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.     

Comprehension of the S(V)LS mechanism growth of silicon-based nanowires

A comprehensive thermodynamic assessment has been carried out on the Au–Si–O system involved in the growth mechanism of Silicon NanoWires (SiNW) via the solid–liquid–solid process. The driving force needed to trigger the SiNW precipitation is supersaturation of liquid alloy Au–Si. Our model demonstrates, for the first time, how and from where supersaturation is reached. Supersaturation is not due to the migration of silicon from the wafer as claimed by many researchers, but to the existence of SiO volatile species resulting from the metastable equilibrium SiO_{2, amorphous}/Si_wafer. More interesting is that the partial pressure P_SiO does impose an initial minimum radius of the first generation of nanowires in the range of 10 nm. After that, other generations of nanowires will grow due to the new metastable equilibrium SiO_{2, amorphous}/Si_nanowire.     

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.     

Eight‐ and Twelve‐membered Cyclosilazoxanes: Structural Investigation of Two N‐Pentafluorophenyl‐substituted Si4N3O and Si6N2O4 Rings

The molecular structures of two N‐pentafluorophenylcyclosilazoxanes have been investigated. X‐Ray crystal structure determinations of (C₆F₅)₃Me₈Si₄N₃O ( 2 ) and (C₆F₅)₂Me₁₂Si₆N₂O₄ ( 3 ) revealed the first structurally authenticated examples of eight‐membered Si₄N₃O and twelve‐membered Si₆N₂O₄ ring systems.     

Ce16Si15O6N32—An Oxonitridosilicate with Silicon Octahedrally Coordinated by Nitrogen

A rarity in solid‐state chemistry is octahedrally coordinated silicon. Ce₁₆Si₁₅O₆N₃₂ is the first nitridosilicate with this structural motif. Most oxosilicates containing octahedrally coordinated silicon are high‐pressure phases. In contrast to that Ce₁₆Si₁₅O₆N₃₂ has been synthesized under ambient pressure. An SiN₆ octahedron that is surrounded by two parallel rings formed from six Si(O,N)₄ tetrahedra is depicted.     

Nitrido-Silicate. I. Hochtemperatur-Synthese und Kristallstruktur von Ca2Si5N8

Nitrido Silicates. I. High Temperature Synthesis and Crystal Structure of Ca₂Si₅N₈ Ca₂Si₅N₈ is obtained by reaction of silicon diimide with metallic calcium under nitrogen atmosphere performed in a specially developed high-frequency furnace at temperatures between 1 500 and 1 600°C. Ca₂Si₅N₈ (Cc, a = 1 435.2(3), b = 561.0(1), c = 968.9(2) pm, β = 112.06(3)°, Z = 4, R = 0.023, wR = 0.018) contains Ca²⁺ ions as well as a three-dimensional covalent network structure of corner-sharing SiN₄ tetrahedra. Two sorts of N occur with molar ratio 1:1 which are bonded to two and three Si, respectively.     

Si X-ray absorption near edge structure (XANES) of Si,SiC, SiO2, and Si3N4 measured by an electron probe X-ray microanalyzer (EPMA)

Silicon K X-ray emission spectra of Si, SiC, Si₃N₄, and SiO₂ are measured using a wavelength dispersive electron probe X-ray microanalyzer. It is shown that the fine structures in the line shape of the low energy tail of the Kα characteristic X-ray emission spectra resemble those of the K X-ray absorption near edge structure (XANES). XANES spectra of 1 μm² area can be obtained by this method.     

