Fluorination Temperatures Research Articles

Study on the Fluorination Process of Sc2O3 by NH4HF2.

Research on rare-earth fluorides is of urgent and critical importance for the preparation and emerging applications of high-purity alloys. The fluorination of Sc2O3 by NH4HF2 to fabricate ScF3 is investigated. The effects of the fluorination temperature, time and mass ratio of reactant on the fluorination rate and fluoride are discussed in this work. The fluorination reaction was first confirmed using thermodynamic calculation. The thermal and mass stability of the fluorination process were analyzed by thermogravimetric and differential scanning calorimetric (TG-DSC). The as-obtained products at different fluorination temperatures were characterized by Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The results indicated that the fluorination began at room temperature (RT) with the formation of (NH4)3ScF6. With the increase of temperature, the reaction proceeded sequentially through the formation of NH4ScF4, (NH4)2Sc3F11, and finally ScF3. The fluorination rate increased with the increase of fluorination temperature and holding time. ScF3 with a purity of 99.997 wt.% could be obtained by fluorination at 400 °C for 2 h.

Carbon fibre fluorination: Surface and structural properties

Knowing that carbon fibre reinforced polymers are sensitive to moisture through their interphases, carbon fibres sized with Bisphenol A diglycidyl ether were fluorinated under a N2/F2 atmosphere, which confers them a hydrophobic behaviour, as highlighted by wettability measurements. Low fluorination temperatures (< 230°C) lead to a sizing perfluorination and even overfluorination without carbon fibre fluorination, while higher temperatures (> 250°C) result in a carbon lattice fluorination with a complete desizing of the fibre through decomposition, that has been evidenced through X-Ray Diffraction and Raman spectroscopy studies. For both fluorination mechanisms, 19F Nuclear Magnetic Resonance, X-Ray Photoelectron and infrared spectroscopies have been used to study the nature of fluorine bonds. Scanning Electron Microscope and Energy-Dispersive X-Ray Spectroscopy have evidenced a core–shell structure for the fluorinated fibres with a fibre degradation at the highest temperatures, also highlighted by tensile tests, Atomic Force Microscopy and Electron Paramagnetic Resonance.

Multi-Metallic Template Fluorination Mmtf for the Preparation of Ternary Metal Fluoride and Their First Use As Cathodes in Solid State Lithium Batteries

