Si-O Bonds Research Articles

A novel atomic removal model for chemical mechanical polishing using developed mesoporous shell/core abrasives based on molecular dynamics.

To improve polishing performance and reduce the environmental pollution of chemical mechanical polishing (CMP) tests, mesoporous shell/core silica abrasives were prepared, and a novel green CMP slurry was developed, including sorbitol, hydrogen peroxide and sodium carbonate. Prior to CMP, fused silica was roughly polished with ceria slurry. Using developed mesoporous abrasives, surface roughness Sa is reduced from 0.347 to 0.253 nm for a scanning area of 200 × 200 μm2, and the material removal rate (MRR) is increased from 70 to 127 nm min-1, compared with traditional solid abrasives. Based on molecular dynamics (MD) simulations, a novel atomic removal model is proposed for mesoporous abrasives through the immediate elastic recovery of atoms. MD simulations suggest that the formation of convex peaks and pits was inhibited by the mesoporous structure, promoting uniform distribution of surface atoms and atomic removal. This is different from a conventional simple increase of polishing times. In addition, more bridge bonds of Si-O-Si and a lower average Si-O bond order are produced in fused silica samples due to their mesoporous structure, contributing to a higher MRR.

Ratiometric fluorescence-based and chromogenic sensors for the detection of fluoride ions and their application in real samples.

This review focuses on the results of synthetic ratiometric fluorescent and colorimetric probes, which have been applied to qualitatively and quantitatively detect fluoride anions in cells, living organisms, and real samples. Primary attention is given to progress made in the working mechanism and applications of these probes to detect fluoride ions in living systems. In addition, design strategies and detection limit for these probes are discussed. This review aims to deliver a comprehensive compilation of the examples reported from 2005 to 2021 on the developments of ratiometric chromogenic and fluorogenic chemosensors for fluoride anions. A total of 20 different ratiometric/colorimetric sensors have been selected for the novelty in their design, sensitivity, detection limit, dynamic range, and speed of detection based on the three fundamental principles of F- ion detection, namely Si-O bond cleavage; excimer emission; and intramolecular charge transfer emission through the B-F monomer, B-F-B bridged dimers, and deprotonation of the amide N-H. Special emphasis has been given to categorize the fluorophores that work in aqueous media, and possible strategies that might be adopted to design green sensors are discussed. Finally, a tabular summary of the comparative studies of all the sensors based on their sensitivity, detection limit, working solvent, and applications is provided. This extensive review may expedite improvements in the development of advanced fluorescent probes for vast and stimulating applications in the future.

A DFT study to unravel the fluorescence mechanisms of APTES-modified carbon dots

Surface passivation is a well-established method for modifying carbon dots (CDs), intended to improve their properties. We present a theoretical study employing density functional theory (DFT) and time-dependent-DFT (TD-DFT) to explain the photoluminescence (PL) mechanism of amine-modified carbon dots (CD-NH2) [CDs modified with (3-Aminopropyl) triethoxy silane (APTES)] considering their local geometry at the terminal ends; the zig-zag (CDZZ-NH2) and armchair (CDAC-NH2) structural orientations. The experimental evidence from our previous report suggests that the amine groups were tethered on the surface of CDs through a Si-O bond realized by the silane coupling reaction between the ethoxy group of APTES and the hydroxyl group of the CDs. The effect of pH in tweaking the PL of these systems is scrutinized in the present study. The influence of pH and structure on the bandgap of CD-NH2 is demonstrated by analyzing the difference in HOMO-LUMO energies, the density of states (DoS) spectra, and electrostatic potentials (ESP).

On the biocatalytic synthesis of silicone polymers.

