Chromatographic Separation of Calcium Isotopes with Polymer-Bound Ligands

Abstract

Calcium isotopes were enriched in chromatography columns containing the polymer-bound ligands cryptand [2_B22], cryptand [2_B21], and 18-Crown-6. The largest equilibrium single stage separation factor, 1.0039 ± 0.0002 for the calcium 40/44 isotope pair, was found in calcium exchange with cryptand [2_B22]. The largest separation factor found for 18-Crown-6 was 1.0025 ± 0.0003 for the 40/44 isotope pair. The size of the isotope effect was found to be highly dependent on the solvent composition. An iminodiacetate resin ion exchange column yielded small enrichments of calcium isotopes. Calcium 40/44 exchange with the chelating functional group iminodiacetate yielded a small equilibrium separation factor of 1.00011 ± 0.00003, which was more typical of calcium ion exchange isotope effects.

Comparisons of the above chemical systems based on separative power showed crown chemical exchange to be an improvement over cryptand and iminodiacetate exchange. The cryptand system was rendered impractical as a consequence of a slow exchange rate and a resulting large stage residence time contribution to the separative power, on the order of 45 min. The separation factor for the iminodiacetate system was too small for practical applicability.     

Contributions of Nuclear Size and Shape,Nuclear Mass,and Nuclear Spin to Enrichment Factors of Zinc Isotopes in a Chemical Exchange Reaction by a Cryptand

ABSTRACT

The isotope effect of zinc in the chemical exchange reaction using a macrocyclic ligand was not found to be ruled by the Bigeleisen-Mayer approximation, which suggested that the enrichment factor is proportional to the mass difference and is inversely proportional to the product of the masses of the isotopes. The separation factors of zinc isotopes in the chemical exchange reaction using cryptand(2_B,2,l) polymer were precisely measured by means of an ICP mass spectrometer equipped with nine collectors as ion detectors. The liquid chromatography of a column packed with the cryptand polymer was used for the separation of the zinc isotopes. The enrichment factor ε_67,66 for ⁶⁷Zn to ⁶⁶Zn was ?3.3329(3) × l0^?4.That for ⁶⁸Zn to ⁶⁶Zn was 1.846(1) × 10 ^?4 and that for ⁷⁰Zn to ⁶⁸Zn was 7.19(2) × 10^?4.They were not scaled with δm/mm′, where δm is the mass difference between the isotope pairs, and m and m′ represent the masses of the isotopes. The isotope effect of zinc is implicated with the isotope shift, and the hyperfine structure shift in the isotopomer of the zinc isotopes. The sum contribution of the vibrational energy shift from one isotope to the other and the nuclear mass shift to the enrichment factor of ⁶⁷Zn was ? 1.05 × 10^?3, and the contribution of the field shift caused by the nuclear size and shape of the isotope was 5.26 × 10^?4. The contribution of the nuclear spin or the hyperfine structure shift to the enrichment factor of ⁶⁷Zn was small: 1.94 × 10 ^?4     

Mechanism of silicon etching in HF-KMnO4-H2O solution

The etching reaction of silicon in HF-KMnO₄-H₂O mixed solution has been studied under various experimental conditions. The etch rates were measured as a function of agitation speed, HF and KMnO4 concentrations, and etching temperature and time. A comprehensive mechanism for the silicon etching and insoluble solid-phase film (K₂SiF₆) formation has been proposed. The holes formed at silicon surface accelerated not only the etch rate of silicon but also the formation rate of K₂SiF₆. With the increase of hole concentration at lower HF concentrations the etch rates decreased because of the deposition of K₂SiF₆ on etched silicon surface. Under the condition of sufficiently high HF concentration, the rate increased with the increase of hole formation and the formation of holes at silicon surface was the rate limiting step of the silicon etching reaction in HF-KMnO₄-H₂O solution. High HF concentration enough for dissolving K₂SiF₆ was apparently essential to obtain high etch rate in the silicon etching reaction.     

