Synthesis and photoluminescence behavior of Bi4Ti3O12 powders obtained by the complex polymerization method

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

(Sr,Tm)ZrO3 powders prepared by the polymeric precursor method: Synthesis, optical properties and morphological characteristics

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Structural and dielectric properties of relaxor Sr0.5Ba 0.5Bi2Nb2O9 ceramic

Sr0.5Ba0.5Bi2Nb2O 9 ceramic was prepared by a conventional solid state reaction method and studied using X-ray powder diffraction and dielectric measurements. At room temperature, an orthorhombic structure was confirmed and their parameters were obtained using the Rietveld method. Dielectric properties were studied in a broad range of temperatures and frequencies. Typical relaxor behaviour was observed with strong dispersion of the complex relative dielectric permittivity. The temperature of the maximum dielectric constant Tm decreases with increasing frequency, and shifts towards higher temperature side. The activation energy Ea≈0·194±0·03 eV and freezing temperature Ta≈371±2 K values were found using the Vogel-Fulcher relationship. Conduction process in the material may be due to the hopping of charge carriers at low temperatures and small polarons and/or singly ionised oxygen vacancies at higher temperatures. © 2010 Maney Publishing.

Microbiological responses to depuration and transport of native and exotic clams at optimal and stressful temperatures

The microbiological responses of two bivalves species from Tagus estuary, Venerupis pullastra (native clam) and Ruditapes philippinarum (exotic clam) were investigated during 48h of depuration and subsequent simulated transport in semi-dry conditions at two temperatures (4 and 22°C) until reaching 50% lethal time (LT50). Regardless of temperature and species, the maintenance of clams in water for 48h (depuration period) did not affect LT50 during transport. R.philippinarum showed higher survival rates than V.pullastra, always reaching LT50 later, especially at 4°C. Significant differences between clams' species were found in almost all microbiological parameters. This can be related with clams' biological activity and habitat environmental conditions since both clams do not coexist in Tagus estuary. Depuration was efficient to reduce the bacterial load, particularly Escherichia coli, but not efficient to remove Vibrio spp. In both species, the growth of Vibrio spp. was inhibited at 4°C, whereas exponential growth occurred at 22°C. Total viable counts significantly increased in most treatments, while E.coli counts significantly decreased to undetected levels, except for non-depurated R.philippinarum simulated transported at 4°C. Thus, this study highlights the importance of clams depuration for at least 24h in polluted estuarine areas, followed by transport at low temperatures (4°C). © 2013 Elsevier Ltd.

Propriedade fotoluminescente dos materiais 'CA''TI''O IND.3' e 'CA IND.1-x''EU IND.x''TI''O IND.3'

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Petrologia magnética dos granodioritos Água Azul e Água Limpa, porção sul do Domínio Carajás - Pará

Os granodioritos Água Azul (GrdAA) e Água Limpa (GrdAL) afloram no extremo sul do Domínio Carajás como dois corpos alongados segundo o trend regional E-W, anteriormente inseridos no Complexo Xingu. O GrdAL é formado essencialmente por biotita-anfibólio granodioritos e muscovita-biotita granodioritos, além de anfibólio-biotita tonalitos subordinados; no GrdAA, epídoto-anfibólio-biotita granodioritos são dominantes, epídoto-anfibólio-biotita tonalitos e (anfibólio)-epídoto-biotita monzogranitos, subordinados. Essas rochas mostram assinaturas geoquímicas afins dos sanukitoides arqueanos. O estudo de suscetibilidade magnética (SM) mostrou valores relativamente baixos para o GrdAL (média de 17,54 × 10^-4 SIv) e o GrdAA (média de 4,19 × 10^-4 SIv). Os estudos dos minerais opacos mostram que a magnetita e a hematita são as fases comuns e que a ilmenita está ausente nessas rochas. O GrdAL contém titanita associada à magnetita, enquanto o GrdAA contém pirita, calcopirita e goethita. No GrdAL, a magnetita é mais abundante e desenvolvida que no GrdAA, justificando, assim, sua SM mais elevada. A martitização da magnetita e a oxidação dos sulfetos, gerando goethita, ocorreram a baixas temperaturas. A correlação positiva entre os valores de SM e os conteúdos modais de opacos, anfibólio, epídoto + allanita e quartzo + K-feldspato, assim como a correlação negativa de SM com biotita e máficos observadas nessas unidades, denunciam uma tendência no aumento de SM no sentido anfibólio tonalitos/anfibólio granodioritos à biotita granodioritos/biotita monzogranitos. Os dados geoquímicos corroboram esse comportamento, com correlação negativa entre os valores de SM e Fe₂O₃T, FeO e MgO, refletindo para as duas unidades uma tendência de aumento nos valores de SM paralelamente à diferenciação magmática. As afinidades geoquímicas e mineralógicas entre essas rochas e os sanukitoides do Domínio Rio Maria sugerem condições de fugacidade de oxigênio entre os tampões HM e FMQ para os granitoides estudados.

