The Experts below are selected from a list of 88200 Experts worldwide ranked by ideXlab platform
Dongmei Zhu - One of the best experts on this subject based on the ideXlab platform.
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highly oriented flake carbonyl iron carbon fiber composite as thin thickness and wide bandwidth microwave absorber
Journal of Alloys and Compounds, 2018Co-Authors: Dandan Min, Fa Luo, Yuchang Qing, Wancheng Zhou, Dongmei ZhuAbstract:Abstract Oriented flake carbonyl iron/carbon fibers (FCI/CF) composite with enhanced microwave Absorption properties were prepared. The effects of orientation, the addition of CF, and the orientation direction of CF on the microwave Absorption properties were investigated, respectively. Compared with the un-oriented FCI composites, higher permeability was obtained due to the FCI orientation. Anisotropic electrical properties of FCI/CF composite were achieved by changing the arranged orientation of CF. A novel double-layer microwave absorber by combining FCI/CF absorbers with different orientation of CF was designed to achieve absorbers with wide Absorption band and optimal microwave Absorption. The calculated Absorption properties indicated that wider Absorption bandwidth and thinner thickness were obtained by the single-layer oriented composites, and the Absorption properties were further enhanced by the double-layer composites. The double-layer FCI/CF composites show a wider Absorption Frequency range of 12.0 GHz from 6 to 18 GHz with reflection loss (RL) below −5 dB at thickness of 0.8 mm.
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greatly enhanced microwave Absorption properties of highly oriented flake carbonyl iron epoxy resin composites under applied magnetic field
Journal of Materials Science, 2017Co-Authors: Dandan Min, Fa Luo, Yuchang Qing, Wancheng Zhou, Dongmei ZhuAbstract:Oriented flake carbonyl iron/epoxy resin (FCI/EP) composites with enhanced microwave Absorption properties were prepared by a magnetic field which was applied to make the plane of FCI parallel to each other. The morphology and the Frequency-dependent electromagnetic and microwave Absorption properties of the composites were investigated. The measurement results showed that the higher permeability and modest permittivity of the composites were obtained after orientation in the Frequency range of 2–18 GHz. The calculated Absorption properties indicated that the orientation plays an important role in decreasing the absorber thickness and broadening the Absorption bandwidths. The oriented FCI/EP composites containing 75 wt% FCI show a wider Absorption Frequency range of 12.5 GHz from 5.5 to 18 GHz with reflection loss below −10 dB at thickness of 1.4 mm, while the bandwidth of the un-oriented one is only in a narrow Frequency range of 1.4 GHz. This work offers a promising approach for the fabrication of magnetic absorbents for thin–thickness and microwave-absorbing materials with adjustable wider working frequencies range simply by magnetic field.
K P Mooley - One of the best experts on this subject based on the ideXlab platform.
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an early peak in the radio light curve of short duration gamma ray burst 200826a
Monthly Notices of the Royal Astronomical Society, 2021Co-Authors: L Rhodes, Rob Fender, D A Williams, K P MooleyAbstract:We present the results of radio observations from the eMERLIN telescope combined with X-ray data from Swift for the short-duration Gamma-ray burst (GRB) 200826A, located at a redshift of 0.71. The radio light curve shows evidence of a sharp rise, a peak around 4-5 days post-burst, followed by a relatively steep decline. We provide two possible interpretations based on the time at which the light curve reached its peak. (1) If the light curve peaks earlier, the peak is produced by the synchrotron self-Absorption Frequency moving through the radio band, resulting from the forward shock propagating into a wind medium and (2) if the light curve peaks later, the turn over in the light curve is caused by a jet break. In the former case, we find a minimum equipartition energy of ~3x10^47 erg and bulk Lorentz factor of ~5, while in the latter case we estimate the jet opening angle of ~9-16 degrees. Due to the lack of data, it is impossible to determine which is the correct interpretation, however, due to its relative simplicity and consistency with other multi-wavelength observations which hint at the possibility that GRB 200826A is in fact a long GRB, we prefer scenario one over scenario two.
Vahe Petrosian - One of the best experts on this subject based on the ideXlab platform.
