Impurity Displacement Research Articles

Quantitative Insights on Impurities in Ice Cores at the Micro‐Scale From Calibrated LA‐ICP‐MS Imaging

AbstractUnderstanding the microscopic variability of impurities in glacier ice on a quantitative level has importance for assessing the preservation of paleoclimatic signals and enables the study of macroscopic deformational as well as dielectric ice properties. Two‐dimensional imaging via laser‐ablation‐inductively‐coupled‐plasma‐mass‐spectrometry (LA‐ICP‐MS) can provide key insight into the localization of impurities in the ice. So far, these findings are mostly qualitative and gaining quantitative insights remains challenging. Recent advances in LA‐ICP‐MS high‐resolution imaging now allow ice grains and grain boundaries to be resolved individually. These resolutions require new adequate quantification strategies and, consequently, accurate calibration with matrix‐matched standards. Here, we present three different quantification methods, which provide a high level of homogeneity at the scale of a few tens of microns and are dedicated to imaging applications of ice cores. One of the proposed methods has a second application, offering laboratory experiments to investigate the displacement of impurities by grain growth, with important future potential to study ice‐impurity interactions. Standards were analyzed to enable absolute quantification of impurities in selected ice core samples. Calibrated LA‐ICP‐MS maps indicate similar spatial distributions of impurities in all samples, while impurity levels vary distinctly: Higher concentrations were detected in glacial periods and Greenland, and lower levels in interglacial periods and samples from central Antarctica. These results are consistent with ranges from complementary meltwater analysis. Further comparison with cm‐scale melting techniques calls for a more sophisticated understanding of the ice chemistry across spatial scales, to which calibrated LA‐ICP‐MS maps now contribute quantitatively.

Diffusion of intruders in granular suspensions: Enskog theory and random walk interpretation.

The Enskog kinetic theory is applied to compute the mean square displacement of impurities or intruders (modeled as smooth inelastic hard spheres) immersed in a granular gas of smooth inelastic hard spheres (grains). Both species (intruders and grains) are surrounded by an interstitial molecular gas (background) that plays the role of a thermal bath. The influence of the latter on the motion of intruders and grains is modeled via a standard viscous drag force supplemented by a stochastic Langevin-like force proportional to the background temperature. We solve the corresponding Enskog-Lorentz kinetic equationby means of the Chapman-Enskog expansion truncated to first order in the gradient of the intruder number density. The integral equationfor the diffusion coefficient is solved by considering the first two Sonine approximations. To test these results, we also compute the diffusion coefficient from the numerical solution of the inelastic Enskog equationby means of the direct simulation Monte Carlo method. We find that the first Sonine approximation generally agrees well with the simulation results, although significant discrepancies arise when the intruders become lighter than the grains. Such discrepancies are largely mitigated by the use of the second Sonine approximation, in excellent agreement with computer simulations even for moderately strong inelasticities and/or dissimilar mass and diameter ratios. We invoke a random walk picture of the intruders' motion to shed light on the physics underlying the intricate dependence of the diffusion coefficient on the main system parameters. This approach, recently employed to study the case of an intruder immersed in a granular gas, also proves useful in the present case of a granular suspension. Finally, we discuss the applicability of our model to real systems in the self-diffusion case. We conclude that collisional effects may strongly impact the diffusion coefficient of the grains.

A theoretical study of the effects of electric field, hydrostatic pressure, and temperature on photoionization cross-section of a donor impurity in (Al, Ga)N/AlN double triangular quantum wells

The aim of this research is to analyze the influence of various factors on the photo-ionization cross-section in (Al, Ga)N/AlN double triangular quantum wells. Using the finite difference method, the effects of the electric field, hydrostatic pressure, temperature, and Ga concentration were investigated within the effective mass and parabolic approximations. Our findings show that the photo-ionization cross-section (PICS) is highly dependent on all the variables under consideration. The optical spectra were blue-shifted with increasing electric field and pressure and red-shifted with increasing temperature and impurity displacement far from the center of the structure. Furthermore, it was found that changes in gallium content and impurity position can increase the PICS amplitude. A comparison of the obtained results with the existing literature as a limiting case of the reported problem is also provided, and excellent agreement is found.