A relationship between structural and electronic order-disorder effects and optical properties in crystalline TiO2 nanomaterials

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Oxygen Interstitial Dynamics in Bi(2)Sr(2)CaCu(2)O(8+delta) Oxides Measured by Mechanical Spectroscopy

Many researchers became interested in the discovery of Bi(2)Sr(2)CaCu(2)O(8+delta) oxides with critical temperature of around 80 K. It is known that the critical temperature is related to the CuO2 planes of the material. For this reason, the study of the interstitial oxygen in these oxides is of great relevance. The samples were prepared by means of conventional solid state reactions, through the stoichiometric mixture of precursory powders. After the sinterization, the samples were submitted to measurements of density, electrical resistivity, x-ray diffraction, scanning electron microscopy and energy dispersion spectroscopy, with the objective of performing their characterization. The measurements of mechanical spectroscopy were performed by a torsion pendulum. The results show three relaxation processes in the temperature range of 200 and 700 K, with activation energy of approximately 0.9 eV, which has been attributed to the dynamics of the interstitial oxygen present in the material.

Nanosized precipitates in H13 tool steel low temperature plasma nitriding

A comprehensive study of pulsed nitriding in AISI H13 tool steel at low temperature (400 degrees C) is reported for several durations. X-ray diffraction results reveal that a nitrogen enriched compound (epsilon-Fe2-3N, iron nitride) builds up on the surface within the first process hour despite the low process temperature. Beneath the surface, X-ray Wavelength Dispersive Spectroscopy (WDS) in a Scanning Electron Microscope (SEM) indicates relatively higher nitrogen concentrations (up to 12 at.%) within the diffusion layer while microscopic nitrides are not formed and existing carbides are not dissolved. Moreover, in the diffusion layer, nitrogen is found to be dispersed in the matrix and forming nanosized precipitates. The small coherent precipitates are observed by High-Resolution Transmission Electron Microscopy (HR-TEM) while the presence of nitrogen is confirmed by electron energy loss spectroscopy (EELS). Hardness tests show that the material hardness increases linearly with the nitrogen concentration, reaching up to 14.5 GPa in the surface while the Young Modulus remains essentially unaffected. Indeed, the original steel microstructure is well preserved even in the nitrogen diffusion layer. Nitrogen profiles show a case depth of about similar to 43 mu m after nine hours of nitriding process. These results indicate that pulsed plasma nitriding is highly efficient even at such low temperatures and that at this process temperature it is possible to form thick and hard nitrided layers with satisfactory mechanical properties. This process can be particularly interesting to enhance the surface hardness of tool steels without exposing the workpiece to high temperatures and altering its bulk microstructure. (c) 2012 Elsevier B.V. All rights reserved.

Low-temperature thermodynamic properties near the field-induced quantum critical point in NiCl2-4SC(NH2)(2)

We present a comprehensive experimental and theoretical investigation of the thermodynamic properties: specific heat, magnetization, and thermal expansion in the vicinity of the field-induced quantum critical point (QCP) around the lower critical field H-c1 approximate to 2 T in NiCl2-4SC(NH2)(2). A T-3/2 behavior in the specific heat and magnetization is observed at very low temperatures at H = H-c1, which is consistent with the universality class of Bose-Einstein condensation of magnons. The temperature dependence of the thermal expansion coefficient at H-c1 shows minor deviations from the expected T-1/2 behavior. Our experimental study is complemented by analytical calculations and quantum Monte Carlo simulations, which reproduce nicely the measured quantities. We analyze the thermal and the magnetic Gruneisen parameters, which are ideal quantities to identify QCPs. Both parameters diverge at H-c1 with the expected T-1 power law. By using the Ehrenfest relations at the second-order phase transition, we are able to estimate the pressure dependencies of the characteristic temperature and field scales.

