Silicon Bond Research Articles

Application of High-Velocity Oxygen-Fuel (HVOF) Spraying to the Fabrication of Yb-Silicate Environmental Barrier Coatings

From the literature, it is known that due to their glass formation tendency, it is not possible to deposit fully-crystalline silicate coatings when the conventional atmospheric plasma spraying (APS) process is employed. In APS, rapid quenching of the sprayed material on the substrate facilitates the amorphous deposit formation, which shrinks when exposed to heat and forms pores and/or cracks. This paper explores the feasibility of using a high-velocity oxygen-fuel (HVOF) process for the cost-effective fabrication of dense, stoichiometric, and crystalline Yb2Si2O7 environmental barrier coatings. We report our findings on the HVOF process optimization and its resultant influence on the microstructure development and crystallinity of the Yb2Si2O7 coatings. The results reveal that partially crystalline, dense, and vertical crack-free EBCs can be produced by the HVOF technique. However, the furnace thermal cycling results revealed that the bonding of the Yb2Si2O7 layer to the Silicon bond coat needs to be improved.

Insights into Molecular Beryllium–Silicon Bonds

We present the synthesis of two silyl beryllium halides HypSiBeX∙(thf) (HypSi = Si(SiMe3)3, X = Cl 2a, I 4a) and the molecular structure of 2a as determined by single crystal X-ray diffraction. Compounds 2a and 4a were characterized via multi-nuclear NMR spectroscopy (1H, 9Be, 13C, 29Si), and the bonding situation was further investigated using quantum chemical calculations (with the addition of further halides X = F 1b, Cl 2b, Br 3b, I 4b). The nature of the beryllium silicon bond in the context of these compounds is highlighted and discussed.

The First Biocatalytic Carbon-Silicon Bond Formation.

Linking two worlds: As a further expansion of the biocatalytic repertoire, carbene insertion into Si-H bonds catalyzed by the heme protein cytochrome c was recently reported. This new biocatalyst holds great promise because it enables the highly selective incorporation of silicon into molecules without prior protection of existing functional groups.

Asymmetric Enzymatic Carbon–Silicon Bond Formation

Key words carbenes - directed evolution - heme protein - carbon–silicon bond formation - cytochrome c - enzyme modification

Gasochromic properties of novel tungsten oxide thin films compounded with methyltrimethoxysilane (MTMS)

Thick tungsten–silicon films with long-term gasochromic performance were synthesized from methyltrimethoxysilane (MTMS) and tungsten oxide sols. The WO3–MTMS films exhibited a stable network with tungsten and silicon bonds.

