Change In Impurity Concentration Research Articles

Observation of tearing mode triggering by sawtooth crash with high-Z impurity accumulation in EAST

The high-Z impurity accumulation in the plasma core could degrade the plasma performance and may affect sawtooth crash and trigger tearing modes (TMs) in experimental advanced superconducting tokamak (EAST). Triggering of m/n = 3/2 TM (m and n are the poloidal and toroidal mode numbers, respectively) and m/n = 2/1 TM by a large sawtooth crash is observed and studied under the condition of high-Z impurity accumulation in core plasma in EAST tokamak. This work mainly focuses on the relationship among impurity, sawtooth crashes, and TMs. When there is a certain amount of impurity in the plasma core, sawtooth crash leads to impurity transport, resulting the changes in impurity concentration, and by inducing the density perturbation near the rational surface, TMs become destabilized. The experimental analyses indicate that the outward impurity transport after a sawtooth crash plays an important role in the destabilization of TMs.

Study on divertor detachment and pedestal characteristics in the DIII-D upper closed divertor

Experiments performed in DIII-D demonstrate that higher plasma current and heating power combined with impurity seeding facilitate the achievement of divertor detachment with a higher pedestal pressure and higher plasma performance in H-mode plasmas with a baffled closed divertor compared with an open divertor. Dedicated experiments were carried out to study the impact of power, plasma current and impurity seeding on divertor detachment with ion directed into the divertor favorable for the L–H transition. With a factor of three variation in heating power and with only D2 puffing, no significant difference in the separatrix density at detachment onset was found. The higher heating power leads to higher impurity concentration and wider scrape-off layer (SOL) width, and reduces the detachment onset density to one similar to that in lower-power plasmas. Higher current requires higher pedestal and line-averaged densities to achieve divertor detachment; however, the increase in separatrix density at increasing plasma current is found to be less pronounced. Initial calculations found that both power scan and plasma current scan datasets are qualitatively consistent with theory after considering the change in impurity concentration and heat flux width. This also motivates the future extensive study of transport and divertor impurity behavior in order to have a quantitative comparison between experiment and theory. Compared with an open divertor, a closed divertor facilitates detachment onset at ∼40% lower line-averaged plasma density. Additional N2 seeding facilitates the achievement of detachment at a lower separatrix density and thus a higher pedestal temperature, which is beneficial for advanced tokamak scenarios. Higher heating power requires a higher N2 puffing rate to achieve the same degree of detachment, while a higher N2 puffing rate leads to lower detachment onset line-averaged density, both of which agree with theory. In contrast to the narrower pedestal in an open divertor approaching detachment, the pedestal density width in a closed divertor increases with density. The density gradient increases with line-averaged density at higher plasma current, but remains nearly unchanged at lower plasma current. In particular, compared with discharges with low power, at high heating power the pedestal density gradient is much weaker, while the SOL density is significantly higher and wider. At the same plasma current, both pedestal pressure gradient and temperature gradient decrease linearly with the line-averaged density but remain similar across different heating powers. Even with different plasma current and heating power, the normalized pressure gradient remains identical. As a result, achievement of divertor detachment with a higher pedestal pressure and higher plasma performance is shown in a closed divertor, which is important for improving core–edge integration as one of the critical issues for future tokamak fusion reactors.

Комп’ютерне прогнозування процесів «фільтрування-регенерація» швидких конусоподібних адсорбційних фільтрів з кусково-однорідними пористими завантаженнями

To predict the processes of «filtration-regeneration» of fast cone-shaped adsorption filters with piecewise homogeneous porous loads, mathematical models of their main technological modes are formed: filtration, backwashing, chemical regeneration and direct washing, taking into account the influence of temperature effects on the internal kinetics of mass transfer while maintaining constant speeds of the respective modes. Given that in the filtration mode the convective components of heat and mass transfer and physical and chemical adsorption prevail over the contribution of diffusion and physical desorption, in the chemical regeneration mode the convective components of heat and mass transfer and physical and chemical desorption prevail over the contribution of diffusion and physical adsorption. modes of reverse and direct washing convective components of heat and mass transfer and physical desorption and chemical adsorption prevail over the contribution of diffusion and physical adsorption, algorithms of numerical-asymptotic approximations of solutions of corresponding nonlinear singularly perturbed boundary value problems, bounded by given, smooth, orthogonal to each other along the edges, two equipotential surfaces and one flow surface and divided into several subregions by some given equipotential surfaces. The proposed models in the complex by taking into account the effect of changes in temperature and filtration flow rate along the filter height on the coefficients that characterize the mass transfer rates during physical and chemical adsorption and desorption, filtration coefficient, allow computer experiments to better investigate changes in impurity concentrations in filtration flow and on the surface of the loading adsorbent, retained by physical and chemical adsorption, filtration flow temperature, filtration coefficient and active porosity in each loading layer along the filter height and based on them predict more optimal use of adsorbents of each loading layer and time intervals of filters, respectively, in filtration, backwash, chemical regeneration and direct washing with constant speeds of the respective modes

