Fluorine Pressure Research Articles

Insight into the Chemical Behaviour of Chromium in CaF2-SiO2-Al2O3-MgO Flux Applied in Aluminium-Assisted Alloying of Carbon Steel in Submerged Arc Welding

Chromium alloying of weld metal is usually accomplished via weld wire. This is done because chromium has a high affinity for oxygen and is therefore not easily transferred across the arc. The formation chromium (VI) is one of the main concerns in welding with chromium containing consumables, especially in open arc processes, and less so in SAW (submerged arc welding). This study investigates the chemical behaviour of chromium in the application of unconstrained metal powders of Al, Cr, Cu and Ti in SAW. The application of aluminium in SAW is used to control the oxygen partial pressure in the process to prevent oxidation of elements of high oxygen affinity, such as chromium. The speciation of chromium was investigated in two-dimensional (2D) and three-dimensional (3D) post-weld slag samples. In the 2D slag samples, the chromium is contained in low concentrations in the oxy-fluoride matrix phase. The 3D samples showed dome structures in which evidence of vapour formation was identified. Chromium presents as distinct chromium spots throughout the oxy-fluoride dome walls, and it is not observed as solute in the oxy-fluoride phase. Chromium presents as chromium-rich Cr-Mn-Mg-O porous particles of less than 20 µm in size, which may agglomerate into larger masses. The calculated thermochemical predominance diagrams show that the Al(l)-Al2O3(s)-AlF(g) equilibrium can maintain the partial oxygen pressure (PO2) at 10−16.5 atmosphere and the partial fluorine pressure (PF2) at 10−11.4 atmosphere at 2500 °C. Under these gas phase conditions in the arc cavity, chromium is present as metallic chromium and may subsequently vaporise as chromium metal and combine with other vaporised elements.

Enhanced dispersion stability and fluidity of rutile TiO2 particles using surface fluorination

Abstract To investigate the effect of surface fluorination on dispersion stability and powder fluidity of rutile TiO2 particles, they were fluorinated using F2 gas under various conditions. Reaction temperature, fluorine pressure and reaction time were set at 25∼200 ℃, 1∼50 kPa, and 1h, respectively. Even at 200℃ of fluorination temperature, the crystal structure did not change in XRD results. Some fluorenes were found on the surface of TiO2 particles in XPS results, and their contents increased with increasing the F2 pressure. From a result of sedimentation test with water, the dispersion stability of TiO2 particles was improved using the surface fluorination. The average size and zeta potentials of fluorinated TiO2 particles were almost 2 times smaller and 2 times larger, respectively, than those (1x103 mm and -20 mV) of untreated TiO2 particles. And in the results of FT4 powder rheometer and repose angle, the powder fluidity of rutile TiO2 particles was improved using surface fluorination. Consequently, the fluorine existed on the surface of TiO2 particles made to improve a dispersion stability and fluidity of TiO2 particles

Effects of Surface Fluorination on the Dispersion Stability and the Electrical Conductivity of SnO2 Particles Using Fluorine Gas

F content in SnO2 powder was adjusted using fluorine gas (F2 gas). From XPS and XRD results of fluorinated SnO2 particles, it proved that the particle surface could be fluorinated without change of crystal structure. Compared with untreated SnO2, the fluorinated SnO2 led to improved dispersion stability. In addition, the surface fluorination of SnO2 particles could improve the electrical conductivity (approximately 75 Sm−1) under a fluorine pressure of 66.7 kPa at 200 °C for 1 h.

Effects of Fluorine Doping and Pressure on the Electronic Structure of LaO1−x F x FeAs Superconductor: a First Principle Study

The effects of fluorine doping on the electronic structure of LaO1−xFxFeAs superconductor have been investigated by ab initio density functional theory using pseudopotential quantum espresso code. Firstly, we have studied the role of fluorine doping on the electronic structure of LaO1−xFxFeAs by calculation of band structure, density of states, and Fermi surfaces at various doping levels x = 0.00, 0.25, and 0.50. The lattice parameters and ionic position have been determined by optimizing crystal structure. Our results show that doping decreases cell volume similar to mechanical pressure and shifts the bands and states near the Fermi level toward the lower energies. According to the Fermi surface calculations, compound with x = 0 has the long-range spin density wave (SDW) like magnetic order but compounds with x = 0.25 and x = 0.50 have no magnetic orders. Therefore, fluorine doping can help creating the superconducting state in these compounds by suppressing their magnetic orders. Finally, in order to study the supplementary effects of fluorine doping and pressure, we have calculated the density of states and band structure of the compound with doping level x = 0.50 under different pressures. The results show there is a certain pressure in which density of states near the Fermi level is maximum.

