Graphene as a diffusion barrier for isomorphous systems: Cu-Ni system

Electrochemical exfoliation technique using the pyrophosphate anion derived from tetra sodium pyrophosphate was employed to produce graphene. As-synthesized graphene was then drop dried over a cold rolled Cu sheet. Ni coating was then electrodeposited over bare Cu and graphene-Cu substrates. Both substrates were then isothermally annealed at 800 degrees C for 3 h. WDS analysis showed substantial atomic diffusion in annealed Ni-Cu sample. Cu-graphene-Ni sample, on the other hand, showed negligible diffusion illustrating the diffusion barrier property of the graphene coating. (C) 2016 Elsevier B.V. All rights reserved.

Graphene as a diffusion barrier for isomorphous systems: Cu-Ni system

Electrochemical exfoliation technique using the pyrophosphate anion derived from tetra sodium pyrophosphate was employed to produce graphene. As-synthesized graphene was then drop dried over a cold rolled Cu sheet. Ni coating was then electrodeposited over bare Cu and graphene-Cu substrates. Both substrates were then isothermally annealed at 800 degrees C for 3 h. WDS analysis showed substantial atomic diffusion in annealed Ni-Cu sample. Cu-graphene-Ni sample, on the other hand, showed negligible diffusion illustrating the diffusion barrier property of the graphene coating. (C) 2016 Elsevier B.V. All rights reserved.

Influence of precipitates on GaN epilayer quality

GaN epilayers grown on pre-nitridated (0001) sapphire substrates by metallorganic vapor phase epitaxy were investigated by wavelength dispersive X-ray spectroscopy and energy dispersive S-ray spectroscopy. Precipitates were observed to mainly consist of O impurity whose strengths were weaker than surrounding matrix. The precipitates were larger in size and distributed more sparsely and inhomogeneously in < 11-20 > directions of the epilayers grown on substrates pre-nitridated for longer periods. The larger precipitates often joined to cracks in the TEM specimens. The crack formation seems to be attributed to the compressive stress concentration at edge angles of the larger precipitates. Yellow luminescence of the epilayers was imaged by cathodoluminescence. The distribution similarity between the cathodoluminescence and the precipitates suggested that the precipitates were responsible for the yellow luminescence band. (C) 2000 Elsevier Science S.A, All rights reserved.

Influence of precipitates on GaN epilayer quality

GaN epilayers grown on pre-nitridated (0001) sapphire substrates by metallorganic vapor phase epitaxy were investigated by wavelength dispersive X-ray spectroscopy and energy dispersive S-ray spectroscopy. Precipitates were observed to mainly consist of O impurity whose strengths were weaker than surrounding matrix. The precipitates were larger in size and distributed more sparsely and inhomogeneously in < 11-20 > directions of the epilayers grown on substrates pre-nitridated for longer periods. The larger precipitates often joined to cracks in the TEM specimens. The crack formation seems to be attributed to the compressive stress concentration at edge angles of the larger precipitates. Yellow luminescence of the epilayers was imaged by cathodoluminescence. The distribution similarity between the cathodoluminescence and the precipitates suggested that the precipitates were responsible for the yellow luminescence band. (C) 2000 Elsevier Science S.A, All rights reserved.