Redefining Biology via Enzyme Engineering

A TV series from the 1970s, “The Six Million Dollar Man,” imagines a former astronaut with bionic implants and superhuman strength acting as a secret agent for the government. Such enhancement of human abilities with artificial components (all at a reasonable price tag) might resonate with some of today’s scientists who are working on a more modest goal of augmenting the properties of cells and biological systems. Their aim is to design new synthetic routes for compounds and even create completely new molecules that may not exist in nature by exploiting the versatility of the life’s systems. Take the current work on silicon. Silicon is a ubiquitous element that sits next to carbon in the periodic table and shares some of its properties. The similarities in the ways silicon and carbon bond with other atoms have led to the idea that silicon might give rise to an alternative, silicon-based life with unique properties. Yet, silicon does not readily partake in biochemical couplings unless nudged to bind carbon via chemical synthesis. Organosilicon compounds that are thus created are highly valued for their ability to bind organic and inorganic substrates and are used as adhesives, sealants, and caulks. Their industrial synthesis is laborious, and thus the interest in bringing silicon into the fold of nature’s biochemistry persists. Now, advances in enzyme engineering have made it possible to incorporate silicon into organic compounds through bacterial biosynthetic pathways. Frances Arnold and her team (Kan et al., 2016Kan S.B.J. Lewis R.D. Chen K. Arnold F.H. Science. 2016; 354: 1048-1051Crossref PubMed Scopus (361) Google Scholar) have discovered that a cytochrome c protein from the thermophilic bacterium Rhodothermus marinus can, under the right conditions, catalyze the formation of a carbon-silicon bond to create short organosilicon compounds. The examination of the enzyme’s structure yields clues to the motifs driving this process, allowing the rapid directed evolution of an enzyme with high activity for making such products. Strikingly, this altered cytochrome c churns out silicon-containing compounds inside of bacteria, suggesting that relatively few tweaks are sufficient to bring silicon into the realm of in vivo synthesis, albeit under highly engineered conditions, and creating compounds that are not yet industrially relevant. Retooling an enzyme for a new production line may, however, require a more comprehensive overhaul of its properties. In a recent work from John Hartwig’s lab (Dydio et al., 2016Dydio P. Key H.M. Nazarenko A. Rha J.Y.-E. Seyedkazemi V. Clark D.S. Hartwig J.F. Science. 2016; 354: 102-106Crossref PubMed Scopus (231) Google Scholar, Key et al., 2016Key H.M. Dydio P. Clark D.S. Hartwig J.F. Nature. 2016; 534: 534-537Crossref PubMed Scopus (268) Google Scholar), myoglobins and cytochrome P450s, heme proteins that transport and react with oxygen, are fitted with iridium, a noble metal not found in biological molecules, to yield an artificial metalloenzyme with new capabilities, promoting reactions not catalyzed by native hemes or other metalloenzymes and with kinetics, productivity, and selectivity that are comparable with those of native enzymes. The widening of the biochemical space achieved with such manipulations is tantalizing, and the next frontier is making such artificial enzymes work inside of a cell, a challenging task, given the complex intracellular environment. Thomas Ward and colleagues (Jeschek et al., 2016Jeschek M. Reuter R. Heinisch T. Trindler C. Klehr J. Panke S. Ward T.R. Nature. 2016; 537: 661-665Crossref PubMed Scopus (264) Google Scholar) tackle this problem by developing a process through which an artificial metathase can be assembled and put to work in the periplasm of a bacterium, bringing new, non-natural metabolic pathways inside cells. On a different spatial scale, other recent studies have endeavored to create large artificial structures within cells. Neil King, Wesley Sundquist, and colleagues (Votteler et al., 2016Votteler J. Ogohara C. Yi S. Hsia Y. Nattermann U. Belnap D.M. King N.P. Sundquist W.I. Nature. 2016; 540: 292-295Crossref PubMed Scopus (75) Google Scholar) designed self-organizing protein nanocages nested inside of membrane vesicles. Such structures can be released from their host eukaryotic cells and taken up by other cells, allowing the transfer of encapsulated cargo and representing a prototype hybrid biological material that can be used to perform complex tasks, such as exchange of metabolites or information, in a controlled and tailored manner. These examples of repurposing natural solutions are still at their proof-of-principle stage; however, they support the idea that biochemical reactions and cellular functions can be augmented to bring new elements to life and create unexpected biological products.

Silica scaling in forward osmosis: From solution to membrane interface

Membrane silica scaling hinders sustainable water production. Understanding silica scaling mechanisms provides options for better membrane process management. In this study, we elucidated silica scaling mechanisms on an asymmetric cellulose triacetate (CTA) membrane and polyamide thin-film composite (TFC) membrane. Scaling filtration showed that TFC membrane was subjected to more severe water flux decline in comparison with the CTA membrane, together with different scaling layer morphology. To elucidate the silica scaling mechanisms, silica species in the aqueous solution were characterised by mass spectrometry as well as light scattering. Key thermodynamic parameters of silica surface nucleation on the CTA and TFC membranes were estimated to compare the surface nucleation energy barrier. In addition, high resolution X-ray photoelectron spectroscopy resolved the chemical origin of the silica-membrane interaction via identifying the specific silicon bonds. These results strongly support that silica scaling in the CTA membrane was driven by the aggregation of mono-silicic acid into large silica aggregates, followed by the deposition from bulk solution onto the membrane surface; by contrast, silica polymerised on the TFC membrane surface where mono-silicic acid interacted with TFC membrane surface, which was followed by silica surface polymerisation.