Long-term stability of fluorine–graphite intercalation compound prepared under high pressure of fluorine

Fluorine–graphite intercalation compounds were prepared from highly oriented pyrolytic graphite (HOPG) and natural graphite under high pressure of fluorine (16MPa) at room temperature and were analyzed by means of X-ray diffraction. The rate of intercalation under high pressure of fluorine was higher than that under ordinary pressure. The compositions of the compounds obtained from HOPG and natural graphite were estimated by the weight change before and after fluorination to be C3.41F and C5.62F, respectively. They were blue–black compounds composed of two different phases. One phase is a conventional stage II CxF with the repeat distance of ca. 0.93–0.96nm which was the same compound as that obtained under ordinary pressure of fluorine, and the other was also a stage II with a different repeat distance of ca. 1.03–1.18nm. The fluorine species intercalated between the graphene layers can easily move between both phases and the phase transition occurs periodically. The decomposition rate of fluorine-intercalated HOPG was extremely low and it shows a long-term stability in air for 26years.

New evidence in an old case: the question of chromium hexafluoride reinvestigated.

The question of whether or not the chromium hexafluoride molecule has been synthesized and characterized has been widely discussed in the literature and cannot, in spite of many efforts, yet be answered beyond doubt. New matrix-isolation experiments can now show, together with state-of-the-art quantum-chemical calculations, that the compound previously isolated in inert gas matrixes, was CrF5 and not CrF6. New bands in the matrix IR spectra can be assigned to the Cr2F10 dimer, and furthermore evidence was found in the spectra for a photodissociation or reversible excitation of CrF5 under UV irradiation. However, even if CrF6 is not stable at ambient conditions, its formation under high fluorine pressures in autoclave reactions cannot be excluded completely.

New inversion and ab initio intermolecular potentials for supercritical fluorine: Calculation of some properties and MD simulation

A new pair-potential energy function of fluorine has been determined via the inversion of reduced viscosity collision integrals. The potential reproduces the second virial coefficient, viscosity, and thermal conductivity of supercritical fluorine in good accordance with experimental data. We have also determined the (F2)2 potential using different ab initio methods at different geometries. Molecular dynamics simulation has been also performed to obtain pressure of supercritical fluorine using the inversion pair-potential supplemented by Feynman–Hibbs quantum correction and the weighted ab initio potential using the CCSD(T)/aug-pvdz method. To consider many-body interactions, the simple three-body potential of Hauschild and Prausnitz [7] used with the inversion potential to improve the properties without requiring an expensive three-body calculation. We have also calculated the three-body interactions for (F2)3 system using the ab initio computations at different orientations. Our results indicated that the three-body potentials improve the two-body supercritical values.

Thermodynamic Study of Sublimation of PbF2 and PbF4 from Torsion-Effusion Vapor Pressure Measurements

The total vapor pressures of lead di- and tetrafluoride were measured by a torsion-effusion method. The vapor above PbF 2 (s) is prevalently PbF 2 (g). The temperature dependence of the vapor pressure is represented, in the temperature range (793 to 951) K, by the expression: log(p/Pa) = (12.76 ± 0.10) - (10940 ± 200)/ (T/K). Treating the vapor pressures by the second and third laws, the sublimation enthalpy of PbF 2 , Δ sub H°(298 K) = (225 ± 5) kJ·mol -1 , was selected. Lead tetrafluoride decomposes according the reaction: PbF 4 (s) → PbF 2 (s) + F 2 (g) (a). In the range (596 to 684) K, the temperature dependence of the fluorine pressure above PbF 4 (s) at near unit activity is represented by the expression: log(p/Pa) = (11.70 ± 0.20) - (7450 ± 200)/ (T/K). In this temperature range, fluorine is partially dissociated so that, in addition to reaction a, the reaction 1/2 PbF 4 (s) → 1/2 PbF 2 (s) + F(g) (b) was also studied. The second-law enthalpies at 650 K, ΔH°(650 K) = (141 and 152) kJ·mol -1 associated to the reactions a and b, respectively, were obtained with an estimated error of 6 kJ·mol -1 for both enthalpies. The entropy of PbF 4 (s), S°(650 K) = (272 ± 15) J·K -1 ·mol -1 , was also evaluated.

Significantly improved charge deposit properties of the cellular polypropylene film modified by gaseous fluorine

Fluorination modification and electret properties were studied on the cellular polypropylene (PP) films modified by gaseous fluorine by attenuated total reflection (ATR) infrared analysis and the measurment of open-circuit thermally stimulated discharge (TSD) current，in situ TSD charge and isothermal charge decay. The results showed that the PP cellular films，especially the preoxidized ones，can be effectively fluorinated by gaseous fluorine，and that charge storage thermal stability of the fluorinated films，especially the fluorinated preoxidized ones，is significantly improved compared with that of the virginal PP film，even under conditions of negative pressure with a low partial pressure of gaseous fluorine，low reaction temperature about 60℃ and short reaction time of 15min.

