NH4F Content Research Articles

Porous Structure Fibers Based on Multi-Element Heterogeneous Components for Optimized Electromagnetic Wave Absorption and Self-Anticorrosion Performance.

The excellent performance of electromagnetic wave absorbers primarily depends on the coordination among components and the rational design of the structure. In this study, a series of porous fibers with carbon nanotubes uniformly distributed in the shape of pine leaves are prepared through electrospinning technique, one-pot hydrothermal synthesis, and high-temperature catalysis method. The impedance matching of the nanofibers with a porous structure is optimized by incorporating melamine into the spinning solution, as it undergoes gas decomposition during high-temperature calcination. Moreover, the electronic structure can be modulated by controlling the NH4F content in the hydrothermal synthesis process. Ultimately, the Ni/Co/CrN/CNTs-CF specimen (P3C NiCrN12) exhibited superior performance, while achieving a minimum reflection loss (RLmin) of -56.18dB at a thickness of 2.2mm and a maximum absorption bandwidth (EABmax) of 5.76GHz at a thickness of 2.1mm. This study presents an innovative approach to fabricating lightweight, thin materials with exceptional absorption properties and wide bandwidth by optimizing the three key factors influencing electromagnetic wave absorption performance.

Solid solution-typed defect concept in forming multivalent cations- and oxygen vacancy-filled MnCo2O4-z spinel bifunctional electrodes for innovative symmetrical supercapacitor

Oxygen vacancies-filled MnCo2O4-z (MCO) nanosheets prepared by a two-step process with varied NH4F content have been investigated. MCO prepared with NH4F at 10 mmol (MCO-10) shows excellent to be used not only for cathode and anode but also for a symmetric supercapacitor cell (SSC). This MCO-10 electrode as a cathode not only reaches a high specific capacitance (Cs) of 7249 F/g at 1 A/g but also performs a high value of 2515 F/g at 1 A/g as an anode. An enlarged SSC of 5 × 5 cm2 is developed. One and five light-emitter-diode (LED) bulbs and commercial 100 F supercapacitor devices are tested to authenticate our reported data. The outstanding performance of nonstoichiometric MCO spinel involving the OH− dipole interaction with oxygen vacancies, chemical redox reaction, multiple cationic valence states, and oxygen vacancies-filled defective structure are all connected with the solid solution-typed defect concept to form bifunctional spinel.

Fabrication of ultra-low porosity plasma electrolytic oxidation coating on Ta-12W alloys and its formation mechanism

The sealing of pores in plasma electrolytic oxidation (PEO) coatings is critical to further enhance the protection of the underlying substrate and maintain the mechanical performance of coatings. This study focuses on the modification of the surface microstructure of PEO coatings on Ta-12W alloy by introducing NH4F additive into a mixed silicate-phosphate electrolyte. As a result, ultra-low porosity coating with a surface porosity of less than 1 % is effectively prepared. Detailed investigations have been conducted to study the effects of NH4F content and treatment duration on various coating characteristics, including microscopic morphology, elemental distribution, thickness, and phase composition. It is found that optimizing the concentration of NH4F eliminates the large pore structures and promotes the development of Ta-rich structures resembling pancakes on the coating surface. Furthermore, the intensities of diffraction peaks from different crystal faces of Ta2O5 in coatings exhibit variations, and selective growth is observed specifically on its (110) crystal plane. After a 20-minute treatment of Ta-12W alloys in the electrolyte containing 4 g/L NH4F, the surface of the coating displays predominantly pancake-like structures with sealed pore/non-pore features. The formation process of these specific structures is discussed based on the evolution of coating morphology with oxidation time. It is proposed that the increase in anodic voltage and gas production in the late stage favors the breakdown of gas bubbles, leading to the creation of non-porous pancake-like structures on the surface. In addition, the breakdown of the dielectric layer at the periphery of gas bubbles results in the influx of film-forming materials into pores, which causes the formation of pancake-like structures with sealed pores.