Polysiloxanes, with poly(dimethyl)siloxane (PDMS) being the most common example, are widely used in various industrial and consumer applications due to the physicochemical properties imparted by their Si-O-Si backbone structure. The conventional synthesis of PDMS involves the hydrolysis of dichlorodimethylsilane, which raises environmental concerns due to the usage of chlorinated compounds. Herein, a biocatalytic approach for PDMS synthesis is demonstrated using silicatein-α (Silα), an enzyme from marine sponges that is known to catalyse the hydrolysis and condensation of Si-O bonds. Using dialkoxysilane precursors, it was found that Silα catalyses the formation of PDMS in non-aqueous media, yielding polymers with higher molecular weights (approximately 1000-2000 Da). However, on prolonged exposure, the gradual degradation of the polymers was also observed. Overall these observations indicate that Silα catalyses the formation polysiloxanes, demonstrating the potential of biocatalysis for more sustainable polysiloxane production.

Influence of aluminium doping on high purity quartz glass properties.

High purity natural quartz is used as raw material for the manufacture of quartz glass crucibles for solar-grade silicon ingots production. One key challenge for cost-effective ingot pulling is to maximise the ability of the crucible to withstand the process conditions (i.e., silicon load and temperature about 1500 °C) without deformation. In order to improve this glass property, aluminium was coated into the raw quartz materials. Our results showed that an addition of up to 1000 wt ppm Al substantially reduces deformation of glass and improves viscosity at high temperatures. This is likely due to the reduction of stability of OH groups in the quartz glass as well as a trapping effect of aluminium on oxygen vacancies. This hypothesis is also supported by atomistic models. In the presence of Al, formation energies of silanol groups (Si-O-H) were much higher than without. Furthermore, the presence of Al in the structure significantly reduces mobility of the oxygen vacancies. It was also found that formation of oxygen vacancies hinders cristobalite crystallisation, on the other hand, Al atoms themselves induce local weakening of the Si-O bond which accelerates the kinetics of the reconstructive phase transition from glassy state to crystalline phase. This was also confirmed experimentally in our study.

Surface Controlled Silicon Nanoparticles Production and Their Effects on High Capacity and High Cyclability of All-Solid-State Lithium-Ion Batteries

Si nanoparticle is expected as anode for high density Lithium-ion batteries as as it facilitates high capacity and high cyclability simultaneously owing to its structural robustness to suppress fracture during cycled (de)lithiation reactions. Direct TEM observation has revealed that the particle size of 150 nm is the threshold for facture and the particles smaller than 150 nm are anticipated to contribute to the structural stability during cycles. However, the surface of silicon nanoparticle is oxidized spontaneously and the specific surface area increases significantly with decreasing the particle size. Accordingly, the specific oxygen content of the Si active material becomes inevitably high for smaller nanoparticles. High oxygen content in Si anode decreases the initial efficiency and capacityas a result of irreversible silicate formation. Therefore, smaller nanoparticle with a limited amount of oxidation would be ideal and necessary to attain high capacity and high cyclability. However, there is few report on the approach to suppress oxidation of smaller silicon nanoparticles and discuss the effect of size and oxygen content separately. Also, no general understanding of the effect of the particle size and oxygen for the case of all-solid-state batteries (ASSBs). In this work, we have produced Si nanoparticles with differently controlled size and oxygen content by plasma flash evaporation (PFE) from low-cost powder feedstock, and the effect of characteristic oxide surface structure on the battery performance of ASSBs with Argyrodite electrolyte has been identified.We have successfully produced the nanoparticles as small as 20 nm and reduced the oxygen content x in SiOx representation from the standard amount of 0.24 to 0.049. As a result, the initial discharged capacity can be improved by 110% while attaining similar cyclabilities. When we compare the nanoparticles with different sizes ranging from 20 nm to 165 nm and having small oxygen content x < 0.05, although the initial discharged capacities are almost the same for these particles, only the particle with 165 nm shows rapid capacity decay in 10 cycles, whereas the 20 nm and 84 nm particles tend to maintain the capacity similarly. This confirms that the particles smaller than 100 nm are effective to maintain high capacity also in ASSBs under a low 0.1 C-rate condition. Furthermore, XPS analysis revealed that the surface of the particles with less oxidation is composed of suboxide Si-O bond oxide preferentially and the amount of Si-O4 bond is less than the conventional particles covered by the oxide shell, with which the Si-O bond ratio is described by the conventional random bond model. No apparent dependence of the capacity with the amount of Si-O4 bond. However, the surface oxide shell thickness estimated by the Si-O bond amount and the total oxygen content has shown a clear linear correlation with the charged capacity, irrespective of the particle size. This suggests that the surface oxide thickness affects significantly the amount of the lithiation reaction and is ideally as thin as possible to attain high capacity, especially for the case of ASSBs.