Influence of fluorination on surface and dielectric characteristics of polyimide thin film

In this study, the effects of fluorine treatment on surface properties, surface free energies, and dielectric constants of polyimide thin films were studied using X-ray photoelectron spectroscopy, contact angles, and dielectric characteristics, respectively. The glass transition temperature of fluorinated polyimide film gradually decreases with increasing fluorine pressure. The surface free energies and dielectric constants of the film decreased with increasing amount of the treatment fluorine gas. It was explained that the replacement of fluorine led to a decrease of the local electron polarization of polyimide and an increase of the free volume, which could be attributed to the relatively large volume of fluorine.

Functionalisation of carbon nanotubes for molecular electronics

Single-walled carbon nanotubes (SWNTs) have been functionalised using a conventional Reactive Ion Etching (RIE) system with CF 4 and SF 6 plasmas. The fluorinated SWNTs have been investigated using X-ray photoelectron spectroscopy. Microscopic plasma parameters including the fluorine pressure and the ion current density within the plasma have been studied using optical emission spectroscopy and a Langmuir probe. It has been observed that a bias is required for successful attachment of fluorine to the SWNTs in the plasma environment. The mechanism of fluorine attachment to SWNTs is evaluated by correlating the microscopic plasma parameters with the F1s binding energy, the fluorine/carbon ratio and also the oxygen/carbon ratio on the SWNTs. The attached fluorine has been seen to remain stable over time, making the plasma process a reliable means of fluorine functionalisation of carbon nanotubes.

Reaction Between Cerium Trifluoride and Elemental Fluorine.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Reaction between cerium trifluoride and elemental fluorine

Reactions between cerium trifluoride and elemental fluorine have been examined by means of the kinetic and crystallographic method. The reactions were performed at 503–573 K under fluorine pressures of 6.7–26.7 kPa for reaction time up to 3 h. Two kinds of CeF 4 were synthesized by controlling the reaction condition. One is the metastable CeF 4-I synthesized at 523–538 K which returns to CeF 3 to release elemental fluorine, and the other is the stable CeF 4-II obtained at temperatures higher than ca. 540 K, which never returns to CeF 3. X-ray diffraction–Rietveld analysis revealed that the structure of CeF 4-I and CeF 4-II were monoclinic structure ( C12/ c1, Z=12, CeF 4-I/CeF 4-II: a 0, 1.26 nm/1.25; b 0, 1.06/1.05; c 0, 0.82/0.82; β, 126.1/126.3°; cell volume, 8.91×10 −28/8.78×10 −28 m 3) and the cell volume of the CeF 4-I was 1.4% larger than that of the CeF 4-II. Bond lengths between Ce and F in CeF 4-I were somewhat longer than those in CeF 4-II. When CeF 4 and NaCl were put separately in the reaction container, CeF 4-I changed to CeF 3 with converting NaCl to NaF, however, CeF 4-II never reacted with NaCl.

Surface properties of oxyfluorinated PAN-based carbon fibers

In this study, the oxyfluorination of PAN-based carbon fibers was undertaken at room temperature using fluorine–oxygen mixtures, and the influence of oxyfluorination on properties such as wettability, surface polarity, surface free energy, conductivity and tensile strength was investigated. As the oxyfluorination time increased at a total pressure of 5 kPa, both the fluorine/carbon and oxygen/carbon ratios increased. The contribution of semicovalent C–F bond to F1s spectra is considerably decreased with increasing total pressure from 5 to 80 kPa, and at the same time, the contribution of covalent C–F bond is increased. As the total pressure of fluorine–oxygen mixtures increased, the contact angle of water significantly decreased and again increased to a similar value to that of as-received carbon fiber. A short oxyfluorination of carbon fibers considerably increased the wettability, that is, hydrophilicity. The electrical conductivity of oxyfluorinated carbon fiber is larger than that of the as-received fiber. This is because the surface region of carbon fiber is fluorinated. An increase in the tensile strength of about 18% after oxyfluorination is observed. The increase in tensile strength of oxyfluorinated carbon fibers can be understood as being due to a decrease in the diameter of the fiber.