There is an urgent need in exploring new cathode materials for Lithium batteries in order to increase their capacities. Conversion type cathodes are relevant replacement candidates and especially metal fluorides such as FeF3 with a theoretical capacity of 712 mAh g-1 for three electrons at a voltage of 2.74 V vs. Li+/Li or CuF2 with a theoretical capacity of 528 mAh g-1 for two electrons at 3.55 V vs. Li+/Li. However, important voltage hysteresis, dissolution in organic electrolytes and poor reversibility hinder their use as reliable cathode materials in Lithium batteries. In this work, compatibility of fluorides with a PEO/LiTFSI (Poly Ethylene Oxide impregnated with Lithium bis(trifluoromethanesulfonyl)imide) solid electrolyte membrane is evaluated to avoid dissolution of active materials in organic liquid electrolytes often encountered with fluorides. Additionally, a general synthesis method is used to prepare ternary metal fluorides. Indeed, ternary metal fluorides were recently explored1–3 as a solution to buffer volume expansion occurring during conversion, hence greatly reducing voltage hysteresis and improving cyclability. These materials are usually prepared by planetary ball milling or by hydrofluoric acid HF-assisted fluorination, both methods not scalable for industrial purpose. In this work, we propose Multi-Metallic Template Fluorination MMTF as a general synthesis way to prepare ternary metal fluorides in a scalable way suitable for industrial applications.Prussian Blue Analogs are particularly relevant fluorination templates: these compounds can be easily prepared in great quantities by coprecipitation, enabling industrial applications. Additionally, a wide variety of compositions can be synthesized while their open structure facilitates gas solid fluorination process used. Multi Metallic Template Fluorination MMTF on PBA therefore enables the preparation of fluoride compounds with desired metallic stoichiometry by tuning it in the pristine PBA. For electrochemical applications considered in this work, Copper based Prussian Blue Analog CuPBA of general formula Cu3[Fe(CN)6]2.xH2O is chosen to combine properties of copper and iron redox centers in a ternary metal fluoride cathode. By monitoring thermal transformations of CuPBA using ThermoGravimetric Analyses coupled to Mass Spectrometry (TGA MS) and XRD, different temperature domains are identified under air, assessing that the CuPBA template can be maintained up to 170 °C while oxides are formed at 350 °C. Fluorination temperatures are selected in consequence: structures of fluorinated products are characterized by combined XAS and XRD analyses. Results indicate the formation of an anti-perovskite [Cu(H2O)4]3.(FeF6)2 phase at low temperatures, showing that Multi Metallic Template Fluorination can proceed in a topotactic manner by maintaining a Multi Metallic Template in the fluorinated product. After a severe change in the local order identified by XAS at 200 °C and especially around copper cations, a biphasic CuF2/FeF3 product is formed at high temperatures.Electrochemical signature of different CuPBA fluorination products are investigated by Cyclic Voltammetry (CV) and galvanostatic measurements with a solid electrolyte membrane. Comparisons with a mechanical milling CuF2/FeF3 reference but also with pristine CuF2 and FeF3 commercial references cycled in the same conditions are presented. Results reveal that CuPBA fluorinated at 350 °C is able to match a CuF2/FeF3 mechanical milling reference and even perform slightly better. Both materials perform much better than the mere addition of CuF2 and FeF3 signatures, confirming the superiority of ternary metal fluorides as cathode materials over binary metal fluorides stated in literature. Better performances of CuPBA Multi Metallic Template Fluorination MMTF product at high temperatures over mechanical milling reference are explained by an homogeneous dispersion of copper and iron redox centers present in the initial CuPBA template resulting in a better distribution of CuF2 and FeF3 domains in the fluorinated product at 350 °C than in the mechanical milling reference.The first discharge capacity approaches the maximum theoretical capacity of 600 mAh g-1 and highlights the possibility to dramatically increase the capacity of Lithium batteries by potentially doubling the capacity reached with classical NMC electrodes. Multi Metallic Template Fluorination therefore enables the preparation of complex fluorinated cathodes in a scalable way for industry with interesting features over a simple mechanical milling.The Multi Metallic Template Fluorination MMTF approach detailed in this work can be adapted to a wide range of multi metallic templates such as Metal Organic Frameworks and Layered Double Hydroxides for the preparation of mixed metal fluorides with unique features such as particular structures and/or morphologies depending on the desired application and in a scalable way.(1) Villa et al. ACS Appl. Mater. Interfaces 2019, 11 (1), 647–654.(2) Wang et al. Nat Commun 2015, 6 (1), 6668.(3) Ding et al. ACS Appl. Mater. Interfaces 2019, 11 (4), 3852–3860. Figure 1

Topotactic fluorination induced stable structure and tunable electronic transport in perovskite barium ferrite thin films

The synthesis of perovskite (ABO3) films with mixed anions has enabled us to constitute a novel class of materials exhibiting intriguing functionalities. In contrast to traditional metallic cation doping, anion doping within easy-to-prepare conditions usually gives rise to tremendous modulations of physical properties with tunable crystal structures. Here, oxyfluoride BaFeO3-xFy (BFO–F) thin films were successfully synthesized via a low-temperature reaction with polyvinylidene fluoride, which exhibit extremely structural stability over time. The existence of fluorine was confirmed by the combination of X-ray photoelectron depth profile and soft X-ray absorption spectroscopy. The oxyfluoride BFO–F films demonstrate a sequence of structural evolutions with reduced out-of-plane lattices, by means of elevating fluorination temperatures. The valence state of iron in BFO–F is gradually changed from 3+ to 4+ for higher fluorination temperatures. Moreover, the resistivity of BFO–F dramatically decreases, compared to that of pristine oxygen-deficient BaFeO3-δ film. Our results provide insight into the modulations on the structures and physical properties of perovskite oxyfluorides through the facile fluorination process and contribute to the fundamental understanding of fluorination.

