Mechanism and Parameters Controlling the Decomposition Kinetics of Na2SiF6 Powder to SiF4

Sodium hexafluorosilicate (Na₂SiF₆) powder has been used as a silicon source for formation of Si₃N₄ coatings by the hybrid precursor system‐chemical vapor deposition (HYSY‐CVD) route. The quantitative effect of processing time, temperature, gas flow rate, and process atmosphere (N₂ and N₂:5% NH₃) upon the fractional weight loss during the decomposition of Na₂SiF₆ was studied using a standard L₉ Taguchi experimental design and analysis of variance. The decomposition kinetics of Na₂SiF₆(s) was studied theoretically and experimentally in the temperature range of 550–650ºC by applying the shrinking core model. It was found that regardless of atmosphere type, the reaction order is n ≈ 0.12 and that a two‐stage mixed mechanism consisting of chemical reaction and boundary layer gas transfer controls the decomposition rate. The determined fractional weight loss during Na₂SiF₆ decomposition in nitrogen atmosphere is about 1.05–1.5 orders of magnitude greater than that in N₂:NH₃. The gas flow rate affects the dissociation activation energy, being of 121, 109, and 94 kJ/mol in N₂ and of 140, 120, and 115 kJ/mol in N₂:NH₃, for the flow rates of 20, 60, and 100 cm³/min, respectively, in both atmosphere types. A good agreement is observed by comparing experimental weight loss data with model predictions.     

Theoretical Investigation of the Solid State Reaction of Silicon Nitride and Silicon Dioxide forming Silicon Oxynitride (Si2N2O) under Pressure

The high‐pressure behavior of Si₂N₂O is studied for pressures up to 100 GPa using density functional theory calculations. The investigation of a manifold of hypothetical polymorphs leads us to propose two dense phases of Si₂N₂O, succeeding the orthorhombic ambient‐pressure polymorph at higher pressures:a defect spinel structure at moderate pressures and a corundum‐type structure at very high pressures. Taking into account the formation of silicon oxynitride from silicon dioxide and silicon nitride and its pressure dependence, we propose five pressure regions of interest for Si₂N₂O within the pseudo‐binary phase diagram SiO₂‐Si₃N₄: (i) stability of the orthorhombic ternary phase of Si₂N₂O up to 6 GPa, (ii) a phase assemblage of coesite, stishovite, and β‐Si₃N₄ between 6 and 11 GPa, (iii) a possible defect spinel modification of Si₂N₂O between 11 and 16 GPa, (iv) a phase assemblage of stishovite and γ‐Si₃N₄ above 40 GPa, and (v) a possible ternary Si₂N₂O phase with corundum‐type structure beyond 80 GPa. The existence of both ternary high‐pressure phases of Si₂N₂O, however, depends on the delicate influence of configurational entropy to the free energy of the solid state reaction.     

Capacitance Polypyrrole‐based Impedimetric Immunosensor for Interleukin‐10 Cytokine Detection

In the present study, we developed a novel label‐free capacitance impedimetric immunosensor based on the immobilization of the human monoclonal antibody anti‐interleukin‐10 (anti‐IL‐10 mAb) onto polypyrrole (PPy)‐modified silicon nitride (Si₃N₄) substrates. The immunosensor was used for the detection of the recombinant interleukin‐10 antigen (rh IL‐10) that may be secreted in patients at the early stage of inflammation. The immunosensor was created by chemical deposition of PPy conducting layer on pyrrole?silane (SPy)‐treated Si/SiO₂/Si₃N₄ substrates (Si/SiO₂/Si₃N₄?SPy), followed by anti‐IL‐10 mAb immobilization through carboxyl‐functionalized diazonium (CMA) protocol and carbodiimide chemistry. The surface characterization and the biofunctionalization steps were characterized by SEM, FTIR and cyclic voltammetry (CV) while the detection process was carried out by using electrochemical impedance spectroscopy (EIS) analyses. The created immunosensor showed two linear fittings (R²=0.999) for the detection of rh IL‐10 within the concentration range from 1–50 pg/mL. It exhibited high sensitivity (0.1128 (pg/mL)^?1) with a very low limit of detection (LOD)=0.347 pg/mL, more particularly, at the low concentration range (1–10 pg/mL). Thus, this developed polypyrrole‐based immunosensor represents a promising strategy for creation of miniaturized label‐free, fast and highly sensitive biosensors for diagnosis of inflammation biomarkers at very low concentrations with reduced cost.     