(Invited) Design of Surface Oxidation and Nitridation Reactions on 4H-SiC for the Advanced SiC Gate Stack Formation Processes

Silicon carbide (SiC) MOSFETs are expected to replace Si bipolar transistors partially in applications such as automotive, railroad, and other high-voltage applications. But the resistance of the MOS inversion layer channel limits the performance of MOSFETs. It has been clarified that the effective carrier density in the inversion channel is significantly reduced by the carrier trapping by the high density of defect states distributed nearby SiO2/SiC interface. In this study, our understanding of oxidation and nitridation reactions of SiC for designing the gate stack formation processes to improve the MOS interface characteristics is presented.Choosing SiO2 as a gate dielectric for SiC is favorable from various aspects including its ability to form a thermodynamically stable interface with SiC, which is free of reactions at the process temperature. However, from a microscopic point of view, Si-O bond alone cannot cover the whole surface of SiC and the excess C atoms on the surface causes the formation of defect states. One of the possible approaches has been employing an appropriate thermal oxidation condition to enhance the excess carbon elimination by forming CO. For example, we have experimentally demonstrated a significant reduction of Dit by employing high-temperature thermal oxidation with a rapid heating furnace [1]. However, such approach seems to bring a serious drawback such as defect formation in the surface region of SiC, experimentally detected by a lattice distortion in the beneath of SiO2 after high-temperature oxidation [2]. One of the possible reasons for such deterioration would be an invasion of activated oxygen atoms into the surface region of SiC [3]. A possible approach to avoid such drawback of high-temperature oxidation process would be utilizing H2O vapor for the formation of SiO2/SiC interface at relatively lower temperature while avoiding the damages on SiC surface [4], since H2O oxidation was found to be advantageous to remove CO more efficiently than O2 oxidation even at low temperature.On the other hand, another approach to passivate SiC surface is the surface nitridation technique, which is the most common way in industry to fabricate SiC MOS interface typically achieved by high-temperature annealing in NO gas [5]. Nitridation can replace the topmost C sites with N without inducing significant distortion in the SiC surface structure, to suppress the C-related defect formation at the interface. The advantage of this reaction is that it proceeds self-limited manner and N atoms are introduced only on the topmost surface of SiC. Note that NO gas triggers the surface nitridation by removing the surface C atoms in the form of CO by oxidation. Actually, a very similar reaction occurs even when SiC is annealed in N2 [6] ambient with a slight addition of O2. We experimentally found that the surface N density saturated with different values when we employed various N2+O2 atmosphere with different O2 concentration. Therefore we setup a simple model to describe the saturation behavior, which is determined by the balance between the rate of N uptake (N+) and the rate of N desorption (N‒) mainly due to the oxidation reaction [7]. This model was found to represent the time-dependent change of surface N density well, by assuming that N+ remained constant during the reaction and N‒ was proportional to both the reaction rate of interface oxidation and the surface N density [8]. Based on this simplified model, it is possible to design a process to tune the N density. By choosing appropriate temperature and oxygen partial pressure we can increase N density at the surface to minimize the interface trap density. Further improvement of surface N density would be achieved by further controlling of the blance between N uptake and N desorption reactions.As we have discussed so far, the role of materials science to clarify the interface phenomena is significantly important to achieve a further improved SiC MOS interface characteristics for the next generation SiC power MOSFET process technology. [1] R. Kikuchi and K. Kita, Appl. Phys. Lett. 105, 032106 (2014).[2] A. D. Hatmanto and K. Kita, Appl. Phys. Express 11, 011201 (2018).[3] A. D. Hatmanto and K. Kita, Jpn. J. Appl. Phys. 59, SMMA02 (2020).[4] H. Hirai and K. Kita, Appl. Phys. Lett. Appl. Phys. Lett. 113, 172103 (2018).[5] J. Rozen et al., J. Appl. Phys. 105, 124506 (2009).[6] A.Chanthaphan et al., AIP Adv. 5, 097134 (2015).[7] T. Yang and K. Kita, Jpn. J. Appl. Phys. 61, SC1077 (2022).[8] T. Yang and K. Kita, SSDM 2023, Chiba.