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synchrotron radiation as the source of gamma ray burst spectra
The Astrophysical Journal, 2000Co-Authors: Nicole M Lloyd, Vahe PetrosianAbstract:We investigate synchrotron emission models as the source of gamma-ray burst (GRB) spectra. We show that including the possibility for synchrotron self-Absorption, a "smooth cutoff" to the electron energy distribution, and an anisotropic distribution for the electron pitch angles produces a whole range of low-energy spectral behavior. In addition, we show that the procedure of spectral fitting to GRB data over a finite bandwidth can introduce a spurious correlation between spectral parameters—in particular, the value of the peak of the νFν spectrum, Ep, and the low-energy photon spectral index α (the lower Ep is, the lower [softer] the fitted value of α will be). From this correlation and knowledge of the Ep distribution, we show how to derive the expected distribution of α. We show that optically thin synchrotron models with an isotropic electron pitch angle distribution can explain the distribution of α below α = - . This agreement is achieved if we relax the unrealistic assumption of the presence of a sharp low-energy cutoff in the spectrum of accelerated electrons and allow for a more gradual break. We show that this low-energy portion of the electron spectrum can be at most flat. We also show that optically thin synchrotron models with an anisotropic electron pitch angle distribution can explain all bursts with - α 0. The very few bursts with low-energy spectral indices that fall above α = 0 may be due to the presence of a synchrotron self-Absorption Frequency entering the lower end of the BATSE window. Our results also predict a particular relationship between α and Ep during the temporal evolution of a GRB. We give examples of spectral evolution in GRBs and discuss how its behavior is consistent with the above models.
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synchrotron radiation as the source of gamma ray burst spectra
The Astrophysical Journal, 2000Co-Authors: Nicole M Lloyd, Vahe PetrosianAbstract:We investigate synchrotron emission models as the source of gamma-ray burst (GRB) spectra. We show that including the possibility for synchrotron self-Absorption, a "smooth cutoff" to the electron energy distribution, and an anisotropic distribution for the electron pitch angles produces a whole range of low-energy spectral behavior. In addition, we show that the procedure of spectral fitting to GRB data over a finite bandwidth can introduce a spurious correlation between spectral parameters—in particular, the value of the peak of the νFν spectrum, Ep, and the low-energy photon spectral index α (the lower Ep is, the lower [softer] the fitted value of α will be). From this correlation and knowledge of the Ep distribution, we show how to derive the expected distribution of α. We show that optically thin synchrotron models with an isotropic electron pitch angle distribution can explain the distribution of α below α = - . This agreement is achieved if we relax the unrealistic assumption of the presence of a sharp low-energy cutoff in the spectrum of accelerated electrons and allow for a more gradual break. We show that this low-energy portion of the electron spectrum can be at most flat. We also show that optically thin synchrotron models with an anisotropic electron pitch angle distribution can explain all bursts with - α 0. The very few bursts with low-energy spectral indices that fall above α = 0 may be due to the presence of a synchrotron self-Absorption Frequency entering the lower end of the BATSE window. Our results also predict a particular relationship between α and Ep during the temporal evolution of a GRB. We give examples of spectral evolution in GRBs and discuss how its behavior is consistent with the above models.
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synchrotron radiation as the source of gamma ray burst spectra
arXiv: Astrophysics, 2000Co-Authors: Nicole M Lloyd, Vahe PetrosianAbstract:We investigate synchrotron emission models as the source of gamma-ray burst spectra. We show that including the possibility for synchrotron self-Absorption, a ``smooth cutoff'' to the electron energy distribution, and an anisotropic distribution for the electron pitch angles produces a whole range of low energy spectral behavior. In addition, we show that the procedure of spectral fitting to GRB data over a finite bandwidth can introduce a spurious correlation between spectral parameters - in particular, the value of the peak of the nu F_nu spectrum, E_p, and the low energy photon spectral index alpha (the lower E_p is, the lower (softer) the fitted value of alpha will be). From this correlation and knowledge of the E_p distribution, we show how to derive the expected distribution of alpha. We show that optically thin synchrotron models with an isotropic electron pitch angle distribution can explain the distribution of alpha below alpha=-2/3. This agreement is achieved if we relax the unrealistic assumption of the presence of a sharp low energy cutoff in the spectrum of accelerated electrons, and allow for a more gradual break. We show that this low energy portion of the electron spectrum can be at most flat. We also show that optically thin synchrotron models with an anisotropic electron pitch angle distribution can explain all bursts with -2/3 < alpha <= 0$. The very few bursts with low energy spectral indices that fall above alpha=0 may be due the presence of a the synchrotron self-Absorption Frequency entering the lower end of the BATSE window. Our results also predict a particular relationship between alpha and E_p during the temporal evolution of a GRB. We give examples of spectral evolution in GRBs and discuss how the behavior are consistent with the above models.
