Gerhard Heide

Schwertmannite formation adjacent to bacterial cells in a mine water treatment plant and in pure cultures of Ferrovum myxofaciens.

Schwertmannite has previously been found in iron- and sulfate-rich mine waters at pH 2.8-4.5. In the present study, schwertmannite (Fe(8)O(8)(OH)(6)SO(4)) was shown to be the major mineral in a mine water treatment plant at pH 3, in which ferrous iron is mainly oxidized by bacteria belonging to the species Ferrovum myxofaciens. Strain EHS6, which is closely related to the type strain of Fv. myxofaciens, was isolated from the pilot plant and characterized as an acidophilic, iron-oxidizing bacterium. In contrast to the pilot plant, the mineral phase formed by a pure culture of Fv. myxofaciens EHS6 was a mixture of schwertmannite and jarosite (KFe(3)(SO(4))(2)(OH)(6)). In contrast to other reports of neutrophilic, iron-oxidizing bacteria, acidophilic microorganisms in the pilot plant and cultures of strain EHS6 did not show encrustation of the cell surface or deposition of minerals inside the cell, though a few cells appeared to be in contact with jarosite crystals. It was concluded that no direct biomineralization occurred in the pilot plant or in laboratory cultures. The lack of encrustation of bacterial cells in the pilot plant is considered advantageous since the cells are still able to get in contact with ferrous iron and the iron oxidation process in the mine water treatment plant can proceed.

Formation and properties of rocksalt-type AlN and implications for high pressure phase relations in the system Si–Al–O–N

Pressure-dependent thermodynamic properties of the ambient and high pressure phases of aluminum nitride (w-AlN and rs-AlN) were calculated from first principles in order to determine their phase boundary in the p− T phase diagram. These predictions were checked by static HP/HT experiments, using a multianvil press and an Al/N/H precursor with low decomposition temperature as educt. The experimental data show that at temperatures between 1000 and 2000 K, the boundary line between the two phases is situated between 11 and 12 GPa, which is ∼1.3 GPa lower than the theoretical result and generally lower than previously assumed. The hardness of rs-AlN – measured for the first time – is ∼30 GPa (Knoop indenter at loads of 25–50 g), twice as hard as w-AlN. Shock wave recovery experiments on nano w-AlN allowed testing of the chemical and thermal stability of rs-AlN, and determination of its infrared absorption and 27Al NMR data. The shock wave technique will eventually enable the synthesis of larger amounts of rs-AlN, making it available for technological use. Finally, implications on the high pressure stability of phases in the Si–Al–O–N system are discussed in the light of thermoelastic properties of AlN.

Shock wave synthesis of aluminium nitride with rocksalt structure

The high pressure phase of aluminium nitride with rocksalt structure (rs) is a ceramic with high potential and a challenging material to investigate. The rs-AlN was synthesised and recovered by shock wave experiments using the flyer-plate method with multiple reflections at peak pressures between 15 and 43 GPa. Successful syntheses were carried out using AlN nanopowder with ambient pressure wurtzite structure (w-AlN) as starting material. The high pressure modification could, however, not be obtained when starting from submicron w-AlN. The recovery of rs-AlN is sensitive to the synthesis conditions as these influence the reconversion of rs-AlN to w-AlN.

Silicophosphates containing SiO6 octahedra – anhydrous synthesis under ambient conditions

Reactions of anhydrous H3PO4 with different tetraalkoxysilanes (Si(OR)4, R = Me, Et, iPr, nBu) in a 1:1 molar ratio in diethylether at room temperature and normal pressure generate silicophosphates (SiPOs). Solid state 29Si NMR investigations clearly indicate that the precipitates contain large amounts of SiO6-species. The substructures of the products are strongly influenced by the alkyl substituents R of the tetraalkoxysilanes, as further supported by 31P MAS, 13C CP/MAS NMR and FTIR spectroscopy. All solid products are amorphous containing P:Si-ratios of 1.2–1.7, which was proved by powder XRD and elemental analysis. Since small amounts of water may be formed in the H3PO4–Si(OR)4-systems upon condensation and esterification reactions (which results in reaction mixtures being not completely anhydrous), the reactivity of P4O10 towards tetraethoxysilane was investigated at room temperature confirming the formation of SiO6-units as well.

