Cadmium Composition Research Articles

Lead and cadmium composition in indoor dust

Pb and Cd levels in different size fractions of indoor dust have been analyzed by differential pulse anodic stripping voltammetry (DPASV) after digestion with HNO 3 and HF using a microwave oven. The relationship between Pb and Cd levels with dust size have been investigated and the metal levels in different sources of indoor dust are compared. In addition, the Pb and Cd composition in different size fractions of dust have been investigated by a sequential extraction method. Dusts are dominated by particulate matter < 500 μm. Pb levels in office dust in the particulate fraction > 75 μm are clearly shown to be higher than that in domestic dust. Generally, for domestic premises, Pb and Cd levels increase with a decrease in the particle size and with an increase in house age, while for office dust, the Pb level increases with an increase of particle size. For domestic houses, most of the metals are bound to carbonate and FeMn oxides in fine dust. For office dust, the situation is more complicated, with a clear increase in Pb in the organic and residue fractions in coarse dust.

The isotopic composition of zinc, palladium, silver, cadmium, tin, and tellurium in acid-etched residues of the Allende meteorite

The isotopic and elemental abundances of Zn, Pd, Ag, Cd, Sn and Te have been measured in three acid-resistant residues extracted from the Allende meteorite. High-efficiency, low-contamination ion-exchange procedures were developed to separate and purify the nanogram amounts of these elements present. Elemental-abundance determinations performed by Mass Spectrometric Isotope Dilution agree with previously published work for similarly derived residues. No isotope anomalies similar to those found for Xe (Xe-HL) in these samples were detected for any of these elements, which is consistent with the residues not being derived directly from the Xe-HL carriers. The lack of major Te-isotope anomalies does not support earlier reports of 126Te and 130Te excesses which were measured by neutron activation in similar samples. Small excesses were detected in the minor isotopes of Sn and Te, but these may be due to measurement problems associated with the small ion currents obtained for these samples. Two of the residue solutions contain Cd with up to several percent excesses for 106Cd and 108Cd. Interpretations of these results are limited by the unknown nature of the carrier minerals in the residues but may indicate the presence of a p-process component in Allende residues.

A liquid-liquid distribution method for the study of complexation of cadmium in natural waters

Liquid-liquid distribution of cadmium between pyridine solution in benzene and aqueous phase containing iodide has been radiometrically investigated, in order to develop a simple method for the study of complexation of cadmium in aqueous solutions. The degree of complexation of cadmium can be determined from the decrease of the distribution ratio in the presence of complexing agents forming non-extractable complexes with this metal. The method was verified for the determination of the composition and stability constants of cadmium with acetate, tartrate, oxalate and glycolate anions and applied for the study of complexation of cadmium in natural waters.

Valence-band barrier formation in graded Hg1−xCdxTe heterojunctions with a valence-band offset included

In two previous papers [J. Appl. Phys. 62, 3267 (1987); 64, 6373 (1988)], we used a highly accurate numerical model to solve the problem of equilibrium barrier formation in graded Hg1−xCdxTe heterojunctions. However, we concentrated solely on narrow-gap p on wide-gap n heterojunctions in which the barrier forms in the conduction band. The present calculation is an extension of that work, but with emphasis on wide-gap p on narrow-gap n heterojunctions. We perform a full parametric study, varying doping concentrations (symmetric and asymmetric), cadmium compositions, and cadmium grading widths. All band profiles are calculated with and without a valence-band offset. Clear trends are observed. Without the valence-band offset, for the larger grading widths, there is a tendency for the valence band to bow down on the n side. The band bowing acts as a potential barrier for the minority carriers. When the valence-band offset is included though, the existence of a barrier to minority carriers depends on the grading width. For the most narrowly graded junctions, a potential well for the minority carriers is present. In the present study, we assume a valence-band offset of 300 meV for HgTe:CdTe. The effects of elevated temperatures and interdiffusion of dopants are also studied, but appear to be inconsequential for the present cases.

Extrinsic p-type doping of HgCdTe grown by organometallic epitaxy

In this letter, we report on the extrinsic p-type doping of mercury cadmium telluride (MCT), during growth by organometallic vapor phase epitaxy. Arsine gas in hydrogen was used as the dopant source. The layers were characterized by Hall effect and by resistivity measurements over the temperature range from 20 to 300 K. The acceptor ionization energy was obtained for different doping concentrations from the Hall coefficient data. Its value decreases with doping concentration, and is about 8.5 meV for a doping of 1.1×1016 cm−3. This is a factor of 2 lower than the ionization energy of mercury vacancies, for layers of this (27–30% cadmium) composition and doping level. Annealing at 205 °C for 15 h in a Hg-rich ambient did not produce significant changes in the measured doping concentration. This indicates that the acceptor level is extrinsic in nature, and that arsenic behaves as a stable acceptor dopant in MCT, grown by organometallic epitaxy.