Polymorphism of Erbium Oxyfluoride: Selective Synthesis, Crystal Structure, and Phase‐Dependent Upconversion Luminescence

Phase‐selective synthesis and structure switching behavior of a functional material are essential to enable comparative studies on the structure–property relationship. Here, we report a controllable fluorination route to phase‐pure erbium oxyfluorides with orthorhombic (O‐ErOF) and rhombohedral (R‐ErOF) structures. This facile method adopts polytetrafluoroethylene (PTFE) as the fluoridizer, and the phase selectivity can be easily achieved at specific fluorination temperatures. The phase evolution and detailed crystal structures of erbium oxyfluoride were characterized by powder X‐ray diffraction (PXRD) at various sintering temperatures and Rietveld refinements, respectively. An irreversible phase transition from O‐ErOF to R‐ErOF was observed under heating around 600 °C. The upconversion (UC) luminescence properties of R‐ErOF and O‐ErOF were studied comparatively by means of photoluminescence, P–I, and UC decay curves. Despite their similar components and crystal structure, R‐ErOF exhibits stronger (more than 20 times) red UC emission than O‐ErOF. The anomalous UC behavior of the two polymorphs of ErOF was associated with the energy transfer processes dependent on their crystal structure.

Core-Shell Ni-NiF2 As Cathode Materials for Secondary Lithium Batteries

Choosing the best cathode material used in a Li-ion battery is one of the most crucial issues in achieving higher energy densities, since the energy density is directly correlated to the specific capacity associated with that cathode material. Conversion based cathode materials tend to exhibit substantially high capacities, due to the fact that essentially all the possible oxidation states of the compound during the redox reaction can be used. Transition metal fluorides have recently been investigated as potential cathode materials because of their high electronegativity. Tarascon’s group first demonstrated that these conversion materials, which were Co3O4, CoO, NiO, and FeO, can exhibit high specific capacities of 600–1000 mAh g− 1 along with good cycling properties. However, these metal oxides are only suitable for use as negative electrodes, due to their low conversion potential, which is lower than 1.0 V. Transition metal fluorides have recently been investigated as potential cathode materials because of their high electronegativity. However, the insulating nature of metal fluorides has limited their electrochemical properties for a long time; for that reason a considerable amount of attention has been devoted by Badway et al., to tailoring their nanostructures to overcome poor electronic conductivity. It has been demonstrated that nanocomposite of carbon and metal fluorides such as FeF2, FeF3, and BiF3 may be utilized as cathodes for next generation high energy lithium ion batteries, revealing a high conversion potential and good cycling properties. However there are very few studies on NiF2 based conversion cathode materials in terms of the conversion mechanism and electrochemical properties, due to its poor electrochemical properties compared to other metal fluorides. In this study, we have chosen to insure some electronic conductivity into NiF2 by partial conversion of nanoparticles of nickel into NiF2 through fluorination under pure molecular fluorine. For such purpose, Ni nanoparticles of median diameter of 50 nm were purchased and fluorinated at temperatures ranged in between room temperature and 450°C. It was then possible to get different proportions of NiF2 shell onto Ni core. The structure of Ni-doped NiF2 was investigated by X-ray diffraction (XRD) and the texture by transmission electron microscopy (MET). Two fluorination mechanisms appears depending on the fluorination temperatures. For temperatures lower than 300°C, the initial spherical shape is preserved but only 73% of Ni can be converted into NiF2. Increasing the fluorination temperature leads to damages into the spherical shape. However these damages can be some preferential way for lithium diffusion. Total conversion of Ni into NiF2 can then be obtained Structural data obtained by XRD combined with the weight uptake evolution got at the end of the fluorination show first the progressive formation of a NiF2 shell which appears as amorphous until 250°C i.e. a weight percentage of NiF2 lower than 24%. Then, the proportion of NiF2 and Ni phases becomes only the same by XRD and weight uptake as soon as the NiF2 phase is perfectly crystallized i.e. for fluorination temperature higher than 400°C. The coherence lengths of each phase evolve with the fluorination temperature and will be discussed owing to the different crystallographic planes of each phase. Electrochemical performances of the various samples partially or totally converted have been performed in different electrolytes chosen for their different solubility of LiF but also at different cycling temperatures and current densities. No carbon additive have been necessary in electrode formulation of partially converted nickel nanoparticles and the consecutive performances will be compared to those of totally converted nickel nanoparticles mixed with classical amount of carbon nanofiber additive. The electrochemical performances are different owing to the electrode composition. For low current density (C/20), when only core-shell particles got by fluorination at 200°C are used together with PDVF as binder, the discharge capacities at the first cycle appears as high but only few of it can be recovered at the next oxidation step. So as soon as the second cycle the capacities are half the theoretical value of NiF2 (554 mAh/g). When totally fluorinated nickel particle must be used as electrode material, some carbon additive is needed for conductive purpose. The first discharge capacity is lower than the one of core shell particle but the performance is easily maintained upon cycle.