Nitrido-silicate. III. Hochtemperatur-Synthese,Kristallstruktur und magnetische Eigenschaften von Ce3[Si6N11]

Nitrido-silicates. III [1] High-Temperature Synthesis, Crystal Structure, and Magnetic Properties of Ce₃[Si₆N₁₁] Pure Ce₃[Si₆N₁₁] was obtained as transparent yellow crystals by reaction of metallic cerium with silicon diimide (Ce:Si = 1:2) under nitrogen atmosphere in a specially developed high-frequency furnace at 1660°C. Ce₃[Si₆N₁₁] (P4bm, a = 1013.7(3), c = 483.9(5) pm, Z = 2, R = 0.034, wR = 0.024) contains Ce³⁺ ions as well as a three-dimensional covalent anionic network structure of corner-sharing SiN₄ tetrahedra. Measurements of the magnetic susceptibility gave no indications for magnetic ordering phenomena in the temperature range between 2 and 300 K. Above 100 K pure Curie-Weiss behaviour (μ_eff = 2,10 μ_B, determined at room temperature) was observed.     

The influence of processing parameters on the fabrication of Si3N4 wires

Silicon nitride (Si₃N₄) wires have been prepared by means of carbothermal reduction followed by the nitridation (CTRN) of silica gel containing ultrafine decomposed saccharose. The influence of temperature of reaction and mass ratio of carbon to silicon $ \left( \frac{C}{Si} \right) $ on the synthesis of Si₃N₄ wires were studied. The presence of nitrogen gas in the pores of gel at high temperature starts the CTRN reaction leading to the formation of Si₃N₄ wires. The results show that the Si₃N₄ was fully formed with two kinds of morphologies including globular and wire with a width of 100–500 nm and length of several microns at sintering temperature of 1,400 °C by employing the mass ratio of $ \frac{C}{Si} \; = \;0.5 $ . The infrared adsorption of the wires exhibits absorption bands related to the absorption peaks of Si–N bond of Si₃N₄. The thermal analysis results reveal that carbothermal nitridation reaction was completed at temperature of 1,400 °C.     

A simple approach to controllably grow network-like branched single-crystalline Si3N4 nanostructures

We reported a simple, large-scale, and controllable growth method for network-like branched single-crystalline Si₃N₄ nanostructures by catalyst-assisted pyrolysis of a polysilazane. The templates were a silicon wafer deposited with a 5 nm Fe film. The processes simply involved in thermal cross-linking of the polymer precursor, crushing of the solidified preceramic polymer chunks into fine powder, and thermal pyrolysis of the powder under the protection of ultra-high purity nitrogen. The collected white network-like branched nanostructures were formed through “metal-absorption on the surface of nanostructures” model by vapor-liquid-solid mechanism. Microstructure characterizations indicate that the nanostructures are single-crystalline hexagonal α-Si₃N₄. The reaction mechanism of Si₃N₄ nanonetworks was also proposed.     

Growth,characterization and interfacial reaction of magnetron sputtered Pt/CeO2 thin films on Si and Si3N4 substrates

Growth of magnetron sputtered Pt/CeO₂ thin films on Si and Si₃N₄ were characterized by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and X‐ray photoelectron spectroscopy (XPS). Interaction of Pt/CeO₂ films with Si on Si and Si₃N₄ substrates was extensively investigated by XPS. XRD studies show that films are oriented preferentially to (200) direction of CeO₂. XPS results show that Pt is mainly present in +2 oxidation state in Pt/CeO₂/Si film, whereas Pt⁴⁺ predominates in Pt/CeO₂/Si₃N₄ film. Concentration of Pt⁴⁺ species is more than four times on Si₃N₄ substrate as compared with that on Si. Ce is present as both +4 and +3 oxidation states in Pt/CeO₂ films deposited on Si and Si₃N₄ substrates, but concentration of Ce³⁺ species is more in Pt/CeO₂/Si film. Interfacial reaction between CeO₂ and Si substrate is controlled in the presence of Pt. Pt/Ce concentration ratio decreases in Pt/CeO₂/Si₃N₄ film upon successive sputtering, whereas this ratio decreases initially and then increases in Pt/CeO₂/Si film. Copyright © 2015 John Wiley & Sons, Ltd.     

