Shifts In Stoichiometry Research Articles

Oxygen stoichiometry shift of irradiated LWR-fuels at high burn-ups: Review of data and alternative interpretation of recently published results

The available oxygen potential data of LWR-fuels by the EFM-method have been reviewed and compared with thermodynamic data of equivalent simulated fuels and mixed oxide systems, combined with the analysis of lattice parameter data. Up to burn-ups of 70–80 GWd/tM the comparison confirmed traditional predictions anticipating the fuels to remain quasi stoichiometric along irradiation. However, recent predictions of a fuel with average burn-up around 100 GWd/tM becoming definitely hypostoichiometric were not confirmed. At average burn-ups around 80 GWd/tM and above, it is shown that the fuels tend to acquire progressively slightly hyperstoichiometric O/M ratios. The maximum derived O/M ratio for an average burn-up of 100 GWd/tM lies around 2.001 and 2.002. Though slight, the stoichiometry shift may have a measurable accelerating impact on fission gas diffusion and release.

Growth of ZnO by chemical vapor transport using [formula omitted] and Zn as a transport agent

ZnO crystal growth by chemical vapor transport (CVT) is carried out using CO 2 and Zn as a new transport agent. ZnO crystals are grown by the reaction: ZnO ( s ) + CO ( g ) = Zn ( g ) + CO 2 ( g ) . The increase in CO 2 / Zn changes the color of as-grown crystals from red to transparent. The color change of the crystals is due to the oxygen partial pressure during the CVT. The red color of crystals is attributed to defects caused by the shift of stoichiometry to zinc-rich (oxygen vacancies, zinc interstitials, or their complexes are created). All of the grown crystals are polycrystals by using the ordinary CVT ampoules. Single crystals are grown reproducibly by using a seed and a CVT ampoule with modified configuration.

Phase shifts in the stoichiometry of rifamycin B fermentation and correlation with the trends in the parameters measured online

Antibiotic fermentation processes are raw material cost intensive and the profitability is greatly dependent on the product yield per unit substrate consumed. In order to reduce costs, industrial processes use organic nitrogen substrates (ONS) such as corn steep liquor and yeast extract. Thus, although the stoichiometric analysis is the first logical step in process development, it is often difficult to achieve due to the ill-defined nature of the medium. Here, we present a black-box stoichiometric model for rifamycin B production via Amycolatopsis mediterranei S699 fermentation in complex multi-substrate medium. The stoichiometric coefficients have been experimentally evaluated for nine different media compositions. The ONS was quantified in terms of the amino acid content that it provides. Note that the black box stoichiometric model is an overall result of the metabolic reactions that occur during growth. Hence, the observed stoichiometric coefficients are liable to change during the batch cycle. To capture the shifts in stoichiometry, we carried out the stoichiometric analysis over short intervals of 8-16 h in a batch cycle of 100-200 h. An error analysis shows that there are no systematic errors in the measurements and that there are no unaccounted products in the process. The growth stoichiometry shows a shift from one substrate combination to another during the batch cycle. The shifts were observed to correlate well with the shifts in the trends of pH and exit carbon dioxide profiles. To exemplify, the ammonia uptake and nitrate uptake phases were marked by a decreasing pH trend and an increasing pH trend, respectively. Further, we find the product yield per unit carbon substrate to be greatly dependent on the nature of the nitrogen substrate. The analysis presented here can be readily applied to other fermentation systems that employ multi-substrate complex media.

Control of saturation magnetization, anisotropy, and damping due to Ni implantation in thin Ni81Fe19 layers

The layer magnetization, the saturation magnetization as well as the magnetic anisotropy, and damping behavior of 20nm thick Ni81Fe19 films have been modified by 30keV Ni ion implantation with fluences up to 1×1016Ni∕cm2 (≈5at.%). With increasing ion fluence a magnetic dead layer of increasing thickness is formed which leads to a reduction of the total magnetization. In addition, the saturation magnetization of the residual ferromagnetic film decreases due to, both, a shift in stoichiometry and radiation damage. Accordingly a reduction of the magnetic anisotropy and a strong enhancement of the magnetic damping parameter are observed. Moreover, ion implantation in an applied magnetic field allows the setting of the uniaxial anisotropy direction irrespective of its original orientation. Static and dynamic magnetic properties of Ni81Fe19 films can be tailored over a wide range after film deposition.