ChemInform Abstract: Carbon—Silicon Bond Formation in the Synthesis of Benzylic Silanes.

Sterically encumbered organosilanes can be difficult to synthesize with conventional, strongly basic reagents; the harsh reaction conditions are often low yielding and not suitable for many functional groups. As an alternative to the typical anionic strategies to construct silanes, the coupling of benzylic halides and arylhalosilanes with sonication has been identified as a high yielding and general strategy to access bulky and functionalized benzylic silanes. This new methodology provides a solution for the synthesis of families of bulky benzylic silanes for study in catalysis and other areas of chemical synthesis.

Low Temperature Fluorinated Silicon FILMS Synthesis

Second-harmonic generation (SHG), photoluminescence (PL) and Fourier-transformed infrared (FT IR) spectra from silicon films with different average size of nanocrystals were analized to develop new possible material for active channel in nonlinear optical devices. Silicon films were prepared with dominant concentrations of silicon bonding with oxygen or fluorine atoms. Also, the volume fraction of nanocrystals was varied from 30% to 70%. It is seen the spectral peak with energy 3.26 eV is related to defects appeared in interface area silicon-silicon dioxide. For films with small silicon crystals (less than 20 nm) the nonlinear optical response contains two spectral peaks. The second peak is caused by optical response from nanocrystal grain boundary that contains oxygen or fluorine atoms incorporated in silicon as dipoles inside film. The optical nonlinear switch device based on the nonlinear optical response of SiOxmedia inside film was proposed. Also, the silicon film with quartz micro-clusters was investigated as material for making the nonlinear optical transmitter device. The structural properties of silicon films can be easily illustrated by using the films morphology which was determined by using atomic-force microscopy. Figure 1 shows the atomic-force microscopic photos for Sample 1, Sample 2 and Sample3, respectively. It is seen, that the nanocrystal grains are in all types of investigated silicon films. The lateral distribution of nanocrystals with dominant orientation (111) are homogeneous for Sample 1 (see Fig. 1 a)), but for the Sample 2 is unhomogeneous due to appeared conglomerates which are created from the amorphous phase separately (see Fig. 1 b)). The Sample 3 has the triangular shape of nanocrystals (111) which are distributed in submicron crystalline (111) domens. It is clear, that there is great difference in morphological properties of three types of films. The HF etching the silicon surface is efficient because at Td=100oC the surface diffusion and desorption from the surface are low. By adding the SiF4 we change the H2 :SiF4 ratio and some amount of fluorine atoms are included into H2SiF6 assembly creation. Because, the etching process is low at [SiF4] =0.25 sccm and [SiF4] =0.375 sccm. After this values of silicon tetra-fluoride flow rate the H2:SiF4 ratio is changed to destruct intensive creation of molecular assemblies. This stage at [SiF4] =0.5 sccm the HF significant etches the silicon and causes the decreasing the grain sizes. It is assumed that there are two possibilities of fluorine removing: the first is hydrogen incorporation with fluorine, and, the second, the H2SiF6 assemble is creation by 300oC. The SHG is forbidden for centrosymmetric crystal such as bulk silicon because the sum dipole moment is zero, but it is possible due to the surface breaking symmetry and quadruple terms contributions. The opposite situation is for nanostructured oxidized (or fluorinated) silicon films for which the surface area of great quantity of nanocrystals is significant, and the breaking of symmetry is permanent and lateral isotropic. The spectral characteristic of optical nonlinear response from the thin silicon film strictly depends on the spectral properties of polarization and susceptibility tensors χ(2) distortion and χ(2) Si-O.F are the nonlinear susceptibilities produced by nonzero dipole moment caused by shape distortion from the spherical shape of nanocrystals and due to the existing of dangling bonds caused by fluorine or oxygen incorporation in grain boundary region, respectively. By these values of densities the dipole moments of such nanoscaled ellipsoidal dipoles covered by surface charges can be estimated as PSiO =0.04 Debye and PSiH =0.015 Debye. Therefore, the concentration of nanocrystals mainly plays a great role in PL and SHG signal intensity of radiation due to their great surface area covered by silicon bonds with contaminants (atoms of O, H, F) in grain boundary but not as a separate nanoscaled dipoles. By increasing the nanocrystal quantity inside the silicon film the ratio S/V grows significant. The size effect in PL from nanocrystalline silicon film can be explained by means of statistical method. We suggested that the ƒ(x) is distribution function of crystallites in their sizes, θ(x) is a quantum efficiency of nanocrystalls. It is supposed, that the large quantity of nanocrystals is Gauss distributed in their sizes. The Nnc-Si value is proportional to the square under the both functions ƒ(x) and θ(x) near the zero values. Accordingly, the quantity of emitted hydrogenized nanocrystals by band-to-band radiative transitions is important to esimate the PL linear optical response. For nanocrystalline silicon films for Pin àSout optical scheme χ(2) xxx component of susceptibility tensor is detected. By using optical scheme Pin àPout SHG spectra contains isotropic and anisotropic component of susceptibility tensor χ(2) zzz, χ(2) zxx,χ(2) xxz , and χ(2) xxx. Figure 1