贵州水银洞卡林型金矿床含金硫化物地球化学与金的赋存状态研究

　　以沉积岩为主要容矿岩石的卡林型金矿，是目前世界上储量最大的金矿类型之一，金“不可见”或其颗粒极细（纳米级）是该类型金矿最重要的特点之一。国内外大量研究已经证实，金主要赋存在含砷黄铁矿中，但金的赋存形式仍然是目前争论的焦点和研究的热点。贵州水银洞金矿床位于灰家堡背斜Au-Hg-Tl矿田的东段，是黔西南地区发现较晚、品位较富（6～18 g/t）的大型（55 t）隐伏卡林型金矿床，并已大规模开采利用，结束了我国卡林型金矿原生矿石不能开发利用的历史。 　　本次研究在系统显微岩相学研究的基础上，采用电子探针（EMPA）背散射电子图像（BSE）、波谱（WDS）和能谱（EDS）分析技术，对水银洞卡林型金矿床原生富矿石中不同类型含砷黄铁矿和毒砂的矿物学、地球化学以及金的赋存状态等进行了较系统的研究，取得的主要认识如下： 1. 水银洞金矿床原生矿石具有去碳酸盐化（Decarbonation）、硅化、硫化物化（Sulfidation）等典型卡林型金矿的热液蚀变特征。原生矿石中的主要载金矿物为含砷黄铁矿，其次为毒砂。 2. 根据含砷黄铁矿和毒砂的形态和结构特征，将含砷黄铁矿分为生物碎屑状含砷黄铁矿、细粒含砷黄铁矿、环带状含砷黄铁矿和细脉状含砷黄铁矿四种类型；毒砂分为板状毒砂和针状毒砂两种类型。毒砂晚于含砷黄铁矿的形成。 3. 电子探针分析表明，含砷黄铁矿中含有较高的Au，一般为300×10-6～3800×10-6，含有As 0.65～14.11 wt%、Sb 0.03～0.12 wt%、Co 0.03～0.08 wt%等元素。毒砂含Au较低，一般为300×10-6～1500 ×10-6。 4. 研究表明，含砷黄铁矿中的As与S含量具有明显的负相关关系，认为As替代S进入含砷黄铁矿和毒砂的结构；Au与As之间不是一种简单的线形正相关关系，而是分布在一个的楔形空间。结合前人的研究结果，认为含砷黄铁矿中的“不可见金”，可能主要以化学结合态金（Au1+）的形式进入含砷黄铁矿和毒砂结构。 5. 金的赋存状态研究表明，水银洞卡林型金矿床发现有两种金的赋存形式：（1）“不可见金”，主要赋存在黄铁矿的含砷环带之中；（2）次显微—显微自然金颗粒（0.1～6μm），主要见于含砷黄铁矿细脉或其集合体。这些自然金颗粒也有两种赋存形式：① 粗粒自然金颗粒（1～6μm），主要分布于早世代粗粒含砷黄铁矿颗粒的表面或晚世代细粒含砷黄铁矿颗粒的边缘，偶见于含Fe碳酸盐矿物的溶蚀空洞中，被解释为热液中的Au局部过饱和的产物；②细粒自然金颗粒（0.1～0.2μm），偶见于早世代粗粒含砷黄铁矿的含砷环带或其溶蚀空洞的边缘，被解释为黄铁矿含砷环带中的Au再溶解过饱和沉淀的产物。 6. 根据原生矿石显微岩相学结构、含砷黄铁矿矿物学与地球化学特征，探讨了含金—砷黄铁矿的形成过程，认为含砷黄铁矿中的Fe，可能主要来源于赋矿围岩中的含Fe碳酸盐矿物（方解石和白云石）溶解而释放的Fe，溶解Fe的硫化物化过程是水银洞卡林型金矿富集成矿的重要机制之一。 7. 结合野外地质观察，认为含Fe碳酸盐赋矿围岩是形成高品位、大型卡林型金矿床最有利的岩性，并提出了与去碳酸盐化有关的碳酸盐脉，是寻找深部隐伏卡林型金矿体的重要找矿标志之一。

Synthesis of molybdenum borides and molybdenum silicides in molten salts and their oxidation behavior in an air-water mixture

The formation of various coatings in molybdenum-boron and molybdenum-silicon systems was investigated. Boronizing and siliciding treatments were conducted in molten salts under inert gas atmosphere in the 850-1050 degrees C temperature range for 7 h. The presence of boride (e.g. Mo2B, MoB, Mo2B5) and silicide (MoSi2, Mo5Si3) phases, formed on the surface of Mo plates, was confirmed by X-ray diffraction analysis. The distribution of elements was determined by means of wavelength dispersive spectroscopy (WDS) spectra of the surface and line-scan analyses from surface to interior. Depending on the process type (diffusional or electrochemical) and temperature, the thickness of the protective layers formed on the substrate ranged from 6 to 40 gm. The oxidation resistance of obtained phases was investigated in an air-water mixture in the temperature range of 500-700 degrees C for a period up to 400 h. An improved oxidation behavior of coated plates in comparison with that of pure molybdenum was observed. (c) 2004 Elsevier B.V. All rights reserved.

