Recurrent Outbursts and Jet Ejections Expected in Swift J1644+57: Limit-Cycle Activities in a Supermassive Black Hole

The tidal disruption event by a supermassive black hole in Swift J1644+57 can trigger limit-cycle oscillations between a supercritically accreting X-ray bright state and a subcritically accreting X-ray dim state. Time evolution of the debris gas around a black hole with mass M=10^{6} {\MO} is studied by performing axisymmetric, two-dimensional radiation hydrodynamic simulations. We assumed the $\alpha$-prescription of viscosity, in which the viscous stress is proportional to the total pressure. The mass supply rate from the outer boundary is assumed to be ${\dot M}_{\rm supply}=100L_{\rm Edd}/c^2$, where $L_{\rm Edd}$ is the Eddington luminosity, and $c$ is the light speed. Since the mass accretion rate decreases inward by outflows driven by radiation pressure, the state transition from a supercritically accreting slim disk state to a subcritically accreting Shakura-Sunyaev disk starts from the inner disk and propagates outward in a timescale of a day. The sudden drop of the X-ray flux observed in Swift J1644+57 in August 2012 can be explained by this transition. As long as ${\dot M}_{\rm supply}$ exceeds the threshold for the existence of a radiation pressure dominant disk, accumulation of the accreting gas in the subcritically accreting region triggers the transition from a gas pressure dominant Shakura-Sunyaev disk to a slim disk. This transition takes place at $t {\sim}~50/({\alpha}/0.1)$ days after the X-ray darkening. We expect that if $\alpha > 0.01$, X-ray emission with luminosity $\gtrsim 10^{44}$ ${\rm erg}{\cdot}{\rm s}^{-1}$ and jet ejection will revive in Swift J1644+57 in 2013--2014.Comment: 6 pages, 4 figures, accepted for publication in PASJ Letter

Covalently bonded interfaces for polymer/graphene composites

The interface is well known for taking a critical role in the determination of the functional and mechanical properties of polymer composites. Previous interface research has focused on utilising reduced graphene oxide that is limited by a low structural integrity, which means a high fraction is needed to produce electrically conductive composites. By using 4,40-diaminophenylsulfone, we in this study chemically modified high-structural integrity graphene platelets (GnPs) of 2–4 nm in thickness, covalently bonded GnPs with an epoxy matrix, and investigated the morphology and functional and mechanical performance of these composites. This covalently bonded interface prevented GnPs stacking in the matrix. In comparison with unmodified composites showing no reduction in electrical volume resistivity, the interface-modified composite at 0.489 vol% GnPs demonstrates an eight-order reduction in the resistivity, a 47.7% further improvement in modulus and 84.6% in fracture energy release rate. Comparison of GnPs with clay and multi-walled carbon nanotubes shows that our GnPs are more advantageous in terms of performance and cost. This study provides a novel method for developing interface-tuned polymer/graphene composites

Development of polymer composites using modified, high-structural integrity graphene platelets

Previous studies on polymer/graphene composites have mainly utilized either reduced graphene oxide or graphite nanoplatelets of over 10 nm in thickness. In this study we covalently modified 3-nm thick graphene platelets (GnPs) by the reaction between the GnPs’ epoxide groups and the end-amine groups of a commercial long-chain surfactant (Mw = 2000), compounded the modified GnPs (m-GnPs) with a model polymer epoxy, and investigated the structure and properties of both m-GnPs and their epoxy composites. A low Raman ID/IG ratio of 0.13 was found for m-GnPs corresponding to high structural integ-rity. A percolation threshold of electrical conductivity was observed at 0.32 vol% m-GnPs, and the 0.98 vol% m-GnPs improved the Young’s modulus, fracture energy release rate and glass transition tem-perature of epoxy by 14%, 387% and 13%, respectively. These significantly improved properties are cred-ited to: (i) the low Raman ID/IG ratio of GnPs, maximizing the structural integrity and thus conductivity, stiffness and strength inherited from its sister graphene, (ii) the low thickness of GnPs, minimizing the damaging effect of the poor through-plane mechanical properties and electrical conductivity of graphene,(iii) the high-molecular weight surfactant, leading to uniformly dispersed GnPs in the matrix, and (iv) a covalently bonded interface between m-GnPs and matrix, more effectively transferring load/electron across interface