Investigation of CMP Process of Silica Substrate by Ceria Abrasive: Insights from Molecular Dynamics Simulation with Neural Network Potential

Chemical mechanical polishing (CMP) is a process of surface planarization by chemical and mechanical driving forces. CMP is expected to solve various problems, including the surface roughness on semiconductor devices. An aqueous slurry, including abrasive grains and chemical components, is used in the CMP process. Development of slurries with high polishing rate, excellent planarity, and superior processability is not only a key challenge, but also a significant opportunity for the future of our industry. In order to achieve the development, it is necessary to understand the detailed mechanism of CMP, which involves complex phenomena accompanied by interfacial chemical reactions and mechanical friction. Computational scientific approaches, such as density functional theory (DFT) and molecular dynamics (MD) simulations, are powerful tools for understanding complex tribochemical phenomena at the atomic scale. However, these methods require much computation time and present difficulties for large-scale simulations. To overcome this limitation, a computational method called Neural Network Potential (NNP) is a powerful tool because it achieves a good balance between accuracy and computational efficiency by constructing potential energy surfaces based on a neural network-based method. We performed NNP-MD simulations of the ceria/silica interface with the development of CMP slurry including ceria abrasive grains, which is typically used on the surfaces of silicon layered devices. As a result, we succeeded in observing polishing phenomena at the interface at the atomic scale, namely Ce-O-Si bond formation between ceria abrasive grains and silica substrate, hydrolysis reaction of Si-O bond, and associated mono-silica dissociation from the substrate. We also performed simulations of systems containing chemical compounds added to the slurry for the purpose of controlling the polishing rate and observed the role of additives on the CMP process. In this presentation, we will show simulation results and discuss the mechanism of CMP of silica substrates with ceria abrasive grains in terms of dependence on surface morphology at the atomic scale and the role of additives. Reference [1] Tran*, R., Lan*, J., Shuaibi*, M., Wood*, B., Goyal*, S., Das, A., Heras-Domingo, J., Kolluru, A., Rizvi, A., Shoghi, N., Sriram, A., Ulissi, Z., & Zitnick, C. (2022). The Open Catalyst 2022 (OC22) Dataset and Challenges for Oxide Electrocatalysis. arXiv preprint arXiv:2206.08917.

B40 and SiB39 fullerenes enhance the physicochemical features of curcumin and effectively improve its anti-inflammatory and anti-cancer activities

