Concentration Of Metal Impurities Research Articles

Effective Kα x-ray excitation rates for plasma impurity measurements

Metal impurity concentrations are measured by the pulse-height-analyzer (PHA) diagnostic from Kα x-ray peak intensities by use of an averaged excitation rate 〈σv〉. Low-Z impurity concentrations are inferred from the continuum enhancement (relative to a pure plasma) minus the enhancement due to metals. Since the PHA does not resolve lines from different charge states, 〈σv〉 is a weighted sum of rates; coronal equilibrium is usually assumed. The 〈σv〉 used earlier omitted the intercombination and forbidden lines from the dominant heliumlike state. The result was an overestimate of metals and an underestimate of low-Z impurities in cases where metals were significant. Improved values of 〈σv〉 using recent calculations for H-, He-, and Li-like Fe range from 10% to 50% larger than the earlier rates and yield metal concentrations in better agreement with those from vacuum ultraviolet (VUV) spectroscopy.

Changes in impurity radiation during ICRF heating of PLT tokamak plasmas

This paper presents the results of a study of the increase in impurity line radiation in the 15-360 Å region during ICRF heating of PLT plasmas, with emphasis on metallic impurity (Ti and Fe) behaviour. Central titanium and iron densities are given for a variety of ICRF heating experiments; total central metallic impurity concentrations of up to about 0.3% of ne(0) are observed at the 2.0 MW RF power level. This study shows that the power radiated by these elements is a small (about 10%) fraction of the total input power to the plasma for the present heating efficiency at 2.0 MW RF power. The impurity line brightnesses scale approximately linearly with RF power up to 2.8 MW. The antenna Faraday shields are shown to be the primary source of metallic impurities during both ICRF heating and Ohmic heating only. The impurity content of discharges heated using a single half-turn antenna and a pair of centre-fed antennas (having the same total surface area but half the poloidal extent of the half-turn antenna) is the same at a relatively low RF power of 350 kW, indicating that the impurity influx does not depend on the poloidal length of the antennas (or that the plasma interacts only with a localized area on the Faraday shields).

Surface conditioning of liners and limiters of textor by plasmachemical carbon deposition

A simple plasmachemical method has been used to deposit in situ thin carbon layers onto the liner and limiters of TEXTOR. The technique leads to the formation of metal carbides on the exposed surfaces. The layer thickness can be controlled easily and the deposit can be removed by thorough RG-discharge cleaning, if desired. Part of the residual oxides which are difficult to reduce by discharges in pure hydrogen are removed as CO. First measurements are presented which indicate a decrease of the oxygen and metal impurity concentration in tokamak discharges when the wall has been carbidized.

Prevention and reversal of oxygen embrittlement in nickel

Abstract Attempts to reverse the oxygen embrittlement caused by high temperature air exposure of nickel of various purities are described. It is shown that reversal can be effected in certain reducing environments in those grades of nickel having very low concentrations of metallic impurities, except in regions where heavy cavitation has occurred owing to gas formation. Oxygen embrittlement can similarly be prevented by prior annealing in hydrogen, but again only in the absence of significant metallic impurities. These results are explained on the basis of three distinct phenomena leading to embrittlement, and their implications are considered for the similar embrittlement observed in superalloys.

Electron correlations in alloys of simple and transition metals

The effect of dynamical electron-electron correlations on the electronic structure of the paramagnetic disordered binary alloy of the transition and simple metals is studied for the Anderson model extended to arbitrary concentration of the transition metal impurities. We use the terminal-point approximation for the many-body quantities, which allows us to solve the random part of the problem within the single-site approximation. The many-body part of the problem is treated within the selfconsistent T-matrix approximation, valid for low, but finite concentration of particles ( ⩽ 0.3/atom/spin). For low concentration of the transition metal impurities we obtain results identical with those for the Anderson model. The theory is illustrated numerically for a simple bandstructure model.

