Direct determination of trace copper and chromium in silicon nitride by fluorinating electrothermal vaporization inductively coupled plasma atomic emission spectrometry with the slurry sampling technique

A method has been developed for the determination of trace impurities in silicon nitride (Si₃N₄) powders by fluorination assisted electrothermal vaporization (ETV) /ICP-AES using the slurry sampling technique. Polytetrafluoroethylene (PTFE) emulsion as a fluorinating reagent not only effectively destroys the skeleton of Si₃N₄, but also carries out selective volatilization between the impurity elements (Cu, Cr) and the matrix (Si). The experimental parameters influencing fluorination reactions were optimized. The detection limits for Cu and Cr are 1.05 ng/mL ( Cu) and 1.58 ng/mL (Cr), the RSDs are in the range of 1.9–4.2%. The proposed method has been applied to the determination of Cu and Cr in Si₃N₄ ceramic powders. The analytical results were compared with those obtained by independent methods.     

Unique solvent effects on selective electrochemical fluorination of organic compounds

Recent studies on solvent effects on electrochemical partial fluorination are reviewed. At first, the historical background and some problems of electrochemical fluorination in organic solvents like acetonitrile (MeCN) are briefly mentioned. Ethereal solvents like dimethoxyethane (DME) and a mixture of DME and MeCN were found to improve both the yield and current efficiency for electrochemical fluorination since these solvent systems effectively suppress anode passivation and overoxidation of fluorinated products once formed during the electrolysis. It was also found that DME stabilizes radical cationic intermediates of 4-arylthio-1,3-dioxolan-2-ones and 3-phenylthiophthalide leading to α-fluorination while dichloromethane (CH₂Cl₂) destabilizes them leading to fluorodesulfurization. On the other hand, imidazolium ionic liquids and liquid fluoride salts like Et₄NF·4HF and Et₃N·5HF exhibited similar effects to CH₂Cl₂. Selective fluorination of hardly oxidizable phthalide was also achieved using a combination of two kinds of ionic liquids (imidazolium triflate and liquid fluoride salts).     

Anhydrous oxygen-free rare earth material preparation and characterization

The present paper addresses the preparation and characterization of anhydrous oxygen-free rare earth materials, such as terbium fluoride (TbF₃). The fluorination of terbium oxide (Tb₄O₇) by ammonium bifluoride (NH₄HF₂) to prepare anhydrous TbF₃ was reported in this work. The parameters affecting fluorination were studied, including the fluorination temperature, excess stoichiometric amount of NH₄HF₂, and time for fluorination. The temperatures of Tb₄O₇ fluorination by NH₄HF₂ determined by thermogravimetric analysis and differential thermal analysis ranged from 350°C to 500°C. The phase structure of the as-prepared products identified by the X-ray diffraction method was indexed to the orthorhombic phase of TbF₃ [space group: Pnma (no. 62)] that exhibited good accordance with the values in the standard cards JCPDS No. 37-1487 for TbF₃. Energy-dispersive X-ray spectroscopy (EDS) methods were used to analyze the elemental compositions in the as-prepared products; the fluorine (F) and terbium (Tb) elemental compositions calculated from the [TbF₃] formula are in good agreement with those calculated from the EDS pattern. The optimum parameters for fluorination were determined from the results, and anhydrous oxygen-free TbF₃ can be used to study the preparation of metallic terbium by the calcinothermic reduction method.     

Innovative Syntheses of Cyano(fluoro)borates: Catalytic Cyanation,Electrochemical and Electrophilic Fluorination

Different types of high-yield, easily scalable syntheses for cyano(fluoro)borates Kt[BF_n(CN)_4−n] (n=0–2) (Kt=cation), which are versatile building blocks for materials applications and chemical synthesis, have been developed. Tetrafluoroborates react with trimethylsilyl cyanide in the presence of metal-free Brønsted or Lewis acid catalysts under unprecedentedly mild conditions to give tricyanofluoroborates or tetracyanoborates. Analogously, pentafluoroethyltrifluoroborates are converted into pentafluoroethyltricyanoborates. Boron trifluoride etherate, alkali metal salts, and trimethylsilyl cyanide selectively yield dicyanodifluoroborates or tricyanofluoroborates. Fluorination of cyanohydridoborates is the third reaction type that includes direct fluorination with, for example, elemental fluorine, stepwise halogenation/fluorination reactions, and electrochemical fluorination (ECF) according to the Simons process. In addition, fluorination of [BH(CN)₂{OC(O)Et}]⁻ to result in [BF(CN)₂{OC(O)Et}]⁻ is described.     

