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At least 361 records · Page 20

Recent developments in semiconductor research

Recent findings drawn from various laboratories focused toward a new understanding of the nature of crystal growth and how impurities are incorporated or excluded from the lattice were considered. From these findings a heuristic approach is given for improving the uniformity and charge carrier concentration of thin semiconductor films. Such improvement is considered essential to achieve respectable yield on large scale integrated circuits. A tentative conclusion is drawn which indicates that if the host compound crystal reactants are provided to the growing crystal surface in a reasonably stoichiometric manner, it is virtually impossible to include impurities into a growing crystal surface held at a sufficiently low temperature; impurities are incorporated into the crystal by gettering action of crystal defects located one and two monlayers beneath the growing surface. Devices in 3-5 semiconductors based on heterojunctions and two dimensional electron gases are noted and reliability concerns are voiced. Alternatives and novel device structures such as truly single crystal high quality silicon on insulator and beta silicon carbide devices are presented.

Yoder, M. N.↗

Gallidation assisted impurity doping

In one embodiment, a product includes a structure comprising a material of a Group-III-nitride having a dopant, where a concentration of the dopant in the structure has a concentration gradient characteristic of diffusion of the dopant inward from at least a portion of a surface of the structure in a direction substantially normal to the portion of the surface. The structure has less than 1% decomposition of the Group-III-nitride at the surface of the structure.

Voss, Lars↗

Separation of cobalt, nickel, and manganese in leach solutions of waste lithium-ion batteries using Dowex M4195 ion exchange resin

Current methods to separate and recover critical metals from waste lithium-ion battery (LIB) scrap require multiple unit operations that involve selective leaching and a combination of separation methods. These methods include impurity removal, solvent extraction, ion exchange, and precipitation, to achieve high purity Co and Ni products. In this paper an efficient extraction strategy was developed to individually separate Ni and Co from a LIB leachate that was generated from an electrochemical leaching process. By using Dowex M4195 resin, as the only method of separation, additional impurity removal operations were not needed. After loading and eluting, three different fractions were obtained: 99.0% Ni concentrate, 98.5% Co concentrate, and a Li/Mn rich concentrate allowing the separation of Ni, Co and Mn. Here, the developed process is then presented as an alternative for metal recovery from leachates from scrap LIBs with minimum chemical addition and pH adjustment.

36 MATERIALS SCIENCE↗

Hydroxylation and Cation Segregation in (La 0.5 Sr 0.5 )FeO 3-$δ$ Electrodes

Simultaneously achieving high activity and stability is the primary challenge when engineering (electro)catalysts. Transition metal perovskite oxides are employed as air electrodes for solid-oxide fuel cells and electrolyzers. However, degradation of oxygen exchange kinetics at the solid–gas interface, often linked to alkaline-earth cation segregation and precipitation, limits widespread commercialization. Here, we systematically investigated the surface degradation mechanism induced by gas-phase impurities in La 0.5 Sr 0.5 )FeO 3-$δ$ (LSF55) thin-film electrodes by varying the concentration of H 2 O, SO 2 , and CO 2 . Degradation of the area-specific resistance in ambient and humidified synthetic air is significantly greater than in dry ambient and dry synthetic air, pointing to the importance of water vapor. Time-resolved, in situ ambient pressure X-ray photoelectron spectroscopy performed in O 2 showed that nonbulk Sr is present on the surface before the exposure to water vapor. Upon introduction of water vapor, neither additional Sr segregation nor precipitation driven by water vapor is a necessary condition for degradation. Rather, hydroxylation of the surface induces irreversible and significant degradation. At the same time, we show that Sr migration driven by water vapor is partially reversible. These fundamental insights can be used for the rational design of electrodes with improved catalytic stability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Phase Stability in U-6Nb Alloy Doped with Ti from the First Principles Theory

First-principles calculations within the density-functional-theory (DFT) approach are conducted in order to explore and explain the effect of small amounts of titanium on phase stability in the U-6Nb alloy. During rapid quenching from high to room temperature, metastable phases α' (orthorhombic), α" (monoclinic), and γ 0 (tetragonal) can form, depending on Nb concentration. Important mechanical properties depend on the crystal structure and, therefore, an understanding of the effect of impurities on phase stability is essential. Insights on this issue are obtained from quantum-mechanical DFT calculations. The DFT framework does not rely on any material-specific assumptions and is therefore ideal for an unbiased investigation of the U-Nb system.