Fluorine intercalation in carbon alloy C xN

Fluorine intercalation in nitrogen-substituted carbon C x N has been investigated at a temperature ranging from room temperature to 300°C. C 16N and C 14N yielded the fluorinated products with low C/ F ratios, C 3F under fluorine pressures of 1.0×10 5 and 0.5×10 5 Pa at room temperature. The intercalated fluorine decreased with decreasing nitrogen content in the host C x N, in particular at 0.25×10 5 Pa. X-ray powder diffraction data revealed that fluorine-intercalated C x N samples consisted of mixtures of several stages or mixtures with unreacted phase, having lower stages than those prepared from carbon. XPS and IR absorption spectra indicated the formation of semi-ionic C–F bonds. It was found that fluorine intercalation was much easier in C x N than in carbon between 50°C and 200°C. XPS and IR absorption spectra showed that C–F bonding varied from semi-ionic to covalent with increasing fluorination temperature from 50°C to 300°C. IR absorption spectrum suggested the formation of NF 2 groups at 300°C.

Rate constant for heterogeneous dissociation of fluorine in a temperature range of 700–900 K on a nickel surface

The aim of the present work was the study of dissociation kinetics of molecular fluorine on the hot surface of a nickel catalyst. The concentration of fluorine atoms was measured by EPR spectroscopy using molecular oxygen as a reference. The fluorine atom concentration was measured with an random error of about 3%. Molecular fluorine dissociation on the walls of a heated nickel tubes was measured. For process simulation and analysis of the experimental curves a one-dimensional kinetic model of the reactor which did not take into account axial and radial diffusion of the gas in the reactor and radial gas speed distribution was used, but account was taken of the pressure drop in the reactor and tubes between the reactor and ESR spectrometer. In the experiments we measured the fluorine atom concentration dependencies on the gas flow velocity and fluorine pressure (10–30 Torr). The dissociation constant on the surface of nickel foil is k d =(2.4±0.7)×10 4 exp [−(19 000±500)/RT] cm/s (at 700–900 K), the probability of heterogeneous recombination on the nickel surface γ = (2.2 ± 0.5) × 10 −4, rate constant of the three-body gas phase recombination on molecular fluorine as `third' body at room temperature k rec = (4.7 ± 1.2) × 10 −34 cm 6/s. This last value coincides with literature value. Our measurements confirmed the value of the equilibrium constant of the fluorine dissociation process: K = 1.11 × 10 25 exp(−37 840/ RT)cm −3 ( K is expressed in cal/mole).

Long pulse operation of an x-ray preionized molecular fluorine laser excited by a prepulse–main pulse system with a magnetic switch

The application of a prepulse–main pulse excitation scheme utilizing a saturable magnetic switch in combination with x-ray preionization has resulted in the generation of long optical pulses from a molecular fluorine laser. Optimum laser pulse durations of 70 ns (full width half maximum) have been obtained in a gas mixture of helium and 3 mbar fluorine at a total pressure of 2 bar. The laser pulse duration is limited by instabilities in the discharge. The laser pulse duration is found to decrease with increasing fluorine pressure and to saturate with increasing current density.

Mass Spectrometry Study of C60 and C60-NiF2 Fluorination with Molecular Fluorine

Fluorination of [60]fullerene and [60]fullerene-NiF2(s) has been studied in situ by Knudsen cell mass spectrometry with admission of molecular fluorine. The fluorination processes were controlled in the temperature range 500 − 800 K at molecular fluorine pressure 10−4 − 10−5 atm. Temperature, time of fluorination and fluorine pressure were varied during the investigation. It was found that variation of these parameters did not lead to the selective fluorination. It was shown that NiF2(s) significantly influences the composition of the product mixture: a large amount of nickel difluoride in the system suppresses the fluorination reaction while the presence of a small amount promotes the selective fluorination. C60Fi18 (50% in the gas phase) was produced in our experiments under the following conditions: 20 h, 720 K, P(F2) = 2 10−4 atm in the prefluorinated (inner surface covered with NiF2(s) layer) Ni-reactor. An important point of the study was verification of the thermodynamic equilibrium in the system. It was concluded that the equilibrium is not established in the system under study, and the fluorination is governed by kinetic factors.

Formation of free radicals in the low-temperature fluorination of polymers

The formation of free radicals in the process of direct fluorination of natural and synthetic polymers at temperatures close to 77 K was studied by ESR. The maximum concentrations of radicals, (1017–5·1018 spin g−1) and their complete oxidation were observed in the temperature interval from 77 to 250 K at a fluorine pressure of 30 Torr. The initiation of chain halogenation, which consists of homolytic breaking of chemical bonds to yield free-radical intermediates. was examined in the framework of the multi-center synchronous transitions model.

Preparation and Characterization of Fluorinated Fullerenes

Abstract Fluorination of pure C60 and a C60/C70 mixture has been studied under different experimental conditions: reaction time, temperature, fluorine pressure. The products obtained have been characterized by IR and NMR spectroscopy, X-ray diffraction, and mass spectrometry analysis by positive and negative ions.