Acidic Fluoride Media Research Articles

Highly hydrophobic zeolite ZSM-8 with perfect framework structure obtained in a strongly acidic medium

The synthesis of zeolites in a strongly acidic medium is a new and nearly unexplored chemistry field. In addition to the previously reported silicalite-1 and the several clathrasils, this paper elaborates that one more material of unique properties could be successfully crystallized in the strongly acidic fluoride medium. A pure-silica zeolite ZSM-8 has been obtained using tetraethylammonium ion as a structure-directing agent. Rietveld refinement proves that ZSM-8 is of the MFI topology, i.e., it is another silicalite-1 but different in framework distortions. Moreover, the discrete and uniform crystals of the acidic-medium synthesized pure-silica ZSM-8 possess a substantially perfect framework structure and thus hydrophobic surfaces. This remarkable character renders the material an ideal adsorbent for discriminating molecules on the polarity basis, which is desired in developing green separation processes. C3H8, CH4, and CO2 are chosen as examples respecting their different polarities and tested for the adsorptive parameters.

Acidic properties of Al-rich ZSM-5 crystallized in strongly acidic fluoride medium

The fluoride medium synthesis of zeolites, especially in the strongly acidic regions, yields products with unique characteristics such as ultra-hydrophobic surfaces and defect-free lattices. However, it is often difficult to incorporate aluminum into the framework of zeolites in this medium, limiting the acquisition and development of catalytic materials. The present paper shows that inter-zeolite transformation is efficient for obtaining ZSM-5 zeolites with lower Si/Al ratios in a strongly acidic fluoride medium. The critical point is that the parent zeolites can retain some locally ordered fragments during their dissolution, including framework Al atoms, and incorporate them into the ZSM-5 product. The acidic properties of the trans-crystallization products obtained in an acidic medium display significant differences from the traditional ZSM-5 with a similar Si/Al ratio in the amounts, strength, and types of the acid sites; thus, they behave differently in typical acid-catalysed reactions, such as dehydration of alcohols.

Analysis on Temperature Dependent Corrosion of Tantalum in Acidic Fluoride Media

Tantalum (Ta) exhibits excellent corrosion resistance in most aqueous media as it forms chemically inert thin oxide layer when exposed to air or water. Due to the high corrosion resistance, Ta is used in chemical industries as lining material for piping, tanks and vessels (1). Although Ta possess excellent corrosion resistance, it dissolve very slowly in acidic fluoride media. In the present work, the corrosion rate of Ta in acidic fluoride media at different temperatures were measured from weight loss experiment and compare with the results of electrochemical Tafel extrapolation.The weight loss experiments were performed with Ta sheets of 2.5 × 2.5 cm with 1 mm thickness. Prior to the experiments, Ta sheets were polished with 0.3 µm alumina powder and cleaned by ultrasonicated in ethanol, rinsed in deionized water and dried with nitrogen gas. The temperature of the HF solution was maintained using a water bath and double-walled polypropylene beaker set up. The weight loss experiments were performed in various concentration of HF from 250 to 1000 mM HF and the experiments were also conducted by adding 250 mM KF/ H2SO4 into 500 mM HF solution to study the key HF dissociated species responsible for the corrosion of metal. The weight loss was measured at different temperature of the solution varied from 30 to 60 °C for any concentration of HF solution. In most of the experiments, the weight difference before and after the experiments were used to calculate the dissolution rate. A few experiments were also conducted wherein the weight was continuously monitored, so that the dissolution rate can be monitored as a function of time.The experimental dissolution rates of Ta in various concentrations of HF at different temperatures are shown in Fig. 1. An analysis of the results shows that corrosion of Ta is assisted by both undissociated HF and species. The activation energy of the reactions were estimated using Arrhenius expression, and the results show that Ta corrosion in acidic fluoride media is very sensitive to temperature and will proceed at rapid rates at high temperatures.The corrosion of Ta in acidic fluoride media at room temperature was also characterized using several electrochemical techniques, viz. Tafel extrapolation, electrochemical impedance spectroscopy and linear polarization. A comparison of the results of weight loss experiments show that Tafel extrapolation over predicts the corrosion rate, in most of the conditions. The EIS spectra obtained at OCP shows multiple loos (capacitive and inductive) in most of the concentrations and equivalent circuit analysis was used to estimate the R P value. The XPS analysis was also performed to understand the different metal oxide species present during the corrosion process. The results indicates that the dissolution process of Ta occurs viz. multiple intermediate steps.

