| 52781. |
- Mahendran, Srinivasan, 1989, et al.
(author)
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Quantitative Predictions of the Thermal Conductivity in Transition Metal Dichalcogenides: Impact of Point Defects in MoS 2 and WS 2 Monolayers
- 2024
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In: Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 128:4, s. 1709-1716
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Journal article (peer-reviewed)abstract
- Transition metal dichalcogenides are investigated for various applications at the nanoscale because of their unique combination of properties and dimensionality. For many of the anticipated applications, heat conduction plays an important role. At the same time, these materials often contain relatively large amounts of point defects. Here, we provide a systematic analysis of the impact of intrinsic and selected extrinsic defects on the lattice thermal conductivity of MoS2 and WS2 monolayers. We combine Boltzmann transport theory and Green’s function-based T-matrix approach for the calculation of scattering rates. The force constants for the defect configurations are obtained from density functional theory calculations via a regression approach, which allows us to sample a rather large number of defects at a moderate computational cost and to systematically enforce both the translational and rotational acoustic sum rules. The calculated lattice thermal conductivity is in quantitative agreement with the experimental data for heat transport and defect concentrations for both MoS2 and WS2. Crucially, this demonstrates that the strong deviation from a 1/T temperature dependence of the lattice thermal conductivity observed experimentally can be fully explained by the presence of point defects. We furthermore predict the scattering strengths of the intrinsic defects to decrease in the sequence VMo ≈ V2S= > V2S⊥ > VS > Sad in both materials, while the scattering rates for the extrinsic (adatom) defects decrease with increasing mass such that Liad > Naad > Kad. Compared with earlier work, we find that both intrinsic and extrinsic adatoms are relatively weak scatterers. We attribute this difference to the treatment of the translational and rotational acoustic sum rules, which, if not enforced, can lead to spurious contributions in the zero-frequency limit.
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| 52782. |
- Mahendran, Srinivasan, 1989, et al.
(author)
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The core structure of screw dislocations with [001] Burgers vector in Mg 2 SiO 4 olivine
- 2021
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In: Comptes Rendus Physique. - 1878-1535 .- 1631-0705. ; 22:S3
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Journal article (peer-reviewed)abstract
- In this study, we report atomistic calculations of the core structure of screw dislocations with [001] Burgers vector in Mg SiO olivine. Computations based on the THB1 empirical potential set for olivine show that the stable core configurations of the screw dislocations correspond to a dissociation in {110} planes involving collinear partial dislocations. As a consequence, glide appears to be favorable in {110} planes at low temperature. This study also highlights the difference between dislocation glide mechanism in {110} versus (010) or (100) for which glide is expected to occur through a locking-unlocking mechanism.
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| 52783. |
- Mahendran, Srinivasan, 1989, et al.
(author)
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The core structure of screw dislocations with [001] Burgers vector inMg(2)SiO(4) olivine
- 2021
-
In: Comptes Rendus Physique. - : Cellule MathDoc/CEDRAM. - 1878-1535 .- 1631-0705. ; 22, s. 7-18
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Journal article (peer-reviewed)abstract
- In this study, we report atomistic calculations of the core structure of screw dislocations with [001] Burgers vector in Mg2SiO4 olivine. Computations based on the THB1 empirical potential set for olivine show that the stable core configurations of the screw dislocations correspond to a dissociation in {110} planes involving collinear partial dislocations. As a consequence, glide appears to be favorable in {110} planes at low temperature. This study also highlights the difference between dislocation glide mechanism in {110} versus (010) or (100) for which glide is expected to occur through a locking-unlocking mechanism.
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| 52784. |
- Mahfouz, MHD Zaher, et al.
(author)
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Impact of Ultra-Narrowband Interference on Wi-Fi Links: An Experimental Study
- 2021
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In: IEEE Transactions on Wireless Communications. - 1558-2248 .- 1536-1276. ; 20:5, s. 3016-3030
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Journal article (peer-reviewed)abstract
- We develop a systematic methodology to experimentally investigate the impact of interference from a non-listen-before-talk ultra-narrowband (UNB) signaling technique on Wi-Fi links. The methodology is based on a worst-case interference scenario, and consists of three investigating steps. This methodology is then applied to a measurement setup to practically study the case of 100 bps UNB signals interfering with an IEEE 802.11n transmission in the 2.4GHz band. Five different Wi-Fi devices are tested. The UNB signal is generated in two modulation schemes, the on-off-keying (OOK) and the Gaussian minimum-shift-keying. Both single and multiple simultaneous UNB interferers are considered. An analysis of the measurement results shows that three of the tested Wi-Fi devices cannot coexist with the considered non-listen-before-talk UNB communication system. The throughput performance analysis of the other tested devices shows that the OOK-modulated UNB signal has the least interfering impact, and the Wi-Fi pilot subcarriers are the most vulnerable to UNB interference. However, if a single UNB interferer avoids these subcarriers and employs the OOK-modulation scheme, then wireless coexistence is possible as long as the signal-to-interference ratio of Wi-Fi to UNB is greater than 30 dB, given that a drop to 75% of the maximum Wi-Fi throughput is acceptable.
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| 52785. |
- Mahfouz, MHD Zaher, et al.