36 MATERIALS SCIENCE↗

Shock tube study of ionization rates of NaCl-contaminated argon

Electron density, electron temperature, and concentration of excited sodium atoms are measured in the weakly ionized regime behind a shock wave in impure argon in a shock tube using microwave techniques and spectrally resolved radiometry. Evidence is presented to show that an apparent increase in the rate of ionization is due to electron detachment of negative chlorine ions produced from sodium chloride vapor contained as an impurity. To be consistent with this chemical model, rate coefficients are found in the temperature range between 5500 and 8600 K for the dissociation of NaCl into an ion pair, dissociation of NaCl into a neutral pair, and electron detachment of a negative chlorine ion. Electron temperature is lower than heavy-particle temperature by roughly 1000 K. The electron-argon impact-ionization rate coefficient is a weak function of electron temperature in contradiction to expectation.

Schneider, K.-P.↗

Fully Deuterated Aliphatic Hydrocarbons Obtained From Iron Carbide Treated with DCl and D2O

According to Oparin, Mendeleev thought that the origin of petroleum was the result of the hydrolysis of iron carbides by superheated steam under pressure from the deep interior of the Earth through geological formations where the metal carbides exist. As early as 1877, Mendeleev described the reaction leading to the synthesis of hydrocarbons according to the general equation 3Fe(sub m)C(sub n) + mH2O yields mFe3O4 + C(sub 3n)H(sub 8m). Other experimental studies on the production of hydrocarbons from cast iron have been reported. Because of the possibility that hydrocarbons may have been trapped within the carbon matrix of the cast iron, which usually has a high content of carbon, we have studied the reaction of pure iron carbide with deuterium chloride and deuterated water. This was done in order to distinguish any newly formed deuterated hydrocarbons from any possible impurities of trapped hydrocarbons. The experiments were carried out by simply allowing iron carbide to react with concentrated deuterium chloride in D2O. The volatile hydrocarbon fraction examined by gas chromatography-mass spectrometry (GC/MS), using a Finnigan 1020/OWA instrument. contained low molecular weight hydrocarbons in a range C3 to C7. Lower molecular weight hydrocarbons were not detected by GC/MS because the MS scanning mode was preset above mass 40 to exclude components of air. The identified hydrocarbons are similar to those obtained under prebiotic conditions using high frequency discharge. The hydrocarbons found in common were propane, butane, pentane, 3-methylpentane, hexane, and heptane. The percent yields decline with increasing carbon number (propane 11%, n-heptane 1%). Similar results have been obtained by the direct treatment of metal carbides by pulse laser vaporization mass spectrometry. These results show that the hydrolysis of iron carbides may have been a significant source of hydrocarbons on the primitive Earth. There appears to be a predominance of straight chain isomers, which may have been important in the prebiotic synthesis of membrane-forming compounds.