Studies on Corrosion of Titanium and Air-Oxidized Titanium in Fluorinated Nitric Acid

Titanium (Ti) is a lustrous transition metal which possesses excellent corrosion resistance in several aggressive environments but is attacked by acidic fluoride media. In this work, the addition of fluoride ion to nitric acid on the corrosion behavior of Ti and air-oxidized Ti was studied. Air oxidation of Ti at 800 °C for 5 h resulted in the formation of an intact rutile TiO2 layer with a thickness of about 35 μm. The corrosion resistance was investigated from the polarization resistance (R_{text{P}}) values which were obtained from linear polarization and electrochemical impedance spectroscopy (EIS) measurements. A decrease in the R_{text{P}} value of Ti with the addition of fluoride ion in nitric acid was observed, and this indicated the formation of an unstable layer, which resulted in accelerated dissolution. The several order increase in the R_{text{P}} value of air-oxidized Ti in nitric acid containing fluoride ions pointed the improved resistance to corrosion. The diffusivity of species through the air-oxidized TiO2 layer was estimated from the electrochemical equivalent circuit (EEC) analysis of EIS data. Even though the diffusivity increased by several orders with the addition of fluoride ions, air-oxidized Ti provided better protection against nitric acid containing fluoride ions than Ti. The corrosion rates of Ti and air-oxidized Ti in boiling nitric acid containing fluoride ions were also estimated from weight loss experiments. Even in boiling fluorinated nitric acid, air-oxidized Ti provided better corrosion protection, with corrosion rates of about 1000 times less than that of Ti.

Mechanistic Analysis of Anodic Dissolution of Zr in Acidic Fluoride Media

Anodic dissolution of Zr in 10 mM HF was investigated using potentiodynamic polarization and electrochemical impedance spectroscopy. The surface state of the electrode was analyzed using X-ray photon spectroscopy, and the surface morphology was characterized using atomic force microscopy. EIS data acquired at multiple dc potentials were subjected to mechanistic analysis. Reaction mechanism analysis approach reveals that at least four intermediates are required to describe the observed results. The intermediates are likely to be Zr sub-oxides, oxyfluorides and ZrO2. The proposed mechanism successfully predicts the major features observed in polarization and impedance spectra. At low overpotentials, the fractional surface coverage of Zr3+ species is higher than that of Zr4+ species, and the electrochemical dissolution rate is higher than the chemical dissolution rate. As the overpotential increases, the surface is covered with Zr4+ species and chemical dissolution rate becomes comparable to electrochemical dissolution rate. Although the surface is covered with Zr4+ species at higher overpotentials, significant chemical and electrochemical dissolution processes continue to occur and hence Zr is not protected in acidic fluoride media under anodic conditions.

Synthesis and characterization of Cr-ZSM-5 composite in fluoride medium and its performance in oxidation reaction of 2-mercaptobenzotiazole

PurposeThe purpose of this paper is to synthesize Cr-ZSM-5 using hydrothermal synthesis in acidic fluoride medium and the investigation of its catalytic performance in heterogeneous oxidation of 2-mercaptobenzothiazole in the presence of H2O2.Design/methodology/approachThe framework was characterized by chemical composition, FT-IR, UV-visible spectroscopy, N2 adsorption (BET), and diffraction x-ray (DRX). FT-IR spectroscopy shows the characteristic band of Si-O-Cr linkage around 960-990 cm−1 and bands around 688-690 and 627 cm−1, indicating the presence of extraframework chromium oxide. UV spectra show a shoulder around 244-257 and bands around 360-380 nm, assigning the tetrahedral coordinated environment in (O2−=Cr6+) − (Cr5+-O−1), charge transfer transition state (CT) of isolated Cr (VI) inside the structure of Cr-ZSM-5.FindingsThe catalytic degradation of 2-mercaptobenzothiazole by H2O2 heterogenous oxidation using catalyst Cr-ZSM-5 showed the formation of bis (benzothiazoyl) disulfide with 95 percent yield.Originality/valueThe application of this process in the environment (catalysis, adsorption, and degradation of organic compounds).