In this paper, density functional theory calculations have been employed to study the interaction of curcumin through β-diketone- and enol- forms with the borospherene molecule (B40) and its silicon-doped variant (SiB39) for application as a nanocarrier for designing a drug delivery system. The obtained results from binding energies analysis indicated that the enol form (state II: −1.32 eV) and β-diketone form (state III: −2.51 eV) of curcumin prefers a strong polar covalent B-O and Si-O bonds through carbonyl groups after the interaction with B40 and SiB39 fullerenes. The boron fullerene doped with silicon atom improves the adsorption of β-diketone form and has higher binding energy values and increased dipole moment. So, SiB39 fullerene is more sensitive to the presence of β-diketone (state III: 76.84 %) and enol form is (state IV: 93.68 %) compared to the B40 fullerene due to the alteration in energy gap. According to molecular docking analysis the β-diketone curcumin conjugated with SiB39 (state III) and B40 fullerenes (state I) revealed the best binding affinity with the tumor necrosis factor alpha (TNF-α) receptor (-8.9 kcal/mol) and the cyclooxygenase-1 (COX-1) receptor (-8.8 kcal/mol). The anticancer activities of β-diketone curcumin conjugated B40 and SiB39 fullerenes improved in comparison with curumin towards Epidermal growth factor and Human Epidermal Growth Factor Receptor 2 receptors. Molecular docking simulation analysis illustrated that the interaction of curcumin conjugated B40 and SiB39 fullerenes have great potential for the anticancer and anti-inflammatory activities in contrast to Curcumin. Thus, the molecular docking and density functional theory analysis confirm that curcumin decorated B40 and SiB39 fullerenes have significant potential to be explored further in cancer and inflammation treatments.

Design, Synthesis, and Cellular Imaging Application of a Fluorescent Probe Based on Fluoride Ion-Induced Cyclization of Phenothiazine Derivatives.

Fluoride is both necessary and potentially harmful in excessive amounts, making its detection crucial. Fluorescent probes provide a sensitive and selective means for this purpose. In this study, we developed and synthesized a fluorescent probe for LDT using phenothiazine derivatives and aryl vinyl nitrile. Initially non-fluorescent, the probe undergoes a Si-O bond breakage in the presence of fluoride ions, resulting in the formation of a larger conjugated system and subsequent fluorescence emission. The probe exhibits superior selectivity and sensitivity towards fluoride ions, with a detection limit of 0.35 µM. Moreover, cellular imaging experiments demonstrated the probe's effectiveness in recognizing fluoride ions within HepG2 cells.

Silica extraction from bamboo leaves using alkaline extraction method

The silica is most common constituent of bamboo leaves mainly varies from 75.90 to 82.86 %.In this study, silica was extracted from bamboo leaves using an alkaline extractionprocess. In proximate analysis, bamboo leaves were found to contain significantly more ash (24.5 %) containing 79.28 % silica and low levels of fixed carbon. However, in ultimate analysis, bamboo leaves were found to contain 57.56 % oxygen.The yield and quality of silica were improved by extracting silica from bamboo leaves ash under various operating conditions.A higher concentration of NaOH (2.5 N) resulted in a greater yield of silica (97.73 %).During the alkaline process, an acid leaching treatment was used toremove metal impurities either from bamboo leaves ash or extracted silica. The silica yield increased from 97.73 % to 98.86 % after acid leaching of feed. In an FTIR analysis, Si-O-Si, Si-O, and Si-OH bonds were identified. Modern characterization techniques such as XRF were used to determine the purity of silica. There is a significant difference between the maximum purity of acid leached silica samples (96.13 %) and that of commercial silica samples (95.52 %).As a result of this study, silica that has a wide range of applications in the food and nutrition industries, the pharmaceutical industry, and as a nano-catalyst could be produced.

Waterborne polyurethane sizing agent with excellent water resistance and thermal stability for improving the interfacial performance of carbon fibers/epoxy resin composites

In order to improve the water-resistance and thermal stability of waterborne polyurethane (WPU), a series of silane and maleimide co-modified waterborne polyurethane emulsions (WPU-M-S) were prepared. Because of the high bond energy of the Si-O bond in silane and the rigid ring in 4-HPM, the WPU-M-S sizing agent achieved better thermal stability. Meanwhile, the excellent hydrophobicity of silane enhances its water resistance. Then, the electrochemical anodic oxidation (EAO) treatment was used to activate the surface of the carbon fibers to introduce carboxyl and hydroxyl groups. Next, WPU-M-S was attached to the carbon fiber by the sizing process. For compound modified carbon fibers, the water contact angle increased, and the single filament tensile strength was also improved. The WPU-M-S sizing agent can react with both the active groups on the fiber surface and the amino groups in the epoxy resin system, thereby achieving a significant improvement in the composite interfacial bonding performance. The interlaminar shear strength (ILSS) of the sizing carbon fibers reinforced epoxy resin composite was promoted to 59.80 MPa, with an increase of 41.61% compared with unsized carbon fiber.