Carbon–Silicon Bond Formation in the Synthesis of Benzylic Silanes

Sterically encumbered organosilanes can be difficult to synthesize with conventional, strongly basic reagents; the harsh reaction conditions are often low yielding and not suitable for many functional groups. As an alternative to the typical anionic strategies to construct silanes, the coupling of benzylic halides and arylhalosilanes with sonication has been identified as a high yielding and general strategy to access bulky and functionalized benzylic silanes. This new methodology provides a solution for the synthesis of families of bulky benzylic silanes for study in catalysis and other areas of chemical synthesis.

Thermal behavior of benzobis(tetraethyldisilacyclobutene)

AbstractThe thermolysis of benzo[1,2:4,5]bis(1,1,2, 2-tetraethyl-1,2-disilacyclobut-3-ene) (1) in the presence of ethylene in an autoclave at 250 °C for 24 h afforded two products, compound2, consisting of one molecule of1and two ethylene molecules, and compound3, arising from two molecules of1and three molecules of ethylene in 60 and 20 % yields, respectively. The reaction of1with phenylacetylene at 150 °C for 24 h gave a mixture of regio-isomers4aand4b, arising from the insertion of the triple bond of phenylacetylene into two silicon–silicon bonds in1in 86 % combined yield. The reaction of1with 1-hexyne at 150 °C for 24 h again produced two regio-isomers5aand5bin 85 % combined yield.

Monolayer-to-bilayer transformation of silicenes and their structural analysis.

Silicene, a two-dimensional honeycomb network of silicon atoms like graphene, holds great potential as a key material in the next generation of electronics; however, its use in more demanding applications is prevented because of its instability under ambient conditions. Here we report three types of bilayer silicenes that form after treating calcium-intercalated monolayer silicene (CaSi2) with a BF4− -based ionic liquid. The bilayer silicenes that are obtained are sandwiched between planar crystals of CaF2 and/or CaSi2, with one of the bilayer silicenes being a new allotrope of silicon, containing four-, five- and six-membered sp3 silicon rings. The number of unsaturated silicon bonds in the structure is reduced compared with monolayer silicene. Additionally, the bandgap opens to 1.08 eV and is indirect; this is in contrast to monolayer silicene which is a zero-gap semiconductor.