Mechanistic Analysis of Zr Dissolution in Acidic Fluoride Media Using Electrochemical Impedance Spectroscopy

Zirconium and its alloys are widely used in chemical industries due to the high resistance to corrosion and H2 embrittlement in many aggressive media [1,2]. These properties are exhibited by the metal due to the formation of stable, protective passive film of ZrO2 [3]. Although Zr metal exhibits excellent corrosion resistance in most of the harsh environments, it readily dissolves in acidic fluoride media [4]. Zr dissolution mechanism in acidic fluoride medium was studied using the reaction mechanism analysis of potentiodynamic polarization data in various HF solutions [5]. A four step mechanism with two adsorbed intermediates predicts the major characteristics of the polarization curves. It was shown that the remaining HF and HF2 - are the key species involved in the dissolution of Zr in acidic fluoride medium. The confidence in the reaction mechanism identification will be more if the model can predict the impedance spectra observed at different dc potentials in different concentrations of HF. Compared to the polarization data, which is obtained at the dclimit, EIS contains more information about the system since it is acquired in a wide range of frequencies. In this work, Zr dissolution in HF was investigated using electrochemical impedance spectroscopy and the data was analyzed in the frame work of reaction mechanism analysis. A rotating disk zirconium electrode (5×4 mm) was used in a standard three electrode cell with Ag/AgCl (in 3.5 M KCl) as reference and Pt wire as counter electrode. PARSTAT 2263 (AMETEK) was used as the electrochemical work station and electrode was rotated at 900 rpm (Pine Instruments). The electrochemical impedance data was obtained by superimposing a small amplitude ac potential perturbation of 10 mV rms on the dc potential in a frequency range of 100 kHz to 0.1398 Hz. EIS measurement was performed on Zr dissolving in 10 HF solutions with 0.1 M Na2SO4 as supporting electrolyte. The EIS spectra was acquired at two different dc potentials in the active region (0.075 and 0.1 V vs. OCP) and at two different dc potentials in the passive region (0.5, 0.8 V vs. OCP). These dc potentials were chosen from the corresponding polarization curves. The morphology of the surface was analyzed using atomic force microscopy (AFM). Figure 1a and Fig. 1b shows the spectra observed in the active and passive region, in 10 mM HF. Impedance spectra in the active region show three loops with an inductive loop in the low frequency end. In the passive region, an inductive loop and a capacitive loop is observed in the mid frequency region and an apparent negative resistance and capacitance in the low frequency end. The observed EIS spectra were validated using Kramer's Kronig Transform (KKT). The spectra were modelled using electrochemical equivalent circuit (EEC) containing three Maxwell elements and the circuit parameters were calculated. A four step mechanism with two intermediates was able to predict most, but not all, of the major features observed in the impedance spectra. The number of Maxwell elements required to model the spectra indicate that at least one more adsorbed intermediate should be incorporated in the mechanism to explain all the features observed.

Effect of Fluoride Concentration on Niobium Anodic Dissolution

Niobium is one of the valve metals [1] and, when exposed to air or water, it readily forms a chemically inert and mechanically strong oxide layer [2]. Nb is used as a popular alloying element in carbon and steel industry [3] due to its high corrosion resistance. Nb is also used for fabricating super conducting radio frequency cavities [2] used in particle accelerators which demand very smooth surfaces. The passive layer on Nb is stable in most aqueous media. However, it can be attacked by hydrofluoric acid [4, 5] or strong alkali [6]. Nb anodic dissolution in HF shows clear active and passive regions, followed by an increase and a large current plateau signifying transpassive dissolution [5]. The polarization and EIS data of Nb in warm alkali and acidic fluoride media was analyzed in the framework of surface charge approach combined with interfacial reactions [4]. The species that are reported to be formed in aqueous HF solutions are HF, H+, F-, HF2 - and H2F3 - [7] and the dissociation of HF involves the following reactions. HF ↔ H+ + F-, HF + F-↔ HF2 -, along with HF + HF2 - ↔ H2F3 - The corresponding equilibrium constants are respectively, 6.84 × 10-4, 5 and 0.58 [7]. In this study, we present the experimental results of anodic dissolution of Nb in different concentrations of HF and propose a kinetic model to match the observed results. All the experiments were carried out using Nb rotating disc electrode (RDE) in a standard three-electrode cell with Ag/AgCl (3.5 M KCl) as reference and Pt wire as counter electrode. HF concentration was changed between 50 mM and 1000 mM. In order to reduce the solution resistance effects, a supporting electrolyte (1 M Na2SO4) was used. The electrode rotational speed was maintained at 900 rpm. Before each experiment, the electrode was mechanically polished with 400, 800 and 1200 grade emery paper followed by ultra-sonication in ethanol and DI water. Potentiodynamic polarization data were acquired by sweeping the potential from open circuit potential (OCP) to 1 V above OCP, at 2 mV/s scan rate. Figure 1A shows the typical anodic behaviour of Nb measured as described above in different concentrations of HF used. The peak current values shift towards right with increase in HF concentration. The potential at which the peak occurs also shifts to more anodic values at higher HF concentration. The concentration of all the constituent species increases with increase in nominal HF concentration [8]. Therefore, few experiments were carried out adding H2SO4and KF to aqueous HF solutions to isolate the effect of different species on the dissolution (Fig 1B). Reaction mechanism analysis was employed to understand the dissolution mechanism of Nb in HF. A four step mechanism involving two dissolution steps, chemical and electrochemical was proposed to explain the results. The model captures all the major characteristics of the polarization curves.

