Thermochemical analyses of the oxidative vaporization of metals and oxides by oxygen molecules and atoms

Equilibrium thermochemical analyses are employed to describe the vaporization processes of metals and metal oxides upon exposure to molecular and atomic oxygen. Specific analytic results for the chromium-, platinum-, aluminum-, and silicon-oxygen systems are presented. Maximum rates of oxidative vaporization predicted from the thermochemical considerations are compared with experimental results for chromium and platinum. The oxidative vaporization rates of chromium and platinum are considerably enhanced by oxygen atoms

Superstar Returns

We study long-term returns on residential real estate in twenty-seven “superstar” cities in fifteen countries over 150 years. We find that total returns in superstar cities are close to 100 basis points lower per year than in the rest of the country. House prices tend to grow faster in the superstars, but rent returns are substantially greater outside the big agglomerations, resulting in higher long-run total returns. The excess returns outside the superstars can be rationalized as a compensation for risk, especially for higher covariance with income growth and lower liquidity. Superstar real estate is comparatively safe

Superstar Returns

We study long-term returns on residential real estate in 27 "superstar" cities in 15 countries over 150 years. We find that total returns in superstar cities are close to 100 basis points lower per year than in the rest of the country. House prices tend to grow faster in the superstars, but rent returns are substantially greater outside the big agglomerations, resulting in higher long-run total returns. The excess returns outside the superstars can be rationalized as a compensation for risk, especially for higher co-variance with income growth and lower liquidity. Superstar real estate is comparatively safe

Combustion of solid carbon rods in zero and normal gravity

In order to investigate the mechanism of carbon combustion, spectroscopic carbon rods were resistance ignited and burned in an oxygen environment in normal and zero gravity. Direct mass spectrometric sampling was used in the normal gravity tests to obtain concentration profiles of CO2, CO, and O2 as a function of distance from the carbon surface. The experimental concentrations were compared to those predicted by a stagnant film model. Zero gravity droptower tests were conducted in order to assess the effect of convection on the normal gravity combustion process. The ratio of flame diameter to rod diameter as a function of time for oxygen pressures of 5, 10, 15, and 20 psia was obtained for three different diameter rods. It was found that this ratio was inversely proportional to both the oxygen pressure and the rod diameter

Directed percolation identified as equilibrium pre-transition towards non-equilibrium arrested gel states

The macroscopic properties of gels arise from their slow dynamics and load-bearing network structure, which are exploited by nature and in numerous industrial products. However, a link between these structural and dynamical properties has remained elusive. Here we present confocal microscopy experiments and simulations of gel-forming colloid–polymer mixtures. They reveal that gel formation is preceded by continuous and directed percolation. Both transitions lead to system-spanning networks, but only directed percolation results in extremely slow dynamics, ageing and a shrinking of the gel that resembles synaeresis. Therefore, dynamical arrest in gels is found to be linked to a structural transition, namely directed percolation, which is quantitatively associated with the mean number of bonded neighbours. Directed percolation denotes a universality class of transitions. Our study hence connects gel formation to a well-developed theoretical framework, which now can be exploited to achieve a detailed understanding of arrested gels

Reproductive isolation in the Aegean Ophrys omegaifera complex (Orchidaceae)

The orchid genus Ophrys operates a system of sexual deception by which high specificity of pollination is attained. Reproductive isolation in Ophrys mainly rests upon prezygotic isolation mechanisms. The level of genetic separateness of Ophrys taxa with different pollinators is therefore likely determined by the fidelity of pollinators. The present study employs genetic fingerprinting to investigate this in the east Aegean Ophrys omegaifera s.l. complex, also including O. dryis, a west Mediterranean species of this complex. Ophrys fleischmannii, O. basilissa, and the west Mediterranean O. dyris, are found to be well-separated genetic entities whereas O. omegaifera s.str. and the putative hybrid taxon, O. sitiaca, are found to be genetically inseparable across their entire range of co-occurrence. This suggests that specific pollinators have high enough fidelity to act as effective isolating factors in east Aegean O. omegaifera s.l. as a whole, but that the situation in the species pair of O. sitiaca and O. omegaifera is likely to be more comple

Evidence for progenitor-derivative speciation in sexually deceptive orchids

Background and Aims Sexually deceptive orchids of the genus Ophrys use mimicry of pollinator females to attract specific pollinators. Pollinator shifts may drive speciation in Ophrys, since novel pollinators may in principle act as isolating factors immediately. It is thus possible that evolution of novel species occurs rapidly and with a progenitor-derivative pattern. The aims of this study are to compare genetic structure and diversity among widespread and geographically restricted Ophrys taxa, to test whether genetic structure is associated with specific pollinators, and to investigate whether any widespread species may have acted as a progenitor for the evolution of more restricted taxa. Methods Genetic differentiation and diversity were investigated in O. leucadica and O. cinereophila, the two taxa of the Ophrys fusca sensu lato complex widespread in the Aegean, and three geographically restricted taxa from Rhodes, O. attaviria, O. parvula and O. persephonae, all differing in their specific pollinators. This was done using amplified fragment length polymorphism (AFLP) DNA fingerprinting, and sequencing of the low-copy nuclear gene LEAFY (LFY). Key Results All taxa were found to be separate genetic entities, with O. leucadica forming two geographic groups from the west and east of the Aegean. Genetic structure was significantly shaped by pollinators and geography, and comparison of sequence and AFLP data revealed ancestral polymorphisms shared among several taxa. Among the sampled taxa, O. leucadica harbours the greatest genetic differentiation and geographic structure, and the highest genetic diversity. Part of the genome of O. parvula, endemic to Rhodes, may be derived from O. leucadica. Conclusions Pollinators probably influence the genetic structure of the investigated Ophrys species. The genetic pattern identified is consistent with O. leucadica being the oldest of the sampled taxa, making O. leucadica a candidate progenitor species from which more restricted taxa such as O. parvula may have evolve