Raman spectroscopy of monoatomic hydrogen at dislocations in silicon

A broad Raman peak at about 2000 cm-1 was found in hydrogenated silicon foils with a regular dislocation network (DN) produced by silicon wafer bonding. It was not observed in the reference foils without a dislocation network. The hydrogenation was performed from acid water solution at room temperature. The peak was ascribed to monoatomic hydrogen in silicon bond centered position that became stable in a close vicinity to the dislocation cores in opposite to the case of the regular lattice where its molecular form is energetically favorable.

Electronic Structure of Silicon Nanowires Matrix from Ab Initio Calculations.

An investigation of the model of porous silicon in the form of periodic set of silicon nanowires has been carried out. The electronic energy structure was studied using a first-principle band method—the method of pseudopotentials (ultrasoft potentials in the basis of plane waves) and linearized mode of the method of combined pseudopotentials. Due to the use of hybrid exchange-correlation potentials (B3LYP), the quantitative agreement of the calculated value of band gap in the bulk material with experimental data is achieved. The obtained results show that passivation of dangling bonds with hydrogen atoms leads to substantial transformation of electronic energy structure. At complete passivation of the dangling silicon bonds by hydrogen atoms, the band gap value takes the magnitude which substantially exceeds that for bulk silicon. The incomplete passivation gives rise to opposite effect when the band gap value decreases down the semimetallic range.

Iridium-Catalyzed, Hydrosilyl-Directed Borylation of Unactivated Alkyl C-H Bonds.

We report the iridium-catalyzed borylation of primary and secondary alkyl C-H bonds directed by a Si-H group to form alkylboronate esters site selectively. The reactions occur with high selectivity at primary C-H bonds γ to the hydrosilyl group to form primary alkyl bisboronate esters. In the absence of such primary C-H bonds, the borylation occurs selectively at a secondary C-H bond γ to the hydrosilyl group, and these reactions of secondary C-H bonds occur with high diastereoselectivity. The hydrosilyl-containing alkyl boronate esters formed by this method undergo transformations selectively at the carbon-boron or carbon-silicon bonds of these products under distinct conditions to give the products of amination, oxidation, and arylation.

Molten silicate reactions with plasma sprayed ytterbium silicate coatings

Abstract The reactions between molten calcium aluminum magnesium silicates (CMAS) at 1300 °C and atmospheric plasma spray (APS) deposited environmental barrier coatings on SiC substrates have been investigated. The tri-layer coatings comprised a silicon bond coat protected by a layer of mullite and either Yb 2 SiO 5 (ytterbium monosilicate, YbMS) or Yb 2 Si 2 O 7 (ytterbium disilicate, YbDS) as the topcoat. The APS deposition process resulted in two-phase top coats; the YbMS coating contained Yb 2 O 3 regions in a matrix of Yb 2 SiO 5 while the YbDS coating contained Yb 2 SiO 5 in a matrix of Yb 2 Si 2 O 7 . Exposure of both coatings to a model CMAS resulted in dissolution of the topcoat accompanied by a rapid rise in the concentration of Yb in the melt, and formation of the same Ca 2 Yb 8 (SiO 4 ) 6 O 2 apatite reaction product phase. The thickness of the apatite layer initially varied with (time) 1/4 , but transitioned to approximately parabolic kinetics after 5–10 h of CMAS exposure. The reaction mechanism on the YbMS layer was consistent with recent observations on Y 2 SiO 5 , wherein molten CMAS transport to the undissolved silicate was controlled by diffusion through thin amorphous films at the apatite grain boundaries. The reaction mechanism for the YbDS layer was more complex, and involved preferential reaction with the YbSiO 5 rich regions, resulting in a reaction zone that contained CMAS, the apatite reaction compound and undissolved Yb 2 Si 2 O 7 . The coating composition and microstructure significantly influenced the mechanism and rate at which the YbDS top coat was consumed by the reaction.