New synthesis methods for fluorinated carbon nanofibres and applications

Several new synthesis methods of fluorinated carbon nanofibres, such as controlled fluorination using fluorinating agent (TbF₄ or XeF₂), or assisted fluorination under UV and gamma irradiation, are reviewed and compared with the direct fluorination using undiluted fluorine gas. The results highlight the different fluorination mechanisms for the direct fluorination and the new methods. The other advantage of those alternative fluorination routes is the possibility to provide fine tuning of the fluorination level, i.e. from F/C atomic ratio close to zero, as a functionalization, to the unity (CF₁) according to the required application, electrochemical or tribological. Two applications are described in this paper as a function of the fluorine content: protection against ozonation and use as solid lubricants.     

Modification of ultra‐high‐molecular weight polyethylene by various fluorinating routes

Chemical–physical properties of ultra‐high‐molecular weight polyethylene (UHMWPE) treated by direct fluorination, direct fluorination accompanied with UV irradiation, by XeF₂ and by TbF₄, were tested by FTIR spectroscopy, visible spectroscopy, 19F and 13C NMR, scanning electron microscopy, XRD, and EPR. Surface energy measurements were carried out. The direct fluorination of UHMWPE is a diffusion‐controlled process, but treatment with XeF₂ is a kinetically controlled one. Direct fluorination and direct fluorination accompanied with UV irradiation results mainly in a formation of ? CF₂? groups. On the contrary, ? CHF? groups are prevailing in UHMWPE treated with XeF₂ and TbF₄. Surface texture of UHMWPE treated with XeF₂ and with F₂ is quite different. Direct fluorination results in a higher polarity of the polymer surface when compared with treatment with XeF₂. For the case of direct fluorination, both long‐lived peroxy and fluoroalkylradicals are formed. For the case of treatment with XeF₂, only fluoroalkylradicals were detected. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49:3559–3573, 2011     

Direct determination of trace copper and chromium in silicon nitride by fluorinating electrothermal vaporization inductively coupled plasma atomic emission spectrometry with the slurry sampling technique

A method has been developed for the determination of trace impurities in silicon nitride (Si₃N₄) powders by fluorination assisted electrothermal vaporization (ETV) /ICP-AES using the slurry sampling technique. Polytetrafluoroethylene (PTFE) emulsion as a fluorinating reagent not only effectively destroys the skeleton of Si₃N₄, but also carries out selective volatilization between the impurity elements (Cu, Cr) and the matrix (Si). The experimental parameters influencing fluorination reactions were optimized. The detection limits for Cu and Cr are 1.05 ng/mL ( Cu) and 1.58 ng/mL (Cr), the RSDs are in the range of 1.9–4.2%. The proposed method has been applied to the determination of Cu and Cr in Si₃N₄ ceramic powders. The analytical results were compared with those obtained by independent methods. Received: 8 December 1998 / Revised: 1 February 1999 / Accepted: 3 February 1999     

Inter- and Intra-Molecular Organocatalysis of SN2 Fluorination by Crown Ether: Kinetics and Quantum Chemical Analysis

We present the intra- and inter-molecular organocatalysis of S_N2 fluorination using CsF by crown ether to estimate the efficacy of the promoter and to elucidate the reaction mechanism. The yields of intramolecular S_N2 fluorination of the veratrole substrates are measured to be very small (<1% in 12 h) in the absence of crown ether promoters, whereas the S_N2 fluorination of the substrate possessing a crown ether unit proceeds to near completion (~99%) in 12 h. We also studied the efficacy of intermolecular rate acceleration by an independent promoter 18-crown-6 for comparison. We find that the fluorinating yield of a veratrole substrate (leaving group = −OMs) in the presence of 18-crown-6 follows the almost identical kinetic course as that of intramolecular S_N2 fluorination, indicating the mechanistic similarity of intra- and inter-molecular organocatalysis of the crown ether for S_N2 fluorination. The calculated relative Gibbs free energies of activation for these reactions, in which the crown ether units act as Lewis base promoters for S_N2 fluorination, are in excellent agreement with the experimentally measured yields of fluorination. The role of the metal salt CsF is briefly discussed in terms of whether it reacts as a contact ion pair or as a “free” nucleophile F⁻.     

