Concentrated HF Solutions Research Articles

Способ кислотного извлечения радионуклидов из увеличенных навесок почвы с использованием автоклавного разложения

The specifics of the nuclear tests carried out on the territory of the former Semipalatinsk test site (STS) led to the emergence of various sites that differ in levels of radioactive contamination and radionuclide composition. This also led to a significant difference in the solubility of radioactive particles. Traditionally, the “Experimental Field” testing ground was characterized by the presence of sparingly soluble, vitrified particles, which was determined by the features inherent in ground tests of nuclear weapons, namely, the interaction of the high-temperature explosion region with soil particles. The presence of such sparingly soluble particles in the soil cover makes it impossible to use 6–8 М HCl and 7–8 М HNO 3 solutions which are most often used radiochemical analysis to determine the content of 90Sr and 239+240Pu. This paper considers methods for the acid extraction of anthropogenic radionuclides from soils of the STS using autoclaving. The aim of the research is to develop a method for the acid extraction of anthropogenic radionuclides from enlarged soil samples using autoclave decomposition. A blank soil sample was used to determine the optimal autoclave decomposition parameters; to test the acid extraction method, a soil sample was taken from the territory of the “experimental field” testing ground. The radionuclide 137Cs has been used as the most optimal indicator of acid extraction due to its strong fixation on clay and mineral soil particles and ease of detection by the gamma radiation. Autoclave decomposition has been carried out by dissolving the studied samples in a mixture of mineral acids (HCl, HNO3, HF, H2SO4). The specific activity of radionuclide has been determined on a BE3830 gamma spectrometer with a high-purity germanium detector (Canberra, USA), the amount of undecomposed soil residue has been determined gravimetrically (PA214C analytical balance, Ohaus, USA). The data obtained indicate that the most complete extraction of 137Cs occurs when using concentrated HF solutions (individually or in a mixture with other mineral acids). The degree of extraction of 137Cs varied from 85 to 100%. The worst results have been shown by the use of a mixture of acids 3HCl:HNO 3. In this case the degree of extraction of 137Cs did not exceed 20%. The efficiency of decomposition of soil samples weighing 10 g with a concentrated solution of HF at a temperature of 120 to 160 ºC was ≥95%.

Ionic equilibria in aqueous organic solvent mixtures. Speciation of hydrofluoric acid in several ethanol/water solutions

The presence of different species derived from hydrofluoric acid, together with the corresponding formation equilibria, has been investigated in 70% (w/w) ethanol/water solutions at analytical concentrations up to 5 mol L−1. In the same way, hydrofluoric acid solutions in 50% (w/w) (up to 2 mol L−1) and in 30% (w/w) ethanol/water (at 0.003–0.005 mol L−1) have been examined too. Conductometric and potentiometric (glass electrode and fluoride-ion-selective electrode) measurements have been used to demonstrate that HF and HF2− predominate in dilute solutions regardless the solvent composition, whereas HF and polyhomoconjugated species prevail in the concentrated ones.Thermodynamic dissociation and homoconjugation constants have been determined in hydrofluoric acid dilute solutions. The more ethanol is contained in the solvent, the higher is the formation constant for the homoconjugated species (between 1.1 and 1.3 as logKhomo). In concentrated HF solutions at 70% and 50% (w/w) ethanol/water medium, the polyhomoconjugated species H2F3− (in 70% and 50% ethanol/water) and H3F4− (in 70% ethanol/water) are formed, with successive formation constants between about 4.1 and 6.9 logarithmic units, being the values obtained for the first homoconjugated species (about 1.2 in logarithmic units), very close to those obtained in dilute HF solutions.