Hydrogen-like donor impurity states in strongly prolate ellipsoidal quantum dot

In the current article, the binding energy and impurity-related photoionization cross-section are investigated for the case of an infinite confinement potential well model using a variational approach for hydrogen-like impurity states in strongly prolate ellipsoidal quantum dot. The binding energy is calculated as a function of ellipsoidal small and large semiaxes and as a function of impurity displacement. The behavior of the oscillator strength for the quantum transitions between the impurity ground state and the first two non-impurity states are considered for different values of incident light angles and geometrical parameters. In the work, we illustrate the results of photoionization cross-section computations depending on the incident photon energy for a range of incident angles and different values of impurity shift. It is revealed that the threshold frequency decreases with the increase of impurity displacement.

Mathematical Modeling of Nucleation and Growth Processes of Ellipsoidal Crystals in Binary Melts

The transient behavior of an ensemble of ellipsoidal particles in a supercooled binary melt is considered. The model laws, based on the Fokker-Planck type kinetic equation for the particle-volume distribution function, the thermal and mass integral balances for the binary melt temperature and solute concentration, as well as the corresponding boundary and initial conditions, are formulated and solved analytically. We show that the temperature and concentration increase with time due to the effects of impurity displacement and latent heat emission by the growing ellipsoidal particles. These effects are also responsible for metastability reduction. As this takes place, increasing the initial solute concentration in a metastable binary melt increases the intensity of its desupercooling. The theory is developed for arbitrary nucleation frequency with special consideration of two important nucleation kinetics according to the Meirs and Weber-Volmer-Frenkel Zel’dovich mechanisms. An analytical solution to the integrodifferential model equations is found in a parametric form. The theory contains all limiting transitions to previously developed analytical approaches. Namely, it contains the growth of spherical crystals in binary melts and ellipsoidal crystals in single-component melts.

Central vacancy creation in icosahedral nanoparticles induced by the displacement of large impurities

We employ metadynamics simulations at room temperature to study the diffusion of large single-atom impurities within otherwise pure icosahedral nanoparticles, for different bimetallic systems (Au-Co, Ag-Co, Ag-Ni, Au-Pt and Au-Rh) and icosahedral sizes. Our simulations reveal that the displacement of the impurity induces the formation of a vacancy in the central part of the structure, as already observed for small icosahedra of Au-Co and Ag-Cu, therefore confirming the generality of this peculiar diffusion process. At the largest size, a new collective displacement mechanism is identified alongside the diffusion of the impurity. For Au-Pt clusters, different diffusion pathways are observed, which are mediated by a partial deformation of the nanoparticle surface. This is due to the lack of stability of fivefold veritices in Pt icosahedral clusters. We prove that the stability of the icosahedral surface is an essential condition for the occurrence of the combined process of impurity diffusion and internal vacancy formation.

Diffusion mechanism in cutting Ni-based alloy containing γ′ phase (Ni3Al) with CBN tool based on MD simulation

Ni-based alloys are widely used in aerospace because of their high strength and high temperature oxidation resistance. CBN tool is suitable for precision machining of Ni-based alloy. Diffusion wear is an important wear form of CBN tool in the process of cutting Ni-based alloy. Therefore, it is of great significance to study the diffusion phenomenon in the process of cutting Ni-based alloy with CBN tool. In this paper, the cutting model of Ni-based alloy containing γ′ phase (Ni3Al) with CBN tool is established based on the molecular dynamics (MD) simulation method. The self diffusion activation energy of all kinds of atoms in the workpiece and the formation energy of several point defects in the tool are calculated, so as to study in depth the atom diffusion mechanism according to the simulation results. The results show that the atoms in the crystal boundary of the workpiece are the most easily diffused, followed by the atoms in the phase boundary, and the atoms in the lattice are the most difficult to diffuse. When the workpiece atoms diffuse into the tool, it is easier to diffuse into the tool grain boundary than to form interstitial impurity atoms or displacement impurity atoms. It is more difficult to form the substitutional impurity atom than to form the interstitial impurity atom.