Detonations in sub-chandrasekhar-mass C+O white dwarfs

Explosions of sub-Chandrasekhar-mass white dwarfs (WDs) are one alternative to the standard Chandrasekhar-mass model of Type Ia supernovae (SNe Ia). They are interesting since binary systems with sub-Chandrasekhar-mass primary WDs should be common and this scenario would suggest a simple physical parameter which determines the explosion brightness, namely the mass of the exploding WD. Here we perform one-dimensional hydrodynamical simulations, associated post-processing nucleosynthesis, and multi-wavelength radiation transport calculations for pure detonations of carbon-oxygen WDs. The light curves and spectra we obtain from these simulations are in good agreement with observed properties of SNe Ia. In particular, for WD masses from 0.97 to 1.15 Msun we obtain 56Ni masses between 0.3 and 0.8 Msun, sufficient to capture almost the complete range of SN Ia brightnesses. Our optical light curve rise times, peak colors, and decline timescales display trends which are generally consistent with observed characteristics although the range of B-band decline timescales displayed by our current set of models is somewhat too narrow. In agreement with observations, the maximum light spectra of the models show clear features associated with intermediate-mass elements and reproduce the sense of the observed correlation between explosion luminosity and the ratio of the Si II lines at ?6355 and ?5972. We therefore suggest that sub-Chandrasekhar-mass explosions are a viable model for SNe Ia for any binary evolution scenario leading to explosions in which the optical display is dominated by the material produced in a detonation of the primary WD. © 2010. The American Astronomical Society.

2D simulations of the double-detonation model for thermonuclear transients from low-mass carbon-oxygen white dwarfs

Thermonuclear explosions may arise in binary star systems in which a carbon-oxygen (CO) white dwarf (WD) accretes helium-rich material from a companion star. If the accretion rate allows a sufficiently large mass of helium to accumulate prior to ignition of nuclear burning, the helium surface layer may detonate, giving rise to an astrophysical transient. Detonation of the accreted helium layer generates shock waves that propagate into the underlying CO WD. This might directly ignite a detonation of the CO WD at its surface (an edge-lit secondary detonation) or compress the core of the WD sufficiently to trigger a CO detonation near the centre. If either of these ignition mechanisms works, the two detonations (helium and CO) can then release sufficient energy to completely unbind the WD. These 'double-detonation' scenarios for thermonuclear explosion of WDs have previously been investigated as a potential channel for the production of Type Ia supernovae from WDs of ~ 1 M . Here we extend our 2D studies of the double-detonation model to significantly less massive CO WDs, the explosion of which could produce fainter, more rapidly evolving transients. We investigate the feasibility of triggering a secondary core detonation by shock convergence in low-mass CO WDs and the observable consequences of such a detonation. Our results suggest that core detonation is probable, even for the lowest CO core masses that are likely to be realized in nature. To quantify the observable signatures of core detonation, we compute spectra and light curves for models in which either an edge-lit or compression-triggered CO detonation is assumed to occur. We compare these to synthetic observables for models in which no CO detonation was allowed to occur. If significant shock compression of the CO WD occurs prior to detonation, explosion of the CO WD can produce a sufficiently large mass of radioactive iron-group nuclei to significantly affect the light curves. In particular, this can lead to relatively slow post-maximum decline. If the secondary detonation is edge-lit, however, the CO WD explosion primarily yields intermediate-mass elements that affect the observables more subtly. In this case, near-infrared observations and detailed spectroscopic analysis would be needed to determine whether a core detonation occurred. We comment on the implications of our results for understanding peculiar astrophysical transients including SN 2002bj, SN 2010X and SN 2005E. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

On the brightness distribution of type ia supernovae from violent white dwarf mergers

We investigate the brightness distribution expected for thermonuclear explosions that might result from the ignition of a detonation during the violent merger of white dwarf (WD) binaries. Violent WD mergers are a subclass of the canonical double degenerate scenario where two carbon-oxygen (CO) WDs merge when the larger WD fills its Roche lobe. Determining their brightness distribution is critical for evaluating whether such an explosion model could be responsible for a significant fraction of the observed population of Type Ia supernovae (SNe Ia). We argue that the brightness of an explosion realized via the violent merger model is mainly determined by the mass of Ni produced in the detonation of the primary COWD. To quantify this link, we use a set of sub-Chandrasekhar mass WD detonation models to derive a relationship between primary WD mass (m) and expected peak bolometric brightness (M). We use this m-M relationship to convert the masses of merging primary WDs from binary population models to a predicted distribution of explosion brightness. We also investigate the sensitivity of our results to assumptions about the conditions required to realize a detonation during violent mergers ofWDs. We find a striking similarity between the shape of our theoretical peak-magnitude distribution and that observed for SNe Ia: our model produces a M distribution that roughly covers the range and matches the shape of the one observed for SNe Ia. However, this agreement hinges on a particular phase of mass accretion during binary evolution: the primary WD gains ~0.15-0.35M? from a slightly evolved helium star companion. In our standard binary evolution model, such an accretion phase is predicted to occur for about 43 per cent of all binary systems that ultimately give rise to binary CO WD mergers. We also find that with high probability, violent WD mergers involving the most massive primaries (?1.3M?, which should produce bright SNe) have delay times ?500 Myr. © 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Constraining type Ia supernova models:SN 2011fe as A test case

