In this study we report on mechanical properties of molded, single component Al 2 O 3 , Ga 2 O 3 , Fe 2 O 3 , and ZrO 2 as well as mixed aerogels, made from yttrium stabilized zirconia, yttrium aluminum garnet, and zinc aluminum spinel. Initially all aerogels were produced equally in molded bodies by a facile epoxy method and were annealed afterward at 300 °C. Then we performed uniaxial pressure tests on cylindrical aerogel monoliths to gain Young's modulus which depends on composition, density, and posttreatment. Already pure aerogels like ZrO 2 show wellpromising Young's modulus of 10.7 MPa, whereas most popular SiO 2 materials display a modulus between 2 and 3 MPa at comparable densities. Moreover we focused on Al 2 O 3 aerogels which exhibit high stability and interesting densification behavior depending on the annealing temperature. On the basis of this observation, we combined the toughness of the Al 2 O 3 scaffold with the extraordinary hardness of ZrO 2 , by adding up to 20 atom % Zr, to increase the specific Young's modulus. For the mixed material with a Zr content of 20 atom %, we reach a record value for compressible aerogels of 125 MPa mL g −1 .

We investigate the influence of microstructure (dislocations, and grain and subgrain boundaries) on the sintering process in compacts of electrolytic and spherical copper powders by means of positron lifetime spectroscopy. We compare the lifetime data obtained to the kinetics of the annealing out of vacancy clusters after low-temperature electron irradiation, and the kinetics of recovery and recrystallization after plastic deformation. The change of powder-particle and grain sizes with temperature is determined in a complementary study by metallography and x-ray line-profile analysis. At the intensive-shrinkage stage, the effective powder-particle size in electrolytic copper powder is and the grain size is . Due to the dendritic morphology of the powder, the effective powder-particle size is much smaller than that determined by particle-size analysis . Because of the small powder-particle and grain sizes, a measurable fraction of positrons annihilate at grain boundaries and in surface states, i.e. at inner pore surfaces. At higher temperatures , grain boundaries are, besides a small surface component for compacts of electrolytic powder, the only detectable lattice defects in both powders. We find that the observed shrinkage rates can be explained - at least qualitatively - by Coble creep, while Nabarro-Herring and Kosevic (dislocation) creep seem to play only a minor role in the systems investigated.

Compacts of tungsten powder with five different powder-particle sizes (from to ) are subjected to pressureless sintering. We investigate the change in microstructure during the sintering process by positron lifetime spectroscopy. So as to be able to distinguish between defects having the same positron lifetime, we investigate their kinetics when the sample is annealed. In particular, we consider the annealing out of vacancy clusters after low-temperature electron irradiation, as well as recovery and recrystallization of a tungsten sheet, in as-manufactured form. Making measurements on uncompacted powder, we find an increasing fraction of positrons annihilating in surface states with decreasing powder-particle size. The powder-particle and grain sizes (influencing the x-ray domain size) are monitored additionally by means of metallography and x-ray diffraction. We find that all of the methods give results in agreement with each other. The small grain sizes at lower temperature, about one fifth of the powder-particle size, cause positrons to annihilate at grain boundaries, leading to vacancy-cluster-like signals. At the intensive-shrinkage stage, there are certainly contributions from different shrinkage mechanisms. The observed shrinkage rates can be explained by Coble creep. It is possible that dislocations also play a role as vacancy sources and sinks, since the intensive-shrinkage stage occurs in a temperature region wherein recrystallization takes place.

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Abstract. In contrast to populations of most dioecious Silene species (which usually are female‐biased), populations of Silene otites have been frequently reported to be male‐biased. We describe sex ratio variation in 34 natural S. otites populations in Central Germany in relation to vegetation cover, population size and fungal infection. The overall sex ratio was unbiased in 1994 and only slightly male‐biased in 1995. Sex ratio varied among the populations from 26.6 % to 72.6 % females. The sex ratio of small populations varied strongly due to stochastic processes. Furthermore, we found that populations in habitats with high vegetation cover contained a higher percentage of females. Hermaphroditic plants, theoretically, could increase male bias as they only produce male or hermaphroditic offspring. Their frequency in the populations, however, was far too low to affect sex ratio. In 1994 12.1 % and in 1995 17.0 % of the plants were infected by the smut fungus Ustilago major. Disease incidence in the population was not related to sex ratio, suggesting equal susceptibility of males and females. The sex ratio of partially infected plants did not deviate from the population sex ratio, both under field conditions and in a greenhouse laboratory experiment. The results suggest that the frequently reported male bias in Silene otites populations is not a general pattern, but is mainly caused by environmental conditions.