HF Dissolution Research Articles

Use of hydrofluoric acid dissolution for the concentration of dickite and nacrite from kaolin deposits: an FTIR study

AbstractA series of kaolin-rich mineral samples was treated with hydrofluoric acid (HF) and the residual material characterized using infrared (IR) spectroscopy, supplemented by scanning electron microscope (SEM) observations. By examining the hydroxyl-stretching region of the IR spectra before and after treatment with hydrofluoric acid, it was possible to identify the three kaolin polytypes – kaolinite, dickite and nacrite – with greater certainty. The SEM observations suggested that the rate of dissolution of the kaolin phase was largely dependent on particle size. In general, dickite and nacrite tend to occur in the coarser clay fractions, and for this reason the finer-grained kaolinite is preferentially dissolved by the HF treatment. However, in the Keokuk kaolinite, which occurs in exceptionally large particles, it was still possible to concentrate a dickitic fraction by HF treatment, suggesting that in some cases kaolinite may be more susceptible to HF dissolution for reasons other than particle size. The IR spectra of disordered kaolinite could be interpreted as arising from a mixture of kaolinite and dickite components. However, both components dissolve at the same rate in HF, supporting the idea that disordered kaolinite consists of an intimate association of randomly stacked dickite-like and kaolinite-like components.

Selective hydrogenation of pentynes over PdZr and PdCuZr prepared from amorphous precursors

The selective semi-hydrogenation of isomeric pentynes was studied in the gas phase in a static circulation system over PdZr and PdCuZr alloys activated by HF dissolution. The catalysts were characterized by X-ray photoelectron spectroscopy and X-ray probe microanalysis. 2-Pentyne was found to be more reactive over PdZr, which was attributed to strong self-poisoning by 1-pentyne. The two reactants, in turn, exhibited similar reactivities over PdCuZr. The reactivity, however, changed drastically in competitive hydrogenations: the compounds reacted more slowly and 1-pentyne was always more reactive. The reactivity difference was particularly pronounced over PdCuZr. These observations could be interpreted by competitive adsorption of the reactants. Attempts to reactivate the self-poisoned (aged) PdZr sample used in the hydrogenation of 1-pentyne by applying various treatments (evacuation and treatment in hydrogen at different temperatures) were unsuccessful. This is due to the accumulation of carbonaceous deposits, Pd dissolution, and crystallization of the amorphous matrix.

Extraction of Pb with artificially too-old ages during stepwise dissolution experiments on Archean zircon

Stepwise HF-dissolution experiments on five Archean zircon populations reveal very systematic patterns of Pb discrimination, releasing at one point Pb with artificially too-old 207Pb/ 206Pb ages. The experiments involved a first HF dissolution step for 1 h at room temperature and evaporation on a hot-plate that produces Pb with young 207Pb/ 206Pb ages, followed by a 4 1/2 h HF dissolution step in an oven at 190°C that liberates the excessively old Pb. The final residue yields in most cases U–Pb data that are consistent with the results obtained by the normal selection, abrasion, and total dissolution procedure. In these examples, the too-old ages cannot be easily explained by the presence of an inherited core component but are more likely to indicate segregation of Pb in zircon sub-domains during thermal annealing episodes early in its history, as has been proposed by other workers. Aside from shedding light on these particular aspects of zircon U–Pb systematics, the combined results also provide geologically relevant information concerning the regional evolution of the western Superior Province in Ontario. An age of 2718±3 Ma for a gabbroic unit from the Quetico Subprovince shows that this was coeval to 2722–2718 Ma ultramafic to felsic igneous rocks in the adjacent Shebandowan greenstone belt, including a gabbro body dated in this study at 2725+17/−11 Ma. These age relationships suggest that volcanic units of the Shebandowan greenstone belt were tectonically imbricated in younger sedimentary rocks of the Quetico basin during late Archean convergence. The other three samples represent felsic intrusive units from Geraldton in the Wabigoon subprovince. An age of 2699±1 Ma for an Au-mineralized feldspar porphyry dyke and identical ages of 2690±1 Ma for two phases of the syn-tectonic Croll Lake stock put constraints on the timing of major deformation and hydrothermal activity in the belt.