Corrosion Behavior of Ti and Zr in HF

Titanium and zirconium are used extensively in the industry due to their exceptional corrosion resistance in most of the harsh environments [1, 2]. Ti is used in aerospace industry, biomedical implantations, and in several chemical industries due to its low density, high mechanical strength and corrosion resistance [1]. Zr is mainly used as a fuel cladding material in nuclear industries and in chemical industries due to its high resistance to corrosion and H2embrittlement [2]. Although these materials exhibit high corrosion resistance in harsh acidic and alkaline environment, they readily dissolve in acidic fluoride media. The anodic polarization curves of Ti and Zr in HF [3,4] show that the active and passive regimes are clearly distinguishable . A comparison of these results also shows that Ti offers better passivation than Zr, in HF media. In this work, the effect of varying the concentration of HF on the corrosion rate was investigated using potentiodynamic polarization, linear polarization techniques and electrochemical impedance spectroscopy at open circuit potential (OCP). A rotating disk electrode of Ti/Zr was used in a three electrode cell with Ag/AgCl (in 3.5 M KCl) as reference and Pt wire as counter electrode. HF concentration was varied from 10 - 1000 mM for the corrosion rate estimation of Ti. Zr corrosion analysis was performed in 5 - 200 mM HF. In all the experiments, Na2SO4was used as the supporting electrolyte and the electrode was rotated at 900 rpm to suppress the mass transport limitations. Potentiodynamic polarization experiments were conducted at a scan rate of 2 mV/s, starting at 250 mV below open circuit potential (OCP) and ending at 250 mV above OCP. In case of Zr electrode dissolving in 100 and 200 mM HF, the scan rate was maintained at 20 mV/s, since a lower scan rate resulted in excessive corrosion of the material during the experiments. The potentiodynamic polarization curves of the Ti and Zr respectively are presented in Fig. 1A and Fig. 1B. The cathodic and anodic branches of potentiodynamic polarization were extrapolated to estimate the corrosion current using Tafel kinetics. HF is a weak acid and it forms multiple species in aqueous solutions. The concentration of these species was correlated and it is difficult to delineate the contribution of each species towards the corrosion current. Hence few experiments with addition of KF/H2SO4 were also carried out. The corresponding changes in the open circuit potential and corrosion current estimates were also analyzed. The concentration of various species, viz. remaining HF, H+, F-, HF2 - and H2F3 -, in the solution were calculated using the equilibrium constants [5]. The value of the polarization resistance (Rp) was calculated from the slope of linear polarization data. Electrical equivalent circuit fitting was used to calculate the polarization resistance from EIS spectra. The corrosion current was estimated using the values of Rp in all solutions. The values of the corrosion current estimated from the different techniques were compared. The correlation between the corrosion current and various solution species was also analyzed. The results indicate that the corrosion current is proportional to and where α and β are close to the exponents obtained from the mechanistic analysis of the polarization curves [3, 4]. The morphology of the electrodes after the passivation was studied using atomic force microscopy (AFM) and it shows less roughness compared to the active region of dissolution. The AFM images showed lesser passivation for Zr metal compared to Ti. These observations provide insight to the difference in the corrosion prevention of the two metals regardless of the same group in the periodic table.