Zone Recrystallization of Zirconium and Hafnium

The work studied the possibility of obtaining of the high-purity samples of zirconium and hafnium by the method of zone recrystallization of round rods with electron-beam heating in a vacuum of 1∙10-4 Pa. Some meltings were carried out in a constant electric field with the variability of its connection. It is shown that the simultaneous passage of several refining processes (evaporation of highly volatile metallic impurities, zone recrystallization with directional displacement of impurities to the end of the sample, electrotransport) made it possible to efficient refining of zirconium both from metallic impurities and from interstitial impurities. The best degree of purification was achieved when zone melting carrying out in an electric field directed opposite to the zone movement. In this case, the displacement of interstitial impurity ions coincided with the direction of movement of the liquid zone. Samples of zirconium with a purity of 99.89 wt. % were obtained (the concentration of aluminum was reduced by 5, iron - 11, copper - 45, chromium - 75, silicon - 10, titanium - 2.5, oxygen - 3.3, nitrogen - 3, carbon - 2 times). The hafnium samples refined by the zone recrystallization method were characterized by a purity of 99.85 wt. %. The concentrations of both all metal impurities and interstitial impurities were significantly reduced (concentration in wt% oxygen was 0.011, carbon - 0.0018, nitrogen - 5∙10-5). A study of gas evolution from samples of iodide hafnium and refined hafnium was carried out. It was found that the maximum gas release peak fell on the temperature range of 500 ... 550 °C. The use of an integrated approach, including high-temperature heating, stages of zone melting at different rates, and thermal cycling in the range of the polymorphic transformation temperature, made it possible to obtain single-crystal hafnium samples. According to X-ray diffraction data, the parameters of the hafnium crystal lattice were determined: а = (0.31950 ± 5·10-5) nm and с = (0.50542 ± 5·10-5) nm (at 298 K), which corresponds to the density ρ = 13.263 g/cm3 and axial ratio с/a = 1.5819.

Displacement to separate host‐cell proteins and aggregates in cation‐exchange chromatography of monoclonal antibodies

An efficient and consistent method of monoclonal antibody (mAb) purification can improve process productivity and product consistency. Although protein A chromatography removes most host-cell proteins (HCPs), mAb aggregates and the remaining HCPs are challenging to remove in a typical bind-and-elute cation-exchange chromatography (CEX) polishing step. A variant of the bind-and-elute mode is the displacement mode, which allows strongly binding impurities to be preferentially retained and significantly improves resin utilization. Improved resin utilization renders displacement chromatography particularly suitable in continuous chromatography operations. In this study we demonstrate and exploit sample displacement between a mAb and impurities present at low prevalence (0.002%-1.4%) using different multicolumn designs and recycling. Aggregate displacement depends on the residence time, sample concentration, and solution environment, the latter by enhancing the differences between the binding affinities of the product and the impurities. Displacement among the mAb and low-prevalence HCPs resulted in an effectively bimodal-like distribution of HCPs along the length of a multi-column system, with the mAb separating the relatively more basic group of HCPs from those that are more acidic. Our findings demonstrate that displacement of low-prevalence impurities along multiple CEX columns allows for selective separation of mAb aggregates and HCPs that persist through protein A chromatography.

Classical Advection-Diffusion in Heterogeneous Media

A method is proposed for solving the problem of impurity transport in heterogeneous medium due to the classical diffusion and advection. The case when advection is absent was analyzed separately. It focuses on distances from the impurity source, which are much larger than the main body of its localization, and the asymptotic approach developed by one of the authors (P.S.K.) is used. The problem is reduced to solving a differential equation of the first order, which determines the linear trajectory of the concentration signal from the source that arises with this approach to the point of observation. The result for concentration is expressed in terms of one-dimensional integrals along the concentration signal line. The solution of the transport problem in the presence of advection is obtained by transition into the coordinate system accompanying advection. The key elements entering the resulting concentration expression are the effective time and impurity displacement, both are caused by advection.

Thermal stability of nonmagnetic Cd and In impurities in Fe3O4

Magnetite (Fe3O4) was doped with radioactive 111mCd and 111In ions as impurities, and their residence sites and thermal stability were investigated by means of time-differential perturbed angular correlation (TDPAC) spectroscopy. Well-defined TDPAC spectra unequivocally show their sites: Cd ions are stably located only in the tetrahedral A site in all the temperature range of the present observation (77 K–873 K); In ions also specifically occupy the A site at low temperature, but at high temperature part of them select the B site in a reversible fashion. The energy difference between the A and B sites for the accommodation of In was experimentally determined to be 0.119 (9) eV by assuming a Boltzmann distribution for the populations of the 111In probe in the respective sites. The element-dependent thermal stabilities observed for Cd and In were well corroborated by density functional theory calculations. The successful observation of thermally activated site-to-site displacement of impurity In ions in Fe3O4 is reported.