Crystal Engineering of Silica Anode Achieving Intrinsic Zero-Strain.

Si-based anodes have large intrinsic volume expansion, which hinders their practicality and commercialization. To address this challenge, the design principle of intrinsic zero-strain anodes (① large intracrystalline cavities and ② strong bonds) is proposed, and silica with large intracrystalline cavities (SLIC) established by strong Si─O bonds ([SiO4 ] coordinate structures) is obtained and acts as an anode, achieving the intrinsic zero-strain feature first in silicon-based anodes. The phase structure of SLIC is maintained and the [SiO4 ] coordinate structure merely shows slight disorder during cycling. The feature stems from lithiation taking place by the solid-solution insertion reaction rather than the conventional conversion/alloying addition reactions, because the solid-solution insertion reaction for the SLIC has the lowest change in the Gibbs free energy. The SLIC anode demonstrates excellent cycling stability and high initial Coulombic efficiency (≈85%). Moreover, owing to the low working voltage (≈0.28V) and relatively high specific capacity, the SLIC anode presents the highest gravimetric energy density among reported zero-/quasi-zero-strain anodes and high volumetric energy density (around twice as much as graphite). The universality of the designing principle is also validated. This work provides design guidelines for zero-strain anodes in next-generation batteries.

Deconstructable and Chemically Recyclable High-Density Polyethylene (HDPE)-Like Materials Based on Si-O Bonds.

Polyethylene (PE) is the most widely produced synthetic polymer. By installing chemically cleavable bonds into the backbone of PE, it is possible to produce chemically deconstructable PE derivatives; to date, however, such designs have primarily relied on carbonyl- and olefin-related functional groups. Bifunctional silyl ethers (BSEs; SiR2(OR'2)) could expand the functional scope of PE mimics as they possess strong Si-O bonds, excellent oxidative stabilities, and facile chemical tunability. Here, we report BSE-containing high-density polyethylene (HDPE)-like materials synthesized through a one-pot catalytic ring-opening metathesis polymerization (ROMP) and hydrogenation sequence. The crystallinity of these materials can be adjusted by varying the BSE concentration or the steric bulk of the Si-substituents, providing handles to control thermomechanical properties while maintaining the high thermal stability of HDPE. Two methods for chemical recycling of HDPE mimics are introduced, including a circular approach that leverages acid-catalyzed Si-O bond exchange with 1-propanol. Additionally, despite the fact that the starting HDPE mimics were synthesized by chain-growth polymerization (ROMP), we show that it is possible to recover the molar mass and dispersity of recycled HDPE products using step-growth Si-O bond formation or exchange, generating high molecular weight recycled HDPE products with mechanical properties similar to commercial HDPE.

The Mechanical Properties and Water-Reducing and Retarding Mechanism of a Xylonic Cement Admixture.