Marquez, C.↗

Modeling Induction Stirring and Particle Tracking in Molten Uranium

Two independent numerical models have been developed to simulate the behavior of carbon impurities in molten uranium metal. Informed by experimental parameters, one model was created using the commercial software Star-CCM+ and compared with another developed using open-source codes, including OpenFOAM, Finite Element Method Magnetics (FEMM) and a First Passage Kinetic Monte Carlo (FPKMC) approach. The target experimental system features a 404g uranium metal charge containing an average carbon concentration of 139 ppm which was melted in a vacuum induction furnace at 1400° C then resolidified. The microstructures of the uranium and its impurities before and after melting have been characterized and reported separately. Prior to simulating the uranium-carbon system described, the numerical models were validated using a previously published nonradioactive experimental system to ensure agreement with expected output values. Focus has been placed on modeling velocity fields under induction stirring and impurity particle trajectories.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Defect chemistry and characterization of Hg(1-x)Cd(x)Te

Undoped mercury cadmium telluride crystals were subjected to high temperature equilibration at temperatures ranging from 400 C to 655 C in various Hg atmospheres. Hall effect and mobility measurements were carried out on the crystals quenched to room temperature subsequent to the high temperature equilibration. The variation of the hole concentration in the cooled crystals at 77 K as a function of the partial pressure of Hg at the equlibration temperatures, together with a comparison of the hole mobility in the undoped samples with that in the copper and phosphorous doped samples yielded a defect model for the undoped crystals, according to which, the undoped crystals are essentially intrinsic at the equilibration temperatures and the native acceptor defects are doubly ionized. Native donor defects appear to be negligible in concentration, implying that the p to n conversion in these alloys is mainly due to residual foreign donor impurities. The thermodynamic constants for the intrinsic excitation process as well as for the incorporation of the doubly ionized native acceptor defects in the undoped crystals were obtained.

Vydyanath, H. R.↗

Elemental fractionation and magnetic properties of melt-based Y1Ba2Cu3Oz containing excess Tb or Pt

Scanning electron microscopy of certain partially melted Y-Ba-Cu-O materials containing minority metal oxide species (Y:Tb:Ba:Cu = 1:0.1:2:3 or Y:Ba:Cu with Pt impurities), accompanied by both EDX and EMP analysis, indicates that the minority species (Tb or Pt) is quantitatively concentrated in a relatively small number of 123-type grains. High magnetic susceptibility and magnetization observed for these materials indicate that such elemental distribution is not detrimental to superconducting behavior.

Hojaji, Hamid↗

Effect of Lithium Doping on MgO Hydroxylation and Carbonation

Recovery of magnesium from brines can potentially be used to source MgO (periclase) as a CO 2 sorbent or for Mg-based cements. However, it is not clear how common impurities in brines, such as lithium, affect the resulting MgO reactivity. Here, to test the effect of lithium incorporation on MgO reactivity for hydration and carbonation, we combined computational simulations with experiments. Experimentally altered (Mg,Li)O with a low dopant concentration (0.012 ± 0.002% w/w Li) was characterized using synchrotron-based X-ray scattering and high-resolution electron microscopy to measure reaction layer formation on (Mg,Li)O. Single-crystal X-ray diffraction analysis of (Mg,Li)O demonstrates that the incorporation of lithium leads to the formation of oxygen vacancies. The presence of vacancies is likely causing faster hydroxylation rates as predicted by ab initio molecular dynamics simulations. However, the faster hydroxylation rates likely lead to faster passivation of the surface because we observe thinner reaction layers on (Mg,Li)O samples both over short time periods (30 days) and over long time periods (28 years). After 28 years, the reaction layer on the (Mg,Li)O sample was less than one-third of the thickness of that of the pure MgO sample. In addition, over 30 days, reaction layers on (Mg,Li)O samples primarily formed at steps rather than on terraces, in contrast to our previous observations on MgO. Based on our results, naturally occurring impurities in MgO modify its reactivity even at very low concentrations and need to be considered for accurate reaction rate prediction for application of MgO as a CO 2 sorbent or in cements.

