Mixed Fluoride Research Articles

A one-pot method for the preparation of mixed matrix polyvinylidene fluoride membranes with in situ synthesized and PEGylated polyethyleneimine particles

In this article, we report a one-pot method for the preparation of mixed matrix polyvinylidene fluoride (PVDF) membranes with in situ synthesized and PEGylated polyethyleneimine (PEI) particles. The key feature of our new membrane preparation route is the in situ synthesis and PEGylation of PEI particles in the dope solutions prior to membrane casting using epichlorohydrin (ECH) and poly(ethylene glycol) diglycidyl ether (PEGDE) as crosslinker and PEGylating reagent, respectively. Using non-solvent induced phase separation (NIPS), we successfully prepared a new family of mixed matrix PVDF membranes with (i) average pore diameters of 16–24nm, (ii) high loadings of PEGylated PEI particles (35–57wt%) and (iii) uniform distributions of particles with average diameters ranging from 600nm to 3.6μm. Preliminary experiments show that our new mixed matrix PVDF membranes with in situ synthesized and PEGylated PEI particles can serve as fouling resistant ultrafiltration membranes for microalgae recovery from culture media of relevance to biofuel production.

Tailoring the Composition of a Mixed Anion Iron-Based Fluoride Compound: Evidence for Anionic Vacancy and Electrochemical Performance in Lithium Cells

Microwave-assisted synthesis allows stabilizing Fe-based fluoride compounds with hexagonal tungsten bronze (HTB) network. The determination of the chemical composition, i.e., FeF2.2(OH)0.8·(H2O)0.33, revealed a significant deviation from the pure fluoride composition, with a high content of OH groups substituting fluoride ions. Rietveld refinement of the X-ray diffraction data and Mössbauer spectroscopy showed that the partial OH/F substitution impact on the structure (interatomic distances, angles, and so on) and the local environment of iron (isomer shift and quadrupole splitting distribution). The thermal behavior of the hydroxyfluoride compound has been thoroughly investigated. From room temperature to 350 °C under Ar flow, the HTB-type structure remains stable without any fluorine loss and only water departure. At T > 350 °C, the structure started to collapse with a partitioning of anions leading to α-FeF3 and α-Fe2O3. Within 200 °C ≤ T ≤ 350 °C, the chemical composition can be tuned with different contents of OH–/O2– and structural water. By an adequate thermal treatment, it has been shown that anionic vacancies formed by dehydroxylation reaction could be stabilized within the HTB network yielding a compound containing three different anions, i.e., FeF2.2(OH)0.8–xOx/2□x/2. XRD Rietveld analysis, atomic pair distribution function, and Mössbauer spectroscopy confirmed the formation of under-coordinated iron FeX5□1 (X = O2–, F–, and OH–) atoms. Different compositions have been prepared by thermal treatment at T ≤ 350 °C and their electrochemical properties evaluated in lithium cell. Structural water seems to block the diffusion of lithium within the hexagonal cavities. Increasing the content of anionic vacancies significantly improves the reversible capacity emphasizing a peculiar role on electrochemical properties. Pair distribution functions obtained on lithiated and delithiated samples indicated that the HTB network was maintained (in the 2–4.2 V voltage range) during the intercalation processes.

EPR study of clusters of rare-earth ions in mixed fluoride crystals

The EPR spectra of the (BaF2)1 − x(CeF3)x system are studied for the concentrations x = 0, 0.001, 0.002, 0.005, 0.01, and 0.02. The appearance of new tetragonal centers is detected beginning from x = 0.002, the intensity of these centers being maximal at x = 0.01. The (CaF2)1 − x − y(CeF3)x(YF3)y double solutions with x = 0.001 and y from 0 to 0.02 are also studied. In addition to the ordinary tetragonal center, beginning from y = 0.001, a new tetragonal center appears with the same structure as in the previously studied mixed crystals based on BaF2—namely, the Ce3+-□-R3+ chain elongated along the fourfold axis substitutes the Ca2+-Ca2+-Ca2+ and Ba2+-Ba2+-Ba2+ chains in regular CaF2 and BaF2 crystals (□ is the cation vacancy, and R3+ is the Ce3+, La3+, or Y3+ trivalent ion).

