Glafka 2004: Spacetime Topology from the Tomographic Histories Approach

The tomographic histories approach is presented. As an inverse problem, we recover in an operational way the effective topology of the extended configuration space of a system. This means that from a series of experiments we get a set of points corresponding to events. The difference between effective and actual topology is drawn. We deduce the topology of the extended configuration space of a non-relativistic system, using certain concepts from the consistent histories approach to Quantum Mechanics, such as the notion of a record. A few remarks about the case of a relativistic system, preparing the ground for a forthcoming paper sequel to this, are made in the end.     

Phonon lifetimes from first-principles self-consistent lattice dynamics

Phonon lifetime calculations from first principles usually rely on time-consuming molecular dynamics calculations, or density functional perturbation theory where the zero-temperature crystal structure is assumed to be dynamically stable. Here is presented a new and effective method for calculating phonon lifetimes from first principles. This method is not limited to crystallographic phases stable at 0 K and provides a scheme more effective than most corresponding molecular dynamics calculations. The method is based on the recently developed self-consistent ab initio lattice dynamical method and is here tested by calculating the bcc phase phonon lifetimes of Li, Na, Ti and Zr as representative examples.     

Sand consolidation with calcium phosphate-polyelectrolyte composites

A new method for the consolidation of loose sand formations has been developed. The method involves in situ precipitation of a composite calcium phosphate-polyelectrolyte salt that binds together with loose sand grains, thus resulting to their consolidation. Three different polyelectrolytes (PE) were tested, i.e., polyacrylic acid (PAA), polyallylamine hydrochloride (PAH), and polyethylenimine (PEI). The effect of PE tested on the thermodynamics and the kinetics of precipitation of calcium phosphate salts was investigated. Three types of experiments were done. Investigation of the adsorption of PE on either hydroxyapatite (Ca(5)(PO(4))(3)OH, HAP) crystals or on sand grains. Measurement of the kinetics of heterogeneous nucleation of HAP on the solid substrates and the mechanical properties of the obtained crystals in batch experiments of low and high supersaturation solutions, respectively. Evaluation of the consolidation in sand packs in order to investigate the effectiveness of the method. The crystallization rates, R(p), on HAP crystals in the presence of the PE tested were found in the order R(p)(PAA)>R(p)(PEI)>R(p)(PAH), while nucleation and crystal growth on silicate sand took place only in the absence of adsorbed PE. PAH favored strongly the consolidation process, whereas PEI and PAA resulted in the formation of poorly consolidated grain agglomerates.     

The effect of high tempered firing cycle on the bioactive behavior of sol–gel derived dental porcelain modified by bioactive glass

The purpose of the present study was to investigate the influence of end-temperature over the structural properties, chemical composition and bioactivity of dental porcelain modified by bioactive glass. In particular, sol–gel derived specimens of bioactive glass dental porcelain underwent firing at two increased end-temperatures. All specimens were characterized using Scanning Electron Microscopy, Fourier Transform Infrared and Raman Spectroscopy, X-ray diffraction and N₂-porosimetry. In vitro bioactivity test was performed too. SEM analysis of both specimens revealed smooth morphology of particles, which were sintered together. Spherical and closed porei were evident. N₂- adsorption isotherms of specimens represented non nano-/meso-porous materials. FTIR and Raman spectroscopy revealed the predominance of b-wollastonite as well as the appearance of a-cristobalite. XRD confirmed the results. In vitro tests evidenced the bioactivity of the specimens regardless of temperature. However, the increased temperature caused delayed apatite precipitation. In conclusion, increased temperature favored the sintering process initiation, along with the surface crystallization, which in turn delayed bioactivity.     

Assessment of T1, T1ρ, and T2 values of the ulnocarpal disc in healthy subjects at 3 tesla

Objective

The purpose of this study was to implement clinically feasible imaging techniques for determination of T₁, T_1ρ, and T₂ values of the ulnocarpal disc and to assess those values in a cohort of asymptomatic subjects at 3 tesla. Resulting values were compared between different age groups, since former histological findings of the ulnocarpal disc indicated frequent early degenerative changes of this tissue starting in the third decade of life, even in asymptomatic subjects.

Materials and methods

Twenty-seven healthy subjects were included in this study. T₁ measurements were performed using 3D spoiled gradient-echo (GRE) sequence with variable flip angle. A series of T_1ρ and T₂-weighted images was acquired by a 3D GRE sequence after suitable magnetization preparation. T_1,T_1ρ, and T₂ maps of the ulnocarpal disc were calculated pixel-wise. Representative mean values from extended regions were analysed.

Results

Mean T₁ values of the ulnocarpal disc ranged from 722 ms in a 39 year-old subject to 1264 ms in a 65 year-old subject, T_1ρ ranged from 9.2 ms (26 year-old subject) to 25.9 ms (65 year-old subject). Calculated T₂ values showed a large range from 4.1 ms to 22.3 ms. T_1ρ and T₁ values tended to increase with age (p < 0.05), whereas T₂ did not.

Conclusions

MR relaxometry of the ulnocarpal disc is feasible, and T_1,T_1ρ, and T₂ values show modest variance in asymptomatic subjects. The potential of relaxation mapping to reveal relevant structural changes in patients has to be investigated in further studies.     

The ring opening of substituted cyclopropylidenes to substituted allenes: the effects of steric and long-range electrostatic interactions

Summary It is shown that all stereospecific preferences found experimentally for the ring opening of substituted cyclopropylidenes are satisfactorily reproduced by adding steric and long-range electrostatic interactions to the cyclopropylidene reaction surface. The corresponding surface for dimethyl cyclopropylidene is mapped out in detail. The surface for 3-methyl- and 2-bromo-3-methyl-cyclopropylidene is explored around the transition region. From the success of this approach it is inferred that short-range covalent interactions are unlikely to be responsible for sterospecific preferences found in these systems.Operated for the U.S. Department of Energy by Iowa State University under Contract No. 7405-ENG-82. This work was supported by the office of Basic Energy Sciences