ICRF Heating in TFR and the problem of impurity release

The efficiency of ICRH in TFR was limited in the 1981 experiments by an important influx ofmetallic impurities in the plasma core. Using carbon limiters and different materials for theantennas Faraday shield leads to a significant decrease of the metallic impurity content : incontrast with the previous results, the increase of temperature associated with the RF pulseremains now constant for more than 0.1 s. However, the power radiated by the metallic ions atthe end of the RF pulse still reaches an appreciable fraction of the total RF power,particularly when using an antenna generating the RF wave from the low magnetic field side ofthe plasma. This effect is attributed to an interaction of the RF wave with the low densityplasma located in the scrape off layer.

Purification of metallurgical-grade silicon by the thermomigration of impurity clusters

A thermomigration technique was applied to purify metallurgical-grade silicon wafers which contain a high concentration ( e.g. more than 100 ppm) of metallic impurities. Wafers cut from the tang end of once-pulled ingots exhibiting a lamellar structure belong to this category. The method requires a high temperature gradient across the wafers with the temperature on the cooler side in excess of 1000 °C. At this temperature the clusters of impurities alloy with the silicon to form droplets. The droplets then migrate toward the hotter side of the wafers. Subsequent etching removes the alloyed impurities from the wafers. The effectiveness of this purification technique was directly analyzed by comparing emission spectroscopy data before and after thermomigration. An impurity reduction of at least one order of magnitude was achieved for such impurities as aluminum, iron, manganese, titanium and vanadium.

Purification of Rice Hulls as a Source of Solar Grade Silicon for Solar Cells

Rice hulls, a by‐product of rice refining, naturally contain both silica and carbon compounds. The metallic impurity content of these hulls can be greatly decreased by appropriate leaching techniques, making the purified material suitable as a source of upgraded metallurgical silicon. The impurity level in the hulls after leaching, and after appropriate coking, is described, as well as techniques for adjusting the carbon to silica ratio in the coked materials.

Allowed and forbidden n=2-2 transitions of the elements Ti, Cr, Fe, Co and Ni in Tokamak discharges

Wavelengths and transition probabilities are presented for the allowed and forbidden n=2-2 transitions belonging to the fluorine I to lithium I isoelectronic sequences which are expected to be observed in Tokamak discharges. The listing is compiled for the elements Ti, Cr, Fe, Co and Ni, these metals being likely contaminants. The emphasis is placed on an accurate compilation of wavelengths from laboratory and solar-flare observations. Where these are inadequate, predictions are given. In the case of the nitrogen-, carbon- and boron-like intercombination lines, these predictions represent a significant improvement on previous data. Theoretical line intensities calculated for conditions typical of a Tokamak plasma are given for each transition. Identifications in Ti of the five strongest forbidden transitions lying in the visible and near-UV spectral regions have been made in spectra from the DITE Tokamak. Such spectral features are useful diagnostic indicators of metallic ion impurity concentrations and mass motions.

‘‘Defect state model’’ and effect of transition metal impurities on metal-free phthalocyanine: Electrical and photoconductive properties

A mechanistic study of the effect of transition metal impurities on trap density, electrical and photoelectrical properties of metal-free phthalocyanine was carried out. The experimental results showed that electrical and photoconductive properties of H2Pc were a function of trap density (nt) and concentration of metallic impurities (particularly iron). The results were analyzed in terms of several theoretical models. Both the hydrogenic-type shallow impurity state model and conduction due to bridging of host molecules via iron impurity atoms were incompatible with the results. An excellent description of the observed electrical and photoconductive behavior of H2Pc as a function of trap density and impurity can be given using a defect state model, analogous to deep state in inorganic system. This model correctly predicts the dependence of specific conductivity, σ, of H2Pc on the inverse of trap density (1/nt) at low concentration and on 1/nt2 at high trap concentration. Similarly, the model offers an explanation of the change in the dependence of conductivity under illumination from 1/Ne to 1/Ne1/2 as a function of increased trap density.

