Phasenbeziehungen und Natriumionenleitung im quasi-binären System Na2SiF6/Na3AlF6

Phase Relations and Sodium Ion Conductivity within the Quasi-binary System Na₂SiF₆/Na₂AIF₆ . The phase diagram of the Na₂SiF₆/Na₃AlF₆ system has been determined by means of x-ray powder diffraction, thermal analysis and conductivity measurements in the sub-solidus region. Na₃AlF₆ accomodates up to 73 mol.-% Na₂SiF₆ maintaining the crystal structure type. The sodium ion conductivity increases by about five orders of magnitude upon doping Na₃AlF₆ with Na₂SiF₆.     

Zum Einfluß der Gasphase auf die thermische Zersetzung von K2[SiF6]

Effect of the Gas Phase on the Thermal Decomposition of K₂[SiF₆] K₂SiF₆ produced in usual ways is contaminated by traces of oxygen and protons. These and traces of water fed by gas atmosphere influence the thermal decomposition reaction. To study the influence of the gas phase definite amounts of H₂O and HF were added. The formation of SiF₄ was determined. The development of a SiO₂ phase in presence of H₂O and other experimental results suggest the construction of a layer on the K₂SiF₆ surface, which hinders further SiF₄ being developped. Temperature and linear velocity of the gas influence the length of a zone of decomposition migrating through the solid. This is explained by sorption and reaction behavior of intermediately formed fluorosiloxanes.     

Kinetics of Silicon Nitride Formation on SiO2‐Derived Rice Husk Ash Using the Chemical Vapor Infiltration Method

Since silicon nitride coatings on silicon dioxide are attractive for the semiconductor and electronics industries, cognizance of their formation kinetics is crucial for optimization of production parameters. In this contribution, the deposition kinetics (rate constant and activation energy) of Si₃N₄ by the hybrid system chemical vapor infiltration route (HYSY‐CVI), starting from N₂:NH₃ and SiF₄ (produced by the decomposition of Na₂SiF₆) has been studied. The deposition rate equation for Si₃N₄ was established from several possible gas‐phase or surface reaction steps involved in the growth of Si₃N₄ coatings onto silica‐derived rice husk ash (RHA). Based on a judicious analysis of four different models, it was found that Freundlich's adsorption model satisfactorily represents the rate of Si₃N₄ deposition process onto RHA.     

Synthesis,Structure, and Properties of Nonlinear Optical Crystal Na2SiF6

Nonlinear optical crystals of fluosilicate Na₂SiF₆ are synthesized via hydrothermal method and its structure is determined by single‐crystal X‐ray diffraction (XRD). The space group of Na₂SiF₆ is P321 with cell parameters a = 8.8715(3) Å, c = 5.0484(5) Å, Z = 3, V = 344.09(4) ų. The properties of the crystal are measured by powder XRD, infrared (IR) spectroscopy, ultraviolet/visible (UV/Vis) near‐infrared (NIR) diffuse reflectance spectroscopy, thermogravimetric (TG), and differential scanning calorimetry (DSC) analysis. The bandgap calculated using CASTEP is 7.41 eV, indicating that the cut‐off edge of the Na₂SiF₆ crystal can be down to deep‐UV energy region. The first‐principles studies were performed to elucidate the structure/property relationship of Na₂SiF₆.     

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.     

Ein neues Doppelsalz, (NH4)2SiF6 · NH4NO3

A New Double Salt, (NH₄)₂SiF₆ · NH₄NO₃. From a aqueous solutions of (NH₄)₂SiF₆ and NH₄NO₃ crystallizes by cooling to room-temperature a new double-salt of the composition (NH₄)₂SiF₆ · NH₄NO₃. Its hexagonal unit containing two molecules has the dimensions: a₀ = 5.868, c₀ = 14.799 Å.     

Spektroskopische Untersuchungen zur Struktur von SiF4·2 Amin-Addukten

Zusammenfassung Die Addukte von SiF₄ mit 2 Molekülen NH₃, ND₃, N₂H₄ und CH₃NH₂ wurden IR- und Raman-spektroskopisch untersucht. Schwingungsspektren,¹⁹F-Breitband-KMR und chemische Eigenschaften stehen mit einercis-oktaedrischen Struktur im Einklang, in der vermutlich durch Dipol—Dipol-Wechselwirkungen einzelne Komplexmoleküle zu polymeren Einheiten zusammentreten.