Biomass Gasification - Process analysis and dimensioning aspects for downdraft units and gas cleaning lines

In such territories where food production is mostly scattered in several small / medium size or even domestic farms, a lot of heterogeneous residues are produced yearly, since farmers usually carry out different activities in their properties. The amount and composition of farm residues, therefore, widely change during year, according to the single production process periodically achieved. Coupling high efficiency micro-cogeneration energy units with easy handling biomass conversion equipments, suitable to treat different materials, would provide many important advantages to the farmers and to the community as well, so that the increase in feedstock flexibility of gasification units is nowadays seen as a further paramount step towards their wide spreading in rural areas and as a real necessity for their utilization at small scale. Two main research topics were thought to be of main concern at this purpose, and they were therefore discussed in this work: the investigation of fuels properties impact on gasification process development and the technical feasibility of small scale gasification units integration with cogeneration systems. According to these two main aspects, the present work was thus divided in two main parts. The first one is focused on the biomass gasification process, that was investigated in its theoretical aspects and then analytically modelled in order to simulate thermo-chemical conversion of different biomass fuels, such as wood (park waste wood and softwood), wheat straw, sewage sludge and refuse derived fuels. The main idea is to correlate the results of reactor design procedures with the physical properties of biomasses and the corresponding working conditions of gasifiers (temperature profile, above all), in order to point out the main differences which prevent the use of the same conversion unit for different materials. At this scope, a gasification kinetic free model was initially developed in Excel sheets, considering different values of air to biomass ratio and the downdraft gasification technology as particular examined application. The differences in syngas production and working conditions (process temperatures, above all) among the considered fuels were tried to be connected to some biomass properties, such elementary composition, ash and water contents. The novelty of this analytical approach was the use of kinetic constants ratio in order to determine oxygen distribution among the different oxidation reactions (regarding volatile matter only) while equilibrium of water gas shift reaction was considered in gasification zone, by which the energy and mass balances involved in the process algorithm were linked together, as well. Moreover, the main advantage of this analytical tool is the easiness by which the input data corresponding to the particular biomass materials can be inserted into the model, so that a rapid evaluation on their own thermo-chemical conversion properties is possible to be obtained, mainly based on their chemical composition A good conformity of the model results with the other literature and experimental data was detected for almost all the considered materials (except for refuse derived fuels, because of their unfitting chemical composition with the model assumptions). Successively, a dimensioning procedure for open core downdraft gasifiers was set up, by the analysis on the fundamental thermo-physical and thermo-chemical mechanisms which are supposed to regulate the main solid conversion steps involved in the gasification process. Gasification units were schematically subdivided in four reaction zones, respectively corresponding to biomass heating, solids drying, pyrolysis and char gasification processes, and the time required for the full development of each of these steps was correlated to the kinetics rates (for pyrolysis and char gasification processes only) and to the heat and mass transfer phenomena from gas to solid phase. On the basis of this analysis and according to the kinetic free model results and biomass physical properties (particles size, above all) it was achieved that for all the considered materials char gasification step is kinetically limited and therefore temperature is the main working parameter controlling this step. Solids drying is mainly regulated by heat transfer from bulk gas to the inner layers of particles and the corresponding time especially depends on particle size. Biomass heating is almost totally achieved by the radiative heat transfer from the hot walls of reactor to the bed of material. For pyrolysis, instead, working temperature, particles size and the same nature of biomass (through its own pyrolysis heat) have all comparable weights on the process development, so that the corresponding time can be differently depending on one of these factors according to the particular fuel is gasified and the particular conditions are established inside the gasifier. The same analysis also led to the estimation of reaction zone volumes for each biomass fuel, so as a comparison among the dimensions of the differently fed gasification units was finally accomplished. Each biomass material showed a different volumes distribution, so that any dimensioned gasification unit does not seem to be suitable for more than one biomass species. Nevertheless, since reactors diameters were found out quite similar for all the examined materials, it could be envisaged to design a single units for all of them by adopting the largest diameter and by combining together the maximum heights of each reaction zone, as they were calculated for the different biomasses. A total height of gasifier as around 2400mm