Evolution of impurity incorporation during ammonothermal growth of GaN

Ammonothermally grown GaN is a promising substrate for high-power optoelectronics and electronics thanks to its scalability and high structural perfection. Despite extensive research, ammonothermal GaN still suffers from significant concentrations of impurities. This article discusses the evolution of impurity incorporation during growth of basic ammonothermal GaN, in specific whether the impurity concentration changes temporally along the growth direction and how the autoclave influences the impurity concentration. The effect of the impurities on the structural, electrical and optical properties of the grown crystal is also discussed. The chemical analysis is carried out by time of flight secondary ion mass spectroscopy (ToF-SIMS) and laser-ablation inductively-coupled plasma mass spectroscopy (LA-ICP-MS). Strain and dislocation generation caused by impurity concentration gradients and steps are studied by synchrotron radiation x-ray topography (SR-XRT). Fourier transform infrared (FTIR) reflectivity is used to determine the effect of the impurities on the free carrier concentration, and the luminescent properties are studied by low temperature photoluminescence (PL). The influence of the autoclave is studied by growing a single boule in multiple steps in several autoclaves. LA-ICP-MS and ToF-SIMS ion intensities indicate that the impurity concentrations of several species vary between different autoclaves by over an order of magnitude. SR-XRT measurements reveal strain at the growth interfaces due to impurity concentration gradients and steps. Oxygen is determined to be the most abundant impurity species, resulting in a high free carrier concentration, as determined by FTIR. The large variation in Mn concentration dramatically affects PL intensity.

Tryptophan oxidation catabolite, N-formylkynurenine, in photo degraded cell culture medium results in reduced cell culture performance.

Chemically defined media have been widely used in the biopharmaceutical industry to enhance cell culture productivities and ensure process robustness. These media, which are quite complex, often contain a mixture of many components such as vitamins, amino acids, metals and other chemicals. Some of these components are known to be sensitive to various stress factors including photodegradation. Previous work has shown that small changes in impurity concentrations induced by these potential stresses can have a large impact on the cell culture process including growth and product quality attributes. Furthermore, it has been shown to be difficult to detect these modifications analytically due to the complexity of the cell culture media and the trace level of the degradant products. Here, we describe work performed to identify the specific chemical(s) in photodegraded medium that affect cell culture performance. First, we developed a model system capable of detecting changes in cell culture performance. Second, we used these data and applied an LC-MS analytical technique to characterize the cell culture media and identify degradant products which affect cell culture performance. Riboflavin limitation and N-formylkynurenine (NFK), a tryptophan oxidation catabolite, were identified as chemicals which results in a reduction in cell culture performance.

Bismuth doping of hydrogenated amorphous germanium thin films

The optoelectronic properties of Bi-doped hydrogenated amorphous germanium (a-Ge:H), with relative impurity concentrations [Nimp/NGe] ranging between 8×10−6 and 5.5×10−3, are reported. The incorporation of Bi produces small changes in the dark conductivity of a-Ge:H. For a three orders of magnitude change in impurity concentration the room-temperature conductivity changes by just one order of magnitude. Within this doping range no, or small, changes were measured in the values of the pseudo-gap, the Urbach energy, and the hydrogen content. The Fermi level always remains far from the conduction band edge, shifting by only 0.1 eV for the sample with the largest doping concentration. The main conclusion is that Bi is a very inefficient active donor in a-Ge:H. The likely reasons for such behavior are discussed.

