Post-Processing of Selective Laser Sintered Polycarbonate Parts

St\ldies were conducted·to det~rmine the effect of press\lre onpost-prpeessingofpolycarbonate coupons. No significant differences betweencou.pons processed at atmospheric •cQndidons.. andcouponsprpeessedin.vacuum were •observed. However, anisotropic shrinkage .was observedin.thez-directionnormal to thesintering plane. Thisphenomenawasfurtl1erinvestigated usingsYIllmetrical polycarbonatecubespr9(iuced by Selective LaserSintering using yariedlas~rpoweryariedpowderlayer thickness, and varied powder bed temperaturesMechanical Engineerin

Laser Sintering Model for Composite Materials

A computer model for the sintering of ceramic/polymer composite materials has been established based on empirical sintering rate data. The model calculates sintering depths which result from variations in the operating parameters which include laser power, beam speed, scan spacing, scan vector length, and initial temperatures of the powder and surroundings. Sintering depths measured in multiple layer parts made of polymer coated ceramic powders are compared to sintering depths calculated by the sintering model.Mechanical Engineerin

Melting as a String-Mediated Phase Transition

We present a theory of the melting of elemental solids as a dislocation-mediated phase transition. We model dislocations near melt as non-interacting closed strings on a lattice. In this framework we derive simple expressions for the melting temperature and latent heat of fusion that depend on the dislocation density at melt. We use experimental data for more than half the elements in the Periodic Table to determine the dislocation density from both relations. Melting temperatures yield a dislocation density of (0.61\pm 0.20) b^{-2}, in good agreement with the density obtained from latent heats, (0.66\pm 0.11) b^{-2}, where b is the length of the smallest perfect-dislocation Burgers vector. Melting corresponds to the situation where, on average, half of the atoms are within a dislocation core.Comment: 18 pages, LaTeX, 3 eps figures, to appear in Phys. Rev.

Schwinger model on a half-line

We study the Schwinger model on a half-line in this paper. In particular, we investigate the behavior of the chiral condensate near the edge of the line. The effect of the chosen boundary condition is emphasized. The extension to the finite temperature case is straightforward in our approach.Comment: 4 pages, no figure. Final version to be published on Phys. Rev.

Density dependent hadron field theory for neutron stars with antikaon condensates

We investigate $K^-$ and $\bar K^0$ condensation in $\beta$-equilibrated hyperonic matter within a density dependent hadron field theoretical model. In this model, baryon-baryon and (anti)kaon-baryon interactions are mediated by the exchange of mesons. Density dependent meson-baryon coupling constants are obtained from microscopic Dirac Brueckner calculations using Groningen and Bonn A nucleon-nucleon potential. It is found that the threshold of antikaon condensation is not only sensitive to the equation of state but also to antikaon optical potential depth. Only for large values of antikaon optical potential depth, $K^-$ condensation sets in even in the presence of negatively charged hyperons. The threshold of $\bar K^0$ condensation is always reached after $K^-$ condensation. Antikaon condensation makes the equation of state softer thus resulting in smaller maximum mass stars compared with the case without any condensate.Comment: 20 pages, 7 figures; final version to appear in Physical Review

Improved perturbation theory in the vortex liquids state of type II superconductors

We develop an optimized perturbation theory for the Ginzburg - Landau description of thermal fluctuations effects in the vortex liquids. Unlike the high temperature expansion which is asymptotic, the optimized expansion is convergent. Radius of convergence on the lowest Landau level is $a_{T}=-3$ in 2D and $a_{T}=-5$ in 3D. It allows a systematic calculation of magnetization and specific heat contributions due to thermal fluctuations of vortices in strongly type II superconductors to a very high precision. The results are in good agreement with existing Monte Carlo simulations and experiments. Limitations of various nonperturbative and phenomenological approaches are noted. In particular we show that there is no exact intersection point of the magnetization curves both in 2D and 3D.Comment: 24 pages, 9 figure

Milk for a girl and bananas for a boy:recipes and reasons for sex-preference practices in a British Internet forum