Erosion protection comparison of stabilised SnF2, mixed fluoride active and SMFP/arginine-containing dentifrices

To investigate the relative erosion protection potential of marketed dentifrices formulated with either stabilised stannous fluoride (SnF2 ), sodium fluoride (NaF) and/or sodium monofluorophosphate (SMFP) using an established laboratory erosion cycling model. Sound enamel cores from extracted, human enamel were cleaned, ground and polished, soaked in pooled saliva (pellicle formation) and treated with a 1:3 slurry of dentifrice and saliva. Specimens were subjected to daily challenges with 1% citric acid, a potentially damaging acid found in common food and drinks. Marketed dentifrices compared were: (1) a stabilised stannous fluoride product formulated with 1,100ppm F as SnF2 ; (2) a cavity protection product containing 1,100ppm F as NaF; (3) a cavity protection product comprising a mixed active fluoride system with 1,000ppm F as SMFP+450ppm F as NaF; and (4) a sensitivity product containing 1,450ppm F as SMFP+8% arginine bicarbonate. Specimens from Group 1 demonstrated an average loss of 5.5 (±1.2)μm of tooth surface enamel; Groups 2, 3 and 4 lost an average of 18.3 (±0.9)μm, 16.0 (±2.0)μm and 17.1 (±1.1)μm, respectively, of tooth surface enamel. Group 1 provided a statistically significant difference in protection compared with the other products. These results suggest that the marketed dentifrice formulated with stabilised SnF2 may provide enhanced protection of exposed tooth surfaces against dietary acid attack compared with the other products tested.

Characterisation of fresh surface films formed on molten Mg–Nd alloy protected by different atmospheres

This study examines the early stages of surface oxidation of liquid Mg–3wt%Nd under UPH argon, dry air, and air mixed with protective fluorine-bearing gases. Each of the gases were introduced as bubbles into solidifying castings. The chemistry and structure of the protective film inside the trapped bubbles were characterized by SEM and EDX analyses. Results show that due to Nd added to Mg alloy under dry air, a dense and wrinkled surface film that contains MgO and Nd2O3 are formed. Under fluorine-bearing gas mixtures, a dense and coherent surface film was found to be a mixed fluoride and oxide. For SF6, the film thickness was 50–100nm thick while for HFC-R134a it was 35–45nm. Needle shaped phases distributed in the Mg matrix and flake-like phases segregated on the inner bubble surface in proximity to the interdendritic regions of the alloy were both identified as Nd rich compounds. These were present under all gas conditions. The results obtained lead to a conclusion that HFC-R134a is capable of providing the most effective melt protection. The integrity and protective capability of the early surface film formation on the liquid Mg–Nd alloy was found to be significantly improved compared to pure Mg under identical gas conditions due to formation of a dense and compact MgO/Nd2O3 layer, regardless of whether fluorine species were also present.

Li0.8Mg2.1B2O5F: the first borate fluoride with magnesium–oxygen–fluorine octahedral chains

The first alkali metal and magnesium mixed borate fluoride, Li(0.8)Mg(2.1)B2O5F, was prepared by the high-temperature solution method. It crystallizes in the monoclinic space group P2(1)/n (no. 14) with lattice constants a = 3.0881(4) Å, b = 13.6020(2) Å, c = 9.6563(1) Å, β = 91.744(9)°, Z = 4. In the structure, a special magnesium-oxygen-fluorine octahedral chain is found in borates for the first time. The octahedral chains are linked into a three-dimensional framework containing tunnels with (1)(∞)[LiO4](7-) chains and isolated B2O5 groups filling in. The differences of octahedral chains in Li(0.8)Mg(2.1)B2O5F and magnesium inosilicate minerals are discussed. The TG-DSC curves, IR spectrum and UV-Vis-NIR diffuse reflectance spectrum were measured.

