Are Neutron-Rich Elements Produced in the Collapse of Strange Dwarfs ?

The structure of strange dwarfs and that of hybrid stars with same baryonic number is compared. There is a critical mass (M~0.24M_sun) in the strange dwarf branch, below which configurations with the same baryonic number in the hybrid star branch are more stable. If a transition occurs between both branches, the collapse releases an energy of about of 3x10^{50} erg, mostly under the form of neutrinos resulting from the conversion of hadronic matter onto strange quark matter. Only a fraction (~4%) is required to expel the outer neutron-rich layers. These events may contribute significantly to the chemical yield of nuclides with A>80 in the Galaxy, if their frequency is of about one per 1500 years.Comment: Accepted for publication in IJMP

Equilibrium configurations for quark-diquark stars and the problem of Her X-1 mass

We report new calculations of the physical properties of a quark-diquark plasma. A vacuum contribution is taken into account and is responsible for the appearance of a stable state at zero pressure and at a baryon density of about 2.2 times the nuclear matter density in this model. The resulting equation of state was used to integrate numerically the Tolman-Oppenheimer-Volkoff equations. The mass-radius relationship has been derived from a series of equilibrium configurations constituted by a mixture of quarks and diquarks. These stellar models, which are representative of a whole class, may be helpful to understand the possible compactness of the X-ray source Her X-1 and related objects.Comment: 15 pp., PlainTex file + 3 figures available upon request at [email protected]. Submitted to Int. Jour. Mod. Phys.

A bright nanowire single photon source based on SiV centers in diamond

The practical implementation of many quantum technologies relies on the development of robust and bright single photon sources that operate at room temperature. The negatively charged silicon-vacancy (SiV-) color center in diamond is a possible candidate for such a single photon source. However, due to the high refraction index mismatch to air, color centers in diamond typically exhibit low photon out-coupling. An additional shortcoming is due to the random localization of native defects in the diamond sample. Here we demonstrate deterministic implantation of Si ions with high conversion efficiency to single SiV- centers, targeted to fabricated nanowires. The co-localization of single SiV- centers with the nanostructures yields a ten times higher light coupling efficiency than for single SiV- centers in bulk diamond. This enhanced photon out-coupling, together with the intrinsic scalability of the SiV- creation method, enables a new class of devices for integrated photonics and quantum science.Comment: 15 pages, 5 figure

Double fermiophobic Higgs boson production at the LHC and LC

We consider the phenomenology of a fermiophobic Higgs boson (h_f) at the Large Hadron Collider (LHC) and a e+e- Linear Collider (LC). At both machines the standard production mechanisms which rely on the coupling h_fVV (V=W,Z) can be very suppressed at large tan beta. In such cases the complementary channels pp to H^\pm h_f, A^0 h_f and e+e- to A^0 h_f offer promising cross-sections. Together with the potentially large branching ratios for H^\pm to h_fW* and A^0 to h_fZ*, these mechanisms would give rise to double h_f production, leading to signatures of gamma gamma gamma gamma, gamma gamma VV and VVVV.Comment: 19 pages, 9 figures, expanded discussion, fig.1 changed slightly, version to appear in Phys.Rev.

Concrete retrofitting using CFRP and geopolymer mortars

A new development in the repair and strengthening of reinforced concrete systems is the use of carbon fiber reinforced polymers (CFRP) strips bonded to concrete substrate with epoxy resins. It has been reported that epoxy adhesive are extremely sensitive to high temperatures. Some authors conclude that the epoxy temperature should not exceed 70 ºC in order to safeguard the adhesiveness of the epoxy and, thus, the integrity and adequate functioning of CFRP. It is noted that even frequently exposure to direct sunlight causes temperatures higher than 70 ºC. Since geopolymers are known to possess high stability at high temperature, these materials can be an alternative to epoxy resins. This papers presents results about the use of metakaolin based geopolymers mortars to insure the adhesion between the CFRP and the concrete substrate. Several compositions of geopolymer mortars were executed by varying the percentage of binder, sand/binder ratio and the concentration of sodium hydroxide. It was found that geopolymer mortars demonstrate very promising performances, having obtained a high mechanical resistance and a good adhesion to concrete. On the other hand the adhesion between CFRP and geopolymer mortars proved to be smaller than expected which could be due, to the fact that the composition of the mortar was not optimized or even to the nature of the CFRP

Mathematical Modeling of an Electromagnetic Forming System with Flat Spiral Coils as Actuator