Quantitative analysis of the volume expansion of nanotubes during constant voltage anodization

The formation process of anodized nanotubes and the reason of volume expansion are still the focus of attention. So far, the traditional field-assisted dissolution and ejection mechanisms have only qualitatively discussed the phenomenon that the growth efficiency of the porous oxide film is about 60% of that of the barrier film, and the corresponding volume expansion factor is about 1.44 (2.4 × 60%). Here, the nanotubes prepared by applying different constant voltages in five kinds of NH4F concentration electrolyte were studied in detail. For the first time, the charges transferred in the process of constant voltage anodization was calculated by integral, thus the volume expansion factors under different conditions were calculated. We found that when the number of charges transferred by the reaction was similar, the total volume hardly changed, and it is independent of the NH4F concentration. The volume expansion factors (about 2.0) did not change with the increase of NH4F content, which is contrary to the expectations of the field-assisted dissolution and ejection. This study will help to promote the qualitative to quantitative study of the volume expansion of anodized TiO2 nanotubes under constant voltage. It is helpful to understand the growth kinetics of porous oxides.

Numerical modelling of anodization reaction kinetics for TiO2 nanotubular-film growth in NH4F-based electrolytes

In most reports that propose mechanisms by which nanostructured TiO2 films are synthesised, little information about chemical reaction kinetics that lead to anodic current density transients is provided. Analyses of the anodic current signal have therefore been mostly experimental, leaving previously proposed nanotube development mechanisms unverified by fundamentals of chemical reaction kinetics. Without customising an existing current/conductivity or data-fitting function, the anodic current density is simulated in this work, using a mathematical model of anodization reaction kinetics. From a system of 14 differential equations, a numerical integration algorithm is formulated and executed. The results show current transients that have a strong correlation with measurements from seven samples grown in ethylene glycol-NH4F electrolytes. Having tested the model at limits of low and high NH4F content, this technique proves that the anodic current measured during TiO2 growth is actually an integral function of the kinetics of the redox reactions of water.

Manipulation of microstructures and the stability of white emissions in NaLuF4:Yb3+, Ho3+, Tm3+ upconversion crystals

A series of Yb3+, Ho3+, Tm3+ doped NaLuF4-based nanostructures and microstructures with controllable crystalline phases and morphologies were designed via adjusting the parameters of the oleic acid-assisted hydrothermal method. It is found that increasing the NH4F content and prolonging the reaction time to conduce the formation of β-NaLuF4 and the possible evolution process was proposed. Besides, the dependence of upconversion emission on sensitizer (Yb3+) concentration and pump power in Yb3+-Ho3+ and Yb3+-Tm3+ codoped β-NaLuF4 was investigated. On this basis, a tunable multicolor was achieved in NaLuF4:Yb3+/Ho3+/Tm3+ microrods. It is worth mentioning that white-light output with the calculated CIE color coordinate of (0.333, 0.330) was realized in the microrods, and the triply doped samples presented an outstanding capacity for color controlling and satisfactory color stability in the white region as well as lower threshold pump power density and lower optimal excitation pump power density for white-emission, which present great potential applications in the field of color display and white light-emitting diodes.

Ultrafast synthesis of bifunctional Er3+/Yb3+-codoped NaBiF4 upconverting nanoparticles for nanothermometer and optical heater

We reported a simple and ultrafast route to synthesize the bifunctional Er3+/Yb3+-codoped NaBiF4 upconverting nanoparticles. It was found that the phase composition and microstructure of the prepared samples were strongly dependent on the NH4F content. When the NH4F content was 14 mmol, after 1 min reaction at room temperature, the resultant compounds exhibited pure single phase and were composed of uniform spherical nanoparticles. Under 980 nm light irradiation, the synthesized nanoparticles emitted visible emissions originating from the intra-4f transitions of Er3+ ions and the involved upconversion luminescence mechanism was associated with the typical two-photon process. With the aid of the fluorescence intensity ratio technique, the optical thermometric behaviors of the studied nanoparticle based on the (2H11/2,4S3/2) thermally-coupled levels in the temperature range of 303–483 K were systematically analyzed and the maximum sensor sensitivity was determined to be about 0.0057 K−1 at 483 K. Furthermore, the internal heating properties of the resultant nanoparticles induced by the laser power source were also studied. With elevating the pump power from 159 to 658 mW, the temperature of the upconverting nanoparticles was improved from 304 to 464 K. These results suggest that the Er3+/Yb3+-codoped NaBiF4 upconverting nanoparticles are promising bifunctional luminescent materials for nanothermometer and optical heater applications.