would be obtained in this case. Besides, by arranging air injecting nozzles at different levels along the reactor, gasification zone could be properly set up according to the particular material is in turn gasified. Finally, since gasification and pyrolysis times were found to considerably change according to even short temperature variations, it could be also envisaged to regulate air feeding rate for each gasified material (which process temperatures depend on), so as the available reactor volumes would be suitable for the complete development of solid conversion in each case, without even changing fluid dynamics behaviour of the unit as well as air/biomass ratio in noticeable measure. The second part of this work dealt with the gas cleaning systems to be adopted downstream the gasifiers in order to run high efficiency CHP units (i.e. internal engines and micro-turbines). Especially in the case multi–fuel gasifiers are assumed to be used, weightier gas cleaning lines need to be envisaged in order to reach the standard gas quality degree required to fuel cogeneration units. Indeed, as the more heterogeneous feed to the gasification unit, several contaminant species can simultaneously be present in the exit gas stream and, as a consequence, suitable gas cleaning systems have to be designed. In this work, an overall study on gas cleaning lines assessment is carried out. Differently from the other research efforts carried out in the same field, the main scope is to define general arrangements for gas cleaning lines suitable to remove several contaminants from the gas stream, independently on the feedstock material and the energy plant size The gas contaminant species taken into account in this analysis were: particulate, tars, sulphur (in H2S form), alkali metals, nitrogen (in NH3 form) and acid gases (in HCl form). For each of these species, alternative cleaning devices were designed according to three different plant sizes, respectively corresponding with 8Nm3/h, 125Nm3/h and 350Nm3/h gas flows. Their performances were examined on the basis of their optimal working conditions (efficiency, temperature and pressure drops, above all) and their own consumption of energy and materials. Successively, the designed units were combined together in different overall gas cleaning line arrangements, paths, by following some technical constraints which were mainly determined from the same performance analysis on the cleaning units and from the presumable synergic effects by contaminants on the right working of some of them (filters clogging, catalysts deactivation, etc.). One of the main issues to be stated in paths design accomplishment was the tars removal from the gas stream, preventing filters plugging and/or line pipes clogging At this scope, a catalytic tars cracking unit was envisaged as the only solution to be adopted, and, therefore, a catalytic material which is able to work at relatively low temperatures was chosen. Nevertheless, a rapid drop in tars cracking efficiency was also estimated for this same material, so that an high frequency of catalysts regeneration and a consequent relevant air consumption for this operation were calculated in all of the cases. Other difficulties had to be overcome in the abatement of alkali metals, which condense at temperatures lower than tars, but they also need to be removed in the first sections of gas cleaning line in order to avoid corrosion of materials. In this case a dry scrubber technology was envisaged, by using the same fine particles filter units and by choosing for them corrosion resistant materials, like ceramic ones. Besides these two solutions which seem to be unavoidable in gas cleaning line design, high temperature gas cleaning lines were not possible to be achieved for the two larger plant sizes, as well. Indeed, as the use of temperature control devices was precluded in the adopted design procedure, ammonia partial oxidation units (as the only considered methods for the abatement of ammonia at high temperature) were not suitable for the large scale units, because of the high increase of reactors temperature by the exothermic reactions involved in the process. In spite of these limitations, yet, overall arrangements for each considered plant size were finally designed, so that the possibility to clean the gas up to the required standard degree was technically demonstrated, even in the case several contaminants are simultaneously present in the gas stream. Moreover, all the possible paths defined for the different plant sizes were compared each others on the basis of some defined operational parameters, among which total pressure drops, total energy losses, number of units and secondary materials consumption. On the basis of this analysis, dry gas cleaning methods proved preferable to the ones including water scrubber technology in al of the cases, especially because of the high water consumption provided by water scrubber units in ammonia adsorption process. This result is yet connected to the possibility to use activated carbon units for ammonia removal and Nahcolite adsorber for chloride acid. The very high efficiency of this latter material is also remarkable. Finally, as an estimation of the overall energy loss pertaining the gas cleaning process, the total enthalpy losses estimated for the three plant sizes were compared with the respective gas streams energy contents, these latter obtained on the basis of low heating value of gas only. This overall study on gas cleaning systems is thus proposed as an analytical tool by which different gas cleaning line configurations can be evaluated, according to the particular practical application they are adopted for and the size of cogeneration unit they are connected to.

