Si Tracer Research Articles

Growth of hafnium and zirconium silicides by reactive diffusion

Diffusion controlled growth of the phases in Hf–Si and Zr–Si systems are studied by bulk diffusion couple technique. Only two phases grow in the interdiffusion zone, although several phases are present in both the systems. The location of the Kirkendall marker plane, detected based on the grain morphology, indicates that disilicides grow by the diffusion of Si. Diffusion of the metal species in these phases is negligible. This indicates that vacancies are present mainly on the Si sublattice. The activation energies for integrated diffusion coefficients in the HfSi2 and ZrSi2 are estimated as 394 ± 37 and 346 ± 34 kJ mol−1, respectively. The same is calculated for the HfSi phase as 485 ± 42 kJ mol−1. The activation energies for Si tracer diffusion in the HfSi2 and ZrSi2 phases are estimated as 430 ± 36 and 348 ± 34 kJ mol−1, respectively.

Reactive diffusion in the Ti–Si system and the significance of the parabolic growth constant

Solid diffusion couple experiments are conducted to analyse the growth mechanism of the phases and the diffusion mechanism of the components in the Ti–Si system. The calculation of the parabolic growth constants and the integrated diffusion coefficients substantiates that the analysis is intrinsically prone to erroneous conclusions if it is based on just the parabolic growth constants determined for a multiphase interdiffusion zone. The location of the marker plane is detected based on the uniform grain morphology in the TiSi2 phase, which indicates that this phase grows mainly because of Si diffusion. The growth mechanism of the phases and morphological evolution in the interdiffusion zone are explained with the help of imaginary diffusion couples. The activation enthalpies for the integrated diffusion coefficient of TiSi2 and the Si tracer diffusion are calculated as 190 ± 9 and 197 ± 8 kJ/mol, respectively. The crystal structure, details on the nearest neighbours of the components, and their relative mobilities indicate that the vacancies are mainly present on the Si sublattice.

Primary production and bacterial carbon metabolism around South Shetland Islands in the Southern Ocean

Phytoplankton and bacterioplankton dynamics were studied around South Shetland Islands (Antarctica) with special emphasis on the Drake Passage region, during austral summer, in order to expand our knowledge on the coupling between the autotrophic and heterotrophic microbial plankton compartments in polar ecosystems. In addition, we directly estimated bacterial growth efficiency in the Drake Passage with the aim of better constraining total bacterial carbon utilization in this important polar ecosystem. Integrated chlorophyll-a concentration (21–86mgm−2), primary production rates (0.7–19.3mgCm−3d−1) and mean water-column photochemical efficiency (0.24–0.60) were significantly correlated with Si⁎ tracer (r2=0.55, 0.46 and 0.64, respectively), which indirectly points to iron as the major limiting factor for phytoplankton growth in the area. Bacterial production was considerably low (0.002–0.3mgCm−3d−1) and was best explained by chlorophyll-a concentration, protein-like fluorescence of dissolved organic matter and temperature (r2=0.53, p<0.001). Water temperature appeared to influence bacterial activity when organic substrate availability is high. Bacterial production accounted on average for only 3.9% of co-occurring primary production, which has been frequently interpreted as an indicator of the marked uncoupling between bacteria and phytoplankton in cold waters. However, using the experimentally derived mean bacterial growth efficiency for the photic zone (6.1±1.3%) the bacterial carbon demand represented on average 63±18% of concomitant primary production, similar to what is found in warmer productive waters. Thus, our study suggests that bacterioplankton and phytoplankton appear to be connected in this polar area.

Plant silicon isotopic signature might reflect soil weathering degree

Plants fractionate Si isotopes which provides a useful Si tracer in the Si soil-plant cycle. This study reports plant Si content and Si-isotopic signatures in mature banana plants grown on soils with different weathering degree, but all developed from basaltic pyroclasts in the Mungo area, Cameroon. The delta Si-30 compositions were determined in various plant parts and soil surface horizons by MC-ICP-MS in dry plasma mode with external Mg doping to a precision of +/- 0.15aEuro degrees (+/- A 2 sigma(SD)). The Si-isotopic compositions in banana plants grown on weathered clayey soils (+0.54 +/- A 0.15aEuro degrees) are heavier than on weakly developed soils rich in fresh ash and pumice (+0.02 +/- A 0.15aEuro degrees). The corresponding bulk soils display lower delta Si-30 value in weathered soil (-1.41aEuro degrees) than in poorly developed soil (-0.41aEuro degrees). We suggest that the dissolved Si source for the plant, governed firstly by dissolution of easily weatherable minerals, was isotopically enriched in heavy isotopes through clay formation over long periods. At seasonal to annual time scale, this source is influenced by a combination of following processes: Si adsorption of light isotopes onto Fe oxides, plant Si uptake and recycling in surface horizons. This would provide an isotopically heavier Si source in the more weathered soil since the Fe oxides content increases with weathering. Plant Si-isotopic signature might thus reflect the soil weathering degree. This study further suggests that in addition to weathering processes, rivers isotopic signatures likely depend on the fate of phytoliths in the soil-plant-river system.

