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Mathur, S.,Gupta, A.,Page, K.,Pogge, R. W.,Krongold, Y.,Goad, M. R.,Adams, S. M.,Anderson, M. D.,Aré,valo, P.,Barth, A. J.,Bazhaw, C.,Beatty, T. G.,Bentz, M. C.,Bigley, A.,Bisogni, S.,Borman, G. American Astronomical Society 2017 The Astrophysical Journal Vol.846 No.1
<P>During the Space Telescope and Optical Reverberation Mapping Project observations of NGC 5548, the continuum and emission-line variability became decorrelated during the second half of the six-month-long observing campaign. Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as part of the campaign. The Swift spectra show that excess flux (relative to a power-law continuum) in the soft X-ray band appears before the start of the anomalous emission-line behavior, peaks during the period of the anomaly, and then declines. This is a model-independent result suggesting that the soft excess is related to the anomaly. We divide the Swift data into on-and off-anomaly spectra to characterize the soft excess via spectral fitting. The cause of the spectral differences is likely due to a change in the intrinsic spectrum rather than to variable obscuration or partial covering. The Chandra spectra have lower signal-to-noise ratios, but are consistent with the Swift data. Our preferred model of the soft excess is emission from an optically thick, warm Comptonizing corona, the effective optical depth of which increases during the anomaly. This model simultaneously explains all three observations: the UV emission-line flux decrease, the soft-excess increase, and the emission-line anomaly.</P>
A. Mathur,S.S. Roy,M. Tweedie,S. Mukhopadhyay,S.K. Mitra,J.A. McLaughlin 한국물리학회 2009 Current Applied Physics Vol.9 No.6
In this study we fabricated a silicon-based stamp with various microchannel arrays, and demonstrated successful replication of the stamp microstructure on poly methyl methacrylate (PMMA) substrates. We used maskless UV lithography for the production of the micro-structured stamp. Thermal imprint lithography was used to fabricate microfeatured fluidic platforms on PMMA substrates, as well as to bond PMMA lids on the fluidic platforms. The microfeature in the silicon-based (silicon wafer coated with SU- 8) stamp includes microchannel arrays of approximately 30 ㎛ in depth and 5 mm in width. We produced various channels without pillars, as well as with SU-8 pillars in the range of 50–100 ㎛ wide and 6 ㎛ in height. PMMA discs of 1 mm thickness were utilized as the molding substrate. We found 10 kN applied force and 100 ℃ embossing temperature were optimum for transferring the microstructure to the PMMA substrate. In this study we fabricated a silicon-based stamp with various microchannel arrays, and demonstrated successful replication of the stamp microstructure on poly methyl methacrylate (PMMA) substrates. We used maskless UV lithography for the production of the micro-structured stamp. Thermal imprint lithography was used to fabricate microfeatured fluidic platforms on PMMA substrates, as well as to bond PMMA lids on the fluidic platforms. The microfeature in the silicon-based (silicon wafer coated with SU- 8) stamp includes microchannel arrays of approximately 30 ㎛ in depth and 5 mm in width. We produced various channels without pillars, as well as with SU-8 pillars in the range of 50–100 ㎛ wide and 6 ㎛ in height. PMMA discs of 1 mm thickness were utilized as the molding substrate. We found 10 kN applied force and 100 ℃ embossing temperature were optimum for transferring the microstructure to the PMMA substrate.
Mathur, R.B.,Bahl, O.P.,Dhami, T.L.,Chauhan, S.K. Korean Carbon Society 2003 Carbon Letters Vol.4 No.3
Carbon/carbon composites were developed using PAN based carbon fibres and phenolic resin as matrix in different volume fractions and heat treated to temperatures between $1000^{\circ}C$ to $2500^{\circ}C$. Although both the starting precursors are nongraphitizing hard carbons individually, their composites lead to very interesting properties e.g. x-ray diffractograms show the development of graphitic phase for composites having fibre volume fractions of 30~40%. Consequently the electrical resistivity of such composites reaches a value of $0.8\;m{\Omega}cm$, very close to highly graphitic material. However, it was found that by increasing the fibre volume fraction to 50~60%, the trend is reversed. Optical microscopy of the composites also reveals the development of strong columnar type microstructure at the fibre (matrix interface due to stress graphitization of the matrix. The study forcasts a unique possibility of producing high thermal conductivity carbon/carbon composites starting with carbon fibres in the chopped form only.