Fluorination rates of polyolefins as a function of structure and gas atmosphere

Polyolefins and fluoropolymers were reacted with elemental fluorine under carefully controlled conditions in a thermobalance adapted to be compatible with fluorine gas. The fluorination reactions were monitored by measuring the mass increase as a result of hydrogen substitution by fluorine. The mass increase was directly proportional to the square root of the fluorination time, which indicates that fluorine gas diffusion to the unreacted surface is the rate determining step. The fluorination rate was increased by increasing the fluorine concentration and the fluorination temperature. The fluorination rate is higher when nitrogen rather than helium is used as diluting gas. The fluorination rate for the reaction in which CO₂ is used as diluting gas is the same as during fluorination with nitrogen as diluting gas, while the presence of oxygen dramatically decreased the fluorination rate. Oxygen is incorporated during fluorination with oxygen as diluting gas, while no functionalization was observed when CO₂ was employed as diluting gas. The effect of polymer structure on fluorination was studied. Poly(vinylfluoride) gained mass during fluorination, while no reaction was observed for poly(vinylidenefluoride). The reaction rate for polypropylene was higher than that of polyethylene. In celebration of the 60^th birthday of Dr. Andrew K. Galwey     

Fluorination of alumino-silicate minerals: The example of lepidolite

The effect of fluorination on silicate and alumino-silicate minerals has been investigated, in particular on the lepidolite [K(Li,Al)₃ [Si₃AlO₁₀] (F,OH)₂] of the mica-type. The fluorination techniques included direct F₂-gas and cold radio-frequency-plasma involving c-C₄F₈ or O₂/CF₄ mixtures. The modifications of the surface properties have been followed mostly by XPS. Depending of the fluorination route used, either a reactive etching process involving M-F bonding occurs (direct F₂-gas; O₂-CF₄ rf-plasma), or a carbon fluoride deposition takes place (c-C₄F₈ rf-plasma).     

Solid-state NMR and EPR study of fluorinated carbon nanofibers

Carbon nanofibers were fluorinated in two manners, in pure fluorine gas (direct fluorination) and with a fluorinating agent (TbF₄ during the so-called controlled fluorination). The resulting fluorinated nanofibers have been investigated by solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR). This underlines that the fluorination mechanisms differ since a (CF)_n structural type is obtained, whatever the temperature, with the controlled reaction, whereas, during the direct process, a (C₂F)_n type is formed over a wide temperature range. Through a careful characterization of the products, i.e. density of dangling bonds (as internal paramagnetic centers), structural type (acting on molecular motion) and specific surface area (related to the amount of physisorbed O₂), the effect of atmospheric oxygen molecules on the spin-lattice nuclear relaxation has been underlined.     

The investigation of fluorination reaction of p-substituted benzenesulfonimides with fluorine–nitrogen mixed gas to synthesize NFSI analogues

This paper studied the fluorination reaction of p-substituted benzenesulfonimides with diluted elemental fluorine to synthesize N-fluoro-benzenesulfonimide (NFSI) analogues. Several synthetic methods were compared and we found that, for many p-substituted benzenesulfonimides, the fluorination of their sodium salts with 10% F₂–N₂ mixed gas in acetonitrile at room temperature could afford NFSI analogues in moderate to good yields.     

Fluorination effect of activated carbon electrodes on the electrochemical performance of electric double layer capacitors

The surface of phenol-based activated carbon (AC) was fluorinated at room temperature with different F₂:N₂ gas mixtures for use as an electrode material in an electric double-layer capacitor (EDLC). The effect of surface fluorination on EDLC electrochemical performance was investigated. The specific capacitance of the fluorinated AC-based EDLC was measured in a 1 M H₂SO₄ electrolyte, in which it was observed that the specific capacitances increased from 375 and 145 F g⁻¹ to 491 and 212 F g⁻¹ with the scan rates of 2 and 50 mV s⁻¹, respectively, in comparison to those of an unfluorinated AC-based EDLC when the fluorination process was optimized via 0.2 bar partial F₂ gas pressure. This enhancement in capacitance can be attributed to the synergistic effect of increased polarization on the AC surface, specific surface area, and micro and mesopore volumes, all of which were induced by the fluorination process. The observed increase in polarization was derived from a highly electronegative fluorine functional group that emerged due to the fluorination process. The increased surface area and pore volume of the AC was derived from the physical function of the fluorine functional group.     