Influence of glass composition on the kinetics of glass etching and frosting in concentrated HF solutions

During etching of a multicomponent glass in concentrated hydrofluoric acid (HF) solutions, a crust can gradually appear on the glass surface, depending on the chemical etching parameters. This crust deposition is the result of the precipitation of hexafluorosilicate anions released by the glass dissolution reaction, with cations coming either from the etching solution or from the glass (like Na+, K+ or Ca2+). To understand the impact of this crust on the overall kinetics of both the etching and frosting process, we have studied the dissolution of four types of commercial glass substrates in etching solutions containing various concentrations of HF. The kinetics of the frosting process was investigated by two independent methods: on the one hand the chemical analysis by ICP-OES of the amount of Si dissolved from the glass as a function of time, and on the other hand the measurement of the glass weight loss with time. These two methods showed that the glass etching rate decreases with time as a result of crust formation which gradually becomes more protective. Both methods also indicated that increasing the amount of HF in the etching solution increases the etching rate in a non-linear way, and that the etching rate increases with the alumina content of the glass. A separate chemical analysis of the amount of Si present in the crust also revealed that the amount of HF in the etching solution has an impact on the amount of crust deposited on the glass surface. Finally, we have been able to rationalize all these kinetic data based on a semi-empirical quantitative etching and frosting model, allowing to extract a characteristic dissolution and precipitation constant (Kdiss and Kdep), respectively.

Controlled crystallization of photocatalytic active Bismuth oxyfluoride/Bismuth fluoride on SrO-Bi2O3-B2O3transparent glass ceramic

SrO-Bi2O3-B2O3 (SBBO) transparent glass ceramics are fabricated via conventional melt-quenching technique. Simply via surface fluorination of these glass ceramics, by using HF solution, we obtained the coating of cubic BiO0.51F1.98over SBBO glass ceramic. Further, with successive fluorination by using concentrated HF solutions the crystal structure of coating tends towards the cubic α-BiF3 structurefrom the cubic BiO0.51F1.98. Both the structures of cubic BiO0.51F1.98 and cubic α-BiF3possess totally different morphologies; first having compact spheres and the other shows densely packed cubes, respectively. Along with fluorination, besides the structural and microstructure transformations, we have attained the improved transparency. Synergy effects of structural transformation on the contact angle and the photocatalytic properties are studied. The fluorinated SBBO transparent glass ceramics can be used as self-cleaning structural application.

Synthesis and tribological property of Ti3C2T X nanosheets

The highly exfoliated Ti3C2T x nanosheets were synthesized by selective etching of the starting material of Ti3AlC2 using 40 % concentrated HF solution at 65 °C for 5 h. The effect of HF treatment temperature on the exfoliation degree was investigated by characterizing the morphology and microstructure of the Ti3C2T x nanosheets using SEM, XRD, and TEM. The results showed that the exfoliation degree of Ti3C2T x was increased with increasing temperature. The role of the termination groups produced during HF treatment on the exfoliation degree was discussed. The tribological property of the prepared Ti3C2T x nanosheets as additives in PAO8 base oil was measured. The maximum reduction of friction and wear was achieved under additive concentration of 0.8 wt% for the highly exfoliated Ti3C2T x nanosheets. The different effects of Ti3C2T x additives on the average friction coefficient and wear volume were analyzed, respectively.

Pseudocapacitive Negative Electrodes Based on Mxene Ti2CT x for High-Power Li-Ion Batteries

Development of rapid charging systems in Li-ion batteries is one of the urgent tasks for the widespread use of environmental-friendly electric vehicles towards a sustainable and green society. However, in spite of numerous efforts devoted to realizing the high-power systems, their power density is not satisfactory, partly due to the poor rate capability of the negative electrodes. A novel family of layered compounds, MXene (M n +1X n T x : M = Ti, V, Nb, etc.; X = C, N; n = 1-3; T = surface termination groups) has attracted increasing attention as negative electrode material with high-rate capability for Li-ion batteries.[1] MXene is the derivative phase of ternary layered compounds, MAX phase (M n AX n : A = Al, Si, S, etc.), first synthesized with concentrated hydrofluoric acid (HF) solution to extract selectively the A element.[2] Due to the tunable compositions and structures in MXene, there are huge choices for the development of the high-power electrode materials. Gogotsi and colleagues have recently discovered that lithium fluoride (LiF) and hydrochloric acid (HCl) solution also selectively extracts the A element from MAX phase, and Ti3C2T x synthesized with the LiF and HCl solution exhibits higher performance as pseudocapactior in aqueous systems than HF-synthesized Ti3C2T x .[3]Therefore, the synthetic method for MXene is a key process to determine its electrochemical property. In this work, we have successfully synthesized Ti2CT x with the LiF and HCl solution. The charge-discharge measurement reveals that the Ti2CT x delivers large reversible capability of 260 mAh/g in 1M LiPF6/EC-DMC (1:1, v:v) electrolyte, which is larger than that of Ti2CT x prepared with concentrated HF solution (180 mAh/g).[4] Reaction mechanism was investigated with combined approach using XANES, XRD, and 7Li-MAS-NMR. We construct the prototype full cell with the Ti2CT x and LiNi1/3Co1/3Mn1/3O2 negative/positive electrodes, and demonstrate that the Ti2CT x is the promising electrode material for the high-power Li-ion batteries. [1] M. R. Lukatskaya, et al. and Y. Gogotsi, Science 341, 1502 (2013). [2] M. Naguib, et al. and M. W. Barsoum, Adv. Mater. 23, 4248 (2011). [3] M. Ghidiu, et al. and Y. Gogotsi, Nature 516, 78 (2014). [4] J. Come, et al. and P. Simon, J. Electrochem. Soc. 159, A1368 (2012). Figure 1. Charge-discharge profile of Ti2CT x in 1M LiPF6/EC-DMC (1:1, v:v) electrolyte and comparison in the rate capability synthesized with different solutions. Figure 1