Radiating Shock Measurements in theZ-Pinch Dynamic Hohlraum

The Z-pinch dynamic hohlraum is an x-ray source for high energy-density physics studies that is heated by a radiating shock to radiation temperatures >200 eV. The time-dependent 300-400 eV electron temperature and 15-35 mg/cc density of this shock have been measured for the first time using space-resolved Si tracer spectroscopy. The shock x-ray emission is inferred from these measurements to exceed 50 TW, delivering >180 kJ to the hohlraum.

Sensitivity of the marine biospheric Si cycle for biogeochemical parameter variations

A systematic quantitative assessment of the marine silicon cycle is presented, based on a prognostic coupled water column‐sediment global biogeochemical ocean general circulation model (HAMOCC). The resulting tracer distributions are compared with a comprehensive marine Si database of measurements. The model parameters which govern the Si cycle within the model world are optimized through a linear response model. The functional relationships between the Si cycle parameters and the Si tracer distributions are derived from a series of sensitivity experiments addressing opal export production, particle flux through the water column, porewater chemistry, and external biogeochemical forcing. The most important parameters for a further quantitative improvement of the simulation are depth‐dependent opal dissolution kinetics, a productivity‐dependent opal settling velocity, a general change in maximum Si uptake velocity Vmaxopal, and the clay as well as the Si input from continental weathering. The modeled Si budget shows a larger global export production, larger opal deposition rates onto the sediment surface and higher diffusive transports of porewater silicic acid into the open water column as estimated by Tréguer et al. [1995].

Emission-depth Dependence of the Signal Photoelectron Energy Spectrum

An analytical expression for the escape probability of signal photoelectrons as a function of the depth of origin, energy and direction of emission from a solid has been found by solving a kinetic equation. The solution is expressed by a series of partial escape probabilities corresponding to contributions of electrons that have suffered a certain number of inelastic collisions before leaving a target. The universal relationship between the probability for an electron to escape from a certain depth after being scattered inelastically n times (n=0, 1, 2, …) and the pathlength distribution has been derived. The photoelectron spectra from an Si(Au) tracer located at different depths in an Au(Si) matrix has been calculated analytically and by means of the Monte Carlo technique for different XPS configurations. The peak and the background regions have been found to be strongly affected by elastic scattering of electrons in the sample. The neglect of the elastic collisions has been shown to over- or underestimate the photoelectron current (up to 100% in the case of the Au sample and the Si tracer located at a depth of the order of several inelastic mean free paths). The analytical theory predictions are compared with the corresponding Monte Carlo data and good agreement in absolute units is observed. The obtained results indicate that the assumption of a simple exponential attenuation of electrons made in non-destructive depth-profile reconstructions seems to be less valid in the case of materials in which elastic scattering is significant. © 1997 by John Wiley & Sons, Ltd.

Emissiondepth Dependence of the Signal Photoelectron Energy Spectrum

An analytical expression for the escape probability of signal photoelectrons as a function of the depth of origin, energy and direction of emission from a solid has been found by solving a kinetic equation. The solution is expressed by a series of partial escape probabilities corresponding to contributions of electrons that have suffered a certain number of inelastic collisions before leaving a target. The universal relationship between the probability for an electron to escape from a certain depth after being scattered inelastically n times (n=0, 1, 2, …) and the pathlength distribution has been derived. The photoelectron spectra from an Si(Au) tracer located at different depths in an Au(Si) matrix has been calculated analytically and by means of the Monte Carlo technique for different XPS configurations. The peak and the background regions have been found to be strongly affected by elastic scattering of electrons in the sample. The neglect of the elastic collisions has been shown to over- or underestimate the photoelectron current (up to 100% in the case of the Au sample and the Si tracer located at a depth of the order of several inelastic mean free paths). The analytical theory predictions are compared with the corresponding Monte Carlo data and good agreement in absolute units is observed. The obtained results indicate that the assumption of a simple exponential attenuation of electrons made in non-destructive depth-profile reconstructions seems to be less valid in the case of materials in which elastic scattering is significant. © 1997 by John Wiley & Sons, Ltd.