Significantly enhanced surface electrical conduction of poly(butylene terephthalate) insulators by direct fluorination

Commercialization of cyclic butylene terephthalate oligomers has opened the door to bulky poly(butylene terephthalate) (PBT) matrix composite parts. In view of the potential applications of PBT in high voltage insulation or other sectors, where surface charge can play an unwanted role, PBT sheets were surface fluorinated in a laboratory vessel using a F2/N2 mixture with 12.5% F2 by volume at the different temperatures of 25, 55, and 85 ° C for the same time of 30 min, to enhance surface electrical conduction and to investigate the effect of fluorination temperature. Conductivity measurements reveal that the fluorinated samples have a much higher surface conductivity than the unfluorinated sample, and surface conductivity significantly increases with fluorination temperature. In accordance with the conductivity measurements, surface potential measurements after corona charging show that the fluorinated samples have a lower or much lower initial surface potential or even no surface potential, strongly depending on fluorination temperature and ambient humidity, compared with a high surface potential of the unfluorinated sample. ATR-IR analyses indicate that fluorination led to substantial changes in surface composition and structure, depending on fluorination temperature. SEM cross section and surface images show an increase of the fluorinated layer thickness with fluorination temperature and a modification of the surface morphology. Contact angle measurements of water show that fluorination also led to a significant increase in surface hydrophilicity, especially at higher fluorination temperatures. The intrinsic increase in surface electrical conduction is considered to be a result of competition between surface compositional changes and surface structural changes or the chemical traps and the physical traps.

Controlled formation of mixed nanoscale domains of high capacity Fe2O3-FeF3 conversion compounds by direct fluorination.

We report a direct fluorination method under fluorine gas atmosphere using a fluidized bed reactor for converting nanophase iron oxide (n-Fe2O3) to an electrochemically stable and higher energy density iron oxyfluoride/fluoride phase. Interestingly, no noticeable bulk iron oxyfluoride phase (FeOF) phase was observed even at fluorination temperature close to 300 °C. Instead, at fluorination temperatures below 250 °C, scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) and X-ray photoelectron spectroscopy (XPS) analysis showed surface fluorination with nominal composition, Fe2O3-xF2x (x < 1). At fluorination temperatures of 275 °C, STEM-EELS results showed porous interconnected nanodomains of FeF3 and Fe2O3 coexisting within the same particle, and overall the particles become less dense after fluorination. We performed potentiometric intermittent titration and electrochemical impedance spectroscopy studies to understand the lithium diffusion (or apparent diffusion) in both the oxyfluoride and mixed phase FeF3 + Fe2O3 composition, and correlate the results to their electrochemical performance. Further, we analyze from a thermodynamical perspective, the observed formation of the majority fluoride phase (77% FeF3) and the absence of the expected oxyfluoride phase based on the relative formation energies of oxide, fluoride, and oxyfluorides.