A method for the formation of Pt metal nanoparticle arrays using nanosecond pulsed laser dewetting

Nanosecond pulsed laser dewetting of Pt thin films, deposited on a dimpled Ta (DT) surface, has been studied here in order to form ordered Pt nanoparticle (NP) arrays. The DT substrate was fabricated via a simple electrochemical anodization process in a highly concentrated H2SO4 and HF solution. Pt thin films (3–5 nm) were sputter coated on DT and then dewetted under vacuum to generate NPs using a 355 nm laser radiation (6–9 ns, 10 Hz). The threshold laser fluence to fully dewet a 3.5 nm thick Pt film was determined to be 300 mJ/cm2. Our experiments have shown that shorter irradiation times (≤60 s) produce smaller nanoparticles with more uniform sizes, while longer times (>60 s) give large nanoparticles with wider size distributions. The optimum laser irradiation time of 1 s (10 pulses) has led to the formation of highly ordered Pt nanoparticle arrays with an average nanoparticle size of 26 ± 3 nm with no substrate deformation. At the optimum condition of 1 s and 500 mJ/cm2, as many as 85% of the dewetted NPs were found neatly in the well-defined dimples. This work has demonstrated that pulsed laser dewetting of Pt thin films on a pre-patterned dimpled substrate is an efficient and powerful technique to produce highly ordered Pt nanoparticle arrays. This method can thus be used to produce arrays of other high-melting-point metal nanoparticles for a range of applications, including electrocatalysis, functionalized nanomaterials, and analytical purposes.

Studies of the products from the reactions of co-acids containing concentrated HF and dilute HNO3 with Zircaloy-4

The pickling of Zircaloy with HF–HNO3 solution as an important surface treatment has been widely used in the nuclear materials industry. However, a thorough knowledge of the products from the reactions of HF–HNO3 system with Zircaloy still needs to be well established. In the present study, the acid solution containing HF (40wt.%) and HNO3 (65wt.%) with four volume ratios, 100:0, 94:6, 89:11 and 80:20, was employed and it reacted with Zircaloy-4 violently with gas releasing at an air pressure of 1bar, a relative humidity of 30±5% and a temperature of 25°C. Meanwhile, white products formed within minutes. These products were identified by using X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The usage of the concentrated HF solution leads to the formation of crystalline ZrF4(H2O), while the usage of co-acids containing concentrated HF and dilute HNO3 results in the formation of crystalline ZrF4(H2O) and NH4ZrF5. In addition, with decreasing the volume fraction of HF and increasing that of HNO3 in the co-acid used, the weight fraction of crystalline ZrF4(H2O) decreases while that of crystalline NH4ZrF5 increases in the products obtained.