MgO↗

Oxygen impurity effects at metal/silicide interfaces - Formation of silicon oxide and suboxides in the Ni/Si system

The effect of implanted oxygen impurities on the Ni/Ni2Si interface is investigated using X-ray photoelectron spectroscopy, He-4(+) backscattering and O(d, alpha)-16 N-14 nuclear reactions. Oxygen dosages corresponding to concentrations of 1, 2, and 3 atomic percent were implanted into Ni films evaporated on Si substrates. The oxygen, nickel, and silicon core lines were monitored as a function of time during in situ growth of the Ni silicide to determine the chemical nature of the diffusion barrier which forms in the presence of oxygen impurities. Analysis of the Ni, Si, and O core levels demonstrates that the formation of SiO2 is responsible for the Ni diffusion barrier rather than Ni oxide or mixed oxides, such as Ni2SiO4. It is determined that 2.2 x 10 to the 16th O/qu cm is sufficient to prevent Ni diffusion under UHV annealing conditions.

Grunthaner, P. J.↗

Study of the Effects of Impurities on the Properties of Silicon Materials and Performance of Silicon Solar Cell

Numerical solutions were obtained from the exact one dimensional transmission line circuit model to study the following effects on the terrestrial performance of silicon solar cells: interband Auger recombination; surface recombination at the contact interfaces; enhanced metallic impurity solubility; diffusion profiles; and defect-impurity recombination centers. Thermal recombination parameters of titanium impurity in silicon were estimated from recent experimental data. Based on those parameters, computer model calculations showed that titanium concentration must be kept below 6x10 to the 12th power Ti/cu cm in order to achieve 16% AM1 efficiency in a silicon solar cell of 250 micrometers thick and 1.5 ohm-cm resistivity.

Sah, C. T.↗

Convective and interfacial instabilities during solidification of succinonitrile containing ethanol

Even though slow convective flow is difficult to detect in solidifying metals, it can readily be observed in transparent materials by observing the motion of small neutrally buoyant particles. Succinonitrile, which solidifies with an unfaceted solid/liquid interface and has well characterized physical properties, is considered an excellent material for such studies. For studies of solute-induced convection, ethanol is a useful addition to succinonitrile since it has a lower density and a somewhat similar molecular structure. Samples of high purity and ethanol-doped succinonitrile are unidirectionally solidified in a vertical temperature gradient. Latex mimcrospheres 2 microns in diameter are suspended in the liquid to reveal the convective flow. Convective and morphological stability is observed as a function of solute concentration and growth velocity. These measurements are compared with theoretical calculations that predict the transition from stability to instability as a function of solidification conditions. The predicted transitions occur at low concentrations and solidification velocities; for this reason, extreme care must be taken in order to eliminate the effects of impurities or thermally induced convection.

Schaefer, R. J.↗

High Purity and Reduced Defects Hard Carbon Synthesis for Sodium-Ion Batteries

Hard carbon (HC) is the state-of-the-art anode material for sodium-ion batteries; however, the high-temperature carbonization of precursors (>1100 °C) often introduces inorganic impurities, an issue that remains largely underexplored. Here, we report a simple synthesis strategy for producing high purity HC by carbonizing a cellulose-derived precursor at 1400 °C under a slightly reducing Ar–H2 atmosphere on a graphite substrate, thereby eliminating the aluminum contamination observed during conventional carbonization on an alumina substrate under Ar. In addition, the modified synthetic condition reduces surface defects and the concentration of oxygen-containing functional groups, thereby altering the interphase formation on the HC surface. At a current density of 20 mA g−1, the impurity-rich HC exhibited an initial coulombic efficiency (ICE) of 74.1% and a reversible capacity of 248.8 mAh g−1. In sharp contrast, the high purity HC delivered a significantly improved ICE of 90.1% and a reversible capacity of 345.1 mAh g−1. These results underscore the critical importance of impurity and defect control during HC synthesis and highlight the clear electrochemical advantages of high purity HC with reduced defects for sodium-ion battery anodes.