Preparation and properties of Ni–Co–P/nano‐sized Si3N4 electroless composite coatings

Ni–Co–P/nano‐sized Si₃N₄ composite coating was successfully fabricated on aluminum alloys by electroless plating in this work. The surface and cross‐sectional morphologies, composition, microstructure, microhardness, friction and wear behavior of deposits were investigated with SEM, EDS, XRD, Vickers hardness and high‐speed reciprocating friction, respectively. It was found that a Ni–Co–P/nano‐sized Si₃N₄ composite coating on aluminum alloy substrate is uniform and compact. The existence of nano‐sized Si₃N₄ particles in the Ni–Co–P alloy matrix causes a rougher surface with a granular appearance, and increases the microhardness but decreases the friction coefficients and wear rate of electroless coatings. Meanwhile, the effects of heat treatment at 200, 300, 400 and 500 °C for 1 h on the hardness and tribological properties were researched. It is revealed that both of the microhardness and tribological properties of Ni–Co–P coatings and Ni–Co–P/Si₃N₄ composite coatings increase with the increase of heating temperature in the range of 200–400 °C, but show different behavior for the two coatings after annealing at 500 °C. Copyright © 2011 John Wiley & Sons, Ltd.     

Fabrication of new polypyrrole/silicon nitride hybrid materials for potential applications in electrochemical sensors: Synthesis and characterization

In this research, an efficient fabrication process of conducting polypyrrole (PPy)/silicon nitride (Si₃N₄) hybrid materials were developed in order to be employed as transducers in electrochemical sensors used in various environmental and biomedical applications. The fabrication process was assisted by oxidative polymerization of pyrrole (Py) monomer on the surface of Si/SiO₂/Si₃N₄ substrate in presence of FeCl₃ as oxidant. To improve the adhesion of PPy layer to Si₃N₄ surface, a pyrrole-silane (SPy) was chemically bonded through silanization process onto the Si₃N₄ surface before deposition of PPy layer. After Py polymerization, Si/SiO₂/Si₃N₄-(SPy-PPy) substrate was formed. The influence of SPy concentration and temperature of silanization process on chemical composition and surface morphology of the prepared Si/SiO₂/Si₃N₄-(SPy-PPy) substrates was studied by FTIR and SEM. In addition, the electrical properties of the prepared substrates were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It was found that the best silanization reaction conditions to get Si/SiO₂/Si₃N₄-(SPy-PPy) substrate with high PPy adhesion and good electrical conductivity were obtained by using SPy at low concentration (4.3 mM) at 90°C. These promising findings open the way for fabrication of new hybrid materials which can be used as transducers in miniaturized sensing devices for various environmental and biomedical applications.     

Mass spectrometric study of NF3 plasma etching of silicon

NF₃ plasma etching is used for dry cleaning of reactors after plasma-enhanced chemical vapor deposition of hydrogenated amorphous silicon from SiH₄. The NF₃ plasma chemistry, in a closed isothermal plasma box with silicon coated walls, is analyzed by mass spectrometry of gases. Silicon is etched as SiF₄ by F atoms produced in the NF₃ dissociation into F+NF₂, or 2F+NF. The NF radicals recombine as N₂ +2F whereas the long-lived NF₂ radicals do not react with Si, but recombine as N₂F₄ This is the main limitation (or fluorine conversion into SiF₄. The pressure increase at the end point of etching is attributed to the sudden increase of F atom concentration in the gas phase and the consequent recombination q( F atoms as F₂.     