Coupled Si and O isotope measurements of meteoritic material by laser fluorination isotope ratio mass spectrometry

We present a procedure for the determination of the isotopic ratios of silicon and oxygen from the same aliquot of anhydrous silicate material. The sample is placed in a bromine pentafluoride atmosphere as it is heated with a CO₂ laser system releasing silicon tetrafluoride and oxygen gasses. The oxygen gas is then purified to remove other reaction by‐products through several liquid nitrogen traps before being captured onto a molecular sieve and transferred to an isotope ratio mass spectrometer. The silicon tetrafluoride gas is then purified using a supplementary line by repeatedly freezing to ?196°C with liquid nitrogen and then thawing with an ethanol slurry at ?110°C through a series of metal and Pyrex traps. The purified gas is then condensed into a Pyrex sample tube before it is transferred to an isotope ratio mass spectrometer for silicon isotope ratio measurements. This system has silicon yields of greater than 90% for pure quartz, olivine, and garnet standards and has a reproducibility of ±0.1‰ (2σ) for pure quartz for both oxygen and silicon isotope measurements. Meteoritic samples were also successfully analyzed to demonstrate this system's ability to measure the isotopic ratio composition of bulk powders with precision. This unique technique allows for the fluorination of planetary material without the need for wet chemistry. Though designed to analyze small aliquots of meteoritic material (1.5 to 3 mg), this approach can also be used to investigate refractory terrestrial samples where traditional fluorination is not suitable.     

Developing organoboranes as phase transfer catalysts for nucleophilic fluorination using CsF

Despite the general high fluorophilicity of boron, organoboranes such as BEt₃ and 3,5-(CF₃)₂C₆H₃–BPin are shown herein for the first time, to our knowledge, to be effective (solid to solution) phase-transfer catalysts for the fluorination of certain organohalides with CsF. Significant (up to 30% e.e.) chiral induction during nucleophilic fluorination to form β-fluoroamines using oxazaborolidine (pre)catalysts and CsF also can be achieved. Screening different boranes revealed a correlation between calculated fluoride affinity of the borane and nucleophilic fluorination reactivity, with sufficient fluoride affinity required for boranes to react with CsF and form Cs[fluoroborate] salts, but too high a fluoride affinity leading to fluoroborates that are poor at transferring fluoride to an electrophile. Fluoride affinity is only one component controlling reactivity in this context; effective fluorination also is dependent on the ligation of Cs⁺ which effects both the phase transfer of CsF and the magnitude of the [Cs⋯F-BR₃] interaction and thus the B–F bond strength. Effective ligation of Cs⁺ (e.g. by [2.2.2]-cryptand) facilitates phase transfer of CsF by the borane but also weakens the Cs⋯F–B interaction which in turn strengthens the B–F bond – thus disfavouring fluoride transfer to an electrophile. Combined, these findings indicate that optimal borane mediated fluorination occurs using robust (to the fluorination conditions) boranes with fluoride affinity of ca. 105 kJ mol⁻¹ (relative to Me₃Si⁺) under conditions where a signficant Cs⋯F–B interaction persists.

Simple boranes with the optimal fluoride ion affinity are effective as catalysts for phase transfer nucleophilic fluorination with CsF.     

Selective formation of C60F18(g) and C60F36(g) by reaction of [60]fullerene with molecular fluorine

Fluorination of C₆₀(s), C₆₀(s)-MnF₂(s), C₆₀(s)-NiF₂(s) and C₆₀(s)-MnF₃(s) mixtures has been studied by Knudsen cell mass spectrometry with admission of molecular fluorine. The fluorination is selective when fullerene reacts with the fluorine chemisorbed on the MnF₂ surface. When the MnF₂ content in the initial mixture is at least 90 mol% both C₆₀F₁₈ and C₆₀F₃₆ are selectively formed. Under certain conditions, mixtures predominantly containing one of three compounds C₆₀F₃₈, C₆₀F₄₀, and C₆₀F₄₂ can be obtained. A consecutive change of the main fluorination products (C₆₀F₁₈ and C₆₀F₃₆) takes place at constant temperature (720 K) and on fluorine admission. A quantitative explanation of this fact is given. Selective fluorination of C₆₀(s) by molecular fluorine is compared with solid-phase fluorination using MnF₃(s) as a fluorinating agent.     

