Horst P. Beck

Sol-Gel Synthesis of Nano-Scaled BaTiO3, BaZrO3 and BaTi0.5Zr0.5O3 Oxides via Single-Source Alkoxide Precursors and Semi-Alkoxide Routes

Sol-gel synthesis of nano-sized BaTiO3, BaZrO3 and BaTi0.5Zr0.5O3 ceramics using alkoxide and semi-alkoxide routes has been investigated and the pervoskites obtained have been compared with respect to crystallisation temperature, crystallite size and compositional purity. Heterometal alkoxides containing two (for BaTiO3 and BaZrO3) and three (for BaTi0.5Zr0.5O3) different metals were used as single-source precursors in the alkoxide route while semi-alkoxide synthesis was performed by reacting barium hydroxide or acetate with Ti and/or Zr alkoxides. Semi-alkoxide synthesis also produces stoichiometric and phase-pure oxides, however, at temperatures higher than 1000°C. At temperatures below 1000°C, BaCO3 and small amounts of other undesired phases (e.g., BaTi2O4) were present in the oxides derived from semi-alkoxide synthesis. Thermal behaviour, studied by TGA/DTA measurements, shows that thermal decomposition occurs in three major steps and depends on the educt composition and the synthesis route. Among alkoxide derived powders, crystalline BaTi0.5Zr0.5O3 phase is formed at 400°C while complete crystallisation of BaMO3 ceramics occurs around 600°C. The cubic to tetragonal phase transition for BaTiO3 is clearly observed at relatively low-temperature of 800°C. The stoichiometry and phase homogeneity of the obtained powders were demonstrated by energy dispersive X-ray analysis and powder diffractometry. The averaged crystallite size of the obtained nano-ceramics was evaluated using the FormFit programme. SEM and TEM observations revealed a high microstructural uniformity.

Staphylococcus aureus ClpC ATPase is a late growth phase effector of metabolism and persistence

Staphylococcus aureus Clp ATPases (molecular chaperones) alter normal physiological functions including an aconitase‐mediated effect on post‐stationary growth, acetate catabolism, and entry into death phase (Chatterjee et al., J. Bacteriol. 2005, 187, 4488–4496). In the present study, the global function of ClpC in physiology, metabolism, and late‐stationary phase survival was examined using DNA microarrays and 2‐D PAGE followed by MALDI‐TOF MS. The results suggest that ClpC is involved in regulating the expression of genes and/or proteins of gluconeogenesis, the pentose‐phosphate pathway, pyruvate metabolism, the electron transport chain, nucleotide metabolism, oxidative stress, metal ion homeostasis, stringent response, and programmed cell death. Thus, one major function of ClpC is balancing late growth phase carbon metabolism. Furthermore, these changes in carbon metabolism result in alterations of the intracellular concentration of free NADH, the amount of cell‐associated iron, and fatty acid metabolism. This study provides strong evidence for ClpC as a critical factor in staphylococcal energy metabolism, stress regulation, and late‐stationary phase survival; therefore, these data provide important insight into the adaptation of S. aureus toward a persister state in chronic infections.

Pitfalls in the synthesis of nanoscaled perovskite type compounds. Part I: Influence of different sol-gel preparation methods and characterization of nanoscaled BaTiO3

Abstract Different sol–gel routes are compared for the synthesis of nanoscaled BaTiO 3 with respect to the reaction paths and the quality of the products. The various precursor systems differ largely in their behaviour during decomposition imposing quite different procedures for their thermal treatment. It is shown that the compositional homogeneity during the process of synthesis, which is the target of sol–gel methods, may well be destroyed by the formation of intermediate compounds. It will only be restored after prolonged heating at higher temperatures where the crystallite sizes will in general have left the nanoregime. Due to such pitfalls the desired properties of the products with narrow size distributions will not always be ensured when working according to such recipes and the products obtained will behave quite differently in fabrication procedures like shaping and sintering of workpieces.

Complexation of europium and uranium by humic acids analyzed by capillary electrophoresis-inductively coupled plasma mass spectrometry.

Investigations of the mobility of radioactive and nonradioactive substances in the environment are important tasks for the development of a future disposal in deep geological formations. Dissolved organic matter (DOM) can play an important role in the mobilization of metal ions due to complexation. In this study, we investigate the complexation behavior of humic acid (HA) as a model substance for DOM and its influence on the migration of europium as homologue for the actinide americium and uranium as the principal component of nuclear fuel. As speciation technique, capillary electrophoresis (CE) was hyphenated with inductively coupled plasma mass spectrometry (ICP‐MS). For the study, 0.5 mg·L−1 of the metals and 25 mg·L−1 of (purified Aldrich) HA and an aqueous solution sodium‐perchlorate with an ionic strength of 10 mM at pH 5 were used. CE–ICP‐MS clearly shows the different speciation of the triple positively charged europium and the double positively charged uranyl cation with HA.