Silicon nitride nanofibers produced by the pyrolysis of SiCl<Subscript>4</Subscript> in H<Subscript>2</Subscript> and N<Subscript>2</Subscript> media

The synthesis of fibrous Si₃N₄ nanostructures by gas-phase chemical deposition during the pyrolysis of silicon tetrachloride in a hydrogen and nitrogen atmosphere was investigated. It was shown that the products contained both amorphous Si₃N₄ nanofibers and their α and β phases both with and without a silicon coating. In addition to the Si₃N₄ nanofibers the products also contained nanoribbons. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 2, pp. 81–84, March–April, 2007.     

Facile Access to an NHC‐Coordinated Silicon Ring Compound with a Si=N Group and a Two‐Coordinate Silicon Atom

Reaction of the bicyclo[1.1.0]tetrasilatetraamide Si₄{N(SiMe₃)Dipp}₄ 1 (Dipp=2,6‐diisopropylphenyl) with 5 equiv of the N‐heterocyclic carbene NHC^Me4 (1,3,4,5‐tetramethylimidazol‐2‐ylidene) affords a bifunctional carbene‐coordinated four‐membered‐ring compound with a Si=N group and a two‐coordinate silicon atom Si₄{N(SiMe₃)Dipp}₂(NHC^Me4)₂(NDipp) 2 . When 2 reacts with 0.25 equiv sulfur (S₈), two sulfur atoms add to the divalent silicon atom in plane and perpendicular to the plane of the Si₄ ring, which confirms the silylone character of the two‐coordinate silicon atom in 2 .     

Formation mechanisms of Si3N4 and Si2N2O in silicon powder nitridation

Commercial silicon powders are nitrided at constant temperatures (1453 K; 1513 K; 1633 K; 1693 K). The X-ray diffraction results show that small amounts of Si₃N₄ and Si₂N₂O are formed as the nitridation products in the samples. Fibroid and short columnar Si₃N₄ are detected in the samples. The formation mechanisms of Si₃N₄ and Si₂N₂O are analyzed. During the initial stage of silicon powder nitridation, Si on the outside of sample captures slight amount of O₂ in N₂ atmosphere, forming a thin film of SiO₂ on the surface which seals the residual silicon inside. And the oxygen partial pressure between the SiO₂ film and free silicon is decreasing gradually, so passive oxidation transforms to active oxidation and metastable SiO(g) is produced. When the SiO(g) partial pressure is high enough, the SiO₂ film will crack, and N₂ is infiltrated into the central section of the sample through cracks, generating Si₂N₂O and short columnar Si₃N₄ in situ. At the same time, metastable SiO(g) reacts with N₂ and form fibroid Si₃N₄. In the regions where the oxygen partial pressure is high, Si₃N₄ is oxidized into Si₂N₂O.     

The analysis of Si3N4, SiC and BN by combustion with elemental fluorine coupled to mass spectrometry

Combustion with elemental fluorine has been used for the decomposition of ceramic powders of Si₃N₄, SiC and BN. Volatile reaction products like N₂, BF₃, SiF₄ and various fluorides of carbon are detected on-line with a quadrupole mass spectrometer. It is shown that thermal separation of the combustion products by controlled release from a cooling trap is useful to decrease limitations by isobaric interferences in quadrupole mass spectrometry. The developed fluorine combustion coupled to a mass spectrometry procedure has been used for the determination of the stoichiometry of Si₃N₄ in which Si could be directly determined.     