Extraction of antimony and niobium with tributyl phosphate from fluoride-ammonium solutions

The purification of niobium from antimony impurity via extraction with tributyl phosphate from solutions of the HF + NH4F mixture has been studied. It has been established that at an HF + NH4F content equal to 7–16 mol/L and an NH4F concentration of 30% in the mixture, Sb extraction hardly changes (70–80% for one stage), and Nb coextraction abruptly decreases to values of less than 0.5%.

Fabrication of Ni-Ti-O nanotube arrays by anodization of NiTi alloy and their potential applications.

Nickel-titanium-oxide (Ni-Ti-O) nanotube arrays (NTAs) prepared on nearly equiatomic NiTi alloy shall have broad application potential such as for energy storage and biomedicine, but their precise structure control is a great challenge because of the high content of alloying element of Ni, a non-valve metal that cannot form a compact electronic insulating passive layer when anodized. In the present work, we systemically investigated the influence of various anodization parameters on the formation and structure of Ni-Ti-O NTAs and their potential applications. Our results show that well controlled NTAs can be fabricated during relatively wide ranges of the anodization voltage (5–90 V), electrolyte temperature (10–50°C) and electrolyte NH4F content (0.025–0.8 wt%) but within a narrow window of the electrolyte H2O content (0.0–1.0 vol%). Through modulating these parameters, the Ni-Ti-O NTAs with different diameter (15–70 nm) and length (45–1320 nm) can be produced in a controlled manner. Regarding potential applications, the Ni-Ti-O NTAs may be used as electrodes for electrochemical energy storage and non-enzymic glucose detection, and may constitute nanoscaled biofunctional coating to improve the biological performance of NiTi based biomedical implants.

Non-chapped, vertically well aligned titanium dioxide nanotubes fabricated by electrochemical etching

This paper reports on the fabrication of non-chapped, vertically well aligned titanium dioxide nanotubes (TONTs) by using electrochemical etching method and further heat treatment. Very highly ordered metallic titanium nanotubes (TNTs) were formed by directly anodizing titanium foil at room temperature in an electrolyte composed of ammonium fluoride (NH4F), ethylene glycol (EG), and water. The morphology of as-formed TNTs is greatly dependent on the applied voltage, NH4F content and etching time. Particularly, we have found two interesting points related to the formation of TNTs: (i) the smooth surface without chaps of the largely etched area was dependent on the crystalline orientation of the titanium foil; and (ii) by increasing the anodizing potential from 15 V to 20 V, the internal diameter of TNT was increased from about 50 nm to 60 nm and the tube density decreased from 403 tubes μm−2 down to 339 tubes μm−2, respectively. For the anodizing duration from 1 h to 5 h, the internal diameter of each TNT was increased from ∼30 nm to 60 nm and the tube density decreased from 496 tubes μm−2 down to 403 tubes μm−2. After annealing at 400 °C in open air for 1 h, the TNTs were transformed into TONTs in anatase structure; further annealing at 600 °C showed the structural transformation from anatase to rutile as determined by Raman scattering spectroscopy.

Photocatalysis and photoelectrochemical properties of tungsten trioxide nanostructured films.

Tungsten trioxide (WO3) possesses a small band gap energy of 2.4–2.8 eV and is responsive to both ultraviolet and visible light irradiation including strong absorption of the solar spectrum and stable physicochemical properties. Thus, controlled growth of one-dimensional (1D) WO3 nanotubular structures with desired length, diameter, and wall thickness has gained significant interest. In the present study, 1D WO3 nanotubes were successfully synthesized via electrochemical anodization of tungsten (W) foil in an electrolyte composed of 1 M of sodium sulphate (Na2SO4) and ammonium fluoride (NH4F). The influence of NH4F content on the formation mechanism of anodic WO3 nanotubular structure was investigated in detail. An optimization of fluoride ions played a critical role in controlling the chemical dissolution reaction in the interface of W/WO3. Based on the results obtained, a minimum of 0.7 wt% of NH4F content was required for completing transformation from W foil to WO3 nanotubular structure with an average diameter of 85 nm and length of 250 nm within 15 min of anodization time. In this case, high aspect ratio of WO3 nanotubular structure is preferred because larger active surface area will be provided for better photocatalytic and photoelectrochemical (PEC) reactions.