Growth of zinc oxide by chemical vapor transport

ZnO crystal growth by chemical vapor transport (CVT) is carried out using carbon as a transport agent. Under the optimum Δ T and growth temperature, a single crystal was grown. The carbon contamination is not detected by SIMS measurements and all the crystals are orange-red colored. It is claimed that the orange-red color is attributed to the shift of stoichiometry to zinc rich atmosphere.

Carbon and nitrogen stoichiometry and nitrogen cycling rates in streams

Stoichiometric analyses can be used to investigate the linkages between N and C cycles and how these linkages influence biogeochemistry at many scales, from components of individual ecosystems up to the biosphere. N-specific NH4+ uptake rates were measured in eight streams using short-term 15N tracer additions, and C to N ratios (C:N) were determined from living and non-living organic matter collected from ten streams. These data were also compared to previously published data compiled from studies of lakes, ponds, wetlands, forests, and tundra. There was a significant negative relationship between C:N and N-specific uptake rate; C:N could account for 41% of the variance in N-specific uptake rate across all streams, and the relationship held in five of eight streams. Most of the variation in N-specific uptake rate was contributed by detrital and primary producer compartments with large values of C:N and small values for N-specific uptake rate. In streams, particulate materials are not as likely to move downstream as dissolved N, so if N is cycling in a particulate compartment, N retention is likely to be greater. Together, these data suggest that N retention may depend in part on C:N of living and non-living organic matter in streams. Factors that alter C:N of stream ecosystem compartments, such as removal of riparian vegetation or N fertilization, may influence the amount of retention attributed to these ecosystem compartments by causing shifts in stoichiometry. Our analysis suggests that C:N of ecosystem compartments can be used to link N-cycling models across streams.

Phosphorylation-induced modulation of pNBC1 function: distinct roles for the amino- and carboxy-termini.

The human NBC1 (SLC4A4) gene encodes the electrogenic sodium bicarbonate cotransporters kNBC1 and pNBC1, which are highly expressed in the kidney and pancreas, respectively. The HCO3-:Na+ stoichiometry of these cotransporters is an important determinant of the direction of ion flux. Recently we showed in a mouse proximal tubule (mPCT) cell line expressing kNBC1, that 8-Br-cAMP shifts the stoichiometry of the cotransporter from 3:1 to 2:1 via protein kinase A (PKA)-dependent phosphorylation of Ser982. pNBC1 has the identical carboxy-terminal consensus phosphorylation PKA site (KKGS1026), and an additional site in its amino-terminus (KRKT49). In this study we determined the potential role of these sites in regulating the function of pNBC1. The results demonstrated that in mPCT cells expressing pNBC1, PKA-dependent phosphorylation of Ser1026 following 8-Br-cAMP treatment shifted the stoichiometry from 3:1 to 2:1. The effect was electrostatic in nature as replacing Ser1026 with Asp resulted in a similar stoichiometry shift. In addition to shifting the stoichiometry, 8-Br-cAMP caused a significant increase in the 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS)-sensitive basolateral membrane conductance (GDS) of cells expressing pNBC1, but not kNBC1. Although, the effect did not involve phosphorylation of Thr49, which was endogenously phosphorylated, replacing this residue with Asp or Ala abolished the 8-Br-cAMP-induced increase in GDS. In the mPEC pancreatic duct cell line, where endogenous pNBC1 functions with a HCO3-:Na+ stoichiometry of 2:1, 8-Br-cAMP increased GDS by ~90 % without altering the stoichiometry or inducing phosphorylation of the cotransporter. The results demonstrate that phosphorylation of Ser1026 mediates the cAMP-dependent shift in the stoichiometry of pNBC1, whereas Thr49 plays an essential role in the cAMP-induced increase in GDS.