Determination of Zirconium, Niobium, Hafnium and Tantalum at ng g‐1 Levels in Geological Materials by Direct Nebulisation of Sample HF Solution into FI‐ICP‐MS

We have developed a rapid and accurate method to determine Zr, Nb, Hf and Ta (denoted as HFSE) in geological samples by inductively coupled plasma‐mass spectrometry fitted with a flow injection system (FI‐ICP‐MS). The method involves sample decomposition by HF followed by HF dissolution of HFSE coprecipitated with insoluble M and Ca fluoride residues formed during the initial HF attack. This HF solution was directly nebulized into an ICP mass spectrometer. An external calibration curve method and an isotope dilution method (ID) were applied for the determination of Nb and Ta, and of Zr and Hf, respectively. Recovery yields of HFSE were > 96% for peridotite, basalt and andesite compositions, apart from Zr and Hf for peridotite (> 85%). No matrix effects for either signal intensities of HFSE or isotope ratios of Zr and Hf were observed in basalt, andesite and peridotite solutions down to a dilution factor of 100. Detection limits in silicate rocks were 40, 2, 1 and 0.1 ng g‐1 for Zr, Nb, Hf and Ta, respectively. This technique required only 0.1 ml of sample solution, and thus is suitable for analysing small and/or precious samples such as meteorites, mantle peridotites and their mineral separates. We also present newly determined data for the Zr, Nb, Hf and Ta concentrations in USGS silicate reference materials DTS‐1, PCC‐1, BCR‐1, BHVO‐1 and AGV‐1, GSJ reference materials JB‐1, ‐2, ‐3, JA‐1, ‐2 and ‐3, and the Smithsonian reference Allende powder.

Kinetic Limitations on Metal Dissolution during Aqueous Silicon Wafer Processing

Metal contamination during aqueous processing has recently been a subject of great interest,1-14 since metal surface contamination can decrease yield in semiconductor manufacturing by causing a wide variety of electrical problems.1,3,5,6 For example, the metal contamination tolerance level for 16 megabyte (MB) dynamic random access memory (DRAM) devices is approximately 1010 metal atoms/cm2.15-17 Deposition has been suggested to occur by a variety of mechanisms,4,6,16,18 including precipitation from a supersaturated cleaning solution and incorporation into the silicon dioxide layer during oxide regrowth. The most problematic is a proposed electroless process whereby metal ion reduction and silicon oxidation occur simultaneously following HF dissolution of the native oxide.4 The physical processes which occur during dissolution and removal have been less studied, although they can also be important to obtaining a contamination-free substrate. For example, during Cu dissolution into an SC-1 process solution, one might expect the electroless dissolution in this basic solution to occur via the following half-reactions

Sequential extraction of selenium from four Scottish soils and a sewage sludge

A sequential extraction procedure was used to assess the availability of selenium (Se) from four Scottish soils which may receive sewage sludge and a sewage sludge that may be applied to agricultural land. The procedure identified two exchangeable (available) and three non‐exchangeable Se fractions. The exchangeable fractions were defined as soluble (0.25M KCl) and ligand exchangeable (0.1M KH2PO4). The non‐exchangeable fractions were defined as acid‐extractable (4M HCl), oxidizable (dissolution in KClO3 and concentrated HNO3), and residual (dissolution in HF and HNO3). The fraction of Se defined as acid‐extractable is not readily available for plant uptake, but has the potential to become available through chemical and microbial mobilization. The total Se was also determined for each soil and the sewage sludge by a one step digestion procedure using HNO3 and HF acids. The determination of Se in the individual fractions was performed by isotope dilution‐mass spectrometry using 76Se. The total Se in the soils ranged from 0.5–0.8 ug g‐1 and less than 5% was available. The sewage sludge had a total Se of 3.55 μg g‐1, but <4% was available for uptake with a further 5% potentially available. The sum of the Se concentrations in the fractions was in good agreement (92–108%) with the total Se concentrations found by the one step digestion. With the likely increase of disposal of sewage sludge to land, the method provides a means of assessing the bioavailability of Se for plant uptake from sewage sludge and sludge amended soil.

Investigation of phase transitions of organic fluids confined in porous silicon

We report differential scanning calorimetry (DSC) experiments which were performed on organic fluids confined in porous silicon samples. The transition temperature of the confined fluid was found to be lower when compared to the bulk fluid. The effect of the doping level of Si substrate, HF dissolution or anodization time on the DSC results are also presented. Some hints for the thermodynamic interpretation of such an experiment are finally given. © 1997 Elsevier Science S.A.