This study presents mathematical modeling and calculation procedure for problems of electromagnetic forming of thin circular metal sheets using flat spiral coil as actuator. The method focuses specifically on the calculation of the electromagnetic field generated by the flat coil and analysis of the circuit that models the electromagnetic forming system. The flat coil is approximated by concentric circles carrying the current discharge from the capacitors. The calculation of electromagnetic force and magnetic couplings between the coil and metal sheet are made to the initial time, before the plastic deformation of the sheet. The method is based on the Biot-Savart law, and the solution of magnetic induction integral equations is performed by numerical methods specifically with the use of Matlab commercial software. A routine calculation, which models the problem as a set of differential equations was implemented in the Matlab, this provides important information that serves as feedback for system design. Free bulging experiments were performed to demonstrate a good relationship with the mathematical model predictions for electrical discharge current in the coil and induced currents in the metal sheet, behavior of the transient electromagnetic force between coil and workpiece and, distribution of magnetic field and electromagnetic density force along the coil. Also, achieved results showed that there is a strong dependence of the back electromagnetic force with respect to plate thickness for the system analyzed. The difference phase between the current induced in the coil and workpiece with higher negative peaks generate the back electromagnetic force

Proposal for a Test Bench for Electromagnetic Forming of Thin Metal Sheets

This paper presents a proposal to build a test bench for electromagnetic forming processes. The project considers the analysis of the electrical circuit and forces involved in the system for selection of low voltage capacitors, resistors, buses, main discharge switch and material choice for actuator s insulation and rigidity, considering also the manufacturing process of actuators and dies. Among the aspects considered for the design, energy efficiency has been prioritized by the use of non-conducting material to the dies. Main switches by mechanical contact and spark gap types were used and its wear and functionality was assessed. Free bulging experiments were performed with aluminium AA1100 plates for a system configured with a flat coil actuator. Test measurements of electric currents in the coil actuator with and without the workpiece as the secondary circuit were performed, as well as an evaluation of wear and functionality of the system. It is observed that the main switch discharge is one of the most critical items of the system

Route to Renewable PET: Reaction Pathways and Energetics of Diels–Alder and Dehydrative Aromatization Reactions Between Ethylene and Biomass-Derived Furans Catalyzed by Lewis Acid Molecular Sieves

Silica molecular sieves that have the zeolite beta topology and contain framework Lewis acid centers (e.g., Zr-β, Sn-β) are useful catalysts in the Diels–Alder and dehydrative aromatization reactions between ethylene and various renewable furans for the production of biobased terephthalic acid precursors. Here, the main side products in the synthesis of methyl 4-(methoxymethyl)benzene carboxylate that are obtained by reacting ethylene with methyl 5-(methoxymethyl)-furan-2-carboxylate are identified, and an overall reaction pathway is proposed. Madon–Boudart experiments using Zr-β samples of varying Si/Zr ratios clearly indicate that there are no transport limitations to the rate of reaction for the synthesis of p-xylene from 2,5-dimethylfuran and ethylene and strongly suggest no mass transport limitations in the synthesis of methyl p-toluate from methyl 5-methyl-2-furoate and ethylene. Measured apparent activation energies for these reaction-limited systems are small (<10.5 kcal/mol), suggesting that apparent activation energies are derived from a collection of parameters and are not true activation energies for a single chemical step. In addition, 13C kinetic isotope effects (KIE) in the synthesis of MMBC and MPT measured by gas chromatography/isotope-ratio mass spectrometry in reactant-depletion experiments support the Madon–Boudart result that these systems are not transport-limited and the KIE values agree with those previously reported for Diels–Alder cycloadditions

Concrete retrofitting using metakaolin geopolymer mortars and CFRP

This paper presents results about the use of metakaolin based geopolymers mortars for retrofitting purposes. Two main situations are addressed, the use of geopolymeric mortars as a repairing layer or as a binding agent to insure the adhesion between CFRP sheets and the concrete substrate. Several compositions of metakaolin geopolymer mortars were executed by varying the percentage of sand/binder mass ratio and the concentration of sodium hydroxide. It was found that metakaolin geopolymer mortars show a high mechanical resistance and a relevant adhesion to the concrete substrate. Although their adhesion strength is lower than the one present by commercial pre-pack repair mortars, they are very cost-effective (5 to 10 times less expensive). On the other hand, the adhesion strength between CFRP and geopolymer mortars proved to be lower than expected which could be due to the fact that the composition of the geopolymeric mortars was not optimized and also to the fact that the CFRP used was not prone to this kind of application