Photoemissions-Elektronenmikroskopie an Adsorbatsystemen

Die Photoemissions-Elektronenmikroskopie ist eine hervorragend geeignete Methode zur Untersuchung dynamischer Vorgänge auf realen polykristallinen Oberflächen im sub-μm Bereich. Bei der Anwendung auf Adsorbatsysteme lassen sich geringe Bedeckungsunterschiede, sowie Adsorbatstrukturen und -phasen unterscheiden. Die Methode erlaubt dabei ein breites Anwendungsspektrum über weite Temperaturbereiche und Systeme unterschiedlichster Bindungsenergie. Bei der Chemisorption von Sauerstoff auf polykristallinen Metallen wird unterschiedliches Aufwachsverhalten in den Helligkeitswerten im Mikroskopbild widergespiegelt. Bei Kupferproben zeigen Oberflächen mit unterschiedlicher kristalliner Richtung aufgrund der Symmetrie des fcc-Gitters ein ähnliches Verhalten. Das hexagonale Gitter des Titans zeigt dagegen große Unterschiede im Adsorptionsverhalten in Abhängigkeit der kristallinen Richtung. Diese Unterschiede konnten auf verschiedene Haftkoeffizienten und Oxidationsstufen der Metalle zurückgeführt werden. In einem Modell zur Photostromanalyse konnte beim Kupfer der Übergang von verschiedenen Überstrukturen bei wachsender Bedeckung gezeigt und die Übergänge ermittelt werden.. Auf den Titanoberflächen wurde so das Wachstum der Oxide TiO, TiO₂ und Ti₂O₃ unterschieden und die Übergänge des unterschiedlichen Wachstums ermittelt. Bei der thermischen Desorption der Schichten konnten unterschiedliche Haftkoeffizient auf einzelnen Kristalliten qualitativ gezeigt werden. Diese erstmalig eingesetzte Analysemethode weist Ähnlichkeiten zur Thermo-Desorptions-Spektroskopie (TDS) auf, zeigt jedoch ortsaufgelöst lokale Unterschiede auf polykristallinen Oberflächen. Bei thermisch gestützten Oberflächenreaktionen ließen sich die Reaktionskeime deutlich identifizieren und mit einer Grauwertanalyse konnte die Oxidation der karbidischen Lagen zu Kohlenmonoxid und die Metalloxidation unterschieden werden. Dabei konnte gezeigt werden, daß die Reaktionskeime nur an Plattengrenzen auftreten, nicht jedoch auf der Oberfläche. Durch die Aufrauhung der Plattengrenzen mit zunehmender Reaktionsdauer nimmt die Zahl der Reaktionskeime kontinuierlich zu, die laterale Ausdehnung der Einzelreaktionen bleibt aber konstant. Bei der Physisorption von Xenon auf Graphit wurde erstmals für die Photoemissionsmikroskopie die resonanten Anregung ausgenutzt. Die verschiedenen Phasen des Adsorbats können dabei deutlich unterschieden werden; bei niedrigen Temperaturen (40K) findet ein gleichmäßiges Wachstum auf der gesamten Oberfläche statt, bei höheren Temperaturen von 60-65K ist dagegen ein Inselwachstum in verschiedenen Phasen zu beobachten. Die zeitliche Entwicklung des Wachstums, die örtliche Lage der Phasen und die Phasenübergänge (gas, fest inkommensurabel, fest kommensurabel) konnten bestimmt werden. Bei der Desorption der Schichten konnten die einzelnen Phasen ebenfalls getrennt werden und das unterschiedliche Desorptionsverhalten sowie die Phasenübergänge selber verifiziert werden.