A facile route to the preparation of mixed matrix polyvinylidene fluoride membranes with in-situ generated polyethyleneimine particles

The development of mixed matrix polymeric membranes with embedded functional particles/nanomaterials has been an active area of research during the last two decades. Such membranes are being designed to carry out multiple functions (e.g. retention, sorption and catalysis) with improved properties and performance over those of commercial membranes. Polymeric particles could provide greater flexibility for the preparation of mixed matrix membranes with improved particle–matrix compatibility, particle loading, flux and permselectivity. In this article, we describe a facile and simple route to the preparation of mixed matrix polyvinylidene fluoride (PVDF) membranes embedded with branched polyethylenimine (PEI) particles. The critical step of our novel methodology is the in-situ generation of crosslinked PEI micro/nanoparticles (with diameters ranging from 400nm to 3μm) in the membrane casting solution using epichlorohydrin as crosslinker. This eliminates the need to utilize inverse emulsion polymerization techniques to synthesize PEI particles prior to the membrane casting. Using non-solvent induced phase separation (NIPS), we successfully prepared mixed matrix PVDF membranes with uniform particle distribution and PEI particle loadings ranging from 27 to 48wt%. Our novel membrane preparation route exhibits many advantages including simplicity, scalability and versatility. Preliminary experiments show that our new mixed matrix PVDF membranes with embedded PEI particles can serve as weak-base membrane absorbers for proteins such as bovine serum albumin.

Alignment of Acentric Units in Infinite Chains: A “Lock and Key” Model

Polar chains built from acentric building units are of importance to investigate the mechanisms driving the polar alignment in the solid state. Our attempts to engineer polar chains in mixed metal oxide fluorides M′(2,2′-bpy)(H2O)2MOxF6–x compounds [M′/M = Cu/Ti, Cu/V, Cu/Nb, Cu/Mo, Zn/Mo, and Zn/W] were successful using a combination of acentric anions [MOxF6–x]2– and acentric cations [M′(2,2′-bpy)(H2O)2]2+. A new general insight is also revealed: the alignment of polar units can be described with a “lock and key” model. The role of both the key (the acentric unit) and the lock (its environment) on the polarity in infinite chains is discussed.

Existence of minimum molar volumes (maximum packing densities) in morphotropic series of mixed oxides and fluorides

Plots of the cubic root of the formula volume, (V/Z)1/3, vs. cation radius have been built for several morphotropic series of mixed oxides and fluorides. These plots are linear for isostructural compounds but exhibit discontinuities at morphotropic transitions. The classical homology rule ‘larger cation – higher its coordination number (CN) – denser the structure’ is only valid (with few exceptions) for substitutions of relatively small cations. Substitutions of large cations usually lead to opposite volume changes, regardless of increase or decrease in CNs. As a result, the densest structure types often appear with ions of somewhat intermediate size in a series. The discovery of the post-perovskite silicate phase, which greatly influenced mineralogy and geophysics, could be predicted four decades earlier, if proper attention had been paid to the small negative deviation of the layered CaIrO3 from the plot for CaMO3 perovskites. An analysis of the volume relations reveals some errors and leads to several predictions.

Effects of nitrogen on properties of oxyfluoronitride bioglasses

Bioglasses are used as bone substitutes and prosthetic coatings. Following implantation, they are predisposed to generate a series of physicochemical reactions at the glass-bone interface. Bioglasses with molar composition: 55SiO2–8.5CaO–31.5Na2O–5CaF2 have been synthesized and characterized. However, because of their poor strength, doping with nitrogen was performed on these glasses to increase their mechanical properties. These glasses were chemically analyzed to verify the amount of nitrogen introduced and structurally characterized by 29Si and 19F MAS NMR. The fluorine complexes with calcium and sodium and is present as mixed calcium sodium fluoride and sodium fluoride species. The addition of fluorine to bioglasses reduces the melting temperature which helps to minimize nitrogen loss and bubble formation. So the fluorine facilitates the dissolution of nitrogen into the melt. Nitrogen substitutes for oxygen in the silicate network and is present as SiO3N and SiO2N2 structural units. The density, glass transition temperature, Young's modulus, hardness and fracture toughness all increase with the content of nitrogen introduced into the glasses. These changes are a result of greater cross-linking of the silicate network due to the higher coordination of nitrogen compared to oxygen.