(author)
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Measurement and Estimation Methodology for EMC and OTA Testing in the VIRC
- 2023
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In: IEEE Transactions on Electromagnetic Compatibility. - 0018-9375 .- 1558-187X. ; 65:1, s. 3-16
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Journal article (peer-reviewed)abstract
- In recent years, promising results have been reported with the vibrating intrinsic reverberation chamber (VIRC) combining performance and cost-efficiency for electromagnetic compatibility (EMC) measurements. This also makes it a potentially attractive solution for over-the-air (OTA) testing, which is yet to be investigated. Therefore, this article proposes the first systematic and thorough methodology to characterize the VIRC for use in EMC and OTA testing of wireless baseband algorithms of narrow-band single-input single-output channels. This methodology has been developed to measure and estimate the channel first- and second-order temporal and spectral characteristics taking into consideration the effect of different carrier frequencies, rotational speeds of VIRC motors, and loading conditions. It is then applied to a channel setup inside a VIRC for a preliminary investigation before the VIRC itself is fully characterized. It is shown that mounting an absorber in a specific location on the hatch significantly improves the rejection rate of the chi-squared goodness-of-fit test for Rician distribution without increasing the K-factor above -10 dB over the frequency range 755-2740 MHz in the VIRC under investigation. However, the proposed methodology has been devised to be universal to any reverberation chamber, and the obtained results can be used to improve EMC testing due to the better understanding of the unique VIRC environment.
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| 52786. |
- Mahiey, S, et al.
(author)
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A Broadband Frequency Tripler for SIS Receivers
- 1998
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In: Ninth International Symposium on Space Terahertz Technology. ; , s. 481-491
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Conference paper (peer-reviewed)abstract
- Considerable success has recently been gained in the design and production of high powered frequency triplers incorporating Schottky varactor diodes. The design approach that was used for this goal has now been turned to the design of fixed tuned, broadband frequency triplers specifically intented for use in SIS receivers.The tripler reported here makes the use of a Heterostructure Barrier Varactor or HBV. A waveguide circuit has been designed that provides more than 50µW of output power for an input power of 10mW over a frequency range of 250 - 300GHz completely fixed tuned and without the need for bias.Further modification of the waveguide circuit should extend the useful fixed tuned bandwidth to greater than 100GHz centered at 300GHz.
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| 52787. |
- Mahlin, D., et al.
(author)
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Influence of polymer molecular weight on the solid-state structure of PEG/monoolein mixtures.
- 2005
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In: Polymer. - : Elsevier BV. - 0032-3861. ; 46:26, s. 12210-12217
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Journal article (peer-reviewed)abstract
- The polar lipid monoolein (MO) and poly(ethylene glycol), PEG, of different molar mass (1500, 4000 and 8000) were melted, mixed and left to solidify at room temperature. Analysis of the solid mixtures by differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) revealed that a phase separation occurs when MO is present in sufficient amounts. The molecular weight of the polymer determines the amount of MO that has to be added before a separate MO phase can be detected. To further understand this behaviour, the folding of the polymers and the thickness of the amorphous domains within the lamellar structure of PEG were determined by calculation of the one-dimensional correlation function from the experimental SAXS data. It revealed that the presence of MO makes the crystalline domains of PEG 1500, which crystallizes unfolded, increase at the expense of the amorphous domains. PEG 4000 and PEG 8000 obtain a higher degree of folding when the MO content in the mixtures increases. Furthermore, a second form of MO was detected when it phase separated from PEG 1500 and 4000. This behaviour was argued to be due to the secondary crystallization of the PEGs.
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| 52788. |
- Mahlin, Denny, et al.
(author)
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The influence of PVP incorporation on moisture-induced surface crystallization of amorphous spray-dried lactose particles
- 2006
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In: International Journal of Pharmaceutics. - : Elsevier BV. - 0378-5173 .- 1873-3476. ; 321:1-2, s. 78-85
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Journal article (peer-reviewed)abstract
- We have recently shown that atomic force microscopy (AFM) may be an appropriate method for characterisation of the re-crystallization of amorphous particles. In this study, spray-dried composite particles consisting of lactose and polyvinyl pyrrolidon (PVP) were characterised by AFM and electron spectroscopy for chemical analysis (ESCA), and their response on increasing the relative humidity (RH) was investigated. The PVP content in the particles used was 0, 5 or 25 wt.% of either PVP K 17 or PVP K90. All composite particles were found to be enriched with PVP at the surface. The incorporation of PVP in the particles influenced the way the particles responded to an increase in RH. The specific RH interval in which the surface of the particles smoothened and the RH where crystallization could be detected, increased with an increase in the amount and molecular weight of the PVP in the particles. The crystallization kinetics of single particles was analysed with AFM and by utilising the JMAK equation. The rate constant for this transformation increased in an exponential manner with increasing RH. Furthermore, above the RH needed for the crystallization to occur, the exponential increase in the crystallization rate was larger for particles with higher polymer content which indicates that the stabilising effect decreases as the water content in the particles becomes higher. In this study we report a method for determination of crystallization kinetics on single composite particles, which is valuable when evaluating the effect of stabilisers in amorphous powders.
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| 52789. |
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| 52790. |
- Mahmood, Ansar, 1975, et al.
(author)
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Collisionless trapped electron and ion temperature gradient modes in an advanced tokamak equilibrium
- 2009
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In: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 16:2, s. 022503-
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Journal article (peer-reviewed)abstract
- The linear stability of coupled collisionless trapped electron (TE) and ion temperature gradient (ITG) modes is investigated in an International Thermonuclear Experimental Reactor [C. Gormezano , Nucl. Fusion 47, S285 (2007)]-like magnetic field configuration. An advanced fluid model in the electrostatic limit and the ballooning mode formalism are used to derive an eigenvalue equation. The growth rates and real frequencies of the most unstable modes and their eigenfunctions are calculated. The coexistence of unstable ITG and TE modes and their relative strengths with respect to the fraction of trapped electrons, wave vector, and local plasma parameters, such as density gradient, electron/ion temperature gradient, and ion to electron temperature ratio are analyzed. The influence of geometrical factors, such as local/global magnetic shear and magnetic field curvature on both ITG and TE modes is also discussed.
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