Effect of fluorine chemistry in the remote plasma enhanced chemical vapor deposition of silicon films from Si2H6-SiF4-H2

SiF₄ was added into Si₂H₆-H₂ to deposit polycrystalline silicon films at low temperatures around 400°C in a remote plasma enhanced chemical vapor deposition reactor. It was found out that the fluorine chemistry obtained from SiF₄ addition had an influence on the chemical composition, crystallinity, and silicon dangling bond density of the film. The fluorine chemistry reduced the amount of hydrogen and oxygen incorporated into the film and also suppressed the formation of powders in the gas phase, which helped the crystallization at low temperatures. Effect of SiF₄ concentration as well as the deposition temperature was also significant.     

Template-free fabrication of silicon micropillar/nanowire composite structure by one-step etching

A template-free fabrication method for silicon nanostructures, such as silicon micropillar (MP)/nanowire (NW) composite structure is presented. Utilizing an improved metal-assisted electroless etching (MAEE) of silicon in KMnO₄/AgNO₃/HF solution and silicon composite nanostructure of the long MPs erected in the short NWs arrays were generated on the silicon substrate. The morphology evolution of the MP/NW composite nanostructure and the role of self-growing K₂SiF₆ particles as the templates during the MAEE process were investigated in detail. Meanwhile, a fabrication mechanism based on the etching of silver nanoparticles (catalyzed) and the masking of K₂SiF₆ particles is proposed, which gives guidance for fabricating different silicon nanostructures, such as NW and MP arrays. This one-step method provides a simple and cost-effective way to fabricate silicon nanostructures.     

Improved Donors for the Separation of the Boron Isotopes by Gas-Liquid Exchange Reactions

Abstract

Chemical exchange between gaseous boron trifluoride (BF₃) and the liquid BF₃·dimethyl ether complex has been used extensively for the commercial fractionation of boron isotopes. Several compounds never before studied as donors in the isotope exchange reaction were examined to determine if they were viable replacements for the dimethyl ether system. For the first time, ketones were studied as donors in the boron isotope exchange reaction. The ketones examined were acetone, methyl isobutyl ketone, and diisobutyl ketone. The ideal single stage separation factors, α, measured for these ketones were between 1.038 and 1.043 at 30°C. The observed separation factor for a fourth donor system, nitromethane, was 1.067 at 30°C, well above that predicted by theory or observed for any known BF₃/donor system. For each of the systems studied, the separation factors were greater than the value of α = 1.027 reported for the dimethyl ether/BF₃ system at 30°C. In view of the experimentally observed separation factors, these donor systems are potential replacements for dimethyl ether in large-scale boron isotope fractionation schemes. It is concluded that plant size could be significantly reduced by using any of these donors rather than dimethyl ether.

     

Differential capacitance of the P-type silicon—electrolyte solution interface

The results of differential capacitance measurements of P(111)-type silicon electrodes (10 Ω cm) placed in 0·1 M Na₂SO₄ solutions with various pH are given in the paper. The capacitance were determined with the pulse techniques in the frequency range from 1 to 100 kHz. The silicon electrode potential, U, relative to an sce was varied from ?0·05 to ?0·9 V. It was found that a dispersion of differential capacitance appeared for potentials up to an U_ss value. For the potentials U < U_ss the experimental differential capacitance values are independent of frequency and of potential variation. The experimental results were interpreted by comparing them with the space charge region capacitance values of silicon.     

NUCLEAR SIZE AND SHAPE EFFECT IN CHEMICAL ISOTOPE EFFECT OF GADOLINIUM USING DICYCLOHEXANO-18-CROWN-6

ABSTRACT

Gadolinium isotopes were fractionated in a liquid-liquid extraction system using dicyclohexano-18-crown-6. The isotope enrichment factors showed breakdowns of the conventional Bigeleisen-Mayer approximation for every condition of the initial phases; the concentrations of gadolinium and hydrochloric acid and the type of organic solvents. The nuclear mass effect and the field shift effect were estimated by use of the isotope pairs ¹⁵²Gd-¹⁶⁰Gd and ¹⁵⁴Gd-¹⁶⁰Gd. Since both isotopes, ¹⁵²Gd and ¹⁵⁴Gd, have the unique characteristics of the nuclear charge radii, <r²>'s, the unusual isotope effects due to the field shifts took place, while the mass effects of the other isotopes canceled the field shift effects. The maximum isotope effect was observed to be ε_160,152 =0·0252 ± 0·0024 whose initial aqueous phase was 0·023 M GdCl₃ in 12 M HC1, and its organic phase was 0·2 M DC18C6 in chloroform: this was 0·00315 ± 0·00030 in terms of the enrichment factor for unit mass.     