Chemical Equilibrium Controlled Etching of MFI-Type Zeolite and Its Influence on Zeolite Structure, Acidity, and Catalytic Activity

Chemical etching with fluoride ions is a new approach for secondary porosity engineering of aluminosilicate zeolite frameworks. We show that diluted HF solutions extract preferentially aluminum from zeolite frameworks. The Brønsted acidity of ZSM-5 treated in such a way decreases, while its structure is unaffected after an HF treatment. With higher HF concentrations, the number of undissociated HF molecules and the concentration of HF2– ions, extracting indiscriminately Al and Si, increase. The addition of NH4F shifts the chemical equilibria to produce more HF2–, avoiding the use of highly concentrated HF solutions; it also suppresses HF dissociation. The etching selectivity of such solutions is concentration-independent and extracts indiscriminately both framework Si and Al. Zeolite dissolution in NH4F-HF solutions starts preferentially at small intergrowth domains and goes deeply in the crystals without a substantial increase of the external surface area. Macropores are produced without altering zeolite acidity. Hierarchical materials obtained by these two approaches are characterized extensively by complementary methods and the catalytic impact illustrated in the m-xylene conversion.

Surface Structure and Reactivity of Anatase TiO2 Crystals with Dominant {001} Facets

Hydrofluoric acid (HF)-assisted hydrothermal/solvothermal methods are widely used to synthesize anatase TiO2 single crystals with a high percentage of {001} facets, which are generally considered to be highly reactive. We have used Density Functional Theory calculations and first principles molecular dynamics simulations to investigate the structure of these facets, which is not yet well understood. Our results suggest that (001) surfaces exhibit the bulk-terminated structure when in contact with concentrated HF solutions. However, (1 × 4)-reconstructed surfaces, as observed in UHV, become always more stable at the typical temperatures, 400–600 °C, used to clean the as-prepared crystals in experiments. Since the (1 × 4)-reconstructed surfaces are only weakly reactive, our results predict that synthetic anatase crystals with dominant {001} facets should not exhibit enhanced photocatalytic activity, consistent with recent experimental observations.

Effects of the initial microstructure of Ti–Cu alloys on final nanoporous copper via dealloying

A Ti60Cu40 ribbon alloy before and after annealing was dealloyed in 0.03M HF and 0.13M HF solutions for 3.6–43.2ks, and a bi-continuous nanoporous copper (NPC) from tenths of nanometers to hundredths of nanometers was obtained. The as-spun amorphous Ti60Cu40 ribbon alloy has a glass transition temperature of 562K and crystallization temperature of 657K. The as-spun ribbon alloy has an amorphous structure, and that after annealing at 633K has a heterogeneous crystalline structure with coexistence of a particle size of 17nm and amorphous phases. The alloy after annealing at 713K has a completely crystalline structure consisting of dendritic Ti2Cu3 and Ti2Cu intermetallic phases. The phase segregation occurs at higher annealing temperatures. The dealloyed crystalline Ti60Cu40 ribbons exhibits multimodal nanoporous structure, which is corresponding to the initial microstructure. The order in uniformity in NPCs is confirmed to be as follows: amorphous alloys>nanocrystalline alloys>completely crystalline alloys, while their characteristic scale length of nanopores and ligaments differs from region to region where the initial microstructure and chemical composition are different. The diffusivity of Ti2Cu3 phase was lower than that of Ti2Cu phase by a factor of 13 due to their difference in chemical composition and microstructure. The difference in the NPC structure became weak in concentrated HF solution due to the higher diffusion in relatively large scales which might exceed the size of nanocrystalline grains. The final NPCs reflected the initial microstructure of the starting materials.

An effective approach for restraining electrochemical corrosion of polycrystalline silicon caused by an HF-based solution and its application for mass production of MEMS devices

This paper presents a novel method to effectively protect the structural material polycrystalline silicon (polysilicon) from electrochemical corrosion, which often occurs when the MEMS device is released in HF-based solutions, especially when the device contains a noble metal. This corrosion seriously degrades the electrical and mechanical performance as well as the reliability of MEMS devices. In this method, a photoresist (PR) is employed to cover the noble metal, which is electrically coupled with the underlying polysilicon layer. This PR cover can effectually prevent an HF-based solution from diffusing through and arriving at the surface of the noble metal, thus cutting off the electrical current of the electrochemical corrosion reaction. The polysilicon is well protected for longer than 80 min in 49% concentrated HF solutions by a 3 µm-thick AZ 6130 PR film. This fabrication process is simple, reliable and suitable for mass production of high-end micromechanical disk resonators. Benefiting from the technology breakthrough mentioned above, a novel low-cost microfabrication method for disk resonators with high performance has been developed, and the VHF polysilicon disk resonators with resonance frequencies around 282 MHz and Q values larger than 2000 at atmosphere have been produced at wafer level.