Anodic Dissolution of Ti in Acidic Fluoride Media

Anodic dissolution kinetics of Ti in HF medium was investigated using potentiodynamic polarization experiments. The concentration of HF was varied and the effect of changing the H+ and F- ions independently by the addition H2SO4 and KF was also investigated. In all the cases, a clear active and passive regions were observed, and the peak current and potential exhibiting a systematic trend with increasing HF concentration. Scanning electron micrographs of electrodes held in the active and passive regions were acquired to characterize the surface morphology. Several mechanisms were evaluated to model the polarization data. A mechanism involving an chemical dissolution step in parallel with an electrochemical dissolution step was proposed to explain the results. The model captures all the major characteristics of the polarization curves. Based on the results, it is proposed that HF2 - is the key species which impacts the electrochemical dissolution and HF is the one which impacts the chemical dissolution process.

Kinetics of anodic dissolution of Zr in acidic fluoride media

The kinetics of anodic dissolution of Zr in hydrofluoric acid (HF) was investigated using potentiodynamic polarization experiments. At lower potentials, an active region with a rapid increase in current with the potential was observed and at higher potentials, a large passivation current plateau was observed. The current decreased only slightly with potential in the passive region. Scanning electron micrographs confirmed that the passivation is incomplete in the region where current exhibits a plateau and x-ray photoelectron spectroscopic analysis showed that oxides and oxyfluorides are present on the surface. A four-step mechanism with two adsorbed intermediate species was evaluated and the model captures the essential characteristics of the polarization plots. The results suggest that \( {HF}_2^{-} \) species participates in the chemical dissolution step and actual HF participates in the electrochemical dissolution step.

Anodic Dissolution of Titanium in Acidic Fluoride Media - Effect of Varying Fluoride Concentration

Ti and Ti alloys are well known for their excellent chemical stability and biocompatibility. Ti metal shows high corrosion resistance due to the presence of protective native oxide film. Although TiO2 layer is stable and protective in action, it can be dissolved in HF [1-3]. Anodic dissolution of Ti in acidic fluoride medium shows three distinct regions, viz. active, passive and transpassive [1,2]. The anodic current increases with potential in the active region and it decreases with potential in the passive region. The current increases slowly with potential in the transpassive region. Transpassive dissolution has been extensively studied [1,3]. Using potentiodynamic polarization and electrochemical impedance spectroscopic data, the anodic dissolution of Ti in 0.1 M HF in the active and passive regime has been investigated recently and a four step mechanism has been proposed [2]. HF is a weak acid and is known to form various species in solution, as described in the equation below. HF ↔ H++F- HF+F-↔HF2 - HF+HF2 -↔H2F3 - The equilibrium constants for the above reactions are K1 = 6.84 × 10-4, K2 = 5 and K3 =0.58 respectively [4]. Although a four step mechanism has been proposed to explain the anodic dissolution of Ti in HF[2], the influence of HF concentration has not been incorporated in the mechanism yet. It is important to quantify each species present in solution and their influence on individual steps of the reaction. In this work, the effect of varying the concentration of HF on Ti dissolution was investigated. A rotating disk Ti electrode was used in a three electrode cell with Ag/AgCl (in 3.5 M KCl) as reference and Pt wire as counter electrode. HF concentration was varied from 25 mM to 1000 mM. In all the experiments, 1 M Na2SO4 was used as the supporting electrolyte. The electrode was rotated at 900 rpm. Potentiodynamic polarization experiments were conducted at a scan rate of 20 mV/s, starting at -250 mV below open circuit potential and ending in the transpassive regime on the anodic side. A few experiments were also conducted at a scan rate of 2 mV/s, in solutions of 50 mM, 500 mM and 1000 mM HF concentrations, and the current response was the same at both 2 mV/s and 20 mV/s scan rates. The results are presented in Fig. A. The anodic polarization curves (Fig. A) show that the active and passive regimes are clearly visible in all the HF concentrations studied here. The inset shows the results at lower concentrations, expanded for clarity. The peak current value increases with an increase in HF concentration. The location of peak potential also shifts to more anodic values at higher HF concentration. The potential and logarithm of the absolute value of current in the cathodic and anodic regime (active region only) are presented in Fig. B. The cathodic and anodic branches of potentiodynamic polarization were extrapolated to estimate the corrosion current. The estimated corrosion current values and the peak current values near active-passive transition were correlated with nominal HF concentration and as well as with the concentration of individual species occurring at equilibrium. In order to identify the effect of individual species unambiguously, a few experiments were also run by adding H2SO4 or KF to the experimental solutions (results not shown here). The corresponding changes in the peak current values and corrosion current estimates were also analysed. The results indicate the corrosion current and peak current can be expressed as proportional to [H+]α and [F-]β , where α is 0.72 and β is 0.92. This suggests that undissociated HF is likely to play a major role in Ti dissolution.