Influence of metal loading and humic acid functional groups on the complexation behavior of trivalent lanthanides analyzed by CE-ICP-MS

The complexation behavior of Aldrich humic acid (AHA) and a modified humic acid (AHA-PB) with blocked phenolic hydroxyl groups for trivalent lanthanides (Ln) is compared, and their influence on the mobility of Ln(III) in an aquifer is analyzed. As speciation technique, capillary electrophoresis (CE) was hyphenated with inductively coupled plasma mass spectrometry (ICP-MS). For metal loading experiments 25 mg L(-1) of AHA and different concentrations (cLn(Eu+Gd)=100-6000 μg L(-1)) of Eu(III) and Gd(III) in 10mM NaClO4 at pH 5 were applied. By CE-ICP-MS, three Ln-fractions, assumed to be uncomplexed, weakly and strongly AHA-complexed metal can be detected. For the used Ln/AHA-ratios conservative complex stability constants log βLnAHA decrease from 6.33 (100 μg L(-1) Ln(3+)) to 4.31 (6000 μg L(-1) Ln(3+)) with growing Ln-content. In order to verify the postulated weaker and stronger humic acid binding sites for trivalent Eu and Gd, a modified AHA with blocked functional groups was used. For these experiments 500 μg L(-1) Eu and 25 mg L(-1) AHA and AHA-PB in 10mM NaClO4 at pH-values ranging from 3 to 10 have been applied. With AHA-PB, where 84% of the phenolic OH-groups and 40% of the COOH-groups were blocked, Eu complexation was significantly lower, especially at the strong binding sites. The log β-values decrease from 6.11 (pH 10) to 5.61 at pH 3 (AHA) and for AHA-PB from 6.01 (pH 7) to 3.94 at pH 3. As a potential consequence, particularly humic acids with a high amount of strong binding sites (e.g. phenolic OH- and COOH-groups) can be responsible for a higher metal mobility in the aquifer due to the formation of dissolved negatively charged metal-humate species.

Influence of geochemical parameters on the sorption and desorption behaviour of europium and gadolinium onto kaolinite

In this study we investigated the sorption and desorption behaviour of europium and gadolinium (homologues of the actinides americium and curium) onto the clay mineral kaolinite KGa-1b. In the model system metal/kaolinite, sorption isotherms and pH-edges were determined in different batch experiments. Calcium and magnesium as competing cations were used to simulate the influence of water hardness on the sorption and desorption processes. After centrifugation, the free metal ions in the supernatant solution were analysed by ICP-MS. With increasing lanthanide concentration, especially the relative desorption of these metals from kaolinite rises significantly before the cation exchange capacity (CEC) of 0.94 meq per 100 g is reached. This indicates that low lanthanide concentrations will result in a relatively lower metal migration due to sorption reactions at higher active sites in comparison with higher lanthanide concentrations. The K(d)-values and sorption isotherms of Eu(iii) and Gd(iii) were determined at pH 5.0 (+/-0.02) and analysed with Freundlich and Langmuir sorption models. The experimental data can best be fitted by Langmuir sorption isotherm. The pH-value has only a minor influence on the sorption onto kaolinite. Only at low pH-values (<pH 4), relevant changes in migration behaviour have to be expected. One important influence factor on lanthanide sorption found in our study is naturally occurring competing cations such as alkaline earth metals. With Ca and Mg concentrations close to those found in nature, lanthanide sorption decreases dramatically. Desorption experiments show that after four consecutive equilibration steps, nearly all the lanthanide has been mobilised from kaolinite to the aquifer.

Sol–gel preparation and characterization of manganese-substituted superconducting YBa2(Cu1−xMnx)4O8 compounds

In this work the effect on the structural and superconducting properties of YBa2Cu4O8 (Y-124) of replacing Cu by Mn was studied. Multimetallic oxide powders with the composition of YBa2(Cu1−xMnx)4O8 (x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05) have been prepared by a simple aqueous sol–gel method at an oxygen pressure of 1 atm starting from an aqueous mixture of the metal acetates. Homogeneous gels were achieved by complexing copper ions by tartaric acid before the gelation process. Thermal decomposition of the gels was studied by thermogravimetry. Effects of manganese substitutions on the properties of compounds were studied by X-ray powder diffraction, TG analysis, IR spectroscopy and resistivity measurements. These data indicated that the transition temperature of superconductivity drastically decreases upon Mn substitution, and that superconductivity is lost at 3% of Mn substitution level. XRD measurements showed that the structure of the Y-124 phase is also largely affected even at low manganese-substitutional levels.