Anthony C Dona - One of the best experts on this subject based on the ideXlab platform.
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kinetics of starch hydrolysis and glucose mutarotation studied by nmr chemical exchange saturation transfer cest
Carbohydrate Polymers, 2011Co-Authors: Anthony C Dona, Guilhem Pages, Philip W KuchelAbstract:The –OH resonance on –C1 of the α and β-anomers of glucose that are normally invisible in 1H NMR spectra, was used to monitor the reaction kinetics of starch hydrolysis in aqueous solutions. The exchangeable protons on glucose were detected indirectly through their exchange with water protons, and hence the water resonance, by using selective radioFrequency (RF) saturation at the Absorption Frequency of the hydroxyl protons. The saturated population of –OH spins exchanged rapidly with the water proton population, leading to the partial suppression of the water signal. Mutarotation of the anomers of glucose and starch digestion kinetics measured this way were consistent with those readily obtained by other more direct, but less sensitive, NMR methods. The chemical exchange saturation transfer (CEST) technique is useful in favourable circumstances when signals from non-exchangeable protons are not resolved in a crowded region of the spectrum. Thus the 1H2O signal is so relatively large that its dynamic range is high and it gives greatly enhanced sensitivity for (indirectly) detecting much less abundant saccharide species.
Dandan Min - One of the best experts on this subject based on the ideXlab platform.
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highly oriented flake carbonyl iron carbon fiber composite as thin thickness and wide bandwidth microwave absorber
Journal of Alloys and Compounds, 2018Co-Authors: Dandan Min, Fa Luo, Yuchang Qing, Wancheng Zhou, Dongmei ZhuAbstract:Abstract Oriented flake carbonyl iron/carbon fibers (FCI/CF) composite with enhanced microwave Absorption properties were prepared. The effects of orientation, the addition of CF, and the orientation direction of CF on the microwave Absorption properties were investigated, respectively. Compared with the un-oriented FCI composites, higher permeability was obtained due to the FCI orientation. Anisotropic electrical properties of FCI/CF composite were achieved by changing the arranged orientation of CF. A novel double-layer microwave absorber by combining FCI/CF absorbers with different orientation of CF was designed to achieve absorbers with wide Absorption band and optimal microwave Absorption. The calculated Absorption properties indicated that wider Absorption bandwidth and thinner thickness were obtained by the single-layer oriented composites, and the Absorption properties were further enhanced by the double-layer composites. The double-layer FCI/CF composites show a wider Absorption Frequency range of 12.0 GHz from 6 to 18 GHz with reflection loss (RL) below −5 dB at thickness of 0.8 mm.
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greatly enhanced microwave Absorption properties of highly oriented flake carbonyl iron epoxy resin composites under applied magnetic field
Journal of Materials Science, 2017Co-Authors: Dandan Min, Fa Luo, Yuchang Qing, Wancheng Zhou, Dongmei ZhuAbstract:Oriented flake carbonyl iron/epoxy resin (FCI/EP) composites with enhanced microwave Absorption properties were prepared by a magnetic field which was applied to make the plane of FCI parallel to each other. The morphology and the Frequency-dependent electromagnetic and microwave Absorption properties of the composites were investigated. The measurement results showed that the higher permeability and modest permittivity of the composites were obtained after orientation in the Frequency range of 2–18 GHz. The calculated Absorption properties indicated that the orientation plays an important role in decreasing the absorber thickness and broadening the Absorption bandwidths. The oriented FCI/EP composites containing 75 wt% FCI show a wider Absorption Frequency range of 12.5 GHz from 5.5 to 18 GHz with reflection loss below −10 dB at thickness of 1.4 mm, while the bandwidth of the un-oriented one is only in a narrow Frequency range of 1.4 GHz. This work offers a promising approach for the fabrication of magnetic absorbents for thin–thickness and microwave-absorbing materials with adjustable wider working frequencies range simply by magnetic field.