Effect of water and fluoride content on morphology and barrier layer properties of TiO2 nanotubes grown in ethylene glycol-based electrolytes

This paper studies the effect of H2O and NH4F content on morphology and barrier layer properties of TiO2 nanotubes grown by potentiostatic anodization in ethylene glycol-based electrolytes. The increase in these two variables leads to an increase in the chemical attack of the formed oxide. However, each of these variables plays a different role in the formation of TiO2 nanotubes. On the one hand, a higher percentage of H2O in the electrolyte leads to a transition from a nanoporous to a nanotubular structure, as well as to a greater diameter of the tubes and a decrease in their length and barrier layer thickness. In contrast, a higher NH4F concentration decreases nanotube diameter and increases their length modifying barrier layer properties due to insertion of F− ions into the lattice. This diminishes the barrier layer resistance, but increases both the adsorption and the diffusion coefficient of F− ions. The different roles of H2O and NH4F in film formation are also associated with the presence of sub-oxides detected by XPS.

Formation of anodic layers on InAs (111)III. Study of the chemical composition

The chemical composition of ∼20-nm-thick anodic layers grown on InAs (111)III in alkaline and acid electrolytes containing or not containing NH4F is studied by X-ray photoelectron spectroscopy. It is shown that the composition of fluorinated layers is controlled by the relation between the concentrations of fluorine and hydroxide ions in the electrolyte and by diffusion processes in the growing layer. Fluorine accumulates at the (anodic layer)/InAs interface. Oxidation of InAs in an acid electrolyte with a low oxygen content and a high NH4F content brings about the formation of anodic layers with a high content of fluorine and elemental arsenic and the formation of an oxygen-free InF x /InAs interface. Fluorinated layers grown in an alkaline electrolyte with a high content of O2− and/or OH− groups contain approximately three times less fluorine and consist of indium and arsenic oxyfluorides. No distinction between the compositions of the layers grown in both types of fluorine-free electrolytes is established.

Rapid microwave reflux process for the synthesis of pure hexagonal NaYF4:Yb3+,Ln3+,Bi3+ (Ln3+ = Er3+, Tm3+, Ho3+) and its enhanced UC luminescence

Pure hexagonal NaYF4:Yb3+,Ln3+ (Ln3+ = Er3+, Tm3+, Ho3+) crystals were obtained for the first time through a facile microwave (MW) reflux method at relatively low temperature (160 °C) and atmospheric pressure within only 50 min. By controllably increasing the NH4F content in the ethylene glycol (EG) solvent, the phase of as-prepared NaYF4:Yb3+,Ln3+ gradually transforms from cubic to hexagonal. Correspondingly, the up-conversion (UC) emission intensities of hexagonal NaYF4:Yb3+,Ln3+ (Ln3+ = Er3+, Tm3+, Ho3+) are increased by 10–12 times compared to those of the cubic phase. A possible growth mechanism for the phase transformation under these MW conditions has been proposed. Moreover, for the first time, we introduced Bi3+ ion into β-NaYF4:20%Yb3+,2%Ln3+ crystals. As expected, the UC emission of β-NaYF4:Yb3+,Ln3+,Bi3+ are about 10–40 times higher than those of Bi3+ free samples. It is found that tri-doping of Bi3+ doesn't change the basic emission of Ln3+ ions. XRD results gives evidence that tri-doping of Bi3+ ions can tailor the local crystal field and dissociate the Yb3+ and Ln3+ ion clusters, which is the main reason for the UC enhancement. This designed MW reflux method for the synthesis of β-NaYF4:20%Yb3+,2%Ln3+ can be applied to prepare other rare earth fluorides. The markedly enhanced UC luminescence through Bi3+ doping also provides an effective way to gain very bright UC emission.

