The growth of zeolites A, X and mordenite in space

Zeolites are a class of crystalline aluminosilicate materials that form the backbone of the chemical process industry worldwide. They are used primarily as adsorbents and catalysts and support to a significant extent the positive balance of trade realized by the chemical industry in the United States (around $19 billion in 1991). The magnitude of their efforts can be appreciated when one realizes that since their introduction as 'cracking catalysts' in the early 1960's, they have saved the equivalent of 60 percent of the total oil production from Alaska's North Slope. Thus the performance of zeolite catalysts can have a profound effect on the U.S. economy. It is estimated that a 1 percent increase in yield of the gasoline fraction per barrel of oil would represent a savings of 22 million barrels of crude oil per year, representing a reduction of$400 million in the United States' balance of payments. Thus any activity that results in improvement in zeolite catalyst performance is of significant scientific and industrial interest. In addition, due to their 'stability,' uniformity, and, within limits, their 'engineerable' structures, zeolites are being tested as potential adsorbents to purify gases and liquids at the parts-per-billion levels needed in today's electronic, biomedical, and biotechnology industries and for the environment. Other exotic applications, such as host materials for quantum-confined semiconductor atomic arrays, are also being investigated. Because of the importance of this class of material, extensive efforts have been made to characterize their structures and to understand their nucleation and growth mechanisms, so as to be able to custom-make zeolites for a desired application. To date, both the nucleation mechanics and chemistry (such as what are the 'key' nutrients) are, as yet, still unknown for many, if not all, systems. The problem is compounded because there is usually a 'gel' phase present that is assumed to control the degree of supersaturation, and this gel undergoes a continuous 'polymerization' type reaction during nucleation and growth. Generally, for structure characterization and diffusion studies, which are useful in evaluating zeolites for improving yield in petroleum refining as well as for many of the proposed new applications (e.g., catalytic membranes, molecular electronics, chemical sensors) large zeolites (greater than 100 to 1000 times normal size) with minimum lattice defects are desired. Presently, the lack of understanding of zeolite nucleation and growth precludes the custom design of zeolites for these or other uses. It was hypothesized that the microgravity levels achieved in an orbiting spacecraft could help to isolate the possible effects of natural convection (which affects defect formation) and minimize sedimentation, which occurs since zeolites are twice as dense as the solution from which they are formed. This was expected to promote larger crystals by allowing growing crystals a longer residence time in a high-concentration nutrient field. Thus it was hypothesized that the microgravity environment of Earth orbit would allow the growth of large, more defect-free zeolite crystals in high yield

Long-Term Survival After Radical Prostatectomy Compared to Other Treatments in Older Men With Local or Regional Prostate Cancer

Background This study aimed to address long-term survival in a large population-based cohort of men with prostate cancer receiving radical prostatectomy compared to other treatments. Methods We studied 5,845 patients diagnosed with local/regional stage prostate cancer at age 65–74 in 1992 with comorbidity score Results Of 5,845 patients, 10-year all-cause survival rates were the highest for patients receiving radical prostatectomy (81.0%; 95% CI: 79.4–82.4%), followed by radical prostatectomy in combination with radiotherapy (67.6%; 62.0–72.5%), radiotherapy (60.5%; 58.3–62.6%), and were the lowest for watchful-waiting (50.7%; 47.5–53.8%). A similar pattern was found for 10-year prostate cancer-specific survivals by treatments. After adjusting for age, ethnicity, region, Gleason Score, comorbidity, median annual household income, hormone therapy and chemotherapy, the hazard ratio of all-cause mortality was 0.31 (95% CI: 0.25–0.37) for radical prostatectomy and 0.38 (95% CI: 0.28–0.52) for radical prostatectomy plus radiation therapy compared to those with watchful-waiting. Conclusions There was a significant long-term survival benefit in men receiving radical prostatectomy compared to those receiving watchful-waiting or radiotherapy. J. Surg. Oncol. 2008;97:583–591. © 2008 Wiley-Liss, Inc

Structural evidence for the partially oxidized dipyrromethene and dipyrromethanone forms of the cofactor of porphobilinogen deaminase: structures of the Bacillus megaterium enzyme at near-atomic resolution.