Corrosion of Stainless Steels and Carbon Steel by Molten Mixtures of Commercial Nitrate Salts

The isothermal corrosion behavior of two stainless steels and a carbon (C) steel in mixtures of NaNO3 and KNO3 was evaluated to determine if the impurities found in commodity grades of alkali nitrates aggravate corrosivity as applicable to an advanced solar thermal energy system. Corrosion tests were conducted for approximately 7000 hours with Types 304 and 316 stainless steels at 570 °C and A36 C steel at 316 °C in seven mixtures of NaNO3 and KNO3 containing variations in impurity concentrations. Corrosion tests were also conducted in a ternary mixture of NaNO3, KNO3, and Ca(NO3)2. Corrosion rates were determined by descaled weight losses while oxidation products were examined by scanning electron microscopy (SEM), electron microprobe analysis (EPMA), and x-ray diffraction (XRD). The nitrate mixtures were periodically analyzed for changes in impurity concentrations and for soluble corrosion products. Results of these tests indicated that the short-term corrosion rates of the stainless steel specimens in many of the mixtures could be described in terms of parabolic kinetics. However, no single rate law could be assigned to the corrosion kinetics resulting from exposure in all of the mixtures. For engineering applications, corrosion rates over the entire exposure period are best described as linear with respect to time. In the binary nitrate mixtures, the annualized rates of metal loss were found to be between 6 and 15 µm/year for the stainless steel specimens at 570 °C depending on the particular mixture. Metal loss for the C steel specimens immersed in these same mixtures at 316 °C extrapolated to approximately 1–4 µm/year. SEM and XRD revealed that the complex, multiphase surface oxides formed on the stainless steel coupons were composed primarily of iron-chromium spinel, iron oxides, and sodium ferrite. Magnetite was the principal corrosion product formed on the carbon steel specimens. Overall, for the typical range of impurities in commercially available nitrate salts, corrosion rates for solar thermal energy applications remained acceptable for all of the materials examined.

Spectroscopic investigation on the impurity influxes of carbon and silicon in the ASDEX upgrade experiment

Emission profiles of carbon and silicon along the heat shield (HS) were measured before and after siliconization of the vessel. Siliconization does not affect the divertor and therefore large changes in impurity concentrations and influxes cannot be attributed to it. A strong transient increase of the silicon concentration was observed immediately after the siliconization lasting for a few discharges. However, no large change was observed in the silicon influx from the HS, excluding it as possible source of the concentration increase. The fast decrease in the silicon concentration can be attributed to strong erosion at the outer limiter. The impurity influxes for C 2+ and Si 2+ from the HS are estimated and compared with the corresponding core concentrations. The relative contribution to the plasma impurity content from divertor, HS and outer protection limiter is discussed. Zeeman splitting analysis permits one to identify the emitting region of carbon radiation. Anomalously high silicon sputtering yields are measured, which do not however cause high core silicon content.

Evolution of a system of nuclei in a supercooled binary melt

Growth of fluctuationally emerging nuclei of a new phase is considered by the example of the process of volume crystallization of a supercooled binary melt. The evolution of the size distribution function for crystals, the kinetics of removal of supercooling, and the change in impurity concentration and temperature in the melt are investigated on the basis of a nonlinear system of mass balance and heat balance equations and the kinetic equation for the crystal size distribution density function. Asymptotic representations for the basic macroscopic characteristics of the process are obtained.

Optimization of the surface impurity concentration of passivated emitter solar cells

The optimum emitter surface dopant concentration (Ns) in passivated silicon solar cells is found for different base resistivities with the help of a computer model. The model takes into account not only the band-gap narrowing, Auger and surface recombination effects on the dark saturation current, but it also considers the variation of series resistance as the impurity concentration changes. The results obtained with the model show different behavior for passivated and nonpassivated solar cells. The open-circuit voltage and the conversion efficiency of nonpassivated solar cells is almost independent of the surface impurity concentration when this parameter is greater than 2×1019 cm−3. On the other hand, for passivated solar cells the optimum value is in the range 2.5×1019 cm−3 ≤Ns<5×1019 cm−3 for typical base resistivities. Furthermore, it is confirmed that even when Ns is high (Ns>1×1020 cm−3) there is an advantage in efficiency of passivated solar cells over those which have a high recombination at the surface. However, in order to increase the efficiency appreciably Ns has to be optimized. The model also gives a procedure to optimize the grid design simultaneously with the surface concentration, assuming the other parameters are optimum.