Using postings from an internet forum, this paper explores the ways in which some women try to influence the sex of a future child. The extensive reproductive work involved give an indication of the women’s commitment to being able to choose a particular sex; in this case a preference for girls rather than boys. The findings revealed stereotypical views of masculinity and femininity at the heart of the preference. The presumption of fixed gendered identities helped to frame this desire as ‘natural,’ lessen the threat to maternal identities, and reinforce the logic of ‘choice,’ and support their reproductive work practices

Color superconducting quark matter core in the third family of compact stars

We investigate first order phase transitions from $\beta$-equilibrated hadronic matter to color flavor locked quark matter in compact star interior. The hadronic phase including hyperons and Bose-Einstein condensate of $K^-$ mesons is described by the relativistic field theoretical model with density dependent meson-baryon couplings. The early appearance of hyperons and/or Bose-Einstein condensate of $K^-$ mesons delays the onset of phase transition to higher density. In the presence of hyperons and/or $K^-$ condensate, the overall equations of state become softer resulting in smaller maximum masses than the cases without hyperons and $K^-$ condensate. We find that the maximum mass neutron stars may contain a mixed phase core of hyperons, $K^-$ condensate and color superconducting quark matter. Depending on the parameter space, we also observe that there is a stable branch of superdense stars called the third family branch beyond the neutron star branch. Compact stars in the third family branch may contain pure color superconducting core and have radii smaller than those of the neutron star branch. Our results are compared with the recent observations on RX J185635-3754 and the recently measured mass-radius relationship by X-ray Multi Mirror-Newton Observatory.Comment: 24 pages, RevTex, 9 figures included; section II shortened, section III elaborated, two new curves in Fig. 9 and acknowledgements added; version to bepublished in Phys. Rev.

Formation of superdense hadronic matter in high energy heavy-ion collisions

We present the detail of a newly developed relativistic transport model (ART 1.0) for high energy heavy-ion collisions. Using this model, we first study the general collision dynamics between heavy ions at the AGS energies. We then show that in central collisions there exists a large volume of sufficiently long-lived superdense hadronic matter whose local baryon and energy densities exceed the critical densities for the hadronic matter to quark-gluon plasma transition. The size and lifetime of this matter are found to depend strongly on the equation of state. We also investigate the degree and time scale of thermalization as well as the radial flow during the expansion of the superdense hadronic matter. The flow velocity profile and the temperature of the hadronic matter at freeze-out are extracted. The transverse momentum and rapidity distributions of protons, pions and kaons calculated with and without the mean field are compared with each other and also with the preliminary data from the E866/E802 collaboration to search for experimental observables that are sensitive to the equation of state. It is found that these inclusive, single particle observables depend weakly on the equation of state. The difference between results obtained with and without the nuclear mean field is only about 20\%. The baryon transverse collective flow in the reaction plane is also analyzed. It is shown that both the flow parameter and the strength of the ``bounce-off'' effect are very sensitive to the equation of state. In particular, a soft equation of state with a compressibility of 200 MeV results in an increase of the flow parameter by a factor of 2.5 compared to the cascade case without the mean field. This large effect makes it possible to distinguish the predictions from different theoretical models and to detect the signaturesComment: 55 pages, latex, + 39 figures available upon reques

Antikaon condensation and the metastability of protoneutron stars

We investigate the condensation of $\bar K^0$ meson along with $K^-$ condensation in the neutrino trapped matter with and without hyperons. Calculations are performed in the relativistic mean field models in which both the baryon-baryon and (anti)kaon-baryon interactions are mediated by meson exchange. In the neutrino trapped matter relevant to protoneutron stars, the critical density of $K^-$ condensation is shifted considerably to higher density whereas that of $\bar K^0$ condensation is shifted slightly to higher density with respect to that of the neutrino free case. The onset of $K^-$ condensation always occurs earlier than that of $\bar K^0$ condensation. A significant region of maximum mass protoneutron stars is found to contain $\bar K^0$ condensate for larger values of the antikaon potential. With the appearance of $\bar K^0$ condensation, there is a region of symmetric nuclear matter in the inner core of a protoneutron star. It is found that the maximum mass of a protoneutron star containing $K^-$ and $\bar K^0$ condensate is greater than that of the corresponding neutron star. We revisit the implication of this scenario in the context of the metastability of protoneutron stars and their evolution to low mass black holes.Comment: 26 pages; Revtex; 8 figures include