Fluoride-containing bioactive glass-ceramics

Fluorapatite glass-ceramics are osteoconductive, and glass-ceramics containing fluorapatite crystals in a bioactive silicate glass matrix can combine the benefits of fluorapatite with the bone-regenerative properties of bioactive glasses. High phosphate content (around 6mol% P2O5) bioactive glasses (SiO2–P2O5–CaO–Na2O–CaF2) were prepared by a melt-quench route. Structural investigation using density measurements and calculations confirmed the presence of phosphorus as orthophosphate. Upon heat treatment, the glasses crystallised to mixed sodium calcium fluoride orthophosphates (sodium-containing compositions) and fluorapatite (sodium-free composition). Fluoride suppressed spontaneous crystallisation, allowing formation of glass-ceramics by controlled crystallisation. A notable feature is that silicate network polymerisation and network connectivity did not change during crystallisation, resulting in orthophosphate and fluorapatite crystals embedded within a bioactive glass matrix. By keeping the phosphate content high and the sodium content low, fluorapatite glass-ceramics can be obtained, while not affecting the structure of the bioactive silicate glass phase.

Raman Spectroscopic Analysis of Perovskite-Structured Alkali Metal Fluorides Containing Nickel and Copper Ions

ABSTRACT The mixed metal fluorides containing alkali metals have a range of important applications in optical and electronic devices. Raman spectrums of two such fluorides were examined. Raman spectrum of KCuF3 at 300 K exhibited bands at 261, 295, 363, 468, 519, and 549 cm−1, indicating site symmetry (orthorhombic) lower than the tetragonal symmetry as observed from the powder X-ray diffraction pattern. Cubic KNiF3 showed bands at 410, 468, and 657 cm−1. The first two bands were attributed to the second-order phonon scattering, and the band at 657 cm−1 was assigned to two-magnon peak.

Structure of Na2O·MO·SiO2·CaF2 (M=Mg, Ca) oxyfluoride glasses

(9−x)CaO·xMgO·15Na2O·60SiO2·16CaF2(x=0, 2, 4, 6, and 9) oxyfluoride glasses were prepared. Utilizing the Raman scattering technique together with 29Si and 19F MAS NMR, the effect of alkaline metal oxides on the Q species of glass was characterized. Raman results show that as magnesia is added at the expense of calcium oxide, the disproportional reaction Q3→Q4+Q2 (Qn is a SiO4 tetrahedron with n bridging oxygens) prompted due to the high ionic field strength of magnesia, magnesium oxide entered into the silicate network as tetrahedral MgO4, and removed other modifying ions for charge compensation. This reaction was confirmed by 29Si MAS NMR. 19F MAS NMR results show that fluorine exists in the form of mixed calcium sodium fluoride species in all glasses and no Si–F bonds were formed. As CaO is gradually replaced by MgO (x=6, 9), a proportion of the magnesium ions combines with fluorine to form the MgF+ species. Meanwhile, some part of Na+ ions complex F− in the form of F–Na(6).

CF4 decomposition over solid ternary mixture NaF-Si-MO (MO = La2O3, CeO2, Pr6O11, Nd2O3, Y2O3)

A solid ternary mixture consisting of NaF, silicon and one metal oxide such as La2O3, CeO2, Pr6O11, Nd2O3, and Y2O3 was prepared and used as de-fluorinated reagent for CF4 decomposition. The results show that 90% conversion of CF4 can be reached initially over NaF-Si-La2O3, NaF-Si-CeO2, NaF-Si-Nd2O3, and NaF-Si-Y2O3 at 850 °C. The fresh and used reagents were characterized using XRD and XPS techniques. It was found that the active components of NaF and metal oxides in NaF-Si-CeO2, NaF-Si-Pr6O11, NaF-Si-Nd2O3, and NaF-Si-Y2O3 were transformed into inert phases of mixed metal fluorides and silicates, respectively, resulting in an ineffective utilization of these de-fluorinated reagents, whereas no inert phases from NaF and La2O3 can be observed in the used NaF-Si-La2O3, indicating the NaF-Si-La2O3 reagent could be utilized more efficiently than the other reagents in CF4 decomposition.