Investigations of spin Hamiltonian parameters and local structures for three Cr(CN)63− centres in NaCl at three temperatures

The spin Hamiltonian parameters (g factors and zero-field splittings (ZFSs)) and local structures of three Cr(CN)63− centres in NaCl are studied at three temperatures based on the perturbation calculations. The signs and magnitudes of the ZFSs D and E are in accordance with those of average bond length discrepancy ΔR‾(=(1/6)∑δRi,i=1–6) and rhombic distortion parameter H(=-(Δφ‾)1/2(ΔR‾)3/D), relevant to average planar bond angle discrepancy Δφ‾(=(1/4)∑δφi,i=1-4) related to an ideal octahedron. The SHPs exhibit the smallest rhombic (or orthorhombic) distortions and the weakest covalency at 4.2K, and the largest rhombic distortions and strongest covalency at 77K with comparable D and E. The magnitude of g-shifts (=gi−gs) obey the tendency 77K<290K⩽4.2K, in consistent with that of covalency factor N. The CT contributions to g-shifts are opposite in sign and about 51–63% in magnitude compared with the CF ones. The microscopic mechanisms of the local structures (i.e., the impurity displacements ΔRX and ΔRY and ligand displacements ΔRP, ΔRP′ and ΔRP″) are analyzed for the various centres in view of the electrostatic interactions arising from the cation vacancies. Unlike conventional rhombic centres I and II for all three temperatures, the new orthorhombic centre III occurs only at low temperatures (4.2K and 77K) and indeed exhibits new structural properties, especially the largest ΔR‾ and H as well as D and E.

Theoretical studies of the g factors and local structure for the rhombic Fe+ center in NaF

The anisotropic g factors and local structure for the rhombic Fe+ center in NaF are theoretically studied using the perturbation formulas of the g factors for a rhombically distorted octahedral 3d7 cluster. The impurity Fe+ is found not to occupy exactly the host Na+ site in NaF but to experience the off-center displacement of about 0.28Å along [110] axis due to size mismatch substitution. The calculated g factors based on the above impurity displacement show reasonable agreement with the experimental data. The local structure of the Fe+ center is discussed.

Хромато-масс-спектрометрическое определение примесей в фосфине высокой чистоты с использованием капиллярных адсорбционных хроматографических колонок

A possibility of the use of capillary adsorption columns in combination with mass-spectrometric detection was investigated with the aim to determine the content of impurities in high-purity phosphine. The effect of the volume of the main substance on separation and determination of impurities was studied. This effect mostly manifests itself with respect to impurities closely located to the front of phosphine. During the change of pressure of the introduced phosphine from 0.1 to 1 atm. the relative retention time of carbon dioxide decreases by 1-5 %. In case of impurity compounds with the retention factor not more than 0.5 the retention time does not depend on the volume of the introduced phosphine. The effect of displacement of impurities is accompanied by the increase in efficiency of a column. In substances eluting after phosphine, e.g., arsine, at pressure input of a sample more than 0.1 atm. there is almost a 10 times decrease is observed in efficiency of the column. The limits of detection of substances of 5∙10 -8 -1∙10 -5 vol. % were attained. They are the best values among those available in literature and for some impurities they by 2-1000 times lower. The content of 35 impurities was detected using the calibrating dependencies and estimates of sensitivity on the values of the total ionization cross sections of the determined substances. The content of impurities was determined in the samples of purified phosphine and in the fractions extracted from the top and the bottom part of rectification column. It was found that carbon dioxide, arsine and propylene are the mostly hard-to-remove impurities. Keywords: phosphine, impurities, gas chromatography–mass spectrometry, adsorption capillary columns, detection limit, trueness (Russian) DOI: http://dx.doi.org/10.15826/analitika.2013.17.4.011 V.A. Krylov 1,2 . O.Iu. Chernova 1 , A.Iu. Sozin 1 , A.P. Kotkov 1,3 , G.V. Pushkarev 2,3 1 G.G. Devyatykh Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences, Russia, Nizhny Novgorod 2 N.I. Lobachevsky Nizhny Novgorod State University, Russia, Nizhny Novgorod 3 FGUP NPP “Saljut”, Russia , Nizhny Novgorod

Theoretical investigation of the spin Hamiltonian parameters and local structure and their concentration dependences for Cd1−xMnxS quantum dots

The spin Hamiltonian parameters (zero-field splitting D, g factors and hyperfine structure constants) and local structure as well as their concentration dependences for Cd1−xMnxS quantum dots are theoretically investigated from the perturbation formulae of these parameters for a 3d5 ion under trigonally distorted tetrahedra. The impurity Mn2+ is found not to occupy exactly the host Cd2+ site in the CdS quantum dots but to experience an inward displacement 0.041 Å towards the ligand triangle along the C3-axis because of the size mismatching substitution and the internal stress. As compared with bulk CdS:Mn2+, the larger impurity displacement and stronger Mn2+ 3d–3s orbital admixture in the Cd1−xMnxS quantum dots suitably account for the higher zero-field splitting and hyperfine structure constant, respectively. The impurity axial displacement and the Mn2+ 3d–3s orbital admixture in the Cd1−xMnxS quantum dots may exhibit an approximately cubic increase with increasing the Mn concentration x from 0.001 to 0.15.