Chemical Vapor Deposition of Silicon Carbide and Silicon Nitride—Chemistry's Contribution to Modern Silicon Ceramics

Pure silicon carbide and silicon nitride have valuable properties in bulk pore-free form; however, their industrial exploitation has hardly been possible so far. Neither compound can be melted or sintered in pure form; hot pressing or sintering at normal pressure requires the presence of additives; and the reaction-sintering process in which only Si and C or Si and N are employed as additives affords porous materials.–The novel process of chemical vapor deposition has partly overcome the drawbacks of the previous methods. In the new process SiC is produced, e.g., by pyrolysis of CH₃SiCl₃, and Si₃N₄ by reaction of SiCl₄ with NH₃. This technique can also be used for pore filling in objects made of SiC and Si₃N₄ (gas phase impregnation) and for producing extremely fine SiC and Si₃N₄ (gas phase impregnation) and for producing extremely fine SiC and Si₃N₄ powder and SiC monofilaments suitable as components for SiC composites. Moreover, gas phase impregnation can also give fiber composites.     

Incorporating Si3N4 into PEEK to Produce Antibacterial,Osteocondutive, and Radiolucent Spinal Implants

Polyetheretherketone (PEEK) is a popular polymeric biomaterial which is primarily used as an intervertebral spacer in spinal fusion surgery; but it is developed for trauma, prosthodontics, maxillofacial, and cranial implants. It has the purported advantages of an elastic modulus which is similar to native bone and it can be easily formed into custom 3D shapes. Nevertheless, PEEK's disadvantages include its poor antibacterial resistance, lack of bioactivity, and radiographic transparency. This study presents a simple approach to correcting these three shortcomings while preserving the base polymer's biocompatibility, chemical stability, and elastic modulus. The proposed strategy consists of preparing a PEEK composite by dispersing a minor fraction (i.e., 15 vol%) of a silicon nitride (Si₃N₄) powder within its matrix. In vitro tests of PEEK composites with three Si₃N₄ variants—β‐Si₃N₄, α‐Si₃N₄, and β‐SiYAlON—demonstrate significant improvements in the polymer's osteoconductive versus SaOS‐2 cells and bacteriostatic properties versus gram‐positive Staphylococcus epidermidis bacteria. These properties are clearly a consequence of adding the bioceramic dispersoids, according to chemistry similar to that previously demonstrated for bulk Si₃N₄ ceramics in terms of osteogenic behavior (vs both osteosarcoma and mesenchymal progenitor cells) and antibacterial properties (vs both gram‐positive and gram‐negative bacteria).     

Sliding friction and wear behaviors of plasma sprayed aluminum–bronze coating in artificial seawater

The friction and wear behaviors of plasma sprayed aluminum–bronze (CuAl) coating sliding against silicon nitride (Si₃N₄) in artificial seawater were investigated and compared with those in pure water and dry sliding. The morphologies of the worn surfaces were analyzed by three‐dimensional non‐contact surface mapping and scanning electron microscopy. Moreover, chemical states of the tribochemical products of CuAl/Si₃N₄ in seawater were characterized by X‐ray photoelectron spectroscopy. Results show that the plasma sprayed CuAl coating possessed a specific wear rate (in order of 10^?7 mm³/Nm) in seawater more than 600 times smaller than that in dry sliding due to the great alleviation in abrasion wear and splats delamination. Besides, the CuAl/Si₃N₄ had a friction coefficient of 0.06 in seawater, significantly lower and more stable than those in pure water and dry sliding. The tribochemical products of CuAl/Si₃N₄ in seawater, which were proved to be silica, alumina, and their hydrates, transformed into a loosened wear‐debris layer under the coagulation effect of the seawater and dominated the excellent lubrication state in artificial seawater. Copyright © 2014 John Wiley & Sons, Ltd.     