Experimental investigation of defect states in amorphous chalcogenide glasses

Our current understanding of the nature of defects in pure and doped amorphous semiconductors has been greatly improved with the advent of several recent models which predict the influence of such defects on the optical and transport properties. We shall describe an experimental characterization of the important defect states in the model binary chalcogenide semiconductor (As 2Se 3) which has been systematically doped (to levels of 10 17−10 20 cm −3) with a variety of predominantly metallic impurities (Mn, Fe, Ni, Cu, Zn, Ga, In, Tl, Li, Na, K, I). Several complementary experimental techniques have been employed including transient hole transport, dc conductivity, electron spin resonance and photoluminescence (PL) measurements. Dramatic changes of the hole drift mobility are observed with the addition of low concentrations of most metallic impurities, but the PL and photo-induced ESR results are generally much less sensitive to these dopants. These findings will be discussed in the context of current models of defects which associate transport traps, paramagnetic and recombination centers with bonding anomalies on the major constituent (As or Se) atoms.

Large grain silicon films on metallurgical silicon substrates for photovoltaic applications

Metallurgical silicon at a cost of about $ 1/kg is an economical substrate for the deposition of silicon films for photovoltaic applications. To prepare the substrate, pulverized metallurgical silicon, purified by acid-extraction and phosphorus pentoxide treatment, was recrystallized on graphite by unidirectional solidification and zone-melting. The structural and crystallographic properties of metallurgical silicon substrates prepared at different rates of solidification were investigated. Silicon films of controlled electrical resistivity were deposited on metallurgical silicon substrates by the thermal reduction of trichlorosilane containing appropriate dopants. The concentration and distribution of metallic impurities, Hall mobilities, potential barriers and minority carrier diffusion length profile in these films were investigated. Solar cells of 9 cm 2 area were prepared from silicon films of 25–30 μm thickness containing a p-n junction. By using a drift field in the surface region and a back surface field at the silicon film/substrate interface, AM1 efficiencies of up to 9.75% have been obtained.

Analysis of trace metal impurities in phthalocyanine pigments

An analytical procedure was developed to determine the content of metallic impurities in metal-free phthalocyanine (H2Pc) pigments. The technique involves dissolving the pigment in concentrated sulfuric acid, followed by oxidative degradation using hydrogen peroxide. Phthalimide was the only organic oxidation product. The remaining trace metals in the solution were determined using plasma atomic emission. The trace metals content in a commercial α polymorph of phthalocyanine pigment (ICI-Monolite Fast Blue GS) was monitored as a function of purification processes. Experiment showed that Monolite Fast Blue contains 2500 ppm Na; 350 ppm Ca, and 370 ppm Fe. Purification of this material by extraction with hot dimethyl-formamide gave β-H2Pc, which contained 300 ppm Na, 110 ppm Ca, and 310 ppm Fe. Sulfuric acid purification (acid pasting) reduced the content of non-transition elements, but in-increased the content of transition metals.These results were compared to the trace metal content in H2Pc pigment which were synthesized.

Effect of Surface Preparation and Heat‐Treatment on Hole Diffusion Lengths in VPE GaAs and GaAs0.6 P 0.4

Chemical etching of a semiconducting surface is a process which can deposit trace amounts of metals that can have significant effects on the bulk lifetime after heat‐treatment. Several frequently used cleaning processes have been shown to be poor in this respect and improved versions are suggested. The effectiveness of an etchant in cleaning a surface of trace metallic impurities is greater for larger etching rates, and smaller for larger metallic impurity content in the etchant.

Time resolved metal impurity concentrations in the dite tokamak using RBS analysis

A carbon disc collector has been inserted into the boundary layer of a tokamak plasma to a position radially inside the limiter. The disc is rotated about an axis parallel to the toroidal field B T during a discharge so that a time resolved measure of the flux of impurities to the collector is obtained. The concentration of impurities on the surface is subsequently analysed using Rutherford backscattering of 3.5 MeV 14N + ions. It has been observed that the flux of metal impurities to the probe rises rapidly at the beginning of all discharges and that in discharges with few low Z impurities the flux remains high throughout the discharge. The flux can be reduced, however, by injecting a low Z impurity, e.g. neon and can be reduced even further by injection of hydrogen gas during a discharge.