Regulation of the sodium bicarbonate cotransporter kNBC1 function: role of Asp(986), Asp(988) and kNBC1-carbonic anhydrase II binding.

The HCO(3)(-) : Na(+) cotransport stoichiometry of the electrogenic sodium bicarbonate cotransporter kNBC1 determines the reversal potential (E(rev)) and thus the net direction of transport of these ions through the cotransporter. Previously, we showed that phosphorylation of kNBC1-Ser(982) in the carboxy-terminus of kNBC1 (kNBC1-Ct), by cAMP-protein kinase A (PKA), shifts the stoichiometry from 3 : 1 to 2 : 1 and that binding of bicarbonate to the cotransporter is electrostaticaly modulated. These results raise the possibility that phosphorylated kNBC1-Ser(982), or other nearby negatively charged residues shift the stoichiometry by blocking a bicarbonate-binding site. In the current study, we examined the role of the negative charge on Ser(982)-phosphate and three aspartate residues in a D986NDD custer in altering the stoichiometry of kNBC1. mPCT cells expressing kNBC1 mutants were grown on filters and mounted in an Ussing chamber for electrophysiological studies. Enhanced green fluorescence protein (EGFP)-tagged mutant constructs expressed in the same cells were used to determine the phosphorylation status of kNBC1-Ser(982). The data indicate that both kNBC1-Asp(986) and kNBC1-Asp(988), but not kNBC1-Asp(989), are required for the phosphorylation-induced shift in stoichiometry. A homologous motif (D887ADD) in the carboxy-terminus of the anion exchanger AE1 binds to carbonic anhydrase II (CAII). In isothermal titration calorimetry experiments, CAII was found to bind to kNBC1-Ct with a K(D) of 160 +/- 10 nM. Acetazolamide inhibited the short-circuit current through the cotransporter by 65 % when the latter operated in the 3 : 1 mode, but had no effect on the current in the 2 : 1 mode. Acetazolamide did not affect the cotransport stoichiometry or the ability of 8-Br-cAMP to shift the stoichiometry. Although CAII does not affect the transport stoichiometry, it may play an important role in enhancing the flux through the transporter when kNBC1-Ser(982) is unphosphorylated.

Phase equilibria in the Li–Ti–O system and physical properties of Li 2TiO 3

The phase equilibria of the pseudo-binary Li 2O–TiO 2 system and the stoichiometry shift of Li 2TiO 3 during the Li transmutation process are investigated. The structural properties, the thermal expansion, enthalpy, heat capacity, enthalpy of transformation and melting, thermal conductivity and the vapourisation behaviour of Li 2TiO 3 are assessed. The ternary Li–Ti–O system is treated with emphasis to the characteristic of Li 2TiO 3 under reducing conditions. The phase is reduced to LiTiO 2 with the Ti valence +3 due to its transition metal character.

Phosphorylation of Ser(982) in the sodium bicarbonate cotransporter kNBC1 shifts the HCO(3)(-) : Na(+) stoichiometry from 3 : 1 to 2 : 1 in murine proximal tubule cells.