The enzyme porphobilinogen deaminase (PBGD; hydroxymethylbilane synthase; EC 2.5.1.61) catalyses an early step of the tetrapyrrole-biosynthesis pathway in which four molecules of the monopyrrole porphobilinogen are condensed to form a linear tetrapyrrole. The enzyme possesses a dipyrromethane cofactor, which is covalently linked by a thioether bridge to an invariant cysteine residue (Cys241 in the Bacillus megaterium enzyme). The cofactor is extended during the reaction by the sequential addition of the four substrate molecules, which are released as a linear tetrapyrrole product. Expression in Escherichia coli of a His-tagged form of B. megaterium PBGD has permitted the X-ray analysis of the enzyme from this species at high resolution, showing that the cofactor becomes progressively oxidized to the dipyrromethene and dipyrromethanone forms. In previously solved PBGD structures, the oxidized cofactor is in the dipyromethenone form, in which both pyrrole rings are approximately coplanar. In contrast, the oxidized cofactor in the B. megaterium enzyme appears to be in the dipyrromethanone form, in which the C atom at the bridging α-position of the outer pyrrole ring is very clearly in a tetrahedral configuration. It is suggested that the pink colour of the freshly purified protein is owing to the presence of the dipyrromethene form of the cofactor which, in the structure reported here, adopts the same conformation as the fully reduced dipyrromethane form

Low angular momentum flow model of Sgr A* activity

Sgr A* is the closest massive black hole and can be observed with the highest angular resolution. Nevertheless, our current understanding of the accretion process in this source is very poor. The inflow is almost certainly of low radiative efficiency and it is accompanied by a strong outflow and the flow is strongly variable but the details of the dynamics are unknown. Even the amount of angular momentum in the flow is an open question. Here we argue that low angular momentum scenario is better suited to explain the flow variability. We present a new hybrid model which describes such a flow and consists of an outer spherically symmetric Bondi flow and an inner axially symmetric flow described through MHD simulations. The assumed angular momentum of the matter is low, i.e. the corresponding circularization radius in the equatorial plane of the flow is just above the innermost stable circular orbit in pseudo-Newtonian potential. We compare the radiation spectrum from such a flow to the broad band observational data for Sgr A*.Comment: Proceedings of the AHAR 2008 Conference: The Universe under the Microscope; Astrophysics at High Angular Resolution, Bad Honef

The Role of Magnetic Field Dissipation in the Black Hole Candidate Sgr A*

The compact, nonthermal radio source Sgr A* at the Galactic Center appears to be coincident with a 2.6 million solar mass point-like object. Its energy source may be the release of gravitational energy as gas from the interstellar medium descends into its potential well. Simple attempts at calculating the spectrum and flux based on this picture have come close to the observations, yet have had difficulty in accounting for the low efficiency in this source. There now appear to be two reasons for this low conversion rate: (1) the plasma separates into two temperatures, with the protons attaining a significantly higher temperature than that of the radiating electrons, and (2) the magnetic field, B, is sub-equipartition, which reduces the magnetic bremsstrahlung emissivity, and therefore the overall power of Sgr A*. We investigate the latter with improvement over what has been attempted before: rather than calculating B based on a presumed model, we instead infer its distribution with radius empirically with the requirement that the resulting spectrum matches the observations. Our ansatz for B(r) is motivated in part by earlier calculations of the expected magnetic dissipation rate due to reconnection in a compressed flow. We find reasonable agreement with the observed spectrum of Sgr A* as long as its distribution consists of 3 primary components: an outer equipartition field, a roughly constant field at intermediate radii (~1000 Schwarzschild radii), and an inner dynamo (more or less within the last stable orbit for a non-rotating black hole) which increases B to about 100 Gauss. The latter component accounts for the observed sub-millimiter hump in this source.Comment: 33 pages including 2 figures; submitted to Ap

Chord distribution functions of three-dimensional random media: Approximate first-passage times of Gaussian processes

The main result of this paper is a semi-analytic approximation for the chord distribution functions of three-dimensional models of microstructure derived from Gaussian random fields. In the simplest case the chord functions are equivalent to a standard first-passage time problem, i.e., the probability density governing the time taken by a Gaussian random process to first exceed a threshold. We obtain an approximation based on the assumption that successive chords are independent. The result is a generalization of the independent interval approximation recently used to determine the exponent of persistence time decay in coarsening. The approximation is easily extended to more general models based on the intersection and union sets of models generated from the iso-surfaces of random fields. The chord distribution functions play an important role in the characterization of random composite and porous materials. Our results are compared with experimental data obtained from a three-dimensional image of a porous Fontainebleau sandstone and a two-dimensional image of a tungsten-silver composite alloy.Comment: 12 pages, 11 figures. Submitted to Phys. Rev.

Combined effects of prevention and quarantine on a breakout in SIR model

Recent breakouts of several epidemics, such as flu pandemics, are serious threats to human health. The measures of protection against these epidemics are urgent issues in epidemiological studies. Prevention and quarantine are two major approaches against disease spreads. We here investigate the combined effects of these two measures of protection using the SIR model. We use site percolation for prevention and bond percolation for quarantine applying on a lattice model. We find a strong synergistic effect of prevention and quarantine under local interactions. A slight increase in protection measures is extremely effective in the initial disease spreads. Combination of the two measures is more effective than a single protection measure. Our results suggest that the protection policy against epidemics should account for both prevention and quarantine measures simultaneously