The nearby supernova SN 2011fe can be observed in unprecedented detail. Therefore, it is an important test case for Type Ia supernova (SN Ia) models, which may bring us closer to understanding the physical nature of these objects. Here, we explore how available and expected future observations of SN 2011fe can be used to constrain SN Ia explosion scenarios. We base our discussion on three-dimensional simulations of a delayed detonation in a Chandrasekhar-mass white dwarf and of a violent merger of two white dwarfs (WDs) - realizations of explosion models appropriate for two of the most widely discussed progenitor channels that may give rise to SNe Ia. Although both models have their shortcomings in reproducing details of the early and near-maximum spectra of SN 2011fe obtained by the Nearby Supernova Factory (SNfactory), the overall match with the observations is reasonable. The level of agreement is slightly better for the merger, in particular around maximum, but a clear preference for one model over the other is still not justified. Observations at late epochs, however, hold promise for discriminating the explosion scenarios in a straightforward way, as a nucleosynthesis effect leads to differences in the Co production. SN 2011fe is close enough to be followed sufficiently long to study this effect. © © 2012 The American Astronomical Society. All rights reserved.

The effect of helium accretion efficiency on rates of Type Ia supernovae: Double detonations in accreting binaries

he double-detonation explosion scenario of Type Ia supernovae (SNe Ia) has gained increased support from the SN Ia community as a viable progenitor model, making it a promising candidate alongside the well-known single degenerate and double degenerate scenarios. We present delay times of double-detonation SNe, in which a sub-Chandrasekhar mass carbon–oxygen white dwarf (WD) accretes non-dynamically from a helium-rich companion. One of the main uncertainties in quantifying SN rates from double detonations is the (assumed) retention efficiency of He-rich matter. Therefore, we implement a new prescription for the treatment of accretion/accumulation of He-rich matter on WDs. In addition, we test how the results change depending on which criteria are assumed to lead to a detonation in the helium shell. In comparing the results to our standard case (Ruiter et al.), we find that regardless of the adopted He accretion prescription, the SN rates are reduced by only ∼25 per cent if low-mass He shells (≲0.05 M⊙) are sufficient to trigger the detonations. If more massive (0.1 M⊙) shells are needed, the rates decrease by 85 per cent and the delay time distribution is significantly changed in the new accretion model – only SNe with prompt (<500 Myr) delay times are produced. Since theoretical arguments favour low-mass He shells for normal double-detonation SNe, we conclude that the rates from double detonations are likely to be high, and should not critically depend on the adopted prescription for accretion of He.

Synthetic light curves and spectra for three-dimensional delayed-detonation models of type ia supernovae

In a companion paper, Seitenzahl et al. have presented a set of three-dimensional delayed detonation models for thermonuclear explosions of near-Chandrasekhar-mass white dwarfs (WDs). Here,we present multidimensional radiative transfer simulations that provide synthetic light curves and spectra for those models. The model sequence explores both changes in the strength of the deflagration phase (which is controlled by the ignition configuration in our models) and the WD central density. In agreement with previous studies, we find that the strength of the deflagration significantly affects the explosion and the observables. Variations in the central density also have an influence on both brightness and colour, but overall it is a secondary parameter in our set of models. In many respects, the models yield a good match to the observed properties of normal Type Ia supernovae (SNe Ia): peak brightness, rise/decline time-scales and synthetic spectra are all in reasonable agreement. There are, however, several differences. In particular, the models are systematically too red around maximum light, manifest spectral line velocities that are a little too high and yield I-band light curves that do not match observations. Although some of these discrepancies may simply relate to approximations made in the modelling, some pose real challenges to the models. If viewed as a complete sequence, our models do not reproduce the observed light-curve width- luminosity relation (WLR) of SNe Ia: all our models show rather similar B-band decline rates, irrespective of peak brightness. This suggests that simple variations in the strength of the deflagration phase in Chandrasekhar-mass deflagration-to-detonation models do not readily explain the observed diversity of normal SNe Ia. This may imply that some other parameter within the Chandrasekhar-mass paradigm is key to the WLR, or that a substantial fraction of normal SNe Ia arise from an alternative explosion scenario.