Development of silicon nitride fiber from Si-containing polymer by radiation curing and its application

A silicon nitride fiber (Si₃N₄) was synthesized from polycarbosilane (PCS) fiber by radiation application. PCS fibers were cured by electron beam (EB) irradiation in a helium gas atmosphere prior to the pyrolysis. The cured PCS fiber was converted to Si₃N₄ ceramic fiber with flexibility by nitridation in ammonia gas at a high temperature of 500–1000°C. The obtained Si₃N₄ fibre showed a high heat resistance up to 1300°C, a high tensile strength of 2 GPa and excellent electrical resistivity of 10¹³ Ω cm. The ceramic fiber was fabricated to cloth and applied for electric wire insulator. The wire cable is flexible and can be applied at a high temperature atmosphere of around 1000°C.     

From cyclotetrasilazane [(CH3)2SiNH]4 via crystalline silicon nitride imide Si2N2NH to α-Si3N4

α-Si₃N₄ is synthesized by an ammonia thermal synthesis using a cyclic oligosilazane, [(CH₃)₂SiNH]₄, as the starting material. [(CH₃)₂SiNH]₄ reacts in the presence of ammonia at 900°C and 80 MPa pressure to give silicon nitride imide (Si₂N₂NH). Subsequently, Si₂N₂NH is converted into α-Si₃N₄ by thermal decomposition at 1500°C and 0.1 MPa nitrogen with the simultaneous loss of NH₃.     

Matrixreaktionen von SiO. IR-spektroskopischer Nachweis der Molekeln SiO2 und OSiCl2

Matrix Reactions of SiO. IR-spectroscopic Identification of the Molecules SiO₂ and OSiCl₂ SiO evaporated from Knudsen cell reacts in argon matrix with atomic oxygen generated by microwave excitation to molecular SiO₂. Bands at 1400 cm^?1 in the IR matrix spectrum are assigned to the ν₃-vibration of molecules Si¹⁶O₂, Si¹⁶O¹⁸O, and Si¹⁸O₂. In an argon matrix SiO can reach with Cl₂ by excitation of a high pressure mercury lamp to OSiCl₂. Isotopic splitting (¹⁶O/¹⁸O, ²⁸Si/²⁹Si, ³⁵Cl/³⁷Cl) and force constant calculations show that the observed frequencies can be assigned to a planar molecule OSiCl₂. The bending mode δ (SiCl₂) could not be observed. The force constant f(SiO) is 9. 10² N m^?1 for SiO₂ and OSiCl₂. According to the SIEBERT rule this valence force constant is expected for a double bond between silicon and oxygen.     

Li4Ca3Si2N6 and Li4Sr3Si2N6 – Quaternary Lithium Nitridosilicates with Isolated [Si2N6]10– Ions

The isotypic nitridosilicates Li₄Ca₃Si₂N₆ and Li₄Sr₃Si₂N₆ were synthesized by reaction of strontium or calcium with Si(NH)₂ and additional excess of Li₃N in weld shut tantalum ampoules. The crystal structure, which has been solved by single‐crystal X‐ray diffraction (Li₄Sr₃Si₂N₆: C2/m, Z = 2, a = 6.1268(12), b = 9.6866(19), c = 6.2200(12) Å, β = 90.24(3)°, wR₂ = 0.0903) is made up from isolated [Si₂N₆]^10– ions and is isotypic to Li₄Sr₃Ge₂N₆. The bonding angels and distances within the edge‐sharing [Si₂N₆]^10– double‐tetrahedra are strongly dependent on the lewis acidity of the counterions. This finding is discussed in relation to the compounds Ca₅Si₂N₆ and Ba₅Si₂N₆, which also exhibit isolated [Si₂N₆]^10– ions.     