Polyol-mediated solvothermal synthesis and luminescence properties of CeF 3, and CeF 3:Tb 3+ nanocrystals

CeF 3 and CeF 3:Tb 3+ nanocrystals were successfully synthesized through a facile and effective polyol-mediated route with ethylene glycol (EG) as solvent. Various experimental techniques including X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and photoluminescence (PL) spectra as well as decay dynamics were used to characterize the samples. The results indicated that the content of NH 4F and reactant concentrations were key factors in the product shape and size. Excessive NH 4F was necessary for the formation of hexagonal nanoplates. The specific morphology of product can be controlled by changing the NH 4F content and reactant concentrations. In addition, Tb 3+ doped-CeF 3 sample shows strong green emission centered at 544 nm corresponding to the 5 D 4– 7 F 5 transition of Tb 3+. Due to the decrease of nonradiative decay rate, the lifetime of 5 D 4 level of Tb 3+ become longer gradually upon increasing the size of product.

Electrochemical Formation and Corrosion Properties of Porous TiO<sub>x</sub> Biomaterials

Formation of porous TiOx layers on Ti during electrochemical etching in H3PO4, CH3COOH electrolytes modified by HF and NH4F was described. The anodization resulted in porous TiOx formation, useful in tissue growth and bone bonding. The pore dimensions increased due to the increase of HF or NH4F content in H3PO4 electrolyte. During anodization at 10 V for 30 min, when the HF content increased from 0.5 to 10%, the pore diameter increased from 30 nm up to 8 m, respectively. Anodization in CH3COOH electrolyte resulted in non-uniform etching with flat hexagonal islands with nanopores inside surrounded by micropores. Corrosion properties of the etched samples were investigated in Ringer’s solution at 37oC and were compared to the unetched sample. The best corrosion resistance showed the samples etched at 10 V for 30 min in 1M H3PO4 + 2% HF and 1M H3PO4 + 10% NH4F, what can be attributed to thick oxide layer. We find, that porous sample presented good biocompatibility with human osteoblasts.

Polyol-Mediated Synthesis of Hexagonal LaF3 Nanoplates Using NaNO3 as a Mineralizer

Hexagonal LaF3 nanoplates have been successfully prepared via a polyol-mediated route with ethylene glycol (EG) as solvent. NaNO3, which was introduced into the reaction system as a mineralizer, played an important role in the formation process of these nanoplates. The size of the as-prepared nanoplates could be tuned by changing the NaNO3 content and the NH4F content. A possible formation mechanism of these nanoplates has been proposed. The effects of other alkali metal nitrates (LiNO3, KNO3, RbNO3 and CsNO3) and solvents (water, methanol, diethylene glycol, and glycerol) on the products have also been explored. The increased chemical potential caused by the addition of NaNO3 in the reaction system made LaF3 nanoplates formed initially unstable and transform into stable regular hexagonal LaF3 nanoplates with a bigger size by a slow dissolution−recrystallization process.

The dependence of etch rate of photo-CVD silicon nitride films on NH 4F content in buffered HF : V. K. Rathi, Manju Gupta and O. P. Agnihotri. Microelectronics Journal, 26, 563 (1995).

The dependence of etch rate of photo-CVD silicon nitride films on NH 4F content in buffered HF : V. K. Rathi, Manju Gupta and O. P. Agnihotri. Microelectronics Journal, 26, 563 (1995).

The dependence of etch rate of photo-CVD silicon nitride films on NH 4F content in buffered HF

The influence was studied of the concentration of ammonium fluoride (NH 4F) on the etch rates of silicon nitride films deposited by the mercury-sensitized photochemical vapour deposition method. The composition of the buffered HF was varied between 0 and 40 weight percent (wt%) NH 4F with 2 to 12wt% hydrofluoric acid (HF). The etch rates as a function of buffered HF composition were measured for films deposited under various process parameters, viz. reactant gas ratio, substrate temperature and chamber pressure. The results of etch rates as a function of process parameters were correlated to variations in material density and silicon content (Si/N ratio) in the films.