Porous silicon formation and other photoelectrochemical effects at silicon electrodes anodized in hydrofluoric acid

Illumination is an important parameter in the electrochemistry of silicon. A moderately doped (<10 18 cm −3) n-type silicon electrode anodized in the dark is passivated against dissolution in HF due to the absence of holes which promote dissolution. Under illumination a current proportional to the light intensity is observed. This dependence of the current on the illumination enabled us to realize relief structures on the electrode surface without the use of standard photolithography. The photoassisted formation of macropores is another aspect of the photoelectrochemistry of silicon reported in this work. Many useful applications such as a silicon based capacitor or photonic band gap materials in the infrared regime are based on photoelectrochemically etched macropores.

Long chain branched celluloses by mild trans-glycosidation

Cellulose dissolved in anhydrous dimethylacetamide (DMAc)/LiCl solution and treated with dialkylaminosulfur trifluoride (DAST) became highly branched and actually gained in molecular weight. The branching was thought to arise from the action of HF which is generated from the consumption of DAST. Subsequent experiments involving the direct addition of HF to anhydrous cellulose/DMAc/ LiCl produced highly branched materials with greater polydispersity and greater molecular weight than the starting cellulose. This branching was likely due to the well known reactivity of glycosyl fluoride endgroups. Although the mechanism is not new, the combination of homogeneous cellulose dissolution and low levels of HF under anhydrous conditions represents a novel method for synthesizing a new class of cellulose derivatives. The potential utility of this method for producing branched cellulose derivatives and blocky cellulosic copolymers is discussed.

Carrier localization in porous silicon investigated by time-resolved luminescence analysis

We analyzed the photoluminescence (PL) mechanisms of porous silicon, and in particular, the origin of the PL high quantum efficiency (QE) at room temperature. For this we used postformation treatments, anodic oxidation, and hydrofluoric acid (HF) etching (known for their strong QE enhancement effect) correlated with a PL time resolved analysis. A third parameter was the temperature which, for heating above room temperature, gave a reversible quenching of the PL. All three parameters give a similar evolution of the PL decay shape, which we consider to originate from the same evolution of the carrier dynamics. Porous silicon is described as an undulating wire. The high QE at room temperature is attributed to carrier localization inside minima of the fluctuating potential along the wire; these considerations are extended to another porous material: amorphous porous silicon. Anodic oxidation and HF dissolution diminish the wire size, giving a reduction of the localization length of the carriers and progressive suppression of the nonradiative recombination channel. A simple model permits one to link the changes of the PL decay shape to the QE evolution. The nonexponential PL decay shape is interpreted as being due to a distribution of nonradiative recombination rates, the value of the nonradiative recombination rate being limited by a tunneling effect. This highly simplified model explains the origin of the nonexponential decay shape, its modification and gives a good description of the QE evolution as a function of temperature, oxidation level, or porosity.

Differential-pulse polarographic determination of chromium and nickel in zircaloys using microsamples

Three methods were developed for the analysis of the alloying elements chromium and nickel in Zircaloy-2 and Zircaloy-4, alloys of zirconium used extensively in the nuclear industry as nuclear fuel sheathing. Zircaloy was dissolved in hydrofluoric acid followed by oxidation of Cr(III) to Cr(VI) by perchloric acid. The polarographic peak of CrO 4 2− in alkaline medium was used to determine Cr. Nickel was determined, after HF dissolution, using the polarographic peak of the nickel-ammonia complex, Ni (NH3) 6 2+ . In an alternative method for Ni, the sample was dissolved in HF and the polarographic peak of the nickel-fluoride complex, NiF 4 2− , was used. Careful control of fluoride ion concentration and the pH eliminated the harmful effect of fluoride on the capillary behaviour of the dropping mercury electrode. The detection limits for chromium, nickel-ammonia and nickel-fluoride in Zircaloys were 0.0016, 0.0033, and 0.0041%, respectively.

Correlation between the Porous Silicon Morphology and the Photoluminescence Efficiency

AbstractSmall‐angle X‐ray scattering is applied to investigate the microstructure and the morphology of three photoluminescent porous silicon samples of the same porosity (80%) but prepared using different electrochemical treatments: (A) as‐prepared, (B) as‐prepared 65% porosity, followed by an oxidation treatment and an oxide dissolution in HF, (C) as‐prepared 65% porosity followed by a HF dissolution. Although the PL spectra are almost similar in energy, there are differences in their intensities: the luminescence intensity of the as‐prepared sample A is a bit greater than the one observed for sample B and much more greater than the one corresponding to sample C. If one assumes the same surface passivation (Si‐H) for the three samples then these differences in luminescence intensity may be due to the different PS morphologies which are evidenced in SAXS curves at low q values.