Effects of TiO<Subscript>2</Subscript> surface fluorination on photocatalytic degradation of methylene blue and humic acid

Photocatalytic degradation (PCD) reactions of cationic methylene blue (MB) and anionic humic acid (HA) were studied in naked TiO₂ and fluorinated TiO₂ (F-TiO₂) suspensions in order to investigate how the modification of the TiO₂ surface functional group influenced PCD reactions. Adsorption behaviors of MB and HA in the naked TiO₂ followed a typical pH-dependent electrostatic interaction mechanism. On the other hand, those in the F-TiO₂ were markedly changed and even showed a reversed dependence in specific pH ranges due to surface fluoride interrupting the interaction of substrates and surface titanol groups. PCD rates of MB (k _MB) and its N-demethylation (Δλ _max) were significantly increased by surface fluorination below circum-neutral pH range, in particular, by a factor of 12 and 54 at pH 2, respectively. In the case of HA, the fluorination had an insignificant effect on its degradation rate but appeared to change its degradation behavior. It has been suggested that, although the primary effect of fluorination enhances the photocatalytic production of hydroxyl radicals, the change in electrostatic interaction with substrates could affect PCD as well.     

Fluorination of poly(p-phenylene) using TbF4 as fluorinating agent

The fluorination using TbF₄ as fluorinating agent was successfully performed on poly(p-phenylene). The method allows the fluorine content of the polymer to be controlled and the formation of structural defects, such as dangling bonds, to be significantly decreased by comparison with the direct fluorination using pure F₂ gas. The aromatic character of the phenyl ring is partly maintained through the fluorination contrary to the direct fluorination (using pure F₂ gas), for which a quasi-perfluorination and a partial decomposition of the polymer occur. Complementary analytical techniques have been used, such as ¹⁹F and ¹³C solid state NMR, FT-IR and EPR to compare the samples as a function of the reaction conditions.     

Fluorination of polyethylene films

Fluorination of HDPE films (100 and 1800 μm thick) and LDPE films (100 μm thick) with elemental fluorine is described. The films were fluorinated at temperatures between 40°C and 100°C. The degree of fluorination was increased by increasing the concentration of fluorine in the F₂/N₂ gas mixture. A diffusion-controlled process was indicated by plotting the measured depth of fluorination against the square root of fluorination time. The chemistry of the process was studied by ESCA measurements or by energy dispersive analyses in the scanning electron microscope.     

Surface modification of carbon anodes for secondary lithium battery by fluorination

Recent results on the surface modification of petroleum cokes and their electrochemical properties as anodes of secondary lithium batteries are summarized. The surface of petroleum coke and those heat-treated at 1860-2800 °C were fluorinated by elemental fluorine (F₂), chlorine trifluoride (ClF₃) and nitrogen trifluoride (NF₃). No surface fluorine was found except only one sample when ClF₃ and NF₃ were used as fluorinating agents while surface region of petroleum coke was fluorinated when F₂ was used. Transmission electron microscopic (TEM) observation revealed that closed edge of graphitized petroleum coke was destroyed and opened by surface fluorination. Raman spectra showed that surface fluorination increased the surface disorder of petroleum cokes. Main effect of surface fluorination with F₂ is the increase in the first coulombic efficiencies of petroleum cokes graphitized at 2300-2800 °C by 12.1-18.2% at 60 mA/g and by 13.3-25.8% at 150 mA/g in 1 mol/dm³ LiClO₄-ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1, v/v). On the other hand, main effect of the fluorination with ClF₃ and NF₃ is the increase in the first discharge capacities of graphitized petroleum cokes by ∼63 mAh/g (∼29.5%) at 150 mA/g in 1 mol/dm³ LiClO₄-EC/DEC.