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Complexes of SiF₄ with 2 molecules of NH₃, ND₃, N₂H₄, and CH₃NH₂ were investigated by IR and Raman spectroscopy. The results as well as¹⁹F broadline nmr and chemical properties support acis-octahedral structure. Probably dipol—dipol interactions are responsible for the association of the complex molecule. *** DIRECT SUPPORT *** A3615111 00003

     

双分子水和氨气催化CF3OH分子裂解的理论研究

The G3 and CBS-QB3 theoretical methods are employed to study the decomposition of CF₃OH into FCFO and HF by water, water dimmer, and ammonia. The decomposition of CF₃OH into FCFO and HF is unlikely to occur in the atmosphere due to the high activated energy of 88.7 kJ/mol at the G3 level of theory. However, the computed results predict that the barrier for unimolecular decomposition of CF₃OH is decreased to 25.1 kJ/mol from 188.7 kJ/mol with the aid of NH₃ at the G3 level of theory, which shows that the ammonia play a strong catalytic effect on the split of CF₃OH. In addition, the calculated rate constants show that the decomposition of CF₃OH by NH₃ is faster than those of H₂O and the water dimmer by 109 and 105 times respectively. The rate constants combined with the corresponding concentrations of these species demonstrate that the reaction CF₃OH with NH₃ via TS4 is of great importance for the decomposition of CF₃OH in the atmosphere.     

Ammonium oxofluorosilicates

The chemical and phase composition of the sublimates and nonvolatiles obtained as a result of the vaporization at 280°C of a mixture of silicon dioxide with (NH₄)₂SiF₆ at ratios of (0.25–10): 1 (mol/mol) is studied. Ammonium oxofluorosilicates of the general formula (NH₄)₂SiF₆ · nNH₄SiOF₃, where n = 1, 2, 4, or 8, are isolated from the gas phase and analyzed. These oxofluorosilicates have the structure of cubic (NH₄)₂SiF₆, which allows them to be treated as solid solutions. The sublimate components (NH₄)₂SiF₆ and NH₄SiOF₃ are in the ratio of 1: 2ⁿ, where n = 0, 1, 2, or 3. In the nonvolatile residue, the starting SiO₂ undergoes deep chemical destruction.     

(NH4)2SiF6 evaporation in the presence of SiO2

The kinetics and mechanism of the solid-phase reaction between (NH₄)₂SiF₆ and silica at molar ratios of (0.5–5): 1 were studied. SiO₂ is bound with (NH₄)₂SiF₆ to form volatile NH₄SiOF₃, which abruptly enhances the ammonium hexafluorosilicate evaporation. The activation energy and rate constants of evaporation were calculated for an (NH₄)₂SiF₆ + SiO₂ (2: 1) mixture in the range 220–300°C. The reaction with crystalline SiO₂ has a higher yield than with an amorphous “white soot.” The role of the SiO₂ surface in the formation of the volatile products is discussed. The phase and chemical composition of the sublimates was studied.     

A comparison of the adsorption of KF and NH4F onto silica gel

Silica gel has been fluorinated with KF, Na₂SiF₆, NH₄F and (NH₄)₂SiF₆, and the resulting reagents have been analysed by ¹⁹F and ²⁹Si magic angle spinning NMR and infrared spectroscopy. Fluorination with NH₄F and (NH₄)₂SiF₆ results in the formation of (SiO)₃SiF groups at the surface, where F has replaced OH, whereas the anion SiF²⁻₆ is formed when silica is fluorinated with KF.     

Thermal stability of ammonium chlorocobaltates(II)

The thermal destruction of ammonium penta-, tetra-, and trichlorocobaltates(II) was studied. Thermogravimetric analysis (TGA) data was used to calculate the thermal destruction activation energy, which was 8.6 kJ/mol for (NH₄)₃CoCl₅, 14.8 kJ/mol for (NH₄)₂CoCl₄, and 117.4 kJ/mol for NH₄CoCl₃. The decomposition enthalpy of 35.9 kJ/mol for (NH₄)₃CoCl₅, 12.1 kJ/mol for (NH₄)₂CoCl₄, and 166.3 kJ/mol for NH₄CoCl₃ was determined from differential scanning calorimetry (DSC) data.     