This study explores the mechanical properties, as well as the water-reducing and setting delay mechanism, of a novel xylonic acid-based water reducer applied to cementitious materials. Four xylonic acid water reducers were synthesized in this study: XACa (PX) from pure xylose, XACa (HS) from hemicellulose hydrolysate, XANa (PX) from pure xylose, and XANa (HS) from hemicellulose hydrolysate. These were generated through the whole-cell catalysis of Gluconobacter oxydans bacteria, using pure xylose and hemicellulose hydrolysate as substrates. The findings indicate that the xylonic acid-based water reducer can attain a water-reducing capability between 14% and 16% when the dosage (expressed as a mass fraction of cement) is roughly 0.2%. In initial and final setting tests, XACa (PX) demonstrated a pronounced retarding influence at admixture levels below 0.15%, reaching its apex at 0.10%. This delayed the initial setting time by 76% and the final setting time by 136% relative to the control group. However, a slight pro-setting effect was noted beyond a 0.2% dosage. In the compressive and flexural tests of concrete, under the same slump, the XA group improved its mechanical properties by 5% to 10% compared to the SodiuM lignosulfonate (SL) group. In the air content and chloride ion migration resistance tests, the XA group reduced the air content by 38% compared to the SL group, but also increased the data of rapid chloride migration (DRCM) by 16%. Characterization studies revealed that the carboxyl and hydroxyl groups in xylonic acid undergo chemisorption with the Si-O bonds on the surface of cement particles. These groups interact with the Si-O bonds on cement particles, contributing to water-reducing effects and delaying the setting process by impeding Ca2+ ion aggregation in the calcium-silicate-hydrate gel. Its significant water-reducing effect, adjustable setting time, and excellent mechanical and durability properties suggest its viability as an alternative to lignosulfonate series water-reducing agents.

Copper crosslinked silicone elastomers in RGB colors: degradable on demand

Increasing demand for fully recyclable polymers has prompted an interest in materials crosslinked via non-covalent interactions. Aggressive depolymerization can cleave backbone SiO bonds to generate linear oils or cyclic monomers from silicone rubbers, but one would rather recover the starting material oils simply by selectively breaking crosslinks. We demonstrate that ligand binding to metals can be used to reversibly crosslink silicone chains. Aminopropylsilicones were crosslinked via complexation with copper (II) acetate to form blue silicone oils. These oils slowly cured in air at room temperature to form soft, green elastomers over a month, or overnight at 50–55 °C in air to give robust, hard red elastomers. Potential explanations for the observed color changes are discussed. Elastomers prepared by either pathway underwent ready degradation by removal of the copper ions via competitive ligand binding using ethylenediamine, allowing for recovery of the unmodified silicone oil; the recovered amine could be reused with more copper to form a (softer) elastomer. This process provides a method for the synthesis of stable elastomeric silicones that are degradable on demand.

The Dissolution Mechanism of Low-Molecular-Weight Organic Acids on the Sillimanite.

The interaction between low-molecular-weight organic acids (LMWOAs) and minerals in nature has been widely studied; however, limited research has been conducted on the dissolution mechanism of sillimanite in the presence of different organic acids. In this study, the interaction between the sillimanite sample and LMWOAs (citric acid, oxalic acid, and citric/oxalic mixture) at the same pH was investigated. The dissolution rate of Si and Al was high during the initial reaction time, then slowed down in the presence of LMWOAs. The dissolution data for Si and Al from sillimanite in the LMWOAs fit well with the first-order equation (Ct = a(1 - exp(-kt))) (R2 > 0.991). The dissolution process of sillimanite in the organic acids was controlled by the surface chemical reaction step. The dissolution concentration of Si in aqueous citric acid was higher than that in oxalic acid. In contrast, the dissolution concentration of Al in oxalic acid was more than that in citric acid. The maximum concentrations of Si and Al in the presence of composite organic acids were 1754 μmol/L and 3904 μmol/L. The sillimanite before and after treatment with LMWOAs were studied using X-ray diffraction (XRD) and scan electron microscopy (SEM). These results are explained by the characterization of the sillimanite. Under the single acid solution, the (210) crystal plane with a high areal density of Al in sillimanite was easily dissolved by the oxalic acid, while the (120) in sillimanite with a high areal density of Si was more easily dissolved by citric acid. In the composite organic acids, the Si-O bond and Al-O bond in sillimanite were attacked alternately, leading to the formation of some deeper corrosion pits on the surface of sillimanite. The results are of interest in the dissolution mechanisms of sillimanite in the low-molecular-weight organic acids and the environmentally friendly activation of sillimanite.