Electrochemical behaviour of stainless steel in acidic fluoride media

Abstract The growth mechanism and electrochemical properties of an oxide film on AISI 304 grade stainless steel were studied in 0.01 and 0.1 mol L−1 fluoride solutions with different pH values (4.5, 5.5, 6.5) by means of electrochemical techniques. The anodic growth and stability of the oxide film on the stainless steel were characterized using cyclic voltammetry. Potentiodynamic analysis suggests that the oxide film growth occurs according to the high-field mechanism. Electric field strength, high-field growth exponential law constants, ionic conductivity through the film and half jump distance were determined. The electrochemical properties of the oxide film, formed spontaneously at the open circuit potential, were studied using electrochemical impedance spectroscopy. The results showed that the fluoride concentration has more considerable influence on the dissolution rate and the resistance of the oxide film than the pH.

Simplified synthesis of isomorphously nickel substituted ZSM-5

Isomorphously nickel-substituted nano-crystalline ZSM-5 is synthesized in the absence of acidic aqueous fluoride medium incorporating simple and low-cost metal inorganic salt precursor NiCl2.6H2O instead of large organic cationic salt like bis (tetraethyl ammonium) tetrachloronickelate (II) with less water quantity to minimize the synthesis waste. PXRD, FT-IR, TG/DTG, XPS, UV–Vis DRS, SEM, TEM, ICP and N2 adsorption-desorption techniques were used to confirm the presence of nano-crystalline material having a MFI structure and heteroatom substitution. The unit cell dimensions increase with increasing levels of nickel substitution. The crystallite size of as synthesized samples was in the range of 60–75 nm, which increased to 60–160 nm after calcination at 550°C. Percentage crystallinity and crystallite size increases with increasing nickel substitution level up to 0.17 mol and beyond that the material becomes amorphous.

Corrosion behavior of a new titanium alloy for dental implant applications in fluoride media

The effect of fluoride ion concentration and pH value on the corrosion behavior of Ti–30Cu–10Ag (wt.%) alloy, which is a new titanium alloy characterized by its low melting point, was examined by electrochemical methods in artificial saliva solutions. The corrosion behavior of pure Ti and Ti–6Al–4V were also examined for comparison. Fluoride ions affect the corrosion behavior of Ti and its alloys and the severity of the attack depends on both the concentration of fluoride ion and the pH value. High corrosion rates were observed in neutral solutions with high fluoride concentrations and in acidic solutions with low fluoride concentrations. Increased fluoride concentration leads to increased thickness and/or porosity of the oxide layer, which reduces its corrosion protection. The presence of Cu and Ag in the Ti–30Cu–10Ag alloy has a beneficial effect on its corrosion behavior, especially in acidic fluoride media.

Fluorine Route Synthesis of Montmorillonites Containing Mg or Zn and Characterization by XRD, Thermal Analysis, MAS NMR, and EXAFS Spectroscopy

EXAFS spectroscopy, combined with X-ray powder diffraction, chemical and thermal analysis, scanning electronic microscopy, 29Si, 27Al, and 19F MAS NMR spectroscopy, are used in the structural analysis of montmorillonites, synthesised in an acidic fluoride medium. Hydrothermal synthesis performed under mild conditions (493 K, autogenous pressure) enables the formation of montmorillonite clays, containing Al and Zn or Mg in the octahedral sheet. It is shown that montmorillonites can only be synthesized in a narrow range of compositions. An accurate value of the Al for Si substitution rate in the tetrahedral sheet is determined by using 27Al MAS NMR spectroscopy performed under definite conditions. Some interatomic distances are determined by EXAFS spectroscopy (RMg−O = 2.11 A, RZn−O = 2.08 A, RZn−Al = 2.98 A, RZn−Zn = 3.11 A) which reveals strong local distortions in the octahedral sheet with respect to the ideal montmorillonite structure. Lengthening of the out-of-plane Zn−Si(Al) distances also shows the swelling of the whole layer in the neighbourhood of Zn. Moreover, the combination of EXAFS and 19F MAS NMR spectroscopic data reveals a clustering of the divalent elements of the octahedral sheet, which is related to a possible local trioctahedral character of these materials.