Characterization of surfacial basic sites of sol gel-prepared alkaline earth fluorides by means of PulseTA®

The CO 2 adsorption properties of a series of different sol gel-prepared metal fluorides of the alkaline earth row and aluminium, among them two mixed (doped) metal fluorides was investigated by applying Pulse Thermal Analysis. The alkaline earth flurorides, which exhibit nanoscopic properties, have not only acid, but basic sites as well. For the first time, these basic sites have been characterized by the exothermicity of the first CO 2 injection pulse. If the basicity is weak or absent, the alternating injection of water can generate OH groups at the surface of the fluorides. They mostly act as basic sites and allow a stronger differentiation of the basic properties, but the formed OH groups can exhibit an acid character as well. The impact of OH groups can affect a different influence on the sorption properties of the solids: the adsorption ability vs. CO 2 can be promoted, suppressed or remain unaffected.

Application of KZnF3 as a Single Source Precursor for the Synthesis of Nanocrystals of ZnO2:F and ZnO:F; Synthesis, Characterization, Optical, and Photocatalytic Properties

Mixed metal fluoride, KZnF3, possessing a cubic perovskite structure has successfully been employed as a single source precursor for the synthesis of fluoride-doped ZnO2 nanocrystals by a simple low-temperature oxidation procedure. Utilizing the fact that ZnO2 is a precursor for ZnO, F–-doped wurtzite ZnO was readily obtained by a straightforward decomposition procedure. The structure, optical, and photocatalytic properties of doped ZnO2:F and ZnO:F were studied and compared with the undoped ones. The preservation of the cubic pyrite structure of ZnO2 by the inclusion of F–-ions was revealed by the powder X-ray diffraction pattern. Uniform cube morphology of the nanocrystals of ZnO2:F was observed in both the scanning electron microscopy and transmission electron microscopy images with the crystallite size of 20 nm. The IR and Raman spectroscopy analysis implied the absence of any Zn–F direct bonding in ZnO2:F. The high-resolution core level X-ray photoelectron spectroscopy (XPS) spectrum of F 1s observed at 687.9 eV confirmed the presence of fluoride ions in the ZnO2 lattice. By fitting the core level F 1s spectrum, the concentration of the F– ion was found to be 8.6%. A red shift in the excitonic absorption was observed on F– doping in ZnO2. A similar trend was also observed in the band edge emission from the photoluminescence spectrum recorded at 300 K. The intensity of the violet emission in the pure ZnO2 (with a decay time of 18 ns) decreased on F– doping (with a decay time of 13 ns). While ZnO2 nanocrystals efficiently degraded methylene blue (MB) solution under UV radiation and moderately under visible radiation, F–-doped samples showed lesser efficiency for the photo degradation of the MB solution. F–-doped ZnO was obtained by decomposing the ZnO2:F in air at 450 °C for 3 h. The symmetry remained hexagonal on F–-doping as revealed by the powder X-ray diffraction pattern. The intensity of the Raman bands of fluoride-doped ZnO nano powders were in general less as compared to the undoped ZnO, except the one observed at 582 cm–1, which indicated the presence of higher oxygen vacancies in ZnO:F. Core level XPS measurements provided conclusive evidence for the doping of fluorine (6.1%) in ZnO. The band gap value of ZnO:F, estimated from the diffuse reflectance spectrum, was 3.0 eV, and it showed broad visible emission. As a consequence of higher oxygen vacancies, ZnO:F exhibited efficient photocatalytic activity under visible irradiation for the degradation of aqueous MB dye solution.