1. Adenosine 3',5'-cyclic monophosphate (cAMP) modulates proximal tubule sodium and bicarbonate absorption by decreasing the rate of apical Na(+)-H(+) exchange and basolateral sodium bicarbonate efflux, through activation of protein kinase A (PKA). The electrogenic sodium bicarbonate cotransporter kNBC1 mediates basolateral sodium and bicarbonate efflux in the proximal tubule by coupling the transport of 1 Na(+) cation to that of 3 HCO(3)(-) anions. In this work we studied the effects of cAMP on the function of kNBC1 expressed heterologously in a proximal tubule cell line. 2. A mouse renal proximal tubule cell line, deficient in electrogenic sodium bicarbonate cotransport function, was transfected with kNBC1. Cells were grown on a permeable support to confluence, mounted in an Ussing chamber and permeabilized apically with amphotericin B. Current through the cotransporter was isolated as the difference current due to the reversible inhibitor dinitrostilbene disulfonate. The HCO(3)(-) : Na(+) stoichiometry of kNBC1 was calculated from its reversal potential by measuring the current-voltage relationships of the cotransporter at different Na(+) concentration gradients. 3. Addition of the potent cAMP agonist 8-Br-cAMP caused the stoichiometry of kNBC1 to shift from 3 HCO(3)(-) : 1 Na(+) to 2 HCO(3)(-) : 1 Na(+). Pretreatment of the cells with the PKA inhibitor H-89 abolished the effect of the agonist on the stoichiometry change. Replacing Ser(982) at the C-terminus consensus PKA phosphorylation site with alanine resulted in a failure of PKA to phosphorylate the transporter and induce a stoichiometry shift. 4. Our data indicate that cAMP modulates the stoichiometry of kNBC1 through activation of PKA. The change in stoichiometry from 3 : 1 to 2 : 1 is predicted to cause a shift in the direction of basolateral membrane sodium bicarbonate transport from efflux to influx. Ser(982) in the C-terminus of kNBC1 is a target for PKA phosphorylation. This is the first example of modulation of the stoichiometry of a membrane transporter by phosphorylation.

Ternary Colquiriite Type Fluorides as Laser Hosts

The Cr3+:LiCaAlF6 (Cr:LiCAF) and Cr3+:LiSrAlF6 (Cr:LiSAF) laser host crystals can be grown by the Czochralski technique. Problems result from the evaporation of lithium-aluminium-fluorides during the growth process and a corresponding stoichiometry shift of the melt towards the CaF2 (SrF2) corner of the ternary concentration triangle. Both Cr:LiCAF, and to a lower extend also Cr:LiSAF, contain micron-sized defects that disturb the lasing efficiency heavily. The defects are assumed to be related to water and/or oxygen impurities within the starting materials and growth atmosphere and to peculiarities of the LiF – CaF2 – AlF3 ternary phase diagram.

Solution structure of the ternary complex between aminoacyl-tRNA, elongation factor Tu, and guanosine triphosphate.

Complex formation between elongation factor Tu (EF-Tu), Phe-tRNAPhe, and GTP was analyzed by small-angle neutron and X-ray scattering methods. Both techniques show that the ternary complex consists of one EF-Tu and one aminoacyl-tRNA. No shift in stoichiometry was detected when the temperature was raised from 5 to 37 degreesC, in contrast to previous observations obtained from RNase A protection experiments [Bilgin and Ehrenberg (1995) Biochemistry34, 715-719]. A small but significant increase in the radius of gyration of the complex was observed when the temperature was decreased from 37 to 5 degreesC. The X-ray solution scattering patterns were compared with those calculated from the crystal structure of the complex formed between EF-Tu from Thermus aquaticus and Phe-tRNAPhe from yeast. The comparison shows that the solution structure of the ternary complex, formed entirely from Escherichia coli components and under translationally optimal buffer conditions, is very close to the crystal structure, formed from heterologous components under very different conditions. Furthermore, for the hybrid complex in solution there is no evidence for the formation of trimers as suggested by the crystal structure.

Simulation of the light-induced oscillations of the membrane potential in Potamogeton leaf cells.

An attempt has been made to simulate the light-induced oscillations of the membrane potential of Potamogeton lucens leaf cells in relation to the apoplastic pH changes. Previously it was demonstrated that the membrane potential of these cells can be described in terms of proton movements only. It is hypothesized that the membrane potential is determined by an electrogenic H(+)-ATPase with a variable H+/ATP stoichiometry. The stoichiometry shifts from a value of two in the dark to a value of one in the light. Moreover, this H+ pump shows the characteristics of either a pump or a passive H+ conductance: the mode of operation of the H+ translocator is considered to be regulated by the external pH. The pump conductance is assumed to be dominant at low or neutral pH, while the passive H+ conductance becomes more significant at alkaline pH. The pH dependence of the transport characteristic is expressed by protonation reactions in the plasma membrane. The proposed model can account for most features of the light-induced oscillations but not for the absolute level of the membrane potential.