Lithium carnallite, LiCl.MgCl2.7H2O.

The title compound, lithium magnesium chloride heptahydrate, LiCl.MgCl(2).7H(2)O, was analyzed in 1988 by powder X-ray diffraction [Emons, Brand, Pohl & Köhnke (1988). Z. Anorg. Allg. Chem. 563, 180-184] and a monoclinic crystal lattice was determined. In the present work, the structure was solved from single-crystal diffraction data. A trigonal structure was found, exhibiting a network structure of Mg(H(2)O)(6) octahedra and Li(H(2)O)Cl(3) tetrahedra connected by H...Cl hydrogen bonds. The [Li(H(2)O)](+) unit is coordinated by distorted edge-connected Cl(-) octahedra.

REECa4O(BO3)3 (REECOB): New Material for High-Temperature piezoelectric applications

Especially in terms of the modern environmental consciousness with demand for more efficient, cleaner and more ecological machinery, processes has to be improved. This is mainly essential for combustion engines, coal fired electrical plants as well as gas heating installations etc. Processes have to be measured and controlled in situ in order to increase efficiency by manipulating parameters. Doing so, sensors which can resist and work under high temperature/high pressure conditions are strongly required. Unfortunately, most piezoelectric sensor materials known today (like quartz, Bi4Ti3O12-and PZT-ceramics) can not proceed at elevated temperatures above 600°C. Others that can, are either expensive and energy-consuming in production (GaPO4) or only available in sufficient size as naturally occurring minerals (tourmaline) with fluctuating properties. In the last few years, the new material REECOB (REECa4O(BO3)3 with REE = rare earth elements: Gd, Y, La, Sm, Nd) emerged to be a promising candidate for high temperature applications, displaying constant piezoelectric properties up to 1,200°C. This material has made a steep career in optical applications (laser host material, nonlinear optics) since the mid-1990s but good properties for sensing applications are only known for a few years (Shimizu et al. 2004; Markiewicz et al. 2006). Investigations on ultra high temperature properties are relatively new (Zhang et al. 2008a, b, c). Unfortunately, thermomechanical data of the material are very rare and partially contradictional.

Boracite Mg3[B7O13Cl] from the Zechstein salt deposits

Abstract Among the borates in the Middle European Zechstein Salt Succession boracite Mg3[B7O13Cl] is the most common mineral in quantity and local distribution. An exceptional enrichment is observed in Stassfurt Serie (Z2) in the Stassfurth seam K2H. Boracite is to be found in two varieties: individual crystals in cubic, tetrahedral or dodecahedral habit on the one hand and fibrous crystals so-called “stassfurtite” on the other hand. The formation conditions such widely spread borates in the salt succession are ambiguous in two respects. First of all the synthetic formation of boracites is to be made by hydrothermal or melt conditions. Both processes can be suspended for the salt succession. Furthermore the cubic modification is stable above 265 ºC for the Mg-boracite. The cubic, tetrahedral or dodecahedral habit could be used as a geothermometer, but such conditions can be exclude by the paragenetic minerals, esp. carnallite (MgKCl3 ⋅ 6 H2O). The chemical composition of orthorhombic, pseudo-cubic boracite depends on the location. Pure Mg-boracite in hexahedral habit and in fibrous habit, so-called “stassfurtite”, occurs in the North Harz region, whereas the Fe-, Mn-, Mg-boracite appears in the South Harz region. Until now the source of boron, the time of formation of crystals, but also the reasons for the differences in habit of the single hexahedral crystals are still unclear. The formation during a diagenetic/metamorphic process is evident. However, the preferred formation in Stassfurt seam could be an indication for the boron enrichment in an early diagenetic process. Furthermore permit the determination of the thermal stability and the volatile content of crystals conclusions to the chemical composition of the fluid. The observed variation suggests that the condition of crystal growth as well as the chemical composition of fluid repeatedly changed over the time. Randomly occuring xenomorpheous anhydrite and magnesite inclusions within single boracite crystals have been interpreted as an indication to factors of chemical milieu during the formation of crystals. The reversible phase transition temperature of the boracite is a linearly function of the iron and manganese content and varies from 265 ºC for Mg-boracite to 330 ºC for Fe(Mn)-boracite. The thermal decomposition of boracite is determined by two processes. The decomposition started with a boronchlorine release (BOCl?), having a maximum rate at 1050 ºC. Additionally to this release one observes a simultaneous emission of H2O, HCl, HF, CO2, N2, SO2, H2, and hydro carbons. The results give evidence for the aged approach of a secondary formation of boracite within the complete Stassfurt seam, possibly in connection with the formation of salt diapirs in the Jura and Cretaceous period. The wider environmental distribution of borates is an indication of chemical transport processes within the salt succession. This should be a more important issue in the discussion about the utilisation of salt diapirs for the storage of nuclear waste.