Ni-ZSM-5 and Cu-ZSM-5 Synthesized Directly from Aqueous Fluoride Gels

Ni-ZSM-5 and Cu-ZSM-5 have been synthesized in a slightly acidic fluoride medium. A low water H-ZSM-5 formula was used to synthesize the metal-substituted systems using bis(tetraethylammonium) tetrachloronickelate(II) [N(CH2CH3)4]2[NiCl4] and bis(tetraethylammonium) tetrachlorocuprate(II) [N(CH2CH3)4]2[CuCl4] as the tetrahedrally coordinated metal species. X-ray diffraction analysis established the presence of highly crystalline and thermally stable material. Unit cell dimensions increased with increasing levels of metal substitution. X-ray fluorescence (XRF) bulk analysis showed increasing levels of metals in the material and corresponding decreases in the moles of silica. The maximum levels of substitution achieved were Ni 3.93 and Cu 4.38 (in wt %). Analysis by scanning electron microscopy (SEM) showed a crystal morphology similar to that of H-ZSM-5 but with smaller crystals having an increasing aspect ratio in the substituted materials. There was evidence of twinned crystals and the appearance of raised faces in all substituted samples. Thermogravimetric and derivative thermogravimetric analysis showed anomalous losses, which could be explained by Jahn-Teller distortions. Fourier transform infrared analysis of the materials showed an increasing level of distortion in significant regions of the spectra. Analysis of the filtrate, from EDTA-exchanged material, by atomic absorbance spectroscopy (AAS) showed that there were tetrahedrally coordinated M2+ species in the material and relatively small amounts were removed by ion exchange.

Use of the bending-beam-method for the study of the anodic oxidation of Si in dilute fluoride media

The chemical dissolution of anodically grown Si oxides in acidic fluoride medium has been studied in-situ with the Bending Beam Method (BBM). Current and deflection transients were recorded after switching the electrode from the given polarisation conditions to zero applied field and monitoring the etchback process. Oxide stress values estimated with this approach are free from contributions due to film electrostriction and to changes in surface tension. Transients recorded after polarisation in the regime of current or potential oscillations show dissolution patterns which contain information on the properties of oxide film along its depth profile.

Electrodissolution of Ti and p-Si in acidic fluoride media: formation ratio of oxide layers from electrochemical impedance spectroscopy

The electrodissolution of Ti and p-Si electrodes in acidic fluoride solutions has been investigated by electrochemical impedance spectroscopy as a function of potential E, fluoride concentration, pH and angular speed of the electrode, with the aim of characterising the oxide layers covering both materials. The impedance diagrams have been analysed to obtain the low frequency capacitance Clf, the high frequency capacitance Chf and the product RhfI (where Rhf is the high frequency resistance and I the steady-state current). In the potential domain where the oxide thickness x is proportional to E, the formation ratio dx/dE has been computed from Clf, Chf and RhfI. The calculated values have been found to be in good mutual agreement and close to the ones found in the literature.

In-situ detection of stress in oxide films during Si electrodissolution in acidic fluoride electrolytes

We report on the in-situ determination of stress in anodic oxide films produced during electrodissolution of silicon crystalline electrodes in acidic fluoride media. The characterization is accomplished with the bending beam method (BBM) using thin (35 μm) Si cantilever electrodes. The oxide stress has been calculated with an improved version of Stoney’s equation which takes into account the difference of Young’s modulus between silicon substrate and oxide layer. The stress is about 220 MPa for very thin oxide films (t<10 nm). Various possible contributions to electrode bending — oxide stress, surface tension effects and film electrostriction — are considered and their relative incidence is then discussed. Analysis of the curvature signal detected during the occurrence of the electrochemical oscillations results in a better understanding of the phenomena.