Kinetics of silicic acid uptake by natural diatom assemblages in two Gulf Stream warm-core rings

We performed Si tracer experiments to measure the uptake rates of silicic acid by natural diatom assemblaged in 2 Gulf Stream warm-core rings as a function of the extracellular silicic acid concentration. The experiments were performed a t and near the centers of rings 81-D and 82-B, and lncluded 5 sampling dates in late spring, summer and autumn, when silicic acid was undetectable to conventional analyses (< 0.2 yM) throughout the upper 20 to 30 m Uptake was measurably substratelimited only at those stations where silicic acid was undetectable. Concentrations of 2.5 to 3 1rM Si(OH)4, which typlfied ring 82-B before the development of the seasonal thermocline, were high enough to support maximum uptake rates. When substrate 1i.mitation was present the dependence of the uptake rate on sllicic acid concentration exhibited hyperbolic saturation kinetics, with half-saturation constants (K,) ranging from 0.53 to 0.90 \&M. This range is considerably lower than that reported previously for natural diatom asselnblages (1.5 to 3.7 pM). A numerical sllnulation indicates that a diatom species with a K, value typical of the ring assemblages would outcompete most diatom species whose silicon kinetics have been examined in culture, not only under conditions of continuously low sll~cic a c ~ d concentration but with the sporadic, pulsed nutrient injections that appear to characterne warm-core ocean eddies. Prevlous studies of silicic acid uptake kinetics in natural waters have all been conducted In relatively nutnent-rlch systems and the lower K, values we report here for 2 Gulf Stream warm-core rings Indicate that diatoms growing in severely nutrient-depleted surface waters have a higher afflnlty for slliclc acid than those growing in more eutrophic habitats. If this is true for other nutrient-depleted environments such as the tropics and mid-ocean gyres, the likelihood of significant slhcon limitation in such systems is diminished.

Diffusion of Sb, Ga, Ge, and (As) in TiSi2

Gallium, Sb, and Ge were implanted into thick (about 400 nm) layers of TiSi2 prepared by metal-silicon reaction. The diffusion of the implanted atoms was analyzed by means of secondary ion mass spectrometry. Gallium was introduced because a former study had shown that the usual p-type dopant B does not diffuse in TiSi2. Germanium was used in lieu of a Si tracer. Its diffusion characteristics are compared to those of P and As (as well as Si) which had been investigated previously. Germanium and Ga diffuse readily above 600 °C, but Sb does not. Its diffusion appears to be limited to grain-boundary effects. Accumulations of the diffusing atoms are observed (except for Sb) at the silicide-silicon interface. These are due to kinetic effects, namely fast diffusion at grain boundaries and interfaces, rather than to real adsorption which is an equilibrium condition. Because diffusion in intermetallic compounds has been shown to be significantly affected by variations in stoichiometry, experiments were conducted with films implanted not only with foreign atoms but with Ti as well. These did not lead to significantly different observations.

A kinetic model of silicate melts (silicon tracer diffusion)

In silicate melts, Si is considered to be always tetrahedrally coordinated by oxygen. These structural units can in principle form polyanionic structures by sharing oxygen atoms between two tetrahedra. Our kinetic model for the description of silicate melts is used to simulate high-temperature Si tracer diffusion experiments in order to compare the results of the computer simulations with experimental data. As basic kinetic events in such systems the diffusive motion of individual polyanions, with a mobility depending on their size, and the interchange of structural units between different polyanions by condensation and splitting reactions are taken into account. The simulations yield a good qualitative agreement with the experiments wherefrom it is concluded that the overall transport of Si may be interpreted as resulting from the individual contributions of differently sized polyanions which exist for a certain lifetime in silicate melts. The results demonstrate that the macroscopically detectable (tracer) diffusivity of Si depends in a rather complicated way on the mobilities of the constituent structural units.

Profile of tracer Si in silicide when Si diffuses by vacancy mechanism

A mathematical model is constructed to interpret the profiles of radioactive Si tracers during silicide formation. This model assumes that only Si moves in the silicide during silicide formation and that the moving Si diffuses in the Si sublattice of the silicide in terms of vacancy mechanism. Analytical solutions of the model for long-time annealing (i.e., asymptotic profiles) are given. The analytical asymptotic profiles are very accurate for the annealing period generally used in experiments. It is shown that the profiles of the Si tracer in the silicide are almost flat. This thus proves that self-diffusion of the tracer atoms cannot be neglected as assumed in some published papers. In fact, several experimental tracer profiles are found to be flat in the silicide. Some numerical solutions for short-time annealing are also given to show how the tracer profile evolves. The result given here can also be used for many intermetallic reactions.