Modulation of surface electrical properties of epoxy resin insulator by changing fluorination temperature and time

In order to systematically investigate the modulating role of fluorination temperature and time on surface electrical properties of the epoxy resin insulator, epoxy resin sheets were surface fluorinated in a laboratory vessel using a F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 2/N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 2 mixture with 12.5% F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 2 by volume at 0.1 MPa and different temperatures of 25–95 °C for different times of 5–120 min. Conductivity measurements have shown that the intrinsic surface conductivity of the epoxy resin insulator could be modulated from an even lower value to a nearly four orders of magnitude higher value than the value prior to fluorination by changing the fluorination temperature and time. Surface conductivity increased with increasing fluorination temperature at the same fluorination time, while decreased to some extent with increasing fluorination time at a given fluorination temperature. Surface potential decay measurements showed consistent results with the surface conductivity measurements. ATR-IR analyses revealed substantial changes in surface chemical composition and structure, depending on the fluorination temperature and time. SEM images clearly showed an increase of the fluorinated layer thickness with increasing fluorination temperature and time. Surface cracks appeared only at elevated fluorination temperatures and increased with fluorination temperature, while the surface became compact with the duration of the fluorination. These results, compared with the previous results on an epoxy resin insulator made of DGEBA epoxy resin with a lower epoxy value, also indicated a significant influence of the epoxy resin insulator itself or the epoxy resin raw material on surface electrical properties and surface physicochemical characteristics after fluorination.

Effect of surface fluorination and conductive additives on the electrochemical behavior of lithium titanate (Li 4/3Ti 5/3O 4) for lithium ion battery

Effect of surface fluorination and conductive additives on the charge/discharge behavior of lithium titanate (Li 4/3Ti 5/3O 4) has been investigated using F 2 gas and vapor grown carbon fiber (VGCF). Surface fluorination of Li 4/3Ti 5/3O 4 was made using F 2 gas (3 × 10 4 Pa) at 25–150 °C for 2 min. Charge capacities of Li 4/3Ti 5/3O 4 samples fluorinated at 70 °C and 100 °C were larger than those for original sample at high current densities of 300 and 600 mA/g. Optimum fluorination temperatures of Li 4/3Ti 5/3O 4 were 70 °C and 100 °C. Fibrous VGCF with a large surface area (17.7 m 2/g) increased the utilization of available capacity of Li 4/3Ti 5/3O 4 probably because it provided the better electrical contact than acetylene black (AB) between Li 4/3Ti 5/3O 4 particles and nickel current collector.

Carbon nanofibres fluorinated using TbF 4 as fluorinating agent. Part I: Structural properties

Fluorination of carbon nanofibres (CNFs) under fluorine gas at 480 °C leads to high fluorine content but also to some partial exfoliation. In order to avoid such phenomenon, an alternative route has been performed at temperatures ranged between 420 and 500 °C using a fluorinating agent, i.e. terbium tetrafluoride. The structural properties of the fluorinated CNFs are discussed taking into account the data of 13C solid state NMR, Raman spectroscopy, SEM, TEM and XRD. Whatever the fluorination temperature, a fluorinated phase of (CF) n structural type, is formed contrary to the direct process using F 2 gas for which a (C 2F) n -type fluorinated phase appeared for fluorination temperatures lower than 450 °C. The progressive release of fluorine atoms from the thermal decomposition of TbF 4 allows an homogenous distribution of the fluorinated part into the CNFs matrix and the formation of a unique (CF) n type structure. Moreover, for high fluorination temperatures (480 and 500 °C), the fluorination leads to some nanofibres breaking but in no way to exfoliation.