Ionic mass transfer rate of CuSO4 in electrodialysis

Ionic mass transfer rate of copper ion in the ion exchange membrane electrodialysis at limiting current density has been studied using a planar flow electrodialyzer consisted of a cation exchange membrane and an anion exchange membrane. Their effective area is 4 × 5 cm². The effects of flow rate, viscosity of electrodialysate solution and thickness of electrodialyzer to the ionic mass transfer rate have been studied. An empirical correlation equation (Nu)_exp. = 4.57 × Re^0.333 Sc^0.307 (de/L)^O.33 is obtained. It fitted well with the theoretical performance equation (NU)_theor.= 3.70 × (Re · Sc · de/L), which is derived from the Nernst-Planck Equation based on the assumption that mass transfer in the concentration boundary layer in the desalting compartment is controlling.     

Electrolytic production of silicon

A critical survey is given of the research and development studies on the electrodeposition of silicon. Among several systems, three are given particular attention, using inorganic baths with SiO₂ or K₂SiF₆ as the source of silicon respectively, or organic baths. The former two appear capable of development to commercial production. Conceptual designs of a commercial-scale cell for silicon production at temperatures above its melting point, and of a pilot plant for plating silicon from an all-fluoride bath are presented.     

Ion‐Exchange Separation of Pu from Macro Concentration of Th in HNO3 Medium

Abstract

Sorption behavior of Th and Pu from anion‐ as well as cation‐exchange resin was investigated from nitric acid medium by both batch and column methods. The anion‐exchange studies involved anionic nitrate complexes of Pu⁴⁺ and Th⁴⁺ sorbed onto DOWEX 1x4 resin (50–100 mesh), and the cation‐exchange studies involved the sorption of Pu³⁺ and Th⁴⁺ onto BIORAD AG 50Wx8 (50–100 mesh) or DOWEX 50Wx4 (50–100 mesh) resin. The batch data gave a separation factor (K _d,Pu/K _d,Th) of 22 for the anion‐exchange method and 0.017 for the cation‐exchange method at 3 and 2 M HNO₃, respectively. A two‐stage ion‐exchange separation method was developed for the quantitative separation of Pu (8 g/L) from a macro amount of Th (200 g/L) in nitric acid medium. The first step involved the quantitative sorption of plutonium from the mixture while about 90% of Th could be washed in 6 column volumes. The plutonium, eluted (as Pu³⁺) using 0.5 M HNO₃ + 0.2 M hydrazinium nitrate (HN) + 0.2 M hydroxyl ammonium nitrate (HAN), and the residual (～10%) Th were subsequently loaded onto a cation‐exchange column in the second step. Greater than 99% Pu was recovered with 2 M HNO₃ (in ～8 column volumes) containing 0.2 M HN + 0.2 M HAN. The final elution of thorium from the cation‐exchange column was achieved in about 6 column volumes of 1 M α‐hydroxy isobutyric acid. A (Pu, Th)O₂ fuel scrap sample was dissolved in 16 M HNO₃ containing 0.005 M HF and was used subsequently as the feed for the anion‐exchange column. The eluted Pu was subsequently loaded onto a cation‐exchange column for final purification. The recovery of plutonium and thorium was found to be >99% and >98%, respectively, while the respective decontamination factors were estimated to be 215 and 292.     

Synthesis of Zirconium Phosphate,HZr2(PO4)3, in Pores of Silica Beads and Some Ion Exchange Separation Properties of the Composite Obtained

Abstract

Zirconium phosphate, HZr₂(PO₄)₃ (HZP), was synthesized in inner pores of highly porous silica beads, and the ion exchange properties of HZP-loaded silica beads (HZP-SiO₂) thus obtained were examined. The ion exchange properties of HZP-SiO₂ were basically equivalent to those of HZP in the form of powders. Chromatographic lithium isotope separation was attempted using HZP-SiO₂ as column packing material. The heavier isotope was effectively accumulated in the frontal part of the lithium chromatogram. However, the tailing phenomenon was observed at the rear boundary of the chromatogram, and no effective accumulation of the isotope separation effect was observed in the tail.     