The growth of mixed alkaline-earth fluorides for laser host applications

A comprehensive analysis is implemented concerning the growth, properties, and applications of doped–co-doped single and mixed alkali earth fluoride systems. Calcium–strontium fluoride solid solutions with a Sr content proportion varying widely between 0.007 and 0.675 mol.% are obtained as a batch of axis-symmetrical boules grown by a Bridgman–Stockbarger (BS) method. The crystallization front (CF) can be controlled to retain a convex CF-shape that is favourable for normal growth of single crystals. This achieved using a broad adiabatic furnace zone (AdZ) independently of the boules’ composition. The influence of the thermal field distribution on the CF and the real crystallization rate (CR), which are both critically decisive in controlling crystal quality, were originally assessed using empirically derived formulas. The optical characteristics of the grown boules were monitored by measuring the external transmittance t and calculating the total losses following light irradiation of optical windows that were prepared from sections of the boules that had been cut parallel to one another. The t-measurements were performed by two different techniques and the comparative analysis of the results reliably indicates any inhomogeneity in the grown boules. A simple supercooling criterion proved to closely relate the morphological stability of the CF enabling one to set up the optimum growth conditions. Thus the normal growth criterion outlines the concentration bounds where the isotropic growth mechanism is replaced by cellular anisotropic growth. A procedure has been established for provisioning researchers with optical quality calcium–strontium fluoride crystals with widely varying composition grown under practically identical conditions. As a consequence one can explore possible reasons that can affect the growth mechanism for this or any other systems with a fluoride structure and so provide scope aimed at the future improvement of the crystal quality thereby enlarging the field of mixed fluoride systems’ applications.

Hydrothermal synthesis of mixed rare earth-alkali metal or ammonium fluorides

The recent results on hydrothermal synthesis of mixed rare earth-alkali or ammonium fluorides were presented. The initial ratios of the starting materials, pH value and reaction temperature were the critical factors for obtaining the single-phase product. Four main types of complex rare earth fluorides, AREF 4, A 2REF 5, ARE 2F 7 and ARE 3F 10 (A=Na +, K +, Rb +, NH 4 +), appeared in the primary hydrothermal reactions. The correlation between cation sizes and the formation of mixed rare earth fluorides under mild hydrothermal condition was given.

Characterisation of fresh surface oxidation films formed on pure molten magnesium in different atmospheres

This paper examines the early stages of surface oxidation of liquid magnesium under argon, air, and air mixed with protective fluorine-bearing gases. Surface film characteristics such as morphology, thickness and composition are determined. In all cases except argon the film was locally uniform with no evidence of specific nuclei. In air, the film thickness was 15–150 nm. Under fluorine-bearing gas mixtures the surface film was a mixed fluoride and oxide and more even: 70–100 nm thick under SF 6 and 30–50 nm under 1,1,1,2-tetrafluoroethane. The latter had a substantially lower O:F ratio. Complete conversion of available fluorine into the film was indicated.

Synthesis of nanocrystalline mixed metal fluorides in nonaqueous medium

Synthesis of mixed metal fluorides of the general formula, KMF3 (M = Mg, Mn, Co, Ni, Cu and Zn), possessing perovskite structure was investigated in non-aqueous medium. The fluorides were characterized by powder X-ray diffraction, FT-IR spectroscopy, thermal analysis, SEM and TEM. Monophasic cubic phases were obtained for the central metal ions such as Mg, Mn, Co, Ni, and Zn and a tetragonally distorted phase was observed for Cu. The usage of non-aqueous medium is advantageous for the bulk synthesis of these fluorides, since it eliminated the generation and handling of the hazardous HF that has usually been encountered during aqueous preparations. The average crystallite size of the fluorides obtained by this approach was estimated to be in the range of 9–30 nm. SEM micrographs of KZnF3 showed cubic morphology of perovskite phases. TEM studies on KCuF3 confirmed the presence of tetragonal distortion. The fluoride content was determined by titrimetry and found to be nearly stoichiometric. Some of these fluorides were found to be thermally stable up to 225°C in air. These fluorides were employed as fluorinating agents in organic fluorination reactions, thereby suggesting their possible utilization for selective fluorination of aliphatic and aromatic hydrofluorocarbons (HFCs) that are industrially relevant.