Photoluminescent properties of porous films of tantalum silicides

The paper reports the first results of observing the visible photoluminescence from the surface of tantalum silicide samples subjected to electrochemical anodization in HF-containing electrolyte. It is shown that at increased concentrations of HF (more than 20%) and low current densities uniform dissolution of the silicide film takes place. A lower HF concentration (10%) yields the same effect at low current densities, whereas higher currents result in formation of a porous passive anodic layer composed of silicon dioxide with small inclusions of elemental silicon and tantalum oxide. These passive layers are efficiently luminescent.

Enthalpies of formation of some solid hafnium nickel compounds and of the Ni-Rich HfNi liquid by direct reaction calorimetry

In the course of a systematic study of binary transition metal alloys, the enthalpies of formation of five Hf-Ni compounds were measured by direct reaction calorimetry: Hf0.17Ni0.83(l/6HfNi5): ΔfH(1323 K) = − 37,000 J/mole of atoms (±3200) Hf0.22Ni0.781/9Hf2Ni7): Δf(1623 K) = −50,700 J/mole of atoms (±2000) Hf0.45Ni0.55(l/2OHf9Ni11): ΔfH(1473 K) = −54,500 J/mole of atoms (±2200) Hf0.50Ni0.50(l/2HfNi): ΔfH(1573 K) = −47,900 J/mole of atoms (±1800) Hf0.67Ni0.33(l/3Hf2Ni): ΔH(1423 K) = −36,700 J/mole of atoms (±1300) with reference to pure metals in their equilibrium states at the reaction temperatures. The Ni-rich Hf-Ni liquid was also studied by the dissolution of Hf in the liquid alloy of variable composition at 1743 and 1633 K. Results show that the enthalpy of formation of the liquid at a given composition depends strongly on temperature, and this point suggests the existence of associations in the liquid. The melting temperature of Hf0.22Ni0.78(Hf2Ni7) which was found (1705 K) is slightly lower than the one (1743 K) given in the literature.

Bestimmung von Spurenelementen in Rein-Titan durch ICP-AES ohne und mit Spuren-Matrix-Trennung

Procedures for the analysis of titanium metal after dissolution in HF and HNO3 are described. The 3 σ-detection limits (DL) for the determination with sequential AES in solutions of 1% titanium are about 100 ng/mL for many analytes. Investigation of the matrix interferences shows that removal of the matrix by one decade to 0.1% Ti improves DL by about 2–3 decades. Using the chelating ion exchanger Cellulose-HYPHAN in combination with hexamethylene dithiocarbamidate (HMDC) in a batch technique, the enrichment rate for the analytes Cu, Fe, Ni, Pb and V is better than 88%. Furthermore the elements Cd, Co and Zn are enriched if Chelex 100 is used in the batch mode. In a column technique 17 elements (Ag, Al, Ba, Ca, Cd, Co, Cu, Fe, Ga, In, Mg, Mn, Ni, Sn, Th, V, Zn) can be determined with enrichment rates better than 89%. Pb and Sr are collected by about 70%. Detection limits of about 1 to 10 ng/mL in the solution can be achieved (dilution factor of the metal sample about 20).

Group separation by solvent extraction from silicate rock matrix of Nb, Mo, Ta, and W at low levels for ICP-MS

The extraction efficiency from silicate rock solutions in HF of Nb, Mo, Ta, and W as a function of fluoride activity in HCl solution, 3 mol/l, was investigated. At pF 3.5, N-benzoyl N-phenylhydroxylamine (BPHA) dissolved in CHCl3 proved to be an ideal system for the separation of the four analyte elements from the rock matrix after HF dissolution. All major rock constituents except for titanium are left behind in the aqueous phase. Values for the distribution ratios are 500±100 for all four elements allowing cross calibration in ICP-MS between enriched isotopes of Mo and W added to the samples before dissolution (isotope dilution) and the neighbouring monoisotopic elements Nb and Ta. Previously such high distribution ratios have been reported for Mo only because Nb, Ta, and W are not normally available as reactive ions due to hydrolysis unless directly released from fluoride complexes by downwards adjustment of fluoride activity of the solution in the presence of BPHA. Analytical data from 20 geochemical reference standards document the practical value of the described analytical procedure. Using ICP-MS and BPHA-pentanol solutions standards, superior detection limits are demonstrated by Nb, Ta, and W values in the sub ppm range obtained for some ultrabasic rock standards for which reference values are not yet available. Using ICP-MS and BPHA-pentanol solutions, superior detection limits are demonstrated by concentration values in the sub ppm range obtained for some ultrabasic rock standards for which reference values are not yet available.