Electroplating silicon and titanium in molten fluoride media

The electrolytic reduction mechanisms of K₂SiF₆ and K₂TiF₆ solutions in LiF-KF and LiF-NaF-KF eutectic mixtures have been studied at temperatures between 550 and 850°C. The reduction of K₂SiF₆ proceeds by two successive electron transfers, $$Si(IV) + 2e \to Si(II) + 2e \to Si$$ coupled with an antidisproportionation reaction $$Si(IV) + Si \underset{{k_b }}{\overset{{k_b }}{\longleftrightarrow}} 2Si(II)$$ Very pure thin silicon layers, up to 300 μm thick, were obtained on a silver substrate. The cathodic reduction of TiF ₆ ^2? ions occurs in two well separated reversible steps, $$TiF_6^{2--} + e \to TiF_6^{3--} + 3e \to Ti + 6F^--$$ Adherent coatings of pure titanium were found to be linked to the copper substrate by an interdiffusion sublayer comprising Ti₂Cu, TiCu, Ti₂Cu₃ and TiCu₄ which were formed in a narrow potential domain preceding titanium deposition.     

Mechanism of electrodeposition of silicon from K2SiF6-flinak

Cyclic voltammetry data for stationary and rotating electrodes are presented for electrodeposition of silicon from solutions of K₂SiF₆ in the LiF/NaF/KF eutectic. It is concluded that electron transfer is the main rate-controlling mechanism but that solute diffusion cannot be neglected. The interface process is preceded by a chemical reaction, probably dissociation of polynuclear Si_nF_2n+2 complexes.     

Sol-gel derived zirconia membrane on silicon carbide substrate

To enhance the high temperature and chemical corrosion resistances of ceramics membrane, a ZrO₂/SiC ceramic membrane was prepared through sol-gel route followed by the dip-coating technique. The substrate layer was made of pure silicon carbide phase by evaporation-condensation process, and the separation layer was made of zirconia phase by solid-phase sintering process. The substrate layer was sintered at 2200 ℃ in the vacuum, and the pores were distributed in a narrow size range from 4.5–6.0 μm. When the membrane was sintered at 700 ℃, a defect-free separation layer formed on the substrate. With the increase of sintering temperature, the average pore size of the separation layer declined from 63 to 48 nm, and the water permeability declined from 355 to 273 L/(m²·h·bar). Our results indicate the ZrO₂/SiC ceramic membrane has potential applications in the separation of high temperature or chemically corrosive wastewaters.     

Separation of Gallium and Indium Isotopes by Cation and Anion Exchange Chromatography

Ion exchange chromatography was applied to study chemical isotope effects of gallium and indium in ligand exchange reactions. A strongly acidic cation and a strongly basic anion exchange resin were used as a solid phase, and aqueous HCl as a liquid phase. On the cation exchanger, the light isotope ⁶⁹Ga was enriched at the front part of the elution band and the heavy isotope ⁷¹Ga at the end part. Instead, the light ¹¹³In isotope was enriched at the end part, and the heavy isotope ¹¹⁵In at the front part. The isotope separation factor ? is equal to 3.3?×?10^–5 for gallium and 2.0?×?10^–4 for indium. On the anion exchanger, the heavy gallium isotope was enriched at the front part, whereas the heavy indium isotope at the end part of the band, with ? equal to ～10^–3 and 1.7?×?10^–4, respectively. This pattern of enrichment is caused by stronger Ga³⁺–OH₂ than Ga³⁺–Cl^? bond, and by inverse order of bond strength for indium. In the displacement method, gallium and indium on anion exchanger also show opposite enrichment of their isotopes, but the ? values (1.5?×?10^–2 for gallium and 5?×?10^–3 for indium) are greater than those found in the elution method, probably due to much higher concentrations of the metals.     