Electrostatic law for interfaces in abrupt semiconductor homostructures

The electrostatic law for interfaces in abrupt semiconductor structures (by which we mean structures in which an abrupt change in impurity concentration occurs) is elaborated and proved. Using this law and knowing only the temperature, the dielectric constant and the carrier concentration, we can determine easily and accurately various important values characterizing the electrical properties of the given structure, such as the electrostatic potential at the interface, the charge carrier concentration at the interface, the maximum internal electric field, the total electric charge of the double layer at the interface, and the thickness of the space charge region in the double layer.

In Situ AES Characterization of Rotating Electrical Contacts

The electrical contact resistance and surface chemical composition of a rotating copper slip ring in contact with two wire brushes were investigated in situ under ultrahigh vacuum (UHV) conditions as a function of the number of revolutions of the slip ring. The initial surface of the copper slip ring was examined by Auger electron spectroscopy (AES) techniques and found to be almost completely covered by surface impurities largely consisting of carbon. As the slip ring was rotated in contact with the brushes, the impurity concentration declined sharply to less than ten at% after several hundred revolutions. A corresponding decrease in electrical contact resistance and a sharp increase in friction and wear were also observed. Regardless of polarity, the brush with the higher contact pressure had the lower contact resistance. The in situ experiments were terminated when the motor turning the slip ring could no longer overcome the adhesive forces between the brushes and slip ring. Subsequent scanning electron microscopy observations of the brush and slip ring surfaces gave supplementary information on the mechanisms of friction, wear and surface cleaning during rotation. For a given experiment the change in contact resistance during rotation divided by the change in impurity concentration on the surface of the slip ring is approximately the same for the positive and negative interfaces. This parallelism of contact resistance and surface impurity concentration suggests that the former is caused to a significant extent by the latter. Since the surface impurities are mostly carbon atoms, it is concluded that carbon impurities are largely responsible for the observed contact resistance.

Optical and electrical properties of CdGeAs2

Large-grained ingots of CdGeAs2 have been grown from near-stoichiometric melts. Resistivity and Hall coefficient (RH) measurements were made on a large number of samples at 77 and 300 K and in some cases up to 450 K. Good fits to the log RH vs 1/T plots are obtained by using a model that assumes three kinds of electronic levels within the energy gap: donors, shallow acceptors, and acceptors with an ionization energy of 0. 30 eV. The deep acceptors are probably native defects, since their concentration varies by nearly four orders of magnitude from ingot to ingot with little change in impurity concentration. Between the intrinsic absorption edge at about 2. 5 Μm and the two-phonon absorption band at 18 Μm, the optical absorption increases with increasing deep acceptor concentration. By using oriented single-crystal samples ∼ 1 cm on a side, conversion efficiencies as high as 27% have been achieved for second-harmonic generation with single-mode pulses from a CO2 TEA laser.

Experimental results and computer simulation of impurity particles motion in N-hexane under D.C. and A.C. conditions

The study of impurity particles motion in a dielectric fluid under the action of an electric field can be of interest in the analysis of conduction and breakdown processes, especially in order to assess the changes in impurity concentrations. To this aim, rather sophisticated models of particle motion are required. In this paper, on the basis of a theory of electro-dielectrophoretic forces previously developed by the authors for spherical particles, a simulation algorithm of impurity particles motion is implemented by proposing models of the charge exchange mechanism at the particle-electrode contact. Experimental identification procedures of some model parameters are outlined and performed, and simulations of particle trajectories under a.c. and d.c. voltages for a sphere plane geometry, in the case of both conducting and dielectric particles in n-hexane, are compared with experimental results obtained by high-speed movie.

Effect of vanadium on the P-C and P-I magnetic transition in chromium alloys

Our experimental studies of electrical resistivity and magnetic susceptibility on Cr+2at.%Si and Cr+3at.%Fe doped with 0–2 at.% vanadium provide evidence that the depression in TN for the same change in impurity concentration is significantly different depending on whether a para-to-incommensurate (P-I) or para-to-commensurate (P-C) spin-density-wave structure is obtained at the onset of antiferromagnetic ordering. This we explain by assuming the observed empirical correspondence between the effect of pressure and the decrease of electron to atom ratio on TN. However, the reason for the two different rates of dTN/dP at the P-I (∼6 K/kbar) and P-C (∼30 K/kbar) type transitions in chromium alloys remain totally unexplained.

