Dirk Walter

The mechanism of the thermal transformation from goethite to hematite

Synthetic pigments of goethite (BayferroxR) of different particle size were investigated by DTA, IR, DSC, TG and X-ray diffraction measurements. It follows that a so-called ‘hydrohematite’ described in the literature does not exist as a discrete intermediate during the dehydration course from goethite to hematite. Instead we observed a dependence of the dehydration mechanism on the particle size. Transformation enthalpies and activation energies for the dehydration process will be given. A plausible dehydration mechanism, which is compatible with our DTA/DSC results, is deduced from TEM investigations.

Thermal analysis of lanthanum hydroxide

Lanthanum oxide (La2O3) is of great interest as catalyst material. When La2O3 particles are prepared from lanthanum hydroxide (La(OH)3) by thermal processes under air, various oxycarbonate phases are formed which are resistant to thermal hydroxylation. This phenomenon has not yet been extensively investigated, even though oxycarbonate phases at the particle surfaces cause a change in lanthanum oxide’s catalytic activity. The carbonate phases formed cannot be detected by means of XRD or REM-EDX investigations due to their detection limits. Thermal analysis, particularly TG-FT-IR, allows not only for the detection of the carbonate phases in La(OH)3, but also for the tracking of the entire dehydration process from La(OH)3 via LaOOH to La2O3 as well as the correct interpretation of mass changes during the thermal transformations. Pursuant to the investigations here carried out, it was determined that carbonate-free lanthanum hydroxide compounds can only be prepared and stored in a CO2-free protective gas atmosphere (e.g., argon).

Lithium ion conductivity in Li2S–P2S5 glasses – building units and local structure evolution during the crystallization of superionic conductors Li3PS4, Li7P3S11 and Li4P2S7

Motivated by the high lithium ion conductivities of lithium thiophosphate glasses, a detailed study is performed on the local chemical nature of the thiophosphate building units within these materials. Using Raman and 31P MAS NMR (Magic Angle Spinning – Nuclear Magnetic Resonance) spectroscopy, the continuous change from dominant P2S74− (di-tetrahedral) anions to PS43− (mono-tetrahedral) anions with increasing Li2S fraction in the (Li2S)x(P2S5)(100−x) glasses is observed. In addition, synchrotron pair distribution function analysis (PDF) of synchrotron X-ray total scattering data is employed to monitor in situ crystallization and phase evolution in this class of materials. Depending on the composition, different crystalline phases evolve, which possess different decomposition temperatures into less conducting phases. The results highlight the critical influence of the local anionic building units on the cation mobility and thermal stability, with PS43− tetrahedra forming the most thermally robust glass ceramics with the highest ionic conductivity.

Standardized Investigation of Percutaneous Absorption of Bitumen Emission in Humans

The potential for percutaneous absorption of aerosols and vapors of bitumen was investigated in human beings using an experimental chamber. Commercial bitumen B 65 was used to generate the bitumen emissions. The system used for air sampling and analysis permitted the separate evaluation of aerosols and vapors as well as the polycyclic aromatic hydrocarbon (PAH) concentrations and contents, respectively. n = 10 male nonsmokers were exposed for 8 h with and two of them in addition without a powered air purifying respirator (PAPR). Quantification of the percutaneous and combined inhaled/percutaneous absorption was related to the biological monitoring of the PAH metabolites pyrene, chrysene, and phenanthrene in urine. The bitumen emissions in chamber were determined to be 20.4 mg/m 3 with a vapor content of about 88%. Two subjects stressed by inhalation and percutaneous contact with bitumen showed PAH absorption values of approximately 57% for pyrene and chrysene and about 50% for phenanthrene.

Various carbon dust particles

Nano-sized carbon dusts are suspected of having negative effects on human health. An exact characterization of such particles is necessary to understand possible toxic effects, i.e. in the lung. Observed by transmission electron microscopy (TEM), the carbon dusts are a composite of very small primary particles and larger agglomerates of these. A differentiation of the primary particles and agglomerates according to source is not possible by TEM, however, thermogravimetry investigations in synthetic air atmosphere are helpful. Standardized carbon black and graphite show a single-step oxidation behaviour, whereas ethene soot and diesel soot, for example, show more complex-reaction mechanisms. The results of ethene soot exemplarily demonstrate the oxidation mechanism. In addition to the oxidation reaction to carbon dioxide, a sintering process takes place. To confirm the oxidation mechanism, thermal behaviour of ethene soot has been simulated by kinetic modulation using a three-step reaction mechanism of n-th order. The reaction order indicates a complex mechanism for the first-reaction step. For the second and third-reaction step, a phase boundary mechanism could be suggested.

Thermal stability of the β″ phase in the ZrO2-Zr3N4 system

Annealing of ZrO2 in a nitrogen atmosphere leads to a nitrogen containing phase called Β″ phase having a trigonally distorted fluorite structure with ordered anion vacancies. DTA/TG investigations indicated that the Β″ phase decomposes intom-ZrO2 above 500‡C releasing nitrogen during the reaction with oxygen. Prevention of oxygen uptake by use of closed tantalum tubes instead of open platinum crucibles enabled the detection of a new reversible phase transition at ∼965‡C. The thermoanalytical results have been confirmed by high temperature-X-ray investigation.

Thermogravimetric and X-ray diffraction investigation on carbonated lanthanum oxide and lanthanum hydroxide formed in humid CO 2 atmosphere

Lanthanum oxide compounds are of great interest because of their wide range of applications (e.g., heterogeneous catalysis). It is well known that lanthanum oxide compounds, like lanthanum oxide, have to be handled carefully in a humid atmosphere due to their hydration affinity. Furthermore, lanthanum hydroxide has a high CO2 affinity and partially forms basic carbonates. The amount of carbonate impurities that are formed in air remains nearly constant over a long time. To investigate the carbonation process in more detail, lanthanum hydroxide and lanthanum oxide were stored in a controlled humid CO2 atmosphere. The results of thermal analysis (TA) and X-ray diffraction (XRD) prove a transformation of lanthanum hydroxide as well as lanthanum oxide to lanthanum carbonate via lanthanum hydroxide carbonate.

In vitro toxicity studies of aluminum compounds

A standardized in vitro system of human lung cells (A459) in RPMI medium is beneficial for investigations concerning the toxicity of inhalable biopersistent fibrous or granular dusts such as asbestos or titanium dioxide. However, the use of non-biopersistent (soluble) dusts such as aluminum sulfate or aluminum chloride revealed an undesirable precipitate in the cell culture system. In contrast, under the same condition, the soluble dust aluminum citrate did not form any precipitate. The precipitate could be identified as aluminum phosphate by the use of electron microscopy, TG and DSC. Further examinations showed that the composition of the precipitate depends on the admitted quantity of aluminum sulfate or aluminum chloride, respectively. For example, at low dosages, besides aluminum phosphate calcium phosphate can also be detected. Furthermore, the composition of the precipitate (especially the content of crystal water) is modified by aging processes. Phosphate as well as calcium ions are ingredients of the RPMI medium (nutrient solution) and hamper the investigation of soluble aluminum-containing dusts in standardized A459 lung cell system.