Interface and Microstructure Development in Carbon/Carbon Composites
Mathur, R.B.,Bahl, O.P.,Dhami, T.L.,Chauhan, S.K.,Dhakate, S.R.,Rand, B. Korean Carbon Society 2004 Carbon Letters Vol.5 No.2
Performance of carbon-carbon composites is known to be influenced by the fibre matrix interactions. The present investigation was undertaken to ascertain the development of microstructure in such composites when carbon fibres possessing different surface energies (T-300, HM-35, P120 and Dialed 1370) and pitch matrices with different characteristics (Coal tar pitch $SP110^{\circ}C$ and mesophase pitch $SP285^{\circ}C$) are used as precursor materials. These composites were subjected to two different heat treatment temperatures of $1000^{\circ}C$ and $2600^{\circ}C$. Quite interesting changes in the crystalline parameters as well as the matrix microstructure are observed and attempt has been made to correlate these observations with the fibre matrix interactions.
Zn-Ion Coated Structural SiO2 Filled LDPE:Effects of Epoxy Resin Encapsulation
C. S. Reddy,C. K. Das,K. Agarwal,G. N. Mathur 한국고분자학회 2005 Macromolecular Research Vol.13 No.3
In the present work, a low-density polyethylene (LDPE) composite, filled with Zn-ion coated structural silica encapsulated with the diglycidyl ether of bisphenol-A (DGEBA), was synthesized using the conventional melt-blending technique in a sigma internal mixer. The catalytic activity of the Zn-ions (originating from the struc tural silica) towards the oxirane group (diglycidyl ether of bisphenol-A (DGEBA): encapsulating agent) was assessed by infrared spectroscopy. Two composites, each with a filler content of 2.5wt% were developed. The first one was obtained by melt blending the Zn-ion coated structural silica with LDPE in a co-rotating sigma internal mixer. The second one was obtained by melt blending the same LDPE, but with DGEBA encapsulated Zn-ion coated structural silica. Epoxy resin encapsulation of the Zn-ion coated structural silica resulted in its having good interfa cial adhesion and a homogeneous dispersion in the polymer matrix. Furthermore, the encapsulation of epoxy resin over the Zn-ion coated structural silica showed improvements in both the mechanical and thermal properties, viz. a 33% increase in the elastic modulus and a rise in the onset degradation temperature from 355 to 371oC, in compar ison to the Zn-ion coated structural silica.
Role of Interface on the Development of Microstructure in Carbon-Carbon Composites
Dhakate, S.R.,Mathur, R.B.,Dhami, T.L.,Chauhan, S.K. Korean Carbon Society 2002 Carbon Letters Vol.3 No.4
Microstructure plays an important role in controlling the fracture behaviour of carbon-carbon composites and hence their mechanical properties. In the present study effort was made to understand how the different interfaces (fiber/matrix interactions) influence the development of microstructure of the matrix as well as that of carbon fibers as the heat treatment temperature of the carbon-carbon composites is raised. Three different grades of PAN based carbon fibres were selected to offer different surface characteristics. It is observed that in case of high-strength carbon fiber based carbon-carbon composites, not only the matrix microstructure is different but the texture of carbon fiber changes from isotropic to anisotropic after HTT to $2600^{\circ}C$. However, in case of intermediate and high modulus carbon fiber based carbon-carbon composites, the carbon fiber texture remains nearly isotropic at $2600^{\circ}C$ because of relatively weak fiber-matrix interactions.