Electrical resistivity of silicon nitride produced by various methods

It has been shown that the grain growth and amount of the glass phase influence the electrical resistivity of pressureless sintered and spark plasma sintered silicon nitride. Sintering additives strongly affect the impurity conductivity of pressureless sintered silicon nitride and slightly influence the intrinsic conductivity due to the longer sintering process as compared with the spark plasma sintering. It was demonstrated that Al₂O₃-Y₂O₃ lead to decrease in the electrical resistivity of SPSed silicon nitride due to increase in the band gap width as opposed to Al₂O₃-MgO. Effect of the sintering additive on the impurity conductivity is practically absent but there is a strong dependence of the sintering temperature for reported spark plasma sintered silicon nitride. However, intrinsic conductivity of SPSed silicon nitride is affected by both sintering temperature and sintering additive. It was also shown that electrical resistivity of produced ceramics is linearly depends on the content of β-Si₃N₄ and microhardness. Electrical resistivity of manufactured silicon nitride varied from 3.16·10⁹ to 1.73·10¹¹ Ω?m. It has been observed strong influence of the sintering additive and sintering temperature on the electrical properties of SPSed and pressureless sintered silicon nitride.     

Recycling photovoltaic silicon waste for fabricating porous mullite ceramics by low-temperature reaction sintering

In this study, porous mullite ceramics with coral-like structures were fabricated at a low temperature of 900 °C by using photovoltaic silicon waste (PSW) as the silicon source directly. The effects of additive content and sintering temperature on the mullitization reaction of green bodies were studied. The results showed that ammonium molybdate tetrahydrate molybdenum (H₂₄Mo₇N₆O₂₄·4H₂O) as an additive could reduce the reaction temperature for mullitization from 1100 °C to 900 °C. The research on the influence of catalyst on material properties showed that porous mullite ceramics with a flexural strength of 52.83 MPa, a 41.78 % porosity, a sintering expansion rate of 0.49 % and an average pore size of 0.23 μm could be fabricated by introducing 7.5 % H₂₄Mo₇N₆O₂₄·4H₂O at the sintering temperature of 1000 °C. This study develops an environment-friendly recycling method of PSW and provides a new idea for the low-cost preparation of porous mullite ceramics with high purity.     

Nitrogen-15 Fractionation by Countercurrent Exchange between Liquid N2O3-N2O4 Mixtures and Their Vapor Phases under Pressured Conditions

Abstract

The nitrogen isotope exchange system for ¹⁵N-fractionation consisting of liquid mixtures of N₂O₃ and N₂O₄ and the vapor phase in equilibrium with the liquid (NO/N₂O₃ system) has been studied at temperatures between ?9 and 20°C under various pressures below 4 atm. A countercurrent isotope exchange apparatus and an associated product refluxer system are described. At 15°C the effective separation factor, α_eff, increases from 1.006 at 1 atm to 1.030 at 2.7 atm and levels off thereafter. The observed α_eff values are in good agreement with recent spectroscopic data. The height equivalent of theoretical plate (HETP) correlates well with the linear flow rate of gas phase, showing a dominant diffusion-control of the overall exchange rate. At 15°C the HETP of the NO/N₂O₃ system under 3～4 atm can be smaller by factors of 7 to 10 than that of the Nitrox (NO/HNO₃ exchange) system having the same product rate, rendering the former system a possibility for significantly smaller plant volume.     

Improving specific stiffness of silicon carbide ceramics by adding boron carbide

A strategy for improving the specific stiffness of silicon carbide (SiC) ceramics by adding B₄C was developed. The addition of B₄C is effective because (1) the mass density of B₄C is lower than that of SiC, (2) its Young’s modulus is higher than that of SiC, and (3) B₄C is an effective additive for sintering SiC ceramics. Specifically, the specific stiffness of SiC ceramics increased from ~142 × 10⁶ m²?s^?2 to ~153 × 10⁶ m²?s^?2 when the B₄C content was increased from 0.7 wt% to 25 wt%. The strength of the SiC ceramics was maximal with the incorporation of 10 wt% B₄C (755 MPa), and the thermal conductivity decreased linearly from ~183 to ~81 W?m^?1?K^?1 when the B₄C content was increased from 0.7 to 30 wt%. The flexural strength and thermal conductivity of the developed SiC ceramic containing 25 wt% B₄C were ~690 MPa and ~95 W?m^?1?K^?1, respectively.