Precipitation and re-solution of impurities at the surface of indium on traversing the melting-point

In the course of preparing a clean indium surface for solid-versus-liquid studies, changes in the surface concentrations of sulphur and oxygen were observed by AES and XPS while the metal was heated and cooled through its melting-point. Both impurities disappeared on melting, and reappeared on solidification, over a very narrow temperature range; the disappearance and reappearance were to a certain extent reproducible. The effect was found to be similar in characteristics to that observed for the behaviour of carbon on a nickel surface by Shelton et al., and the same Bragg-Williams model is invoked to explain the sharpness of the impurity concentration changes with temperature. Although the maximum temperature reached by the indium was only 200°C, traces of platinum were also observed on the indium surface after melting, in both the AES and XPS spectra, probably as a result of solution from the platinum boat.

OUTDIFFUSION THROUGH SILICON OXIDE AND SILICON NITRIDE LAYERS ON GALLIUM ARSENIDE

Backscattering of MeV 4He ions has been used to analyze the Ga and As content in SiO2 and Si3N4 dielectric layers deposited on single-crystal GaAs substrates. Changes in impurity concentrations and distributions are seen after isothermal annealing at both 750 and 800°C. Evidence for Ga outdiffusion is clear.

Switching response of graded-base PN junction diodes

This paper extends the earlier analysis by Kingston of the switching response of a uniform-base diode to a graded-base diode. It concerns the time required to switch a diode from a forward-biased to a reverse-biased condition. The current transient can be separated into two phases: 1) the constant current phase during which the carrier density at the junction changes gradually from a forward-biased to a reverse-biased condition, and 2) the nonconstant current phase during which the injected carriers stored in the base region gradually disappear. In the present analysis, it is found that in a graded-base diode where the impurity concentration decreases from the emitter junction towards the base contact, the time for the constant current phase is greatly shortened because of favorable initial carrier distribution. The effect is already significant if the impurity concentration changes by a factor from 3 to 1 from the emitter junction to the base contact. To shorten the nonconstant current phase, however, a much larger change of impurity concentration, say of the order from 500 to 1, from the emitter junction to the base contact is needed.

SUBSTRUCTURES AND DISLOCATIONS PRODUCED DURING SOLIDIFICATION OF ALUMINUM

The substructure and the dislocation arrangement produced during polycrystalline solidification of 99.99% aluminum has been studied using the decanting method for the interruption of the solidification. Etching techniques to reveal individual dislocations and the method of thin epitaxial layers of aluminum oxide to reveal changes in impurity concentration have been employed simultaneously. The development of the structure towards the interior of the solid was studied by successive electrolytic polishings. In this way it was possible to show that the "cell substructure" formed by dislocations originates in areas where "microsegregation" of impurities has occurred. Towards the interior of the solid a dislocation rearrangement into a "macromosaic" substructure which is closely related to the cellular substructure occurs. At the same time a substructure of "macrosegregation" occurs, arising from the original grain boundaries. The results are interpreted in the light of recent theories of solidification.

A new method of zone leveling for materials having distribution coefficients in the range of 0–0·6

By doping an originally pure crystal at both ends and by subsequently melting it repeatedly back and forth, the calculated impurity distributions, as obtained after each pass, converge to a horizontal straight line. This is, therefore, a useful method for obtaining a uniform impurity distribution along a single crystal. In the last zone, however, the abrupt change in impurity concentration, due to the normal freezing cannot be avoided. The first doping of the crystal has to be done before the first pass and the second doping at the other end of the crystal after the first pass. Graphs are given for different values of k, for the length of the crystal in relation to the zone length and for the minimum number of passes needed to obtain a uniform impurity distribution. The results indicate that the method is extremely well suited for materials with values of k up to 0·6, while ordinary zone leveling fails for materials with a value of k between 0·1 and 1. In the new method the impurity content in any pass is proportional to the introduced impurity content at one end. Thus any desired uniform impurity distribution can be easily obtained.