Reactivity of Carbon Nanofibers with Fluorine Gas

The present study highlights the reactivity of carbon nanofibers (CNFs) with fluorine gas. Highly purified and graphitized CNFs were treated under a stream of fluorine gas for 16 h at temperatures ranging from 380 to 480 °C. Different fluorination temperature zones have been revealed by direct physicochemical analysis such as XRD, Raman spectroscopy, EPR, and solid-state NMR (13C and 19F). The comparison between various parameters such as covalence of C−F bond, T1 spin−lattice nuclear relaxation time, density, and environment of the dangling bonds, among others, allows the fluorination mechanism to be determined, i.e., the formation of (C2F)n type graphite fluoride as the precursor of richer (CF)n compound. This is supported by TEM characterization as the fluorination proceeds from the external parts of the carbon nanofibers and then propagates through the core without a major structural change of the fluorinated parts. A low exfoliation of the sheets is necessary for extended fluorination and conversion into (CF)n; this occurs for fluorination temperatures higher than 472 °C, with concomitant disappearance of the graphitic structure.

ESR spectra of high Tc superconducting oxides treated under various atmospheres

When completely free from parasitic phases, superconducting oxides are characterized by a flat ESR signal and by a strong zero-field microwave absorption. The presence of traces of non-superconducting impurities which often occur at the grain boundaries can be easily detected by ESR, which makes this technique a very powerful tool for the characterization of the quality of superconducting oxides. The influence on the ESR response of the thermal treatments undergone on YBa 2Cu 3O 7− δ and Bi 4(Sr, Ca) 6Cu 4O 16+ δ samples under vacuum or under reactive atmospheres (oxygen, fluorine) has been investigated. The progressive transformation of Cu 2+ in Cu 3+ with increasing fluorination temperatures has been followed by ESR. The results have been compared with oxide and fluoride standards containing Cu 2+ or Cu 3+ species. The superconductivity has been systematically controlled by the zero-field microwave absorption.

Fluorinated Diamond Films, Slabs, and Grit

AbstractDiamond powders, single crystal diamonds (slabs and naturally occurring stones), and CVD diamond films have been fluorinated via direct fluorination and plasma techniques at a variety of temperatures and reaction times. Infrared, ESCA, and mass spectrometric studies verify that the diamond surfaces are altered by fluorination and that fluorine is attached to the surface to some degree in most instances. Based on spectra obtained using the FT-IR techniques of DRIFTS and ATR, the fluorine appears to have an effect on the diamond surfaces at relatively low temperatures in short reaction times by the removal of surface contaminants. A relatively high temperature (600ºC) or long reaction time at slightly lower temperatures is needed to attach a significant number of fluorine atoms onto the diamond surface. Once fluorinated, the diamonds show enhanced resistance to further oxidation. The outward appearance of the diamonds remains unchanged even at fluorination temperatures nearing 800ºC.

Electrochemical Characteristics of Fluorinated Activated Carbon Fibers‐Lithium Cells

Because of their characteristic structural properties such as microcrystallinity with monodispersive micropores and their ability to resist graphitization, activated carbon fibers (ACF's) readily react with elemental fluorine at significantly low temperatures ranging from 20° to 200°C under fluorine pressures of 100–760 torr. The electrochemical characteristics of fluorinated ACF's (F‐ACF's) obtained at the above conditions have been investigated as cathode materials for lithium cells in carbonate solutions. OCV's of F‐ACF cathodes decrease from 3.68 to 3.35V as fluorination temperatures increase from 20° to 200°C. The dependence of OCV's on fluorination temperatures is ascribed to the change in the nature of the C‒F bond which varies from chemisorption through charge transfer to covalent bond, depending on fluorination temperatures. Discharge performance, particularly the stability of the discharge potential and cathode utility, are significantly improved with increasing fluorination temperature. The relation between utilization and x‐ray diffraction patterns of F‐ACF's suggests that to a certain extent the three‐dimensional structure in a F‐ACF is required to be a cathode material with satisfactory discharge performances. F‐ACF's obtained at 200°C but with reduced fluorine pressures have stable discharge potentials (for current densities of 0.5 mA cm−2) which are 0.5V higher than that of conventional graphite fluoride . Cathode utilization is higher than 70%.