Synthesis of Ellipsoidal Hematite/Silica/Polymer Hybrid Materials and the Corresponding Hollow Polymer Ellipsoids

Ellipsoidal trilayer hematite/silica/poly(divinylbenzene) hybrid particles were prepared by distillation precipitation polymerization of divinyl benzene (DVB) in the presence of hematite/3-(methacryloxy)propyl trimethoxysilane (MPS)-modified silica (SiO(2)) core-shell particles as the seeds. The polymerization of DVB was performed in neat acetonitrile with 2,2'-azobisisobtyronitrile (AIBN) as initiator to coat the hematite/MPS-modified SiO(2) seeds through the capture of DVB oligomer radicals with the aid of a vinyl group on the surface of the hematite/MPS-modified silica core-shell particles in the absence of any stabilizer or surfactant. The other hematite/silica/polymer trilayer hybrid particles with different polarity and various functionality, such as hematite/silica/poly(ethylene glycol dimethacrylate) and hematite/silica/poly(divinyl benzene- co-methacrylic acid) could also be prepared by this procedure. Hematite/silica/poly( N, N'-methylenebisacrylamide) composite particles could be prepared with unmodified hematite/silica particles as seeds. Hollow poly(divinyl benzene) (PDVB) and poly( N, N'-methylenebisacrylamide) (PMBAAm) ellipsoids with movable hematite cores were subsequently developed after the selective etching of the silica midlayer in diluted hydrofluoric acid from hematite/silica/PDVB and hematite/silica/PMBAAm trilayer hybrids. Hollow PDVB ellipsoids were obtained by removal of the silica midlayer and hematite core of the trilayer hybrids with concentrated HF solution. The resultant trilayer hybrid particles and hollow polymer ellipsoids were characterized by transmission electron microscopy and vibrating sample magnetometer.

Photoluminescence properties of porous silicon layers prepared by electrochemical etching in extremely dilute HF solutions

Dilute HF solutions with concentrations down to 0.03% have been used to obtain luminescent porous silicon (PSi) layers on p-type Si wafers. The experimental results show that with a constant etching time of 30 min, PSi layers with sufficient luminescence efficiencies can be formed for HF concentrations as low as 0.1%. Because of a significantly lowered critical current density, only very low etching current densities of ≤0.1 mA cm−2 can result in the formation of luminescent PSi samples in 0.1% HF solutions. A notable result is that these low etching current densities cannot be used to form luminescent PSi layers in concentrated ( ≥1%) HF solutions. The behavior of PL intensity as a function of etching current density has been analyzed over a wide range of HF concentration. The PL intensity is determined by the ratio of the etching current density to the critical current density, suggesting that the presence of silicon oxides plays an important role in the formation of luminescent Si nanostructures in PSi layers.

Electrochemical Etching of n-Type 6H-SiC Without UV Illumination

Deep etching of n-type 6H-SiC using a two-step etching process has been studied. First, anodization of 6H-SiC in an HF electrolyte (2 wt.%) without ultraviolet light is applied to form a deep porous layer with the desired dimensions. Then, a thermal oxidation process is used to oxidize this porous layer. The oxidized layer is then removed in a concentrated HF solution. In the experiments, the etching parameters electrolyte concentration and current density are optimized in order to obtain a uniform pore size and hence, a smooth etched surface. After adjusting these parameters, the porous layer formation experiments are carried out at 20/spl deg/C in a 2 wt.% HF electrolyte using a current density of 50 mA/cm/sup 2/. The corresponding porous layer formation rate is about 1.1 /spl mu/m/min. To demonstrate the capabilities of this SiC bulk micromachining process, deep circular cavities are fabricated in n-type 6H-SiC substrates.