Thermodynamic and kinetic parameters of elementary steps in gas-phase hydrolysis of SiF4. Quantum-chemical and FTIR spectroscopic studies

The energies and thermodynamic parameters of elementary steps in the proposed mechanism of silicon tetrafluoride hydrolysis in the gas phase were calculated by the ab initio quantum-chemical method (MP4//MP2/6-311G(2d,2p)) and the density functional theory (B3LYP/6-311G(2d,2p)). The proposed mechanism of gas-phase hydrolysis involves the formation of mono- and dihydroxy derivatives, hexafluorodisiloxane (SiF₃OSiF₃), and linear and cyclic siloxane polymers with the chain length from three to six Si—O and difluorosilanone units. According to the calculations, all reactions considered are endothermic and are characterized by positive Gibbs free energies. The initial hydrolysis steps can be presented with a high accuracy by two parallel processes: formation of trifluorohydroxysilane (SiF₃OH) and SiF₃OSiF₃. These are the most thermodynamically favorable among all reaction channels. The transition states of these elementary steps were found and their kinetic parameters were estimated (G = 132 and 147 kJ mol^–1, respectively). The calculation results were verified using FTIR spectroscopy of a mixture of gas-phase SiF₄ and water vapor. The comparison of the theoretical (absolute) intensities of bands in the IR spectra and integral absorption coefficients in the experimental IR spectrum made it possible to calculate the equilibrium concentrations of the reactants and equilibrium constants of elementary steps of formation of SiF₃OH and SiF₃OSiF₃, which agree with the theoretical values. The role of different derivatives in deep hydrolysis and possibilities of experimental detection of particular intermediates in the gas phase were discussed.     

Preparation of High-Purity Silicon Tetrafluoride by Thermal Dissociation of Na2SiF6

The possibility of preparing high-purity silicon tetrafluoride by the thermal dissociation of pure grade Na₂SiF₆ was studied. The impurity composition of the product was studied by IR and atomic emission spectroscopy and by mass spectrometry.     

A theoretical investigation of the electronic and geometrical structure of silicon fluorides SiF n and their anions SiF n −,n=1−6

The electronic and geometrical structure of the ground and low-lying excited states of the SiF _n and SiF_n ^– series (n = 1-6) are calculated using the density functional method. Energies of fragmentation through different decay channels were evaluated for both series and found to be in good accord with the experimental data and results of nonempirical calculations. The adiabatic electron affinity (EA) of the neutral series is estimated for the first time. The SiF₄ ^– anion is shown to be stable toward dissociation though its neutral precursor possesses adiabatic EA close to zero. The SiF₅ ^– and SiF₆ ^– anions are stable toward dissociation in the gas phase; however, the neutral radical SiF₅ is near the stability threshold and SiF₆ is unstable as regards dissociation to SiF₄+F₂. An interesting peculiarity of the silicon fluoride anions is their similar energy of F-detachment, i.e. the affinities of all the neutral SiF_n, (n = 0-5) for the fluoride anion are similar.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 44–53, January, 1993.     

The dynamics of the SiF4 - sensitized processes initiated by a pulsed CO2 laser

A model enabling the examination of the dynamics of the SiF₄ - sensitized processes initiated by a pulsed CO₂ laser is outlined, taking into account three effects : energy absorption, energeticity of chemical reactions and expansion of hot gas created upon interaction of molecules with the infrared photon field. This approach was used to elucidate some aspects regarding the decomposition of PH₃ and GeH₄.     