Response of ytterbium disilicate–silicon environmental barrier coatings to thermal cycling in water vapor

A preliminary study of a promising bi-layer environmental barrier coating (EBC) designed to reduce the susceptibility of SiC composites to hot water vapor erosion is reported. The EBC system consisted of a silicon bond coat and a pore-free ytterbium disilicate (YbDS; Yb2Si2O7) topcoat. Both layers were deposited on α-SiC substrates using a recently optimized air plasma spray method. The two layers of the coating system had coefficients of thermal expansion (CTE) that were well matched to that of the substrate, while the YbDS has been reported to have a moderate resistance to silicon hydroxide vapor forming reactions in water vapor rich environments. Thermal cycling experiments were conducted between 110 °C and 1316 °C in a flowing 90% H2O/10% O2 atmospheric pressure environment, and resulted in the formation of a thermally grown (silica) oxide (TGO) at the silicon-ytterbium disilicate interface. The TGO layer exhibited linear oxidation kinetics consistent with oxidizer diffusion through the ytterbium silicate layer controlling its thickening rate. The effective diffusion coefficient of the oxidizing species in the YbDS layer was estimated to be 2 × 10−12 m2s−1 at 1316 °C. Slow steam volatilization of the YbDS topcoat resulted in the formation of a thin, partially protective, high CTE ytterbium monosilicate layer on the outside of the YbDS coating. Progressive edge delamination of the coating system was observed with steam exposure time, consistent with water vapor volatilization of the TGO edges that were directly exposed to the environment. This was aided by outward bending of the delaminated region to relax TGO and YbMS surface layer stresses developed during the cooling phase of each thermal cycle.

Confined Polymerization in Highly Ordered Mesoporous Organosilicas.

Hybrid mesoporous organosilica exhibiting crystal-like order in the walls provided an ideal channel reaction vessel for the confined polymerization of acrylonitrile (PAN). The resulting high-molecular-mass PAN fills the channels at high yield and forms an ordered nanostructure of polymer nanobundles enclosed into the hybrid matrix. The in situ thermal transformation of PAN into rigid polyconjugated and, eventually, into condensed polyaromatic carbon nanofibers, retains the periodic architecture. Simultaneously, the matrix evolves showing the fusion of the p-phenylene rings and the cleavage of carbonsilicon bonds: this gives rise to graphitic-carbon/silica nanocomposites containing hyper-oxydrylated silica nanophases. Interestingly, the 3D hexagonal mesostructure survives in the carbonaceous material. The exploitation of porous materials of high capacity and a hybrid nature, for polymerization in the confined state, followed by high temperature treatments, allowed us to achieve unique and precisely fabricated nanostructures, thus paving the way for the construction of fine-tuned electronic and light-harvesting materials.

On the thermodynamically stable amorphous phase of polymer-derived silicon oxycarbide.

A model for the thermodynamic stability of amorphous silicon oxycarbide (SiCO) is presented. It builds upon the reasonably accepted model of SiCO which is conceived as a nanodomain network of graphene. The domains are expected to be filled with SiO2 molecules, while the interface with graphene is visualized to contain mixed bonds described as Si bonded to C as well as to O atoms. Normally these SiCO compositions would be expected to crystallize. Instead, calorimetric measurements have shown that the amorphous phase is thermodynamically stable. In this article we employ first-principles calculations to estimate how the interfacial energy of the graphene networks is favorably influenced by having mixed bonds attached to them. We analyze the ways in which this reduction in interfacial energy can stabilize the amorphous phase. The approach highlights how density functional theory computations can be combined with the classical analysis of phase transformations to explain the behavior of a complex material. In addition we discover a two-dimensional lattice structure, with the composition Si2C4O3 that is constructed from a single layer of graphene congruent with silicon and oxygen bonds on either side.

ChemInform Abstract: Synthesis of Organofluorine Compounds Using α‐Fluorine‐Substituted Silicon Reagents

AbstractReview: [42 refs.