'Super-Chandrasekhar' Type Ia Supernovae at nebular epochs

We present a first systematic comparison of superluminous Type Ia supernovae (SNe Ia) at late epochs, including previously unpublished photometric and spectroscopic observations of SN 2007if, SN 2009dc and SNF20080723-012. Photometrically, the objects of our sample show a diverse late-time behaviour, some of them fading quite rapidly after a light-curve break at ∼ 150-200 d. The latter is likely the result of flux redistribution into the infrared, possibly caused by dust formation, rather than a true bolometric effect. Nebular spectra of superluminous SNe Ia are characterized by weak or absent [Fe III] emission, pointing at a low ejecta ionization state as a result of high densities. To constrain the ejecta and Ni masses of superluminous SNe Ia, we compare the observed bolometric light curve of SN 2009dc with synthetic model light curves, focusing on the radioactive tail after ∼60 d. Models with enough Ni to explain the light-curve peak by radioactive decay, and at the same time sufficient mass to keep the ejecta velocities low, fail to reproduce the observed light-curve tail of SN 2009dc because of too much γ -ray trapping.We instead propose a model with ∼1M of Ni and ∼2 M of ejecta, which may be interpreted as the explosion of a Chandrasekhar-mass white dwarf (WD) enshrouded by 0.6-0.7 M of C/O-rich material, as it could result from a merger of two massive C/O WDs. This model reproduces the late light curve of SN 2009dc well. A flux deficit at peak may be compensated by light from the interaction of the ejecta with the surrounding material.

Deflagrations in hybrid CONe white dwarfs: a route to explain the faint Type Iax supernova 2008ha

Stellar evolution models predict the existence of hybrid white dwarfs (WDs) with a carbon-oxygen core surrounded by an oxygen-neon mantle. Being born with masses similar to 1.1 M-aS (TM), hybrid WDs in a binary system may easily approach the Chandrasekhar mass (M-Ch) by accretion and give rise to a thermonuclear explosion. Here, we investigate an off-centre deflagration in a near-M-Ch hybrid WD under the assumption that nuclear burning only occurs in carbon-rich material. Performing hydrodynamics simulations of the explosion and detailed nucleosynthesis post-processing calculations, we find that only 0.014 M-aS (TM) of material is ejected while the remainder of the mass stays bound. The ejecta consist predominantly of iron-group elements, O, C, Si and S. We also calculate synthetic observables for our model and find reasonable agreement with the faint Type Iax SN 2008ha. This shows for the first time that deflagrations in near-M-Ch WDs can in principle explain the observed diversity of Type Iax supernovae. Leaving behind a near-M-Ch bound remnant opens the possibility for recurrent explosions or a subsequent accretion-induced collapse in faint Type Iax SNe, if further accretion episodes occur. From binary population synthesis calculations, we find the rate of hybrid WDs approaching M-Ch to be of the order of 1 per cent of the Galactic SN Ia rate.

The Unusually Luminous Extragalactic Nova SN 2010U

We present observations of the unusual optical transient SN 2010U, including spectra taken 1.03 days to 15.3 days after maximum light that identify it as a fast and luminous Fe II type nova. Our multi-band light curve traces the fast decline (t 2 = 3.5 ± 0.3 days) from maximum light (MV = -10.2 ± 0.1 mag), placing SN 2010U in the top 0.5% of the most luminous novae ever observed. We find typical ejecta velocities of ≈1100 km s-1 and that SN 2010U shares many spectral and photometric characteristics with two other fast and luminous Fe II type novae, including Nova LMC 1991 and M31N-2007-11d. For the extreme luminosity of this nova, the maximum magnitude versus rate of decline relationship indicates a massive white dwarf (WD) progenitor with a low pre-outburst accretion rate. However, this prediction is in conflict with emerging theories of nova populations, which predict that luminous novae from massive WDs should preferentially exhibit an alternate spectral type (He/N) near maximum light.