Mercury cadmium telluride junctions grown in situ by molecular‐beam epitaxy

The characterization of n‐isotype mercury cadmium telluride heterojunctions made in situ by molecular‐beam epitaxy is reported first. The cadmium composition of each side is 0.3 for the top material and 0.21 for the bottom. Both sides were doped with indium. Strong rectification with an ideality factor varying from 1.8 to 2.5 is shown. The forward bias occurs when the wide‐band‐gap material is biased negatively. The preliminary results of the first homojunctions made by the same technique are then presented. The cadmium composition was 0.27. The bottom p‐type material was doped by stoichiometric deviation, whereas the top n‐type material was doped with silicon. The device is sensitive to infrared radiation and it is found that the doping concentration is not uniform. We suggest that generation–recombination is limiting the device operation at high temperatures.

The Oklo natural reactors: cumulative fission yields and nuclear characteristics of Reactor Zone 9

The isotopic composition of molybdenum, ruthenium, palladium, silver, cadmium, tin, tellurium, neodymium and uranium have been measured by solid source mass spectrometry in eight uraninite samples from Reactor Zone 9 at the Oklo natural reactors. Cumulative fission yields have been derived for most of these elements, after correcting where necessary for the primordial component of the element concerned. Neutron capture reactions on a number of nuclides with significant thermal cross sections, and fission chains in which one of the precursor nuclides has a lengthy half-life, have also been examined to provide information on the relative mobilities of the elements involved. The Pd fission yields have been utilised to calculate the proportions of 235U, 238U and 239Pu fission to be approximately 88%, 8% and 4% respectively in Reactor Zone 9. It has also been shown that nearly 50% of the fissioning 235U nuclides were produced from the α-decay of 239Pu. The integrated neutron flux in this zone was calculated to be approximately 3.6 × 10 20 n cm −2. Estimates of the spectral index show that approximately two-thirds of the neutrons in Zone 9 were thermalised. Although it is unlikely that the Oklo reactors operated continuously, this study has shown that the duration of criticality in Zone 9 was approximately 2.2 × 10 5 years. During this period the average fission density was 0.92 × 10 20 fissions cm −3, which represent a total energy output of 4 × 10 8 J g −1 of sample with an average power output of 8.1 × 10 −5 W g −1 of sample. The accurate analysis of the isotopic abundances of nine elements, has enabled the nuclear characteristics of Reactor Zone 9 to be established, thus providing a base for estimating the nuclear inventory of the various fission products. This will allow a comparison to be made with the present-day elemental abundances and hence an evaluation of the mobility of these elements in the geological environment at Oklo.

Cadmium mass fractionation in unequilibrated ordinary chondrites

Measurements of the isotopic composition of cadmium in whole rock samples of a number of unequilibrated ordinary chondrites have shown the presence of significant fractionation in the abundances of the isotopes. Some meteorites are enhanced in the light isotopes with respect to a laboratory standard, whilst the reverse effect is exhibited in other meteorites. The mass fractionation is linear over the range 106–116 within the limits set by experimental error. The extent of the mass-dependent fractionation varies from −2.1‰ per mass unit for light isotope enrichment for the LL3 chondrite Semarkona, to +2.7‰ per mass unit for the LL3 chondrite Bishunpur. This is the first time that negative mass fractionation has been reported for cadmium in meteoritical material. The cause of the fractionation is not known. If Rayleigh distillation were responsible then some correlation with Cd concentration would be expected, but this is not observed. There does not appear to be a correlation between the magnitude of cadmium fractionation and abundance in whole rock samples of unequilibrated ordinary chondrites.

Barrier formation in graded Hg1−xCdxTe heterojunctions

The problem of equilibrium barrier formation in graded Hg1−xCdxTe heterojunctions is solved with the use of a highly accurate computer model. The present calculation incorporates several features designed to improve on past efforts. The Poisson equation is solved as a nonlinear integro-differential equation. Fermi–Dirac statistics are used to allow for the degeneracy associated with inversion for narrowly graded junctions and the degeneracy of the native defect donors. The band structure is obtained from the numerical solution of the secular equation. Fermi–Dirac statistics are taken to govern the degree of ionization of the acceptors and acceptorlike traps. Acceptors are treated as divalent flaws and their ionization energies as linear functions of the cadmium composition. The results are compared to the recent work of Bratt and Casselman [J. Vac. Sci. Technol. A 3, 238 (1985)], and the earlier work of Migliorato and White [Solid-State Electron. 26, 65 (1983)]. Significant differences are found. The results and differences are analyzed. Further computations are made in order to investigate the impact of elevated temperatures and the interdiffusion of doping concentrations along with compensation effects. It is shown that even a smeared p-n junction can have an advantageous effect.