Defect-free wet etching through pyrex glass using Cr/Au mask

This paper reports the highest etch depth of annealed Pyrex glass achieved by wet etching in highly concentrated HF solution, using a low stress chromium–gold with assistance of photoresist as masking layer. The strategies to achieve that are: increasing the etch rate of glass and simultaneously increasing the resistance of Cr/Au mask in the etchant. By annealing the Pyrex glass and using a highly concentrated HF acid, a high etch rate can be obtained. Furthermore, a method to achieve a good resistance of the Cr/Au masking layer in the etching solution is to control the residual stress and to increase the thickness of Au deposition up to 1 μm. In addition, the presence of a hard baked photoresist can improve the etching performance. As a result, a 500-μm thick Pyrex glass wafer was etched through.

Galvanic effects in Si-based microelectromechanical systems: Thick oxide formation and its implications for fatigue reliability

Nanometer-scale reaction layers have a profound impact on the fracture and fatigue resistance of the Si films used in microelectromechanical systems (MEMS). This letter presents experimental evidence that thick (i.e., greater than 10nm) oxides can form at room temperature during manufacturing due to a galvanic effect between n+-type Si and Au. The growth of such oxides in concentrated HF solutions that are usually associated with oxide dissolution can be predicted from the measured current density–voltage (i–V) behavior and geometry of the galvanic couple. These results can account for unexplained findings in the literature and can be used to improve the performance of MEMS.

Theoretical treatment of the complexation of element 106, Sg, in HF solutions

SummaryFully relativistic density-functional calculations of the electronic structures of various oxo-fluoro-complexes were performed for group-6 elements Mo, W, and element 106, Sg. Based on the electronic density distribution data, relative values of the free energy change of complex formation reactions in HF solutions have been defined. On their basis, sequences in the sorption of Mo, W, and Sg by an anion exchange resin (Kdvalues) have been predicted. The results show a complex situation depending on the acid concentration and pH of the solution. The predicted trends are in agreement with the known data for Mo and W. The following sequence in the sorption of Mo, W, and Sg by an anion exchange resin from 5×10-3HF/0.1 M HNO3solutions is predicted: Mo>Sg>W. A very good separation of Sg from Mo and W is expected at concentrated HF solutions, where MOF5-is formed. The sequence of the sorption in this case should be Sg ≫W > Mo.

Initial stages of porous silicon formation: An in‐situ investigation of the electronic properties and surface morphology

In-situ photovoltage (PV) and photoluminescence (PL) are used to obtain information about the band bending of Si and the PL quenching by nonradiative (nr) surface recombination during the beginning of electrochemical etching of silicon in aqueous fluoride solutions. The onset of the porous silicon formation in diluted NH4F solution is accompanied by an increase of nr surface recombination, by a development of positive charged surface states and by a strong increase of the current density. The evolution of hydrogen and its penetration into near surface regions lead at first to the passivation and with ongoing time to the formation of nr surface defects. The lowest rate of nr surface recombination has been observed in highly concentrated HF solution just in the beginning of the electrochemical etching process. Therefore, intermediates of these reactions are rather of ionic nature than like Si dangling bonds. Scanning electron microscopy reveal the slight roughening of the Si surface and infrared spectroscopy confirm that these surfaces are always hydrogenated. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Band Bending and Nonradiative Recombination at Si Surfaces during Electrochemical Treatment in Aqueous Fluoride Solution

In situ photovoltage (PV) and photoluminescence (PL) are used to obtain information about the band bending of Si and the PL quenching by nonradiative (nr) surface recombination during electrochemical treatments of silicon in aqueous fluoride solutions. The onset of the porous silicon formation in diluted solution is accompanied by a decrease of nr surface recombination. Intensive electrochemical etching leads to an increase of nr surface recombination, to positively charged surface states, and to a strong increase of the current density. The increase of the current density is higher than expected for an ideality factor of 1, which gives evidence for a decrease of the effective barrier height during porous silicon formation. The evolution of hydrogen and its penetration into near surface regions lead to the formation of nr surface defects. The lowest nr surface recombination in the regime of porous silicon formation has been observed in higher concentrated HF solution even at high current densities. Therefore, the intermediates of the reactions which lead to pore formation are more of an ionic nature than like Si dangling bonds. Negatively polarized intermediate states (charged species or dipoles) arise when the electrolyte reaches the Si surfaces during the etchback of oxide layers. © 2002 The Electrochemical Society. All rights reserved.