Melt compounding with graphene to develop functional, high-performance elastomers

Rather than using graphene oxide, which is limited by a high defect concentration and cost due to oxidation and reduction, we adopted cost-effective, 3.56 nm thick graphene platelets (GnPs) of high structural integrity to melt compound with an elastomer—ethylene–propylene–diene monomer rubber (EPDM)—using an industrial facility. An elastomer is an amorphous, chemically crosslinked polymer generally having rather low modulus and fracture strength but high fracture strain in comparison with other materials; and upon removal of loading, it is able to return to its original geometry, immediately and completely. It was found that most GnPs dispersed uniformly in the elastomer matrix, although some did form clusters. A percolation threshold of electrical conductivity at 18 vol% GnPs was observed and the elastomer thermal conductivity increased by 417% at 45 vol% GnPs. The modulus and tensile strength increased by 710% and 404% at 26.7 vol% GnPs, respectively. The modulus improvement agrees well with the Guth and Halpin-Tsai models. The reinforcing effect of GnPs was compared with silicate layers and carbon nanotube. Our simple fabrication would prolong the service life of elastomeric products used in dynamic loading, thus reducing thermosetting waste in the environment

Melt compounding with graphene to develop functional, high-performance elastomers

Rather than using graphene oxide, which is limited by a high defect concentration and cost due to oxidation and reduction, we adopted cost-effective, 3.56 nm thick graphene platelets (GnPs) of high structural integrity to melt compound with an elastomer—ethylene–propylene–diene monomer rubber (EPDM)—using an industrial facility. An elastomer is an amorphous, chemically crosslinked polymer generally having rather low modulus and fracture strength but high fracture strain in comparison with other materials; and upon removal of loading, it is able to return to its original geometry, immediately and completely. It was found that most GnPs dispersed uniformly in the elastomer matrix, although some did form clusters. A percolation threshold of electrical conductivity at 18 vol% GnPs was observed and the elastomer thermal conductivity increased by 417% at 45 vol% GnPs. The modulus and tensile strength increased by 710% and 404% at 26.7 vol% GnPs, respectively. The modulus improvement agrees well with the Guth and Halpin-Tsai models. The reinforcing effect of GnPs was compared with silicate layers and carbon nanotube. Our simple fabrication would prolong the service life of elastomeric products used in dynamic loading, thus reducing thermosetting waste in the environment

A facile approach to chemically modified graphene and its polymer nanocomposites

A scalable approach for the mass production of chemically modifi ed graphene has yet to be developed, which holds the key to the large-scale production of stable graphene colloids for optical electronics, energy conversion, and storage materials, catalysis, sensors, composites, etc. Here, a facile approach to fabricating covalently modifi ed graphene and its polymer nanocompos- ites is presented. The method involves: i) employing a common furnace, rather than a furnace installed with a quartz tube and operated in inert gas as required in previous studies, to treat a commercial graphite intercalation compound with thermal shocking and ultrasonication and fabricate graphene platelets (GnPs) with a thickness of 2.51 ± 0.39 nm that contain only 7 at% oxygen; ii) grafting these GnPs with a commercial, long-chain surfactant, which is able to create molecular entanglement with polymer matrixes by taking advantage of the reactions between the epoxide groups of the platelets and the end amine groups of the surfactant, to produce chemically modi- fi ed graphene platelets ( m - GnPs); and iii) solution-mixing m -GnPs with a commonly used polymer to fabricate nanocomposites. These m -GnPs are well dispersed in a polymer with highly improved mechanical properties and a low percolation threshold of electrical conductivity at 0.25 vol%. This novel approach could lead to the future scalable production of graphene and its nanocomposites