanide Bond Covalency.

We report the use of 29Si NMR spectroscopy and DFT calculations combined to benchmark the covalency in the chemical bonding of s- and f-block metal-silicon bonds. The complexes [M(SitBu3)2(THF)2(THF)x] (1-M: M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(SitBu2Me)2(THF)2(THF)x] (2-M: M = Mg, x = 0; M = Ca, Sm, Eu, Yb, x = 1) have been synthesized and characterized. DFT calculations and 29Si NMR spectroscopic analyses of 1-M and 2-M (M = Mg, Ca, Yb, No, the last in silico due to experimental unavailability) together with known {Si(SiMe3)3}--, {Si(SiMe2H)3}--, and {SiPh3}--substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound 29Si NMR isotropic chemical shifts, δSi, span a wide (∼225 ppm) range when the metal is kept constant, and direct, linear correlations are found between δSi and computed delocalization indices and quantum chemical topology interatomic exchange-correlation energies that are measures of bond covalency. The calculations reveal dominant s- and d-orbital character in the bonding of these silanide complexes, with no significant f-orbital contributions. The δSi is determined, relatively, by paramagnetic shielding for a given metal when the silanide is varied but by the spin-orbit shielding term when the metal is varied for a given ligand. The calculations suggest a covalency ordering of No(II) > Yb(II) > Ca(II) ≈ Mg(II), challenging the traditional view of late actinide chemical bonding being equivalent to that of the late lanthanides.

Structural organization of space polymers

Extraterrestrial polymers of glycine with iron have been characterized by mass spectrometry to have a core mass of 1494 Da with dominant rod-like variants at mass to charge ratios of 1567 and 1639 [McGeoch et al., “Meteoritic proteins with glycine, iron and lithium,” arXiv:2102.10700 (2021)]. Several principal macro-structural morphologies are observed in solvent extracts from a chondritic Vigarano class alteration type 3 meteoritic material. The first is an extended sheet of linked (three-legged) triskelia containing the 1494 Da core entity that encloses gas bubbles in the solvent. A second is of fiber-like crystals found here, via x-ray diffraction, to be multiple-walled nanotubes made from a square lattice of the 1494 Da polymer. A third is a dispersion of floating phantom-like short tubes of up to 100 μm length with characteristic angled bends that suggest the influence of a specific underlying protein structure. Here, it is proposed that the angled tubes are the observable result of a space-filling superpolymerization of 1638 Da polymer subunits guided by the tetragonal symmetry of linking silicon bonds. Distorted hexagonal sheets are linked by perpendicular subunits in a three-dimensional hexagonal diamond structure to fill the largest possible volume. This extended very low-density structure is conjectured to have dominated in a process of chemical selection because it captured a maximum amount of molecular raw material in the ultra-low density of molecular clouds or of the proto-solar nebula. This could have led ultimately to the accretion of the earliest planetary bodies.