Theoretical Investigations of the Defect Structure for Ni&lt;sup&gt;3+&lt;/sup&gt; in ZnO

The defect structure for Ni3+ in ZnO crystal is theoretically investigated using the perturbation formulas of the spin Hamiltonian parameters for a 3d7 ion in trigonally distorted tetrahedra. In view of the significant covalency of the system due to the high valence state of Ni3+, the ligand orbital and spin-orbit coupling contributions are taken into account in a uniform way based on the cluster approach. The impurity Ni3+ is found not to occupy the ideal Zn2+ site in ZnO but to undergo the small axial displacement of about 0.044 Ǻ away from the oxygen triangle along the C3 axis. The theoretical spin Hamiltonian parameters based on the above impurity displacement show good agreement with the experimental data. The defect structure of this impurity center is compared with that for the similar Fe3+ in ZnO.

Studies of the EPR Parameters and Local Structure of Co&lt;sup&gt;2+&lt;/sup&gt; in ZnO

The EPR parameters and the local structure for Co2+ in ZnO are deduced from the perturbation formulas of these parameters for a 3d7 ion in a trigonally distorted tetrahedron. The ligand orbital and spin-orbit coupling contributions are taken into account uniformly from the cluster approach in view of the covalency of the system. The impurity V3+ is found not to locate exactly on the Zn2+ site but to experience a small displacement of 0.04 Ǻ away from the ligand triangle, along the C3 axis. The theoretical EPR parameters based upon the above impurity displacement are in good agreement with the observed values.

Theoretical investigations of the impurity axial displacement for ZnO:V3+

The local structure of the V(3+) center in ZnO crystal is theoretically investigated using the perturbation formulas of the spin Hamiltonian parameters for a 3d(2) ion in trigonally distorted tetrahedra. From the studies, the impurity V(3+) is found not to occupy the ideal Zn(2+) site in ZnO but to suffer an outward displacement of about 0.08 angstrom away from the oxygen triangle along the C(3) axis. The calculated spin Hamiltonian parameters based on the above impurity displacement show good agreement with the experimental data. The outward displacement of the impurity V(3+) in ZnO can be attributed to the local tension due to the size and charge mismatching substitution of the host Zn(2+) by the larger and higher charged V(3+). The studies of this work would be helpful to the investigations of the structure properties of ZnO (or other similar II-VI semiconductors) doped with transition-metal ions. (C) 2011 American Vacuum Society. [DOI: 10.1116/1.3573978]

Spin-Hamiltonian parameters and off-center displacements for Rh4+ and Ir4+ ions in rhombohedral BaTiO3

By means of the formulae based on the first-order theory of resonance for a low-spin 2T2 term in trigonal symmetry, the spin-Hamiltonian parameters (g factors g //, g ⊥ and hyperfine structure constants A //, A ⊥) for Rh4+(4d5) and Ir4+(5d5) ions in the low-temperature rhombohedral phase of BaTiO3 are calculated. The calculated results are in reasonable agreement with the experimental values. From the calculations, the off-center displacements of Rh4+ and Ir4+ in the oxygen octahedra of BaTiO3 crystal are acquired. These impurity displacements are much smaller than that of the replaced host ion Ti4+. This point is analogous to the displacements of 3d n impurities Fe3+ and Co2+ in rhombohedral BaTiO3 obtained by analyzing their spin-Hamiltonian parameters.

Theoretical investigations of the local structures and the g factors for 3d9 ions in CdS

The local structures and the g factors g // and g ⊥ for the isoelectronic 3d9 ions Cu2+ and Ni+ in CdS are theoretically investigated from the perturbation formulas of these parameters for a 3d9 ion under trigonally distorted tetrahedral environments. In consideration of significant covalency of the [MS4] combinations (M = Cu and Ni), the ligand orbital and spin–orbit coupling contributions are taken into account using the cluster approach. Based on the studies, the substitutional impurity Cu2+ (or Ni+) on Cd2+ site is found to undergo a small inward displacement 0.026 A (or a slight outward shift 0.017 A) towards (or away from) the ligand triangle along C 3 axis. The theoretical g factors for both ions based on the above impurity displacements are in good agreement with the experimental data.