Measurement of dilute metallic impurity concentration in lead by differential scanning calorimetry

The traditional method for purity determination by analysis of the peak shape of the melting transition has included fitting to a linear relation between sample temperature ( T) and the reciprocal fraction molten ( 1 7 ). This technique, however, necessitates the use of a series of calibrants to determine the proper choice of limits for 1 7 as a function of purity. In this paper, a non-linear relation between T and 1 7 is developed and applied to the determination of metallic impurities in Pb in the range of a few to 1000 parts per million. The results are found to be independent of the range of 1 7 used. The use of differential scanning calorimety in this determination is discussed, and rate effects are also mentioned.

Crystal growth considerations in the use of “solar grade” silicon

The use of lower purity, lower cost, “solar grade” silicon for crystal growth feed stock has been proposed as one means to reduce the cost of solar cells. The metallic impurities in this material have a deleterious effect on cell efficiency; however, the maximum tolerable impurity concentrations depend not only on the acceptable cell efficiency, but also on the details of the crystal growth process. We wuantitatively analyze the problem using data on relative solar cell efficiency as a function of metallic impurity concentration and the effective segregation coefficients for Czochralski growth. We find, for example, that solar cell performance rather than interface breakdown during crystal growth will most likely determine the permissable impurity content in solar grade silicon. We also show that continuous rather than sequential melt replenishment is advantageous for Czochralski growth.

Defect centers in oxygen-deficient rf-sputtered SiO2 films. I. Electron spin resonance

Two new defect centers have been observed by electron spin resonance (ESR) in rf-sputtered SiO2 films with low metallic impurity content. At 77°K, as-sputtered films have three ESR centers, two HX centers, and an HY center. The HY center, a hole center, has g = 2.008–2.009. The HX center has two components, the HXa center with axial symmetry and with ga∥ = 2.022 and ga⊥ = 2.037; the HXb center with gb = 1.984 which is split by hyperfine interaction with hydrogen. The hydrogen hyperfine interaction has axial symmetry, Ab∥ = 30.2 G and Ab⊥ = 63.6 G. Both the HX and HY centers show hyperfine splitting due to interaction with Si29. Annealing of as-sputtered films in hydrogen-containing atmospheres at 350°C eliminates the HY center, leaving the HX center. Annealing in N2 or O2 at 550°C also eliminates the HY center; the resulting HX resonance is identical to that obtained after hydrogen anneal. The activation energy for the appearance of the HX center is 0.036 eV when samples are cooled to 77°K. From similarities in hyperfine interactions, activation energies, and g values, it is suggested that the HY center is the silicon analog of the Al center in SiO2. It occurs in oxygen-deficient SiO2 and is due to removal of a nonbonding electron from an oxygen which bonds Si+3 to Si+4. The HY center is converted to the HX center by bonding with hydrogen which can be provided by H2 from the annealing ambient or by the H atom in Si–OH incorporated in SiO2 films.

Purification of calcium and barium by reactive distillation

The purification of calcium and barium has been carried out by double ultra high vacuum “reactive distillation” of these metals in a molybdenum column. The distributions of all elemental impurities and molecular species in the starting materials and in various fractions of the distillate were determined with spark source mass spectrographic analysis. The residual gases before, during and after distillation were analyzed by a quadrupole mass spectrometer. The total contents of metallic impurities (carbon and silicon included) of the double distilled metals were, in calcium 124 at. ppm and in barium 69 at. ppm.

Influence of impurities on the surface structures and fault generation in homoepitaxial Si (111) films

In situ LEED studies of the homoepitaxial growth of Si(111) films by uhv sublimation, indicate a strong correlation between the type of surface structure generated and the metallic impurity content of the silicon substrates as estimated from minority carrier lifetimes. The development of the familiar Si (111)−7 × 7 structure is favored by the presence of lifetime-killing impurities in the substrate. Experiments where Fe is introduced on high lifetime substrates prior to annealing and film growth, suggest that this impurity species plays a role in the generation of the 7 × 7 surface structure. Electron microscopy reveals that homoepitaxial Si(111) layers are generally faulted, the number density of which increases progressively as the growth temperature is lowered and the deposition rate increased. Films deposited on high lifetime silicon contain substantially fewer stacking faults than those grown on low lifetime substrates. These results suggest that the faults originate at the substrate surface at microprecipitates consisting of fast diffusing, low solubility impurity species.