Ethylbenzene solubility,diffusivity, and permeability in poly(dimethylsiloxane)

The pure‐gas sorption, diffusion, and permeation properties of ethylbenzene in poly(dimethylsiloxane) (PDMS) are reported at 35, 45, and 55 °C and at pressures ranging from 0 to 4.4 cmHg. Additionally, mixed‐gas ethylbenzene/N₂ permeability properties at 35 °C, a total feed pressure of 10 atm, and a permeate pressure of 1 atm are reported. Ethylbenzene solubility increases with increasing penetrant relative pressure and can be described by the Flory–Rehner model with an interaction parameter of 0.24 ± 0.02. At a fixed relative pressure, ethylbenzene solubility decreases with increasing temperature, and the enthalpy of sorption is −41.4 ± 0.3 kJ/mol, which is independent of ethylbenzene concentration and essentially equal to the enthalpy of condensation of pure ethylbenzene. Ethylbenzene diffusion coefficients decrease with increasing concentration at 35 °C. The activation energy of ethylbenzene diffusion in PDMS at infinite dilution is 49 ± 6 kJ/mol. The ethylbenzene activation energies of permeation decrease from near 0 to −34 ± 7 kJ/mol as concentration increases, whereas the activation energy of permeation for pure N₂ is 8 ± 2 kJ/mol. At 35 °C, ethylbenzene and N₂ permeability coefficients determined from pure‐gas permeation experiments are similar to those obtained from mixed‐gas permeation experiments, and ethylbenzene/N₂ selectivity values as high as 800 were observed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1461–1473, 2000     

Theoretical investigation on identical anionic halide‐exchange SN2 reaction processes on N‐haloammonium cation NH3X+ (X = F,Cl, Br,and I)

CCSD(T) calculations have been used for identically nucleophilic substitution reactions on N‐haloammonium cation, X^? + NH₃X⁺ (X = F, Cl, Br, and I), with comparison of classic anionic S_N2 reactions, X^? + CH₃X. The described S_N2 reactions are characterized to a double curve potential, and separated charged reactants proceed to form transition state through a stronger complexation and a charge neutralization process. For title reactions X^? + NH₃X⁺, charge distributions, geometries, energy barriers, and their correlations have been investigated. Central barriers ΔE for X^? + NH₃X⁺ are found to be lower and lie within a relatively narrow range, decreasing in the following order: Cl (21.1 kJ/mol) > F (19.7 kJ/mol) > Br (10.9 kJ/mol) > I (9.1 kJ/mol). The overall barriers ΔE relative to the reactants are negative for all halogens: ?626.0 kJ/mol (F), ?494.1 kJ/mol (Cl), ?484.9 kJ/mol (Br), and ?458.5 kJ/mol (I). Stability energies of the ion–ion complexes ΔE_comp decrease in the order F (645.6 kJ/mol) > Cl (515.2 kJ/mol) > Br (495.8 kJ/mol) > I (467.6 kJ/mol), and are found to correlate well with halogen Mulliken electronegativities (R² = 0.972) and proton affinity of halogen anions X^? (R² = 0.996). Based on polarizable continuum model, solvent effects have investigated, which indicates solvents, especially polar and protic solvents lower the complexation energy dramatically, due to dually solvated reactant ions, and even character of double well potential in reactions X^? + CH₃X has disappeared. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Effect of MnC2O4 nanoparticles on the thermal decomposition of TEGDN/NC propellant

The effect of MnC₂O₄ nanoparticles on the thermal decomposition of double-base propellant composed of nitrocellulose (NC) and triethylene glycol dinitrate (TEGDN) has been investigated by TG/DSC?CMS?CFTIR coupling technique. The results show that the decomposition of TEGDN/NC propellant has two stages, the first stage is the volatility and decomposition of TEGDN, the second is the decomposition of NC. The addition of MnC₂O₄ nanoparticles gets the onset temperature of first stage higher, and makes the activation energy of decomposition of TEGDN grow by about 20?C30?kJ/mol. The catalytic also accelerates the total weight loss, and makes the peak temperatures of DSC curves higher. The activation energy of the second stage has a decrease of 20?C40?kJ/mol. MS and FTIR analysis show that the catalyst gets the gas products of macromolecular significantly reduce, while small molecules increase significantly. It also results in the decrease of H₂O, N₂O, and NO₂, and the increase of NO and HCN. Above all, the catalytic improves the thermal stability of TEGDN/NC propellant, make it more safety in storage, and make the decomposition easier and more thorough in main reaction zone.