Reaktivität von Dihypersilylhalogenorganylplumbanen

Organylhalogenide RX reagieren formal gemäß einer oxidativen 1,1-Addition mit Dihypersilylplumbylen) PbHyp2 zu Dihypersilyl-halogenorganylplumbanen PbHyp2RX. Diese Umsetzung gelingt mit nahezu allen untersuchten Organylresten, lediglich beispielsweise der Mesitylrest erweist sich als zu sperrig (R = Me, Et, nPr, iPr, tBu, Hx, cHx, Ad, Ph, C6F5, oTo, mTo, pTo, Naph, Anthr).rnrnFür einige Halogenorganyle wird eine analoge Addition auch an das zinnhomologe Dihypersilylstannylen beschrieben.rnrnDie untersuchten Addukte sind thermisch und gegenüber UV-Strahlung, Sauerstoff und Wasser deutlich weniger empfindlich als vergleichbare andere blei- und zinnorganische Verbindungen.rnrnUmfangreiche NMR-Datensätze beschreiben eine markante Hoch-feldverschiebung der Hypersilylprotonen beim Übergang von PbHyp2 hin zu PbHyp2RX, die für Arylreste stärker ausfällt als für Alkylreste.rnrnEin Großteil der Addukte wurde einkristallin erhalten und wird anhand der röntgendiffraktometrisch ermittelten Strukturparameter detailliert charakterisiert. rnrnEs finden sich gegenüber dem idealen Tetraderwinkel auffällig stark aufgeweitete Si Pb Si-Winkel von 130-143°, während die anderen Winkel mehrheitlich unter dem theoretischen Idealwert bleiben (Si Pb X: 94 103°; Si Pb C: 98-112°; C Pb X: 95-103°). Insbesondere größere, weichere Reste und planare Arylplumbane rücken näher an das Halogen heran.rnDie insgesamt recht hohen Blei-Halogen-Abstände nehmen von den Chloriden hin zu den Iodiden zu. Die Iodoplumbane zeigen dabei generell die kleinsten Pb Si und die größten Pb C Abstände. Alkylplumbane weisen längere Pb C Bindungen auf als ansonsten vergleichbare Arylplumbane.rnrnViele der gefundenen Molekülstrukturen zeigen Anzeichen hoher sterischer Spannung in Form von Substituentenverzerrungen. rnrnDie Bildungsgeschwindigkeit der Addukte ist auch bei tiefen Temperaturen hoch. Sie nimmt von X = Cl über Br hin zu I zu und ist für Alkylreste höher als für Aryle. Die Zerfallsgeschwindigkeiten verhalten sich genau entgegengesetzt. Bei den Thermolysen wird regelmäßig Hypersilylhalogenid eliminiert. Dabei entstehen in Abwesenheit koordinierender Solventien Dihypersilyl-diorganylplumbane PbHyp2R2.rnAndere, unbekannte Zerfallskanäle führen zu unerwarteten Produkten, wie dem Iodonium-verbrückten, cyclischen Tetraplumbetan Pb4I(C6F5)Hyp3.rnrnIn Anwesenheit von Lewis-Basen hingegen können sich hetero-leptische Plumbylene bilden, wie am Beispiel eines Bitolyldiylbisplumbylens gezeigt wird. Dieses zeigt auch, dass prinzipiell eine zweifache Addition von Dihalogenorganylen an zwei Äqui-valente Dihypersilylplumbylen möglich ist. Entsprechende Untersuchungen beschäftigen sich ausführlich mit dafür geeigneten und ungeeigneten Organdiylresten.rnrnUnter günstigen reduktiven Bedingungen lassen sich mittels Metal-lierungsreagenz aus den Dihypersilylhalogenorganylplumbanen unter Halogenidentzug Plumbanide erhalten, die in Form getrennter Ionenpaare isolierbar sind. Diese lassen sich in Ana-logie zu den zuvor beschriebenen Plumbanen ebenfalls als Addukt aus Dihypersilylplumbylen und Lithiumorganylen darstellen.rnrnUnter geeigneten speziellen Bedingungen sind neben metallierten auch formal hydridierte Halogenplumbanide zugänglich.rnrnEntsprechend einer geringen Hybridisierung am zentralen Blei-atom, also eines hohen p-AO-Charakters der bindenden Molekülorbitale und s-AO-Charakters des LEP-Orbitals ergeben sich keine trigonal-planaren, sondern Strukturen mit Substituenten-winkeln sogar nahe bei 90°. Die Bindungslängen zum Blei sind deutlich größere als bei den entsprechenden Halogenplumbanen.rnrnEin besonderes Augenmerk der Arbeit liegt auf der Betrachtung von langlebigen und persistenten heteroleptischen Plumbyl-Radikalen, die durch milde Oxidation mittels PbNsi23) aus den Dihypersilylorganylplumbaniden erhalten werden. Während bislang nur homoleptische Vertreter bekannt waren, die aufgrund des sterischen Anspruchs der Substituenten und aufgrund von Hyperkonjugation von axialsymmetrischer nahezu planarer Geometrie sind, findet sich für die heteroleptischen Plumbyle dieser Arbeit eine stärker pyramidale Geometrie. Ausführlich diskutierte EPR-Experimente liefern Spektren, die gut mit Simulationen für die entsprechenden Radikale übereinstimmen.rnrnDie im Zentrum der Betrachtungen dieser Arbeit stehenden Dihypersilylhalogenorganylplumbane stellen somit einen aussichtsreichen und darüber hinaus persistenten und gut zu handhabenden Ausgangspunkt bei der Darstellung neuartiger und interessanter Spezies dar.