Study of Surface Modification of Nanosize Si3N4 with Macromolecular Coupling Agent LMPB-g-GMA in Different Solvents

A new macromolecular coupling agent, low-molecular-weight polybutadiene liquid rubber (LMPB)-glycidyl methacrylate (GMA), was synthesized using solution polymerization initiated by benzoyl peroxide (BPO). The molecular structure was confirmed by FTIR and NMR. This macromolecular LMPB-GMA was used for surface modification of silicon nitride (Si₃N₄) nanopowder in n-heptane, chloroform, ethyl acetate, and DMF, respectively. LMPB-GMA and modified nano-Si₃N₄ were systematically investigated by FTIR, NMR, TGA, and TEM. The results showed that LMPB-GMA bonded and formed an organic coating layer onto the surface of nanosized Si₃N₄ particles. The polarity of the solvents plays an important role in this process. Strong or weak polar both affect the results. The dosage loading of LMPB-GMA is 12 wt% of nanosized Si₃N₄. Nanosized Si₃N₄ modified in ethyl acetate has better dispersibility and stability than that modified in n-heptane or DMF. TEM pictures also reveal that modified nano-Si₃N₄ possesses good dispersibility.     

Fabrication of SiC and Si3N4 Whiskers under Pressurized Ar and N2 Atmosphere through High‐Energy Ball Milling

SiO_{2 (activated or mesoporous silica)}/Mg_{(magnesiothermic or metal sintering aid)}/C_{(activated or polymeric carbon)}/N_{2 (atmosphere)} systems were used in the one‐step synthesis of β‐SiC and β‐Si₃N₄ whiskers. In this study, a mixture of the active precursors was allowed to react via a self‐sustaining reaction (high‐energy ball milling process). Scanning electron micrographs and X‐ray diffraction (XRD ) analysis showed that the rod‐like SiC whiskers (~800 µm) were synthesized in situ by the direct carbothermal reduction of silicon nitride (or silicon) with activated carbon in N₂ (or Ar) atmosphere. The results show that β‐Si₃N₄ (without β‐SiC ) was fully formed after 5 h of milling with four different morphologies, namely whisker tip (droplet/no droplet) and nonuniform whiskers (short hexagonal/rhombohedral/rod‐like) with a length of 0.1–400 µm. By adding metal sintering aids, the liquid phase Mg–Si–O–N and the rate of carbothermal reduction increased (enhanced densification via particle rearrangement) and their hexagonal whiskers tended to assume a rod‐like shape. The effect of the concentration of CO (reduction of α‐Fe₂O₃ to Fe by CO ) on the whisker synthesis suggests that, in addition to the concentration of CO , the nature of the family of mesoporous silica/carbon template is an important factor in the synthesis of β‐SiC and β‐Si₃N₄ whiskers. The possible chemical reactions were investigated by studying the unwanted phases (MgO , Si, SiC , Fe₂O₃ , Fe₃O₄ , FeO , Fe, Fe₃C , MgCO₃ ) of comparable XRD graphs.     

Synthese,Kristallstruktur und festkörper‐NMR‐spektroskopische Untersuchung neuer Oxonitridosilicate der Mischkristallreihe Ba4−xCaxSi6N10O

The new oxonitridosilicates Ba_4?xCa_xSi₆N₁₀O have been synthesized by means of high‐temperature synthesis in a radio‐frequency furnace, starting from calcium, barium, silicon diimide and amorphous silicon dioxide. The maximum reaction temperature was at about 1450 °C. The solid solution series Ba_4?xCa_xSi₆N₁₀O with a phase width 1.81 ≤ x ≤ 2.95 was obtained. The crystal structure of Ba_1.8Ca_2.2Si₆N₁₀O was determined by X‐ray single‐crystal structure determination (P2₁3, no. 198), a = 1040.2(1) pm, Z = 4, wR2 = 0.082). It can be described as a highly condensed network of corner‐sharing SiN₄ and SiON₃ tetrahedra, the voids of which are occupied by the alkaline earth ions. The structure is isotypic with that of BaEu(Ba_0.5Eu_0.5)YbSi₆N₁₁. In the ²⁹Si solid‐state MAS‐NMR spectrum two isotropic resonances at ?50.0 and ?53.6 ppm were observed.     