Improving mechanical properties and durability of polyether sealant in prefabricated buildings with titanium dioxide and graphene

The silane-modified polyether sealant is widely applied to fill the gaps between external walls in prefabricated buildings and to bear the deformation of joints. However, the weak tensile strength and poor ultraviolet and moisture resistance of sealant lead to severe degradation, posing threat to the building waterproofing and safety. To address the problems, titanium dioxide and graphene were adopted to improve the mechanical properties and durability of sealant. The tack-free time, tensile properties, ultraviolet resistance, and hydrophobic performance of pristine and modified sealant were investigated and the enhancement and erosive resistance mechanisms of modified sealant were revealed by microscale characterization. Results indicated that modified sealant with low content of titanium dioxide had greater tensile strength and ultraviolet resistance. The crystalline form of titanium dioxide and produced Si-O bond enhanced the ultraviolet resistance. Moreover, the 0.8% graphene-modified sealant possessed superior moisture resistance with 23% increased contact angle. Graphene increased the surface roughness and generated π-conjugated structure, improving sealant hydrophobicity. It is revealed that the enhanced properties were ascribed to the spatial structure of fillers and chemical bond energy. This work provides a novel sealant with high ultraviolet resistance and hydrophobicity, facilitating the development of durable and low-maintenance prefabricated buildings.

Iron(II)-Catalyzed Activation of Si-N and Si-O Bonds Using Hydroboranes.

We report the activation and functionalization of Si-N bonds with pinacol borane catalyzed by a three-coordinate iron(II) β-diketiminate complex. The reactions proceed via the mild activation of silazanes to yield useful hydrosilanes and aminoboranes. The reaction is studied by kinetic analysis, along with a detailed investigation of decomposition pathways using catecholborane as an analogue of the pinacol borane used in catalysis. We have extended the methodology to develop a polycarbosilazane depolymerization strategy, which generates hydrosilane quantitatively along with complete conversion to the Bpin-protected diamine. The analogous Si-O bond cleavage can also be achieved with heating, using silyl ether starting materials to generate hydrosilane and alkoxyborane products. Depolymerization of poly(silyl ether)s using our strategy successfully converts the polymer to 90% Bpin-protected alcohols.

Effects of temperature on the migration behaviour of arsenic and chromium in tannery sludge under CO2 gasification

To reduce heavy metals (HMs) contamination from tannery sludge, this study investigated the migration behaviour of arsenic (As) and chromium (Cr) at 700–900 °C using CO2 gasification. The HMs enrichment results showed that As contents of ash decreased (6.42→1.87 mg/kg) while Cr contents increased (41.40→78.24 mg/kg) over 700–900 °C. More Si-O bonds and fewer Ca-O bonds with increasing temperature in ash primarily determined this migration behaviour of HMs. Meanwhile, the proportions of toxic As(III) and Cr(VI) declined from 96.02% and 64.26–76.96% and 21.24%, forming As(0) and Cr(III) with less toxicity. This reduction was conducted via two pathways: (i) carbon reduced As(III)/Cr(VI) and (ii) carbon reduced Fe(II)/Fe(III) to Fe(0), then Fe(0) reduced As(III)/Cr(VI) assisted with carbon via Fe(0)→Fe(II)→Fe(III). However, free calcium ions oxidized As(0)/Cr(III) to As(III)/Cr(VI) at 700 ○C. At higher temperatures, silicate glass conversion of ash immobilized free calcium ions and barely oxidized HMs. Furthermore, this study identified the positive effect of increasing temperature on enhancing the stability of HMs in ash by transforming bioavailable HMs into non-bioavailable HMs, which decreased the leaching toxicity and environmental risk. Regarding HMs emissions control and cold gas efficiency, CO2 gasification treatment of tannery sludge is most effective at 800 °C.