Abstract. Cell cultures of Lycopersicon peruvianum L.stressed with CdSO 4 (10-3M) show typical changes in the ultrastructure, starting with the plasmalemma and later on extending to the endoplasmic reticulum and the mitochondrial envelope. Part of the membrane material is extruded, with the formation of osmiophilic droplets which increase in size and number during the stress period. After 4 h, about 20% of the cells are dead. A short heat stress preceeding the heavy-metal stress induces a tolerance effect by preventing the membrane damage. The cells show a normal ultrastructure with one exception: cytoplasmic heat-shock granules are formed. This protective effect can be abolished by cycloheximide. Cadmium uptake is not markedly influenced by the heat stress. Cadmium is found together with sulfur in small deposits in the vacuoles of stressed cells. The precipitates contain an excess of sulfur, evidently due to the stress-induced formation of phytochelatins. The role in heavy-metal tolerance of heat-shock proteins in the plasmalemma (HSP70) and in cytoplasmic heat-stress granules (HSP17, HSP70) is discussed.
The synthesis of trimethylsilyl-substituted poly(titaniumcarbodiimide) as a novel precursor for titanium carbonitride based ceramic materials is described. The precursor and the subsequent processing steps (cross-linking and pyrolysis) are characterized by IR and Raman spectroscopy, thermal gravimetric analysis and simultaneous mass spectroscopy, electron microscopy and powder X-ray diffraction measurements. The novel polymer is formed by the reaction of TiCl 4 or Ti(NEt 2 ) 4 with bis(trimethylsilyl)carbodiimide. Subsequent pyrolysis at 1000°C in argon results in the formation of a ceramic composite material consisting of nanocrystalline TiCN and amorphous SiCN as constituting phases. Using Ti(NEt 2 ) 4 as a starting reagent instead of TiCl 4 , chlorine contamination of the ceramic material can be avoided. The different molecular vibration modes of the metal-nitrogen, metal-carbon and nitrogencarbon bonds in poly(titaniumcarbodiimides) with trimethylsilyl substituents were calculated using quantum mechanical methods, providing a comprehensive understanding of the measured spectra.
Electron energy loss spectra of different silicon compounds, in particular, silicon carbide, dioxide, and oxycarbide, are analysed experimentally as well as theoretically. For this purpose features measured of both the low‐loss region and the SiL23 inner‐shell excitation are compared with results of corresponding calculations. In the theoretical treatment the features of the low‐loss region are interpreted as superpositions of plasmon and interband excitations. On the basis of the energy eigenvalues Em and the occupation densities of the occupied valence states attained by MNDO (modified neglect of diatomic overlap) calculations the scattering function Im {‐1/ϵ(E)} in the low‐loss region is simulated. The low‐loss features attained both experimentally and theoretically agree quite well. The fine structure of the SiL23 ionization edge is interpreted using densities of unoccupied states obtained by MNDO calculations, too. The density of states calculated depends on the chemical bonding as experimentally proved. The calculations enable predictions concerning the electronic structure of the silicon compounds under investigation, whereas the limits of LCAO (linear combination of atomic orbitals) methods have to be considered if inelastic electron scattering processes are described.
This paper presents examples of the chemical bond characterization taken from microanalytical investigations of fibre-reinforced borosilicate glasses. Chemical bonding is examined across the fibre/matrix regions at nanometre resolution by analysing energy loss near edge structures (ELNES), particularly the ELNES. In this context results are presented mainly concerning the chemical bonding of silicon with carbon and oxygen. To identify the bond state of silicon in the interfacial zone the ELNES measured of standard specimens (silicon carbide, dioxide and oxycarbide) was used as a fingerprint. Along a line crossing the interlayer the bonding state of silicon is determined by recording series of EEL spectra. In addition, experimental ELNES results are compared with local densities of unoccupied states calculated by molecular orbital (MO) methods, where the calculations reflect the dependence of fine-structure features on the oxidation state.
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