Experimental and theoretical studies of the basicity and proton affinity of SiF4 and the structure of SiF4H+

A combined experimental and theoretical approach has been employed to establish the basicity and proton affinity of SiF₄ and the structure of SiF₄H⁺. The kinetics and energetics for the transfer of a proton between SiF₄, N₂, and Xe have been explored experimentally in helium at 0.35±0.02 torr and 297±3 K with a selected-ion flow tube apparatus. The results of equilibrium constant measurements are reported that provide a basicity and proton affinity for SiF₄ at 297±3 K of 111.4±1.0 and 117.7±1.2 kcal mol^?1, respectively. These values are more than 2.5 kcal mol^?1 lower than currently recommended values. The basicity order was determined to be GB(Xe)>GB(SiF₄)>GB(N₂), while the proton-affinity order was shown to be PA(Xe)>PA(N₂)>PA (SiF₄). Ab initio molecular orbital computations at MP4SDTQ(fc)/6-311++G(3df,3pd) using geometries from B3LYP/6-31+G(d,p) indicate a value for PA(SiF₄)=118.7 kcal mol^?1 that is in good agreement with experiment. Also, the most stable structure of SiF₄H⁺ is shown to correspond to a core SiF ₃ ⁺ cation solvated by HF with a binding energy of 43. 9 kcal mol^?1. Support for this structure is found in separate SIFT collision induced dissociation (CID) measurements that indicate exclusive loss of HF.     

Reactions of Difluorosilylene with Amines,Phosphines and Halomethanes The First Evidence of the Insertion of Difluorosilylene into Tetrafluorosilane

The reactions of SiF₂/SiF₄ with amines, phosphines and various halomethanes were studied. The results show that in the presence of strong Lewis base (Et₃N, HNEt₂, PR₃, PR₂Cl), SiF₂ inserts initially into SiF₄ to form Si₂F₆, which was followed by subsequent reactions leading to products containing SiF₃ groups. This is the first report of the insertion of SiF₂ into SiF₄. When the reactants were a weaker base (such as RPCl₂, CX₃Br, X=F, Cl), insertion of SiF₂ into P-Cl and C-Br bonds became predominant.     

IPr3Si3Cl5+: A Highly Reactive Cation with Silanide Character

The reaction of a metastable SiCl₂ solution with the sterically less‐demanding carbene N,N‐diisopropylimidazo‐2‐ylidene (IPr) yields the salt [(IPr₃Si₃Cl₅)⁺]Cl^? ( 1 ‐Cl), containing a silyl cation with a Si₃ backbone. Salt 1 is highly reactive, but it can be used as a reagent in deuterated dichloromethane, whereby dehalogenation with Me₃SiOTf (OTf=O₃SCF₃) gives the dicationic silyl halide [(IPr₃Si₃Cl₄)]²⁺ 2 . Quantum chemical calculations show that the HOMO is localized at the negatively charged central silicon atom of 1 and 2 , and thus although both compounds are cations they are better described as silanides, which was also corroborated by NMR investigations.     

Structure,corrosion, and tribological properties of CrSiN coatings with various Si contents in 3.5% NaCl solution

The CrSiN coatings with different silicon contents were deposited on 304L stainless steel and single silicon substrates using medium frequency magnetron sputtering via adjusting the silicon target current. Microstructures, mechanical, corrosion, and tribological performances were systematically investigated by the corresponding equipment. The results showed that the CrSiN coatings were composed of 2 phases: the nanocrystalline CrN coordinated with adjacent Si₃N₄ to form a typical nanocrystalline amorphous structure. With the increase of silicon content, the hardness emerged a rising trend until 12.65 at.%, where the hardness reached its highest value at approximately 22 GPa. At the moment, the friction coefficient and wear rate were approximately 0.27 and 6.9 × 10⁻⁸ mm³/Nm, respectively, which were 18.75% and 61.67% lower than that of the CrN coating, respectively. This mainly attributed to the lubrication of tribochemical products of SiO₂ and Si(OH)₄. In addition, excellent toughness, high hardness, preferable adhesion, and good corrosion resistance also contributed to improve the tribological properties of CrSiN coating.