Thermodynamics of Forming Water Clusters at Various Temperatures and Pressures by Gaussian-2, Gaussian-3, Complete Basis Set-QB3, and Complete Basis Set-APNO Model Chemistries; Implications for Atmospheric Chemistry

The Gaussian-2, Gaussian-3, complete basis set- (CBS-) QB3, and CBS-APNO methods have been used to calculate ΔH° and ΔG° values for neutral clusters of water, (H2O)n, where n = 2−6. The structures are similar to those determined from experiment and from previous high-level calculations. The thermodynamic calculations by the G2, G3, and CBS-APNO methods compare well against the estimated MP2(CBS) limit. The cyclic pentamer and hexamer structures release the most heat per hydrogen bond formed of any of the clusters. While the cage and prism forms of the hexamer are the lowest energy structures at very low temperatures, as temperature is increased the cyclic structure is favored. The free energies of cluster formation at different temperatures reveal interesting insights, the most striking being that the cyclic trimer, cyclic tetramer, and cyclic pentamer, like the dimer, should be detectable in the lower troposphere. We predict water dimer concentrations of 9 × 1014 molecules/cm3, water trimer concentrations of 2.6 × 1012 molecules/cm3, tetramer concentrations of approximately 5.8 × 1011 molecules/cm3, and pentamer concentrations of approximately 3.5 × 1010 molecules/cm3 in saturated air at 298 K. These results have important implications for understanding the gas-phase chemistry of the lower troposphere.

Enhanced proliferation and dopaminergic differentiation of ventral mesencephalic precursor cells by synergistic effect of FGF2 and reduced oxygen tension

Effective numerical expansion of dopaminergic precursors might overcome the limited availability of transplantable cells in replacement strategies for Parkinson's disease. Here we investigated the effect of fibroblast growth factor-2 (FGF2) and FGF8 on expansion and dopaminergic differentiation of rat embryonic ventral mesencephalic neuroblasts cultured at high (20%) and low (3%) oxygen tension. More cells incorporated bromodeoxyuridine in cultures expanded at low as compared to high oxygen tension, and after 6 days of differentiation there were significantly more neuronal cells in low than in high oxygen cultures. Low oxygen during FGF2-mediated expansion resulted also in a significant increase in tyrosine hydroxylase-immunoreactive (TH-ir) dopaminergic neurons as compared to high oxygen tension, but no corresponding effect was observed for dopamine release into the culture medium. However, switching FGF2-expanded cultures from low to high oxygen tension during the last two days of differentiation significantly enhanced dopamine release and intracellular dopamine levels as compared to all other treatment groups. In addition, the short-term exposure to high oxygen enhanced in situ assessed TH enzyme activity, which may explain the elevated dopamine levels. Our findings demonstrate that modulation of oxygen tension is a recognizable factor for in vitro expansion and dopaminergic differentiation of rat embryonic midbrain precursor cells.