Vacuum Annealing Effects on the Surface Stoichiometry of InGaAsP

not Available.

Formation of zintl-ions in alkali-lead alloys

In the last years the liquid alkali-Pb alloys have been studied extensively. The measurements made include resistivity, structure factor, and the specific heat capacity. These alloys show anomalous behavior at either the octet composition with 80 at. % alkalimetal or at the equiatomic composition. From the resistivity measurements follows that a shift in stoichiometry occurs between Na-Pb and K-Pb. Whereas liquid Na-Ph, just as Li-Ph forms an octet compound, the K-Pb, Rb-Pb and Cs-Pb systems form only an equiatomic compound. In this paper we present the results of neutron diffraction and specific heat measurements. Together they constitute evidence for the formation of (Pb{sub 4}){sup 4-}-ions in liquid K-Pb, Rb-Pb and Cs-Pb.

The n-p Transition in YBa2Cu3O6–5+x

ABSTRACTThe role of annealing conditions on the equilibrium defect state of the material was investigated by computer controlled 4 probe d.c. conductivity and thermoelectric power measurements on dense homogeneous ceramics prepared from citrate precursors. At elevated temperatures, the conductivity first decreases and then increases exponentially with decreasing temperature with the conductivity minima shifting to lower temperatures with decreasing PO2. This unusual behavior is described as representing a transition from n-type to p-type conduction. The n-p transition is correlated with the corresponding stoichiometry shift upon cooling from oxygen deficient (Cu1, Cu2) to oxygen excess YBa3Cu3O6–5+x, (Cu2+, Cu3+). A preliminary defect model is proposed which provides a framework for understanding a number of the experimental observations.

High-pressure polymorphism of FeO? An alternative interpretation and its implications for the Earth's core

Summary. It has been reported that no stoichiometric FeO can be formed at temperatures above about 6OO0C regardless of pressure. The stoichiometry of FexO quenched from static high pressure and temperature experiments appears to decrease with increasing pressure in the range 100-300 kbar. On the basis of these observations, we have re-interpreted the recent shock results on FeoawO, and have found that the Hugoniot data can be reasonable interpreted by the same shift in stoichiometry as observed in the static highpressure experiments instead of a first-order transformation as proposed. If oxygen is assumed to be the light component of the outer core, the outer core can be chemically homogeneous only if the stoichiometry of FexO decreases with increasing pressure. A constant stoichiometry would demand that the oxygen content decreases by a factor of 2 from the top to the bottom of the outer core. A chemically homogeneous outer core is consistent with a convecting outer core.

Die radiale sauerstoff-verteilung im brennstoff bestrahlter mischoxidbrennstäbe unterschiedlicher ausgangsstöchiometrie

The radial gradients of the oxygen potential and the oxygen/metal ratio O/M in the fuel of irradiated mixed oxide pins at the end of life have been determined from the Mo and MoO 2 concentrations in the metallic and ceramic inclusions. The measurements were carried out on uranium-plutonium oxides with different initial stoichiometries (O/M) o of 1.96, 1.98 and 1.99, resp., after fast neutron irradiation under identical conditions. Using the measured overall final stoichiometries and the stoichiometry shifts during burnup, the initial stoichiometries of the three investigated oxides have been recalculated, giving (O/M) 2 values of 1.961, 1.978, and >1.985, resp.

X-ray and neutron determination of a so-called Th 2Ni 17-type structure in the lutetium-iron system

The iron-rich lutetium compound crystallises in a hexagonal structure, space group P6 3/ mmc. By X-ray and neutron diffraction single crystal studies we have shown that its structure does not belong to the ideal Th 2Ni 17-type as previously proposed. The structure under investigation is established by ordered substitutions occurring in the CaCu 5 subcells on two of the three chains of lutetium atoms, parallel to the c -axis. These substitutions lead to an increase of the c parameter which allows for some disordered substitutions on the third chain and the stoichiometry shifts from LuFe 8.5 to LuFe 9.5.