High temperature piezoelectric single crystals: Sr 3 NbGa 3 Si 2 O 14 , Sr 3 TaGa 3 Si 2 O 14 and GdCa 4 O(BO 3 ) 3

We have successfully grown high quality piezoelectric single crystals of Sr₃NbGa₃Si₂O₁₄ (SNGS), Sr₃TaGa₃Si₂O₁₄ (STGS) and GdCa₄O(BO₃)₃ (GdCOB) by the Czochralski technique. Dielectric, elastic and piezoelectric constants of SNGS and STGS were measured in a temperature range from 25°C to 400°C. Some parameters of SNGS, STGS and GdCOB crystals important for acoustic applications have been investigated also at temperatures up to 940°C. The strong piezoelectric activity of all investigated materials is kept at least up to this high temperature.

“Shock Wave” Synthesis of Oxygen-Bearing Spinel-Type Silicon Nitride γ-Si3(O,N)4 in the Pressure Range from 30 to 72 GPa with High Purity

Silicon nitride, Si3N4 is a hard, refractory ceramic substance first synthesized more than 100 years ago. In the 1990s, its so-called a-modification was discovered to occur also as an exotic extraterrestrial mineral in certain chondrites and termed Nierite (Lee et al. 1995). The ambient-pressure modifications a-Si3N4 and b-Si3N4 have hexagonal (trigonal) lattice symmetry. b-Si3N4 crystallizes in the space group P63/m (Goodmann and O’Keefe 1980), a-Si3N4 in the space group P31c (Zhou et al. 1995). Both types consist of SiN4 tetrahedra and N3Si groups in a close-to-planar arrangement. In 1999 a high pressure modification of Si3N4 with spinel-type structure (space group Fd-3 m) and sixfold coodination of Si to nitrogen was discovered (ZERR et al. 1999). The g-modification can be quenched to ambient conditions and shows a remarkable metastability and a high bulk modulus (Kuwabara et al. 2008; Jiang et al. 2001; Sekine et al. 2001; Sekine 2002). The excellent thermal and mechanical properties qualify g-Si3N4 as an interesting candidate for high performance ceramics.

Complex Study of E-Glass Corrosion

Both the static and dynamic corrosion tests of E-glass were used for different conditions - temperature, glass surface to solution volume ratio (S/V), solution flow rate (F) and F/S ratio. Results obtained for glass fibres were compared with the ones for glass grains and planar samples. Evaluation of experimental results by kinetic model shows that the change of glass surface should be taken into account in the case of fibres corrosion. The total incongruent process of dissolution could be explained as congruent dissolution of glass accompanied by back precipitation of SiO2 or silicates. In most cases, more then 90% of SiO2 precipitates back. The second possible explanation, i.e. SiO2 network dissolution accompanied by selective leaching of Ca, B and Al, is not very probable. The glass sample shape can influence the estimation of dissolution rate. Up to now, with existing kinetic models, the dissolution rates evaluated from experiments with different shapes of glass (fibres, grains, planar) cannot be used as materials properties.