Harshita, Lohanni↗

Gas-solid reaction pathway for chlorination of rare earth and actinide metals using hydrogen and chlorine gas

Hydriding and chlorination of buttons of metallic cerium have been tested as a head-end step to separating metal impurities from actinides. The objective here was to achieve complete conversion to chlorides while maintaining high surface area via small particles and open porosity. Hydriding by reaction with 100% H 2 gas at 573 K was successful at reducing cm-scale metal buttons into small particles/fragments. This step was followed by reaction with flowing 99.5% anhydrous Cl 2 gas at temperatures ranging from 423 to 573 K for 60 min. In experiments reported here, up to 92% conversion to cerium chloride was achieved from reacting with the anhydrous chlorine gas. The final chloride product has high porosity coupled with small particle sizes (microns to 1600 μm), which makes it suitable for subsequent purification processes. Higher chlorine concentrations in the gas favor higher conversion to chloride. The optimal temperature to achieve high chlorination is 523 K. Oxygen is a minor impurity in the chlorine gas which appears to prevent complete chlorination from being achieved.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗

Oxygen Distribution and Segregation at Grain Boundaries in Nb and Ta-Encapsulated Nb Thin Films for Superconducting Qubits

We report on atomic-scale analyses of oxygen distribution and segregation at grain boundaries (GBs) of Nb and Ta-encapsulated Nb (Ta/Nb) thin films for superconducting qubits using atom probe tomography (APT) and transmission electron microscopy (TEM). We observe oxygen segregation at grain boundaries (GBs) relative to the oxygen concentration within the grains for both Nb and Ta-capped Nb thin films and find that a higher oxygen concentration in the interior of Nb grains leads to greater oxygen segregation levels at GBs. This finding reveals that the formation of a local equilibrium of oxygen concentration between GBs and grain interiors of Nb is the primary driving force of the oxygen segregation behaviors in Nb and Ta-capped Nb. The enrichment factors (CGB/Cgrain) for oxygen segregation at GBs in Nb and Ta-capped Nb range from 2.4 ± 0.3 to 2.7 ± 0.4. The current results also highlight that controlling oxygen impurities in Nb during film deposition and fabrication processing is important to concomitantly reducing the level of oxygen segregation at GBs in Nb. Finally, we find that increases in the oxygen concentration in both Nb grains and GBs correlate with a suppression in the critical temperature for superconductivity (Tc). Together, our comparative chemical and charge transport property analyses provide atomic-scale insights into a potential mechanism, contributing to the decoherence in superconducting qubits.

Lee, Jaeyel [Fermilab] (ORCID:0000000185429612)↗

Probing Heterogeneity in Bovine Enamel Composition through Nanoscale Chemical Imaging using Atom Probe Tomography

Objective: The aim of this study was to determine the heterogeneity in chemical composition of bovine enamel using atom probe tomography, and thereby evaluate the suitability of bovine enamel as a substitute for human enamel in in vitro dental research. Design: Enamel samples from extracted bovine incisor teeth were first sectioned using a diamond saw and then milled into needle-like samples (<100 nm diameter) by focused ion beam (FIB) coupled with a scanning electron microscope (SEM). The samples were then analyzed in the atom probe to acquire three-dimensional (3D) images and quantify the atomic chemistry and distribution in bovine enamel. Results: For the first time, the atomic-level composition and clustering of major constituents and impurities within bovine enamel were determined and imaged. We discovered that the chemical composition of bovine enamel is spatially inhomogeneous at the atomic scale. The average bulk Ca/P ratio, ~1.4, was in agreement with previously reported literature values from alternative conventional methods. When assessed locally at the atomic scale, the Ca/P ratio varied between 1.1 and 2.03. We also discovered that the Mg impurities were significantly segregated throughout the enamel, and such clustering influenced the variation of Ca/P ratios. The increase in Mg concentrations, near the Mg clusters, correlated with increased Ca and decreased P concentrations. Conclusion: In conclusion, the presented findings of variability in local composition should be taken into account when interpreting dental research results from bovine enamel.

59 BASIC BIOLOGICAL SCIENCES↗