A spontaneous solid-state NO donor to fight antibiotic resistant bacteria

This study substantiates the chemical origin of a free-radical-driven antibacterial effect at the surface of biomedical silicon nitride (Si₃N₄) in comparison with the long-known effect of oxygen reduction by oxidized TiO₂ at the surface of biomedical titanium alloys. Similar to the antibacterial effect exerted by reactive oxygen species (ROS; i.e., superoxide anions, hydroxyl radicals, singlet oxygen, and hydrogen peroxide) from TiO₂, reactive nitrogen species (RNS), such as nitrous oxide (N₂O), nitric oxide (NO), and peroxynitrite (^?OONO) in Si₃N₄, severely affect bacterial metabolism and lead to their lysis. However, in vitro experiment with gram-positive Staphylococcus epidermidis (S. epidermidis, henceforth) revealed that ROS and RNS promoted different mechanisms of lysis. Fluorescence microscopy of NO radicals and in situ time-lapse Raman spectroscopy revealed different metabolic responses of living bacteria in contact with different substrates. After 48 h, the DNA of bacteria showed complete destruction on Si₃N₄, while carbohydrates of the peptidoglycan membrane induced bacterial degradation on Ti-alloy substrates. Different spectroscopic fingerprints for bacterial lysis documented the distinct effects of RNS and ROS. Spontaneously activated in aqueous environment, the RNS chemistry of Si₃N₄ proved much more effective in counteracting bacterial proliferation as compared to ROS formed on TiO₂, which requires external energy (photocatalytic activation) to enhance effectiveness. Independent of surface topography, the antibacterial effect observed on Si₃N₄ substrates is due to its unique kinetics ultimately producing NO and represents a new intriguing avenue to fight bacterial resistance to conventional antibiotics.     

Study on optical interference effect of graphene oxide films on SiO2 and Si3N4 dielectric films

The optical interference effect has enabled the visualization of thin layers, even monolayers, of graphene by simple optical microscopy. In this study, we have controlled the optical interference effect by changing the thickness and types of dielectric films, i.e. SiO₂ and Si₃N₄. By investigating differences in RGB parameters between the graphene oxide layer and the dielectric layer, conditions for the highest visibility of the graphene oxide layer were determined. We also studied colors as a function of graphene oxide layer thickness and dielectric layer thickness. These color patterns can be effectively presented as two-dimensional color charts. When comparing SiO₂ and Si₃N₄ as dielectric layers, each layer was found to exhibit different interference fringe patterns, which is due to a mismatch of optical properties between the material layer and dielectric layer. The effects of optical properties (n, k) of the material layer on interference colors were also investigated.     

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.     

LiLa5Si4N10O and LiPr5Si4N10O – Chain‐Type Oxonitridosilicates

The isotypic lithium rare‐earth oxonitridosilicates LiLn₅Si₄N₁₀O (Ln = La, Pr) were synthesized at temperatures of 1200 °C in weld shut tantalum ampoules employing liquid lithium as flux. Thereby, a silicate substructure with a low degree of condensation was obtained. LiLa₅Si₄N₁₀O crystallizes in space group P$\bar{1}$ [Z = 1, LiLa₅Si₄N₁₀O: a = 5.7462(11), b = 6.5620(13), c = 8.3732(17) Å, α = 103.54(3), β = 107.77(3), γ = 94.30(3), wR₂ = 0.0405, 1315 data, 96 parameters]. The nitridosilicate substructure consists of loop branched dreier single‐chains of vertex sharing SiN₄ tetrahedra. Lattice energy calculations (MAPLE) and EDX measurements confirmed the electrostatic bonding interactions and the chemical compositions. The ⁷Li solid‐state MAS NMR investigation is reported.     

Struktur und Eigenschaften von Ba2Mg3Si4, einer Zintl-Phase mit planaren Si6-Einheiten

Structure and Properties of Ba₂Mg₃Si₄, a Zintl Phase with Planar Si₆ Units Within the scope of the investigations on the phase system Ba/Mg/Si a new ternary Zintl phase of the composition Ba₂Mg₃Si₄ was found and structurally characterized. The silicon substructure is built up of Si₂ pairs and a new type of Zintl anion, a planar Si₆ chain. Temperature dependent measurements of the electric conductivity and the magnetic susceptibility show a metallic behavior. Accompanying quantumchemical investigations on the base of the LMTO-ASA method confirm these results and allow an insight in the present bond situation.