Waste Disposal in Clay Formations: Influence of Humic Acid on the Migration of Heavy‐Metal Pollutants

The deep geological disposal of high-level radioactive waste is assumed to be a way of providing adequate protection for humans and the environment. For the safe isolation of radioactive waste, a low permeability and high sorption capacity of the geological barrier is important. Clay minerals play a major role in different concepts for the disposal of high-level nuclear waste in deep geological formations. Geological clay formations are considered to be a host rock for waste repository. Natural clays can contain natural organic matter, for example, humic substances. In most aquatic systems, components of natural organic matter such as humic acids can act as ligands for complexing metal ions and may affect the speciation and thus the mobility of the metal. By forming soluble complexes and colloids with numerous toxic heavy metals, including radio ACHTUNGTRENNUNGnuclides, humic acids can influence the migration behavior of these pollutants in hematite or geological clay formations, as reported, for example, for bentonite, kaolinite, silica, and montmorillonite. Kaolinite is a significant component of many soils and fracture-filling materials of crystalline hard rock. These clay formations are part of the natural barrier protecting the environment from the possible impact of radionuclide contamination. Herein we report the sorption behavior of the trivalent lanthanides europium and gadolinium (as homologues of the actinides americium and curium), under conditions close to those of nature, in a binary system consisting of these heavy metals and well-crystallized kaolinite (KGa-1b) as a model mineral, and in the ternary system metal/kaolinite/humic acid. The influence of the concentration of lanthanide ion, pH value, or concentrations of different competing metal cations (water hardness, represented by calcium and magnesium) on the sorption of the lanthanide ion onto kaolinite in the presence or absence of humic acid was investigated. After the sorption experiments, free metal ions in the supernatant solution were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Speciation of the lanthanide–humic acid complexes are reported elsewhere. The ternary system described in Figure 1 is most suitable to study metal migration under natural conditions. Research on the three binary systems can be seen as preliminary tests to understand partial aspects of the complete system. With humic acid, there is the possibility that metal humates could adsorb on the kaolinite surface through hydrophobic interactions, thus increasing retention of the lanthanide ion. Negatively charged complexes could inhibit sorption of the lanthanide and accelerate migration through clay minerals. Sorption behavior is strongly dependent on the experimental conditions as tests performed in the ternary system clearly show. The experiments that were performed here are based on the system metal/kaolinite with humic acid as an additional influencing factor. With humic acid present, the sorption of the lanthanide ions onto kaolinite is suppressed, especially at low concentrations of Eu and Gd (Figure 2). As lanthanides are complexed by humic acid and as there are not enough positively charged ions present to neutralize the complexes at pH 5, a significant amount of the complexes stays in solution. With rising lanthanide concentration, which also means a higher amount of metal complexed by humic acid, the humic acid partially loses its negative charge and is able to adsorb on the kaolinite surFigure 1. The ternary system, consisting of lanthanide ions, humic acid, and kaolinite, and important influential factors.

Chemiluminescent and Non-Chemiluminescent Ozonations of Selected Electron-Rich Alkynes in Halomethanes

Alkynes of sufficiently high nucleophilicity react with electrophilic O3 under conversion of the alkyne function to a vicinal dicarbonyl function. Contrary to earlier investigations with alkylated or arylated acetylene, products of complete C−C cleavage were not found as primary products, and, beyond that, peroxidic reaction products were absent. Trimethylsilylated alkynes reacted with O3 either by uptake of two or three O-atoms, but again without C−C cleavage or formation of peroxides. Two particularly electron-rich, symmetrically substituted alkynes revealed strong chemiluminescence during ozonation at low temperature, whereas this behavior was not observed with unsymmetrically substituted alkynes. The results are summarized in terms of a mechanistic discussion.

The refinement of α-USe2, twinning in a SrBr2-type structure

The refinement of α-USe2 is reported. It crystallizes with a SrBr2-type structure (space group P4n; a = 1070.0 pm, c = 660.0 pm). It is shown that this high-temperature form is not necessarily restricted to nonstoichiometric compositions and that other descriptions in a space group with higher symmetry and varying U to chalcogen ratios may be due to the fact that the merohedral twinning of this compound was not recognized. The size of the twin domains can be influenced by equilibrating the crystals at elevated temperatures. The small size of crystals grown at very high temperatures leads to pseudoextinctions simulating a higher symmetry.