µm- and nm-Sized Catalytic Structures in Heat Sources for Thermoelectric Generators.

Micro- and nano-structural organization and its influence on the efficiency of catalysts used in the heat sources for permeable thermoelectric generators were investigated. Two types of catalyst were studied­elemental platinum on aluminum oxide granulate Pt/Al2O3 and mixed transition-metal catalyst on fibrous silicon dioxide Co­Cr­Pd­Sr/SiO2. The distribution of active components in catalytic structures which contribute to the maximum combustion completeness of organic fuel in heat sources was investigated. Practically full conversion of hydrocarbons was achieved already with 1 mass.% of platinum in the Pt/Al2O3 catalyst with sub-µm- and nm-sized particles placed at the input of the gas-air mixture into the channel of the permeable thermoelement. The propane-butane conversion rate of 97% for the catalyst Co­Cr­Pd/SiO2 with was further enhanced by addition of 0.5 mass.% of Sr. The catalytic centers are formed by CoCr2O4 nanocrystals (10 to 40 nm in size) with Pd promotor in form of single crystals on the fibrous SiO2 matrix.

Electron holography of biological samples.

In this paper, we summarise the development of off-axis electron holography on biological samples starting in 1986 with the first results on ferritin from the group of Tonomura. In the middle of the 1990s strong interest was evoked, but then stagnation took place because the results obtained at that stage did not reach the contrast and the resolution achieved by conventional electron microscopy. To date, there exist only a few ( approximately 12) publications on electron holography of biological objects, thus this topic is quite small and concise. The reason for this could be that holography is mostly established in materials science by physicists. Therefore, applications for off-axis holography were powerfully pushed forward in the area of imaging, e.g. electric or magnetic micro- and nanofields. Unstained biological systems investigated by means of off-axis electron holography up to now are ferritin, tobacco mosaic virus, a bacterial flagellum, T5 bacteriophage virus, hexagonal packed intermediate layer of bacteria and the Semliki Forest virus. New results of the authors on collagen fibres and surface layer of bacteria, the so-called S-layer 2D crystal lattice are presented in this review. For the sake of completeness, we will shortly discuss in-line holography of biological samples and off-axis holography of materials related to biological systems, such as biomaterial composites or magnetotactic bacteria.

Crystal structure and transport properties of Ba8Ge43square3.

The single phase clathrate-I Ba(8)Ge(43)square(3) (space group Ia3d (no. 230), a = 21.307(1) A) was synthesized by quenching the melt between cold steel plates. Specimens for physical property measurements were characterized by microstructure analysis and X-ray diffraction on polycrystalline samples as well as single crystals. Transport properties including thermopower, electrical resistivity, thermal conductivity and specific heat were investigated in a temperature range of 2-673 K. The electrical resistivity exhibits a metal-like temperature dependence below 300 K turning into a semiconductor-like behaviour above 300 K. The analysis of the specific heat at low temperature indicates a finite density of states at the Fermi level, thus corroborating the metallic character below 300 K. The temperature dependence of the specific heat was modelled assuming Einstein-like localized vibrations of Ba atoms inside the cages of the Ge framework. A conventional crystal-like behaviour of the thermal conductivity with a low lattice contribution (kappa(l)(300 K) = 2.7 W m(-1) K(-1)) has been evidenced.

Superconductivity in the filled cage compounds Ba6Ge25 and Ba4Na2Ge25.

The clathrate compound Ba 6Ge25 and its relatives consist of a rigid germanium skeleton, into which barium or other metal atoms are coordinated. These guest atoms can "rattle" freely at high temperatures, but in Ba 6Ge25 some of them lock randomly into split positions below T(S) approximately 200 K. The resulting bad metal undergoes a BCS-like superconducting transition at T(c) approximately 0.24 K. T(c) increases more than 16-fold, as T(S) is suppressed by hydrostatic pressure p, but changes only slightly with p from T(c) approximately 0.84 K in the undistorted sister compound Ba 4Na2Ge25. The large enhancement of T(c) in Ba 6Ge25 may be attributed mainly to the pressure tuning of strong disorder caused by the random displacement of Ba atoms at T(S).