A hydrofluoric acid dissolution method for determining volcanic glass content of tephra derived soils (andisols)

The determination of volcanic glass content is essential for the classification of many Andisols in Soil Taxonomy. Previously, polarizing microscope and heavy liquid methods have been used to estimate glass content, but we suggest here an alternative method based on gravimetric analysis of glass-bearing samples before and after hydrofluoric acid (HF) dissolution treatment. A series of Bw and C horizons of weakly developed buried paleosols derived from post-c. 15 ka rhyolitic tephra deposits in North Island, New Zealand, was subjected to the HF treatment and glass contents calculated. Our results, corroborated by qualitative examination using a polarizing microscope, indicate that the HF dissolution method for these materials is simple, fast, and accurate, and, with further testing, could be applicable to all types of glass. Slight modifications to the method are probably needed where glass compositions or primary mineralogical assemblages vary significantly between samples. As well as being potentially useful for characterizing and defining Andisols, the method is likely to be applicable wherever estimates of the volcanic glass content of soils, paleosols, or tephra deposits are required.

Last Developments in Luminescence of Porous Silicon

ABSTRACTIt is shown that visible photoluminescence and electroluminescence can be obtained from porous silicon layers. Room temperature photoluminescence is readily obtained from as-formed high porosity samples. Light emission at wavelengths as short as 560 nm can be observed after further thinning of the silicon pore walls by dissolution in HF under illumination. Silicon walls can also be thinned by an electrochemical oxidation process, this method allowing to use layers of rather low porosities (65%) which thus gives good mechanical properties to the samples. The thinning of the already very small size crystallites of porous silicon leads to quantum size effects which are at the origin of the light emission far above the band gap of silicon. Photoluminescence decay characteristics suggest that a tunnelling effect could be involved in the recombination mechanism of photogenerated charge carriers. Bright electroluminescence during anodic oxidation of porous silicon has been also evidenced. The influence of the porosity, of the layer thickness and of the anodic current density on the integrated electroluminescence intensity are described in detail.

Amorphous alloy catalysis: Part 1. Dehydrogenation of 2-propanol on amorphous Cu 61Zr 39 alloy precursors

The dehydrogenation of 2-propanol to acetone on Cu 61Zr 39 alloy has been studied for the first time. The dehydrogenation reaction (flow reactor, 573 K) on as-received or pre- activated (hydrogen, 573 K, 3 h or 473 K, 16 h) alloy brings about a continuous activation of the catalyst. Other types of pretreatment (heat treatment at 800 K for 0.5 h, or treatment with water vapour at 500 K for 24 h) induce an initially high, but unstable, activity. The activity changes parallel changes in the copper surface area (determined via N 2O decomposition), except for samples prepared by HF dissolution, which results in a highly active and stable catalyst with a low surface area. XRD and DSC indicate crystallisation of the amorphous alloy samples under the given reaction conditions and as a result of pretreatment.

Oxygen Isotopic Analysis of Diagenetic Quartz Overgrowths from the Brent Sands: A Comparison of Two Preparation Methods

ABSTRACT Two preparation methods used for determining the 18O of quartz overgrowths are compared from the sandstones of the Middle Jurassic Brent Group, Northern North Sea. Method one uses dilute HF to preferentially attack interfacial materials present between the overgrowth and the detrital core, with subsequent ultrasonic agitation used to break the overgrowths off at the weakened contacts. Size fractionation by sieving should concentrate overgrowths into the smaller grain size separates. In the Brent sands studied, however, little interfacial material is present between overgrowth and core, and the overgrowths are thinly developed (average width 20-50 microns). Concentration of the diagenetic overgrowths is therefore difficult. This difficulty is compounded by large amounts of detrital quartz in the size fractions, as a result of internal fracturing of the grains. The 18O (SMOW) values for the finest size fraction (supposedly overgrowth rich), ranging from 9 to 13.3, are unusually low for diagenetic quartz overgrowths, and result from the high detrital component. The second method derives the 18O values for the overgrowths by the difference in 18O for the quartz grains plus overgrowths and quartz grains from which the overgrowths have been removed by HF dissolution. The mass balance used requires the overgrowth abundance to be known from point counting. The calculated 18O values obtained are typical for authigenic quartz overgrowths and range from 17.1 to 20.9. Direct isolation of quartz overgrowths for 18O measurement is suitable where interfacial material exists between the detrital grain and the overgrowth and where overgrowths are well developed. Otherwise, preferential HF dissolution of the overgrowths and a 18O mass balance calculation is the preferred method.