Lattice parameters and Debye temperatures of AgCdZn(α-phase) alloys

The ternary AgCdZn alloys form solid solutions in the f.c.c. phase up to about 30 at.% Cd and 20 at.% Zn in the silver lattice. Six ternary alloys of varying compositions of silver, cadmium and zinc were prepared from high purity starting materials by melting in an induction furnace. The integrated X-ray intensities of Bragg reflections from these alloys were collected at room temperature using a PW 1730 X-ray generator and a proportional counter. The experimental data were processed to obtain the values of lattice parameters, Debye-Waller factors, Debye temperatures θ m and root-mean-square amplitudes of vibration of atoms of the alloy specimens. The Debye temperature of the alloys varied between 191 and 225 K. It was found that θ m decreased with increasing cadmium concentration irrespective of the percentage of zinc present in the silver lattice.

Cross diffusion of Cd and Zn in Cu2S formed on ZnxCd1−xS thin films

Cadmium and zinc compositions in Cu2S formed on ZnxCd1−xS films (0&amp;lt;x≲0.25) by means of ion exchange have been measured using Auger Electron Spectroscopy (AES), Atomic Absorption Spectroscopy (AAS), and Electron Spectroscopy for Chemical Analysis (ESCA). Net concentrations of Cd and Zn in as-formed Cu2S are generally in the 1018–1019 cm−3 range. Heat treatments in both oxidizing and reducing ambients raise the concentrations by over an order of magnitude, with the Zn concentrations increasing more so than those of Cd. Large increases in Zn at or near the Cu2S surface were measured subsequent to heat treatment, accompanied by increased oxygen. Following heat treatments, Cd and Zn concentrations in the Cu2S ’’bulk’’ are found to be less than 1019 and 1020 cm−3, respectively, for all substrate compositions used.

On the reactivity of metallothioneins with 5,5'-dithiobis-(2-nitrobenzoic acid).

Rat liver and horse kidney metallothioneins react with 5,5'-dithiobis-(2-nitrobenzoic acid) (Nbs2) to release 5-thio-2-nitrobenzoate and metal ions. The reactions are slow and exhibit biphasic kinetics with each process having an empirical rate law of the form: rate - k[RSM] X [Nbs2] + k'[RSM], where RSM represents mental-bound thiolate groups. The pseudo-first-order rates are insensitive to pH but are modified in guanidine hydrochloride solution. Rat liver metallothioneins of variable zinc, copper and cadmium composition react similarly and give observable thiol/total metal ratios in good agreement with stoichiometries of SH/(Cd + Zn) of 3 and SH/Cu of 1. A model complex cadmium-2,3-dimercaptopropanol, resembles the proteins in its reaction with Nbs2.

Composition measurements related to the Cu2S/ZnxCd1−xS heterojunction

Composition of zinc and cadmium have been measured for three key regions related to the formation of Cu2S on ZnxCd1−xX by aqueous ion exchange. These are the ion-exchange solution, the resulting Cu2S, and the ZnxCd1−xS near the Cu2S-ZnxCd1−x interface. It is found that the participation of zinc in the ion-exchange reaction is incomplete. Zinc is thus retained in the p-n junction region, and is found mainly in the ZnxCd1−xS just under the interface. This phenomenon helps to account for certain differences between electro-optical properties of Cu2S/CdS and Cu2S/ZnxCd1−xS solar cells.

Low-temperature phase transitions of Fe 3O 4 with innocuous dopants

Both Zn 2+ and Cd 2+ dopants on the A-sites in Fe 3O 4 lead to a loss in the furcation of the lambda anomaly at the Verwey transition but at markedly different dopant levels. Equilibrium adiabatic calorimetry from 5 to 350 K on new compositions of cadmium- and zinc-doped magneties, e.g. Cd 0.002Fe 2.998O 4 and Zn 0.01Fe 2.99O 4, permits an estimate of the mole ratios of Cd and Zn at which they have equivalent effects on the furcation of the lambda anomaly. These results are consistent with a mechanism in which the furcation of the Verwey transition in doped samples depends linearly on the deviation of the lattice constant from that of pure Fe 3O 4. Further studies on a deliberately oxidized sample demonstrate that the furcated lambda anomaly is characteristic of stoichiometric materials; apparently B-site vacancies have a much greater effect on the transition than A-site dopants. Heat capacities and thermodynamic properties of the three samples are summarized.

The isotopic composition of cadmium in terrestrial minerals

The isotopic composition of cadmium in eight terrestrail minerals, and a Laboratory Standard has been measured by solid source mass spectrometry. Within experimental errors, no natural variations in isotopic abundances were found. The extent of mass discrimination was determined by using the double spike technique. The absolute isotopic abundances of cadmium as determined in this work for 106Cd, 108Cd, 110Cd, 111Cd, 112Cd, 113Cd, 114Cd and 116Cd are 1.25%, 0.894%, 12.51%, 12.81%, 24.13%, 12.22%, 28.71% and 7.47% respectively. Because of the higher precision, these abundances should replace the currently accepted values. We would also recommend that a value of 112.410± 0.005 should be used for the atomic weight of cadmium, rather than the currently accepted figure of 112.40.